In this paper, Al2O3/7075 composites were prepared by mechanical alloying with subsequent hot-pressing sintering, and the effect of Al2O3 nanoparticle on the mechanical and tribological behavior of 7075 was studied. The mechanical property results showed that the hardness and compressive strength of Al2O3/7075 composites first increased and then decreased with Al2O3 amount increasing, and 5 wt.% Al2O3 addition made the material exhibit excellent comprehensive mechanical properties. The tribological properties also indicated that 5 wt.% Al2O3 nanoparticle significantly improved the high-temperature wear resistance of 7075 alloys. Thus, all the mechanical and tribological results confirmed that the addition of Al2O3 nanoparticle was a better strengthening way for 7075 alloys at high temperatures.
A numerical analysis is carried out in order to investigate the effect of the electric field on the performance characteristics of a three-pad hydrostatic squeeze film damper compensated with new electrorheological valve restrictors. The bearing design is composed of three identical hydrostatic bearing pads. Each hydrostatic bearing pad is fed with a negative electrorheological fluid through an electrorheological valve. The numerical analysis showed that the viscosity of a smart fluid inside to each electrorheological valve can be controlled by using an electric field in order to control the static and dynamic characteristics. The results presented in this study can be made more useful to control rotor vibrations and force transmissibility especially around the critical speeds.
The present paper investigates the turbulence effect on the steady-state performance of a new variety of journal bearing, i.e. the noncircular floating ring bearing. This particular bearing consists of the journal, floating ring, as well as lower and upper lobes. The shaft and the floating ring are cylindrical while surfaces of the bearing are noncircular. The classical Navier–Stokes equations and continuity equation in cylindrical coordinates are being satisfactorily adapted with the linearized turbulent lubrication model of Ng and Pan. These improved equations are being solved by the finite element method using Galerkin’s technique and an appropriate iteration strategy. The proposed bearing has a length-to-diameter ratio of 1 and operates over different values of the ratio of clearances (i.e. 0.70 and 1.30). The steady-state performance parameters computed are presented in terms of an inner and outer film eccentricity ratios, load-carrying capacity, attitude angle, speed ratio, friction coefficient variable, oil flow, and temperature rise variable for the Reynolds number up to 9000. The present analysis predicts better performance in the turbulent regime as compared to the laminar regime for the noncircular floating ring bearing.
Fretting wear is a quasi-static process in which repeated relative surface movement of components results in wear and fatigue. Fretting wear is quite significant in the case of spline couplings which are frequently used in the aircraft industry to transfer torque and power. Fretting wear depends on materials, pressure distribution, torque, rotational speeds, lubrication, surface finish, misalignment between spline shafts, etc. The presence of so many factors makes it difficult to conduct experiments for better models of fretting wear and it is the case whenever a mathematical model is sought from experimental data which is prone to noisy measurements, outliers and redundant variables. This work develops a principal component analysis based method, using a criterion which is insensitive to outliers, to realize a better design and interpret experiments on fretting wear. The proposed method can be extended to other cases too.
The metal O-ring is effectively applied to extreme conditions and long-term uses for several decades, where conventional seals cannot meet the requirements. During the long-time service, the stress relaxation of the metal O-ring has been confirmed to be a potential damage to the sealing performance. By analyzing the sealing mechanism and the problem of the stress relaxation of the metal O-ring, a model has been developed to study its stress relaxation. The model includes the contact model, the stress–strain model, and the dynamic equation of stress relaxation. Based on the classical elastic-plastic model for two contact rough surfaces, the contact model of sealing zone is developed, by which the mechanical and geometric parameters of the sealing zone can be obtained for the initial state after the assembling. In the stress–strain model, the determination of stress and strain in the metal O-ring is performed by means of a simplified closed-form solution. The dynamic equation of stress relaxation using the stress-creep rate equation is developed, and is solved by combination of the contact model and the stress–strain model. The equation can be adopted for stress relaxation analysis of the metal O-ring at any time. After a validation of the model by comparison with experiments, a parametric study is presented. Simulation results corresponding with the mathematical models explain how the stress relaxation characteristic of the metal O-ring is influenced by a number of parameters.
The most common failure modes for guideway are wear and contact fatigue, which are significantly influenced by the friction properties of the contact surfaces. In this paper, three-dimensional surface parameters of surface roughness are investigated to evaluate guideway surface. First, an effective three-dimensional surface model is achieved using wavelet transform method and a reverse engineering software (Geomagic Studio). Secondly, effects of the functional surface parameters on friction force, mean pressure, and friction coefficient are studied using the computational fluid dynamics simulation method, and a regression model to predict friction force is achieved. Thirdly, the optimal surface parameters combinations are analyzed looking at friction property index, and the simulation results are compared. Finally, a verification test is conducted to detect the validity of the simulation research. The results show a good agreement between the experimental results and simulations. This study provides theoretical guidance for the manufacture of guideway.
The cylinder liner surface finish, which is commonly produced using the honing technique, is an essential factor of engine performance. The characteristics of the texture features, including the cross-hatch angle, the plateau roughness and the groove depth, significantly affect the performance of the ring pack–cylinder liner system. However, due to the influence of the honed texture features, the surface roughness of the liner is not subject to Gaussian distribution. To simulate the mixed lubrication performance of the ring–liner system with non-Gaussian roughness, the combination of a two-scale homogenization technique and a deterministic asperities contact method is adopted. In this study, a one-dimensional homogenized mixed lubrication model is established to study the influence of groove parameters on the load-carrying capacity and the frictional performance of the piston ring–liner system. The ring profile, plateau roughness, and operating conditions are taken into consideration. The main findings are that for nonflat ring, shallow and wide groove textures are beneficial for friction reduction, and there exists an optimum groove density that makes the friction minimum; for flat ring, wide and sparse grooves help improving the tribological performance, and there exists an optimum groove depth that makes the friction minimum.
Leakage at the piston/cylinder interface of a high-pressure fuel pump for diesel engines becomes more severe due to the increase in delivery pressure. Therefore, a thermal fluid–structure interaction model that can simulate the complex phenomena that take place at the interface is presented in this paper. In the model, the nonisothermal flow, the physical properties of the fluid such as dynamic viscosity and density versus pressure and temperature relationships, the coupled heat transfer between the fluid and structure as well as thermal and pressure-induced elastic deformations of the structure are considered. The calculated leakage rates from the model show good agreement with the experimental results. The impacts of pressure-induced and thermal elastic deformations of the structure on the leakage are discussed. A new direction for reducing the interface leakage is proposed.
In this study, the friction and wear behavior of NiAl alloy containing graphene nanoplatelets (NG) under different sliding velocities were investigated. NG shows the better tribological performance, if compared to NiAl-based alloy without graphene nanoplatelets (NA). In the range of sliding velocities varying from 0.2 to 1.2 m/s, the formation of glaze layer in NG could be easily distinguished. The beneficial effect of graphene nanoplatelets (GNPs) for the formation of glaze layer of NG is obvious, if compared to NA, resulting in the reduction of friction and improvement of wear resistance of NG. Moreover, a suitable higher sliding velocity will be better for the formation of glaze layer, but the wear rate will gradually increase with the increase of sliding velocity. It can be concluded that GNPs hold great application prospect as an effective solid lubricant to improve the tribological performance of NiAl alloys.
We studied the factors that affect the sliding-induced wear of polyamide 66 (PA66) composites. The dimensionless severity parameter Ssc was defined from the minimum equivalent stress in the sliding contact area and calculated by using approximations. Subsequently, we conducted the friction tests for the PA66 composites filled with hard fillers under dry conditions. The hard fillers included rice bran ceramics particles, glass beads, carbon fibers, and glass fibers. According to the relationship between the wear rate and the severity parameter, the specific wear rate increased exponentially with increasing Ssc for all the materials. Obtaining low-wear PA66 required improving the yield strength and reducing the Young’s modulus and the friction coefficient.
Four finite element models of a slewing bearing with different supporting structures are established by the finite element software ANSYS. In these models, the ball–race contact is simplified by the spring element with the same contact stiffness. The nut and bolt head are simplified and coupled with the surface of other connected component. The preload is applied on the bolt by a few pretension elements. The ball–race contact forces, fatigue lives, and carrying capacities of the slewing bearing in the four models are calculated. The effects of supporting stiffness, bolt number, and bolt preload on the contact force and fatigue life of the slewing bearing are studied. Moreover, the effects of supporting stiffness, bolt number, bolt preload, ball–race contact truncation, and bolt–hole backlash on the carrying capacities of the slewing bearing are analyzed. Results show that the fatigue life and carrying capacity of the slewing bearing can be enhanced by appropriately decreasing the supporting stiffness. Optimal values of bolt preload and number for the fatigue life and carrying capacity of a specific slewing bearing are obtained.
Applying cutting tool with longer functioning time is a vital issue in machining of the nickel-based super alloys. However, the experimental analysis of this problem is quite expensive. Thus, three-dimensional numerical simulation of tool wear propagation in turning of Inconel 625 super alloy is taken into account, in this study. The cutting insert with complex geometry is modeled by using a reverse engineering method. Based on the cutting tool and workpiece material, Usui wear rate model is exerted to estimate the tool wear rate. In the first section, characterization of TiAlN-coated carbide tool, which is suggested by catalogue, on wear resistance is evaluated and then simulation results are validated with experiments. As a result, increment of depth of cut is the most effective factor on the generation of temperature and stresses on the tool faces resulting in wear rate acceleration. In the second section, different commercial coatings with multicompositions are applied in the simulation to find the best performance against wear. Finally, TiCN coating outperformed other coatings in turning of Inconel 625.
This study concerns with the theoretical investigation to show the influence of recess geometry and method of compensation on the performance of recessed hydrostatic circular thrust pad bearing operating with couple stress lubricant. Modified Reynolds equation for couple stress lubricant has been derived using Stokes microcontinuum theory. Stokes theory describes the behavior of fluids consisting of long chain polymer additives having sizes comparable to that of the fluid film thickness. Finite element method technique and commercially available software MATLAB 2013 have been used to solve the modified Reynolds equation and, thus, performance characteristics of bearing have been computed. The performance of capillary- and orifice-compensated hydrostatic thrust pad bearings performance have been compared vis-à-vis to different geometric shapes of recess. Each recess shape has identical ratio of bearing pad area to recess area. It has been observed that the couple stress additives have significant positive influence on the static and dynamic performance of the compensated hydrostatic thrust pad bearing provided that restrictors operates with lower values of the restrictor deign parameter
The performance of a noncircular cylindrical floating ring bearing under the laminar flow condition has been analyzed. The noncircular cylindrical floating ring bearing is a modified version of the plain cylindrical floating ring bearing in which the outer bearing of the cylindrical floating ring bearing is changed in the form of lobed pattern. The noncircular patterns are known for its better stability and stiffness behavior according to the magnitude of applied loads. The finite element analysis has been used to solve the classical Navier–Stokes equations in the cylindrical coordinates of the modified form along with the continuity equation that represents the fluid flow field in the clearance space of the noncircular cylindrical floating ring bearing. The numerical data are for a proposed bearing with the length to diameter ratio of 1.0, and values of the ratio of film clearances of 0.70 and 1.30. Performance characteristics of a finite noncircular cylindrical floating ring bearing has been studied in terms of the inner and outer film eccentricity ratio, Sommerfeld number, attitude angle, stiffness and damping coefficients, equivalent support stiffness coefficient, whirl frequency ratio, and critical journal mass at various values of the outer film eccentricity ratios. The proposed analysis disseminates that the dynamic performance parameters of the noncircular cylindrical floating ring bearing are much better than the plain cylindrical floating ring bearing.
To further improve the efficiency of machine components found in automotive engine systems it is important to understand the friction generation in these components. Modelling and simulation of these components are crucial parts of the development process. Accurate simulation of the friction generated in these machine components is, amongst other things, dependent on realistic lubricant rheology and lubricant properties, where especially the latter may change during ageing of the lubricant. Many modern heavy-duty diesel engines are in operation for several hundred hours before the engine oil is changed. In this work, two engine oils, one 10 W-30 and one 5 W-20, have been aged in full heavy-duty diesel engine bench tests for 400 and 470 hours respectively. This roughly corresponds to the amount of ageing these oils are subjected to between oil drains in field conditions. The aged oils were subjected to a number of oil analyses showing, among other things, a maximum increase in oil viscosity of 12.9% for the 5 W-20 oil and 5.5% for the 10 W-30 oil, which is most likely primarily an effect of evaporation and oxidation. The aged oils were tested in a ball-on-disc test rig under elastohydrodynamic conditions where friction was measured and the performance was compared to fresh samples of the same oils. The results show that there is almost no difference in elastohydrodynamic friction when comparing the aged oils with the fresh oils. These results indicate that it is not necessary to include oil ageing in numerical elastohydrodynamic friction models as long as the oil is changed before the ageing has reached a critical level.
Squeeze film dampers are widely applied in high rotating speed machinery to reduce vibration, add damping and increase the stability of the system. The traditional method of squeeze film dampers analysis usually involves a complicated calculation program that is limited to a simplified physical model. In this study, the feasibility of applying commercial code ANSYS-FLUENT to study the characteristics of the squeeze film damper is investigated, where the dynamic mesh method is employed. The Zwart–Gerber–Belamri model is applied to simulate the cavitation phenomenon in squeeze film damper. By numerical simulation, the dynamic pressure distribution and oil film force are obtained and then the damping coefficients are acquired. The effect of the cavitation model on the damping coefficients and inertia coefficients is obtained. The parametric study including the squeeze film damper clearance, length and whirl frequency is carried out. The result indicates that the use of the cavitation model will reduce the amplitude of the dynamic oil film force and the hysteresis phenomenon can be observed. It leaves room for further improvement on this method.
Surface texture technology is a useful method for controlling the wettability. In this paper, ultrasonic-assisted face turning is implemented to create a microscaled texture on the flat faces. The effect of process parameters on the wettability of untextured and textured patterns is investigated by the full factorial method. Experimental results showed that the ultrasonic-assisted face turning is an effective method to create microdimples (periodic ripples) on the flat faces. Considered factors are turning (cutting speed, feed) and ultrasonic (vibration direction and vibration amplitude) parameters, which control the configuration and dimensions of microstructures. Surfaces machined by ultrasonic-assisted face turning have more liquid–solid interfacial areas compared with those processed by conventional turning. By changing the process parameters, expansion of microdimples in the cutting direction changes, hence the ratio between the interfacial and projected area and adsorption between the surface and water increases. So, textured surfaces by the ultrasonic-assisted face turning have better wettability performance in comparison to the untextured surfaces.
Due to the increasing requirements in the aerospace and automotive industries, engine and structural parts must also have an increasing life. This leads to an improvement in the fretting fatigue strength of aluminum alloys over 107 cycles, and sometimes even more. Fretting is a surface degradation process as a result of chemical and mechanical attack between two contacting surfaces by small amplitude oscillatory movement, which is closely related to fatigue, wear, and corrosion. The surface treatments and coatings are expected as a suitable strategy for fretting damage. In this research work, two types of thin solid films TiCr/TiN/CrN (three layers) and Ti/Cr/TiN/Cr/CrN/TiCrN (six layers) were coated onto Al7075-T6 alloy using the physical vapor deposition magnetron sputter technique to enhance the fretting fatigue lives. The result shows that the fretting fatigue lives of three layers deposited substrates were improved at high and low cyclic fatigue of 101% and 34%, respectively. Fretting fatigue lives of the six layers deposited specimens were developed at high and low bending stresses (high and low cyclic fatigue) of 121% and 55%, respectively.
A steady-state performance analysis of two-lobe oil journal bearing has been carried out in this paper. A computer program has been prepared to calculate the steady-state hydrodynamic characteristics. To study the steady-state characteristics of two-lobe bearing, the nondimensional Reynolds’ equation is solved to obtain nondimensional values of load-carrying capacity, flow rate, attitude angle, and friction variable. Parameters of the design are studied for different groove configurations and an optimized groove location is obtained for two-lobe bearing is obtained using genetic algorithm. It has been found that placing the groove on different locations have greater influence on the design parameters such as the nondimensional oil flow rate, frictional variable, and nondimensional load-carrying capacity. From numerical simulation of two-lobe bearing, it has been found that in case of this bearing parameters shows optimum values when the grooves are arranged below the horizontal axis. There is an improvement in nondimensional values of friction variable, nondimensional flow rate, and nondimensional load-carrying capacity at optimum location of the groove. The analysis is carried out for an ellipticity ratio of 0.5 and the results obtained have been reported.
This paper presents a group of elastic Hydrodynamic lubrication (EHL) analyses to evaluate the effects of micro-bottom shape on the performance of misaligned herringbone grooved axial piston bearing. Due to the time-consuming elastic Hydrodynamic lubrication (EHL) tasks introduced in herringbone grooves, complex bottom shape, rotation combined axial movement, and misalignments require high-speed computation. So a new parallel elastic Hydrodynamic lubrication (EHL) numerical algorithm is proposed, which exhibits higher computing speed than the normal successive over relaxation (SOR) method. The results suggest the bottom shapes of herringbone grooves significantly affect the lubrication performance of misaligned herringbone grooved axial piston bearing. Rectangle and left triangle bottom shapes involve a micro-step bearing effect which can significantly improve the lubrication performance, comparing to semiellipse, isosceles triangle, and right triangle bottom shapes.
This paper dealt with the fatigue life of cylindrical roller bearings with several significant error sources that may occur during installations. A four degree-of-freedom quasi-static model for cylindrical roller bearings was developed, which took into account potential error sources such as angular misalignment, axial offset, and radial clearance, together with inertial loading by rotational speed and induced moment loads. A 3D contact model was employed to provide contact pressure distributions in rolling elements. The fatigue life of a cylindrical roller bearing was analyzed as a function of angular misalignment under various loading conditions. Then, the fatigue life analysis was extended to the combined effects of radial clearance, axial offset, and the number of rollers, along with angular misalignment. The computational results showed the significance of each error source on fatigue life. They further showed that cylindrical roller bearing fatigue life maximized when the radial clearances were slightly negative, and that it increased almost linearly with the number of rollers.
An elastohydrodynamic lubrication model is proposed for line contacts under pressurized ambient conditions often encountered in hydraulic pumps, submarine machinery and many other submerged systems. It has been demonstrated that the film forming behavior under such conditions is essentially different from that in conventional elastohydrodynamic lubrication contacts. The numerical simulation results are regressed to develop new central and minimum film thickness equations for Newtonian fluids as functions of ambient pressure, speed, load, and material parameters. An alternative approach is also discussed which involves the use of existing film thickness formulas with ambient viscosity and pressure–viscosity coefficient pertaining to the desired pressure range. A film thickness enhancement of more than 100% over conventional elastohydrodynamic lubrication case is observed. This enhancement is shown to be highly sensitive to the pressure–viscosity coefficient. Besides, the effect of shear-thinning behavior is also investigated and it is found to lower the film thickness enhancement, especially at high ambient pressures.
Leakage clearance between the finger seal and the rotor is influenced by the seal wear, and a large leakage clearance may cause sealing failure. The aim of this study is to develop a predictive method to simulate the progressive wear of C/C composite finger seal. A finite element model is presented for the finger seal dynamic motion. A wear model is built by calculating the local wear rate for the nodes on finger foot bottom and by introducing an Arbitrary Lagrangian Eulerian adaptive meshing strategy. Progressive wear analysis is implemented by integrating the finger dynamic motion and the wear model. The results show that the contact pressure of finger seal is decreased and the leakage clearance is increased with operating time due to progressive wear. Compared to the case that the non-woven cloth is parallel to rotor surface (O1) and the case that the non-woven cloth is perpendicular to rotor surface but the laminated direction is perpendicular to sliding direction (O2), low leakage and low wear are obtained when the non-woven cloth is perpendicular to rotor surface and the laminated direction is the same as sliding direction (O3). In addition, the wear depth of finger foot and the leakage clearance between the finger seal and rotor are both increased with pressure differential and rotor speed.
This article describes the development of unidirectional and multidirectional laminated composites consisting of thermoplastic epoxy resin reinforced with glass/carbon fiber, and studies their solid particle erosion behavior under different operating conditions. The erosion rates of the unidirectional carbon fiber/epoxy composites [0°, 30°, 45°, 60°, 90°] and multidirectional glass fiber/epoxy composites ([0°/–90°/0°], [30°/–60°/30°], [45°/–45°/45°], [60°/–30°/60°], [90°/0°/90°]) were especially scrutinized based on their respective fiber orientations. In addition; the test specimens were evaluated at three different impingement angles of 30°, 60°, and 90° with an impact velocity of 34 m/s. Slightly rounded and irregular Al2O3 particles with an average diameter of 400 µm were used. An optimal fiber orientation combination was determined, which led to minimization of the erosion rate. Moreover, the variation of erosion rates with various laminate orientations were characterized by using X-ray diffraction patterns, 3D digital mapping method, and scanning electron microscopy.
The Inductive Wear Debris Sensor is a relatively new invention that is increasingly being used for the detection of incipient machinery damage or failures by sensing metallic debris in lubrication systems. This type of sensor is typically used in-line and has a superior particle size detection range compared to traditional techniques such as the ubiquitous spectrometric oil analysis. There is, however, very little in the literature regarding the application and interpretation of data arising from this type of sensor. Unlike other condition monitoring sensors, no data will be generated by an Inductive Wear Debris Sensor in an ideal system; however, in real applications it is necessary to discriminate between occasional particles unrelated to a failure and incipient failure particles. Inductive Wear Debris Sensor data could be misinterpreted if a simple cumulative count limit was applied to the data. A short-term rate of particle generation is sometimes used as an alternative; however, it too can be misleading with short succession particles producing high instantaneous rates possibly causing false alarms. The purpose of this work was to develop a robust metric (or group of metrics) that when applied to Inductive Wear Debris Sensor data would reliably identify a failure event and exclude non-failure related particles. The Health Indicator described herein consists of three subordinate Condition Indices that collectively are shown to reliably detect the onset of rolling contact fatigue. The metrics have been applied to bearing test rig data (seeded fault) and data obtained from a non-seeded fault test of a complex helicopter gearbox.
This paper presents the method of plasticity index calculation of isotropic two-process surfaces on the basis of index developed by Greenwood and Williamson for random surfaces of Gaussian ordinate distribution. This method was applied to computer generated isotropic two-process textures. The proposed plasticity index of two-process surface was compared with that of plateau surface and of the whole surface calculated by traditional manner. It was shown that plasticity index and generally the contact properties of two-process surfaces depended not only on plateau but also in a smaller degree on valley roughness. The method of estimation of summit radius of plateau surface from the analysis of two-process surface was presented too.
The article contains mathematical models of Reynolds equation with the effects of hydrophobicity of surface and magnetic field. The first section provides a new mathematical model of the solution of the generalized Reynolds equation and its application for a hydrophilic surface. It also derives a new boundary condition for the contact of a flowing liquid with a hydrophobic surface. This wettability condition is defined in dependence on the adhesion coefficient k. The second part presents mathematical models of Reynolds equation including the effect of hydrophobia and magnetic field. For all problems, the solutions are shown and the definitions of the stiffness and damping matrices of the liquid layer are outlined. From the results, it can be deduced that hydrophobic surface significantly affects the velocity profile of the liquid. It leads to a higher effect of the Lorentz force and thus of the magnetic field in comparison with a hydrophilic surface of the bearing lining.
Surface topography is important as it influences contact load-carrying capacity and operational efficiency through generated friction, as well as wear. As a result, a plethora of machining processes and surface finishing techniques have been developed. These processes yield topographies, which are often non-Gaussian, with roughness parameters that alter hierarchically according to their interaction heights. They are also subject to change through processes of rapid initial running-in wear as well as any subsequent gradual wear and embedding. The stochastic nature of the topography makes for complexity of contact mechanics of rough surfaces, which was first addressed by the pioneering work of Greenwood and Williamson, which among other issues is commemorated by this contribution. It is shown that their seminal contribution, based on idealised Gaussian topography and mean representation of asperity geometry should be extended for practical applications where surfaces are often non-Gaussian, requiring the inclusion of surface-specific data which also evolve through process of wear. The paper highlights a process dealing with practical engineering surfaces from laboratory-based testing using a sliding tribometer to accelerated fired engine testing for high performance applications of cross-hatched honed cylinder liners. Such an approach has not hitherto been reported in literature.
Cam–follower kinematic pairs in the valve trains of engines usually operate in the mixed lubrication regime and work under conditions of wear. The wear mode of cams in mixed lubricated contacts is mild, and the wear value is usually calculated using Archard’s equation in which the wear coefficient is closely related to the lubrication condition. In this study, a nonlinear relationship between the wear coefficient and lubricant oil film thickness under mixed lubrication is proposed with consideration of the rough height distribution of the contact surface. As the cam and flat tappet pair of the valve train in a diesel engine is concerned, the wear coefficient of the cam material for boundary lubrication conditions is measured. After calculating the lubricant oil thickness, the wear depths at different points of the cam are simulated, and results are consistent with the measured values in the practical test.
In this experimental investigation, new sequence of Cu-X wt% SiC (X = 0, 5, 10, and 15) -Y wt% Gr (Y = 5 and 10) metal matrix hybrid composites was effectively manufactured by solid-state mixing (powder metallurgy) technique through a novel microwave sintering process. The influence of hard ceramic reinforcement SiC content and soft self-lubricating reinforcement Gr content on the mechanical and tribological properties of copper hybrid composites was analysed. Using the pin-on-disc tribometer the results of the wear resistance and sliding friction properties of all produced microwave-sintered composites were assessed. Weight percentages of SiC, applied load, sliding distance, and sliding speed are the testing parameters considered for tribological characterization. During dry sliding, the loss in mass was recorded as wear loss. It was observed that with an increase in the SiC weight percentage, the wear resistance enhanced monotonically with hardness. The microwave-sintered cylindrical composites have been characterized by X-ray diffraction. Optical microstructural studies confirmed the distribution of both the reinforcements was homogeneous on the base matrix copper. The tribological performance of the hybrid composite containing 10 wt% graphite and 15 wt% SiC showed better to that of the other manufactured composites. The kind of wear mechanisms was recognized through morphology of the worn-out surfaces and the wear debris by scanning electron microscope. This experimental analysis revealed that the hybridization of both hard reinforcement like SiC and soft reinforcement like Gr get better wear resistance of copper composites considerably through microwave sintering processes.
Flexible risers are used in the offshore oil industry for exporting hydrocarbons from subsea equipment to floating production and storage vessels. The latest research in unbonded flexible pipes aims to reduce weight by replacing metal components with composite materials. This would result in lighter and stiffer flexible risers, which would be well suited for ultra deep water applications. This paper develops a new finite element model used for evaluating the efficiency of anchoring flat unidirectional fiber reinforced tendons in a mechanical grip. It consists two flat grips with the fiber reinforced tendon in between. The grips are pressed against the composite and the pullout force is ensured through friction. The novelty of the paper is represented by the detailed investigation of the influence between the coefficient of friction and the pullout force. By comparing numerical and experimentally obtained results, it is possible to show the importance of friction decay in the grip. Improper contact between the grips and composite is also taken into account and leads to good agreement between numerical and experimental results. This study shows how to avoid over-estimating the efficiency of such grip by using dry friction in finite element models.
Aluminum metal matrix composites have found applications in manufacturing of various components such as piston, cylinder block, and brake drum, in which wear and friction are important phenomenon. This paper presents an overview of diversified reinforcement on aluminum metal matrix composites in terms of tribological aspects. A comprehensive literature review is carried out on aluminum metal matrix composites based on individual reinforcement and multiple reinforcements including various product applications. The research review is summarized in form of tribology wheel for aluminum metal matrix composites, which encompass fields involved in tribology, fabrication processes/parameters, reinforcement(s) and matrix contribution, tribological testing parameter, statistical analytical technique, and product application areas of AMCs. The tribology wheel becomes helpful in selecting parameters with possible permutation and combination for further improvement of tribological properties. Finally, comments are addressed regarding the future work on tribological aspects of aluminum metal matrix composites.
A momentary contact of asperities will generate transient flash temperature phenomena in boundary lubrication. According to the viscosity–temperature characteristics of lubricant, the inhomogeneous distribution of temperature during the process of asperity contact would cause the inhomogeneous distribution of oil viscosity. This paper proposes a coupled Eulerian–Lagrangian based approach to analyze the influence of inhomogeneous distribution of viscosity on the friction coefficient of an asperity junction. The asperity interaction in boundary lubrication with different sliding velocities and different degrees of overlap of the undeformed surfaces were taken into account. Simulation results showed that the friction coefficient is proportional to the overlap and inversely proportional to sliding velocity. The results also showed that the influence of inhomogeneous distribution of oil viscosity on friction coefficient in boundary lubrication is limited.
Schiff base derived from condensation of 4-aminotriazole with indole-3-carboxylaldehyde has been characterized by Fourier-transform infrared and 1H NMR spectroscopic techniques. The tribological properties of synthesized Schiff base have been evaluated using four-ball tester at optimized concentration (1% w/v) and compared with its individual components. The tribological testing has been performed by varying load for 30 min duration and varying test durations at 392 N load. The synergistic action of Schiff base with commercial borate ester effectively enhances the antiwear properties of base oil and its load-carrying capacity. Pronounced reduction in the values of mean wear scar diameter, friction coefficient (µ), mean wear volume (MWV), and wear rates confirm the synergistic interaction between Schiff base and borate ester. The best tribological properties are shown by synergistic mixture, followed by Schiff base, then indole-3-carbaldehyde, and finally 4-aminotriazole. The surface topography of worn surfaces has been studied by scanning electron microscopy and contact mode atomic force microscopy which show the drastic decrease in surface roughness in the presence of synergistic mixture. The energy-dispersive X-ray analysis of worn surface lubricated with synergistic formulation exhibits nitrogen, carbon, boron, and oxygen indicating the adsorption of additive on the metal surface to form in situ protective tribofilms, which prevent direct metal–metal contact thus reducing friction and wear. The quantum chemical calculations studies show that there is significant interaction of Schiff base additive with the metal surface than those of its individual components and the data correlate very well with the obtained tribological results.
This paper concerns with theoretical investigation to predict the combined influence of textured surface and couple stress lubricant behavior on the performance of circular/two-lobe slot-entry hybrid journal bearing. The flow of incompressible couple stress lubricant through the clearance of a textured journal bearing system is governed by the modified Reynolds equation. The modified Reynolds equation has been derived on the basis of Stokes micro-continuum theory and is solved using a finite element method technique for computing the bearing performance characteristics parameters. The static and dynamic characteristics of circular/two-lobe slot-entry hybrid journal bearing have been computed for textured and non-textured bearings with influence of couple stress lubricant. The present study indicates that the influence of couple stress lubricant behavior is significantly more on the values of fluid film stiffness coefficients
The static characteristics of misaligned three-axial water-lubricated journal bearing in the turbulent regime are analyzed for groove angles 36° and 18°. Ng and Pan’s turbulence model is applied to study the turbulence effects in the journal bearing. The static parameters such as load-carrying capacity, friction coefficient, and side leakage are found for different degree of misalignment (DM). The change in flow regime of the lubricant from laminar to turbulent and the increase in misalignment, improved the load capacity of the bearing. For lightly loaded bearings, the friction coefficient of the bearing increased with the increase in Reynolds number.
The author tested the efficiency, load-carrying capacity, and the type of friction in a meshing of a worm gear having the same parameters as worms made of carburized and quenched steel and worm-wheels made of steel, cast iron, and bronze. The type of friction was identified in case of worm gears with steel and cast iron worm-wheel at a volume temperature of the worm-wheel of 100 ℃; these gears did not demonstrate the tendency to seizure. The value of resistance of an oil film formed between the worm and the worm-wheel was accepted as the criterion of the existing type of friction in the meshing during mating of these two elements. The resistance of the oil film in the worm meshing was measured twice each time for two directions of the passage of electric current to avoid the effect of possibly occurring polarization phenomenon. Obtained values were averaged. During the course of the experiments, the image of friction was observed on the screen of an oscilloscope. Obtained values of the oil film resistance prove the low participation of fluid friction in the worm meshing with the worm-wheel made of steel as well as of cast iron. In case of the gear with the steel worm-wheel, the decrease of resistance in comparison with the bronze worm-wheel amounted to approximately 10 times and that of cast iron to almost 100 times. On the basis of the tests one can state that the application of steel worm-wheels and in particularly cast iron worm wheels makes it difficult for the fluid friction to occur while the application of a traditional material mating pair (a steel worm–a bronze worm-wheel) forms conditions for the occurrence of a mixed friction with a very high (approximately 99%) participation of fluid friction.
The physical effects associated with the shape and the scale of regular wavy surface asperities are investigated analytically. A special periodic analytical function, which is a generalization of a sine wave and allows to describe waviness of arbitrary smooth shape is suggested. The formulation and solution of the plane problem of elastic contact of a wavy surface with a half-plane is considered. Asperities of two-scale levels are taken into account—regular waviness with arbitrary shape (small-scale asperities) and regular sine-shaped roughness (large-scale asperities). The obtained pressure distribution for an arbitrary shaped one-scale wave is a generalization of the Westergaard’s solution for a sine wave. The results show that the shape of asperities has significant influence on pressure distribution over the entire range of contact lengths. It is also shown that the elastic coupling of adjacent asperities and asperities of different scales increases the nonlinearity of the contact interaction. But for the small loads the problem can be approximately reduced to linear, and the contact area fraction can be obtained directly from the geometry of contacting surfaces.
Influence of semi-cone angles on the performance of the non-recessed hybrid/hydrostatic hole-entry conical journal bearing compensated with orifice restrictor has been carried out analytically. The analysis comprises two rows of symmetric hole-entry contour in the circumferential direction of hybrid/hydrostatic conical journal bearing. An orifice restrictor is used in the hole-entry contour to allow the restricted flow in the clearance space of a journal and bearing. Well-known finite element method and Newton–Raphson method for restrictor have been used to solve the Reynolds equation, governing the fluid flow in the clearance space of a hybrid/hydrostatic conical journal and bearing. Further, the spherical coordinate system has been employed to solve Reynolds equation instead of cylindrical coordinate system. Performance characteristics in terms of stiffness coefficient, damping coefficient, bearing pressure, bearing flow, and minimum fluid film thickness have been discussed for variable external radial load (
In view of increasingly severe operating conditions and the use of composite (strongly) heterogeneous materials, detailed modelling and optimisation methods are needed to predict the effects of subsurface material topology, either by design or resulting from inclusions and material anisotropy, on rolling-sliding contact fatigue life. In this paper, a method is proposed showing that such predictions can at present be obtained for realistic configurations on small-scale computers by integrating efficient numerical solution of the 3D displacement and stress field in the (heterogeneous) material with fast rough surface-lubricated contact models.
Contact pressures, subsurface stresses and surface fatigue effects are shown for cases of bonded individual or multiple (clusters) of statistically distributed inhomogeneities close to the surface in realistic actual rolling–sliding rough contact geometries. The model contributes to the development of optimised failure criteria for composite/heterogeneous materials close to the surface.
Mixed elastohydrodynamic lubrication of materials with low elastic modulus (soft materials) is investigated. Expressions for prediction of film thickness and the asperity load ratio in soft line-contact elastohydrodynamic lubrication are presented. The traction behavior of soft contact in mixed elastohydrodynamic lubrication regime is also studied in terms of the Stribeck curves.
The area and location of cavitation change with time in the condition of dynamic loading. The cavitation location of sleeve bearing not only depends on axis location and velocity at a certain moment but also is affected by cavitation history, so the boundary condition need to be defined in the solution of lubrication equation. Based on the balance of axial inertia force, oil film force, and dynamic loading, the motion equation, generalized Reynolds equation and oil film thickness equation of spiral oil wedge sleeve bearing are established. The results show that the cavitation location and carrying capacity vary periodically with the change in time. The effect of dynamic loading and rotational speed on the oil film cavitation and carrying capacity is studied.
Investigating the effects of coating on cam/tappet thermal elastohydrodynamic lubrication through numerical simulation has great significance in the design of coated cam/tappet conjunctions. This paper presents a numerical model for the prediction of the thermal elastohydrodynamic lubrication of a coated cam/tappet and the results of a study of the effects of coating parameters on cam/tappet lubrication performance. In the model, the Reynolds equation is solved by the damped Newton method to obtain the pressure distribution, and energy equations are used to obtain the temperature distribution. The total elastic deformation is calculated by the finite element method. The effects of the coating’s mechanical properties on pressure and temperature were found to be significant, as were the effects of the coating’s thermal properties on temperature. These effects were found to increase with increasing coating thickness. A soft coating with low thermal inertia has the greatest ability to reduce friction loss, and the higher the inlet temperature is, the lower the friction loss is. The influence of coating of both the cam and tappet on friction loss is greater than the effect of coating of the tappet only, which is greater than the effect of coating of the cam only.
In aero-engines it is possible for the blades of the compressor, turbine or fan to incur into their casings. At these interfaces a lining of composite abradable material is used to limit damage to components and thereby sustain the efficiency and longevity of the engine as a whole. These composite materials must have good abradability and erosion resistance. Previously, the wear mechanisms at the contact between the blade and the coating have been characterised using stroboscopic imaging and force measurement on a scaled test-rig platform. This work is focused on the characterisation of the wear mechanism for two different hardnesses of abradable lining. The established stroboscopic imaging technique and contact force measurements are combined with sectioning of the abradable material in order to analyse the material’s response during the tests. A measure of the thermal properties and the resulting temperature of the linings during the test have also been made to further understand the effect of coating hardness. The wear mechanism, material response, contact force and thermal properties of the coating have been used to characterise the different material behaviour with different hardness. At low incursion rates, with a soft coating, the blade tip becomes worn after an initial adhesive transfer from the coating. Post-test sectioning showed blade material and significant compaction present in the coating. The harder coating produced adhesion on the blade tip with solidification observed in the coating. Thermal diffusivity measurements and modelling indicated that thermally driven wear observed was as a consequence of the increased number of boundaries between the metal and hBN phases present interrupting heat flow, leading to a concentration of surface heat. At higher incursion rates, the wear mechanism is more similar between the coatings and a cutting mechanism dominates producing negligible adhesion and blade wear.
Lightweight is one of the most important criterion in the optimum design of gear set for motorsport and aerospace application. A tradeoff between optimum weight and failure modes of gear is a subject of interest for researchers and the industry. In the present work weight of a single-stage spur gear set is optimized. This nonlinear constrained optimization formulation has been solved by using differential evolution algorithm. A total of six design variables corresponding to gear geometry and material property are considered. The results obtained are compared with those of published heuristics like genetic algorithm, simulated annealing, and particle swarm optimization algorithm, respectively. The optimization is performed in such a way that the design variables satisfy all constraints at optimum solution. Apart from this, several constraints related to scoring are included in the optimization. The constraint violation study is performed to prioritize the constraints. The sensitivity analysis is carried out to see the effect of manufacturing tolerances of design variables on weight of the gear set. The optimality of the solution has been ensured through the convergence study. The optimization reveals that the reported results are also encouraging in terms of objective function values and CPU time. In addition, the optimum design variables obtained through the weight optimization of spur gear set are used for preparation of a CAD model. Then the stress analysis using finite element analysis is performed on the gear set to identify the critical stress region in the optimized gear set.
Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences.
In the background that the quenchable boron steels have been widely used to manufacture body-in-white by hot stamping because of its ultra-high strength, the understanding of frictional characteristics of the boron steel–tool steel tribopair at high temperature should be deepened. In this work, the friction behaviors of the tool steel H13 against the boron steel 22MnB5 were investigated at different temperatures, sliding velocities, contact pressures, and lubrication conditions by ring-on-disc sliding testing. The tribological characteristics were analyzed through scanning electron microscope and confocal laser scanning. The results show the relationship between the friction coefficient and the hot stamping parameters well. The friction coefficient decreases remarkably with the increasing temperature and contact pressure, but sliding velocity has no noticeable influence on the friction coefficient. The wear mechanism also changes with different process parameters and the main mechanism is a combination of adhesive and abrasive wear. Furthermore, MoS2 lubricant can reduce the friction coefficient effectively and protect the die from severe wear.
Geometric imperfection is a common problem in manufacturing of rolling element bearings. In particular, roller geometric errors frequently occur because a large number of rollers with relatively small size are engaged in a bearing. However, computational tools for rolling bearing characteristics take into account ideal bearings without any geometric error. In this study, the stiffness and fatigue life of tapered roller bearings were investigated with consideration for the effects of roller diameter errors possibly induced during manufacturing process. To this end, a general model for tapered roller bearings having rollers with diameter error (or defective rollers) was developed that can reflect the time-varying stiffness due to the roller error effects. The effects of the number of defective rollers, error magnitude, and position of defective rollers on the stiffness and fatigue life were investigated. Computational results showed that even small roller diameter errors appreciably alter the tapered roller bearings internal load distributions and therefore the stiffness and fatigue life of tapered roller bearings.
The characteristic of gas film is a key factor in the performance of the aerostatic bearing. Because the gas film flow is in the slip regime, influence of the rarefied effect is significant. The modified Reynolds equation suitable for compressible gas in the rarefied effect is deduced through introducing the flow factor in the rarefied effect to the Reynolds equation. Pressure distribution, capacity, and stiffness of the gas film under the rarefied effect are analyzed. With the increase of gas pressure, the gas film capacity and stiffness of bearing would also increase. However, the greater the gas supply pressure, the more intense the gas film vibration, so it was important to select a reasonable gas supply pressure for achieving the optimal gas film characteristic. Finally, the gas rarefied effect is verified by the experiment indirectly, which agreed well with the analytical results and provided a theoretical guidance for the machining accuracy of the machine tool.
The multilayer protuberant foil bearing, as a new type of compliant surface foil bearing, shows a great and wide application promise. Six pads multilayer protuberant foil thrust bearings with different configurations were designed and fabricated in this study. The static characteristics of these bearings and the effects of their key configuration parameters including the thickness of top foil, the thickness of protuberant foil, and the layers of protuberant foil are investigated. The experimental results reveal that the bearings show nonconstant structural stiffness, and the stiffness mainly depends on both the load force and the configuration of the bearings. In the airborne regime, the torque of the bearing is mainly dependent on the load force rather than the rotational speed, which can be interpreted by the proportional relationship between the bearing clearance and the rotational speed. Furthermore, the experimental results also show that the maximum load capacities of the bearing are also greatly affected by the bearing configuration. With more layers of the protuberant foils and thinner top foil, the bearing shows larger maximum load capacity. The work provides some insights about the relationships between the characteristics and the configuration of the bearings.
Surface texturing is a widely used method to improve the tribological performance of the mechanical systems. To introduce the surface texture into the mechanical components, this study experimentally examined the frictional behaviors of grease lubricated spherical plain bearings under mixed lubrication conditions on an electro-hydraulic servo tribological test rig. The effects of the surface texture parameters and roughness parameters on the frictional properties of bearings were investigated. The results showed that higher dimple depths and lower dimple densities would result in a distinct improvement on the friction coefficients. A maximum reduction of 55% was gained for the textured sample under running conditions compared with the untextured one. In addition, the surface roughness parameters also influence the frictional behaviors. In the high load conditions, the friction coefficient decreases while the Ssk value gets more negative, even though the Sa value is much higher. On the other hand, in the low load conditions, when the value of Ssk varies between –5 and –1, the influence of the value of Sa on the friction coefficient becomes obvious, a higher Sa value results in a higher friction coefficient.
This paper presents an investigation of the hot judder phenomenon of multidisc clutches, which takes place during the engagement process. Depending on the results of finite element analysis, a pressure distribution function is defined and a contact pressure equation is established to demonstrate the non-uniformity of the contact pressure distribution on the friction interfaces due to frictional heat. The relationship between the coefficient of friction and the temperature is analyzed. A 4 degrees of freedom power-train model is developed to evaluate the clutch judder behavior. The paper indicates that the clutch judder is influenced by the non-uniformity of the interface contact pressure distribution, which is excited by frictionally induced thermal load. The non-uniform contact pressure distributions along the radial direction have a slight influence on the clutch judder, while the uneven contact pressure distributions along the circumference contribute to the judder substantially. Furthermore, the results in this work can be used to study the operation instability and the thermal failure of clutches.
This paper reports the synthesis and tribo-performance investigation of microfluids developed using silane functionalized µ-molybdenum disulphide (µ-MoS2) particles for improved tribological performance. The functionalized µ-MoS2 particles were characterized by analytical techniques like Fourier-transform infrared, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The µ-MoS2 particles were then blended with mineral base oil in varying concentrations. The lubrication characteristics in terms of anti-friction, anti-wear, and extreme pressure properties of the µ-MoS2 particle blended microfluids were evaluated under different lubrication regimes. The results showed an excellent improvement in anti-wear, anti-friction, and extreme pressure properties. The tribo-performance of the MoS2 microfluids strongly depends on the compatibility and concentration of particles with base oils. Functionalization of the µ-MoS2 particles by trichlorooctadecylsilane allows forming stable dispersion in base oil. The MoS2 microfluids can be used as lubricants in advance tribological applications.
Transient behaviour of tribo-characteristics caused by transversely oriented ridges on point contact surfaces was investigated based on a thermal elastohydrodynamic lubrication analysis. The ridges were assumed to exist on both the contact surfaces with different velocities. Results show that the interaction of ridges gives a large influence on the local film thickness, pressure, friction coefficient, temperatures on both the solid surfaces and temperature in the oil film. It is also pointed out that the size of the contact bodies brings strong effect on the temperature distribution and shear rate as well as on the friction coefficient. Furthermore, it is revealed that under rolling-sliding conditions, the shear-thinning property of the lubricant is negligible when the size of the contact body is large enough. However, shear-thinning effect plays an important role when the size is extremely small.
The main purpose of this paper is to investigate the tribological performance of the zinc-phosphate- and chromium-coated spheroidal cast iron piston rings in reciprocating tribotest rig with conventional commercial lubricant. Results of the experiments showed that tribofilm was formed both on ring and liner surfaces. Zinc phosphate coating was removed from the ring surface after the experiments. Wear rate of liner-chromium-coated ring pair was lower than the liner-phosphate-coated ring. The friction coefficient of the liner-phosphate-coated ring pair ranged from 0.104 to 0.124, lower than the liner-chromium-coated ring, which ranged from 0.114 to 0.129. Change of average surface roughness was also lower in chromium-coated ring (31%) than the phosphate-coated ring (72%). Test results showed that hard chromium-coated ring had better tribological performance and coating efficiency than the phosphate-coated ring due to wear resistance and lower change of average surface roughness.
The cylindrical spur tractive gear of the electric locomotive has been conducted using the author’s method of the calculation of maximum contact pressures, teeth wear and durability at technological correction of engagement at changeable, in the result of teeth wear and conditions of their contact. The regularities of tribocontact pressures change in engagement has been established after reaching the permissible wear depending on the shift coefficients in the phases of double–single–double tooth engagement. Considerable decrease of maximum tribocontact pressures is observed in the result of teeth wear in the entry phase of double-tooth engagement in comparison with their initial values. Depending on the shift coefficients, maximum (permissible) wear of wheel teeth would arise in different contact points: at the entrance into single-tooth engagement in non-corrected gear and at the exit in the presence of correction. Gear durability has its optimum at
The objective of this paper is to investigate the high-speed wheel/rail adhesion under interfacial liquids contamination using a numerical model. This model considers the rheological property of interfacial liquids, elastic-plastic deformation of microasperities contact and the temperature across the film thickness. The pressure and the temperature fields can be obtained. The effects of train speed, surface roughness parameters, characteristic shear stress, and the slip ratio are investigated. Furthermore, the present model is compared with the elastic model and the elastic-plastic model without considering the thermal effect. The numerical results show that the train speed and temperature affects the wheel/rail adhesion significantly.
This paper analyses the steady-state performance behaviour of a new type of journal bearing, i.e. the non-circular cylindrical floating ring journal bearing. It consists of a floating ring in between the shaft and the upper and lower lobes of a two-lobe bearing. The journal and the inner surface of the ring are cylindrical while bearing surfaces are non-circular. The classical Navier–Stokes equations in the modified form together with the continuity equation are being solved by the finite element method. The cylindrical coordinates form of the Navier–Stokes equation and continuity equation are used in the present analysis to compute the important proposed bearing characteristics. In this analytical study, the finite bearing approximation (L/D=1) with a C2/C1 value of 0.70 and 1.30 are being used to simulate the behaviour of non-circular cylindrical floating ring journal bearing. The Reynold’s boundary condition is used to enumerate the performance of the proposed bearing. In the present analysis, the steady-state parameters in terms of an inner and outer film eccentricity ratio, a speed ratio, attitude angle, load capacity, friction coefficient parameter, axial oil flow and rise in temperature variable are determined. The results reveal that the steady-state performance of the non-circular floating ring journal bearing is superior to a plain cylindrical floating ring journal bearing.
In this paper, the mechanical behaviors of the tubular rubber seal under the curved surface loading are studied by the experimental and finite element methods. First, both the compressive behaviors and the frictional performance of the tubular rubber seal under the plane loading are studied experimentally. Second, two-dimensional finite element model about the tubular rubber seal is established and the constitutive model parameters of the rubber seal material are extracted. Finally, the mechanical behavior of the tubular rubber seal under the curved surface loading is investigated by means of the approximation analysis method and the finite element simulation. The results indicate that the physical parameters based on the experiment and finite element method simulation of the tubular rubber seal under the plane loading can describe the mechanical behavior under the curved surface loading. Both the strain distribution of the tubular rubber seal and the driving torque are slightly influenced by the friction coefficient between the tubular rubber seal and the curved surfaces. The influence factor of the friction coefficient on the driving torque is proposed, which plays an important role in the design and evaluation of advanced tubular rubber seal.
The purpose of this investigation was to analyze the effect of adding different weight fraction of nanozirconia on wear characteristics of resin-based dental composite. The dental composites were fabricated by adding (0–3 wt.%) of silane-modified nanozirconia particles into a monomer system (50 wt.% bisphenol-A glycidyl methacrylate, 49 wt.% tri-ethylene glycol dimethacrylate, 0.2 wt.% Camphorquinone, and 0.8 wt.% ethyl 4 dimethyl amino benzoate). The experiments were conducted based on steady state condition and Taguchi’s orthogonal design in a dental wear simulator. A theoretical model was proposed for wear assessment and validated with the experimental results. The findings of the experiments indicated that the composite with 3 wt.% nanozirconia demonstrated lowest volumetric wear rate.
This paper investigates the relationship between the predicted interface flash temperature and the experimentally measured temperature when two solids are in contact sliding. To reveal this, both pin-on-disc experiments and finite element simulations were carried out systematically. The infrared technique was used to experimentally measure the interface temperature. Bulk metallic glass was especially selected as the pin material because this material can have microstructural changes to record its temperature history experienced. Transmission electron microscopic analysis showed that nanocrystallisation occurred during the contact sliding, showing that the temperature at the real contact area must have exceeded glass transition temperature, Tg of the bulk metallic glass and reached its onset temperature of crystallisation Tx. However, the temperature measured was much less than the glass transition temperature Tg. On the other hand, the finite element analysis showed that the temperature at the pin surface was beyond Tg when the flash temperature reached Tx. The study thus concluded that there exists a thermal resistance due to the complex surface morphology of rough surfaces, which alters the thermal conductance in the neighbourhood of the contact interface.
This article presents a novel calculation method for the bearing performance of the aerostatic thrust bearing. This method is provided while taking the fluid-structure coupling effect into account. High pressure air film leads to the structure deformation, thus the bearing clearance after deformation is utilized to estimate the real bearing performance. Furthermore, the influencing factors on the bearing performance, the thrust plate’s thickness, and the orifices’ location, are investigated on the basis of the fluid-structure coupling effect. The calculation method of the best gas film thickness is deduced and presented. Experimental results for validation highlight the reliability of this method.
In this paper, a complete engineering model for particle entrapment in rolling bearings and consequences in raceway indentations is presented. The attention is focussed on the conditions for particle entrapment once the particles are in contact with the two rolling surfaces. After the entrapment, the model follows the particle trajectory within the contact. The particle deformation is then calculated and finally an elastic-plastic contact model is applied to simulate the eventual raceway indentation. Laboratory experiments with a single contact device as well as full bearing tests were performed to validate the models. The comparisons between model and experiments show good agreement in the number and type of indentations.
A numerical method to generate bifractal surfaces due to a modification of the slope of the power spectral density function in the low- or high-frequency range is proposed. The method has been applied to simulate real surfaces of Ginkgo Biloba leaf scanned at two different magnifications by matching the corresponding experimental power spectral densities. Slight differences have been found in the statistical distributions of the asperity heights and curvatures for the lowest magnification that had marginal influence on the frictionless normal contact response of the surface. For highest magnification, however, the statistics of the simulated numerical surface were quite different from those of the real one, leading also to a significant difference in the normal contact results.
The environmental problems that are being caused by the mineral-based lubricants has prompted for exploring the possibility of using vegetable-based oil as a lubricant and this has become a worldwide trend. In the present study, the tribological behavior of the chemically modified rapeseed oil has been studied by using a four-ball tester. The CuO and CeO2 nanoparticles of size less than 50 nm were added to the chemically modified rapeseed oil ranging from 0.1 to 1 wt%. And the friction and antiwear properties were analyzed as per the ASTM standard D4172. The size of the nanoparticles was calculated qualitatively by using Scherrer equation with the help of X-ray diffraction analysis. Tribological results showed that the chemically modified rapeseed oil has good antiwear and friction-reducing properties than the raw rapeseed oil. The addition of oxide nanoparticles at an optimum concentration in the chemically modified rapeseed oil showed the good friction-reducing property nonetheless not good antiwear property. The surface topography of the worn ball surfaces was also studied by using a scanning electron microscope.
In the current study, in order to obtain a thick film thickness under zero entrainment velocity at low surface velocity, the effects of ambient viscosity, pressure–viscosity index of the lubricant, and the surface waviness are investigated numerically based on a thermal elastohydrodynamic lubrication mathematical model. The increasing ambient viscosity and modest waviness can deepen the dimple by a stronger "temperature-viscosity wedge" effect. With the combined effect of ambient viscosity, pressure–viscosity index, and surface waviness, a small centralized dimple in smooth contact evolves into a big classical one together with the disappearance of the former thin droopy film thickness.
The erosion behaviors of aluminum alloy have been evaluated practically at different test conditions under ambient temperature. Irregular silica sand (SiO2) is used as an erodent within the range of 300–600 µm. The impact velocity within 30–50 m/s, impact angle 15–90°, and stand-off distance 15–25 mm considered as related parameters. The maximum level of erosion is obtained at impact angle 15° which indicates the ductile manner of the tested alloy. The higher the impact velocity, the higher the erosion rate as almost linear fashion is observed. Mass loss of aluminum alloy reduces with the increase of stand-off distance. A dimensional analysis, erosion efficiency () and relationship between friction and erosion indicate the prominent correlation. The test results are designated using Taguchi’s concept to ensure the minimization of observations for clarification of results in alternative process. ANOVA data analysis is considered to signify the interaction of tested parameters as well as identifying most influencing operating parameter. S/N ratio indicates that there are 2.92% deviations estimated between predicted and experimental results. To elaborately analyze the results, GMDH method is mentioned. After erosion process of the tested composite, the damage propagation on the surfaces is examined using SEM for confirming wear mechanisms. The elemental composition of eroded test samples at varying percentage of aluminum is analyzed by energy dispersive X-ray spectroscopy analysis.
In this article, the finite difference lattice Boltzmann method (FDLBM) is successfully applied to analyze the hydrodynamic properties of the wedge-shaped gas film lubrication for the high speed micro gas bearings by comparing with the macroscopic methods (solving the modified Reynolds equation coupled with the simplified energy (modified Reynolds equation) and the Navier–Stokes equations coupled with the energy equation). By comparison, it is found that the vertical flow across the gas film can weaken the gas backflow and thus improves the gas film pressure, as the Navier–Stokes equation and FDLBM are used to analyze the wedge-shaped film lubrication. The continuum assumption in the macroscopic methods leads to a larger gas film pressure, compared with the value predicted by the FDLBM. And, the high temperature and speed enlarge this difference between them. Furthermore, the FDLBM provides a good warm-up for the multiscale simulation on the complex flow in the micro gas bearings.
The study reported here deals with elastohydrodynamic point contacts and it is focused on the influence of contact ellipticity. In five velocity–load reference cases, ellipticity was varied from slender to wide configurations, including the circular contact. For each case, Hertzian pressure, Hertzian area, load, and entrainment velocity were kept constant while the ellipticity was varied by changing the curvature radii. In this context, the maximum central film thickness did not occur for the infinitely wide contact, but for a slender configuration close to the circular case. Moreover, the minimum film thickness reached its optimum for a wide but finite elliptical contact. For low ellipticity ratios, specific film thickness features were obtained. In particular, very high central/minimum film thickness ratios are found. The cause of these behaviors was found in the change of the convergent shape. When the ellipticity was varied, the Poiseuille flows parallel and transverse to the entrainment direction were significantly modified and these modifications were quantitatively analyzed for the different cases. The competition between the Couette and the Poiseuille flows was totally different between the narrow and the wide elliptical contact, and this change was responsible for the film thickness variations with ellipticity. Ellipticity also had an effect on friction as it influenced the maximum pressure which in turn impacts the fluid viscosity.
Mine excavator buckets are made of austenitic manganese steel and subjected to wear while excavating ore, minerals and hard soil etc. Wear bars are used to reduce the wear in original bucket plate. A mathematical model based on several processes and design parameters has been developed to analyze the mechanism of wear generation of the bottom plate in the excavator bucket. An experimental setup has also been developed to study the wear in the excavator bucket. Different angular orientations of wear bars have been used to find out the minimum volume of wear on the bottom plate. It has been found that the wear bar with 90° orientation causes least wear. It has also been observed that the mathematical model has a good agreement with the experimental results.
The modified Reynolds equation for active lubrication has been the cornerstone around which the theoretical investigations regarding actively lubricated bearings have evolved over the years. Introduced originally in 1994, it enables to calculate in a simplified manner the bearing pressure field as a function of servovalve controlled pressurized oil injection. This article deals with a preliminary critical review of the simplificatory assumptions that are introduced into the modified Reynolds equation in order to model the phenomena taking place in the interface between the injection nozzle and the bearing clearance. The analysis is performed by means of direct comparison of the results of the modified Reynolds equation model versus benchmark CFD calculations, applied to a geometry representative of the system analyzed. The results show that the modified Reynolds equation mathematical simplicity comes at the cost of reduced accuracy regarding the description of the oil velocity field in the vicinities of the injection nozzle. On the other hand, the modified Reynolds equation results provide sufficient accuracy regarding global magnitudes, such as resulting loads and injected flows, which are essential for designing and operating bearings featuring the active lubrication system.
This article deals with the tribological study of solution-treated and peak aged AZ91 Mg alloy. Tribological studies were carried out using reciprocating sliding contact under lubricated conditions. The effect of load, sliding speed, and microstructure on wear response of AZ91 Mg alloy was investigated. The wear response was characterized using optical microscopy, scanning electron microscope, and energy dispersive X-ray spectroscopy mapping. The peak aged alloy showed excellent wear resistance against Al6351 Al alloy as compared with solution-treated alloy. The formation of -Mg17Al12 precipitates in aged sample was found to increase the wear resistance. The specific wear rate for peak aged alloy was almost one-third than that of solution-treated alloy. In both the samples, i.e. solution-treated and peak aged, the coefficient of friction decreases with increase in sliding speed at low load, while the behavior of peak aged sample was found to be opposite at higher load. At low load, the wear mode for peak aged sample was oxidative which further shifted to adhesive at higher loads.
Acoustic emission is an emerging technique for condition monitoring of rolling element bearings and potentially offers advantages for detection of incipient damage at an early stage of failure. Before such a technique can be applied with confidence for health monitoring, it is vital to understand the variation of acoustic emission generation with operating conditions in a healthy bearing. This paper investigates the effects of increased speed and load on the generation of acoustic emission within cylindrical roller bearings, and it was found that the root mean square signal level increased significantly with increasing speed whereas increasing load had a far weaker effect. The AERMS value for each experiment was compared with the trend of the Lambda value. The bearing was operating under full film lubrication regime, so it was determined that increases in AERMS were not caused by asperity contact. By consideration of trends in frequency energy amplitude, it was determined that excitation of the bearings resonant frequencies were responsible for an increase of energy in the frequency range of 20–60 kHz. The excitation energy at 330 kHz (the acoustic emission sensor’s resonant frequency) increased with load, indicating a link between high-frequency emission and stress at the contact zone. Following characterisation of the bearing under normal operating conditions, an accelerated life test was conducted in order to induce fatigue failure. The frequency response demonstrated that throughout a period of constant wear, the energy amplitude at the bearings resonant frequency increased with time. As the bearing failure became more significant, the energy of the high-frequency components above 100 kHz was spread over a broader frequency range as multiple transient bursts of energy were released simultaneously by fatigue failure of the raceways. This paper demonstrates the potential of acoustic emission to provide an insight into the bearing’s behaviour under normal operation and provide early indication of bearing failure.
A new dynamic mesh algorithm is developed in this paper to realize the three-dimensional (3D) computational fluid dynamics (CFD) method for studying the small clearance transient flow field of tilting pad journal bearings (TPJBs). It is based on a structured grid, ensuring that the total number and the topology relationship of the grid nodes remain unchanged during the dynamic mesh updating process. The displacements of the grid nodes can be precisely recalculated at every time step. The updated mesh maintains high quality and is suitable for transient calculation of large journal displacement in FLUENT. The calculation results, such as the static equilibrium position and the dynamic characteristic coefficients, are consistent with the two-dimensional (2D) solution of the Reynolds equation. Furthermore, in the process of transient analysis, under conditions in which the journal is away from the static equilibrium position, evident differences appear between linearized and transient oil film forces, indicating that the nonlinear transient calculation is more suitable for studying the rotor-bearing system.
Some observations from acoustic emissions recorded during a yield test of a bearing raceway compressed into plasticity using a rolling element are presented. The general objective of the study is to establish whether there is enough evidence of the onset of sub-surface plasticity in the acoustic emissions signature. It is discussed here how acoustic emissions monitoring during compression could indicate the onset of subsurface plasticity as a precursor to damage propagation to the surface. Some comparisons are drawn between the acoustic emissions activity levels and time-frequency response during elastic deformation and at yield loads.
In this paper, a comparison is made between calculated and measured displacements from a complete contact fretting test device. An experimental technique based on digital image correlation was used to measure the local displacement field at the contact interface. The material of the fretting specimen and pads was quenched and tempered steel. The effect of test device compliances and rigid body movement was minimized by measuring displacements very close to the contact interface. The measured displacements were successfully compared to the computed displacements of a corresponding finite element model. The relative slip amplitude in partial slip conditions, slip distribution across the contact, length of the slip region, and accumulated slip distribution, were compared. Relative slip decreases markedly with increasing normal load and friction coefficient. The friction coefficient was calibrated and determined as a function of loading cycles of fretting fatigue tests with two normal loads. The friction coefficient was found to increase at the beginning of tests and stabilize after about 1000 cycles, which is in agreement with general observations.
The tribology of the grinding process can be considered in the context of a tribosystem, in which the main structural elements (grinding wheel, workpiece, grinding fluid, and environment) are interrelated and interdependent. One of the most important factors influencing the contact conditions of these listed elements of the grinding process tribosystem are the proper selection of grinding fluids and anti-adhesive substances, as well as careful consideration of how they are applied. This article describes a new zonal centrifugal coolant provision system as well as the results of experimental studies conducted into its use. The aim of these experiments was to determine the impact of the system of grinding fluids provision on grinding wheel radial wear in the surface grinding process of steel CrV12. Reference methods within the described studies consisted of dry grinding and grinding using the flood method, among others. The obtained results of the experiment revealed that the use of a zonal centrifugal coolant provision system enabled the acquisition of a similar radial wear rate of the grinding wheel at 90% reduction of grinding fluids flow rate, compared with flooding method. It was also demonstrated that in the most favorable conditions, it is even possible to significantly reduce the intensity of the radial wheel wear, compared with the conventional method.
Friction tests using ZrO2 (Y-PSZ: yttria partially stabilized zirconia) pin slid against polymer-like carbon film of bilayer and multilayer structures under H2 gas environment are conducted. It will be shown that friction coefficients of the level of 0.0001 (friction fade-out) is stably realized by adding alcohol vapors to H2 gas during run-in stage, then by stepping up the load from 19.8 N to 63.7 N after run-in stage. Four kinds of vapors of alcohol aqueous solutions are tested using bilayer samples, and ethanol-vapored H2 gas shows the longest friction fade-out duration. Polymer-like carbon/diamond-like carbon multilayer sample shows long-life friction fade-out of 4 h, and it will be shown that the friction trace of 4 h reflects wear process of the first layer of polymer-like carbon and the second layer of diamond-like carbon. ZrO2 surface is observed by an optical microscope and scanning electron microscopy and measured by surface profiler after friction fade-out test, and it is shown that flat contact area at the central region has many blisters and crimps, and is surrounded by peripheral bumps. It is also shown that the sliding marks are seen only at the top of crimps at the central region. Raman measurements indicate that short-chain carbons are predominant at blisters and ring carbons of small clusters are predominant at bumps. With these observations friction fade-out mechanism is discussed.
Chromium plated cylinder has been wildly used in large bore diesel engines due to its light weight, good durability, and low induced liner wear. Deposits accumulated in the grooves and micro-crack on chromium plated cylinder could smooth the running surface, reduce the oil retention capability, and increase the engine’s oil consumption. In the present research, deposit formation on chromium-plated cylinder in fully formulated oil under different loads and temperatures was investigated by using a reciprocating sliding tester. Surface analytical techniques such as SEM, EDX, and XPS have been used to gain the knowledge about deposits’ origin, distribution, and factors affecting the deposit formation. The deposits are mainly derived from C, O, and S in the lubricating oil and anti-wear additive ZDDP (Zn and P). Deposits only accumulated in the depression (honing grooves and micro-cracks) on chromium plated cylinder surface, and there were no deposits found on the plateaus. The deposit formation increased with the increasing of load and temperature, and increased sharply over 60 MPa and 150℃. The deposit formation on liner surface was significantly different between chromium-plated coating and cast iron in component and distribution.
This paper presents the identified dynamic coefficients of a lightly loaded actively lubricated bearing under three lubrication regimes: passive, hybrid and feedback-controlled. The goal is to experimentally demonstrate the feasibility of modifying the bearing dynamic properties via active lubrication. Dominated by the latest two regimes, the bearing properties become adjustable or controllable due to the injection of either a constant or variable pressurized oil flow. Such a flow is regulated by a hydraulic control system composed of (a) a high-pressure oil supply unit, (b) servovalves, (c) radial injection nozzles, (d) displacement sensors and (e) well-tuned digital controllers. A scaled-down industrial rotor featuring active lubrication, composed of a flexible rotor supported by a four-rocker load-between-pads tilting-pad bearing under light load condition, is used for this objective. The experimental identification is performed by means of measured frequency response functions and a rotor finite element model. Predicted coefficients are also provided for benchmarking. Comparing results between the different regimes, presented along with their expanded uncertainty, provides the experimental evidence of the bearing properties modification via active lubrication.
A water–gas turbulent lubrication model is established by coupling the generalized Reynolds equation and bubble force equilibrium equation, which includes surface friction force and surface tension of bubbles. The above coupled-equations are solved to obtain load capacity, bubble concentration, and dynamic characteristic for journal bearing. The available experimental results reveal that the load capacity is well predicted by the present model. The theoretical results show that bubble surface tension and low gas volume fraction (β ≤ 0.1) are beneficial in increasing the load capacity of journal bearing, while high gas volume fraction (β ≥ 0.2) have opposite effects. The results also show that water–gas turbulent lubrication can decrease the dynamic characteristic parameters of journal bearing, while increasing the instability whirl frequency of journal bearing.
Extensive investigations have been carried out to understand the effect of gear material, gear parameters and service conditions over polymer gear durability. However, the effect of the mating gear surface condition over test gear performance has not been completely understood. In this study, injection-molded polypropylene gears were paired with stainless steel gears and evaluated in the power absorption gear test rig. This study considered steel gears manufactured through the wire-cut electric discharge machining with different surface roughness (3.8–4.1 µm, 2.5–2.8 µm and 1.9–2.2 µm). The bearing ratio curves of the steel gear surfaces were obtained with the aid of non-contact profiler. During testing, the surface temperature of polymer gear increases due to the gear material hysteresis and surface interaction. The surface temperature of the gear increased by 5–15℃ due to the increase of surface roughness of the mating steel gear tooth (1.9–4.1 µm). The worn-out gear tooth surface also confirmed the significance of the mating gear surface condition. Further, stainless steel discs with different surface roughness (4.7–5.4 µm, 2.6–3.2 µm) were manufactured through the wire-cut electric discharge machining process. Injection-molded polypropylene pins were slid against these discs. Due to the increase in surface roughness, coefficient of friction was found to increase from 0.38 to 0.45 for the chosen test condition. The measured net surface temperature of the test specimen also increased from 52 to 59℃ due to the increase in surface roughness.
Emerging clearances caused by out-of-round cylinder bores alter the tribological performance of piston compression rings. This change in performance intensifies during engine warm-up. To analyze the development of tribo-characteristics, a 2D mixed lubrication model is considered in the simulation of piston ring conjunction with a distorted bore during the entire warm-up period of a four-stroke spark ignition engine. Piston ring axial dynamics and conformability analyses are incorporated in the simulation. Oil film thickness, frictional loss, asperity interaction, and lubricant flow to the combustion chamber are investigated. The effect of the magnitude of bore distortion on ring conformability, oil transport, and energy loss per warm-up is also evaluated. Variations in oil transport and energy loss per warm-up per unit change in bore distortion are subsequently analyzed. Results show that despite the high film thickness, frictional losses remain substantially high during the initial warm-up phase. An uneven lubricating film resulting from bore non-circularity reduces this energy loss, but drastically increases the already augmented oil transport during the cold engine start-up. The temperature-dependent variation in performance is sensitive to bore distortions.
Skidding may cause wear and incipient failure of rollers and races of cylindrical roller bearings when the roller is loaded. The previous skidding studies on bearings are mainly focused on steady state condition. However, bearings often operate under different accelerations. This paper proposes an analytical model to investigate skidding between the maximum loaded roller and the races taking account of contact force and friction force between the roller, the cage and the races, fluid pressure and centrifugal force of roller. This model consists of nonlinear first-order ordinary differential equations which can be solved using Euler method. The results of this model are compared to the results of Harris model (steady-state condition) to analyze the effect of acceleration on skidding. A mixed elastohydrodynamic lubrication model which is capable of handling practical case with 3D machined roughness is used to study the lubricating performance between the maximum loaded roller and the inner race during acceleration. The results indicate that acceleration has a great impact on hydrodynamic pressure, friction coefficient, and contact stress, however, the acceleration can hardly affect oil film thickness and contact area ratio.
A pull-off force between a sphere and a flat plate is precisely investigated using a newly developed surface force apparatus (SFA). In this system, (1) a pull-off force between a spherical glass probe and a sample plate is measured in vacuum, (2) the probe is directly pulled off from the sample by an electromagnetic force, and (3) the pull-off force and displacement of the probe are measured with ultra-high resolution of 0.4 nN and 0.3 nm, respectively, which are electrical noise levels. The pull-off process, which appears as a part of force curve, is clearly measured by this system, and pull-off force is measured with high reproducibility and accuracy. Pull-off force distributions on flat surfaces of Si, SrTiO3, glassy carbon, and diamond-like carbon are measured. It is shown that despite the differences between these materials, for all four for them, the distribution strongly depends on the surface roughness, such that the relative standard deviation of pull-off force is proportional to the surface roughness (Ra and Rz). The Greenwood and Williamson (GW) model is then used for the analysis of the pull-off force. Comparing with SFA experiments, it is shown that the pull-off force and separation displacement can be predicted by the GW model and that the pull-off force strongly depends on the standard deviation of surface height, causing the broad distribution of pull-off force at rough surfaces.
A tribo-dynamic model for spur gear pairs combined dynamic loading with elastohydrodynamic lubrication theory is proposed in this paper. The integrated model incorporates a transverse-torsional dynamic model and a mixed elastohydrodynamic lubrication model to predict the dynamic behavior and lubricating performance of a spur gear pair under typical operating conditions. The dynamic model considering time-varying mesh stiffness, backlash as well as friction force generated from contact surfaces is built up to predict the dynamic tooth forces and surface velocities for the elastohydrodynamic lubrication model. In return, the elastohydrodynamic lubrication model taking into account the transient effect provides the dynamic model with viscous damping, friction forces and moments that play a non-negligible role in free and forced vibrations. In view of the convergence and efficiency, an iterative calculation program combined Runge-Kutta method and the multigrid method is developed to implement the tribo-dynamic model orderly to investigate the mutual relationship between the two models. The elastohydrodynamic lubrication predictions of the proposed model under dynamic conditions are compared to those obtained from a quasi-static elastohydrodynamic lubrication model to demonstrate the influence of the coupling effect on the elastohydrodynamic lubrication behavior. Simulation results indicate that the vibration caused by time-varying gear mesh force may enhance the friction coefficient at high speed.
The paper describes a theoretical study of friction at elastic-plastic contact of rough surfaces in n-point asperity model framework and based on accurate finite element analysis of elastic-plastic deformation of single asperity contact. Plasticity index, a well-defined nondimensional parameter is used to study the prospective situations arising out of variation in material properties and surface roughness. Using practical values of material properties and surface roughness parameters, results are obtained for the behavior of applied load, friction force, and coefficient of friction as a function plasticity index and mean separation of surfaces. It is observed that with increase in amount of plastic deformation, the load required to be applied for maintaining a particular level of separation between contacting surfaces increases while the frictional force decreases. Coefficient of friction is independent of applied load in predominantly plastic domain of deformation but varies nonlinearly in other domains of deformation. Also, for the same applied load the coefficient of friction increases with increase in elastic deformation.
This article studies the static load capacity, dynamic stiffness, and damping coefficient of the externally pressurized annular porous gas thrust bearings. The Reynolds equation for the bearing is established using linear perturbation; for the first time, three degrees of freedom are considered in the modeling. The steady state equations are solved to obtain the static load capacity, and the first-order perturbation equations are solved for the dynamic characteristics. The influences of various bearing parameters on the static and dynamic characteristics are evaluated. Results from the numerical simulation prove that the proposed method is a valid mean of estimating the static and dynamic characteristics of the porous gas thrust bearings.
Damping factors and natural frequencies of a flexible rotor supported by a gas bearing with piezoelectrically adjusted flow, are theoretically determined using a rotor finite element model coupled with the modified Reynolds equation. An extra term is added to the standard formulation of Reynolds equation aiming at incorporating the effect of the adjustable external pressurized inlet flow. Two different configurations are theoretically as well as experimentally studied: (a) the air is injected from a single orifice positioned at the bottom of the bearing and (b) the air is injected through four radial injectors equally pressurized. For the two configurations, the theoretical results are experimentally validated as a function of the piezoactuators input voltage and the journal angular velocity. Results show a good agreement for natural frequencies and damping factors. Theoretical and experimental results show qualitatively as well as quantitatively that the injectors position and the injection flow (dependent on the piezoactuator input voltage) have an important influence on the dynamic characteristics of the rotor-bearing system. By using one single injector positioned at the bearing bottom, the damping factor associated with the first mode shape can be increased by 10 times when compared to four injectors equally pressurized.
This paper proposes a new method for measuring normal interfacial stiffness. The contact interface is traditionally equivalent to some springs, but this general equivalent method is proved not sufficiently accurate in processing the elastic-plastic rough contact. The improved interfacial equivalent method is presented to remove the inaccuracy by introducing a contact layer, which considers the plastic zone beneath the rough surface. On the basis of the new method, the image method is proposed for measuring the improved interfacial stiffness. This method is fulfilled by using an image registration method and a sub-pixel algorithm to obtain the deformation of the contact layer and the smooth layer. Simultaneously, the ultrasonic measurement is carried out to measure the ultrasonic interfacial stiffness. The comparative results reveal that the measurement data of the improved interfacial stiffness show better agreement with the numerical results.
Based on ferrofluid flow model given by R.E. Rosensweig, a general equation for different slider squeeze film-bearing design systems, formed by solid upper surface and lower porous plate, is theoretically derived considering the effects of porosity, permeability, squeeze velocity, tangential velocity and oblique variable magnetic field. While deriving the general equation, continuity equation and Darcy’s law are also considered. Expressions for pressure and load-carrying capacity for different squeeze film-bearing design systems are obtained. The results for dimensionless load-carrying capacity are computed and compared with previous results in some cases. The results indicate the better performance of different bearing systems when ferrofluid is used as lubricant. Further, some important conclusions are also made. Two permeability models – globular sphere and capillary fissures are discussed. The variable magnetic field is considered because uniform magnetic field does not enhance bearing performances.
Many industries require contactless transport of delicate or clean products such as silicon wafers, flat foodstuffs and freshly painted objects. In this study, a contactless air film conveyor for flat objects is introduced. The object is supported by a thin film formed between the object and the conveyor surface and transported by viscous traction which is generated by controlled airflow underneath the object. Experimental and theoretical investigations are conducted on the basic characteristics (flow rate characteristics, film pressure distribution and viscous force). A theoretical model based on Reynolds equation coupling with incompressible Navier–Stokes equation is established to help analyze and understand these characteristics. The results show that air film pressure is symmetrically distributed along the direction perpendicular to the airflow in the actuating cells and non-symmetrically distributed along the airflow direction. The viscous force obtained by differentiating the pressure distribution implies that it consists of an actuating force produced by airflow in the pockets and a drag force resulted from airflow across the dam area. In addition, the influences on the viscous force from gap thickness, suction flow rate, inner bulge and depth of the pocket are discussed.
Copper is added in non-asbestos organic (NAO) friction materials (FM) to increase the thermal conductivity and to enhance the lubricity at high temperature apart from the well-accepted fact that it influences tribological performance also. Copper is being phased out slowly in FM formulations because of its proven threat to the aquatic life. Nowadays, the research in this area is being focussed on finding alternatives to Cu.
Hexa-boron nitride (hBN) also known as white graphite is a ceramic powdery filler, having crystalline structure similar to that of graphite along with various desirable properties (such as higher thermo-oxidative stability and thermal conductivity) which could be possibly suitable for developing Cu-free FMs. Hence, it was thought worthwhile to explore hBN as a potential ingredient in FM formulation for replacing copper. Five FMs in the form of brake shoes were developed, keeping all other ingredients identical and varying combinations of two solid lubricants viz. graphite and hBN systematically. The amount of these two solid lubricants was purposefully kept high (20%) so that the effect on performance properties could be visible clearly. Samples were tribo-evaluated using chase test rig as per IS 2742 standard. The paper discusses on the influence of these solid lubricants on performance properties of the series of five FMs.
The friction and wear phenomena in metal–polymer gears are directly connected to the kinematic and load conditions at each location on the flank and face of the tooth. This study investigates the characterization of the distribution of friction, temperature, and wear in the regions close to the pitch point of the contact between the teeth of a polyamide–steel gear. Twin-disc test configuration is considered using polyamide (PA66) disc and nonalloyed structural steel C45 disc with different diameters. The effect of the slip ratios (4%, 12%, 20%, and 28%) were studied at a constant pressure of 34 MPa, and a constant rotational speed of 300 r/min. The contact conditions were controlled with the measurements of the friction coefficient and the two discs surface temperatures using a thermal imager. The results have proven that the wear and friction are closely related to the contact temperature generated by the sliding phenomenon. For points closer to the pitch point, the coefficients of friction and temperature are characterized by a quasi-linear increase with time, and have a similar pattern at both flank and face sides. For these points, the wear increases slowly. For points far from the pitch point, the coefficients of friction and temperature reached a steady state after an initial linear increase. For these points, the friction coefficient and temperature evolutions with the test duration show a gap between the flank and face sides of the tooth. The wear increases dramatically at the flank side, whereas it increases slowly at the face side of the tooth. During the test, a transferred film of PA66 was formed on the steel surface causing the development of adhesive interactions between the contacting discs which increase the friction coefficient and the contact temperature.
In this work, the influences of flexibility and surface texture on the performances of a finite-long journal bearing have been investigated theoretically. Reynolds equation has been solved by using finite difference method. In this study, Winkler foundation model has been used to calculate the elastic deformation of the bearing bush or housing. The steady-state characteristics of a lubricated finite-long journal bearing are studied in the textured and untextured bearing surface, respectively. The simulation results of the present study indicate that the surface texture can appreciably change the shaft center-line locus and oil film pressure distribution comparatively to the smooth case. The texture arrangements on the bearing surface, the texture number in the circumferential or axial directions, and the texture depth in the radial direction, all of them have significant influences on the static characteristics of a flexible journal bearing.
The slip surface has a significant effect on the tribological performances of a slider bearing and a hydrodynamic journal bearing. Less attention has been paid to the influence of a boundary slip on a hybrid journal bearing. However, hybrid journal bearings are increasingly used due to their intrinsic advantages. The effects of slip surface on the performances of a hybrid journal bearing are studied in this work. The affecting rule of both the slip-region location and the size on the load-carrying capacity is obtained. Only a well-designed slip surface could improve the tribological performances of hybrid journal bearings. This work could provide a valuable guide for the design of a slip surface in high-speed hybrid journal bearings.
Reliability of the traditional single screw compressor (SSC) is poor since its discharge capacity decreases sharply. This is because that the lubrication between the screw and the gate-rotor meshing pair is poor, which leads to a rapid abrasion of the meshing pair. In this paper, a simulation model is established to analyze the lubrication problem of the meshing pair in the SSC. Based on the traditional single straight line enveloped meshing pair profile and the new proposed multi-columns-enveloped meshing pair profile, the hydrodynamic lubricating characteristics of the water-flooded single screw compressor are investigated. Results show that the water film can keep the gate-rotor tooth from contacting the screw thread flank during the meshing process in the optimized multi-columns-enveloped meshing pair, but the water film cannot keep the gate-rotor tooth from contacting the screw thread flank during the meshing process in the single straight line enveloped meshing pair. A prototype of the water flooded SSC with the optimized multi-columns-enveloped meshing pair was developed and has passed a 2000-h endurance test without discharge capacity reduction.
A steady-state hydrodynamic analysis of two-axial groove oil journal bearing is investigated in this paper. A computer program is developed to calculate the steady-state hydrodynamic characteristics. To study the various steady-state hydrodynamic characteristics of oil journal bearing, the nondimensional Reynolds’ equation is solved to obtain nondimensional values of load-carrying capacity, flow rate, attitude angle, and friction variable. Nature of design parameters for different groove configurations is studied and using genetic algorithm, an optimized groove location is obtained for two-axial grooved bearing. It has been found that placing the groove on different locations have greater influence on the design parameters such as nondimensional oil flow rate, frictional variable, and nondimensional load-carrying capacity. From numerical simulation of two-axial groove bearing it has been found that the frictional variable and nondimensional load-carrying capacity is better for up–horizontal configuration and flow rate shows a maximum value for some up–up configuration.
This research article deals with numerical study of micropolar lubrication in constant flow valve compensated hole entry hybrid journal bearing by considering the thermal effects. The bearing performance characteristics are computed by the solution of modified Reynolds, three-dimensional energy, and three-dimensional conduction equations. The results obtained numerically indicate that bearing performance is significantly affected by increase in temperature. Hence it is imperative to consider the thermal effects for bearing operating with micropolar lubrication to generate realistic bearing characteristic data.
This paper presents a unified mathematical model spanning the entire mixed lubrication for rolling-sliding line contacts. A load-sharing parameter is defined to divide the lubrication spectrum into three regions with increasing proportion of solid load sharing: light-mixed lubrication, significant-mixed lubrication and near-boundary lubrication. The main effort of the work is to develop a model for significant-mixed lubrication and integrate it with two previously developed governing-equation based models for light-mixed and near-boundary lubrication. A preliminary model is first developed by means of smooth, monotonically increasing interpolations of key contact variables obtained by the light-mixed and near-boundary lubrication models. The variables include surface temperature in the contact, friction coefficient, friction power intensity and proportion of solid contact area to the area of the line contact. The model is then refined by factoring in a potential transition of surface tribofilm breakdown which may take place in the second half of the region of significant-mixed lubrication. A basic parametric analysis is presented in the paper to show the sensibility of the model in analyzing the effects of operating conditions on contact stability. The model may be used to perform basic design and analysis calculations for line contacts with various degrees of mixed lubrication. It may also be implemented into a gear-meshing model to study the gear performance under various temperature and lubrication conditions.
In the automation filed, low-backlash gearboxes are required to guarantee precise positioning. For such kind of applications, planetary speed reducers represent one of the most attractive solutions. This type of gearing ensures at the same time high power density and reduction ratios. On the other side, the compactness of the solutions leads to high operating temperatures. For this reason, it is important to be able to quantify the power dissipation and the operating temperatures already in the design stage, therefore to be able to find the best compromise between the load carrying capacity and the maximum transmittable power due to thermal limitations. For this reason, an innovative calculation method capable to quantify the efficiency under different operating conditions and the related operating temperatures was developed. Experimental tests were performed under different operating conditions to validate the predictions. The comparison shows good agreement.
A 2.5 MW wind turbine gearbox design was considered to perform a power loss prediction using different wind turbine gear oil formulations. A gearbox power loss model, previously validated with experimental results, was used to predict the efficiency of a full wind turbine planetary gearbox. The power loss model account the gears and rolling bearing losses using well established models calibrated with a method proposed by the author. The calculations clearly showed that significant energy savings can be achieved by selecting different base oils, modifying gear tooth geometry, or combining both.
The article aims to investigate the relationship between eccentricity ratio and the Sommerfeld number based on hydrodynamic water-lubricated journal bearings. Considering the differences between water and oil, especially the difference in vapor pressure, the effect of cavitation on pressure distribution of water film is analyzed based on three boundary conditions and a cavitation model by computational fluid dynamics (CFD). The result shows that the three boundary conditions are all not suitable to replace cavitation model in numerical analysis of water-lubricated journal bearings. Then numerical analysis of journal bearings with different dimensions is undertaken with the cavitation model under different working conditions. Based on the analysis, a reference is produced for designing water-lubricated journal bearings under hydrodynamic lubrication. At last, the reference is verified by experiments.
The improvement of environment efficiency of automotive internal combustion engine becomes a fundamental objective. The cylinder engine surface texture considerably influences the functional performances of the ring-pack tribo-system. These surfaces are obtained by honing process that was pioneered in the last decade. Several innovative honing techniques were developed resulting in new surface textures with different cross-hatch angle obtained after several stages: the rough and finish honing and final stage. The aim of this study is to investigate the effect of variable cross-hatch angle generated at a final stage. These measured obtained surfaces are measured by a 3D white light interferometer and used as input data of the numerical model to estimate the friction. The results show the importance of this ‘residual’ angle.
A formula based on Winkler foundation and generalized viscoelastic Maxwell model of belt backing material has been given by Thomas J Rudolphi to determine the indentation rolling resistance. However, this formula must be solved by the iteration. As a result, the influence of some common working conditions can be reflected indirectly on this formula that results in the formula not being useful practically. On the other hand, the formula which is developed by Jonkers can reflect firstly the effect of working condition directly except for belt speed, but this formula has an inherent deficiency where Jonkers assumes that the indentation depth is independent of the belt speed. However, if the vertical load is assumed to be constant then the indentation depth and the contact will decrease with increasing belt speed in nature. Therefore, it offers an advantageous reference for simplifying the Rudolphi’s formula to get an analytic formula which can exhibit the influence of working condition on the rolling resistance. In this paper, author analyzes the effect of common working conditions on rolling resistance with the help of Rudolphi’s formula firstly. Then, in the premise of satisfying the suitable precision, author simplifies the Rudolphi’s formula to get an analytic formula. Finally, a verification example based on experiment is given to validate the rationality of analytic formula which may be of interest to belt-conveyor designers.
This article investigated the tribological performance of the specially formulated chlorine-free lubricant in strip drawing of advanced high strength steel. Four different lubrication conditions (dry, chlorine-free lubricant, chlorine additive lubricant, and mineral base lubricant) at two sliding speeds (10 and 100 mm/min) were carried out to observe the friction coefficients of the die-workpiece interface in the strip drawing test. The main difference among these lubricants was the contents of chlorine and sulfur additives. The die and workpiece materials were SKD11 and JSH780R, respectively. The results showed that the combination of chlorine and sulfur additives provided the best tribological behaviors. In addition, only the small amount of sulfur content could establish a bond with metal surfaces. However, the higher sulfur content could interact with metal surfaces, because it was influenced by the increased temperature (higher sliding speed) and adsorption.
Gas foil bearings have bright application prospects in oil-free turbomachinery such as aircraft air cycle machines, compressors and gas turbines. To extend the applicability of foil bearings to high-speed and heavy-load systems, the axial force produced primarily by the pressure difference between the turbine and compressor sides must be taken into consideration. The thrust disc of the rotor is typically used to sustain the axial force and maintain the attitude of the rotor in a rotor-bearing system. In an analytical model used to predict the performance of gas foil thrust bearings (GFTBs), fluid–structure interaction must be considered because of the coupling effects of hydrodynamic lubrication and the compliance of bearing surface. In this study, a link-spring structural model was employed to calculate the equivalent vertical stiffness of the bump foil. This model exhaustively considered the effect of three factors: flexibility of the bump foil, interactions between bumps, and frictional forces at the contact surfaces. In addition, top foil deflection that directly affects film thickness is calculated using a finite-element shell model. The static and dynamic characteristics of GFTBs were predicted after solving the steady-state Reynolds equation and two linearized dynamic coefficient equations which were obtained with perturbation method by using finite-difference method. The structural deformation equation was substituted into the Reynolds equations in the calculation process. The calculated static load was compared with the published experimental data, and the results validated the theoretical analysis. Thereafter, the static and dynamic characteristics including impact of rotational speed, initial minimum film thickness and the ratio of the inlet to outlet film thicknesses were analyzed. Parametric studies were also conducted to evaluate the effect of different structural parameters of the bump foil and journal tilting on the static characteristics of GFTBs. Because the proposed analytical model is completely algebraic, it can be easily programmed for accurately and effectively investigating the performance of GFTBs.
The influence of high-frequency vibration parallel to the sliding direction on friction force was analyzed by taking dry sliding friction pair of polymer composite and metal as the object of the research. A model of friction reduction with in-plane high-frequency vibration was proposed. The friction coefficient as a function of scale and direction of relative sliding velocity was discussed. The friction coefficient as a function of the amplitude of in-plane vibration velocity and sliding velocity was experimentally studied with a self-developed high-frequency vibration tribometer. The experimental results are in agreement with theoretical predictions. The results show that in-plane high-frequency vibration has obvious effect on reducing friction force under certain conditions.
Mechanical engraving surface texturing methods are proposed, and pin-on-disc configuration tribological tests are carried out to investigate the effect of microgrooves, with different microgrooved direction and area ratio, on tribological performance of rock bit sliding bearing (RBSB). The RBSB, always working under low-speed and heavy-load conditions, is made of 20CrNiMo /beryllium bronze. Compared with untextured samples, wavy microgrooved tribopairs with area ratio of 5% and reference angle of 15° exhibit lower friction coefficient and wear rate. Experimental results show a reduction of 54% in friction and 78.8% reduction for wear rate, revealing that microgrooves show great potential in improving the service life of RBSB.
It is difficult to evaluate the characteristics of traveling wave ultrasonic motors (TWUSMs) during the whole life of friction material (FM) because the wear of FM is quite slow. To solve this problem, an accelerated life test method was proposed and the mechanical and electrical characteristics of TWUSMs in the whole life of FM were analyzed systematically. The results show that the mechanical characteristics including speed and efficiency have a decline trend after an ascent with the increase of FM thickness, especially when the load torque is great. When the load torque is greater than 0.4 Nm, the optimum range of FM thickness is 0.10–0.20 mm. When the residual FM thickness is less than 0.17 mm, holding torque is greater than stall torque. Electrical characteristics of TWUSM show monotonicity in the whole life of FM. Therefore, the variations of electrical characteristics could be the wear indicator of FM. Based on experiment analysis, mechanism of interactions and couplings in the energy transfer process of the TWUSM system was explored. The energy transfer process of TWUSM was divided into four subprocesses. The wear of FM is in the intermediate subprocess, which has influence on both output performance and drive circuit of TWUSM. The four subprocesses interact and couple with each other closely. Consequently, the mechanical part and electrical part of TWUSM should be not isolated, and not only the forward action, but also the backward action in energy transfer process should be considered for a more realistic theoretical model of TWUSM.
Cryogenic technology is used in various engineering and industrial applications such as aerospace, automotive, defence and medical fields due to its better cooling and /or lubrication action. Conventional materials used in cryo-techniques are substituted with metal matrix composites (MMCs) due to their high strength-to-weight ratio. Aluminium-reinforced titanium carbide particulate (Al-TiCp) composite has gained attention due to its superior thermomechanical and tribological characteristics, but the effect of cryogenic temperature technology on Al-TiCp MMC, have not been studied substantially. So an effort has been made in this current research work to bridge the gap. To determine the cryogenic tribological behaviour of Al-TiCp composite, sliding wear tests are conducted. A customised cryogenic-tribo setup has been developed by attaching cryogenic system to pin-on-disc configuration. Nozzle arrangement to the cryogenic-tribo setup feeds cryogenic fluid (liquid nitrogen: LN2) exactly into sliding contact with a restricted flow rate and constant pressure for better cooling and lubrication. Wear studies have been carried out under different applied loading and sliding velocities under LN2, cryogenic chilled air and dry environment. The present analysis reveals that the weight loss of composite sample increases linearly with increase in normal load and sliding distance whereas decreases with increase in sliding velocity. Auxiliary observations revels that presence of LN2 in sliding contact offers significant reduction in friction and wear values when compared to other environments. The performance improvement with LN2 could be due to the hydraulic pressure of trapped LN2 into the sliding contact which takes away a part of normal load resulting in reduced apparent friction coefficient and wear rate. Microscopic images of worn surfaces showed the cracks and fracture in the wear track over sliding surfaces is high in dry than compared to LN2 condition.
Although the spiral grooved opposed-hemisphere gas bearings have been widely applied in supporting the spin axis of inertial grade gyroscopes, the accurate prediction of the performance characteristics of the bearings is still very difficult because of their structural complexity. In this paper, the static characteristics of the bearings considering the coupling effects between the journal and two hemisphere bearings are obtained theoretically. The Reynolds equations modeling the journal and hemisphere bearings are solved simultaneously by the finite element method satisfying the continuous boundary conditions between these three bearings. The fluctuations of the bearing load and reaction torque caused by rotating grooves are shown and can be neglected at large number of grooves. The results show that neglecting the coupling effects between the journal and two hemisphere bearings will result in large discrepancies at high rotational speeds and large eccentricity ratios. And the hydrodynamic effect can be divided into two regions for spiral grooved gas bearings: the groove effect domain region and the converged wedge effect domain region. A parametric study provides efficient guidance for the optimal design of the opposed-hemisphere gas bearing.
Surfactants are important molecules to decide the lubricating characteristics in aqueous lubrication. In metal working fluids, stabilization of oil droplets in continuous water phase is aided by repulsive force created by surfactants at oil–water interface. Present study explores the possibility of participation of surfactants molecules in lubrication process. Effect of surfactant hydrocarbon chain length and type of head group in aqueous lubrication is explored. Aqueous solutions were prepared using Oleic acid, Tween 20 and Tween 80 surfactants. Goniometer was used for recording surface and interfacial effects of surfactants in water and on steel substrate. Lubricating response of aqueous solutions of surfactants was recorded using Tribometer. Ability of a surfactant to adsorb the steel surface along with longer hydrocarbon chain length is well suited for aqueous lubrication. Solubility of surfactant in water is another important parameter.
Advanced engineering polymers of the polyaryletherketone (PAEK) family with carbon fibre reinforcement are finding application in engineering systems as tribological bearing surfaces under severe operating conditions that cyclically move the polymer into and beyond the glass transition temperature region. To support such an application, the friction in high-speed and low-load PAEK-steel sliding contacts was investigated both unlubricated and lubricated with a trinonyl trimellitate ester, a base fluid for high temperature industrial lubricants. Four polymers in the PAEK family, PEEK, PEK, PEKEKK and PEKK, with 30 wt% of carbon fibre whiskers were tested against an AISI 4140 steel disc. When unlubricated, low friction depended upon the formation of a PAEK transfer film on the steel disc and when this became unstable in the glass transition region the friction increased to much higher levels with associated polymer surface damage. Frictional heating due to the high sliding speed dominated the differences in glass transition behaviour between the four PAEKs. When lubricated, the lubricant film controlled friction and there was no significant effect of the glass transition of any of the PAEKs. The irreversible nature of the glass transition in PAEKs in such tribological applications, due to high surface damage at high temperature, means that it is essential to ensure effective lubrication in both fluid film and boundary lubrication.
Acoustic emission (AE) signal generated from artificial defects in rolling element bearings are investigated using experimental measurements in this paper. Rolling element bearings are crucial parts of many machines and there has been an increasing demand to find effective and reliable health monitoring technique and advanced signal processing to detect and diagnose the size and location of incipient defects. Condition monitoring of rolling element bearings, comprises four main stages which are, statistical analysis, faults diagnostics, defect size calculation, and prognostics. In this paper, the effect of defect size, operating speed, and loading conditions on statistical parameters of AE signals, using design of experiment method, have been investigated to select the most sensitive parameters for diagnosing incipient faults and defect growth on rolling element bearings.
The application of abradable seal coating is an effective method to improve the efficiency of turbine engines. The AlSi-polyester is a typical one, which has been widely used in mediate and low pressure compressors. To evaluate abradable performance and investigate wear mechanisms of these seal coatings, a high-temperature and high-velocity tester was constructed, with 520 m/s maximum blade tip speed and 5–2000 μm/s incursion speed and 1200℃ maximum heating temperature. Abrasion tests with blade tip velocities between 150 and 300 m/s and incursion rates between 13 and 500 μm/s have been conducted under 350℃ to investigate the wear behavior between Ti6Al4V blade and an AlSi-polyester coating sample. The coating abradability was evaluated after abrasion tests through analyzing the mass losses, wear morphology, and interaction force of the coating and blade. It is found that the AlSi-polyester seal coating used in our research has good abradability to protect the Ti6Al4V blade tip, and abrasion wear is its predominant wear mechanism with furrow surface morphology. Incursion per pass has significant impact on the variation tendency of interaction force, with tests at low incursion per pass showing stable tendency and lower force, whereas tests at high incursion per pass showing gradual rise before relatively stable trend and higher force. This work provides a new basic data for the preliminary study of the interaction between blade tips and abradable coatings in aero-engine applications.
The performance characteristics of capillary compensated tilted pad hydrostatic bearing operating with Newtonian lubricant and cubic law lubricant are studied numerically considering the flexibility of pad. The two-dimensional Reynolds equation coupled with three-dimensional equilibrium equations governing the deformation of thrust pad has been solved by using finite element method. The results indicate the influence of tilt parameter and bearing deformation on bearing performance characteristics parameters. In the analysis, four different types of recess shapes such as rectangular, circular, elliptical, and square have been chosen for the analysis. The results show that the consideration of bearing pad deformation significantly increases the value of fluid-film stiffness coefficient and decreases the value of fluid-film damping coefficient. The fluid-film pressure distributions also compared for circular, elliptical, square, and rectangular bearing.
By means of numerical simulation the elastohydrodynamic lubricating film formation during start-up of motion of a line contact from rest is studied. The transition from solid contact to lubricated contact is of importance in investigating the start-up process and its effects on bearing performance. The numerical results of Holmes et al. (2002) and others, concerning point contact and Newtonian lubricant, are not in good agreement with the known experimental data. The numerical film thickness is significantly smaller than the experimental values and the experimental velocity at which lubricant travels through the conjunction does not equal the entrainment velocity of numerical results. In the current paper, a lubricant with viscoelastic properties is considered. The Maxwell model of a lubricant at pure rolling of elastic cylinders is used. Also, as well as in work of Usov (2008) the model assumes the division of the whole area of contact into three zones: the dry contact of surfaces, the transient zone and the zone in which the surfaces are separated by a liquid lubricant layer. The rigid body model is assumed for the lubricant in the transient zone. The effects of viscoelasticity and size of transient zone on film thickness and the velocity at which the lubricant moves through the contact are studied. It is shown that viscoelasticity raises film thickness in a start of the motion and lowers the velocity at which the lubricant moves through the contact. This velocity is lower than the entrainment velocity in the first half of the contact, as in the experimental results with point contact. In the second half of the contact, this velocity may be lower or higher than the entrainment velocity depending on the size of the transient zone. The film thickness may increase considerably at the start of motion when the size of the transient zone increases.
Two main effects control the clearances and pressures in rough elastohydrodynamically lubricated contacts. The first is the local attenuation produced by any relative sliding of the surfaces, the second is the clearance variation that is generated in the inlet and which moves through the conjunction at the mean surface velocity – the complementary wave. The first component, which is absent in rolling contacts, is relatively easy to estimate. The second is more complex, particularly in soft contacts, where, for sinusoidal waveforms, it may have a wavelength that differs from the original profile. In addition it will generally decay in amplitude during its transit through the conjunction. The paper explores this component and outlines some of the factors that determine its behaviour with both sinusoidal and more general roughness profiles.
In this work, the effects of preload factor on the stability performance of noncircular two and three lobe journal bearings with incompressible micropolar lubricant are presented based on the linear and nonlinear dynamic models. Assuming that the rotor is rigid, the governing Reynolds equation for the hydrodynamic lubrication of finite length lobed journal bearings has been modified using micropolar theory. Then, the linear and nonlinear dynamic models which include a certain harmonic disturbance and a time dependent trajectory of rotor center are applied to analyze the stability performance of the considered bearings. The generalized differential quadrature method and fourth-order Runge–Kutta technique have been used to solve the governing Reynolds equation and time-dependent dynamic equations of rotor motion, respectively. Finally, the numerical results for the critical mass parameter and the whirl frequency ratio of the rotor as the stability characteristics of the lobed bearings are obtained for different values of preload factor and are compared together. Results show that the dynamic stability of the noncircular bearings is enhanced by decreasing the preload factor i.e. increasing the amount of bearing noncircularity. These enhancements are in terms of increase of the critical mass parameter and decrease for the whirl frequency ratio. Also, by comparing the two dynamic analysis methods, it is seen that the results of linear model are more conservative in different investigated cases. The results of nonlinear analysis reveal that by changing the preload factor the dynamic responses of the lobed bearings appear in different manners. From the responses, it is observed that the type of dynamic trajectory of rotor center varies from stable fixed point to limit cycle periodic motions and also contact between rotor and bearing’s shell.
The application of magnetic fluid in ferrography has improved the technique for separating nonferrous wear debris by magnetic means. Brass debris was used as an example to analyze the magnetization mechanism based on ferrographic experiments and molecular analysis. The results show that the nanomagnetic particles in magnetic fluid attach to the surface of the micro nonferrous debris by the functional group of surfactant molecule so as to develop in that debris a positive susceptibility and make it responsive to the magnetic field. The magnetic action of the magnetic particles is selective as to the property of the debris surface to which it attaches, and the magnetization time depends on how readily of the attachment. Mathematical derivation based on mechanical models shown that the magnetic force exerted on the debris is strong enough for its precipitating on ferrogram.
Noise from the cavitation around propellers is the main noise source of marine vehicles. In our experiment, we found that the material of the propeller also affected the cavitation noise, especially on their frequencies. Three most commonly used metal propeller materials, including brass (B), aluminum bronze (AlB), and stainless steel (SS), were placed in an ultrasonic cavitation system. They presented different spectral peaks in the measured cavitation noise spectrum, and the one with higher elastic modulus has a higher noise frequency. According to the numerical validation, this phenomenon is attributed to the elastic response of different metals under the impaction of the collapsing bubbles, and the elastic modulus of the metals is the dominant mechanical factor that affects the frequency.
High-speed aerostatic spindles are essential for ultra-precision micro-milling machine tools. The distinguishing functional characteristics of the spindle system are achieved through innovative design and integration of thermal, fluidic, and electromagnetic physics on the spindle structure and system. These physics, together with the shaft dynamics, interact dynamically and collectively to determine the spindle’s performance. It is thus required to have a comprehensive analysis and scientific understanding on the multiple physics interactions within the spindle, which is essential for design of the spindle working at much higher speeds and accuracy under various increasingly stringent engineering conditions. This paper presents an integrated multiphysics simulation approach to design and analysis of high-speed aerostatic spindles. Based on the proposed approach, an integrated multiphysics simulation platform is developed with application to micro-milling machines. Experiments have been designed and carried out to validate the integrated multiphysics simulation method and evaluate the simulation results, which show that the integrated multiphysics model is able to predict and analyze the performance characteristics of the high-speed aerostatic spindle. The integrated multiphysics model and the corresponding virtual spindle simulation can be used as a powerful design aid for supporting the design and analysis of next generation high-speed aerostatic spindles at nanometric precision.
The present study deals with numerical simulation of fluid flow within a single-groove journal bearing under laminar regime and steady-state condition. The lattice Boltzmann method was employed to simulate lubricant flow in the bearing. A linear interpolation method was implemented to satisfy the no-slip condition on the bearing boundaries. The validation with previously published data reveals that the present methodology can evaluate the flow field of journal bearings with an acceptable accuracy. Also, since the present approach was entirely local in behavior it can be adapted for parallelization.
Since many years, the pioneering work of Hu and Tonder is used to generate rough surfaces with prescribed statistical moments (skewness and kurtosis) along with spatial properties (correlation lengths). The present work enlightens the drawbacks of this method and it proposes an original approach based on a hybrid analytical/numerical method. Simulations are conducted on very different surface specimens and the method is validated over a wide range of statistical moments. The results are obtained with high accuracy (beyond what is usually needed) and very short computing times (the order of a second).
In large two-stroke marine diesel engines, bearings are designed to last the lifetime of the engine. The design has shown very good service experiences. The design parameters of the main bearings are, among others, based on the average maximum specific load which the bearing should operate under. In general, the frictional loss is less than 1% of the nominal power of the engine but is still a target for optimization. Fatigue mechanisms of bearing lining material are not fully understood and the design limits with regards to minimum oil film thickness, max oil film pressure and oil film pressure gradient are not established. Large two-stroke journal bearings are not suitable for fatigue test due to the size, the low rotational speed and the complexity of such a test-rig. The disc fatigue test rig was designed with the purpose to test white metal coatings under realistic bearing conditions, in a confined time-frame. The test-rig simulates a scale model of a thrust bearing, in contrary to standard design, the bearing lining material is applied to the rotating collar. Parameters, such as bearing load, rotational speed, oil temperature, oil contamination is controlled/monitored in order to achieve repeatability and a systematic approach to the experiments. Test performed on the test-rig shows good correlation on the fatigue cracks with those experienced on large two-stroke journal bearings.
In this paper, the influence of surface roughness on the local tribological load with a dry sliding contact is studied. First, three artificial rough surfaces with similar structure but different asperity heights are generated and projected on a smooth ball. After that, a contact pattern is determined between a rough ball and a smooth surface taking into account the elastic only as well as the linear elastic-perfectly plastic material description. On the basis of the calculated contact pressure distribution, the subsurface stresses and a three-dimensional temperature distribution in the sliding contact are calculated. The solutions show that a low surface roughness not necessarily results in low local tribological load of the surface.
To improve operational safety and/or achieve a higher load capacity of turbine tilting-pad bearings, an axially concave pad profile is presented. The thermal and mechanical stress of the loaded pads of a test bearing in load between pivot configuration has been analysed. Both film thickness and pressure distribution have been measured at a very high resolution. A fluid film calculation program in combination with a finite-volume-based structural mechanics program is used to simulate the deformation of a single pad under high circumferential speeds. In this context, the axial and tangential heat transfer coefficients of the pad surface, which act as boundary conditions for the calculation of the 3D temperature distribution, are determined using an optimization process. Herein, the match of predicted and measured pad temperatures is the goal. It can be shown that there must be a huge difference in heat transfer in axial and tangential direction in order to match the large measured temperature gradient in circumferential direction. Based on the measured deformed profile the program code is used to derive a concave pad profile, which will result in an axially non-arched sliding surface under the expected thermal load. Therefore, an iterative simulation procedure is used. By decreasing the axial arching of the pad and thus the large film thickness at the axial ends using an improved profile designed for a specific operation point, the minimum film thickness and maximum pad temperature can be influenced beneficially. The comparison of measurement data and calculation results shows very good agreement regarding the pad deformations. The results indicate that by axially concave profiling of the loaded pads of a large tilting-pad bearing for a specific operation point, the static characteristics in the form of temperature, film thickness and load capacity can be improved.
In order to gain a fundamental understanding of material behaviour at elevated temperatures a unique tribometer was designed which can operate at temperatures up to 1000℃. The test takes place in vacuum in order to avoid oxidation and the normal load range varies from 10 to 500 N. It is thus possible to describe the evolution of hardness over a broad range of temperatures and loads. This can give indications of possible microstructural modifications, which can be investigated afterwards on cross sections. For the characterisation of single abrasive phenomena on a very fundamental level, scratch tests at variable loads are proposed. The interaction of sliding surfaces can be simulated by adhesion testing. To this end an application specific counter body, e.g. taken from a field specimen, can be slid over the specimen surface at variable loads. Finally, it can be stated that this newly designed tribometer offers an enormous potential for deeper understanding of fundamental wear phenomena like ploughing, micro-breaking or adhesion occurring at high temperature. The possibilities of high temperature scratch and hardness testing with the new measurement system are shown on the common austenitic stainless steel 1.4301 (AISI 304).
The continuously variable transmission investigated in this paper works with contacts in the elastohydrodynamic regime of lubrication, thus the tangential forces between elements are transmitted through the shearing the lubricant film. The behaviour of the lubricant film when subjected to shear depends of the nature of the lubricant and the relative motion between the contacting surfaces. In this paper, a non-Newtonian behaviour is assumed for the lubricant while the relative motion is determined for every point on the contact area by kinematic methods. The net tractive force in the sliding direction and the spin torque are evaluated and from these the power losses in the contacts are calculated. The dynamic behaviour of the device is evaluated taking into account the behaviour of the lubricant as extracted from traction measurements. A simplified method for the evaluation of the dynamic response of the device to a rapid variation of the resistive torque is presented. The results show the importance of the rheological behaviour of the lubricant, measured by the slope of the linear region of lubricant traction curves.
The first operating period of machine elements is subject to a running-in phase. Thereby, stresses applied on the contacting surfaces are influenced by the running-in conditions including the lubricant used and can strongly affect the frictional behavior and load-carrying capacity of lubricated contacts after running-in. This study investigates the influences of operating conditions and lubricant additives on the running-in behavior of lubricated line contacts of discs and gears. Results show a strong influence of the lubricant additive on the coefficient of friction during running-in as well as on the surface roughness and surface condition after running-in. The specific boundary friction power, supplemented by considering load carrying capacity limits, has been introduced as a criterion for defining the running-in conditions of gears.
This study presents experimental results on the effect of out-of-contact lubricant channeling on the tribological performance of nonconformal contacts under starved lubrication. Channeling of lubricant was carried out by adding a slider with a limited slot for scraping the displaced lubricant on one of mating surfaces (ball). Thus, the scraped lubricant is forced to flow back into the depleted track through the limited slot resulting in robust replenishment. The measurements have been conducted using optical tribometer (ball-on-disc) equipped with a digital camera and torque sensor. The effect of lubricant channeling was compared to the original contact condition by means of measuring friction and film thickness. The results show that the out-of-contact lubricant channeling leads to a significant enhancement of film thickness and friction reduction under starved conditions. Indeed, the starved elastohydrodynamic lubrication contacts transformed to the fully flooded regime after introducing the flow reconditioning. Moreover, the film thickness decay over time, which is common with starved elastohydrodynamic lubrication contacts, has not been observed in the case of lubricant channeling. However, the beneficial effect of lubricant channeling diminishes as the original contact condition tends to the fully flooded regime. The results of this study can be easily implemented in practical applications such as radial and thrust rolling-element bearings.
Conformal surfaces in parallel sliding lack a macroscopic hydrodynamic pressure and fluid film formation mechanism. However, such a mechanism still exists on a microscopic level due to roughness. It is common to translate roughness into a variation of fluid film thickness which in turn yields a hydrodynamic pressure distribution resulting in a net hydrodynamic lift. Reynolds equation and a suitable cavitation algorithm suffice to describe this effect mathematically. In case one surface consists of a compliant material with low modulus of elasticity, the deformation of asperities due to pressures and shear stresses in the fluid cannot be neglected—in fact, besides cavitation, it significantly contributes to the net hydrodynamic lift. Therefore, a coupling between fluid dynamics and elastic solid body deformations needs to be introduced. An additional complication arises when the hydrodynamic lift and the subsequent separation of the mean lines of the contacting rough surfaces is not enough to prevent asperity contacts completely. This situation is known as mixed lubrication where part of the normal load is transmitted at asperity contacts. These contacts are commonly treated as solid body contacts with a Coulomb-like friction law or more sophisticated solid friction models. However, when considering asperities as contraformal Hertzian contacts, elastic deformation may allow for the existence of thin micro-elastohydrodynamic lubricant films preventing direct solid body contact even at speeds which otherwise would be regarded as deep within the mixed lubrication regime close to boundary lubrication. These films may not be able to prevent wear completely, but may reduce friction significantly in comparison to dry friction. In this paper, the existence of such effects is demonstrated both by simulation and by experiments with elastomeric radial lip seals.
This paper is part of a project aiming at optimizing the cylinder-liner/piston-ring contact performance: oil consumption, friction and wear. The surface micro-geometry has a major influence on these characteristics. Classical cylinder-liners display cross-hatched patterns. Grooves modify contact pressure distributions and act as lubricant reservoirs and pipes redistributing oil. The load-carrying capacity is greatly influenced by the number of grooves and their geometry. An automatic groove geometry identification (depth, width, angle) is performed on cylinder-liner surface measurements. The surfaces were measured at two instants: new and after a fired engine test. The micro-geometry evolution is discussed.
Lubricated friction and wear behaviour of Al–25Zn–3Cu–3Si alloy was investigated on a block-on-disc test machine in comparison with 60/40 brass and SAE 65 bronze. T6 heat treatment increased the hardness, strength and wear resistance of the Al–25Zn–3Cu–3Si alloy. This alloy exhibited much higher wear resistance than the 60/40 brass and SAE 65 bronze in both as-cast and heat-treated conditions. The results obtained from the experimental alloys are discussed in terms of their microstructure, mechanical properties and wear behaviour.
The effect of surface pattern (orientation) on film thickness, asperity load ratio, and traction coefficient in line-contact EHL is studied. Numerical simulations results of the modified Reynolds—which takes into account the effect of surface roughness and its orientation—bulk deformation of the surfaces, and statistical elasto-plastic deformation of the surface asperities are presented. It is shown that surface pattern influences the behavior of lubricant flow, which affects the film thickness and the asperity load. The results of more than 2000 simulations are used to develop expressions to quantify the effect of surface roughness pattern in estimating the film thickness and the asperity load ratio. As an illustrative example, the contact in spur gear teeth is studied to show the utility of the developed formulas. Moreover, the traction behavior is investigated by utilizing a thermo-elastohydrodynamic approach and it is shown how the surface pattern affects the hydrodynamic and asperity parts of the traction coefficient.
The effect of the centrifugal force of the fluid film on the performance of the high speed hydrostatic thrust bearing lubricated by low viscosity fluid cannot be neglected. Considering the centrifugal inertia effect, concentrated inertia effect at the recess edge, and the misalignment effect, the thermo-hydrodynamic lubrication model is established by using the reduced Navier–Stokes equation, the energy equation, and the bulk flow theory. The effects of centrifugal forces on dynamic performances of the high speed thrust bearing with different supply pressures and rotational speeds have been analyzed systematically. The results show that the centrifugal force reduces the axial and angular stiffness and hardly affects the damping coefficients; the centrifugal effects decrease with the increasing of eccentricity ratio and misalignment angle.
Wear-debris characterization using ferrography, which is of importance in machine condition monitoring and fault diagnosis, remains a challenge. The newly developed on-line visual ferrograph can provide digital wear-debris images and perform on-line analysis during wear monitoring. In this article, image projection transformation was utilized for extracting the overall characteristics of the wear-debris chains according to the fundamental feature of wear-debris arraying along the horizontal direction. Moreover, Full Binary Tree Based Image Division was also proposed to analyse the regional features on different scales in on-line visual ferrographic images. Several descriptive parameters including thinning ratio, chain length and chain width were proposed. In the experiments, four types of images with different wear-debris groups were compared and a group of time-sequence on-line visual ferrographic images of an inline four-cylinder gasoline engine was studied. It is found that more comprehensive wear information can be acquired through the proposed multi-parameter description method. Meanwhile, the image projection transformation method extracts macrocharacteristics of images rapidly and efficiently and the thinning ratio measures wear-debris width in an intuitive way.
Dry-sliding tribological performance of in-situ Fe3Al-20 wt.%Al2O3, (4 wt.%Cr)/Fe3Al-20 wt.%Al2O3, and (4 wt.%Cr, 5 wt.%Mo)/Fe3Al-20 wt.%Al2O3 composites, produced by mechanically induced self-propagating mechanism and plasma-activated sintering, was investigated against Inconel718 from 25℃ to 800℃ under air/argon atmosphere. It can be found that (4 wt.%Cr, 5 wt.%Mo)/Fe3Al-20 wt.%Al2O3 composite exhibits more excellent comprehensive tribological properties with the friction coefficient decrease nearly 67% compared to Fe3Al-20 wt.%Al2O3 composite above 350℃ due to the balance between strength and lubricity. At 800℃, the dominant wear mechanism is oxidation wear under air condition, whereas under argon atmosphere, the actual wear mechanism is mainly transferred to microploughing.
Wear tests were carried out using a pin-on-disc tester. The sliding friction experiments were made using textured and untextured discs of 50 HRC hardness, under starved lubricated conditions in unidirectional sliding. Two batches of tests were done. The special construction was made to obtain conformal contact in the first type of tests. In the second test type in non-conformal initial point contact condition, a steel disc was put in contact with a ball from bearing steel of 3.175 mm radius. Two kinds of textured discs were tested; the oil pockets were positioned in spiral and radial arrays the number of oil pockets within friction track also varied. It was found that in conformal contact conditions the presence of dimples resulted in considerable decrease of the friction force compared to untextured samples. Surface texturing caused transition from point to conformal contact during test, which resulted in decrease of friction force but increase of wear of balls. The tribological behaviour of assembly with spiral dimples layout was better than that with radial array of oil pockets.
A laminar skin simulant was constructed to study the incidence of friction blisters. The skin simulant consists of a thin polyurethane top layer and textured gum foam rubber bottom layer adhered to an acrylic-backing plate to emulate the layered structure of the human skin. Friction blisters were produced on the skin simulant by using a dual-axis tribometer. The effect of the applied normal load on the number of cycles required to produce a blister was investigated. The skin simulant was also analyzed as an adhesive-bonded laminar composite to determine the relationship between the applied normal load and the number of cycles for blistering. The normal load and the number of cycles were found to be inversely related and vary by a power law function, as observed in previous work on human subjects in Naylor’s pioneering study. The results obtained from the experimental data and the fracture mechanics modeling of the skin simulant indicate the potential of elastomeric skin simulant in providing useful insight into blister mechanics and other tribological properties of skin.
Experiments are conducted using a 10-MW Kaplan hydropower machine which is outfitted with an extensive array of sensors to determine oil film thickness, pad load and oil temperature in all three guide bearings as well as motion of the shaft in relation to both the bearing housings and the concrete foundation. Test results for all journal bearings are compared to a commercial rotor dynamics model and results for the central journal bearing are compared to a multi-physics model to provide insight into the machine's steady state and dynamic characteristics and their variations during normal operation.
The range of applications for self-structured surfaces is growing. They are used to increase wear resistance, reduce friction and corrosion, and also used in design of biosensors and innovative coatings. However, to effectively manufacture these surfaces on a large scale, methods for their texture characterisation/description are required. Currently, we do not have any effective and accurate methods to characterise these surfaces. This is severely hindering their wider applications and further developments in this area. The texture of self-structured surfaces, like any texture of any other surface, would need to be characterised/described during the formation (production) process and for specific applications. During formation process, the self-structured surfaces change texture roughness and directionality. These changes are gradual, complex and occur over many scales. In this work, a recently developed method, called an augmented blanket with rotating grid method, is applied to microscopic images of real self-structured surface textures. Groups of isotropic and anisotropic texture images were analysed. In the first group the textures were formed through the growth of nanorods on indium oxide substrates while in another the laser beam irradiation was used to treat the polymer films. Results obtained showed that the augmented blanket with rotating grid method accurately quantifies minute decreases in roughness of isotropic surfaces and changes in roughness with directions of anisotropic surfaces observed during the formation process.
Direct application of vegetable oil as automotive lubricants is less favorable due to its poor thermo-oxidative stability, cold flow behavior, and poor anti-wear characteristics. The investigation detailed in this paper pertains to chemical modification of rapeseed oil through epoxidation, hydroxylation, and esterification processes in order to improve its thermo-oxidative stability and cold flow behavior. The copper oxide (CuO) nanoparticles of size ~40–70 nm was dispersed in chemically modified rapeseed oil (CMRO) in three different concentrations (0.1 wt%, 0.5 wt%, and 1 wt%) as an anti-wear additive using an ultrasonic sonicator. The tribological behavior of cylinder liner against piston ring segment was tested with CMRO with and without CuO nano-additive using high frequency reciprocating rig tribometer. The morphological characterization of CuO nanoparticles dispersed in CMRO was done using scanning electron microscopy and X-ray diffraction analysis. The results were also compared with commercial synthetic lubricant under the same operating conditions. The atomic force microscopy and scanning electron microscopic analyses proved that the surface morphology of tested cylinder liner surfaces are smooth with optimum concentration of 0.5 wt% CuO nanoparticles dispersed in CMRO.
Biodiesel is a promising alternative among the different fuels put forward as a substitute for diesel fuel. One important issue in internal combustion engines is the lubrication of the equipments which are lubricated by the fuel itself. Previous studies have shown that biodiesel possesses excellent inherent lubricity. However, biodiesel is susceptible to oxidation reactions due to the presence of unsaturation in its moiety. These reactions are accelerated in the presence of oxygen and high temperatures and may alter the lubrication characteristics. In this particular research work, an investigation has been made to assess the impact of load, temperature, and biodiesel concentration upon friction and wear in the four-ball wear machine. The effect of individual parameter utilizing other biodiesels has already been presented elsewhere. The current study demonstrates the response of biodiesel and oxidized biodiesel with combined variation of operating parameters. The selected temperatures and loads were 45–60–75 ℃ and 40–50–60 kg, respectively. The tests were conducted with diesel (B0), B20 (20% jatropha methyl ester by volume), B40, B100, and oxidized biodiesel. It was found that these variables not only individually but also collectively affect the tribological performance of biodiesel.
To investigate the lubricating characteristics between the rolling piston and the vane, as well as the characteristics of rotating motion of the rolling piston in a rotary compressor, we perform a numerical analysis of the rolling piston and vane motion. The analysis considers mixed lubrication of the vane sliding surface to calculate the friction force between the vane and the vane slot. We find that the relative sliding velocity between the rolling piston and the vane can be reduced by reducing the rolling piston weight, and that the maximum PV value during one crank revolution between the rolling piston and the vane decreases due to the increased vane slot length in high-rotating-frequency regions.
Increasing interest has been paid to surface texturing in order to improve tribological properties of sliding surfaces. Currently, the patterns of micro-dimples have attracted more attention since such closed texture cells are supposed to generate additional hydrodynamic pressure easily. It has been proven that the area density of the dimple pattern is a critical parameter for hydrodynamic pressure generation. However, the optimal values of area density obtained by theoretical models are usually different to that obtained from experiments, which show material dependence. In order to understand the design principles of the area density for mixed lubrication regime, a brief review on the studies related to the area density issue of dimple patterns is carried out; the phenomena during contacting and sliding are analyzed numerically in this paper. It is found that the stress concentration and deformation will occur on the contacting and sliding area, and influence the tribological properties of the textured surface significantly depending on the area density and material properties. These negative influences should be considered carefully during surface texture design.
This paper proposes a numerical approach for the modeling and prediction of wear at revolute clearance joint in flexible multibody mechanical systems by integrating the procedures of wear prediction with multibody dynamics. In the approach, the flexible component was modeled by using absolute nodal coordinate formulation-based element, while the rigid component was described by employing the natural coordinate formulation. The clearance joint was modeled as a dry contact pair, in which the continuous contact force model proposed by Lankanrani and Nikravesh was applied to evaluate the normal contact force, and the friction effect was considered using the LuGre friction model. The calculation of wear was performed by an iterative wear prediction procedure based on Archard’s wear model. Using this approach, a planar slider–crank mechanism including a flexible rod and clearance joint was numerically investigated as a demonstrative example. Furthermore, the effects of the flexibility of the mechanism and the clearance size on the wear at clearance joint were also studied.
Plasma-sprayed molybdenum coatings are used in automotive, aerospace, pulp and paper industries in order to guard the machine parts against wear and corrosion. In this work, an attempt has been made to study the effect of particle size on wear behaviour of molybdenum coated steel. Two types of Mo coatings were deposited on American Iron and Steel Institute (AISI) 1020 steel substrate, one is with particle size of 15–40 µm (type I) and another one is with 40–90 µm (type II) using plasma spraying. The microstructures and worn surface morphologies of the coatings were analysed by means of X-ray diffractometer and scanning electron microscopy. It was found that the as-sprayed type I and type II coatings consist of molybdenum (Mo) as the major phase and molybdenum oxide (MoO2) as the minor phases. For the evaluation of influence of coating powder particle size on wear characteristics, the sliding wear tests were performed on a pin-on-disc apparatus. The variation of volume loss with applied load, sliding speed and sliding distance was monitored. The type I sprayed coating exhibits better wear resistance. Tribological testing was also supported by metallographic examination for the identification of wear mechanisms. It was verified that the wear of coating is dominated by fracture of splats, crack propagation, delamination and plowing.
For chemical-mechanical polishing of epitaxial gallium nitride (GaN), a two-step experiment method with two kinds of abrasives, aluminum oxide (Al2O3) and colloidal silica (SiO2), was put forward. The average material removal rates of GaN by the slurry with Al2O3 and SiO2 abrasives were 594.79 and 66.88 nm/h, respectively. An atomically flat surface with roughness (Ra) of 0.056 nm was obtained after the second chemical-mechanical polishing process with SiO2-based slurry, which presented an atomic step-terrace structure. The material removal characteristics of GaN surfaces were investigated in detail. A model was proposed to describe the different behaviors of the two kinds of abrasive during chemical-mechanical polishing process.
In this paper, the mechanical and tribological properties of synthesized ortho- and meta-cresol novalac epoxy resin composites have been explored. Three microsize fillers SiC, Al2O3, and ZnO particulates (10 wt%) have been used as reinforcement for the composites. The dry sliding wear tests were conducted on the pin-on-disc apparatus at sliding velocities of 0.9 m/s, 1.8 m/s, 2.7 m/s, and 3.6 m/s) and applied normal loads of 10 N, 20 N, 30 N, and 40 N. By evaluating the mechanical and tribological properties, positive synergetic effect was found. It was also found that the coefficient of friction increases and specific wear rate decreases with an increase in the sliding velocity, whereas specific wear rate increases with an increase in the applied normal and decreases with an increase in the sliding velocity. Also, the worn surfaces of novalac epoxy composites were analyzed with the help of scanning electron microscope.
Plasma-sprayed thermal barrier coatings are frequently subjected to impacts by solid particles, which induce surface erosion. This article depicts the solid particle erosion response of plasma sprayed nanostructured yttria-stabilized zirconia (YSZ) coatings on Inconel 718 substrates. The influence of five operating parameters i.e. impact velocity, erodent size, erodent temperature, impingement angle, and stand-off distance with four different level each, on performance output (i.e. erosion rate) are studied using Taguchi’s L16 orthogonal array design. Out of the five parameters, impact velocity has been found to be most significant factor followed by erodent temperature and impingement angle influencing erosion. Maximum erosion takes place at an impingement angle of ~60° showing the semi-ductile response of the coating to solid particle erosion. The morphology of the eroded surface also showed micro-cutting and small crater formation in the binder matrix caused by the repetitive impacts of erodent particles. It was observed that coating with nano-YSZ grains exhibited higher erosion resistance compared to conventional YSZ coating.
Viscoelastic contacts are present in countless industrial components including tires, dampers and rubber seals. The effective design of such components requires a full knowledge of viscoelastic contact mechanics in terms of stresses, strains and hysteric dissipation. To assess some of these issues, this paper describes a series of experiments on the contact area and penetration in a rolling contact between a nitrile rubber ball and a glass disk. The experimental results are compared with the theory proposed by Carbone and Putignano1 showing close agreement at low speeds. However, discrepancies arise at speeds above 100 mm/s because of the frictional heating. In order to evaluate this effect, the temperature of the sliding interface is measured for different rolling speeds using infrared microscopy. Thermal results showed that interfacial temperature remained constant at low rolling speeds before rising significantly when speeds above 100 mm/s were reached. These temperature effects are incorporated into the numerical simulations by means of an approximated approach, which corrects the viscoelastic modulus based on the mean measured temperature in the contact. The result of this approach is to extend the region of agreement between experimental and numerical outcomes to higher speeds.
The supporting stiffness and coulomb damping in a bearing play significant roles in the smooth operation of rotor-bearing system. The performance of multi-decked protuberant gas foil journal bearing is evaluated experimentally in a high-speed turboexpander. The effect of radial clearance on the bearing performance is analyzed based on the relationship between rotor speed and supply pressure in the speed-up and speed-down processes. The maximal speed of the 25 mm diameter rotor reached as high as 100 kr/min, and subsynchronous vibrations are suppressed in the tests. For the bearings with 0.05 mm protuberant foils, there will be thermal runaway problem with –20 µm clearance, while unstable operation appears with 80 µm clearance. For the bearing with 0.07 mm protuberant foil, the vibration amplitude is constrained within smaller amplitude due to stiffer supporting structure. The test results indicate that the bearing can operate stably under different gas film thickness and supporting stiffness, and that this kind of foil bearing can be applied in high-speed turbomachinery due to its stability and adaptability.
This contribution deals with a modelling of the tangential velocity slip problem in terms of variational inequalities. In particular, various technical situations for which the slippage problem appears to play an important role are first reviewed. Then, a mathematical formulation in terms of variational inequalities is developed where the critical shear stress criterion is considered. The theoretical conditions under which a unique solution exists are also discussed and an algebraic description based upon a complementarity approach is presented. Preliminary numerical results end the paper and a validation versus an analytical solution is proposed.
Human skin is characterised by a complex and highly variable friction behaviour. Although the variation of friction coefficients measured for skin depends on numerous parameters related to the skin itself, the surface in contact as well as contact conditions, water or sweat – either bound in the stratum corneum and manifest as skin hydration or in form of liquid films lubricating the interface – is the most important factor. Here, we analyse the variation of previous experimental data on the basis of the adhesion friction model and show how lower and upper bounds, i.e. envelope functions, can be derived for measured skin-friction coefficients. From these envelope functions, essential tribological parameters such as the interfacial shear strength and the real contact area of skin are estimated.
The tribological properties and hydrolytic stabilities of two novel N-containing long chain alkyl phenylborate esters, 2-(4-dodecylphenyl)-6-octadecyl-1,3,6,2-dioxazaborate (DBDB), and bis(2-(2-((E)-heptadec-8-enyl)-4,5-dihydro-1H-imidazol-1-yl) ethyl)-4-dodecylphenylborate (OMDB), as lubricating additives in mineral oil (HVI WH150) were evaluated. The results confirm that the two additives possess good anti-wear property and excellent hydrolytic stability, which demonstrates that the new approach by introducing long chain alkyl phenyl group is an effective way to improve the hydrolytic stability of borate esters. X-ray absorption near edge structure (XANES) spectroscopy was adopted to analyze the thermo-oxidative films and tribofilms generated from the synthesized additives in base oil to further understand their tribological behaviors on the metal surfaces. The XANES spectra reveal that the thermal films are mainly composed of trigonal coordination boron, while the tribofilms consist of trigonal and tetrahedral coordination borons in comparison with the model compounds, which suggests that the additive can generate the tetrahedral boron species or the trigonal boron species partially undergoing transformation to produce tetrahedral boron species during the vigorous condition of rubbing process.
In healthy natural synovial joints, the extremely low friction and minimum wear are maintained by their superior load-carrying capacity and lubricating ability. This superior lubricating performance appears to be actualized not by single lubrication mode but by synergistic combination of multimode mechanisms such as fluid film, biphasic, hydration, gel film and/or boundary lubrication. On the contrary, in most artificial joints composed of ultra-high molecular weight polyethylene against metal or ceramic-mating material, boundary and/or mixed lubrication modes prevail and thus local direct contact brings down high friction and high-wear problems. To extend the durability of artificial joint, the reduction in friction and wear by improvement in lubrication mechanism is required as an effective design solution. In this paper, at the start, the mechanism of superior lubricity for articular cartilage is examined from the viewpoints of biphasic and boundary lubrication mechanism. Subsequently, the proposal of biomimetic artificial hydrogel cartilage is put forward to improve the lubricating modes in artificial joints. The tribological behaviours in two kinds of poly(vinyl alcohol) hydrogels are compared with that of natural cartilage. The importance in lubrication mechanism in artificial hydrogel cartilage is discussed.
Experimental verifications of cam–follower contacts are very important owing to the difficulties faced during a reliable simulation due to the continuous variation of load, speed and geometry of the lubricated contact. Some experiments have been carried out with a new apparatus, specifically designed and realised for investigation on cam–follower and gear teeth contacts, in order to test its capability to measure film thickness and contact forces. Circular eccentric cams have been used because they feature lower transient effects and comparison of the results with the theoretical/numerical ones is easier. The tests have been performed using cams with two different eccentricities and surface roughness, and two different followers, one made of steel and one made of glass. The behaviour of the cam–follower contacts at several different rotational speeds, ranging from 50 to 500 r/min, and different pre-loads have been investigated. Lubrication regimes ranged from boundary to complete, with most of the tests being performed under mixed lubrication conditions. Localised wear has occurred during some tests. Data of all contact force and moment components as well as of the cam shaft driving torque have been acquired at high acquisition frequency. Some methodologies for numerical data elaboration have been identified. Optical interference images have been correctly recorded at the desired frequency. The trends of the normal and friction forces measured in the different tests as well as the evaluated trends of the friction coefficient are presented in the paper. Some sample optical interference images are also shown. The results furnish encouraging indications about the capabilities of the experimental apparatus.
The layered structure of transition metal dichalcogenides makes them promising materials for self-lubricating films. Transition metal dichalcogenide films can be considered as substitute for carbon-based self-lubricating films in several varieties of environmental conditions. The tribological properties of these films at high load have been studied extensively. However, the tribological behaviour of these films in the milli-Newton load range relevant for micro-electro-mechanical systems has hardly been reported. In the present work, the microtribological response of W-S-C coatings deposited by reactive sputtering is investigated. For that purpose, W-S-C coatings with various concentrations of carbon were deposited on steel substrates using magnetron sputtering. The friction and wear of these coatings are determined as function of applied load and carbon content. The results show that, even though the wear of these films increases with applied load and decreases with carbon content, the friction coefficient is minimum for the films containing the highest amount of carbon at low and intermediate load. It is maximum for the films containing minimum carbon at the highest load. There is no evidence for a transfer layer on the worn surfaces.
Influences of system parameters of the piston-crankshaft system in an internal combustion engine on the main journal vibration are investigated with the proposed systematic method considering oil film forces. In doing so, oil film forces of the lubricated piston pack and main journal were solved and then incorporated into the dynamic equations for the components in the system, through which the system parameter influences on the main journal vibration are simulated, and further verified. The numerical results show that there exists an optimal rotational speed of the internal combustion engine to keep the main journal stable. Moreover, the varied ratio of the connecting-rod length to piston diameter, unbalance mass of crankshaft, and damping coefficient of the main journal can change the main journal’s stability, and its vibration amplitude.
The effect of contaminants has been overlooked and yet plays a significant role in driver safety and road maintenance. A laboratory test method is developed to reproduce the deposit of contaminant particles on the road surface and measure the friction coefficient on dry and wet-contaminated surfaces. It simulates in this way the variation of skid resistance of the road surface due to contaminants during a dry period–precipitation event and the washoff effect of the rain. Protocols are described with respect to the contaminant collection on site and the subsequent preparation in the laboratory, the spreading of contaminant particles on the road specimen and their compaction to simulate the effect of the traffic, the wetting of the test surface to simulate precipitations, and the friction measurement. Values of friction coefficient on clean and dirty dry surfaces as well as during the surface wetting (simulation of precipitations) are plotted. Comparison with the literature shows that the developed test method reproduces remarkably well qualitative graphs used to highlight the loss of skid resistance with time during a rain preceded by a long dry period. The effect of contaminant concentrations and traffic is shown. Explanations are given in terms of the masking of the road surface microtexture; they are supported by visual observation of the road surface before and after the contaminant deposit.
This paper describes a numerical simulation model for prediction of the tribological effects of an oil control ring running against an out-of-round cylinder liner in a heavy duty diesel engine. The model considers the full three-dimensional geometry of the oil control ring and includes the effect of both surface roughness and global deformation. Results that test the model’s ability to do this under stationary conditions are presented. Furthermore, stationary results for prediction of the friction reduction possible by reduced ring tension in combination with reduced out-of-roundness are given. The model predicts the friction for the oil control ring at mid-stroke to be 78% larger in an out-of-round cylinder liner compared to a perfectly cylindrical one.
Hydrodynamic thrust bearings, used to carry axial loads in heavily loaded shafts of water power plants hydro turbines, can reach outer diameters even exceeding 5 m. In such large objects scale effect could be observed. According to this, allowable bearing specific load assuring safe operation of the bearings has to be decreased, which increases thrust bearing dimensions. This effect is caused by excessive thermal deflections of bearing pads, which significantly change oil gap geometry, and in consequence, decreases bearing load-carrying ability. Design of hydrodynamic thrust bearing of large dimensions seems to be a demanding engineering challenge, and additional difficulty comes from limited possibilities of experimental testing of these systems due to high costs. Theoretical investigations, carried out with the use of specially developed computer models, remain a feasible alternative for experimental research. But the accuracy of the models is not often directly validated, because of the lack of appropriate experimental data coming from large objects. In this paper, results of calculations carried out for a large hydrodynamic thrust bearing are shown and compared to measurement data obtained at bearing commissioning stage. Pad temperatures profile sliding surface, oil pressure in hydrodynamic gap and film geometry are compared to the measured values. According to the presented comparisons, some conclusions are drawn with respect to the accuracy of models used to predict large thrust bearing performance.
This review is mainly concerned with investigations devoted to periodic contact problems involving frictional effects, which were carried out by scientists from the former Soviet Union and CIS countries. The following class of problems is considered: 2D and 3D problems of frictional sliding contact between elastic and viscoelastic bodies with regular relief; stick-slip contact of bodies with wavy and periodically grooved surfaces; thermoelastic interaction between bodies with regular relief with regard for frictional heating; sliding contact between a surface with regular relief and a viscoelastic layer bonded to a rigid or an elastic foundation; effect of adhesion or incompressible fluid in the gaps on contact parameters in sliding contact between a textured surface and a viscoelastic layer.
Solution of a plane frictionless contact problem for two rough elastic solids is considered. An exact solution of the problem resulting in a singly connected contact region is considered, and it is conveniently expressed in the form of a series in Chebyshev polynomials. A sufficient (not necessary) condition for a contact of the solids to be singly connected is derived. The singly connected contact condition depends on the surface micro-topography, material effective elastic modulus, solid shapes, and applied load. It is determined that under certain conditions, a normal contact of three times differentiable rough surfaces with sufficiently small asperity amplitude and/or sufficiently large applied load is singly connected.
In this work, three kinds of phenolic resin bonded abrasive tool specimens containing β-cyclodextrin or its two complexes with dialkyl pentasulfide and sulfurized isobutylene at different contents were prepared by a cold compression method, respectively. The tribological properties of these abrasive tool specimens at different speeds and loads were investigated under water lubrication. The results indicated that filling of the complexes could improve the friction and wear performance of the abrasive tools. The minimum friction coefficient and wear rate were obtained when the content of the filler was about 15–20 wt.%. The friction coefficient of the abrasive tool decreased with the increase of speed and load, while the wear rate showed a reverse trend. X-ray photoelectron spectroscopy (XPS) analysis was employed to investigate the self-lubricating mechanism of the abrasive tool. It was found that the additives were released along with the decomposition of the complexes. The enhanced tribological properties of these abrasive tools were believed to stem from the formation of a self-lubricating layer that constituted by sulfide film and carbon deposited film, which had significant influence on the phenolic resin transfer process from substrate to the counter steel surface, and also had functions of anti-friction, anti-wear, and an improvement of surface quality.
In this study, the mechanical design and analysis of a magnetic levitating linear bearing suitable for working in the non-hysteretic range of forces is presented. The semi-cylindrical design of the superconductor provides stable equilibrium positioning and restoring forces in all degrees of freedom except for two with a cylindrical magnet floating along the axis of revolution/displacement. Using finite element analysis, it has been proven that the magnet can float stably and passively in a complete non-hysteretic Meissner state. This non-hysteretic passive linear bearing could be suitable for long-stroke precision positioning. The high translational symmetry of the magnetic field seen by the superconductor assures a usable long stroke of around ±90 mm with full performance and ±150 mm with reduced performance. This linear bearing in combination with an actuating system for only one degree of freedom could be used for accurate precision positioning systems for cryogenic environments with zero hysteresis in the movement.
The harsh environment rolling element bearings are exposed to in iron-mining industries is replicated in a laboratory scale in this work. Bearings (SKF 7204BEP) were tested both with and without magnetite oxide (Fe3O4) contamination. In order to study the interaction between contaminants and extreme pressure additives, the rolling element bearings were lubricated with two different greases: Grease without extreme pressure and grease containing sulphur-based extreme pressure additives. Further, the effect of the contamination–additive interaction on rolling bearing performance and monitoring signals (vibration and acoustic emission) was investigated. The obtained results indicate an advantage of extreme pressure additive in case of the investigated operating conditions. Furthermore, the use of extreme pressure additives decreased wear, surface roughness, vibration and acoustic emission for both test durations of 24 h and 168 h. The decrease of the acoustic emissions and the surface roughness parameter Rq in case of the tests with a duration of 168 h as high as 70% and 60%, respectively using extreme pressure additives in comparison with the plain grease was observed. The major cause for this reduction seems to be the interaction between contaminants and extreme pressure additives.
The aim of this study is to develop a thermal modeling of a grease lubricated rolling bearing in order to analyze the heat evacuated through its solid parts and the lubricant. The system under consideration in this study is a clutch thrust bearing and more precisely a sealed single row angular contact ball bearing. To analyze heat transfer inside the bearing, two models have been developed using the thermal network method. These models have been compared to measurements performed on a specific test rig. As some heat transfers appear to be negligible, in the end a simplified model is proposed.
Wear is one of the important tribological parameter that increases the revision rate of artificial hip implantation. Silicon nitride (Si3N4) is one of the promising ceramic materials with low wear rate and has been implanted in patients for the past 3 years. The objective of this paper is to analyse the effect of radial clearance over the contact pressure and wear of Si3N4 bearing couple, i.e. acetabulam cup and femoral head, using finite element concepts. The 3D force values for normal walking cycle of hip joint are used to predict the contact pressure and 3D rotation angles are used to calculate the sliding distance in determining the wear of bearing couple. The finite element results are validated initially with the previous experimental results. Then the analysis is extended to various radial clearance values. The increase in radial clearance values resulted in increase in linear wear but there is not much change in the volumetric wear.
Prosthetic devices are used to restore as much as possible not only the functionality, but also the self-esteem of patients who have been submitted to amputation surgeries. Typically, lower limb prostheses need a socket to act as a link with the human stump, so the contact stresses at the socket–stump interface are critical for the recovery process and the subsequent comfort perception of the patient as well. In this work, a broad experimentation to establish the coefficient of friction (COF) between socket material (polypropylene) and human skin was developed with the aid of an instrumented sclerometer, which was adapted to put in contact a polypropylene probe with human forearms. Seven factors were considered, but only sweat and hair skin were found to have a significant effect on COF, which varied from 0.22 to 0.45 in the tests. Lower values of COF were obtained when sweat was present at the interface, while the absence of both sweat and hair skin led to the highest value. The results are believed to be relevant for developing reliable finite element (FE) models for socket–stump interaction since the relation between normal and shear stresses at the interface of two interacting bodies is strongly determined by the COF.
The performance of hydrodynamic journal bearings is affected by the conditions under which the lubricant is fed to the bearing gap. Axial grooves are often used and, depending on their location relatively to the load line, they might substantially interfere with the hydrodynamic pressure generation and the thermal behaviour of the bearing. However, many of the existing tools for predicting bearing performance are not able to suitably predict bearing behaviour under varying load angle given the oversimplified way under which they treat lubricant feeding conditions. The present work proposes a detailed thermohydrodynamic approach which realistically incorporates these conditions into the bearing analysis. Special care is put on the mass and energy-conserving models of the ruptured film region and on a detailed treatment of lubricant mixing within the vicinity of grooves. This includes the first full modelling of the effect of negative flow rate in a groove, a phenomenon originally described experimentally in detail by the authors in previous publications, and which happens for a broad range of load/groove angles. An extensive investigation on the influence of loading direction on the performance of twin groove journal bearings has been performed. This parameter is found to affect deeply all major performance parameters due to the interference of groove regions in the hydrodynamic pressure generation and in the flow rates at each groove.
The formation of white etching cracks in the 1 mm zone beneath the contact surface in steel rolling element bearings causes a premature wear failure mode called white structure flaking. The formation drivers of white etching cracks are contested, as are the initiation and propagation mechanisms of the cracks. Hydrogen diffusion into bearing steel sourced from the hydrocarbon lubricant or water contamination and transient operating conditions have been suggested as formation drivers. Extensive work has been conducted at Southampton to further understanding of white structure flaking and this paper summarises these evidences and the conclusions made. Serial sectioning has been used to map subsurface wear volumes of wind turbine gearbox bearings from service and large-scale test rigs, test specimens/bearings from laboratory under hydrogen charged conditions and non-hydrogen charged conditions. The process involves polishing of cross sections of test specimens/bearings at ~3–5 µm material removal intervals typically over hundreds of slices, and this was used to map white etching cracks in their entirety for the first time. Serial sectioning has allowed a comprehensive investigation of the initiation and propagation mechanisms of white etching cracks and thresholds for their formation with respects to concentration of diffusible hydrogen, contact pressure and number of rolling cycles. From these studies it has been found that white etching cracks can form by subsurface crack initiation at inclusions under hydrogen charged and non-hydrogen charged conditions; hence it has been confirmed that this is one mechanism of WEC formation. Small/short sized sulfide inclusions, globular manganese sulfide + oxide inclusions and small globular oxide inclusions between ~1 µm and 20 µm in diameter/length predominated as crack initiators. In addition, detailed focused ion beam/transmission electron microscopic studies have been conducted to enhance the understanding of butterfly crack and white etching area formation mechanisms.
A skid damage test rig was developed to simulate the dynamic contact between bearing ring and roller, to reveal the reason why the skid damage of roller bearing sometimes occurs and/or sometimes does not under high-speed and light-load condition at the same slip rate. The mechanism of skid damage to a roller bearing was examined by changing experimental conditions such as the surface topography, slip, inner ring speed, radial load and lubrication. An orthogonal experiment design was used, and the results showed that the slip affected the temperature rise the most followed by the radial load, lubrication and inner ring speed. The relationship between the temperature rise and skid damage was examined; skid damage occurred when the temperature increased rapidly and continually, and non-skid damage occurred when the temperature remained constant or increased very slowly. The temperature varied directly with the inner ring speed. Skid damage is mostly thermal failure where metal-to-metal contact occurs in the contact area of the bearing ring and roller after the oil film is broken; this can result in a sharp temperature rise and material transfer, and the damage can quickly extend to the whole contact area. The preliminary test result showed that the unbalanced loading and muddy oil lubrication are prone to accelerate the damage process and intensify the degree of skid damage.
Among prevalent tribological failures, notably in rolling element bearings for wind turbines, an unusual rolling/sliding contact fatigue failure mode has been identified as white etching cracks. White etching cracks are broad subsurface three-dimensional branching crack networks bordered by white etching microstructure, eventually leading to flaking. Reproduction of the failure mode on standard rolling element bearings test rigs has not been mastered yet except with artificial hydrogen charging. Even though these failures have been reported for several decades, there is no evident common denominator in different occurrences. Hence, initiation and propagation mechanisms are not yet fully understood in application. Analyses of the contact conditions of a standard rolling element bearings test rig reproducing white etching cracks on standard and hydrogen precharged inner rings reveal that hydrogen charging seems to modify the white etching cracks initiation mechanism. Based on fractographs, serial sectioning, and scanning electron microscopic analyses, surface initiation and propagation mechanisms are proposed, including influent drivers and possible preventive techniques.
The generalized Reynolds equation is established considering cavitation mechanism based on modified Elrod method in theory. The cavitation characteristics of spiral oil wedge hydrodynamic bearings are investigated experimentally using transparent bearing and high-speed camera. The influence of speed of rotation and supply pressures on the shape of the cavitation and the location of the cavitation is analyzed, and the experimental results are compared with the modified Elrod calculation results. The results show that the cavitation of oil film is strip-shaped. With the increase in supply pressure, the rupture location of the oil film moves along the direction of rotation of the axis, the reformation location of the oil film moves along the opposite direction of rotation of the axis, the rupture area of the oil film decreases and the speed of rotation of the bearing increases when cavitation begins to occur; with the increase in speed of rotation, the reformation location of the oil film moves along the direction of rotation of the axis and the rupture area of the oil film increases. The experimental cavitation region is larger than the theoretical cavitation region; however, the theoretical results generally coincide with experimental results.
This article investigates one of the important parameters when designing for feel, namely the friction coefficient. An experiment was performed to evaluate how fringe projection could be used to investigate the topography of the fingertip, especially while in contact and sliding on a smooth surface. By allowing this smooth surface to be a small sheet of glass, a topographic camera could take pictures through it. The glass was also connected to a universal force gauge to measure normal and tangential forces from which the coefficient of friction could be calculated. The intention was to get dependable data on the forces, coefficient of friction, apparent contact area and actual contact area. This set-up was tested using 66 students who used one and three fingers in both dry and wet conditions and with a rubber glove. In order to measure natural everyday friction, they were not given any particular instructions on how to clean or slide their fingers. This method resulted in a much higher variation in friction coefficients than has been found in previous research. In particular, many higher values were noticed. This illustrates that the friction coefficient is a very hard parameter to rely on when it comes to designing surfaces for feel.
Tool wear and tool failure are some of the main critical problems in industrial manufacturing fields since they affect the quality of the machined part and raise production costs. Improving our knowledge of wear mechanisms and capabilities of wear prediction are therefore of great importance in the machining process. Abrasion, adhesion and diffusion are usually identified as the three main wear modes at the tool–chip and the tool–workpiece interfaces. From an experimental point of view, the analysis of mechanisms that govern the wear process is still difficult to conduct. The objective of this research work is then to develop a wear modeling focusing on the abrasive wear mode at the tool–workpiece interface. This wear phenomenon is assumed to be closely linked to the microstructure’s material workpiece and caused by hard conical particles trapped into the contact between the cutting tool and the workpiece. The proposed model is based on an analytical approach including a statistical description governing the distribution of particles with conical shape embedded in the contact area. The volume of the removed material per unit time was chosen in this study as the main parameter to describe abrasive flank wear mode. A parameter
A taper roller bearing can support both radial and axial loads and is available for the differential gear mechanism of a vehicle. Since the inner and outer rings can be separated, the axial force should be loaded to some extent as preload even if the bearing is used for supporting the radial load. However, when the axial force is loaded, the rotational torque becomes larger owing to the sliding friction between the roller end face and the large flange surface of the inner ring. In this research, to reduce the sliding friction inside the taper roller bearing, precision powder shot peening was performed on the large flange surface of the inner ring. Thus the bearing torque was reduced considerably by increasing the surface hardness and residual compressive stress and by smoothing the surface protrusion by grinding Shot Machine A. one Polish treatment. Furthermore, it was confirmed that the wear of the taper roller end face was the same as before and that of the large flange surface could be inhibited.
Foil bearings have been attracting considerable attention for their application in achieving low power consumption in small-sized turbo machines, excellent stability at high speeds, and durability at high temperature. Foil bearings with bump foils are one of the most suitable candidates for these applications as indicated in recent studies. However, it was reported that current small foil thrust bearings had insufficient load capacity to support thrust forces in practical high-speed turbo machines, and were needed to improve the load capacity at high speeds. As a first step in improving the load capacity of small foil thrust bearings, the load capacity and rotational torque of this type of foil thrust bearings that operate at up to 350,000 r/min were investigated experimentally and numerically in this paper. In the numerical calculation, the Reynolds and elasticity equations for the top foil were solved simultaneously by using the finite difference and the finite element method, respectively. It was experimentally and numerically found that the foil thrust bearings treated in this paper have the load capacity coefficient of 5.36 x 10–6 N/(mm)3 kr/min, which was comparable to that of the second-generation foil thrust bearings.
The understanding of the tactile perception mechanism implies the reproduction and measurement of friction forces and vibrations induced by the contact between the skin of human fingers and object surfaces. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify surfaces and information about their properties. The information concerning the object surface is affected by the forces and vibrations induced by the friction between the skin and the rubbed object. The vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Because of the low amplitude of the induced vibrations, it results quite hard to reproduce the tactile surface scanning and measuring it without affecting measurements by external noise coming from the experimental test-bench. In fact the reproduction of the sliding contact between two surfaces implies the relative motion between them, which is obtained by appropriate mechanisms having a more or less complicated kinematics and including several sliding surfaces (bearings, sliders, etc.). It results quite difficult to distinguish between the vibrations coming from the reproduced sliding and the parasitic noise coming from the other sliding contact pairs. This paper presents the design and validation of a tribometer, named TRIBOTOUCH, allowing for reproducing and measuring friction forces and friction induced vibrations that are basilar for a clear understanding of the mechanisms of the tactile sense.
In this study, fundamental aspects and mechanisms of acoustic levitation together with governing equations are presented first. Then, the acoustic levitation phenomenon is considered as a new way to design air suspension systems capable of self-levitation. A particular emphasis is laid on journal bearings and their specific geometrical configuration. A practical feasibility of using acoustic levitation to separate contacting surfaces is supported and illustrated by results of experimental testing of a number of prototype devices.
A new roughness perception test was designed to evaluate cutaneous sensibility. Blindfolded subjects explored a sample of sandpaper using one of two methods: stroking the sample (dynamic method) or applying pressure (static method). A range of samples of various grades were presented in a random order, and subjects scored each one in terms of perceived roughness. Each subject performed the test in three conditions – wearing latex and nitrile examination gloves and bare-handed. Mean normalised scores for each combination of sandpaper grade and hand condition were calculated. The coefficient of friction (COF) between each sample and glove (or bare finger) was measured, and the topography of each sample was analysed using a profilometer. It was found that the COF did not vary significantly across the samples, and so could not be related to perceived roughness. However, there were strong correlations between perceived roughness and surface topography (roughness average, particle diameter, particle spacing), particularly in the dynamic method. In the static method, most subjects did not perceive roughness differences below 800 µm particle spacing. In both methods, there was a significant reduction in perceived roughness when gloves were donned, but no significant difference between the two glove types could be found. It was concluded that the dynamic method was a useful test for evaluating cutaneous sensibility. Further investigation of the relationship between friction and perceived roughness was recommended, using a wider range of sample materials.
Modern machine elements are required to work under increasingly severe conditions (thinner lubricating films, higher temperatures, less lubricant, etc.). Therefore, in heavily loaded lubricated contacts the effect of surface topography is now more relevant than say, 40 years ago. Yet, in the year 1977 the 4th Leeds-Lyon Symposium on Tribology was held with the theme ‘Surface Roughness Effects in Lubrication’! How much have we learnt since then? The objective of this paper is to discuss in detail the progress in micro-elastohydrodynamic lubrication (micro-EHL) and to give an overview of mainly modelling progress and understanding in the last 40 years. The paper starts with a quick review of the main methods, models and challenges in micro-elastohydrodynamic lubrication, from the days of the 4th Leeds-Lyon Symposium to current times. Then the focus shifts towards discussing in more detail the existing semi-analytical methodologies developed in recent times to model the micro-elastohydrodynamic lubrication problem. The global and local effects of roughness, the effects of sliding and the transition to dry contact will be included. Finally, examples of industrial applications will be presented where roughness plays a major role in function; at the end, new challenges in the modelling of surface roughness effects in elastohydrodynamic lubrication will be outlined, e.g. interactions with the subsurface material.
In the sliding contact between the fingerpad and a rough surface when touching a product’s surface, friction plays a role in the perception of roughness, slipperiness and warmth. For product engineers who aim to control and optimize the sensorial properties of a product surface interacting with the skin, it is essential to understand this frictional behaviour. However, the friction of skin is yet poorly understood. The variation that is observed within or between skin friction studies can be assigned to gender, age and orientation of the finger. Analysing data collected from literature shows some consistent trends. The coefficient of friction increases considerably with increasing hydration level of the skin, due to softening of the top layer of the skin. The coefficient of friction of the fingerpad decreases with normal load to a constant value, which can be attributed to effects of normal adhesion and the deformation behaviour of the fingerpad. There is no consistent effect of velocity on the coefficient of friction. Friction decreases with increasing Ra roughness. When the Ra roughness increases further, the contribution of deformation causes an increase in the friction after which it remains constant. Some influence of the finishing method is reported. The type of material has a smaller influence than the surface roughness of the sample or the condition of the skin. Even though the coefficient of friction of the fingerpad shows some consistent trends, examining the friction behaviour at a more detailed level might explain the contribution of friction to tactile perception. The measuring signal contains relevant information and should be analysed thoroughly as opposed to taking the average coefficient of friction of the steady state part of the signal. Future work should involve the study of local friction behaviour at the scale of the surface roughness.
Safety and reliability are the main requirements for brake devices in the mining winding installations. Therefore, selection of the right materials for the friction brake elements (pads and discs) is the most challenging task for brake system designers. The friction coefficient for such friction couples should be relatively high but, above all it should be stable. In order to achieve the desired brake friction couple performance, a new approach to the prediction of the tribological processes versus friction materials formulation is needed. The paper shows that the application of the artificial neural network can be productive in modelling complex, multi-dimensional functional relationships directly from experimental data. The artificial neural network can learn to produce the model of friction brake behaviour.
Research is being focused on to find the influence of surface variations such as texturing or grooving on the bearing performances. Though, earlier it was restricted to the theoretical aspects alone, however, with the help of advance manufacturing techniques (laser surface texturing, etching, special grinding process, etc.), it has become possible to produce micro-textures or grooves on the surface of journal bearing. The objective of the present study is to numerically compare the influence of spherical texture and micro-grooving (longitudinal and transverse) on friction coefficient and average temperature of journal bearing at low and high eccentricity ratios. From the findings, it has been observed that the micro-grooving reduces the friction coefficient and average temperature is maximum in comparison with spherical texture.
The main failure mode of cylindrical roller element bearings is localized surface defects such as spalls and pits on the surfaces of its races or rollers.1 However, it is difficult to describe the time-varying deflection excitation generated by a defect and the time-varying contact stiffness excitation due to the changes in contact conditions between a roller and the defect when the roller passes over the defect by using the previous defect models for the cylindrical roller bearing. In this work, a new dynamic analysis method is proposed to formulate a localized surface defect more accurately for a cylindrical roller bearing dynamic modeling. A two-degree of freedom dynamic model for a cylindrical roller bearing with a localized surface defect on its races is proposed, which considers both the time-varying deflection excitation and the time-varying contact stiffness excitation produced by the defect. The load-deflection relationship between the roller and the race is considered as non-Hertzian one, which can be used to determine the load-deflection relationship between the logarithmic-profile roller and the races of the cylindrical roller bearing. The numerical results are compared with the available results from the previous defect models in the literature. The effects of the defect width, depth, and types are investigated. To validate the proposed model, an experiment is also presented. The results show that the proposed method describes a more accurate in describing the real excitation produced by the defect and provides a new method to simulate the effects of a localized surface defect on the vibration response of a cylindrical roller bearing.
A numerical model of plastic indentation and abrasion of elastohydrodynamic contacts by debris particles previously developed by the author is extended to include the dependence of material flow stress on strain rate. Using the Johnson–Cook viscoplasticity model, the flow stress of all materials involved in the indentation process is expressed as a function of plastic strain, strain rate and temperature. This complements other elements of the model, including strain-gradient plasticity, work-hardening, frictional heating from particle extrusion, thermal softening, melting and material loss due to adhesion. Following a laborious programme of experimental validation and numerical comparisons, the predictions of the model are shown to be in excellent agreement with the experimental results on soft and hard particles in rolling and rolling–sliding elastohydrodynamic contacts. The incorporation of strain-rate effects further improved the agreement between theoretical and experimental results previously established with simpler versions of the model that ignored the strain-rate factor. Strain rate is also shown to affect several parameters in the process of surface damage, including the magnitude of contact stresses and flash temperatures, as well as the behaviour of a particle in a concentrated contact. It is also shown that for an optimum contact velocity linked to strain-rate effects and fluid film thickness in lubricated contacts, surface damage is minimised, particularly for large and hard particles.
The behavior of regular wavy surfaces in contact is important for the study of rough surface contact, and therefore has applications to the prediction of friction, wear, surface fatigue, and electrical and thermal contact resistance. This work characterizes the average surface separation between a sinusoidal surface and a flat rigid surface as a function of the average contact pressure or load using the finite element method (FEM). The FEM results also agree well with the limiting spherical based solutions at low contact pressures and the other asymptotic solution at high contact pressures. Empirical fits are provided that can approximately predict the average surface separation for both elastic and elastic–plastic sinusoidal contacts over a wide range of material properties. Contact stiffness can also be predicted from surface separation results.
Dynamics of small free swings of a pendulum supported by two spherical balls pre-rolling on a flat basis are considered to investigate the occurrence of micro-slippage at the contact interface between the balls and the flat surface. The basic mechanisms of this micro-slippage are investigated, showing how geometric conditions could initiate and magnify micro-slippage rather than assuming that these effects are core characteristics in any rolling system. This article also discusses the energy dissipation of a ball slipping on a flat surface at pre-rolling caused by three mechanisms and the conditions enabling the decrease of energy dissipation of the pendulum and the influence of the basis vibrations.
In this work, experimental and theoretical analyses of the power losses in an industrial planetary speed reducer were performed in order to define the weight of the different types of power losses in the global efficiency as well as to understand the appropriateness and the effectiveness of the models and tools available in the literature for the prediction of these power losses. From this investigation, it has emerged that even if for all the kinds of power losses improvements are in principle possible, those for which, at the same time, more improvements seem possible and effective models are lacking are the load-independent power losses. For this reason, a computational fluid dynamics methodology able to evaluate load-independent gear power losses in planetary gearboxes is proposed. The other types of power losses were determined by means of analytical models available in the literature and, together with the calculated load-independent gear power losses, compared with the experimentally determined power losses showing the effectiveness of the adopted calculation models.
Hybrid bearings have been used in high-speed machine tools due to their inherent advantages of low temperature rise, high load-carrying capacity, and high stability. The objective of this study is to investigate the influence of turbulence and thermal effects on the performance of water-lubricated hybrid bearings with circumferential grooves and stepped recesses used to support high-speed spindles. The governing equations are solved using the finite element method with appropriate boundary conditions. A bulk-flow thermohydrodynamic model has been developed incorporating the turbulence effect. The results indicate that the turbulence effect appreciably affects the bearing performance. Besides, the thermal effect should also be taken into account especially when the rotating speed is high. The study gives a useful guide for the future hybrid bearing design and operation of high-speed spindles supported by water-lubricated hybrid bearings.
The current work studies the evolution of the load carrying capacity and friction forces as a function of the level of starvation for spherical hydrodynamically lubricated contacts. Unfortunately, the parabolic approximation of the geometry cannot be applied, as it leads to infinite friction forces for the fully flooded case. Thus, this work uses the complete spherical geometry but this introduces an additional parameter which is the ratio between the minimum film thickness h0 and the radius R. Two viscous forces are distinguished: the Couette term associated with sliding and the Poiseuille term generated by the pressure gradient. The load and friction forces are computed numerically as a function of the geometry and the level of starvation. Finally, some simple curve fitted equations are given for load and friction as a function of the degree of starvation A and the ratio .
The present paper is aimed to investigate theoretically the effect of non-linear behavior of the lubricant on the performance of an orifice compensated two lobe non recessed worn hybrid journal bearing. A non-linear behavior of lubricating oil is modeled by using the cubic shear stress law. Dufrane’s abrasive model has been used for the analysis of wear damage on the bearing surface. A modified form of Reynolds' equation is solved by using finite element method (FEM) taking into account the relation between shear stress and shear strain rate. The influence of non-linearity factor (
A series of tests were conducted to study the effect of the strip inclination angle on the friction and wear behavior of a contact strip rubbing against a contact wire with electric current using a ring-block-type test machine. The arc voltage and electric current were measured throughout the tests to evaluate the scaled accumulated arc energy. The variations of the wear rate of the carbon strip and the scaled accumulated arc energy with strip inclination angle were obtained. Experimental results show that the strip inclination angle has an effect on the friction and wear behavior of contact strip against contact wire with electric current, and an appropriate strip inclination angle can decrease wear of the contact strip.
Fretting fatigue behavior of Al 7075-T651 is investigated at temperatures of 20 ℃, 150 ℃, 250 ℃ and 350 ℃ and at different stress levels. The results show that: (a) fatigue life increases from 15% to 160% for low stresses and reduces from –20% to –40% for higher stresses and (b) fretting fatigue life increases from 155% to 290% for low stress levels and reduces from –12% to –65% for high stress levels. All life changes have been measured with respect to the lives at ambient temperature. It is believed that under low stresses, the effect of glaze oxide layer created through surface oxygen absorption prevails and gives rise to improvement of plain and fretting fatigue of material at elevated temperatures. Precipitation and solid solution hardening and good lubrication of the oxide layer are the main reasons for this improvement. For the higher stresses, 180, 200 and 280 MPa, the oxide layer breaks down and the degradation due to reduction in ultimate strength dominates the fatigue behavior of material and diminishes the fatigue lives. Fractography of specimens reveals brittle fracture mode in the fracture surface regardless of the test temperature. Optical microscopy of specimens shows more germination of precipitates at lower temperatures and shorter aging durations and more growth of precipitates for higher temperatures and longer aging durations.
Pure aluminum oxide (Al2O3), ZrO2 toughened Al2O3 (ZTA; 25–30% ZrO2), and ZTA (55–60% ZrO2) ceramic particles reinforced Cr25 matrix composites are successfully fabricated by the infiltration casting process. The volume fraction of ceramic particles in the composites is 47–55%. The interface structure between ceramic particles and the matrix was analyzed by digital probing microscopy and was found to be continuous mechanical bonding. The wear behavior of the composites was studied by a three-body abrasive wear tester. ZTA (55–60% ZrO2) reinforced Cr25 composite has excellent wear resistance which is approximately six times higher than that of the high chromium cast iron matrix. It is discovered that the wear mechanism of composites is micro-cutting and fatigue stripping. A correlation exists between the wear resistance of composites and the strength of ceramic particles.
In this study, the influences of wax content on the tribological and mechanical performance of nylon-polytetrafluoroethylene blend were evaluated. Wear tests were carried out using nylon 6 + 10% polytetrafluoroethylene + wax pins on AISI 316 L disc material. The wax percentages in the blend are 2%, 4% and 6% by weight. Tests were carried out at room temperature with applied loads of 50–100 N and sliding speeds of 0.4–1.6 m/s. These conditions are equvilent to a combined pressure and speed factor values of 0.141–5.664 MPa m/s. Although the results showed a slight drop in mechanical properties, the friction coefficients and the wear rates of nylon blend were significantly reduced by the addition of wax filler. The friction coefficient was reduced from 0.29 to 0.13 and the reduction in the wear rate was up to 73%. Furthermore, the results also showed that the change in pressure and speed factor values did not have a significant influence on the specific wear rate values. Finally, micrographs of the the worn surfaces showed the presence of transfer film, deformation and adhesive wear mechanisms.
Soluble oils are the largest class of oils used in metal cutting industry. Flood cooling involves problems related to its treatment and disposal. Minimum quantity lubrication in the form of mist application can be used to replace flood cooling. But as less amount of cutting fluid is used in minimum quantity lubrication, its capacity to carry away heat and providing adequate lubrication is limited. Hence, the heat-carrying and lubricating ability of soluble oil has to be enhanced. Graphite has better lubricating and cooling properties and hence inclusion of graphite nanoparticles in cutting fluid may help in formulating a better coolant in machining operation. This article compares the performance of mist application of nanographite-soluble oil with dry lubrication, flood lubrication and mist application of soluble oil without nanographite in turning AISI 1040 steel. Performance is evaluated based on experimental measurement of average chip–tool interface temperature, tool wear and cutting forces. The results showed that use of nanographite-soluble oil in mist application has greatly improved the cutting conditions by lowering the temperature generated, reducing the tool wear and reducing the cutting forces.
In this work, we derive a modified slip model for gas lubrication based on the effective viscosity concept and a kinetic-theory-based slip-boundary condition. A more accurate expression for the effective mean-free path is proposed, which can overcome the deficiency of other slip models in predicting effective mean-free path. In comparison with the existing slip models, such as the first, second, and 1.5-order slip models, the current slip model agrees well with F-K model over the entire flow regimes and therefore is expected to be more accurate for ultra-thin gas film lubrication at head-disk interface.
Wear over time affects engine’s reliability and efficiency. On-line wear monitoring could provide timely information about engine health condition. In the current study, on-line monitoring of engine wear via an on-line visual ferrograph was performed in reliability tests of gasoline engines, and a wavelet-analysis-based differential method of data analysis for wear condition estimation was proposed. The tests were designed for 220 h, which consist of a running-in stage of 20 h and a thermal shock cycle test stage of 200 h. One of the tests was terminated because of failures in the main bearings and crankshaft journal at 146th hour of thermal shock cycle test, while the other two completed successfully without failures. Index of particle coverage area, which represents wear-debris concentration in lube oil, was studied, and piecewise trend-extraction of index of particle coverage area was achieved by wavelet decomposition and reconstruction. The first-order differential of the index of particle coverage area trend was used to represent the wear rate or the generation rate of debris for health condition assessment of engines. Off-line oil analyses were performed in laboratory via an analytical ferrograph, and engine disassembly results of the engines were given to determine the causes of engine failures if it happened. It is found that favorable trend extraction from index of particle coverage area could be achieved by the segmented wavelet de-noising. Moreover, on-line visual ferrograph monitoring estimated the engine wear at macro-levels effectively, and it provided an advance warning for the failures after the continued deterioration of the engine wear. The study and application of this method can make early failure prediction of engine and avoid serious fault.
The objective of this study is to investigate the adhesion characteristics of wheel/rail under oil contamination with consideration of surface roughness using a three-dimensional model of wheel/rail in rolling contact. A partial elastohydrodynamic lubrication theory is employed in the model. An under-relaxation revision on the film thickness is used to keep the simulation procedure stable. The dependence of the wheel/rail adhesion coefficient on train speed, surface roughness amplitude, parameter of roughness orientation and axle load is studied under oil contamination. Moreover, the numerical solutions of a two-dimensional model are compared with those of the three-dimensional model. In addition, a good agreement has been found between the numerical results and the experimental results obtained by a JD-1 wheel/rail simulation facility, which consists of a small wheel roller serving as locomotive or rolling stock wheel and a large wheel roller serving as rail.
The piston ring pack is the single greatest contributor to mechanical losses in a heavy duty diesel engine, accounting for 1.1–6.8% of the total losses. Therefore, the piston ring-cylinder liner contact is potentially the most rewarding area to study when attempting to reduce mechanical losses in a heavy duty diesel engine. In this work, four different heavy duty diesel engine cylinder liner variants have been tested to evaluate the lubricating conditions that occur when a section of top compression ring is reciprocated against them in a lubricated environment. Two of the cylinder liners were traditional grey cast iron and plateau honed with different honing angles, one had ANS Triboconditioning® applied and the last was plasma sprayed with a stainless steel and ceramic coating, then honed. An experimental test rig was used where friction and film thickness was recorded, by means of an ultrasonic technique. A numerical model was also developed to calculate the friction and film thickness. Comparisons are made between the simulation and experiment, and the four cylinder liner variants are also evaluated. It was found that both simulation and experiment could differentiate between all surfaces and the results from the model and experiment also correlated well with each other. A lower plateau average surface roughness, as exhibited by the ANS Triboconditioning® and plasma liners, led to a significant reduction in friction.
A novel method was presented to prevent surface defects, mainly burrs generation, in micro milling process. Burr formation mechanism was analysed in machining and it was found that negative shear plays a critical role in burr generation, especially near the workpiece boundaries. In order to eliminate its effect, low melting point alloy was selected to extend the boundary of the workpiece as auxiliary support. Therefore, burrs were formed on low melting point alloy, which were removed by melting it off afterward and the burrs-free component was thus achieved. Experiments were carried out on a micro milling machine and aluminum alloy was machined with micro end milling tools. The results revealed that most burrs were prevented and there was no contamination on the workpiece surface. The paper concluded with further discussions on the potential applications of the method.
The effect of contact geometry on the rotary bending fretting fatigue of Al 7075-T6 has been investigated. Two types of contact geometries, conforming and non-conforming, have been considered in the investigation. Three different characteristic lengths have been used for each type of contact. In the conforming type, three different contact areas and in the non-conforming type three different fillet radii has been considered in this work. The results show that bending fretting fatigue reduces the life of the material by about 90% in some cases. The reduction varies with the contact type and characteristic length. In the conforming type of contact, fretting fatigue life increases with the increase of characteristic length for low bending stresses but at higher bending stresses the fatigue life converges to the same cycles, regardless of the characteristic length. In the non-conforming type, fretting fatigue life reaches a minimum at the fillet radius of 1.5 mm. Since, contact pressure, shear stress, and slip amplitude have great effects on fretting fatigue life, the distribution of these parameters have been determined using Abaqus software for each contact type. The numerical simulations are quite consistent with the experimental results. The numerical results could reasonably explain the manner of variations observed in S–N curves for different contact geometries considered in this work.
Anti-corrosion phosphate ionic liquids were synthesized and evaluated as lubricants for the contact of steel/aluminum by using an Optimol SRV oscillating friction and wear tester under ambient conditions. Phosphate ionic liquids yield commendable tribological performance and are superior to the conventional ionic liquid lubricant 1-methy-3-hexylimidazolium hexafluorophosphate in terms of friction-reducing performance and anti-wear capacity. Contrastive experiments demonstrate that the simple mixing of two compounds (butylamine and dibutylphosphate) does not yield a mixture lubricant with better tribological properties as compared to the ionic liquid synthesized using these two compounds. The worn surfaces of aluminum discs were analyzed by using JSM-5600LV scanning electron microscope and PHI-5702 multifunctional X-ray photoelectron spectrometer. The friction reducing and anti-wear mechanisms originate from the layered structures of ionic liquids under boundary lubrication conditions and from the tribochemical interactions of ionic liquids with the fresh surface of the substrates.
High-strength aluminum alloys are widely used in the aeronautic industry because of their excellent combined performance including good corrosion resistance. In this paper, an experiment-based study on the surface characteristics is presented including residual stress and surface defects induced by end milling processes. Salt-spray corrosion tests are carried out on the end machined surfaces and corrosion morphologies during different corrosion periods are achieved. Observations from microscopy and energy dispersive X-ray spectrometer (EDS) analysis support the existence of micro-cracks, leading to occluded corrosion cells, which accelerate the corrosion. It is found that residual stress, micro-cracks and their combination are important to the corrosion resistance performance of machined aluminum alloy parts. Experiments show that while feed speed vf = 750 mm/min and rotational speed n = 4500 rpm, larger compressive stresses and lower corrosion depths are achieved.
Polymer-based additives such as poly isobutylene are blended with lubricating oil to enhance the lubricating performance of the base oil. The Rabinowitsch fluid model effectively describes the influence of polymer additives on the lubricating performance of a non-Newtonian (pseudoplastic) lubricant. This article deals with the analysis of a capillary compensated hydrostatic circular thrust pad bearing using Rabinowitsch fluid model. For a circular recess hydrostatic thrust bearing operating with pseudoplastic lubricant, the closed form expressions for load-carrying capacity, lubricant flow rate, fluid film stiffness coefficient and damping coefficient have been obtained using a small perturbation method. The close form solutions for different performance characteristic parameters have been compared with the results obtained from finite element method formulation. The numerically simulated results indicate that pseudoplastic parameter significantly affects the parameters of the bearing performance characteristics such as pocket pressure, lubricant flow rate, fluid film stiffness coefficient and damping coefficient. The value of the fluid film stiffness coefficient gets substantially increased with the pseudoplastic parameter whereas the value of damping coefficient gets reduced. The analytical results presented in this study can be used to obtain an optimum performance of a hydrostatic circular thrust pad bearing.
In the hollow glass industry, specifically in the luxury perfume glass bottle industry, the success of the forming process depends on preventing the sticking at the glass/mold interface to make it easier to release the mold and prevent defects on the glass surface. This study concerns a new way to analyze the impact of lubrication on the glass/tool mechanical behavior. It is based on successive forming cycles performed on the experimental glass/tool interaction platform in the TEMPO Laboratory (Valenciennes, France). Each forming cycle combines a pressing phase, when the hot glass is pressed by the punch and a punch rotation phase in order to estimate the friction force. This article presents the analysis of the flint glass forming cycles using different punch lubrication conditions (i.e. bare punch, swabbed punch, coated punch, and coated/swabbed punch). The friction analysis is based on the evolution of the friction coefficient during forming cycles on the glass/tool interaction platform. The effect of the initial temperature of the glass on the friction coefficient for the glass forming is presented using the swabbed punch.
Paper-based composite friction material is a kind of key functional material for automatic transmission. In this study, five types of paper-based composite friction materials with different compound mineral fiber contents were prepared using the paper-making process. The effect of compound mineral fiber content on the properties such as shear strength, thermal property, friction torque curves, dynamic friction coefficient and wear resistance was studied. Meanwhile, the sensitivity of the friction coefficients was also investigated. Worn surfaces of samples were analyzed using scanning electron microscope. Experimental results revealed that the thermal properties of the samples were improved as the compound mineral fiber content increased and the shear strength increased initially and then decreased. The friction torque curve of the sample with 20% compound mineral fiber contents was the highest among all the samples and the sample with higher compound mineral fiber contents possessed the more flat friction torque curve during the mixed asperity contact phase of the engagement process. The dynamic friction coefficient increased with the increase of compound mineral fiber content and decreased as contact pressure and rotating speed increased. The wear resistance of the sample was highest when the compound mineral fiber content was about 15%.
Sintered small-module gears have two important aspects that readily follow from the name: they are sintered and with a small module of 1.5875 mm (DP 16). Both aspects are to date insufficiently described for gear designers despite an increased need for sintered small-module gears. This article reviews a decade of systematic experimental investigations with regard to pitting resistance of sintered small-module gears. Pitting resistance levels are listed for the common pressing/sintering/surface densification technologies of today. Currently the highest pitting resistance of 1300 MPa is achieved for pressed, sintered, rolled, re-sintered and case carburized low-alloy gears in comparison with 1800 MPa for the reference ground case-carburized 16MnCr5-wrought gears. This offset can, however, be compensated by selection of a relatively higher viscosity and lower operating temperature of the lubricating oil, if the application allows it. Thus, powder metal technology can once again contribute to a competitive total cost with high material utilization.
Mechanical energy alters chemical reactions. Although the mechanisms of mechanochemical reactions are not understood well, they have been utilized for alloying, polishing, lubrication and so on. On the other hand, tribochemical reactions which involve the interactions between frictional materials and the environment accelerate the decomposition of solid and surrounding molecules. It results in the emission of small particles and gases, bringing problems to environment and health. Mechanochemical and tribochemical reactions are often studied in different fields and in different societies. Even if similar reactions from each field are named differently, the same physics and chemistry operate in both reactions. Therefore, this study aimed at better understanding of some tribochemical reactions from the mechanochemical point of view. The main mechanism is proposed to be the emission of electrons, which is enhanced by the energy input onto the surface and surrounding molecules due to the breakage of chemical bonds and chemisorption of environmental molecules.
Aluminium and its alloys are widely used in a wide variety of applications. Aluminium’s main advantages include: lightness, high specific strength, high thermal and electrical conductivities, good formability, excellent machinability, diversity of aluminium alloys, extensive range of forming and processing options (e.g. rolling, extrusions, stampings, forgings and castings) and suitability for a diverse range of joining techniques, surface treatments and recyclability. A number of surface treatment technologies are available which produce thicker oxide coating layers that can be used to combat corrosion and wear of aluminium alloys under aggressive environments, such as in petroleum extraction environments. Coating processes for surface modification of aluminium alloys include plasma electrolytic oxidation, plasma-sprayed ceramic and hard anodising. In this article, erosive wear characteristics of coatings produced using the aforesaid three processes are compared with each other and benchmarked against the uncoated aluminium substrate. This article investigates the extent of erosion resistance, in particular impingement due to sand loading, of these coatings taking into consideration the effect of material properties such as adhesion, ductility and roughness.
Modelling deep valleys from surface micro-geometry in elastohydrodynamic lubrication has always been a challenge, since the solution of the Reynolds equation meets the cavitation condition p ≥ 0 also in that part of the contact. Full numerical methods in principle can handle situations where the pressure is nearly zero, although the convergence can be affected. However, simplified approaches such as ‘amplitude reduction’ are in principle not designed to handle this problem. The present article shows some modifications to the amplitude reduction approach to handle pressures in deep valleys in a better way and compares with full-numerical solutions and with the unmodified amplitude reduction technique. At the end, some validity limits are established.
The frictional performance of the brake material used in mine hoisters varies abnormally under some extreme braking conditions. Its friction coefficient runs the danger of falling suddenly in such a braking. Based on the catastrophe theories and the energy transforming ideas, this article established a cusp catastrophe model to describe and predict the friction catastrophe behaviors of the brake material in some continuous repeated brakings of mine hoisters. The predictive capability of the model was verified by some braking experiments. It is shown that the catastrophe phenomena of the friction catastrophe in braking can be described by the catastrophe theories as a cusp catastrophe. The friction coefficient can be taken as its state variable. The initial braking velocity and braking frequency can be taken into account for one control variable while another control variable may contain the braking pressure and braking time. Moreover, the friction catastrophe of the brake material has a critical braking state. The cusp model built in this article can properly predict its critical braking frequency. However, in the present state, the exact time that the friction catastrophe occurs in a critical braking cannot be accurately located.
The modification of surface layer of machine elements used for engineering applications requires modern tools and techniques for improving its functional aspect and enhanced life. In this study, modification of surface layer was done by combining laser texturing and filling the textured surface by hard wear-resistant coating. This study aimed at investigating the friction and wear behavior of nano aluminum chromium nitride (AlCrN) composite coating over modified titanium alloy (Ti6Al4V) surfaces under dry sliding contact condition. The surface of as-received titanium alloy substrate was lapped and laser surface texturing was performed on the lapped surface. Magnetron-sputtered physical vapor deposition technique was preferred for coating. Scanning electron microscope, energy dispersive X-ray and microhardness tests were performed to characterize the coating. Further, to evaluate the bonding strength of coating against modified surfaces, scratch tests were performed by a scratch tester. Using pin-on-disc tribometer, the coated samples were subjected to tribological investigation for dry sliding contact conditions. The effect of normal load on friction and wear characteristics were examined and discussed. The performance of these coatings on textured and lapped surfaces was compared. Critical parameters such as variation in coefficient of friction and specific wear rate were reported. The influence of textured surface on coating performance was discussed.
The rotary table of the heavy machine tools demands higher stiffness to provide substantial load capacity as well as a lower cost. The hydrostatic rotary table supplied with constant flow rate pump is not often used due to its high cost; meanwhile, the system with constant pressure pump suffers from a relatively lower stiffness problem. In this study, an alternative type of rotary table supplied by a constant flow pump and a constant pressure one is proposed to satisfy the requirements. The mathematical model is presented on the basis of hydrostatic theory to predict the axial static performance of the table. A comparison of the performances between constant pressure supply table, constant flow supply table and constant flow and constant pressure supply table has been conducted; the results show that constant flow and constant pressure supply table is the best in static behavior. A multicriteria optimization technique based on the weighted ideal point method is better than the single criteria optimization in achieving the comprehensive property of the table. A variation rate method is used to calculate the weighted factors. The optimization problem is formulated with the maximum stiffness, the minimum power dissipation and the minimum supply pressure.
Metal matrix composites are being increasingly used in aerospace and automobile industries attributed to their improved properties such as elastic modulus, hardness, tensile strength at room and elevated temperatures, wear resistance combined with significant weight savings over unreinforced alloys. Because of these capabilities it can be applied for aviation industries, where the most critical areas of an aircraft to be affected by corrosion are engine inlets, control surfaces landing gear doors, radome, and aerodynamic fairings. They are subjected to wear and corrosion processes, which can occur simultaneously. This article critically reviews the present and past state of understanding of the erosive wear behavior of metals and alloy. First of all, the different types of reinforcing with different coated metal matrix composites are reviewed. The accuring failure mechanisms are discussed. This is followed by discussion of the essential features of erosive wear. Various predictions and models developed by different investigators describing the erosion rate are presented. Finally, the relevant areas for future studies are indicated.
This paper discusses about the wear behaviour of as-received and laser hardened EN25 low alloy steel performed in dry sliding condition using a pin-on-disc method. A 2 kW continuous wave neodymium yttrium-aluminium-garnet laser source is used for transformation hardening to improve the hardness and wear resistance. The laser transformation hardened steel samples are characterized by optical microscope, x-ray diffractometer and microhardness tester. The sliding wear study is conducted for different loads (10 N, 25 N, 40 N), sliding distances (1000 m, 2000 m, 3000 m) at various elevated temperatures (200°C, 400°C and 600°C) with constant sliding speed of 0.15 m/s. The study at room temperature is also carried out for comparison. The friction and wear characteristics in sliding contact are evaluated and the worn surfaces are analysed through a scanning electron microscope. The experimental work indicates that wear resistance of laser hardened steel is five times higher than the as-received steel. The results also indicate that the wear resistance increases at 400°C due to the oxide layer formation and decreases at 600°C due to fracture of the oxide layer.
In this research, the tribological characteristics of PEEK filled with short carbon fiber/PTFE/Graphite sliding against AISI630 stainless steel under water lubrication were investigated. The friction experiments were conducted on MCF-10 ring-on-ring test rig. Based on SEM analysis of worn surfaces, an attempt was made to approach the tribological mechanism of friction pairs. The results showed that the matching materials achieved the best tribological performance at velocity of 1 m/s and load of 1.66 MPa. When load applied on the specimens was increased to 2.49–3.32 MPa, cracks and delamination wear occurred in the surface of PEEK. The effect of sliding velocity on triobological performance of matching materials was complicated. At higher sliding velocity, hydrodynamic lubrication was easier to form. However, the rapid rising temperature with the increase of sliding velocity had adverse effect on water lubrication film. Under load of 1.66 MPa and sliding velocity of 1.5–2 m/s, adhesive wear was the dominant factor in the sliding process. The matching materials performed higher friction coefficient and wear rates under insufficient water lubrication than that under circulating water lubrication. It is attributed to that the friction heat accumulated gradually. The research results will lay the foundation for the materials screening of key friction pairs in the water hydraulic components.
An experimental method for estimating the tangential contact stiffness of contact interface with controlled contact asperities is discussed. The tangential contact stiffness is analyzed from the shift of certain eigenmode frequency of the system resulting from the changes in the density of contact asperities and the application of normal force. The results show that for the case of contact combination with 189 contact asperities, the corresponding eigenmode frequency shifted from 1.01 kHz at normal force of 19 N to 1.64 kHz at normal force of 148 N. This shift corresponds to the increase of tangential contact stiffness from 34.08 MN/m at normal force of 19 N to 67.44 MN/m at normal force of 148 N. It is found that the corresponding eigenmode frequency and its shift were affected by the density of contact asperities as well as the normal force. In the case of contact combination with 397 contact asperities, the corresponding eigenmode frequency shifted from 2.36 kHz at normal force of 49 N to 3.9 kHz at normal force of 153 N, indicating the increase of the tangential contact stiffness from 93.91 to 140.73 MN/m. A simple power relationship is proposed to explain the quantitative relationship between the eigenmode frequency shift and the tangential contact stiffness value.
A numerical thermoelastohydrodynamic simulation method is proposed in this article with the considerations of the topography of sealing lip and shaft, interference of sealing lip, the elastic deformation of sealing lip and the influence of temperature on the viscosity of lubricant. The proposed method is applied to WR auto water pump bearing seal to study the lubrication and sealing property of lip seal, and the relationships between the lubrication property of lip seal and interference, rotation speed in given working condition are obtained.
The hydrodynamic lubrication theory founded by Osborne Reynolds in 1886 is based on several assumptions, including that the contact surfaces are perfectly smooth, and the lubricant flow regime is laminar. However, in certain special cases these hypotheses may not be valid, and it is seen that surface roughness and turbulence affect bearing performance. The approach adopted in the present work is based on the application of the homogenization method to the turbulent Reynolds equation. In homogenization, multiscale expansion of the fluid pressure terms leads to a system of four partial differential equations governing two types of problems (local and global). The solutions are, respectively, periodic functions, and the homogenized pressure. As an illustrative example, a journal bearing with a rough surface operating in the turbulent regime is analyzed. Numerical simulations are performed by imposing periodic isotropic roughness patterns over the stationary outer cylinder surface. The homogenized solutions are compared to direct solutions of the deterministic problem. Homogenization is shown to be a powerful but practical method to attack problems of rough surfaces and is applied to turbulent lubrication for the first time. In the plain journal bearing example, the effect of roughness is to increase the load significantly in many cases. The effect of turbulence is to greatly increase the load, in both the rough and smooth cases.
This article presents an outline of a method of planning an experiment developed by Taguchi for the optimisation of processes and as an example of its application in the optimisation of parameters of a ball-cratering method. Abrasive wear tests were performed on chosen antiwear physical vapour deposition coatings with the optimum work parameters of friction node. (The test friction node is created by the rotating ball and the immobile sample disc that is pressed against it and an abrasive slurry is drip fed into the contact zone.) Then the factor of resistance to abrasive wear was calculated based on the evaluation of their resistance to abrasive wear. The Taguchi optimisation method was used in order to devise and verify the methodology of the abrasive wear research of physical vapour deposition coatings deposited using cathodic arc evaporation on two differently prepared substrates. A series of resistance tests to abrasive wear performed using ball-cratering method was conducted in accordance with the test plan based on the Taguchi design. The variable test parameters were load, the speed of ball rotation and sliding distance. A measuring device, an optical microscope, was used to take pictures and measure the diameters of the wear traces visible in the form of craters received after the performance of tribological tests. The purpose of this research was to devise a method for testing the resistance of antiwear physical vapour deposition coatings deposited on tools and machine parts to abrasive wear (performed using the ball-cratering method) by applying the Taguchi optimisation method. The experiments were carried out using the combination of tribological test parameters based on the nine experiments (L9) using Taguchi orthogonal design with variable three test parameters of load, the speed of ball rotation and sliding distance. The results of the abrasive wear test performed using ball-cratering method on duplex and non-duplex coatings were successfully verified using the Taguchi optimisation program. Optimisation of the tribological test parameters based on the Taguchi method has been found to be very efficient and convenient for the investigation of the abrasive wear rate of the duplex and non-duplex coatings.
In this article, a study of elastic–plastic contact of statistical rough surfaces is presented. The contact behaviors of a single asperity with deformable substrate are studied by finite element method. The influences of material properties and substrate deformation on the single asperity contact behaviors are investigated. An elastic–plastic contact model of rough surfaces is established based on the proposed single asperity contact model and statistical rough surface contact model. The surface topography plasticity index and material plasticity index are defined to indicate the influences of surface topography and material properties on the contact behaviors of rough surfaces. Comparisons with some exiting models and finite element results show that the proposed formulations can model the single asperity contact behaviors well for a wide interference range. The rough surface contact model in this work is consistent with some existing models for small plasticity indices, and it is slightly different from the existing models for large plasticity indices. The increase of surface topography plasticity index and material plasticity index would result in larger contact area and contact load for given separations.