Spline couplings are mechanical elements used to transmit power by joining two coaxial rotating shafts. The main damaging phenomenon on these elements is fretting wear, which can occur on teeth surfaces when there is a relative movement. Therefore, specific design criteria to evaluate teeth damage of spline couplings subjected to fretting wear are needed. In this work, the criterion proposed by Ruiz is considered and criticalities of the first Ruiz parameter are analysed by performing an uncertainty evaluation. The specific working misaligned conditions of a dedicated test rig were considered in this analysis. An uncertainty table allows the comparison of main geometrical and mechanical factors, evidencing their relevant effects. It shows the need of a deeper analysis on the coefficient of friction between the contact surfaces of spline couplings working in misaligned conditions.