Miscalculation of film thickness, friction and contact efficiency by ignoring tangential tractions in elastohydrodynamic contacts

Traction in elastohydrodynamic contacts causes normal surface displacements, affecting the lubricant film thickness. This is ignored in the vast majority of published theoretical studies and film-thickness regression formulae. This article analytically formulates traction effects on normal displacements, establishing the most contributing factors and the associated displacement error when tractions are ignored, which can exceed 20%. Numerical tests on a rolling-sliding-spinning elastohydrodynamic elliptical contact and comparison with well-known published formulae show that the central film thickness is unaffected by the traction factor simplification but the minimum film thickness is occasionally significantly overestimated. Traction and contact efficiency are also in error but usually by less than 1%. Correspondingly, wear may also be miscalculated, particularly in contacts with sliding and/or spinning motion.