Two-surface slip in EHL contacts: Influence on film thickness and friction

This work examines symmetric shear-limited slip at the two fluid–solid interfaces in a fully flooded, isothermal elastohydrodynamic line contact using a fully coupled Navier–Stokes and elastic deformation framework. This formulation computes the velocity field and wall shear rate directly, allowing straight identification of the slip start position and the slip-active zone, which cannot be resolved unequivocally with Reynolds-based models. Unlike earlier single-surface or limiting shear stress approaches, this study resolves the slip start position and the associated active zone relative to the Hertzian contact. The study establishes the mechanical conditions under which slip reduces the friction while either reducing or preserving the film thickness and quantifies the influence of the slip yield stress and the slip length on the evolution of the wall shear rate and the wall shear stress across the contact. Slip initiated before the Hertzian contact lowers the effective entrainment velocity, which reduces the film thickness. The largest friction reduction occurs when the mean slip start position is located near the Hertzian contact half-width, where the slip weakens the viscous resistance inside the high-pressure region with only a minor influence on the inlet flow. When the slip is confined to the Hertzian contact, the film thickness remains unchanged, and friction reductions up to 48% are obtained. Increasing the slip length continuously reduces the wall shear stress and thereby the friction until the fluid layer adjacent to the interface approaches the far-field fluid velocity in the film. • Slip start position controls EHL film thickness and friction • Film thickness decreases when slip starts before the Hertzian contact • Film thickness remains unchanged when slip occurs within the Hertzian contact • Maximum friction reduction occurs when slip starts at the Hertzian half-width