Elastohydrodynamic Lubrication Model With Limiting Shear Stress Behavior Considering Realistic Shear Thinning and Piezo Viscous Response

Abstract

This paper addresses a largely ignored aspect pertaining to the elastohydrodynamic lubrication (EHL) traction behavior of fragile lubricants which undergo transition to glassy state at typical EHL contact zone pressures. For such lubricants, a conventional EHL model predicts extremely high and unrealistic values of traction coefficient, especially under near pure rolling conditions where thermal effect is negligible. Therefore, an EHL model incorporating the effect of limiting shear stress and the associated wall slip phenomenon is presented herein. Unlike the other such investigations involving limiting shear stress behavior, the present model employs Carreautype powerlaw based models to describe the rheology of lubricants below the limiting shear stress along with realistic pressureviscosity relationships (WLF and DoolittleTait). The use of Carreautype shearthinning model in this analysis allows the simultaneous prediction of minimum film thickness and traction coefficient for lubricants which shearthin in the inlet zone and exhibit limiting shear stress behavior in the contact zone, a feature absent in the existing EHL models utilizing ideal viscoplastic or some other unrealistic rheological model. Using published experimental data pertaining to the shearthinning and pressureviscosity response of two fragile lubricants (L100 and LVI260), it has been demonstrated that the present model can explain the appearance of plateau in the experimental traction curve. Also, the influence of shearthinning parameters and the pressureviscosity coefficient on the predicted limiting shear stress zone has been studied.