Numerical Analysis of Grease Film Thickness and Thickener Concentration in Elastohydrodynamic Lubrication of Point Contacts

Abstract

This article presents a numerical analysis of elastohydrodynamic lubrication (EHL) with grease in point contacts to predict the grease film thickness at low speeds, which can increase with decreasing speed and exceed the base oil film thickness by orders of magnitude. The grease film thickness is obtained by solving the EHL problem, taking grease flow as a single-phase flow with a non-Newtonian viscosity. The continuity equation for thickener is derived, taking grease flow as a two-phase flow of base oil and thickener and the network structure of the thickener as both a porous medium and a non-Newtonian fluid. The flow rate of the thickener network is obtained as the grease flow rate subtracted by the Darcy flow rate of the base oil. The thickener concentration is obtained by solving the continuity equation for the thickener. Numerical results show that the increase in film thickness at low speeds is due to an increase in the equivalent viscosity in the inlet of the contact, which is due to a decrease in the shear rate and an increase in the thickener concentration. At low speeds, base oil is more easily squeezed out of the inlet of the contact than the thickener network, which leads to an increase in the thickener concentration, because the shear stress due to the Newtonian flow of base oil decreases with decreasing the shear rate, whereas the yield stress of the thickener network is independent of the shear rate. A reasonable agreement between the numerical and experimental results validates the present model.