Non-Newtonian Effects of Multigrade Oils on Journal Bearing Performance

Abstract

This study is aimed at predicting the non-Newtonian effects of multigrade oils on journal bearing performance. The governing differential equation, similar to the Reynolds equation, is derived using a general apparent-viscosity function as the constitutive equation in order to be able to accommodate different rheological equations. Treatment of the non-Newtonian effects is confined to nonlinear viscous behavior. Oil elasticity and normal stress effects are not considered. The problem formulation resembles that of the THD problem, and the governing differential equation contains non-Newtonian factors based on the cross-film integrals of viscosity. The problem is solved by the finite difference technique, and the results show reasonable agreement with the experimental data available in the literature. In order to represent the rheology of multigrade engine oils, a new non-linear viscous constitutive equation is proposed. This equation is fitted to experimental data on the rheology of two multigrade engine oils. The results using this new constitutive equation demonstrate that Newtonian models cannot accurately predict journal bearing performance with multigrade oils, even when an effective high-shear-rate viscosity is used. With the two sample oils at typical operating conditions, the present non-Newtonian model predicted up to 40 percent reduction in loud capacity and power loss due to shear thinning. It is also shown that the shear stabilities of two, otherwise similar, multigrade oils have important effects on journal bearing performance. The effects of bearing geometry and oil temperature on promoting non-Newtonian behavior are also indicated. Shortcomings of using the power law, a simple and widely used constitutive equation, to represent the rheology of multigrade engine oils are demonstrated.