An Optical Study Of Grease Rheology in an Elastohydrodynamic Point Contact Under Fully Flooded and Starvation Conditions

Abstract
An interferometric study of grease rheology in a fully flooded and deliberately starved elastohydrodynamic (EHD) point contact has been carried out under isothermal conditions at 30 °C and various rolling speeeds up to 1 m/s for a viscous liquid lubricant and greases made with lithium, calcium and polyurea thickeners. The measured grease film thickness (GFT) is always greater than that generated by their base oils under fully flooded conditions. The enhanced GFT generated by the lithium and calcium thickened greases can be described by Newtonian flow behaviour using an effective grease viscosity. The results are consistent with the thickener for these greases being fully degraded in passage through the EHD inlet region. The polyurea grease exhibited non-Newtonian fluid flow behaviour (shear thinning) and gave the greatest GFT enhancement (over that exhibited by the base oil). A model to predict the degree of grease starvation in a rolling EHD contact is proposed which is based on determining the inlet meniscus position. Comparison of the results predicted by this model with experiment indicate that the heavily worked grease in the contact inlet exhibits Newtonian behaviour under constant speed conditions. This suggests that the role of the degraded thickener is to increase the effective viscosity of the grease base oil.