An experimental investigation of grease lubricated EHD contact subjected to normal sinusoidally variable loading

Abstract

Oil lubrication of the contacts between high-elastic modulus, non-conformal surfaces is well understood in fully-flooded, steady state conditions, when there is generous supply of lubricant in the inlet and the parameters that govern the film thickness or the friction force do not vary in time. Semi-analytical or full numerical formulas have been derived which can predict the lubricant film thickness in terms of the working parameters and the properties of the materials involved. In case of lubricating greases, their semi-solid nature given by the presence of a solid phase makes the prediction of the film thickness and thus the design of various systems difficult. The equations developed for oil lubricants can be used, using the properties of the base oil however it was found experimentally that the actual film thickness deviates strongly from theoretical values, for values of the entrainment speed above a certain critical value. Grease-lubricated contacts are very important because over 90% of the second most numerous machine components that is, the rolling element bearings are lubricated with greases. An added factor of uncertainty is the variable load on the contacts between the rolling elements and the raceways, due to on one hand the passage of the former through the loaded region of the bearing and on the other due to vibrations likely to be experienced by the bearing. In this paper film thickness measurements of grease lubricated EHD contacts subjected to harmonic load variation are reported for the first time. The experiments were carried in a test rig based on optical interferometry under both steady-state and variable loading conditions. It was found that severely starved contacts, which run under very thin films, show a degree of recovery of the film thickness towards fully flooded values, depending of the frequency of the variable load and the properties of the grease.