Shear-degradation of grease and base oil availability in starved EHL lubrication

Experimental studies of starved grease lubrication show that, initially, the film thickness decays with time, as each overrolling displaces lubricant from the track. In earlier work the authors have modelled this condition and, assuming zero reflow, have obtained a power-law decay of film thickness with the number of overrollings. In the early stages the decay is as predicted, however as the test proceeds the film thickness sometimes recovers to a much higher value. This recovery of the film has important implications for the long-term operation of starved grease lubricated contacts. Film recovery is attributed to the shear degradation of the grease at the side of the track releasing ‘base oil’ which is thus available for reflow. In the starved condition the film thickness ultimately obtained is a function of the amount of lubricant present in the track and this amount is determined by the competing loss and replenishment mechanisms. The current paper proposes a model which links the shear degradation of grease at the side of the track with the amount of ‘free’ oil released. Close to the contact the grease experiences very severe conditions and the thickener matrix is progressively destroyed with each overrolling. This degradation results in base oil containing dispersed particles with a predominantly viscous character and thus provides the ‘free’ lubricant for reflow. In this paper starved film decay and recovery has been measured for three greases and the shear degradation behaviour characterised in a rotational viscometer. A model has been developed to describe the shear degradation of the grease and this has been coupled to an analysis of the shear rate conditions at the side of the Hertzian contact during overrolling. The predictions from the numerical model are compared to experimental measurements of starved film recovery.