Lubrication and reflow properties of thermally aged greases

The lubricating life of a grease in rolling element bearings is limited by operation at high temperatures. The thermal and mechanical stresses imposed result in gross physical and chemical changes to the grease which contribu te, eventually, to failure both of the lubricant and the bearing. The probl em is very complex as both grease and bearing parameters contribute to fail ure and these are difficult to disentangle. Most of the research work in th is area has been with bearing tests where samples of bulk grease have been removed for analysis after failure. Although this approach will yield resul ts as to grease life and the condition of the grease at the end of the test it provides little insight into the fundamental mechanisms of failure. This paper approaches the problem from a simpler perspective. The aim is to develop experiments to artificially age greases under controlled condition s of thermal and mechanical stress, to characterize the changes that occur and to relate these changes to lubricating ability. This paper concentrates on thermal ageing: in future work mechanical working will also be studied. Thermal ageing tests have been carried out on simple lithium hydroxystearat e greases and the resulting changes in their chemistry characterized by inf rared spectroscopy. The lubricating performance of the aged greases has bee n assessed by measuring film thickness and oil release in a rolling contact under starved conditions.