Thermal degradation of greases and the effect on lubrication performance

The lubricating life of a grease in a rolling element bearing is reduced by operation at high temperatures and this can result in premature failure of the bearing. A grease experiences severe conditions in an operating bearing where the combination of high temperatures and sustained mechanical working result in gross physical and chemical changes to the grease. These changes have a significant effect on the ability of the grease to replenish the contact and maintain a lubricating film, particularly under starved inlet conditions. Extended operation at high temperatures promotes evaporation of low molecular weight base oil components (1) and oxidation of one or both of the grease components (2)(3). The presence of small amounts of transition metals and their oxides can accelerate these processes and such material is commonly found as wear debris in bearings (4). It is difficult to disentangle these effects and this paper concentrates on the effects of thermal ageing on the lubricating ability of the grease. Simple thermal ageing tests have been carried out on two lithium hydroxystearate greases and the resulting changes in their chemistry characterised by infrared spectroscopy. The lubricating performance of the aged greases has been assessed by measuring film thickness and oil release in a rolling contact under starved conditions. Results from infrared analysis have shown that the oxidation process is accelerated at a temperature of 120 degrees C forming carboxylic acids and related species. The film thickness results showed that the aged greases give a lower equilibrium film thickness and this correlates with reduced oil release.