Thin-film grease lubrication

Abstract

One of the most important examples of the thin-film regime is in the grease lubrication of rolling element bearings. Grease lubrication is not usually considered in this context and yet there is considerable evidence that many bearings operate under starved conditions where the film thickness drops to a fraction of the fully flooded value. Thus it is the mechanisms of lubricant supply and film formation in the starved regime that are important in ensuring the successful operation of a bearing. The aim of this paper, therefore, was to explore grease lubrication from a thin-film perspective and to examine the many factors that could influence starved film formation. Much of the current understanding in this area is derived from studies of starved fluid film lubrication and this provides a convenient starting point for the discussion. Grease lubrication is considered as a replenishment problem and recent experimental work is presented illustrating some of the aspects discussed in the text. The effect of grease properties, bearing operation and design on film formation is examined. Grease lubrication in the thin-film regime is a complex problem. It includes aspects of non-Newtonian rheology, surface chemistry, capillary flow and starved elastohydrodynamic lubrication (EHL). All of these aspects need to be resolved in any future model that will be capable of explaining and predicting grease lubricating behaviour in bearings.