An experimental study into the influence of grease composition on friction in EHL contacts

Abstract

Rolling bearings are among the most widely used machine elements. As a result, reducing bearing frictional losses can have a significant impact on the overall energy efficiency of the vast majority of engineering systems. Given that rolling bearings are typically lubricated with grease, one of the most effective ways to improve bearing efficiency is through a suitable grease formulation. However, at present there is a lack of understanding of grease frictional behaviour in typical non-conformal, highly stressed bearing contacts. This limits the development of low-friction bearing greases, and thus further improvements in bearing efficiency. The current research work attempts to address this topic through an experimental investigation. Friction and film thickness tests were conducted on a wide selection of greases in ball-on-disc tribometers, under conditions broadly representative of rolling bearing contacts. Initial work was carried out with a series of commercially available greases, selected to cover a wide spectrum of thickener types, base oil types and viscosities, and practical bearing applications. These tests served to identify the most influential aspects of grease formulation on friction, and provide a reference line in terms of the current state of the art in low-friction bearing greases. Subsequently, the work was focussed on the study of a series of custom lithium-thickened greases. These were designed and manufactured with the purpose of isolating and investigating the effects of particular aspects of interest in grease formulation, including base oil type, base oil viscosity and the presence of friction modifiers. In addition, the performance of the corresponding bled and base oils was also studied to help shed some light on the prevailing mechanisms. The results show that two distinct operating regions, at high and low speeds, can be identified in the tribological performance of greases. At high speed greases behave approximately as their base oils. In this region, oil type was found to be the major parameter of influence on friction, with synthetic oils (particularly PAO) providing the lowest friction coefficients. In the same high speed region, the influence of base oil viscosity is that expected from a classical Stribeck relationship for oils, i.e. higher base oil viscosities in the grease formulation tend to produce higher friction as a result of thicker films. In contrast, at low speed grease behaviour is much more complex and is strongly influenced by both base oil and thickener properties. In this region the combination of lithium and PAO was shown to provide both thick films and low friction. No benefit in friction reduction was observed when a friction modifier, namely oleic acid, was added to the formulation of the greases. The influence of mechanical degradation of greases on their tribological performance was also evaluated. It was established that mechanical degradation has the effect of diminishing the friction and film thickness benefits provided by some of the greases at low speeds. A remarkable exception was found in the tribological behaviour of greases formulated with lithium and PAO oil, which was overall practically unaffected by mechanical degradation. Finally, based on the observed results, the combination of a low viscosity PAO base oil and lithium thickener was suggested as the most suitable formulation for a low-friction rolling bearing grease.