The evolution of lubricant film defects in the starved regime

Surface damage or failure of rolling element bearings is often due to inadequate lubricant replenishment of the contact. Our understanding of the mechanisms of starved elastohydrodynamic lubrication and the behaviour of thin fluid films has advanced significantly in recent years and, thus, provides a basis for a fundamental study of different bearing failure modes. In this paper a possible explanation for the surface damage observed in grease lubricated spherical roller bearings operating at low temperatures is advanced. This type of bearing failure is characterized by 'brown bands' that form identical patterns on every element of the bearing: the roller set, inner and outer ring. In this case damage is attributed to a single defect in the lubricant film, which is then propagated around the bearing. Such a defect might be initiated by local scraping, and removal, of the lubricant film by a bearing cage. It is the propagation of the film defect and the role of lubricant replenishment that is studied in this paper. In the starved regime insufficient lubricant replenishment of the defect results in a locally depleted film which, combined with surface roughness, leads to surface damage and possibly brown band formation. The numerical work studies the influence of a local lubricant defect on the central film thickness in the contact. An established starvation model is used to explore the relationship between oil inlet film thickness and the maximum defect size that can be replenished. In parallel experimental work a local defect has been deliberately created in the lubricant film in a rolling contact. The behaviour of the defect has been studied under different levels of lubricant replenishment. This work confirms qualitatively the effects predicted by the numerical study.