Analysis of grease lubrication of a ball bearing using acoustic emission measurement

The behaviour of grease lubrication in a rolling bearing is less known than that of oil in the same type of tribological system. In recent years the research of grease lubrication has shown several features of the grease lubrication that differ from the classical elastohydrodynamic lubrication theory of oil lubrication, notably that starvation is more prominent under grease-lubricated conditions. The goal of the present investigation was to clarify, by using a real bearing installation and realistic running conditions, how the fundamental properties of the lubricating grease influence the lubrication in a rolling bearing. The fundamental properties of the greases considered in the study were thickener concentration, base oil viscosity, base oil bleeding rate and consistency. Results obtained with grease lubrication were verified with pure oil lubrication. The analysis of grease lubrication in the present investigation is based on the relative differences in the pulse count rate of the acoustic emission (AE) generated by the running of the bearing. In order to make the correct decisions about the lubrication situation of a bearing, the fundamentals of the behaviour of grease lubrication in rolling bearings and its influence on the AE signal have to be known. Moreover, AE has proven to be a good tool to study grease lubrication in situ in a bearing. The results show that starvation of the lubrication situation increases with increasing thickener content and decreases with lowered base oil viscosity. This is in agreement with film thickness measurements made in ball-and-disc type equipment.