THE ADVERSE EFFECT OF STEEL PARTICLE CONTAMINANTS ON FATIGUE LIFE OF GREASE LUBRICATED BALL BEARINGS

Abstract

The rating life of ball bearings is reduced when hard particle contaminants are present in the lubricant. Usually, this life reduction is taken into account in the calculation of modified rating life by using the contamination factor through a general characterization of the lubrication conditions. However, the impact of contaminant’s variables such as size, hardness and concentration level has to be specified in detail. This need is resolved by the present work, where greases contaminated with hard steel particles of different sizes are tested with the purpose of finding a pattern in the relationship between steel particle sizes and the progress of wear inside the bearings. A laboratory rig is utilized for these tests and vibration analysis is performed to estimate the condition and the residual life of the bearings. After the tests, optical inspections performed in a stereoscope verify the predictions of vibration analyses. It was found that large contaminating particles, after their initial deformation, produce high stresses and therefore higher wear of the bearings and that the wear mechanism is closely related to the interruption of the lubricating film in such a way that local overheating caused point melting and adhesion of the particles adhered on the raceways. In case of grease contaminated with steel particles, vibration analysis can indicate the severity of wear and monitor its progress. From the conducted tests of the present work it is proved that the size of the contaminant particles affect strongly the wear process in such a way that the larger steel particle contaminants cause greater damage to the bearing. The vibration levels were higher and the damage was greater as particle size increased. It is difficult to establish an equation that describes and quantifies the wear progress involving all the parameters of size and concentration levels of the steel contaminant particles due to the stochastic nature of the wear mechanisms and therefore presently a statistical approach of the wear and the residual life expectancy seems to be more practical. The collection of a large number of experimental results is underway.