An Investigation of wear and fatique mechanisms of rolling bearings

In this study, a new bearing test rig was designed and built to investigate wear and fatigue mechanisms in rolling bearings. Wear and fatigue behaviour of a number of ball bearings were investigated under different operating conditions of loading, speed and lubrication. The temperature of the bearings were monitored as a function of radial loading, rotational speed and oil supply rate. Microstructure of the bearing components were investigated using optical light and electron metallography, X-ray microanalysis techniques and microhardness measurements. The temperatures of both bearings and lubricating oil were found to be proportional to the applied radial load and rotational speed of the shaft, but inversely proportional to the oil flow rate. It was also observed that, at high speeds the failure of the bearings started from the cage. Microstructural changes were observed in the surface material of the bearings operated under extreme running conditions. These changes resulted in the formation of a hard marten- sitic layer and a overtempered martensite on some surface areas of the bearing components. Thus, It was concluded that the observed structural changes, related to thermal effects can upset the hardness balance between the bearing components and result in excessive wear leading to failure. In addition, some stress-induced structural changes including band formation in dark etching regions, and butter fly formation around non metallik inclusions were observed in both rolling elements and inner rings of the bearings at the depths to maximum shear stresses. These subsurface, microstructural changes resulted in initiation of cracks. It was concluded that, the observed structural changes and initiation of cracks may seriously shorten the life of the rolling bearings by accelerating the rolling contact fatigue. Key words: Rolling bearings, wear, rolling contact fatigue, bearing test rig.