Investigation of damaged rolling-element bearings and deterioration of lubricants under the influence of electric fields

The present work deals with investigations carried out on bearings operating under the influence of electric fields in controlled operating conditions on a roller bearing test machine and also on non-insulated bearings used in electric motors. Surface damage to bearing races and rollers, caused by the electric current, was studied. It was found that the basic types of microscopic damage are craters on the bearing races that reduce the actual contact area of the rolling surfaces, leading to the development of corrugations due to the increase in contact pressure. The pitch of corrugations on the various damaged bearings was measured and theoretically analysed on the basis of kinematics, quality, positions of plane of action of radial loading, number of rolling elements in the loaded zone and frequency of rotation of bearing. The measured pitch of corrugations on the test bearing and various motor bearings is found to be close to that derived from the theoretical investigations. The width of the corrugations was also determined analytically and found to be in line with the measurement. This paper gives a diagnosis, using SRV (Schmierstoff-Lubricant-Material Test System) and atomic absorption spectro-photometry techniques, of the periodic changes occurring in the greases normally used in non-insulated rolling-element bearings when a potential drop exists across the inner and outer races. The paper also highlights the behaviour of bearings and mechanism of bearing failure under the influence of electrical fields.