Occurrence of brittle flaking on bearings used for automotive electrical instruments and auxiliary devices

Abstract The higher transmission efficiency of engine accessory drive belts has resulted in a new type of bearing damage, called “brittle flaking” by NTN. This new type of bearing damage has been observed in bearings used in automotive electrical instruments and auxiliary devices. This paper summarizes recent test results which clarify the initiation mechanism of the brittle flaking damage. the type of grease used in the bearing was found to have a significant effect on the occurrence of brittle flaking. Thus, brittle flaking can be avoided by using specific greases. The amount of hydrogen measured in the inner rings, outer rings and balls of bearing run under acceleration-deceleration tests, was highest in the stationary ring, where brittle flaking was likely to occur. Brittle flaking was reproduced in rolling contact fatigue tests when the test specimens were charged with a sufficient amount of hydrogen prior to testing. The test results concluded that brittle flaking is caused by the diffusion of hydrogen into the steel due to the decomposition of the grease by a tribochemical reaction.