Case 55: Failure of a Sealed Ball Bearing

One of the rotor bearings in an electric motor failed, producing excessive vibrate. The bearing was removed and disassembled, revealing craters and bruises on the inner ring raceway and balls along with evidence of melting and burning of metal. Scanning electron microscopy revealed metal particles near the craters, and energy-dispersive x-ray analysis showed that slivers recovered from the grease had the same composition as the bearing raceway and balls. Based on these observations, it was concluded that the bearing failed due to electrostatic discharge, which would have led to seizure if it continued. The report recommends the use of electrically conductive grease and proper grounding practices.