Failure analysis of the deep groove ball bearings of an electric motor

Abstract

The deep groove ball bearings of an electric motor, driving an oil-injected compressor, were periodically monitored for vibrations. The ball bearings were replaced when before the expected lifetime of the bearing the measured vibration exceeded the tolerated level. The bearing on the axle side of the motor was the locating bearing while the ball bearing of the fan side (where the fan of the electric motor was placed) was mounted as a (spring-loaded) floating bearing. The outer rings of the fan side bearing showed signs of fretting wear. The outer ring of both bearings was ground through to disassemble the bearings. Visual inspection, optical microscopy and scanning electron microscopy of the raceways indicated that the bearings were subjected to an axial load in the range of the permissible axial load, but for each bearing in a different direction. The placement of the wear marks on the raceways of both bearings indicated an axial loading from the inside towards the outside of the electric motor. Also the wear marks on the raceways of both ball bearings showed the typical washboard pattern of the passage of electric current >0.5 A, although the rotor was already equipped with a grounding brush on the non-drive side. While replacing the deep groove ball bearing by a cylindrical roller bearing can easily prevent the fretting wear on the fan side bearing, the origin of the bearing currents merits further investigation.