Fracture of Ball-Bearing Components by Rolling-Contact Fatigue Because of Subsurface Nonmetallic Inclusions

An aircraft was grounded when illumination of the transmission oil-pressure light and an accompanying drop in pressure on the oil-pressure gage was reported by the pilot. No discrepancy in the bearing assemblies and related components was revealed by teardown analysis of the transmission. The center bearing of the transmission input-shaft ball-bearing stack had a broken cage and one ball was found to have been split into several pieces. Several scored balls and flaking damage in the raceways of the inner and outer rings was observed. The origin (area in rectangle) was oriented axially in the raceway and was flanked by areas of markedly different-textured flaking damage. Stringers of nonmetallic inclusions were revealed at the origin during metallographic examination of a section parallel to the axially oriented origin. Thus it was concluded that the failure was caused by contact fatigue mechanism (flaking) activated by the subsurface nonmetallic inclusions.