Failure of a Bearing for a Jet Engine Because of Misalignment Between the Bearing and a Shaft

Abstract

The engine on a jet aircraft was shut down immediately as it produced excessive vibration. Complete failure of the cage in one of the two main-shaft ball bearings (placed side by side in the engine) was revealed in the dismantled engine. The ball bearings (made of vacuum-melted 52100 steel) were both of the single-row deep-groove type with split inner rings and were designed to operate at a maximum temperature of 175 deg C. Overtempering of the rings was indicated by the reduced hardness in comparison to unfailed rings. Severe damage to approximately 20% of the load-bearing surface, with more damage on one of the shoulders of the groove, was revealed during examination of the outer raceway of the bearing which indicated misalignment of the bearing. No damage other than spalling cavities in the inner-ring raceway, caused by the elongated subsurface inclusion revealed by metallographic examination of circumferential section of the largest cavity, was exhibited by the second bearing. It was concluded that the fracture of the cage was caused by overheating and misalignment caused excessive stressing of the bearing on the main shaft.