Failure of an Aero Engine Ball Bearing Due to Axial Loading

Abstract

During a flight of an aircraft, the rpm of one engine dropped to 96% with frequency 390 cycles per second (CPS). The pilot tried to stabilize the rpm of the engine using standard procedures but failed and soon the rpm further dropped to 95%. The engine was switched off and a safe landing was made with remaining three engines. During initial investigation, cage of one of the rear bearing of governor in the valve housing was found broken, whereas second bearing of this set was found unserviceable due to abnormal play. Valve housing was a part of propeller control assembly. It controls flow of pressurized hydraulic fluid toward pitch changing mechanism in the dome assembly through cams, valves, and switches, to change the propeller blades angle. The ball bearing under consideration was supposed to run for indefinitely long period it was to be replaced mandatorily during overhaul of the valve housing after 6000 h of its life. In this particular case, failed ball bearing had consumed only 1938.4 h. The failure analysis of the bearing was carried out, and the factors found to be contributing in the failure of bearing were (a) Improper mounting and/or excessive axial load, (b) Entrapment of the foreign particles leading to wear and stresses at localized regions, (c) Contact fatigue under excessive axial loads leading to formation of spalling; the formation of debris during spalling might also enhance the wear.