Failure analysis of collector gear of a helicopter

A tooth of the collector gear of a helicopter has failed by fatigue. Multiple fatigue cracks were found to have initiated at the spalled region on the working flank of the tooth and propagated simultaneously across the thickness to critical lengths before giving way to final overload failure. Examination revealed spalling, destructive pitting and wear on all the teeth of the sector of the gear provided for investigation. The damages were predominantly observed below the pitch line. Severe wear and sliding marks were observed over a band on the dedendum of the flank suggesting unusual loading during the tooth withdrawal cycle