Failure analysis of MGB I stage pinions of a helicopter

Investigation showed that the pinion AHF-12516 had failed by fatigue leading to subsequent damages to the main gearbox. The chips detected by the magnetic plug were resulted from the crushed fragments of the pinion teeth. Pinion AHF-12516 had failed by tooth bending fatigue. Evidences suggest that tooth bending fatigue was caused due to misalignment in the assembly. The pinions were found to conform to design specification in terms of dimension, effective case depth, surface hardness, core hardness and microstructure. No metallurgical and/or mechanical abnormalities were responsible for fatigue crack initiation and the fatigue was stress related. From the components provided for investigation, it was not possible to identify the cause(s) for misalignment in the pinion assembly. It is recommended that the affected MGB be subjected to detailed examination in an effort to establish as to what could cause the misalignment