Analysis of spalled collector pinion of a helicopter

The damaged collector pinion (LH) of a helicopter was forwarded to the laboratory for investigation. The damage was in the form of spalling on the working flank of one of the teeth. The spalling had initiated at the dedendum of the tooth and propagated upward progressively. Analysis showed that the spalling was caused due to high surface contact stresses. The situation was further aggravated due to sliding of the contact surfaces. No metallurgical abnormalities were found responsible for the failure. The microstructure, hardness and the effective case depth were found to be uniform on all the teeth, and they were found to be within specification. . A detailed analysis of the failure is presented in this report. It may be noted that a number of failed collector pinions of another helicopter were investigated in this laboratory in the past (refer NAL Report Nos. MT-FA-917-05-2005, MT-FA-943-10-2005 and MT-FA-1078-12-2007). The present failure is exactly similar to those investigated earlier