Case Study: Failure of Propeller Governor Idler Gear
A 250 hp aircraft engine suffered an in flight failure during a training flight due to the over speed of the propeller. During the inspection of the engine, four teeth (two of them next to each other and the others at different parts) from the propeller governor idler and 14 out of 34 teeth from the camshaft gear were found to be broken.
The fracture surfaces of the governor idler gear teeth were examined by using a Scanning Electron Microscope, and that revealed the crack initiation, crack propagation and final fracture zone on the fracture surface of gear teeth. The presence of microscopic fatigue striations at higher magnifications showed the progress of fatigue cracks were consistent with the mechanical fatigue failure of idler gear teeth which were subjected to severe fatigue loading.
Also during the inspection of the engine, the head of two cam followers had broken away from its shaft and three cam followers were found to be broken into pieces. The cam lobes were also damaged. The cam followers failure and damages on the cam lobes were caused by the broken governor idler gear teeth getting into the cam follower and the cam lobes area. Ultimately, this damage on the cam followers and cam lobes resulted in the loss of power in the engine.