Failure investigation of gears on a tyre manufacturing machine

This failure investigation deals with an apparently straightforward case of gear failure that was undertaken to see whether it contained any cracking that pre-existed the final fracture of a tooth. Several interesting features were observed in this case before reaching a conclusion that the gears would have failed prematurely from manufacturing details. These observations included chevron markings on the ductile fracture surface that pointed back to the local fracture origin, and the observation of very localised intergranular oxidation in the carburised case of the gear that occurred during heat treatment. This led to pre-existing defects that could readily initiate fatigue cracks that greatly reduced the impact resistance of the gear.