Failure analysis of a gear wheel of a marine azimuth thruster

A failure analysis of two helical gear wheels of a ducted azimuth thruster is presented. The research work consisted of a fracture examination of the material in order to determine the damage root causes. The samples for the failure analysis were obtained from two broken teeth of two helical gear wheels. An analysis through the scanning electron microscope (SEM) was carried out close to the crack initiation. It was found that the damage in the bevel gears were by the fatigue fracture mode. The SEM analysis showed that the gear teeth were under severe contact stress during the operation aggravated by an inappropriate lubricating. A possible misalignment between the pinions and the gear wheels teeth could also contribute for the premature failure.