Gearbox Failure Analysis

An analysis of the failures of gearbox units after about 10,000 service hours when the design service life of 45,000 hours was expected was performed to establish the cause of failure. The gear tooth characteristics, namely, beam strength, maximum allowable dynamic load, allowable static load and limiting tooth wear load were analysed for ability to cope with the duty load. The failed gear components were examined visually and metallurgically. Lubrication oil used in the gearboxes was examined for consistency with manufacturer’s standards. The observed failures were due to design and manufacturing errors: for instance, the duty load on the gear teeth exceeded the maximum tooth beam strength, allowable dynamic load and limiting wear load. The out-of-tolerance surface finish also contributed as it resulted in the misalignment of pinions and gears. To prevent these failures required adequate sizing of the gear teeth and face width as well as better choice of materials with higher allowable static stresses to bear the loads imposed. The surface roughness should be within tolerance.