Investigation of a failed axle of a reduction gearbox

After 11,300 h of operation, one of the axles of the reduction gearbox of the hoist mechanism of a dockside crane failed due to the propagation of a fatigue crack. The axle contained two gears, the largest of which was mounted swith a key, while the smallest gear was an integral part of the axle. On both ends of the axle a bearing was mounted: one cylindrical bearing and one spherical roller bearing (locating bearing).The axle was made from 17CrNiMo6 steel, and the machined gear teeth were case carburised. The failure of the axle occurred in the middle between both bearings of the axle, on a non-contacting part of the gear machined on the axle.During visual inspection striations were observed on the fracture surface and three separate crack initiation zones could be observed on three neighbouring gear teeth. The ratio of final fracture versus fatigue fracture was low, indicating a low nominal stress on the axle.The three initiation sites were investigated with optical microscopy and SEM, and clear indications of a ductile overload fracture were found. In an etched longitudinal section of one of the gear teeth where initiation took place several cracks could be observed in the hardened case of the tooth.The hardness at the initiation site was found to be 777 HV, while the hardness at the final fracture (diametric position relative to the initiation of the crack) was found to be lower, namely 722 HV.It was concluded that at the initiation site during the case carburisation quenching cracks were formed, which then propagated through the whole section of the axle under relatively low operation load.