Fatigue Failure of a Metal-Sprayed Crankshaft

In the course of a general overhaul, the crankpins and main journals (3 in. diam) of the crankshaft of a four-cylinder oil engine were built up by metal spraying. Four weeks later, the shaft broke through the pin remote from the flywheel (driving) end. The fracture was of the fatigue type. A creeping crack originated in the fillet at the inside surface of the pin and extended parallel to the plane of the web across practically the entire section before complete rupture occurred. The sprayed metal on the fractured pin had very poor adhesion. The surfaces of the main journals had not been grooved but appeared to have been roughened by shot or grit-blasting prior to spraying and the deposit was more firmly adherent to these surfaces than in the case of the pins. It is doubtful, however, whether the adhesion of sprayed metal to a surface prepared even in this manner would always be satisfactory under severe loading conditions, such as those to which a crankpin is subjected in service.