Failure analysis of overrunning clutch spring for main transmission system of helicopter

Abstract

In the present work, the premature rupture of a spring used in the aviation sprag overrunning clutch of the helicopter was analyzed based on the general method for failure analysis by use of the investigations of the fracture surface, mechanical and chemical performance as well as the microstructure of the spring material. The results showed that spring breakage is mainly caused by mechanical fatigue. The cracks and fracture originated from internal surface of the spring, and the cracks extended from outside surface of the spring toward the external surface of the spring, which was consistent with the direction of tensile residual stress on the internal surface of the spring. The marks of the shot peening were obvious on the outer surface, while the inner surface had obvious metal flow lines, indicating that the residual tensile stress formed on the internal surface of spring. Moreover, the oxide inclusions were also detected on the internal surface of spring. It can be noted that the stress concentration on the spring internal surface and oxide inclusions are responsible for the fracture of the spring.