Fatigue failure of an idle gear shaft of a gearbox

Abstract

This paper reports the results of failure analysis of an idle gear shaft of a gearbox in a hot re-rolling steel mill in Thailand. The shaft failed prematurely after only about 6,000 hours of service which was very much lower than the expected working life of 40,000-50,000 hours. The results showed that the shaft failed by fatigue fracture. Beach marks on the fracture surface were clearly visible. Fatigue cracks were initiated at two corners of the keyway of the shaft. Relatively large final fracture area of the fracture surface indicated that the shaft was under a very high stress at the time of failure. The root cause of the fracture is poor design and poor machining of the keyway. Too small fillet radii at the bottom of the keyway gave rise to stress concentration resulting in excessive stresses at the keyway corners. These high stresses led to fatigue crack initiation at the keyway corners followed by crack growth, and final fracture.