Failure Analysis of a Two High Gearbox Shaft

This paper reports the results of failure analysis of a two high gearbox shaft of a gearbox in a hot steel rolling mill in Thailand which fail prematurely after about 15,000 hours of service. Standard procedures for failure analysis were employed in this investigation. The results showed that the shaft failed by fatigue fracture. Beach marks on the fracture surface were clearly visible. Fatigue cracks were initiated at the corners of the wobbler. Relatively small final fracture area of the fracture surface indicated that the shaft was under a low stress at the time of failure.It is concluded that the shaft failed by fatigue fracture and that premature failure occurred due to high stress concentration at the corners of the wobbler of the shaft which led eventually to fatigue crack initiation, crack growth, and final fracture. Improved design and machining practice suggested that this would help prolong service life of the component.