Recently we were asked to examine a large 5 1/4 inch diameter shaft failure. The shaft was a long, cylindrical rod that rotates and transmits power. In this case, the shaft rod transported industrial products on a conveyor. The shaft failure halted the day-to-day operations of the client. An overall view of the failed shaft is shown in Photograph A.
Photograph A Severely deformed 5¼ inch conveyor shaft
The steel shaft was manufactured from high strength steel, tensile strength 150,000 psi. The shaft strength was such that it could support the weight of about 810 pickup trucks. Yes, 810 pickup trucks! As can be seen in photograph A, the shaft was bent and distorted. The shaft also exhibited galling and heating at the shaft bearing surfaces.
Later in the investigation, a photograph of the failed shaft was observed to show a flange on the bearing housing. That photograph is shown in Photograph B.
Photograph B Shaft bearing housing with arrows showing failed studs
It should be noted that the studs attaching the flange to the bearing housing had all failed. The failed studs are denoted with red arrows in Photograph B. Photographic enlargements of the failed studs are shown in Photograph C.
Photograph C Enlargement of failed studs.
When the steel fracture photographs were examined with magnification, it appears as if each stud fracture is flat and has a “velvety” surface texture. Flat fractures in steel studs, which also exhibit a velvety surface texture, are almost assuredly fatigue failures.
The problem is that these studs were not recovered and sent to J.E.I. for failure analysis. Stud failures resulting from metal fatigue in the bearing flange studs would explain the failure sequence and that the root cause of the shaft failure was the result of over (or under) torquing of these bearing flange studs.