CRM Engineering

A client was experiencing failure in a cantilevered shaft which supported a heavy spool that was subject to frequent rapid starts and stops. The initial design incorporated a a 3″ diameter shaft that was supported by two bearings with a drive gear halfway between the bearings. A larger, 5″ diameter shaft was fillet welded onto the 3″ shaft just outboard of the outer bearing.

The massive spool was accelerated from rest to 60 rpm in a fraction of a second and braked several times an hour, 24/7. Thus the number of stress reversals seen in a year required a design in which the peak stress is well below the fatigue limit. The shaft would break at the fillet weld within a few weeks of service at the fillet weld (no surprise).

Examining the problem from first principles (beam theory and stress concentration factors) showed that the expected fatigue life was measured in hours. A detailed finite element analysis of the system corroborated the diagnosis.

Our recommendations:

Use a material with a fatigue limit much higher than the mild steel used in the original design