A shaft failure is a serious fault. I once observed one which did not trip the drive and resulted in a whipping jackshaft. Causes include:
- Rotational bending fatigue
- Impact loading
- Torsional loading
An excellent article describing motor-shaft faults including photos is in IEEE Industry Applications, Vol. 18, No. 6, "Root Cause Methodology For Induction Motors" by Bonnet, Austin H.