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A motor shaft almost never breaks under the motor's own torque; the source of the fracture is usually a load applied to the shaft from outside, repeated over and over. If the new motor is connected to the same belt, coupling and base arrangement, the same shaft end meets the same strain.
What does the fracture face tell you?
Photographing the fracture face before cleaning it is worthwhile. If the surface shows lines spreading out in waves and ends in a small, rough final rupture zone, the crack grew over a long period; this indicates a fatigue fracture and points to repeated bending or torsion. If the surface looks coarse from end to end, as if it snapped in one blow, a sudden jam or impact is likely. The fracture usually starts where the keyway ends or at the shaft shoulder.
What to examine on the machine
- Belt drive: Excessive tension, the motor pulley fitted away from the shaft shoulder towards the shaft end, or a motor pulley smaller than it should be. A small pulley means a higher belt force for the same torque.
- Coupling: Angular or parallel misalignment bends the shaft on every turn; the load grows as the flexible element hardens.
- Shock load: On crushers, shredders and presses, the sudden stop when the machine jams puts a heavy torsional load on the shaft.
- Base: Loose bolts or a flexing chassis increase vibration and speed up fatigue.
- Pulley fitting: A pulley hammered onto the shaft damages both the bearing and the shaft shoulder.
When choosing the new motor
Shaft diameter and length depend on frame size. For applications with heavy belt loads, state the belt type, pulley diameter and the pulley's position on the shaft when ordering, and ask beforehand whether the shaft end will carry that load. The pulley and belt calculator can be used to recalculate the pulley diameter.
For heavy belt-driven machines, application pages such as belt conveyor motors give a useful idea; compare frame and shaft size with the old motor and pick the right model in the shop.