A broken coupling can play both roles: most often it is the result of misalignment, shock loading or a bearing that has started to fail, but a cracked or disintegrated coupling can also cause new damage to the motor shaft and bearings. To tell which came first, look at how the coupling broke and at the condition of both shafts.

DRG-200L-4 30 kW 1500 rpm cast iron frame DRG three-phase motor – broken coupling

Common reasons couplings break

  • Misalignment: If the motor and machine shafts are not in line, the coupling flexes on every turn; the flexible element heats up and cracks, and the metal parts fatigue.
  • Shock and jamming: A sudden lock-up of the machine creates more torque than the coupling can carry.
  • No axial clearance: If no gap is left between the two hubs, thermal expansion squeezes the coupling.
  • Aged flexible element: Rubber hardened by oil, heat and time can no longer damp.
  • Wrong size: If motor power was increased but the coupling left unchanged, the coupling becomes the weak link.

Could the motor be to blame?

When a motor bearing begins to fail, the shaft moves off centre and vibration increases, which can strain the coupling. If the shaft is bent, the coupling is loaded at the same point on every turn. So before replacing the coupling, turn the motor shaft by hand to check for play and noise, and measure the run-out of the shaft end.

Effect of a broken coupling on the motor

A disintegrating coupling runs out of balance and delivers blows to the motor shaft and drive-end bearing. Broken pieces can be flung around inside the guard. If the coupling has separated completely, the motor runs unloaded; this usually does no harm to the motor, but the fault is noticed because the machine stops.

Checks after replacement

When the new coupling is fitted, alignment should be checked with a dial indicator or laser tool, and the shims under the feet recorded. If the motor is also being replaced, compare the new motor's shaft height and shaft diameter with the old one; if they differ, the alignment must be done from scratch.

For applications where coupled drives are common, see the centrifugal pump motors page, and compare models with a matching shaft and shaft height in the shop.