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Electrical Testing

Tip of the Week: Motors and Vibration, Part 2

The causes of vibration are nearly always mechanical.

Excess vibration destroys bearings, windings, and even the cases of motors. Not only that, the vibration can travel through the pipes, couplings, and other parts of the system to damage gearboxes, pumps, and other equipment.

The causes of vibration are nearly always mechanical. Common culprits:

  • Excessive belt tension, in a belt-driven system.
  • Bent or misaligned pulley, in a belt-driven system.
  • Defective ball or sleeve bearings in the motor, nearly always because of lubrication errors.
  • Misalignment of the motor with its load.
  • Imbalance caused by a weak motor base or a weak motor pad.
  • Gearbox problems (e.g., lubrication defects or excess wear or gear damage) feeding vibration back to the motor.

A very common cause of vibration is an unbalanced rotating member. That member might be the rotor, rotating load, or some component in the drive train. The motor rotor is the least likely culprit. If you uncouple the motor from the load and run it, you’re likely to find the problem isn’t there but in the load.

Don’t wait for your vibration tester to indicate “excess vibration.” It's something that’s hard to define correctly for a specific installation (there are rules of thumb, but use them as starting points).

Trend your vibration measurements and start checking these mechanical sources when vibration shows an increase over time. For critical motors, use a vibration monitoring system that can alert you to changes in vibration.

By trending, you can see the start of a problem that hasn’t yet become full blown. A bearing that is merely galled is much less of a problem than one that has been galled for a long time and finally has spun apart.

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