June 20, 2017


By Stan Riddle

Maint Strat part 4 photo3

Electric motors are almost always well-balanced when they leave the motor manufacturer.  But occasionally, motor unbalance problems are picked up with vibration monitoring tools, such as the OneProd Hawk.

Here are some easy troubleshooting methods to determine the cause of unbalance in the motor.

An electric motor’s rotor, like any rotating mass, cannot become out of balance, unless something is either added to it, or taken away from it.  But the components attached to the rotor can become unbalanced, such as:

  1. The cooling fan. If motor unbalance is diagnosed, inspect the cooling fan for dirt build up, or broken fan blades.  If the cooling fan is dirty, clean it, and recheck the vibration.  If the fan is broken, it should be replaced.
  2. The coupling hub. Many, but not all, coupling hubs are balanced.  More likely, the hub could either be experiencing:
  • Eccentric bore.  Use a dial indicator to check for runout on the coupling hub.  If the hub is bored out of center by more than 2-3 mils, it should be replaced.
  • Improper key stock length. There are a handful of methods to determine proper key stock length, but the easiest is:

Hub length + key slot length ÷ 2 = length of key stock

In addition, the coupling should be assembled with the keys opposed 180 degrees, to help counterbalance any residual unbalance forces.

An easy field method to narrow down the unbalance problem, if vibration levels indicate motor unbalance:

  • Uncouple the motor, and re-check vibration. If the vibration drops substantially, the problem could be misalignment, not unbalance.  Or, the coupling element is unbalanced.
  • If this does not reduce the unbalance, remove the coupling hub from the motor shaft, and re-measure. If vibration is reduced, check to make sure the hub is concentric, and the key length is sized correctly.

If motor vibration is still excessive, inspect the cooling fan.  If no problem is found, the motor should be sent to a motor shop for balancing.


  1. Rick Diese on June 29, 2017 at 8:52 pm

    In addition, the coupling should be assembled with the keys opposed 180 degrees, to help counterbalance any residual unbalance forces.

    I’m not sure I agree with this statement. One could easily make it worse no better. One would be assuming both keys are too long, not sure on the odds of this. Could you have just as easily said “try different coupling positions,, relative to each other, and see if one is better than the others. Or am I missing something?

  2. Stan Riddle on June 29, 2017 at 9:30 pm

    Thanks for your comment Rick! It is an interesting discussion.

    If a shaft is well-machined, and not bent, it should be well-balanced. But if it is machined for a key slot, removing metal would slightly unbalance it. Broaching the coupling hub for a key would do the same thing as machining the shaft. If the hub is factory-balanced, no problem. If the key was cut to the proper length, it should counteract residual unbalance forces from the key slots. It would not matter if the keys were opposed or not.

    But keys usually are generally not cut to length with the same precision as machining the slots. It could be too long, too short, or just right. It’s random, so you are correct. But at least in the industries I’ve worked in, it is standard procedure to oppose the keys. You certainly could try positioning them differently, and find a good counter-balance point – if you have the time.

    In my experience, I have reduced vibration a few times by opposing they keys. But I’ve never increased vibration by doing it.

  3. Chad Knecht on December 4, 2018 at 7:00 am

    As a former machinist I always here the key way removes stuck causing unbalance. I guess everyone forgets the key being installed replaces the weight. Key to it is to calculate the combined slot of motor key way and coupling times it by .5 that’s the length the key needs to be to balance out material loss.

  4. ODP Variable frequency drives on June 6, 2019 at 3:42 am

    Unbalances in motor parts can lead to troubleshooting. It is quite necessary to take care of that. If troubleshooting and minor problems with the motors is not taken into consideration, it may lead to motor failure. Thanks a lot! I would like to see more good information on this!