A new motor was installed on a pump application. The original motor had failed electrically but had otherwise run with no vibration. The new motor was supplied by the OEM, and was claimed to be balanced at the factory.

Upon start-up, higher vibration than normally acceptable for a new motor was experienced. Several attempts were made to balance it in place. This turns out to have been futile. The vibration data did not appear stable. What could be wrong? Where to start looking?

a)    Review factory balance data and weight locations.
b)    Remove the motor and make a thorough inspection of all rotor components.
c)    Check the coupling.


It is doubtful that a balance data review will produce any solution. Any OEM is careful to make certain any information supplied to the customer is reliable.

Option b) might have to be considered but first inspect the coupling. The male side of the centering fit is normally provided with a radius. In that case the female fit must be provided with a chamfer larger than the radius. If not, the coupling faces will not properly engage. This causes a vibration situation where the amplitude and/or phase angle will vary from start to start.

A simple check is to back of the coupling bolts and tighten just enough to ensure that the bolts are engaged. Check to see if there is any gap at the coupling faces. Any gap is clear evidence of a problem in the centering fit. The answer is (c).