High Vibration of large motor

[tolk]

We have high vibrations on a 1000hp motor and the motor shaft is moving back and forth with very high resonance and the pump shaft is steady not moving at all. Any ideas as to what could be causing the motor shaft to move back and forth?

 

[JJPellin]

It sounds like a motor fighting to find magnetic center. Traditionally, this is believed to be caused by an incorrectly set distance between shaft ends (DBSE). And when we see this, that is often the case. But, in some cases, we seem to find that DBSE is not the root cause. If the coupling is a disk-pack style, it will have a certain spring rate and an axial natural frequency. I think that some machines tend to excite the natural frequency of the coupling in an axial oscillation mode. If the pump shaft is not moving in the same manner, it is probably because the pump has rolling element bearings with much lower axial clearances, or it has a strong and stable thrust that resists this motion better than the axially soft motor rotor.

But, start with the easy answers and work to the harder ones. Verify that you have a good alignment in hot, running condition s. Verify that the DBSE is set correctly including allowance for axial growth of the running shafts. A certain about of cold pre-stretch may be needed to achieve a proper DBSE on the running machine train.


Johnny Pellin

 

[electricpete]

We have axial shuttling of motor shaft a family of horizontal 2500 hp 1800 rpm sleeve bearing motors driving centrifugal pumps through Thomas shim pack coupling.

In our case it occurs only at low pump flow. It looks like about 1/8" of movement.

I picture the system as follows. Pump acts like ground (very little clearance in thrust bearing). Coupling acts like spring. Motor acts like mass. You have a simple single degree of freedom system with a low stiffness and high mass giving low resonant frequency on the order of 1hz.

The excitation that I imagine must be broadband axial thrust variation which occurs only at low flow and causes slight movement within the pump thrust bearing clearance.. the broadband excitation excites the resonant frequency.

=====================================
(2B)+(2B)' ?