High Non-Synchronous vibration with harmonics on Vertcal motor

[Hoosierken]

We are running a 1500HP vertical motor with an upper guide bearing and a lower ball bearing. Running uncoupled we see the dominant vibration at .55 of running speed (1800) and also harmonics of the .55 peak. Even sidebands at .55 x RPM surround the running speed peak. 1X RPM vibration appears very low at .01 IPS RMS. the .55 sideband is at .13 IPS RMS.
Any thoughts on this?

Thanks

Ken

 

[NotecA]

What caused the vibration ?
Did the vibration increase smoothly over time, or did it occur suddenly?
Did you carry out any repair work or install a new unit?
How is it secured to the frame and foundation? Can you take some pictures?
At first glance I would inspect the structure for resonances, cracks and loose bolt connections. Check for play in the bearings. Also check the anchor mounts, if any.


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[Hoosierken]

Thanks for the reply.
This motor is actually in our shop for recondition.
Answers to your questions below.

What caused the vibration ? Motor came to our shop due to pump only problems. Simple recondition.
Did the vibration increase smoothly over time, or did it occur suddenly? Incoming test showed no unusual peaks. After reassembly we see the odd frequencies
Did you carry out any repair work or install a new unit? Repairs were cleaning up the guide bearing and replacing bottom ball bearing.
How is it secured to the frame and foundation? Can you take some pictures? Motor is bolted to our test pad and secured and not possible soft foot.

At first glance I would inspect the structure for resonances, cracks and loose bolt connections. Check for play in the bearings. Also check the anchor mounts, if any.

The only thing we did was cleanup and measure clearances on guide bearing and replace lower ball bearing. All vibration is from the upper housing.

Thanks

Ken

 

[GregLocock]

.55 sounds suspiciously like half speed whirl, due to misbehavior of oil films and the like.

https://www.sciencedirect.com/topics/engineering/oil-whirl

Cheers

Greg Locock


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[electricpete]

So the upper bearing is a sliding bearing? ... Is it sleeve or it tilt pad?

The lower bearing you described as ball bearing - is that deep groove or some combination of angle contact bearings? And it takes the weight of the rotor?

If the upper bearing is a simple cylindrical sleeve design then Greg's suggestion oil whirl is plausible (although typically it's would occur at a frequency less than 0.5x). Tilt pad shouldn't be nearly as susceptible to that. If it is whirl, it may be sensitive to oil temperature among other things.

Also the bottom bearing can be a source of problem. It may be sensitive to thrust loading especially if there is an angle contact bearing. Then the problem might possibly be lack of external downthrust (yes I realize it didn't vibrate incoming without thrust load, but see my note below (*)). More likely there might be assembly factors which establishes preload of angle contact bearing even before external thrust is applied (how tightly is the angle contact clamped axially).

A little bit on the unlikely side, being a vertical motor with oil lubricated upper bearings you probably have a standpipe forming the inner diameter of the upper bearing oil reservoir, and then some piece (maybe called a runner or collar) which mounts to the shaft at elevation above the standpipe and provides a surface to interface with the bearing stationary components at an elevation below the standpipe. If that piece (runner or collar) is too loose on the shaft (and not well clamped axially) then I believe that can cause non-sync vibration as the collar itself tilts in a whirling pattern with respect to the shaft.

On the far-out unlikely side, if inner ring is spinning on the shaft you might get non-sync frequencies and probably worsening vibration, heating and failure in short order... I assume shaft fits to inner ring were checked during the maintenance?

Also on the far-out side, did you check the FTF (cage fault frequency) associated with the lower bearing part number to see if it is 0.45 orders?

(*) We have one family of vertical motors with rolling element thrust bearings which give non-sync frequencies (not assocociated with any fault pattern) which show only intermittently during uncoupled run, but always go away coupled.

Fwiw, we also have one family of rolling bearing vertical motors which has and unusual non-sync vibration not identifiable as a bearing fault patterns and in this particular case it doesn't go away coupled or uncoupled, but comes and goes for reasons we can't even narrow down. We have never figured out a definitive cause (even with lots of help from our motor shop). After spending a lot of time and money I finally gave up trying to figure it out. It has been that way for decades and never associated with any degradation or reliability problems on those motors so it's not worth any further investigation.