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This is half case study, half question.

This is a 7,000hp 3,600rpm 13.2kv horizontal tilt-pad radial bearing motor, driving a centrifugal pump.
The service is mostly standby with very short intermittent periods of running/

Attached waterfall (slide 1) shows that between Nov 2010 reading and November 2011 reading, the outboard bearing axial (1A position) 1x vib jumped from 0.13 ips to approx 0.4 ips (above the scale of the waterfall).
None of the other frequencies increased.
Slides 2 thru 10 show all other motor positions have no significant change in vibration (except possibly motor inboard vertical = 2H which had slight increase in 2x).

What happened in between Nov 2010 and Nov 11 when we saw the jump:
1 – We removed/reinstalled endcovers for inspection.
2 – We re-aligned since the pump was found 0.003 offset. Our target is 0. We have no thermal growth targets established, even though the pump fluid is hot (we do plan on going back to do Acculign checks).

Slide 11 = Overview of the motor outboard end.
Slide 12 = closer view, labeled with vibration measurements. While the bearing cap reads 0.4 ips, the upper endshield reads approx 1.2 ips. By pressing on the upper endshield, you can make the vibration go down.

Slide 13 = First vibration reducing device. Simply placing this loosely inside the two endshield handles cause bearing axial vibration to decrease to 0.25 ips. Then tightening the screw to “stiffen” the endplate (which was our original intention) only caused vib to go back up.

Slide 14 = 2nd vibration reducing device. Placing this heavy bar tightly inside the two endshield handles (it was a tight fit... had to hammer it in) caused bearing axial vib to decrease to approx 0.25 ips. Clamping the device to the outside of the handles caused bearing vib to decrease to 0.19 to 0.2 ips. It appears adding mass is more effective than our stiffening attempts.

3 questions:

1 – Do you think the observed increase in bearing housing axial vibration is more likely due to
A: Change in resonant properties when we removed/reinstalled the endshield including sealant
or
B: Re-alignment of the machine (note there was not much change in vibration at other locations).
(We will of course investigate both further, but just interested in your thoughts and opinions)

2 – We won’t have any more opportunity to inspect this machine while running. If we do bump test on that endplate, do you think we'd see near 3600rpm resonance and be able to tune it without machine running based on bump test? Or do you think the off-line to on-line change in resonance due to bearing oil-film stiffness will invalidate this approach? (fwiw I tend to think it is something like a drum-head mode.

3 - Is this similar to endshield resonance you have seen in the past? I remember there have been a lot of discussions about 2*LF endshield resonance, but not 1x. Also I dont' remember anyone saying that in those cases they could de-tune by pushing on the endshield. Were your experiences with endshield resonance a drum-head type mode or something different?

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EP,
It has to be a localized resonance based on your data. If you thought the motor had this much vibration in the axial direction, the axial on the driven equipment dispells that.
I have witnessed customers going around the bolts like they are tuning a turn of the century radio in order to minimize the resonance vibration in the end bracket. My advice to you is let it run. The end bracket vibration isn't causing any degradation to the bearings.
And, yes, it depends on the motor size. There are smaller motors where the natural frequency lines up with 2x line freq, but on motors as big as this one, it is normally near 1x.
Good Luck!
Hi EP,
1. A
2. Yes I think you will see a weakness near 3600 rpm. I don't think the on-line off-line will have much effect in this case. I would try loosening the top endshield bolts that connect to the bottom bearing plate. Assumimng you are allowed. Smiler
3. Have seen several motors with this type of drum head vibration.

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