Sudden rotor unbalance / TIR for no apparent reason 9

[electricpete]

This is a 2500hp 1800rpm horizontal sleeve bearing motor, driving a single stage hot water pump vis a shim back coupling.

We had a motor that was fine on vibration in August < 0.1ips.
It ran from August until December.

In December we shut it down for 2 days maintenance (filter change, electrical testing) and restarted it (DOL, no vfd). Within 1 day after restart we noticed a very loud noise. Vibration had undergone a dramatic increase.

We did a variety of troubleshooting, ended up swapping the rotor and sending it to repair shop (the position of the motor within the plant does not support rigging out the motor as a unit). With the new rotor, vibration was back to normal.

Repair shop evaluated the removed rotor.

TESTS THAT WERE NORMAL:
Rotor core loop test thermography did not show any hotspots. Injected current thermography did not show any hotspots. Paint blasted from end ring and inspected – no anomalies. Rotor bar tap test indicated all bars very tight (which is not necessarily normal, this rotor has tighter bars than normal).

TESTS THAT WERE ABNORMAL:

Repair shop found significant unbalance on the rotor (“7 mils” unbalance on inboard end is how they described the unbalance – it corresponds to a calculated eccentricity).

They measured TIR with machine supported in lathe at the ends. Found very low TIR on outboard end, TIR at the inboard end started at 0 at the tip of the shaft and slowly increase dto 6 mils at the winding-side lab seal surface (the last machined surface available on that side of the motor, I’ve asked to check TIR further toward the core on non-machined surface). Slide 1 shows the TIRs recorded (note the graphic upon which this info is recorded is a standard form, it does not represent the scale of this rotor).

We checked the rotor cold and hot (heated in over), there was no change in unbalance condition or TIR (of course this does not recreate non-uniform heating of a motor start).

=== ==== ===

These unbalance and TIR measurements are very abnormal and suggest a smoking gun cause of the elevated vibration (which again was fine in August). The question is what caused a sudden abnormal rotor TIR.

There were no unusual plant transients recorded or unusual maintenance done on the motor following the August normal vibration.

The rotor has a fabricated spider with 6 spokes welded to the shaft along the entire length of the spider. Slides 2-4 show the rotor or similar rotors, as labeled. The bars are swaged once in the axial middle of the rotor. The bars are tight in the slot with a copper liner remaining in the slot (that was used to facilitate original bar installation). Slide 5 shows the liner.


Additional tests/inspections planned:
- Check TIR as close to the spider as we can get on the inboard end (to attempt to localize the bend)
- Inspect accessible spider welds

What do you think could be the cause of the sudden appearance of rotor bend?
What additional checks would you suggest?

My objective: I would like to understand the cause well enough that I can have confidence in selecting a rotor repair or replacement strategy that will prevent the problem from recurring. Shop had proposed simply bend the shaft extension using heat to restore the balance, then balance, and check balance / tir repeatability including after heating. That doesn’t give me a lot of confidence the problem will not recur when we reinstall this rotor, considering we don't know why it bent to begin with.


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

 

[dArsonval]

Quite a few questions entangled within this thread.

One thing in your favor in diagnosing the motor mystery is... at least it's a four-pole motor.

In viewing the portion of the PDF labeled [Faulty Rotor TIR Map] I have at least one observation.

If the "in-board" (drive end) shaft end was centered in a lathe, the reading would be "0"as shown on the TIR Map.

The numbers represented at the drive end elude the rotor was held fast at each shaft-end-center
when "indicated" in for documentation, and not necessarily measured with the rotor resting on both its journals.
(With both rotor journals rotating on balancing ways, it allows each shaft end to reflect their true run out in relation to the journals.)
The drive end shaft extension would then show a more pronounced out-of-tolerance
measurement then what's represented on the rotor drawing.

I have to believe a competent repair shop measured the total run out correctly, yet make the observation anyway.

Focusing on Pete's single question of, "What do you think could be the cause of the sudden
appearance of rotor bend?

Somewhat of a hunch... I believe the rotor shaft was out of tolerance all along.
What ever happened during the jostling of the apparatus during its shut down for maintenance
nudged the motor past the negligible cusp it was teetering on.

Permissible shaft extension run out on a NEMA Frame Size machine discussed here would
be .003".

If the shaft on the suspected rotor is indeed bent, I'd be leery of straightening it with any kind of heat tactic.
Yes, It can be done... "but" a motor that size with a shaft issue may warrant an outright new machined shaft to correct the problem.

A new shaft would also instill confidence the issue would not reoccur.

John

 

[electricpete]

Thanks for all the input.

What did noise sound like, how long did it last, and did sound event occur at same time of vibration event?

Inspect both bearings for Babbitt condition and loose or crack liner or housing.


Inspect shaft coupling for cracked discs and sheared bolts.

Inspect for rotor contact with stator.

Inspect for loose/missing balance weights.

Conduct Growler test for cracked rotor bar (or any rotor electrical circuit fault)

To keep the thread focused, I left out most of the details of what we did at the plant before we concluded the rotor was the problem. In December 2018 (when the noise/vibration appeared), we checked bearing-to-shaft clearances, bearing to housing clearances, bearing wear pattern all good except for recent surface indications attributable to elevated vibration. Replaced the bearings. We did uncoupled run, vibration remained (ruled out misalignment and bearing problems).

We also measured shaft extension TIR during that evolution. It was 0.002” at the end with shaft sitting in bearings. We didn’t consider that unusual at the time and we didn’t map out the rest of the shaft at that time.

We did decide to replace the rotor (Feb ’19) because we had checked everything else and we do have some history of rotor problems on sister motors (one electrical rotor problem at braze joints, one mechanical rotor problem long ago, limited details). Replacing the rotor seemed to be what cured the vibration.

Other misc: noise was growling. The first high vibration was noted the first time we measured vibration when investigating the abnormal noise (all of this within about 3 days in December '18). Coupling was inspected, no anomalies.

Conduct rotor natural frequency (bending modes) test to identify possible cracked shaft or change in NF caused by rotor structure change.

We have no other rotor available for comparison. Nor do we have machine frame available for testing in the shop. I’m sure it is a rigid rotor by design at running speed (on rigid supports any resonance should be far above running speed).
It might be interesting to try to position the rotor in different orientations rotating it by 60 degrees (corresponding to distance between spider arms) and see if bump test results change as we rotate it. I’ll talk to the shop about exactly how we would do that and what do we think it would buy us.

Panter and Bill mentioned possible transport damage (which presumably would be after we pulled the motor to send it to the shop. I can’t 100.0% rule that out since we didn’t map the TIR across other portions of the rotor or check rotor balance before we shipped the rotor to the shop. But it seems unlikely since changing the rotor seemed to cure the problem.

This size motor with sleeve bearings should have received an alignment before startup, there should be an alignment report that would have indicated a bent shaft before start-up had the motor been damaged during transport or installation. Having said that, it's hard to imagine the shaft being bent in service under load, my experience is that a high torsional load would shear the shaft. Is there any way that the shaft could have taken a large hit after alignment but before startup?

There was no alignment performed recently before the vibration increase. The motor was operating fine and we saw a sudden increase following motor shutdown for filter change / electrical test and restart. BUT it does suggest that it would be useful to go back and find the last alignment record and see if that same 0.002” mils was present (assuming they checked at the same location on the shaft extension). I will check that.

Going back to non-uniform thermal expansion is it possible that the shaft was prevent from growing longitudinally along its axis,

That is not an option in this application. The motor has only sleeve journal bearings (no thrust bearings). They do have temporary-duty thrust bumpers which restrain motion to 0.5” while uncoupled. The axial position is established via flexible coupling via the driven machine and we always ensure it is at least 1/8” from either limit of travel. There was no sign of contact at the thrust bumpers.

If there are large temperature differences caused by not rotating the rotor during cooling it can lead to temporary distortion of massively stiff parts. This could lead to some movement between press-fit or clamped parts which involves a lot of friction. If motion occurs in a press fit it will not return to the same shape due to the hysteresis caused by friction. Just conjecture.

We discussed heating a little above. Since then, I have verified space heater is located along the axial center of the rotor, but outside the stator core. The stator winding temperature is in the neighborhood of 20F-30F above ambient temperature while the machine is secured. The rotor which is further from the space heater would be cooler and would see less directional heating (the stator core would spread out the heat more compared to direct exposure to a heater). So I don't think the space heater is causing any thermal warping of the rotor.

How hot is the water? Is the coupling conducive to transmitting heat to the motor shaft?

I think the water is around 400F when the pump is running and doesn’t cool much when the pump is placed in standby (there is small backflow in idle pump intended to keep it near running temperature). The coupling is shim pack style. I really don’t think this is likely (apart from the long distance, surely mechanics would complain about the heat while uncoupling), but I’ll grab a thermography gun and check the motor shaft extensioin temperature on the running and idle pumps.

In viewing the portion of the PDF labeled [Faulty Rotor TIR Map] I have at least one observation.

If the "in-board" (drive end) shaft end was centered in a lathe, the reading would be "0"as shown on the TIR Map.

The numbers represented at the drive end elude the rotor was held fast at each shaft-end-center
when "indicated" in for documentation, and not necessarily measured with the rotor resting on both its journals.
(With both rotor journals rotating on balancing ways, it allows each shaft end to reflect their true run out in relation to the journals.)
The drive end shaft extension would then show a more pronounced out-of-tolerance
measurement then what's represented on the rotor drawing.

I’ll ask the shop to rotate carefully with journals on vee blocks or rollers and see if they can recreate the 0.002” that we saw at the shaft extension when we rotated the rotor on its bearings within the machine.

A new shaft would also instill confidence the issue would not reoccur.

So, (assuming we don’t narrow the problem any further), you’re saying you feel reasonably confident that the shaft or shaft-to-spidder welds are the cause (and replacing the shaft with existing spider and bars would cure the problem)?


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

 

[Compositepro]

Could a rat, a snake, or some other critter have gotten into the rotor while it was stationary, causing an unbalance?

 

[desertfox]

Hi electrcpete

Found this link:-

https://pdfs.semanticscholar.org/1a44/a820be1528d1202d55f2fdacd1ab5d231622.pdf

might not be anything here you didn't already know but I chucked it in anyway.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[rhatcher]

Pete,

It sounds like you have thoroughly covered all of the possibilities based on the information at hand. It will be informative to see the results of the second rotor TIR with the journals on the rollers. Please share this when you get it.

 

[PhilCorso]

Cepsicon (Electrical)
Details on motor enclosure, please: drip-proof; TEFC; air-cooled with air from non-hazardous-area, If latter, what is shaft-bushing material ?
Regards, Phil Corso

I do not presume to have a solution, but, perhaps an experience will shed some light! Coincident change in amplitude and phase-shift could be indicative of a vibration phenomenon referred to as the “Morton Effect!” It is associated with tilting-pad bearings or seal-to-shaft rub. My experience was associated with the latter phenomenon.
A 21,000 Hp induction-motor, driving a refinery compressor, would trip randomly. There was no recognizable pattern… it happened day or night, weekdays or weekends, rain or shine, during all phases of the moon, the ocean tides, space-shuttle launches, sun-spot cycles, etc, etc.
A 15-man team was organized, at least one from each engineering branch, including Dr. Den Hartog, then known as the “father” of vibration analysis! Also on site were the motor's designer and a major competitor. Testing even included “bouncing” of the shaft while it was in operation! Also measured the tidal impact on its foundation!
After 3-weeks, the problem was accidently observed when force was applied to its fiberglass enclosure where its shaft passed through a wooden (yes, wooden) bushing! A slight unbalance of the motor’s rotor produced the rub, which, in turn generated enough heat to bow the motor's 6-8" dia. shaft. It seemed impossible to the team assigned to solve the problem. Additional details will be furnished on request, Regards, Phil Corso