Alignment Targets for Large TEFC Motors 6

[JJPellin]

I posted a question back in August about motor shaft axial shuttling in some newer pump trains with relatively large TEFC motors. Update: We have largely resolved our shuttling problem. Our motor supplier and motor repair shop establish mag center on these large TEFC motors by running them solo with the cooling fan removed. If we have to establish axial running position in the field, we place the rotor in the center of mechanical float. We have less axial shuttling problems, but have another more serious problem.

On at least four occasions, we have had coupling disk pack failures on these machines. The failures are characteristic of severe misalignment. For the rest of my comments on this post, I will use one machine train as an example. Energy control policy does not allow us to perform a true hot alignment. We align these machines cold using alignment targets based on predicted thermal growth. For a large pump train, the pump OEM predicted that the pump would grow 0.016” vertically and the 800 HP TEFC motor would grow 0.006” vertically. Based on this, we set the motor 0.010” high for our cold alignment. Over the last six months, we have failed disks in the coupling twice. We recognized a need to validate the alignment targets.

We placed dial indicators on the coupling hubs with the pump in hot condition and monitored as it cooled off. The pump hub dropped 0.003” and the motor dropped 0.018”. Next we performed a very detailed temperature survey of all pump and motor supports. This showed that the motor supports averaged 95 F on the drive end and 60 F on the non-drive end. The cooling fan arrangement on these TEFC motors is not cooling the motor housing uniformly. Similarly, the pump supports were averaging 95 F on the NDE and 70 F on the DE. The air from the motor fan is blowing hard against the inboard supports. Using these temperatures, we would predict that the motor needs to be set about 0.010” low for a cold alignment. Recall, the OEM had us setting the motor high by 0.010”. We were running with 0.020” parallel offset misalignment. No surprise we were having coupling problems.

We will adjust our alignment targets and realign the motors next week. I am still curious if anyone else has particular experience with uneven vertical growth on large TEFC motors.


Johnny Pellin

 

[electricpete]

Sorry to beat a dead horse, but after thinking some more, I wanted to add two more things that make me doubt that corrosion would affect coupling flexibility (i.e. I doubt that the coupling relies on sliding between shims):
1 - there is no requirement for lub of shim pack couplings, regardless of shim material (if sliding was expected you'd think they would be lubed). That was the aspect Mike asked about.
2 - According to Piatrowski's Shaft Alignment Handbook, thermography can detect misalignment of gear type couplings and grid type couplings, but not disk pack couplings or diaphram type couplings. If sliding between disks was involved in accommodating misalignment, you'd think it would show up on thermography.



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(2B)+(2B)' ?

 

[CouplingGuru]

Let me pound on that horse as well

I think you may be hung up on this "sliding of the shims" thinking. The danger that I have witnessed first hand, is that when the shims get real rusty. They can stick together, that in itself isn't bad from them not being able to move relative to one another, but effectively shortens the chord length. (As you correctly stated, they aren't lubricated, maybe very lightly upon assembly)

Now to my example, we have underwater operation on dredging applications that we frequently change out couplings on, mainly they upgrade to our coupling with SS shims. I have seen other manufacturers carbon steel shims rusted together, this effectively shortens the chord length. That chord shortening is essentially reducing the misalignment ability of the pack. Now this is a special application, but I don't see why the same thing couldn't happen in other applications where severe corrosion is present. I have also seen the same "rust together" phenomenon on seasonal irrigation pump applications.

When it comes to couplings we are always here to help.

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[electricpete]

I think you may be hung up on this "sliding of the shims" thinking.

I personally am very interested to distinguish between problems that can affect the long-term reliability of the coupling only (these don't concern me very much since these couplings are generally low-maintenance / high reliability based on experience) and problems that might reduce the flexibility of the coupling and thereby increase loading on the bearings in presence of modest misalignment (rolling bearings tend to be higher maintenance items on some of our machines).

Can you explain a little how corrosion decreases effective cord length?


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(2B)+(2B)' ?

 

[CouplingGuru]

I am talking severe corrosion, of which I have only observed on carbon steel shims. Basically right next to the washer if the shims start corroding together it is in effect making that section act like a block instead of a flexible element. This shortens the effective flexing range (the chord) and increases the strain on the shims assuming the same amount of misalignment. And as you stated this will also increase the restoring force of the coupling which goes directly back to your bearings and seals. (i.e. it will take more force to bend the coupling out of alignment then amount of force required prior to it corroding)

But in the bigger picture, if you are concerned about extending your bearing a seal life your best bet is to invest in a lower restoring force coupling. The characteristics of a low restoring force disc coupling are a very thin shim pack with high performance stainless spring steel. Although you may spend more upfront, you save a lot on bearing and seal life. Going hand in hand with that is power density. This is essentially the amount of torque that a coupling can drive in comparison to its outer diameter. Why is that important? The discs act like a resisting lever during misalignment, the closer the lever is to the axis of rotation (i.e. smaller coupling diameter) the less leverage that restoring force has.

On top of all that power density directly effects the coupling weight. Smaller couplings weigh less typically, so a 50lb hub spinning on the end of your stub shaft is a lot less desirable then a 25lb hub. Less weight decreases vibratory loads due to coupling imbalance. So even if two couplings are equally balanced the one weighing half as much will impart half as much induced vibratory loads on your bearings and seals.




When it comes to couplings we are always here to help.

http://WWW.PSCCOUPLINGS.COM

 

[Tmoose]

Hi JJPellin,

Might just be the camera angle, but In the picture you provided 2 May 17 12:16 I don't see radiuses on the washer faces that touch the discs. That is a mighty important detail specifically for increased shim life.

Other shim styles are contoured, which, whether by design or coincidence, would bring some additional reduced stress concentration as well.

 

[JJPellin]

Thanks Tmoose.

This coupling model does not use the washers with the large radius. The standard washers for this coupling have a slight radius on both sides. We only use the factory original washers for the rebuild of these couplings. I am certain that we are using the correct washers and installing them correctly. Rather than installing a coupling that can accommodate a greater amount of misalignment, I would very much prefer to reduce the misalignment using improved alignment targets.

I reviewed the coupling drawing and the coupling rebuild procedures (OEM) to verify that there are no notes to suggest that the washers need to be turned a particular way (larger radius toward the disk pack) which is common in some other designs. The drawings and the instructions suggest that the washers are symmetrical, with the same radius on both sides.

Johnny Pellin