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

 

[CouplingGuru]

I wouldn't call it serious to this point, but it is definitely something to keep an eye on.

On a side note, the Rexnord Thomas Series 71 375-8 really hasn't been upgraded or optimized in over 30 years. The new stuff that is out there now is so much better for application longevity. Look at the attached comparison, we are just one supplier, but look at the stark contrast in power density. Our 57-1708 coupling will transmit the same amount of torque with almost half as much weight spinning around on your bearings and seals. Not to say the Thomas Legacy product you have is bad, it is just that there is stuff out there that is so much better and a lot less costly.



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[electricpete]

fwiw - here are some photos of a coupling that ejected pieces of shims, which caught our attention laying below the coupling guard. The cracks are adjacent to where the shim pack would be clamped by the bolt force (similar to yours) which Thomas' literature suggests misalignment as you mentioned. The machine continued to run fine, just a small increase in running-speed vibration (although I can't predict how much longer it would have continued). These particular machines are 3600rpm 100hp motor driving overhung hot water pump (400F).

While we are working to improve our thermal corrections and alignment, we have also added periodic inspections/replacement of shim packs on some of our machines (5 years on one set of machines where we do other work every five years anyway, and 10 years on another set of machines). As I recall, all the cracking we have seen is on coupling shim packs that have been in service more than 10 years. Purists might view periodic shim replacement as some kind of "workaround", but we don't use it as an excuse to lower our standards on alignment, rather it is just a relatively small extra effort to ensure one portion of the machine is more robust in the event of misalignment.

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[CouplingGuru]

electricpete,

That is great picture. unfortunately I don't think that is a misalignment failure, those looks like corrosion induced cracking. If you switched to stainless disc packs you may see a significant increase in product life.
Is that a Thomas DBZ?


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

http://WWW.PSCCOUPLINGS.COM

 

[electricpete]

At the risk of hijacking the thread, here is something else we see when inspecting shim packs... in this case on motor driven hermetic chiller compressors. They are found with a greasy film and when the greasy film is removed, permanent pits remain. I think there are two contributors: 1 - lubricant from the motor (oil) or compressor (mixture of refrigerant/oil) that escapes into the coupling guard to cause some kind of chemical action. 2 - fretting appears to be a contributor because the pits only occur on the inside surface between shims... the outer shims don't show this pattern. It illustrates another reason to inspect your shim packs (cracking from misalignment isn't the only degradation mechanism).

Once again, sorry for the detour.

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[CouplingGuru]

Another nice picture, trapped particle fretting. This occurs when particles (loose rust) get trapped in between the disc shims. the constant back and forth movement of the discs grind those particles into the shims and create little pits just as you described, also why you haven't seen any pits on that exterior disc shims.

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

http://WWW.PSCCOUPLINGS.COM

 

[electricpete]

That is great picture. unfortunately I don't think that is a misalignment failure, those looks like corrosion induced cracking. If you switched to stainless disc packs you may a significant increase in product life.
Is that a Thomas DBZ?

Thanks for the comments.
It is a Thomas DBZ.
I won't rule out corrosion as a contributor.
The location of the crack does correspond to the location that the literature tells us would result from misalignment.
Maybe the corrosion accelerated after things had loosened up due to the ejection.
Maybe there is a degree of fretting involved (see the chiller compressor example just posted).
Maybe corrosion is a root cause (as I think you suggest).
I didn't know there were options on shim material. I'll keep an open mind and check the price of stainless steel shims.




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[CouplingGuru]

I should restate that a little, the strain is coming from misalignment, but the corrosion is creating a stress concentration. What clues me into the corrosion as a primary contributor is the shape of the crack itself. The jagged crack line indicates the crack propagation following the stress concentration pits. As you correctly indicated, better alignment will lower the strain and increase life, but eventually the pits will get so deep that they will eventually crack leading to even higher stress concentrations at the crack tip. Also, when ever you have shims cracking on the inside of the pack first, that typically points to sub symptom of misalignment. Classic misalignment failures work their way in from the outside first (where strain is highest) and typically crack right next to the washer actually following the curvature of the washer.

Oh and if the Thomas 71 is old, the DBZ is Ancient. That iron horse was developed back in the 1930s I believe.

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

http://WWW.PSCCOUPLINGS.COM

 

[MikeHalloran]

Johnny, what is the stuff I'm seeing between discs?
I'd expect to find air, and maybe some airborne contaminants, but not a solid fill, which would change the stress environment of the discs.


Mike Halloran
Pembroke Pines, FL, USA

 

[JJPellin]

There is nothing between the disks. They are directly stacked one upon the other. There are no gaps, no spacers or any other material.

Johnny Pellin

 

[MikeHalloran]

That way, when rust bonds the discs together, you are flexing one thick laminated disc, not a plurality of individual thin discs.

I'd expect to see thin shim washers between the discs at the bolts, or maybe chem-milled areas between the bolts, or maybe a film of moly grease between the discs.
Do the couplings arrive with any such feature from the manufacturer?







Mike Halloran
Pembroke Pines, FL, USA

 

[CouplingGuru]

Mike,

Johnny's Thomas Series 71-8 Size 375 coupling has stainless discs, so rusting together is most likely not a concern. But with non-stainless discs the rusting together phenomenon that you described is a real concern especially if there are periods of inactivity in the application, like seasonal operation.

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

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[MikeHalloran]

Stainless is only rust-resistant, not rust-proof.
Even the most exotic stainless is still ~60 pct iron.
... and will rust if it's not passivated to chemically remove available iron from the surface proper.

Do I see a little rust on the discs in the foreground in Johnny's photo?



Mike Halloran
Pembroke Pines, FL, USA

 

[CouplingGuru]

Yes, but I think there is a wide range between "rust proof" and "rusting together". So "most likely" not going to be an issue for a very long time.

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

http://WWW.PSCCOUPLINGS.COM

 

[electricpete]

If I understand correctly, you guys are suggesting that corrosion would interfere with flexing of the coupling. It is a subject of interest to me because (obviously) we've seen some rusty coupling shims.

I'm not saying it's wrong, but I'd like to explore it.

I understand the fact that a LOOSE stack of (example geometry long thin rectangular shims can bend easier than a solid bar of same dimensions because they will slide relative to each other. If we try to bend them in U shape, it is easy because the top/inside shim will end up sticking up higher at the end/top of the U than the bottom/outside shim as a result of the relative sliding. BUT, if you bolt the long shims together on each end of the stack and then bend them into a U, it will be a lot harder to bend because all the shim ends are now constrained to be the same position at top of the U (I think it now effectively acts like a solid piece, right?). It seems to me the shim pack coupling is more like the bolted-shim situation although it's a complex geometry and there may be multiple ways the shims have to bend.

Let's take a look at the coupling inspection guide:

http://www.rexnord.com/ContentItems/TechLibrary/Documents/CP-Monitor_Manual

B. Detection of trouble with equipment out of service and guard removed:
Here are some of the more evident visual inspection criteria and recommended corrective procedures.
1. Reddish brown color bleeding out between disc laminations at the O.D. of the pack. This is an indication of fretting and/or chemical attack of the disc material. See Figure 2…..


IV. ANALYSIS OF FAILED DISC COUPLINGS
In the event of a coupling failure, a thorough investigation should be made to determine the cause. The most common causes of failure are improper coupling selection, improper assembly, excessive misalignment, and corrosive attack…..

4. Disc broken adjacent to washer face with fretting corrosion present in the area of crack usually indicates excessive shaft misalignment during operation. Also, iron oxide will most likely be evident on outside of disc pack. See Figures 2 & 9. This is sometimes associated with a coupling that has been in service for several years and/or operating in a corrosive atmosphere. Breaks will first appear in outer discs and progress through the disc pack. Replace both disc packs and realign the equipment. Different disc pack material may also be considered.

Hmmm. It seems the context of their discussion of corrosion is primarily limited to being a consequence of fretting or chemical attack. In the failure analysis section, corrosion is only mentioned in the section on broken disk from misalignment (not as a separate cause of failure). They certainly don't come out and say this interferes with coupling flexibility (I'd think it would be more prominently discussed if that were the case). I'm still unsure about this. If it is true it will certainly drive us quicker toward different shim material.


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[JJPellin]

We have thousands of these couplings running outside in the elements for decades. We rarely see a problem with corrosion on shims leading to failure. If I see corrosion between the shims, but not on the outside, I classify this as a poor alignment resulting in excessive movement resulting in fretting corrosion. Improve the alignment. Changing shim material would not solve the problem.

Johnny Pellin

 

[electricpete]

Do I see a little rust on the discs in the foreground in Johnny's photo?

It seems roughly the same area adjacent to the bolt that fretting products would bleed out as discussed above B...1... Figure 2

We have thousands of these couplings running outside in the elements for decades. We rarely see a problem with corrosion on shims leading to failure. If I see corrosion between the shims, but not on the outside, I classify this as a poor alignment resulting in excessive movement resulting in fretting corrosion. Improve the alignment. Changing shim material would not solve the problem.

That's a great perspective, thanks!


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[CouplingGuru]

Not following your thought electricpete. . . .

IV. ANALYSIS OF FAILED DISC COUPLINGS
In the event of a coupling failure, a thorough investigation should be made to determine the cause. The most common causes of failure are improper coupling selection, improper assembly, excessive misalignment, and corrosive attack…..

Corrosion is one of the more common modes of failure with non stainless material.

and Johnny if you are running all stainless shims currently, you already have upgraded them significantly from plain carbon steel. In order to get even better protection you would have to go to 316, Inconel or Monel, but as you stated you haven't seen any corrosion issues.
Plus API-610/671 requires Stainless Disc shims probably for the concerns we are discussing, so I would be surprised if you were seeing corrosion issues.


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

http://WWW.PSCCOUPLINGS.COM

 

[Njinear]

Wow, I kind of stumbled on to this thread, but there is a lot of great disc coupling information in here.

CouplingGuru, you have certainly demonstrated your knowledge of couplings. I have never heard of PSC Couplings though.

Thanks for the great discussion, I will have to save this one for future reference.

JJPellin, Mike and ElectricPete. Thanks as well.

 

[electricpete]

Not following your thought electricpete. . . corrosive attack

Yes, I saw that part and quoted it… it comes down to trying to parse the intent of the authors (subective). I can understand corrosion as a long-term failure mechanism, but I have never seen any references singling out corrosion as inhibiting the flexibility of a shim-pack coupling. And this particular reference (while very detailed on other aspects, and also discussing various aspects of corrosion) certainly doesn't mention anything about corrosion affecting flexibility of the coupling. That's the particular aspect I was questioning. I looked at API 671 2nd edition (I don't have current edition) and it does recommends corrosion resistant flexible elements for couplings (paragraph 2.6.4), but doesn't say why. It's all still murky for me and I don't really expect a black and white resolution, but I'm glad to have had heard the discussion (it gives me a broader range of things to think about with these couplings).

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[djs]

I wonder if the temperature difference between the front of the motor and the back of the motor would result in an angular misalignment sufficient to cause the issue shown?