Alignment Targets for Large TEFC Motors
Alignment Targets for Large TEFC Motors
(OP)
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
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





RE: Alignment Targets for Large TEFC Motors
Muthu
www.edison.co.in
RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
What is the distance between the disk packs?
I'd rig up something to allow viewing the coupling in operation, or take high speed pictures.
The intent is to view the coupling in operation to reveal if the disks are puckering or bending?
Do you loosen all the disk pack clamp bolts in the aligned state, and then tighten them, to allow the disks to start life fairly "neutral?
3 down 17 to go.
RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
... and someone's selection criteria need adjusting.
I assume that moving motor or pump farther apart and using a longer coupling assembly is now a huge deal.
I wonder if the coupling people might offer disc packs with a greater number of thinner discs for improved fatigue life in your application. Are they working on that problem, or are you left on your own?
The problem is a little extra messy because of the motor's asymmetrical expansion.
If it were a little closer to symmetrical, i.e., just a changing parallel offset, I'd look at Schmidt couplings.
Mike Halloran
Pembroke Pines, FL, USA
RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
We don't have any TEFC motors above 200hp. Where we do have challenging thermal growth situations (turbine driven pumps with turbine and pump on separate/independent platforms), we attempt to measure off-line-to-running change in alignment using Permalign. I see you attempted to determine something similar with dial indicators but your efforts were limited because safety procedures prevented you from performing a true/full alignment check hot… instead just monitored change in vertical position at each shaft. With the Permalign, we install it with equipment tagged out (typically cold) and then get the useful data during the next transition to hot conditions.
fwiw I could easily imagine that TEFC motor grows more on the inboard end (DE) than the outboard end (NDE) based on the external cooling pattern where the outboard endbell is heavily cooled, inboard endbell has no cooling airflow, and the cooling air applied to the frame fins warms as it flows from outboard toward inboard. Infrared thermal image while loaded might give a reasonable basis for developing a thermal correction estimate. But again we've never measured thermal growth on a TEFC.
I agree with Tmoose on this (I think I learned it from him on the other board). For Thomas Shim pack couplings, if the machine is aligned to a non-zero target cold then you should if possible loosen/retighten coupling bolts hot to relieve stresses you induced into the coupling through the growth. It is recommended in the Thomas / Rexnord literature. Attached poweropint is my attempt to illustrate the reasoning.
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RE: Alignment Targets for Large TEFC Motors
You are correct. Base on the numbers you mentioned of 8" distance between shaft ends that should allow you a 0.046" parallel offset with out any problems. But that is assuming perfect angular alignment and zero axial movement. Based on your failure description, the disc coupling manufacture is correct, you have a strain problem. The manufacturer has pointed to mis-alignment but that generally is a summation of many alignments. Angular-Parallel-Axial alignment. Now all those alignments resolve back to angular deflection in the disc pack. I would be willing to bet that you actually have some twisting going on that is actually providing some angular misalignment as well as parallel misalignment. Since you have an API coupling, I would take a measurement across the flex element from adapter to center member flange at 90 degree intervals around the coupling. This measurement will give you the exact angular misalignment that the packs are seeing. It would be best if you did this measurement in the hot condition and measure again when the pump cools down. The manufacturer should have these values defined in their manual, if not I can walk you through how to calculate them base on the coupling manufacturers misalignment rating.
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
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RE: Alignment Targets for Large TEFC Motors
I am surprised at the comments about loosening shim pack (disk pack) bolts to relax stresses. I think this may be a result of me providing inadequate information. I referring to Thomas Series 71 couplings with removable spacers. We rebuild these spacers in the shop and tighten the bolts with no stress and no imposed misalignment and then install them as a complete assembly. There are other types of couplings (Series 42, Series 52, AMR) where you assemble the disk packs and associated bolting in the field. In these, you have no choice but to tighten the bolts with the coupling under some amount of misalignment. For those types, it makes sense to consider loosening and tightening bolting in a hot condition once the misalignment is at a minimum. Our energy control practices would make this impractical in most situations, but I can still see the value. If I needed to address this with a field-assembled coupling, I would probably do it differently. I would ask the mechanics to align the machine to zero-zero, assemble the coupling, and then make the moves needed to achieve the cold-offset targets. This should have the coupling disk packs in a relaxed condition with a nearly perfect alignment.
I agree that I should embrace the technology and get the laser tools to be able to measure alignment changes on a running machine. We have tried this with our laser systems and have hired outside contractors to come in and do this for us. In all cases, I was disappointed with the results. I think the lasers are very capable to take these measurements. But the bracketry needed to mount them is tricky. Unless the bracketry is perfect, it can distort the results. I need to revisit this technology and get current on its capability.
The coupling failures we have had were not complete failures. We have caught all of them when the outer 1 or 2 disks cracked. But, we still have to go in and rebuild the coupling. We have viewing windows in our coupling guards and inspect the couplings with a strobe if we see evidence in the vibration that we have coupling issues.
I am generally skeptical of coupling manufacturers’ values for allowable misalignment. They have a strong incentive to rate their product capabilities higher. I would never willingly run a coupling this large with an offset more than about 0.015”, no matter what the coupling manufacturer claimed. Even if the coupling could take this for a period of time, I would be concerned about the loads imposed on the driver and driven machine and the resulting vibration.
We have a thermal imaging camera in our group. I should use that to get a better picture (literally) of the temperature distributions on the running equipment. I don’t know why I didn’t think of that. Thanks for the suggestion.
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
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RE: Alignment Targets for Large TEFC Motors
I agree with almost everything you have stated. I am very familiar with the Thomas product, although you have seen first hand the detriments of running with misalignment, I have seen first hand what it takes for a coupling of that nature to fail. While I don't contend you should run it at rated misalignment I do know that somehow you are seeing excess misalignment in order to get that failure. In terms of coupling design really the only way to achieve better misalignment rating is to have a higher tensile material as the disc pack(i.e. more springy) or a longer chord length (4 bolt vs 6 bolt vs 8 bolt) These high performance materials are generally way too costly to incorporate since most OEMs are drawn to the lowest cost product. Anyway, getting back to your problem, if you see cracking of the discs next to the washer it is 100% a strain induced failure, this is only cause by angular misalignment across the flex element. So I am fairly positive you have some additional misalignment sources aside from parallel misalignment. Or you may have compensated so greatly in the cold condition, that at start up you are running above rated misalignment. Either way, best way to verify is to measure the distance from the adapter flange to the center spool flange at 90 degree intervals around the coupling. This will give you the minimum and maximum spacing that the flex element is seeing. Then you can use the outer diameter of the coupling in the following formula INV-SIN[((Max-Min)/2)/(Spool Dia/2)]= Angluar element flex. This angular element flex has to be less that 0.25 degrees, if that is the case the disc will last a very very very long time.
Also Technically for API compliance those center-member assemblies are suppose to be factory tightened and balanced, maybe you are doing all that but switching disc packs on a factory balanced and match marked center member assembly could create some serious vibration issues, I am sure you are on top of that, but just giving you a heads up on that.
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
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RE: Alignment Targets for Large TEFC Motors
You are correct, pure parallel misalignment can cause this, but base on the numbers JJPellin described he is well with in the prescribed parallel misalignment rating. However, how do you know the shafts stay perfectly parallel to one another through-out the thermal growth range? Plus I know for a fact that it impossible to have "perfectly" parallel shafts, just like it is virtually impossible to achieve "perfect" alignment. That is why I always recommend measuring the flex element gap, it is pure angular misalignment, it takes into account face run-outs, bolt location deviation, angular shaft misalignment, parallel shaft misalignment and even axial misalignment if the nominal flex gap value is known (or measured prior to install). Either way the disc pack failure is from over flexing the element. Now we just have to figure out where it is coming from.
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
The better way is by getting that "N" dimension. Measure that, that automatically combines both of them into angular deflection of the pack.
So your flex element gap has to be between .70" and .68", again that it highly conservative, because their rating of 1/3 degree per pack would allow.
SIN[.333]*[7.62/2]+Nominal gap & SIN[.333]*[7.62/2]-Nominal gap (Nominal gap = (.70+.68)/2 = .69)
Or a gap tolerance +\- 0.022" so with their published 1/3 degree rating that "N" gap needs to fall with in 0.712" to 0.668"
The "N" is the measurable distance between the adapter and center member flange.
Also you mentioned chord length on the 71 vs 71-8, that is why the angular misalignment rating of the 71 (6 bolt) is 1/2 degree and and 71-8 (8 bolt) is 1/3 degree.
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
Mike Halloran
Pembroke Pines, FL, USA
RE: Alignment Targets for Large TEFC Motors
When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM
RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
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.
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
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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RE: Alignment Targets for Large TEFC Motors
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?
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RE: Alignment Targets for Large TEFC Motors
Once again, sorry for the detour.
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(2B)+(2B)' ?
RE: Alignment Targets for Large TEFC Motors
When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM
RE: Alignment Targets for Large TEFC Motors
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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(2B)+(2B)' ?
RE: Alignment Targets for Large TEFC Motors
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.
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RE: Alignment Targets for Large TEFC Motors
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
RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
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
RE: Alignment Targets for Large TEFC Motors
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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RE: Alignment Targets for Large TEFC Motors
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
RE: Alignment Targets for Large TEFC Motors
When it comes to couplings we are always here to help.
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RE: Alignment Targets for Large TEFC Motors
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/Do...
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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RE: Alignment Targets for Large TEFC Motors
Johnny Pellin
RE: Alignment Targets for Large TEFC Motors
That's a great perspective, thanks!
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RE: Alignment Targets for Large TEFC Motors
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.
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RE: Alignment Targets for Large TEFC Motors
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.
RE: Alignment Targets for Large TEFC Motors
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RE: Alignment Targets for Large TEFC Motors
RE: Alignment Targets for Large TEFC Motors
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)' ?
RE: Alignment Targets for Large TEFC Motors
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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RE: Alignment Targets for Large TEFC Motors
Can you explain a little how corrosion decreases effective cord length?
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RE: Alignment Targets for Large TEFC Motors
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.
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RE: Alignment Targets for Large TEFC Motors
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.
RE: Alignment Targets for Large TEFC Motors
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