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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

RE: Alignment Targets for Large TEFC Motors

I have seen a few 1700 KW ID fan motors having a vertical thermal growth and such motors were cold aligned with the motor low. One theory was since the motor bottom is bolted, the only way for it to grow was upwards.

Muthu
www.edison.co.in

RE: Alignment Targets for Large TEFC Motors

(OP)
I should have mentioned that the pump I was referring to runs at about 650 F. In theory, the pump should grow much more than the motor, even though it is centerline mounted (API pump). The fact that we need to install the motor low of the pump is very unusual and is based on the uneven growth of the motor and pump. I was taught that motors grow straight up with approximately equal growth on the inboard and outboard ends. And, that seems to be true for WPII motors. But, for TEFC, it does not appear to be true.

Johnny Pellin

RE: Alignment Targets for Large TEFC Motors

Do you have some pictures of the failed coupling disks?

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

(OP)
The disks were broken at the outermost disks at the edge of the washers. The coupling manufacturer's manual indicates this is suggestive of misalignment. The coupling spacer is built as an assembly with a hub-to-hub gap of 8 inches. The distance between disk packs is probably about 6.5 inches. The center members with disk packs assembled are built up on the bench. I have not heard of any benefit from loosening bolts in the field.

Johnny Pellin

RE: Alignment Targets for Large TEFC Motors

Next time of course, you want much longer coupling spacers.
... 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

(OP)
The coupling spacing meets API and our internal design criteria. The coupling is a standard design with a service factor greater than 1.5. We have approximately 1500 centrifugal pumps that use this same coupling design and have coupling spacing shorter than this. Failures of these couplings are extremely rare. Moving the pump or the motor is impractical at this point.

Johnny Pellin

RE: Alignment Targets for Large TEFC Motors

Thanks for the feedback on the axial shuttling.

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.

Quote:

I have not heard of any benefit from bolt loosening...
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.


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

RE: Alignment Targets for Large TEFC Motors

2
JJPellin,

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

Typically these shim pack couplings give warning before failure. If you are inspecting them, you can see the cracks early. If you are not inspecting them, then eventually (at a later stage), you will see pieces of individual shim packs ejected somewhere below coupling guard (but machine continues to run fine at this point). We have seen both the above, but never experienced a complete failure of the coupling that actually stopped the machine. I'm curious what you saw on your four failures.

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

RE: Alignment Targets for Large TEFC Motors

(OP)
Thank you for several good points and suggestions.

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

I'm not familiar with that 71 type. I agree with you that my comments about loosening probably don't apply. They wouldn't apply if the capscrews through the hub thread directly into the adapter (nomenclature per figure 2 here ) rather than a loose fit in a non-threaded hole with nut on the other side where the bolt can end up held off-center within the hole.

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

RE: Alignment Targets for Large TEFC Motors

JJPellin,

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

Quote:

So I am fairly positive you have some additional misalignment sources aside from parallel misalignment.
I'm not sure why you say that. Parallel misalignment of the shafts translate to an equal/opposite angular misalignment at each of the two flex planes as you know. So couldn't pure parallel misalignment of shafts cause this? (I'm not saying it was pure parallel misalignment in this particular case, I'm just checking the logic).

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(2B)+(2B)' ?

RE: Alignment Targets for Large TEFC Motors

electricpete,

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

(OP)
I think what electricpete is trying to say is that any angular or offset misalignment will result in pure angular misalignment across an individual disk pack. In that regard, you are correct, if we could directly measure the angular misalignment across each disk pack, we would know for certain if it was within OEM specifications or not. But, it would not matter if we achieved that amount of angular misalignment across that disk pack by having a purely offset, purely angular or combination misalignment between the driver and driven shaft. You mentioned a good point about the chord length affecting the allowable misalignment. This coupling is a Series 71-8 which is a smaller OD with 8 bolts through the disk packs. This has a shorter chord length than the old Series-71 original design. According to the OEM manual for this coupling, the maximum misalignment is 0.015" TIR angular and 0.012” TIR offset. I have not found out how to interpret these numbers. If these are truly TIR readings from indicator sweeps, then we are only allowed 0.006” parallel offset and an angular misalignment of 0.11 degrees. Based on our readings, we had almost zero angularity, but 0.020” to 0.030” parallel offset. Am I interpreting this incorrectly? Our coupling is a size 375.

Johnny Pellin

RE: Alignment Targets for Large TEFC Motors

Using the right side of the chart is a combination measurement, it defines the mid-point of the angular and parallel misalignment relationship.

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

Did you all mean to say "chord length"?

Mike Halloran
Pembroke Pines, FL, USA

RE: Alignment Targets for Large TEFC Motors

Yes on the chord, auto correct cell phone

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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.

When it comes to couplings we are always here to help.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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.

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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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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(2B)+(2B)' ?

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

Quote:

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.




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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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

RE: Alignment Targets for Large TEFC Motors

(OP)
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

RE: Alignment Targets for Large TEFC Motors

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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

2
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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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/Do...

Quote:


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.


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

RE: Alignment Targets for Large TEFC Motors

(OP)
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

RE: Alignment Targets for Large TEFC Motors

Quote:

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

Quote:

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!


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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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.

RE: Alignment Targets for Large TEFC Motors

Quote:

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).

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

RE: Alignment Targets for Large TEFC Motors

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?

RE: Alignment Targets for Large TEFC Motors

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.



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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

Quote:

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?

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

RE: Alignment Targets for Large TEFC Motors

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.
WWW.PSCCOUPLINGS.COM

RE: Alignment Targets for Large TEFC Motors

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.

RE: Alignment Targets for Large TEFC Motors

(OP)
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

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