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Repeated Shim Pack Coupling Failure

Repeated Shim Pack Coupling Failure

Dear All
We are facing problem of repeated shimpack coupling failure on recycle gas compressor train.

Equipment History
The equipment was originally commissioned in 2009. In 2011 there was damage to LP compressor rotor (a piece at discharge of 1st stage 1st impeller broke) while in operation and rotor had to be replaced. In 2014 compressor was overhauled again on preventive basis and at that time subject coupling (LP to HP) was found damaged “two of the flexible-disc screws being found sheared off and the disc (shim pack) had a fracture”. In 2014 rotor was replaced with repaired rotor (same that got damaged in operation in 2011) due to fouling on 1st and 2nd stage impeller

May 2017 Failure
In April 2017 compressor train was shut down due to some other problem and On May 4th 2017, compressor was started. Following sequence of events observed during startup
Within 8 to 10 seconds of reaching the full speed on 13,300 rpm, the LP compressor tripped on High Vibration .
Subsequently, the compressor train was started four times and operated each time for about 40 to 70 minutes, during 4 and 5 May 2017.
On 5 May the it was observed that the System1 was not receiving any vibration data for HP Compressor after the first start up and trip. It was also revealed that HPC was not developing pressure during the last four startups.
Upon investigation, it was observed that the HP Compressor was not rotating during the last four startups since key-phasor was not showing any speed indication and the bearings were not indicating any vibration amplitudes.
Further it was noticed that during the first start up, HPC compressor reached only about 12,600 rpm and then coasted down to zero speed, meanwhile, LPC reached the rated speed of 13,300 rpm and with 8 to 10 seconds tripped on high vibration.
Based on this coupling between LP and HP Compressor was inspected and found badly damaged. Coupling transmission unit was replaced and compressor was started. Alignment readings could not be taken

Oct 2017 Failure

In June 2017, nearly one month after coupling replacement vibration values increased on LP compressor. In October last week compressor train was shut down again due to some other problem and subject coupling was inspected. Coupling was again found damaged. 5 out of 6 bolts on LP compressor side shim pack found broken. Also LP compressor shims were found damaged.As found alignment was checked and found as follows
Parallel: HP compressor 0.1mm down
Angular: 0.1mm/100mm close at bottom
Parallel: 0.2 mm
Angular: 0.0mm/100mm

Coupling Manufacturer Feed Back
The couplings damaged in May 2017 and Oct 2017 were sent to coupling manufacturer and as per their feed back torsional vibrations cause loss of pre tension in drive bolt which eventually results in coupling damage. In Oct 2017 damage the HP compressor side drive bolts and shim pack were in tact and as per finding from coupling manufacturer tightening torque on these bolts was found to be 4 NM against design/set value of 10 NM. Refer to attached zip folder for required data.

what can be possible source/cause of torsional vibrations ?
Please note that the stripper bolts no 4 in coupling drawing which attach transmission unit to hubs are also torqued to 10 NM. Why these bolts are not loosened due to torsional vibrations ?

Any reply will be a great favor

RE: Repeated Shim Pack Coupling Failure

With so many faying surfaces within the grip of the bolt I worry about loss of preload due to embeddment.

Calculate the elongation of the bolts at 10 NM installation torque and compare that to the potential possible embeddment of the joint. Refer to VDI 2230 for potential embeddment per interfacing surface.

I also find it hard to believe that any investigation following a destructive failure could actually determine the install torque was 4 NM vs a requirement of 10 NM.

RE: Repeated Shim Pack Coupling Failure

I cannot open your attachment. My computer does not recognize that file type. The most likely source of torsional vibration would be a torsional resonance. If the machine had run successfully for many years, then I would look for something that has changed. It is not clear to me if this is a motor driven or turbine driven train. I would expect this service to be turbine driven. I would look for any changes in the start-up procedures. The speeds at which they hold for warm up or the ramp rates passing through critical speeds could have been changed from original start-up procedure. The original analysis and testing at the time of manufacturer should have included a torsional analysis including a transient analysis. I would review that data with the manufacturer and make certain that adequate separation margins exist. I would review the coupling design and make sure it has an adequate service factor. I would want a service factor of 1.5 for a machine of this importance.

Johnny Pellin

RE: Repeated Shim Pack Coupling Failure

1. Please confirm that the 'Train Layout' is in error, or at least somewhat simplified, and that each coupling is in fact a double coupling with a long spacer.

2. The coupling drawing shows a taper inside the coupling ends which attach to each compressor shaft.
Please confirm that the tapered male shaft on each compressor was not distorted/bent as a result of the first coupling failure.

3. It looks like radial disassembly/removal of the couplings is not actually possible, unless one goes to the trouble of removing one or both compressors from their foundations, allowing axial disassembly,
OR, one compresses the disk packs axially, bending the shims in a way that can't be good for them, in order to shorten the assembly enough to allow radial assembly. Have you PERSONALLY witnessed assembly of the couplings to the compressors for the entire operation, from unpacking the couplings to full speed rotation?

Mike Halloran
Pembroke Pines, FL, USA

RE: Repeated Shim Pack Coupling Failure

Dear JJ Pelin
The attachment is a zip file. It can be opened with winrar. This is motor driven compressor train. The vibration data collected at system 1 shows low amplitude spikes at gearbox with frequency in range of 1st torsional critical. The coupling manufacturer has highlighted that HP rotor critical speed is very close to 3rd torsional critical. However considerable separation margin exists at operating speed. I have attached critical speed data for your review.

Dear Mike
You are correct each coupling is double coupling with spacer. As for disassembly is concerned, there is a limit to which shims can be compressed. Refer to note 4 in attached drawing.
The coupling damaged in Oct has been found over compressed as highlighted by coupling manufacturer. I have note witnessed coupling assembly but I these couplings also failed in 2014 and Oct 2017. On both these occasions coupling was assembled by Compressor manufacturer FSE.

RE: Repeated Shim Pack Coupling Failure

For an application with compressors the service factor is usually just 1, those machines running very smoothly. On the drawing, the nominal Sf is given to 1.5, the actual to 1.82. This strikes me as odd.
And even so, the coupling does not serve to satisfaction. The failures are of a nature that is, by design, unlikely to happen with disc couplings!

It is remarkable that there is an additional intermediate flange to connect the coupling to the end flanges. The connecting bolts of this interm. flange are not body fitted (dia. 8.2 in drwg.), i. e. there is a space for play once the friction grip has been overcome. This seems to me an unusual design, given that it might be difficult to check the torqueing in operation (is that so?).
Furthermore, if i read the pictures correctly, the main element washers seem not to be crowned but rather chamfered on the inside.

With the coupling having run a long time (2 + 3 years) w/o trouble (? really?), there might have been a significant change in the setup or deciding parameters of operation.
Malfunction of one of the connected compressors, at high rev's? Interference from nearby machinery through coupled supports?
If the current cplg supplier is unable to establish the core reason, then it might be time to turn to an external advisor, with a better troubleshooting competence. Why not involve someone like Falk (no rel.)?


Roland Heilmann

RE: Repeated Shim Pack Coupling Failure

Dear Roland

Thanks for your reply. As for additiional intermediate flange is concerned, I have always seen this in shim pack couplings for high speed applications. Refer to attached Dwg from some other manufacturer. I am unable to understand reason for this.

Shim Pack Coupling Failure Analysis suggests shim fracture through bolt hole is an indication of loose drive bolts. Same has been confirmed by coupling manufacturer but they have attributed this to torsional vibrations.

I tend to believe that as highlighted by Mint tightening torque of 10NM may not be sufficient because the stripper bolts at addiitonal interm. flange highlighted by you ( Pos 4 dia 8.2 on Dwg) also have Tightjening Torque of 10 NM. These bolts are Torqued in field
However as per my understanding these bolts are subject to same Torque Transmission during operation as main drive bolts but being at greater radius from shaft center these (8.2 mm Dia bolts in Dwg at Pos 4) will be subected to less force. Also these bolts (Pos 4 Dia 8.2 in Dwg) are in Qty 04 each side in contrast to main drive bolts whicha re 03 on each side.

RE: Repeated Shim Pack Coupling Failure


Just one more point, putting last failure a side, the previous two failures were after nearly 4 years of service life. Train was comissioned by End of 2009 and first coupling failure was in Jan 2014 and second was in May 2017. The last failure which occured after 04 months only.
At a fertilizer plant, I have seen bigger (greater torque transmission)shim pack couplings by same manufacturer in service for more than 20 years on high speed steam turbine driven centrifugal compressor applications.

Although I am a maintenance guy and don't have any deisgn experience but based on field experience my gut feeling is that this is not good selection for this service.

One thing that I have noted is Dwg states max axial misalignment capability is 1.1 mm for cont. service and 1.7 mm for Transient. The shaft Thermal Growth data requires 1.1 mm prestretch in cold condition while assembling. This implies every time we are starting this train with axial prestretch beyond coupling max continuous capability although still within max transient capability.
The value of Tightening Torque of drive bolts also seems inadequate when compared with bolts at interm. joint as mentioned in previous message

RE: Repeated Shim Pack Coupling Failure

while I am familiar with this type of coupling, I am having a hard time visulizing...BUT

just a comment, Is it possible the coupling is being installed using the disc pack bolts to provide some freedom? and thus the disc are being flexed prior to full bolt tensioning.

RE: Repeated Shim Pack Coupling Failure


That looks like a pricey coupling. Not sure what was driving the selection, but a connection between two compressors even though they are centrifugal can create slight harmonics. For this type of application I would recommend a minimum of 2.5 service factor. Looks like they are at 1.81 with a 1.5 target spec.

Also pretty sure the reason the stripper bolts didn't loosen is because they are at a bigger diameter, so the amount of drive torque through them is less.

Are those hubs hydraulically fitted as well?

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

RE: Repeated Shim Pack Coupling Failure

Thanks byrdj for feedback.

Please note that the disc pack is factory assembled.
During maintenance we only remove complete Transmission Unit Item No 1 in Coupling Dwg. by removing and reinstalling stripper bolts item No 4 highlighted green in attached Dwg.
The entire disc pack hardware i.e. disc pack, disc pack bolts (highlighted yellow in coupling Dwg) washers and spacers etc. are factory assembled. In fact disc pack bolts tightening Torque is not given by manufacturer in any data. Although this time it is given in failure analysis report.

Another surprising observation is that the material used for disc pack bolts is high tens steel equivalent to BS 970817 M 40 X. Based on material properties this is equivalent to property class 12.9 and 10 NM tightening torque for M 8 bolt seems to be far below what is mentioned in standard torque charts

I have attached coupling Dwg again for convenience

RE: Repeated Shim Pack Coupling Failure

another rambling thought...
I shaded/colored the shim pack bolting trying to grasp its details.
that drawing implies the bolts are sleeved (red). Is it possible the sleeve's length was preventing full clamping of the pack?

RE: Repeated Shim Pack Coupling Failure

I still cannot open your original attachment. Is the motor a synchronous design? If so, it would be expected that all torsional resonances below two times running speed will necessarily be excited during start-up. There was a very good paper presented at the International Turbomachinery Symposium in 2015 that may be helpful. I have included a link below:

The full paper is available on-line at no cost.

Johnny Pellin

RE: Repeated Shim Pack Coupling Failure

very interesting paper, thanks a lot!!

my point at those intermediate flanges is just there's designs without such intermediate flange. However, imo disc couplings are in general a correct choice for this application. Details t. b. c., of course.
my point at the fixation bolts main to intermediate flange is, that the bolt is surely M8 with an 8 mm dia. shank, so once the friction grip is loosened wearout & fretting may start from the play in the dia. 8,2 mm bore. The damage to the disc pack & disc pack bolts would develop as secondary effect to the primary cause, from dynamical effects due to the play.
my point at the drive train: I'd seek if there's drive torque spikes in transient modes or external excitation. The root cause is imo a change in the loading of the coupling, comparing the original situation with that leading to the failures.

Roland Heilmann

RE: Repeated Shim Pack Coupling Failure

Thanks for your thoughts and putting effort to grasp bolt details. Its difficult for me also as I never disassembled these. However I will attach some pictures showing details. I believe the sleeve length is not preventing clamping of disc pack because in pictures in Annex-I show embedding/grooving on mating surfaces(Guard ring and spacer)

JJ Pelin
Motor is 3 phase Induction Motor. I will reattach the files of original attachment

Annex- I Oct 2017 Damage and Manuf. Comemnts
Annex-II Train Layout
Annex-III Coupling Dwg
Annex-IV Coupling Manuf Comments

Also I have attached motor data in Annex-V

RE: Repeated Shim Pack Coupling Failure

Dear RolMEc

Thanks for the thoughts. Please note that disc pack drive bolts (Highlighted yellow in coupling Dwg)and fixation bolts main to intermediete flange (Stripper bolts high lighted green) are M6 thread siz not M8. I have physicaly verified from spare coupling. Dwg mentions 6.2 which may not be clear. Regarding your concern about stripper bolts (Fixation bolts main to Interm flange, we never had any problem with these. Infact in all failures they remained intact.

But still the material BS 970817 M 40 X seems to be close to property class 12.9. I am still wondering if the tightening torque of 10 NM is sufficient ? Torque chart for general applications suggest 19 NM Dry

Dear Coupling Guru
The hubs are hydraulically fitted.

RE: Repeated Shim Pack Coupling Failure

We have couplings of this same type driving 25MB10 and 25MB6 compressors for more than 20 years and have not seen any failures. Both of these machine trains are turbine driven. I would focus on two possibilities. The shop that is rebuilding these couplings could be assembling them incorrectly. Or, something in the machine train has changed the torsional characteristics resulting in torsional resonance.

Johnny Pellin

RE: Repeated Shim Pack Coupling Failure

Maybe it's time for strain gages to be applied to the coupling shaft to measure the torsion. The report indicates that the fatigue is high-cycle, so the change in amplitude is small. Is it possible there is a beat frequency between the two compressors?

RE: Repeated Shim Pack Coupling Failure


In regards to the bolts that are failing vs. the bolts of that aren't failing.
You mentioned that the stripper bolts are smaller M6 compared to M8 but are the same torque 10NM. This will make smaller M6 bolts have more clamping force then the M8 bolts.
When comparing bolts of equal applied torque, the smaller diameter bolt has higher clamping force. (All other variables constant) This higher clamping force resists vibration. Plus the fact the smaller bolts are operating at a larger diameter helps them resist vibration even more.

check this calculator

So this may lead you to think that M8 bolt is not torqued high enough from the manufacturer, but with this design coupling, torquing that bolt higher may induce disc pack distortion, which probably won't be acceptable at your operating speeds. This type of disc coupling is designed for constant torque at high speed, it isn't really designed to handle vibratory torque. The only way to accomplish that is get a different designed coupling or oversize the selection of this coupling so that the vibratory torque doesn't exceed the friction carrying capability of the bolted joint. All that should have been done by the manufacturer.

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

RE: Repeated Shim Pack Coupling Failure

Coupling Guru

Thanks for the thoughts. Just be noted that I mentioned both stripper bolts (that are not failing highlighted green in drawing) and disc pack drive bolts (that are failing high lighted yellow in Coupling Dwg)are of same size M6 and have same tightening torque 10 NM.
As highlighted by you earlier bolts that are not failing are at greater radius and therefore subject to less force. Also not that bolts that are not failing (highlighted green) are 04 in Quantity
On the other hand disc pack drive bolts which are of same size and have same tightening torque are 03 each side

This tends me to believe what you have mentioned that an over sized coupling having larger dia drive bolts with thicker shim pack and greater drive bolt tightening torque can solve this issue but the coupling manufacturer is insisting on torsional vibration measurements before proposing any solution.

JJ Pelin

Thanks for sharing useful info. The G Box to LP Compressor Coupling on this train is operating fine since commissioning. This is similar coupling from same manufacturer but with greater Torque transmission capability and higher service factor.
Another machine at our plant having a motor driving 15 MB 8 compressor casing through Gear Box has a coupling with same torque rating but handling more torque i.e. operating at less service factor. In fact that coupling has same stripper bolts and disc pack drive bolts size and shim pack thickness as of this coupling which is being damaged.
Symptoms of Torsional resonance are there as we are observing intermittent spikes at G Box with frequency in range of 1st Torsional Critical

RE: Repeated Shim Pack Coupling Failure

Actually there is no exposed shaft on this train so Torsional vibration measurements will be an intricate activity and may involve significant downtime.
I am confused whether to go for this or ask coupling manufacturer to propose a oversize coupling with greater service factor based on the fact that what ever torsional vibrations are there are not causing anything else to damage. In fact stripper bolts having same size and tightening torque but at larger radius remain intact ?

RE: Repeated Shim Pack Coupling Failure


I find it odd, that manufacture is requesting torsional vibration measurement. It seems obvious that there is some vibratory torque in the system. Analyzing the system will provide a data set, but once you change the coupling inertia/stiffness, that entire data set will change. Unless they are proposing a complete Torsional vibration analysis of the entire drive train, which should be able to be done without taking the system down, but that is a theoretical analysis. I would think that theoretical analysis was already done prior to commissioning, which lead to the current coupling selection. All that being said, you are having problems with the current selection, In my opinion the manufacturer should replace your coupling with one that can withstand vibratory torque, I would also lobby to increase your minimum service factor on this spec to 2.5

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

RE: Repeated Shim Pack Coupling Failure

Coupling Guru

I believe you have seen the clarification I mentioned that both stripper bolts (that are not being damaged) and disc pack drive bolts have same size M6 and Tightening Torque. (I mistakenly selected inappropriate text color in previous reply)
Torsional vibration study report is available

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