There has been some previous discussions of the need to loosen/re-tighten coupling bolts for Thomas Shim Pack couplings if the bolts were tightened while misaligned, and then the machine later moves into alignment (due to alignment moves or thermal growth):

To view these previous discussions:
See last three posts here:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...701020533#8701020533

See first 6 posts here:
http://maintenanceforums.com/eve/forums/a/tpc/f/3751089...34381?r=6301034381#6

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Now I am wondering to what extent this concept applies to other types of couplings. I am particularly interested in a diaphgram coupling. It has rigid hubs attached to each shaft a flexible spoolpiece bolted between those hubs. The spool piece derives it's flexibility from flexibile internal diaghrams at each end connecting the large diameter of the flange to the small diameter of the center tube.

Attached is a pictorial view of why I think this type coupling could experience similar problems. In particular the scenario we are looking at is when coupling is installed/tightened in an intentionally misaligned condition and then grows into alignment. What do you think?

InitialMisalignmentMemory.ppt (64 Kb, 48 downloads)

One thing that is probably very relevant is whether the coupling has a "boss fit".

A boss fit would force the coupling spoolpiece to bend to accomodate misalignment present during tightening, rather than bolts to move within holes during tightening. In other words, if boss fit were present, the relationship between the boltage flange and hub is fixed, regardless of conditions at time of tightening (slop in bolt holes would be irrelevant).

I don't think the diagphram coupling on the machine I am interested in has a boss fit. I will check some more.

I looked in the diaphram coupling manufacturer's literature (Bendix) and found out that this is a "fitted bolt pilot". So the bolts have close tolerances and act (just like a boss fit would) to force the spacer/hub joint into alignment so that flexible spacer accomodates misalignment at time of bolt tightening, rather than bolt holes.

I guess that eliminates any need for loosening/retightening coupling bolts on these diaphram couplings. As far as I know, this concern only applies to shim Pack couplings.

Of course we have a bunch of Thomas Shim pack couplings in applications where we intentionally misalign cold to achieve alignment when hot.

Question: I would be interested to know how many people do and don't loosen/retighten shim pack coupling bolts when the machine heats up after coupling had been reassembled cold.

Question: Has anyone seen vibration or distortion on shim pack couplings that remains after heatup which is attributed to not loosening/retightening?

E-Pete,

You seem to be a one-man show on this posting, Pete. I would like to contribute more...but the cold fact is that I have never heard of doing this. I am, however, very interested...and hope to keep this in my hat next time we confront vibration issues around shim-pack couplings.

My list of troubleshooting for shim-pack couplings includes, primarily, inspecting for "oil-canning" of the shims. I've heard this term around our craft...and believe it describes a buckling or separation of the shims between the bolts. Also search for improper axial spacing between the hubs...as I believe these type of couplings are pretty intollerant of axial mispositioning.

Thanks for your hint.

George

George,

The term oil canning is used also in the metal building industry to describe the same effect you describe. Metal building siding comes with a label that says "Not Responsible for Oil Canning".

EPete,

I loosen the bolts on them when I hot align a machine with the shim pack couplings (mostly NH3 compressors and a few pumps). I have seen what I believed to be visual evidence of the pack itself becoming "thinner" when the bolts were loosened and then retightened. I have never mic'd the pack, but god's mics said it had an effect. This would be the reduction of the "oil canning" mentioned above.

George,

You are right about the Thomas not liking any axial deflection. The best thing I have found to determine the running condition of these couplings is a strobe light. You can definately see them gyrating around.

I don't guess I've thought of it before, but sometime if I can, I'll look at them right after cold startup, and then after hot alignment and retightening. Maybe a movie.

Will have to be on the new sight though

Dave

On these shim pack couplings; I'm assuming you're not loosening and tightening the shim pack itself? It should be tight in the perfect aligned state.

Now for assembly. The coupling deviates from perfect to an acceptable tolerance. It is a must to keep the movement within the tolerance box. This keeps the discs from cracking prematurely. And, to my thinking if the starts are excessive (or, what I deem as too many starts in a month) the tolerance box has to decrease. One must engineer the design to be within the tolerance box through colt to hot but the set of the coupling is designed to deviate from perfect IMHO.

A bit of an aside, but the large orange Omega couplings are the worst I've seen about taking a 'set' when operating, especially if misaligned. I always replace the element when I align one of these; if you don't, vibration can actually be worse than before you align it.

By extension, I suspect that most resilient element couplings that operate in shear (twisting) develop a 'set' when operating.

The dodge paraflex is also bad about having a memory. We always change the element when aligning them also.