Question:
What is "locked coupling" and could it be related to a reading showing a peak at 4 times turning speed?

Choices:
I just don't know.

 

Normally this refers to a gear type coupling in which less than desirable things can occur that lock up one or both "flex" points; hence the flexibility is lost. This can happen from deteriorated grease or in a continuous lube type coupling the oil be leave behind deposits at the teeth where it can literally be centrifuged.

Essentially the sysmptoms resemble mislaignment.

Thanks for the quick reply.

Some more general comments about locked gear couplings:
1 - You would expect to see 180 phase shift of fundamental vibration axially across the coupling.
2 - Coupling lockup also depends on the load. In some cases it can be relieved by shutting down and restarting the unit. (We see this on our horizontal turbine driven feedwater pumps).

Now to your specific question about 4x vibration. It would be helpful to narrow down whether the predominant vibration really has a FUNDAMENTALl frequency of 1x or 4x. You can do this with time waveform. You might see (A) impacting at 1x and the ringing afterwards is roughly 4x. Or you might see (B) sinusoidal or impacting at 4x.

You can check the same thing with your spectrum on log scale. If you see a series of peaks 1x, 2x, 3x, 4x, 5x, 6x etc which rises to a peak at 4x, then likely you have pattern A. If you don't see this but see more of a pattern 4x, 8x, 12x etc, you likely have pattern B.

Pattern A seems like might be associated with gear coupling lockup among other things. Can you tell us the coupling type? While you're at it the speed and type of machine?

Pattern B would not be gear coupling lockup I don't think. If you have pattern B check closely to see if it is really 4x or perhaps a non-syncronous bearing fault frequency. If truly 4x and not a bearing fault frequency, think about what do you have 4 of in the machine. Perhaps 4 vanes on pump impeller. For Thomas Shim Pack Coupling I'm thinking each shim pack is spanned by 4 bolts.... maybe not.

A locked coupling can refer to a coupling where the gap is not set properly. The hubs could be set too far apart or too close(touching each other). With lubed type coupling, such as a gear coupling, locked could either refer to the coupling being dry & rusty from no lube at all, or over greased caused by being hydraulically locked and the outer coupling flange assembly cannot float over the hubs. I have seen over greased and locked couplings produce a dominant 3x's, dry couplings produce a dominant 4x's, and on a few occasions a dry rusty one produces 1 & 2x's. You will usually see the best indicator in the axial directions. 3 jaw couplings are usually seen with a predominant 3x's. Exactly what type coupling are you talking about? Different types produce different characteristics.

Where's all the members from the old board? Man I miss it. Any way to get an old post from the old board to this one? I have a good follow up coming up in April with Pkvue vs. Velocity data.

quote:

Where's all the members from the old board? Man I miss it. Any way to get an old post from the old board to this one? I have a good follow up coming up in April with Pkvue vs. Velocity data.

I think it may take awhile for all the old members to find it. Probably an important thing during this initial period is to keep the board active so people who stop by will want to stay.

As far as transferring an old post... the only way I know to search for an old post is on Oli's board.
http://www.vtab.se/PHP-NBoard/html/index.php

Then you can perhaps cut/paste relevant portions or else save it as html or mht, or else post a link directly to the old thread.

The effect of a locked coupling is derived from the fact that any misalignment present between shafts becomes now very pronounced as two shafts bend, thus producing one "S"-shaped shaft.

When a S-shape shaft is trying to rotate, it oveloads the bearings on both sides of the coupling driving vibration response into non-linear region and, therefore, generating few higher harmonics (mainly 2x and 3x, may be 4x as well).

If the above is correct, then appearance of 4x only, in general is not representative of a locked coupling.