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MSHtech

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Reply with quote  #1 
Hi All,

Using vibration analysis, is it possible to detect flexible element wear in a jaw or spider coupling ?  Assuming the coupling is aligned correctly, will wear in the flexible element be detectable through vibration readings?

Our plant uses these types of couplings on our Chilled Water System pump and motor sets. The motors are in the 37kW to 90kW range.

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Stan.
Noknroll

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G'day Stan
coupling wear can show up as 1X in radial and or axial direction, if you can get a strobe light and see the coupling this is an easy way to verify coupling wear
a spider or jaw coupling can also show up as 1/2 the number of rubber lobes on the element. Most common is 6 or 8 elements or 3 or 4 dogs per each side of the coupling so you will see 3X or 4X
Edwin

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Right. I was just typing something like that. It will show as the number of claws per site, including higher harmonics of it as there is a clearance in it. It will show on both driver and driven and will sound as a rattle. 

Coupling.PNG 



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VibraMac

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Reply with quote  #4 
The coupling in question is typically known as a Holse/Renold couping (manufacturer's name) in these parts.  Typical application is small power generation units.

My experience is always at 1X (introduced by a cranking effect as the rubbers wear/collaspe).  Many times experienced a vibration problem (1X) after replacing new rubbers (with good alignment).  Some times the units are run up/run down to attempt to 'settle the rubbers' and hence the vibraiton.

Would like some suggestions how to set these rubbers correctly on the first attempt.  Tried many assembly procedures to ensure this (even as per the manufacturer's recommendation) without achieving the desired result.

Rubber selection (hardness and damping qualities) are critical.

 
arie mol

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Stan,
The proof of the pudding is in the eating...:
Obtain worn out coupling elements, mount these in a coupling. And look how vibration signature changes compared to signature with non-worn out elements.
RustyCas

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Reply with quote  #6 
There are many types of couplings with "flexible" elements and vibration characteristics vary. A 3-jaw coupling will show high 3x if misaligned, but not if simply "worn" - if aligned well they last a very long time. But they are mostly not well aligned, and wear quickly. I often just see low-frequency broadband vibration that increases over time. I typically "diagnose" them with a strobe light, or when I can't see them, a digital camera (with flash) taking a blind photo from below.

For others (Omega, Woods single or split element, etc) I usually just see high or increasing 1X as the elements start to deform and create imbalance.

My experience is that, with the exception of 3-jaw couplings, you can pretty much ignore what you've heard about diagnosing misalignment using vibration, unless the misalignment is severe.

Visual inspection of a flexible element is the best, and perhaps the only reliable way to diagnose a "bad" element.

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RRS_Dave

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Reply with quote  #7 
When you see dust the color of the element laying under the guard, you can rest assured there is wear, and the coupling is misaligned.
I agree with Rusty on diagnosing the misalignment with vibes. Unless it's so bad you don't need a meter, it's hard to tell with one.

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plongson

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Reply with quote  #8 
Rusty/Dave...I agree 100% You saved me a lot of typing.

Throw a shim pack type coupling in the mix and angular misalignment detection with vibration and essentially impossible, although you will see offset.

A strobe is your friend if the machine can't be shut off...

Just remember you need a forcing function of some kind to see response. If it bangs, rattles, hops or in some way moves from a reference it'll show up on a meter. With a great alignment, wear will be minimal for a LONG time...possibly as long as the driver or driven if sized correctly.

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stace1g

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Reply with quote  #9 
I quite often detect misalignment using vibration via the 'traditional' symptoms. I.e. 1x/2x radial, High 1x axial, phase shift across coupling etc. However I will agree that the symptom set is not fixed across differing coupling designs and I guess there will be machinery out on site that is misaligned that is showing no symptoms at all. 

We have detected misalignment on shim pack couplings quite successfully although a recent occurrence had me scratching my head. A multi stage HFO feed pump was displaying the classic signs of misalignment after being returned to service following overhaul so I asked for it to be removed from service for an alignment check. Figures came back that indicated an almost perfect alignment so I started to consider other possibilities - until I talked to the engineer in charge. Turns out the fitters had been checking the alignment with the coupling bolted up! Sure enough when they split the coupling and re checked they found a 7 thou error. Correction of this did indeed reduce vibration levels.

Gary 
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Hi Gary, was the .007" error offset or angular?
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Danny Harvey

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Reply with quote  #11 
I believe that the by-the-book calls on alignment by means of vibration hold true as long as there are not other forces such as imbalance and pipe strain at work which means only on occasion.  

I have seen pumps that exhibit horizontal vibe at running speed when they are horizontally misaligned; vertical vibes for vertical misalignment and axial for angular with all the phase shifts that would be anticipated.

But that is the exception, not the rule.


stace1g

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Reply with quote  #12 
It was an offset error as I recall

Gary
jvoitl

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Reply with quote  #13 
   Several months ago I found out that about 3 days after a motor had passed vibe analysis with flying colors I found out the coupling flexible element had to be replaced.  I investigated and found this near the .
machine.

DSCN0276.JPG   

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