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KVib

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Reply with quote  #1 
I wanted to verify misalignment by using phase analysis and was wondering if anyone enters 2x runspeed instead of 1x run speed. Im getting most of my ampltude at 2x and thought phase differences would show up better. Has anyone had any success checking phase this way?
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RustyCas

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Reply with quote  #2 
My experience is that unless you have severe misalignment, diagnosing misalignment with vibration data is just a "guess" and misalignment is nearly always a good guess unless the units are routinely aligned to tight specs (i.e., there is a formal alignment program).  By that I mean it will be misaligned, but correcting the alignment may make little difference in your vibration spectrum.  Alignment often gets the credit for fixing the problem, but often a number of other things are done as well that are just as likely to be the solution.

I've not really taken a lot of phase data to diagnose misalignment. Usually it has to be pretty bad to cause vibration, and there are so many other problems that create a spectrum that looks like misalignment.   If I were you, I would take phase data both ways (1X & 2X) and compare.  Then check and correct alignment and repeat your measurements.

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Beatnik

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Reply with quote  #3 
The theory tells us that we expect a 180 deg phase difference between the 2 machines at 1x. On each machine the 2x and 1x should be in phase. 2x will therefore "hit" at each 180 deg period of the 1x. Considering this, I would expect the 2x to be in phase with each other. 
RustyCas

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"The theory"... so tell me again what misalignment causes to actually, physically happen to cause the two shafts to be 180 out of phase....
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Beatnik

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Reply with quote  #5 
I understand it visually, it's hard to explain.

In parrallel misalignement, one shaft will be under the axe of rotation and the other on will be over it. Therefore when rotating their 1x vibrations should be out of phase by 180 deg in the radial direction.

In angular misalignement the shaft are pushed/pulled in opposite direction by the misalignement, it's why there SHOULD be a 180 deg out of phase mesurement axially.

Of course the real world is a different animal.

To quote Mr.Tranter :
"Mobius Institute’s Tranter offers one more important comment to consider. “There is no guarantee that the vibration will change if the machine is misaligned,” he notes (Figure 2). “Experiments have been performed, on test rigs but also on larger real machines, where the machines have been deliberately misaligned, and yet the vibration did not change either at all or in a significant enough way that would alert a vibration analyst. Therefore, the golden rule is to employ precision alignment practices and not to assume that your alignment practices are acceptable just because the vibration readings do not indicate that a fault exists.”"

http://www.plantservices.com/articles/2014/vibration-analysis-detect-alignment-problems/?start=2
plongson

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Reply with quote  #6 
MANY moons ago, like about 35 years...I worked for a #2 major brewery who was hell bent on being #1...they were all about PdM best practices. The subject of misalignment came up and we were having a tough time correctly identifying misalignment with our vibration tools.

We did a an in-depth study using IR. Using lab-like protocol and setup we got very interesting results. We photographed and documented misalignment in various degrees, both offset and angular and using several different couplings and speeds.

We printed the study for in-house and inter-plant resources, but it was never published it.

Bottom line...IR is probably the best tool for identifying and quantifying misalignment (short of using a Rota-line...LOL). I still have a hard copy in my files. It would be great to post but I'd have to scan each page.

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John from PA

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Reply with quote  #7 
Quote:
Originally Posted by plongson
Bottom line...IR is probably the best tool for identifying and quantifying misalignment (short of using a Rota-line...LOL). I still have a hard copy in my files. It would be great to post but I'd have to scan each page.


There are reliable resources on the web...http://reliableplant.com/Read/28638/using-thermography 
stace1g

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Reply with quote  #8 
Best explanation i have ever come up with for why misalignment causes a radial/axial 180 phase change is to do with machining or mounting errors in the coupling, which too a degree always exist. As we know with a flexible coupling an alignment error is a static force hence why 2x?.

If you have an alignment face error then this may be enough to take up any clearance and the couplings may (or may not) touch or lock allowing any face error (swash) to attempt to push the couplings apart hence a 180 shift across the coupling. Similar effects for a radial alignment issue where there is an eccentricity in one or both coupling halves. 

hope this makes sense?

Gary
RustyCas

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Reply with quote  #9 
Quote:
As we know with a flexible coupling an alignment error is a static force hence why 2x?


My point exactly.  What type couplings do you suppose the wall charts showing misalignment spectra were referencing?  I've never understood how a flexible coupling generates 2X vibration, or why there'd be 180 deg phase shift across the coupling.  Just never made sense to me.

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OLi

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Reply with quote  #10 
Couplings whatever design are not that flexible at full load IMHO. I do think that the asymmetric rotor stiffness is the reason for the behavior.  If you have one stiffness one way and another 180 DEG turned that explain it to me anyway and many rotors do look like that or similar. Olov
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stace1g

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Reply with quote  #11 
Sorry this was meant to say 'so why the phase shift across the coupling?' not 'so why the 2x?' written in a hurry!

the 2x is in my opinion as the result of asymmetry as mentioned by Oli. this can be the coupling but also the keyway.. Apply a static load (misalignment) and you get a stiffness variance twice per revolution hence 2x.

Gary
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