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Greeting to all the Vibration Gents,

I recently got a set of data from a Motor which drive a screw compressor. The dominant peak in radial (both horizontal and vertical) of the NDE and DE is 1xRPM, the dominant peak in axial direction of DE is 2xRPM (there is no data for axial of NDE, might be because of the cooling fan). Things is when I'm working with the phase data, some data point toward unbalance, but some to misalignment as in the pic below:

1. For unbalance symptoms the data shows:

- Dominant 1X RPM

- Horizontal phase difference closely matches the vertical phase difference (V1V2 ~ H1H2).

But one thing is that the radial phase differences (V1H1, V2H2) are not close to 90 deg. Is that if unbalance occured, the radial phase difference should close to 90 deg or there will be case that even if the unbalance exits the radial phase difference can be in phase.

2. For Misalignment symptom the data shows:

- Dominant 1xRPM in radial (should 2xRPM must be also present?)

- Dominant 2xRPM in axial Motor DE

- Vertical phase difference across coupling is 133 deg

- Radial phase differences (V1H1, V2H2) are close to 0 deg

But one thing is that the horizontal phase difference closely matches the vertical phase difference. Is that if misalignment occurred, the horizontal phase difference and the vertical phase difference should not be consistent (for example if H1H2 = 0 deg, the V1V2 can be expected to show 180 deg). Or there will be case when misalignment occurred, the horizontal phase difference can be close to vertical phase difference.

Could we do a discussion on this matter? I am really exciting to be enlighten by your experiences. I've also attached the spectrum and motor datasheet for your reference.

P/S: This case is not in my company, just a case I've been share with, so I will have very limited information. So sorry for that. 


Images (1)
  • mceclip0
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Discussion about phase behavior show both misalignment and unbalance
Tags: phase, Motor, vibration, Discussion

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What is the motor speed?

One thing that can make the phase difference close to 0 degrees (or 180) is rsonance. The force driving a resonance can be at 1X.

If one is looking for a solution by balancing, the question to ask is whether the response to balance weights will work, i.e. do you get enough response, the right response--the right response in some systems has to do with whether you have sufficent balance planes and planes in a good place.

In a resonant system balance weights can respond with a 0 degree difference between vertical and horizonal. In fact 0 or 180 degrees might be expected.

Where are you with respect to the natural frequency.


Hi William,

Thank so much for your response,

The motor speed is 2986 rpm.

If resonance to be suspected in 1xrpm, that means the natural frequencies should have 1 frequency close to 1xrpm right? I'm not sure about the current state of resonance identify for this case (as it's not in my company). But should I be concern about resonance if there are significantly highly directional vibration? (i.e much higher horizontal vibration than vertical vibration or vice versa).


Hi fburgos,

Well I guess maybe it is because of the convenient for using cross channel measurement compare to the use of phototach (which might require the machine to be stop to set up the reflective tape, and a lot of facility want to avoid stop machine for checking). From your experience, do you think that bubble charts will be more precise than cross phase? I would like to know more about it.


Directional vibration does not have to be higher in either the horizontal or vertical direction..

What is directinal vibration? Loosely, vibration mostly in one direction. From a measurement view, vibration at a natural frequency should be close to a modeshape (pedantic examples excepted). For many modes of which people speak, normal modes, modeshapes the measurements are either inphase or out of phase. Complex modes would have other phase relations.


Here is an early post along the lines of your questions. Looks like it might be from 2010 (12 years ago)

It has a few more members responding. But appears to say, more or less what the current replies say.

In my opinion, in most cases, phase is not an absolute, "fix-all"  tool. Just another tool used with an open-mind when in the search for the real problem.



BTW: The balance might be from the driven machine and not from the motor, if you have it coupled up, and based on the "misalignment" statements, it sounds as thought you do have it coupled up. Being a screw compressor, I would lean toward alignment or mounting base problems.

Last edited by Registered Member
@Registered Member posted:

The solo was not in the same base

Probably the balance of the motor is good.

I just was curious if the same base and lower vibration was due to uncoupled or different base.

Anyway, I had experience with a screw compressor, improving alignment improved the axial vibration, I could not find a solution, I think the problem was in the screws itself, but the motor had lot higher vibration than the compressor side


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