High vertical vibration amplitude

[indoldrums]

Hello all,

We are trying to troubleshoot a vibration issue with our motor-compressor drivetrain. The issue is that when we run a coast-down from 3600rpm down to the min speed, we get high amplitdue vertical vibrations (7mm/s RMS) at about 1500rpm and 2400rpm. Horizontal and axial vibration levels at these modes are low, less than 4mm/s RMS.

Does anyone have any ides why we may be seeing high amplitudes in one direction only, and what could we do to lower the vertical amplitude down? Thank you!

 

[amanuensis]

Re #2 :
I try to explain why you should have 180° phase shift.

First, if you have 180° phase shift between force F (due to impact hammer) and velocity V, that means F = -K.V. Where K is just a constant (because cos(180°)=-1). F and V are in opposite direction.

This can be understood from physical point of view. As you know, the effect of damping is to create a force which is opposite to the displacement (displacement which is in the same direction as the force) in view to decrease it.

Have you only ploted the velocity ? Because the 180° phase shift should only appear on the plot of FRF=Velocity/Force and not on Velocity.

Last point : At the resonance frequency, Mass and Spring effects cancel each other. Only the damping effect exists (because damping is in quadrature with Mass and Spring).

Re #1 :
In the first case, when the excitation is due to the motor, you can have access only to the outputs : Velocities. You have to do Operational Modal Analysis.
In the second case , when the excitation is due to the impact hammer, you have access both to the input and outputs : Force and Velocities. You can do Experimental Modal Analysis.

In the first case, you can only plot Transmissibility : V2/V1 (ratio between two velocities takent at two différents points).
In the second case, you can plot FRF = V/F.

In general transmissibility and FRF are not the same and can lead to misinterpretation.

 

[Strong]

I am still waiting for photos of the components and support along with some nameplate data.

Walt

 

[indoldrums]

Still collecting data and I plan on sharing what we find out. Will be back with more info.

 

[stunner]

What is the reason for more axial vibration in constant speed centrifugal horizontal pump apart from alignment.