High vertical vibration amplitude
High vertical vibration amplitude
(OP)
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!
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!





RE: High vertical vibration amplitude
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RE: High vertical vibration amplitude
RE: High vertical vibration amplitude
Walter Strong
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RE: High vertical vibration amplitude
We are measuring the vibration on the NDE motor foot. We are also measuring the base skid that the drivetrain is attached to, near the NDE. Base skid vibrations are not nearly as high as the motor foot vibrations. This makes me think that isopads are doing their job.
Based on the bump test and modal fea analysis, we can see there are modes aligning with the 1500rpm(25Hz) and 2400rpm(40Hz) and some others but I am mostly interested in these two modes as they exhibit high, vertical only vibrations. Operating range of the motor is 1200rpm (20Hz) to 3600rpm(60Hz).
I don't know if it is even feasible to move these two modes out of the operating range, or if there is anything that can be done to lower the levels.
RE: High vertical vibration amplitude
Walter Strong
www.LinkedIn.com
RE: High vertical vibration amplitude
You are correct. I have some data, and an idea of what the problem is but I think I need to investigate more in order to come up with the right corrective action.
In this case, the motor is tied to the isolation pads and the isolation pads are tied to the steel frame which sits on the floor. Unfortunately, I don't have any flexibility to change that structure.
Thanks for your help!
RE: High vertical vibration amplitude
Can you spend some time on this to understand the problem, or do you just want the quick,very expensive, fix?
Cheers
Greg Locock
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RE: High vertical vibration amplitude
Thanks for your feedback. I would like to invest some time into this problem and do it right and learn something in the process.
I am attaching the plot we took at the NDE motor foot. We have a pretty good match with the bump test results and this coast-down plot. This tells me that my issue is likely to be a structural one. As Walter mentioned it above, I have mass, stiffness and damping to play with but I am not sure what's my next most logical step should be in addressing this. You guys can tell I don't have much experience in applied vibration but I do enjoy problem solving.
RE: High vertical vibration amplitude
What make and model of coupling?
What type of compressor?
How much do the isolators deflect when the motor is placed on them?
Was the motor originally bolted hard to the steel frame?
Is the compressor bolted to the steel frame?
How is the steel frame attached to the floor?
Pictures are always nice.
RE: High vertical vibration amplitude
I don't have access to the machine right this moment for pictures but I can briefly explain the setup:
The motor is bolted to the rotary compressor with a steel adapter piece. There are four isolation pads, two on the compressor and two on the motor side. This entire assembly is then bolted to the steel base frame. Base frame, however, is not bolted to the floor. I will have to check to see the make and model of the coupling.
Isolators deflect about 0.003-0.005in. Overall height of the isolator is about 2in. I need to check the transmissibility ratio again.
Thanks for your help!
RE: High vertical vibration amplitude
Walt
RE: High vertical vibration amplitude
This will give you an idea of the x y z deflections along the major axis of the machine, and confirm what we all suspect, that it is just rigid body motion of the two major components, moving as one.
It is probably worth measuring the frame side vibration at the isolators as well, just to make sure you have a rigid foundation, this may be your 'Base V' plot. I agree with Walt, is A axial? If so and that is what you want to cure then it is more complex - 42 Hz looks to be purely axial.
Cheers
Greg Locock
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RE: High vertical vibration amplitude
Also, the Base V plot is the vibration on the base frame, in the vertical direction, near the isolation pads.
We ran the machine up to 4000rpm with a VFD. I believe the idea was to see vibrations 10% beyond the normal operating range.
I will take a closer look at the force-hammer tests we ran on all three axes and report back.
Thanks for all the help!
RE: High vertical vibration amplitude
TTFN
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RE: High vertical vibration amplitude
For a simple system That would be too stiff (too little deflection) to function as an isolator ( reducing transmitted vibration, at the expense of somewhat higher equipment vibration ).
Table 1, page 5 here -
file:///C:/Users/Robyn%20Timberlake/Downloads/20070727_simmons.pdf
0.003" static deflection translates to about 3400 cpm / 57 Hz resonant frequency.
With the mounts directly under the equipment feet the "foot print" is kind of narrow, inviting other degrees of freedom like horizontal rocking to join the party.
Vertical and horizontal Amplitude and phase info from all over the machine, steel frame, and floor at the various test speeds would be what I'd do next.
“Data!data!data!" he cried impatiently. "I can't make bricks without clay.”
Sherlock Holmes in, The Adventure of the Copper Beeches
RE: High vertical vibration amplitude
Tmoose, you are correct that the isolation pads are too stiff; I think that is the case even with the ones that have 0.065in deflection. The article you attached suggests fd/fn to be a minimum of 3.5 at the lowest fd. In this case, we are at fd=20Hz (minimum driving frequency)and fn=12.5Hz (isolation pad frequency), which is 1.6. Basically, we are hardly transmitting anything to the base until we reach a driving frequency of 45Hz, probably.
I think the reason the isolation pads are the way they are is because there was some effort to reduce the noise being transmitted to the base frame, to lower the overall noise level of the machine. Not sure if that was the right approach but I think it's wort re-testing with softer isolation pads.
Thanks for everyone's ideas and suggestions. I will be collecting more data next week.
RE: High vertical vibration amplitude
Does anyone want to bet 10$ on the contrary (that frequencies will remain the same) ?
RE: High vertical vibration amplitude
Also, just a general question; when I look at a velocity vs. frequency plot along with the bode plot from a hammer test, is it fair to say that for a frequency to be a resonant one, there must be a 180deg phase change?
RE: High vertical vibration amplitude
Cheers
Greg Locock
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RE: High vertical vibration amplitude
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.
RE: High vertical vibration amplitude
Walt
RE: High vertical vibration amplitude
RE: High vertical vibration amplitude