Variation of amplification factor - Vibration shaker testing
Variation of amplification factor - Vibration shaker testing
(OP)
We were trying to do a vibration test of compressor assembly in a electrodynamic shaker at an external lab. Initially, the test lab personnel did a sine sweep from 5 to 1000Hz at an input acceleration of 1g and a sweep rate of 1 oct/min. Thereafter we need to do a resonance dwelling at the identified natural frequencies (first two).
On sweep testing, we identified the first two natural frequencies (less than 100Hz in this particular test direction) and the amplification factors. When we subsequently did a dwelling for an input acceleration of 4g at the resonant frequency, the amplification factors are all off (pretty low in a particular direction which we were testing). The positions of control accelreometer in the shaker table and the response acclr on the compressor assembly are the same.The fixture (in this case was just a solid plate) resonant frequencies are established to be much more than 1000Hz.
Kindly for your suggestions on possible points I need to check at the laboratory.
Regards
On sweep testing, we identified the first two natural frequencies (less than 100Hz in this particular test direction) and the amplification factors. When we subsequently did a dwelling for an input acceleration of 4g at the resonant frequency, the amplification factors are all off (pretty low in a particular direction which we were testing). The positions of control accelreometer in the shaker table and the response acclr on the compressor assembly are the same.The fixture (in this case was just a solid plate) resonant frequencies are established to be much more than 1000Hz.
Kindly for your suggestions on possible points I need to check at the laboratory.
Regards





RE: Variation of amplification factor - Vibration shaker testing
Tobalcane
"If you avoid failure, you also avoid success."
“Luck is where preparation meets opportunity”
RE: Variation of amplification factor - Vibration shaker testing
> what form factor is the UUT, i.e., is it tall and skinny?
TTFN
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RE: Variation of amplification factor - Vibration shaker testing
Tobalcane : In fact, in the initial trial, we had some slipping between the bolt and chasis. This was then torqued and was ensured through wrench for later tests.
TTFN : The frequencies ar 18.9Hz and 59Hz. The first freuency is very lighly damped and hence I can understand as to why amplification factor is more sensitive. But even for the second nat frequency, the amp factor is quite varying! Whether the form factor has role in this? In this case, it is pretty much like a cantilevered load. I have attached the schematic of the setup and the resposne data.
Kindly for your comments
Regards
Jeyaselvan
RE: Variation of amplification factor - Vibration shaker testing
Were any of these resonances predicted beforehand?
I had thought there might be a problem arising from the slip table itself, given the cantilever, but your structure appears to be ill-suited for a vibration environment.
TTFN
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RE: Variation of amplification factor - Vibration shaker testing
RE: Variation of amplification factor - Vibration shaker testing
Cheers
Greg Locock
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RE: Variation of amplification factor - Vibration shaker testing
BrianE22: Yes, that was a real concern. Unfortunately, that is how it is installed in the application.
Greg : You are right.In the real installation, there is another stage compressor(first stage), which is not shown,(only mounting holes are shown). And that is supported. But this stage (second stage, which is of concern) still remains a cantilever.
The drive end is coupled to the engine flywheel housing. This is a two stage engine driven compressor and is typically skid mounted and is used in off road conditions. Our concern is that while the vehicle is moving in offroad, the assembly probably gets into transient excitations (no vibration mounts at the moment) from the road terrain and excites the inherent cantilever type of installation. We are trying to see whether we can prove a failure mode of the bearing. The bearing failures corresponds to those of the bearings at the free end of the cantilever.The bearing failures are related to static failure (as analyzed by bearing manfr), hence not on running condition (operational excitations).
Regards
RE: Variation of amplification factor - Vibration shaker testing
Cheers
Greg Locock
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RE: Variation of amplification factor - Vibration shaker testing
While frequency shift is not your problem in this case, the same nonlinear effects with different input amplitudes my be behind your problem
I like Twoballcane's explanation better though. I once ran into a chassis slipping on its vibration fixture causing the response amplitudes to just drop away after a certain amplitude was reached.
Have you found a cause yet?
RE: Variation of amplification factor - Vibration shaker testing
It would be interesting to look at the phase spectrum.