Bump test
Bump test
(OP)
Has anyone heard of being able to do a "Bump Test" to determine natural frequencies with the equipment running?
Is this possible?
Is this possible?
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Bump test
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Bump test
Then either subract them mentally (put both on a log scale with identical axis scaling and compare) or numerically.
I'm sure there are some better ways (as mentioned Greg coherence... not sure exactly how to apply it here), but this has worked for us.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
The more energy you can [safely] put in with your impacting, the better your s/n.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Bump test
Keeping the machine at steady operating conditions, take 5 spectra (no-averages) at say 2 second intervals - then start impacting: begin with light taps using a hard rubber/plastic mallet at 5 taps/second) and take another 10 spectra also at 2 second intervals, followed by another 5 spectra also taken at 2 second intervals with no impacting.
Plot the specta in a waterfall format and you can see the effects of any excitation of a natural frequency (unless it is always visible due to being excited during during normal operation). There's a bit of trial and error involved, but once you can see that you are getting results, you can fine tune the procedure.
I have carried out this test with Adre and Zonic book systems, both of which allow you to automatically schedule spectrum capture at fixed time intervals. With a bit of thought and planning you could also do this with a hand-held datcollector system.
Of course, this will only tell you if there is a natural frequency present close to your measurement point and not its location (foundation/bearing housing/casing/shaft or piping).
A better way of identifying natural frequencies associated with rotating equipment is to measure 1X (and nX) amplitude and phase during a shutdown (or slow startup) and inspect the bode and polar plots - for that you would definitely need an Adre or Zonic book and fit the machine with a keyphasor (once per rev tacho/reference pulse)
RE: Bump test
RE: Bump test
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
No we didn't, we vector summed 6 frames of complex spectral data and divided the magnitudes by 6 to get one average spectrum.
etc
Not that it really matters!
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Bump test
Another one that used to bug me was FFT vs DFT. Coming out of school we knew well that the DFT was what we were computing, the FFT was simply the result. If I want to compute DFT by brute force on my computer I can do it and have done it using the definition (without the FFT). But it's a helluva lot slower.
So DFT is the result, FFT is simply the algorithm. But the term FFT is so universal now that we all use it to refer to the result. DFT is gone from the vocabulary and most vib analyst would look at you funny if you used that term.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
of course should have been
"..the FFT was simply the algorithm...:
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
Obviously it is very very close, but is it mathematically identical?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Bump test
h
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Bump test
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Bump test
M
--
Dr Michael F Platten