Natural frequencies from transmissibility
Natural frequencies from transmissibility
(OP)
Suppose a compact machine is placed on vibration isolators on the ground. You want to check the natural frequencies of the machine-isolator modes corresponding to the isolators being compressed and the machine moving up and down. The proper way to do this is probably a modal analysis from transfer functions between force excitation (hammer or shaker) and vibration response. Or operational modal analysis based entirely on responses.
But, simplifying the procedure: How far could you get looking just at accelerometer transfer functions, eg H1 between measurements above and below an individual isolator?
But, simplifying the procedure: How far could you get looking just at accelerometer transfer functions, eg H1 between measurements above and below an individual isolator?





RE: Natural frequencies from transmissibility
If yes, I think you have a reasonable chance at discoverying the resonant frequencies.
If no, then you'll need some calculations to supplement your measurement.
=====================================
Eng-tips forums: The best place on the web for engineering discussions.
RE: Natural frequencies from transmissibility
If your isolators are linear then the force is k(x1-x2), and x1 (etc) is easily derived from the accleration spectrum.
Rather than using running excitation, switch the machine off and belt it with a screwdriver/fist/hammer.
Cheers
Greg Locock
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Natural frequencies from transmissibility
GregLocock - By looking at the phase, you mean looking for phase shifts in the order of magnitude 90 degr, right?
How about Nyquist circles? If the reference measurement had been force, I would look for neat circles to identify natural frequencies. Any idea if this applies when using only vibration responses?
RE: Natural frequencies from transmissibility
Ideally it would pass through and excite the resonant frequency at some point in coast-down.
RE: Natural frequencies from transmissibility
However, I'm wondering about methods that could apply also to "objects" placed on vibration isolators in cases where you for some reason can't use the "object" itself as an exciter. An example would be vibration sensitive equipment to be protected from outside vibrations.
RE: Natural frequencies from transmissibility
TTFN
FAQ731-376: Eng-Tips.com Forum Policies