Natural frequencies from transmissibility

[Vibac]

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?

 

[electricpete]

I'm not sure I understand the question. Is the isolator to be installed in its final configuration under the machine when you do your test?

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.

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[GregLocock]

You can usually get a good feel for it, especially if you look at phase.

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 for tips on how to make the best use of Eng-Tips.

 

[Vibac]

Electricpete - Suppose an existing, installed machine which is to be examined.

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?

 

[Rob45]

Record and analyze acceleration vs time as you shut the machine off and let it coast down.

Ideally it would pass through and excite the resonant frequency at some point in coast-down.

 

[Vibac]

Rob45, I see what you mean, run-up/down is probably the simplest method for rotating machinery.
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.

 

[IRstuff]

People use nylon hammers for this sort of excitation

TTFN

FAQ731-376