Why do a bare fixture eval?

[floattuber]

We run aerospace parts ranging in size from baseball size up to 1000 pound engines on our shakers. Some customers want to do a bare fixture evaluation and some don't. I've heard some reasons to do it and some reasons for why it's a waste of time but I haven't been able to really delve into it and decide for myself.

So if you do a bare fixture eval you find all the natural frequencies of the fixture. I got that. But as soon as you put the test unit on there, you've changed the natural frequencies, so what did you gain by doing the bare fixture eval?

 

[GregLocock]

It could be a quick check that there are no existing resonances/buzzes in the frequency range of interest. As you say it seems of limited use.

Cheers

Greg Locock


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[IRstuff]

I agree that it's of marginal utility. In fact, a top-heavy UUT can cause a conventional slip table to rock on the oil film, thereby inducing a resonance that does not exist with either UUT or table by themselves. We once wasted a couple of weeks because of that. We had to contract with a completely different testing facility that had a hydrostatic bearing table to complete the testing.

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[floattuber]

Ok thanks for validating my thoughts.

Greg, the best response that I've heard so far is that bare fixture response is close enough to real test conditions and fixing any resonances in the region of interest is better than doing nothing.

 

[drawoh]

floattuber,

Could it be a QA check to ensure the fixture was designed properly?

If the mount fixture has a mode within your test range, it has to be re-designed.

--
JHG

 

[boyze]

as others have said it's usually to gather data to see if the test fixture will influence the response of the actual test item. You are correct that the test item may change the fixture frequencies. But the more important question is whether the test fixture will change the test item response of interest.

Have Fun!

James A. Pike

http://www.xl4sim.com

http://www.erieztechnologies.com

 

[btrueblood]

We used to do fixture qualification tests with dummy masses. This was for aerospace "one off", expensive hardware, i.e. breaking the flight hardware due to a test fixture problem would not be good for the company or customer. Bare fixture tests were usually done too, just because we could, but I don't recall any useful information being generated as a result.

 

[GregLocock]

I can see the point with a dummy test article in there, that sounds sensible if the IUT is a proto or unique.

Cheers

Greg Locock


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