Uncontrollable Shaker Table Resonance

[Noah Lopez]

Hello,
I am trying to conduct random vibration testing using a electrodynamic shaker table. In testing there is a resonance (~1975 Hz), caused by the shaker assembly itself, within the the desired spectrum (10-2000 Hz). This resonance cannot be controlled and is preventing our response PSD from being within tolerance. I have heard that this is a common problem with vibration testing, but cannot find much literature specifically addressing this issue. I would like to know if there is a standard approach for fixture design which could address this resonant issue. I have heard that using a magnesium fixture may help (we are currently using an aluminum fixture)due to its superior damping characteristics. Any advice would be appreciated, thanks.

 

[IRstuff]

How flimsy is the fixture?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Noah Lopez]

Fairly flimsy, it is only 0.75" thick. However, its first resonant mode is around ~3000Hz (simulated).

 

[IRstuff]

Have you run the shaker without the fixture? Does it still have the resonance?
N.B. simulations are based on a truckload of assumptions, which, if not true, will result in radically different results. The fact that the measured resonance is close to 2 kHz would suggest that it's likely from your fixture, simulation not withstanding.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Noah Lopez]

I have run tests with the bare table only. Its resonance is ~2150 Hz. I believe adding our fixture decreases it to 1975 Hz.

 

[GregLocock]

3000 Hz indicates your FEA is missing the essential details.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[IRstuff]

Was your shaker always like that? If so, then it's not appropriate for your application. If not, the you need to fix it.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[Noah Lopez]

Its had this resonance issue since we got it. I was able to recreate the shaker armature and perform FEA on it. I believe the resonant mode in question is a swaying/twisting motion of the armature. I can't think of a way to stiffen/dampen this mode through any changes to the fixture on top.

 

[IRstuff]

Does it meet its specifications? If this is outside the manufacturer's specifications, then trying to fix it will likely be an impossibility; otherwise, the manufacturer would/could have done it to sell it with a higher frequency performance.

If it's not meeting its specifications, one would think you should talk to the manufacturer about getting it fixed or replaced.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[btrueblood]

I can recall having a similar high frequency resonance with our shaker head. It turned out that the threaded holes in the top of the head had become worn, changing the effective (grip) length of hold-down bolts, and there was some wear on some of the faying surfaces near the bolt holes that created a non-planar bolt face. We had our machine shop mill the surface flat and replace the threaded inserts which fixed the issue.

 

[nicomedian]

Have you tried to notch down the input around those frequencies?

I would first do a low level sine test and get the transfer function. Then, do the notching in such a way that my vibration will still be in the limits of my desired application and its tolerances.