Vibration Table Setup Sanity Check

[Transient1]

I've got a shaker table, with a plate mounted on it. This plate is the mounting plate for units under test. On this plate an accelerometer is placed. Am I correct to think that the accelerometer on the mounting plate, should reflect almost identically the input from the shaker table?

Basically, my vibe testing guy is telling me that it is natural to see system resonances in the shaker table under 2000 Hz. That seems erroneous. There should be no resonances in the frequency range of my tests. If there are , I would have to increase the number of mounting points on the test plate or increase the plates stiffness. Thanks for your input.

 

[btrueblood]

You shouldn't have any resonances in the mounting fixture below 2000 Hz (or whatever your upper test limit is) ... but, sometimes they creep in. Make sure you have all the bolts tight, and no burrs or debris under the bolt heads.

I can recall conducting production tests with a fixture that had a nasty resonance, which, uncompensated, would damage the DUT during qual-level testing. We had accel's mounted near the DUT, and used those as the feedback for the test levels; it worked, but was kinda kludgey. One of my first tasks as a young engineer for that company was to help design a new vib. fixture for those devices.

Yes, increase the mounting points if possible, but don't just make the fixture bigger to pick up the next outer ring of bolt holes. Making the structure more monolithic helps. Finally, the fixtures should ideally be made of magnesium alloy - it has good stiffness/mass (lighter fixtures reduce the amplifier power requirements, and helps keep the output stages of your amplifier intact longer) and (more importantly) magnesium has a high natural damping coefficient, which helps to control the accel of any resonances.

 

[Transient1]

Thanks for the input, that sounds like valuable advice. When you talk about the magnesium alloys high natural damping, your talking about material damping, right? Is there any specific product you would suggest?

 

[btrueblood]

Yes, material (internal? natural?) damping. Sorry, can't help you with a specific grade...maybe ask in the metallurgy forum.

 

[Transient1]

One last question:

Has anyone read IEST-RP-DTE013.1: SHOCK AND VIBRATION FIXTURING?

Is it a document worth getting?

 

[floattuber]

Whether you see any natural frequencies below 2000hz depends on how big your shake table is. We have some pretty big ones where the slip plate is about 36"x42"x2" and we have a resonance at around 1300 Hz (if I remember correctly).

The slip table is actually dancing around a lot more than you think.

Just a tip, grease up the interface between the transfer plate and the slip table. This will provide a much more solid interface between the two plates and also filter out some of the high frequency noise. We use this white grease called Lubriplate.

 

[Transient1]

Hmm, I think I've heard rumors around hear about a 1300 Hz resonance as well. That slip plate dimensions seems a bit larger than my table though. I'm not sure about the oil we use but its a dark oil (Slightly darker than motor oil).

 

[floattuber]

No, not the slip plate oil bath. I'm talking about greasing the mating surfaces between the slip plate and your fixture plate.

 

[Transient1]

Interesting, I will look into that. Thanks.

 

[VibeAC]

I'm not sure I understand your test setup, but might some modal impact testing provide some information as to the modal frequencies of the test rig?

Andrew Gorton, MSc
Noise & Vibration Consultant

http://www.PapadimosGroup.com

 

[GregLocock]

That'd be my preference. A morning spent with a hammer and an accelerometer will give you enough bad news to keep you busy for weeks. I'd run the table at 120 Hz while taking the data so that you have the right boundary conditions.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Transient1]

Hi Greg,

I suppose the hammer and accelerometer setup, makes small resonances more prounounced? My vibration fixture is just a flat plate. Also, what is the purpose of the 120 Hz background vibration. Why 120 and not another number?

Is there a good practical reference for vibration testing? Thanks for the help.

 

[VibeAC]

Here are a few resources:

(1) Modal Testing: Theory and Practice by D. J. Ewins

(2)

http://www.bksv.com/Library/Primers.aspx

(3)

http://www.sdrl.uc.edu/academic-course-info

The B&K primers are very good a taking complex concepts and making them very easy to understand. For more in depth information, the Modal Testing book or the course notes from the University of Cincinnati (my Alma Mater) are very good. The Vibrations III course notes will probably tell you more than you need to know.

Good luck...

Andrew Gorton, MSc
Noise & Vibration Consultant

http://www.PapadimosGroup.com

 

[GregLocock]

My experience is that you won't get much useful information at mains electricity frequency and harmonics, so running the table at that will not lose much more info.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[btrueblood]

Greg will correct this, I probably have mistaken something, but: ... the purpose of the background vibration is to keep the slip table "slipping", which can/may affect the boundary condition on the fixture base, and thus affect the resonances. 120 Hz is low enough that it is well below any reasonable resonances in your fixture, high enough to put the fluid dynamics of the slip table oil bath into the right flow regime...or at least closer to the same flow regime that it would see in testing...like everything else, it's a trade-off.

 

[Transient1]

btrueblood,

That makes sense.

Andrew,

Those course notes are a great reference.

 

[IRstuff]

Slip tables can also produce resonances as a function of the UUT placement above the slip plate. A top-heavy system that we shook popped out a 35-Hz resonance, which was completely unpredicted by our MEs. Took about 2 weeks of thrashing before concluding that the slip table was actually rocking on the oil film. In the meantime, we had installed a number of cross braces in the UUT, hoping to get rid of the resonance.

TTFN

FAQ731-376

 

[Transient1]

Did the stiffness of the cross braces end up reducing the rocking of the unit+fixture? I've seen a lot of these types of issues you've guys have been talking about. I kind of wish I got working with the vibration guys earlier.

 

[btrueblood]

Argh. Yes, IRstuff, I recall similar problems with test fixture designers that didn't at least try to keep the c.g. of the system in line with the shaker head center (for the slip table, although the same rule is appropriate for vertical-axis shake tests too). One such bad fixture test resulted in having to send the slip table adapter head out for remachining, the flexure and resulting fretting(?) and wear at the fixture interface was so bad...

We also instituted a regular fixture inspection and maintenance program, to make sure that dings/dents in any contact surfaces for the bolt joints were kept smooth, counterbore faces perpendicular, joints square and tight. This resulted in a lot less downtime and bottlenecking of schedules for the vib test lab, less time spent by various people trying to figure out why things were breaking or vib spectra not being to spec.

 

[IRstuff]

We never found out. We made the cross-bracing changes, but also switched to a hydrostatic-bearing table. The hydrostatic bearings supposedly constrains the slip table from angular motions.

However, I suspect that the crossbracing wouldn't have helped on the original table. You can imagine the system as an inverted pendulum mounted on an angularly unconstrained cart. A linear input to the cart results in a torque to the entire system. Without some sort of constraint, the system will rock, unless the cart

TTFN

FAQ731-376