Vibration Table Setup Sanity Check
Vibration Table Setup Sanity Check
(OP)
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.
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.





RE: Vibration Table Setup Sanity Check
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.
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
Has anyone read IEST-RP-DTE013.1: SHOCK AND VIBRATION FIXTURING?
Is it a document worth getting?
RE: Vibration Table Setup Sanity Check
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.
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
Andrew Gorton, MSc
Noise & Vibration Consultant
www.PapadimosGroup.com
RE: Vibration Table Setup Sanity Check
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Vibration Table Setup Sanity Check
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.
RE: Vibration Table Setup Sanity Check
(1) Modal Testing: Theory and Practice by D. J. Ewins
(2) 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
www.PapadimosGroup.com
RE: Vibration Table Setup Sanity Check
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
btrueblood,
That makes sense.
Andrew,
Those course notes are a great reference.
RE: Vibration Table Setup Sanity Check
TTFN
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RE: Vibration Table Setup Sanity Check
RE: Vibration Table Setup Sanity Check
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.
RE: Vibration Table Setup Sanity Check
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
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RE: Vibration Table Setup Sanity Check
unless the cart is substantially more substantial than the pendulum.
TTFN
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RE: Vibration Table Setup Sanity Check
I say depending on the levels because I've seen some extremely robust tests which have pushed out all the oil faster than the pump can replenish it.
RE: Vibration Table Setup Sanity Check
TTFN
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