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jcrisco65
07-18-2002, 09:59 PM
Previously I asked about vibration damping a servohydraulic testing
machine while we were fatiguing specimens. I received several
suggestions and requests for info. We solved the problem fairly
inexpensively (there were much more expensive solutions). Below is a
summary:



When we moved into the 4th floor of an office building, we located
the load frame (Instron Bi-axial Servohydraulic 8521S) along an
outside wall near a load-bearing column. The frame was placed on the
floor (vinyl tiles on a five inch thick concrete floor) with the
factory-supplied rubber pads under the feet.

When we ran the machine in fatigue, we found that the floor would
vibrate around the machine and even as far as about 40 feet away.
The vibration was not confined to our floor as people above and below
us complained that their computer monitors were shaking in an
annoying fashion. We were using a sinusoidal waveform with an
amplitude of approximately 3 mm and a frequency of 16 Hz, although
the excess vibration did not tail off completely until the frequency
was below 5 Hz.

We looked at air cushion isolators from Instron, MTS, and McMaster
Carr as well as a spring isolator from Kinetics Noise Control. We
settled on four "single tire" air cushions from McMaster Carr ($100
each, made by Goodyear) and found that this virtually eliminated the
transmission of vibration from the frame to the floor. However,
especially at lower air pressures with a low frequency (e.g. 5 Hz)
waveform, the frame "bounces" during fatigue testing. This bouncing
produces a marked inertial loading on the load cell. We have
overcome this problem by maintaining the air cushions at high
pressure (100 psi) and running with a faster frequency (e.g. 16 Hz)
waveform.


NOTE: we also did some investigations into inertial loading during
fatigue testing and found that there was significant inertial
loading. So a word of caution - if your are testing in fatigue be
very careful about interpreting your load readings!

--


__________________________________________________ _________________________
J.J. Trey Crisco, Ph.D.
Dir. Bioengineering Laboratory
Assoc. Professor, Dept. of Orthopaedics
Brown Medical School / Rhode Island Hospital
Adjunct Assoc. Prof.
Division of Engineering, Brown University
Research Dir. NOCSAE

1 Hoppin Street
Coro West, Suite 404
Providence, RI 02903
Tel: 401-444-4231
Fax: 401-444-4418
email: joseph_crisco@brown.edu
__________________________________________________ _________________________

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