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Vibration and Shock Isolation
Trends and Solutions (cont.)
by Wayne Tustin
COTScon West 2001
San Diego, California
December 4-5, 2001
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Fig. 11 (courtesy
Milton F. Meeds, NUWC Keyport)
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Figures 11 and 12 show preparation
for an explosive barge test, as in Video Clip 4.
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Fig. 12 (courtesy
NUWC Keyport)
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The shocks received by that
equipment resemble (to a degree) the shocks that would be
received in actual service aboard a surface vessel or a submarine
in the event of a "near miss" explosion, with energy traveling
through the water and through the ship's structure.
The degree of that resemblance
is often described by SRS - the Shock Response Spectrum. Are
you involved with SRS? Perhaps you'd like to telephone me.
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 Click
here to watch video clip 4
If you do not have Real Player to watch the video above, just
click
here to download it for free
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Fig. 13 |
Let's talk about Isolator
Selection Suppose that an assembly within the "cocooned"
equipment resonates at 10 Hz,  =
10 Hz.
Suppose also that your vehicle
shakes at 10 Hz,  =
10 Hz. Resonance. That's bad.
Suppose further that the
only isolators available result in natural frequencies
of
100, 10, 5 and 2 Hz. Which should we use?
4 diagrams, much pointing, discussion.
What have we learned?
100 Hz too stiff,
10 Hz compounds the problem,
5 Hz would work,
2 Hz much too soft. d
about 2.5" or 60 mm. Any maneuver would cause impact with
other structure.
To summarize:
Isolators that are too stiff are useless.
Isolators that are too soft bring on new problems.
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Fig. 14 (courtesy
Unholtz-Dickie) |
Let's discuss isolating
active equipment - machinery. Is a shaker used in your
lab? Undesirable vibration is introduced into the buildings
in which shakers are used.
Let's isolate our shaker,
using elastomeric springs. Perhaps we need to test down to
10 Hz. Isolators with static deflection d
of 0.4 inch or 10 mm will give us a natural frequency
of 5 Hz. This should work fairly well.
Suppose that test specs
change. Now test down to 5 Hz.
If test
matches 5 Hz isolation system ,
the shaker body will move out of phase (with the load) with
large relative displacement. Shaker stroke may be reduced
thereby.
We could use air bags for
greater static deflection d as
in picture. That would not be very stable.
A better remedy: increase
shaker body mass. Attach the shaker body to a 10X or 100X
concrete mass (in a below-floor pit) which in turn is isolated
from the building.
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Fig. 15 |
Let's talk a little more about damping.
Let's diagram our hardware. We have a "sprung
mass" M and a spring with stiffness K.
We also have a friction or damping element C.
C is not always visible, but is always
present. No system exists without some damping.
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Fig. 16 (courtesy
Ace Control) |
Consider the suspension of your automobile,
supporting the body mass. You have four springs.
You also have four friction elements, variously
called dampers or dash pots or shock absorbers. Don't try
to drive without them!
Here are some friction elements - dampers -
that you can see.
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Fig. 17 |
Let's say a little more about "cable" isolators.
Visualize strands of stainless steel wire twisted
into a cable and then wrapped into a helix. Rubbing (friction
or damping) accompanies flexing of the isolator assembly.
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Fig. 18 |
Figure 18 shows a transportation application.
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Fig. 19
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I promised that if I had time, I'd explain those C/
ratios we saw earlier in Figure 3, identifying members of
the family of transmissibility graphs. C represents how much
friction or damping we have, while
represents a specific amount of friction or damping, called
"critical damping".
Attempts to show what we mean by the phrase "critical damping".
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 Click
here to watch video clip 5
If you do not have Real Player to watch the video above, just
click
here to download it for free
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Video
Clip 5 brings you the best example (that I know of) of critical
damping. Immediately after the howitzer fires, expanding gasses
and the projectile exit to our left. The reaction force immediately
drives the barrel to our right, from which it returns to its
starting position. We don't want it to "spring back" immediately,
however, because it would then oscillate about the original
position. Instead, a "dash pot" extracts energy, converting
motion into heat, and greatly slowing down the return journey.
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Fig. 20 (courtesy Ericsson) |
BG-PWB
pads pulled away by sudden shock. This weakness not observable
by any known test. |
Fig. 21 (courtesy Ericsson) |
Close-up
of ductile rupture from previous slide . Estimate that only
10% was a real solder joint. |
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To summarize,
We reviewed the purpose of isolators and demonstrated isolation
to you. First I hand-activated a spring mass model. The you
saw a video clip of a demonstration on a shaker.
That led to the concept of transmissibility
or magnification.
We need isolators and first discussed elastomers
e.g. rubber. Better: metallic isolators with much better hot
and cold behavior.
We saw the role of damping or friction in limiting
the magnification "Q" value to say 10 if we must pass through
resonance in coming up to speed.
Following Wayne's presentation, several people
asked for CD-ROM copies of Wayne's PowerPoint slides. Hence
we are posting them on our Web site.
back to the top
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You can easily and inexpensively advertise in this space.
Please visit our ads page for more information. |
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Check our Vibration
and Shock Glossary. You will find important words and
their definitions. This list evolved from Wayne's 50 years
of work experience and it's updated frequently.
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Are you interested in dynamics (vibration, mechanical
shock, noise, acoustics, automotive buzz, squeak, rattle, etc.)? Please
click here
to visit vibrationandshock.com, the other ERI web site.
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ERI is seeking expertise and a desire
to teach about Life Cycle Environmental Profiling, as described
near the front of MIL-STD-810F. Please contact Wayne
Tustin (805/564-1260).
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A number of organizations shortsightedly
laid off their shaker operators. Now they are trying to resume
testing, and find (surprise!) that no one knows how to operate
their shaker. Or even understand the manuals. Or maintain
the shaker system. Or interpret test specs. On a short-term
assignment, can you teach those subjects? Please contact Wayne
Tustin (805/564-1260).
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Electro Static Discharge (ESD) to consult
and present short training courses. Please, email
Wayne Tustin, sending bio and web address, if possible.
Thanks!
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