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Vibration and Shock Isolation
Trends and Solutions
by Wayne Tustin
COTScon West 2001
San Diego, California
December 4-5, 2001
This paper introduces various forms of practical
vibration and shock isolators that modify commercial-off-the-shelf (COTS)
equipment so that it can withstand MIL vibration and shock environments.
Fig. 1 |
Are there any bicycle riders in
the audience?
Here's an example of passive
isolation: road vibrations pass from wheels to handle bars,
possibly damaging your hands and arms.
Consider soft, cushioned
gloves to "isolate" your hands from the handlebars.
At higher road-generated
frequencies, the handlebars still vibrate, but displacements
are smaller, less troubling, less likely to damage you.
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Fig. 2 (courtesy
Sorbothane) |
Consider bonded isolators.
Metallic or plastic pieces were placed in the mold before
the mold was filled with liquid elastomer.
Once (many years ago) all
electronic "black boxes" were thus isolated from aircraft,
automobiles, etc. because components such as vacuum tubes,
relays, etc. were so delicate.
It is difficult to visualize
early car radios in the automobile trunk, or racks of radio
equipment in aircraft and tanks, "floating" on soft isolators.
Some in the audience had a hard
copy of my slides, I asked them to please look ahead at Figure
15, while I performed a little demonstration using a spring
and mass. I wanted to illustrate the words "isolate" and "isolation".
(Demonstrated)
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Then I suggested that we
instead use a controllable source of vibration (an electrodynamic
shaker - similar in principle to an electrodynamic loudspeaker)
to drive a simple one spring, one mass load.
I pointed out when we drive
the load at a very low forcing frequency,
<< 
sprung mass (response) displacement D = input D. Since they
are equal, their ratio (called transmissibility or magnification
factor or gain) = 1.
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If we advance the forcing
frequency until it matches the "natural frequency" of the
spring-mass system, so that
= 
sprung mass (response) D >> input D.
We call this condition Resonance.
How much greater is sprung
mass (response) D than input D? That ratio is often called
"Q".
Today's discussion, however,
centers on isolation. Watch this next video clip, in which
we have further increased .
>>
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Now sprung mass (response)
D << input D.
We call this situation ISOLATION,
our subject today.
Now that we understand what isolation
does for us, let's examine the engineering involved.
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Fig. 3 |
Consider the SDoF "behavior
chart" family of transmissibility curves. Note that
the "region of isolation" commences where
=
1.414
(where all the curves cross)
and extends to the right.
At the end, Wayne had time to explain C /  .
When we employ passive vibration
isolators, we select isolators with soft spring rate K (giving
us a large static deflection d)
so that our is
higher than 1.414 times our natural frequency .
For example, =
2
or =
3 .
This is possible when we
know the forcing frequency. (slide off)
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Fig. 4 |
Please guess: which system
has the higher natural frequency. The two springs are identical
- have the same stiffness K.
I hope you all recognize
that B is the stiffer, with the lesser static deflection d
and the higher natural frequency. Conversely, A is the softer,
with greater static defection d and
the lower natural frequency.
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Fig. 5 |
Let's discuss Elastomeric and Helical Isolators
See cross-sections of three bonded rubber-to-metal isolators.
Natural rubber was long used for engine mounts and other passive
isolators. Neoprene, however, better withstands oil and grease.
Back on Figure 3, a typical C/
is 5%, so that resonant magnification "Q", a brief experience
while a piece of rotating machinery is coming up to speed,
is perhaps a tolerable 10.
(return to fig. 5) However, elastomers are not
good at temperature extremes.
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Fig. 6 |
Steel coil springs make excellent isolators,
but their "Q" can exceed 100. Thus the spring at left is surrounded
by an air bag that "breathes" through an orifice. Air
friction lowers "Q".
Alternately, right, springs can be packed with
stainless steel mesh. This provides damping (friction).
Lowers "Q".
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Fig. 7 |
We have been discussing a concentrated mass
on a single spring - that oversimplifies the "real world".
Let's mentally shift to a "black box" that
can be described as a volume of many springs, many masses
and many dampers. How will we isolate that?
With several isolators. Wayne used Figure 7
to discuss where and how to locate the several isolators.
Over time, big old radio receivers and transmitters
and other electronic hardware went "solid state", they got
smaller and more rugged, so that isolation today is seldom
used in commercial and personal electronics.
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Fig. 8 (courtesy
Sorbothane) |
One exception: the hard disk drive (HDD) inside
your computer generates some vibration, may trouble other
HDDs nearby. Certainly vibration radiates from the computer
as sound.
In the other direction, HDDs are vulnerable
to vibration and shock (such as a laptop computer being dropped
or the extremely severe shock of the HDD being "clicked" into
place in a rack or chassis - likened to an ammunition clip
being clicked into a gun).
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Fig. 9 (courtesy
Sorbothane) |
A
solution: cushioning between the HDD and its housing. We'll
look, a bit later, at "air bags" and inherently-damped cable
isolators. But for HDD problems, small pieces of elastomeric
material are inexpensively molded and quite easily inserted. |
Fig. 10 (courtesy
Martin Testing Labs) |
When will Wayne discuss COTS
Equipment? Now.
In a sense, history is repeating.
MIL services seek COTS equipment (NDE or non-developmental
equipment) in military applications that (among other damaging
environments) often reach temperature extremes and that involve
severe vibration and/or shock.
Computers and other equipment
may be "cocooned" inside temperature-controlled, vibration-and-shock-isolated
boxes.
Observe the "cocooned" unit, ready
for a vibration test. Note the "cable" vibration isolators,
discussed later.
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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