Presentations by Wayne Tustin
Vibration and Shock Isolation Trends and Solutions
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.
Figure 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.
Figure 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)
Click here to watch video clip 1
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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,
Ff << Fn
sprung mass (response) displacement D = input D. Since they
are equal, their ratio (called transmissibility or magnification
factor or gain) = 1.
Click here to watch video clip 2
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If we advance the forcing
frequency until it matches the "natural frequency" of the
spring-mass system, so that
Ff = 
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 Ff.
Ff >> Fn
Click here to watch video clip 3
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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.
Figure 3
Consider the SDoF "behavior chart" family of transmissibility curves. Note that the "region of isolation" commences where
Ff= 1.414 Fn
(where all the curves cross) and extends to the right. At the end, Wayne had time to explain C / CC.
When we employ passive vibration isolators, we select isolators with soft spring rate K (giving us a large static deflection d) so that our Ffis higher than 1.414 times our natural frequency Fn.
For example, Ff= 2 Fn or Ff= 3 Fn.
This is possible when we know the forcing frequency. (slide off)
Figure 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 deflection d and the lower natural frequency.
Figure 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/CC 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.
Figure 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".
Figure 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.
Figure 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).
Figure 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.
Figure 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.
