Presentations by Wayne Tustin
Vibration and Shock Isolation Trends and Solutions (page 2)
Figure 11
(courtesy Sorbothane)
Figures 11 and 12 show preparation for an explosive barge test, as in Video Clip 4.
Figure 12
(courtesy NUWC Keyport)
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.
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.
Figure 13
(courtesy NUWC Keyport)
Let's talk about Isolator Selection Suppose that an assembly within the "cocooned" equipment resonates at 10 Hz, Fn=10 Hz.
Suppose also that your vehicle shakes at 10 Hz, Ff= 10 Hz. Resonance. That's bad.
Suppose further that the
only isolators available result in natural frequencies
Fn of
100, 10, 5 and 2 Hz. Which should we use?
4 diagrams, much pointing, discussion.
What have we learned?
Fn 100 Hz too stiff, Fn 10 Hz compounds the problem, Fn 5 Hz would work, Fn 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.
Figure 14
(courtesy Unholtz-Dickie)
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 Fn of 5 Hz. This should work fairly well.
Suppose that test specs change. Now test down to 5 Hz.
If test Ff matches 5 Hz isolation system Fn, 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.
Figure 15
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.
Figure 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.
Figure 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.
Figure 18
Figure 18 shows a transportation application.
Figure 19
I promised that if I had time, I'd explain those C/CC 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 C/CC represents a specific amount of friction or damping, called "critical damping".
Attempts to show what we mean by the phrase "critical damping
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.
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.
Figure 20
(courtesy Ericsson)
BG-PWB pads pulled away by sudden shock. This weakness not observable by any known test.
Figure 21
(courtesy Ericsson)
Close-up of ductile rupture from previous slide. Estimate that only 10% was a real solder joint.
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.
