Best Isolation HRS? SRA? Zoethecus? Symposium?

Geoffkait

Nice to know that four course on vibration I took in university can be boiled down to one easy sentence Geoff. It is far more complex in study than what you have stated. It has everything to do with inertia.

"The heavy mass used in Vibraplane is employed to lower the resonant freq. of the "mass on spring" system and has nothing whatsoever to do with inertia, as you suggested. System resonant frequency is a (strict) function of the Sq. Root of spring rate over the mass."

What you talk of is a simple machine, a text book example not a real world application. What you talk of is a single degree of freedom spring and mass system. It does not particularly apply. Yes, additonal mass will lower the resonant frequency of the platform but is not the sole or most important reason for the mass. You could simply damp, clamp and change the resonant frequency with out the mass. You have completely ignored Newton and Kinetics all in one foul swoop. How about your transmission ratios, harmonic ground motion, forced periodic motion etc.?

"Thus, if vibraplane employed an even heavier mass, all things being equal, it would be an even better isolator. But not because of the reason you gave."

Beg to differ here sir. A larger mass requires larger surface areas and will not necessarily translate to better isolation. It introduces another set of issues. It is a large reason as to why it operates the way it does. Mass limits movement the isolators decouple from the source of the vibration. It is similar in operation to an inertia base under large rotating pieces of equipment, conversly though they are trying to limit the vibration transmitted into a building. Mass is key in keeping things in place and limiting both movement and transmission of vibration.

j-badov, the equivalent performance (resonant frequency) for the VP would be obtained by simultaneously *reducing* the mass and reducing the spring rate of the thin-skin bladders.

Therefore, by inspection, mass cannot be the *key* to the design of the mass-on-spring isolating system you seem to think it is.

Also, a larger mass doesn't necessarily require more surface area as higher density of mass will solve that particular problem.

Maybe dust off the old university textbooks, eh? :-)

Geoffkait, or you could partially fill the bladders with fluids as Kworks does. Filling the center with sand would also lower the resonant frequency.

I would use abs. minimum no. of bladders (of correct geometry) -- that is one sure way to lower resonant frequency and improve performance...fluids in bladders could overdamp things... haven't tried it, might be worth a shot, but generally don't like fluids or sand...

Hey Geoff with out getting into a pissing match here. We may be talking about two different things.

I am not discussing the resonant frequency here. I am discussing the use of mass to keep the platform in place. Things at rest tend to stay at rest. The higher the mass the more extenal force it takes to move the object.

If the platform were directly coupled to a stand via cones etc. the seismic churning of the earth will disturb the platform. The better the isolation the less the transmitted force. The larger mass simply keeps in place. With out the mass the transmitted forces will allow the platform to move sympathetically with the seismic vibrations.

As for the density of the materials used, VP utilizes a large billet of steel underneath to make up the 150 lbs this thing weighs. A denser material would be much more expensive and in many case more difficult to work with.

Design of industrial and institutional vibration platforms is not the simple mass-on-spring you seem to think it is. They employ over 15 PE's on staff. Check out many of the vibration and acoustic control companies around and you will find that most of these companies are made up of PHD's. Not because they want to look important but because it is a very difficult field to understand.

If it were as easy as you seem to think it is they would just employ a couple simple dense slabs of Corian separted by a couple of springs. Or every one would just use two pieces of MDF with a Thermarest mat in between.

Rest assured my textbooks are well dusted. I am PE Mechanical Engineer and work with a number of Vibration consultants on my projects on a daily basis.