Pneumatic Isolators for OEM Applications
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Gimbal Piston™ Isolators
The Gimbal Piston™ utilizes proprietary pneumatic damping techniques, which include air flow restrictors and a unique geometry. It is lightly damped and highly responsive to typical, low-amplitude ambient floor vibrations, yet achieves very high damping for gross transient disturbances, such as sudden load changes or bumping the top plate. The result is that Gimbal Piston Isolators provide superior isolation yet will virtually eliminate any gross disturbance within a few seconds. The Gimbal Piston™ can also stabilize
isolated systems with relatively high centers of gravity without
compromising isolation. |
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The
Gimbal Piston™ Air Isolator provides outstanding
isolation in all directions for even the lowest input levels. It
is lightly damped and highly responsive to typical, low-amplitude
ambient floor vibrations, yet achieves very high damping for gross
transient disturbances, such as sudden load changes or bumping the
top plate. The result is that Gimbal Piston Isolators provide superior
isolation yet will virtually eliminate any gross disturbance within
a few seconds. It can also stabilize isolated systems with relatively
high centers of gravity without compromising isolation. The Gimbal Piston Isolator design is unique in its ability to maintain its stated resonant frequency and high level of attenuation in even the most quiet, real, floor environments. The performance is linear to such low amplitudes because the design is virtually free of friction and therefore able to avoid rolling friction to static friction transitions. Every other system that we have tested at levels typical for floor vibration exhibits either a higher resonant frequency than claimed or a substantial increase in transmission through the isolator mount. We stress the importance of performance specifications
at low levels because we have repeatedly observed, in our own testing
and in many as-used installations, that better performance is much
easier to achieve at greater amplitudes and higher frequencies.
Back to
top Our innovative Isolator allows a thin-wall, rolling diaphragm seal to accommodate horizontal displacement by acting as a gimbal. Instead of using a cable-type pendulum suspension, the Gimbal Piston Isolator carries the load on a separate top plate that has a rigid rod extending down into a well in the main piston. The bottom of the rod has a ball-end that bears on a hard, flat seat. The result is an inherently flexible coupling which allows horizontal flexure in the isolator as the ball simply rocks (without sliding or rolling) very slightly on the seat. The approach works extremely well, even with sub-microinch levels of input displacement, because the static friction is virtually the same as the rolling friction. Horizontal motion is simply converted to the usual vertical diaphragm flexure but out of phase: one side of the piston up, the other down, in a gimballike motion.
Back
to top Thick- Wall Rubber Diaphragms. Most commercial isolators employ an inexpensive, thick-walled rubber diaphragm in the piston to achieve vertical isolation. Because of the relative inflexibility of these elements, low amplitude vibration isolation performance is compromised. Though such a system feels “soft” to gross hand pressure, typical low-level floor vibration causes the rubber to act more like a rigid coupling than a flexible isolator. Sealed Pneumatic Isolators (Passive). Sealed air isolators do not automatically adjust to load changes. The primary limitation of such systems is that they must be made too stiff to be effective isolators. For example, a passive isolator with a true 1.5 Hz resonant frequency would drift several inches vertically in response to small changes in load, temperature, or pressure and require constant manual adjustment. Thus, no practical sealed isolators are designed with such low resonant frequencies. Bearing Slip Plates. In theory, bearing slip plates should allow horizontal isolation by their decoupling effect. In practice, for such a design to work at low amplitudes, it would require precision ground, hardened bearings with impossibly small tolerances. The commercially available versions cannot overcome the static frictional forces at low amplitudes to get the bearings rolling at all. In addition, all such systems are difficult to align initially and easily drift out of calibration. Homemade Assemblies. Homemade isolation systems - often a steel or granite slab placed on rubber pads, tennis balls, or air bladders - will work only if the disturbing vibrations are high frequency and minimal isolation is required. While all isolators use the principle of placing a mass on a damped spring, their performance is differentiated primarily by spring stiffness: the stiffer the spring, the higher the resonant frequency. Thus, homemade solutions are limited by their high resonant frequency. A Gimbal Piston™ Isolator with a 1.5 Hz
vertical resonant frequency begins to isolate at 2 Hz and can reduce
vibration by over 95% at 10 Hz. A tennis ball under a steel plate
with a 7 Hz resonant frequency begins to isolate above 10 Hz and
reduces vibrations by 90% at 30 Hz. But most building floors exhibit
their highest vibrational displacements between 5 and 30 Hz, so
that a tennis ball or rubber pad actually makes the problem worse
by amplifying ambient frequencies between 5 and 10 Hz.
  Photo courtesy of Argonne National Laboratory Gimbal PIston™ Isolators
are routinely used for the most demanding electron microscope installations. |
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