The greatest challenge in designing an effective
isolator is to maintain good performance at the low vibration amplitude
inputs typical of ambient building floor vibration. Isolator specifications
are often based on measurements done with the isolator placed on
a “shaker table”
with very high amplitude input levels. Such testing,
with input amplitudes on the order of millimeters, yields unrealistic
performance expectations and is misleading as results will not reflect
the actual performance in use.
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
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.
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.
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.
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.
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.