24
AIRMOUNT
®
VIBRATION ISOLATION
At design height and without an auxiliary reservoir, the single
and double convoluted parts follow this pattern; i.e., the lateral
rate varies from
1
/
5
to
1
/
2
of the vertical rate (only the larger
high strength parts get as high as
1
/
2
). Notice the #22 is
approximately
1
/
4
(
)
. Going back to the original example of
a vibrating screen which weighs 16,400 Ibs. mounted on four
#22’s (@ 9.5 inches), a side load of 1,300 pounds (325 x 4)
would deflect the entire suspended mass by one inch.
TRIPLE CONVOLUTED AND
REVERSIBLE SLEEVE TYPE PARTS
Both of these types are unstable laterally (except for the
1M1A). Due to low natural frequencies, both can be excellent
isolators; however, do not use these two types as Airmount
isolators without consulting Firestone (for special guidelines
and precautions).
DESIGN ENVELOPE
Adequate clearance should be provided around the Airmount to
prevent puncturing or rubbing of the bellows. The maximum
diameter @ 100 psig for each Airmount (bellows) is shown just
above the cross sectional view of the air spring.
SAFETY STOPS
It is normally recommended that positive stops be installed
in all
directions; i.e., into compression, extension, and laterally.
Positioning of the vertical stops depends upon the amplitude of
movement, both during normal operation and startup and shut-
down. A good “rule of thumb” is ±
1
/
2
inch from design height for
vertical stops and also ±
1
/
2
inch (horizontally) for lateral stops.
INITIAL INSTALLATION
NEVER use Airmounts to lift equipment into place, due to the lat-
eral instability at lower air spring heights as discussed
previously. Equipment should be rested on stops set
slightly
below design height and raised into position for isolation.
STARTUP AND SHUTDOWN /
RESONANCE AND AMPLIFICATION
Resonance is the condition where the forced frequency of the
vibrating system is at the natural frequency of the suspension.
When this happens, AMPLIFICATION of movement occurs.
Going back to our vibrating screen example again, if the nor-
mal stroke is
5
/
16
of an inch, during startup and shutdown (as
the machine goes through resonance), the amplitude of
movement will be multiplied somewhat. So, while the machine
is building up to speed and slowing down, the stroke may be
amplified in the range of
1
/
2
to 1
1
/
2
inches. The longer the
machine takes to go through resonance (to build up to, or
slow down from full operating speed), the larger the amplitude
of movement.
ISOLATING AN UNBALANCED MASS
The primary concern in this case is the amplitude of move-
ment. It is dependent on:
1) The ratio of the unbalanced moving mass to the total
suspended mass, and
2) The ratio of the speed of the unbalanced moving mass
(forced frequency) to the natural frequency of the Airmounts.
The addition of damping to the isolation system (shock
absorbers) will reduce the large amplitude of movement
experienced during resonance.
If the amplitude of movement is too great, one possible solu-
tion would be to add an inertia base in order to increase the
ratio of the total suspended mass to the moving unbalanced
mass. A good “rule of thumb” is 10:1, respectively.
LOW PRESSURE OPERATION
The lateral rate of a single and double convoluted style
Airmount
decreases with decreasing internal air pressure
(becomes unstable). Consult Firestone if you plan on oper-
ating an Airmount at less than 40 psig.
EFFECT OF AN AUXILIARY RESERVOIR
There is a direct relationship between natural frequency
and isolation effectiveness. Generally, the lower the natural
frequency, the better the isolator (or higher percentage of
isolation). As previously mentioned, a double convoluted
Airmount has a lower natural frequency than a single con-
voluted type (of the same size) because it has more
internal air volume. We can use this principle to lower the
natural frequency of an air spring by adding an auxiliary
reservoir (pressure vessel) externally to the Airmount. This
effectively increases the air spring volume and reduces its
natural frequency.
In order for the reservoir to work properly, there must be a
free flow of air between the Airmount and reservoir.
Therefore, it should be mounted as close as possible to the
Airmount. Additionally, a bead ring attachment is the best
end closure choice as the hole in the upper mounting plate
can be sized as large as the inside diameter of the bellows
(at the top). A
3
/
4
" NPT air inlet will restrict the flow of air
somewhat, but can be used as long as it is understood that
there is some throttling effect.
Going back to the #22 example, an auxiliary reservoir of
three times the internal volume of the air spring at design
height (approximately 10 gallons) will reduce the natural
frequency from 106.8 cpm to 90.2 cpm. The spring rate
also decreases, from 1,324 Ibs./inch to 944 Ibs./inch.
325
1,373
