As engineers search for the best
answer to their particular NVH issues
(noise, vibration and harshness), the
first question they have to ask is
whether they need an active isolation system or a
passive system.
This applies whether they are mounting the
source of the NVH on isolators to prevent it
transmitting problems to nearby equipment (a
stamping press, metal machining installation,
heavy motor, compressor or conveyor, for
example), or where a sensitive piece of equipment
is being mounted on isolators to remove it from
surrounding NVH (such as a control panel, or even
a complete computer room).
Both active and passive systems involve isolators
that join two entities yet allow each to remain in its
natural state without influence extending to the
other.
Active systems are usually more expensive
and more complex. They involve a system that
supports and isolates an object by providing
motion that counteracts the disturbing motion
that needs to be isolated. This requires extremely
fast-acting actuators and high-gain feedback
devices to sense the motion and control the
actuators.
Passive systems are more straightforward.
Passive isolators are mounts that support and
isolate an object without the contribution of any
outside energy source.
Passive systems are often therefore an ideal lowmaintenance
solution to isolation of equipment
such as vibratory conveyors and vibration tables,
large drying machines, centrifugal separators,
measuring tables and machinery, commercial
laundry machines, compressors and electronic
equipment involved in production automation.
A traditional solution to passive isolation needs
has long been provided by metal coil springs.
Coil springs also have a constant spring rate.
This means that they will only achieve their
published isolation effectiveness at a single
operating point. Any variation of load or input
frequency will dramatically change the isolation
achieved. Equipment mounted on coil springs
can react quite violently to start-up and shutdown
conditions. Also, coil springs are only able
to provide isolation in one axis. If there is any
lateral loading, coil springs will not provide good
isolation.
Metal can also fail quickly in harsh environments,
deteriorating through rust and corrosion. Metal
construction does not allow for much overload
capacity. Overloading a coil spring will most
generally cause it to fail. Because coil springs can
cause great damage if they do fail, spring cages or
other means must be used to protect equipment
and personnel from this possibility.
Alternative solutions for high efficiency
isolation include tough rubber Airmount
pneumatic isolators and fabric-reinforced
rubber MarshMellow springs, both of which are
manufactured by Firestone and proven globally.
They are distributed by Air Springs Supply, in sizes
from those which will each support a few dozen
kilograms, to large models that will each support
as much as 40 tons.
AIRMOUNT ISOLATORS
Airmounts are, in effect, tremendously
tough, highly engineered balloons identical
in construction to the air springs used in the
suspension of semi-trailers, luxury coaches,
trains and all-terrain vehicles. Their high rate of
isolation is exactly why they are used for vehicular
suspension .
Airmounts are the only passive isolator that
operates on the principle of compressing a gas
rather than deflecting a solid. Because of this,
Airmounts are the passive isolator with the lowest
natural frequency (and the lower the frequency,
the lower the transmission). They are so effective,
in fact, that they are used beneath some hospital
operating beds and to support weapons
guidance. Using Airmounts severe occupational
health and safety issues have been overcome
through isolation efficiencies of 90-99%.
Quiet, stable and ultra-effective
Airmount isolators
UNIQUE QUALITIES OF
AIRMOUNTS
Airmounts are also the only passive isolator that
can have a reduction in the natural frequency.
With the use of an auxiliary reservoir, the natural
frequency can be lowered to improve isolation
effectiveness.
Airmounts are also unique in that they have a
variable spring rate, which allows the natural
frequency to remain nearly constant with changes
in pressure and load. This allows for the use of the
same Airmount at different mounting points on
unevenly loaded equipment.
Because Airmounts are using air as the isolation
media rather than a solid material, they provide
less of a pathway for transmitting high frequency
vibration. Therefore, they reduce structurally
transmitted vibration, and hence reduce noise
transmission. Also, Airmounts do not exhibit the
chatter that conventional coil springs do.
Further, Airmounts are also adaptable to handling
changing loads or overload conditions. Any given
Airmount can easily handle a substantial change
in load by simply adjusting the air pressure,
whereas coil springs are designed for a very
narrow opening range.
And finally, because of the nearly constant natural
frequency, Airmounts react in a much less violent
manner than coil springs during start-up and shutdown
conditions as the input frequency changes.
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