Conflicting Mounting Requirements
An ideal mounting system for diesel generators and other running machinery on-board watercrafts
should:
1. support the weight without excessive static deflection
2. be soft while the vessel is not moving, i.e., the
propulsion engines are off and peace, tranquility and quiet are needed.
This will prevent the transmission of vibration and noise,
through the hull, to the living quarters.
3. be stiff when the vessel is moving or in rough waters and the isolated machine need to be
securely attached to the structure
4. be damped around the resonant frequency (to mitigate the resonance problem)
and highly underdamped at high frequencies to avoid transmitting noise and high frequency
vibration to the hull
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When sailing, noise and vibration produced by diesel generators and other machinery are normally
masked by the noise and vibration created by the propulsion engines,
propellers induced turbulence, etc. Thus, noise and vibration isolation
attributes of diesel generator mounting system have a lower priority than
its shock isolation attributes requiring mounts with large stiffness and damping.
On the other hand, while the vessel is docked (and not on shore power) or anchored,
diesel generators are the main source of noise and vibration, which if not isolated
properly, will transmit their noise and vibration through the hull
to the living quarters, disturbing the occupants.
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No one passive solution quite satisfies all requirements
listed above. Even the popular
double mounting,
while effective at high frequencies, has less than desirable low-frequency isolation
effectiveness. Shock isolation of double mounting
is also inferior to that of single mounting. In addition, double mounting
imposes unfavorable weight penalty and space penalty. The complexity and
difficulties of converting an existing mounting system to double mounting
does not help the appeal of double mounting in retrofit applications.
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Why Air Isolation
The reasons for choosing air isolation are: 1) softness providing low natural frequency enabling air
to provide the highest degree
of low-frequency isolation of any type vibration isolator, 2) large load-bearing without, despite its
low stiffness, excessive static deflection,3) negligible overall damping enhancing
high-frequency isolation, 4) adjustability, and 5) light weight. The low stiffness and low damping which give air its
legendary vibration and noise isolation, are also what make
shock isolation attributes of air less than desirable.
DEICON’s Computer Controlled Air Isolation System enhances shock isolation capabilities of
air mounts while maintaining their highly attractive vibration isolation, noise isolation,
and light weight attributes.
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Types of Air Mounts/Isolators
An air mount (air spring) is simply an enclosed compressed air. They have been used in vibration
isolation for the last 70 years. Commercially available air mounts are of two basic types known as
convolution (bellow) type and pneumatic-elastomeric type.
Figure (a) depicts a single convolution air mount. The envelope is made of two or three
relatively thin plies of fabric reinforced rubber, sealed to hold pressure typically up to
100 or 150 Psi (7 to 10 Bars) depending the number of plies. The metal plates on the top
and bottom are for locating and loading the air spring. The shape and relatively thin wall
of convolution type air springs are mainly meant to hold the air and do not provide much
lateral stiffness.
Figure (b) depicts a pneumatic-elastomeric mount. These springs have a thick-walled
cylindrical body and the top shaped as a diaphragm coupling the body to the top plate
(where the load sits). This thick wall in conjunction with 3 steel rings built into the
make up of the mount prevents the bulging of the wall providing a transverse stiffness
about equal to the axial stiffness of the mount. Isolators of this type, although more
effective than a elastomeric isolators but do not provide as much isolation effectiveness as
convolution type air mount.
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(a)
(b)
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Control System
Automatic control of DEICON's Computer Controlled Air Isolation System
is performed by a collection of solenoid and servo-valve subsystems
under the constant supervision of a computer,
all housed in a control cabinet. In addition to feedback control of either pressure in individual
mounts or mounting height of the machine, the control system switches (on-demand) between
different pre-programmed isolation states.
Various degrees of automation can be incorporated into the control system
to initiate the change in the state of the
isolation system. At one end of this automation spectrum, a selector switch can be used for
commanding the computer to switch the states. At the other end of the spectrum, the control
computer monitors the accelerations and on detecting
accelerations beyond a certain level, switching of the states gets enabled.
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The two time traces in the figure shows the experimentally measured acceleration of a 500 lb (240 Kg) machine
mounted on an uncontrolled and controlled air mount. Clear from this figure, active
tuned damping can introduce an appreciable amount of damping into the system.
Comparison of the two traces in this figure indicates the increase in damping form
2.5% to 21%, i.e. in excess of 8 fold increase, more than adequate for taming the machine
bounce at resonance contributing to shock isolation.
Active Tuned Damping of Air Mounted Systems
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Active Stiffness Control
With optional active stiffness control, the stiffness of the mount can be lowered (softened) or
increased (stiffened) without physically
changing the mounting arrangement or connecting the mount to a bulky and heavy
auxiliary air reservoir.
The three traces in the figure show the experimentally measured power spectrum of acceleration of
a 500 lb (240 Kg) machine mounted on a stiffness-controlled air mount for 3 different
controller gains.
Evidient from the figure, the resonant frequency has changed from 2.25 Hz to 4.5 Hz, i.e.
a factor of 2 change in resonant frequency indicating a factor of 4 (400%)
change in stiffness.
It should be pointed out that stiffness variation occurs in a matter of milli-seconds.
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Lowering the natural frequency (softening the mount) further enhances the
isolation performance of the air mounted system beyond what the uncontrolled system provides.
Increasing the natural frequency (stiffening the mount) prevents the genset from
undergoing excessive motion in response to abrupt disturbances such as starting up or shutting down the engine.
Active Stiffness Control of Air Mounted Systems
Vibration Isolation Effectiveness
The effectiveness of rubber (blue trace) and air mounts (red trace) in isolating
a 175 KVA diesel generator on-board a luxury vessel is compared in the figure
depicting power spectrums of acceleration at a location on the engine room floor.
Clear from the figure, the vibration isolation effectiveness of air
mounts by far exceeds that of rubber mounts.
More performance data.
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Shock Isolation and Structural Integrity
In addition to or in place of
active damping , stiffening in
lateral and heave directions are used to enhance the shock isolation
attributes of DEICON's air isolation system.
Considering that air mounts do not provide sufficient lateral
stiffness needed to secure the diesel generator while the boat is in
motion, additional lateral support is provided by a set of smaller
mounts which will be engaged, when needed. These mounts not only
provide lateral stiffness, they also increase the heave stiffness of
the mounting system. Even more heave stiffness is realized by replacing
part of the air in the main mounts with liquid. Thus, by selecting any of
the three states, via a selector switch or automatically, the mounting system
under computer control changes from soft
with small lateral stiffness (providing excellent noise and vibration isolation),
to somewhat stiff with lateral stiffness, and stiff with lateral stiffness
(providing excellent shock isolation). The figure shows one mounting foot of a
diesel generator isolated from the structure by 1 ‘main’ and 2 ‘lateral’ air mounts.
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