- In the process of deciding on a vibration isolator for a
particular application, there are a number of critical pieces
of information which are necessary to define the desired functionality
of the isolator. Some terms are more critical than others but
all should be considered in order to select, or design the appropriate
product.
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- Some of the factors which must be considered are:
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- Weight, size, center-of-gravity of the equipment to be
isolated
- Obviously, the weight of the unit will have a direct bearing
on the type and size of the isolator. The size, or shape of the
equipment can also affect the isolator design since this may
dictate the type of attachment and the available space for the
isolator. The center-of-gravity location is quite important in
that isolators of different load capacities may be necessary
at different points on the equipment due to weight distribution.
The locations of the isolators relative to the center-of-gravity,
at the base of the equipment versus in the plane of the c.g.,
for example, could also affect the design of the isolator.
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- Types of dynamic disturbances to be isolated
- This is basic to the definition of the problem to be addressed
by the isolator selection process. In order to make an educated
selection or design of a vibration/shock isolator, this type
of information must be defined as well as possible. Typically,
sinusoidal and/or random vibration spectra will be defined for
the application. In many installations of military electronics
equipment, random vibration tests have become commonplace and
primary military specifications for the testing of this type
of equipment (such as MIL-STD-810) have placed heavy emphasis
on random vibration, tailored to the actual application. Other
equipment installations, such as in shipping containers, may
still require significant amounts of sinusoidal vibration testing.
Disk Jockeys have a special requirement to be met.
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- Shock tests are often required for many types of equipment.
Such tests are meant to simulate those operational (e.g., dance
floor movement) or handling (e.g., bench handling or drop) conditions
which lead to impact loading of the equipment.
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- Static loadings other than supported weight
- In addition to the weight and dynamic loadings which isolators
must react to, there are some static loads which can impact the
selection of the isolator. An example of such loading is that
imposed by an aircraft in a high speed turn. This maneuver loading
must be reacted to by the isolator and can, if severe enough,
cause an increase in the isolator size. These loads are often
superposed on the dynamic loads.
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- Allowable system response
- This is another basic bit of information. In order to appropriately
isolate a piece of equipment, the isolator selector must know
the response side of the problem. The equipment manufacturer
or user should have some knowledge of the fragility of the unit.
This fragility, related to the specified dynamic loadings will
allow the selection of an appropriate isolator. This may be expressed
in terms of the vibration level versus frequency or the maximum
shock loading which the equipment can endure without malfunctioning
or breaking. If the equipment manufacturer or installer is somewhat
knowledgeable about vibration/shock isolation, this allowable
response may be simply specified as the allowable natural frequency
and maximum transmissibility allowed during a particular test.
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- The specifications of allowable system response should include
the maximum allowable motion of the isolated equipment. This
is important to the selection of an isolator since it may define
some mechanical, motion limiting feature which must be incorporated
into the isolator design. It is fairly common to have an incompatibility
between the allowable "sway space" and the motion necessary
for the isolator to perform the desired function. In order to
isolate to a certain degree, it is required that a definite amount
of motion be allowed. Problems in this area typically arise when
isolators are not considered early enough in the process of designing
the equipment or the structural location of the equipment. DJ
equipment must support horizontal and vertical movement.
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- Ambient environment
- The environment in which the equipment is to be used is very
important to the selection of an isolator. Within the topic of
environment, temperature is by far the most critical item. Variations
in temperature can cause variations in the performance of many
typical vibration/shock isolators. Thus, it is quite important
to know the temperatures to which the system will be exposed.
The majority of common isolators are elastomeric. Elastomers
tend to stiffen and gain damping at low temperatures and to soften
and lose damping at elevated temperatures. The amounts of change
depend on the type of elastomer selected for a particular installation.
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- Other environmental effects - from humidity, ozone, atmospheric
pressure, altitude, etc. - are minimal and may be typically ignored.
Some external factors that may not be thought of as environmental
may impact on the selection of an isolator. Such things as fluids
(oils, fuels, coolants, etc.) which may be in the area of the
isolators may cause a change in the material selection or the
addition of some form for protection of the isolators.
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- Service life
- The length of time for which an isolator is expected to function
effectively is another strong determining factor in the selection
or design process. Vibration isolators, like other engineering
structures have finite lives. Those lives depend on the loads
imposed on them. The prediction of the life of a vibration/shock
isolator depends on the distribution of loads over the typical
operating spectrum of the equipment being isolated. Typically,
the longer the desired life of the isolator, the larger that
isolator must be for a given set of operating parameters. The
definition of the isolator operating conditions is important
to any semi- reliable prediction of life.
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- What
are we talking about?
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- There are a number of terms which should be understood before
entering into a discussion of vibration and shock theory. Some
of these are quite basic and may be familiar to the users of
this catalog. However, a common understanding should exist for
maximum effectiveness.
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- Acceleration - rate of change of velocity with time.
Usually along a specified axis, usually expressed in "g"
or gravitational units. It may refer to angular motion.
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- Amplitude - the maximum displacement from its zero
value position.
- Compression - when specified as a direction for loading
- a deformation caused by squeezing the layers of an object in
a direction perpendicular to the layers.
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- Damping (c) - the mechanism in an isolation system
which dissipates a significant amount of energy. This mechanism
is important in controlling resonance in vibratory systems.
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- Disturbing frequency (fd) - the number of oscillations
per unit time of an external force or displacement applied to
a vibrating system. fd = disturbing frequency.
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- Durometer (hardness) - an arbitrary numerical value
which measures the resistance to the penetration of the durometer
meter indenter point; value may be taken immediately or after
a very short specified time.
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- Fragility - is the highest vibration or shock level
that can be withstood without equipment failure.
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- "G" level - an expression of the vibration
shock acceleration level being imposed on a piece of equipment
as a dimensionless factor times the acceleration due to gravity.
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- Isolation - the protection of equipment from vibration
and/or shock. The degree (or percentage) of isolation necessary
is a function of the fragility of the equipment.
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- Load deflection curve - the measured and recorded
displacement of a mounting plotted versus an applied load.
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- Natural frequency (fn) - the number of cycles (expressed
as Hertz or cycles per second) at which a structure will oscillate
if disturbed by some force and allowed to come to rest without
any further outside influence.
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- Random vibration - non-sinusoidal vibration characterized
by the excitation of a broad band of frequencies at random levels
simultaneously.
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- Resonance - A vibratory system is said to be operating
at resonance when the frequency of the disturbance (vibration
or shock) coincides with the system natural frequency.
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- Set - is the amount of deformation never recovered
after removal of a load. It may be in shear or compression.
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- Shear - when specified as a direction for loading
- a deformation caused by sliding layers of an object past each
other in a direction parallel to the layers.
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- Shock Pulse - a shock pulse is a transmission of kinetic
energy to a system, which takes place in a relatively short length
of time compared to the natural period of this system. It is
followed by a natural decay of the oscillatory motion. Shock
pulses are usually displayed as plots of acceleration vs. period
of time.
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- Spring rate - is the force required to induce a unit
deflection of spring. A steel spring has a very linear relationship
between force and deflection. Elastomeric springs may or may
not be linear depending on the amount of deflection due to the
load.
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- Static deflection (ds) - the deflection of the isolator
under the static or deadweight load of the mounted equipment.
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- Transmissibility (T) - is a dimensionless unit expressing
the ratio of the response vibration output to the input condition.
It may be measured as motion, force, velocity or acceleration
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