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LEARN SOME TERMS AND DEFINITIONS  !!!

Vibration Basics                           

          Definition                                                              

Before giving the details of the calculations required for suspensions able to provide vibration and shock protection, the vibration terminology used in this catalogue is defined.

Coupling
The natural frequencies of a system are interdependent. If the natural frequency for a given degree of freedom is excited, it may cause a response at the same frequency in the other degrees of freedom due to the coupling effect. When designing a suspension for a particular application, coupling must be taken into consideration. The stiffness and damping of the mountings in each axis should be considered as well as the moments of inertia for the rotational behavior.

Creep
Irreversible change with time of the deformation under constant load.

Critical damping
Damping value for which the return to equilibrium is asymptotic and without oscillation.

Damping
Damping is the force that reduces the amplitude of a vibration. There are two main types of damping :
- Frictional damping (static friction): the force is independant of the relative speed of the suspended mass relative to the supporting base. For a given displacement the force is constant. For movement, a force has to be applied that is at least as great as the damping force. - Viscous damping (characteristic of shock absorbers) : the force is dependent on the speed of the suspended assembly relative to the supporting base. Viscous damping is dynamic and does not change the static equilibrium position.

Decibels
Dimensionless measure of the ratio of two parameters of a vibration system. dB = 10 log₁₀ X₁/X₂ where X₁ and X₂ may be, for example, the amplitudes of vibration above and below the mounting system.

Deflection
Displacement associated with the application of a static or dynamic force or a combination of forces.

Degrees of freedom
The number of degrees of freedom is the number of independent parameters that determine the position of the system at a given moment. - Linear movement along an axis. - Rotation round an axis. Freely suspended assemblies have 6 degrees of freedom -3 mutually perpendicular and 3 rotational.

Elastic mountings
Elastic mountings have a wide range of both elastic and damping properties.

Elasticity
Elasticity describes the reversible deformation of an elastic mounting where the deformation is approximately proportional to the load.

Elastomer
Elastomer is a general term for any natural or synthetic rubber. The choice of elastomer to be used for a mounting system depends on the environment and application.

Forced vibration
Vibration imposed on the system either by its own operation or by its surroundings.

Matching
For guidance and radar systems, it may be necessary to use a set of mountings with matched dynamic characteristics (stiffness and damping) to avoid coupling and improve stability.

Natural frequency
When a system is displaced from its position of equilibrium and released, it oscillates at its natural frequency. The natural frequency of a system is usually expressed in Hertz. In theory, there are as many natural frequencies as degrees of freedom.

Natural vibration
Vibration at the natural frequency in the absence of external influences.

Noise (not to be confused with sound)
Noise is a random vibration. It can be broken down into a set of unrelated, elementary components. Vibration can cause the emission of audible noise.

Noise is usually either airborne or propagated through the structure. An elastic mounting system only reduces structure borne noise.

Q factor
A quantity which is a measure of the sharpness of resonance or frequency selectivity of a resonant oscillatory system having a single degree of freedom. Note: Q = 1 / (2C/Cc) when C/Cc is a not too high damping ratio.

Random vibration
Random vibration is the most commonly occurring form of vibration. It is characterised by the fact that neither the amplitude nor frequency of the vibration are constant, as opposed to sinusoidal vibration.

Resilience
Ability of a system to return to its initial position after being subjected to one or several forces.

Resonance
Resonance of a system in forced oscillation exists when any change however small, in the frequency of excitation causes a decrease in a response of the system.

Spectral power density
Expression used to describe random vibration. It is expressed in g² /Hz. The power spectral density of a quantity is the mean-square value of that part of the quantity passed by a narrow-band filter of contrefrequency f, per unit bandwidth, in the limit as the bandwidth approaches zero and the averaging time approaches infinity.

Stiffness
Characteristics of a mounting system. The force applied divided by the deflection. An equifrequent mounting system has the same stiffness in all directions.

- Linear stiffness :
Kx along the X axis : longitudinal movement
Ky along the Y axis : transverse movement
Kz along the Z axis : vertical movement

For each axis, the linear stiffness of the suspension is the sum of the linear stiffnesses of all the mountings.
Kx = S Kx     Ky = S Ky     Kz = S Kz
Linear stiffness is expressed in N/m or lb/in.

- Torsional stiffness :
Cx along the X axis = roll
Cy along the Y axis = pitch
Cz along the Z axis = yaw