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Damping and Isolation -

There are two facets of vibration management: isolation and damping. Isolation is theprevention of vibrations from entering a system. Damping is the absorption of the

vibration energy that is entering the system and dissipating it by changing the kinetic

energy of vibration into a different form of energy. The two forms of vibrationmanagement are different from each other, but often are used in conjunction to achieve

the desired performance.

Transmissibility

In order to understand what isolation and damping are and how to apply them, we mustfirst understand transmissibility and natural frequency. Transmissibility is a measurement

used in the classification of materials for vibration management characteristics. It is a

ratio of the vibrational force being measured in a system to the vibrational force enteringa system.

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Graphs may be plotted among transmissibility and frequency ratio. Figure below

illustrates these graphs. This may be noted down that all of the curves pass through (0, 1)& ( 2, 1). The transmissibility is more than 1 for frequency ration 0 < / n < 2.Throughout this range of frequencies damping is most effectual in dropping the

transmissibility. For frequency ratio more than 2 transmissibility is always < 1. For thisfrequency range damping enhances transmissibility. The frequency range 2< /n < is very good for maintain transmissibility low however damping must be as low as

possible.

Region of Isolation

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This is important in design of machine mounts so that vibration is exactly isolated. Theunbalanced force is a machine is transmitted to the ground like a shaking force through

its mount on the foundation. Hence, appropriate isolators must be used to drop or

eliminate shaking forces transmitted to the ground.

(i) For > 2n , output is smaller than the input. Hence, for effective isolation thenatural frequency of the system must be less than 0.707 times the operational speed or

harmonic frequency i.e. to be isolated.

(ii) For > 2n , damping enhance transmissibility therefore the damping must bekept as low as possible.

(iii) If < 2n , more and more damping must be utilized to maintain transmissibilityas low as possible.

Isolation

Earlier we defined isolation as the prevention of vibrations from entering a system; we

also defined transmissibility as the percent of vibrational energy that is being transmittedthrough a structure.

Mathematically these two terms are simply related.

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In the region where Transmissibility percent is greater than 100%. there is no isolation;

this is known as the region of amplification. Also take note that this region ofamplification is centered around the natural frequency where, as we mentioned earlier,

large amplitudes of deflection occur. See the region of isolation in the fig below

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Damping and isolation are very different approaches to vibration management, but are

often used interchangeably in conversations and specifications. It is important to recall

the definitions mentioned above and understand that isolation and damping have differenteffects on the system. In fact, having a large amount of damping has a negative effect onisolation and vice versa. In Figure below, it can be seen that as the damping ratio

increases the region of isolation decreases.

Damping and Isolation - Selection

The easiest way to determine whether a damper or an isolator is needed is to identify thefrequencies that are of concern. Selecting the correct isolator will shift the naturalfrequency lower and move the frequencies of concern into the region of isolation,

preventing them from penetrating the system. When the systems natural frequency can

not be shifted lower and the frequencies of concern are located near or at the naturalfrequency, damping is the appropriate method of vibration management.