Vibration Isolation

Vibration problems are solved by considering the system as a number of springs and masses with damping. It is sometimes possible to reduce the problem to a single mass supported by a spring and a damper.

If the vibration is produced by a motor inside a machine, it is usually desirable to ensure that the frequency of motor oscillations (the forcing frequency) is well above the frequency of the natural resonance of the machine on its support. This is achieved by altering the mass or stiffness of the system as appropriate.

The method of vibration isolation is very easy to demonstrate with a weight held from a rubber band. As the band is moved up and down very slowly the suspended weight will move by the same amount. At resonance the weight will move much more, but as the frequency is increased still further the weight will become almost stationary. In practical circumstances springs are more likely to be used in compression than tension, but the principles are exactly the same.

A further method of vibration control is to attempt to cancel the forces involved using a Dynamic Vibration Absorber. Here an additional "tuned" mass-spring combination is added so that it exerts a force equal and opposite to the unwanted vibration. They are only appropriate when the vibration is of a fixed frequency.

Active vibration control, using techniques akin to active noise control, is now coming into use.

Important:- Intuitive attempts to reduce vibration from machinery can sometimes instead aggravate the problem. This is especially true when care was originally taken to minimize vibration at the time of design, manufacture and installation.

See also: Foundation Stiffness, Six Degrees of Freedom, Vibration Isolator.

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Subjects: Noise & Vibration