SHOCK & VIBRATION ISOLATION

Shock perturbations are rather abrupt and thus possess energy over a wide frequency range. They can excite many (including high-frequency) resonances in an isolated system. Therefore an ideal shock isolation system should low-pass filter the high-frequency components of the shock input, transmitting only a negligible amount of shock’s high-frequency energy. Considering that a soft vibration isolation system, with its low natural frequency, behaves like a mechanical low-pass filter with a low corner frequency, it would hinder the transmission of most of the high-frequency components of shock inputs.

With low natural frequencies, an isolated system subject to shock perturbation experiences excessive motion at those frequencies. Introduction of damping into isolators will alleviate the excessive motion at the natural frequencies. Examples of damped isolators include wire rope springs and coil springs combined with viscous dampers, which effectively limit oscillations while maintaining isolation performance.

(a)

(b)

Wire rope vibration/shock isolator (a) and springs plus viscous damper vibration/shock isolator (b)

Even with the presence of damping in shock isolators, there must be adequate free deflection in the system to prevent the isolators from bottoming out.  Refer to the shock and vibration isolation primer, for more information on shock isolation.

Unlike shock, which is a sudden and impulsive perturbation applied to machinery or equipment, vibration refers to periodic mechanical oscillation around an equilibrium state, typically occurring at frequencies associated with the equipment’s operating conditions.

An industrial fan isolated on coil springs

Vibration isolation solutions serve two primary purposes:

1) to reduce the transmission of vibrations from the support structure to the isolated equipment, and

2) to limit the transfer of vibration energy from the equipment back to the support structure,

while controlling the motion of the isolated object. This isolation is typically achieved by placing resilient elements—such as elastomeric (rubber) mounts, coil springs, or air springs—between the equipment and its supporting structure.

As shown in the shock and vibration isolation primer, passive systems, do not address all the requirements/concerns of an ideal isolation scheme, simultaneously; their effectiveness in addressing one requirement is normally realized at the expense of either not being effective in or adversely affecting other vibration isolation requirements. This is also true for the more elaborate ‘two-stage mounting’ (also known as ‘double mounting’) scheme.

As an alternative to passive solutions, the parameters of which are fixed and can not be changed once such solutions are put in place, one can use adjustable-passive (also known as semi-active) vibration isolation schemes, such as DEICON’s Computer Controlled Air Isolation System. Such systems allow for in-situ adjustment of the mounting parameters modifying the isolation system such that it best suits the instantaneous vibration isolation and shock isolation requirements on hand, while ensuring the integrity of installation.