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 resonant motion of the isolated device. Introduction of damping into isolators will alleviate the excessive resonant motion.  Wire rope springs and coil springs paired with viscous dampers are examples of isolators with damping.

(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.

Contrary to shock which is an impulsive, sudden perturbation applied to a machine/equipment, vibration is a periodic mechanical oscillation occurring around an equilibrium state, with frequencies related to the operating conditions of the equipment.

An industrial fan isolated on coil springs

Vibration isolation schemes are to

1. reduce the propagation of support structure vibration to the isolated object (equipment), and
2.  abate the transmission of vibration energy of the equipment to the support structure while limiting the motion of the isolated object (equipment).

Vibration isolation is achieved by placing resilient elements (isolators/mounts), typically elastomeric (rubber), springs, and/or pneumatic (cushion of compressed air) between the isolated object and its support 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.