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Mechanical Equipment Isolation: Isolator Selection Guide

Ciaran Spillane

In the past, unwanted machine vibration could be isolated by placing the equipment on top of cork or felt. However, with greater expectations and expanded knowledge, such methods of isolation are now inadequate, both in large building structures and in the general industrial environment. 

In building construction, improved design allows lighter but inherently more flexible structures to be used. At the same time, increasingly powerful air-conditioning and auxiliary equipment is needed and may often be installed in upper level plant-rooms. Equipment vibration can therefore be a major problem in such flexible structures. Similarly, in general industry, the output of vibration producing equipment is increasing whilst the community and workforce demand improved living and working conditions, free from vibration or structure-borne noise. Both situations require engineered vibration control systems which give predictable results appropriate to the surrounding environment. 

Isolation Efficiency Chart 

This chart illustrates the theoretical relationship between isolation efficiency, disturbing frequency and static deflection (or dynamic natural frequency) for a simple isolation system on a rigid foundation. It is also grouped into zones suggesting isolation efficiency ranges appropriate to different applications. 

To use the chart, determine the lowest rotation speed of the equipment and consider this to be the disturbing frequency. Move vertically to intersect the diagonal line corresponding to the percentage isolation required; then move horizontally left and read the dynamic natural frequency, or right to read the static deflection required of the mount. 

It is sufficient to use static deflection to select the correct isolator for spring mounted systems; but for rubber and pad mounts, selection should be made on the basis of dynamic natural frequency. 


Isolation Selection Guide

This chart provides a selection guide for isolator type according to given values of operating speed, and suggested levels of isolation efficiency in certain operating locations. 

Two values for each isolation efficiency are given, one for basement or on-grade installations, the other for upper level installations where some allowance is made for flexibility of the supporting structure.