Vibration Control of Equipment

Since vibration in the audible frequency range can easily be transformed directly into noise, or propagated to some other part of the building and then radiated as noise, it is of extreme importance to control all vibration in the mechanical room to within tolerable limits. Ideally, the mechanical room should be located well away from critical areas in the building, but when this is not possible, even greater care must be taken with the control of vibration. Although it might seem that the best approach should be to furnish all pieces of equipment with vibration isolators, this is not necessarily so. The first line of attack should be to ensure that each piece of equipment is selected to produce minimum noise and is operated under its specified conditions. For example, one should choose, where possible, rotating equipment units in preference to the equivalent reciprocating ones since, in general, the latter type produces much more objectionable noise. Furthermore, it is extremely important to see that each piece of equipment is balanced, both statically and dynamically, to within the recommended tolerances [27].

A major concern to the design engineer is to avoid resonances both in the equipment, in its supports, and in the building structure. At resonance, a large vibrational amplitude is developed, which may be accompanied by excessive radiated noise and stress. The stresses set up during the resonance may ultimately lead to fatigue in equipment or its support, or – even more disastrously – in the building structure (i.e. the floor)! One should, therefore, carefully check whether any equipment is to be operated close to (within ±25%) any critical speed in the machine, structural resonance in its support, or building structural resonance. Since the floor resonances and machine critical speeds are almost fixed, the operating speeds must be chosen to be well away from both of these. The only concern, then, is the support. In general, support resonances may be avoided by making the support very much less flexible than the flexibility that would result in resonance or by using a separate isolator with much higher flexibility. Such an increase in flexibility automatically leads to increased static deflection.