Planned Isolation

The isolation of machinery to prevent the transmission of vibration and noise has become one of the important phases of modern building engineering. Light weight construction and locating mechanical equipment on upper floors, adjacent to quiet areas, increases the requirement for vibration control. The use of isolation is primarily for reducing the effect of the dynamic forces generated by moving parts in a machine into the surrounding structure.

Isolation Theory

Every machine by its very function of operation creates a vibration or shock of varying intensity or amplitude. The requirements for isolating this vibration depend upon the local conditions of installation. Three principle factors control the selection of an isolator for a particular machine. The first is the weight to be supported, the second is the disturbing frequency of the machine and the third is the rigidity of the structure supporting the machine.

Vibration is a force and establishing an opposed force can effectively reduce its transmission. This is accomplished by incorporating a truly resilient material, which when subjected to a static load, deflects and by so doing establishes the natural frequency of the isolation system. When the natural frequency of the isolation system is lower than the operating or disturbing frequency of the supported machine, each cycle of vibratory force finds the resilient material in the returning phase of its cycle. The effectiveness of the isolation then, is a function of the distance of return travel remaining at the time of impact.

This is best explained by visualizing each cycle as an individual blow. This blow drives the isolator into dynamic deflection. When the force of the blow is spent, the isolator starts its return at its own frequency. Since the frequency is slower than that of the blows, it is obvious the return will be only partial before next impact. Because the isolator possessed the energy with which to complete its return to equilibrium, the unaccomplished portion of travel represents the amount of opposed energy that will absorb the next impact. Therefore, the greater the ratio of disturbing to natural frequency the more efficient the isolation, subject to diminishing returns. It is evident any truly resilient material capable of the required static deflection, operating within its elastic limits will produce the required results. The essential factor of an isolator is it must be truly resilient. It must have the ability to return to its original height when loads or forces are removed. Such a material, when loaded within its elastic limits, will have a long effective life.

In the case where vibrations are present due to a constant steady-state oscillation of imbalance in a machine a precise formula may be applied with reasonable certainty of attaining desired results. in substance, this formula is based on the ration of the operating frequency of the machine or other equipment to be isolated, to the natural frequency of the isolated system. The disturbing frequency f d of a machine can be readily determined either by measurement or by the known operating characteristics of the equipment. Generally the lowest R.P.M. in the system is used as the disturbing frequency.

The natural frequency f n of a machine set on resilient material is a function of the static deflection of the resilient material under the imposed load. For practical purposes the natural frequency f n is described by the formula:

Vibration Elimination

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