Vibration Isolation

Shock Isolation

Shock can be described as a motion in which the velocity changes very suddenly causing a single or multiple impact into the ground or surrounding structure. Consequently, the theory of isolation outlined above is not applicable as the forcing (disturbing) frequency is usually low (less than 100 cpm). However, best results are obtained with isolators providing low natural frequencies. The design and selection of an isolation system for impact machines such as drop hammers and punch presses, must take into account that the energy of each blow is dissipated before the next impact occurs. when necessary, use of inertia mass and damping devices may be employed to attain desired stability.

Noise

An additional benefit of machinery isolation is the associated noise reduction. Introduction of a resilient medium between the equipment and the structure acts to break the path of structural borne noise. This results in noise reduction in areas outside the machinery room and to limited extent, in the area around the machine. This is achieved by reducing the sounding board effect normally associated with machinery mounted solidly to the structure. use of isolation does not reduce sir-borne noise if found to be above allowable levels. Air-borne noise must be treated acoustically with acoustic enclosures or other sound absorbing devices.

Location

In evaluating and selecting an effective isolation system, consideration must be given to identifying critical areas. In buildings where equipment is located directly over or adjacent to quiet areas, offices, libraries, studios, operating rooms, etc., a very critical condition exists, requiring extremely high isolation efficiency. Basement locations and areas with high ambient noise such as factories, warehouses, power plants, etc., are considered less critical and can tolerate larger transmitted forces. In addition, the geographic location of the building is also important. if located in a designated earthquake zone, the potential seismic forces produced will dictate the incorporation of restraints to protect the resilient isolated equipment.

Materials

After the desired static deflection has been established the isolating medium is selected. Steel springs and elastomers are generally accepted material that meet the requirements of resiliency for isolation.

Elastomers describes natural rubber and the various synthetic materials such as neoprene. It possesses inherent damping and sound deadening characteristics and can be molded into any shape or hardness and vulcanized to metal. The term rubber-in-shear is used to describe the deflection in shear rather than compression. Elastomers limit static deflection to a maximum of 1/2 inch and can be incorporated to provide damping in spring isolators.

Metal springs become preferable when the required static deflection exceeds 1/2 inch. they can be designed to provide large static deflection and depending on the application are available either free standing, housed or vertically restrained. Free standing springs are usually unrestrained devices which must be laterally stable meeting a minimum of 0.8 ration of spring diameter to compressed height. housed springs are used to counteract the effect of lateral forces, while vertically restrained springs are used for equipment whose weight varies with the addition or removal of large amounts of water and for rooftop equipment subject to wind loads. Since steel springs will transmit noise, introducing an elastomer pad to break the direct structural borne noise path is necessary.

Definitions

Frequency is defined as the number of complete cycles of oscillation that occur per unit of time.

Natural frequency is the number of complete cycles of oscillation a mass will vibrate in a given unit of time if a force displaces it from its equilibrium position and allows it to vibrate freely.

Disturbing frequency is the frequency of vibration produced by an unbalanced, rotating or reciprocating movement in mass.

Resonance is when the disturbing frequency equals the natural frequency of the isolation system resulting in an amplification of vibration producing an excessive violent movement. This is common as the system passes through resonance.

Static deflection is the distance an isolator will deflect under the static or dead weight of the equipment.

Spring rate is the weight required to induce a spring to deflect a given distance.

Damping is the ability of an isolation system to absorb and dissipate a significant amount of energy and convert it to heat. The importance of damping is to reduce the tendencies of the isolated equipment to "bounce." This occurs during acceleration or slowing of the equipment when the speed reaches resonance and the amplification of the unbalanced forces occurs. It will be noted, damping reduces the efficiency of the isolator. However, this reduction is so minor it can well be ignored or compensated, by a very small increase in the static deflection requirements for the isolation system.