In the process of deciding on a vibration isolator for a particular application, there are a number of critical pieces of information which are necessary to define the desired functionality of the isolator. Some items are more critical than others, but all should be considered in order to select or design the appropriate product.
Some of the factors which must be considered are:
Weight, Size, Center-of-Gravity of the Equipment to be Isolated — The weight of the unit will have a direct bearing on the type and size of the isolator. The size or shape of the equipment can also affect the isolator design since this may dictate the type of attachment and the available space for the isolator. The center-of-gravity location is also important as isolators of different load capacities may be necessary at different points on the equipment due to weight distribution. The locations of the isolators relative to the center-of-gravity — at the base of the equipment versus in the plane of the C.G., for example — could also affect the design of the isolator.
Types of Dynamic Disturbances to be Isolated — This is basic to the definition of the problem to be addressed by the isolator selection process. In order to make an educated selection or design of a vibration/shock isolator, this type of information must be defined as well as possible. Typically, sinusoidal and/or random vibration spectra will be defined for the application. In many installations of military electronics equipment, random vibration tests have become commonplace and primary military specifications for the testing of this type of equipment (such as MIL-STD-810) have placed heavy emphasis on random vibration, tailored to the actual application. Other equipment installations, such as in shipping containers, may require significant amounts of sinusoidal vibration testing.
Shock tests are often required by many types of equipment. Such tests are meant to simulate those operational (e.g., carrier landing of aircraft) or handling (e.g., bench handling or drop) conditions which lead to impact loading of the equipment.
Static Loadings Other than Supported Weight — In addition to the weight and dynamic loadings which isolators must react, there are some static loads which can impact the selection of the isolator. An example of such loading is the load imposed by an aircraft in a high-speed turn. This maneuver loading must be reacted by the isolator and can, if severe enough, necessitate an increase in the isolator size. These loads are often superposed on the dynamic loads.
Allowable System Response — This is another basic piece of information. In order to appropriately isolate a piece of equipment, the response side of the problem must be known. The equipment manufacturer or user should have some knowledge of the fragility of the unit. This fragility, related to the specified dynamic loadings will allow the selection of an appropriate isolator. This may be expressed in terms of the vibration level versus frequency or the maximum shock loading which the equipment can endure without malfunctioning or breaking. If the equipment manufacturer or installer is experienced with vibration/shock isolation, this allowable response may be specified as the allowable natural frequency and maximum transmissibility allowed during a particular test.
The specification of allowable system response should include the maximum allowable motion of the isolated equipment. This is important to the selection of an isolator since it may define some mechanical motion limiting feature which must be incorporated into the isolator design. It is fairly common to have an incompatibility between the allowable “sway space” and the motion necessary for the isolator to perform the desired function. In order to isolate to a certain degree, it is required that a definite amount of motion be allowed. Problems in this area typically arise when isolators are not considered early enough in the process of designing the equipment or the structural location of the equipment.