chapter  7
56 Pages

Vibration Damping

Damping is the phenomenon by which mechanical energy is dissipated (usually converted into internal thermal energy) in dynamic systems. Some knowledge of the level of damping in a dynamic system is important in the utilization, analysis, and testing of a system. For example, a device having natural frequencies within the seismic range (i.e., less than 33Hz) and having relatively low damping could produce damaging motions under resonance conditions when subjected to a seismic disturbance. Also, the device motions could be further magnified by low-frequency support structures and panels having low damping. This illustrates that knowledge of damping in constituent devices, components, and support structure is particularly useful in the design and operation of a complex mechanical system. The nature and the level of component damping should be known in order to develop a dynamic model of the system and its peripherals. Knowledge of damping in a system is also important in imposing dynamic environmental limitations on the system (i.e., the maximum dynamic excitation the system could withstand) under in-service conditions. Furthermore, some knowledge of its damping could be useful in order to make design modifications in a system that has failed the acceptance test. However, the significance of knowledge of damping level in a test object, for the development of test excitation (input), is often overemphasized. Specifically, if the response-spectrum method is used to represent the required excitation in a vibration test, it is not necessary that the damping value used in the development of the required response spectrum specification be equal to the actual damping in the test object. It is only necessary that the damping used in the specified response spectrum be equal to that used in the test-response spectrum. The degree of dynamic interaction between the test object and the shaker table, however, will depend on the actual level of damping in these systems. Furthermore, when testing near the resonant frequency of a test object, it is desirable to know the level of damping in the test object. In characterizing damping in a dynamic system, it is important, first, to understand

the major mechanisms associated with the dissipation of mechanical energy in the system. Then, a suitable damping model should be chosen to represent the associated energy dissipation. Finally, damping values (model parameters) are determined, for example, by testing the system or a representative physical model, by monitoring the system response under transient conditions during normal operation, or by employing already available data.