Concepts of Vibration Isolation

A motor-driven pump assembly shows a high vibration behaviour at motor drive endbearing as well as the front-bearing of the pump. Figure below shows the schematic of the system. The salient features of the vibration data are as follows.

Motor drive end-bearing horizontal vibration 95 µ pk–pk, vertical 20 µ pk–pk, axial 10 µ pk–pk.

Pump bearing 1 horizontal 85 µ pk–pk, vertical 15 µ pk–pk, and axial µ 20 pk–pk.

What inference can you draw from the above data? Suggest suitable further investigations. The pump is driven by a 3000 rpm AC induction motor.

Another important aspect of vibration analysis is vibration isolation. In these exercises, we shall include some problems on vibration isolation although the basics of the same have been covered elsewhere in this book.

Concepts of Vibration Isolation

The emphasis in this chapter is on diagnosis and control of practical vibration problems. We concluded that for resolving the problem, we must identify the perturbation forces and eliminate them or at least reduce their intensity to the extent possible. We also discussed that in some practical situations, we may not be able to handle the issue of perturbation forces and thus, we need to alter the structural behavior of the component/structure and/or introduce additional damping devices. The other important aspect of this problem is vibration isolation which is procedure by which the undesirable effects of vibration are reduced. The isolation of vibrating system is also extremely important as in absence of it; the vibrations can harm the personnel operating the machine as well as they may harm the surroundings in which the machine is operating.

The vibration-isolation system could be passive in which we utilize a resilient member (stiffness) such as metal springs, pneumatic springs and an energy dissipater (damping) such as cork, felt and elastomers. We dealt with such systems in earlier chapters of this book (chapters 2, 3 and 4). The vibration-isolation system can also be active and is comprised of a servomechanism with a sensor, signal processor and an actuator. Active-vibration isolation is a very complex subject and hence, it is beyond the scope of the present book.

The effectiveness of an isolator is stated in terms of its transmissibility. We had defined this as the ratio of the amplitude of the force transmitted to that of excitation force. We, in this exercise shall deal with some complex problems of passive-vibration isolation.

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