Vibration isolation is a procedure through which the transmission of oscillating disturbances or forces is reduced. The ideal isolator is one that will support the equipment being isolated without transmitting any dynamic forces. An isolator with infinite static stiffness and zero dynamic stiffness will achieve this goal. Although this ideal isolation cannot be obtained in practice, it can be approximated through a wide range of devices. This approximation occurs over a limited frequency band and methods of increasing this band were investigated. The goal of this thesis was to further our understanding of mechanical systems that can approximate the ideal isolator behaviour. To compare the various devices the blocked transfer dynamic stiffness was defined. This value was found to represent the isolator properties without the additional complication of the equipment being isolated as happens in traditional transmissibility methods. Three classes of devices were distinguished namely isolators, vibration-absorbing isolators (VAl) and amplified vibration-absorbing isolators (AVAI). The last two types exploit nodalisation to reduce the dynamic stiffness over a limited frequency range. The focus of this work is the broadening of the effective low stiffness bandwidth of amplified vibration-absorbing isolators by adapting system characteristics. If the excitation is tonal time-varying these devices can be used successfully. Two novel adaptive amplified vibration-absorbing isolators were introduced and studied in the time and frequency domains. The type I AVAI uses flexible reservoir walls to vary the isolation frequency. The type II device incorporates a heavy metal slug. Both devices use variable pressure air springs to change their stiffness. The use of air springs are convenient, offers low damping and can be used in an application such as a pneumatic rock drill handle to eliminate the need for a control system. Conceptual design methodologies for both damped and un-damped fixed and adaptive isolation frequency AVAls are presented. To determine the effects of tuning the equations were transformed in terms of constant frequency ratios and the variable stiffness ratio. The devices can be controlled using an optimisation approach, but care should be taken since the method could be unsuccessful in some cases. The design was then applied to a pneumatic rock drill. This application was particularly demanding because the stiffness had to be large enough for the operator to remain in control of the drill, yet low enough to offer isolation. Extensive measurements of drill vibration at a test facility found that the maximum acceleration values were 18.72 m/s2. The maximum allowed under the proposed European Union legislation is 10 m/s2 for short durations. The excitation consisted of a large tonal component and wide-band noise. The tonal component contributed ~50% of the total weighted equivalent acceleration experienced by the operator and a vibration absorbing isolator should therefore be an ideal solution. The measurements also showed that the excitation frequency is a function of the supply air pressure. By using the supply air pressure to feed the air spring the device could be made self-¬tuning. Numerical simulation showed that there is only a slight difference between using the supply pressure and forcing coincidence of the excitation and isolation frequencies. It was also found that the vibration levels could be reduced to below 10 m/s2<./sup> in some cases.
Thesis (PhD (Mechanical Engineering))--University of Pretoria, 2006.