Abstract:
Vibrating machinery like rock drills and compactors are becoming more prominent in modem industry. The vibrations of these machines can damage surrounding structures and foundations and be harmful to their operators. Hand arm vibration syndrome is one example of serious injuries suffered by operators of these machines. Due to the fact that these machines need to vibrate, vibration absorbers that minimise the vibrations of the machines cannot be used. In such cases vibration isolators are necessary to isolate the vibration between the vibrating machine and other bodies like the handle or foundations. A tuned vibration isolator is a type of isolator that is able to isolate a certain frequency very effectively. These isolators can retain low mass and high stiffness compared to traditional isolators and can obtain complete isolation at the isolation frequency if no damping is present. The liquid inertia vibration eliminator (LIVE) is such a tuned vibration isolator that makes use of hydraulic amplification, which result in a very compact design. A LIVE isolator was designed incorporating the variable stiffness spring and a variable damping mechanism. Equations for the damped natural and isolation frequency of the LIVE isolator were also derived. The reason for changing the stiffness was to be able to adjust the isolation frequency of the isolator to coincide with the excitation frequency that resulted in a more effective isolator. The variable stiffness spring consisted of two leaf springs mounted on top of each other and separated at the centre to stiffen the whole spring assembly. The leaf springs were separated by a wax actuator that was controlled with a closed loop displacement control system to form a smart actuator. A stiffness change of 2.7 times the original stiffness was obtained by separating the springs. The variable damping mechanism was to be able to control the amount of amplification of noise at the natural frequency. An experimental isolator was built and tested and resulted in a tunable vibration isolator. The isolation frequency of the isolator could be shifted from 22.8 Hz to 36.2 Hz and a transmissibility of 10% was achieved over that whole range. The variable damping mechanism increased the viscous damping ratio from 0.001 to 0.033. A control system was designed and implemented that tuned the isolator automatically to the excitation conditions. It incorporated an optimisation algorithm to determine the optimum settings and then kept the isolator at that setting until the excitation conditions change. The whole process was then repeated. A tunable vibration isolator was therefore successfully developed that can be used to isolate tonal vibrations very effectively. The isolation frequency and damping of the isolator can be changed while in operation and a transmissibility of 10% can be achieved at the isolation frequency.
