Antiresonance vibration isolation has long been a well known, studied and applied method for alleviating vibrations in stiff structures where small static deflection and a low transmissibility is needed, making it ideal for use in the rotor-craft industry. Most prior arts focus on passive single frequency antiresonance vibration isolation, while some, most notably liquid inertia vibration isolators, are adapted to actively isolate vibrations at more than one frequency. Very little literature is found on the adaptation of mechanical pendulum antiresonance vibration isolators for in-flight tunable multiple frequency isolation, and although these systems predate the more modern liquid inertia type isolator, there is merit in their further development and use as low cost, robust and low maintenance isolators. A feasibility study on the performance of changing each fundamental design variable to achieve antiresonance tuning concludes, that for the antiresonance frequency shift range of interest in this dissertation, no specific design variable change quantifiably outperforms another with respect to tuning the antiresonance. Concept designs are created and investigated, finding the superior method of tuning the vibration isolator based on other criteria like overall weight, design simplicity, practicality, robustness and reliability. Shifting the tuning mass on the pendulum arm is deemed to be the superior concept, with respect to the helicopter being developed, and a tunable multi-frequency pendulum antiresonance vibration isolation system with a sliding concentrated mass is developed with ADAMS multi-body dynamics software and SolidWorks. The isolation system along with a full scale dummy fuselage and transmission-rotor assembly is manufactured and experimentally tested. Initial experimental results show antiresonance frequencies 10Hz higher than the design targets, this phenomenon is later discovered to be related to friction in the pin joints of the pendulum hinges, increasing the system overall stiffness. Needle roller bearings are inserted to eliminate the friction, and experimental and ADAMS model results are again compared showing good correlation, with experimental results isolating close to the three target frequencies within 3% error. An astonishing level of vibration isolation is observed with the largest transmissibility obtained at the three frequencies being 0:5%. This dissertation proves the concept of a tunable mechanical pendulum vibration isolator, and its design methodology, particularly with respect to shifting the position of the tuning mass. Suggestions for further work are: to implement this system with an actuation mechanism, further research on the effects of friction in isolators and the use of said phenomenon as a tuning method, development of isolators implementing the other concept of changing the design variables and a comparison between the effect of normal damping and friction damping on vibration isolation.
Dissertation (MEng)--University of Pretoria, 2017.