High levels of vibration are essential for the proper operation of vibrating screens. However, this motion imparts high dynamic loads on their support structures leading to premature failure or costly construction. Various methods exist for the attenuation of these forces, but they require undesirable addition of weight to the screen assembly, which can be as much as 130% of the screen mass. More appropriate methods are pendulum, hydraulic and liquid inertia vibration absorbers. These devices can provide similar isolation at only a fraction of the weight increase of current screen isolation methods. The liquid inertia vibration absorber's unique properties make it ideal for the attenuation of screen forces, as this study will show. A mathematical model describing the motion for the vibration absorber was derived. This led to an equation describing the force transmissibility, which was used to show which parameters influence the absorber's performance. The model was extended to take into account the effect of conical port inlets/outlets, which were used to reduce the viscous damping. The effect of viscous damping was quantified using computational fluid dynamics. The mathematical model was used to show how an optimal set of parameters could be found. Two design procedures were developed for the vibration absorber and were then used to design an experimental absorber. The experimental absorber was used to validate the mathematical model. Several practical considerations for the design were discussed and solutions suggested. The stiffness of the absorber was estimated using finite element modelling. Two elastomeric springs of different hardnesses were fitted to the absorber. The softer spring achieved a transmissibility of 16% by 42 Hz. The main stumbling block in reducing the transmissibility even further is the reduction of the damping. The experience gained from the experimental absorber was used to suggest how an absorber could be applied to a screen. An absorber isolating at 12.5 Hz was designed for this purpose. A theoretical design study investigated two possible configurations of absorber fitment. When the absorber was fitted directly to the screen the force transmitted was reduced 7.2 times. Fitting the absorber to the sub-frame gave similar transmissibility results to that of a screen fitted with a sub-frame only, but the mass ratio was only 15%. The outcome of this study is a thorough understanding of liquid inertia vibration absorbers as well as a procedure for their optimal design.
Dissertation (M Eng (Mechanical Engineering))--University of Pretoria, 2006.