Bearing Optimization to Minimize Vibrations

In most hydropower applications fluid film journal bearings are used. The bearings are often design to be stiff and with high damping, i.e. low bearing clearances to increase the fluid film pressure. In this thesis a hydropower unit that suffers from poor stability due to low damping is studied. The focus of this thesis will be to investigate how the bearing parameters affect the bearing stiffness and damping coefficients and also how these coefficients affect the hydropower units dynamics.Simulations of the system and bearings are preformed in a rotordynam- ics software called RAPPID which also includes a Navier-Stokes based solver for tilting pad journal bearings.From simple mechanical systems it is shown that high damping coeffi- cients in the bearings will not guarantee maximal stability, but instead decrease it. The same behaviour is also observed for the more com- plex hydropower unit. With high stiffness and damping in the bearing the motion in the bearing that generates damping for the system will be almost zero for some modes. Instead a bearing designs that lowers the stiffness in order to allow larger motion in the bearing, resulting in higher damping for the system are found to be the best solution in order to maximize the stability for the hydropower unit.Validerat; 20130611 (global_studentproject_submitter