Analysis of the performances of a tilting pad thrust bearing with polymeric layered pads using multiphysics modeling

Hydrodynamic bearings are usually installed in rotating machines which are characterized by heavy working conditions owing to high rotational speeds and loads. Examples of these applications are machines for electrical power generation, oil extraction and for auxiliary devices. Oil-film tilting pad thrust bearings are generally used for supporting the high axial load of the turbine shaft in vertical hydroelectric units or the axial loads in turbo machines. In these applications, the pad coating material plays an important role. Usually the white metaln(Babbitt) is used as pad coating material. However, the present trend is to replace the Babbitt with a polymeric material layer such as PTFE or PEEK, improving the bearing performances and extending its operating conditions. This leads to a reduction of the bearing overall dimensions as a consequence of the load capacity increase. Apart the friction and the resistance to chemical attacks properties of the polymeric layer, the main cause on the improved performances of the bearing is the compliance of the pad layer. In particular the polymeric layer reduces the typical pad crowning allowing a more uniform pressure distribution over the pad and a reduction of its maximum value with respect to Babbitt metal pads. Therefore, the design of layered pad requires a deeper investigation such as thermoelastohydrodynamic (TEHD) analysis, including oil thermal effects and bearing thermal deformation. In the paper, the performances of Babbitt metal and polymeric layered pads of a standard size offset-pivoted tilting pad thrust bearings of a vertical axis unit are compared using multiphysics software able to manage simultaneously the mechanical, the thermal and the fluid aspects. Layer and pad deformation, oil-film thickness, temperature and pressure distributions have been obtained for different operating conditions in order to validate the model using experimental data available in literature.