Dynamic Performance and Design Aspects of Compliant Fluid Film Bearings

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Due to government regulations together with health and safety reasons, there are increasing demands on reducing hazardous polluting chemicals from fossil fuel power plants. Therefore, more efforts are imposed on using renewable resources such as water, wind, solar and tide to produce clean/green electricity. On top of that, there is another increasing demand from Original Equipment Manufacturers (OEMs) to operate power plants with higher load while keep the power loss to the minimum. These requirements drive conventional fluid film bearings to its mechanical and temperature limits. This calls for the development of new bearing system designs. An outstanding tribological performance such as low start-up and break-away friction, excellent resistance to chemical attack and anti-seizure properties, can be achieved by introducing compliant polymer liners. At the same time, bearings with compliant liners may alter rotor-bearing system dynamic behaviour compared to the systems with conventional white metal bearings. The research approach of this thesis is to implement compliant liner on bearing surface, impose synchronous shaft excitation and investigate the effect of bearing design parameters on bearing dynamic response.Plain cylindrical journal bearings with different compliant liner thicknesses were analysed using a FEM approach. The numerical model was compared with an in-house developed code based on the finite difference method (FDM) for a bearing operated at steady state conditions. Results obtained by the numerical models showed good agreement. After verification of the numerical model for fixed geometry journal bearings, models for tilting pad journal bearings were developed. Dynamic behaviour of the tilting pad journal bearing with three pads with line pivot geometry was compared with published data. A good agreement was obtained between the two numerical models. The effect of pad pivot geometry on bearing dynamic response was investigated. Vertical and horizontal shaft configurations were compared in terms of the effect of preload factor, radial clearance, pivot offset, and pad inclination angles. Influence of the elastic properties of compliant liners was also studied. All these factors significantly affect bearing dynamic response. It is shown how these factors should be selected to control the journal orbit sizes. Misalignments in compliant tilting pad journal bearings were analysed for load between pivots and load on pivots with consideration of thermal effects. Significant improvements in bearing performance were obtained with compliant bearings compared to white metal bearings. Furthermore, different polymer materials (PTFE, UHMWPE, pure PEEK and PEEK composite) were characterized using Frequency Response Function (FRF). It was shown that as the excitation frequency increased the equivalent stiffness was more or less constant while equivalent damping decreased exponentially. PTFE had similar equivalent stiffness compared to PEEK. As for equivalent damping, PTFE had slightly higher damping compared to PEEK or UHMWPE. Oil film thickness, oil film temperature and loads on tilting pad journal bearing were measured on 10 MW Kaplan hydroelectric power machine. Test results were compared to FEM model. It was shown that stiffness of the supporting structure may be more important to machine performance than the stiffness of the bearing alone.