Nonlinear Isoviscous Behaviour of Compliant Journal Bearings

Sammanfattning: Plans to shut down nuclear power plants in some European countries as well as increased electricity production by wind and solar power will increase the work load on hydroelectric power plants in the future. Also, due to the power grid regulations, hydroelectric power plants undergo more frequent start-ups and shut-downs. During such transient periods, a large amplitude shaft motion can occur, especially in the power plants with vertical shafts. Large shaft motion is not desirable because it can lead to a machine failure. Furthermore, performance limitations of conventional white metal or babbitted bearings call for the development of new bearing designs. An outstanding tribological performance 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 employ nonlinear analysis to provide further understanding of the compliant bearing dynamic response to synchronous shaft excitation.Plain cylindrical journal bearings with different compliant liner thicknesses were analysed using a nonlinear approach. The numerical model was verified with an in-house developed code 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. Results for the tilting pad journal bearing with three pads with line pivot geometry were compared with published data in dynamic conditions. 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, pivot offset, tapers and pad inclination angles. Influence of the viscoelastic 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. It was also shown that the compliant liner provides lower maximum oil film pressure and thicker minimum oil film thickness in the bearing mid-plane in both static and dynamic operating conditions.