UBC Theses and Dissertations

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UBC Theses and Dissertations

Dynamic behavior of fluid film bearings, applications in the flexible rotor instability analysis Hemmati, Farzad


Fluid film bearings are widely used in many different industrial applications such as high-speed rotating machinery. Many of such rotating machinery suffer from excessive sub-synchronous whirl motion when rotating speed of the shaft exceeds “threshold speed of instability”. In this situation the rotor-bearing system experiences sub-synchronous whirling known as “oil whirl/whip” which is the most common type of rotor instability. Existing nonlinear stability models fall short in predicting the nature of sub-synchronous instabilities in flexible rotor supported by journal bearings. In this thesis, linear and non-linear stability of a flexible rotor-bearing system supported on short and long journal bearings is studied for both laminar and turbulent operating conditions. The turbulent pressure distribution and forces are calculated analytically from the modified Reynolds equation based on two turbulent models. Hopf bifurcation theory was utilized to estimate the local stability of periodic solutions near bifurcating operating points. The shaft stiffness was found to play an important role in bifurcating regions on the stable boundaries. It was found that for shafts supported on short journal bearings with shaft stiffness above a critical value, the dangerous subcritical region can be eliminated from a range of operating conditions with high static load. By increasing the Reynolds number, under shear effect assumption, stable operating region expands at high Sommerfeld numbers. The results presented have been verified by published outcomes in the open literature. It was found that, for a specific rotor bearing system having a stiffness lower than the critical stiffness of the shaft, there exist two transition system characteristic numbers α₁ and α₂. The operating system undergoes supercritical bifurcation for the rotor bearing system with intermediate system characteristic numbers (α₁ < α < α₂); hence, to avoid hysteresis phenomenon in a rotor bearing system fluid film viscosity shall be maintained within the range of μ'₁ < μ < μ'₂, where μ'₁ and μ'₂ correspond to system characteristic numbers α₁ and α₂ respectively. Stable operating region of flexible shafts supported journal bearings were shown to squeeze in size slightly by increasing the oil inlet pressure at low Sommerfeld numbers. Fluid film pressure distribution was found to be a strong function of oil inlet position.

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