Linear and nonlinear vibrational characteristics of oil lubricated journal bearings

This thesis is concerned with an investigation of the vibrational characteristics of oil lubricated journal bearings and, in particular, the instability known as oil whirl, which is a self-excited oscillation induced by the hydrodynamic forces generated in the bearing. The thesis
is divided into two parts. In Part 1, linear stability theory is used to determine the critical parameter values at which oil whirl is initiated. This information is conveniently expressed on a two dimensional stability
chart, separating regions of stability from those of instability. The vibrational characteristics of a variety of bearing models are contrasted with particular emphasis on the effect of adopting different cavitation
boundary conditions at film rupture and the effect of oil film behaviour during journal vibration. An analysis of the single axial groove journal bearing is presented and the effects of groove location and oil supply pressure on the vibrational characteristics of the bearing are examined.

In Part 2, attention is focused on the nonlinear aspects of oil whirl, one objective being to determine the motion of the journal beyond its stability threshold. Several nonlinear techniques are employed to analyse the nonlinear equations of motion and to identify different features
(bifurcation theory, multiple scaling, the method of averaging and numerical integration). Particular emphasis is placed on examining the structure of periodic solutions of the equations of motion at and close to the position of neutral stability. It is shown that the onset of oil whirl
is a bifurcation phenomenon in which the equilibrium position gives way to a small amplitude whirl orbit (limit cycle). Two different types of bifurcation behaviour are possible, depending on the operating parameters of the bearing. Results obtained from the different nonlinear techniques are contrasted and an assessment is made of how suitable these methods are for examining the phenomenon of oil whirl.