Dynamic behaviour of oil lubricated journal bearings

Abstract

This thesis is concerned with the dynamic behaviour of oil lubricated journal bearings and particularly with the small vibrations about the equilibrium position known as 'oil whirl'. The importance of shaft flexibility and oil film cavitation to this phenomena are investigated. Several authors have shown that by the use of linear techniques it is possible to derive a stability borderline which can be used for design purposes to ascertain whether or not a bearing is stable. These linear techniques are used to examine journal bearings with flexible shafts operating under a range of cavitation boundary conditions. It is demonstrated that these boundary conditions, particularly the behaviour of a lubricant during a vibration, play a crucial role in determining the predicted stability of the bearing. The effect of shaft flexibility is to make the bearing less stable, but the extent of this change is also governed by the oil film behaviour. Nonlinear analytical techniques are used to carry out an investigation into the behaviour of a journal bearing operating with a rigid shaft close to the stability borderline for a particular set of cavitation boundary conditions. It is found that two types of behaviour are possible: (i) supercritical, in which small stable whirl orbits are possible at speeds just above the threshold speed (the speed above which the bearing is unstable according to linear theory). (ii) subcritical, in which small unstable orbits exist at speeds just below the threshold speed. The parameter space is split into two regions, one subcritical and the other supercritical. Several methods are used in the investigation; it is shown that the methods give identical results, but only if they are applied correctly. These results are subsequently confirmed by a numerical integration of the equations of motion. The thesis concludes with an investigation of the application of nonlinear techniques to a variety of cavitation boundary condition