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 conditions

In recent years, sewing technology has witnessed dramatic increases in machines speeds, new types of materials, new sewing threads and evaluation methods – but the principal type of sewing machine remains the lockstitch type and this is likely to remain the most common and versatile for the foreseeable future, particularly for sewing woven fabrics. Sewing machine speed increases lead to a loss of control of the sewing process due largely to an increase of the dynamic forces and consequently to problems such as seam pucker. In this research computer-based instrumentation and high-speed digital image and signal acquisition systems were developed to study the dynamic effects of the sewing machine, fabric and thread on seam pucker. Needle thread tension, needle bar pressure/tension, presser-foot pressure and displacement signals were acquired simultaneously by 4 strain-gage/piezo-quartz sensors mounted on a Pfaff-563 machine and results were related to pucker measured by a CCO colour-video camera system integrated with the sewing machine. A series of experiments conducted on various types of fabrics provided illustrative examples of the characteristics of each signal acquired (distinctive shape, amplitudes-peaks valleys locations, duration etc), and also enabled their characteristics to be compared and the interactions among the signals to be studied. For example it was found that the increases in magnitude of the dynamic forces in relation to sewing machine speed increases from 200rpm to 5500rpm were lower than expected (needle thread tension 2.6 fold, presser-foot 1.2 fold and needle penetration force 3.2 fold). A comparison of the needle thread tension for standard woven fabrics and micro-fibre fabrics showed a significant difference in the signal shape, location and amplitudes. The instrumentation developed is located at the Institute of Textiles and Clothing of the Hong Kong Polytechnic University in Hong Kong.

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