The work described here concerns the possibility that textured surfaces may reduce coefficients of friction and increase load support within hydrodynamic bearings. This topic is of particular interest in the context of the potential of texturing to reduce friction in internal combustion engines using techniques adapted to suit high volume production, such as Laser Honing. Despite the existence of a number of theoretical explanations (which include shear rate reduction, inlet suction, effect of the presence of cavitation and the mitigation of starvation) and numerous experimental reports as to the effect of texture on friction, there has been no simultaneous study of friction, cavitation and oil film thickness in textured hydrodynamic bearings. This is because such studies are difficult. One must address certain barriers which include the large range of oil film thicknesses encountered and the need for accurately known feature dimensions on the surfaces in question. A new test rig has been designed and constructed that enables the simultaneous measurement of friction, load and oil film thickness. Novel features include a rotating glass disc on a silicon pad, a high-sensitivity fluorescence imaging microscope system to determine the oil film thickness and a non-contact displacement sensor for friction measurement. The process of photolithography was used to accurately produce predetermined textures on silicon surfaces. Tests were conducted on convergent plain and textured silicon pads whilst concurrently monitoring friction, load, cavitation (if present) and oil film thicknesses. The results were compared to theoretical predictions based upon the solution of Reynolds equation, with cavitation, in two dimensions. Results show broad agreement with the theoretical predictions and suggest that textured surfaces may be either beneficial or detrimental according to both their geometry and the operating conditions.

The research concerns improvement of the power efficiency of lubricated components in automotive drivelines such as transmissions and axles. Meshed gear pairs, rolling bearings, seals and oil churning by rotating components immersed in the oil are studied. The purpose of the research is to explore the most effective way of improving the efficiency in drivelines, focusing on an axle comprising hypoid gears and a differential assembly. First, a study of the nature of losses affecting the efficiency of a simple spur gear box was carried out, and a model of friction and churning in a simple transmission was developed. Next, a detailed experimental study of oil churning losses in a hypoid axle from a four wheel drive road vehicle was carried out using the inertia run-down technique. To perform the above experiments, a new test rig for measuring oil churning losses was designed, manufactured and commissioned. The test rig allowed a wide range of speed and lubricant parameters to be explored and was designed by the author for tests available at different roll and pitch attitudes. In addition, an "extended" housing, consisting of a modified gear case that accepted the same internal components as the production axle but which had much greater internal clearances, was designed and manufactured. This enabled the effects of different casing geometry and of internal baffles to be studied. Additionally, the extension housing was modified to investigate the oil flow inside the housing through its one transparent side for the understanding of the effect of oil flow on churning losses. An investigation of design-related parameters influencing churning losses was carried out using the new test rig. Empirical equations for the churning losses, based on dimensional analysis, were developed to describe the test results. It was found that some combinations of baffles gave a significant reduction in losses. Supplementary tests were carried out using transparent windows to visualise the oil flow. These identified some of the mechanisms responsible for the reduction in churning loss and suggested a number of practical methods by which churning could be reduced without compromising the lubricant supply to remote components. It is argued that these innovations can contribute to improving fuel efficiency and limiting oil temperature rise in all-wheel-drive vehicles.

This thesis is concerned with the characterization of layered contact problems. The research project has been prompted by an investigation into creep, and ultimately, gross sliding, of rubber tyres fitted around steel wheels in earth moving equipment. In general, slippage between tyre and rim is experienced in common engineering applications employing tyred systems. A general and systematic approach for investigating the interfacial behaviour of tyred systems has consequently been proposed. Classical techniques together with novel numerical approaches based on advanced mathematical programming have been implemented to support the investigation. Creep between mating surfaces, frictional shakedown and measuring friction in partial slip condition are the main objects of investigation. The analytical and numerical models developed by the author have been complemented by experimental work, whose detailed description is also included in this thesis. Finally, further studies have been performed to shed light on some of the design issues offered by the threedimensional full-scale engineering application. The numerical approach based on finite element modelling used to tackle these aspects of the project and the experimental work carried out by the author to corroborate the numerical findings are also presented.

A major challenge in silicon devices based on micro-electro-mechanical systems (MEMS) is the provision of effective lubrication for sliding parts. This greatly limits the development and exploitation of MEMS devices, with current designs avoiding sliding contacts where possible. This thesis describes research aimed at lubricating high sliding MEMS devices. A micro-scaled tribometer has been constructed to obtain measurements of friction between two sliding silicon surfaces. This work focuses on lubricating MEMS with liquids, a self-replenishing lubrication method which had been dismissed previously as they were assumed to carry too much viscous drag. The major finding is that ferromagnetic fluids make excellent lubricants for sliding MEMS surfaces. These fluids provide low friction at high speeds, and reduce the boundary friction at low speeds when the hydrodynamic film is absent. The properties of such fluids allow the liquid to be contained in the presence of a magnetic field, meaning that only a small, localised amount is required. Low viscosity liquids were also shown to provide acceptably low friction at high speeds. These results agreed reasonably well with theory. Friction modifier (FM) additives were added to low viscosity liquids in order to reduce boundary friction by forming boundary films, when no hydrodynamic film is generated at low speeds. Drag has also been shown to be insignificant. A study of the wear of silicon surfaces under prolonged sliding was conducted. Previous studies have focussed on dry coatings and apparently untreated surfaces. In this thesis, the effects of different surface preparations, the use of low viscosity liquids and vapour phase lubrication on wear have been studied. This thesis concludes that it is feasible to use liquids to lubricate sliding MEMS. High sliding MEMS is possible and practicable in future if self-replenishing methods, such as those studied in this work, are employed in real devices.

Bladed disk vibrations in turbomachinery can lead to failure due to High Cycle Fatigue. One way in which vibration may be reduced is by dry friction damping. Frictional damping originates from micro and macro slip in the contacting interfaces (“joints”) and is controlled by the relationship of the applied load and tangential displacement. In order to predict the dynamic response of the structure, knowledge of the coefficient of friction and the tangential contact stiffness of the contact are crucial. Vibration induced slip and the consequent damage in contacting surfaces has been widely studied and is usually called fretting. However, little is known about the effect of the changing interface during fretting on the coefficient of friction and the tangential contact stiffness, which is required when trying to predict these parameters. This study seeks an improved understanding of the effects of surface topography, surface chemistry, and elastic and plastic material properties on the friction and damping performance of joints under fretting conditions. In the present study experiments were conducted to measure the coefficient of friction and the tangential contact stiffness of different metals under different test conditions. Fretting damage mechanisms were investigated using metallography, SEM, EBSD, TEM and XRD techniques. The evolution of roughness and conformity was investigated by using interferometric profiling systems and image registration via cross correlation. An infrared radiation measuring system was employed to measure the dissipated radiation and frictional power in fretting which was then compared with calculated energy dissipation maps. Experimental results were used to validate models predicting contact stiffness which have been developed throughout the project by collaborating researchers. This study highlighted real contact conditions and their dependence on running time, which need to be taken into account when modelling friction contacts.

No description available.

No description available.

No description available.

No description available.

No description available.