Browsing by Author "Parkins, D. W."
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Item Open Access Design and development of a high-speed test facility and the measurement of the fluid film characteristics of journal bearings(Cranfield University, 1998-10) Rowan, D; Parkins, D. W.In the theoretical analysis of high speed rotor bearing systems, it is common to use four displacement and four velocity based coefficients, which characterise the behaviour of the lubricating fluid film. Although a great deal of work has been published establishing theoretical models of all types of hydrodynamic journal bearings, the large amount of experimental work has centred on relatively low speed conditions. This work presents a contribution to the experimental study of the static and dynamic characteristics of oil films in journal bearings used in high-speed rotating machinery. The main objectives of the work are: • To devise new experimental techniques for the measurement of dynamic coefficients suitable for use at high rotational speeds • To design, manufacture, assemble and commission a test facility to measure the static and dynamic characteristics of journal bearings at speeds up to 30000 rpm • To determine the static and dynamic characteristics of a 5 Pad Tilting Pad Journal Bearing Unit of 80 mm diameter at speeds up to 25 000 rpm using the said test facility. New techniques are particularly necessary for the measurement of velocity coefficients because these invoke the necessity of imposing a velocity on to the bearing housing and previous techniques have utilised synchronous motion of the bearing. Consequently a new experimental procedure for measuring the four velocity or damping coefficients of an oil film journal bearing from imposed dynamic "orbits" has been devised called the "double pulse" technique. All four velocity coefficients are derived from one imposed journal centre dynamic orbit and, therefore may be regarded as being obtained at the same time. The method requires the production of a "cross- over" point similar to that of a "figure of eight" shaped orbit and utilises the "cross-over" point therein. Coefficients are initially evaluated in a co-ordinate system, which is chosen to align with the designated parts of the measured orbit. Each coefficient is then evaluated from single values of instantaneous imposed force and resulting journal centre velocity. Coefficients are them converted into any other desired axes system. The result is a simpler experimental procedure, with reduced uncertainty compared to hitherto existing methods. The use of non-sinusoidal excitation of the oil film was explored, in the form of applying a step-pulse train load pattern to produce a cross-over pattern in the journal displacement ·orbit'. Experimental tests were completed on a tilting pad bearing at speeds up to 15 000 rpm inclusive. At speeds above this, the bearing exhibited a vibrational response, which precluded the accurate measurement of journal centre displacement.Item Open Access Static and dynamic characteristics of an hydrodynamic journal bearing(1976-05) Parkins, D. W.Hydrodynamic oil film bearings exhibit lateral flexibility which influences the dynamics of rotors they support. This lateral flexibility is specified by coefficients which relate forces generated by the oil film to the instantaneous journal centre velocity and its displacement from an equilibrium position. Previous investigators adopted a linear treatment by taking uniform viscosity with small displacement and velocity increments. Relatively large journal centre velocities are possible in rotating machinery. Therefore, this thesis investigates the non-linear behaviour of these oil film coefficients. Coefficient calculations allowed viscosity to vary with temperature and pressure rendering the governing Reynolds Equation non-linear. A range of positive and negative displacement and velocity increments were examined. Novel experimental techniques have been developed which allow determination of coefficient variation with respective displacement and velocity. Coefficients were deduced from specially chosen, imposed vibration orbits arising from two mutually perpendicular external oscillating forces of variable relative magnitude and phase. Journal centre displacement and velocity were measured using high speed data logging equipment. A unique feature was the ability to obtain, experimental displacement coefficients from the results of both dynamic and incremental loading. It was found necessary to establish the bearing centre separately for each warm-up/load combination. Journal clearance in the hot rotating condition could not be measured to the precision required by its sensitivity to calculated load. Clearance and cavitation zone pressures were deduced from simultaneous predictions of the measured vertical load and attitude angle. Theoretical oil film tensile forces were necessary, a proposition supported by recently published experimental findings. Theoretical results for an equivalent uniform viscosity combined with experimental data gave a simple static locus design procedure. A temperature profile was assumed for theoretical work but choice thereof was found to be not critical. Coefficients are defined in terms of a "zero" value and linear gradient. Using realistic criteria, measured coefficient variation was found to be significant at eccentricity ratios greater than 0.78. Theory adequately predicted most "zero" values but not gradients. It is concluded that improvement in the coefficient prediction willItem Open Access Using ferro-magnetic fluids to lubricate non-conformal isothermal contacts(Cranfield University, 1993-06) Noronha De Souza, Petronio.; Parkins, D. W.Magnetic fluids are synthetic materials that combine in a single medium the properties of fluids and magnetic materials. The action of a magnetic field on the magnetic fluid allows the design of new types of bearings and also provides conventional ones with new properties. This work presents a contribution to the study of the lubrication of linear isothermal non-conformal contacts with magnetic fluids under the action of a non uniform magnetic field perpendicular to the plane of the contact. The fluid is assumed to present extrinsic superparamagnetism. The regime of lubrication considered is the isoviscous-rigid. The analytical approach adopted considers small perturbations of the classical analytical solution already known for this type of contact. The main objectives of the work are: To establish the equations and assumptions that are necessary to treat the proposed lubrication problem To analyze the impact of the change in the apparent viscosity of the magnetic fluid on the thickness of the lubricating lm due to the presence of the magnetic field ° To develop an experimental technique able to deal with the problems that arise with the use of magnetic fluids in the lubrication of a two-disc machine The mathematical treatment of the proposed problem brings together the analytical methods adopted in fluid lm lubrication to the analysis of long isothermal hydrodynamic bearings and those from the magnetic-liquid fluid dynamics (ferrohydrodynamics) usually adopted to study the flow of magnetic fluids that present extrinsic superparamagnetism. The result is a coherent system of equations supported by a well defined set of assumptions. The solution obtained allows the study of the eventual change of the lm thickness that can be achieved by the application of a magnetic field across the contact. The work also discusses the experimental technique necessary to study the problem in a two-disc machine that runs with minute amounts of lubricant, applies a magnetic field across the contact and measures the lm thickness by a capacitance method. The work concludes by showing theoretically that the lm thickness can be either increased or reduced depending on the magnetic and viscous properties of the fluid and the intensity of the magnetic field. These conclusions are supported by experiments with a magnetic oil in a two-disc machine.