Development of porous ceramic air bearings

Porous air bearings enjoy some important advantages over conventional air bearing types such as increased load carrying capacity, higher stiffness and improved damping. However, these types of bearings have yet to find widespread acceptance due to problems with obtaining materials with consistent permeability, instability issues relating to the volume of gas trapped at the bearing surface in the pores, and manufacturing the bearing without altering the permeability. Using a series of fine grades of alumina powder to minimise surface pore volume it has been demonstrated that it is possible to consistently and reproducibly manufacture porous bearings by injection moulding and slip casting. The relationship between powder size, processing conditions, porosity, mechanical properties and fluid flow characteristics were experimentally determined. The temperature of processing and the green density were found to be the controlling parameters in the resulting fluid flow properties for a given powder size, Test bearings were produced from the range of processing conditions investigated. It was found that the fine powder size bearings were stable over the entire range of test conditions irrespective of their initial manufacturing route. The most important consideration for the bearing performance was the quality of manufacture. The bearings were found to be sensitive to the flatness of their working surface and quality of fit in their test holder. The bearings were compared with published theories for load capacity and stiffness. A reasonable agreement was found with load carrying capacity once a correction for surface roughness was incorporated. Stiffness predictions provided a useful tool for the analysis and prediction of properties such as optimum values of permeability for a given geometry, if certain allowances are made

Development of lumped parameters models for aerostatic gas bearings

L'abstract è presente nell'allegato / the abstract is in the attachmen

Static, dynamic and levitation characteristics of squeeze film air journal bearing: Designing, modelling, simulation and fluid solid interaction

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Bearings today need to be able to run at very high speed, providing high positional accuracy for the structure that it supports, and requiring very little or no maintenance. For this to happen, bearings must have tight tolerances and very low or zero friction during operation. This pushes many traditional contact-type bearings to their limits as they often fail due to friction, generating heat and causing wear. By comparison, existing non-contact bearings fare better because of their very low or zero friction. But some have their own problem too. For example, the fact that aerostatic bearings require an air supply means having to use a separate air compressor and connecting hoses. This makes the installation bulky. Aerodynamic and hydrodynamic bearings cannot support loads at zero speed. Both hydrodynamic and hydrostatic bearings may cause contamination to the work-pieces and the work environment because of the use of lubricating fluid. A potential solution to the above-mentioned problems is the new squeeze film air bearing. It works on the rapid squeeze action of an air film to produce separation between two metal surfaces. This has the benefit of being compact with a very simple configuration because it does not require an external pressurized air supply, can support loads at zero speed and is free of contamination. For this research, two squeeze film air journal bearings, made from material of Al 2024 – T3 and Cu - C101 with the same geometry, were designed. The bearing is in the shape of a round tube with three fins on the outer surface and the journal, a round rod. When excited at a certain normal mode, the bearing shell flexes with a desirable modal shape for the squeeze film action. The various modes of vibration of Al bearing were obtained from a finite-element model implemented in ANSYS. Two Modes, the 13th and 23rd, at the respective frequencies of 16.320 kHz and 25.322 kHz, were identified for further investigation by experiments with respect to the squeeze film thickness and its load-carrying capacity. For Cu bearing, the two Modes are also 13th and 23rd at the respective frequencies of 12.184 kHz and 18.459 kHz. In order to produce dynamic deformation of the bearings at their modes, a single layer piezoelectric actuator was used as a driver. The maximum stroke length and the maximum blocking force of the single layer piezoelectric actuator were determined using manual calculation and ANSYS simulation. In the coupled-field analysis, the single layer piezoelectric actuator was mounted on the outside surface of the bearing shell and loaded with an AC and a DC voltage in order to produce the static and dynamic deformation. For the static analysis, the maximum deformation of Al bearing shell is 0.124 μm when the actuators are driven at the DC of 75 V. For the dynamic analysis, the actuators are driven at three levels of AC, namely 55, 65 and 75V with a constant DC offset of 75V and the driving frequency coincided with the modal frequency of the bearing. The maximum dynamic deformation of Al bearing shell is 3.22μm at Mode 13 and 2.08μm at Mode 23 when the actuators were driven at the AC of 75 V and the DC of 75 V. Similarly, the FEA simulation was used for analyzing Cu bearing. Furthermore, the dynamic deformation of both Al and Cu bearing at Mode 13 and 23 are validated by experiments. This research developed two theoretical models that explain the existence of a net pressure in a squeeze film for the levitation. The first model uses the ideal gas law as first approximation whilst the second uses the CFX simulation to provide a more exact explanation. In terms of the load-carrying capacity, Mode 13 was identified to be better than Mode 23 for both bearings. However, at Mode 13, Al bearing has a higher load-carrying capacity than Cu bearing. This is due to Al bearing having a higher modal frequency and amplitude. Finally, the coupled-field analysis for fluid solid interaction (FSI) was studied at both Mode 13 and 23 for Al bearing. The findings are that: a) the fluid force in the squeeze film can affect the dynamic deformation of the bearing shell, especially at high oscillation frequency, more at Mode 13 than at Mode 23 due to the relatively high pressure end-leakage in the latter; b) the dynamic deformation of the bearing shell increases with the gap clearance in a logarithmic manner at Mode 13; and c) the micron levels of gap clearance provide a damping effect on the dynamic deformation of the bearing shell at Mode 13 and at Mode 23, though much less dominant

Tribology and Rotordynamics of Small High-Speed Cryogenic Turboexpander

Turboexpander is considered as the heart of present-day cryogenic process plants such as helium, hydrogen and nitrogen liquefiers, low-temperature refrigerators and air separation units, . The operational objective of a turboexpander is to refrigerate a gas stream, by removing work from the gas, and expanding the gas nearly isentropically. The turbine based cryogenic process plants in recent years are low-pressure system and have the advantage of high thermodynamic efficiency and high reliability. The high efficiency is possible at highspeed of the turboexpander, and these turboexpanders in a typical cryogenic refrigerator or liquefier run at high-speed greater than 50,000 rpm without contaminating the process gas. Such operating condition imposes rigorous constraints on tribo-pair design. Oil-free gas bearings have advanced as the most acceptable solution for supporting small and high-speed cryogenic turboexpander rotors. An inherent issue with classic gas bearing is its lower dynamic properties such as stiffness and damping because of its low viscosity. Low stiffness and damping are prone to instability at high rotational speed. So gas foil bearings (GFBs) have received much attention for research, development, and experiment over past three decades for its ability to tailor the stiffness and damping with the use of compliant foils. Bump type compliant foil gas being is quite popular among researchers for various turbomachines for its high load carrying capacity, simplified numerical analysis, and easy fabrication methodology compared to other types. In the present work, a modest attempt is made to understand, standardize and document the numerical analysis, design methodology and fabrication methodology. It evaluates the rotor bearing performance to determine the feasibility of bump type gas foil bearings for axial and radial support of cryogenic turboexpanders. The work presented in current dissertation classified into five parts. The first part includes the status of research and development in the field of gas bearings in turboexpanders and a broad literature review of gas foil bearings. The outcome of the literature review directs that extensive research is essential for designing and development of gas bearing for a more advanced cryogenic system which is technically and economically better than present gas bearings. The second part deals with the design and numerical analysis of gas foil journal and thrust bearings and its feasibility to apply in a small and high-speed cryogenic turboexpander. The numerical analysis helps to fix the dimensions of foils such as its thickness, bump length, and pitch. for a previously designed rotor and its load carrying capacity. The dynamic properties of the bearings are determined to be used in the rotordynamics analysis. Finally, a step by step detailed design procedure itemized for both the gas foil bearings. Transverse vibration being a major issue for high speed rotating machinery such as a cryogenic turboexpander, a detailed vibration analysis completed in part three. The vibration analysis includes determination of critical speeds, mode shapes and unbalanced response for the desired configuration of the rotor-bearing system with determined stiffness and damping from previous part. A small clearance between the gas bearings and the rotor is maintained in order of 10 to 40 m; This makes a cryogenic turboexpander with gas foil bearing a precision equipment. All precision equipment demands micron scale manufacturing tolerance, so fourth part of the dissertation explains the details design methodology for gas foil bearings, the rotor and other associated parts of turboexpander. A broad analysis is done on bump forming methodology for fabrication of bump foil of the desired dimension. A Finite Element Method (FEM) simulation of forming process carried to simplifies the die design process. Special attention is given to the material selection of bearing components, balancing of the rotor, tolerance analysis, fabrication, coating of solid lubricant and assembly of the turboexpander. The last part includes performance study of the fabricated turboexpander with gas foil journal and thrust bearing. Several issues are encountered during this phase, and most of them are rectified either by modification of design process or rectification in fabrication methodology. A vibration study is done using accelerometers on the bearing housing close to the journal bearings. The vibration analysis reveals gas foil bearings can be an alternative rotor bearing system for a high-speed small sized cryogenic turboexpander. A satisfactory operation is carried out for the duration of 30 hrs with an achievable speed of 81,000 rpm with multiple starts and stops

Elements for the design of precision machine tools and their application to a prototype 450mm Si-wafer grinder

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 155-160).Next generation precision machines will require ever more rigid elements to achieve the required machining tolerances. The presented work focuses on the application of ultra stiff servo-controllable kinematic couplings and hydrostatic bearings to minimize the structural loop of multi-axis precision grinding machines while reducing complexity. The fundamental importance of these ultra stiff, adjustable machine elements is demonstrated in the design of a grinding machine for 450mm diameter silicon wafers. A new generation of silicon wafer grinding machines is needed to back-grind large (450mm diameter) wafers from the production thickness of up to 1 mm down to less than 50pm so as to reduce the cost of Si-wafer based components. The grinding process needs to be done in about 90 sec (fine-grinding, e.g. -200 micron) to 160 sec (coarse grinding, e.g. -600 micron). After completion of the fine grinding process the wafer must be flat to 0.1 pm/o45mm and parallel to 0.6pm/450mm diameter. The surface roughness must be less than Rymax 0.1 pm and Ra 0.01 pm. Even though the required machining forces are 1 N/nm is required, which is many times stiffer than a typical machine tool (0.1 to 0.3 N/nm). In cooperation with industry, this work had the aim of creating a new machine design philosophy, with an example application that focuses on nano-adjustable kinematic coupling and feedback controlled water hydrostatic bearing technology. This new design philosophy is needed to enable the design of a relatively small footprint, compact precision machines. In particular, a ball screw preloaded height adjustable kinematic coupling and a magnetically preloaded hydrostatic thrust bearing were designed and built. The adjustable kinematic coupling allows for up to 8mm of vertical height adjust and 7N/nm stiffness at 26 kN preload. By varying the preload on the coupling by +/- 10%, in-process nm to micron height and tilt adjustment at >95% of the nominal stiffness is possible. Under the assumption of a constant flow supply, the hydrostatic bearing achieves a theoretical stiffness of 1 N/ nm at a 20 micron bearing gap and 7000 N combined gravitational and magnetic preload. In practice, the stiffness is limited by the pressure flow characteristics of the supplying pumps. To increase the bearing stiffness to a required 4N/ nm, various control loops have been developed and tested.by Gerald Rothenhöfer.Ph.D

International Workshop on MicroFactories (IWMF 2012): 17th-20th June 2012 Tampere Hall Tampere, Finland

This Workshop provides a forum for researchers and practitioners in industry working on the diverse issues of micro and desktop factories, as well as technologies and processes applicable for micro and desktop factories. Micro and desktop factories decrease the need of factory floor space, and reduce energy consumption and improve material and resource utilization thus strongly supporting the new sustainable manufacturing paradigm. They can be seen also as a proper solution to point-of-need manufacturing of customized and personalized products near the point of need

An Experimental Study of Gas Lubricated Foil Journal Bearings Using an Instrumented Rotor with Wireless Telemetry

In order to increase their efficiency and power-density, turbomachines are continuously pushed to run faster, and hotter rotors. These requirements create enormous engineering challenges that affect the design of turbomachines down to the component level. Among these challenges is the choice of an adequate bearing technology. Gas lubricated foil bearings showed competency to support several high-speed turbomachinery applications. The foil bearing performance is governed by the properties of the gas film and the underlying compliant structure. A significant amount of research is dedicated to analyze the latter. However, the gas film was addressed only once in the experimental research efforts on foil bearings extending from the 1960s. This gap in the literature is due to the complexity of the foil bearing structure that hinders the placement of sensors through the bearing surface. As a consequence, the pressure profile inside the gas film of compliant foil journal bearings were never measured. The lack of such experimental data is hampering the conclusive validation of foil bearing models using pressure as the fundamental variable. The goal of this thesis is to provide pressure profile measurements within the gas film of compliant foil journal bearings at different rotational speeds. The experimental data will be a step towards the validation of foil bearing models using gas film measurements. An instrumented rotor with embedded pressure probes and a wireless telemetry is used to execute that mission. The designed rotor is capable of measuring the pressure profile at two different axial planes inside the bearing. The developed embedded pressure probes consisted of pressure transducers, and pneumatic channels to connect them to the measurement point on the surface of the rotor. Such layout required a special calibration procedure in order to account for the dynamics of the pneumatic channel that influences the pressure signal. A Siren Disk was designed and manufactured to produce periodic pressure signals with a controlled frequency and amplitude. Such signal was used to excite the pressure probes, and consequently identity their transfer functions, which are used to correct the pressure signals afterwards. As a proof of concept, the instrumented rotor was tested on externally pressurized gas journal bearings up to a speed of 37.5 krpm. The test bearings were equipped with pressure taps to measure the spatially sampled pressure profiles from the bearing side. The two measurements were compared and were in good agreement at quasi-static conditions. The bearing side measurement was considered as a reference signal (input), and once compared to the rotor side measurement (output), an in-situ calibration and system identification is performed. The pressure measurements were used to validate an externally pressurized bearing model based on the compressible Reynolds equation at different rotational speeds and supply pressures. The developed transfer function was subjected to several fitness tests before placing the instrumented rotor on foil bearings and measuring the pressure profiles at different rotational speeds. The developed transfer functions were used to correct the measured signal within the gas film of the foil bearing. Finally, the pressure profiles were compared to a foil bearing model based on the compressible Reynolds equation

Gas Flows in Microsystems

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