Fluid Bearing

6 Key excerpts on "Fluid Bearing"

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  eBook - PDF

  Engineering Tribology

  • John Williams(Author)
  • 2005(Publication Date)
  • Cambridge University Press

    (Publisher)

  However, it is clear that since the viscosity of air is so low (about 0.01% of typical oils) the speed of a gas-lubricated bearing must be many times that of its oil lubricated counterpart to generate either a similar Sommerfeld number or the same degree of load support. Air bearings are widely used in engineering devices in which there are high peripheral sliding speeds such as machine tool spindles, turbo-machinery, instrument and gyroscope bearings, and in high-speed precision machinery such as dental drills and textile processing machines. In Section 8.2 the ideas already in connection with conventional hydrodynamic bearings are extended to cover a number 302 Non-Newtonian fluids, and ehl of these more specialized, but nevertheless technologically important, cases. While many conventional lubricants can be considered to exhibit Newto-nian behaviour under a wide range of service conditions, there are a number of circumstances in which predictions based on this simple model are inade-quately accurate. It is well known, for example, that liquids with a complex structure (such as polymer solutions or melts, soap solutions, and solid suspensions or slurries) can respond to applied loads or strain rates in rather surprising ways, and that such fluids are not always at all well modelled by the constitutive equation for Newtonian behaviour. Section 8.3 gives a brief outline of some aspects of non-Newtonian behaviour which are relevant to tribology. Under conditions of extreme pressure the behaviour of conven-tional mineral lubricating oils can also become distinctly non-Newtonian. The branch of tribology that considers these effects, together with the asso-ciated elastic deformation of the bounding solid surfaces, is known as elasto-hydrodynamics and is treated in Section 8.4. 8.2 Lubrication by gases and vapours; air bearings Gas-lubricated bearings operate on the same principles as those using liquid lubricants.

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  12th International Conference on Vibrations in Rotating Machinery

  Proceedings of the 12th Virtual Conference on Vibrations in Rotating Machinery (VIRM), 14-15 October 2020

  • Institute of Mechanical Engineers(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  Characteristics of a high speed thin film fluid lubricated bearing N.Y. Bailey Department of Mechanical Engineering, University of Bath, UK

  ABSTRACT

  A fluid lubricated bearing model is derived for operation under extreme operating conditions, including velocity slip boundary conditions appropriate for very small bearing face separation and retention of centrifugal inertia effects. Both compressible and incompressible Reynolds equations are formulated to model the fluid film and the fluid flow characteristics are examined for the steady state case.

  Coupling the fluid flow to the bearing structure, where the rotor and stator are modelled as spring-mass-damper systems, allows the dynamics to be examined when the bearing is subject to an external harmonic force. This replicates forces the bearing may be subject to when situated within a larger complex dynamical system.

  1     Introduction

  Fluid lubricated bearing and seal technology comprises two structural components; namely a rotor and stator, which are separated by a thin fluid film that experiences relative rotational motion. The set up is of a thrust/axial bearing with radial flow and this type of technology is also described as non-contacting, gas-lubricated or film-riding and those containing an air film termed air-riding seal. Next generation bearing and seal technology aims to provide a considerable improvement in efficiency for applications characterised by higher rotational speeds and smaller operating clearances, possibly down to the order of several microns.

  To completely capture the dynamics of a fluid-lubricated bearing, the fluid flow and bearing structure need to be appropriately coupled together. If an external axial force is imposed on the bearing, a hydrodynamic force is typically generated by the normal motion of the faces, enhancing the local fluid film pressure, causing the fluid film to be maintained. Etison modelled a fluid lubricated device, identifying the hydrodynamic and hydrostatic components of the air film pressure and showed that the squeeze film behaviour (incorporated in the hydrostatic component) was potentially able to maintain the air film between the rotor and stator [1 ]. For highly vibrating operations, Salbu examined the effect of significant axial disturbances by describing the rotor-stator clearance with oscillatory motion and confirmed squeeze films have a load carrying capacity [2 ]. The dynamics of a coupled high speed air lubricated bearing were examined by Garratt et al., where the effects of centrifugal inertia were retained and one bearing face underwent small prescribed amplitudes, and the other responded to the induced film dynamics [3

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  Thermohydrodynamic Instability in Fluid-Film Bearings

  • J. K. Wang, M. M. Khonsari, J. K. Wang(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  1 Fundamentals of Hydrodynamic Bearings Hydrodynamic (fluid film) bearings are used extensively in different kinds of rotating machinery in the industry. Their performance is of utmost importance in chemical, petrochemical, automotive, power generation, oil and gas, aerospace turbo-machinery, and many other process industries around the globe. Hydrodynamic bearings are generally classified into two broad categories: jour- nal bearings (also called sleeve bearings) and thrust bearings (also called slider bearings). In this book, we exclusively focus our attention on journal bearings. Figure 1.1a shows a schematic illustration of a rotor bearing system, which con- sists of a shaft with a central disk symmetrically supported by two identical journal bearings at both ends. Figure 1.1b shows the geometry and system coordinates of the journal rotating in one of the two identical journal bearings. To easily identify the bearing’s physical wedge effect and annotate the multiple parameters of a rotor bearing system, the clearance between the journal and the bearing bushing is exag- gerated. θ is the circumferential coordinate starting from the line going through the centers of the bearing bushing and the rotor journal. ϕ is defined as the system atti- tude angle. e is the rotor journal center eccentricity from the center of the bearing bushing. W represents the vertical load imposed on the shaft and supported by the bearing. p is the hydrodynamic pressure applied by the thin fluid film onto the jour- nal surface. f is the hydrodynamic force obtained by integrating the hydrodynamic pressure p generated around the journal circumference. Thermohydrodynamic Instability in Fluid-Film Bearings, First Edition. J. K. Wang and M. M. Khonsari. © 2016 John Wiley & Sons, Ltd. Published 2016 by John Wiley & Sons, Ltd. In most cases, except in a floating ring configuration, the bearing bushing is fixed and the rotor rotates at the speed of ω inside the bearing bushing.

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  Mechanical Design Engineering Handbook

  • Peter Childs, Peter R. N. Childs(Authors)
  • 2013(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Chapter 5

  Journal Bearings

  Abstract

  The purpose of a bearing is to support a load, typically applied to a shaft, whilst allowing relative motion between two elements of a machine. The two general classes of bearings are journal bearings, also known as sliding or plain surface bearings, and rolling element bearings. The aims of this chapter are to describe the range of bearing technology, to outline the identification of which type of bearing to use for a given application, and to introduce journal bearing design with specific attention to boundary lubricated bearings and full-film hydrodynamic bearings. The selection of rolling element bearings is considered in Chapter 6 .

  Keywords

  Bearings; Boundary; Film; Friction; Full; Hydrodynamic; Journal; Lubricants; Lubricated; Plain; Selection; Sliding; Surface

  Chapter Outline

  5.1 Introduction 5.2 Sliding Bearings 5.2.1 Lubricants 5.3 Design of Boundary-Lubricated Bearings 5.4 Design of Full-Film Hydrodynamic Bearings 5.4.1 Reynolds Equation Derivation 5.4.2 Design Charts for Full-Film Hydrodynamic Bearings 5.4.3 Alternative Method for the Design of Full-Film Hydrodynamic Bearings 5.5 Conclusions References Further Reading Nomenclature

  5.1 Introduction

  The term “bearing” typically refers to contacting surfaces through which a load is transmitted. Bearings may roll or slide or do both simultaneously. The range of bearing types available is extensive, although they can be broadly split into two categories: sliding bearings, also known as journal or plain surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combination of rolling motion and lubrication. Lubrication is often required in a bearing to reduce friction between surfaces and to remove heat. Figure 5.1 illustrates two of the more commonly known bearings: a journal bearing and a deep groove ball bearing. A general classification scheme for the distinction of bearings is given in Figure 5.2

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  Principles and Applications of Tribology

  Pergamon International Library of Science, Technology, Engineering and Social Studies: International Series in Materials Science and Technology

  • Desmond F. Moore, D. W. Hopkins(Authors)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  1 In most cases, the lubricating fluid is a gas (normally air), and the overall design is commonly referred to in this case as an externally pressurized, gas-lubricated bearing. As we might anticipate, the question of positional or vertical stability is of much greater significance for gas or air bearings than for liquid bearings. This can be attributed both to the smaller values of film thickness h for gas bearings, and to the virtual absence of internal damping in gases compared with liquids. Small deviations in h from the operational or design value have a far greater probability of amplification (with the ultimate catastrophe of bearing seizure) if the working fluid is a gas rather than a viscous liquid. The applications of hydrostatic lubrication are many and diverse. Usually heavy equip-ment, where the speed of relative motion between the components is small, depends on some form of externally pressurized lubricant to separate the parts in the manner of Fig. 6.19. Radio and astronomical telescopes, water-wheel generators, large-scale thrust bearings, and vertical turbo-generators are typical examples. 6.7 Cavitation The phenomenon of cavitation has been referred to briefly in Section 6.4 and Table 6.2, since it gives rise to a net load support action particularly on rough surfaces. It can be attributed directly to a reduction of local pressure in a liquid to a level approaching its vapour pressure. In such circumstances, tiny bubbles filled with gas or vapour appear and grow rapidly. When they move subsequently to a region of increased pressure, the bubbles collapse or implode, thereby releasing tremendous energy and causing severe erosion in submerged surfaces. The phenomenon of cavitation was anticipated by Euler as early as 1754 in his theory on hydraulic turbines, and it was first observed experimentally in connection with ships' propellers in 1895.

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  Mechanical Design

  • P.R.N. Childs, T.H.C. Childs(Authors)
  • 2003(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  d 39 4 BEARINGS The purpose of a bearing is to support a load, typically applied to a shaft, whilst allowing rela-tive motion between two elements of a machine. The aims of this chapter are to describe the range of bearing technology, to outline the iden-tification of which type of bearing to use for a given application, to introduce journal bearing design and to describe the selection of standard rolling element bearings. LEARNING OBJECTIVES At the end of this chapter you should be able to: • distinguish what sort of bearing to use for a given application; • specify when to use a boundary lubricated bearing and select an appropriate bearing material to use for given conditions; • determine the principal geometry for a full-film boundary lubricated bearing; • determine the life of a rolling element bear-ing using the life equation; • select an appropriate rolling element bearing from a manufacturer’s catalogue; • specify the layout for a rolling bearing sealing and lubrication system. 4.1 Introduction The term ‘bearing’ typically refers to contacting surfaces through which a load is transmitted. Bearings may roll or slide or do both simultan-eously. The range of bearing types available is extensive, although they can be broadly split into two categories: sliding bearings also known as plain surface bearings, where the motion is facilitated by a thin layer or film of lubricant, and rolling element bearings, where the motion is aided by a combin-ation of rolling motion and lubrication. Lubrica-tion is often required in a bearing to reduce friction between surfaces and to remove heat. Figure 4.1 illustrates two of the more commonly known bearings: a deep groove ball bearing and a journal Bearings 40 Chapter 4 41 Bearings 42 common term is ‘plain surface bearings’.This sec-tion is principally concerned with bearings for rotary motion and the terms ‘journal’ and ‘sliding’ bearing are used interchangeably. There are three regimes of lubrication for slid-ing bearings: 1.

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