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Über dieses Buch

Gas bearings have been used to support rotating parts in a wide range of applications - from magnetic recording heads in computer disk drives to gyroscopes and special machine tools. The advantage of gas bearings is the very low viscosity of air compared to that of most oils used in lubrication. As a result, not only is there much less frictional heat to dissipate, but the bearing remains very nearly isothermal. Gas bearings can thus support rotors spinning at much higher rotational velocities than those lubricated with liquids. This book discusses models for the behavior of gas bearings, particularly of the aspects affecting the stability of the system. It begins with a discussion of the mathematical models, identifying the stiffness and damping coefficients, and describing the behavior of the models in unstable regions. It then turns to apply these results to bearings: static characteristics and stability of various rotor systems and an extensive discussion of air rings.

Inhaltsverzeichnis

Frontmatter

Introduction

Introduction

Abstract
Gas bearings have been used to support rotors in machines since the 1960. They have been designed for several applications, including gyros, supports for magnetic heads in computer hard disc’s, and special machine tools. Gas bearings are particularly valuable when they are used to support high-speed rotors in precision machines. Gas-lubricated films are nearly isothermal, because the ability of the bearing materials to dissipate heat is greater than the heat-generating capacity of gas films, which have very low friction losses, so no thermal effects appear during gas bearing operation. These advantages of gas bearings are due to the fact that the surfaces of the journal and bush are separated by a gas (mainly air) layer characterized by a very low (compared with oil) viscosity. Gas bearings retain their advantages at high rotational velocities, which significantly exceed the maximum rotational velocities admissible for oil bearings and rolling bearings.
Krzysztof Czolczynski

Theory

Frontmatter

1. Mathematical Model of a Gas Journal Bearing

Abstract
The object of considerations in this chapter is a radial, externally pressurized gas bearing with a cylindrical bush, shown in Figure 1.1. The description of the bearing is given in the system of Cartesian coordinates(x-y-z)related to the bush; the forceF Z means the external loading of the journal, andFis the load capacity.
Krzysztof Czolczynski

2. Identification of Stiffness and Damping Coefficients

Abstract
The dynamic characteristics of gas bearings can be represented by a set of stiffness and damping coefficients, which are functions of the static load, the rotating speed and the whirl frequency of the bearing shaft. These coefficients can be used directly in a critical speed calculation or an unbalance response calculation. In addition, the coefficients can be used in a stability investigation. Values of the coefficients are computed mostly by the perturbation method in the Reynolds equation, which is the basic equation of the mathematical model of gas bearings. This method is restricted to the self-acting or porous bearings; in general it enables us to determine only the eight linear stiffness and damping coefficients.
Krzysztof Czolczynski

3. Mathematical Model of Rotor — Gas Bearing System

Abstract
The object of the first part of considerations in this chapter is the rigid symmetrical rotor supported in two gas bearings (Figure 3.1). Between the joint bushes and the casing, an isotropic system of the linear springsK p and the viscous dampers Cpis mounted. The external static loading of the rotor (for example, the weight of the rotor) acts on half of a rotor length. The equations that describe the motion of the rotor and bushes around their static equilibrium positions are written in the Cartesian coordinates x, y, and z. The forces 2FzandF z act in thex-zplane.
Krzysztof Czolczynski

Applications

Frontmatter

4. Gas Bearings

Abstract
The objects of consideration here are the bearings, the data of which as follow:
  • lengthL= 0.11 m
  • adiusR1= 0.055 m
  • radial clearance c1= 30 x 10-6m, and
  • gas viscosity σ = 18.2 x 10-6kg m-1s-1(air).
Krzysztof Czolczynski

5. Stability of Rotor — Gas Bearing System

Abstract
Figure 5.1 shows the stability map of the reduced system supported in the rigidly mounted (K p = ∞) externally pressurized (p 0 =7) bearing with direct feeding system.
Krzysztof Czolczynski

6. Air Rings

Abstract
In his first investigations, the author computed the stiffness and damping coefficients of the air rings with the simplest direct feeding system (the same as the direct feeding system of the bearings). The length of the ringL =0.11 m was equal to the length of the bearing (Figure 6.1).
Krzysztof Czolczynski

7. Stability of the Rotor — Bearing — Air Rings System (Applications)

Abstract
When the boundaries of the unstable regions are defined, the main problem to be solved is to design such a support of the joint bushes to ensure the required values of the parametersCpandKp. This book proposes an external gas ring surrounding the bearing bush.
Krzysztof Czolczynski

Backmatter

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