Gas lubrication is a form of fluid lubrication, which uses air or working medium gas as a lubricant to separate two friction pairs moving in relation to each other. It has the advantages of low friction resistance, high working accuracy, and a wide range of applicable temperatures, and is widely used in the design of gas-eous static pressure bearings, high-speed thrust bearings, foil bearings, mechanical seals, and other mechanical parts and equipment under extreme conditions such as extremely high or low temperatures, ultra-high or -low speeds, and ultra-precision.
In 1854, Hirm came up with the idea of using gas as a lubricant. In 1886, Reynolds deduced the Reynolds equation describing the pressure distribution of fluid lubrication film, which raised peoples understanding of the principle of fluid lubrication to a theoretical height. In the 1950s, research on gas lubrication theory based on bearing design requirements developed rapidly. In 1959, Elrod and Burgdorfer theoretically explained that the temperature rise effect inside the lubrication gas film could be ignored under general working conditions, and the isothermal hypothesis was generally accepted in gas lubrication analysis. After the 1980s, the development of magnetic storage technology pushed the film thick-ness of gas bearings from microns to nanometers, and microscopic effects such as the gas thinning effect and surface roughness were widely studied, which pro-moted the development of gas thin film lubrication theory.
With an increase of rotation speed in mechanical equipment and the appear-ance of new bearing structures, the problem of gas thermodynamic lubrication became increasingly prominent. For example, in a gas hydrostatic bearing under the conditions of a film thickness of 20 m, pressure of 0.7 MPa, and rotation speed of 20,000 rpm, shear heat can cause the gas film lubrication temperature to rise above 30C. The increase of rotor temperature rise and gas viscosity can improve the stiffness and damping coefficient of the bearing, but in the absence of sufficient cooling, the bearing is prone to thermal instability, which is particu-larly prominent in a high-speed bearing design.
Compared with bearing gas lubrication dominated by shear flow, there is also pressure flow caused by seal pressure in the gas seal lubrication area. In the pro-cess of gas leakage flow from the high-pressure side to the low-pressure side, gas film temperature decreases because of rapid volume expansion, resulting in ther-mal distortion and other gas thermohydrodynamic lubrication problems.
In 1968, the John Crane Company first developed the circular arc surface spiral groove gas-lubricated seal and introduced plane spiral groove gas-lubricated seal products. With the development of gas sealing technology to high-temperature, high-pressure, high-speed, and other high parameters; the diversification of sealing media; and the continuous expansion of application fields, the surface thermal dis-tortion, supersonic flow, media phase change, and other gas thermohydrodynamic lubrication problems are increasing.
Preface
Based on the research methods and results of elastohydrodynamic lubrication theory of Prof. Wen Shizhu, this book summarizes the research results of the authors in recent years. Grounded in the application background of lubrication design of high-speed gas bearings and high-pressure gas seals, theory and analysis methods of gas thermohydrodynamic lubrication are systematically expounded. In this book, theoretical model and lubrication characteristics of gas lubrication are discussed for typical bearings and seals.
The book is divided into 10 chapters.
In Chapter 1, Properties of gases, properties of gases are introduced. Based on the principle of energy equipartition, the ideal gas state equation is decomposed into two independent gas equations.
In Chapter 2, Gas lubrication equations, the derivation of the Reynolds equa-tion, energy equation, heat conduction equation, interface equation, and other basic lubrication equations are explained. The lubrication analysis of force bal-ance and flow conservation problems are also discussed.
In Chapter 3, Isothermal gas lubrication, we introduce the modeling method and basic lubrication characteristics of isothermal gas lubrication of typical struc-tures such as the slider bearing, radial bearing, thrust bearing, and face seal.
In Chapter 4, Gas thermohydrodynamic lubrication of rigid surfaces, the gas thermodynamic lubrication law of gas-lubricated bearings and low-pressure gas seals under high-speed working conditions are analyzed without consideration of surface distortions.
Chapter 5, Gas thermoelastichydrodynamic lubrication of face seals, discusses the high-pressure gas face seal, the modeling method of TEHL, and fundamental characteristics of choked flow effect, thermoelastic distortion of seal faces, and the temperature distribution of gas film.
In Chapter 6, Transient thermoelastichydrodynamic gas lubrication of face seals, targets the gas face seal, dynamic load characteristics of the seal gas film under the conditions of isothermal lubrication, rigid surface thermal lubrication, and TEHL, exploring these in detail.
Chapter 7, Vapor-condensed gas lubrication of face seals, explores the face seal of high-pressure gas, the law of condensation and precipitation of water vapor in the seal gas film, and the movement of liquid drops in the seal gap under TEHL conditions.
Chapter 8, Cryogenic gas lubrication of face seals, uses the inclined ellipse dimpled gas face seal as an example and natural gas as the sealing medium to introduce the modeling method of cryogenic gas lubrication and the fundamental characteristics of phase change in seal film and thermoelastic distortion of seal faces.
In Chapter 9, Surface grooves of gas face seals and testing technology, typical seal grooves and their laser processing methods and experimental measurement methods are presented. Opening characteristics, hydrodynamic characteristics, and friction characteristics of gas face seals are discussed by taking the micro-dimpled face seal as an example.
Preface III
In Chapter 10, Design of gas face seals, an internal flow single-face spiral groove gas seal is used as an example to introduce the basic method and design process of gas face seals.
In the process of compiling this book, Prof. Huang Ping has given great sup-port and help. Here I would like to express my sincere gratitude. At the same time, I would like to express my heartfelt thanks to my colleagues and graduate students who have given me warm support and help in the preparation of this book.
Bai Shaoxian
Hangzhou