- Email: [email protected]
Gas film lubrication
Int. J. Mech. Sci. P e r g a m o n Press Ltd. 1(.}64. Vol. 6, p. 151. P r i n t e d in Great Britain
BOOK R E V I E W W. A. GROSS: Gas Film Lubrication. John Wiley, New York, 1962. 413 pp., 105s. THIS book is an outgrowth of nine I.B.M. Research Reports dated 1958 to 1960 and this conveniently brings them together within a single volume. Its purpose is to review the fundamentals of fluid lubricating films and apply these to a variety of examples ; experimental results are introduced to corroborate the theory. This is the first book which deals largely with gas lubricated bearings b u t normal incompressible fluid lubrication theory and experiment is extensively discussed as under conditions of low velocity a good approximation to the behaviour of a gas film is obtained from a solution, assuming the film to be incompressible. Thus the book also serves as an introduction to the whole field of lubrication theory. Differences and similarities between the properties of compressible gas and incompressible lubricating films are constantly brought out. For example, a popular justification for using gas bearings is that friction is low; however, although the frictional force of the gas bearing is lower by perhaps three orders of magnitude, the coefficient of friction is larger than that of a corresponding liquid bearing. Gas bearings are being increasingly used where there are large temperature variations (the viscosity of a gas does not change much with temperature), in nuclear applications (because lubricating gases are comparatively free from adverse effects due to radioactivity), in precision instruments and for high-speed bearings. They have been used for journal and thrust bearings and for hydrostatic as well as hydrodynamic bearings. After a general introduction to the field of lubrication, the momentum, energy and continuity equations of fluid flow are presented, limiting conditions explained and by the order of magnitude approach, the equations are reduced to their simplest form. The general Reynolds equation for unsteady, compressible and polytropic flow is derived although subsequent treatment deals almost exclusively with isothermal films. For a gas film pressure boundary conditions are simple as compared with liquid films which m a y cavitate. The next chapter deals with infinitely long steady hydrodynamic films for m a n y oil film geometries, followed by a chapter dealing with finite hydrodynamic bearings. Because of the non-linear form of the Reynolds equation for compressible films analytic solutions are possible only for the simplest film shapes. Recourse is made to simplifications to make the equation linear, perturbation methods, relaxation methods and computer solutions. I t is in the latter approach that the author and his associates at I.B.M. have made valuable contributions to the understanding of gas lubrication. Many facets of steady externally pressurized bearings are then discussed and a final chapter deals with unsteady films and bearing systems. The book concludes with an excellent clearly presented bibliography. Only minor errors have been picked up b y the reviewer, e.g.p. 308, the third line of Section 6.1 should refer to equation 2.6.10 and not 2.5.10; equation 2.6.13 (on p. 308) should contain a sigma in the last item. This book can be recommended to anyone interested in the field of lubrication. A special plea is made for the standardization of symbols. Another excellent book on this subject has recently been published. This is Theory of Hydrodynamic Lubrication by O. Pinkus and B. Sternlicht (McGraw-Hill). Although there are m a n y similarities, uniform symbols would be desirable. C. F . KETTLEBOROUGH
BOOK R E V I E W W. A. GROSS: Gas Film Lubrication. John Wiley, New York, 1962. 413 pp., 105s. THIS book is an outgrowth of nine I.B.M. Research Reports dated 1958 to 1960 and this conveniently brings them together within a single volume. Its purpose is to review the fundamentals of fluid lubricating films and apply these to a variety of examples ; experimental results are introduced to corroborate the theory. This is the first book which deals largely with gas lubricated bearings b u t normal incompressible fluid lubrication theory and experiment is extensively discussed as under conditions of low velocity a good approximation to the behaviour of a gas film is obtained from a solution, assuming the film to be incompressible. Thus the book also serves as an introduction to the whole field of lubrication theory. Differences and similarities between the properties of compressible gas and incompressible lubricating films are constantly brought out. For example, a popular justification for using gas bearings is that friction is low; however, although the frictional force of the gas bearing is lower by perhaps three orders of magnitude, the coefficient of friction is larger than that of a corresponding liquid bearing. Gas bearings are being increasingly used where there are large temperature variations (the viscosity of a gas does not change much with temperature), in nuclear applications (because lubricating gases are comparatively free from adverse effects due to radioactivity), in precision instruments and for high-speed bearings. They have been used for journal and thrust bearings and for hydrostatic as well as hydrodynamic bearings. After a general introduction to the field of lubrication, the momentum, energy and continuity equations of fluid flow are presented, limiting conditions explained and by the order of magnitude approach, the equations are reduced to their simplest form. The general Reynolds equation for unsteady, compressible and polytropic flow is derived although subsequent treatment deals almost exclusively with isothermal films. For a gas film pressure boundary conditions are simple as compared with liquid films which m a y cavitate. The next chapter deals with infinitely long steady hydrodynamic films for m a n y oil film geometries, followed by a chapter dealing with finite hydrodynamic bearings. Because of the non-linear form of the Reynolds equation for compressible films analytic solutions are possible only for the simplest film shapes. Recourse is made to simplifications to make the equation linear, perturbation methods, relaxation methods and computer solutions. I t is in the latter approach that the author and his associates at I.B.M. have made valuable contributions to the understanding of gas lubrication. Many facets of steady externally pressurized bearings are then discussed and a final chapter deals with unsteady films and bearing systems. The book concludes with an excellent clearly presented bibliography. Only minor errors have been picked up b y the reviewer, e.g.p. 308, the third line of Section 6.1 should refer to equation 2.6.10 and not 2.5.10; equation 2.6.13 (on p. 308) should contain a sigma in the last item. This book can be recommended to anyone interested in the field of lubrication. A special plea is made for the standardization of symbols. Another excellent book on this subject has recently been published. This is Theory of Hydrodynamic Lubrication by O. Pinkus and B. Sternlicht (McGraw-Hill). Although there are m a n y similarities, uniform symbols would be desirable. C. F . KETTLEBOROUGH