It is hard to imagine any machine that could operate over a prolonged length of time without a lubricant. Thus, it is fortuitous that air, with its ubiquitous abundance, can function in this capacity. This is not intuitively obvious, particularly when one deals with parallel surfaces in a thrust bearing. Late Professor Fuller—in his book on the theory and practice of lubrication for engineers, also published by Wiley in 1984—shows the picture of a small thrust bearing with three shoes that can support a 4 lb (17.8 N) thrust runner 5 in. (12.7 cm) in diameter. By simply spinning the runner by hand, one can show that bearing can ride on a thin layer of air for a long time. Running this simple experiment in a classroom has become an eye-opening experience for our engineering students.
Professor Al-Bender provides an excellent preface in which he describes his experience with air bearings and the long-lasting research and educational career that eventually led him to write this comprehensive treatise on air bearings. The book provides a very methodical and unique approach to presenting the material. For example, Chapter 1 is devoted to a very nice, easily digestible, and informative synopsis of all 17 chapters. Then—in contrast to many other books that tend to jump to the derivation of the Reynolds equation and its solution—he first devotes Chapters 2 and 3 to general flow considerations in a bearing. With clear illustrative diagrams, he shows the flow behavior outside the entrance to the bearing, how the flow can be treated as the so-called inviscid flow away from the surface, and how its velocity profile changes as air enters the clearance space of a bearing. Having established the flow characteristics, in Chapter 4, a full derivation of the Reynolds equation is presented with various types of solutions for self-acting and externally fed bearings. The topic of externally fed (aerostatic) bearings and full solutions are presented in Chapter 5 and continued in Chapter 6 with axial tilt and misalignment considerations. Chapter 7 uses the linearized Reynolds equation to provide insight into the stability and dynamic behavior of aerostatic circular bearings. Chapter 8 is particularly interesting since it provides a very succinct description and formulation of the effect of narrow grooves in a bearing and derivation of herringbone-type configurations to enhance the pumping of air in or out of the bearing. All these chapters are primarily concerned with thrust-type bearings. Having familiarized the reader with the concepts, Chapter 8 picks up the subject of journal bearings and continues in Chapters 9 and 10 with discussions on different types of grooves, high-speed application, and whirl instability. Chapter 11 introduces foil bearings—a subject that is of significant interest in both academia and industry. Chapter 12 is on a specialty field of porous gas bearings. There are interesting details and complexities associated with slip flow and boundary conditions. Here, analytical predictions are validated with experimental measurements. Chapters 13 and 14 deal with tilting pad bearings and a so-called hanging air bearing, respectively. Chapter 15 discusses active compensation, active milling spindles, and squeeze-film/acoustic bearings. Chapter 16 describes an application of active compensation in ultra-high precision positioning (linear slide). Chapter 17—the final chapter—covers thermal effects. In liquid-lubricated bearings, thermal effects tend to be detrimental to the generation of load-carrying capacity since viscosity drops exponentially with the temperature. Thus, in many books, the subject of thermal effects is covered early on. In gas bearings, the increase in temperature has the opposite effect on the gas viscosity. So, its impact on the load-carrying capacity is not dangerous.
All chapters are written in an authoritative yet easy-to-read manner. The introduction of similarity parameters and scale effects in different chapters and a nice blend of experimental comparisons to theoretical analyses sprinkled throughout will appeal to graduate students and researchers.
In summary, this comprehensive book on air bearings is a carefully written, methodical, insightful, and welcome contribution to the tribology literature.
