The book is aimed at providing the engineering profession with a unified approach to the discipline of lubrication. Whereas, up to the present, the field consisted of several noncontiguous modes of operation – boundary, fluid-film, and dry and solid lubrication – the book introduces a new concept wherein all these disparate modes are shown to be but particular phases of a tribological continuum. In this approach, bearings, seals, gear teeth, dampers, and other tribological devices operate at all times on a syndrome of confluence of all of hte above-mentioned mechanisms. The understanding and practical application of this tribological continuum approach should enable the engineer to design and calculate the performance of seals, dampers, tilting pad, bearings, piston rings, and other parallel surface systems for which pervious theory was incapable of providing solutions.
A particularly important aspect of the book is the space allotted to the newest branch of interface media — powder lubrication. There are two important aspects to this new technology. One is that the debris produced by wear at the mating surfaces, instead of being harmful and something to be avoided, can actually be made to act as a lubrication. Given the small size of wear particles hey can be made to form a quasi-hydrodynamic film at the interface, facilitating the smooth performance of the machine in question. The other aspect of this new technology is that of deliberately feeding into tribological devices a supply of powder lubricant. This is a field in which the author has been its foremost proponent over the last dozen years, both as a researcher and product designer. It is a technology of tremendous potential through-out industry.
Throughout the history of conventional lubrication theory and experience, it has been known to workers in the field that many phenomena did not conform to the postulates of hydrodynamic theory, as indicated by the Reynolds equation. In particular, the ability of parallel surfaces, such as thrust bearings with centrally pivoted pads, flat plate sliders and seals, and other parallel surface devices, were known to be capable of supporting a load, in contradiction of the requirement of a convergent film, as postulated by hydrodynamic theory. many attempts were made to explain or “justify” this b3ehavior, most of them imparting to such surfaces some semblance of convergence, either via surface deformation, thermal distortion, the action of surface roughness, or of such perturbations as variations in lubricant viscosity or density. However, they all proved unsatisfactory, either because the calculated effects were minute o because nondeformable, polished surfaces still showed themselves capable of producing a load capacity.
In a 1988 paper, the author and two of his colleagues for the first time offered the hypothesis that there is an overlap or a continuum between boundary lubrication and hydrodynamic in the sense that both mechanisms contribute to load capacity through the weights carried by the two regimes depend on the geometric and operating conditions of the tribological device. A tribological continuum was proposed for the parallel convergent films or the boundary hydrodynamic action with the total load capacity and tribological performance compounded of the simultaneous interaction of the two mechanisms of surface interaction. This understanding was later extended with the introduction of powder lubrication. this form of lubrication can arise either form the unintended accumulation of debris due to normal wear between two surfaces or as a deliberate mode of facilitation relative motion by feeding a properly selected layer of triboparticulates between interacting surfaces. In the course of extensive experimental work with such triboparticulates, it was ascertained that such films exhibit all the basic characteristics of hydrodynamic lubrication. The main difference of powder films stems from the different rheological properties, similar to the differences with any other viscoelastic or pseudoplastic substance. These experimentally verified characteristics extended the previously noted overlap of boundary and hydrodynamic regimes to that of powder lubrication.
The above experimentally observed relations provide the basis of the postulate of an overall tribological continuum spanning a wide array of material lubricants. This synthesis is schematically portrayed in Figure I.1, which can be labeled as a neo-Stribeck curve in which the various regimes of lubrication from boundary to gas-liquid-powder interfaces are seen to overlap, providing conjointly for the generation of load capacity at surfaces moving at some relative motion with respect to each other. It is only necessary to assign to each material lubricant – whether a gas, liquid, powder, or even discrete particles — the proper rheological model to be able to employ the generalized form of equations of motion to obtain the quasi-viscous forces produced by these various kinds of lubricants or, as they are often referred to, “third bodies.”
The present book is concerned with this new interdisciplinary approach to the subject of tribological interaction of various third-body lubricants. In addition to postulating the general theory of interdisciplinary tribology, the text provides specific methods of solving powder lubrication problems encountered in bearings, seals, dampers, and a host of other machine components in which debris particles provide a lubricant layer considerably affecting the dynamics of the machine. In addition to the conceptual and theoretical explication of this new approach to tribology, the book offers experimental data on the rheology of triboparticulate films, as well as design guidelines based on extensive experience and other advanced systems.