ABSTRACT
Guillaume Amontons
This chapter introduces the essential tribological concepts of surfaces, lubrication, friction and wear, each of which plays a critical role in the performance and reliability of machine elements. Surfaces are investigated in detail, including common definitions to quantify roughness and a general discussion of their physical nature. Conformal contacts such as in journal bearings, and non-conformal contacts such as gears and rolling-element bearings are differentiated. The field of Hertzian contact for non-conformal surfaces is presented. Lubrication theory is developed in terms of the film parameter and the regimes of lubrication, including boundary, partial, and hydrodynamic lubrication. Boundary lubrication typically occurs at low speeds, with inviscid lubricants, and/or high loads, and is always a concern during startup. In boundary lubrication, load is transferred between the asperities of surfaces in contact. At the other extreme, hydrodynamic lubrication involves transfer of load across a pressurized lubricant film that completely separates the surfaces. Surfaces that transfer load through a combination of direct asperity contact and pressurized lubricants involve partial lubrication. Elastohydrodynamic lubrication is a condition that is common in machine elements and is therefore also included. Basically, elastohydrodynamic lubrication involves circumstances where the surfaces are separated by a fluid film and where elastic deformation of the surfaces are large compared to the film thickness. Friction and wear theory are also summarized, including the important Archard wear law used to predict the life of machine elements subjected to adhesive or abrasive wear.
