Introduction

Not all sliding tribological pairs are designed to operate in the presence of a generous supply of lubricant. The nature of the working environment may make it impossible, or impracticable, to arrange for the contact to be lubricated by a full hydrostatic or hydrodynamic fluid film, for example, in deep space or satellite applications where any liquid lubricant would be lost or degraded by evaporation, or in food processing or chemical plant where contamination of either the product or the environment by any escape of a lubricating fluid would be unacceptable. In other, simpler applications the design constraint may simply be the cost of the lubricants supply and handling equipment. Generally, the aim in the design of machinery is to minimize friction; however, in some devices (clutches, brakes, friction drives, and so on) friction is beneficial, indeed essential, and often components of this sort are operated unlubricated. In this chapter, as well as the tribology of dry sliding (taking as examples both brakes and clutches as well as dry rubbing bearings) we shall also consider the tribological aspects of some ‘marginally’ lubricated contacts. These are only intermittently lubricated by a less than complete fluid film and so rely for their success on a combination of hydrodynamic, elasto-hydrodynamic, and boundary lubrication. Bearings and bushes designed to run completely dry are often manufactured as monolithic solid components and involve at least one nonmetallic material, while marginally lubricated bearings often make use of porous, sintered metals (usually bronze) impregnated by an appropriate mineral oil, grease, or solid boundary lubricant.