ABSTRACT
This chapter outlines the main types of lubricant and covers some of the more important lubricant properties and tests, finishing with engine tests. Lubricants can be fluids (gases or liquids) or solids. Gases are not always considered as lubricants, but air-lubricated bearings are quite widely used for special purposes. However, air for specialist applications may not be a cheap lubricant. For hydrostatic spindle bearings on machine tools, for instance, air cleaned and dried to adequate standards can cost several times as much as lubricating oil. The first, obvious, property of a lubricant is its ability to keep moving surfaces apart, in all the conditions of pressure and temperature, and in the presence of contaminants etc., to which they may be subjected. Measurement of the properties of lubricants can show lubricant composition and can indicate, to some extent, lubricant performance, but hydrocarbon oils are too complex for this type of testing to be a complete guide to behaviour in practice.
