TABLE OF CONTENTS Lubricants minimise friction and wear in rubbing contacts by reducing metal-metal contact, removing wear debris, and carrying away frictional heat. They may also prevent rusting and with liquid lubricants remove heat. Lubricants may be solid, such as graphite, molybdenum disulphide, poly-tetrafluoroethylene and talc; or gaseous, commonly air; but the principal lubricants are liquids such as mineral oil, or the semi-solid greases formed from liquids by the use of thickening agents.
A viscous fluid film between two solid surfaces moving in very close proximity to one another becomes pressurised and holds those surfaces apart, often against considerable loads. During the process, the surface may be deformed elastically (EHL elastohydrodynamic lubrication) or plastically.
In this case, two solid surfaces are separated by a thick fluid film supplied from an external pressure source, e.g. by an oil pump system.
When the contact of asperities on sliding couples increases as the load increases, the sliding speed decreases or the fluid viscosity decreases, the friction significantly increases and the load is mainly supported by the asperity contact. Such lubrication condition is known as boundary lubrication.
This is the situation when several lubrication modes, such as HL and boundary lubrication, coexist.
In hydrodynamic lubrication friction is purely viscous and is directly dependent on the area of the film, the rate of shear and the viscosity of the lubricant. Coefficients of friction are as...
