From Smithells Metals Reference Book, Eighth Edition

24.1 Introduction

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.

24.1.1 Main Regimes of Lubrication


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.

24.2 Lubrication Condition, Friction and Wear

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...

Products & Services
Industrial Lubricants
Industrial lubricants are oils, fluids, greases and other compounds designed to reduce friction, binding or wear and exclude moisture. Specialized characteristics may enhance thermal conduction across thermal interfaces or reduce electrical resistivity across electrical joints.
Metalworking Lubricants, Coolants, and Fluids
Metalworking lubricants, coolants and fluids are specialized coatings and carriers for metal forming, metal cutting, lapping, polishing, and grinding applications.
Hydrostatic and Hydrodynamic Bearings
Hydrostatic bearings and hydrodynamic bearings are fluid film bearings that rely on a film of oil or air to create a clearance between the moving and stationary elements.
Oil Additives and Fluid Additives
Oil additives and fluid additives are chemical substances that are added to oils and industrial fluids to impart or improve certain properties. They are used with lubricants, coolants, thermal oils, greases, metal working fluids, and other application-specific fluids.
Plain Bearings and Sleeve Bearings
Plain bearings and sleeve bearings (also referred to as bushings or journal bearings) are used to constrain, guide or reduce friction in rotary or linear applications. They function via a sliding action.

Topics of Interest

The friction and wear characteristics of materials are not intrinsic properties but, rather, depend on a large number of variables including the physical, chemical and mechanical properties of the...

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