Lubricant
Whenever one body moves over the surface of another, it encounters a resistance to its movement from the other surface. This resistance is called friction. Although friction may exist between a solid and fluid surface, friction is primarily a surface phenomenon that arises between solids.
Lubricants are used for one or more of the following purposes: to reduce friction, to prevent wear, to prevent adhesion, to aid in distribution of a load, to cool moving parts, and/or to prevent corrosion. Besides oils and greases, many plastics, solids, and even gases are now used as lubricants. The chief limitations on many of these materials is their ability to replenish themselves, to dissipate frictional heat, to withstand high temperature environments, and to remain stable in working environments. Types of lubricants include petroleum fluids, synthetic fluids, greases, solid films, working fluids, gases, plastics, animal fat, metallic and mineral films, and vegetable oils.
When a small amount of lubricant is placed between two solid surfaces, the surfaces will remain in contact, but the friction between the two surfaces will be reduced. In this regime, lubricants act by reducing the shear strength at the surface through the formation of a surface film held by physical adsorption to, chemisorption to, or chemical reaction with the solid surface.
When larger amounts of a viscous lubricant are placed between two sliding surfaces, the separation between the surfaces increases as the sliding speed increases or as the vertical load on the surfaces decreases. As the distance between the surfaces increases, the amount of solid-solid contact between the surfaces decreases. In this case, the resistance to sliding is produced by harder asperities on the surfaces and by adhesion between points of solid-solid contact. At the point that the solid surfaces is completely separated by the lubricating film, the lubrication becomes hydrodynamic, i.e., governed only by the lubricant, and the frictional resistance becomes very small, being determined by the fluid shear of the lubricant. Hydrodynamic lubrication has been achieved in thrust bearings using mineral oil, synthetic lubricants, gas, water, and other lubricants.
The coefficient of friction is defined as the ratio of the frictional force between two bodies in contact parallel to the contacting surfaces to the force at right angles to contact that presses the bodies together. Clean metals in vacuum have a coefficient of friction between themselves of about unity. Lubrication by liquids can give a coefficient of friction for roller bearings of about 10(-3-4), for ball bearings of about 10(-5), and for hydrostatically pumped oil pads of about 10(-7). Solids such as graphite, molybdenum disulfide, Teflon, lead, babbit, silver, and metal oxides, with coefficients of friction of about 10(-1-2), are sometimes used as dry film lubricants, often in conjunction with fluid or grease lubricants. Lubricating greases are usually petroleum oils thickened with dispersions of soap; synthetic oils with soap or inorganic thickeners; or oils containing silicon-based dispersions.
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