LUBRICATION

introduction of any of various substances between sliding surfaces to reduce wear and friction. Nature has been applying lubrication since the evolution of synovial fluid, which lubricates the joints and bursas of vertebrate animals. Prehistoric people used mud and reeds to lubricate sledges for dragging game or timbers and rocks for construction. Animal fat lubricated the axles of the first wagons and continued in wide use until the petroleum industry arose in the 19th century, after which crude oil became the chief source of lubricants. The natural lubricating capacity of crude oil has been steadily improved through the development of a wide variety of products designed for the specific lubricating needs of the automobile, the airplane, the diesel locomotive, the turbojet, and power machinery of every description. The improvements in petroleum lubricants have in turn made possible the increase in speed and capacity of industrial and other machinery. There are three basic varieties of lubrication: fluid-film, boundary, and solid.