Use
lubrication: The use of a substanceto prevent contact between solid surfacesin relative motion in order toreduce friction, wear, overheating,and rusting. Liquid hydrocarbons(oils), either derived from petroleumor made synthetically, are the mostwidely used lubricants as they arerelatively inexpensive, are goodcoolants, provide the appropriaterange of viscosities, and are thermallystable. Additives include polymericsubstances that maintain thedesired viscosity as the temperatureincreases, antioxidants that prevent the formation of a sludge, and alkaline-earth phenates that neutralizeacids and reduce wear.At high temperatures, solid lubricants,such as graphite or molybdenumdisulphide, are often used.Semifluid lubricants (greases) areused to provide a seal against moistureand dirt and to remain attachedto vertical surfaces. They are madeby adding gelling agents, such asmetallic soaps, to liquid lubricants.