Fundamental Properties of
Lubricants are various substances placed between two rubbing surfaces in order
to reduce friction and wear. Lubricants can be liquids or solids, and even gas films
have important applications. Solid lubricants are often used to reduce dry or
boundary friction, but we have to keep in mind that they do not contribute to the
heat transfer of the dissipated friction energy. Greases and waxes are widely used
for light-duty bearings, as are solid lubricants such as graphite and molybdenum
disulphide (MoS2). In addition, coatings of polymers such as PTFE (Teflon) and
polyethylene can reduce friction and are used successfully in light-duty applica-
However, liquid lubricants are used in much larger quantities in industry
and transportation because they have several advantages over solid lubricants.
The most important advantages of liquid lubricants are the formation of hydro-
dynamic films, the cooling of the bearing by effective convection heat transfer,
and finally their relative convenience for use in bearings.
Currently, the most common liquid lubricants are mineral oils, which are
made from petroleum. Mineral oils are blends of base oils with many different
additives to improve the lubrication characteristics. Base oils (also referred to as
mineral oil base stocks) are extracted from crude oil by a vacuum distillation
process. Later, the oil passes through cleaning processes to remove undesired
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
components. Crude oils contain a mixture of a large number of organic
compounds, mostly hydrocarbons (compounds of hydrogen and carbon). Various
other compounds are present in crude oils. Certain hydrocarbons are suitable for
lubrication; these are extracted from the crude oil as base oils.
Mineral oils are widely used because they are available at relatively low
cost (in comparison to synthetic lubricants). The commercial mineral oils are
various base oils (comprising various hydrocarbons) blended to obta