The introduction of a thin-film of lubricant – usually a semiviscous
fluid – between two surfaces moving relative to each other, in order
to minimize friction and abrasive wear.
In particular, bearings are lubricated in
engines and other machinery. Liquid lubricants are most common, usually
petroleum fractions, being cheap, easy
to introduce, and good at cooling the parts. The viscosity
is tailored to the load, being made high enough to maintain the film yet
not so high that power is lost. Multigrade oils cover a range of viscosity.
The viscosity index represents the constancy of the viscosity over the usual
temperature range – a desirable feature.
|Lubricants are used to reduce wear between surfaces
in rubbing contact, such as bearings. A film of oil between the bearing
surfaces prevents metal contacting metal and reduces friction (A).
Refined mineral oils have small quantities of fatty acids (1) added
as boundary lubricants; the reactive carboxyl group at one end of
the molecule (2) attracts it to the metal surface so that it stands
on end. Oils for different uses usually have different vicosities,
but multigrade oils contain additives which allow them to withstand
the range of temperatures and pressures encountered as bearings change
speed. However, with time, oil molecules degrade, breakdown, or undergo
chemical change and become less effective and should be replaced or
Synthetic oils, including silicones, are
used for high-temperature and other special applications. Greases –
normally oils thickened with soap,
fats, or waxes –
are preferred where the lubricant has to stay in place without being sealed
in. Solid lubricants, usually applied with a binder, are soft, layered solids
including graphite, molybdenite,
talc, and boron
nitride. Teflon, with its uniquely low coefficient of friction, is used
for self-lubricating bearings. Rarely air or another gas is used as a lubricant.
Additives to liquid lubricants include antioxidants, detergents, pour-point
depressants (increasing low-temperature fluidity), and polymers to improve
the viscosity index.