Lubrication,
introduction of a substance between the contact surfaces of moving parts to
reduce friction and to dissipate heat. A lubricant may be oil, grease,
graphite, or any substance—gas, liquid, semisolid, or solid—that permits free
action of mechanical devices and prevents damage by abrasion and
"seizing" of metal or other components through unequal expansion
caused by heat. In machining processes lubricants also function as coolants to
forestall heat-caused deformities.
Types of Lubricants
Lubricants can be
classified by their origin—animal (e.g., sperm oil, goose grease), vegetable
(e.g., soybean oil, linseed oil), or mineral (e.g., petroleum, molybdenum
sulfide). From ancient times until the late 19th cent. lubricants were obtained
from vegetable oils or animal fats and oils. Today most are derived from
mineral oils, such as petroleum and shale oil, which can be distilled and
condensed without decomposition. Synthetic lubricants, such as silicones, are
of great value in applications involving extreme temperatures. In certain types
of high-speed machinery films of gas under pressure have been successfully used
as lubricants.
Types of Lubrication
Differing widely in
viscosity, specific gravity, vapor pressure, boiling point, and other
properties, lubricants also offer a wide range of selection for the
increasingly varied needs of modern industry. But whatever their derivation or
properties, the purpose of lubricants is to replace dry friction with either
thin-film or fluid-film friction, depending on the load, speed, or intermittent
action of the moving parts. Thin-film lubrication, in which there is some
contact between the moving parts, usually is specified where heavy loads are a
factor. In fluid, or thick-film, lubrication a pressure film is formed between
moving surfaces and keeps them completely apart. This type of lubrication
cannot easily be maintained in high-speed machinery and therefore is used where
reciprocating or oscillating conditions are moderate.
Application of
Lubricants
Efficient operation of
machinery largely depends not only on the lubricant selected but also on its
method of application. Lubricants formerly were applied by hand, but modern
machinery requires exact methods that can be precisely controlled. For most
machinery, different methods of lubrication and types of lubricants must be
employed for different parts. In an automobile, for example, the chassis is
lubricated with grease, the manual transmission and rear-axle housings are
filled with heavy oil, the automatic transmission is lubricated with a
special-grade light oil, wheel bearings are packed with a grease that has a
thickener composed of long fibers, and the crankcase oil that lubricates engine
parts is a lightweight, free-flowing oil.
Application of Liquid
Lubricants
Mechanical devices to
supply lubricants are called lubricators. A simple form of lubricator is a
container mounted over a bearing or other part and provided with a hole or an
adjustable valve through which the lubricant is gravity-fed at the desired rate
of flow. Wick-feed oilers are placed under moving parts, and by pressing
against them they feed oil by capillary action. Horizontal bearings are frequently
oiled by a rotating ring or chain that carries oil from a reservoir in the
bearing housing and distributes it along the bearing through grooves or
channels. Bath oiling is useful where an oil-tight reservoir can be provided in
which the bearing journal may be submerged; the pool of oil helps to carry away
heat from contact surfaces. Splash-oiling devices are used where gears,
bearings, or other parts contained in housings have moving parts that dip into
the lubricant and splash it on the bearings or into distribution channels.
Centralized oiling systems usually consist of a reservoir, pump, and tubes
through which oil is circulated, while heaters or coolers may be introduced to
change the viscosity of the lubricant for various parts of the system. Many
oiling operations are automatically synchronized to start and stop with the
machinery.
Application of
Semisolid and Solid Lubricants
Grease lubricants are
semisolid and have several important advantages: They resist being squeezed
out, they are useful under heavy load conditions and in inaccessible parts
where the supply of lubricant cannot easily be renewed, and they tend to form a
crust that prevents the entry of dirt or grit between contact surfaces. Grease
is a mixture of a lubricant and a thickener; often it is made from a mineral
oil and a soap. It may be applied in various ways: by packing enclosed parts
with it, by pressing it onto moving parts from an adjacent well, by forcing it
through grease cups by a spring device, and by pumping it through pressure
guns. Solid lubricants are especially useful at high and low temperatures, in
high vacuums, and in other applications where oil is not suitable; common solid
lubricants are graphite and molybdenum disulfide. So if you are looking to
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