Lubrication:-
is the process or
technique employed to reduce friction between, and wear of one or both,
surfaces in proximity and moving relative to each other, by interposing a
substance called a lubricant in between them.
The lubricant can be a solid,a solid/liquid dispersion, a
liquid such as oil or water, a liquid-liquid dispersion (a grease) or a gas.
With fluid lubricants the applied load is either carried by
pressure generated within the liquid due to the frictional viscous resistance
to motion of the lubricating fluid between the surfaces, or by the liquid being
pumped under pressure between the surfaces.
Lubrication can also describe the phenomenon where reduction
of friction occurs unintentionally, which can be hazardous such as hydroplaning
on a road.
The science of friction, lubrication and wear is called
tribology.
Adequate lubrication allows smooth continuous operation of
equipment, reduces the rate of wear, and prevents excessive stresses or
seizures at bearings. When lubrication breaks down, components can rub
destructively against each other, causing heat, local welding, destructive
damage and failure.
The regimes of
lubrication:-
As the load increases on the contacting surfaces three
distinct situations can be observed with respect to the mode of lubrication,
which are called regimes of lubrication:
Fluid film lubrication is the lubrication regime in which,
through viscous forces, the load is fully supported by the lubricant within the
space or gap between the parts in motion relative to one another object (the
lubricated conjunction) and solid–solid contact is avoided.
Hydrostatic lubrication is when an external pressure is
applied to the lubricant in the bearing, to maintain the fluid lubricant film
where it would otherwise be squeezed out.
Hydrodynamic lubrication is where the motion of the
contacting surfaces, and the exact design of the bearing is used to pump
lubricant around the bearing to maintain the lubricating film. This design of
bearing may wear when started, stopped or reversed, as the lubricant film
breaks down.
Lubrication in simple worlds, this is a medium which use to
control heat , friction, to safe machine parts, this is very good , without
this some operation can not be possible.
In market many brands are available , cheap and expensive due to their
capacity of observe heat, and safe parts to damage because of sliding over each
other.
Elastohydrodynamic
lubrication:
Mostly for nonconforming surfaces or higher load conditions,
the bodies suffer elastic strains at the contact. Such strain creates a
load-bearing area, which provides an almost parallel gap for the fluid to flow
through. Much as in hydrodynamic lubrication, the motion of the contacting
bodies generates a flow induced pressure, which acts as the bearing force over
the contact area. In such high pressure regimes, the viscosity of the fluid may
rise considerably. At full elastohydrodynamic lubrication the generated
lubricant film completely separates the surfaces. Contact between raised solid
features, or asperities, can occur, leading to a mixed-lubrication or boundary
lubrication regime.
Boundary
lubrication (also called boundary film lubrication):
The bodies come into closer contact at their asperities; the
heat developed by the local pressures causes a condition which is called
stick-slip and some asperities break off. At the elevated temperature and
pressure conditions chemically reactive constituents of the lubricant react
with the contact surface forming a highly resistant tenacious layer, or film on
the moving solid surfaces (boundary film) which is capable of supporting the
load and major wear or breakdown is avoided. Boundary lubrication is also
defined as that regime in which the load is carried by the surface asperities
rather than by the lubricant.
Besides supporting the load the lubricant may have to
perform other functions as well, for instance it may cool the contact areas and
remove wear products. While carrying out these functions the lubricant is
constantly replaced from the contact areas either by the relative movement
(hydrodynamics) or by externally induced forces.
Lubrication is required for correct operation of mechanical
systems pistons, pumps, cams, bearings, turbines, cutting tools etc. where
without lubrication the pressure between the surfaces in close proximity would
generate enough heat for rapid surface damage which in a coarsened condition
may literally weld the surfaces together, causing seizure.
In some applications, such as piston engines, the film
between the piston and the cylinder wall also seals the combustion chamber,
preventing combustion gases from escaping into the crankcase.
A lubricant :- is a substance introduced to reduce
friction between surfaces in mutual contact, which ultimately reduces the heat
generated when the surfaces move. It may also have the function of transmitting
forces, transporting foreign particles, or heating or cooling the surfaces. The
property of reducing friction is known as lubricity.
In addition to industrial applications, lubricants are used
for many other purposes. Other uses include cooking (oils and fats in use in
frying pans, in baking to prevent food sticking), bio-medical applications on
humans (e.g. lubricants for artificial joints), ultrasound examination, and medical
examination.
Properties:-
A good lubricant generally possesses the following
characteristics:
·
high boiling point and low freezing point (in
order to stay liquid within a wide range of temperature)
·
high viscosity index
·
thermal stability
·
hydraulic stability
·
demulsibility
·
corrosion prevention
·
high resistance to oxidation.
·
Formulation
Typically lubricants contain 90% base oil (most often
petroleum fractions, called mineral oils) and less than 10% additives.
Vegetable oils or synthetic liquids such as hydrogenated polyolefins, esters,
silicones, fluorocarbons and many others are sometimes used as base oils.
Additives deliver reduced friction and wear, increased viscosity, improved
viscosity index, resistance to corrosion and oxidation, aging or contamination,
etc.
Non-liquid lubricants include grease, powders (dry graphite,
PTFE, molybdenum disulfide, tungsten disulfide, etc.), PTFE tape used in
plumbing, air cushion and others. Dry lubricants such as graphite, molybdenum
disulfide and tungsten disulfide also offer lubrication at temperatures (up to
350 °C) higher than liquid and oil-based lubricants are able to operate.
Limited interest has been shown in low friction properties of compacted oxide
glaze layers formed at several hundred degrees Celsius in metallic sliding
systems, however, practical use is still many years away due to their
physically unstable nature.
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