Lubrication and Lubricants
Introduction
No doubt that the friction between the moving surfaces of the machines
causes loss in the mechanical energy and rapid damage for the machines,
and thus lubricants should be used to decrease this friction. Lubricants are
also useful in internal combustion engines since they prevent gas leakage
from the minute distances between the cylinder and the piston rings.
The lubricating oil also helps in carrying some of the solid combustion
products which will cause erosion if not removed. It also absorbs the extra
heat in the machine and sends it to the radiator.
Usually the lubricating oil passes through a cycle; oil is pumped from a
ground storage tank to channels for lubrication locations. The oil is then
collected and filtered to remove any impurities; the oil then falls back by
gravity to the ground storage tank.
Choice of a lubricant depends on the speed, load and physical conditions of
the moving metal surfaces inside the engines. In the case of using lubricating
oils, its viscosity should be reasonable and should not extremely vary with
temperature. The oil should also resist all harmful chemical reactions (like
reaction with oxygen or molecular cracking) during its operation. This last one
disperses its viscosity.
Viscosity of the lubricating oil:
Although the lubricating oil is used to reduce friction, oil viscosity itself acts as
a friction force that prevents the movement of the machine. The equation
that shows the movement resistance in the ideal cases is (F N ), where F
is the movement resistance, N is the number of revolutions per unit time
(velocity) and is the oil viscosity. Rapid revolution of a cylindrical body
covered with oil results in a repelling force that repels the oil from the surface
of the body. The centrifugal force is directly proportional to N2, which means
that the oil should be viscous enough to resist the centrifugal force and to
remain stuck to the body surface. The suitable oil is that oil whose viscosity
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(attraction) is slightly higher than the centrifugal force (repulsion) at the
operating temperature at the higher possible velocity. It may be preferable to
add high molecular weight compounds that contain attracting groups which
may cover the whole body or combines with the solid surface.
Lubricating oils:
In the previous period, vegetable and animal oils were used for the lubricating
purposes, as olive and castor oils; and then these oils were replaced by the
heavy paraffinic compounds resulting from oil refinery whose boiling points
range from 300 oC to 350 oC, which are produced under vacuum (30-50 mm
Hg). These products are divided into 3 products: heavy, medium and light;
each is treated to reach the desired product requirements.
Each of these products passes through some processes aiming at:
First:
Removal of the oxidizable materials (naphthenes). The oxidation results in the
formation of water and acidic compounds which have harmful effects on the
oil viscosity change with respect to temperature; expressed as viscosity index.
Second:
Removal of the compounds which can solidify at the low temperatures (waxy
compounds). These waxy compounds act as a gelatinous mass which resists
the motion of the machines.
Third:
Improvement of the color and flow properties and removal of the glue
forming compounds.
Addition of certain compounds that expand the life time of the oil.
Lubricating oil industry passes through the following processes:
1. Deasphalting
This is accomplished by using solvents that extract the naphthenic
compounds from the oil, where the solubility of the naphthenic
compounds in these solvents is higher than the solubility of the
paraffinic oil in it.
The most commonly used solvents are phenol and ferforal.
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When the oil is mixed with one of these solvents, the naphthenes
dissolve in it and are precipitated in the bottom; then the naphthenes-
free oil floats above.
2. Dewaxing:
The deasphaled oil is then cooled to a low temperature (-10oC), the
wax solidifies and can be filtered from the oil through rotary filters
where the wax is skimmed from it.
3. Finishing
Hydrogen treatment is important to react with the unsaturated
compounds (which may be converted to heavy polymers causing oil
blockage) and convert them to saturated compounds. Hydrogen also
reacts with any sulpher present in the oil and converts it to hydrogen
sulphide.
Activated clay is added to the treated oil to remove any color or
residual wax and naphthenes. The activated clay containing the
undesired compounds is filtered, and then some special additives are
mixed with the oil.
Oil improvement additives:
To expand the life time of the lube oil and improve its efficiency, oil is mixed
with some additives to improve its viscosity index. Some polymers whose
viscosity is increased with the increase in temperature are added to
compensate the decrease of oil viscosity resulting from high temperature.
Other additives are added to coat any solidified pellets to prevent the
solidification of the whole oil. This enhances the oil efficiency in the cold
conditions.
Other additives are added to prevent oil oxidation and erosion, and to react
with metal surfaces and cover them.
Lubricating Oil Tests:
Many tests are applied to the treated oil to test its efficiency.
1. Viscosity measurement:
To measure the oil viscosity, the time (seconds) required for a specific
oil volume to spill through a standard hole of a specific instrument is
measured. The most famous instrument is Saybolt; and the viscosity
unit is called Saybolt Universal Seconds S.U.S.. Oil viscosity is usually
measured at 100, 130, 210oF. Oil viscosity decreases as the
temperature increases, the oil is ideal when the rate of change of its
viscosity decreases with the increase in temperature.
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2. Viscosity Index (V.I.):
Viscosity index expresses the rate of change of the oil viscosity w.r.t.
change in temperature. The value of this index is high when the oil is
perfect; which means that this change is low.
The rate of change of the paraffinic oil viscosity w.r.t. temperature is
very low (Pennsylvanian oil), and its viscosity index is taken as 100,
while this rate for a naphthenic oil (Gulf oil) is very high and its
viscosity index is taken as 0.
To evaluate the V.I. of an oil, its viscosity is measured at 210 and
100oF. The Gulf and Pennsylvanian oils that have the same viscosity at
210 oF as the tested oil are then determined. Their viscosities at 100 oF
are then measured. The V.I. is then calculated from the following
equation:
3. Oxidation Resistance test:
An oil sample is taken and then its oxygen absorption is determined.
Oxygen passes over the sample in a standard way, when the oxidizing
ability of the oil is high, the oil turned turbid due to the oxidation of its
components. As a result of this oxidation, water is formed which hang
with the oil and increase its friction. The oil then is classified as a poor
oil.
4. Sediments test
An oil sample is taken and diluted with alcohol, then it enters to a
centrifuge machine to assure the removal of any solids or remaining
activated clays or additives.
Greases:
Greases are used in the locations where it is hard to make an oil cycle or
when the lubricated locations are exposed to water and dust. Greases are
made of lubricating oils dispersed in a soap material (مادة صابونيتSodium
soap, Calcium soap and Lithium soap). During operation, the soap acts as a
velvety surface which carries the lube oil. Both the soap and the lube oil
prevent the friction.
Greases differ in the kind of oil used and the acidic and mineral radicals of the
used soap and their percentages.
The most commonly used grease is the calcium grease, which is composed of
calcium soap; since it resists the wet medium and thus it is used in water
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pumps and tractors. But this kind of grease cannot bare high temperatures
(higher than 60oC). When it is exposed to high temperatures, the soap loses
its water content and thus it loses its coherence.
Sodium grease can bare up to 80 oC, but it dissolves in water. Lithium grease
has both advantages of calcium and sodium greases and it can bare very low
temperatures (-5 oC). Its high production cost limits its usage and thus it is
used in lubricating some plane parts.
Silicone grease:
This grease is composed of lithium soap and silicone oil. This grease can be
used from -7 oC to 300 oC for 1000 working hours. It is used in lubricating
some spacecrafts parts and ultrasonic planes.
Solid Lubricants:
At high temperatures, powders of solid materials are used. Molybdenum
sulphide and graphite are the most commonly used lubricants. Molybdenum
sulphide is preferred upon graphite at temperatures till 900 oC, while at higher
temperatures graphite predominates. These powders are sprayed over the
lubricated locations from its bottle عبوة. In this bottle عبوة, the powder is
mixed with compressed gas or liquid, and when the atomizer is pressed the
powder and the gas spreads where the atomizer is directed.
Solid lubricants can also be used in the form of coatings that coat the
supports formed from sintered materials to fill the surface spaces using
Molybdenum sulphide or graphite.
Spacecrafts use a mixture of Molybdenum sulphide 70%, graphite 7% and
Silicone oil 23%.
In some cases, some coats of plastics are used as dry lubricants replaced
every few years. Teflon is usually used for these purposes.
Lubricating emulsions:
During cutting and lathing processes, emulsion of lube oil in water is used.
This oil minimizes the friction between the metal and the cutting equipment
and thus decreases the cutting cost due to the decrease of the required
energy in addition to the softness of the cut surface. Water also absorbs the
heat energy resulting from cutting due to its high heat capacity.
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Brake Fluid Oil
Brake oil should be lubricating and should not attack the metal or the piston.
The piston should not also absorb the oil to prevent its expansion which will
cause its damage. Its surface tension should also be reasonable to prevent
the leakage from the piston. Its V.I. should be high.
Castor oil is very satisfactory, especially when diluted to 25% (by volume)
with a diluents which does not spoil the previously mentioned properties. This
diluents might be amyl or butyl alcohol. An erosion inhibitor can also be
added.
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Solvent 2
Solvent 1 Solvent 2
Asphalt
separation
solvent 1
heating Wax
solvent 2
Oil + solvent
separation
Solvent 1
heating
Oil
mixing
Deasphalting wax + solvent wax
cooling
separation
Raw oil
heating
heating filtration
separation
Treated
oil
Asphalt
