2. PURPOSE AND NECESSITY OF LUBRICATION
Lubrication is the control of friction and wear by the introduction of a friction-
reducing film between moving surfaces in contact. The lubricant used can be a
fluid, solid, or plastic substance. Many different substances can be used to
lubricate a surface.
• Although this is a valid definition, it fails to realize all that lubrication actually achieves.
• Many different substances can be used to lubricate a surface. Oil and grease are the most common.
• Grease is composed of oil and a thickening agent to obtain its consistency, while the oil is what actually
lubricates. Oils can be synthetic, vegetable or mineral-based as well as a combination of these.
3. The primary functions of a lubricant are to:
• Reduce friction
• Prevent wear
• Protect the equipment from corrosion
• Control temperature (dissipate heat)
• Control contamination (carry contaminants to a filter or sump)
• Transmit power (hydraulics)
• Provide a fluid seal
Sometimes the functions of reducing friction and preventing wear are used interchangeably.
However, friction is the resistance to motion, and wear is the loss of material as a result of friction, contact
fatigue and corrosion.
There is a significant difference. In fact, not all that causes friction (e.g., fluid friction) causes wear, and not
all that causes wear (e.g., cavitational erosion) causes friction.
Reducing friction is a key objective of lubrication, but there are many other benefits of this
process. Lubricating films can help prevent corrosion by protecting the surface from water and
other corrosive substances. In addition, they play an important role in controlling contamination
within systems.
4. Types of Lubrication
There are three different types of lubrication: boundary, mixed and full film. Each
type is different, but they all rely on a lubricant and the additives within the oils to
protect against wear.
Full-film lubrication or Hydrodynamic lubrication can be broken down into two forms:
hydrodynamic and elastohydrodynamic. Hydrodynamic lubrication occurs when two
surfaces in sliding motion (relative to each other) are fully separated by a film of
fluid.
Elastohydrodynamic lubrication is similar but occurs when the surfaces are in a
rolling motion (relative to each other). The film layer in elastohydrodynamic
conditions is much thinner than that of hydrodynamic lubrication, and the pressure
on the film is greater. It is called elastohydrodynamic because the film elastically
deforms the rolling surface to lubricate it.
Even on the most polished and smooth surfaces, irregularities are present. They
stick out of the surface forming peaks and valleys at a microscopic level. These
peaks are called asperities. In order for full-film conditions to be met, the lubricating
film must be thicker than the length of the asperities. This type of lubrication protects
surfaces the most effectively and is the most desired
5. Boundary lubrication is found where there are frequent starts and stops, and where shock-loading conditions
are present. Some oils have extreme-pressure (EP) or anti-wear (AW) additives to help protect surfaces in the
event that full films cannot be achieved due to speed, load or other factors.
These additives cling to metal surfaces and form a sacrificial layer that protects the metal from wear. Boundary
lubrication occurs when the two surfaces are contacting in such a way that only the EP or AW layer is all that is
protecting them. This is not ideal, as it causes high friction, heat and other undesirable effects.
6. Mixed lubrication is a cross between boundary and
hydrodynamic lubrication. While the bulk of the surfaces are
separated by a lubricating layer, the asperities still make
contact with each other. This is where the additives again
come into play.
With a better understanding of this process, it should be
easier to define what lubrication actually is. It is a process of
either separating surfaces or protecting them in a manner to
reduce friction, heat, wear and energy consumption. This can
be accomplished by using oils, greases, gases or other fluids.
So the next time you change the oil in your car or grease a
bearing, realize there is more going on than meets the eye.
7. • Sliding between clean solid surfaces is generally characterized by a high coefficient of friction and severe wear
due to the specific properties of the surfaces, such as low hardness, high surface energy, reactivity, and
mutual solubility.
• Clean surfaces readily adsorb traces of foreign substances, such as organic compounds, from the
environment.
• The newly formed surfaces generally have a much lower coefficient of friction and wear than the clean
surface.
• The presence of a layer of foreign material at an interface cannot be guaranteed during a sliding process;
therefore, lubricants are deliberately applied to produce low friction and wear.
• The term “lubrication” is applied to two different situations: solid lubrication and fluid (liquid or gaseous) film
lubrication.
8. • A solid lubricant is any material used as a powder or a thin, solid film on a surface to
provide protection from damage during a relative movement by reducing friction and
wear.
• Solid lubricants are used for applications in which any sliding contact occurs, for
example, a bearing operating at high loads and low speeds and a hydrodynamically
lubricated bearing requiring start/stop operations.
• The term solid lubricants embrace a wide range of materials that provide low friction
and wear.
• Hard materials are also used for low wear and/or under extreme operating conditions.
Friction
9. • A thin film on the order of surface roughness of moving surfaces, results in relatively
low friction and wear, as compared to solid–solid contact.
• A thick fluid film between two surfaces in relative motion prevents solid–solid contact
and can provide very low friction (in the range of 0.001–0.003) and negligible wear.
• Fluid can be liquid or gaseous; even a thick film of air transposed between two
moving surfaces is a method of good lubrication.
10. Regimes of Fluid Film Lubrication
A regime of lubrication, in which a thick film is maintained
between two surfaces with little or no relative motion by an
external pumping agency, is called hydrostatic lubrication.
A summary of the lubrication regimes observed in fluid
lubrication without an external pumping agency (self-acting) can
be found in the familiar Stribeck curve
11. This plot for a hypothetical fluid-lubricated bearing system presents the coefficient of
friction as a function of the product of absolute viscosity (η) and rotational speed in
revolutions per unit second (N) divided by the load per unit projected bearing area (P).
The curve has a minimum, which immediately suggests that more than one lubrication
mechanism is involved. The regimes of lubrication are sometimes identified by a
lubricant film parameter equal to h/σ – (mean film thickness)(composite standard
deviation of surface heights of the two surfaces).
13. Liquid Lubricants:
Liquid lubricants or Lubricating oils: Lubricating oils also known as liquid lubricants and further classified into three
categories; (i) Animal and Vegetables oils, (ii) Mineral or Petroleum oils and (iii) blended oils.
Characteristic of good lubricating oils: (1) high boiling point, (2) low freezing point, (3) adequate viscosity for proper
functioning in service, (4) high resistance to oxidation and heat, (5) non-corrosive properties and (6) stability to
decomposition at the operating temperatures.
(i) Animal and Vegetables oils: Animal oils are extracted from the crude fat and vegetables oils such as cotton seed oil and
caster oils. These oils possess good oiliness and hence they can stick on metal surfaces effectively even under elevated
temperatures and heavy loads. But they suffer from the disadvantages that they are costly, undergo easy oxidation to give
gummy products and hydrolyze easily on contact with moist air or water. Hence they are only rarely used these days for
lubrication. But they are still used as blending agents in petroleum based lubricants to get improved oiliness.
14. Liquid Lubricants:
(ii) Mineral or Petroleum oils: These are basically lower molecular weight hydrocarbons with about 12 to 50 carbon
atoms. As they are cheap, available in abundance and stable under service conditions, hence they are widely used. But the
oiliness of mineral oils is less, so the addition of higher molecular weight compounds like oleic acid and stearic acid
increases the oiliness of mineral oil.
(iii) Blended oils: No single oil possesses all the properties required for a good lubricant and hence addition of proper
additives is essential to make them perform well. Such additives added lubricating oils are called blended oils. Examples:
The addition of higher molecular weight compounds like oleic acid, stearic acid, palmetic acid, etc or vegetables oil like
coconut oil, castor oil, etc increases the oiliness of mineral oil.
15. Semi-solid Lubricants (Grease):
Semi-solid Lubricants are formed by emulsifying oil and fat with thickening agents like soap of sodium, calcium, lithium,
aluminum at higher temperature.
Classification
Soda based: In this case sodium soaps are used as a thickening agent in mineral or petroleum oil. They are slightly soluble in
water. They can be used up to 175 C.
Lithium based: In this case lithium soaps are emulsifying with petroleum oil. They are water resistance and used up to 15 C.
Calcium based: In this case calcium soaps are emulsifying with petroleum oil. They are also water resistant and used up to
80C. At higher temperature soap and petroleum oil are separate from each other.
16. Solid Lubricants:
Graphite, molybdenum disulphide (MoS2), boron nitride (BN)x are predominantly
used as a solid lubricants. They are used under high temperature and high load
(pressure).
i) Graphite:
It is most widely used as a solid lubricant. Graphite has layer structure; layers are
held together with the help of weak Vander Waals’ forces which facilitate the
easy sliding of one layer on the other layer. It is very soapy to touch, non-
inflammable. It is used at higher temperature (around 450 C) condition. They
are either used as powder form or mixed with oil or water.
17. ii) Molybdenum disulphide (MoS2):
It is sandwich like structure in which hexagonal layer of molybdenum
(Mo) lies between two hexagonal layers of sulfur (S) atom. Like
graphite each layers are held together with weak Vander Waals’ forces.
It is stable up to 400 C. It is differ from graphite because it is used in
high vacuum unlike graphite (graphite is mixed with water or oil). It
adheres even more strongly to the metal or other surface.
18. Solid Lubricants:
Solid lubricants: They are preferred where (1) the operating conditions are such that a lubricating film cannot be
secured by the use of lubricating oils or grease
(2) contamination (by the entry of dust particles) of lubricating oils or grease is unacceptable
(3) the operating temperature or load is too high, even for grease to remain in position and
(4) combustible lubricants must be avoided.
They are used either in the dry powder form or with binders to make them stick firmly to the metal surfaces while in
use. They are available as dispersions in nonvolatile carriers like soaps, fats, waxes, etc and as soft metal films. The
most common solid lubricants are graphite, molybdenum disulphide, tungsten disulphide and zinc oxide. They can
withstand temperature up to 650° C and can be applied in continuously operating situations. They are also used as
additives to mineral oils and greases in order to increase the load carrying capacity of the lubricant. Other solid
lubricants in use are soapstone (talc) and mica.
19. Properties of Lubricants
a) Cloud Point:
The temperature at which lubricating oil becomes cloudy in appearance is called cloud point.
b) Pour Point:
The lowest temperature at which the lubricant oil become semi-solid and ceases to flow is called pour point. It indicates the
suitability of lubricants used in cold condition. Good lubricant should possess low pour point.
d) Flash point:
The flash point of a volatile material is the lowest temperature at which vapors of the material will ignite for a moment
when an ignition source brought near to it. The lubricating oil should have flash point reasonably above its working
temperature.
e) Fire point:
The fire point of a fuel is the lowest temperature at which the vapour of that fuel will continue to burn for at least 5 seconds
when an ignition source brought near to it. Fire point is around 10oC higher than flash point.
20. Properties of Lubricants
e) Viscosity:
Viscosity is the property of a fluid that determines its
resistance to flow. It is an indicator of flow ability of
lubricating oil. The lower viscosity greater will be the flow
ability. If temperature increases viscosity of the lubricating
oil decreases and pressure increases viscosity of lubricating
oil increases. In short we can say that good lubricating oil is
that whose viscosity does not change with temperature.
f) Viscosity Index:
The variation of viscosity of a liquid with temperature is
called viscosity index. A relatively small change in viscosity
with temperature is indicated by high viscosity index
whereas, a low viscosity index shows, a relatively large
change in viscosity with temperature.
21. Properties of Lubricants
f) Aniline Point:
• Aniline point of the lubricant oil is defined as the minimum equilibrium solution temperature for equal volumes of
aniline and lubricant oil samples.
• It gives an indication of the possible deterioration of the lubricant oil in contact with rubber sealing; packing, etc.
Aromatic hydrocarbons have a tendency to dissolve natural rubber and certain types of synthetic rubbers. Consequently,
low aromatic content in the lubricant oil is desirable.
• A higher aniline point means a higher percentage of paraffinic hydrocarbons and hence, a lower percentage of aromatic
hydrocarbons.
22. Properties of Lubricants
g) Corrosion Stability:
• Corrosion stability of the lubricant oil is estimated by carrying out corrosion test. A polished copper strip is placed in the
lubricant oil for a specified time at a particular temperature.
• After the stipulated time, the strip is taken out and examined for corrosion effects. If the copper strip has tarnished, it
shows that the lubricant oil contains any chemically active substances which cause the corrosion of the copper strip.
Good lubricating oil should not affect the copper strip.
• To retard corrosion effects of the lubricant oil, certain inhibitors are added to them. Commonly used inhibitors are
organic compounds containing P, As, Cr, Bi or Pb.
23. Lubrication Methods
• For bearing lubrications, either grease or oil is used. It is important to choose the appropriate lubrication
method that suits bearing operating conditions and purpose for the bearing to perform well.
• Oil lubrication is generally better than grease lubrication in many respects, but grease lubrication is also
widely used, because they have merits in that bearings have the available inside spaces for grease and that it
is comparatively quite simple to use them.
24. Grease Lubrication
Lubricating Grease
• Grease can be defined as the lubricant of solid or semi-solid state that contains the thickener and some grease contain
various special ingredients.
• Because various kinds of greases have their own distinct characteristics, and sometimes even the same kind of greases
produce quite different performance results, one has to be careful when selecting the greases.
(1) Base Oil
• Base oil in the grease is the main ingredient which actually provides lubricating function, and it forms 80 to 90% of
grease.
• So, it is important to select the right kind of base oil and its viscosity. There are two main types of base oil, mineral base
oils and compound base oils. Mineral oils from low to high viscosity are widely used.
• Generally, the mineral oils with higher viscosity are used for the locations requiring the lubrications of high load, low
speed, and high temperature, and the ones with lower viscosity for the locations requiring the lubrications of low load,
low speed, and low temperatures.
25. Grease Lubrication
• Compound base oils are generally very expensive and used for the locations requiring the lubrications of extremely high
or low temperatures, or wide temperature ranges, and fast speed and high precision.
• Compound base oils of mainly ester, poly--olefine, or silicon series are generally used, but the use of fluorine compound
oils are increasing nowadays.
(2) Thickener
• Thickener is one of the most important elements in deciding the properties of the grease, and the thickness of
grease depends on how much thickener is mixed in the grease.
• There are mainly three kinds of thickeners, namely, metal soap, non-organic non-soap, and organic non-soap, but
the metal soap thickeners are mostly used, and the non-organic non-soap thickeners are generally used only for
the special cases, such as operation in high temperature.
• Generally speaking, the grease with high dropping point can be used in high temperatures, and the water-
resistance of grease depends on that of thickener. Also for the bearings that come in contact with water or are
operated under the high humidity level, the Na soap grease or the grease that contains Na soap cannot be used,
because they deteriorate quickly when in contact with the water or moisture.
26. Grease Lubrication
(3) Additives
• Various kinds of additives are used to enhance the grease performance and to meet the customers’ demands for different
functions.
• These additives enhance the physical or chemical properties of grease, and/or minimize the wear, corrosion, or rust to
the lubricated metals.
• There are various kinds of additives used for prevention of oxidization, wear and tear, or rust. The appropriate grease
containing right kind of additives to the applied location has to be used.
(4) Worked Penetration
• Worked penetration is used to represent the hardness of grease, and it is shown as the penetrated depth(1/10mm) to
grease by the pendulum of specified weight, and the greater the value is, the softer the grease is.
27. Grease Lubrication
Polymer Grease
• Polymer grease of hardened lubricant mixed with polyamid is generally used, and it allows to supply the grease for a
long period.
• It is widely used for the bearings to which the strong centrifugal force is applied, such as the ones in wire stranding
machines or compressors, or to which leaking and pollution to the environment or insufficient lubrication is easy to
happen.
Life Span of Grease
• The life span of grease is a period from the start of bearing operation to bearing failure due to its insufficient lubricating
action. The life span of grease with 10% of bearing failure possibility.
• Refilling interval is set considerably shorter than the lubrication interval, so as to provide stability.
• Reliability can be increased sufficiently even for the greases barely meeting the minimum requirements, if lubricated in
accordance to specification.
28. Oil Lubrication
Lubricants
• Lubricants can be largely divided into two groups, namely mineral oil base lubricants and synthetic lubricants.
• When selecting a lubricant, its viscosity is one of the most important factors to be considered. If its viscosity is too low at
its operating temperature, oil film cannot be sufficiently formed, causing abrasion and/or burning-and-sticking.
• And, if it is too high, its viscosity resistance becomes higher, causing temperature/friction rise and subsequent abnormal
power loss. In general, lubricants with low viscosity are used when it runs at high speed and low load, and ones with high
viscosity when at low speed and high load.
• Lubricants should be selected in accordance with viscosity specified by ISO, and its viscosity index can be used
conveniently for references.
• Although it depends on viscosity indices, its viscosity gets reduced by half whenever the temperature of lubricant
increases by 10.
Oil Lubrication Methods
(1) Oil Sump Lubrication
• It is the most generally used lubrication method, especially for low or medium speed operations.
29. Oil Lubrication
• Oil surface should be, in principle, placed at the center of lowest rolling element, and it is better to be able to confirm the
location of oil surface by using the oil gauge.
(2) Drip Feed Lubrication
• This method is widely used for small bearings that operate at a relatively high speed, and oil supply is controlled by
adjusting the volume of oil drip.
(3) Throwaway Lubrication
• This is a method that utilizes gear or circulation ring to supply oil to bearings. It is widely used for automotive
transmissions or gears.
(4) Circulation Lubrication
It is widely used when it is necessary to cool the bearing parts that revolve at a high speed or that with high surrounding
temperature. Oil is fed through feed pipe and recovered through recovery pipe, which is cooled down and re-fed again.
The diameter of recovery pipe should be bigger than that of feed pipe, so as to prevent back pressure from occurring to
the oil inside a bearing.
30. Oil Lubrication
(5) Jet lubrication
• It is widely used for high speed revolution bearings and oil is jet-sprayed through one or several nozzles under constant
pressure into the inside of a bearing.
• In general, jet stream speed should be faster than 1/5 of circumferential speed of inner ring outer surface because air wall
formed by surrounding air revolving with bearing tends to weaken the jet stream.
• Provided that total volume of lubricant is same, the more the number of nozzles are, the smoother and the greater the
cooling effect is.
(6) Spray Lubrication
• Spray lubrication is a method that vaporizes the lubricant by blowing in the air to be sprayed into bearing. It has
following merits.
• - Due to small volume of lubricant required, its churning resistance gets smaller, which in return makes it suitable for
high speed revolution bearings. - Because it minimizes volume of discharged lubricant, the pollution to the
equipment can be also kept to the minimum. - Because fresh lubricant is fed all the time, bearing life can be
extended. Therefore, it is widely used for various machining.
36. • Oxidation stability is a chemical reaction that occurs with a combination of the lubricating oil and
oxygen.
• The rate of oxidation is accelerated by high temperatures, water, acids and catalysts such as copper.
• The rate of oxidation increases with time. The service life of a lubricant is also reduced with increases in
temperature. Oxidation will lead to an increase in the oil's viscosity and deposits of varnish and sludge.
• The rate of oxidation is dependent on the quality and type of base oil as well as the additive package
used. Some synthetics, such as polyalphaolefins (PAO), have inherently better oxidation stability than do
mineral oils.
• This improved oxidation stability accounts for the slightly higher operating temperatures that these
synthetic oils can accommodate.
• Generally, oxidation will reduce the service life of a lubricant by half, for every 10 degrees C (18 degrees
F) increase in fluid temperature above 60degrees C (140 degrees F).
• This concept is based on the Arrhenius rate rule, which is named for the 19th-century Swedish chemist
Svante Arrhenius.
Oxidation stability of Lubricants
37. • There is a little controversy concerning the oxidation stability of natural mineral base oils as determined
by the refining method.
• There is one school of thought that suggests that hydrotreated base stocks have superior oxidation
resistance and thermal stability than does solvent-refined base oil.
• This is based on the misconception that since hydrotreating removes all of the potentially undesirable
compounds, the base oil tends to automatically reduce deposit-forming tendencies and thereby will
better resist oxidation. However, removing all of the compounds considered undesirable can in fact be
detrimental.
• Solvent refining results in the production of base oils, which retain some sulfur compounds that are
natural antioxidants.
• These base oils retain a natural ability to prevent oxidation, while hydrotreated base oils must be further
fortified with antioxidants in order to maintain thermal and oxidation stability.
Oxidation stability of Lubricants
38. • Once the antioxidants are weakened or depleted, oxidation of some hydrotreated oils can occur
very rapidly.
• Severely hydrotreated base oils also have poor solubility characteristics. Without proper
formulation, additives may not remain suspended, and some additive drop out could occur.
• Several methods may be used to determine or evaluate the oxidation stability of an oil, which is
usually regarded as the number of hours until a given increase in viscosity is noted or until there is
a given increase in the acid number (AN).
Oxidation stability of Lubricants
39. • Thermal stability can have a dramatic impact on how well a lubricant performs under different operating
conditions. At low temperatures,
• some lubricants may start to congeal which can affect pour point characteristics and flow rate. This can starve
machinery of vital lubrication and result in increased resistance, friction and in a worst case scenario, seizure.
• In hydraulic circulating systems, congealed lubricants can trigger a drop in static pressure.
• This can create vaporous air bubbles which implode when compressed by pumps and can intensify vibrations,
accelerate wear and compromise the efficiency of filters.
• High temperatures can also cause issues, including the destruction of hydrodynamic lubrication properties.
• Exposure to high temperatures can accelerate wear and oxidation, which can release contaminants into a system
and increase the risk of water contamination.
• Additive depletion is also an issue, with high temperatures actively degrading chemical components used to
improve lubrication performance. These can include antioxidants, corrosion inhibitors, anti-wear agents,
detergents and dispersants
Thermal stability of Lubricants
40. • Thermal conductivity and viscosity are two of the most important characteristics of engine oil as they have a
significant impact on overall performance of a motor.
• Thermal conductivity describes a material’s ability to transfer heat. Materials with a high thermal conductivity,
such as copper or silver, can facilitate rapid heat transfer whereas insulative materials, such as foam or cotton,
are slow to take up and transfer heat from their environment.
• Engine oil will absorb heat from contact surfaces and transport it to another location such as an oil stump
where is can be safely dispersed.
• Most oils are engineered to help cool a number of engine parts including piston assembly, heads and valves.
Generally, most formulated oils consist of either a mineral, semi or fully synthetic base material combined with
a varying number of additives.
• The quality of an engine oil depends on the base stock as well as the properties of the additives. Engine oil is
available at a range of SAE grades designed to best suit the climate where it is being used.
• The relationship between thermal conductivity and efficiency of the engine is strongly correlated as engine oils
with a higher thermal conductivity value will have greater efficiency and minimize friction loss.
Thermal Conductivity of Lubricants
41. • The additive content in lubricating oils ranges from just a few parts per million to several percentage points.
Depending on the function that the additives develop they may be classed as:
• Substances intended to improve the intrinsic characteristics of the base oils (viscosity index modifiers and pour point
improvers).
• Lubricant protective substances (antioxidants).
• Substances giving new properties and protecting the metal surfaces of engines (detergents, dispersants, friction
modifiers, anti-wear/Extreme Pressure (EP) additives, rust and corrosion inhibitors).
• The additives added to improve lubricating oils include:
✓ Viscosity Index Modifiers - Hydrogenated ethylene-propylene copolymers (also called OCP, Olefin Co-Polymers).
✓ Pour point improvers – (polymethacrylates, ethylene-vinyl acetate copolymers and polyfumarates)
✓ Antioxidants – (alkylated aromatic amines, sterically hindered phenols, zinc dialkyl dithiophosphates, and
derivatives of dialkyl dithiocarbamic acid)
✓ Detergents and dispersants – (Metallic detergents & Dispersants). [The combination of detergent/dispersant
additives allows more acid compounds to be neutralized and more contaminant particles to stay suspended].
✓ Friction modifiers – (long amphiphilic organic molecules or of metal-organic compounds (generally with a
molybdenum base))
✓ Anti-wear/EP additives – (zinc dialkyl dithiophosphates)
✓ Anti-corrosives/rust-inhibitors – (etoxylate alcohols, long-chain carboxylic acids, phosphoric esters, amines,
imidazoline and thioderivatives) https://www.tribonet.org/wiki/lubricant-additives/
Types of additives of Lubricants
