4. Introduction
• All material surfaces, even if smooth show irregularities
in the form of peaks (asperities) and valleys--- these are
considered large on a molecular scale
• Sliding Friction: Two materials of different hardness
slide over one another, peaks of softer metal get broken
more easily than peaks of harder metal
• Sliding friction results in
• Ploughing (cutting) out the softer material by peaks of harder
metals
• Frictional heat at rubbing surfaces resulting in high local
temperature--formation of welded junctions
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5. Frictional Resistance
Applied force or load
Coeffcient of friction = ____________________
Friction is nothing but, “ the force of resistance to the relative motion of
two solid surfaces in contact” and coefficient of friction is, “ the ratio of
force of friction to the applied force or load.”
When the applied force is more than the resistance between two moving
surfaces, wear occurs
When lubricating substances are applied on the moving surfaces, friction
reduces and thereby wear also gets minimized
Helps to reduce maintenance cost of machines, because a thin film of a
Lubricant reduces friction and thus controls wear
Friction
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6. Lubricants and Lubrication
• Any substance introduced between two
moving/sliding surfaces to reduce frictional
resistance between them is known as lubricant
• Process of reducing frictional resistance between
moving/sliding surfaces by introduction of lubricants
in between them is called lubrication
• To keep sliding/moving surfaces apart, so that
frictional resistance and consequent destruction
of materials is minimized
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7. Functions of a lubricant
• Reduces surface deformation, wear & tear because
direct contact between the rubbing surfaces is avoided
• Reduces loss of energy in form of heat --- lubricant
acts like a coolant --- reduces frictional heat thereby
controlling expansion of metals --- maintains shape,
size & dimensions of metal parts in contact
• Reduces waste of energy-- Efficiency of machine
enhanced
• Acts as a sealent --- preventing leakage of gases under
high pressure
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8. • Controls corrosion of moving machine parts ---
prevents attack due to moisture
• Cleaning and Suspending—Lubricant facilitates
smooth operation of the equipment by removing &
suspending potentially harmful products, such as carbon,
dirt and wear debris
Functions of a lubricant
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9. Mechanism of lubrication
The mechanism of lubrication is nothing but, “application of
lubricant on sliding/moving surfaces and its action towards
reducing frictional resistance”
Friction depends upon,
a) The nature of metal
b) The load on the metal
c) The temperature developed during operation
d) The pressure exerted during operation/working
Three types of mechanisms most widely used under various
conditions:
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11. Hydrodynamic lubrication
• Moving/Sliding surfaces
are separated from each
other by a thick lubricant
film (1000A0 thick)
• This prevents direct
surface to surface
contact so that the small
peaks and valleys do not
interlock
• Sewing machines, clocks,
scientific instruments
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12. • Lubricant is applied in the
annular space
• Stationary position – Two
surfaces remain in contact
• In Motion – Shaft begins to
rotate and also the lubricant film
rotates between metallic surfaces
• As shaft gains velocity, liquid film
flows at greater rate
• Due to thick oil film, all asperities
of metal surfaces are filled up and
pressure is developed that keeps
the two surfaces separated----
thereby reducing wear
Operation of a Journal
Bearing
Bearing consists of shaft
rotating at a fair speed with
moderate load
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13. Boundary lubrication
• Thick film cannot persist when
machines are on low speed and
high load with a lubricant of low
viscosity
• Thin layer of lubricant is
adsorbed (physical or chemical
forces) on the metallic surfaces
which avoids direct metal to metal
contact
• Soaps of vegetable oil or animal
oil, mineral oils blended with fatty
acids, solid lubricants like
graphite, MoS2, greases
Applied in machines like gears,
tractors, rollers, rail axle boxes
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14. Extreme pressure lubrication
• Under heavy load, high speed operating conditions,
machines attain higher temperatures
• Due to frictional heat, liquid lubricants fail to stick to the
surfaces & may decompose or vaporize
• Additives like P, S or chloro compounds are added to
mineral oils. Such additives improve specific characteristics
of lubricating oil---Extreme pressure additives
• Additives react with metal surfaces => form durable films
like phosphides, sulfides that act as good lubricant under
extreme pressure & temperature
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15. Classifications of Lubricants
• Solid lubricants- used at high speed and moderate load
or at very high load and low speed e.g. solid
organic/inorganic substances such as graphite, mica,
MoS2, Teflon etc.
• Semisolid lubricants- low speed and high pressure, and
high temperature up to 80oC e.g. various types of
greases
• Liquid lubricants- e.g. vegetable & animal oils,
mineral or petroleum oils; blended or compounded oils
and synthetic oils
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16. Semisolid lubricants (Greases)
Further classified on the basis of soaps used in
their manufacturing:
i) Calcium soap grease
ii) Sodium soap grease
iii) lithium soap grease
iv)Barium soap grease
v) Aluminium soap grease
The characteristic of greases are influenced by the
soap and properties such as:
i) Cloud and pour point
ii) Resistance to oxidation
iii) Consistency of finished greases
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18. Properties of lubricants
Viscosity and Viscosity Index
• Property by virtue of which a liquid or fluid (oil) offers
resistance to its own flow Viscosity
• Rate at which viscosity of oil changes with temperature is
measured by an empirical number, known as Viscosity
Index
• Oils become thin on heating– viscosity decreases;
Decrease in viscosity is rapid ---oil has low viscosity index
& vice versa
• Oils should possess adequate viscosity to function as a
good lubricant
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20. Flash and Fire point
• Measures of flammability
• Flash Point : Minimum temperature at which liquid gives
off vapours that will ignite for a moment when a small
flame is brought near it
• Fire Point : Minimum temperature at which vapors of the
oil burn continuously for at least 5 seconds when a small
flame is brought near it
• Used to indicate fire hazards of petroleum products and
evaporation losses under high temperature operations
• Knowledge of flash and fire points in lubricating oil helps
to take preventive measures against fire hazards
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22. Cloud and Pour Point
• Indicate the suitability of lubricants in cold conditions
• Cloud Point: Temperature at which oil becomes cloudy
or hazy in appearance; temperature at which solidifiable
compounds, like paraffin wax, present in oil begin to
crystallize or separate from solution
• Pour Point : Temperature at which oil ceases to flow or
pour
• A good lubricating oil must have low cloud and pour
point or else it may solidy at lower temp causing
jamming of machine parts affecting speed of working
machine
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24. • Oiliness: Ability of lubricant oil to stick to the
machine parts under heavy load or pressure
• Poor oiliness causes easy squeeze out of lubricants when
machines are operated under heavy load
• Petroleum oil has poor oiliness, but Vegetable oils have
good oiliness
• Petroleum oils are mixed with animal/vegetable oils or
fatty acids like oleic acid, stearic acid, etc
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25. Saponification & Acid Value
• Saponification value: Number of milligrams of potassium
hydroxide required to saponify one gram of oil
• Characteristic property of vegetable/animal oil;
Mineral/synthetic oils do not undergo saponification
• Alkaline hydrolysis of pure oil giving soap and glycerol
• Helps us to estimate stability of an oil in aqueous/alkaline
medium, if machine parts face such conditions
• Signifies composition of oil, thereby checks suitability of
oils for lubrication--- Drying property of oil is a harmful
phenomena which can also be checked
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26. • A known quantity of oil (W gm) is mixed with known excess
of alcoholic KOH (0.5 N)
• Stirred vigorously and reflux mixture for 2 hr in water bath
using water condenser
• Fatty acids form potassium salts (soaps) & glycerol is
released
Oil + alc. KOH = Soap + glycerol + unreacted KOH
• Unreacted KOH titrated against HCl (0.5N). Amount of
unreacted KOH is known from amount of HCl consumed
thus quantity of KOH consumed can be calculated
• Saponification value = Amount of KOH (blank – back)
x NKOH (0.5) x 56/W of oil (Wg)
• Expressed in mg of KOH
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27. • Acid value: Number of milligrams of potassium
hydroxide required to neutralize free fatty acids present
in one gram of oil
• To determine content of free acids in an oil; their
presence can harm machines during lubrication
• Mineral oils on prolonged exposure to oxygen, results in
oxidation--- formation of carboxylic acids--- making oil
unsuitable for lubrication
• Acid value should be minimum
• Higher the acid value, greater the corrosion of machine
parts, more wear & tear----------- more maintenance
cost
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28. • Accurately weigh 5 g of the oil under test into a 250 ml
conical flask and add 50 ml of neutral alcohol.
• Heat the flask over a water bath for about 30 minutes.
• Cool the flask and the contents to room temperature and
add a few drops of phenolphthalein indicator.
• Titrate with the standard KOH solution until a pale
permanent pink colour appears at end point.
• Acid Value = Amount of KOH x No of ml of N/10
KOH used x 56/ Weight of oil (W gm)
• Expressed in mg of KOH
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