Transcript 11801_lubricants-final-1

LUBRICANTS
Lubricants and lubrication
 Lubricant is a substance which when
introduced between two moving parts
reduces friction by forming a slippery film
between the two surfaces and thus improving
efficiency and decreasing wear
 So, lubrication is the process of reducing
friction and wear between two moving
surfaces by applying lubricating substances
between the parts
Friction
 Friction is defined as opposing force that is set
up between the surfaces of contact, when one
body moves or tends to move over the surface
of other body.
Sliding or kinetic friction: Friction between two moving/ sliding surfaces
Rolling frictions: Friction due rolling of spherical and cylindrical objects on
other surface
Principle of lubrication
 All material have irregularities in form of peak (asperities)
and valleys
 When peak of upper surface come in contact with peak of
lower surface, these asperities may crush under small load,
until the contact area is large enough to support the load and
is known as wear
 And also when the upper surface and lower surface move
against each other that lead to friction which further reduces
the efficiency of the machine
 Therefore to reduce wear and to reduce friction we use
lubricants that increases the efficiency of the machines
Purpose/functions of lubrication
 To keep moving parts apart: by forming thin layer between
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the surfaces
To reduce friction: lubricant to surface friction is lesser than
surface to surface friction
To protect against wear: lubricant prevent wear by keeping
moving part apart
To transfer heat: liquid lubricants are effective in heat
circulation due to high specific capacity
To prevent corrosion: as surface of metal do not come in
contact with air (Oxygen)
To seal gapes: lubricant seal the space between moving parts
through capillary force
To carry away contaminants and debris
 Reduces chance of seizure of two moving surfaces
 Reduces the maintenance & running cost of machine
Mechanism of lubrication: thick film or hydrodynamic
lubrication
 This also known as fluid film lubricant
 If the surface slide over the other lead to increased friction and
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asperities tend to break leading to wear
An excellent method to prevent this is to use lubricant film to
separate the surfaces and is known as hydrodynamic lubrication
(hydro= liquid, dynamic = relative motion)
The lubricant film ensure that no wear is possible between moving
parts due to reduced friction
The hydrodynamic film is formed when the geometry, motion of
moving surface and lubricant viscosity combine to increase the
lubricant pressure to support the load as increased pressure forces
the surface apart
In hydrodynamic lubrication one surface floats over the other
surface and increased fluid pressure forces the surface apart
The four essential elements to consider in hydrodynamic
lubrication: liquid (hydro); relative motion (dynamic); viscous
properties of liquid and geometry of surfaces
 The heat generated between moving parts has to be
dissipated effectively to decrease the temperature
dependence of viscosity
 Delicate instruments such as watches, clocks, guns,
scientific instruments are provided with hydrodynamic
lubrication
 Hydrocarbons are considered to be best for fluid film
lubrication
 Other mechanism:
 Thin film of boundary lubrication
 Extreme pressure lubrication
Classification of lubricants
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Gaseous lubricants: (lower viscosity, and higher
compressibility eg air, steam or liquid metal vapors
Liquid lubricants:
water (used as such or in combination with other
oils and used in engineering processes such as
milling and lathe turning.
Mineral oil (derived from crude petroleum oil) eg
polyalphaolefins, naphthenics, polyalkyleneglycos
Lanolin (derived from sheep wool and are
corrosion inhibitor, protection against rust, salt and
acid)
Vegetable oil (triglyceride esters, eg castor oil,
palm oil, sunflower oil, whale oil etc)
III.
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Solid lubricants: used under high temperature conditions in
form of dry powder
Graphite (planes of polycyclic carbon atom and hexagonal
orientation. Higher the carbon atom content and higher the
crystallization the better is the lubrication action
Molybdenum disulphide (M0S2) and Tungsten disulphide:
hexagonal structure and lubrication performance is better than
graphilte, but lubrication is limited to 400 oC as above 400 oC it
undergoes degradation
Boron nitride (BN): ceramic powder ( used at high temp. up to
1200 oC; exist in tow forms cubic (very hard cant used as
lubricant) and hexagonal used for lubrication
Teflon or poly tetrafluoroethane: PTFE does not have layered
structure but the molecules slip easily along each other and
hence the lubrication is achieved
Mica and chalk
Note MoS2, PTFE and graphite are antifriction, antiwear
additives are often mixed with polymers to give self lubrication
composites
Structure of graphite
Molybdenum disulphide
 Sandwich structure: Mo atoms lie between 2
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layers of S atoms
Poor interlaminar interaction and hence very low
friction between layers
Stable in air upto 4000C.
Also used with solvents & greases.
Uses:
 Where greases and oils cant be used
 Work at high temperature
 Combustible lubricants must be avoided.
IV. Semisolid lubricants (grease): contain thickening
agents like soaps of sodium, calcium or aluminium
mixed with mineral oil (80% mineral oil+10%
soaps+ 10 % additives)
 Grease are named after soap used in manufacture
eg
 Lithium based grease (water and dust resistance
and used up to 15-30oC)
 Calcium based grease or cup grease (water
resistant and used up to 70oC)
 Aluminium based grease(high water and acid
resistance and used up to 80oC)
 Sodium based grease (sparingly soluble in water
and cant be used in wet condition and used up to
120oC)
V. Metal alloys: some metal and alloy can also
be used as lubricant or grease additives eg
lead, tin, zinc alloys are used in bearings and
metal powders are used to lubricate sliding
surfaces
Cup greases
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 Uses of greases:
 Where oils flow away due to heavy load, low speed and
sudden jerks.
 Where bearings produce high temperature.
 Where bearings needs to be sealed due to entry of
dust, moisture etc.
 Where oil drops spoil the machine.
Additives of lubricants
 Additives are employed to impart desired characteristics to
lubricants and some additive used in lubricant formulation
are:
 Antioxidants: as at elevated temperature lubricant oil get
oxidized or deteriorated so antioxidants are used to reduce
the rate of oxidation reaction of oil. Compounds used as
antioxidants are alkyl and aromatic sulphides, aromatic
amines, zinc dithiophosphate (ZDP) and hindered phenols.
 Corrosion Inhibitors: These help by forming a barrier film on
the surface of the machine part thus preventing it from the
attack of O2, H2O and other chemicals. Materials used to
prevent corrosion and rust formation are: esters, carboxylic
acid, alkaline compounds and amino acid derivatives.
 Viscosity Index Improvers: Viscosity of lubricants decreases
with increase in temperature and hence their performance
declines. Additives are: acrylate polymers. It makes stable oil
film even at high temperature.
Properties of lubricants
 Viscosity
 Flash point
 Fire point
 Cloud point
 Pour point
 Aniline point (number)
Viscosity
 Viscosity is the most important property of lubricants that
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determines the operating condition under which it is used
Viscosity is the property of fluid which is a measure of its
resistance to flow
The steady state of flow of a liquid can be visualized to
consist of series of parallel layers moving one above other
The function of lubricating oil is to form a liquid film
between two moving or sliding surfaces.
If the viscosity of oil is too low, the resistance to flow
between individual layer of lubrication is low and
consequently the excessive wear take place
On the other hand if the viscosity is too high resistance to
flow between different layers of later is high and friction
between layer increases and hence decreases efficiency
Therefore viscosity of lubricant should be optimum
according to need
Factors affecting viscosity
 Temperature: Viscosity of liquid decreases with temperature as the
lubricant oil become thinner as operating temperature increases
 It is desirable that viscosity of a lubricant should be consistent over
a range of temperature so that it can be used at different
temperature and we measure it by viscosity index (V.I.), higher the
viscosity index, smaller the change with temperature and lower the
viscosity index higher will be viscosity change with temperature
 Pressure: Viscosity increases with increasing temperature of
operating system
 Molecular structure: in general an oil with high molecular weight
possesses a high boiling point and high viscosity
 V.I of the lubricant can be increased by adding certain sparingly
soluble polymers, at low temp. polymer is less soluble but at high
temp. solubility is high which increases the V.I. of lubricant.
Fire and flash points
 Flash point is the lowest temperature at which the lubricant oil
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gives off enough vapors to ignite (spark) but not burn when small
flame is brought near it
Fire point is the lowest temperature at which the vapors of
lubricant oil burn continuously at least for 5 seconds when a small
flame is brought near it
Flash and fire points determine the volatility and fire resistance
of lubricant
Flash point is important in determining shipping, storage, and
safety regularity of oil
A drop in flash point shows the presence of volatile impurities in
oil
A good lubricant should have higher flash point than the
temperature it is used (usually fire point are 8-10% higher than
flash points)
Flash and fire points is determined by using PENSKY MARTENS
APPARATUS
Cloud and pour points
 Cloud point: When lubricant is slowly cooled, the
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temperature at which it become cloudy or hazy in
appearance is called its cloud point
Pour point: When the lubricant is slowly cooled, the
temperature at which the oil ceases to flow or pour
is called pour point
Cloud and pour point indicate the suitability of
lubricant in cold conditions
They are also useful in identifying the source of
lubricating oil as oil derived from petroleum source
contain paraffin wax and other high melting fraction
which solidify on cooling
Lubricant used in machine working at lower
temperature should posses low pour point to avoid
solidification of oil.
Cloud Point
Pour Point
Aniline Point (number)
Aniline point gives an indication of the possible
deterioration of the oil in contact with rubber used in
sealing, packing, etc. Aromatic hydrocarbons have
the tendency to dissolve natural rubber and certain
types of synthetic rubber, and hence their low value is
desirable in lubricants.
The lowest temperature at which equal parts of
aniline and sample of oil are completely miscible or
the temperature at which the mixture becomes
turbid or cloudy.
A product with a high aniline point will be low in
aromatics .
Aniline Point
 Aniline point is the minimum equilibrium solution
temperature for equal volume of aniline and oil
sample.
 It is determines by mixing mechanically equal
volumes of aniline and oil sample.
 A lower aniline point of an oil means a higher
percentage of aromatic hydrocarbons in it.
 Aromatic hydrocarbons have a tendency to dissolve
natural rubber and certain types of synthetic
rubbers.
 Thus good lubricating oil should have high aniline
point.
Determination of
aniline point
 Aniline point is determined by mixing mechanically
equal volume of oil sample and aniline , the mixture is
heated till homogenous solution is obtained then it is
allowed to cool, the temperature at which oil and
aniline separate out is recorded as aniline value.
 The lower the aniline point, the greater is the content
of aromatic compounds in the oil as obviously a lower
temperature is needed to ensure miscibility.
Selection of lubrication
 Selection of lubrication depend upon Lubrication
properties
 System of application and service condition
requirement such as type of motion speed,
temperature, load and environment
 In engineering domain lubricants are selected on
basis of tribology (study of friction) and wear,
chemical compatibility with machine parts and
environmental impact
 The most important criteria for selection include
the system on which lubricant is applied and
service conditions (Application based)
Lubricant selection based on
application
 Lubricants for cutting tools: For this application cutting
fluids in combination with lubricant and coolants are
used and main functions are
(a) to cool cutting tool edge at appropriate temperature
(b) to cool metal work piece to prevent distortion
(c) to reduce power consumption by lubrication action
(d) preventing rusting of machine parts
 For heavy cutting low viscosity mineral oil blended
with fatty oils
 For light cutting, the emulsion of oil in aqueous soap
solution is most effective
 For grinding and turning operation water may be used
as coolant and lubricant
 Lubricant for fragile/ delicate equipment: For
delicate machines (watches, clocks, scientific
equipments which do not operate at high temp.
and press. Mild lubricant are required (vegetable
or mineral oil)
 Lubricant for combustion engines: Lubricant in
internal combustion exp0se to high temperature
has high temp. Therefore lubricant should
posses high thermal stability and viscosity and
hence petroleum oils (with additives) which are
stable oxidation and temp. are used
 Lubricant for gears: gears are subjected to
extreme pressure and centrifugal force; require
thick viscous lubricants.
 Lubricant for transformers: Require such type
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of lubricant which can insulate the winding
and carry away the heat generated when it is
on load.
Lubricants for refrigeration system: Lubricant
should have low cloud and pour point.
Lubricants in food and pharmaceutical
industry: Lubricant should be inert, free from
contaminations, tasteless and odorless.
Lubricant for different loads and speed
Lubricant for textile industry