Transcript MEL 417 Lubrication Semester 2, 2010-`11

MEL 417 Lubrication
Semester 2, 2010-’11
Course co-instructor: Dr. Rahul Ribeiro
Email: [email protected]
Tel: 011-26591034
Mobile: 9711010305
Web: http://web.iitd.ac.in/~rribeiro
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Lubrication and Lubricants
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Almost every relatively moving component in an assembly
requires LUBRICANT
Standard lock
window
lifting
mechanism
Turning key slides bolt into a notch
on door frame.
Pendulum Clock
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Lubrication
• Reduction of frictional force by application of a
3rd. material between surfaces, or by surface
treatment (e.g. coatings)
• Improves efficiency and reduces wear
• Decreases running cost
• Removes heat
• Removes wear particles
• Prevents corrosion
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History of Lubrication
 Bone joints have a natural lubricant in Synovial fluid
 Mesopotamia (3000 BC)- Wheeled carriages and
potters wheel with bearings
 Egypt (~2000 BC)- Hydrated calcium sulphate was
used to form a lubricant for placing pyramid stones
 Egypt (~2000 BC)- Chariot wheels with grease
lubricated bearings were found in pharoh’s tomb
 Chinese (400 BC)- Bronze bearings on war chariots
 Europe (1400s AD)- Lubrication by animal fats
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Desired properties of lubricants
 Adequate film strength
 Chemical stability
 Adequate lubricity
 Purity (freedom from contaminants)
 Non-corrosive
 Good sealing properties
 High VI (change in viscosity with temperature)
 Minimum volatility or out-gassing
 Adhesiveness to bearing surfaces
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What do we expect from a lubricant
Lubricant between cylinder liner and rings
•Lubricant must form a film
to separate the surfaces
Combustion
gases
Lubricant
injection
holes
Piston
rings
•Needs to adhere to the
surfaces
• Must neutralize the
corrosive products of
combustion
• Withstand high
temperature inside the
cylinder
Piston
Piston rod
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Lubricant as a cleaning agent
Smooth surface 1
Object 1
Lubricant
Object 2
Wear particles
Smooth surface 2
After running-in of surfaces, the wear particles that are
worn off the sufraces are trapped between the surfaces
and need to be removed by the lubricant
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Lubrication in journal bearings
• Must be able to
supports shaft+ loads
• Absence of contact
stresses
• Damp vibrations
Which are replaced more
often- Bearing or shaft?
Ans: Actually bearing shell
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Lubrication in Bone joints
SYNOVIAL FLUID
• Contain proteins that stick
to cartilage layer resulting
in smooth sliding
Femur
• coefficient of friction ~ 0.01
Articular cartilage
Joint capsule
Ligament
• minerals that nourish the
cartilage cells
• Increase viscosity with
increase in applied
pressure
Joint (synovial) fluid
space
Tibia
NOTE: Lubricant properties are specific to the application.
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Applications of lubrication
COF ~ 0.01
www.lifemodeler.com
Artificial hip joint
Synovial bone joint
Requirements: Biocompatible
Low fiction, low wear prticulates
Shoppingtrolley.net/images
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Lubricants in machining
• Acts as a coolant be keeping
the temperature stable
between the tool and
workpiece
• Maximizes the life of the
cutting tip by lubricating the
working edge and reducing
tip welding.
• Prevent the growth of
bacteria or fungi.
• Washes away contaminants
and wear debris
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Applications of lubrication
Shaft/bearings
www.tpub.com
Cylinder
Piston/liner
Lubricant
application
Piston
www.moorecells.com
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Applications of lubrication
Lubrication during
machining
Micro eletromechanical systems
actuator
Vapour lubricants
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www.memx.com
Lubrication in turbines
• High rotational frequency and turbulence
• Should have good cleaning property to remove
particles from small clearances in bearings
Steam turbines- require high oxidation stability due to
leakage of steam into oil
Gas turbines
• Should withstand high temperatures (204oC to
316oC)
• Should withstand thermal cycling without
deteriorating
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Thick & Thin Lubrications
• Understanding thick lubrication by
Reynolds, lead to removal of an oil from
the load line of railway axle bearings
(1890) Lesser oil quantity, lesser friction.
• Thin lubrication is far more complex.
Requires scientific study at nano- to microlevel.
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Types of friction
Frictional force
Direction of motion of
top object
Sliding friction- Resists relative motion between sliding solid
bodies
Frictional force
Direction of motion of
top object
Rolling friction- Resists relative motion between two solid
bodies when one or both roll over the surface of the other
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Types of friction (contd.)
Moving body
Maximum velocity
Zero velocity
Velocity profile of
fluid
Stationary body
Fluid friction- Resistance to motion between layers of fluid
Boundary friction- Surfaces barely separated by a layer of
lubricant (micron range)
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Lubricant properties and tests
Flash point- Temperature at which an oil vaporizes
sufficiently to sustain momentary ignition when
exposed to a flame under atmospheric conditions
Fire point- Temperature at which an oil will sustain
ignition continually when exposed to a flame under
atmospheric conditions
Pour point- Lowest temperature at which an oil will
flow when tested under certain specified conditions
Viscosity- Intrinsic friction during flow. Determines
speed of flow.
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Lubricant properties and tests (contd.)
Viscosity index (VI)- change in viscosity with temperature
Carbon residue- percentage of carbon residue left by an oil
upon evaporation under specified test conditions
Friction and wear test- pin-on-disc, four ball lubricant
Oxidation stability- oxidation resistance of lubricant
Bearing load analysis- pressure distribution in a bearing
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Boundary Lubrication
 English Biologist “Sir Hardy”. 1922.
“Very thin adsorbed layers, about 10 A° thick, were sufficient to
cause two glass surfaces to slide over each other”.
 A very thin layer of lubricant (a few molecules thick 1
to 10 nm) separates sliding surfaces, i.e. no direct contact
of the sliding parts.
 Engineering equipment such as steel gears, piston-rings and
metal -working tools depend on one or more of these lubrication
modes, to prevent severe wear or high coefficients of friction and
seizure.
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How thin layer is able to separate surfaces?
Surface Surface
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1
Surface2 2
Surface
Boundary
lubricants: “Oiliness
additives”
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Characteristics required for Thin Film Lubrication
• Long chain molecules with an
active end group.
– Attaching itself to the solid surface
& building a surface layer
• Dissolvable in mineral/lubricating
oils.
• Temperature stability:
– Typical fatty acids decompose at
temperature above 200C and lose
their effectiveness.
Why boundary lubricants are required
when metals are covered with natural
protective layer of oxide?
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Effect of Length of Molecule on Boundary lubrication
• Longer hydrocarbon chain, more effective separation
between solid surfaces.
– High degree of slip. Low lubricant friction
• Sir William Hardy’s experimental results:
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Mechanisms of Boundary Lubrication
• Physical adsorption (Physisorption)
– All petroleum and synthetic lubricants have some potential
for forming boundary films under mild sliding conditions.
– Useful under light load and low temperature conditions.
• Chemical adsorption (Chemisorption):
– Higher bond energies than physisorption (> 40kJ/mol)
– With polar and paraffinic molecules, chemisorbed lubricants
can be very closely packed on a surface.
– Most effective boundary lubricants combine a chemical
reaction with the surface and a cohesion interaction
between lubricant species.
Mechanisms of boundary lubrication are usually controlled by
additives present in the oil.
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Physisorption
• Physisorption or “physical adsorption’ (physical
bonding by van der Waals force)
2 nm
• Surface active molecules of oiliness additives are
attracted to surface by electrostatic (dipole) forces.
– Energy is lowered when the molecules adsorb on the
surface.
– Molecules of adsorbate attach or detach from a surface
without any irreversible changes to the surface or the
adsorbate.
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Physisorption
Solvent tends to dissolve solute again,
– a process encouraged by dilute concentrations, but
– hindered by high concentration of polar molecules
Lubricant
Friction Coefficient
Pure mineral oil
2% oleic acid in mineral oil
10% oleic acid in mineral oil
50% oleic acid in mineral oil
Pure oleic acid
0.360
0.249
0.198
0.198
0.195
solute  em pty_ site  adsorbed_ sites
a1  a2  a3
 equilibrium constantk 
a3
a1a2

T otalGibb' s free energy G  H  TS   RT log K

C 1   
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Chemisorption
• Physically absorbed boundary additive decompose or melt
at high temperature
• Is a form of corrosion
• To form a chemically bound layer three things are needed:
– Surfactant must be chemically active
– Metal surface also must be reactive
– Surface must be free enough of physisorbed meterial for the
chemical reaction to take place. Temperature Gap between
physical and chemical is known as “Temperature Distress Gap”
• During each contact the chemical layer is rubbed off the
surface and has to be reformed before next contact come
round. Surface is therefore slowly worn away so the
additive must be chosen with care.
• Must be active enough to protect the surface, but not so active that
it corrodes violently at high temp.
NOTE: At a somewhat higher temperature physically absorbed molecules get desorbed. In
other words molecules still present on the surface but lose their attachment. Consequently
wherever the surfaces come together the lubricant molecules are pushed away and
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intimate metal-metal contact is able to occur.
Desirable properties from a boundary lubricant
• Following properties are required to get the best protection of
tribo-surface using boundary lubricants:
– strong attraction between the chains to resist penetration by surface
asperities,
– low shear strength to give a low friction,
– high melting point so that it provides solid-film protection up to a high
temperature.
• The best materials are the long chain alcohols, amines and fatty
acids since these can be dissolved in small quantities in an ordinary
lubricating oil and will then attach themselves to the metal surface.
Fatty acids have the additional merit that, if
– "the metal is reactive, they will react with the metal to form the metal
soap. They not only have desirable shear properties, they also have
melting points considerably higher than that of the original fatty acid.
For example, the melting point of stearic acid is 69°C; of copper
stearate, about 120°C.
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