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MEL 417 LubricationSemester 2, 2010-’11

Course co-instructor: Dr. Rahul RibeiroEmail: rribeiro@mech.iitd.ac.in

Tel: 011-26591034Mobile: 9711010305

Web: http://web.iitd.ac.in/~rribeiro

Lubrication and Lubricants

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window lifting mechanism Pendulum Clock

Almost every relatively moving component in an assembly requires LUBRICANT

Standard lockTurning key slides bolt into a notch on door frame.

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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

What do we expect from a lubricant

Lubricant between cylinder liner and rings

Piston rings

Piston

Combustion gases

Piston rod

•Lubricant must form a film to separate the surfaces

•Needs to adhere to the surfaces

• Must neutralize the corrosive products of combustion

• Withstand high temperature inside the cylinder

Lubricant injection

holes

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Lubricant as a cleaning agent

Object 1

Object 2

Lubricant

Smooth surface 1

Smooth surface 2Wear particles

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

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• 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

Articular cartilage

Femur

Joint capsule

Tibia

Joint (synovial) fluid space

Ligament

SYNOVIAL FLUID

• Contain proteins that stick to cartilage layer resulting in smooth sliding

• coefficient of friction ~ 0.01

• minerals that nourish the cartilage cells

• Increase viscosity with increase in applied pressure

NOTE: Lubricant properties are specific to the application. 10

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Applications of lubrication

www.lifemodeler.com

Shoppingtrolley.net/images

Synovial bone joint

COF ~ 0.01

Artificial hip jointRequirements: BiocompatibleLow fiction, low wear prticulates

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

Lubricant application

Piston

Cylinder

Shaft/bearings

Piston/liner

www.tpub.com

www.moorecells.com

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Lubrication during machining

Applications of lubrication

Micro eletro-mechanical systems actuator

www.memx.com

Vapour lubricants

Lubrication in turbines• High rotational frequency and turbulence• Should have good cleaning property to remove

particles from small clearances in bearingsSteam turbines- require high oxidation stability due to

leakage of steam into oilGas turbines• Should withstand high temperatures (204oC to

316oC)• Should withstand thermal cycling without

deteriorating15

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 micro- level.

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Types of friction

Sliding friction- Resists relative motion between sliding solid bodies

Rolling friction- Resists relative motion between two solid bodies when one or both roll over the surface of the other

Direction of motion of top object

Frictional force

Frictional force Direction of motion of top object

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Types of friction (contd.)

Velocity profile of fluid

Moving body

Stationary body

Fluid friction- Resistance to motion between layers of fluid

Zero velocity

Maximum velocity

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.

How thin layer is able to separate surfaces?

Surface 1

Surface 2

Surface 1

Surface 2

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 200C 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)

• 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.

2 nm

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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

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3321

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

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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 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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