Lubricants 1

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LUBRICANTS

Friction:

Resistance observed between two

moving or sliding surfaces creating wear

and tear.

Lubricant:

Any substance introduced between twomoving or sliding surfaces with a view to

reduce the frictional resistance between

them is know as a lubricant.

Lubrication:The process of introducing lubricant

between moving/sliding parts is known

as lubrication


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Functions of Lubricant

oTo reduce frictional resistance between surfaces and

reduce deformation, wear and tear between moving/slidingsurfaces.

o To reduce loss of energy in the form of heat (Coolant).

o To reduce waste of energy i.e., to increase efficiency of

machines.

o To reduce irregular expansion of metals.

o To reduce welding of the two surfaces.

o To reduce or avoid rough relative motions of moving /

sliding parts.

o To reduce running and maintenance cost of the machine.o To reduce the leakage of gases under high pressure like a

seal or Teflon.


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Mechanisms of Lubrication

1. Fluid-film (or) Thick-film (or)

Hydrodynamic lubrication

2. Boundary lubrication (or)

Thin-film lubrication

3. Extreme pressure lubrication


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1.Fluid Film/Thick-Film/Hydrodynamic Lubrication(~ 1000 )

Characteristics:

o The surfaces are separated by a thick-film (at least

1000 thick) and hence there is no direct surface tosurface contact.

o No welding of junctions.

o Since thick film lubricant covers/fills the irregularitieson the both surfaces, there is no direct contact

between material surfaces and so the wear is reduced.


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In Fluid Film Lubrication,

the Lubricating properties depend on:

oViscosity of lubricant (Lubricant should beof low viscosity).

o Thickness of lubricant layer.

o Relative velocity and area of moving/slidingsurfaces.


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Example: Shaft running (Journal bearings)

Lubricant oil covers the irregularities of the shaft as well as thebearing surfaces.


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o Examples where fluid film lubrication is used are :

i) Sophisticated instruments

ii) Light machines like watches, guns, sewing machines etc.

o Examples for fluid-film lubricants are:

Hydrocarbon oils are considered to be satisfactory lubricants.

To maintain viscosity throughout lifecycles long chain polymers are

used as blenders with normal hydrocarbons oils.

Small amount of unsaturated hydrocarbons present in hydrocarbon

oils produced from petroleum fractions, which causes oxidation and

produce gummy like products. Hence, anti-oxidant like aminophenolare used in journal bearings with lubricant


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Mechanism of fluid film lubrication


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2. Boundary Lubrication/Thin-film Lubrication

Characteristics of thin film lubrication are:

oHigh viscosity-index.

o Resistance to heat and oxidation.

o Good Oiliness.

o

Low pour-point.


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For boundary lubrication, the lubricant molecule

should have:

(i) Long hydrocarbon chain with polar groups.

(ii) Polar groups promote spreading and orientationover the metallic surfaces at high pressure.

(iii) Lateral attraction between the chains.

(iv) Active groups or atoms, which can form chemicallinkages metal or other surfaces.


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3. Extreme-pressure LubricationWhen moving/sliding surfaces are under very high pressure

and speed, a high local temperature is attained. In such conditions, liquid lubricants fail to stick and may

decompose and even vaporize. To avoid this, special additives are added to mineral oils. These

are called extreme-pressure additives.

MechanismThe extreme-pressure additives form on metal surfaces

more durable films, capable of withstanding very high loadsand high temperatures.

Examples:Organic compounds containing chlorine, sulphur and phosphorus.

o These compounds react with metallic surfaces, at prevailing hightemperatures, to form metallic chlorides, sulphide or phosphides.

o These metallic compounds possess high melting points.


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CLASSIFICATION OF LUBRICANT

Based on Physical state, lubricants are

classified as:

a) Lubricating oils or liquid lubricants

b) Semi solid lubricants or greases

c) Solid lubricants


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Types of Liquid Lubricants

a) Animal and Vegetable oils:

- Usable under very high temperature and heavy load. Disadvantages of its usages are

1. Costly

2. Undergo oxidation easily in contact with air and forms

gummy and acidic products, and get thickened.

3. Tendency to hydrolyze in contact with moist-air or aqueousmedium.

So, they are used as blending agents with other mineral oils.

b) Mineral oils or petroleum oils:

- They are obtained by distillation of petroleum.o Length of hydrocarbon chain varies between 12 to 50 carbon atoms.

o Shorter- chain oils have lower viscosity than the longer- chainhydrocarbons.


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Liquid lubricants are most widely used lubricants

because they are

1. Cheap2. Available in abundance

3. Quite stable under service conditions.

oBut they havepoor oiliness charactercompared to

animal and vegetable oils.

o So, high molecular weight compounds like oleicacid, stearic acid are used to over come this problem.

c) Blended oils:

o No single oil serves as the most satisfactory lubricantfor many of the modern machines. Hence, additivesare used to improve the properties. These blendedoils give desired lubricating property required for a

machinery.


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Additives used are:

a) Oiliness- carriers:

o Coconut oil, caster oil, and palmitic, stearic and oleic acids.

b) Extreme-Pressure additives such as:

o Fatty esters or acids which form oxide film with metal surface.

o Organic materials containing sulphur.

o Organic chlorine compounds.

o Organic phosphorous compounds.

o Some times lead (Pb) compounds could be used as high pressure

lubricants.c) Pour-point depressing additives:

o Phenol, condensation product of chlorinated wax with

naphthalene.


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2. Semi-Solid Lubricants or Greases

oSemi solid consisting of a soap dispersed throughout

a liquid lubricating oil.

- May be Petroleum oil or synthetic oil with a specific additive.

Preparation:Saponification of fat (such as tallow or fatty acid) with alkali

(like lime, caustic soda etc.,)

Addition to hot lubricating oil under agitation

oTo increase the heat resistance of grease, inorganic solid

thickening agents ( like finely divided clay, bentonite, colloidalsilica, carbon block etc.,) are added.

o Have higher shear or frictional resistance than oils and hence

support much heavier load at lower speeds.


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Applications of Greases:

o When oil cannot remain in place due to high load, low

speed, intermittent operation, sudden jerks etc.

o Work at high temperature

o When external contamination may create problem

o When dripping or spurting of oil is undesirable

Types of greases:

o Calcium based greases or cup-greases

o Soda-based greases

o Lithium-based greaseso Axle- greases lime with resin and fatty acids

o Graphite greases

o Soap stone


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Examples of Solid Lubricants

1.Graphite:oVery soapy in toucho Non inflammable

o Not oxidized in air below 375C

o Oil + graphite oildag

o Water + Graphite aquadag

- Emulsifying agent (tannin)

o Grease + graphite graphite

-greases


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Examples of Solid Lubricants

2. Molybdenum disulphide:

o Low coefficient of frictiono Stable in air up to 400 C

Soapstone, talc or mica are also used as solid lubricants.


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3. Oiliness:

A measure of its capacity to stick on to the surfaces of

machine parts under conditions of heavy pressure or load.

o For high pressure - high oiliness oil should be used.

o Important for extreme Pressure lubrication

4. Cloud and Pour points:

When an oil is cooled slowly, the temperature at which itbecomes cloudy or hazy in appearance, is called

its CLOUD POINT.

The temperature at which the oil ceases to flow or pour, is

called its POUR POINT.

5. Volatility:

o Good lubricant should have low volatility.

o

It is measured by vaporimeter.


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6. Emulsification:o The property of oils to get intimately mixed with water forming

an emulsion.

o Emulsions have a tendency to collect dirt, girt, foreign materialetc., causing abrasion and wearing out of the lubricating partsof the machinery.

o A good lubricating oil should form an emulsion with water

which breaks off quickly.7. Carbon residue:

Normally lubricants consist of high % of carbon containingcompounds.

o Lubricants decompose due to raise in temp. and depositcarbon creating problems to :a) IC engines and b) Air compressors.

o A good lubricant should deposit least amount of the carbon .


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8. Corrosion stability:

Corrosion Test:

o

A polished copper strip is placed inside a lubricating oil fora specified time and temperature and then checked for any

tarnishing .

o To prevent or retard corrosion effect of lubricating oils,

additives such as Phosphorous, Arsenic, Antimony,Chromium, Bismuth or Lead are added.

9. Decomposition stability:o Lubricating oils must be stable to decomposition at the

operating temperatures by :

a. Oxidation: To prevent it anti oxidant or inhibitor are used.

b. Hydrolysis: Moisture in oils causes hydrolysis of esters

c. Pyrolysis : At high temperature


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10. Aniline point: (A.P.)

o The minimum equilibrium solution temp. for equal volumes of

aniline and oil sample.

o A good lubricating oil should have higher aniline points (A.P)

o Higher A.P means higher % of paraffinic hydrocarbons and

hence lower % of aromatic HC.(Aromatic HC dissolves natural rubbers and few synthetic

ones)

11. Precipitation Number:

o The percentage of asphalt present in oil.

o Precipitation Number is used to differentiate the different

classes of lubricants.


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12. Specific Gravity:

A.P.I. ( American petroleum Institute) degreeA.P.I degree = 141.5/sp. gr. at temp(60F) -131.5

where 141.5 modulus of the A.P.I scale.

13. Ash Point:

o For used oil it is important to get an idea as to how much

abrasion and wear it may cause

14. Saponification number:

oNumber of milligrams of KOH required to saponify1g of oil.


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15. Mechanical stability:

o At very high pressures of operation, the stability of a

lubricant is judged by four balls extreme pressure

lubricant test.

16. Neutralization Number:

o Is a scale to determine the amount of acidic or basicconstituents of an oil.

o Acid Number:Amount of KOH required in milligramsto neutralize the fatty acids in 1g of oil.

o Good lubricating oil acid number value < 0.1

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