LUBRICANTS

Chapter 3: Lubricants

3.1 Introduction

3.2 Terminology

3.3 Function of Lubricants

3.4 Characteristics of an Ideal Lubricants

3.5 Classification of Lubricants

3.5.1 Solid Lubricants

3.5.2 Semi-solid Lubricants

3.5.3 Liquid Lubricants

3.6 Mechanism of Lubricants

3.6.1 Fluid Film or Thick Film or Hydrodynamic Lubrication

3.6.2 Thin Film or Boundary Lubrication

3.6.3 Extreme Pressure Lubrication

3.7 Testing of Lubricants

3.7.1 Viscosity and Viscosity Index

3.7.2 Flash and Fire Point

3.7.3 Cloud and Pour Point

3.7.4 Saponification Value

3.7.5 Acid Value

3.8 Additives for lubricants

3.9 Selection of lubricant

3.10 Quick Recap

3.11 Solved Examples

3.12 University Questions (with hints)

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3.1. Concept of Lubrication and Lubricants:

In our daily life we come across so many machines, in which surfaces of moving or sliding parts rub

against each other. For example movement of piston in cylinder of IC engines. Due to mutual

rubbing, a resistance is offered to the movement of piston. This resistance is called Friction. Friction

causes a lot of wear and tear of the surface of cylinder and piston as well. In fact; even highly

polished metal surfaces, at molecular level; show many irregularities in the form of peaks and

valleys. Due to this, surfaces get corroded also. Corrosion further enhances roughness and

irregularities of the surfaces. This roughness is the reason behind the friction developed during

motion. Due to this friction large amounts of energy are dissipated in the form of heat, thereby

causing loss in the efficiency of machine. More over, the moving parts get heated up damaged and

even sometimes results in seizure.

These ill effects of friction can be minimized, if not cancelled out, by applying a thin layer of suitable

Lubricants in between the moving surfaces. Thus we can define lubricants as follows –

“Lubricants are the chemical substances which reduce friction between two sliding/ moving metal

surfaces & thereby reduce wear & tear of machines.”

The main purpose of a lubricant is to keep the sliding / moving surfaces apart so that the frictional

resistance and consequent destruction of metal can be minimized.

The process of minimization of frictional resistance and consequent destruction of two sliding/

moving metal surfaces, by the introduction of lubricants is called as Lubrication.

3.2. Terminology: Before we start with lubricants we must have a glimpse and understanding of

commonly encountering terms. They are as follows-

(a) Viscosity: is the property of a fluid by the virtue of which a fluid (liquid or gas) offer

resistance to its own flow. We know that, highly viscous fluids flow slowly. Lubricants must

have adequate viscosity, for proper functioning. Too low viscosity (lubricant may leave the

surface easily) and too high viscosity (may offer some resistance) will not serve the purpose.

(b) Viscosity index: It is the rate of change of viscosity against temperature raise. It is in fact the

measurement of tendency to maintain its viscosity against temperature increase. Oil becomes

thin on heating i.e. their viscosity decreases. If the decrease in viscosity is less, the oil is said

to have high viscosity index and vice versa. A good lubricant should have high viscosity

index.

(c) Flash point: is the temperature at which the oil gives out enough vapors that ignite for a

moment when a small flame is brought near it. It must be comfortably high above the room

temperature or working area temperature, to avoid any risk of fire.2

(d) Fire point: is the temperature at which the oil gives out enough vapors which burn

continuously at least for five seconds, when a small flame is brought near it. Obviously it

must be comfortably high above the room temperature or working area temperature, to avoid

any risk of fire.

(e) Cloud point: is the temperature at which the oil becomes cloudy or hazy in appearance. It

must be comfortably below the working area temperature. Other wise viscosity and

lubricating property will be affected.

(f) Pour point: is the temperature at which oil ceases to flow or pour. At this point most of the

fraction of oil solidifies. This oil may harm the machine if used as lubricants due to friction.

So this point also must be below the working area temperature.

(g) Saponification Value: Saponification value of an oil is the number of milligrams of KOH,

required to saponify one gram of oil. It is the characteristic property of vegetable/ animal oils.

Since they are triglycerides of mixed fatty acids they can be saponified to glycerol and soap.

But mineral / synthetic oils can not be saponified, because they are not glycerides but simply

hydrocarbons. In Oils these fatty acids are mostly unsaturated e.g. Oleic acid, linoleic and

linolenic acid but in Fats they are mostly saturated e.g lauric, myristic palmitic etc.

(h) Acid value: is defined as the number of milligrams of KOH required to neutralize free fatty

acids present in one gram of oil.

The vegetable / animal oils contain fatty acids in combined form as triglycerides of mixed

fatty acids. Unsaturated sites in fatty acids tend to absorb oxygen on exposure to air and form

carboxylic acids. These acids if present even in small quantity harm machine during

lubrication.

3.3. Functions of lubricants: The main purpose of a lubricant is to keep the sliding / moving

surfaces apart so that the frictional resistance and consequent destruction of metal can be minimized.

Along with that following purposes are also solved.

(i) It reduces wastage of energy and there by increases the efficiency of machines.

(ii) It acts as coolants, by reducing the frictional heat generated. This reduces expansion of metal

by local frictional heat.

(iii) Acts as sealant (in IC engines) as it does not allow the escape of gases from engine under high

pressure.

(iv) It prevents the attack of moisture on machine surfaces. This helps in preventing corrosion of

the moving machine parts.

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(v) Acts as cleaning agent, by washing off solid particles produced due to combustion or wear. It

helps in preventing corrosion of the moving machine parts.

3.4. Characteristics of an Ideal Lubricant: An Ideal lubricant prevents the machine surface

from wear & tears due to friction under all severe working conditions. For that, Ideal lubricant should

have following characteristics. (As an assignment you should try to find out the reasons for these

characteristics)

(i) High oiliness

(ii) Adequate viscosity & high viscosity index.

(iii) Flash & fire points should be higher than the working temperature of the machine in

which the lubricant is used.

(iv) The pour point should be lower than the working temperature of the machine in which the

lubricant is to be used.

(v) Acid value should be very low.

(vi) Saponification value should be low.

(vii) Emulsion made by oil with water should be unstable & should break easily at work

(viii) Oxidation stability should be high.

3.5. Classification of Lubricants: On the basis of their physical state lubricants may be

classified as follows: (1) Solid lubricants (2) Semi solid lubricants or greases (3) Liquid lubricants or

lubricating oils.

3.5.1. Solid lubricants: We are familiar with liquid (for example castrol engine oil) and semi solid

lubricants (grease). Question arises what are solid lubricants? Where they are used? How do they

work? Let us have a look on it.

Graphite, Mica, Teflon, Molybdenum disulphide, chalk, talc etc. are the examples of solid lubricants.

Where they are used?

a. When machinery is to be operated at high speed and moderate load or high load and low

speed.

b. When the design of machine is intrinsic i.e. machine parts are not easily accessible.

c. These can be used at high working temperature and pressure.

d. Contamination of lubricating oil or grease is unacceptable, e.g. in commutator bushes of

electrical generators and motors.

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e. Combustible lubricants must be avoided.

Most commonly used are Graphite, Mica, Teflon, Molybdenum disulphide, chalk, talc etc.

How do they work?

If we look at the structure of Graphite, Mica, Teflon, Molybdenum disulphide, chalk, talc etc. it

becomes very clear that their lubricating action is due to their layered structure lattice. Let us discuss

in detail.

Graphite: is an allotrope of carbon consists of meshwork of hexagonal carbon rings arranged on

each other. Each carbon is linked by covalent bonds to three other carbon atoms. The distance from

the fourth carbon is almost more than double to that in between rest three carbon atoms. Due to which

this fourth valency is flexible & keeps moving about; thereby weakening the bonds between different

layers. Graphite when applied between uneven surfaces fills in to valleys, thereby making surfaces

more even .When machine is in operation the graphite particles slide over each other, with the motion

of machine. As a result graphite is soft & has lubricating properties. Fig 3.1

Molybdenum disulphide possesses a sand witch like structure in which a layer of molybdenum

atoms lie between two layers of sulphur atoms. These layers slide over each other when used as

lubricants in a machine. Fig 3.2

Aquadag: It is a dispersion of graphite in water. Mostly used in air compressors, & in equipments

used for food processing.

Oildag: It is a dispersion of graphite in oil .Commonly used in internal combustion engines.

Teflon: It is a polymer of tetra flouro-ethylene and has a very low coefficient of friction. It is widely

used in gasoline gear pumps, underwater machineries, oxygen valves, etc

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3.5.2. Semi solid lubricants: We are very much familiar with semi-solid lubricants greases,

waxes etc.

Where they are used?

They are used under working conditions, such as,

(i) Low speed & high pressure

(ii) Machineries used in textiles mills, paper & food products manufacturing etc. where spilling &

spurting of lubricant is harmful to the product.

(iii) Machines where liquid lubricants cannot be maintained in position due to intermittent operation

of machine parts such as shaft etc

(iv) Where bearing has to be sealed against entry of dirt, water, dust & grit.

How they are manufactured? Greases & Vaseline are most widely used semi-lubricants. Greases are

manufactured by Saponification of Fatty Oils with an Alkali. For manufacturing, fatty oil is mixed

with Mineral oil to form a homogenous solution which is then subjected to saponification with

respective alkalies.

Properties of a good Grease:

(a) Pour point should be as low as possible

(b) Resistance to oxidation should be as high as possible

(c) Consistency of greases should not alter to a great extent at working temperature.

Properties of various Greases:

Grease Optimum

temperature

Properties Uses

Calcium

Soap

650 C good resistant to oxidation water pumps, tractors etc

Sodium

soap

1750 C high temperature sustenance have high affinity for water

Aluminum

soap

900 C good oxidation stability where high adhesiveness is

desired i.e. where speed &

load is high

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Lithium

soap

1500 C good oxidation stability, high

temperature sustenance & high

mechanical stability

Special applications only i.e.

Air crafts

Barium

soap

- High M.P., high oxidation &

high consistency

Automobiles & farm

equipment

Axle

grease

- Cheap & water insoluble &

high temperature sustenance

In delicate equipments

working at low speed under

high load

3.5.3. Liquid lubricants: Most commonly used around us.

They are further classified on the basis of their origin

3.5.3.1. Vegetable or Animal Oils- are extracted from plants/animals .eg. tallow, castor, olive,

coconut, palm, neem, linseed, rosin, whale, cod-liver etc. Pure fatty acids such as Oleic acid are also

used as lubricants.

Usefulness: these oils possess high Oiliness due to which oil gets adhered to the machine surface &

help to reduce friction. They are generally used for blending mineral oils for desired oiliness.

Limitations:

i. They are very costly.

ii. They tend to undergo oxidation, in presence of oxygen or moisture, or emulsification with water at

working temperature.

iii. They also acquire dust & dirt from atmosphere & form grit on the surface of machine.

iv. They get hydrolyzed in presence of moist / humid air on prolonged exposure liberating free acids

which cause corrosion to metal surfaces.

3.5.3.2. Mineral Oils- These oils normally have long hydrocarbon chain of 12-50 carbon atoms. Oils

with lower number of carbon atoms show lower viscosity.

Obtained by fractional distillation of crude oil in petroleum industries.

Usefulness: They are abundantly available & stable under working conditions & are of low cost.

Limitation: their oiliness is low as compared to animal/ vegetable oils. To overcome these problems,

they are blended with suitable animal/ vegetable oils, oleic/ steric acid etc

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3.5.3.3. Blended oils: Essential, because no single pure lubricant serves satisfactorily. Animal oils

and mineral oils are also having so many limitations. So blended oils are prepared keeping end use in

mind.

Blending agents are used to improve properties such as viscosity index, oxidation stability, oiliness,

pour point, flash point etc. Corrosion / abrasion inhibitors, antifoaming agents etc are also added

A detailed list of various additives in lubricating oils is given in section 3.7

3.5.3.4. Synthetic Lubricating Oils:

Why Needed? For machines working under severe working condition i.e. in Aircraft engines, there is

the need of a lubricant which can be useful between very wide range of temperatures such as -500 C

to + 1500 C.

Pre-requisites: For such lubricants viscosity index, flash and fire points should be as high as possible

where as their pour point should be as low as possible. The resistance to oxidation & corrosion

should be high & their oiliness & thermal stability should be very high. Since lubricants discussed so

far can not serve this purpose, we need synthetic oils having all necessary properties.

Examples:

(i) Chlorinated and fluorinated hydrocarbons (chlorinated diphenyl compounds)

(ii) Siliconised oils(silicones): can be used up to 2000 C

(iii) Derivatives of polyhydroxy alcohols(polyethylene glycols, their ethers and esters): can be

used up to 3000 C

(iv) Organo-phosphatic esters (tricresyl phosphate) can be used up to 1500C.

Limitations:

(i) Highly inflammable

(ii) High cost

3.6. Mechanism of lubricants: Basically depending on the requirement of various machines

different kind of lubrication mechanism are possible.

3.6.1. Fluid or Hydrodynamic Lubrication- (liquid lubricant) In this mechanism a thick film (at

least 1000 A0) of a liquid lubricant (vegetable oil or blended oil) is applied between two moving

surfaces. It reduces the coefficient of friction (μ) to about 0.001 to 0.03, which is much lower as

compared to unlubricated surface (0.5 to 1.5).

How it works? Let us take an example of journal bearing. The bearing consists of a shaft rotating at a

fair speed, with moderate load (Fig 3.3). The lubricant is applied in annular space. When journal

bearing is stationary two surfaces remain in contact, but as the shaft begin to rotate, the film of 8

lubricant also rotate between the two metallic surfaces. Due to the presence of a thick oily layer, all

the valleys of the metal surfaces are filled up & a pressure is developed which practically keeps the

two surfaces away from each other, thereby reducing ‘wear’.

Hydrocarbon lubricants blended with long chain polymers (to maintain viscosity at working

temperature) with anti-oxidants (amino phenol), and organo-metallic compounds (to keep carbon

particles away from machine surface) are used for better results.

Fig 3.3. ………………

3.6.2. Thin film or Boundary lubrication:

When it is used? The machine is to be operated at comparatively low speed or during operation a

shaft starts moving from rest at fixed intervals, or the machine is operated under high load. Because

under such working conditions a thick continuous film of liquid lubricant can not persist between two

moving surfaces.

How do they work? A thick film of lubricating oil is applied in the clearing space of the moving

surfaces. This film gets adsorbed on metal surfaces due to physical or chemical or both the forces &

covers all irregularities of metal surfaces (Fig 3.4). It remains there & can bear high load as well as

high temperature. The coefficient of friction is reduced to the extent of 0.05 to 0.15.

Examples:

(i) Soaps of Vegetable or animal oils, as they possesses a great tendency of adsorption on

surfaces. But at high temperature they get decomposed, hence can not be used in internal

engines.

(ii) At high temperature mineral oils blended with fatty oils or fatty acids are used as their

thermal stability is high.

(iii) Graphite, molybdenum disulphide either alone or their stable suspension in oil is also

used.

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Fig 3.4…

3.6.3. Extreme pressure lubrication:

When they are used? At working condition where sliding / moving surfaces are under high pressure

and speed and surfaces normally attain high temperature. Since liquid lubricants may not stick to the

surfaces & it may decomposes at that high temperature, and losing its lubricating capacity. There

might be some loss of lubricant due to vaporization.

Generally mineral oils with special additives (known as extreme pressure additives) are used to

improve specific characteristics of lubricating oil. Substances such as Chlorinated esters, sulphurised

oils or phosphates (like tri cresyl phosphate) are mostly used as additives.

How do they work? The metal underlying the film of lubricant reacts with these additives & form

metal chlorides, sulphides, or phosphide which sticks to metal surfaces with greater force of

attraction. These metal compounds have very high melting points. Hence the film of lubricant

remains on the surface of metal providing adequate lubrication under such extreme pressure &

temperature.

3.7. Testing of Lubricants:

The lubricating oils are tested for their physical & chemical properties. Knowledge of these help to

choose the appropriate lubricant, most suitable under giving working conditions.

Table 1.0

Physical tests Method used

1 Viscosity & Viscosity index Redwood viscometer 1 & 2

2 Flash & fire point a) Abel’s flash point apparatus

b) Pensky-Marten’s apparatus

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3 Cloud & pour point Cloud & pour point

4 Volatile matter content Vaporimeter

5 Oxidation stability Sligh’s oxidation test

6 Carbon residue a) Conradson method

b) Ramsbottom method

Table 2.0

Chemical testing Method used

1 Acid value Titrimetry

2 Saponification Value Titrimetry

3 Aniline point Aniline point apparatus

4 Emulsification Steam emulsion number (SEN)

3.7.1. Viscosity & Viscosity Index:

Viscosity is the property of a fluid by the virtue of which a fluid (liquid or gas) offer resistance to its

own flow. We know that, highly viscous fluids flow slowly. Lubricants must have adequate viscosity,

for proper functioning. Too low viscosity (lubricant may leave the surface easily) and too high

viscosity (may offer some resistance) will not serve the purpose.

Since it is impossible to select oil having same viscosity over a wide range of operating temperatures,

we can select oil whose variation in viscosity with temperature is minimum. For this we must know

the concept of Viscosity index.

Viscosity index is the rate of change of viscosity against temperature raise. It is in fact the

measurement of tendency to maintain its viscosity against temperature increase. Viscosity index is

the numerical expression of the average slope of the viscosity temperature curve of a lubricating oil

between 1000F to 2000F.

Oil becomes thin on heating i.e. their viscosity decreases. If the decrease in viscosity is less, the oil is

said to have high viscosity index and vice versa. A good lubricant should have high viscosity index.

Generally oils with higher molecular weight show higher viscosity.

Viscosity and Viscosity index are measured by Redwood viscometers in commonwealth countries (in

India also) and by Saybolt viscometer in USA. Redwood viscometers no 1(for thin oils) & 2(for

thicker oils) are used to measure the viscosity & it is expressed in terms of seconds of the respective

apparatus because the viscosity is measured as time taken for a fixed volume of oil to flow through

orifice of the oil cup of the apparatus.

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Redwood Viscometer no 1:

It consists of following essential parts: Oil Cup, Heating bath, Stirrer, Spirit level, Leveling screws,

and Kohlraush flask. Fig 3.5

(i) Oil cup: It is silver plated cylinder, open at top; 90 mm in height 46.5 mm in diameter. The agate

jet with bore of diameter 1.62 mm & 10 mm in length is fitted at the bottom of oil cup. The jet can be

opened or closed with a valve rod, which is a small silver platted brass ball, fixed to a long stout wire.

The cup at its upper end is fitted with a pointer to check the level of oil. The cup is provided with a

lid having an opening for thermometer, to record the temperature of oil.

(ii) Heating Bath: is a cylindrical copper container filled with water, surrounding the oil cup. Heater

is provided at the bottom to heat the water.

(iii) Stirrer: A stirrer with four blades is provided, to heat the water uniformly. To prevent the

splashing of water a shield is provided with it.

(iv) Spirit level: The lid of oil cup is provided with spirit level for vertical leveling of the jet.

(v) Leveling screws: are provided to level the apparatus.

(vi) Kohlrausch flask: It is a wide mouth flask measuring definite quantity of oil (generally 50 ml)

Working / Procedure:

(i) The oil cup is cleaned, leveled & filled up to the pointer mark. The agate jet is closed with valve

rod and water bath is filled with water at room temperature.

(ii) A clean Kohlrausch flask is arranged under the agate jet. The temperature of oil is recorded. The

valve rod is shifted to open jet so that oil starts flowing out of the oil cup.

(iii) The stop watch is started at this moment. The time in seconds is recorded for 50 ml of oil to flow

out. The agate jet is closed with valve rod & oil cup is again filled with oil.

(iv) Water is heated using electric heater and different readings at various temperatures ( differing by

10oC ) are taken for 50 ml of oil each time. Similarly with falling temperature also the procedure is

repeated.

(v) Then a graph is plotted between temperature v/s time (Redwood seconds)

(vi) It is a linear curve, because as the oil on heating become thin, their viscosity decreases & hence

time taken for definite quantity of oil to flow out of the agate jet reduces.

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→ Temp 0C

3.7.2. Flash & Fire points:

Both are very important properties of a lubricant oil, because these help in knowing the highest

temperature up to which an oil can be used as lubricant. These are determined by using Pensky-

Marten’s flash point apparatus.

Pensky-Marten’s flash point apparatus: Its essential parts are – Oil cup, Shutter, flame exposure

device, Air bath, Pilot burner Fig 3.6

(i) Oil cup: It is about 5 cm in diameter and 5.5 cm deep, with oil level pointer. The cup lid is

provided with four openings of standard sizes. One for thermometer, and second for introducing test

flame. Through third opening passes a stirrer carrying two brass blades. The fourth is meant for

admission of air.

(ii) Shutter: is a lever mechanism provided at the top of the cup. It helps in bringing the flame over

the oil surface.

(iii) Flame exposure device: is connected to the shutter by lever mechanism.

(iv) Air bath: Oil cup is supported by its flange over an air bath which is heated by a gas burner.

(v) Pilot burner:

Working / Procedure:

i) Oil under examination is filled up to the mark & heated by heated air bath.

ii) At every 10C rise of temperature, flame is introduced for a moment by working of shutter.

iii) The temperature at which a distinct flash (a combination of flame & sound) appears inside the

cup, is recorded as the Flash point.

iv) The heating is continued there after & the test flame is applied as before. When the oil ignites

& continues to burn for at least 5 seconds, the temperature reading is recorded as the Fire point of

the oil.

3.7.3. Cloud & Pour points:

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Time inRedwood Seconds

These characteristic physical constants indicate the suitability of oils at lower temperature, or in cold

regions. Both the points should be lower than the working temperature. Otherwise the oil may get

solidified at working temperature & this may cause jamming of the machine parts.

These are measured by using a special apparatus as shown in Fig 3.7

Apparatus consists of a broad jar, which is used as a cold bath. The oil is taken in a flat bottomed

hard glass test tube. This test tube is kept in an air jacket.

Two thermometers are suspended, one in oil tube & other in cooling mixture in the jar to note the

respective temperatures.

Working:

i) The oil tube is filled with oil up to the level generally half the length of tube. The jar is filled

with cooling mixture surrounding the air jacket. Two thermometers are suspended through the

wooden corks & the temperature of oil & cooling bath is noted.

j) Initially for every 20C fall in temperature of oil, the tube is taken out of air jacket, wiped with

filter paper from outside & the oil is viewed for checking the transparency. The temperature at

which oil looses its transparency ie it appears cloudy is recorded as cloud point.

k) The cooling is continued further. Now with every 10C fall in temperature, the oil tube is taken

out and tilted to check the flow of the oil. The temperature at which oil hardens ie. ceases to

flow along the sides of the tube is recorded as Pour point.

3.7.4. Saponification value:

Saponification value of an oil is the number of milligrams of KOH, required to saponify one gram

of oil. It is the characteristic property of vegetable/ animal oils. Since they are triglycerides of

mixed fatty acids they can be saponified to glycerol and soap. But mineral / synthetic oils can not

be saponified, because they are not glycerides but simply hydrocarbons. In Oils these fatty acids

are mostly unsaturated e.g. Oleic acid, linoleic and linolenic acid but in Fats they are mostly

saturated e.g lauric, myristic palmitic etc.

Determination of Saponification value: A known quantity of oil (W gram) is mixed with known

excess of alcoholic KOH solution (0.5N). The mixture is shaken vigorously and is refluxed for

about two hours, on watre bath, using water condenser. Following reaction takes place:

Oil + alcoholic KOH → Soap + Glycerol + un reacted KOH

In the above reaction, un reacted KOH is titrated against dil. HCl (0.5N). This reading is called

Back reading. A blank set is prepared (with out adding oil sample) and titrated against same HCl

(0.5N). This is called Blank reading. Now the difference of these two readings will give us the

amount of KOH consumed in saponification of oil sample.

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Saponification value = Amount of KOH consumed (Blank - Back) × (N KOH × 56)/ W

Significance: This value help us to know the stability of oil in aqueous / alkaline medium if in

case machine parts face such conditions.

It must be low for better suitability as lubricating oil.

3.7.5. Acid Value: is defined as the number of milligrams of KOH required to neutralize free

fatty acids present in one gram of oil.

The vegetable / animal oils contain fatty acids in combined form as triglycerides of mixed fatty

acids. Unsaturated sites in fatty acids tend to absorb oxygen on exposure to air and form

carboxylic acids. These acids if present even in small quantity harm machine during lubrication.

Thus it is important to determine the content of free acids in oil. Ideally it should be minimum.

Determination:

The oil sample is weighed (W g.) and mixed with absolute alcohol (ideally 50 ml for 1 gm of

sample). The mixture is warmed for 10-15 minutes on water bath. The mixture is then titrated

against 0.1 N KOH solution, using phenolphthalein indicator.

Acid value = Volume of KOH (ml) × (NKOH × 56) / W gm

3.8. Additives and their functions:

Sr.

no.

Additive Chemicals Functions

1 Detergents Calcium and barium phosphates

And sulphates

Reducing deposits in Engines

Working at high temperature

2 Dispersants Nitrogen containing polymers

ex. Polymethacrylates, alkyl

succinimides

Reducing sludge formation and

deposition in machines operated

under low temperature

3 Anti-oxidants Phenols ,Amines , Organic

sulphides and phosphides

Reducing the oxidation of

lubricating oils and there by

minimizing formation of resins

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acids and sludge

4 Corrosion

inhibitors

Organometallic compounds ex.

Sulphurised terpens

Protecting metal surfaces from

corrosion

5 Oiliness Fatty acids, vegetable oils

6 Viscosity and

index improvers

Long chain polymers such as

polystyrene, polyisobutylene

Increasing viscosity and viscosity

index

7 Pour point

depressant

Wax alkylated phenols and

naphthalene

Lowering pour point of the oil

8 Anti foam

additives

Glycerol and glycols Preventing formation of stable

foam

9 Emulsifiers Sodium salts of organic acids,

monoesters of polyhydric

alcohols.

Help the formation of emulsion of

oil with water

3.9. Selection of Lubricating Oils:

3.10. Quick Recap:

Solved Examples with step by step solving strategy

Problems on Acid value:

Example1. 2.5 gm of an oil sample require 2.5 ml of N/100 KOH to neutralize fatty acids in oil. Find

the acid value.

Solution: Step by step solution require following attention:

(1) First of all carefully read the problem and write all given data.

(2) Then write the formula to determine Acid value.

(3) Now we can easily trace the required quantity.

(4) Important quantities in this problem are volume and normality of KOH.

These will be given either directly, as in this example or indirectly.

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(5) Generally weight of oil sample is given directly. Some times volume of oil

sample taken is given. In this case density will also be given. We can get the

weight of sample just by simple calculation. Weight = density × volume

Let us start:

Acid value = Volume of KOH (ml) × (NKOH × 56) / W gm

Given data: Volume of KOH = 2.5 ml, NKOH = N/100, W = 2.5 gm

Acid value = 2.5 × (1/100 × 56) /2.5

= 0.56 mg/g

Example 2. 20 ml of an oil was dissolved in alcohol solution was titrated against 0.1 N KOH solution.

At the end point burette reading was 2.5 ml. Calculate the acid value of the oil. (Density = 0.86 g/ml)

Solution: Given data: volume of oil = 20 ml, density = 0.86g/ml, Volume of KOH consumed

in titration = 2.5 ml, NKOH = 0.1N

Acid value = Volume of KOH (ml) × (NKOH × 56) / W gm

Weight of oil = 20 × 0.86 = 17.20 g

Acid value = 2.5 × 0.1 × 56 / 17.20

= 0.814 mg of KOH / g of oil

Problems on Saponification value:

Step by step solution require following attention:

(1) First of all carefully read the problem and write all given data.

(2) Then write the formula to determine Saponification value.

(3) Now we can easily trace the required quantity.

(4) Important quantities in this problem are volume and normality of KOH.

These will be given either directly, as in this example or indirectly.

(5) Indirectly these figures may be given in terms of dilute HCl acid used in

titration of un reacted alkali (corresponds to Back reading). From Acid- base

equivalency we need to calculate the normality of KOH solution as well.

(These calculations are easy. But should not be forgotten.)

(6) Maximum care should be taken for evaluation of Blank and Back readings.

We need the difference of both these readings.

(7) Generally weight of oil sample is given directly. Some times volume of oil

sample taken is given. In this case density will also be given. We can get the

weight of sample just by simple calculation. Weight = density × volume17

Example 3. 2.5 g. of oil was saponified by alcoholic KOH (0.25 N). The blank reading with 0.5N

HCl was 40 ml, while back was 20 ml of same HCl. Find saponification value.

Solution:

Given data: W = 2.5 g, N KOH = 0.25 N, Blank = 40 ml with 0.5 N HCl, Back = 20

ml with 0.5 N HCl.

* All data are given. Only Blank and Back readings of KOH are given in terms of HCl. We should

convert them in volumes of KOH only as follows:

In Blank reading,

40 ml of 0.5 N HCl ≡ 40 ml of 0.5 N; KOH (both are 1: 1 electrolyte)

≡ 80 ml of 0.25 N KOH

Thus blank = 80 ml KOH

Similarly for Back reading,

20 ml of 0.5 N HCl ≡ 20 ml of 0.5 N; KOH (both are 1: 1 electrolyte)

≡ 40 ml of 0.25 N KOH

Back reading = 40 ml of KOH

Thus KOH consumed in saponification = 80 – 40 = 40 ml

Now,

Saponification value = Amount of KOH consumed (Blank - Back) × (N KOH × 56)/ W

= 40 × 0.25 × 56 / 2.5

= 224 mg. KOH

18