Lubricants and Greases

Shibu G. Pillai

Contents

• Introduction

• Functions of lubricants

• Mechanism of lubrication

• Classification of lubricants

• Properties of lubricants

• Selection of lubricants

INTRODUCTION• In All types of machines ,the

surface of moving, rolling orsliding parts rub against eachother, So resistance is producedto their movement called asFRICTION.

EFFECTS OF FRICTION

• Causes lot of wear and tear to surfaces of moving/rolling/sliding parts .

• Large amount of energy is dissipated in form of heat

• Ultimately reduces the efficiency of machine.

Solution is LUBRICANTS

DEFINITION:-Any substance which reduces the friction when introduced between two surfaces known as lubricant and the process is

termed as lubrication..

FUNCTIONS OF LUBRICANTS

1.Reduces the maintenance and running cost of the machines as it prevents from rust and corrosion.

2.Prevents foreign matter(dirt) from entering bearing and damaging the smooth surface of the journal and bearing.

3.Reduces wear, tear and surface deformation:

The direct contact between thesliding/moving surface is avoided. Inabsence of lubricants, some of the smallpeaks would be sheared off as one blockmoves over the other.

4.As sealing Agent in Internal Combustion Engine:

In the engine the lubricant used between the piston and the

cylinder wall acts as a seal. This seal prevents the leakage of

gases under high pressure from the cylinder.

5.Lubricant acts as a CoolantDuring a motion of body on surface, a considerableamount of frictional heat energy is evolved at rubbingsurfaces. So work of lubricant over here is to simply flowover the heated surfaces and absorb heat from there.

6.Lubricants reduces the frictionOn examining a smooth metal surface under powerful microscope we may observe peaks and valleys of different heights and depths called surface roughness.

So on applying load on these surfaces due to presence of these valleys and depths there is hinderance caused due the movement.

• But when lubricating oil is kept in between two surfaces, small peaks and valleys do not interlock and top part easily slides over each other i.e. reduces friction.

MECHANISM OF LUBRICATION-

Considering the nature of motion between moving or sliding

surfaces, there are different types of mechanisms by which

the lubrication is done. They are:

Hydrodynamic lubrication or thick film lubrication.

Boundary lubrication or thin film lubrication.

Extreme pressure lubrication.

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Hydrodynamic Lubrication or Thick Film Lubrication-

Hydrodynamic lubrication is said to exist when the moving

surfaces are separated by the pressure of a continuous

unbroken film or layer of lubrication. In this type of

lubrication, the load is taken completely by the oil film.

The basis of hydrodynamic lubrication is the formation of an

oil wedge. When the journal rotates, it creates an oil taper or

wedge between the two surfaces, and the pressure build up

with the oil film supports the load.

Hydrodynamic lubrication depends on the relative speed

between the surfaces, oil viscosity, load, and clearance

between the moving or sliding surfaces.

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In hydrodynamic lubrication the lube oil film thickness is greater than outlet, pressure

at the inlet increases quickly, remains fairly steady having a maximum value a little to

the outside of the bearing center line, and then decreases quickly to zero at the outlet..

The coefficient of friction is as low as 0.001 - 0.003 for fluid lubricated system in

comparison to 0.5 to 1.5 for unlubricated surfaces

Hydrocarbon petroleum fraction contain small quantities of unsaturated hydrocarbons

,which get oxidized under operating conditions , forming gummy products. Hence it is

essential that antioxidants (aminophenols) to be blended with hydrocarbon oils.

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Application of hydrodynamic lubrication-

Delicate instruments.

Light machines like watches, clocks, guns, sewing machines.

Scientific instruments.

Large plain bearings like pedestal bearings, main bearing of diesel engines.

Hydrocarbon oils are considered to be satisfactory lubrication for fluid film lubrication.

In order to maintain the viscosity of the oil in all seasons of the year, ordinary

hydrocarbon lubricants are blended with selected long chain polymers.

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Hydrodynamic Lubrication-

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Boundary Lubrication or Thin Film Lubrication-

Boundary lubrication exists when the operating condition are such that it is not possible to

establish a full fluid condition, particularly at low relative speeds between the moving or sliding

surfaces.

The oil film thickness may be reduced to such a degree that metal to metal contact occurs between the

moving surfaces. The oil film thickness is so small that oiliness becomes predominant for boundary

lubrication.

Boundary lubrication happens when

A shaft starts moving from rest.

The speed is very low.

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The load is very high.

Viscosity of the lubricant is too low.

Examples for boundary

lubrication:

Guide and guide shoe in two stroke engine.

Lubrication of the journal bearing in diesel engines (mainly during starting and

stopping of engine).

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Lubricants used for boundary lubrication are:

Graphite and MoS2

• Use in the form of solids or as stable emulsion , in oil.

• It reduces friction between metallic surfaces by forming films on the surfaces and they can bear compression as well as high temperature

Mineral oils

• These are thermally stable and their adhesion property is improved by adding addingsmall amount of fatty acids or oils

Vegetable and mineral oils and their soap

• Posses greater oiliness compared to mineral oils.

• They either physically adsorbed to metal surfaces or react chemically at the metal surfaces ,forming a thin film of metallic soap,which acts as lubricants.

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Extreme Pressure Lubrication-

• When the moving or sliding surfaces are under very high pressure and

speed, a high local temperature is attained. Under such condition, liquid

lubricant fails to stick to the moving parts and may decompose and even

vaporize. To meet this extreme pressure condition, special additives are

added to the minerals oils. These are called “extreme pressure additives.”

These additives forms more durable films on the metal surfaces, capable of

withstanding high loads and high temperature. Additives are organic

compounds like chlorine (as in chlorinated esters), sulphur (as in

sulphurized oils), and phosphorus (as in tricresyl phosphate).

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Classification of Lubricants

• Solid lubricants (graphite, MoS2)

• Semi-Solid Lubricants (greases, vaselines)

• Liquid Lubricants (all oils)

– Animal and Vegetable oils

– Mineral oils

– Blended Oils

– SYNTHETIC Oils

• Cutting Fluids (O/W and W/O Emulsions)

Solid lubricants are used

at high temperature and heavy load

where a liquid or a semi-solid lubricant film

cannot be maintained

contamination of oil is unacceptable

where combustible lubricants must be avoided

Graphite and Molybdenum Disulphide

Oil dag (suspension of graphite in oil)

Aqua dag (suspension of graphite in water)

CHARACTERISTICS:

1. Soapy to touch.2. Inflammable.3. Doesn’t oxidizes under 375 C.4. Dispersion in oil is called ‘oildag’.

Dispersion in water is called ‘aquadag’.

USES:

1. As lubricants in air-compressors , lathes , foodstuffs industry , open gears , chains , railway track-joints etc.

2. Graphite mixed with grease (graphite-grease) is used at higher temperatures.

3. Oildag is used in internal combustion engines & Aquadag is used in foodstuffs industries.

Semi-Solid lubricants are used

machinery parts are subjected to slow speed

Heavy load and produce high temperature

Spilling of the oil is undesirable

Greases and Vaselines are most important

Grease

Semi-solid lubricant obtained by thickening of alubricating oil by the addition of metallic soap

The properties of grease depend upon the

the nature and mount of thickener used

the characteristics of the base oil used

the way in which the grease is prepared

Most of the common grease are made with soapthickener (3% - 50%)

Non-soap thickeners: carbon black, asphalten

Important functions of a SOAP in a grease it act as a thickener

it enables the grease to stick to the metal surface

the nature of the soap determines thetemperature up to which grease can be used andits consistency

Ca-Based Grease/Cup-GreasesEmulsion of petroleum oil with calcium soap.1. Insoluble in water2. They are the cheapest3. They can even be used at low temperatures because

above 80° C , soap starts to separateUSES: They are used in tractors , water pumps , etc.

Na-Based1. They are not water resistant.2. They can be used upto 175 °CUSES:They are used in ball bearings.

Li-BasedThey are petroleum oils thickened by mixing lithium soap 1. Water Resistant2. Expensive3. Thermally stableUSES: In Aircraft Applications.

Axle GreaseThey are cheap resin greases prepared by adding lime to resin & fatty oils.• They are water-resistant.USES:• They are used in the equipment which work under

high load & low speed

Liquid lubricants

• Mostly used

• Used in delicate & light machines which works at high speed but under low pressure

Animal and vegetable oil

Mineral oil

Blended oil

Synthetic oil

Animal and Vegetable Oil

• Extracted directly from nature

• They are very costly

• They can undergo oxidation very easily

• They are used where there is heavy load

• They are triglycerides of higher fatty acids

Possess Good Oiliness Property

Examples : Olive Oil , Sunflower Oil , Coconut Oil, Castor oil, Cod-liver oil

Mineral/petroleum Oil • Obtained by distillation of petroleum

• These are basically lower molecular weighthydrocarbons with 12-50 C atoms

• Shorter chain have lower viscosity

• Most widely used

• Cheap, available in abundance and quite stableunder any conditions

• Possess Poor Oiliness Property

• Oiliness can be improved by adding high molecular weight compunds like oleic acid, stearic acid or triglycerides of higher fatty acids

Blended Oil

• Additives are added to the lubricating oils inorder to further improve their properties of therefined lubricating oils.

• They are added in amounts upto 10%.

Cutting Fluid/Oil/Emulsion

• In various machining operations such as drilling,cutting, threading, turning the friction is veryhigh and this generates large amount of heat

• Due to overheating, the tool may lose itstemper and hardness

• In order to prevent this, efficient lubricationand cooling have to be provided

• This is done by employing cutting oils or fluidsor cutting emulsion

In order to provide satisfactory service, thecutting fluid should possess followingcharacteristics:

• good lubricating properties

• low viscosity, so that lubricant can easily fill inthe cracks formed on the work-piece

• chemical stability

• non-corrosive nature

• high thermal conductivity

Properties of Lubricants

FOR OILS• Viscosity and Viscosity Index

• Cloud and Pour-Point

• Flash and Fire-Point

• Aniline Point

• Oiliness

• Saponification number

• Steam Emulsion Number (SEN)

• Carbon Residue

FOR GREASE

• Penetration Index

• Drop Point

Viscosity & Viscosity Index

• Property of a liquid by virtue of which it opposerelative motion between its different layers isknown as viscosity or internal friction of liquid

• Measure of internal resistance to flow

• “Thickness” of fluid (in laymen terms)

• Important in lubricant selection

• Two Types:

– Kinematic Viscosity

– Absolute Viscosity

Viscosity-Temperature Relation

• viscosity of liquids decreases with increasingtemperature

• consequently, the lubricating oil becomesthinner as the operating temperature increases

Hence, viscosity of a good lubricating oil shouldnot change much with change in temperature

Viscosity Index• The rate at which the viscosity of an oil changes

with temperature is measured by an arbitraryscale

• If the viscosity of an oil falls rapidly as thetemperature is raised, it has low viscosity index

• If the viscosity of an oil slightly affected onrising the temperature, it has high viscosityindex

For a good lubricating oil viscosity-index shouldbe HIGH

Determination of Viscosity Index

• Two types of standard oils, Paraffinic-basePennsylvanian oils (VI =100) and Naphthanic-base Gulf oils (VI =0) are used

• Against each of these is marked their viscositiesat 100°F and 210°F

• Former are known as H-oils and latter as L-oils

• Following steps involved:

• The viscosities of the oil under test at 100°F andalso at 210°F are first found out, let thesevalues be U and V respectively. The differencebetween the two values should be low, if the oilis good.

• Now from the list of H-oils (VI =100), the oilwhich as the same viscosity at 210°F as the oilunder-test is selected, and its correspondingviscosity at 100°F is determine: say H

• Then, from the list of L-oils (VI = 0), the oilwhich as same viscosity at 100°F is determine:say L

U: viscosity at 100°F of the test oil (sample)

L: viscosity at 100°F of the low-viscosity standard oil

H: viscosity at 100°F of the high-viscosity standard oil

𝑽𝑰 =𝑳 −𝑼

𝑳 −𝑯× 𝟏𝟎𝟎

Viscosity-Temperature Graph

Redwood ViscometerParameter Redwood Viscosity No. 1 Redwood Viscosity No.2

Dimension of orifice Length:10mmDiameter:1.62mm

Length:50mmDiameter:3.80mm

Receive flask has Smaller mouth Larger mouth

Useful for Low viscous oil such as lubricating oils

High viscous oil such as fuel oil

Efflux time 2000 sec or less Approximately 1th/10 of that obtained with No.1 instruments.

Cloud and Pour Point

• When an oil is cooled slowly, the temperatureat which it becomes cloudy or hazy inappearance, is called its CLOUD POINT

• While the temperature at which the oil ceasesto flow or pour, is called its POUR POINT

Good lubricant should have low pour point.

Polyalkylbenzene, is a pour point depressant

• It gives the suitability of lubricants in coldenvironments

• The presence of waxes in the lubricating oilincreases the pour point.

• Pour point determiners the suitability of alubricant oil for low temperature.

Flash and Fire Point • The minimum temperature at which the oil

gives off sufficient vapours to ignite for amoment (flash), when a flame is brought nearthe surface of the oil, called its FLASH POINT

• The lowest at which the vapours of the oil burncontinuously for atleast 5 seconds, called itsFIRE POINT

Good lubricant should have higher flash pointw.r.t working temperature

Aniline Point

• gives an indication of the possible tendency ofdeterioration of an oil when it comes intocontact with packing, rubber sealing etc.

• Aromatic hydrocarbon have a tendency todissolve rubber

• defined as the minimum equilibrium solutiontemperature for equal volumes of aniline andoil sample

• Should be HIGH

Determination of Aniline Point

• Mixing equal volumes of aniline and the oilsample in a tube

• Heating the mixture until a homogeneoussolution is obtained

• Allowed to cool at a specific rate, until the twophases (oil & aniline) separate out

• Temperature corresponding to observation isreported as ANILINE POINT

Saponification Number

• The number of milligram of KOH require toneutralize the fatty acids resulting from the

complete hydrolysis of 1g of fat

• Unitless

• To indicate the presence of fatty materials inoil

Steam Emulsion Number (SEN)

• It is number of second require to demulsifiedLubricant

• Unit: seconds

• For good lubricant it should be less

• To indicates the formation of stable emulsionor not

• Good lubricating oils never form stableemulsion

Penetration Index/Consistency value

• Consistency of the grease

• Defined as the distance in tenth of mm that astandard cone penetrates vertically into thesample, under the condition of load, temp andtime

• Instrument: Penetrometer

Drop Point

• Defined as the temperature at which greasepasses from the semi-solid to the liquid state

• Upper limit temp for the applicability of thegrease