16791_Lubricants.ppt

7/27/2019 16791_Lubricants.ppt

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LUBRICANTS


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Lubricants and lubrication

Lubricant is a substance which whenintroduced between two moving partsreduces friction by forming a slippery filmbetween the two surfaces and thus improving

efficiency and decreasing wear

So, lubrication is the process of reducingfriction and wear between two moving

surfaces by applying lubricating substancesbetween the parts


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Principle of lubrication

All material have irregularities in form of peak (asperities)and valleys

When peak of upper surface come in contact with peak oflower surface, these asperities may crush under small load,until the contact area is large enough to support the load andis known as wear

And also when the upper surface and lower surface move

against each other that lead to friction which further reducesthe efficiency of the machine

Therefore to reduce wear and to reduce friction we uselubricants that increases the efficiency of the machines


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Purpose of lubrication

To keep moving parts apart: by forming thinlayer between the surfaces

To reduce friction: lubricant to surface friction islesser than surface to surface friction

To protect against wear: lubricant prevent wear

by keeping moving part apart To transfer heat: liquid lubricants are effective in

heat circulation due to high specific capacity

To prevent corrosion: as surface of metal do not

come in contact with air (Oxygen) To seal gapes: lubricant seal the space between

moving parts through capillary force

To carry away contaminants and debris


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Mechanism of lubrication: thick film or hydrodynamic

lubrication

This also known as fluid film lubricant

If the surface slide over the other lead to increased friction andasperities tend to break leading to wear

An excellent method to prevent this is to use lubricant film toseparate the surfaces and is known as hydrodynamic lubrication(hydro= liquid, dynamic = relative motion)

The lubricant film ensure that no wear is possible between movingparts due to reduced friction

The hydrodynamic film is formed when the geometry, motion ofmoving surface and lubricant viscosity combine to increase thelubricant pressure to support the load as increased pressure forcesthe surface apart

In hydrodynamic lubrication one surface floats over the othersurface and increased fluid pressure forces the surface apart

The four essential elements to consider in hydrodynamiclubrication: liquid (hydro); relative motion (dynamic); viscousproperties of liquid and geometry of surfaces


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The heat generated between moving parts has to bedissipated effectively to decrease the temperaturedependence of viscosity

Delicate instruments such as watches, clocks, guns,

scientific instruments are provided with hydrodynamiclubrication

Hydrocarbons are considered to be best for fluid filmlubrication

Other mechanism:

Thin film of boundary lubrication

Extreme pressure lubrication


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Classification of lubricants

i. Gaseous lubricants: (lower viscosity, and highercompressibility eg air, steam or liquid metal vapors

ii. Liquid lubricants:

water (used as such or in combination with otheroils and used in engineering processes such asmilling and lathe turning.

Mineral oil (derived from crude petroleum oil) egpolyalphaolefins, naphthenics, polyalkyleneglycos

Lanolin (derived from sheep wool and arecorrosion inhibitor, protection against rust, salt andacid)

Vegetable oil (triglyceride esters, eg castor oil,palm oil, sunflower oil, whale oil etc)


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III. Solid lubricants: used under high temperature conditionsin form of dry powder

Graphite (planes of polycyclic carbon atom andhexagonal orientation. Higher the carbon atom content

and higher the crystallization the better is the lubricationaction

Molybdenum disulphide (M0S2): hexagonal structure andlubrication performance is better than graphilte, butlubrication is limited to 400 oC as above 400 oC it

undergoes degradation Boron nitride (BN): ceramic powder ( used at high temp.

up to 1200 oC; exist in tow forms cubic (very hard cantused as lubricant) and hexagonal used for lubrication

Teflon or poly tetrafluoroethane: PTFE does not have

layered structure but the molecules slip easily along eachother and hence the lubrication is acheived

Note MoS2, PTFE and graphite are antifriction, antiwearadditives are often mixed with polymers to give selflubrication composites


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IV. Semisolid lubricants (grease): contain thickeningagents like soaps of sodium, calcium or aluminiummixed with mineral oil (80% mineral oil+10%

soaps+ 10 % additives) Grease are named after soap used in manufacture

eg

Lithium based grease (water and dust resistance

and used up to 15-30o

C) Calcium based grease (water resistant and used up

to 70oC)

Aluminium based grease(high water and acid

resistance and used up to 80o

C) Sodium based grease (sparingly soluble in water

and cant be used in wet condition and used up to120oC)


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V. Metal alloys: some metal and alloy can alsobe used as lubricant or grease additives eg

lead, tin, zinc alloys are used in bearings andmetal powders are used to lubricate slidingsurfaces


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Additives of lubricants

Additives are employed to impart desired characteristics tolubricants and some additive used in lubricant formulationare:

Antioxidants: as at elevated temperature lubricant oil getoxidized so antioxidants are used to reduce the rate ofreaction

Detergents: detergents additive maintain a suspension ofcarbon particle and extend lubricant life

Antifoaming additives: to provide proper flooding oflubricant at point of contact antifoaming additives are used

Alkalinity additives: certain fuels which have high sulfurcontent lead to SO2 and SO3 gases which lead to acidcorrosion and hence to avoid that we add alkalinity additives

Demulsibility (water-shedding) additives: in marine relatedequipments lubricant get contaminated with water and these

additive force the quick separation of water from lubricants


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Properties of lubricants

Viscosity

Flash point

Fire point

Cloud point

Pour point


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Viscosity

Viscosity is the most important property of lubricants that

determines the operating condition under which it is used Viscosity is the property of fluid which is a measure of its

resistance to flow

The steady state of flow of a liquid can be visualized toconsist of series of parallel layers moving one above other

The function of lubricating oil is to form a liquid filmbetween two moving or sliding surfaces.

If the viscosity of oil is too low, the resistance to flowbetween individual layer of lubrication is low andconsequently the excessive wear take place

On the other hand if the viscosity is too high resistance toflow between different layers of later is high and frictionbetween layer increases and hence decreases efficiency

Therefore viscosity of lubricant should be optimumaccording to need


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Factors affecting viscosity

Temperature: Viscosity of liquid decreases with

temperature as the lubricant oil become thinner asoperating temperature increases

It is desirable that viscosity of a lubricant should beconsistent over a range of temperature so that it canbe used at different temperature and we measure itby viscosity index, higher the viscosity index, smallerthe change with temperature and lower the viscosityindex higher will be viscosity change withtemperature

Pressure: Viscosity increases with increasingtemperature of operating system

Molecular structure: in general an oil with highmolecular weight possesses a high boiling point andhigh viscosity


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Fire and flash points

Flash point is the lowest temperature at which the lubricant

oil gives off enough vapors to ignite (spark) but not burnwhen small flame is brought near it

Fire point is the lowest temperature at which the vapors oflubricant oil burn continuously at least for 5 seconds when asmall flame is brought near it

Flash and fire points determine the volatility and fireresistance of lubricant

Flash point is important in determining shipping, storage,and safety regularity of oil

A drop in flash point shows the presence of volatileimpurities in oil

A good lubricant should have higher flash point than thetemperature it is used (usually fire point are 8-10% higherthan flash points)


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Cloud and pour points

Cloud point: When lubricant is slowly cooled, the

temperature at which it become cloudy or hazy inappearance is called its cloud point

Pour point: When the lubricant is slowly cooled, thetemperature at which the oil ceases to flow or pouris called pour point

Cloud and pour point indicate the suitability oflubricant in cold conditions

They are also useful in identifying the source oflubricating oil as oil derived from petroleum source

contain paraffin wax and other high melting fractionwhich solidify on cooling

Lubricant used in machine working at lowertemperature should posses low pour point to avoidsolidification of oil.


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Selection of lubrication

Selection of lubrication depend upon Lubrication properties

System of application and service conditionrequirement such as type of motion speed,

temperature, load and environment In engineering domain lubricants are selected on

basis of tribology (study of friction) and wear,chemical compatibility with machine parts and

environmental impact The most important criteria for selection include

the system on which lubricant is applied andservice conditions (Application based)


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Lubricant selection based on

application

Lubricants for cutting tools: For this application cuttingfluids in combination with lubricant and coolants areused and main functions are

(a) to cool cutting tool edge at appropriate temperature

(b) to cool metal work piece to prevent distortion(c) to reduce power consumption by lubrication action

(d) preventing rusting of machine parts

For heavy cutting low viscosity mineral oil blendedwith fatty oils

For light cutting, the emulsion of oil in aqueous soapsolution is most effective

For grinding and turning operation water may be usedas coolant and lubricant


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Lubricant for fragile equipment: For delicatemachines (watches, clocks, scientific equipementswhich do not operate at high temp. and press. Mild

lubricant are required (vegetable or mineral oil) Lubricant for combustion engines: Lubricant in

internal combustion exp0se to high temperature hashigh temp. Therefore lubricant should posses high

thermal stability and viscosity and hence petroleumoils (with additives) which are stable oxidation andtemp. are used

Lubricant for gears: gears are subjected to extremepressure and centrifugal force so posses followingproperties (a) good viscosity(b) good oiliness (c )good adhesion and drip (d) chemical stability andhigh resistance to oxidation (e) high load carryingcapacity


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Lubricant for different loads and speed: (based oncondition temperature, pressure and speed) Hightemperature (low volatility and good oxidationresistance), High pressure or load and speed (lubricant

with extreme pressure additives are used), heavy loadwith low speed (greases and blended thick oil), low loadat high speed (low viscosity oil and gases as lubricants)

Lubricant for transformers (lubricant in transformer areused to insulate winding and carry away heat

generated , therefore lubricant with good dielectricconstant, good heat transfer properties and lowviscosity are used . For example highly refined mineraloil with excellent electric and insulating properties andalso chemical stability

Lubricant for textile industry: (involves parts moving athigh speed, mineral and synthetic oil are used for knitmachines and spindles, semi synthetic and syntheticgrease that do not stain are used in bearing


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Lubricants for refrigeration system (lubricantwith low cloud and pour point, eg naphtalene

based oils Lubricants in food and pharmaceutical

industry: Food grade lubricant must perform

same function as other lubricant ie provideprotection against wear, friction, corrosionand oxidation, dissipate heat but comply withhealth and safety norms (inert, tasteless and

odorless). They should resist growth ofmicroorganism to avoid contamination

Eg castor oil, white oil, methyl silicone withadditives and thickners

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