unit 1-lubricants and propellants modified

8/7/2019 Unit 1-Lubricants and Propellants Modified

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LUBRICANTS

Friction:

Resistance observed between two

moving or sliding surfaces creating wear

and tear.

Lubricant:

Any substance introduced between two

moving or sliding surfaces with a view tore uce t e r ct ona res stance etween

them is know as a lubricant.

Lubrication:

The process of introducing lubricant

between moving/sliding parts is known

as u r ca on

http://in.groups.yahoo.com/group/dedicatedchemgr/


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FunctionsofLubricant

o To reduce frictional resistance between surfaces and

reduce deformation, wear and tear between moving/sliding

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 the leakage of gases under high pressure like a

seal or Teflon.


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.

Hydrodynamic lubrication

2. Boundary lubrication (or)Thinfilm lubrication

3.Extremepressurelubrication


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(~1000)

Characteristics:

o The surfaces are separated by a thickfilm (at least

1000 thick) and hence there is no direct surface to

surface contact.

o No welding of junctions.

on the both surfaces, there is no direct contact

.


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,theLubricatingpropertiesdependon:

o Viscosity of lubricant (Lubricant should be

.

.

surfaces.


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Example: Shaft running (Journal bearings)

Lubricant oil covers the irregularities of the shaft as well as theear ng sur aces.


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o Exampleswherefluidfilmlubricationis usedare:

i) Sophisticatedinstruments

ii) Lightmachineslikewatches,guns,sewingmachinesetc.

o Examples for fluidfilm 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.

ma amoun o unsa ura e y rocar ons presen n y rocar on

oils produced from petroleum fractions, which causes oxidation and

produce gummy like products. Hence, antioxidant like aminophenol

are used in journal bearings with lubricant


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Mechanismoffluidfilmlubrication


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2.BoundaryLubrication/ThinfilmLubrication

Characteristics of thin film lubrication are:

o Resistance to heat and oxidation.

o oo ness.

o Low pourpoint.


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MechanismofBoundaryLubrication/Thinfilm

This Lubrication takes place due to:

o Adsorption of lubricating oils to both surfaces by

.

o The adsorbed layers on the both metal surfaces

carry the applied load.

o Coefficient of friction, f = 0.05 0.15 and distance

between surfaces is to be the order of the distance

of the asperities.


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or oun ary u r cat on, t e u r cant

molecule should have:

(i) Long hydrocarbon chain with polar groups.

(ii) Polar groups promote spreading and orientation

over the metallic surfaces at high pressure.

(iii) Lateral attraction between the chains.

,chemical linkages metal or other surfaces.


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ExamplesofBoundarylubrication Vegetable and animal oils (glycerides of higher fatty acids

& their soaps).

o These oils either physically adsorbed to metal surfaces or react

chemically at the metal surfaces.

o oug ese o s posses grea er a es on proper y an o ness, ye

they tend to breakdown at high temperatures. Hence, fatty acids are

added to mineral oils to improve the oiliness.

Graphite and Molybdenum disulphide alone or oilsuspension may be used because:

o TheyhaveLowinternalfriction

o Theycanbear/withstandcompression

o Theyarethermallystable


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MechanismofBoundaryLubrication


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3.ExtremepressureLubrication When 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

ecompose an even vapor ze.

To avoid this, special additives are added to mineral oils. Theseare called extreme-pressure additives.

Mechanism The extreme-pressure additives form on metal surfaces

more durable films, capable of withstanding very high loadsan g empera ures.

Fe-Chloride = 650oC, Fe-Sulphide= 1100oC

Exam les:

Organic compounds containing chlorine, sulphur and phosphorus.

o These compounds react with metallic surfaces, at prevailing high

temperatures, to form metallic chlorides, sulphide or phosphides.

o These metallic compounds possess high melting points.


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BasedonPhysicalstate,lubricantsareclassifiedas:

a) Lubricatingoilsorliquidlubricantsb) Semisolidlubricantsorgreases

c) Solidlubricants


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1.Lubricatingoilsorliquidlubricants

Purpose:o Provideacontinuousfluidfilm.

o Provideacoolingbetweenthesurfaces.

o Actasasealingagent.

o Actascorrosionpreventingmaterials.

Propertiesofliquidlubricants:

o Lowpressurei.e.,highboilingpoint.

o Adequateviscosityforparticularserviceconditions.

o Lowfreezingpoint.

.

o Stabilitytodecompositionattheoperatingterms.

o Highoxidationresistance.

o Noncorrosiveproperties.


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TypesofLiquidLubricants

a) AnimalandVegetableoils: Usableunderveryhightemperatureandheavyload.

D sa vantageso tsusagesare

1.Costly2.Undergooxidationeasilyincontactwithairandforms

gummyandacidicproducts,andgetthickened.

3.Tendencytohydrolyzeincontactwithmoistairoraqueous

medium.So,theyareusedasblendingagentswithothermineraloils.

b) Mineraloilsorpetroleumoils:

Theyareobtainedbydistillationofpetroleum.o Lengthofhydrocarbonchainvariesbetween12to50carbonatoms.

o Shorter chainoilshavelowerviscositythanthelonger chainhydrocarbons.


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Liquid lubricants are most widely used lubricants

because they are

1. Cheap

.

3. Quite stable under service conditions.

animal and vegetable oils.

o So hi h molecular wei ht com ounds 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, additives

are used to improve the properties. These blended

oils give desired lubricating property required for a

machinery.


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

lubricants.

c)Pour-point depressing additives:

naphthalene.


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d)Viscosity index improvers : hexanol

e)Thickeners : Polystyrene or polystyers

f)Antioxidants or inhibitors : Aromatic phenolic or amino compounds

g)Corrosion preventers : Phosphorous or Antimony organic

compounds

h)Abrasion inhibitors : tricresyl phosphate

i)Antifoaming agents : glycols and glycerol

k)Deposit inhibitors : detergents such as salts of phenol and

carboxylic acids


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2.SemiSolidLubricantsorGreases

o Semisolidconsistingofasoapdispersedthroughout

MaybePetroleumoilorsyntheticoilwithaspecificadditive.

Pre aration:

Saponificationoffat(suchastalloworfattyacid)withalkali

(likelime,causticsodaetc.,)

Additionoftheabovetohotlubricatingoilunderagitation

thickening agents ( like finely divided clay, bentonite, colloidal

silica, 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 lace due to hi h load, low s eed,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 Ca based greases or cup-greases (up to 80oC & water resistant)

-

o Lithium-based greases (up to 15oC and water resistant)

o Axle- greases lime with resin and fatty acids (water resistant &

less delicate equipment under high loads and at low speed)o Graphite greases (rail axle boxes, machine bearings, tractor

& wire ro es

o Soap stone (Steatite is relatively soft and may feel soapy when

touched, hence the name and rich in Mg).


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Solidlubricantsareusedwhen:

o Otherlubricantscannotbeused

o Toohightemperatureorloadareinvolved

o Combustiblelubricantsnotacceptable

a) Graphite(or)

c) sandstone,mica,talc,etc.


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ExamplesofSolidLubricants

1.Graphite:Talc

o Noninflammableo Notoxidizedinairbelow375C

o Oil+graphite oildag(internal

combustionengines)o Water+Graphite aquadag

Emulsifyingagent(tannin)

o Grease+graphite

graphitegreases(athighertemp.)


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ExamplesofSolidLubricants Mo

2.Molybdenumdisulphide:

o Lowcoefficientoffriction(0.0050.01)

S

kestr

308 pm Mo

313 pm

dwitc

h-li

Sa

Soapstone,talcormicaarealsousedassolidlubricants.


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4. Synthetic Lubricants

e ro eum u r can s + spec c a ves mee s mos ras c

and severe conditions

. -

Low freezing point

-

Non-inflammable

Hi h flash oint

High temp. stability

High chemical stability

Examples:

Pol merized h drocarbons: ol ro lene ol but lene

polyethylene ~ molecular ranges of 500 to 50,000 Da


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Polyglycols & related compounds: polyethylene glycol,

,

oxides in rubber-bearings and joints.

of sheet glass manufacturing machines

Oxides are not used at hi h tem . due to thermal decom osition

Organic amines, imines & amides:

- Low pour points

- High viscosity-index

- Temp. range: -50oC to 250oC

Silicones:

-Up to 200oC and applicable at low temp. lubrication process

- Starts cracking at 230oC

- High viscosity-index


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Fluorocarbons:

- Not decomposable by heat i.e. temp. resistance

- Chemicall resistant- Non-oxidizable- But reacts with molten sodium

A fairly dispersion of two immiscible liquids, the one being dispersed as

fine dro lets in the other

The disperse phase/the internal liquid that is broken into droplets (1-6m)

The dispersing phase/external/continuous phase is the surrounding liquid

Emulsifier/emulsifying agent:

Oil in water emulsion: 74.02% water + 25.98% oil (3-20% water-soluble

emulsifying agent as sodium soap; sodium & potassium salts of

sulphonic acids) for cooling of cutting tools and for rust protection

Water in oil emulsion: 74.02% oil + 25.98% water (1-10% water soluble

Emulsifying agent like alkaline-earth soap e.g., calcium stearate)


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Properties ofLubricants1. Viscosity:

Thepropertyofaliquidorfluidbyvirtueofwhichitoffersresistancetoitsownflow.

.

(viscosityisinverselyproportionaltotemperature)2.Flash PointsandFire Points :

FlashPoint:

vapour that ignites for a moment, when a tiny flame is brought nearit.

re o n :

Thelowesttemperatureatwhichthevapouroftheoilburncontinuouslyforatleastfiveseconds,whenatinyflameisbroughtnearit.Inmostcasesthefirepointsare5to40oChigherthanthe

flashpoints.ItismeasuredbyusingPenskymartensapparatus.


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3. Oiliness:

measure o s capac y o s c on o e sur aces o

machine parts under conditions of heavy pressure or load.

- .

o Important for extreme Pressure lubrication

When an oil is cooled slowly, the temperature at which it

becomes cloudy or hazy in appearance, is calleds .

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

.

5. Volatility:

o oo u r can s ou ave ow vo a y.

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.

, ,

etc., causing abrasion and wearing out of the lubricating partsof the machinery.

o A ood lubricatin oil should form an emulsion with water

which breaks off quickly.

7. Carbon residue:Normally lubricants consist of high % of carbon containing

compounds.

.

carbon 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 for

tarnishing .

o To revent or retard corrosion effect of lubricatin 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

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

b. H drol sis: Moisture in oils causes h drol sis of esters

c. Pyrolysis : At high temperature


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. . .o The minimum equilibrium solution temp. for equal volumes of

aniline and oil sam le.

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 e percen age o asp a presen n o a er cen r ug ng osolution in petroleum ether.

o Precipitation Number is used to differentiate the different

classes of lubricants.


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12. Specific Gravity: (useful to identify unknown oil)

A.P.I. ( American petroleum Institute) degree:

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

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

API hydrometer for oil starts at 10 which is equiv. to 1, sp. Gravity

of water at 60oF

Useful for interconversion of wt and volume measurements of oils

13. Ash Point:

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

abrasion and wear it may cause

.

o Number of milligrams of KOH required to saponify

1g 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

.

To check maximum load that can be carried safely by alubricant

16. Neutralization Number:

constituents of an oil.

o Acid Number: Amount of KOH re uired in milli rams

to neutralize the fatty acids in 1g of oil.

o Good lubricating oil acid number value < 0.1o More than 0.1 indicates oil gets oxidized and

corrodes the engine


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C.Pro ellantsHighoxygencontainingfuelsormixtureof

EvolutionofHu evolumeofhot ases(temp=3000oC & pressure:300kg/cm2)

supersonicvelocity.


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Characteristicsofagoodpropellanto Havehighspecificimpulse

o Depositlowmolecularweightproducts(H2,CO2,CO,N2)

o Havelowignitiondelay(inmilliseconds)o Havehighdensity

o BeStableoverawiderangeoftemperatures

o BeSafetohandleandstoreatordinaryconditions

. ., ,

o Bereadilyignitableatpredictableburningrate

o BeNoncorrosiveandnonhygroscopic

o Leavenosolidresidueafterignition

o Notreleaseanytoxicproductsonburning

o Givehightemperatureoncombustion


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Classification of propellants:

1) Solid propellants: Homo enous or com osite

***Homogeneous solid propellants:

colloidal stateI.Single-base propellant from single P. ou e- ase prope an --- one so w c con a ns wo

materials

Examplesnitrocellulose

Ballistite

65% nitrocelluloseor e ro g ycer n

5% Petroleum jelly( as plasticizer)

C i i f D B P (D bl b ll )


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Composition of D-B-P (Double-base propellant)

- 40-45% Nitroglycerine Diethyl phthalate: up to 5% Di hen l amine u to 1% is added as stabilizer Temperature: 27000 C

Volume of the product gases = 1500 fold

*** Heterogeneous or composite:

Fuel + oxidising agent

Example:

Gun powder oldest composite propellant -

Volume 400 fold

46% of ammonium picrate + 48% NaNO3 + 8% plastic resin binder

80% Ammonium perchlorate + 20% resin binder


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Properties of oxidizing substances:

Non hygroscopic

Stable in contact with fuel

Does not form an corrosive roducts..

Potassium perchlorate forms KCl (white powder) Ammonium perchlorate forms no residue but combustions products

HCl and water which are toxic and corrosive fog.

2. Li uid Pro ellants:

More versatile

Engine calibration is easier

Two Types

. ono prope an s2. Bi propellants


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Types:Monopropellants

Fuel+oxidizerareinthesamemoleculeorinthe

samesolution.

Properties:

o Safetostore

o Burningshouldbesmooth

Examples:i)H2O2,ii)nitromethane,

iii)ethyleneoxide,

v y raz ne,

v)propylnitrate

vi amixtureof21.4%MeOHand78.4%

H2O2 solution

Bi ll


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Bipropellants

injected separately to the combustion chamber from

Fuels: Liquidhydrogen,hydrazine,ethylalcohol,aniline kerosene.

Ethanolmixedwith25%ofwater

Oxidizers: Li uidox en ozone H O fumin HNOliquidfluorine

o Liquid O2 is non toxic but need high pressure insulatedcon a ner or s or ng.

o Liquid O3 is also very good but forms toxic products andcan explode at high concentration

o Liquid fluorine is volatile, toxic, corrosive, very reactive,

difficult to store and handle.


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Comparativefeaturesofsolidandliquid

prope an sSolidpropellants Liquidpropellants

Lowspecificimpulse High

Easilyhandlingandstoring Noteasy

Simpleengine Delicateenginedesign

Moreeconomical Lesseconomical

Notversatile Versatile


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Specific Impulse:

rus n g per secon per g o e ue urn . e va ue o rus or

propulsive force due to momentum of the exiting gases and is given by:

F= mv/g + (Pe - Pa)Ae

F= thrust ( kg/m/kg); m = mass flow (kg);

g = acceleration due to gravity (9.81 m2/s);

v = exhaust velocity (m/s); Pe = exit pressure (kg/m2);

Pa = ambient gas pressure (kg/m2) and

Ae = nozzle exit area (m2)

unit 1-lubricants and propellants modified

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