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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)