DE G513: TRIBOLOGYDE G513: TRIBOLOGY

Topic‐I: Physical Properties of Lubricants and their Composition:Lubricants and their Composition:

Part‐1

Dr. Srinivasa Prakash Regalla

Professor & HeadProfessor & Head

Department of Mechanical Engineering

Scope• What is tribology? 

• Friction, wear and lubrication. 

• Cost of friction and wear.

What is Tribology?• Tribology focuses on friction, wear and lubrication of interacting surfaces in relative 

i h l f h i dmotion among the elements of mechanisms and machines

• 'Tribology' is derived from the Greek word 'tribos’, meaning rubbing or sliding

• Tribology is a field of science & engineering, which applies an operational analysis to problems of great economic significance such as reliabilityof great economic significance such as reliability, maintenance and wear of technical equipment ranging from household appliances to spacecraft.

FrictionFriction

• Friction is the resistance to relative sliding ofFriction is the resistance to relative sliding of surfaces in contact, mainly due to the interacting roughness textures and anyinteracting roughness textures and any solid/liquid interposed between them

WearWear

• Wear is the major cause of material wastage and loss j gof mechanical performance and any reduction in wear can result in considerable savingsF i ti i i i l f d• Friction is a principal cause of wear and energydissipation.

• Considerable savings can be made by improved frictionConsiderable savings can be made by improved friction control, both in terms of material and energy

• It is estimated that one‐third of the world's energyresources in present use is needed to overcome frictionin one form or another.

LubricationLubrication

• Lubrication is an effective means of controlling wear gand reducing friction.

• It may be a liquid, gas or solid that is interposedb t t ti f t t l f i ti dbetween contacting surfaces to control friction, andhence wear

• General thickness of lubricant film ranges from 1 μm toGeneral thickness of lubricant film ranges from 1 μm to 100 μm: thinner or thicker are possible though

• Formal definition of lubrication: The knowledge that is related to enhancing or diagnosing the effectiveness of these films in preventing damage in solid contacts is known as “Lubrication”known as  Lubrication

Gas, Liquid or Solid Lubrication?Gas, Liquid or Solid Lubrication?

• Gaseous films are preferred for low contactGaseous films are preferred for low contact stress situations

• Solid films are preferred for low sliding speed• Solid films are preferred for low sliding speed contact situations

I ll h i li id fil b d• In all other occasions liquid films may be used

Hydrodynamic LubricationHydrodynamic Lubrication

• It is the detailed analysis of gaseous or liquidIt is the detailed analysis of gaseous or liquid films

• Possible only when the relative movement of• Possible only when the relative movement of surfaces is sufficiently high

A ffi i l hi k fil f l b i i• A sufficiently thick film of lubricant is generated and maintained as long as the 

d i i i dspeed is maintained

Elastohydrodynamic LubricationElastohydrodynamic Lubrication

• Specialized form of hydrodynamic lubricationSpecialized form of hydrodynamic lubrication

• Takes into account the elastic deformation of contacting surfaces and involves physicalcontacting surfaces and involves physical interaction of contacting bodies 

I i i ll i ifi• It is practically very significant

Hydrostatic LubricationHydrostatic Lubrication

• A liquid or gaseous lubricant is forced into theA liquid or gaseous lubricant is forced into the space between contacting bodies

• Forcibly thus separation of the contacting• Forcibly, thus, separation of the contacting bodies is maintained to enable relative movementmovement

• Necessary when the relative speed of rotation i l f h d d i l b i iis too low for hydrodynamic lubrication to sustain

Magnetic BearingsMagnetic Bearings

• Application of magnetic forces to separate theApplication of magnetic forces to separate the surfaces

• It is called as the principle of magnetic• It is called as the principle of magnetic levitation or maglev

Hi hl i li d h l d ill h• Highly specialized technology and still at the stage of development

Boundary and Extreme Pressure bLubrication

• Involves both high pressure mechanical andInvolves both high pressure mechanical and chemical interactions between contacting bodies and lubricantbodies and lubricant

Solid lubricationSolid lubrication

• It is lubrication by solid films either separatelyIt is lubrication by solid films, either separately interposed or applied as coatings on one or both surfacesboth surfaces 

Why “the interacting surfaces in l ”relative motion” are important?

• The answer is that surface interaction dictates or controls the f ti i f ti ll d i d l d bfunctioning of practically every device developed by man.

• Everything that man makes wears out, almost always as a result of relative motion between surfaces.

• An analysis of machine break downs shows that in the majority of• An analysis of machine break‐downs shows that in the majority of cases failures and stoppages are associated with interacting moving parts

• Examples are: gears, bearings, couplings, sealings, cams, clutches,Examples are: gears, bearings, couplings, sealings, cams, clutches, etc.

• The majority of problems accounted for are tribological.• Our human body also contains interacting surfaces, e.g., human y g , g ,

joints, which are subjected to lubrication and wear.• The lubrication of human joints is still not clear despite so much 

research in Tribology

Friction and wear, do we always aim to minimize? 

• No, not always.

Limitations of Liquid LubricationLimitations of Liquid Lubrication

• Degradation at high temperaturesDegradation at high temperatures

• Low load carrying capacity at hightemperaturestemperatures

• Lack of environmental sustainability

Types of Wear

• Adhesive wear• Abrasive wear• Erosive wear• Corrosive wear• Oxidation wear• Fretting wear• Fatigue wear• Impact wear• Melting wear• Diffusive wear

COST OF FRICTION AND WEARCOST OF FRICTION AND WEAR

• Per machine it may appear small but cost toPer machine, it may appear small, but cost to nation due to all the machines running in the country can be very highcountry can be very high

SAVINGS EXAMPLESAVINGS EXAMPLE

• Power rating of a worm gear drive = 7.5 kWg g• 5% savings in power due to tribology practice = 7.5 * 0.05 = 0.375 kW

• Annual power saving per worm gear drive = 0.375 * 365 *10 duty cycle per day = 1369 kilo‐watt‐hourshours

• If there are 30 lakh worm gears running in the country, annual power savings = 1369 * 30 00 000 41070 l kh kWh30,00,000 = 41070 lakh kWh

• At a rate of 5 rupees per kWh, savings = 41070 * 5 = 205350 lakhs = 2053 5 crores5 = 205350 lakhs = 2053.5 crores

Further Evidences of Economic f b lImportance of Tribology

• Peter Jost report, 1966, UK, 515 million poundsp , , , p• West Germany, 1976, 10 billion DM per annum (50% due to abrasive wear)

• USA, estimate: 11% of total annual energy savings in transportation, turbo machinery, power generation, industrial processesgeneration, industrial processes

• USA, 18.6% of total energy consumed by cars=$14.3 billion per annum

• Most governments have introduced legislations to implement good tribological practices

Importance of Tribology in Designing f h lof Mechanical Systems

• Erosive wear is 50 times more in bends than in straight gsections in pneumatic transportation pipes

• Non‐abrasive materials such as wood chips, sugar cane h b i i hi h dmash can cause abrasive wear in high speed 

transportation pipes• Bearing failures in generator sets in USA can costBearing failures in generator sets in USA can cost $25000  per day

• To replace a £200,000 bearing  in an oil rig, an expense of £1 million is required!

• The cost of wear of a US naval aircraft is #243 per day

Material Selection for TribologyMaterial Selection for Tribology

Requirements of LubricantsRequirements of Lubricants

• Required properties of lubricants:• Required properties of lubricants:–Control friction and wear–Not degrade in service–Not corrode surfaces in service

Physical Properties of LubricantsPhysical Properties of Lubricants

• ViscosityViscosity• Viscosity temperature dependence• Viscosity index• Viscosity index• Pour pointFl h i t• Flash point

• Volatility• Oxidation stability• Thermal stability

Oil ViscosityOil Viscosity

• Fundamental roleFundamental role• Oil viscosity depends on:• Tempearture• Tempearture• Shear rateP• Pressure

• Thickness of oil film ∝ viscosity• Performance of lubricant is NOT directly proportional to viscosity, why?

Dynamic Viscosity

Kinematic ViscosityKinematic Viscosity

Density of mineral oil is 850 kg/m3./ 3Most lubricating oils have densities between 700 and 1200 kg/m3.

Why the above relation and the discussion of kinematic viscosity are important?

Viscosity‐Temperature RelationshipViscosity Temperature Relationship

• With increasing temperature the viscosity ofWith increasing temperature, the viscosity of oil drops drastically

• Two ways to determine viscosity at a given• Two ways to determine viscosity at a given temperature (other than room temperature)

U i i it t t ti– Using viscosity temperature equations

– Using ASTM charts

Viscosity‐Temperature RelationshipsViscosity Temperature Relationships

ASTM ChartsASTM Charts

• The basis for ASTM charts is the Walther’sThe basis for ASTM charts is the Walther sequation

Taking 10 base log on both sides two timessuccessively, we obtain the following relation usingwhich the ASTM charts were prepared.

VISCOSITY INDEX

Problem: Find the viscosity index of an oilwhose v40 = 120 cS and v100 is 16.4 cS.

Solution: From table, for v100 is 16.4 cS,L=346.6 and H=170.7. U=120. Therefore theVI for this oil is VI=[(346.6‐120)/(346.6‐170.7)]*100 = 128.8.

Viscosity‐pressure relationshipViscosity pressure relationship• Lubricant viscosity increases  tremendously with pressure: for example asphalt or bitumenwith pressure: for example asphalt or bitumen or tar

B i f dBarus equation for moderate pressures:

For higher pressure, Chu & Cameron’s equation:

The pressure‐viscosity coefficient , α:The Wooster’s equation:The Wooster s equation:

Alternatively So & Klaus’s empirical express ion can be used:Alternatively, So & Klaus’s empirical express ion can be used:

The ASTM slope, b

Effect of shear rate on αEffect of shear rate on α

• No concrete work available to account for the shear rate effect on pressure‐viscosity coefficient

• For paraffins, α is between 1.5 to 2.4×10‐8 [m2/N]• For aromatic oils, α is between 2.5 to 3.5×10‐8 [m2/N]• However,  value of α for aromatic oils/naphthanic oils is poorly affected at high temperatureis poorly affected at high temperature

• Paraffins are fairly unaffected by temperature hence are more suitable choice

• Water can function as lubricant in some situations but only marginal increase occurs in viscosity with pressure

Alternative computational formula for l hviscosity‐pressure relationship

• One more expression is provided by RoelandsOne more expression is provided by Roelands

DynamicDynamic viscosity and αvalues for some common lubricants

Viscosity‐Shear relationshipViscosity Shear relationship• For Newtonian fluids, viscosity and shear rate are directly proportionalare directly proportional

This linearity is vaid upto a shear rate of 106 1/sec, beyond that fluids become non‐Newtonian.Water, benzene and light oils are Newtonian for even higher shear rates., g gWater‐oil emulsions, polymer thickened oils and greases become non‐Newtonian at high shear rates.

Two most important non‐Newtonian behaviours:(1) Pseudo plastic(2) Thixotropic

Pseudo plastic BehaviourPseudo plastic Behaviour

It is also called shear thinning.

Thixotropic BehaviourThixotropic Behaviour• It is also known as shear duration thinning, which is the loss

of consistency of the fluid as the duration of shearincreases.

• The reverse of thixotropy is called as inverse of thixotropyor rheopectic behaviour, where thickening occurs. Synovialfluids are examples.

VISCOSITY MEASUREMENTSVISCOSITY MEASUREMENTS

• Capillary viscometersCapillary viscometers– Suitable for Newtonian fluids

• Rotational viscometers• Rotational viscometers– Suitable for non‐Newtonian fluids

Capillary Viscometers( )(ASTM D445, ASTM D2161)

• The time for a specified volume of fluid toThe time for a specified volume of fluid to flow through the capillary tube is recorded

• This time gives the kinematic viscosity• This time gives the kinematic viscosity

• The flow must be laminar because Poiseuille’sl f d i fl i i i dlaw for steady viscous flow in a pipe is used

• Saybolt viscometer

• Redwood viscometer

• Engler degrees

Rotational Viscometers (ASTM D2983)Rotational Viscometers (ASTM D2983)

• Principle: The fluid viscosity is related to thePrinciple: The fluid viscosity is related to the force required to generate shear between two surfaces sepearated by a film of fluidsurfaces sepearated by  a film of fluid

• Speed change for a constant torque is measuredmeasured

• Torque change for a constant speed is dmeasured

• “Dynamic viscosity” is measured

Rotating Cylinder ViscometerRotating Cylinder Viscometer

Cone on plate Viscometer

Falling ball viscometerviscometer

Viscosity of mixtures:using ASTM viscosity paper

Oil Viscosity ClassificationsOil Viscosity Classifications

• Most commonly used:Most commonly used:– SAE viscosity classification

ISO classification– ISO classification

– Military classification

SAE viscosity classification

ISO Classification of industrial oilsISO Classification of industrial oils

Density, Specific Gravity and API unitDensity, Specific Gravity and API unit

• Density is mass per unit volumey p• Specific gravity is the ratio of mass of a given volume of oil at temperature  t1 to the mass of an equal volume of pure water at tequal volume of pure water at t1

• American Petroleum Institute (API) unit = (141.5 / s) – 131.5s)  131.5

s = specific gravity 15.6 oC.Density of mineral oil is about 850 kg/m3, density of y g/ , ywater is 1000 kg/m3, hence the specific gravity of mineral oil is t ypically 0.85.

THERMAL PROPERTIES OF LUBRICANTSTHERMAL PROPERTIES OF LUBRICANTS

• The most important thermal properties are:The most important thermal properties are:

• Specific heatV i li l ith t t– Varies linearly with temperature

– Specific heat of mineral oil is roughly half that of waterwater

• Thermal conductivity

• Thermal diffusivity 

Specific Heat of Mineral Oils and h lSynthetic Oils

• Ranges from 1800 J/kgK at 0 oC to 3300 J/kgKRanges from 1800 J/kgK at 0  C to 3300 J/kgKat 400 oC.

• For rought estimate the following equationFor rought estimate the following equation can be used:

Density of typical mineral oil is about 850 kg/m3, density of water is 1000 kg/m3,hence the specific gravity of mineral oils is typically 0.85.

Thermal ConductivityThermal Conductivity

• For mineral oils and synthetic oils, thermalFor mineral oils and synthetic oils, thermal conductivity varies in the range of 0.14 W/mKat 0oC to 0.11 W/mK

• Rough estimate can be obtained from:

Thermal DiffusivityThermal Diffusivity• Property that describes the heat propagation into the solids and is defined as:into the solids and is defined as:

Temperature Characteristics of bLubricants

• Pour pointPour point

• Flash point

l ili• Volatility

• Oxidation

• Thermal stability

Pour Point (ASTM D97, D2500)Pour Point (ASTM D97, D2500)

• It is the lowest temperature at which oil willIt is the lowest temperature at which oil will flow when cooled, plus 3 oC

• Particularly important for western countries• Particularly important for western countries, russia and northern china (cold countries)

Vi i f il i i hi h• Viscosity of oil at pour point is very high, serveral hundred Pas

Cloud Point and Flock Point• Again important in only cold countries• Cloud point is the temperature at whichCloud point is the temperature at which paraffin wax in the oil begins to precipitate

• Wax Pour Point: If cloud point is more thanWax Pour Point:  If cloud point is more than pour point

• Viscosity Pour Point: If pour point is reachedViscosity Pour Point: If pour point is reached without cloud point

• Flock Point: only for refrigerants; it is theFlock Point: only for refrigerants; it is the temperature at which oil separates from refrigerant

Flash Point and Fire Point (ASTM D92, D93, D56, D1310)

• Flash Point: Temperature atwhich oil’s vapour ignites

• Fire Point: Temperature atwhich the oil’s vapoursustains burning aftersustains burning afterigniting

• They are important fromy psafety point of view

• For most lubricating oils,the flash point is around210 oC and fire point isaround 230 oCaround 230 C

Volatility (ASTM D2715)• It is the loss of lubricant due to evaporation

• In the testing equipment , the oil is aerated at air flow rate of 2 litres/min for 22 hours

• The difference of weight gives volatility

Oxidation Stability (ASTM D943, )D2272, D2893, D1313, D2446)

• Resistance of oil to molecular breakdownResistance of oil to molecular breakdown

• Refinement of oil by removing sulphur, oxygen etc helps improve oxidation stabilityetc. helps improve oxidation stability

Thermal stability• Resistance of lubricant to molecularbreakdown at elevated temperaturep

• When heated, mineral oils breakdown tomethane, ethane and ethylene.methane, ethane and ethylene.

• Thermal stability can be improved by refiningthe oil NOT by additivesthe oil, NOT by additives

OTHER CHARACTERISTICS OF LUBRICANTS

• Surface tensionSurface tension

• Neutralization number

C b id• Carbon residue

Surface tension (ASTM D971, D2285)• It is resistance to wetting and spreading of lubricant oils; 

• Lower the surface tension better it is• Additives can reduce the surface tension; lessAdditives can reduce the surface tension; less than 0.1% of silicone in mineral oil reduces the surface tension of oil to that of silicone

Measurement of Surface tension of oils

Du Noy Ring Method

Neutralization Number (ASTM D974, D664)It i th tit f t i h d id (KOH) i• It is the quantity of potassium hydroxide (KOH) in milligrams per gram oil necessary to neutralize acidic or alkaline compounds present in the lubricantp p

• For acidic oils, it is indicated by Total Acid Number (TAN)

• For alkaline oils, it is indicated by Total Base Number (TBN).

• These paramters of the oil are monitored in condition• These paramters of the oil are monitored in condition monitoring of machines like steam turbine generators, compressors, gears

• A limiting neutralization number serves as a criterion for indicating when oil should be changed

Carbon Residue (ASTM D189, D524)Carbon Residue (ASTM D189, D524)

• It is an indication of degradation of oil atIt is an indication of degradation of oil atelevated temperature, 300 oC or more

• It is determined by weighing the residue after• It is determined by weighing the residue after the oil has been heated to a high temperature in the absence of airin the absence of air

OPTICAL PROPERTIES OF LUBRICANTS: R f i i d (ASTM D1218 D1747)Refractive index (ASTM D1218, D1747)

• It is the ratio of velocity of a specified wavelength  of light in air to that in the oil being testedlight in air to that in the oil being tested

• Abbe refractometer is used to measure it• For most mineral oils it is 1.51• Refractive index is needed to experimentally determine 

film thickness using optical interferometry• It can also be estimated from:It can also be estimated from: 

Nature of AdditivesNature of Additives

• Additive compatibilityAdditive compatibility

ddi i S l bili• Additive Solubility

Lubricant Impurities and ContaminantsLubricant Impurities and Contaminants• Water content (ASTM D95, D1744, D1533, D96)D96)– Important from corrosion point of view

• Sulphur content (ASTM D1266 D129 D1662)• Sulphur content (ASTM D1266, D129, D1662)– Excess can be corrosive

A h t t (ASTM D482 D874)• Ash content (ASTM D482, D874)– Can result in secondary wear

Chl i t t (ASTM D808 D1317)• Chlorine content (ASTM D808, D1317)– Excess can be corrosive

Dissolved gases in OilsDi l d ff t f f• Dissolved gases can affect performance oflubricants

• Ostwald coefficient for mineral oils:

General Formula for Ostwald ff f lCoefficient for any Oil

Example:Estimate the q antit of o gen that co ld beEstimate the quantity of oxygen that could be dissolved in 1 litre of Methyl Phenyl Silicone at 120 oC.

Solution: From table, ∂1 = 18.41 and ∂2 = 7.75.T = 120 + 273 = 393 oC.lnCo = [0.0395(18.41 – 7.75)2 – 2.66]×(1 – 273/393) – (0.303)(18.41) – 0.0241 (17.6 – 7.75)2 + 5.731 = ‐1 6271.627

Co = 0.1965In every litre of Methyl Phenyl Silicone oil,In every litre of Methyl Phenyl Silicone oil,approximately 196.5 ml of oxygen can bedissolved at 120 oC.

Different OilsDifferent Oils

• Straight mineral oilsStraight mineral oils– From natural fossil fuels

• Synthetic lubricants• Synthetic lubricants– Invented to serve better than straight mineral oils

• Emulsions and aqueous lubricants– Useful for combined cooling and lubricating

• Greases– Mixtures of lubricating oils and thickenersg

Grease RheologyH h l B lkl ti• Hershel‐Bulkley equation:

The value of n is close to 1 When the n is exactly unityThe value of n is close to 1. When the n is exactly unity, the behaviour or grease is that of a bingham fluid.