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TribologyinMechanicalEngineering

MAE493N/593T

Dr.Konstantinos

A.

Sierros

WestVirginiaUniversity

Mechanical&AerospaceEngineering

ESBAnnex

263

[email protected]

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Projectlist

Eachteammustchooseoneproject

Deadlinefor

choosing

project

and

joining/forming

ateam

IsNEXTWEEK(weekstartingSep13th

)

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Frictionofsolids

Frictionistheresistanceencounteredbyonebodyinmovingoveranother

Theworkexpendedagainstfrictionisredundant(i.e.makesnouseful

contributiontotheoveralloperationofthedevice)

Workbecauseoffrictionisdissipatedaswasteheat

Inmostcasesweneedtoreducefriction

Exceptions Automobiletiresonroadways

Vehiclebrakes

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Examples frictionalinteraction

A

nail

hammered

into

a

piece

of

wood

is

held

in

place

by

frictional

effects

alongitslength

Gripbetweenanutandaboltdependsonadequatefrictionbetweenthem

Wovenfabricsdependonfrictionbetweenadjacentthreads

http://www.snakerobots.com/simulation.html
http://www.snakerobots.com/simulation.htmlhttp://www.snakerobots.com/simulation.html

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Qualitativelawsoffriction

A

light

object

can

be

slid

easier

across

a

horizontal

surface

than

a

much

heavier

one

Thematerialofwhichtheobjectismade,isalsoimportant

(eg.)Ablockoficeslidesoverthefloormuchmoreeasierthan

ablockofwood

ofthe

same

weight

Itiseasiertomoveaheavyweightsupportedonrollers/wheels

Forcesofrollingfrictionareingeneralmuchlessthanthoseassociatedwith

slidingfriction

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Facts

15

20%

of

the

power

produced

by

a

modern

internal

combustion

engine is

neededinordertoovercomefrictionbetweenengineparts

Aeroenginesexhibitmuchsmallerlossesduetofrictionbutstillasignificant

amountoffueliswastedbecauseoffriction

Wearis

ausual

companion

of

friction

Higherfrictionmeanshighersurfacetemperature(s)

whichleadtohigher

surfacedamageandwear

http://www.fueleconomy.gov/feg/atv.shtml

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Amontons lawsoffriction Two

empirical

laws

of

friction Theselawsarenotglobalandoftentheirprecisionisnothigh

andthereforeinsomecasestheydonotapply

GuillaumeAmontons

(August31,1663 October11,1705)

LeonardoDaVinci

April15,1452 May2,1519

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Amontons lawsoffriction1st

Law

FrictionforceF

betweenapairofloadedslidingsurfacesisproportionaltothe

normalloadWthattheycarry

Constantof

proportionality

between

F

and

W is

known

as

the

coefficient

of

friction

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Amontons lawsoffriction2nd

Law

ThefrictionforceF

betweenthetwosolidbodiesis

independent

oftheapparent

areaofcontact

betweenthem(Thislawwastestedexperimentallytwice!)

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Physicalbasisoflawsoffriction

Whymovingamassoveranapparentlyflathorizontalsurface requiresa

significant

expenditure

of

energy?

Amontonsrecognizedthatsurfacesconsistofasperitiesandpostulatedthat

frictionaleffectsarelinkedwiththeworkneededtobedonefor;

Lifting

the

load

over

the

surface

asperitiesor

Bendingthesurfaceasperities

or

Breakingthe

asperities

JohnDesagulierssuggestedthatadhesionwithinthegeneralcontactzone

plays

arole

Coulombalsoconsideredthepossibilityofadhesion

Euleradoptedthemodelofinterlockingasperities

Heassumed

the

asperities

to

be

of

triangular

shape andofslope.Then,coefficientoffrictionis;

=tan

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ReexaminingAmontons

Laws

Totalresistance

to

tangential

motion

= resistancesofalltruecontactareas

= AFTotalforce Truearea

ofcontact

Resistance

toshearing

InordertoachieveAmontons

Lawswealso

need;

WA

Thevalidity

of

the

2nd

Law

depends

on

the

statistical

distribution

of

the

surface

asperities

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Coefficientoffriction

= AF

WA

Ifboththeseexpressionsholdthen;

F/W=

=

constant

Numericalvaluesofcoefficientoffriction

dependson;

Pairofmaterialsincontact

Physicalconditionsofsliding(surroundingsetc)

(eg.1)Coefficientoffrictionbetweentireanddry

roadisaround1

Coefficientoffrictionbetweentireandwet

roadisaround0.5

(eg.2)Coefficientoffrictionbetweenbrassblockandsteelplateinatmosphere

isaround0.5

Coefficientoffrictionbetweenbrassblockandsteel

plateinvacuumis

around10

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Mechanismsoffriction

Mechanisms

of

friction

Adhesion Deformation

Outermostatomsofsurface

asperitiesareinveryclose

proximity

Sufficientforcemustbeapplied

topullorslidethemapart

Surfacesmustbeclean

Particularlytrue

for

ductile

metals

(eg.)Twogoldsurfacesincontact

willformonepiece(coldwelded)

Whentwosurfacesslideagainst

eachother,surfacedamageis

observedinatleastoneofthem

Ploughingdeformation

Thisisaddedtotheadhesional

effects

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Adhesionaltheoryoffriction

HAW Indentationhardnessofsoftersurface

TrueareaofcontactLoad

Asaresult

of

this,

severe

localized

plasticdeformation,strongadhesive

junctionsareformed

hAF =Shearstrengthof

softermaterial

H

h

W

F==

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H

h

W

F==

Adhesionaltheoryoffriction

Coefficientoffrictionisassociatedwithratiooftwostrengthpropertiesofweaker

solid

is

aconstant,

independent

of

load

(Amontons)

FisindependentofA(Amontons)

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Coefficientoffriction

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Theoryofjunctiongrowth

Previoustheorysuggeststhat

dependsonlyontwostrengthpropertiesofthe

weakermaterial

Inpracticeweobservethattheothersurfacealsoinfluencesthevalueof

Thevalueof

dependsontheslidingenvironment(vacuum,atmosphereetc)and

canincreasebyanorderofmagnitude

Therefore,the

true

area

of

contact

must

also

increase

Weneedamechanismbywhichtheislandsoftruecontactcanenlarge

Sothattheshearingstrengthwillincrease

Mechanismofjunctiongrowth

Willdiscuss

this

in

more

detail

later!

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Staticandkineticfriction:stickslip

Thefrictionalforcerequiredtoinitiate

dryslidingmotionismore

thanthatneeded

tomaintain

the

surfaces

in

the

subsequent

relative

sliding

Whichmeansthat;static

>kinetic

(3rd

AmontonsLaw)

Leadstostickslipmotion

Modeling

the

slidingsystem

Elasticityofany

mechanical

system

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Stickslipphenomenon

Naturalelasticityofloadingmechanism

Inherentdamping

Whenlowerspecimenmovesfromlefttoright,theblockwillbe carriedwithitandthespringwillbestretched

Thiscontinuesuntilthespringforce(kx)equalstatic

.Theblockwillthenstartto

slip

The

system

will

then

start

to

execute

damped

simple

harmonic

motion

until

the

velocityoftheblockwillmatchthatofthelowersurface

Then,thesurfaceswillagainstick

andsoon

lower specimen

S i k li h

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Stickslipphenomenon

M f f i i

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Measurementoffriction

Simplestmethod:Inclinedplanetest

fs tan=

M t f f i ti

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Measurementoffriction

Pi di f i ti l h ti

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Pinondisc frictionalheating

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Summary

Introductiontofriction

Lawsoffriction


Friction=Adhesion

+Deformation

(To

be

continued)

Junctiongrowth(Tobecontinued)

Staticandkineticfriction

Stickslipphenomenon

Measurementof

friction

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