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TribologyinMechanicalEngineering
MAE493N/593T
Dr.Konstantinos
A.
Sierros
WestVirginiaUniversity
Mechanical&AerospaceEngineering
ESBAnnex
263
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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.html8/8/2019 Mae 493n 593t Lec6
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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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Physicalbasisoflawsoffriction
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
Physicalbasisoflawsoffriction
Friction=Adhesion
+Deformation
(To
be
continued)
Junctiongrowth(Tobecontinued)
Staticandkineticfriction
Stickslipphenomenon
Measurementof
friction
Recommended