friction measurement
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Introduction:
THE MEASUREMENT OF FRICTION FORCE and the
calculation of the coecient of friction are often
warranted for man tri!osstems" Such testin# is
$articularl critical for !ra%es& clutches& and similar
$ower' and motion'control mechanisms where the
friction force must !e %nown and controlled"
In machine desi#n& friction characteristics must !e
%nown for friction dri(es& for si)in# motors& for
determinin# wor% forces& and for desi#nin# an $art of amachine where chan#es in forces !etween slidin#
mem!ers could a*ect o$eration" Friction is not a
measure of wear or the tendenc to wear+ therefore&
it does not determine ser(ice life" Frictional heatin#
sometimes causes a machine $art to fail& !ut this
usuall occurs when circumstances create a!normal
friction e*ects" This re$ort will descri!e the numerous
techni,ues used to measure friction"
1. Tribometers:
Tri!ometers& or de(ices to measure friction and
wear& are the !asic technolo# used in most of the
tri!olo#ical in(esti#ations" In the followin# I will discuss
the t$es of tri!ometers"
1.1. Tribometers for dry or partially lubricatedsliding contacts.
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Fig (1) Basic sample configurations used in simulations of dry or partially lubricated
sliding contacts1.1.1. Four-Ball Tester:
In this a$$aratus four ./"0 1mm2 diameter !alls areused& one of which is held in the chuc% rotatin# at.003 4 53 1r$m2 while three others are held ri#idl ina $ot " 6urin# the tests the $ot is 7lled with thelu!ricant and the 78ed !alls are then $resseda#ainst the rotatin# !all o(er a ten second inter(al"
Testin# in(ol(es an a$$lication of a series of tenseconds runs at $re'selected and successi(el hi#herloads until weldin# of the four !alls occurs" Themeasurements of the wear scars are made on eachof the three lower !alls" From the measurements theload wear scar cur(e is o!tained and the weld $oint"
1.1.2. Falex Tester:
The Fale8 tester& since it allows lar#e contact stressesto !e de(elo$ed without the need for a stron#mechanical su$$ort& is widel used in industrialresearch la!oratories and elsewhere" In this a$$aratus9 contact forces !alance each other so that theonl force re,uired is a :'!loc% closure force andtor,ue to dri(e the $in" The tor,ue $ro(ides a directmeasure of friction force without an error of friction#enerated in !earin#s" Hi#h contact $ressures
achie(ed durin# the tests render this a$$aratussuita!le for sei)ure and scun# studies" This instrument
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$ro(ides the data on the (alue of coecients of friction fora $articular material;lu!ricant com!ination& ma8imumload !efore sei)ure and wear scar diameter after testin#"
1.1.3. Timken pparatus:
In this a$$aratus a rin# rotatin# at ?331r$m2 is $ressed a#ainst a stationar sla! in the $resence of the lu!ricant tested "The testis conducted for ten minutes at successi(el hi#herloads until =O@= and failure loads are determined" The=O@= load is the ma8imum load which can !e a$$liedon the le(er arm without $roducin# scorin# mar%s on
the sla!" The load on the le(er arm is used as theindication of the stren#th of the lu!ricatin# 7lm"The four'!all& Fale8 and Tim%en a$$aratus are usedfor standard lu!ricant tests that are s$eci7ed !either scienti7c institutions or ! industrialor#ani)ations" These tri!orneters& while fre,uentlused for commercial testin# and some researchwor%& ha(e the disad(anta#es of (arin# contact stressdurin# tests and $oorl controlled frictional
tem$erature" The si)e of the contact areas !etween!alls in a four'!all test can (ar from the small area ofHert)ian contact to a much #reater si)e de$endin# onthe scale of the wear scars formed on the !alls" TheFale8 machine is also su!ect to chan#es in contactforce if se(ere wear occurs durin# a test"
1.1.!. "ill-on-#isc pparatus
This a$$aratus is $erha$s the most widel used" A$in is $ressed a#ainst a rotatin# disc either onits Bat surface"
1.1.$. "in-on-%lab:
It is a slidin# reci$rocatin# of studin# means a con(enient$ro(ides this a$$aratus o$$osed to unidirectional slidin#which Is studied on the $in'an'disc a$$aratus" Theowden'De!en machine is $ro!a!l the most famousa$$lication of the $in' on'sla! $rinci$le and has !een
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used in a lar#e num!er of research $roects"
1.2. Tribometers for &ombined 'olling and %liding:
Man mechanical com$onents contain dnamic contactswhich function under a com!ination of rollin# and slidin#and e8$eriments conducted under conditions of either$ure slidin# or $ure Tonin# do not alwas accuratelsimulate the friction and wear $henomena occurrin#"
The most commonl used desi#n of the tri!ormeter tosimulate the com!ined rollin# and slidin# is the two'disca$$aratus"In this a$$aratus discs in contact are dri(en at
di*erent an#ular (elocities which su$erim$osessome slidin# on Tollin# at the dnamic contact " Theamount of slidin# is controlled ! mani$ulatin# the(elocit of one"One of the discs is mounted in a ri#id structurewhile the other disc is free to $i(ot and it can!e loaded a#ainst the 78ed disc" The slidin#com$onent of (elocit can !e o!tained ! usin##ears to im$ose a ratio of an#ular (elocities on the
discs or ! (arin# the ratio of disc diameters
Fig (2) Schematic diagram of the two-disc apparatus
1.3. Tribometers for non-ambient conditions:
1.3.1. (ig) Temperature Tribometers
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Ceramics are (er attracti(e materials from the
en#ineerin# (iew $oint due to their hi#h stren#th&
heat and wear resistance& chemical inertness& low
thermal conducti(it and maintenance of #ood
mechanical $ro$erties at ele(ated tem$eratures" Thus
there is an increasin# interest in the use of
ad(anced ceramics in internal com!ustion en#ines and
#as tur!ines where tem$eratures u$ to .333C and
.933C are e8$ected res$ecti(el " As con(entional
li,uid lu!ricants !rea% down at much lower
tem$eratures& ceramic lu!rication !ased on lu!ricious
solid coatin#s is usuall in(esti#ated at tem$eratures
a!o(e ?33C" To attain the tem$eratures ran#e where
ad(anced ceramics are intended to re$lace metals in
man en#ineerin# a$$lications and conduct the
necessar tri!olo#ical tests a s$eciall desi#ned and
!uilt tri!ormeter is re,uired " Some of the
tri!ometers ori#inall desi#ned for am!ient
tem$erature a$$lications& such as $in'on'disc
machine or $in'on'sla! a$$aratus& can !e
ada$ted to o$erate at moderate tem$erature !
7ttin# the heatin# elements and insulatin# the test
cham!er" These modi7cations can !e introduced
without much e*ort& are relati(el ine8$ensi(e and
allow testin# at tem$eratures u$ to G33C to !e
conducted" For hi#her tem$erature ran#es $ur$ose
!uilt tri!ometers !ecome necessar" These tri!ometers
can !e used for wear and friction measurements at
tem$eratures as hi#h as ./33'.S33C"
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Fig (3) Schematic diagram of the high temperature tribormeter
1.3.2. Tribometers for *peration in +acuum:
The latter studies are of #reat im$ortance to s$ace
(ehicle technolo#" A tri!ormeter for tests under (acuum
usuall consists of a (acuum cham!er which contains the
dnamic contact and surroundin# ancillar e,ui$ment
such as (acuum $um$s and a dri(e sstem for the
dnamic contact" The shaft dri(in# the dnamic contact$asses throu#h the wall of the (acuum cham!er (ia a
seal to the electric motor" Friction and wear
transducers can !e 7tted either inside the (acuum
sstem as the lac% of ail= does not usuall interfere
with the transducers or outside the (acuum sstem" A
schematic dia#ram of tri!ormeter for (acuum o$eration
is shown in Fi#ure "?"
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Fig (4) Schematic diagram of tribormeter for operation in acuum
1.!. Tribometers for %tudies of (ydrodynamic
,ubrication
1.!.! Friction easurements in )ydrodynamic
Bearings
In most cases the !earin#s used in these t$es of
e8$eriments were the same as used in realen#ineerin# e,ui$ment" The a$$aratus usuall
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consisted of a ournal !earin# with a rotatin# shaft and
!ush loaded ! a han#in# wei#ht" Friction forces were
measured from a side arm connected to the !earin#
that $ulled on a s$rin# or !alancin# wei#ht" Friction force
is measured ! a force transducer" The electrical
contact resistance !etween shaft and ournal can also
!e monitored in order to determine when
hdrodnamic 7lm failure occurs"
Fig (!) "est for studying pressure and friction coefficient in #ournal bearing
2. easurement of friction coecient:
2.1. Tec)ni/ues of Friction Force easurement:
There are two !asic t$es of de(ice commonl
used for the measurement of friction force and a
su!se,uent determination of a friction coecient"
These de(ices are the $ie)oelectric force #au#e and the
strain #au#e transducer"
2.1.1. "ie0oelectric Force auges:
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-ie)oelectric force #au#es $ro(ide a direct record of
frictional force as an electrical im$ulse which can !e
recorded electronicall" Earlier (ersions of
$ie)oelectric force transducers ori#inall contained a
,uart) crstal which has the $ro$ert of emittin#
electric char#e when it is com$ressed or stretched"
Fig ($) %iagram of mounting of a pie&oelectric force transducer in a tribormeter
2.1.2. %train auged Beams:
Strain #au#ed !eams are considera!l chea$er and can
!e desi#ned to suit almost an le(el of friction force"
Friction force is usuall measured from the !endin# of
a !eam arran#ed $er$endicularl to the direction of
the friction force" Measurement of friction force usin# the
$rinci$le of a Be8i!le !eam is schematicall illustrated in
Fi#ure 9
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Fig (') ombination of strain gauges and a fleible beam*
2.1.3. iscellaneous met)od:
There are (arious method used to determine friction force
without transducers li%e in the followin# 7#ure
Fig (+) ,ethods used to determine friction coefficient without using transducers
3. Factors aecting test:
If the sstem under stud will wear& and it is the
desired #oal of the stud to determine the frictioncharacteristics under conditions of stead'state wear&
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friction should !e measured in a wear test" If the
sstem of concern will not chan#e with time& a
friction test that does not in(ol(e wear can !e
conducted"
Fig () .ffect of system wear on friction force* (a) System that does not
eperience any wear or changes in behaior when wear occurs* (b)
System where friction force increases with time until reaching a steady-
state condition* (c) System where friction force aries with each eent in the
wear process
Full'7lm se$aration will not occur unless the 7lmthic%ness is #reater than the surface rou#hness"
E(en with hdrodnamic lu!rication& there are
some in(esti#ators who claim that the sstem
friction is a*ected ! the surface rou#hness and
la see followin# 7#ure"
Fig (1/) .ffect of surface roughness on the friction coefficient of a
lubricated sliding system*
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!. &onclusion:
In man life as$ects friction is an im$ortant rule !ut
in other it must !e reduced in order not to a*ect
sstem function"
Tri!ometers are im$ortant for measurin# tri!olo#ical
characteristics of an tri!olo# sstem" Measurements de$end on o$eratin# en(ironment li%e
tem$erature& humidit& (acuum and surface
condition"
For an tri!olo#ical sstem
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