Friction Measurement

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Friction measurement

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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Friction Measurement