MECH2400 5400 Belt Drives Flat Belt Drives 2014

8/10/2019 MECH2400 5400 Belt Drives Flat Belt Drives 2014

1/16


2/16


3/16

FlatBelting

isstill

the

logical

selection

to

agreat

many

basic

driving

problems.Itstillremainsthemajormeansoftransmittingpowerintraditionalolderstyle

ofmachinery.

ThetwokeyadvantagesofusingFlatBeltingare:

1. Maybeusedonverylongcentre distancearrangements.2. Relativelyinexpensive.

FlatBeltDrives

SometypicalarrangementsinFlatBeltdrivesinclude:


4/16

GeneralTheoryonDesignofBeltDrives

Difference

in

Belt

Tensions

(Effective

Pull)

NotethatthebelttensionTlispullingthepulleyinananticlockwisedirectionwhilethebelttensionT2ispullingthepulleyintheclockwisedirection,butasTl islargerthanT2thepulleywillrotateinan

anti

clockwise

direction.

1

2

=(T1T2)

Indesigningabelttheaimisto

maketheturningeffectandhencethedifference(T1T2)aslargeaspossible.

Thedifferenceinbelttension(T1T2)isknownastheeffectivepullTEofthebelt.


5/16


Sum

of

Tensions

Thepulleyisheldinpositionbyashaftmountedinbearings.

TheloadontheshaftwillbethesumofthetensionsT1+T2whentheslacksideandtightsideofthebeltareparallel(evenwhenthesidesarenotparallel;thisisroughlytrue).

HenceitisdesirabletokeepthesumofthetensionsTl +T2assmallaspossible.


6/16


Tension

RatioOfcourseT1+T2isalwayslargerthanTlT2,butthesetworequirements(T1T2largeandTl+T2small)arefulfilledwhenthetensionratioT1/T2ismadeaslargeas

possible.Considertwoexamples,bothhavingthesameeffectivepull:

Example1:

T1/T2=2

LetT1=6 T2=3

T1 T2=3T1+T2=9

Example2:

T1/T2=4

LetT1=4 T2=1

T1 T2=3T1+T2=5

Itcanbeseenthatthesetwodriveswouldtransmitthesamepoweratthesamespeed(sinceT

lT

2isthesameinboth

cases).But,inExample1,theloadontheshaftsandbearingsisnearlytwiceasgreatasin2,alsoin1themaximumtensioninthebeltT1is50%greater. Hence,whenthe

tensionratio

islower

(Ex.

1)

the

shafts,

bearings,belts,andperhapspulleyswouldallneedtobestrongerandmoreexpensive.

NOTE:

Shouldalways

be

kept

large.


7/16


General

FormulaThetensionratiowhenthebeltisjustonthepointofslippingis,

Where e=2.718(baseofnaturallogs)=Coefficientoffrictionbetweenbeltandpulley

=Arcofcontact(rads)=2arcos

Forthetensionratio

tobelarge,mustbelarge;

Thismeansthatbothandshouldbeaslargeapossible.

iscontrolledbythematerialsofthebeltandpulley,itcanbeslightlyincreasedforleatherbeltsbyusingabeltdressing.

isalways

smaller

on

the

small

pulley

so

the

belt

will

slip

there

first.


8/16

Thearcofcontact()canbeimprovedby:1. Havingasmallratiobetweenthetwopulleys2. Havingalongcentredistancebetweenpulleys3. Usingajockeypulleyarrangement

4. Usingacrossedbeltarrangement


Arrangements

Arcof

Contact

( )


9/16


Arrangements

PowerThepowertransmittedbyabeltisproportionaltotheproductoftorqueandspeed(R.P.M.).i.e..

P=

Henceatlowspeeds,thefasterthebeltruns,

themorepoweritwilltransmit.

Athigherspeeds,theeffectofthecentrifugaltensionbecomessolargethatthepowertransmittedbeginstodecreasesharplyasthe

beltspeedincreases.


10/16

Wherethespeedofbeltingisabove900m/min allowanceshouldbemadefortheeffectofcentripetalforce.Thecentrifugalforcepermetre lengthofarcwillbegivenby;

f=

TheseradialforceswillhavearesultantRdirectedtowardstheleftandwillbebalancedbytensionsTcinthebelt,whichareinadditiontothoserequiredfordrivingpurposes.

Thecaseisanalogoustoaboilershellsubjectedtointernalpressure.

R=f2r=

2r=2mv2

Also2Tc=R,

Tc=mv2

GeneralTheoryonDesignofBeltDrivesArrangements DeterminationofTc


11/16


Power

Formula for

a

Flat

Belt

1

=0

i.e.

1 3=0

i.e.mv2 =

1

orTc=

1

P= 1 1

v

Thepowerformulaforaflatbeltdrivemaybewritten:

Thispowerisamaximumwhen: ServiceFactors(S.F.):

DesignPower =motorpowerxS.F.

Prime

Mover Uniform Moderate HeavyShock Shock

ElectricalMotor 1.20 1.40 1.60MultiCylinderICengine 1.25 1.50 2.00SingleCylinderICengine 1.50 1.75 2.25

Careshouldbetakentokeepmaximumbelttensionduringtheaccelerationperiodwithinacceptablelimits.

Theapplicationofthecorrectservicefactoratthedesignstagehelpsinavoidinganyproblem.

Ifthestartingtorqueissevere,thebeltmanufacturercanprovidemaximumpermissibleaccelerationtensionsformoreprecisecalculations.


12/16


Good

Practice1. Theslacksideofthebeltshouldbekepttothetopsothattheforceofgravity

increasesthewrapofthebeltaroundthepulleys.

2. Becauseofbeltslipandcreepthedrivenpulleywillrotateataslightlyslowerspeedthanthepulleyratioswouldindicate.

3. Thevelocityratiodoesnotremainquiteconstant.

4. Allowanceforbeltslipandcreepcanbemadebysmallchangeinoneofthepulleydiameters.


13/16


Length

of

Belt

LengthofBelt(exact)=2Ccos+

1 2 +

1 2

LengthofBelt(approx)=

1 2 +2C+


14/16

GeneralTheoryonDesignofBeltDrivesLimitsonSizes


15/16

Generallywithtransmissionbeltingalonglengthofunsupportedbeltapproachesthepulley.Undertheseconditionscrowninghelpsthebelttotrackcorrectly.

Thiseffectisduetothetendencyofthetwosidesofthebeltsection,viewedseparately,torideintowardsthehighpointonthepulley,whichisthecrownof

thecamber.

Wherepulleycamberistoogreat,thebeltwillintimeacquireaglazedsurfaceinthecentre,asaresultofcarryingthewholeloadandslipping,becausetheedgesofthebeltarenotincontactwiththepulley.

BeltDrives CrowningofPulleys

Note:Excessivecrowningisdetrimentaltothebeltandresultsinuneventensiondistribution.Withsometypesofbeltingmaterials,suchasnylon,flatfacedpulleys(orpulleyswithaverysmallamountofcrowning)arepreferred.Alwayscheckwiththe

beltmanufacturer.


16/16

BeltDrives PossibleModesofMisalignment

Documents

MECH2400 5400 Belt Drives Flat Belt Drives 2014