iii

Air Brake Endorsement · permits the holder to drive vehicles

equipped with air brakes in class of vehicle for which the driver is licenced.

· To adjust manual slack adjusters, the operator must hold an “E” brake endorsement.

Requirements for Air Brake Endorsement

· Must complete an Air Brake Written Test.

· Must complete an Air Brake Practical Test.

Foreward

The Air Brake Manual has been prepared by the Department of Public Safety (Motor Vehicle Branch) to assist drivers in understanding the basic operation and function of an air brake system. The study of this manual, together with practical instruction, is recommended for a driver who is preparing for the air brake examination. A large illustration of a complete dual air brake system is located on the inside cover and can be folded out and referred to when studying this manual. Study questions are included at the end of each section so that readers may self-test their understanding of the subject matter. Drivers who have qualified and are authorized to operate air brake equipped vehicles are encouraged to review this manual on a periodic basis to ensure they are fully aware of the proper method of inspecting an air brake system and identifying problems that can occur when the system malfunctions.

The illustrations and explanations of various types of brake system designs are provided for instructional purposes only. Most air gauges measure in imperial units. Therefore the measurements used and relating to the air brake system will be in imperial units. This manual has no legislative sanction. For interpreting and applying the law, consult the Motor Vehicle Act and its regulations.

We gratefully acknowledge the contributions of all jurisdictions, particularly Manitoba.

ISBN 1-55396-034-3 10230

www.gnb.ca/publicsafety

Air Brake Manual

Ce document est disponible en français.

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34

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30

6 6

Legend

blue-supply/wet green-primary/dry red-secondary/dry yellow-springparking brakesystem darkgreen-trailersystem In-cabportionishighlighted

1. Compressor 2. Governor 3. Airdryer 4. Safetyvalve 5. Supply/wetreservoir 6. Drainvalves 7. One-waycheckvalves 8. Primary/dryreservoir 9. Lowpressureindicator10. Secondary/dryreservoir11. Rearservicebrakechambers12. Springparkingbrakechambers13. Tractorrelayvalves14. Trailerservicebrakechamber15. Trailerspringparkingbrakechamber16. Trailerreservoirs17. Trailerrelayvalve18. Trailerspringparkingbrakevalve19. Anti-compoundlines20. Gladhands

21. Supply(emergency)line22. Control(service)line23. Springbrakemodulatorvalve24. Tractorprotectionvalve25. Stoplampswitch26. Two-waycheckvalves27. Springparkingbrakecontrolvalve28. Trailersupplyvalve29. Reservoirairpressuregauges30. Trailerbrakehandvalve31. Footvalve32. Frontservicebrakechambers33. Quickreleasevalve34. Automaticfrontbrakelimitingvalve

Thisillustrationhasanautomaticfrontbrakelimitingvalve(34),andthereforethecontrolvalve(35),foramanualfrontbrakelimitingvalve(36)arenotshownhere,butappearlaterinthemanual.

CHECK THE SLACK!It is up to YOU,

the DRIVER, to ensure that

your vehicle has safe, properly

adjusted brakes.

Dual Air Brake System

1

Foreward FoldoutiAirBrakeEndorsement FoldoutiiRequirementsforAirBrakeEndorsement FoldoutiiDualAirBrakeSystemIllustration FoldoutiiiSafetyTips 02MakingAppointmentsforTests 0 3SectionOne-BrakesandBraking 0 5 Heat-Energy-Traction-Friction 06 Speed-Weight-Distance 07 BrakingForce 07 StoppingDistance 08 SectionSummaryQuestions 09SectionTwo-TheComponentsofanAirBrakeSystem 1 1 TheComponentsofanAirBrakeSystem 12 CompressorandGovernor 12 Reservoirs 14 AirDryer 15 SafetyValve 16 FootValve 16 BrakeChambers,SlackAdjustersandBrakeLining 16 WedgeBrakes 20 DiscBrakes 21 Air-Over-HydraulicBrakeSystems 21 SectionSummaryQuestions 24SectionThree-HowtheBasicSystemWorks 2 5 BasicAirBrakeSystem 26 One-wayCheckValve 26 AirPressureGauge 27 BrakeApplicationGauge 27 LowPressureWarningDevice 27 StopLightSwitch 27 QuickReleaseValve 28 RelayValve 28 ManualFrontBrakeLimitingValve 28 AutomaticFrontBrakeLimitingValve 29 TandemRearAxles 30 SectionSummaryQuestions 30SectionFour-SpringParkingBrakes 3 1 SpringParkingBrakeSystems 32 UsingaSpringParkingBrake 33 MechanicalRelease(Caging) 35 SectionSummaryQuestions 35

SectionFive-TrailerSystem 3 7 GladHands 38 ApplicationLine 38 TrailerBrakeHandValve 39 Two-wayCheckValves 40 TractorProtectionSystem 41 TractorProtectionValve 42 TrailerSupplyValve 43 AutomaticTrailerSupplyValveSystem 44 TractorandTrailerCoupled 46 ChargingtheTrailerSystem 47 FootorHandValveBrakeApplication 47 EmergencyApplication 48 Supply(Emergency)LineRupture 49 Control(Service)LineRupture 49 LossofReservoirAirPressure 50 ManualTrailerSupplyValve 51 TrailerSpringParkingBrakes 52 SectionSummaryQuestions 52SectionSix-DualAirBrakeSystem 5 3 DualAirBrakeSystemwithSpringParkingBrakes 56 SpringParkingBrakeswithModulatorValve 57 CombinationTractorandTrailerwithSpringParkingBrakes 58 SectionSummaryQuestions 59SectionSeven-ElectronicControlledBrakingandTractionSystems 6 1 Anti-lockBrakeSystem(ABS) 62 AutomaticTractionControl(ATC) 64 SectionSummaryQuestions 64SectionEight-BrakeAdjustmentandIn-ServiceCheck 6 5 BrakeAdjustment 66 S-camBrake 66 Strokevs.Force 67 S-camBrakeAdjustmentwithManualSlackAdjuster 68 S-camBrakewithAutomaticSlackAdjuster 68 DiscBrakeAdjustment 68 WedgeBrakeAdjustment 69 AfteraBrakeAdjustment 69 In-serviceChecks 69 MaintenanceandServicingoftheAirBrakeSystem 70 SectionSummaryQuestions 70SectionNine-Pre-tripAirBrakeInspection 7 1 SingleUnit(Notforairoverhydraulicbrakesystems) 72 CombinationUnit 73 AirOverHydraulic(AirActuated)BrakeSystem 75 SectionSummaryQuestions 76MetricConversionTable 7 7

Table of Contents

2

Safety Tips

1. Reminder-isyourcommercialtrailerequippedwiththemandatoryretro-reflectivemarkings?InJanuary2002,underthemotorvehicleinspectionprogram,alltrailersmustnowbeequippedwithretro-reflectivemarkings.Beseen-besafe.

2. SeatbeltsSaveLives-PleaseBuckleUp-TheLifeyouSaveMayBeYourOwn

Theproperuseofoccupantrestraintshasbecomethemostcost-effectivemethodtoreducedeathandinjuriesresultingfrommotorvehiclecollisions.

3. AnimalsontheHighways-SlowDown-PleaseBeAlert.

-Driversshouldusecautionespeciallyatdawnandduskwhentheanimalsareonthemove.

-Animalsareunpredictablesoreduceyourspeed. -Stayalertandscanbothsidesoftheroad,notjustthe

pavementinfrontofyourvehicle.

4. CellularPhones-Cellulartelephonesareanimportantsafetyaidfordrivers.Manypeopleusetheircellulartelephonetoreportaccidentsandcrimesandfortheirpersonalsafetywhentheirvehiclebreaksdownortheyarelost.

-Useahands-freedevicetomakeiteasiertokeepbothhandsonthewheel.

-Whendiallingmanually,dialonlywhenstopped,orhaveapassengerdialforyou.

-Avoidunnecessarycallsandkeepconversationstoaminimum.

-Befamiliarwiththevariousfunctionsofyourcellularphoneandprogramfrequentlydiallednumbers.

-Donotuseyourcellularphonewhendrivingconditionsarehazardous.

-RememberitisanoffenceundertheMotor Vehicle Act todrivewithoutduecareandattention.

5. SEEANDBESEEN!-Turnonyourheadlights.

6. DRIVERDISTRACTIONS-Manyeverydayhabitsofdriversaredangerousandcanleadtocrashes.Distractionssuchaseatingfastfood,drinkingcoffee,changingtheradiostation,switchingCDsortapes,talkingonacellularphoneortryingtokeepaneyeonayoungchildinthevehicleincreasestheriskofbeinginvolvedinacollision.Alldriversshoulddrivedefensivelyandbepreparedfortheunsafeactionsofothermotoristsorforpoordrivingconditions.Expecttheunexpected.

7. OperationLifesaverremindsalldriverstostayalertatalltimesandespeciallywhencrossingarailwaytrack.

-Becareful-lowslungtrailerunitscangetstuckonraisedcrossings.

-Knowthelengthofyourtruckandtrailer.Whenyouseeasignalorstopsignbecertainyouhaveenoughroomtocompletelycleartherailwaytracksbeforecrossing.

TakeCareofYourself!Themostimportantpartofamovingtruckorbusisthedriver!Getplentyofrestbeforegettingbehindthewheel.Eatwellandstayfit.Remember,hoursofserviceviolationsareseriousandcanthreatenyourlivelihoodorevenyourlife.Stayhealthyandwellrested,ordon’tdrive.AlwaysMaintainYourVehicleInspectyourvehiclebeforeeachtripandcheckyourbrakesregularly.Learnhowtoinspectyourbrakes,identifysafetydefects,andgetthemrepairedbeforeriskingyourlifeandothersonthehighway.SlowDowninWorkZonesWatchoutforhighwayconstruction.Stayalert.Workzonecrashesaremorelikelytohappenduringtheday.Almostone-thirdoffatalcrashesinworkzonesinvolvedlargetrucks.Takeyourtimegoingthroughworkzonesandgiveyourselfplentyofroom.Expecttheunexpected.AlwaysKeepYourDistanceAlwaysleaveenoughspacebetweenyouandthevehicleinfrontofyou.Ifyouhitsomeonefrombehind,youaretypicallyconsidered“atfault”,regardlessofthesituation.Largetrucksrequiremorestoppingdistancesthanothervehicles.Takeadvantageofyourdrivingheight,andanticipatebrakingsituations.

3

AlwaysDriveDefensivelyAvoidaggressivedrivers!Itisestimatedthateachyear,two-thirdsofalltrafficfatalitiesarecausedbyaggressivedrivingbehavioursKeepyourdistanceandmaintainasafespeed.Theonlythingspeedwillincreaseisyourchanceforacrash.WorktoHelpYourselvesBetheprofessionalonthehighwayandatsafetyevents!Helpstrandedmotorists;notifytrafficsafetyagenciesofcrashes,unsafedrivers,unsaferoadwayconditions,andothersituationsthatcanleadtocrashes.yourparticipationinpublicsafetyeventsandyourperformanceonthehighwaycanchangepublicperception!YOURARELYRUNOUTOFBRAKES,BUTYOURUNOUTOFADJUSTMENT.(Thebrakecomponentscouldallbenewbutiftheadjustmentisnotdone,thebrakeswillnotdotheirjob.)Checkthesteeringbrakeairline-it’swellworththetime.Itisrecommendedthattheairlinethatfeedsthesteeringbrakesbeinspectedforbulges,flatspots,cracksandloosenessatthefitting.Thisisanimportantsafetyissueasablownairlinehosewillresultinrapidlossofairpressureanddecreasedabilitytostop.Ensuringproperbrakeoperationandsafetyistheresponsibilityofthedriver.Taketimeduringthepre-tripinspectiontocheckthebrakes-itcouldpreventaseriouscollision.Brakerelateddefectscontinuetobethemostfrequentreasoncommercialvehiclesareputout-of-service.Thedriver/carriercanmakeadifferencebya)increasingknowledgeofbrakecomplianceandvehiclebrakeperformance,andb)makingsureallapplicablebrakesysteminspectionrequirementsarefollowed.

Note:· Makesurethatyourbrakesareproperlyadjusted· Donotmismatchairchamberinsizeonthesameaxle.·Aproperlyinstalledairchamberandslackadjustershould

nothavemorethana90degreeanglebetweenthecomponents.

· Donotmismatchslackadjustersinlengthonthesameaxle.

Making Appointments for Tests

ContactyourlocalServiceNewBrunswickofficetoarrangeforanappointmentandanyadditionalinformationregardingtestingprocedures.

4

5

SECTIONONE-

BRAKESANDBRAKING

6

Tostopthevehiclein1/10thetimeittooktoacceleratewouldrequireastoppingforceof10timestheaccelerationforce–theequivalentofapproximately2,000horsepower.Ifthevehiclehadsixwheels,eachwheelwouldhaveto

provide1/6thebrakingforce.Ifoneortwoofthewheelshadbrakesthatwerenotproperlyadjusted,theotherwheelswouldhavetodomorethantheirshareofthebraking,andthatmightbemorethantheirbrakeswereconstructedtostand.Excessiveuseofthebrakeswouldthenresultinabuildupofheatgreaterthanthebrakedrumscouldabsorbanddissipate.Toomuchheatresultsinbrakedamageandpossiblefailure.Mostbrakeliningsoperatebestataround250°Candshouldnotexceed425°C.It’simportanttounderstandthatthepowerneededtostopgeneratesheatwhichcoulddamagethebrakes.

Heat-Energy-Traction-FrictionForavehicletomovealongthehighway,aninternalcombustionenginemustconvertitsheatenergyintomechanicalenergy.Thismechanicalenergygoesfromtheenginetothedrivingwheeltiresbymeansofasystemofconnectingrods,shaftsandgears.Thefinalfactorthatmovesthevehicleistheamountoftractionitstireshaveontheroadsurface.Frictionistheforcethatresistsmovementbetweentwosurfacesincontactwitheachother.Tostopavehicle,thebrakeshoeliningsareforcedagainsthemachinedsurfacesofthebrakedrums,creatingfriction.Thisfrictionproducesheat.Theengineconvertstheenergyofheatintotheenergyofmotion;thebrakesmustconvertthisenergyofmotionbackintotheenergyofheat.Thefrictionbetweenbrakedrumsandliningsgeneratesheatwhilereducingthemechanicalenergyoftherevolvingbrakedrumsandwheels.Theheatproducedisabsorbedbythemetalbrakedrums,whichdissipatetheheatintotheatmosphere.Theamountofheatthebrakedrumscanabsorbdependsonthethicknessofthemetal.Whenenoughfrictioniscreatedbetweenthebrakeliningandthedrums,thewheelsstopturning.Thefinalfactorthatstopsthevehicleisthetractionbetweenthetiresandtheroadsurface.Ifa200-horsepowerengineacceleratesavehicleto100km/hinoneminute,imaginethepowerneededtostopthissamevehicle.Also,considerthatthevehiclemighthavetostopinanemergencyinaslittleassixseconds(just1/10thetimeittooktoreach100km/h).

250°C Normal

425°C Maximum

1100°C Panic!

BrakeDrums

100km/h

10X

7

Speed-weight-distanceThedistancerequiredtostopavehicledependsonitsspeedandweight,inadditiontoenergy,heatandfriction.Thebrakingforcerequiredtostopavehiclevariesdirectlywithitsweightandspeed.Forexample,iftheweightisdoubled,thebrakingforcemustbedoubledtobeabletostopinthesamedistance.Ifthespeedisdoubled,thebrakingforcemustbeincreasedfourtimestobeabletostopinthesamedistance.Whenweightandspeedarebothdoubled,thebrakingforcemustbeincreasedeighttimestobeabletostopinthesamedistance.Forexample,avehiclecarryingaloadof14,000kgat16km/hisbroughttoastopin30metreswithnormalapplicationofthebrakes.Ifthissamevehiclecarried28,000kgat32km/h,itwouldrequireeighttimesthebrakingforcetostopthevehiclein30metres.Thiswouldbemorebrakingforcethanthebrakescouldprovide.Novehiclehasenoughbrakingforcewhenitexceedsitslimitations.

BrakingForceMechanicalBrakingsystemsusedevicestogainamechanicaladvantage.Themostcommondeviceforthispurposeisleverage.Aleverisplacedonapivotcalledthefulcrum.AsthedistancefromAtoCisfourfeet,andfromCtoBisonefoot,theratioisfourtoone(4:1).Forcehasbeenmultipliedbytheleverageprinciple.Lookatthissimpleleversystem:

Ifa100lbdownwardforceisappliedatpointA,thentheupwardforceatpointBis400lb.

S-cambrake

Applied force = 100 lb

Delivered force = 400 lb

A

BC

4 feet 1 foot

A B

C

A 100 lb

B 400 lb

B 400 lb

C

B = 400 lb

A = 100 lb

8

UseofAirPressureForcecanalsobemultipliedbytheuseofairtogainfurthermechanicaladvantage.Everyonehasfelttheforceofaironawindyday.Aircanbecompressed(squeezed)intoamuchsmallerspacethanitnormallywouldoccupy,forinstance,aircompressedintirestosupporttheweightofavehicle.Thesmallerthespaceintowhichairissqueezed,thegreatertheair’sresistancetobeingsqueezed.Thisresistancecreatespressure,whichisusedtogainmechanicaladvantage.Ifaconstantsupplyofcompressedairisdirectedthroughapipethatisoneinchsquare,andifaoneinchsquareplugwereplacedinthepipe,thecompressedairwouldpushagainsttheplug.Ascalecanbeusedtomeasurehowmanypoundsofforcearebeingexertedbytheairagainsttheplug.

Ifthescaleregisters10pounds,forexample,thenitcouldbesaidtheforceis10poundsontheonesquareinchsurfaceoftheplugor10poundspersquareinch(psi).Themorecompressedtheairinthesupplyreservoir,thegreatertheforceexertedonthefaceoftheplug.

LeverageandAirPressureInactualoperation,pipesareroundandplugsarediaphragmsofflexiblematerialactingagainstpushrods.Ifcompressedairof120psiactsonadiaphragmof30squareinches,3,600lbofforceisproduced(120x30).Applythisforcetoapushrodtomovea6-inchslackadjusteroperatingacamandthetotalforceequals21,600inchpoundstorque(3,600x6),or1,800footpoundstorque(21,600÷12).Itrequires25to30footpoundsoftorquetotightenthewheelonacar.Thiscomparisonillustratestheforceobtainedfromusingmechanicalleverageandairpressurecombined.

StoppingDistanceStoppingdistanceconsistsofthreefactors:·driver’sreactiontime·brakelag·brakingdistanceDriver’sreactiontime:Reactiontimeisoftencalled“thinkingtime.”Thetimeittakesfromthemomentahazardisrecognizedtothetimethebrakeisapplied,approximately3/4ofasecond.Brakelag:Asairishighlycompressible,itrequiresarelativelylargevolumeofairtobetransmittedfromthereservoirtothebrakechamberbeforethereisenoughpressureforthebrakestoapply.Itcanbesaidthatbrakelagisthetimeittakestheairtotravelthroughaproperlymaintainedairbrakesystem(approximately4/10ofasecond).Brakingdistance:Theactualdistancethevehicletravelsafterthebrakeisapplieduntilthevehiclestops.

1 square inch

10 psi

120 psi

30 square inches

6 inches

1 inch

9

SectionSummaryQuestions1. Whatisthefinalfactorthatwilldetermineifthe

vehiclewillmove?2. Whatisthefinalfactorthatwilldetermineifthe

vehiclewillstop?3. Howistheheatthatisgeneratedbythebrakes

dissipated?4. Ifonesetofbrakeshoesispoorlyadjusted,whateffect

couldithaveontheremainingsetsofbrakeshoesinthesystem?

5. Whatismeantbytheterm“friction?”6. Iftheweightofthevehicleisdoubled,howmany

timesmustthestoppingpowerbeincreased?7. Ifthespeedofthevehicleisdoubled,howmanytimes

mustthestoppingpowerbeincreasedtobeabletostopatthesamedistance?

8. Ifbothweightandspeedofthevehiclearedoubled,howmanytimesmustthestoppingpowerbeincreasedtostopatthesamedistance?

9. Whatiscompressedair?10. Whatdoestheabbreviation“psi”standfor?11. If40psiisexertedagainstadiaphragmof30square

inchesinarea,whatarethetotalpoundsofforcethatcouldbeexerted?

12. Stoppingdistanceconsistsofwhatthreefactors?13. Definethefollowingterms? “Driver’sReactionTime”-“BrakingDistance”-“Brake

Lag.”

Thedistancedependsontheabilityofthebrakeliningtoproducefriction,thebrakedrumstodissipateheatandthetirestogriptheroad.Driversshouldnevertaketheirbrakesforgranted.Thebrakingsystemmustbetestedandtheadjustmentcheckedbeforeplacingthevehicleintoservice.Driversmustunderstandthebrakingsystem,realizeitscapabilitiesandlimitations,andlearntousethemtothebestadvantage.Heavyvehiclesrequirepowerfulbrakingsystemsthatareobtainedbyuseofmechanicalleverageandairpressure.Brakesmustbeusedkeepinginmindtheheatgeneratedbyfriction.Iftheheatbecomestoogreat,brakingeffectivenesswillbelost.Theheaviertheloadandthefasterthespeed,thegreatertheforceneededtostop.Itisimportanttorememberthatanairbrakeequippedvehicle,evenwithproperlyadjustedbrakes,willnotstopasquicklyasapassengercar.

Brakes applied

Passenger car

Loaded truck

Actual stop

Actual stop

ComparativeStoppingDistances

10

11

SECTIONTWO-

THECOMPONENTSOFANAIRBRAKESYSTEM

12

SectionOneofthismanualhasexplainedthatitispossibletogainamechanicaladvantagethroughtheuseofleversandthatairunderpressurecanbeusedtogainamechanicaladvantage.SectionTwowillexplainhowairunderpressurecanbeusedtooperatetheairbrakesofavehicle.Pipingillustrationshavebeenkeptsimpleinordertobeeasilyunderstood.Thepipingarrangementsfoundonvehiclesinactualuseonthehighwaymightdiffersomewhatfromtheillustrationsinthismanual.

TheComponentsofanAirBrakeSystemAbasicairbrakesystemcapableofstoppingavehiclehasfivemaincomponents:1. Acompressortopumpairwithagovernortocontrolit.2. Areservoirortanktostorethecompressedair.3. Afootvalvetoregulatetheflowofcompressedair

fromthereservoirwhenitisneededforbraking.4. Brakechambersandslackadjusterstotransferthe

forceexertedbythecompressedairtomechanicallinkages.

5. Brakeliningsanddrumsorrotorstocreatethefrictionrequiredtostopthewheels.

Itisnecessarytounderstandhoweachofthesecomponentsworkbeforestudyingtheirfunctionsintheairbrakesystem.

CompressorandGovernorCompressedairisusedtotransmitforceinanairbrakesystem.Thesourceofthecompressedairisacompressor(1).Acompressorisdesignedtopumpairintoareservoirwhichresultsinpressurizedair.Thecompressorisdrivenbythevehicle’sengine,eitherbybeltsandpulleysorshaftsandgears.Invehicleswherethecompressorisdrivenbybelts,theyshouldbecheckedregularlyforcracksandtension.Also,checkthecompressorforbrokenmountingbracketsorloosebolts.Thecompressorisinconstantdrivewiththeengine.Whenevertheengineisrunning,soisthecompressor.Whenpressureinthesystemisadequate,anywherefromalowof80psitoahighof135psiitisnotnecessaryforthecompressortopumpair.Agovernor(2)controlstheminimumandmaximumairpressureinthesystembycontrollingwhenthecompressorpumpsair.Thisisknownasthe“loading”or“unloading”stage.Mostcompressorshavetwocylinderssimilartoanengine’scylinders.Whenthesystempressurereachesitsmaximum,whichisbetween115and135psi,thegovernorplacesthecompressorinthe“unloading”stage.

Pressure setting spring

Reservoir port

Reservoir port

Unload port

Exhaust port

Governor

Exhaust port

Unload port

13

Thecompressormustbeabletobuildreservoirairpressurefrom50to90psiwithinthreeminutes.Ifunabletodosothecompressorrequiresservicing.Acompressormaynotbeabletobuildairpressurefrom50to90psiwithinthreeminutesiftheairfilterispluggedorifthebeltisslipping.Ifthesewerenotatfaultthecompressorcouldbefaulty.Placingthecompressorintheunloadingstageisdonebydirectingairpressuretotheinletvalvesofthecompressor,holdingthemopen,allowingtheairtobepumpedbackandforthbetweenthetwocylinders,insteadofcompressingtheair.Whenthepressureinthesystemdrops,theinletvalvesclose,returningthecompressortothe“loading”stage.Thegovernormustplacethecompressorinthe“loading”stageatnolowerthan80psi.Duringthe“unloading”stage,thecompressorisabletocool.

Usuallycompressorsarelubricatedfromtheenginelubricationsystem,althoughsomecompressorsareself-lubricatingandrequireregularchecksofthelubricantlevel.

Itisveryimportanttheairthatentersthesystembekeptascleanaspossible.Theairmustfirstpassthroughafiltertoremoveanydustparticles.Theairfiltermustbecleanedregularly.Adirtyfilterwillrestricttheflowofairintothecompressor,reducingitsefficiency.Somevehicleshavetheinletportofthecompressorconnectedtotheintakemanifoldandreceiveairthathasbeenfilteredbytheengineaircleaner.Apistontypecompressoroperatesonthesameprincipleastheintakeandcompressionstrokesofanengine.· Intakestroke:Thedownwardstrokeofthepistoncreates

avacuumwithinthecylinderwhichcausestheinletvalvetoopen.Thiscausesatmosphericairtoflowpasttheinletvalveintothecylinder.

P i s t o n

From governor

Intake air filter

Unload plunger

Inlet valve

Discharge valve

Compressor(Unloadingstage)

P i s t o n

Unload plunger

Inlet valve

Compressor(Intakestroke)

Intake air filterDischarge valve

14

· Compressionstroke:Theupwardmotionofthepistoncompressestheairinthecylinder.Therisingpressurecannotescapepasttheinletvalve(whichthecompressedairhasclosed).Asthepistonnearsthetopofthestroke,thepressurizedairisforcedpastthedischargevalveandintothedischargelineleadingtothereservoir.

ReservoirsReservoirsortanksholdasupplyofcompressedair.Thenumberandsizeofthereservoirsonavehiclewilldependonthenumberofbrakechambersandtheirsize,alongwiththeparkingbrakeconfiguration.Mostvehiclesareequippedwithmorethanonereservoir.Thisgivesthesystemalargervolumeofmainreservoirair.Thefirstreservoirafterthecompressorisreferredtoasthesupplyorwet(5)reservoir.Theotherreservoirsareknownasprimary(8)andsecondary(10)ordry(8)(10)reservoirs.Whenairiscompressed,itbecomeshot.Theheatedaircoolsinthereservoir,formingcondensation.Itisinthisreservoirthatmostofthewateriscondensedfromtheincomingair.Ifoilleakspastthepistonringsofthecompressorandmixeswiththismoisture,itformssludge,whichaccumulatesinthebottomofthereservoir.Ifallowedtoaccumulate,thissludge(waterandoil)wouldenterthebrakingsystemandcouldcausetroublewithvalvesandotherparts.Inwinter,waterinthesystemmayfreeze,causingthemalfunction

P i s t o n

Intake air filter

Unload plunger

To reservoir

Inlet valve

Discharge valve

Compressor(Compressionstroke)

ofvalvesorbrakechambers.Reservoirsareequippedwithdrainvalvessothatanymoistureorsludgethatmayhaveaccumulatedcanbedrained.Ifyounoticesludgewhendrainingyoursystem,haveitinspectedbyamechanic.Tominimizetheamountofwatercollection,allreservoirsmustbedraineddaily.Underextremeconditions,reservoirsmayhavetobedrainedmorethanonceaday.Todrainthereservoirsalwaysstartwiththewetreservoironthetractor.Allowallairpressuretoescape,whichwillthenpermitthemoisturecollectedinthereservoirtodrain.Somereservoirshavemorethanonecompartmentandeachcompartmenthasitsowndrainvalve,whichmustbedrainedindividually.Brieflyopeningthevalvejusttoallowsomeoftheairtoescapedoesnotdrainthemoisture!Itisnotsafetoassumethatthewetreservoir,orthepresenceofanairdryerisreasontoneglecttheotherreservoirsonthepowerunit,trailersordollies.Theyshouldallbecompletelydraineddaily.Somereservoirsmaybeequippedwithautomaticreservoirdrainvalves(spittervalves).Thesevalveswillautomaticallyexhaustmoisturefromthereservoirwhenrequired,althoughtheyshouldbecheckeddailyanddrainedperiodicallytoensurethemechanismisfunctioningproperly.Anylooseordisconnectedwiresassociatedwiththevalveheatersshouldberepairedimmediately.

Reservoir

15

AirDryerAnairdryer(3)maybeinstalledbetweenthecompressorandthewetreservoirtohelpremovemoisturefromthecompressedair.Itmaybepartiallyfilledwithahighmoisture-absorbentdesiccantandanoilfilter,oritmaybehollowwithbafflesdesignedtoassistinseparatingthemoisturefromtheair.Bothtypesofairdryersuseairpressuretopurgeorejecttheaccumulatedcontaminantsfromtheirdesiccantbed.Thepurgevalvehasaheaterelement,whichpreventsthemoisturefromfreezingincoldclimateoperation.Thewiringconnectedtotheheatershouldbeinspectedforlooseordisconnectedwires.Theyarealsoequippedwithasafetyvalve.

One-way check valve

Control port

Supply port

Cut-off piston

Exhaust

Purge valve Delivery port

AirDryer(Purgecycle)

One-way check valveOrifice

Desiccant bed

Control port

Exhaust

Delivery portCompressor

Reservoir

Governor

Supply port

Purge valveHeater

element

Sump

Check valve assembly

Dried air

Oil separator

Cut-off piston

Desiccant cartridge

Air Dryer

Supply port

Control port

AirDryer(Dryingcycle)

16

SafetyValveAsafetyvalve(4)protectsreservoirsfrombecomingoverpressurizedandburstingifthegovernormalfunctionedanddidnotplacethecompressorintheunloadingstage.Thevalveconsistsofaspring-loadedballthatwillallowairtoexhaustfromthereservoirintotheatmosphere.Thevalve’spressuresettingisdeterminedbytheforceofthespring.Asafetyvalveisnormallysetat150psi.Ifthepressureinthesystemrisestoapproximately150psi,thepressurewouldforcetheballoffitsseat,allowingthepressuretoexhaustthroughtheexhaustportinthespringcage.Whenreservoirpressureissufficientlyreducedtoapproximately135psi,thespringwillforcetheballbackontoitsseat,sealingoffthereservoirpressure.Notallsafetyvalveshaveamanualreleasefeature.

Ifthesafetyvalvehastorelievepressure,thegovernororcompressorrequiresadjustment,serviceorrepair.Thisshouldbedonebyaqualifiedmechanic.

FootValveThefoot-operatedvalve(31)isthemeansofapplyingairtooperatethebrakes.Thedistancethetreadleofthefootvalveisdepressedbythedriverdeterminestheairpressurethatwillbeapplied,butthemaximumapplicationwillnotexceedthepressureinthereservoir.Releasingthefootvalvetreadlereleasesthebrakes.

Whenthedriverappliesthebrakes,depressingthetreadlepartway,thefootvalvewillautomaticallymaintaintheapplicationairpressurewithoutthedriverhavingtoadjustthepressureofhisfootonthetreadle.Releasingthetreadleallowstheapplicationairtobereleasedthroughtheexhaustportsintotheatmosphere.Airtreadlesarespringloaded,producingadifferent“feel”fromhydraulicbrakeapplications.

BrakeChambers,SlackAdjustersandBrakeLining

Air pressure greater than 150 psi

SafetyValve

Treadle

Treadle spring

Exhaust port

To brake chambers

Supply from reservoir

To brake chambers

FootValve

Push rod

Brake chamber

Diaphragm Diaphragm return spring

Air inlet

Mounting bolts Clevis and pin

Slack adjuster

BrakeChamberandSlackAdjuster(Brakesoff)

17

Abrakechamber(11)(14)(32)isacircularcontainerdividedinthemiddlebyaflexiblediaphragm.Airpressurepushingagainstthediaphragmcausesittomoveawayfromthepressure,forcingthepushrodoutwardagainsttheslackadjuster.Theforceexertedbythismotiondependsonairpressureanddiaphragmsize.Ifaleakoccursinthediaphragm,airisallowedtoescape,reducingtheeffectivenessofthebrakechamber.Ifthediaphragmiscompletelyruptured,brakesbecomeineffective.

Frontbrakechambers(32)areusuallysmallerthanthoseintherearbecausefrontaxlescarrylessweight.Abrakechamberisusuallymountedontheaxle,nearthewheelthatistobeequippedforbraking.Airpressureisfedthroughaninletport.Theairpushesagainstthediaphragmandthepushrod.Thepushrodisconnectedbyaclevisandpintoacrankarm-typelevercalleda“slackadjuster.”ThisconvertsthepushingmotionofthepushrodfromthebrakechambertoatwistingmotionofthebrakecamshaftandS-cams.Whentheairisexhausted,thereturnspringinthebrakechamberreturnsthediaphragmandpushrodtothereleasedposition.Asindicatedbyitsname,theslackadjusteradjuststhe“slack”orfreeplayinthelinkagebetweenthepushrodandthebrakeshoes.Thisslackoccursasthebrakeliningswear.Iftheslackadjustersarenotadjustedwithinthelimitations,effectivebrakingisreducedandbrakelagtimeisincreased.Iftoomuchslackdevelops,thediaphragmwilleventually“bottom”inthebrakechamber,andthebrakeswillnotbeeffective.

Push rod

Brake chamber

Diaphragm Diaphragm return spring

Air inlet

Mounting bolts Clevis and pin

Slack adjuster

BrakeChamberandSlackAdjuster(Brakeson)

ManualSlackAdjusters

BallIndentSlackAdjuster PositiveLockSlackAdjuster

Lock screw

Adjusting bolt

Worm shaft

Worm gear

Locking collar

Spline Spline

Grease fitting

Adjusting bolt

18

Thrust washer

Clevis

Clevis pin (large)Clevis pin (small)

Actuator rod

Hairpin clip

Boot and strap

Actuator piston

Roller (pin)

Actuator (adjusting sleeve)

Pressure relief capscrew (pull pawl)

Pawl spring

Adjusting pawl

Worm

Worm seal

Adjusting bolt

Grease groove

Grease fitting

Housing

Worm gear

Automatic Slack Adjuster

Previously illustrated are two common types of manual slack adjusters, showing the worm adjusting gear. When the brakes are fully applied, the angle between the push rod and the arm of the slack adjuster should be no more than 90° (at a right angle).

On manual slack adjusters, the adjusting worm bolt is turned until the brake linings touch the drums and then backed off, normally 1/4° to 1/2° a turn. A locking device, which may be a spring loaded collar over the head of the adjusting bolt, must be depressed when the wrench is

slipped over the bolt head, this is known as a positive lock slack adjuster. Or they could use a spring-loaded internal check ball to lock the adjustment, and it must be removed to make any adjustment. This is known as a ball indent slack adjuster. The more often the driver checks the “slack,” the less the probability of brake failure. Vehicles rarely “lose” their brakes because of air loss; it is usually because they are out of adjustment.When conducting a pre-trip air brake inspection look for worn or damaged components, also ensure that the slack adjuster and push rod are at 90° with the brakes applied, as illustrated. If more than 90° there is a drastic loss in braking efficiency, less than 90° may indicate an over adjustment and brakes could be dragging.It is the driver’s responsibility to ensure that brakes are adjusted correctly. A simple service brake application at low speed to check brake adjustment is not adequate. Braking at highway speed causes brake drum expansion due to heat, which in turn requires greater push rod travel to maintain the same braking force. If a brake is out of adjustment there would not be enough reserve stroke of the push rod travel to compensate for drum expansion. This would cause a brake fade and would greatly extend stopping distance. If travelling down a hill, this could cause complete brake loss.Note: Detailed brake adjustment procedures are outlined in Section Eight.

90°Air

inlet

Slack adjuster

Brake Chamber and Slack Adjuster (Brakes on)

Pushrod

19

Somesystemshaveautomaticslackadjustersthatadjustautomaticallytocompensateforbrakeliningwear,usuallymaintainingthecorrectclearancebetweenthebrakelininganddrum.Automaticslackadjustersmustbecheckedregularlytoensurethatcorrectadjustmentisbeingmaintained.Therearevariousmakesandmodelsofautomaticslackadjustersinuse.Primarily,theyareeitherstroke-sensingorclearance-sensing.Astroke-sensingadjusterwilladjusttheslackwhenitsensesthesetstrokeisexceeded.Aclearance-sensingadjusterwilladjustwhentheproperclearancebetweenthebrakedrumandbrakeshoeisnotmaintained.Someautomaticslackadjustershavetheabilitytoback-offorincreasetheslackwhenithasoveradjustedthebrake.Ifavehicleisequippedwithautomaticslackadjusters,itshouldnotbetakenforgrantedthatthebrakeswillalwaysbeinadjustment.Thesystemisnotfoolproof.Anumberoffactorscouldresultintheautomaticslackadjusternotmaintainingproperslack.Therecouldbeimproperinstallation,inadequatemaintenance,deformedbrackets,worncambushings,bentpushrods.Evenpoorvisualinspectioncanresultinproblemsunrelatedtoadjusterfunction.Automaticslackadjusterscanmalfunctionandnotkeepthebrakeinadjustment,especiallywhenithasbeeninserviceforalongperiodoftime.Thetwomostcommonproblemsareexcessiveprematurewearandinternalcontamination.Asanautomaticslackadjusteragesinservice,thecomponentswearthatsensewhenanadjustmentisrequired.Theresultismorestrokeisrequiredfortheliningtocontactthebrakedrum,andifnotcheckedthebrakecouldbeoutofadjustment.Ifevenasmallamountofwaterissuckedintoanautomaticslackadjustermechanismitcancause

corrosionor,inwinter,itcanfreezetheinternalsensingcomponentsandinhibitorpreventadjustment.Also,undercertainconditions,anautomaticslackadjusterthatdoesnothavetheabilitytoback-offorincreaseslack,mayoveradjustabrakecausingittodrag.Forexamplethiscouldtakeplacewhenatractor-trailerisnegotiatingalong,curvingdowngrade.Thedrivershould“snub”thebrakes,whichisrepeatedlyapplyingthebrakesmoderatelytomaintainsafecontrolofthevehicle.Howeveritwouldnottakelonginthisseverebrakingconditionforoneormoreofthebrakedrumstooverheatandexpand.Theoverheatingwillphysicallyincreasethebrakedrumsdiameter,andinextremeandprolongedconditionswillleadtolongerpush-rodstrokestoachievethebrakingforcerequired.Theautomaticslackadjusterinterpretsthisasaneedforadjustmentandwilltakeupslack.Whenthebrakedrumcoolsdownandreturnstonormalsizethebrakesareoveradjustedanddragging.Atthattimethedrivershouldstopandcheckthebrakesforadjustment.Anumberoffullbrakeapplicationsperdaymayberequiredtokeeptheautomaticbrakeadjustersinadjustment(seepage68formoreinformation).Becauseautomaticslackadjustersarenotfoolproof,itisimportanttheoperatorofavehicleequippedwithautomaticslackadjustersbeabletomanuallyadjustthem.Forinformationonmanuallyadjustingtheautomaticslackadjustersonyourvehicleconsultthemanufacturer.Illustratedisacommontypeofbrakeassemblyusedontruckrearaxlesandtraileraxles.Afrontaxleassemblyhasthebrakechamberandslackadjustermountedonthebacking-platebecauseofthesteeringaction.

Brake drum

Brake chamber

Push rod, clevis and pin

Slack adjuster

S-cam

Brake lining

BrakeAssembly

20

Brakeliningmaterialisattachedtotheshoes.Thematerialuseddependsonthebrakingrequirementsofthevehicle.Brakeliningmustgiveuniformoutputofbrakeeffortwithminimumfadeathightemperatures.Fadingorreductioninbrakingeffortoccurswhentheheateddrumsexpandawayfromthebrakelinings.Thebrakeliningsalsolosetheireffectivenesswithoverheating.ThetwistingactionofthebrakecamshaftandS-camforcesthebrakeshoesandliningsagainstthedrums.Thebrakeliningsgenerateheatfromfrictionwiththebrakedrumsurface.Thethicknessofthedrumsdeterminestheamountofheattheyareabletoabsorbanddissipateintotheatmosphere.Drumswornthinwillbuildupheattooquickly.Dangerouslyundependablebrakeperformancewillresultfromdistorteddrums,weakreturnsprings,improperlining,pooradjustment,orgreaseordirtonthelining.Drumsmustneverbemachinedorwornbeyondthemanufacturer’sspecification.

WedgeBrakesThisisanotherexampleofabrakeassemblyusedonsomeairbrake-equippedvehicles.Theactionofthebrakechamberpushrodforcesawedge-shapedpushrodbetweenthebrakeshoerollers.Thisforcesthebrakeshoeliningagainstthebrakedrum.

Thevehiclemaybeequippedwithasingleordualchambersoneachwheel,dependingonthevehicle’ssizeandstyle.Thesebrakesmaybeequippedwithaself-adjustingmechanismorwithamanual“starwheel”adjuster.Thestarwheeladjustmentismadewiththevehiclejackedup,toinsurethatthebrakeliningsdonotdrag.Manualadjustmentofwedgebrakesisusuallydonebyaqualifiedmechanic.

Brake chamber

Adjusting wheel

Brake shoe roller

Push rod

Shoe return spring

Brake lining

Brake shoe

WedgeBrake-SingleChamber

Brake chamber

Singlechamber Dualchamber

Shoe return springs

Brake lining

Adjusting wheel

Brake chambers

Adjusting wheel

WedgeBrakes

21

DiscBrakesTheair-activatedheavytruckdiscbrakeissimilarinprincipletothatusedonpassengervehicles.Airpressureactsonabrakechamberandslackadjuster,activatingthebrakes.Insteadofthecamorwedgeusedinconventionalheavytruckdrumbrakes,a“powerscrew”isused.ApowerscrewworkslikeaC-clamp,sothattheliningpadsexertequalforcetobothsidesofthediscorrotor.Sometypesofdiscbrakeshaveabuilt-inautomaticadjuster.DiscbrakesthatrequiremanualadjustmenthaveadjustmentspecificationsthatdifferfromconventionalS-cambrakingsystems.Alwayscheckthemanufacturer’sspecificationsbeforeadjusting.Discbrakeassembliesmayhaveaspringparkingbrakeunitattachedtotheservicebrakechamber.

Air-Over-HydraulicBrakeSystemsAiroverhydraulicbrakesystemsweredevelopedformediumweightvehiclesbecause:· dieselenginesdonothaveasourceforvacuumboosting

unlesstheyareequippedwithavacuumpump.· mediumweightvehiclesdonotrequireafullairbrake

system.· itgivestheoptionofpullinganairbrakeequippedtrailer.Thesesystemscombinethebestfeaturesofanairandhydraulicbrakesystem.Theyusehydraulicbrakesateachwheelwiththeirreliableselfadjustersandlimitedmaintenance.Onthesesystemstheairisusedtoeitheractuatethehydraulicbrakesorboostthehydraulicbrakepressureasexplainedinthefollowing.

AirActuatedHydraulicBrakeSystem(AirBrakeEndorsementRequired)Anairactuatedsystemusuallyhasthesamecomponentsofastandardairsupplysystemincludingawarningbuzzerandlight,compressor,governor,wetanddryreservoirs,andafootvalvethatcouldbeasingleordualtype.Thesecomponentsarefoundusuallyinthesameplacesasonafullairbrakesystem.Alsothereareoneortwoairactuatedhydraulicpressureconvertersdependingonifthesystemisasingleoradualsystem.Thissystemconsistsofanairchamberorcylinderattachedtoahydraulicmastercylinder.Whenthefootvalveisdepressed,theairpressureactuatesthepushrodfromtheairunitthatpushesagainstthemastercylinderpiston,producinghydraulicpressuredirectedthroughtubingtothewheelcylindersactuatingthefrontandrearaxleservicebrakes.

DiscBrake

22

Itisessentialthattheoperatorofsuchavehiclehaveknowledgeofairpressurebuilduptime,governorloadingandunloadingpressure,warningdeviceoperation,andhowtodrainairreservoirsproperly(seeSectionNine;Pre-TripAirBrakeInspection).Ifanair-actuatedhydraulicbrakesystemwastoloseitsairsupply,thevehiclewouldhavenoservicebrakes.Onlytheparkingbrakewouldbeoperatingasitismechanicalandrequiresnoairpressuretooperate.Eachvehiclemanufacturermayhavedifferentparkingbrakeapplications,eitherautomaticallywhenairpressureisreducedinthereservoir,ormechanicallybyabrakeontherearofthetransmission,orwiththerearbrakesystem.Sincehydraulicbrakesystemsactuatedbyairpressureareregardedasanairbrakesystem,yourdriver’slicencemusthaveanairbrakeendorsementforyoutooperatevehiclesequippedwithair-activatedhydraulicbrakes.Astherearemanydifferentsystemsinuse,refertotheoperator’smanual.

Air-boostHydraulicBrakeSystem(AirBrakeEndorsementnotRequired)Anair-boosthydraulicbrakesystemusesairpressuretoassistbrakeforce.Thisissimilartovacuum-assistedbrakesonmostpassengervehicles.Anair-boostsystemusuallyhasthesamecomponentsofastandardairsupplysystemincludingacompressor,governor,wetanddryreservoirs.Thesecomponentsarefoundusuallyinthesameplacesasonafullairbrakesystem.Thebrakepedallinkageoperatesahydraulicmastercylinderthatsendshydraulicpressuretotheboosterunit.Initially,atlowpressurethehydraulicfluidpassesthroughtheboosterandbeginstopressurizethewheelcylindersmovingthebrakeshoesouttothedrums.Theseboosterunitsaresimilarinoperationto“Hypower”or“Hydrovac”vacuumboostersfoundonmostlightandmediumweightvehicles,butairpressureisusedtointensifythehydraulicpressuregeneratedbythemastercylinderratherthanvacuum.Builtintotheboosterunitisahydraulicallyoperatedaircontrolvalve.

Air linesReservoirsCompressor

Foot valveHydraulic lines

Air brake chamber

Hydraulic wheel cylinders

Hydraulic wheel

cylinders

Air-actuatedhydraulicBrakeSystem

Air lines

Air brake chamber

Hydraulic master cylinder

Hydraulic master cylinder

23

Thisiswhereairfromthereservoirisdirected.Asthepressurefromthemastercylinderincreases,theaircontrolsectionintheboosterwillopenandbegintodeliverairpressuretotherearoftheaircylinder.Theaircylinderpushrodtransferspressureonapistoninthehydraulicsectionofthebooster,increasingthehydraulicpressureatthewheelcylinders.

Thedriverhasfullcontrolofthebrakingforceastheaircontrolsectionmodulatestheboostpressureinproportiontothemastercylinderpressure.Ifthevehiclewastolosealloftheairpressurethebrakesystemwouldlosetheairassistboost,howeverthehydraulicsystemwouldcontinuetoworkbutatreducedeffectiveness.Anairbrakeendorsementonadriver’slicenceisnotrequiredtooperateavehiclewiththisbrakesystem.Consulttheoperator’smanualforthevehicleyoudriveformaintenancerequirements.

Air lines

Hydraulic wheel cylinders

Compressor

Reservoir Hydraulic master cylinder

Brake pedal

Hydraulic lineBooster unit

Air-boostHydraulicBrakeSystem

Hydraulic wheel

cylinders

Air lines Booster unitHydraulic

line

24

SectionSummaryQuestions1. Whatarethefivebasiccomponentsofanairbrake

system?2. Atwhatpressureshouldthegovernorcausethe

compressortoreturntoits“loading”stage?3. Atwhatpressurewillthegovernorplacethe

compressorinthe“unloading”stage?4. Howisapluggedairfilterlikelytoaffecttheair

compressor?5. Whatcausesmoisturetoformintheairbrakesystem?6. Whenisthecompressorabletoaccomplishmostofits

cooling?7. Howaremostcompressorslubricated?8. Howoftenshouldthereservoirsbedrained?9. Isitnecessarytoallowallthepressuretoescapefrom

thereservoirinordertoremovethemoistureandsludgewhichmayhaveaccumulated?

10. Whatisthemaximumpressureavailableforafullbrakeapplicationatanygiventime?

11. Whatwillresultifthebrakedrumsarewornthinorturnedtoofar?

12. Ifthegovernorvalvefailedto“unload”thecompressor,whatwouldprotectthereservoirsfrombecomingoverpressurizedandbursting?

13. Whatisthepurposeofhavingmorethanonereservoir?

14. Whataretwofunctionsoftheslackadjusters?15. Doestheamountofslackinthebrakelinkageshave

anyeffectonthebrakingefficiencyofthevehicle?16. Whatistheadvantageofkeepingthebrakechamber

pushrodtraveladjustedwithinlimitations?17. Whatisthemostcommoncauseoflossofeffective

brakinginanairbrakesystem?18. DoautomaticslackadjustersonS-cambrakesrequire

checking?

19. Cantheadjustmentonair-operateddiscbrakesdifferfromS-cambrakes?

20. Whatoccurswhendrumbrakesbecomeoverheated?21. Whatcausesbrakefade?22. Whatisthemainfunctionofthefootvalve?23. Whydoesthe“feel”ofanair-operatedfootvalvediffer

fromahydraulicbrakepedal?24. Onwhatprincipledoesadiscbrakeoperate?25. Whattypeofairoverhydraulicbrakesystemrequires

theoperatortoholdanairbrakeendorsement?

25

SECTIONTHREE-

HOWTHEBASICSYSTEMWORKS

26

BasicAirBrakeSystemAirispumpedbythecompressor(1)tothewetreservoir(5),whichisprotectedfromoverpressurizationbyasafetyvalve(4).Thegovernor(2)controlsthepressureinthereservoirtothebottomofthefootvalve(31).Thedriverpushesthefootvalvetreadledownandairpressureflowstothefrontandrearbrakechambers(32&11).Thebrakechamberpushrodsmovetheslackadjusters.TheslackadjustersrotatetheS-cams,forcingthebrakeshoesagainstthedrums.Thiscausesfrictionthatstopsthewheels.Thedriverreleasesthefootvalvetreadleandtheairinthebrakechambersisallowedtoexhaustthroughthefootvalve,releasingthebrakes.Thefollowingexplainstheadditionalcomponentsofabasicairbrakesystem.Othervalveswhicharenecessarytoensuresmoothandefficientoperationsarenotincludedinthissimpledrawing.Theywillbediscussedlaterinthemanual.Note:Anairdryer(3)hasbeenaddedtoreducetheamountofmoistureinthesystem.

One-wayCheckValveInthediagrambelow,tworeservoirsareshown(5)(10).Topreventairfromflowingbackwardsinthesystemtowardthecompressor,aone-waycheckvalve(7)isinstalledbetweenthereservoirs.Thisvalveallowstheairtoflowinonedirectiononly.Thevalveisspringloaded.Pressureat

theinletsideovercomesthespringpressureandliftsthecheckvalveball,ordisc,offitsseat.Airpassesthroughthevalvetotheoutlet.Whenpressureattheoutletbecomesgreaterthanattheinlet-togetherwiththespringpressure-thecheckdeviceseats,preventingairfromflowingbackthroughthevalve.

Ball

Spring

Body

Cap nut

One-wayCheckValve

1

2

3 4 5 7 9 1032 11

32 31 11

BasicAirBrakeSystem

27

AirPressureGaugeVehicleswithanairbrakesystemareequippedwithareservoirairpressuregauge(29).Thisgaugeismountedinthecab,usuallyonthedashboardandindicatestheairpressureintheprimaryandsecondaryordryreservoirs.Thesupplyorwetreservoirdoesnotusuallyhaveanairpressuregauge.Commonoperatingpressuresare80to135psi,dependingonthesystem.Monitoringthegaugewillalertthedrivertoanyunusualchangesinairpressure.

BrakeApplicationGaugeAnadditionalgaugecanbeinstalledonthedashtoindicatetheapplicationairpressurewhenthebrakesareapplied.Thisgaugecanbepipedtoindicatethepressureofeitherafootorhandapplication.(Handapplicationwillbeexplainedlaterinthemanual.)

LowPressureWarningDeviceAllvehiclesequippedwithanairbrakesystemmusthaveadevicetowarnthedriveriftheairpressureinthesystemdropstoadangerouslevel.Thisdevicemustbecomprisedoftwosystems-visualandaudible-consistingofaredwarninglightandabuzzerorawigwag.Duetooveruseorleaks,thelowpressureindicatorswitch(9)willturnonaredwarninglightonthedashorcauseabuzzertosoundatorbefore55psi.Somevehiclesareequippedwithbothalightandabuzzertowarnthedriverofalowairpressurecondition.Wig-wagsarenotfoundinmodernvehicleshavingbeenreplacedwitharedwarninglightandbuzzer.Theymaystillbeinuseonoldervehicles.Therearetwotypesofwig-waglowpressurewarningdevicesthatmaybeused.Bothtypeswilldropintothedriver’sviewshouldthesystempressuredropto55psi.Theautomaticwarningdevicewillriseoutofthedriver’sviewwhenthepressureinthesystemrisesabove55psi.Themanualresettypemustbeplacedinthe“outofview”positionmanuallyandwillnotstayinplaceuntilthepressureinthesystemgoesabove55psi.Whicheverwarningsystemisused,buzzer-lightsorwig-wag,thedrivermuststopthevehicleandfindthecauseoftheairloss.Theairpressureremaininginthesystem(approximately55psi)isenoughforabrakeapplicationifthedriveractspromptly.

StopLightSwitchAnydriverfollowingyourvehiclemustbewarnedwhenreducingspeedorstoppingthevehicle.Thestoplightswitch(25)isanair-operatedelectricswitchthatturnsonthebrakelightsattherearofthevehiclewhenabrakeapplicationisbeingmade.

28

QuickReleaseValveTheapplicationofthebrakesinthebasicsystemwasdescribedearlier.Inabasicsystem,whenthedriverreleasesthefootvalve,itwouldbenecessaryfortheairunderpressureinthebrakechamberstoreturntothefootvalvetoreleasethebrakes.Thisreleasingactionwouldbeslowedinlongwheelbasevehiclesbecauseofthelongerlinesbetweenthefootvalveandtherearbrakechambers.Toallowthebrakestoreleasequicklyandfullybydischargingtheapplicationairnearthebrakechambers,aquickreleasevalve(33)maybeinstalled.

RelayValveThefootvalveisusuallylocatedclosertothefrontwheelsthantotherearwheels.Thelongerthedistancefromthefootvalvetotherearchambers,themoretimeitwilltakebeforetherearbrakesapply.Thisisknownasbrakelag.Tocorrectthisconditiononalongwheelbasevehicle,arelayvalve(13)isinstalledneartherearbrakechambers.Alargerdiameterpipeisconnectedbetweenthemainreservoirandtherelayvalve.Theairlinefromthefootvalvetotherelayvalvenowbecomesa“controlline.”(Theairinthecontrolline“deadends”attherelayvalve.)Whenthefootvalveisdepressed,theairpressureinthecontrollineactsonthetopsectionoftherelayvalve,relayingreservoirairdirectlytotherearbrakechambersthroughthelargerdiameterpipe.Thepressureofthereservoirairdeliveredinthiswaywillbethesameasthecontrolpressuredeliveredbythefootvalve.

Releasingthefootvalveexhauststhecontrolairtotherelayvalve,allowingittocutofftheflowofreservoirairtotherearchambers.Thisinturnexhauststheairinthebrakechambersbythequickreleasefeatureoftherelayvalve.

ManualFrontBrakeLimitingValveForbettersteeringcontrolonaslipperyroadsurface,itcanbeanadvantagetoreducethebrakingefforttothefrontwheels.Thiscanbeaccomplishedbyinstallingacontrolvalve(35)inthecab,andafrontbrakelimitingvalve(36)onthefrontaxle.Thecontrolvalveissetinthe“normal”positionfordryroadsurfacesandthefrontbrakingapplicationairpressureisnormal.Onaslipperyroadsurface,thecontrolvalve(35)issettothe“slipperyroad”position.Inthisposition,thecontrolvalvewillcausethelimitingvalve(36)tooperate.Applyingairpressuretothefrontbrakesisthenreducedto50percentoftheapplicationairpressurebeingdeliveredtotherearbrakechambers.

RelayValve

QuickReleaseValve

Delivery ports not shown

ManualFrontBrakeLimitingValve

DashMountedControlValve

Service port Lever

Exhaust port Service port

To limiting valve

From liniting

valve

29

Somesystemsareequippedwithanautomaticlimitingvalve(34).Thisvalvewillholdoffbrakeapplicationtothefrontwheelsfrom0to10psi,dependingonhowithasbeenpreset.Betweenthepresetpressureand40psiofbrakeapplication,thereductionisapproximately50percent.

Brakeapplicationsbetween40psiand60psiarereducedbylessthan50percent.Brakeapplicationsmorethan60psiarenotreducedandfullapplicationisdirectedtothefrontwheels.

Service port

Delivery port

Piston spring

Inlet-exhaust valve spring

Lower piston

assembly

Service port

Delivery port

AutomaticFrontBrakeLimitingValve

1

2

3 4

5

6 7 6 8 932 11

1135

31

32

29

1336

BasicAirBrakeSystemwithManualFrontBrakeLimitingValve

30

Theairbrakesystemdiscussedpreviouslyisforavehiclewithasinglerearaxle.Thediagramillustratesanairbrakesystemforavehicleequippedwithanautomaticfrontbrakelimitingvalve(34),aquickreleasevalve(33)andatandemsetofrearaxles.Bothaxlesofthetandemsetareequippedwithbrakes.Arelayvalve(13)hastwouses:toprovideaquickerapplicationofairpressuretothetandemrearaxlebrakeswhenabrakeapplicationismade,andtoreleasethebrakesquickerwhenabrakeapplicationisreleased.

SectionSummaryQuestions1. Howcanthedrivertellhowmuchairpressureisinthe

mainreservoirs?2. Whatmustthedriverdowhenalowpressurewarning

systemactivates?3. Whatisthepurposeofaquickreleasevalve?4. Whatisthepurposeofarelayvalve?5. Whatisthepurposeofusingalargerdiameterpipe

betweenthereservoirandtherelayvalve?6. Ifthefrontbrakelimitingvalveisinthe“slipperyroad”

position,andthefootvalveisdepressedtomakeabrakeapplicationof30psi,howmuchpressurewillbeappliedinthefrontbrakechambers?

7. Howisthereservoirprotectedfromoverpressurization?

8. Whatstopspressurizedairfromflowingfromthedryreservoirbackintothecompressor?

9. Atwhatpressureshouldthelowpressurewarningdeviceactivate?

10. Howis“brakelag”totherearwheelsminimized?11. Whenshouldadriverusethefrontbrakelimiting

valve?

34

33 13

TandemRearAxles

31

SECTIONFOUR-

SPRINGPARKINGBRAKES

32

SpringParkingBrakeSystems(Singlecircuitsystemonly)Theinstallationofspringparkingbrakesandtheirpipingarrangementsintoavehicleairbrakesystemwillvarydependingonthevehiclemake.Springparkingbrakesmaybeinstalledonanairbrake-equippedvehicleforuseasareliableparkingbrakesystem.Intheservicebrakesystem,thebrakesareappliedbyairpressureandretractedbysprings.Inthespringparkingbrakesystem,thebrakesareappliedbyspringpressureandretractedbyairpressure.Thespringparkingbrakechambersareattachedtotheservicebrakechambersandoperatethroughthesamelinkage,thereforetheeffectivenessofthespringparkingbrakedependsontheservicebrakeadjustment.Acontrolvalve(operatedbyasquare,yellowbutton)locatedinthecaballowsthedrivertoexhaustairoutofthespringparkingbrakecircuittoapplythebrakes,orpressurizethecircuittoreleasethem.Somesystemsmayhaveanadditionalvalvecontrolledbyabluebuttonthatappliesonlythetractorspringparkingbrakesandnotthetrailerspringparkingbrakes.Thesystemcanalsoactasanemergencybrake.Lossofairfromthesystemmayautomaticallyapplythebrakes,dependingonhowthesystemispiped.

Controlvalveswillvary,dependingonthemanufacturerandtypeofpipingarrangements.Aspring-loadedvalverequiresthatthevalvebepushedintoreleasethespringparkingbrakes.Thisvalvecannotbeleftinthereleasedpositionbelowapproximately35psiinthesystem.Anytimethereservoirpressuredropstoapproximately35psi,thisvalvewillexhaustautomatically,placingthespringparkingbrakesintofullapplication.Onsomeoldervehiclestheremaybeasingletypeofpush-pullcontrolvalvethatdoesnothaveanautomaticreleasefeature.Toapplythespringparkingbrakes,thevalvemustbeoperatedmanually,eventhoughthereservoirpressurehasbeendepleted.Duringnormaloperation,airpressurecages(compresses)thespring,holdingitreadyforparkingoremergencybraking.

Mounting bolts

Spring parking brake chamber

Clevis and pin

Slack adjuster

Push rodDiaphragmDiaphragm

return spring

Dust cap

BrakesOffParking spring

brakeService brake

chamber

33

Onthepre-tripairbrakeinspection(Section9),youmustensurethattheparkingbrakespringisnotmanuallycagedoritwillnotexpandandapplythebrake.Thebrakechambersshouldbecheckedforcracksanddamage.Thebrakechambershouldbefittedwithadustcaptoensuredebriswillnotenterthechamber.Duringnormalservicebrakeoperation,theparkingbrakespringdoesnotexpand.Airpressurekeepsthespringcaged.

12

12

27

UsingaSpringParkingBrake

Springparkingbrakes(12),addedtothebrakechambersoftherearaxleonthesingleunitvehicle,areillustrated.Acontrolvalve(27)ismountedinthecab.

Asupplylineofreservoirairispipedfromthedryreservoirtothecontrolvalve.Openingthecontrolvalveallowsreservoirairpressuretoflowtothespringparkingbrakechambers,releasingthem.

ServiceBrakesAppliedBrakeOn

Mounting bolts

Spring parking brake chamber

Clevis and pinSlack adjuster

Push rodDiaphragmDiaphragm

return spring

Dust cap

Parking brake spring

Service brake chamber

34

Closingthecontrolvalveshutsoffthesupplyofreservoirairpressureandexhauststheexistingpressureinthespringparkingbrakechambers.Thismotionallowsthespringtoexpand,applyingthebrakes.

Caution:Parkingbrakesshouldbeinthereleasepositionbeforemakingaservicebrakeapplication.Afull-brakeapplication,madewhentheparkingbrakesareapplied,cancompoundtheforceexertedontheslackadjustersandlinkageandresultindamageorbrakefailure.Compoundingisthecombinationoftwoforces:theforceappliedbythespringbrakesandtheservicebrake.Springbrakesareprimarilyusedasaparkingbrake,butintheeventoflossofairpressureinthesystem,theycanassistinstoppingthevehicle.Howquicklytheywillstopthevehicledependsonsuchfactorsas:· theweightandspeedofthevehicle;· thesteepnessofthegrade;· thespringforceofthespringbrakesthathavebeen

installed;and,· theadjustmentoftheservicebrakes.

Ifthebrakeshaveoverheated,suchasduringmountaindrivingorhardhighwaybraking,caremustbetakenwhenparkingthevehicle.Ifthespringparkingbrakesareappliedwhenthebrakedrumhasexpandedbecauseofextremeheating,whenthebrakedrumstartstocoolandcontract,thepressureexertedbythespringparkingbrakemaycausethebrakedrumtocrackorwarp.Whenparkingavehiclewithoverheatedbrakes,parkonlevelground,stoptheengineandleavethetransmissioninthelowestgearandblockthewheels.Donotsetthespringparkingbrakesuntilyouhaveverifiedthebrakedrumiscooltothetouch.

SpringParkingBrakesAppliedBrakesOn

Mounting bolts

Spring parking brake chamber

Clevis and pinSlack adjuster

Push rodDiaphragmDiaphragm

return spring

Dust cap

Parking brake spring

Service brake chamber

35

MechanicalRelease(Caging)Somespringparkingbrakescanbereleasedmechanicallyby“windingthemoff”or“caging”them.Cagingmeansthebrakesarebeingreleased.Thisisachievedwithaboltthatrunsthroughthecentreofthechamberbody,whichisturnedtocompressthespring.Itmaybenecessarytofirstremovealockplateandstudtogainaccesstotheheadofthebolt.Othertypeshaveadustcapthatmustfirstberemovedandaboltinserted.Insomecases,aspecialwrenchisrequired.Instructiononhowto“cage”isusuallyonthebodyoftheparkingbrakechamber.Ifallairislostandthevehiclehastobetowed,theparkingbrakescanbereleasedbycagingthem.Alwaysblockthewheelswhencagingtheparkingbrakespring.

WarningSpringparkingbrakechambersshouldneverbedisassembledwithoutfirstcompressingthespringwithacagingbolt.Thesespringsareunderextremepressureandcouldcauseseriouspersonalinjuryifdisassemblyisattemptedbyanyonenotexperiencedinservicingtheseunits.Disassemblyofaspringbrakechambershouldonlybepreformedbyaqualifiedmechanicortechnician.

Mounting bolts

Spring parking brake chamber

Service brake chamber

Clevis and pin

Slack adjuster

Push rodDiaphragmDiaphragm

return spring

Parking brake spring

Caging bolt

ParkingBrakeSpringCagedBrakesOff

SectionSummaryQuestions1. Whatismeantby“compounding”thebrakes?

2. Whyarespringbrakesareliabletypeofparkingbrake?

3. Howareparkingbrakesheldinthereleasedposition?

4. Whatarethefunctionsofthecab-mountedparkingbrakecontrolvalve?

5. Willparkingbrakesapply“automatically”inallbrakingsystems?

6. Whatisthereasonforreleasingtheparkingbrakesbeforemakingafullbrakeapplicationtest?

7. Whymustyoubecarefulparkingavehiclewithoverheatedbrakes?

8. Howcansometypesofparkingbrakesbereleasedwithouttheuseofairpressure?

9. Whatisthedangerofdisassemblingaspringparkingbrakeunit?

36

37

SECTIONFIVE-

TRAILERSYSTEM

38

Uptothispoint,thesystemdiscussedistheairbrakesystemofatruckortractor.Ifatrailerwascoupledtoatruckortractor,thebrakesofthetrailerwouldhavetobeoperatedfromthetruckortractor.Inthefollowingpagesthepowerunitofacombinationvehiclewillbereferredtoasatractor.

GladHandsThistermreferstothecouplingdeviceusedtoconnectthecontrol(service)andsupply(emergency)linesofthetrailertothetractor.Thesecouplersconnecttogetherandlockinposition.Theyhaverubbergasketsthatpreventairfromescapingattheconnections.

Beforeconnectionismade,couplersshouldbecleanandfreeofdirtandgrit.Whenconnectingthegladhands,startwiththetwogasketstogetherandthecouplersata90°angletoeachother.Aquicksnapdownwardswilljoinandlockthecouplers.Vehiclesequippedwith“dead-end”couplersshouldusethemwheneverthevehicleisusedwithoutatrailertopreventwateranddirtfromenteringthecouplerandlines.

Iftheunitisnotequippedwithdead-endcouplers,thegladhandofthecontrol(service)linecanbelockedtothegladhandofthesupply(emergency)linetokeepwateranddirtfromenteringtheunusedlines.Thecleanertheairsupplyiskept,thelesschanceofbrakeproblems.Gladhandsandlinesshouldalsobesecuredtopreventthelinesfromchafingagainstvehiclecomponentsorbouncingoffthevehicle.Thiscouldseriouslydamagethegladhandsorlines.

ApplicationLineTheapplicationlineisreferredtoasacontrol(service)line.Thislineisconnectedtothefootandhandvalve.Whenthedriverdepressesthefootvalvetreadleapplicationairwillbedeliveredtothetractorbrakechambersandtothetrailerbrakechambers.Whenthedriverreleasesthefootvalvetreadle,theapplicationairtothetrailerbrakechambersmustreturntothefootvalvetobeexhaustedtotheatmosphere.Thedisadvantagesofthissystemare:· ifthetrailerbrokeawayfromthetractor,thetrailer

wouldnothavebrakes.· ifthecontrol(service)linepartedorruptured,thetrailer

brakeswouldnotbeapplied,andtheapplicationairwouldbelostfromthetractorifthebrakeswereapplied.

· iftheairpressureinthereservoirsislost,therewouldbenowaytoapplythebrakesofthetractororthetrailer.

· thetrailerbrakescannotbeappliedindependentlyfromthetractorandthereisnowaytosetthetrailerbrakeswhencouplingtothetractor.

· theapplicationandreleaseofthetrailerbrakeswouldbeslowerthanthoseofthetractor.

Thesedisadvantagesareovercomebytheadditionofthesupply(emergency)lineandvalvesdiscussedinthefollowingpages.Theillustrationshowsthepipingofaunitwithbrakesapplied,similartothetandemaxlesofthetractor.Alsowithbrakesapplied,thetrailerhastandemaxlesequippedwithbrakechambers.Theapplicationlinehasa“T”insertedbetweenthefootvalve(31)andthetractor’srelayvalve(13).Anairlinehasbeenconnectedfromthis“T”tothetrailerbyasetofcouplers(gladhands)(20).

Air line

Rubber seal

Air line

39

Thepurposeofthetrailerbrakehandvalve(30)istoallowthedrivertocontrolindependentlytheamountofapplicationairpressuretobedirectedtothetrailerbrakes.Italsoprovidesamethodofapplyingthetrailerbrakeswhencouplingthetrailertothetractor.Thevalve,alsoallows

thedrivertoapplythetrailerbrakesindependentlyofthetractor.Theamountofapplicationairpressuredelivereddependsontheamountthevalveisopenedbythedriver.(Itcannotexceedthereservoirairpressure.)Somevalvesareequippedwithselfreturninghandles.

13 20“T”

31

ApplicationLine

30

31

TrailerBrakeHandValve

40

Thedriverhasappliedthebrakesbyusingthefootvalve(31).Applicationairisdirectedtothebrakechambersofthetractorandtothetrailerbrakesthroughatwo-waycheckvalve(26).Theshuttlehasshiftedtothelowpressureside,closingoffany

Note:Thetrailerbrakehandvalveisnottobeusedforparking,asairmaybleedoffiftheengineisstoppedorthehandvalvemovestothereleasedposition.

Two-wayCheckValvesThepurposeofatwo-waycheckvalve(26)istodirectairflowintoacommonlinefromeitheroftwosources.Thisvalvewillpermitairtoflowfromthesourcethatissupplyingthehigherapplicationpressures.Theshuttlewillshiftsothatthehigherpressurewillbedirectedtothetrailerthroughthecontrol(service)line.Thisvalveislocatedbetweenthefoot-operatedvalveandthehand-operatedvalve.

Air from hand valve

Air from foot valve

To trailer

Shuttle

airflowtowardthehandvalveside.Thehandvalve(30)isintheclosedpositionandequalpressureisbeingappliedtothebrakechambersofthetractorandthebrakechambersofthetrailer.

30 26

31

FootValveApplication

41

Inthisillustrationwiththefootvalve(31)releasedandthehandvalve(30)opened,applicationairisdirectedfromthehandvalvethroughatwo-waycheckvalve(26),tothebrakechambers.Thetwo-waycheckvalveinthisapplicationhasshiftedtothelowpressureside,closingoffanyairflowtowardthefootvalveside.Anytimeatrailerbrakeapplicationismadeusingthehandvalve,thedrivermaydepressthefootvalvetreadle.Ifthefootvalveapplicationisofahigherpressurethanthatofthehandvalve,thetwo-waycheckvalvewillshifttothelowerpressureside,allowingthehigherpressuretobedirectedtothetractorandtrailerbrakes.Duringafootvalveapplication,ifthedrivermakesahandvalveapplicationofahigherairpressure,thetwo-waycheckvalvewilldirectthehigherhandvalveairpressuretothetrailerbrakes.Althoughthetrailerbrakesmaybeappliedindependentlybymeansofthehandvalve,themaximumapplicationpressurecanonlybethesameas,orslightlylessthan,reservoirpressure.

TractorProtectionSystemAtractorprotectionsystempreventstotallossofairfromthetractorifthetrailerbreaksaway,oriftheconnectingairlinesbetweentractorandtrailerbecomeseparatedorruptured.Thetractorprotectionsystemconsistsoftwovalves:thetractorprotectionvalveandthetrailersupplyvalve.Othernamesforthetrailersupplyvalveare“trailerparkingcontrol”and“emergencyvalve.”Therearetwotypesoftrailersupplyvalves.Themostcommonistheautomatictrailersupplyvalve.Thisisaspring-loadedvalvethatisopenedmanuallyandheldopenbyairsystempressure.Theotherisamanualtrailersupplyvalve,whichmaybeatoggle-typeswitchorapush/pull-typevalve.Tounderstandthefunctionofthetrailersupplyvalveandthetractorprotectionvalveinthesystem,itisimportanttounderstandhowtheyoperate.

30 26

31

HandValveApplication

42

Italsohastwoairlines:onecomingfromthesupply(emergency)line(21),andonecomingfromthecontrol(service)line(22).Thetractorprotectionvalveisspring-loadedandrequiresapproximately45psiinthesupply(emergency)linetoopenthevalve,thisallowscontrol(service)airtopassthroughthecontrol(service)linetothetrailerwhilemakingabrakeapplication.Whenairlinesfromthetractorarecoupledtoatrailer,theopeningorclosingofthetrailersupplyvalveopensorclosesthetractorprotectionvalve.Disconnectingthesupply(emergency)linebetweenthetractorandtrailerwhilethetrailerischargedwillcauseanimmediatelossofpressureinthesupply(emergency)lineonthetractorcausingthetractorprotectionvalvetoclose,whichinturnclosesoffairflowtothecontrol(service)line.Intheeventofacontrol(service)lineruptureordisconnectionbetweenthetractorandtrailernoactionorairlosswilltakeplaceuntilabrakeapplicationismade.Serviceairwillbelostoutofthedisconnectedorrupturedlinecausingthetractor’sairpressuretodrop.Atapproximately45psithetrailersupplyvalvewillclosecausinganemergencyapplicationofthetrailerbrakesandthetractorprotectionvalvetoclose.Thiswillstopthelossofserviceairatthedisconnectedline.Thetractorprotectionvalvealsoprotectsthetractor’sserviceairfrombeinglostduringnormalbrakeapplicationswhileoperatingthetractorwithoutatrailer.

Atractorprotectionvalve(24)isnormallylocatedneartherearofthetractorcabandhastwoairlinesrunningtoit,onefromthetrailersupplyvalve(28)andtheotherfromthetwo-waycheckvalve(26)fedbythehandorfootvalve.

Air from foot valve

Air from hand valve

Two-way check valve

(26)Control

(service) line (22)

Supply (emergency)

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab)

(28)

Air from reservoir

TractorProtectionSystem(open)(trailercharged)

TractorProtectionValve

To the supply (emergency)

glad hand

To the control

(service) glad hand

43

Totesttheproperfunctionofthetractorprotectionvalve,hookthetractor’sairlinestoatrailer,ensurethevehicleissecureandthewheelsareblockedandreleasetheparkingbrake.Verifythatthesystemisatfullpressure,chargethetrailerbyopeningthetrailersupplyvalve,makeabrakeapplicationandholdit,(useahandbrakevalveifalone.)Disconnectthecontrol(service)line(therewillbealossofair.)Nowdisconnectthesupply(emergency)line.Control(service)airwillstopimmediatelyandsupply(emergency)airshouldcontinuetobelost,dependingonthesystem.Reconnectthesupply(emergency)lineandagainserviceairwillbelost.Thisteststheproperopeningandclosingoperationofthespringportionofthevalve.Atractorprotectionvalvewithabrokenreturnspringwillnotclosethevalveandwillcauselossofairduringnormalbrakeapplicationsifoperatingthetractorwithoutatrailer.

TrailerSupplyValveThisvalve(usuallyaredoctagonalbutton)ismountedinthecabofthetractor.Thedriveropensthevalvebypushingorpullingthebutton,dependingonthetypeused.Openingthevalvepermitsreservoirairpressuretoflowthrough.Thisairpressureispipedtothetractorprotectionvalveandthentothesupply(emergency)line.Thevalveisspring-loadedandwillbeheldintheopenpositionwhensufficientpressureisreached.Iftheairpressuredropstoarangebetween45and20psi,thevalvewillcloseautomaticallybyspringpressure,openingtheexhaustport.Thedrivercanclosethevalvemanually.Thiswillopentheexhaustportwithoutthepressurebeingdepletedfromthetractorreservoirs.Thespringparkingbrakeswillalsobeapplied.

Air from foot valve

Air from hand valve

Two-way check valve

(26)

Supply (emergency)

line (21)

Tractor protection valve (24)

Trailer supply valve (mounted in cab)

(28)

TractorProtectionsystem(closed)(trailernotcharged)

Air from reservoir

Control (service) line

(22)

44

30 28

31

2624

20

22 21

28 26

24

22 21

20

AutomaticTrailerSupplyValveSystemThediagrambelowillustratesairbeingpipedfromthedryreservoirlinetothetrailersupplyvalve(28).Thetractorprotectionvalve(24)isfedbytwolines:onefromthetrailersupplyvalve(28)andonefromthetwo-waycheckvalve

(26).Leadingofffromthetractorprotectionvalvearetwolines,eachwithagladhandcoupler(20).Thesetwolinesarereferredtoasthecontrol(service)line(22)andthesupply(emergency)line(21).

Inthediagramstheupperlineisthecontrol(service)line(22)andthelowerlineisthesupply(emergency)line(21).Illustratedisatractorequippedwithatrailersupplyvalve(28)andatractorprotectionvalve(24).Thetrailer

isnotcoupledandthetractorisbeingoperatedalone(“bobtailing”).Thedriverhasnotopenedthetrailersupplyvalve(28)andthehandvalve(30)isclosed.

45

31

2624

30 28

20

31

2624

30 28

20

Inthisillustration,thedriverhasmadeabrakeapplicationwiththefootvalve(31)andapplicationairisbeingdeliveredtothetractorbrakechambers.Thetwo-waycheckvalve(26)hasshiftedtothelowpressureside,allowingapplicationairtoreachthetractorprotectionvalve(24).

Thereisnoairlossfromthetractorthroughthedisconnectedgladhands(20).

Ifthedriver,bymistake,appliedthehandvalve(30)withthetrailerdisconnected,theapplicationairdirectedtothetractorprotectionvalvewouldalsobedead-ended.Again,

noairlosswouldoccurifthetrailersupplyvalve(28)isintheclosedposition.

46

Inthisillustration,thetrailerhasbeencoupledtothetractor,andthecontrol(service)andsupply(emergency)lineshavebeencoupledusinggladhands(20).Thetrailerunithasareservoir(16)installed.Thisreservoirwillprovideavolumeofairnearthetrailerchambersfornormaloremergencybraking.Thetrailerreservoirisfittedwithadrainvalve(6),thesameasatractorreservoir.Arelayemergencyvalve(39)ismountedonthetrailerreservoirortothetrailerframenearthebrakechambers.Therelayemergencyvalveservesthreemainfunctionsinthesystem:1. Itrelaysairfromthetrailerreservoirtothetrailerbrake

chambersduringabrakeapplication.Thispartofthevalveoperatesliketherelayvalvepreviouslydiscussed.Italsoprovidesaquickreleaseofthetrailerbrakes.

2. Itdirectstrailerreservoirairpressuretothetrailerbrakes,causinganemergencyapplicationofthebrakes.Thisactionwilloccurautomaticallyintheeventofrupturedorpartedairlinesbetweentractorandtrailer,orlossofairpressurefromthepowerunitservicesystem.Arupturedcontrol(service)linewouldnotcauseanemergencybrakeapplicationonthetraileruntilabrakeapplicationwasmade,andthiswouldcausearapidlossofairpressurefromthesystem.Thedriveratanytimemayoperatethecab-mountedtrailersupplyvalve(28)tocauseanemergencyapplicationofthetrailerbrakes.

3. Ithasaone-waycheckvalvewhichstopsairpressureinthereservoirfromgoingbacktothesourceofthesupply.

20

16

6

39

28

TractorandTrailerCoupled

47

Intheillustration,thecompressorhasraisedthereservoirairpressuretomaximum.Thedriverhasopenedthetrailersupplyvalve(28)toallowthereservoirairpressuretobedirectedthroughthetractorprotectionvalve(24)tothetrailer.Theairpressurepassesthroughtherelayemergencyvalve(39)tothereservoiron

thetrailer.Pressurewillbuildupinthetrailerreservoirtothesamepressureasthereservoirsonthetractor.Thisisknownas“charging”thetrailersystem.Thetrailersupplyvalveshouldbeopenwhenthetractorpressurehasbuiltuptoapproximately90psi,dependingonthevalve.

Thisandthenextillustrationaretoonlyshowwhichbrakecomponentsandlinesareusedforafootvalveandhandvalveapplication.· Applicationwiththefootvalve(31)inorange.· Applicationwiththehandvalve(30)inbrown.Orangeandbrownareusedtoshowwhereairisappliedanddoesnotrepresentthesourceoftheair.Whenthefoot

valveisdepressed,theapplicationairwillbeappliedtothetractorbrakesandthetrailerbrakestogether.Aspreviouslyexplained,thetwo-waycheckvalve(26)hasshifted,andapplicationairisbeingdirectedthroughthetractorprotectionvalve(24)tothecontrol(service)line.Ifthefootvalveisreleasedandthehandvalveisapplied,theshuttleinthetwo-waycheckvalvewouldshiftandapplicationairwouldapplythetrailerbrakesonly.

39

2428

ChargingtheTrailerSystem

31

24

30

26

FootorHandValveBrakeApplication

48

14

39

14

16

30

31

Applicationairdirectedfromeitherthefootorhandvalvecausescontrolpressuretomovethroughthecontrol(service)lineandactontherelayemergencyvalve(39).Thiscontrolpressurewillcausetherelayemergencyvalvetodirectairfromthetrailerreservoir(16)tothetrailerbrakechambers(14).Thepressuredirectedbytherelayemergencyvalvetothetrailerbrakechamberswillbethesameasthepressuredirectedtothetractorbrakechambers.Insuchasystem,brakelagtimehasbeenminimizedwiththeadditionofthetrailerreservoirandrelayemergencyvalve.

Releaseofthefootorhandvalvestopstheflowofapplicationair.Therelayportionofthevalvereturnstoitsoriginalposition,stoppingtheflowofairpressure.Theexhaustingportsofthevalveexhausttheairpressurefromthebrakechambers,releasingthebrakes.Inthissystem,thebrakesofboththetractorandtrailercanbereleasedquickly.Caution:Trailerbrakesmustnotbeusedtoholdaparkedvehiclethatisleftunattended.Lossofpressurewillresultinlossofbrakes.Alwayssettheparkingbrake.

14

39

14

22 21

16

EmergencyApplication

49

21

14

14

39

16

Ifatrailerthatisnotequippedwithspringparkingbrakesweretoseparatefromthetractor,thecontrol(service)line(22)andthesupply(emergency)line(21)woulddetachfromthetractor.Thesuddenlossofairpressureinthesupply(emergency)linewilltriggertherelayemergencyvalve(39)whichcausesthetrailerreservoir(16)todumpitsairdirectlytothetrailerbrakechambers(14).Thisplacesthetrailerbrakesintoemergencyapplication.Thecheckvalveintherelayemergencyvalvewillclose,preventing

anybleedbackofpressurefromthetrailerreservoir.Thelossofpressureinthetrailersupply(emergency)linewillcausethetractorprotectionsystemtosealoffthetractorleavingsufficientairpressuretobrakethetractor.Thetrailerbrakeswillremainapplieduntileitherthepressureinthetrailerreservoirandlinesisdrainedoff,orthesupply(emergency)lineisrepairedandthesystemisrecharged.

Control(Service)LineRuptureIfthecontrol(service)line(22)isrupturedordisconnected,noactionwilltakeplaceuntilabrakeapplicationismade.Whenitismadebyeitherthefootorhandvalve,theairlossfromthecontrol(service)linewilllowertheairpressureinthetractor’sreservoirsquickly,dependingontheamountofbrakingdemanded.Thisairlosswilleventuallyactivatethetractorprotectionsystemwhichinturnwillexhaustthetrailersupply(emergency)lineandcausethetrailerrelayemergencyvalvetoapplythetrailerbrakes.Itshouldbenotedthatanyproblemthatcausesaseveredropinreservoirpressureonthetractorsystem,willcausethelowwarningdevicetosignalthedriver.

Inthefollowingillustration,thecontrol(service)line(22)hasrupturedandthedriverismakingabrakeapplicationwiththefootvalve(31).Thetractorwillhavebrakesappliedbutthetrailerwillhavenobrakeaction.Ifthebrakeapplicationisheld,theairpressureinthetractorsystemwilllowertoadangerouslylowlevelandthenthetractorprotectionsystemwillplacethetrailerbrakesintoanemergencyapplication.

Supply(Emergency)LineRuptureAruptureofthesupply(emergency)line(21)oranuncouplingofthesupply(emergency)linegladhandswilltriggerthesameactionasdescribedabove.

50

Ruptureofthecompressordischargelinewouldresultinlossofairpressurefromthesupply/wetreservoir.Whentheairpressureinthesupply/wetreservoir(5)ofthetractorfallsbelowthewarninglevel,duetoacompressorfailureorexcessiveleakageonthetractor,thewarningdeviceswillstarttooperate.Inthediagram,theone-waycheckvalve(7)haspreventedthereservoirairpressureintheprimary/dryreservoir(8)fromescapingbacktothesupply/wetreservoirandtherupturedline.

Thereissufficientreservoirairpressureintheprimary/dryreservoirforalimitednumberofbrakeapplicationstostopthevehiclebeforethespringparkingbrakesareactivated.(Thiswilldependonhowthespringparkingbrakesarepipedinthesystem.)

22

31

Control(Service)LineRupture

5 7 8

LossofReservoirAirPressure

51

Inthisillustrationthepressurehasbeenloweredtoapproximately45-20psiandthetractorprotectionsystemhasclosedautomatically,placingthetrailerbrakesintoanemergencyposition.Also,thespringparkingbrakessystemhashadtheairpressurereleasedactivatingthespringparkingbrakes.

Thetruckprotectionsystemdescribedisanexampleofatractorequippedwithatypeofcab-mountedtrailersupplyvalve(28)whichwillcloseautomaticallywhentheairpressureinthesupply(emergency)line(21)dropsbelow45-20psi.Thevalvemayalsobeclosedmanually.

ManualTrailerSupplyValveSomeoldertractorsmaybeequippedwithadifferenttypeofcab-mountedtrailersupplyvalvewhichmustbeoperatedmanuallybythedriver.Ithastwopositions:normalandemergency.Thetractorwillbeequippedwithatractorprotectionvalve,andthetrailerunitwitharelayemergencyvalve,asintheprevioussystem.Thefunctionsofthetrailersupplyvalve,tractorprotectionvalveandtherelayemergencyvalvewillbesimilartothoseexplainedpreviously.However,thereisoneimportantdifference.Intheeventoftractorreservoirairloss,thetrailersupplyvalvemustbeshiftedtotheemergencypositionmanuallytosealoffthetractor.

Anytimethedrivershiftsthecab-mountedtrailersupplyvalvetotheemergencyposition,andthetrailersystemischarged,thetrailersupplyvalvewillexhaustthesupply(emergency)line,whichcausesthetrailerreservoirtodumpitsairdirectlytothetrailerbrakechambers.Thetrailerbrakeswillremainappliedonlyaslongasairpressureremainswithinthetrailersystem.Howlongtheairinthesystemwillholdthebrakesapplieddependsonhowairtightthesystemis.Asasafetyprecaution,parkedtrailerswithoutspringparkingbrakesshouldalwayshavethewheelsblockedtopreventapossiblerunaway.Tomoveatrailerthathasbeenparkedwiththebrakesinanemergencyapplication,itisnecessarytochargethesystemtoreleasethetrailerbrakes.

2128

LossofReservoirAirPressure

52

22

21

18 17

15

16

TrailerSpringParkingBrakesSpringparkingbrakesarenowcommonlyfoundontrailers.Springparkingbrakesserveasameansofsecuringaparkedtrailerwhetheritisattachedtoatractorornot.Thespringparkingbrakesareappliedwithspringpressureandnotairpressure,sothereisnoriskoftheparkingbrakesreleasingandthetrailermoving.Theyalsoactasanemergencybrakingsystemifthetrailerweretobreakawayfromthetractororifthetractorlostadequateairpressure.Atrailerwithspringparkingbrakeshasthesecomponents:· frontandrearservicereservoirs(16)· trailerspringparkingbrakevalve(18)· relayvalve(17)(thesameasonatractor-notan

emergencyrelayvalveusedontrailerswithoutspringparkingbrakes)

· springparkingbrakechambers(15)(sameasonatractor)

Thetrailerspringparkingbrakevalveisresponsibleforseveralimportantfunctions:1) Itcontrolstheapplicationandreleaseofthetrailer’s

springbrakes.2) Itprotectsandisolatesthefrontservicereservoir

fromtherearservicereservoir.Thisisimportantasitpreventsanautomaticapplicationofthetrailerspringbrakesifoneofthereservoirsweretoloseairpressure.

3) Itpreventsautomaticspringbrakeapplicationifthetrailersupplylinehasagradualleak.

4) Itwillautomaticallyapplythespringparkingbrakesifsupplypressureisrapidlylost(ie:trailerbreakaway).

SectionSummaryQuestions1. Whatisthepurposeofatwo-waycheckvalve?2. Whyshouldthegladhandsbeprotectedwhennotin

use?3. Howcanadrivercontrolthetrailerbrakes

independently?4. Shouldthehandvalvebeusedforparking?5. Whatisthepurposeofthetractorprotectionvalve?6. Howcanyoutesttheproperfunctionofthetractor

protectionvalve?7. Whatisthepurposeofthetrailersupplyvalve?8. Namethreefunctionsoftherelayemergencyvalve.9. Describethefunctionofthesupply(emergency)line.10. Describethefunctionofthecontrol(service)line.

11. Whatwilloccurifthesupply(emergency)lineruptures?

12. Whatwilloccurifthecontrol(service)lineruptures?13. Whatwilloccurifabrakeapplicationismadewitha

rupturedcontrol(service)line?14. Ifthefootvalveandthehandvalveareoperatedatthe

sametime,cantheapplicationairpressurebegreaterthanthereservoirairpressure?

15. Whyisatrailerequippedwithspringparkingbrakes?16. Whatbrakecomponentsareonatrailerequippedwith

springparkingbrakes?17. Whatarethefourfunctionsofthetrailerspring

parkingbrakevalve?

53

SECTIONSIX-

DUALAIRBRAKESYSTEM

54

Moreandmoreheavydutyvehiclesontheroadtodayareusingadualairbrakesystem.Thesystemhasbeendevelopedtoaccommodateamechanicallysecuredparkingbrakethatcanbeappliedintheeventofservicebrakefailure.Italsoaccommodatestheneedforamodulatedbrakingsystemshouldeitheroneofthetwosystemsfail.Itisactuallytwobrakesystemsinone,withmorereservoircapacityresultinginamuchsafersystem.Atfirstglance,thedualsystemmightseemcomplicated,butifyouunderstandthebasicairbrakesystemdescribedsofar,andifthedualsystemisseparatedintobasicfunctions,itbecomesquitesimple.

Asitsnamesuggests,thedualsystemistwosystemsorcircuitsinone.Therearedifferentwaysofseparatingthetwopartsofthesystem.Onatwo-axlevehicle,onecircuitoperatestherearaxleandtheothercircuitoperatesthefrontaxle.Ifonecircuithasafailure,theothercircuitisisolatedandwillcontinuetooperate.

BasicDualAirBrakeSystem

55

Intheillustration,airispumpedbythecompressor(1)tothesupply/wetreservoir(5)(blue),whichisprotectedfromoverpressurizationbyasafetyvalve(4).Pressurizedairmovesfromthesupply/wetreservoirtotheprimary/dryreservoir(8)(green)andthesecondary/dryreservoir(10)(red)throughone-waycheckvalves(7).Atthispoint,thedualcircuitsstart.Airfromtheprimary/dryreservoirisdirectedtothefootvalve(31).Airisalsodirectedfromthesecondary/dryreservoirtothefootvalve.Thefootvalveissimilartotheonedescribedearlierinthebasicairbrakesystem,butisdividedintotwosections(twofootvalvesinone).Onesectionofthisdualfootvalvecontrolstheprimarycircuitandtheothercontrolsthesecondarycircuit.Whenabrakeapplicationismade,airisdrawnfromtheprimaryreservoirthroughthefootvalveandispassedontotherearbrakechambers.Atthesametime,airisalsodrawnfromthesecondaryreservoir,passesthroughthefootvalveandispassedontothefrontbrakechambers.Ifthereisairlossineithercircuit,theotherwillcontinuetooperateindependently.Unlessairislostinbothcircuits,thevehiclewillcontinuetohavebrakingability.Theprimaryandsecondarycircuitsareequippedwithlowairpressurewarningdevices,whicharetriggeredbythelowairpressureindicatorswitch(9)andreservoirairpressuregauges(29)locatedonthedashofthevehicle.

Treadle

Supply ports

Exhaust port

Delivery ports

Dual-circuitFootValve

1 4 5 7 98 10

31

29DualAirBrakeSystem

56

Whenspringbrakesareaddedtoadualairbrakesystem,thesametypeofdashcontrolvalvediscussedpreviouslyisused.Blendedairisusedtosupplythespringparkingbrakecontrolvalve(27).Blendedairisairtakenfromtheprimaryandsecondarycircuitsthroughatwo-waycheckvalve(26).

Withthispipingarrangementthevehiclecanhaveafailureineithercircuitwithoutthespringbrakesapplyingautomatically.Ifairislostinbothcircuits,thespringbrakeswillapply.

27 26

DualAirBrakeSystemwithSpringParkingBrakes

57

Springparkingbrakesinthissystemservetwopurposes:first,asaparkingbrake,andsecondasanemergencybrakingsystem.Ifafailureoccursintheprimarycircuit(green),andabrakeapplicationismade,controlairfromthefootvalveisdirectedtoaspringbrakemodulatorvalve(23).Asthereisnosupplyairtomaintainbalanceinthemodulatorvalve,becauseoftheprimarycircuitfailure,themodulatorvalvethenexhaustsairpressurefromthespringparkingbrakecircuit.Theamountofairreleasedisequaltotheamountofairappliedbythefootvalve.Thereleaseofairinthespringparkingbrakecircuitcausesthedriveaxletobrakeusingspringpressure(12).Whenthebrakesarereleased,supply

airfromthesecondarycircuit(red)returnsthespringparkingbrakestoanoffposition.Brakeapplicationscanberepeateduntilalltheairfromthesecondarycircuitislost.Howeverastheairpressuredropsbelow85psi,thespringparkingbrakeswon’treturntothefulloffposition,infacttheywillstarttodrag.Atapproximately35psi,thespringparkingbrakecontrolvalve(27)onthedashwillexhausttheremainingairinthesecondarycircuit,andthespringparkingbrakesarefullyapplied.Theonlywaythevehiclecanbemovedafterallairislostistorepairthedamagedcircuitandrechargethesystem,orcagethespringparkingbrakesystem.

27 23 12

12

SpringParkingBrakeswithModulatorValve

58

28 18 15

15

17

2426

CombinationTractorandTrailerwithSpringParkingBrakesThetrailersystemissuppliedbyblendedtractorairtakenfromtheprimaryandsecondarycircuitsthroughatwo-waycheckvalveaspreviouslydescribed.Thesystemischargedbyopeningthetrailersupplyvalve(28),allowingairfromthetractortopassthroughthetractorprotectionvalve(24)andthetrailerspringparkingbrakevalve(18)directlyintothetrailerspringparkingbrakechambers(15).Whenairenters,thepressureprotectionpartofthetrailerspringparkingbrakevalveopens,allowingtheairtofillthetrailerreservoirs.Thetrailerspringbrakeswillnotreleaseuntilthereservoirpressureonthetrailerisadequate.Whenabrakeapplicationismade,blendedcontrolairactsontherelayvalve(17),whichreleasesairfromthetrailerreservoirtothebrakechambers.Inadualairbrakesystem,ifonecircuitdevelopsaleak,theothercircuitwouldbeprotectedfromairpressurelossbythetwo-waycheckvalve(26).Ifthetractorbreaksawayfromthetrailer,thecontrol(service)andsupply(emergency)lineswillbepulledapart.Thesuddenlossofairinthesupply(emergency)linewillcausethetractorprotectionvalvetoclose,preventingairfromescapingoutofeitherbrokenconnection.Theair

supplyinthetractorissealedoffandisavailabletocontrolthetractorbrakes.Atthesameinstant,thesuddenlossofairinthesupply(emergency)linecausesthetrailerspringparkingbrakevalvetoexhausttheairfromthetrailerspringparkingbrakechambers,applyingthetrailerbrakes.Thetrailerbrakescannotbereleasedundertheseconditionsunlessthelinesarere-coupledandthetrailerreservoirsrecharged.Ifonlythesupply(emergency)linebreaksbetweentractorandtrailer,thesamesequenceofeventswilloccur.Abreakorruptureinthecontrol(service)linewillnotaffectthetraileruntilabrakeapplicationismade.Alossofpressureinthetractorsystemwillthenresult,causingthesameemergencybrakeapplicationdescribedabove.However,thedriverwillbeabletoreleasethespringparkingbrakesbyreleasingthefootvalve,rebuildingairpressureandopeningthetrailersupplyvalve.Toapplythespringparkingbrakes,thespringparkingbrakecontrolvalve(27)isclosed,causingalossofairpressureinthelinewhichappliesthespringparkingbrakesasdescribedabove.Theoldandnewtractorandtrailersystemsarefullyinterchangeable,whethertheyareadualairbrakesystemorbasicairbrakesystem,andwhethertheyaresystemswithorwithoutspringparkingbrakes.

59

SectionSummaryQuestions1. Whatisthebasicprincipleofthedualairbrakesystem?

2. Whatvalveisusedtoprotecttheprimarycircuitfromthesecondarycircuit?

3. Inadualairbrakesystem,willthevehiclecontinuetohavebrakingabilityifonecircuitfails?

4. Isthereadifferencebetweenthefootvalveusedinabasicairbrakesystemandthefootvalveusedinthedualairbrakesystem?

5. Nametwofunctionsofthespringparkingbrakesinadualairbrakesystem.

6. Describethefunctionsofthespringparkingbrakemodulatorvalve.

7. Ifthetrailerbreaksawayfromthetractoronadualairbrakesystem,whatappliesthebrakesonthetrailer?

8. Whatisblendedair?

9. Canatrailerwithabasicairbrakesystembetowedbyatractorwithadualairbrakesystem?

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61

SECTIONSEVEN-

ELECTRONICCONTROLLEDBRAKINGANDTRACTION

SYSTEMS

62

Anti-lockBrakeSystem(ABS)Ananti-lockbrakesystemisanelectronicsystemthatmonitorswheelspeedatalltimesandcontrolswheelspeedwhilebraking.Ifitdetectsawheellocking-upduringabrakeapplication,thesystemreleasesbrakepressuretothatwheelonly.Thiskeepsthewheelfromskiddingandincreasesvehiclestabilityandcontrolduringemergencystopsandinadversesituationssuchasonwetoriceslickedroads,throughcurvesorduringlanechanges.TheairbrakesystemremainsthesamewiththeadditionoftheABScomponents.AnABSbasicallyconsistsof:· Anelectroniccontrolunit(ECU)· Awheelsensorandtoothwheel· ABSvalves

TheECUisthebrainofthesystem.Thewheelsensorsarelocatedonaminimumoftwo,oruptoallsixsetsofwheels,dependingonthesystem.ThewheelsensorsconstantlysendinformationtotheECU.Whenabrakeapplicationismadeandthesystemdetectsawheellocking-up,theECUsendsamessagetotheABSvalvestoreleasebrakepressureonthatparticularwheelpreventinglockup.Usuallythereisawarninglightlocatedonthedashtoinformthedriverofthesystemoperation.Everydrivershouldhavefullunderstandingofthewarninglightoperationforthevehicletheydrive.WhendrivingavehiclewithABSapplythebrakesnormally.WhentheABSstartsworking,donotreleasethepressureyouhaveappliedtothebrakepedal.AvoidpumpingthebrakepedalastheABSautomaticallyappliesandreleasesthebrakesuptofivetimesasecondwhichismuchfasterthanyoucanpumpthebrakepedal.ForoptimumABSoperation,thedrivershouldmakeabrakeapplicationandhold,allowingtheABStocontrolbrakingpressureateachwheel.Whenusinganenginebrakeandyouencounteraslipperyroadsurface,theABSwilldetectthewheellockupandautomaticallyturnofftheenginebrakeuntiltractionisregained,thenresumeenginebraking.Forinformationspecifictothevehicleyoudrive,refertotheoperator’smanual.TrailersmayalsobeequippedwithABSandthesystemwilloperatemuchthesameasonthetractor.However,trailerswithABSwillhaveasystemwarninglighttypicallymountedonthetrailer’sleftfrontcorner,visibleintheleftsidemirror.Onsomesystemstheymayhaveanextrawarninglightonthedashofthetractor.TractorsandtrailerswithorwithoutABScanbeconnectedtooneanotherwithoutaffectingtheoperationoftheABS.

Sensor block

Tooth wheel

ToothWheelandSensorBlock

Wheel

Axle

63

Foursensor-fourmodulatorvalveanti-lockbrakesystem

(E.C.U.)

Relay block

Tooth wheel and sensor block

Tooth wheel and sensor block

Modulator valves

Modulator valves

Brake chambers

Brake chambers

Brake pedalBrake

chambers

Dash lamp

Tooth wheel and sensor block

Tooth wheel and sensor block

64

AutomaticTractionControl(ATC)Automatictractioncontrolisanelectronicsystemthatmonitorswheelspinonaccelerationandusescontrolledbrakingtogaintraction.Thissystemreducesthepotentialofpoweringintoajackknifecausedbywheeloverspinduringacceleration,alongwithimprovingtheabilityofthedrivertomanoeuvrethevehicleonslipperysurfacesthroughcurvesandlanechanges.ATCisanoptiononlyavailableonvehiclesequippedwithABS.Thesystemusestwofunctions:differentialbrakingandelectronicenginecontrol.DifferentialbrakingoccurswhenadrivewheelbeginstospincausingtheATCtoautomaticallyapplythebrakeonthatwheelandtransferpowertotheotherdrivingwheels.Differentialbrakingisactivatedwhenthesystemdetectsawheelslippingbecausethewheelsoperatingondifferentsurfacessuchasonewheelondrypavementandtheotheronice.ElectronicenginecontrolisactivatedautomaticallybytheATCifallthewheelsbegintoslip.Enginepowerisreducedtoattainoptimumtire-to-roadtractionwithoutdriverintervention.Whenavehicleisbeingoperatedwiththecruisecontrolsetandwheelslipisdetected,theATCwillautomaticallycancelthecruisecontrolsetting.Thiswillaidthedriverinmaintainingcontrol.Cruisecontrolshouldnotbeusedonslipperysurfaces.Awarninglightislocatedonthedashtoinformthedriverwhenwheelslipisdetected.TheATCsystemusesmanyofthesamesensingandcontrolcomponentsastheanti-lockbrakesystem.Forinformationspecifictothevehicleyoudrive,refertotheoperator’smanual.

SectionSummaryQuestions1. Whatisthepurposeoftheanti-lockbrakesystem?

2. Whatthreecomponentsdoesananti-lockbrakesystemconsistof?

3. Howshouldyouapplythebrakeswhenbrakingonwetoricyroadsinavehicleequippedwithanti-lockbrakes?

4. Canatrailerwithanti-lockbrakesbeconnectedtoatractorwithoutanti-lockbrakesystem?

5. Whatisthepurposeoftheautomatictractioncontrolsystem?

6. Whattwofunctionsdoesthesystemusetocontroltraction?

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SECTIONEIGHT-

BRAKEADJUSTMENTANDIN-SERVICECHECK

66

BrakeAdjustmentOnvehiclesequippedwithhydraulicbrakesitispossibletopumpthebrakepedaltocompensateforbrakesthatareoutofadjustment.Thisisnotpossiblewithavehicleequippedwithanairbrakesystemwithmanualslackadjusters.Whenthebrakesareadjustedbyslackadjusters,theshoesaremovedoutwards.Thisbringsthemasclosetothedrumsaspossibleminimizingtheamountoffreetravelwhenthebrakesareapplied.Italsoreducesthevolumeofairusedtoapplythebrakes.Brakeadjustment(pushrodtravel)mustbecheckedaspartofthepre-tripairbrakeinspection(Section9).

S-camBrakeThefollowingarerecommendedstepstodetermineifanS-cambrakewithmanualorautomaticslackadjustersrequiresadjustment.· Ensurevehicleissecureandwheelsareblocked.· Shutofftheengine,leavingthetransmissioninalow

gearorparkthenreleasethespringparkingbrakes.· Makeachalkmarkwhereeachpushrodentersthebrake

chamber.· Reapplythespringparkingbrakesandmeasurethe

distancefromthebrakechambertothechalkmark.Ensuretheslack(pushrodtravel)iswithin3/4and11/2inchesorwithinmanufacturer’sspecificationsandtheanglebetweentheslackadjusterandpushrodis90°orascloseaspractical.Ifnot,thenabrakeadjustmentisrequired.

Whenthebrakesareoutofadjustment,brakingefficiencyisreducedbythreefactors:1. Brakelagincreasesbecauseadditionalairisrequired

tofillandpressurizetheincreasedchambervolumecausedbytheincreasedstrokeofthepushrod.

2. Theanglebetweentheslackadjusterarmandthepushrodbecomesmorethan90°,whichresultsinalossofforcebetweentheliningsandthedrum.(seediagramonpage18)

3. Theeffectivenessofdiaphragmbrakechambersdropsoffsignificantlyifthestrokeexceeds75%ofitsdesignedtravel.Foratype30chamber(30squareinchesofeffectivediaphragmarea)thathasadesignstrokeof21/2inches,thebrakesshouldbeadjustedatorbefore11/2inchesoftravel.Withaworkingpressureof100psithischamberwillproduceaforceof3,000lbat11/2inchesoftravel,butonly2,500lbofforceatatravelof21/4inches.Remember,whenachamberbottomsout,theforcereducestozero.

Chalk mark

Brakeoff

BrakeonChalk mark

Ruler

67

Otherfactorsthataffectthebrakingabilityofthevehicle,include:· overheatedbrakes;· wornlinings;· oversizedrums;· malfunctioningvalves;· valveswithabovenormalreleasepressure;· seizedorpartiallyseizedbrakecamshaftsorshoe

anchors;· excessivemoistureinreservoirs;· contaminatedbrakelinings.

Brakefailuresareseldomtheresultofcatastrophicsystemfailure.Theyareusuallytheresultofbrakesbeingoutofadjustment,oftentothepointthatthereisnobrakingactiononsomewheels.Underfrequentbrakingconditionsanddependingontheseverity,therewillbesomeexpansionofthebrakedrumbecauseofheatandtheamountwillvarywiththethicknessofthedrum.Thisexpansionofthedrumwillalsoaddtotheoverallpushrodtravel.Thedrivershouldbetrainedinbrakeadjustmentpriortoattemptingthepracticalexamination.

Strokevs.ForceTheamountofforceavailableatthepushrodisconsistentouttotwoinchesofstroke.Aftertwoinches,push-rodforcedropsveryquickly.

1 1 - 1/2 2 2 - 1/20 1/2 3

4000

3000

2000

1000

0

Recommended max. stroke at 100 psi before readjustment

Air chamber bottom-out

Push

-rod

Forc

e (in

lb) a

t 100

psi

Push-rod Stroke (in inches)

Don’tbeFooled-ChecktheSlack

Itisuptoyou,thedriver,toensurethatyourvehiclehassafe,properlyadjustedbrakes.

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S-camBrakeAdjustmentwithManualSlackAdjuster

· Ensurevehicleissecureandwheelsblocked,releasetheparkingbrake.

· Verifysystemisatfullpressure.· Useaproperwrenchtodisengagetheexternallocking

device(ifsoequipped)fromtheadjustmentboltoftheslackadjuster.

· Turntheadjustmentboltuntiltheliningcontactsthedrumandtheadjustmentboltwillnotturnanyfurther.Ifpossible,visuallychecktoseethatthebrakeliningsareincontactwiththebrakedrum.Pullorpryontheslackadjuster.Thereshouldbenomovementorfreeplay.Ifthereisfreeplay,youhaveturnedtheadjustingnutthewrongway.

· Whenturningtheadjustmentboltontheslackadjuster,theS-camshouldturninthesamedirectionasifabrakeapplicationwerebeingmade.

· Backofftheadjustmentboltabout1/4to1/2ofaturnandensurethelockingdevicereengages.

· Nowrecheckslackadjustertravel.Thetravelshouldnowbebetween1/2to3/4inchesfreetravelwhilepullingonthelinkage,orwithinmanufacturer’sspecifications.

Note:ApplicantsconductinganairbrakeexaminationwillberequiredtosatisfactorilyadjustS-cambrakeswithmanualslackadjusters.

S-camBrakewithAutomaticSlackAdjusterAutomaticslackadjustersadjustthemselvesduringbrakeapplicationstoaccommodateforbrakelininganddrumwear.However,theymustbecheckeddailytoensuretheyaremaintainingproperpushrodtravelwhichnormallyistwoincheswhenthebrakeisapplied.Iftheyarebadlyoutofadjustment,aqualifiedmechanicshouldinspectthem.

Adjustingautomaticslackadjustersbyhandisnotrecommendedunlessyouarethoroughlytrainedontheproperadjustmentproceduresasdictatedbythemanufacturer.Ifanautomaticslackadjusterrequiresadjustingbyhandtheslackadjustermaynotbeworkingproperlyandshouldbeinspectedorreplacedbyaqualifiedmechanicassoonaspossible.

DiscBrakeAdjustmentThereareanumberofmakesandmodelsofairdiscbrakes,eachwithadifferentadjustmentprocedure.Itisthereforerecommendedthatyouconsultthemanufacturer’smanualforadjustmentorservice.

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WedgeBrakeAdjustmentManualadjusters:· Hoistorjackwheelsofftheground.· Removedustcoverfromadjustingslotsattwoplaceson

eachbrake.Ontwinchamberunitstheadjustingslotsareinthebackingplatebelowtheforwardchamberandabovetherearchamber.Ifstarwheeladjustingboltsarenotfoundatthesepositions,thebrakehasbeenassembledonthewrongsideofthevehicle.Thesinglechamberunitshaveadjustingslotsinthebackingplateoneachsideofthechamber.

· Adjustingboltshaveright-handthreads.Turningthewheelbyhand,useanadjustingspoontoturnthestarwheeluntiltheshoedevelopsaheavydragonthedrum.Thenbackoffthebolttoaverylightdragonthedrum.

· Repeatfortheothershoeonthatbrakeassembly.· Replacethedustcoversintheadjustingslots.· Repeattheabovestepsontheotherbrakeassemblies.

AfteraBrakeAdjustmentAtthefirststopafteradjustingthebrakes,checkeachbrakedrumorrotorfortemperature.Anextrahotbrakedrumindicatesthatyouhaveoveradjustedit.Onlywhenallbrakesareproperlyadjustedwillthesystembeabletoabsorbtherequiredamountofenergytobringtheloadedvehicletoasafestopunderextremeconditions.Rememberthatpoorlyadjustedairbrakesmayappeartobeworkingatslowspeedsonlightairapplication.Theonlywaytobesureyourvehiclebrakesareproperlyadjustedistophysicallycheckthebrakesateachwheel.

In-serviceChecksInsomeareassignsarepostedinadvanceofsteeporlongdowngrades.Thesesignsindicatethatthedrivermuststopthevehicleinthepulloutareaandinspectthebrakingsystembeforeproceeding.Checkthat:· thecompressorismaintainingfullreservoirairpressure;

· pushrodtraveliswithinlimitationsonallchambers;· therearenoairleaks;· thegladhandsandthelinesaresecure;· thedrums,bearingsandtiresarenotoverheating;· thetrailersupplyvalveisoperatingproperly.

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Thisshouldbeusedasanin-servicecheckonlyandnotmistakenasadailypre-tripairbrakeinspection.Thedrivermustbeawareoftheconditionofthevehicle’sbrakingsystematalltimes.Thiscanbedoneby:· watchingthepressuregauges;· hearingthewarningsignals;and· feelingthebrakingresponseofthevehicle.

Bythesemethodsofobservation,thedrivershouldbeabletonoticeanydefectsdevelopinginthebrakingsystemandbeawarethatserviceoradjustmentisrequired.

MaintenanceandServicingoftheAirBrakeSystemEnsuringthatthevehicleisinproperconditiontodriveistheresponsibilityofeveryoneinvolved.Preventivemaintenancechecksaredesignedtopreventmechanicalfailuresthatcouldresultfromneglect.Thesecheckscutrepaircostsbydetectingminormechanicaldefectswhichwouldotherwisedevelopintomajorbreakdowns.Preventivemaintenancemaybeassignedtoamaintenancecrewortothedriver.Howeverthedrivermustensurethatthebrakingsystemisinoperatingconditionbeforethevehiclemoves.

SectionSummaryQuestions1. ShouldalldriversbeabletoadjustS-cambrake

systems?

2. Whattestsmustbemadeata“truckstophere”signpostedbeforeasteeporlongdowngrade?

3. DothebrakeadjustmentspecificationsdifferbetweentheS-camandthediscbrakessystem?

4. Afteranadjustmenthasbeenmadewhenshouldthebrakesbechecked?

5. Whatisbrakefade?

6. Whatisthemaincauseofbrakefailure?

7. Whatarethreewaystochecktheconditionofavehicle’sbrakingsystem?

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SECTIONNINE-

PRE-TRIPAIRBRAKEINSPECTION

72

SingleUnit(Notforairoverhydraulicbrakesystems)Whenupgradingyourdriver’slicencefromsingletocombinationvehicleswithairbrakes,youwillberequiredtosatisfactorilydemonstratethecompletepre-tripairbrakeinspectionforcombinationvehicles.Thepre-tripairbrakeinspectionshouldbecompletedwithin20minutes.Failuretocompletethetestwithin20minutesmayresultinthetestbeingdiscontinued.Practicalexaminationwillbeconductedineitherthemetricorimperialsystemsofmeasurement,whichevercoincideswiththeequipmentbeingoperated.

VehicleSecure· Setthespringparkingbrakeonthevehicle.· Blockthewheels;thevehicleshouldbeonlevelgroundif

possible.· Checkthatthecompressorissecure.· Checkthedrivebeltandpulleysforwear,cracking,

slippageandtension(ifequipped).· Drainallreservoirscompletely(wet/supplyreservoir

first).Whenreservoirsareemptyclosethedrainvalves.“Fortestingpurposesonly,theapplicantwillnotberequiredtomanuallydrainthereservoirs.Instead,theapplicantwillverballyexplainthestepsthattheymustfollowwhendrainingthereservoirsandthenfanthebrakepedaltoemptythereservoir.”

· Checkbrakechambers,airlinesandslackadjustersforsecurityandwear.

CompressorandWarningDevicesCheck· Starttheengineandrunatfastidletobuildairpressure.· Warningdevice(s)mustoperateto55psiorhigher.· Ensurereservoirairpressurebuildsfrom50to90psi

withinthreeminutes.· Releasethespringparkingbrakeat90psioraboveto

preventcompoundingthebrakepressure.· Continuebuildingreservoirairpressure.Checkthatthe

governorplacesthecompressorintheunloadingstagebetween115psiminimumand135psimaximum.

· Fanthebrakepedaluntilthepressuredropsto80psi;stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(thereadingontheairpressuregaugeshouldincrease).

· Continuefanningthebrakepedal.Warningdevice(s)mustoperateatorbefore55psiandthespringparkingbrakeshouldapplyatorbeforethereservoirpressuredropsto20psi.

CheckforLeakage· Rebuildreservoirairpressureatfastidle.Whenthe

pressurereaches90psireleasethespringparkingbrake.Continuetobuildpressuretobetween115psiminimumand135psimaximum.

· Shutofftheengine.· Makeafullbrakeapplicationwiththefootvalveand

hold.Observethereservoirairpressuregauge.· Thereadingshouldnotdropmorethan3psiperminute.

Lightlytapthegaugetoensureitisnotsticking.Foranaccuratereadingbrakeapplicationshouldbeheldforatleasttwominutes.

· Listenforaudibleairleaks.· Releasefullbrakeapplication.

BrakeAdjustment(Adrivershouldbetrainedinbrakeadjustmentpriortoattemptingpracticalexaminationandsupplytheequipmentrequiredtodoabrakeadjustment.)ThefollowingarerecommendedstepstodetermineifanS-cambrakewithmanualslackadjustersrequiresadjustment.· Ensurevehicleissecureandwheelsareblocked.Verifythe

systemisatfullpressure.· Shutofftheengine.Leavingthetransmissioninalow

gearorpark.· Releasethespringparkingbrakes.· Makeachalkmarkwherethepushrodsenterthebrake

chambers.

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· Reapplyspringparkingbrakesandmeasurethedistancefromthebrakechambertothechalkmark.Ensuretheslack(pushrodtravel)iswithin1/2-3/4inchesorwithinmanufacturer’sspecificationsandtheanglebetweentheslackadjusterandpushrodis90°orascloseaspractical.Ifnot,thenabrakeadjustmentisrequired.

ThefollowingarerecommendedadjustmentstepsforS-cambrakeswithmanualslackadjusters.· Ensurevehicleissecureandwheelsblocked,releasethe

springparkingbrake.· Verifyreservoirsareatfullpressure.· Useawrenchtodisengagetheexternallockingdevice

(ifsoequipped)fromtheadjustmentboltoftheslackadjuster.

· Turntheadjustmentboltuntilthebrakeliningcontactsthebrakedrum.Ifpossible,visuallychecktoseethatthebrakeliningsareincontactwiththebrakedrum.

· Whenturningtheadjustmentboltontheslackadjuster,theS-camshouldturninthesamedirectionasifabrakeapplicationwerebeingmade.

· Backofftheadjustmentboltabout1/4to1/2ofaturn.Checkthatthepushrodfreetravelisnowbetween1/2to3/4inches,orwithinmanufacturer’sspecifications.

BrakeTest· Reapplythespringparkingbrake,removewheelblocks.· Gentlytugagainstthespringparkingbrakesinlowgear,

itshouldholdthevehicle.· Releasethespringparkingbrakes.· Movevehicleaheadslowlyandmakeaservicebrake

applicationtocheckbrakeresponse.

CombinationUnitWhenupgradingyourdriver’slicencefromsingletocombinationvehicleswithairbrakes,youwillberequiredtosatisfactorilydemonstratethepre-tripairbrakeinspectionforcombinationvehicles.Thepre-tripairbrakeinspectionshouldbecompletedwithin30minutes.Failuretocompletethetestwithin30minutesmayresultinthetestbeingdiscontinued.Practicalexaminationwillbeconductedineitherthemetricorimperialsystemsofmeasurement,whichevercoincideswiththeequipmentbeingoperated.

VehicleSecure· Setthespringparkingbrakeontractorandtrailer.· Blockthewheels;thevehicleshouldbeonlevelgroundif

possible.· Checkthatthecompressorissecure.· Checkthedrivebeltandpulleysforwear,cracking,

slippageandtension(ifequipped).· Drainallreservoirscompletelyonthetractor(wet/supply

reservoirsfirst)thenthetrailer.Whenreservoirsareemptyclosethedrainvalves.“Fortestingpurposesonly,theapplicantwillnotberequiredtomanuallydrainthereservoirs.Instead,theapplicantwillverballyexplainthestepsthattheywouldfollowwhendrainingthereservoirsandthenfanthebrakepedaltoemptythereservoir.”

· Checkbrakechambers,airlinesandslackadjustersforsecurityandwear.

· Ensuretheairbrakegladhandsandtrailerelectricalcordisconnectedproperly.

CompressorandWarningDevicesCheck· Starttheengineandrunatfastidletobuildupair

pressure.· Warningdevice(s)mustoperateto55psiorhigher.· Ensurereservoirairpressurebuildsfrom50to90psi

withinthreeminutes.· At90psiorabove,releasethespringparkingbrakesto

preventcompoundingthebrakepressureandchargethetrailersystembyopeningthetrailersupplyvalve.Applyandreleasethetrailerbrakestoensuretheyarereleasing.(Failuretoreleasecouldindicatesupply(emergency)andcontrol(service)linesarecrossed).

74

· Continuebuildingreservoirairpressure.Checkthatthegovernorplacesthecompressorintheunloadingstagebetween115psiminimumand135psimaximum.

· Fanthebrakepedaluntilthepressuredropsto80psi.Stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(thereadingontheairgaugeshouldincrease).

· Continuefanningthebrakepedal.Warningdevice(s)mustoperateatorbefore55psiandthetrailersupplyvalvewillcloseatorbefore20psi.Continuefanningthebrakepedal,thespringparkingbrakeshouldalsoapplyatorbeforethereservoirpressuredropsto20psi.

· Rebuildreservoirairpressureatfastidletobetween115psiminimumand135psimaximum.At90psichargethetrailersystembyopeningthetrailersupplyvalve.

· Setthetractorspringparkingbrake(ifpossible).

TractorandTrailerEmergencySystemCheck· Disconnectsupply(emergency)linegladhand.· Trailerbrakesshouldapplyimmediately.· Thereshouldbenoairlossfromtrailergladhand.· Airfromthetractorshould: (a)stopimmediatelyor, (b)bleeddownnolowerthan20psiandstop. Note:Undernocircumstancesshouldthetractorpressure

drainbelow20psi.· Reconnectsupply(emergency)lineandchargetrailer.

Rebuildpressureifnecessary.

TractorProtectionValveCheck· Disconnectcontrol(service)linegladhand.· Noairshouldescapefromthetractorortrailerglad

hands.· Releasethespringparkingbrake.· Makefullbrakeapplication.Tractorbrakeswillfunction

normally.Trailerservicebrakeswillnotfunctionatall.Withbrakeapplicationmaintained,airwillcontinuetoescapefromthecontrol(service)lineuntilairpressureinthereservoirsisdepletedtonolowerthan20psi.Thetrailersupplyandthetractorprotectionvalvewillcloseandthetractorandtrailerbrakeswillapply.Airlossfromcontrol(service)linewillstop.

· Reconnectcontrol(service)line.

CheckforLeakage· Rebuildreservoirairpressureatfastidletobetween115

psiminimumand135psimaximum,releasethespringparkingbrakeandchargethetrailerat90psi.

· Shutofftheengine.· Makeafullbrakeapplicationwiththefootvalveand

hold.Observethereservoirairpressuregauge.· Thereadingshouldnotdropmorethan4psiperminute.

Lightlytapthegaugetoensureitisnotsticking.Foranaccuratereadingbrakeapplicationshouldbeheldforatleasttwominutes.

· Listenforaudibleairleaks.· Releasefullbrakeapplication.

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BrakeAdjustment(Adrivershouldbetrainedinbrakeadjustmentpriortoattemptingpracticalexaminationandsupplytheequipmentrequiredtodoabrakeadjustment.)ThefollowingarerecommendedstepstodetermineifanS-cambrakewithmanualslackadjustersrequiresadjustment.· Ensurevehicleissecureandwheelsblocked.· Verifyreservoirsareatfullpressure.· Shutofftheengine,leavingthetransmissioninalow

gearorpark.· Releasethespringparkingbrakes.· Makeachalkmarkwherethepushrodentersthebrake

chamber,repeatthisonallthebrakechambers.· Reapplythespringparkingbrakesandmeasurethe

distancefromthebrakechambertothechalkmark.Ensuretheslack(pushrodtravel)iswithin1/2and3/4inchesorwithinmanufacturer’sspecificationsandtheanglebetweentheslackadjusterandpushrodis90°orascloseaspractical.Ifnot,thenabrakeadjustmentisrequired.

ThefollowingarerecommendedadjustmentstepsforS-cambrakeswithmanualslackadjusters.· Ensurethevehicleissecureandthewheelsareblocked.· Releasethespringparkingbrakes.· Verifyreservoirsareatfullpressure.· Useawrenchtodisengagetheexternallockingdevice

(ifsoequipped)fromtheadjustmentboltoftheslackadjuster.

· Turntheadjustmentboltuntilthebrakeliningcontactsthebrakedrum.Ifpossible,visuallychecktoseethatthebrakeliningsareincontactwiththebrakedrum.

· Whenturningtheadjustmentboltontheslackadjuster,theS-camshouldturninthesamedirectionasifabrakeapplicationwerebeingmade.

· Backofftheadjustmentboltabout1/4to1/2ofaturn.· Nowrecheckslackadjustertravel.Checkthatthepush

rodfreetravelisnowbetween1/2to3/4inches,orwithinmanufacturer’sspecifications.

BrakeTest· Reapplythespringparkingbrake,removewheelblocks.· Gentlytugagainstthespringparkingbrakesinlowgear,

itshouldholdthevehicle.· Releasethespringparkingbrakesandsettrailerbrakes

withthehandvalve.· Gentlytugagainsttrailerbrakesinlowgear.· Movevehicleaheadslowlyandmakeaservicebrake

applicationtocheckbrakeresponse.

AirOverHydraulic(AirActuated)BrakeSystemThepre-tripairbrakeinspectionshouldbecompletedwithin20minutes.Failuretocompletethetestwithin20minutesmayresultinthetestbeingdiscontinued.Practicalexaminationwillbeconductedineitherthemetricorimperialsystemsofmeasurement,whichevercoincideswiththeequipmentbeingoperated.

VehicleSecure· Applytheparkingbrakeonthevehicle.· Blockthewheels;thevehicleshouldbeonlevelgroundif

possible.· Checkhydraulicfluidlevelinthereservoir.· Checkunderthehoodforhydraulicfluidleaks.· Checkthatthecompressorissecure.· Checkthedrivebeltandpulleysforwear,cracking,

slippageandtension(ifequipped).· Drainallairreservoirscompletely(wet/supplyair

reservoirfirst).Whenairreservoirsareemptyclosethedrainvalves.“Fortestingpurposesonly,theapplicantwillnotberequiredtomanuallydrainthereservoirs.Instead,theapplicantwillverballyexplainthestepsthattheywouldfollowwhendrainingthereservoirsandthenfanthebrakepedaltoemptythereservoir”.

· Checkwheelcylindersandlinesforsignsofhydraulicfluidleaks.

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CompressorandWarningDevicesCheck· Starttheengineandrunatfastidletobuildairpressure.· Warningdevice(s)mustoperateto55psiorhigher.· Ensurereservoirpressurebuildsfrom50to90psiwithin

threeminutes.· Ifequippedwithspringparkingbrakesreleasethemto

preventcompoundingthebrakepressure.· Continuebuildingreservoirairpressure.Checkthatthe

governorplacesthecompressorintheunloadingstagebetween115psiminimumand135psimaximum.

· Fanthebrakepedaluntilthepressuredropsto80psi;stopfanningtoseeifthegovernorhasplacedthecompressorintheloadingstage(needleontheairgaugeshouldstarttoclimb).

· Continuefanningthebrakepedal.Warningdevice(s)mustoperateatorbefore55psi.

CheckforLeakage· Rebuildreservoirairpressuretobetween115psi

minimumand135psimaximum(ifequippedreleasethespringparkingbrakes).

· Shutofftheengine.· Makeafullbrakeapplicationwiththefootvalveand

hold.Observethereservoirairpressuregauge.· Thereadingshouldnotdropmorethan3psiperminute.

Lightlytapthegaugetoensureitisnotsticking.Foranaccuratereadingapplicationshouldbeheldforatleasttwominutes.

· Listenforaudibleairleaks.· Releasefullbrakeapplication.

BrakeTest· Reapplytheparkingbrake,removewheelblocks.· Gentlytugagainsttheparkingbrakesinlowgear,it

shouldholdthevehicle.· Releasetheparkingbrakes.· Movevehicleaheadslowlyandmakeaservicebrake

applicationtocheckbrakeresponse.

SectionSummaryQuestions1. Whatisthemaximumtimepermittedforthe

compressortobuildfrom50to90psi?

2. Whatisthemaximumairpressurelosspermittedonafullbrakeapplicationwiththemotorshutoff?

3. Howcantheholdingpowerofthetrailerbrakesbetested?

4. Whatisthefinalbraketestthatshouldbemadebeforethevehicleisputintoservice?

5. IsabrakeadjustmentpartoftheairbraketestgivenbytheDivisionofDriverandVehicleLicencing?

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Metric Conversion Table

Assometruckshaveairgaugesindicatingpressureinkilopascals(kPa),thefollowingconversionchartmayhelp

kPatopsi psitokPa5 0.72 1 6.8910 1.45 2 13.7815 2.17 3 20.6820 2.90 4 27.5725 3.62 5 34.4730 4.35 6 41.3635 5.07 7 48.2640 5.80 8 55.1545 6.52 9 62.0550 7.25 10 68.9460 8.70 15 103.4270 10.15 20 137.8980 11.60 25 172.3690 13.05 30 206.84100 14.50 35 241.31150 21.75 40 275.78200 29.00 45 310.26250 36.29 50 344.73300 43.51 55 379.20310 44.96 60 413.68350 50.76 65 448.15400 58.01 70 482.62415 60.19 75 517.10450 65.26 80 551.57500 72.51 85 586.04550 79.77 90 620.52585 84.84 95 654.99600 87.02 100 689.47650 94.27 105 723.94700 101.52 110 758.41725 105.15 115 792.89750 108.77 120 827.36800 116.03 125 861.83850 123.28 130 896.31900 130.53 135 930.78950 137.78 140 965.251000 145.03 145 999.731050 152.29 150 1034.20

78

METRIC CONVERTER

0 10 20 40 50 70 80 90 100 110

0 20 70 110 130 150 170

MAXIMUM

30MAXIMUM

60

MAXIMUM

50MAXIMUM

90km/hkm/hkm/h

mph

Weight

1gram(g) = .035ounce

1kilogram(kg) = 2.2pounds

1metricton = 1.102ton

Length 1centimetre(cm) = .39inch 1metre(m) = 39.3inchesor3.27feet 1metre(m) = 1.09yards 1kilometre(km) = .621mile

km = kilometres

km/h = kilometresperhour

m/h = milesperhour

79

Organ Transplantation and Donation

TransplantationAdvancesinmedicalsciencenowmakeitpossibletoreplacefailinghumanorgans.Infact,replacementofakidney,cornea,heartorliverisnolongerconsideredanexperimentbutatreatmentchoice.Organsandtissuesthatcanbeeffectivelytransplantedarekidneys,hearts,lungs,livers,corneas,bones,joints,skin,pancreas,bonemarrowandbowel.Therefore, one person’s decision to be an organ donor could benefit as many as 10 people.However,manyCanadiansdieeachyearbecausedonororgansarenotavailable.

OrganDonationFormanypatients,transplantationofferstheonlyhopeforleadingahealthyandproductivelife,or,insomecases,forlifeatall.Bysigninganorgandonorcardandurgingyourfamilyandfriendstodothesame,youcouldensurethefreedomandqualityoflifethatyouenjoyispassedontosomeoneelse.Yourdonorcardconfirmsyourwishtogivethegiftofanyoralltissuesandorgansafteryourdeath.Althoughitisimportanttodiscussyourfeelingsabouttransplantationwithyourfamily,friendsordoctor,theinformationonthecardmayhelpthemwithdecisionsatadifficulttime.

AgeAgeisneveranabsolutebarriertosomeformoforgandonation.Ifyouareover19,youmaydirectthatanytissueororganmaybeusedafteryourdeath.Forpersonsunder19years,yourparentorguardianshouldsignthedonorcardalso.DonorcardsareavailablefromTheKidneyFoundationortheCNIB.

FormoreinformationFormoreinformationontransplantationandorgandonation,contactNB Organ and Tissue Procurement Program at (506)643-6848.