Trailer Air Suspension - System Description
4
Air Suspension System Description General The number of vehicles with air suspension will increase in the next few years, because this system has considerable advantages. According to conservative estimates 85% of trucks, 100% of heavy buses and 60% of fifthwheel trailers and 95% of semitrailers are equipped with air suspension. We will look at the general theoretical principles of air suspension, design and function of the components and additional control possibilities, e.g. swapbody systems, lift axle controls and special systems t increase ease of operation and safety. Dualcircuit: Right and left side of the vehicle is separated by the levelling valve and COLAS (cross restriction active) In almost all modern trailer suspensions the excellent driving behaviour and the good load protection is achieved by a law spring rate of the air bags. The spring rate o bags and the spring axle assembly (mechanic stabiliser) form the roll stiffness of the suspension axles, which is important for the roll stability and thus for the safety of trailer during cornering. The special feature of modern air suspensions is the use of a single air valve, which exhibits a "pseudodual circuit air installation". At this dual versions with cross restriction in the levelling valve the air bags on the left and right sides of the vehicle are pneumatically separated and only connected together by th restriction. The air bag of the left and right side of the vehicle is supplied through the connections 21 and 22. During cornering, the air can only equalise itself slowly be the two sides of the vehicle. As a result, the air bags additionally assist in supporting the rolling motion of the vehicle body. Within the air suspension, approx. 80% of th stiffness is provided by the springaxle assembly and approx. 20% by the air bags. In relation to the whole system made up of tyres and air suspension, the share in th stiffness of the air bags is only about 10 %, as the spring rate of the tyres has a considerable share in the roll angle of the build up. Singlecircuit: Right and left side of the vehicle is direct linked together (no cross restriction) At single circuit versions without cross restriction the air bags on the left and right hand sides of the vehicle are pneumatically connected. There is no transverse flow restrictor. During cornering, the air can equalise itself more quickly between the two sides of the vehicle. This means the rolling motion is not supported by the air bags roll stability and, therefore, road safety is reduced in comparison with a dual circuit air installation. As well as the reduction in road safety, the mechanical loads in the a suspension unit are higher. Since there is no roll stabilisation of the air bags, the axle and trailing arm combination has to undertake this portion of the stabilisation as w The use of single circuit air suspension installations can lead to damage to the vehicle as a result of the higher loads. For this reason, some axle manufacturers canno any warranty for chassis and suspension damage resulting from this effect. To achieve optimum function and the greatest possible road safety, in particular in critical situations, it is recommended to use dual circuit air suspension installations with a cross restriction. Lifting axles are the only exception to this recommendation. In this is permitted for at most one lifting axle in a three or four axle unit to be installed with one circuit. Advantages of air suspension systems A constant ground clearance between the road surface and the vehicle body gives an unchanging height for boarding and loading. The headlamp settings remain constant, irrespective of the load condition.
-
Upload
christianson87 -
Category
Documents
-
view
9 -
download
2
description
General description
Transcript of Trailer Air Suspension - System Description
-
07.05.2015 TrailerApplicationGuide:SystemDescription
data:text/htmlcharset=utf8,%3Cdiv%20id%3D%22content%22%20style%3D%22margin%3A%200px%3B%20padding%3A%200px%3B%20color%3A%2 1/4
AirSuspensionSystemDescription
GeneralThenumberofvehicleswithairsuspensionwillincreaseinthenextfewyears,becausethissystemhasconsiderableadvantages.Accordingtoconservativeestimates,some85%oftrucks,100%ofheavybusesand60%offifthwheeltrailersand95%ofsemitrailersareequippedwithairsuspension.Wewilllookatthegeneraltheoreticalprinciplesofairsuspension,designandfunctionofthecomponentsandadditionalcontrolpossibilities,e.g.swapbodysystems,liftaxlecontrolsandspecialsystemstoincreaseeaseofoperationandsafety.
Dualcircuit:RightandleftsideofthevehicleisseparatedbythelevellingvalveandCOLAS(crossrestrictionactive)
Inalmostallmoderntrailersuspensionstheexcellentdrivingbehaviourandthegoodloadprotectionisachievedbyalawspringrateoftheairbags.Thespringrateoftheairbagsandthespringaxleassembly(mechanicstabiliser)formtherollstiffnessofthesuspensionaxles,whichisimportantfortherollstabilityandthusforthesafetyofthetrailerduringcornering.Thespecialfeatureofmodernairsuspensionsistheuseofasingleairvalve,whichexhibitsa"pseudodualcircuitairinstallation".Atthisdualcircuitversionswithcrossrestrictioninthelevellingvalvetheairbagsontheleftandrightsidesofthevehiclearepneumaticallyseparatedandonlyconnectedtogetherbythecrossrestriction.Theairbagoftheleftandrightsideofthevehicleissuppliedthroughtheconnections21and22.Duringcornering,theaircanonlyequaliseitselfslowlybetweenthetwosidesofthevehicle.Asaresult,theairbagsadditionallyassistinsupportingtherollingmotionofthevehiclebody.Withintheairsuspension,approx.80%oftherollstiffnessisprovidedbythespringaxleassemblyandapprox.20%bytheairbags.Inrelationtothewholesystemmadeupoftyresandairsuspension,theshareintherollstiffnessoftheairbagsisonlyabout10%,asthespringrateofthetyreshasaconsiderableshareintherollangleofthebuildup.
Singlecircuit:Rightandleftsideofthevehicleisdirectlinkedtogether(nocrossrestriction)
Atsinglecircuitversionswithoutcrossrestrictiontheairbagsontheleftandrighthandsidesofthevehiclearepneumaticallyconnected.Thereisnotransverseflowrestrictor.Duringcornering,theaircanequaliseitselfmorequicklybetweenthetwosidesofthevehicle.Thismeanstherollingmotionisnotsupportedbytheairbags.Therollstabilityand,therefore,roadsafetyisreducedincomparisonwithadualcircuitairinstallation.Aswellasthereductioninroadsafety,themechanicalloadsintheairsuspensionunitarehigher.Sincethereisnorollstabilisationoftheairbags,theaxleandtrailingarmcombinationhastoundertakethisportionofthestabilisationaswell.Theuseofsinglecircuitairsuspensioninstallationscanleadtodamagetothevehicleasaresultofthehigherloads.Forthisreason,someaxlemanufacturerscannotofferanywarrantyforchassisandsuspensiondamageresultingfromthiseffect.Toachieveoptimumfunctionandthegreatestpossibleroadsafety,inparticularincriticalsituations,itisrecommendedtousedualcircuitairsuspensioninstallationswithacrossrestriction.Liftingaxlesaretheonlyexceptiontothisrecommendation.Inthiscase,itispermittedforatmostoneliftingaxleinathreeorfouraxleunittobeinstalledwithonecircuit.
AdvantagesofairsuspensionsystemsAconstantgroundclearancebetweentheroadsurfaceandthevehiclebodygivesanunchangingheightforboardingandloading.Theheadlampsettingsremainconstant,irrespectiveoftheloadcondition.
-
07.05.2015 TrailerApplicationGuide:SystemDescription
data:text/htmlcharset=utf8,%3Cdiv%20id%3D%22content%22%20style%3D%22margin%3A%200px%3B%20padding%3A%200px%3B%20color%3A%2 2/4
Higherridingcomfort.Thisgivessmoothertransportofthegoodsbeingtransported,nojumpingwithanemptytrailer,andlesswearontheroadsurface.Stabledrivingcharacteristicsduetofirmcontactbetweentyresandroadsurface.Stablecornering.Accuratecontrolofloaddependentcontrol(LSVloadsensing,EBS).Busescanbeloweredononeside(kneeling).Swapbodysystemspossibleforadaptingtoloadingplatforms(COLAS,COLAS+)Electroniccontrolpossible,e.g.memoryfunctions,differentrideheightscanbestored.Liftaxlecontrolispossible.Increasedeaseofuse.Quickhitchingandunhitchingofarticulatedtrailers.Lessmechanicalwearoncomponents.Easytorepairandmaintain,leadingtocostsavings.
Springcharacteristiccurvemaindiagram(Fig.1)
Theoreticalprinciples
Definitionofairsuspension:
Asuspensionsystemisregardedasairsuspensionifatleast75%ofthespringactionisprovidedbypneumaticdevices.
Task
Thetaskofavehiclesuspensionistodamptheshockscausedduringdrivingbybumpsintheroadsurfacewhentransmittingthemtothevehiclebody.Thistaskcanbeconvenientlyfulfilledbyairsuspension.Allkindsofairspringsareemployedastheloadbearingspringsforthebodyloadinmotorvehiclesandtrailers.Adistinctionismadebetweenprimaryandsecondarysuspension.Theprimarysuspensioncarriestheweightofthevehiclebodypermanently,whilethesecondarysuspensiononlycomesintoeffectintheeventofextremelyhardroadshocksorfailureoftheprimarysuspension.Theprimaryandsecondarysuspensionmustbewelltunedtoeachother,withprioritybeinggiventoridecomfortinpassengervehiclesandtohandling(liftingandloweringthevehicle)ingoodsvehicles.Secondarysuspensionusuallyconsistsofrubber,hollowrubberorplasticsprings.Sinceairsuspensionpossessesalmostnointrinsicdamping,damperswithhigherforcesinthetensionandcompressionstagesthanthoseforleafsprungaxlesarerequired.
Furthermore,theairspringbellowscanonlyabsorblateralforcestoalimitedextent.Consequently,thesprungwheelsoraxlesmustbeheldbylinks,arms,antirollbarsandthelike.
Springratesgeneral
Aspringcharacteristiccurvecanbelinear,progressiveordegressive,inwhichlattercaseaninstablespringwouldresult.Fig.1showsthestaticanddynamicworkingrangeofaspringtogetherwithitsworkcapacity,whichisshownastheareaboundedbythespringcharacteristiccurve,thelimits(3)and(4),andtheXaxis.Thedynamicworkingrangeoverlapsthestatic,dependingontheavailabledynamicspringtravel(paths13and24).
Important:Thetotalspringdeflectionthereforeconsistofthestaticspringtravel,whichoccurswhenthevehicleisloadedwhileatrest,andthedynamicspringtravel,whichisthesameasthebottomingtravel.Thesmallerthespringrateandthelargerthe(vehicle)mass,thesmallerthe(body)accelerationandthebettertheridecomfortis.
Ridecomfort(Fig.2)
-
07.05.2015 TrailerApplicationGuide:SystemDescription
data:text/htmlcharset=utf8,%3Cdiv%20id%3D%22content%22%20style%3D%22margin%3A%200px%3B%20padding%3A%200px%3B%20color%3A%2 3/4
Thefollowingconclusionscannowbedrawn:
toobtainahighdegreeofridecomfortatalltimes,theratioofaxlespringconstanttobodyweightmustremainthesame.Thenaturalfrequencyofthebodymustbeassmallaspossibleifgoodridecomfortistobeachieved.Thenaturalfrequencycanbeinfluencedbyboththespringrateandthebodymass.Thenaturalfrequencyhasaverygreateffectonridecomfort.Sinceridecomfortisverydifficulttorepresentobjectively,subjectiveperceptionsmustbeemployedtodescribethecomfortmathematicallyorgraphically.Itcanbeseenclearlythatthezoneexperiencedbypeopleascomfortablenotonlypresupposesarelativelyverysmallnaturalfrequency,butalsodependsonthevibrationamplitude.Ifthenaturalfrequencyistobekeptasconstantaspossibleundervaryingloads,thespringratemustbechangedinsuchawaythattheratioofspringratetoweightremainsconstant.Theincreasingimportanceoflightweightconstructionandtheassociatedlargedifferenceinweightbetweenanemptyandaloadedvehiclemaketheneedforacontrollablespringevermoreurgent.
Compressedairgenerationandstorage
Forsafetyreasons,thecompressedairforthebrakingsystemmustbekeptseparateandisolatedfromthatoftheairsuspension.Circuit1and2(connections21and22onthequadruplecircuitprotectionvalve)isalwaysallocatedtotheSB.
Onairsuspensionsystemsfortrailersorsemitrailers,thepressurelevelcorrespondstothatofthebrakingsystem,sincetheairsuspensionsystemissuppliedfromthebrakingsystemsupplyline.
Varioussystemscanbedifferentiatedintractorunits:
pressureintheairsuspensionsystem=pressureinthebrakingsystem(correspondstotrailer/semitrailer).pressureintheairsuspensionsystemisgreaterorsmallerthanthepressureinthebrakingsystem(trucksandbuses).Inthefirstinstance,thecompressedairfortheairsuspensionistakenfromtheancillaryconsumercircuit(connection23or24onthequadruplecircuitprotectionvalve)afterthequadruplecircuitprotectionvalve,inwhichcasethequadruplecircuitprotectionvalvemustbedesignedtogiveprioritytofillingbrakecircuits21and22(alegalrequirementsasfrom1stOctober1994).Inmostinstances,theairsuspensionsystemisprovidedwithahigherpressurethanthebrakingsystem.Here,thecompressedairistakenbetweenthegovernorvalveandthequadruplecircuitprotectionvalve,andfedtotheairsuspensionairtankthroughapressureprotectionvalvewithlimitedbackflow(ornobackflowatall)settoahigherpressurethanthequadruplecircuitprotectionvalve.Thiskindofdesignhastheadvantagethatsmallerairspringsupportbellowsandairtankscanbeinstalled.Italsoallowsshorterfillingandexhaustingtimes,sincethepressuredifferencebetweenthestoragetankandtheairspringsupportbellowsisgreater.Ontrailervehicles,thesupplyfortheairsuspensionistakenfromtheservicebrakesupply.Bothcircuitsmustbeisolatedfromeachotherbyapressureprotectionvalve(withorwithoutlimitedbackflow).Theopeningpressureisabout6bar,sincetheisolatingpressureislaiddownbylawasatleast5.2bar.
LSVloadsensing/EBScontrol
Onemajoradvantageofairsuspensionisthatthebellowspressure,whichdependsontheloadconditionatanygiveninstant,canbeusedtocontroltheautomaticloaddependentloadsensingvalve/EBS.Afunctionofthiskindshowsthefigureonthetopforthedualcircuitairsuspension,whenusingatwincircuitautomaticallycontrolledloadsensingvalve,theintactbellowsstillensuresloaddependentcontrolofthebrakingpressure,evenintheevenofalinebreakage.
Dependingonthedesign,anadequatebrakingeffectcanstillbeobtainedinvehicleswithfullorpartialairsuspensionintheeventoffaultsintheairsuspensionsystem.
Airsupplyexample(Fig.3)
AS=AirsuspensionSB=ServiceBrake
-
07.05.2015 TrailerApplicationGuide:SystemDescription
data:text/htmlcharset=utf8,%3Cdiv%20id%3D%22content%22%20style%3D%22margin%3A%200px%3B%20padding%3A%200px%3B%20color%3A%2 4/4
