Miller Nitro 7380 - Propel System; Auxiliary Circuit

7-MAINTENANCE
Propelsystem
Thepropelsystemcontains2electronicallycontrolledhydrostaticpumpsand4electronicallycontrolledwheelmo-
tors.Thepropelsystemiscontrolledbyanelectronicdrivelinecontrollerwhichdeterminesthedisplacementofthe
hydrostaticpumpsandmotorsbasedontheoperatorscommand.Thesystemisaclosedloopcircuitwhichcombines
theowofbothhydrostaticpumpsanddistributesittothefourwheelmotors.Eachhydrostaticpumphasanintegral
chargepump.Thepurposeofthechargepumpistosupplyoilintotheclosedlooppropelcircuitwhichislostdueto
inefcaciesofthepumps,motorsandprovidemake-upoilwhichisremovedbyaloopushingvalveineachmotor.
Theoilinthehydraulicreservoirispulledintothechargepumpwhereitispressurizedtoapproximately3447kPa
(500psi).Theoilisthenpushedthroughthehighpressureltersmountedonthebacksideofthepropelpumps.
Fromthehighpressureltersitgoesintothelowpressuresideofthehydrostaticpump(returnow).Fromthepropel
pumps(37921kPa(5500psi)orgreater)theoilispumpedaccordingtodemand(theamountofdisplacementbythe
joystickinthecab)tothewheeldrivemotors.Thedisplacementoftheelectronicallycontrolwheelsmotorsiscontrol
bythejoystick.
Thepressureinthechargepressurecircuitsisalsomonitoredwithtwoidenticalpressureswitches.Theseswitches
triggerlightsandalarmsinthecabifthepressuredropsbelow1600kPa(232psi)intheirrespectivecircuits.Loss
inchargepressurecouldbeduetoanumberofitems:abursthose,orafailureinachargepump,avalveorpropel
pump.Propelsystemdamagecanoccurifachargepressurewarninglampisignored.
Extraowfromthechargepumps(astheyarepositivedisplacement)isemptiedintothecaseofeachhydrostatic
pumps(propel).Asetamountofoilisalsodrainedoffthepropelcircuit(lowpressureside)toallownewoilintothe
system(thisisaccomplishedbytheloopushingvalveineachwheelmotor).Thisallowsclean,lteredoiltoenter
thesystemviathechargepump.Theoilwhichisdrainedoffthepropelloopisthendirectedthroughaheatexchanger
andthencombineswiththehydrostaticpumpscaseowstoareturnlterlocatedinsidetheoilreservoir.
Sincetheparkbrakesarepressurereleased,aminimumof1379kPa(200psi)inrequiredintheparkingbrakecircuit
topreventtheparkbrakesfromengaging.Whenthepressuredropsbelowthisvalue,theparkingbrakeswillstartto
engage.Continuedusewiththeparkbrakelightonmaydamagethebrakesandothercomponents.Thebrakesare
protectedwithapressureswitchwhichmonitorspressureinthebrakecircuit.Theparkingbrakecircuitisfedbythe
parkingbrake/laddervalvewhichinpartoftheauxiliaryhydrauliccircuit.
Auxiliarycircuit
Theauxiliarycircuitoperatesat176Kg/cm²(2500psi).Thepumpsenseshowmuchowisneededfortheauxiliary
circuit.Oilisdrawnfromtheoilreservoirintothepump,whichmaintains176Kg/cm²(2500psi)atalltimes.Theoil
owsfromthepumptothepriorityvalve.Thepriorityvalvegivesthesteeringsystempriorityoverallotherfunctions.
Onlyafterthesteeringcircuitoilneedsaresatised,isanyoilpassedthroughthepriorityvalvetotheotherfunctions:
spraypumpmotor,parkbrake,laddercylinder,liftandboomcylinders,legwidening,etc.Alloftheoilinthiscircuit
returnstoareturnlterlocatedintheoilreservoir.
7-18