Groundsmaster 4500--D/4700--D Hydraulic SystemPage 4 -- 15
Traction Circuit: High Speed (2WD)
The traction circuit piston pump is a variable displace-
ment pump that is directly coupled to the engine fly-
wheel. Pushing the traction pedal engages a hydraulic
servo v alve which c ontrols the variable displacement
piston pump swash plate to create a flow of oil. This oil
isdirectedtothefrontwheelandrearaxlemotors.Oper-
ating pressure on the high pressure side of the closed
traction circuit loop is determined by the amount of load
developedatthefixeddisplacementwheelandaxlemo-
tors. As the load increases, circuit pressure can in-
creasetoreliefvalvesettings:5000PSI(345bar)inboth
forward and reverse. If pressure exceeds the relief set-
ting,oilflowsthroughthereliefvalve tothelowpressure
sideoftheclosedlooptractioncircuit.Thetractioncircuit
provides operation in either low speed (4WD) or high
speed (2WD).
Traction circuit pressure (forward and reverse) can be
measuredattestports located on the traction circuithy-
draulic tubes of the machine.
The traction circuit pump and motors use a small
amount of hydraulic fluid for internal lubrication. Fluid is
designed to leak across traction pump and motor com-
ponents into the case drain. This leakage results in the
lossofhydraulicfluidfromtheclosedlooptractioncircuit
thatmustbereplaced. The charge circuit is designed to
replace this traction circuit leakage.
The gear pump section that supplies oil to the steering
and lift/lower circuits also provides charge oil for the
traction circuit. This gear pump is driven directly off the
tractionpump.Itprovidesaconstantsupplyofchargeoil
to the tractioncircuit to make up for oil that is lost due to
internal leakage in the traction pump and motors.
Chargepump flowisdirected throughtheoil filter andto
the low pressure side of the closed loop traction circuit.
A filter bypass valve allows charge oil flowto the closed
loopifthefilterbecomesplugged.Chargepressure isli-
mited to 207 PSI (14.3 bar) by a relief valve located in
the filtration/charge control manifold. Charge pressure
canbe measured at the testportonthefiltration/charge
control manifold.
NOTE: When in high speed (2WD), the cutting decks
are prevented from being lowered and the mow circuit
cannot be engaged.
Forward Direction
Withthe Hi--Lospeed controlswitchis inthehigh speed
(2WD) position, solenoid valve (SV) in the 4WD/2WD
control manifold is energized. The solenoid valve spool
shifts to direct charge pressure that shifts the PD1 and
PD2 control valve spools. The shifted PD1 valve pre-
vents hydraulic flow from the piston pump to the rear
axle motor. With flow blocked to the rear axle motor, all
pistonpumpflow is directedtothefrontwheel motors to
allow a higher transport speed in the forward direction.
Without flow to the rear axle motor, the rotating rear
wheels drive the axle motor so it acts like a pump. Inlet
oiltotheaxlemotor is provided by a check valve that al-
lows charge oil into the rear axle motor circuit. Oil leav-
ing the axle motor enters the 4WD/2WD control
manifold at port M2 and is directed back to the axle mo-
tor through the shifted PD1 cartridge and manifold port
M1.Toallowforrearwheelloopcoolingwheninforward
highspeedoperation,asmallamountofoilexitsthrough
the shifted PD1 and PD2 cartridges that returns to the
reservoir.
Reverse Direction
The traction circuit operates essentially the same in re-
verse high speed as it does in the forward direction.
However, the flow through the c ircuit is reversed. The
shifted solenoid valve (SV) and directional valves PD1
andPD2inthe4WD/2WDmanifoldpreventoilflowfrom
the rear axle motor. Oil flow from the piston pump is
thereforedirectedtoonlythefrontwheelmotors.Thisoil
drives the front wheel motors in the reverse direction
and then returns to the piston pump. Oil circulation
through the rear axle motor loop is the same as in the
2WD forward direction.
Hydraulic
System
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