Toro 30857 - Traction Circuit: Low Speed (4 Wd)

Groundsmaster 4500--D/4700--D Hydraulic SystemPage 4 -- 13
Traction Circuit: Low Speed (4WD)
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
andlift/lower circuits also providesoil for the charge c ir-
cuit. This gear pump is driven directly off the traction
pump. It provides a constant supply of charge oil to
make up for oil that is lost due to internal leakage in the
traction pump and motors.
Pump flow for the charge circuit is directed through the
oil filter and to the low pressure side of the closed loop
traction circuit. A filter bypass valve allows charge oil
flow to the closed loop if the filter becomes plugged.
Chargepressure islimitedto207 PSI(14.3bar)by are-
liefvalvelocatedinthefiltration/chargecontrolmanifold.
Chargepressure can be measured atthecharge circuit
pressure test port on the filtration/charge control man-
ifold.
To enhance traction control, the lift/lower circuit is
equippedwith a counterbalance system.Aproportional
reliefvalve(TS)locatedintheliftcontrolmanifoldallows
cutting deck weight to be transferred to the machine for
traction improvement (see Counterbalance in Lower
Cutting Decks in this section).
Forward Direction
WhentheHi--Lospeedcontrolswitchisinthelowspeed
(4WD) position and the traction pedal is pushed in the
forwarddirection, oil from the piston pumpis directed to
thefrontwheelmotorsand4WD/2WDcontrolmanifold.
Oilflowtothefrontwheelmotorsdrivesthemotorsinthe
forward direction and then returns to the piston pump.
Oilflowtothe4WD/2WDcontrolmanifoldenterstheP1
port and then is directed to the PD1 cartridge and out of
the manifold M1 port to drive the rear axle motor in the
forward direction. Oil returning from the rear motor re--
enters the 4WD/2WD control manifold at the M2 port.
Flow passes through the PD2 cartridge, through the
check v alve (CV), out manifold port P2 and back to the
piston pump.
Whengoingdownahill,themachinebecomesanover--
running load that drives the wheel and axle motors. In
this condition, the rear axle motor could lock up as the
oil pumped from the motor increases pressure as it re-
turns to the piston pump. To preventrear wheel lock up,
an adjustable relief valve (RV) in the 4WD/2WD control
manifold reduces rear axle motor pressure created in
down hill, dynamic braking conditions.
Reverse Direction
The traction circuit operates essentially the same in re-
verse4WDas it does in theforwarddirection.However,
theflow through the circuit isreversed. Oil flow from the
piston pump is directed to the front wheel motors and
also to the 4WD/2WD control manifold. The oil to the
front wheel motors drives them in the reverse direction
and then returns to the piston pump. The oil to the
4WD/2WD control manifold enters the manifold at port
P2 and flows through pressure reducing v alve (PR)
which limits the down stream pressure to the rear axle
motorto450PSI(31bar)sotherearwheelswillnotscuff
theturfduringreverseoperation.Thisreducedpressure
flows through the PD2 cartridge and out port M2 to the
rearaxlemotor.Returnoilfromtherearmotorre--enters
the 4WD/2WD control manifold at port M1, flows
through the PD1 cartridge, exits the manifold at port P1
and returns to the piston pump.
Hydraulic
System