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 valve which controls 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-
crease to relief valve settings: 4000 PSI (274 bar) in
forward and 5000 PSI (343 bar) in reverse. If pressure
exceeds the relief setting, oil flows through the piston
pump relief valve to the low pressure side of the closed
loop traction circuit. The traction circuit provides opera-
tion in either Hi speed (2WD) or Low speed (4WD).
Forward Direction
WhentheHi/LowswitchisintheLowspeed(4WD)posi-
tionandthetractionpedalispushedintheforwarddirec-
tion, oil from the piston pump is directed to the front
wheel motors and 4WD manifold. Oil flow to the front
wheel motors drives the motors in the forward direction
and then returns to the hydrostat. Oil flow to the 4WD
manifold enters the P1 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 manifold at the
M2 port. Flow passes through the PD2 cartridge,
through the CV check valve, out manifold port P2 and
back to the hydrostat.
When going down a hill, the tractor becomes an over--
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 prevent rear wheel lock up,
an adjustable relief valve (RV) in the 4WD manifold re-
duces rear axle motor pressure created in down hill, dy-
namic braking conditions.
Traction circuit pressure (forward and reverse) can be
measured at test ports in hydraulic tubes. The forward
traction port is on the left side of the machine and the re-
verse traction port is on the right side 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
that must be replaced. The charge circuit is designed to
replace this traction circuit leakage.
Reverse Direction
The traction circuit operates essentially the same in re-
verse Low speed (4WD) as it does in the forward direc-
tion. However, the flow through the circuit is reversed.
Oil flow from the piston pump is directed to the front
wheel motors and also to the 4WD manifold. The oil to
the front wheel motors drives them in the reverse direc-
tion and then returns to the piston pump. The oil to the
4WD manifold enters the manifold at port P2 and flows
through pressure reducing valve (PR) which limits the
down stream pressure to the rear axle motor to 650 PSI
(45 bar) so the rear wheels will not scuff the turf during
reverse operation. This reduced pressure flows through
the PD2 cartridge and outport M2 to therear axle motor.
Return oil from the rear motor re--enters the 4WD man-
ifold at port M1, flows through the PD1 cartridge, exits
the manifold at port P1 and returns to the piston pump.
The gear pump section that supplies oil to the steering
and lift/lower circuits also provides oil for the charge cir-
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.
Charge pressure is limited to 250 PSI (17 bar) by a relief
valve located in the oil filter manifold. Charge pressure
can be measured at the charge circuit pressure test port
on the oil filter manifold.
Groundsmaster 4100--D/4110--D
Page 4 -- 11
Hydraulic System
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