Traction Circuit: Hi 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 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
With the Hi/Low speed switch in the Hi speed position,
solenoid valve (SV) in the 4WD control manifold is ener-
gized. The solenoid valve spool shifts to direct charge
pressure that shifts the PD1 and PD2 control valve
spools. The shifted PD1 valve prevents piston pump hy-
draulic flow from reaching the rear axle motor. With flow
blocked to the rear axle motor, all traction pump flow is
directedtothefrontwheelmotorstoallowahighertrans-
port 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
oil to the axle motor is provided by a check valve that al-
lows charge circuit oil into the rear axle motor circuit. Oil
leaving the axle motor enters the 4WD manifold at port
M2 and is directed back to the axle motor through the
shifted PD1 cartridge and manifold port M1. To allow for
rear wheel loop cooling when in forward transport op-
eration, a small amount of oil exits through the shifted
PD1 and PD2 cartridges that returns to the reservoir.
This oil loss is replaced by charge circuit oil.
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-
versetransport(2WD)asitdoesintheforwarddirection.
However, the flow through the circuit is reversed. The
shifted solenoid valve (SV) and directional valves PD1
and PD2 in the 4WD manifold prevent oil flow to the rear
axle motor. Oil flow from the hydrostat is therefore di-
rected to only the front wheel motors. This oil drives the
front wheel motors in the reverse direction and then re-
turns to the hydrostat. Oil circulation through the rear
axlemotorloopisthesameasintheHispeed(2WD)for-
ward direction.
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 traction circuit to make up for oil that is lost due to
internal leakage in the traction pump and motors.
Charge pump flow 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 li-
mited 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 man-
ifold.
Groundsmaster 4100--D/4110--D
Page 4 -- 13
Hydraulic System
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