Traction Circuits
Forward
Atwosectiongearpumpisdirectlycoupledtothethehy-
drostat. Gear pump section (P2) supplies hydraulic flow
formaintaining100to150PSI(6.9to10.3bar)tothelow
pressure side of the traction circuit (charge), raising and
lowering the cutting units and operating the steering
system. The pump replenishes the closed loop traction
circuit with fluid from the hydraulic tank. The charge re-
lief valve in the hydrostat maintains sufficient pressure
so that pump flow is guided to the low pressure side of
the traction circuit through one of two check valves.
Pump flow in excess of replenishment requirements is
relieved through the charge relief valve back to gear
pump (P1) inlet.
The traction circuit of the hydraulic system consists of a
hydrostatconnectedinaclosedloopcircuittotwoorbital
rotor wheel motors.
The engine drives traction pump (P3) indirectly through
pulleysandaV--belt.Thetractionpumpisavariabledis-
placement piston pump. The traction pedal connects
through a cable to the trunnion shaft and swash plate of
the pump. With the engine running and the traction ped-
al in the neutral position, traction pump (P3) supplies no
flow to the wheel motors. When the traction pedal is
pressedtotheforward position,thecablefromthepedal
positions the swash plate in the traction pump so oil
flows out of the lower port. Oil flow out of the lower port
goes to the wheel motors and turns them in the forward
direction. Oil flowing out of the wheel motors returns to
the upper port of the hydrostat and is continuously
pumped out of the lower port.
Reverse
The traction circuit operates essentially the same in re-
verse as it does in forward. However, there are a few dif-
ferences in operation.
When the reverse traction pedal is depressed, the cable
from the pedal positions the swash plate in the traction
pump (P3) so oil flows out of the upper port. Oil flow out
of the upper port goes to the wheel motors and turns
theminthereversedirection.Oilflowingoutofthewheel
motors returns to the lower port of the hydrostat and is
continuously pumped out of the upper port.
As the load increases, forward traction circuit pressure
can increase to the relief valve setting of 3000 PSI (207
bar). If pressure exceeds the relief setting, oil flows
through the relief valve in the hydrostat to the low pres-
sure side of the closed loop traction circuit.
Thetractionpumpusesasmallamountofhydraulicfluid
for internal lubrication. Fluid is designed to leak across
pump parts into the case drain. This leakage results in
the loss of hydraulic fluid from the closed loop traction
circuit that must be replenished.
Groundsmaster 3505--D
Page 4 -- 9
Hydraulic System
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