Textron JACOBSEN GP400, Technical/Repair Manual

Page: 145 / 354

HYDROSTATIC POWER TRAIN
4252490-Rev A 5-27
5
Traction Circuit—Reverse Schematic
and Theory of Operation
(3WD Models)
System Conditions:
• Engine running
• Park brake released
• Traction pedal in the reverse position
Charge Pressure Circuit
The charge pump and traction pump are both driven by
the engine. The charge pump is mounted between the
rotating group and rear valve plate assembly in the
traction pump housing. It supplies oil to the traction
pump/circuit. Oil is drawn from the hydraulic tank through
the charge filter and enters the charge pump.
The charge filter uses a 10-micron filter to remove debris
from the oil. If the charge filter becomes plugged, a 29 psi
(2.0 bar) pressure switch will activate the “Filter Blocked”
LED on the instrument panel. The charge filter will be
bypassed when the pressure exceeds 51 psi (3.5 bar).
Oil from the charge pump is routed to the 150–210 psi
(10–15 bar) charge relief valve, the 800 psi (55 bar)
implement relief valve, and the two check valves.
The charge relief valve regulates the charge pressure
applied to the traction pump/circuit. When the incoming
charge pressure oil reaches 150–210 psi (10–15 bar),
the charge pressure relief valve will open, venting excess
flow back to the hydraulic tank via the steering unit and
oil cooler.
The excess charge pump supply is used to operate the
steering circuit. The 800 psi (55 bar) implement relief
valve is used to allow intermittent operation of the charge
pump at 800 psi (55 bar).
Traction Drive Circuit
The traction pedal is linked to the traction pump by cable.
Movement of the traction pedal results directly in a
corresponding movement of the traction pump
swashplate. When the traction pedal is moved to the
reverse position, the swashplate is moved to the reverse
position, stroking the traction pump and producing oil
flow against the check valve and to port “B.”
Operating pressure oil from port “B” of the traction pump
is routed to the rear wheel motor and the “RW2” port of
the flushing valve.
Oil exiting the rear motor is routed in parallel to the front
wheel motors, driving the motors in the reverse direction.
Oil from the “RW2” port of the flushing valve flows
against the flushing valve, through the check valve, and
exits the “RW1” port of the flushing valve. It is then routed
in parallel to the front and rear wheel motors. Since
pressure is equal on each side of the rear wheel motor,
the rear wheel motor does not drive in reverse and is
effectively bypassed.
Oil from the front wheel motors returns to the traction
pump, entering port “A” and replenishing the intake side
of the pump.
Flushing Valve
The flushing valve is held closed when there is operating
pressure in the reverse position.
Circuit Relief
The traction circuit is not protected by any form of shock
or relief valve. This is considered unnecessary, because
overloading the system will result in spinning the slick
tires.