Textron JACOBSEN GP400, Technical/Repair Manual

Page: 141 / 354

HYDROSTATIC POWER TRAIN
4252490-Rev A 5-23
5
Traction Circuit—Forward Schematic
and Theory of Operation
(3WD Models)
System Conditions:
• Engine running
• Park brake released
• Traction pedal in the forward 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
forward position, the swashplate is moved to the forward
position, stroking the traction pump and producing oil
flow against the check valve and to port “A.” Operating
pressure oil from port “A” of the traction pump is supplied
in parallel to the front wheel motors, then in series to the
rear wheel motor, resulting in full-time three-wheel drive.
Oil from the rear wheel motor returns to the traction
pump, entering port “B” and replenishing the intake side
of the pump.
Flushing Valve
The flushing valve is used to remove hot oil from the
closed-loop traction drive circuit when moving forward.
Oil lost due to venting through the flushing valve is
replaced within the traction pump by cooler, filtered oil
from the charge pump. When driving forward, operating
pressure oil is routed to the pilot line of the flushing valve,
shifting the valve spool and opening the valve. Hot oil
from the rear wheel motor flows through the flushing
valve, through the 0.040 in. (1.0 mm) orifice, and is
routed back to the hydraulic tank.
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.