Service Manual

Groundsmaster 7200/7210 Hydraulic SystemPage 4 – 11
Traction Circuits
The Groundsmaster traction system consists of a trans-
mission that is directly coupled to, and driven by, the en-
gine flywheel. The transmission provides two identical
and independent closed loop traction circuits: one for
the right wheel motor and one for the left wheel motor.
Each of these transmission circuits include a variable
displacement, slipper foot design piston pump which
provides hydraulic flow for one wheel motor. The swash
plate in each of the piston pumps is controlled by one of
the operator traction levers.
The angle of the swash plate determines pump flow and
ultimately traction speed. When a traction lever is
moved a small amount, a small swash plate rotation re-
sults in low pump output and lower traction speed. When
the traction lever is moved fully, the pump swash plate
rotates fully to provide maximum pump output flow and
traction speed.
Because hydraulic flow to each rear wheel is deter-
mined by traction lever movement and subsequent
swash plate position, steering is accomplished by mov-
ing the two traction levers a different amount or direc-
tion.
With the engine running and the traction levers in the
neutral position, the piston pump swash plates are held
in the vertical position, providing no flow to either wheel
motor and the machine remains stationary. The trans-
mission reverse check valves include an orifice which
makes it easier to locate the transmission neutral posi-
tion.
Forward
When a traction lever is pushed forward, the rod con-
nected to the traction lever positions the swash plate in
the piston pump to provide oil flow from the transmission
(port A for the RH side or C for the LH side). This oil flows
to the wheel motor and turns the motor in the forward
direction. Oil flow from the wheel motor returns to the
transmission (port B or D) and is continuously pumped
as long as the traction lever is pushed forward.
As the traction load increases, forward traction circuit
pressure can increase to the relief valve setting of 4000
PSI (276 bar). If pressure exceeds the relief setting, oil
flows through the forward relief valve in the transmission
to the low pressure side of the closed loop traction cir-
cuit.
The piston pumps use a small amount of hydraulic fluid
for internal lubrication. Fluid is designed to leak across
pump parts into the transmission case drain. This leak-
age results in the loss of hydraulic fluid from the closed
loop traction circuits that must be replenished.
A charge pump in the transmission supplies hydraulic
flow for maintaining 70 to 100 PSI (4.8 to 6.9 bar) to the
low pressure side of both of the traction circuits. The
charge pump also provides pressure for the transmis-
sion PTO drive system. The charge pump replenishes
the closed loop traction circuits with fluid from the hy-
draulic tank. The charge relief valve in the transmission
maintains sufficient pressure so that charge pump flow
is guided past check valves to the low pressure side of
each traction circuit. Charge pump flow in excess of sys-
tem requirements is relieved through the charge relief
valve back to the hydraulic tank.
Both of the transmission traction circuits include a flush-
ing valve that bleeds off a small amount of hydraulic fluid
for cooling of the closed loop traction circuits. The flush-
ing valves are opened only during forward direction op-
eration. When the flushing valves are unseated, some
oil from the low pressure side of the traction loop is re-
turned to the hydraulic tank. Oil loss from the closed loop
due to the flushing valve is replenished by the charge
pump.
Reverse
The traction circuit operates essentially the same in re-
verse as it does in forward.
When a traction lever is pulled rearward, the rod con-
nected to the lever positions the swash plate in the trans-
mission piston pump to provide oil flow from the
transmission (port B for the RH side or D for the LH side).
This oil flows to the wheel motor and turns the motor in
the reverse direction. Oil flow from the wheel motor re-
turns to the transmission (port A or C) and is continuous-
ly pumped as long as the traction lever is pushed
rearward.
As the traction load increases, reverse traction circuit
pressure can increase to the relief valve setting of 4000
PSI (276 bar). If pressure exceeds the relief setting, oil
flows through the reverse relief valve in the transmission
to the low pressure side of the closed loop traction cir-
cuit.
The charge circuit functions the same in reverse as it
does in the forward direction. The check valve placed in
the reverse side of the traction circuit includes an orifice
which allows the traction neutral position to be easier to
adjust. The flushing valve remains seated during re-
verse operation.
Hydraulic
System