caution - Toro

Traction Circuit: 4WD (Mow)
The traction circuit piston pump is a variable displacement
pump that is directly coupled to the engine flywheel.
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
is directed to the front wheel and rear axle motors. Operating
pressure on the high pressure side of the closed
traction circuit loop is determined by the amount of load
developed at the fixed displacement wheel and axle motors.
As the load increases, circuit pressure can increase
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 relief
valve to the low pressure side of the closed loop traction
circuit. The traction circuit provides operation in either
4WD (mow) or transport (2WD).
Traction circuit pressure (forward and reverse) can be
measured at test ports on the sides 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 components
into the case drain. This leakage results in the
loss of hydraulic fluid from the closed loop traction circuit
that must be replaced. The charge circuit is designed to
replace this traction circuit leakage.
The gear pump section that supplies oil to the steering
and lift/lower circuits also provides oil for the charge circuit.
This gear pump is driven directly off the traction
pump. It provides a constant supply of charge oil to
make up for oil that is lost due to internal leakage in the
traction pump and motors.
Pump flow for the charge circuit 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 limited 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 manifold.
Forward Direction
When the transport/4WD switch is in the 4WD (mow)
position and the traction pedal is pushed in the forward
direction, oil from the piston pump is directed to the front
wheel motors and 4WD manifold. Oil flow to the front
wheel motors drives the motors in the forward direction
and then returns to the hydrostat. Oil flow to the 4WD
manifold enters the P1 port and then is directed to the
PD1 cartridge and out of the manifold M1 port to drive
the rear axle motor in the forward direction. Oil returning
from the rear motor re--enters the 4WD manifold at the
M2 port. Flow passes through the PD2 cartridge,
through the CV check valve, out manifold port P2 and
back to the hydrostat.
When going down a hill, the tractor becomes an over--
running load that drives the wheel and axle motors. In
this condition, the rear axle motor could lock up as the
oil pumped from the motor increases pressure as it returns
to the hydrostat. To prevent rear wheel lock up, an
adjustable relief valve (RV) in the 4WD manifold reduces
rear axle motor pressure created in down hill, dynamic
braking conditions.
Reverse Direction
The traction circuit operates essentially the same in reverse
4WD (mow) as it does in the forward direction.
However, the flow through the circuit is reversed. Oil flow
from the hydrostat is directed to the front wheel motors
and also to the 4WD manifold. The oil to the front wheel
motors drives them in the reverse direction and then returns
to the hydrostat. The oil to the 4WD manifold enters
the manifold at port P2 and flows through pressure
reducing valve (PR) which limits the down stream pressure
to the rear axle motor to 650 PSI (45 bar) so the rear
wheels will not scuff the turf during reverse operation.
This reduced pressure flows through the PD2 cartridge
and out port M2 to the rear axle motor. Return oil from
the rear motor re--enters the 4WD manifold at port M1,
flows through the PD1 cartridge, exits the manifold at
port P1 and returns to the hydrostat.
Hydraulic
System
Groundsmaster 4100--D
Page 4 - 11
Hydraulic System