Traction Circuit: 4Wd (Mow) - Toro Groundsmaster 4000-D Service Manual

Traction Circuit: 4WD (Mow)

The traction circuit piston pump is a variable displace-
ment pump that is directly coupled to the engine fly-
wheel. 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. Oper-
ating 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 mo-
tors. As the load increases, circuit pressure can in-
crease 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 com-
ponents 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 cir-
cuit. 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.
A flow divider is incorporated into the traction circuit and
is located in the traction manifold. When in 4WD (mow),
the operator can momentarily engage this flow divider
when low traction situations could lead to wheel spin.
Depressing the flow divider switch energizes the sole-
noid valve in the traction manifold. This energized sole-
noid valve causes traction pump hydraulic flow to split
between the front wheel motors (approximately 45%)
and rear axle motor (approximately 55%) to reduce the
chance that excessive flow goes to a spinning wheel.
Groundsmaster 4000--D
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 passes through the
traction manifold. Oil flow from traction manifold port M1
drives the front wheel motors in the forward direction
and then returns to the hydrostat. Oil flow from traction
manifold port M2 is routed to the P1 port of the 4WD
manifold where it is directed to the PD1 cartridge and out
of the manifold 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 re-
turns to the hydrostat. To prevent rear wheel lock up, an
adjustable relief valve (RV) in the 4WD manifold re-
duces rear axle motor pressure created in down hill, dy-
namic braking conditions.
Reverse Direction
The traction circuit operates essentially the same in re-
verse 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 re-
turns to the hydrostat through the traction manifold. 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 car-
tridge, exits the manifold at port P1 and returns to the hy-
drostat.
Page 4 - - 11 Hydraulic System