Traction Circuits - Toro 30821 Service Manual

Traction Circuits

Forward
The traction circuit of the hydraulic system consists of a
hydrostat connected in a closed loop circuit to three or-
bital vane wheel motors.
The engine drives traction pump (P3) indirectly through
pulleys and a V- belt. The traction pump is a variable dis-
placement piston pump. The traction pedal connects
through a cable to the trunnion shaft and swash plate of
the pump. With the engine running and the traction ped-
al in the neutral position, traction pump (P3) supplies no
flow to the wheel motors. When the traction pedal is
pressed to the forward position, the cable from the pedal
positions the swash plate in the traction pump so oil
flows out of the lower port. Oil flow out of the lower port
goes to the wheel motors and turns them in the forward
direction. Oil flowing out of the wheel motors returns to
the upper port of the hydrostat and is continuously
pumped out of the lower port.
The rear wheel motor has a small check valve across its
ports that allows the rear motor to over run during tight
turns in the forward direction.
The traction pump uses a small amount of hydraulic fluid
for internal lubrication. Fluid is designed to leak across
pump parts into the case drain. This leakage results in
Groundsmaster 3500- D
the loss of hydraulic fluid from the closed loop circuit that
must be replenished.
The charge pump (P2) is a fixed displacement gear
pump. It is driven directly off the traction pump. The
pump replenishes the closed loop traction circuit with
fluid from the hydraulic tank. The charge relief valve
supplies sufficient pressure so that charge pump flow is
guided to the low pressure side of the traction circuit
through one of two check valves. Pump flow in excess
of replenishment requirements is relieved through the
charge relief valve back to the gear pump inlet.
Reverse
The traction circuit operates essentially the same in re-
verse as it does in forward. However, there are a few dif-
ferences in operation.
When the reverse traction pedal is depressed, the cable
from the pedal positions the swash plate in the traction
pump (P3) so oil flows out of the upper port. Oil flow out
of the upper port goes to the wheel motors and turns
them in the reverse direction. Oil flowing out of the wheel
motors returns to the lower port of the hydrostat and is
continuously pumped out of the upper port. Oil by-
passes the rear motor in reverse because of the check
valve inside the rear wheel motor.
Page 4 - 13 Hydraulic System (Rev. G)