Traction Circuit - Toro Greensmaster TriFlex 3320/3420 Service Manual

Traction Circuit

The traction circuit piston pump is a variable displace-
ment pump that is directly coupled to the engine. Push-
ing the traction pedal controls the variable displacement
piston pump swash plate to create a flow of oil. This oil
is directed to the wheel 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 motors. As the load in-
creases, circuit pressure can increase to relief valve set-
tings: 3000 PSI (207 bar) in both forward and 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 pump and wheel 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 that supplies oil to the steering and lift
circuits also provides charge circuit oil. This gear pump
is driven directly off the piston (traction) pump. It pro-
vides a constant supply of charge oil to the traction cir-
cuit to make up for oil that is lost due to internal leakage
in the traction pump and wheel motors.
Charge pump flow is directed to the low pressure side
of the closed loop traction circuit. Charge circuit pres-
sure (110 to 150 PSI / 7.6 to 10.3 bar) is maintained by
the charge relief valve that is attached to the piston
pump back plate.
The functional control lever on Greensmaster TriFlex
Hybrid machines has positions for neutral, mow and
transport. When the functional control lever is in the neu-
tral position, the traction pedal is secured in the neutral
position. The mow position allows traction pedal inputs
that are appropriate for mow speeds by limiting the
movement of the piston pump swash plate. The trans-
port position allows full movement of the traction pedal
so complete pump swash rotation is possible.
Forward
The piston (traction) pump is driven directly by the en-
gine. The hydraulic traction circuit acts essentially as a
closed loop. Taking its suction directly from the return
side of the wheel motors of the traction circuit, the piston
pump supplies oil flow to the wheel motors through the
supply side of the traction circuit.
Greensmaster 3320/3420
With the engine running and traction pedal in the neutral
position, the piston pump supplies no flow to the wheel
motors. When the traction pedal is pressed to the for-
ward position, the linkage from the pedal positions the
swash plate in the piston pump so oil flows out the top
port of the pump. Oil flow out of the top port goes to the
wheel motors and turns them in the forward direction.
Oil flowing out of the wheel motors returns to the bottom
port of the piston pump and is continuously pumped out
the top port.
Reverse
The traction circuit operates essentially the same in re-
verse as it does in the forward direction. However, the
flow through the circuit is reversed.
With the engine running and traction pedal in the neutral
position, the piston pump supplies no flow to the wheel
motors. When the traction pedal is pressed to the re-
verse position, the linkage from the pedal positions the
swash plate in the piston pump so oil flows out the bot-
tom port of the pump. Oil flow out of the bottom port goes
to the wheel motors and turns them in the reverse direc-
tion.
Oil flowing out of the wheel motors returns to the top port
of the piston pump and is continuously pumped out the
bottom port.
Traction Circuit Cooling
The piston pump includes a shuttle valve that bleeds off
a small amount of hydraulic fluid for cooling of the closed
loop traction circuit. This valve allows a small amount of
hydraulic oil to pass from the low pressure side of the
traction circuit while operating the traction unit in either
direction. A relief valve in the piston pump back plate
prevents shuttle valve bleed off until the relief valve
opens (70 to 100 PSI / 4.9 to 6.8 bar). The charge circuit
replenishes oil that is bled from the traction circuit by the
shuttle valve.
NOTE: The shuttle valve assembly is located in the pis-
ton pump back plate. Access to the shuttle valve re-
quires removal of the back plate from the piston pump.
Page 5 - - 13 Hydraulic System