Toro 07511AA, Service Manual

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A two (2) section gear pump is coupled directly to the tandem piston (traction)
pump. The gear pump section furthest from the piston pump, gear pump section
(P4), supplies fluid to the steering, hitch lift and traction charge circuits. Gear
pump section (P4) also supplies flow to the 2 speed drive motor control, brake
release, differential lock and PTO clutch circuits.
Hydraulic flow from gear pump section (P4) is directed to the appropriate circuits
by a steering priority valve (EC1) located in the main manifold. The steering
priority valve ensures the steering circuit always has enough hydraulic flow to
steer the machine, no matter what the hydraulic needs of the other circuits are at
the time.
The steering wheel is directly attached to the steering valve. The steering valve
is a closed center, load sensing rotary valve. The steering valve senses when
fluid flow is needed for steering and the steering priority valve (EC1) shifts to
supply hydraulic flow. Gear pump (P4) fluid flow that is not needed for the
steering circuit is available for the remaining circuits.
Two (2) steering cylinders are used on the machine, one for steering the front
wheels and one for steering the rear wheels. The cylinders are connected to
the steering valve in series. To allow for synchronization of the front and rear
steering cylinders, both the steering cylinders include an internal re-phasing
check valve assembly in the cylinder piston. While rotating the steering wheel for
a turn (either right or left), the cylinder check valve will open when the cylinder is
fully extended or retracted. After both cylinders have moved fully, the 4 wheel
steering system has been synchronized. If the steering wheel continues to be
turned, the check valves will stay open and will allow flow through the circuit.
Neutral Position
When the steering wheel is in the neutral position (at rest) and the engine is
running, hydraulic fluid from gear pump (P4) enters the main manifold port P,
flows through the steering priority valve (EC1) and is routed to the steering valve
where it dead heads at the control valve spool. Fluid is also sent to both ends
of the steering priority valve (EC1) spool. On one end of the spool, fluid is
directed to both the steering relief valve (RV1), and also to orifice OR1 then out
the manifold LS/OR1 port to the steering valve. This flow provides steering load
sensing that is directed through a small passage in the steering valve spool and
sleeve before returning to the hydraulic tank. As long as this load sense flow is
returning to the tank due to no steering input, the steering priority valve (EC1)
spool is shifted to direct fluid to the flow regulator valve (EC2) for other uses.
Left Turn
When a left turn is made with the engine running, the turning of the steering
wheel positions the steering valve spool so that the load sense flow is blocked.
Without load sense flow, pressures on the ends of the steering priority valve
(EC1) start to equalize causing (EC1) to move toward its un-shifted “neutral”
position which directs the fluid needed to turn the machine to the steering valve.
Fluid is routed out manifold port ST, into steering valve port P, through the
steering valve spool, passes through the rotary meter and out the L port to the
steering cylinders. The rotary meter ensures that the fluid flow to the cylinders is
proportional to the amount that the steering wheel is turned. Fluid leaving the
cylinders flows back through steering valve R port, the valve spool, out the T port
and is then returned to the hydraulic reservoir. The steering valve returns to the
neutral position when the steering wheel is released or turning is completed.
Right Turn
When a right turn is made with the engine running, the turning of the steering
wheel positions the steering valve spool so that the load sense flow is blocked.
Hydraulic System: Hydraulic Flow Diagrams Page 5–20 Outcross 9060
18234SL Rev B