Service Manual User Manual

Hydraulic System
9-19
9.8 HYDRAULIC CIRCUITS
This section describes each hydraulic circuit and
includes, on the following pages, a circuit schematic and
a discussion of the hydraulic function involved.
The hydraulic system consists of several components
that operate with each other and in conjunction with the
electrical system to produce the desired function via
input from operator controls or automated vehicle sys-
tems (such as the patented Stabil-TRAK system).
Components of the hydraulic system include the hydrau-
lic fluid, reservoir, filters, cooler, pump, unloader valve,
the main control valve, auxiliary function valve, pilot
select manifold, joystick control valve, cylinders, steering
select valve, brake valve, steering unit, hoses and fittings.
The gear pump produces flow of the hydraulic fluid for the
system. When the gear pump is operating, the atmo-
spheric pressure condition on the inlet side of the pump
is lower and hydraulic fluid flows from the reservoir,
through a strainer (filter) and hose, to the pump. The
pump sends the fluid through a hose to the unloader
valve and from there to various places within the system.
The unloader valve acts as a pressure regulator and as a
pressure relief for the hydraulic system.
The main control valve directs flow of fluid to the appro-
priate circuit via spool valve movement.
The auxiliary function valve contains pressure-reducing
valves and a pressure-relief valve to regulate circuit pres-
sure in the pilot pressure system, brake system, and
steering system.
The pilot select manifold contains solenoids which, when
activated, open passages in the manifold to direct pilot
pressure flow to operate the spool valves in the main
control valve.
The joystick control valve contains four valves that are
activated by moving the joystick, plus a reservoir return
valve. Activated valves result in fluid flow being directed
to the pilot select manifold. The joystick contains switches
that, when pressed, activate solenoids in the pilot select
manifold, producing a combination of functions.
The steer select valve is comprised of two electrical sole-
noids that act on a spool valve to direct flow to the steer-
ing cylinders, providing steering action.
The service brake valve is comprised of two identical
brake valves mounted next to each other in the same
casting. As force is applied to the brake pedal, the valve
sends oil to the service brakes at pressure that is propor-
tional to the force that is applied to the pedal.
In the manual mode of operation (ignition keyswitch OFF,
engine OFF), the brake valve operates as a two-stage
pump. The first stage pumps a high volume of oil at a rel-
atively low amount of pressure. The second stage then
pumps a low volume of oil for higher braking pressure,
ultimately resulting in braking action.
The power steering unit consists of a control valve and
metering sections. The control valve section contains
mechanically actuated linear spool that is torsion bar
centered and is of the open-center type. It directs fluid to
and from the metering section and steering cylinders and
regulates the flow supplied to the steering cylinders. The
metering section of the steering unit consists of a com-
mutator and a bi-directional gear-rotor element. It meters
the amount of fluid sent to the steering cylinders.
The main control valve contains four working sections,
plus an inlet section, and two end caps. Operating or
working sections for the hoist/lower, extend/retract, frame
tilt, and attachment tilt circuits are mounted between the
end cap sections, forming a valve stack. The main control
valve directs fluid to and from the various hydraulic cir-
cuits.
Model 3606 • Origin 10/99