Group 30, Hydraulic Control Valve/Lift Circuit
30-1-2 • Hydraulic Control Valve/Lift Circuit Specifications and Description SM 765, Nov ’06
Description
The following description focuses primarily on hydraulic
circuitry controlled by the main hydraulic control valve,
that is, the lift/tilt/aux circuit. Various other hydraulic sys-
tems come into play, however, and are mentioned. The
entire hydraulic system is depicted in the schematics in
Section 2 (next page).
Descriptions of the braking and steering circuits are given
in Groups 23 and 25.
The main hydraulic pump (described in Group 29) is
driven by the pump motor and draws fluid from the sump
through a particle-blocking suction screen.
The main hydraulic control valve features an open-center,
parallel-circuit type modular design. It has the main (lift/
tilt) pressure relief valve (steering pressure relief valve
located in the steering control assembly (Orbitrol)), a sec-
ondary pressure relief valve for optional auxiliary compo-
nents, a lift spool, a tilt spool with an integral
counterbalance valve, optional auxiliary spools, and
adjustable pressure-compensated flow controls. All spools
are low-leakage design.
The main hydraulic valve has from two to four valve sec-
tions. Each section performs a separate function; standard
two spool assemblies have a inlet/lift section (with fluid
inlet port), a tilt section, and an outlet section. A third and
fourth section may be added to control auxiliary compo-
nents.
When lift attachments are used, an auxiliary section may
be added to the outer (RH) side of the standard (lift/tilt
only) main valve. The optional auxiliary sections also
have an adjustable relief valve and can be assembled with
optional flow control levels.
The valve spools are arranged in standard sequence (from
the operator’s position) to first provide lift control, then
tilt, and finally auxiliary control. The control levers are
spring-loaded (by the valve spool centering springs) to
return them to neutral when released. Oil flow is con-
trolled by the amount or distance the control handles are
moved. Excess oil flow is returned to the sump. A check
valve prevents reverse flow.
When all the control valve spools are in neutral, the micro
switch turns the pump motor OFF and not fluid flows.
When a spool is partially shifted and the associated cylin-
der or other actuator has not reached its end-of-travel,
some
of the fluid flows to the cylinder (actuator) and the rest
flows to the sump line. In both cases, the pressure in the system
should be less than the amount required to open the relief valves.
The main relief valve vents flow to the sump when one of
the following conditions is present:
• The operator continues to hold the lift control in
the lift position after the lift mechanism reaches its
endof-travel.
• Too heavy a load is being lifted.
• The operator continues to hold the tilt control in
the tilt position after the tilt mechanism has
reached its end-of-travel (This is called “tilt
bypass.“)
• Auxiliary relief fails to operate.
The auxiliary relief valve vents flow to the sump when the
operator continues to hold the attachment control in the
operated position after the attachment reaches its end-of-
travel.
Main and auxiliary relief pressure settings can be checked
through a gauge port on the main valve.
A tilt-lock valve built into the main control valve assem-
bly locks the upright into its current tilt position when the
truck is turned off. A load lowering flow valve mounted
on the upright limits the speed at which the operator can
lower a load, decreasing the speed for heavier loads. A
velocity fuse built into one of the lift cylinder ports pre-
vents the upright from falling rapidly should a hydraulic
line rupture or be disconnected.
N
TE
Hydraulic plumbing arrangement is illustrated in Group 29.
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