valve in the forward position, allowing the fluid to flow
You may recall that this type of system is called an
in the opposite direction back to the reservoir.
open-center system. In an open-center system, when
the actuating mechanisms are idle, there is fluid flow
To tilt the upright, the tilt control valve lever is
but no pressure. The pump circulates the fluid from the
pulled back toward the operator. This directs the flow of
reservoir, through the control valves, and back to the
fluid from the pump to one of the manifolds, where it is
reservoir. The control valves in an open-center system
split into two lines and flows to the corresponding ends
are always connected in series, an arrangement
of the two tilt cylinders. The fluid from the opposite
whereby the pressure line goes through each control
ends of the actuating cylinders flows through the other
valve.
manifold, through the control valve, and back to the
reservoir.
Fluid is always allowed free passage through the
control valves and back to the reservoir until one of the
The shift and telescopic boom systems operate
control valves is positioned to operate a mechanism.
much like the tilt system. The quick-disconnect fittings
Fluid is then directed from the pump through one of the
are provided for convenient attachment and detachment
working lines to the actuator. Pressure builds up in the
of the telescopic boom.
system and provides the necessary force to move the
There is a filter located in the return line. The filter
actuator. The fluid from the other end of the actuator
is a full-flow type; that is, all fluid flows through the
returns to the control valve through the opposite
filter element. If the filter element becomes clogged
working line and flows back to the reservoir.
and causes pressure to build up in the return line, a relief
The pump (fig. 13-11) is a constant-displacement
valve in the top of the filter assembly opens and allows
pump. On engine-driven forklifts, the pump is usually
the fluid to bypass the element and flow directly to the
mounted on the transmission assembly and receives its
reservoir.
power from the engine through the transmission. On
electrically driven forklifts, an electric motor drives the
MAINTENANCE
pump. Gear pumps are normally used in these systems;
however, vane pumps are used on some models. With
The maintenance of the forklift hydraulic system
the engine running, the pump provides a flow of fluid to
includes periodic inspections, servicing, and repair.
the control valves.
The inspection intervals and procedures are included in
the operation and servicing instructions. Inspection
The control valves are actually four individual
consists mainly of checking for external leaks,
control valves. The lift control valve and the tilt control
damaged tubing and flexible hose, loose fittings, etc.
valve are constructed as one assembly, and the shift
You should pay particular attention to the type of fluid
control valve and the telescopic boom control valve are
used for servicing the system. Consult the technical
also constructed as one assembly. Each valve operates
manual for the correct fluid specification.
independently of the others, and each control valve
contains a relief valve. The relief valve protects the
Suppose one of the subsystems, like the lift system,
pump from any overload of pressure. For example, if
fails to operate. Before assuming it is defective, first
the pressure increases due to an overload, or due to an
check the operation of one or more of the other
actuator extending or retracting to the end of its stroke,
subsystems. If more than one fails to operate, the
the relief valve opens and fluid is allowed to bypass and
trouble is probably in the power system. The power
return to the reservoir.
system includes those components necessary to supply
fluid to the control valves. In this case, the power
To raise the fork, the lever of the lift control valve is
system includes the reservoir and the pump. Check the
moved aft. This directs fluid from the pump to the
fluid level in the reservoir first, and then check the
bottom of the lift cylinder. The orifice check valve
output of the pump.
allows free flow during the raising operation. However,
during the lowering operation, the orifice check valve
If the other subsystems operate normally, the
restricts the flow, thus preventing the load on the fork
malfunction is probably due to an internal leak in the
from forcing the piston of the lift cylinder to lower too
lift actuating cylinder or the lift control valve. Similar
fast. Flow regulators are used in some systems for this
procedures may be used to determine the cause of
purpose. The fork is lowered by positioning the control
malfunctions in the other subsystems.
13-18
