2000 SRM 1233
Description and Operation
Operation
The main control valve controls the lift, tilt, and aux-
iliary functions. In these valves, the spools can be op-
erated without preventing the flow of oil to the other
spools.
The two-spool section (tilt/lift section) of the main
control valve receives oil from the flow amplifier of
the steering system. The single spool (lift section) re-
ceives oil directly from the hydraulic pump (tandem).
The main control valve has four passages through
each section. When the spools are in the NEUTRAL
position, the oil flows through the open-center pas-
sage and returns to the hydraulic tank.
A spool
makes a restriction in the open-center passage when
the spool is moved from the NEUTRAL position.
This restriction causes an increase in pressure in the
parallel passage. This passage is common to all sec-
tions of the valve, but oil cannot flow freely through
it. The pressure in the parallel passage causes the
oil to flow through the check valve into a supply
cavity in the valve body. The spool opens a path
from the supply cavity to the hydraulic cylinders to
do work, and also opens the tank line.
Auxiliary Section
The single spool control valve will, depending on the
function selected by the operator, cause either out-
put Aux A or output Aux B to supply hydraulic oil to
carriage/attachment. When output Aux A supplies
the oil, the solenoid valve provides the return to
hydraulic tank. When output Aux B supplies the oil,
the solenoid valve provides the return to the tank.
When no function is selected by the operator, both
solenoid valves provide return to hydraulic tank.
Oil that is not used to operate auxiliary functions is
added to the flow of the lift functions. The pressure
of the oil of the single spool main control valve can
be checked at check port MP.
Tilt Section
Each port in the tilt section of the single spool con-
trol valve is connected to the tilt cylinders. When
the spool is moved to the forward tilting position,
the spool makes a restriction in the open-center pas-
sage. The increased pressure in the parallel passage
causes oil to flow to the tilt cylinders and pressure
to the impact relief valve. The impact relief valve is
set on 25 MPa (3625 psi) and will be activated dur-
ing contact. This allows the operator to tilt the mast
backwards. When the spool is in the backward tilt-
ing position, the spool opens the paths from the tilt
cylinders to the hydraulic tank. Tilt lock relief valves
prevents the mast from tilting forward too fast by
keeping some back pressure at the rod ends of the
tilt cylinders. The pressure of the oil of the two-spool
main control valve can be checked at check port M3.
Lift Section
Each port in the lift section of the control valve is con-
nected to the lift cylinders. The main control valve
has two lift sections, and will not start together. One
section is delayed. See Figure 3. When the spool is
moved to the LIFT position, the spool makes a re-
striction in the open-center passage. The increases
pressure in the parallel passage causes oil to flow
through the check valve to the supply cavity. When
the spool is in the LOWER position, the spool opens
the paths from the lift cylinders to the drain cav-
ity. The maximum LOWER speed is controlled by
the lower control valves in the lift cylinders.
Oil
from the pump can flow through the open-center pas-
sage when the spool is in the LOWER position or the
NEUTRAL position. The NEUTRAL position of the
spool closes the passages to the ports for the lift cylin-
ders.
The pressure of the oil of the single spool main control
valve can be checked at check port M2.
RETURN MANIFOLD, SECTION 3 OF
HYDRAULIC PLATE
The return manifold regulates and directs the flow
of oil from all components to the hydraulic plate and
the cab manifold to the hydraulic tank, except cooling
flow of the brakes. See Figure 3.
Two hydraulic lines are connected from the return
manifold to the hydraulic tank. Oil from the man-
ifold returns through two filters (10 micron) with a
maximum pressure of 300 kPa (43.5 psi) to the hy-
draulic tank.
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