SUB-SECTION 4D
CONTROL VALVES
Figure 4D-4. Control Valve (36Z725)
CONTROL VALVE (36Z726)
TROUBLESHOOTING
Table 4D-3 lists the difficulties which may be experienced
DESCRIPTION
with the valve and hydraulic system. It indicates the cause
Control valve 36Z726 is used to control the operation of
and remedy for each of the troubles listed. It should always
the boom hoist cylinders. This is a four-way, spring
be remembered that pressure and delivery are factors which
centered directional control valve. The valve is a balanced
am usually dependent upon each other. Adequate pressure
sliding spool type with open-centered circuitry, which
gauge equipment and a thorough understanding of the
allows free flow from inlet to outlet when the spool is in
operation of the complete hydraulic system are essential to
the neutral position. The operation of the valve is illus-
diagnosing improper valve operation.
tratad in Figure 4D-5.
OVERHAUL
A main relief valve is provided to limit circuit pressure to a
preset maximum. When the circuit is operating at less than
GENERAL. Overhaul of this valve is limited to the replace-
the relief valve setting, there is no flow over the by-pass
ment of the relief valve assembly and seals. Replace, the
orifice of the relief valve, so full circuit pressure is sensed at
complete valve when the required overhaul is beyond these
the spring end of the valve, as well as the opposite end. The
limitations.
valve is thus hydraulically balanced and the large spring
During disassembly, particular attention should be given to
holds it closed.
identification and orientation of parts for reassembly. The
Maximum circuit pressure is determined by the small spring
spool is selectively fitted to the valve body and must be
inside the relief valve assembly. When circuit pressure is
returned to the same body from which it was removed.
high enough to overcome this small spring, the poppet
inside the assembly is forced off its seat. Fluid immediately
Figure 4D-6 is an exploded view of the valve showing the
flows past the poppet to the tank passage. This flow creates
proper relationship of the parts for reassembly. Refer to
this figure while disassembling and reassembling the valve.
a pressure drop across the sensing orifice and the relief valve
is no longer hydraulically balanced. When this pressure
differential is great enough to overcome the large spring,
DISASSEMBLY. Disassemble the valve as follows:
the valve shifts, permitting flow to the tank passage.
1. Be sure the valve is not subjected to pressure. Discon-
RELIEF VALVE ADJUSTMENT
nect and cap all lines, and disconnect the linkage from the
spool. Remove the valve from the machine.
The setting of the main relief valve is determined by a
spring installed in the relief valve at the factory, and no
2. Remove the screws which secure the end cap, and
provisions have been made to adjust this relief valve.
remove the end cap.
4D-6
