SUB-SECTION 4D
CONTROL VALVES
Figure 4D-9. Control Valve Operation (36Z986)
Maximum circuit pressure is determined by the small spring
operation of the complete hydraulic system are essential to
inside the relief valve assembly. When circuit pressure is
diagnosing improper valve operation.
high enough to overcome this small spring, the poppet
inside the assembly is forced off its seat. Fluid immediately
OVERHAUL
flaws past the poppet to the tank passage. This flow creates
GENERAL. Overhaul of this valve is limited to the replace-
a pressure drop across the sensing orifice end the relief valve
ment of the relief valve assembly and seals. Replace the
is no longer hydraulically balanced. When this pressure
complete valve when the required overhaul is beyond these
differential is great enough to overcome the large spring,
limitations.
the valve shifts, permitting flow to the tank passage.
RELIEF VALVE ADJUSTMENT
During disassembly, particular attention should be given to
identification and orientation of parts for reassembly. The
The setting of the main relief valve is determined by a
spool is selectively fitted to the valve body and must be
spring installed in the relief valve at the factory, and no
returned to the same body from which it was removed.
provisions have been made to adjust this relief valve.
Figure 4D-10 is en exploded view of the valve showing the
TROUBLESHOOTING
proper relationship of the parts for reassembly. Refer to
this figure while disassembling and reassembling the valve.
Table 4D-5 lists the difficulties which may be experienced
with the valve and hydraulic system. It indicates the cause
DISASSEMBLY. Disassemble the valve as follows:
and remedy for each of the troubles listed. It should always
be remembered that pressure and delivery are factors which
1. Be sure the valve is not subjected to pressure. Discon-
are usually dependent upon each other. Adequate pressure
nect and cap all lines, and diconnect the linkage from the
gauge equipment and a thorough understanding of the
spool. Remove the valve from the machine.
4D-12
