3-22. CONTROL VALVE ASSEMBLY MAINTENANCE
THEORY OF OPERATION
The control valve assembly is a three spool valve with an open center passage. This
open center passage starts at control valve assembly inlet port and stops at the
outlet port. Return oil from bucket, clam, and lift cylinders flows to the outlet
port through return passages in top and bottom of the control valve assembly.
A supply passage starts at inlet port and stops at bucket spool of control valve as-
sembly. This supply passage permits operation of bucket and clam cylinders at the
same time. Cylinders with least amount of resistance will move first.
When clam spool or bucket spool is actuated, oil flow to lift spool is stopped. Lift
cylinders will not move until both clam and bucket spools are returned to neutral.
Load check valves are connected between the open center passage and A port and B
port for each spool. The load check valves prevent reverse flow of oil between cyl-
inders and control valve assembly as a spool is moved into a power position.
Circuit relief valves give protection to circuits when a cylinder is made to move
with associated spool in neutral. Circuit relief valves are connected between A port
and B port and return passage for bucket spool and clam spool.
An adjustable main relief valve is located at the inlet of the control valve assem-
bly. If inlet pressure reaches main relief valve setting while a spool is actuated,
the main relief valve opens to give protection to the hydraulic system.