Hitachi ZW180 Technical Manual page 278

During Metering or Pressure Decrease (Output
Diagram: Between C and D)
1. When the control lever is further tilted and pusher
(2) is pushed down, hole part (7) on spool (6)
reaches port P and pressure oil of port P flows
into the output port.
2. Pressure at the output port acts on the bottom of
spool (6) and tends to push spool (6) upward.
3. In case the force acting on spool (6) is smaller
than the spring force of balance spring (4),
balance spring (4) is not compressed. Therefore,
spool (6) is not pushed upward, and pressure at
the output port keeps rising.
4. When pressure at the output port rises further, the
force pushing spool (6) upward increases. If this
force becomes larger than the force of balance
spring (4), spool
compressing balance spring (4).
5. When spool (6) moves upward, hole part (7)
closes, pressure oil stops flowing from port P to
the output port, and pressure increase at the
output port is stopped.
6. In this way, balance spring (4) is compressed by
the amount spool (6) is pushed downward, and
pressure at the output port is the balanced
pressure acting on spool (6) and the spring force.
COMPONENT OPERATION / Pilot Valve
(6) moves upward
Pilot
Pressure
by
T3-6-20
E
D
C
A B
Lever Stroke
Output Diagram
F
T523-02-05-001