Hitachi ZW180 Technical Manual page 268

During Metering or Pressure Decrease
1. When the control lever is further tilted, the ouput
port is connected to port P via the fine control hole
on spool (7).
2. Pressure oil of port P flows into the output port
through spool (7) and pressure at the output port
is raised.
3. Pressure at the output port acts on spool (7) and
tends to push spool (7) upward.
4. In case the force pushing spool (7) upward is
smaller than the spring force of balance spring (5),
balance spring (5) is not compressed. Therefore,
port P and the output port remain connected, and
pressure at the output port keeps rising.
5. When pressure at the output port rises further, the
force pushing spool (7) upward increases. If this
force becomes larger than the force of balance
spring (5), spool
compressing balance spring (5).
6. When spool (7) moves upward, the output port is
not connected to port P and pressure oil stops
flowing from port P to the output port.
And pressure increase at the output port is
stopped.
7. In this way, balance spring (5) is compressed by
the amount spool (7) is pushed downward, and
pressure at the output port is the balanced
pressure acting on spool (7) and the spring force.
COMPONENT OPERATION / Pilot Valve
(7) moves upward
Full Stroke
1. When the control lever is fully stroked, disc (7)
pushes push rod (2) and piston (3) downward,
and spring guide (4) pushes spool (7) downward.
2. The output port is connected to port P via the fine
control hole on spool (7).
3. As spool (7) is pushed down by spring guide (4),
the output port remains connected via the fine
control hole of spool (7) although pressure at the
output port is raised and as spool (7) does not
move upward.
4. Consequently, pressure at the output port side is
equal to pressure at port P.
by
T3-6-10