Hitachi ZW180 Technical Manual page 262

During Metering or Pressure Decrease (Pusher
Stroke: Between C and D)
1. When the control lever is further tilted, the ouput
port is connected to port P via 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 surface B of
spool (7) and tends to push spool (7) upward.
4. In case the force push spool (7) upward is smaller
than the spring force of balance spring (5),
balance spring (5) is not compressed. Therefore,
port P and 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 (Pusher Stroke: Between E and F)
1. When the control lever is fully stroked, pusher (3)
moves downward until spring guide (4) contacts
the shoulder part of the casing.
2. At this time, spool (7) is directly pushed by the
bottom of pusher (3). Therefore, the output port
remains connected to port P through the notch
part of spool (7) although pressure at the output
port is raised, and as spool (7) does not move
upward.
3. Consequently, pressure at the output port side is
equal to pressure at port P.
Stroke amount C of the pusher determines the
total stroke of the lever.
Pilot
by
Pressure
T3-6-4
E
D
C
Pusher Stroke
A B
Output Diagram
F
T505-02-07-006