Hitachi ZW180 Technical Manual page 224

Bottom Side Circuit of Lift Arm and Bucket
Relief Operation
1. Pressure at port HP (the front circuit) acts on the
shaft through the seat.
2. When the pressure at port HP reaches the set
force of spring A, the shaft moves and pressure
oil flows to port LP.
3. Consequently, the pressure in the front circuit
decreases.
4. If pressure in the front circuit decreases to the set
pressure, the shaft is moved by the force of spring
A and the oil passage is closed.
Make-Up Operation
1. When pressure at port HP (the front circuit)
decreases lower than the pressure at port LP (the
hydraulic oil tank), the sleeve moves to the right.
2. Hydraulic oil in port LP flows into port HP and
cavitation is prevented.
3. When the pressure at port HP reaches the set
pressure, the sleeve is closed by the force of
spring B.
COMPONENT OPERATION / Control Valve
T3-2-16