Overload Relief Valve - Hitachi ZW180 Technical Manual

OVERLOAD RELIEF VALVE

(With Make-Up Function)
The overload relief valve is provided with the circuit in
the bottom side of the lift arm and the bottom and rod
side of the bucket. The overload relief valve controls
pressure in each front circuit in order not to rise
abnormally when each front attachment is operated by
external force. It also makes make-up operation by
refilling oil from the hydraulic oil tank in order to prevent
cavitation when pressure in the front circuit decreases.
Rod Side Circuit of Bucket
Relief Operation
1. Pressure at port HP (the front circuit) acts on the
pilot poppet through the orifice of the piston.
2. When pressure at port HP reaches the set force of
spring B, the pilot poppet opens and pressure oil
flows to port LP (the hydraulic oil tank) through
passage A and the periphery of the sleeve.
3. At this time, the pressure difference arises
between port HP and the spring chamber due to
the orifice.
4. When this pressure difference reaches the set
force of spring A, the piston and main poppet open,
and pressure oil in port HP flows to port LP.
5. Consequently, pressure in the front circuit
decreases.
6. If the front circuit pressure decreases to the set
pressure, the piston and main poppet are closed
by the force of spring A.
COMPONENT OPERATION / Control Valve
Make-Up Operation
1. When the 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 rises to the set
pressure, the sleeve is closed by the force of
spring C.
T3-2-14