Steering Overload Relief Valve - Hitachi ZW180 Technical Manual

STEERING OVERLOAD RELIEF VALVE

The steering overload relief valve is installed in the left
and right steering circuits. The overload relief valve
controls pressure in the respective steering circuits
from rising abnormally high when the steering cylinder
is moved by an external force.
Relief Operation
1. Pressure at port HP (steering cylinder circuit) acts
on the pilot poppet through the orifice in the
piston.
2. When pressure at port HP reaches the set force of
spring B, the pilot poppet opens. Pressure oil
flows to port LP (hydraulic oil tank) through
passage A and the periphery of the sleeve.
3. At this time, pressure difference arises between
port HP and the spring chamber by the orifice.
4. When this pressure difference reaches the set
force of spring A, the piston and the main poppet
open, and pressure oil at port HP flows to port LP.
5. Consequently, pressure in the steering cylinder
circuit decreases.
6. If pressure in the steering cylinder circuit
decreases to the set pressure value, the piston
and the main poppet are closed by the force of
spring A.
COMPONENT OPERATION / Steering Valve
Make-Up Operation
1. When pressure at port HP (steering cylinder
circuit) becomes lower than pressure at port LP
(hydraulic oil tank), the sleeve moves to the right.
2. Hydraulic oil flows into port HP and cavitation is
prevented.
3. If pressure at port HP rises to the set pressure
value, the sleeve is closed by the force of spring
C.
T3-5-8