Hitachi ZW180 Technical Manual page 120

Priority Valve Circuit
•
At stop of the engine, the priority valve spool is
pushed leftward by the spring force.
•
When the engine is started, pressure oil from the
main pump flows to the steering valve through the
priority valve spool, and also flows to ports LS1
and LS2 through orifices 1 and 2 respectively.
•
At neutral of the steering valve, as pressure oil to
port LS2 flows to the hydraulic oil tank through
orifice 3 and the steering valve spool, port LS2 is
not pressurized.
•
As pressure at port LS1 is larger than the spring
force, the priority valve spool moves rightward,
and all pressure oil from the main pump is
supplied to the control valve.
•
The priority valve spool is provided with a notch to
lead pressure oil from the main pump to the
steering valve and a notch to lead pressure oil
from the main pump to the control valve, which
are both connected to the main pump delivery
port constantly.
•
When the priority valve spool moves rightward,
the notch to lead pressure oil from the main pump
to the steering valve moves until the delivery port
on the steering valve side in the priority valve is
closed. When pressure balance is obtained and
the spool stops moving.
•
When the steering valve spool moves, the tank
port connected to port LS2 is closed.
•
At this time, as port LS2 is connected to the main
circuit through the steering valve spool, pressure
corresponding to movement of the steering valve
spool arises at port LS2.
SYSTEM / Hydraulic System
T2-3-4
•
When pressure at port LS2 and the spring force
overcome pressure at port LS1, the priority valve
spool moves leftward.
•
Larger the movement of the steering valve spool
is, the higher the pressure at port LS2 rises, the
larger the priority valve spool moves leftward, and
the more pressure oil from the main pump is
supplied to the steering valve.
NOTE: Orifice 2 of the priority valve is installed in
order to warm up the circuit by flowing
pressure oil to the hydraulic oil tank from
port LS2 at neutral of the steering valve.
Although diameter of port 2 is small and
temperature of the oil passing through it
rises rapidly, pressure is not raised enough
to influence movement of the priority valve
spool.