IV-58
HYDRAULIC UNITS
CONTROL VALVE (HIGH FLOW)
Unload Relief Valve
When Unloading (When the Solenoid Valve is
Not Powered On):
The hydraulic oil from the port P partially flows
through the small hole (2) of the plunger (1) and
from the solenoid valve switching section into the
port T. This makes the pressure in the chamber A
lower than the pressure in the port P, moving the
plunger (1) to the left, and the hydraulic oil from the
port P flows through the passage (3) and the port P,
and returns to the tank.
When Relief (When the Solenoid Valve is Pow-
ered On):
The circuit of the solenoid switching section is closed
to generate the pressure in the chamber A. As the
pressures in the port P and the chamber A become
equivalent, the plunger (1) is moved to the right by
the spring force and pressed against the seat.
F1 (force to the right direction) = A1 × P + spring
force
F2 (force to the left direction) = A2 × P
Therefore, F1 > F2 is established, and this makes the
plunger (1) to be pressed against the seat (1).
When the pressure in the port P becomes higher than
the set value, the needle valve (4) in the chamber B
moves to the left, and the oil in the chamber B flows
through the passage (5) into the port T. The relations
between the pressures in the port P and the chamber
A becomes F1 < F2, and the plunger (1) moves to
the left to let the oil flow into the port T.
When the pressure in the port P becomes equal to
or less than the set value for the relief, the needle
valve (4) is pressed against the seat by the spring
to generate the pressure in the chamber A. Then the
plunger (1) shuts off the circuit from the port P to
the port T to keep the circuit pressure to the one set
for the relief.
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