Maeda MK0003, Service Manual, Page: 103 / 348

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[2] Operation of flow control valve
1. When each switching valve is in its neutral position
When each switching valve is in its neutral position, the hydraulic oil supplied from pump passes through the
central bypass path and tank path including this control valve, and returns to the tank through the discharge port
of the supply section.
2. When a switching valve is operated
(when the solenoid of negative magnetic proportional reducing
valve is not excited)
When any of the switching valves is switched, the central bypass path is closed, and the hydraulic oil pushes
detecting valve (1) upward to flow the oil into the parallel path. In this stage, pump pressure is applied to the
primary side chamber (a) of flow control spool (2), while load pressure (downstream pressure of the detecting
valve) is applied to the secondary side chamber (b) and the detecting valve spring chamber (c). Note that since
the primary side piston (3) and secondary side piston (4) have the same diameter, the front and rear differential
pressure of detecting valve (1) is applied to the rightward of the flow control spool (2).
On the other hand, the secondary pressure of negative magnetic proportional reducing valve (5) is applied to the
control valve’s spring chamber (d), and works in the leftward direction on the flow control spool (2) by overcoming
the spring force.
When the negative magnetic proportional reducing valve solenoid (SOL10) is not excited, the secondary
pressure reaches its maximum value, and the leftward force overcomes the rightward force acting on flow control
spool (2), and thus the flow control spool (2) is positioned at the extreme left end. Consequently, the control
notch (e) remains closed, and thus the hydraulic oil from pump totally passes the parallel path and flows into the
switching valve.
SOL10
Actuator
From pump
U turn cover
Central bypass path of
each switching valve
U turn cover
Tank path of each
switching valve
Tank path
Tank
Central bypass path
Parallel path
Central bypass path shut off
1. Detecting valve
2. Flow control spool
3. Primary side piston A
4. Secondary side piston B
5. Negative magnetic proportional reducing valve