Takeuchi TB 250, Workshop Manual

Page: 91 / 352

3-10
3
CONTROL VALVE
FUNCTION
When the Boom Lower operation is performed
When the Boom Lower operation is performed, the pilot
pressure is led to the port P1 to push down the spool (3).
This allows the oil in the spring chamber (5) to flow through
the passage (4) and into the port Dr (tank port). As a result,
the poppet (1) is kept opened and the return passage is
secured.
Switch valve
This valve switches among the circuits of bucket, arm,
boom, swing and service when any of them is activated
during traveling. The combined control of straight travel
and other operation can be done, because the oil supplied
by the pump P3 is directed to the section other than the
travel section.
When the switch valve is activated
When the traveling operation and other operation are per-
formed at the same time, the pilot pressure is directed tot
the port P1 to push up the spool (1). This allows the oil
entered the port P to flow through the parallel passage (2),
the spool head and the second parallel passages A and B.
Since the second parallel passage A is connected to the
boom and the bucket sections, while the second parallel
passage B is connected to the arm and the swing sections,
the combined operation of traveling and other operation
can be achieved.
When the switch valve is not activated
The spool (1) is not switched because the pilot passage
(2) is open to the tank passage and thus the pressure in
the pilot chamber (3) is equal to the pressure in the tank.
Thus, the oil entered the port P flows into the center bypass
passage (3). Also, part of the oil flows through the parallel
passage (2), dozer blade and slew sections.
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