S0709455K
Page 3
Main Control Valve (Toshiba)
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GENERAL DESCRIPTION
The main control valve controls the operation of the bucket, boom, and option cylinders. The bucket spool
has priority over the boom spool.
Pump output flows to port P on the main control valve. See Figure 1. Maximum pressure is prevented from
exceeding the desired level by the main relief valve (1). See hydraulic schematic (Figure 2). When all
spools on the main control valve are in the neutral position (when the pilot valve is not being operated), oil
passes through port P, the bucket spool, the boom spool, the option spool, and passes out T, the tank port.
Oil that passes out the tank port passes through a filter and returns to tank.
Two overload relief valve (2) are installed in the bucket circuit to protect circuit components in the event of
abnormally high pressure. One valve works as a relief valve while the other valve works as an
anticavitation valve to provide additional oil flow to the opposite side of the bucket cylinders.
Boom spool has two important roles in loader work at the floating position. One is that it is possible to lower
the boom by its weight without oil supply from the pump, and that, at the same time, the oil from the pump
makes it possible to combine with the bucket crowd at the max. height of bucket through the bucket line.
Another is that boom and bucket are positioned at the level as the cylinder ports A and B are open as a
leveling function of ground. There is a suction check valve (3) in the boom power port. When the boom is
dropped suddenly by its weight, the cylinder bottom has a cavitation with a lack of oil supply. The check
valve supplements oil to prevent the cavitation.
Figure 1
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