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HYDRAULIC CIRCUIT
CASE TRAINING CENTER
AUGUST 2000
Boom Down Load Holding Valve Circuit
When boom down is operated, pilot pressure is supplied at 4a2 and boom (1) spool is changed over.
At the same time, pilot pressure, which is supplied from Ps2, acts on the poppet of the load holding valve,
and the poppet is opened. Then, the hydraulic oil in the spring chamber of the load holding valve check is
returned to the tank line via the load holding valve poppet.
The pressure in the spring chamber of the load holding valve check decreases and the check is released.
The hydraulic oil at the bottom of the boom cylinder is returned to the tank line, the rod of the boom cylinder
shrinks and the boom is lowered.
Boom Down Recycle Circuit
When boom down is operated, pilot pressure is supplied at 4a2 and boom (1) spool is changed over. The
boom load holding valve is opened and oil discharged at the bottom side of the cylinder is recycled to the
head side via the boom (1) spool.
A part of returned oil from the bottom side enters the central bypass, and makes up for the shortage of
pressure without the pump's full operation in order to maintain the negative control pressure at a certain
level, the engine output can be efficiently used.
Boom Down Extra Vibration Prevention Circuit
Due to the bleeding oil passage of the central bypass, a high pressure occurrence is restricted and extra
vibration is reduced even when boom down is operated very quickly, since hydraulic oil is bled.
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