This section describes boom lower operation.
(1) Boom down pilot circuit
(2) Prevention of natural fall by lock valve and
actuation at lowering
(3) Constant recirculation function of boom down main
circuit
22.7.3
BOOM DOWN PILOT CIRCUIT
(1) Operation:
1.
If the boom down operation is performed, the
secondary pilot proportional pressure comes out of
portaC of the right pilot valve (9) and acts upon
the low pressure sensor (SE-4).
At the same time, the pressure acts upon the
PBb port of C/V (2).
2.
The voltage output of the low pressure sensor
(SE-4) enters the mechatro controller and
processed in it.
3.
Then, the proportionl secondary pressure fed into
C/V (2) PBb port and branches off in two lines
and switches boom spool and releases boom lock
valve.
22.7.4
PREVENTION OF NATURAL FALL
BY LOCK VALVE AND ACTUATION
AT LOWERING
(1) Purpose:
Prevention of natural fall when the lever is neutral.
(2) Principle:
The oil is prevented from returning to the boom
spool by the poppet seat of the boom lock valve.
(3) Operation:
In the boom down action, the selector valve is
changed over by the secondary proportional
pressure of PBb port. Then the poppet spring
chamber of the lock valve CRb gets through the
drain line (Dr) and makes the lock valve poppet
open.
When the boom lever is at neutral, the drain line
on the lock valve CRb poppet spring chamber is
closed which causes the poppet closed.
The result is that the oil returning from the boom
cylinder head (H) to the boom spool is held and
makes the leak from the boom spool zero.
Thus the boom cylinder is prevented from
making a natural fall.
22.7.5
CONSTANT RECIRCULATION
FUNCTION OF BOOM DOWN MAIN
CIRCUIT
(1) Purpose:
Prevention of cavitation during boom lowering
motion.
(2) Principle:
The oil returning from the boom cylinder head (H)
is recirculated to the rod (R).
(3) Operation:
When the oil is supplied to the boom cylinder
rod (R) side during boom down operation, the
boom moves faster than it should do in some
cases by the self weight of the attachment.
On that occasion, the circuit pressure on the rod
(R) side is on the negative side.
The oil supplied to the boom cylinder rod (R)
flows into the A1 port of the P1 pump and the
P1 port of C/V. The oil then passes through the
boom spool and goes out of the Bb port.
On that occasion, the oil returning from the head
(H) goes through the recirculation path in the
boom spool, pushes the check valve in the spool
open, is recirculated to the Bb port and is
supplied to the rod (R). When the (R) pressure
is larger than the head (H) pressure, the check
valve in spool closes.
Thereupon, the recirculation is stopped.
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22. HYDRAULIC SYSTEM
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22-16
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