22.6
BUCKET CIRCUIT
This section describes the following.
(1) Bucket digging pilot circuit
(2) Auto accel operation
(3) Flow control in the work mode
(4) Bucket digging travel straight main circuit
(5) Bucket spool stroke limiter
22.6.1
PILOT CIRCUIT FOR BUCKET
DIGGING
(1) Mechatronics:
1.
When the operation for bucket digging is
performed, the pilot proportional secondary
pressure is delivered through port 1 of the right
pilot V (9), flows to PAc port, and acts on the
low pressure sensor (SE-1), and at the same time
the bucket spool is switched.
2.
The voltage output by low pressure sensor
inputs in mechatro controller. The mechatro
controller performs signal processing and outputs
current corresponding to the increase of pump
flow rate to pump proportional valves (PSV-P1)
and (PSV-P2) on the P1 and P2 pump sides, and
at the same time the command current is output
to P1 unloading valve (PSV-D), P2 unloading
valve (PSV-B) and travel straight proportional
valve (PSV-C).
In the following pages, the relation of operation of
low pressure sensor to both the increase of pump
flow rate and unloading proportional valve is the
same. Therefore the explanation is omitted.
3.
The secondary pressure output by P1 pump
proportional valve (PSV-P1) and P2 pump
proportional valve (PSV-P2) exerts on pump
regulator, actuating the pump onto the delivery
flow increase side.
4.
The secondary pressure delivered in P1 unloading
valve (PSV-D) and P2 unloading valve (PSV-B)
is led to PCb and PCa ports provided in control
valve (2).
The secondary pressure from proportional valve
which has exerted on PCb and PCa ports
switches the unloading spool in CLOSE side.
5.
The secondary pressure delivered in travel straight
proportional valve (PSV-C) is led to PTb port
provided in control valve (2) and shifts the travel
straight valve.
22.6.2
AUTO ACCELERATION OPERATION
(1) Principle:
Auto acceleration actuates according to signals
from low pressure sensor.
(2) Operation:
<When lever is set to neutral position>
In the event where the sensor does not receive
signal for 4 seconds or more even though the
acceleration dial is set to MAX position, the
engine speed should be raised to 1000 rpm.
<When lever is operated>
When the pressure 0.6 MPa (87 psi) is input to
low pressure sensor in STD specification (travel,
bucket, swing, arm), proportional voltage from
low pressure sensor inputs in mechatro
controller, and then the engine speed returns to
the dial set position corresponding to the lever
operation.
22.6.3
STAND-BY CURRENT CONSTANT
CONTROL
(1) Principle:
When the engine speed is intermediate speed or
lower, command current value to have the pump
delivery rate constant is output.
(2) Operation:
Even if the engine speed specified by acceleration
potentiometer is low speed, as the delivery rate
corresponds to the intermediate speed, the
actuator moves earlier than the movement
equivalent to the control lever angle rate in light
load operation.
22.6.4
BUCKET DIGGING AND TRAVEL
STRAIGHT CONFLUENCE (CONFLUX)
MAIN CIRCUIT
(1) Operation:
1.
The oil delivered through A1 port of P1 pump
goes into P1 port of C/V (2), opens load check
valve LCc through parallel circuit and enters in
bucket spool.
2.
On the other hand, the oil delivered through P2
port of P2 pump goes into P2 port of C/V (2),
confluences with oil from P1 pump through travel
straight valve, and goes into bucket spool.
(Confluence of oil from 2 pumps)
3.
With the function of pilot circuit, the oil flow is
switched from bucket spool, and is fed into
bucket cylinder head side through AC port of
C/V (2). On the other hand, the return oil from
cylinder rod (R) side is throttled by bucket spool
and returns to tank circuit from BC port of C/V
(2).
References:
・
In bucket dumping operation, the travel straight
valve also switches like digging operation,
resulting in confluence with travel straight.
・
When the bucket operation and other operation
are simultaneously performed, the travel straight
valve does not actuate, resulting in single flow of
bucket circuit.
22.6.5
BUCKET SPOOL STROKE LIMITER
(1) Purpose:
To secure simultaneous operability of boom and
arm and to prevent cavitations at low engine
speed.
(2) Operation:
On boosting up attachment pressure, pilot
primary pressure exerts on PCc port of C/V(2)
through attachment booster solenoid valve,
operates stroke limiter of bucket spool, and
throttle oil path of spool. As a result, P1 pump
pressure increases, making the simultaneous
operation of boom up and bucket digging possible.
Even though the engine speed is low, similarly
the cavitations can be prevented by actuating the
stroke limiter.
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22. HYDRAULIC SYSTEM
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22-12
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