Excavating – Ditcher – Condition responsive
Reexamination Certificate
2001-12-04
2002-09-24
Novosad, Christopher J. (Department: 3671)
Excavating
Ditcher
Condition responsive
C417S213000, C417S216000, C060S443000, C060S445000
Reexamination Certificate
active
06453585
ABSTRACT:
TECHNICAL FIELD
This invention relates to a hydraulic drive system for a work machine such as a hydraulic excavator, and especially to a hydraulic drive system for a work machine, said hydraulic drive system being provided with a flow-combining valve for combining flows of pressure fluid from two hydraulic pumps and being adapted to perform overall power control such that a total value of all torques including input torques to the two hydraulic pumps does not exceed an output torque from an engine.
BACKGROUND ART
FIG. 5
is a hydraulic circuit diagram showing the construction of a conventional hydraulic drive system for a work machine.
The conventional technique illustrated in
FIG. 5
is applied, for example, to a hydraulic excavator, and is provided with an engine
30
and a first and second hydraulic pumps
15
,
18
both of which are of the variable displacement type and are driven by the engine
30
. To the first hydraulic pump
15
, a first group of directional control valves consisting of plural center-bypassed directional control valves is connected. To the second hydraulic pump
18
, a second group of directional control valves consisting of plural center-bypassed directional control valves is connected likewise. In the second group of directional control valves, a flow-combining directional control valve
4
for changing over and controlling a combined-flow-driven actuator
20
is included. To a directional control valve
1
positioned most downstream of the first group of directional control valves connected to the above-mentioned first hydraulic pump
15
, a flow-combining valve
2
is connected via a center bypass passage
3
such that pressure fluid from the first hydraulic pump
15
can be supplied, in combination with pressure fluid from the second hydraulic pump
18
, to the aforementioned flow-combining directional control valve
4
. The flow-combining valve
2
and a supply port of the flow-combining directional control valve
4
are connected to each other by a flow-combining circuit
5
.
The aforementioned flow-combining valve
2
is arranged such that, depending on the magnitude of a pilot pressure in a pilot line
7
through which the pilot pressure is guided to change over the flow-combining directional control valve
4
, the flow-combining valve
2
is changed over from an open position at which the center bypass passage
3
and a reservoir
17
are communicated with each other to a closed position at which the center bypass passage
3
and the reservoir
17
are cut off from each other or conversely, from the closed position to the open position.
An attachment which is driven by the aforementioned combined-flow-driven actuator
20
comprises a predetermined attachment mounted on a free end of an arm of the hydraulic excavator, for example, a breaker. On the free end of the arm, a bucket is generally mounted. By removing the bucket, this breaker is mounted instead.
FIG. 5
also illustrates a parallel line
21
via which the individual directional control valves included in the second group of directional control valves are connected parallel to the second hydraulic pump
18
, a reservoir passage
19
communicating the center bypass passage of the second group of directional control valves and the reservoir
17
with each other, a check valve
22
for preventing pressure oil in the flow-combining line
5
from flowing toward the parallel line
21
, and a check valve
6
for preventing the pressure fluid in the flow-combining line
5
from flowing toward the center bypass passage
3
.
According to the conventional technique constructed as described above, when any one of the individual directional control valves is changed over except for a change-over operation that the flow-combining directional control valve
4
is changed over to the right position of
FIG. 5
, no pilot pressure is developed in the pilot line
7
, and the flow-combining valve
2
is thus held in the open position by the force of a spring. Namely, the center bypass passage
3
is maintained in communication with the reservoir
17
. In this state, change-over of one or more of the directional control valves included in the first group of directional control valves makes it possible to supply the pressure fluid from the first hydraulic pump
15
to the corresponding directional control valve(s) only, and change-over of one or more of the directional control valves included in the second group of directional control valves makes it possible to supply the pressure fluid from the second hydraulic pump
18
to the corresponding directional control valve(s) only.
When the pilot pressure is guided into the pilot line
7
upon driving the combined-flow-driven actuator
20
, the flow-combining directional control valve
4
is changed over to the right position of FIG.
5
and at the same time, the flow-combining valve
2
is changed over to the closed position against the force of the spring. As a consequence, the center bypass passage
3
and the reservoir
17
are cut off from each other.
Accordingly, the pressure fluid from the first hydraulic pump
15
is supplied, in combination with the pressure fluid from the second hydraulic pump
18
, to the supply port of the flow-combining directional control valve
4
via the center bypass passage
3
, the flow-combining circuit
5
and the check valve
6
. The combined pressure fluid of the pressure fluid from the first hydraulic pump
15
and the pressure fluid from the second hydraulic pump
18
is supplied from the flow-combining directional control valve
4
to the combined-flow-driven actuator
20
. The combined-flow-driven actuator
20
is, therefore, actuated to drive the unillustrated breaker so that breaking work or the like of rocks is performed.
To perform combined operation of the breaker and an unillustrated arm and/or boom or combined operation of the breaker and running and/or revolving, the corresponding one or more of the directional control valves included in the first group of directional control valves, for example, may also be changed over at the same time. In this case, the pressure fluid from the first hydraulic pump
15
is supplied to the corresponding one or more directional control valves. At this time, the center bypass passage(s) of the corresponding one or more directional control valves, in many instances, is(are) not completely closed in actual work, so that there is also a tendency that a portion of the pressure fluid from the first hydraulic pump
15
is also supplied to the flow-combining line
5
. In other words, the combined-flow-driven actuator
20
tends to be brought into such a situation that it is driven by the portion of the pressure fluid from the first hydraulic pump
15
and the pressure fluid from the second hydraulic pump
18
.
While these operations are carried out, overall power control is performed such that a total value of input torques to the first hydraulic pump
15
and the second hydraulic pump
18
does not exceed an output torque from the engine
30
to avoid stalling.
In the above-described conventional technique, a load pressure on the combined-flow-driven actuator
20
may become high for a certain reason in the course of work that the unillustrated breaker is driven by a combined flow of the pressure fluid from the first hydraulic pump
15
and that from the second hydraulic pump
18
. Corresponding to the load pressure, a delivery pressure on the side of the second hydraulic pump
18
then becomes high, and a delivery pressure on the side of the first hydraulic pump
15
also becomes high. As a result, a total value of an input torque to the first hydraulic pump
15
and an input torque to the second hydraulic pump
18
becomes large, and an output torque from the engine
30
also increases.
When the load pressure on the combined-flow-driven actuator
20
becomes high as mentioned above, there is a situation that force is required more than speed. Combining the pressure fluid from the first hydraulic pump
15
with the pressure fluid from the second hydraulic pump
18
in such a situat
Nishimura Yoshizumi
Nozawa Yusaku
Takahashi Kinya
Tougasaki Mitsuhisa
Crowell & Moring LLP
Hitachi Construction Machinery Co. Ltd.
Novosad Christopher J.
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