Excavating – Ditcher – Condition responsive
Reexamination Certificate
2002-11-19
2004-11-23
Will, Thomas B. (Department: 3671)
Excavating
Ditcher
Condition responsive
C037S414000, C060S445000, C060S452000, C091S392000, C701S050000
Reexamination Certificate
active
06820355
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a hydraulic control circuit for a boom cylinder in construction and/or civil engineering machinery, such as a hydraulic excavator that is used in various types of work machines.
2. Description of Related Art
Generally, in some construction and civil engineering machinery, such as a hydraulic excavator, there is a type in which a front attachment mounted on the machinery body is composed of a boom whose base end portion is supported so as to swing upward and downward, an arm that is supported at the tip end portion of the boom so as to swing forward and backward, and a work attachment, such as a bucket attached to the tip end portion of the arm. In such a piece of machinery, the above-described boom moves upward and downward in line with extension and contraction of a boom cylinder, wherein, conventionally, a hydraulic control circuit, such as shown in
FIG. 3
, has been known as one of the hydraulic control circuits for the boom cylinder.
That is, in
FIG. 3
, reference number
8
denotes a boom cylinder, reference numbers
9
,
10
denote other hydraulic actuators in addition to an arm cylinder and a bucket cylinder, which are attached to a hydraulic excavator. Reference number
11
denotes a pressurized oil supply source for the boom cylinder
8
and other hydraulic actuators. Reference number
12
denotes an oil reservoir. Reference number
13
denotes a boom control valve for controlling the supply and discharge of pressurized oil with respect to the boom cylinder
8
. Reference numbers
14
,
15
denote other hydraulic actuator control valves for controlling the supply and discharge of pressurized oil with respect to the other hydraulic actuators. Reference number
16
denotes a pilot valve for outputting pilot pressure to pilot lines C, D at the elevation side and descent side on the basis of the operation of a boom operating lever
17
. In addition, reference letter A denotes the head side line for connecting the boom control valve
13
to the head side oil chamber
8
a
of the boom cylinder
8
. Reference letter B denotes the rod side line for connecting the boom control valve
13
to the rod side oil chamber
8
b
of the boom cylinder. Also, reference letter G denotes a recycling line for communicating the above-described head side line A to the rod side line B, and a recycling valve
27
, provided with a check valve
27
a
, is provided in the recycling line G.
In this type of hydraulic control circuit, when the boom operating lever
17
is operated to the descent side, with the pilot pressure outputted from the pilot valve
16
to the descent side pilot line D, the boom control valve
13
is changed to the descent side position Y. At the same time, the recycling valve
27
is changed to the second position Y that opens the recycling line G, wherein, while pressurized oil from the pressurized oil supply source
11
is supplied to the rod side oil chamber
8
b
of the boom cylinder
8
via the boom control valve
13
, oil discharged from the head side oil chamber
8
a
is discharged into the oil reservoir
12
, and is further supplied to the rod side oil chamber
8
b
via a recycling valve
27
. That is, while the pressure of the head side oil chamber
8
a
is higher than the pressure of the rod side oil chamber
8
b
when the boom is lowered, the oil discharged from the head side oil chamber
8
a
can be provided to the rod side oil chamber
8
b
as recycling oil, whereby the recycling oil is supplied into the rod side oil chamber
8
b
in addition to the pressurized oil of the pressurized oil supply source
11
, which is provided from the above-described boom control valve
13
. Therefore, the rod side oil chamber
8
b
is not placed into a pressure-reduced state, wherein the operation speed of the boom cylinder
8
can be accelerated. Also, when other hydraulic actuators (for example, arm cylinder and bucket cylinder) for which the pressurized oil supply source of the boom cylinder
8
is concurrently used are operated in combination with a descending operation of the boom, because surplus pump flow volume obtained by the recycling can be provided to the other hydraulic actuators, it is possible to prevent the operation speed of the other hydraulic actuators from being lowered when such a complex operation is carried out, wherein the surplus pump flow volume obtained by the recycling can contribute to an improvement in work efficiency.
However, because a force against the descent of the boom operates where the above-described boom is lowered for execution of rolling work by descent of the boom and scraping work of an inclined plane, it is necessary to supply highly pressurized oil into the rod side oil chamber. To the contrary, when the boom is lowered in the air (where the boom is lowered in a state where the front attachment is not grounded), because a weight applied to the boom (the total weight of the front attachment) operates as a force for contraction of the boom, the pressurized oil supplied into the rod side oil chamber may be low. In addition, as the head side area of a piston of the boom cylinder is larger than the rod side area thereof, only the recycling oil from the above-described head side oil chamber may be sufficient.
Accordingly, where the boom is lowered with the above-described related art hydraulic circuit, not only is recycling oil provided but also pressurized oil from the pressurized oil supply source is provided via the boom control valve even if the boom is lowered in the air. Therefore, there is a problem in that, in cases of operating the arm and bucket while the boom descends in the air, pressurized oil from the pressurized oil supply source is shared among the boom cylinder, arm cylinder and bucket cylinder, wherein the movement of the arm and bucket becomes slow in comparison with independent operations thereof, resulting in a worsening in work efficiency. Further, in spite of only the recycling oil from the head side oil chamber being sufficient where the boom is independently lowered in the air, because pressurized oil is supplied from the pressurized oil supply source to the rod side oil chamber, considerable surplus oil is discharged from the head side oil chamber
8
a
to the oil reservoir
12
via the boom control valve
13
, wherein an energy loss is produced, which hinders a decrease in fuel consumption. These and other problems are objects to be solved by the invention.
Further, in a work machine provided with a front attachment consisting of the above-described boom, arm, work attachment, etc., for example, in a case where work for raking up debris is carried out while moving the bucket forward and backward with the bucket bottom grounded, it is necessary that three operations of the boom, arm and bucket so as to cause the bucket bottom to depict a roughly horizontal locus are simultaneously carried out. The operations are delicate, and skilled operations are required. In addition, where the ground is hardened with the bucket bottom by continuously repeated operations of descent and elevation of the boom, that is, bumping work is carried out, unless the boom elevation operation is timely carried out at the instant when the bucket bottom is grounded, the ground is excessively bumped by a reaction caused at the time of descent of the boom, or the front part of a machine body is raised. Therefore, in order to continuously carry out bumping, a highly skilled operation is required. The work requiring such skill is difficult for a beginner, and even a skilled operator is obliged to pay meticulous attention to the work. Accordingly, another problem to be solved exists in that, in such situations, the maneuverability and work efficiency are made worse.
SUMMARY OF THE INVENTION
In view of the above-described situations, the invention was developed to solve these and other problems. It is therefore an object of the invention to provide a hydraulic control circuit for a boom cylinder, which is provided with a boom cylinder extending and contracting so as to
Shimada Yoshiyuki
Yoshino Tetsuya
Beach Thomas A
Oliff & Berridg,e PLC
Shin Caterpillar Mitsubishi Ltd.
Will Thomas B.
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