Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2000-01-21
2001-09-04
Ryznic, John E. (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S444000, C060S445000, C060S464000
Reexamination Certificate
active
06282890
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a hydraulic circuit for construction machines, and more particularly to a cutoff function-carrying hydraulic circuit for construction machines, adapted to improve the suspension capacity thereof by disengaging the cutoff function and increasing a maximum discharge pressure of the hydraulic circuit when the power and speed of a working unit are required at once in load suspending work, and stump digging work or boulder raising work.
2. Description of the Related Art
A hydraulic excavator of the related art is used to suspend a load in some cases by a boom extending operation. First related art hydraulic circuit for construction machines shown in
FIG. 7
will now be described.
A discharge pipe line
1
a
of a variable displacement pump
1
is connected to a boom cylinder
3
via a direction change-over valve
2
for a boom, and a relief valve
4
for setting a maximum discharge pressure (set pressure P
1
) to the discharge pipe line
1
a
of the variable displacement pump
1
. A discharge oil of a control pump
5
is set to a predetermined pressure by a fixed relief valve
6
, and sent to a volume control unit
9
for the variable displacement pump
1
via a torque variable control valve
7
from a cutoff valve
8
. The torque variable control valve
7
receives a discharge pressure P of the variable displacement pump
1
, controls a discharge oil of the control pump
5
, which has been set to a predetermined level, to such a control pressure that permits volume V (cc/rev) of the variable displacement pump
1
to set constant the torque K
2
shown by P·V, and outputs the resultant control pressure. Namely, the control pressure becomes rectangularly hyperbolic as shown by the torque K
2
in FIG.
8
. The cutoff valve
8
receives a control pressure outputted from the torque variable control valve
7
, and outputs when the discharge pressure P of the variable displacement pump
1
reaches a level in the vicinity of a set level P
1
(i.e. Pa) of the relief valve
4
such a control pressure that makes the volume V of the variable displacement pump
1
decrease gradually and attain a minimum level V
1
as shown by a curve C in FIG.
8
. The volume control unit
9
comprises a volume control valve and a volume control cylinder (neither of which is shown) which are adapted to receive a control pressure outputted from the cutoff valve
8
, and control the volume V of the variable displacement pump
1
as shown by K
2
and C in FIG.
8
.
The operation of the first related art hydraulic circuit will now be described with reference to
FIGS. 7 and 8
.
The volume V of the variable displacement pump
1
is controlled as shown by the torque curves K
2
and C in
FIG. 8
, in accordance with the discharge pressure P of the variable displacement pump
1
which is determined depending upon a load pressure working on the boom cylinder
3
. In
FIG. 8
, the volume V of the pump is taken in the direction of the lateral axis. When a discharge rate Q (m
3
/min) of the pump is taken on the same axis, the torque curves shown by K
2
and C turn into horsepower curves. Therefore, loss torque K
1
(i.e. P
1
·V
1
) which the variable displacement pump
1
relieves at a point A at the cutoff time decreases as compared with that K
2
(i.e. P
1
·V
2
) which the variable displacement pump
1
relieves at a point B at the cutoff operation stopping time. Consequently, the loss horsepower which the variable displacement pump
1
relieves at the point A decreases as compared with that which the variable displacement pump
1
relieves at the point B, so that the saving of energy is attained.
Second related techniques (disclosed, for example, in Japanese Patent Publication No. 72437/1994) shown in
FIG. 9
will now be described. In the parentheses shown after the names of constituent elements of this related art hydraulic circuit, the names of corresponding constituent elements of the present invention will be inserted once each, and the descriptions will thereafter be given by referring to the name of constituent elements of the present invention with the constituent elements of this related art hydraulic circuit equivalent to those of the first related art hydraulic circuit designated by the same reference numerals to omit the descriptions thereof.
In a hydraulic circuit for a working unit of a hydraulic excavator, a return oil passage
10
passing through each direction change-over valve is connected to a tank
12
via a restriction
11
. A discharge oil of a hydraulic pump (control pump)
5
the pressure in which is set at a predetermined level is sent to a pilot pressure receiving member
14
b
of a variable relief valve (relief valve)
14
via a solenoid valve
13
, and further from one side of a shuttle valve
15
to a volume control unit
16
of a variable displacement hydraulic pump (variable displacement pump)
1
. The other side of the shuttle valve
15
is connected to an upstream side of the restriction
11
provided in the return oil passage
10
. The solenoid valve
13
is excited when a switch
19
provided on a free end of an operating lever
18
is pressed, and it is thereby shifted to a position b to cause the control pump
5
to be connected to the pilot pressure receiving member
14
b
and the first-mentioned side of the shuttle valve
15
. When the hand pressing the switch
19
is removed therefrom, the solenoid valve is deexcited to be shifted to a position a, and connect the pilot pressure receiving member
14
b
and the first-mentioned side of the shuttle valve
15
to the tank
12
. The relief valve
14
is set to a normal set level P
1
when a pilot pressure is not supplied to the pilot pressure receiving member
14
b
, and shifted to a higher set level P
2
, which is higher than the normal set level P
1
, when a pilot pressure is supplied thereto.
The operation of the second related art hydraulic circuit will now be described. When regular excavation work is carried out by the hydraulic excavator, the switch
19
is not pressed, so that the solenoid valve
13
is deexcited, and takes the position a. Accordingly, the pilot pressure receiving member
14
b
and the first-mentioned side of the shuttle valve
15
are drained. Consequently, the relief valve
14
comes to have a normal set pressure P
1
, and a pressure oil in the portion of the return oil passage
10
which is on the upstream side of the restriction
11
works from the second-mentioned side of the shuttle valve
15
on the volume control unit
16
. Owing to this operation, the variable displacement valve
1
is controlled so that, when none direction change-over valves, such as a direction change-over valve
2
for a boom is operated, a flow rate in the return oil passage
10
increases to cause the volume V of the variable displacement pump
1
to become minimal, and so that, when any one of the direction change-over valves, such as the direction change-over valve
2
for a boom is operated, a flow rate in the return oil passage
10
becomes zero to cause the volume V of the variable displacement pump
1
to become maximal.
In order to use the hydraulic excavator as a crane for load suspending work, the switch
19
of the operating lever
18
is pressed, so that the solenoid valve
13
is excited to take the position b. Accordingly, a pilot oil from the control pump
5
flows to the pilot pressure receiving member
14
b
to increase the oil pressure to a higher set level P
2
as shown in
FIG. 10
, so that lifting power increases. Moreover, since a control pressure of the control pump
5
is applied from the first-mentioned side of the shuttle valve
15
to the volume control unit
16
, the volume V of the variable displacement pump
1
is set to a lower level V
3
as shown in FIG.
10
.
However, these related techniques have the following problems.
(1) When load suspending work is carried out according to the first related techniques, the discharge pressure P of the pump increases, i.e., the load suspending work is necessarily carried out at a dis
Komatsu Ltd.
Ryznic John E.
Varndell & Varndell PLLC
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