Hydraulic system having tandem hydraulic function

Pump ports 9p, 10p, 11p, 13p of boom, arm, bucket and first travel directional control valves 9-11, 13 are connected to first and second hydraulic pumps 1a, 1b through feeder lines 93a, 93b; 103a, 103b; 113a, 113b; 133a, 133b. Auxiliary valves 91a, 91b; 101a, 101b; 111a, 101b; 131a, 131b controlled respectively by proportional solenoid valves 31a, 31b; 32a, 32b; 33a, 33b; 34a, 34b are disposed in those feeder lines. The auxiliary valves each have a function as a reverse-flow preventing function and a variable resisting function including a flow cutoff function, whereby a joining circuit and a preference circuit can be realized in the closed center circuit with a simple structure and further, a preference degree and metering characteristics can be set independently of each other during the combined operation of plural actuators.

TECHNICAL FIELD 
The present invention relates to a hydraulic system for driving a plurality 
of actuators by a plurality of pumps in a hydraulic excavator or the like. 
BACKGROUND ART 
Hydraulic systems for driving a plurality of actuators by a plurality of 
pumps comprise so-called open center circuits as disclosed in 
JP-B-2-16416, for example, and so-called closed center circuits as 
disclosed in JP-A-4-194405. The open center circuit is a circuit having a 
center bypass line, and a pump delivery flow is bled to a reservoir 
through the center bypass line when each directional control valve is in a 
neutral condition. An opening of the center bypass line located in each 
directional control valve is gradually throttled as the directional 
control valve is shifted by a larger amount, whereupon a pump pressure is 
produced and a hydraulic fluid is supplied to each corresponding actuator 
through a meter-in circuit. 
In the open center circuit, independence of plural actuators is maintained 
by providing a preference circuit in the form of a so-called tandem 
connection or arranging a plurality of hydraulic pumps so that hydraulic 
fluids are joined together selectively. 
On the other hand, the closed center circuit is a circuit having no center 
bypass line. As disclosed in the above-cited JP-A-4-194405, spools are 
connected to a hydraulic pump in parallel. There are also known a load 
sensing system for controlling a differential pressure between a pump 
pressure and a load pressure to be fixed when each directional control 
valve is in a neutral condition, and a system for reducing a pump delivery 
rate through a bleed circuit including a bleed valve as disclosed in 
JP-A-7-63203 when each directional control valve is in a neutral position. 
DISCLOSURE OF THE INVENTION 
In the open center circuit, as mentioned above, independence of plural 
actuators is maintained by providing a preference circuit in the form of a 
so-called tandem connection or by arranging a plurality of hydraulic pumps 
so that hydraulic fluids are joined together selectively. However, it is 
required not only to form a center bypass line in each directional control 
valve, but also to provide a plurality of directional control valves for 
one actuator. The valve structure is, therefore, complicated and increased 
in size. Also, because the preference circuit is made up by using the 
center bypass line, a preference degree and metering characteristics 
cannot be set independently of each other during the combined operation of 
actuators. 
In the closed center circuit, the valve structure is relatively simple 
because the center bypass line is not necessary and only one directional 
control valve is usually required for one actuator. However, the closed 
center circuit is basically a parallel circuit and hence has a difficulty 
in realizing a preference circuit. 
A first object of the present invention is to provide a hydraulic system in 
which a joining circuit and a preference circuit are realized in a closed 
center circuit with a simple structure. 
A second object of the present invention is to provide a hydraulic system 
in which a preference degree and metering characteristics can be set 
independently of each other during the combined operation of actuators in 
a closed center circuit. 
(1) To achieve the above first object, the present invention is constituted 
as follows. A hydraulic system comprises first and second hydraulic pumps, 
first and second actuators, a first directional control valve of closed 
center type connected to the first and second hydraulic pumps for 
controlling a flow rate of a hydraulic fluid supplied to the first 
actuator, and a second directional control valve of closed center type 
connected to at least the first hydraulic pump for controlling a flow rate 
of a hydraulic fluid supplied. The second actuator, the hydraulic system 
further comprises first and second feeder lines respectively connecting 
the first and second hydraulic pumps to a pump port of the first 
directional control valve, and first and second reverse-flow preventing 
valves disposed respectively in the first and second feeder lines for 
preventing the hydraulic fluids from reversely flowing to the first and 
second hydraulic pumps. 
In the present invention constructed as set forth above, when the first 
actuator is solely driven, the hydraulic fluids from the first and second 
hydraulic pumps are joined together through the first and second feeder 
lines (joining circuit). Also, the first and second reverse-flow 
preventing valves serve to prevent the hydraulic fluids from reversely 
flowing to the pumps from the actuator when the load pressure of the first 
actuator is higher than the delivery pressures of the first and second 
hydraulic pumps (load check valves). 
When the first and second actuators are both simultaneously driven, it is 
always ensured in a hydraulic system where the load pressure of the first 
actuator is higher than the load pressure of the second actuator that the 
first actuator can be operated by the hydraulic fluid from the second 
hydraulic pump and the second actuator can be operated by the hydraulic 
fluid from the first hydraulic pump. At this time, even with the load 
pressure of the second actuator being lower than the load pressure of the 
first actuator, the hydraulic fluid from the second hydraulic pump is 
prevented from flowing into the second actuator by the presence of the 
first reverse-flow preventing valve (preference circuit). 
(2) In the above (1), preferably, a first auxiliary valve with a flow 
cutoff function of selectively cutting off a flow of the hydraulic fluid 
supplied from the first hydraulic pump is disposed, in addition to the 
first reverse-flow preventing valve, in at least the first feeder line of 
the first and second feeder lines. 
When the first actuator is solely driven, the hydraulic fluids from the 
first and second pumps can be joined together and supplied to the first 
actuator through the first and second feeder lines, as with the above 
case, by holding the flow cutoff function of the first auxiliary valve 
turned off (joining circuit). 
When the first and second actuators are both simultaneously driven, the 
flow cutoff function of the first auxiliary valve is turned on upon 
detecting an operation of the second directional control valve, causing 
the first hydraulic pump to be connected to the second actuator 
preferentially (i.e., in tandem). Regardless of the load pressures of the 
first and second actuators, therefore, the first actuator can be operated 
by the hydraulic fluid from the second hydraulic pump and the second 
actuator can be operated by the hydraulic fluid from the first pump 
independently of each other (preference circuit). 
(3) In the hydraulic system of the above (1) wherein the second directional 
control valve is connected to the first and second hydraulic pumps, 
preferably, the hydraulic system further comprises third and fourth feeder 
lines respectively connecting the first and second hydraulic pumps to a 
pump port of the second directional control valve, and third and fourth 
reverse-flow preventing valves disposed respectively in the third and 
fourth feeder lines for preventing the hydraulic fluids from reversely 
flowing to the first and second hydraulic pumps, wherein a first auxiliary 
valve with a flow cutoff function of selectively cutting off a flow of the 
hydraulic fluid supplied from the first hydraulic pump is disposed, in 
addition to the first reverse-flow preventing valve, in at least the first 
feeder line of the first and second feeder lines, and a fourth auxiliary 
valve with a flow cutoff function of selectively cutting off a flow of the 
hydraulic fluid supplied from the second hydraulic pump is disposed, in 
addition to the fourth reverse-flow preventing valve, in at least the 
fourth feeder line of the third and fourth feeder lines. 
When the first actuator is solely driven, the hydraulic fluids from the 
first and second hydraulic pumps can be joined together and supplied to 
the first actuator, as with the above case, by holding the flow cutoff 
function of the first auxiliary valve turned off (joining circuit). 
When the second actuator is solely driven, the hydraulic fluids from the 
first and second hydraulic pumps can be joined together and supplied to 
the second actuator, as with the above case, by holding the flow cutoff 
function of the fourth auxiliary valve turned off (joining circuit). 
When the first and second actuators are both simultaneously driven, the 
flow cutoff functions of the first and fourth auxiliary valves are turned 
on upon detecting operations of the first and second directional control 
valves, respectively, causing the first hydraulic pump to be connected to 
the second actuator preferentially and the second hydraulic pump to be 
connected to the first actuator preferentially. Regardless of the load 
pressures of the first and second actuators, therefore, the first actuator 
can be operated by the hydraulic fluid from the second hydraulic pump and 
the second actuator can be operated by the hydraulic fluid from the first 
hydraulic pump independently of each other (preference circuit). 
(4) In the above (3), preferably, each of the first and fourth auxiliary 
valves is constructed to further have a variable resisting function 
including said flow cutoff function. 
(5) In the above (4) having such a feature, preferably, the variable 
resisting function of the first auxiliary valve increases line resistance 
depending on an operation amount of the second directional control valve, 
and the variable resisting function of the fourth auxiliary valve 
increases line resistance depending on an operation amount of the first 
directional control valve. 
When the first actuator is solely driven with only the first directional 
control valve fully operated, the variable resisting function of the first 
auxiliary valve is fully opened and the variable resisting function of the 
fourth auxiliary valve is fully closed. Therefore, the hydraulic fluids 
from the first and second hydraulic pumps can be joined together and 
supplied to the first actuator, as with the above case (joining circuit). 
When the second directional control valve is half-operated from the above 
state, the variable resisting function of the first auxiliary valve is 
gradually restricted depending on the shift amount of the second 
directional control valve and the first hydraulic pump is connected to the 
second actuator preferentially depending on an extent by which the 
variable resisting function of the first auxiliary valve is restricted. 
When the variable resisting function of the fourth auxiliary valve is 
fully closed with the first directional control valve fully operated, the 
second hydraulic pump is connected to the first actuator preferentially to 
a full extent (adjustment of preference degree). Therefore, all of the 
hydraulic fluid from the second hydraulic pump plus part of the hydraulic 
fluid from the first hydraulic pump are supplied to the first actuator, 
and most of the hydraulic fluid from the first hydraulic pump is supplied 
to the second actuator, enabling the first and second actuators to be 
simultaneously driven (preference circuit). Further, when the second 
directional control valve is fully operated, the variable resisting 
function of the first auxiliary valve is fully closed and the first 
hydraulic pump is connected to the second actuator preferentially to a 
full extent. Therefore, all of the hydraulic fluid from the second 
hydraulic pump is supplied to the first actuator and all of the hydraulic 
fluid from the first hydraulic pump is supplied to the second actuator, 
enabling the first and second actuators to be simultaneously driven 
(preference circuit). Also, if the variable resisting function of the 
first auxiliary valve is abruptly turned on/off when it is restricted, 
there would occur a shock because of the circuit being closed at the 
moment the second directional control valve is operated. But such a shock 
can be suppressed in this case because the variable resisting function of 
the first auxiliary valve is gradually restricted depending on the valve 
operation amount. 
When the first actuator is solely driven with the first directional control 
valve half-operated, the variable resisting function of the first 
auxiliary valve is fully opened and the variable resisting function of the 
fourth auxiliary valve is throttled. Therefore, the hydraulic fluids from 
the first and second pumps can be joined together and supplied to the 
first actuator (joining function). 
When the second directional control valve is half-operated from the above 
state, the variable resisting function of the first auxiliary valve is 
gradually restricted depending on the shift amount of the second 
directional control valve and the first hydraulic pump is connected to the 
second actuator preferentially depending on an extent by which the 
variable resisting function of the first auxiliary valve is restricted. At 
the same time, since the variable resisting function of the fourth 
auxiliary valve is restricted with the first directional control valve 
half-operated, the second hydraulic pump is connected to the first 
actuator preferentially depending on an extent by which the variable 
resisting function of the fourth auxiliary valve is restricted (adjustment 
of preference degree). Therefore, most of the hydraulic fluid from the 
second hydraulic pump plus part of the hydraulic fluid from the first 
hydraulic pump are supplied to the first actuator, and most of the 
hydraulic fluid from the first hydraulic pump plus part of the hydraulic 
fluid from the second hydraulic pump are supplied to the second actuator, 
enabling the first and second actuators to be simultaneously driven 
(preference circuit). Further, when the second directional control valve 
is fully operated, the variable resisting function of the first auxiliary 
valve is fully closed and the first hydraulic pump is connected to the 
second actuator preferentially to a full extent. Therefore, most of the 
hydraulic fluid from the second hydraulic pump is supplied to the first 
actuator and all of the hydraulic fluid from the second first hydraulic 
pump plus part of the hydraulic fluid from the hydraulic pump are supplied 
to the second actuator, enabling the first and second actuators to be 
simultaneously driven (preference circuit). In this case, it is also 
possible to suppress a shock otherwise occurred at the moment the second 
directional control valve is operated. 
The transition from the sole operation of the second actuator to the 
combined operation of the first and second actuators is performed in a 
like manner to the above. 
(6) In the above (5), preferably, the variable resisting function of at 
least one of the first and fourth auxiliary valves changes line resistance 
depending on a load pressure of one of the first and second auxiliary 
valves. 
By thus changing the line resistance controlled by the variable resisting 
function depending on not only the operation amount of the directional 
control valve, but also the load pressure, the actuator can be driven with 
small throttling loss by utilizing the load pressure. 
(7) Also, to achieve the above second object, the present invention is 
constituted as follows. The hydraulic system of the above (4) further 
comprises first and second bleed valves disposed respectively between the 
first and second hydraulic pumps and a reservoir, and reducing opening 
areas thereof depending on operation amounts of the first and second 
directional control valves. 
In control of the first and second bleed valves, the operation amounts of 
the first and second directional control valves may be determined as a 
total of both the operation amounts or a maximum value thereof, or may be 
calculated by using any function. As an alternative, it is also possible 
to calculate proportions of the flow rate demanded for the first hydraulic 
pump and the flow rate demanded for the second hydraulic pump from the 
extent by which respective flows are throttled by the variable resisting 
functions, divide a total of the operation amounts by the calculated 
proportions, and determine part of the total amount associated with the 
first hydraulic pump and part of the total amount associated with the 
second hydraulic pump. 
When the first or second actuator is solely driven, or when the first and 
second actuators are simultaneously driven, the first and second bleed 
valves are throttled to gradually increase the pump delivery pressures 
depending on the operation amounts of the directional control valves, 
thereby supplying the first and second actuators with the hydraulic fluids 
at flow rates corresponding to the pump delivery pressures (bleed 
control). By changing the respective extent by which the first and second 
bleed valves are throttled, therefore, flow rate characteristics (metering 
characteristics) of the hydraulic fluids supplied to the first and second 
actuators through meter-in openings of the first and second directional 
control valves can be changed. In this way, preference circuits 
constituted by the first to fourth reverse-flow preventing valves or the 
first and fourth auxiliary valves and bleed circuits constituted by the 
first and second bleed valves are separated from each other, a preference 
degree and metering characteristics can be set independently of each 
other. Further, even if the first and second directional control valves 
are abruptly operated at the start-up of the first or second actuator, the 
pump delivery pressure is gradually increased because of a time lag 
occurring before the pump delivery pressure rises due to throttling of the 
bleed valve. As a result, abrupt driving of the actuator can be avoided. 
(8) In the above (4), preferably, a second auxiliary valve with a variable 
resisting function including a flow cutoff function is disposed, in 
addition to the second reverse-flow preventing valve, in the second feeder 
line as with the first feeder line, and a third auxiliary valve with a 
variable resisting function including a flow cutoff function is disposed, 
in addition to the third reverse-flow preventing valve, in the third 
feeder line as with the fourth feeder line. 
With this feature, the circuit can be freely selected as follows, and 
design change of the circuit per model and product is facilitated. 
(1) When the variable resisting functions of the first to fourth auxiliary 
valves are all turned off, the first and second hydraulic pumps are each 
connected to the first and second actuators in parallel. 
(2) When the variable resisting functions of the first and third auxiliary 
valves are turned off and the variable resisting function of the fourth 
auxiliary valve is throttled depending on the operation amount of the 
first directional control valve, the first hydraulic pump is connected to 
the first and second actuators in parallel and the second hydraulic pump 
is connected to the first actuator preferentially. 
(3) When the variable resisting functions of the first and third auxiliary 
valves are turned off and the variable resisting function of the second 
auxiliary valve is throttled depending on the operation amount of the 
second directional control valve, the first hydraulic pump is connected to 
the first and second actuators in parallel and the second hydraulic pump 
is connected to the second actuator preferentially. 
(4) When the variable resisting functions of the second and fourth 
auxiliary valves are turned off and the variable resisting function of the 
third auxiliary valve is throttled depending on the operation amount of 
the first directional control valve, the first hydraulic pump is connected 
to the first actuator preferentially and the second hydraulic pump is 
connected to the first and second actuators in parallel. 
(5) When the variable resisting functions of the second and fourth 
auxiliary valves are turned off and the variable resisting function of the 
first auxiliary valve is throttled depending on the operation amount of 
the second directional control valve, the first hydraulic pump is 
connected to the second actuator preferentially and the second hydraulic 
pump is connected to the first and second actuators in parallel. 
(9) In the above (8), preferably, each of the first to fourth auxiliary 
valve is a single valve including a function as each of the first to 
fourth reverse-flow preventing valves. 
(10) In the above (9), preferably, the first to fourth auxiliary valves are 
poppet type flow control valves comprising respectively poppet valves 
disposed in the first to fourth feeder lines and pilot valves for 
controlling the poppet valves. 
By so constructing the auxiliary valves by utilizing poppet type flow 
control valves, a valve apparatus including a reverse-flow preventing 
function and a variable resisting function can be easily realized without 
making the valve structure complicated. 
(11) Further, to achieve the above object, the present invention is 
constituted as follows. A hydraulic system for a hydraulic excavator 
comprises at first and second hydraulic pumps, a plurality of actuators 
including a boom cylinder, an arm cylinder, a bucket cylinder, a swing 
motor and first and second travel motors, and a plurality of directional 
control valves of closed center type including a boom directional control 
valve, an arm directional control valve, a bucket directional control 
valve, a swing directional control valve and first and second travel 
directional control valves for controlling respective flow rates of 
hydraulic fluids supplied to the boom cylinder, the arm cylinder, the 
bucket cylinder, the swing motor and the first and second travel motors. 
The hydraulic system further comprises first and second feeder lines and 
third and fourth feeder lines respectively connecting the first and second 
hydraulic pumps to pump ports of at least two of the plurality of 
directional control valves of closed center type, first and second 
reverse-flow preventing valves disposed respectively in the first and 
second feeder lines for preventing the hydraulic fluids from reversely 
flowing to the respective first and second hydraulic pumps, first and 
second auxiliary valves disposed respectively in the first and second 
feeder lines and having variable resisting functions of subsidiarily 
controlling flows of the hydraulic fluids from the respective first and 
second hydraulic pumps, third and fourth reverse-flow preventing valves 
disposed respectively in the third and fourth feeder lines for preventing 
the hydraulic fluids from reversely flowing to the respective first and 
second hydraulic pumps, and third and fourth auxiliary valves disposed 
respectively in the third and fourth feeder lines and having variable 
resisting functions of subsidiarily controlling flows of the hydraulic 
fluids supplied from the respective first and second hydraulic pumps. 
By so providing the feeder lines, the reverse-flow preventing valves, and 
the auxiliary valves each having a variable resisting function, a joining 
circuit and a reference circuit can be realized with a simple structure by 
employing a closed center circuit, as mentioned above, in a hydraulic 
system for a hydraulic excavator. 
(12) In the above (11), by way of example, the directional control valves 
are the boom directional control valve and the arm directional control 
valve, the first and second feeder lines are first and second boom feeder 
lines, the third and fourth feeder lines are first and second arm feeder 
lines, the first and second reverse-flow preventing valves are first and 
second boom reverse-flow preventing valves, the first and second auxiliary 
valves are first and second boom auxiliary valves, the third and fourth 
reverse-flow preventing valves are first and second arm reverse-flow 
preventing valves, and the third and fourth auxiliary valves are first and 
second arm auxiliary valves. 
(13) Preferably, the hydraulic system of the above (12) further comprises 
control means for controlling the variable resisting function so as to 
throttle the first arm auxiliary valve when boom operating means for 
instructing the boom cylinder to be driven is operated. 
With this feature, during the simultaneous operation of the boom and the 
arm, most of the hydraulic fluid from the first hydraulic pump is sent to 
the boom cylinder because the first arm auxiliary valve is throttled, and 
the hydraulic fluid from the second hydraulic pump is primarily sent to 
the arm cylinder. 
(14) Also, the hydraulic system of the above (12) further comprises, by way 
of example, first and second bucket feeder lines respectively connecting 
the first and second hydraulic pumps to a pump port of the bucket 
directional control valve, first and second bucket reverse-flow preventing 
valves disposed respectively in the first and second bucket feeder lines 
for preventing the hydraulic fluids from reversely flowing to the 
respective first and second hydraulic pumps, and first and second bucket 
auxiliary valves disposed respectively in the first and second bucket 
feeder lines and having variable resisting functions of subsidiarily 
controlling flows of the hydraulic fluids supplied from the respective 
first and second hydraulic pumps. 
(15) Preferably, the hydraulic system of the above (14) further comprises 
control means for controlling the variable resisting function so as to 
throttle the first arm auxiliary valve when boom operating means and/or 
bucket operating means for respectively instructing the boom cylinder and 
the bucket cylinder to be driven is operated. 
With this feature, during the simultaneous operation of the boom or the 
bucket and the arm, most of the hydraulic fluid from the first hydraulic 
pump is sent to the boom cylinder or the bucket cylinder because the first 
arm auxiliary valve is throttled, and the hydraulic fluid from the second 
hydraulic pump is primarily sent to the arm cylinder. 
(16) In the above (15), preferably, the control means controls the variable 
resisting function when the boom operating means, the bucket operating 
means, and arm operating means for instructing the arm cylinder to be 
driven are operated, such that the first and second boom auxiliary valves 
are opened, the first bucket auxiliary valve is throttled, and the second 
bucket auxiliary valve is closed when the boom operating means instructs 
boom-up, and the first boom auxiliary valve and the first bucket auxiliary 
valve are opened and the second boom auxiliary valve and the second bucket 
auxiliary valve are closed when the boom operating means instructs 
boom-down. 
With this feature, during the combined operation of three members of the 
front working equipment in which the boom (boom-up), the arm and the 
bucket are simultaneously driven, the first arm auxiliary valve and the 
first bucket auxiliary valve are controlled to be throttled, the first and 
second boom auxiliary valves and the second arm auxiliary valve are all 
controlled to be opened, and the second bucket auxiliary valve is 
controlled to be closed. Because a load pressure in the operation of each 
of the arm and the bucket is lower than that in the boom-up operation, 
most of the hydraulic fluid from the second hydraulic pump is sent to the 
arm cylinder through the arm directional control valve after passing the 
second arm auxiliary valve, whereas most of the hydraulic fluid from the 
first hydraulic pump is sent to the boom cylinder and the bucket cylinder 
through the boom directional control valve and the bucket directional 
control valve after passing the first boom auxiliary valve and the first 
bucket auxiliary valve, thereby enabling the combined operation of three 
members of the front working equipment to be performed. 
Also, during the combined operation of three members of the front working 
equipment in which the boom (boom-down), the arm and the bucket are 
simultaneously driven, the first arm auxiliary valve is controlled to be 
throttled, the first boom auxiliary valve, the second arm auxiliary valve 
and the first bucket auxiliary valve are all controlled to be opened, and 
the second boom auxiliary valve and the second bucket auxiliary valve are 
controlled to be closed. Therefore, the hydraulic fluid from the second 
hydraulic pump is sent to the arm cylinder through the arm directional 
control valve after passing the second arm auxiliary valve, whereas most 
of the hydraulic fluid from the first hydraulic pump is sent to the boom 
cylinder and the bucket cylinder through the boom directional control 
valve and the bucket directional control valve after passing the first 
boom auxiliary valve and the first bucket auxiliary valve, thereby 
enabling the combined operation of three members of the front working 
equipment to be performed. 
(17) The hydraulic system of the above (12) further comprises, by way of 
example, first and second travel feeder lines respectively connecting the 
first and second hydraulic pumps to a pump port of the first travel 
directional control valve, a third travel feeder line connecting the first 
hydraulic pump to a pump port of the second travel directional control 
valve, first and second reverse-flow preventing valves disposed 
respectively in the first and second travel feeder lines for preventing 
the hydraulic fluids from reversely flowing to the respective first and 
second hydraulic pumps, and first and second travel auxiliary valves 
disposed respectively in the first and second travel feeder lines and 
having variable resisting functions of subsidiarily controlling flows of 
the hydraulic fluids supplied from the respective first and second 
hydraulic pumps. 
(18) Preferably, the hydraulic system of the above (17) further comprises 
control means for controlling the variable resisting functions so as to 
close the first travel auxiliary valve and open the second travel 
auxiliary valve when only first-and-second travel operating means for 
instructing the first and second travel motors to be driven is operated. 
With this feature, during the sole operation of travel, the first travel 
auxiliary valve is controlled to be closed and the second travel auxiliary 
valve is controlled to be opened. Therefore, the hydraulic fluid from the 
first hydraulic pump is sent to the second travel motor through the second 
travel directional control valve, and the hydraulic fluid from the second 
hydraulic pump is sent to the first travel motor through the second travel 
auxiliary valve and the first travel directional control valve. 
(19) Preferably, the hydraulic system of the above (17) further comprises 
control means for controlling the variable resisting functions such that 
the first travel auxiliary valve is opened and the second travel auxiliary 
valve is throttled when at least boom operating means and/or arm operating 
means for respectively instructing the boom cylinder and the arm cylinder 
to be driven is operated, and at least one of the first boom auxiliary 
valve and the first arm auxiliary valve is throttled when the second 
travel operating means is operated. 
With this feature, during the combined operation of plural modes including 
travel, for example, during the simultaneous operation of the boom and 
travel, the first boom auxiliary valve is controlled to be throttled as 
the second travel directional control valve is operated, the second travel 
auxiliary valve is controlled to be throttled as the boom directional 
control valve is operated, and the second boom auxiliary valve and the 
first travel auxiliary valve are both controlled to be fully opened. 
Therefore, most of the hydraulic fluid from the first hydraulic pump is 
supplied to the first and second travel motors and part thereof is also 
supplied to the boom cylinder after being throttled by the first boom 
auxiliary valve, whereas most of the hydraulic fluid from the second 
hydraulic pump is supplied to the boom cylinder through the second boom 
auxiliary valve and the boom directional control valve. As a result, 
sufficient forces to perform the travel and boom operations are ensured, 
and the combined operation including travel is implemented while 
preventing the excavator from traveling askew. This is equally applied to 
the simultaneous operation of travel combined with any other mode or 
member. 
(20) The hydraulic system of the above (17) further comprises, by way of 
example, first and second bucket feeder lines respectively connecting the 
first and second hydraulic pumps to a pump port of the bucket directional 
control valve, first and second bucket reverse-flow preventing valves 
disposed respectively in the first and second bucket feeder lines for 
preventing the hydraulic fluids from reversely flowing to the respective 
first and second hydraulic pumps, first and second bucket auxiliary valves 
disposed respectively in the first and second bucket feeder lines and 
having variable resisting functions of subsidiarily controlling flows of 
the hydraulic fluids supplied from the respective first and second 
hydraulic pumps, and control means for controlling the variable resisting 
functions such that the first travel auxiliary valve is closed and the 
second travel auxiliary valve is opened when only first-and-second travel 
operating means for instructing the first and second travel motors to be 
driven is operated, that the first travel auxiliary valve is opened and 
the second travel auxiliary valve is throttled when at least one of boom 
operating means, arm operating means, bucket operating means and swing 
operating means for respectively instructing the boom cylinder, the arm 
cylinder, the bucket cylinder and the swing motor to be driven is 
operated, and that at least one of the first boom auxiliary valve, the 
first arm auxiliary valve and the first bucket auxiliary valve is 
throttled when the second travel operating means is operated. 
This feature enables the hydraulic system to effect the sole operation of 
travel mentioned in the above (18) and the combined operation of travel 
with the boom, the arm, the bucket or swing mentioned in the above (19). 
(21) The hydraulic system of the above (12) further comprises, by way of 
example, a swing feeder line for connecting the second hydraulic pump to a 
pump port of the swing directional control valve. 
(22) Preferably, the hydraulic system of the above (21) further comprises 
control means for controlling the variable resisting function so as to 
throttle the arm auxiliary valve when swing operating means for 
instructing the swing motor to be driven is operated. 
With this feature, during the simultaneous operation of the arm and swing, 
for example, the first arm auxiliary valve is controlled to be opened and 
the second arm auxiliary valve is controlled to be throttled. Therefore, a 
sufficient pressure for the swing operation is ensured and the operability 
in the combined operation of plural modes including swing is improved. 
(23) Preferably, the hydraulic system of the above (21) further comprises 
control means for controlling the variable resisting functions when the 
boom operating means for instructing the boom cylinder to be driven is 
operated, such that the first and second boom auxiliary valves are both 
opened when the boom operating means instructs boom-up, and the first boom 
auxiliary valve is opened and the second boom auxiliary valve is closed 
when the boom operating means instructs boom-down. 
With this feature, during the simultaneous operation of swing and boom-up, 
for example, the first and second auxiliary valves are both controlled to 
be fully opened so that the boom cylinder and the swing motor are 
connected to the first and second hydraulic pumps in parallel. As a 
result, the pressure for the swing operation is ensured by a boom driving 
pressure and the boom can be satisfactorily raised by a swing load 
pressure. 
Also, during the simultaneous operation of swing and boom-down, the first 
boom auxiliary valve is controlled to be fully opened and the second boom 
auxiliary valve is controlled to be fully closed so that the boom cylinder 
is connected to the first hydraulic pump alone. As a result, the pressure 
for the swing operation is ensured without being affected by a low load 
pressure during boom-down, and the operability in the combined operation 
including swing is improved. 
(24) Further, to achieve the above second object, the present invention is 
constituted as follows. The hydraulic system of the above (11) further 
comprises first and second bleed valves disposed respectively between the 
first and second hydraulic pumps and a reservoir, and reducing opening 
areas thereof depending on operation amounts of at least two directional 
control valves. 
By so providing the first and second bleed valves, a preference degree and 
metering characteristics can be set independently of each other during the 
combined operation of plural actuators by employing a closed center 
circuit, as mentioned above, in a hydraulic system for a hydraulic 
excavator.

BEST MODE FOR CARRYING OUT THE INVENTION 
Hereunder, embodiments of the present invention will be described with 
reference to the drawings. 
In FIG. 1, a hydraulic system of one embodiment comprises two first and 
second variable displacement hydraulic pumps 1a, 1b, and regulators 2a, 2b 
for controlling respective capacities of the hydraulic pumps 1a, 1b. A 
plurality of actuators are provided, including a boom cylinder 3, an arm 
cylinder 4, a bucket cylinder 5, a swing motor 6 and first and second 
travel motors 7, 8, along with a boom directional control valve 9, an arm 
directional control valve 10 and a bucket directional control valve 11, 
each being of closed center type, connected to the first and second 
hydraulic pumps 1a, 1b for controlling respective flow rates of hydraulic 
fluids supplied to the boom cylinder 3, the arm cylinder 4 and the bucket 
cylinder 5. A swing directional control valve 12 of closed center type is 
connected to the second hydraulic pump 1b for controlling a flow rate of a 
hydraulic fluid supplied to the swing motor 6, a first travel directional 
control valve 13 of closed center type is connected to the first and 
second hydraulic pumps 1a, 1b for controlling a flow rate of hydraulic 
fluids supplied to the first travel motor 7, and a second travel 
directional control valve 14 of closed center type is connected to the 
first hydraulic pump 1a for controlling a flow rate of a hydraulic fluid 
supplied to the second travel motor 8. 
The boom, arm, bucket, swing, and first and second travel directional 
control valves 9-14 are pilot-operated valves having respective pairs of 
pilot hydraulic driving sectors 9da, 9db; 10da, 10db; 11da, 11db; 12da, 
12db, 13da, 13db; 14da, 14db, and controlled by respective pilot pressure 
signals 92a, 92b; 102a, 102b; 112a, 112b; 122a, 122b; 132a, 132b; 142a, 
142b in a switchable manner. 
The boom, arm, bucket, swing, and first and second travel directional 
control valves 9-14 have pump ports 9p, 10p, 11p, 12p, 13p, 14p, reservoir 
ports 9t, 10t, 11t, 12t, 13t, 14t, and two actuator ports 9a, 9b; 10a, 
10b; 11a, 11b; 12a, 12b; 13a, 13b; 14a, 14b, respectively. The reservoir 
ports are all connected to a reservoir 29, and the actuator ports are 
connected to the corresponding hydraulic actuators. Counterbalancing 
valves 27, 28 are disposed respectively between the actuator ports 13a, 
13b of the first travel directional control valve 13 and the first travel 
motor 7 and between the actuator ports 14a, 14b of the second travel 
directional control valve 14 and the second travel motor 8. 
Also, the pump port 9p of the boom directional control valve 9 is connected 
to the first and second hydraulic pumps 1a, 1b through first and second 
pump lines 30a, 30b and first and second boom feeder lines 93a, 93b. The 
pump port 10p of the arm directional control valve 10 is connected to the 
first and second hydraulic pumps 1a, 1b through the first and second pump 
lines 30a, 30b and first and second arm feeder lines 103a, 103b. The pump 
port 11p of the bucket directional control valve 11 is connected to the 
first and second hydraulic pumps 1a, 1b through the first and second pump 
lines 30a, 30b and first and second bucket feeder lines 113a, 113b. The 
pump port 12p of the swing directional control valve 12 is connected to 
the second hydraulic pump 1b through the second pump line 30b and a swing 
feeder line 123b. The pump port 13p of the first travel directional 
control valve 13 is connected to the first and second hydraulic pumps 1a, 
1b through the first and second pump lines 30a, 30b and first and second 
travel feeder lines 133a, 133b. The pump port 14p of the second travel 
directional control valve 14 is connected to the first hydraulic pump 1a 
through the first pump line 30a and a travel feeder line 143a. 
First and second boom auxiliary valves 91a, 91b are disposed respectively 
in the first and second boom feeder lines 93a, 93b. Likewise, first and 
second arm auxiliary valves 101a, 101b, first and second bucket auxiliary 
valves 111a, 111b, and first and second travel auxiliary valves 131a, 131b 
are disposed respectively in the first and second arm feeder lines 103a, 
103b, the first and second bucket feeder lines 113a, 113b, and the first 
and second travel feeder lines 133a, 133b. These auxiliary valves are 
driven by respective control pressures generated from proportional 
solenoid valves 31a, 31b; 32a, 32b; 33a, 33b; 34a, 34b. 
The auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 131a, 131b are 
poppet type valves each having both a function as a check valve to prevent 
the hydraulic fluids from reversely flowing back to the first and second 
hydraulic pumps 1a, 1b, and a variable resisting function of subsidiarily 
controlling flows of the hydraulic fluids supplied from the first and 
second hydraulic pumps 1a, 1b. The variable resisting function includes a 
flow cutoff function of selectively cutting off flows of the hydraulic 
fluids supplied from the first and second hydraulic pumps 1a, 1b. The 
principles of a poppet valve having such a variable resisting function are 
well known (see JP-A-58-501781, for example) and the disclosed poppet 
valve is applied as each of the auxiliary valves in this embodiment. 
Details of the auxiliary valve will be described below. 
Disposed in the swing feeder line 123b is a load check valve 16 for 
preventing the hydraulic fluid from reversely flowing back to the second 
hydraulic pump 1b from the swing motor 6 when a load of the swing motor 6 
is high. A fixed throttle 17 for limiting a bucket speed is disposed in 
the second bucket feeder line 113b upstream of the second auxiliary valve 
111b. 
First and second bleed lines 25a, 25b for connecting the first and second 
hydraulic pumps 1a, 1b to the reservoir 29 are branched from the first and 
second pump lines 30a, 30b, and first and second bleed valves 15a, 15b are 
disposed respectively in the first and second bleed lines 25a, 25b. The 
bleed valves 15a, 15b are pilot-operated valves having hydraulic driving 
sectors 15ad, 15bd and driven by control pressures generated from 
proportional solenoid valves 24a, 24b, respectively. 
In FIG. 2, denoted by 19, 20 and 21 are control lever units provided with 
pilot valves for generating pilot pressure signals 92a, 92b; 102a, 102b; 
112a, 112b; 122a, 122b; 132a, 132b; 142a, 142b. The control lever unit 19 
is associated with the boom and the bucket and, when its control lever is 
operated, the pilot valves built therein generate the pilot pressure 
signals 92a, 92b; 112a, 112b depending on the direction and amount in and 
by which the control lever is operated. The control lever unit 20 is 
associated with the arm and the swing motor and, when its control lever is 
operated, the pilot valves built therein generate the pilot pressure 
signals 102a, 102b; 122a, 122b depending on the direction and amount in 
and by which the control lever is operated. The control lever unit 21 is 
associated with the first and second travel motors and, when its control 
lever is operated, the pilot valves built therein generate the pilot 
pressure signals 132a, 132b; 142a, 142b depending on the direction and 
amount in and by which the control lever is operated. Denoted by 22 is a 
hydraulic source used for generating the pilot pressure signals. 
Also, as control means for the auxiliary valves 91a, 91b; 101a, 101b; 111a, 
111b; 131a, 131b, the bleed valves 15a, 15b, and the regulators 2a, 2b, 
there are provided pilot pressure sensors 41a, 41b; 42a, 42b; 43a, 43b; 
44a, 44b; 45a, 45b; 46a, 46b for detecting pressures of the pilot pressure 
signals, and a controller 23. The controller 23 executes predetermined 
steps of processing based on signals from the pilot pressure sensors and 
outputs command signals to the proportional solenoid valves 31a, 31b-34a, 
34b; 24a, 24b and the regulators 2a, 2b. 
As shown in FIG. 3, the controller 23 comprises an input portion 23a for 
receiving detection signals from the pilot pressure sensors 41a, 41b-46a, 
46b after A/D-conversion, a storage portion 23b for storing preset 
characteristics, a processing portion 23c for reading the preset 
characteristics from the storage portion 23b and executing predetermined 
steps of processing to calculate command signals for the proportional 
solenoid valves 31a, 31b-34a, 34b; 24a, 24b and the regulators 2a, 2b, and 
an output portion 23d for converting the command signals calculated by the 
processing portion 23c into driving signals and outputting the converted 
driving signals. 
The hydraulic system of this embodiment is equipped on a hydraulic 
excavator as shown in FIG. 4. The hydraulic excavator comprises a boom 50 
driven by the boom cylinder 3, an arm 51 driven by the arm cylinder 4, a 
bucket 52 driven by the bucket cylinder 5, an upper structure (swing) 53 
driven by the swing motor 6, and left and right traveling devices (tracks) 
54, 55 driven by the first and second travel motors 7, 8. The boom 50, the 
arm 51 and the bucket 52 make up a front working equipment 56 with which 
the excavator perform work in front of the upper structure 53. The left 
and right traveling devices 54, 55 make up an undercarriage 57. 
The operating principles of the hydraulic system of this embodiment will be 
described with reference to FIGS. 5 to 15. 
FIGS. 5 to 12 illustrate, in the form of circuit models, respective minimum 
units of the hydraulic system shown in FIG. 1 divided per function. In 
these drawings, pumps P1, P2 correspond to the first and second hydraulic 
pumps 1a, 1b; actuators A, B correspond to any two of the hydraulic 
actuators 3-5 and 7; valves VA, VB correspond to any two of the 
directional control valves 9-11 and 13; ports PA, PB correspond to any two 
of the pump ports 9p-11p and 13p, lines FA1, FA2, and FB1, FB2 correspond 
to any two pairs of the feeder lines 93a, 93b; 103a, 103b; 113a, 113b; and 
133a, 133b, check valves CA1, CA2 and CB1, CB2 represent functions of any 
two pairs of the auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; and 
131a, 131b as valves for preventing reverse flow (hereinafter referred to 
simply as reverse-flow preventing functions), on/off valves DA1, DB2 
represent flow cutoff functions of any two of the auxiliary valves 91a, 
91b; 101a, 101b; 111a, 111b; 131a, and 131b; variable throttle valves EA1, 
EA2 and EB1, EB2 represent variable resisting functions of any two pairs 
of the auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 131a; 131b; 
valves B1, B2 correspond to the first and second bleed valves 15a, 15b, 
regulators R1, R2 correspond to the regulators 2a, 2b; and sensors SA1, 
SA2 and SB1, SB2 correspond to any two pairs of the pilot pressure sensors 
41a, 41b-46a, 46b, respectively. 
Note that while, in FIGS. 6 to 12, the check valves CA1, etc. are disposed 
in relatively upstream positions and the on/off valves DA1, etc. or the 
variable throttle valves EA1, etc. are disposed in relatively downstream 
positions in the same feeder line, the order in which those valves are 
disposed in the same feeder line may be reversed. 
A: Reverse-flow preventing function of the auxiliary valve (FIG. 5) 
(1) When the actuator A is solely driven, hydraulic fluids from the two 
pumps P1, P2 can be joined together and supplied to the actuator A through 
the feeder lines FA1, FA2 (joining circuit). Also, when the load pressure 
of the actuator A is higher than the delivery pressures of the pumps P1, 
P2, the check valves (the reverse-flow preventing functions of the 
auxiliary valves) CA1, CA2 prevent the hydraulic fluids from reversely 
flowing from the actuator to the pumps (load check function). 
(2) When the actuators A, B are simultaneously driven, it is always ensured 
in a hydraulic system where the load pressure of the actuator A is higher 
than the load pressure of the actuator B that the actuator A can be 
operated by the hydraulic fluid from the pump P2 and the actuator B can be 
operated by the hydraulic fluid from the pump P1 (preference circuit). At 
this time, even with the load pressure of the actuator B being lower than 
the load pressure of the actuator B, the hydraulic fluid from the pump P2 
is prevented from flowing into the actuator B by the presence of the check 
valve CA1. 
B: Reverse-flow preventing function+flow cutoff function 1 of the auxiliary 
valve (FIG. 6) 
(1) When the actuator A is solely driven, the hydraulic fluids from the two 
pumps P1, P2 can be joined together and supplied to the actuator A, as 
with the above case, by holding the on/off valve (the flow cutoff function 
of the auxiliary valve) DA1 turned off (joining circuit). 
(2) When the actuators A, B are simultaneously driven, the on/off valve DA1 
is turned on upon the sensors SB1, SB2 detecting an operation of the 
directional control valve VB, causing the pump P1 to be connected to the 
actuator B preferentially (i.e., in tandem). Regardless of the load 
pressures of the actuators A, B, therefore, the actuator A can be operated 
by the hydraulic fluid from the pump P2 and the actuator B can be operated 
by the hydraulic fluid from the pump P1 independently of each other 
(preference circuit). 
C: Reverse-flow preventing function+flow cutoff function 2 of the auxiliary 
valve (FIG. 7) 
(1) When the actuator A is solely driven, the hydraulic fluids from the two 
pumps P1, P2 can be joined together and supplied to the actuator A, as 
with the above case, by holding the on/off valve (the flow cutoff function 
of the auxiliary valve) DA1 turned off (joining circuit). 
(2) When the actuator B is solely driven, the hydraulic fluids from the two 
pumps P1, P2 can be joined together and supplied to the actuator B, as 
with the above case, by holding the on/off valve (the flow cutoff function 
of the auxiliary valve) DB2 turned off (joining circuit). 
(3) When the actuators A, B are simultaneously driven, the on/off valves 
DA1, DB2 are turned on upon the sensors SA1, SA2 and SB1, SB2 detecting 
operations of the directional control valves VA, VB, respectively, causing 
the pump P1 to be connected to the actuator B preferentially and the pump 
P2 to be connected to the actuator A preferentially. Regardless of the 
load pressures of the actuators A, B, therefore, the actuator A can be 
operated by the hydraulic fluid from the pump P2 and the actuator B can be 
operated by the hydraulic fluid from the pump P1 independently of each 
other (preference circuit). 
D: Reverse-flow preventing function+variable resisting function of the 
auxiliary valve (FIG. 8) 
(1) An opening area of the variable throttle valve (the variable resisting 
function of the auxiliary valve) EB2 and an opening area of the variable 
throttle valve (the variable resisting function of the auxiliary valve) 
EA1 are set such that, when the directional control valves VA, VB are 
operated, the opening areas of the variable throttle valves EB2, EA1 are 
each changed from a maximum value in the fully open state to a minimum 
value in the fully closed state, as indicated by X1 in FIG. 13, depending 
on respective operation amounts of the directional control valves VA, VB. 
In FIG. 13, X0 indicates a corresponding change in opening area of each 
meter-in throttle depending on the operation amounts of the directional 
control valves VA, VB. The operation amounts of the directional control 
valves VA, VB are detected by the sensors SA1, SA2 and SB1, SB2. 
(2) When the actuator A is solely driven with only the directional control 
valve VA fully operated, the variable throttle valve EA1 is fully opened 
and the variable throttle valve EB2 is fully closed. Therefore, the 
hydraulic fluids from the two pumps P1, P2 can be joined together and 
supplied to the actuator A, as with the above case (joining circuit). 
(3) When the directional control valve VB is half-operated from the state 
of (2), the variable throttle valve EA1 is gradually throttled depending 
on the operation amount of the directional control valve VB and the pump 
P1 is connected to the actuator B preferentially depending on an extent by 
which the variable throttle valve EA1 is throttled. When the variable 
throttle valve EB2 is fully closed with the directional control valve VA 
fully operated, the pump P2 is connected to the actuator A preferentially 
to a full extent (adjustment of preference degree). Therefore, all of the 
hydraulic fluid from the pump P2 plus part of the hydraulic fluid from the 
pump P1 are supplied to the actuator A, and most of the hydraulic fluid 
from the pump P1 is supplied to the actuator B, enabling the actuators A, 
B to be simultaneously driven (preference circuit). Further, when the 
directional control valve VB is fully operated, the variable throttle 
valve EA1 is fully closed and the pump P1 is connected to the actuator B 
preferentially to a full extent. Therefore, all of the hydraulic fluid 
from the pump P2 is supplied to the actuator A and all of the hydraulic 
fluid from the pump P1 is supplied to the actuator B, enabling the 
actuators A, B to be simultaneously driven (preference circuit). Also, if 
the variable throttle valve EA1 is abruptly turned on/off when it is 
throttled, there would occur a shock because of the circuit being closed 
at the moment the directional control valve VB is operated. But such a 
shock can be suppressed in this case because the variable throttle valve 
EA1 is gradually throttled depending on the operation amount of the 
directional control valve VB. 
(4) When the actuator A is solely driven with the directional control valve 
VA half-operated, the variable throttle valve EA1 is fully opened and the 
variable throttle valve EB2 is throttled. Therefore, the hydraulic fluids 
from the two pumps P1, P2 can be joined together and supplied to the 
actuator A (joining function). 
(5) When the directional control valve VB is half-operated from the state 
of (4), the variable throttle valve EA1 is gradually throttled depending 
on the operation amount of the directional control valve VB and the pump 
P1 is connected to the actuator B preferentially depending on an extent by 
which the variable throttle valve EA1 is throttled. At the same time, 
since the variable throttle valve EB2 is throttled with the directional 
control valve VA half-operated, the pump P2 is connected to the actuator A 
preferentially depending on an extent by which the variable throttle valve 
EB2 is throttled (adjustment of preference degree). Therefore, most of the 
hydraulic fluid from the pump P2 plus part of the hydraulic fluid from the 
pump P1 are supplied to the actuator A, and most of the hydraulic fluid 
from the pump PI plus part of the hydraulic fluid from the pump P2 are 
supplied to the actuator B, enabling the actuators A, B to be 
simultaneously driven (preference circuit). Further, when the directional 
control valve VB is fully operated, the variable throttle valve EA1 is 
fully closed and the pump P1 is connected to the actuator B preferentially 
to a full extent. Therefore, most of the hydraulic fluid from the pump P2 
is supplied to the actuator A and all of the hydraulic fluid from the pump 
P1 plus part of the hydraulic fluid from the pump P2 are supplied to the 
actuator B, enabling the actuators A, B to be simultaneously driven 
(preference circuit). In this case, it is also possible to suppress a 
shock otherwise occurring at the moment the directional control valve VB 
is operated. 
(6) The transition from the sole operation of the actuator B to the 
combined operation of the actuators A, B is performed in a like manner to 
the above (5). 
(7) In the above description, the opening areas of the variable throttle 
valves EB2, EA1 are set such that they are each changed from a maximum 
value in the fully open state to a minimum value in the fully closed 
state, as indicated by X1 in FIG. 13, depending on the operation amounts 
of the directional control valves VA, VB. However, the setting may be 
modified such that the opening area of at least one of the variable 
throttle valves EB2, EA1 is changed depending on the load pressure of the 
actuator A or B. For example, the opening area of the variable throttle 
valve EB2 may be set to have a larger value as the load pressure of the 
actuator B increases (see FIG. 33). This may result in a smaller 
throttling loss produced when the hydraulic fluid from the pump P2 passes 
the variable throttle valve EB2, and hence smaller energy loss. Such a 
modification is equally applied to the following cases shown in FIGS. 9 to 
12 as well. That modified embodiment will be described later with 
reference to FIGS. 31 to 33. 
E: Reverse-flow preventing function+variable resisting function of the 
auxiliary valve+bleed control function (FIG. 9) 
(1) Opening areas of the bleed valves B1, B2 are set such that, when the 
directional control valves VA, VB are operated, the opening areas of the 
bleed valves B1, B2 are each changed from a maximum value in the fully 
open state to a minimum value in the fully closed state, as indicated by 
X2 in FIG. 14, depending on respective operation amounts of the 
directional control valves VA, VB. At this time, the operation amounts of 
the directional control valves VA, VB may be determined as a total of both 
the operation amounts or a maximum value thereof, or may be calculated by 
using any function. As an alternative, it is also possible to calculate 
proportions of the flow rate demanded for the first pump 1a and the flow 
rate demanded for the second pump 1b from the extent by which respective 
flows are throttled by the variable resisting functions, divide a total of 
the operation amounts by the calculated proportions, and determine part of 
the total amount associated with the pump P1 and part of the total amount 
associated with the pump P2. In FIG. 14, X0 indicates a corresponding 
change in opening area of each meter-in throttle depending on the 
operation amounts of the directional control valves VA, VB when solely 
operated. 
(2) When the actuator A or B is solely driven, or when the actuators A and 
B are simultaneously driven, the bleed valves B1, B2 are throttled to 
gradually increase the pump delivery pressures depending on the operation 
amounts of the directional control valves VA, VB, thereby supplying the 
actuators A, B with the hydraulic fluids at flow rates corresponding to 
the pump delivery pressures (bleed control). By changing the respective 
extent by which the bleed valves 15a, 15b are throttled, therefore, flow 
rate characteristics (metering characteristics) of the hydraulic fluids 
supplied to the actuators A, B through meter-in openings of the 
directional control valves VA, VB can be changed. Further, since the pump 
delivery pressure is gradually increased when the actuator A or B is 
started up, abrupt driving of the actuator can be avoided. 
F: Reverse-flow preventing function+variable resisting function of the 
auxiliary valve+bleed control function+pump control 1 (FIG. 10) 
(1) Target delivery rates of the pumps P1, P2 are set such that, when the 
directional control valves VA, VB are operated, the target pump delivery 
rates are each increased, as shown in FIG. 15, depending on respective 
operation amounts of the directional control valves VA, VB. At this time, 
the operation amounts of the directional control valves VA, VB may be 
calculated similarly to the above case. Tiltings (displacements) of the 
pumps P1, P2 are then controlled by the regulators R1, R2 so that the 
target pump delivery rates are obtained. 
(2) When the actuator A or B is solely driven, or when the actuators A and 
B are simultaneously driven, the delivery rates of the pumps P1 and/or P2 
are gradually increased depending on the operation amounts of the 
directional control valves VA, VB, thereby delivering the hydraulic fluids 
at flow rates required (positive control). 
G: Reverse-flow preventing function of the auxiliary valve+variable 
resisting function of each feeder line (FIG. 11) 
The circuit can be freely selected as follows, and design change of the 
circuit per model and product is facilitated. 
(1) When the variable throttle valves (variable resisting functions of the 
auxiliary valves) EA1, EA2 and EB1, EB2 are all turned off, the pumps P1, 
P2 are each connected to the actuators A, B in parallel. 
(2) When the variable throttle valves EA1, EB1 are turned off and the 
variable throttle valve EB2 is throttled as indicated by X1 in FIG. 13 
depending on the operation amount of the directional control valve VA, the 
pump P1 is connected to the actuators A, B in parallel and the pump P2 is 
connected to the actuator A preferentially. 
(3) When the variable throttle valves EA1, EB1 are turned off and the 
variable throttle valve EA2 is throttled as indicated by X1 in FIG. 13 
depending on the operation amount of the directional control valve VB, the 
pump P1 is connected to the actuators A, B in parallel and the pump P2 is 
connected to the actuator B preferentially. 
(4) When the variable throttle valves EA2, EB2 are turned off and the 
variable throttle valve EB1 is throttled as indicated by X1 in FIG. 13 
depending on the operation amount of the directional control valve VA, the 
pump P1 is connected to the actuator A preferentially and the pump P2 is 
connected to the actuators A, B in parallel. 
(5) When the variable throttle valves EA2, EB2 are turned off and the 
variable throttle valve EA1 is throttled as indicated by X1 in FIG. 13 
depending on the operation amount of the directional control valve VB, the 
pump P1 is connected to the actuator B preferentially and the pump P2 is 
connected to the actuators A, B in parallel. 
H: Reverse-flow preventing function+variable resisting function of the 
auxiliary valve+bleed control function+pump control 2 (FIG. 12) 
(1) The load pressures of the actuators A, B are detected respectively in 
the directional control valves VA, VB. A higher one of the load pressures 
(maximum load pressure) is detected through shuttle valves M1, M2, and the 
regulators R1, R2 control tiltings (displacements) of the pumps P1, P2 so 
that the pump delivery pressure is held higher than the maximum load 
pressure by a predetermined value. Also, the auxiliary valves disposed in 
the feeder lines FA1, FB2 are constructed to have, in addition to the 
above-stated variable resisting functions (the variable throttle valves 
EA1, EB2), functions as on/off valves LA1, LB2 capable of selectively 
communicating and cutting off the load pressures detected in the 
directional control valves VA, VB. 
(2) When the actuator A or B is solely driven, or when the actuators A and 
B are simultaneously driven, the delivery rates of the pumps P1 and/or P2 
are increased depending on the operation amounts of the directional 
control valves VA, VB so that a differential pressure between the maximum 
load pressure and the pump delivery pressure is held at the predetermined 
value, thereby delivering the hydraulic fluids at flow rates required 
(load sensing control). In this way, the load sensing control can also be 
applied to the circuit shown in FIG. 1. 
The hydraulic system of this embodiment shown in FIG. 1 has all of the 
above functions A to G, thus making it possible to easily construct a 
joining circuit and a preference circuit in the hydraulic circuit using 
valves of closed center type. Also, comparing the conventional open center 
circuit, a preference degree and metering characteristics can be set 
independently of each other because preference circuits constituted by the 
auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 113a, 113b and bleed 
circuits constituted by the bleed valves 15a, 15b are separated from each 
other. 
Processing steps executed in the processing portion 23c of the controller 
23 in the hydraulic system of this embodiment will now be described with 
reference to FIGS. 16 to 21. 
As shown in FIG. 16, the processing portion 23c of the controller 23 
receives the detection signals of the pilot pressure sensors 41a, 41b-46a, 
46b (step 100) and, based on the received signals, carries out control of 
the first and second hydraulic pumps 1a, 1b, control of the first and 
second bleed valves 15a, 15b and control of the auxiliary valves 91a, 91b; 
101a, 101b; 111a, 111b; 113a, 113b (steps 200, 300 and 400). 
In the control of the hydraulic pumps 1a, 1b, as described in the above F, 
target delivery rates of the hydraulic pumps 1a, 1b are preset such that 
they are each increased, as shown in FIG. 15, depending on respective 
operation amounts of the directional control valves 9-14. The processing 
portion 23c calculates the target delivery rates of the first and second 
hydraulic pumps 1a, 1b corresponding to the operation amounts of the 
directional control valves 9-14 from the detection signals of the pilot 
pressure sensors 41a, 41b-46a, 46b, and then calculates and outputs 
command signals for the regulators 2a, 2b to achieve the target delivery 
rates. At this time, as described in the above E, the operation amounts of 
the directional control valves 9-14 may be determined as a total of the 
operation amounts or a maximum value thereof, or may be calculated by 
using any function. As an alternative, it is also possible to calculate 
proportions of the flow rate demanded for the pump 1 and the flow rate 
demanded for the pump 2 from the extent by which the auxiliary valves 91a, 
91b; 101a, 101b; 111a, 111b; 131a, 131b are throttled, divide a total of 
the operation amounts by the calculated proportions, and determine part of 
the total amount associated with the first pump 1a and part of the total 
amount associated with the second pump 1b. 
In the control of the bleed valves 15a, 15b, as described in the above E, 
target opening areas of the first and second bleed valves 15a, 15b are 
preset such that they are each decreased, as shown in FIG. 14, depending 
on respective operation amounts of the directional control valves 9-14. 
The processing portion 23c calculates the target opening areas of the 
first and second bleed valves 15a, 15b corresponding to the operation 
amounts of the directional control valves 9-14 from the detection signals 
of the pilot pressure sensors 41a, 41b-46a, 46b, and then calculates and 
outputs command signals for the proportional solenoid valves 24a, 24b to 
achieve the target opening areas. At this time, the operation amounts of 
the directional control valves 9-14 may be determined similarly to the 
above case. One example of such control is described in the above-cited 
JP-A-7-63203. 
In the control of the auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 
131a, 131b, the processing portion 23c judges the operating conditions of 
the traveling devices (travel), the upper structure (swing), the boom, the 
arm and the bucket based on the detection signals of the pilot pressure 
sensors 41a, 41b-46a, 46b, determines the operation positions of the 
auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 131a, 131b (i.e., 
whether the auxiliary valves are to be fully opened, fully closed or 
throttled, or to what degree they are to be throttled if so) in accordance 
with the judged operating conditions, and then calculates and outputs 
command signals for the proportional solenoid valves 31a, 31b-34a, 34b to 
achieve the determined operation positions. 
The relationship between the valve operation amount and the target pump 
delivery rate as shown in FIG. 15 that is employed in the control of the 
hydraulic pumps 1a, 1b, the relationship between the operation amount and 
the opening area as shown in FIG. 14 that is employed in the control of 
the bleed valves 15a, 15b, and the relationships between the operating 
conditions and the auxiliary valve operation positions that are employed 
in the control of the auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 
131a, 131b, are all stored in the storage portion 23b of the controller 
23. 
The relationships between the operating conditions and the auxiliary valve 
operation positions that are employed in the control of the auxiliary 
valves are set, by way of example, as shown in FIGS. 17 to 21. FIG. 17 
shows the operation positions of the auxiliary valves during the sole 
operation, FIG. 18 shows the operation positions of the auxiliary valves 
during the combined operation of two and three modes including travel, 
FIG. 19 shows the operation positions of the auxiliary valves during the 
combined operation of two and three modes including swing, FIG. 20 shows 
the operation positions of the auxiliary valves during the combined 
operation of two members of the front working equipment, and FIG. 21 shows 
the operation positions of the auxiliary valves during the combined 
operation of three members of the front working equipment, respectively. 
In tables of these drawings, .smallcircle. implies that the auxiliary 
valve is fully opened, x implies that it is fully closed, and .DELTA. 
implies that it is throttled. Also, ()represents the operation position in 
a standby state. 
The settings of FIGS. 17 to 21 are intended to, in the hydraulic system 
shown in FIG. 1, realize a circuit equivalent to a conventional open 
center circuit, called OHS, shown in FIG. 22 and achieve the functions 
which are not obtained by the conventional open center circuit. The 
conventional open center circuit shown in FIG. 22 is the same as shown in 
FIG. 1 of the above-cited JP-B-2-16416. In FIG. 22, hydraulic pumps and 
actuators are denoted by the same reference numerals as in FIG. 1 of the 
drawings attached to this application. Directional control valves are 
divided into two valve groups 83, 84 corresponding to two hydraulic pumps 
1a, 1b and are denoted by the same reference numerals as the directional 
control valves in FIG. 1, but affixed with A, B corresponding to the two 
valve groups. Denoted by 60, 61 are pump lines, 62, 63 are center bypass 
lines, 64 is an on/off valve for travel, 86, 88, 90, 94, 102, 104 are 
bypass lines, and 92, 96 are fixed throttles. 
In the open center circuit shown in FIG. 22, a joining circuit is realized 
by providing two directional control valves belonging respectively to the 
two valve groups 83, 85 for one actuator. Also, in each valve group, a 
preference circuit is selectively realized in a combination of a tandem 
connection by which pump ports of the directional control valves are 
connected to only the center bypass lines 62, 63, and a parallel 
connection by which the pump ports of the directional control valves are 
connected to only the center bypass lines 62, 63 through the bypass lines 
86, 88, 90, 94, 102. Then, a preference degree is adjusted by providing 
the fixed throttles 92, 96 in the bypass lines. Furthermore, the 
preference circuit is set as follows. In the valve group 83, connection is 
made such that front actuators 3-5 are driven by the pump 1a more 
preferentially than a travel motor 7. In the valve group 85, connection is 
made such that a travel motor 8 is driven by the pump 1b more 
preferentially than the front actuators 3-5. A travel directional control 
valve 13A and a travel directional control valve 14B are connected to each 
other through the bypass line 104 and, when the front actuators 3 -5 are 
driven, the on/off valve 64 disposed in the bypass line 104 is opened to 
supply a hydraulic fluid from the pump 1b to the two travel motors 7, 8 in 
parallel. 
The hydraulic system of this embodiment shown in FIG. 1 operates as 
described below based on the settings of FIGS. 17 to 21 to realize a 
circuit equivalent to the conventional open center circuit and achieve the 
functions which are not obtained by the conventional open center circuit. 
First, a description will be made on the sole operation of travel, the sole 
operation of boom-up, and the combined operation of travel and boom-up. 
During the sole operation of travel, the auxiliary valve 131a is controlled 
to be fully closed and the auxiliary valve 131b is controlled to be fully 
opened (FIG. 17), so that the hydraulic fluid from the first hydraulic 
pump 1a is sent to the second travel motor 8 through the directional 
control valve 14 and the hydraulic fluid from the second hydraulic pump 1b 
is sent to the first travel motor 7 through the auxiliary valve 131b and 
the directional control valve 13. 
Next, during the sole operation of boom-up, the auxiliary valves 91a, 91b 
are both controlled to be fully opened (FIG. 17), so that the hydraulic 
fluids from the hydraulic pumps 1a, 1b are joined together and sent to the 
boom cylinder 3 through the directional control valve 9. 
During the combined operation of travel and boom-up, the auxiliary valve 
91a is controlled to be throttled as the travel directional control valve 
14 is operated, the auxiliary valve 131b is controlled to be throttled as 
the boom directional control valve 9 is operated, and the auxiliary valves 
91b, 131a are both controlled to be fully opened (FIG. 18). At this time, 
when the sole operation of travel is changed to the combined operation of 
travel and boom-up, it is preferable to provide some time lag in the 
transition because there occurs a large shock on the travel if the 
auxiliary valve 131b is abruptly throttled. Also, the auxiliary valve 131b 
is only required to be throttled to such an extent as producing a pressure 
enough to surely raise the boom cylinder 3, and is not required to be 
fully closed. Further, in order to avoid the effect of a low travel load 
pressure as experienced, e.g., when the excavator travels over a 
downslope, the auxiliary valve 131b may be fully closed after the lapse of 
a predetermined time. The auxiliary valve 131a is fully opened at the same 
time as when the boom is operated. By controlling the auxiliary valves in 
that way, during the combined operation of travel and boom-up, most of the 
hydraulic fluid from the hydraulic pump 1a is supplied to the travel 
motors 7, 8 and part thereof is also supplied to the boom cylinder 3 after 
being throttled by the auxiliary valve 91a, whereas most of the hydraulic 
fluid from the hydraulic pump 1b is supplied to the boom cylinder 3 
through the auxiliary valve 91b and the directional control valve 9. As a 
result, it is possible not only to ensure sufficient forces to perform the 
travel and boom operations, but also to prevent the excavator from 
traveling askew. 
During the combined operation of travel combined with another mode, the 
auxiliary valves are likewise controlled such that the auxiliary valve 
131a is opened, the auxiliary valve 131b is throttled, and the auxiliary 
valve located on the same side as the hydraulic pump 1a and associated 
with the directional control valve for an operation other than travel is 
throttled (FIG. 18). 
As described above, during the combined operation of travel and boom-up, 
the auxiliary valve 131b is throttled as the boom directional control 
valve 9 is operated, the auxiliary valve 131a is fully opened, and the 
auxiliary valve 91a is throttled as the travel directional control valve 
14 is operated. In this process, the throttling operation of the auxiliary 
valve 131b corresponds to the operation of throttling an opening of the 
center bypass line 62 of the boom directional control valve 9A in the 
conventional open center circuit shown in FIG. 22, and the throttling 
operation of the auxiliary valve 91a corresponds to the operation of 
throttling an opening of the center bypass line 63 of the travel 
directional control valve 14B in the conventional open center circuit. 
These throttling operations each have a function of determining a 
preference degree in the combined operation. The opening operation of the 
auxiliary valve 131a corresponds to the opening operation of the on/off 
valve 64 in the conventional open center circuit. 
Here, in the conventional open center circuit, characteristics (opening 
curves) of the openings of the center bypass lines versus the operation 
amounts of the boom directional control valve 9A and the travel 
directional control valve 14B have functions of determining both a 
preference degree in the combined operation and metering characteristics 
developed when the respective directional control valves are operated. 
Thus, the characteristics (opening curves) of the openings of the center 
bypass lines versus the operation amounts of the directional control 
valves are determined based not on operability in the combined operation, 
but on the metering characteristics of the respective directional control 
valves. Accordingly, when the boom and the traveling devices are 
half-operated, it has sometimes occurred that the travel speed change is 
so large as to pose an inconvenience in operation of the excavator. 
In the present invention, since the preference circuit made up of the 
auxiliary valves 91a, 131b and the bleed circuit made up of the first and 
second bleed valves 15a, 15b are separated from each other, metering 
characteristics developed when the directional control valves 9, 13, 14 
are operated are determined by the relationships between respective 
meter-in and meter-out throttles provided in the directional control 
valves and opening areas of the bleed valves 15a, 15b, and a preference 
degree in the combined operation is determined by the extent by which the 
auxiliary valves 91a, 131b are throttled. Therefore, the metering 
characteristics in the sole operation and the preference degree in the 
combined operation can be optimally determined independently of each 
other, and operability in the combined operation can be improved. Without 
being limited to the combined operation of travel and boom-up, this is 
also equally applied to the combined operation of other modes described 
later. 
During the combined operation of the bucket and travel, since there is no 
demand for moving the bucket cylinder 5 fast, the auxiliary valve 111b is 
not required to be fully opened. To this end, the fixed throttle 17 may be 
disposed in series with respect to the auxiliary valve 111b as shown in 
FIG. 1. Alternatively, a maximum opening of the auxiliary valve 111b may 
be restricted. 
A description will now be made on the sole operation of swing, the sole 
operation of the arm, and the simultaneous operation of the arm and swing. 
During the sole operation of swing, the hydraulic fluid from the hydraulic 
pump 1b is supplied to the swing motor 6 through the directional control 
valve 12. On this occasion, the hydraulic fluid is not throttled in this 
embodiment because the swing directional control valve 12 is provided with 
no auxiliary valve, but with an ordinary load check valve 16 alone. Of 
course, an auxiliary valve may be associated with the travel directional 
control valve. 
During the sole operation of the arm, the auxiliary valves 101a, 101b are 
both controlled to be fully opened (FIG. 17), so that the hydraulic fluid 
from the hydraulic pump 1a is sent to the directional control valve 10 and 
the arm cylinder 4 through the auxiliary valve 101a and the hydraulic 
fluid from the hydraulic pump 1b is joined with the hydraulic fluid from 
the hydraulic pump 1a after passing the auxiliary valve 101b. 
During the simultaneous operation of the arm and swing, the arm auxiliary 
valve 101a is controlled to be fully opened and the arm auxiliary valve 
101b is controlled to be throttled (FIG. 19). With this control, a 
sufficient pressure for the swing operation is ensured during the combined 
operation of the arm and swing, and the operability in the combined 
operation including swing is improved. The auxiliary valve 101b may be 
throttled by restricting a maximum opening, or depending on the operation 
amount of the swing directional control valve 12. Additionally, the arm 
operation is divided into arm crowding and arm dumping. Since the arm 
crowding is performed under a relatively light load, the extent by which 
the auxiliary valve 101b is throttled is changed between the arm crowding 
and arm dumping so that it is throttled to a larger extent in the arm 
crowding. 
The sole operation of the boom and the simultaneous operation of the boom 
and swing will now be described. 
During the sole operation of boom-up, the auxiliary valves 91a, 91b are 
both controlled to be fully opened (FIG. 17), so that the hydraulic fluids 
from the hydraulic pumps 1a, 1b are joined together after passing the 
auxiliary valves 91a, 91b and then sent to the directional control valve 9 
and the boom cylinder 3. During the sole operation of boom-down, the flow 
supplied from only one pump is sufficient for the operation. Therefore, 
the auxiliary valve 91a is controlled to be fully opened and the auxiliary 
valve 91b is controlled to be fully closed (FIG. 17), so that the 
hydraulic fluid from the hydraulic pump 1a is sent to the directional 
control valve 9 and the boom cylinder 3 through the auxiliary valve 91a. 
During the simultaneous operation of swing and boom-up, the auxiliary 
valves 91a, 91b are both controlled to be fully opened (FIG. 19) similarly 
to the sole operation of boom-up, so that the boom cylinder 3 and the 
swing motor 6 are connected to the two hydraulic pumps 1a, 1b in parallel. 
As a result, the pressure for the swing operation can be ensured by a boom 
driving pressure and the boom can be satisfactorily raised by a swing load 
pressure. 
During the simultaneous operation of swing and boom-down, the auxiliary 
valve 91a is controlled to be fully opened and the auxiliary valve 91b is 
controlled to be fully closed (FIG. 19) similarly to the sole operation of 
boom-down, so that the boom cylinder 3 is connected to the hydraulic pump 
1a alone. As a result, the pressure for the swing operation is ensured 
without being affected by a low load pressure during boom-down, and the 
operability in the combined operation including swing is improved. That 
function of enabling the boom cylinder to be connected to the hydraulic 
pumps 1a, 1b in different ways between boom-up and boom-down is not 
provided in the conventional open center circuit. 
The simultaneous operation of the boom and the arm will now be described. 
The sole operation of the boom and the sole operation of the have been 
described above. During the simultaneous operation of the arm and boom-up, 
the auxiliary valves 91a, 91b, 101b are all controlled to be fully opened 
and the auxiliary valve 101a is controlled to be throttled depending on 
the operation amount of the boom directional control valve 9 (FIG. 20). 
Because a boom-up load pressure is high during the simultaneous operation 
of the arm and boom-up, the hydraulic fluid from the hydraulic pump 1b is 
primarily sent to the arm cylinder 4 through the auxiliary valve 101b and 
the directional control valve 10. Most of the hydraulic fluid from the 
hydraulic pump 1a is sent to the boom cylinder 3 because the auxiliary 
valve 101a is throttled. 
During the simultaneous operation of the arm and boom-down, the auxiliary 
valves 91a, 101b are both controlled to be fully opened, the auxiliary 
valve 91b is controlled to be fully closed, and the auxiliary valve 101a 
is controlled to be throttled depending on the operation amount of the 
boom directional control valve 9 (FIG. 20). Because a boom-down load 
pressure is low during the simultaneous operation of the arm and 
boom-down, the hydraulic fluid from the hydraulic pump 1b is sent to the 
arm cylinder 4 by fully closing the auxiliary valve 91b. Most of the 
hydraulic fluid from the hydraulic pump 1a is sent to the boom cylinder 3 
because the auxiliary valve 101a is throttled. 
The sole operation of the bucket and the combined operation including the 
bucket will now be described. 
During the sole operation of the bucket, when the bucket is solely operated 
in a bucket crowding mode, the auxiliary valves 111a, 111b are both 
controlled to be fully opened (FIG. 17), so that the hydraulic fluid from 
the hydraulic pump 1a is sent to the bucket cylinder 5 through the 
directional control valve 11 after passing the auxiliary valve 111a, and 
the hydraulic fluid from the hydraulic pump 1b is joined therewith after 
passing the fixed throttle 17 and the auxiliary valve 111b and then also 
sent to the bucket cylinder 5 through the directional control valve 11. 
When the bucket is solely operated in a bucket dumping mode, the auxiliary 
valve 111a is controlled to be fully opened and the auxiliary valve 111b 
is controlled to be fully closed, so that the hydraulic fluid from the 
hydraulic pump 1a is sent to the bucket cylinder 5 through the directional 
control valve 11 after passing the auxiliary valve 111a. 
During the simultaneous operation of the arm and the bucket, the auxiliary 
valve 101a is controlled to be throttled depending on the operation amount 
of the bucket directional control valve 11, and the auxiliary valves 101b, 
111a, 111b are all controlled to be fully opened (FIG. 20). Therefore, 
most of the hydraulic fluid from the hydraulic pump 1a is sent to the 
bucket cylinder 5 through the directional control valve 11 after passing 
the auxiliary valve 111a, whereas most of the hydraulic fluid from the 
hydraulic pump 1b is sent under an action of the fixed throttle 17 to the 
arm cylinder 4 through the directional control valve 10 after passing the 
auxiliary valve 101b, thereby enabling the simultaneous operation to be 
performed. 
During the combined operation of three members of the front working 
equipment in which the boom (boom-up), the arm and the bucket are 
simultaneously driven, the auxiliary valve 101a is controlled to be 
throttled depending on the operation amounts of the boom directional 
control valve 9 and the bucket directional control valve 11, the auxiliary 
valve 101a is controlled to be throttled depending on the operation 
amounts of the boom directional control valve 9 and the arm directional 
control valve 10, the auxiliary valves 91a, 91b, 101b are all controlled 
to be fully opened, and the auxiliary valve 111b is controlled to be fully 
closed (FIG. 21). Because a load pressure in the operation of each of the 
arm and the bucket is lower than that in the boom-up operation, most of 
the hydraulic fluid from the hydraulic pump 1b is sent to the arm cylinder 
4 through the directional control valve 10 after passing the auxiliary 
valve 101b, whereas most of the hydraulic fluid from the hydraulic pump 1a 
is sent to the boom cylinder 3 and the bucket cylinder 5 through the 
directional control valves 9, 11 after passing the auxiliary valves 91a, 
111a, thereby enabling the combined operation of three members of the 
front working equipment to be performed. 
During the combined operation of three members of the front working 
equipment in which the boom (boom-down), the arm and the bucket are 
simultaneously driven, the auxiliary valve 101a is controlled to be 
throttled depending on the operation amount of the boom directional 
control valve 9, the auxiliary valves 91a, 101b, 111a are all controlled 
to be fully opened, and the auxiliary valves 91b, 111b are controlled to 
be fully closed (FIG. 21). Therefore, the hydraulic fluid from the 
hydraulic pump 1b is sent to the arm cylinder 4 through the directional 
control valve 10 after passing the auxiliary valve 101b, whereas most of 
the hydraulic fluid from the hydraulic pump 1a is sent to the boom 
cylinder 3 and the bucket cylinder 5 through the directional control 
valves 9, 11 after passing the auxiliary valves 91a, 111a, thereby 
enabling the combined operation of three members of the front working 
equipment to be performed. 
In this way, the combined operation of three members of the front working 
equipment, which has been difficult to realize in the conventional open 
center circuit, can be easily implemented. 
Next, an embodiment of a valve apparatus including the directional control 
valves 9-14, the auxiliary valves 91a, 91b; 101a, 101b; 111a, 111b; 131a, 
131b, and the bleed valves 15a, 15b will be described with reference to 
FIGS. 23 to 29. 
FIG. 23 shows an appearance of the valve apparatus, FIG. 24 is a sectional 
view taken along line XXIV--XXIV in FIG. 23, including the boom 
directional control valve 9 and the auxiliary valves 91a, 91b, FIG. 25 is 
an enlarged view of a portion including the auxiliary valve, FIG. 26 is a 
sectional view taken along line XXVI--XXVI in FIG. 23, including the 
bucket directional control valve 11 and the auxiliary valves 111a, 111b, 
FIG. 27 is a sectional view taken along line XXVII--XXVII in FIG. 23 
including the swing directional control valve 12, FIG. 28 is a sectional 
view taken along line XXVIII--XXVIII in FIG. 23 including the travel motor 
directional control valve 14, and FIG. 29 is a sectional view taken along 
line XXIX--XXIX in FIG. 23, including the bleed valves 15a, 15b. 
In FIG. 23, denoted by 200 is the valve apparatus including the directional 
control valves 9-14, the auxiliary valves 91a, 91b; 101a, 101b; 111a, 
111b; 131a, 131b, and the bleed valves 15a, 15b. The valve apparatus 200 
has a common housing 201 in which the first and second pump lines 30a, 30b 
are defined as shown in FIGS. 24 to 29. 
As shown in FIG. 24, the boom directional control valve 9 has a spool 202 
slidable in the housing 201, the spool 202 having notches 203a, 203b; 
204a, 204b formed therein. Also, the housing 201 has formed therein the 
first and second boom feeder lines 93a, 93b, the pump port 9p of the boom 
directional control valve 9, the actuator ports 9a, 9b, and the reservoir 
port 9t. The notches 203a, 203b make up meter-in variable throttles for 
communicating the pump port 9p with the actuator ports 9a, 9b, and the 
notches 204a, 204b make up meter-out variable throttles for communicating 
the actuator ports 9a, 9b with the reservoir port 9t. The hydraulic 
driving sectors 9da, 9db are provided at opposite ends of the spool 202. 
The auxiliary valves 91a, 91b of poppet type comprise respectively poppet 
valves 210a, 210b being slidable in the housing 201 to selectively open 
and close the feeder lines 93a, 93b, and pilot spools (pilot valves) 212a, 
212b being slidable in blocks 211a, 211b fixed to the housing 210 and 
operating the poppet valves 210a, 210b. 
As shown in FIG. 25 in an enlarged scale, the poppet valve 210a of the 
auxiliary valve 91a has a poppet 210 slidably inserted into a bore 213 
defining the feeder line 93a and a bore 215 defining a back pressure 
chamber 214. In a portion of the poppet 210 which is inserted into the 
bore 213, there is formed an opening 216 for flow rate control which 
changes an opening area established between the pump line 30a and the pump 
port 9p depending on a stroke through which the poppet 210 is moved. The 
poppet 210 has a pressure bearing portion 217 for bearing the pressure at 
the pump port 9p, a pressure bearing portion 218 for bearing the pressure 
in the pump line 30a, and a pressure bearing portion 219 for bearing the 
pressure in the back pressure chamber 214. Assuming that the effective 
pressure bearing area of the pressure bearing portion 217 is Ap, the 
effective pressure bearing area of the pressure bearing portion 218 is Az, 
and the effective pressure bearing area of the pressure bearing portion 
219 is Ac, the relationship of Ac=Az+Ap holds. Further, in a portion of 
the poppet 210 which is inserted into the bore 215, there is formed a 
feedback slit 220 which changes an opening area communicated with the back 
pressure chamber 214 depending on a stroke through which the poppet 210 is 
moved. Also, the poppet 210 has an inner passage 221 formed therein for 
communicating the feedback slit 220 with the pump port 30a, and a load 
check valve 222 for preventing a reverse flow from the load side is 
disposed in the inner passage 221. 
The pilot spool 212a has a notch 230 formed therein, the notch 230 
constituting a pilot variable throttle whose opening area is changed 
depending on a stroke through which the pilot spool 212a is moved. Also, a 
passage 231 for communicating the back pressure chamber 214 with a space 
including the notch 230 is formed in the block 211a, and passages 232, 233 
for communicating the space including the notch 230 with the pump port 9p 
are formed in the block 211a and the housing 201, respectively. A pilot 
flow rate through a pilot line made up of the back pressure chamber 214, 
the feedback slit 220, the inner passage 221 and the passages 231, 232, 
233 is varied by changing the opening area of the pilot variable throttle. 
On the side of one end of the pilot spool 212a, there is provided a 
hydraulic driving sector 234 to which a control pressure is introduced 
from the proportional solenoid valve 31a. The pilot spool 212a is moved by 
the hydraulic driving sector 234 in accordance with the control pressure. 
The poppet valve 210b and the pilot spool 212b on the side of the auxiliary 
valve 91b are similarly constructed. 
The principles of the auxiliary valve 91a of poppet type constructed as 
described above are known in the art. Assuming that the ratio of the 
effective pressure bearing area Ac of the pressure bearing portion 219 of 
the poppet 210 on the side of the back pressure chamber 214 to the 
effective pressure bearing area Ap of the pressure bearing portion 218 of 
the poppet 210 on the side of the pump line 30a (or 30b) is K, the 
pressure in the pump line 30a (or 30b) (i.e., the pump pressure) is Pp, 
and the pressure at the pump port 9p (i.e., the pressure on the entry side 
of the meter-in variable throttle) is Pz, the pressure Pc in the back 
pressure chamber 214 is expressed by a function of K, Pp and Pz. Thus, the 
poppet 210 is moved so that the opening area established by the feedback 
slit 220 is held in a predetermined relationship depending on K with 
respect to the opening area established by the notch 230 of the pilot 
spool 212a (or 212b). Given Ac:Ap=2:1 and K 1/2, by way of example, 
Pc=(Pp+Pz)/2 results and the poppet 210 is moved so that the opening area 
established by the feedback slit 220 is equal to the opening area 
established by the notch 230 of the pilot spool 212a (or 212b). At this 
time, by properly selecting the size of the opening 216, the opening area 
communicated from the pump line 30a (or 30b) to the pump port 9p can be 
optionally controlled by moving the pilot spool 212a (or 212b). Since the 
pilot spool 212a (or 212b) is controlled by the proportional solenoid 
valve 31a (or 31b), the opening area communicated from the pump line 30a 
(or 30b) to the pump port 9p can be eventually controlled by the 
controller 23 (variable resisting function). 
Further, when the pump port 9p is subjected to a higher pressure load than 
the pump line 30a (or 30b), the load pressure is exerted on the pressure 
bearing portion 217 of the poppet 210 on the side of the pump port 9p and, 
simultaneously, the same pressure acts on the pressure bearing portion 219 
of the poppet 210 on the side of the back pressure side 214 through the 
passages 233, 232, the notch 230 and the passage 231. Here, the pressure 
bearing portion 219 of the poppet 210 has a larger effective pressure 
bearing area than the pressure bearing portion 217 thereof. Therefore, the 
poppet 210 is pushed toward the pump port 9p and hence serves as a load 
check valve (reverse-flow preventing function). 
Another set of the arm directional control valve 10 and the auxiliary 
valves 101a, 101b and still another set of the first travel directional 
control valve 13 and the auxiliary valves 131a, 131b are also constructed 
similarly to the above set of the boom directional control valve 9 and the 
auxiliary valves 91a, 91b. 
The bucket directional control valve 11 and the auxiliary valves 111a, 111b 
are also constructed almost similarly to the boom directional control 
valve 9 and the auxiliary valves 91a, 91b. As shown in FIG. 26, however, 
the opening 216A for flow rate control defined in the poppet 210 of the 
auxiliary valve 91b is formed to have a small opening area so that it 
functions as the fixed throttle 17. 
The swing directional control valve 12 and the second travel directional 
control valve 14 are also constructed, as shown in FIGS. 27 and 28, 
similarly to the boom directional control valve 9. In the swing 
directional control valve 12, however, the load check valve 16 is disposed 
in the feeder line 123b as shown in FIG. 27. The pump line 30a is not 
connected to the pump port 12p. In the second travel directional control 
valve 14, the feeder line 143a is merely a passage and the pump line 30b 
is not connected to the pump port 14p. 
As shown in FIG. 29, the bleed valves 15a, 15b have spools 302a, 302b 
slidable in the housing 201, the spools 302a, 302b having notches 303a, 
303b formed therein, respectively. Also, passages 304a, 305a; 304b, 305b 
serving as the first and second bleed lines 25a, 25b are formed in the 
housing 201. The notches 303a, 303b constitute bleed-off variable 
throttles for communicating the passages 304a, 304b with the passages 
305a, 305b. 
Further, the hydraulic driving sectors 15ad, 15bd are provided respectively 
at opposite outer ends of the spools 302a, 302b. Denoted by 306a, 306b are 
pump connection ports through which the first and second hydraulic pumps 
1a, 1b are connected to the pump lines 30a, 30b. 
By utilizing poppet valves as described above, the valve apparatus in which 
the auxiliary valves including the reverse-flow preventing function and 
the variable resisting function are built in can be easily realized 
without making the valve structure complicated. 
Another embodiment of the present invention will be described with 
reference to FIG. 30. In FIG. 30, equivalent members to those shown in 
FIG. 1 are denoted by the same reference numerals. In the foregoing 
embodiment, the auxiliary valve is constructed as a poppet type valve to 
have a function of a reverse-flow preventing valve as well, an electric 
command signal is output from the controller to the proportional solenoid 
valve, and the auxiliary valve is driven by the control pressure output 
from the proportional solenoid valve. By contrast, in this embodiment, a 
reverse-flow preventing valve and an auxiliary valve having a variable 
resisting function (including a flow cutoff function) are constituted as 
separate valves, and the auxiliary valve is directly driven by the pilot 
pressure signal from the control lever unit. 
In FIG. 30, a check valve 500a is disposed in the first boom feeder line 
93a, and a check valve 500b and an auxiliary valve 501b of spool type are 
disposed in the second boom feeder line 93b. The check valve 500a has a 
function as a reverse-flow preventing valve for preventing the hydraulic 
fluid from reversely flowing to the first hydraulic pump 1a from the 
feeder line 93a, the check valve 500b has a function as a reverse-flow 
preventing valve for preventing the hydraulic fluid from reversely flowing 
to the second hydraulic pump 1b from the feeder line 93b, and the 
auxiliary valve 501b has a flow cutoff function of selectively cutting off 
the flow of the hydraulic fluid supplied to the feeder line 93b from the 
second hydraulic pump 1b. 
A check valve 510a and an auxiliary valve 511a of spool type are disposed 
in the first arm feeder line 103a and a check valve 510b is disposed in 
the second arm feeder line 103b. The check valve 510a has a function as a 
reverse-flow preventing valve for preventing the hydraulic fluid from 
reversely flowing to the first hydraulic pump 1a from the feeder line 
103a, and the auxiliary valve 511b has a variable resisting function 
(including a flow cutoff function) of subsidiarily controlling the flow of 
the hydraulic fluid supplied to the feeder line 103a from the first 
hydraulic pump 1a. Also, the check valve 500b has a function as a 
reverse-flow preventing valve for preventing the hydraulic fluid from 
reversely flowing to the second hydraulic pump 1b from the feeder line 
103b. 
The auxiliary valve 501b and the auxiliary valve 511a are pilot-operated 
valves having respective hydraulic driving sectors 501c, 511c which 
operate in the direction to close the valves. The pilot pressure signal 
92b in the boom-down direction is supplied to the hydraulic driving sector 
501c through pilot lines 531, 532, and the pilot pressure signal 92a in 
the boom-up direction or the pilot pressure signal 92b in the boom-down 
direction is supplied to the hydraulic driving sector 511c through pilot 
lines 530, 531, a shuttle valve 53 and a pilot line 534. 
During the sole operation of boom-up, the pilot pressure signal 92b is not 
output and the auxiliary valve 501b is held in a fully open position as 
shown. Therefore, the hydraulic fluids from the hydraulic pumps 1a, 1b are 
joined together after passing the check valves 500a, 500b and then sent to 
the directional control valve 9 and the boom cylinder 3 (joining circuit). 
During the sole operation of boom-down, since the pilot pressure signal 
92b is output, the auxiliary valve 501b is operated to a fully closed 
position by the pilot pressure signal 92b, whereupon the hydraulic fluid 
from the hydraulic pump 1a is sent to the directional control valve 9 and 
the boom cylinder 3 through the check valve 500a. 
During the simultaneous operation of the arm and boom-up, the auxiliary 
valve 501b is controlled to be fully opened and the auxiliary valve 511a 
is controlled to be throttled depending on the boom-up pilot pressure 
signal 92a (the operation amount of the boom directional control valve 9). 
Because a boom-up load pressure is high during the simultaneous operation 
of the arm and boom-up, the hydraulic fluid from the hydraulic pump 1b is 
primarily sent to the arm cylinder 4 through the check valve 510b and the 
directional control valve 10 (preference circuit). Most of the hydraulic 
fluid from the hydraulic pump 1a is sent to the boom cylinder 3 because 
the auxiliary valve 511a is throttled (preference circuit and adjustment 
of a preference degree). 
During the simultaneous operation of the arm and boom-down, the auxiliary 
valve 501b is controlled to be fully closed by the boom-down pilot 
pressure signal 92b and the auxiliary valve 511a is controlled to be 
throttled depending on the boom-down pilot pressure signal 92b (the 
operation amount of the boom directional control valve 9). Because a 
boom-up load pressure is low during the simultaneous operation of the arm 
and boom-down, the hydraulic fluid from the hydraulic pump 1b is sent to 
the arm cylinder 4 by fully closing the auxiliary valve 501b (preference 
circuit). Most of the hydraulic fluid from the hydraulic pump 1a is sent 
to the boom cylinder 3 because the auxiliary valve 511a is throttled 
(adjustment of a preference degree). 
As described above, with this embodiment wherein the auxiliary valve having 
a variable resisting function is constituted as a spool-type valve, the 
reverse-flow preventing valve and the auxiliary valve are constituted as 
separate valves, and the auxiliary valve is directly driven by the pilot 
pressure signal from the control lever unit, the joining circuit and the 
preference circuit can also be realized with a simple structure by 
employing a closed center circuit as with the first embodiment. 
Still another embodiment of the present invention will be described with 
reference to FIGS. 31 to 33. In these drawings, equivalent members to 
those shown in FIGS. 1 and 3 are denoted by the same reference numerals. 
While in the foregoing embodiments the opening area of the auxiliary valve 
developing a variable resisting function is changed depending on only the 
operation amount of the directional control valve, it is changed depending 
on not only the operation amount of the directional control valve, but 
also the load pressure of the actuator in this embodiment. 
In FIG. 31, a load pressure sensor 600 for detecting a load pressure of the 
arm cylinder 4 in the extending direction (arm crowding direction) is 
disposed in an actuator line on the arm crowding side connected to the 
actuator port 10a of the arm directional control valve 10. In FIG. 32, a 
detection signal of the load pressure sensor 600 is also applied to the 
input portion 23a of a controller 23A in addition to the detection signals 
of the pilot pressure sensors 41a, 41b-46a, 46b. Also, when the 
simultaneous operation of boom-up and arm crowding is detected, the 
processing portion 23c of the controller 23A calculates a target opening 
area of the auxiliary valve 101a based on the detection signal of the 
boom-up pilot pressure sensor 41a and the detection signal of the load 
pressure sensor 600, and computes a command signal for the proportional 
solenoid valve 32a for the auxiliary valves 101a. 
FIG. 33 shows the relationship among the operation amount of the boom 
directional control valve 9 (i.e., the pilot pressure signal) in the 
boom-up direction, the arm crowding load pressure, and the target opening 
area of the auxiliary valve 101a. As indicated by X3 in the graph of FIG. 
33, the relationship is set such that as the operation amount of the boom 
directional control valve 9 in the boom-up direction increases, the 
opening area of the auxiliary valve 101a is changed from a maximum value 
in the fully open state to a minimum value in the fully closed state, and 
as the arm crowding load pressure increases, the opening area of the 
auxiliary valve 101a has a larger value at the same operation amount of 
the boom directional control valve 9 in the boom-up direction. 
In this embodiment thus constructed, during the simultaneous operation of 
boom-up and arm crowding, the auxiliary valves 91a, 91b, 101b are 
controlled to be fully opened as mentioned above. Also, the auxiliary 
valve 101a is controlled to be throttled depending on the operation amount 
of the boom directional control valve 9 (FIG. 20), and to have a larger 
opening area as the arm-crowding load pressure increases (FIG. 33). 
Because the boom-up load pressure is high during the simultaneous 
operation of boom-up and arm crowding, the hydraulic fluids are supplied 
basically in a like manner to the above. Thus, the hydraulic fluid from 
the hydraulic pump 1b is primarily sent to the arm cylinder 4 through the 
auxiliary valve 101b and the directional control valve 10 and most of the 
hydraulic fluid from the hydraulic pump 1a is sent to the boom cylinder 3 
because the auxiliary valve 101a is throttled. Further, since the 
arm-crowding load pressure varies to a large extent depending on an arm 
angle, the opening area of the auxiliary valve 101a is set to have a 
smaller value at the same valve operation amount in the boom-up direction 
when the arm-crowding load pressure is low and the difference between the 
arm-crowding load pressure and the boom-up load pressure is large, causing 
most of the hydraulic fluid from the hydraulic pump 1a to be sent to the 
boom cylinder 3 because the auxiliary valve 101a is throttled. On the 
other hand, the opening area of the auxiliary valve 101a is set to have a 
larger value at the same valve operation amount in the boom-up direction 
when the arm-crowding load pressure is increased and the difference 
between the arm-crowding load pressure and the boom-up load pressure 
becomes small, causing most of the hydraulic fluid from the hydraulic pump 
1a to be sent to the boom cylinder 3 under a combination of throttling of 
the auxiliary valve 101a and the arm-crowding load pressure. Therefore, 
when part of the hydraulic fluid from the hydraulic pump 1a is supplied to 
the arm cylinder 4 through the auxiliary valve 101a, the extent by which 
the flow is throttled by the auxiliary valve 101a is small (i.e., the 
auxiliary valve 101a has a large opening area). As a result, the 
throttling loss produced when the hydraulic fluid passes auxiliary valve 
101a is reduced and hence the energy loss is reduced. 
With this embodiment, as described above, a hydraulic system having a 
structure capable of reducing the energy loss and saving energy, in 
addition to the advantages of the first embodiment, can be provided. 
Industrial Applicability 
According to the present invention, a joining circuit and a preference 
circuit can be realized in a closed center circuit with a simple 
structure. 
Also, it is possible in a closed center circuit to set a preference degree 
and metering characteristics independently of each other during the 
combined operation of plural actuators, and to improve the operability in 
the combined operation.