# Patent

## Title
Hydraulic drive system for construction machine

## Abstract

In a hydraulic drive system performing the load sensing control by using a pump device having two delivery ports whose delivery flow rates are controlled by a single pump controller, surplus flow is prevented and energy loss at an unload valve and a pressure compensating valve is reduced in combined operations in which two actuators are driven at the same time while producing a relatively large supply flow rate difference therebetween. A boom cylinder 3a is connected so that the hydraulic fluids delivered from delivery ports P1 and P2 of a pump device 1a are merged and supplied to the boom cylinder 3a. An arm cylinder 3h is connected so that the hydraulic fluids delivered from delivery ports P3 and P4 of a pump device 1b are merged and supplied to the arm cylinder 3h. A travel motor 3d is connected so that the hydraulic fluid delivered from one (delivery port P2) of the delivery ports of the pump device 1a and the hydraulic fluid delivered from one (delivery port P4) of the delivery ports of the pump device 1b are merged and supplied to the travel motor 3d. A travel motor 3e is connected so that the hydraulic fluid delivered from the other (delivery port P1) of the delivery ports of the pump device 1a and the hydraulic fluid delivered from the other (delivery port P3) of the delivery ports of the pump device 1b are merged and supplied to the travel motor 3e.

## Background
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for a construction machine such as a hydraulic excavator. In particular, the invention relates to a hydraulic drive system for a construction machine comprising a pump device which has two delivery ports whose delivery flow rates are controlled by a single pump regulator (pump controller), and a load sensing system which controls delivery pressures of the pump device to be higher than the maximum load pressure of actuators.
BACKGROUND ART
For example, Patent Literature 1 describes a hydraulic drive system for a construction machine comprising a pump device which has two delivery ports whose delivery flow rates are controlled by a single pump regulator, and a load sensing system which controls delivery pressures of the pump device to be higher than the maximum load pressure of actuators. In the Patent Literature 1, a hydraulic pump of the split flow type is used as the pump device having two delivery ports. The split flow type hydraulic pump, including only one pump regulator and only one swash plate (displacement control mechanism), controls the delivery flow rates of the two delivery ports by adjusting the tilting angle of the single swash plate (displacement) with the single pump regulator, thereby implementing a pump function of two pumps with a compact structure.
PRIOR ART LITERATURE
Patent Literature
Patent Literature 1: JP, A 2012-67459
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
For example, such a split flow type hydraulic pump is used in a hydraulic drive system comprising a load sensing system, and the hydraulic circuit is configured so that hydraulic fluids delivered from the two delivery ports are separately led to different actuators. In this example, for a combined operation in which two actuators are driven at the same time while producing a relatively large supply flow rate difference therebetween (e.g., leveling operation performed by a hydraulic excavator by use of a boom and an arm), the demanded flow rate on the high flow rate actuator's side (arm cylinder's side) is given high priority and the swash plate of the hydraulic pump is controlled to increase the tilting angle.
In such a case, a surplus flow occurs in the pump flow delivered from the delivery port on the low flow rate actuator's side. The surplus flow is drained to a tank by an unload valve, causing part of the energy consumption by the hydraulic pump.
As above, in cases where a split flow type hydraulic pump is used in a hydraulic drive system comprising a load sensing system and the hydraulic circuit is configured so that the hydraulic fluids delivered from the two delivery ports are separately led to different actuators, a surplus flow occurs in such a combined operation in which two actuators are driven at the same time while producing a relatively large supply flow rate difference therebetween. The surplus flow is equivalent to energy loss. The load sensing system's original function of preventing the surplus flow is impaired in such a combined operation.
In the Patent Literature 1, in combined operations other than those using a traveling unit and/or a dozer unit, the delivery flows from the two delivery ports of the split flow type hydraulic pump are merged together so that the two delivery ports function as one pump. Therefore, the delivery flow rate of the hydraulic pump is controlled without causing the surplus flow in combined operations such as the leveling operation performed by use of the boom and the arm. However, in combined operations in which two actuators are driven at the same time, the load pressures of the actuators differ from each other in many cases. For example, in the leveling combined operation performed by use of the boom and the arm, the boom cylinder operates as the high load pressure side and the arm cylinder operates as the low load pressure side. When such a combined operation driving a high load pressure actuator and a low load pressure actuator in combination is carried out by a hydraulic drive system having a load sensing system, the delivery pressures of the hydraulic pump are controlled to be higher than the high load pressure of the boom cylinder by a certain preset pressure. In this case, a pressure compensating valve that is provided for driving the arm cylinder and preventing excessive flow to the arm cylinder at the low load pressure is throttled. Thus, energy loss is caused by the pressure loss at the pressure compensating valve.
It is therefore the primary object of the present invention to provide a hydraulic drive system for a construction machine that performs the load sensing control by using a pump device having two delivery ports whose delivery flow rates are controlled by a single pump controller and that is capable of preventing the surplus flow and reducing the energy loss at the unload valve and the pressure compensating valve in combined operations in which two actuators are driven at the same time while producing a relatively large supply flow rate difference therebetween.
Means for Solving the Problem
To achieve the above object, the present invention provides a hydraulic drive system for a construction machine, comprising: a first pump device having first and second delivery ports; a second pump device having third and fourth delivery ports; and a plurality of actuators which are driven by hydraulic fluid delivered from the first and second delivery ports of the first pump device and hydraulic fluid delivered from the third and fourth delivery ports of the second pump device. The first pump device includes a first pump controller which is provided for the first and second delivery ports as a common controller. The second pump device includes a second pump controller which is provided for the third and fourth delivery ports as a common controller. The first pump controller includes a first load sensing control unit which controls displacement of the first hydraulic pump device so that delivery pressures of the first and second delivery ports of the first hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the first and second delivery ports by a prescribed pressure and a first torque control unit which performs limiting control of the displacement of the first hydraulic pump device so that absorption torque of the first hydraulic pump device does not exceed a prescribed value. The second pump controller includes a second load sensing control unit which controls displacement of the second hydraulic pump device so that delivery pressures of the third and fourth delivery ports of the second hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the third and fourth delivery ports by a prescribed pressure and a second torque control unit which performs limiting control of the displacement of the second hydraulic pump device so that absorption torque of the second hydraulic pump device does not exceed a prescribed value. The plurality of actuators include first and second actuators which are driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween. The first actuator is connected so that hydraulic fluids delivered from the first and second delivery ports of the first pump device are merged and supplied to the first actuator. The second actuator is connected so that hydraulic fluids delivered from the third and fourth delivery ports of the second pump device are merged and supplied to the second actuator.
In the above configuration, the hydraulic drive system comprises two pump devices each having two delivery ports. Each of the first and second pump devices is equipped with a pump controller. One of the first and second actuators driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween (first actuator) is connected so that hydraulic fluids delivered from the first and second delivery ports of the first pump device are merged and supplied to the actuator. The other actuator (second actuator) is connected so that hydraulic fluids delivered from the third and fourth delivery ports of the second pump device are merged and supplied to the actuator. With this configuration, in the simultaneous driving of the first and second actuators, the load sensing control by the first/second load sensing control unit and the constant absorption torque control by the first/second torque control unit can be performed on the first pump device's side and on the second pump device's side independently of each other. In combined operations in which the two actuators need a high flow rate and a low flow rate, respectively (e.g., leveling combined operation), each of the first and second pump devices delivers only the necessary flow rates, no surplus flow is caused, and energy loss can be reduced.
Further, when a combined operation driving a high load pressure actuator and a low load pressure actuator at the same time in the leveling combined operation is performed, the delivery pressure of the pump device on the low load pressure actuator's side can be controlled independently. Consequently, energy loss caused by the pressure loss at pressure compensating valves of the low load pressure actuator can be reduced.
Preferably, the plurality of actuators include third and fourth actuators which are driven at the same time in another operation of the construction machine while achieving a prescribed function by their supply flow rates becoming equivalent to each other. The third actuator is connected so that hydraulic fluid delivered from one of the first and second delivery ports of the first pump device and hydraulic fluid delivered from one of the third and fourth delivery ports of the second pump device are merged and supplied to the third actuator. The fourth actuator is connected so that hydraulic fluid delivered from the other of the first and second delivery ports of the first pump device and hydraulic fluid delivered from the other of the third and fourth delivery ports of the second pump device are merged and supplied to the fourth actuator.
In the above configuration, one of the third and fourth actuators driven at the same time while achieving a prescribed function by their supply flow rates capable of becoming equivalent to each other (third actuator) is connected so that hydraulic fluid delivered from one of the first and second delivery ports of the first pump device and hydraulic fluid delivered from one of the third and fourth delivery ports of the second pump device are merged and supplied to the actuator. The other actuator (fourth actuator) is connected so that hydraulic fluid delivered from the other of the first and second delivery ports of the first pump device and hydraulic fluid delivered from the other of the third and fourth delivery ports of the second pump device are merged and supplied to the actuator. With this configuration, even when the load pressure of one of the third and fourth actuators changed, the average delivery pressure of the first and second delivery ports and that of the third and fourth delivery ports are equal to each other. Thus, even when the constant absorption torque control by the first and second torque control units is in operation, the delivery flow rate of the first and second delivery ports and that of the third and fourth delivery ports become equal to each other and the third and fourth actuators can achieve the intended prescribed function.
Further, thanks to the above-described connection of the third and fourth actuators, even when a delivery flow rate difference occurred between the first and second delivery ports and the third and fourth delivery ports, the supply flow rate of the third actuator and that of the fourth actuator become equal to each other, by which the third and fourth actuators are allowed to achieve the intended prescribed function.
Furthermore, even in cases where the displacements of the first and second pump devices are designed to be different from each other, optimum design of the first and second pump devices becomes possible since the supply flow rates of the third and fourth actuators are kept equal to each other and the third and fourth actuators are allowed to achieve the intended prescribed function.
Preferably, the hydraulic drive system in accordance with the present invention further comprises: a first travel communication valve which is arranged between the first and second delivery ports of the first pump device, situated at an interrupting position for interrupting communication between the first and second delivery ports at the time other than combined operation in which the third and fourth actuators and at least one of other actuators related to the first pump device are driven at the same time, and switched to a communicating position for communicating the first and second delivery ports to each other at the time of the combined operation in which the third and fourth actuators and at least one of other actuators related to the first pump device are driven at the same time; and a second travel communication valve which is arranged between the third and fourth delivery ports of the second pump device, situated at an interrupting position for interrupting communication between the third and fourth delivery ports at the time other than combined operation in which the third and fourth actuators and at least one of other actuators related to the second pump device are driven at the same time, and switched to a communicating position for communicating the third and fourth delivery ports to each other at the time of the combined operation in which the third and fourth actuators and at least one of other actuators related to the second pump device are driven at the same time.
With this configuration, when the combined operation driving the third and fourth actuators and another actuator at the same time is performed, the supply flow rate of the third actuator and that of the fourth actuator are kept equal to each other, by which the third and fourth actuators are allowed to achieve the intended prescribed function.
Preferably, the construction machine is a hydraulic excavator having a front work implement, the first actuator is a boom cylinder for driving a boom of the front work implement, and the second actuator is an arm cylinder for driving an arm of the front work implement.
With this configuration, no surplus flow is caused and flow rate control with no energy loss becomes possible in combined operations in which the arm cylinder needs a high flow rate and the boom cylinder needs a low flow rate as in the leveling operation by use of the boom and the arm.
Preferably, the construction machine is a hydraulic excavator having a lower track structure equipped with left and right crawlers, the third actuator is a travel motor for driving one of the left and right crawlers, and the fourth actuator is a travel motor for driving the other of the left and right crawlers.
With this configuration, the vehicle is allowed to travel straight without meandering even when the load pressure of one of the left and right travel motors becomes high in the straight traveling operation for the reasons such that one of the left and right crawlers has run on an obstacle.
Further, the vehicle is allowed to travel straight without meandering even when a traveling combined operation is performed.
Preferably, each of the first and second pump devices is a hydraulic pump of the split flow type having a single displacement control mechanism.
A hydraulic pump of the split flow type, including only one pump controller and only one swash plate that is a displacement control element, is capable of implementing a pump function of two pumps with a compact structure. By configuring the first and second pump devices by using two hydraulic pumps of the split flow type, a pump function of four pumps can be implemented with a compact structure.
Preferably, the first pump torque control unit of the first pump device controls the displacement of the first hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the first and second delivery ports of the first hydraulic pump device related to itself but also the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device, and the second pump torque control unit of the second pump device controls the displacement of the second hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device related to itself but also the delivery pressures of the first and second delivery ports of the first hydraulic pump device.
With this configuration, the engine stall is prevented when an actuator related to the first pump device and an actuator related to the second pump device are driven at the same time. Further, the output torque of the prime mover can be fully utilized while preventing the stall of the prime mover in cases where only actuators related to the first pump device are driven and in cases where only actuators related to the second pump device are driven.
Effect of the Invention
According to the present invention, in a hydraulic drive system performing the load sensing control by using a pump device having two delivery ports whose delivery flow rates are controlled by a single pump controller, the surplus flow can be prevented and the energy loss can be reduced in combined operations in which two actuators are driven at the same time while producing a relatively large supply flow rate difference therebetween.
According to the present invention, in a combined operation in which two actuators are driven at the same time while achieving a prescribed function by their supply flow rates becoming equivalent to each other, even when the load pressure of one of the two actuators gets high, the supply flow rates to the two actuators become equal to each other and the intended prescribed function can be achieved.
According to the present invention, when a combined operation driving the third and fourth actuators and another actuator at the same time is performed, the supply flow rate of the third actuator and that of the fourth actuator become equal to each other and the third and fourth actuators are allowed to achieve the intended prescribed function.
According to the present invention, the surplus flow can be prevented and the energy loss can be reduced in combined operations in which the arm cylinder needs a high flow rate and the boom cylinder needs a low flow rate as in the leveling operation by use of the boom and the arm.
According to the present invention, the vehicle is allowed to travel straight without meandering even when the load pressure of one of the left and right travel motors becomes high in the straight traveling operation for the reasons such that one of the left and right crawlers has run on an obstacle).
According to the present invention, the vehicle is allowed to travel straight without meandering even when the traveling combined operation is performed.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a first embodiment of the present invention.
FIG. 2A is a torque control diagram of a first torque control unit of a first pump device.
FIG. 2B is a torque control diagram of a second torque control unit of a second pump device.
FIG. 3 is a schematic view showing the external appearance of the hydraulic excavator.
FIG. 4 is a schematic view summarizing the inventive concept of the first embodiment.
FIG. 5 is a schematic view showing a comparative example.
FIG. 6 is a schematic view showing circuitry in the first embodiment in contrast with the comparative example of FIG. 5.
FIG. 7 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a second embodiment of the present invention.
FIG. 8A is a torque control diagram of a first torque control unit of a first pump device in the second embodiment of the present invention.
FIG. 8B is a torque control diagram of a second torque control unit of a second pump device in the second embodiment of the present invention.
FIG. 9 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a third embodiment of the present invention.
FIG. 10 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a fourth embodiment of the present invention.



MODE FOR CARRYING OUT THE INVENTION
Referring now to the drawings, a description will be given in detail of preferred embodiments of the present invention.
First Embodiment
Configuration
FIG. 1 shows a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a first embodiment of the present invention.
Referring to FIG. 1, the hydraulic drive system according to the first embodiment comprises a first pump device 1a of the variable displacement type having two delivery ports of a first delivery port P1 and a second delivery port P2, a second pump device 1b of the variable displacement type having two delivery ports of a third delivery port P3 and fourth delivery port P4, a prime mover 2, a plurality of actuators 3a-3h, and a control valve 4. The prime mover 2 is connected to the first and second pump devices 1a and 1b to drive the first and second pump devices 1a and 1b. The actuators 3a-3h are driven by hydraulic fluid delivered from the first and second delivery ports P1 and P2 of the first pump device 1a and hydraulic fluid delivered from the third and fourth delivery ports P3 and P4 of the second pump device 1b. The control valve 4 is arranged between the first through fourth delivery ports P1-P4 of the first and second pump devices 1a and 1b and the actuators 3a-3h in order to control the flow of the hydraulic fluid supplied from the first through fourth delivery ports P1-P4 to the actuators 3a-3h. 
The displacement of the first pump device 1a and that of the second pump device 1b are equal to each other. However, the displacement of the first pump device 1a and that of the second pump device 1b may also be designed to differ from each other.
The first pump device 1a is equipped with a first pump controller 5a which is provided for the first and second delivery ports P1 and P2 as a common controller. Similarly, the second pump device 1b is equipped with a second pump controller 5b which is provided for the third and fourth delivery ports P3 and P4 as a common controller.
The first pump device 1a is a hydraulic pump of the split flow type having a single displacement control mechanism (swash plate). The first pump controller 5a controls the delivery flow rates of the first and second delivery ports P1 and P2 by driving the single displacement control mechanism and controlling the displacement of the first pump device 1a (tilting angle of the swash plate). Similarly, the second pump device 1b is a hydraulic pump of the split flow type having a single displacement control mechanism (swash plate). The second pump controller 5b controls the delivery flow rates of the third and fourth delivery ports P3 and P4 by driving the single displacement control mechanism and controlling the displacement of the second pump device 1b (tilting angle of the swash plate).
Each of the first and second pump devices 1a and 1b may also be formed by a combination of two variable displacement hydraulic pumps each having one delivery port. In this case, the first pump controller 5a may be used for driving the two displacement control mechanisms (swash plates) of the two hydraulic pumps of the first pump device 1a, and the second pump controller 5b may be used for driving the two displacement control mechanisms (swash plates) of the two hydraulic pumps of the second pump device 1b. 
The prime mover 2 is implemented by a diesel engine, for example. As is publicly known, a diesel engine is equipped with an electronic governor or the like which controls the fuel injection quantity. The revolution speed and the torque of the diesel engine are controlled through the control of the fuel injection quantity. The engine revolution speed is set by use of operation means such as an engine control dial. The prime mover 2 may also be implemented by an electric motor.
The control valve 4 includes flow control valves 6a-6m of the closed center type, pressure compensating valves 7a-7m, first and second shuttle valve sets 8a and 8b, and first through fourth unload valves 10a-10d. Each pressure compensating valve 7a-7m is connected upstream of each flow control valve 6a-6m to control the differential pressure across the meter-in throttling portion of the flow control valve 6a-6m. The first shuttle valve set 8a is connected to the load pressure ports of the flow control valves 6a-6f to detect the maximum load pressure of the actuators 3a-3e. The second shuttle valve set 8b is connected to the load pressure ports of the flow control valves 6g-6m to detect the maximum load pressure of the actuators 3d-3h. The first and second unload valves 10a and 10b are connected respectively to the delivery ports P1 and P2 of the first pump device 1a. When the delivery pressure of the delivery port P1, P2 exceeds a pressure as the sum of the maximum load pressure and a preset pressure (unload pressure) of a spring 9a, 9b, the unload valve 10a, 10b shifts to an open state, returns the hydraulic fluid delivered from the delivery port P1, P2 to a tank, and thereby limits the increase in the delivery pressure. The third and fourth unload valves 10c and 10d are connected respectively to the delivery ports P3 and P4 of the second pump device 1b. When the delivery pressure of the delivery port P3, P4 exceeds a pressure as the sum of the maximum load pressure and a preset pressure (unload pressure) of a spring 9c, 9d, the unload valve 10c, 10d shifts to an open state, returns the hydraulic fluid delivered from the delivery port P3, P4 to the tank, and thereby limits the increase in the delivery pressure. The preset pressures of the springs 9a-9d of the first through fourth unload valves 10a-10d have been set equal to or slightly higher than a target differential pressure of the load sensing control which will be explained later.
Although not shown in FIG. 1, the control valve 4 further includes first through fourth relief valves. The first and second relief valves are connected respectively to the delivery ports P1 and P2 of the first pump device 1a to function as safety valves. The third and fourth relief valves are connected respectively to the delivery ports P3 and P4 of the second pump device 1b to function as safety valves.
The first pump controller 5a includes a first load sensing control unit 12a and a first torque control unit 13a. The first load sensing control unit 12a controls the swash plate tilting angle (displacement) of the first pump device 1a so that the delivery pressures of the first and second delivery ports P1 and P2 of the first pump device 1a become higher by a prescribed pressure than the maximum load pressure of the actuators 3a-3e that are the actuators driven by the hydraulic fluid delivered from the first and second delivery ports P1 and P2. The first torque control unit 13a performs limiting control of the swash plate tilting angle (displacement) of the first pump device 1a so that the absorption torque of the first pump device 1a does not exceed a prescribed value.
The second pump controller 5b includes a second load sensing control unit 12b and a second torque control unit 13b. The second load sensing control unit 12b controls the swash plate tilting angle (displacement) of the second pump device 1b so that the delivery pressures of the third and fourth delivery ports P3 and P4 of the second pump device 1b become higher by a prescribed pressure than the maximum load pressure of the actuators 3d-3h that are the actuators driven by the hydraulic fluid delivered from the third and fourth delivery ports P3 and P4. The second torque control unit 13b performs the limiting control of the swash plate tilting angle (displacement) of the second pump device 1b so that the absorption torque of the second pump device 1b does not exceed a prescribed value.
The first load sensing control unit 12a includes a shuttle valve 15a, a load sensing control valve 16a, and a load sensing control piston 17a. The shuttle valve 15a detects the delivery pressure of one of the first and second delivery ports P1 and P2 that is on the high pressure side. The output pressure of the control valve 16a is led to the load sensing control piston 17a. The load sensing control piston 17a changes the swash plate tilting angle of the first pump device 1a according to the output pressure of the control valve 16a. 
The second load sensing control unit 12b includes a shuttle valve 15b, a load sensing control valve 16b, and a load sensing control piston 17b. The shuttle valve 15b detects the delivery pressure of one of the third and fourth delivery ports P3 and P4 that is on the high pressure side. The output pressure of the control valve 16b is led to the load sensing control piston 17b. The load sensing control piston 17b changes the swash plate tilting angle of the second pump device 1b according to the output pressure of the control valve 16b. 
The control valve 16a of the first load sensing control unit 12a includes a spring 16a1 for setting the target differential pressure of the load sensing control, a pressure receiving part 16a2 situated opposite to the spring 16a1, and a pressure receiving part 16a3 situated on the same side as the spring 16a1. The delivery pressure of one of the first and second delivery ports P1 and P2 on the high pressure side detected by the shuttle valve 15a is led to the pressure receiving part 16a2. The maximum load pressure of the actuators 3a-3e detected by the first shuttle valve set 8a is led to the pressure receiving part 16a3. When the delivery pressure of one of the first and second delivery ports P1 and P2 on the high pressure side which is led to the pressure receiving part 16a2 exceeds a pressure as the sum of the maximum load pressure of the actuators 3a-3e led to the pressure receiving part 16a3 and the target differential pressure (prescribed pressure) set by the spring 16a1, the control valve 16a moves leftward in FIG. 1 and increases its output pressure. When the delivery pressure of one of the first and second delivery ports P1 and P2 on the high pressure side led to the pressure receiving part 16a2 falls below the pressure as the sum of the maximum load pressure of the actuators 3a-3e led to the pressure receiving part 16a3 and the target differential pressure (prescribed pressure) set by the spring 16a1, the control valve 16a moves rightward in FIG. 1 and decreases its output pressure. With the increase in the output pressure of the control valve 16a, the load sensing control piston 17a decreases the swash plate tilting angle of the first pump device 1a and thereby decreases the delivery flow rates of the first and second delivery ports P1 and P2. With the decrease in the output pressure of the control valve 16a, the load sensing control piston 17a increases the swash plate tilting angle of the first pump device 1a and thereby increases the delivery flow rates of the first and second delivery ports P1 and P2. With the above configuration, the first load sensing control unit 12a controls the swash plate tilting angle (displacement) of the first pump device 1a so that the delivery pressures of the first and second delivery ports P1 and P2 of the first pump device 1a become higher by the prescribed pressure than the maximum load pressure of the actuators 3a-3e driven by the hydraulic fluid delivered from the first and second delivery ports P1 and P2. The target differential pressure of the load sensing control that is set by the spring 16a1 is approximately 2 MPa, for example.
The control valve 16b of the second load sensing control unit 12b includes a spring 16b1 for setting the target differential pressure of the load sensing control, a pressure receiving part 16b2 situated opposite to the spring 16b1, and a pressure receiving part 16b3 situated on the same side as the spring 16b1. The delivery pressure of one of the third and fourth delivery ports P3 and P4 on the high pressure side detected by the shuttle valve 15b is led to the pressure receiving part 16b2. The maximum load pressure of the actuators 3d-3h detected by the second shuttle valve set 8b is led to the pressure receiving part 16b3. The control valve 16b and the control piston 17b operate similarly to the control valve 16a and the control piston 17a of the first load sensing control unit 12a explained above. With the above configuration, the second load sensing control unit 12b controls the swash plate tilting angle (displacement) of the second pump device 1b so that the delivery pressures of the third and fourth delivery ports P3 and P4 of the second pump device 1b become higher by the prescribed pressure than the maximum load pressure of the actuators 3d-3h driven by the hydraulic fluid delivered from the third and fourth delivery ports P3 and P4.
The first torque control unit 13a includes a first torque control piston 18a to which the delivery pressure of the first delivery port P1 is led and a second torque control piston 19a to which the delivery pressure of the second delivery port P2 is led. When the average delivery pressure (P1p+P2p)/2 of the first and second delivery ports P1 and P2 of the first pump device 1a exceeds a prescribed pressure Pa, the first torque control unit 13a executes control so as to decrease the swash plate tilting angle of the first pump device 1a with the increase in the average delivery pressure.
The second torque control unit 13b includes a third torque control piston 18b to which the delivery pressure of the third delivery port P3 is led and a fourth torque control piston 19b to which the delivery pressure of the fourth delivery port P4 is led. When the average delivery pressure (P3p+P4p)/2 of the third and fourth delivery ports P3 and P4 of the second pump device 1b exceeds the prescribed pressure Pa, the second torque control unit 13b executes control so as to decrease the swash plate tilting angle of the second pump device 1b with the increase in the average delivery pressure.
FIG. 2A is a torque control diagram of the first torque control unit 13a. FIG. 2B is a torque control diagram of the second torque control unit 13b. In each torque control diagram, the vertical axis represents the tilting angle (displacement) q. If the vertical axis is replaced with the delivery flow rate, these diagrams become power control diagrams.
Referring to FIG. 2A, the first torque control unit 13a does not operate when the average delivery pressure of the first and second delivery ports P1 and P2 is Pa or less. In this case, the swash plate tilting angle (displacement) of the first pump device 1a is controlled by the first load sensing control unit 12a with no limitation by the first torque control unit 13a and can increase up to the maximum tilting angle qmax of the first pump device 1a according to the operation amount of the control lever device (demanded flow rate).
When the average delivery pressure of the first and second delivery ports P1 and P2 exceeds Pa, the first torque control unit 13a operates. With the increase in the average delivery pressure, the first torque control unit 13a performs the limiting control of the maximum tilting angle (maximum displacement) of the first pump device 1a so as to decrease the maximum tilting angle (maximum displacement) along the characteristic lines TP1 and TP2. In this case, due to the limiting control by the first torque control unit 13a, the first load sensing control unit 12a cannot increase the tilting angle of the first pump device 1a over a tilting angle specified by the characteristic lines TP1 and TP2.
The characteristic lines TP1 and TP2 have been set by two springs S1 and S2 (represented by one spring in FIG. 1 for simplicity of illustration) to approximate a constant absorption torque curve (hyperbolic curve). The setup torque of the characteristic lines TP1 and TP2 is substantially constant. Accordingly, the first torque control unit 13a executes constant absorption torque control (or constant power control) by decreasing the maximum tilting angle of the first pump device 1a along the characteristic lines TP1 and TP2 with the increase in the average delivery pressure.
The second torque control unit 13b also operates in the same way as the first torque control unit 13a. As shown in FIG. 2B, the second torque control unit 13b operates when the average delivery pressure of the third and fourth delivery ports P3 and P4 exceeds Pa. With the increase in the average delivery pressure, the second torque control unit 13b executes the limiting control so as to decrease the maximum tilting angle of the second pump device 1b along the characteristic lines TP3 and TP4 of the two springs S3 and S4 (represented by one spring in FIG. 1 for simplicity of illustration). By decreasing the maximum tilting angle as above, the second torque control unit 13b carries out the constant absorption torque control (or the constant power control).
Incidentally, the setup torque of the characteristic lines TP1 and TP2 and the setup torque of the characteristic lines TP3 and TP4 have been set to be lower than 1⁄2 of the output torque TEL of the engine 2. The first torque control unit 13a performs the limiting control of the swash plate tilting angle (displacement) of the first pump device 1a so that the absorption torque of the first pump device 1a does not exceed a prescribed value (1⁄2 of TEL). The second torque control unit 13b performs the limiting control of the swash plate tilting angle (displacement) of the second pump device 1b so that the absorption torque of the second pump device 1b does not exceed the prescribed value (1⁄2 of TEL). Accordingly, even when an actuator related to the first pump device 1a and an actuator related to the second pump device 1b are driven at the same time, the total absorption torque of the first pump device 1a and the second pump device 1b remains within the output torque TEL of the engine 2, by which the engine stall is prevented.
Returning to FIG. 1, each pressure compensating valve 7a-7m is configured to set the differential pressure between the pump delivery pressure and the maximum load pressure as a target compensation differential pressure. Specifically, the delivery pressure of the first delivery port P1 is led to the opening-direction actuation side of the pressure compensating valves 7a-7c, while the maximum load pressure of the actuators 3a-3e detected by the first shuttle valve set 8a is led to the closing-direction actuation side of the pressure compensating valves 7a-7c. Each pressure compensating valve 7a-7c performs control so that the differential pressure across the meter-in throttling portion of the corresponding flow control valve 6a-6c becomes equal to the differential pressure between the delivery pressure and the maximum load pressure. The delivery pressure of the second delivery port P2 is led to the opening-direction actuation side of the pressure compensating valves 7d-7f, while the maximum load pressure of the actuators 3a-3e detected by the first shuttle valve set 8a is led to the closing-direction actuation side of the pressure compensating valves 7d-7f. Each pressure compensating valve 7d-7f performs control so that the differential pressure across the meter-in throttling portion of the corresponding flow control valve 6d-6f becomes equal to the differential pressure between the delivery pressure and the maximum load pressure. The delivery pressure of the third delivery port P3 is led to the opening-direction actuation side of the pressure compensating valves 7g-7i, while the maximum load pressure of the actuators 3d-3h detected by the second shuttle valve set 8b is led to the closing-direction actuation side of the pressure compensating valves 7g-7i. Each pressure compensating valve 7g-7i performs control so that the differential pressure across the meter-in throttling portion of the corresponding flow control valve 6g-6i becomes equal to the differential pressure between the delivery pressure and the maximum load pressure. The delivery pressure of the fourth delivery port P4 is led to the opening-direction actuation side of the pressure compensating valves 7j-7m, while the maximum load pressure of the actuators 3d-3h detected by the second shuttle valve set 8b is led to the closing-direction actuation side of the pressure compensating valves 7j-7m. Each pressure compensating valve 7j-7m performs control so that the differential pressure across the meter-in throttling portion of the corresponding flow control valve 6j-6m becomes equal to the differential pressure between the delivery pressure and the maximum load pressure. Accordingly, in each of the first and second pump devices 1a and 1b, in the combined operation in which two or more actuators are driven at the same time, appropriate flow rate distribution according to the opening area ratio among the flow control valves becomes possible irrespective of the magnitude of the load pressure of each actuator. Further, even in the saturation state in which the delivery flow rate of the first through fourth delivery ports P1-P4 is insufficient, it is possible to secure excellent operability by decreasing the differential pressure across the meter-in throttling portion of each flow control valve according to the degree of the saturation.
The actuators 3a-3h are a boom cylinder, a swing cylinder, a bucket cylinder, left and right travel motors, a swing motor, a blade cylinder and an arm cylinder of the hydraulic excavator, respectively.
The boom cylinder 3a (first actuator) is connected to the first and second delivery ports P1 and P2 of the first pump device 1a via the flow control valves 6a and 6e and the pressure compensating valves 7a and 7e so that the hydraulic fluid delivered from the first delivery port P1 and the hydraulic fluid delivered from the second delivery port P2 are supplied to the boom cylinder 3a after merging together. The arm cylinder 3h (second actuator) is connected to the third and fourth delivery ports P3 and P4 of the second pump device 1b via the flow control valves 6h and 6l and the pressure compensating valves 7h and 7l so that the hydraulic fluid delivered from the third delivery port P3 and the hydraulic fluid delivered from the fourth delivery port P4 are supplied to the arm cylinder 3h after merging together.
The left travel motor 3d (third actuator) is connected to the second delivery port P2 (one of the first and second delivery ports P1 and P2 of the first pump device 1a) and the fourth delivery port P4 (one of the third and fourth delivery ports P3 and P4 of the second pump device 1b) via the flow control valves 6f and 6j and the pressure compensating valves 7f and 7j so that the hydraulic fluid delivered from the second delivery port P2 and the hydraulic fluid delivered from the fourth delivery port P4 are supplied to the left travel motor 3d after merging together. The right travel motor 3e (fourth actuator) is connected to the first delivery port P1 (the other of the first and second delivery ports P1 and P2 of the first pump device 1a) and the third delivery port P3 (the other of the third and fourth delivery ports P3 and P4 of the second pump device 1b) via the flow control valves 6c and 6g and the pressure compensating valves 7c and 7g so that the hydraulic fluid delivered from the first delivery port P1 and the hydraulic fluid delivered from the third delivery port P3 are merged and supplied to the right travel motor 3e. 
The swing cylinder 3b is connected to the first delivery port P1 of the first pump device 1a via the flow control valve 6b and the pressure compensating valve 7b so that the hydraulic fluid delivered from the first delivery port P1 is supplied to the swing cylinder 3b. The bucket cylinder 3c is connected to the second delivery port P2 of the first pump device 1a via the flow control valve 6d and the pressure compensating valve 7d so that the hydraulic fluid delivered from the second delivery port P2 is supplied to the bucket cylinder 3c. 
The swing motor 3f (second actuator) is connected to the third delivery port P3 of the second pump device 1b via the flow control valve 6i and the pressure compensating valve 7i so that the hydraulic fluid delivered from the third delivery port P3 is supplied to the swing motor 3f. The blade cylinder 3g is connected to the fourth delivery port P4 of the second pump device 1b via the flow control valve 6k and the pressure compensating valve 7k so that the hydraulic fluid delivered from the fourth delivery port P4 is supplied to the blade cylinder 3g. 
The flow control valve 6m and the pressure compensating valve 7m are used as spares (accessory). For example, when a bucket 308 that has been attached to the hydraulic excavator is replaced with a crusher, an open/close cylinder of the crusher is connected to the fourth delivery port P4 via the flow control valve 6m and the pressure compensating valve 7m. 
FIG. 3 shows the external appearance of the hydraulic excavator.
Referring to FIG. 3, the hydraulic excavator comprises an upper swing structure 300, a lower track structure 301, and a front work implement 302. The upper swing structure 300 is mounted on the lower track structure 301 to be rotatable. The front work implement 302 is connected to the front end part of the upper swing structure 300 via a swing post 303 to be rotatable vertically and horizontally. The lower track structure 301 is equipped with left and right crawlers 310 and 311, as well as a vertically movable earth-removing blade 305 attached to the front of a track frame 304. The upper swing structure 300 includes a cabin (operating room) 300a. Operating means such as control lever devices 309a and 309b for the front work implement and the swinging (only one is illustrated in FIG. 3) and control lever/pedal devices 309c and 309d for the traveling (only one is illustrated in FIG. 3) are arranged in the cabin 300a. The front work implement 302 is formed by connecting a boom 306, an arm 307 and a bucket 308 by using pins.
The upper swing structure 300 is driven and rotated with respect to the lower track structure 301 by the swing motor 3f. The front work implement 302 is rotated horizontally by rotating the swing post 303 with the swing cylinder 3b (see FIG. 1). The left and right crawlers 310 and 311 of the lower track structure 301 are driven and rotated by the left and right travel motors 3d and 3e. The blade 305 is driven vertically by the blade cylinder 3g. The boom 306, the arm 307 and the bucket 308 are vertically rotated by the expansion/contraction of the boom cylinder 3a, the arm cylinder 3h and the bucket cylinder 3c, respectively.
Operation
Next, the operation of this embodiment will be described below.
<Single Driving>
<<Single Driving of Actuator on First Pump Device 1a's Side>>
When one of the actuators connected to the first pump device 1a's side, e.g., boom cylinder 3a, is driven solely to perform the boom operation, the flow control valves 6a and 6e are switched over according to the operator's operation on the boom control lever and the hydraulic fluid delivered from the first delivery port P1 and the hydraulic fluid delivered from the second delivery port P2 are merged and supplied to the boom cylinder 3a. In this case, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control by the first torque control unit 13a as explained above.
When the swing cylinder 3b or the bucket cylinder 3c is driven solely to perform the swing operation or the bucket operation, the flow control valve 6b or the flow control valve 6d is switched over according to the operator's operation on the swing control lever or the bucket control lever and the hydraulic fluid delivered from one of the first and second delivery ports P1 and P2 is supplied to the swing cylinder 3b or the bucket cylinder 3c. Also in this case, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control by the first torque control unit 13a. The hydraulic fluid delivered from the delivery port P2 or P1 on the side not supplying the hydraulic fluid to the swing cylinder 3b or the bucket cylinder 3c is returned to the tank via the unload valve 10b or 10a. 
<Single Driving of Actuator on Second Pump Device 1b's Side>
When one of the actuators connected to the second pump device 1b's side, e.g., arm cylinder 3h, is driven to perform the arm operation, the flow control valves 6h and 6l are switched over according to the operator's operation on the arm control lever and the hydraulic fluid delivered from the third delivery port P3 and the hydraulic fluid delivered from the fourth delivery port P4 are merged and supplied to the arm cylinder 3h. In this case, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control by the second torque control unit 13b as explained above.
When the swing motor 3f or the blade cylinder 3g is driven solely to perform the swinging or the blade operation, the flow control valve 6i or the flow control valve 6k is switched over according to the operator's operation on the swing control lever or the blade control lever and the hydraulic fluid delivered from one of the third and fourth delivery ports P3 and P4 is supplied to the swing motor 3f or the blade cylinder 3g. Also in this case, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control by the second torque control unit 13b. The hydraulic fluid delivered from the delivery port P4 or P3 on the side not supplying the hydraulic fluid to the swing motor 3f or the blade cylinder 3g is returned to the tank via the unload valve 10d or 10c. 
<Simultaneous Driving of Actuator on First Pump Device 1a's Side and Actuator on Second Pump Device 1b's Side>
<<Simultaneous Driving of Boom Cylinder and Arm Cylinder>>
When the boom cylinder 3a and the arm cylinder 3h are driven at the same time to perform the combined operation of the boom 306 and the arm 307, the flow control valves 6a and 6e and the flow control valves 6h and 6l are switched over according to the operator's operation on the boom control lever and the arm control lever. In this case, the hydraulic fluid delivered from the first delivery port P1 and the hydraulic fluid delivered from the second delivery port P2 are merged and supplied to the boom cylinder 3a, while the hydraulic fluid delivered from the third delivery port P3 and the hydraulic fluid delivered from the fourth delivery port P4 are merged and supplied to the arm cylinder 3h. On the first pump device 1a's side, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control by the first torque control unit 13a as explained above. On the second pump device 1b's side, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control by the second torque control unit 13b as explained above.
<Simultaneous Driving of Boom Cylinder and Swing Motor>
When the boom cylinder 3a and the swing motor 3f are driven at the same time to perform the combined operation of the boom 306 and the upper swing structure 300 (swinging), the flow control valves 6a and 6e and the flow control valve 6l are switched over according to the operator's operation on the boom control lever and the swing control lever. In this case, the hydraulic fluid delivered from the first delivery port P1 and the hydraulic fluid delivered from the second delivery port P2 are merged and supplied to the boom cylinder 3a, while the hydraulic fluid delivered from the third delivery port P3 is supplied to the swing motor 3f. On the first pump device 1a's side, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control by the first torque control unit 13a as explained above. On the second pump device 1b's side, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control by the second torque control unit 13b as explained above. The hydraulic fluid delivered from the fourth delivery port P4 on the side where the flow control valves 6i-6m are closed is returned to the tank via the unload valve 10d. 
<<Simultaneous Driving of Other Combinations of Actuator on First Pump Device 1a's Side and Actuator on Second Pump Device 1b's Side>>
Also in other combined operations in which at least one of the actuators connected only to the first and second delivery ports P1 and P2 of the first pump device 1a (boom cylinder 3a, swing cylinder 3b, bucket cylinder 3c) and at least one of the actuators connected only to the third and fourth delivery ports P3 and P4 of the second pump device 1b (swing motor 3f, blade cylinder 3g, arm cylinder 3h) are driven at the same time, the delivery flow rates of the first and second delivery ports P1 and P2 and the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control and the constant absorption torque control and the hydraulic fluid delivered from the delivery port on the side where the flow control valves are closed is returned to the tank via the corresponding unload valve similarly to the above example.
<Simultaneous Driving of Two Actuators on First Pump Device 1a's Side>
In a combined operation in which at least one of the actuators connected to the first delivery port P1 of the first pump device 1a (boom cylinder 3a, swing cylinder 3b, right travel motor 3e) and at least one of the actuators connected to the second delivery port P2 of the first pump device 1a (boom cylinder 3a, bucket cylinder 3c, left travel motor 3d) are driven at the same time, the delivery flow rates of the first and second delivery ports P1 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control (or the constant power control) by the first torque control unit 13a similarly to the case of the boom operation in which only the boom cylinder 3a is driven. In this case, when there is a difference in the demanded flow rate, the surplus hydraulic fluid flow from the delivery port on the low demanded flow rate side is returned to the tank via the unload valve.
Also in combined operations of actuators connected to the first delivery port P1 of the first pump device 1a (boom cylinder 3a, swing cylinder 3b, right travel motor 3e) and combined operations of actuators connected to the second delivery port P2 of the first pump device 1a (boom cylinder 3a, bucket cylinder 3c, left travel motor 3d), the delivery flow rates of the first and second delivery ports P1 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control (or the constant power control) by the first torque control unit 13a similarly to the case of the boom operation in which only the boom cylinder 3a is driven. In this case, the hydraulic fluid delivered from the delivery port on the side where the flow control valves are closed is returned to the tank via the corresponding unload valve.
<Simultaneous Driving of Two Actuators on Second Pump Device 1b's Side>
Also in combined operations in which two actuators on the second pump device 1b's side are driven at the same time, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control (or the constant power control) by the second torque control unit 13b similarly to the aforementioned case of the combined operation in which two actuators on the first pump device 1a's side are driven at the same time. The surplus hydraulic fluid flow from the delivery port on the low demanded flow rate side or the hydraulic fluid delivered from the delivery port on the side where the flow control valves are closed is returned to the tank via the unload valve.
<Traveling Operation>
When the left travel motor 3d and the right travel motor 3e is driven to perform the traveling operation, the flow control valves 6f and 6j and the flow control valves 6c and 6g are switched over according to the operator's operation on the left and right travel control levers/pedals. In this case, the hydraulic fluid delivered from the second delivery port P2 of the first pump device 1a and the hydraulic fluid delivered from the fourth delivery port P4 of the second pump device 1b are merged and supplied to the left travel motor 3d, while the hydraulic fluid delivered from the first delivery port P1 of the first pump device 1a and the hydraulic fluid delivered from the third delivery port P3 of the second pump device 1b are merged and supplied to the right travel motor 3e. On the first pump device 1a's side, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the load sensing control by the first load sensing control unit 12a and the constant absorption torque control by the first torque control unit 13a as explained above. On the second pump device 1b's side, the delivery flow rates of the third and fourth delivery ports P3 and P4 are controlled by the load sensing control by the second load sensing control unit 12b and the constant absorption torque control by the second torque control unit 13b as explained above.
<<Straight Traveling Operation>>
When straight traveling is performed in the traveling operation, the operator operates the left and right travel control levers/pedals by the same amount. Accordingly, the flow control valves 6f and 6j and the flow control valves 6c and 6g are switched over so that the stroke amount (opening area) of the flow control valve 6f/6j equals the stroke amount (opening area) of the flow control valve 6c/6g, by which the demanded flow rate of the flow control valves 6f and 6j and that of the flow control valves 6c and 6g become equal to each other. In this case, the hydraulic fluid delivered from the second delivery port P2 of the first pump device 1a and the hydraulic fluid delivered from the fourth delivery port P4 of the second pump device 1b are merged and supplied to the left travel motor 3d, while the hydraulic fluid delivered from the first delivery port P1 of the first pump device 1a and the hydraulic fluid delivered from the third delivery port P3 of the second pump device 1b are merged and supplied to the right travel motor 3e. Therefore, even when the load pressure of one of the left and right travel motors becomes high for the reasons such that one of the left and right crawlers 310 and 311 has run on an obstacle, the supply flow rate of the left travel motor 3d and that of the right travel motor 3e become equal to each other and the vehicle is allowed to travel straight without meandering (details will be explained later).
FIG. 4 is a schematic view summarizing the inventive concept of this embodiment which has been described above. As shown in FIG. 4, in this embodiment, for the combined operation of the boom and the arm, each of the first and second pump devices 1a and 1b performs independent load sensing control and constant absorption torque control (power control). For the traveling operation, the first and second pump devices 1a and 1b perform linking constant absorption torque control (power control).
Effect
Next, effects achieved by this embodiment will be explained below.
1. Combined Operation of Boom and Arm
Combined operation for the leveling is an example of the combined operation of the boom 306 and the arm 307. In the leveling combined operation, the arm cylinder 3h is controlled at a high flow rate, while the boom cylinder 3a is controlled at a low flow rate. In other words, in the leveling combined operation, the boom 306 and the arm 307 operate as the first and second actuators that are driven at the same time while producing a relatively large supply flow rate difference therebetween.
In hydraulic drive systems equipped with a conventional load sensing system employing one split flow type hydraulic pump having two delivery ports and separately connecting the boom cylinder and the arm cylinder to the two delivery ports, when the leveling operation is performed, a the demanded flow rate on the high flow rate actuator's side (arm cylinder's side) is given high priority in the load sensing control and the swash plate tilting angle of the pump device is controlled to increase the displacement. In this case, since the same swash plate is used for the two delivery ports in the split flow type hydraulic pump, the delivery port on the low flow rate actuator's side (boom cylinder's side) also delivers a high flow rate and that causes a surplus flow. The surplus flow is drained to the tank by the unload valve as part of the energy consumption by the pump device, causing energy loss.
In hydraulic drive systems equipped with a conventional load sensing system that merges the delivery flows of two delivery ports of a split flow type hydraulic pump and drives the boom cylinder and the arm cylinder by use of the merged delivery flow, the delivery flow rates of the hydraulic pump are controlled without causing the surplus flow when the leveling operation is performed. However, in the leveling combined operation which is performed by using the boom and the arm, the boom cylinder operates as the high load pressure side and the arm cylinder operates as the low load pressure side, and the delivery pressures of the hydraulic pump are controlled to be higher than the high load pressure of the boom cylinder by a certain preset pressure. In this case, the pressure compensating valve provided for driving the arm cylinder and preventing excessive flow to the low load pressure arm cylinder is throttled. Thus, energy loss is caused by the pressure loss at the pressure compensating valve.
In contrast to such conventional systems, the system of this embodiment employs two split flow type hydraulic pumps each having two delivery ports. The boom cylinder 3a is connected so that hydraulic fluids delivered from the two delivery ports (first and second delivery ports P1 and P2) of one (first pump device 1a) of the two hydraulic pumps (pump devices 1a and 1b) are merged and supplied to the boom cylinder 3a. The arm cylinder 3h is connected so that hydraulic fluids delivered from the two delivery ports (third and fourth delivery ports P3 and P4) of the other hydraulic pump (second pump device 1b) are merged and supplied to the arm cylinder 3h. With this configuration, in the simultaneous driving of the boom cylinder 3a and the arm cylinder 3h, the load sensing control and the constant absorption torque control are performed on the first pump device 1a's side and on the second pump device 1b's side independently of each other. Consequently, in combined operations in which the two actuators need a high flow rate and a low flow rate, respectively, as in the leveling combined operation, each of the first and second pump devices 1a and 1b delivers only the necessary flow rates, no surplus flow is caused, and flow rate control with no energy loss becomes possible. Further, since the delivery pressures of the second pump device 1b on the arm cylinder 3h's side (low load pressure side) are controlled to be higher than the load pressure of the arm cylinder 3h by a certain preset pressure, energy loss caused by the pressure loss at the pressure compensating valves 7h and 7l of the arm cylinder 3h can also be reduced.
2. Straight Traveling Operation
By employing two split flow type hydraulic pumps each having two delivery ports and connecting the boom cylinder 3a and the arm cylinder 3h respectively to the two hydraulic pumps (pump devices 1a and 1b) so that the hydraulic fluids delivered from the two delivery ports are merged and supplied to each actuator of the boom cylinder 3a and arm cylinder 3h, even in combined operations in which a flow rate difference occurs between the two actuators as in the leveling operation, no surplus flow is caused and flow rate control with no energy loss becomes possible as explained above. However, it is necessary to add an idea to the connection of the actuators to the two hydraulic pumps in cases where such a hydraulic system employing two split flow type hydraulic pumps is used for driving two actuators such as the left and right travel motors that achieve a prescribed function (e.g., straight traveling function) by their supply flow rates becoming equivalent to each other.
FIG. 5 is a schematic view showing a comparative example. In this comparative example employing two split flow type hydraulic pumps, the left travel motor 3d is connected to the first and second delivery ports P1 and P2 of the first pump device 1a, while the right travel motor 3e is connected to the third and fourth delivery ports P3 and P4 of the second pump device 1b. The first pump controller 5a and the second pump controller 5b are configured in the same way as in the system of this embodiment. Power control diagrams of the first and second pump devices 1a and 1b are shown at the bottom.
In the configuration shown in FIG. 5, when the load pressure of one of the left and right travel motors becomes high for the reasons such that one of the left and right crawlers has run on an obstacle, the delivery flow rates of the first and second delivery ports P1 and P2 are controlled by the constant absorption torque control of the first and second torque control units 13a and 13b as shown in the power control diagrams below the first and second pump controllers 5a and 5b in FIG. 5. Specifically, when the load pressure of the left travel motor 3d is low and the load pressure of the right travel motor 3e is high, on the first pump device 1a's side, the first torque control unit 13a does not operate, the swash plate tilting angle does not undergo the limitation by the constant absorption torque control, and the delivery flow rates of the first and second delivery ports P1 and P2 do not decrease. On the second pump device 1b's side, the swash plate tilting angle is decreased by the constant absorption torque control by the second torque control unit 13b and the delivery flow rates of the third and fourth delivery ports P3 and P4 decrease. Consequently, assuming that the delivery flow rates of the first through fourth delivery ports P1-P4 are Q1-Q4, the delivery flow Q1+Q2 supplied to the left travel motor 3d and the delivery flow Q3+Q4 supplied to the right travel motor 3e satisfy the relationship Q1+Q2>Q3+Q4. In this case, the supply flow to the right travel motor 3e drops in spite of the straight traveling operation, causing the meandering of the vehicle.
FIG. 6 is a schematic view showing the circuitry in this embodiment in contrast with the comparative example of FIG. 5. Power control diagrams of the first and second pump devices are shown below the pump devices.
In this embodiment, the travel motors 3d and 3e are connected to the first through fourth delivery ports P1-P4 so that the hydraulic fluid delivered from the second delivery port P2 of the first pump device 1a and the hydraulic fluid delivered from the fourth delivery port P4 of the second pump device 1b are merged and supplied to the left travel motor 3d and the hydraulic fluid delivered from the first delivery port P1 of the first pump device 1a and the hydraulic fluid delivered from the third delivery port P3 of the second pump device 1b are merged and supplied to the right travel motor 3e. Therefore, the average delivery pressure of the first and second delivery ports P1 and P2 and that of the third and fourth delivery ports P3 and P4 are equal to each other. Specifically, assuming that the delivery pressures of the first through fourth delivery ports P1-P4 are P1p-P4p, the average delivery pressure of the first and second delivery ports P1 and P2 can be expressed as (P1p+P2p)/2 and that of the third and fourth delivery ports P3 and P4 can be expressed as (P3p+P4p)/2. Since the conditions P1p=P3p and P2p=P4p hold, the following relationship is satisfied:

(P1p+P2p)/2=(P3p+P4p)/2

Therefore, even when the load pressure of one of the left and right travel motors becomes high for the reasons such that one of the left and right crawlers has run on an obstacle, the load pressure is controlled by both the first torque control unit 13a of the first pump controller 5a and the second torque control unit 13b of the second pump controller 5b and the relationship (P1p+P2p)/2=(P3p+P4p)/2 is maintained. Consequently, even if the swash plate tilting angles of the first and second pump devices 1a and 1b are decreased by the constant absorption torque control by the first and second torque control units 13a and 13b and the delivery flow rates of the first and second delivery ports P1 and P2 and those of the third and fourth delivery ports P3 and P4 decreased, the tilting angles (delivery flow rates) of the first and second pump devices 1a and 1b are kept equal to each other as shown in FIG. 6, by which the vehicle is allowed to travel straight without meandering.
Further, since the travel motors 3d and 3e in this embodiment are connected to the first through fourth delivery ports P1-P4 so that the hydraulic fluid delivered from the second delivery port P2 of the first pump device 1a and the hydraulic fluid delivered from the fourth delivery port P4 of the second pump device 1b are merged and supplied to the left travel motor 3d and the hydraulic fluid delivered from the first delivery port P1 of the first pump device 1a and the hydraulic fluid delivered from the third delivery port P3 of the second pump device 1b are merged and supplied to the right travel motor 3e, the supply flow rate of the left travel motor 3d and that of the right travel motor 3e remain equal to each other even supposing the swash plate tilting angles of the first and second pump devices 1a and 1b has become different from each other and a delivery flow rate difference has occurred between the first and second delivery ports P1 and P2 and the third and fourth delivery ports P3 and P4. Consequently, the vehicle is allowed to travel straight without meandering.
Specifically, assuming that the delivery flow rates of the first through fourth delivery ports P1-P4 are Q1-Q4 similarly to the case of FIG. 5, the supply flow rate to the left travel motor 3d and that to the right travel motor 3e are expressed as follows:

left travel supply flow rate: Q2+Q4

right travel supply flow rate: Q1+Q3

where relationships Q1=Q2 (due to the use of the same swash plate) and Q3=Q4 (due to the use of the same swash plate) hold. Thus, even supposing Q1=Q2≠Q3=Q4, the following relationship is satisfied and the supply flow rates of the left and right travel motors 3d and 3e become equal to each other:

Q2+Q4=Q1+Q3

As above, even when a delivery flow rate difference occurred between the first and second delivery ports P1 and P2 and the third and fourth delivery ports P3 and P4, the supply flow rates of the left and right travel motors 3d and 3e become equal to each other and the vehicle is allowed to travel straight without meandering.
Incidentally, such cases where a delivery flow rate difference occurs between the first and second delivery ports P1 and P2 and the third and fourth delivery ports P3 and P4 even when the average delivery pressure of the first and second delivery ports P1 and P2 and that of the third and fourth delivery ports P3 and P4 are equal to each other and the constant absorption torque control is ON include a case where a difference in the displacement occurs between the first and second pump devices 1a and 1b due to manufacturing errors or secular change, a case where a difference in the delivery flow rate occurs due to a difference in transient responsiveness, and so forth.
While the displacements of the first and second pump devices 1a and 1b are set equal to each other in this embodiment, the displacements of the pump devices 1a and 1b may also be intentionally designed to be different from each other. Even with such a design, the vehicle is allowed to travel straight since the aforementioned relationship Q2+Q4=Q1+Q3 is maintained. Optimum design of the first and second pump devices 1a and 1b becomes possible by setting the displacements of the first and second pump devices to be different from each other based on the maximum demanded flow rate on the first pump device 1a's side and that on the second pump device 1b's side.
Second Embodiment
FIG. 7 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a second embodiment of the present invention, wherein part of the circuit elements are unshown for the simplicity of illustration. In this embodiment, total power control is performed by feeding back the delivery pressures of all the ports to the first and second pump torque control units of the first and second pump devices.
Referring to FIG. 6, a first torque control unit 113a of a first pump controller 105a in this embodiment includes not only the first and second torque control pistons 18a and 19a to which the delivery pressures of the first and second delivery ports P1 and P2 of the first hydraulic pump device 1a related to itself are led, but also fifth and sixth torque control pistons 20a and 21a to which the delivery pressures of the third and fourth delivery ports P3 and P4 of the second hydraulic pump device 1b are led. When the average delivery pressure (P1p+P2p+P3p+P4p)/4 of the first and second delivery ports P1 and P2 of the first pump device 1a and the third and fourth delivery ports P3 and P4 of the second hydraulic pump device 1b exceeds a prescribed pressure P1, the first torque control unit 113a performs control so as to decrease the swash plate tilting angle of the first pump device 1a with the increase in the average delivery pressure. By this control, the swash plate tilting angle (displacement) of the first hydraulic pump device 1a is controlled so that the total absorption torque of the first and second hydraulic pump devices 1a and 1b does not exceed a prescribed value.
Similarly, a second torque control unit 113b of a second pump controller 105b includes not only the third and fourth torque control pistons 18b and 19b to which the delivery pressures of the third and fourth delivery ports P3, P4 of the second pump device 1b related to itself is led, but also seventh and eighth torque control pistons 20b and 21b to which the delivery pressures of the first and second delivery ports P1 and P2 of the first hydraulic pump device 1a are led. When the average delivery pressure (P1p+P2p+P3p+P4p)/4 of the first and second delivery ports P1 and P2 of the first pump device 1a and the third and fourth delivery ports P3 and P4 of the second hydraulic pump device 1b exceeds the prescribed pressure P1, the second torque control unit 113b performs control so as to decrease the swash plate tilting angle of the second pump device 1b with the increase in the average delivery pressure. By this control, the swash plate tilting angle (displacement) of the second hydraulic pump device 1b is controlled so that the total absorption torque of the first and second hydraulic pump devices 1a and 1b does not exceed a prescribed value.
FIG. 8A is a torque control diagram of the first torque control unit 113a. FIG. 8B is a torque control diagram of the second torque control unit 113b. In each torque control diagram, the vertical axis represents the tilting angle (displacement) q. If the vertical axis is replaced with the delivery flow rate, these diagrams become power control diagrams.
In FIG. 8A, the characteristic lines TP5 and TP6 have been set by two springs S5 and S6 (represented by one spring in FIG. 7 for simplicity of illustration) to approximate a constant absorption torque curve (hyperbolic curve). The setup torque of the characteristic lines TP5 and TP6 is substantially constant. Accordingly, the first torque control unit 113a executes the constant absorption torque control (or the constant power control) by decreasing the maximum tilting angle of the first pump device 1a along the characteristic lines TP5 and TP6 with the increase in the average delivery pressure (P1p+P2p+P3p+P4p)/4.
In FIG. 8B, the characteristic lines TP7 and TP8 have been set by two springs S7 and S8 (represented by one spring in FIG. 7 for simplicity of illustration) to approximate a constant absorption torque curve (hyperbolic curve). The setup torque of the characteristic lines TP7 and TP8 is substantially constant. Accordingly, the second torque control unit 113b executes the constant absorption torque control (or the constant power control) by decreasing the maximum tilting angle of the second pump device 1b along the characteristic lines TP7 and TP8 with the increase in the average delivery pressure (P1p+P2p+P3p+P4p)/4.
Incidentally, the setup torque of the characteristic lines TP5 and TP6 has been set to be higher than the setup torque of the characteristic lines TP1 and TP2 shown in FIG. 2A and lower than the output torque TEL of the engine 2. The setup torque of the characteristic lines TP7 and TP8 has been set to be higher than the setup torque of the characteristic lines TP3 and TP4 shown in FIG. 2B and lower than the output torque TEL of the engine 2. The first torque control unit 113a performs the limiting control of the swash plate tilting angle (displacement) of the first pump device 1a so that the absorption torque of the first pump device 1a does not exceed a prescribed value (TEL). The second torque control unit 113b performs the limiting control of the swash plate tilting angle (displacement) of the second pump device 1b so that the absorption torque of the second pump device 1b does not exceed the prescribed value (TEL). Accordingly, when an actuator related to the first pump device 1a and an actuator related to the second pump device 1b are driven at the same time, the total absorption torque of the first and second pump devices 1a and 1b remains within the output torque TEL of the engine 2, by which the engine stall is prevented. Further, the output torque TEL of the engine 2 can be fully utilized while preventing the engine stall in cases where only actuators related to the first pump device 1a are driven and in cases where only actuators related to the second pump device 1b are driven.
Third Embodiment
FIG. 9 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a third embodiment of the present invention, wherein part of the circuit elements are unshown for the simplicity of illustration.
In this embodiment, the first and second pump devices 1a and 1b are provided with separate diesel engines 2a and 2b as the prime mover connected to the first and second pump devices 1a and 1b for driving them.
Also by this embodiment, effects similar to those of the first embodiment can be achieved.
Further, when an actuator related to the first pump device 1a and an actuator related to the second pump device 1b are driven at the same time, the total absorption torque of the first and second pump devices 1a and 1b remains within the output torque TEL of each engine 2a, 2a, by which the engine stall is prevented. Further, in each of the first and second pump devices 1a and 1b, the output torque TEL of each engine 2a, 2a can be fully utilized while preventing the engine stall.
Fourth Embodiment
FIG. 10 is a schematic view showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with a third embodiment of the present invention. This embodiment allows the vehicle to travel straight without meandering even in combined operation of the travel motors and another actuator.
Referring to FIG. 10, the hydraulic drive system in this embodiment comprises a control valve 204, a first pump controller 205a, and a second pump controller 205b instead of the control valve 4, the first pump controller 5a, and the second pump controller 5b in the first embodiment shown in FIG. 1.
The control valve 204 includes first through fourth shuttle valve sets 208a-208d instead of the first and second shuttle valve sets 8a and 8b in the first embodiment shown in FIG. 1. The first shuttle valve set 208a is connected to the load pressure ports of the flow control valves 6a-6c to detect the maximum load pressure of the actuators 3a, 3b and 3e. The second shuttle valve set 208b is connected to the load pressure ports of the flow control valves 6d-6f to detect the maximum load pressure of the actuators 3a, 3c and 3d. The third shuttle valve set 208c is connected to the load pressure ports of the flow control valves 6g-6i to detect the maximum load pressure of the actuators 3e, 3f and 3h. The fourth shuttle valve set 208d is connected to the load pressure ports of the flow control valves 6j-6m to detect the maximum load pressure of the actuators 3d, 3g and 3h and a spare actuator when the spare actuator has been connected to the flow control valve 6m. 
The control valve 204 is not equipped with the shuttle valves 15a and 15b employed in the first embodiment shown in FIG. 1. Instead, the control valve 204 is equipped with a first travel communication valve 215a (communication valve) and a second travel communication valve 215b (communication valve). The first travel communication valve 215a is arranged between the delivery hydraulic lines of the first and second delivery ports P1 and P2 of the first pump device 1a and between the output hydraulic lines of the first and second shuttle valve sets 208a and 208b. The first travel communication valve 215a is set at an interrupting position (upper position in FIG. 10) at the time other than combined operation driving the travel motors 3d and 3e and at least one of other actuators related to the first pump device 1a (boom cylinder 3a, swing cylinder 3b, bucket cylinder 3c) at the same time (hereinafter referred to as “at the time other than the traveling combined operation”). The first travel communication valve 215a is switched to a communicating position (lower position in FIG. 10) at the time of the combined operation driving the travel motors 3d and 3e and at least one of the aforementioned other actuators at the same time (hereinafter referred to as “at the time of the traveling combined operation”). The second travel communication valve 215b is arranged between the delivery hydraulic lines of the third and fourth delivery ports P3 and P4 of the second pump device 1b and between the output hydraulic lines of the third and fourth shuttle valve sets 208c and 208d. The second travel communication valve 215b is set at an interrupting position (upper position in FIG. 10) at the time other than combined operation driving the travel motors 3d and 3e and at least one of other actuators related to the second pump device 1b (swing motor 3f, blade cylinder 3g, arm cylinder 3h) at the same time (hereinafter referred to as “at the time other than the traveling combined operation”). The second travel communication valve 215b is switched to a communicating position (lower position in FIG. 10) at the time of the combined operation driving the travel motors 3d and 3e and at least one of the aforementioned other actuators at the same time (hereinafter referred to as “at the time of the traveling combined operation”).
At the interrupting position (upper position in FIG. 10), the first travel communication valve 215a interrupts the communication between the delivery hydraulic lines of the first and second delivery ports P1 and P2 of the first pump device 1a. When switched to the communicating position (lower position in FIG. 10), the first travel communication valve 215a brings the delivery hydraulic lines of the first and second delivery ports P1 and P2 of the first pump device 1a to communicate to each other.
Similarly, the second travel communication valve 215b at the interrupting position (upper position in FIG. 10) interrupts the communication between the delivery hydraulic lines of the third and fourth delivery ports P3 and P4 of the second pump device 1b. When switched to the communicating position (lower position in FIG. 10), the second travel communication valve 215b brings the delivery hydraulic lines of the third and fourth delivery ports P3 and P4 of the second pump device 1b to communicate to each other.
The first travel communication valve 215a includes a shuttle valve. At the interrupting position (upper position in FIG. 10), the first travel communication valve 215a interrupts the communication between the output hydraulic lines of the first and second shuttle valve sets 208a and 208b while communicating each of the output hydraulic lines to the downstream side. When switched to the communicating position (lower position in FIG. 10), the first travel communication valve 215a brings the output hydraulic lines of the first and second shuttle valve sets 208a and 208b to communicate to each other via the shuttle valve while leading out the maximum load pressure on the high pressure side to the downstream side of each of the output hydraulic lines.
Similarly, the second travel communication valve 215b includes a shuttle valve. At the interrupting position (upper position in FIG. 10), the second travel communication valve 215b interrupts the communication between the output hydraulic lines of the third and fourth shuttle valve sets 208c and 208d while communicating each of the output hydraulic lines to the downstream side. When switched to the communicating position (lower position in FIG. 10), the second travel communication valve 215b brings the output hydraulic lines of the third and fourth shuttle valve sets 208c and 208d to communicate to each other via the shuttle valve while leading out the maximum load pressure on the high pressure side to the downstream side of each of the output hydraulic lines.
When the first travel communication valve 215a is at the interrupting position (upper position in FIG. 10), on the first delivery port P1's side of the first pump device 1a, the maximum load pressure of the actuators 3a, 3b and 3e detected by the first shuttle valve set 208a is led to the first unload valve 10a and the pressure compensating valves 7a-7c. Based on the maximum load pressure, the first unload valve 10a limits the increase in the delivery pressure of the first delivery port P1 and each pressure compensating valve 7a-7c controls the differential pressure across the meter-in throttling portion of each flow control valve 6a-6c. On the second delivery port P2's side of the first pump device 1a, the maximum load pressure of the actuators 3a, 3c and 3d detected by the second shuttle valve set 208b is led to the second unload valve 10b and the pressure compensating valves 7d-7f. Based on the maximum load pressure, the second unload valve 10b limits the increase in the delivery pressure of the second delivery port P2 and each pressure compensating valve 7d-7f controls the differential pressure across the meter-in throttling portion of each flow control valve 6d-6f. 
When the first travel communication valve 215a is switched to the communicating position (lower position in FIG. 10), on the first delivery port P1's side of the first pump device 1a, the maximum load pressure of the actuators 3a-3e detected by the first and second shuttle valve sets 208a and 208b is led to the first unload valve 10a and the pressure compensating valves 7a-7c. Based on the maximum load pressure, the first unload valve 10a limits the increase in the delivery pressure of the first delivery port P1 and each pressure compensating valve 7a-7c controls the differential pressure across the meter-in throttling portion of each flow control valve 6a-6c. On the second delivery port P2's side of the first pump device 1a, the maximum load pressure of the actuators 3a-3e detected by the first and second shuttle valve sets 208a and 208b is similarly led to the second unload valve 10b and the pressure compensating valves 7d-7f. Based on the maximum load pressure, the second unload valve 10b limits the increase in the delivery pressure of the second delivery port P2 and each pressure compensating valve 7d-7f controls the differential pressure across the meter-in throttling portion of each flow control valve 6d-6f. 
When the second travel communication valve 215b is at the interrupting position (upper position in FIG. 10), on the third delivery port P3's side of the second pump device 1b, the maximum load pressure of the actuators 3e, 3f and 3h detected by the third shuttle valve set 208c is led to the third unload valve 10c and the pressure compensating valves 7g-7i. Based on the maximum load pressure, the third unload valve 10c limits the increase in the delivery pressure of the third delivery port P3 and each pressure compensating valve 7g-7i controls the differential pressure across the meter-in throttling portion of each flow control valve 6g-6i. On the fourth delivery port P4's side of the second pump device 1b, the maximum load pressure of the actuators 3d, 3g and 3h detected by the fourth shuttle valve set 208d is led to the fourth unload valve 10d and the pressure compensating valves 7j-7m. Based on the maximum load pressure, the fourth unload valve 10d limits the increase in the delivery pressure of the fourth delivery port P4 and each pressure compensating valve 7j-7m controls the differential pressure across the meter-in throttling portion of each flow control valve 6j-6m. 
When the second travel communication valve 215b is switched to the communicating position (lower position in FIG. 10), on the third delivery port P3's side of the second pump device 1b, the maximum load pressure of the actuators 3d-3h detected by the third and fourth shuttle valve sets 208c and 208d is led to the third unload valve 10c and the pressure compensating valves 7g-7i. Based on the maximum load pressure, the third unload valve 10c limits the increase in the delivery pressure of the third delivery port P3 and each pressure compensating valve 7g-7i controls the differential pressure across the meter-in throttling portion of each flow control valve 6g-6i. On the fourth delivery port P4's side of the second pump device 1b, the maximum load pressure of the actuators 3d-3h detected by the third and fourth shuttle valve sets 208c and 208d is similarly led to the fourth unload valve 10d and the pressure compensating valves 7j-7m. Based on the maximum load pressure, the fourth unload valve 10d limits the increase in the delivery pressure of the fourth delivery port P4 and each pressure compensating valve 7j-7m controls the differential pressure across the meter-in throttling portion of each flow control valve 6j-6m. 
The first pump controller 205a includes a first load sensing control unit 212a. The first load sensing control unit 212a includes load sensing control valves 216a and 216b and a low pressure selection valve 221a instead of the load sensing control valve 16a. The low pressure selection valve 221a selects the output pressure of the load sensing control valve 216a or 216b on the low pressure side and outputs the selected output pressure.
The control valve 216a includes a spring 216a1 for setting the target differential pressure of the load sensing control, a pressure receiving part 216a2 situated opposite to the spring 216a1, and a pressure receiving part 216a3 situated on the same side as the spring 216a1. The delivery pressure of the first delivery port P1 is led to the pressure receiving part 216a2. When the first travel communication valve 215a is at the interrupting position (upper position in FIG. 10), the maximum load pressure of the actuators 3a, 3b and 3e detected by the first shuttle valve set 208a is led to the pressure receiving part 216a3 of the control valve 216a. When the first travel communication valve 215a is switched to the communicating position (lower position in FIG. 10), the maximum load pressure of the actuators 3a-3e detected by the first and second shuttle valve sets 208a and 208b is led to the pressure receiving part 216a3 of the control valve 216a. The control valve 216a slides according to the balance among the delivery pressure of the first delivery port P1 which is led to the pressure receiving part 216a2, the maximum load pressure of the actuators 3a, 3b and 3e or the actuators 3a-3e which is led to the pressure receiving part 216a3, and the biasing force of the spring 216a1 and thereby increases/decreases the output pressure. The operation of the control valve 216a in these cases is substantially the same as the operation of the control valve 16a in the first embodiment.
The control valve 216b includes a spring 216b1 for setting the target differential pressure of the load sensing control, a pressure receiving part 216b2 situated opposite to the spring 216b1, and a pressure receiving part 216b3 situated on the same side as the spring 216b1. The delivery pressure of the second delivery port P2 is led to the pressure receiving part 216b2. When the first travel communication valve 215a is at the interrupting position (upper position in FIG. 10), the maximum load pressure of the actuators 3a, 3c and 3d detected by the second shuttle valve set 208b is led to the pressure receiving part 216b3 of the control valve 216b. When the first travel communication valve 215a is switched to the communicating position (lower position in FIG. 10), the maximum load pressure of the actuators 3a-3e detected by the first and second shuttle valve sets 208a and 208b is led to the pressure receiving part 216b3 of the control valve 216b. The control valve 216b slides according to the balance among the delivery pressure of the second delivery port P2 which is led to the pressure receiving part 216b2, the maximum load pressure of the actuators 3a, 3c and 3d or the actuators 3a-3e which is led to the pressure receiving part 216b3, and the biasing force of the spring 216b1 and thereby increases/decreases the output pressure. The operation of the control valve 216b in these cases is substantially the same as the operation of the control valve 16a in the first embodiment.
The low pressure selection valve 221a selects the output pressure of the load sensing control valve 216a or 216b on the low pressure side and outputs the selected output pressure to the load sensing control piston 17a. According to the output pressure, the load sensing control piston 17a changes the swash plate tilting angle of the first pump device 1a and thereby increases/decreases the delivery flow rates of the first and second delivery ports P1 and P2. The operation of the load sensing control piston 17a in this case is substantially the same as the operation of the load sensing control piston 17a in the first embodiment.
The second pump controller 205b includes a second load sensing control unit 212b. The second load sensing control unit 212b includes load sensing control valve 216c and 216d and a low pressure selection valve 221b instead of the load sensing control valve 16b. The low pressure selection valve 221b selects the output pressure of the load sensing control valve 216c or 216d on the low pressure side and outputs the selected output pressure.
The control valve 216c includes a spring 216c1 for setting the target differential pressure of the load sensing control, a pressure receiving part 216c2 situated opposite to the spring 216c1, and a pressure receiving part 216c3 situated on the same side as the spring 216c1. The delivery pressure of the third delivery port P3 is led to the pressure receiving part 216c2. When the second travel communication valve 215b is at the interrupting position (upper position in FIG. 10), the maximum load pressure of the actuators 3e, 3f and 3h detected by the third shuttle valve set 208c is led to the pressure receiving part 216c3 of the control valve 216c. When the second travel communication valve 215b is switched to the communicating position (lower position in FIG. 10), the maximum load pressure of the actuators 3d-3h detected by the third and fourth shuttle valve sets 208c and 208d is led to the pressure receiving part 216c3 of the control valve 216c. The control valve 216c slides according to the balance among the delivery pressure of the third delivery port P3 which is led to the pressure receiving part 216c2, the maximum load pressure of the actuators 3e, 3f and 3h or the actuators 3d-3h which is led to the pressure receiving part 216c3, and the biasing force of the spring 216c1 and thereby increases/decreases the output pressure. The operation of the control valve 216c in these cases is substantially the same as the operation of the control valve 16b in the first embodiment.
The control valve 216d includes a spring 216d1 for setting the target differential pressure of the load sensing control, a pressure receiving part 216d2 situated opposite to the spring 216d1, and a pressure receiving part 216d3 situated on the same side as the spring 216d1. The delivery pressure of the fourth delivery port P4 is led to the pressure receiving part 216d2. When the second travel communication valve 215b is at the interrupting position (upper position in FIG. 10), the maximum load pressure of the actuators 3d, 3g and 3h detected by the fourth shuttle valve set 208d is led to the pressure receiving part 216d3 of the control valve 216d. When the second travel communication valve 215b is switched to the communicating position (lower position in FIG. 10), the maximum load pressure of the actuators 3d-3h detected by the third and fourth shuttle valve sets 208c and 208d is led to the pressure receiving part 216d3 of the control valve 216d. The control valve 216d slides according to the balance among the delivery pressure of the fourth delivery port P4 which is led to the pressure receiving part 216d2, the maximum load pressure of the actuators 3d, 3g and 3h or the actuators 3d-3h which is led to the pressure receiving part 216d3, and the biasing force of the spring 216d1 and thereby increases/decreases the output pressure. The operation of the control valve 216d in these cases is substantially the same as the operation of the control valve 16b in the first embodiment.
The low pressure selection valve 221b selects the output pressure of the load sensing control valve 216c or 216d on the low pressure side and outputs the selected output pressure to the load sensing control piston 17b. According to the output pressure, the load sensing control piston 17b changes the swash plate tilting angle of the second pump device 1b and thereby increases/decreases the delivery flow rates of the third and fourth delivery ports P3 and P4. The operation of the load sensing control piston 17b in this case is substantially the same as the operation of the load sensing control piston 17b in the first embodiment.
Next, the operation of this embodiment will be described below.
The operations from the <Single Driving> to the <Traveling Operation> (traveling sole operation) explained in the first embodiment are operations at the time other than the traveling combined operation. Since the first and second travel communication valves 215a and 215b are at the interrupting positions (upper positions) in these cases, these operations in this embodiment are basically equivalent to those in the first embodiment. However, this embodiment differs from the first embodiment in that the maximum load pressure is detected separately by the first and second shuttle valve sets 208a and 208b on the first delivery port P1's side and the second delivery port P2's side of the first pump device 1a and separately by the third and fourth shuttle valve sets 208c and 208d on the third delivery port P3's side and the fourth delivery port P4's side of the second pump device 1b and the detected maximum load pressures are respectively led to corresponding pressure compensating valves, unload valves and load sensing control valves.
Specifically, in the above operations, the maximum load pressure of the actuators on the first delivery port P1's side of the first pump device 1a is detected by the first shuttle valve set 208a, the maximum load pressure of the actuators on the second delivery port P2's side is detected by the second shuttle valve set 208b, each maximum load pressure is led to the corresponding load sensing control valve 16a or 16a, pressure compensating valves 7a-7c or 7d-7f and unload valve 10a or 10b, and the load sensing control and the control of the pressure compensating valves and the unload valves are performed according to the maximum load pressure. The second pump device 1b's side also operates in a similar manner; the load sensing control and the control of the pressure compensating valves and the unload valves are performed by detecting the maximum load pressure separately on the third delivery port P3's side and on the fourth delivery port P4's side.
In the case where the combined operation driving at least one of the actuators connected to the first delivery port P1 of the first pump device 1a (boom cylinder 3a, swing cylinder 3b, right travel motor 3e) and at least one of the actuators connected to the second delivery port P2 of the first pump device 1a (boom cylinder 3a, bucket cylinder 3c, left travel motor 3d) at the same time is performed in the <Simultaneous Driving of Two Actuators on First Pump Device 1a's Side>, the load pressure (maximum load pressure) of the actuators on the first delivery port P1's side detected by the first shuttle valve set 208a is led to the pressure compensating valves 7a-7c and the first unload valve 210a, the load pressure (maximum load pressure) of the actuators on the second delivery port P2's side detected by the second shuttle valve set 208b is led to the pressure compensating valves 7d-7f and the second unload valve 210b, and the control of the pressure compensating valves and the unload valves is performed separately on the first delivery port P1's side and on the second delivery port P2's side. Accordingly, when a surplus flow occurred in a delivery port on the low load pressure side, the increase in the pressure in the delivery port is limited based on the low load pressure by the unload valve on the same side as the delivery port. Therefore, the pressure loss at the unload valve when the surplus flow returns to the tank is reduced and operation with less energy loss is made possible.
The same applies to the case where the combined operation driving at least one of the actuators connected to the third delivery port P3 of the second pump device 1b (right travel motor 3e, arm cylinder 3h, swing motor 3f) and at least one of the actuators connected to the fourth delivery port P4 of the second pump device 1b (left travel motor 3d, blade cylinder 3g, arm cylinder 3h) at the same time is performed in the <Simultaneous Driving of Two Actuators on Second Pump Device 1b's Side>; the pressure loss at the unload valve on the low load pressure side when the surplus flow through the unload valve returns to the tank is reduced and operation with less energy loss is made possible.
<Traveling Combined Operation>
The traveling combined operation in which the travel motors 3d and 3e and at least one of the other actuators, e.g., boom cylinder 3a, are driven at the same time will be explained below.
When the left and right travel control levers/pedals and the boom control lever are operated by the operator intending the traveling combined operation, the flow control valves 6f and 6j, the flow control valves 6c and 6g, and the flow control valves 6a and 6e are switched over, and at the same time, the first travel communication valve 215a is switched to the communicating position (lower position in FIG. 10). Accordingly, to the left travel motor 3d, the hydraulic fluids delivered from the first and second delivery ports P1 and P2 are merged and supplied from the first pump device 1a's side, while the hydraulic fluid delivered from the fourth delivery port P4 is supplied from the second pump device 1b's side. To the right travel motor 3e, the hydraulic fluids delivered from the first and second delivery ports P1 and P2 are merged and supplied from the first pump device 1a's side, while the hydraulic fluid delivered from the third delivery port P3 is supplied from the second pump device 1b's side. To the boom cylinder 3a, the rest of the hydraulic fluid from the first and second delivery ports P1 and P2 supplied to the travel motor 3d or 3e is supplied.
In this case, on the first pump device 1a's side, the first travel communication valve 215a is switched to the communicating position (lower position in FIG. 10). Therefore, the maximum load pressure of the actuators 3a-3e detected by the first and second shuttle valve sets 208a and 208b is led to the load sensing control valves 216a and 216b, the pressure compensating valves 7a-7c and 7d-7f, and the unload valves 10a and 10b, and the load sensing control and the control of the pressure compensating valves and the unload valves are performed according to the maximum load pressure. In contrast, on the second pump device 1b's side, the second travel communication valve 215b is held at the interrupting position (upper position in FIG. 10). Therefore, the maximum load pressure is detected separately on the third delivery port P3's side and on the fourth delivery port P4's side, each maximum load pressure is led to the corresponding load sensing control valve 216c or 216d, pressure compensating valves 7g-7i or 7j-7m and unload valve 10c or 10d, and the load sensing control and the control of the pressure compensating valves and the unload valves are performed according to each maximum load pressure.
Here, the case where the straight traveling is performed in the traveling combined operation will be explained.
When the left and right travel control levers/pedals are operated by the same amount by the operator intending the straight traveling in the traveling combined operation, the flow control valves 6f and 6j and the flow control valves 6c and 6g are switched over so that the stroke amount (opening area) of the flow control valve 6f/6j equals the stroke amount (opening area−demanded flow rate) of the flow control valve 6c/6g. As mentioned above, to the left travel motor 3d, the hydraulic fluids delivered from the first and second delivery ports P1 and P2 are merged and supplied from the first pump device 1a's side, while the hydraulic fluid delivered from the fourth delivery port P4 is supplied from the second pump device 1b's side. To the right travel motor 3e, the hydraulic fluids delivered from the first and second delivery ports P1 and P2 are merged and supplied from the first pump device 1a's side, while the hydraulic fluid delivered from the third delivery port P3 is supplied from the second pump device 1b's side. Accordingly, also in the traveling combined operation, the supply flow rate of the left travel motor 3d and that of the right travel motor 3e become equal to each other and the vehicle is allowed to travel straight without meandering.
Specifically, assuming that the delivery flow rates of the first through fourth delivery ports P1, P2, P3 and P4 are Q1, Q2, Q3 and Q4, respectively, and the flow rates of the hydraulic fluid supplied to the left and right travel motors 3d and 3e are Qd and Qe, respectively, and the flow rate of the hydraulic fluid supplied to the boom cylinder 3a that is the actuator other than the travel motors is Qa, the flow rates Qd and Qe of the hydraulic fluid supplied to the left and right travel motors 3d and 3e can be determined as explained below.
From the first pump device 1a's side, 1⁄2 of Q1+Q2−Qa that is total delivery flow rate Q1+Q2 of the first and second delivery ports P1 and P2 minus the flow rate Qa of the hydraulic fluid supplied to the boom cylinder 3a is supplied to each of the left and right travel motors 3d and 3e. Here, Q1+Q2−Qa is multiplied by 1⁄2 since the stroke amount (opening area) of the flow control valve 6f and the stroke amount (opening area−demanded flow rate) of the flow control valve 6c are equal to each other. From the second pump device 1b's side, 1⁄2 of the total delivery flow rate Q3+Q4 of the third and fourth delivery ports p3 and p4 is supplied to each of the left and right travel motors 3d and 3e. Also in this case, Q3+Q4 is multiplied by 1⁄2 since the stroke amount (opening area) of the flow control valve 6j and the stroke amount (opening area−demanded flow rate) of the flow control valve 6g are equal to each other. Therefore, the flow rates Qd and Qe of the hydraulic fluid supplied to the left and right travel motors 3d and 3e are expressed as follows:



  
    
      right
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      travel
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      supply
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      ⁢
      flow
      ⁢
      
        
      
      ⁢
      rate
      ⁢
      
        
      
      ⁢
      Qd
    
    =
    
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              1
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              2
            
            -
            Qa
          
          )
        
        /
        2
      
      +
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              3
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              4
            
          
          )
        
        /
        2
      
    
  




  
    
      left
      ⁢
      
        
      
      ⁢
      travel
      ⁢
      
        
      
      ⁢
      supply
      ⁢
      
        
      
      ⁢
      flow
      ⁢
      
        
      
      ⁢
      rate
      ⁢
      
        
      
      ⁢
      Qe
    
    =
    
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              1
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              2
            
            -
            Qa
          
          )
        
        /
        2
      
      +
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              3
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              4
            
          
          )
        
        /
        2
      
    
  



Since Qd=Qe is satisfied as above, the vehicle is allowed to travel straight without meandering.
The above example of the traveling combined operation is about the case where the travel motors 3d and 3e and the boom cylinder 3a are driven at the same time. As another example of the traveling combined operation, there is a traveling combined operation in which the travel motors 3d and 3e and an actuator driven by the hydraulic fluid delivered from only one of the first and second delivery ports P1 and P2 of the first pump device 1a (swing cylinder 3b, bucket cylinder 3c) or an actuator driven by the hydraulic fluid delivered from only one of the third and fourth delivery ports P3 and P4 of the second pump device 1b (swing motor 3f, blade cylinder 3g) are driven at the same time. In this embodiment, the vehicle is allowed to travel straight without meandering even when such a traveling combined operation is performed.
As an example of such a traveling combined operation, a traveling combined operation in which the travel motors 3d and 3e and the bucket cylinder 3c are driven at the same time will be considered below. The flow rate of the hydraulic fluid supplied to the bucket cylinder 3c is assumed to be Qc. Since the delivery flow of the first delivery port P1 and that of the second delivery port P2 are merged and supplied in this embodiment, the flow rates Qd and Qe of the hydraulic fluid supplied to the left and right travel motors 3d and 3e are expressed as follows also in such a traveling combined operation similarly to the case of the traveling combined operation in which the travel motors 3d and 3e and the boom cylinder 3a are driven at the same time:



  
    
      right
      ⁢
      
        
      
      ⁢
      travel
      ⁢
      
        
      
      ⁢
      supply
      ⁢
      
        
      
      ⁢
      flow
      ⁢
      
        
      
      ⁢
      rate
      ⁢
      
        
      
      ⁢
      Qd
    
    =
    
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              1
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              2
            
            -
            Qc
          
          )
        
        /
        2
      
      +
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              3
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              4
            
          
          )
        
        /
        2
      
    
  




  
    
      left
      ⁢
      
        
      
      ⁢
      travel
      ⁢
      
        
      
      ⁢
      supply
      ⁢
      
        
      
      ⁢
      flow
      ⁢
      
        
      
      ⁢
      rate
      ⁢
      
        
      
      ⁢
      Qe
    
    =
    
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              1
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              2
            
            -
            Qc
          
          )
        
        /
        2
      
      +
      
        
          (
          
            
              Q
              ⁢
              
                
              
              ⁢
              3
            
            +
            
              Q
              ⁢
              
                
              
              ⁢
              4
            
          
          )
        
        /
        2
      
    
  



The relationship Qd=Qe is satisfied also in this case.
As explained above, in this embodiment, the vehicle is allowed to travel straight without meandering in any type of traveling combined operation.
Incidentally, while the fourth embodiment is configured by providing the first through fourth shuttle valve sets 208a-208d, the first and second travel communication valves 215a and 215b, the load sensing control valves 216a-216d and the low pressure selection valves 221a and 221b and having the first and second travel communication valves 215a and 215b perform the communication/interruption on both the delivery ports and the output hydraulic lines of the maximum load pressure, it is also possible to configure the first and second travel communication valves 215a and 215b to perform the communication/interruption on the delivery ports only, while configuring the rest of the circuitry in the same way as the first embodiment. Also in this case, the effect of securing the straight traveling performance can be achieved by the switching of the first and second travel communication valves 215a and 215b to the communicating positions at the time of the traveling combined operation.
Other Examples
The above embodiments have been described by taking a hydraulic excavator as an example of the construction machine and the boom cylinder for driving the boom of the front work implement of the hydraulic excavator and the arm cylinder for driving the arm of the front work implement as an example of the first and second actuators that are driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween. However, the first and second actuators can also be actuators other than the boom cylinder or the arm cylinder as long as the actuators are those driven at the same time in a certain combined operation while producing a relatively large supply flow rate difference therebetween. For example, the boom cylinder and the swing motor are actuators driven at the same time in a combined operation of the swinging and the boom elevation while producing a relatively large supply flow rate difference therebetween (boom cylinder flow rate≧swing motor flow rate). By modifying the hydraulic circuit to connect the swing motor to both the third and fourth delivery ports, effects similar to those in the case of the leveling operation by use of the boom and the arm can be achieved.
While the above embodiments have been described by taking the travel motors for driving the left and right crawlers as an example of the third and fourth actuators that are driven at the same time in another operation of the construction machine while achieving a prescribed function by their supply flow rates becoming equivalent to each other, the third and fourth actuators can also be actuators other than the travel motors as long as the actuators are those driven at the same time in a certain operation while achieving a prescribed function by their supply flow rates becoming equivalent to each other.
Further, the present invention is applicable also to construction machines other than hydraulic excavators as long as the construction machine comprises actuators satisfying such operational conditions of the first and second actuators or the third and fourth actuators.
Furthermore, the load sensing system described in the above embodiments is just an example and can be modified in various ways. For example, the target compensation differential pressure may also be set by providing a differential pressure reducing valve that outputs the differential pressure between the pump delivery pressure and the maximum load pressure as the absolute pressure and leading the output pressure of the differential pressure reducing valve to the pressure compensating valve. It is also possible to feed back the output pressure of the differential pressure reducing valve to the load sensing control valve. The target differential pressure of the load sensing control may also be set by providing a differential pressure reducing valve that outputs pressure varying depending on the engine revolution speed as the absolute pressure and leading the output pressure of the differential pressure reducing valve to the load sensing control valve.
DESCRIPTION OF REFERENCE CHARACTERS


    1a first pump device
    1b second pump device
    2 prime mover (diesel engine)
    3a-3h actuator
    3a boom cylinder
    3d left travel motor
    3e right travel motor
    3h arm cylinder
    4 control valve
    5a first pump controller
    5b second pump controller
    6a-6m flow control valve
    7a-7m pressure compensating valve
    8a first shuttle valve set
    8b second shuttle valve set
    9a-9d spring
    10a-10d unload valve
    12a first load sensing control unit
    12b second load sensing control unit
    13a first torque control unit
    13b second torque control unit
    15a, 15b shuttle valve
    16a, 16b load sensing control valve
    17a, 17b load sensing control piston
    18a first torque control piston
    19a second torque control piston
    18b third torque control piston
    19b fourth torque control piston
    204 control valve
    205a first pump controller
    205b second pump controller
    208a-208d shuttle valve set
    215a first travel communication valve
    215b second travel communication valve
    212a first load sensing control unit
    212b second load sensing control unit
    216a, 216b load sensing control valve
    221a low pressure selection valve
    216c, 216d load sensing control valve
    221b low pressure selection valve

## Claims

1. A hydraulic drive system for a construction machine comprising:
a first pump device having first and second delivery ports;
a second pump device having third and fourth delivery ports; and
a plurality of actuators which are driven by hydraulic fluid delivered from the first and second delivery ports of the first pump device and hydraulic fluid delivered from the third and fourth delivery ports of the second pump device, wherein:
the first pump device includes a first pump controller which is provided for the first and second delivery ports as a common controller, and
the second pump device includes a second pump controller which is provided for the third and fourth delivery ports as a common controller, and
the first pump controller includes a first load sensing control unit which controls displacement of the first hydraulic pump device so that delivery pressures of the first and second delivery ports of the first hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the first and second delivery ports by a prescribed pressure and a first torque control unit which performs limiting control of the displacement of the first hydraulic pump device so that absorption torque of the first hydraulic pump device does not exceed a prescribed value, and
the second pump controller includes a second load sensing control unit which controls displacement of the second hydraulic pump device so that delivery pressures of the third and fourth delivery ports of the second hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the third and fourth delivery ports by a prescribed pressure and a second torque control unit which performs limiting control of the displacement of the second hydraulic pump device so that absorption torque of the second hydraulic pump device does not exceed a prescribed value, and
the plurality of actuators include first and second actuators which are driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween, and
the first actuator is connected so that hydraulic fluids delivered from the first and second delivery ports of the first pump device are merged and supplied to the first actuator, and
the second actuator is connected so that hydraulic fluids delivered from the third and fourth delivery ports of the second pump device are merged and supplied to the second actuator, and
the plurality of actuators include third and fourth actuators which are driven at the same time in another operation of the construction machine while achieving a prescribed function by their supply flow rates becoming equivalent to each other, and
the third actuator is connected so that hydraulic fluid delivered from one of the first and second delivery ports of the first pump device and hydraulic fluid delivered from one of the third and fourth delivery ports of the second pump device are merged and supplied to the third actuator, and
the fourth actuator is connected so that hydraulic fluid delivered from the other of the first and second delivery ports of the first pump device and hydraulic fluid delivered from the other of the third and fourth delivery ports of the second pump device are merged and supplied to the fourth actuator.

2. The hydraulic drive system for a construction machine according to claim 1, further comprising:
a first travel communication valve which is arranged between the first and second delivery ports of the first pump device, situated at an interrupting position for interrupting communication between the first and second delivery ports at the time other than combined operation in which the third and fourth actuators and at least one of other actuators related to the first pump device are driven at the same time, and switched to a communicating position for communicating the first and second delivery ports to each other at the time of the combined operation in which the third and fourth actuators and at least one of other actuators related to the first pump device are driven at the same time; and
a second travel communication valve which is arranged between the third and fourth delivery ports of the second pump device, situated at an interrupting position for interrupting communication between the third and fourth delivery ports at the time other than combined operation in which the third and fourth actuators and at least one of other actuators related to the second pump device are driven at the same time, and switched to a communicating position for communicating the third and fourth delivery ports to each other at the time of the combined operation in which the third and fourth actuators and at least one of other actuators related to the second pump device are driven at the same time.

3. The hydraulic drive system for a construction machine according to claim 1, wherein:
the construction machine is a hydraulic excavator having a front work implement, and
the first actuator is a boom cylinder for driving a boom of the front work implement, and
the second actuator is an arm cylinder for driving an arm of the front work implement.

4. The hydraulic drive system for a construction machine according to claim 1, wherein:
the construction machine is a hydraulic excavator having a lower track structure equipped with left and right crawlers, and
the third actuator is a travel motor for driving one of the left and right crawlers, and
the fourth actuator is a travel motor for driving the other of the left and right crawlers.

5. The hydraulic drive system for a construction machine according to claim 1, wherein each of the first and second pump devices is a hydraulic pump of the split flow type having a single displacement control mechanism.

6. The hydraulic drive system for a construction machine according to claim 1, wherein:
the first pump torque control unit of the first pump device controls the displacement of the first hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the first and second delivery ports of the first hydraulic pump device related to itself but also the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device, and
the second pump torque control unit of the second pump device controls the displacement of the second hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device related to itself but also the delivery pressures of the first and second delivery ports of the first hydraulic pump device.

7. A hydraulic drive system for a construction machine, comprising:
a first pump device having first and second delivery ports;
a second pump device having third and fourth delivery ports; and
a plurality of actuators which are driven by hydraulic fluid delivered from the first and second delivery ports of the first pump device and hydraulic fluid delivered from the third and fourth delivery ports of the second pump device, wherein:
the first pump device includes a first pump controller which is provided for the first and second delivery ports as a common controller, and
the second pump device includes a second pump controller which is provided for the third and fourth delivery ports as a common controller, and
the first pump controller includes a first load sensing control unit which controls displacement of the first hydraulic pump device so that delivery pressures of the first and second delivery ports of the first hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the first and second delivery ports by a prescribed pressure and a first torque control unit which performs limiting control of the displacement of the first hydraulic pump device so that absorption torque of the first hydraulic pump device does not exceed a prescribed value, and
the second pump controller includes a second load sensing control unit which controls displacement of the second hydraulic pump device so that delivery pressures of the third and fourth delivery ports of the second hydraulic pump device become higher than maximum load pressure of the actuators driven by the hydraulic fluid delivered from the third and fourth delivery ports by a prescribed pressure and a second torque control unit which performs limiting control of the displacement of the second hydraulic pump device so that absorption torque of the second hydraulic pump device does not exceed a prescribed value, and
the plurality of actuators include first and second actuators which are driven at the same time in a certain combined operation of the construction machine while producing a relatively large supply flow rate difference therebetween, and
the first actuator is connected so that hydraulic fluids delivered from the first and second delivery ports of the first pump device are merged and supplied to the first actuator, and
the second actuator is connected so that hydraulic fluids delivered from the third and fourth delivery ports of the second pump device are merged and supplied to the second actuator, and
the first pump torque control unit of the first pump device controls the displacement of the first hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the first and second delivery ports of the first hydraulic pump device related to itself but also the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device, and
the second pump torque control unit of the second pump device controls the displacement of the second hydraulic pump device so that total absorption torque of the first and second hydraulic pump devices does not exceed a prescribed value by feeding back not only the delivery pressures of the third and fourth delivery ports of the second hydraulic pump device related to itself but also the delivery pressures of the first and second delivery ports of the first hydraulic pump device.