Hydraulic drive system for construction machine

In addition to a main pump 102 having two delivery ports 102a and 102b and performing the load sensing control, two subsidiary pumps 202 and 302 for the load sensing control for respectively performing assist driving on a boom cylinder 3a and an arm cylinder 3b are provided. When driving the boom cylinder 3a or the arm cylinder 3b, a selector valve 141 or 241 is switched and flows of hydraulic fluid are merged together and supplied to the boom cylinder 3a or the arm cylinder 3b. When driving actuators other than the boom cylinder 3a or the arm cylinder 3b, only the hydraulic fluid from the main pump is supplied to the actuators. In short, the hydraulic drive system is configured so that two specific actuators having great demanded flow rates and tending to have a great load pressure difference between each other when driving at the same time can be driven with hydraulic fluid delivered from separate delivery ports. With this configuration, wasteful energy consumption due to pressure loss in a pressure compensating valve can be suppressed, and in cases of driving an actuator of a low demanded flow rate, the hydraulic pump can be used at a point where the volume efficiency is high.

TECHNICAL FIELD

The present invention relates to a hydraulic drive system for a construction machine such as a hydraulic excavator. In particular, the present invention relates to a hydraulic drive system for a construction machine comprising a pump device and a load sensing system, the pump device having two delivery ports whose delivery flow rates are controlled by a single pump regulator (pump control unit), the load sensing system controlling delivery pressures of the pump device to be higher than the maximum load pressure of actuators.

BACKGROUND ART

A hydraulic drive system having a load sensing system for controlling the delivery flow rate of a hydraulic pump (main pump) so that the delivery pressure of the hydraulic pump becomes higher by a target differential pressure than the maximum load pressure of a plurality of actuators as described in Patent Document 1 is widely used today as the hydraulic drive systems for construction machines such as hydraulic excavators.

There has also been known a two-pump load sensing system as an example of the load sensing system, in which two hydraulic pumps are arranged associated with a first actuator group and a second actuator group as described in Patent Document 2 and Patent Document 3.

In the two-pump load sensing system described in Patent Document 2, a separation/confluence selector valve is arranged between delivery hydraulic lines of the two hydraulic pumps. When the load pressure difference among the actuators included in the first and second actuator groups is small, the delivery flow rates of the first and second hydraulic pumps are controlled on the basis of the maximum load pressure of the first and second actuator groups, and the delivery flows from the two hydraulic pumps are merged together and supplied to the actuators.

In the two-pump load sensing system described in Patent Document 3, the maximum displacement of one of the two hydraulic pumps (first hydraulic pump) is set larger than the maximum displacement of the other hydraulic pump (second hydraulic pump). The maximum displacement of the first hydraulic pump is set at a displacement enough for driving an actuator whose demanded flow rate is the highest (assumed to be an arm cylinder). A specific actuator (assumed to be a boom cylinder) is driven by the delivery flow from the second hydraulic pump. Further, a confluence valve is arranged on the first hydraulic pump's side, by which the delivery flow from the second hydraulic pump can be merged with the delivery flow from the first hydraulic pump and the merged delivery flow can be supplied to the specific actuator (assumed to be the boom cylinder).

Further, Patent Document 4 describes a load sensing system in which a hydraulic pump of the split flow type having two delivery ports is employed instead of two hydraulic pumps. In the system, the delivery flow rates of first and second delivery ports can be controlled independently of each other on the basis of the maximum load pressure of a first actuator group and the maximum load pressure of a second actuator group, respectively. Also in this system, the separation/confluence selector valve (travel independent valve) is arranged between the delivery hydraulic lines of the two delivery ports. In cases like performing the traveling only or using the dozer equipment while traveling, the separation/confluence selector valve is switched to a separation position and the delivery flows from the two delivery ports are supplied independently to the actuators. In cases of driving actuators not for the traveling or the dozer (e.g., boom cylinder, arm cylinder, etc.), the separation/confluence selector valve is switched to a confluence position so that the delivery flows from the two delivery ports can be merged together and supplied to the actuators.

PRIOR ART DOCUMENT

Patent Documents

SUMMARY OF THE INVENTION

Problem to be Solved by the Invention

In hydraulic drive systems having an ordinary type of load sensing system like the one described in Patent Document 1, the delivery pressure of the hydraulic pump is controlled to be constantly higher by a certain preset pressure than the maximum load pressure of a plurality of actuators. When an actuator of a high load pressure and an actuator of a low load pressure are driven in combination (e.g., when the boom raising operation (load pressure: high) and the arm crowding operation (load pressure: low) are performed at the same time like the so-called “leveling”), the delivery pressure of the hydraulic pump is controlled to be higher by a certain preset pressure than the high load pressure of the boom cylinder. In this case, a pressure compensating valve for driving the arm cylinder and for preventing excessive inflow into the arm cylinder of the low load pressure is throttled, and thus pressure loss in the pressure compensating valve leads to wasteful energy consumption.

In hydraulic drive systems having the two-pump load sensing system described in Patent Document 2, the wasteful energy consumption as the problem with the load sensing system of Patent Document 1 can be suppressed since the system comprises two hydraulic pumps (first and second hydraulic pumps) and the delivery flow rates of the first and second hydraulic pumps can be controlled independently of each other on the basis of the maximum load pressure of the first actuator group and the maximum load pressure of the second actuator group, respectively.

However, the two-pump load sensing system described in Patent Document 2 has another problem.

In construction machines such as hydraulic excavators, the necessary flow rate (demanded flow rate) of each actuator can vary greatly depending on the type of the actuator and the status of the operation. In the case of hydraulic excavators, for example, the arm cylinder and the boom cylinder tend to need higher flow rates than the other actuators such as the travel motors and the bucket cylinder.

In such cases, if the displacements (maximum displacements) of the first and second hydraulic pumps are set to suit the demanded flow rates of the arm cylinder and the boom cylinder, the displacement of each pump becomes extremely large. Thus, the volume efficiency of the hydraulic pumps deteriorates since the first or second hydraulic pump is driven at a small displacement in the variable-displacement range at times of driving an actuator of a low demanded flow rate (e.g., bucket cylinder).

Incidentally, if the two-pump load sensing system of Patent Document 2 is configured to drive the boom cylinder and the arm cylinder by merging together the delivery flows from the two hydraulic pumps, a problem like the problem with the one-pump load sensing system of Patent Document 1 arises since wasteful energy consumption in the combined operation of the boom cylinder and the arm cylinder increases.

In the two-pump load sensing system described in Patent Document 3, in cases where there is a great difference between the necessary flow rate of the boom cylinder and the arm cylinder and the necessary flow rate of the other actuators (travel motors, bucket cylinder, etc.), the displacements of the two hydraulic pumps are set on the basis of the necessary flow rate of the boom cylinder and the arm cylinder. Thus, the two-pump load sensing system of Patent Document 3 shares the same problem with Patent Document 2 in that the hydraulic pumps are driven at a small displacement in comparison with the entire displacement (entire volume) in cases like driving an actuator of a low flow rate and the volume efficiency of the hydraulic pumps is deteriorated.

In the load sensing system described in Patent Document 4, in cases other than the traveling or using the dozer equipment, the delivery flows from the two delivery ports are merged together and the two delivery ports are made to function as one pump. Therefore, this load sensing system has the same problem as Patent Document 1: wasteful energy consumption occurs due to the pressure loss in a pressure compensating valve in the combined operation like performing the boom raising (load pressure: high) and the arm crowding (load pressure: low) at the same time). Further, since the hydraulic fluid flows delivered from the two delivery ports are merged together and supplied to the actuators, this load sensing system shares the same problem with Patent Document 2 in that the hydraulic pumps are driven at a small displacement in comparison with the entire displacement (volume) in cases like driving an actuator of a low flow rate and the volume efficiency of the hydraulic pumps is deteriorated.

The object of the present invention is to provide a hydraulic drive system for a construction machine capable of suppressing the wasteful energy consumption due to the pressure loss in a pressure compensating valve by making it possible to drive two specific actuators (having great demanded flow rates and tending to have a great load pressure difference between each other when driven at the same time) with hydraulic fluid delivered from separate delivery ports, and also capable of using each hydraulic pump at a point where the volume efficiency is high in cases of driving an actuator of a low demanded flow rate other than the two specific actuators.

Means for Solving the Problem

(1) 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 plurality of actuators which are driven by hydraulic fluid delivered from the first and second delivery ports; a plurality of flow control valves which control the flow rates of the hydraulic fluid supplied from the first and second delivery ports to the actuators; a plurality of pressure compensating valves each of which controls the differential pressure across each of the flow control valves so that the differential pressure becomes equal to a target differential pressure; and a first pump control unit including a first load sensing control unit which controls the displacement of the first pump device so that the delivery pressures of the first and second delivery ports become higher by a target differential pressure than the maximum load pressure of actuators driven by the hydraulic fluid delivered from the first and second delivery ports. The plurality of actuators include a first actuator group and a second actuator group, the first actuator group including a first specific actuator, the second actuator group including a second specific actuator. The first and second specific actuators are actuators having greater demanded flow rates than other actuators and tending to have a great load pressure difference between each other when driven at the same time. The actuators of the first actuator group other than the first specific actuator and the actuators of the second actuator group other than the second specific actuator are actuators having less demanded flow rates than the first and second specific actuators. The actuators of the first actuator group other than the first specific actuator are connected to the first delivery port of the first pump device via associated pressure compensating valves and flow control valves. The actuators of the second actuator group other than the second specific actuator are connected to the second delivery port of the first pump device via associated pressure compensating valves and flow control valves. The hydraulic drive system further comprises: a second pump device having a third delivery port to which the first specific actuator of the first actuator group is connected via an associated pressure compensating valve and flow control valve; a third pump device having a fourth delivery port to which the second specific actuator of the second actuator group is connected via an associated pressure compensating valve and flow control valve; a second pump control unit including a second load sensing control unit which controls the displacement of the second pump device so that the delivery pressure of the third delivery port becomes higher by a target differential pressure than the load pressure of the first specific actuator; a third pump control unit including a third load sensing control unit which controls the displacement of the third pump device so that the delivery pressure of the fourth delivery port becomes higher by a target differential pressure than the load pressure of the second specific actuator; a first selector valve which interrupts communication between the first delivery port and the third delivery port when only one or more actuators other than the first specific actuator are driven among the actuators of the first actuator group, while establishing communication between the first delivery port and the third delivery port when at least the first specific actuator is driven among the actuators of the first actuator group; and a second selector valve which interrupts communication between the second delivery port and the fourth delivery port when only one or more actuators other than the second specific actuator are driven among the actuators of the second actuator group, while establishing communication between the second delivery port and the fourth delivery port when at least the second specific actuator is driven among the actuators of the second actuator group.

By providing the second and third pump devices as assist pumps specifically for driving the first and second specific actuators as described above, it becomes possible to drive the first and second specific actuators (having great demanded flow rates and tending to have a great load pressure difference between each other when driven at the same time) with hydraulic fluid delivered from separate delivery ports.

Therefore, when an actuator of a high load pressure (first specific actuator) and an actuator of a low load pressure (second specific actuator) are driven in combination (e.g., the so-called “leveling operation” in which the boom and the arm are operated at the same time), the delivery pressure of the delivery port on the low load pressure actuator's side can be controlled independently. Consequently, the wasteful energy consumption in the pressure compensating valve for the low load pressure actuator is prevented and operation with high efficiency becomes possible.

Further, since the actuators of the first actuator group other than the first specific actuator are driven by the hydraulic fluid delivered from the first delivery port of the first pump device and the actuators of the second actuator group other than the second specific actuator are driven by the hydraulic fluid delivered from the second delivery port of the first pump device, the first pump device can be used at a point of higher efficiency in cases of driving an actuator of a low demanded flow rate.

(2) Preferably, in the above hydraulic drive system (1) for a construction machine, the actuators of the first actuator group other than the first specific actuator include a third specific actuator, the actuators of the second actuator group other than the second specific actuator include a fourth specific actuator, and the third and fourth specific actuators are actuators achieving a prescribed function by having supply flow rates equivalent to each other when driven at the same time. The hydraulic drive system further comprises a third selector valve which interrupts communication between the first delivery port and the second delivery port of the first pump device at times other than when the third and fourth specific actuators and at least another actuator are driven at the same time, while establishing communication between the first delivery port and the second delivery port of the first pump device when the third and fourth specific actuators and at least another actuator are driven at the same time.

With this configuration, when the third and fourth specific actuators and one of the first and second actuators (three actuators) are driven at the same time, flows of the hydraulic fluid from the first and second delivery ports of the first pump device and one of the third and fourth delivery ports of the second and third pump devices (three delivery ports) are merged together and supplied to the three actuators. When the third and fourth specific actuators and an actuator of the first actuator group other than the first or third specific actuator or an actuator of the second actuator group other than the second or fourth specific actuator are driven at the same time, flows of the hydraulic fluid from the first and second delivery ports of the first pump device (two delivery ports) are merged together and supplied to the actuators. Therefore, when the third and fourth specific actuators and at least another actuator are driven at the same time, equal amounts of hydraulic fluid can be supplied to the third and fourth specific actuators by operating the control levers of the third and fourth specific actuators at equal input amounts (operation amounts). Consequently, excellent operability in the combined operation can be provided.

(3) Preferably, the above hydraulic drive system (1) or (2) for a construction machine further comprises a control pressure generation circuit which generates pressure for controlling hydraulic devices including the pressure compensating valves, the first pump control unit, the second pump control unit, and the third pump control unit. When only one or more actuators other than the first specific actuator are driven among the actuators of the first actuator group, a differential pressure between the delivery pressure of the first delivery port of the first pump device and the maximum load pressure of the actuators other than the first specific actuator is lead as the target differential pressure to the first pump control unit and the pressure compensating valves related to the actuators other than the first specific actuator. When at least the first specific actuator is driven among the actuators of the first actuator group, a differential pressure between the delivery pressure of the first delivery port of the first pump device or the fourth delivery port of the second pump device and the maximum load pressure of the first actuator group is led as the target differential pressure to the first pump control unit and the pressure compensating valves related to the second pump device and the first actuator group. When only one or more actuators other than the second specific actuator are driven among the actuators of the second actuator group, a differential pressure between the delivery pressure of the second delivery port of the first pump device and the maximum load pressure of the actuators other than the second specific actuator is led as the target differential pressure to the first pump control unit and the pressure compensating valves related to the actuators other than the second specific actuator. When at least the second specific actuator is driven among the actuators of the second actuator group, a differential pressure between the delivery pressure of the second delivery port of the first pump device or the third delivery port of the third pump device and the maximum load pressure of the second actuator group is lead as the control pressure generation circuit leads the target differential pressure to the first pump control unit and the pressure compensating valves related to the third pump device and the second actuator group.

With this configuration, the load sensing control and the control of the pressure compensating valves can be performed appropriately according to the load pressures of the currently driven actuators.

(4) Preferably, any one of the above hydraulic drive systems (1)-(3) for a construction machine further comprises: a first unload valve which shifts to the open state and returns the hydraulic fluid delivered from the first delivery port of the first pump device to a tank when the delivery pressure of the first delivery port of the first pump device becomes higher by a prescribed pressure than the maximum load pressure of the actuators other than the first specific actuator when only one or more actuators other than the first specific actuator are driven among the actuators of the first actuator group; a second unload valve which shifts to the open state and returns the hydraulic fluid delivered from the first delivery port of the first pump device or the third delivery port of the second pump device to the tank when the delivery pressure of the first delivery port of the first pump device or the third delivery port of the second pump device becomes higher by a prescribed pressure than the maximum load pressure of the first actuator group when at least the first specific actuator is driven among the actuators of the first actuator group; a third unload valve which shifts to the open state and returns the hydraulic fluid delivered from the second delivery port of the first pump device to the tank when the delivery pressure of the second delivery port of the first pump device becomes higher by a prescribed pressure than the maximum load pressure of the actuators other than the second specific actuator when only one or more actuators other than the second specific actuator are driven among the actuators of the second actuator group; and a fourth unload valve which shifts to the open state and returns the hydraulic fluid delivered from the second delivery port of the first pump device or the fourth delivery port of the second pump device to the tank when the delivery pressure of the second delivery port of the first pump device or the fourth delivery port of the third pump device becomes higher by a prescribed pressure than the maximum load pressure of the second actuator group when at least the second specific actuator is driven among the actuators of the second actuator group.

With this configuration, it becomes possible to appropriately control the pressures of the first and second delivery ports of the first pump device and the third and fourth delivery ports of the second and third pump devices independently of one another according to the load pressures of the currently driven actuators in any case of single driving or combined driving of actuators.

Further, as a result, when an actuator of a high load pressure (first specific actuator) and an actuator of a low load pressure (second specific actuator) are driven in combination (e.g., the so-called “leveling operation” in which the boom and the arm are operated at the same time), the wasteful energy consumption in the pressure compensating valve on the low load pressure actuator's side is prevented and operation with high efficiency becomes possible.

(5) Preferably, in the above hydraulic drive system (1) or (2) for a construction machine, the first pump control unit further includes a torque control unit having a first torque control actuator to which the delivery pressure of the first delivery port is led, a second torque control actuator to which the delivery pressure of the second delivery port is led, and a third torque control actuator to which average pressure of the delivery pressures of the third and fourth delivery ports is led. The first and second torque control actuators are configured to decrease the displacement of the first pump device with the increase in average pressure of the delivery pressures of the first and second delivery ports. The third torque control actuator is configured to decrease the displacement of the first pump device with the increase in the average pressure of the delivery pressures of the third and fourth delivery ports.

With this configuration, even when the load pressure of one actuator increases significantly in a combined operation of driving an actuator of the first actuator group and an actuator of the second actuator group (two actuators, for example) at the same time, the displacement of the first pump device is controlled by torque control with the average pressure of the delivery pressures of the first and second delivery ports and the average pressure of the delivery pressures of the third and fourth delivery ports. Consequently, the drop in the driving speed of the actuator due to a significant decrease in the displacement of the first pump device can be prevented and excellent operability in the combined operation can be secured.

(6) Preferably, in any one of the above hydraulic drive systems (1)-(5) for a construction machine, the first and second specific actuators are a boom cylinder and an arm cylinder for driving a boom and an arm of a hydraulic excavator, and one of the actuators of one of the first and second actuator groups is a bucket cylinder for driving a bucket of the hydraulic excavator.

With this configuration, the wasteful energy consumption due to the pressure loss in a pressure compensating valve can be suppressed in the so-called leveling operation in which the boom and the arm are operated at the same time. Further, in cases of driving the bucket cylinder whose demanded flow rate is lower than those of the boom cylinder and the arm cylinder, the first pump device can be used at a point where the volume efficiency is high.

(7) Preferably, in any one of the above hydraulic drive systems (2)-(6) for a construction machine, the third and fourth specific actuators are left and right travel motors for driving a track structure of a hydraulic excavator.

With this configuration, when the left and right travel motors and at least another actuator are driven at the same time, flows of the hydraulic fluid from two delivery ports or three delivery ports are merged together and supplied to the actuators. Therefore, equal amounts of hydraulic fluid can be supplied to the left and right travel motors by operating the control levers of the left and right travel motors at equal input amounts (operation amounts). This makes it possible to drive the other actuator(s) while maintaining the straight traveling property and to achieve excellent travel combined operation.

Effect of the Invention

According to the present invention, it becomes possible to drive two specific actuators (having great demanded flow rates and tending to have a great load pressure difference between each other when driven at the same time) with hydraulic fluid delivered from separate delivery ports. Therefore, the delivery pressure of the delivery port on the low load pressure actuator's side can be controlled independently. Consequently, the wasteful energy consumption in the pressure compensating valve for the low load pressure actuator is prevented and operation with high efficiency becomes possible. Further, the first pump device can be used at a point of higher efficiency in cases of driving an actuator of a low demanded flow rate.

When actuators achieving a prescribed function by having supply flow rates equivalent to each other when driven at the same time and at least another actuator are driven at the same time, flows of the hydraulic fluid from the first and second delivery ports and one of the third and fourth delivery ports (three delivery ports) or from the first and second delivery ports (two delivery ports) are merged together and supplied to the actuators. Therefore, when the third and fourth specific actuators and at least another actuator are driven at the same time, equal amounts of hydraulic fluid can be supplied to the third and fourth specific actuators by operating the control levers of the third and fourth specific actuators at equal input amounts (operation amounts). Consequently, excellent operability in the combined operation can be provided.

The displacement of the first pump device is controlled by torque control with the average pressure of the delivery pressures of the first and second delivery ports and the average pressure of the delivery pressures of the third and fourth delivery ports. Therefore, even when the load pressure of one actuator increases significantly in the combined operation, the drop in the driving speed of the actuator due to a significant decrease in the displacement of the first pump device can be prevented and excellent operability in the combined operation can be secured.

In the so-called leveling operation in which the boom and the arm are operated at the same time, the wasteful energy consumption due to the pressure loss in a pressure compensating valve can be suppressed, and the first pump device can be used at a point where the volume efficiency is high in cases of driving the bucket cylinder whose demanded flow rate is lower than those of the boom cylinder and the arm cylinder.

When the left and right travel motors and at least another actuator are driven at the same time, flows of the hydraulic fluid from two delivery ports or three delivery ports are merged together and supplied to the actuators. Therefore, equal amounts of hydraulic fluid can be supplied to the left and right travel motors by operating the control levers of the left and right travel motors at equal input amounts. This makes it possible to drive the other actuator(s) while maintaining the straight traveling property and to achieve excellent operability in the travel combined operation.

MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, a description will be given in detail of a preferred embodiment of the present invention.

Configuration

FIG. 1is a schematic diagram showing a hydraulic drive system for a hydraulic excavator (construction machine) in accordance with an embodiment of the present invention.

Referring toFIG. 1, the hydraulic drive system according to this embodiment comprises a prime mover1, a main pump102(first pump device), a subsidiary pump202(second pump device), a subsidiary pump302(third pump device), actuators3a,3b,3c,3d,3e,3f,3gand3h, a control valve unit4, a regulator112(first pump control unit), a regulator212(second pump control unit), and a regulator312(third pump control unit). The prime mover1(e.g., diesel engine) drives the main pump102, the subsidiary pumps202and302, and a pilot pump30(explained later). The main pump102(first pump device) is a variable displacement pump of the split flow type having first and second delivery ports102aand102b. The subsidiary pump202(second pump device) is a variable displacement pump having a third delivery port202a. The subsidiary pump302(third pump device) is a variable displacement pump having a fourth delivery port302a. The actuators3a,3b,3c,3d,3e,3f,3gand3hare driven by hydraulic fluid delivered from the first and second delivery ports102aand102bof the main pump102, the third delivery port202aof the subsidiary pump202and the fourth delivery port302aof the subsidiary pump302. The control valve unit4controls the flow of the hydraulic fluid supplied from the first and second delivery ports102aand102bof the main pump102, the third delivery port202aof the subsidiary pump202and the fourth delivery port302aof the subsidiary pump302to the actuators3a,3b,3c,3d,3e,3f,3gand3h. The regulator112(first pump control unit) is used for controlling the delivery flow rates of the first and second delivery ports102aand102bof the main pump102. The regulator212(second pump control unit) is used for controlling the delivery flow rate of the third delivery port202aof the subsidiary pump202. The regulator312(third pump control unit) is used for controlling the delivery flow rate of the fourth delivery port302aof the subsidiary pump302.

The hydraulic drive system further comprises a pilot pump30, a prime mover revolution speed detection valve13, a pilot relief valve32, a gate lock valve100, and control lever units122,123,124aand124b(FIG. 2). The pilot pump30is a fixed displacement pump which is driven by the prime mover1. The prime mover revolution speed detection valve13is connected to a hydraulic fluid supply line31aof the pilot pump30and detects the delivery flow rate of the pilot pump30as absolute pressure Pgr. The pilot relief valve32is connected to a pilot hydraulic fluid supply line31bdownstream of the prime mover revolution speed detection valve13and generates a fixed pilot pressure in the pilot hydraulic fluid supply line31b. The gate lock valve100is connected to the pilot hydraulic fluid supply line31band connects a hydraulic fluid supply line31cdownstream of the gate lock valve100with the pilot hydraulic fluid supply line31bor a tank (switching) depending on the position of the a gate lock lever24. The control lever units122,123,124aand124b(FIG. 2) include pilot valves (pressure-reducing valves) that are connected to the pilot hydraulic fluid supply line31cdownstream of the gate lock valve100for generating operating pilot pressures for controlling flow control valves6a,6b,6c,6d,6e,6f,6gand6h(explained later).

The actuators3a-3hinclude a first actuator group (actuators3a,3c,3dand3f) including a first specific actuator3aand a second actuator group (actuators3b,3e,3gand3h) including a second specific actuator3b. The first and second specific actuators3aand3bare actuators having greater demanded flow rates than other actuators and tending to have a great load pressure difference between each other when driven at the same time. The actuators of the first actuator group other than the first specific actuator3a(the actuators3c,3dand3f) and the actuators of the second actuator group other than the second specific actuator3b(the actuators3e,3gand3h) are actuators having less demanded flow rates than the first and second specific actuators3aand3b. The actuators of the first actuator group other than the first specific actuator3a(the actuators3c,3dand3f) include a third specific actuator3f. The actuators of the second actuator group other than the second specific actuator3b(the actuators3e,3gand3h) include a fourth specific actuator3g. The third and fourth specific actuators3fand3gare actuators achieving a prescribed function by having supply flow rates equivalent to each other when driven at the same time.

Specifically, the first and second specific actuators3aand3bare a boom cylinder for driving a boom of the hydraulic excavator and an arm cylinder for driving an arm of the hydraulic excavator, for example. The actuators3c,3dand3fof the first actuator group (having less demanded flow rates than the first and second specific actuators3aand3b) are a swing motor for driving a swing structure of the hydraulic excavator, a bucket cylinder for driving a bucket of the hydraulic excavator, and a left travel motor for driving a left crawler of a lower track structure of the hydraulic excavator. The actuators3e,3gand3hof the second actuator group (having less demanded flow rates than the first and second specific actuators3aand3b) are a swing cylinder for driving a swing post, a right travel motor for driving a right crawler of the lower track structure, and a blade cylinder for driving a blade. The third and fourth specific actuators3fand3gare the left and right travel motors.

The control valve unit4includes the flow control valves6a,6b,6c,6d,6e,6f,6gand6h, pressure compensating valves7a,7b,7c,7d,7e,7f,7gand7h, and operation detection valves8a,8b,8c,8d,8e,8f,8gand8h. The flow control valves6a-6hcontrol the flow rates of the hydraulic fluid supplied to the actuators3a-3hfrom the first and second delivery ports102aand102bof the main pump102, the third delivery port202aof the subsidiary pump202and the fourth delivery port302aof the subsidiary pump302. Each pressure compensating valve7a-7hcontrols the differential pressure across each flow control valve6a-6hso that the differential pressure becomes equal to a target differential pressure. Each operation detection valve8a-8hstrokes together with the spool of each flow control valve6a-6hin order to detect the switching of each flow control valve.

The flow control valves6a,6c,6dand6fare valves for controlling the flow rates of the hydraulic fluid supplied to the actuators3a,3c,3dand3fof the first actuator group. Among the flow control valves6a,6c,6dand6f, the flow control valves6c,6dand6fassociated with the actuators3c,3dand3fother than the first specific actuator3aare connected to a first hydraulic fluid supply line105(which is connected to the first delivery port102aof the main pump102) via the pressure compensating valves7c,7dand7f. The flow control valve6aassociated with the first specific actuator3ais connected to a third hydraulic fluid supply line305(which is connected to the third delivery port202aof the subsidiary pump202) via the pressure compensating valve7a.

The flow control valves6b,6e,6gand6hare valves for controlling the flow rates of the hydraulic fluid supplied to the actuators3b,3e,3gand3hof the second actuator group. Among the flow control valves6b,6e,6gand6h, the flow control valves6e,6gand6hassociated with the actuators3e,3gand3hother than the second specific actuator3bare connected to a second hydraulic fluid supply line205(which is connected to the second delivery port102bof the main pump102) via the pressure compensating valves7e,7gand7h. The flow control valve6bassociated with the second specific actuator3bis connected to a fourth hydraulic fluid supply line405(which is connected to the fourth delivery port302aof the subsidiary pump302) via the pressure compensating valve7b.

The control valve unit4further includes main relief valves114and214, unload valves115,215,315and415, and selector valve141,241and40. The main relief valve114is connected to the first hydraulic fluid supply line105of the main pump102and controls the pressure in the first hydraulic fluid supply line105so that the pressure does not exceed a preset pressure. The main relief valve214is connected to the second hydraulic fluid supply line205of the main pump102and controls the pressure in the second hydraulic fluid supply line205so that the pressure does not exceed a preset pressure. The unload valve115(first unload valve) is connected to the first hydraulic fluid supply line105via the selector valve141when the boom cylinder3ais not driven. When the pressure in the first hydraulic fluid supply line105becomes higher by a prescribed pressure (which is set by a spring) than the maximum load pressure of the actuators3c,3dand3fof the first actuator group other than the boom cylinder3a, the unload valve115shifts to the open state and returns the hydraulic fluid in the first hydraulic fluid supply line105to the tank. The unload valve215(third unload valve) is connected to the second hydraulic fluid supply line205via the selector valve241when the arm cylinder3bis not driven. When the pressure in the second hydraulic fluid supply line205becomes higher by a prescribed pressure (which is set by a spring) than the maximum load pressure of the actuators3e,3gand3hof the second actuator group other than the arm cylinder3b, the unload valve215shifts to the open state and returns the hydraulic fluid in the second hydraulic fluid supply line205to the tank. The unload valve315(second unload valve) is connected to the third hydraulic fluid supply line305. At times of driving the boom cylinder3a, when the pressure in the third hydraulic fluid supply line305becomes a prescribed pressure or more higher than the maximum load pressure of the actuators3a,3c,3dand3fof the first actuator group, the unload valve315shifts to the open state and returns the hydraulic fluid in the third hydraulic fluid supply line305to the tank. Also when an actuator3c,3dor3fof the first actuator group other than the boom cylinder3ais driven at times of not driving the boom cylinder3a, the unload valve315shifts to the open state and returns the hydraulic fluid in the third hydraulic fluid supply line305to the tank when the pressure in the third hydraulic fluid supply line305becomes higher by the prescribed pressure (which is set by a spring) than the tank pressure. The unload valve415(fourth unload valve) is connected to the fourth hydraulic fluid supply line405. At times of driving the arm cylinder3b, when the pressure in the fourth hydraulic fluid supply line405becomes higher by a prescribed pressure than the maximum load pressure of the actuators3b,3g,3eand3hof the second actuator group, the unload valve415shifts to the open state and returns the hydraulic fluid in the fourth hydraulic fluid supply line405to the tank. Also when an actuator3e,3gor3hof the second actuator group other than the arm cylinder3bis driven at times of not driving the arm cylinder3b, the unload valve415shifts to the open state and returns the hydraulic fluid in the fourth hydraulic fluid supply line405to the tank when the pressure in the fourth hydraulic fluid supply line405becomes higher by the prescribed pressure (which is set by a spring) than the tank pressure. The selector valve141(first selector valve) is positioned at a first position (lower position inFIG. 1) when the boom cylinder3ais not driven. At the first position, the selector valve141interrupts communication between the first hydraulic fluid supply line105of the main pump102and the third hydraulic fluid supply line305of the subsidiary pump202and connects the first hydraulic fluid supply line105of the main pump102to the unload valve115. When the boom cylinder3ais driven, the selector valve141switches to a second position (upper position inFIG. 1). At the second position, the selector valve141establishes communication between the first hydraulic fluid supply line105of the main pump102and the third hydraulic fluid supply line305of the subsidiary pump202and interrupts communication between the first hydraulic fluid supply line105of the main pump102and the unload valve115. The selector valve241(second selector valve) is positioned at a first position (lower position inFIG. 1) when the arm cylinder3bis not driven. At the first position, the selector valve241interrupts communication between the second hydraulic fluid supply line205of the main pump102and the fourth hydraulic fluid supply line405of the subsidiary pump302and connects the second hydraulic fluid supply line205of the main pump102to the unload valve215. When the arm cylinder3bis driven, the selector valve241switches to a second position (upper position inFIG. 1). At the second position, the selector valve241establishes communication between the second hydraulic fluid supply line205of the main pump102and the fourth hydraulic fluid supply line405of the subsidiary pump302and interrupts communication between the second hydraulic fluid supply line205of the main pump102and the unload valve215. The selector valve40(third selector valve) is positioned at a first position (interrupting position) when a travel combined operation is not performed. The travel combined operation is an operation in which the left travel motor3fand/or the right travel motor3gand at least one of the other actuators are driven at the same time. At the first position, the selector valve40interrupts communication between the first hydraulic fluid supply line105and the second hydraulic fluid supply line205. When the travel combined operation is performed, the selector valve40switches to a second position (communicating position) and establishes communication between the first hydraulic fluid supply line105and the second hydraulic fluid supply line205.

The control valve unit4further includes shuttle valves9c,9d,9e,9f,9g,9h,9iand9jand selector valves145,146,245and246. The shuttle valves9c,9dand9fare connected to load detection ports of the flow control valves6a,6c,6dand6fassociated with the actuators3a,3c,3dand3fconnected to the first and third hydraulic fluid supply lines105and305and detect the maximum load pressure Plmax1of the actuators3a,3c,3dand3f. The shuttle valves9e,9gand9hare connected to load detection ports of the flow control valves6b,6e,6gand6hassociated with the actuators3b,3e,3gand3hconnected to the second and fourth hydraulic fluid supply lines205and405and detect the maximum load pressure Plmax2of the actuators3b,3e,3gand3h. The selector valve145is positioned at a first position (lower position inFIG. 1) when the boom cylinder3ais not driven. At the first position, the selector valve145leads the tank pressure to the unload valve315which is connected to the third hydraulic fluid supply line305and to a differential pressure reducing valve311which will be explained later. When the boom cylinder3ais driven, the selector valve145switches to a second position (upper position inFIG. 1) and leads the maximum load pressure Plmax1of the actuators3a,3c,3dand3fto the unload valve315and the differential pressure reducing valve311. The selector valve245is positioned at a first position (lower position inFIG. 1) when the arm cylinder3bis not driven. At the first position, the selector valve245leads the tank pressure to the unload valve415which is connected to the fourth hydraulic fluid supply line405and to a differential pressure reducing valve411which will be explained later. When the arm cylinder3bis driven, the selector valve245switches to a second position (upper position inFIG. 1) and leads the maximum load pressure Plmax2of the actuators3b,3e,3gand3hto the unload valve415and the differential pressure reducing valve411. The selector valve146is positioned at a first position (lower position inFIG. 1) when the travel combined operation (driving the left travel motor3fand/or the right travel motor3gand at least one of the other actuators at the same time) is not performed. At the first position, the selector valve146outputs the tank pressure. When the travel combined operation is performed, the selector valve146switches to a second position (upper position inFIG. 1) and outputs the maximum load pressure Plmax1of the actuators3a,3c,3dand3fconnected to the first and third hydraulic fluid supply lines105and305. The shuttle valve9jdetects the higher pressure from the output pressure of the selector valve146and the load pressure of the right travel motor3gand leads the detected higher pressure to the shuttle valve9g. The selector valve246is positioned at a first position (lower position inFIG. 1) when the travel combined operation is not performed. At the first position, the selector valve246outputs the tank pressure. When the travel combined operation is performed, the selector valve246switches to a second position (upper position inFIG. 1) and outputs the maximum load pressure Plmax2of the actuators3b,3e,3gand3hconnected to the hydraulic fluid supply lines205and405. The shuttle valve9idetects the higher pressure from the output pressure of the selector valve246and the load pressure of the left travel motor3fand leads the detected higher pressure to the shuttle valve9f.

The control valve unit4further includes a boom operation detection hydraulic line52, an arm operation detection hydraulic line54, a travel combined operation detection hydraulic line53, and differential pressure reducing valves111,211,311and411. The boom operation detection hydraulic line52is a hydraulic line whose upstream side is connected to the pilot hydraulic fluid supply line31bvia a restrictor42and whose downstream side is connected to the tank via the operation detection valve8a. When the boom cylinder3ais driven, the communication of the boom operation detection hydraulic line52to the tank is interrupted by the operation detection valve8astroking together with the flow control valve6a, and thus the pressure generated by the pilot relief valve32is led to the selector valves141,145and146as operation detection pressure, by which the selector valves141,145and146are pushed downward inFIG. 1and switched to the second positions. When the boom cylinder3ais not driven, the boom operation detection hydraulic line52is connected to the tank via the operation detection valve8a, by which the operation detection pressure becomes equal to the tank pressure and the selector valves141,145and146are switched to the first positions (lower positions inFIG. 1). The arm operation detection hydraulic line54is a hydraulic line whose upstream side is connected to the pilot hydraulic fluid supply line31bvia a restrictor44and whose downstream side is connected to the tank via the operation detection valve8b. When the arm cylinder3bis driven, the communication of the arm operation detection hydraulic line54to the tank is interrupted by the operation detection valve8bstroking together with the flow control valve6b, and thus the pressure generated by the pilot relief valve32is led to the selector valves241,245and246as operation detection pressure, by which the selector valves241,245and246are pushed downward inFIG. 1and switched to the second positions. When the arm cylinder3bis not driven, the arm operation detection hydraulic line54is connected to the tank via the operation detection valve8b, by which the operation detection pressure becomes equal to the tank pressure and the selector valves241,245and246are switched to the first positions (lower positions inFIG. 1). The travel combined operation detection hydraulic line53is a hydraulic line whose upstream side is connected to the pilot hydraulic fluid supply line31bvia a restrictor43and whose downstream side is connected to the tank via the operation detection valves8a,8b,8c,8d,8e,8f,8gand8h. When the travel combined operation (driving the left travel motor3fand/or the right travel motor3gand at least one of the other actuators at the same time) is performed, the communication of the travel combined operation detection hydraulic line53to the tank is interrupted by the operation detection valve8fand/or the operation detection valve8gand at least one of the operation detection valves8a,8b,8c,8d,8eand8hstroking together with associated flow control valves, and thus the pressure generated by the pilot relief valve32is led to the selector valve40as operation detection pressure, by which the selector valve40is pushed downward inFIG. 1and switched to the second position (communicating position). When the travel combined operation is not performed, the travel combined operation detection hydraulic line53is connected to the tank via the operation detection valve8fand/or the operation detection valve8gand the operation detection valves8a,8b,8c,8d,8eand8h, by which the operation detection pressure becomes equal to the tank pressure and the selector valve40is switched to the first position as the lower positions inFIG. 1(interrupting position). The differential pressure reducing valve111outputs the difference between the pressure in the first hydraulic fluid supply line105of the main pump102(i.e., pump pressure P1) and the maximum load pressure Plmax1of the actuators3a,3c,3dand3fconnected to the first and third hydraulic fluid supply lines105and305(LS differential pressure) as absolute pressure Pls1. The differential pressure reducing valve211outputs the difference between the pressure in the second hydraulic fluid supply line205of the main pump102(i.e., pump pressure P2) and the maximum load pressure Plmax2of the actuators3b,3e,3gand3hconnected to the second and fourth hydraulic fluid supply lines205and405(LS differential pressure) as absolute pressure Pls2. The differential pressure reducing valve311outputs the difference between the pressure in the third hydraulic fluid supply line305of the subsidiary pump202(i.e., pump pressure P3(=pump pressure P1)) and the maximum load pressure Plmax3of the actuators3a,3c,3dand3f(LS differential pressure) as absolute pressure Pls3when the boom cylinder3ais driven. When the boom cylinder3ais not driven, the differential pressure reducing valve311outputs the pressure in the third hydraulic fluid supply line305(=pressure equivalent to the prescribed pressure set by the spring of the unload valve315) as the absolute pressure Pls3. The differential pressure reducing valve411outputs the difference between the pressure in the fourth hydraulic fluid supply line405of the subsidiary pump302(i.e., pump pressure P4(=pump pressure P2)) and the maximum load pressure Plmax4of the actuators3b,3e,3gand3h(LS differential pressure) as absolute pressure Pls4when the arm cylinder3bis driven. When the arm cylinder3bis not driven, the differential pressure reducing valve411outputs the pressure in the fourth hydraulic fluid supply line405(=pressure equivalent to the prescribed pressure set by the spring of the unload valve415) as the absolute pressure Pls3.

The prime mover revolution speed detection valve13includes a flow rate detection valve50which is connected between the hydraulic fluid supply line31aof the pilot pump30and the pilot hydraulic fluid supply line31band a differential pressure reducing valve51which outputs the differential pressure across the flow rate detection valve50as absolute pressure Pgr.

The flow rate detection valve50includes a variable restrictor part50awhose opening area increases with the increase in the flow rate through itself (delivery flow rate of the pilot pump30). The hydraulic fluid delivered from the pilot pump30passes through the variable restrictor part50aof the flow rate detection valve50and then flows to the pilot hydraulic line31b's side. At this time, a differential pressure increasing with the increase in the flow rate occurs across the variable restrictor part50aof the flow rate detection valve50. The differential pressure reducing valve51outputs the differential pressure across the variable restrictor part50aas the absolute pressure Pgr. Since the delivery flow rate of the pilot pump30changes according to the revolution speed of the engine1, the delivery flow rate of the pilot pump30and the revolution speed of the engine1can be detected by the detection of the differential pressure across the variable restrictor part50a.

The regulator112of the main pump102includes a low-pressure selection valve112a, an LS control valve112b, and tilting control pistons112c,112d,112eand112f. The low-pressure selection valve112aselects the lower pressure from the LS differential pressure outputted by the differential pressure reducing valve111(absolute pressure Pls1) and the LS differential pressure outputted by the differential pressure reducing valve211(absolute pressure Pls2). The LS control valve112boperates according to differential pressure between the selected lower LS differential pressure and the output pressure (absolute pressure) Pgr of the prime mover revolution speed detection valve13. When the LS differential pressure is higher than the output pressure (absolute pressure) Pgr, the LS control valve112bincreases the output pressure by connecting its input side to the pilot hydraulic fluid supply line31b. When the LS differential pressure is lower than the output pressure (absolute pressure) Pgr, the LS control valve112bdecreases the output pressure by connecting its input side to the tank. The tilting control piston112cis a piston for LS control which is supplied with the output pressure of the LS control valve112band operates in the direction of decreasing the tilting (displacement) of the main pump102with the increase in the output pressure. The tilting control pistons112eand112dare pistons for torque control (power control) which respectively operate in the direction of decreasing the tilting (displacement) of the main pump102according to the pressures in the first and second hydraulic fluid supply lines105and205of the main pump102. The tilting control piston112fis a piston for total torque control (total power control) which operates in the direction of decreasing the tilting (displacement) of the main pump102according to the output pressure of a pressure reducing valve112gto which the pressure of the third hydraulic fluid supply line305of the subsidiary pump202and the pressure of the fourth hydraulic fluid supply line405of the subsidiary pump302are led via restrictors112hand112i, respectively.

The regulator212of the subsidiary pump202includes an LS control valve212aand tilting control pistons212cand212d. The LS control valve212aoperates according to differential pressure between the LS differential pressure (absolute pressure Pls3outputted by the differential pressure reducing valve311and the output pressure (absolute pressure) Pgr of the prime mover revolution speed detection valve13. When the LS differential pressure is higher than the output pressure (absolute pressure) Pgr, the LS control valve212aincreases the output pressure by connecting its input side to the pilot hydraulic fluid supply line31b. When the LS differential pressure is lower than the output pressure (absolute pressure) Pgr, the LS control valve212adecreases the output pressure by connecting its input side to the tank. The tilting control piston212cis a piston for the LS control which is supplied with the output pressure of the LS control valve212aand operates in the direction of decreasing the tilting (displacement) of the subsidiary pump202with the increase in the output pressure. The tilting control piston212dis a piston for the torque control (power control) which operates in the direction of decreasing the tilting (displacement) of the subsidiary pump202according to the pressure in the third hydraulic fluid supply line305of the subsidiary pump202.

The regulator312of the subsidiary pump302includes an LS control valve312aand tilting control pistons312cand312d. The LS control valve312aoperates according to differential pressure between the LS differential pressure (absolute pressure Pls4outputted by the differential pressure reducing valve411and the output pressure (absolute pressure) Pgr of the prime mover revolution speed detection valve13. When the LS differential pressure is higher than the output pressure (absolute pressure) Pgr, the LS control valve312aincreases the output pressure by connecting its input side to the pilot hydraulic fluid supply line31b. When the LS differential pressure is lower than the output pressure (absolute pressure) Pgr, the LS control valve312adecreases the output pressure by connecting its input side to the tank. The tilting control piston312cis a piston for the LS control which is supplied with the output pressure of the LS control valve312aand operates in the direction of decreasing the tilting (displacement) of the subsidiary pump302with the increase in the output pressure. The tilting control piston312dis a piston for the torque control (power control) which operates in the direction of decreasing the tilting (displacement) of the subsidiary pump302according to the pressure in the fourth hydraulic fluid supply line405of the subsidiary pump302.

The low-pressure selection valve112a, the LS control valve112band the tilting control piston112cof the regulator112(first pump control unit) constitute a first load sensing control unit which controls the displacement of the main pump102(first pump device) so that the delivery pressures of the first and second delivery ports102aand102bbecome higher by a target differential pressure than the maximum load pressure of the actuators driven by the hydraulic fluid delivered from the first and second delivery ports102aand102b. The LS control valve212aand the tilting control piston212cof the regulator212(second pump control unit) constitute a second load sensing control unit which controls the displacement of the subsidiary pump202(second pump device) so that the delivery pressure of the third delivery port202abecomes higher by a target differential pressure than the maximum load pressure of the actuators driven by the hydraulic fluid delivered from the third delivery port202a. The LS control valve312aand the tilting control piston312cof the regulator312(third pump control unit) constitute a third load sensing control unit which controls the displacement of the subsidiary pump302(third pump device) so that the delivery pressure of the fourth delivery port302abecomes higher by a target differential pressure than the maximum load pressure of the actuators driven by the hydraulic fluid delivered from the fourth delivery port302a.

The tilting control pistons112dand112e, the restrictors112hand112i, the pressure reducing valve112gand the tilting control piston112fof the regulator112(first pump control unit) constitute a torque control unit which decreases the displacement of the main pump102(first pump device) with the increase in the average pressure of the delivery pressures of the first and second delivery ports102aand102band decreases the displacement of the main pump102(first pump device) with the increase in the average pressure of the delivery pressures of the third and fourth delivery ports202aand302a. The tilting control piston212dof the regulator212(second pump control unit) constitutes a torque control unit which decreases the displacement of the subsidiary pump202(second pump device) with the increase in the delivery pressure of the third delivery port202a. The tilting control piston312dof the regulator312(third pump control unit) constitutes a torque control unit which decreases the displacement of the subsidiary pump302(third pump device) with the increase in the delivery pressure of the fourth delivery port302a.

The pilot pump30, the prime mover revolution speed detection valve13, the pilot relief valve32, the operation detection valves8a-8h, the shuttle valves9c-9j, the selector valves145,146,245and246, the boom operation detection hydraulic line52, the arm operation detection hydraulic line54, the travel combined operation detection hydraulic line53and the differential pressure reducing valves111,211,311and411constitute a control pressure generation circuit which generates pressure for controlling hydraulic elements such as the pressure compensating valves7a-7h, the unload valves115,215,315and415, the selector valves141,241and40, the regulator112(first pump control unit), the regulator212(second pump control unit) and the regulator312(third pump control unit).

FIG. 2is a schematic diagram showing the external appearance of the hydraulic excavator in which the hydraulic drive system explained above is installed.

Referring toFIG. 2, the hydraulic excavator (well known as an example of a work machine) comprises a lower track structure101, an upper swing structure109, and a front work implement104of the swinging type. The front work implement104is made up of a boom104a, an arm104band a bucket104c. The upper swing structure109can be rotated (swung) with respect to the lower track structure101by a swing motor3c. A swing post103is attached to the front of the upper swing structure109. The front work implement104is attached to the swing post103to be movable vertically. The swing post103can be rotated (swung) horizontally with respect to the upper swing structure109by the expansion and contraction of the swing cylinder3e. The boom104a, the arm104band the bucket104cof the front work implement104can be rotated vertically by the expansion and contraction of the boom cylinder3a, the arm cylinder3band the bucket cylinder3d, respectively. A blade106which is moved vertically by the expansion and contraction of the blade cylinder3h(seeFIG. 1) is attached to a center frame of the lower track structure101. The lower track structure101carries out the traveling of the hydraulic excavator by driving left and right crawlers101aand101bby the rotation of the travel motors3fand3g.

The upper swing structure109is provided with a cab108of the canopy type. Arranged in the cab108are a cab seat121, the left and right front/swing control lever units122and123(only the left side is shown inFIG. 2), the travel control lever units124aand124b, a swing control lever unit (unshown), a blade control lever unit (unshown), the gate lock lever24, and so forth. The control lever of each of the control lever units122and123can be operated in any direction with reference to the cross-hair directions from its neutral position. When the control lever of the left control lever unit122is operated in the longitudinal direction, the control lever unit122functions as a control lever unit for the swinging. When the control lever of the left control lever unit122is operated in the transverse direction, the control lever unit122functions as a control lever unit for the arm. When the control lever of the right control lever unit123is operated in the longitudinal direction, the control lever unit123functions as a control lever unit for the boom. When the control lever of the right control lever unit123is operated in the transverse direction, the control lever unit123functions as a control lever unit for the bucket.

Operation

The operation of this embodiment will be explained below by referring toFIG. 1.

First, the hydraulic fluid delivered from the fixed displacement pilot pump30driven by the prime mover1is supplied to the hydraulic fluid supply line31a. The hydraulic fluid supply line31ahas the prime mover revolution speed detection valve13. The prime mover revolution speed detection valve13uses the flow rate detection valve50and the differential pressure reducing valve51and thereby outputs the differential pressure across the flow rate detection valve50(which changes according to the delivery flow rate of the pilot pump30) as the absolute pressure Pgr. The pilot relief valve32connected downstream of the prime mover revolution speed detection valve13generates a fixed pressure in the pilot hydraulic fluid supply line31b.

(a) When all Control Levers are at Neutral Positions

All the flow control valves6a-6hare positioned at their neutral positions since all the control levers are at their neutral positions. The operation detection valves8aand8bare also positioned at their neutral positions since the flow control valves6aand6bare at their neutral positions.

The pilot hydraulic fluid in the pilot hydraulic fluid supply line31bis discharged to the tank via the restrictors42and44and the operation detection valves8aand8bat the neutral positions. Therefore, the pressures in the boom operation detection hydraulic line52and the arm operation detection hydraulic line54situated downstream of the restrictors42and44become equal to the tank pressure, and the pressures led to the selector valves141,241,145and245also become equal to the tank pressure. Each of the selector valves141,241,145and245is pushed upward inFIG. 1by a spring and held at the first position. The hydraulic fluid supplied from the first delivery port102aof the main pump102to the first hydraulic fluid supply line105is led to the unload valve115via the selector valve141. The hydraulic fluid supplied from the second delivery port102bof the main pump102to the second hydraulic fluid supply line205is led to the unload valve215via the selector valve241.

The pilot hydraulic fluid in the pilot hydraulic fluid supply line31bis discharged to the tank via the restrictor43and the operation detection valves8f,8g,8b,8h,8e,8d,8cand8aat the neutral positions. Therefore, the pressure in the travel combined operation detection hydraulic line53situated downstream of the restrictor43becomes equal to the tank pressure, and the pressures led to the selector valves40,146and246also become equal to the tank pressure. Each of the selector valves40,146and246is pushed upward inFIG. 1by the function of the spring and held at the first position.

By the selector valves146and246, the tank pressure is led to hydraulic lines downstream of the shuttle valves9fand9gvia the shuttle valves9iand9j.

The unload valve115is supplied with the maximum load pressure Plmax1of the actuators3a,3c,3dand3fvia the shuttle valves9c,9dand9f. The unload valve215is supplied with the maximum load pressure Plmax2of the actuators3b,3h,3eand3gvia the shuttle valves9e,9gand9h.

When all the flow control valves6a-6hare at their neutral positions, their load detection ports are connected to the tank. In this case, the shuttle valves9c,9dand9fand the shuttle valves9e,9gand9hdetect the tank pressure as the maximum load pressure Plmax1and the maximum load pressure Plmax2, respectively, and thus both of Plmax1and Plmax2are equal to the tank pressure. Accordingly, the pressures P1and P2in the first and second hydraulic fluid supply lines105and205are kept by the unload valves115and215at a prescribed pressure (spring-set pressure) Pun0that is set by the spring of each unload valve115,215(P1=Pun0, P2=Pun0). The spring-set pressure Pun0is generally set slightly higher than the output pressure Pgr of the prime mover revolution speed detection valve13(Pun0>Pgr).

The differential pressure reducing valve111outputs the differential pressure between the pressure P1in the first hydraulic fluid supply line105and the maximum load pressure Plmax1of the actuators3a,3c,3dand3f(LS differential pressure) as the absolute pressure Pls1. The differential pressure reducing valve211outputs the differential pressure between the pressure P2in the second hydraulic fluid supply line205and the maximum load pressure Plmax2of the actuators3b,3h,3eand3g(LS differential pressure) as the absolute pressure Pls2. When all the control levers are at the neutral positions, both of Plmax1and Plmax2are equal to the tank pressure as mentioned above, and thus relationships Pls1=P1−Plmax1=P1=Pun0>Pgr and Pls2=P2−Plmax2=P2=Pun0>Pgr are satisfied assuming that the tank pressure is 0. The lower pressure is selected by the low-pressure selection valve112afrom the LS differential pressures Pls1and Pls2and the selected lower pressure is led to the LS control valve112b.

Since Pls1or Pls2=Pun0>Pgr is satisfied when all the control levers are at the neutral positions, the LS control valve112bis pushed leftward inFIG. 1and switched to the right-hand position. At the right-hand position, the LS control valve112bleads the fixed pilot pressure generated by the pilot relief valve32to the load sensing control piston112c. Since the hydraulic fluid is led to the load sensing control piston112c, the displacement of the main pump102is maintained at the minimum level.

Meanwhile, the hydraulic fluid delivered from the subsidiary pumps202and302is led to the third and fourth hydraulic fluid supply lines305and405, respectively. Since the boom and arm flow control valves6aand6bare at the neutral positions and the operation detection valves8aand8bare also at the neutral positions as mentioned above, the selector valves145and245are pushed upward inFIG. 1by the springs and held at the first positions. To the unload valves315and415connected to the third and fourth hydraulic fluid supply lines305and405, the tank pressure is led as the load pressure. When all the control levers are at the neutral positions as mentioned above, the pressures P3and P4in the third and fourth hydraulic fluid supply lines305and405are kept by the unload valves315and415at the prescribed pressure Pun0set by the spring of each unload valve315,415(P3=Pun0, P4=Pun0). The prescribed pressure Pun0is generally set slightly higher than the output pressure Pgr of the prime mover revolution speed detection valve (Pun0>Pgr).

The differential pressure reducing valve311outputs the differential pressure between the pressure P3in the third hydraulic fluid supply line305and the tank pressure (LS differential pressure) as the absolute pressure Pls3. The differential pressure reducing valve411outputs the differential pressure between the pressure P4in the fourth hydraulic fluid supply line405and the tank pressure (LS differential pressure) as the absolute pressure Pls4. When all the control levers are at the neutral positions, relationships Pls3=P3−0=P3=Pun0>Pgr and Pls4=P4−0=P4=Pun0>Pgr are satisfied. The LS differential pressures Pls3and Pls4are led to the LS control valves212aand312a.

Since Pls3or Pls4>Pgr is satisfied when all the control levers are at the neutral positions, the LS control valves212aand312aare pushed leftward inFIG. 1and switched to the right-hand positions. At the right-hand positions, the LS control valves212aand312alead the fixed pilot pressure generated by the pilot relief valve32to the load sensing control pistons212cand312c. Since the hydraulic fluid is led to the load sensing control pistons212cand312c, the displacements of the subsidiary pumps202and302are maintained at the minimum level.

(b) When Boom Control Lever is Operated

When the boom control lever is operated in the direction of expanding the boom cylinder3a(i.e., boom raising direction), for example, the flow control valve6afor driving the boom cylinder3ais switched upward inFIG. 1. In response to the switching of the flow control valve6a, the operation detection valve8ais also switched, by which the hydraulic line for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictor42and the operation detection valve8ais interrupted and the pressure in the boom operation detection hydraulic line52rises to the pressure in the pilot hydraulic fluid supply line31b. Accordingly, the selector valves141and145are pushed downward inFIG. 1and switched to the second positions. When the selector valve141is switched to the second position, the hydraulic fluid in the first hydraulic fluid supply line105merges with the hydraulic fluid in the third hydraulic fluid supply line305via the selector valve141.

When the selector valve145is switched to the second position, the maximum load pressure Plmax1of the actuators3a,3c,3dand3fis led to the unload valve315and the differential pressure reducing valve311. In the single operation of the boom cylinder3a, the load pressure of the boom cylinder3ais led in the direction of closing the unload valve315via the internal channel and the load detection port of the flow control valve6a, the shuttle valve9cand the selector valve145. Accordingly, the set pressure of the unload valve315rises to the load pressure of the boom cylinder3aplus spring force and the hydraulic line for discharging the hydraulic fluid in the third hydraulic fluid supply line305to the tank is interrupted. Consequently, the merged hydraulic fluid from the first hydraulic fluid supply line105and the third hydraulic fluid supply line305is supplied to the boom cylinder3avia the pressure compensating valve7aand the flow control valve6a.

Meanwhile, the load pressure of the boom cylinder3ais led also to the differential pressure reducing valve111via the internal channel and the load detection port of the flow control valve6aand the shuttle valve9c, and to the differential pressure reducing valve311via the internal channel and the load detection port of the flow control valve6a, the shuttle valve9cand the selector valve145.

The differential pressure reducing valve111outputs the differential pressure between the pressure in the first hydraulic fluid supply line105and the load pressure of the boom cylinder3a(LS differential pressure) as the absolute pressure Pls1. The pressure Pls1is led to the left end face (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102.

The pressure Pls1is approximately 0 (Pls1≅0) since the difference between the pressure in the first hydraulic fluid supply line105and the load pressure of the boom cylinder3abecomes almost 0 just after the control lever is operated for activating the boom cylinder3a.

The LS differential pressure of each actuator driven by the second hydraulic fluid supply line205(i.e., Pls2) acts on the right end face (inFIG. 1) of the low-pressure selection valve112a. Since Pls2=P2=Pun0>Pgr holds as explained in the chapter (a), the low-pressure selection valve112aoutputs the pressure Pls1≅0 to the LS control valve112bas the lower pressure. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls1. Since the relationship Pls1≅0<Pgr holds just after the control lever is operated at the start of the boom raising, the LS control valve112bperforms the control so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement. The increase in the displacement continues until Pls1=Pgr is satisfied.

Meanwhile, the differential pressure reducing valve311outputs the differential pressure between the pressure P3in the third hydraulic fluid supply line305and the load pressure of the boom cylinder3a(LS differential pressure) as the absolute pressure Pls3. The pressure Pls3is led to the LS control valve212a. The LS control valve212acompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls3. Since the relationship Pls3≅0<Pgr holds just after the control lever is operated at the start of the boom raising, the LS control valve212aperforms the control so as to discharge the hydraulic fluid in the load sensing control piston212cto the tank. As the hydraulic fluid in the load sensing control piston212cis discharged to the tank, the subsidiary pump202increases its displacement. The increase in the displacement continues until Pls3=Pgr is satisfied.

As above, at times of the boom lever operation, the displacements of the main pump102and the subsidiary pump202are controlled appropriately by the functions of the regulators112and212of the main pump102and the subsidiary pump202so that the flow rate of the merged hydraulic fluid from the main pump102and the subsidiary pump202becomes equal to the demanded flow rate of the flow control valve6a.

(c) When Arm Control Lever is Operated

When the arm control lever is operated in the direction of expanding the arm cylinder3b(i.e., arm crowding direction), for example, the flow control valve6bfor driving the arm cylinder3bis switched upward inFIG. 1. In response to the switching of the flow control valve6b, the operation detection valve8bis also switched, by which the hydraulic line for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictor44and the operation detection valve8bis interrupted and the pressure in the arm operation detection hydraulic line54rises to the pressure in the pilot hydraulic fluid supply line31b. Accordingly, the selector valves241and245are pushed downward inFIG. 1and switched to the second positions. When the selector valve241is switched to the second position, the hydraulic fluid in the second hydraulic fluid supply line205merges with the hydraulic fluid in the fourth hydraulic fluid supply line405via the selector valve241.

When the selector valve245is switched to the second position, the maximum load pressure Plmax2of the actuators3b,3e,3gand3his led to the unload valve415and the differential pressure reducing valve411. In the single operation of the arm cylinder3b, the load pressure of the arm cylinder3bis led in the direction of closing the unload valve415via the internal channel and the load detection port of the flow control valve6b, the shuttle valve9hand the selector valve245. Accordingly, the set pressure of the unload valve415rises to the load pressure of the arm cylinder3bplus spring force and the hydraulic line for discharging the hydraulic fluid in the fourth hydraulic fluid supply line405to the tank is interrupted. Consequently, the merged hydraulic fluid from the second hydraulic fluid supply line205and the fourth hydraulic fluid supply line405is supplied to the arm cylinder3bvia the pressure compensating valve7band the flow control valve6b.

Meanwhile, the load pressure of the arm cylinder3bis led also to the differential pressure reducing valve211via the internal channel and the load detection port of the flow control valve6band the shuttle valve9h, and to the differential pressure reducing valve411via the internal channel and the load detection port of the flow control valve6b, the shuttle valve9hand the selector valve245.

The differential pressure reducing valve211outputs the differential pressure between the pressure in the second hydraulic fluid supply line205and the load pressure of the arm cylinder3b(LS differential pressure) as the absolute pressure Pls2. The pressure Pls2is led to the right end face (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102.

The pressure Pls2is approximately 0 (Pls2≅0) since the difference between the pressure in the second hydraulic fluid supply line205and the load pressure of the arm cylinder3bbecomes almost 0 just after the control lever is operated for activating the arm cylinder3b.

The LS differential pressure of each actuator driven by the first hydraulic fluid supply line105(i.e., Pls1) acts on the left end face (inFIG. 1) of the low-pressure selection valve112a. Since Pls1=P1=Pun0>Pgr holds as explained in the chapter (a), the low-pressure selection valve112aoutputs the pressure Pls2≅0 to the LS control valve112bas the lower pressure. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls2. Since the relationship Pls2≅0<Pgr holds just after the control lever is operated at the start of the arm crowding, the LS control valve112bis switched so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement. The increase in the displacement continues until Pls2=Pgr is satisfied.

Meanwhile, the differential pressure reducing valve411outputs the differential pressure between the pressure P4in the fourth hydraulic fluid supply line405and the load pressure of the arm cylinder3b(LS differential pressure) as the absolute pressure Pls4. The pressure Pls4is led to the LS control valve312a. The LS control valve312acompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls4. Since the relationship Pls4≅0<Pgr holds just after the control lever is operated at the start of the arm crowding, the LS control valve312aperforms the control so as to discharge the hydraulic fluid in the load sensing control piston312cto the tank. As the hydraulic fluid in the load sensing control piston312cis discharged to the tank, the subsidiary pump302increases its displacement. The increase in the displacement continues until Pls4=Pgr is satisfied.

As above, at times of the arm lever operation, the displacements of the main pump102and the subsidiary pump302are controlled appropriately by the functions of the regulators112and312of the main pump102and the subsidiary pump302so that the flow rate of the merged hydraulic fluid from the main pump102and the subsidiary pump302becomes equal to the demanded flow rate of the flow control valve6b.

(d) When Bucket Control Lever is Operated

When the bucket control lever is operated in the direction of expanding the bucket cylinder3d(i.e., bucket crowding direction), for example, the flow control valve6dfor driving the bucket cylinder3dis switched upward inFIG. 1. In response to the switching of the flow control valve6d, the operation detection valve8dis also switched. Since the operation detection valves8fand8gfor the flow control valves6fand6gfor driving the travel motors are at the neutral positions, the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor43is discharged to the tank. Accordingly, the pressure in the travel combined operation detection hydraulic line53becomes equal to the tank pressure. Consequently, the selector valve40is pushed upward inFIG. 1by the function of the spring and held at the first position and the first and second hydraulic fluid supply lines105and205are kept in the interrupted state.

The pressure in the boom operation detection hydraulic line52becomes equal to the tank pressure and the selector valves141and145are pushed upward inFIG. 1by the functions of the springs and held at the first positions since the boom control lever is not operated, the operation detection valve8ais at the neutral position and the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor42and the operation detection valve8ais discharged to the tank via the operation detection valve8a. Accordingly, the first hydraulic fluid supply line105is connected to the unload valve115and the tank pressure is led to the unload valve315and the differential pressure reducing valve311as the load pressure.

Similarly, the pressure in the arm operation detection hydraulic line54becomes equal to the tank pressure and the selector valves241and245are pushed upward inFIG. 1by the functions of the springs and held at the first positions since the arm control lever is not operated, the operation detection valve8bis at the neutral position and the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor44and the operation detection valve8bis discharged to the tank via the operation detection valve8b. Accordingly, the second hydraulic fluid supply line205is connected to the unload valve215and the tank pressure is led to the unload valve415and the differential pressure reducing valve411as the load pressure.

The load pressure of the bucket cylinder3dis led in the direction of closing the unload valve115via the internal channel and the detection port of the flow control valve6dand the shuttle valves9f,9dand9c. Accordingly, the set pressure of the unload valve115rises to the load pressure of the bucket cylinder3dplus spring force and the hydraulic line for discharging the hydraulic fluid in the first hydraulic fluid supply line105to the tank is interrupted. Consequently, the hydraulic fluid in the first hydraulic fluid supply line105is supplied to the bucket cylinder3dvia the pressure compensating valve7dand the flow control valve6d.

The load pressure of the bucket cylinder3dis led also to the differential pressure reducing valve111. The differential pressure reducing valve111outputs the differential pressure between the pressure in the first hydraulic fluid supply line105and the load pressure of the bucket cylinder3d(LS differential pressure) as the absolute pressure Pls1.

The pressure Pls1is led to the left end face (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102.

The pressure Pls1is approximately 0 (Pls1≅0) since the difference between the pressure in the first hydraulic fluid supply line105and the load pressure of the bucket cylinder3dbecomes almost 0 just after the control lever is operated for activating the bucket cylinder3d.

The LS differential pressure of each actuator driven by the second hydraulic fluid supply line205(i.e., Pls2) acts on the right end face (inFIG. 1) of the low-pressure selection valve112a. Since Pls2=P2=Pun0>Pgr holds as explained in the chapter (a), the low-pressure selection valve112aoutputs the pressure Pls1≅0 to the LS control valve112bas the lower pressure. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls1. Since the relationship Pls1≅0<Pgr holds just after the control lever is operated for activating the bucket cylinder3d, the LS control valve112bperforms the control so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement. The increase in the displacement continues until Pls1=Pgr is satisfied.

As above, at times of the bucket lever operation, the displacement of the main pump102is controlled appropriately by the function of the regulator112of the main pump102so that the flow rate of the hydraulic fluid delivered from the main pump102becomes equal to the demanded flow rate of the flow control valve6d.

Meanwhile, since the flow control valve6afor driving the boom cylinder3aand the flow control valve6bfor driving the arm cylinder3bare not switched, the tank pressure is led to the unload valves315and415and the differential pressure reducing valves311and411as the load pressure of each actuator. Accordingly, the hydraulic fluid in the third and fourth hydraulic fluid supply line305and405is discharged to the tank by the unload valves315and415. At this time, the pressures P3and P4in the third and fourth hydraulic fluid supply lines305and405are maintained at the pressure Pun0slightly higher than the pressure Pgr (target LS differential pressure) by the functions of the springs of the unload valves315and415.

Meanwhile, the outputs Pls3and Pls4of the differential pressure reducing valves311and411satisfy Pls3=P3=Pun0>Pgr and Pls4=P4=Pun0>Pgr. The pressures Pls3and Pls4are led to the right end faces (inFIG. 1) of the LS control valves212aand312a, respectively. The output pressure Pgr of the prime mover revolution speed detection valve13is led to the left end faces (inFIG. 1) of the LS control valves212aand312a. Since the above relationships hold, the LS control valves212aand312aare pushed leftward inFIG. 1and switched to the right-hand positions. At the right-hand positions, the LS control valves212aand312alead the pressure in the pilot hydraulic fluid supply line31bto the load sensing control pistons212cand312c. As the hydraulic fluid is led to the load sensing control pistons212cand312c, the subsidiary pumps202and302are controlled in the direction of decreasing the displacement and are maintained at the minimum displacement.

As above, at times of driving the bucket cylinder3dwhose demanded flow rate is low, the main pump102can be used at a point of higher efficiency since the bucket cylinder3dcan be driven by the main pump102alone.

(e) When Boom and Arm Control Levers are Operated at the Same Time

A case of performing the level smoothing operation (combined operation of the boom cylinder (high load, low flow rate) and the arm cylinder (low load, high flow rate)) will be explained below.

When the boom control lever is operated in the direction of expanding the boom cylinder3a(i.e., boom raising direction) and the arm control lever is operated in the direction of expanding the arm cylinder3b(i.e., arm crowding direction), the flow control valve6afor driving the boom cylinder3ais switched upward inFIG. 1and the flow control valve6bfor driving the arm cylinder3bis also switched upward inFIG. 1.

In response to the switching of the flow control valves6aand6b, the operation detection valves8aand8bare also switched, the hydraulic lines for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictors42and44and the operation detection valves8aand8bare interrupted, and the pressures in the boom operation detection hydraulic line52and the arm operation detection hydraulic line54rise to the pressure in the pilot hydraulic fluid supply line31b. Accordingly, the selector valves141,145,241and245are pushed downward inFIG. 1and switched to the second positions. When the selector valves141and241are switched to the second positions, the hydraulic fluid in the first hydraulic fluid supply line105merges with the hydraulic fluid in the third hydraulic fluid supply line305via the selector valve141and the hydraulic fluid in the second hydraulic fluid supply line205merges with the hydraulic fluid in the fourth hydraulic fluid supply line405via the selector valve241. When the selector valve145is switched to the second position, the maximum load pressure Plmax1of the actuators3a,3c,3dand3fis led to the unload valve315and the differential pressure reducing valve311. When the selector valve245is switched to the second position, the maximum load pressure Plmax2of the actuators3b,3e,3gand3his led to the unload valve415and the differential pressure reducing valve411.

In the combined operation of the boom cylinder3aand the arm cylinder3b, the load pressure of the boom cylinder3ais led in the direction of closing the unload valve315via the internal channel and the load detection port of the flow control valve6a, the shuttle valve9cand the selector valve145. Accordingly, the set pressure of the unload valve315rises to the load pressure of the boom cylinder3aplus spring force and the hydraulic line for discharging the hydraulic fluid in the third hydraulic fluid supply line305to the tank is interrupted. Meanwhile, the load pressure of the arm cylinder3bis led in the direction of closing the unload valve415via the internal channel and the load detection port of the flow control valve6b, the shuttle valve9hand the selector valve245. Accordingly, the set pressure of the unload valve415rises to the load pressure of the arm cylinder3bplus spring force and the hydraulic line for discharging the hydraulic fluid in the fourth hydraulic fluid supply line405to the tank is interrupted. Consequently, the merged hydraulic fluid from the first hydraulic fluid supply line105and the third hydraulic fluid supply line305is supplied to the boom cylinder3avia the pressure compensating valve7aand the flow control valve6a, and the merged hydraulic fluid from the second hydraulic fluid supply line205and the fourth hydraulic fluid supply line405is supplied to the arm cylinder3bvia the pressure compensating valve7band the flow control valve6b.

The load pressure of the boom cylinder3ais led to the differential pressure reducing valve111via the internal channel and the load detection port of the flow control valve6aand the shuttle valve9c, and also to the differential pressure reducing valve311via the selector valve145. The load pressure of the arm cylinder3bis led to the differential pressure reducing valve211via the internal channel and the load detection port of the flow control valve6band the shuttle valve9h, and also to the differential pressure reducing valve411via the selector valve245.

The differential pressure reducing valve111outputs the differential pressure between the pressure in the first hydraulic fluid supply line105and the load pressure of the boom cylinder3a(LS differential pressure) as the absolute pressure Pls1. The pressure Pls1is led to the left end face (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102. The differential pressure reducing valve211outputs the differential pressure between the pressure in the second hydraulic fluid supply line205and the load pressure of the arm cylinder3b(LS differential pressure) as the absolute pressure Pls2. The pressure Pls2is led to the right end face (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102.

The low-pressure selection valve112aoutputs the lower pressure selected from Pls1and Pls2to the LS control valve112b. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls1or Pls2. Since the relationship Pls1=Pls2≅0<Pgr holds just after the control levers are operated at the start of the boom raising and the arm crowding, the LS control valve112bis switched so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement and the delivery flow rates of the first and second delivery ports102aand102b.

In the level smoothing operation, Pls1>Pls2holds since a high flow rate is generally necessary for the arm cylinder as mentioned above. Therefore, when the delivery flow rates of the first and second delivery ports102aand102bincrease and the relationship Pls1>Pls2is satisfied, the low-pressure selection valve112aoutputs the lower pressure Pls2to the LS control valve112band increases the delivery flow rates of the first and second delivery ports102aand102bof the main pump102until Pls2=Pgr is satisfied.

The differential pressure reducing valve311outputs the differential pressure between the pressure in the third hydraulic fluid supply line305and the load pressure of the boom cylinder3a(LS differential pressure) as the absolute pressure Pls3. The pressure Pls3is led to the LS control valve212a. Since the flow rate of the boom cylinder is allowed to be low in the level smoothing operation, a flow higher than that required by the boom cylinder flows from the main pump102into the first hydraulic fluid supply line105, and thus the pressure Pls3increases above the target LS differential pressure Pgr. Since Pls3>Pgr is satisfied, the LS control valve212ais pushed leftward inFIG. 1and switched to the right-hand position, by which the hydraulic fluid is led from the pilot hydraulic fluid supply line31bto the load sensing control pistons212cand312c, the subsidiary pump202is controlled in the direction of decreasing the displacement, and the delivery flow rate of the subsidiary pump202is maintained at a low level.

From the unload valve315, unnecessary hydraulic fluid corresponding to the difference between the flow supplied from the main pump102and the subsidiary pump202and the flow supplied to the boom cylinder (remainder) is discharged to the first and third hydraulic fluid supply lines105and305.

Meanwhile, the differential pressure reducing valve411outputs the differential pressure between the pressure in the fourth hydraulic fluid supply line405and the load pressure of the arm cylinder3b(LS differential pressure) as the absolute pressure Pls4. The pressure Pls4is led to the LS control valve312a. The LS control valve312acompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls4, performs the control so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank as explained above, and increases the displacement of the subsidiary pump302until Pls4=Pgr is satisfied.

The pressure P1in the first hydraulic fluid supply line105of the main pump102and the pressure P3(=P1) in the third hydraulic fluid supply line305of the subsidiary pump202are maintained by the unload valve315at a pressure that is higher than the load pressure of the boom cylinder3aby the pressure Pun0set by the spring of the unload valve315(i.e., at a pressure that is the pressure Pun0higher than the load pressure of the boom cylinder3a). The pressure P2in the second hydraulic fluid supply line205of the main pump102and the pressure P4(=P2) in the fourth hydraulic fluid supply line405of the subsidiary pump302are maintained by the unload valve415at a pressure that is higher than the load pressure of the arm cylinder3bby the pressure Pun0set by the spring of the unload valve415(i.e., at a pressure that is the pressure Pun0higher than the load pressure of the arm cylinder3b).

In the level smoothing operation, P1=P3>P2=P4holds since the boom cylinder3aoperates at a high load and a low flow rate and the arm cylinder3boperates at a low load and a high flow rate as mentioned above.

As above, when the boom and arm control levers are operated at the same time (e.g., leveling operation), the boom cylinder of a high load pressure and the arm cylinder of a low load pressure are driven by hydraulic fluid flows supplied separately from the delivery ports102aand202aand the delivery ports102band302a. Therefore, the delivery pressures of the delivery ports102band302aon the arm cylinder3b's side (i.e., on the low load pressure actuator's side) can be controlled independently, by which the wasteful energy consumption due to the pressure loss in the pressure compensating valve7bof the arm cylinder (low load pressure actuator) can be suppressed.

Further, since the delivery flow rate of the subsidiary pump202specifically for the boom cylinder3aof a low demanded flow rate is maintained at a low level and the flow rate of the hydraulic fluid discharged from the unload valve315on the boom cylinder3a's side to the tank is low, the bleed-off loss of the unload valve315can be reduced and operation with still higher efficiency becomes possible.

The pressures P1and P2in the first and second hydraulic fluid supply lines105and205of the main pump102are led to the tilting control pistons112eand112dfor the torque control (power control), respectively, and the power control is performed with the average pressure of the pressures P1and P2. Meanwhile, the pressure P3in the third hydraulic fluid supply line305of the subsidiary pump202and the pressure P4in the fourth hydraulic fluid supply line405of the subsidiary pump302are led to the pressure reducing valve112gvia the restrictors112hand112i, respectively, and the output pressure of the pressure reducing valve112gis led to the tilting control piston112ffor the total torque control (total power control). In this case, the pressure led to the pressure reducing valve112gvia the restrictors112hand112iis the average pressure (intermediate pressure) of the pressures P3and P4and the power control is performed with the average pressure of the pressures P3and P4. As above, the torque control is performed on the main pump102of the split flow type not only with the average pressure of the pressures P1and P2but also with the average pressure of the pressures P3and P4. Therefore, when the delivery pressure of the first delivery port102aon the boom cylinder's side of the main pump102rises in the level smoothing operation and the total torque consumption of the main pump102and the subsidiary pumps202and302is about to exceed a prescribed value, the tilting control pistons112d,112eand112ffunction more preferentially than the load sensing control, restrict the increase in the displacement of the main pump102, and perform the control so that the total torque consumption of the main pump102and the subsidiary pumps202and302does not exceed the prescribed value. Consequently, even when the load pressure of the boom cylinder3ais high, the drop in the driving speed of the arm cylinder3bdue to a significant decrease in the displacement of the main pump102can be prevented and excellent operability in the combined operation can be secured.

Incidentally, while the above explanation has been given of the level smoothing operation in which the boom cylinder3aand the arm cylinder3bare driven, also when the load pressure of one actuator increases significantly in a combined operation of simultaneously driving two or more actuators arbitrarily selected from the actuators3a,3c,3dand3fof the first actuator group and the actuators3b,3e,3gand3hof the second actuator group, the displacement of the main pump102is controlled by the torque control not only with the average pressure of the pressures P1and P2but also with the average pressure of the pressures P3and P4, by which the drop in the driving speed of the actuator due to a significant decrease in the displacement of the main pump102can be prevented and excellent operability in the combined operation can be secured.

(f) When Left and Right Travel Control Levers are Operated

When the left and right travel control levers are operated, for example, the flow control valves6fand6gfor driving the travel motors3fand3gare switched upward inFIG. 1.

In response to the switching of the flow control valves6fand6g, the operation detection valves8fand8gare also switched. However, the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor43is discharged to the tank via the operation detection valves8b,8h,8e,8d,8cand8asince the operation detection valves8b,8h,8e,8d,8cand8afor the flow control valves6b,6h,6e,6d,6cand6afor driving the other actuators3b,3h,3e,3d,3cand3aare at the neutral positions. Accordingly, the pressure in the travel combined operation detection hydraulic line53becomes equal to the tank pressure, the selector valves40,146and246are pushed upward inFIG. 1by the functions of the springs and held at the first positions, the first and second hydraulic fluid supply lines105and205are interrupted (isolated from each other), and the tank pressure is led to the shuttle valves9jand9ivia the selector valves146and246, respectively.

Meanwhile, the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor42and the operation detection valve8ais discharged to the tank via the operation detection valve8a. Accordingly, the pressure in the boom operation detection hydraulic line52becomes equal to the tank pressure and the selector valves141and145are pushed upward inFIG. 1by the functions of the springs and held at the first positions. Therefore, the first hydraulic fluid supply line105is connected to the unload valve115and the tank pressure is led as the load pressures of the unload valve315and the differential pressure reducing valve311.

The hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor44and the operation detection valve8bis discharged to the tank via the operation detection valve8b. Accordingly, the pressure in the arm operation detection hydraulic line54becomes equal to the tank pressure and the selector valves241and245are pushed upward inFIG. 1by the functions of the springs and held at the first positions. Therefore, the second hydraulic fluid supply line205is connected to the unload valve215and the tank pressure is led as the load pressures of the unload valve415and the differential pressure reducing valve411.

The load pressure of the travel motor3fis led in the direction of closing the unload valve115via the internal channel and the detection port of the flow control valve6fand the shuttle valves9f,9dand9c. The load pressure of the travel motor3gis led in the direction of closing the unload valve215via the internal channel and the detection port of the flow control valve6gand the shuttle valves9g,9eand9h. Accordingly, the set pressure of each unload valve115/215rises to the load pressure of the travel motor3f/3gplus spring force and the hydraulic lines for discharging the hydraulic fluid in the first and second hydraulic fluid supply lines105and205to the tank are interrupted. Consequently, the hydraulic fluid in the first hydraulic fluid supply line105is supplied to the travel motor3fvia the pressure compensating valve7fand the flow control valve6f, while the hydraulic fluid in the third hydraulic fluid supply line305is supplied to the travel motor3gvia the pressure compensating valve7gand the flow control valve6g.

The load pressure of the travel motor3fis led also to the differential pressure reducing valve111via the internal channel and the detection port of the flow control valve6fand the shuttle valves9f,9dand9c, while the load pressure of the travel motor3gis led also to the differential pressure reducing valve211via the internal channel and the detection port of the flow control valve6gand the shuttle valves9g,9eand9h. The differential pressure reducing valve111outputs the differential pressure between the pressure in the first hydraulic fluid supply line105and the load pressure of the travel motor3f(LS differential pressure) as the absolute pressure Pls1, while the differential pressure reducing valve211outputs the differential pressure between the pressure in the second hydraulic fluid supply line205and the load pressure of the travel motor3g(LS differential pressure) as the absolute pressure Pls2. The pressures Pls1and Pls2are respectively led to the left and right end faces (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102.

Suppose that the load pressures of the left and right travel motors3fand3gare equal to each other just after the control levers are operated for activating the left and right travel motors3fand3g, Pls1=Pls2≅0 holds since the difference between the pressure in the first/second hydraulic fluid supply line105/205and the load pressure of the right/left travel motor3g/3gbecomes almost 0. The low-pressure selection valve112aoutputs Pls1=Pls2≅0 to the LS control valve112b. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls1or Pls2. Since Pls1=Pls2≅0<Pgr holds just after the control levers are operated for activating the travel motors3fand3g, the LS control valve112bperforms the control so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement. The increase in the displacement continues until Pls1or Pls2coincides with Pgr.

As above, at times of the travel lever operation, the displacement of the main pump102is controlled appropriately by the function of the regulator112of the main pump102so that the flow rate of the hydraulic fluid delivered from the main pump102becomes equal to the demanded flow rate of the flow control valves6fand6g.

Meanwhile, since the flow control valve6afor driving the boom cylinder3aand the flow control valve6bfor driving the arm cylinder3bare not switched, the tank pressure is led to the unload valves315and415and the differential pressure reducing valves311and411as the load pressure of each actuator. Accordingly, the hydraulic fluid in the third and fourth hydraulic fluid supply line305and405is discharged to the tank by the unload valves315and415. At this time, the pressures P3and P4in the third and fourth hydraulic fluid supply line305and405are maintained at the pressure Pun0slightly higher than the pressure Pgr (target LS differential pressure) by the functions of the springs of the unload valves315and415.

Meanwhile, the outputs Pls3and Pls4of the differential pressure reducing valves311and411satisfying Pls3=P3=Pun0>Pgr and Pls4=P4=Pun0>Pgr are led to the right end faces (inFIG. 1) of the LS control valves212aand312a, respectively. The output pressure Pgr of the prime mover revolution speed detection valve13is led to the left end faces (inFIG. 1) of the LS control valves212aand312a. Since the above relationships hold, the LS control valves212aand312aare pushed leftward inFIG. 1and switched to the right-hand positions. At the right-hand positions, the LS control valves212aand312alead the pressure in the pilot hydraulic fluid supply line31bto the load sensing control pistons212cand312c. As the hydraulic fluid is led to the load sensing control pistons212cand312c, the subsidiary pumps202and302are controlled in the direction of decreasing the displacement and are maintained at the minimum displacement.

As above, at times of the travel lever operation, the displacement of the main pump102is controlled appropriately so that the flow rate of the hydraulic fluid delivered from the main pump102becomes equal to the demanded flow rate of the flow control valves6fand6g. Therefore, when the left and right travel levers are operated at equal operation amounts with the intention of straight traveling, equal amounts of hydraulic fluid are supplied to the left and right travel motors from the first and second delivery ports102aand102bof the main pump102, by which the straight traveling property can be secured.

Further, the main pump102is a pump of the split flow type, the pressures P1and P2in the first and second hydraulic fluid supply lines105and205of the main pump102are led to the tilting control pistons112eand112dfor the torque control (power control), and the power control is performed with the average pressure of the pressures P1and P2. Therefore, the drop in the steering speed due to a significant decrease in the displacement of the main pump102(when the load pressure of one travel motor increased significantly in the travel steering operation) can be prevented and an excellent steering feel can be secured.

(g) When Travel Control Levers and Boom Control Lever are Operated at the Same Time

When the left and right travel control levers and the boom control lever (for the boom raising operation) are operated at the same time, for example, the flow control valves6fand6gfor driving the travel motors3fand3gand the flow control valve6afor driving the boom cylinder3aare switched upward inFIG. 1. In response to the switching of the flow control valves6fand6g, the operation detection valves8fand8gare also switched. In response to the switching of the flow control valve6a, the operation detection valve8ais also switched. By the switching of the operation detection valves8fand8g, the hydraulic lines for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictor43and the operation detection valves8aand8bare interrupted and the hydraulic line for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictor43and the operation detection valve8ais also interrupted. Accordingly, the pressure in the travel combined operation detection hydraulic line53becomes equal to the pressure in the pilot hydraulic fluid supply line31b, the selector valves40,146and246are pushed downward inFIG. 1and switched to the second positions, the first and second hydraulic fluid supply lines105and205are brought into communication with each other, the maximum load pressure Plmax1of the actuators3a,3c,3dand3fis led to the downstream side of the shuttle valve9gvia the shuttle valve9j, and the maximum load pressure Plmax2of the actuators3g,3eand3his led to the downstream side of the shuttle valve9fvia the shuttle valve9i.

By the switching of the operation detection valve8a, the hydraulic line for leading the hydraulic fluid in the pilot hydraulic fluid supply line31bto the tank via the restrictor42and the operation detection valve8ais interrupted, by which the pressure in the boom operation detection hydraulic line52becomes equal to the pressure in the pilot hydraulic fluid supply line31band the selector valves141and145are pushed downward inFIG. 1and switched to the second positions. Accordingly, the first hydraulic fluid supply line105connects with the third hydraulic fluid supply line305and the maximum load pressure of the actuators3a,3b,3c,3d,3f,3g,3eand3his led to the unload valve315and the differential pressure reducing valve311.

Meanwhile, since the hydraulic fluid supplied from the pilot hydraulic fluid supply line31bvia the restrictor44and the operation detection valve8bis discharged to the tank via the operation detection valve8b, the pressure in the arm operation detection hydraulic line54becomes equal to the tank pressure and the selector valves241and245are pushed upward inFIG. 1by the functions of the springs and held at the first positions. Accordingly, the second and fourth hydraulic fluid supply lines205and405are interrupted (isolated from each other), the second hydraulic fluid supply line205is connected to the unload valve215, and the maximum load pressure of the actuators3a,3b,3c,3d,3f,3g,3eand3his led to the unload valve215and the differential pressure reducing valve211.

Further, since the tank pressure is led to the unload valve415and the differential pressure reducing valve411connected to the fourth hydraulic fluid supply line405, the hydraulic fluid in the fourth hydraulic fluid supply line405is discharged to the tank by the unload valve415. At this time, the pressure P4in the fourth hydraulic fluid supply line405is maintained at the pressure Pun0slightly higher than the pressure Pgr (target LS differential pressure) by the function of the spring of the unload valve415. Thus, the output Pls4of the differential pressure reducing valve411satisfies Pls4=P4=Pun0>Pgr.

Suppose that the load pressures of the travel motors3fand3gare higher than the load pressure of the boom cylinder3a(e.g., the load pressures of the travel motors3fand3gare 10 MPa and the load pressure of the boom cylinder3ais 5 MPa) when the left and right traveling and the boom raising operation are performed, the load pressures 10 MPa of the travel motors3fand3g(as the maximum load pressure) are led in the directions of closing the unload valves315and215. Accordingly, the set pressure of each unload valve315/215rises to the load pressure of the travel motor3f/3gplus spring force and the hydraulic lines for discharging the hydraulic fluid in the hydraulic fluid supply lines105,205and305to the tank are interrupted. Consequently, the merged hydraulic fluid from the first hydraulic fluid supply line105, the second hydraulic fluid supply line205and the third hydraulic fluid supply line305is supplied to the travel motors3fand3gvia the pressure compensating valve7f, the flow control valve6f, the pressure compensating valve7gand the flow control valve6g, and to the boom cylinder3avia the pressure compensating valve7aand the flow control valve6a.

Meanwhile, each differential pressure reducing valve111/311/211outputs the difference between the pressure P1=P2=P3in the first/second/third hydraulic fluid supply line105/205/305and the maximum load pressure 10 MPa as the absolute pressure Pls1=Pls2=Pls3. The pressures Pls1and Pls2are respectively led to the left and right end faces (inFIG. 1) of the low-pressure selection valve112ain the regulator112of the main pump102. In this case, Pls1=Pls2=Pls3≅0 holds since the difference between the pressure in the first/second/third hydraulic fluid supply line105/205/305and the load pressure of the travel motors3gand3gbecomes almost 0 just after the control levers are operated for activating the travel motors3fand3gand the boom cylinder3a. The low-pressure selection valve112aoutputs the pressure Pls1=Pls2≅0 to the LS control valve112b. The LS control valve112bcompares the output pressure Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls1or Pls2. Since Pls1=Pls2≅0<Pgr holds just after the control levers are operated for activating the travel motors3fand3gand the boom cylinder3a, the LS control valve112bperforms the control so as to discharge the hydraulic fluid in the load sensing control piston112cto the tank. As the hydraulic fluid in the load sensing control piston112cis discharged to the tank, the main pump102increases its displacement. The increase in the displacement continues until Pls1or Pls2coincides with Pgr.

Assuming that Pgr=2 MPa, for example, when Pls1=Pls2=2 MPa is satisfied, the pressure P1/P2/P3in the first/second/third hydraulic fluid supply line105/205/305is controlled to be equal to the load pressure of the travel motors3fand3g(10 MPa+2 MPa=12 MPa). The pressure compensating valve7aconnected to the boom cylinder3acompensates for the difference (=12 MPa−5 MPa=7 MPa) between the pressure 12 Mpa in the third hydraulic fluid supply line305and the load pressure 5 MPa of the boom cylinder3a(pressure compensation) by controlling its own opening (aperture).

Meanwhile, in the regulator212of the subsidiary pump202, the aforementioned pressure Pls3≅0 is led to the right end face (inFIG. 1) of an LS control valve212b. The LS control valve212bcompares the output Pgr of the prime mover revolution speed detection valve13(target LS differential pressure) with the pressure Pls3. Since the relationship Pls3≅0<Pgr is satisfied, the LS control valve212bperforms the control so as to discharge the hydraulic fluid in the load sensing control piston212cto the tank. As the hydraulic fluid in the load sensing control piston212cis discharged to the tank, the subsidiary pump202increases its displacement. The increase in the displacement continues until Pls3=Pgr is satisfied.

As explained above, the displacements of the main pump102and the subsidiary pump202are controlled appropriately by the functions of the regulator112of the main pump102and the regulator212of the subsidiary pump202so that the flow rate of the hydraulic fluid delivered from the main pump102and the subsidiary pump202becomes equal to the sum total of the demanded flow rates of the flow control valves6a,6fand6g.

As above, in the combined operation of the traveling and the boom, three delivery ports (the first and second delivery ports102aand102bof the main pump102and the third delivery port202aof the subsidiary pump202) function as one delivery port and the flows of the hydraulic fluid from the three delivery ports are merged together and supplied to the left and right travel motors and the boom cylinder. Therefore, equal amounts of hydraulic fluid can be supplied to the left and right travel motors by operating the control levers of the left and right travel motors at equal input amounts (operation amounts). This makes it possible to drive the boom cylinder while maintaining the straight traveling property and to achieve excellent travel combined operation.

While the above explanation has been given of the combined operation of the traveling and the boom, excellent travel combined operation can be achieved similarly also in the combined operation of the traveling and the arm. In other combined operations in which the travel actuators and an actuator (other actuator) not for the boom or the arm are driven, the two delivery ports102aand102bof the main pump102function as one delivery port and the flows of the hydraulic fluid from the two delivery ports are merged together and supplied to the left and right travel motors and the other actuator. Also in such cases, it is possible to drive the other actuator while maintaining the straight traveling property and to achieve excellent travel combined operation.

Effects

As described above, the following effects can be achieved by this embodiment:

(1) When the boom and arm control levers are operated at the same time (e.g., leveling operation), the boom cylinder of a high load pressure and the arm cylinder of a low load pressure are driven by hydraulic fluid flows supplied separately from the delivery ports102aand202aand the delivery ports102band302a. Therefore, the delivery pressures of the delivery ports102band302aon the arm cylinder3b's side (i.e., on the low load pressure actuator's side) can be controlled independently, by which the wasteful energy consumption due to the pressure loss in the pressure compensating valve7bof the arm cylinder (low load pressure actuator) can be suppressed. Further, since the delivery flow rate of the subsidiary pump202specifically for the boom cylinder3aof a low demanded flow rate is suppressed to a low level and the flow rate of the hydraulic fluid discharged from the unload valve315of the boom cylinder3ato the tank is reduced, the bleed-off loss of the unload valve315can be reduced and operation with still higher efficiency becomes possible.

(2) At times of driving the bucket cylinder3dwhose demanded flow rate is low, the main pump102can be used at a point of higher efficiency since the bucket cylinder3dcan be driven by the main pump102alone without placing a burden on the subsidiary pump202or302.

(3) In the combined operation of the traveling and the boom, the flows of the hydraulic fluid from three delivery ports (the first and second delivery ports102aand102bof the main pump102and the third delivery port202aof the subsidiary pump202) are merged together and supplied to the left and right travel motors and the other actuator (e.g., boom cylinder). Therefore, equal amounts of hydraulic fluid can be supplied to the left and right travel motors by operating the control levers of the left and right travel motors at equal input amounts (operation amounts). This makes it possible to drive the other actuator (e.g., boom cylinder) while maintaining the straight traveling property and to achieve excellent travel combined operation.

(4) The displacement of the main pump102is controlled by the torque control with the average pressure of the delivery pressures of the first and second delivery ports102aand102band the average pressure of the delivery pressures of the third and fourth delivery ports202aand302a. Therefore, even in a combined operation in which the load pressure of one actuator increases significantly, the drop in the driving speed of the actuator due to a significant decrease in the displacement of the main pump102can be prevented and excellent operability in the combined operation can be secured. Especially, even when the load pressure of one travel motor increased significantly in the travel steering operation, the drop in the steering speed due to a significant decrease in the displacement of the main pump102can be prevented and an excellent steering feel can be secured.

Other Examples

While the above explanation of the embodiment has been given of a case where the construction machine is a hydraulic excavator and the first and second specific actuators are the boom cylinder3aand the arm cylinder3b, respectively, the first and second specific actuators can be actuators other than the boom cylinder or the arm cylinder as long as the actuators are those having greater demanded flow rates than other actuators and tending to have a great load pressure difference between each other when driven at the same time.

While the above explanation of the embodiment has been given of a case where the left and right travel motors3fand3gare the third and fourth specific actuators, the third and fourth specific actuators can be actuators other than the travel motors as long as the actuators are those achieving a prescribed function by having supply flow rates equivalent to each other when driven at the same time.

The present invention is applicable also to construction machines other than hydraulic excavators as long as the construction machine comprises actuators satisfying the above-described operating condition of the first and second specific actuators or the third and fourth specific actuators.

While the above explanation of the embodiment has been given of a case where the first pump device having the first and second delivery ports is the hydraulic pump102of the split flow type having the first and second delivery ports102aand102b, the first pump device may also be implemented by combining two variable displacement hydraulic pumps each having a single delivery port and driving two displacement control mechanisms (swash plates) of the two hydraulic pumps by use of the same regulator (pump control unit).

Furthermore, the load sensing system in the above embodiment is just an example and can be modified in various ways. For example, while the target differential pressure of the load sensing control is set in the above embodiment by arranging the differential pressure reducing valves for outputting the pump delivery pressures and the maximum load pressures as absolute pressures and leading the output pressures of the differential pressure reducing valves to the pressure compensating valves (to set a target compensation pressure) and to the LS control valves, it is also possible to lead the pump delivery pressures and the maximum load pressures to pressure control valves and LS control valves via separate hydraulic lines.

DESCRIPTION OF REFERENCE CHARACTERS

1: prime mover102: variable displacement main pump (first pump device)102a,102b: first and second delivery ports112: regulator (first pump control unit)112a: low-pressure selection valve112b: LS control valve112c: tilting control piston for LS control112d,112e: tilting control piston for torque control (power control)112g: pressure reducing valve112h,112i: restrictor112f: tilting control piston for total torque control (total power control)202: variable displacement subsidiary pump (second pump device)202a: third delivery port212: regulator (second pump control unit)212a: LS control valve212c: tilting control piston for LS control212d: tilting control piston for torque control (power control)302: variable displacement subsidiary pump (third pump device)302a: fourth delivery port312: regulator (third pump control unit)312a: LS control valve312c: tilting control piston for LS control312d: tilting control piston for torque control (power control)105: first hydraulic fluid supply line205: second hydraulic fluid supply line305: third hydraulic fluid supply line405: fourth hydraulic fluid supply line115: unload valve (first unload valve)215: unload valve (third unload valve)315: unload valve (second unload valve)415: unload valve (fourth unload valve)141: selector valve (first selector valve)241: selector valve (second selector valve)111,211,311,411: differential pressure reducing valve145,146,245,246: selector valve3a-3h: actuator3a: boom cylinder (first specific actuator)3b: arm cylinder (second specific actuator)3f,3g: left and right travel motors (third and fourth specific actuators)4: control valve unit6a-6h: flow control valve7a-7h: pressure compensating valve8a-8h: operation detection valve9c-9j: shuttle valve13: prime mover revolution speed detection valve24: gate lock lever30: pilot pump31a,31b,31c: pilot hydraulic fluid supply line32: pilot relief valve40: selector valve (third selector valve)52: boom operation detection hydraulic line53: travel combined operation detection hydraulic line54: arm operation detection hydraulic line42,43,44: restrictor100: gate lock valve122,123,124a,124b: control lever unit