When hydraulic fluid discharged from a hydraulic actuator is to be recovered for driving a different hydraulic actuator, the recovery frequency is increased to achieve further energy saving. To this end, a pressure increasing circuit 36 is provided in which a communication pressure increasing valve 12 is disposed in a communication passage 26 that connects a bottom side line 23 of and a rod side line 24 a boom cylinder 4. A recovery control valve 11 is controlled such that, when a first operation unit 5 is operated in a boom lowering direction (own weight falling direction of the boom) and a second operation unit 6 is operated simultaneously, only if the pressure at the bottom side of the boom cylinder 4 is higher than the pressure at the arm cylinder side that is a recovery destination of hydraulic fluid, the recovery control valve 11 is opened to recover the flow rate discharged from the bottom side of the boom cylinder 4 to the arm cylinder side.

Provided is a hydraulic shovel capable of moving a boom, an arm, and a bucket at respective adequate speeds even during complex operations thereof, without a significant pressure loss. This hydraulic shovel includes: a first pump (31) connected to a boom actuator (24) and a bucket actuator (28); a second pump (32) connected to an arm actuator (26) and the boom actuator (24); a third pump (33) connect to the arm actuator (26); a boom control valve (54) interposed between the first pump (31) and the boom actuator (24); an arm control valve (56) interposed between the second pump (32) and the arm actuator (26); a bucket control valve (58) interposed between the first pump (31) and the bucket actuator (28); a boom merging valve (55) for speed increase interposed between the second pump (32) and the boom actuator (24); and an arm merging valve (57) for speed increase interposed between the third pump (33) and the arm actuator (26).

Provided is a hydraulic drive system for a work machine configured with a single solenoid proportional valve for a regeneration circuit, wherein substantially the same actuator speed can be secured irrespective of whether or not hydraulic fluid discharged from a hydraulic actuator is regenerated for driving of another hydraulic actuator. The hydraulic drive system includes: a regeneration line that connects a bottom-side hydraulic chamber of a hydraulic cylinder 4 to a portion between a hydraulic pump device 50 and a second hydraulic actuator 8, and a regeneration flow rate adjustment device that supplies, at an adjusted flow rate, at least part of the discharged hydraulic fluid to a portion between the hydraulic pump device 50 and the second hydraulic actuator; a discharge flow rate adjustment device that discharges, at an adjusted flow rate, the discharged hydraulic fluid to a tank; one electric drive device 22 that simultaneously controls the regeneration flow rate adjustment device and the discharge flow rate adjustment device; and a control unit 27 that outputs a control command to the electric drive device in such a manner that falling speed of a first driven body does not vary significantly, irrespective of the magnitude of the regeneration flow rate caused by the regeneration flow rate adjustment device.

To provide a hydraulic drive system for a work machine capable of securing a favorable operability in the case where hydraulic fluid discharged from a hydraulic actuator is regenerated for driving other hydraulic actuator. The hydraulic drive system for a work machine includes: a regeneration line that connects a bottom-side hydraulic chamber of a hydraulic cylinder to a portion between a hydraulic pump and a second hydraulic actuator; a regeneration flow rate adjustment device that supplies at least part of the hydraulic fluid discharged from the bottom-side hydraulic chamber to a portion between the hydraulic pump and the second hydraulic actuator through the regeneration line; a differential pressure calculating section that reads a pressure in the bottom-side hydraulic chamber of the hydraulic cylinder detected by a first pressure sensor and a pressure between the hydraulic pump and the second hydraulic actuator detected by a second pressure sensor, and calculates a differential pressure, or a differential pressure sensor; and a control unit that controls the regeneration flow rate adjustment device such as to gradually increase the flow rate of the hydraulic fluid flowing through the regeneration line according to an increase in the differential pressure calculated by the differential pressure calculation section or in the differential pressure detected by the differential pressure sensor.

Provided is a hydraulic drive system including: first and second pumps; a first main supply fluid line and an optional supply fluid line leading to the first pump; a second main supply fluid line leading to the second pump; a main manipulating device for the main actuator; an optional manipulating device for the optional actuator; a bleed-off flow rate regulating section to change the bleed-off flow rate for the second pump; and a bleed-off control section to operate the bleed-off flow rate regulating section to change the discharge flow rate of the second pump in accordance with a control operation applied to the main manipulating device and to make a bleed-off flow rate in a specific combined manipulation state with simultaneous performance of a specific main control operation and an optional control operation be smaller than that in a single main manipulation state.

A hydraulic circuit of a construction machine including center bypass passages, into which a pressurized oil from hydraulic pumps is supplied, includes a directional control valve group in tandem with the center bypass passages; a bleed-off valve on a downstream side of each center bypass passage; and a merging circuit that merges the pressurized oil supplied into one center bypass passage and that in another center bypass passage, wherein each directional control valve includes a first internal passage that flows the pressurized oil into the center bypass passages, and a second internal passage that supplies the pressurized oil to a hydraulic actuator, wherein the center bypass passages and the first internal passage form a parallel passage, wherein an opening area of the bleed-off valve is changed, wherein the merging circuit includes a merging directional control valve that controls an inflow direction of the pressurized oil to be merged.

Disclosed is a hydraulic construction machinery capable of operating a travel apparatus and a swing apparatus, when same are to be operated in combination, by means of working fluids supplied at the maximum discharge flow rate from a plurality of hydraulic pumps. A hydraulic construction machinery according to the present invention comprises: first and second hydraulic pumps; travel and swing motors connected to the first hydraulic pump; a hydraulic actuator connected to the second hydraulic pump; a main control valve comprising a first control valve and a second control valve, the first control valve controlling a working fluid supplied from the first hydraulic pump to the travel and swing motors, and the second control valve controlling the working fluid supplied from the second hydraulic pump to the hydraulic actuator; a first regulating valve, disposed on the most upstream side of a supply channel of the second hydraulic pump, for switching when the travel and swing motors are operated simultaneously so that the working fluid of the first hydraulic pump is supplied to the travel motor and the working fluid of the second hydraulic pump is supplied to the swing motor; a second regulating valve, disposed on the most downstream side of a supply channel of the second hydraulic pump, for switching when the travel and swing motors are operated simultaneously so as to close the most downstream opening of the supply channel of the second hydraulic pump; and an electronic proportional valve for outputting a control signal, corresponding to a control signal from a controller, to the first and second regulating valves when a manipulation signal, sensing whether the travel and swing motors are manipulated, is entered in the controller.

To provide a work machine that makes it possible to drive actuators faster and more accurately by supplying flows to the actuators accurately at target rates without depending on load variations in a case where the machine body is controlled automatically by command inputs of a controller, while high operability is ensured for manual operation by an operator. In a case where a machine control function is cancelled via a machine control switch, a controller cancels limitation of the flow rate of a hydraulic fluid supplied to a plurality of directional control valves, the limitation being performed by the auxiliary flow rate control devices, and in a case where the machine control function is selected via the machine control switch, the controller causes the auxiliary flow rate control devices to limit the flow rate of the hydraulic fluid supplied to the plurality of directional control valves.

A fluid pressure control device for a power shovel includes: a first switching valve configured to permit or prohibit communication between a first pump and a first cylinder; a second switching valve configured to permit or prohibit communication between a second pump and a second cylinder; a third pump configured to allow to supply working fluid to the first cylinder and the second cylinder; a first junction control valve configured to permit or prohibit communication between the third pump and the first cylinder or the second cylinder; and a communication control valve configured to prohibit communication between the third pump and the first cylinder to guide the working fluid discharged by the third pump to the second cylinder by controlling the first junction control valve in a case where the second pump is communicated with the second cylinder by the second switching valve regardless of whether or not the first pump is communicated with the first cylinder by the first switching valve.

Disclosed is a hydraulic system for performing land preparation works by means of a simultaneous boom-up and arm-in operation. The hydraulic system according to the present invention includes: an arm cylinder and a boom cylinder that are connected to first and second hydraulic pumps, respectively; a first boom control valve that is disposed in the discharge flow path of the second hydraulic pump; a second boom control valve that is disposed in the discharge flow path of the first hydraulic pump and causes the working fluid of the first hydraulic pump to converge with the working fluid which is supplied from the second hydraulic pump to the boom cylinder; a first arm control valve that is disposed in the discharge flow path of the first hydraulic pump; a second arm control valve that is disposed in the discharge flow path of the second hydraulic pump and causes the working fluid of the second hydraulic pump to converge with the working fluid which is supplied from the first hydraulic pump to the arm cylinder; a recycle valve that is disposed in the flow path between the working fluid inlet port of the first arm control valve and a hydraulic tank; and a second boom control valve spool having a parallel pressure section in which the boom-up pilot pressure does not increase with respect to the boom-up strokes during the simultaneous boom-up and arm-in operation.