Work machine

Abstract

To provide a work machine that can avoid pressure the generation of a pressure loss in a switching valve at a position where return oil is not divided and avoid unexpected drive of another actuator when dividing the return oil, in a technique for reducing the pressure loss by dividing the return oil from an actuator into a pipe line of the other actuator by the switching valve.SOLUTION: The work machine includes: an openable / closable first switching valve 38 arranged on a connection pipeline 48 connecting a first pipeline of a pair of first actuator pipelines 43, 44 to a second pipeline of a pair of second actuator pipelines 45, 46; an openable / closable second switching valve 39 arranged on a tank pipeline 49 connecting the first pipeline to a tank 34; a pressure sensor 51 for detecting a pressure in the first pipeline; and a control device 60 for controlling the first and second switching valves. If there is no drive instruction for a first operation device 56 and there is a drive instruction for a second operation device 57, when a load index mark of a first hydraulic actuator based on a detected value by the pressure sensor satisfies predetermined conditions, the control device controls the first and second switching valves to be in opened states.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a work machine, and more particularly to a work machine configured so that return oil discharged from each hydraulic actuator that operates a driven body is returned to a hydraulic oil tank. [Background technology]

[0002] Some working machines, such as hydraulic excavators and cranes, are equipped with a hydraulic system (open circuit) configured to return return oil discharged from a plurality of hydraulic actuators that operate working devices, etc., to a hydraulic oil tank. Each hydraulic actuator is connected to a hydraulic pump and a tank via a directional control valve. The directional control valve controls the flow of pressurized oil supplied from the hydraulic pump to the hydraulic actuator, and also controls the flow of return oil discharged from the hydraulic actuator to the tank. Each directional control valve is connected to the hydraulic actuator by a pair of pipes, and is connected to the tank by one pipe. When the return oil discharged from the hydraulic actuator returns to the tank, if the pipe connecting the hydraulic actuator to the tank via the control valve becomes longer, the pressure loss occurring in the return oil increases accordingly.

[0003] As a technique for reducing the pressure loss of return oil discharged from a hydraulic actuator, for example, a hydraulic circuit control system described in Patent Document 1 has been proposed. In this hydraulic circuit control system, a common switching valve is installed on a first pipe connecting a first actuator and a first directional control valve and on a second pipe connecting a second actuator and a second directional control valve, and a sub-switching valve is installed on a sub-pipe connecting the second pipe and a tank. When the first actuator is operated and the second actuator is not operated or in an equivalent situation, the switching valve switches the first pipe and the second pipe from a blocked state to a communicating state, and the sub-switching valve switches the second pipe and the tank from a blocked state to a communicating state. The return oil discharged from the first actuator is divided into the first pipe and the second pipe by the switching valve and discharged to the tank via the first directional control valve or the sub-switching valve. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-219118 A Summary of the Invention [Problem to be solved by the invention]

[0005] In the hydraulic circuit control system described in Patent Document 1, a switching valve is installed on both a first pipe line connecting a first actuator and a first directional control valve and a second pipe line connecting a second actuator and a second directional control valve. When the switching valve is in a position to block both pipe lines, the switching valves located on both pipe lines connect the respective pipe lines, so there is a concern that excessive pressure loss will occur when the pressure oil supplied to the first actuator or the second actuator or the return oil discharged from the first actuator or the second actuator passes through the switching valve.

[0006] In order to avoid the occurrence of pressure loss caused by the pressure oil passing through the switching valve when the switching valve is in a position that blocks both lines, it is possible to consider not locating the switching valve on both lines. However, in this configuration, when the sub-switching valve switches the second line to a state where it is connected to the tank in order to discharge part of the return oil from the first actuator from the second line to the tank, it is assumed that the second actuator will also be connected to the tank from the second line via the sub-line. In this case, there is a concern that the second actuator will be unexpectedly actuated by its own weight without being operated by the operating device.

[0007] The present invention has been made to solve the above problems, and its object is to provide a work machine that, in a technology in which a portion of the return oil from one actuator is diverted to a pipeline of another actuator using a switching valve to reduce pressure loss of the return oil, can avoid the occurrence of pressure loss due to a switching valve in a position that does not divert the return oil, while avoiding unintended operation of another actuator when diverting the return oil. [Means for solving the problem]

[0008] The present application includes a plurality of means for solving the above-mentioned problems. One example thereof is a work machine including a hydraulic pump that discharges pressure oil, a first hydraulic actuator driven by pressure oil supplied from the hydraulic pump, a second hydraulic actuator driven by pressure oil supplied from the hydraulic pump, a tank that stores return oil discharged from the first hydraulic actuator and the second hydraulic actuator, a first control valve that controls the flow of pressure oil supplied from the hydraulic pump to the first hydraulic actuator and the flow of return oil discharged from the first hydraulic actuator to the tank, a second control valve that controls the flow of pressure oil supplied from the hydraulic pump to the second hydraulic actuator and the flow of return oil discharged from the second hydraulic actuator to the tank, a pair of first actuator lines connecting the first hydraulic actuator and the first control valve, a pair of second actuator lines connecting the second hydraulic actuator and the second control valve, a return line connecting the first control valve and the second control valve to the tank, a first operating device that instructs the operation of the first hydraulic actuator, and a second operating device that instructs the operation of the second hydraulic actuator, a connecting pipe connecting a first pipe which is one of the pair of first actuator pipes and a second pipe which is one of the pair of second actuator pipes; a tank pipe branching off from the first pipe and connected to the tank; a first switching valve arranged on the connecting pipe and capable of switching the connecting pipe between a blocked state and a connected state; a second switching valve arranged on the tank pipe and capable of switching the tank pipe between a blocked state and a connected state; and a pressure detection device which detects a pressure in at least the first pipe of the pair of first actuator pipes. and a control device that controls the first switching valve and the second switching valve, wherein when there is no drive command from the first operating device and the second operating device commands the second hydraulic actuator to be driven so as to discharge return oil to the second pipeline, when an index of the load acting on the first hydraulic actuator obtained based on the detection value of the pressure detection device satisfies a predetermined condition, the control device controls the first switching valve so that the connecting pipeline is connected, and controls the second switching valve so that the tank pipeline is connected. Effect of the Invention

[0009] According to the present invention, by disposing the first switching valve on the connecting pipeline, it is possible to avoid disposing the first switching valve on the pair of first actuator pipelines and the pair of second actuator pipelines, and therefore it is possible to avoid the occurrence of pressure loss due to the first switching valve when the return oil is not diverted. Also, only when the load index of the first hydraulic actuator satisfies a predetermined condition, the tank pipeline is opened by the second switching valve, so that the first actuator is not unintentionally driven at this time. That is, in the technology for reducing the pressure loss of the return oil by diverting a part of the return oil of the second actuator to the pipeline of the first actuator by the first switching valve, it is possible to avoid the occurrence of pressure loss due to the first switching valve in a position where the return oil is not diverted, and to avoid the unintentional driving of the first actuator when diverting the return oil. Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a side view showing a hydraulic excavator as a work machine according to a first embodiment of the present invention. [Diagram 2] 1 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a first embodiment. [Diagram 3] FIG. 3 is a schematic diagram showing the overall length of actuator pipes connected to each hydraulic cylinder in the hydraulic system of the work machine according to the first embodiment shown in FIG. 2 and the connection positions of connection pipes connected to both actuator pipes. [Figure 4] 3 is a hydraulic circuit diagram showing the switching positions of a first changeover valve and a second changeover valve when return oil is diverted in the hydraulic system of the work machine according to the first embodiment shown in FIG. 2. FIG. [Diagram 5] 3 is a block diagram showing an example of control logic for a first switching valve and a second switching valve during bucket operation in the control device for the work machine according to the first embodiment shown in FIG. 2. FIG. [Figure 6]FIG. 5 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a second embodiment of the present invention. [Figure 7] 7 is a block diagram showing an example of control logic for a first switching valve and a second switching valve during bucket operation in the control device for the work machine according to the second embodiment shown in FIG. 6. [Figure 8] FIG. 11 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a third embodiment of the present invention. [Figure 9] 9 is a block diagram showing an example of control logic for a first switching valve and a second switching valve during bucket operation in the control device for the work machine according to the third embodiment shown in FIG. 8. FIG. [Figure 10] FIG. 4 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a first example of other embodiments of the present invention. [Figure 11] FIG. 11 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a second example of other embodiments. [Figure 12] FIG. 11 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a third example of other embodiments. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a work machine according to the present invention will be described with reference to the drawings. In this embodiment, a hydraulic excavator will be described as an example of a work machine.

[0012] [First embodiment] First, a schematic configuration of a hydraulic excavator as a work machine according to a first embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a side view showing a hydraulic excavator as a work machine according to a first embodiment of the present invention. Here, the description will be given using the direction as seen by an operator seated in the driver's seat.

[0013] In Fig. 1, a hydraulic excavator as a work machine includes a self-propelled running body 1, a rotating body 2 rotatably mounted on the running body 1, and a working device 3 provided at the front of the rotating body 2 so as to be capable of being raised and lowered. The rotating body 2 is configured to be rotated relative to the running body 1 by a swing hydraulic motor 5 which is a hydraulic actuator.

[0014] The traveling body 1 has, for example, crawler-type traveling devices 11 (only the left one is shown in FIG. 1) on both the left and right sides. The left and right traveling devices 11 are each operated to travel by a traveling hydraulic motor (not shown) which is a hydraulic actuator.

[0015] The rotating body 2 includes a rotating frame 13 as a support structure rotatably mounted on the traveling body 1, a cab 14 installed on the left front side of the rotating frame 13, a counterweight 15 provided at the rear end of the rotating frame 13, and a machine room 16 provided between the cab 14 and the counterweight 15. Inside the cab 14, a driver's seat (not shown) where an operator sits, and operation devices 56, 57 (see FIG. 2 described later) described below are arranged. The counterweight 15 is for achieving a weight balance with the working device 3. The machine room 16 houses various devices such as a prime mover (not shown), a hydraulic pump 31 described later, and an assembly of multiple control valves 35, 36 (see FIG. 2 described later).

[0016] The working device 3 is for performing various operations such as excavation work, and is, for example, an articulated device configured by connecting a plurality of link members so as to be rotatable in the vertical direction. The plurality of link members are, for example, configured with a boom 18, an arm 19, and a bucket 20 as a working tool. The base end of the boom 18 is supported on the front part of the revolving frame 13 of the revolving body 2 so as to be rotatable in the vertical direction. The base end of the arm 19 is supported on the tip of the boom 18 so as to be rotatable in the vertical direction. The base end of the bucket 20 is supported on the tip of the arm 19 so as to be rotatable in the vertical direction. The boom 18, the arm 19, and the bucket 20 are driven by hydraulic actuators, namely a boom cylinder 21, an arm cylinder 32, and a bucket cylinder 33 (see also FIG. 2), respectively.

[0017] Next, the configuration of the hydraulic system provided in the work machine according to the first embodiment will be described with reference to Figures 2 and 3. Figure 2 is a hydraulic circuit diagram showing the configuration of the hydraulic system provided in the work machine according to the first embodiment. Figure 3 is a schematic diagram showing the overall length of actuator lines connected to each hydraulic cylinder in the hydraulic system of the work machine according to the first embodiment shown in Figure 2, and the connection positions of connection lines connected to both actuator lines.

[0018] The hydraulic excavator is equipped with a hydraulic system 30 that hydraulically operates a traveling body 1, a rotating body 2, and a working implement 3 (all see FIG. 1). The hydraulic system 30 shown in FIG. 2 shows only a hydraulic circuit for operating the arm 19 and bucket 20 of the working implement 3 as a hydraulic circuit that can clearly explain the present invention. Other hydraulic circuits are omitted.

[0019] 2, the hydraulic system 30 includes a hydraulic pump 31 that discharges pressure oil, an arm cylinder 32 and a bucket cylinder 33 as a plurality of hydraulic actuators driven by the pressure oil supplied from the hydraulic pump 31, and a tank 34 that stores return oil discharged from the arm cylinder 32 and the bucket cylinder 33. The hydraulic system 30 is configured to be controlled by a control device 60.

[0020] The hydraulic pump 31 also supplies pressure oil to other hydraulic actuators (not shown). The hydraulic pump 31 is, for example, a variable displacement pump, and has a regulator (not shown) that adjusts the pump volume. The hydraulic pump 31 is configured so that the pump volume (discharge flow rate) is adjusted in response to a command from the control device 60.

[0021] The arm cylinder 32 and the bucket cylinder 33 have bottom side oil chambers 32b, 33b (hereinafter referred to as bottom chambers) and rod side oil chambers 32r, 33r (hereinafter referred to as rod chambers), respectively. The arm cylinder 32 as a first hydraulic actuator is connected to the hydraulic pump 31 and a tank 34 via a first directional control valve 35. The bucket cylinder 33 as a second hydraulic actuator is connected to the hydraulic pump 31 and a tank 34 via a second directional control valve 36. The first directional control valve 35, the second directional control valve 36, and directional control valves (not shown) for the other hydraulic actuators are accommodated in the machine room 16 (see FIG. 1).

[0022] The first directional control valve 35 controls the flow of pressure oil supplied from the hydraulic pump 31 to the arm cylinder 32 and the flow of return oil discharged from the arm cylinder 32 to the tank 34, and is, for example, a three-position control valve. When in the neutral position (N), the first directional control valve 35 blocks the supply of pressure oil from the hydraulic pump 31 to the arm cylinder 32 and blocks the discharge of return oil from the arm cylinder 32 to the tank 34. When in the first position (A), the first directional control valve 35 supplies pressure oil from the hydraulic pump 31 to the bottom chamber 32b of the arm cylinder 32 and guides the return oil discharged from the rod chamber 32r of the arm cylinder 32 to the tank 34. When in the second position (B), the first directional control valve 35 supplies pressure oil from the hydraulic pump 31 to the rod chamber 32r of the arm cylinder 32 and guides the return oil discharged from the bottom chamber 32b of the arm cylinder 32 to the tank 34. The first directional control valve 35 is configured so that its position (flow direction) and stroke (flow rate) are controlled in response to a drive command (electrical signal) from the control device 60.

[0023] The first directional control valve 35 has a pump port connected to the hydraulic pump 31 via a discharge line 41, and a tank port connected to the tank 34 via a return line 42. The first directional control valve 35 also has a pair of actuator ports connected to the arm cylinder 32 via a pair of first actuator lines 43, 44. One line 43 of the pair of first actuator lines (hereinafter, bottom side line) is connected to the bottom chamber 32b of the arm cylinder 32, and the other line 44 (hereinafter, rod side line) is connected to the rod chamber 32r of the arm cylinder 32.

[0024] The second directional control valve 36 controls the flow of pressure oil supplied from the hydraulic pump 31 to the bucket cylinder 33 and the flow of return oil discharged from the bucket cylinder 33 to the tank 34, and is, for example, a three-position control valve. When in the neutral position (N), the second directional control valve 36 blocks the supply of pressure oil from the hydraulic pump 31 to the bucket cylinder 33 and blocks the discharge of return oil from the bucket cylinder 33 to the tank 34. When in the first position (A), the second directional control valve 36 supplies pressure oil from the hydraulic pump 31 to the bottom chamber 33b of the bucket cylinder 33 and guides the return oil discharged from the rod chamber 33r of the bucket cylinder 33 to the tank 34. When in the second position (B), the second directional control valve 36 supplies pressure oil from the hydraulic pump 31 to the rod chamber 33r of the bucket cylinder 33 and guides the return oil discharged from the bottom chamber 33b of the bucket cylinder 33 to the tank 34. The second directional control valve 36 is configured so that its position (flow direction) and stroke (flow rate) are controlled in response to a command Cd2 (electrical signal) from the control device 60.

[0025] The second directional control valve 36 has a pump port connected to the hydraulic pump 31 via a discharge pipe 41, and a tank port connected to the tank 34 via a return pipe 42. The second directional control valve 36 also has a pair of actuator ports connected to the bucket cylinder 33 via a pair of second actuator pipes 45, 46. One pipe 45 of the pair of second actuator pipes (hereinafter, bottom side pipe) is connected to the bottom chamber 33b of the bucket cylinder 33, and the other pipe 46 (hereinafter, rod side pipe) is connected to the rod chamber 33r of the bucket cylinder 33.

[0026] The bottom-side pipe 45 of the pair of second actuator pipes 45, 46 is connected to the bottom-side pipe 43 of the pair of first actuator pipes 43, 44 via a connecting pipe 48. That is, one end 48a of the connecting pipe 48 is connected to the bottom-side pipe 45 of the second actuator pipe, and the other end 48b is connected to the bottom-side pipe 43 of the first actuator pipe. As shown in FIG. 3, the connecting pipe 48 is connected to the bottom-side pipe 43 of the first actuator pipe and the bottom-side pipe 45 of the second actuator pipe at a position closer to the arm cylinder 32 and the bucket cylinder 33 than the first directional control valve 35 and the second directional control valve 36 that constitute a part of the assembly of control valves accommodated in the machine room 16 (see FIG. 1). This is intended to shorten the distance from the bottom chamber 33b of the bucket cylinder 33 in the bottom-side pipe 45 of the second actuator pipe to the connecting part 48a with the connecting pipe 48 as much as possible.

[0027] A first switching valve 38 capable of switching the connecting pipe 48 between a blocked state and a communicating state is disposed on the connecting pipe 48. The first switching valve 38 is, for example, a two-position electromagnetic proportional valve, and is configured as a normally closed type in which the normal position is a closed position (S) in a blocked state. The first switching valve 38 is configured such that the opening area is variable according to the stroke when in an open position (C) in a communicating state, for example, and the open / closed state and the stroke are controlled according to a command Cs1 (electrical signal) from the control device 60.

[0028] Of the pair of first actuator pipes 43, 44, the bottom-side pipe 43 connected to the other end 48b of the connecting pipe 48 is connected to the tank 34 via a tank pipe 49 branching off from the bottom-side pipe 43. That is, the tank pipe 49 connects the bottom-side pipe 43 to the tank 34 without passing through the first directional control valve 35. One end 49a of the tank pipe 49 is connected to a position closer to the first directional control valve 35 than the connecting portion 48b of the bottom-side pipe 43 of the first actuator pipe with the connecting pipe 48.

[0029] A second switching valve 39 capable of switching the tank pipe 49 between a blocked state and a connected state is disposed on the tank pipe 49. The second switching valve 39 is, for example, a two-position electromagnetic proportional valve, and is configured as a normally closed type in which the normal position is a closed position (S) in a blocked state. The second switching valve 39 is configured such that the opening area is variable according to the stroke when in an open position (C) in a connected state, for example, and the open / closed state and the stroke are controlled according to a command Cs2 (electrical signal) from the control device 60.

[0030] A pressure sensor 51 is provided in the bottom-side pipe 43 of the first actuator pipe line to which the other end 48b of the connection pipe line 48 and one end 49a of the tank pipe line 49 are connected, for detecting the pressure of the bottom-side pipe line 43. The pressure sensor 51 constitutes a pressure detection device that detects the pressure of at least the bottom-side pipe line 43 to which the connection pipe line 48 and the tank pipe line 49 are connected, among the pair of first actuator pipe lines 43, 44. The pressure of the bottom-side pipe line 43 corresponds to the pressure of the bottom chamber 32b of the arm cylinder 32 (hereinafter, sometimes referred to as bottom pressure). The pressure sensor 51 outputs a detection signal corresponding to the detected pressure value to the control device 60.

[0031] The control device 60 is electrically connected to the arm operating device 56 and the bucket operating device 57, and controls the hydraulic system 30 in response to the operation of the operating devices 56, 57. The arm operating device 56 and the bucket operating device 57 instruct the drive (operation) of the arm cylinder 32 (arm 19) and the bucket cylinder 33 (bucket 20), respectively, and output operation signals to the control device 60 in response to the operation (operation direction and operation amount) of the operating devices 56, 57.

[0032] The control device 60 is electrically connected to the first directional control valve 35 and the second directional control valve 36, and controls the positions and strokes of the first directional control valve 35 and the second directional control valve 36 in response to the operation of the operating devices 56, 57. The control device 60 is also electrically connected to the first switching valve 38 and the second switching valve 39, and controls the first switching valve 38 and the second switching valve 39 based on the operation of the operating devices 56, 57 and the detection value of the pressure sensor 51.

[0033] The control device 60 includes, as a hardware configuration, a storage device 60a including a RAM, a ROM, etc., and a processing device 60b including a CPU, an MPU, etc. The storage device 60a stores in advance programs and various information required for controlling the operation of the first directional control valve 35, the second directional control valve 36, the first switching valve 38, and the second switching valve 39 in the hydraulic system 30. The processing device 60b reads the programs and various information from the storage device 60a as appropriate, and also takes in operation signals from the operating devices 56, 57 and detection signals from the pressure sensor 51, and executes processing according to the programs to realize various functions. The control of the second directional control valve 36, the first switching valve 38, and the second switching valve 39 by the control device 60 will be described in detail later.

[0034] Next, the operation of the hydraulic system of the work machine according to the first embodiment will be described with reference to Figures 2 to 4. Figure 4 is a hydraulic circuit diagram showing the switching positions of the first and second changeover valves when the return oil is diverted in the hydraulic system of the work machine according to the first embodiment shown in Figure 2.

[0035] First, the operation of the hydraulic system 30 when the dump operation of the arm 19 (see FIG. 1) is performed alone will be described. When the arm operating device 56 shown in FIG. 2 is operated to perform the arm dump operation, the first directional control valve 35 is switched to the second position (B) side by a command from the control device 60. As a result, the pressure oil discharged from the hydraulic pump 31 passes through the rod side line 44 of the pair of first actuator lines 43, 44 from the discharge line 41 via the first directional control valve 35 and is supplied to the rod chamber 32r of the arm cylinder 32. As a result, the arm cylinder 32 contracts, and the arm 19 performs the dump operation. At this time, the return oil is discharged from the bottom chamber 32b of the arm cylinder 32 to the bottom side line 43 of the first actuator line. The flow rate of the return oil discharged to the bottom side line 43 is larger than the flow rate of the pressure oil supplied to the arm cylinder 32 by the amount that the cylinder cross-sectional area of ​​the bottom chamber 32b is larger than that of the rod chamber 32r. Therefore, the relatively large flow rate of return oil passing through the bottom-side pipe 43 tends to cause a relatively large pressure loss.

[0036] Therefore, in this embodiment, the second switching valve 39 is switched from the closed position (S) shown in FIG. 2 to the open position (C) side by a command from the control device 60. As a result, the bottom chamber 32b of the arm cylinder 32 communicates with the tank 34 via the bottom side pipe 43 of the first actuator pipe and the tank pipe 49. Therefore, a part of the return oil flowing out to the bottom side pipe 43 is discharged from the tank pipe 49 to the tank 34 via the second switching valve 39. In addition, the rest of the return oil flowing out to the bottom side pipe 43 is discharged to the tank 34 through the return pipe 42 via the first directional control valve 35. In this way, the return oil discharged from the bottom chamber 32b of the arm cylinder 32 to the bottom side pipe 43 of the first actuator pipe is divided into the first directional control valve 35 side and the second switching valve 39 side from the connection part 49a between the bottom side pipe 43 and the tank pipe 49 as a starting point, and is discharged to the tank 34. Therefore, the flow rate of the return oil passing through each pipe downstream of the connecting portion 49a is reduced by the amount of the return oil that is diverted, so that the pressure loss occurring in the return oil can be reduced.

[0037] Secondly, the operation of the hydraulic system 30 when the dump operation of the bucket 20 (see FIG. 1) is performed alone will be described. When the bucket operation device 57 shown in FIG. 2 is operated to dump the bucket, the second directional control valve 36 is switched to the second position (B) side by a command from the control device 60, as shown in FIG. 4. As a result, the pressure oil discharged from the hydraulic pump 31 passes through the rod side line 46 of the pair of second actuator lines 45, 46 from the discharge line 41 via the second directional control valve 36 and is supplied to the rod chamber 33r of the bucket cylinder 33. As a result, the bucket cylinder 33 contracts, and the bucket 20 performs the dump operation. At this time, the return oil is discharged from the bottom chamber 33b of the bucket cylinder 33 to the bottom side line 45 of the second actuator line. The flow rate of the return oil discharged to the bottom side line 45 is larger than the flow rate of the pressure oil supplied to the bucket cylinder 33 by the amount that the cylinder cross-sectional area of ​​the bottom chamber 33b is larger than that of the rod chamber 33r. Therefore, the relatively large flow rate of return oil passing through the bottom-side pipe 45 tends to cause a relatively large pressure loss.

[0038] In this embodiment, as shown in Fig. 4, both the first switching valve 38 and the second switching valve 39 are switched from the closed position (S) shown in Fig. 2 to the open position (C) side by a command from the control device 60. As a result, the bottom chamber 33b of the bucket cylinder 33 is communicated with the tank 34 through the tank pipe 49 from the bottom side pipe 45 of the second actuator pipe via the connecting pipe 48 and the bottom side pipe 43 of the first actuator pipe. Therefore, a part of the return oil discharged to the bottom side pipe 45 flows from the connecting pipe 48 through the first switching valve 38 in the open state into the bottom side pipe 43 of the first actuator pipe, and is discharged from the tank pipe 49 through the second switching valve 39 in the open state to the tank 34. The rest of the return oil discharged to the bottom side pipe 45 passes through the return pipe 42 via the second directional control valve 36 and is discharged to the tank 34. In this way, the return oil discharged from the bottom chamber 33b of the bucket cylinder 33 to the bottom-side pipe 45 of the second actuator pipe is diverted starting from the connection part 48a between the bottom-side pipe 45 and the connecting pipe 48, and is discharged to the tank 34. Therefore, since the flow rate passing through each pipe downstream of the connection part 48a is reduced by the amount of the return oil diverted, it is possible to reduce pressure loss occurring in the return oil.

[0039] In this embodiment, in addition to the bottom side pipe 43 of the first actuator pipe connected to the bottom chamber 32b of the arm cylinder 32 being connected to the tank 34 via the tank pipe 49, the bottom side pipe 45 of the second actuator pipe connected to the bottom chamber 33b of the bucket cylinder 33 is connected to the bottom side pipe 43 of the first actuator pipe via the connection pipe 48. Furthermore, the connection pipe 48 and the tank pipe 49 can be switched to a blocked state or a communicating state by the first switching valve 38 and the second switching valve 39, respectively. Therefore, not only the return oil from the bottom chamber 32b of the arm cylinder 32, but also the return oil from the bottom chamber 33b of the bucket cylinder 33 can be divided into two flow paths and discharged to the tank 34. This makes it possible to reduce pressure losses occurring in the return oil from the bottom chamber 32b of the arm cylinder 32 and the return oil from the bottom chamber 33b of the bucket cylinder 33.

[0040] In this embodiment, as shown in Fig. 3, a connection part 48a on the bottom-side pipe 45 side of the second actuator pipe in a connecting pipe 48 in which the first switching valve 38 is arranged is located closer to the bucket cylinder 33 side than the second directional control valve 36, and a connection part 48b on the bottom-side pipe 43 side of the first actuator pipe is located closer to the arm cylinder 32 side than the first directional control valve 35. In a hydraulic excavator, the pipes from the hydraulic cylinders that drive the working equipment to the directional control valves are generally long. For this reason, by positioning the branch point of the return oil from the bottom chamber 33b of the bucket cylinder 33 upstream in the flow direction of the return oil, the effect of reducing pressure loss is particularly large.

[0041] In the present embodiment, the first switching valve 38 is disposed on a connecting line 48 connected to the bottom side line 45 of the second actuator line and the bottom side line 43 of the first actuator line. That is, the first switching valve 38 is not disposed on a pair of first actuator lines 43, 44 connected to the arm cylinder 32, and is not disposed on a pair of second actuator lines 45, 46 connected to the bucket cylinder 33. For this reason, when the first switching valve 38 is in the closed position (S) which blocks the connecting line 48, even if pressure oil from the hydraulic pump 31 is supplied to the arm cylinder 32 or the bucket cylinder 33, no pressure loss occurs due to the first switching valve 38.

[0042] However, in the configuration of the hydraulic system 30 according to the present embodiment, when a part of the return oil from the bottom chamber 33b of the bucket cylinder 33 is discharged from the connecting pipe 48 to the tank 34 via the bottom side pipe 43 of the first actuator pipe and the tank pipe 49, the bottom chamber 33b of the arm cylinder 32 communicates with the tank 34 via the bottom side pipe 43 and the tank pipe 49. At this time, in a state where a load, for example, its own weight, acts on the arm cylinder 32 from the rod chamber 32r side toward the bottom chamber 32b side, there is a concern that the arm cylinder 32 may unintentionally fall due to its own weight even without an operation input of the arm operating device 56. Therefore, in the present embodiment, the control device 60 is configured to control the first switching valve 38 and the second switching valve 39 so as to avoid such unintentional driving of the arm cylinder 32.

[0043] Next, control of the bucket cylinder by the control device for the work machine according to the first embodiment will be described with reference to Figures 4 and 5. Figure 5 is a block diagram showing an example of control logic for the first and second selector valves during bucket operation in the control device for the work machine according to the first embodiment shown in Figure 2.

[0044] Here, the control of the control device 60 when the bucket 20 (see FIG. 1) is caused to perform a dump operation will be described. That is, the control when the bucket operating device 57 issues a command to drive the bucket cylinder 33 so as to discharge return oil to the bottom-side pipe 45 of the pair of second actuator pipes 45, 46 will be described. The control device 60 of this embodiment generally performs the following controls.

[0045] 4, the control device 60 controls the second directional control valve 36 to switch to the second position (B) side, thereby supplying pressure oil from the hydraulic pump 31 to the rod chamber 33r of the bucket cylinder 33 via the second directional control valve 36, and discharging a portion of the return oil discharged from the bottom chamber 33b of the bucket cylinder 33 from the bottom side pipe 45 of the second actuator pipe through the second directional control valve 36 and the return pipe 42 to the tank 34. Furthermore, by controlling both the first switching valve 38 and the second switching valve 39 to the open position (C), the remaining return oil discharged from the bottom chamber 33b of the bucket cylinder 33 is discharged from the connecting pipe 48 through the bottom side pipe 43 of the first actuator pipe and the tank pipe 49 to the tank 34 without passing through the second directional control valve 36.

[0046] 5 in response to the input of the operation signal Ibk of the bucket operating device 57 and the operation signal Iam of the arm operating device 56, and the input of the detection signal Pam_b of the pressure sensor 51. As a result, the control device 60 outputs a second control valve command Cd2 that commands the drive of the second directional control valve 36, a first switching valve command Cs1 that commands the drive of the first switching valve 38, and a second switching valve command Cs2 that commands the drive of the second switching valve 39.

[0047] Specifically, in FIG. 5 , the control device 60 has a function generator 61, a function generator 62, and a function generator 63 to which are input an operation signal Ibk (bucket operation signal) of the bucket operation device 57, an operation signal Iam (arm operation signal) of the arm operation device 56, and a detection signal Pam_b (detection signal corresponding to the bottom pressure of the arm cylinder 32) of the pressure sensor 51, respectively, an integrator 64 that integrates the output of the function generator 61 and the output of the function generator 62, an integrator 65 that integrates the output of the function generator 63 and the output of the integrator 64, and an output conversion unit 66, an output conversion unit 67, and an output conversion unit 68 that output commands to the second directional control valve, the first switching valve 38, and the second switching valve 39, which are to be controlled, respectively.

[0048] The function generator 61 calculates the opening of the second directional control valve 36 (see FIG. 4) according to the operation signal Ibk (the operation amount of the bucket operation device 57) input from the bucket operation device 57. The function generator 61 outputs the opening of the second directional control valve 36 as the calculation result to the integrator 64 and the output conversion unit 66. Note that in this case, this is a function generator for the case where an instruction is given to drive the bucket cylinder 33 so as to discharge the return oil to the second pipeline.

[0049] The function generator 62 calculates "1" when the operation signal Iam (operation amount of the arm operation device 56) input from the arm operation device 56 is smaller than the upper limit n of the dead zone of the arm operation device 56. The dead zone is a region in which the operation is invalid when the operation amount is smaller than a preset range. Also, the function generator 62 calculates "0" when the operation signal Iam is equal to or larger than the upper limit n of the dead zone. That is, the calculation result "1" corresponds to the case where there is no command to drive the arm cylinder 32, while the calculation result "0" corresponds to the case where there is a command to drive the arm cylinder 32. This prevents the first switching valve 38 and the second switching valve 39 from dividing the return oil of the bucket cylinder 33 into the bottom side pipe 43 of the first actuator pipe when controlling to drive the arm cylinder 32 in response to a drive command from the arm operation device 56. In this way, the function generator 62 has a function of judging the presence or absence of a drive command to the arm cylinder 32 based on the operation signal Iam of the arm operating device 56, and when there is a drive command to the arm cylinder 32, it outputs "0" which does not allow or prohibits the first switch valve 38 and the second switch valve 39 from switching to the open position (C), whereas when there is no drive command to the arm cylinder 32, it outputs "1" which allows the first switch valve 38 and the second switch valve 39 to switch to the open position (C). The function generator 62 outputs the calculation result of "0" or "1" to the integrator 64.

[0050] The function generator 63 calculates "0" when the detection signal Pam_b (arm cylinder bottom pressure detection signal) input from the pressure sensor 51 is equal to or greater than a positive threshold value α that is slightly larger than 0. On the other hand, when the detection signal Pam_b of the pressure sensor 51 is smaller than the threshold value α, the function generator 63 calculates "1."

[0051] When the pressure in the bottom-side conduit 43 of the first actuator conduit detected by the pressure sensor 51, i.e., the pressure in the bottom chamber 32b of the arm cylinder 32, is relatively higher than the pressure in the rod chamber 32r, as described above, there is a risk that the arm cylinder 32 will be unintentionally driven due to its own weight or the like when the second switching valve 39 is switched to the open position (C). When there is a risk of unintentional driving of the arm cylinder 32, it is necessary not to execute the diversion of the return oil of the bucket cylinder 33 by the switching control of the first switching valve 38 and the second switching valve 39. For this reason, the pressure in the bottom-side conduit 43 of the first actuator conduit detected by the pressure sensor 51 is useful as an index of the load, such as its own weight, acting on the arm cylinder 32, and can be used as a criterion for determining whether or not there is a possibility of unintentional driving of the arm cylinder 32 due to its own weight or the like.

[0052] Therefore, the function generator 63 is provided with a function for judging the presence or absence of the possibility of unintended actuation due to the weight of the arm cylinder 32 or the like based on the arm cylinder bottom pressure detection signal Pam_b of the pressure sensor 51. When the function generator 63 judges that there is a possibility of unintended actuation of the arm cylinder 32 from the comparison of the detection signal Pam_b of the pressure sensor 51 with the threshold value α, it outputs "0" which prohibits the first switching valve 38 and the second switching valve 39 from switching to the open position (C). On the other hand, when the function generator 63 judges that there is a low possibility of unintended actuation of the arm cylinder 32 from the comparison of the detection signal Pam_b of the pressure sensor 51 with the threshold value α, it outputs "1" which permits the first switching valve 38 and the second switching valve 39 to switch to the open position (C). The function generator 62 outputs the calculation result "0" or "1" to the integrator 65.

[0053] The integrator 64 integrates the opening degree of the second directional control valve 36, which is the calculation result output from the function generator 62, with "0" or "1". When the output of the function generator 62 is "0", the integrator 64 calculates "0" which does not allow or prohibits the first switching valve 38 and the second switching valve 39 from switching to the open position (C). On the other hand, when the output of the function generator 62 is "1", the integrator 64 calculates the opening degree of the second directional control valve 36, which is the output of the function generator 61. That is, when there is a drive command to the arm cylinder 32, the integrator 64 prevents the first switching valve 38 and the second switching valve 39 from switching to the open position (C). The integrator 64 outputs the calculation result "0" or the opening degree of the second directional control valve 36 to the integrator 65.

[0054] The integrator 65 integrates the calculation result "0" or "1" output from the function generator 63 with the calculation result "0" output from the integrator 64 or the opening degree of the second directional control valve 36. When the output of the integrator 64 is "0" or when the output of the function generator 63 is "0", the integrator 65 calculates "0" which does not allow or prohibits the switching control of the first switching valve 38 and the second switching valve 39 to the open position (C). On the other hand, when the output of the function generator 63 is "1" and the output of the integrator 64 is the opening degree of the second directional control valve 36, the integrator 65 calculates the opening degree of the second directional control valve 36. That is, even when there is no drive command for the arm cylinder 32, when there is a possibility of unintended driving due to the weight of the arm cylinder 32 or the like, the integrator 65 prevents the diversion of the return oil of the bucket cylinder 33 by the switching control of the first switching valve 38 and the second switching valve 39. Therefore, only when there is no command to drive the arm cylinder 32 and there is low possibility of unintended drive due to the arm cylinder 32's own weight or the like, the integrator 65 outputs the opening degree of the second directional control valve 36 in accordance with the operation of the bucket operating device 57. The integrator 65 outputs the calculation result of "0" or the opening degree of the second directional control valve 36 to the output conversion units 67, 68.

[0055] The output conversion unit 66 converts the output from the function generator 61 (the opening degree of the second directional control valve 36) into a command (electrical signal) to be input to the second directional control valve 36. The output conversion unit 66 outputs the converted command (electrical signal) to the second directional control valve 36 as a second control valve command Cd2. As a result, as shown in Fig. 4, the second directional control valve 36 is controlled to switch to the second position (B) in response to the operation of the bucket operating device 57 (the opening degree of the second directional control valve 36).

[0056] The output conversion unit 67 converts the output from the integrator 65 into a command (electrical signal) to be input to the first switching valve 38. The output conversion unit 67 outputs the converted command (electrical signal) to the first switching valve 38 as a first switching valve command Cs1. The integrator 65 outputs the opening degree of the second directional control valve 36 according to the operation of the bucket operation device 57 only when there is no drive command for the arm cylinder 32 and the possibility of unintended drive due to the arm cylinder 32's own weight or the like is low. For this reason, as shown in FIG. 4, the first switching valve 38 is switched to the open position (C) side according to the operation of the bucket operation device 57 only when there is no drive command for the arm cylinder 32 and the possibility of unintended drive due to the arm cylinder 32's own weight or the like is low. In other cases, the first switching valve 38 is maintained in the closed position (S) which is the normal position.

[0057] The output conversion unit 68 converts the output from the integrator 65 into a command (electrical signal) to be input to the second switching valve 39. The output conversion unit 68 outputs the converted command (electrical signal) to the second switching valve 39 as a second switching valve command Cs2. The integrator 65 outputs the opening degree of the second directional control valve 36 according to the operation of the bucket operation device 57 only when there is no drive command for the arm cylinder 32 and the possibility of unintended drive due to the arm cylinder 32's own weight or the like is low. For this reason, as shown in FIG. 4, the second switching valve 39 is switched to the open position (C) side according to the operation of the bucket operation device 57 only when there is no drive command for the arm cylinder 32 and the possibility of unintended drive due to the arm cylinder 32's own weight or the like is low. In other cases, the second switching valve 39 is maintained in the closed position (S) which is the normal position.

[0058] The control device 60 according to this embodiment configured as described above does not switch the first switching valve 38 and the second switching valve 39 to the open position (C) when the pressure in the bottom side pipe 43 of the first actuator pipe (the bottom pressure of the arm cylinder 32) detected by the pressure sensor 51 is equal to or higher than a predetermined positive threshold value α. This makes it possible to avoid unintended actuation of the arm cylinder 32 due to its own weight or the like caused by switching the first switching valve 38 and the second switching valve 39 to the open position (C).

[0059] The hydraulic excavator (work machine) according to the first embodiment described above includes a hydraulic pump 31 that discharges pressurized oil, an arm cylinder 32 as a first hydraulic actuator driven by the pressurized oil supplied from the hydraulic pump 31, a bucket cylinder 33 as a second hydraulic actuator driven by the pressurized oil supplied from the hydraulic pump 31, a tank 34 that stores return oil discharged from the arm cylinder 32 (first hydraulic actuator) and the bucket cylinder 33 (second hydraulic actuator), a first directional control valve 35 (first control valve) that controls the flow of pressurized oil supplied from the hydraulic pump 31 to the arm cylinder 32 (first hydraulic actuator) and the flow of return oil discharged from the arm cylinder 32 (first hydraulic actuator) to the tank 34, and a second directional control valve 36 (second control valve) that controls the flow of the pressurized oil supplied from the hydraulic pump 31 to the bucket cylinder 33 (second hydraulic actuator). the arm cylinder 32 (first hydraulic actuator) and the first directional control valve 35 (first control valve); a pair of first actuator lines 43, 44 connecting the arm cylinder 32 (first hydraulic actuator) and the first directional control valve 35 (first control valve); a pair of second actuator lines 45, 46 connecting the bucket cylinder 33 (second hydraulic actuator) and the second directional control valve 36 (second control valve); a return line 42 connecting the first directional control valve 35 (first control valve) and the second directional control valve 36 (second control valve) to the tank 34; an arm operating device 56 (first operating device) that instructs the operation of the arm cylinder 32 (first hydraulic actuator); and a bucket operating device 57 (second operating device) that instructs the operation of the bucket cylinder 33 (second hydraulic actuator).The system further includes a connecting line 48 that connects a bottom side line 43 (either one of the first lines) of the pair of first actuator lines 43, 44 and a bottom side line 45 (either one of the second lines) of the pair of second actuator lines 45, 46, a tank line 49 that branches off from the bottom side line 43 (first line) and is connected to the tank 34, a first switching valve 38 that is arranged on the connecting line 48 and can switch the connecting line 48 between a blocked state and a connected state, a second switching valve 39 that is arranged on the tank line 49 and can switch the tank line 49 between a blocked state and a connected state, a pressure sensor 51 as a pressure detection device that detects the pressure of at least the bottom side line 43 (first line) of the pair of first actuator lines 43, 44, and a control device 60 that controls the first switching valve 38 and the second switching valve 39. The control device 60 is configured to control the first switching valve 38 so that the connecting pipe 48 is in a connected state, and to control the second switching valve 39 so that the tank pipe 49 is in a connected state, when an index of the load acting on the arm cylinder 32 (first hydraulic actuator) obtained based on the detection value of the pressure sensor 51 (pressure detection device) satisfies a predetermined condition when there is no drive command from the arm operating device 56 (first operating device) and the bucket operating device 57 (second operating device) commands the driving of the bucket cylinder 33 (second hydraulic actuator) so as to discharge return oil to the bottom side pipe 45 (second pipe).

[0060] According to this configuration, by arranging the first switching valve 38 on the connecting pipe 48, it is possible to avoid arranging the first switching valve 38 on the pair of first actuator pipes 43, 44 and the pair of second actuator pipes 45, 46, and therefore it is possible to avoid the occurrence of pressure loss due to the first switching valve 38 when it is in a position where it does not divert return oil. Also, only when the load index of the arm cylinder 32 (first hydraulic actuator) satisfies a predetermined condition, the tank pipe 49 is brought into communication by the second switching valve 39, so that unintended actuation of the arm cylinder 32 (first hydraulic actuator) does not occur at this time. That is, in a technology for reducing pressure loss of the return oil by diverting a portion of the return oil of the bucket cylinder 33 (second hydraulic actuator) to the first actuator line 43 of the arm cylinder 32 (first hydraulic actuator) by the first switching valve 38, it is possible to avoid unintended operation of the arm cylinder 32 (first hydraulic actuator) when diverting the return oil, while avoiding the occurrence of pressure loss due to the first switching valve 38 being in a position that does not divert the return oil.

[0061] Also, in this embodiment, an indicator of the load acting on the arm cylinder 32 (first hydraulic actuator) is the pressure in the bottom side line 43 (first line) detected by the pressure sensor 51 (pressure detection device), and the specified condition is when the pressure in the bottom side line 43 (first line) detected by the pressure sensor 51 (pressure detection device) is lower than a preset positive threshold value α.

[0062] According to this configuration, the load index of the arm cylinder 32 (first hydraulic actuator) is calculated using only the pressure in the bottom side line 43 (first line) of the first actuator lines 43, 44, so the pressure detection device required to calculate the load index of the arm cylinder 32 (first hydraulic actuator) requires only the pressure sensor 51, thereby simplifying the pressure detection device.

[0063] In this embodiment, the connection part 48b of the connecting line 48 with the bottom side line 43 (first line) is located closer to the arm cylinder 32 (first hydraulic actuator) than the first directional control valve 35 (first control valve), and the connection part 48a of the connecting line 48 with the bottom side line 45 (second line) is located closer to the bucket cylinder 33 (second hydraulic actuator) than the second directional control valve 36 (second control valve).

[0064] According to this configuration, by positioning the point where the return oil from the bucket cylinder 33 (second hydraulic actuator) is diverted upstream in the flow direction of the return oil, the length of the pipeline downstream from the diverting point is increased, thereby enhancing the effect of reducing pressure loss of the return oil.

[0065] [Second embodiment] Next, a second embodiment of a work machine of the present invention will be described with reference to Figures 6 and 7. In Figures 6 and 7, the same reference numerals as those in Figures 1 to 5 denote similar parts, and detailed descriptions thereof will be omitted. Figure 6 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to the second embodiment.

[0066] The work machine according to the second embodiment differs from the first embodiment in that the hydraulic system 30A shown in Fig. 6 further includes a pressure sensor 52 installed in the rod side pipe 44 in addition to the pressure sensor 51 installed in the bottom side pipe 43 of the pair of first actuator pipes 43, 44, and that the control device 60A executes control of the first switching valve 38 and the second switching valve 39 using the detection value of the additional pressure sensor 52 in addition to the detection value of the pressure sensor 51. The other configurations of the work machine according to the second embodiment are similar to those of the work machine according to the first embodiment, and therefore description thereof will be omitted.

[0067] The pressure sensor 52 detects the pressure in the rod side pipe 44 of the pair of first actuator pipes 43, 44. The pressure sensor 52, together with the pressure sensor 51, constitutes a pressure detection device that detects the pressure in the pair of first actuator pipes 43, 44 connected to the arm cylinder 32. The pressure in the rod side pipe 44 corresponds to the pressure in the rod chamber 32r of the arm cylinder 32 to which the rod side pipe 44 is connected (hereinafter, may be referred to as rod pressure). The pressure sensor 52 outputs a detection signal according to the detected pressure value (corresponding to rod pressure) to the control device 60A.

[0068] Next, control of the bucket cylinder by the control device for a work machine according to the second embodiment will be described with reference to Figures 6 and 7. Figure 7 is a block diagram showing an example of control logic for the first and second switching valves during bucket operation in the control device for a work machine according to the second embodiment shown in Figure 6.

[0069] Here, too, a description will be given of the control of the control device 60A when the bucket 20 (see FIG. 1) is caused to execute a dump operation. That is, a description will be given of the control when the bucket operating device 57 shown in FIG. 6 instructs the bucket cylinder 33 to be driven so as to discharge return oil to the bottom-side pipe 45 of the pair of second actuator pipes 45, 46.

[0070] The control of the control device 60A for the work machine according to the second embodiment shown in FIG. 7 differs from the control of the control device 60 for the work machine according to the first embodiment in that an index of a new load acting on the arm cylinder 32 is calculated using the detection value of the pressure sensor 52 in addition to the detection value of the pressure sensor 51, and whether or not there is a possibility that the arm cylinder 32 will be unintentionally driven due to its own weight or the like is determined based on the calculated index of the new load. The pressure of the bottom side pipe 43 detected by the pressure sensor 51 corresponds to the bottom pressure of the arm cylinder 32, while the pressure of the rod side pipe 44 detected by the pressure sensor 52 corresponds to the rod pressure of the arm cylinder 32. The thrust of the arm cylinder 32 can be estimated from the bottom pressure and rod pressure of the arm cylinder 32. When the thrust of the arm cylinder 32 as a result of the calculation is in the opposite direction to the acting direction of its own weight (the direction in which the arm cylinder 32 extends), it can be determined that there is a low possibility that the arm cylinder 32 will be unintentionally driven due to its own weight or the like. Since the presence or absence of unintended driving of the arm cylinder 32 is determined based on the thrust of the arm cylinder 32, a more accurate determination is possible than in the first embodiment in which the determination is made based only on the detected pressure of the pressure sensor 51.

[0071] Specifically, the control device 60A includes a gain calculator 63b and a gain calculator 63c to which detection signals from the pressure sensors 51 and 52 are input, instead of the function generator 63 (see FIG. 5) of the control device 60 according to the first embodiment, a subtractor 63d that subtracts the output of the gain calculator 63b from the output of the gain calculator 63c, and a function generator 63e to which the calculation result of the subtractor 63d is input. The gain calculators 63b and 63c, the subtractor 63d, and the function generator 63e constitute a switching determiner 63A that sets "1" to permit switching control of the first switching valve 38 and the second switching valve 39 to the open position (C) side or "0" to prohibit switching control based on the detection values ​​of the pressure sensors 51 and 52.

[0072] The gain calculator 63b multiplies the detection signal Pam_b (pressure corresponding to the bottom pressure of the arm cylinder 32) input from the pressure sensor 51 by the pressure receiving area Kb on the bottom chamber 32b side of the arm cylinder 32. This calculation calculates the force acting on the bottom chamber 32b side. The calculation result of the gain calculator 63b is output to a subtractor 63d.

[0073] The gain calculator 63c multiplies the detection signal Pam_r (pressure equivalent to the rod pressure of the arm cylinder 32) input from the pressure sensor 52 by the pressure receiving area Kr of the rod chamber 32r of the arm cylinder 32. This calculation calculates the force acting on the rod chamber 32r side. The calculation result of the gain calculator 63c is output to the subtractor 63d.

[0074] The subtractor 63d calculates the difference by subtracting the thrust on the bottom chamber 32b side, which is the calculation result of the gain calculator 63b, from the thrust on the rod chamber 32r side, which is the calculation result of the gain calculator 63c, to calculate the thrust of the arm cylinder 32 (the direction in which the arm cylinder 32 extends is the positive direction). The thrust of the arm cylinder 32, which is the difference of the calculation result of the subtractor 63d, is output to a function generator 63e.

[0075] The function generator 63e judges whether or not there is a possibility of unintended actuation of the arm cylinder 32 due to its own weight or the like by switching the second switching valve 39 to the open position (C) side based on the thrust of the arm cylinder 32 which is the calculation result of the subtractor 63d, and outputs "1" which permits switching of the first switching valve 38 and the second switching valve 39 or "0" which prohibits switching according to the judgment result. In detail, when the thrust of the arm cylinder 32 which is the calculation result of the subtractor 63d is larger than the positive threshold value β, it judges that there is no possibility of unintended actuation of the arm cylinder 32 due to its own weight or the like, and calculates "1" which permits switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side. On the other hand, if the thrust of the arm cylinder 32, which is the calculation result of the subtractor 63d, is negative, or if the thrust is positive but below the threshold value β, it is determined that there is a possibility of unintended actuation due to the arm cylinder 32's own weight, etc., and calculates "0" to prohibit switching of the first switching valve 38 and the second switching valve 39. If the thrust of the arm cylinder 32 is positive but extremely small, it takes into consideration a situation in which the possibility of unexpected actuation of the arm cylinder 32 due to the action of a momentary force caused by a disturbance, etc. cannot be denied. The function generator 63e outputs the calculation result of "0" or "1" to the integrator 65.

[0076] As described above, the control device 60A according to this embodiment has a switching determiner 63A that is configured with the gain calculator 63b, the gain calculator 63c, the subtractor 63d, and the function generator 63e. The switching determiner 63A uses the detection value of the pressure sensor 51 and the detection value of the pressure sensor 52 to calculate the thrust of the arm cylinder 32 as an index of the load acting on the arm cylinder 32. If the thrust of the arm cylinder 32 as a result of the calculation satisfies a predetermined condition (if it is greater than the positive threshold value β), it determines that there is no possibility of unintended driving due to the arm cylinder 32's own weight, etc., and calculates "1" that permits switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side. On the other hand, if the thrust of the arm cylinder 32 resulting from the calculation does not satisfy the specified condition (if it is equal to or less than the positive threshold value β), it is determined that there is a possibility of unintended operation of the arm cylinder 32, and a value of "0" is calculated to prohibit switching of the first switching valve 38 and the second switching valve 39 to the open position (C).

[0077] Other functional units of the control device 60A according to the second embodiment are similar to the functional units of the control device 60 according to the first embodiment, and therefore the description thereof will be omitted.

[0078] According to the hydraulic excavator (work machine) of the second embodiment described above, as in the first embodiment, in a technology for reducing pressure loss of the return oil by diverting a portion of the return oil of the bucket cylinder 33 (second hydraulic actuator) to the first actuator line 43 of the arm cylinder 32 (first hydraulic actuator) by the first switching valve 38, it is possible to avoid unintended operation of the arm cylinder 32 (first hydraulic actuator) when diverting the return oil while avoiding the occurrence of pressure loss due to the first switching valve 38 being in a position that does not divert the return oil.

[0079] In the present embodiment, the pressure detection device has a pressure sensor 51 (first pressure sensor) that detects the pressure of the bottom side pipe 43 (first pipe) of the first actuator pipe 43, and a pressure sensor 52 (second pressure sensor) that detects the pressure of a rod side pipe (the other pipe) different from the bottom side pipe 43 (first pipe) of the pair of first actuator pipes 43, 44. In addition, an index of the load acting on the arm cylinder 32 (first hydraulic actuator) is calculated using the detection value of the pressure sensor 51 (first pressure sensor) and the detection value of the pressure sensor 52 (second pressure sensor).

[0080] According to this configuration, the detection value of pressure sensor 52 in addition to the detection value of pressure sensor 51 is used to calculate an index of the load acting on arm cylinder 32 and determine the possibility of unintended actuation due to the arm cylinder 32's own weight, etc., so that a more accurate determination is possible than in the first embodiment in which the determination is made only from the detection value of pressure sensor 51.

[0081] Also, in this embodiment, the first hydraulic actuator is the arm cylinder 32 (hydraulic cylinder), and the index of the load of the first hydraulic actuator is the thrust of the arm cylinder 32 (hydraulic cylinder) calculated using the detection value of the pressure sensor 51 (first pressure sensor) and the detection value of the pressure sensor 52 (second pressure sensor).

[0082] According to this configuration, when the first hydraulic actuator is the arm cylinder 32 (hydraulic cylinder), by using the thrust of the arm cylinder 32 (hydraulic cylinder) as an indicator of the load of the first hydraulic actuator, it is possible to accurately determine the possibility of unintended actuation of the arm cylinder 32 (hydraulic cylinder).

[0083] [Third embodiment] Next, a work machine according to a third embodiment of the present invention will be described with reference to Figures 8 and 9. In Figures 8 and 9, the same reference numerals as those shown in Figures 1 to 7 denote similar parts, and detailed descriptions thereof will be omitted. Figure 8 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to the third embodiment.

[0084] The work machine according to the third embodiment differs from the second embodiment in that a hydraulic system 30B shown in Fig. 8 changes the arm cylinder 32 of the hydraulic system 30A shown in Fig. 6 to a swing hydraulic motor 5, and in that a control device 60B partially changes the method of switching control of the first changeover valve 38 and the second changeover valve 39 in accordance with the change from the arm cylinder 32 to the swing hydraulic motor 5. Other configurations of the work machine according to the third embodiment are similar to those of the work machine according to the second embodiment, and therefore description thereof will be omitted.

[0085] Specifically, the swing hydraulic motor 5 has a first port 5f and a second port 5s. In the swing hydraulic motor 5, if the leakage amount is ignored, the flow rate of the inflow and outflow to the first port 5f and the flow rate of the inflow and outflow to the second port 5s are the same. That is, the volume of the first port 5f and the volume of the second port 5s are the same. This is a part different from the hydraulic cylinder (arm cylinder 32) having the bottom chamber 32b and the rod chamber 32r with different pressure receiving areas. The first port 5f and the second port 5s of the swing hydraulic motor 5 are connected to one side pipe 43 and the other side pipe 44 of the first actuator pipes 43 and 44, respectively. The driving of the swing hydraulic motor 5 is controlled by the control device 60B via the first directional control valve 35B. The first directional control valve 35B is controlled by the control device 60B in response to the operation of the swing operation device 56B. The pressure in one side line 43 of the first actuator line detected by the pressure sensor 51 corresponds to the pressure on the first port 5f side of the swing hydraulic motor 5. The pressure in the other side line 44 of the first actuator line detected by the pressure sensor 52 corresponds to the pressure on the second port 5s side of the swing hydraulic motor 5.

[0086] When the hydraulic excavator (see FIG. 1) is on a slope, the swing hydraulic motor 5 supports the weight of the rotating body 2 on one side in the inclination direction, so a load including the weight of the rotating body 2 acts on the swing hydraulic motor 5. For this reason, there is a risk that unintended rotational drive of the swing hydraulic motor 5 will occur due to the weight of the rotating body 2 without the operation of the operating device 56B when the second switching valve 39 is switched to the open position (C), and it is necessary to prevent such an unexpected swing operation of the rotating body 2 from occurring.

[0087] Therefore, the control device 60B of this embodiment, as in the second embodiment, calculates an index of the load acting on the swing hydraulic motor 5 using the detection values ​​of the pressure sensors 51 and 52, and determines whether or not there is a possibility of unintended rotational driving of the swing hydraulic motor 5 due to its own weight, etc., based on the load index obtained as a result of the calculation.

[0088] Next, control of the bucket cylinder by the control device for a work machine according to the third embodiment will be described with reference to Figures 8 and 9. Figure 9 is a block diagram showing an example of control logic for the first and second switching valves during bucket operation in the control device for a work machine according to the third embodiment shown in Figure 8.

[0089] Here, too, a description will be given of the control of the control device 60B when the bucket 20 (see FIG. 1) is caused to execute a dump operation. That is, a description will be given of the control when the bucket operating device 57 shown in FIG. 8 instructs the bucket cylinder 33 to be driven so as to discharge return oil to the bottom-side pipe 45 of the pair of second actuator pipes 45, 46.

[0090] The control of the control device 60B for the work machine according to the second embodiment shown in FIG. 9 differs from the control of the control device 60A for the work machine according to the second embodiment mainly in that an index of the load acting on the swing hydraulic motor 5 is calculated using the detection values ​​of the pressure sensors 51 and 52, and whether or not there is a possibility of unintended driving of the swing hydraulic motor 5 due to its own weight or the like is determined based on the load index of the calculation result. The pressure of the bottom side pipe 43 detected by the pressure sensor 51 corresponds to the pressure acting on the first port 5f of the swing hydraulic motor 5 (hereinafter sometimes referred to as the first port pressure), while the pressure of the rod side pipe 44 detected by the pressure sensor 52 corresponds to the pressure acting on the second port 5s of the swing hydraulic motor 5 (hereinafter sometimes referred to as the second port pressure). From the first port pressure and the second port pressure of the swing hydraulic motor 5, it is possible to calculate an index of the load corresponding to the torque acting on the swing hydraulic motor 5. When the direction of the load index of the calculation result is opposite to the direction of the weight (the direction in which rotation occurs such that return oil of the swing hydraulic motor 5 flows out from the second port 5s), it is possible to determine that there is no possibility of unintended driving due to the weight of the swing hydraulic motor 5. Note that in this embodiment, this determination is made based on the detection values ​​of both the pressure sensors 51 and 52, but it is also possible to make this determination using only the detection value of the pressure sensor 51, as in the first embodiment. In this way, it is possible to make a determination corresponding to the hydraulic motor rather than the hydraulic cylinder.

[0091] Specifically, the switching determination unit 63B of the control device 60B does not require the gain calculator 63b and the gain calculator 63c in the switching determination unit 63A of the control device 60A according to the second embodiment shown in Fig. 7, and has a subtractor 63g to which detection signals from the pressure sensors 51 and 52 are input instead of the subtractor 63d of the switching determination unit 63A, and a function generator 63h to which the calculation result of the subtractor 63g is input instead of the function generator 63e of the switching determination unit 63A. The switching determination unit 63B, which is composed of the subtractor 63g and the function generator 63h, determines whether or not there is a possibility of unintended driving of the swing hydraulic motor 5 by using the difference (pressure difference) between the detection value of the pressure sensor 51 and the detection value of the pressure sensor 52 as an index of the load acting on the swing hydraulic motor 5, and sets "1" which permits switching control of the first switching valve 38 and the second switching valve 39 to the open position (C) side, or "0" which prohibits it.

[0092] The subtractor 63g of the control device 60B subtracts the first port pressure (pressure in one side pipe 43) of the swing hydraulic motor 5, which is the detection value of the pressure sensor 51, from the second port pressure (pressure in the other side pipe 44) of the swing hydraulic motor 5, which is the detection value of the pressure sensor 52. The pressure difference between the second port pressure and the first port pressure of the swing hydraulic motor 5, which is the difference obtained by subtracting the detection value of the pressure sensor 51 from the detection value of the pressure sensor 52, corresponds to the torque (load) acting on the swing hydraulic motor 5 and can be used as an index of the load acting on the swing hydraulic motor 5. The calculation result of the subtractor 63g is output to a function generator 63h.

[0093] The function generator 63h judges whether or not there is a possibility of the swing hydraulic motor 5 being unintentionally driven to rotate due to its own weight or the like by switching the second switching valve 39 to the open position (C) side based on the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5, which is the calculation result of the subtractor 63g, and outputs "1" which permits the switching of the first switching valve 38 and the second switching valve 39 or "0" which prohibits it according to the judgment result. In detail, when the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5, which is the calculation result of the subtractor 63g, is larger than the positive threshold value γ, it judges that there is no possibility of the swing hydraulic motor 5 being unintentionally driven due to its own weight or the like, and calculates "1" which permits the switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side. On the other hand, if the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5, which is the calculation result of the subtractor 63g, is negative, or if the pressure difference is positive but below the threshold value γ, it is determined that there is a possibility of unintended driving due to the weight of the swing hydraulic motor 5, etc., and calculates "0" to prohibit switching of the first switching valve 38 and the second switching valve 39. If the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5 is extremely small even if it is positive, it takes into consideration a situation in which the possibility of unexpected driving of the swing hydraulic motor 5 due to the action of a momentary force caused by a disturbance, etc. cannot be denied. The function generator 63h outputs the calculation result "0" or "1" to the integrator 65.

[0094] As described above, the control device 60B according to this embodiment has a switching determiner 63B composed of a subtractor 63g and a function generator 63h. The switching determiner 63B uses the detection value of the pressure sensor 51 and the detection value of the pressure sensor 52 to calculate the pressure difference between the ports 5f, 5s of the swing hydraulic motor 5 as an index of the load acting on the swing hydraulic motor 5. If the calculated pressure difference between the ports 5f, 5s of the swing hydraulic motor 5 satisfies a predetermined condition (if it is greater than the positive threshold value γ), it is determined that there is no possibility of unintended driving due to the weight of the swing hydraulic motor 5 or the like, and calculates "1" which permits switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side. On the other hand, if the calculated pressure difference between both ports 5f, 5s of the swing hydraulic motor 5 does not satisfy the specified condition (if it is less than the positive threshold value γ), it is determined that there is a possibility of unintended operation of the swing hydraulic motor 5, and the value "0" is calculated to prohibit switching of the first switching valve 38 and the second switching valve 39 to the open position (C).

[0095] Other functional units of the control device 60B according to the third embodiment are similar to the functional units of the control device 60A according to the second embodiment (see FIG. 7), and therefore the description thereof will be omitted.

[0096] According to the hydraulic excavator (work machine) of the third embodiment described above, as in the second embodiment, in a technology for reducing pressure loss of the return oil by diverting a portion of the return oil of the bucket cylinder 33 (second hydraulic actuator) to the first actuator line 43 of the arm cylinder 32 (first hydraulic actuator) by the first switching valve 38, it is possible to avoid unintended driving of the swing hydraulic motor 5 (first hydraulic actuator) when diverting the return oil while avoiding the occurrence of pressure loss due to the first switching valve 38 being in a position where the return oil is not diverted.

[0097] In addition, in this embodiment, the first hydraulic actuator is a swing hydraulic motor 5 (hydraulic motor), and an index of the load of the first hydraulic actuator is the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5, calculated from the difference between the detection value of the pressure sensor 51 (first pressure sensor) and the detection value of the pressure sensor 52 (second pressure sensor).

[0098] According to this configuration, when the first hydraulic actuator is a swing hydraulic motor 5 (hydraulic motor), by using the pressure difference between both ports 5f, 5s of the swing hydraulic motor 5 (hydraulic motor) as an indicator of the load on the first hydraulic actuator, it is possible to accurately determine the possibility of unintended driving of the swing hydraulic motor 5 (hydraulic motor).

[0099] [Other embodiments] In the above-described first to third embodiments, an example has been shown in which the present invention is applied to a hydraulic excavator. However, the present invention can be widely applied to various types of work machines configured so that return oil discharged from each of the hydraulic actuators 21, 32, 33, and 5 is returned to the tank 34.

[0100] Furthermore, the present invention is not limited to the above-described embodiment, and various modified examples are included. The above-described embodiment has been described in detail to easily explain the present invention, and is not necessarily limited to those having all of the configurations described. For example, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. It is also possible to add, delete, or replace a part of the configuration of each embodiment with another configuration.

[0101] For example, in the above-described embodiment, the first directional control valve 35 and the second directional control valve 36 of the hydraulic systems 30, 30A, 30B are driven by commands (electrical signals) from the control devices 60, 60A, 60B. However, the first directional control valve 35 and the second directional control valve 36 can also be configured as hydraulic pilot type control valves driven by pilot pressures generated by the operating devices 56, 56B and 57.

[0102] In addition, the control devices 60, 60A, and 60B according to the above-described embodiments are configured such that the processing device 60b interprets and executes a program to realize each functional unit. Information such as programs, tables, and files that realize each function can be stored in the storage device 60a, as well as in recording media such as IC cards, SD cards, and DVDs. However, it is also possible to realize a part or all of the software in hardware by designing it as an integrated circuit, for example.

[0103] In the above-described embodiment, the connecting pipe 48 of the hydraulic system 30, 30A, 30B is connected to the bottom-side pipe 43 of the pair of first actuator pipes 43, 44, and is also connected to the bottom-side pipe 45 of the pair of second actuator pipes 45, 46. However, the connecting pipe may be any pipe that connects a first pipe, which is one of the pair of first actuator pipes 43, 44, and a second pipe, which is one of the pair of second actuator pipes 45, 46. In this case, the tank pipe is configured to branch off from the first pipe, which is one of the pair of first actuator pipes 43, 44. The combination of the connecting pipes and the tank pipes configured in this way will be described with reference to Figs. 10 to 12.

[0104] Fig. 10 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a first example of another embodiment of the present invention. In the hydraulic system 30C shown in Fig. 10, a connecting line 48C is connected to the rod side line 44 of the pair of first actuator lines 43, 44, and is also connected to the rod side line 46 of the pair of second actuator lines 45, 46. In this case, one end 49a of the tank line 49C is connected to the rod side line 44 of the first actuator line to which the other end 48b of the connecting line 48C is connected. As a result, by switching the first switching valve 38 and the second switching valve 39 to the open position (C), a part of the return oil discharged from the rod chamber 33r of the bucket cylinder 33 to the rod side line 46 can be discharged from the connecting line 48C through the rod side line 44 of the first actuator line through the tank line 49C to the tank 34.

[0105] In this case, the control device 60C can be configured to calculate an index of the load of the arm cylinder 32 using only the detection value (rod pressure of the arm cylinder 32) of the pressure sensor 52, as in the first embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the calculation result. This makes it possible to avoid unintended driving due to the weight of the arm cylinder 32, etc. Also, the control device 60C can be configured to calculate the thrust of the arm cylinder 32 using the detection value (rod pressure of the arm cylinder 32) of the pressure sensor 52 and the detection value (bottom pressure of the arm cylinder 32) of the pressure sensor 51, as in the second embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the thrust of the calculation result. However, the positive direction of the thrust of the arm cylinder 32 is set in the opposite direction to that of the second embodiment. This makes it possible to reliably avoid unintended driving due to the weight of the arm cylinder 32, etc.

[0106] 11 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a second example of another embodiment. In a hydraulic system 30D shown in FIG. 11, a connecting line 48D is connected to the rod side line 44 of a pair of first actuator lines 43, 44, and is also connected to the bottom side line 45 of a pair of second actuator lines 45, 46. In this case, one end 49a of a tank line 49D is connected to the rod side line 44 of the first actuator line to which the other end 48b of the connecting line 48D is connected. This allows a part of the return oil discharged from the bottom chamber 33b of the bucket cylinder 33 to the bottom side line 45 to be discharged from the connecting line 48D through the rod side line 44 of the first actuator line and through the tank line 49D to the tank 34 by switching the first switching valve 38 and the second switching valve 39 to the open position (C).

[0107] In this case, the control device 60D can be configured to calculate an index of the load of the arm cylinder 32 using only the detection value (rod pressure of the arm cylinder 32) of the pressure sensor 52, as in the first embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the calculation result. This makes it possible to avoid unintended driving due to the weight of the arm cylinder 32, etc. Also, the control device 60D can be configured to calculate the thrust of the arm cylinder 32 using the detection value (rod pressure of the arm cylinder 32) of the pressure sensor 52 and the detection value (bottom pressure of the arm cylinder 32) of the pressure sensor 51, as in the second embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the thrust of the calculation result. However, the positive direction of the thrust of the arm cylinder 32 is set in the opposite direction to that of the second embodiment. This makes it possible to reliably avoid unintended driving due to the weight of the arm cylinder 32, etc.

[0108] 12 is a hydraulic circuit diagram showing the configuration of a hydraulic system provided in a work machine according to a third example of other embodiments. In the hydraulic system 30E shown in FIG. 12, a connection line 48E is connected to the bottom side line 43 of the pair of first actuator lines 43, 44, and is also connected to the rod side line 46 of the pair of second actuator lines 45, 46. In this case, one end 49a of the tank line 49 is connected to the bottom side line 43 of the first actuator line to which the other end 48b of the connection line 48D is connected. As a result, by switching the first switching valve 38 and the second switching valve 39 to the open position (C), a part of the return oil discharged from the rod chamber 33r of the bucket cylinder 33 to the rod side line 46 can be discharged from the connection line 48E through the bottom side line 43 of the first actuator line, through the tank line 49, and into the tank 34.

[0109] In this case, the control device 60E can be configured to calculate an index of the load of the arm cylinder 32 using only the detection value of the pressure sensor 51 (bottom pressure of the arm cylinder 32) as in the first embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the calculation result. This makes it possible to avoid unintended driving due to the weight of the arm cylinder 32 or the like. Also, the control device 60E can be configured to calculate the thrust of the arm cylinder 32 using the detection value of the pressure sensor 52 (rod pressure of the arm cylinder 32) and the detection value of the pressure sensor 51 (bottom pressure of the arm cylinder 32) as in the second embodiment, and set the allowance of switching of the first switching valve 38 and the second switching valve 39 to the open position (C) side based on the thrust of the calculation result. In this case, the positive direction of the thrust of the arm cylinder 32 is set to the same direction as in the second embodiment. This makes it possible to reliably avoid unintended driving due to the weight of the arm cylinder 32 or the like.

[0110] As described above, the work machines according to the first to third embodiments and the first to third examples of the other embodiments include the connection pipelines 48, 48C, 48D, 48E that connect a first pipeline, which is one of the pair of first actuator pipelines 43, 44, to a second pipeline, which is one of the pair of second actuator pipelines 45, 46, and the tank pipeline branching off from the first pipeline 43 or 44, which is one of the pair of first actuator pipelines 43, 44, and connected to the tank 34. The pressure detection device includes: pipelines 49, 49C, 49D; a first switching valve 38 arranged on connecting pipelines 48, 48C, 48D, 48E; a second switching valve 39 arranged on tank pipelines 49, 49C, 49D; a pressure sensor 51 or a pressure sensor 52 constituting a pressure detection device for detecting the pressure of at least the first pipeline 43 or 44 of a pair of first actuator pipelines 43, 44; and control devices 60, 60A, 60B, 60C, 60D, 60E for controlling the first switching valve 38 and the second switching valve 39. The control devices 60, 60A, 60B, 60C, 60D, 60E are configured to control the first switching valve 38 so that the connecting pipes 48, 48C, 48D, 48E are in a connected state, and to control the second switching valve 39 so that the tank pipes 49, 49C, 49D are in a connected state, when an index of the load acting on the first hydraulic actuator 32, 5 obtained based on the detection value of the pressure sensor 51 or 52 (pressure detection device) satisfies a predetermined condition, when there is no drive command from the operating device 56, 56B (first operating device) and the bucket operating device 57 (second operating device) commands the driving of the bucket cylinder 33 (second hydraulic actuator) so as to discharge return oil to the second pipe 45 or 46.

[0111] According to this configuration, by arranging the first switching valve 38 on the connecting pipes 48, 48C, 48D, 48E, it is possible to avoid arranging the first switching valve 38 on the pair of first actuator pipes 43, 44 and the pair of second actuator pipes 45, 46, thereby making it possible to avoid the occurrence of pressure loss due to the first switching valve 38 when it is in a position where it does not divert return oil. In addition, only when the load index of the first hydraulic actuators 32, 5 satisfies a predetermined condition, the tank pipes 49, 49C, 49D are brought into communication by the second switching valve 39, so that unintended actuation of the first hydraulic actuators 32, 5 does not occur at this time. That is, in a technology for reducing pressure loss of the return oil by diverting a portion of the return oil from the bucket cylinder 33 (second hydraulic actuator) to the first actuator line 43 or 44 of the first hydraulic actuator 32, 5 by the first switching valve 38, it is possible to avoid unintended operation of the first hydraulic actuator 32, 5 when diverting the return oil, while avoiding the occurrence of pressure loss due to the first switching valve 38 being in a position that does not divert the return oil. [Explanation of symbols]

[0112] 5...swing hydraulic motor (first hydraulic actuator), 31...hydraulic pump, 32...arm cylinder (first hydraulic actuator), 33...bucket cylinder (second hydraulic actuator), 34...tank, 35...first directional control valve (first control valve), 36...second directional control valve (second control valve), 38...first switching valve, 39...second switching valve, 42...return line, 43...first actuator line (first line, other line), 44...first actuator line (other line, first line), 45, 46...pair of second actuator lines (second lines), 48, 48C, 48D, 48E...connecting lines, 48a...one end (connecting portion), 48b...other end (connecting portion), 49, 49C, 49D...tank lines, 51...pressure sensor (pressure detection device, first pressure sensor, second pressure sensor), 52...pressure sensor (pressure detection device, second pressure sensor, first pressure sensor), 56...arm operation device (first operation device), 57...bucket operation device (second operation device), 60, 60A, 60B, 60C, 60D, 60E...control device

Claims

1. A hydraulic pump that discharges pressure oil; a first hydraulic actuator driven by pressure oil supplied from the hydraulic pump; a second hydraulic actuator driven by pressure oil supplied from the hydraulic pump; a tank that stores return oil discharged from the first hydraulic actuator and the second hydraulic actuator; a first control valve for controlling a flow of pressure oil supplied from the hydraulic pump to the first hydraulic actuator and a flow of return oil discharged from the first hydraulic actuator to the tank; a second control valve for controlling a flow of pressure oil supplied from the hydraulic pump to the second hydraulic actuator and a flow of return oil discharged from the second hydraulic actuator to the tank; a pair of first actuator lines connecting the first hydraulic actuator and the first control valve; a pair of second actuator lines connecting the second hydraulic actuator and the second control valve; a return line connecting the first control valve and the second control valve to the tank; A first operating device that instructs driving of the first hydraulic actuator; a second operating device that instructs driving of the second hydraulic actuator, a connecting pipe that connects a first pipe, which is one of the pair of first actuator pipes, and a second pipe, which is one of the pair of second actuator pipes; a tank pipeline branching off from the first pipeline and connected to the tank; a first switching valve that is disposed in the connecting pipe and is capable of switching the connecting pipe between a blocked state and a communicating state; a second switching valve that is disposed in the tank pipeline and is capable of switching the tank pipeline between a blocked state and a connected state; a pressure detection device for detecting a pressure in at least the first of the pair of first actuator lines; a control device that controls the first switching valve and the second switching valve, When there is no drive command from the first operating device and the second operating device commands the second hydraulic actuator to be driven so as to discharge return oil to the second pipeline, and when an index of a load acting on the first hydraulic actuator obtained based on a detection value of the pressure detection device satisfies a predetermined condition, the control device controls the first switching valve so that the connection pipeline is in a communicated state and controls the second switching valve so that the tank pipeline is in a communicated state. A working machine characterized by:

2. 2. The work machine according to claim 1, the indicator of the load is a pressure in the first line detected by the pressure detection device, The predetermined condition is that the pressure in the first pipeline detected by the pressure detection device is lower than a preset positive threshold value. A working machine characterized by:

3. 2. The work machine according to claim 1, The pressure detection device is A first pressure sensor that detects a pressure in the first pipeline; a second pressure sensor that detects a pressure in the other of the pair of first actuator pipelines that is different from the first pipeline; The load index is calculated using the detection value of the first pressure sensor and the detection value of the second pressure sensor. A working machine characterized by:

4. 4. The work machine according to claim 3, the first hydraulic actuator is a hydraulic cylinder; The load index is the thrust of the hydraulic cylinder calculated using the detection value of the first pressure sensor and the detection value of the second pressure sensor. A working machine characterized by:

5. 4. The work machine according to claim 3, the first hydraulic actuator is a hydraulic motor; The load index is a pressure difference calculated from a difference between the detection value of the second pressure sensor and the detection value of the first pressure sensor. A working machine characterized by:

6. 2. The work machine according to claim 1, The connecting pipe has a connection portion with the first pipe located closer to the first hydraulic actuator than the first control valve, and a connection portion with the second pipe located closer to the second hydraulic actuator than the second control valve. A working machine characterized by:

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