Work machine

Abstract

The purpose of the present invention is to enable a work device to perform work suitably and efficiently regardless of the operation state of the work device and even when a hydraulic cylinder of the work device is in an overload state. This work machine comprises: a boom cylinder and an arm cylinder; a hydraulic pump; a boom-direction control valve and an arm-direction control valve; and a first communication pipe that connects a bottom-side oil chamber of the boom cylinder, which is an extension-side oil chamber when the boom cylinder is extended by an excavation operation, to a bottom-side oil chamber of the arm cylinder, which is an extension-side oil chamber when the arm cylinder is extended by the excavation operation.

Description

Work machinery

[0001] The present invention relates to a work machine.

[0002] A hydraulic excavator, a typical example of a work machine, is equipped with a work device having an articulated structure. This type of work device is configured, for example, with a boom, an arm, a bucket, and a boom cylinder, an arm cylinder, and a bucket cylinder that drive these.

[0003] When performing excavation work, the operator must properly perform the following excavation operations: boom-raising, which rotates the boom upward; arm-crowding, which rotates the arm in the crowding direction; and bucket-crowding, which rotates the bucket in the crowding direction. The arm-crowding operation is the main operation in excavation work, and the boom-raising and bucket-crowding operations are auxiliary operations.

[0004] If the operator does not raise the boom during excavation work, the excavation resistance will become excessive, causing the boom cylinder, arm cylinder, or bucket cylinder to become overloaded, which may result in high-pressure hydraulic oil being discharged from the overload relief valve into the hydraulic oil tank or the vehicle body floating up, which may prevent the work machine from working properly and efficiently.

[0005] Patent Document 1 discloses a technology for preventing the vehicle body from lifting up during a combined excavation operation involving arm crowding or bucket crowding and boom raising by controlling the pressure in the bottom oil chamber of the boom cylinder or the pressure in the bottom oil chamber of the arm cylinder so that the pressure in the bottom oil chamber of the boom cylinder does not exceed the maximum allowable pressure due to excavation resistance. With the technology disclosed in Patent Document 1, when the operator's amount of boom raising operation is small and the vehicle body would lift up, the pilot pressure related to the boom raising operation is controlled to be equal to or greater than the operator's amount of operation, thereby increasing the boom raising operation speed. With the technology disclosed in Patent Document 1, when the operator's amount of arm crowding operation is large and the vehicle body would lift up, the pilot pressure related to the arm crowding operation is controlled to be equal to or less than the operator's amount of operation, thereby slowing down the arm crowding operation speed. In this way, the technology disclosed in Patent Document 1 prevents the vehicle body from lifting up.

[0006] Patent No. 5969379

[0007] However, in the technology disclosed in Patent Document 1, the above-mentioned control to prevent the vehicle body from lifting up is performed only during a combined excavation operation. That is, in the technology disclosed in Patent Document 1, when the operator is not performing a boom-raising operation, the pilot pressure control related to the boom-raising operation is not performed, and when the operator is not performing an arm-crowding operation, the pilot pressure control related to the arm-crowding operation is not performed. Therefore, for example, when one of the hydraulic cylinders becomes overloaded due to an arm-crowding operation alone, it is not possible to discharge high-pressure hydraulic oil from the overload relief valve or prevent the vehicle body from lifting up.

[0008] Furthermore, in the technology disclosed in Patent Document 1, when controlling the pilot pressure related to the boom-raising operation, part of the hydraulic oil discharged from the hydraulic pump is diverted to the circuit related to the boom-raising operation, so in a work machine equipped with a control valve device in which the circuit related to the arm crowding operation or the circuit related to the bucket crowding operation and the circuit related to the boom-raising operation form a tandem circuit, the arm crowding operation speed or the bucket crowding operation speed will slow down.

[0009] In view of the above circumstances, an object of the present invention is to enable a working device to perform work appropriately and efficiently even if the hydraulic cylinder of the working device is in an overloaded state, regardless of the operating status of the working device.

[0010] In order to solve the above problem, the working machine of the present invention is characterized by comprising: a working device including a boom rotatably connected to a vehicle body, an arm rotatably connected to the boom, a boom cylinder that drives the boom, and an arm cylinder that drives the arm; a hydraulic pump that supplies hydraulic oil to the boom cylinder and the arm cylinder; a control valve device that includes a boom direction control valve that is provided between the hydraulic pump and the boom cylinder and controls the flow direction of the hydraulic oil supplied from the hydraulic pump to the boom cylinder, and an arm direction control valve that is provided between the hydraulic pump and the arm cylinder and controls the flow direction of the hydraulic oil supplied from the hydraulic pump to the arm cylinder; and a first communicating pipe that communicates between a bottom-side oil chamber of the boom cylinder that is an extension-side oil chamber when the boom cylinder is extended by an excavation operation, and a bottom-side oil chamber of the arm cylinder that is an extension-side oil chamber when the arm cylinder is extended by the excavation operation.

[0011] According to the present invention, regardless of the operating state of the working device, the working device can perform work appropriately and efficiently even if the hydraulic cylinder of the working device is in an overload state.

[0012] FIG. 1 is a right side view of a work machine. FIG. 2 is a diagram showing the configuration of a hydraulic drive circuit for a work device. FIG. 3 is a diagram explaining the operation of the hydraulic drive circuit when the arm cylinder is overloaded when an arm crowd sole operation is performed. FIG. 4 is a diagram explaining the operation of the hydraulic drive circuit when the bucket cylinder is overloaded when an arm crowd sole operation is performed. FIG. 5 is a diagram explaining the operation of the hydraulic drive circuit when the arm cylinder is overloaded when an arm bucket combined operation is performed. FIG. 6 is a diagram explaining the operation of the hydraulic drive circuit when the bucket cylinder is overloaded when an arm bucket combined operation is performed. FIG. 7 is a diagram showing another example of a work machine.

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Components with the same reference numerals in each embodiment have similar components in each embodiment unless otherwise specified, and description thereof will be omitted.

[0014] FIG. 1 is a right side view of a work machine 1. As shown in FIG.

[0015] The work machine 1 is a work machine that performs excavation work, such as a hydraulic excavator. In this embodiment, the work machine 1 will be described by taking a hydraulic excavator as an example.

[0016] The work machine 1 shown in Figure 1 is a hydraulic excavator equipped with an extra-large backhoe-type bucket 14. The work machine 1 is equipped with a self-propelled crawler-type undercarriage 2, an upper rotating body 3 rotatably provided on the upper part of the undercarriage 2, and a working device 11 having a multi-joint structure rotatably connected to the front part of the upper rotating body 3. The undercarriage 2 and the upper rotating body 3 constitute the vehicle body of the work machine 1.

[0017] The work device 11 is configured to include work machines 12 to 14 and hydraulic cylinders 15 to 17 that drive the work machines 12 to 14. The work machines 12 to 14 include a boom 12 that is rotatably connected to the vehicle body, an arm 13 that is rotatably connected to the boom 12, and a bucket 14 that is rotatably connected to the arm 13. The hydraulic cylinders 15 to 17 include a boom cylinder 15 that drives the boom 12, an arm cylinder 16 that drives the arm 13, and a bucket cylinder 17 that drives the bucket 14.

[0018] The boom 12 has its base end connected to the revolving frame 5 of the upper revolving body 3, and rotates relative to the revolving frame 5 by extension and contraction of a boom cylinder 15. The arm 13 has its base end connected to the tip of the boom 12, and rotates relative to the boom 12 by extension and contraction of an arm cylinder 16. The bucket 14 has its base end connected to the tip of the arm 13, and rotates relative to the arm 13 by extension and contraction of a bucket cylinder 17.

[0019] The boom cylinder 15, arm cylinder 16, and bucket cylinder 17 extend and retract using hydraulic oil supplied from the hydraulic pump 33. This changes the posture of the work implement 11. The boom cylinder 15, arm cylinder 16, and bucket cylinder 17 extend and retract by a stroke amount corresponding to the amount of operation of a left operating lever device and a right operating lever device, which will be described later.

[0020] A cab 6 provided on the upper rotating body 3 is provided inside a driver's seat where an operator sits. On both the left and right sides of the driver's seat, a left operation lever device and a right operation lever device are provided as operation devices operated by the operator. These operation lever devices are operated to perform the swing operation of the upper rotating body 3 and the rotation operation of the working implement 11. The left operation lever device is composed of, for example, a swing operation lever device that commands the operation of the swing hydraulic motor of the swing implement 4, and an arm operation lever device that commands the operation of the arm cylinder 16 of the working implement 11. The right operation lever device is composed of, for example, a boom operation lever device that commands the operation of the boom cylinder 15 of the working implement 11, and a bucket operation lever device that commands the operation of the bucket cylinder 17 of the working implement 11.

[0021] The left and right operating lever devices are connected to a control device (not shown) and output operation signals indicating commands in response to operator operation to the control device. The control device controls multiple proportional solenoid valves (not shown) in response to the operation signals output from these operating lever devices. As a result, hydraulic oil discharged from the pilot pump is output to a control valve device 38 (boom direction control valve 38A, arm direction control valve 38B, and bucket direction control valve 38C) via the proportional solenoid valves as pilot pressure in response to operator operation. This allows the operator to drive the hydraulic cylinders 15 to 17, such as the boom cylinder 15, arm cylinder 16, and bucket cylinder 17, of the working device 11.

[0022] Fig. 2 is a diagram showing the configuration of a hydraulic drive circuit 31 for the working implement 11. Note that Fig. 2 does not show the circuit related to the opening and closing cylinder that opens and closes the open-close bucket 14 provided on the working implement 11 of the loading hydraulic excavator.

[0023] The work machine 1 is equipped with a hydraulic drive circuit 31 that drives the work implements 12 to 14 of the work implement 11. In addition to the hydraulic drive circuit 31 for the work implement 11 shown in Figure 2, the work machine 1 is also equipped with a hydraulic drive circuit for the traveling device that drives the traveling device that causes the undercarriage 2 to travel, and a hydraulic drive unit for the swing device that drives the swing device 4.

[0024] In addition to the hydraulic cylinders 15-17, the left operating lever device, and the right operating lever device, the hydraulic drive circuit 31 for the working device 11 includes the following components: a hydraulic pump 33 that supplies working oil to the hydraulic cylinders 15-17, a prime mover 32 that is the power source for the hydraulic pump 33, a hydraulic oil tank 34 that stores hydraulic oil, a control valve device 38 provided between the hydraulic pump 33 and the hydraulic cylinders 15-17, a discharge pipe 36 that communicates between the hydraulic pump 33 and the control valve device 38, first oil passages 39 and 40, second oil passages 41 and 42, and third oil passages 43 and 44.

[0025] The hydraulic pump 33 is rotationally driven by the prime mover 32. When rotationally driven by the prime mover 32, the hydraulic pump 33 draws hydraulic oil from a hydraulic oil tank 34 and discharges the drawn hydraulic oil into a discharge line 36, also referred to as a delivery line. A control valve device 38 is connected to the tip of the discharge line 36. The hydraulic oil discharged from the hydraulic pump 33 to the discharge line 36 is supplied to the control valve device 38.

[0026] The control valve device 38 distributes the hydraulic oil supplied from the hydraulic pump 33 to the hydraulic cylinders 15 to 17 in response to the operation of the left and right operation lever devices. The control valve device 38 includes a boom directional control valve 38A provided between the hydraulic pump 33 and the boom cylinder 15, an arm directional control valve 38B provided between the hydraulic pump 33 and the arm cylinder 16, and a bucket directional control valve 38C provided between the hydraulic pump 33 and the bucket cylinder 17. The control valve device 38 is configured by a tandem circuit in which the arm directional control valve 38B and the bucket directional control valve 38C are connected in parallel to each other downstream of the boom directional control valve 38A.

[0027] The boom directional control valve 38A is configured by a pilot-operated directional control valve, for example, a 5-port 3-position (or 6-port 3-position or 4-port 3-position) pilot-operated directional control valve.

[0028] The boom direction control valve 38A controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the boom cylinder 15 in response to an operation signal output from the boom operation lever device. Specifically, a control device controls a proportional solenoid valve in response to the operation signal output from the boom operation lever device, and pilot pressure generated by controlling the proportional solenoid valve is supplied to the boom direction control valve 38A. The boom direction control valve 38A switches the spool position in the boom direction control valve 38A in response to the pilot pressure, thereby switching between supplying and discharging hydraulic oil to the boom cylinder 15. This changes the hydraulic oil in the bottom-side oil chamber 15A and the rod-side oil chamber 15B of the boom cylinder 15, causing the boom cylinder 15 to extend or retract. In this way, the boom direction control valve 38A can control the hydraulic oil supplied to the boom cylinder 15 in response to an operation signal output from the boom operation lever device.

[0029] Like the boom direction control valve 38A, the arm direction control valve 38B controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the arm cylinder 16 in response to an operation signal output from the arm operation lever device. Like the boom direction control valve 38A, the bucket direction control valve 38C controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the bucket cylinder 17 in response to an operation signal output from the bucket operation lever device. The arm direction control valve 38B and the bucket direction control valve 38C are similar to the boom direction control valve 38A except for the destination (hydraulic cylinders 15 to 17) of the hydraulic oil to be supplied, and therefore further description will be omitted.

[0030] During the excavation operation, the boom raising operation, the arm crowding operation, and the bucket crowding operation are performed individually or in combination. That is, during the excavation operation, it is assumed that any one of the hydraulic cylinders 15 to 17 among the boom cylinder 15, the arm cylinder 16, and the bucket cylinder 17 will be extended.

[0031] Therefore, the bottom-side oil chamber 15A of the boom cylinder 15 is the oil chamber that extends when the boom cylinder 15 is extended by an excavation operation, and is also referred to as the "extension-side oil chamber" of the boom cylinder 15. The rod-side oil chamber 15B of the boom cylinder 15 is the oil chamber that contracts when the boom cylinder 15 is extended by an excavation operation, and is also referred to as the "retraction-side oil chamber" of the boom cylinder 15. Similarly, the bottom-side oil chamber 16A of the arm cylinder 16 is the extension-side oil chamber when the arm cylinder 16 is extended by an excavation operation, and the rod-side oil chamber 16B of the arm cylinder 16 is the retraction-side oil chamber when the arm cylinder 16 is extended by an excavation operation. The bottom-side oil chamber 17A of the bucket cylinder 17 is the extension-side oil chamber when the bucket cylinder 17 is extended by an excavation operation, and the rod-side oil chamber 17B of the bucket cylinder 17 is the retraction-side oil chamber when the bucket cylinder 17 is extended by an excavation operation.

[0032] The first oil passages 39, 40 are pipes that communicate between the boom direction control valve 38A and the boom cylinder 15. The first oil passages 39, 40 include a BMCB pipe 39 that communicates between the boom direction control valve 38A and the bottom-side oil chamber 15A of the boom cylinder 15, and a BMCR pipe 40 that communicates between the boom direction control valve 38A and the rod-side oil chamber 15B of the boom cylinder 15.

[0033] The second oil passages 41, 42 are passages that communicate between the arm direction control valve 38B and the arm cylinder 16. The second oil passages 41, 42 include an AMCB passage 41 that communicates between the arm direction control valve 38B and the bottom side oil chamber 16A of the arm cylinder 16, and an AMCR passage 42 that communicates between the arm direction control valve 38B and the rod side oil chamber 16B of the arm cylinder 16. The AMCB passage 41 is provided with an overload relief valve 41A that releases hydraulic oil from the bottom side oil chamber 16A to the hydraulic oil tank 34 when the bottom side oil chamber 16A of the arm cylinder 16 is overloaded. The AMCR passage 42 is provided with an overload relief valve 42A that releases hydraulic oil from the rod side oil chamber 16B to the hydraulic oil tank 34 when the rod side oil chamber 16B of the arm cylinder 16 is overloaded.

[0034] The third oil lines 43, 44 are lines that communicate between the bucket direction control valve 38C and the bucket cylinder 17. The third oil lines 43, 44 include a BKCB line 43 that communicates between the bucket direction control valve 38C and the bottom-side oil chamber 17A of the bucket cylinder 17, and a BKCR line 44 that communicates between the bucket direction control valve 38C and the rod-side oil chamber 17B of the bucket cylinder 17. The BKCB line 43 is provided with an overload relief valve 43A that releases hydraulic oil from the bottom-side oil chamber 17A to the hydraulic oil tank 34 when the bottom-side oil chamber 17A of the bucket cylinder 17 is overloaded. The BKCR line 44 is provided with an overload relief valve 44A that releases hydraulic oil from the rod-side oil chamber 17B to the hydraulic oil tank 34 when the rod-side oil chamber 17B of the bucket cylinder 17 is overloaded.

[0035] In the technology disclosed in the aforementioned Patent Document 1, during a combined excavation operation of an arm crowding operation or a bucket crowding operation and a boom raising operation, if the pressure in the bottom-side oil chamber of any of the boom cylinder, arm cylinder, or bucket cylinder increases due to excavation resistance and the condition for the vehicle body to float up is met, control is performed to make the pilot pressure for the boom raising operation equal to or greater than the amount of operation by the operator, or control is performed to make the pilot pressure for the arm crowding operation equal to or less than the amount of operation by the operator. This makes it possible to prevent each hydraulic cylinder from becoming overloaded, preventing high-pressure hydraulic oil from being discharged from the overload relief valve to the hydraulic oil tank and preventing the vehicle body from floating up.

[0036] However, with the technology disclosed in Patent Document 1, for example, if one of the hydraulic cylinders becomes overloaded due to an arm crowding operation alone, the pilot pressure related to the boom-raising operation is not controlled, and it is not possible to prevent high-pressure hydraulic oil from being discharged from the overload relief valve or the vehicle body from floating up. As a result, the technology disclosed in Patent Document 1 has low energy efficiency and the working equipment cannot perform its work properly.

[0037] Furthermore, in the technology disclosed in Patent Document 1, if the control valve device configures a tandem circuit with a circuit related to arm crowding operation or a circuit related to bucket crowding operation and a circuit related to boom raising operation, when control is performed to make the pilot pressure related to boom raising operation equal to or greater than the amount of operation by the operator, the arm crowding operation speed may slow down, and work efficiency may decrease.

[0038] Therefore, the work machine 1 of this embodiment is equipped with a hydraulic drive circuit 31 for the work implement 11 that is configured to automatically perform a boom-raising operation in which the arm 13 rotates upward when the bottom-side oil chambers 16A, 17A of either the arm cylinder 16 or the bucket cylinder 17 become overloaded, regardless of whether a single operation or a combined operation is being performed as an excavation operation. Furthermore, the work machine 1 of this embodiment is equipped with a hydraulic drive circuit 31 for the work implement 11 that is configured to prevent a slowdown in the arm crowding operation speed or bucket crowding operation speed due to the diversion of hydraulic oil at the control valve device 38 associated with the boom-raising operation.

[0039] Specifically, the hydraulic drive circuit 31 includes a first communication pipe 20 that communicates between a bottom-side oil chamber 15A, which is an extension-side oil chamber of the boom cylinder 15, and a bottom-side oil chamber 16A, which is an extension-side oil chamber of the arm cylinder 16. The first communication pipe 20 communicates between the bottom-side oil chamber 15A of the boom cylinder 15 and the bottom-side oil chamber 16A of the arm cylinder 16 without passing through the control valve device 38. A first valve 20A is provided on the first communication pipe 20. The first valve 20A opens when the pressure in the bottom-side oil chamber 16A of the arm cylinder 16 exceeds a first threshold, thereby allowing hydraulic oil to flow from the bottom-side oil chamber 16A of the arm cylinder 16 to the bottom-side oil chamber 15A of the boom cylinder 15.

[0040] A return circuit 21 is connected to the rod-side oil chamber 15B of the boom cylinder 15. The return circuit 21 includes a first conduit 24 that connects the rod-side oil chamber 15B, which is the retraction-side oil chamber of the boom cylinder 15, with the rod-side oil chamber 16B, which is the retraction-side oil chamber of the arm cylinder 16. The first conduit 24 is connected to the rod-side oil chamber 15B of the boom cylinder 15 and an AMCR conduit 42, and connects the rod-side oil chamber 15B of the boom cylinder 15 with the rod-side oil chamber 16B of the arm cylinder 16 via the AMCR conduit 42. A third valve 21A is provided on the first conduit 24 and opens to allow hydraulic oil to flow from the rod-side oil chamber 15B of the boom cylinder 15 to the rod-side oil chamber 16B of the arm cylinder 16 when the pressure in the bottom-side oil chamber 16A of the arm cylinder 16 exceeds a first threshold, thereby entering an overload state. A check valve 24A is provided on the first line 24 between the third valve 21A and the AMCR line 42 to restrict the flow direction of the hydraulic oil. The first line 24 between the third valve 21A and the check valve 24A branches and communicates with the hydraulic oil tank 34.

[0041] The hydraulic drive circuit 31 also includes a second communication pipe 22 that communicates between a bottom-side oil chamber 15A, which is an extension-side oil chamber of the boom cylinder 15, and a bottom-side oil chamber 17A, which is an extension-side oil chamber of the bucket cylinder 17. The second communication pipe 22 communicates between the bottom-side oil chamber 15A of the boom cylinder 15 and the bottom-side oil chamber 17A of the bucket cylinder 17 without passing through the control valve device 38. A second valve 22A is provided on the second communication pipe 22. The second valve 22A opens when the pressure in the bottom-side oil chamber 17A of the bucket cylinder 17 exceeds a second threshold, thereby allowing hydraulic oil to flow from the bottom-side oil chamber 17A of the bucket cylinder 17 to the bottom-side oil chamber 15A of the boom cylinder 15.

[0042] A return circuit 23 is connected to rod-side oil chamber 15B of boom cylinder 15. Return circuit 23 includes a second pipe 25 that connects rod-side oil chamber 15B, which is a retraction-side oil chamber of boom cylinder 15, with rod-side oil chamber 17B, which is a retraction-side oil chamber of bucket cylinder 17. Second pipe 25 is connected to rod-side oil chamber 15B of boom cylinder 15 and BKCR pipe 44, and connects rod-side oil chamber 15B of boom cylinder 15 with rod-side oil chamber 17B of bucket cylinder 17 via BKCR pipe 44. A fourth valve 23A is provided on second pipe 25. When the pressure in bottom-side oil chamber 17A of bucket cylinder 17 exceeds a second threshold, an overload state occurs, and hydraulic oil flows from rod-side oil chamber 15B of boom cylinder 15 to rod-side oil chamber 17B of bucket cylinder 17. A check valve 25A is provided on the second line 25 between the fourth valve 23A and the BKCR line 44 to restrict the flow direction of the hydraulic oil. The second line 25 between the fourth valve 23A and the check valve 25A branches and communicates with the hydraulic oil tank 34.

[0043] The first valve 20A, the second valve 22A, the third valve 21A, and the fourth valve 23A are each configured as a sequence valve. The first valve 20A and the third valve 21A are opened by the same pilot pressure. The second valve 22A and the fourth valve 23A are opened by the same pilot pressure. Note that the first valve 20A, the second valve 22A, the third valve 21A, and the fourth valve 23A may each be configured as a solenoid valve.

[0044] FIG. 3 is a diagram for explaining the operation of the hydraulic drive circuit 31 when the arm cylinder 16 is overloaded during an arm crowding operation alone.

[0045] The arm crowding operation alone is a type of excavation operation in which only the arm 13 of the boom 12, arm 13, and bucket 14 is operated in the crowding direction, without operating the boom 12 or the bucket 14. The overload state of the arm cylinder 16 means that the pressure in the bottom-side oil chamber 16A, which is the extension-side oil chamber of the arm cylinder 16, exceeds a first threshold value. The first threshold value is smaller than the allowable value of the pressure in the bottom-side oil chamber 16A of the arm cylinder 16.

[0046] 3, hydraulic oil is supplied from the hydraulic pump 33 to the bottom-side oil chamber 16A of the arm cylinder 16 via the arm direction control valve 38B. In the case shown in FIG. 3, the second valve 22A and the fourth valve 23A are closed, but the first valve 20A and the third valve 21A are open.

[0047] When the first valve 20A and the third valve 21A are opened, at least a portion of the hydraulic oil supplied from the hydraulic pump 33 to the bottom-side oil chamber 16A of the arm cylinder 16 via the arm direction control valve 38B is supplied to the bottom-side oil chamber 15A of the boom cylinder 15 via the first communicating pipe 20. As a result, a boom-raising operation is performed in which the boom 12 rotates upward, even though no boom-raising operation is being performed in the work machine 1. The hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 is discharged to the hydraulic oil tank 34 via the first pipe 24.

[0048] When the overload state of the arm cylinder 16 is eliminated by the boom-up operation, the first valve 20A and the third valve 21A are closed.

[0049] FIG. 4 is a diagram illustrating the operation of the hydraulic drive circuit 31 when the bucket cylinder 17 is overloaded during an arm crowding operation alone.

[0050] The overload state of the bucket cylinder 17 means that the pressure in the bottom-side oil chamber 17A of the bucket cylinder 17 exceeds the second threshold value. The second threshold value is smaller than the allowable value of the pressure in the bottom-side oil chamber 17A of the bucket cylinder 17.

[0051] 4, hydraulic oil is supplied from the hydraulic pump 33 to the bottom-side oil chamber 16A of the arm cylinder 16 via the arm direction control valve 38B. In the case shown in FIG. 4, the first valve 20A and the third valve 21A are closed, but the second valve 22A and the fourth valve 23A are open.

[0052] When the second valve 22A and the fourth valve 23A are opened, the hydraulic oil in the bottom-side oil chamber 17A of the bucket cylinder 17 is supplied to the bottom-side oil chamber 15A of the boom cylinder 15 via the second communicating pipe 22 due to an overload. As a result, in the work machine 1, a boom-raising operation is performed in which the boom 12 rotates upward, even though no boom-raising operation is being performed. At this time, based on the amount of hydraulic oil that has flowed out of the bottom-side oil chamber 17A of the bucket cylinder 17 due to the overload, the hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 flows into the rod-side oil chamber 17B of the bucket cylinder 17 via the second pipe 25. As a result, the bucket cylinder 17 retracts.

[0053] When the overload state of the bucket cylinder 17 is eliminated by the boom-up operation, the second valve 22A and the fourth valve 23A are closed.

[0054] FIG. 5 is a diagram illustrating the operation of the hydraulic drive circuit 31 when the arm cylinder 16 is overloaded during combined arm-bucket operation.

[0055] The combined arm-bucket operation is a type of excavation operation, and means that both the arm 13 and the bucket 14 are operated in the cloud direction, and the boom 12 is not operated.

[0056] 5, hydraulic oil is supplied from the hydraulic pump 33 to the bottom-side oil chamber 16A of the arm cylinder 16 via the arm direction control valve 38B, and hydraulic oil is also supplied from the hydraulic pump 33 to the bottom-side oil chamber 17A of the bucket cylinder 17 via the bucket direction control valve 38C. In the case shown in FIG. 5, the second valve 22A and the fourth valve 23A are closed, but the first valve 20A and the third valve 21A are open.

[0057] 3, hydraulic oil in the bottom-side oil chamber 16A of the arm cylinder 16 is supplied to the bottom-side oil chamber 15A of the boom cylinder 15 via the first communicating pipe 20. As a result, a boom-raising operation is performed in which the boom 12 rotates upward even though no boom-raising operation is being performed in the work machine 1. Hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 is discharged to the hydraulic oil tank 34 via the first pipe 24.

[0058] Moreover, even though the control valve device 38 is configured with a tandem circuit as described above, because the boom raising operation is not performed, the boom direction control valve 38A does not divert hydraulic oil to the boom cylinder 15. As a result, in the work machine 1, the amount of hydraulic oil supplied to the bucket cylinder 17 is not affected, so the boom raising operation can be performed without the bucket crowding operation speed slowing down compared to when the boom raising operation is performed.

[0059] Note that there are cases where the load increase effect on the bottom-side oil chamber 16A of the arm cylinder 16 due to the bucket crowding operation outweighs the load reduction effect on the bottom-side oil chamber 16A of the arm cylinder 16 due to the boom-raising operation. In this case, all of the hydraulic oil supplied to the bottom-side oil chamber 16A from the hydraulic pump 33 via the arm direction control valve 38B and the hydraulic oil flowing out of the bottom-side oil chamber 16A due to overload join together, flow out of the bottom-side oil chamber 16A, and are supplied to the bottom-side oil chamber 15A of the boom cylinder 15, thereby performing the boom-raising operation. At this time, based on the amount of hydraulic oil flowing out of the bottom-side oil chamber 16A of the arm cylinder 16, the hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 flows into the rod-side oil chamber 16B of the arm cylinder 16 via the first pipe 24. This causes the arm cylinder 16 to retract.

[0060] When the overload state of the arm cylinder 16 is eliminated by the boom-up operation, the first valve 20A and the third valve 21A are closed.

[0061] FIG. 6 is a diagram illustrating the operation of the hydraulic drive circuit 31 when the bucket cylinder 17 is overloaded during combined arm-bucket operation.

[0062] 6, hydraulic oil is supplied from the hydraulic pump 33 to the bottom-side oil chamber 16A of the arm cylinder 16 via the arm direction control valve 38B, and hydraulic oil is supplied from the hydraulic pump 33 to the bottom-side oil chamber 17A of the bucket cylinder 17 via the bucket direction control valve 38C. In the case shown in FIG. 6, the first valve 20A and the third valve 21A are closed, but the second valve 22A and the fourth valve 23A are open.

[0063] 4 , when the second valve 22A and the fourth valve 23A are opened, hydraulic oil in the bottom-side oil chamber 17A of the bucket cylinder 17 is supplied to the bottom-side oil chamber 15A of the boom cylinder 15 via the second communicating pipe 22. As a result, a boom-raising operation is performed in which the boom 12 rotates upward, even though no boom-raising operation is being performed in the work machine 1. Hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 is discharged to the hydraulic oil tank 34 via the second pipe 25.

[0064] Moreover, even though the control valve device 38 is configured with a tandem circuit as described above, because the boom raising operation is not performed, the boom direction control valve 38A does not divert hydraulic oil to the boom cylinder 15. As a result, in the work machine 1, the amount of hydraulic oil supplied to the arm cylinder 16 is not affected, so the boom raising operation can be performed without the arm crowding operating speed slowing down compared to when the boom raising operation is performed.

[0065] Note that there are cases where the effect of increasing the load on the bottom-side oil chamber 17A of the bucket cylinder 17 due to the arm crowding operation outweighs the effect of reducing the load on the bottom-side oil chamber 17A of the bucket cylinder 17 due to the boom-raising operation. In this case, all of the hydraulic oil supplied to the bottom-side oil chamber 17A from the hydraulic pump 33 via the bucket directional control valve 38C and the hydraulic oil flowing out of the bottom-side oil chamber 17A due to overload join together, flow out of the bottom-side oil chamber 17A, and be supplied to the bottom-side oil chamber 15A of the boom cylinder 15, thereby performing the boom-raising operation. At this time, based on the amount of hydraulic oil flowing out of the bottom-side oil chamber 17A of the bucket cylinder 17, the hydraulic oil in the rod-side oil chamber 15B of the boom cylinder 15 flows into the rod-side oil chamber 17B of the bucket cylinder 17 via the second pipeline 25. This causes the bucket cylinder 17 to retract.

[0066] When the overload state of the bucket cylinder 17 is eliminated by the boom-up operation, the second valve 22A and the fourth valve 23A are closed.

[0067] FIG. 7 is a diagram showing another example of the work machine 1.

[0068] The work machine 1 does not have to be a backhoe-type hydraulic excavator as shown in Fig. 1, but may be a hydraulic excavator having a loading-type bucket 14 as shown in Fig. 7. The work machine 1 shown in Fig. 7 basically comprises the same hydraulic drive circuit 31 as the work machine 1 shown in Fig. 1.

[0069] 7, the excavation operation involves either a boom raising operation, an arm pushing operation (an operation for rotating the arm 13 forward), and a bucket crowding operation, either individually or in combination. That is, even in a loading-type hydraulic excavator, it is expected that any one of the hydraulic cylinders 15 to 17, the boom cylinder 15, the arm cylinder 16, and the bucket cylinder 17, will be extended during the excavation operation.

[0070] 7 , the bottom-side oil chamber 15A of the boom cylinder 15 is the extension-side oil chamber when the boom cylinder 15 is extended by an excavation operation, and the rod-side oil chamber 15B of the boom cylinder 15 is the retraction-side oil chamber when the boom cylinder 15 is extended by an excavation operation. Similarly, the bottom-side oil chamber 16A of the arm cylinder 16 is the extension-side oil chamber when the arm cylinder 16 is extended by an excavation operation, and the rod-side oil chamber 16B of the arm cylinder 16 is the retraction-side oil chamber when the arm cylinder 16 is extended by an excavation operation. The bottom-side oil chamber 17A of the bucket cylinder 17 is the extension-side oil chamber when the bucket cylinder 17 is extended by an excavation operation, and the rod-side oil chamber 17B of the bucket cylinder 17 is the retraction-side oil chamber when the bucket cylinder 17 is extended by an excavation operation.

[0071] In the work machine 1 shown in Figure 7, the first communication pipe 20 communicates between the bottom oil chamber 15A of the boom cylinder 15 and the bottom oil chamber 16A of the arm cylinder 16. The second communication pipe 22 communicates between the bottom oil chamber 15A of the boom cylinder 15 and the bottom oil chamber 17A of the bucket cylinder 17.

[0072] 7 , the first pipeline 24 communicates between the rod-side oil chamber 15B of the boom cylinder 15 and the rod-side oil chamber 16B of the arm cylinder 16. The second pipeline 25 communicates between the rod-side oil chamber 15B of the boom cylinder 15 and the rod-side oil chamber 17B of the bucket cylinder 17.

[0073] In the work machine 1 shown in Figure 7, the overload state of the arm cylinder 16 means a state in which the pressure in the bottom-side oil chamber 16A, which is the extension-side oil chamber of the arm cylinder 16, exceeds a first threshold value. The overload state of the bucket cylinder 17 means a state in which the pressure in the bottom-side oil chamber 17A, which is the extension-side oil chamber of the bucket cylinder 17, exceeds a second threshold value.

[0074] 7, if the arm cylinder 16 becomes overloaded when an arm-pushing operation alone is performed or an arm-bucket combined operation is performed, hydraulic oil from the bottom-side oil chamber 16A, which is the extension-side oil chamber of the arm cylinder 16, is supplied to the bottom-side oil chamber 15A, which is the extension-side oil chamber of the boom cylinder 15, via the first communicating pipe 20, causing the boom 12 to rotate upward even when no boom-raising operation is being performed.If the bucket cylinder 17 becomes overloaded, hydraulic oil from the bottom-side oil chamber 17A, which is the extension-side oil chamber of the bucket cylinder 17, is supplied to the bottom-side oil chamber 15A, which is the extension-side oil chamber of the boom cylinder 15, via the second communicating pipe 22, causing the boom 12 to rotate upward even when no boom-raising operation is being performed.

[0075] As described above, the work machine 1 of this embodiment is equipped with a working device 11 including the boom 12 rotatably connected to the vehicle bodies 2, 3, the arm 13 rotatably connected to the boom 12, the boom cylinder 15 that drives the boom 12, and the arm cylinder 16 that drives the arm 13. The work machine 1 is equipped with a hydraulic pump 33 that supplies hydraulic oil to the boom cylinder 15 and the arm cylinder 16. The work machine 1 is equipped with a control valve device 38 that includes a boom direction control valve 38A that is provided between the hydraulic pump 33 and the boom cylinder 15 and controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the boom cylinder 15, and an arm direction control valve 38B that is provided between the hydraulic pump 33 and the arm cylinder 16 and controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the arm cylinder 16. The work machine 1 is equipped with first oil passages 39, 40 that connect the boom direction control valve 38A to the boom cylinder 15, and second oil passages 41, 42 that connect the arm direction control valve 38B to the arm cylinder 16. The work machine 1 is provided with a first communication pipe 20 that connects the bottom-side oil chamber 15A of the boom cylinder 15, which is the extension-side oil chamber when the boom cylinder 15 is extended by an excavation operation, and the bottom-side oil chamber 16A of the arm cylinder 16, which is the extension-side oil chamber when the arm cylinder 16 is extended by an excavation operation.

[0076] As a result, the work machine 1 can automatically perform a boom-raising operation if the arm cylinder 16 or the bucket cylinder 17 becomes overloaded, and can eliminate the overload state, regardless of whether a single operation or a combined operation is being performed as an excavation operation. Therefore, the work machine 1 can prevent high-pressure hydraulic oil from being discharged from the overload relief valves 41A to 44A to the hydraulic oil tank 34 and the vehicle bodies 2, 3 from floating up. Therefore, the work machine 1 can allow the work device 11 to perform work appropriately and efficiently, regardless of the operating status of the work device 11, even if the hydraulic cylinders 16, 17 of the work device 11 become overloaded.

[0077] Furthermore, in the work machine 1 of this embodiment, the work device 11 includes a bucket 14 rotatably connected to the arm 13, and a bucket cylinder 17 that drives the bucket 14. The hydraulic pump 33 supplies hydraulic oil to the bucket cylinder 17. The control valve device 38 includes a bucket directional control valve 38C that is provided between the hydraulic pump 33 and the bucket cylinder 17 and controls the flow direction of hydraulic oil supplied from the hydraulic pump 33 to the bucket cylinder 17. The bucket directional control valve 38C and the bucket cylinder 17 are communicated by third oil passages 43, 44. A bottom-side oil chamber 17A of the bucket cylinder 17, which is an extension-side oil chamber when the bucket cylinder 17 is extended during an excavation operation, and a bottom-side oil chamber 15A of the boom cylinder 15 are communicated by a second communicating pipe 22. A first valve 20A is provided on the first communicating pipe 20. The first valve 20A opens when the pressure in the bottom-side oil chamber 16A of the arm cylinder 16 exceeds a first threshold value, allowing hydraulic oil to flow from the bottom-side oil chamber 16A of the arm cylinder 16 to the bottom-side oil chamber 15A of the boom cylinder 15. A second valve 22A is provided on the second communicating pipe 22. The second valve 22A opens when the pressure in the bottom-side oil chamber 17A of the bucket cylinder 17 exceeds a second threshold value, allowing hydraulic oil to flow from the bottom-side oil chamber 17A of the bucket cylinder 17 to the bottom-side oil chamber 15A of the boom cylinder 15.

[0078] As a result, the work machine 1 can reliably perform a boom-up operation when the arm cylinder 16 or the bucket cylinder 17 is overloaded, and can reliably eliminate the overload state, regardless of whether a single operation or a combined operation is being performed as an excavation operation. Therefore, the work machine 1 can reliably prevent high-pressure hydraulic oil from being discharged from the overload relief valves 41A to 44A to the hydraulic oil tank 34 and the vehicle bodies 2, 3 from floating up. Therefore, the work machine 1 can allow the work implement 11 to perform work more appropriately and efficiently even if the hydraulic cylinders 16, 17 of the work implement 11 are overloaded.

[0079] Furthermore, the work machine 1 of this embodiment is provided with a first pipeline 24 that connects the rod-side oil chamber 15B of the boom cylinder 15, which is the retracted-side oil chamber when the boom cylinder 15 is extended by an excavation operation, and the rod-side oil chamber 16B of the arm cylinder 16, which is the retracted-side oil chamber when the arm cylinder 16 is extended by an excavation operation. The work vehicle 1 is provided with a second pipeline 25 that connects the rod-side oil chamber 17B of the bucket cylinder 17, which is the retracted-side oil chamber when the bucket cylinder 17 is extended by an excavation operation, and the rod-side oil chamber 15B of the boom cylinder 15.

[0080] As a result, regardless of whether a single operation or a combined operation is being performed as an excavation operation, the work machine 1 can perform the boom-raising operation described above without supplying hydraulic oil from the hydraulic pump 33 to the extension-side oil chamber of the boom cylinder 15 via the control valve device 38. Therefore, the work machine 1 can prevent high-pressure hydraulic oil from being discharged from the overload relief valves 41A to 44A to the hydraulic oil tank 34 and the vehicle bodies 2, 3 from floating up, while preventing a slowdown in the arm crowding operation speed or bucket crowding operation speed during, for example, an arm-bucket combined operation. Therefore, the work machine 1 can allow the work implement 11 to perform work appropriately and efficiently even if the hydraulic cylinders 16, 17 of the work implement 11 are in an overload state.

[0081] Furthermore, in the work machine 1 of the present embodiment, a third valve 21A is provided on the first pipeline 24. The third valve 21A opens when the pressure in the bottom-side oil chamber 16A of the arm cylinder 16 exceeds a first threshold value, allowing hydraulic oil to flow from the rod-side oil chamber 15B of the boom cylinder 15 to the rod-side oil chamber 16B of the arm cylinder 16. The second pipeline 25 is provided with a fourth valve 23A. The fourth valve 23A opens when the pressure in the bottom-side oil chamber 17A of the bucket cylinder 17 exceeds a second threshold value, allowing hydraulic oil to flow from the rod-side oil chamber 15B of the boom cylinder 15 to the rod-side oil chamber 17B of the bucket cylinder 17.

[0082] As a result, regardless of whether a single operation or a combined operation is being performed as an excavation operation, the work machine 1 can reliably perform the boom raising operation described above without supplying hydraulic oil from the hydraulic pump 33 to the rod-side oil chamber 15B of the boom cylinder 15 via the control valve device 38. Therefore, the work machine 1 can prevent high-pressure hydraulic oil from being discharged from the overload relief valves 41A to 44A to the hydraulic oil tank 34 and the vehicle bodies 2, 3 from floating up, while reliably preventing a slowdown in the arm crowding operation speed or bucket crowding operation speed during combined arm-bucket operation, for example. Therefore, the work machine 1 can allow the work implement 11 to perform work appropriately and efficiently even if the hydraulic cylinders 16, 17 of the work implement 11 are overloaded.

[0083] Furthermore, in the work machine 1 of this embodiment, the first valve 20A, the second valve 22A, the third valve 21A, and the fourth valve 23A are configured as sequence valves. The first valve 20A and the third valve 21A are opened by the same pilot pressure. The second valve 22A and the fourth valve 23A are opened by the same pilot pressure.

[0084] This allows the work machine 1 to drive the first valve 20A to the fourth valve 23A mechanically and hydraulically. Furthermore, the work machine 1 can determine the open state of the first valve 20A or the second valve 22A, which allows hydraulic oil to flow into the bottom-side oil chamber 15A of the boom cylinder 15, and the open state of the third valve 21A or the fourth valve 23A, which allows hydraulic oil to flow out of the rod-side oil chamber 15B, under the same pressure conditions, eliminating the need to monitor the establishment of these pressure conditions through electrical control using a sensor or the like. This minimizes electrical control of the first valve 20A to the fourth valve 23A, ensuring the robustness of the hydraulic drive circuit 31, thereby enabling the boom-raising operation described above to be performed more reliably. This allows the work machine 1 to operate the work implement 11 more appropriately and efficiently, even when the hydraulic cylinders 16, 17 of the work implement 11 are overloaded.

[0085] Furthermore, in the work machine 1 of this embodiment, the first valve 20A, the second valve 22A, the third valve 21A and the fourth valve 23A are configured as solenoid valves.

[0086] As a result, even if the work machine 1 is a model in which it is difficult to arrange a pilot circuit, the hydraulic drive circuit 31 can be installed and the boom-raising operation described above can be performed. Therefore, in various models of work machine 1, the work implement 11 can perform work appropriately and efficiently even if the hydraulic cylinders 16, 17 of the work implement 11 are overloaded.

[0087] Furthermore, in the work machine 1 of this embodiment, the control valve device 38 is configured as a tandem circuit in which the arm directional control valve 38B and the bucket directional control valve 38C are connected in parallel to each other downstream of the boom directional control valve 38A.

[0088] As a result, even if the control valve device 38 is configured with this tandem circuit, the work machine 1 can prevent a slowdown in the arm crowding operation speed or bucket crowding operation speed during combined arm-bucket operation, for example, while preventing high-pressure hydraulic oil from being discharged from the overload relief valves 41A to 44A to the hydraulic oil tank 34 and preventing the vehicle bodies 2, 3 from floating up. Therefore, even if the hydraulic cylinders 16, 17 of the work implement 11 of the work machine 1 are in an overload state, the work machine 1 can allow the work implement 11 to perform work appropriately and more efficiently.

[0089] Furthermore, the work machine 1 of this embodiment is a backhoe-type hydraulic excavator. When an arm-crowding operation, in which only the arm 13 of the boom 12, arm 13, and bucket 14 is operated in the crowding direction, is performed as an excavation operation, or when an arm-bucket combined operation, in which both the arm 13 and the bucket 14 are operated in the crowding direction, is performed as an excavation operation, if the pressure in the bottom-side oil chamber 16A of the arm cylinder 16 exceeds the first threshold value and the arm cylinder 16 enters an overload state, the hydraulic oil in the bottom-side oil chamber 16A of the arm cylinder 16 is supplied to the bottom-side oil chamber 15A of the boom cylinder 15 via the first communicating pipe 20, and the boom 12 rotates upward even though a boom-raising operation is not being performed. When the work machine 1 is in an overload state of the bucket cylinder 17, where the pressure in the bottom oil chamber 17A of the bucket cylinder 17 exceeds the second threshold value, the hydraulic oil in the bottom oil chamber 17A of the bucket cylinder 17 is supplied to the bottom oil chamber 15A of the boom cylinder 15 via the second communicating pipe 22, causing the boom 12 to rotate upward even though no boom-raising operation is being performed.

[0090] As a result, the work machine 1 can automatically perform a boom-up operation if the arm cylinder 16 or the bucket cylinder 17 becomes overloaded, and can eliminate the overload state, regardless of whether a single operation or a combined operation is being performed as an excavation operation. Regardless of the operating status of the work device 11, the work machine 1 can allow the work device 11 to perform work appropriately and efficiently even if the hydraulic cylinders 16, 17 of the work device 11 become overloaded.

[0091] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to these embodiments and various modifications can be made without departing from the spirit of the present invention. In the present invention, elements of one embodiment can be added to elements of another embodiment, elements of one embodiment can be replaced with elements of another embodiment, or some of the elements of one embodiment can be deleted.

[0092] REFERENCE SIGNS LIST 1...work machine, 2...undercarriage, 3...upper rotating body, 4...swivel device, 5...swivel frame, 6...cab, 11...work device, 12...boom, 13...arm, 14...bucket, 15...boom cylinder, 15A...bottom side oil chamber, 15B...rod side oil chamber, 16...arm cylinder, 16A...bottom side oil chamber, 16B...rod side oil chamber, 17...bucket cylinder, 17A...bottom side oil chamber, 17B...rod side oil chamber, 20...first communicating pipe, 20A...first valve, 21...return circuit, 21A...third valve, 22...second communicating pipe, 22A...second valve, 23...return circuit, 23A...fourth valve, 24...first Pipe, 24A...check valve, 25...second pipe, 25A...check valve, 31...hydraulic drive circuit, 32...prime mover, 33...hydraulic pump, 34...hydraulic oil tank, 36...discharge pipe, 38...control valve device, 38A...boom directional control valve, 38B...arm directional control valve, 38C...bucket directional control valve, 39...BMCB pipe, 40...BMCR pipe, 41...AMCB pipe, 41A...overload relief valve, 42...AMCR pipe, 42A...overload relief valve, 43...BKCB pipe, 43A...overload relief valve, 44...BKCR pipe, 44A...overload relief valve

Claims

1. A work machine comprising: a working device including a boom rotatably connected to a vehicle body, an arm rotatably connected to the boom, a boom cylinder that drives the boom, and an arm cylinder that drives the arm; a hydraulic pump that supplies hydraulic oil to the boom cylinder and the arm cylinder; a control valve device including a boom direction control valve that is provided between the hydraulic pump and the boom cylinder and controls the flow direction of the hydraulic oil supplied from the hydraulic pump to the boom cylinder, and an arm direction control valve that is provided between the hydraulic pump and the arm cylinder and controls the flow direction of the hydraulic oil supplied from the hydraulic pump to the arm cylinder; and a first communicating pipe that communicates between the bottom-side oil chamber of the boom cylinder, which is an extension-side oil chamber when the boom cylinder is extended by an excavation operation, and the bottom-side oil chamber of the arm cylinder, which is an extension-side oil chamber when the arm cylinder is extended by the excavation operation.

2. The working device includes a bucket rotatably connected to the arm, and a bucket cylinder that drives the bucket, the hydraulic pump supplies the hydraulic oil to the bucket cylinder, the control valve device includes a bucket direction control valve that is provided between the hydraulic pump and the bucket cylinder and controls the flow direction of the hydraulic oil supplied from the hydraulic pump to the bucket cylinder, a bottom-side oil chamber of the bucket cylinder that is an extension-side oil chamber when the bucket cylinder is extended by the excavation operation and the bottom-side oil chamber of the boom cylinder are communicated by a second communicating pipe, and a first valve is provided on the first communicating pipe that opens when the pressure in the bottom-side oil chamber of the arm cylinder exceeds a first threshold value to allow the hydraulic oil to flow from the bottom-side oil chamber of the arm cylinder to the bottom-side oil chamber of the boom cylinder, 2. The work machine according to claim 1, wherein a second valve is provided on the second communicating pipe, the second valve opening when the pressure in the bottom-side oil chamber of the bucket cylinder exceeds a second threshold value to allow the hydraulic oil to flow from the bottom-side oil chamber of the bucket cylinder to the bottom-side oil chamber of the boom cylinder.

3. The work machine according to claim 2, further comprising: a first pipeline connecting the rod-side oil chamber of the boom cylinder, which is the retracted-side oil chamber when the boom cylinder is extended by the excavation operation, and the rod-side oil chamber of the arm cylinder, which is the retracted-side oil chamber when the arm cylinder is extended by the excavation operation; and a second pipeline connecting the rod-side oil chamber of the bucket cylinder, which is the retracted-side oil chamber when the bucket cylinder is extended by the excavation operation, and the rod-side oil chamber of the boom cylinder.

4. A work machine as described in claim 3, characterized in that a third valve is provided on the first pipeline that opens when the pressure in the bottom-side oil chamber of the arm cylinder exceeds the first threshold value, allowing the hydraulic oil to flow from the rod-side oil chamber of the boom cylinder to the rod-side oil chamber of the arm cylinder, and a fourth valve is provided on the second pipeline that opens when the pressure in the bottom-side oil chamber of the bucket cylinder exceeds the second threshold value, allowing the hydraulic oil to flow from the rod-side oil chamber of the boom cylinder to the rod-side oil chamber of the bucket cylinder.

5. The work machine according to claim 4, wherein the first valve, the second valve, the third valve and the fourth valve are configured as sequence valves, the first valve and the third valve are opened by the same pilot pressure, and the second valve and the fourth valve are opened by the same pilot pressure.

6. A work machine according to claim 4, characterized in that the first valve, the second valve, the third valve and the fourth valve are constituted by solenoid valves.

7. A work machine according to claim 2, wherein the control valve device is configured by a tandem circuit in which the arm directional control valve and the bucket directional control valve are connected in parallel to each other downstream of the boom directional control valve.

8. The work machine according to claim 2, wherein the bucket is a backhoe-type bucket, and when an arm-crowd single operation in which only the arm of the boom, arm, and bucket is operated in the crowding direction is performed as the excavation operation, or an arm-bucket combined operation in which both the arm and the bucket are operated in the crowding direction is performed as the excavation operation, if the pressure in the bottom-side oil chamber of the arm cylinder exceeds a first threshold value and the arm cylinder enters an overload state, the hydraulic oil in the bottom-side oil chamber of the arm cylinder is supplied to the bottom-side oil chamber of the boom cylinder via the first communicating pipe, causing the boom to rotate upward even though a boom-raising operation for operating the boom upward is not being performed, and when the pressure in the bottom-side oil chamber of the bucket cylinder exceeds a second threshold value and the bucket cylinder enters an overload state, the hydraulic oil in the bottom-side oil chamber of the bucket cylinder is supplied to the bottom-side oil chamber of the boom cylinder via the second communicating pipe, causing the boom to rotate upward even though the boom-raising operation is not being performed.

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