Hydraulic system, hydraulic excavator, and control method for hydraulic excavator

Abstract

To warm up a hydraulic device mounted on a hydraulic shovel.SOLUTION: A hydraulic system comprises: a main hydraulic circuit composed of an open circuit for driving a work machine cylinder 24; a swing hydraulic circuit composed of a closed circuit for driving a swing motor 30; a back pressure valve 18 that is arranged in a tank flow path 27 between a main valve 26 and a tank 28, and adjusts the back pressure of the main valve 26; a warm-up flow path 76 arranged around a spool of a neutral holding valve 33; a supply channel 78 connecting an upstream portion of a tank flow path 17 between the main valve 26 and the back pressure valve 18 and an inlet port of the warm-up flow path 76; and a discharge flow path 79 connecting a downstream portion of the tank flow path between the back pressure valve 18 and the tank 28 and an outlet port of the warm-up flow path 76. When the temperature is low, the back pressure valve 18 is set to a second set pressure, and warm oil warmed by relief operation of the main valve 26 is supplied to a warm circuit of the neutral holding valve 33, thereby promoting warming up of the neutral holding valve 33.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present disclosure relates to a hydraulic system, a hydraulic excavator, and a control method for a hydraulic excavator.

Background Art

[0002] In the technical field related to hydraulic excavators, a hydraulic excavator as disclosed in Patent Document 1 is known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] A hydraulic excavator has a plurality of hydraulic devices. When the hydraulic excavator starts, the hydraulic devices may be warmed up. For example, if high-temperature hydraulic oil is suddenly supplied to a hydraulic device that has not been warmed up, malfunction of the hydraulic device may occur.

[0005] An object of the present disclosure is to warm the hydraulic devices mounted on a hydraulic excavator.

Means for Solving the Problems

[0006] According to the present disclosure, there is provided a hydraulic system including: a main hydraulic circuit configured by an open circuit having a main pump, a work implement cylinder, a main valve that controls the inflow of hydraulic oil from the main pump to the work implement cylinder, and a tank to which the hydraulic oil flowing out from the work implement cylinder is discharged through the main valve; a swing hydraulic circuit configured by a closed circuit having a swing pump, a swing motor, and a neutral holding valve that controls the passage of hydraulic oil between the swing pump and the swing motor; a back pressure valve disposed in a tank flow path between the main valve and the tank to adjust the back pressure of the main valve; a warm-up flow path disposed around a spool of the neutral holding valve; a supply flow path connecting an upstream portion of the tank flow path between the main valve and the back pressure valve and an inlet port of the warm-up flow path; and a discharge flow path connecting a downstream portion of the tank flow path between the back pressure valve and the tank and an outlet port of the warm-up flow path.

Advantages of the Invention

[0007] According to the present disclosure, the hydraulic equipment mounted on the hydraulic excavator is warmed up.

Brief Description of the Drawings

[0008]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Modes for Carrying Out the Invention

[0009] Hereinafter, embodiments according to the present disclosure will be described with reference to the drawings, but the present disclosure is not limited to the embodiments. The components of each embodiment described below can be combined as appropriate. Also, there may be cases where some components are not used.

[0010] [Hydraulic Excavator] FIG. 1 is a perspective view showing a hydraulic excavator 1 according to an embodiment. As shown in FIG. 1, the hydraulic excavator 1 includes an upper swing body 2, a working machine 3, a lower traveling body 4, and an operation lever 5.

[0011] The upper swing body 2 is rotatably supported by the lower traveling body 4. The upper swing body 2 supports the working machine 3. The upper swing body 2 has a driver's cab 6. The operator of the hydraulic excavator 1 boards the driver's cab 6. A driver's seat 7 on which the operator sits is provided in the driver's cab 6.

[0012] The working machine 3 is connected to the upper swing body 2. The working machine 3 includes a boom 8, an arm 9, and a bucket 10. The boom 8 is rotatably connected to the upper swing body 2. The arm 9 is rotatably connected to the boom 8. The bucket 10 is rotatably connected to the arm 9.

[0013] The lower traveling body 4 supports the upper swing body 2. The lower traveling body 4 has a drive wheel 11 and a crawler 12. When the drive wheel 11 rotates, the crawler 12 rotates. When the crawler 12 rotates, the lower traveling body 4 travels.

[0014] The operation lever 5 is operated by the operator of the hydraulic excavator 1. The operation lever 5 is operated to operate each of the upper swing body 2, the working machine 3, and the lower traveling body 4. The operation lever 5 is disposed in the driver's cab 6.

[0015] [Hydraulic System] FIG. 2 is a diagram showing a hydraulic system 13 according to an embodiment. The hydraulic system 13 is mounted on the hydraulic excavator 1. As shown in FIGS. 1 and 2, the hydraulic system 13 includes an engine 14, a power take-off 15, a main hydraulic circuit 16, a swing hydraulic circuit 17, a back pressure valve 18, a relief valve 19, a temperature sensor 20, and a controller 21.

[0016] The engine 14 is a power source of the hydraulic excavator 1. As an example of the engine 14, a diesel engine is exemplified. The engine 14 is connected to the power take-off 15.

[0017] The main hydraulic circuit 16 operates at least one of the work implement 3 and the lower traveling body 4. Working oil for operating at least one of the work implement 3 and the lower traveling body 4 flows through the main hydraulic circuit 16. The main hydraulic circuit 16 includes a main pump 22, a pump passage 23, a work implement cylinder 24, a travel motor 25, a main valve 26, a tank passage 27, and a tank 28. The main hydraulic circuit 16 is configured by an open circuit.

[0018] The swing hydraulic circuit 17 operates the upper swing body 2. Working oil for operating the upper swing body 2 flows through the swing hydraulic circuit 17. The swing hydraulic circuit 17 includes a swing pump 29, a swing motor 30, a first main passage 31, a second main passage 32, and a neutral holding valve 33. The swing hydraulic circuit 17 is configured by a closed circuit.

[0019] The power take-off 15 distributes the power generated by the engine 14 to each of the main pump 22 and the swing pump 29.

[0020] <Main Hydraulic Circuit> The main pump 22 is a variable displacement hydraulic pump. The main pump 22 is connected to the power take-off 15. The main pump 22 is driven by the engine 14. The main pump 22 discharges working oil for driving each of the work implement cylinder 24 and the travel motor 25.

[0021] The pump passage 23 is connected to the discharge port 22A of the main pump 22. The hydraulic oil discharged from the discharge port 22A of the main pump 22 flows through the pump passage 23.

[0022] The work implement cylinder 24 is a hydraulic cylinder. The work implement cylinder 24 generates power for operating the work implement 3. The work implement cylinder 24 is driven based on the hydraulic oil discharged from the main pump 22. The work implement cylinder 24 includes a boom cylinder 35, an arm cylinder 36, and a bucket cylinder 37. The boom cylinder 35 operates the boom 8. The arm cylinder 36 operates the arm 9. The bucket cylinder 37 operates the bucket 10.

[0023] The travel motor 25 is a hydraulic motor. The travel motor 25 generates power for operating the lower travel body 4. The travel motor 25 is driven based on the hydraulic oil discharged from the main pump 22. The travel motor 25 rotates the drive wheels 11.

[0024] The main valve 26 is connected to the main pump 22 via the pump passage 23. The main valve 26 controls the inflow of hydraulic oil from the main pump 22 to the work implement cylinder 24. Also, the main valve 26 controls the inflow of hydraulic oil from the main pump 22 to the travel motor 25. The main valve 26 has a boom spool 38, an arm spool 39, a bucket spool 40, and a travel spool 41.

[0025] The boom spool 38 controls the inflow of hydraulic oil from the main pump 22 to the boom cylinder 35. The boom spool 38 controls the flow rate and direction of the hydraulic oil supplied from the main pump 22 to the boom cylinder 35.

[0026] The arm spool 39 controls the inflow of hydraulic oil from the main pump 22 to the arm cylinder 36. The arm spool 39 controls the flow rate and direction of the hydraulic oil supplied from the main pump 22 to the arm cylinder 36.

[0027] The bucket spool 40 controls the inflow of hydraulic oil from the main pump 22 to the bucket cylinder 37. The bucket spool 40 controls the flow rate and direction of the hydraulic oil supplied from the main pump 22 to the bucket cylinder 37.

[0028] The travel spool 41 controls the inflow of hydraulic oil from the main pump 22 to the travel motor 25. The travel spool 41 controls the flow rate and direction of the hydraulic oil supplied to the travel motor 25.

[0029] The tank flow path 27 is connected to the tank 28. The hydraulic oil flowing out from the work implement cylinder 24 flows through the main valve 26 and then through the tank flow path 27.

[0030] The tank 28 discharges the hydraulic oil from the tank flow path 27. The hydraulic oil flowing out from the work implement cylinder 24 is discharged to the tank 28 via the main valve 26.

[0031] The main valve 26 has a pump port 38A, a first inlet / outlet port 38B, a second inlet / outlet port 38C, a tank port 38D, a neutral inlet port 38E, and a neutral outlet port 38F. The pump port 38A is connected to the pump flow path 23 via the inflow flow path 42. The first inlet / outlet port 38B is connected to the bottom chamber 35A of the boom cylinder 35 via the bottom flow path 43. The second inlet / outlet port 38C is connected to the rod chamber 35B of the boom cylinder 35 via the rod flow path 44. The tank port 38D is connected to the tank flow path 27 via the outflow flow path 45. The neutral inlet port 38E is connected to the neutral flow path 46. The neutral outlet port 38F is connected to the neutral flow path 47.

[0032] The boom spool 38 moves to the bottom position, the rod position, and the neutral position. When the boom spool 38 is disposed at the bottom position, the hydraulic oil discharged from the main pump 22 is supplied to the bottom chamber 35A of the boom cylinder 35 through the inflow passage 42, the boom spool 38, and the bottom passage 43, and the hydraulic oil discharged from the rod chamber 35B of the boom cylinder 35 is discharged to the tank 28 through the rod passage 44, the boom spool 38, and the outflow passage 45. When the boom spool 38 is disposed at the rod position, the hydraulic oil discharged from the main pump 22 is supplied to the rod chamber 35B of the boom cylinder 35 through the inflow passage 42, the boom spool 38, and the rod passage 44, and the hydraulic oil discharged from the bottom chamber 35A of the boom cylinder 35 is discharged to the tank 28 through the bottom passage 43, the boom spool 38, and the outflow passage 45. When the boom spool 38 is disposed at the neutral position, the passage of the hydraulic oil through the boom spool 38 is blocked.

[0033] The main valve 26 has a pump port 39A, a first inlet / outlet port 39B, a second inlet / outlet port 39C, a tank port 39D, a neutral inlet port 39E, and a neutral outlet port 39F. The pump port 39A is connected to the pump passage 23 through the inflow passage 48. The first inlet / outlet port 39B is connected to the bottom chamber 36A of the arm cylinder 36 through the bottom passage 49. The second inlet / outlet port 39C is connected to the rod chamber 36B of the arm cylinder 36 through the rod passage 50. The tank port 39D is connected to the tank passage 27 through the outflow passage 51. The neutral inlet port 39E is connected to the neutral passage 47. The neutral outlet port 39F is connected to the neutral passage 52.

[0034] The arm spool 39 moves to the bottom position, the rod position, and the neutral position. When the arm spool 39 is disposed at the bottom position, the hydraulic oil discharged from the main pump 22 is supplied to the bottom chamber 36A of the arm cylinder 36 via the inflow passage 48, the arm spool 39, and the bottom passage 49, and the hydraulic oil discharged from the rod chamber 36B of the arm cylinder 36 is discharged to the tank 28 via the rod passage 50, the arm spool 39, and the outflow passage 51. When the arm spool 39 is disposed at the rod position, the hydraulic oil discharged from the main pump 22 is supplied to the rod chamber 36B of the arm cylinder 36 via the inflow passage 48, the arm spool 39, and the rod passage 50, and the hydraulic oil discharged from the bottom chamber 36A of the arm cylinder 36 is discharged to the tank 28 via the bottom passage 49, the arm spool 39, and the outflow passage 51. When the arm spool 39 is disposed at the neutral position, the passage of the hydraulic oil through the arm spool 39 is blocked.

[0035] The main valve 26 has a pump port 40A, a first inlet / outlet port 40B, a second inlet / outlet port 40C, a tank port 40D, a neutral inlet port 40E, and a neutral outlet port 40F. The pump port 40A is connected to the pump passage 23 via the inflow passage 53. The first inlet / outlet port 40B is connected to the bottom chamber 37A of the bucket cylinder 37 via the bottom passage 54. The second inlet / outlet port 40C is connected to the rod chamber 37B of the bucket cylinder 37 via the rod passage 55. The tank port 40D is connected to the tank passage 27 via the outflow passage 56. The neutral inlet port 40E is connected to the neutral passage 52. The neutral outlet port 40F is connected to the neutral passage 57.

[0036] The bucket spool 40 moves to the bottom position, the rod position, and the neutral position. When the bucket spool 40 is disposed at the bottom position, the hydraulic oil discharged from the main pump 22 is supplied to the bottom chamber 37A of the bucket cylinder 37 via the inflow passage 53, the bucket spool 40, and the bottom passage 54, and the hydraulic oil discharged from the rod chamber 37B of the bucket cylinder 37 is discharged to the tank 28 via the rod passage 55, the bucket spool 40, and the outflow passage 56. When the bucket spool 40 is disposed at the rod position, the hydraulic oil discharged from the main pump 22 is supplied to the rod chamber 37B of the bucket cylinder 37 via the inflow passage 53, the bucket spool 40, and the rod passage 55, and the hydraulic oil discharged from the bottom chamber 37A of the bucket cylinder 37 is discharged to the tank 28 via the bottom passage 54, the bucket spool 40, and the outflow passage 56. When the bucket spool 40 is disposed at the neutral position, the passage of the hydraulic oil through the bucket spool 40 is blocked.

[0037] The main valve 26 has a pump port 41A, a first inlet / outlet port 41B, a second inlet / outlet port 41C, a tank port 41D, a neutral inlet port 41E, and a neutral outlet port 41F. The pump port 41A is connected to the pump passage 23 via the inflow passage 58. The first inlet / outlet port 41B is connected to the first suction port 25A of the travel motor 25 via the first motor passage 59. The second inlet / outlet port 41C is connected to the second suction port 25B of the travel motor 25 via the second motor passage 60. The tank port 41D is connected to the tank passage 27 via the outflow passage 61. The neutral inlet port 41E is connected to the pump passage 23. The neutral outlet port 41F is connected to the neutral passage 46.

[0038] The traveling spool 41 moves to a forward position, a reverse position, and a neutral position. When the traveling spool 41 is disposed at the forward position, the hydraulic oil discharged from the main pump 22 is supplied to the first suction port 25A of the traveling motor 25 via the inflow passage 58, the traveling spool 41, and the first motor passage 59, and the hydraulic oil discharged from the second suction port 25B of the traveling motor 25 is discharged to the tank 28 via the second motor passage 60, the traveling spool 41, and the outflow passage 61. When the traveling spool 41 is disposed at the reverse position, the hydraulic oil discharged from the main pump 22 is supplied to the second suction port 25B of the traveling motor 25 via the inflow passage 58, the traveling spool 41, and the second motor passage 60, and the hydraulic oil discharged from the first suction port 25A of the traveling motor 25 is discharged to the tank 28 via the first motor passage 59, the traveling spool 41, and the outflow passage 61. When the traveling spool 41 is disposed at the neutral position, the passage of the hydraulic oil through the traveling spool 41 is blocked.

[0039] In the embodiment, the inflow passage 42, the inflow passage 48, the inflow passage 53, and the inflow passage 58 are connected in parallel to the pump passage 23. The outflow passage 45, the outflow passage 51, the outflow passage 56, and the outflow passage 61 are connected in parallel to the tank passage 27.

[0040] In the example shown in FIG. 2, each of the boom spool 38, the arm spool 39, the bucket spool 40, and the traveling spool 41 is disposed at the neutral position. The neutral passage 57 is connected to a negative control mechanism 62 that negatively controls the capacity of the main pump 22. When each of the boom spool 38, the arm spool 39, the bucket spool 40, and the traveling spool 41 is disposed at the neutral position, the hydraulic oil discharged from the main pump 22 is discharged to the tank 28 via the pump passage 23, the traveling spool 41, the neutral passage 46, the boom spool 38, the neutral passage 47, the arm spool 39, the neutral passage 52, the bucket spool 40, the neutral passage 57, the negative control mechanism 62, and the tank passage 27.

[0041] A boom check valve 63 is arranged in the inflow passage 42. An arm check valve 64 is arranged in the inflow passage 48. A bucket check valve 65 is arranged in the inflow passage 53. A travel check valve 66 is arranged in the inflow passage 58.

[0042] The boom check valve 63 suppresses the reverse flow of the hydraulic oil from the boom cylinder 35 to the main pump 22 via the boom spool 38. The arm check valve 64 suppresses the reverse flow of the hydraulic oil from the arm cylinder 36 to the main pump 22 via the arm spool 39. The bucket check valve 65 suppresses the reverse flow of the hydraulic oil from the bucket cylinder 37 to the main pump 22 via the bucket spool 40. The travel check valve 66 suppresses the reverse flow of the hydraulic oil from the travel motor 25 to the main pump 22 via the travel spool 41.

[0043] <Slewing hydraulic circuit> The slewing pump 29 is a variable displacement hydraulic pump. The slewing pump 29 is connected to the power take-off 15. The slewing pump 29 is driven by the engine 14. The slewing pump 29 discharges the hydraulic oil for driving the slewing motor 30.

[0044] The slewing motor 30 is a hydraulic motor. The slewing motor 30 generates the power for slewing the upper slewing structure 2. The slewing motor 30 is driven based on the hydraulic oil discharged from the slewing pump 29.

[0045] The first main passage 31 connects the first discharge port 29A of the slewing pump 29 and the first suction port 30A of the slewing motor 30. The second main passage 32 connects the second discharge port 29B of the slewing pump 29 and the second suction port 30B of the slewing motor 30. The first main passage 31, the second main passage 32, the slewing pump 29, and the slewing motor 30 constitute a closed circuit.

[0046] The neutral holding valve 33 controls the passage of the hydraulic oil between the swivel pump 29 and the swivel motor 30. The neutral holding valve 33 is arranged to be interposed between the first main flow path 31 and the second main flow path 32. The neutral holding valve 33 has a spool 67. The neutral holding valve 33 operates based on the control signal output from the controller 21.

[0047] The neutral holding valve 33 has a first pump port 67A, a second pump port 67B, a first motor port 67C, and a second motor port 67D. The first pump port 67A is connected to the first discharge port 29A of the swivel pump 29 via the first main flow path 31. The second pump port 67B is connected to the second discharge port 29B of the swivel pump 29 via the second main flow path 32. The first motor port 67C is connected to the first suction port 30A of the swivel motor 30 via the first main flow path 31. The second motor port 67D is connected to the second suction port 30B of the swivel motor 30 via the second main flow path 32.

[0048] The spool 67 moves to a first position, a second position, and a neutral position. When the spool 67 is arranged at the first position, the passage of the hydraulic oil between the first pump port 67A and the first motor port 67C is blocked, and the passage of the hydraulic oil from the second motor port 67D to the second pump port 67B is allowed. When the spool 67 is arranged at the second position, the passage of the hydraulic oil between the second pump port 67B and the second motor port 67D is blocked, and the passage of the hydraulic oil from the first motor port 67C to the first pump port 67A is allowed. When the spool 67 of the neutral holding valve 33 is arranged at the neutral position, the passage of the hydraulic oil between the first pump port 67A and the first motor port 67C is blocked, and the passage of the hydraulic oil between the second pump port 67B and the second motor port 67D is blocked.

[0049] The neutral holding valve 33 has a first bypass passage 68 and a second bypass passage 69 that are arranged to bypass the spool 67. The first bypass passage 68 connects the first discharge port 29A of the swivel pump 29 and the first suction port 30A of the swivel motor 30. The second bypass passage 69 connects the second discharge port 29B of the swivel pump 29 and the second suction port 30B of the swivel motor 30.

[0050] A check valve 70 is arranged in the first bypass passage 68. A check valve 71 is arranged in the second bypass passage 69. The check valve 70 and the check valve 71 allow the working oil to pass from the swivel pump 29 to the swivel motor 30 and prevent the working oil from passing from the swivel motor 30 to the swivel pump 29.

[0051] <Back pressure valve> The back pressure valve 18 is arranged in the tank passage 27 between the main valve 26 and the tank 28. The back pressure valve 18 adjusts the back pressure of the main valve 26. The back pressure valve 18 adjusts, as the back pressure of the main valve 26, at least the pressure in the tank passage 27 between the main valve 26 and the back pressure valve 18.

[0052] The back pressure valve 18 lowers the back pressure of the main valve 26 by opening the tank passage 27 and allowing the working oil from the main valve 26 to pass through. The back pressure valve 18 raises the back pressure of the main valve 26 by narrowing the tank passage 27 and restricting the passage of the working oil from the main valve 26.

[0053] The spool of the back pressure valve 18 moves to an open position that opens the tank passage 27 and a throttle position that narrows the tank passage 27. In the embodiment, the back pressure valve 18 is of a normally closed type. An electromagnetic part 18A is connected to a part of the spool of the back pressure valve 18, and a spring part 18B is connected to another part. When the electromagnetic part 18A is energized, the spool of the back pressure valve 18 moves to the open position by the action of the electromagnetic part 18A. When the electromagnetic part 18A is de-energized, the spool of the back pressure valve 18 moves to the throttle position by the action of the spring part 18B.

[0054] The back pressure valve 18 adjusts the back pressure of the main valve 26 to the first back pressure by opening the tank flow path 27. The back pressure valve 18 adjusts the back pressure of the main valve 26 to the second back pressure by narrowing the tank flow path 27. The second back pressure is higher than the first back pressure. As an example, the first back pressure is 0 [kg / cm 2 in gauge pressure. The second back pressure is 8 [kg / cm 2 in gauge pressure.

[0055] In the following description, the tank flow path 27 between the main valve 26 and the back pressure valve 18 is appropriately referred to as the first part (upstream part) 27A of the tank flow path 27, and the tank flow path 27 between the back pressure valve 18 and the tank 28 is appropriately referred to as the second part (downstream part) 27B of the tank flow path 27.

[0056] <Relief valve> The relief valve 19 is connected to the pump flow path 23 between the main pump 22 and the main valve 26 (travel spool 41) via the relief flow path 74. The relief valve 19 is also connected to the first part 27A of the tank flow path 27 via the relief flow path 75. In the embodiment, the relief flow path 75 is connected to the outflow flow path 61. The outflow flow path 61 is connected to the first part 27A of the tank flow path 27. The relief valve 19 is connected to the first part 27A of the tank flow path 27 via the relief flow path 75 and the outflow flow path 61.

[0057] The relief valve 19 is opened when the discharge pressure of the main pump 22 is equal to or higher than a predetermined relief pressure. The discharge pressure of the main pump 22 includes the pressure of the hydraulic oil in the pump flow path 23. When the relief valve 19 is opened, the hydraulic oil flows out from the relief valve 19 into the relief flow path 75. The hydraulic oil flowing out from the relief valve 19 is supplied to the first part 27A via the outflow flow path 61.

[0058] <Temperature sensor> The temperature sensor 20 detects the temperature of the hydraulic oil in the tank flow path 27. In the embodiment, the temperature sensor 20 detects the temperature of the hydraulic oil in the second part 27B. Note that the temperature sensor only needs to be able to detect the temperature of the hydraulic oil in the tank flow path 27, and the installation position is not limited. The temperature sensor 20 may be provided on the low-pressure circuit side such as the suction pipe of the pump.

[0059] <Controller> FIG. 3 is a block diagram showing the controller 21 according to the embodiment. The controller 21 includes a computer system. The controller 21 has a processor 21A such as a CPU (Central Processing Unit), a main memory 21B including a non-volatile memory such as a ROM (Read Only Memory) and a volatile memory such as a RAM (Random Access Memory), a storage 21C, and an interface 21D including an input / output circuit. The functions of the controller 21 are stored in the storage 21C as a computer program. The processor 21A reads the computer program from the storage 21C and expands it in the main memory 21B, and executes prescribed processing according to the computer program. Note that the computer program may be distributed to the controller 21 via a network.

[0060] The controller 21 controls at least the back pressure valve 18. The controller 21 controls the back pressure valve 18 so that the back pressure of the main valve 26 changes from the first back pressure to a second back pressure higher than the first back pressure. That is, the controller 21 controls the back pressure valve 18 so that the tank flow path 27 changes from an open state to a constricted state. As described above, in the embodiment, the back pressure valve 18 is of a normally closed type. The controller 21 can move the spool of the back pressure valve 18 from the open position to the throttle position by outputting a control signal so that the electromagnetic part 18A is energized.

[0061] Based on the detected value of the temperature sensor 20, the controller 21 controls the back pressure valve 18. When the detected value of the temperature sensor 20 is equal to or lower than a predetermined specified value, the controller 21 adjusts the back pressure to the second back pressure. The specified value is, for example, 40 [°C].

[0062] In addition, the controller 21 controls the neutral holding valve 33. The controller 21 controls the neutral holding valve 33 based on the operation signal of the operation lever 5 operated to turn the upper swing body 2. The controller 21 can move the spool 67 to the first position, the second position, and the neutral position by adjusting the pilot pressure acting on the pressure chamber at the end of the spool 67.

[0063] The operation lever 5 is operated by the operator of the hydraulic excavator 1. When the operation lever 5 is operated to turn the upper swing body 2, an operation signal corresponding to the operation amount and operation direction of the operation lever 5 is output from the operation lever 5.

[0064] Based on the operation signal from the operation lever 5, the controller 21 outputs a control signal corresponding to the operation amount and operation direction of the operation lever 5 to the neutral holding valve 33.

[0065] When the operation lever 5 is disposed at the neutral position, the tilt angle of the swash plate of the swing pump 29 becomes zero. When the tilt angle of the swash plate is zero, even if the engine 14 is started and the swing pump 29 is driven, the discharge amount of the hydraulic oil from the swing pump 29 is zero, so the swing motor 30 does not rotate.

[0066] When the neutral holding valve 33 is disposed at the neutral position, the passage of the hydraulic oil from the swing motor 30 to the swing pump 29 is blocked. In addition, check valves 70 and 71 for blocking the passage of the hydraulic oil from the swing motor 30 to the swing pump 29 are provided in the first bypass passage 68 and the second bypass passage 69. Therefore, for example, even if the hydraulic excavator 1 stops on a slope and an external force for turning the upper swing body 2 acts on the upper swing body 2, the stopped state of the swing motor 30 is maintained, so that the upper swing body 2 is prevented from turning inadvertently.

[0067] When the operation lever 5 is operated by the operator to rotate the upper swing body 2, a control signal corresponding to the operation amount and operation direction of the operation lever 5 is output from the controller 21 to the neutral holding valve. The tilt angle of the swash plate of the swing pump 29 is changed, and the spool 67 of the neutral holding valve 33 moves from the neutral position to the first position or the second position.

[0068] For example, when hydraulic oil is discharged from the first discharge port 29A of the swing pump 29 and the spool 67 of the neutral holding valve 33 moves to the first position, the hydraulic oil discharged from the first discharge port 29A of the swing pump 29 to the first main flow path 31 is supplied to the first suction port 30A of the swing motor 30 via the first bypass flow path 68. The hydraulic oil supplied to the swing motor 30 is discharged from the second suction port 30B to the second main flow path 32 and then supplied to the second discharge port 29B of the swing pump 29 via the neutral holding valve 33. In this case, the upper swing body 2 rotates, for example, clockwise.

[0069] For example, when hydraulic oil is discharged from the second discharge port 29B of the swing pump 29 and the spool 67 of the neutral holding valve 33 moves to the second position, the hydraulic oil discharged from the second discharge port 29B of the swing pump 29 to the second main flow path 32 is supplied to the second suction port 30B of the swing motor 30 via the second bypass flow path 69. The hydraulic oil supplied to the swing motor 30 is discharged from the first suction port 30A to the first main flow path 31 and then supplied to the first discharge port 29A of the swing pump 29 via the neutral holding valve 33. In this case, the upper swing body 2 rotates, for example, counterclockwise.

[0070] [Warming up of the neutral holding valve] FIG. 4 is a diagram schematically showing the neutral holding valve 33 according to the embodiment. As shown in FIG. 4, the neutral holding valve 33 has a spool 67 and a body 77 disposed around the spool 67. As shown in FIGS. 2 and 4, a warm-up flow path 76 is formed in the body 77 around the spool 67 of the neutral holding valve 33. The body 77 movably supports the spool 67. The warm-up flow path 76 is disposed in the body 77. In the embodiment, the warm-up flow path 76 is formed inside the body 77. Note that the warm-up flow path 76 may be a flow path of a tube disposed so as to contact the outer surface of the body 77.

[0071] The warm-up flow path 76 has an inlet port 76A and an outlet port 76B. Warm hydraulic oil flows into the warm-up flow path 76 through the inlet port 76A and flows through the warm-up flow path 76, whereby the neutral holding valve 33 is warmed up. The hydraulic oil that has flowed through the warm-up flow path 76 flows out from the outlet port 76B. The warm-up flow path 76 is formed at a position different from the position where the first pump port 67A, the second pump port 67B, the first motor port 67C, and the second motor port 67D of the body 77 are formed, and is not connected to the first pump port 67A, the second pump port 67B, the first motor port 67C, and the second motor port 67D.

[0072] As shown in FIGS. 2 and 4, the hydraulic system 13 includes a supply flow path 78 connected to the inlet port 76A of the warm-up flow path 76 and a discharge flow path 79 connected to the outlet port 76B of the warm-up flow path 76. The supply flow path 78 connects the first part 27A of the tank flow path 27 between the main valve 26 and the back pressure valve 18 and the inlet port 76A of the warm-up flow path 76. The discharge flow path 79 connects the second part 27B of the tank flow path 27 between the back pressure valve 18 and the tank 28 and the outlet port 76B of the warm-up flow path 76.

[0073] As described above, the relief valve 19 is opened when the discharge pressure of the main pump 22 is equal to or higher than the relief pressure. When the relief valve 19 is opened, the hydraulic oil flows out from the relief valve 19. When the hydraulic oil flows out from the relief valve 19, the pressure energy of the hydraulic oil is converted into thermal energy, and the temperature of the hydraulic oil rises. That is, the hydraulic oil flowing out from the relief valve 19 is warm.

[0074] The warm hydraulic oil flowing out from the relief valve 19 is supplied to the first portion 27A via the outflow passage 61. That is, the warm hydraulic oil is supplied to the first portion 27A. The warm hydraulic oil flowing out from the relief valve 19 is supplied to the warm-up passage 76 via the first portion 27A, the supply passage 78, and the inlet port 76A. Since the warm hydraulic oil flows through the warm-up passage 76, the neutral holding valve 33 is warmed up. The hydraulic oil that has flowed through the warm-up passage 76 flows out from the outlet port 76B and is discharged to the tank 28 via the second portion 27B.

[0075] [Control method] FIG. 5 is a flowchart showing a control method of the hydraulic excavator 1 according to the embodiment. With reference to FIG. 5, a warm-up method for the neutral holding valve 33 will be described as a control method of the hydraulic excavator 1.

[0076] When the hydraulic excavator 1 is keyed on and the hydraulic excavator 1 starts up, the temperature sensor 20 detects the temperature of the hydraulic oil. The controller 21 acquires the detection value of the temperature sensor 20 (step S1).

[0077] The controller 21 determines whether or not the detection value of the temperature sensor 20 indicating the temperature of the hydraulic oil is equal to or lower than a predetermined specified value (step S2).

[0078] In step S2, when it is determined that the detection value of the temperature sensor 20 is equal to or lower than the specified value (step S2: Yes), the controller 21 determines that warming up of the neutral holding valve 33 is necessary.

[0079] The controller 21 controls the back pressure valve 18 so that the tank passage 27 is throttled. When the tank passage 27 is closed, the back pressure of the main valve 26 is adjusted to the second back pressure (step S3).

[0080] FIG. 6 is a diagram showing the hydraulic system 13 when the neutral holding valve 33 according to the embodiment is being warmed up. As shown in FIG. 6, when warming up the neutral holding valve 33, the controller 21 adjusts the back pressure of the main valve 26 to the second back pressure so that the hydraulic oil is supplied from the first portion 27A to the warm-up passage 76. When the back pressure of the main valve 26 increases, the supply of the hydraulic oil from the first portion 27A of the tank passage 27 to the warm-up passage 76 is promoted.

[0081] In the embodiment, the warm-up of the main valve 26 is carried out in parallel with the warm-up of the neutral holding valve 33. When carrying out the warm-up of the neutral holding valve 33 and the warm-up of the main valve 26, the discharge pressure of the main pump 22 is increased to be equal to or higher than the relief pressure.

[0082] As shown in FIG. 6, the operator of the hydraulic excavator 1 operates the operation lever 5 so that, for example, the piston of the bucket cylinder 37 reaches the end (stroke end) of the movable range of the piston. In the example shown in FIG. 6, the operator of the hydraulic excavator 1 operates the operation lever 5 so that the piston is arranged at the most bottom side within the movable range of the piston. When the piston is arranged at the most bottom side, the bucket cylinder 37 is contracted the most. When the hydraulic oil continues to be supplied from the main pump 22 to the rod chamber 37B of the bucket cylinder 37 in the state where the piston is arranged at the most bottom side, the pressures of the hydraulic oil in the pump passage 23, the inflow passage 53, the rod passage 55, and the rod chamber 37B increase.

[0083] When the pressure of the hydraulic oil rises and the pressure of the hydraulic oil in each of the pump passage 23, the inflow passage 53, the rod passage 55, and the rod chamber 37B becomes equal to or higher than the relief pressure, the relief valve 19 is opened. When the relief valve 19 is opened, the hydraulic oil flows out from the relief valve 19 to the outflow passage 61 through the relief passage 75. As described above, the hydraulic oil flowing out from the relief valve 19 is warm. The warm hydraulic oil flowing out from the relief valve 19 is supplied to the warm-up passage 76 through the first portion 27A and the supply passage 78. Thereby, the neutral holding valve 33 is warmed up.

[0084] The hydraulic oil flowing through the warm-up passage 76 flows out from the outlet port 76B and is supplied to the second portion 27B through the discharge passage 79. The pressure of the second portion 27B is lower than the second back pressure. Therefore, the hydraulic oil flowing through the warm-up passage 76 can flow to the second portion 27B through the outlet port 76B and the discharge passage 79. The hydraulic oil supplied to the second portion 27B is discharged to the tank 28.

[0085] Also, as shown in FIG. 6, the warm hydraulic oil flowing out from the relief valve 19 is supplied to each of the outflow passage 45, the outflow passage 51, the outflow passage 56, the first motor passage 59, and the second motor passage 60. Further, when the hydraulic oil flows out from the bottom chamber 37A of the bucket cylinder 37, the pressure energy of the hydraulic oil is converted into thermal energy, and the temperature of the hydraulic oil rises. That is, the hydraulic oil flowing out from the bottom chamber 37A of the bucket cylinder 37 is warm. The hydraulic oil flowing out from the bottom chamber 37A of the bucket cylinder 37 is supplied to the bottom passage 54. The main valve 26 is warmed up by the hydraulic oil flowing out from the relief valve 19 and the hydraulic oil flowing out from the bucket cylinder 37.

[0086] In FIG. 6, the passage indicated by the thick solid line is the passage through which the warmed hydraulic oil passes.

[0087] Note that Fig. 6 shows a state where the piston of the bucket cylinder 37 has reached the stroke end. In order to make the discharge pressure of the main pump 22 equal to or higher than the relief pressure, the operation lever 5 may be operated so that the piston of the boom cylinder 35 reaches the stroke end, or the operation lever 5 may be operated so that the piston of the arm cylinder 36 reaches the stroke end.

[0088] The controller 21 controls the back pressure valve 18 so that the tank flow path 27 continues to be closed until the detected value of the temperature sensor 20 exceeds the specified value.

[0089] The controller 21 acquires the detected value of the temperature sensor 20 in a state where the tank flow path 27 is throttled (step S1), and determines whether or not the detected value of the temperature sensor 20 is equal to or less than the specified value (step S2).

[0090] In step S2, when it is determined that the detected value of the temperature sensor 20 is not equal to or less than the specified value (step S2: No), the controller 21 determines that warming up of the neutral holding valve 33 is unnecessary. When warming up of the neutral holding valve 33 is unnecessary, control of the back pressure valve 18 by the controller 21 is not performed. When a control signal is output from the controller 21 to the back pressure valve 18, the back pressure valve 18 operates so that the tank flow path 27 is opened. By opening the tank flow path 27, the back pressure of the main valve 26 is adjusted to the first back pressure.

[0091] When the neutral holding valve 33 is being warmed up, the operator of the hydraulic excavator 1 can operate the operation lever 5 to operate the hydraulic excavator 1 in a state where the back pressure of the main valve 26 is adjusted to the first back pressure.

[0092] When the back pressure of the main valve 26 is adjusted to the first back pressure and the working machine 3 is operated, the fuel consumption of the hydraulic excavator 1 is reduced. The working machine 3 is driven by the thrust generated by the pressure difference between the rod pressure of the cylinder and the pressure on the head side. When the back pressure of the main valve 26 is adjusted to the first back pressure, since the back pressure becomes low, even if the discharge pressure of the main pump 22 is low, the cylinder differential pressure required for driving the working machine 3 can be obtained. As a result, when the back pressure of the main valve 26 is low, even if the output of the engine 14 is low and the discharge pressure of the main pump 22 is low, hydraulic oil can be supplied to the working machine cylinder 24 with the pressure required for the operation of the working machine 3, so the fuel consumption of the hydraulic excavator 1 is reduced.

[0093] When the operation lever 5 is operated so that the travel motor 25 is driven and the lower travel body 4 is traveling, and the operation lever 5 is returned to the neutral position, the travel spool 41 is arranged at the neutral position, and the inflow of hydraulic oil from the main pump 22 to the travel motor 25 is restricted. On the other hand, even if the operation lever 5 is returned to the neutral position, the lower travel body 4 continues to travel by inertia, so the travel motor 25 continues to drive. If the travel motor 25 continues to drive while the inflow of hydraulic oil from the main pump 22 to the travel motor 25 is restricted, at least a part of the travel motor 25 may become negative pressure. When at least a part of the travel motor 25 becomes negative pressure, cavitation may occur in the travel motor 25. When cavitation occurs, malfunction of the travel motor 25 may occur.

[0094] In the embodiment, the controller 21 determines whether or not the inflow of hydraulic oil from the main pump 22 to the travel motor 25 is restricted and the travel motor 25 is in an inertia travel state in which it is driving when the back pressure of the main valve 26 is adjusted to the first back pressure (step S4).

[0095] In step S4, when it is determined that the inflow of the hydraulic oil to the travel motor 25 is restricted and the travel motor 25 is in the inertial travel state where it is being driven (step S4: Yes), the controller 21 controls the back pressure valve 18 so that the tank flow path 27 is closed while the main pump 22 is being driven. By closing the tank flow path 27 while the main pump 22 is being driven, the back pressure of the main valve 26 is adjusted to the second back pressure (step S5).

[0096] When the back pressure of the main valve 26 increases, for example, the hydraulic oil in the tank flow path 27 is supplied to the travel motor 25 via the outflow flow path 61, the travel spool 41, and the first motor flow path 59 or the second motor flow path 60. That is, when the back pressure of the main valve 26 increases, the supply of the hydraulic oil from the tank flow path 27 to the travel motor 25 is promoted. Thereby, the travel motor 25 is prevented from becoming negative pressure, and the occurrence of cavitation is suppressed.

[0097] In step S4, when it is determined that the travel motor 25 is not being driven (step S4: No), the tank flow path 27 is opened (step S6).

[0098] Hereinafter, the controller 21 repeats the above-described processing until the hydraulic excavator 1 is turned off for the control related to the back pressure valve 18.

[0099] [Effect] As described above, according to the embodiment, the hydraulic system 13 includes a main hydraulic circuit 16 configured by an open circuit and a swing hydraulic circuit 17 configured by a closed circuit. The main hydraulic circuit 16 includes a main pump 22, a work implement cylinder 24, a main valve 26 that controls the inflow of hydraulic oil from the main pump 22 to the work implement cylinder 24, and a tank 28 to which the hydraulic oil flowing out from the work implement cylinder 24 is supplied via the main valve 26. The swing hydraulic circuit 17 includes a swing pump 29, a swing motor 30, and a neutral holding valve 33 that controls the passage of hydraulic oil between the swing pump 29 and the swing motor 30. Further, the hydraulic system 13 includes a back pressure valve 18 disposed in a tank passage 27 between the main valve 26 and the tank 28 to adjust the back pressure of the main valve 26, a warm-up passage 76 disposed around a spool 67 of the neutral holding valve 33, a supply passage 78 connecting a first portion 27A of the tank passage 27 between the main valve 26 and the back pressure valve 18 and an inlet port 76A of the warm-up passage 76, and a discharge passage 79 connecting a second portion 27B of the tank passage 27 between the back pressure valve 18 and the tank 28 and an outlet port 76B of the warm-up passage 76.

[0100] Thereby, the warm hydraulic oil supplied from the main hydraulic circuit 16 flows through the warm-up passage 76. Since the warm hydraulic oil flows through the warm-up passage 76, the neutral holding valve 33, which is a type of hydraulic device, is warmed up.

[0101] If high-temperature hydraulic oil is suddenly supplied to the neutral holding valve 33 when the neutral holding valve 33 is not warmed up, there is a possibility that a malfunction of the neutral holding valve 33 may occur. For example, when the heat capacity of the spool 67 and the heat capacity of the body 77 are different, if high-temperature hydraulic oil is suddenly supplied to the non-warmed neutral holding valve 33, a phenomenon may occur in which the spool 67 and the body 77 are fixed due to the difference between the thermal expansion amount of the spool 67 and the thermal expansion amount of the body 77. If the spool 67 and the body 77 are fixed, there is a possibility that a malfunction of the neutral holding valve 33 may occur.

[0102] According to the embodiment, the neutral holding valve 33 is warmed up by the warm hydraulic oil supplied from the main hydraulic circuit 16. Therefore, the occurrence of the phenomenon that the spool 67 and the body 77 stick together is suppressed. Accordingly, the occurrence of malfunction of the neutral holding valve 33 is suppressed.

[0103] In the embodiment, the warm-up passage 76 is disposed in the body 77 of the neutral holding valve 33. Thereby, the neutral holding valve 33 is properly warmed up.

[0104] The main hydraulic circuit 16 is connected to a pump passage 23 between the main pump 22 and the main valve 26, and has a relief valve 19 that is opened when the discharge pressure of the main pump 22 exceeds a predetermined relief pressure. When the hydraulic oil flows out from the relief valve 19, the pressure energy of the hydraulic oil is converted into thermal energy, and the temperature of the hydraulic oil rises. The warm hydraulic oil flowing out from the relief valve 19 is supplied to the warm-up passage 76 via the first portion 27A and the supply passage 78, thereby warming up the neutral holding valve 33.

[0105] In the embodiment, the controller 21 controls the back pressure valve 18 so that the back pressure of the main valve 26 changes from the first back pressure to a second back pressure higher than the first back pressure. By increasing the back pressure of the main valve 26, the supply of the hydraulic oil from the first portion 27A of the tank passage 27 to the warm-up passage 76 is promoted. Accordingly, the neutral holding valve 33 is properly warmed up.

[0106] In the embodiment, the temperature of the hydraulic oil in the tank passage 27 is detected by the temperature sensor 20. The controller 21 raises the back pressure of the main valve 26 to the second back pressure when the detected value of the temperature sensor 20 is equal to or lower than a predetermined specified value. Thereby, the neutral holding valve 33 is properly warmed up when warm-up is required.

[0107] In the embodiment, the controller 21 raises the back pressure of the main valve 26 to the second back pressure when the inflow of the hydraulic oil to the travel motor 25 is restricted and the travel motor 25 is in an inertial travel state in which it is driven. Thereby, the occurrence of cavitation in the travel motor 25 is suppressed.

[0108] [Other Embodiments] In the above-described embodiment, the back pressure valve 18 may be incorporated in the main valve 26.

Description of Reference Numerals

[0109] 1…Hydraulic excavator, 2…Upper slewing body, 3…Working equipment, 4…Lower traveling body, 5…Operation lever, 6…Operator's cab, 7…Driver's seat, 8…Boom, 9…Arm, 10…Bucket, 11…Drive wheel, 12…Track, 13…Hydraulic system, 14…Engine, 15…Power take-off, 16…Main hydraulic circuit, 17…Slewing hydraulic circuit, 18…Back pressure valve, 18A…Electromagnetic part, 18B…Spring part, 19…Relief valve, 20…Temperature sensor, 21…Controller, 21A…Processor, 21B…Main memory, 21C…Storage, 21D…Interface, 22…Main pump, 22A…Discharge port, 23…Pump flow path, 24…Working equipment cylinder, 25…Traveling motor, 25A…First suction port, 25B…Second suction port, 26…Main valve, 27…Tank flow path, 27A…First part, 27B…Second part, 28…Tank, 29…Slewing pump, 29A…First discharge port, 29B…Second discharge port, 30…Slewing motor, 30A…First suction port, 30B…Second suction port, 31…First main flow path, 32…Second main flow path, 33…Neutral holding valve, 35…Boom cylinder, 35A…Bottom chamber, 35B…Rod chamber, 36…Arm cylinder, 36A…Bottom chamber, 36B…Rod chamber, 37…Bucket cylinder, 37A…Bottom chamber, 37B…Rod chamber, 38…Boom spool, 38A…Pump port, 38B…First inlet / outlet port, 38C…Second inlet / outlet port, 38D…Tank port, 38E…Neutral inlet port, 38F…Neutral outlet port, 39…Arm spool, 39A…Pump port, 39B…First inlet / outlet port, 39C…Second inlet / outlet port, 39D…Tank port, 39E…Neutral inlet port, 39F…Neutral outlet port, 40…Bucket spool, 40A…Pump port, 40B…First inlet / outlet port, 40C…Second inlet / outlet port, 40D…Tank port, 40E…Neutral inlet port, 40F…Neutral outlet port, 41…Traveling spool, 41A…Pump port, 41B…First inlet / outlet port, 41C…Second inlet / outlet port, 41D…Tank port, 41E…Neutral inlet port, 41F…Neutral outlet port, 42…Inflow path, 43…Bottom path, 44…Rod path, 45…Outflow path, 46…Neutral path, 47…Neutral path, 48…Inflow path, 49…Bottom path, 50…Rod path, 51…Outflow path, 52…Neutral path, 53…Inflow path, 54…Bottom path, 55…Rod path, 56…Outflow path, 57…Neutral path, 58…Inflow path59…First motor passage, 60…Second motor passage, 61…Outlet passage, 62…Negative control mechanism, 63…Boom check valve, 64…Arm check valve, 65…Bucket check valve, 66…Travel check valve, 67…Spool, 67A…First pump port, 67B…Second pump port, 67C…First motor port, 67D…Second motor port, 68…First bypass passage, 69…Second bypass passage, 70…Check valve, 71…Check valve, 74…Relief passage, 75…Relief passage, 76…Warm-up passage, 76A…Inlet port, 76B…Outlet port, 77…Body, 78…Supply passage, 79…Discharge passage.,

Claims

1. A main hydraulic circuit configured by an open circuit having a main pump, a work machine cylinder, a main valve that controls the inflow of hydraulic oil from the main pump to the work machine cylinder, and a tank into which the hydraulic oil flowing out from the work machine cylinder is discharged via the main valve; A swing hydraulic circuit configured by a closed circuit having a swing pump, a swing motor, and a neutral holding valve that controls the passage of hydraulic oil between the swing pump and the swing motor; A back pressure valve disposed in a tank flow path between the main valve and the tank, for adjusting the back pressure of the main valve; A warm-up flow path disposed around the spool of the neutral holding valve; A supply flow path connecting an upstream portion of the tank flow path between the main valve and the back pressure valve and an inlet port of the warm-up flow path; A discharge flow path connecting a downstream portion of the tank flow path between the back pressure valve and the tank and an outlet port of the warm-up flow path; A hydraulic system.

2. The neutral holding valve has a body disposed around the spool, The warm-up flow path is disposed in the body, The hydraulic system according to claim 1.

3. A relief valve connected to a pump flow path between the main pump and the main valve and opened when the discharge pressure of the main pump is equal to or higher than a predetermined relief pressure, The hydraulic oil flowing out from the relief valve is supplied to the warm-up flow path via the upstream portion and the supply flow path, The hydraulic system according to claim 1 or claim 2.

4. A controller for controlling the back pressure valve so that the back pressure changes from a first back pressure to a second back pressure higher than the first back pressure, The controller adjusts the back pressure to the second back pressure so that the hydraulic oil is supplied from the upstream portion to the warm-up flow path, The hydraulic system according to any one of claims 1 to 3.

5. A temperature sensor for detecting the temperature of the hydraulic oil in the tank flow path, The controller adjusts the back pressure to the second back pressure when the detected value of the temperature sensor is equal to or lower than a predetermined specified value, The hydraulic system according to claim 4.

6. The main hydraulic circuit has a travel motor, The main valve controls the inflow of hydraulic oil from the main pump to the travel motor, When the inflow of hydraulic oil to the travel motor is restricted and the travel motor is driving, the controller adjusts the back pressure to the second back pressure. The hydraulic system according to claim 4 or claim 5.

7. A hydraulic system according to any one of claims 1 to 6, An upper swing body swung by the swing motor, A working machine connected to the upper swing body and operated by the working machine cylinder. Hydraulic excavator.

8. A main hydraulic circuit composed of an open circuit having a main pump, a working machine cylinder, a main valve for controlling the inflow of hydraulic oil from the main pump to the working machine cylinder, and a tank into which the hydraulic oil flowing out from the working machine cylinder is discharged through the main valve, A swing hydraulic circuit composed of a closed circuit having a swing pump, a swing motor, and a neutral holding valve for controlling the passage of hydraulic oil between the swing pump and the swing motor, A control method for a hydraulic excavator, comprising a back pressure valve disposed in a tank flow path between the main valve and the tank for adjusting the back pressure of the main valve, Allowing hydraulic oil to flow into a warm-up flow path disposed around the spool of the neutral holding valve from an upstream portion of the tank flow path between the main valve and the back pressure valve, Allowing hydraulic oil to flow out from the warm-up flow path to a downstream portion of the tank flow path between the back pressure valve and the tank. A control method for a hydraulic excavator.

9. Increasing the back pressure of the main valve by the back pressure valve to allow hydraulic oil to flow into the warm-up flow path. The control method for a hydraulic excavator according to claim 8.

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