Work machine and method for controlling work machine

Abstract

To provide a work machine capable of reducing costs and improving fuel efficiency.SOLUTION: A work machine 1 includes a hydraulic steering device 61, a refill oil passage 62, a charge pump 63, a discharge oil passage 64, a bypass oil passage 69, a variable relief valve 70, and a controller 51. The refill oil passage 62 refills oil to the hydraulic steering device 61. The charge pump 63 is disposed in the refill oil passage 62 and supplies oil to the hydraulic steering device 61. The discharge oil passage 64 discharges oil from the hydraulic steering device 61. The bypass oil passage 69 connects between a portion of the refill oil passage 62 downstream of the charge pump 63 and the discharge oil passage 64, and bypasses the hydraulic steering device 61. The variable relief valve 70 is disposed in the bypass oil passage 69. The controller 51 determines whether or not the vehicle is not turning, and when it determines that the vehicle is not turning, controls the variable relief valve 70 so that oil discharged from the charge pump 63 can pass through the bypass oil passage 69 at a pressure value lower than that during turning.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a work machine and a method for controlling a work machine. [Background technology]

[0002] Hydraulic steering devices such as HSS (Hydrostatic Steering System) are used in work machines such as bulldozers (see, for example, Patent Document 1). Such hydraulic steering devices have a closed circuit including a pump and a motor, and a charge pump is mounted on the work machine to replenish oil lost due to leakage in the closed circuit. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 11-181823 Summary of the Invention [Problem to be solved by the invention]

[0004] However, when the vehicle is not rotating, no hydraulic oil flows in the closed circuit, so there is no need to replenish hydraulic oil with the charge pump, and all the work of the charge pump becomes lost horsepower, resulting in poor fuel economy. To reduce lost horsepower, it is possible to use a variable displacement pump as the charge pump, but variable displacement pumps are expensive and have a complex structure, which requires a lot of maintenance work.

[0005] Furthermore, when a variable displacement pump is used as the charge pump, the resistance of the hydraulic oil increases during low temperature start-up, requiring a large drive torque, which deteriorates startability.

[0006] An object of the present disclosure is to provide a work machine and a control method for a work machine that are capable of reducing costs and improving fuel efficiency. [Means for solving the problem]

[0007] A work machine according to a first aspect of the present disclosure includes a hydraulic steering device, a refill oil passage, a fixed displacement pump, a discharge oil passage, a first bypass oil passage, a control valve, and a controller. The refill oil passage refills oil to the hydraulic steering device. The fixed displacement pump is disposed in the refill oil passage and supplies oil to the hydraulic steering device. The discharge oil passage discharges oil from the hydraulic steering device. The first bypass oil passage connects a portion of the refill oil passage downstream of the fixed displacement pump and the discharge oil passage, and bypasses the hydraulic steering device. The control valve is disposed in the first bypass oil passage. The controller determines whether or not the vehicle is not turning, and when it determines that the vehicle is not turning, controls the control valve so that oil discharged from the fixed displacement pump can pass through the first bypass oil passage at a pressure value lower than that during turning.

[0008] A working machine according to a second aspect of the present disclosure includes a hydraulic steering device, a refill oil passage, a fixed displacement pump, a discharge oil passage, a first bypass oil passage, a control valve, and a controller. The refill oil passage refills oil to the hydraulic steering device. The fixed displacement pump is disposed in the refill oil passage and supplies oil to the hydraulic steering device. The discharge oil passage discharges oil from the hydraulic steering device. The first bypass oil passage connects a portion of the refill oil passage downstream of the fixed displacement pump and the discharge oil passage, and bypasses the hydraulic steering device. The control valve is disposed in the first bypass oil passage. The controller determines whether or not the hydraulic steering device is in a low-temperature start state, and when it determines that the hydraulic steering device is in a low-temperature start state, controls the control valve so that oil discharged from the fixed displacement pump can pass through the first bypass oil passage at a pressure value lower than that during a start other than a low-temperature start state.

[0009] A control method for a work machine according to a third aspect of the present disclosure includes a determination step and a control step. The work machine includes a hydraulic steering device, a refill oil passage, a fixed displacement pump, a discharge oil passage, a first bypass oil passage, and a control valve. The refill oil passage refills oil to the hydraulic steering device. The fixed displacement pump is disposed in the refill oil passage and supplies oil to the hydraulic steering device. The discharge oil passage discharges oil from the hydraulic steering device. The first bypass oil passage connects between a portion of the refill oil passage downstream of the fixed displacement pump and the discharge oil passage, and bypasses the hydraulic steering device. The control valve is disposed in the first bypass oil passage. The determination step determines whether or not the machine is not turning. When it is determined that the machine is not turning, the control step controls the control valve so that oil discharged from the fixed displacement pump can pass through the first bypass oil passage at a pressure value lower than that during turning.

[0010] A control method for a work machine according to a fourth aspect of the present disclosure includes a determination step and a control step. The work machine includes a hydraulic steering device, a refill oil passage, a fixed displacement pump, a discharge oil passage, a first bypass oil passage, and a control valve. The refill oil passage refills oil to the hydraulic steering device. The fixed displacement pump is disposed in the refill oil passage and supplies oil to the hydraulic steering device. The discharge oil passage discharges oil from the hydraulic steering device. The first bypass oil passage connects between a portion of the refill oil passage downstream of the fixed displacement pump and the discharge oil passage, and bypasses the hydraulic steering device. The control valve is disposed in the first bypass oil passage. The determination step determines whether or not a low temperature start has occurred. When it is determined that a low temperature start has occurred, the control step controls the control valve so that oil discharged from the fixed displacement pump can pass through the first bypass oil passage at a pressure value lower than that during a start other than at low temperature. Effect of the Invention

[0011] According to the present disclosure, it is possible to provide a work machine and a method for controlling a work machine that are capable of reducing costs and improving fuel efficiency. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view of a work machine in a first embodiment according to the present disclosure. [Diagram 2] FIG. 1 is a top view of a work machine in a first embodiment according to the present disclosure. [Diagram 3] 1 is a block diagram showing a drive system of a work machine in a first embodiment according to the present disclosure. FIG. [Figure 4] 1 is a block diagram showing a steering mechanism and a control system of a work machine in a first embodiment according to the present disclosure. FIG. [Diagram 5] FIG. 3 is a flow chart showing a control method for a work machine in the first embodiment according to the present disclosure. [Figure 6] FIG. 11 is a block diagram showing a steering mechanism and a control system of a work machine in a second embodiment according to the present disclosure. [Figure 7] FIG. 11 is a flow chart showing a control method for a work machine in a second embodiment according to the present disclosure. [Figure 8] FIG. 11 is a flow diagram showing a steering mechanism of a work machine in a third embodiment according to the present disclosure. [Figure 9] FIG. 11 is a block diagram showing a steering mechanism of a work machine in a modified example of the second embodiment according to the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a control system and a control method for a work machine according to an embodiment will be described with reference to the drawings.

[0014] (Embodiment 1) Fig. 1 is a perspective view showing a work machine 1 according to a first embodiment. Fig. 2 is a top view of the work machine 1. The work machine 1 according to this embodiment is a bulldozer. The work machine 1 includes a vehicle body 11, a traveling device 12, and a work implement 13. The vehicle body 11 has a driver's cab 14 and a power room 15. A driver's seat (not shown) is disposed in the driver's cab 14. The power room 15 is disposed in front of the driver's cab 14.

[0015] The traveling gear 12 is attached to the lower part of the vehicle body 11. The traveling gear 12 includes a left driving wheel 16, a right driving wheel 17, a left crawler belt 18, and a right crawler belt 19. The left driving wheel 16 and the right driving wheel 17 are rotatably supported by the vehicle body 11. The left crawler belt 18 is wound around the left driving wheel 16. The right crawler belt 19 is wound around the right driving wheel 17. The left and right driving wheels 16, 17 rotate to drive the left and right crawlers 18, 19, causing the work machine 1 to travel.

[0016] The work machine 13 is attached to the vehicle body 11. The work machine 13 includes lift frames 21, 22, a blade 23, lift cylinders 24, 25, and tilt cylinders 26, 27. The lift frames 21, 22 are attached to the vehicle body 11 so as to be movable up and down. The lift frames 21, 22 support the blade 23.

[0017] The blade 23 is disposed in front of the vehicle body 11. The lift cylinders 24, 25 are connected to the vehicle body 11 and the blade 23. The lift frames 21, 22 move up and down in accordance with the up and down movement of the blade 23 by the lift cylinders 24, 25. Alternatively, the lift cylinders 24, 25 may be connected to the vehicle body 11 and the lift frames 21, 22. In this case, the lift cylinders 24, 25 extend and retract, and the blade 23 moves up and down in accordance with the up and down movement of the lift frames 21, 22. The tilt cylinders 26, 27 are connected to the lift frames 21, 22 and the blade 23. The tilt cylinders 26, 27 extend and retract, and the left and right ends of the blade 23 tilt up and down.

[0018] The work machine 1 includes a drive system 2 that drives the left track 18 and the right track 19, and a control system 3. Figure 3 is a block diagram showing the configuration of the drive system 2 of the work machine 1.

[0019] (Drive system 2) As shown in Fig. 3, the drive system 2 includes an engine 31 and a drive unit 32. The engine 31 and the drive unit 32 are mounted on the vehicle body 11. The drive unit 32 rotates the left drive wheel 16 and the right drive wheel 17 by the driving force from the engine 31. The drive system 2 includes a hydraulic pump (not shown) for the work machine 13. The hydraulic pump for the work machine 13 is driven by the engine 31, and discharges hydraulic oil for driving the above-mentioned cylinders 24-27.

[0020] The drive device 32 includes a PTO (Power Take Off) 33, a transmission 34, a transfer 35, and a steering mechanism 36. The PTO 33 distributes driving force from the engine 31 to the transmission 34 and the steering mechanism 36. The transmission 34 is connected to the engine 31 via the PTO 33. The transmission 34 includes, for example, a clutch and a plurality of speed change gears. The transmission 34 is connected to the left driving wheel 16 and the right driving wheel 17 via the transfer 35.

[0021] The drive device 32 includes a left clutch 37, a left brake 38, and a left link mechanism 39. The left clutch 37, the left brake 38, and the left link mechanism 39 are connected to the left driving wheel 16. The left clutch 37 switches between transmitting and cutting off the driving force from the transmission 34 to the left driving wheel 16. The left brake 38 brakes the left driving wheel 16. The left link mechanism 39 connects the left driving wheel 16 and the steering mechanism 36. The left link mechanism 39 includes, for example, a planetary gear mechanism.

[0022] The drive device 32 includes a right clutch 41, a right brake 42, and a right link mechanism 43. The right clutch 41, the right brake 42, and the right link mechanism 43 are connected to the right driving wheel 17. The right clutch 41 switches between transmitting and cutting off the driving force from the transmission 34 to the right driving wheel 17. The right brake 42 brakes the right driving wheel 17. The right link mechanism 43 connects the right driving wheel 17 and the steering mechanism 36. The right link mechanism 43 includes, for example, a planetary gear mechanism.

[0023] The steering mechanism 36 assists the turning of the vehicle body 11 by generating a rotational speed difference between the left driving wheel 16 and the right driving wheel 17. For example, the steering mechanism 36 assists the turning of the vehicle body 11 to the right by increasing the speed of the left driving wheel 16 more than that of the right driving wheel 17. The steering mechanism 36 assists the turning of the vehicle body 11 to the left by increasing the speed of the right driving wheel 17 more than that of the left driving wheel 16. The steering mechanism 36 has a hydraulic steering device 61 including a hydraulic pump 81 and a hydraulic motor 82.

[0024] The hydraulic pump 81 is connected to the engine 31 via the PTO 33. The hydraulic pump 81 is driven by the engine 31 to discharge hydraulic oil. The hydraulic motor 82 is connected to the hydraulic pump 81 via a hydraulic circuit 83. The hydraulic motor 82 is driven by the hydraulic oil discharged from the hydraulic pump 81. The hydraulic motor 82 can be connected to the left driving wheel 16 via the left link mechanism 39. The hydraulic motor 82 can be connected to the right driving wheel 17 via the right link mechanism 43. The hydraulic motor 82 is driven by the hydraulic oil from the hydraulic pump 81 to increase the speed of the left driving wheel 16 and the right driving wheel 17.

[0025] (Steering mechanism 36) 4 is a hydraulic circuit diagram showing the configuration of the steering mechanism 36. The steering mechanism 36 is an HSS (Hydrostatic Steering System). The steering mechanism 36 has a hydraulic steering device 61, a refill oil passage 62, a charge pump 63 (fixed displacement pump), a discharge oil passage 64, a shuttle valve 65, a charge relief valve 66 (discharge valve), a charge safety oil passage 67 (second bypass oil passage), a charge safety valve 68 (safety valve), a bypass oil passage 69 (first bypass oil passage), and a variable relief valve 70 (control valve).

[0026] The hydraulic steering device 61 includes the above-mentioned hydraulic pump 81 (steering pump), hydraulic motor 82 (steering motor), and hydraulic circuit 83. The hydraulic circuit 83 includes a first oil passage 83a and a second oil passage 83b. The first oil passage 83a connects the hydraulic pump 81 and the hydraulic motor 82. The second oil passage 83b connects the hydraulic pump 81 and the hydraulic motor 82. The hydraulic pump 81, the first oil passage 83a, the hydraulic motor 82, and the second oil passage 83b form a closed circuit.

[0027] The replenishment oil passage 62 replenishes hydraulic oil from a hydraulic oil tank 71 in which hydraulic oil is stored to the hydraulic steering device 61. The replenishment oil passage 62 has a first replenishment oil passage 62a, a second replenishment oil passage 62b, and a third replenishment oil passage 62c. One end of the first replenishment oil passage 62a is connected to a first oil passage 83a. The other end of the first replenishment oil passage 62a is connected to one end of the second replenishment oil passage 62b. The other end of the second replenishment oil passage 62b is connected to a second oil passage 83b. The third replenishment oil passage 62c connects the connection point between the first replenishment oil passage 62a and the second replenishment oil passage 62b and the hydraulic oil tank 71.

[0028] The charge pump 63 is disposed in the third replenishment oil passage 62c of the replenishment oil passage 62. The charge pump 63 is a fixed displacement pump. The charge pump 63 is, for example, a gear pump with a fixed volume. The charge pump 63 is driven in response to the rotation of the engine 31, and replenishes hydraulic oil to the hydraulic steering device 61 through the replenishment oil passage 62. A filter 84 is disposed downstream of the charge pump 63 in the third replenishment oil passage 62c, and removes impurities from the hydraulic oil. A filter relief passage 75 is disposed between the filter 84 and the charge pump 63 in the third replenishment oil passage 62c, and connects the hydraulic oil tank 71 to the hydraulic oil tank 71. A relief valve 76 is disposed in the filter relief passage 75. When the pressure becomes high due to clogging of the filter 84, the relief valve 76 is opened, and the hydraulic oil supplied from the charge pump 63 is returned to the hydraulic oil tank 71 through the filter relief passage 75.

[0029] A check valve 85a is disposed in the first refill oil passage 62a. The check valve 85a allows the hydraulic oil to flow from the third refill oil passage 62c toward the first oil passage 83a, and prevents the hydraulic oil from flowing in the opposite direction. A check valve 85b is disposed in the second refill oil passage 62b. The check valve 85b allows the hydraulic oil to flow from the third refill oil passage 62c toward the second oil passage 83b, and prevents the hydraulic oil from flowing in the opposite direction. The steering mechanism 36 has a relief oil passage 86a, a relief valve 87a, a relief oil passage 86b, and a relief valve 87b. The relief oil passage 86a branches from the upstream side of the check valve 85a of the first refill oil passage 62a and joins on the downstream side of the check valve 85a. The relief valve 87a is disposed in the relief oil passage 86a. When a steering operation is performed and the oil pressure in the first oil passage 83a becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the first oil passage 83a to the second oil passage 83b through the relief valve 87a. The relief oil passage 86b branches from the upstream side of the check valve 85b of the second replenishment oil passage 62b and joins with the check valve 85b downstream side. The relief valve 87b is disposed in the relief oil passage 86b. When a steering operation is performed and the oil pressure in the second oil passage 83b becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the second oil passage 83b to the first oil passage 83a through the relief valve 87b.

[0030] The oil discharge passage 64 discharges hydraulic oil from the hydraulic steering device 61 toward the hydraulic oil tank 71. The oil discharge passage 64 includes a first oil discharge passage 64a, a second oil discharge passage 64b, and a third oil discharge passage 64c. One end of the first oil discharge passage 64a is connected to the first oil passage 83a, and the other end is connected to the shuttle valve 65. One end of the second oil discharge passage 64b is connected to the second oil passage 83b, and the other end is connected to the shuttle valve 65. The third oil discharge passage 64c connects the shuttle valve 65 and the hydraulic oil tank 71.

[0031] The shuttle valve 65 has a valve body that moves due to the pressure difference between the first oil passage 83a and the second oil passage 83b, and connects the third discharge oil passage 64c to the first discharge oil passage 64a or the second discharge oil passage 64b. Also, a connecting oil passage 72 is provided to connect the third replenishment oil passage 62c of the replenishment oil passage 62 and the shuttle valve 65. When the hydraulic pressures of the first oil passage 83a and the second oil passage 83b are equal, the valve body of the shuttle valve 65 does not move, and the shuttle valve 65 connects the connecting oil passage 72 to the third discharge oil passage 64c.

[0032] The charge relief valve 66 is disposed in the third discharge oil passage 64c. The charge relief valve 66 closes the third discharge oil passage 64c when the input oil pressure is less than a predetermined relief pressure value, and opens the third discharge oil passage 64c when the input oil pressure is equal to or greater than the relief pressure value.

[0033] When the steering operation increases the oil pressure in either the first oil passage 83a or the second oil passage 83b, moving the shuttle valve 65 and exceeding the relief pressure value of the charge relief valve 66, hydraulic oil is discharged from the side with the higher pressure into the hydraulic oil tank 71.

[0034] The charge safety oil passage 67 connects between the third replenishment oil passage 62c of the replenishment oil passage 62 and the third discharge oil passage 64c of the discharge oil passage 64. A branching portion of the charge safety oil passage 67 from the third replenishment oil passage 62c is disposed downstream of a branching portion of the connection oil passage 72 from the third replenishment oil passage 62c.

[0035] The charge safety valve 68 is disposed in the charge safety oil passage 67. The charge safety valve 68 is a valve for preventing the hydraulic steering device 61 from being damaged due to the hydraulic pressure from the charge pump 63 becoming too high. The charge safety valve 68 closes the charge safety oil passage 67 when the input hydraulic pressure is less than a predetermined relief pressure value, and opens the charge safety oil passage 67 when the input hydraulic pressure becomes equal to or greater than the relief pressure value. The relief pressure value (first pressure value) of the charge safety valve 68 is higher than the relief pressure value (second pressure value) of the charge relief valve 66.

[0036] The bypass oil passage 69 connects between the third replenishment oil passage 62c of the replenishment oil passage 62 and the third discharge oil passage 64c of the discharge oil passage 64. A branching portion of the bypass oil passage 69 from the third replenishment oil passage 62c is disposed upstream of a branching portion of the connection oil passage 72 from the third replenishment oil passage 62c.

[0037] The variable relief valve 70 is disposed in the bypass oil passage 69. When the input hydraulic pressure is less than a predetermined relief pressure value, the variable relief valve 70 closes the bypass oil passage 69, and when the input hydraulic pressure is equal to or greater than the relief pressure value, the variable relief valve 70 opens the bypass oil passage 69. The relief pressure value of the variable relief valve 70 can be changed by a command from a controller 51, which will be described later. The variable relief valve 70 has, for example, a spring that biases the valve body in a closing direction, and a solenoid that changes the biasing force of the spring. The biasing force of the spring can be changed by moving the solenoid, and the relief pressure value can be changed. In this embodiment, when a steering operation is performed and the vehicle body 11 is turning, the variable relief pressure is set to a first relief pressure value, and when a steering operation is not performed and the vehicle body 11 is not turning, the variable relief pressure is set to a second relief pressure value. The second relief pressure value is smaller than the first relief pressure value. When the variable relief pressure value is set to the second relief pressure value rather than the first relief pressure value, the bypass oil passage 69 is opened at a lower pressure. The first relief pressure value also includes a state in which the biasing force of the spring by the solenoid is not changed and the adjustable relief valve 70 is simply closed. For example, the adjustable relief valve 70 can be set to the first relief pressure value by setting the current output to the adjustable relief valve 70 to 0 mA in response to a command from the controller 51. In addition, the solenoid can be driven to set the adjustable relief valve 70 to a second relief pressure value lower than the first relief pressure value by outputting a predetermined value of current to the adjustable relief valve 70 in response to a command from the controller 51.

[0038] The first relief pressure value is set higher than the relief pressure value of the charge safety valve 68. The second relief pressure value is set lower than the relief pressure value of the charge relief valve 66. The second relief pressure value is set to 0 kg / cm 2 More than 10kg / cm2 Less than or equal to 0 kg / cm 2 More than 5kg / cm 2 The following is even more preferred:

[0039] (Control System 3) The control system 3 includes a controller 51. The controller 51 is programmed to control the work machine 1. The controller 51 includes memories such as RAM and ROM, and a processor such as a CPU. The controller 51 may include an auxiliary storage device such as an SSD or an HDD. The controller 51 records programs and data for controlling the work machine 1. The controller 51 executes processing for controlling the work machine 1 based on the programs and data. For example, the controller 51 is disposed in the cab 4. The controller 51 is connected to at least the steering mechanism 36 via a signal line (L).

[0040] As described in more detail below, the controller 51 controls the relief pressure value of the variable relief valve 70 when the vehicle is not turning to a second relief pressure value that is lower than the first relief pressure value when the vehicle is not turning.

[0041] The control system 3 includes a steering lever 52 (operation member), a lever position sensor 53 (operation detection unit), a key cylinder 54, and a rotation sensor 55 (operation detection unit).

[0042] The steering lever 52 can be operated by an operator to turn the vehicle body 11 to the left or right. The steering lever 52 is disposed inside the cab 14. Operation of the steering lever 52 results in a steering operation, which turns the vehicle body 11. The steering lever 52 does not have to be limited to a lever, and may be another member such as a switch.

[0043] The lever position sensor 53 detects the operation position of the steering lever 52, and outputs an operation signal including operation position information to the controller 51. A potentiometer, for example, can be used as the lever position sensor 53. The controller 51 controls a hydraulic pump 81 in response to the operation position signal to drive a hydraulic motor 82 and assist turning.

[0044] In the key cylinder 54, a key is inserted and rotated to drive the engine 31. The key cylinder 54 is disposed in the driver's cab 14. When the key is inserted into the key cylinder 54 and rotated, the state changes to accessory key on, ignition key on, and cranking on, in that order, depending on the stage of rotation. At each stage, a signal is transmitted from the starter unit including the key cylinder to the controller 51. When the key is rotated to the accessory key on position, an accessory key on signal is transmitted from the starter unit to the controller 51. When the key is rotated to the ignition key on position, an ignition key on signal is transmitted from the starter unit to the controller 51. When the key is rotated to the cranking on position, a start cranking signal is transmitted from the starter unit to the controller 51. In this embodiment, an ignition key including a key cylinder and a key has been described, but this is not limited thereto, and other configurations such as a rotary switch that does not use a key, a push switch, and the like may be used.

[0045] The rotation sensor 55 detects the rotation speed of the engine 31. The rotation sensor 55 outputs a rotation speed signal including information on the detected engine rotation speed to the controller 51. The rotation sensor 55 is not limited to detecting the rotation speed of the engine 31, and may detect the rotation speed in the driving force transmission system and output a rotation speed signal including information on the rotation speed to the controller 51.

[0046] The controller 51 determines whether or not the vehicle is turning based on the rotation speed signal from the rotation sensor 55 and the operation signal from the lever position sensor 53 or the key-on signal from the key cylinder 54. If the controller 51 determines that the vehicle is not turning, it sets the relief pressure of the adjustable relief valve 70 to the second relief pressure. If the controller 51 determines that the vehicle is turning, it sets the relief pressure of the adjustable relief valve 70 to the first relief pressure.

[0047] (Method of controlling a work machine) Next, a method for controlling the work machine will be described with reference to Fig. 5. Fig. 5 is a flow chart showing a method for controlling the work machine according to this embodiment.

[0048] In step S10, the controller 51 determines whether the engine 31 is operating. Specifically, the controller 51 determines that the engine 31 is operating when the rotation speed is equal to or greater than a predetermined threshold based on a rotation speed signal from a rotation speed sensor 55 that detects the rotation speed of the engine 31 or the rotation speed in the driving force transmission system. If it is determined that the engine 31 is operating, the control proceeds to step S11.

[0049] In step S11, the controller 51 judges whether the steering lever 52 has been operated. Specifically, the controller 51 judges whether the steering lever 52 has been operated by a predetermined amount or more based on an operation signal from the lever position sensor 53. Here, the predetermined amount is a value for judging whether the steering operation has been performed or not, and therefore, for example, it can be set to the amount of play of the steering lever 52. Also, the predetermined amount may be set to the amount at which the hydraulic motor 82 of the hydraulic steering device 61 starts to operate. If the steering lever 52 is not operated by the predetermined amount or more, the controller 51 judges that the steering lever 52 has not been operated, and the control proceeds to step S12. In this way, when the engine 31 is running and the steering lever 52 is not operated, the controller 51 judges that the vehicle body 11 is not turning. Steps S10 and S11 correspond to the judgment step. In addition, in step S11, the controller 51 may determine that the steering lever 52 is not being operated if the state in which the steering lever 52 is not being operated is maintained for a predetermined period of time or more, in order to prevent the controller 51 from determining that the steering lever 52 is not being operated, contrary to the operator's intention, based on the operation of the steering lever 52 caused by erroneous operation or noise such as vibration.

[0050] In step S12, the controller 51 outputs an operation signal to the variable relief valve 70 to set the relief pressure value of the variable relief valve 70 to the second relief pressure value, and the control ends. When the vehicle is not turning, there is no pressure difference between the first oil passage 83a and the second oil passage 83b, so the shuttle valve 65 is in the neutral position and connects the connection oil passage 72 to the third discharge oil passage 64c. Here, the second relief pressure value of the variable relief valve 70 is lower than the relief pressure value of the charge relief valve 66 arranged in the third discharge oil passage 64c, so the hydraulic oil supplied toward the hydraulic steering device 61 by the drive of the charge pump 63 flows into the bypass oil passage 69 without passing through the hydraulic steering device 61, and is returned to the hydraulic oil tank 71. Step S12 corresponds to a control step.

[0051] On the other hand, if it is determined in step S10 that the engine 31 is not running, the controller 51 outputs an operation signal to the variable relief valve 70, sets the relief pressure value of the variable relief valve 70 to the first relief pressure value, and ends the control.

[0052] Furthermore, if it is determined in step S11 that the steering lever 52 has been operated, then in step S13 the controller 51 outputs an operation signal to the variable relief valve 70, sets the relief pressure value of the variable relief valve 70 to the first relief pressure value, and ends the control. When the engine 31 is operating and the steering lever 52 is operated, the controller 51 determines that the vehicle body 11 is turning, and sets the relief pressure value of the variable relief valve 70 to a value greater than the relief pressure value of the charge relief valve 66, thereby enabling the hydraulic oil supplied from the charge pump 63 to be supplied to the hydraulic steering device 61 without passing through the bypass oil passage 69. Note that surplus hydraulic oil is returned to the hydraulic oil tank 71 via the third discharge oil passage 64c by the movement of the shuttle valve 65. In addition, in step S11, in order to prevent a determination that the steering lever 52 has been operated against the operator's intention based on the operation of the steering lever 52 caused by erroneous operation or noise such as vibration, the controller 51 may determine that the steering lever 52 has been operated if the operation of the steering lever 52 is maintained for a predetermined period of time or more.

[0053] (Features, etc.) In the work machine of this embodiment 1, the controller 51 controls the variable relief valve 70 so that the hydraulic oil discharged from the charge pump 63 can pass through the bypass oil passage 69 at a lower pressure value when the vehicle is not turning than when the vehicle is turning.

[0054] By lowering the relief pressure value of variable relief valve 70 when the vehicle is not turning in this way, the hydraulic oil supplied from charge pump 63 can be returned to hydraulic oil tank 71, bypassing hydraulic steering device 61. This reduces horsepower loss and improves fuel efficiency. In addition, because a fixed displacement pump is used for charge pump 63, costs can be reduced and durability can be improved.

[0055] The work machine of the first embodiment is equipped with a charge relief valve 66 arranged in the discharge oil passage 64. The second relief pressure value set in the adjustable relief valve 70 when not turning is lower than the relief pressure value of the charge relief valve 66.

[0056] When the steering lever 52 is not being used for steering (when not turning), no hydraulic oil flows in the hydraulic steering device 61, so the shuttle valve 65 connects the connecting oil passage 72 to the third discharge oil passage 64c. For this reason, if the bypass oil passage 69 and the variable relief valve 70 were not provided, the hydraulic oil discharged from the charge pump 63 would be returned to the hydraulic oil tank 71 via the connecting oil passage 72 and the third discharge oil passage 64c, but a torque according to the relief pressure value of the charge relief valve 66 would be required. In contrast, in this embodiment, the relief pressure value of the variable relief valve 70 is set lower than the relief pressure value of the charge relief valve 66 when not turning, so the hydraulic oil discharged from the charge pump 63 is returned to the hydraulic oil tank 71 via the bypass oil passage 69. In this way, the hydraulic oil discharged from the charge pump 63 can be returned to the hydraulic oil tank 71 through the variable relief valve 70 without passing through the charge relief valve 66, so that horsepower loss can be reduced and fuel efficiency can be improved.

[0057] (Embodiment 2) Next, a working machine 1 according to the second embodiment will be described. The working machine 1 according to the second embodiment has a different control system configuration from that according to the first embodiment, and during a cold start, the relief pressure value of the adjustable relief valve 70 is set to the second relief pressure value. Therefore, the configuration according to the second embodiment that differs from that according to the first embodiment will be mainly described.

[0058] Fig. 6 is a diagram showing a control system 3' and a steering mechanism 36 of a work machine according to the present embodiment 2. As shown in Fig. 6, compared to the control system 3 of the embodiment 1, the control system 3' of the present embodiment 2 further includes an outside air temperature sensor 56 (low temperature detection unit) and a hydraulic oil temperature sensor 57 (low temperature detection unit).

[0059] The outside air temperature sensor 56 measures the outside air temperature of the vehicle body 11, and outputs a measurement signal including information about the outside air temperature to the controller 51. The outside air temperature sensor 56 is disposed in the vehicle body 11.

[0060] The hydraulic oil temperature sensor 57 measures the temperature of the hydraulic oil, and outputs a measurement signal including information about the hydraulic oil temperature to the controller 51.

[0061] The controller 51 determines whether or not the engine is in a low temperature start state based on a rotation speed signal from the rotation sensor 55 (start detection unit), a measurement signal from the outside air temperature sensor 56, and a measurement signal from the hydraulic oil temperature sensor 57. If the controller 51 determines that the engine is in a low temperature start state, the controller 51 sets the relief pressure of the adjustable relief valve 70 to the second relief pressure value, and if the controller 51 determines that the engine is not in a low temperature start state, the controller 51 sets the relief pressure of the adjustable relief valve 70 to the first relief pressure value.

[0062] (Control method during cold start) FIG. 7 is a flow chart showing a control method for a work machine according to the second embodiment.

[0063] First, in step S20, an ignition key-on signal is input to the controller 51 from a starter unit including the key cylinder 54. The ignition key-on signal indicates that the ignition key has been turned on or that the ignition key is in an on state.

[0064] Next, in step S21, the controller 51 determines whether the temperature is low. The controller 51 determines whether the temperature is low based on the measurement signal of the outside air temperature sensor 56 and the measurement signal of the hydraulic oil temperature sensor 57. The controller 51 determines that the temperature is low when the measurement value of the outside air temperature sensor 56 is equal to or lower than a predetermined threshold temperature and the measurement value of the hydraulic oil temperature sensor 57 is equal to or lower than a predetermined threshold temperature. Specifically, for example, when the measurement value of the outside air temperature sensor 56 is equal to or lower than -5°C and the measurement value of the hydraulic oil temperature sensor 57 is equal to or lower than -5°C, the controller 51 determines that the temperature is low. If it is determined that the temperature is low, the control proceeds to step S22.

[0065] In step S22, the controller 51 determines whether or not starting (cranking) of the engine 31 is being performed. The controller 51 determines whether or not starting of the engine 31 is being performed based on a rotation speed signal from a rotation speed sensor 55 that detects the rotation speed of the engine 31. For example, when the rotation speed of the engine 31 is not 0 rpm, the controller 51 determines that starting of the engine 31 is being performed. Also, when the rotation speed of the engine 31 is 0 rpm, the controller 51 determines that starting of the engine 31 is not being performed. In addition, without being limited to this, for example, the controller 51 may determine that starting of the engine 31 is being performed when the rotation speed of the engine 31 is 50 rpm or more, and may determine that starting of the engine 31 is not being performed when the rotation speed of the engine 31 is less than 50 rpm. When it is determined that starting of the engine 31 is being performed, the control proceeds to step S23.

[0066] In step S23, the controller 51 determines whether the engine 31 is inoperable or to abort the start, based on a rotation speed signal from the rotation sensor 55 that detects the rotation speed of the engine 31. The ... determination whether the engine 31 is inoperable or to abort the start is made when the rotation speed is 0 rpm after the start of the engine 31 in step S22, or is not equal to or higher than a predetermined rotation speed.

[0067] Specifically, when the rotation speed of the engine 31 is not 0 rpm, the controller 51 determines that the engine 31 is operating and there is no need to abort the starting. On the other hand, when the rotation speed of the engine 31 is 0 rpm, the controller 51 determines that the engine 31 is not operating and therefore cannot operate, and therefore aborts the starting.

[0068] Furthermore, without being limited to this, for example, when the rotation speed of the engine 31 is 50 rpm or more, the controller 51 may determine that the engine 31 is running and there is no need to stop starting, and when the rotation speed of the engine 31 is less than 50 rpm, the engine 31 is not running at a predetermined rotation speed or more, so that it is necessary to stop starting the engine 31.

[0069] In addition, the controller 51 may determine to stop starting the engine 31 when a predetermined time has elapsed in a state where the rotation speed of the engine 31 is not 0 rpm or is equal to or greater than a predetermined rotation speed (e.g., 50 rpm) but does not satisfy the condition of step S25 described below.

[0070] If it is determined that the engine 31 is running and there is no need to stop starting, the control proceeds to step S24. Steps S21, S22, and S23 correspond to determination steps.

[0071] In step S24, the controller 51 outputs an operation signal to the adjustable relief valve 70 to set the relief pressure value of the adjustable relief valve 70 to the second relief pressure value. This allows the hydraulic oil supplied toward the hydraulic steering device 61 by the drive of the charge pump 63 during cold start to be returned to the hydraulic oil tank 71 through the bypass oil passage 69 without passing through the hydraulic steering device 61. Step S24 corresponds to a control step.

[0072] Next, in step S25, the controller 51 determines whether the rotation speed of the engine 31 has reached a predetermined value or more, based on the rotation speed signal from the rotation sensor 55. The rotation speed can be set to, for example, 300 rpm. If the rotation speed is less than the predetermined value, control returns to step S22, and the relief pressure value of the adjustable relief valve 70 is maintained at the second relief pressure value. On the other hand, if the rotation speed of the engine 31 reaches or exceeds the predetermined value, control proceeds to step S26. In step S26, the controller 51 outputs an operation signal to the adjustable relief valve 70, sets the relief pressure value of the adjustable relief valve 70 to the first relief pressure value, and the control ends.

[0073] On the other hand, if it is determined in step S21 that the temperature is not low, in step S26, the controller 51 outputs an operation signal to the adjustable relief valve 70, sets the relief pressure value of the adjustable relief valve 70 to the first relief pressure value, and the control ends.

[0074] Also, if it is determined in step S22 that engine start-up is not being performed, in step S26, the controller 51 outputs an operation signal to the variable relief valve 70, sets the relief pressure value of the variable relief valve 70 to the first relief pressure value, and the control ends.

[0075] Also, if it is determined in step S23 that the engine 31 cannot be operated or that starting is to be stopped, in step S26, the controller 51 outputs an operation signal to the variable relief valve 70, sets the relief pressure value of the variable relief valve 70 to the first relief pressure value, and the control ends.

[0076] (Features, etc.) The working machine of the second embodiment controls the variable relief valve 70 so that oil discharged from the fixed displacement pump can pass through the bypass oil passage 69 at a lower pressure during a low temperature start than during a start other than at low temperatures.

[0077] By lowering the relief pressure value of the variable relief valve 70 during cold start in this manner, the hydraulic oil supplied from the charge pump 63 can be returned to the hydraulic oil tank 71, bypassing the hydraulic steering device 61. This reduces the load on the charge pump 63, improves fuel efficiency, and further improves cold startability. In addition, since a fixed displacement pump is used for the charge pump 63, the drive torque can be reduced during cold start compared to when a variable displacement pump is used. Furthermore, since a fixed displacement pump is used for the charge pump 63, costs can be reduced and durability can be improved.

[0078] (Embodiment 3) In the above first and second embodiments, an HSS that assists turning is used for the drive device 32, but in the work machine of the third embodiment, an HST (Hydro Static Transmission) is used as the drive device 232.

[0079] 8 is a diagram showing a drive device 232 which is an HST. The drive device 232 has two hydraulic steering devices 161, 261, a refill oil passage 162, a charge pump 163 (fixed displacement pump), a discharge oil passage 164, shuttle valves 165, 265, charge relief valves 166, 266 (discharge valves), a charge safety oil passage 167 (second bypass oil passage), a charge safety valve 168 (safety valve), a bypass oil passage 169 (first bypass oil passage), and a variable relief valve 170 (control valve).

[0080] The hydraulic steering device 161 includes a hydraulic pump 181 (steering pump), a hydraulic motor 182 (steering motor), and a hydraulic circuit 183. The hydraulic circuit 183 includes a first oil passage 183a and a second oil passage 183b. The hydraulic pump 181 is driven by the engine 31. The hydraulic motor 182 is connected to the left driving wheel 16, and the left driving wheel 16 is rotated by driving the hydraulic motor 182. The first oil passage 183a connects the hydraulic pump 181 and the hydraulic motor 182. The second oil passage 183b connects the hydraulic pump 181 and the hydraulic motor 182. The hydraulic pump 181, the first oil passage 183a, the hydraulic motor 182, and the second oil passage 183b form a closed circuit.

[0081] The hydraulic steering device 261 includes a hydraulic pump 281 (steering pump), a hydraulic motor 282 (steering motor), and a hydraulic circuit 283. The hydraulic circuit 283 includes a first oil passage 283a and a second oil passage 283b. The hydraulic pump 181 is driven by the engine 31. The hydraulic motor 282 is connected to the right driving wheel 17, and the right driving wheel 17 is rotated by driving the hydraulic motor 282. The first oil passage 283a connects the hydraulic pump 281 and the hydraulic motor 282. The second oil passage 283b connects the hydraulic pump 281 and the hydraulic motor 282. The hydraulic pump 281, the first oil passage 283a, the hydraulic motor 282, and the second oil passage 283b form a closed circuit. The left driving wheel 16 is rotated by the hydraulic steering device 161, and the right driving wheel 17 is rotated by the hydraulic steering device 261. By changing the rotation speed of the left driving wheel 16 and the right driving wheel 17, the vehicle body 11 can be turned left or right.

[0082] The refill oil passage 162 refills hydraulic oil from a hydraulic oil tank 171 in which hydraulic oil is stored to the hydraulic steering device 161, 261. The refill oil passage 162 has a first refill oil passage 162a, a second refill oil passage 162b, a third refill oil passage 162c, a fourth refill oil passage 162d, and a fifth refill oil passage 162e. One end of the first refill oil passage 162a is connected to the first oil passage 183a. One end of the second refill oil passage 162b is connected to the second oil passage 183b. The other end of the first refill oil passage 162a is connected to the other end of the second refill oil passage 162b. One end of the third refill oil passage 162c is connected to the first oil passage 283a. One end of the fourth refill oil passage 162d is connected to the second oil passage 283b. The other end of the third replenishment oil passage 162c is connected to the other end of the fourth replenishment oil passage 162d. The fifth replenishment oil passage 162e connects between the connection point of the first replenishment oil passage 162a and the second replenishment oil passage 162b, the connection point of the third replenishment oil passage 162c and the fourth replenishment oil passage 162d, and the hydraulic oil tank 171.

[0083] The charge pump 163 is disposed in the fifth replenishment oil passage 162e of the replenishment oil passage 162. The charge pump 163 is a fixed displacement pump. The charge pump 163 is a gear pump with a fixed volume. The charge pump 163 is driven in response to the rotation of the engine 31, and replenishes hydraulic oil to the hydraulic steering devices 161, 261 via the replenishment oil passage 162. A filter 184 is disposed downstream of the charge pump 163 in the fifth replenishment oil passage 162e, and removes impurities from the hydraulic oil.

[0084] A check valve 185a is disposed in the first refill oil passage 162a. The check valve 185a allows the hydraulic oil to flow from the fifth refill oil passage 162e toward the first oil passage 183a, and blocks the hydraulic oil from flowing in the opposite direction. A check valve 185b is disposed in the second refill oil passage 162b. The check valve 185b allows the hydraulic oil to flow from the fifth refill oil passage 162e toward the second oil passage 183b, and blocks the hydraulic oil from flowing in the opposite direction. The drive unit 232 has a relief oil passage 186a, a relief valve 187a, a relief oil passage 186b, and a relief valve 187b. The relief oil passage 186a branches from the upstream side of the check valve 185a of the first refill oil passage 162a and merges with the downstream side of the check valve 185a. The relief valve 187a is disposed in the relief oil passage 186a. When a steering operation is performed and the oil pressure in the first oil passage 183a becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the first oil passage 183a to the second oil passage 183b through the relief valve 187a. The relief oil passage 186b branches from the upstream side of the check valve 185b of the second refill oil passage 162b and joins with the check valve 185b downstream side. The relief valve 187b is disposed in the relief oil passage 186b. When a steering operation is performed and the oil pressure in the second oil passage 183b becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the second oil passage 183b to the first oil passage 183a through the relief valve 187b.

[0085] A check valve 285a is disposed in the third refill oil passage 162c. The check valve 285a allows the hydraulic oil to flow from the fifth refill oil passage 162e toward the first oil passage 283a, and blocks the hydraulic oil from flowing in the opposite direction. A check valve 285b is disposed in the fourth refill oil passage 162d. The check valve 285b allows the hydraulic oil to flow from the fifth refill oil passage 162e toward the second oil passage 283b, and blocks the hydraulic oil from flowing in the opposite direction. The drive device 232 has a relief oil passage 286a, a relief valve 287a, a relief oil passage 286b, and a relief valve 287b. The relief oil passage 286a branches from the upstream side of the check valve 285a of the third refill oil passage 162c and joins the downstream side of the check valve 285a. The relief valve 287a is disposed in the relief oil passage 286a. When a steering operation is performed and the oil pressure in the first oil passage 283a becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the first oil passage 283a to the second oil passage 283b through the relief valve 287a. The relief oil passage 286b branches from the upstream side of the check valve 285b of the fourth refill oil passage 162d and joins with the downstream side of the check valve 285b. The relief valve 287b is disposed in the relief oil passage 286b. When a steering operation is performed and the oil pressure in the second oil passage 283b becomes equal to or higher than a predetermined pressure, the hydraulic oil flows from the second oil passage 283b to the first oil passage 283a through the relief valve 287b.

[0086] The discharge oil passage 164 discharges hydraulic oil from the hydraulic steering device 161, 261 toward the hydraulic oil tank 171. The discharge oil passage 164 includes a first discharge oil passage 164a, a second discharge oil passage 164b, a third discharge oil passage 164c, a fourth discharge oil passage 164d, a fifth discharge oil passage 164e, a sixth discharge oil passage 164f, and a seventh discharge oil passage 164g. One end of the first discharge oil passage 164a is connected to the first oil passage 183a, and the other end is connected to the shuttle valve 165. One end of the second discharge oil passage 164b is connected to the second oil passage 183b, and the other end is connected to the shuttle valve 165. One end of the third discharge oil passage 164c is connected to the shuttle valve 165, and the other end is connected to one end of the seventh discharge oil passage 164g. The other end of the seventh discharge oil passage 164g is connected to the hydraulic oil tank 171. One end of the fourth discharge oil passage 164d is connected to the first oil passage 283a, and the other end is connected to the shuttle valve 265. One end of the fifth discharge oil passage 164e is connected to the second oil passage 283b, and the other end is connected to the shuttle valve 265. One end of the sixth discharge oil passage 164f is connected to the shuttle valve 265, and the other end is connected to the seventh discharge oil passage 164g.

[0087] The shuttle valve 165 has a valve element that moves due to the pressure difference between the first oil passage 183a and the second oil passage 183b, and connects the third discharge oil passage 164c to the first discharge oil passage 164a or the second discharge oil passage 164b. The shuttle valve 265 has a valve element that moves due to the pressure difference between the first oil passage 283a and the second oil passage 283b, and connects the sixth discharge oil passage 164f to the fourth discharge oil passage 164d or the fifth discharge oil passage 164e.

[0088] The charge relief valve 166 is disposed in the third discharge oil passage 164c. The charge relief valve 166 closes the third discharge oil passage 164c when the input oil pressure is less than a predetermined relief pressure value, and opens the third discharge oil passage 164c when the input oil pressure is equal to or greater than the relief pressure value. The charge relief valve 266 is disposed in the sixth discharge oil passage 164f. The charge relief valve 266 closes the sixth discharge oil passage 164f when the input oil pressure is less than a predetermined relief pressure value, and opens the sixth discharge oil passage 164f when the input oil pressure is equal to or greater than the relief pressure value.

[0089] The charge safety oil passage 167 connects between the fifth replenishment oil passage 162e of the replenishment oil passage 162 and the seventh discharge oil passage 164g of the discharge oil passage 164. The charge safety valve 168 is disposed in the charge safety oil passage 167. The charge safety valve 168 is a valve for preventing the hydraulic steering device 161, 261 from being damaged due to the hydraulic pressure from the charge pump 163 becoming too high. The charge safety valve 168 closes the charge safety oil passage 167 when the input hydraulic pressure is less than a predetermined relief pressure value, and opens the charge safety oil passage 167 when the input hydraulic pressure is equal to or greater than the relief pressure value. The relief pressure value of the charge safety valve 168 is higher than the relief pressure values ​​of the charge relief valves 166, 266.

[0090] The bypass oil passage 169 connects between the fifth replenishment oil passage 162e of the replenishment oil passage 162 and the seventh discharge oil passage 164g of the discharge oil passage 164. A branching portion of the bypass oil passage 169 from the fifth replenishment oil passage 162e is disposed upstream of a branching portion of the charge safety oil passage 167 from the fifth replenishment oil passage 162e.

[0091] The variable relief valve 170 is disposed in the bypass oil passage 169. When the input oil pressure is less than a predetermined relief pressure value, the variable relief valve 170 closes the bypass oil passage 169, and when the input oil pressure becomes equal to or greater than the relief pressure value, the variable relief valve 170 opens the bypass oil passage 169. The relief pressure value of the variable relief valve 170 can be changed by a command from the controller 51, as in the first and second embodiments.

[0092] The control method during cold start is the same as in the second embodiment, and therefore the explanation will be omitted.

[0093] In the above configuration, by setting the relief pressure value of variable relief valve 170 to the second relief pressure value during cold start, similar to embodiment 2, the hydraulic oil supplied from charge pump 163 can be returned to hydraulic oil tank 171 without passing through hydraulic steering devices 161, 261. This reduces the load on charge pump 163 and improves cold startability.

[0094] (Other embodiments) Although one embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the invention.

[0095] (A) In the above-mentioned first embodiment, the operation of the engine 31 is determined based on the rotation speed of the engine 31 or the rotation speed of the driving force transmission system, but this is not limiting. For example, the operation of the engine may be determined by detecting the flow rate or pressure of the intake and exhaust of the engine, or the flow rate or pressure of the engine cooling system. The operation of the engine 31 may also be determined based on both the rotation speed of the engine 31 and the rotation speed of the driving force transmission system. The controller 51 may also determine that the engine is operating based on a key-on signal (e.g., a cranking key-on signal, etc.) from the key cylinder 54.

[0096] (B) In the above-mentioned second embodiment, when the measured value of the outside air temperature sensor 56 is equal to or lower than the predetermined threshold temperature and the measured value of the hydraulic oil temperature sensor 57 is equal to or lower than the predetermined threshold temperature, it is determined that the temperature is low, but this is not limiting and it is also possible to determine that the temperature is low when either one of the conditions is satisfied. Note that, since the load does not increase when the hydraulic oil temperature is high, it is preferable to determine whether the temperature is low using at least the condition that the measured value of the hydraulic oil temperature sensor 57 is equal to or lower than the predetermined threshold temperature.

[0097] (C) In the above-described second embodiment, the threshold temperature for the measurement value of the outside air temperature sensor 56 and the threshold temperature for the measurement value of the hydraulic oil temperature sensor 57 are the same, but they may be different.

[0098] (D) In the above-mentioned second embodiment, the controller 51 determines whether or not the engine is being started based on the rotation speed of the engine 31, but this is not limiting, and the controller 51 may determine whether or not the engine is being started based on a starter cranking signal. When the key is inserted into the key cylinder 54 and rotated to the cranking on position, a starter cranking signal is transmitted from the starter unit to the starter motor, and at the same time, a starter cranking signal is transmitted from the starter unit to the controller 51. When the starter cranking signal is input, the controller 51 determines that the engine is being started (cranked), and when the starter cranking signal is not input, the controller 51 can determine that the engine is not being started.

[0099] Furthermore, the controller 51 may determine that the engine is being started when both the rotation speed of the engine 31 and the starter cranking signal satisfy a condition, or when either one of them satisfies a condition.

[0100] (E) In the above first and second embodiments, the charge safety valve 68 and the adjustable relief valve 70 are separate, but the adjustable relief valve 70 may also function as the charge safety valve 68. When the charge safety valve 68 and the charge safety oil passage 67 are not provided and the adjustable relief valve 70 also functions as the charge safety valve 68, the first relief pressure value of the adjustable relief valve 70 is set to the relief pressure value set for the charge safety valve 68.

[0101] (F) In the above first and second embodiments, the variable relief valve 70 is disposed in the bypass oil passage 69, but it is not limited to the variable relief valve 70, and an on-off valve may be disposed. The on-off valve is opened when the engine is not turning or during a low-temperature start, and is closed when the engine is turning or during a start at a temperature other than low temperature. The open state can be said to mean that the on-off valve is set to the second relief pressure value, and the closed state can be said to mean that the on-off valve is set to the first relief pressure value.

[0102] (G) In the above first and second embodiments, the first relief pressure value is changed to the second relief pressure value by outputting a current to the variable relief valve 70, but the present invention is not limited thereto. The first relief pressure value may be changed to the second relief pressure value by moving a solenoid of the variable relief valve using pilot hydraulic pressure. As shown in Fig. 9, the controller 51 controls the opening and closing of an EPC (Electromagnetic Proportional Control) valve 79, and adjusts the opening of the EPC valve 79 to supply a PPC pressure (pilot hydraulic pressure) to a solenoid of a PPC relief valve 370, thereby changing the relief pressure value of the PPC relief valve 370. The pump that supplies the PPC pressure may be provided separately from the charge pump, or may also serve as the charge pump.

[0103] (H) In the above-mentioned first and second embodiments, the relief pressure value of the adjustable relief valve 70 is changed between the first relief pressure value and the second relief pressure value, but the relief pressure value may be changed in a stepwise manner. This makes it possible to suppress water hammer and piping vibration.

[0104] (I) In the above embodiments 1 to 3, it has been described that the relief pressure value of the variable relief valve 70 is set to the second relief pressure value when the engine is not turning, and that the relief pressure value of the variable relief valve 70 is set to the second relief pressure value when the engine is not turning and when the engine is started at low temperature. However, different relief pressure values ​​may be set when the engine is not turning and when the engine is started at low temperature.

[0105] (J) In the above-mentioned second embodiment, when it is determined in step S23 that the engine 31 cannot be operated and starting is to be stopped, the control proceeds to step S26, but this is not limited thereto. When it is determined in step S23 that the engine 31 cannot be operated and starting is to be stopped, the control may proceed to step S22. In this case, the engine 31 can be started again from cranking without turning the key on again. Also, in this case, when the condition of step S23 is not satisfied even after steps S22 and S23 are repeated for a predetermined time or a predetermined number of times, the control may proceed to step S26.

[0106] (K) Although a bulldozer has been described as an example of the working machine in the above embodiment, other working machines such as a forklift, a wheel loader, and a hydraulic excavator may also be used. [Industrial Applicability]

[0107] INDUSTRIAL APPLICABILITY The work machine and the control method for the work machine disclosed herein have the effect of reducing costs and horsepower losses, and are useful in bulldozers and the like. [Explanation of symbols]

[0108] 1: Work machine 51: Controller 61:Hydraulic steering device 62: Replenishment oil path 63: Charge pump 64:Drainage oil path 69: Bypass oil passage 70: Variable relief valve

Claims

1. A hydraulic steering system that assists in turning or performs turning, The hydraulic steering system includes a replenishment oil passage for supplying oil, A constant-capacity pump is arranged in the aforementioned oil supply passage and supplies oil to the hydraulic steering device, A discharge oil passage for discharging oil from the hydraulic steering system, A first bypass oil passage connects the portion of the replenishment oil passage downstream of the constant-capacity pump and the discharge oil passage, bypassing the hydraulic steering device. A control valve located in the first bypass oil passage, The system includes a controller that determines whether or not the pump is not swirling, and if it is determined that the pump is not swirling, controls the control valve so that the oil discharged from the constant-capacity pump can pass through the first bypass oil passage at a lower pressure than when the pump is swirling, A type of machinery used for industrial work.

2. A hydraulic steering system that assists in turning or performs turning, The hydraulic steering system includes a replenishment oil passage for supplying oil, A constant-capacity pump is arranged in the aforementioned oil supply passage and supplies oil to the hydraulic steering device, A discharge oil passage for discharging oil from the hydraulic steering system, A first bypass oil passage connects the portion of the replenishment oil passage downstream of the constant-capacity pump and the discharge oil passage, bypassing the hydraulic steering device. A control valve located in the first bypass oil passage, The system includes a controller that determines whether or not it is a cold start, and if it is determined to be a cold start, controls the control valve so that the oil discharged from the constant-capacity pump can pass through the first bypass oil passage at a lower pressure than during a non-cold start, A type of machinery used for industrial work.

3. The control valve is a variable relief valve, The relief pressure value during non-turning is lower than the relief pressure value during turning. The work machine according to claim 1.

4. The control valve is a variable relief valve, The relief pressure value during low-temperature startup is lower than the relief pressure value during startup at temperatures other than low temperature. The working machine according to claim 2.

5. A second bypass oil passage connects the portion of the replenishment oil passage downstream of the constant-capacity pump and the discharge oil passage, bypassing the hydraulic steering device. The system further comprises a safety valve located in the second bypass oil passage and opened at a predetermined first pressure value, The aforementioned low pressure value is lower than the first pressure value. The working machine according to claim 1 or 2.

6. The oil discharge passage is further provided with a discharge valve that is opened at a predetermined second pressure value, The aforementioned low pressure value is lower than the aforementioned second pressure value. The working machine according to claim 1 or 2.

7. The aforementioned constant-capacity pump is a gear pump that is driven according to the rotation of the engine. The working machine according to claim 1 or 2.

8. The hydraulic steering device is A steering pump driven by an engine, A steering motor driven by oil discharged from the steering pump, A first oil passage connecting the steering pump and the steering motor, It has a second oil passage connecting the steering pump and the steering motor, The steering pump, the steering motor, the first oil passage, and the second oil passage constitute a closed circuit. The working machine according to claim 1 or 2.

9. The control valve also functions as a safety valve for the hydraulic steering system. The relief pressure value during the aforementioned rotation is set to the pressure value of the safety valve. The work machine according to claim 3.

10. The control valve also functions as a safety valve for the hydraulic steering system. The relief pressure value during startup other than the aforementioned low temperature is set to the pressure value of the safety valve. The work machine according to claim 4.

11. The controller controls the control valve to change the relief pressure value in steps between the relief pressure value during rotation and the relief pressure value when not rotating. The work machine according to claim 3.

12. An operating member for operating the hydraulic steering device, An operation detection unit for detecting the operating state of the operating member, It includes an operation detection unit that detects the operating status of the engine, The controller determines that a turning operation has occurred when it detects, based on the detection signals from the operation detection unit and the operation detection unit, that the engine has been operated and the operating member has been operated. The work machine according to claim 1.

13. A start detection unit that detects the engine's starting status, It includes a low-temperature detection unit for detecting low-temperature conditions, The controller, based on the detection signals from the start detection unit and the low temperature detection unit, determines that the low temperature start has occurred when it detects that the engine has started and is at a low temperature. The working machine according to claim 2.

14. A hydraulic steering system that assists in turning or performs turning, The hydraulic steering system includes a replenishment oil passage for supplying oil, A constant-capacity pump is arranged in the aforementioned oil supply passage and supplies oil to the hydraulic steering device, A discharge oil passage for discharging oil from the hydraulic steering system, A first bypass oil passage connects the portion of the replenishment oil passage downstream of the constant-capacity pump and the discharge oil passage, bypassing the hydraulic steering device. A control method for a work machine comprising a control valve arranged in the first bypass oil passage, A determination step to determine whether or not the vehicle is not turning, The control step includes controlling the control valve so that, when it is determined that the pump is not swirling, the oil discharged from the constant-capacity pump can pass through the first bypass oil passage at a lower pressure than when swirling, A method for controlling industrial machinery.

15. A hydraulic steering system that assists in turning or performs turning, The hydraulic steering system includes a replenishment oil passage for supplying oil, A constant-capacity pump is arranged in the aforementioned oil supply passage and supplies oil to the hydraulic steering device, A discharge oil passage for discharging oil from the hydraulic steering system, A first bypass oil passage connects the portion of the replenishment oil passage downstream of the constant-capacity pump and the discharge oil passage, bypassing the hydraulic steering device. A control method for a work machine comprising a control valve arranged in the first bypass oil passage, A determination step to determine whether or not it is a cold start, The control step includes, when it is determined that a low-temperature start has occurred, controlling the control valve so that the oil discharged from the constant-capacity pump can pass through the first bypass oil passage at a lower pressure than during a start other than a low-temperature start, A method for controlling industrial machinery.

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