Work machine and method for controlling work machine

Abstract

To provide a work machine having both mechanical transmission and HST, which can improve a fuel efficiency while suppressing a reduction in durability of a component of the HST.SOLUTION: A work machine includes a drive source, a first drive wheel, a mechanical transmission, a second drive wheel, an HST, and a controller. The mechanical transmission transmits a drive force of the drive source to the first drive wheel. The HST includes a hydraulic pump, a hydraulic motor, and a drive circuit. The hydraulic pump is driven by the drive source. The hydraulic motor is driven by the work fluid discharged from the hydraulic pump. The drive circuit connects the hydraulic pump and the hydraulic motor. The HST transmits the drive force of the drive source to the second drive wheel. The controller acquires the current speed gear of the mechanical transmission. The controller controls the HST such as to restrict the hydraulic pressure of the drive circuit to be equivalent to or less than a cutoff pressure depending on the current speed gear.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] TECHNICAL FIELD The present disclosure relates to work machines and methods for controlling work machines. [Background technology]

[0002] Some work machines are equipped with both a mechanical transmission and a hydrostatic transmission. For example, in the work machine disclosed in Patent Document 1, the driving force from the engine is transmitted to the rear wheels via a mechanical transmission. The driving force from the engine is also transmitted to the front wheels via a hydrostatic transmission. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] US Patent US5474147 Summary of the Invention [Problem to be solved by the invention]

[0004] In the above-described work machines, the driving performance of the mechanical transmission may be excessively high compared to the driving performance of the hydrostatic transmission. In such cases, the load on the components of the hydrostatic transmission, such as the hydraulic motor, increases, raising concerns about a decrease in the durability of the components.

[0005] On the other hand, hydrostatic transmissions have a cutoff function that limits the hydraulic pressure in the drive circuit to a predetermined cutoff pressure or less to protect the components. Therefore, if the driving performance achieved by the mechanical transmission becomes excessive compared to the driving performance achieved by the hydrostatic transmission, the cutoff function limits the hydraulic pressure in the drive circuit to a pressure or less than the cutoff pressure, thereby reducing the load on the components.

[0006] However, the balance between the driving performance of the mechanical transmission and the driving performance of the hydrostatic transmission varies depending on the speed stage of the mechanical transmission. Therefore, if the cutoff pressure is too low relative to the balance between the driving performance of the mechanical transmission and the driving performance of the hydrostatic transmission, there is a concern that fuel economy will decrease. An object of the present disclosure is to prevent a decrease in durability of components of the hydrostatic transmission and improve fuel economy in a work machine equipped with both a mechanical transmission and a hydrostatic transmission. [Means for solving the problem]

[0007] A work machine according to one aspect of the present disclosure includes a drive source, a first drive wheel, a mechanical transmission, a second drive wheel, a hydrostatic transmission, and a controller. The mechanical transmission has a plurality of speed stages. The mechanical transmission transmits drive force from the drive source to the first drive wheel. The hydrostatic transmission includes a hydraulic pump, a hydraulic motor, and a drive circuit. The hydraulic pump is driven by the drive source. The hydraulic motor is driven by hydraulic fluid discharged from the hydraulic pump. The drive circuit connects the hydraulic pump and the hydraulic motor. The hydrostatic transmission transmits drive force from the drive source to the second drive wheel. The controller acquires a current speed stage of the mechanical transmission. The controller controls the hydrostatic transmission to limit the hydraulic pressure in the drive circuit to a cutoff pressure corresponding to the current speed stage or lower.

[0008] A method according to another aspect of the present disclosure is a method for controlling a work machine. The work machine includes a drive source, first drive wheels, a mechanical transmission, a second drive wheel, and a hydrostatic transmission. The mechanical transmission has a plurality of speed stages. The mechanical transmission transmits drive power from the drive source to the first drive wheel. The hydrostatic transmission includes a hydraulic pump, a hydraulic motor, and a drive circuit. The hydraulic pump is driven by the drive source. The hydraulic motor is driven by hydraulic fluid discharged from the hydraulic pump. The drive circuit connects the hydraulic pump and the hydraulic motor. The hydrostatic transmission transmits drive power from the drive source to the second drive wheel. The method according to this aspect includes obtaining a current speed stage of the mechanical transmission, and controlling the hydrostatic transmission to limit hydraulic pressure in the drive circuit to a cutoff pressure corresponding to the current speed stage or lower. [Effects of the Invention]

[0009] According to the present disclosure, the hydraulic pressure in the drive circuit of the hydrostatic transmission is limited to a level equal to or lower than the cutoff pressure corresponding to the current speed stage of the mechanical transmission, thereby preventing deterioration in the durability of components of the hydrostatic transmission and improving fuel economy. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a side view of a work machine according to an embodiment. [Figure 2] FIG. 2 is a schematic diagram showing the configuration of a drive system of the work machine. [Figure 3] FIG. 2 is a schematic diagram showing a control system of a work machine. [Figure 4] 4 is a flowchart showing the process of controlling the HST in the AWD mode. [Figure 5] 10 is a flowchart showing a process of high-pressure cutoff control. [Figure 6] 10 is a table showing an example of threshold data. [Figure 7] FIG. 10 is a diagram showing changes in the driving pressure of the HST and the displacement of the travel pump during high-pressure cutoff control. [Figure 8] 10 is a flowchart showing a process for controlling an HST in an AWD mode according to another embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] An embodiment of the present disclosure will now be described with reference to the drawings. FIG. 1 is a side view of a work machine 1 according to the embodiment. The work machine 1 according to the embodiment is a motor grader. As shown in FIG. 1, the work machine 1 includes a vehicle body 2, drive wheels 3A, 4A, 4B, and a work implement 5. The vehicle body 2 includes a front frame 11, a rear frame 12, a cab 13, and a power compartment 14. The drive wheels 3A, 4A, 4B include a front wheel 3A (second drive wheel) and rear wheels 4A, 4B (first drive wheels). Note that the drive wheels 3A, 4A, 4B are provided on the left and right, respectively, but FIG. 1 shows only the left front wheel 3A and rear wheels 4A, 4B.

[0012] The rear frame 12 is connected to the front frame 11. The front frame 11 can be articulated left and right relative to the rear frame 12. A cab 13 and a power compartment 14 are disposed on the rear frame 12. A driver's seat is disposed inside the cab 13. The power compartment 14 is disposed behind the cab 13. The front frame 11 extends forward from the rear frame 12. A front wheel 3A is attached to the front frame 11. Rear wheels 4A, 4B are attached to the rear frame 12.

[0013] The work implement 5 is movably connected to the vehicle body 2. The work implement 5 includes a support member 15, a blade 16, and multiple hydraulic actuators 17, 18. The blade 16 is disposed between the front wheel 3A and the rear wheel 4A. The blade is disposed below the front frame 11. The support member 15 is movably connected to the front frame 11 of the vehicle body 2. The support member 15 supports the blade 16. The multiple hydraulic actuators 17, 18 operate the support member 15 and the blade 16. The multiple hydraulic actuators 17, 18 include, for example, hydraulic cylinders. The multiple hydraulic actuators 17, 18 may include hydraulic motors.

[0014] Figure 2 is a diagram showing the configuration of a drive system mounted on the work machine 1. As shown in Figure 2, the work machine 1 is equipped with a drive source 21, a torque converter 22, a mechanical transmission 23, a work implement pump 24, a work implement control valve 25, and a hydrostatic transmission (hereinafter referred to as "HST") 26.

[0015] The drive source 21 is, for example, an internal combustion engine. The torque converter 22 is connected to the drive source 21. The mechanical transmission 23 is connected to the drive source 21 via the torque converter 22. The mechanical transmission 23 includes a plurality of speed change gears G1-G8 corresponding to a plurality of speed stages, and a clutch (not shown). The mechanical transmission 23 changes the speed of the drive force from the drive source 21 and transmits it to the rear wheels 4A, 4B.

[0016] The work implement pump 24 is connected to the drive source 21. The work implement pump 24 is a hydraulic pump, and is driven by the drive source 21 to discharge hydraulic oil. The work implement pump 24 is connected to the hydraulic actuators 17, 18 via a work implement control valve 25. The hydraulic oil discharged from the work implement pump 24 is supplied to the hydraulic actuators 17, 18. This drives the work implement 53. The work implement pump 24 is a variable displacement hydraulic pump. A capacity control device 27 is connected to the work implement pump 24. The capacity control device 27 of the work implement pump 24 controls the capacity of the work implement pump 24. The capacity control device 27 of the work implement pump 24 includes, for example, a cylinder and a control valve.

[0017] The HST 26 transmits the driving force of the drive source 21 to the front wheels 3A. An HST circuit may be provided independently for each of the left and right front wheels 3A (a so-called two-pump, two-motor circuit). Alternatively, the HST circuit may be common to both the left and right front wheels 3A (a so-called one-pump, two-motor circuit or one-pump, one-motor circuit). In the embodiment shown in FIG. 2, an HST circuit is provided for each of the left and right front wheels 3A. However, FIG. 2 shows only one HST circuit; the other HST circuit has the same circuit configuration and is therefore not shown. The HST 26 includes a travel pump 31, drive circuits 32A and 32B, and a travel motor 33. The travel pump 31 is connected to the drive source 21. The travel pump 31 is a hydraulic pump. The travel pump 31 discharges hydraulic oil when driven by the drive source 21. The travel pump 31 is a variable displacement hydraulic pump. The hydraulic oil discharged from the travel pump 31 is sent to the travel motor 33 through drive circuits 32A and 32B.

[0018] The drive circuits 32A and 32B connect the travel pump 31 and the travel motor 33. The drive circuits 32A and 32B include a first drive circuit 32A and a second drive circuit 32B. The travel pump 31, the travel motor 33, the first drive circuit 32A, and the second drive circuit 32B form a closed circuit. Hydraulic oil is supplied from the travel pump 31 to the travel motor 33 via the first drive circuit 32A, thereby driving the travel motor 33 in one direction (for example, the forward direction). In this case, the hydraulic oil returns from the travel motor 33 to the travel pump 31 via the second drive circuit 32B. Furthermore, hydraulic oil is supplied from the travel pump 31 to the travel motor 33 via the second drive circuit 32B, thereby driving the travel motor 33 in the other direction (for example, the reverse direction). In this case, the hydraulic oil returns from the travel motor 33 to the travel pump 31 via the first drive circuit 32A.

[0019] The travel motor 33 is a variable displacement hydraulic motor. The travel motor 33 is driven by hydraulic oil discharged from the travel pump 31 and generates driving force for travel. The travel motor 33 is connected to the front wheels 3A via a clutch 34. When the clutch 34 is engaged, the rotation of the travel motor 33 is transmitted to the front wheels 3A. A capacity control device 35 is connected to the travel motor 33. The capacity control device 35 of the travel motor 33 controls the capacity of the travel motor 33.

[0020] The motor displacement control device includes a motor cylinder 36 and a motor control valve 37. The motor cylinder 36 is connected to the travel motor 33. The motor cylinder 36 is hydraulically driven to change the tilt angle of the travel motor 33. The motor control valve 37 is an electromagnetic proportional control valve that is controlled based on a command signal input to the motor control valve 37. The motor control valve 37 operates the motor cylinder 36, thereby changing the displacement of the travel motor 33.

[0021] The HST 26 includes a charge pump 38 and a charge circuit 39. The charge pump 38 is connected to the drive source 21. The charge pump 38 is driven by the drive source 21 to supply hydraulic oil to the charge circuit 39. The charge circuit 39 is connected to the charge pump 38. The charge circuit 39 is connected to the first drive circuit 32A via a first relief valve 41. The charge circuit 39 is connected to the second drive circuit 32B via a second relief valve 42. The charge circuit 39 is provided with a charge relief valve 43. The charge relief valve 43 opens when the hydraulic pressure in the charge circuit 39 exceeds a predetermined relief pressure. This limits the hydraulic pressure in the charge circuit 39 so that it does not exceed the predetermined relief pressure.

[0022] A displacement control device 44 is connected to the travel pump 31. The displacement control device 44 controls the displacement of the travel pump 31. The displacement of the travel pump means the amount of hydraulic oil discharged per rotation (cc / rev). The displacement control device 44 also controls the discharge direction of the travel pump 31. The displacement control device 44 includes a pump control cylinder 46 and a pump control valve 47.

[0023] The pump control cylinder 46 is connected to the travel pump 31. The pump control cylinder 46 is hydraulically driven to change the tilt angle of the travel pump 31. In this way, the pump control cylinder 46 changes the capacity of the travel pump 31. The pump control cylinder 46 is connected to the charge circuit 39 via a pump pilot circuit 48.

[0024] The pump control valve 47 is an electromagnetic proportional control valve that is controlled based on a command signal input to the pump control valve 47. The pump control valve 47 switches the supply direction of hydraulic oil to the pump control cylinder 46. The pump control valve 47 switches the discharge direction of the travel pump 31 by switching the supply direction of hydraulic oil to the pump control cylinder 46. This changes the drive direction of the travel motor 33, and switches the working machine 1 between forward and reverse travel.

[0025] Furthermore, the pump control valve 47 controls the pressure of hydraulic oil (hereinafter referred to as "pump pilot pressure") supplied to the pump control cylinder 46 via a pump pilot circuit 48. Specifically, the pump control valve 47 adjusts the tilt angle of the travel pump 31 by changing the pump pilot pressure supplied to the pump control cylinder 46. This controls the capacity of the travel pump 31.

[0026] The HST 26 includes a cutoff mechanism 49. The cutoff mechanism 49 includes a relief valve 51 and a shuttle valve 52. The pump pilot circuit 48 is connected to a hydraulic oil tank via the relief valve 51. A pilot port of the relief valve 51 is connected to the first drive circuit 32A and the second drive circuit 32B via the shuttle valve 52. The shuttle valve 52 introduces the larger of the hydraulic pressure of the first drive circuit 32A and the hydraulic pressure of the second drive circuit 32B (hereinafter referred to as the "drive pressure") into the pilot port of the relief valve 51.

[0027] The relief valve 51 opens when the drive pressure reaches or exceeds a predetermined first cutoff pressure. This causes the pump pilot circuit 48 to communicate with the hydraulic oil tank. This reduces the hydraulic pressure in the pump pilot circuit 48, thereby reducing the capacity of the travel pump 31. As a result, the drive pressure is limited to or below the first cutoff pressure.

[0028] Figure 3 is a schematic diagram showing the control system of the work machine 1. As shown in Figure 3, the control system of the work machine 1 includes a first rotational speed sensor 61, a second rotational speed sensor 62, a third rotational speed sensor 63, a first pressure sensor 64, and a second pressure sensor 65. The first rotational speed sensor 61 detects the output rotational speed of the drive source 21. The first rotational speed sensor 61 outputs a signal indicating the output rotational speed of the drive source 21. The second rotational speed sensor 62 detects the output rotational speed of the travel motor 33. The second rotational speed sensor 62 outputs a signal indicating the output rotational speed of the travel motor 33. The third rotational speed sensor 63 detects the output rotational speed of the mechanical transmission 23. The third rotational speed sensor 63 outputs a signal indicating the output rotational speed of the mechanical transmission 23.

[0029] The first pressure sensor 64 is connected to the first drive circuit 32A. The first pressure sensor 64 detects the oil pressure of the first drive circuit 32A. The second pressure sensor 65 is connected to the second drive circuit 32B. The second pressure sensor 65 detects the oil pressure of the second drive circuit 32B. The first pressure sensor 64 outputs a signal indicative of the oil pressure of the first drive circuit 32A. The second pressure sensor 65 outputs a signal indicative of the oil pressure of the second drive circuit 32B.

[0030] The control system of the work machine 1 includes a travel operation member 71, a gear change operation member 72, a work implement operation member 73, a work implement lock member 74, and an AWD setting member 75. The travel operation member 71 can be operated by an operator to travel the work machine 1. The travel operation member 71 includes, for example, a pedal. Alternatively, the travel operation member 71 may include other members such as a lever or a switch. The travel operation member 71 outputs a signal indicating operation of the travel operation member 71.

[0031] The gearshift operating member 72 can be operated by an operator to change the gear position of the mechanical transmission 23. The gearshift operating member 72 includes, for example, a lever. Alternatively, the gearshift operating member 72 may include other members such as a switch. The gearshift operating member 72 outputs a signal indicating the operation of the gearshift operating member 72.

[0032] The work implement operation member 73 can be operated by an operator to operate the work implement 5. The work implement operation member 73 includes, for example, a lever. Alternatively, the work implement operation member 73 may include other members such as a switch. The work implement operation member 73 outputs a signal indicating an operation to the work implement operation member 73.

[0033] The work implement locking member 74 can be operated by the operator to lock the operation of the work implement 5. The work implement locking member 74 includes, for example, a lever. Alternatively, the work implement locking member 74 may include other members such as a switch or a touch panel. The work implement locking member 74 outputs a signal indicating an operation to the work implement locking member 74. Note that the travel operating member 71, the speed change operating member 72, the work implement operating member 73, and the work implement locking member 74 may output hydraulic pressure instead of an electrical signal in response to an operation of each member.

[0034] The AWD setting member 75 can be operated by an operator to switch between enabling and disabling an all-wheel drive mode (hereinafter referred to as AWD mode). The AWD mode will be described later. The AWD setting member 75 includes, for example, a lever. Alternatively, the AWD setting member 75 may include other members such as a switch or a touch panel. The AWD setting member 75 outputs a signal indicating the operation of the AWD setting member 75.

[0035] The control system of the work machine 1 includes a controller 66. The controller 66 includes a processor 67 and a storage device 68. The processor 67 is, for example, a CPU. The storage device 68 includes memory such as ROM and RAM. The storage device 68 may include an auxiliary storage device such as an HDD or SSD. The storage device 68 stores programs and data for controlling the work machine 1. The controller 66 controls the work machine 1 by executing processing in accordance with the programs and data.

[0036] For example, the controller 66 controls the drive source 21 in response to the operation of the travel operation member 71. This controls the output rotation speed of the drive source 21 in response to the operation of the travel operation member 71. The controller 66 controls the mechanical transmission 23 in response to the operation of the gear change operation member 72. This changes the gear position of the mechanical transmission 23 in response to the operation of the gear change operation member 72.

[0037] The controller 66 controls the work implement control valve 25 and the capacity control device 27 of the work implement pump 24 in response to the operation of the work implement operation member 73. This controls the operation of the work implement 5 in response to the operation of the work implement operation member 73. The controller 66 also locks the operation of the work implement 5 in response to the operation of the work implement lock member 74. Alternatively, the work implement lock member 74 may mechanically lock the operation of the work implement operation member 73.

[0038] The controller 66 switches the AWD mode between enabled and disabled in response to operation of the AWD setting switch. When the AWD mode is disabled, the controller 66 does not drive the front wheels 3A via the HST 26, and causes the work machine 1 to travel only by driving the rear wheels 4A, 4B via the mechanical transmission 23. In this case, the clutch 34 of the HST 26 is disengaged, and the driving force of the drive source 21 is not transmitted to the front wheels 3A via the HST 26.

[0039] When the AWD mode is enabled, the clutch 34 is brought into an engaged state. The controller 66 causes the work machine 1 to travel by both driving the front wheels 3A via the HST 26 and driving the rear wheels 4A, 4B via the mechanical transmission 23. In the AWD mode, the controller 66 controls the HST 26 so that the front wheels 3A rotate at a rotational speed that corresponds to the rotational speed of the rear wheels 4A, 4B. For example, in the AWD mode, the operator can adjust the rotational speed of the front wheels 3A so that it becomes a predetermined ratio to the rotational speed of the rear wheels 4A, 4B. Control of the HST 26 in the AWD mode will now be described. Figure 4 is a flowchart showing the control process of the HST 26 in the AWD mode.

[0040] 4, in step S101, the controller 66 obtains the current output rotation speed of the driving source 21. The controller 66 obtains the current output rotation speed of the driving source 21 based on a signal from the first rotation speed sensor 61. In step S102, the controller 66 obtains the current output rotation speed of the traveling motor 33. The controller 66 obtains the current output rotation speed of the traveling motor 33 based on a signal from the second rotation speed sensor 62.

[0041] In step S103, the controller 66 acquires the current output rotation speed of the mechanical transmission 23. The controller 66 acquires the current output rotation speed of the mechanical transmission 23 based on a signal from the third rotation speed sensor 63. In step S104, the controller 66 acquires the current gear stage of the mechanical transmission 23. The controller 66 acquires the current gear stage of the mechanical transmission 23 based on a signal from the gear shift operating member 72.

[0042] In step S105, the controller 66 determines the target rotation speed of the traveling motor 33. The controller 66 determines the target rotation speed of the traveling motor 33 based on the current output rotation speed of the mechanical transmission 23. In other words, the controller 66 determines the target rotation speed of the traveling motor 33 in accordance with the rotation speed of the rear wheels 4A, 4B. For example, the controller 66 may determine the target rotation speed of the traveling motor 33 so that the rotation speed of the front wheel 3A is the same as the rotation speed of the rear wheels 4A, 4B.

[0043] In step S106, the controller 66 determines a displacement command for the traveling motor 33. The controller 66 determines the displacement command for the traveling motor 33 based on the current speed stage and the current output rotation speed of the traveling motor 33. In step S107, the controller 66 controls the displacement of the traveling motor 33. The controller 66 controls the displacement control device 35 for the traveling motor 33 based on the displacement command for the traveling motor 33.

[0044] In step S108, the controller 66 determines a displacement command for the travel pump 31. The controller 66 determines a displacement command for the travel pump 31 based on the current output rotation speed of the drive source 21, the current speed stage, the target rotation speed of the travel motor 33, and the current output rotation speed of the travel motor 33 so that the current output rotation speed of the travel motor 33 becomes the target rotation speed of the travel motor 33.

[0045] In step S109, the controller 66 controls the displacement of the travel pump 31. The controller 66 controls the displacement control device 44 of the travel pump 31 based on the displacement command of the travel pump 31. This controls the displacement of the travel motor 33 and the displacement of the travel pump 31 so that the front wheel 3A rotates at a rotation speed corresponding to the rotation speed of the rear wheels 4A, 4B.

[0046] In the AWD mode described above, while the work machine 1 is traveling, the front wheels 3A rotate at a rotational speed that corresponds to the rotational speed of the rear wheels 4A, 4B. However, it is conceivable that an excessive increase in drive pressure will increase the load on the components of the HST 26, such as the travel motor 33, relative to the traveling performance of the HST 26. To suppress this increase in load on the components of the HST 26, in the work machine 1 according to this embodiment, the controller 66 executes high-pressure cutoff control. The processing of high-pressure cutoff control will be described below.

[0047] Figure 5 is a flowchart showing the high-pressure cutoff control process. As shown in Figure 5, in step S201, the controller 66 acquires the driving pressure. The controller 66 acquires the driving pressure based on signals from the first pressure sensor 64 and the second pressure sensor 65. In step S202, the controller 66 acquires the current gear position. The controller 66 acquires the current gear position based on a signal from the gear shift operating member 72.

[0048] In step S203, the controller 66 determines a high-pressure threshold. In step S204, the controller 66 determines a low-pressure threshold. The controller 66 determines the high-pressure threshold and the low-pressure threshold based on the current speed stage. The controller 66 stores threshold data that defines the relationship between the current speed stage and the high-pressure threshold and the low-pressure threshold. Figure 6 is a table showing an example of the threshold data.

[0049] As shown in FIG. 6, the mechanical transmission 23 has forward first through eighth gears F1-F8, reverse first through eighth gears R1-R8, parking P, and neutral N. The threshold data assigns a first high-pressure threshold A1 and a first low-pressure threshold B1 to forward first, second, sixth, and eighth gears F1, F2, and F6-F8. The threshold data assigns a second high-pressure threshold A2 and a second low-pressure threshold B2 to forward third through fifth gears F3-F5. Similarly, the threshold data assigns a first high-pressure threshold A1 and a first low-pressure threshold B1 to reverse first, second, sixth, and eighth gears R1, R2, and R6-R8. The threshold data assigns a second high-pressure threshold A2 and a second low-pressure threshold B2 to reverse third through fifth gears R3-R5. The parking P and neutral N are assigned a first high pressure threshold A1 and a first low pressure threshold B1.

[0050] The second high-pressure threshold A2 is smaller than the first high-pressure threshold A1. The second low-pressure threshold B2 is smaller than the first low-pressure threshold B1. Furthermore, the second high-pressure threshold A2 is smaller than the first cutoff pressure. The first high-pressure threshold A1 is larger than the first cutoff pressure. The controller 66 references the threshold data to determine the high-pressure threshold and the low-pressure threshold corresponding to the current speed stage.

[0051] For example, for the highest forward speed stage (eighth speed F8) and the lowest forward speed stage (first speed F1), the controller 66 determines a first high-pressure threshold A1 and a first low-pressure threshold B1 as the high-pressure threshold and low-pressure threshold corresponding to the current speed stage. For intermediate forward speed stages (third to fifth speeds F3-F5), the controller 66 determines a second high-pressure threshold A2 and a second low-pressure threshold B2 as the high-pressure threshold and low-pressure threshold corresponding to the current speed stage. That is, when the current speed stage is an intermediate speed stage, the controller 66 reduces the high-pressure threshold and low-pressure threshold compared to when the current speed stage is the highest speed stage or the lowest speed stage.

[0052] Returning to FIG. 5, in step S205, the controller 66 determines whether the drive circuit is in a high-pressure state. FIG. 7 is a diagram showing changes in the drive pressure in the HST 26 and the capacity of the travel pump 31 during high-pressure cutoff control. As shown in FIG. 7, the controller 66 determines that the drive circuit is in a high-pressure state when the drive pressure remains equal to or higher than the high-pressure threshold for a predetermined period of time or longer (time T1-T2). If the drive circuit is in a high-pressure state, the process proceeds to step S206.

[0053] In step S206, the controller 66 reduces the displacement of the travel pump 31 (time T2-T3). As shown in FIG. 7, the controller 66 gradually reduces the displacement of the travel pump 31 at a predetermined reduction rate. In step S207, the controller 66 determines whether the drive circuit is in a low-pressure state. If the drive pressure remains below the low-pressure threshold for a predetermined period of time or longer, the controller 66 determines that the drive circuit is in a low-pressure state (time T3-T4). If the drive circuit is not in a low-pressure state, in step S206, the reduction in the displacement of the travel pump 31 continues. If the drive circuit is in a low-pressure state, the process proceeds to step S208.

[0054] In step S208, the controller 66 increases the displacement of the travel pump 31. As shown in Fig. 7, the controller 66 gradually increases the displacement of the travel pump 31 at a predetermined increase rate (time T4-T5). In step S209, the controller 66 determines whether an end condition for the high-pressure cutoff control is satisfied. The end condition includes, for example, that the displacement of the travel pump 31 has reached a predetermined return target displacement, or that a predetermined time has elapsed since the drive circuit entered a low-pressure state.

[0055] The controller 66 stores the target return capacity. The target return capacity may be variable. Alternatively, the target return capacity may be a fixed value. The controller 66 determines that the termination condition is satisfied (time T5) when the capacity of the travel pump 31 reaches the target return capacity or when a predetermined time has elapsed since the drive circuit entered a low-pressure state.

[0056] If the termination condition is satisfied, the controller 66 terminates the high-pressure cutoff control in step S210. In this case, the controller 66 controls the displacement of the travel motor 33 and the displacement of the travel pump 31 by controlling the HST 26 in the AWD mode described above.

[0057] In the work machine 1 according to the present embodiment described above, when the current speed stage is a predetermined speed stage (third to fifth speeds F3-F5, R3-R5), the second high-pressure threshold A2 is determined as the high-pressure threshold. Then, when the drive pressure is equal to or greater than the second high-pressure threshold A2, the controller 66 reduces the capacity of the travel pump 31. This limits the drive pressure to equal to or less than the second high-pressure threshold A2, i.e., the second cutoff pressure.

[0058] On the other hand, if the current speed stage is a speed stage other than the predetermined speed stages (first, second, sixth to eighth forward speeds F1, F2, F6-F8, and first, second, sixth to eighth reverse speeds R1, R2, R6-R8), the controller 66 determines the first high-pressure threshold A1 as the high-pressure threshold. The first high-pressure threshold A1 is greater than the first cutoff pressure described above. Therefore, if the current speed stage is a speed stage other than the predetermined speed stages, the capacity of the travel pump 31 is not reduced, but the cutoff mechanism 49 limits the drive pressure to equal to or less than the first cutoff pressure.

[0059] As described above, in the work machine 1 according to this embodiment, when the current speed stage is a speed stage other than a predetermined speed stage, the driving pressure is limited to a value equal to or lower than the first cutoff pressure. Furthermore, when the current speed stage is a predetermined speed stage, the driving pressure is limited to a value equal to or lower than the second cutoff pressure, which is lower than the first cutoff pressure. This prevents a decrease in the durability of the components of the HST 26 and improves fuel efficiency.

[0060] 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.

[0061] The work machine 1 is not limited to a motor grader, but may be another machine such as a wheel loader. The configuration of the work machine 1 is not limited to that of the above embodiment and may be modified. For example, the number of speed stages of the mechanical transmission 23 is not limited to eight forward and eight reverse stages and may be modified.

[0062] In the above embodiment, the drive pressure is limited to a value equal to or lower than the first cutoff pressure by the cutoff mechanism 49. However, the controller 66 may limit the drive pressure to a value equal to or lower than the first cutoff pressure by reducing the displacement of the travel pump 31.

[0063] The high-pressure cutoff control process is not limited to that of the above embodiment and may be modified. For example, FIG. 8 is a flowchart showing the control process of the HST in AWD mode according to another embodiment. As shown in FIG. 8, in step S100, the controller 66 may determine whether the operation of the work implement 5 has been locked by operation of the work implement locking member 74. The controller 66 may execute the high-pressure cutoff control when the operation of the work implement 5 has not been locked by operation of the work implement locking member 74. [Industrial Applicability]

[0064] According to the present disclosure, in a work machine 1 equipped with both a mechanical transmission and a hydrostatic transmission, deterioration in the durability of components of the hydrostatic transmission is suppressed and fuel economy is improved. [Explanation of symbols]

[0065] 3A: Front wheels (second drive wheels), 4A, 4B: Rear wheels (first drive wheels), 5: Work equipment, 21: Drive source, 23: Mechanical transmission, 25: Hydrostatic transmission, 31: Travel pump, 32A: First drive circuit, 32B: Second drive circuit, 33: Travel motor, 64: First pressure sensor, 65: Second pressure sensor, 66: Controller, 74: Work equipment locking member

Claims

1. A driving source; a first drive wheel; a mechanical transmission having a plurality of speed stages and transmitting the driving force of the driving source to the first driving wheel; A second drive wheel; a hydrostatic transmission including a hydraulic pump driven by the drive source, a hydraulic motor driven by hydraulic oil discharged from the hydraulic pump, and a drive circuit connecting the hydraulic pump and the hydraulic motor, and transmitting the drive force of the drive source to the second drive wheel; a controller that acquires a current speed stage of the mechanical transmission and controls the hydrostatic transmission so as to limit the hydraulic pressure of the drive circuit to a cut-off pressure or less corresponding to the current speed stage; A work machine comprising:

2. the hydrostatic transmission includes a relief valve connected to the drive circuit; the relief valve opens when the hydraulic pressure in the drive circuit is equal to or higher than a first cutoff pressure, thereby limiting the hydraulic pressure in the drive circuit to equal to or lower than the first cutoff pressure; When the current speed stage is a predetermined speed stage, the controller controls the hydrostatic transmission so as to limit the hydraulic pressure of the drive circuit to a second cut-off pressure or less that is lower than the first cut-off pressure.

2. The work machine according to claim 1.

3. the hydrostatic transmission includes a pressure sensor connected to the drive circuit to detect hydraulic pressure in the drive circuit; The controller Acquire the hydraulic pressure of the drive circuit; When the current speed stage is the predetermined speed stage and the hydraulic pressure of the drive circuit is equal to or higher than the second cut-off pressure, the hydraulic pump is reduced in capacity to limit the hydraulic pressure of the drive circuit to equal to or lower than the second cut-off pressure.

3. The work machine according to claim 2.

4. a work machine driven by the driving force of the drive source; a work machine locking member that is operated by an operator to lock the operation of the work machine; Furthermore, When the current speed stage is a predetermined speed stage in a state in which the operation of the work implement is not locked by operation of the work implement lock member, the controller controls the hydrostatic transmission to limit the hydraulic pressure of the drive circuit to not more than the second cut-off pressure.

3. The work machine according to claim 2.

5. A method for controlling a work machine comprising: a drive source; a first drive wheel; a mechanical transmission having a plurality of speed stages and transmitting drive force of the drive source to the first drive wheel; a second drive wheel; a hydraulic pump driven by the drive source; a hydraulic motor driven by hydraulic oil discharged from the hydraulic pump; and a drive circuit connecting the hydraulic pump and the hydraulic motor, and a hydrostatic transmission that transmits drive force of the drive source to the second drive wheel, Obtaining a current speed stage of the mechanical transmission; Controlling the hydrostatic transmission so as to limit the hydraulic pressure of the drive circuit to a cut-off pressure or less corresponding to the current speed stage; A method for providing the above.

6. the hydrostatic transmission includes a relief valve connected to the drive circuit; the relief valve opens when the hydraulic pressure in the drive circuit is equal to or higher than a first cutoff pressure, thereby limiting the hydraulic pressure in the drive circuit to equal to or lower than the first cutoff pressure; When the current speed stage is a predetermined speed stage, the hydrostatic transmission is controlled so as to limit the hydraulic pressure of the drive circuit to a second cut-off pressure or less that is lower than the first cut-off pressure. The method of claim 5.

7. the hydrostatic transmission includes a pressure sensor connected to the drive circuit to detect hydraulic pressure in the drive circuit; acquiring hydraulic pressure of the drive circuit; When the current speed stage is the predetermined speed stage and the hydraulic pressure of the drive circuit is equal to or higher than the second cut-off pressure, the hydraulic pump is reduced in displacement to limit the hydraulic pressure of the drive circuit to equal to or lower than the second cut-off pressure. The method of claim 6.

8. The work machine includes: a work machine driven by the driving force of the drive source; a work machine locking member that is operated by an operator to lock the operation of the work machine; Furthermore, and when the current speed stage is a predetermined speed stage in a state in which the operation of the work machine is not locked by operation of the work machine lock member, controlling the hydrostatic transmission to limit the hydraulic pressure of the drive circuit to not more than the second cut-off pressure. The method of claim 6.

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