Hydraulic drive system for traveling work machine

Abstract

To provide a hydraulic drive device of a trave-type work machine capable of suppressing deterioration in workability during curved advancing regardless of a material of the portion of the traveling device in contact with the ground and the condition of the ground.SOLUTION: In a hydraulic drive device of a trave-type work machine, when a travel state is switched from a straight advancing state to a curved advancing state, a controller 70 maintains a motor capacity to a second capacity when a pump load is less than a first switching threshold, and switches the motor capacity from the second capacity to a first capacity when the pump load is equal to or higher than the first switching threshold. In a state where the motor capacity is set to the first capacity, when the travel state is the curved advancing state, the controller 70 maintains the motor capacity at the first capacity. When the travel state is switched from the curved advancing state to the straight advancing state, the controller 70 maintains the motor capacity to the first capacity when the pump load is equal to or higher than a second switching threshold, and switches the motor capacity from the first capacity to the second capacity when the pump load is less than the second switching threshold.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a hydraulic drive system for a traveling work machine such as a hydraulic excavator. [Background technology]

[0002] In the construction machinery travel drive system of Patent Document 1, when a controller recognizes a pivot turn based on the detection values ​​of an operation state detection means that detects the operation state of an operating means that drives the variable displacement hydraulic travel motor and the detection values ​​of a load pressure detection means that detects the load pressure of the variable displacement hydraulic travel motor, it operates the two-position switch valve associated with the variable displacement hydraulic travel motor on the stop side to increase the displacement. This strengthens the hydraulic braking force of the variable displacement hydraulic travel motor on the stop side. With the technology of Patent Document 1, the displacement is changed by comparing the travel load pressure with a set value, which can lead to hunting, a phenomenon in which the travel motor's motor displacement changes frequently.

[0003] The hydraulic motor control device in Patent Document 2 sets the following pressure settings for the motor displacement switching valve, which switches the displacement of the travel motor: a low-speed switching pressure; a high-speed switching pressure that is lower than the low-speed switching pressure; and a threshold value that is even lower than the high-speed switching pressure and prevents the motor drive pressure from exceeding the low-speed switching pressure even when the motor displacement switching valve is switched from the low-speed position to the high-speed position. This control device maintains the travel motor in a low-speed state, i.e., the motor displacement at a large capacity, after the motor displacement switching valve switches to the low-speed position until the motor drive pressure falls below the threshold, thereby preventing hunting. However, if the motor displacement is set to a large capacity, the motor displacement is maintained at a large capacity until the motor drive pressure falls below the threshold, preventing high-speed travel, which can reduce workability.

[0004] Patent Document 3 discloses a technique for suppressing the occurrence of hunting, in which the motor capacity of a variable displacement travel motor frequently switches between first-speed mode (large capacity) and second-speed mode (small capacity) depending on the traveling load when an operation to change the traveling direction is performed while rotating the left and right crawlers in the same direction. That is, the excavator control device in Patent Document 3 uniformly switches the motor capacity to a low-speed setting (large capacity) in order to suppress the occurrence of hunting depending on the traveling load when an operation to change the traveling direction of the undercarriage is performed. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 5-156666 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-004369 [Patent Document 3] International Publication No. 2019 / 009198 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the traveling load and the magnitude of fluctuations in the traveling load when a traveling work machine travels in a curve at a work site vary depending on the material of the portion of the traveling device that contacts the ground (for example, the crawler belt) and the condition of the ground. Therefore, as in the excavator of Patent Document 3, when an operation is performed to change the direction of travel of the lower traveling body, that is, when the lower traveling body is switched from a straight traveling state to a curved traveling state, if the motor capacity is uniformly switched from small to large, high-speed traveling will not be possible, and work efficiency at the work site may decrease.

[0007] The present disclosure aims to provide a hydraulic drive system for a traveling work machine that can suppress a decrease in workability during turning travel from when the traveling state of the traveling body is switched from a straight-line state to a curved state until it returns to the straight-line state, regardless of the material of the part of the traveling device that comes into contact with the ground and the condition of the ground. [Means for solving the problem]

[0008] What is provided is a hydraulic drive system for a traveling work machine equipped with a traveling body including a traveling device having a portion that contacts the ground, the system comprising: at least one hydraulic pump that discharges hydraulic oil; variable displacement first and second traveling motors that are each operated to drive the traveling device by receiving a supply of hydraulic oil from the at least one hydraulic pump and that can switch the traveling state of the traveling body from one of a straight traveling state and a curved traveling state to the other; and a controller that switches the motor capacities of the first and second traveling motors between a first capacity and a second capacity that is smaller than the first capacity, and when the traveling state of the traveling body is switched from the straight traveling state to the curved traveling state with the motor capacity set to the second capacity, the controller When the pump load is less than a first switching threshold which is a preset threshold, the controller maintains the motor displacement at the second displacement, and when the pump load is equal to or greater than the first switching threshold, the controller switches the motor displacement from the second displacement to the first displacement, and when the traveling state of the traveling object is the turning state with the motor displacement set to the first displacement, the controller maintains the motor displacement at the first displacement, and when the traveling state of the traveling object is switched from the turning state to the straight state with the motor displacement set to the first displacement, the controller maintains the motor displacement at the first displacement when the pump load is equal to or greater than a second switching threshold which is a preset threshold, and switches the motor displacement from the first displacement to the second displacement when the pump load is less than the second switching threshold.

[0009] In this hydraulic drive system, by the controller performing the control as described above, it is possible to suppress the occurrence of hunting while generating an appropriate traveling torque while the traveling body is traveling in a curve, regardless of the material of the portion of the traveling device that contacts the ground and the condition of the ground. This makes it possible to enable high-speed traveling when a large traveling torque is not required, suppressing a decrease in workability, and also suppressing a decrease in workability due to the occurrence of hunting. Specifically, this is as follows.

[0010] In this hydraulic drive system, even when the traveling state of the traveling vehicle is switched from a straight state to a turning state, the motor displacement is maintained at the second displacement until the pump load becomes equal to or greater than the first switching threshold. That is, when the traveling state of the traveling vehicle is switched from a straight state to a turning state, the controller maintains the motor displacement at the second displacement when the pump load is less than the first switching threshold, and switches the motor displacement from the second displacement to the first displacement when the pump load is equal to or greater than the first switching threshold. This prevents the generation of excessive traveling torque that is incommensurate with the pump load when the traveling state is switched to a turning state, allowing the traveling vehicle to travel at high speed even in a turning state and suppressing a decrease in workability. Furthermore, after the motor displacement is set to the first displacement, the motor displacement is maintained at the first displacement while the traveling state of the traveling vehicle is in a turning state. This suppresses hunting, a phenomenon in which the motor displacement frequently switches between the first and second displacements when the traveling state is in a turning state, where the pump load is likely to fluctuate. Furthermore, with this hydraulic drive system, even when the traveling state of the traveling vehicle is switched from a turning state to a straight state, in which the pump load fluctuates less than in the turning state, the motor displacement is maintained at the first displacement until the pump load becomes less than the second switching threshold. That is, when the traveling state of the traveling vehicle is switched from a turning state to a straight state, the controller maintains the motor displacement at the first displacement when the pump load is equal to or greater than the second switching threshold, and switches the motor displacement from the first displacement to the second displacement when the pump load is less than the second switching threshold. This makes it possible to switch from a turning state to a straight state while ensuring the generation of a traveling torque appropriate for the pump load, regardless of the material of the portion of the traveling device that contacts the ground and the condition of the ground. As described above, the hydraulic drive system according to the present disclosure can suppress a decrease in workability during turning traveling from when the traveling state of the traveling vehicle is switched from a straight state to a turning state until it returns to the straight state, regardless of the material of the portion of the traveling device that contacts the ground and the condition of the ground.

[0011] Preferably, the at least one hydraulic pump includes a first hydraulic pump capable of supplying hydraulic oil to the first travel motor and a second hydraulic pump capable of supplying hydraulic oil to the second travel motor, and the hydraulic drive system further includes a first pressure sensor that detects a first load pressure corresponding to a load on the first hydraulic pump and a second pressure sensor that detects a second load pressure corresponding to a load on the second hydraulic pump, and when the traveling state of the traveling vehicle is switched from the straight state to the turning state with the motor displacement set to the second displacement, the controller switches the motor displacement from the second displacement to the first displacement if the larger of the first load pressure and the second load pressure is equal to or greater than the first switching threshold. With this configuration, the larger of the first load pressure and the second load pressure, i.e., the pump load at which the motor rotates faster during turning, is used to determine whether to switch the motor displacement. This allows the motor displacement to be switched more appropriately without being affected by the smaller load pressure, making it possible to generate more appropriate traveling torque.

[0012] Preferably, the controller prohibits switching of the motor displacement from one of the first displacement and the second displacement to the other until a preset switching prohibition time has elapsed. With this configuration, switching of the motor displacement is temporarily prohibited based on the switching prohibition time, so that the occurrence of hunting can be more effectively suppressed and workability can be further improved.

[0013] Preferably, the at least one hydraulic pump includes a first hydraulic pump capable of supplying hydraulic oil to the first travel motor and a second hydraulic pump capable of supplying hydraulic oil to the second travel motor, and the hydraulic drive system further includes a first pressure sensor that detects a first load pressure which is a pressure corresponding to the load on the first hydraulic pump, and a second pressure sensor that detects a second load pressure which is a pressure corresponding to the load on the second hydraulic pump, and when the travel state of the travel object is the straight travel state, the controller switches the motor displacement from the second displacement to the first displacement when both the first load pressure and the second load pressure are equal to or greater than a third switching threshold which is a preset threshold, or when an average value of the first load pressure and the second load pressure is equal to or greater than the third switching threshold, and maintains the motor displacement at the second displacement when at least one of the first load pressure and the second load pressure is less than the third switching threshold or the average value is less than the third switching threshold. In this configuration, if one of the first load pressure and the second load pressure temporarily increases due to, for example, ground conditions, i.e., if there is no need to switch the motor displacement from the second displacement to the first displacement, the motor displacement is maintained at the second displacement. This enables high-speed traveling when large traveling torque is not required, more effectively suppressing deterioration in workability while also more effectively suppressing deterioration in workability due to the occurrence of hunting.

[0014] Preferably, the hydraulic drive system of the work machine further includes a first travel operating device to which a first operation, which is an operation by an operator to operate the first travel motor, is given, a second travel operating device to which a second operation, which is an operation by an operator to operate the second travel motor, a first operation amount detector that detects a first operation amount that is an operation amount of the first operation, and a second operation amount detector that detects a second operation amount that is an operation amount of the second operation, and the controller determines the travel state of the travel body based on the first operation amount and the second operation amount. With this configuration, the travel state is appropriately determined based on the operation amounts of the first operation and the second operation given to the first travel operating device and the second travel operating device. [Effects of the Invention]

[0015] According to the present disclosure, a hydraulic drive system for a traveling work machine is provided that can suppress a decrease in workability during turning travel from when the traveling state of the traveling body is switched from a straight state to a curved state until it returns to the straight state, regardless of the material of the part of the traveling device that comes into contact with the ground and the condition of the ground. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a side view of a traveling work machine according to an embodiment of the present disclosure. [Figure 2] 2 is a hydraulic circuit diagram showing a hydraulic circuit mounted on the traveling work machine and a controller connected thereto. FIG. [Figure 3] 4 is a flowchart showing a control operation executed by the controller. [Figure 4] 10 is a time chart showing an example of the relationship between signals input to a controller, load during traveling, and motor capacity of a traveling motor when the traveling work machine travels in a manner including turning and straight traveling. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments of the present disclosure will now be described with reference to the drawings.

[0018] Fig. 1 shows a hydraulic excavator 100, which is an example of a mobile work machine equipped with a hydraulic drive system according to this embodiment. Fig. 2 shows a hydraulic circuit equipped in the hydraulic excavator 100. Note that the hydraulic drive system according to the present disclosure is not limited to the hydraulic excavator 100 shown in Fig. 1, but can also be widely applied to other mobile work machines that have left and right travel motors and operate using hydraulic pressure as the main power source.

[0019] The hydraulic excavator 100 includes a lower traveling body 1 capable of traveling on ground G, an upper rotating body 2 supported on the lower traveling body 1 so as to be rotatable about a vertical axis Z, and a working device 3 supported by the upper rotating body 2. The lower traveling body 1 is an example of a traveling body in the present disclosure. The upper rotating body 2 includes an upper frame supported by the lower traveling body 1, a driver's cab located in the front portion of the upper frame, and a machine room located in the rear portion of the upper frame. The machine room houses multiple components including an engine. The working device 3 is supported by the front portion of the upper frame.

[0020] The lower traveling body 1 includes a left traveling unit 1L and a right traveling unit 1R spaced apart in the left-right direction, and a not-shown lower frame supported by the traveling units 1L and 1R. In this embodiment, each of the left and right traveling units 1L and 1R is a crawler traveling unit having a shape extending in the front-rear direction. Each of the traveling units 1L and 1R includes a traveling frame 1A extending in the front-rear direction, wheels 1B and 1C rotatably supported at the front and rear ends of the traveling frame 1A, and a crawler belt 1D, which is an endless belt looped around the wheels 1B and 1C. Note that in FIG. 1, the right traveling unit 1R is located behind the left traveling unit 1L. The left and right traveling units 1L and 1R are driven independently in the forward or backward direction, as described below. The crawler belt 1D is an example of a portion of the traveling unit in this disclosure that comes into contact with the ground.

[0021] The material of the crawler belt 1D (the portion of the traveling device that comes into contact with the ground) may be, for example, metal, rubber, or other materials. The traveling load and the magnitude of fluctuations in the traveling load when the hydraulic excavator 100 travels in a curve at a work site vary depending on the material of the crawler belt 1D of the traveling devices 1L, 1R and the condition of the ground. The condition of the ground varies depending on, for example, the material of the ground, the water content of the ground, etc.

[0022] The work device 3 has a boom 4, an arm 5, and a bucket 6. The boom 4 has a base end and a tip end on the opposite side. The base end of the boom 4 is supported on the front portion of the upper rotating body 2 so that the boom 4 can be raised and lowered, i.e., can rotate around a horizontal axis. The arm 5 has a base end and a tip end on the opposite side. The base end of the arm 5 is connected to the tip of the boom 4 so that the arm 5 can rotate around the horizontal axis. The bucket 6 is a tip attachment that is attached to the tip of the arm 5 so that it can rotate around the horizontal axis. Note that the tip attachment is not limited to the bucket 6 and may be other devices such as a grapple, crusher, breaker, or fork.

[0023] 2 includes a plurality of hydraulic pumps, a plurality of hydraulic actuators, a plurality of actuator operators, a plurality of actuator control valves, a motor displacement switching valve 44, and a tank T. The hydraulic circuit is capable of supplying hydraulic oil to the plurality of hydraulic actuators and adjusting the direction and flow rate of the supply. The hydraulic excavator 100 is equipped with a controller 70 for controlling the operation of the hydraulic circuit.

[0024] The plurality of hydraulic pumps are driven by an engine (not shown) to discharge hydraulic oil from a tank T. The plurality of hydraulic pumps include a first main pump 21, a second main pump 22, and a pilot pump 23.

[0025] The first and second main pumps 21, 22 discharge hydraulic oil for operating hydraulic actuators to be driven among the multiple hydraulic actuators, and correspond to the first hydraulic pump and the second hydraulic pump, respectively, according to the present disclosure. The pilot pump 23 discharges pilot oil for supplying pilot pressure to the multiple actuator control valves. Each of the first and second main pumps 21, 22 according to this embodiment is a variable displacement hydraulic pump. The hydraulic excavator 100 includes a first pump regulator (not shown) that is a regulator for adjusting the pump displacement of the first main pump 21, and a second pump regulator (not shown) that is a regulator for adjusting the pump displacement of the second main pump 22. The pump displacements of the first and second main pumps 21, 22 are adjusted by the first and second pump regulators based on a pump displacement command output from the controller 70.

[0026] The plurality of hydraulic actuators include a plurality of work actuators, a swing motor 11 (see FIG. 1), a right traveling motor 31, and a left traveling motor 32 (see FIG. 2). As shown in FIG. 1, the plurality of work actuators include a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9.

[0027] The boom cylinder 7 is a hydraulic cylinder for raising and lowering the boom 4 relative to the upper rotating body 2, the arm cylinder 8 is a hydraulic cylinder for rotating the arm 5 relative to the boom 4, and the bucket cylinder 9 is a hydraulic cylinder for rotating the bucket 6 relative to the arm 5. The swing motor 11 is a hydraulic motor for swinging the upper rotating body 2 relative to the lower traveling body 1.

[0028] Each of the right traveling motor 31 and the left traveling motor 32 is a variable displacement hydraulic motor for propelling the undercarriage 1, and has an output shaft that rotates when hydraulic oil is supplied to it. Each of the right traveling motor 31 and the left traveling motor 32 has a pair of ports, and when hydraulic oil is supplied to one of these ports, the output shaft rotates in the direction corresponding to that port, and the hydraulic oil is discharged from the other port.

[0029] The right traveling motor 31 is connected to the right traveling device 1R so as to move the right traveling device 1R in the forward and backward directions. Specifically, the output shaft of the right traveling motor 31 is connected to the wheel 1B or wheel 1C of the right traveling device 1R, for example, via a not shown reducer, so as to rotate the wheel 1B or wheel 1C of the right traveling device 1R in the forward and backward directions. The left traveling motor 32 is connected to the left traveling device 1L so as to move the left traveling device 1L in the forward and backward directions. Specifically, the left traveling motor 32 is connected to the wheel 1B or wheel 1C of the left traveling device 1L, for example, via a not shown reducer, so as to rotate the wheel 1B or wheel 1C of the left traveling device 1L in the forward and backward directions.

[0030] The hydraulic excavator 100 is equipped with a right traveling regulator 31A that is a regulator for adjusting the motor capacity of the right traveling motor 31, and a left traveling regulator 32A that is a regulator for adjusting the motor capacity of the left traveling motor 32.

[0031] The motor displacement switching valve 44 is an electromagnetic switching valve that operates in response to a motor displacement command input from the controller 70. For example, the motor displacement switching valve 44 operates to connect the pilot pump 23 to the right traveling regulator 31A and the left traveling regulator 32A when a motor displacement command is input from the controller 70, and operates to disconnect the pilot pump 23 from the right traveling regulator 31A and the left traveling regulator 32A when a motor displacement command is not input from the controller 70. The right traveling regulator 31A and the left traveling regulator 32A control the motor displacement of the right traveling motor 31 and the left traveling motor 32 by adjusting the swash plate tilt angle of the right traveling motor 31 and the left traveling motor 32 in accordance with the pilot pressure supplied through the motor displacement switching valve 44.

[0032] For example, the right traveling motor 31 and the left traveling motor 32 each have a motor displacement set to a first displacement when a motor displacement command is not input to the motor displacement switching valve 44, and each have a motor displacement set to a second displacement smaller than the first displacement when a motor displacement command is input to the motor displacement switching valve 44. However, the right traveling motor 31 and the left traveling motor 32 each may have a motor displacement set to the second displacement when a motor displacement command is not input to the motor displacement switching valve 44, and each have a motor displacement set to the first displacement when a motor displacement command is input to the motor displacement switching valve 44. When set to the first displacement, the right traveling motor 31 and the left traveling motor 32 each generate a larger traveling torque than when set to the second displacement, causing the undercarriage 1 to travel at a slower speed.

[0033] Each of the multiple actuator operators has an operation receiver that receives an operation to move the hydraulic actuator corresponding to that actuator operator, and a remote control valve that outputs a pilot pressure corresponding to that operation to a pilot port of the actuator control valve corresponding to that actuator operator. The operation receiver may be configured as, for example, an operation lever or an operation pedal. The multiple actuator operators are respectively arranged between the pilot pump 23 and the multiple actuator control valves, and reduce the pilot primary pressure output from the pilot pump 23 to a degree corresponding to the magnitude of the operation applied to the operation receiver, to generate a pilot secondary pressure that is input to the pilot port of the actuator control valve.

[0034] Each of the multiple actuator control valves operates to control the supply and discharge of hydraulic oil to the hydraulic actuator corresponding to that actuator control valve in accordance with a pilot pressure input from a remote control valve corresponding to that actuator control valve. In this embodiment, each of the multiple actuator control valves is configured as a pilot-operated hydraulic change-over valve having a pair of pilot ports. Each of the multiple actuator control valves receives a supply of pilot pressure to one of the pair of pilot ports and opens at a stroke corresponding to the magnitude of the pilot pressure, thereby allowing a flow rate of hydraulic oil corresponding to the stroke to be supplied to the hydraulic actuator. In other words, each of the multiple actuator control valves allows the hydraulic actuator to operate in a direction corresponding to the pilot port to which the pilot pressure is supplied.

[0035] The multiple actuator controllers include a right travel controller 61 and a left travel controller 62. The multiple actuator controllers also include a boom controller, an arm controller, a bucket controller, and a swing controller, which are not shown.

[0036] The multiple actuator control valves include a right travel control valve 41, a left travel control valve 42, a travel switching valve 43, and actuator control valves 51 to 54, 56 to 58. The actuator control valves 51 to 54, 56 to 58 include a boom control valve, an arm control valve, a bucket control valve, and a swing control valve.

[0037] The right travel operation controller 61 is an operation controller that receives a right travel operation, which is a travel operation for operating the right travel motor 31. The left travel operation controller 62 is an operation controller that receives a left travel operation, which is a travel operation for operating the left travel motor 32. The right travel operation controller 61 and the left travel operation controller 62 are examples of a first travel operation controller and a second travel operation controller. The right travel operation and the left travel operation are examples of a first operation and a second operation. The right travel operation controller 61 includes a right operation pedal, which is an operation receiver that receives a stepping operation as a right travel operation, and a right travel remote control valve, which is a remote control valve that generates a travel pilot pressure, which is a pilot pressure corresponding to the stepping operation applied to the right operation pedal, and inputs the generated travel pilot pressure to the pilot port of the right travel control valve 41. The left travel operation controller 62 includes a left operation pedal, which is an operation receiver that receives a stepping operation as a left travel operation, and a left travel remote control valve, which is a remote control valve that generates a travel pilot pressure, which is a pilot pressure corresponding to the stepping operation applied to the left operation pedal, and inputs the generated travel pilot pressure to the pilot port of the left travel control valve 42.

[0038] Specifically, when a forward or reverse pedal operation is applied to the right operating pedal of the right traveling controller 61, the traveling remote control valve of the right traveling controller 61 inputs a traveling pilot pressure to a pilot port (a forward pilot port or a reverse pilot port, described later) of the right traveling control valve 41, such that the right traveling motor 31 rotates in the forward or reverse direction at a speed corresponding to the magnitude of the forward or reverse pedal operation. The mechanism of the left traveling controller 62 is the same as that of the right traveling controller 61. Note that the traveling operation is not limited to the above-described depression operation, but may also be a rotation operation applied to the traveling operation lever.

[0039] When a boom operation (boom-raising operation or boom-lowering operation) to extend or retract the boom cylinder 7 is applied to the boom operator, the remote control valve of the boom operator inputs a pilot pressure corresponding to the magnitude of the boom operation to the pilot port of the boom control valve. Similarly, when an arm operation (arm-pushing operation or arm-pulling operation) to extend or retract the arm cylinder 8 is applied to the arm operator, the remote control valve of the arm operator inputs a pilot pressure corresponding to the magnitude of the arm operation to the pilot port of the arm control valve. When a bucket operation (bucket-pushing operation or bucket-pulling operation) to extend or retract the bucket cylinder 9 is applied to the bucket operator, the remote control valve of the bucket operator inputs a pilot pressure corresponding to the magnitude of the bucket operation to the pilot port of the bucket control valve. When a swing operation (right swing operation or left swing operation) to operate the swing motor 11 is applied to the swing operator, the remote control valve of the swing operator inputs a pilot pressure corresponding to the magnitude of the swing operation to the pilot port of the swing control valve. Boom operation, arm operation, bucket operation, and swing operation are each an example of a work operation.

[0040] Each of the multiple actuator control valves according to this embodiment belongs to either a first group G1 or a second group G2. In this embodiment, the right travel control valve 41 and the actuator control valves 51-54 belong to the first group G1, and the left travel control valve 42 and the actuator control valves 56-58 belong to the second group G2. The right travel control valve 41 and the actuator control valves 51-54, which belong to the first group G1, are supplied with hydraulic oil discharged from the first main pump 21 when an independent operation is being performed. A state in which an independent operation is being performed is a state in which one of the travel operation and the work operation is being performed, and the other of the travel operation and the work operation is not being performed. The left travel control valve 42 and the actuator control valves 56-58, which belong to the second group G2, are supplied with hydraulic oil discharged from the second main pump 22 when an independent operation is being performed.

[0041] Specifically, a first center bypass line CL1 leading to the tank T is connected to the discharge port of the first main pump 21, and the right travel control valve 41 and the actuator control valves 51-54 are arranged in tandem along the first center bypass line CL1. Similarly, a second center bypass line CL2 leading to the tank T is connected to the discharge port of the second main pump 22, and the left travel control valve 42 and the actuator control valves 56-58 are arranged in tandem along the second center bypass line CL2.

[0042] A first supply line SL1 is connected to the discharge port of the first main pump 21 in parallel with the first center bypass line CL1. The first supply line SL1 further branches off corresponding to the actuator control valves 51-54 and is connected to these actuator control valves so as to distribute the hydraulic oil discharged from the first main pump 21 to each of the actuator control valves 51-54. Similarly, a second supply line SL2 is connected to the discharge port of the second main pump 22 in parallel with the second center bypass line CL2. The second supply line SL2 further branches off corresponding to the actuator control valves 56-58 and is connected to these actuator control valves so as to distribute the hydraulic oil discharged from the second main pump 22 to each of the actuator control valves 56-58.

[0043] The right travel control valve 41 is interposed between the first main pump 21 and the right travel motor 31, and operates to control the direction and flow rate of hydraulic oil supplied from the first main pump 21 to the right travel motor 31. The left travel control valve 42 is interposed between the second main pump 22 and the left travel motor 32, and operates to control the direction and flow rate of hydraulic oil supplied from the second main pump 22 to the left travel motor 32.

[0044] The right travel control valve 41 has a pair of pilot ports, a forward pilot port and a reverse pilot port, and is maintained in a neutral position when the pilot pressure supplied to these pilot ports is zero or very low. In this neutral position, the right travel control valve 41 disconnects the right travel motor 31 from the first main pump 21 and opens the first center bypass line CL1. When a pilot pressure equal to or higher than a certain level is supplied to the forward pilot port or the reverse pilot port in response to a right travel operation applied to the right travel operating device 61, the right travel control valve 41 shifts from the neutral position in a direction corresponding to the pilot port and with a stroke corresponding to the magnitude of the pilot pressure. As a result, the right travel control valve 41 communicates between the first main pump 21 and the port of the pair of ports of the right travel motor 31 that corresponds to the pilot port with an opening area corresponding to the stroke, allowing the right travel motor 31 to operate in the direction corresponding to the stroke.

[0045] The left travel control valve 42 has a pair of pilot ports, a forward pilot port and a reverse pilot port, and is maintained in a neutral position when the pilot pressure supplied to these pilot ports is zero or very low. In this neutral position, the left travel control valve 42 disconnects the left travel motor 32 from the second main pump 22 and opens the second center bypass line CL2. When a pilot pressure equal to or greater than a certain level is supplied to the forward pilot port or the reverse pilot port in response to a left travel operation applied to the left travel controller 62, the left travel control valve 42 shifts from the neutral position in a direction corresponding to the pilot port and by a stroke corresponding to the magnitude of the pilot pressure. As a result, the left travel control valve 42 communicates between the second main pump 22 and the port of the pair of ports of the left travel motor 32 that corresponds to the pilot port with an opening area corresponding to the stroke, allowing the left travel motor 32 to operate in the direction corresponding to the stroke.

[0046] The boom control valve is interposed between the first main pump 21 or the second main pump 22 and the boom cylinder 7, and operates to control the direction and flow rate of hydraulic oil supplied from the first main pump 21 or the second main pump 22 to the boom cylinder 7. The arm control valve is interposed between the first main pump 21 or the second main pump 22 and the arm cylinder 8, and operates to control the direction and flow rate of hydraulic oil supplied from the first main pump 21 or the second main pump 22 to the arm cylinder 8. The bucket control valve is interposed between the first main pump 21 or the second main pump 22 and the bucket cylinder 9, and operates to control the direction and flow rate of hydraulic oil supplied from the first main pump 21 or the second main pump 22 to the bucket cylinder 9. The swing control valve is interposed between the first main pump 21 or the second main pump 22 and the swing motor 11, and operates to control the direction and flow rate of hydraulic oil supplied from the first main pump 21 or the second main pump 22 to the swing motor 11.

[0047] The travel switching valve 43 is a switching valve for switching the flow path of the hydraulic oil discharged from the first main pump 21 and the second main pump 22. The travel switching valve 43 is configured to be switchable between a neutral position and a straight travel position. The travel switching valve 43 may be, for example, a two-position four-port switching valve having two pump ports and two actuator ports. The travel switching valve 43 is switched by, for example, the secondary pressure of an electromagnetic proportional travel switching proportional valve (not shown) based on a command from the controller 70.

[0048] The controller 70 receives an operation signal (e.g., a signal from a pressure sensor detecting the secondary pressure of a remote control valve) corresponding to the amount of operation applied to the operation receivers of the multiple actuator operation devices. The controller 70 determines whether an independent operation or a combined operation is being performed based on the input operation signal. As described above, an independent operation is being performed when one of the traveling operation and the work operation is being performed, and the other of the traveling operation and the work operation is not being performed. A combined operation is being performed when the traveling operation and the work operation are being performed simultaneously. When an independent operation is being performed, the controller 70 controls the traveling selector valve 43 to be in the neutral position. In this case, hydraulic oil from the first main pump 21 is supplied to the first group G1, and hydraulic oil from the second main pump 22 is supplied to the second group G2.

[0049] On the other hand, when combined operation is being performed, the controller 70 controls the travel switching valve 43 to the straight travel position. In this case, the hydraulic oil from the first main pump 21 is distributed and supplied to the right travel motor 31 and the left travel motor 32, and the hydraulic oil from the second main pump 22 is supplied to the hydraulic actuators other than the right travel motor 31 and the left travel motor 32. In this case, the right travel motor 31 and the left travel motor 32 are driven by the common first main pump 21, ensuring straight travel.

[0050] The cut valve 55 operates to switch between an unloaded position, as shown in Fig. 2, which connects the center bypass line CL1 to the tank T, and a blocked position, as shown in Fig. 2, which cut valve 59 operates to switch between an unloaded position, as shown in Fig. 2, which connects the center bypass line CL2 ... Of these cut valves 55, 59, the cut valve 55 of the first group G1 is controlled so that, in a state in which hydraulic oil from the first main pump 21 is supplied to the left travel control valve 42 and hydraulic oil from the second main pump 22 is supplied to the actuator control valves 56-58 belonging to the second group G2, the left travel control valve 42 and any of the actuator control valves 56-58 are operated, or the travel switching valve 43 is set to the straight travel position, and hydraulic oil from the first main pump 21 is supplied to the left travel control valve 42 and any of the actuator control valves 56-58 belonging to the second group G2, and the cut valve 55 switches to the block position when the left travel control valve 42 and any of the actuator control valves 56-58 are operated. In a state in which hydraulic oil from the first main pump 21 is supplied to the right travel control valve 41 and the actuator control valves 51 to 54 belonging to the first group G1, the cut valve 59 of the second group G2 switches to the blocking position when the right travel control valve 41 or any of the actuator control valves 51 to 54 is operated, thereby ensuring operation of the hydraulic actuators in both groups G1 and G2.

[0051] The hydraulic drive system of the hydraulic excavator 100 further includes a plurality of sensors. The plurality of sensors include a first pressure sensor 81, a second pressure sensor 82, a right travel pilot pressure sensor 83, and a left travel pilot pressure sensor 84. Each of the plurality of sensors inputs an electric signal corresponding to the detected pressure as a detection signal to the controller 70. Specifically, the first pressure sensor 81 and the second pressure sensor 82 input pump pressure detection signals, which are detection signals corresponding to the first pump pressure P1 of the first main pump 21 and the second pump pressure P2 of the second main pump 22, to the controller 70. The right travel pilot pressure sensor 83 inputs a right travel operation detection signal, which is a detection signal corresponding to the amount and direction of right travel operation applied by the operator to the operation receiver of the right travel controller 61, to the controller 70. Similarly, the left travel pilot pressure sensor 84 inputs a left travel operation detection signal, which is a detection signal corresponding to the amount and direction of left travel operation applied by the operator to the operation receiver of the left travel controller 62, to the controller 70. The operation direction is either a forward operation direction or a reverse operation direction. The right traveling pilot pressure sensor 83 and the left traveling pilot pressure sensor 84 are examples of a first operation amount detector and a second operation amount detector.

[0052] The first pressure sensor 81 detects a first pump pressure P1, which is the discharge pressure of the first main pump 21. The second pressure sensor 82 detects a second pump pressure P2, which is the discharge pressure of the second main pump 22. The first pump pressure P1 is an example of a first load pressure, and the second pump pressure P2 is an example of a second load pressure. The right travel pilot pressure sensor 83 detects the pilot secondary pressure output from the remote control valve of the right travel operation device 61, i.e., the pilot pressure input to the pilot port of the right travel control valve 41. The left travel pilot pressure sensor 84 detects the pilot secondary pressure output from the remote control valve of the left travel operation device 62, i.e., the pilot pressure input to the pilot port of the left travel control valve 42. As shown in the specific example of the hydraulic circuit in FIG. 2, each of the right travel pilot pressure sensor 83 and the left travel pilot pressure sensor 84 may detect the pilot pressure corresponding to either the forward operation direction or the reverse operation direction via a shuttle valve. In this case, each of the right traveling operation device 61 and the left traveling operation device 62 may be configured to input a signal indicating either the forward operation direction or the reverse operation direction to the controller 70.

[0053] The controller 70 controls the motor displacement of the right traveling motor 31 and the motor displacement of the left traveling motor 32 based on detection signals input from a plurality of sensors. The controller 70 includes a computer including a processing unit such as a CPU and a memory. The controller 70 includes a determination unit 71 and a command unit 72. The determination unit 71 and the command unit 72 are realized by the processing unit executing a program.

[0054] The determination unit 71 determines the traveling state of the lower traveling body 1. The traveling state of the lower traveling body 1 includes a straight traveling state and a turning state. In this embodiment, the determination unit 71 determines the traveling state of the lower traveling body 1 based on a right traveling operation detection signal input from the right traveling pilot pressure sensor 83 to the controller 70 and a left traveling operation detection signal input from the left traveling pilot pressure sensor 84 to the controller 70. The right traveling pilot pressure sensor 83, the left traveling pilot pressure sensor 84, and the determination unit 71 constitute a traveling state determiner that determines the traveling state of the lower traveling body 1.

[0055] The straight traveling state is a state in which the traveling direction of the lower traveling body 1 is constant or almost constant and the lower traveling body 1 is moving forward or backward. The turning state is a state in which the traveling direction of the lower traveling body 1 is changing while the lower traveling body 1 is moving. The turning state may be a first turning state in which the right traveling motor 31 and the left traveling motor 32 have the same rotation direction but different rotation speeds. The turning state may also be a second turning state (spin turn state) in which the rotation directions of the right traveling motor 31 and the left traveling motor 32 are opposite. The turning state may also be a third turning state (pivot turn state) in which one of the right traveling motor 31 and the left traveling motor 32 is rotating and the other of the right traveling motor 31 and the left traveling motor 32 is stopped.

[0056] The determination unit 71 may determine that the running state of the lower running body 1 is a straight-ahead state based on the right running operation detection signal and the left running operation detection signal if the operation directions of the right running operation and the left running operation are the same and the operation amounts of the right running operation and the left running operation are the same or the difference between the operation amounts of the right running operation and the left running operation is smaller than a preset threshold value.

[0057] The determination unit 71 can determine that the traveling state of the lower traveling body 1 is a turning state based on the right traveling operation detection signal and the left traveling operation detection signal. Specifically, the determination unit 71 may determine that the traveling state of the lower traveling body 1 is a first turning state, for example, when the operation directions of the right traveling operation and the left traveling operation are the same and the difference between the operation amount of the right traveling operation and the operation amount of the left traveling operation is equal to or greater than a preset threshold. The determination unit 71 may also determine that the traveling state of the lower traveling body 1 is a second turning state, for example, when the operation directions of the right traveling operation and the left traveling operation are opposite. Furthermore, the determination unit 71 may determine that the traveling state of the lower traveling body 1 is a third turning state, for example, when the operation amount of one of the right traveling operation and the left traveling operation is greater than a preset threshold and the operation amount of the other of the right traveling operation and the left traveling operation is zero or negligible.

[0058] The command unit 72 is configured to be able to switch the motor displacement of the right traveling motor 31 and the left traveling motor 32 between a first displacement and a second displacement. The command unit 72 outputs a motor displacement command, which is a command for setting the motor displacement of the right traveling motor 31 and the motor displacement of the left traveling motor 32, to the motor displacement switching valve 44. The motor displacement switching valve 44 operates to allow pilot pressure corresponding to the input motor displacement command to be input to the right traveling regulator 31A and the left traveling regulator 32A, thereby controlling the motor displacement of the right traveling motor 31 and the left traveling motor 32. The motor displacement switching valve 44, the right traveling regulator 31A, the left traveling regulator 32A, and the command unit 72 constitute a motor displacement switch that switches the motor displacement of the right traveling motor 31 and the left traveling motor 32 between the first displacement and the second displacement.

[0059] When the traveling state of the undercarriage 1 is switched from the straight traveling state to the curved traveling state with the motor displacements of the right traveling motor 31 and the left traveling motor 32 set to the second displacement, the command unit 72 maintains the motor displacements of the right traveling motor 31 and the left traveling motor 32 at the second displacement when the pump load is less than a first switching threshold, which is a preset threshold, and switches the motor displacements of the right traveling motor 31 and the left traveling motor 32 from the second displacement to the first displacement when the pump load is equal to or greater than the first switching threshold. The pump load is at least one of a first pump pressure P1 (first load pressure) detected by a first pressure sensor 81 and a second pump pressure P2 (second load pressure) detected by a second pressure sensor 82.

[0060] When the motor capacity of the right traveling motor 31 and the left traveling motor 32 is set to the first capacity and the traveling state of the lower traveling body 1 is the turning state, the command unit 72 maintains the motor capacity of the right traveling motor 31 and the left traveling motor 32 at the first capacity regardless of the pump load.

[0061] When the running state of the lower running body 1 is switched from the turning state to the straight running state with the motor capacities of the right running motor 31 and the left running motor 32 set to the first capacity, the command unit 72 maintains the motor capacities of the right running motor 31 and the left running motor 32 at the first capacity when the pump load is equal to or greater than a second switching threshold, which is a preset threshold, and switches the motor capacities of the right running motor 31 and the left running motor 32 from the first capacity to the second capacity when the pump load is less than the second switching threshold.

[0062] An example of the control operation executed by the controller 70 of the hydraulic drive system according to this embodiment will be described below with reference to Figures 3 and 4. Figure 3 is a flowchart showing the control operation executed by the controller 70. Figure 4 is a time chart showing an example of the relationship between the signal input to the controller 70, the load during traveling, and the motor capacity of the traveling motor when the undercarriage 1 travels in a manner including curved traveling and straight traveling.

[0063] In this embodiment, the second switching threshold is set to a value smaller than the first switching threshold. In the third graph from the top in Fig. 4, the threshold indicated as "Threshold: High Speed ​​→ Low Speed" is the first switching threshold, and the threshold indicated as "Threshold: Low Speed ​​→ High Speed" is the second switching threshold. In addition, the notation "Steering ON" indicated at the top of Fig. 4 indicates that the traveling state of the lower traveling body 1 has been switched from a straight traveling state to a turning traveling state, and the notation "Steering OFF" indicates that the traveling state of the lower traveling body 1 has been switched from a turning state to a straight traveling state.

[0064] The determination unit 71 of the controller 70 determines whether at least one of a right travel operation and a left travel operation is being applied to the right travel operation device 61 and the left travel operation device 62 based on the right travel operation detection signal and the left travel operation detection signal (step S1 in FIG. 3). If a right travel operation and a left travel operation are not being applied to the right travel operation device 61 and the left travel operation device 62 (NO in step S1), the command unit 72 of the controller 70 controls the motor displacement switching valve 44 so that the motor displacement of the right travel motor 31 and the left travel motor 32 becomes the first displacement (first travel speed) (step S10).

[0065] When at least one of the right travel operation and the left travel operation is applied to the right travel operating device 61 and the left travel operating device 62 (YES in step S1), the determination unit 71 determines whether the travel second speed switch is ON or OFF (step S2). The travel second speed switch accepts an operator's input for switching the control mode that controls the motor displacement. When the travel second speed switch is ON (YES in step S2), the control mode is set to a variable mode in which the motor displacement of the right travel motor 31 and the left travel motor 32 can be switched between a first displacement (first travel speed) and a second displacement (second travel speed). On the other hand, when the travel second speed switch is OFF (NO in step S2), the control mode is set to a fixed mode in which the motor displacement of the right travel motor 31 and the left travel motor 32 is fixed to the first displacement (first travel speed). The setting of the control mode may be managed, for example, by a flag.

[0066] If the second-speed travel switch is OFF (NO in step S2), the command unit 72 controls the motor displacement switching valve 44 so that the motor displacement of the right traveling motor 31 and the left traveling motor 32 becomes the first displacement (first traveling speed) (step S10). If the second-speed travel switch is ON (YES in step S2), the determination unit 71 determines whether the traveling state of the lower traveling body 1 is a turning state based on the right traveling operation detection signal and the left traveling operation detection signal (step S3). Note that the "steering state" in FIG. 3 indicates the turning state.

[0067] In the specific example shown in the time chart of FIG. 4, at time t0, a left travel operation with the maximum operation amount in the forward operation direction is applied to the operation receiver of the left travel operation controller 62, and a right travel operation with the maximum operation amount in the forward operation direction is applied to the operation receiver of the right travel operation controller 61. Thereafter, the left travel operation for the left travel operation controller 62 is maintained at the maximum operation amount, while the right travel operation for the right travel operation controller 61 is changed to an intermediate operation amount that is an operation amount between the maximum operation amount and the minimum operation amount. In this case, the undercarriage 1 is turning right. Based on the right travel operation detection signal and the left travel operation detection signal, the determination unit 71 determines at time t1 that the operation amount of the right travel operation has changed and determines that the traveling state has switched from a straight state to a turning state (YES in step S3 of FIG. 3, time t1 in FIG. 4).

[0068] If the determination unit 71 determines that the traveling state is a cornering state (YES in step S3), the determination unit 71 determines whether the pump pressure is equal to or greater than a specified value, i.e., whether the pump load is equal to or greater than a first switching threshold (step S4). In this embodiment, the determination unit 71 determines whether the larger of the first pump pressure P1 and the second pump pressure P2 is equal to or greater than the first switching threshold (step S4).

[0069] 4, during the time period from time t1 to time t2, the second pump pressure P2 (pump load for left traveling) is greater than the first pump pressure P1 (pump load for right traveling) but is less than the first switching threshold. Therefore, during this time period, the determination unit 71 determines that the pump load is less than the first switching threshold (NO in step S4), and the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right traveling motor 31 and the left traveling motor 32 are maintained at the second displacement (second traveling speed) (step S9).

[0070] As shown in FIG. 4, at time t2, the second pump pressure P2 (pump load for left-hand travel) is greater than the first pump pressure P1 (pump load for right-hand travel) and reaches the first switching threshold. Therefore, the determination unit 71 determines that the pump load is equal to or greater than the first switching threshold (YES in step S4), and the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right travel motor 31 and the left travel motor 32 are switched from the second displacement (second traveling speed) to the first displacement (first traveling speed) (step S6). Note that in FIG. 4, there is a slight time difference between time t2 when the second pump pressure P2 (pump load for left-hand travel) becomes equal to or greater than the first switching threshold and the time when the motor displacement is switched to the first displacement (low speed setting). This time difference (time delay) is caused by the time required for various hydraulic devices in the hydraulic circuit to actually operate based on commands from the controller 70.

[0071] Next, with the motor capacities of the right traveling motor 31 and the left traveling motor 32 set to the first capacity, the determination unit 71 determines whether the traveling state of the lower traveling body 1 is a turning state (steering state) based on the right traveling operation detection signal and the left traveling operation detection signal (step S7). If the determination unit 71 determines that the traveling state is a turning state (YES in step S7), the command unit 72 maintains the motor capacities of the right traveling motor 31 and the left traveling motor 32 at the first capacity (first traveling speed) regardless of the pump load (step S6).

[0072] 4, in the time period from time t2 to time t3, a left travel operation with the maximum operation amount in the forward operation direction is applied to the left travel operation device 62, while a right travel operation with the intermediate operation amount is applied to the right travel operation device 61. Furthermore, after the motor displacement is switched to the first displacement at time t2, the second pump pressure P2 (pump load for left traveling) is lower than the second pump pressure P2 at time t2.

[0073] 4, at time t3, a left travel operation with the maximum operation amount in the forward operation direction is applied to the left travel operation controller 62, and a right travel operation with the maximum operation amount in the forward operation direction is applied to the right travel operation controller 61. In this case, the lower traveling structure 1 is traveling straight. Therefore, based on the right travel operation detection signal and the left travel operation detection signal, the determination unit 71 determines that the traveling state of the lower traveling structure 1 is not a turning state (NO in step S7).

[0074] Next, the determination unit 71 determines whether the pump pressure is equal to or greater than a specified value, i.e., whether the pump load is equal to or greater than a second switching threshold (step S8). In this embodiment, the determination unit 71 determines whether the larger of the first pump pressure P1 and the second pump pressure P2 is equal to or greater than the second switching threshold (step S8).

[0075] 4, during the time period from time t3 to time t4, the second pump pressure P2 (pump load for left traveling) is greater than the first pump pressure P1 (pump load for right traveling) and is equal to or greater than the second switching threshold. Therefore, during this time period, the determination unit 71 determines that the pump load is equal to or greater than the second switching threshold (YES in step S8), and the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right traveling motor 31 and the left traveling motor 32 are maintained at the first displacement (first traveling speed) (step S10).

[0076] 4, at time t4, the second pump pressure P2 is greater than the first pump pressure P1 and less than the second switching threshold. Therefore, the determination unit 71 determines that the pump load is less than the second switching threshold (NO in step S8), and the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right traveling motor 31 and the left traveling motor 32 are switched from the first displacement to the second displacement (second traveling speed) (step S9).

[0077] In this embodiment, the controller 70 may prohibit switching of the motor displacement regardless of the pump load until a preset switching prohibition time has elapsed from the time when the motor displacement is switched from one of the first displacement and the second displacement to the other (e.g., time t2). As shown in Fig. 4, the elapsed time t from time t2 to time t4 is equal to or greater than the switching prohibition time. Therefore, at time t4, the controller 70 permits switching of the motor displacement, and the command unit 72 controls the motor displacement switching valve 44 so that the motor displacement is switched from the first displacement to the second displacement (step S9). If the elapsed time t from time t2 to time t4 is shorter than the switching prohibition time, the controller 70 may prohibit switching of the motor displacement at time t4, and the command unit 72 may control the motor displacement switching valve 44 so that the motor displacement is maintained at the first displacement.

[0078] 3, if the determination unit 71 determines that the vehicle is not in a turning state (steering state), i.e., that the vehicle is in a straight-ahead state (NO in step S3), the determination unit 71 determines whether the pump pressure is equal to or greater than a specified value, i.e., whether the pump load is equal to or greater than a third switching threshold, which is a preset threshold (step S5). The third switching threshold may be the same value as the first switching threshold, or may be a value different from the first switching threshold.

[0079] In this embodiment, when the traveling state of the lower traveling body 1 is a straight traveling state (NO in step S3), and both the first pump pressure P1 and the second pump pressure P2 are equal to or greater than the third switching threshold, or when the average value of the first pump pressure P1 and the second pump pressure P2 is equal to or greater than the third switching threshold (YES in step S5), the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right traveling motor 31 and the left traveling motor 32 are switched from the second displacement (second traveling speed) to the first displacement (first traveling speed) (step S10). On the other hand, when the traveling state of the lower traveling body 1 is a straight traveling state (NO in step S3), and at least one of the first pump pressure P1 and the second pump pressure P2 is less than the third switching threshold, or when the average value is less than the third switching threshold (NO in step S5), the command unit 72 controls the motor displacement switching valve 44 so that the motor displacements of the right traveling motor 31 and the left traveling motor 32 are maintained at the second displacement (step S9).

[0080] The present disclosure includes other aspects, for example, the following.

[0081] (A) First and second traction motors In the above embodiment, the right running motor 31 corresponds to the first running motor and the left running motor 32 corresponds to the second running motor, but conversely, the left running motor 32 may correspond to the first running motor and the right running motor 31 may correspond to the second running motor.

[0082] (B) Motor capacity In the above embodiment, the motor capacity of each traction motor is switched between two stages, a first capacity and a second capacity, but it may be configured to be switchable between three or more stages.

[0083] (C) Travel control device In the above embodiment, each actuator operator includes an operation receiver that receives operation by an operator and a remote control valve, but the present invention is not limited to this. Each actuator operator may be configured to receive operation by the operator and cause the actuator control valve to open a valve corresponding to the operation. Specifically, each actuator operator may be configured to combine, for example, an electric operation receiver such as an electric lever device that converts a given operation into an operation signal, which is an electric signal, an electromagnetic proportional valve that changes the pilot pressure input to the actuator control valve, and a pilot pressure command unit that inputs a pilot pressure command corresponding to the operation signal to the electromagnetic proportional valve so that the actuator control valve performs a valve opening operation corresponding to the operation signal.

[0084] (D) Hydraulic pump The hydraulic drive system according to the embodiment includes the first main pump 21 and the second main pump 22, but it is also possible to include only one of the first main pump 21 and the second main pump 22, and omit the other.

[0085] (E) About the controller In the above embodiment, when the traveling state of the undercarriage 1 is switched from a straight traveling state to a curved traveling state with the motor displacement set to the second displacement, the controller 70 switches the motor displacement from the second displacement to the first displacement when the larger of the first load pressure (first pump pressure P1) and the second load pressure (second pump pressure P2) is equal to or greater than the first switching threshold. However, in the hydraulic drive system according to the present disclosure, the controller 70 may be configured to switch the motor displacement from the second displacement to the first displacement when, for example, both the first load pressure and the second load pressure are equal to or greater than the first switching threshold, or when the average value of the first load pressure and the second load pressure is equal to or greater than the first switching threshold, when the traveling state of the undercarriage 1 is switched from a straight traveling state to a curved traveling state with the motor displacement set to the second displacement.

[0086] In addition, in the above embodiment, the controller 70 prohibits switching of the motor capacity from the time when the motor capacity is switched from one of the first capacity and the second capacity to the other until a predetermined switching prohibition time has elapsed, but control based on this switching prohibition time can be omitted.

[0087] In the above embodiment, when the traveling state of the undercarriage 1 is a straight traveling state, the controller 70 switches the motor displacement from the second displacement to the first displacement when both the first load pressure and the second load pressure are equal to or greater than the third switching threshold or when the average value of the first load pressure and the second load pressure is equal to or greater than the third switching threshold, and maintains the motor displacement at the second displacement when at least one of the first load pressure and the second load pressure is less than the third switching threshold or when the average value is less than the third switching threshold. However, the control based on the third switching threshold can be omitted. [Explanation of symbols]

[0088] 1: Lower running body 21: First main pump 22: Second main pump 31: Right drive motor 32: Left driving motor 61: Right travel control 62: Left travel controller 70: Controller 81: First pressure sensor 82: Second pressure sensor 83: Right travel pilot pressure sensor 84: Left travel pilot pressure sensor 100: Hydraulic excavator P1: First pump pressure P2: Second pump pressure

Claims

1. A hydraulic drive system for a traveling work machine having a traveling body including a traveling device having a portion in contact with the ground, At least one hydraulic pump that discharges hydraulic oil; a variable displacement first travel motor and a variable displacement second travel motor that are each operated to drive the travel device by receiving a supply of hydraulic oil from the at least one hydraulic pump and are capable of switching the travel state of the travel body from one of a straight travel state and a curved travel state to the other; a controller that switches the motor displacements of the first traveling motor and the second traveling motor between a first displacement and a second displacement that is smaller than the first displacement; When the traveling state of the traveling object is switched from the straight traveling state to the curved traveling state with the motor displacement set to the second displacement, the controller maintains the motor displacement at the second displacement when a pump load that is a load on the at least one hydraulic pump is less than a first switching threshold that is a preset threshold, and switches the motor displacement from the second displacement to the first displacement when the pump load is equal to or greater than the first switching threshold; When the traveling state of the traveling object is the turning state with the motor displacement set to the first displacement, the controller maintains the motor displacement at the first displacement, a hydraulic drive system for a work machine, wherein when the traveling state of the traveling body is switched from the turning state to the straight traveling state with the motor displacement set to the first displacement, the controller maintains the motor displacement at the first displacement when the pump load is equal to or greater than a second switching threshold that is a threshold that is set in advance to a value smaller than the first switching threshold, and switches the motor displacement from the first displacement to the second displacement when the pump load is less than the second switching threshold.

2. A hydraulic drive system for a work machine according to claim 1, the at least one hydraulic pump includes a first hydraulic pump capable of supplying hydraulic oil to the first travel motor and a second hydraulic pump capable of supplying hydraulic oil to the second travel motor, The hydraulic drive device is a first pressure sensor that detects a first load pressure that is a pressure corresponding to a load on the first hydraulic pump; a second pressure sensor that detects a second load pressure that is a pressure corresponding to a load of the second hydraulic pump, a hydraulic drive system for a work machine, wherein when the traveling state of the traveling body is switched from the straight traveling state to the curved traveling state with the motor capacity set to the second capacity, the controller switches the motor capacity from the second capacity to the first capacity when the larger of the first load pressure and the second load pressure is equal to or greater than the first switching threshold.

3. The hydraulic drive system for a work machine according to claim 1 or 2, a hydraulic drive system for a work machine, wherein the controller prohibits switching of the motor displacement until a switching prohibition time, which is a preset time, has elapsed from the time the motor displacement is switched from one of the first displacement and the second displacement to the other.

4. A hydraulic drive system for a work machine according to claim 1, the at least one hydraulic pump includes a first hydraulic pump capable of supplying hydraulic oil to the first travel motor and a second hydraulic pump capable of supplying hydraulic oil to the second travel motor, The hydraulic drive device is a first pressure sensor that detects a first load pressure that is a pressure corresponding to a load on the first hydraulic pump; a second pressure sensor that detects a second load pressure that is a pressure corresponding to a load of the second hydraulic pump, When the traveling state of the traveling body is the straight traveling state, the controller switches the motor displacement from the second displacement to the first displacement when both the first load pressure and the second load pressure are equal to or greater than a third switching threshold that is a preset threshold, or when an average value of the first load pressure and the second load pressure is equal to or greater than the third switching threshold, and maintains the motor displacement at the second displacement when at least one of the first load pressure and the second load pressure is less than the third switching threshold or the average value is less than the third switching threshold.

5. A hydraulic drive system for a work machine according to any one of claims 1 to 4, a first travel operation device to which a first operation, which is an operation by an operator to operate the first travel motor, is given; a second travel operation device to which a second operation, which is an operation by an operator to operate the second travel motor, is given; a first operation amount detector that detects a first operation amount that is an operation amount of the first operation; a second operation amount detector that detects a second operation amount that is an operation amount of the second operation, The controller determines the traveling state of the traveling body based on the first operation amount and the second operation amount.

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