work machine

Abstract

A working machine comprises a lower carriage; an upper pivoting body configured to pivot on the lower carriage; a hydraulic actuator; a hydraulic pump configured to supply hydraulic oil to the hydraulic actuator; a first control valve located in a first oil channel connecting the hydraulic pump and a hydraulic oil tank; a second control valve located in a second oil channel connecting the hydraulic pump and the hydraulic actuator; and a third control valve located between the hydraulic pump and the second control valve in the second oil channel. The third control valve is configured to close the second oil channel when a load on a drive source is intentionally increased.

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The present disclosure relates to a working machine. 2. Description of the state of the art Conventionally, a working machine (a hydraulic excavator) comprises a hydraulic pump driven by a power source (an engine) (see patent document 1). The hydraulic excavator is configured to intentionally increase the engine load to perform a specific process. This specific process might be, for example, increasing the exhaust gas temperature to burn off particulate matter collected in a filter of an exhaust aftertreatment system using the heat of the exhaust gas. Furthermore, the hydraulic excavator is configured to increase the engine load by increasing the delivery pressure of a hydraulic pump when a control lever is not being operated. The hydraulic excavator described above is configured to increase the hydraulic pump's delivery pressure by causing a control valve (a control valve for regulating the supply and discharge of hydraulic oil into and out of the hydraulic cylinder), located in an oil channel connecting a hydraulic cylinder and the hydraulic pump, to block the flow of hydraulic oil in that oil channel. Therefore, in the hydraulic excavator described above, there is a possibility that the hydraulic oil pressure on the upstream side of the control valve will inevitably rise and, over a period during which a process to intentionally increase the load on the engine is carried out, will exceed the hydraulic oil pressure on the downstream side of the control valve. As a result, leakage of hydraulic oil from the upstream to the downstream side of the control valve cannot be prevented. In light of the foregoing, it is desirable to provide a working machine that can intentionally increase the load on a drive source while simultaneously suppressing an increase in a pressure differential across a control valve, wherein a difference between the pressure of hydraulic oil on the upstream side of the control valve and the pressure of hydraulic oil on the downstream side of the control valve is, when the pressure of the hydraulic oil on the upstream side is greater than the pressure of the hydraulic oil on the downstream side. RELATED STATE OF THE ART PATENT DOCUMENTS Patent document 1: Japanese unexamined patent publication no. 2020-051482 SUMMARY OF THE INVENTION A working machine according to one embodiment of the present disclosure comprises a lower carriage; an upper pivoting body configured to be pivotably mounted on the lower carriage; a hydraulic actuator; a hydraulic pump configured to supply hydraulic oil to the hydraulic actuator; a first control valve provided in a first oil channel connecting the hydraulic pump and a hydraulic oil tank; a second control valve provided in a second oil channel connecting the hydraulic pump and the hydraulic actuator; and a third control valve provided between the hydraulic pump and the second control valve in the second oil channel. The third control valve is configured to close the second oil channel when a load on a drive source is intentionally increased. The machine described above can intentionally increase the load on a drive source, while an increase in the pressure differential is suppressed via a control valve. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side view of a working machine according to an embodiment of the present disclosure; Fig. 2 is a schematic diagram illustrating an example configuration of a hydraulic drive system installed in the working machine from Fig. 1; Fig. 3 is a flowchart illustrating an example of the sequence of a forced high-load process; Fig. 4 is a diagram illustrating an example state of the hydraulic drive system illustrated in Fig. 2; Fig. 5 is a diagram illustrating another example state of the hydraulic drive system illustrated in Fig. 2; Fig. 6 is a diagram illustrating yet another example state of the hydraulic drive system illustrated in Fig. 2; Fig. 7 is a diagram illustrating yet another example state of the hydraulic drive system illustrated in Fig. 2; and Fig.Figure 8 is a schematic diagram illustrating an example configuration of yet another hydraulic drive system installed in the working machine of Fig. 1. DESCRIPTION OF PREFERRED EXECUTION FORMS First, a working machine 100 according to an embodiment of the present disclosure is described with reference to Fig. 1. Fig. 1 is a side view of the working machine 100. The working machine 100 illustrated in Fig. 1 is an excavator (an excavating machine) and comprises a lower carriage 1, a pivoting mechanism 2, and an upper pivoting body 3. The upper pivoting body 3 is pivotably mounted on the lower carriage 1 via the pivoting mechanism 2. A boom 4, which serves as a working element, is attached to the upper pivoting body 3. An arm 5, which serves as a working element, is attached to the tip of the boom 4, and a bucket 6, which serves as a working element and an end attachment, is attached to the tip of the arm 5. The boom 4, the arm 5, and the bucket 6 form an excavating attachment, which is an example of an attachment.The boom 4 is driven by a boom cylinder 7, the arm 5 is driven by an arm cylinder 8, and the bucket 6 is driven by a bucket cylinder 9. The upper swing body 3 is equipped with a cabin 10 and a drive source 11. The working machine 100 can be any other working machine that includes hydraulic actuators capable of intentionally increasing the load on the drive source 11, such as a wheel loader, a crane, an asphalt paver, or a forklift. Fig. 2 is a diagram illustrating an example configuration of a hydraulic drive system installed in the working machine 100 from Fig. 1. In Fig. 2, a mechanical power transmission system is indicated by a double line, a hydraulic oil line by a solid line, a pilot line by a dashed line, and an electrical control line by a dash-dotted line. The hydraulic drive system of the working machine 100 essentially comprises the drive source 11, a pump controller 13, a main pump 14, a pilot pump 15, an operating device 26, a discharge pressure sensor 28, an operating sensor 29, a control 30 and the like. The drive source 11 is a drive source for the working machine 100. The drive source 11 can be an electric motor powered by an external energy source, such as a battery, fuel cell, or the like; an internal combustion engine powered by gasoline, diesel fuel, hydrogen, biofuel, or the like; or a hybrid energy source combining an internal combustion engine and an electric motor. In the illustrated example, the drive source 11 is a diesel engine operating to maintain a predetermined rotational speed. An output shaft of the drive source 11 is coupled to a drive shaft of the main pump 14 and a drive shaft of the pilot pump 15. The load on the drive source 11 is intentionally increased when regeneration control of an exhaust aftertreatment device such as a diesel particulate filter (DPF), defrosting and heating control of urea solution, frost protection control of a blow-by gas line, air conditioning control (heating control), or warm-up control of a hydraulic drive system is performed. This is because the various processes described above are carried out using thermal energy generated by the increased load on the drive source 11. Furthermore, it is desirable to intentionally increase the load on the drive source 11 without actuating a hydraulic actuator installed in the working machine 100.Therefore, in the illustrated example, the working machine 100 increases the load on the drive source 11 by blocking an oil channel through which hydraulic oil supplied by the main pump 14 flows, in order to increase the delivery pressure of the main pump 14 and increase the hydraulic load (pump torque) without operating the attachment. The main pump 14 is an example of a hydraulic pump and is configured to supply hydraulic oil to a control valve unit 17. In the illustrated example, the main pump 14 is a swashplate hydraulic pump with variable displacement and comprises a left main pump 14L and a right main pump 14R. The pump controller 13 is configured to control the delivery rate of the main pump 14. In the illustrated example, the pump controller 13 controls the delivery rate of the main pump 14 by adjusting the tilt angle of the swashplate of the main pump 14 in response to a command from the controller 30. The pump controller 13 can output information regarding the tilt angle of the swashplate to the controller 30. Specifically, the pump controller 13 includes a left pump controller 13L, configured to control the delivery rate of the left main pump 14L, and a right pump controller 13R, configured to control the delivery rate of the right main pump 14R. The pilot pump 15 is configured to supply hydraulic oil to various hydraulic devices, including the control device 26. In the illustrated example, the pilot pump 15 is a fixed-displacement hydraulic pump. However, the pilot pump 15 can be omitted. In such a case, the function of the pilot pump 15 can be implemented by the main pump 14. That is, in addition to supplying hydraulic oil to the control valve unit 17, the main pump 14 can also supply hydraulic oil to the control device 26 and the like, after the hydraulic oil pressure has been reduced by a throttle or similar device. The control valve unit 17 accommodates several control valves, enabling their actuation. In the illustrated example, the control valve unit 17 comprises several control valves 170 to 177, configured to control the flow of hydraulic oil delivered by the main pump 14. The control valve unit 17 is configured to selectively direct the hydraulic oil delivered by the main pump 14 to one or more hydraulic actuators via the control valves 170 to 177. The multiple control valves 170 to 177 control the flow rate of the hydraulic oil flowing from the main pump 14 to the hydraulic actuators and the flow rate of the hydraulic oil flowing from the hydraulic actuators to a hydraulic oil tank T1. The hydraulic actuators can be hydraulic cylinders or hydraulic motors. The hydraulic cylinders can be single-rod or double-rod hydraulic cylinders. In the illustrated example, the hydraulic actuators include the boom cylinder 7, the arm cylinder 8, the bucket cylinder 9, a travel hydraulic motor 20, and a swing hydraulic motor 21. The travel hydraulic motor 20 includes a left-hand travel hydraulic motor 20L and a right-hand travel hydraulic motor 20R. The swivel hydraulic motor 21 is a hydraulic motor that swivels the upper swivel body 3. An oil channel 21P, connected to a port of the swivel hydraulic motor 21, is connected to an oil channel 44 via a relief valve 22 and a control valve 23. Specifically, the oil channel 21P comprises a left oil channel 21PL and a right oil channel 21PR. The relief valve 22 comprises a left relief valve 22L and a right relief valve 22R. The control valve 23 comprises a left control valve 23L and a right control valve 23R. The left relief valve 22L opens to release hydraulic oil from the left oil channel 21PL to the oil channel 44 when the hydraulic oil pressure in the left oil channel 21PL reaches a predetermined relief pressure. Furthermore, the right relief valve 22R opens to release hydraulic oil from the right oil channel 21PR to the oil channel 44 when the hydraulic oil pressure in the right oil channel 21PR reaches a predetermined relief pressure. The left relief valve (control valve 23L) opens to discharge hydraulic oil from oil channel 44 to the left oil channel 21PL when the hydraulic oil pressure in the left oil channel 21PL becomes lower than the hydraulic oil pressure in oil channel 44. The right control valve 23R opens to discharge hydraulic oil from oil channel 44 to the right oil channel 21PR when the hydraulic oil pressure in the right oil channel 21PR becomes lower than the hydraulic oil pressure in oil channel 44. This configuration allows control valve 23 to supply hydraulic oil to the inlet-side port when the swivel hydraulic motor 21 is braked. The control device 26 is a device used by an operator to actuate the hydraulic actuators. In the illustrated example, the control device 26 is of a hydraulic type and supplies hydraulic oil delivered by the pilot pump 15 via a pilot line to a pilot port of a control valve corresponding to each hydraulic actuator. A pilot pressure, which is the pressure of the hydraulic oil supplied to each pilot port, is a pressure corresponding to the direction and amount of action of an operating lever or pedal of the control device 26 corresponding to the respective hydraulic actuator. The control device 26 can be of an electric type. In particular, the operating device 26 comprises a left operating lever, a right operating lever, a left-hand drive operating lever, a right-hand drive lever, a left-hand drive operating pedal, and a right-hand drive operating pedal. The left operating lever functions as an arm operating lever and a swivel operating lever. The right operating lever functions as a boom operating lever and a bucket operating lever. Hereinafter, at least one of the left operating levers or the right operating lever may be referred to as an “attachment operating device,” and at least one of the left-hand drive lever, the right-hand drive lever, the left-hand drive pedal, or the right-hand drive pedal may be referred to as a “driving operating device.” The discharge pressure sensor 28 is configured to detect the discharge pressure of the main pump 14 and output a detected value to the controller 30. In the illustrated example, the discharge pressure sensor 28 comprises a left discharge pressure sensor 28L, which detects the discharge pressure of the left main pump 14L, and a right discharge pressure sensor 28R, which detects the discharge pressure of the right main pump 14R. The operating sensor 29 is a device for detecting details of the operator's actions using the operating device 26. Examples of operating details include the direction and extent of the action (the operating angle). In the illustrated example, the operating sensor 29 is a pressure sensor that detects the direction and extent of action of a lever or pedal of the operating device 26, each corresponding to a hydraulic actuator, in the form of pressure, and outputs a detected value to the controller 30. However, the operating details of the operating device 26 can be detected using the output signal of a device other than a pressure sensor, such as an operating angle sensor, an accelerometer, an angular velocity sensor, a resolver, a voltmeter, or an ammeter. The controller 30 is an example of a processing circuit and functions as a control device for controlling the working machine 100. In the illustrated example, the controller 30 is configured with a computer comprising a CPU, volatile memory, and non-volatile memory. An electromagnetic valve 31 is provided in an oil channel connecting the pilot pump 15 to a left pilot port of a control valve 175L in the control valve unit 17, and is configured to change the flow range of the oil channel. In the illustrated example, the electromagnetic valve 31 is an electromagnetic proportional control valve that operates in response to a control command issued by the controller 30. With this configuration, the controller 30 can automatically actuate the control valve 175L regardless of whether a boom lowering operation is performed by the operator. Furthermore, in the illustrated example, the left pilot port of the control valve 175L is configured so that the higher of a pilot pressure generated by the electromagnetic valve 31 and a pilot pressure generated by the boom control lever acts on the left pilot port. An electromagnetic valve 32 is provided in an oil channel that connects the pilot pump 15 to a left pilot port of the control valve 177 in the control valve unit 17, and is configured to change the flow range of the oil channel. In the illustrated example, the electromagnetic valve 32 is an electromagnetic proportional control valve that operates in response to a control command issued by the controller 30. With this configuration, the controller 30 can automatically actuate the control valve 177. A middle bypass oil channel 40 is a hydraulic oil line that passes through the control valves arranged in the control valve unit 17 and includes a left middle bypass oil channel 40L and a right middle bypass oil channel 40R. The control valve 170 is a spool valve that serves as a straight-ahead travel valve. In the illustrated example, the control valve 170 is configured to switch the hydraulic oil flow so that hydraulic oil is supplied from the main pump 14 to the travel hydraulic motor 20 to improve the straight-ahead travel of the lower chassis 1. Specifically, the valve position of the control valve 170 is configured to switch between a first valve position (left valve position) and a second valve position (right valve position) in response to a control command from the controller 30. In particular, the control valve 170 is in the first valve position when only the drive control device or only the attachment control device is operated, and is in the second valve position when the drive control device and the attachment control device are operated simultaneously. The first valve position is a valve position in which the left main pump 14L and the left-hand hydraulic motor 20L communicate with each other, and the right main pump 14R and the right-hand hydraulic motor 20R communicate with each other. In this state, the left main pump 14L can supply hydraulic oil to the left-hand hydraulic motor 20L, and the right main pump 14R can supply hydraulic oil to the right-hand hydraulic motor 20R. The second valve position is a valve position in which the left main pump 14L communicates with both the left-hand hydraulic motor 20L and the right-hand hydraulic motor 20R. In this state, the left main pump 14L can supply hydraulic oil to both the left-hand hydraulic motor 20L and the right-hand hydraulic motor 20R. The control valve 171 is a spool valve that switches the hydraulic oil flow to supply the hydraulic oil delivered by the main pump 14 to the drive hydraulic motor 20 and to discharge the hydraulic oil delivered by the drive hydraulic motor 20 to the hydraulic oil tank T1. Specifically, the control valve 171 comprises a control valve 171L and a control valve 171R. The control valve 171L switches the hydraulic oil flow to supply the hydraulic oil delivered by the left-hand main pump 14L to the left-hand drive hydraulic motor 20L and to discharge the hydraulic oil delivered by the left-hand drive hydraulic motor 20L to the hydraulic oil tank T1. The control valve 171R switches the hydraulic oil flow to supply the hydraulic oil delivered by the left main pump 14L or the right main pump 14R to the right-hand hydraulic motor 20R and to deliver the hydraulic oil delivered by the right-hand hydraulic motor 20R to the hydraulic oil tank T1. The control valve 172 is a spool valve that switches the hydraulic oil flow to supply hydraulic oil delivered by the left main pump 14L to an optional hydraulic actuator and to discharge hydraulic oil delivered by the optional hydraulic actuator to the hydraulic oil tank T1. The optional hydraulic actuator is, for example, a grapple drive cylinder or a crusher drive cylinder. The control valve 173 is a spool valve that switches the hydraulic oil flow to supply the hydraulic oil delivered by the left main pump 14L to the swivel hydraulic motor 21 and to discharge the hydraulic oil delivered by the swivel hydraulic motor 21 to the hydraulic oil tank T1. The control valve 174 is a spool valve that switches the hydraulic oil flow to supply the hydraulic oil delivered by the right main pump 14R to the bucket cylinder 9 and to discharge the hydraulic oil in the bucket cylinder 9 to the hydraulic oil tank T1. The control valve 175 is a spool valve that switches the hydraulic oil flow to supply the hydraulic oil delivered by the main pump 14 to the boom cylinder 7 and to discharge the hydraulic oil in the boom cylinder 7 to the hydraulic oil tank T1. Specifically, the control valve 175 comprises the control valve 175L and a control valve 175R. In the illustrated example, the control valve 175L is configured to switch its valve position in response to a control command from the controller 30 between a first valve position (left valve position), a second valve position (right valve position), and a third valve position (center valve position). The control valve 175L is in the first valve position when a boom lowering operation is performed, in the second valve position when a boom raise operation is performed, and in the third valve position (center valve position), which serves as an output valve position when neither a boom raise nor a boom lower operation is performed. Specifically, when a boom lowering operation is performed, the control valve 175L is in the first valve position and can cause hydraulic oil in a bottom-side oil chamber of the boom cylinder 7 to flow into the left center bypass oil channel 40L.Therefore, the control valve 175L can cause hydraulic oil flowing from the bottom oil chamber of boom cylinder 7 to flow into (regenerate) arm cylinder 8 when boom lowering operation is performed. It should be noted that an oil channel in the control valve 175L, which causes hydraulic oil in the bottom oil chamber of boom cylinder 7 to flow into the left center bypass oil channel 40L, may be omitted. A holding valve (not illustrated) is provided between boom cylinder 7 and control valve 175L to prevent the boom 4 from moving due to its own weight or the like when boom operation is not performed (a valve to prevent hydraulic oil from flowing out of boom cylinder 7). The holding valve does not prevent hydraulic oil from flowing into boom cylinder 7.Furthermore, control valve 175L is in its second valve position when a boom lifting operation is performed and can cause hydraulic oil delivered by the left main pump 14L to flow into the bottom oil chamber of the boom cylinder 7. Similarly, control valve 175R can cause hydraulic oil delivered by the right main pump 14R to flow into the bottom oil chamber of the boom cylinder 7 when a boom lifting operation is performed. Furthermore, control valve 175R can cause hydraulic oil delivered by the right main pump 14R to flow into a rod-side oil chamber of the boom cylinder 7 when a boom lowering operation is performed. The control valve 176 is a spool valve that switches the hydraulic oil flow to supply the hydraulic oil delivered by the main pump 14 to the arm cylinder 8 and to discharge hydraulic oil in the arm cylinder 8 to the hydraulic oil tank T1. Specifically, the control valve 176 comprises a control valve 176L and a control valve 176R. The control valve 177 is a spool valve that restricts the flow rate of the hydraulic oil through a left parallel oil channel 42L, which will be described later, to the control valve 176L and is referred to as a "sub-spool valve" or "shut-off valve". The "sub-spool valve" is used in contrast to a "main spool valve" such as the control valves 170 to 176 provided in the middle bypass oil channel 40. In the illustrated example, the control valve 177 is configured such that, in response to a control command from the controller 30, the valve position switches between a first valve position (left valve position), a second valve position (right valve position), and a third valve position (middle valve position). In particular, the control valve 177 is in the first valve position, which serves as an output valve position, when no opening / closing operation of the arm 5 is performed or when an opening / closing operation of the arm 5 is performed independently. Furthermore, the control valve 177 is in the second valve position when a process to intentionally increase the load on the drive source 11 is performed, and it is in the third valve position when a combined operation of an opening / closing operation of the arm 5 and a pivoting operation or the like is performed. In particular, the control valve 177 is configured such that it is in the third valve position and acts as a pivot priority valve when combined operation is performed, which includes an opening / closing operation of the arm 5 and a pivot operation. Therefore, the cross-sectional area of ​​a third fixed throttle AP3, provided in an oil channel connecting the upstream and downstream sides of the control valve 177 in the third valve position (middle valve position), is smaller than the cross-sectional area of ​​a first fixed throttle AP1, provided in an oil channel connecting the upstream and downstream sides of the control valve 177 in the first valve position (left valve position).With this configuration, for example, if the arm cylinder 8 and the slewing hydraulic motor 21 are operated simultaneously and the load pressure of the arm cylinder 8 is lower than that of the slewing hydraulic motor 21, the third fixed throttle AP3 prevents most of the hydraulic oil delivered by the left main pump 14L from flowing into the arm cylinder 8 with the lower load pressure. This is because the third fixed throttle AP3 can increase the pressure of hydraulic oil on its upstream side when hydraulic oil flows through the control valve 177 into the arm cylinder 8. Therefore, the hydraulic drive system, including the third fixed throttle AP3, can reliably operate not only the arm cylinder 8 with the lower load pressure but also the slewing hydraulic motor 21 with the higher load pressure, even if, for example, the arm cylinder 8 with the lower load pressure and the slewing hydraulic motor 21 with the higher load pressure are operated simultaneously.In the following, a function implemented by such a pivot priority valve will also be referred to as a "pivot priority function". Furthermore, the first fixed throttle AP1 and the third fixed throttle AP3 can be implemented by a variable throttle. For example, the oil channel in the first valve position (left valve position) can be equipped with a variable throttle that functions as both the first fixed throttle AP1 and the third fixed throttle AP3. In this case, the third valve position can be omitted. In the illustrated example, the control valves 170 to 176 are pilot-type spool valves, and each pilot port is hydraulically connected to the operating device 26, which serves as a hydraulic control device. However, not every pilot port of the control valves 170 to 176 is necessarily hydraulically connected to the operating device 26 in a case where the operating device 26 is of an electric type. In particular, in a case where an operating lever serving as the operating device 26 is an electric operating lever, the lever operating amount and lever operating direction are input to the controller 30 as electrical signals. In this case, an electromagnetic control valve is typically arranged between the pilot pump 15 and a pilot port of each control valve. The electromagnetic valve is configured to operate in response to an electrical signal from the controller 30.With this configuration, when the control lever is operated manually, the controller 30 controls the electromagnetic valve based on an electrical signal corresponding to the lever's movement, increasing or decreasing the pilot pressure so that each control valve is moved (actuated). Each control valve can be configured as an electromagnetic spool valve. In this case, the electromagnetic spool valve operates in response to an electrical signal from the controller 30 that corresponds to the movement of the electric control lever. A return oil channel 41 is a hydraulic oil line arranged in the control valve unit 17 and comprises a middle return oil channel 41C, a left return oil channel 41L, and a right return oil channel 41R. In the illustrated example, the middle return oil channel 41C is a return oil channel that connects a relief valve 55 and the hydraulic oil tank T1. The left return channel 41L is a return channel that connects each of the control valves 171L, 172, 173, 175L, and 176L to the hydraulic oil tank T1. The right return channel 41R is a return channel that connects each of the control valves 170, 171R, 174, 175R, and 176R to the hydraulic oil tank T1. The relief valve 55 is a device for maintaining the pressure of hydraulic oil in the hydraulic drive system at or below a predetermined relief pressure. In the illustrated example, an upstream port of the relief valve 55 is connected via a control valve to both the left center bypass oil channel 40L and the right center bypass oil channel 40R, and a downstream port of the relief valve 55 is connected via the center return oil channel 41C to the hydraulic oil tank T1. The relief valve 55 is configured to be held in a closed position when the pressure of hydraulic oil on the upstream side (the pressure of the hydraulic oil in the hydraulic drive system) is lower than the predetermined relief pressure, and is configured to be open when the pressure of hydraulic oil on the upstream side is higher than or equal to the predetermined relief pressure.In the illustrated example, the relief valve 55 is a fixed relief valve where the relief pressure is fixed, but the relief valve 55 can be a variable electromagnetic relief valve where the relief pressure can be adjusted. A parallel oil channel 42 is a hydraulic oil line that runs parallel to the center bypass oil channel 40. In the illustrated example, the parallel oil channel 42 includes the left parallel oil channel 42L, which runs parallel to the left center bypass oil channel 40L, and a right parallel oil channel 42R, which runs parallel to the right center bypass oil channel 40R. If the hydraulic oil flow through the left center bypass oil channel 40L is restricted or blocked by the control valve 171L, 172, 173, or 175L, the left parallel oil channel 42L can supply hydraulic oil to a control valve located further downstream. If the flow of hydraulic oil through the right middle bypass oil channel 40R is restricted or blocked by the control valve 171R, 174 or 175R, the right parallel oil path 42R can supply hydraulic oil to a control valve further downstream. A throttle 60 is a fixed throttle valve provided in the right parallel oil path 42R, that is, on the upstream side of the control valve 176R and on the downstream side of a branch point where an oil path connecting the right parallel oil path 42R and the control valve 175R branches off from the right parallel oil path 42R. In the illustrated example, the throttle valve 60, for instance, has the function of preventing most of the hydraulic oil delivered by the right main pump 14R from flowing into the arm cylinder 8 at a low load pressure when the arm cylinder 8 is operating at a low load pressure and a hydraulic actuator (at least one from the boom cylinder 7, the bucket cylinder 9, or the right-hand hydraulic motor 20R) is simultaneously actuated at a high load pressure.This is because the throttle 60 can increase the pressure of hydraulic oil on its downstream side when hydraulic oil flows through the control valve 176R into the boom cylinder 8. Therefore, the hydraulic drive system, which includes the throttle 60, can reliably operate not only the boom cylinder 8 at low load pressure but also the boom cylinder 7 at high load pressure, even if, for example, the boom cylinder 8 is operated simultaneously at low load pressure and the bucket cylinder 9 or the right-hand travel hydraulic motor 20R is operated simultaneously at high load pressure. The same applies if the boom cylinder 8 is operated simultaneously at low load pressure and the bucket cylinder 9 or the right-hand travel hydraulic motor 20R is operated simultaneously at high load pressure. Here, the negative control used in the hydraulic drive system of Fig. 2 is described. A throttle 18 is arranged in the central bypass oil channel 40 between the most downstream control valve 176 and the hydraulic oil tank T1. The hydraulic oil flow delivered by the main pump 14 is restricted by the throttle 18. The throttle 18 generates a control pressure (negative control pressure) for controlling the pump governor 13. Specifically, the throttle 18 is a fixed throttle with a fixed opening range and comprises a left throttle 18L and a right throttle 18R. However, the throttle 18 can also be a variable throttle with a variable opening range. The throttle 18 tends to increase stability against a sudden change in control pressure as the opening range increases. Furthermore, the throttle 18 tends to increase the responsiveness to a control pressure as the opening range decreases.The hydraulic oil flow delivered by the left main pump 14L is restricted by the left throttle 18L. The left throttle 18L then generates a control pressure to control the left pump governor 13L. Similarly, the hydraulic oil flow delivered by the right main pump 14R is restricted by the right throttle 18R. The right throttle 18R then generates a control pressure to control the right pump governor 13R. A control pressure sensor 19 is a sensor that detects a control pressure (negative control pressure) generated upstream of the throttle 18 and comprises a left control pressure sensor 19L and a right control pressure sensor 19R. In the illustrated example, the control pressure sensor 19 is configured to output a detected value to the controller 30. The controller 30 issues a command corresponding to the control pressure to the pump controller 13. The pump controller 13 controls the delivery rate of the main pump 14 by adjusting the tilt angle of the swashplate of the main pump 14 in response to the command. Specifically, the pump controller 13 decreases the delivery rate of the main pump 14 when the control pressure increases and increases the delivery rate of the main pump 14 when the control pressure decreases. Due to the negative control, the hydraulic drive system of Fig. 2 can control unnecessary energy consumption in the main pump 14 when none of the hydraulic actuators are actuated. This unnecessary energy consumption includes pump losses caused by the hydraulic oil delivered by the main pump 14 into the central bypass oil channel 40. The hydraulic drive system according to Fig. 2 ensures that when a hydraulic actuator is actuated, the necessary and sufficient hydraulic oil is supplied from the main pump 14 to the actuated hydraulic actuator. The central bypass oil channel 40 and the return oil channel 41 are connected to an oil channel 43 downstream of the throttle 18 via a junction. In the illustrated example, the oil channel 43 branches downstream of the junction to connect to an oil channel 45 and an oil channel 46 outside the control valve unit 17. This means that the hydraulic oil flowing through the central bypass oil channel 40 and the hydraulic oil flowing through the return oil channel 41 merge in the oil channel 43 and then flow through the oil channel 45 and the oil channel 46 to the hydraulic oil tank T1. Furthermore, the oil channel 43 is connected to the swivel hydraulic motor 21 via the oil channel 44, which is a hydraulic oil line for compensating for a hydraulic oil shortage on the supply side of the swivel hydraulic motor 21. Oil channel 45 is a hydraulic oil line that connects oil channel 43 and hydraulic oil tank T1. A control valve 50, an oil cooler 51, and a filter 53 are located in oil channel 45. The control valve 50 is a valve that opens when the pressure difference between the primary and secondary sides exceeds a predetermined valve opening pressure difference. In the illustrated example, the control valve 50 is a spring-loaded control valve and opens to cause hydraulic oil in the control valve unit 17 to flow to the oil cooler 51 when the inlet pressure is higher than the outlet pressure and the pressure difference exceeds the valve opening pressure difference. This configuration allows the control valve 50 to maintain the pressure of hydraulic oil in oil channel 43 and oil channel 44 at a level higher than the valve opening pressure, ensuring that the lack of hydraulic oil on the supply side of the swivel hydraulic motor 21 is compensated for. In this case, the valve opening pressure is the lower limit of a back pressure against the throttle 18.The back pressure against the throttle 18 increases when the flow rate of the hydraulic oil flowing through the control valve 50 increases. The control valve 50 may be integrated into the control valve unit 17 or may be omitted. In the case where the control valve 50 is omitted, a pressure drop in each of the oil channel 45, the control valve 50, the oil cooler 51, and the filter 53 results in back pressure against the throttle 18. The back pressure against the throttle 18 increases when the flow rate of the hydraulic oil flowing through the oil channel 45 increases. The oil cooler 51 is a device for cooling hydraulic oil circulating in the hydraulic drive system. In the illustrated example, the oil cooler 51 is enclosed in a heat exchanger unit, which is cooled by a cooling fan driven by the drive source 11. The heat exchanger unit includes a radiator, an intercooler, the oil cooler 51, and the like. Furthermore, in the illustrated example, the oil channel 45 includes an oil channel section 45a, which connects the control valve 50 and the oil cooler 51, and an oil channel section 45b, which connects the oil cooler 51 and the hydraulic oil tank T1. The filter 53 is arranged in the oil channel section 45b. Oil channel 46 is a bypass oil channel that bypasses the oil cooler 51. In the illustrated example, oil channel 46 has one end connected to oil channel 43 and the other end connected to hydraulic oil tank T1. One end of oil channel 46 can be connected to oil channel 45 between control valve 50 and oil cooler 51. Furthermore, a control valve 52 is located in oil channel 46. Similar to control valve 50, control valve 52 is a valve that opens when the pressure difference between the primary and secondary sides exceeds a predetermined valve opening pressure difference. In the illustrated example, control valve 52 is a spring-loaded control valve and opens to cause hydraulic oil in control valve unit 17 to flow to the hydraulic oil tank T1 when the upstream pressure is higher than the downstream pressure and the pressure difference exceeds the valve opening pressure difference. The valve opening pressure difference for control valve 52 is greater than the valve opening pressure difference for control valve 50. Therefore, hydraulic oil in control valve unit 17 initially flows through control valve 50 and subsequently, when the pressure exceeds the valve opening pressure due to resistance during flow through the oil cooler 51, it flows through control valve 52.The control valve 52 can be integrated into the control valve unit 17. Next, with reference to Fig. 3, a process for intentionally increasing the load on the drive source 11 (hereinafter referred to as the "forced high-load process") is described. Fig. 3 is a flowchart illustrating an example of the forced high-load process. In the illustrated example, the controller 30 is configured to repeatedly execute the forced high-load process in a predetermined control cycle while the drive source 11 is operating. Hereinafter, a function implemented by the forced high-load process is also referred to as a "forced high-load function." First, the controller 30 determines whether a predetermined condition for increasing the load at the drive source 11 is met (step ST1). In the illustrated example, the controller 30 determines that the predetermined condition is met when a predetermined button is pressed. The predetermined button is a graphic image that serves as a software button, displayed on a touch panel monitor provided as a display device in the cabin 10. The predetermined button can be a hardware button attached to the display device provided in the cabin 10 or a hardware button provided at another location in the cabin 10. Alternatively, the control unit 30 can determine that the predetermined condition is met when a predetermined voice command is entered via a voice input device, such as a microphone provided in cabin 10. Alternatively, the control unit 30 can determine that the predetermined condition is met when the working machine 100 is placed in a non-operable state by a door locking lever or the like, and the boom control lever is operated in the lowering direction. The non-operable state refers to a condition in which a hydraulic actuator cannot be operated even if the control device 26 is operated, for example, a condition in which an oil channel between the pilot pump 15 and a pilot port of each control valve is blocked. Alternatively, the controller 30 can determine that the predetermined condition is met if the operating device 26 is not operated and a predetermined time has also arrived, the outside air temperature is lower than or equal to a predetermined lower limit temperature, the outside air temperature is higher than or equal to a predetermined upper limit temperature, or the operating time of the drive source 11 exceeds a predetermined time period. If the controller 30 determines that the predetermined condition is not met (NO at step ST1), the controller 30 terminates the current forced high-load process. This is because it determines that there is no need to intentionally increase the load on the drive source 11. If, however, the controller 30 determines that the predetermined condition is met (YES at step ST1), the controller 30 increases the output pressure of the main pump 14 (step ST2). In the illustrated example, the controller 30 increases the output pressure of the left main pump 14L by closing the left center bypass oil channel 40L. Specifically, the controller 30 issues a control command to the electromagnetic valve 31 to increase the control pressure acting on the left pilot port of the control valve 175L. As the control pressure acting on the left pilot port increases, the valve position of the control valve 175L switches to the first valve position (left valve position), and the hydraulic oil flow from the left main pump 14L to the hydraulic oil tank T1 through the left center bypass oil channel 40L can be closed. At this time, no hydraulic oil flows out of the bottom oil chamber of the boom cylinder 7.This means that the lowering process of the boom 4 is not carried out. This is because a holding valve (not illustrated) prevents the hydraulic oil from flowing out of the bottom-side oil chamber of the boom cylinder 7. Fig. 4 is a diagram illustrating a state of the hydraulic drive system when the controller 30 switches the valve position of the control valve 175L from the third valve position (center valve position) to the first valve position (left valve position). For clarity, a thick solid line in Fig. 4 indicates an increase in hydraulic oil pressure in the left center bypass oil channel 40L, which is blocked by the control valve 175L. When the left center bypass oil channel 40L is closed by the control valve 175L, the hydraulic oil pressure in the left parallel oil channel 42L upstream of the control valve 176L also increases, as illustrated in Fig. 5. Fig. 5 is a diagram illustrating a state of the hydraulic drive system when the hydraulic oil pressure in the left parallel oil channel 42L increases. For clarity, thick solid lines in Fig. 5 indicate an increase in hydraulic oil pressure in the left parallel oil channel 42L in addition to the increase in hydraulic oil pressure in the left center bypass oil channel 40L. Furthermore, if the hydraulic oil pressure in the left central bypass oil channel 40L and the hydraulic oil pressure in the left parallel oil channel 42L increase—that is, if the discharge pressure of the left main pump 14L increases—the hydraulic oil pressure in oil channel 47, which connects the left main pump 14L to the relief valve 55, also increases, as illustrated in Fig. 6. Fig. 6 is a diagram illustrating a state of the hydraulic drive system when the hydraulic oil pressure in oil channel 47 increases. For clarity, thick solid lines in Fig. 6 indicate an increase in hydraulic oil pressure in oil channel 47 in addition to the increase in hydraulic oil pressure in the left central bypass oil channel 40L and the increase in hydraulic oil pressure in the left parallel oil channel 42L. When the pressure of hydraulic oil in the left central bypass oil channel 40L, the pressure of hydraulic oil in the left parallel oil channel 42L, and the pressure of hydraulic oil in oil channel 47 each exceeds a predetermined relief pressure, the hydraulic oil is discharged via the relief valve 55 into the central return oil channel 41C. The hydraulic oil discharged by the relief valve 55 then flows via the central return channel 41C, oil channel 43, and oil channel 45 to the hydraulic oil tank T1. In this way, the controller 30 can maintain the hydraulic load (pump torque) of the main pump 14 in a high state for a desired period of time and can consequently maintain the load at the drive source 11 in a high state. With reference to Fig. 3, the description of the forced high-load process continues. In the illustrated example, after increasing the discharge pressure of the main pump 14, the controller 30 switches the valve position of the shut-off valve (step ST3). In the illustrated example, the controller 30 issues a control command to the electromagnetic valve 32 to increase the pilot pressure acting on the pilot port of the control valve 177. When the pilot pressure acting on the pilot port is increased, the valve position of the control valve 177, as illustrated in Fig. 2, switches to the second valve position (right valve position), and the left parallel oil channel 42L can be closed.At this point, an oil channel section PS1 of the left parallel oil channel 42L, which connects control valve 177 and control valve 176L, is connected to an oil channel PS2, which connects control valve 177 and the left return oil channel 41L. As a result, some of the hydraulic oil in oil channel section PS1 is discharged via oil channel PS2 to the left return oil channel 41L, and the pressure of the hydraulic oil in oil channel section PS1 is reduced. Oil channel PS2 is also referred to as a leakage oil channel. Fig. 7 is a diagram illustrating a state of the hydraulic drive system when the controller 30 switches the valve position of the control valve 177 to the second valve position (right valve position). For clarity, in Fig. 7 a thick dotted line indicates the oil channel section PS1, and thick arrows indicate that some of the hydraulic oil in oil channel section PS1 is discharged via oil channel PS2 to the left return oil channel 41L. When some of the hydraulic oil in oil channel section PS1 is discharged to the left return channel 41L, and the hydraulic oil pressure in oil channel section PS1 decreases, the difference between the hydraulic oil pressure upstream of control valve 176L and the hydraulic oil pressure downstream of control valve 176L (the pressure differential across control valve 176L) decreases. The pressure differential across control valve 176L is the difference between the hydraulic oil pressure upstream of control valve 176L and the hydraulic oil pressure downstream of control valve 176L when the hydraulic oil pressure upstream is greater than the hydraulic oil pressure downstream.The pressure differential across the control valve 176L provides a negative pressure effect on the outflow of hydraulic oil from the upstream to the downstream side of the control valve 176L through a substantially annular gap between a substantially cylindrical spool section of the control valve 176L and a valve block section that has a substantially cylindrical space for receiving the spool section. Furthermore, the negative pressure on the outflow of hydraulic oil also provides a negative pressure effect on the inflow of hydraulic oil into each of the rod-side oil chambers and the bottom-side oil chambers of the arm cylinder 8, thus preventing the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber from becoming substantially equal.Furthermore, preventing the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the arm cylinder 8 from becoming substantially equal has the effect of minimizing the unintended extension of the arm cylinder 8 by the operator, that is, it suppresses the occurrence of an unintended arm closing operation. This is because, when the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the arm cylinder 8 become substantially equal, a piston in the arm cylinder 8, which is a single-rod hydraulic cylinder, moves in a direction in which the bottom-side oil chamber expands due to a difference in the pressure-bearing surfaces. In the illustrated example, the pressure-bearing surface of the piston facing the bottom-side oil chamber is approximately twice the size of the pressure-bearing surface of the piston facing the rod-side oil chamber.Therefore, if the pressure of the rod-side oil chamber and the pressure of the bottom-side oil chamber of the arm cylinder 8 are essentially equal to each other, the thrust required to move the piston towards the cylinder extension side (the arm closing side) is approximately twice the thrust required to move the piston towards the cylinder retraction side (the arm opening side). A holding valve (not illustrated) may be provided between the arm cylinder 8 and the control valve 176 to prevent the arm 5 from moving due to its own weight or the like when arm operation is not performed (a valve to prevent hydraulic oil from flowing out of the arm cylinder 8). As described above, the hydraulic drive system described above provides an effect of minimizing unintentional extension of the arm cylinder 8. This is achieved by ensuring that the hydraulic oil pressure on the upstream side of the control valve 176L is greater than the hydraulic oil pressure on the downstream side of the control valve 176L, for example, during the forced high-load process, which is carried out when the operating device 26 is not actuated. This is because the control 30 can cause some of the hydraulic oil in oil channel section PS1 to flow back to the hydraulic oil tank T1, thus preventing the hydraulic oil pressure in oil channel section PS1 from remaining high and preventing the pressure differential across the control valve 176L from remaining large during the forced high-load process. Furthermore, in the configuration described above, even if hydraulic oil escapes through the substantially annular gap between the substantially cylindrical spool section of the control valve 177 and the valve block section from the upstream side to the downstream side of the control valve 177, the escaped hydraulic oil is discharged via oil channel PS2 to the left return oil channel 41L. Therefore, the hydraulic oil escaping through the substantially annular gap around the control valve 177 does not increase the pressure differential across the control valve 176L. In the example described above, the controller 30 prevents a large pressure differential across the control valve 176L from being maintained by switching the valve position of the control valve 177 to the second valve position (right valve position) after the valve position of the control valve 175L has switched to the first valve position (left valve position). However, the controller 30 can prevent an increase in the pressure differential across the control valve 176L by switching the valve position of the control valve 177 to the second valve position (right valve position) at the same time that the valve position of the control valve 175L is switched to the first valve position (left valve position).Alternatively, the controller 30 can prevent an increase in the pressure differential across the control valve 176L by switching the valve position of the control valve 177 to the second valve position (right valve position) before the valve position of the control valve 175L is switched to the first valve position (left valve position). Furthermore, the control valve 177 is configured to suppress any increase in the differential pressure across the control valve 177 by connecting a port on the downstream side of the control valve 177 to the return oil channel 41. However, the control valve 177 can also be configured so that the port on the downstream side of the control valve 177 is not connected to the return oil channel 41. This is because, for example, if the valve position of the control valve 177 switches to the second valve position (right valve position) at the same time as the valve position of the control valve 175L switches to the first valve position (left valve position), an increase in the differential pressure across the control valve 177 can be prevented.The same applies if the valve position of control valve 177 is switched to the second valve position (right valve position) before the valve position of control valve 175L is switched to the first valve position (left valve position). Furthermore, even if the valve position of control valve 177 is switched to the second valve position (right valve position) after the valve position of control valve 175L has been switched to the first valve position (left valve position), control valve 177 may be configured such that the port on the downstream side of control valve 177 is not connected to the return oil channel 41. This is because, even if the left parallel oil channel 42L is blocked only by control valve 177, the hydraulic drive system can prevent a large pressure differential from being maintained across control valve 176L for an extended period. Returning to Fig. 8, an example configuration of another hydraulic drive system installed in the working machine 100 of Fig. 1 will now be described. Fig. 8 is a diagram illustrating the configuration example of the further hydraulic drive system installed in the working machine 100 of Fig. 1 and corresponds to Fig. 2. The hydraulic drive system illustrated in Fig. 8 differs from the hydraulic drive system illustrated in Fig. 2 in that the hydraulic drive system illustrated in Fig. 8 includes a control valve 177A instead of the control valve 177, and the control valve 177A is not connected to the return oil channel 41. However, the hydraulic drive system illustrated in Fig. 8 is otherwise the same as the hydraulic drive system illustrated in Fig. 2. Therefore, the description of the common parts is omitted, and differences are described in detail below. The control valve 177A is configured to limit the flow rate of hydraulic oil flowing through the left parallel oil channel 42L to the control valve 176L. In the illustrated example, the control valve 177A is configured to switch the valve position in response to a control command from the controller 30 between a first valve position (left valve position), a second valve position (right valve position), and a third valve position (center valve position). In particular, the control valve 177A is in the first valve position, which serves as an output valve position when no opening / closing operation of arm 5 is performed, or when an opening / closing operation of arm 5 is performed independently. The control valve 177A is in the second valve position when a process to intentionally increase the load on the drive source 11 is performed, and the control valve 177A is in the third valve position when a combined operation consisting of an opening / closing operation of arm 5 and a swivel operation, or the like, is performed. In the illustrated example, the control valve 177A is configured such that it is in the third valve position and acts as a swivel priority valve when a combined operation is performed that includes an opening / closing operation of arm 5 and a swivel operation. When a predetermined condition for intentionally increasing the load at the drive source 11 is met and the valve position of the control valve 177A is switched to the second valve position, the control valve 177A closes the left parallel oil channel 42L. Since at this time the oil channel section PS1 of the left parallel oil channel 42L, which connects the control valve 177 and the control valve 176L, is not connected to the return oil channel 41, the pressure of the hydraulic oil in the oil channel section PS1 is maintained at the same pressure as when the valve position of the control valve 177A is switched to the second valve position. Therefore, the control unit 30 can limit the increase in hydraulic oil pressure in oil channel section PS1 to a relief pressure by switching the valve position of control valve 177A to the second valve position before switching the valve position of control valve 175L to the first valve position (left valve position). That is, the control unit 30 can prevent a large pressure differential from being maintained across control valve 176L during the forced high-load process. As described above, the hydraulic drive system illustrated in Fig. 8, similar to the hydraulic drive system illustrated in Fig. 2, provides the effect of minimizing unintentional extension of the arm cylinder 8 caused by an increase in the pressure differential across the control valve 176L during the forced high-load process. In the hydraulic drive system illustrated in Fig. 2, the control valve 177 is configured such that the swivel priority function is implemented by switching from the first valve position to the third valve position, and the forced high-load function is implemented by switching from the first valve position to the second valve position. However, the swivel priority function and the forced high-load function can also be implemented by independent, separate control valves. That is, the third valve position of the control valve 177 can be omitted. In this case, the control valve 177 can be located upstream of a branch point BP1 (see Fig. 2), upstream of a branch point BP2 (see Fig. 2), or upstream of a branch point BP3 (see Fig. 2) in the left parallel oil channel 42L.Alternatively, the control valve 177 can be arranged upstream of the control valve 170 in the left parallel oil channel 42L. Furthermore, the swivel priority function can be implemented by a different control valve that does not have a second valve position. The same applies to the control valve 177A of the hydraulic drive system illustrated in Fig. 8. In other words, the control valve 177 can be implemented by adding a valve position to an existing control valve, such as a pivot priority valve (a control valve already integrated into the hydraulic drive system). Furthermore, the existing control valve can be a different type of control valve than a pivot priority valve, such as a regeneration valve or a regenerative valve. Furthermore, in the embodiment described above, the hydraulic drive system increases the load on the drive source 11 by blocking an oil channel through which hydraulic oil delivered by the left main pump 14L flows, thereby increasing the delivery pressure of the left main pump 14L and increasing the hydraulic load (pump torque). However, the hydraulic drive system can also increase the load on the drive source 11 by blocking an oil channel through which hydraulic oil delivered by the right main pump 14R flows, thereby increasing the delivery pressure of the right main pump 14R and increasing the hydraulic load (pump torque). In this case, the control valve 177 is located in the right parallel oil channel 42R. As described above and illustrated in Fig. 1 and Fig. 2, a working machine 100 according to an embodiment of the present disclosure comprises a lower carriage body 1, an upper pivoting body 3 pivotably mounted on the lower carriage body 1, a hydraulic actuator (arm cylinder 8), a hydraulic pump (main pump 14) configured to supply hydraulic oil to the hydraulic actuator (arm cylinder 8), a first control valve (control valve 175L) provided in a first oil channel (central bypass oil channel 40) connecting the hydraulic pump (main pump 14) and a hydraulic oil tank T1, a second control valve (control valve 176L) provided in a second oil channel (parallel oil channel 42) connecting the hydraulic pump (main pump 14) and the hydraulic actuator (arm cylinder 8), and a third control valve (control valve 177).which is provided between the hydraulic pump (main pump 14) and the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42). The third control valve (control valve 177) is configured to block the second oil channel (parallel oil channel 42) when a load on a drive source 11 is intentionally increased. For example, the third control valve (control valve 177) is configured to block the second oil channel (parallel oil channel 42) when the first oil channel (central bypass oil channel 40) is blocked by the first control valve (control valve 175L) and the second oil channel (parallel oil channel 42) is blocked by the second control valve (control valve 176L). This configuration can block the first control valve (control valve 175L) in the first oil channel (middle bypass oil channel 40) and can block the third control valve (control valve 177), which is located upstream of the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42). Therefore, this configuration can increase the hydraulic load (pump torque) of the hydraulic pump (main pump 14). Furthermore, this configuration can intentionally increase the load on the drive source 11 while suppressing an increase in the pressure differential across the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42). That is, the forced high-load process can be carried out.Since this configuration can further suppress an increase in the pressure differential across the second control valve (control valve 176L), it can suppress the flow of hydraulic oil from the upstream side to the downstream side of the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42). Therefore, this configuration makes it less likely that the hydraulic oil flowing onto the downstream side of the second control valve (control valve 176L) will enter the hydraulic actuator (arm cylinder 8), and consequently minimizes unintended movement of the hydraulic actuator (arm cylinder 8) by the operator. As illustrated in Fig. 2, the third control valve (control valve 177) can further be configured to switch between a first state and a second state. In the first state, hydraulic oil flow from the hydraulic pump (main pump 14) to the second control valve (control valve 176L) can be permitted. In the second state, the flow of hydraulic oil from the hydraulic pump (main pump 14) to the second control valve (control valve 176L) can be blocked, and hydraulic oil flow from the oil channel section PS1, located between the third control valve (control valve 177) and the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42), to the hydraulic oil tank T1 can be permitted.Furthermore, the third control valve (control valve 177) can be configured to adjust the flow rate of the hydraulic oil flowing from the hydraulic pump (main pump 14) to the second control valve (control valve 176L) when the third control valve (control valve 177) is in the first state. For example, the third control valve (control valve 177) can be configured to switch between a first valve position (left valve position) and a second valve position (right valve position). In this case, in the first valve position (left valve position), the flow of hydraulic oil from the hydraulic pump (main pump 14) to the second control valve (control valve 176L) can be permitted.Furthermore, in the second valve position (right valve position), the flow of hydraulic oil from the hydraulic pump (main pump 14) to the second control valve (control valve 176L) can be blocked, and the flow of hydraulic oil from oil channel section PS1, located between the third control valve (control valve 177) and the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42), to the hydraulic oil tank T1 can be permitted. In the illustrated example, control valve 177 has three valve positions, and the first and second valve positions are a left and a right valve position, respectively. However, the first and second valve positions can be any of the three valve positions.For example, the control valve 177 can be configured such that the first valve position and the second valve position are the left valve position and a middle valve position, or the middle valve position and the left valve position. This configuration can provide the effect of executing the forced high-load process while further suppressing an increase in the pressure differential across the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42). This is because this configuration can connect the oil channel section PS1 upstream of the second control valve (control valve 176L) in the second oil channel (parallel oil channel 42) to the return oil channel 41. Specifically, this is because this configuration can reduce the pressure of the hydraulic oil in oil channel section PS1. Furthermore, this is because this configuration can discharge the leaked hydraulic oil to the return oil channel 41, even if the hydraulic oil leaks from the upstream side to the downstream side of the third control valve (control valve 177) in the second oil channel (parallel oil channel 42).This means that this configuration can suppress an increase in hydraulic oil pressure in oil channel section PS1 caused by hydraulic oil leaking from the upstream side to the downstream side of the third control valve (control valve 177). Furthermore, the hydraulic actuator (arm cylinder 8) can be a single-rod hydraulic cylinder, as illustrated in Fig. 1 and Fig. 2. This configuration provides an effect by minimizing unintended movement of the single-rod hydraulic cylinder. This is because this configuration can suppress the flow of hydraulic oil that has exited the downstream side of the second control valve (control valve 176L) into both the bottom-side and rod-side oil chambers of the single-rod hydraulic cylinder, thus preventing the hydraulic oil pressure in the bottom-side oil chamber and the hydraulic oil pressure in the rod-side oil chamber from becoming equal.In particular, this configuration can minimize extension of the single-rod hydraulic cylinder caused by the difference in pressure between the pressure-receiving surfaces on either side of a piston in the cylinder when the hydraulic oil pressure in the bottom-side oil chamber and the hydraulic oil pressure in the rod-side oil chamber become equal. In the illustrated example, the hydraulic drive system is configured to minimize unintended movement of the arm cylinder 8, which is an example of a single-rod hydraulic cylinder; however, it can be configured to minimize unintended movement of the boom cylinder 7 or the bucket cylinder 9. Furthermore, the hydraulic drive system can be configured to minimize unintended movement of a double-rod hydraulic cylinder, or it can be configured to minimize unintended movement of a hydraulic motor. Furthermore, as illustrated in Fig. 2, the third control valve (control valve 177) can be a pivot priority valve. That is, the third control valve (control valve 177) can be located downstream of branch point BP2. By adding a valve position to the pivot priority valve, which is an existing control valve, this configuration minimizes unintended movement of the hydraulic actuator (arm cylinder 8) during the forced high-load process. In other words, this configuration allows for the simple integration of a function into the existing hydraulic drive system that minimizes unintended movement of the hydraulic actuator (arm cylinder 8) during the forced high-load process. As further illustrated in Fig. 2, the hydraulic pump (main pump 14) can comprise a first main pump (left main pump 14L) and a second main pump (right main pump 14R). In this case, the first oil channel (middle bypass oil channel 40) can comprise a first middle bypass oil channel (left middle bypass oil channel 40L) and a second middle bypass oil channel (right middle bypass oil channel 40R). Furthermore, the second oil channel (parallel oil channel 42) can comprise a first parallel oil channel (left parallel oil channel 42L) running parallel to the first middle bypass oil channel (left middle bypass oil channel 40L), and a second parallel oil channel (right parallel oil channel 42R) running parallel to the second middle bypass oil channel (right middle bypass oil channel 40R).The first control valve (control valve 175L) can be provided in the first central bypass oil channel (left central bypass oil channel 40L), and the second control valve (control valve 176L) and the third control valve (control valve 177) can be provided in the first parallel oil channel (left parallel oil channel 42L). This configuration can block the control valve 175L in the left middle bypass oil channel 40L and can block the control valve 177, located upstream of the control valve 176L, in the left parallel oil channel 42L. Therefore, this configuration can increase the hydraulic load (pump torque) of the left main pump 14L. Furthermore, this configuration can selectively increase the load on the drive source 11 while suppressing an increase in the pressure differential across the control valve 176L in the left parallel oil channel 42L. That is, the forced high-load process can be carried out. Since this configuration can also suppress an increase in the pressure differential across the control valve 176L, the escape of hydraulic oil from the upstream to the downstream side of the control valve 176L in the left parallel oil channel 42L can be suppressed.Therefore, this configuration can suppress the flow of hydraulic oil that has escaped to the downstream side of control valve 176L into the arm cylinder 8 and can consequently minimize any movement of the arm cylinder 8 that the operator does not intend. Furthermore, as illustrated in Figs. 1 and 2, the hydraulic actuator can comprise the boom cylinder 7 and the arm cylinder 8. In this case, the first control valve (control valve 175L) can be a spool valve configured to control the flow rate of the hydraulic oil flowing into the boom cylinder 7. The second control valve (control valve 176L) can also be a spool valve configured to control the flow rate of the hydraulic oil flowing into the arm cylinder 8. This configuration can cause control valve 175L, configured to control the flow of hydraulic oil to drive boom cylinder 7, to close the left center bypass oil channel 40L. Furthermore, this configuration can cause control valve 177, located upstream of control valve 176L, configured to control the flow of hydraulic oil to drive arm cylinder 8, to close the left parallel oil channel 42L, thus suppressing any increase in the pressure differential across control valve 176L in the left parallel oil channel 42L. Therefore, this configuration can prevent hydraulic oil from flowing from the upstream to the downstream side of control valve 176L in the left parallel oil channel 42L.Furthermore, this configuration can suppress the flow of hydraulic oil that has escaped to the downstream side of control valve 176L into arm cylinder 8 and can consequently minimize any movement of arm cylinder 8 that the operator does not intend. The embodiment of the present disclosure has been described above. However, the present disclosure is not limited to the embodiment described above. Various modifications, substitutions, and the like are applicable to the embodiment described above without deviating from the scope of the present disclosure. Furthermore, all features described with reference to the embodiment described above can be suitably combined, provided no technical contradiction arises. DESCRIPTION OF REFERENCE MARKS 1 Lower travel body 2 Slewing mechanism 3 Upper slewing body 4 Boom 5 Arm 6 Bucket 7 Boom cylinder 8 Arm cylinder 9 Bucket cylinder 10 Cab 11 Drive source 13 Pump controller 14 Main pump 15 Pilot pump 17 Control valve unit 18 Throttle 19 Control pressure sensor 20 Travel hydraulic motor 20L Left-hand travel hydraulic motor 20R Right-hand travel hydraulic motor 21 Slewing hydraulic motor 21P Oil channel 22 Relief valve 23 Control valve 26 Operating device 28 Output pressure sensor 29 Operating sensor 30 Control 31 Electromagnetic valve 32 Electromagnetic valve 40 Central bypass oil channel 41 Return oil channel 42 Parallel oil channel 43 to 47 Oil channel 45a, 45b Oil channel section 50 Control valve 51 Oil cooler 52 Control valve 53 Filter 55 Relief valve 60 Throttle 100 Working machine 170 to 177, 177A Control valve BP1, BP2, BP3 Branch point PS1 Oil channel section PS2 Oil channel QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature JP 2020-051482

[0005]

Claims

Working machine comprising: a lower carriage; an upper pivoting body configured to be pivotably mounted on the lower carriage; a hydraulic actuator; a hydraulic pump configured to supply hydraulic oil to the hydraulic actuator; a first control valve provided in a first oil channel connecting the hydraulic pump and a hydraulic oil tank; a second control valve provided in a second oil channel connecting the hydraulic pump and the hydraulic actuator; and a third control valve provided between the hydraulic pump and the second control valve in the second oil channel, the third control valve being configured to close the second oil channel when a load on a drive source is intentionally increased. Working machine according to claim 1, wherein the third control valve has a first state and a second state, in the first state a flow of hydraulic oil from the hydraulic pump to the second control valve is permitted, and in the second state the flow of hydraulic oil from the hydraulic pump to the second control valve is blocked, and a flow of hydraulic oil from an oil channel section located between the third control valve and the second control valve in the second oil channel to the hydraulic oil tank is permitted. Working machine according to claim 1, wherein the hydraulic actuator is a single-rod hydraulic cylinder. Working machine according to claim 1, wherein the hydraulic pump comprises a first main pump and a second main pump, the first oil channel comprises a first central bypass oil channel and a second central bypass oil channel, the second oil channel comprises a first parallel oil channel parallel to the first central bypass oil channel and a second parallel oil channel parallel to the second central bypass oil channel, the first control valve is provided in the first central bypass oil channel, and each of the second control valve and the third control valve is provided in the first parallel oil channel. Working machine according to claim 1, wherein the hydraulic actuator comprises a boom cylinder and an arm cylinder, the first control valve is a spool valve configured to control a flow rate of the hydraulic oil flowing into the boom cylinder, and the second control valve is a spool valve configured to control a flow rate of the hydraulic oil flowing into the arm cylinder. Working machine according to claim 1, wherein the third control valve is a pivot priority valve.

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