Hydraulic drive device

Abstract

The purpose of the present invention is to provide a hydraulic drive device capable of improving hydraulic actuator operability by suppressing shock occurring when switching from a state in which pressure has accumulated in a pressure accumulation device via a hydraulic pump to a state of driving a hydraulic actuator via the hydraulic pump. Therefore, when the detection value from a first pressure sensor falls below a predetermined first threshold in a state in which no operation signal has been inputted, a control device causes the hydraulic pump and the pressure accumulation device to communicate with each other in a state in which the hydraulic pump and the hydraulic actuator are cut off from each other and the hydraulic pump and a tank are cut off from each other, and when an operation signal has been inputted in a state in which pressure oil has been supplied from the hydraulic pump to the pressure accumulation device, the control device reduces the opening of the pressure accumulation flow rate control valve and causes the hydraulic pump and the tank to communicate with each other in a state in which the hydraulic pump and the hydraulic actuator are cut off from each other, and causes the hydraulic pump and the hydraulic actuator to communicate with each other after a predetermined time has elapsed or after the pressure of the hydraulic pump has decreased.

Description

Hydraulic drive unit

[0001] The present invention relates to a hydraulic drive system mounted on a construction machine such as a hydraulic excavator.

[0002] Patent Document 1, for example, is a prior art document disclosing a hydraulic drive system mounted on a construction machine such as a hydraulic excavator. Patent Document 1 discloses a hydraulic drive system including a first hydraulic pump, a first hydraulic actuator driven by a supply of pressurized oil from the first hydraulic pump, a first pipeline supplying pressurized oil from the first hydraulic pump to the first hydraulic actuator, a first pump flow rate control device installed in the first pipeline and controlling the flow of pressurized oil from the first hydraulic pump to the first hydraulic actuator, a first accumulator, and a first accumulator flow rate control device controlling the flow of pressurized oil from the first hydraulic pump to the first accumulator. The hydraulic drive system further includes a first accumulator flow rate supply device that supplies pressurized oil stored in the first accumulator to the first hydraulic actuator via a second pipeline separate from the first pipeline. In this hydraulic drive system, the hydraulic actuator can be driven by the hydraulic pump or the accumulator, and a pressure-reduced accumulator can be charged (accumulated) by the hydraulic pump.

[0003] International Publication No. 2018 / 061165

[0004] However, in the hydraulic drive system described in Patent Document 1, when switching from a state in which the hydraulic pump is accumulating pressure in the accumulator to a state in which the hydraulic pump is driving the hydraulic actuator, a shock larger than the pressure difference between the hydraulic pump and the hydraulic actuator occurs, which raises concerns that the good operability desired by the operator cannot be ensured.

[0005] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a hydraulic drive system that can improve the operability of a hydraulic actuator by suppressing the shock that occurs when switching from a state in which a hydraulic pump accumulates pressure in an accumulator device to a state in which the hydraulic pump drives a hydraulic actuator.

[0006] In order to achieve the above object, the present invention provides a hydraulic drive system comprising a tank for storing hydraulic oil, a hydraulic pump for drawing in and discharging hydraulic oil from the tank, a hydraulic actuator, an actuator flow control valve for controlling the flow rate of pressurized oil supplied from the hydraulic pump to the hydraulic actuator, a tank flow control valve for controlling the flow rate of pressurized oil discharged from the hydraulic pump to the tank, a pressure accumulator for accumulating pressure in the pressure accumulator, a first pressure sensor for detecting pressure in the pressure accumulator, an operating device for operating the hydraulic actuator, and a control device that receives an operation signal from the operating device and a detection value of the first pressure sensor and outputs control signals to the actuator flow control valve, the tank flow control valve, and the pressure accumulator flow control valve, When the detection value of the first pressure sensor falls below a predetermined first threshold value with no operation signal input, the actuator flow control valve disconnects the hydraulic pump from the hydraulic actuator and the tank flow control valve disconnects the hydraulic pump from the tank, while the accumulator flow control valve communicates the hydraulic pump with the accumulator device, thereby supplying pressurized oil from the hydraulic pump to the accumulator device, and when an operation signal is input from the operating device with pressurized oil being supplied from the hydraulic pump to the accumulator device, the actuator flow control valve disconnects the hydraulic pump from the hydraulic actuator, while the tank flow control valve communicates the hydraulic pump with the tank, and after a predetermined time has elapsed or the pressure of the hydraulic pump has decreased, the actuator flow control valve communicates the hydraulic pump with the hydraulic actuator.

[0007] According to the present invention, when an operation signal is input from an operating device while the hydraulic pump is accumulating pressure in the accumulator, the hydraulic pump and the hydraulic actuator are connected via the actuator flow control valve after a predetermined time has elapsed or after the pressure of the hydraulic pump has decreased, so it is possible to prevent the hydraulic pump and the hydraulic actuator from connecting via the actuator flow control valve in a state where the pressure of the hydraulic pump greatly exceeds the pressure of the hydraulic actuator. This suppresses shock when switching from a state where the hydraulic pump is accumulating pressure in the accumulator to a state where the hydraulic pump is driving the hydraulic actuator, making it possible to improve the operability of the hydraulic actuator.

[0008] 1 is a perspective view of a hydraulic excavator equipped with a hydraulic drive system according to a first embodiment of the present invention; FIG. 2 is a hydraulic circuit diagram of the hydraulic drive system according to the first embodiment of the present invention; FIG. 3 is a flowchart showing processing by a control device relating to the operation of each control valve according to the first embodiment of the present invention; FIG. 4 is a time chart showing the operation of each control valve according to the first embodiment of the present invention in comparison with a case where the control of this embodiment is not applied (prior art); FIG. 5 is a modified example of the hydraulic circuit diagram of the hydraulic drive system according to the first embodiment of the present invention; FIG. 6 is a hydraulic circuit diagram of a hydraulic drive system according to a second embodiment of the present invention; and FIG. 7 is a flowchart showing processing by a control device relating to the operation of each control valve according to the second embodiment of the present invention.

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In each drawing, the same reference numerals are used to designate the same components, and redundant description will be omitted where appropriate.

[0010] 1 is a perspective view of a hydraulic excavator equipped with a hydraulic drive system according to a first embodiment of the present invention. Note that the application of the hydraulic drive system according to the present invention is not limited to hydraulic excavators, but can also be applied to other construction machines such as crawlers and wheel loaders.

[0011] 1 , a hydraulic excavator 100 includes a lower traveling structure 101, an upper rotating structure 102, and a working device 103. The upper rotating structure 102 is driven by a traveling motor (hydraulic actuator) 104. The upper rotating structure 102 is provided on the lower traveling structure 101 so as to be able to rotate, and is driven by a swing motor (hydraulic actuator) not shown.

[0012] The work device 103 has a boom 105, an arm 106, and a bucket 107. The boom 105 is rotatably supported on the upper rotating body 102 and is driven by a boom cylinder 108 (hydraulic actuator). The arm 106 is rotatably supported on the boom 105 and is driven by an arm cylinder 109 (hydraulic actuator). The bucket 107 is rotatably supported on the arm 106 and is driven by a bucket cylinder 110 (hydraulic actuator).

[0013] Fig. 2 is a hydraulic circuit diagram of a hydraulic drive system mounted on the hydraulic excavator 100. Note that Fig. 2 shows only the parts related to driving the boom cylinder 108, and omits the parts related to driving the other actuators.

[0014] 2, the hydraulic drive system 200 includes an engine 1 as a power source, a hydraulic pump 2 driven by the power of the engine 1, a boom cylinder 108 driven by pressure oil supplied from the hydraulic pump 2, a center bypass type control valve 3 (actuator flow control valve) that controls the flow rate of pressure oil supplied from the hydraulic pump 2 to the boom cylinder 108 and the flow rate of pressure oil discharged from the boom cylinder 108 to the tank 8, an accumulator 5 (pressure accumulator) that accumulates pressure oil discharged from the hydraulic pump 2, a control valve 6 (accumulator flow control valve) that controls the flow rate of pressure oil supplied from the hydraulic pump 2 to the accumulator 5, and an accumulator. The hydraulic system is equipped with a control valve 7 that controls the flow rate of pressurized oil supplied from the accumulator 5 to the boom cylinder 108 and the flow rate discharged from the boom cylinder 108 to the tank 8, a control valve 9 (tank flow rate control valve) that controls the flow rate of pressurized oil discharged from the hydraulic pump 2 to the tank 8, a boom operation lever 10 (operation device) that instructs the operation of the boom cylinder 108, a pressure sensor 11 (first pressure sensor) that detects the pressure of the accumulator 5, and a control device 12 that adjusts the discharge flow rate of the hydraulic pump 2, the rotation speed of the engine 1, and the flow rate or opening of the control valves 3, 6, 7, and 9 according to the amount of operation of the boom operation lever 10 and the detection value of the pressure sensor 11.

[0015] The discharge port of hydraulic pump 2 is connected to tank 8 via center bypass passage 13. Control valves 3 and 9 are arranged in center bypass passage 13, in this order from upstream to downstream. Control valve 3 is an open-center type flow control valve, and control valve 9 is a center bypass cut valve. Control valve 3 is connected to boom cylinder 108 via actuator passages 14 and 15. The meter port of control valve 3 is connected to center bypass passage 13 via check valve 16. Check valve 16 prevents backflow of pressure oil from boom cylinder 108 to center bypass passage 13.

[0016] The control device 12 is equipped with a computing device such as a CPU, storage devices such as ROM and RAM, and an input / output interface for inputting and outputting signals to and from external devices, and realizes the following functions by executing programs stored in the ROM or the like. When an operation signal is input from the boom operation lever 10, the control device 12 increases the discharge flow rate of the hydraulic pump 2 in accordance with the amount of lever operation and switches at least one of the control valves 3, 7 to a position corresponding to the direction of lever operation. As a result, pressure oil is supplied from the hydraulic pump 2 or the accumulator 5 to one side (the bottom side or the rod side) of the boom cylinder 108, and pressure oil is discharged from the other side (the rod side or the bottom side) of the boom cylinder 108 to the tank 8, thereby extending or retracting the boom cylinder 108. Furthermore, when the pressure in the accumulator 5 (the value detected by the pressure sensor 11) decreases, the control device 12 causes the control valve 6 to communicate between the hydraulic pump 2 and the accumulator 5, and the pressure oil discharged from the hydraulic pump 2 is accumulated in the accumulator 5.

[0017] Figure 3 is a flowchart showing the processing of the control device 12 regarding the operation of the control valves 3, 6, and 9. The flow shown in Figure 3 starts when a key switch (not shown) is turned to the ON position. The normal position of the control valve 3 (the position when no control signal is input) is the neutral position (closed position), the normal position of the control valve 6 is the closed position, and the normal position of the control valve 9 is the open position.

[0018] First, in step S101, it is determined whether or not an operation signal has been input from boom operation lever 10. If the determination result in step S101 is Yes, control valve 9 is held in the open position, control valve 6 is held in the closed position, and the opening of control valve 3 is adjusted in accordance with the signal from boom operation lever 10 (step S102). As a result, pressure oil at a flow rate corresponding to the signal from boom operation lever 10 is supplied from hydraulic pump 2 via control valve 3 to one side (bottom side or rod side) of boom cylinder 108, an excess flow rate of pressure oil discharged from hydraulic pump 2 is discharged to tank 8 via control valves 3 and 9, and pressure oil discharged from the other side (rod side or bottom side) of boom cylinder 108 is discharged to tank 8 via control valve 3. After step S102 is completed, the process returns to step S101.

[0019] If the determination result in step S101 is No, it is determined whether the detection value of the pressure sensor 11 (the pressure of the accumulator 5) is below a predetermined threshold value (first threshold value) (step S103). If the determination result in step S103 is No, the control valve 9 is held in the open position, the control valve 6 is held in the closed position, and the control valve 3 is held in the neutral position (step S104). As a result, when the pressure of the accumulator 5 is high, pressure is not accumulated in the accumulator 5, and the pressure oil discharged from the hydraulic pump 2 is discharged to the tank 8 via the control valves 3 and 9, thereby preventing an unnecessary pressure increase in the hydraulic pump 2. After step S104 is completed, the process returns to step S101.

[0020] If the determination result in step S103 is Yes, the control valve 3 is held in the neutral position, the control valve 9 is switched to the closed position, and the control valve 6 is switched to the open position (step S105). As a result, the pressure oil discharged from the hydraulic pump 2 is accumulated in the accumulator 5.

[0021] Following step S105, it is determined whether or not an operation signal has been input from the boom operation lever 10 (step S106). If the determination result in step S106 is No, the process returns to step S103. As a result, in a state in which an operation signal has not been input from the boom operation lever 10, pressure continues to accumulate in the accumulator 5 until the pressure in the accumulator 5 reaches the first threshold value.

[0022] If the determination result in step S106 is Yes, the control valve 3 is held in the neutral position, the control valve 6 is switched to the closed position, and the control valve 9 is switched to the open position (step S107). As a result, the accumulation of pressure in the accumulator 5 stops and the pressure oil discharged from the hydraulic pump 2 is discharged to the tank 8 via the control valves 3 and 9, thereby reducing the pressure of the hydraulic pump 2. Note that in step S107, the control valve 6 does not necessarily have to be fully closed, and it is sufficient if it is reduced to an opening that is sufficiently small compared to the opening of the control valve 9.

[0023] Following step S107, the process waits for a predetermined time (step S108), and then proceeds to step S102. As a result, the hydraulic pump 2 and boom cylinder 108 are connected via control valve 3 after the pressure in hydraulic pump 2 has dropped, thereby suppressing shock when boom cylinder 108 starts to operate. The predetermined time here refers to the time from when a control signal for switching control valve 9 to the open position is output until control valve 9 actually switches to the open position, and is determined, for example, by trial and error using an actual machine, but is thought to be approximately the same as the response time of a typical solenoid valve (around 100 msec).

[0024] 4 is a time chart illustrating the operation of the control valves 3, 6, and 9 in this embodiment, compared with a case where the control of this embodiment is not applied (prior art). In the prior art, the control valve 6 is switched to the open position at time t1 to store pressure in the accumulator 5. An operation signal is input from the boom control lever 10 at time t2, and immediately thereafter, the control valve 3 is switched to the open position. As a result, the hydraulic pump 2 and the boom cylinder 108 are communicated in a state where the pressure of the hydraulic pump 2 (pump pressure) greatly exceeds the pressure of the boom cylinder 108 (cylinder pressure). This causes a momentary increase in cylinder pressure (between time t2 and time t3), resulting in a shock. In contrast, in this embodiment, the control valve 3 is not switched to the open position between time t2 and time t3 (the predetermined time in step S108), and is switched to the open position at time t3 after the pump pressure has dropped. This prevents the cylinder pressure from rising, and therefore no shock occurs.

[0025] (Summary) In the first embodiment, the hydraulic drive system 200 includes a tank 8 that stores hydraulic oil, a hydraulic pump 2 that absorbs and discharges hydraulic oil from the tank 8, a hydraulic actuator 108, an actuator flow control valve 3 that controls the flow rate of pressure oil supplied from the hydraulic pump 2 to the hydraulic actuator 108, a tank flow control valve 9 that controls the flow rate of pressure oil discharged from the hydraulic pump 2 to the tank 8, an accumulator 5 that accumulates pressure in the pressure accumulator 5, an accumulator flow control valve 6 that controls the flow rate of pressure oil supplied from the hydraulic pump 2 to the accumulator 5, a first pressure sensor 11 that detects the pressure of the accumulator 5, an operation device 10 for operating the hydraulic actuator 108, and a control device 12 that receives an operation signal from the operation device 10 and a detection value of the first pressure sensor 11 and outputs control signals to the actuator flow control valve 3, the tank flow control valve 9, and the accumulator flow control valve 6. When the detection value of the first pressure sensor 11 falls below a predetermined first threshold value with no operation signal input from the operation device 10, the actuator flow control valve 3 isolates the hydraulic pump 2 from the hydraulic actuator 108 and the tank flow control valve 9 isolates the hydraulic pump 2 from the tank 8, while the accumulator flow control valve 6 connects the hydraulic pump 2 to the accumulator 5, thereby supplying pressurized oil discharged from the hydraulic pump 2 to the accumulator 5, and when an operation signal is input from the operation device 10 with pressurized oil being supplied from the hydraulic pump 2 to the accumulator 5, the actuator flow control valve 3 isolates the hydraulic pump 2 from the hydraulic actuator 108, while the opening of the accumulator flow control valve 6 is reduced and the tank flow control valve 9 connects the hydraulic pump 2 to the tank 8, and after a predetermined time has elapsed or the pressure of the hydraulic pump 2 has decreased, the actuator flow control valve 3 connects the hydraulic pump 2 to the hydraulic actuator 108.

[0026] According to the first embodiment configured as described above, when an operation signal is input from the operating device 10 while the hydraulic pump 2 is accumulating pressure in the accumulator 5, the hydraulic pump 2 and the hydraulic actuator 108 communicate with each other via the actuator flow control valve 3 after a predetermined time has elapsed or after the pressure of the hydraulic pump 2 has dropped, so it is possible to prevent the hydraulic pump 2 from communicating with the hydraulic actuator 108 via the actuator flow control valve 3 in a state where the pressure of the hydraulic pump 2 greatly exceeds the pressure of the hydraulic actuator 108. This suppresses shock when switching from a state where the hydraulic pump 2 is accumulating pressure in the accumulator 5 to a state where the hydraulic pump 2 drives the hydraulic actuator 108, making it possible to improve the operability of the hydraulic actuator 108.

[0027] In the first embodiment, the actuator flow control valve 3 is an open-center type flow control valve provided in the center bypass flow path 13, and the tank flow control valve 9 is a center bypass cut valve provided in the center bypass flow path 13. As a result, in the hydraulic drive system 200 having an open-center type hydraulic circuit configuration, shock is suppressed when switching from a state in which the hydraulic pump 2 accumulates pressure in the accumulator 5 to a state in which the hydraulic pump 2 drives the hydraulic actuator 108, thereby improving the operability of the hydraulic actuator 108. Note that the actuator flow control valve is not limited to the open-center type flow control valve 3, and may be a closed-center type flow control valve 3A as shown in the modified example of FIG. 5 . In this case, a bleed-off valve 9A that discharges excess flow from the hydraulic pump 2 to the tank 8 corresponds to the tank flow control valve.

[0028] The second embodiment of the present invention will be described, focusing on the differences from the first embodiment.

[0029] 6 is a hydraulic circuit diagram of the hydraulic drive system 200 in this embodiment. A pressure sensor 17 (second pressure sensor) that detects the pressure of the hydraulic pump 2 is provided at the most upstream of the center bypass flow path 13, and a pressure sensor 18 (third pressure sensor) that detects the bottom side pressure of the boom cylinder 108 is provided in the actuator flow path 14 that connects the control valve 3 and the bottom side of the boom cylinder 108. The detected values ​​of the pressure sensors 17 and 18 are input to the control device 12.

[0030] 7 is a flowchart showing the process of the control device 12 in this embodiment. In FIG. 7, the difference from the first embodiment (shown in FIG. 4) is that step S108A is executed instead of step S108.

[0031] In step S108A, it is determined whether the value obtained by subtracting the value detected by pressure sensor 18 (the bottom-side pressure of boom cylinder 108) from the value detected by pressure sensor 17 (the pressure of hydraulic pump 2) is below a predetermined threshold (second threshold). The second threshold is a value that indicates how much the pressure of hydraulic pump 2 can exceed the bottom-side pressure of boom cylinder 108 without causing a shock, and is determined, for example, by trial and error using an actual machine. If the determination result in step S108A is No, the process returns to step S107, and if the determination result in step S108A is Yes, the process proceeds to step S102. As a result, the hydraulic pump 2 and the boom cylinder 108 communicate with each other via control valve 3 at the timing when the pressure of hydraulic pump 2 drops to near the bottom-side pressure of boom cylinder 108, making it possible to suppress a response delay of the boom cylinder 108.

[0032] (Summary) In the second embodiment, the hydraulic drive system 200 includes a second pressure sensor 17 that detects the pressure of the hydraulic pump 2 and a third pressure sensor 18 that detects the pressure of the hydraulic actuator 108, and when an operation signal is input from the operating device 10 while pressurized oil is being supplied from the hydraulic pump 2 to the pressure accumulator 5, the control device 12 reduces the opening of the accumulator flow control valve 6 while the actuator flow control valve 3 blocks the hydraulic pump 2 from the hydraulic actuator 108, and causes the tank flow control valve 9 to communicate with the hydraulic pump 2 and the tank, and after the value obtained by subtracting the detection value of the second pressure sensor 17 from the detection value of the third pressure sensor 18 falls below a predetermined second threshold, causes the actuator flow control valve 3 to communicate with the hydraulic pump 2 and the hydraulic actuator 108.

[0033] In the second embodiment configured as described above, as in the first embodiment, shock is suppressed when switching from a state in which the hydraulic pump 2 accumulates pressure in the accumulator 5 to a state in which the hydraulic pump 2 drives the hydraulic actuator 108, thereby improving the operability of the hydraulic actuator 108. Furthermore, the hydraulic pump 2 and the hydraulic actuator 108 communicate with each other via the actuator flow control valve 3 at the timing when the pressure of the hydraulic pump 2 drops to near the pressure of the hydraulic actuator 108, so that a response delay of the hydraulic actuator 108 can be suppressed.

[0034] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments are presented to clearly explain the present invention, and are not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0035] 1...engine, 2...hydraulic pump, 3...control valve (flow control valve, actuator flow control valve), 3A...flow control valve (actuator flow control valve), 5...accumulator (pressure accumulation device), 6...control valve (pressure accumulation flow control valve), 7...control valve, 8...tank, 9...control valve (center bypass cut valve, tank flow control valve), 9A...bleed-off valve (tank flow control valve), 10...boom operation lever (operation device), 11...pressure sensor (first pressure sensor), 12...control device, 13...center bypass flow path, 14, 15...A Actuator flow path, 16...check valve, 17...pressure sensor (second pressure sensor), 18...pressure sensor (third pressure sensor), 100...hydraulic excavator, 101...lower traveling body, 102...upper rotating body, 103...working device, 104...travel motor (hydraulic actuator), 105...boom, 106...arm, 107...bucket, 108...boom cylinder (hydraulic actuator), 109...arm cylinder (hydraulic actuator), 110...bucket cylinder (hydraulic actuator), 200...hydraulic drive unit.

Claims

1. A hydraulic drive system comprising: a tank for storing hydraulic oil; a hydraulic pump for absorbing and discharging hydraulic oil from the tank; a hydraulic actuator; an actuator flow control valve for controlling the flow rate of pressurized oil supplied from the hydraulic pump to the hydraulic actuator; a tank flow control valve for controlling the flow rate of pressurized oil discharged from the hydraulic pump to the tank; an accumulator for accumulating the pressurized oil discharged from the hydraulic pump; a pressure accumulator flow control valve for controlling the flow rate of pressurized oil supplied from the hydraulic pump to the accumulator; a first pressure sensor for detecting the pressure of the accumulator; an operating device for operating the hydraulic actuator; and a control device which receives an operating signal from the operating device and a detection value of the first pressure sensor, and outputs control signals to the actuator flow control valve, the tank flow control valve and the pressure accumulator flow control valve, a pressure accumulator flow control valve communicating with the hydraulic pump and the tank when an operation signal is input from the operating device, the actuator flow control valve disconnects the hydraulic pump from the hydraulic actuator and the tank flow control valve disconnects the hydraulic pump from the tank when the hydraulic pump is connected to the accumulator, thereby supplying pressurized oil from the hydraulic pump to the accumulator; and when an operation signal is input from the operating device, the actuator flow control valve disconnects the hydraulic pump from the hydraulic actuator and the tank flow control valve connects the hydraulic pump and the tank when the hydraulic pump is connected to the accumulator, the actuator flow control valve connects the hydraulic pump and the tank when the hydraulic pump is connected to the accumulator.

2. A hydraulic drive system as claimed in claim 1, further comprising a second pressure sensor which detects the pressure of the hydraulic pump, and a third pressure sensor which detects the pressure of the hydraulic actuator, wherein when an operation signal is input from the operating device while pressurized oil is being supplied from the hydraulic pump to the accumulator device, the control device reduces the opening of the accumulator flow control valve while isolating the hydraulic pump from the hydraulic actuator with the actuator flow control valve and connects the hydraulic pump to the tank with the tank flow control valve, and connects the hydraulic pump to the hydraulic actuator with the actuator flow control valve after a value obtained by subtracting the detection value of the second pressure sensor from the detection value of the third pressure sensor falls below a predetermined second threshold value.

3. A hydraulic drive system as claimed in claim 1, characterized in that the actuator flow control valve is an open centre type flow control valve provided in a centre bypass flow passage, and the tank flow control valve is a centre bypass cut valve provided in the centre bypass flow passage.

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