CN111936751B

CN111936751B - Hydraulic drive system for construction machine

Info

Publication number: CN111936751B
Application number: CN201880092456.1A
Authority: CN
Inventors: 村冈英泰; 能势知道; 木下敦之; 陵城孝志; 坂本守行
Original assignee: Kawasaki Jukogyo KK
Current assignee: Kawasaki Motors Ltd
Priority date: 2018-05-21
Filing date: 2018-05-21
Publication date: 2023-08-01
Anticipated expiration: 2038-05-21
Also published as: CN111936751A; GB202017037D0; WO2019224877A1; GB2585813B; GB2585813A

Abstract

A hydraulic drive system for a construction machine is provided with: the control device controls the swing direction switching valve by switching the swing direction switching valve from the neutral position to the operating position at which the discharge side supply and discharge line and the tank line communicate with each other when the swing speed detected by the speed detector is lower than a first threshold value and by switching the swing direction switching valve from the operating position to the neutral position when the swing speed detected by the speed detector is lower than a second threshold value.

Description

Hydraulic drive system for construction machine

Technical Field

The present invention relates to a hydraulic drive system for a construction machine.

Background

A hydraulic drive system including a swing motor for swinging a swing body is mounted in a construction machine such as a hydraulic excavator (shodel) or a hydraulic crane (crane) (for example, refer to patent document 1). The working fluid is supplied from the pump to the swing motor through the swing direction switching valve.

Specifically, the swing direction switching valve is connected to the swing motor through a pair of supply and discharge lines. The turning direction switching valve switches (shift) between a neutral position in which a pair of supply and discharge lines are blocked (block) and an operating position in which one of the pair of supply and discharge lines communicates with the pump line and the other of the pair of supply and discharge lines communicates with the tank line. The turning direction switching valve is switched from the neutral position to the operating position when the lever of the turning operation device is tilted, and has an opening corresponding to the tilting angle (turning operation amount) of the lever.

However, when the revolving operation lever is returned to the neutral state in order to stop the revolving body during revolving, the pair of supply and discharge lines are blocked by the revolving direction switching valve. Therefore, when the revolution speed of the revolving unit is reduced to zero, there may be a so-called rollback phenomenon in which a pressure difference remains between the discharge-side supply/discharge line, which is in a closed state, and the supply-side supply/discharge line, which directly flows from the reservoir into the working fluid during revolution reduction, and the revolving unit is reversed by the pressure difference, and the magnitude of the pressure difference between the supply/discharge lines is reversed by the reversal, so that the revolving unit is further reversed.

In the hydraulic drive system disclosed in patent document 1, an anti-reverse valve is used to prevent such a hunting phenomenon.

Prior art literature:

patent literature:

patent document 1: japanese patent laid-open No. 9-310701.

Disclosure of Invention

Problems to be solved by the invention:

however, in the case of using an anti-reverse valve as in patent document 1, the size of the swing circuit may be large and the cost may be high.

Therefore, an object of the present invention is to prevent the hunting phenomenon at the time of stopping the swing without using an anti-reverse valve.

Means for solving the problems:

in order to solve the above-described problems, the present invention provides, from a first aspect, a hydraulic drive system for a construction machine, comprising: a rotation motor for rotating the rotation body; a swing operation device including an operation lever for outputting a swing operation signal corresponding to a tilting angle of the operation lever; a swing direction switching valve connected to the swing motor through a pair of supply and discharge lines, the swing direction switching valve including a spool (spool) and a driving portion that receives a command current and drives the spool, the swing direction switching valve switching between a neutral position that blocks the pair of supply and discharge lines and an operating position that communicates one of the pair of supply and discharge lines with a pump line and the other with a tank line; a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger; and a speed detector for detecting a rotation speed of the rotator; the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to a neutral state, the swing direction switching valve is switched from a neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the swing speed detected by the speed detector is lower than a first threshold value, and is switched from the operating position to the neutral position when the swing speed detected by the speed detector is lower than a second threshold value smaller than the first threshold value.

According to the above configuration, when the swing is stopped (when the operation lever of the swing operation device is returned to the neutral state), the swing direction switching valve is switched to the operation position when the swing speed is lower than the first threshold value. This reduces the pressure in the discharge-side supply/discharge line rapidly, thereby reducing the pressure difference between the supply/discharge lines and preventing the rotation body from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve.

The present invention also provides, from a second side, a hydraulic drive system for a construction machine, comprising: a rotation motor for rotating the rotation body; a swing operation device including an operation lever for outputting a swing operation signal corresponding to a tilting angle of the operation lever; a swing direction switching valve connected to the swing motor through a pair of supply and discharge lines, the swing direction switching valve including a spool and a driving portion that receives a command current and drives the spool, the swing direction switching valve switching between a neutral position that blocks the pair of supply and discharge lines and an operating position that communicates one of the pair of supply and discharge lines with a pump line and the other with a tank line; a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger; a speed detector for detecting a rotation speed of the rotator; and a pair of pressure sensors for detecting inflow pressure and outflow pressure of the swing motor; the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to a neutral state, the swing direction switching valve is switched from a neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the outflow pressure of the swing motor detected by one of the pair of pressure sensors is lower than a first threshold value, and is switched from the operating position to the neutral position when the swing speed detected by the speed detector is lower than a second threshold value.

According to the above configuration, when the swing is stopped (when the operation lever of the swing operation device is returned to the neutral state), the swing direction switching valve is switched to the operation position when the outflow pressure of the swing motor is lower than the first threshold value. This reduces the pressure in the discharge-side supply/discharge line rapidly, thereby reducing the pressure difference between the supply/discharge lines and preventing the rotation body from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve.

In a third aspect, the present invention provides a hydraulic drive system for a construction machine, comprising: a rotation motor for rotating the rotation body; a swing operation device including an operation lever for outputting a swing operation signal corresponding to a tilting angle of the operation lever; a swing direction switching valve connected to the swing motor through a pair of supply and discharge lines, the swing direction switching valve including a spool and a driving portion that receives a command current and drives the spool, the swing direction switching valve switching between a neutral position that blocks the pair of supply and discharge lines and an operating position that communicates one of the pair of supply and discharge lines with a pump line and the other with a tank line; a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger; a speed detector for detecting a rotation speed of the rotator; and a pair of pressure sensors for detecting inflow pressure and outflow pressure of the swing motor; the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to the neutral state, the swing direction switching valve is switched from the neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the swing speed detected by the speed detector is lower than a threshold value, and then the swing direction switching valve is set to an opening degree corresponding to a pressure difference between the inflow pressure and the outflow pressure of the swing motor detected by the pair of pressure sensors.

According to the above configuration, when the swing is stopped (when the operation lever of the swing operation device is returned to the neutral state), the swing direction switching valve is switched to the operation position when the swing speed is lower than the threshold value. This reduces the pressure in the discharge-side supply/discharge line rapidly, thereby reducing the pressure difference between the supply/discharge lines and preventing the rotation body from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve. In the above configuration, the opening degree corresponding to the differential pressure between the inflow pressure and the outflow pressure of the swing motor is set when the swing direction switching valve is switched to the operating position, so that the differential pressure between the supply and discharge pipes can be suppressed to be as small as possible.

Further, according to a fourth aspect of the present invention, there is provided a hydraulic drive system for a construction machine, comprising: a rotation motor for rotating the rotation body; a swing operation device including an operation lever for outputting a swing operation signal corresponding to a tilting angle of the operation lever; a swing direction switching valve connected to the swing motor through a pair of supply and discharge lines, the swing direction switching valve including a spool and a driving portion that receives a command current and drives the spool, the swing direction switching valve switching between a neutral position that blocks the pair of supply and discharge lines and an operating position that communicates one of the pair of supply and discharge lines with a pump line and the other with a tank line; a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger; and a pair of pressure sensors for detecting inflow pressure and outflow pressure of the swing motor; the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to the neutral state, if the outflow pressure of the swing motor detected by one of the pair of pressure sensors is lower than a threshold value, the swing direction switching valve is switched from the neutral position to an operating position at which the discharge-side supply/discharge line and the tank line communicate, and then the swing direction switching valve is set to an opening degree corresponding to the differential pressure between the inflow pressure and the outflow pressure of the swing motor detected by the pair of pressure sensors.

According to the above configuration, when the swing is stopped (when the operation lever of the swing operation device is returned to the neutral state), the swing direction switching valve is switched to the operation position when the outflow pressure of the swing motor is lower than the threshold value. This reduces the pressure in the discharge-side supply/discharge line rapidly, thereby reducing the pressure difference between the supply/discharge lines and preventing the rotation body from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve. In the above configuration, the opening degree corresponding to the differential pressure between the inflow pressure and the outflow pressure of the swing motor is set when the swing direction switching valve is switched to the operating position, so that the differential pressure between the supply and discharge pipes can be suppressed to be as small as possible.

In the third and fourth side surfaces, the control device may control the turning direction switching valve as follows: the control lever of the swing control device returns to a neutral state, and after the swing direction switching valve is switched to an operating position in which the discharge-side supply/discharge line and the tank line are connected, the swing direction switching valve is switched to an operating position in which the supply-side supply/discharge line and the tank line are connected when the inflow pressure of the swing motor is higher than the outflow pressure. According to this structure, even if the rotation body is reversed when the rotation is stopped, further reverse rotation can be prevented thereafter.

The invention has the following effects:

according to the present invention, the backlash phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve.

Drawings

Fig. 1 is a schematic configuration diagram of a hydraulic drive system of a construction machine according to a first embodiment of the present invention;

fig. 2 is a side view of an excavator as an example of a construction machine;

fig. 3A to 3D are graphs showing the time-dependent change in valve body displacement of the turning direction switching valve in the first embodiment, 3A in fig. 3 showing the time-dependent change in turning speed in fig. 3B, 3C in fig. 3 showing the time-dependent change in inflow pressure of the turning motor, and 3D in fig. 3 showing the time-dependent change in outflow pressure of the turning motor;

fig. 4 is a schematic configuration diagram of a hydraulic drive system according to a modification of the first embodiment;

fig. 5A to 5D are graphs showing the time-dependent change in the spool displacement of the turning direction switching valve in the second embodiment, 5A in fig. 5 shows the time-dependent change in the turning speed in fig. 5B, 5C in fig. 5 shows the time-dependent change in the inflow pressure of the turning motor, and 5D in fig. 5 shows the time-dependent change in the outflow pressure of the turning motor.

Detailed Description

(first embodiment)

Fig. 1 shows a hydraulic drive system 1 of a construction machine according to a first embodiment of the present invention, and fig. 2 shows a construction machine 10 on which the hydraulic drive system 1 is mounted. The construction machine 10 shown in fig. 2 is a hydraulic excavator, but the present invention is also applicable to other construction machines such as a hydraulic crane.

The construction machine 10 shown in fig. 2 is self-propelled, and includes a traveling body 75 and a revolving body 76 rotatably supported by the traveling body 75. The revolving unit 76 is provided with a cabin (cabin) including a steering seat, and is connected to a boom (boom). An arm (arm) is connected to the tip end of the boom, and a bucket (bucket) is connected to the tip end of the arm. However, the construction machine 10 may not be self-propelled.

The hydraulic drive system 1 includes a boom cylinder 71, an arm cylinder 72, and a bucket cylinder 73 (actuator) shown in fig. 2, and includes a swing motor 4 and a pair of left and right travel motors, not shown, shown in fig. 1 as hydraulic actuators. The turning motor 4 turns the turning body 76. As shown in fig. 1, the hydraulic drive system 1 includes a pump 2 that supplies the hydraulic fluid to the actuators. In fig. 1, hydraulic actuators other than the swing motor 4 are omitted for simplicity of the drawing.

The hydraulic drive system 1 further includes a turning direction switching valve 3 for controlling supply and discharge of the working fluid to and from the turning motor 4, a turning operation device 5 including an operation lever 51 for receiving a turning operation, and a control device 6.

The pump 2 is a variable displacement pump with a changeable tilting angle. The pump 2 may be a swash plate pump or a swash shaft pump. The tilting angle of the pump 2 is adjusted by a regulator (regulator) 21. The regulator 21 may be driven by an electric signal or by a pilot pressure.

The pump 2 is connected to the turning direction switching valve 3 via a pump line 11. A non-return valve 12 is provided in the pump line 11. The discharge pressure of the pump 2 is maintained at or below the first upper limit pressure by a relief valve, not shown. The turning direction switching valve 3 is connected to a tank via a tank line 13.

The turning direction switching valve 3 is connected to the turning motor 4 through a pair of supply and discharge lines 41 and 42. A release line 43 branches off from the supply and discharge lines 41, 42, respectively, the release line 43 being connected to the tank. A relief valve 44 is provided in each release line 43. That is, the pressure of each of the supply and discharge lines 41, 42 is maintained below the second upper limit pressure by the relief valve 44. The second upper limit pressure may be equal to or different from the first upper limit pressure.

The supply and drain lines 41 and 42 are connected to the tank via a supply line 45. A check valve 46 is provided in each of the supplemental lines 45 to permit flow to the supply and discharge line (41 or 42) but to prohibit flow in the opposite direction. The replenishment line 45 serves as a supply/discharge line (41 or 42) for directly flowing the working fluid from the reservoir to the supply side during rotational deceleration.

The turning direction switching valve 3 switches between a neutral position where both the supply and discharge lines 41 and 42 are blocked, a first operating position (left position in fig. 1) where the supply and discharge line 41 communicates with the pump line 11 and the supply and discharge line 42 communicates with the tank line 13, and a second operating position (right position in fig. 1) where the supply and discharge line 42 communicates with the pump line 11 and the supply and discharge line 41 communicates with the tank line 13.

The turning direction switching valve 3 is driven by an electrical signal. Specifically, the turning direction switching valve 3 includes a valve body 31 and a driving portion 32 that receives the command current and drives the valve body 31. For example, the driving unit 32 may be configured as a pair of electromagnetic proportional valves that output secondary pressures that act against the valve body 31 in opposite directions, or may be a linear motion mechanism including an electric motor, a ball screw, and the like, which is coupled to the valve body 31. The turning direction switching valve 3 increases the opening degree as the command current to the driving unit 32 increases, and increases the amount of the working fluid supplied to the turning motor 4 and the amount of the working fluid discharged from the turning motor 4.

The swing operation device 5 outputs a swing operation signal (right swing operation signal or left swing operation signal) corresponding to the flip angle (swing operation amount) of the operation lever 51. That is, the swing operation signal outputted from the swing operation device 5 increases as the tilting angle of the operation lever 51 increases. In the present embodiment, the swing operation device 5 is an electric joystick that outputs an electric signal as a swing operation signal.

The swing operation signal (electrical signal) output from the swing operation device 5 is input to the control device 6. For example, the control device 6 has a memory such as a ROM or a RAM and a CPU, and a program stored in the ROM is executed by the CPU.

The control device 6 transmits a command current that increases as the swing operation signal increases to the driving unit 32 of the swing direction switching valve 3. Thus, the valve body 31 of the turning direction switching valve 3 moves more and more as the tilting angle of the operation lever 51 of the turning operation device 5 increases.

The control device 6 is electrically connected to a speed detector 65 that detects the rotation speed of the rotator 76. The speed detector 65 is, for example, a gyro sensor provided in the rotator 76. However, the speed detector 65 may be, for example, an encoder (encoder) or a resolver (resolver) attached to the swing motor 4.

When the lever 51 of the swing operation device 5 returns to the neutral state, the control device 6 controls the swing direction switching valve 3 so as to prevent the swing back phenomenon at the time of stopping the swing. Specifically, when the operation lever 51 of the swing operation device 5 returns to the neutral state, the control device 6 sets the command current to the driving unit 32 of the swing direction switching valve 3 to zero. Thereby, as shown by 3A in fig. 3, the turning direction switching valve 3 is switched from the operating position (first operating position or second operating position) to the neutral position. As a result, the inflow pressure (pressure of the supply-side supply/discharge line) of the swing motor 4 is zero as shown in 3C of fig. 3, and the outflow pressure (pressure of the discharge-side supply/discharge line) of the swing motor 4 increases as shown in 3D of fig. 3.

When the turning direction switching valve 3 is switched to the neutral position, the turning speed of the turning body 76 is gradually reduced by the braking (brake) action of the discharge-side relief valve 44 as shown in fig. 3B. When the revolution speed of the revolution body 76 detected by the speed detector 65 is lower than the first threshold value α, the control device 6 transmits the command current to the driving unit 32 of the revolution direction switching valve 3 so as to switch the revolution direction switching valve 3 from the neutral position to the operating position where the discharge-side supply/discharge line and the tank line 13 communicate with each other. Thereby, the outflow pressure of the swing motor 4 is reduced to zero.

After that, when the revolution speed of the revolution body 76 detected by the speed detector 65 is lower than the second threshold value β smaller than the first threshold value α, the control device 6 sends the command current to the driving portion 32 of the revolution direction switching valve 3 so as to switch the revolution direction switching valve 3 from the operating position to the neutral position. Thereby, the rotator 76 is completely stopped.

The first threshold value α and the second threshold value β may be fixed values set in advance, or may be calculated by multiplying the coefficient by the revolution speed before the revolution is stopped. For example, when the first threshold value α and the second threshold value β are fixed values, the first threshold value α is 5 to 20% of the maximum speed, and the second threshold value β is 1 to 10% of the maximum speed.

As described above, in the present embodiment, when the turning is stopped, the turning direction switching valve 3 is switched to the operating position when the turning speed is lower than the first threshold value α. This rapidly reduces the pressure in the discharge-side supply/discharge line, thereby reducing the pressure difference between the supply/discharge lines 41 and 42, and preventing the rotation body 76 from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve. For reference, fig. 3C and 3D show pressure changes when the backswing phenomenon occurs without performing the control of the present embodiment in dotted lines.

In the present embodiment, the timing of decreasing the rotation speed or the degree of decrease thereof can be freely set by electronically controlled adjustment. Therefore, calibration (calibration) for compensating for the influence of the temperature of the working fluid per body becomes easy, and the range of adjustment of the operability of the swing stop, such as adjustment in accordance with the preference of the operator, is widened.

< modification >

As shown in fig. 4, a pair of pressure sensors 61, 62 may be provided in the supply and discharge lines 41, 42. One of the pressure sensors 61 and 62 detects the inflow pressure of the rotary motor 4, and the other of the pressure sensors 61 and 62 detects the outflow pressure of the rotary motor 4.

In the example shown in fig. 3A, when the turning direction switching valve 3 is switched to the operating position after the operation lever 51 of the turning operation device 5 returns to the neutral state, the opening degree (displacement of the valve body 31) of the turning direction switching valve 3 is constant. In contrast, when the pressure sensors 61 and 62 are provided, the rotation direction switching valve 3 may be controlled so that the rotation direction switching valve 3 has an opening degree corresponding to the difference between the inflow pressure and the outflow pressure of the rotation motor 4 detected by the pressure sensors 61 and 62 when the rotation direction switching valve 3 is switched to the operating position after the operation lever 51 of the rotation operation device 5 returns to the neutral state. With this configuration, the pressure difference between the supply and drain pipes 41 and 42 can be suppressed to be as small as possible.

When the pressure sensors 61 and 62 are provided, after the operation lever 51 of the swing operation device 5 returns to the neutral state, the control device 6 may send the command current to the driving unit 32 of the swing direction switching valve 3 so as to switch the swing direction switching valve 3 from the neutral position to the operating position where the discharge-side supply/discharge line and the tank line 13 communicate with each other when the outflow pressure of the swing motor 4 detected by one of the pressure sensors 61 and 62 is lower than the first threshold α'. The same effects as those of the above embodiment can be obtained with this configuration.

The first threshold value α (or α') may be divided into a threshold value α1 for the swing alone operation and a threshold value α2 (< α1) for the swing composite operation in which the swing operation and other operations (for example, the boom operation, the arm operation, or the bucket operation) are simultaneously performed.

(second embodiment)

Next, a hydraulic drive system for a construction machine according to a second embodiment of the present invention will be described with reference to fig. 5A to 5D. The configuration of the hydraulic drive system according to the second embodiment is the same as that of the modification of the first embodiment shown in fig. 4.

In the present embodiment, the control device 6 controls the turning direction switching valve 3 so as to prevent the hunting phenomenon at the time of turning stop when the operation lever 51 of the turning operation device 5 returns to the neutral state. Specifically, when the operation lever 51 of the swing operation device 5 returns to the neutral state, the control device 6 sets the command current to zero, which is sent to the driving unit 32 of the swing direction switching valve 3. As a result, as shown by 5A in fig. 5, the turning direction switching valve 3 is switched from the operating position (first operating position or second operating position) to the neutral position. As a result, the inflow pressure (pressure of the supply-side supply/discharge line) of the swing motor 4 is zero as shown by 5C in fig. 5, and the outflow pressure (pressure of the discharge-side supply/discharge line) of the swing motor 4 increases as shown by 5D in fig. 5.

When the turning direction switching valve 3 is switched to the neutral position, the turning speed of the turning body 76 is gradually reduced by the braking action of the discharge-side relief valve 44 as shown in fig. 5B. When the revolution speed of the revolution body 76 detected by the speed detector 65 is lower than the threshold value γ, the control device 6 transmits a command current to the driving unit 32 of the revolution direction switching valve 3 so as to switch the revolution direction switching valve 3 from the neutral position to the operating position where the discharge-side supply/discharge line and the tank line 13 communicate with each other. Thereby, the outflow pressure of the swing motor 4 is reduced to zero. The threshold value γ may be a fixed value set in advance, or may be calculated by multiplying the coefficient by the revolution speed before the revolution stop.

Thereafter, the control device 6 sends the command current to the driving unit 32 of the turning direction switching valve 3 so that the turning direction switching valve 3 has an opening degree corresponding to the differential pressure between the inflow pressure and the outflow pressure of the turning motor 4 detected by the pressure sensors 61 and 62. More specifically, the control device 6 increases the opening degree of the turning direction switching valve 3 as the differential pressure between the inflow pressure and the outflow pressure of the turning motor 4 increases.

After the operation lever 51 of the swing operation device 5 returns to the neutral state and the swing direction switching valve 3 is switched to the operating position for communicating the discharge side supply/discharge line with the tank line 13, the control device 6 controls the swing direction switching valve 3 so that the swing direction switching valve 3 is switched to the operating position for communicating the supply side supply/discharge line with the tank line 13 when the inflow pressure of the swing motor 4 is higher than the outflow pressure. That is, the turning direction switching valve 3 is switched from the first operating position to the second operating position, or from the second operating position to the first operating position.

As described above, in the present embodiment, when the turning is stopped, the turning direction switching valve 3 is switched to the operating position when the turning speed is lower than the threshold value γ. This rapidly reduces the pressure in the discharge-side supply/discharge line, and thus the pressure difference between the supply/discharge lines 41 and 42 decreases, preventing the rotation body 76 from reversing. Therefore, the hunting phenomenon at the time of stopping the swing can be prevented without using the anti-reverse valve. In the present embodiment, the opening degree corresponding to the pressure difference between the inflow pressure and the outflow pressure of the swing motor 4 is set when the swing direction switching valve 3 is switched to the operation position, so that the pressure difference between the supply and drain pipes 41 and 42 can be suppressed to be as small as possible.

In the present embodiment, the timing of decreasing the rotation speed or the degree of decrease thereof can be freely set by electronically controlled adjustment. Therefore, the adjustment for compensating the influence of the temperature of the working fluid for each machine body becomes easy, and the adjustment range of the operability of the swing stop, such as adjustment in accordance with the preference of the operator, is enlarged.

In the present embodiment, when the inflow pressure of the swing motor 4 is higher than the outflow pressure, the operation position of the swing direction switching valve 3 is switched, so that even if the swing body 76 is reversed at the time of stopping the swing, further reverse rotation can be prevented thereafter.

< modification >

When the outflow pressure of the swing motor 4 detected by one of the pressure sensors 61 and 62 is lower than the threshold value γ' after the operation lever 51 of the swing operation device 5 returns to the neutral state, the control device 6 may send the command current to the driving unit 32 of the swing direction switching valve 3 so as to switch the swing direction switching valve 3 from the neutral position to the operating position where the discharge-side supply/discharge line and the tank line 13 communicate with each other. This structure can also provide the same effects as those of the above embodiment. In this case, the speed detector 65 may be omitted.

When the swing direction switching valve 3 is switched to the operation position after the operation lever 51 of the swing operation device 5 returns to the neutral position, the opening degree of the swing direction switching valve 3 may be adjusted according to the rate of change of the inflow pressure and/or the outflow pressure of the swing motor 4.

The threshold value γ (or γ') may be divided into a threshold value γ1 for the swing alone operation and a threshold value γ2 (< γ1) for the swing composite operation in which the swing operation and other operations (for example, the boom operation, the arm operation, or the bucket operation) are simultaneously performed.

(other embodiments)

The present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the spirit of the present invention.

For example, the swing operation device 5 does not necessarily have to be an electric lever, and may be a pilot operated valve that outputs a pilot pressure as a swing operation signal. In this case, the pilot pressure output from the swing operation device 5 is detected by a pressure sensor and input to the control device 6.

Symbol description:

1. hydraulic drive system

11. Pump pipeline

13. Storage tank pipeline

3. Rotary direction switching valve

31. Valve core

32. Drive unit

4. Rotary motor

5. Rotary operation device

51. Operating lever

6. Control device

61. 62 pressure sensor

65. A speed detector.

Claims

1. A hydraulic drive system for a construction machine is characterized by comprising:

a rotation motor for rotating the rotation body;

a swing operation device including an operation lever for outputting a swing operation signal corresponding to a tilting angle of the operation lever;

a swing direction switching valve connected to the swing motor through a pair of supply and discharge lines, the swing direction switching valve including a spool and a driving portion that receives a command current and drives the spool, the swing direction switching valve switching between a neutral position that blocks the pair of supply and discharge lines and an operating position that communicates one of the pair of supply and discharge lines with a pump line and the other with a tank line;

a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger; and

a speed detector for detecting a rotation speed of the rotator;

the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to the neutral state, the swing direction switching valve is switched from the neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the swing speed detected by the speed detector is lower than a first threshold value, and is switched from the operating position to the neutral position when the swing speed detected by the speed detector is lower than a second threshold value smaller than the first threshold value and larger than zero.

2. A hydraulic drive system for a construction machine is characterized by comprising:

a rotation motor for rotating the rotation body;

a control device for transmitting a command current to the swing direction switching valve, the command current being larger as the swing operation signal is larger;

a speed detector for detecting a rotation speed of the rotator; and

a pair of pressure sensors for detecting inflow pressure and outflow pressure of the rotary motor;

the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to a neutral state, the swing direction switching valve is switched from a neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the outflow pressure of the swing motor detected by one of the pair of pressure sensors is lower than a first threshold value, and is switched from the operating position to the neutral position when the swing speed detected by the speed detector is lower than a second threshold value.

3. A hydraulic drive system for a construction machine is characterized by comprising:

a rotation motor for rotating the rotation body;

a speed detector for detecting a rotation speed of the rotator; and

the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to the neutral state, the swing direction switching valve is switched from the neutral position to an operating position in which the discharge-side supply/discharge line and the tank line are connected when the swing speed detected by the speed detector is lower than a threshold value, and then the opening degree between the discharge-side supply/discharge line and the tank line and the opening degree between the supply-side supply/discharge line and the pump line are set to the opening degrees corresponding to the differential pressures of the inflow pressure and the outflow pressure of the swing motor detected by the pair of pressure sensors.

4. The hydraulic drive system according to claim 3, wherein,

the control device controls the turning direction switching valve in the following manner: the control lever of the swing control device returns to a neutral state, and after the swing direction switching valve is switched to an operating position in which the discharge-side supply/discharge line and the tank line are connected, the swing direction switching valve is switched to an operating position in which the supply-side supply/discharge line and the tank line are connected when the inflow pressure of the swing motor is higher than the outflow pressure.

5. A hydraulic drive system for a construction machine is characterized by comprising:

a rotation motor for rotating the rotation body;

the control device controls the turning direction switching valve in the following manner: when the operation lever of the swing operation device returns to a neutral state, if the outflow pressure of the swing motor detected by one of the pair of pressure sensors is lower than a threshold value, the swing direction switching valve is switched from a neutral position to an operating position at which the discharge side supply/discharge line and the tank line communicate with each other, and then the opening between the discharge side supply/discharge line and the tank line and the opening between the supply side supply/discharge line and the pump line are set to openings corresponding to the differential pressures between the inflow pressure and the outflow pressure of the swing motor detected by the pair of pressure sensors.

6. The hydraulic drive system according to claim 5, wherein,