JP6333713B2 - Hydraulic drive system - Google Patents

Description

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

  A construction machine such as an industrial vehicle including a plurality of wheels (for example, a wheel loader) or a hydraulic excavator including a crawler is equipped with a hydraulic drive system. This hydraulic drive system includes a working hydraulic actuator and a variable displacement hydraulic pump, and the tilt angle of the hydraulic pump is changed by a regulator (see, for example, Patent Document 1).

  For example, the tilt angle of the hydraulic pump is adjusted by negative control. Specifically, a circulation line constituting a center bypass line of a work circuit for driving a work hydraulic actuator extends from the hydraulic pump to the tank, and the circulation line has a throttle on the downstream side of the work circuit. Provided. Then, a negative control pressure, which is a pressure upstream of the throttle in the circulation line, is introduced to the regulator through the pilot line.

  In an industrial vehicle, the hydraulic drive system includes a pair of steering cylinders for changing the traveling direction, and hydraulic oil is distributed to the steering cylinders from the upstream side of the work circuit in the circulation line.

JP-A-6-117411

  In negative control, the negative control pressure is usually set when the operator is not operating the operating means (operating lever or steering wheel) for moving the working hydraulic actuator or steering cylinder (only for industrial vehicles). And the discharge flow rate of the hydraulic pump can be kept small. On the other hand, when the operator operates the operation means, the negative control pressure decreases and the discharge flow rate of the hydraulic pump increases. As a result, a large amount of hydraulic oil is supplied to the hydraulic actuator and steering cylinder for work that is about to move.

  However, when the temperature of the hydraulic oil is low in a cold region or the like, even if the operator operates the operation means, the working hydraulic actuator or the steering cylinder may not immediately move (so-called response delay).

  Therefore, an object of the present invention is to provide a hydraulic drive system that can improve response delay and the like when the temperature of hydraulic oil is low.

  In order to solve the above-mentioned problem, the inventors of the present invention, as a result of intensive research, have found that the response delay when the temperature of the hydraulic oil is low is that the operating means is operated due to the low viscosity of the hydraulic oil. Immediately after that, it was found that the discharge pressure of the hydraulic pump was slightly reduced. The present invention has been made from such a viewpoint.

  That is, the hydraulic drive system of the present invention is a hydraulic drive system mounted on a construction machine, and includes a hydraulic pump whose tilt angle is changed by a regulator, and a working hydraulic actuator extending from the hydraulic pump to the tank. A circulation line constituting a center bypass line of the working circuit for driving, a circulation line provided with a throttle on the downstream side of the working circuit, and a portion between the working circuit and the throttle in the circulation line; A pilot line connecting the regulator, a switching valve provided in the pilot line, and an external pilot higher than a minimum value of a negative control pressure which is a pressure upstream of the throttle in the circulation line to the switching valve An external pilot line that guides pressure, and the switching valve has the negative controller. A first position for guiding the Le pressure to the regulator is switched between the second position for guiding the external pilot pressure to the regulator, characterized in that.

  According to the above configuration, if the switching valve is maintained at the second position when the temperature of the hydraulic oil is low, the discharge pressure of the hydraulic pump can be kept high to some extent before the operator operates the operating means. Can do. As a result, when the operator operates the operating means, a large amount of hydraulic oil is supplied from the beginning to the working hydraulic actuator and the steering cylinder, and they immediately move. As a result, response delay when the temperature of the hydraulic oil is low can be improved.

  The construction machine is an industrial vehicle including a plurality of wheels, and the hydraulic drive system includes a steering circuit for driving a pair of steering cylinders for changing a traveling direction, and a control device for controlling the switching valve. The control device is during the operation of the hydraulic pump, the steering operation and the specific operation for driving the hydraulic actuator are not performed, and the temperature of the hydraulic oil is When the temperature is lower than the predetermined temperature, the switching valve may be maintained in the second position. According to this configuration, response delay when the temperature of the hydraulic oil is low can be improved.

  The control device may switch the switching valve from the second position to the first position after a predetermined time has elapsed since the steering operation or the specific operation was performed. According to this configuration, it is possible to switch from uniform control based on the external pilot pressure to negative control.

  The construction machine includes a boom, the hydraulic actuator is a boom cylinder that operates the boom, and the cargo handling circuit is a control valve for the boom cylinder, and during the detent operation that continues to raise or lower the boom Including a control valve that is returned to the neutral position when the boom reaches the highest or lowest position, and the hydraulic drive system further includes a control device that controls the switching valve, and the control device includes: The switching valve may be switched from the first position to the second position before the boom reaches the highest position or the lowest position.

  By the way, when the working hydraulic actuator is a boom cylinder, depending on the construction machine, a detent operation may be performed in which the boom continues to be raised or lowered until it reaches the highest or lowest position. For example, the highest position and the lowest position can be arbitrarily set within the range of the mechanical stroke of the boom cylinder. In this detent operation, when the boom reaches the highest rise position or the lowest drop position, the control valve is immediately returned to the neutral position. As a result, the supply of hydraulic oil to the boom cylinder is suddenly stopped, and a stop shock may be caused. The detent operation is often performed when raising the boom.

  On the other hand, according to the above configuration, the discharge flow rate of the hydraulic pump can be forcibly reduced before the boom reaches the highest position or the lowest position. As a result, the stop shock when the boom reaches the highest position or the lowest position can be mitigated.

  The external pilot line is provided with an electromagnetic proportional valve that outputs the external pilot pressure as a secondary pressure. After the control device switches the switching valve to the second position, the external pilot pressure is The electromagnetic proportional valve may be controlled so as to increase gradually. According to this configuration, the boom can be smoothly stopped toward the highest position or the lowest position.

  The hydraulic pump is a main pump, and the hydraulic drive system further includes an accessory pump driven by an engine common to the main pump, and the external pilot line guides hydraulic oil discharged from the accessory pump. It may be connected to a supply line. According to this configuration, the discharge pressure of the accessory pump can be rationally used as a pressure source.

  The external pilot line may be connected to an upstream portion of the work circuit in the circulation line, and the external pilot line may be provided with a check valve and an accumulator in order from the upstream side. According to this configuration, the highest pressure in the pressure history in the circulation line can be rationally used as a pressure source.

  According to the present invention, response delay or the like when the temperature of the hydraulic oil is low can be improved.

It is a figure showing typically the schematic structure of the construction machine carrying the hydraulic drive system concerning one embodiment of the present invention. It is a side view of the construction machine shown in FIG. It is a graph which shows a time-dependent change of the discharge pressure of the main pump at the time of steering operation or cargo handling operation. It is a schematic block diagram of a cargo handling circuit. It is a flowchart regarding control of the switching valve at the time of steering operation or cargo handling operation. It is a flowchart regarding control of the switching valve at the time of raising detent operation. It is a figure which shows typically schematic structure of the construction machine carrying the hydraulic drive system of a 1st modification. It is a figure which shows typically the schematic structure of the construction machine carrying the hydraulic drive system of a 2nd modification. It is a figure which shows typically the schematic structure of the construction machine carrying the hydraulic drive system of a 3rd modification. It is a figure which shows typically schematic structure of the construction machine carrying the hydraulic drive system of a 4th modification.

  FIG. 2 shows a construction machine 1 equipped with a hydraulic drive system 2A according to an embodiment of the present invention. FIG. 1 schematically shows a schematic configuration of the construction machine 1. Although the construction machine 1 shown in FIG. 2 is a wheel loader which is one of industrial vehicles, the present invention can be applied to other industrial vehicles including wheels such as an excavator loader, a forklift and a truck crane. For example, the present invention is also applicable to construction machines other than industrial vehicles such as hydraulic excavators including crawlers.

<Configuration>
As shown in FIG. 2, the construction machine 1 includes a front vehicle body 10A and a rear vehicle body 10B that are connected to each other so as to be swingable in the horizontal direction. A front wheel 15A is attached to the front vehicle body 10A, and a rear wheel 15B is attached to the rear vehicle body 10B. Although illustration is omitted, a pair of left and right steering cylinders 28 for changing the traveling direction is disposed between the front vehicle body 10A and the rear vehicle body 10B.

  The rear vehicle body 10B is provided with a cab 17 and an engine 11 is mounted. The boom 18 is connected to the front vehicle body 10A so as to be swingable in the vertical direction, and the bucket 19 is connected to the tip of the boom 18 so as to be swingable in the vertical direction. The front vehicle body 10 </ b> A is provided with a pair of left and right boom cylinders 24 that operate the boom 18 and a bucket cylinder 25 that operates the bucket 19. The boom cylinder 24 and the bucket cylinder 25 correspond to the working hydraulic actuator of the present invention.

  The boom cylinder 24, bucket cylinder 25, and steering cylinder 28 are components of the hydraulic drive system 2A shown in FIG. The output of the engine 11 is used for running and as a power source for the hydraulic drive system 2A.

  Specifically, as shown in FIGS. 1 and 2, the engine 11 is connected to wheels 15A and 15B via a torque converter 12, a transmission 13, and axles 14A and 14B. The transmission 13 can change the speed ratio between the input shaft and the output shaft, and whether the rotation direction of the output shaft is the same as the input shaft or the reverse direction in order to switch the vehicle forward and reverse. Can be switched.

  A hydraulic drive system 2 </ b> A shown in FIG. 1 includes a main pump (hydraulic pump) 21 and an accessory pump 23 connected to the torque converter 12. That is, the main pump 21 and the accessory pump 23 are driven by the common engine 11. The hydraulic drive system 2 </ b> A includes a cargo handling circuit (corresponding to a work circuit of the present invention) 34 for driving the boom cylinder 24 and the bucket cylinder 25, and a steering circuit 36 for driving the steering cylinder 28.

  The hydraulic oil discharged from the main pump 21 is supplied to the cargo handling circuit 34 and the steering circuit 36. Specifically, a circulation line 31 extends from the hydraulic pump 21 to the tank 32. The circulation line 31 constitutes a center bypass line 34 a of the cargo handling circuit 34. A branch line 35 branches from the circulation line 31 on the upstream side of the cargo handling circuit 34, and this branch line 35 is connected to the steering circuit 36.

  The main pump 21 is a variable displacement pump, and its tilt angle is changed by a regulator 22. The main pump 21 may be a swash plate pump or an oblique shaft pump.

  As shown in FIG. 4, the cargo handling circuit 34 includes a control valve 72 for the boom cylinder 24 and a control valve 71 for the bucket cylinder 25. The center bypass line 34a described above extends so as to penetrate these control valves 71 and 72. A pilot pressure is output from the boom operation valve 82 to the control valve 72 for the boom cylinder 24, and a pilot pressure is output from the bucket operation valve 81 to the control valve 71 for the bucket cylinder 25. The boom operation valve 82 and the bucket operation valve 81 are disposed in the cab 17. Further, the boom operation valve 82 and the bucket operation valve 81 include an operation lever, and output a pilot pressure corresponding to the operation amount of the operation lever.

  In the present embodiment, each of the boom operation valve 82 and the bucket operation valve 81 is a hydraulic pilot valve that directly outputs a pilot pressure corresponding to the operation amount of the operation lever. However, each of the boom operation valve 82 and the bucket operation valve 81 includes a pair of electromagnetic proportional valves connected to the pilot port of the control valve (72 or 71), and an electric signal corresponding to the operation amount of the operation lever is electromagnetic proportional. It may be an electric pilot valve fed to the valve.

  Although not shown, the steering circuit 36 includes a steering valve that controls supply and discharge of hydraulic oil to and from the steering cylinder 28. For example, a steering signal (pilot flow) is output to the steering valve from a steering signal output device such as Orbit Roll (registered trademark). The steering signal output device is connected to a steering wheel disposed in the cab 17 via a steering shaft.

  Returning to FIG. 1, the circulation line 31 is provided with a throttle 33 on the downstream side of the cargo handling circuit 34. Although not shown, the circulation line 31 is connected to a bypass line that bypasses the throttle 33, and a relief valve is provided in the bypass line. A portion of the circulation line 31 between the cargo handling circuit 34 and the throttle 33 is connected to the regulator 22 by a pilot line 51.

  When the boom cylinder 24, the bucket cylinder 25, and the steering cylinder 28 are not driven, the entire amount of hydraulic oil discharged from the main pump 21 passes through the throttle 33, and therefore the pressure on the upstream side of the throttle 33 in the circulation line 31. Negative control pressure Pn increases. On the other hand, when at least one of the boom cylinder 24, the bucket cylinder 25, and the steering cylinder 28 is driven, only part of the hydraulic oil discharged from the main pump 21 passes through the throttle 33, so that the negative control pressure Pn is Lower.

  The regulator 22 decreases the tilt angle of the main pump 21 when the pressure guided through the pilot line 51 is high, and increases the tilt angle of the main pump 21 when the pressure guided through the pilot line 51 is low. When the tilt angle of the main pump 21 decreases, the discharge flow rate of the main pump 21 decreases, and when the tilt angle of the main pump 21 increases, the discharge flow rate of the main pump 21 increases.

  A switching valve 52 is provided in the pilot line 51. The switching valve 52 will be described in detail later.

  The hydraulic oil discharged from the accessory pump 23 is guided to the fan circuit 42, the traveling brake 45 and the parking brake 46 through the supply line 4. The accessory pump 23 is a fixed displacement pump in this embodiment, but may be a variable displacement pump.

  The fan circuit 42 is for driving the fan motor 26 (see FIG. 2) that sends wind to the radiator 16 for the engine 11 (see FIG. 2). That is, the fan circuit 42 is a circuit for cooling the engine 11.

  The supply line 4 is provided with an unloader valve 41, and hydraulic oil discharged from the accessory pump 23 by the unloader valve 41 is distributed to the brakes 45 and 46 side and the fan circuit 42 side. That is, the supply line 4 includes a main flow path 4 a that connects the accessory pump 23 to the unloader valve 41, a first branch 4 b that connects the unloader valve 41 to the traveling brake 45 and the parking brake 46, and the unloader valve 41 to the fan circuit 42. A second branch 4c to be connected is included.

  The unloader valve 41 preferentially supplies hydraulic oil to the first tributary 4b when the pressure of the first tributary 4b falls below the set pressure Pb by the operation of the traveling brake 45 or the parking brake 46, and the pressure of the first tributary 4b. When the pressure exceeds the set pressure Pb, the hydraulic oil is supplied to the second branch 4c.

  The first branch 4b is provided with a check valve 43 and an accumulator 44 in order from the upstream side. The downstream portion from the position where the accumulator 44 of the first tributary 4 b is provided is further divided into a first end flow path 4 d connected to the traveling brake 45 and a second end flow path 4 e connected to the parking brake 46.

  The second end passage 4e is provided with a switching valve 47 for switching whether or not the parking brake 46 is operated. The switching valve 47 is electrically connected to a parking brake switch (not shown) disposed in the cab 17. When the parking brake switch is off, the switching valve 47 opens the second end passage 4e to keep the parking brake 46 in a brake release state, and when the parking brake switch is turned on, the second end passage 4e is interrupted and the parking brake 46 is operated.

  From the supply line 4, hydraulic oil is also guided to the boom operation valve 82 and the bucket operation valve 81 described above.

  An external pilot line 53 is connected to the switching valve 52 provided in the pilot line 51 described above. The external pilot line 53 guides the external pilot pressure Po higher than the minimum value (for example, 0.8 MPa) of the negative control pressure Pn to the switching valve 52. In the present embodiment, the external pilot line 53 is connected to the supply line 4. If the external pilot line 53 is connected to the supply line 4 in this way, the discharge pressure of the accessory pump 23 can be rationally used as a pressure source.

  More specifically, the external pilot line 53 is connected to the downstream portion of the check valve 43 in the first tributary 4b. In the illustrated example, the external pilot line 53 is connected to the second terminal flow path 4e on the parking brake 46 side of the first branch 4b, but the external pilot line 53 is connected to the first terminal flow path 4d on the traveling brake 45 side. It may be connected, or may be connected to a portion between the check valve 43 and the accumulator 44 in the first tributary 4b.

  The discharge pressure of the accessory pump 23 is set much higher than the minimum value of the negative control pressure Pn. For this reason, in the present embodiment, an electromagnetic proportional valve 54 is provided in the external pilot line 53 to output the discharge pressure of the accessory pump 23 as a primary pressure and the above-described external pilot pressure Po as a secondary pressure. However, instead of the electromagnetic proportional valve 54, a fixed throttle or a pressure reducing valve may be provided.

  The switching valve 52 is switched between a first position where the downstream portion of the pilot line 51 communicates with the upstream portion and a second position where the downstream portion of the pilot line 51 communicates with the external pilot line 53. That is, the switching valve 52 guides the negative control pressure Pn to the regulator 22 at the first position, and guides the external pilot pressure Po to the regulator 22 at the second position.

  In this embodiment, the switching valve 52 is normally maintained at the first position, and is switched to the second position when the solenoid is excited. However, the switching valve 52 is normally maintained in the second position, and may be switched to the first position when the solenoid is excited.

  The switching valve 52 and the electromagnetic proportional valve 54 are connected to the control device 6. In addition, a temperature sensor 61, a pressure sensor 62, and an angle sensor 63 are connected to the control device 6. The temperature sensor 61 is for detecting the temperature of the hydraulic oil, and is provided in the tank 32, for example. The pressure sensor 62 is for detecting the discharge pressure of the main pump 21, and is provided, for example, at a portion upstream of the branch point of the branch line 35 in the circulation line 31. The angle sensor 63 is for detecting the angle of the boom 18.

  By controlling the switching valve 52 by the control device 6, an operator operates an unillustrated steering wheel (hereinafter, steering operation) or an operator operates an operation lever of at least one of the boom operation valve 82 and the bucket operation valve 81 (hereinafter, referred to as “steering operation”). The response delay when the temperature of the hydraulic oil is low when a cargo handling operation (corresponding to the specific operation of the present invention) is performed can be improved.

  Further, the hydraulic drive system 2A of the present embodiment has a configuration capable of performing a detent operation for continuing to raise the boom 18 and a detent operation for continuing to lower the boom 18 as will be described in detail later. In this embodiment, the stop shock when the boom 18 reaches the highest position during the raising detent operation can be mitigated by the control of the switching valve 52 by the control device 6.

  Hereinafter, the control of the switching valve 52 by the control device 6 will be described in detail for a steering operation or a cargo handling operation and a lifting detent operation.

<During steering operation or cargo handling operation>
The control device 6 controls the switching valve 52 when the main pump 21 is in operation, the steering operation and the cargo handling operation are not performed, and the temperature of the hydraulic oil is lower than the predetermined temperature T1. Maintain in the second position. As a result, the external pilot pressure Po output from the electromagnetic proportional valve 54 is guided to the regulator 22.

  Specifically, as shown in FIG. 5, the control device 6 determines the temperature of the hydraulic oil detected by the temperature sensor 61 to a predetermined temperature T1 (in order to determine whether or not the temperature of the hydraulic oil is low. For example, it is compared with 0 ° C. (step S1). When the temperature of the hydraulic oil is lower than the predetermined temperature T1 (YES in step S1), the process proceeds to step S2. On the other hand, when the temperature of the hydraulic oil is equal to or higher than the predetermined temperature T1 (NO in step S1), the control device 6 holds the switching valve 52 in the first position without exciting the solenoid of the switching valve 52, and negative The control pressure Pn is guided to the regulator 22. That is, when the temperature of the hydraulic oil is not low, the tilt angle of the hydraulic pump 21 is adjusted by normal negative control when a steering operation or a cargo handling operation is performed.

  In step S2, the control device 6 excites the solenoid of the switching valve 52 to switch the switching valve 52 to the second position. Thereafter, the control device 6 maintains that state until a steering operation or a cargo handling operation is performed (NO in step S3). In the meantime, if the temperature of the hydraulic oil detected by the temperature sensor 61 is equal to or higher than the predetermined temperature T1 (NO in step S1), the control device 6 demagnetizes the solenoid of the switching valve 52 and moves the switching valve 52 to the first position. Return to.

  In step S2, the control device 6 uses the electromagnetic proportional valve 54 to generate an external pilot pressure Po, which is a pump capacity capable of securing a flow rate required to start the movement of the steering cylinder 28 or the cargo handling cylinder (the boom cylinder 24 and / or the bucket cylinder 25). Is supplied to the electromagnetic proportional valve 54. For example, the external pilot pressure Po is a minimum value (for example, 0.8 MPa) of the negative control pressure Pn that maximizes the pump capacity, and a maximum value (for example, 3.12 MPa) of the negative control pressure Pn that minimizes the pump capacity. Is a value in the central region when the range between and is divided into three equal parts (for example, 2.2 MPa).

  If the steering operation or the cargo handling operation is performed while the switching valve 52 is maintained at the second position (YES in step S3), the control device 6 determines a predetermined time Δt (for example, after the steering operation or the cargo handling operation is performed). , 0.7 to 1.2 seconds), the solenoid of the switching valve 52 is demagnetized and the switching valve 52 is switched from the second position to the first position (step S4).

  Whether the steering operation or the cargo handling operation has been performed is determined by detecting the pilot line pressure between the steering signal output device (not shown) and the steering valve, or the pilot line pressure between the boom operation valve 82 and the control valve 72. Further, it can be determined by detecting the pressure of the pilot line between the bucket operation valve 81 and the control valve 71. Alternatively, whether or not the steering operation and the cargo handling operation are performed can also be determined by detecting the pressure of the hydraulic oil supplied to the steering cylinder 28 and the boom cylinder 24.

  The timing for switching the switching valve 52 from the second position to the first position is not necessarily after a predetermined time Δt has elapsed since the steering operation or the cargo handling operation was performed. For example, the switching valve 52 may be switched from the second position to the first position when the discharge pressure of the main pump 21 detected by the pressure sensor 62 becomes a specific pressure. Further, the external pilot pressure Po may be changed after the steering operation or the cargo handling operation is performed.

  The change over time of the discharge pressure of the main pump 21 when the control shown in FIG. 5 is performed is shown by a solid line in FIG.

  When the switching valve 52 is maintained in the first position without performing the control as shown in FIG. 5 and the negative control pressure Pn is always led to the regulator 22, if the temperature of the hydraulic oil is low, As shown by the one-dot chain line in FIG. 3, immediately after the operating means (the operating levers of the operating valves 81 and 82 and the steering wheel not shown) are operated, the discharge pressure of the main pump 21 slightly decreases.

  On the other hand, when the control as shown in FIG. 5 is performed, since the switching valve 52 is maintained at the second position when the temperature of the hydraulic oil is low, before the operator operates the operating means, The discharge pressure of the main pump 21 can be kept high to some extent. Thus, when the operator operates the operation means, a large amount of hydraulic oil is supplied from the beginning to the cargo handling cylinder (the boom cylinder 24 and / or the bucket cylinder 25) and the steering cylinder 28, and they immediately move. As a result, response delay when the temperature of the hydraulic oil is low can be improved.

  Further, since the switching valve 52 is returned to the first position after a predetermined time Δt has elapsed since the steering operation or the cargo handling operation is performed, it is possible to switch from uniform control based on the external pilot pressure Po to negative control.

<When raising detents>
The boom operation valve 82 shown in FIG. 4 has a raising detent coil 83 for maintaining the boom operation valve 82 in the boom raising operation position, and a lowering detent coil 84 for keeping it in the boom lowering operation position. More specifically, the boom operation valve 82 has a pair of push rods that move up and down in opposite directions following the operation of the operation lever, and when the operation lever is tilted to the maximum on the ascending side or the descending side, The plate fixed to one push rod is attracted to the raising detent coil 83 or the lowering detent coil 84.

  The control device 6 excites the raising detent coil 83 when the raising detent operation is performed, and demagnetizes the raising detent coil 83 when the boom 18 reaches the highest position during the raising detent operation. As a result, the control valve 72 for the boom cylinder 24 is immediately returned to the neutral position. Further, the control device 6 excites the lower detent coil 84 when the lower detent operation is performed, and demagnetizes the lower detent coil 84 when the boom 18 reaches the lowest position during the lower detent operation. . As a result, the control valve 72 for the boom cylinder 24 is immediately returned to the neutral position.

  In the control device 6, the most elevated position and the most lowered position of the boom 18 are stored in advance as the angle of the boom 18. The highest position and the lowest position can be arbitrarily set within the range of the mechanical stroke of the boom cylinder 24.

  As described above, in the present embodiment, the stop shock when the boom 18 reaches the highest position during the raising detent operation is alleviated by the control of the switching valve 52 by the control device 6. That is, the control device 6 switches the switching valve 52 from the first position to the second position before the boom 18 reaches the highest position.

  More specifically, as shown in FIG. 6, the control device 6 waits until the raising detent operation is performed (NO in step S11). When the raising detent operation is performed (YES in step S11), the control device 6 continues until the boom angle detected by the angle sensor 63 is equal to or larger than the predetermined angle θ smaller than the highest ascending position (NO in step S12). The switching valve 52 is held in the first position without exciting the solenoid of the switching valve 52. The predetermined angle θ may be calculated by multiplying the maximum angle θmax indicating the highest ascending position by a certain ratio, or may be calculated by subtracting a certain value from the maximum angle θmax.

  When the boom angle detected by the angle sensor 63 is equal to or greater than the predetermined angle θ (YES in step S12), the control device 6 excites the solenoid of the switching valve 52 to switch the switching valve 52 to the second position (step S13). . At this time, for example, the control device 6 supplies a current to the electromagnetic proportional valve 54 so that the electromagnetic proportional valve 54 outputs an external pilot pressure Po equal to the negative control pressure Pn.

  After switching the switching valve 52 to the second position, the control device 6 controls the electromagnetic proportional valve 54 so that the external pilot pressure Po gradually increases (step S14). Step S14 is continued until the boom 18 reaches the highest position (that is, the boom angle detected by the angle sensor 63 becomes the maximum angle θmax) (NO in step S15). For example, the external pilot pressure Po is increased so that the pump displacement is minimized when the boom 18 reaches the highest position.

  When the boom 18 reaches the highest position (YES in step S15), the control device 6 demagnetizes the solenoid of the switching valve 52 and returns the switching valve 52 to the first position.

  If the control as shown in FIG. 6 is performed, the discharge flow rate of the main pump 21 can be forcibly reduced before the boom 18 reaches the highest position. As a result, the stop shock when the boom 18 reaches the highest position can be alleviated.

  Further, after the switching valve 52 is switched to the second position, the electromagnetic proportional valve 54 is controlled so that the external pilot pressure Po gradually increases, so that the boom 18 is smoothly stopped toward the highest position. be able to.

  If the control shown in FIG. 6 is applied during the lowering detent operation, the stop shock when the boom 18 reaches the lowest lowered position during the lowering detent operation can be reduced. Specifically, the control device 6 may switch the switching valve 52 from the first position to the second position before the boom 18 reaches the lowest position.

  When the boom operation valve 82 is an electric pilot valve including a pair of electromagnetic proportional valves, the operation lever is moved upward or downward by a predetermined angle or more (for example, the operation lever of the operation lever) instead of the detent coils 83 and 84. What is necessary is just to employ | adopt the program which maintains the electric signal sent to an electromagnetic proportional valve until the boom 18 reaches | attains the highest ascending position or the most descending position when it tilts to 80% or more of the maximum operating angle.

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

  For example, the external pilot line 53 may be connected to the second branch 4c of the supply line 4 as in the hydraulic drive system 2B of the first modification shown in FIG. Alternatively, the external pilot line 53 may be connected to the main flow path 4a of the supply line 4 as in the hydraulic drive system 2C of the third modified example shown in FIG. In this case, the external pilot line 53 is preferably provided with a check valve 55 and an accumulator 56 in order from the upstream side.

  Furthermore, the external pilot line 53 is not necessarily connected to the supply line 4 extending from the accessory pump 23. For example, the external pilot line 53 may be connected to the upstream side portion of the cargo handling circuit 34 in the circulation line 31 as in the hydraulic drive system 2D of the fourth modified example shown in FIG. In this case, the external pilot line 53 is provided with a check valve 55 and an accumulator 56 in order from the upstream side.

  Although the pressure in the circulation line 31 varies, the check valve 55 and the accumulator 56 are provided in the external pilot line 53, so that the highest pressure in the circulation line 31 is accumulated in the accumulator 56. Thereby, the highest pressure in the pressure history in the circulation line 31 can be rationally used as a pressure source.

  Further, like the hydraulic drive system 2E of the fifth modification shown in FIG. 10, the hydraulic drive system 2E includes an auxiliary pump 27 in addition to the main pump 21 and the accessory pump 23, and the external pilot line 53 includes the auxiliary pump 27. It may be connected to.

  In the configuration shown in FIG. 10, a relief line 57 is branched from the external pilot line 53, and a relief valve 58 is provided in the relief line 57. The set pressure of the relief valve 58 is set higher than the minimum value of the negative control pressure Pn.

  In the configuration shown in FIG. 10, the discharge pressure of the auxiliary pump 27 can be set to be small to some extent, so that the electromagnetic proportional valve 54 is not necessarily provided in the external pilot line 53.

  The hydraulic drive system of the present invention is useful for various construction machines.

DESCRIPTION OF SYMBOLS 1 Construction machine 2A-2E Hydraulic drive system 21 Main pump (hydraulic pump)
22 Regulator 23 Accessory pump 24 Boom cylinder (hydraulic actuator)
25 Bucket cylinder (hydraulic actuator)
28 Steering cylinder 31 Circulation line 32 Tank 33 Throttle 34 Cargo handling circuit (work circuit)
34a Center bypass line 4 Supply line 51 Pilot line 52 Switching valve 53 External pilot line 54 Proportional solenoid valve 55 Check valve 56 Accumulator 6 Controller 61 Temperature sensor 62 Pressure sensor 63 Angle sensor

Claims (6)

A hydraulic drive system mounted on an industrial vehicle including a plurality of wheels ,
A hydraulic pump whose tilt angle is changed by a regulator;
A circulation line constituting a center bypass line of a work circuit for driving a working hydraulic actuator extending from the hydraulic pump to the tank, wherein a circulation line provided with a throttle on the downstream side of the work circuit;
A pilot line connecting the regulator and the part between the working circuit and the throttle in the circulation line;
A switching valve provided in the pilot line;
An external pilot line that leads to the switching valve an external pilot pressure that is higher than the lowest negative control pressure that is the pressure upstream of the throttle in the circulation line;
A steering circuit for driving a pair of steering cylinders for changing the traveling direction;
A control device for controlling the switching valve ,
The switching valve is switched between a first position for guiding the negative control pressure to the regulator and a second position for guiding the external pilot pressure to the regulator ;
In the control device, a steering operation and a specific operation for driving the hydraulic actuator are not performed while the hydraulic pump is operating, and the temperature of the hydraulic oil is lower than a predetermined temperature. Sometimes the hydraulic drive system maintains the switching valve in the second position . 2. The hydraulic drive system according to claim 1 , wherein the control device switches the switching valve from a second position to a first position after a predetermined time has elapsed since the steering operation or the specific operation. The industrial vehicle includes a boom;
The hydraulic actuator is a boom cylinder that operates the boom,
The cargo handling circuit is a control valve for the boom cylinder, and is returned to a neutral position when the boom reaches the highest lift position or the lowest drop position during a detent operation for continuously raising or lowering the boom. Including
A control device for controlling the switching valve;
2. The hydraulic drive system according to claim 1, wherein the control device switches the switching valve from a first position to a second position before the boom reaches the highest lift position or the lowest drop position. The external pilot line is provided with an electromagnetic proportional valve that outputs the external pilot pressure as a secondary pressure,
The hydraulic drive system according to claim 3 , wherein the control device controls the electromagnetic proportional valve so that the external pilot pressure gradually increases after switching the switching valve to the second position. The hydraulic pump is a main pump;
An accessory pump driven by an engine common to the main pump;
The hydraulic drive system according to claim 1 , wherein the external pilot line is connected to a supply line that guides hydraulic oil discharged from the accessory pump. The external pilot line, said being connected to the upstream portion of the working circuit in the circulation line, the outside pilot line includes, in order from the upstream side, the check valve and the accumulator is provided, in claim 1 The described hydraulic drive system.

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