JP2013044398A - Hydraulic drive system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydraulic drive system capable of reducing loss in consumption power in a charge pump and also capable of preventing an increase in cost.SOLUTION: In the hydraulic drive system 1, a hydraulic fluid passage 15 constitutes a closed circuit between a main pump 10 and a hydraulic actuator 14. A charge circuit 16 replenishes the hydraulic fluid passage 15 with a hydraulic fluid when a hydraulic pressure in the hydraulic fluid passage 15 is lower than a hydraulic pressure in a charge passage 35. When the hydraulic actuator 14 is not working, charge hydraulic pressure reducing parts 24d and 37 reduce the hydraulic pressure in the charge passage 35 to be lower than the hydraulic pressure in the charge passage 35 when the hydraulic actuator 14 is working.

Description

  The present invention relates to a hydraulic drive system.

  Work machines such as hydraulic excavators and wheel loaders include hydraulic actuators such as hydraulic cylinders and hydraulic motors. The hydraulic oil discharged from the hydraulic pump is supplied to the hydraulic actuator via the hydraulic circuit. For example, Patent Document 1 proposes a work machine including a hydraulic closed circuit for supplying hydraulic oil to a hydraulic actuator. Since the hydraulic circuit is a closed circuit, the kinetic energy and potential energy of the member driven by the hydraulic actuator are regenerated. As a result, the fuel consumption of the prime mover that drives the hydraulic pump can be reduced.

  In many cases, a charge circuit is added to the hydraulic closed circuit. The charge circuit is provided, for example, to replenish hydraulic oil in an amount corresponding to oil leaked from the hydraulic pump. The charge circuit is provided with a charge pump and a relief valve. The charge pump is usually a fixed capacity pump and is driven by a drive source such as an engine. The relief valve defines the hydraulic pressure of the charge circuit (hereinafter referred to as “charge pressure”). When the flow rate of the hydraulic oil supplied to the hydraulic pump is insufficient and the hydraulic pressure of the hydraulic closed circuit falls below the charge pressure, the hydraulic oil is supplied from the charge circuit to the hydraulic closed circuit.

Special table 2009-511831

  The hydraulic closed circuit as described above is preferably provided in a hydraulic circuit where kinetic energy and potential energy are sufficiently regenerated. For this reason, the hydraulic closed circuit is often provided independently of the normal hydraulic circuit. For example, in the case of a hydraulic excavator, the boom cylinder is driven by a hydraulic closed circuit. Alternatively, in the case of a wheel loader, the lift cylinder is driven by a hydraulic closed circuit. In these cases, the closed hydraulic circuit is not operating when the vehicle is running. In the case of a wheel loader or bulldozer equipped with HST (Hydro Static Transmission), a hydraulic motor is driven by a hydraulic closed circuit. In this case, the hydraulic closed circuit is not operating when the vehicle is stopped. In such a case, the power consumed by the charge pump is almost lost.

  In order to reduce the loss of power consumption in the charge pump as described above, it is conceivable to use a variable displacement pump as the charge pump. In this case, when the hydraulic closed circuit is not operated, the loss of power consumed by the charge pump can be reduced by changing the discharge flow rate of the charge pump to zero. However, the variable displacement pump is more expensive than the fixed displacement pump. For this reason, when a variable displacement pump is used as a charge pump, there is a problem that the cost of the work machine increases.

  The subject of this invention is providing the hydraulic drive system which can suppress the increase in cost while being able to reduce the loss of the power consumption in a charge pump.

  The hydraulic drive system according to the first aspect of the present invention includes a hydraulic pump, a hydraulic actuator, a hydraulic fluid passage, a charge circuit, an operation member, an operation state determination unit, and a charge hydraulic pressure reduction unit. The main pump discharges hydraulic oil. The hydraulic actuator is driven by hydraulic oil discharged from the main pump. The hydraulic oil flow path connects the main pump and the hydraulic actuator, and forms a closed circuit between the main pump and the hydraulic actuator. The charge circuit has a charge flow path and a charge pump. The charge channel is connected to the hydraulic fluid channel. The charge pump discharges hydraulic oil into the charge flow path. The charge circuit replenishes the hydraulic oil passage with hydraulic oil when the hydraulic pressure of the hydraulic oil passage becomes smaller than the hydraulic pressure of the charge passage. The operation member is a member for operating the hydraulic actuator, for example, by controlling the discharge flow rate of the main pump. The operation state determination unit determines whether the hydraulic actuator is being operated or not being operated. The charge hydraulic pressure reduction unit reduces the hydraulic pressure in the charge channel when the hydraulic actuator is not operated, compared to the hydraulic pressure in the charge channel when the hydraulic actuator is operated.

  The hydraulic drive system according to the second aspect of the present invention is the hydraulic drive system according to the first aspect, and further includes a hydraulic oil tank. The hydraulic oil tank stores hydraulic oil. The charge hydraulic pressure reduction unit further includes a charge flow path opening / closing unit and a charge hydraulic pressure control unit. The charge channel opening / closing part can be switched between a connection state in which the charge channel is connected to the hydraulic oil tank and a closed state in which the space between the charge channel and the hydraulic oil tank is closed. The charge hydraulic pressure control unit sets the charge flow path opening / closing unit to a closed state when the hydraulic actuator is being operated. The charge hydraulic pressure control unit sets the charge flow path opening / closing unit to the connected state when the hydraulic actuator is not being operated.

  A hydraulic drive system according to a third aspect of the present invention is the hydraulic drive system according to the first aspect, and the charge hydraulic pressure reduction unit includes a relief valve and a charge hydraulic pressure control unit. The relief valve is adjusted so that the hydraulic pressure in the charge channel does not exceed a predetermined set pressure. The relief valve can switch the set pressure between the first set pressure and the second set pressure. The second set pressure is lower than the first set pressure. The charge hydraulic pressure control unit sets the set pressure of the relief valve to the first set pressure when the hydraulic actuator is being operated. The charge hydraulic pressure control unit sets the set pressure of the relief valve to the second set pressure when the hydraulic actuator is not operated.

  A hydraulic drive system according to a fourth aspect of the present invention is the hydraulic drive system according to the first aspect, wherein the operation state determination unit is configured to perform hydraulic pressure when the operation member is held at the neutral position for a predetermined time or more. It is determined that the actuator is not operating.

  In the hydraulic drive system according to the first aspect of the present invention, when the hydraulic actuator is not operated, the hydraulic pressure in the charge channel is reduced below the hydraulic pressure in the charge channel when the hydraulic actuator is being operated. . For this reason, loss of power consumption in the charge pump can be reduced without using a variable displacement hydraulic pump as the charge pump. Therefore, it is possible to reduce the loss of power consumed by the charge pump and to suppress an increase in cost.

  In the hydraulic drive system according to the second aspect of the present invention, when the hydraulic actuator is being operated, the space between the charge flow path and the hydraulic oil tank is closed. Further, when the hydraulic actuator is not operated, the charge flow path is connected to the hydraulic oil tank. Thereby, when the hydraulic actuator is not operated, the hydraulic pressure of the charge channel is reduced from the hydraulic pressure of the charge channel when the hydraulic actuator is operated.

  In the hydraulic drive system according to the third aspect of the present invention, when the hydraulic actuator is being operated, the set pressure of the relief valve is set to the first set pressure. Further, when the hydraulic actuator is not being operated, the set pressure of the relief valve is set to the second set pressure. Thereby, when the hydraulic actuator is not operated, the hydraulic pressure of the charge channel is reduced from the hydraulic pressure of the charge channel when the hydraulic actuator is operated.

  In the hydraulic drive system according to the fourth aspect of the present invention, it is determined that the hydraulic actuator is not operated when the operating member is held at the neutral position for a predetermined time or more. Therefore, it is possible to prevent erroneous determination that the hydraulic actuator is not operated when the hydraulic actuator is not being operated, such as when the operating member temporarily passes through the neutral position.

It is a block diagram which shows the structure of a hydraulic drive system. It is a block diagram which shows the structure of the hydraulic drive system which concerns on other embodiment. It is a block diagram which shows the structure of the hydraulic drive system which concerns on other embodiment. It is a block diagram which shows the structure of the hydraulic drive system which concerns on other embodiment. It is a block diagram which shows the structure of the hydraulic drive system which concerns on other embodiment. It is a block diagram which shows the structure of the hydraulic drive system which concerns on other embodiment.

  Hereinafter, a hydraulic drive system according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a hydraulic drive system 1 according to an embodiment of the present invention. The hydraulic drive system 1 is mounted on a work machine such as a hydraulic excavator, a wheel loader, or a bulldozer. The hydraulic drive system 1 includes an engine 11, a main pump 10, a hydraulic actuator 14, a hydraulic oil flow path 15, and a pump controller 24.

  The engine 11 drives the first hydraulic pump 12 and the second hydraulic pump 13. The engine 11 is, for example, a diesel engine, and the output of the engine 11 is controlled by adjusting the fuel injection amount from the fuel injection device 21. The fuel injection amount is adjusted by the fuel injection device 21 being controlled by the engine controller 22. The actual rotational speed of the engine 11 is detected by the rotational speed sensor 23, and the detection signal is input to the engine controller 22 and the pump controller 24, respectively.

  The engine controller 22 controls the output of the engine 11 by controlling the fuel injection device 21. The engine controller 22 maps and stores engine output torque characteristics set based on the set target engine speed and work mode. The engine output torque characteristic indicates the relationship between the output torque of the engine 11 and the rotation speed. The engine controller 22 controls the output of the engine 11 based on the engine output torque characteristics.

  The main pump 10 is driven by the engine 11 and discharges hydraulic oil. The hydraulic oil discharged from the main pump 10 is supplied to the hydraulic actuator 14. The main pump 10 includes a first hydraulic pump 12 and a second hydraulic pump 13.

  The first hydraulic pump 12 is a variable displacement hydraulic pump. By controlling the tilt angle of the first hydraulic pump 12, the discharge flow rate of the first hydraulic pump 12 is controlled. The tilt angle of the first hydraulic pump 12 is controlled by the first pump flow control device 25. The first pump flow rate control device 25 controls the discharge flow rate of the first hydraulic pump 12 by controlling the tilt angle of the first hydraulic pump 12 based on the command signal from the pump controller 24. The first hydraulic pump 12 is a two-way discharge type hydraulic pump. Specifically, the first hydraulic pump 12 has a first pump port 12a and a second pump port 12b. The first hydraulic pump 12 can be switched between a first discharge state and a second discharge state. In the first discharge state, the first hydraulic pump 12 is supplied with hydraulic oil to the second pump port 12b and discharges the hydraulic oil from the first pump port 12a. In the second discharge state, the first hydraulic pump 12 is supplied with hydraulic oil to the first pump port 12a and discharges hydraulic oil from the second pump port 12b.

  The second hydraulic pump 13 is a variable displacement hydraulic pump. By controlling the tilt angle of the second hydraulic pump 13, the discharge flow rate of the second hydraulic pump 13 is controlled. The tilt angle of the second hydraulic pump 13 is controlled by the second pump flow control device 26. The second pump flow rate control device 26 controls the discharge flow rate of the second hydraulic pump 13 by controlling the tilt angle of the second hydraulic pump 13 based on the command signal from the pump controller 24. The second hydraulic pump 13 is a two-way discharge type hydraulic pump. Specifically, the second hydraulic pump 13 has a first pump port 13a and a second pump port 13b. Similar to the first hydraulic pump 12, the second hydraulic pump 13 can be switched between a first discharge state and a second discharge state. In the first discharge state, the second hydraulic pump 13 is supplied with hydraulic oil to the second pump port 13b and discharges the hydraulic oil from the first pump port 13a. In the second discharge state, the second hydraulic pump 13 is supplied with hydraulic oil to the first pump port 13a and discharges hydraulic oil from the second pump port 13b.

  The hydraulic actuator 14 is driven by hydraulic oil discharged from the first hydraulic pump 12 and the second hydraulic pump 13. The hydraulic actuator 14 is a hydraulic cylinder, and drives a working machine such as a boom, an arm, or a bucket, for example. The hydraulic actuator 14 has a cylinder rod 14a and a cylinder tube 14b. The cylinder rod 14a partitions the inside of the cylinder tube 14b into a first chamber 14c and a second chamber 14d. The hydraulic actuator 14 expands and contracts by switching between supply and discharge of hydraulic oil to and from the first chamber 14c and the second chamber 14d. Specifically, the hydraulic oil is supplied to the first chamber 14c and the hydraulic oil is discharged from the second chamber 14d, whereby the cylinder rod 14a extends. When the hydraulic oil is supplied to the second chamber 14d and the hydraulic oil is discharged from the first chamber 14c, the cylinder rod contracts. The pressure receiving area in the first chamber 14c of the cylinder rod 14a is larger than the pressure receiving area in the second chamber 14d of the cylinder rod 14a. Therefore, when the hydraulic actuator 14 is extended, a larger amount of hydraulic oil than the hydraulic oil discharged from the second chamber 14d is supplied to the first chamber 14c. When the hydraulic actuator 14 is contracted, a larger amount of hydraulic oil than the hydraulic oil supplied to the second chamber 14d is discharged from the first chamber 14c.

  The hydraulic fluid passage 15 is connected to the first hydraulic pump 12, the second hydraulic pump 13, and the hydraulic actuator 14. The hydraulic oil flow path 15 connects the first chamber 14c and the first pump port 12a, and connects the second chamber 14d and the second pump port 12b. The hydraulic fluid passage 15 forms a closed circuit between the main pump 10 and the hydraulic actuator 14. Specifically, the hydraulic oil flow path 15 includes a first flow path 31 and a second flow path 32. The first flow path 31 connects the first chamber 14 c of the hydraulic actuator 14 and the first pump port 12 a of the first hydraulic pump 12. The first flow path 31 is a flow path for supplying hydraulic oil to the first chamber 14 c of the hydraulic actuator 14 or for collecting the hydraulic oil from the first chamber 14 c of the hydraulic actuator 14. The first flow path 31 is also connected to the first pump port 13 a of the second hydraulic pump 13. Therefore, the hydraulic fluid from both the first hydraulic pump 12 and the second hydraulic pump 13 is supplied to the first flow path 31. The second flow path 32 is connected to the second chamber 14 d of the hydraulic actuator 14 and the second pump port 12 b of the first hydraulic pump 12. The second flow path 32 is a flow path for supplying hydraulic oil to the second chamber 14 d of the hydraulic actuator 14 or for collecting the hydraulic oil from the second chamber 14 d of the hydraulic actuator 14. Note that the second pump port 13 b of the second hydraulic pump 13 is connected to the hydraulic oil tank 27. The hydraulic oil tank 27 stores hydraulic oil. Therefore, the hydraulic fluid from the first hydraulic pump 12 is supplied to the second flow path 32. The hydraulic fluid passage 15 forms a closed circuit between the main pump 10 and the hydraulic actuator 14 by the first passage 31 and the second passage 32.

  The hydraulic drive system 1 further includes a charge circuit 16. The charge circuit 16 includes a charge channel 35 and a charge pump 28. The charge pump 28 is a hydraulic pump for replenishing the working oil passage 15 with working oil. The charge pump 28 corresponds to the second hydraulic pump of the present invention. The charge pump 28 is driven by the engine 11 to discharge hydraulic oil to the charge flow path 35. The charge pump 28 is a fixed displacement hydraulic pump. The charge flow path 35 connects the charge pump 28 and the hydraulic oil flow path 15. Specifically, the charge flow path 35 is connected to the first flow path 31 via the check valve 41a. The check valve 41a is opened when the hydraulic pressure of the first flow path 31 becomes lower than the hydraulic pressure of the charge flow path 35 (hereinafter referred to as “charge pressure”). The charge flow path 35 is connected to the second flow path 32 via the check valve 41b. The check valve 41b is opened when the hydraulic pressure in the second flow path 32 becomes lower than the charge pressure. Thus, the charge circuit 16 replenishes the hydraulic oil passage 15 with hydraulic oil when the hydraulic pressure in the hydraulic oil passage 15 becomes smaller than the charge pressure. The charge flow path 35 is connected to the hydraulic oil tank 27 via a charge relief valve 42. The charge relief valve 42 maintains the charge pressure at a predetermined set pressure. When the hydraulic pressure of the first flow path 31 or the second flow path 32 becomes lower than the charge pressure, the hydraulic oil from the charge pump 28 is supplied to the first flow path 31 or the second flow path 32 via the charge flow path 35. The Thereby, the hydraulic pressure of the first flow path 31 and the second flow path 32 is maintained at a predetermined value or more.

  In addition, a charge channel opening / closing part 37 is connected to the charge channel 35. The charge flow path opening / closing part 37 is a so-called bypass valve, and can be switched between a connected state Pa and a closed state Pb. The charge channel opening / closing part 37 connects the charge channel 35 to the hydraulic oil tank 27 in the connection state Pa. The charge flow path opening / closing part 37 closes between the charge flow path 35 and the hydraulic oil tank 27 in the closed state Pb. The charge flow path opening / closing part 37 is an electromagnetic switching valve, and is switched between a connection state Pa and a closed state Pb by a command signal from the pump controller 24. Specifically, when the command signal from the pump controller 24 is OFF, the charge flow path opening / closing part 37 is set to the closed state Pb by the urging force of the urging member 37a. The charge flow path opening / closing part 37 is set to the connection state Pa when the command signal from the pump controller 24 is ON.

  The hydraulic oil passage 15 further has a relief passage 36. The relief channel 36 is connected to the first channel 31 via a check valve 41c. The check valve 41c is opened when the hydraulic pressure of the first flow path 31 becomes higher than the hydraulic pressure of the relief flow path 36. The relief flow path 36 is connected to the second flow path 32 via a check valve 41d. The check valve 41d is opened when the hydraulic pressure of the second flow path 32 becomes higher than the hydraulic pressure of the relief flow path 36. The relief flow path 36 is connected to the charge flow path 35 via a relief valve 43. The relief valve 43 maintains the pressure of the relief flow path 36 below a predetermined relief pressure. Thereby, the hydraulic pressure of the first flow path 31 and the second flow path 32 is maintained below a predetermined relief pressure.

  When the hydraulic actuator 14 is extended, the first hydraulic pump 12 and the second hydraulic pump 13 are driven in the first discharge state. As a result, the hydraulic oil discharged from the first pump port 12 a of the first hydraulic pump 12 and the first pump port 13 a of the second hydraulic pump 13 passes through the first flow path 31 and the first of the hydraulic actuator 14. It is supplied to one chamber 14c. Further, the hydraulic oil in the second chamber 14 d of the hydraulic actuator 14 passes through the second flow path 32 and is collected in the second pump port 12 b of the first hydraulic pump 12. As a result, the hydraulic actuator 14 extends.

  When the hydraulic actuator 14 is contracted, the first hydraulic pump 12 and the second hydraulic pump 13 are driven in the second discharge state. As a result, the hydraulic oil discharged from the second pump port 12 b of the first hydraulic pump 12 is supplied to the second chamber 14 d of the hydraulic actuator 14 through the second flow path 32. Further, the hydraulic oil in the first chamber 14 c of the hydraulic actuator 14 passes through the first flow path 31 and is collected in the first pump port 12 a of the first hydraulic pump 12 and the first pump port 13 a of the second hydraulic pump 13. The As a result, the hydraulic actuator 14 contracts.

  The hydraulic drive system 1 further includes an operation device 46. The operation device 46 includes an operation member 46a and an operation detection unit 46b. The operation member 46a is a member for operating the hydraulic actuator 14, and is operated by an operator to command various operations of the work machine. For example, when the hydraulic actuator 14 is a boom cylinder that drives a boom, the operation member 46a is a boom operation lever for operating the boom. That is, the operation member 46a is operated by the operator in order to operate the hydraulic actuator 14. The operation member 46a can be operated in two directions: a direction in which the hydraulic actuator 14 is extended from the neutral position, and a direction in which the hydraulic actuator 14 is contracted. The operation detection unit 46b detects an operation amount and an operation direction of the operation member 46a. The operation detection unit 46b is a sensor that detects the position of the operation member 46a, for example. When the operation member 46a is located at the neutral position, the operation amount of the operation member 46a is zero. A detection signal indicating the operation amount and operation direction of the operation member 46a is input from the operation detection unit 46b to the pump controller 24.

  The pump controller 24 controls the first hydraulic pump 12 and the second hydraulic pump 13 according to the operation amount of the operation member 46a. The pump controller 24 includes a pump control unit 24a, a storage unit 24b, an operation state determination unit 24c, and a charge hydraulic pressure control unit 24d. The pump control unit 24a, the operation state determination unit 24c, and the charge hydraulic pressure control unit 24d are realized by an arithmetic device such as a CPU. The storage unit 24b is realized by a recording device such as a RAM, a ROM, a hard disk, and a flash memory. The pump control unit 24a calculates a target flow rate of the hydraulic oil supplied to the hydraulic actuator 14 according to the operation amount of the operation member 46a. The storage unit 24 b stores information for controlling the first hydraulic pump 12 and the second hydraulic pump 13.

  The operation state determination unit 24c determines whether the hydraulic actuator 14 is being operated or not being operated based on the detection signal from the operation detection unit 46b. Specifically, the operation state determination unit 24c determines that the hydraulic actuator 14 is not being operated when the operation member 46a is held at the neutral position for a predetermined time or more. The operation state determination unit 24c determines that the hydraulic actuator 14 is being operated when the holding time at the neutral position of the operation member 46a is less than a predetermined time. The operation state determination unit 24c determines that the hydraulic actuator 14 is being operated when the operation member 46a is in a position other than the neutral position.

  When the hydraulic actuator 14 is not in operation, the charge hydraulic pressure control unit 24d controls the charge flow path opening / closing unit 37 described above to make the charge pressure higher than the charge pressure when the hydraulic actuator 14 is in operation. Reduce. Specifically, when the hydraulic actuator 14 is in operation, the charge hydraulic pressure control unit 24d turns off the command signal to the charge flow channel opening / closing unit 37, thereby bringing the charge flow channel opening / closing unit 37 into the closed state Pb. Set. Thereby, the charge pressure is defined by the set pressure of the charge relief valve 42. Further, when the hydraulic actuator 14 is not in operation, the charge hydraulic pressure control unit 24d sets the charge flow path opening / closing part 37 to the connection state Pa by turning on a command signal to the charge flow path opening / closing part 37. . Thereby, when the hydraulic actuator 14 is not operated, the charge pressure is reduced from the charge pressure when the hydraulic actuator 14 is operated. The charge hydraulic pressure control unit 24d and the charge flow path opening / closing unit 37 constitute a charge hydraulic pressure reduction unit of the present invention.

  The hydraulic drive system 1 according to the present embodiment has the following features.

  When the hydraulic actuator 14 is not operated, the charge pressure is reduced below the charge pressure when the hydraulic actuator 14 is operated. For this reason, loss of power consumption in the charge pump 28 can be reduced without using a variable displacement hydraulic pump as the charge pump 28. Therefore, it is possible to reduce power consumption loss in the charge pump 28 and to suppress an increase in cost.

  When the operation member 46a is held at the neutral position for a predetermined time or more, it is determined that the hydraulic actuator 14 is not being operated. Therefore, it is possible to prevent erroneous determination that the hydraulic actuator 14 is in operation when the hydraulic actuator 14 is not in operation, such as when the operation member 46a temporarily passes through the neutral position. it can.

  As mentioned above, although one Embodiment of this invention was described, this invention is not limited to the said embodiment, A various change is possible in the range which does not deviate from the summary of invention.

  For example, in the above embodiment, the charge flow path opening / closing part 37 is set to the closed state Pb when the command signal from the pump controller 24 is OFF. Moreover, the charge flow path opening / closing part 37 is set to the connection state Pa when the command signal from the pump controller 24 is ON. However, contrary to the above, as shown in FIG. 2, when the command signal from the pump controller 24 is OFF, the charge flow path opening / closing part 37 is set to the connection state Pa by the urging force of the urging member 37a. May be. In this case, the charge flow path opening / closing part 37 is set to the closed state Pb when the command signal from the pump controller 24 is turned ON. The pump controller 24 turns on a command signal to the charge flow path opening / closing part 37 when the hydraulic actuator 14 is being operated. Thereby, the charge pressure is defined by the set pressure of the charge relief valve 42. The pump controller 24 turns off the command signal to the charge flow path opening / closing part 37 when the hydraulic actuator 14 is not being operated. Thereby, the charge pressure when the hydraulic actuator 14 is not operated is reduced from the charge pressure when the hydraulic actuator 14 is operated.

  In the above embodiment, the charge flow path opening / closing part 37 is a bypass valve, but as shown in FIG. 3, a charge relief valve 38 may be used as the charge flow path opening / closing part. The charge relief valve 38 connects the charge flow path 35 and the hydraulic oil tank 27 when the charge pressure exceeds a predetermined set pressure. Thereby, the charge relief valve 38 adjusts so that the charge pressure does not exceed a predetermined set pressure. The set pressure of the charge relief valve 38 can be switched between the first set pressure and the second set pressure. The second set pressure is smaller than the first set pressure. When the pilot pressure is applied to the relief pilot port 38a, the set pressure of the charge relief valve 38 is set to the first set pressure. When the pilot pressure is not applied to the relief pilot port 38a of the charge relief valve 38, the set pressure of the charge relief valve 38 is set to the second set pressure. The set pressure of the charge relief valve 38 is switched by a set pressure switching unit 39. The set pressure switching unit 39 is, for example, an electromagnetic control valve, and is switched between the first position state P1 and the second position state P2 in response to a command signal from the pump controller 24. When the command signal from the pump controller 24 is OFF, the set pressure switching unit 39 is set to the first position state P1 by the urging force of the urging member 39a. The set pressure switching unit 39 is set to the second position state P2 when the command signal from the pump controller 24 is ON. The set pressure switching unit 39 connects the relief pilot flow path 44 to the hydraulic pressure source 40 in the first position state P1. The hydraulic source 40 is a minute capacity fixed capacity pump, for example, a gear pump. The relief pilot channel 44 is connected to a relief pilot port 38 a of the charge relief valve 38. Accordingly, the pilot pressure is applied to the relief pilot port 38a of the charge relief valve 38, and the set pressure of the charge relief valve 38 is set to the first set pressure. The set pressure switching unit 39 connects the relief pilot flow path 44 to the hydraulic oil tank 27 when in the second position state P2. For this reason, the pilot pressure is not applied to the relief pilot port 38a of the charge relief valve 38, and the set pressure of the charge relief valve 38 is set to the second set pressure. The charge hydraulic pressure control unit 24d turns off the command signal to the set pressure switching unit 39 when the hydraulic actuator 14 is being operated. Thereby, the set pressure of the charge relief valve 38 is set to the first set pressure. The charge hydraulic pressure control unit 24d turns on a command signal to the set pressure switching unit 39 when the hydraulic actuator 14 is not being operated. Thereby, the set pressure of the charge relief valve 38 is reduced to the second set pressure. For this reason, the charge pressure when the hydraulic actuator 14 is not operated is lower than the charge pressure when the hydraulic actuator 14 is operated.

  In the hydraulic drive system shown in FIG. 3, the set pressure switching unit 39 is set to the first position state P1 when the command signal from the pump controller 24 is OFF. Further, the set pressure switching unit 39 is set to the second position state P2 when the command signal from the pump controller 24 is ON. However, contrary to the above, as shown in FIG. 4, the set pressure switching unit 39 may be set to the first position state P <b> 1 when the command signal from the pump controller 24 is ON. In this case, the set pressure switching unit 39 is set to the second position state P2 when the command signal from the pump controller 24 is OFF. The pump controller 24 turns on a command signal to the charge flow path opening / closing part 37 when the hydraulic actuator 14 is being operated. Thereby, the set pressure of the charge relief valve 38 is set to the first set pressure. The pump controller 24 turns off the command signal to the set pressure switching unit 39 when the hydraulic actuator 14 is not being operated. Thereby, the set pressure of the charge relief valve 38 is reduced to the second set pressure. For this reason, the charge pressure when the hydraulic actuator 14 is not operated is lower than the charge pressure when the hydraulic actuator 14 is operated.

  In the hydraulic drive system shown in FIG. 3 or 4, the pilot pressure is applied to the relief pilot port 38 a by the hydraulic oil from the hydraulic source 40. However, as shown in FIG. 5, the pilot pressure may be applied to the relief pilot port 38a by the hydraulic oil from the charge pump 28.

  In the above embodiment, the present invention is applied to a two-pump hydraulic drive system in which two hydraulic pumps 12 and 13 are connected to the hydraulic actuator 14. However, one hydraulic pump is connected to the hydraulic actuator 14. The present invention may be applied to a one-pump hydraulic drive system.

  In the above embodiment, the hydraulic actuator 14 is a hydraulic cylinder, but other types of hydraulic actuators may be used. For example, as shown in FIG. 6, a hydraulic motor 49 may be used as a hydraulic actuator. The hydraulic motor 49 is, for example, a traveling hydraulic motor that constitutes an HST (Hydro Static Transmission). Alternatively, the hydraulic motor 49 may be a hydraulic motor that rotates the upper swing body.

  ADVANTAGE OF THE INVENTION According to this invention, while being able to reduce the loss of the power consumption by a charge pump, the hydraulic drive system which can suppress the increase in cost can be provided.

DESCRIPTION OF SYMBOLS 1 Hydraulic drive system 10 Main pump 14 Hydraulic actuator 15 Hydraulic oil flow path 16 Charge circuit 24c Operation state determination part 24d Charge hydraulic pressure control part 27 Hydraulic oil tank 28 Charge pump 35 Charge flow path 37 Charge flow path opening / closing part 38 Relief valve 46a Operation Element

Claims (4)

A main pump that discharges hydraulic oil;
A hydraulic actuator driven by hydraulic oil discharged from the main pump;
A hydraulic fluid passage connecting the main pump and the hydraulic actuator, and forming a closed circuit between the main pump and the hydraulic actuator;
A charge flow path connected to the hydraulic oil flow path; and a charge pump that discharges the hydraulic oil to the charge flow path, wherein the hydraulic pressure of the hydraulic oil flow path is smaller than the hydraulic pressure of the charge flow path. A charge circuit for replenishing hydraulic oil to the hydraulic oil flow path sometimes;
An operating member for operating the hydraulic actuator;
An operation state determination unit for determining whether the hydraulic actuator is being operated or not being operated;
A charge hydraulic pressure reduction unit that reduces the hydraulic pressure of the charge flow path when the hydraulic actuator is not operated, than the hydraulic pressure of the charge flow path when the hydraulic actuator is operated;
Hydraulic drive system with A hydraulic oil tank for storing hydraulic oil;
The charge hydraulic pressure reduction unit is
A charge flow path opening and closing portion that is switchable between a connection state in which the charge flow path is connected to the hydraulic oil tank and a closed state in which the charge flow path and the hydraulic oil tank are closed.
A charge hydraulic pressure that sets the charge flow path opening / closing part to the closed state when the hydraulic actuator is in operation, and sets the charge flow path opening / closing part to the connected state when the hydraulic actuator is not operated. A control unit;
Having
The hydraulic drive system according to claim 1. The charge hydraulic pressure reduction unit is
A relief valve capable of adjusting the hydraulic pressure of the charge flow path so as not to exceed a predetermined set pressure, and switching the set pressure between a first set pressure and a second set pressure lower than the first set pressure;
When the hydraulic actuator is being operated, the set pressure of the relief valve is set to the first set pressure, and when the hydraulic actuator is not being operated, the set pressure of the relief valve is set to the second set pressure. A charge hydraulic pressure controller to be set;
Having
The hydraulic drive system according to claim 1. The operation state determination unit determines that the hydraulic actuator is not operated when the operation member is held at a neutral position for a predetermined time or more.
The hydraulic drive system according to claim 1.

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