Hydraulic circuit for construction machine

Abstract

To provide a hydraulic circuit for a construction machine which can supply work fluid to a close circuit without a charge pump.SOLUTION: An open circuit 2a of a hydraulic circuit 2 includes: a hydraulic pump 4; a hydraulic actuator 6; a directional switch valve 8; and a return check valve 12 provided on a return line 10 for connecting the directional switch valve 8 and a work fluid tank 14. A close circuit 2b of the hydraulic circuit 2 includes: a hydraulic motor 30 operated by the work fluid ejected by a bidirectional hydraulic pump 28; a first line 32 for connecting a first port 28a of the bidirectional hydraulic pump 28 and the hydraulic motor 30; and a second line 34 for connecting a second port 28b of the bidirectional hydraulic pump 28 and the hydraulic motor 30. A charge line 44 is provided between the open circuit 2a and the close circuit 2b which connects the return line 10 and the first line 32 via a first check valve 46, and connects the return line 10 and the second line 34 via a second check valve 48.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hydraulic circuit for a construction machine that includes an open circuit and a closed circuit. [Background technology]

[0002] The hydraulic circuits of hydraulic excavators and wheel loaders often employ a closed circuit consisting of a bidirectional hydraulic pump and a hydraulic actuator. In such closed circuits, a charge pump is usually provided to supply hydraulic oil to the closed circuit in order to replenish the oil when the hydraulic actuator is stopped, cool the hydraulic oil in the closed circuit, and so on (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-173025 Summary of the Invention [Problem to be solved by the invention]

[0004] Adopting a closed circuit can lead to problems such as increased hydraulic circuit costs and wasted energy. When a charge pump is installed, it is generally recommended to use an external filter, so the cost of the charge pump and the external filter are included in the cost of the entire hydraulic circuit. Furthermore, the charge pump is driven by a drive source such as an engine, and is constantly driven as long as the drive source is running, even when there is no need to charge hydraulic oil into the closed circuit. This wastes energy.

[0005] An object of the present invention is to provide a hydraulic circuit for a construction machine that can supply hydraulic oil to a closed circuit without a charge pump. [Means for solving the problem]

[0006] According to the present invention, there is provided the following hydraulic circuit for a construction machine that solves the above-mentioned problems: "A hydraulic circuit of a construction machine having an open circuit and a closed circuit, The open circuit is a variable displacement hydraulic pump that discharges hydraulic oil drawn from a hydraulic oil tank; a hydraulic actuator that operates using hydraulic oil discharged by the hydraulic pump; a directional control valve that switches the flow direction of hydraulic oil from the hydraulic pump to the hydraulic actuator; a pump line connecting the hydraulic pump and the directional control valve; a return line connecting the directional control valve and the hydraulic oil tank; a return check valve installed in the return line; a bypass line connecting the pump line and the return line; an electromagnetic proportional bypass valve installed in the bypass line, The closed circuit is a variable displacement bidirectional hydraulic pump having a first port and a second port; a hydraulic motor operated by the hydraulic oil discharged by the bidirectional hydraulic pump; a first line connecting the first port of the bidirectional hydraulic pump and the hydraulic motor; a second line connecting the second port of the bidirectional hydraulic pump and the hydraulic motor; Between the open circuit and the closed circuit, a charge line is provided that connects a portion of the return line upstream of the return check valve to the first line via a first check valve, and that connects a portion of the return line upstream of the return check valve to the second line via a second check valve.

[0007] The closed circuit may include a first relief valve that releases hydraulic oil in the first line to the second line via the second check valve, and a second relief valve that releases hydraulic oil in the second line to the first line via the first check valve.

[0008] It is preferable that throttles are arranged between the first relief valve and the second check valve and between the second relief valve and the first check valve, that third relief valves are arranged between the first relief valve and the hydraulic oil tank and between the second relief valve and the hydraulic oil tank, and that the set pressure of the third relief valve is lower than the set pressure of the first relief valve and also lower than the set pressure of the second relief valve.

[0009] The closed circuit may include a first relief valve that releases hydraulic oil in the first line to the hydraulic oil tank, and a second relief valve that releases hydraulic oil in the second line to the hydraulic oil tank.

[0010] The first check valve and the second check valve may be mounted on a housing of the bidirectional hydraulic pump, and the charge line may be connected to the housing, or the first check valve and the second check valve may be mounted on a housing of the hydraulic motor, and the charge line may be connected to the housing.

[0011] The hydraulic circuit of the present invention preferably includes a pressure sensor that detects the pressure in the return line upstream of the return check valve, and a controller that increases the discharge rate of the hydraulic pump when the pressure detected by the pressure sensor is lower than the required pressure.

[0012] The controller preferably sets the required pressure based on an operation amount of a motor operating tool that outputs a signal for operating the hydraulic motor of the closed circuit. The controller may also set the required pressure based on a rotational speed of an object operated by the hydraulic motor of the closed circuit.

[0013] The hydraulic circuit of the present invention may include a regulator that controls the discharge amount and discharge direction of the bidirectional hydraulic pump, an electromagnetic proportional regulator switching valve that switches the flow direction of pilot hydraulic oil to the regulator, a pilot line that branches off from the pump line and extends to the regulator switching valve, and a pressure reducing valve installed in the pilot line.

[0014] In the hydraulic circuit of the present invention, a charge line is provided between the open circuit and the closed circuit, connecting the portion of the return line upstream of the return check valve to the first line via a first check valve, and connecting the portion of the return line upstream of the return check valve to the second line via a second check valve, so that hydraulic oil can be supplied to the closed circuit without a charge pump. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a hydraulic circuit diagram according to a first embodiment of the present invention. [Figure 2] 2 is a flowchart of control executed by the controller shown in FIG. 1; [Figure 3] FIG. 4 is a hydraulic circuit diagram according to a second embodiment of the present invention. [Figure 4] FIG. 6 is a hydraulic circuit diagram according to a third embodiment of the present invention. [Figure 5] FIG. 10 is a hydraulic circuit diagram according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] (First embodiment) First, a first embodiment of a hydraulic circuit for a construction machine according to the present invention will be described with reference to the drawings.

[0017] (Hydraulic circuit 2, open circuit 2a) 1 shows a hydraulic circuit 2 according to a first embodiment. The hydraulic circuit 2 includes an open circuit 2a and a closed circuit 2b. The open circuit 2a includes a hydraulic pump 4, a hydraulic actuator 6, a directional control valve 8, a return line 10, and a return check valve 12.

[0018] (Hydraulic pump 4) The hydraulic pump 4 has a suction port 4a and a discharge port 4b, and draws hydraulic oil from a hydraulic oil tank 14 through the suction port 4a and discharges the hydraulic oil from the discharge port 4b to a pump line 16. The hydraulic pump 4 is driven by a drive source 18 such as an engine or an electric motor, and while the drive source 18 is running, hydraulic oil is constantly discharged from the hydraulic pump 4 to the pump line 16. However, the hydraulic pump 4 is of a variable displacement type, and the amount of hydraulic oil discharged to the pump line 16 can be changed as appropriate depending on the operating conditions of the construction machine in which the hydraulic circuit 2 is installed.

[0019] (Hydraulic Actuator 6) The hydraulic actuator 6 is operated by the hydraulic oil discharged by the hydraulic pump 4. Although FIG. 1 shows a hydraulic cylinder as the hydraulic actuator 6, the hydraulic actuator 6 may also be a hydraulic motor. The number of hydraulic actuators 6 is arbitrary. For example, the hydraulic actuators 6 of the open circuit 2a may include a plurality of hydraulic cylinders and a plurality of hydraulic motors.

[0020] (Directional switching valve 8) The directional control valve 8 switches the flow direction of hydraulic oil from the hydraulic pump 4 to the hydraulic actuator 6. The directional control valve 8 is connected to the hydraulic pump 4 via a pump line 16, and is also connected to a hydraulic oil tank 14 via a return line 10. The directional control valve 8 is also connected to the hydraulic actuator 6 via a pair of lines 20a, 20b. The directional control valve 8 may be of an electromagnetic proportional type or a hydraulic pilot type. A plurality of directional control valves 8 may be provided corresponding to the number of hydraulic actuators 6.

[0021] When the directional control valve 8 is opened, the pump line 16 is connected to one of the pair of lines 20a, 20b, and the other of the pair of lines 20a, 20b is connected to the return line 10. This activates the hydraulic actuator 6. When the directional control valve 8 is closed, the pump line 16 and the return line 10 are cut off from the pair of lines 20a, 20b, and the operation of the hydraulic actuator 6 stops.

[0022] (Return line 10, return check valve 12) The return line 10 is a line that connects the directional control valve 8 and a hydraulic oil tank 14. A return check valve 12 installed in the return line 10 generates back pressure in the return line 10. The back pressure generated by the return check valve 12 increases as the amount of hydraulic oil passing through the return line 10 (return flow rate) increases. In addition, a pressure sensor 22 is provided in the return line 10 to detect the pressure in the return line 10 upstream of the return check valve 12.

[0023] (Bypass line 24, bypass valve 26) The open circuit 2a is provided with a bypass line 24 that connects the pump line 16 and the return line 10. The bypass line 24 is provided with an electromagnetic proportional bypass valve 26 that adjusts the amount of hydraulic oil flowing from the pump line 16 to the return line 10.

[0024] (Closed circuit 2b) The closed circuit 2b includes a bidirectional hydraulic pump 28, a hydraulic motor 30, a first line 32, and a second line 34.

[0025] (Bidirectional hydraulic pump 28) The bidirectional hydraulic pump 28 has a first port 28a and a second port 28b, and draws hydraulic oil through the first port 28a and discharges it through the second port 28b, or draws hydraulic oil through the second port 28b and discharges it through the first port 28a. That is, in the bidirectional hydraulic pump 28, the first port 28a may be the suction port and the second port 28b may be the discharge port, or conversely, the second port 28b may be the suction port and the first port 28a may be the discharge port. Note that the closed circuit 2b is a closed circuit formed by the bidirectional hydraulic pump 28 and the hydraulic motor 30, and therefore, unlike the hydraulic pump 4 of the open circuit 2a, the bidirectional hydraulic pump 28 does not draw in and discharge hydraulic oil from the hydraulic oil tank 14.

[0026] The bidirectional hydraulic pump 28 is driven by the drive source 18 in the same way as the hydraulic pump 4 of the open circuit 2a. For this reason, the bidirectional hydraulic pump 28 is configured to rotate constantly while the drive source 18 is operating. However, since the bidirectional hydraulic pump 28 is a variable displacement type and the discharge amount (capacity) and discharge direction are controlled by the regulator 36, there may be cases where hydraulic oil is not discharged from either the first port 28a or the second port 28b even if the bidirectional hydraulic pump 28 is rotating.

[0027] The regulator 36 of the bidirectional hydraulic pump 28 is of a hydraulic pilot type, and the flow direction of the hydraulic oil to the regulator 36 is switched by an electromagnetic proportional regulator switching valve 38. The regulator switching valve 38 is connected to the pump line 16 via a pilot line 40. A pressure reducing valve 42 is installed in the pilot line 40 to reduce the pressure of the hydraulic oil in the pump line 16 to a predetermined level.

[0028] The housing 28c of the bidirectional hydraulic pump 28 is indicated by a dashed line in Fig. 1. The components arranged within the rectangular area defined by the dashed line (for example, first and second check valves 46, 48 and first and second relief valves 50, 52, which will be described later) are assumed to be attached to the housing 28c of the bidirectional hydraulic pump 28.

[0029] (Hydraulic motor 30, first and second lines 32, 34) The hydraulic motor 30 is operated by hydraulic oil discharged from the bidirectional hydraulic pump 28. The hydraulic motor 30 has a first port 30a and a second port 30b. The first port 30a of the hydraulic motor 30 is connected to a first port 28a of the bidirectional hydraulic pump 28 via a first line 32. The second port 30b of the hydraulic motor 30 is connected to a second port 28b of the bidirectional hydraulic pump 28 via a second line 34.

[0030] (Charge Line 44) A charge line 44 is provided between the open circuit 2a and the closed circuit 2b to supply hydraulic oil from the open circuit 2a to the closed circuit 2b. The open circuit 2a side of the charge line 44 is connected to a portion of the return line 10 upstream of the return check valve 12. On the other hand, the closed circuit 2b side of the charge line 44 is connected to the first line 32 via a first check valve 46 and to the second line 34 via a second check valve 48. In addition, the closed circuit 2b side of the charge line 44 is connected to the housing 28c of the bidirectional hydraulic pump 28.

[0031] (First and second relief valves 50, 52) The closed circuit 2b of the first embodiment includes a first relief valve 50 that releases the hydraulic oil in the first line 32 to the second line 34 via the second check valve 48, and a second relief valve 52 that releases the hydraulic oil in the second line 34 to the first line 32 via the first check valve 46. The set pressures of the first and second relief valves 50, 52 may be, for example, 30 MPa to 40 MPa.

[0032] The hydraulic circuit 2 also includes an actuator operating device 54 , a motor operating device 56 , and a controller 58 .

[0033] (Actuator operating tool 54, motor operating tool 56) The actuator operating device 54 outputs a signal for operating the hydraulic actuator 6. The motor operating device 56 outputs a signal for operating the hydraulic motor 30. The actuator operating device 54 and the motor operating device 56 may be configured to include input devices (for example, a lever that can be operated in the forward and backward directions, a joystick that can be operated in a cross direction, a slide switch, a pedal, etc.) that output signals with increasing strength as the amount of operation increases. While FIG. 1 shows an example in which the operating devices 54, 56 output electrical signals, the operating devices 54, 56 may also output hydraulic signals.

[0034] (Controller 58) The controller 58 executes circuit control based on signals output from the operating tools 54, 56. The controller 58 is composed of a computer having a processing device and a storage device. When the signals output from the operating tools 54, 56 are hydraulic signals, the output hydraulic signals are detected by a pressure sensor (not shown), and the detection result of the pressure sensor is input to the controller 58.

[0035] (Operation of hydraulic circuit 2) Next, the operation of the hydraulic circuit 2 as described above, particularly the supply of hydraulic oil from the open circuit 2a to the closed circuit 2b, will be described. First, the supply of pilot hydraulic oil to the regulator 36 of the closed circuit 2b will be described, and then the replenishment of hydraulic oil to the first and second lines 32, 34 of the closed circuit 2b will be described. Note that the following description will be given assuming that the directional control valve 8 is an electromagnetic proportional type and is controlled by an electrical signal from the controller 58.

[0036] (Supply of pilot oil to regulator 36) Pilot hydraulic oil can be supplied to the regulator 36 of the bidirectional hydraulic pump 28 from the hydraulic pump 4 of the open circuit 2a. As described above, the regulator 36 is connected to the hydraulic pump 4 of the open circuit 2a via the pump line 16 and the pilot line 40. Furthermore, while the drive source 18 is operating, hydraulic oil is constantly discharged from the hydraulic pump 4 to the pump line 16. Therefore, in the first embodiment, pilot hydraulic oil can be supplied to the regulator 36 of the closed circuit 2b even without a charge pump.

[0037] However, pilot hydraulic oil is actually supplied to the regulator 36 of the bidirectional hydraulic pump 28 only when the motor operating device 56 is operated. When the motor operating device 56 is operated, a signal is output from the motor operating device 56 in accordance with the amount of operation. In response to the signal output from the motor operating device 56, the controller 58 opens the regulator switching valve 38 and adjusts the aperture of the regulator switching valve 38. Therefore, the hydraulic oil discharged from the hydraulic pump 4 to the pump line 16 is reduced to a predetermined pressure by the pressure reducing valve 42 in the pilot line 40 and supplied to the regulator 36. As a result, hydraulic oil is discharged from the first port 28a or the second port 28b of the bidirectional hydraulic pump 28 in accordance with the operation direction of the motor operating device 56, and an amount of hydraulic oil corresponding to the amount of operation of the motor operating device 56 is discharged from the bidirectional hydraulic pump 28. As a result, the hydraulic motor 30 rotates in a direction corresponding to the operation direction of the motor operating device 56 and at a rotational speed corresponding to the operation amount of the motor operating device 56.

[0038] On the other hand, when the motor operating device 56 is not operated, pilot hydraulic oil is not supplied to the regulator 36. When the motor operating device 56 is not operated, no signal is output from the motor operating device 56. In this case, the controller 58 positions the regulator selector valve 38 in the closed position and causes the regulator selector valve 38 to block the pilot line 40, so that pilot hydraulic oil is not supplied to the regulator 36. As a result, hydraulic oil is not discharged from the bidirectional hydraulic pump 28 and the hydraulic motor 30 does not operate.

[0039] (Replenishing hydraulic oil to the first and second lines 32 and 34) Next, the replenishment of hydraulic oil to the first and second lines 32, 34 of the closed circuit 2b will be described.

[0040] In the hydraulic circuit 2, when the actuator operating device 54 is operated, the hydraulic actuator 6 is actuated. When the actuator operating device 54 is operated, a signal is output from the actuator operating device 54 in accordance with the amount of operation. In response to the signal output from the actuator operating device 54, the controller 58 adjusts the discharge rate of the hydraulic pump 4 and the opening of the bypass valve 26, thereby increasing the pressure in the pump line 16. Furthermore, in accordance with the signal output from the actuator operating device 54, the controller 58 opens the directional control valve 8 and adjusts the opening of the directional control valve 8. As a result, hydraulic oil is supplied from the hydraulic pump 4 to the hydraulic actuator 6 via the pump line 16, and hydraulic oil is returned from the hydraulic actuator 6 to the hydraulic oil tank 14 via the return line 10. As a result, the hydraulic actuator 6 is actuated.

[0041] When the hydraulic actuator 6 is operating, hydraulic oil is discharged from the hydraulic actuator 6 to the return line 10, causing the return check valve 12 to generate back pressure in the return line 10. As a result, a pressure equal to or greater than a predetermined value acts on the charge line 44, which branches off from the return line 10. As a result, hydraulic oil can be replenished from the charge line 44 to the first line 32 via the first check valve 46, or hydraulic oil can be replenished from the charge line 44 to the second line 34 via the second check valve 48. Therefore, in the first embodiment, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44 even without a charge pump.

[0042] On the other hand, when the actuator operating device 54 is not operated, the hydraulic actuator 6 does not operate and hydraulic oil is not discharged from the hydraulic actuator 6 to the return line 10. When the actuator operating device 54 is not operated, no signal is output from the actuator operating device 54. In this case, the controller 58 positions the directional control valve 8 in the closed position. Therefore, hydraulic oil is not supplied from the pump line 16 to the hydraulic actuator 6, and hydraulic oil is not discharged from the hydraulic actuator 6 to the return line 10.

[0043] However, even when hydraulic oil is not discharged from the hydraulic actuator 6 to the return line 10, hydraulic oil is still sent to the return line 10. When the actuator operating device 54 is not being operated, the controller 58 adjusts the discharge rate of the hydraulic pump 4 to a relatively small standby flow rate and adjusts the opening of the bypass valve 26 to a predetermined opening that is not fully closed. This maintains the pressure in the pump line 16 at a predetermined standby pressure (for example, 3 MPa to 4 MPa). Furthermore, the hydraulic oil discharged from the hydraulic pump 4 to the pump line 16 is sent to the return line 10 through the bypass line 24.

[0044] As a result, a pressure equal to or greater than a predetermined value also acts on the charge line 44 that branches off from the return line 10, allowing hydraulic oil to be replenished from the charge line 44 to the first line 32 or the second line 34. Therefore, in the first embodiment, even if hydraulic oil is not being discharged from the hydraulic actuator 6 to the return line 10, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44.

[0045] (Adjusting the pressure in the return line 10) It is desirable that the controller 58 be configured to increase the discharge rate of the hydraulic pump 4 when the pressure in the return line 10 detected by the pressure sensor 22 is lower than the required pressure. This is because, depending on the operating conditions of the construction machine, the pressure in the return line 10 may not rise to the value (required pressure) necessary to replenish hydraulic oil in the closed circuit 2b.

[0046] In order to reduce energy consumption, the discharge rate (standby flow rate) of the hydraulic pump 4 when the hydraulic actuator 6 is not operating is less than the discharge rate of the hydraulic pump 4 when the hydraulic actuator 6 is operating. That is, the flow rate in the return line 10 when the hydraulic actuator 6 is not operating is less than the flow rate in the return line 10 when the hydraulic actuator 6 is operating. Therefore, the pressure in the return line 10 when the hydraulic actuator 6 is not operating is lower than the pressure in the return line 10 when the hydraulic actuator 6 is operating. As a result, depending on the operating conditions of the construction machine, the pressure in the return line 10 may not rise to the required pressure when the hydraulic actuator 6 is not operating. However, even when the hydraulic actuator 6 is operating, the pressure in the return line 10 may not rise to the required pressure depending on the operating conditions of the construction machine.

[0047] Therefore, it is desirable that the controller 58 increases the discharge rate of the hydraulic pump 4 to raise the pressure in the return line 10 to the required pressure when the pressure in the return line 10 detected by the pressure sensor 22 is lower than the required pressure, regardless of whether the hydraulic actuator 6 is operating. This makes it possible to supply hydraulic oil to the closed circuit 2b regardless of the operating conditions of the construction machine.

[0048] Regarding the above-mentioned "required pressure," it is preferable that the controller 58 sets the required pressure based on the amount of operation of the motor operating tool 56.

[0049] After the motor operating device 56 is operated to operate the hydraulic motor 30, when the motor operating device 56 is returned to the neutral position (non-operating position), the supply of hydraulic oil from the bidirectional hydraulic pump 28 to the hydraulic motor 30 is cut off. When the motor operating device 56 is returned to the neutral position, the signal from the motor operating device 56 is discontinued, and the controller 58 stops the discharge of hydraulic oil from the bidirectional hydraulic pump 28 via the regulator selector valve 38. As a result, the supply of hydraulic oil from the bidirectional hydraulic pump 28 to the hydraulic motor 30 is cut off.

[0050] However, even if the supply of hydraulic oil to the hydraulic motor 30 is cut off, the hydraulic motor 30 may continue to operate, which may result in cavitation. Even if the supply of hydraulic oil to the hydraulic motor 30 is cut off, the hydraulic motor 30 may continue to operate due to the inertia of the object operated by the hydraulic motor 30 (for example, in a hydraulic excavator, the upper rotating body rotated by a hydraulic swing motor). In this case, the hydraulic motor 30 functions as a pump. That is, the hydraulic motor 30 draws hydraulic oil from one of the first and second ports 30a and 30b and discharges it from the other of the first and second ports 30a and 30b. Therefore, cavitation may occur in the line into which the hydraulic oil is drawn (the first line 32 or the second line 34).

[0051] To prevent cavitation from occurring, a required pressure must be applied to the return line 10. This required pressure varies depending on the inertial force of the workpiece. That is, the greater the inertial force of the workpiece, the greater the required pressure, and the smaller the inertial force of the workpiece, the smaller the required pressure. The inertial force of the workpiece depends on the rotation speed of the hydraulic motor 30 just before it stops. The rotation speed of the hydraulic motor 30 just before it stops depends on the amount of hydraulic oil supplied from the bidirectional hydraulic pump 28 to the hydraulic motor 30. The amount of hydraulic oil supplied from the bidirectional hydraulic pump 28 to the hydraulic motor 30 is controlled by the controller 58 in accordance with the amount of operation of the motor operating device 56. Therefore, it is preferable that the controller 58 set the required pressure based on the amount of operation of the motor operating device 56.

[0052] Hereinafter, the control executed by the controller 58 to apply the required pressure to the return line 10 will be described with reference to FIG.

[0053] (Step S1) First, the controller 58 executes step S1 of calculating the amount of hydraulic oil supplied from the bidirectional hydraulic pump 28 to the hydraulic motor 30 (the flow rate of the hydraulic motor 30) based on a signal output from the motor operating device 56 in accordance with the amount of operation of the motor operating device 56. A first map indicating the relationship between the amount of operation of the motor operating device 56 and the flow rate of the hydraulic motor 30 is registered in advance in the controller 58. Therefore, the controller 58 refers to the first map to calculate the flow rate of the hydraulic motor 30 from the signal output from the motor operating device 56.

[0054] (Step S2) After executing step S1, the controller 58 executes step S2 in which the controller 58 determines the rotation speed of the hydraulic motor 30 from the flow rate of the hydraulic motor 30 determined in step S1. A second map indicating the relationship between the flow rate of the hydraulic motor 30 and the rotation speed of the hydraulic motor 30 is registered in advance in the controller 58. Therefore, the controller 58 determines the rotation speed of the hydraulic motor 30 from the flow rate of the hydraulic motor 30 by referring to the second map.

[0055] (Step S3) After executing step S2, the controller 58 executes step S3 of determining the required pressure to be applied to the return line 10 from the rotation speed of the hydraulic motor 30 determined in step S2. A third map indicating the relationship between the rotation speed of the hydraulic motor 30 and the required pressure to be applied to the return line 10 is registered in advance in the controller 58. The third map takes into account the rotation speed of the object operated by the hydraulic motor 30, the mass of the object, etc. The controller 58 then refers to the third map and determines the required pressure to be applied to the return line 10 based on the rotation speed of the hydraulic motor 30.

[0056] (Step S4) After executing step S3, the controller 58 executes step S4 to determine whether the pressure in the return line 10 is smaller than the required pressure. The pressure in the return line 10 is input to the controller 58 as the detection result of the pressure sensor 22. The required pressure is the pressure calculated in step S3. If the pressure in the return line 10 is smaller than the required pressure (if the determination result in step S4 is Yes), the process proceeds to step S5. On the other hand, if the pressure in the return line 10 is greater than the required pressure (if the determination result in step S4 is No), the process returns to step S1.

[0057] (Step S5) When the process proceeds from step S4 to step S5, the controller 58 determines the actual return flow rate from the detection result of the pressure sensor 22. As described above, the return line 10 is provided with the return check valve 12 that generates back pressure, and the back pressure generated by the return check valve 12 increases as the amount of hydraulic oil passing through the return line 10 (return flow rate) increases. A fourth map created based on the characteristics of the return check valve 12 is registered in advance in the controller 58. The fourth map is a map that shows the relationship between the flow rate passing through the return line 10 (return flow rate) and the back pressure generated in the return line 10 by the return check valve 12. The controller 58 then determines the actual return flow rate from the detection result of the pressure sensor 22 (actual pressure in the return line 10) by referring to the fourth map.

[0058] (Step S6) After executing step S5, the controller 58 executes step S6, in which the controller 58 refers to the fourth map to determine the return flow rate corresponding to the required pressure determined in step S3. Note that step S6 may be executed before step S5, or steps S5 and S6 may be executed in parallel.

[0059] (Step S7) After executing step S6, the controller 58 calculates the corrected flow rate using the following formula 1 from the actual return flow rate (flow rate before correction) calculated in step S5 and the return flow rate corresponding to the required pressure calculated in step S6. Equation 1 Corrected flow rate = Return flow rate corresponding to required pressure - Actual return flow rate

[0060] (Step S8) After executing step S7, the controller 58 executes step S8 to determine a correction current from the correction flow rate determined in step S7. A fifth map indicating the relationship between the current sent to the hydraulic pump 4 and the discharge rate of the hydraulic pump 4 is registered in advance in the controller 58. The controller 58 then refers to the fifth map to determine the correction current corresponding to the correction flow rate.

[0061] (Step S9) After executing step S8, the controller 58 executes step S9 to correct the discharge rate of the hydraulic pump 4 based on the correction current calculated in step S8. Specifically, the controller 58 sends to the hydraulic pump 4 a current that is the sum of the correction current calculated in step S8 and the current that was sent to the hydraulic pump 4 before the correction. This causes the hydraulic pump 4 to discharge a flow rate that is the discharge rate of the hydraulic pump 4 before the correction plus the correction flow rate. As a result, the pressure in the return line 10 rises to the required pressure, making it possible to replenish the necessary amount of hydraulic oil to the closed circuit 2b via the charge line 44, thereby preventing the occurrence of cavitation. When correcting the discharge rate of the hydraulic pump 4, an adjustment current as a safety margin may be added to the current before the correction along with the correction current.

[0062] In this way, it is desirable that the controller 58 sets the required pressure based on the amount of operation of the motor operating device 56, and when the pressure in the return line 10 detected by the pressure sensor 22 is lower than the required pressure, increase the discharge volume of the hydraulic pump 4 to raise the pressure in the return line 10 to the required pressure.

[0063] Alternatively, the controller 58 may set the required pressure based on the rotational speed of the working object operated (rotated) by the hydraulic motor 30 of the closed circuit 2b. The rotational speed of the working object is related to the inertial force of the working object, and the inertial force of the working object is related to the required pressure to be applied to the return line 10. In other words, the required pressure depends on the rotational speed of the working object. Therefore, the controller 58 can set the required pressure based on the rotational speed of the working object.

[0064] In the case where the controller 58 sets the required pressure based on the rotational speed of the actuated object, a rotational speed detection means (for example, a rotational angle sensor that detects the rotational angle of the actuated object) for detecting the rotational speed of the actuated object is provided, and the detection result of the rotational speed detection means is sent to the controller 58. Furthermore, a sixth map indicating the relationship between the rotational speed of the actuated object and the required pressure is registered in advance in the controller 58. The controller 58 then sets the required pressure from the rotational speed of the actuated object by referring to the sixth map, and, when the pressure in the return line 10 detected by the pressure sensor 22 is lower than the required pressure, increases the discharge rate of the hydraulic pump 4 to raise the pressure in the return line 10 to the required pressure.

[0065] As described above, in the first embodiment, hydraulic oil can be replenished from the charge line 44 to the first line 32 via the first check valve 46, or hydraulic oil can be replenished from the charge line 44 to the second line 34 via the second check valve 48. That is, in the first embodiment, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44 even without a charge pump.

[0066] Therefore, in the first embodiment, there is no need for a charge pump, an external filter that should be attached to the charge pump, or piping components for incorporating the charge pump and external filter into the hydraulic circuit 2, so there is no increase in the cost of the hydraulic circuit 2. In addition, the reduction in the number of parts makes maintenance easier. Furthermore, in the first embodiment, hydraulic oil (return oil) in the return line, which does not use the horsepower of the drive source, is used to replenish hydraulic oil to the closed circuit 2b, so horsepower consumption of the drive source can be reduced. In other words, energy consumption can be suppressed.

[0067] (Second embodiment) Next, a second embodiment of a hydraulic circuit for a construction machine according to the present invention will be described with reference to Fig. 3. In the second embodiment, components that may be the same as those in the first embodiment are given the same reference numerals as those in the first embodiment, and descriptions thereof will be omitted.

[0068] 3, throttles 60 are disposed between the first relief valve 50 and the second check valve 48 and between the second relief valve 52 and the first check valve 46. In addition, third relief valves 62 are disposed between the first relief valve 50 and the hydraulic oil tank 14 and between the second relief valve 52 and the hydraulic oil tank 14. The set pressure of the third relief valve 62 is lower than the set pressure of the first relief valve 50 and is also lower than the set pressure of the second relief valve 52. In addition, a third check valve 64 is provided in the charge line 44, which allows flow from the open circuit 2a to the closed circuit 2b and prevents flow from the closed circuit 2b to the open circuit 2a.

[0069] In the second embodiment, when the pressure in the first line 32 exceeds the set pressure of the first relief valve 50, the hydraulic oil in the first line 32 passes through the first relief valve 50, then flows through the throttle 60 and the second check valve 48 to the second line 34, and also flows through the third relief valve 62 to the hydraulic oil tank 14. Similarly, when the pressure in the second line 34 exceeds the set pressure of the second relief valve 52, the hydraulic oil in the second line 34 passes through the second relief valve 52, then flows through the throttle 60 and the first check valve 46 to the first line 32, and also flows through the third relief valve 62 to the hydraulic oil tank 14.

[0070] As described above, in the second embodiment, a portion of the hydraulic oil relieved from the first and second lines 32, 34 flows into the hydraulic oil tank 14. Therefore, in the second embodiment, a larger amount of hydraulic oil is replenished from the open circuit 2a to the closed circuit 2b. That is, in the second embodiment, the hydraulic oil in the closed circuit 2b is more easily replaced. Generally, the temperature of hydraulic oil rises more easily in a closed circuit than in an open circuit. However, in the second embodiment, the hydraulic oil in the closed circuit 2b is more easily replaced, which prevents the temperature of the hydraulic oil from rising excessively. Note that in the second embodiment, as in the first embodiment, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44 without the need for a charge pump.

[0071] (Third embodiment) Next, a third embodiment of a hydraulic circuit for a construction machine according to the present invention will be described with reference to Figure 4. In the third embodiment, components that may be the same as those in the first and second embodiments are given the same reference numerals as those in the first and second embodiments, and descriptions thereof will be omitted.

[0072] The closed circuit 2b of the third embodiment includes a first relief valve 50' that releases the hydraulic oil in the first line 32 to the hydraulic oil tank 14, and a second relief valve 52' that releases the hydraulic oil in the second line 34 to the hydraulic oil tank 14. As can be understood by referring to FIG. 4 , the first relief valve 50' of the third embodiment differs from the first relief valve 50 of the first and second embodiments in that it is not connected to the second check valve 48. Furthermore, the second relief valve 52' of the third embodiment differs from the second relief valve 52 of the first and second embodiments in that it is not connected to the first check valve 46.

[0073] In the third embodiment, all of the hydraulic oil that passes through the first and second relief valves 50', 52' flows into the hydraulic oil tank 14. Therefore, in the third embodiment, the hydraulic oil in the closed circuit 2b is replaced more easily than in the second embodiment, and temperature increases in the hydraulic oil are further suppressed. Note that in the third embodiment, as in the first and second embodiments, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44 without the need for a charge pump.

[0074] (Fourth embodiment) Finally, a fourth embodiment of a hydraulic circuit for a construction machine according to the present invention will be described with reference to Figure 5. In the fourth embodiment, components that may be the same as those in the first to third embodiments are given the same reference numerals as those in the first to third embodiments, and descriptions thereof will be omitted.

[0075] In the fourth embodiment, the first and second check valves 46, 48 are mounted on the housing 30c of the hydraulic motor 30, and the charge line 44 is also connected to the housing 30c of the hydraulic motor 30.

[0076] In the fourth embodiment, for example, when the hydraulic circuit 2 is mounted on a hydraulic excavator and the hydraulic motor 30 of the closed circuit 2b is a swing motor that swings the upper swing body of the hydraulic excavator, the layout of the charge line 44 connecting the open circuit 2a and the closed circuit 2b is simplified. Generally, in a hydraulic excavator, the swing motor is disposed closer to the directional control valve than the hydraulic pump. Therefore, connecting the charge line 44 to the housing 30c of the hydraulic motor 30 (swing motor) as in the fourth embodiment is easier to lay out than connecting the charge line 44 to the housing 28c of the bidirectional hydraulic pump 28 as in the third embodiment. In the fourth embodiment, hydraulic oil can be supplied from the open circuit 2a to the closed circuit 2b via the charge line 44, even without a charge pump, as in the first to third embodiments. [Explanation of symbols]

[0077] 2: Hydraulic circuit 2a: open circuit 2b: Closed circuit 4: Hydraulic pump 6: Hydraulic actuator 8: Directional valve 10: Return Line 12: Return check valve 14: Hydraulic oil tank 16: Pump line 22: Pressure sensor 24: Bypass line 26: Bypass valve 28: Bidirectional hydraulic pump 28a: First port of bidirectional hydraulic pump 28b: Second port of bidirectional hydraulic pump 28c: Bidirectional hydraulic pump housing 30: Hydraulic motor 30a: First port of hydraulic motor 30b: Second port of hydraulic motor 30c: Hydraulic motor housing 32: First line 34: Second line 36: Regulator 38: Regulator switching valve 40: Pilot Line 42: Pressure reducing valve 44: Charge Line 46: First check valve 48: Second check valve 50: First relief valve (first and second embodiments) 50': First relief valve (third and fourth embodiments) 52: Second relief valve (first and second embodiments) 52': Second relief valve (third and fourth embodiments) 56: Motor operating tool 58: Controller 60:Aperture 62: Third relief valve

Claims

1. A hydraulic circuit of a construction machine having an open circuit and a closed circuit, The open circuit is a variable displacement hydraulic pump that discharges hydraulic oil drawn from a hydraulic oil tank; a hydraulic actuator that operates using hydraulic oil discharged by the hydraulic pump; a directional control valve that switches the flow direction of hydraulic oil from the hydraulic pump to the hydraulic actuator; a pump line connecting the hydraulic pump and the directional control valve; a return line connecting the directional control valve and the hydraulic oil tank; a return check valve installed in the return line; a bypass line connecting the pump line and the return line; an electromagnetic proportional bypass valve installed in the bypass line, The closed circuit is a variable displacement bidirectional hydraulic pump having a first port and a second port; a hydraulic motor operated by the hydraulic oil discharged by the bidirectional hydraulic pump; a first line connecting the first port of the bidirectional hydraulic pump and the hydraulic motor; a second line connecting the second port of the bidirectional hydraulic pump and the hydraulic motor; A hydraulic circuit for a construction machine is provided with a charge line between the open circuit and the closed circuit, the charge line connecting the upstream portion of the return line from the return check valve to the first line via a first check valve, and the charge line connecting the upstream portion of the return line from the return check valve to the second line via a second check valve.

2. The closed circuit is a first relief valve that releases hydraulic oil in the first line to the second line via the second check valve; 2. The hydraulic circuit for a construction machine according to claim 1, further comprising: a second relief valve that releases hydraulic oil in the second line to the first line via the first check valve.

3. a restriction is disposed between the first relief valve and the second check valve, and between the second relief valve and the first check valve; a third relief valve is disposed between the first relief valve and the hydraulic oil tank, and between the second relief valve and the hydraulic oil tank; 3. The hydraulic circuit for a construction machine according to claim 2, wherein the set pressure of the third relief valve is lower than the set pressure of the first relief valve and is lower than the set pressure of the second relief valve.

4. The closed circuit is a first relief valve that releases hydraulic oil in the first line to the hydraulic oil tank; 2. The hydraulic circuit of claim 1, further comprising: a second relief valve that releases hydraulic oil in the second line to the hydraulic oil tank.

5. 2. The hydraulic circuit for a construction machine according to claim 1, wherein the first check valve and the second check valve are mounted on a housing of the bidirectional hydraulic pump, and the charge line is connected to the housing.

6. 2. The hydraulic circuit for a construction machine according to claim 1, wherein the first check valve and the second check valve are mounted on a housing of the hydraulic motor, and the charge line is connected to the housing.

7. a pressure sensor that detects a pressure in the return line upstream of the return check valve; 2. The hydraulic circuit for a construction machine according to claim 1, further comprising: a controller that increases the discharge rate of the hydraulic pump when the pressure detected by the pressure sensor is lower than a required pressure.

8. 8. The hydraulic circuit for a construction machine according to claim 7, wherein the controller sets the required pressure based on an amount of operation of a motor operating tool that outputs a signal for operating the hydraulic motor of the closed circuit.

9. 8. The hydraulic circuit for a construction machine according to claim 7, wherein the controller sets the required pressure based on a rotational speed of an object operated by the hydraulic motor in the closed circuit.

10. a regulator for controlling the discharge amount and discharge direction of the bidirectional hydraulic pump; an electromagnetic proportional regulator switching valve that switches the flow direction of pilot hydraulic oil to the regulator; a pilot line branching from the pump line and extending to the regulator switching valve; 2. The hydraulic circuit of claim 1, further comprising a pressure reducing valve installed in the pilot line.

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