Hydraulic circuit for construction machinery

Abstract

Task: A hydraulic circuit for a construction machine having good energy efficiency is provided. A hydraulic circuit (2) includes a motor generator (4), a pump motor (6) connected to the motor generator (4), a boom cylinder (8), a rod-side line (10) connecting the pump motor (6) and a rod-side port (8c) of the boom cylinder (8), a cap-side line (12) connecting the pump motor (6) and a cap-side port (8d) of the boom cylinder (8), and an operation device (18) outputting a signal for operating the boom cylinder (8). And a controller (20) that performs loop control on the basis of a signal output from the operation device (18). When a rod push-out signal of a boom cylinder (8) is output from an operation device (18), hydraulic oil flows from a rod-side conduit (10) to a cap-side conduit (12) via a pump motor (6). When a rod retraction signal of a boom cylinder (8) is output from an operation device (18), hydraulic oil flows from a cap-side conduit (12) to a rod-side conduit (10) via a pump motor (6).

Description

Technical Field

[0001] The invention relates to a hydraulic circuit for an engineering machine having a boom. Background Art

[0002] A hydraulic excavator as a typical example of a construction machine having a boom includes a lower traveling body, an upper rotating body supported by the lower traveling body, and a front attachment attached to the upper rotating body. The front attachment includes a boom swingably supported by the upper rotating body, an arm swingably supported at the top end of the boom, and a bucket swingably supported at the top end of the arm.

[0003] A hydraulic circuit for a hydraulic excavator includes a hydraulic pump, a hydraulic actuator (such as a boom cylinder, an arm cylinder, and a bucket cylinder) operated by oil discharged from the hydraulic pump, an operating device that outputs a signal for operating the hydraulic actuator, and a directional switching valve for switching the hydraulic oil supply direction from the hydraulic pump to the hydraulic actuator based on the signal output from the operating device.

[0004] In a hydraulic excavator, oil discharged from a hydraulic pump is supplied to a hydraulic actuator via a directional switching valve based on a signal output from an operating device, and the hydraulic actuator is operated to swing the front attachment (see, for example, Patent Document 1).

[0005] Related technical literature

[0006] Patent Literature

[0007] Patent Document 1: JP-A-2013-23821 Summary of the invention

[0008] Problems to be solved by the present invention

[0009] However, in the hydraulic excavator of the related art, the pressure loss in the directional switching valve is large, and there is room for improvement in energy efficiency.

[0010] When the bucket leaves the ground, the weight of the boom, arm and bucket acts on the boom cylinder. Therefore, with the boom lowered, the pressure used to operate the boom cylinder (retracting the boom cylinder's piston rod into the cylinder tube) is lower than the pressure used to operate the arm cylinder and bucket cylinder.

[0011] Therefore, in the case of operating other hydraulic actuators such as the arm cylinder together with the boom cylinder, although the discharge pressure of the hydraulic pump needs to be increased to the extent that the other hydraulic actuators can be operated, the pressure of the hydraulic oil supplied to the boom cylinder is reduced by the directional switching valve for the boom. Therefore, there is a problem that a large pressure loss occurs in the directional switching valve and it is desired to improve energy efficiency.

[0012] The object of the present invention is to provide a hydraulic circuit for a construction machine having good energy efficiency.

[0013] Means of solving the problem

[0014] According to the present invention, a hydraulic circuit for construction machinery is provided, which solves the above-mentioned problems as described below. That is, it provides:

[0015] A hydraulic circuit for an engineering machine having a boom, the hydraulic circuit comprising:

[0016] Motor generator,

[0017] a variable displacement bidirectional pump motor connected to the motor generator,

[0018] A boom cylinder, which is used to operate the boom,

[0019] a rod-side pipeline connecting the pump motor and the rod-side port of the boom cylinder,

[0020] a cap-side pipeline connecting the pump motor and the cap-side port of the boom cylinder,

[0021] an operating device that outputs a signal for operating the boom cylinder, and

[0022] A controller performs loop control based on a signal output from the operating device, wherein

[0023] When a rod push-out signal for the boom cylinder is output from the operating device, hydraulic oil flows from the rod-side pipeline to the cap-side pipeline via the pump motor, and

[0024] When a rod retraction signal for the boom cylinder is output from the operating device, hydraulic oil flows from the cap-side pipeline to the rod-side pipeline via the pump motor.

[0025] Preferably, the hydraulic circuit also includes a cap side pressure sensor, which detects the pressure in the cap side pipeline, and when a rod retraction signal for the boom cylinder is output from the operating device, when the pressure detected by the cap side pressure sensor is less than a predetermined value, the controller causes the electric generator to drive the pump motor, and when the pressure detected by the cap side pressure sensor is equal to or higher than a predetermined value, the controller allows the electric generator to be driven by the pump motor.

[0026] The hydraulic circuit may also include a rod side return line connecting the rod side line and the hydraulic oil tank, and an electromagnetic proportional switching valve arranged in the rod side return line, and when a rod retraction signal for the boom cylinder is output from the operating device, the controller may open the electromagnetic proportional switching valve to discharge excess hydraulic oil from the rod side return line.

[0027] Preferably, the controller adjusts the opening degree of the electromagnetic proportional switch valve according to the signal output from the operating device, and controls the amount of excess hydraulic oil discharged from the rod-side return line.

[0028] The hydraulic circuit may also include a cap side return line connecting the cap side line and the hydraulic oil tank, and an electromagnetic proportional release valve arranged in the cap side return line, and when a rod retraction signal for the boom cylinder is output from the operating device, the controller may reduce the release pressure of the electromagnetic proportional release valve to discharge excess hydraulic oil from the cap side return line.

[0029] Ideally, the hydraulic circuit also includes a rod side replenishment line connected to the rod side line, a rod side check valve arranged in the rod side replenishment line, a cap side replenishment line connected to the cap side line, and a cap side check valve arranged in the cap side replenishment line, and when a rod push-out signal for the boom cylinder is output from the operating device, the rod side check valve opens to replenish the rod side line with hydraulic oil, or the cap side check valve opens to replenish the cap side line with hydraulic oil.

[0030] The cap side pipeline is provided with a drift reduction valve that allows flow from the pump motor to the cap side port and prevents flow from the cap side port to the pump motor, and when a rod retraction signal for the boom cylinder is output from the operating device, the controller can open the drift reduction valve to allow flow from the cap side port to the pump motor.

[0031] Preferably, the controller adjusts the rotation speed of the motor generator and the displacement of the pump motor according to a signal output from the operating device.

[0032] Advantages of the present invention

[0033] In the hydraulic circuit for construction machinery according to the present invention, in the case where a rod push-out signal for a boom cylinder is output from an operating device, hydraulic oil flows from a rod-side pipeline to a cap-side pipeline via a pump motor, and in the case where a rod retract signal for a boom cylinder is output from an operating device, hydraulic oil flows from a cap-side pipeline to a rod-side pipeline via a pump motor. Therefore, a directional switching valve for switching the operating direction of the boom cylinder is not required. Therefore, no pressure loss occurs in the directional switching valve, and good energy efficiency is achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 : is a circuit diagram showing the hydraulic circuit of the present invention.

[0035] Figure 2 Is equipped with Figure 1 Schematic diagram of a hydraulic excavator showing a hydraulic circuit (a state in which an external force acts on the boom cylinder in a rod retracting direction).

[0036] Figure 3 It is shown in Figure 2 This is a circuit diagram of the flow of hydraulic oil when a rod push signal is output in the state shown.

[0037] Figure 4 It is shown in Figure 2 This is a circuit diagram of the flow of hydraulic oil when a rod retraction signal is output in the state shown.

[0038] Figure 5 It is shown Figure 2 The schematic diagram shown is a state in which the hydraulic excavator uses the front attachment to tilt the machine body (a state in which an external force acts on the boom cylinder in the rod pushing direction).

[0039] Figure 6 It is shown in Figure 5 This is a circuit diagram of the flow of hydraulic oil when a rod push signal is output in the state shown.

[0040] Figure 7 It is shown in Figure 5 This is a circuit diagram of the flow of hydraulic oil when a rod retraction signal is output (when the electromagnetic proportional switch valve is open) in the state shown.

[0041] Figure 8 It is shown in Figure 5 The circuit diagram of the flow of hydraulic oil when the rod retraction signal is output in the state shown (when the electromagnetic proportional relief valve is open). DETAILED DESCRIPTION

[0042] Embodiments of a hydraulic circuit for a construction machine according to the present invention will be described below with reference to the accompanying drawings.

[0043] Hydraulic circuit 2

[0044] Figure 1 The hydraulic circuit 2 shown may be installed on a construction machine having a boom (eg, a hydraulic excavator). The hydraulic circuit 2 includes a motor generator 4, a variable displacement bidirectional pump motor 6 connected to the motor generator 4, and a boom cylinder 8 for operating the boom.

[0045] Motor generator 4, pump motor 6

[0046] A battery (not shown) is connected to the motor generator 4. When electric power is supplied from the battery to the motor generator 4, the motor generator 4 functions as a motor, and the pump motor 6 driven by the motor generator 4 functions as a hydraulic pump.

[0047] When the motor generator 4 rotates forward to rotate the pump motor 6 forward, low-pressure hydraulic oil is sucked from the first port 6a of the pump motor 6, and high-pressure hydraulic oil is discharged from the second port 6b of the pump motor 6. On the other hand, when the motor generator 4 rotates reversely to rotate the pump motor 6 reversely, low-pressure oil is sucked from the second port 6b, and high-pressure oil is discharged from the first port 6a.

[0048] Furthermore, in the case where the motor generator 4 functions as a generator, the pump motor 6 functions as a hydraulic motor that drives the motor generator 4 .

[0049] When high-pressure oil flows into the first port 6a of the pump motor 6 and low-pressure oil flows out from the second port 6b, the pump motor 6 rotates forward to rotate the motor generator 4 forward. On the other hand, when high-pressure oil flows into the second port 6b of the pump motor 6 and low-pressure oil flows out from the first port 6a, the pump motor 6 rotates reversely to rotate the motor generator 4 reversely. Electric power generated by the forward rotation or reverse rotation of the motor generator 4 is accumulated in the battery.

[0050] Note that the oil discharged from the pump motor 6 is supplied only to the boom cylinder 8, and hydraulic actuators (arm cylinder, bucket cylinder, etc.) other than the boom cylinder 8 are supplied with hydraulic oil from other hydraulic pumps other than the pump motor 6. The other hydraulic pumps are driven by a drive source (motor, engine, etc.) other than the motor generator 4. Therefore, in the hydraulic circuit 2, there is no interference between the circuit of the boom cylinder 8 and the circuit of the other hydraulic actuators, and operability is excellent.

[0051] Boom cylinder 8, rod side pipeline 10, cap side pipeline 12

[0052] The boom cylinder 8 includes a cylinder tube 8a and a piston rod 8b. A rod-side port 8c of the boom cylinder 8 is connected to a first port 6a of the pump motor 6 via a rod-side conduit 10. A cap-side port 8d of the boom cylinder 8 is connected to a second port 6b of the pump motor 6 via a cap-side conduit 12.

[0053] A rod-side pressure sensor 14 for detecting the pressure in the rod-side pipeline 10 is provided in the rod-side pipeline 10. A cap-side pressure sensor 16 for detecting the pressure in the cap-side pipeline 12 is provided in the cap-side pipeline 12.

[0054] like Figure 1 As shown, the hydraulic circuit 2 includes an operating device 18 that outputs a signal for operating the boom cylinder 8 , and a controller 20 that performs circuit control based on the signal output from the operating device 18 .

[0055] Operating device 18

[0056] The operating device 18 may have an input device (such as a joystick or a slide switch) that increases the output strength of the electric signal as the amount of operation increases. The operating device 18 may output a signal for operating the boom cylinder 8 and a signal for operating a hydraulic actuator other than the boom cylinder 8.

[0057] Controller 20

[0058] The controller 20 includes a computer having a processing device and a storage device. Figure 1 As shown, the operating device 18 and the motor generator 4 are electrically connected to the controller 20. Although not shown, the pump motor 6, the rod-side pressure sensor 14, and the cap-side pressure sensor 16 are also electrically connected to the controller 20.

[0059] The hydraulic circuit 2 of the illustrated embodiment includes a rod-side return line 24 connecting the rod-side line 10 and the hydraulic oil tank 22, an electromagnetic proportional switching valve 26 arranged in the rod-side return line 24, a cap-side return line 28 connecting the cap-side line 12 and the hydraulic oil tank 22, and an electromagnetic proportional release valve 30 arranged in the cap-side return line 28.

[0060] The rod-side return line 24 and the cap-side return line 28 merge before the hydraulic oil tank 22, and are connected to the hydraulic oil tank 22 via a merged return line 32. Another return line 34 for returning the hydraulic oil discharged from the hydraulic actuator other than the boom cylinder 8 to the hydraulic oil tank 22 is connected to the merged return line 32. In addition, the merged return line 32 is provided with a back pressure check valve 36 for replenishment.

[0061] Although not shown, the electromagnetic proportional switching valve 26 and the electromagnetic proportional release valve 30 are electrically connected to the controller 20. The electromagnetic proportional switching valve 26 closes the oil path when no signal is output from the controller 20, and opens the oil path when a signal is sent from the controller 20. The opening degree of the electromagnetic proportional switching valve 26 is adjusted according to the signal from the controller 20. In addition, the release pressure of the electromagnetic proportional release valve 30 is maintained at an initial value when no signal is output from the controller 20, and is changed from the initial value according to the signal from the controller 20.

[0062] like Figure 1 As shown, the hydraulic circuit 2 further includes a rod-side supplementary line 38 connected to the rod-side line 10 , a rod-side check valve 40 arranged in the rod-side supplementary line 38 , a cap-side supplementary line 42 connected to the cap-side line 12 , and a cap-side check valve 44 arranged in the cap-side supplementary line 42 .

[0063] The rod-side replenishment line 38 connects the rod-side line 10 and the hydraulic oil tank 22. The rod-side check valve 40 allows flow from the hydraulic oil tank 22 to the rod-side line 10 and blocks flow from the rod-side line 10 to the hydraulic oil tank 22.

[0064] The cap-side supplementary line 42 in the illustrated embodiment is connected to the cap-side return line 28 in a manner that bypasses the electromagnetic proportional release valve 30, and a cap-side check valve 44 is arranged in parallel with the electromagnetic proportional release valve 30. The cap-side check valve 44 allows flow from the cap-side supplementary line 42 to the cap-side line 12, and blocks flow from the cap-side line 12 to the cap-side supplementary line 42.

[0065] In the illustrated embodiment, the cap-side line 12 is provided with a drift reduction valve 46. The drift reduction valve 46 allows flow from the second port 6b of the pump motor 6 to the cap-side port 8d of the boom cylinder 8, and in principle blocks flow from the cap-side port 8d to the second port 6b.

[0066] Although not shown, the drift reduction valve 46 is electrically connected to the controller 20. The controller 20 opens the drift reduction valve 46 when a rod retract signal for the boom cylinder 8 is output from the operating device 18 to allow flow from the cap-side port 8d to the second port 6b.

[0067] Figure 2 A hydraulic excavator 50 is shown as an example of a construction machine on which the above-described hydraulic circuit 2 can be mounted. The hydraulic excavator 50 includes a lower traveling body 52 , an upper rotating body 54 rotatably supported by the lower traveling body 52 , and a front attachment 56 attached to the upper rotating body 54 .

[0068] The front attachment 56 includes a boom 58 swingably supported by the upper rotating body 54, an arm 60 swingably supported at the top end of the boom 58, and a bucket 62 swingably supported at the top end of the arm 60. In addition, the front attachment 56 is provided with a boom cylinder 8 for swinging the boom 58, an arm cylinder 64 for swinging the arm 60, and a bucket cylinder 66 for swinging the bucket 62.

[0069] The bottom end of the cylinder tube 8a of the boom cylinder 8 is connected to the upper rotating body 54, and the top end of the piston rod 8b is connected to the boom 58. Then, when the piston rod 8b is pushed out from the cylinder tube 8a, the boom 58 moves along the Figure 2 In addition, when the piston rod 8b is retracted into the cylinder tube 8a, the boom 58 moves along the Figure 2 Swing in the direction shown by the dotted arrow B.

[0070] Next, the operation of the hydraulic circuit 2 for a construction machine as described above will be described.

[0071] When the operating device 18 is not operated, no signal is output from the operating device 18 to the controller 20. In this case, the motor generator 4 and the pump motor 6 do not operate, and the hydraulic oil does not flow in the rod-side pipeline 10 and the cap-side pipeline 12. Therefore, the boom cylinder 8 does not operate.

[0072] like Figure 2 As shown, when the boom cylinder 8 stops and the bucket 62 leaves the ground, the weight of the front attachment 56 acts on the boom cylinder 8, that is, an external force acts on the boom cylinder 8 in the rod retracting direction. Therefore, the pressure in the cap-side pipeline 12 increases.

[0073] However, in the illustrated embodiment, unless the operating device 18 outputs a rod retract signal, the drift reduction valve 46 blocks the flow from the cap-side port 8d to the second port 6b of the pump motor 6. Therefore, the position of the piston rod 8b of the boom cylinder 8 is maintained, and the natural lowering of the boom 58 is prevented. In the above state, no pressure is generated in the rod-side pipeline 10.

[0074] 1. When external force acts on the rod retracting direction, the rod is pushed out

[0075] exist Figure 2 In the state shown, there is no pressure in the rod-side pipeline 10, and the pressure detected by the rod-side pressure sensor 14 is less than a predetermined value. Then, when a rod push-out signal for the boom cylinder 8 is output from the operating device 18, the controller 20 supplies power from the battery to the motor generator 4 to rotate the motor generator 4 in the forward direction. This causes the pump motor 6 to rotate in the forward direction. That is, the motor generator 4 functions as a motor, and the pump motor 6 functions as a hydraulic pump.

[0076] Then, if Figure 3 As shown, the low-pressure hydraulic oil in the rod-side pipeline 10 is sucked into the first port 6a of the pump motor 6, and the high-pressure hydraulic oil is discharged from the second port 6b to the cap-side pipeline 12. That is, the hydraulic oil flows from the rod-side pipeline 10 to the cap-side pipeline 12 via the pump motor 6. Figure 3 In the diagram, the flow of high-pressure hydraulic oil is represented by the thick black line, while the flow of low-pressure hydraulic oil is represented by the thick gray line. Figure 4 and 6 To 8.

[0077] Furthermore, since the hydraulic oil flows out from the rod-side port 8c of the boom cylinder 8 into the rod-side pipeline 10, and the hydraulic oil flows from the cap-side pipeline 12 into the cap-side port 8d, the piston rod 8b is pushed out from the cylinder tube 8a. Figure 2 Swing (rise) in the direction indicated by the solid arrow A.

[0078] As described above, since the drift reduction valve 46 allows flow from the second port 6 b of the pump motor 6 to the cap side port 8 d , the drift reduction valve 46 opens when the discharge pressure of the pump motor 6 exceeds the pressure of the cap side port 8 d .

[0079] When the piston rod 8b is pushed out of the cylinder tube 8a, the amount of hydraulic oil flowing out of the rod side port 8c is smaller than the amount of hydraulic oil flowing into the cap side port 8d by the volume of the piston rod 8b. Therefore, the pressure in the rod side pipeline 10 becomes lower than the pressure in the hydraulic oil tank 22, and as Figure 3 As shown, the rod-side check valve 40 is opened, and the rod-side pipeline 10 is replenished with hydraulic oil from the hydraulic oil tank 22 via the rod-side replenishing pipeline 38. Therefore, cavitation is prevented.

[0080] 2. When external force acts in the direction of rod retraction, the rod retracts

[0081] exist Figure 2 In the state shown, the pressure in the cap-side pipe 12 increases as described above, so that the pressure detected by the cap-side pressure sensor 16 becomes equal to or higher than the predetermined value. Therefore, when the rod retract signal for the boom cylinder 8 is output from the operating device 18, the controller 20 allows the motor generator 4 to be driven by the pump motor 6.

[0082] In addition, the controller 20 opens the drift reduction valve 46 to allow the hydraulic oil to flow from the cap-side port 8d to the second port 6b of the pump motor 6. As a result, Figure 4 As shown, high-pressure hydraulic oil flows from the cap-side pipeline 12 into the second port 6b, the pump motor 6 rotates in the reverse direction, and low-pressure hydraulic oil is discharged from the first port 6a. That is, the hydraulic oil flows from the cap-side pipeline 12 to the rod-side pipeline 10 via the pump motor 6. At this time, the pump motor 6 functions as a hydraulic motor that drives the motor generator 4. In addition, the motor generator 4 functions as a generator to generate electric power.

[0083] Then, the hydraulic oil flows out from the cap side port 8d of the boom cylinder 8 into the cap side pipeline 12, and the hydraulic oil flows from the rod side pipeline 10 into the rod side port 8c. As a result, the piston rod 8b is retracted into the cylinder tube 8a, and the boom 58 moves along the Figure 2 Swing (descend) in the direction indicated by the dashed arrow B.

[0084] When the piston rod 8b is retracted into the cylinder tube 8a, the amount flowing out from the cap side port 8d is larger than the amount flowing into the rod side port 8c by the volume of the piston rod 8b. Therefore, when the rod retraction signal for the boom cylinder 8 is output from the operating device 18, as shown in FIG. Figure 4 As shown, the controller 20 opens the electromagnetic proportional switching valve 26 to discharge the excess hydraulic oil from the rod-side return line 24 to the hydraulic oil tank 22 .

[0085] It is desirable that the controller 20 adjusts the opening of the electromagnetic proportional switch valve 26 according to the signal output from the operating device 18, and controls the amount of excess hydraulic oil discharged from the rod-side return line 24 to the hydraulic oil tank 22. Thus, the pressure of the rod-side line 10 is adjusted, and the retraction speed of the piston rod 8b can be appropriately controlled according to the signal from the operating device 18.

[0086] Machine lifting

[0087] In the hydraulic excavator 50, as Figure 5 As shown, there is a situation where the bucket 62 is pushed against the ground to tilt the machine body (lower traveling body 52 and upper rotating body 54) (sometimes referred to as machine lifting). In this case, an external force acts on the boom cylinder 8 in the rod push-out direction, and the pressure in the rod side pipeline 10 increases.

[0088] 3. When the external force acts in the direction of the rod pushing out, the rod pushes out

[0089] exist Figure 5 In the state shown, the pressure of the rod side pipeline 10 rises, and the pressure detected by the rod side pressure sensor 14 is equal to or greater than a predetermined value. At this time, the predetermined value of the pressure detected by the rod side pressure sensor 14 does not need to be the same as the predetermined value of the pressure detected by the cap side pressure sensor 16 (the predetermined value for allowing the motor generator 4 to be driven by the pump motor 6 when the rod retraction signal is output).

[0090] Then, in Figure 5 In the state shown, when the rod push-out signal for the boom cylinder 8 is output from the operating device 18, the controller 20 supplies power from the battery to the motor generator 4 to rotate the motor generator 4 in the forward direction. As a result, the pump motor 6 rotates in the forward direction, and the hydraulic oil flows into the first port 6a of the pump motor 6, and the hydraulic oil flows out from the second port 6b. In other words, the hydraulic oil flows from the rod-side pipeline 10 to the cap-side pipeline 12 via the pump motor 6.

[0091] In addition, since the hydraulic oil flows out from the rod-side port 8c of the boom cylinder 8 into the rod-side pipeline 10, and the hydraulic oil flows from the cap-side pipeline 12 into the cap-side port 8d, the piston rod 8b is pushed out from the cylinder tube 8a. Figure 5 Swing in the direction shown by the solid arrow C.

[0092] As mentioned above, in Figure 5 In the state shown, the pressure in the rod-side pipeline 10 increases, so that the hydraulic oil flowing into the first port 6a is at high pressure. Therefore, when the pump motor 6 rotates forward and generates a flow from the rod-side pipeline 10 to the cap-side pipeline 12, the controller 20 can stop the power supply from the battery to the motor generator 4, the pump motor 6 can be rotated forward by the high-pressure hydraulic oil flowing into the first port 6a, and the motor generator 4 can be rotated forward by the rotation of the pump motor 6.

[0093] That is, when an external force acts in the rod pushing direction (when the pressure detected by the rod side pressure sensor 14 is equal to or higher than a predetermined value), when the rod pushing signal is output, the controller 20 first causes the electric generator 4 to drive the pump motor 6 serving as a hydraulic pump, and after a predetermined period of time has passed since the pump motor 6 starts to be driven, the controller 20 can switch the pump motor 6 to a hydraulic motor and cause the electric generator 4 to generate electricity.

[0094] As described above, in the case where the piston rod 8b is pushed out from the cylinder tube 8a, the outflow amount from the rod side port 8c is smaller than the inflow amount into the cap side port 8d by the volume of the piston rod 8b, but when the external force acts in the rod pushing direction, the pressure in the rod side pipeline 10 becomes higher than the pressure in the hydraulic oil tank 22. Therefore, the rod side check valve 40 does not open.

[0095] However, in the illustrated embodiment, since the back pressure check valve 36 is provided in the combined return line 32 connected to the cap side return line 28, the pressure in the cap side line 12 becomes lower than the pressure in the cap side return line 28, and the cap side check valve 44 is as shown. Figure 6 As shown, the cap-side line 12 is opened, and the cap-side line 12 is replenished with hydraulic oil from the cap-side replenishment line 42. Thus, cavitation is prevented.

[0096] 4. When external force acts in the direction of the rod pushing out, the rod retracts

[0097] exist Figure 5 In the state shown, there is no pressure in the cap-side pipeline 12, and the pressure detected by the cap-side pressure sensor 16 becomes less than a predetermined value. Therefore, when a rod retraction signal for the boom cylinder 8 is output from the operating device 18, the controller 20 supplies power from the battery to the motor generator 4 to reversely rotate the motor generator 4. This causes the pump motor 6 to reversely rotate. In this way, the motor generator 4 functions as a motor, and the pump motor 6 functions as a hydraulic pump.

[0098] In addition, the controller 20 opens the drift reduction valve 46 to allow the hydraulic oil to flow from the cap-side port 8d to the second port 6b of the pump motor 6. As a result, Figure 7 As shown, low-pressure hydraulic oil in the cap-side line 12 is sucked into the second port 6b, and high-pressure hydraulic oil is discharged from the first port 6a to the rod-side line 10. That is, hydraulic oil flows from the cap-side line 12 to the rod-side line 10 via the pump motor 6.

[0099] As a result, the hydraulic oil flows out from the cap-side port 8d of the boom cylinder 8 into the cap-side pipeline 12, and the hydraulic oil flows from the rod-side pipeline 10 into the rod-side port 8c. As a result, the piston rod 8b is retracted into the cylinder tube 8a, and the machine body (the lower traveling body 52 and the upper rotating body 54) moves along the Figure 5 Swing in the direction shown by the dotted arrow D.

[0100] Furthermore, in the case where a rod retracting signal for the boom cylinder 8 is output from the operating device 18, the controller 20 opens the electromagnetic proportional switching valve 26 in the same manner as when an external force acts in the rod retracting direction on the boom cylinder 8. As a result, the excess hydraulic oil is discharged from the rod-side return line 24 to the hydraulic oil tank 22.

[0101] Also in this case, it is desirable that the controller 20 adjusts the opening of the electromagnetic proportional switch valve 26 according to the signal output from the operating device 18, and controls the amount of excess hydraulic oil discharged from the rod-side return line 24 to the hydraulic oil tank 22. Thus, the pressure of the hydraulic oil flowing into the rod-side port 8c can be adjusted, and the retraction speed of the piston rod 8b can be appropriately controlled according to the signal from the operating device 18.

[0102] Alternatively, in the case where an external force acts on the boom cylinder 8 in the rod push-out direction and a rod retracting signal for the boom cylinder 8 is output from the operating device 18, the controller 20 may reduce the release pressure of the electromagnetic proportional release valve 30 to discharge the excess hydraulic oil from the cap-side return line 28 to the hydraulic oil tank 22, as shown in FIG. Figure 8 As shown. As a result, the amount of inflow from the cap side conduit 12 to the pump motor 6 can be reduced, and power loss can be suppressed. At this time, the electromagnetic proportional switch valve 26 can be opened or closed.

[0103] Furthermore, the controller 20 adjusts the rotation speed of the motor generator 4 and the displacement of the pump motor 6 according to the signal output from the operating device 18, regardless of whether the rod push-out signal or the rod retracting signal is output. Therefore, the push-out speed or retracting speed of the piston rod 8b of the boom cylinder 8 can be appropriately controlled.

[0104] As described above, in the hydraulic circuit 2, in the case where the rod push-out signal for the boom cylinder 8 is output from the operating device 18, the hydraulic oil flows from the rod-side pipeline 10 to the cap-side pipeline 12 via the pump motor 6, and in the case where the rod retract signal for the boom cylinder 8 is output from the operating device 18, the hydraulic oil flows from the cap-side pipeline 12 to the rod-side pipeline 10 via the pump motor 6. Therefore, a directional switching valve for switching the operating direction of the boom cylinder 8 is not required. Therefore, pressure loss does not occur in the directional switching valve, and good energy efficiency is achieved.

Claims

1. A hydraulic circuit for an engineering machine having a boom, the hydraulic circuit comprising: Motor generator; a variable displacement bi-directional pump motor connected to the motor-generator; A boom cylinder, used to operate the boom; a rod-side pipeline connecting the pump motor and a rod-side port of the boom cylinder; a cap-side pipeline connecting the pump motor and the cap-side port of the boom cylinder; An operating device, which outputs a signal for operating the boom cylinder; as well as A controller performs loop control based on a signal output from the operating device, wherein When a rod push-out signal for the boom cylinder is output from the operating device, hydraulic oil flows from the rod-side pipeline to the cap-side pipeline via the pump motor, and When a rod retraction signal for the boom cylinder is output from the operating device, hydraulic oil flows from the cap-side pipeline to the rod-side pipeline via the pump motor.

2. The hydraulic circuit for engineering machinery according to claim 1, further comprising: The cap side pressure sensor detects the pressure in the cap side pipeline, wherein In a case where a rod retraction signal for the boom cylinder is output from the operating device, when the pressure detected by the cap side pressure sensor is less than a predetermined value, the controller causes the electric generator to drive the pump motor, and when the pressure detected by the cap side pressure sensor is equal to or higher than the predetermined value, the controller allows the electric generator to be driven by the pump motor.

3. The hydraulic circuit for engineering machinery according to claim 1, further comprising: A rod-side return pipeline connecting the rod-side pipeline and a hydraulic oil tank; as well as An electromagnetic proportional switching valve is arranged in the rod side return line, wherein When the rod retract signal for the boom cylinder is output from the operating device, the controller opens the electromagnetic proportional switching valve to discharge excess hydraulic oil from the rod-side return line.

4. The hydraulic circuit for construction machinery according to claim 3, wherein the controller adjusts the opening of the electromagnetic proportional switch valve according to the signal output from the operating device and controls the amount of excess hydraulic oil discharged from the rod-side return line.

5. The hydraulic circuit for engineering machinery according to claim 1, further comprising: A cap side return pipeline, which connects the cap side pipeline and the hydraulic oil tank; as well as An electromagnetic proportional release valve is arranged in the cap side return line, wherein When the rod retraction signal for the boom cylinder is output from the operating device, the controller reduces the release pressure of the solenoid proportional release valve to discharge excess hydraulic oil from the cap-side return line.

6. The hydraulic circuit for engineering machinery according to claim 1, further comprising: a rod-side supplementary pipeline connected to the rod-side pipeline; a rod-side check valve disposed in the rod-side supplementary line; A cap-side supplementary pipeline connected to the cap-side pipeline; as well as A cap-side check valve is arranged in the cap-side replenishment line, wherein In a case where the rod push-out signal for the boom cylinder is output from the operating device, the rod-side check valve opens to replenish the rod-side pipeline with hydraulic oil, or the cap-side check valve opens to replenish the cap-side pipeline with hydraulic oil.

7. The hydraulic circuit for construction machinery according to claim 1, wherein The cap-side conduit is provided with a drift reduction valve that allows flow from the pump-motor to the cap-side port and blocks flow from the cap-side port to the pump-motor, and The controller opens the drift reduction valve to allow flow from the cap-side port to the pump motor in a case where the rod retract signal for the boom cylinder is output from the operating device.

8. The hydraulic circuit for a construction machine according to claim 1, wherein the controller adjusts the rotation speed of the motor generator and the displacement of the pump motor according to the signal output from the operating device.

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