Hydraulic system

Abstract

To provide a hydraulic system capable of physically preventing occurrence of confining of pressure in either a rod side or a head side lower in pressure in a configuration in which a second bidirectional pump is added to a first bidirectional pump.SOLUTION: A hydraulic system 1 according to an embodiment includes a variable displacement type first bidirectional pump 2 connected to a head side chamber 4h of a single rod cylinder 4 by a head side line 21 and connected to a rod side chamber 4r of the single rod cylinder 4 by a rod side line 22, and a variable displacement type second bidirectional pump 3 connected to the head side line 21 by a supply and discharge line 31. The hydraulic system 1 further includes a low pressure selection valve 6 connected to the head side line 21 by a first discharge line 61 and connected to the rod slide line 22 by a second discharge line 62, and a position detector 9 for detecting a position of the low pressure selection valve 6.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a hydraulic system for a single rod cylinder. [Background technology]

[0002] Conventionally, a hydraulic system for a single-rod cylinder including a bidirectional pump connected to the single-rod cylinder so as to form a closed circuit has been known. For example, Patent Document 1 discloses a hydraulic system 100 as shown in FIG.

[0003] This hydraulic system 100 includes a first bidirectional pump 110 and a second bidirectional pump 120. The first bidirectional pump 110 is connected to a head side chamber 210 of a single rod cylinder 200 by a head side line 111, and is connected to a rod side chamber 220 of the single rod cylinder 200 by a rod side line 112. The second bidirectional pump 120 is for eliminating the flow rate difference (difference between the inflow flow rate and the outflow flow rate) between the head side chamber 210 and the rod side chamber 220 of the single rod cylinder 200, and is connected to the head side line 111 by a supply and discharge line 121.

[0004] The first bidirectional pump 110 and the second bidirectional pump 120 are driven in the same direction by the electric motor 130. More specifically, the electric motor 130 drives the first bidirectional pump 110 and the second bidirectional pump 120 in a first direction when the single-rod cylinder 200 is extended (i.e., when the rod is moved forward), and drives the first bidirectional pump 110 and the second bidirectional pump 120 in a second direction opposite to the first direction when the single-rod cylinder 200 is contracted (i.e., when the rod is moved backward). When the first bidirectional pump 110 and the second bidirectional pump 120 are driven in the first direction, they discharge the working fluid to the head side chamber 210, and when the first bidirectional pump 110 and the second bidirectional pump 120 are driven in the second direction, they suck the working fluid from the head side chamber 210. Furthermore, in hydraulic system 100 , second bidirectional pump 120 is a variable displacement pump, and the displacement of second bidirectional pump 120 is changed by regulator 140 .

[0005] In the hydraulic system 100, if the capacity of the second bidirectional pump 120 when there is no excess or deficiency in the inflow rate to the single-rod cylinder 200 or the outflow rate from the single-rod cylinder 200 is taken as the reference capacity, when the capacity of the second bidirectional pump 120 is greater than the reference capacity during the extension of the single-rod cylinder 200, pressure trapping occurs on the lower pressure side of the rod side (rod side chamber 220 and rod side line 112) and the head side (head side chamber 210 and head side line 111). Conversely, when the capacity of the second bidirectional pump 120 is less than the reference capacity during the extension of the single-rod cylinder 200, cavitation occurs on the lower pressure side of the rod side and the head side. Also, when the single-rod cylinder 200 is contracted, when the capacity of the second bidirectional pump 120 is greater than the reference capacity, cavitation occurs on the lower pressure side of the rod side and the head side, and when the capacity of the second bidirectional pump 120 is less than the reference capacity, pressure trapping occurs on the lower pressure side of the rod side and the head side. The regulator 140 is controlled based on the pressure in the head side chamber 210 or the rod side chamber 220 of the single rod cylinder 200 so that trapping or cavitation of these pressures is suppressed. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2013-245740 A [Patent Document 2] JP 2002-54602 A Summary of the Invention [Problem to be solved by the invention]

[0007] As described above, in the hydraulic system 100 of Patent Document 1, the occurrence of pressure trapping on the lower pressure side of the rod side or the head side is controlled to be suppressed. In response to this, there is a demand to physically prevent the occurrence of pressure trapping on the lower pressure side of the rod side or the head side.

[0008] 2 of Patent Document 2 discloses, as a conventional technique, a hydraulic circuit in which only one bidirectional pump is used for a single-rod cylinder and a low-pressure selection valve (called a "flushing valve" in Patent Document 2) is employed that enables the discharge of hydraulic fluid from either the head side line or the rod side line, whichever has the lower pressure, into a tank. However, this hydraulic circuit does not have a second bidirectional pump added to the first bidirectional pump, as in hydraulic system 100 of Patent Document 1.

[0009] Therefore, an object of the present disclosure is to provide a hydraulic system that can physically prevent pressure trapping from occurring on the lower pressure side, either the rod side or the head side, in a configuration in which a second bidirectional pump is added to a first bidirectional pump. [Means for solving the problem]

[0010] The present disclosure relates to a hydraulic system for a single-rod cylinder including a head side chamber and a rod side chamber, the hydraulic system including: a first bidirectional variable displacement pump connected to the head side chamber by a head side line and connected to the rod side chamber by a rod side line; a second bidirectional variable displacement pump connected to the head side line by a supply / discharge line; an electric motor that drives the first bidirectional pump and the second bidirectional pump in the same direction; and a second bidirectional variable displacement pump connected to the head side line by a first discharge line and connected to the rod side line by a second discharge line, the first bidirectional variable displacement pump being driven by a first discharge line and a second discharge line when a pressure difference between a pressure in the head side chamber and a pressure in the rod side chamber is less than a threshold value. Provided is a hydraulic system comprising: a low pressure selection valve that is positioned in a neutral position to block a discharge line and the second discharge line, and that is switched to a rod side discharge position to communicate the second discharge line with a relief line provided with a relief valve when the pressure in the head side chamber is higher than the pressure in the rod side chamber by at least the threshold value, and that is switched to a head side discharge position to communicate the first discharge line with the relief line when the pressure in the rod side chamber is higher than the pressure in the head side chamber by at least the threshold value, and a position detector that detects whether the position of the low pressure selection valve is the neutral position, the rod side discharge position, or the head side discharge position. Effect of the Invention

[0011] According to the present disclosure, a hydraulic system is provided that can physically prevent pressure trapping from occurring on the lower pressure side, either the rod side or the head side, in a configuration in which a second bidirectional pump is added to a first bidirectional pump. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a schematic configuration diagram of a hydraulic system according to an embodiment. [Diagram 2] 11 is a flowchart showing a process for extending a cylinder. [Diagram 3] 13 is a diagram showing the flow of hydraulic fluid when the pressure difference between the head side chamber and the rod side chamber is less than a threshold value when the cylinder is extended. FIG. [Figure 4]13 is a diagram showing the flow of hydraulic fluid when the pressure difference between the head side chamber and the rod side chamber is equal to or greater than a threshold value when the cylinder is extended. FIG. [Diagram 5] 13 is a flowchart showing a procedure when the cylinder is retracted. [Figure 6] 13 is a diagram showing the flow of hydraulic fluid when the pressure difference between the head side chamber and the rod side chamber is less than a threshold value when the cylinder is retracted. FIG. [Figure 7] 13 is a diagram showing the flow of hydraulic fluid when the pressure difference between the head side chamber and the rod side chamber is equal to or greater than a threshold value when the cylinder is retracted. FIG. [Figure 8] FIG. 13 is a schematic configuration diagram of a hydraulic system according to a modified example. [Figure 9] FIG. 1 is a schematic diagram of a conventional hydraulic system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] 1 shows a hydraulic system 1 according to one embodiment. The hydraulic system 1 is a hydraulic system for a single-rod cylinder 4 including a head side chamber 4h and a rod side chamber 4r. The hydraulic fluid used in the hydraulic system 1 is typically oil.

[0014] Specifically, the hydraulic system 1 includes a first bidirectional pump 2 connected to the single-rod cylinder 4 to form a closed circuit, and a second bidirectional pump 3 for eliminating the flow rate difference (the difference between the inflow flow rate and the outflow flow rate) between the head side chamber 4h and the rod side chamber 4r of the single-rod cylinder 4.

[0015] The first bidirectional pump 2 is connected to the head side chamber 4h of the single-rod cylinder 4 by a head side line 21, and is connected to the rod side chamber 4r of the single-rod cylinder 4 by a rod side line 22. The second bidirectional pump 3 is connected to the head side line 21 by a first supply / discharge line 31, and is connected to the tank by a second supply / discharge line 32.

[0016] A first lock valve 23, which is an on-off valve, is provided in the head side line 21, and a second lock valve 24, which is an on-off valve, is provided in the rod side line 22. The first lock valve 23 and the second lock valve 24 are controlled by a control device 8. Note that in FIG. 1, some signal lines are omitted for simplification of the drawing.

[0017] The control device 8 opens the first lock valve 23 and the second lock valve 24 when the single-rod cylinder 4 is operated (extended or retracted), and closes the first lock valve 23 and the second lock valve 24 at other times. By closing the first lock valve 23 and the second lock valve 24 in this manner, the operation of the single-rod cylinder 4 can be locked.

[0018] A head side relief line 51 branches off from the head side line 21 and is connected to a tank. A head side relief valve 52 having a relief pressure set to a relatively high value (for example, 25 to 35 MPa) is provided in the head side relief line 51.

[0019] Furthermore, a first bypass line 55 that bypasses the head side relief valve 52 branches off from the head side line 21 and is connected to the tank. The first bypass line 55 is provided with a first check valve 56 that permits flow from the tank to the head side line 21 but prohibits flow in the opposite direction.

[0020] In the illustrated example, the head side relief line 51 and the first bypass line 55 branch off from the head side line 21 between the first lock valve 23 and the first bidirectional pump 2, but the head side relief line 51 and the first bypass line 55 may branch off from the head side line 21 between the first lock valve 23 and the single-rod cylinder 4. In addition, the first bypass line 55 does not necessarily have to branch off from the head side line 21, and may branch off from the head side relief line 51 upstream of the head side relief valve 52.

[0021] Similarly, a rod-side relief line 53 branches off from the rod-side line 22, and this rod-side relief line 53 is connected to a tank. The rod-side relief line 53 is provided with a rod-side relief valve 54 whose relief pressure is set relatively high (for example, 25 to 35 MPa).

[0022] Furthermore, a second bypass line 57 that bypasses the rod side relief valve 54 branches off from the rod side line 22 and is connected to a tank. The second bypass line 57 is provided with a second check valve 58 that allows a flow from the tank to the rod side line 22 but prohibits a flow in the opposite direction.

[0023] In the illustrated example, the rod side relief line 53 and the second bypass line 57 branch off from the rod side line 22 between the second lock valve 24 and the first bidirectional pump 2, but the rod side relief line 53 and the second bypass line 57 may branch off from the rod side line 22 between the second lock valve 24 and the single-rod cylinder 4. In addition, the second bypass line 57 does not necessarily have to branch off from the rod side line 22, and may branch off from the rod side relief line 53 upstream of the rod side relief valve 54.

[0024] If the head side relief line 51 and the rod side relief line 53 are provided in this manner, the pressure Ph in the head side chamber 4h and the pressure Pr in the rod side chamber 4r can be kept equal to or lower than the set pressures of the head side relief valve 52 and the rod side relief valve 54. Furthermore, if the first bypass line 55 and the second bypass line 57 are provided, cavitation in the lower pressure of the head side (head side chamber 4h and head side line 21) and the rod side (rod side chamber 4r and rod side line 22) can be suppressed by supplying working fluid to the head side line 21 or the rod side line 22 via the first check valve 56 or the second check valve 58.

[0025] The first bidirectional pump 2 and the second bidirectional pump 3 are, for example, axial piston pumps (swash plate pumps or bent-axis pumps), and the first bidirectional pump 2 and the second bidirectional pump 3 are variable displacement pumps.

[0026] The displacement of first bidirectional pump 2 is changed by first regulator 25, and the displacement of second bidirectional pump 3 is changed by second regulator 35. First regulator 25 and second regulator 35 are controlled by control device 8. For example, when first bidirectional pump 2 is a swash plate pump, first regulator 25 may be a device that electrically changes the hydraulic pressure acting on a servo piston connected to the swash plate of first bidirectional pump 2, or may be an electric actuator connected to the swash plate of first bidirectional pump 2. Similarly, when second bidirectional pump 3 is a swash plate pump, second regulator 35 may be a device that electrically changes the hydraulic pressure acting on a servo piston connected to the swash plate of second bidirectional pump 3, or may be an electric actuator connected to the swash plate of second bidirectional pump 3.

[0027] The first bidirectional pump 2 and the second bidirectional pump 3 are driven in the same direction by an electric motor 7. The first bidirectional pump 2 and the second bidirectional pump 3 may be arranged so that their central axes are aligned coaxially (tandem type), or may be arranged so that their central axes are aligned in parallel (parallel type).

[0028] The electric motor 7 is controlled by a control device 8. The electric motor 7 drives the first bidirectional pump 2 and the second bidirectional pump 3 in a first direction when the single-rod cylinder 4 is extended, and drives the first bidirectional pump 2 and the second bidirectional pump 3 in a second direction opposite to the first direction when the single-rod cylinder 4 is retracted. When the first bidirectional pump 2 and the second bidirectional pump 3 are driven in the first direction, they discharge working fluid to the head side chamber 4h, and when the first bidirectional pump 2 and the second bidirectional pump 3 are driven in the second direction, they suck working fluid from the head side chamber 4h.

[0029] Furthermore, in this embodiment, a low pressure selection valve 6 is employed which is connected to the head side line 21 by a first discharge line 61 and is connected to the rod side line 22 by a second discharge line 62. The low pressure selection valve 6 is connected to a tank by a low pressure relief line 65. The low pressure relief line 65 is provided with a low pressure relief valve 66 whose relief pressure is set relatively low (for example, 0.1 to 2 MPa).

[0030] The low pressure selection valve 6 is switched among a neutral position (center position in FIG. 1), a head side discharge position (right side position in FIG. 1), and a rod side discharge position (left side position in FIG. 1). In the neutral position, the low pressure selection valve 6 blocks the first discharge line 61 and the second discharge line 62. In the head side discharge position, the low pressure selection valve 6 blocks the second discharge line 62 while connecting the first discharge line 61 to the low pressure relief line 65. In the rod side discharge position, the low pressure selection valve 6 blocks the first discharge line 61 while connecting the second discharge line 62 to the low pressure relief line 65.

[0031] The pressure Ph of the head side chamber 4h is introduced to the low pressure selection valve 6 through a first pilot line 63, and the pressure Pr of the rod side chamber 4r is introduced through a second pilot line 64. The first pilot line 63 is for switching the low pressure selection valve 6 from the neutral position to the rod side discharge position, and the second pilot line 64 is for switching the low pressure selection valve 6 from the neutral position to the head side discharge position. In the illustrated example, the first pilot line 63 and the second pilot line 64 are branched off from the first discharge line 61 and the second discharge line 62, respectively, but the first pilot line 63 may be branched off from the head side line 21, and the second pilot line 64 may be branched off from the rod side line 22.

[0032] The low-pressure selection valve 6 is positioned at the neutral position when the differential pressure ΔP (ΔP = |Ph - Pr|) between the pressure Ph in the head-side chamber 4h and the pressure Pr in the rod-side chamber 4r is less than the threshold value Pt. On the other hand, the low-pressure selection valve 6 is switched from the neutral position to the rod-side discharge position when the pressure Ph in the head-side chamber 4h is higher than the pressure Pr in the rod-side chamber 4r by the threshold value Pt or more, and is switched from the neutral position to the head-side discharge position when the pressure Pr in the rod-side chamber 4r is higher than the pressure Ph in the head-side chamber 4h by the threshold value Pt or more. For example, the threshold value Pt is in the range of 0.5 to 3 MPa. Note that the threshold value Pt may be different between when the cylinder extends and when the cylinder contracts.

[0033] Furthermore, the hydraulic system 1 includes a position detector 9 that detects whether the position of the low-pressure selection valve 6 is in the neutral position, the rod-side discharge position, or the head-side discharge position. The position detector 9 is electrically connected to the control device 8.

[0034] In this embodiment, the position detector 9 includes a pressure sensor 91 that measures the pressure Ph in the head-side chamber 4h, a pressure sensor 92 that measures the pressure Pr in the rod-side chamber 4r, and a calculation unit. The calculation unit determines that the position of the low-pressure selection valve 6 is in the neutral position when |Ph - Pr| < Pt, determines that the position of the low-pressure selection valve 6 is in the rod-side discharge position when Ph - Pr ≥ Pt, and determines that the position of the low-pressure selection valve 6 is in the head-side discharge position when Pr - Ph ≥ Pt. Note that the calculation unit may be incorporated into the control device 8.

[0035] In the illustrated example, the pressure sensors 91 and 92 are provided on the head-side line 21 and the rod-side line 22, but the pressure sensors 91 and 92 may be provided in the head-side chamber 4h and the rod-side chamber 4r. Also, the position detector 9 may include a stroke sensor provided on the low-pressure selection valve 6 instead of the two pressure sensors.

[0036] Next, the control of the electric motor 7, the first regulator 25, and the second regulator 35 by the control device 8 will be described in detail. The functions of the elements disclosed herein with respect to the control device 8 can be performed using circuits or processing circuits, including general purpose processors, special purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuits. In this disclosure, a circuit, unit, or means is hardware that performs the recited functions or hardware that is programmed to perform the recited functions. The hardware may be hardware disclosed herein or other known hardware that is programmed or configured to perform the recited functions. In the case where the hardware is a processor, which is considered a type of circuit, the circuit, means, or unit is a combination of hardware and software, and the software is used to configure the hardware and / or the processor.

[0037] A first operation signal which is an extension command for the single rod cylinder 4 and a second operation signal which is a contraction command for the single rod cylinder 4 are input to the control device 8. In this embodiment, an operation device 11 including an operation lever operated by a user is employed, and the first operation signal and the second operation signal are input from this operation device 11 to the control device 8. However, when unmanned operation is performed, the operation device 11 may be omitted and the first operation signal and the second operation signal may be input to the control device 8 from the unmanned system.

[0038] In this embodiment, the operating speed of the single-rod cylinder 4 is determined by the angle of the operating lever of the operating device 11. For this reason, the first operating signal and the second operating signal include a speed command for the single-rod cylinder 4. However, the first operating signal and the second operating signal may not include a speed command, and the single-rod cylinder 4 may operate at a constant speed during extension and contraction.

[0039] (Cylinder extension) Fig. 2 is a flowchart for extending the single rod cylinder 4. As shown in Fig. 3, when the operating lever of the operating device 11 is tilted in the direction A, a first operating signal, which is an extension command for the single rod cylinder 4, is input from the operating device 11 to the control device 8.

[0040] When the first operation signal is input to the control device 8, the control device 8 rotates the electric motor 7 in the first direction (step S1). As a result, the first bidirectional pump 2 and the second bidirectional pump 3 are driven in the first direction, and the first bidirectional pump 2 draws in the working fluid through the rod side line 22, while the second bidirectional pump 3 draws in the working fluid through the second supply / discharge line 32. The first bidirectional pump 2 discharges the working fluid through the head side line 21, and the second bidirectional pump 3 discharges the working fluid through the first supply / discharge line 31. That is, the working fluids discharged from the first bidirectional pump 2 and the second bidirectional pump 3 join together and are supplied to the head side chamber 4h. As described above, since the first operation signal includes a speed command for the single-rod cylinder 4, the control device 8 rotates the electric motor 7 at the rotation speed N according to the speed command.

[0041] Next, the control device 8 acquires the position of the low pressure selection valve 6 detected by the position detector 9 (step S2). When the position of the low pressure selection valve 6 is the neutral position, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 becomes the first reference capacity qr1 (q1=qr1), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the second reference capacity qr2 (q2=qr2) (step S3). For example, when almost no load is applied to the single rod cylinder 4 (for example, when the operating lever of the operating device 11 is moved slowly), the position of the low pressure selection valve 6 becomes the neutral position.

[0042] The first reference capacity qr1 is a capacity determined from the operating speed V of the single-rod cylinder 4, the pressure-receiving area Ar of the rod side chamber 4r, and the rotation speed N of the electric motor 7 (qr1=V×Ar / N). The second reference capacity qr2 is obtained by multiplying the first reference capacity qr1 of the first bidirectional pump 2 by the ratio of the cross-sectional area Ac of the rod of the single-rod cylinder 4 to the pressure-receiving area Ar of the rod side chamber 4r (qr2=Ac / Ar×qr1). In other words, when q1=qr1 and q2=qr2, theoretically there is no excess or deficiency in the inflow flow rate Qh to the single-rod cylinder 4 or the outflow flow rate Qr from the single-rod cylinder 4.

[0043] For this reason, unless there is a factor that disrupts the balance between the discharge flow rate Q1 of the first bidirectional pump, the discharge flow rate Q2 of the second bidirectional pump 3, and the inflow flow rate Qh and outflow flow rate Qr of the single-rod cylinder 4, the hydraulic fluid will not be replenished from the tank through the first bypass line 55 or the second bypass line 57, and the hydraulic fluid will not be discharged to the tank through the first discharge line 61 or the second discharge line 62. Factors that disrupt the balance between the discharge flow rates Q1, Q2, the inflow flow rate Qh and the outflow flow rate Qr include the performance of the first regulator 25 and the second regulator 35, the efficiency of the first bidirectional pump 2 and the second bidirectional pump 3, external leakage of the hydraulic fluid, aging, etc.

[0044] That is, the inflow flow rate Qh into the head side chamber 4h is the sum of the discharge flow rate Q1 of the first bidirectional pump 2 and the discharge flow rate Q2 of the second bidirectional pump 3. The discharge flow rate Q1 [L / min] of the first bidirectional pump 2 is obtained by multiplying the capacity q1 [L / r] of the first bidirectional pump 2 by the rotation speed N [rpm], and the discharge flow rate Q2 [L / min] of the second bidirectional pump 3 is obtained by multiplying the capacity q2 [L / r] of the second bidirectional pump 3 by the rotation speed N [rpm]. The outflow flow rate Qr from the rod side chamber 4r is equal to the suction flow rate Q1 of the first bidirectional pump 2.

[0045] Thereafter, when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S6), the control device 8 stops the electric motor 7 (step S7). For example, the time when the stroke of the single-rod cylinder 4 reaches a predetermined amount includes the time when the input of the first operation signal from the operation device 11 to the control device 8 is stopped and the time when the single-rod cylinder 4 operates to the stroke end.

[0046] On the other hand, when the position of the low pressure selection valve 6 is the rod side discharge position in step S2, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 is smaller than the first reference capacity qr1 by the first predetermined amount Δq1 (q1=qr1-Δq1), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 is larger than the second reference capacity qr2 by the first predetermined amount Δq1 (q2=qr2+Δq1) (step S4). The first predetermined amount Δq1 is determined in consideration of factors that cause the balance of the discharge flow rates Q1, Q2, the inflow flow rate Qh, and the outflow flow rate Qr to be lost. For example, Δq1 is within a range of 1 to 10% of qr2.

[0047] For example, when a load is applied to the single-rod cylinder 4 in the contraction direction, the pressure Ph in the head side chamber 4h becomes higher than the pressure Pr in the rod side chamber 4r by more than the threshold value Pt, and the low pressure selection valve 6 is switched to the rod side discharge position.

[0048] Since the sum of the discharge flow rate Q1 of the first bidirectional pump 2 and the discharge flow rate Q2 of the second bidirectional pump 3 is the same as when the low pressure selection valve 6 is in the neutral position, the operating speed of the single-rod cylinder 4 is equal to the reference speed when q1=qr1 and q2=qr2. Meanwhile, since the capacity q1 of the first bidirectional pump 2 is less than the first reference capacity qr1 by the first predetermined amount Δq1, an excess amount Qfr according to the first predetermined amount Δq1 is discharged from the rod side line 22 through the second discharge line 62, the low pressure selection valve 6, and the low pressure relief line 65, as shown in FIG.

[0049] For example, when q1 = qr1 and q2 = qr2, assume that Qh = 100, Qr = 50, Q1 = 50, and Q2 = 50. If Q1 = 40 and Q2 = 60 when the position of the low pressure selection valve 6 is the rod side discharge position, then Qh = 100, Qr = 50, and Qfr = 10.

[0050] Conversely, when the position of the low pressure selection valve 6 is the head side discharge position in step S2, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 becomes the first reference capacity qr1 (q1=qr1), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes larger than the second reference capacity qr2 by a second predetermined amount Δq2 (q2=qr2+Δq2) (step S5). The second predetermined amount Δq2 is determined in consideration of factors that cause the balance between the discharge flow rates Q1 and Q2, the inflow flow rate Qh, and the outflow flow rate Qr to be lost. The second predetermined amount Δq2 may be the same as or different from the first predetermined amount Δq1 described above. For example, Δq2 is within a range of 1 to 10% of qr2.

[0051] For example, when a load is applied to the single-rod cylinder 4 in the extension direction, the pressure Pr in the rod side chamber 4r becomes higher than the pressure Ph in the head side chamber 4h by the threshold value Pt or more, and the low pressure selection valve 6 is switched to the head side discharge position.

[0052] Since the suction flow rate Q1 of the first bidirectional pump 2 is the same as when the low pressure selection valve 6 is in the neutral position, the operating speed of the single-rod cylinder 4 is equal to the reference speed when q1=qr1 and q2=qr2. On the other hand, since the capacity q2 of the second bidirectional pump 3 is larger than the second reference capacity qr2 by the second predetermined amount Δq2, an excess amount Qfh corresponding to the second predetermined amount Δq2 is discharged from the head side line 21 through the first discharge line 61, the low pressure selection valve 6, and the low pressure relief line 65, as shown in Fig. 4.

[0053] For example, as above, when q1=qr1 and q2=qr2, assume that Qh=100, Qr=50, Q1=50, and Q2=50. If Q1=50 and Q2=60 when the position of the low pressure selection valve 6 is the head side discharge position, then Qr=50, Qh=100, and Qfh=10.

[0054] Regardless of whether the low pressure selection valve 6 is in the rod side discharge position or the head side discharge position, when the stroke of the single rod cylinder 4 reaches a predetermined amount (YES in step S6), the control device 8 stops the electric motor 7 (step S7).

[0055] (Cylinder shortening) Fig. 5 is a flowchart for retracting the single rod cylinder 4. As shown in Fig. 6, when the operation lever of the operation device 11 is tilted in the B direction, a second operation signal, which is a retraction command for the single rod cylinder 4, is input from the operation device 11 to the control device 8.

[0056] When the second operation signal is input to the control device 8, the control device 8 rotates the electric motor 7 in the second direction (step S11). As a result, the first bidirectional pump 2 and the second bidirectional pump 3 are driven in the second direction, and the first bidirectional pump 2 draws in the working fluid through the head side line 21, while the second bidirectional pump 3 draws in the working fluid through the first supply / discharge line 31. In other words, the working fluid flowing out from the head side chamber 4h is distributed to the first bidirectional pump 2 and the second bidirectional pump 3. In addition, the first bidirectional pump 2 discharges the working fluid through the rod side line 22, and the second bidirectional pump 3 discharges the working fluid through the second supply / discharge line 32. As described above, since the second operation signal includes a speed command for the single-rod cylinder 4, the control device 8 rotates the electric motor 7 at the rotation speed N according to the speed command.

[0057] Next, the control device 8 acquires the position of the low pressure selection valve 6 detected by the position detector 9 (step S12). When the position of the low pressure selection valve 6 is the neutral position, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 becomes the first reference capacity qr1 (q1=qr1), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the second reference capacity qr2 (q2=qr2) (step S13). For example, when almost no load is applied to the single rod cylinder 4 (for example, when the operation lever of the operation device 11 is moved slowly), the position of the low pressure selection valve 6 becomes the neutral position.

[0058] As described above, the first reference capacity qr1 is a capacity determined by the operating speed V of the single-rod cylinder 4, the pressure-receiving area Ar of the rod side chamber 4r, and the rotation speed N of the electric motor 7 (qr1=V×Ar / N). The second reference capacity qr2 is obtained by multiplying the first reference capacity qr1 of the first bidirectional pump 2 by the ratio of the cross-sectional area Ac of the rod of the single-rod cylinder 4 to the pressure-receiving area Ar of the rod side chamber 4r (qr=Ac / Ar×qr1). In other words, when q1=qr1 and q2=qr2, theoretically there is no excess or deficiency in the inflow flow rate Qh to the single-rod cylinder 4 or the outflow flow rate Qr from the single-rod cylinder 4.

[0059] For this reason, unless there is a factor that disrupts the balance between the discharge flow rate Q1 of the first bidirectional pump, the discharge flow rate Q2 of the second bidirectional pump 3, and the inflow flow rate Qr and outflow flow rate Qh of the single-rod cylinder 4, the hydraulic fluid will not be replenished from the tank through the first bypass line 55 or the second bypass line 57, and the hydraulic fluid will not be discharged to the tank through the first discharge line 61 or the second discharge line 62. Factors that disrupt the balance between the discharge flow rates Q1, Q2, the inflow flow rate Qr and the outflow flow rate Qh include the performance of the first regulator 25 and the second regulator 35, the efficiency of the first bidirectional pump 2 and the second bidirectional pump 3, external leakage of the hydraulic fluid, aging, etc.

[0060] That is, the inflow rate Qr into the rod side chamber 4r is equal to the discharge rate Q1 of the first bidirectional pump 2. The outflow rate Qh from the head side chamber 4h is the sum of the suction flow rate Q1 of the first bidirectional pump 2 and the suction flow rate Q2 of the second bidirectional pump 3.

[0061] Thereafter, when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S16), the control device 8 stops the electric motor 7 (step S17). For example, the time when the stroke of the single-rod cylinder 4 reaches a predetermined amount includes the time when the input of the second operation signal from the operation device 11 to the control device 8 is stopped and the time when the single-rod cylinder 4 operates to the stroke end.

[0062] On the other hand, when the position of the low pressure selection valve 6 is the head side discharge position in step S12, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 becomes the first reference capacity qr1 (q1=qr1), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes less than the second reference capacity qr2 by a third predetermined amount Δq3 (q2=qr2-Δq3) (step S15). The third predetermined amount Δq3 is determined in consideration of factors that cause the balance between the discharge flow rates Q1 and Q2, the inflow flow rate Qh, and the outflow flow rate Qr to be lost. The third predetermined amount Δq3 may be the same as one of the first predetermined amount Δq1 and the second predetermined amount Δq2 described above, or may be different from both the first predetermined amount Δq1 and the second predetermined amount Δq2. For example, Δq3 is within a range of 1 to 10% of qr2.

[0063] For example, when a load is applied to the single-rod cylinder 4 in the extension direction, the pressure Pr in the rod side chamber 4r becomes higher than the pressure Ph in the head side chamber 4h by the threshold value Pt or more, and the low pressure selection valve 6 is switched to the head side discharge position.

[0064] Since the discharge flow rate Q1 of the first bidirectional pump 2 is the same as when the low pressure selection valve 6 is in the neutral position, the operating speed of the single-rod cylinder 4 is equal to the reference speed when q1=qr1 and q2=qr2. On the other hand, since the capacity q2 of the second bidirectional pump 3 is less than the second reference capacity qr2 by the third predetermined amount Δq3, an excess amount Qfh corresponding to the third predetermined amount Δq3 is discharged from the head side line 21 through the first discharge line 61, the low pressure selection valve 6, and the low pressure relief line 65, as shown in Fig. 7.

[0065] For example, when q1=qr1 and q2=qr2, assume that Qh=100, Qr=50, Q1=50, and Q2=50. If Q1=50 and Q2=40 when the position of the low pressure selection valve 6 is the head side discharge position, then Qr=50, Qh=100, and Qfh=10.

[0066] Conversely, when the position of the low pressure selection valve 6 is the rod side discharge position in step S2, the control device 8 controls the first regulator 25 so that the capacity q1 of the first bidirectional pump 2 is larger than the first reference capacity qr1 by a fourth predetermined amount (q1=qr1+Δq4), and controls the second regulator 35 so that the capacity q2 of the second bidirectional pump 3 is smaller than the second reference capacity qr2 by a fourth predetermined amount Δq4 (q2=qr2-Δq4) (step S14). The fourth predetermined amount Δq4 is determined in consideration of factors that cause the balance between the discharge flow rates Q1 and Q2, the inflow flow rate Qh, and the outflow flow rate Qr to be lost. The fourth predetermined amount Δq4 may be the same as any of the first predetermined amount Δq1, the second predetermined amount Δq2, and the third predetermined amount Δq3 described above, or may be different from any of the first predetermined amount Δq1, the second predetermined amount Δq2, and the third predetermined amount Δq3. For example, Δq4 is within the range of 1 to 10% of qr2.

[0067] For example, when a load is applied to the single-rod cylinder 4 in the contraction direction, the pressure Ph in the head side chamber 4h becomes higher than the pressure Pr in the rod side chamber 4r by more than the threshold value Pt, and the low pressure selection valve 6 is switched to the rod side discharge position.

[0068] Since the sum of the suction flow rate Q1 of the first bidirectional pump 2 and the suction flow rate Q2 of the second bidirectional pump 3 is the same as when the low pressure selection valve 6 is in the neutral position, the operating speed of the single-rod cylinder 4 is equal to the reference speed when q1=qr1 and q2=qr2. Meanwhile, since the capacity q1 of the first bidirectional pump 2 is larger than the first reference capacity qr1 by a fourth predetermined amount Δq4, an excess amount Qfr according to the fourth predetermined amount Δq4 is discharged from the rod side line 22 through the second discharge line 62, the low pressure selection valve 6, and the low pressure relief line 65, as shown in FIG.

[0069] For example, as above, when q1=qr1 and q2=qr2, assume that Qh=100, Qr=50, Q1=50, and Q2=50. If Q1=60 and Q2=40 when the position of the low pressure selection valve 6 is the rod side discharge position, then Qh=100, Qr=50, and Qfr=10.

[0070] Regardless of whether the low pressure selection valve 6 is in the rod side discharge position or the head side discharge position, when the stroke of the single rod cylinder 4 reaches a predetermined amount (YES in step S16), the control device 8 stops the electric motor 7 (step S17).

[0071] As described above, in hydraulic system 1 of the present embodiment, low pressure selection valve 6 is provided, so that it is possible to physically prevent pressure trapping from occurring on the lower pressure of the rod side or head side in a configuration in which second bidirectional pump 3 is added to first bidirectional pump 2. Moreover, since the position of low pressure selection valve 6 is detected by position detector 9, various controls can be performed based on the position of the low pressure selection valve.

[0072] Moreover, in this embodiment, since control is performed as shown in the flowchart in FIG. 2, even if the direction of the load is reversed during extension of the single-rod cylinder 4 and the low pressure selection valve 6 switches from the rod side discharge position to the head side discharge position, or from the head side discharge position to the rod side discharge position, the operating speed of the single-rod cylinder 4 can be kept constant.

[0073] Furthermore, in this embodiment, since control is performed as shown in the flowchart in FIG. 5, even if the direction of the load is reversed when the single-rod cylinder 4 is contracted and the low pressure selection valve 6 switches from the rod side discharge position to the head side discharge position, or from the head side discharge position to the rod side discharge position, the operating speed of the single-rod cylinder 4 can be kept constant.

[0074] (Modification) The present disclosure is not limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present disclosure.

[0075] For example, as in a modified hydraulic system 1A shown in Fig. 8, a first on-off valve 67 may be provided in the first pilot line 63, and a second on-off valve 68 may be provided in the second pilot line 64. According to this configuration, by closing the first on-off valve 67 and the second on-off valve 68 in a situation where the magnitude relationship between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r is inverted, hunting of the low pressure selection valve 6 can be prevented.

[0076] (summary) In a first aspect, the present disclosure provides a hydraulic system for a single-rod cylinder including a head side chamber and a rod side chamber, the hydraulic system including: a first bidirectional variable displacement pump connected to the head side chamber by a head side line and connected to the rod side chamber by a rod side line; a second bidirectional variable displacement pump connected to the head side line by a supply / discharge line; an electric motor that drives the first bidirectional pump and the second bidirectional pump in the same direction; and an electric motor connected to the head side line by a first exhaust line and connected to the rod side line by a second exhaust line, the electric motor being configured to operate in a manner such that when a pressure difference between a pressure in the head side chamber and a pressure in the rod side chamber is less than a threshold value, a low pressure selection valve that is located in a neutral position where it blocks the first discharge line and the second discharge line, and that is switched to a rod side discharge position where the second discharge line communicates with a relief line provided with a relief valve when the pressure in the head side chamber is higher than the pressure in the rod side chamber by at least the threshold value, and that is switched to a head side discharge position where the first discharge line communicates with the relief line when the pressure in the rod side chamber is higher than the pressure in the head side chamber by at least the threshold value, and a position detector that detects whether the position of the low pressure selection valve is the neutral position, the rod side discharge position, or the head side discharge position.

[0077] According to the above configuration, since the low pressure selection valve is provided, it is possible to physically prevent the trapping of pressure in the lower pressure side of the rod side or the head side in a configuration in which a second bidirectional pump is added to a first bidirectional pump. Moreover, since the position of the low pressure selection valve is detected by the position detector, various controls can be performed based on the position of the low pressure selection valve.

[0078] As a second aspect, in the first aspect, the hydraulic system includes a first regulator that changes the capacity of the first bidirectional pump, a second regulator that changes the capacity of the second bidirectional pump, and a control device that controls the electric motor, the first regulator, and the second regulator, and the control device controls the capacity of the first bidirectional pump to a first reference capacity and the capacity of the second bidirectional pump to a second reference capacity when the position of the low pressure selection valve detected by the position detector is the neutral position when the single rod cylinder is extended. The first regulator and the second regulator may be controlled so that when the position of the low pressure selection valve detected by the position detector is the rod side discharge position, the displacement of the first bidirectional pump is less than the first reference displacement by a first predetermined amount and the displacement of the second bidirectional pump is greater than the second reference displacement by the first predetermined amount, and when the position of the low pressure selection valve detected by the position detector is the head side discharge position, the displacement of the first bidirectional pump becomes the first reference displacement and the displacement of the second bidirectional pump becomes greater than the second reference displacement by a second predetermined amount. With this configuration, the operating speed of the single rod cylinder can be kept constant even if the direction of the load is reversed during extension of the single rod cylinder and the low pressure selection valve is switched from the rod side discharge position to the head side discharge position or from the head side discharge position to the rod side discharge position.

[0079] As a third aspect, in the first or second aspect, the hydraulic system includes a first regulator that changes the displacement of the first bidirectional pump, a second regulator that changes the displacement of the second bidirectional pump, and a control device that controls the electric motor, the first regulator, and the second regulator, and the control device is configured to control a displacement of the first bidirectional pump to a first reference displacement and a displacement of the second bidirectional pump to a second reference displacement when the position of the low pressure selection valve detected by the position detector is the neutral position when the single rod cylinder is contracted. The first regulator and the second regulator may be controlled so that when the position of the low pressure selection valve detected by the position detector is the head side discharge position, the displacement of the first bidirectional pump becomes the first reference displacement and the displacement of the second bidirectional pump becomes less than the second reference displacement by the third predetermined amount, and when the position of the low pressure selection valve detected by the position detector is the rod side discharge position, the displacement of the first bidirectional pump becomes greater than the first reference displacement by a fourth predetermined amount and the displacement of the second bidirectional pump becomes less than the second reference displacement by the fourth predetermined amount. With this configuration, the operating speed of the single rod cylinder can be kept constant even if the direction of the load is reversed during contraction of the single rod cylinder and the low pressure selection valve is switched from the rod side discharge position to the head side discharge position or from the head side discharge position to the rod side discharge position.

[0080] As a fourth aspect, in the second or third aspect, for example, the second reference capacity may be the first reference capacity multiplied by a ratio of a pressure-receiving area of ​​the rod side chamber to a cross-sectional area of ​​the rod of the single-rod cylinder.

[0081] As a fifth aspect, in any of the first to fourth aspects, the hydraulic system may further include a first lock valve which is an on-off valve provided in the head side line, and a second lock valve which is an on-off valve provided in the rod side line. According to this configuration, the operation of the single-rod cylinder can be locked by closing the first lock valve and the second lock valve.

[0082] As a sixth aspect, in any of the first to fifth aspects, the low pressure selection valve may be guided to the pressure of the head side chamber through a first pilot line and to the pressure of the rod side chamber through a second pilot line, and the hydraulic system may further include a first on-off valve provided in the first pilot line and a second on-off valve provided in the second pilot line. According to this configuration, by closing the first on-off valve and the second on-off valve in a situation where the magnitude relationship between the pressure of the head side chamber and the pressure of the rod side chamber is reversed, hunting of the low pressure selection valve can be prevented.

[0083] As a seventh aspect, in any of the first to sixth aspects, the above hydraulic system may further include a head side relief valve provided in a head side relief line branching off from the head side line, a first check valve provided in a first bypass line bypassing the head side relief valve, a rod side relief valve provided in a rod side relief line branching off from the rod side line, and a second check valve provided in a second bypass line bypassing the rod side relief valve. According to this configuration, the pressure in the head side chamber and the pressure in the rod side chamber can be kept equal to or lower than the set pressure of the relief valve, and cavitation on the head side or rod side, whichever has the lower pressure, can be suppressed by supplying hydraulic fluid to the head side line or the rod side line via the check valve. [Explanation of symbols]

[0084] 1,1A Hydraulic System 2. First two-way pump 21 Head side line 22 Rod side line 23 First lock valve 24 Second lock valve 25 First regulator 3. Second bidirectional pump 31,32 Supply and exhaust lines 35 Second regulator 4 Single Rod Cylinder 4h Head Chamber 4r Rod side chamber 51 Head side relief line 52 Head side relief valve 53 Rod side relief line 54 Rod side relief valve 55 First Bypass Line 56 First check valve 57 Second Bypass Line 58 Second check valve 6 Low Pressure Selector Valve 61 First discharge line 62 Second discharge line 63 First Pilot Line 64 2nd Pilot Line 65 Low Pressure Relief Line 66 Low Pressure Relief Valve 67 First opening and closing valve 68 Second opening and closing valve 7 Electric motor 8 Control device 9 Position detector

Claims

1. A hydraulic system for a single rod cylinder including a head side chamber and a rod side chamber, a first bidirectional pump of a variable displacement type connected to the head side chamber by a head side line and connected to the rod side chamber by a rod side line; a second bidirectional pump of a variable displacement type connected to the head side line by a supply / discharge line; an electric motor that drives the first bidirectional pump and the second bidirectional pump in the same direction; a low pressure selection valve which is connected to the head side line by a first discharge line and to the rod side line by a second discharge line, which is located in a neutral position for blocking the first discharge line and the second discharge line when a pressure difference between the pressure in the head side chamber and the pressure in the rod side chamber is less than a threshold value, which is switched to a rod side discharge position for communicating the second discharge line with a relief line provided with a relief valve when the pressure in the head side chamber is higher than the pressure in the rod side chamber by at least the threshold value, and which is switched to a head side discharge position for communicating the first discharge line with the relief line when the pressure in the rod side chamber is higher than the pressure in the head side chamber by at least the threshold value; a position detector that detects whether the low pressure selection valve is in the neutral position, the rod side discharge position, or the head side discharge position; A hydraulic system comprising:

2. a first regulator that changes the displacement of the first bidirectional pump; a second regulator that changes the displacement of the second bidirectional pump; a control device that controls the electric motor, the first regulator, and the second regulator, When the control device extends the single rod cylinder, when the position of the low pressure selection valve detected by the position detector is the neutral position, the capacity of the first bidirectional pump becomes a first reference capacity and the capacity of the second bidirectional pump becomes a second reference capacity, when the position of the low pressure selection valve detected by the position detector is the rod side discharge position, the capacity of the first bidirectional pump becomes smaller than the first reference capacity by a first predetermined amount, and the capacity of the second bidirectional pump becomes larger than the second reference capacity by the first predetermined amount, when the position of the low pressure selection valve detected by the position detector is the head side discharge position, the capacity of the first bidirectional pump becomes the first reference capacity, and the capacity of the second bidirectional pump becomes larger than the second reference capacity by a second predetermined amount, The hydraulic system of claim 1 , further comprising: a first regulator and a second regulator.

3. a first regulator that changes the displacement of the first bidirectional pump; a second regulator that changes the displacement of the second bidirectional pump; a control device that controls the electric motor, the first regulator, and the second regulator, When the control device contracts the single rod cylinder, when the position of the low pressure selection valve detected by the position detector is the neutral position, the capacity of the first bidirectional pump becomes a first reference capacity and the capacity of the second bidirectional pump becomes a second reference capacity, when the position of the low pressure selection valve detected by the position detector is the head side discharge position, the capacity of the first bidirectional pump becomes the first reference capacity, and the capacity of the second bidirectional pump becomes smaller than the second reference capacity by the third predetermined amount, when the position of the low pressure selection valve detected by the position detector is the rod side discharge position, the capacity of the first bidirectional pump is larger than the first reference capacity by a fourth predetermined amount, and the capacity of the second bidirectional pump is smaller than the second reference capacity by a fourth predetermined amount; The hydraulic system according to claim 1 or 2, further comprising: a control unit for controlling the first regulator and the second regulator.

4. 3. The hydraulic system according to claim 2, wherein the second reference capacity is obtained by multiplying the first reference capacity by a ratio of a pressure receiving area of ​​the rod side chamber to a cross-sectional area of ​​a rod of the single rod cylinder.

5. a first lock valve which is an on-off valve provided in the head side line; The hydraulic system according to claim 1 , further comprising: a second lock valve that is an on-off valve provided in the rod side line.

6. The pressure of the head side chamber is introduced to the low pressure selection valve through a first pilot line, and the pressure of the rod side chamber is introduced to the low pressure selection valve through a second pilot line, a first on-off valve provided in the first pilot line; The hydraulic system according to claim 1 , further comprising: a second on-off valve provided in the second pilot line.

7. a head side relief valve provided in a head side relief line branching off from the head side line; a first check valve provided in a first bypass line that bypasses the head side relief valve; a rod-side relief valve provided in a rod-side relief line branching off from the rod-side line; 2. The hydraulic system according to claim 1, further comprising: a second check valve provided in a second bypass line that bypasses the rod-side relief valve.

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