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 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 allowing the second discharge line 62 or the first discharge line 61 to communicate with a relief line 65 equipped with a relief valve 66.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Conventionally, a hydraulic system for a single-rod cylinder including a bidirectional pump connected to form a closed circuit with the single-rod cylinder is known. For example, Patent Document 1 discloses a hydraulic system 100 as shown in FIG. 10.

[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 the head-side chamber 210 of the single-rod cylinder 200 by a head-side line 111 and to the 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 (the difference between the inflow rate and the outflow 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 / drain line 121.

[0004] The first bidirectional pump 110 and the second bidirectional pump 120 are driven in the same direction by an electric motor 130. More specifically, when extending the single-rod cylinder 200 (i.e., when advancing the rod), the electric motor 130 drives the first bidirectional pump 110 and the second bidirectional pump 120 in the first direction, and when shortening the single-rod cylinder 200 (i.e., when retracting the rod), the electric motor 130 drives the first bidirectional pump 110 and the second bidirectional pump 120 in a second direction opposite to the first direction. When the first bidirectional pump 110 and the second bidirectional pump 120 are driven in the first direction, they discharge the hydraulic fluid into 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 hydraulic fluid from the head-side chamber 210. Further, in the hydraulic system 100, the second bidirectional pump 120 is a variable displacement pump, and the capacity of the second bidirectional pump 120 is changed by a regulator 140.

[0005] In the hydraulic system 100, if the capacity of the second bidirectional pump 120 is taken as the reference capacity when there is no excess or deficiency in the inflow rate into or outflow rate from the single-rod cylinder 200, then when the single-rod cylinder 200 is extended, if the capacity of the second bidirectional pump 120 is greater than the reference capacity, pressure conflation will occur on the lower pressure side of either the rod side (rod side chamber 220 and rod side line 112) or the head side (head side chamber 210 and head side line 111). Conversely, when the single-rod cylinder 200 is extended, if the capacity of the second bidirectional pump 120 is less than the reference capacity, cavitation will occur on the lower pressure side of either the rod side or the head side. Also, when the single-rod cylinder 200 is retracted, if the capacity of the second bidirectional pump 120 is greater than the reference capacity, cavitation will occur on the lower pressure side of either the rod side or the head side, and if the capacity of the second bidirectional pump 120 is less than the reference capacity, pressure conflation will occur on the lower pressure side of either the rod side or 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 as to suppress the containment or cavitation of these pressures. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-245740 [Patent Document 2] Japanese Patent Publication No. 2002-54602 [Overview of the project] [Problems that the invention aims to solve]

[0007] As described above, in the hydraulic system 100 of Patent Document 1, pressure trapping on the lower pressure side of the rod side and the head side is suppressed by control mechanisms. In contrast, there is a desire to physically prevent pressure trapping on the lower pressure side of the rod side and the head side.

[0008] Furthermore, Figure 2 of Patent Document 2 discloses a hydraulic circuit in which, as prior art, only one bidirectional pump is used for a single-rod cylinder, and a low-pressure selector valve (referred to as a "flushing valve" in Patent Document 2) is employed to allow the discharge of working fluid to the tank from the line with lower pressure between the head-side line and the rod-side line. However, this hydraulic circuit does not have a second bidirectional pump added to the first bidirectional pump, as is the case with the hydraulic system 100 in Patent Document 1.

[0009] Therefore, the present disclosure aims to provide a hydraulic system that can physically prevent pressure trapping from occurring on the side with lower pressure, 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] This disclosure provides a hydraulic system for a single-rod cylinder including a head side chamber and a rod side chamber, comprising: a first bidirectional 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 variable displacement bidirectional pump connected to the head side line by a supply / discharge line; an electric motor for driving the first bidirectional pump and the second bidirectional pump in the same direction; and a low-pressure selector valve connected to the head side line by a first discharge line and connected to the rod side line by a second discharge line, which connects the second discharge line to a relief line equipped with a relief valve when the pressure in the rod side chamber is lower than the pressure in the head side chamber, and connects the first discharge line to the relief line when the pressure in the head side chamber is lower than the pressure in the rod side chamber. [Effects of the Invention]

[0011] According to this disclosure, a hydraulic system is provided that can physically prevent pressure trapping from occurring on the side with the lower pressure, 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 explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of a hydraulic system according to one embodiment. [Figure 2] This is a flowchart showing the cylinder extension process. [Figure 3] This diagram shows the flow of the working fluid when the differential pressure between the head-side chamber and the rod-side chamber is below a threshold during cylinder extension. [Figure 4] This diagram shows the flow of the working fluid when the differential pressure between the head-side chamber and the rod-side chamber exceeds a threshold during cylinder extension. [Figure 5] This is a flowchart showing the cylinder shortening process. [Figure 6] This diagram shows the flow of the working fluid when the differential pressure between the head-side chamber and the rod-side chamber is below a threshold when the cylinder is retracted. [Figure 7] This diagram shows the flow of the working fluid when the differential pressure between the head-side chamber and the rod-side chamber is above a threshold when the cylinder is retracted. [Figure 8] This is a schematic diagram of a modified hydraulic system. [Figure 9] This is a schematic diagram of a hydraulic system in another modified configuration. [Figure 10] This is a schematic diagram of a conventional hydraulic system. [Modes for carrying out the invention]

[0013] Figure 1 shows a hydraulic system 1 according to one embodiment. This 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 working fluid used in the hydraulic system 1 is typically oil.

[0014] Specifically, the hydraulic system 1 includes a first bidirectional pump 2 connected to form a closed circuit with the single-rod cylinder 4, and a second bidirectional pump 3 for eliminating the flow rate difference (the difference between the inflow rate and the outflow 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 the head side line 21 and is connected to the rod side chamber 4r of the single-rod cylinder 4 by the rod side line 22. The second bidirectional pump 3 is connected to the head side line 21 by the first supply / drain line 31 and is connected to the tank by the second supply / drain 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 the control device 8. In FIG. 1, for the sake of simplifying the drawing, the drawing of some signal lines is omitted.

[0017] When operating (extending or shortening) the single-rod cylinder 4, the control device 8 opens the first lock valve 23 and the second lock valve 24, and closes the first lock valve 23 and the second lock valve 24 otherwise. By closing the first lock valve 23 and the second lock valve 24 in this way, the operation of the single-rod cylinder 4 can be locked.

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

[0019] Furthermore, a first bypass line 55 branches off from the head-side line 21, bypassing the head-side relief valve 52, and this first bypass line 55 is connected to the tank. The first bypass line 55 is equipped with a first check valve 56 that allows flow from the tank to the head-side line 21 but prohibits flow in the reverse 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. However, the head-side relief line 51 and the first bypass line 55 may also branch off from the head-side line 21 between the first lock valve 23 and the single-rod cylinder 4. Furthermore, the first bypass line 55 does not necessarily need to branch off from the head-side line 21; it 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 equipped with a rod-side relief valve 54 set to a relatively high relief pressure (for example, 25-35 MPa).

[0022] Furthermore, a second bypass line 57 branches off from the rod-side line 22, bypassing the rod-side relief valve 54, and this second bypass line 57 is connected to the tank. The second bypass line 57 is equipped with a second check valve 58 that allows flow from the tank to the rod-side line 22 but prohibits flow in the reverse 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. However, the rod-side relief line 53 and the second bypass line 57 may also branch off from the rod-side line 22 between the second lock valve 24 and the single-rod cylinder 4. Furthermore, the second bypass line 57 does not necessarily need to branch off from the rod-side line 22; it may branch off from the rod-side relief line 53 upstream of the rod-side relief valve 54.

[0024] With the head-side relief line 51 and rod-side relief line 53 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 below the set pressures of the head-side relief valve 52 and the rod-side relief valve 54. Furthermore, with the first bypass line 55 and the second bypass line 57 provided, cavitation can be suppressed in the lower pressure side 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) 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 (swashplate pumps or oblique shaft pumps). In this embodiment, the first bidirectional pump 2 is a fixed-displacement pump, and the second bidirectional pump 3 is a variable-displacement pump. However, the first bidirectional pump 2 may also be a variable-displacement pump.

[0026] The capacity of the second bidirectional pump 3 is changed by a regulator 35. The regulator 35 is controlled by a control device 8. For example, if the second bidirectional pump 3 is a swashplate pump, the regulator 35 may electrically change the hydraulic pressure acting on a servo piston connected to the swashplate of the second bidirectional pump 3, or it may be an electric actuator connected to the swashplate of the 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 they may be arranged so that their central axes are aligned parallel to each other (parallel type).

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

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

[0030] The low-pressure selector valve 6 can be switched between a neutral position (center position in Figure 1), a head-side discharge position (right position in Figure 1), and a rod-side discharge position (left position in Figure 1). In the neutral position, the low-pressure selector valve 6 blocks the first discharge line 61 and the second discharge line 62. In the head-side discharge position, the low-pressure selector 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 selector valve 6 blocks the first discharge line 61 while connecting the second discharge line 62 to the low-pressure relief line 65.

[0031] The low-pressure selector valve 6 receives pressure Ph from the head-side chamber 4h via a first pilot line 63, and pressure Pr from the rod-side chamber 4r via a second pilot line 64. The first pilot line 63 is for switching the low-pressure selector valve 6 from the neutral position to the rod-side discharge position, and the second pilot line 64 is for switching the low-pressure selector 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 branch off from the first discharge line 61 and the second discharge line 62, respectively, but the first pilot line 63 may branch off from the head-side line 21, or the second pilot line 64 may branch off from the rod-side line 22.

[0032] The low-pressure selector valve 6 is positioned in the rod-side discharge position when the pressure Pr in the rod-side chamber 4r is lower than the pressure Ph in the head-side chamber 4h, connecting the second discharge line 62 to the low-pressure relief line 65. When the pressure Ph in the head-side chamber 4h is lower than the pressure Pr in the rod-side chamber 4r, it is positioned in the head-side discharge position, connecting the first discharge line 61 to the low-pressure relief line 65. In this embodiment, 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 exceeds the threshold Pt, the low-pressure selector valve 6 is switched from the neutral position to either the rod-side discharge position or the head-side discharge position. For example, the threshold Pt is in the range of 0.5 to 3 MPa. Note that the threshold Pt may differ between cylinder extension and cylinder retraction.

[0033] Next, the control of the electric motor 7 and regulator 35 by the control device 8 will be described in detail. With respect to the control device 8, the functions of the elements disclosed herein can be performed using circuits or processing circuits, including general-purpose processors, dedicated 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 enumerated functions, or hardware programmed to perform the enumerated functions. The hardware may be hardware disclosed herein, or other known hardware that is programmed or configured to perform the enumerated functions. If the hardware is a processor, which is considered a type of circuit, then 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.

[0034] The control device 8 receives a first operation signal, which is an extension command for one rod cylinder 4, and a second operation signal, which is a shortening command for one rod cylinder 4. In this embodiment, an operating device 11 including an operating lever operated by the user is employed, and the first and second operation signals are input from this operating device 11 to the control device 8. However, in the case of unmanned operation, the operating device 11 may be omitted, and the first and second operation signals may be input from the unmanned system to the control device 8.

[0035] 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 single-rod cylinder 4 may operate at a constant speed during extension and retraction even if the first and second operating signals do not include a speed command.

[0036] Furthermore, the control device 8 is electrically connected to a differential pressure gauge 9 that detects the differential pressure ΔP between the pressure Ph in the head-side chamber 4h and the pressure Pr in the rod-side chamber 4r. In this embodiment, the differential pressure gauge 9 includes a pressure sensor 91 provided on the head-side line 21 and a pressure sensor 92 provided on the rod-side line 22. However, the pressure sensors 91 and 92 may be provided in the head-side chamber 4h and the rod-side chamber 4r, respectively. Also, the differential pressure gauge 9 does not necessarily have to include two pressure sensors; it may be a single sensor.

[0037] (Cylinder extension) Figure 2 is a flowchart for extending the single-rod cylinder 4. As shown in Figure 3, when the operating lever of the operating device 11 is tilted in 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.

[0038] 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, with the first bidirectional pump 2 drawing in hydraulic fluid through the rod-side line 22 and the second bidirectional pump 3 drawing in hydraulic fluid through the second supply / discharge line 32. The first bidirectional pump 2 also discharges hydraulic fluid through the head-side line 21, and the second bidirectional pump 3 discharges hydraulic fluid through the first supply / discharge line 31. In other words, the hydraulic fluid discharged from the first bidirectional pump 2 and the second bidirectional pump 3 merges and is 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 a rotational speed N corresponding to that speed command.

[0039] Next, the control device 8 compares the differential pressure ΔP detected by the differential pressure gauge 9 with a threshold Pt (step S2). If the differential pressure ΔP is less than the threshold Pt (NO in step S2), the control device 8 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the reference capacity qr (q2=qr) (step S3). For example, if there is almost no load on the single-rod cylinder 4 (for example, if the operating lever of the operating device 11 is moved slowly), the differential pressure ΔP will be less than the threshold Pt.

[0040] The standard capacity qr is obtained by multiplying the capacity q1 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 × q1). That is, if q2 = qr, theoretically there is no excess or deficiency in the inflow flow rate Qh into the single-rod cylinder 4 or the outflow flow rate Qr from the single-rod cylinder 4.

[0041] Therefore, 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, no replenishment of working fluid from the tank via the first bypass line 55 or the second bypass line 57, and no discharge of working fluid to the tank via the first discharge line 61 or the second discharge line 62 will occur. Factors that disrupt the balance between the discharge flow rates Q1, Q2, inflow flow rate Qh and outflow flow rate Qr include the performance of the regulator 35, the efficiency of the first bidirectional pump 2 and the second bidirectional pump 3, external leakage of working fluid, and aging.

[0042] In other words, the inflow rate Qh into the head side chamber 4h is the sum of the discharge rate Q1 of the first bidirectional pump 2 and the discharge rate Q2 of the second bidirectional pump 3. Note that the discharge 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 rotational speed N [rpm], and the discharge 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 rotational speed N [rpm]. The outflow rate Qr from the rod side chamber 4r is equal to the suction rate Q1 of the first bidirectional pump 2.

[0043] Subsequently, the control device 8 stops the electric motor 7 (step S6) when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S5). For example, when the stroke of the single-rod cylinder 4 reaches a predetermined amount, this includes when the input of the first operation signal from the operating device 11 to the control device 8 is stopped, and when the single-rod cylinder 4 has moved to the end of its stroke.

[0044] On the other hand, if the differential pressure ΔP is greater than or equal to the threshold Pt (YES in step S2), the control device 8 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 is greater than the reference capacity qr by a first predetermined amount Δq1 (q2 = qr + Δq1) (step S4). The first predetermined amount Δq1 is determined by considering the factors that disrupt the balance between the discharge flow rates Q1, Q2, inflow flow rate Qh, and outflow flow rate Qr described above. For example, Δq1 is within the range of 1 to 10% of qr.

[0045] For example, when a load is applied to one rod cylinder 4 in the shortening direction, the pressure Ph in the head side chamber 4h becomes higher than the pressure Pr in the rod side chamber 4r by a threshold Pt or more, and when a load is applied to one rod cylinder 4 in the extension direction, the pressure Pr in the rod side chamber 4r becomes higher than the pressure Pr in the head side chamber 4h by a threshold Pt or more.

[0046] As shown in Figure 4, when the pressure Ph in the head-side chamber 4h becomes higher than the pressure Pr in the rod-side chamber 4r by a threshold Pt or more, the low-pressure selector valve 6 is switched to the rod-side discharge position. As a result, the excess Qfr corresponding to the first predetermined amount Δq1 is discharged from the rod-side line 22 through the second discharge line 62, the low-pressure selector valve 6, and the low-pressure relief line 65. Note that the operating speed of the single-rod cylinder 4 at this time is faster than the reference speed when q2=qr by an amount corresponding to the first predetermined amount Δq1.

[0047] Conversely, if the pressure Pr in the rod-side chamber 4r becomes higher than the pressure Pr in the head-side chamber 4h by a threshold Pt or more, the low-pressure selector valve 6 is switched to the head-side discharge position. As a result, the excess Qfh corresponding to the first predetermined amount Δq1 is discharged from the head-side line 21 through the first discharge line 61, the low-pressure selector valve 6, and the low-pressure relief line 65. Note that the operating speed of the single-rod cylinder 4 at this time is equal to the reference speed when q2 = qr.

[0048] For example, if q2=qr, then assume Qh=100, Qr=50, Q1=50, and Q2=50. When Ph-Pr≧Pt, if Q1=50 and Q2=60, then Qh=110, Qr=55, and Qfr=5. On the other hand, when Pr-Ph≧Pt, if Q1=50 and Q2=60, then Qr=50, Qh=100, and Qfh=10.

[0049] Even if the differential pressure ΔP is greater than or equal to the threshold Pt, the control device 8 stops the electric motor 7 (step S6) when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S5).

[0050] (Cylinder shortening) Figure 5 is a flowchart for shortening the single-rod cylinder 4. As shown in Figure 6, when the operating lever of the operating device 11 is tilted in direction B, a second operating signal, which is a shortening command for the single-rod cylinder 4, is input from the operating device 11 to the control device 8.

[0051] 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, with the first bidirectional pump 2 drawing in hydraulic fluid through the head-side line 21 and the second bidirectional pump 3 drawing in hydraulic fluid through the first supply / discharge line 31. In other words, the hydraulic fluid flowing out from the head-side chamber 4h is distributed between the first bidirectional pump 2 and the second bidirectional pump 3. The first bidirectional pump 2 also discharges hydraulic fluid through the rod-side line 22, and the second bidirectional pump 3 discharges hydraulic fluid through the second supply / discharge line 32. As described above, the second operation signal includes a speed command for the single-rod cylinder 4, so the control device 8 rotates the electric motor 7 at a rotational speed N corresponding to that speed command.

[0052] Next, the control device 8 compares the differential pressure ΔP detected by the differential pressure gauge 9 with a threshold Pt (step S12). If the differential pressure ΔP is less than the threshold Pt (NO in step S12), the control device 8 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the reference capacity qr (q2=qr). For example, if there is almost no load on the single-rod cylinder 4 (for example, if the operating lever of the operating device 11 is moved slowly), the differential pressure ΔP will be less than the threshold Pt.

[0053] As described above, the standard capacity qr is obtained by multiplying the capacity q1 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 × q1). That is, if q2 = qr, theoretically there is no excess or deficiency in the inflow flow rate Qr into the single-rod cylinder 4 or the outflow flow rate Qh from the single-rod cylinder 4.

[0054] Therefore, 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, no replenishment of hydraulic fluid from the tank via the first bypass line 55 or the second bypass line 57, and no discharge of hydraulic fluid to the tank via the first discharge line 61 or the second discharge line 62 will occur. Factors that disrupt the balance between the discharge flow rates Q1, Q2, inflow flow rate Qr, and outflow flow rate Qh include the performance of the regulator 35, the efficiency of the first bidirectional pump 2 and the second bidirectional pump 3, external leakage of hydraulic fluid, and aging.

[0055] In other words, 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 rate Q1 of the first bidirectional pump 2 and the suction rate Q2 of the second bidirectional pump 3.

[0056] Subsequently, the control device 8 stops the electric motor 7 (step S16) when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S15). For example, when the stroke of the single-rod cylinder 4 reaches a predetermined amount, this includes when the input of the second operation signal from the operating device 11 to the control device 8 is stopped, and when the single-rod cylinder 4 has moved to the end of its stroke.

[0057] On the other hand, if the differential pressure ΔP is greater than or equal to the threshold Pt (YES in step S12), the control device 8 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 is less than the reference capacity qr by a second predetermined amount Δq2 (q2 = qr - Δq2) (step S14). The second predetermined amount Δq2 is determined by considering the factors that disrupt the balance between the discharge flow rates Q1, Q2, inflow flow rate Qr, and outflow flow rate Qh described above. 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 the range of 1 to 10% of qr.

[0058] For example, when an extensional load is applied to one rod cylinder 4, the pressure Pr in the rod-side chamber 4r becomes higher than the pressure Ph in the head-side chamber 4h by a threshold Pt or more. When a shortening load is applied to one rod cylinder 4, the pressure Ph in the head-side chamber 4h becomes higher than the pressure Pr in the rod-side chamber 4r by a threshold Pt or more.

[0059] As shown in Figure 7, when the pressure Pr in the rod-side chamber 4r becomes higher than the pressure Pr in the head-side chamber 4h by a threshold Pt or more, the low-pressure selector valve 6 is switched to the head-side discharge position. As a result, the excess 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 selector valve 6, and the low-pressure relief line 65. Note that the operating speed of the single-rod cylinder 4 at this time is equal to the reference speed when q2 = qr.

[0060] Conversely, if the pressure Ph in the head chamber 4h becomes higher than the pressure Pr in the rod chamber 4r by a threshold Pt or more, the low-pressure selector valve 6 is switched to the rod-side discharge position. As a result, the excess Qfr corresponding to the second predetermined amount Δq2 is discharged from the rod-side line 22 through the second discharge line 62, the low-pressure selector valve 6, and the low-pressure relief line 65. Note that the operating speed of the single-rod cylinder 4 at this time is slower than the reference speed when q2=qr by an amount corresponding to the second predetermined amount Δq2.

[0061] For example, if q2=qr, then assume Qr=50, Qh=100, Q1=50, and Q2=50. When Pr-Ph≧Pt, if Q1=50 and Q2=40, then Qr=50, Qh=100, and Qfh=10. On the other hand, when Ph-Pr≧Pt, if Q1=50 and Q2=40, then Qh=90, Qr=45, and Qfr=5.

[0062] Even if the differential pressure ΔP is greater than or equal to the threshold Pt, the control device 8 stops the electric motor 7 (step S16) when the stroke of the single-rod cylinder 4 reaches a predetermined amount (YES in step S15).

[0063] As described above, in the hydraulic system 1 of this embodiment, a low-pressure selector valve 6 is provided, so in a configuration in which a second bidirectional pump 3 is added to the first bidirectional pump 2, it is possible to physically prevent pressure conflation from occurring on the side with lower pressure, either the rod side or the head side.

[0064] Furthermore, in this embodiment, when the single-rod cylinder 4 is extended, the regulator 35 is controlled by comparing the differential pressure ΔP detected by the differential pressure gauge 9 with a threshold Pt. Therefore, when the differential pressure ΔP between the pressure Ph in the head-side chamber 4h and the pressure Pr in the rod-side chamber 4r is small, the capacity q2 of the second bidirectional pump 3 can be maintained at the reference capacity qr. Moreover, in this case, the low-pressure selector valve 6 is in the neutral position, so the single-rod cylinder 4 can be operated in a state where 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 balanced with the inflow flow rate Qh into the head-side chamber 4h of the single-rod cylinder 4, and the outflow flow rate Qr from the rod-side chamber 4r of the single-rod cylinder 4 is balanced with the suction flow rate Q1 of the first bidirectional pump 2. This state is usually desired when operating the single-rod cylinder 4 slowly under no load, thus enabling smoother operation with less shock. Furthermore, by keeping the low-pressure selector valve 6 in the neutral position, hunting of the low-pressure selector valve 6 can be suppressed.

[0065] Furthermore, in this embodiment, when the single-rod cylinder 4 is shortened, the regulator 35 is controlled by comparing the differential pressure ΔP detected by the differential pressure gauge 9 with a threshold Pt. Therefore, when the differential pressure ΔP between the pressure Ph in the head-side chamber 4h and the pressure Pr in the rod-side chamber 4r is small, the capacity q2 of the second bidirectional pump 3 can be maintained at the reference capacity qr. Moreover, in this case, the low-pressure selector valve 6 is in the neutral position, so the discharge flow rate Q1 of the first bidirectional pump 2 and the inflow flow rate Qr into the rod-side chamber 4r of the single-rod cylinder 4 are balanced, and the single-rod cylinder 4 can be operated in a state where the outflow flow rate Qh from the head-side chamber 4h of the single-rod cylinder 4 is balanced with 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. This state is usually desired when operating the single-rod cylinder 4 slowly under no load, so a smoother operation with less shock can be achieved. In addition, by keeping the low-pressure selector valve 6 in the neutral position, hunting of the low-pressure selector valve 6 can be suppressed.

[0066] (modified version) This disclosure is not limited to the embodiments described above, and various modifications are possible without departing from the gist of this disclosure.

[0067] For example, in the embodiment described above, when the differential pressure ΔP between the pressure Ph in the head-side chamber 4h and the pressure Pr in the rod-side chamber 4r becomes greater than or equal to a threshold Pt, the low-pressure selector valve 6 is switched from the neutral position to the rod-side discharge position or the head-side discharge position. In such cases, instead of the differential pressure gauge 9, a position detector (e.g., a stroke sensor) 95 is used to detect whether the position of the low-pressure selector valve 6 is the neutral position, the head-side discharge position, or the rod-side discharge position, as in the modified hydraulic system 1A shown in Figure 8. The control device 8 may then control the regulator 35 based on the detection result of this position detector 95. Whether a differential pressure gauge 9 is used or a position detector 95 is used, the function that the low-pressure selector valve 6 performs can be determined from the detection results, and the regulator 35 can be controlled according to the function of the low-pressure selector valve 6.

[0068] Furthermore, the control device 8 may control the regulator 35 such that, regardless of the differential pressure ΔP and the position of the low-pressure selector valve 6, when the single-rod cylinder 4 is extended, the capacity q2 of the second bidirectional pump 3 is always greater than the reference capacity qr by a first predetermined amount Δq1, and when the single-rod cylinder 4 is retracted, the capacity q2 of the second bidirectional pump 3 is always less than the reference capacity qr by a second predetermined amount Δq2. However, in this case, there is a risk of shock occurring due to the switching of the low-pressure selector valve 6 when the electric motor 7 starts rotating, so it is desirable to control the regulator 35 based on the differential pressure ΔP or the position of the low-pressure selector valve 6.

[0069] Furthermore, as shown in the modified hydraulic system 1B in Figure 9, 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. With this configuration, if the first on-off valve 67 and the second on-off valve 68 are closed in a situation where the relative pressure relationship between the pressure Ph in the head side chamber 4h and the pressure Pr in the rod side chamber 4r is reversed, hunting of the low-pressure selector valve 6 can be prevented.

[0070] (summary) In a first aspect, the present disclosure provides a hydraulic system for a single-rod cylinder including a head chamber and a rod chamber, comprising: a first bidirectional pump connected to the head chamber by a head line and to the rod chamber by a rod line; a second variable-displacement bidirectional pump connected to the head line by a supply / discharge line; an electric motor for driving the first bidirectional pump and the second bidirectional pump in the same direction; and a low-pressure selector valve connected to the head line by a first discharge line and to the rod line by a second discharge line, which connects the second discharge line to a relief line equipped with a relief valve when the pressure in the rod chamber is lower than the pressure in the head chamber, and connects the first discharge line to the relief line when the pressure in the head chamber is lower than the pressure in the rod chamber.

[0071] With the above configuration, since a low-pressure selector valve is provided, it is physically possible to prevent pressure conflation from occurring on the side with the lower pressure, either the rod side or the head side, in a configuration where a second bidirectional pump is added to a first bidirectional pump.

[0072] In a second embodiment, in the first embodiment, the hydraulic system comprises a regulator for changing the capacity of the second bidirectional pump and a control device for controlling the electric motor and the regulator, wherein the control device may control the regulator so that when the single-rod cylinder is extended, the capacity of the second bidirectional pump is greater than the reference capacity by a first predetermined amount. With this configuration, when the single-rod cylinder is extended, if the pressure in the head side chamber is higher than the pressure in the rod side chamber, the excess amount corresponding to the first predetermined amount is discharged from the rod side line through the second discharge line, the low-pressure selector valve and the relief line, and if the pressure in the rod side chamber is higher than the pressure in the head side chamber, the excess amount corresponding to the first predetermined amount is discharged from the head side line through the first discharge line, the low-pressure selector valve and the relief line.

[0073] In a third embodiment, in the first or second embodiment, the hydraulic system comprises a regulator for changing the capacity of the second bidirectional pump and a control device for controlling the electric motor and the regulator, wherein the control device may control the regulator such that when shortening the single-rod cylinder, the capacity of the second bidirectional pump becomes less than the standard capacity by a second predetermined amount. With this configuration, when shortening the single-rod cylinder, if the pressure in the rod-side chamber is higher than the pressure in the head-side chamber, the excess amount corresponding to the second predetermined amount is discharged from the head-side line through the first discharge line, low-pressure selector valve and relief line, and if the pressure in the head-side chamber is higher than the pressure in the rod-side chamber, the excess amount corresponding to the second predetermined amount is discharged from the rod-side line through the second discharge line, low-pressure selector valve and relief line.

[0074] In a fourth embodiment, in the second or third embodiment, for example, the reference capacity may be obtained by multiplying the capacity of the first bidirectional pump by the ratio of the cross-sectional area of ​​the rod of the single-rod cylinder to the pressure-receiving area of ​​the rod-side chamber.

[0075] In a fifth embodiment, in any of the second to fourth embodiments, the low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position that connects the first discharge line to the relief line, or to a rod-side discharge position that connects the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold value. The control device may then control the regulator such that when the low-pressure selector valve is in the neutral position, the capacity of the second bidirectional pump becomes the standard capacity, and when the low-pressure selector valve is in the head-side discharge position or the rod-side discharge position, the capacity of the second bidirectional pump becomes the first predetermined amount greater than the standard capacity. With this configuration, the capacity of the second bidirectional pump can be maintained at the standard capacity when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber is small. Furthermore, since the low-pressure selector valve is in the neutral position at this time, the sum of the discharge flow rates of the first and second bidirectional pumps is balanced with the inflow flow rate into the head side chamber of the single-rod cylinder, and the outflow flow rate from the rod side chamber of the single-rod cylinder is balanced with the suction flow rate of the first bidirectional pump, allowing the single-rod cylinder to be operated. This condition is usually desired when operating the single-rod cylinder slowly under no load, so a smoother operation with less shock can be achieved. In addition, by keeping the low-pressure selector valve in the neutral position, hunting of the low-pressure selector valve can be suppressed.

[0076] In a sixth embodiment, in any of the third to fifth embodiments, the low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position connecting the first discharge line to the relief line, or to a rod-side discharge position connecting the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold value. The control device may then control the regulator such that when the low-pressure selector valve is in the neutral position, the capacity of the second bidirectional pump becomes the standard capacity, and when the low-pressure selector valve is in the head-side discharge position or the rod-side discharge position, the capacity of the second bidirectional pump becomes less than the standard capacity by a predetermined second amount. With this configuration, the capacity of the second bidirectional pump can be maintained at the standard capacity when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber is small. Furthermore, since the low-pressure selector valve is in the neutral position at this time, the discharge flow rate of the first bidirectional pump and the inflow flow rate into the rod side chamber of the single-rod cylinder are balanced, and the single-rod cylinder can be operated in a state where the outflow flow rate from the head side chamber of the single-rod cylinder is balanced with the sum of the suction flow rates of the first bidirectional pump and the second bidirectional pump. This state is usually desired when operating the single-rod cylinder slowly under no load, so a smoother operation with less shock can be achieved. In addition, by keeping the low-pressure selector valve in the neutral position, hunting of the low-pressure selector valve can be suppressed.

[0077] In a seventh aspect, in any of the first to sixth aspects, the hydraulic system may further include a first lock valve, which is an on / off valve, provided on the head-side line, and a second lock valve, which is an on / off valve, provided on the rod-side line. With this configuration, the operation of one rod cylinder can be locked by closing the first lock valve and the second lock valve.

[0078] In an eighth aspect, in any of the first to seventh aspects, the pressure of the head side chamber is supplied to the low-pressure selector valve through a first pilot line, and the pressure of the rod side chamber is supplied to the low-pressure selector valve through a second pilot line, and the hydraulic system may further include a first on-off valve provided on the first pilot line and a second on-off valve provided on the second pilot line. With this configuration, hunting of the low-pressure selector valve can be prevented by closing the first on-off valve and the second on-off valve in a situation where the relative magnitudes of the pressures of the head side chamber and the rod side chamber are reversed.

[0079] In a ninth aspect, in any of the first to eighth aspects, the hydraulic system may further include a head-side relief valve provided in a head-side relief line branching 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 from the rod-side line, and a second check valve provided in a second bypass line bypassing the rod-side relief valve. With this configuration, the pressure in the head-side chamber and the pressure in the rod-side chamber can be kept below the set pressure of the relief valve, and cavitation can be suppressed in the head-side or rod-side, whichever has lower pressure, by supplying working fluid to the head-side line or the rod-side line via the check valve.

[0080] In a tenth embodiment, in any of the first to ninth embodiments, the hydraulic system may further include a differential pressure gauge for detecting the differential pressure between the pressure in the head chamber and the pressure in the rod chamber. With this configuration, the function of the low-pressure selector valve can be determined from the detection result of the differential pressure gauge, and the regulator for changing the capacity of the second bidirectional pump can be controlled according to the function of the low-pressure selector valve.

[0081] In an eleventh embodiment, in any of the first to ninth embodiments, the low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position that connects the first discharge line to the relief line, or to a rod-side discharge position that connects the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold, and the hydraulic system further includes a position detector that detects whether the low-pressure selector valve is in the neutral position, the rod-side discharge position, or the head-side discharge position. With this configuration, it is possible to determine what function the low-pressure selector valve performs from the detection result of the position detector, and therefore the regulator that changes the capacity of the second bidirectional pump can be controlled according to the function of the low-pressure selector valve. [Explanation of symbols]

[0082] 1,1A,1B Hydraulic System 2. First bidirectional pump 21 Head-side line 22 Rod-side line 23. First lock valve 24. Second locking valve 3. Second bidirectional pump 31,32 Supply and discharge lines 35 Regulator 4 Single-rod cylinder 4h Head side 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 Emission Line 62 Second Emission Line 63 First Pilot Line 64. Second Pilot Line 65 Low-pressure relief line 66 Low-pressure relief valve 67 First shut-off valve 68 Second shut-off valve 7 Electric motor 8 Control device 9. Differential pressure gauge 95 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 is connected to the head side chamber by a head side line and to the rod side chamber by a rod side line, A variable displacement second bidirectional 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, A low-pressure selector valve is provided, which is connected to the head-side line by a first discharge line and to the rod-side line by a second discharge line, and when the pressure in the rod-side chamber is lower than the pressure in the head-side chamber, the second discharge line is connected to a relief line equipped with a relief valve, and when the pressure in the head-side chamber is lower than the pressure in the rod-side chamber, the first discharge line is connected to the relief line. A regulator for changing the capacity of the second bidirectional pump, The system comprises a control device for controlling the electric motor and the regulator, The low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position that connects the first discharge line to the relief line, or to a rod-side discharge position that connects the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold. The control device controls the regulator such that, when extending the single-rod cylinder, the capacity of the second bidirectional pump becomes the standard capacity when the low-pressure selector valve is in the neutral position, and the capacity of the second bidirectional pump becomes the standard capacity by a first predetermined amount when the low-pressure selector valve is in the head-side discharge position or the rod-side discharge position.

2. A hydraulic system for a single-rod cylinder, including a head side chamber and a rod side chamber, A first bidirectional pump is connected to the head side chamber by a head side line and to the rod side chamber by a rod side line, A variable displacement second bidirectional 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, The first discharge line is connected to the head-side line, and the second discharge line is connected to the head-side line. A low-pressure selector valve is connected to the rod-side line, and when the pressure in the rod-side chamber is lower than the pressure in the head-side chamber, it connects the second discharge line to a relief line equipped with a relief valve, and when the pressure in the head-side chamber is lower than the pressure in the rod-side chamber, it connects the first discharge line to the relief line. A regulator for changing the capacity of the second bidirectional pump, The system comprises a control device for controlling the electric motor and the regulator, The low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position that connects the first discharge line to the relief line, or to a rod-side discharge position that connects the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold. The control device controls the regulator such that, when shortening the single-rod cylinder, the capacity of the second bidirectional pump becomes the standard capacity when the low-pressure selector valve is in the neutral position, and the capacity of the second bidirectional pump becomes less than the standard capacity by a second predetermined amount when the low-pressure selector valve is in the head-side discharge position or the rod-side discharge position.

3. The hydraulic system according to claim 1 or 2, wherein the standard capacity is obtained by multiplying the capacity of the first bidirectional pump by the ratio of the cross-sectional area of ​​the rod of the single-rod cylinder to the pressure-receiving area of ​​the rod-side chamber.

4. A first lock valve, which is an on / off valve, is provided in the head-side line, The hydraulic system according to claim 1 or 2, further comprising a second lock valve, which is an on / off valve, provided on the rod-side line.

5. The pressure of the head side chamber is introduced to the low-pressure selector valve through a first pilot line, and the pressure of the rod side chamber is introduced through a second pilot line. A first on / off valve provided in the first pilot line, The hydraulic system according to claim 1 or 2, further comprising a second on-off valve provided in the second pilot line.

6. A head-side relief valve is provided in the head-side relief line that branches off from the head-side line, A first check valve is provided in the first bypass line that bypasses the head-side relief valve, A rod-side relief valve is provided in the rod-side relief line that branches off from the aforementioned rod-side line, The hydraulic system according to claim 1 or 2, further comprising a second check valve provided in a second bypass line that bypasses the rod-side relief valve.

7. The hydraulic system according to claim 1 or 2, further comprising a differential pressure gauge for detecting the differential pressure between the pressure in the head chamber and the pressure in the rod chamber.

8. The low-pressure selector valve is switched from a neutral position blocking the first and second discharge lines to a head-side discharge position that connects the first discharge line to the relief line, or to a rod-side discharge position that connects the second discharge line to the relief line, when the differential pressure between the pressure in the head-side chamber and the pressure in the rod-side chamber exceeds a threshold. The hydraulic system according to claim 1 or 2, further comprising a position detector for detecting whether the position of the low-pressure selector valve is the neutral position, the rod-side discharge position, or the head-side discharge position.

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