Hydraulic system

Abstract

A hydraulic system (1) according to one embodiment includes: a first bidirectional pump (2) connected to a head-side chamber (4h) of a single-rod cylinder (4) through a head-side line (21) and connected to a rod-side chamber (4r) of the single-rod cylinder (4) through a rod-side line (22); and a variable-capacity second bidirectional pump (3) connected to the head-side line (21) via a feed / discharge line (31). Furthermore, the hydraulic system (1) is provided with a low-pressure selection valve (6) which is connected to the head-side line (21) by a first discharge line (61), is connected to the rod-side line (22) by a second discharge line (62), and connects the second discharge line (62) or the first discharge line (61) to a relief line (66) in which a relief valve (66) is provided.

Description

Technical Field

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

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

[0003] The hydraulic system 100 includes a first two-way pump 110 and a second two-way pump 120. The first two-way pump 110 is connected to a head side chamber 210 of a single-rod cylinder 200 through a head side line 111, and is connected to a rod side chamber 220 of the single-rod cylinder 200 through a rod side line 112. The second two-way pump 120 is used to eliminate the flow difference (the difference between the inflow flow and the outflow flow) 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 through a supply and discharge line 121.

[0004] The first two-way pump 110 and the second two-way pump 120 are driven in the same direction by the electric motor 130. More specifically, the electric motor 130 drives the first two-way pump 110 and the second two-way pump 120 in the first direction when the single-rod cylinder 200 is extended (i.e., when the rod is advanced); and drives the first two-way pump 110 and the second two-way pump 120 in the second direction opposite to the first direction when the single-rod cylinder 200 is shortened (i.e., when the rod is retracted). When the first two-way pump 110 and the second two-way pump 120 are driven in the first direction, they discharge the working fluid to the head side chamber 210; when the first two-way pump 110 and the second two-way pump 120 are driven in the second direction, they suck the working fluid from the head side chamber 210. In addition, in the hydraulic system 100, the second two-way pump 120 is a variable capacity pump, and the capacity of the second two-way pump 120 is changed by the regulator 140.

[0005] In the hydraulic system 100, if the capacity of the second two-way pump 120 when the inflow flow rate to or outflow flow rate from the single-rod cylinder 200 is not excessive or insufficient is set as the reference capacity, when the capacity of the second two-way pump 120 is larger than the reference capacity when the single-rod cylinder 200 is extended, pressure stagnation occurs on 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), whichever has a lower pressure. On the contrary, when the capacity of the second two-way pump 120 is smaller than the reference capacity when the single-rod cylinder 200 is extended, cavitation occurs on the rod side and the head side, whichever has a lower pressure. Furthermore, when the capacity of the second two-way pump 120 is larger than the reference capacity when the single-rod cylinder 200 is shortened, cavitation occurs on the rod side and the head side, whichever has a lower pressure, and when the capacity of the second two-way pump 120 is smaller than the reference capacity, pressure stagnation occurs on the rod side and the head side, whichever has a lower pressure. The regulator 140 is controlled based on the pressure of the head side chamber 210 or the rod side chamber 220 of the single-rod cylinder 200 to suppress stagnation of these pressures or cavitation.

[0006] Prior art literature: Patent literature: Patent Document 1: Japanese Patent Application Publication No. 2013-245740 Patent Document 2: Japanese Patent Application Publication No. 2002-54602 Summary of the invention

[0007] Problems to be solved by the invention: As described above, in the hydraulic system 100 of Patent Document 1, pressure stagnation on the rod side or the head side, whichever has the lower pressure, is controlled and suppressed. In contrast, it is desirable to physically prevent pressure stagnation on the rod side or the head side, whichever has the lower pressure.

[0008] In addition, in Patent Document 2 Figure 2 As a prior art, a hydraulic circuit is disclosed that uses only one bidirectional pump for a single-rod cylinder and adopts a low-pressure selector valve (referred to as a "flush valve" in Patent Document 2) that can discharge the working fluid from the lower pressure of the head side line and the rod side line to a tank. However, this hydraulic circuit does not add a second bidirectional pump to the first bidirectional pump like the 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 from stagnation on the rod side or the head side, whichever has a lower pressure, in a structure in which a second two-way pump is added to a first two-way pump.

[0010] Means of solving the problem: The present disclosure provides a hydraulic system, which is a hydraulic system for a single-rod cylinder including a head side chamber and a rod side chamber, and comprises: a first two-way pump, which is connected to the head side chamber through a head side line and is connected to the rod side chamber through a rod side line; a second two-way pump of a variable capacity type, which is connected to the head side line through a supply and discharge line; an electric motor, which drives the first two-way pump and the second two-way pump in the same direction; and a low-pressure selection valve, which is connected to the head side line through a first discharge line and is connected to the rod side line through a second discharge line, 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 the overflow line provided with an overflow 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 overflow line.

[0011] Effect of the invention: According to the present disclosure, there is provided a hydraulic system capable of physically preventing pressure from stagnation on the rod side or the head side, whichever has a lower pressure, in a structure in which a second two-way pump is added to a first two-way pump. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a schematic structural diagram of a hydraulic system of an embodiment; Figure 2 This is the flow chart when the cylinder is extended; Figure 3 is a diagram showing the flow of the working fluid when the pressure difference between the head side chamber pressure and the rod side chamber pressure is smaller than a threshold value when the cylinder is extended; Figure 4 is a diagram showing the flow of the working fluid when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber is equal to or larger than a threshold value when the cylinder is extended; Figure 5 This is the flow chart when the cylinder is shortened; Figure 6 is a diagram showing the flow of the working fluid when the pressure difference between the head side chamber pressure and the rod side chamber pressure is smaller than a threshold value when the cylinder is shortened; Figure 7 is a diagram showing the flow of the working fluid when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber is equal to or larger than a threshold value when the cylinder is shortened; Figure 8 is a schematic structural diagram of a hydraulic system of a modified example; Fig. 9 is a schematic structural diagram of a hydraulic system of another modified example; Fig.10 This is a schematic diagram of the structure of a conventional hydraulic system. DETAILED DESCRIPTION

[0013] Figure 11 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 working fluid used in the hydraulic system 1 is typically oil.

[0014] Specifically, the hydraulic system 1 includes: a first two-way pump 2, which is connected to the single-rod cylinder 4 in the form of a closed loop; and a second two-way pump 3, which is used to eliminate the flow difference (the difference between the inflow flow and the outflow flow) 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 through the head side line 21, and is connected to the rod side chamber 4r of the single rod cylinder 4 through the rod side line 22. The second bidirectional pump 3 is connected to the head side line 21 through the first supply and discharge line 31, and is connected to the tank through the second supply and discharge line 32.

[0016] A first lock valve 23 as an on-off valve is provided in the head side line 21, and a second lock valve 24 as 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. Figure 1 In order to simplify the drawings, some signal lines are omitted.

[0017] The control device 8 opens the first lock valve 23 and the second lock valve 24 when the single-rod cylinder 4 is actuated (extended or shortened), and otherwise closes the first lock valve 23 and the second lock valve 24. In this way, by closing the first lock valve 23 and the second lock valve 24, the action of the single-rod cylinder 4 can be blocked.

[0018] A head-side overflow line 51 is branched from the head-side line 21 and connected to the tank. The head-side overflow line 51 is provided with a head-side overflow valve 52 whose overflow pressure is set relatively high (for example, 25 to 35 MPa).

[0019] A first bypass line 55 bypassing the head-side relief valve 52 branches 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 allows flow from the tank to the head-side line 21 but prohibits reverse flow.

[0020] In the example, the head-side overflow line 51 and the first bypass line 55 are branched from the head-side line 21 between the first lock valve 23 and the first two-way pump 2, but the head-side overflow line 51 and the first bypass line 55 may be branched 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 be branched from the head-side line 21, and may be branched from the head-side overflow line 51 on the upstream side of the head-side overflow valve 52.

[0021] Similarly, a rod-side relief line 53 branches off from the rod-side line 22 and is connected to the 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] A second bypass line 57 bypassing the rod-side relief valve 54 branches from the rod-side line 22 and is connected to the tank. The second bypass line 57 is provided with a second check valve 58 that allows flow from the tank to the rod-side line 22 but prohibits reverse flow.

[0023] In the example, the rod-side overflow line 53 and the second bypass line 57 are branched from the rod-side line 22 between the second lock valve 24 and the first two-way pump 2, but the rod-side overflow line 53 and the second bypass line 57 may be branched 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 be branched from the rod-side line 22, and may be branched from the rod-side overflow line 53 on the upstream side of the rod-side overflow valve 54.

[0024] Thus, if the head-side relief line 51 and the rod-side relief line 53 are provided, the pressure Ph of the head-side chamber 4h and the pressure Pr of the rod-side chamber 4r can be maintained below the set pressure 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, the cavitation phenomenon of the lower pressure of the head side (the head-side chamber 4h and the head-side line 21) and the rod side (the rod-side chamber 4r and the rod-side line 22) can be suppressed by supplying the 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). 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 the regulator 35. The regulator 35 is controlled by the control device 8. For example, when the second bidirectional pump 3 is a swash plate pump, the regulator 35 may be a regulator that electrically changes the hydraulic pressure acting on the servo piston connected to the swash plate of the second bidirectional pump 3, or may be an electric actuator connected to the swash plate 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 the electric motor 7. The first bidirectional pump 2 and the second bidirectional pump 3 may be arranged in a form where their central axes are coaxially arranged (series type) or in a form where their central axes are arranged in parallel (parallel type).

[0028] The electric motor 7 is controlled by the control device 8. The electric motor 7 drives the first two-way pump 2 and the second two-way pump 3 in the first direction when the single-rod cylinder 4 is extended, and drives the first two-way pump 2 and the second two-way pump 3 in the second direction opposite to the first direction when the single-rod cylinder 4 is shortened. If the first two-way pump 2 and the second two-way pump 3 are driven in the first direction, they discharge the working fluid into the head side chamber 4h; if the first two-way pump 2 and the second two-way pump 3 are driven in the second direction, they inhale the working fluid from the head side chamber 4h.

[0029] Furthermore, in the present embodiment, a low-pressure selector valve 6 is used which is connected to the head-side line 21 via the first discharge line 61 and is connected to the rod-side line 22 via the second discharge line 62. The low-pressure selector valve 6 is connected to the tank via a low-pressure relief line 65. A low-pressure relief valve 66 having a relatively low relief pressure (for example, 0.1 to 2 MPa) is provided on the low-pressure relief line 65.

[0030] The low pressure selector valve 6 is in the neutral position ( Figure 1 central position), head side discharge position ( Figure 1 right side position), rod side discharge position ( Figure 1 In the neutral position, the low pressure selector valve 6 closes the first discharge line 61 and the second discharge line 62. In the head side discharge position, the low pressure selector valve 6 closes the second discharge line 62 and communicates the first discharge line 61 with the low pressure overflow line 65. In the rod side discharge position, the low pressure selector valve 6 closes the first discharge line 61 and communicates the second discharge line 62 with the low pressure overflow line 65.

[0031] The pressure Ph introduced into the head side chamber 4h through the first pilot line 63 and the pressure Pr introduced into the rod side chamber 4r through the second pilot line 64 on the low pressure selector valve 6. The first pilot line 63 is used to switch the low pressure selector valve 6 from the neutral position to the rod side discharge position, and the second pilot line 64 is used to switch the low pressure selector valve 6 from the neutral position to the head side discharge position. In the example, the first pilot line 63 and the second pilot line 64 are respectively branched from the first discharge line 61 and the second discharge line 62, but the first pilot line 63 may be branched from the head side line 21, and the second pilot line 64 may be branched from the rod side line 22.

[0032] The low pressure selector valve 6 is located at the rod side discharge position when the pressure Pr of the rod side chamber 4r is lower than the pressure Ph of the head side chamber 4h, so that the second discharge line 62 is connected to the low pressure overflow line 65; when the pressure Ph of the head side chamber 4h is lower than the pressure Pr of the rod side chamber 4r, the low pressure selector valve 6 is located at the head side discharge position, so that the first discharge line 61 is connected to the low pressure overflow line 65. In this embodiment, when the pressure difference ΔP (ΔP=|Ph-Pr|) between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r reaches or exceeds the threshold value 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. For example, the threshold value Pt is in the range of 0.5~3MPa. In addition, the threshold value Pt may be different when the cylinder is extended and when the cylinder is shortened.

[0033] Next, the control of the electric motor 7 and the regulator 35 by the control device 8 is described in detail. With respect to the control device 8, the functions of the elements disclosed in this specification can be performed using a circuit or processing circuit including a general-purpose processor, a dedicated processor, an integrated circuit, an ASIC (Application Specific Integrated Circuits), an existing circuit, and / or a combination thereof, which is configured or programmed to perform the disclosed functions. The processor includes transistors and other circuits, so it can be regarded as a processing circuit or circuit. In the present disclosure, a circuit, a unit, or a means is hardware that performs the listed functions, or hardware that is programmed to perform the listed functions. The hardware may be the hardware disclosed in this specification, or it may be other known hardware that is programmed or configured to perform the listed functions. When the hardware is a processor that is considered to be a type of circuit, the circuit, means, or unit is a combination of hardware and software, and the software is used to constitute the hardware and / or the processor.

[0034] The first operation signal as an extension command to the single-rod cylinder 4 and the second operation signal as a contraction command to the single-rod cylinder 4 are input to the control device 8. In the present embodiment, an operation device 11 including an operation lever operated by a user is used, and the first operation signal and the second operation signal are input to the control device 8 from the operation device 11. However, in the case of unmanned driving, 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 an unmanned system.

[0035] In the present embodiment, the operating speed of the single-rod cylinder 4 is determined by the angle of the operating rod of the operating device 11. Therefore, 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.

[0036] In addition, the control device 8 is electrically connected to a differential pressure gauge 9 that detects a pressure difference ΔP between a pressure Ph of the head side chamber 4h and a pressure Pr of the rod side chamber 4r. In the present embodiment, the differential pressure gauge 9 includes a pressure sensor 91 provided in the head side line 21 and a pressure sensor 92 provided in 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. In addition, the differential pressure gauge 9 does not necessarily have to include two pressure sensors, and may be a single sensor.

[0037] (Cylinder extension) Figure 2 This is a flow chart for extending the single-rod cylinder 4. Figure 3 As shown, when the operating rod of the operating device 11 is tilted in the direction A, a first operating signal as an extension command to the single-rod cylinder 4 is input from the operating device 11 to the control device 8 .

[0038] If 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 two-way pump 2 and the second two-way pump 3 are driven in the first direction, the first two-way pump 2 sucks the working fluid through the rod side line 22, and the second two-way pump 3 sucks the working fluid through the second supply and discharge line 32. In addition, the first two-way pump 2 discharges the working fluid through the head side line 21, and the second two-way pump 3 discharges the working fluid through the first supply and discharge line 31. That is, the working fluids discharged from the first two-way pump 2 and the second two-way pump 3 merge with each other and are supplied to the head side chamber 4h. As described above, since the first operation signal includes a speed instruction for the single-rod cylinder 4, the control device 8 rotates the electric motor 7 at a rotation speed N corresponding to the speed instruction.

[0039] Next, the control device 8 compares the pressure difference ΔP detected by the differential pressure gauge 9 with the threshold value Pt (step S2). When the pressure difference ΔP is smaller than the threshold value 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, when the load is hardly applied to the single-rod cylinder 4 (for example, when the operating rod of the operating device 11 is slowly moved), the pressure difference ΔP is smaller than the threshold value Pt.

[0040] The reference 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, when q2=qr, theoretically, the inflow flow Qh to the single-rod cylinder 4 or the outflow flow Qr from the single-rod cylinder 4 is neither too much nor too little.

[0041] Therefore, unless there is a factor causing an imbalance in the discharge flow rate Q1 of the first two-way pump, the discharge flow rate Q2 of the second two-way pump 3, the inflow flow rate Qh and the outflow flow rate Qr of the single-rod cylinder 4, the supply of the working fluid from the tank through the first bypass line 55 or the second bypass line 57 and the discharge of the working fluid to the tank through the first discharge line 61 or the second discharge line 62 are performed. In addition, the factors causing an imbalance in the discharge flow rates Q1, Q2, the inflow flow rate Qh and the outflow flow rate Qr include the performance of the regulator 35, the efficiency of the first two-way pump 2 and the second two-way pump 3, external leakage of the working fluid, aging, and the like.

[0042] That is, the inflow flow rate Qh to the head side chamber 4h is the sum of the discharge flow rate Q1 of the first two-way pump 2 and the discharge flow rate Q2 of the second two-way pump 3. In addition, the discharge flow rate Q1 [L / min] of the first two-way pump 2 is obtained by multiplying the capacity q1 [L / r] of the first two-way pump 2 by the rotation speed N [rpm], and the discharge flow rate Q2 [L / min] of the second two-way pump 3 is obtained by multiplying the capacity q2 [L / r] of the second two-way 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 two-way pump 2.

[0043] After that, 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, the input of the first operation signal from the operation device 11 to the control device 8 stops, and when the single-rod cylinder 4 moves to the end of the stroke.

[0044] On the other hand, when the pressure difference ΔP is greater than the threshold value Pt ("YES" in step S2), the control device 8 controls the regulator 35 in such a manner that the capacity q2 of the second bidirectional pump 3 is greater than the reference capacity qr by the first predetermined amount Δq1 (q2=qr+Δq1) (step S4). The first predetermined amount Δq1 is determined by taking into account the imbalance of the discharge flow rates Q1, Q2, the inflow flow rate Qh, and the 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 in the shortening direction is applied to the single-rod cylinder 4, the pressure Ph in the head side chamber 4h is higher than the pressure Pr in the rod side chamber 4r by more than the threshold value Pt. When a load in the extending direction is applied to the single-rod cylinder 4, the pressure Pr in the rod side chamber 4r is higher than the pressure Pr in the head side chamber 4h by more than the threshold value Pt.

[0046] like Figure 4As shown, when the pressure Ph of the head side chamber 4h is higher than the pressure Pr of the rod side chamber 4r by a threshold value Pt or more, the low pressure selector valve 6 is switched to the rod side discharge position. As a result, the remaining amount 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. In addition, 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] On the contrary, when the pressure Pr of the rod side chamber 4r is higher than the pressure Pr of the head side chamber 4h by a threshold value Pt or more, the low pressure selector valve 6 is switched to the head side discharge position. As a result, the remaining amount 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. In addition, the operating speed of the single rod cylinder 4 at this time is equal to the reference speed when q2=qr.

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

[0049] When the pressure difference ΔP is equal to or larger than the threshold value 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 shortened) Figure 5 This is a flow chart for shortening the single-rod cylinder 4. Figure 6 As shown, when the operating lever of the operating device 11 is tilted in the B direction, the second operating signal as a contraction command for the single-rod cylinder 4 is input from the operating device 11 to the control device 8 .

[0051] If 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 two-way pump 2 and the second two-way pump 3 are driven in the second direction, the first two-way pump 2 sucks the working fluid through the head side line 21, and the second two-way pump 3 sucks the working fluid through the first supply and discharge line 31. That is, the working fluid flowing out of the head side chamber 4h is distributed into an amount toward the first two-way pump 2 and an amount toward the second two-way pump 3. In addition, the first two-way pump 2 discharges the working fluid through the rod side line 22, and the second two-way pump 3 discharges the working fluid through the second supply and discharge line 32. As described above, since the second operation signal includes a speed instruction for the single-rod cylinder 4, the control device 8 rotates the electric motor 7 at a rotation speed N corresponding to the speed instruction.

[0052] Next, the control device 8 compares the pressure difference ΔP detected by the differential pressure gauge 9 with the threshold value Pt (step S12). When the pressure difference ΔP is smaller than the threshold value 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, when the load is hardly applied to the single-rod cylinder 4 (for example, when the operating rod of the operating device 11 is slowly moved), the pressure difference ΔP is smaller than the threshold value Pt.

[0053] As described above, the reference 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, when q2=qr, theoretically, the inflow flow rate Qr to the single-rod cylinder 4 or the outflow flow rate Qh from the single-rod cylinder 4 is neither excessive nor insufficient.

[0054] Therefore, unless there is a factor causing an imbalance in the discharge flow rate Q1 of the first two-way pump, the discharge flow rate Q2 of the second two-way pump 3, the inflow flow rate Qr and the outflow flow rate Qh of the single-rod cylinder 4, the supply of the working fluid from the tank through the first bypass line 55 or the second bypass line 57 and the discharge of the working fluid to the tank through the first discharge line 61 or the second discharge line 62 are performed. In addition, the factors causing an imbalance in the discharge flow rates Q1, Q2, the inflow flow rate Qr and the outflow flow rate Qh include the performance of the regulator 35, the efficiency of the first two-way pump 2 and the second two-way pump 3, external leakage of the working fluid, aging, and the like.

[0055] That is, the inflow flow rate Qr into the rod side chamber 4r is equal to the discharge flow rate Q1 of the first bidirectional pump 2. The outflow flow 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.

[0056] After that, when the stroke of the single-rod cylinder 4 reaches a predetermined amount ("YES" in step S15), the control device 8 stops the electric motor 7 (step S16). For example, when the stroke of the single-rod cylinder 4 reaches a predetermined amount, it includes when the input of the second operation signal from the operating device 11 to the control device 8 stops, and when the single-rod cylinder 4 moves to the end of the stroke.

[0057] On the other hand, when the pressure difference ΔP is greater than the threshold value Pt ("YES" in step S12), the control device 8 controls the regulator 35 in such a manner that the capacity q2 of the second bidirectional pump 3 is less than the reference capacity qr by the second predetermined amount Δq2 (q2=qr-Δq2) (step S14). The second predetermined amount Δq2 is determined by taking into account the imbalance of the discharge flow rates Q1, Q2, the inflow flow rate Qr, and the 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 in the range of 1 to 10% of qr.

[0058] For example, when a load in the extension direction is applied to the single-rod cylinder 4, the pressure Pr of the rod side chamber 4r is higher than the pressure Ph of the head side chamber 4h by more than the threshold value Pt. When a load in the shortening direction is applied to the single-rod cylinder 4, the pressure Ph of the head side chamber 4h is higher than the pressure Pr of the rod side chamber 4r by more than the threshold value Pt.

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

[0060] On the contrary, when the pressure Ph of the head side chamber 4h is higher than the pressure Pr of the rod side chamber 4r by a threshold value Pt or more, the low pressure selector valve 6 is switched to the rod side discharge position. As a result, the remaining amount 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. In addition, 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, when q2=qr, it is assumed that Qr=50, Qh=100, Q1=50, Q2=50. When Pr-Ph≧Pt, if Q1=50, Q2=40, then Qr=50, Qh=100, Qfh=10. On the other hand, when Ph-Pr≧Pt, if Q1=50, Q2=40, then Qh=90, Qr=45, Qfr=5.

[0062] When the pressure difference ΔP is equal to or larger than the threshold value 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, since the low pressure selector valve 6 is provided, it is possible to physically prevent pressure from stagnating on the rod side or the head side, whichever has a lower pressure, in a configuration in which the second two-way pump 3 is added to the first two-way pump 2 .

[0064] In addition, in the present embodiment, when the single-rod cylinder 4 is extended, the regulator 35 is controlled by comparing the pressure difference ΔP detected by the differential pressure gauge 9 with the threshold value Pt, so that the capacity q2 of the second two-way pump 3 can be maintained at the reference capacity qr when the pressure difference ΔP between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r is small. Moreover, at this time, the low-pressure selector valve 6 is in the neutral position, so the sum of the discharge flow rate Q1 of the first two-way pump 2 and the discharge flow rate Q2 of the second two-way pump 3 is balanced with the inflow flow rate Qh to 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 two-way pump 2, and the single-rod cylinder 4 can be operated. Generally, such a state is when it is desired to operate the single-rod cylinder 4 slowly under no load, so smoother operation with less shock can be achieved. In addition, the low-pressure selector valve 6 is maintained in the neutral position, thereby suppressing hunting of the low-pressure selector valve 6.

[0065] Furthermore, in the present embodiment, when the single-rod cylinder 4 is shortened, the regulator 35 is controlled by comparing the pressure difference ΔP detected by the differential pressure gauge 9 with the threshold value Pt, so that when the pressure difference ΔP between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r is small, the capacity q2 of the second two-way pump 3 can be maintained at the reference capacity qr. Moreover, at this time, 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 discharge flow rate Q1 of the first two-way pump 2 and the inflow flow rate Qr to the rod side chamber 4r of the single-rod cylinder 4 are balanced, and 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 two-way pump 2 and the suction flow rate Q2 of the second two-way pump 3. Generally, such a state is when it is desired to operate the single-rod cylinder 4 slowly under no load, so smoother operation with less shock can be achieved. In addition, the low-pressure selector valve 6 is maintained in the neutral position, thereby suppressing the vibration of the low-pressure selector valve 6.

[0066] (Variation) The present disclosure is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present disclosure.

[0067] For example, as in the above-mentioned embodiment, when the pressure difference ΔP between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r reaches or exceeds the threshold value Pt, the low pressure selector valve 6 may be switched from the neutral position to the rod side discharge position or the head side discharge position. Figure 8As in the hydraulic system 1A of the modified example shown, a position detector (e.g., stroke sensor) 95 for detecting 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 is used instead of the differential pressure gauge 9, and the control device 8 controls the regulator 35 based on the detection result of the position detector 95. In either case of using the differential pressure gauge 9 or the position detector 95, since it is known from the detection results of the two, what function the low pressure selector valve 6 performs, 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 in such a manner that the capacity q2 of the second two-way pump 3 is always greater than the reference capacity qr by the first predetermined amount Δq1 when the single-rod cylinder 4 is extended, and the capacity q2 of the second two-way pump 3 is always less than the reference capacity qr by the second predetermined amount Δq2 when the single-rod cylinder 4 is contracted, regardless of the pressure difference ΔP and the position of the low-pressure selector valve 6. However, in this case, when the electric motor 7 starts to rotate, there is a possibility that a shock is generated due to the switching of the low-pressure selector valve 6, and therefore, it is desirable to control the regulator 35 based on the pressure difference ΔP or the position of the low-pressure selector valve 6.

[0069] In addition, you can also Fig. 9 As in the hydraulic system 1B of the modified example shown in the figure, the first on-off valve 67 is provided on the first pilot line 63, and the second on-off valve 68 is provided on the second pilot line 64. According to this structure, in the case 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 reversed, the hunting of the low pressure selector valve 6 can be prevented by closing the first on-off valve 67 and the second on-off valve 68.

[0070] (Summarize) As a first aspect, the present disclosure provides a hydraulic system, which is a hydraulic system for a single-rod cylinder including a head side chamber and a rod side chamber, and comprises: a first two-way pump, which is connected to the head side chamber through a head side line and is connected to the rod side chamber through a rod side line; a second two-way pump of a variable capacity type, which is connected to the head side line through a supply and discharge line; an electric motor, which drives the first two-way pump and the second two-way pump in the same direction; and a low-pressure selection valve, which is connected to the head side line through a first discharge line and is connected to the rod side line through a second discharge line, 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 the overflow line provided with an overflow 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 overflow line.

[0071] According to the above configuration, since the low pressure selector valve is provided, it is possible to physically prevent pressure from stagnation on the rod side or the head side where the pressure is lower in the configuration in which the second two-way pump is added to the first two-way pump.

[0072] As a second aspect, in the first aspect, the hydraulic system includes: a regulator that changes the capacity of the second two-way pump; and a control device that controls the electric motor and the regulator, and the control device controls the regulator in such a manner that the capacity of the second two-way pump is greater than the reference capacity by a first specified amount when the single-rod cylinder is extended. According to this structure, when the single-rod cylinder is extended, when the pressure of the head side chamber is higher than the pressure of the rod side chamber, the remaining amount corresponding to the first specified amount is discharged from the rod side line through the second discharge line, the low pressure selector valve, and the overflow line, and when the pressure of the rod side chamber is higher than the pressure of the head side chamber, the remaining amount corresponding to the first specified amount is discharged from the head side line through the first discharge line, the low pressure selector valve, and the overflow line.

[0073] As a third aspect, in the first or second aspect, the hydraulic system may include: a regulator that changes the capacity of the second two-way pump; and a control device that controls the electric motor and the regulator, wherein the control device controls the regulator so that the capacity of the second two-way pump is less than the reference capacity by a second predetermined amount when the single-rod cylinder is shortened. According to this structure, when the single-rod cylinder is shortened, when the pressure of the rod side chamber is higher than the pressure of the head side chamber, the remaining amount corresponding to the second predetermined amount is discharged from the head side line through the first discharge line, the low pressure selector valve, and the relief line, and when the pressure of the head side chamber is higher than the pressure of the rod side chamber, the remaining amount corresponding to the second predetermined amount is discharged from the rod side line through the second discharge line, the low pressure selector valve, and the relief line.

[0074] As a fourth aspect, in the second or third aspect, 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] As a fifth aspect, in any one of the second to fourth aspects, the low-pressure selector valve, when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber reaches or exceeds a threshold value, switches from a neutral position that closes the first discharge line and the second discharge line to a head side discharge position that connects the first discharge line with the overflow line or a rod side discharge position that connects the second discharge line with the overflow line, and the control device, when extending the single-rod cylinder, controls the regulator in the following manner: when the low-pressure selector valve is in the neutral position, the capacity of the second two-way pump becomes the reference 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 two-way pump is greater than the reference capacity by only the first specified amount. According to this structure, when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber is small, the capacity of the second two-way pump can be maintained at the reference capacity. Moreover, at this time, the low-pressure selector valve is in a neutral position, so the single-rod cylinder can be operated in a state where the sum of the discharge flow rates of the first two-way pump and the second two-way pump is balanced with the inflow flow rate to 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 two-way pump. Usually, such a state is a situation where it is desired to operate the single-rod cylinder slowly under no-load conditions, so smoother operation with less impact can be achieved. In addition, the low-pressure selector valve is kept in a neutral position, thereby suppressing the oscillation of the low-pressure selector valve.

[0076] As a sixth aspect, it can also be that in any one of the third to fifth aspects, the low-pressure selector valve, when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber reaches or exceeds a threshold value, switches from a neutral position that closes the first discharge line and the second discharge line to a head side discharge position that connects the first discharge line to the overflow line or a rod side discharge position that connects the second discharge line to the overflow line, and the control device controls the regulator in the following manner when shortening the single-rod cylinder: when the low-pressure selector valve is in the neutral position, the capacity of the second two-way pump becomes the reference 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 two-way pump is less than the reference capacity by only the second specified amount. According to this structure, the capacity of the second two-way pump can be maintained at the reference capacity when the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber is small. Moreover, at this time, the low-pressure selector valve is in a neutral position, so the single-rod cylinder can be operated in a state where the discharge flow of the first two-way pump is balanced with the inflow flow to the rod side chamber of the single-rod cylinder, and the outflow flow from the head side chamber of the single-rod cylinder is balanced with the sum of the suction flow of the first two-way pump and the second two-way pump. Usually, such a state is a situation where it is desired to operate the single-rod cylinder slowly under no-load conditions, so smoother operation with less impact can be achieved. In addition, the low-pressure selector valve is kept in a neutral position, thereby suppressing the oscillation of the low-pressure selector valve.

[0077] As a seventh aspect, in any one of the first to sixth aspects, the hydraulic system further comprises: a first lock valve as an on-off valve provided in the head side circuit; and a second lock valve as an on-off valve provided in the rod side circuit. According to this structure, by closing the first lock valve and the second lock valve, the action of the single rod cylinder can be blocked.

[0078] As an eighth aspect, in any one of the first to seventh aspects, the pressure of the head side chamber is introduced into the low pressure selector valve through the first pilot line, and the pressure of the rod side chamber is introduced into the low pressure selector valve through the second pilot line, and the hydraulic system further comprises: a first opening and closing valve provided in the first pilot line; and a second opening and closing valve provided in the second pilot line. According to this structure, 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, the oscillation of the low pressure selector valve can be prevented by closing the first opening and closing valve and the second opening and closing valve.

[0079] As a ninth aspect, in the first to eighth aspects, the hydraulic system further comprises: a head-side relief valve disposed in the head-side relief line branched from the head-side line; a first check valve disposed in the first bypass line bypassing the head-side relief valve; a rod-side relief valve disposed in the rod-side relief line branched from the rod-side line; and a second check valve disposed in the second bypass line bypassing the rod-side relief valve. According to this structure, the pressure of the head-side chamber and the pressure of the rod-side chamber can be maintained below the set pressure of the relief valve, and the cavitation phenomenon of the head side and the rod side, whichever has a lower pressure, can be suppressed by supplying the working fluid to the head-side line or the rod-side line via the check valve.

[0080] As a tenth aspect, in any one of the first to ninth aspects, the hydraulic system further comprises: a differential pressure gauge that detects the pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber. According to this structure, it is known from the detection result of the differential pressure gauge what function the low pressure selector valve performs, so it is possible to control the regulator that changes the capacity of the second bidirectional pump according to the function of the low pressure selector valve.

[0081] As an eleventh aspect, in any one of the first to ninth aspects, the low-pressure selector valve switches from a neutral position that closes the first discharge line and the second discharge line to a head-side discharge position that connects the first discharge line with the overflow line or a rod-side discharge position that connects the second discharge line with the overflow line when the pressure difference between the pressure of the head-side chamber and the pressure of the rod-side chamber reaches or exceeds a threshold value, and the hydraulic system further comprises: a position detector that detects whether the position of the low-pressure selector valve is the neutral position, the rod-side discharge position, or the head-side discharge position. According to this structure, it is known from the detection result of the position detector what function the low-pressure selector valve performs, and therefore it is possible to control the regulator that changes the capacity of the second bidirectional pump according to the function of the low-pressure selector valve.

Claims

1. A hydraulic system, characterized in that: It is a hydraulic system for a single-rod cylinder consisting of a head side chamber and a rod side chamber. It has: a first bidirectional pump connected to the head side chamber via a head side line and connected to the rod side chamber via a rod side line; A second two-way pump of a variable capacity type connected to the head-side circuit via a supply and discharge circuit; an electric motor driving the first bidirectional pump and the second bidirectional pump in the same direction; as well as A low-pressure selector valve is connected to the head-side line through a first discharge line and to the rod-side line through a second discharge line. 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 an overflow line provided with an overflow valve. 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 overflow line.

2. The hydraulic system according to claim 1, characterized in that: have: a regulator that varies the capacity of the second bidirectional pump; and a control device, which controls the electric motor and the regulator, The control device controls the regulator so that the capacity of the second bidirectional pump is greater than a reference capacity by a first predetermined amount when the single-rod cylinder is extended.

3. The hydraulic system according to claim 1 or 2, characterized in that: have: a regulator that varies the capacity of the second bidirectional pump; and a control device, which controls the electric motor and the regulator, The control device controls the regulator so that the capacity of the second bidirectional pump is reduced by a second predetermined amount from a reference capacity when the single-rod cylinder is shortened.

4. The hydraulic system according to claim 2, characterized in that: The reference 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.

5. The hydraulic system according to claim 2, characterized in that: The low-pressure selector valve switches from a neutral position that closes the first discharge line and the second discharge line to a head-side discharge position that connects the first discharge line with the overflow line or to a rod-side discharge position that connects the second discharge line with the overflow line when the pressure difference between the pressure of the head-side chamber and the pressure of the rod-side chamber reaches or exceeds a threshold value. The control device controls the regulator in the following manner when extending the single-rod cylinder: when the low-pressure selection valve is located in the neutral position, the capacity of the second two-way pump is made to become the reference capacity; when the low-pressure selection valve is located in the head-side discharge position or the rod-side discharge position, the capacity of the second two-way pump is made to be greater than the reference capacity by the first specified amount.

6. The hydraulic system according to claim 3, characterized in that: The low-pressure selector valve switches from a neutral position that closes the first discharge line and the second discharge line to a head-side discharge position that connects the first discharge line with the overflow line or to a rod-side discharge position that connects the second discharge line with the overflow line when the pressure difference between the pressure of the head-side chamber and the pressure of the rod-side chamber reaches or exceeds a threshold value. The control device controls the regulator in the following manner when shortening the single-rod cylinder: when the low-pressure selection valve is located at the neutral position, the capacity of the second two-way pump becomes the reference capacity; when the low-pressure selection valve is located at the head-side discharge position or the rod-side discharge position, the capacity of the second two-way pump becomes less than the reference capacity by the second specified amount.

7. The hydraulic system according to claim 1, characterized in that: The hydraulic system also has: a first lock valve as an on-off valve provided in the head-side line; and A second lock valve is provided as an on-off valve on the rod side line.

8. The hydraulic system according to claim 1, characterized in that: In the low-pressure selector valve, the pressure of the head side chamber is introduced through the first pilot line, and the pressure of the rod side chamber is introduced through the second pilot line. The hydraulic system also has: A first on-off valve provided in the first pilot line; and The second on-off valve is provided in the second pilot line.

9. The hydraulic system according to claim 1, characterized in that: Also available: a head-side overflow valve, which is provided on a head-side overflow line branched from the head-side line; a first check valve disposed in a first bypass line bypassing the head-side relief valve; a rod-side relief valve provided in a rod-side relief line branched from the rod-side line; as well as A second check valve is provided in a second bypass line bypassing the rod-side relief valve.

10. The hydraulic system according to claim 1, characterized in that: Also available: A differential pressure gauge detects a pressure difference between the pressure of the head side chamber and the pressure of the rod side chamber.

11. The hydraulic system according to claim 1, characterized in that: The low-pressure selector valve switches from a neutral position that closes the first discharge line and the second discharge line to a head-side discharge position that connects the first discharge line with the overflow line or to a rod-side discharge position that connects the second discharge line with the overflow line when the pressure difference between the pressure of the head-side chamber and the pressure of the rod-side chamber reaches or exceeds a threshold value. The hydraulic system also has: A position detector detects 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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