hydraulic system

Abstract

To provide a hydraulic system having constitution in which a second bidirectional pump is imparted to a first bidirectional pump, the hydraulic system enabling prevention in a simple manner, of such a problem that pressure confinement occurs on the rod side or the head side of the bidirectional pump, whichever pressure is lower.SOLUTION: A hydraulic system 1A in one embodiment includes: a first bidirectional pump 2 connected to a single rod cylinder 4 via a head side line 21 and a rod side line 22; a second bidirectional pump 3 connected to the head side line 21 via a first supply / discharge line 31; and an accumulator 7 connected to the second bidirectional pump 3 by a second supply / discharge line 71. The hydraulic system 1A further includes a low pressure selection valve 6 connected to the head side line 21, the rod side line 22, and the second supply / discharge line 71. The second supply / discharge line 71 is connected by a junction line 72, to a portion between a pair of check valves 55, 56 in a bridge line 54 connecting the head side line 21 and the rod side line 22.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 two-way pump connected to form a closed loop with the single-rod cylinder is known. For example, Patent Document 1 discloses a hydraulic system 100 as shown in FIG. 7.

[0003] This 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 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 two-way pump 120 is for eliminating the flow rate difference between the head-side chamber 210 and the rod-side chamber 220 of the single-rod cylinder 200, that is, the difference between the inflow rate and the outflow rate, and is connected to the head-side line 111 by a supply / discharge line 121.

[0004] The first two-way pump 110 and the second two-way pump 120 are driven in the same direction by an electric motor 130. More specifically, when the electric motor 130 extends the single-rod cylinder 200, that is, when the rod is advanced, the first two-way pump 110 and the second two-way pump 120 are driven in the first direction, and when the single-rod cylinder 200 is shortened, that is, when the rod is retracted, the first two-way pump 110 and the second two-way pump 120 are driven in the second direction opposite to the first direction. When the first two-way pump 110 and the second two-way pump 120 are driven in the first direction, they discharge the hydraulic fluid into the head-side chamber 210, and when the first two-way pump 110 and the second two-way 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 two-way pump 120 is a variable-displacement pump, and the capacity of the second two-way 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 confinement occurs on the side with the lower pressure, either the rod side or the head side. The rod side is the rod side chamber 220 and the rod side line 112, and the head side is the head side chamber 210 and the 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 occurs on the side with the lower pressure, either the rod side or the head side. Furthermore, when the single-rod cylinder 200 is shortened, if the capacity of the second bidirectional pump 120 is greater than the reference capacity, cavitation occurs on the lower pressure side of the rod side or the head side, and if the capacity of the second bidirectional pump 120 is less than the reference capacity, pressure confinement occurs on the lower pressure side of 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 these pressure confinement or cavitation. [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 Initiative] [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 simply 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 easily 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 relates to 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 bidirectional pump connected to the head side line by a first supply / discharge line; an accumulator connected to the second bidirectional pump by a second supply / discharge line; an electric motor for driving the first bidirectional pump and the second bidirectional pump in the same direction; and a third discharge line connected to the head side line by a first discharge line and connected to the rod side line by a second discharge line. The hydraulic system is provided, comprising: a low-pressure selector valve connected to the second supply and discharge line by an outlet line, which connects the second discharge line to the third discharge line 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 third discharge line when the pressure in the head side chamber is lower than the pressure in the rod side chamber; and a pair of check valves facing in opposite directions, provided on a bridge line connecting the head side line and the rod side line, wherein the second supply and discharge line is connected to the portion of the bridge line between the pair of check valves by a relay line. [Effects of the Invention]

[0011] According to this disclosure, a hydraulic system is provided that can easily prevent pressure entanglement 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. [Brief explanation of the drawing]

[0012] [Figure 1] This is a schematic diagram of the hydraulic system according to the first embodiment. [Figure 2] This is a schematic diagram of the hydraulic system according to the second embodiment. [Figure 3] This is a schematic diagram of the hydraulic system according to the third embodiment. [Figure 4] This is a schematic diagram of the hydraulic system according to the fourth embodiment. [Figure 5]This diagram shows a modified low-pressure selector valve. [Figure 6] This diagram shows another modified example of a low-pressure selector valve. [Figure 7] This is a schematic diagram of a conventional hydraulic system. [Modes for carrying out the invention]

[0013] <First Embodiment> Figure 1 shows a hydraulic system 1A according to the first embodiment. This hydraulic system 1A 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 1A is typically oil.

[0014] Specifically, the hydraulic system 1A includes a first bidirectional pump 2 connected to a single-rod cylinder 4 to form a closed loop, and a second bidirectional pump 3 for eliminating the flow rate difference between the head-side chamber 4h and the rod-side chamber 4r of the single-rod cylinder 4, i.e., the difference between the inflow flow rate and the outflow flow rate. The hydraulic system 1A also includes an accumulator 7 for storing pressurized working fluid. The single-rod cylinder 4, the first bidirectional pump 2, the second bidirectional pump 3, and the accumulator 7, together with several lines described later, form a closed circuit.

[0015] More specifically, 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 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 / discharge line 31 and to the accumulator 7 by the second supply / discharge line 71.

[0016] The set pressure of the accumulator 7 is, for example, within the range of 0.1-2 MPa. Here, "set pressure of the accumulator 7" refers to the pressure at which working fluid can flow into the accumulator 7.

[0017] The head-side line 21 and the rod-side line 22 are respectively provided with lock valves 23, 24 which are on-off valves. The lock valves 23, 24 are controlled by the control device 10. In FIG. 1, for the sake of simplifying the drawing, the drawing of some signal lines is omitted. When the control device 10 operates the single-rod cylinder 4, that is, when extending or shortening it, the lock valves 23, 24 are opened, and when it is not in such an operation state, the lock valves 23, 24 are closed.

[0018] The head-side line 21 and the rod-side line 22 are connected to each other by a first bridge line 51 and a second bridge line 54. A pair of relief valves 52, 53 are provided in the first bridge line 51 in opposite directions to each other, and a pair of check valves 55, 56 are provided in the second bridge line 54 in opposite directions to each other.

[0019] In the illustrated example, the first bridge line 51 and the second bridge line 54 are connected to the head-side line 21 and the rod-side line 22 between the lock valves 23, 24 and the first bidirectional pump 2, but the first bridge line 51 and the second bridge line 54 may be connected to the head-side line 21 and the rod-side line 22 between the lock valves 23, 24 and the single-rod cylinder 4. Also, a part of the first bridge line 51 and a part of the second bridge line 54, for example, both ends and the central parts thereof, may merge with each other to form a common flow path.

[0020] The relief valve 52 opens when the pressure in the head-side line 21 becomes too high, and the relief valve 53 opens when the pressure in the rod-side line 22 becomes too high. The relief pressures of the relief valves 52, 53 are set relatively high, for example, within the range of 25 - 35 MPa.

[0021] The check valve 55 allows the flow from the center of the second bridge line 54 to the head-side line 21 but prohibits the reverse flow. The check valve 56 allows the flow from the center of the second bridge line 54 to the rod-side line 22 but prohibits the reverse flow.

[0022] The portion between relief valves 52 and 53 in the first bridge line 51 and the portion between check valves 55 and 56 in the second bridge line 54 are connected to the second supply and discharge line 71 by a relay line 72.

[0023] The first bidirectional pump 2 and the second bidirectional pump 3 are, for example, axial piston pumps. An axial piston pump is, for example, a swashplate pump or a slanted shaft pump. 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 be a variable-displacement pump. Alternatively, both the first bidirectional pump 2 and the second bidirectional pump 3 may be fixed-displacement pumps.

[0024] The capacity of the second bidirectional pump 3 is changed by a regulator 35. The regulator 35 is controlled by a control device 10. 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.

[0025] The first bidirectional pump 2 and the second bidirectional pump 3 are driven in the same direction by an electric motor 9. The first bidirectional pump 2 and the second bidirectional pump 3 may be arranged so that their central axes are coaxial, or they may be arranged so that their central axes are parallel. The former is a tandem type, and the latter is a parallel type.

[0026] The electric motor 9 is controlled by the control device 10. When extending the single-rod cylinder 4, the electric motor 9 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.

[0027] 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 second supply and discharge line 71 by a third discharge line 63.

[0028] In this embodiment, a relief valve 64 is provided in the third discharge line 63. The relief valve 64 plays a role in maintaining a relatively high low pressure in the closed loop consisting of the single-rod cylinder 4, the head-side line 21, the rod-side line 22, and the first bidirectional pump 3. The relief pressure of the relief valve 64 is set relatively low, for example, within the range of 0.1-2 MPa. The low pressure in the closed loop is the sum of the set pressure of the accumulator 7 and the set pressure of the relief valve 64, for example, 1-2 MPa. Note that if the set pressure of the accumulator 7 is relatively high, the relief valve 64 can be omitted.

[0029] The low-pressure selector valve 6 can be switched between a neutral position, a head-side discharge position, and a rod-side discharge position. In Figure 1, the center position is the neutral position, the right position is the head-side discharge position, and the left position is the rod-side discharge position. 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 third discharge line 63. 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 third discharge line 63.

[0030] In this embodiment, the low-pressure selector valve 6 is a pilot-operated three-position valve. Therefore, the pressure Ph of the head-side chamber 4h is introduced to the low-pressure selector valve 6 through the first pilot line 65, and the pressure Pr of the rod-side chamber 4r is introduced through the second pilot line 66. The first pilot line 65 is for switching the low-pressure selector valve 6 from the neutral position to the rod-side discharge position, and the second pilot line 66 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 65 and the second pilot line 66 branch off from the first discharge line 61 and the second discharge line 62, respectively, but the first pilot line 65 may branch off from the head-side line 21, or the second pilot line 66 may branch off from the rod-side line 22.

[0031] The low-pressure selector valve 6 is switched to 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, and to the head-side discharge position when the pressure Ph in the head-side chamber 4h is lower than the pressure Pr in the rod-side chamber 4r. In this embodiment, the low-pressure selector valve 6 is switched from the neutral position to the rod-side discharge position or the head-side discharge position 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, i.e., the absolute value of the deviation between pressure Ph and pressure Pr, becomes greater than or equal to the threshold Pt. For example, the threshold Pt is in the range of 0.5-3 MPa. Note that the threshold Pt may differ between cylinder extension and cylinder retraction.

[0032] Next, an example of the control performed by the control device 10 will be described. However, the control performed by the control device 10 is not limited to that described below, and the control device 10 may perform other control.

[0033] With respect to the control device 10, 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 10 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. The control device 10 also receives the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r, measured by a pressure sensor, or the differential pressure ΔP between the pressure Ph of the head side chamber 4h and the pressure Pr of the rod side chamber 4r, measured by a differential pressure gauge.

[0035] 1. Cylinder extension When a first operation signal is input to the control device 10, the control device 10 rotates the electric motor 9 in a first direction. This drives the first bidirectional pump 2 and the second bidirectional pump 3 in the first direction, causing the first bidirectional pump 2 to draw in working fluid through the rod-side line 22 and the second bidirectional pump 3 to draw in working fluid through the second supply / discharge line 71. The first bidirectional pump 2 also discharges working fluid through the head-side line 21, and the second bidirectional pump 3 discharges working fluid through the first supply / discharge line 31.

[0036] If 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 less than the threshold Pt, the control device 10 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the reference capacity qr. 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, if q2 = qr, theoretically there is no excess or deficiency in the inflow flow rate to the single-rod cylinder 4 or the outflow flow rate from the single-rod cylinder 4. For this reason, the supply of working fluid from the accumulator 7 to the head-side line 21 or the rod-side line 22 through part of the second bridge line 54 and part of the relay line 72 and the second supply / discharge line 71, and the discharge of working fluid from the head-side line 21 or the rod-side line 22 through the first discharge line 61 or the second discharge line 62 are not performed.

[0037] On the other hand, if the differential pressure ΔP is greater than or equal to the threshold Pt, the control device 10 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. For example, Δq1 is within the range of 1-10% of qr.

[0038] 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.

[0039] 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 amount corresponding to the first predetermined amount Δq1 is discharged from the rod-side line 22 to the accumulator 7 through the second discharge line 62, the low-pressure selector valve 6, the third discharge line 63, and a portion of the second supply and discharge line 71.

[0040] 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 amount corresponding to the first predetermined amount Δq1 is discharged from the head-side line 21 to the accumulator 7 through the first discharge line 61, the low-pressure selector valve 6, the third discharge line 63, and a portion of the second supply and discharge line 71.

[0041] 2. Cylinder shortening When a second operation signal is input to the control device 10, the control device 10 rotates the electric motor 9 in the second direction. As a result, the first bidirectional pump 2 and the second bidirectional pump 3 are driven in the second direction, causing the first bidirectional pump 2 to draw in working fluid through the head-side line 21 and the second bidirectional pump 3 to draw in working fluid through the first supply / discharge line 31. The first bidirectional pump 2 also discharges working fluid through the rod-side line 22, and the second bidirectional pump 3 discharges working fluid through the second supply / discharge line 71.

[0042] If 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 less than the threshold Pt, the control device 10 controls the regulator 35 so that the capacity q2 of the second bidirectional pump 3 becomes the reference capacity qr. That is, if q2 = qr, theoretically there is no excess or deficiency in the inflow flow rate to the single-rod cylinder 4 or the outflow flow rate from the single-rod cylinder 4. For this reason, no working fluid is supplied from the accumulator 7 to the head-side line 21 or the rod-side line 22 through part of the second bridge line 54, the relay line 72, and part of the second supply / discharge line 71, and no working fluid is discharged from the head-side line 21 or the rod-side line 22 through the first discharge line 61 or the second discharge line 62.

[0043] On the other hand, if the differential pressure ΔP is greater than or equal to the threshold Pt, the control device 10 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. 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.

[0044] 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.

[0045] 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 amount corresponding to the second predetermined amount Δq2 is discharged from the head-side line 21 to the accumulator 7 through the first discharge line 61, the low-pressure selector valve 6, the third discharge line 63, and a portion of the second supply and discharge line 71.

[0046] 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 amount corresponding to the second predetermined amount Δq2 is discharged from the rod-side line 22 to the accumulator 7 through the second discharge line 62, the low-pressure selector valve 6, the third discharge line 63, and a portion of the second supply and discharge line 71.

[0047] As described above, in the hydraulic system 1A 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 easily prevent pressure conflation from occurring on the lower pressure side of the rod side or the head side. The rod side consists of the rod side chamber 4r and the rod side line 22, and the head side consists of the head side chamber 4h and the head side line 21.

[0048] Furthermore, since the second bidirectional pump 3 is connected to the accumulator 7 by the second supply / discharge line 71, cavitation due to a pressure drop on the suction side of the second bidirectional pump 3 can be prevented when the second bidirectional pump 3 starts rotating in the direction of discharging the working fluid through the first supply / discharge line 31. In addition, when the pressure on the suction side drops due to a delay in the return of the working fluid to the suction side at the start of rotation of the first bidirectional pump 2, pressurized working fluid from the accumulator 7 is supplied to the suction side via the check valve 55 or 56, thus preventing cavitation on the suction side of the first bidirectional pump 2.

[0049] Furthermore, in this embodiment, since a relief valve 64 is provided in the third discharge line 63, the pressure at which the working fluid is discharged from the head-side line 21 or the rod-side line 22 through the first discharge line 61 or the second discharge line 62 can be set separately from the set pressure of the accumulator 7.

[0050] <Second Embodiment> Figure 2 shows the hydraulic system 1B according to the second embodiment. In this embodiment, as well as in the third and fourth embodiments described later, the same reference numerals are used for components identical to those in the first embodiment, and redundant explanations are omitted.

[0051] The difference between the hydraulic system 1B and the hydraulic system 1A of the first embodiment is that the hydraulic system 1B further includes a tank 11 and a charge pump 8. The charge pump 8 is connected to the tank 11 by a suction line 81 and to a second supply / discharge line 71 by a discharge line 82. However, the discharge line 82 does not necessarily have to be connected to the second supply / discharge line 71; it may be connected to a third discharge line 63 between the relief valve 64 and the second supply / discharge line 71, or to an intermediate line 72.

[0052] The discharge line 82 is equipped with a check valve 83 that allows flow from the charge pump 8 to the second supply / discharge line 71 but prohibits flow in the reverse direction. Additionally, a relief line 84 branches off from the discharge line 82 between the charge pump 8 and the check valve 83, and the relief line 84 is connected to the tank 11.

[0053] A relief valve 85 is provided in the relief line 84. The relief pressure of the relief valve 85 is, for example, in the range of 0.1-2 MPa.

[0054] As in this embodiment, if a charge pump 8 is provided, the amount of working fluid in the closed circuit, which consists of a single-rod cylinder 4, a first bidirectional pump 2, a second bidirectional pump 3, and an accumulator 7, can be replenished when the amount of working fluid decreases. For example, if the first bidirectional pump 2 and the second bidirectional pump 3 are of the external drain type, the amount of working fluid in the closed circuit will gradually decrease.

[0055] The charge pump 8 may be operated manually, but if the control device 10 controls the charge pump 8, for example, if the amount of working fluid in the accumulator 7 falls below a predetermined value, or if the pressure at a predetermined position in the closed circuit falls below a predetermined value, the control device 10 may activate the charge pump 8.

[0056] Furthermore, the discharge line 82 does not necessarily need to be constantly connected to the second supply / discharge line 71; it may be connected to the second supply / discharge line 71 only when replenishing the working fluid. Also, as described in the first embodiment, the relief valve 64 can be omitted when the set pressure of the accumulator 7 is relatively high. The fact that the relief valve 64 can be omitted is also the same in the third and fourth embodiments described later.

[0057] <Third Embodiment> Figure 3 shows the hydraulic system 1C according to the third embodiment. The differences between the hydraulic system 1C and the hydraulic system 1B of the second embodiment are that the discharge line 82 is connected to the third discharge line 63 between the relief valve 64 and the second supply / discharge line 71, and that the relief line 84 branches off from the second supply / discharge line 71. In other words, the relief line 84 branches off from the second supply / discharge line 71 on the opposite side of the discharge line 82, with a portion of the third discharge line 63 and a portion of the second supply / discharge line 71 in between.

[0058] However, the relief line 84 may branch off from the relay line 72 on the opposite side of the discharge line 82, with a portion of the third discharge line 63, a portion of the second supply / discharge line 71, and a portion of the relay line 72 in between. Alternatively, contrary to Figure 3, the discharge line 82 may be connected to the second supply / discharge line 71 or the relay line 72, and the relief line 84 may branch off from the third discharge line 63 between the relief valve 64 and the second supply / discharge line 71.

[0059] In this embodiment, the charge pump 8 operates continuously. That is, when the second bidirectional pump 3 is stopped, the working fluid discharged from the charge pump 8 flows into the accumulator 7, as shown by the solid arrows in Figure 3. After the accumulator 7 is full, the working fluid in the tank 11 circulates in the following order: suction line 81, charge pump 8, discharge line 82, part of the third discharge line 63, part of the second supply / discharge line 71, and relief line 84.

[0060] On the other hand, when the second bidirectional pump 3 rotates in the direction of discharging the working fluid through the first supply / discharge line 31, the second bidirectional pump 3 is supplied with working fluid from the charge pump 8 and the accumulator 7, as shown by the dashed arrows in Figure 3.

[0061] In this embodiment, the charge pump 8 can circulate the working fluid from the discharge line 82 through a portion of the closed circuit consisting of the single-rod cylinder 4, the first bidirectional pump 2, the second bidirectional pump 3, and the accumulator 7 to the relief line 84. This allows the working fluid within the closed circuit to be cooled.

[0062] <Fourth Embodiment> Figure 4 shows the hydraulic system 1D according to the fourth embodiment. The differences between the hydraulic system 1D and the hydraulic system 1C of the third embodiment are that the accumulator 7 is located near the second bidirectional pump 3, a cooler 12 is interposed between the charge pump 8 and the accumulator 7, and the relief line 84 branches off from the relay line 72.

[0063] In this embodiment, the cooler 12 is located in the third discharge line 63. Also in this embodiment, the relief valve 85 is located on the opposite side of the accumulator 7 from the cooler 12. In other words, the working fluid discharged from the charge pump 8 and passing through the cooler 12 flows through the portion of the second supply / discharge line 71 adjacent to the accumulator 7, and then flows into the relief line 84 via a portion of the relay line 72.

[0064] In this embodiment, the cooling effect of the working fluid in the closed circuit described in the third embodiment can be improved by the cooler 12. Moreover, since the accumulator 7 is located between the cooler 12 and the relief valve 85, the working fluid stored in the accumulator 7 can also be cooled as the working fluid circulates.

[0065] Furthermore, in this embodiment, since the cooler 12 is provided in the third discharge line 63, the working fluid is also cooled when it is returned to the accumulator 7 through the third discharge line 63. However, from the viewpoint of improving the cooling effect during the circulation of the working fluid, the cooler 12 may be provided in the discharge line 82 or the like.

[0066] <Other Embodiments> 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, instead of the pilot-operated low-pressure selector valve 6, a solenoid three-position low-pressure selector valve 6A, as shown in Figure 5, may be used. Alternatively, a low-pressure selector valve 6B, which includes a first solenoid valve 6a and a second solenoid valve 6b, as shown in Figure 6, may be used. The first solenoid valve 6a is connected to the first discharge line 61 and the third discharge line 63, and the second solenoid valve 6b is connected to the second discharge line 62 and the third discharge line 63.

[0068] <Summary> In a first aspect, the present disclosure relates to 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 bidirectional pump connected to the head line by a first supply / discharge line; an accumulator connected to the second bidirectional pump by a second supply / discharge line; an electric motor driving the first bidirectional pump and the second bidirectional pump in the same direction; and a second discharge line connected to the head line and to the rod line by a second discharge line. The present invention provides a hydraulic system comprising: a low-pressure selector valve connected to the second supply and discharge line by a third discharge line, which connects the second discharge line to the third discharge line 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 third discharge line when the pressure in the head side chamber is lower than the pressure in the rod side chamber; and a pair of check valves facing in opposite directions, provided on a bridge line connecting the head side line and the rod side line, wherein the second supply and discharge line is connected to the portion of the bridge line between the pair of check valves by a relay line.

[0069] With the above configuration, since a low-pressure selector valve is provided, it is possible to easily prevent pressure conflation on the lower pressure side of the rod side or head side in a configuration in which a second bidirectional pump is added to the first bidirectional pump. Moreover, since the second bidirectional pump is connected to the accumulator by a second supply and discharge line, cavitation due to a pressure drop on the suction side of the second bidirectional pump can be prevented when the second bidirectional pump starts rotating in the direction of discharging the working fluid through the first supply and discharge line. Furthermore, when the pressure on the suction side drops due to a delay in the return of the working fluid to the suction side at the start of rotation of the first bidirectional pump, pressurized working fluid from the accumulator is supplied to the suction side via a check valve, thus preventing cavitation on the suction side of the first bidirectional pump as well.

[0070] In a second embodiment, the hydraulic system described above may further include a charge pump connected to a tank by a suction line and connected to any of the second supply / discharge line, the third discharge line, and the relay line by a discharge line. With this configuration, the working fluid can be replenished when the amount of working fluid in the closed circuit decreases.

[0071] In a third embodiment, in the second embodiment, for example, the hydraulic system may further include a relief valve provided in a relief line that branches off from the discharge line and connects to the tank.

[0072] In a fourth embodiment, in the second embodiment, the hydraulic system may further include a relief valve provided in a relief line that branches off from any of the second supply / discharge line, the third discharge line, and the relay line and connects to the tank. With this configuration, the charge pump can circulate the working fluid from the discharge line through a portion of the closed circuit to the relief line. This allows the working fluid in the closed circuit to be cooled.

[0073] In a fifth embodiment, in the fourth embodiment, the hydraulic system further comprises a cooler interposed between the charge pump and the accumulator, and the relief valve may be located on the opposite side of the accumulator from the cooler. With this configuration, the cooler can improve the cooling effect of the working fluid in the closed circuit. Moreover, since the accumulator is located between the cooler and the relief valve, the working fluid stored in the accumulator can also be cooled as the working fluid circulates.

[0074] In a sixth embodiment, in any of the first to fifth embodiments, the hydraulic system may further include a relief valve provided in the third discharge line. With this configuration, the pressure at which the working fluid is discharged from the head-side line or the rod-side line through the first discharge line or the second discharge line can be set separately from the set pressure of the accumulator.

[0075] In the seventh aspect, in any of the first to sixth aspects, for example, the low-pressure selector valve may be an electromagnetic three-position valve.

[0076] In an eighth aspect, in any of the first to seventh aspects, for example, the low-pressure selector valve may include a first solenoid valve and a second solenoid valve, which are electromagnetic two-position valves, with the first solenoid valve being connected to the first discharge line and the third discharge line, and the second solenoid valve being connected to the second discharge line and the third discharge line. [Explanation of Symbols]

[0077] 1A to 1D Hydraulic Systems 11 tanks 12 Cooler 2. First bidirectional pump 21 Head-side line 22 Rod-side line 3. Second bidirectional pump 31. First supply and discharge line 4 Single-rod cylinder 4h Head side chamber 4r rod side chamber 51, 54 Bridge Line 55, 56 Check valve 6, 6A, 6B Low-pressure selector valve 61 First Emission Line 62 Second Emission Line 63 Third Emission Line 64 Relief valve 7. Accumulator 71 Second supply and discharge line 72 relay lines 8 Charge pump 81 Intake line 82 Discharge Line 84 Relief Line 85 Relief valve 9 Electric motor

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 second bidirectional pump connected to the head-side line by a first supply / discharge line, An accumulator connected to the second bidirectional pump by a second 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, to the rod-side line by a second discharge line, and to the second supply / discharge line by a third 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 the third discharge line, 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 third discharge line. A pair of check valves facing opposite directions are provided on the bridge line connecting the head-side line and the rod-side line, A hydraulic system in which the second supply and discharge line is connected by a relay line to the portion of the bridge line between the pair of check valves.

2. The hydraulic system according to claim 1, further comprising a charge pump connected to a tank by an intake line and connected to any of the second supply / discharge line, the third discharge line, and the relay line by a discharge line.

3. The hydraulic system according to claim 2, further comprising a relief valve provided in a relief line that branches off from the discharge line and connects to the tank.

4. The hydraulic system according to claim 2, further comprising a relief valve provided in a relief line that branches off from any of the second supply / discharge line, the third discharge line, and the relay line and connects to the tank.

5. The system further comprises a cooler interposed between the charge pump and the accumulator, The hydraulic system according to claim 4, wherein the relief valve is located on the opposite side of the accumulator from the cooler.

6. The hydraulic system according to any one of claims 1 to 5, further comprising a relief valve provided in the third discharge line.

7. The hydraulic system according to any one of claims 1 to 5, wherein the low-pressure selector valve is an electromagnetic three-position valve.

8. The hydraulic system according to any one of claims 1 to 5, wherein the low-pressure selector valve includes a first solenoid valve and a second solenoid valve, which are electromagnetic two-position valves, the first solenoid valve is connected to the first discharge line and the third discharge line, and the second solenoid valve is connected to the second discharge line and the third discharge line.

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