Hydraulic system and method for controlling a hydraulic system

Abstract

Hydraulic system which comprises a tandem hydraulic actuator with a first, second and third pressure chamber and a hydraulic pump. A first pressure line connects a first port of the hydraulic pump to the first pressure chamber. A second pressure line connects the first port of the hydraulic pump to the second pressure chamber. A third pressure line connects a second port of the hydraulic pump to the first pressure chamber and a fourth pressure line connects the second port of the hydraulic pump to the third pressure chamber. A first pilot operated check valve of the pilot-to-open configuration is arranged in the first pressure line allowing hydraulic fluid flow towards the first port of the hydraulic pump and blocking hydraulic fluid flow in the opposite direction unless a control port of the first check valve is pressurized with high system pressure taken from the third and / or fourth pressure line. A second check valve is arranged in the third pressure line allowing a hydraulic fluid flow towards the second port of the hydraulic pump and blocking hydraulic fluid flow in the opposite direction.

Description

[0001] Description

[0002] Hydraulic System and Method for Controlling a Hydraulic System

[0003] The invention relates to hydraulic systems, in particular to hydraulic systems for operating a hydraulic actuator. The invention is further related to a method for controlling such hydraulic systems.

[0004] Hydraulic actuators are used in various applications. Tandem hydraulic actuators, also known as tandem hydraulic cylinders, are a specific type of hydraulic actuators.

[0005] Tandem hydraulic cylinders are frequently used in heavy-duty industrial applications, like construction, manufacturing, or agriculture applications. E.g., tandem cylinders are often used in forklift trucks to raise the forks. Also, some of the largest and most powerful agricultural vehicles, including combine harvesters and crop sprayers, use tandem cylinders. These cylinder systems are also frequently used in mining and excavating equipment. Another field of application are deep drawing devices and hydraulic presses.

[0006] In general, tandem hydraulic cylinders comprise two cylinders which are arranged in-line adjacent to each other, i.e. end to end. One piston of a two-piston subassembly is arranged in each of the cylinders, wherein the pistons of the two-piston subassembly are joined by a common piston rod. The pistons form pressure chambers in combination with the cylinders that can be pressurized by supplying hydraulic fluid to the pressure chambers.

[0007] The hydraulic fluid is selectively supplied to and drained from the pressure chambers by means of a control arrangement comprising various components, e.g. flow valves or directional control valves. Due to the high operating pressures of the supplied hydraulic fluid and the complex control setup, the known control arrangements are bulky and require expensive components.

[0008] It is therefore an objective of the invention to provide a hydraulic system comprising a tandem hydraulic actuator which requires less mounting space than the solutions known in the prior art. The hydraulic system and control shall be reliable be simple to manufacture. Furthermore and inexpensive. The objective of the invention is solved by a hydraulic system according to claim 1 , by a deep drawing device or press according to claim 15 and a method for controlling a hydraulic system according to claim 16. Preferred embodiments are presented in the subclaims dependent thereon.

[0009] A hydraulic system according to the invention includes a tandem hydraulic actuator. The tandem hydraulic actuator comprises a first cylinder, a second cylinder and a two-piston subassembly. A first piston of the two-piston subassembly is arranged in the first cylinder, delimiting in the first cylinder a first pressure chamber from a second pressure chamber. A second piston of the two-piston subassembly is arranged in the second cylinder, forming a third pressure chamber in the second cylinder. The pistons are joined by a common piston rod. From at least one of the pistons which are joined by the common piston rod, an actuator bar extends on the opposite side of the common piston rod in axial prolongation of the common piston rod through one of the first or the second cylinder and protrudes with a first end out of the respective cylinder.

[0010] The hydraulic system further comprises a hydraulic pump. A first pressure line connects a first port of the hydraulic pump to the first pressure chamber. A second pressure line connects the first port of the hydraulic pump to the second pressure chamber. A third pressure line connects a second port of the hydraulic pump to the first pressure chamber and a fourth pressure line connects the second port of the hydraulic pump to the third pressure chamber.

[0011] Further, a first pilot operated check valve of pilot-to-open configuration is arranged in the first pressure line allowing hydraulic fluid flow towards the first port of the hydraulic pump and / or towards the second pressure line. The first pilot operated check valve blocks hydraulic fluid flow in the opposite direction unless a first control port of the first check valve is pressurized. By nature of pilot-to-open check valves hydraulic fluid flow through the check valve is enabled when pilot pressure is present, e.g. moving a valve body in the check valve against a spring force, for instance, to a position enabling flow-through through the connected line.

[0012] A second check valve is arranged in the third pressure line allowing a hydraulic fluid flow towards the second port of the hydraulic pump and / or towards the fourth pressure line, and blocking the hydraulic fluid flow in the opposite direction.

[0013] In this configuration, the two-piston subassembly is movable bi-directionally along its longitudinal axis. For moving the two-piston subassembly in a first direction, e.g. an extending direction of the hydraulic actuator, the pump supplies high pressure at the first port and sucks in hydraulic fluid at low pressure at the second port. High pressure is present in the first pressure line up to the first check valve, which blocks the flow of pressurized hydraulic fluid towards the first pressure chamber as no pilot pressure is present at the first check valve. High pressure is present also in the second pressure line. Thus, hydraulic fluid is guided towards the second pressure chamber via the second pressure line. The first pressure chamber is connected to the second port of the hydraulic pump via the third pressure line, wherein the second check valve allows flow from the first pressure chamber towards the second port of the hydraulic pump. The third pressure chamber is also connected to the low pressure port of the hydraulic pump via the fourth pressure line. In consequence, the high pressure in the second pressure chamber causes the second pressure chamber to expand as the high pressure in the second pressure chamber generates a force on the piston which is higher than the force generated by the low pressure in the first pressure chamber and simultaneously in the third pressure chamber. Thus, the two-pis- ton subassembly moves towards the first pressure chamber as the second pressure chamber expands, wherein the fluid in first pressure chamber and in the third pressure chamber is pressed towards the second port of the hydraulic pump, here the low pressure port. Due to the high pressure difference between the pressure present in the first and second pressure chamber, the two-piston subassembly is moved with a high force, e.g. in a hydraulic actuator expansion direction in which the actuator bar, when located at the first piston , moves out of the first cylinder.

[0014] In a second mode, e.g., in a retracting mode of the two-piston subassembly, the pump is operated with inversed conveying direction and hydraulic fluid at a high pressure is provided at the second port of the pump, wherein fluid is sucked in at a low pressure via the first port. The first and second pressure chambers are connected to the low pressure second port of the hydraulic pump via the first and second pressure line as the first check valve allows hydraulic fluid flow through the first pressure line towards the low pressure port of the hydraulic pump, here the first port. The third pressure chamber is connected to the second port via the fourth pressure line. Hydraulic fluid flow from the second port via the third pressure line to the first pressure chamber is blocked due to the second check valve being operated in closing direction. Therefore, only the third pressure chamber is pressurized, and the first and second pressure chambers remain at low pressure.

[0015] In consequence, the two-piston subassembly is moved due to the force generated by the high pressure in the third pressure chamber. As the first check valve is of pilot to open configuration and high pressure is guided to the control port of the first check valve via a pilot pressure line connected to the second port of the hydraulic pump (here the high pressure port), the first check valve opens and therefore enables a connection between the first pressure chamber and the second pressure chamber as both pressure lines are connected with each other. Thus, fluid from the shrinking pressure chamber in the first cylinder can be directly displaced towards the expanding other pressure chamber in the first cylinder. Excess fluid, that does not enter the expanding pressure chamber, is supplied to the first pressure port of the hydraulic pump, here the suction port.

[0016] In comparison to flow valves, which are frequently used in the state of the art to conduct pressurized hydraulic fluid to and from pressure chambers of a hydraulic actuator, check valves require less mounting space and are less expensive. Therefore, the hydraulic system according to the invention requires less space and is less-expensive to manufacture, compared to the systems known from the prior art.

[0017] Preferably, the first control port of the first checkvalve is fluidly connected to the hydraulic pump and / or to the third pressure line and / or to the fourth pressure line via a first pilot line. Thus, in the retracting mode of the actuator bar (assumed to protrude out of the first cylinder), hydraulic fluid at high pressure present in the third and / or fourth pressure line is guided as a pilot pressure to the first control port of the first check valve and due to the pilot-to-open configuration of the first check valve, the high pilot pressure level causes the first check valve to open. Thereby, a fluid connection between the first and the second pressure chamber is established without the necessity of providing a further switchable flow valve.

[0018] In a preferred embodiment, the second check valve is a second pilot operated check valve. Optionally, a second pilot line may be provided which connects a control port of the second check valve to the first pressure line and / or second pressure line and or first port of the hydraulic pump. Further preferred, the second pilot operated checkvalve is of pilot-to-close configuration.

[0019] In this embodiment, a third operation mode of the hydraulic system, i.e. a rapid speed extending mode, can be realized. In this operation mode, the hydraulic pump supplies high pressure at the first pressure port and supplies low pressure at the second pressure port. Hence the control port of the second pilot operated check valve is supplied with a pilot pressure level such that the second check valve is closed due to its pilot-to-close configuration. High pressure is also present in the second pressure line connected to the second pressure chamber. On the other hand, low pressure is present in the third and fourth pressure lines and thus also at the third pressure chamber, wherein the closed second check valve provides a barrier between the high pressure side and the low pressure side. Thus, the high pressure in the second pressure chamber causes the second pressure chamber to extend and the two-piston subassembly is moved towards the first pressure chamber in the first cylinder. Thereby, hydraulic fluid is pressed out of the first pressure chamber into the first pressure line. From there, as the third pressure line is blocked by means of the closed second check valve, the hydraulic fluid is conducted via the first check valve into the second pressure line and further-on into the second pressure chamber. In consequence, the system is "flow-balanced". Like in the retracting mode, the pump is only required to supply a low amount of hydraulic fluid, as the fluid of the contracting first pressure chamber is guided into the expanding second pressure chamber, such that the hydraulic pump needs only to compensate the volume flow rate caused by different diameters of the piston rod arranged in the second pressure chamber and the actuator bar located in the first pressure chamber. In comparison to the solutions according to the prior art, the system can therefore be driven by a smaller pump which is less expensive.

[0020] Preferably, a control valve is arranged in the second pilot line adapted to guide a pilot pressure to the control port of the second pilot operated check valve. According to the state of the art, flow valves or directional valves are used for guiding pressurized fluid to the pressure chambers of a hydraulic actuator. Therefore, these flow or directional valves are required to withstand high pressure forces exerted by the pressure of the hydraulic fluid, which makes them expensive. By contrast, according to the invention the control valve is arranged in a control or pilot line. Therefore, the control valve is only required to withstand the pressure forces generated by a relatively low pilot pressure and is therefore less expensive and less bulky.

[0021] In a preferred embodiment, the control valve is movable between an open position in which flow of hydraulic fluid through the second pilot line is enabled, and a closed position in which flow of hydraulic fluid through the second pilot line is at least partially blocked, preferably completely blocked. Therefore, when high pressure is present at the first port of the hydraulic pump and / or in the first and / or second pressure line, the high pressure may or may not be guided towards the control port of the second check valve as a pilot pressure depending on whether the check valve should be in the open or closed position.

[0022] The control valve may be hydraulically, electro-magnetically, electrically, mechanically or manually operable. The control valve may be held in one of its positions, preferably in the closed position, by means of a pre-tensioning element if the control valve is not operated.

[0023] In a preferred embodiment of the hydraulic system, the piston rod comprises a diameter which is smaller than the diameter of the actuator bar attached to one of the pistons s on the opposite side of the piston rod. As the diameters of the piston rod and the actuator bar influence the area of the pistons on which pressure can act, providing a piston rod with multiple sections of different diameters allows adjusting the force that can be generated when pressurized fluid is supplied to the various pressure chambers in order to move the two-piston subassembly.

[0024] Preferably, the piston rod and the actuator bar are arranged on opposite sides of the first piston. Further preferred the actuator bar extends through the first pressure chamber, and the piston rod extends through the second pressure chamber and, optionally, through the third pressure chamber. Here it is preferred that the first cylinder comprises a bigger diameter than the second cylinder. In this configuration the second pressure chamber sealed by the bigger, first piston connected to the piston rod - having the smaller diameter than the actuator bar on the other side - can be used as the high hydraulic force generating pressure chamber, as the available piston ring surface in the second pressure chamber is the biggest of the four piston surfaces which could be pressurized by hydraulic fluid under pressure.

[0025] This configuration is especially preferred for the high or maximum speed extending mode with low hydraulic force, e.g. Due to different diameters of the piston rod and the actuator bar, the first pressure chamber comprises a smaller hydraulic fluid volume, or a smaller ring cross section on which hydraulic fluid under pressure can act. The corresponding ring section of the second pressure chamber is bigger as the piston rod diameter is smaller than the actuator bar diameter. According to the invention the first pressure chamber can be connected with the second pressure chamber via the first and the second pressure line, the first check valve. For achieving this the second check valve must be held in its closed position to avoid that hydraulic fluid displaced from the first pressure chamber is guided via the third pressure line to the suction port of the hydraulic pump. For this the control valve is switch to open position such that pilot pressure can be guided to pilot-to-close type second checkvalve. In this configuration and switching state of the control valve hydraulic fluid from the contracting smaller first pressure chamber can be displaced directly to the expanding bigger second pressure chamber, when pressurized fluid is supplied from the second now the high pressure port of the hydraulic pump to the second pressure chamber. Hence, the hydraulic pump only needs to supply / compensate the additional differential fluid volume flow that is required due to the higher volume / flow cross section of the second pressure chamber. Therefore, a high speed expanding mode can be achieved with simple means, and additionally the pump can be designed smaller and therefore cost-effective. In the above describe operational mode the hydraulic fluid from third pressure chamber is pressed out to the suction port of the hydraulic pump when the two-piston subassembly is moving towards the first pressure chamber. In one embodiment the diameters of the second cylinder and the second piston as well as the diameter of the piston rod running through the third pressure chamber are selected to that size that the volume flow needed to fill the volume difference between the first and the second pressure chamber due to the moving first piston having different ring cross section on either side, i.e. the hydraulic fluid displaced from the first pressure chamber is not sufficient to fill the expanding second pressure chamber. The volume difference has to be supplied by the hydraulic pump being fed, e.g. by the hydraulic fluid displaced from the third pressure chamber.

[0026] In case the hydraulic fluid volume displaced from the third pressure chamber does not equal the volume difference needed to fill the second pressure chamber a hydraulic fluid accumulator connected to the first port of the hydraulic pump can be used to compensate a deviation of hydraulic fluid volume in either direction, i.e. receiving hydraulic fluid in extending operation of the hydraulic actuator of the invention, and dispensing hydraulic fluid in retracting operation, or vice versa. The filling or discharging in one the two hydraulic actuator main moving direction depends on the design of the ring cross section of the third pressure chamber in comparison with the ring cross section difference in the other two pressure chambers. So the hydraulic fluid accumulator can be designed either to support the filling of the second pressure chamber or the third pressure chamber with hydraulic fluid.

[0027] In this preferred embodiment, the diameter of the first piston is different from the diameter of the second piston, in particular the diameter of the first piston is bigger than the diameter of the second piston. Choosing different diameters for the pistons further enhances the design possibilities for a system designer. In particular, a smaller piston leads to a smaller diameter of the corresponding cylinder, such that the volume of the pressure chambers enclosed by the piston and the cylinder can be tailored towards a specific application and in dependency of whether the focus should lie on quick moveability with low force (=small diameter) or high force with slow piston speed (= large diameter or large ring surface).

[0028] Providing a second piston that has a smaller diameter than the first piston is especially advantageous as, in this case, the volume of the third pressure chamber may be smaller than the volume of the first and / or second pressure chamber. Thus, in the retracting mode of the hydraulic system, the amount of hydraulic fluid that must be conducted to the third pressure chamber is small to achieve a fast retracting movement of the two-piston subassembly and the actuator bar. In consequence, the hydraulic pump can be designed smaller and is less expensive.

[0029] In one embodiment, the diameter of the actuator bar is equal to or even bigger than the diameter of the second piston. This allows an efficient redistribution of hydraulic fluid between the pressure chambers and therefore enables the use of a pump with small displacement volume as well as the use of a hydraulic accumulator with small volume.

[0030] The volume of the hydraulic accumulator can be further reduced when the diameter of the second piston is equal to the diameter of actuator rod.

[0031] Preferably, the hydraulic pump is bi-directionally operable, i.e. the hydraulic fluid conveying direction through the closed circuit of the hydraulic system is reversible. For this, e.g., the pump may comprise a displacement element that is tiltable in two opposite directions with respect to a neutral position of the displacement element. However, it is also possible that the pump may be rotatable / drivable in opposite directions or that the hydraulic system comprises a pump with a single conveying direction in combination with a valve arrangement that enables an inversion of the high pressure and low pressure side of the hydraulic system.

[0032] The hydraulic pump may be driven by an electric motor or an internal combustion engine. Preferably, due to the required small pump size and the possibility to operate the pump in two directions of rotation, an electric motor may be used preferably.

[0033] In a preferred embodiment the used hydraulic pump is a constant displacement pump whose displacement volume during one revolution is constant and non-adaptable. When using such a constant displacement pump, e.g. a gear pump, a gerotor pump, a vane pump, a screw pump or an axial or radial piston pump with fixed displacement volume, the flow rate conveyed to the second or third pressure chamber can be modified by varying the rotational speed. However, hydraulic pump with continuously variable displacement can be used according to the invention as well.

[0034] Further preferably, the hydraulic system comprises an accumulator which is fluidly connected to the first pressure line, the second pressure line, the third pressure line or the fourth pressure line. A hydraulic accumulator may be additionally or alternatively be connected to the first port of the hydraulic pump or to the second port of the hydraulic pump or to a tank of the hydraulic pump. The accumulator may be capable of compensating of volume flow rate differences between the pressure chambers as mentioned above, of compensating leaked hydraulic fluid and / or of absorbing hydraulic shocks, e.g. when the movement of the two-piston subassembly is interrupted abruptly. Additionally, the accumulator may ensure that the suction side of the hydraulic pump is always pressurized, i.e. that always a minimum pressure is present at the pump's suction side.

[0035] In a preferred embodiment, the hydraulic system comprises a position measurement system for measuring the position of the two-piston subassembly and / or actuator bar with respect to the first cylinder and / or the second cylinder. The measured position of the two-piston subassembly and / or actuator bar may be provided to an electronic control unit, which, further, may be capable of controlling the motor / engine driving the hydraulic pump and / or the displacement or the rotational speed of the hydraulic pump and / or the control valve of the hydraulic system in order to adjust the movement of the two-piston subassembly and the attached actuator bar or the force with which the two-piston subassembly and the actuator bar is moved. In other words, a control circuit may be provided, wherein the position and / or velocity and / or - in general - the movement of the two-piston subassembly or the actuator bar is measured, and is fed back to a control unit which is capable of providing a control signal to at least one of the controllable components of the hydraulic system, in order to minimize an error between the measured movement signal and a commanded movement signal.

[0036] The hydraulic system may be used, e.g., in a deep drawing device or a press. As the use of the presented hydraulic system is advantageous for nearly any technical field. Due to the low necessary mounting space and the low costs of the system, the hydraulic system may also be used in other heavy-duty industrial applications like construction, manufacturing, or agriculture industries.

[0037] Further, a method for controlling a hydraulic system as disclosed above is presented, in particular with one control valve for providing a pilot pressure signal. The method comprises at least one of the following steps:

[0038] Sucking in hydraulic fluid via the second port of the hydraulic pump and supplying pressurized hydraulic fluid via the first port of the hydraulic pump, and switching the control valve to the open position, in order to enable a fast forward movement of the piston rod and the actuator bar. Thus, this step corresponds to the above disclosed fast / rapid speed extending mode.

[0039] Sucking in hydraulic fluid via the second port of the hydraulic pump and supplying pressurized hydraulic fluid via the first port of the hydraulic pump, and switching the control valve to the closed position, in order to enable a creeping movement of the piston rod and the actuator bar. Thus, this step corresponds to the above disclosed creep motion of the two-piston subassembly in extending direction. Due to the high pressure difference between the pressures in the first and in the second pressure chamber, the two-piston subassembly is moved with a high force.

[0040] Sucking in hydraulic fluid via the first port of the hydraulic pump and supplying pressurized hydraulic fluid via the second port of the hydraulic pump, and switching the control valve to the closed position, in order to enable a retracting movement of the piston rod and the actuator bar. As disclosed above, in some embodiments, the retracting movement of the two-piston subassembly may also be considered as "fast" or "rapid" retraction due to hydraulic connection between the first and the second pressure chamber and due to the small volume of the third pressure chamber.

[0041] The hydraulic system may further comprise a fourth pressure chamber which is arranged opposite to the third pressure chamber with respect to the second piston. The fourth pressure chamber may be fluidly connected to the above disclosed hydraulic pump of the hydraulic system. Alternatively or additionally, the fourth pressure chamber may be connected to an additional pump of the hydraulic system, to a gas (pressure) accumulator, a liquid pressure accumulator or simply to the environment.

[0042] The fourth pressure chamber may be discharge compressed gas, e.g. nitrogen gas or any other fluid to a gas or fluid accumulator when the actuator bar is retracted, i.e. when the fourth pressure chamber is compressed. The fourth pressure chamber may be connected via a line to an additional gas accumulator, as mentioned before. When the two-piston subassembly is moved due to an external force, e.g. gravity, compressed gas can be displaced to the gas accumulator, and / or may be used to drive pneumatic driven means, such as a generator or a fan. In this manner energy can be regenerated from the movement of the two-piston subassembly. Vice versa energy can be discharged / recuperated from the gas accumulator when the fourth pressure chamber expands, i.e. when the actuator bar extends. In this manner the gas accumulator discharges stored potential energy when the fourth pressure chamber expands. Needless to say that the potential energy stored in the gas accumulator can be used for a plurality of pneumatic applications know in the art.

[0043] In order to further increase the force, with which the two-piston subassembly is moved, e.g., in the creep speed extending mode and / or in the rapid speed extending mode, the fourth pressure chamber may be supplied with hydraulic fluid or pressurized gas from an additional pressure source, e.g. a gas or fluid accumulator, at high pressure level. In consequence, the fluid in the second pressure chamber and the fluid in the fourth pressure chamber are under high pressure, whereas the fluid in the first pressure chamber and the fluid in the third pressure chamber are under low pressure. Thus, due to the principle of superposition of forces, the area on which high pressure can act, is further increased. As the force corresponds to the product of the pressure level with the area on which the pressure acts, the increase of force directly proportional to the increase of the area on which pressure can act.

[0044] With the help of the enclosed Figures preferred embodiments of a system according to the invention are explained in more detail in order to enhance the understanding of the basic idea of the invention. The present embodiments do not limit the scope of the idea of the invention, but only represent possible design alternatives, to which within the knowledge of a person with skills in the relevant art modifications can be made without leaving the scope of the invention. Therefore, all those modifications and changes are covered by the claimed invention. In the Figures it is shown in:

[0045] Figure 1 a schematic hydraulic circuit diagram of a hydraulic system;

[0046] Figure 2 a schematic hydraulic circuit diagram of the hydraulic system in a first operating state;

[0047] Figure 3 a schematic hydraulic circuit diagram of the hydraulic system in a second operating state; and

[0048] Figure 4 a schematic hydraulic circuit diagram of the hydraulic system in a third operating state.

[0049] In the Figures same reference numerals are used for same components of different embodiments throughout the description to improve readability.

[0050] Figure 1 discloses a schematic hydraulic circuit diagram of a hydraulic system 1 . The hydraulic system 1 comprises a hydraulic actuator 5. The hydraulic actuator 5 is designed as a tandem hydraulic cylinder and comprises a first cylinder 10 and a second cylinder 20 which are arranged adjacent to each other. This means that the first cylinder 10 and the second cylinder 20 are separated by a common separating element 7. The first cylinder 10 and the second cylinder 20 share a common central axis 8 which is identical to the central axis of a piston rod 33 of a two-piston subassembly30. The two-piston subassembly30 comprises a first piston 31 which is arranged in the first cylinder 10. A second piston 32 is accommodated in the second cylinder 20. The first head 31 and the second piston 32 are connected by means of the piston rod 33. In other words, the two- piston subassembly 30 is movably accommodated in the first cylinder 10 and the second cylinder 20 by means of the first piston 31 and the second piston 32. In particular, the two-piston subassembly 30 is longitudinally slidable in the first cylinder 10 and the second cylinder 20 along the central axis 8.

[0051] The first piston 31 delimits in combination with the first cylinder 10 a first pressure chamber 11 and a second pressure chamber 12, wherein the second pressure chamber 12 is arranged opposite to the first pressure chamber 11 with respect to the first piston 31 . The second piston 32 encloses in combination with the second cylinder 20 a third pressure chamber 23 which is arranged adjacent to the separating element 7.

[0052] Thus, seen in longitudinal direction of the central axis 8 from below to the top an actuator bar 37 extending from the first piston 31 through the first pressure chamber 11 which is adjacent to the first piston 31 . On the other side of piston 31 the second pressure chamber 12 arranged around the piston 33. The second pressure chamber 12 is closed by the separating element 7 separating the first cylinder 10 from the second cylinder 20. Next to the separating element 7 the third pressure chamber 23 is located, wherein the third pressure chamber 23 is delimited by the adjacent second piston 32. Although not shown in the Figures, a fourth pressure chamber may be formed in the second cylinder 20 opposite to the third pressure chamber 23 with respect to the second piston 32.

[0053] The first pressure chamber 11 is formed around the actuator bar 37 which protrudes with a first end 39 from the first cylinder 10. The piston rod 33 extend between the first piston 31 towards the second piston 32. The piston rod 33 comprises a smaller diameter than the actuator bar 37. Therefore, the surface area on which pressure can act on the first piston 31 is bigger in the second pressure chamber 12 than in the first pressure chamber 11 . This means that if hydraulic fluid with identical pressure would be supplied to the first and second pressure chambers 11, 12, the two-piston subassembly 30 would move towards the first pressure chamber 11 .

[0054] The second piston 32 has a smaller diameter than the first piston 31 . Therefore, the diameter of the third pressure chamber 23 is smaller in comparison to the diameter of the first pressure chamber 11 and the second pressure chamber 12. In consequence, if the pressure chambers 11, 12, 23 have a comparable length, as in the present embodiment, the volume of the third pressure chamber is smaller than the volume of the other pressure chambers 11 , 12, in particular than the volume of the second pressure chamber 12. Or expressed in other words the piston ring surface in the second pressure chamber 12 is the biggest, followed by the piston ring surface in the first pressure chamber 11 and the piston ring surface in the third pressure chamber 23.

[0055] In the shown embodiment, the diameter of the second piston 32 is identical to the diameter of the actuator bar 37. The benefit of this dimensioning of the components is explained at a later stage.

[0056] The hydraulic system 1 further includes a hydraulic pump 80 with a first port 81 and a second port 82, wherein the first port is in fluid connection with the first and second pressure chamber 11 & 12 of the first cylinder 10, and the second port is in fluid connection with the third pressure chamber in the second cylinder 20 and with the first pressure chamber 11 in the first cylinder 10. In the shown embodiment, the hydraulic pump 80 is driven by an electric motor 83 which is capable of bi-directional rotation. The pump may comprise a fixed displacement volume or an adjustable displacement volume. In consequence, the hydraulic pump 80 may convey hydraulic fluid under high pressure from the first port 81 towards the first cylinder 10 and sucks in hydraulic fluid under low pressure at the second port 82 in actuator expanding operation modes, or -vice versa - may sucks in hydraulic fluid under low pressure at the first port 81 and convey hydraulic fluid under high pressure at the second port 82 in actuator retraction operation modes. The specific flow and pressure levels of high and low system pressure which are provided by the pump may thereby be adjustable.

[0057] The first port 81 of the hydraulic pump 80 is fluidly connected to the first pressure chamber 11 by means of a first pressure line 41 . The first port 81 is fluidly connected also to the second pressure chamber 12 by means of a second pressure line 42. Starting at the first port 81, the first pressure line 41 and the second pressure line 42 form one single line portion before they branch off each other to form the first pressure line 41 distinct from the second pressure line 42. In other words: the first pressure line 41 and the second pressure line 42 share at least for a section of their extension one common pressure line segment. Thus, without considering the check valves 50, 60, in some operation states, hydraulic fluid may be conducted from the first pressure chamber 11 to the second pressure chamber 12 via the first pressure line 41 and the second pressure line 42, or vice versa.

[0058] The second pressure port 82 is fluidly connected to the first pressure chamber 11 by means of a third pressure line 43. The second pressure port 82 is fluidly connected to the third pressure chamber 23 by means of a fourth pressure line 44. Like the first pressure line 41 and the second pressure line 42, the third pressure line 43 and the fourth pressure line 44 share at least for a section of their extension one common pressure line segment. This means that, e.g., starting at the second port 82 of the hydraulic pump 80, the third pressure line 43 and the fourth pressure line 44 form one common line up to the point where they branch-off from each other. Thus, not considering the check valves 50, 60, hydraulic fluid may be conducted from the first pressure chamber 11 to the third pressure chamber 23 via the third pressure line 43 and the fourth pressure line 44, or vice versa.

[0059] A first check valve 50 is arranged in the first pressure line 41 , wherein the check valve 50 allows hydraulic flow from the first pressure chamber 11 towards the first port 81 of the pump 80 and / or towards the second pressure chamber 12 via the second pressure line. The check valve 50 blocks, in a closed state, flow in the opposite direction, i.e., hydraulic flow towards the first pressure chamber 11 .

[0060] The first check valve 50 is of pilot-to-open configuration. This means that if a high pilot pressure is present at a first control port 52 of the first check valve 50, the first check valve 50 is switched into an open position in which hydraulic fluid can pass the first check valve 50 in both flow directions. Thus, in the open position, not only hydraulic flow from the first pressure chamber 11 towards the first port 81 of the pump 80 or towards the second pressure chamber 12 is enabled, but also flow in the opposite direction, i.e., hydraulic flow towards the first pressure chamber 11 is possible.

[0061] The first control port 52 of the first pilot valve 50 is connected to the common line segment of the third pressure line 43 and the fourth pressure line 44, i.e. to the pressure level at second port of the hydraulic pump 80 via a first pilot line 51 . Thus, the pilot pressure at the first control port 52 is identical to the pressure in the third pressure line 43 and to the pressure in the fourth pressure line 44.

[0062] A second check valve 60 is arranged in the third pressure line 43. The second check valve 60 is of pilot-to-close configuration, such that the second check valve 60 is closed, when a high pilot pressure is present at a second control port 62 of the second check valve 60. Due to its orientation, the second check valve 60 allows flow from the first pressure chamber 11 towards the second port 82 of the pump 80 and / or towards the third pressure chamber 23, when no or a low pilot pressure is present at the second control port 62. When a high pilot pressure is present at the second control port 62 or when high pressure flow in the third pressure line 43 is directed towards the first pressure chamber 11 , the second pilot valve 60 blocks the hydraulic flow. The control port 62 of the second check valve 60 is fluidly connected to the first and / or second pressure line 41 , 42, i.e. to the pressure level present at the first port 81 of the hydraulic pump 80 via a second pilot line 61. In the second pilot line 61 a control valve 70 is arranged for selectively blocking the fluid connection between the first and / or second pressure line 41, 42, respectively the first pump port 81 , and the second control port 62.

[0063] The control valve 70 comprises at least two positions, wherein in a closed position no hydraulic flow through the control valve 70 can occur. In an open position of the control valve 70, hydraulic flow between first pressure port 81 and / or the first and / or second pressure line 41, 42 and the second control port 62 is enabled.

[0064] The hydraulic system 1 further comprises a hydraulic accumulator 90 connected to the second port 82 of the hydraulic pump 80 or to a tank of the pump 80 for compensating volume flow and / or pressure differences between the first and second cylinder 10 and 20 and / or for replacing leaked hydraulic fluid and / or for absorbing pressure shocks.

[0065] In the Figures, the first, second, third and fourth pressure line 41 to 44 are represented by solid lines. The dashed lines represent the first and second pilot lines 51, 61 . In order to visualize the pressure levels in the pressure lines 41 to 44 and in the pressure chambers 11 , 12, 23, in the Figures 2 to 4, pressure lines / pilot lines conducting high pressure are indicated by bold lines. Pressure / pilot lines conducting low pressure are indicated by thin lines. Pressure chambers at a high pressure level are hatched, whereas pressure chambers at a low pressure level are filled with a dotted pattern. Further, the arrows at the pressure lines 41 to 44 indicate the flow direction of hydraulic fluid to and from the pressure chambers 11 , 12 & 23, respectively the conveying direction of the hydraulic pump 80.

[0066] Figure 2 shows a first operational state of the hydraulic system 1. The first operation state may, e.g., be called fast forward movement or rapid extending mode of the hydraulic actuator 5. The hydraulic pump 80 supplies pressurized fluid at a high working pressure at its first port 81 . At the second port 82 hydraulic fluid is sucked in at a low working pressure level.

[0067] Thus, hydraulic fluid at high pressure is guided via the second pressure line 42 towards the second pressure chamber 12. Fluid flow from the first port 81 via the first pressure line 41 towards the first pressure chamber 11 is blocked by means of the first check valve 50. The low pressure level of the second port 82 is present at the control port 52 of first check valve 50, as the first pilot line 51 establishes a connection between the first control port 52 and the second port 82 of the hydraulic pump 80. Therefore, the pilot pressure at the control port 52 is not sufficient to open the first check valve 50 and allow a fluid flow towards the first pressure chamber 11 through the first check valve 50.

[0068] Further, low pressure is present in the fourth pressure line 44 and in the third pressure chamber 23. Thus, the force which is generated by the fluid under high pressure in the second pressure chamber 12 is higher than the opposite force generated on the second piston 32 by the fluid under low pressure in the third pressure chamber 23. The balance of forces causes the two-piston subassembly 30 to longitudinally move towards the protruding first end 39 of actuator bar 37, and cause the second pressure chamber 12 to expand. Simultaneously, hydraulic fluid at high pressure is supplied via the first check valve 50 into the second pressure line 42.

[0069] However, hydraulic fluid pressed out of the first pressure chamber 11 into the first pressure line 41 into the third pressure line 43 is blocked by the second check valve 60 which is forced into its closed position by a high pilot pressure guided to the control port 62 via the control valve 70 in the pilot line 61 connected to the first pump port 81 , here the high pressure port. Hence, pilot pressure is guided to the control port 62 from the first port 81 of the pump 80 via the second pilot line 61, wherein the control valve 70 is switched into its open position such that the high pilot pressure can hold the second check valve 60 closed.

[0070] In consequence all fluid pressed out of the first pressure chamber 11 is conducted directly towards the second pressure chamber 12 via the first pressure line 41 and the second pressure line 42. As the volume / ring cross section of the second pressure chamber 12 is larger than the volume / ring cross section of the first pressure chamber 11 , the pump 80 needs to provide only for the differential amount of hydraulic fluid that is required in addition to fill the larger second pressure chamber 12. As the required differential amount of hydraulic fluid is low, the hydraulic system 1 can be operated by a small pump 80 with a low displacement size. Nevertheless, a fast forward movement of the two-piston subassembly 30 can be realized due to the balanced hydraulic flows and the recirculating fluid between the pressure chambers 11, 12.

[0071] Figure 3 shows a second operational state of the hydraulic system 1. The second operation state may, e.g., be called creeping movement or extending of the hydraulic actuator 5 with creep speed and / or high force. The hydraulic pump 80 supplies pressurized fluid at a high working pressure at its first port 81 . At the second port 82 hydraulic fluid is sucked in at a low working pressure level. However, in this operating mode, the control valve 70 is in its closed position. Therefore, the pilot pressure present at the control port 62 of the second check valve 60 is not high enough to hold the second check valve 60 in its closed position. Thus, when the two-piston subassembly 30 is moved towards its first end 39 due to the pressure difference in the second pressure chamber 12 and in the third pressure chamber 23, the fluid in the first pressure chamber 11 is pressed into the first pressure line 41 and into the third pressure line 43. However, as the second check valve 60 is not forced into its closing position, the fluid from the first pressure chamber 11 is relieved into the low pressure side of the hydraulic system 1 via the third pressure line 43 and the second check valve 60. Simultaneously the first check valve 50 is held in its closed state due to the high pressure which is present at the first port 81 of the hydraulic pump 80 and in the second pressure line 42.

[0072] Thus, the second pressure chamber 12 is only filled by the pressurized hydraulic fluid provided by the pump 80. However, in comparison to the first mode, i.e., the fast forward mode, the force, with which the piston is moved, is higher, as the fluid from the first pressure chamber 11 is directly relieved to the low pressure side of the hydraulic system 1 . In the former mode, the fast forward movement mode of the hydraulic actuator 5, the fluid in the first pressure chamber 11 was pressed at high pressure level to the second pressure chamber 12 due to the direct fluid connection to the first pressure port 81 . In consequence, in the creep movement mode, the two- piston subassembly 30 travels with a lower velocity, but is moved by a higher force as the counter force in the first pressure chamber is lower.

[0073] In this mode pressure chambers 11 and 23 act as hydraulic fluid reservoirs for the hydraulic fluid conveyed with high pressure to the second pressure chamber 12. In case the flow rate of hydraulic fluid entering the hydraulic pump 80 at port 82 does not coincide with the flow rate which is pressed into the second pressure chamber 12 the accumulator 90 can compensate for equal flow rates. Thereby it is imaginable that the accumulator 90 is filled or emptied. E.g., this can be necessary if there is leakage of hydraulic fluid or if the diameter of the second piston is not equal to the diameter of the actuator rod 37.

[0074] Figure 4 shows a third operational state of the hydraulic system 1 . The third operation state may, e.g., be called retracting movement or fast / rapid retracting of the hydraulic actuator 5. In this working mode, the hydraulic pump 80 is operated with inversed fluid flow as in the extending mode of the hydraulic system 1 . This means that the hydraulic pump 80 supplies pressurized fluid at a high working pressure at its second port 82 towards the third pressure chamber 23. At the first port 81 hydraulic fluid is sucked in at a low working pressure level.

[0075] From the second port 82 of the hydraulic pump 80 now the high pressure port fluid under high pressure is conducted into the third pressure line 43 and into the fourth pressure line 44. The flow through the third pressure line 43 is blocked by the second check valve 60 which is charged in closing direction such that all pressurized fluid provided by the hydraulic pump 80 is supplied to the third pressure chamber 23 via the fourth pressure line 44.

[0076] In this mode the second pressure chamber 12 is connected to the second pressure port 82 via the second pressure line 42, such that low pressure is present in the second pressure chamber 12. Therefore, the force generated on the two-piston subassembly 30 by the high pressure in the third pressure chamber 23 surpasses the opposite force generated on the two-piston subassembly 30 by the low pressure in the second pressure chamber 12, and the two-piston subassembly 30 moves towards the third pressure chamber 23.

[0077] In order to enable a faster movement of the two-piston subassembly 30 and to avoid negative pressure in the expanding first pressure chamber 11 , the first check valve 50 is pushed into the open position by a high pilot pressure present at the control port 52 of the first check valve 50. The pilot pressure is guided to the control port 52 via the first control line 51 which is connected to the second port 82 of the hydraulic pump 80. The open position of the first check valve 50 enables a recirculating hydraulic fluid flow from the first pressure port 81 and from the second pressure chamber 12 towards the first pressure chamber 11 . Therefore, fluid from the contracting second pressure chamber 12 can be pushed directly into the smaller first pressure chamber 11 . The remaining fluid, or excess fluid, is sucked into the first port 81 of the hydraulic pump 80 and is pumped under high pressure towards the third pressure chamber 23. In case hydraulic pump 80 conveys more hydraulic fluid than pressure chamber 23 can receive, e.g. because the flow rate out of the biggest pressure chamber 12 is too large the excess of hydraulic fluid can be passed into accumulator 90. In a later hydraulic actuator 5 expanding operation with high force (creep mode) or high speed (low force) the hydraulic fluid stored in the accumulator can be used again to compensate lack of hydraulic fluid volume flow rate, e.g.

[0078] The ring cross-section / volume of the first pressure chamber 11 is smaller than the ring cross section / volume of the second pressure chamber 12 due to the bigger diameter of the actuator bar 37 in comparison to the piston rod 33 diameter. As the diameter of the actuator bar 37 can be equal to the diameter of the second piston 32, the amount of fluid required to fill the third pressure chamber 23 is equal to the difference between the fluid drained from the second pressure chamber 12 and the fluid supplied to the first pressure chamber 11 . Thus, in this configuration, the fluid in the hydraulic system 1 is used efficiently and the required size of the accumulator 90 can be minimized, or an accumulator 90 can be even suppressed. Due to the small volume of the third pressure chamber 23, the fluid flow pressed into the third pressure chamber 23 causes a retracting movement of the two-piston subassembly 30 with a high velocity.

[0079] The check valves 50, 60 arranged in the pressure lines 41, 43 comprise a simple and reliable design and are in-expensive in comparison to flow valves or direction control valves. There is no continuous flow through the control valve 70 as it is just connecting and disconnecting the control port 62 of pilot valve 60. Thus, the control valve 70 is not required to withstand high flow forces, and it can be designed smaller and less bulky. In consequence, the hydraulic system 1 is less costly than known alternatives while providing all the necessary functions and controls to move a hydraulic actuator reliably and efficiently.

[0080] Reference List

[0081] 1 Hydraulic system 44 Fourth pressure line

[0082] 5 Hydraulic actuator 45 Fourth pressure chamber

[0083] 7 Separating element

[0084] 8 Central axis 50 First checkvalve

[0085] 51 First pilot line

[0086] 10 First cylinder 52 Control port

[0087] 11 First pressure chamber

[0088] 12 Second pressure chamber

[0089] 60 Second checkvalve

[0090] 20 Second cylinder 61 Second pilot line

[0091] 23 Third pressure chamber 62 Control port

[0092] 30 Two-piston subassembly 70 Control valve

[0093] 31 First piston

[0094] 32 Second piston 80 Hydraulic pump

[0095] 33 Piston rod 81 First port

[0096] 37 Actuator bar 82 Second port

[0097] 39 First end 83 Electric motor

[0098] 41 First pressure line

[0099] 42 Second pressure line 90 Accumulator

[0100] 43 Third pressure line 100 Gas or liquid accumulator

Claims

Claims1 . Hydraulic system (1 ) comprising:• a tandem hydraulic actuator (5) comprising a first cylinder (10), a second cylinder (20) and a two-piston subassembly (30), wherein a first piston (31 ) of the two-piston subassembly (30) is arranged in the first cylinder (10), delimiting a first pressure chamber (11 ) from a second pressure chamber (12), and wherein a second piston (32) of the two-piston subassembly (30) is arranged in the second cylinder (20), forming a third pressure chamber (23), the first and the second pistons (31, 32) being joined by a common piston rod (33), wherein an actuator bar (37) extends on the opposite side of one of the pistons (31, 32) in axial prolongation through one of the first or the second cylinder (10, 20) and protrudes with a first end (39) out of the respective cylinder,• a hydraulic pump (80),• a first pressure line (41) connecting a first port (81 ) of the hydraulic pump (80) to the first pressure chamber (11 ),• a second pressure line (42) connecting the first port (81 ) of the hydraulic pump (80) to the second pressure chamber (12),• a third pressure line (43) connecting a second port (82) of the hydraulic pump (80) to the first pressure chamber (11 ),• a fourth pressure line (44) connecting the second port (82) of the hydraulic pump (80) to the third pressure chamber (23),• a first pilot operated checkvalve (50) of pilot-to-open configuration arranged in the first pressure line (41 ) allowing hydraulic fluid flow towards the first port (81 ) of the hydraulic pump (80) and / or towards the second pressure line (42), and blocking hydraulic fluid flow in the opposite direction unless a first control port (52) of the first check valve (50) is pressurized with high system pressure taken from the third and / or fourth pressure line,• a second check valve (60) arranged in the third pressure line (43) allowing a hydraulic fluid flow towards the second port (82) of the hydraulic pump (80) and / or towards the fourth pressure line (44), and blocking hydraulic fluid flow in the opposite direction.

2. Hydraulic system (1) according to claim 1 , wherein the first control port (52) of the first check valve (50) is fluidly connected to the second port (82) of the hydraulic pump (80) and / or to the third pressure line (43) and / or to the fourth pressure line (44) via a first pilot line (51 ).

3. Hydraulic system (1) according to claim 1 or 2, wherein the second check valve (60) is a second pilot operated check valve and a second pilot line (61) connects a second control port (62) of the second check valve (60) to the first port (81) of the hydraulic pump (80) and / or to the first pressure line (41 ) and / or to the second pressure line (42).

4. Hydraulic system (1 ) according to claim 3, wherein the second pilot operated check valve (60) is of pilot-to-close configuration.

5. Hydraulic system (1) according to claim 3 or 4, wherein a control valve (70) is arranged in the second pilot line (61 ) adapted to guide a pilot pressure to the second control port (62) of the second pilot operated check valve (60).

6. Hydraulic system (1) according to claim 5, wherein the control valve (70) is hydraulically, electro-magnetically, electrically, mechanically or manually operable.

7. Hydraulic system (1) according to claims 5 or 6, wherein the control valve (70) is movable at least between an open position in which hydraulic fluid flow of hydraulic fluid through the second pilot line (61) is enabled, and a closed position in which hydraulic fluid flow of hydraulic fluid through the second pilot line (61) is at least partially blocked, preferably completely blocked.

8. Hydraulic system (1) according to any of the preceding claims, in which the diameter of the piston rod (33) is smaller than the diameter of the actuator bar (37).

9. Hydraulic system (1) according to claim 8, wherein the actuator bar (37) and the piston rod (33) are arranged on opposite sides of the first piston (31).

10. Hydraulic system (1) according to any of the preceding claims, wherein the diameter of the first piston (31) is different from the diameter of the second piston (32), in particular the diameter of the first piston (31 ) is bigger than the diameter of the second piston (32).

11. Hydraulic system (1 ) according to any of claims 8 to 10, wherein the diameter of the second piston (32) is equal to the diameter of actuator rod (37).

12. Hydraulic system (1) according to any of the preceding claims, wherein the hydraulic pump (80) is a bi-directional pump with reversible hydraulic conveying directions.

13. Hydraulic system (1) according to any of the preceding claims, wherein the hydraulic pump (80) is driven by an electric motor (83) or an internal combustion engine.

14. Hydraulic system (1 ) according to any of the preceding claims, wherein the hydraulic system (1 ) comprises an accumulator (90) which is fluidly connected to the first port (81 ) of the hydraulic pump (80), to the second port (82) of the hydraulic pump (80), to a tank of the hydraulic pump (80), to the first pressure line (41), to the second pressure line (42), to the third pressure line (43) or to the fourth pressure line (44).

15. Hydraulic system (1 ) according to any of the preceding claims, wherein the hydraulic system (1) comprises a position measurement system for measuring the position of the two-piston subassembly (30) with respect to the first cylinder (10) and / or the second cylinder (20).

16. Hydraulic system (1) according to any of the preceding claims, wherein a fourth pressure chamber (45) arranged opposite to the third pressure chamber (23) is connected to a gas or liquid pressure accumulator (100) configured to be filled with compressed gas or liquid when the fourth pressure chamber (45) is compressed and to discharge stored potential energy when the fourth pressure chamber (45) expands.

17. Deep drawing device or press with a hydraulic system (1 ) according to any of the preceding claims.

18. Method for controlling a hydraulic system (1) according to one of claims 5 to 15, comprising at least one of:• Sucking in hydraulic fluid via the second port (82) of the hydraulic pump (80) and supplying pressurized hydraulic fluid via the first port (81) of the hydraulic pump (80), and switching the control valve (70) to the open position, in order to enable a fast forward movement of the actuator bar (37), or• Sucking in hydraulic fluid via the second port (82) of the hydraulic pump (80) and supplying pressurized hydraulic fluid via the first port (81) of the hydraulic pump (80), and switching the control valve (70) to the closed position, in order to enable a creeping movement of the actuator bar (37), or• Sucking in hydraulic fluid via the first port (81 ) of the hydraulic pump (80) and supplying pressurized hydraulic fluid via the second port (82) of the hydraulic pump (80), and switching the control valve (70) to the closed position, in order to enable a retracting movement of the actuator bar (37).

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