Hydraulic actuator capable of recuperation

Abstract

The invention relates to a hydraulic actuator (1) comprising at least one bidirectionally operable hydraulic pump (2) designed for delivering hydraulic fluid and having two connectors (6a, 6b), and comprising a hydraulic actuator component (3) having two hydraulic fluid chambers (4a, 4b) which can be loaded with hydraulic fluid in order to actuate the hydraulic actuator component (3), wherein the hydraulic fluid chambers (4a, 4b) are each connected via a line (7a, 7b) to the connectors (6a, 6b) of the hydraulic pump (2), wherein a first line (7a) connects the first chamber (4a) to the hydraulic pump (2) and a second line (7b) connects the second chamber (4b) to the hydraulic pump (2), with the result that, when one of the hydraulic fluid chambers (4a, 4b) is filled with hydraulic fluid, a displacement of hydraulic fluid from the other hydraulic fluid chamber (4b, 4a) is brought about at the same time and a volume of the hydraulic fluid is moved by the hydraulic pump (2), - wherein a second chamber cross section (8b) of the second chamber (4b) differs from a first chamber cross section (8a) of the first chamber (4a), and the first line (7a) is connected to a pressure reservoir (5) in which a differential volume (28) can be received and provided, which occurs as a result of the different chamber cross sections (8a, 8b) of the chambers (4a, 4b), wherein a compressible element (17) exists in the pressure reservoir (5), which compressible element can be compressed in order for the pressure reservoir (5) to receive pressurized hydraulic fluid, and which expands when hydraulic fluid is removed from the pressure reservoir (5), - wherein the first line (7a) also has a supply connector (15), via which the hydraulic actuator (1) is supplied with hydraulic fluid, wherein a hydraulic fluid supply (16) is connected to the supply connector (15), which hydraulic fluid supply maintains a minimum pressure in the first line (7a), - wherein a controllable shut-off valve (35) is arranged between the first line (7a) and the pressure reservoir (5), by means of which shut-off valve the pressure reservoir (5) can be disconnected from the first line (7a).

Description

[0001] Hydraulic actuator capable of recuperation

[0002] The invention relates to a hydraulic actuator capable of recuperation, comprising at least one hydraulic actuator with differential volume and a hydraulic pump unit.

[0003] Hydraulic actuators with differential volume are, in particular, so-called differential cylinders. The hydraulic differential cylinder is used for a specific task – for example, for lifting and / or lowering a load. A typical application for such hydraulic differential cylinders would be in an excavator arm, the arm of a telehandler, or for the front end of a tractor. In such applications, hydraulic differential cylinders are used, for example, for raising and lowering the arm of a telehandler or the front end of a tractor. There are many other applications for such hydraulic differential cylinders, particularly in the areas of construction and agricultural machinery, municipal and forestry machinery. Differential cylinders are frequently used in such applications due to their ability to deliver large forces.

[0004] When operating construction and agricultural machinery, many tasks are carried out using hydraulic differential cylinders. When digging a construction pit, the excavator arm is raised and lowered with great regularity. The same applies to the arm of a telehandler, which is used, for example, for stacking and / or loading loads. It is often only necessary to drive the hydraulic differential cylinder in one direction of movement (upwards). In the other direction, the energy from the movement process must be dissipated in a controlled manner. A desired goal in hydraulics in such situations is to operate it in such a way that recuperation is possible. In particular, with hydraulic differential cylinders, it is desirable to operate them in such a way that energy from the movement can be recuperated. If the work performed with a hydraulic differential cylinder, e.g. lifting a load (e.g.When a load (e.g., an arm of a telehandler) is directed against the force of gravity, the movement typically increases potential energy. Conversely, this potential energy must be dissipated when a load is lowered using a hydraulic differential cylinder.

[0005] In principle, it is desirable if this potential energy can be used or recovered and then made available again for the further operation of the hydraulic actuator or for other applications. This energy can be recovered using a hydraulic pump unit coupled to the hydraulic differential cylinder. Lowering the load preferably creates a volume flow of hydraulic fluid, which flows with a pressure gradient into the other chamber of the differential cylinder or into a tank. This volume flow can be throttled to prevent uncontrolled lowering of the load. Alternatively, the controlled lowering of the load can be achieved using the hydraulic pump unit. The potential energy is recovered via torque and speed.

[0006] Hydraulic differential cylinders are double-acting hydraulic cylinders. They are typically designed with a piston guided within a cylinder body, with a chamber on either side of the piston. Hydraulic fluid can be forced into this chamber to move the piston within the cylinder in one direction or the other. As double-acting hydraulic cylinders, hydraulic differential cylinders are generally suitable for both pushing and pulling.

[0007] As the piston moves, the respective chambers of the hydraulic cylinder expand or contract. In hydraulic differential cylinders, a rod connected to the piston extends through one of the two chambers on one side of the hydraulic differential cylinder. This rod allows the force exerted by the hydraulic fluid on the piston to be transferred outwards. The end of the cylinder body opposite the rod and the end of the rod opposite the cylinder body each form the ends of the differential cylinder, which are pushed apart by the differential cylinder when the differential cylinder is extended and retracted again when the differential cylinder is retracted.

[0008] The side of the piston on which the rod extends through the chamber is regularly referred to as the ring side or rod side because the chamber on this side is ring-shaped around the rod. The chamber through which the rod extends is accordingly referred to here as the ring chamber. The other side of the piston is regularly referred to as the bottom side or piston side. The chamber on this other side of the piston is accordingly referred to here as the bottom chamber. The rod can be extended and retracted from the hydraulic differential cylinder, depending on which of the two chambers hydraulic fluid flows into or out of. When hydraulic fluid is pumped into the ring chamber, the rod is retracted. The differential cylinder then exerts a tensile force. When hydraulic fluid is pumped into the bottom chamber, the rod is extended.The differential cylinder then exerts a compressive force.

[0009] Due to the design of this rod, the cross-sectional areas of the two chambers are different. The cross-sectional area of ​​the annular chamber is smaller than the cross-sectional area of ​​the bottom chamber by the cross-sectional area of ​​the rod.

[0010] Due to this difference in the cross-sectional areas of the hydraulic fluid chambers, there is a difference in the volume flows that flow into and out of the chambers when the piston moves. If the bottom chamber with the larger cross-sectional area is pressurized with hydraulic fluid, the volume flow of hydraulic fluid exiting the annular chamber is smaller than the volume flow of hydraulic fluid entering the bottom chamber. Conversely, if the annular chamber is pressurized with hydraulic fluid, the volume flow of hydraulic fluid exiting the bottom chamber is greater than the volume flow of hydraulic fluid entering the annular chamber. The difference in the volume flows, or the total differential volume that occurs for a specific displacement of the piston, is called the differential volume and is the reason why such a hydraulic cylinder is called a "differential cylinder."The term “differential volume” is used in the following as a general term - both for a difference in the volume flows entering and exiting the annular chamber and the bottom chamber and for the actual differential volume that occurs when the piston is displaced by a certain distance.

[0011] Differential cylinders must be distinguished from so-called synchronous cylinders, in which the two chambers have the same cross-sectional area. With synchronous cylinders, a difference in volume flow or a differential volume does not occur. However, special concepts are required for the design of synchronous cylinders. With synchronous cylinders, measures must be implemented to equalize the cross-sectional areas of the chambers. One possible design for a synchronous cylinder, for example, has two rods, each of which extends through both chambers of the synchronous cylinder. Another possible design for synchronous cylinders is the counter-rotating connection of two differential cylinders to form one actuator. Such designs are not practical in many applications. For this reason, so-called differential cylinders are more widely used.

[0012] The differential volume of differential cylinders presents a challenge when designing hydraulic actuators capable of recuperation. This is especially true if the differential cylinders' ability to act as double-acting cylinders with the ability to act both pulling and pushing is to be utilized.

[0013] The technical implementation of the recuperation capability of hydraulic actuators is achieved, for example, by connecting the two chambers of the hydraulic cylinder via the hydraulic pump unit. The hydraulic pump unit can then preferably pump hydraulic fluid in both directions. This means that it can either pump from the annular chamber into the base chamber to extend the hydraulic cylinder, or it can pump from the base chamber into the annular chamber to retract the hydraulic cylinder. Furthermore, the hydraulic pump unit can preferably counteract the hydraulic fluid with a torque in both directions in order to extract energy from the hydraulic fluid or to recover energy and make it usable for other components.The energy obtained from the flow of hydraulic fluid by the hydraulic pump unit can be stored, for example, in a hydraulic or electrical energy storage device.

[0014] Connecting the two chambers via the hydraulic pump unit requires additional measures for differential cylinders due to the differential volume. When the differential cylinder is extended, the differential volume must also be provided because the bottom chamber is larger than the annular chamber, and the volume flow of hydraulic fluid escaping from the annular chamber cannot completely fill the bottom chamber. When the differential cylinder is retracted, the differential volume must be drained because the volume flow flowing out of the bottom chamber is greater than the volume flow that can be absorbed by the annular chamber. Circuits with double pumps as the hydraulic pumps used are particularly recommended for this problem. Such double pumps have two separate pump chambers.A first pump chamber serves to pump the hydraulic fluid volume from the annular chamber between the annular chamber and the base chamber. A second pump chamber simultaneously serves to pump the differential volume between an accumulator and the base chamber. This requires very precise coordination of the double pump with the connected differential cylinders. In particular, it is essential that the ratio of the pump chambers is precisely matched to the ratio of the chamber volumes of the base chamber and the annular chamber and the differential volume. Due, among other things, to the compressibility of the oil and the resulting movements of the cylinder during load changes, these circuits also often require so-called intermediate circuits.

[0015] Such an intermediate circuit is described, for example, in patent DE102011056894B4. The intermediate circuit ensures that the pressures in the lines to the annular chamber and the base chamber remain within a specified pressure range. If an upper pressure limit is exceeded in the respective chamber and the associated lines, hydraulic fluid volume is diverted to the intermediate circuit. If a lower limit is exceeded, the respective line is refilled. The intermediate circuit and its connection to the lines to the annular chamber and the base chamber require a large number of additional hydraulic components.

[0016] If the ability to recuperate and the use of hydraulic differential cylinders are established as boundary conditions for the design of a hydraulic actuator, there is another alternative to the use of an intermediate circuit and two hydraulic pumps, namely the use of a pressure reservoir that is firmly and permanently connected to the annular chamber and that is capable of absorbing the differential volume that flows from the bottom chamber towards the annular chamber when the differential cylinder is retracted. In the static state, this pressure reservoir always has exactly the same pressure as the annular chamber because there is an open connection here and pressure equalization always occurs. The term "static" state in this context describes that temporary pressure differences resulting from the hydraulic flows in the lines are not taken into account here.

[0017] With such a circuit, the hydraulic actuator, or rather the differential cylinder in the hydraulic actuator, loses the ability to act in both directions—that is, to both pull and push. With such a circuit, only pushing is possible, not pulling. However, such a circuit makes it possible to recover energy from the pressure of the hydraulic fluid when the hydraulic cylinder retracts.

[0018] The reason that hydraulic actuators connected in this way are not suitable for pulling action is due to the effects of the pressure reservoir, which is permanently connected to the annular chamber. This pressure reservoir is designed to absorb the differential volume of hydraulic fluid that is displaced from the base chamber when the differential cylinder is retracted and which cannot be passed on to the annular chamber. The pressure reservoir is also designed to make this differential volume available again for pumping into the base chamber when the differential cylinder is extended again. As already explained, the pressure in the pressure reservoir and in the annular chamber are always equal in the static state. By absorbing the differential volume of hydraulic fluid in the pressure reservoir, the pressure in the pressure reservoir and in the annular chamber increases.Conversely, the pressure in the pressure reservoir and the annular chamber decreases when differential volume is provided for delivery into the bottom chamber.

[0019] Such a pressure reservoir is pressurized with a base pressure before the hydraulic actuator is first put into operation, for example, by providing a gas bubble with a defined gas volume and a defined pressure in the pressure reservoir. The gas bubble in the pressure reservoir is compressed or expanded by the inflow and outflow of the differential volume when the differential cylinder retracts and extends. The pressure in the pressure reservoir is determined by the amount of gas and the pressure of the gas or the remaining volume of the pressure reservoir available for the gas. The pressure in the pressure reservoir is continued via the connecting hydraulic fluid line into the annular chamber.

[0020] Such a pressure reservoir fulfills some of the functions of the intermediate circuit mentioned above and the use of two parallel hydraulic pumps for separate treatment of the fluid volume flow exchanged between the two chambers and the differential volume. The pressure reservoir eliminates the need for an intermediate circuit. However, the limitation is that the pressure in the annular chamber (and in the pressure reservoir itself) is dependent on the position of the piston and is therefore no longer precisely controllable. Controlled pressure application to the annular chamber, and thus pulling with the differential cylinder or hydraulic actuator, is no longer possible.

[0021] The limitation that a hydraulic actuator can only push when extending and recuperate when retracting, but not pull, is acceptable for many applications. Hydraulic actuators with differential cylinders that raise and lower an arm operate predominantly against gravity in their normal operation. However, since many machines frequently move large masses and have a high deadweight, the recuperation capability of such hydraulic actuators is very useful (as described above). Large amounts of energy can be recovered.

[0022] However, sometimes situations arise where, even in applications where hydraulic actuators are normally only able to push, it is desirable to also be able to pull, at least temporarily, for certain special applications. Telehandlers, for example, can push off the ground with their arm to raise themselves or a machine axle, or to press a load onto the ground. This is extremely advantageous for changing tires, as the machine does not need to be jacked up. Although this application is rare, foregoing this capability in favor of recuperation limits the machine's universal usability.

[0023] In such applications, hydraulic actuators with the described intermediate circuit must be used if both the full operational capability of the hydraulic actuator (pull and push) and the ability to recuperate are to be achieved. Due to the much more complex circuitry, this is usually not desirable.

[0024] The object of the present invention is to propose a novel hydraulic actuator capable of recuperation in most situations and suitable for both pulling and pushing. At the same time, the design should be significantly simplified compared to hydraulic actuators with an intermediate circuit and dual pump, which can pull and push and are fully capable of recuperation both during retraction and extension.

[0025] Described here is a hydraulic actuator comprising at least one bidirectionally operable hydraulic pump for conveying hydraulic fluid with two connections, as well as a hydraulic actuator component with two hydraulic fluid chambers which can be supplied with hydraulic fluid to actuate the hydraulic actuator component, wherein the hydraulic fluid chambers are each connected to the connections of the hydraulic pump via a line, wherein a first line connects the first chamber to the hydraulic pump and a second line connects the second chamber to the hydraulic pump, so that when one of the hydraulic fluid chambers is filled with hydraulic fluid, hydraulic fluid is simultaneously displaced from the other hydraulic fluid chamber and a volume of the hydraulic fluid is moved through the hydraulic pump,wherein a second chamber cross-section of the second chamber differs from a first chamber cross-section of the first chamber, and the first line is connected to a pressure reservoir in which a differential volume can be accommodated and provided, which differential volume occurs due to the different chamber cross-sections of the chambers, wherein a compressible element exists in the pressure reservoir, which element is compressible to accommodate pressurized hydraulic fluid in the pressure reservoir and which expands when hydraulic fluid is withdrawn from the pressure reservoir, wherein the first line further comprises a supply connection via which the hydraulic actuator is supplied with hydraulic fluid, wherein a hydraulic fluid supply is connected to the supply connection, which hydraulic fluid supply maintains a minimum pressure in the first line, wherein a controllable shut-off valve is arranged between the first line and the pressure reservoir,with which the pressure reservoir can be separated from the first line.,

[0026] It is particularly preferred if the hydraulic actuator component is at least one differential cylinder.

[0027] The basic design of differential cylinders, with a piston that can be moved within the cylinder and a rod that runs through a first chamber of the differential cylinder, has already been described in the introduction. Reference is made to this in full here. Differential cylinders are a particularly frequently used form of hydraulic actuator, with different chamber cross-sections and a differential volume. Differential cylinders are particularly frequently used in applications that involve lifting, pushing, and / or pulling loads. Differential cylinders are particularly widespread in construction machinery, agricultural machinery, municipal machinery, and forestry machinery. Particularly in applications where differential cylinders are used, it is advantageous if energy recovery is possible and, at the same time, it is also possible to generate forces in both directions (pushing and pulling) with the differential cylinder.At the same time, with differential cylinders in such applications, there is usually one direction of movement (pushing or pulling, especially pushing) that is used most frequently, while in the opposite direction (preferably when retracting the differential cylinder), there are particularly many situations in which recuperation is possible. For this reason, the application of the hydraulic actuator with differential cylinders described here is particularly advantageous. However, the principle of a hydraulic actuator described here is not limited to differential cylinders; it can be applied and transferred to other hydraulic actuators with differential volume.

[0028] The hydraulic actuator preferably has at least one displaceable actuator element, which is arranged between the hydraulic fluid chambers and is displaceable by pressure or pressure differences of the hydraulic fluid in the hydraulic fluid chambers. In the case of a differential cylinder, the displaceable actuator element is in particular a piston that is displaceable in a cylinder body. As explained above, in a differential cylinder, the first chamber is an annular chamber through which a rod connected to the piston extends, while the second chamber is a bottom chamber. The second chamber then has a second chamber cross-section that is larger than a first chamber cross-section of the first chamber.

[0029] The hydraulic actuator described here has the described pressure reservoir, which is connected to the first line. During operation, the pressure reservoir serves to absorb or release the differential volume described above when the hydraulic fluid is pumped into one chamber of the hydraulic actuator component and flows out of the other chamber of the hydraulic actuator component. This pressure reservoir contains a compressible element that compresses or expands to provide the variable volume in the pressure reservoir for absorbing and / or providing the differential volume of the hydraulic fluid. The change in volume and the compression / decompression of the compressible element result in different total volumes of the lines and chambers of the hydraulic actuator, depending on the position of the hydraulic actuator component.

[0030] The pressure reservoir and the compressible element also impose limitations on the ability of the hydraulic pump to pump hydraulic fluid from the second line and the second chamber into the first line and the first chamber, for example, to operate the hydraulic actuator component in a pulling manner. By providing the pressure reservoir, hydraulic fluid pumped from the second line or the second chamber into the first line or the first chamber is also pumped into the pressure reservoir. The pumped hydraulic fluid cannot contribute, or can only partially contribute, to increasing the pressure and causing movement against a load.

[0031] For this reason, it is proposed to provide a controllable shut-off valve between the first line and the pressure reservoir. With this controllable shut-off valve, the pressure reservoir can be separated from the first line. When the pressure reservoir is separated from the first line, the first line and the first chamber can be pressurized with hydraulic fluid using the hydraulic pump and / or via the supply connection. The closed shut-off valve prevents the flow of hydraulic fluid to the pressure reservoir. Separation means that neither hydraulic fluid can pass from the first line to the pressure reservoir, nor hydraulic fluid from the pressure reservoir to the first line. The controllable shut-off valve has a closed switching position, in which a blocking effect is produced in both flow directions. In this way, movements against a load can be generated. When such movements are stopped orare no longer required, the shut-off valve can be opened again and in regular operation with recuperation the pressure reservoir is again available for recuperation. The hydraulic actuator described here can therefore be operated in two different operating modes. In a first (usual) operating mode with the shut-off valve open, the hydraulic actuator cannot execute forces and movements that require pressurising the first chamber, or can only do so to a limited extent. In return, the possibilities for recuperation are particularly good and the control of the hydraulic actuator is greatly simplified. In a second operating mode, the described restrictions on executing forces and movements are lifted. In return, the possibilities for recuperation are restricted or reduced and the control of the hydraulic actuator is more complex in this operating mode.

[0032] A hydraulic fluid supply is connected to the supply port, which can be used to supply the hydraulic actuator (in particular the lines, the chambers, and the hydraulic pump of the hydraulic actuator) with hydraulic fluid. In the hydraulic actuator described, the hydraulic fluid supply is preferably used to maintain a minimum pressure in the hydraulic fluid in the first line by specifically supplying hydraulic fluid to the supply port.

[0033] The hydraulic fluid supply is also referred to as a charging circuit, with which the hydraulic actuator can be charged.

[0034] The hydraulic supply includes in particular the supply connection with which the hydraulic actuator can be supplied with hydraulic fluid.

[0035] The hydraulic actuator described here is usually part of a more complex overall hydraulic system, which includes a plurality of hydraulic actuators.

[0036] The hydraulic fluid supply preferably has a central feed pump with a main hydraulic drive, wherein the main hydraulic drive is preferably configured to operate the central feed pump. The central feed pump supplies hydraulic fluid to the hydraulic actuator described here via the supply connection. As explained, the main hydraulic pump also supplies other hydraulic actuators with hydraulic fluid. The hydraulic actuator described here, which is capable of recuperation and for this purpose additionally has the hydraulic pump with the pump drive described above, can be supplied by the hydraulic fluid supply alongside other hydraulic actuators, wherein these additional hydraulic actuators can be constructed differently. In particular, it is not necessary for these other hydraulic actuators to have their own hydraulic pumps and pump drives.These additional hydraulic actuators can also be operated directly with the hydraulic fluid pressure provided by the hydraulic fluid supply.

[0037] The hydraulic fluid supply preferably additionally has a main reservoir (also called a central hydraulic tank) from which the main hydraulic pump constantly delivers and supplies the hydraulic fluid. There is preferably a first pressure relief valve, via which hydraulic fluid is drained from the first line when a pressure in the first line exceeds a first limit pressure. There is preferably a second pressure relief valve, via which hydraulic fluid is drained from the second line when a pressure in the second line exceeds a second limit pressure. The first limit pressure and the second limit pressure can differ from one another. The supply connection preferably also comprises a pressure relief valve, with which the pressure in the hydraulic actuator and in particular in the first line can be specifically released, e.g.at the end of an operating phase of the hydraulic actuator described here upon deactivation of a machine in which the hydraulic actuator described here is provided.

[0038] It is particularly preferred if the hydraulic fluid supply can either deliver hydraulic fluid to the supply connection or take in hydraulic fluid from the supply connection.

[0039] The hydraulic fluid supply preferably also includes a central hydraulic pump, which can be used to provide pressurized hydraulic fluid. The hydraulic fluid supply preferably allows for precise adjustment of the pressure level and the volume flow of hydraulic fluid that is supplied (discharged into the hydraulic actuator) or taken in at the supply connection.

[0040] This pressurized hydraulic fluid is preferably distributed to various consumers using a power distribution system (also called power split for short). The hydraulic actuator described here is preferably a consumer connected to the power distribution system. From the perspective of the hydraulic actuator described here, the power distribution system is part of the hydraulic fluid supply system from which the hydraulic actuator is supplied with hydraulic fluid.

[0041] The principle of power splitting is a basic element of complex hydraulic systems with various hydraulic actuators, such as those widely used in tractors, excavators, and work machines. Hydraulic power from a central hydraulic pump is fed to a block and from there branched to multiple consumers using valves. In detail, a distinction is made between many types of power splitting. Of particular note here, and generally suitable for the hydraulic actuator described, are so-called open center systems, which use valves with an open center position and without individual pressure compensators. Unused oil, which is made available to the power split by the central hydraulic pump, flows back through the block to the tank. Due to the lack of individual pressure compensators, the volume flow to a consumer depends on load changes in other consumers in the same circuit.Power branches designed as closed center systems use valves with closed center positions and individual pressure compensators on the individual consumers.

[0042] Power branches that are designed to measure the pressure of the respective consumer (in this case the pressure at the supply connection) and to take this into account when providing hydraulic fluid are particularly suitable for supplying the hydraulic actuator described here. When the hydraulic actuator described here is operated with a closed shut-off valve, the differential volume cannot be absorbed by the pressure reservoir. For this reason, it is advantageous if the power branch or the hydraulic fluid supply is not only suitable for providing hydraulic fluid at the supply connection. Preferably, the power branch is also suitable for taking hydraulic fluid from the supply connection and, in doing so, also specifically maintaining or adjusting a pressure in the first line. The hydraulic fluid supply orIn this operating situation, the power split preferentially takes over the task of the pressure reservoir, providing the differential volume (depending on the delivery direction of the hydraulic pump) or absorbing it.

[0043] When operating the shut-off valve to disconnect and later reconnect the pressure reservoir to the first line, it is advantageous to avoid sudden pressure increases and sudden compensating flows of the hydraulic fluid between the first line and the pressure reservoir as much as possible. This is more important when opening the shut-off valve than when closing the shut-off valve because, when the shut-off valve is closed, different pressures regularly develop in the pressure reservoir and the first line, since disconnecting with the shut-off valve closed means preventing the flow of hydraulic fluid in both directions. The hydraulic actuator is preferably configured so that the pressure in the first line with the hydraulic fluid supply is set to the actual value of the pressure in the accumulator before the shut-off valve is opened.

[0044] It is particularly preferred if the first line and the second line are connected to one another by a bypass line, wherein the bypass line prevents an undersupply of hydraulic fluid in the hydraulic pump by establishing a bypass flow from the first line into the second line through the bypass line when a pressure in the second line falls to a threshold value, wherein a deactivation circuit is arranged in the bypass line, with which the bypass line can be interrupted when the pressure in the second line is above a threshold value. In the hydraulic actuator described here, a distinction is made between two lines or two line systems. A first line system (referred to here as the first line or, in the case of a differential cylinder as the hydraulic actuator, also as the annular chamber line) connects the first chamber or the annular chamber to the hydraulic pump.A second line system (referred to here as the second line or, in the case of a differential cylinder as a hydraulic actuator, also as the bottom chamber line) connects the second chamber or bottom chamber with the hydraulic pump. When referring to lines or line systems here, this always refers to both the first lines or first line systems (on the ring side / ring lines) and the second lines or second line systems (on the bottom side / bottom lines).

[0045] The first line and the second line are normally separated from each other and at different pressure levels. Hydraulic fluid exchange between the first line and the second line (and thus between the first chamber or annular chamber and the second chamber or bottom chamber) occurs only via the hydraulic pump.

[0046] The hydraulic actuator comprises exactly one hydraulic actuator component, which may also consist of several individual elements (e.g., several differential cylinders connected in parallel) and which interacts with exactly one hydraulic pump. This interaction between the actuator component and the hydraulic pump essentially enables recuperation.

[0047] A bypass line exists here, which connects the lines, if necessary, thus breaking the separation between them. The bypass flow is the flow of hydraulic fluid through the bypass line.

[0048] The bypass line is a protective measure provided due to the pressure accumulator connected to the first chamber. It serves to prevent a possible undersupply of hydraulic fluid to the hydraulic pump and thus protects the hydraulic pump from cavitation. Cavitation can occur if, when hydraulic fluid is being pumped, the hydraulic pump is undersupplied with hydraulic fluid at the connection where the hydraulic pump draws in hydraulic fluid or where hydraulic fluid flows into the hydraulic pump. This undersupply creates low pressures and thus gas bubbles form. This phenomenon is called cavitation. Cavitation can cause significant damage to the hydraulic pump. When the cylinder retracts, hydraulic oil is pumped from the second chamber into the first chamber.If mechanical resistance occurs at the cylinder or a holding valve in the second line closes, an immediate undersupply would occur in the second line, resulting in cavitation, which could damage or destroy the hydraulic pump. When reference is made to cavitation in the following, this also means that the hydraulic pump is undersupplied with hydraulic fluid, which then leads to cavitation.

[0049] Preferably, such a closable holding valve is arranged between the second chamber and the second line, with which the second chamber can be closed in order to hold the hydraulic actuator component in a defined position.

[0050] Such a lockable holding valve safely contains the hydraulic fluid in the second chamber (in the base chamber in the case of a differential cylinder), so that the hydraulic actuator is held in a certain position even in the event of very large external forces. Such a lockable holding valve can, for example, be a valve that closes when de-energized, which is open when energized and, in the event of a system failure, prevents uncontrolled movement of the hydraulic actuator (and thus, for example, the uncontrolled lowering of the component driven by the hydraulic actuator, e.g. an arm of a telehandler). Such a lockable holding valve is regularly a mandatory safety element. However, such a lockable holding valve increases the risk of cavitation in the hydraulic pump, because closing this holding valve prevents hydraulic fluid from flowing out of the second chamber orfrom the floor chamber into the hydraulic pump is immediately prevented.

[0051] In order to prevent cavitation in the pump, the bypass line described is provided, which usually contains a check valve that opens towards the second line or the bottom side and closes towards the first line or the ring side. Through this bypass line, a circular flow with the hydraulic pump from the first line to the second line preferably occurs as soon as the pressure in the second line falls below the pressure in the first line. This circular flow reliably prevents cavitation in the pump. At the same time, however, it is also not possible to reduce the pressure in the second line below the pressure in the first line. It is practically impossible to generate pulling forces with the differential cylinder when retracting the hydraulic cylinder.

[0052] Furthermore, the hydraulic actuator generally has a tendency to be moved in one direction (towards the first chamber). In the case of a differential cylinder, this is a tendency to move the piston towards the annular chamber and thus extend the differential cylinder. Due to the larger cross-sectional area of ​​the second chamber or the bottom chamber, a situation with the same pressures in the first chamber or the annular chamber and in the second chamber or the bottom chamber always leads to a pressure force building up or pushing the hydraulic actuator component of the hydraulic actuator. In typical applications, however, this is not a major factor because, for example, the weight of a telehandler arm or a similar load always acts on the differential cylinder.

[0053] The deactivation circuit proposed here specifically deactivates the bypass line when the pressure in the second line is sufficiently high or above a threshold value.

[0054] The threshold value is a fixed pressure value related to an absolute pressure level and / or an ambient pressure level and in particular not a relative pressure value related to a reference pressure obtained at another position in the hydraulic actuator.

[0055] It is particularly preferred if the threshold value of the pressure maintained in the second line is in a range between 2 bar and 20 bar.

[0056] Such a deactivation circuit represents an improved alternative to a check valve in the bypass line. A check valve in the bypass line prevents the pressure in the second line (on the bottom side) from falling below the pressure in the first line (on the ring side). This provides good protection against cavitation. A check valve in the bypass line essentially couples the minimum pressure in the second line to the pressure in the first line.

[0057] The deactivation circuit in the bypass line proposed here is a novel approach to meeting the aforementioned need for cavitation protection. Preferably, the deactivation circuit checks the pressure in the second line shortly before the connection to the hydraulic pump. It has been found that the bypass line can be dispensed with if sufficient pressure is still present at this position to reliably prevent cavitation. However, the deactivation circuit becomes ineffective, or rather, it activates the bypass line, as soon as this pressure in the second line is no longer present. Ideally, it regulates the pressure at the second connection of the hydraulic pump to a predetermined, fixed value, independent of the pump speed or the pressure at the first connection of the hydraulic pump.

[0058] The deactivation circuit makes it possible for the pressure in the second line to be reduced below the pressure in the first line, provided that the pressure in the second line is at least above the threshold value. The ability to generate forces towards the first chamber (pulling forces in the case of a differential cylinder) with a hydraulic actuator has thus been created for the first time. The deactivation circuit enables the hydraulic pump to reduce the pressure in the second line and the base chamber below the pressure in the first line and the annular chamber. Only when the pressure in the second line falls to or below the threshold value does the bypass line engage and a connection is established between the first line and the second line.By lowering the pressure in the bottom chamber below the pressure in the annular chamber, a pulling force can be generated with the hydraulic actuator component, which would not be possible without the described deactivation circuit, because lowering the pressure in the annular chamber below the pressure in the bottom chamber would be prevented in principle without the described deactivation circuit.

[0059] The deactivation circuit in the bypass line provides the basis for the pressure in the first line to be increased above the pressure in the second line by separating the pressure reservoir from the first line with the aid of the shut-off valve, without a bypass flow occurring through the bypass line. The bypass line is provided in the hydraulic actuator as a protective measure to protect the hydraulic pump from cavitation. The deactivation circuit for deactivating the bypass line essentially makes it possible for the pressure in the first line to be increased above the pressure in the second line, thus enabling the hydraulic pump to pressurize the first chamber, allowing pulling forces to be generated with the hydraulic actuator component.By isolating the pressure reservoir with the shut-off valve, a further increase in the pressure in the first line and the first chamber is possible, allowing pressures above the maximum permissible pressure of the pressure reservoir to be generated in the first line and the first chamber, and allowing particularly strong pulling forces to be generated with the hydraulic actuator or hydraulic actuator component. In the closed switching position that causes the isolating action, the shut-off valve prevents hydraulic fluid from flowing from the first line to the pressure reservoir, thus protecting the pressure reservoir from excessive pressure.It is particularly advantageous if, in addition to the deactivation circuit, a check valve is provided in the bypass line, which can be opened in the direction from the first line to the second line, so that even if the pressure in the second line is below the threshold value, it is ensured that the pressure in the second line does not fall below the pressure in the first line.

[0060] The check valve and the deactivation circuit are preferably connected in series in the bypass line so that, on the one hand, it is ensured that the pressure in the second line or at the second connection of the hydraulic pump is always above the threshold value and, in the event that the pressure in the second line or at the second connection of the hydraulic pump should fall below the threshold value, it is also ensured that the pressure in the second line does not fall below the pressure in the first line.

[0061] In other words: As long as the pressure in the second line or at the second connection of the hydraulic pump is above the threshold value, the pressure in the second line or at the second connection of the hydraulic pump can be below the pressure in the first line. Below the threshold value, the pressure in the second line or at the second connection of the hydraulic pump would be raised to the pressure in the first line. In practical application, however, the pressure in the second line or at the second connection of the hydraulic pump would not fall below the threshold value because this is already reliably prevented by the deactivation circuit and there is always sufficient hydraulic fluid pressure available in the first line to keep the pressure level in both lines overall above the threshold value.

[0062] As described above, the deactivation circuit proposed here in the bypass line is an alternative to the check valve in the bypass line. A pure check valve in the bypass line prevents cavitation in the hydraulic pump by preventing the pressure in the second line from falling below the pressure in the first line. Since the pressure in the first line cannot usually drop suddenly due to the closing of a holding valve, and circulation from the first line to the second line through the bypass line occurs when the check valve opens, this ensures that cavitation does not occur.

[0063] The combination of deactivation circuit and check valve ensures particularly good safety that no cavitation occurs in the hydraulic pump.

[0064] It is particularly advantageous if, in the direction from the second line to the first line in the bypass line, the check valve is arranged first and the deactivation circuit is arranged behind it.

[0065] In most operating situations of the hydraulic actuator, the pressure in the second line is higher than the pressure in the first line. This is especially true because the hydraulic actuator is preferably used so that the pressure in the second line acts against gravity—for example, in a differential cylinder mounted on the arm of a telehandler, where the second line is pressurized on the bottom side to raise the telehandler arm.

[0066] The pressure in the second line thus acts preferentially on the check valve in the bypass line as long as the pressure in the second line is higher than in the first line. The pressure in the second line only continues until the deactivation circuit when the pressure in the second line drops below the pressure in the first line, e.g., because the hydraulic pump is used to pump hydraulic fluid out of the second chamber (bottom chamber) to generate pulling forces with the hydraulic actuator.

[0067] The deactivation circuit may have a design-related (small) leak. By arranging the deactivation circuit from the second line downstream of the check valve, this design-related (small) leak can be reduced. It is also advantageous if the hydraulic pump is suitable for both generator and motor operation.

[0068] Various types of hydraulic pumps are suitable for this purpose, such as vane pumps, gear pumps, axial piston pumps, etc. In principle, there are no restrictions on the type of pumps that can be used for the hydraulic actuator. Pumps that can be operated in both directions, both as generators and motors, are also referred to as "four-quadrant capable." The use of such four-quadrant capable pumps as hydraulic pumps for the actuator described here is particularly preferred.

[0069] It is particularly preferred if the hydraulic pump is connected to a drive with which energy from the pressure and the volume flow of the hydraulic fluid can be converted by the hydraulic pump and stored in an energy storage device.

[0070] The drive serves to drive the hydraulic pump and thus operate the hydraulic actuator component of the hydraulic actuator. In the case of a differential cylinder as the hydraulic actuator component, this can be extended or retracted by driving the hydraulic pump with its drive.

[0071] In preferred embodiments, the drive is an electric drive. With an electric drive, electrical energy can be recuperated into an electrical energy storage device (e.g., a battery or a capacitor) if the hydraulic pump is driven as a generator, acting as a hydraulic motor.

[0072] The drive is preferably designed as an electric drive that operates at variable speed and is powered by a DC circuit via an inverter. In generator mode, the inverter feeds the converted energy back into the DC circuit, where it is absorbed by a battery or capacitors or drawn directly from simultaneously operating loads. In other design variants, the drive can also include additional hydraulic pumps, with which the recovered energy is stored hydraulically or as pressure in an additional hydraulic accumulator.

[0073] It is particularly advantageous if the deactivation circuit comprises a separating valve with which the bypass line can be interrupted, which closes proportionally depending on a pressure in the second line, so that a pressure above a threshold value is maintained in the second line.

[0074] The isolating valve of the deactivation circuit preferably opens up a flow-through line cross-section depending on the pressure in the second line. This flow-through line cross-section preferably expands the more the pressure drops. There is preferably a control pressure range of the isolating valve which lies in the range of the threshold value and which, for example, begins slightly above the threshold value with an upper spreader pressure value and ends slightly below the threshold value with a lower control pressure value. As soon as the pressure in the second line reaches the control pressure value from the top, the isolating valve begins to open. If the pressure in the second line drops further, the isolating valve opens further so that when the lower control pressure value is reached, the flow-through line cross-section of the isolating valve is fully open. The isolating valve behaves proportionally within the control pressure range.

[0075] Such a circuit regulates the pressure in the second line to the threshold value. If a check valve is arranged in the second line upstream of the deactivation circuit, this only occurs when the pressure in the second line is below the pressure in the first line.

[0076] It is particularly advantageous if the isolating valve (directly or indirectly piloted) is clamped between an active surface on which the pressure in the second line acts and a pre-tensioning element, wherein the pre-tensioning element is adjusted such that the isolating valve is opened in such a way that pressure in the second line is kept above the threshold value.

[0077] The preload element is, for example, a spring, which is preferably preloaded such that the control pressure range with the upper control pressure value and the lower control pressure value is maintained. However, the preload element can also be implemented electronically or with magnets, and in any other possible design. The preload element is designed to provide a force comparable to the pressure acting on the active surface from the second line.

[0078] It is also advantageous if the threshold value of the pressure maintained in the second line is in a range between 2 bar and 20 bar.

[0079] Depending on the application and the design of the hydraulic actuator, the threshold value is set to reliably prevent cavitation in the hydraulic pump. A suitable threshold value can be determined for the specific circuit and application through simulations and / or tests. Generally, a lower threshold value is desirable to further reduce the pressure in the second line and thus generate stronger forces toward the second chamber (in the case of a differential cylinder, toward the bottom chamber or pulling forces) with the hydraulic actuator.

[0080] In variant designs, the deactivation circuit and the check valve can also be integrated into a common hydraulic component, which is implemented as a reverse pressure relief valve for limiting a pressure drop in the second line below the threshold value with a check function.

[0081] A possible design of the pressure reservoir explained above, which is connected to the first line and can be separated from the first line by the shut-off valve, will be described in more detail here. It is advantageous if the pressure reservoir contains a compressible element that can be compressed to hold pressurized hydraulic fluid in the pressure reservoir and expands when hydraulic fluid is withdrawn from the pressure reservoir.

[0082] The compressible element is typically designed as a gas bladder contained within a pressure vessel. Alternatively, the compressible element can also be implemented, for example, as a spring-loaded membrane or any other design variant.

[0083] The minimum pressure provided by the hydraulic fluid supply represents the lower pressure level that can exist in the first line, the first chamber, and the pressure reservoir. In the case of a differential cylinder as a hydraulic actuator component, for example, this pressure level occurs when the differential cylinder is fully extended. This is when the total internal volume of the hydraulic actuator is at its maximum because the chamber with the larger cross-section is large and the chamber with the smaller cross-section is small. In the case of a gas bubble as a spring element in the pressure reservoir, this gas bubble is expanded. The highest pressure in the pressure reservoir occurs when the differential cylinder is fully retracted. This is when the total internal volume of the hydraulic actuator is at its minimum because the chamber with the smaller cross-section is large and the chamber with the larger cross-section is small.In the case of a gas bubble as a compressible element in the pressure reservoir, this gas bubble is compressed within the pressure reservoir. In this situation, the maximum differential volume of the hydraulic actuator, or differential cylinder, is absorbed by the pressure reservoir.

[0084] Preferably, the compressible gas bubble is formed with nitrogen, which is filled into the pressure reservoir during initial commissioning of the hydraulic actuator. Particularly preferably, the gas bubble and the hydraulic fluid in the pressure reservoir are separated from each other by a separating element.

[0085] A diaphragm or a piston can be used as the separating element. During operation of the hydraulic actuator, the pressure in the pressure accumulator varies within a pressure range, for example, between 20 bar and 45 bar. When the cylinder is fully extended, the second chamber with the (larger) second chamber cross-section is large, and the pressure in the pressure reservoir is, for example, on the order of 20 bar. When the cylinder is fully retracted, the second chamber with the (larger) second chamber cross-section is particularly small, and the differential volume that must be absorbed by the pressure reservoir is particularly large. The pressure in the pressure reservoir is then, for example, on the order of 45 bar.

[0086] In principle, the pressure in the pressure reservoir is subject to various influences. Temperature changes, in particular, can cause changes in the pressure level due to thermal expansion.

[0087] The size of the gas bubble can be used to adjust the pressure characteristic of the pressure reservoir. During initial commissioning, the pressure reservoir is preferably (fully) filled with nitrogen (or another gas) such that the gas bubble pressure is below the minimum pressure to which the hydraulic actuator is subjected by the hydraulic fluid supply. The pressure reservoir must always be able to supply hydraulic fluid in the working pressure range. The minimum pressure from the hydraulic fluid supply then compresses the gas bubble in the pressure reservoir, and a quantity of hydraulic fluid enters the pressure reservoir. The initial filling pressure, with which the gas bubble is created in the pressure reservoir before the hydraulic actuator is commissioned, must always be lower than the lower working pressure.In the case described above, where the working pressure range is between 20 bar and 45 bar, the filling pressure can be, for example, 18 bar.

[0088] It is particularly preferred if at least one pressure sensor is arranged on the first line, with which a pressure in the first line can be measured, wherein the hydraulic actuator has a control unit with which the hydraulic fluid supply can be controlled such that the pressure in the first line can be regulated to a desired value. The control unit can be designed independently for the described actuator or be part of a higher-level control component on which, for example, a special software component for the operation of the hydraulic actuator described here is stored.

[0089] The control unit uses a pressure signal from a pressure sensor on the first line, preferably in combination with control signals received from a controller for controlling the hydraulic actuator. The controller includes, for example, a control lever with which the hydraulic actuator can be controlled by an operator. In operating situations of the hydraulic actuator in which the first line or the first chamber is pressurized with hydraulic fluid and in which the shut-off valve is closed to separate the pressure reservoir from the first line, the pressure in the first line is preferably monitored.

[0090] In such situations, pressure is built up in the first line and the first chamber, particularly with the help of the hydraulic pump, in order to pump hydraulic fluid from the second chamber into the first chamber. This occurs, for example, when a pulling force is to be generated with the hydraulic actuator component. Pressure reduction in the first line and the first chamber in such a situation is preferably also carried out with the hydraulic pump, by pumping the hydraulic fluid back into the second line or the second chamber.

[0091] If the desired pressure build-up and / or pressure reduction in the first line or the first chamber cannot be generated with the hydraulic pump alone, or if the pressure builds up faster than desired, the amount of hydraulic fluid in the first line and the first chamber is preferably adjusted via the supply connection. Additional hydraulic fluid can optionally be supplied to the hydraulic actuator via the supply connection and excess hydraulic fluid is discharged via the supply connection. When hydraulic fluid is pumped from the second chamber to the first chamber, a portion of the hydraulic fluid is usually discharged via the supply connection because the volume of the second chamber is greater than the volume of the first chamber by the differential volume. If the shut-off valve were open, this differential volume would be absorbed by the pressure reservoir.Because the shut-off valve is closed, the differential volume must be discharged in another way. This is preferably done via the supply port. When hydraulic fluid is pumped back from the first chamber to the second chamber, additional hydraulic fluid is usually supplied to the hydraulic actuator through the supply port. The volume of the second chamber is larger than the volume of the first chamber by the differential volume. Because the shut-off valve is closed, the differential volume cannot be provided from the pressure reservoir. The differential volume is preferably provided via the supply port.

[0092] The hydraulic fluid supply connected to the supply connection preferably has (as already described above) a power branch, with which pressurized hydraulic fluid can be provided to the hydraulic actuator in a targeted manner from a central feed pump. This power branch preferably has a valve drive with which the supply or intake of hydraulic fluid can be controlled. The valve drive can preferably be used to regulate the pressure in the first line of the hydraulic actuator with the power branch. The described control of the hydraulic actuator is preferably configured to actuate the valve drive of the power branch in order to regulate the pressure in the first line of the hydraulic actuator.

[0093] Controlling the hydraulic actuator in a second operating mode with the shut-off valve closed is more complex than in the (usual) first operating mode with the shut-off valve open. Furthermore, in the second operating mode with the shut-off valve closed, recuperation with the hydraulic actuator is not possible or only possible to a limited extent. This is especially true because (as described) hydraulic fluid must be regularly added or removed via the supply connection. This leads to power losses that cannot be channeled to recuperation.

[0094] It is particularly preferred if the hydraulic actuator has a changeover switch with which the controllable shut-off valve can be shut off when an operation of the hydraulic actuator is provided in which the first chamber of the hydraulic actuator component is to be subjected to a pressure which can be greater than an operating pressure of the accumulator.

[0095] The changeover switch can, for example, be designed as a switch for an operator. If the hydraulic actuator is installed in a work machine, an operator can activate this switch, for example, if they want to generate particularly large pulling forces with the hydraulic actuator that cannot be generated during normal operation (with a pressure reservoir connected to the first line). This can be the case, for example, in the situation described above, in which the operator wants to support the work machine with the hydraulic actuator to initiate a tire change.

[0096] Preferably, the shut-off valve can also be opened again using the changeover switch if the second operating mode with the shut-off valve closed is to be ended. Preferably, the changeover switch does not act directly on the shut-off valve when switching back to an open shut-off valve. Preferably, during this switchback, the pressure in the first line is first adjusted to the pressure in the pressure reservoir. Before the shut-off valve is opened again, the target and actual pressure in the first line should again be close to the actual pressure in the pressure reservoir. A signal from the changeover switch to switch back or open the shut-off valve is preferably first transmitted to a control system. The control system preferably controls the hydraulic fluid supply (e.g. the valve drive of a power branch) in order to adjust the actual pressure in the first line to the actual pressure in the pressure reservoir.If a pressure sensor on the first line detects that the actual pressure in the first line corresponds to the actual pressure in the pressure reservoir, the signal to switch back or to open the shut-off valve is only implemented. A value for the actual pressure in the pressure reservoir can preferably be stored in a controller. The actual pressure is preferably measured with the pressure sensor on the first line before the shut-off valve is closed. When the shut-off valve is open, the pressures in the first line and the pressure reservoir correspond, so that the pressure measured with the pressure sensor on the first line can be considered the pressure in the pressure reservoir. As soon as the shut-off valve is closed, the pressure in the pressure reservoir can no longer change.Before the shut-off valve is reopened, the pressure in the first line is preferably adjusted to the pressure that existed in the first line when the shut-off valve was closed and which, for this reason, is also present in the pressure reservoir.

[0097] By adjusting the pressure in the first line to the pressure in the pressure reservoir before opening the shut-off valve, a sudden adjustment of the pressures in the pressure reservoir and the first line can be prevented. This can prevent noise, shocks, or even damage to the hydraulic actuator.

[0098] Particularly preferably, the hydraulic actuator comprises a controller for actuating the hydraulic actuator component and a control unit, wherein the control unit is configured to close the controllable shut-off valve when a control command is received via the controller, according to which the first chamber of the hydraulic actuator component is to be subjected to a pressure which may be greater than a maximum pressure of the accumulator.

[0099] It is particularly preferred if at least one pressure sensor is arranged on the second line, which is connected to a control unit with which a pressure drop in the second line and at the second connection can be monitored, wherein the control unit is configured to close the controllable shut-off valve in response to a pressure drop in the second line.

[0100] It is also preferred if a torque sensor is arranged on a pump drive of the hydraulic pump, which is connected to a control unit with which a torque applied to the hydraulic pump by the pump drive can be monitored, wherein the control unit is designed to close the controllable shut-off valve in response to an increase in the torque when hydraulic fluid is delivered from the second line (7b).

[0101] A torque sensor can be designed in various ways. The torque sensor can be a physical torque sensor that measures the torque of the pump drive, for example, via strain gauges on an output shaft of the pump drive. A torque sensor can also be configured virtually, for example, via current and voltage sensors on an electrical supply of the pump drive. Preferably, the torque in the pump drive can be determined based on the operating voltage and / or the operating current.

[0102] In this embodiment, preferably no (manually operable) switch is provided to close the shut-off valve. Instead, the control unit is configured to use the control commands to detect a situation in which closing the shut-off valve is required and then first control the shut-off valve accordingly, close the shut-off valve, and then switch to a second operating mode with the shut-off valve closed. Subsequently, the control unit operates the other components (hydraulic pump, hydraulic fluid supply, etc.) in such a way that the control commands specified by the controller are implemented.

[0103] Particularly preferred in this design variant is that the switch back to the first operating mode occurs automatically. For this purpose, it must be detected that the operator is no longer sending control commands that require the second operating mode. For this detection, the torque generated by the pump drive can also be used, for example, especially in combination with a direction of rotation signal.

[0104] In further embodiments, pressure signals can also be evaluated to detect a situation that allows a switch back to the first operating mode. Preferably, a pressure sensor is located on the second line. This pressure sensor is preferably also connected to the control unit to monitor the pressure in the second line.

[0105] If a torque on the pump drive and / or a pressure in the second line detects that the hydraulic actuator is again generating forces that require hydraulic fluid to be pumped into the second chamber, it is possible to switch back to the first operating mode or the shut-off valve can be opened again.

[0106] In further preferred embodiments of the hydraulic actuator, a closable holding valve is arranged between the first chamber and the first line, with which the first chamber can be closed in order to hold the hydraulic actuator component in a defined position.

[0107] With a closable holding valve located here, the pressure built up in the first chamber can be maintained without the need for the constant application of force with the hydraulic pump. A holding valve for closing the first chamber is advantageous in conjunction with the shut-off valve for isolating the pressure reservoir because the shut-off valve can generate higher pressures in the first chamber, and these pressures can be maintained with the holding valve.

[0108] Furthermore, a mobile work machine with a hydraulic actuator according to the invention is described here. The mobile work machine can be a construction machine, agricultural machine, municipal machine, or forestry machine. The hydraulic actuator can be used in particular for raising and lowering an excavator arm or the arm of a telehandler.

[0109] Also to be described here is a method for generating a force with a described hydraulic actuator by pressurizing the first chamber of the hydraulic actuator component, comprising the following steps: a) isolating the pressure reservoir from the first line by closing the controllable shut-off valve; b) regulating an increased pressure beyond the designed operating pressure of the accumulator in the chamber; and c) operating the hydraulic pump to pump hydraulic fluid from the second chamber and the second line into the first chamber and the first line.

[0110] It should be noted that the particular advantages and design features described in connection with the hydraulic actuator component described above are also applicable to the method. The method describes the operation of the described hydraulic actuator component. In step a), a change is made from a first operating mode (with a connected pressure reservoir) to a second operating mode in which the pressure reservoir is separated from the first line. Steps b) and c) take place during operation of the hydraulic actuator component in the second operating mode, wherein the hydraulic actuator is used to generate forces that require a pressure in the first chamber that is above the designed operating pressure of the accumulator. The designed operating pressure is, in particular, the maximum pressure for which the accumulator is designed.Such forces are in particular (larger) pulling forces that are generated with the hydraulic actuator component.

[0111] In the second operating mode, the pressure adjustment according to step b) and the operation of the hydraulic pump according to step c) preferably occur at least partially overlapping in time. Preferably, in step b) and / or step c), the hydraulic fluid supply is additionally operated to regulate the pressure in the first line. In this case (depending on the operating situation), hydraulic fluid is preferably supplied from the hydraulic fluid supply to the hydraulic actuator and / or taken up from the hydraulic actuator (from the first line).

[0112] Particularly preferably, the operating method further comprises switching back to a first operating mode with a pressure reservoir connected to the first line, comprising the following steps: d) adjusting the pressure in the first line to the pressure in the pressure reservoir; and e) opening the shut-off valve.

[0113] Preferably, the operating method for operating the described hydraulic actuator is controlled by a control unit of the hydraulic actuator. In embodiments (as also described above in connection with the hydraulic actuator), individual steps of the method are initiated manually by an operator – e.g., switching from the first operating mode to the second operating mode by actuating a changeover switch. In embodiments, it is also possible for the described method steps and the described switching to take place automatically with the aid of a control unit if control commands are specified by an operator or a higher-level controller that require a change from the first operating mode to the second operating mode and, if necessary, back to the first operating mode.

[0114] The invention and the technical context of the invention are explained in more detail below with reference to the figures. The figures show preferred embodiments to which the invention is not limited. It should be noted in particular that the figures, and in particular the proportions shown in the figures, are only schematic. Figs. 1 to 6 show, by way of example, a circuit diagram of the hydraulic actuator described here in various operating states. First, the structure applicable to all figures is explained. Subsequently, the operating states shown in the figures are discussed in detail.

[0115] Figures 1 to 4 show different design variants of the described hydraulic actuator 1.

[0116] Figures 5 to 8 illustrate the operation of the described hydraulic actuator 1 with a differential cylinder as the hydraulic actuator component 3 in various operating situations. Figure 9 shows an alternative embodiment of a described hydraulic actuator 1.

[0117] The basic structure of the described hydraulic actuator 1 is similar to all of Figs. 1 to 9. In particular, Figs. 1 to 4 illustrate various concepts for controlling the described hydraulic actuator 1. The mode of operation of the described hydraulic actuator 1 is explained in generalized form (for all embodiments illustrated in Figs. 1 to 4) with reference to Figs. 5 and 8. Fig. 9 shows an alternative embodiment which differs from the embodiments shown in Figs. 1 to 8, particularly with regard to the hydraulic layout. The features of the embodiment shown in Fig. 9 are transferable to all embodiments shown in Figs. 1 to 8.

[0118] The hydraulic actuator 1, designed as a differential cylinder according to all embodiments, has a piston 26 guided in a cylinder body 25, to which a rod 27 is connected on one side. Above the piston 26, in the cylinder body 25, there is a first chamber 4a, also referred to as an annular chamber. Below the piston 26, in the cylinder body 25, there is a second chamber 4b, also referred to as a bottom chamber. The first chamber / annular chamber 4a has a first chamber cross-section / annular chamber cross-section 8a. The second chamber / bottom chamber 4b has a second chamber cross-section / bottom chamber cross-section 8b. The second chamber cross-section 8b is larger than the first chamber cross-section 8a, resulting in a differential volume 28 that is defined by the thickness or cross-section of the rod 27. The differential volume 28 is shown as an example in Fig. 1.

[0119] The hydraulic actuator 1 has a hydraulic pump 2, which has a first connection 6a and a second connection 6b and which is preferably four-quadrant capable, i.e., can be operated both as a motor and as a generator in both directions from the first connection 6a to the second connection 6b and vice versa. The annular chamber 4a is connected to the first connection 6a of the hydraulic pump 2 via a first line 7a. The bottom chamber 4b is connected to the second connection 6b of the hydraulic pump 2. Connected to the first line 7a is a pressure reservoir 5, in which a compressible element 17 is present, which is formed in particular by a gas bubble in the pressure reservoir, which was initially filled with gas pressure (before initial commissioning of the hydraulic actuator 1).

[0120] In the second line 7b there is a holding valve 22 which can be closed to interrupt the second line 7b and maintain a pressure in the bottom chamber 4b without having to maintain a pressure difference between the second port 6b and the first port 6a on the hydraulic pump 2.

[0121] A supply connection 15 is arranged on the first line 7a and is connected to a hydraulic fluid supply 16. The hydraulic fluid supply 16 here comprises a central feed pump 21 with which pressurized hydraulic fluid can be provided. The hydraulic fluid supply 16 preferably further comprises a power branch 36 with which the pressurized hydraulic fluid provided by the central feed pump 21 can be delivered in a targeted manner to the supply connection 15. The figures each show the part of a power branch 36 which is responsible for supplying the hydraulic actuator 1 described here with hydraulic fluid. A plurality of hydraulic consumers are preferably connected to the power branch 36. The power branch 36 preferably comprises a plurality of valves or stages, which can, for example, be arranged next to one another.The hydraulic actuator 1 described here is preferably one of these consumers and is connected to a stage / valve of the power branch 36. The power branch 36 preferably has a separate valve assembly for each connected consumer, whereby the valve assembly of the power branch 36 responsible for the hydraulic actuator 1 described here is shown schematically here. The power branch 36 has two connections A and B for each consumer, at which hydraulic fluid can be optionally supplied or taken in. The two connections A and B can, for example, be connected directly to the two chambers of a differential cylinder, which can then be controlled with the power branch 36. In the circuit shown here, only one of the two connections (here, for example, connection B) is connected to the supply connection 15 of the hydraulic actuator 1.With the power branch 36, the pressure in the first line 7a and thus in the first chamber 4a can be specifically adjusted. This is achieved by selectively supplying or receiving hydraulic fluid at the supply port 15 in order to control the pressure in the first line 7a and in the first chamber 4a.

[0122] The hydraulic fluid supply 16 or the central feed pump 21 accesses a main reservoir 24 to provide hydraulic fluid. Excess hydraulic fluid from all components of the overall system (in particular also from all components of the hydraulic actuator 1) is preferably returned to this main reservoir 24. The power splitter 36 preferably has a port P, at which the power splitter 36 is supplied with pressurized hydraulic fluid provided by the feed pump 21. The power splitter 36 preferably has a port T, which is connected to the main reservoir and to which excess hydraulic fluid can be discharged. Leakage flows 29 from components, such as the hydraulic pump 2, also preferably flow into the main reservoir 24.The hydraulic fluid supply 16 fills the lines 7a, 7b and the chambers 4a, 4b and the pressure reservoir 5 of the hydraulic actuator 1. After an initial filling for operation (which occurs, for example, upon activation of the work machine in which the hydraulic actuator 1 is provided), normally (in a regular first operating mode of the hydraulic actuator) no further hydraulic fluid is transferred via the supply connection 15 into the lines 7a, 7b and the chambers 4a, 4b of the hydraulic actuator 1. The hydraulic actuator 1 operates with the amount of hydraulic fluid present in the lines 7a, 7b, the pressure reservoir 15 and the chambers 4a, 4b. However, a minimum pressure is maintained in the first line 7a via the supply connection 15.If the pressure in the first line 7a falls below a predetermined value, the line 7a (and thus also the other components, the second line 7b, as well as the chambers 4a, 4b, and the pressure reservoir 5) is refilled. Furthermore, a controllable pressure relief valve 18 is preferably arranged on the first line 7a, via which hydraulic fluid can be drained from the first line 7a if the pressure in the first line 7a is too high or if the hydraulic actuator 1 is deactivated. Both lines 7a and 7b each have pressure relief valves 20a, 20b, via which hydraulic fluid can be drained into the main reservoir 24 if there is excess pressure in the respective line 7a, 7b.The pressure relief valves 20a, 20b are preferably arranged on the lines 7a, 7b in the immediate vicinity of the chambers 4a, 4b, so that no further components (in particular no valves blocking the lines 7a, 7b) are arranged between the pressure relief valves 20a, 20b and the chambers 4a, 4b, and the pressure relief valves 20a, 20b also prevent overpressure in the chambers 4a, 4b.

[0123] The first line 7a and the second line 7b are connected to each other via the bypass line 9. The connection of the bypass line 9 to the first line 7a and the second line 7b is preferably arranged at connection points A, B, which enables a rapid and reliable flow of hydraulic fluid from the first line 7a, through the bypass line 9 and the second line 7b to the hydraulic pump 2 when the bypass line 9 is open, in order to prevent cavitation in the hydraulic pump 2. Located in the bypass line 9 is the deactivation circuit 11, which includes a separating valve 12 that interrupts the bypass line 9 as long as a pressure tapped at the second line 7b is greater than the threshold value. The pressure tap X is preferably made on the second line 7b near the second connection 6b of the hydraulic pump 2.As soon as the pressure at the second port 6b of the hydraulic pump 2 falls below the threshold value, the deactivation of the bypass line 9 with the deactivation circuit 11 is ended and the bypass line 9 is released. Hydraulic fluid from the first line 7a flows through the bypass line 9 in order to keep the pressure in the second line 7b above the threshold value. The pressure in the second line 7b preferably acts on an active surface 13 of the isolating valve 12 and works there against a preload element 14. The opening and closing of the isolating valve 12 is determined by the interaction of the pressure acting on the active surface 13 and a force of the preload element 14 working against this pressure. As soon as the pressure falls below an upper limit value starting from the top, the isolating valve 12 begins to open. When a lower limit value is reached, the isolating valve 12 is then fully open.Between the upper limit and the lower limit, the isolating valve 12 preferably opens proportionally, so that the flow-through cross-section of the isolating valve 12 expands increasingly. The limit values ​​and the proportional opening behavior, together with the structure of the lines 7a, 7b, 9, are coordinated so that the threshold value of the pressure in the second line 7b is maintained as intended. The interaction of the active surface 13 and the biasing element 14 for controlling the isolating valve 12 of the deactivation circuit 11 can be implemented directly on a valve element for closing and opening a flow-through cross-section itself or on a pilot, wherein the pilot in turn controls a valve element of the isolating valve 12. Such control is called "piloted control." In principle, a wide variety of approaches are possible for implementing the deactivation circuit 11.

[0124] In the bypass line 9, a check valve 10 is preferably provided in addition to the deactivation circuit 11. The check valve 10 generally closes when the pressure in the second line 7b is greater than the pressure in the first line 7a. In the direction from the second line 7b to the first line 7a, the check valve 10 is preferably arranged upstream of the deactivation circuit 11. The deactivation circuit 11 is therefore only subjected to pressure when the pressure in the second line 7b is reduced by the hydraulic pump 2 below the pressure in the first line 7a, i.e., when hydraulic fluid is pumped out of the annular chamber 4a and into the bottom chamber 4b in order to generate a pulling force 33 with the hydraulic actuator 1 of the hydraulic actuator component 3, which is designed here as a differential cylinder. Due to its design, the deactivation circuit 11 also regularly has a leakage flow 29.

[0125] In the event that particularly high pulling forces are to be generated with the described hydraulic actuator 1, a shut-off valve 35 is provided with which the pressure reservoir 5 can be separated from the first line 7a. The shut-off valve 35 is designed as a 2 / 2-way valve. In a first open switching position of the shut-off valve 35, the first line 7a is connected to the pressure reservoir 5 to enable a flow of hydraulic fluid from the first line 7a to the pressure reservoir 5 and a flow of hydraulic fluid from the pressure reservoir 5 to the first line 7a.In a second closed switching position of the shut-off valve 35, the first line 7a is separated from the pressure reservoir 5 in such a way that neither hydraulic fluid from the first line 7a can pass to the pressure reservoir 5 nor hydraulic fluid from the pressure reservoir 5 to the first line 7a, thus preventing the flow of hydraulic fluid in both directions. By separating the pressure reservoir 5 from the first line 7a, the hydraulic actuator 1 can be switched to a second operating mode and operated as described above.

[0126] In the embodiment according to Fig. 1, a changeover switch 37 is provided with which the shut-off valve 35 can be closed and opened again. If an operator wishes to generate movements and forces with the hydraulic actuator 1 that require pressurization of the first chamber 4a, the changeover switch 37 must first be actuated. The controller 38 can then be actuated to generate corresponding movements and forces with the hydraulic actuator 1. Control commands from the controller 38 are preferably converted by the control unit 39 into control signals to the pump drive 19 and to a valve drive 41 of the power branch 36. To generate the control signals, the control unit 39 preferably also takes into account the pressure in the first line 7a, which is monitored by a pressure sensor 40 on the first line 7a, wherein the pressure sensor 40 outputs a sensor signal to the controller 38.Control commands, sensor signals and control signals are shown in the figures as dashed arrows.

[0127] In the embodiment according to Fig. 2, the changeover switch 37 is omitted. In this embodiment, the control unit 39 is configured to detect when a control command is generated by the controller 38 which requires pressurization of the first chamber 4a, which in turn requires separation of the pressure reservoir 5 from the first line 7a. In this case, the control unit 39 is configured to transmit control signals to the controllable shut-off valve 35, which enable the shut-off valve 35 to be closed. For the automatic control of the controllable shut-off valve 35 according to the embodiment according to Fig. 2, additional information about the state of the hydraulic actuator 1 is helpful. In particular, for suitable control of the hydraulic pump 2 and the valve drive 41 of the power branch 36, it is helpful to monitor the pressures acting in the lines 7a, 7b and at the hydraulic pump 2.Based on these pressures, external loads acting on the hydraulic actuator component 3 can be detected. The shut-off valve 35 is preferably closed when the available control commands from the controller 38 and the pressures in the lines 7a, 7b indicate that closing the shut-off valve 35 is necessary to implement the control commands. An additional pressure sensor 40 is provided on the second line 7b to monitor the pressure in the second line 7b.

[0128] In the embodiment variant shown in Fig. 3, instead of the pressure sensor 40 on the second line 7b, a torque sensor 23 is arranged on the pump drive 19. This torque sensor 23 is configured to transmit sensor signals to the control unit 39. By monitoring the torque of the pump drive 19, the resistances against which the hydraulic pump 2 delivers hydraulic fluid can be detected. The torque sensor 23 on the pump drive 19 is an alternative to the pressure sensor 40 on the second line 7b shown in Fig. 2.

[0129] In the embodiment variant according to Fig. 4, a holding valve 22 is arranged in the first line 7a. This holding valve 22 can separate the first chamber 4a from the first line 7a. This can be done in particular if pressures have built up in the first chamber 4a and the hydraulic actuator component 3 or the hydraulic actuator 1 is intended to thereby hold a force.

[0130] 1 to 4 each show an operating situation in which the hydraulic actuator 1 is filled with the hydraulic fluid supply 16. In each case, a conveying direction of hydraulic fluid at the supply connection 15 into the hydraulic actuator 1 or in particular into the first line 7a is shown. Further operating situations of the hydraulic actuator 1 are now shown in FIGS. 5 to 8, wherein these figures are generalized with regard to the special features for controlling the shut-off valve 35, the power branch 36 and the hydraulic pump 2. The operating situations shown in FIGS. 5 to 8 can be transferred to all hydraulic actuators 1 according to FIGS. 1 to 4 and 9. Arrows indicate conveying directions 30 and flows 32 driven by pressure gradients. Lines 7a, 7b and chambers 4a, 4b under pressure are each marked.

[0131] According to the situation shown in Figs. 1 to 4, in particular an initial filling of the lines 7a, 7b and chambers 4a, 4b as well as the pressure reservoir 5 takes place via the supply connection 15. Hydraulic fluid is conveyed into the system at the supply connection 15 in the conveying direction 30. The compressible element 17, preferably configured as a gas bubble, in the pressure reservoir 5 is compressed so that the pressure reservoir 5 is filled and a predetermined initial pressure is established in the line 7a. The level of the predetermined initial pressure is preferably dependent on the respective position of the hydraulic actuator component 3. When the hydraulic actuator component 3 is fully retracted, the total volume of the annular chamber 4a and the base chamber 4b is particularly small and the differential volume 28 must be completely introduced into the pressure reservoir 5 so that the initial pressure is high.When the hydraulic actuator component 3 is fully extended, the total volume of the annular chamber 4a and the bottom chamber 4b is particularly large, and the differential volume 28 is entirely contained in the bottom chamber 4b. Not much hydraulic fluid needs to be introduced into the pressure reservoir 5, so the initial pressure is low.

[0132] Fig. 5 shows a situation in which the hydraulic pump 2 is motor-driven by the pump drive 19, and hydraulic fluid is pumped from the annular chamber 4a into the bottom chamber 4b to extend the hydraulic actuator component 3. The hydraulic pump 2 operates as a pump. The pressure in the bottom chamber 4b or the second line 7b is higher than the pressure in the first annular chamber 4a or the first line 7a. The check valve 10 blocks the bypass line 9. The deactivation circuit 11 is not loaded. Flow through the bypass line 9 is prevented.

[0133] Fig. 6 shows a situation in which the hydraulic actuator component 3 is held in a specific position. For this purpose, the holding valve 22 is closed. The pressure in section 31 of the second line 7b between the holding valve 22 and the hydraulic pump 2 can now drop below the pressure in the first line 7a. Significant flows or pumping of hydraulic fluid do not occur in the situation illustrated in Fig. 6. If the pump were inadvertently activated in this state to pump hydraulic fluid from 7a to 6a, it would be protected from cavitation.

[0134] Fig. 7 now shows a situation in which the hydraulic pump 2 is driven in a generator-like manner by a flow 32 of hydraulic fluid from the bottom chamber 4b into the annular chamber 4a. An external load 34 presses the hydraulic actuator component 3, which is designed here as a differential cylinder. This creates a pressure in the bottom chamber 4b, which drives the flow 32 into the annular chamber 4a. Energy from the pressure of the hydraulic fluid is converted into a generator by the hydraulic pump 2 and the pump drive 19 and stored in an energy storage device (not shown here).

[0135] According to Fig. 8, the deactivation circuit 11 in the bypass line 9 and the controllable shut-off valve 35 now come into play in order to generate a pulling force 33 with the hydraulic actuator component 3 designed as a differential cylinder. Hydraulic fluid is actively pumped out of the bottom chamber 4b and the second line 7b and into the first line 7a and the annular chamber 4a by the hydraulic pump 2. The pressure in the bottom chamber 4b and the second line 7b is reduced well below the pressure in the annular chamber 4a and the first line 7a. This creates a pulling force 33 on the hydraulic actuator component 3. The check valve 10 in the bypass line 9 is ineffective. However, the pressure in the second line 7b is still above the threshold value, so that the deactivation circuit 11 keeps the bypass line 9 closed.Due to the closed shut-off valve 35, the pressure in the first line 7a and the first chamber 4a can be increased beyond the maximum permissible pressure for the pressure reservoir 5. The pump drive 19 of the hydraulic pump 2 and the valve drive 41 of the power branch 36 are controlled as described above, particularly in connection with Figs. 1 to 4.

[0136] Fig. 9 shows an alternative embodiment of a described hydraulic actuator 1. All details of operation described in connection with Figs. 1 to 8 can be transferred to the embodiment of a hydraulic actuator 1 according to Fig. 9. In the embodiment according to Fig. 9, the ring side and the base side are interchanged. The first chamber 4a is the base chamber and the second chamber 4b is the ring chamber. The first chamber cross-section 8a is larger than the second chamber cross-section 8b. The pressure reservoir 5 is connected to the first line 7a, which here forms the base chamber line, in the same way as in the embodiment according to Figs. 1 to 6.

[0137] A differential volume 28, which results from the difference between the first chamber cross-section 8a and the second chamber cross-section 8b and which flows out of the first chamber 4a when the rod 27 of the differential cylinder is extended, does not flow at all through the hydraulic pump 2 into the pressure reservoir 5 in this embodiment. Instead, the differential volume 28 passes from the first chamber 4a via the sections of the first line 7a directly into the pressure reservoir 5. In such a hydraulic actuator 1, the hydraulic actuator component 3 can primarily generate pulling forces 33 in the differential cylinder when the hydraulic actuator component 3 or the rod 27 of the differential cylinder is retracted. In contrast, recuperation is possible when an external load pulls on the hydraulic actuator component 3, i.e. pulls out the rod 27 of the differential cylinder.Energy from the pressure of the hydraulic fluid can then be converted into a generator using the hydraulic pump 2 and the pump drive 19.

[0138] In this embodiment of the described actuator 1, the bypass line 9 also prevents undersupply of the hydraulic pump 2 and thus cavitation if the pressure in the second line 7b drops too sharply and reaches a threshold value from above, in which a bypass flow from the first line 7a into the second line 7b is established.

[0139] The deactivation circuit 11 and the controllable shut-off valve 35 make it possible for the hydraulic actuator 1 according to Fig. 9 to bring the pressure in the second line 7b below the pressure in the first line 7a despite the bypass line 9, and to increase the pressure in the first line 7a beyond the maximum pressure of the pressure reservoir 5. Therefore, the hydraulic actuator 1 according to Fig. 9 can also generate pushing forces.

[0140] List of reference symbols for hydraulic actuators

[0141] Hydraulic pump hydraulic actuator components a first chamber, annular chamber b second chamber, bottom chamber

[0142] Pressure reservoir a first connection b second connection a first line, annular chamber line b second line, bottom chamber line a first chamber cross-section, annular chamber cross-section b second chamber cross-section, bottom chamber cross-section

[0143] Bypass line 0 Check valve 1 Deactivation circuit 2 Isolating valve 3 Effective area 4 Preload element 5 Supply connection 6 Hydraulic fluid supply 7 Compressible element 8 Pressure relief valve 9 Pump drive 0a First pressure relief valve 0b Second pressure relief valve 1 Central feed pump 2 Holding valve 3 Torque sensor 4 Main reservoir 5 Cylinder body 6 (guided) piston 27 Rod

[0144] 28 Differential volume

[0145] 29 Leakage current

[0146] 30 Conveying direction

[0147] Section 31

[0148] 32 Current

[0149] 33 pulling force

[0150] 34 external load

[0151] 35 controllable shut-off valve

[0152] 36 Power split

[0153] 37 switches

[0154] 38 Control

[0155] 39 Control unit

[0156] 40 pressure sensor

[0157] 41 Valve actuator

[0158] A second connection point

[0159] B first connection point

[0160] X pressure tap

Claims

Claims 1. Hydraulic actuator (1) comprising at least one bidirectionally operable hydraulic pump (2) for conveying hydraulic fluid with two connections (6a, 6b), as well as with a hydraulic actuator component (3) with two hydraulic fluid chambers (4a, 4b) which can be supplied with hydraulic fluid to actuate the hydraulic actuator component (3), wherein the hydraulic fluid chambers (4a, 4b) are each connected via a line (7a, 7b) to the connections (6a, 6b) of the hydraulic pump (2), wherein a first line (7a) connects the first chamber (4a) to the hydraulic pump (2) and a second line (7b) connects the second chamber (4b) to the hydraulic pump (2), so that when one of the hydraulic fluid chambers (4a, 4b) is filled with hydraulic fluid, hydraulic fluid is simultaneously displaced from the other hydraulic fluid chamber (4b,4a) is effected and a volume of hydraulic fluid is moved by the hydraulic pump (2), - wherein a second chamber cross-section (8b) of the second chamber (4b) deviates from a first chamber cross-section (8a) of the first chamber (4a) and the first line (7a) is connected to a pressure reservoir (5) in which a differential volume (28) can be accommodated and provided, which occurs due to the different chamber cross-sections (8a, 8b) of the chambers (4a, 4b), wherein a compressible element (17) exists in the pressure reservoir (5), which element is compressible to accommodate pressurized hydraulic fluid in the pressure reservoir (5) and which expands when hydraulic fluid is withdrawn from the pressure reservoir (5), - wherein the first line (7a) further comprises a supply connection (15) via which the hydraulic actuator (1) is supplied with hydraulic fluid, wherein a hydraulic fluid supply (16) is connected to the supply connection (15), which maintains a minimum pressure in the first line (7a), - wherein a controllable shut-off valve (35) is arranged between the first line (7a) and the pressure reservoir (5), with which the pressure reservoir (5) can be separated from the first line (7a).

2. Hydraulic actuator (1) according to claim 1, wherein the hydraulic actuator component (3) is at least one differential cylinder.

3. Hydraulic actuator (1) according to one of the preceding claims, wherein the hydraulic fluid supply (16) can selectively deliver hydraulic fluid to the supply connection (15) or receive hydraulic fluid from the supply connection (15).

4. Hydraulic actuator (1) according to one of the preceding claims, wherein the first line (7a) and the second line (7b) are further connected to each other by a bypass line (9), wherein an undersupply of hydraulic fluid in the hydraulic pump (2) is prevented by the bypass line (9) by a bypass flow from the first line (7a) into the second line (7b) through the bypass line (9) when a pressure in the second line (7b) from above reaches a threshold value, - wherein a deactivation circuit (11) is arranged in the bypass line (9), with which the bypass line (9) can be interrupted when the pressure in the second line (7b) is above a threshold value.

5. Hydraulic actuator (1) according to one of the preceding claims, wherein a closable holding valve (22) is arranged between the second chamber (4b) and the second line (7b), with which the second chamber (4b) can be closed in order to hold the hydraulic actuator component (3) in a defined position.

6. Hydraulic actuator (1) according to one of the preceding claims, wherein in the bypass line (9) in addition to the deactivation circuit (11) a check valve (10) is provided, which can be opened in the direction from the first line (7a) towards the second line (7b), so that even at a pressure in the second line (7b) below the threshold value it is ensured that a pressure in the second line (7b) does not fall below the pressure in the first line (7a).

7. Hydraulic actuator (1) according to one of the preceding claims, wherein the hydraulic pump (2) is suitable for generator as well as motor operation.

8. Hydraulic actuator (1) according to one of the preceding claims, wherein the hydraulic pump (2) is connected to a pump drive (19) with which energy from the pressure and the volume flow of the hydraulic fluid through the hydraulic pump (2) can be converted and stored in an energy storage device.

9. Hydraulic actuator (1) according to one of the preceding claims, wherein at least one pressure sensor (40) is arranged on the first line (7a), with which a pressure in the first line (7a) can be measured, wherein the hydraulic actuator (1) has a control unit (39) with which the hydraulic fluid supply (16) can be controlled such that the pressure in the first line (7a) can be regulated to a desired value.

10. Hydraulic actuator (1) according to one of the preceding claims, comprising a changeover switch (37) with which the controllable shut-off valve (35) can be shut off when an operation of the hydraulic actuator (1) is provided in which the first chamber (4a) of the hydraulic actuator component (3) is to be subjected to a pressure which can be greater than an operating pressure of the pressure reservoir (5).

11. Hydraulic actuator (1) according to one of the preceding claims, comprising a control (38) for actuating the hydraulic actuator component (3) and a control device (39) which is designed to close the controllable shut-off valve (35) when a control command is received via the control (38), according to which the first chamber (4a) of the hydraulic actuator component (3) is subjected to a pressure which may be greater than a maximum pressure of the pressure reservoir (5).

12. Hydraulic actuator (1) according to one of the preceding claims, wherein at least one pressure sensor (40) is arranged on the second line (7b), which is connected to a control unit (38) with which a pressure drop in the second line (7b) and at the second connection (6b) can be monitored, wherein the control unit (38) is configured to close the controllable shut-off valve (35) in response to a pressure drop in the second line (7b).

13. Hydraulic actuator (1) according to one of the preceding claims, wherein a torque sensor (43) is arranged on a pump drive (19) of the hydraulic pump (2), which torque sensor is connected to a control unit (38) with which a torque applied to the hydraulic pump (2) by the pump drive (19) can be monitored, wherein the control unit (38) is configured to close the controllable shut-off valve (35) in response to an increase in the torque when hydraulic fluid is delivered from the second line (7b).

14. Mobile work machine with a hydraulic actuator (1) according to one of the preceding claims.

15. A method for generating a force with a hydraulic actuator (1) according to one of claims 1 to 13 by pressurizing the first chamber (4a) of the hydraulic actuator component (3), comprising the following steps: a) separating the pressure reservoir (5) from the first line (7a) by closing the controllable shut-off valve (35); b) regulating an increased pressure beyond the designed operating pressure of the pressure reservoir (5) into the first chamber (4a); and c) operating the hydraulic pump (2) to pump hydraulic fluid (2) from the second chamber (4b) and the second line (7b) into the first chamber (4a) and the first line (7a).

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