Hydraulic circuit and method for hydraulically controlling gripper arms of an attachment for an industrial truck

Abstract

The invention relates to a hydraulic circuit and a corresponding method for the hydraulic control of gripper arms of an attachment for an industrial truck. The hydraulic circuit 100 has a first hydraulic line L1 for connecting a first chamber 22 of a clamping cylinder 220 to the hydraulic system of the industrial truck. A pressure reducing valve DM1 built into the first hydraulic line L1 serves to limit the pressure on the front side of the pressure reducing valve and in the first chamber 22 of the clamping cylinder 220 to a predetermined pre-clamping pressure. In order to ensure that the attachment and the hydraulic circuit can be connected to the hydraulic supply of the industrial truck during readjustment orIn order to make re-clamping of the goods to be picked up unnecessary, a first pressure accumulator DS1 is installed in the first hydraulic line L1 for receiving and storing pressurized hydraulic fluid from the first hydraulic line L1 and for providing the pressurized hydraulic fluid into the first hydraulic line at a later time.

Description

[0001] The present invention relates to a hydraulic circuit and a corresponding method for hydraulically controlling gripper arms of an attachment for an industrial truck (FFZ). Furthermore, the invention relates to such an industrial truck and a computer program product containing software code elements with which the steps of the method can be carried out.

[0002] Industrial trucks, such as forklifts, are known in the art. Attachments typically have at least two gripper arms mounted on them. The gripper arms are actuated to grip a product to be picked up by the industrial truck with a clamping force. The product to be picked up can be, for example, paper rolls, tissue rolls, cardboard goods, concrete pipes, paper bales, cellulose bales, or household appliances, etc.

[0003] The gripper arms can be arranged on the attachment so that they can move linearly and / or pivot or rotate.

[0004] To ensure secure gripping of the goods, a specific clamping force must be set. Only with the appropriate clamping force can the goods be prevented from slipping during lifting and damage to the goods caused by excessive force be avoided.

[0005] This can be problematic when the industrial truck is used to handle goods of varying weights. For safety reasons, the clamping force is traditionally adjusted so that the heaviest possible goods can be safely handled. However, this has the disadvantage that all lighter goods are potentially gripped with excessive clamping force, potentially resulting in damage. Excessive clamping force, which may be exerted by the gripper arms on the goods, can lead to severe deformation of the goods and / or damage to their surface.

[0006] In view of this problem, various hydraulic valve circuits are known in the prior art which are designed to determine an automatically variable gripping force adapted to the weight of the goods to be picked up and to apply this force during handling of the goods to be picked up; see, for example, European patent specifications EP 3 524 568 B1 or EP 0 995 557 B1. The latter discloses an electrical control arrangement for operating a load handling clamp with selectively closable and openable load engagement surfaces. At least one of these is controllable by means of a fluid drive which applies a gripping force to a load as a result of a manually operated load clamp selector valve. The control arrangement includes a sensor for sensing a weight of the goods to be picked up, at least one fluid valve arrangement for regulating the magnitude of said gripping force, and an electrical control device for controlling said fluid valve arrangement.The control device is operable to cause said valve arrangement to controllably increase said gripping force as a result of an increase in the weight to be picked up without simultaneous manual activation of the load clamp selector valve and without simultaneous exertion of a lifting force on said load.

[0007] However, the known solutions in the prior art have the disadvantage that they always require an active connection to the hydraulic supply of the industrial truck for the aforementioned readjustment of the clamping force depending on the weight of the goods to be picked up. This makes little sense from an energy perspective.

[0008] The invention is based on the object of developing a known hydraulic circuit, a known method and a known computer program for the hydraulic control of gripper arms of an attachment for an industrial truck as well as a corresponding known industrial truck itself in such a way that a coupling of the attachment and the hydraulic circuit to the hydraulic supply of the industrial truck becomes unnecessary during the readjustment or re-clamping of the goods to be picked up.

[0009] This object is achieved for the hydraulic circuit by the subject matter of patent claim 1. Accordingly, the hydraulic circuit according to the invention is characterized by a first pressure accumulator installed in the first hydraulic line for receiving and storing pressurized hydraulic fluid from the first hydraulic line and for providing the pressurized hydraulic fluid into the first hydraulic line with a time delay.

[0010] The pressure accumulator, which is part of the hydraulic circuit, makes it possible to decouple the attachment and the hydraulic circuit from the industrial truck's hydraulic supply during readjustment of the clamping force. The hydraulic pressure required for readjusting the clamping force is then no longer supplied from the industrial truck's hydraulic supply, as is traditionally the case, but from the first pressure accumulator, and the second pressure accumulator collects the draining hydraulic fluid.

[0011] On the one hand, this saves energy because the hydraulic supply of the FV, which is energy-intensive during operation, can be shut down during the readjustment. Furthermore, the readjustment does not require any intervention in the manufacturer's (internal) hydraulic system of the FV (safety aspect). Definitions:

[0012] The industrial truck (FFZ) can be designed either for driverless operation, in which case it is operated with a corresponding remote control device (hereinafter referred to simply as the control device). Alternatively or additionally, it can be designed for operation by a human operator.

[0013] The “hydraulic circuit”, for example, is an electro-hydraulic circuit because at least some of the hydraulic components, such as the valves, are controlled by means of a control device that is at least partially electronic.

[0014] The term "clamp cylinder" refers to a piston-cylinder unit for moving one or more gripper arms of the attachment, particularly for clamping, gripping, or gripping a load. The first chamber of the clamp cylinder refers to the rod-side or piston-side chamber of the piston-cylinder unit. The second chamber of the clamp cylinder refers to the other chamber, also called the counter chamber, i.e., the piston-side or rod-side chamber of the piston-cylinder unit.

[0015] The term “lifting cylinder” refers to a piston-cylinder unit for moving, in particular for lifting and lowering the attachment with at least one gripper arm, possibly also with the picked-up goods, on a lifting mast of the industrial truck.

[0016] The term “front of the pressure reducing valve” refers to the section of the first hydraulic line between the pressure reducing valve and one of the chambers of the clamp cylinder.

[0017] The term “back of the pressure reducing valve” refers to the section of the first hydraulic line between the clamp control spool of the industrial truck and the pressure reducing valve.

[0018] The term “front side” of the clamp control spool or the “lift cylinder control spool” refers to the side facing away from the FFZ or its hydraulic supply.

[0019] The term “rear side” of the clamp control spool or the “lift cylinder control spool” refers to the side facing the FFZ or its hydraulic supply.

[0020] Not all of the two or more gripper arms required need to be movable. To build up the (pre-)clamping pressure, it may be sufficient if at least one of the gripper arms is movable or controllable.

[0021] The term “pre-clamping pressure” is also synonymous with a pre-clamping pressure value. End of definitions

[0022] According to a first embodiment, a second pressure accumulator is installed in the second hydraulic line for receiving, storing, and providing pressurized hydraulic fluid. This second pressure accumulator serves to absorb hydraulic fluid accumulating in a counter chamber in the clamp cylinder during re-clamping or readjustment of the clamping force. In the prior art, it was necessary to drain the pressure or pressurized hydraulic fluid accumulating in the counter chamber of the clamp cylinder during re-clamping into a tank on the industrial truck. Thanks to the second pressure accumulator, this connection to the industrial truck is no longer necessary; instead, the pressurized hydraulic fluid from the second hydraulic line or from the counter chamber is absorbed by the second pressure accumulator during readjustment.

[0023] According to another embodiment of the invention, the variable clamping force is adjusted during readjustment by a variably adjustable through-pass valve connected in parallel to the pressure-reducing valve, which directs the volume flow of hydraulic fluid past the pressure-reducing valve into the first chamber of the clamping cylinder. In this respect, the through-pass valve enables a different adjustment of the clamping pressure, in particular an increase of the clamping pressure, on the front side of the pressure-reducing valve. The through-pass valve can be a proportional valve, for example.

[0024] According to a further embodiment, the clamping pressure is controlled using a control device integrated into the hydraulic circuit. Specifically, the actual pressure of the hydraulic fluid in the first hydraulic line is detected using a first pressure sensor and controlled as a controlled variable to a predetermined target clamping pressure by appropriately varying the volume flow of hydraulic fluid into the first chamber of the clamping cylinder using the passage valve as an actuator. During readjustment, control is carried out to a continuously increasing target clamping pressure. Once a final target clamping pressure has been reached, the provision of the control device advantageously makes it possible to keep the final target clamping pressure exerted on the goods to be lifted constant.

[0025] The above-mentioned object of the invention is further achieved by an industrial truck according to patent claim 6, a method for controlling the gripper arms of the attachment for the industrial truck according to patent claim 8 and by a corresponding computer program product according to patent claim 17. The advantages of these solutions correspond to the advantages previously mentioned with regard to the claimed hydraulic circuit.

[0026] Further advantageous embodiments of the invention are the subject of the dependent claims.

[0027] Four figures are attached to the description, where Fig. 1 an attachment with two gripping arms for clamping a product to be picked up; Fig. 2 the electro-hydraulic circuit according to the invention; Fig. 3 the pressure curves in the clamp cylinder and the lifting cylinder during different operating modes; and Fig. 4 shows the control device according to the invention.

[0028] The invention is described in detail below with reference to the figures mentioned in the form of exemplary embodiments. In all figures, identical technical elements are designated by identical reference numerals.

[0029] Fig. 1 shows an attachment 200 with two gripper arms 210, such as can be connected, for example, to the lifting mast of an industrial truck, in particular a forklift truck. The goods to be picked up are symbolized by the circle shown. The gripper arms are actuated by means of clamp cylinders 220, in particular by applying them to or moving them away from the goods to be picked up. By appropriate control using the hydraulic circuit 100 according to the invention, the gripper arms 210 are not only actuated in the manner described, but the clamping pressure with which the gripper arms 210 press against the goods to be picked up is also adjusted.

[0030] Fig. 2 illustrates in particular the hydraulic circuit 100 according to the invention for hydraulically controlling the gripper arms 210 of the attachment 200 for the industrial truck 300. The attachment 200 is designed as previously described with reference to Fig. 1 described.

[0031] The hydraulic circuit 100 has a first hydraulic line L1 for connecting a first chamber 22 of the clamping cylinder 220 to the internal hydraulic system 300' of the industrial truck. A pressure reducing valve DM1 is installed in this first hydraulic line L1 for limiting the pressure of the hydraulic fluid on the front side of the pressure reducing valve and in the first chamber 22 of the clamping cylinder 220 to a predetermined pre-clamping pressure. According to the invention, a first pressure accumulator DS1 is installed in the first hydraulic line L1 on the back side of the pressure reducing valve DM1 for receiving and storing pressurized hydraulic fluid from the first hydraulic line L1 and for providing the pressurized hydraulic fluid to the first hydraulic line L1 at a time delay.

[0032] According to the invention, a controllable passage valve PV1 is connected in parallel to the said pressure reducing valve DM1 for variably adjusting a volume flow of the hydraulic fluid past the pressure reducing valve DM1 into the first chamber 22 of the clamp cylinder 220.

[0033] Furthermore, the hydraulic circuit 100 comprises a first pressure sensor S1 for detecting the respective current actual pressure of the hydraulic fluid in the first hydraulic line L1 on the front side of the pressure reducing valve DM1, including the pressure in the first chamber 22 of the clamping cylinder 220. In addition, a control device 120 is optionally provided for controlling the actual pressure of the hydraulic fluid as a controlled variable to a predetermined target clamping pressure as a reference variable by suitably varying the volume flow of the hydraulic fluid in the line L1 into the first chamber 22 with the aid of the passage valve PV1 as an actuator.

[0034] In addition to the first hydraulic line L1, the hydraulic circuit 100 has a second hydraulic line L2 for connecting a second chamber 24 (counter chamber) of the clamp cylinder 220 to the hydraulic system 300', in particular the hydraulic supply 320 of the industrial truck. A second pressure accumulator DS2 is also preferably installed in this second hydraulic line L2 for receiving, storing, and supplying pressurized hydraulic fluid from or into the second hydraulic line L2.

[0035] Below the horizontal line in Fig. Figure 2 shows the internal hydraulic system 300' of the industrial truck. This consists of a hydraulic network 310 with a clamp control spool 32 and a lifting cylinder control spool 34. It also includes a hydraulic supply 320 on the rear side of the two control spools 32, 34 for applying system pressure to the hydraulic line network 310.

[0036] In addition to the aforementioned internal hydraulic system 300', the industrial truck 300 comprises the hydraulic circuit 100 according to the invention and the attachment 200, which is connected to the hydraulic system 300' of the industrial truck 300 via the hydraulic circuit 100. Specifically, the connection is made via the first and second hydraulic lines L1, L2, which are connectable or connected to the clamp control slide 32 of the industrial truck 300 on the front side. On the attachment 200 side, the two hydraulic lines L1 and L2 of the hydraulic circuit 100 according to the invention are each connected to one of the chambers 22, 24 of the clamp cylinder 220. Through this connection, the gripper arms 210 can be actuated to grip the goods to be picked up by the industrial truck 300 with a clamping pressure.

[0037] The industrial truck 300 further comprises a lifting mast (not shown in the figures) on which the attachment 200 can be moved with the aid of a lifting cylinder 330. Typically, the attachment 200, together with its gripper arms 210, can be raised or lowered along the lifting mast. The lifting cylinder control spool 34 is installed in the hydraulic network 310 between the lifting cylinder 330 and the hydraulic supply 320 of the industrial truck 300. A second pressure sensor S2 serves to detect the actual pressure on the front side of the lifting cylinder control spool 34 and in a first chamber of the lifting cylinder 330. The second pressure sensor S2 generates a pressure signal representing the actual lifting cylinder pressure for the control device 120 in the hydraulic circuit 100 as an input signal.

[0038] When operating the above-described industrial truck, a distinction is made between three operating modes: "clamping," "lifting," and "lowering." The method according to the invention for controlling the gripper arms 210 of the attachment 200 in the individual operating modes is described below. Operating mode “Clamps”:

[0039] The prerequisite for this operating mode is that the attachment 200 is connected to the clamp control spool 32 and to the hydraulic line network 310 of the industrial truck via the hydraulic circuit 100 and is filled with hydraulic fluid without pressure. In this initial state, the clamp control spool 32 is in its central position, i.e., in its locked position.

[0040] Starting from this initial state, the “Clamping” operating mode then has the following steps: First, the industrial truck FFZ is driven towards the goods to be picked up.

[0041] During or after this, the clamp control spool 32 is moved into its cross position, either by a control device of the FFZ, particularly if it is operated driverless, or by an operator of the FFZ actuating a lever accordingly. In the cross position, the first hydraulic line L1 and the connected first chamber 22 of the clamp cylinder 220 are connected to the hydraulic supply 320 of the industrial truck. This hydraulic supply 320 also makes the hydraulic system pressure of the industrial truck available to the hydraulic line L1. As a result, the pressure builds up in the first hydraulic line L1 and in the first chamber 22 of the clamp cylinder 220 because the pressure reducing valve DM1 is initially open to the pressurized hydraulic fluid. This pressure in the hydraulic line L1 is measured by the pressure sensor S1.However, the pressure build-up in the first hydraulic line is monitored by the pressure reducing valve and limited to a predetermined pre-clamping pressure at its front. In this way, the pressure reducing valve DM1 prevents the high system pressure of the industrial truck from passing through to the clamping cylinder 220 and thus, via the gripper arms 210, to the goods to be lifted. As soon as the pressure reducing valve detects that the predetermined pre-clamping pressure has been reached, it is therefore at least partially, but typically completely, closed to allow the flow of hydraulic fluid. However, the system pressure of the industrial truck is still present at the back of the pressure reducing valve DM1. This system pressure is therefore also present in the first pressure accumulator DS1, which is connected to the first hydraulic line on the back of the pressure reducing valve DM1.

[0042] The pressure curves described during the operating mode “clamps” are shown in Fig. 3 shown.

[0043] During the said pressure build-up in the first chamber 22 of the clamp cylinder 220, the pressure in the counter chamber 24 of the clamp cylinder 220 and in the second pressure accumulator DS2, which is connected to the second hydraulic line L2, would theoretically also increase if this pressure could not or would not be discharged into the tank 35 of the industrial truck due to the cross position of the clamp control slide 32, which is the case.

[0044] Therefore, the counter chamber 24 and the second pressure accumulator DS2 are depressurized during the initial pressure buildup in the clamp cylinder 220. However, the pressure buildup in the clamp cylinder 220 is still possible due to the counter pressure that the gripper arms 210 experience when they enclose the goods.

[0045] After the pressure on the front side of the pressure reducing valve DM1 and in the clamping cylinder 220 has risen to the pre-clamping pressure and the goods are gripped with this pressure, the method according to the invention further provides that the clamp control slide 32 is brought into its middle blocking position by the control device or the operator, in which the hydraulic connection of the first and second hydraulic lines L1, L2 to the hydraulic line network 310 of the FFZ is blocked.

[0046] Following the “clamping” operating mode, the “lifting” operating mode, which may also include transporting the goods, follows with the following steps: - Activating the lifting cylinder 330 to lift the attachment with the gripping arms 210 and with the goods by appropriately actuating the lifting cylinder control slide 34 into its parallel position, wherein the pressure in the lifting cylinder 330 measured by a second pressure sensor S2 continues to rise until the goods are completely lifted from the ground; - during lifting: readjustment, in particular increasing the clamping pressure on the goods starting from the pre-clamping pressure proportional to the pressure increase in the lifting cylinder 330 until a final target clamping pressure is reached by appropriately opening the passage valve PV1, whereby the pressure increase is fed by the pressure accumulator DS1 previously pressurized with the system pressure of the FFZ.

[0047] As the chamber pressure in the clamp cylinder 220 increases, the hydraulic fluid is forced out of its counter chamber 24 into the second pressure accumulator, whereby the pressure in the latter increases further because, as previously described, the hydraulic connection to the tank 35 of the hydraulic fluid of the industrial truck is blocked due to the blocking position of the clamp control slide 32.

[0048] The target clamping pressure with which the gripper arms 210 are to enclose the goods to be picked up is calculated from input values ​​E, in particular the diameter of the clamping cylinder 220, a given mechanical transmission, a correction factor, the efficiency of the attachment 200 and the pressure in the lifting cylinder 330 measured by means of the second pressure sensor S2.

[0049] At the beginning of the lifting process, the final target clamping pressure for each item to be lifted must first be determined successively, depending on its weight. This is done by initially increasing the clamping pressure exerted by the gripper arms 210 on the item - starting from the specified pre-clamping pressure - slowly or gradually as an increasing proportion of the weight of the item is lifted from the ground during the lifting of the item, until the item is finally completely lifted from the ground. This lifting of the item is accompanied by a change in the pressure in the lifting cylinder 330, which is continuously detected by the second pressure sensor S2. The pressure measured there represents the increasing proportion of the weight of the item picked up by the attachment; it reaches its maximum when the item is completely lifted from the ground and loads the attachment and the lifting cylinder 330 with its full weight.

[0050] The pressure curves during the “Lift” operating mode are shown in Fig. 3 shown.

[0051] The target clamping pressure, with which the gripper arms 210 press on the goods even during the initial lifting thereof, is preferably calculated in real time and as a function of the respective lifted weight portion of the goods to be picked up. After the pre-clamping pressure has been reached or during the described increase in the load for the clamping cylinder 220 and the lifting cylinder 330, the actual clamping pressure measured by the first pressure sensor S1 in the first hydraulic line L1 is controlled or regulated. The control or regulation is carried out on the target clamping pressure, which, as described, increases as a function of the pressure in the lifting cylinder 330, by appropriately varying the volume flow or the pressure of the hydraulic fluid in the hydraulic line L1 and in the chamber 22 of the clamping cylinder 220 with the aid of the passage valve PV1 as an actuator.

[0052] Once the final target clamping pressure is reached, it no longer changes as long as the goods remain fully lifted. This final target clamping pressure is then maintained by adjusting the actual clamping pressure measured by the first pressure sensor S1 in the first hydraulic line L1 as a control or measured variable to the specified final target clamping pressure by appropriately varying the volume flow or pressure of the hydraulic fluid in the hydraulic line L1 using the flow valve PV1 as the actuator.

[0053] The corresponding control scheme, which illustrates the operation of the control device 120, is shown in Fig.4. First, the target clamping pressure is calculated in the calculation device 122, depending on the current operating state, from the measurement signal of the pressure sensor S2 and the input values ​​E. This target clamping pressure is compared in a comparison device 123 with the actual clamping pressure currently measured by the first pressure sensor S1, typically in the form of a difference, to determine a control deviation e. This control deviation serves as an input variable for a controller 124. This controller generates a control signal MS1 for the passage valve PV1 as an actuator based on the control deviation e as an output signal. The passage valve PV1 acts on the controlled system 126 or the entire system through the aforementioned variation in the pressure in the hydraulic fluid in the hydraulic line L1 on the front side of the pressure reducing valve DM1 and in the first chamber 22 of the clamping cylinder 210.The variation in pressure is detected by the first pressure sensor S1, thereby closing the control loop of the control device 120.

[0054] The "lifting" operating mode is followed by a "lowering" operating mode. This operating mode is initiated by the control device or the operator of the FFZ moving the lifting cylinder control spool 34 into its cross position. List of reference symbols 100 hydraulic circuit 120 Control device 122 Calculation device for target clamp pressure 123 Comparator device 124 controllers 126 Control system 22 first chamber of the clamp cylinder 24 second chamber of the clamp cylinder 200 attachments 210 gripper arm(s) 220 clamp cylinders 32 clamp control slides of the FFZ 34 lifting cylinder control spool FFZ 35 Tank for hydraulic fluid of the FFZ 300 industrial trucks 300' internal hydraulic system of the FFZ 310 Hydraulic pipeline network 320 Hydraulic supply 330 lifting cylinders DM1 pressure reducing valve DS1 first pressure accumulator DS2 second pressure accumulator e pressure control deviation E Input values FFZ industrial truck L1 first hydraulic line L2 second hydraulic line PV1 through-flow valve S1 first pressure sensor S2 second pressure sensor QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] EP 3 524 568 B1

[0006] EP 0 995 557 B1

[0006]

Claims

[1] Hydraulic control system (100) for hydraulically actuating gripping arms (210) of an attachment (200) for a forklift truck (300), wherein the attachment has at least two gripping arms (210) and at least one clamping cylinder (220) for actuating the gripping arms (210) to grip goods to be picked up by the forklift truck with a clamping pressure; wherein the control system (100) has: a first hydraulic line (L1) for connecting a first chamber (22) of the clamp cylinder (220) to the hydraulic system of the FFZ; a pressure reducing valve (DM1) installed in the first hydraulic line (L1) to limit the pressure of a hydraulic fluid in the first hydraulic line (L1) on the front of the pressure reducing valve and in the first chamber (22) of the clamp cylinder (220) to a predetermined pre-clamping pressure; and a second hydraulic line (L2) for connecting a second chamber (24) of the clamp cylinder (220) to the hydraulic system of the FFZ; characterized by a first pressure accumulator (DS1) installed in the first hydraulic line (L1) for receiving and storing pressurized hydraulic fluid from the first hydraulic line (L1) and for supplying the pressurized hydraulic fluid into the first hydraulic line (L1) at a later time. [2] Circuit according to claim 1, characterized by a second pressure accumulator (DS2) installed in the second hydraulic line (L2) for receiving and storing pressurized hydraulic fluid from the second hydraulic line (L2) and for supplying the pressurized hydraulic fluid into the second hydraulic line (L2) at a later time. [3] Circuit according to one of the preceding claims, characterized bya flow control valve (PV1) connected in parallel to the pressure reducing valve (DM1) for variably adjusting a volume flow of the hydraulic fluid past the pressure reducing valve (DM1) into the first chamber (22) of the clamp cylinder (220). [4] Circuit according to claim 3, characterized by a first pressure sensor (S1) for detecting the current actual pressure of the hydraulic fluid in the first hydraulic line (L1) on the front of the pressure reducing valve (DM1) including in the first chamber (22) of the clamping cylinder (220); and a control device (120) for controlling the actual pressure of the hydraulic fluid as a control variable to a predetermined target clamp pressure as a reference variable by suitable variation of the volume flow of the hydraulic fluid into the first chamber (22) using the through-valve (PV1) as an actuator. [5] Circuit according to any of the preceding claims, characterized bythat the first and second hydraulic lines (L1, L2) can be connected to a clamp control valve (32) of the FFZ. [6] Industrial truck (300) having: an internal hydraulic system (300') with a hydraulic line network (310) with a clamp control valve (32) and a lifting cylinder control valve (34) and with a hydraulic supply (320) on the rear side of the two control valves (32, 34) for pressurizing the hydraulic line network (310) with a system pressure; the hydraulic circuit (100) according to one of the preceding claims, wherein the first and the second hydraulic lines (L1, L2) are connected to the front of the clamp control valve (32) of the industrial truck; and an attachment (200) hydraulically connected to the forklift truck (300) via the hydraulic circuit (100) with at least one piston-cylinder unit (220) for actuating gripping arms (210) of the attachment (200) for gripping goods to be picked up by the forklift truck (300) with a clamping pressure. [7] Industrial truck according to claim 6, characterized by a lifting mast with a lifting cylinder (330) for moving, in particular raising and lowering the attachment (200) with the gripping arms (210); the lifting cylinder control valve (34) connected to the hydraulic line network (310), which is connected between the lifting cylinder (330) and the hydraulic supply (320); and a second pressure sensor (S2) for detecting the actual pressure on the front of the lifting cylinder control valve (34) and in a first chamber of the lifting cylinder (330) and for generating a pressure signal representing the actual pressure of the lifting cylinder for the control device (120) in the hydraulic circuit (100) as an input signal. [8] Method for controlling gripping arms (210) of an attachment (200) for a forklift truck (300) according to claim 6 or 7 for lifting a product, wherein the attachment (200) is connected via the hydraulic circuit (100) to the clamp control valve (32) and to the hydraulic line network (310) of the forklift truck (300) and is filled with hydraulic fluid without pressure, wherein the clamp control valve (32) is initially in its locked position, and wherein the method in a “clamping” operating mode comprises the following steps: -Approaching the goods to be received by the forklift truck, - Setting the clamp control slide (32) into its cross position, causing the gripping arms (210) to begin gripping the goods; - Building up pressure in the first hydraulic line (L1) and in a first chamber (22) of the clamping cylinder (220) with the pressure reducing valve (DM1) initially open using the hydraulic supply (320) of the industrial truck, whereby this pressure build-up is limited to a predetermined pre-clamping pressure by at least partially closing the pressure reducing valve (DM1) on its front side, while the system pressure of the industrial truck is present on the back side of the pressure reducing valve (DM1); characterized by , that the pressure in the first pressure accumulator (DS1), which is connected to the first hydraulic line (L1) on the back of the pressure reducing valve (DM1), is also increased to the system pressure of the FFZ during the pressure build-up. [9] Method according to claim 8, characterized by, that the pressure in the second pressure accumulator (DS2), which is connected to the second hydraulic line (L2), is discharged into the tank (35) of the FFZ during the pressure build-up in the first hydraulic line (L1) - due to the pressure coupling via the clamp cylinder (220). [10] Method according to claim 8 or 9, characterized by , - after completion of gripping the goods with the pre-chamber pressure: adjusting the clamp control slide (32) by the control device or the operator into a locking position in which the hydraulic connection of the first and second hydraulic lines (L1, L2) to the hydraulic line network (310) of the FFZ is shut off. [11] Method according to claim 10; characterized by , that the "clamping" operating mode is followed by a "lifting" operating mode with the following steps: - Activating the lifting cylinder (330) to lift the attachment with the gripping arms (210) and optionally with the goods encompassed by the gripping arms by appropriately actuating the lifting cylinder control slide (34) into its parallel position, whereby the pressure in the lifting cylinder (330) measured by a second pressure sensor (S2) increases progressively until the goods are completely lifted from the ground; - during lifting: Readjusting, in particular increasing the clamping pressure on the goods proportionally to the pressure increase in the lifting cylinder (330) until a final target clamping pressure is reached by appropriately opening the through-valve (PV1), whereby the pressure increase is supplied by the pressure accumulator (DS1) previously pressurized with the system pressure of the FFZ. [12] Method according to claim 11, characterized by, that with the increase in chamber pressure in the clamp cylinder (220) hydraulic fluid is forced out of its counter chamber and into the second pressure accumulator (DS2), causing the pressure in this to increase further because the clamp control valve (32) has been closed after setting its locking position. [13] Method according to claim 11 or 12, characterized by , that the target clamping pressure is calculated from input values ​​(E), in particular the diameter of the clamping cylinder (220), a given mechanical transmission, a correction factor, the efficiency of the attachment (200) and the measured pressure in the lifting cylinder (330); and that the target clamping pressure during lifting is preferably calculated in real time and depending on the respective weight fraction of the goods to be lifted, represented by the measured pressure in the lifting cylinder (330). [14] Method according to any one of claims 11 to 13, characterized by After reaching the pre-clamp pressure: Control or regulate the actual clamp pressure measured by the first pressure sensor (S1) in the first hydraulic line (L1) as a control or measured variable to a target clamp pressure that increases depending on the pressure in the lifting cylinder by suitable variation of the volume flow or pressure of the hydraulic fluid in the hydraulic line (L1) and the chamber (22) of the clamp cylinder (220) using the flow control valve (PV1) as an actuator. [15] Method according to any one of claims 11 to 14, characterized by - After reaching the final target clamp pressure: Maintaining it by controlling the actual clamp pressure measured by the first pressure sensor (S1) in the first hydraulic line (L1) as a control or measured variable to the specified final target clamp pressure by appropriately varying the volume flow or pressure of the hydraulic fluid using the flow control valve (PV1) as an actuator. [16] Method according to any one of claims 11 to 15, characterized by , that after the operating mode "lifting" an operating mode "lowering" of the goods takes place with the following step: - Initiating the lowering of the goods by the control device or the operator of the FFZ bringing the lifting cylinder control slide (34) into its cross position. [17] Computer program product that can be loaded into the internal memory of a computer and that contains software code elements with which the steps of the method according to any one of claims 8 to 16 are carried out when the computer program product is running on the computer.

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