Holders for processes during punching and / or riveting

Abstract

The present invention relates to a clamp for a coupling drive and a coupling drive having the clamp. The clamp comprises a clamping cylinder, wherein the clamping cylinder provides a clamping force by applying pressure to a pressure chamber. Preferably, the pressure chamber is composed of the clamping cylinder and a punch of the coupling drive. The pressure loading required to provide the clamping force is provided by means of a hydraulic circuit of the coupling drive. During coupling operation, the pressure chamber of the clamp is connected to a pressure storage device of the clamp, so that a volume change of the pressure chamber of the clamp has no significant effect on the pressure in the pressure chamber.

Description

Technical Field Background Art

[0001] The riveting process or riveting described under the riveting process on Wikipedia is a production method belonging to the group of joining by deformation, which includes the production of riveted connections. During riveting, the auxiliary joining parts, namely the rivet(s), are plastically deformed at the connection holes. Riveting is a joining method used in particular for sheet metal and similar low-strength semi-finished products.

[0002] The purpose of punch riveting is to connect sheet metal parts indirectly and non-detachably without pre-drilling as in the case of conventional solid rivets or blind rivets. For this purpose, a rivet element (auxiliary joining piece) is used, which at the same time acts as a punch. Depending on the rivet element used, in principle two punch riveting methods are important: punch riveting with solid rivets or punch riveting with semi-tubular rivets. Both methods have in common that they require that the components are accessible from both sides and that the establishment of the connection takes place in a single-stage installation process.

[0003] In order to achieve the desired joining quality, the two sheets are pressed together with a certain force before the joining process begins. This prevents the sheets from moving sideways due to the transverse forces that occur, for example. The force is applied by a hold-down device. The hold-down device applies the force before the rivet is pushed into the sheet with the joining force. The hold-down device is advantageously guided into the workpiece by the same linear drive that also applies the riveting force. A spring system generates and limits the holding-down force.

[0004] For example, punch riveting devices and methods are known from DE 10 2018 200 012 A1 and DE 10 2015 213 433. The punch riveting device has a punch. A bottom die is assigned to the punch, wherein the bottom die and the punch are arranged on opposite sides of the components to be joined. In order to detect the positioning of the punch, a positioning sensor is provided. The punch is coaxially surrounded by a clamp. The clamp is connected to a clamp via a spring element. Thus, the force from the drive is transmitted to the punch via the clamp. In addition, the pressing force acting on the two components to be joined is transmitted from the drive via the clamp and the spring element to the clamp.

[0005] EP 1 294 505 B1 discloses a riveter and a method for riveting, wherein a pressure device and a riveting punch are loaded with pressure via a common pressure chamber. The forces acting on the rivet and the pressure device and their ratio are set by the surface ratio. Disadvantageously, the two forces acting on the workpiece are always determined by the surfaces of the punch piston and the pressure device piston loaded with pressure via the pressure chamber.

[0006] A method and a device for producing a punch-riveted connection are known from EP 1 034 055 B1. It is provided that a clamp is coupled or decoupled with a punch depending on the punch force and / or the punch stroke. The coupling of the piston of the clamp with the piston of the punch is achieved via a fluid chamber, wherein the fluid in the fluid chamber is incompressible and the pressure in the chamber is variable. The piston of the punch and the piston of the clamp are arranged in a common housing. The punch piston is constructed as a synchronous cylinder and can be axially moved by applying pressure to the individual chambers. All hydraulic chambers are arranged in a common housing.

[0007] DE 201 06 207 U1 discloses a drive device for a press-in tool with a clamp. The punch is driven by a spindle drive. The clamp is operatively connected to the punch by means of a pneumatic chamber and is driven by the movement of the punch until it hits a blocking member. After the impact, only the punch still moves further axially by the spindle drive. Here, the pressure chamber of the clamp has the effect of a spring. The pressure chamber is connected to a pressure control device via a flow connection. The pressure control device has a pressure regulator, a check valve, a reversing valve and a pressure booster. The pressure in the pressure chamber of the clamp can be adjusted during each riveting process by means of the pressure control device.

[0008] DE 10 2011 002 058 A1 discloses a hydraulically operated mounting device and a joining method with a hydraulic unit. By making targeted and full use of the volume flow of the hydraulic fluid in the forward stroke chamber of the piston combined with the punch and the return stroke chamber of the piston, as well as the compression chamber combined with the submerged piston and the compression device, a shorter cycle time can be achieved. For this purpose, a tank hose and a pump hose made of flexible material are provided. A valve is arranged in the course of the hose. By switching the valve, the pressurized hydraulic medium can be closed and also released. As a result, the hose section can be used as an energy storage for the pressurized hydraulic fluid volume to shorten the cycle time.

[0009] DE 10 2009 040 126 A1 discloses an electric hydraulic drive for an installation device with a clamp. The hydraulic drive has three hydraulic interfaces. A piston rod with a piston can be axially moved into a hydraulic cylinder by means of a spindle drive. Thus, hydraulic volumes with different pressures can be provided. By switching the valves provided, the chambers of the installation device can therefore be loaded with different pressures. The hydraulic drive is constructed as a hydraulic closed system together with the installation device. In order to enable the hydraulic drive to have enough hydraulic force for the operation of the installation device and to be able to absorb the hydraulic force output by the installation device again, two hydraulic pressure volume storage devices are provided. This storage device or the stored hydraulic volume can be accessed during each installation process. The clamp piston of the installation device is arranged in a coaxial auxiliary cylinder, and the main piston is arranged in a coaxial main cylinder.

[0010] Another known joining technique is bite-joining. Bite-joining is also known as bite-joining. Another synonym is: Press Joining. This joining technique is a method for joining sheets without the use of additional material such as rivets. From Wikipedia it can be learned that the static strength is about 2 / 3 to 1.5 times that of a similar spot welded connection. Due to the absence of a notch effect (in the case of non-piercing connections) and the absence of a heat-affected zone, the fatigue strength is higher than that of a spot welded connection. In particular, bite-joining has great potential when sheets of different thicknesses have to be joined. When following the "thick to thin" joining direction, the static strength can exceed one and a half times the strength of a spot welded connection. Another advantage is that different types of materials and / or coated sheets can also be joined.

[0011] The clinching tool consists of a punch and a die. The sheet metal to be connected is pressed into the die by the punch under plastic deformation, similar to deep drawing. The special shaping of the die creates a snap-like shape that connects the sheets to one another in a form-fitting and force-fitting manner. Depending on the system, either a depression in the bottom of the rigid die or a yielding of the movable die section leads to an overlap of the sheets. During clinching, the hold-down performs the same function as during riveting or punch riveting. Summary of the invention

[0012] The object of the present invention is to provide a compact hold-down in which pressure fluctuations during the joining process are reduced and which has a simple construction.

[0013] Furthermore, the object of the present invention is to enable a simple method for setting or readjusting the contact force to a predetermined value.

[0014] An advantageous development of the invention is based on the object of providing a hold-down device in which the holding-down force can be set to a predetermined value.

[0015] According to the invention, this object is achieved by a holding down device according to the invention for a joining drive and a method according to the invention for operating a rivet drive having a joining drive and a holding down device.

[0016] In the clamping device for joining the drive according to the present invention, a clamping cylinder is provided, and the applied force of the clamping cylinder is set by applying pressure to the pressure chamber. The pressure chamber is hydraulically connected to the pressure storage device. Advantageously, no valve is required between the pressure chamber and the pressure storage device. As the pressure in the pressure storage device is set, the pressure in the pressure chamber is also set at the same time. The pressure storage device provides a volume with a constant pressure, so that the set pressure can be reliably applied. The volume change of the pressure chamber during the clamping process causes slight pressure fluctuations, which have no effect on the joining process.

[0017] Large short-term pressure fluctuations during the joining process are thereby reduced.

[0018] The contact force can be adjusted completely independently of the joining force of the joining device.

[0019] In a preferred embodiment, the pressure storage device has a supply line with a valve. The valve is configured to be connected to the hydraulic unit of the associated engagement drive. It can be achieved that the pressure medium supply provided by the hydraulic unit of the engagement drive is configured to set the pressure in the pressure storage device of the clamp. Therefore, the clamp does not need its own pressure medium supply. Therefore, on the one hand, the clamping force of the clamp can be set, but the clamp is cheap, because the pressure medium supply of the engagement drive can be used. In addition, it is also advantageous for the required structural space.

[0020] In a preferred embodiment, at least one travel sensor for detecting the relative positioning of the holding cylinder relative to the punch and / or a pressure sensor for detecting the pressure in the pressure reservoir / pressure chamber of the holding device and / or a temperature sensor for detecting the temperature of the hydraulic medium of the holding device is provided. Depending on the detected signals and by comparison with predetermined signal values, the holding device can be precisely controlled.

[0021] In one embodiment, a unit consisting of a clamping device with a clamping cylinder and a punch is provided. A particularly compact structural space is achieved by a pressure chamber arranged radially between the clamping cylinder and the punch. It has been found to be particularly advantageous if the pressure chamber is arranged coaxially with the punch. This enables a particularly compact structure.

[0022] In a preferred embodiment, the clamping cylinder is supported on the punch in an axially movable manner. By applying pressure to the pressure chamber, the clamping device is subjected to relative movement relative to the punch. The clamping device can also apply a force different from the punch to a component also referred to as a workpiece.

[0023] It has proven to be advantageous to provide a pressure accumulator. The pressure chamber is connected to the pressure accumulator during the engagement operation. Even in the event of a volume change in the pressure chamber, pressure fluctuations can be reduced by the pressure accumulator.

[0024] In a preferred embodiment, the punch is configured to have at least one radially protruding flange to form a stop. The stop predetermines the boundaries of the relative positioning of the presser and the punch. By arranging the stop directly on the punch, a particularly compact pressing unit is provided.

[0025] In a preferred embodiment, it is provided that the clamping device has at least one cover, preferably two covers, wherein the cover / covers are detachably connected to the clamping cylinder. This allows the clamping device and the punch to be assembled. The punch can also be replaced in a simple manner.

[0026] In an alternative embodiment, the clamping cylinder is supported on the piston rod of the engagement drive instead of on the punch. By arranging the clamp to be connectable to the clamping cylinder, the clamping force can be transmitted to the workpiece. Instead of the punch, the cylinder of the engagement drive can then be formed with a flange for providing an axial stop.

[0027] In a preferred embodiment, a joining drive with a piston rod is used, wherein the piston rod can be driven by a hydraulic drive. Preferably, a differential cylinder is provided, wherein the force required for the joining process is provided by the piston chamber. In the reverse direction, no great force is required, since in the reverse direction the punch is only pulled out of the tool and brought into the starting position.

[0028] In a preferred embodiment, it is provided that the piston chamber can be connected to the hydraulic circuit via a valve. It is thus possible in particular to use the hydraulic unit for applying pressure to the reservoir independently of applying pressure to the piston chamber. It is in particular possible to apply hydraulic medium to the reservoir, wherein neither the annular chamber nor the piston chamber is applied with hydraulic medium. Thus, the hydraulic unit can be used to set a desired pressure in the reservoir independently of applying pressure to the engagement drive.

[0029] In a preferred embodiment, it is provided that the reservoir of the holding-down device can be hydraulically connected to the engagement drive in a switchable manner via at least one valve. Preferably, the reservoir can be connected via a first valve to a supply line leading to the line between the piston chamber and the hydraulic unit and via a second valve to a supply line leading to the annular chamber. Thus, the pressure reservoir can be supplied with hydraulic medium from the hydraulic unit both via the supply line leading to the annular chamber and via the supply line leading to the piston chamber.

[0030] In a preferred embodiment, it is provided that the hydraulic unit comprises a pump with a reversible delivery direction, preferably a four-quadrant pump. As a result, no valve is required in the delivery to the annular space.

[0031] It has proven to be advantageous to dimension the contact force to 5% to 20% of the maximum joining force that occurs. A corresponding configuration can be achieved by dimensioning the surface of the pressure chamber that points in the direction of the components to be joined. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Further advantageous embodiments of the present invention are explained with reference to the accompanying drawings based on exemplary embodiments. The mentioned features can be advantageously realized not only in the combination shown, but also individually in combination with one another. Among them:

[0033] Figure 1 Detailed illustration of a coupling drive with a controllable and / or adjustable hold-down device

[0034] Figure 2 Detailed view of the hold-down cylinder with the pressure chamber

[0035] Figure 3 Detailed view of the clamp with the clamping cylinder

[0036] These figures are described in more detail below. DETAILED DESCRIPTION

[0037] Figure 1 1 shows a coupling device 1 with a coupling drive 2, which has a clamping device 10. The coupling drive 2 has a differential piston with a piston chamber 4 and an annular chamber 5. The piston chamber 4 is connected to the hydraulic unit 3 via a supply line 7. A valve 6 is arranged in the supply line 7. In the embodiment shown, a reversing valve V3 is provided as the valve 6, by which the connection between the hydraulic unit 3 and the piston chamber 4 can be established and disconnected.

[0038] The annular chamber 5 is connected to the hydraulic unit via a supply line 8. No valve is provided in the supply line 8. In order to detect the position of the piston, a position sensor 55 is provided. The pressure P provided by the hydraulic circuit is detected via a pressure sensor 52. A . Temperature of hydraulic medium T AThe temperature sensor 51 is used to detect the pressure P in the supply line 8 to the annular chamber 5. B , a pressure sensor 53 is provided. A hydraulic branch leading to the reservoir 13 of the clamp 10 is provided between the valve 6 and the hydraulic unit. Another valve is arranged in the supply line, which is a reversing valve 15. The supply line from the hydraulic unit 3 to the reservoir 13 or the pressure chamber 12 of the clamp 10 can be switched by the reversing valve. The pressure P in the pressure chamber 12 of the clamp 10 is X The temperature T of the hydraulic medium is detected by the pressure sensor 54. X It can be detected by the provided temperature sensor 57. The positioning of the clamp or the clamping cylinder 11 relative to the punch 22 can be detected by providing a travel sensor 56. The reservoir 13 is connected to the pressure chamber 12. The reservoir 13 can be used to apply an almost constant pressure in the pressure chamber regardless of the positioning of the clamping cylinder 11.

[0039] Clamping force F X The pressure force P is generated by a hydraulic cylinder, also referred to as a pressure cylinder 11. X The pressure of is ready in the storage 13. By changing the storage pressure P X To achieve the settable force. To set the storage pressure P X The operating pressure P at the hydraulic actuator 20 is used for the joining process. A , P B In this case, the accumulator pressure is set outside the joining process. In this case, a differential cylinder is provided as actuator 20 .

[0040] The hydraulic coupling drive represents a linear movement on the piston rod 21. As an alternative to the hydraulic actuator shown here, an electromechanical drive with a stroke spindle or a combination of the two can also be provided.

[0041] A punch 22, also called an engagement punch, is fastened to the piston rod. The clamping cylinder 11 is designed as an annular cylinder. The active movement ("downward") is limited by a flange 26 on the engagement punch 22. Likewise, the return movement of the clamp in passive positioning is limited by the cylinder bottom 27 and the flange 25. The clamping force is generated by the pressure in the pressure chamber 12, which is fed via the pressure connection 24.

[0042] A clamp 30 is fastened to the clamp cylinder 11. When the joining punch 22 moves downward, it first strikes a first sheet as a first component 32, below which a second sheet as a second component 33 to be joined is arranged. When the joining punch 22 moves further toward the sheet, the pressure in the pressure chamber 12 causes a force in the direction of the sheet to act on the clamp cylinder 11 and the clamp 30 fastened thereto.

[0043] Force = Ring area * Pressure in pressure chamber

[0044] Ring area = (D I 2 *π / 4)-(D A 2*π / 4)

[0045] D I =Inner diameter of the clamping cylinder

[0046] D A = Outer diameter of the joining punch

[0047] As the hold-down device 30 is placed on the sheet metal, the hollow rivet 31 is pushed into the sheet metal 32, 33 and a joining connection is established. In the return stroke, the joining punch 22 uses the flange 26 to drive the holding cylinder 11 upwards again.

[0048] The piston rod 21 of the riveting drive as the joining drive 1 is driven by the pressure P in the pressure chambers 4, 5. A and P B The pressure generation takes place in a hydraulic unit H with reference numeral 3 . The hydraulic unit 3 can be a throttle control with a pressure generator and a throttle valve, or a displacement control, in which a pumping assembly acts directly on the pressure chambers 4 and 5 .

[0049] The sensor 55 measures the position S of the piston rod 21 of the riveting drive 2 A The sensors 52 and 53 measure the pressure P in the pressure chambers 4 and 5 of the riveting drive 2. A and P B The sensor 51 measures the temperature T of the fluid in the piston chamber 4. A Additional pressure sensors and temperature sensors can detect additional conditions of the system. A not shown CNC / PLC control unit collects the sensor signals and uses them for condition monitoring and controlling the riveting process.

[0050] The cylinder holding cylinder 11 is arranged coaxially with the engaging punch 22 and moves with the engaging punch. The engaging punch is arranged on the end of the piston rod 21 on the active side and moves with the piston rod 21. Therefore, the pressing cylinder 11 also moves with the piston rod 21. The pressure chamber 12 is fed with pressure C by the pressure storage 13. X The clamping cylinder 11 is at the lower stop held by the flange 26. The pressure P in the pressure chamber 12 X and temperature T X The position S of the clamping cylinder 11 relative to the joining punch 22 is measured using the sensors 54, 57. X Can be measured using sensor 56 .

[0051] In a first embodiment, the pressure chamber 12 can be connected via a valve V1 (reference numeral 15 ) to the line 7 leading to the hydraulic unit 3 . This first embodiment can be supplemented by a valve V3 (reference numeral 6 ) which can disconnect the pressure chamber / piston chamber 4 from the hydraulic unit 3 .

[0052] In the second embodiment, the pressure chamber 12 can be connected via a valve V2 (reference numeral 16) to the line 8 leading to the hydraulic unit 3. This second embodiment can be usefully supplemented with a valve V4 (not shown in the sketch), wherein the pressure chamber / annular chamber 5 can be disconnected from the hydraulic unit 3 via this valve.

[0053] In another embodiment, Figure 1 As shown, two valves 15 and 16 can also be provided.

[0054] By measuring the positioning S of the clamping cylinder 11 X , it is possible to detect at which position of the piston rod 21 the system hits the sheet 32, 33. With the known geometry / positioning of the sheets 32, 33 and the piston rod 21, process monitoring is possible. Incorrect sheet thickness or incorrect number of sheets as well as damage to the holding cylinder 11 / holding device 10, the bottom die 34 or the joining punch 22 can also be detected.

[0055] According to the present invention, instead of the positioning signal S X It can also be observed by observing the pressure signal P X This is achieved by the impact of the holding cylinder 11 on the sheet metal 32. The impact of the holding cylinder 11 leads to a small but detectable pressure increase in the pressure chamber 12 of the holding device.

[0056] Observe the pressure P X Observed pressure P A This is more advantageous because the force range of the hold-down device 10 is only 5% to 20% of the force range of the riveting drive / joining drive 2. Therefore, in the range of small forces, the sensor 54 has a higher resolution than the pressure sensor 52 of the piston chamber, which is beneficial to the accuracy of the detection. For example, when indirectly measuring the thickness of the sheet according to this method, it is desirable to reliably identify possible small impact forces.

[0057] The following briefly describes the methods for setting the pressure P X and a method for operating the compactor.

[0058] The method is described for a first embodiment of a joining device 1 having a valve 15 in the above-mentioned embodiment. The valve 15 is activated so that the pressure chambers 4 and 12 are connected. Now, the hydraulic unit is controlled so that the desired pressure appears in PA and PX. One of the pressure sensors 52, 54 for measuring the pressure PA or PX can be considered for this setting. During this process, when the pressure PX to be set is large enough, the piston rod of the joining drive can move out. After reaching the desired pressure, the valve 15 is deactivated and the piston chamber 4 is disconnected from the pressure chamber 12 again. The set pressure PX in the pressure chamber 12 of the clamping device 10 is maintained by the memory 13. The hydraulic unit 3 can now be used to control the joining drive, while the clamping device 10 provides the desired clamping force. Due to temperature fluctuations T X Changes in pressure PX caused by leakage or leakage are disadvantageous. These can be confirmed by sensors 54 or 57 during the operation of the riveting drive. In particular, once P X If the tolerance limit is exceeded, the ongoing riveting process can be suspended and the pressure P in the storage 13 can be reset again using the method described. X Replenish to the required value.

[0059] In some cases, the movement of the piston rod 21 during the pressure setting may be disadvantageous and should be avoided. In these cases, in the present embodiment, the piston chamber 4 can be disconnected from the hydraulic unit 3 by setting the valve 6 (in which the valve 6 is placed in the closed position). Only then is the valve 15 opened. Subsequently, the pressure in the pressure chamber 12 is set by the hydraulic unit 3, or the pressure setting in the pressure chamber 12 is completed. After the hydraulic unit 3 has set the pressure in the reservoir 13 / pressure chamber 12 and closed the valve 15, the valve 6 can be opened again in order to control the desired movement of the piston rod 21 with the hydraulic unit 2. The valve 6 is normally open, wherein the valve 15 is normally closed. Therefore, it is not necessary to actively switch one of these valves during the riveting operation.

[0060] The method described can also be used in a second arrangement, in which the pressure chamber PX is not connected to the piston chamber 4 by means of the valve 6, but is connected to the annular chamber 5 via the valve 16. If a movement of the piston rod 21 is to be avoided during the pressure setting, an additional valve V4 can disconnect the annular chamber 5 from the hydraulic unit 3 and the pressure chamber 12 during the pressure setting.

[0061] Since the pressure chamber is bounded by the punch and the holding cylinder of the holding device, it should be more accurately said that it is bounded by the holding unit. The holding unit is composed of the holding device and the punch 22. Due to the axial extension of the punch, the holding device can also be arranged in the area of ​​the piston 21 differently from the illustrated figures, wherein the axial length of the holding device 30 (see Figure 3 ) to make adjustments.

[0062] Reference numerals list

[0063]

[0064]

Claims

1. A clamping device (10) for engaging a drive (2), the clamping device comprising a clamping cylinder (11) and a pressure chamber (12) for providing a clamping force and / or positioning the clamping device, It is characterized in that The holding-down device (10) has a pressure accumulator (13), and the pressure chamber (12) is permanently hydraulically connected to the pressure accumulator (13) during the joining process, wherein: The pressure reservoir (13) has a supply line (14) with valves (15, 16) for connecting to a hydraulic unit (3) of an associated coupling drive (2), wherein the hydraulic unit (3) comprises a pump with a reversible delivery direction, wherein the coupling drive (2) has a differential piston with a piston chamber (4) and an annular chamber (5), and wherein no valve is required in the delivery section to the annular chamber, wherein the pressure cylinder (11) together with a punch or a piston rod (21) delimits a pressure chamber (12) which is designed to at least partially retain a hydraulic fluid in the pressure chamber.

2. The pressing device (10) according to claim 1, It is characterized in that At least one travel sensor (56) is associated for detecting the relative positioning of the clamping cylinder (11) relative to the punch and / or a pressure sensor for detecting the pressure in the pressure storage / pressure chamber (13, 12) of the clamping device (10) and / or a temperature sensor (57) for detecting the temperature of the hydraulic medium of the clamping device (10).

3. The pressing device (10) according to claim 1, It is characterized in that The pump is a four-quadrant pump.

4. A pressing unit consisting of the pressing device (10) according to claim 1 and a punch (22), It is characterized in that A pressure chamber (12) is formed by the pressing device (10) and the punch (22).

5. The pressing unit according to claim 4, It is characterized in that The holding cylinder (11) is mounted on the punch (22) in an axially displaceable manner, and the pressure chamber (12) is permanently connected to a pressure reservoir (13) of the holding device.

6. The pressing unit according to claim 4 or 5, It is characterized in that The punch (22) is configured with at least one radially protruding flange (25, 26) for forming a stop.

7. The pressing unit according to claim 4 or 5, It is characterized in that The holding down device (10) has at least one cover (17, 18), wherein the cover (17, 18) is / are detachably connected to the holding down cylinder (11).

8. The pressing unit according to claim 4, It is characterized in that A pressure chamber (12) is formed by the pressure cylinder (11) and the punch (22), wherein the pressure chamber (12) is radially configured between the pressure cylinder and the punch (22).

9. The pressing unit according to claim 7, It is characterized in that The hold-down device (10) has two covers (17, 18).

10. A joining device (1) for producing a punched / riveted connection, comprising a joining drive (2) with a hydraulic unit (3) and an axially displaceable piston rod (21) and a holding-down device (10) according to any one of claims 1 to 3, It is characterized in that The coupling device has a differential cylinder for driving the piston rod (21), the differential cylinder having a piston chamber (4) and an annular chamber (5), wherein the annular chamber (5) is connected to the hydraulic unit via a second supply line (8), and wherein no valve is arranged in the second supply line (8).

11. The joining device according to claim 10, It is characterized in that The piston chamber (4) can be connected to the hydraulic unit (3) via a valve (6).

12. The joining device according to claim 10 or 11, It is characterized in that The reservoir (13) of the clamping device (10) can be hydraulically connected to the hydraulic unit (3) of the engagement drive in a switchable manner via at least one valve (15, 16) in order to set a predetermined pressure in the reservoir (13) of the clamping device (10).

13. The joining device according to claim 10 or 11, It is characterized in that The pressure cylinder (11) is supported on the piston rod (21) in an axially displaceable manner, wherein the piston rod (21) and the pressure cylinder (11) delimit a pressure chamber (12) for providing a pressure force.

14. The joining device (1) according to claim 13, It is characterized in that The piston rod (21) has a radially protruding flange (26, 26a) for providing a stop for limiting the axial mobility of the clamping cylinder (11).

15. The joining device according to claim 10 or 11, It is characterized in that The holding down device (10) has at least one cover (17, 18), wherein the cover (17, 18) is / are detachably connected to the holding down cylinder (11).

16. The joining device according to claim 15, It is characterized in that The hold-down device (10) has two covers (17, 18).

17. Method for operating a rivet drive having a joining drive and a holding-down device according to claim 1, wherein: The engagement operation is interrupted depending on the pressure detected in the pressure storage (13) or the pressure chamber (12) to set a predetermined pressure, and the hydraulic unit (3) provided in the engagement drive (2) is controlled to load the pressure chamber (12) / pressure storage (13) with pressure or release the pressure to set the predetermined pressure, and the engagement operation is continued after the predetermined pressure is set, wherein the hydraulic unit (3) includes a pump with a reversible delivery direction.

18. The method according to claim 17, It is characterized in that The pump is a four-quadrant pump.

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