processing machine

Abstract

Processing machine (11), in particular a router, with a shaft (33) which can be driven to rotate about a rotation axis (R), a tool holder (17) which is drivingly coupled to the shaft (33) or is formed integrally therewith, and a clamping device (19) for releasably fastening a tool (18) in the tool holder (17), wherein the clamping device (19) comprises: at least one clamping member (35) held on the tool holder (17) and having a central passage (42) for passing through the tool (18) to be inserted into the tool holder (17), wherein the clamping member (35) is deformable by axial loading and the width of the passage (42) is variable by this deformation, and a clamping element (45) mounted on the tool holder (17) which is displaceable in the axial direction between a clamping position and a release position, wherein the clamping member (35) can be axially loaded by displacing the clamping element (45) in the direction of the clamping position, characterized by that the clamping device (19) has a pivotable actuating lever (23) by means of which the clamping element (45) can be adjusted between the clamping position and the release position, that a conversion device is provided for converting a pivoting movement of the actuating lever (23) into a sliding movement of the clamping element (45), and that the transfer device comprises an eccentric shaft (65) rotatable by the actuating lever (23) for directly or indirectly acting on the clamping element (45).

Description

The present invention relates to a machining machine, in particular a routing cutter, having a shaft which can be driven to rotate about an axis of rotation, a tool holder which is coupled to the shaft in a drive-effective manner or is formed integrally therewith, and a clamping device for the detachable fastening of a tool in the tool holder according to the preamble of claim 1.Such machining tools are used with different tools, for example with milling cutters or drills of different dimensions, which can be exchanged by means of the clamping device. The clamping device can be a collet chuck which is opened and closed by means of a hand tool such as a fork wrench.The handling of a separate hand tool is time consuming and inconvenient. In particular in the case of a router which has a milling table connected to the housing via guide columns, the rotary actuation of a collet chuck is awkward because the hand tool strikes against the guide columns and must therefore be repeatedly implemented. In addition, the hand tool can be easily laid. A fundamental challenge in clamping devices of processing machines is the ensuring of an adequate rotational accuracy and a sufficiently strong frictional connection.DE 10 2013 212 250 A1 discloses a router with a spindle locking device which serves to secure the spindle against a rotational restriction if required. The tool holder provided on the spindle is designed as a collet chuck.DE 33 11 389 A1 discloses a method for machining the raceway of a rolling bearing raceway, in which the raceway to be machined is inserted into the chuck of a grinding machine and is radially clamped from the outside by means of a clamping device. The clamping device comprises a packet of disk spring-like disks, which can be axially loaded by displacing a push rod and a pressure piece.It is an object of the invention to simplify the clamping of tools on machine tools without impairing the connection strength and the rotational accuracy of the clamped tool.The object is achieved by a processing machine having the features of claim 1.According to the invention, the clamping device comprises at least one clamping element held on the tool holder, which clamping element has a central passage for passing a tool to be inserted into the tool holder, wherein the clamping element can be deformed by axial loading and the width of the passage can be changed by this deformation, and a clamping element mounted on the tool holder, which clamping element can be displaced in the axial direction between a clamping position and a release position, wherein the clamping element can be axially loaded by a displacement of the clamping element in the direction of the clamping position.According to the invention, the clamping device has a pivotable actuating lever, by means of which the clamping element can be adjusted between the clamping position and the release position. This enables simple tool-free operation of the clamping device.According to the invention, a converting device is provided for converting a pivoting movement of the actuating lever into a displacement movement of the clamping element. For the user, the pivoting of a lever is simpler and more convenient than, for example, the axial displacement of an adjusting element.According to the invention, the conversion device comprises an eccentric shaft rotatable by the actuating lever for the direct or indirect application of force to the clamping element. The actuating lever can in this case engage directly on the eccentric shaft in a simple manner.The clamping of the tool in the tool holder is thus effected by a narrowing of the passage as a result of a deformation of the clamping element. This deformation can be brought about in a simple and quick manner by the clamping element being displaced relative to the tool holder and the clamping element thereby being axially acted upon-either by the clamping element itself or by a transmission element which is in mechanical connection with the latter. It has been found that a comparatively small axial stroke of the clamping element is sufficient in this case to enable a high transmission of frictional torque from the tool holder to the tool. A clamping device according to the invention can therefore be actuated particularly easily and quickly. In particular, the use of a tool is not absolutely necessary, that is to say clamping without tools is possible.A further advantage of the invention is that the tool holder is independent of the rotational position of the tool. The tool consequently does not need to be aligned for clamping. It is also not necessary to lock the tool prior to the clamping process, i.e. to secure it against twisting, as is necessary, for example, in the prior art when using a fork wrench.In the context of the present disclosure, the terms "axial" and "radial" are to be understood as referring to the axis of rotation of the rotary drive shaft.Preferably, the clamping device is exclusively effective in a force-locking manner, so that the clamping can take place in any rotational position of the tool holder and of the tool. In contrast, in the case of a form-fitting tool fastening, an alignment of the tool relative to the tool holder is required.The clamping element can have a conical ring surrounding the central passage. By means of such a ring, a particularly uniform radial pressure can be exerted on a tool shank to be clamped, as a result of which a high rotational accuracy results.The ring preferably has an outer edge region and an inner edge region, wherein by displacing the clamping element in the direction of the clamping position one of the edge regions of the clamping element can be axially loaded with support of the other edge region in order to flatten the clamping element. In the case of a flattened portion, the central recess of the ring narrows and abuts against a tool shank which is passed through the latter, as a result of which the latter is held reliably.The clamping element can comprise at least one flat conical clamping disk, in particular at least one star disk. Flat conical clamping disks make it possible to exert a radial clamping force when acted upon by an axial adjusting force. In order to ensure adequate deformability, star disks are provided with recesses arranged distributed along the circumference. In particular, a star disk can be formed by a meandering web.One embodiment of the invention provides that the clamping device comprises at least one clamping disk pack comprising a plurality of clamping disks arranged in a stacked manner, which clamping disks are flattened together when acted upon by the clamping element. By means of such a clamping disk stack, an increased clamping force can be exerted. In addition, the clamping force is effective in an extended range.A special embodiment of the invention provides that the clamping device comprises two or more separate clamping disk packs each consisting of a plurality of clamping disks arranged in a stacked manner, wherein a spacer is arranged in each case between directly successive clamping disk packs. In this embodiment, the tool is fixed by clamping at at least two clamping points spaced apart from one another, which ensures particularly secure retention and high rotational accuracy.It can be provided that the spacer comprises an arrangement of two or more coaxial spacer sleeves that are axially displaceable relative to one another, wherein an inner spacer sleeve is assigned to the inner edge regions of the clamping disks abutting against it and an outer spacer sleeve is assigned to the outer edge regions of the clamping disks abutting against it. In this embodiment, the clamping disk packs are supported against one another both on the inside and on the outside, and specifically also when the clamping disks are flattened.According to a further embodiment of the invention, an abutment for the clamping element formed on the tool holder is arranged in the axial direction with spacings between two abutments of the clamping element which can be varied in the opposite sense by the displacement of the clamping element. This results in an axial movement clearance for the clamping element on one side of the abutment provided for the clamping element and an additional axial movement clearance on the other side of the abutment provided for the clamping element, which may be used for a prestressing device, for example. When the clamping element is displaced, the distance between the abutment provided for the clamping element and one of the abutments of the clamping element increases, while the distance between the abutment provided for the clamping element and the other abutment of the clamping element decreases by the same amount. For example, in this embodiment a spring can be relaxed and thereby axially compress the clamping element, while conversely the spring is tensioned during the axial expansion of the clamping element.The clamping element can comprise a sleeve which engages around the tool holder at least for supporting the clamping element. This enables a particularly simple construction. The sleeve can also have a section which forms an abutment for a prestressing device.Preferably, the clamping element is non-rotatably mounted on the tool holder. This means that it is particularly preferred that a rotary drive is provided between the tool holder and the clamping element, whereas an axial movement of the clamping element relative to the tool holder is made possible. The clamping element can then be used together for torque transmission.In particular, the clamping element for an axially displaceable and non-rotatable mounting can be coupled to the tool holder via at least one positively locking driver device such as a polygonal profile, a splined shaft arrangement or a feather key arrangement. This enables a particularly reliable rotary driving.The tool holder can expand radially toward the free end to a holder into which the clamping element is inserted. This ensures a particularly compact construction. The receptacle can be sleeve-like, for example, wherein a depression provided in the sleeve base serves to receive an end section of the tool. The clamping element can form a sleeve section encompassing the receptacle.The clamping element can be prestressed into the clamping position or into the release position. The user then applies force either only when clamping in or only when releasing a tool. The clamping element is preferably prestressed into the clamping position, so that an active exertion of force is required for releasing.A pretensioning device for the clamping element can comprise a spring device, which preferably comprises at least one disk spring, preferably a disk spring pack made of a plurality of disk springs arranged in a stacked manner. Disk springs have a particularly high spring hardness, i.e. a relatively small axial deflection is associated with a large force. This counteracts a compact construction of the clamping device and a small axial stroke of the clamping element.The disk spring can be supported directly on the tool holder and / or directly on the clamping element, e.g. on an abutment of the clamping element and / or on an abutment of the tool holder. The tool holder and / or the clamping element can be provided with suitable extensions or flanges for this purpose.Preferably, the actuating lever is captively mounted on a housing of the processing machine, so that the clamping device can be actuated at any time.The actuating lever can be pivotable about a pivot axis running transversely to the axis of rotation. In the case of a router placed on a workpiece surface, such an actuating lever is to be pivoted up and down, which is convenient for the user.The actuating lever can extend at least substantially parallel to the axis of rotation when the clamping element is placed in the clamping position, and / or extend perpendicular or oblique to the axis of rotation when the clamping element is placed in the release position. When working with a clamped tool, the actuating lever thus does not protrude from the housing of the processing machine, but rather, as it were, fits tightly against it.A special embodiment of the invention provides that the eccentric shaft is divided into two individual shafts, between which there is a free space. The shaft or the tool holder can be passed through the free space. The loading of the clamping element on both sides of the shaft or of the tool holder is favorable with regard to the force transmission.The individual shafts can be coupled to respective lever legs, which together with a handle connecting the lever legs form the actuating lever and preferably run parallel to one another. The user can grasp the handle for actuating the clamping device and swivel down the actuating lever on the handle.According to a further embodiment of the invention, the transfer device for acting on the clamping element comprises a radially displaceable clamping wedge which can be acted on by an eccentric portion of the actuating lever. The clamping wedge can be slidingly guided in a guide section of the tool holder and can cooperate with a section of the clamping element. The inclined surface of the clamping wedge can be designed in such a way that self-locking is present. The clamping wedge can be acted upon by a pressure pin.A processing machine according to the invention can be designed as a hand-guided router, which comprises a machine unit and a milling table, wherein the milling table is connected to the machine unit via at least one guide column. Because the actuation of the clamping device takes place via an axial displacement of the clamping element and not, for example, by rotating it by several full revolutions as in the case of a collet chuck, an undesired striking of a hand tool against the guide column does not occur.Preferably, a processing machine according to the invention is electrically driven.Further developments can also be found in the dependent claims, the description and the appended drawings.The invention will be described below by way of example with reference to the drawings. FIG. 1 is a simplified perspective view of a routing cutter according to the invention. FIG. 2 shows a clamping device of the routing cutter according to FIG. 1 with a clamped milling tool in a lateral sectional view. FIG. 3 shows a clamping disk of the clamping device according to FIG. 2 in a plan view. FIG. 4 shows a section through the line A-A in FIG. 2. FIG. 5 shows the clamping device of the routing device shown in FIG. 2 in a relaxed state without a milling tool. FIG. 6 shows the routing device according to FIG. 1 with an actuating lever pivoted down for a tool change.The routing cutter 11 shown in FIG. 1 comprises a milling table 13 and a machine unit 15. A preferably electric motor, which is not visible and by means of which a tool holder 17 can be driven to rotate about an axis of rotation R, is accommodated in the machine unit 15. A milling tool 18 is detachably fastened in the tool holder 17 by means of a clamping device 19 to be described in more detail below. An actuating lever 23 is provided for actuating the clamping device 19.The machine unit 15 is coupled in a generally known manner to the milling table 13 via guide columns 25 in such a way that a rectilinear insertion movement of the machine unit 15 relative to the milling table 13 along the axis of rotation R is possible. In order to enable the milling tool 18 to be inserted into a workpiece, a recess 26 is provided in the milling table 13.As can be seen in the enlarged sectional illustration according to FIG. 2, the tool holder 17 has a first holder section 28 and a second holder section 29 which is radially widened with respect to the first holder section 28. The width of the first receiving section 28 is adapted to the diameter of a shank 30 of the milling tool 18. In the exemplary embodiment shown, the tool holder 17 is formed integrally with a motor shaft 33.Inserted into the second receiving section 29 of the tool receiver 17 is a clamping element 35 which is capable of fixing the shank 30 of the milling tool 18 in a force-fit manner in the tool receiver 17. The clamping element 35 comprises two clamping disk assemblies 37, which are each formed from a plurality of ring-shaped clamping disks 38 arranged in a stacked manner and are separated from one another in the axial direction by a spacer 39. The clamping disks 38 are preferably designed as star disks. An exemplary clamping disk 38 is shown in plan view in FIG. 3.The spacer 39 comprises an inner spacer sleeve 40 and an outer spacer sleeve 41, which are arranged coaxially with respect to one another and are axially displaceable with respect to one another. The annular clamping disks 38 form a passage 42 for the shaft 30.The tool holder 17 is surrounded by a clamping element 45 which here comprises two oppositely aligned sleeve elements 46, 47 firmly connected to one another. The clamping element 35 is supported in the axial direction on the one hand on the bottom of the second receiving section 29 and on the other hand on a lower inner wall section of the clamping element 45. The clamping element 45 is mounted on the tool holder 17 in an axially displaceable but non-rotatable manner. In order to accomplish the corresponding rotary driving, the front end of the tool holder 17 is provided with a polygonal shaft profile which cooperates with a polygonal hub profile of the clamping element 45, as can be seen in FIG. 4.At the axial height of the first receiving section 28, there is a disk spring pack 55 (FIG. 2 ) comprising a plurality of disk springs 56 arranged in a stacked manner, which is supported in the axial direction on one side on a shoulder 57 of the tool receptacle 17 and on the other side on an upper inner wall section of the clamping element 45.The clamping element 45 is prestressed axially by the disk spring pack 55 into the clamping position shown in FIG. 2. The prestressing force provided by the disk spring pack 55 can be more than 3 kN. In the clamping position, the clamping element 35 is axially acted upon by the clamping element 45, so that the clamping disks 38 of the clamping disk packs 37 are flattened and press radially against the shank 30 of the milling tool 18, so that the latter is fixed in a clamping manner in the tool receptacle 17. The torque is transmitted from the tool holder 17 via the upper clamping disk package 37 in the illustration to the shank 30 on the one hand and from the tool holder 17 via the clamping element 45 and the lower clamping disk package 37 in the illustration to the shank 30 on the other hand.To release the milling tool 18 from the tool holder 17, the clamping element 45 is displaced axially in a direction facing away from the machine unit 15 counter to the prestressing force of the disk spring pack 55, until it has reached the release position shown in FIG. 5. The disk spring pack 55 is then tensioned, whereas the clamping disk packs 37 are relaxed and the clamping disks 38 assume a flat conical basic shape in this case. This is associated with a slight radial widening of the bushing 42 and a release of the clamping connection. Because the spacer 39 comprises two spacer sleeves 40, 41 which are displaceable relative to one another, the inner edge regions 60 of the clamping disk packs 37 and the outer edge regions 61 of the clamping disk packs 37 are in each case held at a distance from one another independently.The axial displacement of the clamping element 45 from the clamping position into the release position is effected by means of the actuating lever 23 shown in FIG. 1. This is mounted on the machine unit 15 so as to be pivotable about a pivot axis S running transversely to the axis of rotation R and is coupled to an eccentric shaft 65 rotatable about the pivot axis S. As can be seen in FIGS. 2 and 4, the eccentric shaft 65 is divided into two individual shafts 67, between which the tool holder 17 is passed. As shown in FIG. 6, the single shafts 67 are coupled to respective lever legs 69, which are connected to each other by a handle 70.When the user grasps the handle 70 and pivots it from the position shown in FIG. 1 into the position shown in FIG. 6, the eccentric shaft 65 acts on the upper side of the clamping element 45 and presses it axially away from the machine unit 15 into the release position. The clamping is then released, so that a tool change can take place. The clamping element 45 could also be acted upon by the eccentric shaft 65 indirectly via one or more transmission elements.Because the actuating lever 23 extends substantially parallel to the axis of rotation R when the clamping element 45 is set in the clamping position, the handling of the router 11 with the clamped milling tool 18 is practically not impaired by the actuating lever 23.It has been found that it is possible by means of the clamping disks 38 to transmit a torque of 8 Nm and more, wherein the axial stroke of the clamping element 45 is extraordinarily small. For example, the axial stroke of the clamping element 45 between the clamping position and the release position can be less than 1 mm.In the concept presented here, the user can consequently release a tool 18 to be replaced by means of the actuating lever 23 against a prestressing force of the disk spring pack 55 of, for example, approximately 3.5 kN, this prestressing force being sufficient to compress the two packs 37, each comprising a plurality of clamping disks 38, by means of an axial stroke of the clamping element 45 of approximately 1 mm in such a way that all clamping disks 38 jointly transmit a torque of approximately 8 Nm to the tool 18 clamped in this way.The torque of about 8 Nm mentioned here refers to a shank diameter of the tool 18 of 8 mm. For larger or smaller shank diameters, other torques to be transmitted may be required. The number, size, and / or other characteristics of the clamping washers 38 may then be varied accordingly. In particular, it can be provided that different structural units are provided for different shank diameters as replaceable tool holders or replaceable structural groups of the tool holder, which are installed depending on the respective shank diameter of the tool.An embodiment of a router according to the invention, not shown, provides a clamping device in which a radially displaceable clamping wedge is provided for converting the pivoting movement of the actuating lever into an axial movement of the clamping element, which clamping wedge can be acted upon by an eccentric portion of the actuating lever.A clamping device 19 as described above can also be advantageous in the case of processing machines other than routing machines, for example drilling machines or milling motors.List of reference numbers:11 Machining machine, router 13 Milling table 15 Machine unit 17 Tool holder 18 Tool, milling tool 19 Clamping device 23 Actuating lever 25 Guide column 26 Recess 28 First receiving portion 29 Second receiving portion 30 Shaft 33 Shaft, motor shaft 35 Clamping element 37 Package, clamping disk package 38 Clamping disk 39 Spacer 40 Inner spacer sleeve 41 Outer spacer sleeve 42 Feedthrough 45 Clamping element 46, 47 Sleeve element 55 Disk spring package 56 Disk spring 57 Shoulder 60 Inner edge region 61 Outer edge region 65 Eccentric shaft 67 Individual shaft 69 Lever limb 70 Handle R Axis of rotation S Pivot axis

Claims

Machining machine (11), in particular routing cutter, having a shaft (33) which can be driven in rotation about an axis of rotation (R), a tool holder (17) which is coupled to the shaft (33) in a drive-effective manner or is formed integrally therewith, and a clamping device (19) for the releasable fastening of a tool (18) in the tool holder (17), wherein the clamping device (19) comprises: at least one clamping element (35) which is held on the tool holder (17) and has a central passage (42) for passing the tool (18) to be inserted into the tool holder (17), wherein the clamping element (35) can be deformed by axial loading and the width of the passage (42) can be changed by this deformation, and a clamping element (45) which is mounted on the tool holder (17) and can be displaced in the axial direction between a clamping position and a release position, wherein the clamping element (35) can be axially acted upon by a displacement of the clamping element (45) in the direction of the clamping position, characterized in that the clamping device (19) has a pivotable actuating lever (23), by means of which the clamping element (45) can be adjusted between the clamping position and the release position, that a converting device is provided for converting a pivoting movement of the actuating lever (23) into a displacement movement of the clamping element (45), and that the converting device comprises an eccentric shaft (65) rotatable by the actuating lever (23) for directly or indirectly acting upon the clamping element (45).Processing machine (11) according to claim 1, characterised in that the clamping member (35) has a conical ring surrounding the central passage (42).Processing machine (11) according to claim 2, characterised in that the ring has an outer edge region (61) and an inner edge region (60), wherein by displacing the clamping element (45) in the direction of the clamping position one of the edge regions (61) of the clamping element (35) can be axially loaded with support of the other edge region (60) in order to flatten the clamping element (35).Processing machine (11) according to at least one of the preceding claims, characterized in that the clamping element (35) comprises at least one flat conical clamping disc (38), in particular at least one star disc.Processing machine (11) according to claim 4, characterised in that the clamping device (19) comprises at least one clamping disc pack (37) made of a plurality of clamping discs (38) arranged in a stacked manner, which are flattened together when acted upon by the clamping element (45).Processing machine (11) according to claim 5, characterised in that the clamping device (19) comprises two or more separate clamping disc packs (37) each consisting of a plurality of clamping discs (38) arranged in a stacked manner, wherein a spacer (39) is arranged in each case between directly successive clamping disc packs (37).Processing machine (11) according to claim 6, characterised in that the spacer (39) comprises an arrangement of two or more coaxial spacer sleeves (40, 41) which are axially displaceable relative to one another, wherein an inner spacer sleeve (40) is assigned to the inner edge regions (60) of the clamping discs (38) which bear against it and an outer spacer sleeve (41) is assigned to the outer edge regions (61) of the clamping discs (38) which bear against it.Machining machine (11) according to at least one of the preceding claims, characterized in that an abutment for the clamping member (35) formed on the tool holder (17) is arranged in the axial direction with spacings between two abutments of the clamping element (45) which can be varied in the opposite sense by the displacement of the clamping element (45).Machining machine (11) according to at least one of the preceding claims, characterized in that the clamping element (45) comprises a sleeve element (46) which engages around the tool holder (17) at least for supporting the clamping element (35).Machining machine (11) according to at least one of the preceding claims, characterized in that the clamping element (45) is mounted non-rotatably on the tool holder (17).Machining machine (11) according to claim 10, characterised in that the clamping element (45) is coupled to the tool holder (17) for axially displaceable and non-rotatable mounting via at least one positively locking driver device, in particular a polygonal profile or a splined shaft arrangement.Machining tool (11) according to at least one of the preceding claims, characterized in that the tool holder (17) widens radially towards the free end to form a holder into which the clamping element (35) is inserted.Processing machine (11) according to at least one of the preceding claims, characterized in that the clamping element (45) is prestressed into the clamping position or into the release position.Processing machine (11) according to claim 13, characterised in that a prestressing device for the clamping element (45) comprises a spring device which preferably comprises at least one disk spring (56), preferably a disk spring pack (55) made of a plurality of disk springs (56) arranged in a stacked manner.Machining tool (11) according to claim 14, characterised in that a disk spring (56) of the prestressing device is supported directly on the tool holder (17) and / or directly on the clamping element (45).Processing machine (11) according to at least one of the preceding claims, characterized in that the actuating lever (23) is mounted in a captive manner on a housing of the processing machine (11).Processing machine (11) according to at least one of the preceding claims, characterized in that the actuating lever (23) is pivotable about a pivot axis (S) running transversely to the axis of rotation (R).Processing machine (11) according to at least one of the preceding claims, characterized in that the actuating lever (23) extends parallel to the axis of rotation (R) when the clamping element (45) is set in the clamping position, and / or extends perpendicular or obliquely to the axis of rotation (R) when the clamping element (45) is set in the release position.Processing machine (11) according to at least one of the preceding claims, characterized in that the eccentric shaft (65) is divided into two individual shafts (67), between which there is a free space.Processing machine (11) according to claim 19, characterised in that the individual shafts (67) are coupled to respective lever legs (69), which together with a handle (70) connecting the lever legs (69) form the actuating lever (23) and preferably run parallel to one another.Processing machine (11) according to at least one of the preceding claims, characterized in that the transfer device for acting on the clamping element (45) comprises a radially displaceable clamping wedge which can be acted on by an eccentric portion of the actuating lever (23).Processing machine (11) according to at least one of the preceding claims, characterized in that the processing machine (11) is designed as a hand-guided router which comprises a machine unit (15) and a milling table (13), wherein the milling table (13) is connected to the machine unit (15) via at least one guide column (25).Machining tool (11) according to at least one of the preceding claims, characterized in that the clamping element (35) is designed to transmit a torque in the range from 6 to 10 Nm, in particular of approximately 8 Nm, to the tool (18) to be fastened, wherein in particular the diameter of the tool (18) or shank (30) of the tool (18) interacting with the clamping element (35) is 8 mm.Processing machine (11) according to at least one of the preceding claims, characterized in that an axial stroke of the clamping element (45) of 0.5 mm to 3 mm, in particular of approximately 1 mm, is provided for displacing the clamping element (45) between the clamping position and the release position.Processing machine (11) according to at least one of the preceding claims, characterized in that the clamping element (45) is prestressed into the clamping position or into the release position with a prestressing force of 2 kN to 5 kN, in particular of approximately 3.5 kN.

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