Clamping device for a blind rivet and method for determining a setting force curve of a blind rivet

Abstract

Clamping device for a blind rivet (1) with a clamping element (19) designed to engage a mandrel (8) of the blind rivet (1) and a tension element (15) separate from the clamping element (19) and designed to engage a rivet head (11) of a hollow rivet body (7) of the blind rivet (1), characterized in that the clamping element (19) comprises a clamping device (20) designed such that clamping forces exerted by it on the mandrel (8) increase with increasing load on the blind rivet (1).

Description

[0001] The present invention relates to a clamping device for a blind rivet and a method for determining a setting force curve of a blind rivet using such a clamping device.

[0002] A rivet is a form-fitting fastener that is plastically deformed during the setting process. Rivets are typically used to join multiple components, especially sheet metal parts. To do this, the rivet is inserted through overlapping openings in the components and then plastically deformed at its end to form a positive-locking collar for the components. Usually, one end of the rivet is deformed, while the other end has a pre-formed rivet head. The overlapping openings in the components can be created before the rivet is set or by the rivet itself during the setting process.

[0003] A blind rivet, also known as a pull-type rivet, is a special type of hollow rivet that allows installation with access to the joint from only one side. A blind rivet consists of a hollow rivet body with a rivet head formed at one end and a mandrel with a thickened mandrel head inserted through the rivet body from the other end. Joining components is achieved by inserting the blind rivet into overlapping openings in the components until the rivet head is flush. Then, using a setting tool, the rivet head is simultaneously forced against the components, and the mandrel protruding from the rivet head is subjected to tensile stress. This first deforms the other end of the rivet body radially, and then the mandrel is released from the rivet body. This release usually occurs when the mandrel head snaps off the remaining part of the mandrel at a predetermined breaking point.

[0004] Rivet guns for setting blind rivets are available that record and display the force applied to each rivet during setting, the so-called setting force curve. However, the usefulness of this generated setting force curve is limited, as it is retrospective and therefore cannot be used to influence the setting process itself.

[0005] Devices of this type are known from the publications DE 10 2012 022 622 A1 and DE 10 2010 015 325 A1.

[0006] It is therefore an object of the present invention to provide a way to optimally adapt a joining of parts produced by means of a blind rivet before the blind rivet is set.

[0007] The foregoing problem is solved by a method having the features according to claim 11. A clamping device usable for carrying out this method is the subject of claim 1. Further features and details of the invention will become apparent from the further claims, the description and the drawings.

[0008] The invention is based on the idea of ​​examining the setting behavior of a blind rivet of a blind rivet type or of a production batch before setting the blind rivets of the same blind rivet type or of the same production batch intended for the actual joining of components on a tensile testing device and using the results obtained, which may be in the form of a setting force curve in particular, to adjust setting parameters as necessary.

[0009] According to the invention, a method for determining a setting force curve of a blind rivet provides that the blind rivet is clamped in a tensile testing device by means of a suitable clamping device and loaded until the mandrel is released from the rivet body of the blind rivet, and the forces applied are recorded.

[0010] A particular advantage of this approach lies in the ability to influence the force applied to the blind rivet and to evaluate the resulting effects on the rivet. This evaluation, possibly after conducting several setting tests, can be used to optimize the setting parameters for a subsequent real-world setting process for joining components. This is not possible with currently available blind rivet setting devices.

[0011] A clamping device suitable for carrying out the inventive method for a blind rivet has at least one clamping element designed to engage a mandrel of the blind rivet and a tension element separate from the clamping element, engaging a head of a hollow rivet body of the blind rivet and preferably surrounding a section of the mandrel.

[0012] The term "separate" means that the clamping element and the pulling element are not connected in such a way that this connection is functionally necessary for setting the blind rivet, as is the case, for example, with a setting pliers or a rivet gun.

[0013] To enable the clamping device to be fixed within the tensile testing device, the clamping element and the tensile element preferably each have fastening means for fixing them to a crossbeam of the tensile testing device. In a simple embodiment, these fastening means can be designed as one or more through-holes for screw or plug connections, or as parallel contact surfaces intended for clamping contact with a collet of a crossbeam of the tensile testing device.

[0014] According to the invention, the clamping element comprises a clamping device whose clamping forces exerted on the mandrel increase with increasing tensile load on the blind rivet. Such a "self-reinforcing" clamping device is easy to handle and simultaneously ensures a secure connection with the mandrel of the blind rivet.

[0015] Preferably, such a self-reinforcing clamping device may have at least two, preferably three or more, clamping jaws that engage the mandrel (each by partially gripping it) and are held axially movable with respect to the longitudinal axis of the mandrel in a jaw sleeve. The contact surface pairings of the clamping jaws and the jaw sleeve, which guide the relative movement between the clamping jaws and the jaw sleeve, are oriented obliquely with respect to the longitudinal axis of the mandrel. This represents a structurally simple and reliable design for the self-reinforcing clamping device.

[0016] In a further preferred embodiment of the clamping device according to the invention, the clamping device may have at least two, preferably three or more, clamping jaws that engage the mandrel and are acted upon by at least one pre-tensioned spring element against the mandrel. Such an embodiment of the clamping device is particularly advantageous when combined with a self-reinforcing clamping device, since in this way an initial force-fit between the clamping jaws and the mandrel, required for self-reinforcing, can be generated.

[0017] In particular, to adapt the clamping device to different blind rivets and especially mandrel diameters, it may be provided that the preload force of the spring element is adjustable.

[0018] For this purpose, a hollow adjusting screw arranged coaxially to the mandrel of the blind rivet can particularly preferably be provided, positioned such that the mandrel can be moved through the adjusting screw after the clamping device has been released. This movement of the mandrel through the adjusting screw can be particularly gravity-assisted, i.e., the mandrel falls through the adjusting screw in the operating position of the clamping device. This allows for advantageous removal of the mandrel from the clamping device after a setting test on a blind rivet, without having to detach the clamping device or at least the clamping element from the corresponding crosshead of the tensile testing device. Preparing the clamping device for a subsequent setting test of another blind rivet can therefore be done with relatively little effort.

[0019] In order to avoid as much as possible the transmission of transverse forces to the blind rivet by the tensile testing device, which may distort the determination of a setting force curve, it is preferably provided that the tensile element is mounted radially (with respect to the longitudinal axis of the mandrel) movable to the mandrel.

[0020] To make the clamping device adaptable to a large number of blind rivets, particularly those of different dimensions, the pulling element can be designed to have a base body and an adapter arranged between the base body and the head of the blind rivet. This adapter is preferably detachably attached to the base body. The various adapters that can be used with the clamping device could, for example, be existing nozzles of a setting device, such as a rivet gun, adapted to the different blind rivets being tested.

[0021] In a preferred embodiment of the clamping device according to the invention, a displacement sensor can be provided for measuring a change in distance between the clamping element and the tensile element. Such a displacement sensor can be particularly advantageous if a displacement measurement carried out using the tensile testing device proves to be too inaccurate.

[0022] For the evaluation of a blind rivet joint, information on how far the mandrel head was displaced into the rivet body, and thus relative to the rivet head, before the mandrel releases from the rivet body, can be important. To determine this displacement more precisely, which only approximates (i.e., without considering the elongation of the mandrel body) the change in distance between the clamping element and the tensioning element at the time of release, a (possibly additional) displacement sensor can be provided in a further preferred embodiment of the clamping device according to the invention to measure the distance traveled by the mandrel head relative to the rivet head.

[0023] The displacement sensor(s) can be of any design and in particular can be based on a non-contact (e.g. optical or acoustic) measurement of the distance (change) or a contact measurement, in which the respective displacement sensor is directly or indirectly connected to the elements whose relative movement to each other is to be determined.

[0024] The invention is explained in more detail below with reference to an embodiment illustrated in the drawings. These show: Fig. 1 schematically the implementation of a method according to the invention for determining the setting force curve of a blind rivet; Fig. 2 a first embodiment of a clamping device that can be used to carry out the method in a longitudinal section; Fig. 3 a second embodiment of a clamping device that can be used to carry out the method in a perspective view; and Fig. 4 the second embodiment according to Fig. 3 in another perspective view.

[0025] The Fig. Figure 1 schematically shows the execution of a method according to the invention for determining the setting force curve of a blind rivet 1 using a conventional tensile testing device 2 and a clamping device according to the invention.

[0026] In a known manner, the tensile testing device 2 comprises two crossbeams 3, 4, one (3), here the lower one, is fixed (3) and the other (4) is movable. The crossbeams 3, 4 are connected to each other via four hydraulic cylinders 5. By controlled extension and retraction of the hydraulic cylinders 5, the movable crossbeam 4 can be moved relative to the fixed crossbeam 3.

[0027] Each of the crossbeams 3, 4 has fastening means that allow direct or indirect fixation of one end of a test specimen to be clamped between the crossbeams and subjected to tensile stress. In this case, the stationary crossbeam 3 is provided with a system of parallel, trapezoidal grooves in which slides with internal threads are slidably mounted. The fastening means of the slidable crossbeam 4 include a hydraulically actuated collet 6.

[0028] By means of a clamping device according to the invention, which in a first embodiment is described in detail in Fig. As shown in Figure 2, a blind rivet 1 can advantageously be clamped between the crossbeams 3, 4 to simulate the setting of the blind rivet 1. The forces required for the movement of the movable crossbeam 4, and thus for the plastic deformation of a rivet body 7 of the blind rivet 1 and the release of a mandrel 8 of the blind rivet 1 from the rivet body 7, are recorded and output by means of an evaluation device 10.

[0029] As can be seen from the Fig. As shown in Figure 2, the blind rivet 1 comprises the hollow rivet body 7, which forms a rivet head 11 at a first longitudinal end with a widened outer diameter. Starting from the second longitudinal end of the rivet body 7, the mandrel 8 is inserted into the rivet body 7 in a movement along its longitudinal axis 9, whereby a mandrel head 12, whose outer diameter is larger than the inner diameter of the rivet body 7, abuts the second end of the rivet body 7, while the leading end of the mandrel 8 protrudes from the rivet body 7. A constriction 14 is formed between the mandrel head 12 and the remainder of the mandrel 8, the mandrel body 13, which serves as a predetermined breaking point.

[0030] As can be seen from Fig. As shown in section 2, the clamping device comprises a tension element 15. This element rests against the rivet head 11 on the side facing away from the second end of the rivet body 7. The tension element 15 comprises a base body 16 and an adapter 17 arranged between the base body 16 and the rivet head.

[0031] The base body 16 of the tension element 15, which is also hollow in the longitudinal axial direction, surrounds a section of the mandrel body 13, with a relatively large inner diameter. This is intended to make it suitable for testing even comparatively large blind rivets 1. When testing a relatively small blind rivet 1, as is the case in Fig. As shown in Figure 2, the adapter 17 serves to ensure reliable force transmission between the pulling element 15 and the rivet head 11. Thus, when carrying out the method according to the invention, it is possible to use an adapter adapted to the geometry and dimensions of each type of blind rivet to be tested, but always the same pulling element 15. Preferably, the nozzles of a setting device suitable for setting various blind rivets 1 are used as the adapter 17.

[0032] The intention is that, regardless of the size of the adapter 17 used, a relatively large annular gap is always formed between the wall of the longitudinal opening formed by the base body 16 and the cylindrical section of the adapter 17 arranged in the longitudinal opening. This is intended to ensure a defined movement of the blind rivet 1 in the radial direction, which should guarantee that the blind rivet 1 is clamped in the clamping device with as little transverse force as possible.

[0033] On its outer side, the tensile element 15 forms two parallel contact surfaces 18 on which the collet 6 of the movable traverse 4 of the tensile testing device 2 can engage.

[0034] The clamping device further comprises a clamping element 19, which is designed for a force-fit engagement with the end of the mandrel body 13 projecting from the rivet body 7. For this purpose, the clamping element 19 includes a clamping device 20 with three clamping jaws 21, each having a partially cylindrical inner surface and a partially conical outer surface. The three clamping jaws 21 together surround a section of the mandrel body 13, with their inner surfaces bearing against the mandrel body 13. Their outer surfaces define a truncated cone with a circumference that tapers or decreases towards the rivet body 7. A gap is formed between adjacent clamping jaws 21.

[0035] The clamping jaws 21 are received in a jaw sleeve 22, which has a conical inner surface corresponding to the truncated cone formed by the outer surfaces of the clamping jaws 21. Movement of the jaw sleeve 22 relative to the clamping jaws 21 in a direction away from the rivet body 7, as occurs when a tensile load is applied to the blind rivet 1 using the tensile testing device 2, thus causes the clamping jaws 21 to compress radially and consequently increases the contact pressure between the inner surfaces of the clamping jaws 21 and the section of the mandrel body 13 held therein. The clamping device is therefore self-reinforcing.

[0036] The jaw sleeve 22 is connected via an internal thread to an external thread of a main body 23 of the clamping element 19. The main body 23 is also hollow and has an annular shoulder at its lower end. Several bores 24 are provided in the annular shoulder. Screws (not shown) can be inserted through these bores 24 and engage in the internal threads of the slides of the stationary crossbeam 3. In this way, the clamping element 19 can be screwed to the stationary crossbeam 3.

[0037] An adjusting screw 25 is screwed into an internal thread formed in the main body 23. The adjusting screw 25 is hollow and has a circumferential shoulder within the cavity. A (hollow) guide sleeve 26 rests against this shoulder. The guide sleeve 26 serves to guide and stabilize a helical spring 27, which is supported between the free end of the adjusting screw 25 and the undersides of the clamping jaws 21. The helical spring 27 is pre-tensioned, and the pre-tension can be adjusted by screwing the adjusting screw 25 into or out of the main body 23. The helical spring 27 serves to move the clamping jaws 21 within the jaw sleeve 22 sufficiently so that their inner surfaces bear securely against the mandrel body 13 of the blind rivet 1 with a defined initial contact pressure.

[0038] To determine the setting force curve of the blind rivet 1, the movable crossbeam 4 is moved away from the stationary crossbeam 3 at a defined speed. The rivet body 7 is carried along by the pulling element 15, while the end of the mandrel body 13, held in the clamping element 19, is held against the stationary crossbeam 3. This causes the mandrel head 12 to be drawn into the second end of the rivet body 7, where it is plastically deformed and forms an outer bead. In the case of a riveted joint, this bead would rest against the back of the components and, in conjunction with the rivet head 11, would form a positive-locking riveted joint. Upon reaching a critical force, the mandrel body 13 breaks away from the mandrel head 11 at the predetermined breaking point 14, and the test is complete.

[0039] The broken mandrel body 13 can fall out of the tensile testing device 2 after the clamping jaws 21 are manually moved against the preload of the coil spring 27 through the guide sleeve 26 and the adjusting screw 25. For this purpose, the stationary crossbeam 3 is provided with an opening (not visible) that also accommodates the lower section of the adjusting screw 25. Disassembly of the clamping element 19 to remove the broken mandrel body 13 is therefore not necessary.

[0040] The rivet body 7, optionally together with the adapter 17, can be easily removed upwards from the tensile testing device 2. Conversely, a new blind rivet 1, optionally in combination with a suitable adapter 17, can then be inserted into the tensile element 15 of the clamping device and subsequently tested.

[0041] A second embodiment of a clamping device according to the invention is described in the Fig. 3 and Fig. 4 shown. This is based on the clamping device according to Fig. 2 and therefore comprises a corresponding clamping element 19 and a corresponding tensioning element 15. The clamping device according to the Fig. 3 and Fig. 4 is supplemented by two displacement measuring sensors 28, 29, the first of which (28) is provided for the exact measurement of the relative movement between the clamping element 19 and the tensioning element 15. The second displacement measuring sensor 29 serves to measure the distance traveled by a relative movement between the mandrel head 12 and the rivet head 11 of the clamping element 19. Fig. 3 and Fig. 4 blind rivets not shown 1.

[0042] For the integration of the displacement measuring sensors 28, 29, both the tension element 15 and the clamping element 19 are rigidly connected to adapter plates 30, 31, respectively. The adapter plates 30, 31 are in turn rigidly connected to support elements 32, 33, each of which is designed for attachment to one of the crossbeams 3, 4 of the tensile testing device 2. In the embodiment shown here, conventional clamping tools are used as support elements 32, 33. The adapter plates 30, 31 are not fixed by clamping them in the clamping tools, but rather by screwing them to their respective support elements 32, 33 (clamping tools). This prevents the test result from being affected by slippage of a clamping device held in clamping tools.Furthermore, this fixed connection between the clamping device and the support elements 32, 33 (clamping tools) offers the advantage of simplified integration of the displacement measuring sensors 28, 29, since the support elements 32, 33 are thereby available for fastening or supporting parts of the displacement measuring sensors 28, 29.

[0043] Each of the displacement measuring sensors 28, 29 comprises a conventional sensor unit. This unit includes a sensor piston 35 guided longitudinally in a sensor guide 34, whereby a relative movement between the sensor piston 35 and the sensor guide 34 (here: capacitively) is evaluated. The sensor guides 34 of both displacement measuring sensors 28, 29 are connected to the support element 33 associated with the clamping element 19 and the corresponding adapter plate 31.

[0044] The sensor piston 35 of the first displacement sensor 28 is guided through a through-opening in the adapter plate 31 associated with the clamping element 19 and rests against a contact surface of the adapter plate 30 associated with the tension element 15, under the load of a pre-tensioned spring (not visible) arranged within the associated sensor guide 34. This contact surface can be formed by the flat-ground head of a screw (not visible) screwed into a threaded bore 36 of this adapter plate 30. When the support element 32 associated with the tension element 15 is moved away from the support element 33 associated with the clamping element 19 during a setting test of a blind rivet 1 held in the clamping device using a tensile testing device 2, the sensor piston 35 of the first displacement sensor 28 follows this movement due to the spring action.The resulting relative movement of the sensor piston 35 to the sensor guide is detected and transmitted to an evaluation unit 10 (see . Fig. 1) issued.

[0045] The sensor piston 35 of the second displacement sensor 29 rests against a contact surface of a cantilever 37 of the second displacement sensor 29, biased by a pre-tensioned spring (not visible) arranged within the associated sensor guide 34. Here, too, the contact surface can be formed by a flat-ground head of a screw (not visible) screwed into a threaded bore of the cantilever 37. The cantilever 37 is guided in a linear guide 38 and is thus movably mounted in a direction corresponding to the longitudinal axis 9 of the mandrel 8 or the tensile direction of the tensile testing device 2 relative to the unit consisting of the tensile element 15 (with rivet head 11 and rivet body 7 received therein), adapter plate 30, and support element 32. The cantilever 37 is connected by means of a Fig. 3 and Fig.4 not shown, pre-tensioned spring (which generates a greater spring force than the spring acting on the sensor piston 35 of the second displacement sensor 29) in the direction of the tension element 15.

[0046] A holding device 39 for a test needle (not shown) is arranged at the free end of the arm 37. The test needle is designed to rest with its tip against the mandrel head 12 of the blind rivet 1, the force exerted on the arm 37 by the spring of the linear guide contributing to a secure resting position of the test needle against the mandrel head 12.

[0047] If the support element 32 associated with the tension element 15 is moved away from the support element 33 associated with the clamping element 19 during a setting test of a blind rivet 1, the arm 37 follows this movement to the same extent as the mandrel head 12. However, since the mandrel head 12 is pulled into the rivet body 7 against the direction of pull of the tensile testing device 2 during the setting test, the movement of the support element 32 (or the elements associated with it: adapter plate 30, tension element 15, rivet body 7 and rivet head 11) relative to the clamping element 19 is greater than that of the arm 37 or the mandrel head 12. The distance traveled by the arm 37 is measured by detecting the distance of the relative movement of the sensor piston 35 in the sensor guide 34 of the second displacement sensor 29.

[0048] Using the second displacement sensor 29, a shorter distance is measured during a setting test than with the first displacement sensor 28. Specifically, the distance measured by the second displacement sensor 29 corresponds to the distance of the relative movement of the two crossbeams 3, 4 of the tensile testing device 2 (measured by the first displacement sensor 28), minus the distance of the relative movement between the mandrel head 12 and the movable crossbeam 3 (and thus the rivet head 11), which results from the mandrel head 12 being drawn into the rivet body 7. By simply calculating the difference between the two measured distances, the distance of the relative movement of the mandrel head 12 relative to the rivet head 11 can be determined.

[0049] The need to determine the distance traveled by the relative movement of the mandrel head 12 to the rivet head 11 by means of a displacement sensor is due to the fact that, because of the elasticity of the elements involved, in particular the comparatively long and narrow mandrel body 13, this relative movement cannot be equated with the relative movement of the crossbeams 3, 4 of the tensile testing device 2. Reference symbol list 1 blind rivet 2 Tensile testing device 3 fixed crossbeams 4 movable crossbeams 5 hydraulic cylinders 6 collet 7 rivet bodies 8 Dorn 9 Longitudinal axis of the mandrel 10 Evaluation unit 11 Rivet head 12 Thornhead 13 spine bodies 14 Constriction 15 Pull element 16 basic shapes 17 adapters 18 contact surfaces 19 Clamping element 20 clamping device 21 clamping jaws 22 Jaw sleeve 23 Main body 24 bore 25 Adjusting screw 26 Guide sleeve 27 coil spring 28 first displacement sensor 29 second displacement sensor 30 adapter plate 31 Adapter plate 32 Support element 33 Support element 34 Sensor guidance 35 sensor pistons 36 threaded holes 37 outriggers 38 Linear guide 39 Holding device

Claims

[1] Clamping device for a blind rivet (1) comprising a clamping element (19) designed to engage a mandrel (8) of the blind rivet (1) and a tension element (15) separate from the clamping element (19) and designed to engage a rivet head (11) of a hollow rivet body (7) of the blind rivet (1), characterized by , that the clamping element (19) comprises a clamping device (20) which is designed such that the clamping forces exerted by it on the mandrel (8) increase in tension with increasing load on the blind rivet (1). [2] Clamping device according to claim 1, characterized by , that the clamping device (20) has at least two clamping jaws (21) engaging the mandrel (8), which are held axially movable in a jaw sleeve (22) with respect to the longitudinal axis (9) of the mandrel (8), wherein the contact surface pairings of clamping jaws (21) and jaw sleeve (22) which guide the relative movement are oriented obliquely with respect to the longitudinal axis (9) of the mandrel (8). [3] Clamping device according to claim 1 or 2, characterized by , that the clamping device (20) has at least two clamping jaws (21) engaging the mandrel (8), which can be acted upon against the mandrel (8) via at least one pre-tensioned spring element (27). [4] Clamping device according to claim 3, characterized by , that the preload force of the spring element (27) is adjustable. [5] Clamping device according to claim 4, characterized by , that the preload force of the spring element (27) is adjustable by means of a hollow adjusting screw (25) arranged coaxially to the longitudinal axis (9) of the mandrel (8), wherein the adjusting screw (25) is arranged such that the mandrel (8) can be moved through the adjusting screw (25) after its fixation has been released by means of the clamping device (20). [6] Clamping device according to one of the preceding claims, characterized by , that the tension element (15) is mounted to be radially movable relative to the mandrel (8). [7] Clamping device according to one of the preceding claims, characterized by , that the tension element (15) has a base body (16) and an adapter (17) arranged between the base body (16) and the rivet head (11). [8] Clamping device according to one of the preceding claims, characterized by , that the clamping element (19) and the tension element (15) each have fastening means for fixing to a crossbeam (3, 4) of a tensile testing device (2). [9] Clamping device according to one of the preceding claims, characterized by a displacement sensor (28) for measuring a change in distance between the clamping element (19) and the tension element (15). [10] Clamping device according to one of the preceding claims, characterized by a displacement sensor (29) for measuring the distance traveled by a movement of the mandrel head (12) relative to the rivet head (11). [11] Method for determining a setting force curve of a blind rivet (1), characterized by, that the blind rivet (1) is clamped in a tensile testing device (2) by means of a clamping device according to one of the preceding claims and is loaded until the mandrel (8) is released from the rivet body (7) of the blind rivet (1) and the forces applied are recorded.

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