Abstract:
A method of feeding a stud to a joining head including a stud shank clamp and operable to join the stud to a workpiece, and wherein the method of feeding the stud comprises the steps of providing a stud holding device including a securing sleeve and a plurality of collet arms; feeding the stud into the stud holding device to a first position wherein the flange is arranged above the free end of the plurality of arms and below the securing sleeve; and securing the stud in the first position by engaging the securing sleeve behind the flange.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 12/986,616, filed Jan. 7, 2011 which is a continuation of PCT Application No. PCT/EP2009/003503, filed May 16, 2009 which claims the benefit of German Application No. 10 2008 033 371.9, filed on Jul. 8, 2008, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a holding device for studs which have a radially projecting flange section, comprising a collet component which has a clamping section and a hollow insertion section, it being possible for a stud to be inserted via the insertion section to the clamping section, and the clamping section being designed in order to exert a radially inwardly directed clamping force for clamping the stud in place, a securing device, past which a flange section of the stud can be directed, being arranged on the insertion section and being designed with respect to the clamping section in such a way that a flange section which is directed past the securing device is mounted in a secured manner in a securing position between the securing device and the clamping section. 
         [0003]    Such a holding device is disclosed by document EP 1 495 828 B1. 
         [0004]    The present invention also relates to a joining head for joining studs to workpieces, comprising such a holding device, and also relates to a method of feeding a stud to a joining head. 
         [0005]    In the field of joining technology, it is known to join studs to the surface of a workpiece. This includes “stud welding”, in which a stud is welded to the surface of a workpiece. Alternative joining techniques include, for example, the adhesive bonding of a stud to the surface of a workpiece. 
         [0006]    Such processes are frequently carried out in an automated manner, for example in the motor vehicle industry, where a multiplicity of such studs are joined to the vehicle sheet in order to provide anchors for fastening means, paneling, etc. The automated joining of studs to workpieces as a rule includes the provision of a joining head on a robot. The joining head is in this case connected to a supply unit which provides, for example, the electric welding current and other control signals. Furthermore, it is preferred to feed the studs to the joining head in an automated manner. As a rule, this is effected by compressed air through feed tubes. For high cycle times, it is appropriate in this case to feed the studs from the rear, as it were, into a holding device of the joining head. The holding device serves to shift the stud into a defined ready position, starting from which a joining process can be initiated. 
         [0007]    The diameter of the feed passages is as a rule slightly larger than the diameter of the flange section in order to make it possible to transport the stud therein with an easy motion. In the holding device, too, the inside diameter of the insertion section is as a rule slightly larger than the outside diameter of the flange section. This may result in a stud fed to the holding device coming into an oblique position in the holding direction and first having to be oriented (centered) again with a welding axis before the stud can be transferred into the ready position. In some situations, it may be the case that this centering does not succeed, the result of which is that the fed stud then has to be ejected from the holding device. In the process, the stud falls downward in an uncontrolled manner, either onto the floor or also, for example in automobile construction, into the body. The ejected studs then lie distributed on the floor and are swept up and thrown away. Studs left lying in a body may subsequently lead to disturbing noises during driving. 
         [0008]    The holding device which has been disclosed by document EP 1 495 828 B1 has a securing device in the form of a collet element arranged inside a collet cover. The collet element has a plurality of axially extending arms which can be expanded radially relative to one another. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to specify an improved holding device, an improved joining head and an improved feed method, the reliability of the feeding in particular being increased. 
         [0010]    This object is achieved in the holding device mentioned at the beginning in that the securing device has at least two securing-sleeve sections which are movable radially relative to one another and which are designed as separate components and together form a securing sleeve. 
         [0011]    Due to the securing device being formed by two securing-sleeve sections which are movable radially relative to one another and which are designed as separate components, the securing device can be produced in a simple manner from the structural point of view and fitted in the holding device. In addition, the reliability of the feeding of the stud can be increased. Furthermore, the securing device is more flexible, since the force which is required for passing the flange section through the securing device into the securing position can be lower or can be established so as to be adapted to the respective stud type. In addition, the securing device can be made in a cost-effective manner, for example of steel. Due to the securing device, which is preferably arranged in the interior of the collet component, wear of the collet component can also be reduced. 
         [0012]    The expression “secure mounting” of the flange section is intended in this case to mean that the stud cannot fall back through the insertion section (in particular if the holding device is held “overhead”) and/or that the stud is secured against an extreme oblique position inside the holding device. 
         [0013]    It is generally conceivable here that the flange section is held between the securing device and the insertion section in such a secured manner that the stud is already centered in the holding device in the securing position. In this case, it may be sufficient, for example, to press the stud in the axial direction with its flange section into the clamping section. Alternatively, the securing position can also be formed in such a way that the stud, the flange section of which is arranged between the securing device and the clamping section, is already held in the correct position in the holding device, such that a subsequent process (such as a stud welding process) can be initiated (the securing position is then the ready position). 
         [0014]    However, it is preferred if a centering device is assigned to the holding device, which centering device can be inserted from above into the insertion section and into the securing device and is designed in order to orient or center or take hold of a shank section of the stud. In this embodiment, the stud, in the securing position, need not be oriented completely with the joining axis, and therefore the holding device can be designed with comparatively large tolerances. However, the securing device prevents the stud in the holding device from passing into an extreme oblique position in which the centering device is also not able to orient the shank section of the stud. In this case, too, without the securing device according to the invention, the stud would have to be ejected. 
         [0015]    However, the provision of the securing device can ensure that the centering device can always readily take hold of the stud and consequently orient or center it. A situation in which studs are needlessly ejected can therefore be avoided. Consequently no disturbing noises occur, for example, in a vehicle body. Contamination in the production cell is also avoided. A cost saving is also obtained, since ejected studs generally have to be discarded (risk of damage). 
         [0016]    In addition, it is possible with the holding device according to the invention to reliably feed what are referred to as large-flange studs having a comparatively short shank. 
         [0017]    Further and according to the invention, the above objective at the initially mentioned holding device is solved according to a second aspect in that the securing device has a plurality of radially elastic support elements distributed about a perimeter of the collet component. 
         [0018]    By providing radially elastic support elements distributed about a perimeter of the collet component, substantially the same effect as by the two securing-sleeve sections which are movable radially relative to one another is achieved. When introducing a stud, the support elements are pushed outwards by the flange section so that the flange section of the stud can pass by the support elements. Afterwards, the support elements move back into their original position. Now, the flange section cannot pass by the support elements in an opposite direction any more. Thereby, the elasticity of the support elements has to be chosen correspondingly so that for example the weight of a stud during “overhead”-working is not sufficient to push the support elements apart. 
         [0019]    For example, wire elements which are guided through the longitudinal slots between single arms of the collet component can be provided as support elements. The wire elements can be bent so that bend sections project into the collet component in the initial position and prevent the shaft of the stud completely abutting the collet component. In this alternative manner, a secure mounting in the meaning of the invention is also achieved. 
         [0020]    Furthermore, the object is achieved according to the invention by a method of feeding a stud to a joining head which serves to join a stud to a workpiece, the stud having a radially projecting flange section, comprising the steps: feeding the stud into a holding device of the joining head until the flange section is arranged in front of a clamping section of the holding device, and securing the stud in this position by a securing device engaging behind the flange section, which securing device has at least two securing-sleeve sections which are movable radially relative to one another and which are designed as separate components and together form a securing sleeve. 
         [0021]    Finally, the above object is achieved by a joining head for joining studs to workpieces, comprising a holding device according to the invention and comprising a shank-clamping device which can be inserted into the holding device and is designed in order to accommodate the shank section of a stud in the securing position, the shank-clamping device being designed in order to shift the stud from the securing position into a ready position. 
         [0022]    The object is therefore completely achieved. 
         [0023]    The securing device of the holding device according to the invention can generally be arranged outside the insertion section and can extend with a lug or the like into the interior of the insertion section in order to establish the securing position. 
         [0024]    However, it is especially advantageous if the securing device is arranged inside the insertion section. 
         [0025]    This results in a simple and compact construction. 
         [0026]    According to a further preferred embodiment, the securing sleeve is designed in such a way that the flange section can be passed through the securing sleeve. 
         [0027]    In this embodiment, the securing sleeve preferably directly adjoins a feed passage of the joining head, such that a smooth transition can be ensured. 
         [0028]    In this case, it is especially advantageous if the securing sleeve has a securing section which is arranged adjacent to the clamping section, the securing section being designed to be radially expandable in such a way that the flange section, in the securing position, is held between the at least not completely expanded securing section and the clamping section in a secured manner. 
         [0029]    As a result, the stud is secured in the securing position by the securing section engaging behind the flange section when it is not completely expanded. 
         [0030]    In this case, it is preferred that the radial force exerted on the flange section by the clamping section be greater, to be precise in particular markedly greater, than the radial force which is exerted on the flange section by the expandable securing section. 
         [0031]    It is generally conceivable for the securing section, on account of structural boundary conditions, to automatically or inevitably fall back behind the flange section after it has been expanded. However, it is especially preferred if the securing section is compressed into the non-expanded position by elastic means. The elastic means are in this case preferably designed in such a way that only a very low force is exerted on the flange section when the latter travels through the securing device. This can ensure that the stud can also be reliably inserted into the securing position by conventional means, such as by compressed air for example. 
         [0032]    Consequently, the securing-sleeve sections, in the region of their ends adjacent to the clamping section, are preferably prestressed toward one another in the radial direction by means of a prestressing device. 
         [0033]    The prestressing device can be formed, for example, from an O-ring made of heat-resistant material or also from a spring washer made of metal. 
         [0034]    The flange section is generally preferably surrounded by the securing sleeve when passing through the securing device, such that a situation can be avoided in which the flange section (which can be of polygonal design for example) catches on contours of the securing device. 
         [0035]    In this case, it is especially advantageous if the securing-sleeve sections have, in the region of their end opposite the clamping section, a collar section which is movably mounted on a housing section in such a way that the securing-sleeve sections can be expanded relative to one another in the radial direction at least in the region of their securing ends adjacent to the clamping section. 
         [0036]    In this embodiment, the securing device can be constructed in a very simple manner, in the ideal case only by the two securing-sleeve sections which are mounted such as to be movable inside the holding device, to be more precise such as to be movable inside the insertion section. 
         [0037]    In this case, it is especially preferred if the securing-sleeve sections can be pivoted relative to one another about a tilting axis which is oriented transversely to the insertion direction and which is arranged between the collar section and the securing ends. 
         [0038]    In this embodiment, the securing sleeve preferably has its smallest inside diameter, adapted to the flange section, in the region of the tilting axis. Since this section having the smallest inside diameter is arranged in the region between the collar section and the securing ends, a prestressing device for radially prestressing the securing-sleeve sections can be dimensioned to be relatively small on account of the relatively short lever forces. It is consequently possible overall for the flange section to be passed through the securing sleeve with relatively low resistance, such that the reliability of the feeding is increased. 
         [0039]    Furthermore, it is advantageous in this embodiment if the securing-sleeve sections, on their sides pointing toward one another, are tapered from the tilting axis toward the securing ends. 
         [0040]    Consequently, the inside diameter which is formed by the securing-sleeve sections in the region of the securing ends is smaller than the inside diameter of the initial sleeve from which the tapered securing-sleeve sections have been produced. 
         [0041]    This consequently makes it possible in a simple manner from the structural and production point of view for the securing ends to be able to engage behind the flange section, passed through the securing sleeve, in order to establish the securing position. 
         [0042]    Consequently, when the securing sleeve is closed (when the securing ends bear against one another), a wedge space forms in the region from the tilting axis toward that end of the securing sleeve which faces the feed passage. This results in a relatively large insertion opening into the securing sleeve in the region of the feed passage, such that the reliability of the feeding can likewise be increased. 
         [0043]    Furthermore, it is advantageous overall if the collet component has clamping arms, which form the clamping section, and stop arms, which are axially longer than the clamping arms. 
         [0044]    The stop arms can prevent a stud that is shifted from the securing position into a ready position from falling out of the holding device on account of an insufficient build-up of clamping forces when the flange section is pushed past the clamping arms. 
         [0045]    This applies in particular when the joining head has a shank-clamping device which can be inserted into the holding device and is designed in order to accommodate the shank section of a stud in the securing position. Here, the radial clamping force for holding the stud in the ready position is produced by a radial clamping force of the clamping arms from outside on the shank-clamping device. This force, when the flange section is pushed axially past the clamping arms, is still not sufficient in some cases in the prior art in order to securely hold the stud. In this case, the stop arms prevent the stud from falling out. The clamping arms then come to bear on the outside against the shank-clamping device and exert a radial force on the latter in order to clamp the stud in place. The flange section of the stud is then also pushed axially past the stop arms in order to shift the stud into the ready position. 
         [0046]    Accordingly, it is especially advantageous in the feed method according to the invention if the stud is taken hold of and centered in the securing position by a clamping device before the clamping device is inserted into the clamping section in order to shift the stud into the ready position. 
         [0047]    Alternatively, the collet element can be provided for having a slotted cylindrical front portion forming the clamping section. 
         [0048]    In this alternative, the arms of the collet element consequently all have the same length and each arm has a clamping surface so that overall kind of a cylindrical clamping collet is formed. When pushing the stud through the slotted cylindrical front portion, the shank section of the stud is clamped in the clamping surface first and it is avoided that the stud falls out. Then, when the flange section is pushed past the clamping surfaces, the clamping surfaces abut the shank-clamping device and push the shank-clamping device to the shank of the component which is then placed in the ready position. 
         [0049]    Due to the common length layout of the arms, the clamping device can be manufactured more economically. Further, the clamping surfaces having the same length and abutting about the whole perimeter avoid that melt enters the interior of the clamping device through the gaps between the ends of the arms during a welding process. 
         [0050]    It goes without saying that the abovementioned features and the features still to be explained below can be used not only in the respectively specified combination but rather also in other combinations or on their own without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0051]    Exemplary embodiments of the invention are shown in the drawing and are described in more detail below. In the drawing: 
           [0052]      FIG. 1  shows a cross-sectional view of a holding device according to an embodiment of the present invention; 
           [0053]      FIG. 2  shows part of a joining head with the holding device of  FIG. 1 ; 
           [0054]      FIG. 3  shows a cross-sectional view of a holding device according to a second embodiment of the present invention; 
           [0055]      FIG. 4  shows a cross-sectional view of a holding device according to a third embodiment of the present invention; and 
           [0056]      FIG. 5  shows a welding system with a welding head. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0057]    A joining system in the form of a stud welding system is designated generally by  10  in  FIG. 5 . The welding system  10  contains a robot  12 , which can be designed, for example, as a robot having multi-jointed arms. A welding head  14  is secured to the end of a processing arm of the robot  12 . The welding head  14  is connected via an electrical supply  18  and via a stud feed  20  to a supply unit  16 . The supply unit  16  typically contains power electronics for providing a welding current, which is fed to the welding head  14 . Furthermore, the supply unit  16  typically contains a control device, by means of which the robot  12  can also be activated, to be precise via a robot control  19 . Furthermore, the supply unit  16  contains a stud supply, from which studs are fed separately to the stud feed  20 . 
         [0058]    The welding head  14  contains a feed passage  22  which is connected to the stud feed  20  and via which a respective stud  26  can be directed into the welding head  14 . The stud  26  fed to the welding head  14  is to be welded to a workpiece  24 , for example a metal sheet, by means of the welding system  10 . In the present case, the welding system  10  is designed as a stud welding system which can work, for example, according to the drawn arc method. In this method, the stud  26  is first of all set down on the workpiece  24  by means of the welding head  14 . A “pilot current” is then directed through the stud  26  and the workpiece  24 . After that, the stud  26  is lifted slightly from the workpiece, in the course of which an arc is struck. The current is then increased to a welding current, in the course of which those surfaces of the stud  26  and of the workpiece  24  which are opposite one another are fused. Finally, the stud  26  is lowered onto the workpiece  24  again and the welding current is switched off. The connected molten pools of stud  26  and workpiece  24  solidify, such that the stud  26  is finally fixedly connected to the workpiece  24 . Such stud welding systems are widely used in the motor vehicle industry. 
         [0059]    Alternatively, however, the joining system  10  may also be designed to adhesively bond or braze the stud  26  to the workpiece  24 . 
         [0060]    The stud  26  has a shank section  28 , which can be provided, for example, with an external thread. Furthermore, the stud  26  has a flange section  30  which projects radially relative to the shank section  28  and which may be of polygonal design, for example, at its outer circumference. Finally, the stud  26  has a welding section  32  which is to be connected to the workpiece  24 . 
         [0061]    The joining head  14  contains a holding device  100  for the stud  26 , this holding device  100  serving to shift the stud fed via the feed passage  22  into a ready position, in which the stud  26  has a defined position with respect to the joining head  14  and which the stud  26  assumes before the joining process described above is initiated. 
         [0062]    The holding device  100  contains a housing section  34  which is rigidly connected to the joining head  14 . Furthermore, the holding device  100  contains an insertion section  36  which is designed as a hollow section and is connected to one end of the feed passage  22 . The holding device  100  also contains a clamping section  38  which is designed to clamp the stud  26  frictionally in the ready position (by inwardly acting radial forces). The insertion section  36  and the clamping section  38  are in this case formed by an individual collet component  39  which has an annular section (not designated in any more detail) in the top region of the insertion section  36 . The annular section is connected to the feed passage  22 . Extending from the opposite end of the annular section are a plurality of circumferentially distributed clamping arms  40 , on the end of which respective clamping lugs  42  are formed, said clamping lugs  42  extending from the clamping arms  40  radially inward at an angle. 
         [0063]    Furthermore, a plurality of stop arms  44  extend from the annular section. The stop arms  44  are generally longer than the clamping arms  40  and are provided at their ends with stop lugs  46  which are directed radially inward. The stop arms  44  and the clamping arms  40  are arranged alternately, for example, over the circumference of the holding device. The stop arms  44  not only perform the function of a limit stop, as described below, but also a clamping function. The stop arms  44  therefore also form second clamping arms. 
         [0064]    The clamping arms  40  and the stop arms  44  can be elastically expanded in the radial direction. The inside diameter defined by the clamping lugs  42  or the stop lugs  46  is in each case smaller than the outside diameter of the flange section  30 . A stud  26  fed into the holding device  100  is shown in  FIG. 3 , which stud  26  has been fed from above via the stud feed  20  and the feed passage  22  through the insertion section  36 . The stud  26  in this case is shown in a marked oblique position, which generally cannot be avoided with this type of holding device. This is due to the fact that the flange section  30  inside the insertion section  36  comes into contact with the inner side of the clamping arms  40  or of the stop arms  44 . The fact that the outer circumference of the flange section  30  is often of polygonal design and/or the fact that longitudinal grooves are formed between the arms  40 ,  44  can result in such an oblique position. 
         [0065]    Furthermore, a shank-clamping device  50 , which is formed on the joining head  14  such as to be movable in the axial direction, as shown at  52 , is assigned to the holding device. The shank-clamping device  50  has a shank-locating section  54  which contains a plurality of radially expandable arms extending in the axial direction. Furthermore, the shank-clamping device  50  has a centering section  56 . The centering section  56  is formed on the ends of the arms of the shank-locating section  54 , the arms in the region of the centering section running obliquely outward, such that they jointly form an insertion cone. 
         [0066]    To shift the stud  26  into a ready position, the shank-clamping device  50  is moved downward (the specification “downward” relates to the illustration in  FIG. 1  but should not be understood as being restrictive), to be precise into the holding device  100 . In the process, the centering section  56  takes hold of the top end of the shank section  28  and sets the stud  26  upright, the shank section  28  being inserted into the shank-locating section  54  until a top end of the shank section  28  strikes a limit stop of the shank-clamping device  50 . In the process, the stud  26  is supported on the clamping lugs  42 . 
         [0067]    The shank-clamping device  50  is then moved further downward, the flange section  30  first of all being pushed past the clamping lugs  42 . In the process, the clamping arms  40  expand radially outward. The stop lugs  46  of the stop arms  44  prevent the stud  26  from subsequently falling out of the holding device  100  by virtue of the fact that the stop lugs  46  act on the underside of the flange section  30 . Proceeding therefrom, the shank-clamping device  50  is moved further downward, the clamping lugs  42  acting on the outer circumference of the shank-clamping device  50  and thereby clamping the stud  26  fixedly in the shank-clamping device  50 . 
         [0068]    The shank-clamping device  50  is then moved still further downward until the flange section  30  is also pushed past the stop lugs  46 . A ready position is reached when the flange section  30  lies slightly below the bottom end of the stop lugs  46 . A radial force continues to be exerted on the shank-clamping device  50  in this state by the clamping arms  40  and the stop arms  44 , such that the stud  26  is held or clamped frictionally in this position in the holding device  100 . Proceeding therefrom, the joining process described above can be initiated. When the stud  26  is rigidly joined to the workpiece  24 , the joining head  14  is retracted (in which case the shank-clamping device  50  can first of all be moved back inside the joining head  14 ). As a result, the stud  26  is released from the holding device  100 . A further stud  26  can then be fed via the feed passage  22  and a further joining process can be initiated. 
         [0069]    The stud feed can be realized in a largely reliable manner by the double clamping system set up in this way. Nonetheless, a situation may occur in which the stud  26  is in such an oblique position or is jammed in such a way in the holding device  100  that the centering section  56  can also no longer orient the stud  26 . In this case, the stud  26  is ejected downward via the holding device  100 , a factor which leads to the problems mentioned at the beginning. 
         [0070]    An improved embodiment of a holding device according to the invention or of a joining head is shown in  FIGS. 1 and 2 , with which embodiment these problems can be further reduced. 
         [0071]    With regard to construction and functioning, the holding device shown in  FIGS. 1 and 2  generally corresponds to the holding device of  FIG. 5 . The same elements are therefore designated by the same designations. Only the differences are dealt with below. Furthermore, the joining head  14  on which the holding device  60  of  FIGS. 1 and 2  is to be used corresponds to the joining head  14  of  FIG. 5 . The general method sequence during the feeding and readying of studs and the subsequent joining sequence are otherwise identical, insofar as nothing else is mentioned. 
         [0072]    The holding device  60  shown in  FIGS. 1 and 2  is generally constructed like the holding device  100 , namely with an insertion section  36  and a clamping section  38 . However, the holding device  60  additionally has a securing device  62  which prevents a stud  26  fed into the holding device  60  from becoming jammed therein or from assuming such an oblique position that it can no longer be set upright by the centering section  56 . Furthermore, the securing device  62  can alternatively or additionally prevent a stud  26  fed into the holding device  60  from falling back against the feed direction (which could be the case, for example, if a stud  26  is to be joined to a workpiece  24  against the force of gravity, for example “overhead”). 
         [0073]    The securing device  62  is formed in the interior of the holding device  60 . For this purpose, the inside diameter of the clamping arms  40  and of the stop arms  44  is greater than a diameter D 1  of the feed passage  22 . The securing device  62  is formed by a plurality of securing-sleeve sections  64  (in the present case two securing-sleeve sections  64   a ,  64   b ) which together roughly define a sleeve shape and are arranged inside the insertion section  36  concentrically thereto. 
         [0074]    At their top end, the securing-sleeve sections  64  have a respective circumferential section  66 a,  66 b which is accommodated in an inner circumferential groove  68  of the housing section  34 . The collar section  66  is in this case movably accommodated inside the groove  68 . 
         [0075]    The securing-sleeve sections  64  have a respective bearing section  70  which faces the feed passage  22  and contains the collar section  66 . Furthermore, the securing-sleeve sections  64  have a respective securing section  72  which faces the clamping section  38 . 
         [0076]    The securing-sleeve sections  64   a ,  64   b  define an inner passage, which in the region of the bearing section  70  has a diameter which corresponds approximately to the diameter D 1  of the feed passage  22 . At a bottom end of the securing section  72 , the inner passage has a diameter D 2  which is smaller than the diameter D 1  and smaller than the outside diameter D B , shown in  FIG. 2 , of the stud  26 . In particular on account of the movable mounting of the collar section  66 , the securing sections  72  of the securing-sleeve sections  64  can be expanded radially, as shown at  74   a  and  74   b , respectively. Consequently, a stud fed via the feed passage  22  can be directed with its flange section  30  through the securing device  62 . The securing sections  72  expand in the process. Finally, the flange section  30  passes into a position above the clamping lugs  42  of the clamping arms  40 . As soon as the securing sections  72  fall back again into the position shown in  FIG. 1 , the bottom ends of the securing sections  72  engage behind the flange section, as shown in  FIG. 2 . In this case, a clearance space  76  between a bottom end of the securing sections  72  and a top end of the clamping lugs  42  is selected in such a way that the stud  26  can no longer assume any extreme oblique position. Furthermore, the stud  26  is prevented from being able to fall back again in the direction of the feed passage  22 . Consequently, at most an oblique position as shown in  FIG. 2  may arise. In this position, the shank section  28  is generally at a distance from the inner circumference of the securing device  62 , such that the shank-clamping device  50  can take hold of the shank section  28  without any problems (via its centering section  56 ). 
         [0077]    Although the securing device  62  in the present case is formed by two securing-sleeve sections  64  in the interior of the holding device  60 , it goes without saying that such a securing device can also be arranged outside the holding device  60  and can project, for example, via apertures in the holding device at a suitable point into the inner circumference of the holding device  60 . It is also generally conceivable for the securing device  62  to be formed by elastically deformable means at the inner circumference of the holding device  60 . In any case, the securing device  62  is designed to hold the flange section  30  between the clamping section  38  and the securing device  62  in such a secured manner that the stud  26  does not assume an extreme oblique position and cannot fall back into the feed passage  22 . 
         [0078]    In the present case, the securing-sleeve sections  64  are formed from two halves of a cylindrical sleeve which is formed with the collar section  66  at its top end. The two sleeves are tapered in the region of the securing section  72 , as shown at  78   a  and  78   b , respectively, in  FIG. 1 . The result of this is that, when these tapers  78  bear against one another, the securing-sleeve sections  64  in the region of the bearing section  70  than define a wedge space  80  between them. The line of the transition between the bearing section  70  and the securing section  72  then forms a type of tilting axis or pivot bearing  81  for the securing-sleeve sections  64 . 
         [0079]    At their bottom end facing the clamping section  38 , the securing-sleeve sections  64  can be provided with an outer groove  82 . Elastic pressure means  84 , such as, for example, an O-ring made of heat-resistant material or a metallic spring washer, can be arranged in this outer groove  82 . However, an O-ring is preferred, since it can exert a more uniform circumferential force on the securing-sleeve sections  64 . Elastic pressure means may also be arranged, for example, between the groove  68  and the collar section. 
         [0080]    However, it is generally also conceivable for the securing-sleeve sections  64  to also function without such elastic pressure means  84  by virtue of the fact that, depending on position, at least one of the securing-sleeve sections  64  falls back into the position shown in  FIG. 1  after the flange section  30  has been passed through the securing device  62 . 
         [0081]    An alternative second embodiment of the holding device depicted in  FIGS. 1 and 2  is shown  FIG. 3  and, with regard to construction and functioning, generally corresponds to the holding device of  FIGS. 1 and 2 . The same elements are designated by the same designations. Only the differences are dealt with below. Furthermore, the joining head  14  on which the holding device  60  of  FIG. 3  is to be used corresponds to the joining head  14  of  FIG. 5 . The general method sequence during the feeding and readying of studs and the subsequent joining sequence are otherwise identical, insofar as nothing else is mentioned. 
         [0082]    In the holding device  60 ′ shown in  FIG. 3 , no stop arms  44  are provided. Instead, clamping arms  40  are provided which all have the same length and form a cylindrical front portion  41  of the collet component  39 . The collet component  39  has longitudinal slots in the known manner, to provide an elasticity in a radial direction to the clamping arms  40  and achieve the desired radially directed clamping effect. The insertion of the stud  26  takes place as already described in connection with the embodiment shown in  FIGS. 1 and 2  until the stud  26  has reached the securing position with its flange section  30 . 
         [0083]    Subsequently, the shank section  28  of the stud  26  is picked by the shank-clamping device  50  and pushed through the slotted cylindrical front portion  41 . At the same time, the shank-clamping device  50  is constructed so that its outer diameter is slightly smaller than the outer diameter of the flange section  30  of the stud  26 . Thus, the arms  40  are expanded and, at first, clamp the flange section  30  of the stud  26  with their clamping surfaces  43  in place so that the stud  26  cannot fall downwards. Thereby “downwards” refers to the orientation shown in  FIG. 3 . When the flange section  30  is pushed through the slotted cylindrical front portion  41 , the clamping surfaces  43  snap together and clamp the shank-clamping device  50  in place on its outer surface. Then, the shank section  28  of the stud  26  is held by the shank-clamping device  50  so that the stud  26  cannot slip out downwards. The stud  26  is now arranged in the ready position and can be handed over. 
         [0084]    In  FIG. 4  a holding device  60 ″ in an alternative third embodiment is shown. With regard to construction and functioning, the holding device  60 ″ shown in  FIG. 4  generally corresponds to the holding device of  FIG. 5 . The same elements are designated by the same designations. Only the differences are dealt with below. Furthermore, the joining head  14  in which the holding device  60 ″ of  FIG. 4  is to be used corresponds to the joining head  14  of  FIG. 5 . The general method sequence during the feeding and readying of studs and the subsequent joining sequence are otherwise identical, insofar as nothing else is mentioned. 
         [0085]    Contrary to the holding device  60  shown in  FIGS. 1 and 2 , the holding device  60 ″ shown in  FIG. 4  has no securing-sleeve sections to hold the stud  26  in a securing position. The collet component  39  has a plurality of arms  42 ,  44  and is slotted in a longitudinal direction to provide a certain elasticity in a radial direction to the arms  42 ,  44 . A support element  90  in the form of a wire is guided through each of the longitudinal slots between the arms  42 ,  44 , wherein a bend section  92  is positioned within the collet component  39  in a resting position of the support elements  90  thereby reducing a free cross-section within the collet component  39 . 
         [0086]    If now a stud  26  is inserted into the collet component  39 , a certain pressure on the stud  26  is sufficient so that a flange section  30  of the stud  26  can expand the support elements  90  and pass by the bend sections  92 . When the flange section  30  has passed by the bend sections  92 , the radially elastic support elements  90  move back into their initial position shown in  FIG. 4 . Now, the flange section  28  cannot move back any more, wherein the elasticity of the support elements  90  has to be chosen such that a stud  26  cannot expand the support elements  90  solely by its weight. The shank section  28  of the stud  26  is supported at the support elements  90  so that the shank section  28  does not abut the collet component  39  and can reliably be picked by the shank-clamping device  50 . 
         [0087]    In  FIG. 4 , the collet component is provided with clamping arms  40  and stop arms  44  like it is the case in the embodiment shown in  FIGS. 1 and 2 . Of course, it is conceivable that, as an alternative, the collet component  39  has a slotted cylindrical front portion like it is the case in the embodiment shown in  FIG. 3 . 
         [0088]    Although exemplary embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.