Abstract:
A fastening system is providing, the fastening system has a weld stud welded to a sheet metal surface at a weldment portion to form a weld joint. The system additionally has a fracturable nut coupled to the weld stud. The fracturable nut and stud construction is configured to fail under torsional load prior to the failure of the sheet metal or the weld joint.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT International Application PCT/EP02/12468 filed on Nov. 8, 2002, which claims the benefit of German Application DE 101 56 403.1, filed Nov. 13, 2001. The disclosure of the above applications is incorporated herein by reference. 
     
    
     BACKGROUND AND SUMMARY  
       [0002]     The present invention relates to a fastening system for fastening a member to a structural metal part, in particular for fastening a member to sheet metal, such as the sheet metal of the body of a motor vehicle, with a threaded metal stud that is fastened to the structural part in short-time arc welding, and a lock nut that is screwed onto the stud and by which the member is fastened to the structural part. Such a fastening system is known from U.S. Pat. No. 5,579,986 A. The fastening system is frequently used in the automobile industry. It is used there chiefly to fasten elements of the interior fittings to the vehicle body.  
         [0003]     The threaded stud is welded onto a metal sheet of the body in so-called short-time arc welding. Short-time arc welding is also known as stud welding. There a metal stud (threaded stud) is placed on the sheet metal of the body. A pilot current is then turned on and the metal stud is again slightly lifted off from the sheet metal of the body. At the same time, an arc is drawn. Then a welding current is turned on, so that the facing surfaces of metal stud and body sheet metal are fused. The metal stud is then again lowered onto the sheet metal of the body, so that the melts combine. The welding current is turned off and the whole fused mass solidifies.  
         [0004]     A system for stud welding is disclosed in for example the brochure “Neue TUCKER Technologie. Bolzenschweissen mit System!” [New Tucker Technology. Stud Welding with System!], Emhard Tucker, September 1999. A lock nut is then screwed onto the stud, thus projecting from the sheet metal of the body. The nut acts to fix the member to the sheet metal. As a rule, the lock nut is made of synthetic material. The stud may be a coarse-pitch threaded stud or a fine-pitch threaded stud. A matching thread is provided on the lock nut. In the case of a coarse-pitch thread, it is alternatively possible that only one hole is provided on the lock nut. The coarse-pitch thread then cuts a corresponding counter-thread into the hole. Steel studs are welded onto conventional sheet steel. Aluminum studs are welded onto aluminum sheets or other aluminum carriers, recently also frequently used.  
         [0005]     Stud welding is a high-tech process. Frequently, hundreds of such studs are used per vehicle. Individual welding operations are frequently performed by a robot. The total welding time may lie in the range of milliseconds per welding operation in this context. Like any other process, the stud welding process is also subject to failures. Uncovering these is the aim and object of routine quality control. In quality control, the studs are tested for strength. A torque or tension wrench is used for this purpose. Quality controls by torque or tension wrench occasionally find fractures in the stud and fractures of the sheet metal of the body in the region of the welded joint. The reasons for the failures may lie in faulty welded joints, but also in faulty lock nuts. In addition, it may also be that the torque or tension wrench was incorrectly adjusted. Fractures of threaded studs on the one hand and of metal body sheet on the other occur in undefined fashion. It is hard to establish what the reason for the failure was. In addition, reworking of the fractured sheet metal of a car body requires a considerably greater expenditure than reworking in the case of a fractured stud. In a fracture of the stud, a new stud can be welded at the same spot, without the strength of the sheet metal suffering.  
         [0006]     The threaded stud known from U.S. Pat. No. 5,579,986 A mentioned at the beginning has between two threaded sections a weakened area that serves to remove an upper threaded section while a lower threaded section remains on the stud. It is also known, from DE 38 02 798 A1, to provide a stud with a predetermined breaking point wherein the strength of the predetermined breaking point is adapted to the metal sheets to be joined, and excessive deformation of the metal sheets is avoided. The predetermined breaking point is always used for removing the undesired shaft of the stud. Lastly, the document DE 100 04 720 C1 describes a device and a method for testing the attachment point of an externally threaded stud for torsional strength. In order to test the weld point for torsional strength, a driving member is chucked in a rotary driver by the clamping stud and the driver is set to a specified torque. Then a threaded part is screwed onto the external thread of the weld stud being tested. If its weld point does not withstand the specified torque, it separates.  
         [0007]     Against this background, the problem underlying the invention is to indicate an improved fastening system of generic type, which in particular requires little reworking. This object is accomplished in the fastening system mentioned at the beginning in that the strength of the welded joint between the structural part and the threaded stud and the strength of the stud itself are adapted to one another so that, upon application of a torque that exceeds that torque which is applied per specification when the lock nut is screwed onto the threaded stud, it is ensured that the stud fractures before the structural part fractures.  
         [0008]     According to another aspect, the above object is accomplished by the fastening system mentioned at the beginning in that the strength of the welded joint between the structural part and the threaded stud and the strength of the thread of the stud itself are adapted to one another so that, upon application of a torque that exceeds that torque which is applied per specification when the lock nut is screwed onto the threaded stud, it is ensured that the thread of the stud is damaged before the structural part fractures. This ensures that whenever too high a torque is applied to a threaded stud having a “good” welded joint, in every case the stud fractures or its thread is damaged, and not the structural part. In this way, reworking costs due to incorrectly adjusted torque or tension wrenches are reduced. Even when an incorrect (too strong a) lock nut is used, it is ensured that damage of the structural part is largely ruled out when the welded joint between the stud and the part is “good.” 
         [0009]     In this connection, a “fracture” is intended to mean any damage to an element (lock nut, stud, structural part) in which a torque applied to the respective element can no longer be transmitted to a following element of the fastening chain. A fracture of the structural part generally is intended to signify that the part is structurally damaged, and in particular, that it pulls out in the region of the welded joint. In this way, the object is fully accomplished.  
         [0010]     It is of special advantage when the threaded stud is weakened at one spot and when the weakening is designed so that the stud fractures at the point of weakening before the structural part fractures in the region of the welded joint between the structural part and the stud. This embodiment has the advantage that strengthening of the structural part (sheet metal of the body of the vehicle) is unnecessary to ensure that, upon application of an excessively high torque, the stud will fracture before the part fractures. There weakening may be effected in many ways, for example, by the selection of material, by the construction of the stud, etc. The case in which the thread of the stud becomes unusable, i.e., is no longer able to transmit torque, should also be understood as a fracture. Alternatively, by a fracture it is to be understood that the threaded stud as a whole breaks off against its foot, substantially without damaging the welded joint structurally.  
         [0011]     It is of special advantage when the stud has a weakening recess, in particular a peripheral groove. Such a weakening recess makes it possible to ensure, in structurally simple fashion, that according to the invention first the stud fractures before the structural part fractures when an excessive torque is applied. The weakening recess may be produced by for example machining. A useful example embodiment of such a weakening recess is disclosed in GB 2 153 948 A, the disclosure of which is incorporated in the present application by reference.  
         [0012]     According to another preferred embodiment, the threaded stud has a flange section that is arranged in the neighborhood of the welded joint and against which the member is screwed by the lock nut or against which the lock nut itself is screwed. This measure likewise contributes to the fact that, when too high a torque is applied, the stud in every case fractures in the region of the welded joint before the structural part fractures. This ensure that the tensile forces occurring when the lock nut is screwed on bear on the stud and not on the part. It is therefore possible to concentrate the weakening of the stud in such a way that weakening takes place with regard to the torque or the torsional force that is applied by the lock nut to the stud. At the same time, it is especially preferred when the weakened spot is arranged in the neighborhood of the flange section. In this way, weakening can be produced relatively easily in the region of the transition between flange section and the actual threaded section (shaft section). In the simplest case, weakening is already produced in that a relatively sharp-edged transition is provided from the actual threaded section to the flange section.  
         [0013]     According to an additional preferred embodiment, the stud is a coarse-pitch threaded stud whose external thread, when the lock nut is screwed on, cuts a thread into its hole. According to an alternative embodiment, the threaded stud has a fine-pitch thread such as a metric thread and the lock nut has a corresponding internal thread. In addition, it is preferable when the strength of the threaded stud and the strength of the lock nut are adapted to one another in such a way that, upon application of a torque to the lock nut that exceeds that torque which per specification is applied when the lock nut is screwed onto the threaded stud, it is ensured that the lock nut is structurally damaged before the stud is structurally damaged. As a rule, the lock nut is made of synthetic material and is an element that is comparatively inexpensive to produce. In this respect, it is of special advantage when, upon application of too high a torque, in every case the nut breaks before the stud breaks or its function is adversely affected in any way.  
         [0014]     On the whole, in this way a closed process chain is obtained in which the predetermined breaking moment of the lock nut is smaller than the pre-determined breaking moment of the threaded stud, which in turn is smaller than the predetermined breaking moment of the structural part and/or of the welded joint between the structural part and the stud. It goes without saying that the features mentioned above and to be explained below are usable not only in the combination indicated in each instance, but are also usable in other combinations or standing alone, without exceeding the scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     Examples of the invention are represented in the drawing and are described in detail in the following description. Shown in  
         [0016]      FIG. 1  is a schematic sectional view of a first embodiment of a fastening system according to the invention;  
         [0017]      FIG. 2 , a detailed view of a modified embodiment of a fastening system, in section;  
         [0018]      FIG. 3 , a sectional representation of an additional embodiment of a fastening system according to the invention; and  
         [0019]      FIG. 4 , a diagram with a qualitative representation of a variety of relevant torques of the fastening system of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     In  FIG. 1 , a first embodiment of a fastening system of the present invention is labeled generally  10 . The fastening system  10  acts to fasten a member  12 , in the case represented a part of synthetic material traversed by an aperture  13 , to a structural part  14 , in the present case the sheet metal  14  of a car body. The fastening system  10  includes a threaded stud  16 , which is welded onto the sheet metal  16  [sic; should be:  14 ] of the car body in the stud welding process. In addition, the fastening system  10  contains a lock nut  18  made of synthetic material, which is capable of being screwed onto the stud  16 . The stud  16  contains a flange section  20 . In the present case, a flange section is intended to mean a section with a fairly great diameter that is at least twice as great as the shaft section of the stud. The threaded stud  16  is welded in the stud welding process by the underside of its flange section  20  onto an upper side of the car body sheet metal  14 . The welded joint  22  is shown schematically in  FIG. 1 . On the opposing side of the flange section  20  there is provided a shaft section  24 , on which is formed a coarse-pitch thread  26 .  
         [0021]     In the region of the transition between the coarse-pitch thread  26  and the flange section  20 , the threaded stud  16  in addition has a weakened section  28 , which in the present case is formed by a peripheral groove  30 . The peripheral groove  30  represents a predetermined breaking point of the stud, as will be explained below in detail. The lock nut  18  has a hole  32  and the diameter of the hole  32  is adapted to the diameter of the shaft section  24 . The coarse-pitch thread  26  is designed as a self-cutting thread and therefore an internal thread is cut into the hole  32  when the lock nut  18  is screwed onto the stud  16 . As can be seen in  FIG. 1 , the aperture  13  of the member  12  is slipped onto the threaded stud  16 . Then the lock nut  18  is screwed on, so that the member  12  is held between the upper side of the flange section  20  and the lower side of the lock nut  18 . In  FIG. 1 , it is indicated schematically how a torque M applied to the lock nut  18  is converted in the region of the thread  26  into an axial force A, which produces a tensile force on the stud  16 , and into a tangential force T, which in turn exerts a corresponding moment on the threaded stud  16 .  
         [0022]     A modification  10 ′ of the fastening system  10  is shown in  FIG. 2 . In the fastening system  10 , the threaded stud  16 ′ is designed with a flange section  20 ′, which lies between a shaft section  24 ′ and a welded section  34 . When a threaded stud  16 ′ is welded onto the sheet metal of a car body  14 , a welded joint  22 ′ is produced between the welded section  34  and the sheet metal  14 . Therefore, a space  36  remains between the upper side of the sheet metal  14  and the underside of the flange section  20 ′. The diameter of the welded section  34  is selected greater than the diameter of the shaft section  24 ′. On the whole, therefore, a welded joint  22 ′ can be obtained with a strength that is greater than that strength which is obtainable when the diameter of the welded section  34  is equal to the—specified—diameter of the shaft section  24 ′. Owing to the space  36 , back ventilation is obtained, so that corrosion problems are avoided. Otherwise, the fastening system  10 ′ does not differ from the fastening system  10 , so that reference is made to the description of the latter.  
         [0023]      FIG. 3  shows an additional embodiment of a fastening system  40 .  
         [0024]     The fastening system  40  acts to fasten a member  42  in the form of a metal tube to a structural part  44 , such as the sheet metal of a car body.  
         [0025]     The fastening system  40  has a threaded stud  46 , which is welded by a stud-welding process to the sheet metal  44  of a car body. In addition, the fastening system  40  includes a lock nut  48  in the form of a clip of synthetic material. The threaded stud  46  has a flange section  50 , which corresponds to the flange section  20 ′ of the fastening system  10 ′ of  FIG. 2 . A welded joint between the threaded stud  46  and the sheet metal  14  of a car body is shown at  52 . A shaft section  54  of the stud  46  is provided with a metric thread  56 .  
         [0026]     The threaded stud  46  is weakened in the region of the transition between the shaft section  54  and the flange section  50 , as is shown schematically at  58 . In the fastening system  40 , weakening is effected only in that the diameter of the shaft section  54  is distinctly smaller than the diameter of the flange section  50  and a welded section lying under the latter and not described in detail. In addition, the transition between the shaft section  54  and the flange section  50  is designed as a sharp-edged corner. The lock nut  48  has a hole  60 , which is provided with an internal metric thread  62 . Therefore, the lock nut  48  (the clip of synthetic material) can be screwed onto the threaded stud  46 . In the present case, the clip of synthetic material is screwed onto the threaded stud  46  until an underside of the clip  48  strikes an upper side of the flange section  50 . The member  42 , in the form of a metal tube, is fixed exclusively to the clip  48  of synthetic material. In the embodiment shown, a recess  64  is provided for the accommodation of the metal tube  42 . In addition, the clip  48  of synthetic material has a flexibly seated locking strap  66 , which is designed for the purpose of closing off the recess  64  and so accommodating the metal tube  42  form-lockingly in the clip  48 .  
         [0027]     It is understood that in all three embodiments of FIGS.  1  to  3 , the threaded studs  16 ,  46  and the sheet metal  14 ,  44  of a car body may in each instance consist of steel or a steel alloy or of aluminum or an aluminum alloy. It is also understood that the lock nuts  18 ,  48  may be made of a material other than synthetic material, provided that the strength requirements explained below with reference to  FIG. 4  are met. The member  12  may alternatively be a metal element. Correspondingly, the member  42  may alternatively be an element of synthetic material. In all three embodiments, the strengths of the separate elements are adapted to one another, as is shown schematically in  FIG. 4 .  
         [0028]     A torque M, which in the representation of  FIG. 1  is applied to the lock nut  18  in order to fasten the member  12  to the sheet metal of a car body, is plotted on the abscissa in  FIG. 4 . In order to obtain proper fastening of the member  12 , the lock nut  18  is screwed on with a given rated torque M N , which in  FIG. 4  is represented qualitatively as greater than zero. The rated torque M N  is assigned a tolerance region T N , within which the rated torque M N  typically applied by a torque wrench or tension wrench varies. Upon application of the rated torque M N , assuming failure-free parts and a failure-free welded joint  22 , proper fastening of the member  14  is obtained. A predetermined breaking moment of the lock nut  18  is additionally shown at M M  in  FIG. 4 . The predetermined breaking moment M M  is qualitatively higher than the rated torque M N . The predetermined breaking moment M M  is assigned a tolerance region T M , within which the lock nut  18  fractures or its thread is destroyed. At the same time, care should be taken to see that the tolerance regions T M  and T N  do not intersect, but preferably adjoin one another.  FIG. 4  additionally shows a predetermined breaking moment M G  of the threaded stud  16 . The predetermined breaking moment M G  is qualitatively higher than the predetermined breaking moment M M  of the lock nut  18 . The pre-determined breaking moment M G  is assigned a tolerance region that does not intersect with the tolerance region T M  of the lock nut  18 , but directly adjoins it.  
         [0029]     Lastly, a predetermined breaking moment of the welded joint  22  is shown at M S  in  FIG. 4 . The predetermined breaking moment M S  is distinctly greater than the predetermined breaking moment M G  of the stud  16 . The predetermined breaking moment M S  of the welded joint  22  is likewise assigned a tolerance region T S . The tolerance region T S  of the predetermined breaking moment M S  of the welded joint  22  does not intersect with the tolerance region T G  but, rather, lies at a considerable distance apart from it. It is therefore ensured that the maximum predetermined breaking moment M G  still capable of being borne by a threaded stud (the upper limit of the tolerance region T G ) is distinctly smaller than the minimum predetermined breaking moment M S , at which the welded joint  22  could fracture. For purposes of simple representation, only one fracture of the welded joint  22  has been mentioned regarding  FIG. 4 . However, it is understood that this is intended to mean a fracture of the welded joint and/or of the sheet metal of a car body.  
         [0030]     This “closed process and fastening chain” of rated torque and pre-determined breaking moments ensures that, in every operating condition, the element whose replacement results in the lowest costs is always the one that fractures. If, when the lock nut  18  is screwed onto the member  12 , too high a torque M (greater than the upper limit of the tolerance region T N ) is inadvertently applied, the nut fractures or its thread strips in every case, since the pre-determined breaking moment M M  of the nut is distinctly smaller than the predetermined breaking moment M G  of the threaded stud  16 , and because of the fact that the tolerance regions T M  and T G  do not intersect. If, in the representation of  FIG. 1 , an incorrect lock nut  18  (a lock nut with too high a strength) has inadvertently been selected, the distinct distance apart of the tolerance regions T G  and T M  in every case ensures that first the stud  16  fractures (usually at its predetermined breaking point  30  or by destruction of its thread), and therefore no damage to the welded joint  22  or to the sheet metal  14  of the car body occurs. For all sources of error that may occur in the fastening system  10 , it is therefore ensured that the welded joint  22  and the sheet metal  14  of the car body are not unnecessarily damaged.  
         [0031]     In quality control of the threaded stud before the lock nut  18  is screwed on, a test moment that is equal to the predetermined breaking moment M M  of the specified lock nut  18  is usually applied to the stud. A fiberglass-reinforced test nut is usually used for this purpose. If, in this testing, too high a torque is inadvertently applied, the distance between the tolerance regions T G  and T S  ensures that in every case the stud  16  fractures and the welded joint  22  and the sheet metal  14  of the car body are not damaged. The above description of the various moments and the closed process chain is correspondingly applicable to the embodiments of  FIGS. 2 and 3 . In the case of the embodiment of  FIG. 3 , the clip  48  of synthetic material represents the lock nut. It is understood that the thread match between the studs  16 ,  46  and the lock nuts  18 ,  48  should be selected so that, in case of destruction of the thread of the lock nuts  18 ,  48 , unscrewing should nevertheless be possible, so as to prevent unnecessarily high torques from being applied to the studs  16 ,  46  upon unscrewing. Because of the closed process chain, the lock nut  18 ,  48  (which usually is made of synthetic material) is the “weakest link.” The next weakest link is the fastening stud  16 . The welded joint  22  or  52  has the greatest strength.