Patent Application
20050008427
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.
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.
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.
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.
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.
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.
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.”
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.
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.
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.
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.
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.
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.
Examples of the invention are represented in the drawing and are described in detail in the following description. Shown in
In
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
A modification 10′ of the fastening system 10 is shown in
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.
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
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.
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
A torque M, which in the representation of
Lastly, a predetermined breaking moment of the welded joint 22 is shown at MS in
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 TN) is inadvertently applied, the nut fractures or its thread strips in every case, since the pre-determined breaking moment MM of the nut is distinctly smaller than the predetermined breaking moment MG of the threaded stud 16, and because of the fact that the tolerance regions TM and TG do not intersect. If, in the representation of
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 MM 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 TG and TS 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
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 101 56 403.1 | Nov 2001 | DE | national |
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.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP02/12468 | Nov 2002 | US |
| Child | 10850565 | May 2004 | US |
