BACKGROUND OF THE INVENTION 
   The invention relates to a wheel nut having a nut body with a widened radial collar and with a thrust washer which is held rotatably and captively on the nut body by a locking means, and having a cap arranged on the nut body. The invention relates, furthermore, to a fastening element of this type. 
   Wheel nuts or fastening elements of this type, such as, for example, wheel screws with a decorative cap, are known in the field of motor vehicle technology. They serve, in particular, for the fastening of wheel rims, and above all of high-quality aluminum rims, but also steel rims, on a wheel hub. The caps are produced from highly polished stainless steel and are placed onto the engagement part of a nut or of a fastening element, said engagement part being in most cases of hexagonal design. 
   It is customary to connect such a cap firmly to the fastening element or to the nut body of a wheel nut by means of electrical ring welding. It is also customary for the cap to be fastened on the nut body by adhesive bonding. 
   When the cap is being welded to the fastening element or to the nut body of the wheel nut, on the one hand, a complicated welding appliance is required. On the other hand, damage to the visible surface of the cap may occur. The highly polished surface of the cap can be dulled by an electrode being applied. The bearing surface of the nut body or of the fastening element, said bearing surface being in engagement with the surface of a thrust washer, may also be damaged. This damage arises due to the fact that the electrodes necessary for the welding operation are applied at these points. Furthermore, satisfactory welding between the cap and the nut body or fastening element is possible only when the surfaces of the individual components are electrically conductive. This means that, for welding purposes, the surface of the components must, for example, be galvanized. 
   Since then, however, other surface coatings, such as the application of a lacquer layer in the form of a zinc lamellar coating, have also been employed, but these are, in part, not electrically conductive and therefore rule out a welded connection. 
   SUMMARY OF THE INVENTION 
   The German laid-open publication DE 101 23 854 A1 discloses a wheel fastening arrangement, in which a thrust ring is held rotatably and captively on the flange of a nut body by means of a beaded edge, with a thrust washer being interposed. However, a decorative cap is merely slipped onto the hexagonal portion of the nut body, without being fastened in a particular way. Such a static-frictional connection between the cap and the nut body may easily lead to the loss of the cap. 
   The object on which the invention is based is to provide a wheel nut or a fastening element of the abovementioned type, which has an uncomplicated construction and the individual components of which can be assembled and held together in a simple way. 
   The solution lies in the wheel nut having the features of patent claim  1  and in a fastening element having the features of claim  2 . The wheel nut according to the invention or the fastening element according to the invention is distinguished in that the thrust washer and the cap are held jointly on the nut body by the locking means. 
   The result of the wheel nut or of the fastening element being designed according to the invention is that the three components, nut body or body, thrust washer and cap, are held in their assembled position reliably and in a functionally appropriate manner as single locking means and in a single operation. Furthermore, there is no longer any need for a complicated and inefficient welding or adhesive bonding of the cap on the nut body or body. Above all, even surface treatments of the individual components can be carried out which are no longer suitable for the welding of the cap and nut body or body because of their electrical conductivity. 
   Advantageous developments may be gathered from the subclaims. 
   In particular, the invention is such that the locking means is designed as an edge which runs around on the thrust washer and which engages both over the collar and over a flange formed at the free end of the cap. Purely as a result of a design of this type, the cap is held jointly with the thrust washer on the nut body or body. 
   The three components, nut body or body, thrust washer and cap, are assembled in a simple way in that a free region at the edge of the thrust washer is provided, which can be oriented radially inward after the assembly of the nut body or body, the thrust washer and the cap. In this case, the free region of the edge can be oriented radially inward by means of a beading method. 
   Assembly of this type has the advantage that all three components, such as nut body or body, thrust washer and cap, can be set in their mutually assigned position simply by the beading of the free region of the edge of the locking means. In this case, it proves advantageous for the flange of the cap to lie on the collar of the nut body or body. 
   In a further embodiment, the nut body or body has, above the collar, a step-shaped shoulder running around, the cap being provided, above the flange, with a step adapted to the shape of the shoulder. 
   This design of the subject of the invention contributes decisively to ensuring that, as a result of the meander-like design, no or virtually no moisture can penetrate between the cap and the nut body or body. The useful life of the wheel nut or of the fastening element is thereby prolonged considerably because of the absence of corrosion. 
   This advantage of leaktightness with respect to moisture is reinforced considerably in that, with the nut assembled or the fastening element assembled, the free region of the edge of the thrust washer is contiguous, flush and in a leaktight manner, to the step which is formed on the cap. 
   The measure whereby the cap sits on the nut body or body by means of a press fit also contributes to ensuring that no or little moisture can penetrate between the cap and nut body or body. 
   In a further embodiment of the invention, at least one of the surfaces of the nut body or body and of the thrust washer which lie on one another is provided with a central recess running around, in such a way that the nut body or body and the thrust washer lie on one another only in their outer radial region. 
   This design has, on the one hand, the advantage that, in the case of any angular tolerances between the bearing surfaces of the nut body or body and of the thrust washer, not only linear bearing contact takes place between these components, but essentially also two-dimensional bearing contact. Moreover, this embodiment results in a more uniform prestressing force of the connection between nut and screw or between fastening element and screw. On the other hand, the recess may be utilized for receiving a suitable lubricant. 
   In a development of the invention, a sealing means may be provided between the locking means and the collar and the cap. Expediently, the sealing means may be provided between the free region of the edge of the thrust washer and the collar or the flange of the cap. This design has the advantage that the penetration of moisture is prevented or at least reduced even between the nut body or body and the thrust washer. 
   In particular, the development may be such that the sealing means is a sealing ring or a sealing washer which is arranged between the flange of the cap, said flange lying on the collar of the nut body or body, and the radially inward-oriented free region of the edge of the thrust washer. 
   An embodiment of this type can be produced in a simple way, reliably in terms of the process and cost-effectively and has a positive effect on the useful life and functional reliability of the connection between screw and nut or screw and fastening means. 


   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention are explained in more detail below with reference to the accompanying drawings in which: 
       FIG. 1  shows an exemplary embodiment of the fastening element according to the invention in the form of a wheel nut, in a diagrammatic sectional illustration which is not true to scale; 
       FIG. 2  shows a further exemplary embodiment according to  FIG. 1 , in a diagrammatic sectional illustration which is not true to scale; 
       FIG. 3  shows a top view of the wheel nut according to  FIG. 1  in a diagrammatic illustration which is not true to scale; 
       FIG. 4  shows a further exemplary embodiment of the fastening element according to the invention in the form of a wheel screw, in a diagrammatic sectional illustration which is not true to scale. 
   


   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   As illustrated in  FIG. 1 , a wheel nut  10  of the invention, which serves for fastening a wheel rim on a wheel hub, consists of a nut body  11  having a radially widened collar  12  running around. The nut body  11  is provided with an axial threaded bore  24  for receiving a threaded bolt. 
   For reasons of decorative appearance, the nut body  11  is encased by a cap  14  which consists of polished stainless steel. In this case, the free open end of the cap  14  terminates in a flange  18  which lies on the radially widened collar  12  of the nut body  11 . Above the collar  12 , the nut body  11  is provided with a shoulder  19  running around, and the cap  14  has a step  20  adapted to the shape of the shoulder  19 . This meander-like design largely prevents the penetration of moisture between the nut body  11  and the cap  14 . In order to optimize this effect, the cap  14  sits on the nut body  11  by means of a press fit. 
   A thrust washer  13  bears rotatably with its surface  28  on a surface  27  of the nut body  11 . The thrust washer  13  is provided with a passage bore  25  and with a locking means  16  which, in the exemplary embodiment, consists of an axially extending edge  17  terminating in the free region  23 . 
   This edge  17  with its free region  23  surrounds the collar  12  of the nut body  11  and, at the same time, the flange  18  of the cap  14 , said flange lying on the collar  12 . In this way, the thrust washer  13  is held captively and rotatably on the nut body  11  and also the cap  14  is held on the nut body  11 . 
   By virtue of this inventive design, the three components, nut body  11 , thrust washer  13  and cap  14 , can be connected to one another in a single operation by the free region  23  of the edge  17  being beaded radially inward. 
   Of course, in addition to the wheel nut  10  being designed according to the invention, the cap  14  can be secured to the nut body  11  by means of an adhesive bond or a weld. According to the invention, however, this is not necessary in terms of the reliable functioning of the wheel nut. 
   Finally, a central radially extending recess  21  is provided in that surface  27  of the nut body  11  which lies on the surface  28  of the thrust washer  13 . The recess is also possible in the thrust washer. This recess  21  prevents the situation where, in the event of angular tolerances between the surfaces  27  and  28 , only linear bearing contact occurs between the surfaces. Shifting of the bearing contact of the surfaces  27  and  28  radially outward reduces the dispersion of the prestressing force between the wheel nut  10  and a screwed-in screw bolt. The recess  21  may be utilized additionally for receiving a lubricant. 
     FIG. 2  shows a development of the wheel nut  10 . Here, the same reference numerals designate components identical to those in the exemplary embodiment described with reference to FIG.  1 . 
   The difference from the exemplary embodiment illustrated in  FIG. 1  is the arrangement of a sealing ring  22  between the flange  18  of the cap  14  and the free region  23  of the edge  17  of the locking means  16 . 
   This embodiment largely prevents a penetration of moisture between the nut body  11  and the thrust washer  13 , so that the formation of corrosion on these components is virtually ruled out. 
   The wheel nut according to the invention thus constitutes a component which can be produced simply and cost-effectively and has high process reliability and which ensures a reliable and long-term functioning. 
     FIG. 4  shows a further exemplary embodiment of a fastening element according to the invention in the form of a wheel screw  10 ′. The wheel screw  10 ′ serves for fastening a wheel rim on a wheel hub and has a screw body  11 ′ with a radially widened collar  12 ′ running around. The screw body  11 ′ is provided, in the axial extension of the collar  12 ′ with a threaded bolt  24 ′. 
   For reasons of decorative appearance, the screw body  11 ′ is encased by a cap  14 ′ which consists of polished stainless steel. In this case, the free open end of the cap  14 ′ terminates in a flange  18 ′ which lies on the radially widened collar  12 ′ of the screw body  11 ′. Above the collar  12 ′, the screw body  11 ′ is provided with a shoulder  19 ′ running around, and the cap  14 ′ has a step  20 ′ adapted to the shape of the shoulder  19 ′. This meander-like design largely prevents the penetration of moisture between the screw body  11 ′ and the cap  14 ′. In order to optimize this effect, the cap  14 ′ sits on the screw body  11 ′ by means of a press fit. 
   A thrust washer  13 ′ bears rotatably with its surface  28 ′ on the surface  27 ′ of the screw body  11 ′. The thrust washer  13 ′ is provided with a passage bore  25 ′ and with a locking means  16 ′ which, in the exemplary embodiment, consists of an axially extending edge  17 ′ which terminates in the free region  23 ′. 
   This edge  17 ′ with its free region  23 ′ surrounds the collar  12 ′ of the screw body  11 ′ and, at the same time, the flange  18 ′ of the cap  14 ′, said flange lying on the collar  12 ′. In this way, the thrust washer  13 ′ is held captively and rotatably on the screw body  11 ′ and the cap  14 ′ is also held on the screw body  11 ′. 
   By virtue of this inventive design, the three components, screw body  11 ′, thrust washer  13 ′ and cap  14 ′, can be connected to one another in a single operation by the free region  23 ′ of the edge  17 ′ being beaded radially inward. 
   Of course, in addition to the wheel screw  10 ′ being designed according to the invention, the cap  14 ′ can be secured to the screw body  11 ′ by means of an adhesive bond or a weld. According to the invention, however, this is not necessary in terms of reliable functioning of the wheel screw. 
   Finally, a central radially extending recess  21 ′ is provided in that surface  27 ′ of the nut body  11 ′ which lies on the surface  28 ′ of the thrust washer  13 ′. It is also possible for the recess to be arranged in the thrust washer. This recess  21 ′ prevents the situation where, in the event of angular tolerances between the surfaces  27 ′ and  28 ′, only linear bearing contact occurs between the surfaces. Shifting the bearing contact of the surfaces  27 ′ and  28 ′ radially outward reduces the dispersion of the prestressing force between the wheel screw  10 ′ and a screwed-on nut. The recess  21 ′ may additionally be utilized for receiving a lubricant. 
   As with the wheel nut  10  illustrated in  FIGS. 1  to  3 , in the wheel screw  10 ′ illustrated in  FIG. 4  a sealing ring may be arranged between the flange  18 ′ of the cap  14 ′ and the free region  23 ′ of the edge  17 ′ of the locking means  16 ′. 
   This embodiment largely prevents a penetration of moisture between the screw body  11 ′ and the thrust washer  13 ′, so that the formation of corrosion on these components is virtually ruled out. 
   The wheel screw according to the invention thus constitutes a component which can be produced simply and cost-effectively and has high process reliability and which ensures reliable and long-term functioning.
The present invention relates to a wheel nut ( 10) having a nut body ( 11) with a widened radial collar ( 12) and a thrust washer ( 13) which is held rotatably and captively on the nut body ( 11) by a locking means ( 16), and having a cap ( 14) arranged on the nut body ( 11). In this case, the locking means ( 16) is designed as an edge ( 17) which runs around on the thrust washer ( 13) and which engages both over the collar ( 12) and over a flange ( 18) formed at the free end of the cap ( 14). The present invention relates, moreover, to a further fastening element of this type, for example a wheel screw.