CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the U.S. national stage application pursuant to 35 U.S.C. §371 of International Patent Application No. PCT/EP2011/061204, filed Jul. 4, 2011, which claims priority from German Patent Application No. 10 2010 047 932.2, filed Oct. 8, 2010, which applications are incorporated herein by reference in their entireties. 
     FIELD OF THE INVENTION 
     The invention relates to a wheel bearing unit, in particular for commercial vehicles, having a wheel hub which is mounted rotatably on a bearing journal by means of two tapered roller bearings which are spaced apart from one another axially with in each case one outer ring, one inner ring and one roller crown ring which is arranged between the former and is formed from tapered rollers and a bearing cage for them, the tapered roller bearings being received by means of their outer rings on in each case one axial stop of the wheel hub such that they are directed counter to one another, and the inner rings of the tapered roller bearings being spaced apart axially from one another by means of a sleeve. 
     BACKGROUND OF THE INVENTION 
     A wheel bearing unit of the generic type for use in a commercial vehicle is known, for example, from U.S. Pat. No. 6,149,244. Here, a wheel hub which carries a wheel is received rotatably on a bearing journal. Two tapered roller bearings which are received on the wheel hub such that they are spaced apart from one another axially serve for this purpose. Here, the axes of inclination of the tapered rollers are inclined with respect to one another and the outer rings are positioned axially with respect to one another, on corresponding axial stops with respect to one another. In the installed state of the wheel bearing unit, the axial end sides of the inner rings, which end sides point away from one another, are fixed in each case axially on the bearing journal or axially on the lock nut which is screwed onto said bearing journal. The end faces which face one another are positioned axially by means of a sleeve which is arranged between them. 
     In the non-installed state, the inner rings and the roller crown rings are captively secured and positionally fixed to a limited extent. In particular, as a result of a partly great axial play, the sleeve is not held between the inner rings and can therefore be displaced radially and therefore make mounting of the wheel bearing unit on the bearing journal more difficult. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the invention is to develop a radial bearing unit of simple production and mounting on the bearing journal. 
     The object is achieved by a wheel bearing unit, in particular for commercial vehicles, having a wheel hub which is mounted rotatably on a bearing journal by means of two tapered roller bearings which are spaced apart from one another axially with in each case one outer ring, one inner ring and one roller crown ring which is arranged between the former and is formed from tapered rollers and a bearing cage for them, the tapered roller bearings being received by means of their outer rings on in each case one axial stop of the wheel hub such that they are directed counter to one another, the inner rings of the tapered roller bearings being spaced apart axially from one another by means of a sleeve, the wheel hub, tapered roller bearings and sleeve forming a self-retaining assembly in the non-installed state of the wheel bearing unit, and both tapered roller bearings being secured axially by means of in each case one securing means which is received directly on the wheel hub and covers the roller crown ring radially at least partially. 
     A simple self-retaining action of the tapered roller bearings and of the sleeve can be achieved during the time up to mounting of the assembly by way of two securing means which are arranged in each case on those sides of the tapered roller bearings which face away from one another. To this end, in particular, a play between the roller crown ring or the inner rings of the tapered roller bearings and the securing means is kept small, in order to limit the axial displacement thereof in the non-installed state. Contact or even a prestress of the securing means with respect to the roller crown ring or the inner ring is avoided at least in the installed state of the wheel bearing unit, in order to avoid frictional wear between components of the tapered roller bearing and the securing means which are loaded with a differential rotational speed. It can be provided here to clamp the securing means with respect to the roller crown ring or the inner ring in the non-installed state and to form a play between the securing means and the tapered roller bearing during mounting. To this end, for example, a securing means can be provided which is configured as a disk spring, is clamped axially between the axial stop and the inner ring or roller crown ring, and the prestress of which is canceled by means of a nut which is screwed onto the bearing journal in order to secure the wheel bearing unit, by it clamping between inner ring and nut and lifting up from the axial stop. 
     In one embodiment, at least one of the two securing means can be formed from a securing ring which is arranged in a radial groove of the wheel hub. Here, the securing ring can be stretched radially to the inside to such an extent that it engages radially over the roller crown ring and forms an axial stop for the inner ring. Here, the securing ring is provided with a small amount of play with respect to the inner ring, in order to avoid friction in the installed state. 
     As an alternative to a self-retaining action of the inner ring by way of the securing ring, the latter can form an axial stop for the roller crown ring, the inner ring being secured axially in the direction of the securing ring with respect to the roller crown ring by means of an annular rim. Here, the bearing cage of the roller crown ring is positionally secured axially, it also being possible here for a minimum play of the securing ring with respect to the bearing cage to be maintained. 
     In another embodiment, the securing ring is configured as a wave ring with a wave profile which is formed over the circumference. The wave ring can have an outer and inner profile formed in the circumferential direction, the outer profile engaging into the radial groove and the roller crown ring being secured axially by means of the inner profile thereof. 
     As an alternative or in addition, at least one of the tapered roller bearings can be fixed positionally by means of a securing ring which is of substantially U-shaped configuration with two limbs which are connected by means of a clip. Here, the free ends thereof of the limbs and corners between limb and clip engage radially into the radial groove, the roller crown ring being axially secured and positionally fixed by means of the limbs in the non-installed state of the wheel bearing unit. In order to increase the bearing area of the securing ring with respect to the roller crown ring, the bearing cage of the roller crown ring, a washer which is recessed in a substantially U-shaped manner can be arranged between the securing ring and the tapered roller bearing. By way of the cutout in the washer, the nut, as central nut, which is screwed onto the bearing journal for axial fixing of the wheel bearing unit can be clamped with the inner ring of the outer tapered roller bearing without removal of the washer. 
     As an alternative to a securing ring for the self-retaining action of the tapered roller bearings, on at least one side which faces a tapered roller bearing, the securing means can be formed from a seal which is arranged between the wheel hub and the inner ring and forms an axial stop for the roller crown ring, and the inner ring can be secured axially in the direction of the securing ring with respect to the roller crown ring by means of an annular rim. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention taken with the accompanying drawing figures, in which: 
         FIG. 1  is a partial section through a wheel bearing unit of the invention with two different securing means for each tapered roller bearing; 
         FIG. 2  is a tapered roller bearing in partial section with a securing ring which engages completely over the roller crown ring radially in accordance with  FIG. 1 ; 
         FIG. 3  is a partial section of a tapered roller bearing with a securing ring which is modified with respect to  FIG. 2  and engages over the roller crown ring partially; 
         FIG. 4  is a partial section through a tapered roller bearing with a securing means which is configured as a seal and is arranged between the wheel hub and inner ring; 
         FIG. 5  is a plan view of a tapered roller bearing with a securing means which is configured as a wave ring; 
         FIG. 6  is a plan view of a tapered roller bearing with a securing ring of U-shaped configuration; 
         FIG. 7  is a diagrammatic view of a washer for the securing ring of  FIG. 6 , and, 
         FIG. 8  is a partial section through a tapered roller bearing with a securing means which is configured as a spring element and is clamped axially between the wheel hub and the inner ring. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. 
     Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and, as such, may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
       FIG. 1  shows wheel bearing unit  100  in a diagrammatic illustration in partial section. The upper part of wheel bearing unit  100  is arranged around rotational axis  102 . In the state in which it is removed from bearing journal  103 , wheel bearing unit  100  forms assembly  101  which is formed from wheel hub  104  which receives a wheel, two tapered roller bearings  105 ,  106  which are spaced apart axially and are spaced apart axially from one another on different diameters on wheel hub  104 , and sleeve  107  which is arranged between tapered roller bearings  105 ,  106 . 
     Tapered roller bearings  105 ,  106  in each case have outer ring  109 ,  110  which is received on wheel hub  104 , inner ring  111 ,  112  which is provided for receiving wheel bearing unit  100  on bearing journal  103 , and roller crown rings  113 ,  114  comprising tapered rollers  117 ,  118  which are received in bearing cage  115 ,  116  such that they are distributed over the circumference. 
     Rotational axes  102   a ,  102   b  of tapered rollers  117 ,  118  are in each case inclined with respect to rotational axis  102  so as to face one another, with the result that wheel hub  104  is mounted radially and axially with respect to bearing journal  103 . Tapered roller bearings  105 ,  106  are placed on axial stops  121 ,  122  by means of outer rings  109 ,  110  on their end sides  119 ,  120  which lie opposite one another. 
     In the installed state of assembly  101 , end sides  123 ,  124 , lying opposite one another, of inner rings  111 ,  112  are clamped on one side by means of a central nut (not shown) which is screwed onto the end side of bearing journal  103  and on the other side by means of axial stop  125 , sleeve  107  spacing inner rings  111 ,  112  apart. 
     In the removed state of assembly  101 , inner rings  111 ,  112  are not prestressed axially against one another, with the result that they are not of captive configuration per se as a consequence of inclined rotational axes  102   a ,  102   b  of tapered rollers  117 ,  118 . In order to produce the positional fixing of inner rings  111 ,  112  and therefore of roller crown rings  113 ,  114  in the removed state of radial bearing unit  100  in the form of assembly  101 , in each case one axial securing means  108 ,  126  which in each case positionally fixes inner ring  111 ,  112  axially is provided on the outer sides of wheel bearing unit  100 . Securing means  108 ,  126  are shown in detail individually in the following figures. 
       FIG. 2  shows a detail of wheel bearing unit  100  of  FIG. 1  with tapered roller bearing  105 . Here, securing means  108  is configured as securing ring  127  which is fitted into radial groove  128  of wheel hub  104  and engages radially over roller crown ring  113  and loads inner ring  111  axially. Here, a contact with inner ring  111  can be produced, which contact limits the axial play of inner ring  111  in such a way that sleeve  107  remains centered between inner rings  111  and  112  ( FIG. 1 ). Here, friction of securing ring  127  with respect to inner ring  111  is kept low, in order, in the installed state, to avoid wear and frictional moments between securing ring  127  which rotates with wheel hub  104  and fixed inner ring  111 . As an alternative, a small axial gap can be provided between securing ring  127  and inner ring  111 . 
       FIG. 3  shows a variant of securing ring  127  which is shown in  FIG. 2 , in the form of securing ring  227  of radial bearing unit  200 , which securing ring  227  is received in radial groove  228  which is made in wheel hub  204  at a small spacing from outer ring  209 . Securing ring  227  engages over roller crown ring  213  partially radially and secures it against axial displacement. Here, axial gap  229  is formed in order to prevent friction and wear between roller crown ring  213  and securing ring  227 . As a consequence of annular rims  230 ,  231  which are provided on inner ring  211 , inner ring  211  is mounted substantially without axial play with respect to tapered rollers  217 , with the result that the inner ring is prevented by annular rim  231  from being deflected axially in the case of axially fixed roller crown ring  213 . The axial fixing of roller crown ring  213  by way of securing ring  227  can be effected at tapered rollers  217  or at the bearing cage (not shown). 
       FIG. 4  shows a detail of wheel bearing unit  100  with tapered roller bearing  106  which faces away from the free end of the bearing journal and axial securing means  126  of which is configured as seal  132 . Seal  132  is introduced between wheel hub  104  and inner ring  112  and, by way of its friction, prevents an axial displacement of the inner ring with respect to wheel hub  104 , as a result of which it positionally fixes inner ring  112  and also roller crown ring  114  in the removed state of wheel bearing unit  100 . 
       FIG. 5  shows wheel bearing unit  300  with wheel hub  304  and tapered roller bearing  305  with inner ring  311  and roller crown ring  313 , which tapered roller bearing  305  is received by means of outer ring  309  on wheel hub  304 . The axial positional securing of inner ring  311  and of roller crown ring  313  is effected by means of securing ring  327  which is provided in this embodiment as wave ring  333  with a wave profile which is configured in the form of radially inwardly and outwardly displaced waves  334 ,  335 . Here, waves  335  engage in each case into radial groove  328  which is provided so as to be circumferential, whereas radially inwardly directed waves  334  axially position roller crown ring  313  and therefore, inner ring  311  axially, since inner ring  311 , in accordance with inner ring  211  of  FIG. 3 , has an annular rim (not shown) which corresponds to annular rim  230 . 
       FIG. 6  shows a further alternative of wheel bearing unit  400  with securing ring  427  which is configured to be open on one side and U-shaped. Here, two limbs  436 ,  437  are connected by means of clip  438 , free ends  439 ,  440  which are rolled up here and corners  441 ,  442  being latched into radial groove  428  of wheel hub  404 . The middle regions of limbs  436 ,  437  sweep over inner ring  411  and form the positional securing for the latter. 
     With reference to  FIG. 6 ,  FIG. 7  diagrammatically shows washer  443  with cutout  444 , as a result of which the contour of washer  443  is adapted to securing ring  427 . Washer  443  can be arranged between securing ring  427  and inner ring  411  and, as a consequence of cutout  444 , permits access of the central nut for loading inner ring  411  during the mounting of the radial bearing unit on the bearing journal. 
       FIG. 8  shows a partial section of wheel bearing unit  500  with tapered roller bearing  505 . In the embodiment shown, securing means  508  is configured as spring element  545 , for example, as a disk spring, which is clamped axially between inner ring  511  and axial stop  546  of wheel hub  504 , with the result that, in the non-installed state of wheel bearing unit  500 , inner ring  511  is positionally fixed and is clamped axially with respect to sleeve  507 , with the result that, in the non-installed state, sleeve  507  remains centered between the two inner rings. During mounting of wheel bearing unit  500  on bearing journal  503 , central nut  547  is screwed onto the bearing journal and therefore, as the dashed illustration of spring element  545 ′ shows, spring element  545  is lifted up from axial stop  546 , with the result that, in the installed state of wheel bearing unit  500 , the prestress of spring element  545  with respect to inner ring  511  and its contact with the latter are canceled. Additional washer  548  can be provided between central nut  547  and the spring element. 
     Thus, it is seen that the objects of the present invention are efficiently obtained, although modifications and changes to the invention should be readily apparent to those having ordinary skill in the art, which modifications are intended to be within the spirit and scope of the invention as claimed. It also is understood that the foregoing description is illustrative of the present invention and should not be considered as limiting. Therefore, other embodiments of the present invention are possible without departing from the spirit and scope of the present invention. 
     LIST OF REFERENCE NUMBERS 
     
         
           100  Wheel bearing unit 
           101  Assembly 
           102  Rotational axis 
           102   a  Rotational axis 
           102   b  Rotational axis 
           103  Bearing journal 
           104  Wheel hub 
           105  Tapered roller bearing 
           106  Tapered roller bearing 
           107  Sleeve 
           108  Securing means 
           109  Outer ring 
           110  Outer ring 
           111  Inner ring 
           112  Inner ring 
           113  Roller crown ring 
           114  Roller crown ring 
           115  Bearing cage 
           116  Bearing cage 
           117  Tapered roller 
           118  Tapered roller 
           119  End side 
           120  End side 
           121  Axial stop 
           122  Axial stop 
           123  End side 
           124  End side 
           125  Axial stop 
           126  Securing means 
           127  Securing ring 
           128  Radial groove 
           132  Seal 
           200  Wheel bearing unit 
           204  Wheel hub 
           209  Outer ring 
           211  Inner ring 
           213  Roller crown ring 
           217  Tapered roller 
           227  Securing ring 
           228  Radial groove 
           229  Axial gap 
           230  Annular rim 
           231  Annular rim 
           300  Wheel bearing unit 
           304  Wheel hub 
           305  Tapered roller bearing 
           309  Outer ring 
           311  Inner ring 
           313  Roller crown ring 
           327  Securing ring 
           328  Radial groove 
           333  Wave ring 
           334  Wave 
           335  Wave 
           400  Wheel bearing unit 
           404  Wheel hub 
           411  Inner ring 
           427  Securing ring 
           428  Radial groove 
           436  Limb 
           437  Limb 
           438  Clip 
           439  Free end 
           440  Free end 
           441  Corner 
           442  Corner 
           443  Washer 
           444  Cutout 
           500  Wheel bearing unit 
           503  Bearing journal 
           504  Wheel hub 
           505  Tapered roller bearing 
           507  Sleeve 
           508  Securing means 
           511  Inner ring 
           545  Spring element 
           545 ′ Spring element 
           546  Axial stop 
           547  Central nut 
           548  Washer
A wheel bearing unit having a wheel hub which is mounted rotatably on a bearing journal by means of two tapered roller bearings which are spaced apart axially. The wheel bearing includes one outer ring, one inner ring and one roller crown ring which is formed from tapered rollers and a bearing cage. The tapered roller bearings are received by their outer rings on one axial stop of the wheel hub such that they are directed counter to one another, and the inner rings of the tapered roller bearings are spaced apart axially from one another by means of a sleeve wherein the wheel hub, tapered roller bearing and sleeve form a self-retaining assembly in the non-installed state of the wheel bearing unit. Both tapered roller bearings are secured axially by one securing means which is received directly on the wheel hub and covers the roller crown ring radially.