RELATED APPLICATION 
     This application claims priority to German Patent Application No. 10120432.9-12 filed Apr. 26, 2001, which application is herein expressly incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a centered double universal joint, especially for driving, or for drives in, agricultural implements and tractors. 
     BACKGROUND OF THE INVENTION 
     U.S. Pat. No. 3,470,712 describes a centered double universal joint. Two outer joint yokes are each articulatably connected via a cross member to two inner joint yokes. Each inner joint yoke is provided with a bearing housing portion. The bearing housing portions are bolted to one another. The bearing housing portions delimit an annular guiding recess which radially displaceably supports a guiding disc. Floatingly arranged annular guiding plates are provided between both sides of the guiding plates and the inner joint yokes in the guiding recess. The guiding disc has a centrally arranged guiding projection which, on both sides, projects from the guiding disc. A bore starts from each end face of the guiding projection. The bore is initially cylindrical and then hollow-spherical. The outer joint yoke arms of each outer joint yoke are connected to one another by a welded-in bridge. Each bridge has a journal projection with a spherical face that engages the associated bearing bore of the guiding projection of the guiding disc. At high torque values and large articulation angles, this design leads to a concentrated load. Thus, the concentrated load leads to an increase in wear in the region of contact between the spherical journal, associated with the bridge, and the respective cylindrical bearing bore which is engaged by the spherical journal. The purpose of the above-described centring means is to control the connected universal joints onto half the articulation angle between an input shaft and an output shaft in order to achieve constant velocity. If wear leads to play, the relationship is disturbed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a centered double universal joint which ensures accurate control of the universal joints that form the double universal joint onto half the articulation angle. This achieves a long service life. 
     In accordance with the invention, a centered double universal joint comprises: A first outer joint yoke has first yoke arms and a first bridge that connects the two first yoke arms. The first bridge carries a first cylindrical bearing journal. A first inner joint yoke includes a first bearing housing portion. A first cross member articulatably connects the first outer joint yoke to the first inner joint yoke. A second outer joint yoke has second yoke arms and a second bridge that connects the two second yoke arms. The second bridge carries a second cylindrical bearing journal. A second inner joint yoke includes a second bearing housing portion. The second bearing housing portion is connected to the first bearing housing portion and forms an annular guiding recess. A second cross member articulatably connects the second outer joint yoke to the second inner joint yoke. A guiding disc has a central guiding projection which projects on both sides of the guiding disc towards the first and the second bearing journal. The guiding projection has a continuous cylindrical bearing bore. The two bearing journals extend into the bearing bore from different ends. The guiding disc is adjustable in the guiding recess. A bearing ball is on each bearing journal. The bearing ball has the shape of an outer spherical zone and has a through-bore. The through-bore enables the bearing ball to be positioned on the bearing journal. The bearing ball is held at least axially with reference to the axis of the bearing journal. The bearing ball has a spherical outer face. A bearing race for each bearing ball is supported in the cylindrical bearing bore. The bearing race supports the spherical outer face of the bearing ball. The bearing race includes a corresponding hollow spherical bore that enables the bearing ball to pivot in all directions. The bearing race and the bearing ball form a preassembled unit. The bearing race has a cylindrical outer face which is adjustably positioned in the cylindrical bearing bore of the guiding disc. 
     An advantage of this design is that a lower surface pressure and, in consequence, greatly reduced wear is achieved between the guiding disc bearing bore and the bearing race, on the one hand, and between the bearing race and the bearing ball, on the other hand. This is achieved by a surface to surface contact between the conforming surfaces. In addition, it is possible to provide at least one of the components with a friction-reducing coating or to produce it from a material with advantageous friction value. The bearing ball and the bearing race can be pre-assembled to form one unit. The bearing ball and race are then connected to the associated bearing journal of the outer joint yoke. Finally, the inner joint yoke can be assembled with the cross member and the outer joint yoke. These units can then be connected to the guiding disc. 
     According to a further embodiment of the invention, the bearing race is formed as one piece and the bearing ball is pressed into the hollow-spherical bore in the bearing race. The axis of the through-bore extends perpendicular relative to the longitudinal axis of the cylindrical outer face of the bearing race. To achieve as large a joint articulation angle as possible, the bearing race is axially delimited by two end faces. The bearing race defines a longitudinal axis on which the cylindrical outer face is centered. The hollow-spherical bore is axially and eccentrically arranged between the two end faces. This embodiment is additionally advantageous since the process of fitting the bearing ball in the bearing race has been facilitated. This is due to, towards one end face, a larger aperture is obtained in the bearing race relative to the hollow-spherical bore. 
     The two bearing housing portions are preferably bolted to one another. Accordingly, a releasable unit is obtained which enables the exchange of components. However, it is also possible to connect the two bearing housing portions to one another after assembly, using a material-locking connection, such as a low-heat welding process. To achieve the largest possible articulation angle and to keep the amount of wear in the bearing region of the guiding disc in the guiding recess to a minimum, annular guiding plates are arranged in the annular guiding recess on both sides of the guiding disc. Also, the guiding recess is closed on the radial outside and open on the radial inside. Further, the outer diameter of the guiding plates is greater than the inner diameter of the guiding recess and smaller than the greatest diameter of the guiding recess. Furthermore, the inner diameter of the guiding plates is greater than the outer diameter of the guiding projection of the guiding disc and smaller than the outer diameter of the guiding disc. 
     An advantageous situation, with respect to strength, is achieved if the bridge forms an integral part of the associated outer joint yoke. 
     Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a side view, partially in section, of an inventive double universal joint. 
     FIG. 2 is an enlarged section view of a detail Z of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     FIG. 1 illustrates a double universal joint with two individual universal joints which are connected to one another by a centring mechanism in accordance with the invention. Both joints each accommodate half the articulation angle when an input shaft is articulated relative to an output shaft in order to ensure constant velocity conditions. The two individual joints are substantially identical in design. 
     The double universal joint according to FIG. 1 comprises a first outer joint yoke  1  with two first yoke arms  2  connected to one another at their free ends by a first bridge  3 . The first bridge  3  carries a first bearing journal  4 . The first bearing  4  journal defines an axis  5  and has a cylindrical outer face. A first bearing ball  6 , with a spherical zone shape, is non-displaceably held on the first bearing journal  4  along the first axis  5 . The spherical outer face of the first bearing ball  6  is given the reference number  7 . The first outer joint yoke  1  is articulatably connected to the first inner joint yoke  8  by a first cross member  10 . Via its two yoke arms  2 , the first inner joint yoke  8  is integrally formed onto the first bearing housing portion  9 . 
     The second joint includes a second outer joint yoke  11  with two second yoke arms  12  of which, again, only one yoke arm  12  is visible. The two second yoke arms  12  are connected to one another by a second bridge  13 . The second bridge  13  carries a second bearing journal  14 . The second bearing journal  14  has a cylindrical outer face and is centered on the second axis  15 . A second bearing ball  16  is non-displaceably held on the second bearing journal  14  along the second axis  15 . The second bearing ball  16  includes a second spherical outer face  17 . The second outer joint yoke  11  is articulatably connected to the yoke arms of a second inner joint yoke  18  by a second cross member  20 . The yoke arms of the second inner joint yoke  18  are being integral with a second bearing housing portion  19 . 
     The first bearing housing portion  9  and the second bearing housing portion  19  are removably connected to one another by bolts  38 . The two bearing housing portions  9 ,  19 , together, form an annular guiding recess  21 . The guiding recess  21  is closed on the radial outside and open on the radial inside. A guiding disc  22  is radially adjustably received together with guiding plates  27  arranged on both sides of guiding disc  22  in the guiding recess  21 . The diameters D 1  to D 6  of the two annular guiding plates  27  of the guiding disc  22  and of the guiding recess  21  are adjusted to one another such that it is possible to carry out the adjustment of the guiding disc  22  in the annular guiding recess  21 . The diameters D 1  to D 6  are set according to the adjustment necessitated by the articulation requirement, and to hold the guiding disc  22  securely held in the recess  21 . 
     In consequence, the diameter D 1  of the guiding recess  21  is greater than the outer diameter D 3  of the two annular guiding plates  27  and greater than the outer diameter D 5  of the guiding disc  22 . The inner diameter D 4  of the two annular guiding plates  27 , however, is smaller than the outer diameter D 5  of the guiding disc  22 . The guiding disc  22  includes a guiding projection  23 . A cylindrical bearing bore  24  is provided in the guiding projection  23 . The guiding projection  23  projects from the planar faces on both sides of the guiding disc  22 . For this reason, the smallest diameter D 2  of the annular guiding recess  21  is greater than the outer diameter D 6  of the guiding projection  23 . Furthermore, the inner diameter D 4  of the annular guiding plates  27  has to be greater than the outer diameter D 6  of the guiding projection  23 . 
     A first bearing race  25  is positioned on the first bearing ball  6 . The hollow spherical bore of the first bearing race  25  is adapted to the spherical outer face  7  of the first bearing ball  6 . The first bearing race  25  includes a cylindrical outside which is displaceably guided in the bearing bore  24  of the guiding projection  23 . The second bearing ball  16 , by its second spherical outer face  17 , is received in a corresponding hollow-spherical bore of a second bearing race  26 . The second bearing race  26 , via its cylindrical outer face, is also displaceably received in the bearing bore  24  of the guiding projection  23 . 
     FIG. 2 is in an enlarged view of the second bearing ball  16  arranged on the second bearing journal  14 . The second bearing ball  16  is provided with a cylindrical through-bore  28 . The second bearing race  26  together with the second bearing ball  16  is received on the cylindrical seat face  29  which represents the outer face of the second bearing journal  14 . The second bearing ball  16 , by means of an end face, is contactingly held against a shoulder  30  of the second bearing journal  14 . The second bearing ball  16  is axially secured along the axis  15  at the second bearing journal  14  by deformation regions  31  distributed along the circumference of the second bearing journal  14  in the region of the end face. The deformation regions  31  lead to an accumulation of material in the region of the other end face of the second bearing ball  16 . 
     Furthermore, with reference to the second bearing race  26  it can be seen that the hollow-spherical bore  32 , with its center on the longitudinal axis  37  of the second bearing race  26 , is offset away from the end face  35  towards the end face  36 . As a result, a collar is obtained towards the end face  35 . In the region of the end face  36 , the hollow spherical bore  32  ends in a circular-shaped aperture. In consequence, in the region of the end face  36 , a sufficiently large articulation relative to the second bearing journal  14  and the second bridge  13  is achieved in order to ensure the desired large articulation angle. The collar formed towards the end face  35  provides the second bearing race  26  with the required stiffness. In this region, sufficient free space exists which enables the second bearing ball  16  to project beyond the end face  35  under articulation conditions. 
     Furthermore, it can be seen that the second bearing race  26  includes a cylindrical outer face  33  which includes a continuous groove  34 . The second bearing race  26  is displaceably supported by the cylindrical outer face  33  in the bearing bore  24  of the guiding projection  23 . The longitudinal axis  37 , on which the hollow spherical bore  32  is centered, simultaneously forms the axis on which the cylindrical outer face  33  and thus the bearing bore  24  are centered. The first bearing ball  6  and the first bearing journal  4  are designed and arranged as described above in connection with the second bearing ball  16  and the second bearing journal  14 . 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.
A centered double universal joint has a first outer joint yoke ( 1) carrying a first cylindrical bearing journal ( 4). A unit including a first bearing ball ( 6) and a first bearing race ( 25) are secured on the journal ( 4). Via a cylindrical outer face ( 33), first bearing race ( 25), the unit is positioned in a bearing bore ( 24) of a guiding projection ( 23) of a guiding disc ( 22). Accordingly, a surface to surface contact is achieved between the cylindrical outer face ( 33) and the bearing bore ( 24) as well as between the hollow spherical bore ( 32) of the bearing race ( 26) and the spherical outer face ( 17) of the bearing ball ( 16). The other outer joint yoke is provided with a similar bearing ball and a bearing race arrangement.