Abstract:
A method for axially preloading a bearing-hub assembly having a rotatable hub, provided with rolling tracks and a bearing unit having a stationary radially outer ring, provided with rolling tracks, and a double row of rolling bodies disposed between the radially outer ring and the hub, wherein the method requires the use of a biasing means and provides the following steps: a) feeding to an assembly station a hub-bearing assembly, already assembled, b) verifying the value of the interference (C) between the radially outer ring and the biasing means, c) exerting an axial force (F) radially on the outer ring for the biasing means, to deform under compression the radially outer ring that in turn will transmit a force to the rolling bodies, by means of the raceways.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Italian patent application no. 102015000013555 filed on Apr. 29, 2015, the contents of which are fully incorporated herein by reference. 
       TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method for preloading a bearing unit—flanged hub, i.e. the so-called third generation rolling bearing. The unit in question is suitable for applications where the outer ring of the bearing is stationary, or for applications at a driving wheel of a motor vehicle. 
       TECHNICAL BACKGROUND 
       [0003]    They are already known to the state of the technique bearing groups—the flanged hub for driving wheels of motor vehicle applications. The document EP 2602123 A1, for example, describes a hub bearing unit, in this case asymmetrical, for the wheel of a motor vehicle, which includes a flanged hub rotatable around a rotation axis, a flange integral with the hub flanged and transverse to the axis of rotation, a stationary ring disposed radially outside of the flanged hub and provided with rolling tracks axially spaced from one another, and two rolling bodies crowns (for example, balls) arranged between the stationary ring and the flanged hub. The flanged hub integrally form a radially inner raceway for the ball bearing axially outer, while the radially inner raceway for the balls axially inner crown is formed on an inner ring of the bearing, radially outer planted on flanged hub. 
         [0004]    Such a realization, especially in the case of heavy duty applications in terms of load transmitted, entails considerable local loads between the bearing rings and rolling bodies; also does not permit to obtain large values of resistance of the bearing and its greater duration in the time. Finally, usually it presents important axial dimensions, due to the presence of a flange integral to the flange portion and the hub transverse to the axis of rotation. 
         [0005]    To increase the performance and especially the stiffness of the bearing is required to increase the distance of the pressure centers. This can be achieved by increasing the diameter of the circumference of the centers of the rolling bodies (the so-called “pitch diameter” or more simply “pitch”) of the bearing. Such solutions are already known and are developed in order to significantly improve the performance. The disadvantage connected to the increase of the “pitch” is that consequently also the volume and therefore weight dramatically increase with the “pitch-squared value”. This increase in weight can usually not be accepted by car manufacturers. 
         [0006]    A further improvement is to increase even more the diameter of the circumference of the centers of the rolling bodies so as to be able to enter inside the bearing constant velocity joint and integrate in a single piece the so-called bell of the joint with the hub, or with the inner ring of the bearing. Evidently, the integration of both components allows the reduction of weight and cost of the entire unit. And ‘possible to further reduce weight and costs by integrating also the small inner ring of the bearing, the axially internal, with the bell of the joint. In other words, the hub also assumes the function of single inner ring of the bearing and the bell of the joint at a constant speed. 
         [0007]    The concept of single inner ring is already known in so-called third-generation bearings. The main difference compared to the other known solutions consists in the fact that the bearing has an axial clearance, which is not axially preloaded. This feature, in the past has been accepted for standard applications with no particularly heavy load conditions, and also because the bearing design did not allow the generation of no axial preload. With the development of applications that require high performance, this situation is no longer acceptable and the bearing must necessarily be axially preloaded. 
         [0008]    There exists, therefore, the need to develop a method for preloading a bearing-hub unit with high performance. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide a method for axially preloading a bearing-hub unit for application of the driving wheel of a motor vehicle. The above method is applicable both in the case of bearings with symmetrical internal form, or with the same “pitch” for the two crowns of rolling bodies, both in the case of asymmetric bearings, or with different “pitch” between the two crowns of bodies of rolling. 
         [0010]    According to the present invention it is described a method for axially preloading a bearing-hub assembly, having the characteristics set out in the appended independent claim. 
         [0011]    Additional embodiments of the invention, preferred and/or particularly advantageous, are described according to the characteristics set forth in the appended dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    The invention will now be described with reference to the accompanying drawings, which illustrate some examples of non-limiting embodiments, in which: 
           [0013]      FIG. 1  is an axisymmetric sectional view of a hub bearing assembly of symmetric inner shape, 
           [0014]      FIG. 2  is an axisymmetric sectional view of a hub bearing assembly of asymmetric internal form, 
           [0015]      FIG. 3  illustrates the geometrical conditions to be imposed to the tool that generates the preload on the bearing, 
           [0016]      FIG. 4  schematically shows the step of generation of the preload. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring now to  FIG. 1 , a hub-bearing assembly according to a preferred embodiment of the invention is indicated as a whole with  10 . 
         [0018]    The assembly  10  includes a rotatable hub  20  and a bearing unit  30 . The hub  20 , as will be seen better hereinafter, is configured to also take the inner rolling loop function of the bearing bell and a joint of constant speed drive. Throughout the present description and in the claims, terms and expressions indicating positions and orientations such as “radial” and “axial” refer to the axis X of the central bearing rotation unit  30 . Expressions such as “axially outer” and “axially inner” are, however, referred to the mounted condition, and in this case, preferably, refer to a wheel side and, respectively, to a side opposite to the wheel side. 
         [0019]    The bearing unit  30  comprises a radially outer ring  31 , stationary and two crowns of rolling bodies  32 ,  33 , in this example balls, interposed between the radially outer ring  31  and the hub  20  with radially inner ring function. For simplicity of graphic representation references  32  and  33  will be attributed both to individual spheres, both to the crown of spheres and in particular with  32  will indicate the ring of balls or the single axially outer sphere, while with  33  will indicate the ring of balls or the single axially inner sphere. Again for simplicity, we often use the term “sphere” used by way of example in the present description and in the accompanying drawings instead of the more generic term “rolling body” (and will also be using the same numerical references). It will be understood always that in place of the spheres it may be used any other rolling I cover (for example, rollers, tapered rollers, needles, etc.). 
         [0020]    In  FIG. 1  is shown the case of a symmetrical bearing, ie with the same “pitch” for both crowns of rolling bodies. What will be said in the following description is also applicable to the case of the asymmetrical bearing, shown in  FIG. 2 , where for simplicity we have used the same references. As can be seen the only substantial difference between the two types of bearing is that, while in the case of symmetrical bearing the radii R, R′ of the circumferences of the centers of the rolling bodies of the corresponding crowns  32 ,  33  assume the same value, in case of asymmetrical bearing the same rays differ between them. In the example in  FIG. 2 , in particular, the radius R of the circle of the centers of the rolling bodies of the crown  32 , axially outer, is greater than the radius R′ of the circumference of the crown of the rolling bodies of the centers  33 , axially inner. 
         [0021]    The radially outer ring  31 , preferably shaped in tubular form axially extended, defines internally the raceways  34 ,  35  to the corresponding rolling bodies of the crowns  32 ,  33 . 
         [0022]    The radially outer ring  31  has a radially outer cylindrical surface  31   a  adapted to mate with a cylindrical seat formed in a fixed member of the vehicle, such as a pillar of a suspension, of a known type and not shown in FIG. The cylindrical surface  31   a  extends for the entire axial dimension of the radially outer ring  31  and has an axial dimension of the order of magnitude of that of the cylindrical seat of the upright. On top of the raceways  34 ,  35 , the outer ring also presents a tubular portion  31 ′. 
         [0023]    The rolling bodies of the crowns  32 ,  33  rotate, as well as on the ring radially outer  31 , on a central tubular portion  21  of the hub  20  which defines a rolling track  36  for the rolling bodies of the axially outer crown  32  and a track of  37  for the rolling of rolling bodies of the crown  33 , the axially inner. The crowns  32 , rolling bodies  33  are kept in position by corresponding cages  38 ,  39 , namely a first cage  38 , axially outer, for the rolling bodies  32  and a second cage  39 , axially inner, for the rolling bodies  33 . The pockets of the cage  38  housing the balls are directed in an axially outer direction (as shown in  FIGS. 1 and 2 ), even if the cage  38  can be mounted according to an alternative procedure in order to have the pockets facing in the direction axially inner. The cage pockets  39  are addressed according to the axially outward direction. 
         [0024]    A first sealing means  40  seals the bearing unit from the axially outer side, being planted on a cylindrical surface  21   a,  a radially outer, of the tubular portion  21  of the hub  20  and a second sealing means  41  seals the bearing unit from the axially inner side. 
         [0025]    The hub  20  also defines a shoulder  22 , on the axially inner side, and a flange portion  23  axially outer. The flange portion has a plurality of axial fixing holes  24 , the axes of which are arranged along a circumference of radius r, with respect to the axis of symmetry X. These holes are the seats for the same number of fixing means (such as captive bolts, not shown in the figure) that connect in a known manner an element of the motor vehicle wheel, for example the brake disc (also of known type and not shown in the figure), to the hub  20 . 
         [0026]    Advantageously, a bushing  50  for centering the wheel of the motor vehicle can be realized by means of a metal sheet, for example of steel, and coupled to the flange portion  23  of the hub, preferably by means of a simple pressing-fit operation. 
         [0027]    The bearing unit  30  is made so as to obtain the radii R, R′ of the circumferences of the centers of the rolling bodies of the corresponding crowns  32 ,  33  slightly lower than or substantially equal to the radius r of the circumference of the fixing axes of the holes  24 . This substantial geometrical equality, in the case of asymmetrical bearing, will be verified at least with the radius R of the circle of the centers of the rolling bodies  32 , axially external, ie closest to the wheel of the motor vehicle brake disc. By means of this solution, the flanged hub assumes a conformation in the manner of a rotor, i.e. its flange portion is much reduced. The above solution is obtained by increasing the radius R, R′ of the circumferences of the centers of the rolling bodies and of course, leaving unaltered, since the case of a constraint of the motor design, the radius r of the circumference of the fixing axes of the holes  24 . 
         [0028]    The transmission of the motion from a drive shaft of a motor vehicle transmission assembly (not shown) is guaranteed by a constant velocity joint  60 . In particular, the outer bell of the coupling  60  is integrated in the hub  20  which in its axially inner terminal part  61  He defines the track  62  of the joint. 
         [0029]    The procedure for mounting the bearing assembly—hub illustrated in  FIGS. 1 and 2  was defined ex-novo as it requires special care. Two possible solutions for assembling the assembly have been described in two different and parallel patent applications, which will become accessible to the public almost simultaneously to the content of the present patent application. Anyway, all the mounting procedures for bearings having a double ball bearing with only one inner ring and one outer ring, including new methods mentioned above, are suitable for bearings with game. Therefore, it is necessary to generate the preload on the bearing-hub assembly already assembled. 
         [0030]    The following will describe the new method for the generation of the axial preload on the bearing, with the aid of  FIGS. 3 and 4 . This method is based on the ring deformability radially outer  31 . 
         [0031]    In more detail, as visible in  FIG. 3 , the radially outer ring  31  comprises the tubular portion  31  on which exert a pressure from outside. The effect of the pressure tends to compress the ring  31 . Appropriately calibrating the value of the pressure, the outer ring compression causes a pressure is generated also between the rolling bodies  32 , axially outside, and the respective tracks  34 , radially outer, and  36 , the radially inner. The same pressure will also be generated between the rolling bodies  33 , axially inner, and the respective tracks  35 , radially external, and  37 , the radially inner. This state of stress between the rolling elements and respective tracks is precisely the desired axial preload. 
         [0032]    In order to generate this state of stress, it is possible to act on the interference between the radially outer ring  31 , and a biasing means  70 , which advantageously may be the upright of a suspension or any other tool of annular shape. In practice, said A, the ring outer radius of said radially outer  31  and B the inner radius of the means of annular thrust, the C interference will be given by: 
         [0000]    
       
      
       C=A−B  
      
     
         [0033]    The greater the interference C, the higher the axial preload generated on the crowns of the rolling bodies  32 ,  33 . 
         [0034]    The extent of the deformation of the outer ring  31  depends on the ratio between the diameter of the circumference of the rolling bodies (the “pitch”) and the outer ring thickness, or rather of its tubular portion  31 ′. In the case of the solutions described above (therefore both in the case of symmetrical bearing, which in the case of asymmetrical bearing), with high value of the “pitch”, the outer ring deformation is greater than that of a bearing with small value of “pitch”. So the thickness of the outer ring will properly be sized to avoid excessive deformations. On the contrary, in the case of small values of “pitch”, you will have to use a radially outer ring with smaller thickness of the tubular portion, so as to ensure the sufficient axial preload the bearing unit. 
         [0035]    The radially outer ring  31  must not necessarily have a completely cylindrical shape. For example, protrusions or grooves may be present to improve the locking of the strut of the suspension in the correct position. What is important is that both the cylindrical portion immediately above the rolling tracks. 
         [0036]    This solution, allows generation of the axial bearing preload on the unit, even if one is forced to utilize a mounting procedure that leaves the bearing with axial play. In particular, without this solution it would be impossible to generate a preload and, therefore, the solution with single inner ring would not be usable, since the mounting procedures devised by the same Applicant leave the bearing with axial play. 
         [0037]    So, thanks to the axial preload generation method, as defined, you can use these new buffer solutions whose performance is greatly improved compared to the standard solution. Since it is increased the “pitch” of the spheres, is also increased the distance between the pressure centers, with very positive effects in terms of stiffness. 
         [0038]    The useful life of the bearing is not subject to penalties, since the number of balls used is comparable with that required by the standard solutions. On the contrary, the weight of these solutions was reduced greatly, having regard to the compactness of the realized solution, considered as a whole. In fact, while they are bearing-coupling constant speed remains substantially the same weight, to the upright of the suspension can delete a remarkable amount of material. 
         [0039]    Finally, not negligible are also the benefits for having reduced the costs of the solution and the total number of components. 
         [0040]    In addition to the embodiments of the invention, as described above, it is to be understood that there are many further variants. It must be understood that these embodiments of implementation are only illustrative and do not limit the invention or its applications, nor its possible configurations. On the contrary, although the description above makes it possible to man craft of the implementation of the present invention at least one of its second configuration example, it should be understood that numerous variations are conceivable of the components described, without that for this will come out from the object of ‘invention, as defined in the appended claims, interpreted literally and/or according to their legal equivalents.