Abstract:
A motor vehicle sheet-steel wheel rim ( 30, 50 ), including axially, from the inner side to the outer side, an inner hook ( 4 ), an inner seat ( 5 ), a connecting zone ( 7 ), a mounting groove ( 9 ), a hump ( 11 ), an outer seat ( 5 ) and an outer hook ( 13 ), the rim being intended to be joined to a wheel disc ( 2 ) under the mounting groove ( 9 ), wherein with the rim having a given thickness E in mm at the mounting groove ( 9 ), the connecting zone ( 7 ) includes a zone (Z 5,  Z′ 4 ) of thickness E1 in mm such that:  
     E/3+0.5 mm≧E1≧E/3 mm; and  
     E1≧0.7 mm.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application is a continuation of international application PCT/EP02/00545 filed Jan. 21, 2002, which was published in French on Aug. 1, 2002 as publication number WO 02/058946 and which claims priority to French application number 01/00968 filed Jan. 23, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to sheet-steel wheels including a disc and a rim with a mounting groove which are joined under the mounting groove, and relates more particularly to the profile of the section of the rims of such wheels.  
           [0004]    2. The Related Art  
           [0005]    The wheels of vehicles are subjected to repeated stresses in operation which may lead to degredation through fatigue. To measure the fatigue strength of the wheels, they are subjected to endurance tests on vehicles or on machines. The test machines reproduce in a simplified and accelerated manner the stresses of the wheels in actual use. The stresses of the wheels depend principally on the load of the vehicle for which the wheels are intended.  
           [0006]    Standard sheet-steel wheel rims are produced by a series of operations for shaping a hoop of constant thickness. These rims thus have a substantially constant thickness. The zones of fatigue degradation of such wheels are usually located at the radii of the groove bottom, at the joining weld and at the valve hole (see FIG. 1).  
           [0007]    The thickness of the original sheet-metal blanks intended for the manufacture of the rims is thus a function of the stresses to which the wheels are subjected in operation.  
           [0008]    U.S. Pat. No. 5,579,578 (the disclosure of which is hereby incorporated by reference for all purposes) describes a method of manufacturing a wheel rim including one or more reverse cylindrical flow-turning operations in order to obtain a rim profile of variable thickness and, thereby, to reduce the weight of the rim. This patent gives no information about the profiles of the sections of the rims to be obtained.  
         SUMMARY OF THE INVENTION  
         [0009]    The object of the invention is to define the section of a rim which is optimized to have a high degree of lightening while retaining an acceptable fatigue strength.  
           [0010]    According to a first aspect, the invention relates to motor vehicle sheet-steel wheel rim, which includes axially, from the inner side to the outer side, an inner hook, an inner seat, a connecting zone, a mounting groove, a hump, an outer seat and an outer hook, such rim being intended to be joined to a wheel disc under the mounting groove. The rim is characterized in that, with the rim having a given thickness E (in mm) at the mounting groove, the connecting zone includes a zone of thickness E1 such that:  
           E/3+0.5 mm≧E1≧E/3 mm; and 
           E1≧0.7 mm. 
           [0011]    The whole of the substantially cylindrical and/or frustoconical zone of the connecting zone can have a thickness E1.  
           [0012]    These relationships show that the thickness of the rim, over all or part of the connecting zone, can be reduced by up to substantially a third of the thickness at the mounting groove. This very great thickness reduction produces a very substantial lightening of the rim, even if a rim thus optimized has a thickness at the mounting groove greater than that of a comparable single-thickness rim.  
           [0013]    Advantageously, the thickness E2 (in mm) at the hump adjacent to the outer seat includes a zone such that:  
           E2≧E−0.6 mm; and 
           E2≧1.4 mm. 
           [0014]    The whole of the hump may have the thickness E2.  
           [0015]    It is found that the thickness of the hump zone, adjacent on the outside to the mounting-groove flank through which the valve hole passes, has only a small effect on the amplitude of the stresses in the mounting-groove zone. However, the proximity of the valve-hole zone imposes a limited lack of parallelism in this zone in order to preserve good leak-tightness of the valve after it has been fitted. The minimum thickness must be of the order of 1.4 mm.  
           [0016]    Advantageously, the wheel rim according to the invention has a thickness E3 (in mm) at the inner seat which includes a zone which satisfies the following relationships:  
           E3≧E/3+0.45 mm; and 
           E3≧1.2 mm. 
           [0017]    The zone of thickness E3 may extend over the whole of the inner seat, including, where appropriate, to the hump adjacent to this seat.  
           [0018]    These relationships show that the thickness in this zone can also be very substantially reduced, by 40 to 50% relative to the thickness in the mounting-groove zone. It is advisable, however, not to reduce this thickness below 1.2 mm, so as not to run the risk, for example, of the stresses, such as those linked to the hooks of the mounting-machine clamps for clamping the wheels, leading to damage which may be harmful in fatigue.  
           [0019]    Preferably, E3≧E/3+0.9 mm. This contributes to the lightening of the rim.  
           [0020]    Advantageously, the thickness of the wheel rim at the inner hook includes a zone, the thickness E4 (in mm) of which satisfies the following relationship:  
           E4≧E/3+0.75 mm 
           [0021]    And preferably,  
           E4≦E/3+1.2 mm 
           [0022]    The zone of thickness E4 may include the zone of the hook with an orientation perpendicular to the axis of rotation of the wheel.  
           [0023]    The thickness reduction in this zone may thus amount to around 40 to 27% as a function of the thickness of the mounting-groove zone. It is important to keep to the lower limits as specified so as not to impair the behavior in the fatigue tests of the mounting groove of the wheel. This is because the thickness in this zone conditions the rigidity of the rim hook.  
           [0024]    Finally, the outer seat of the wheel may include a zone, the thickness E5 (in mm) of which satisfies the following relationship:  
           E5≧E/3+0.5 mm 
           [0025]    Preferably, with the aim of lightening the rim, also:  
           E5≦E/3+0.9 mm 
           [0026]    The zone of thickness E5 may extend substantially over the whole of the outer seat.  
           [0027]    Preferably, the thickness E at the mounting groove is between 1.9 and 2.75 mm. These thicknesses are normally used for sheet-steel wheel rims of passenger cars.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    Particular embodiments of the invention are now described, by way of nonlimiting examples, with the aid of the attached drawings, in which:  
         [0029]    [0029]FIG. 1 is a schematic view of a conventional single-thickness wheel indicating the possible fatigue fracture zones;  
         [0030]    [0030]FIG. 2 shows the various steps of the process for producing a rim according to the invention;  
         [0031]    [0031]FIGS. 3, 4 and  5  illustrate schematically different steps of the production process shown in FIG. 2;  
         [0032]    [0032]FIG. 6 is meridian sectional view of a first embodiment of a rim hoop according to the invention at the conclusion of a set of flow-turning operations;  
         [0033]    [0033]FIG. 7 is a meridian sectional view of the rim corresponding to the hoop of FIG. 6, after the roll-forming operations;  
         [0034]    [0034]FIG. 8 is a meridian sectional view of a second embodiment of a rim hoop according to the invention at the conclusion of a set of flow-turning operations; and  
         [0035]    [0035]FIG. 9 is a meridian sectional view of the rim corresponding to the hoop of FIG. 8, after the roll-forming operations have shaped it. 
     
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0036]    [0036]FIG. 1 shows a partial section of an ordinary wheel made of sheet steel. The wheel  1  comprises a disc  2  and a rim  3 .  
         [0037]    The rim  3  is produced by a set of operations for rolling a hoop of constant thickness, and has a substantially constant thickness. The rim  3  has an inner hook  4 , an inner seat  5 , an inner hump  6 , a transition zone  7 , a mounting groove  9  with an inner flank  8  and an outer flank  10 , an outer hump  11 , an outer seat  12  and an outer hook  13 . The disc  2  is joined to the rim  3  by fitting under the mounting groove  9 . FIG. 1 also illustrates, partially folded over, the valve hole  14  which passes through the outer flank  10  of the mounting groove  9 .  
         [0038]    Hereinbelow, the same reference numbers will be used for similar parts of the embodiments of a wheel rim according to the invention.  
         [0039]    The steps of a process for producing a wheel rim according to the invention are illustrated schematically in FIGS.  2 - 5 . Initially, a blank of sheet metal (not shown), of steel, aluminum or alloys, is bent to give it a generally cylindrical hoop shape  21  with two free edges. Next, the hoop  21  is welded by a flash welding, resistance welding or other welding process. This hoop  21  has a constant thickness (FIG. 3). The hoop  21  is then, preferably, calibrated in expansion with the aid of a calibrating tool shown schematically in FIG. 4. The expansion is obtained by the displacement of a cam  22  which separates sectors  23  around which the hoop  21  is fitted. FIG. 5 illustrates the following step which consists in obtaining by cylindrical flow turning the flat profile which is sought for the rims according to the invention. The flow-turning process used is reverse flow turning. The hoop  21  is mounted on a mandrel  24  and comes to bear against a wall of the system  25  for locking the hoop  21 . The mandrel  24  is then set in rotation and at least two rolls  26  roll on the radially outer surface of the hoop  21  in the zones in which the thickness is to be reduced. Relative to the mandrel  24 , the rolls  26  are displaced axially in the direction of the X axis while applying a radial and tangential force so that the material flow flows in the direction of the arrows Y. This material flow takes place in the opposite direction to the displacement of the rolls  26 . FIG. 5 illustrates schematically an example of the hoop  21  profile thus obtained. This profile comprises five zones of constant thicknesses and three transition zones of variable thicknesses.  
         [0040]    It should be noted that this reverse flow-turning process does not permit a reduction of the thickness of the entire hoop. Indeed, a zone which is not flow-turned is left on each side of the hoop: on the side of engagement of the rolls, that could destabilize the hoop and damage the rolls; on the side of the device for fixing the hoop, forming a stop, a zone is left so that this device can grip the edge of the hoop and drive it in rotation during the flow-turning operation.  
         [0041]    After completion of the flow-turning operations, one or both of the edges of the hoop can be cut, if necessary, and then the hoop is flared, hoop-rolling operations are performed to obtain the rim profile which is sought (see FIG. 7), the rim is calibrated and suitable discs are fitted.  
         [0042]    In FIG. 1, the possible zones of fatigue degradation of the rim  3  of the wheel  1  are also indicated schematically. These zones are the following: the zone  15  situated in the inner flank  8  of the mounting groove  9  or at the connection between this flank  8  and the groove  9 ; the zone  16  situated under the mounting groove  9 , close to the disc-rim weld seam; the zone  17  situated at the connection between the mounting groove  9  and the outer flank  10  of the groove; and the zones  18  at the edge of the valve hole  14 . In all these zones, cracks are shown as appearing in FIG. 1.  
         [0043]    [0043]FIG. 6 illustrates an example of a hoop obtained by a flow-turning process, corresponding to the profiles of the sections according to the invention.  
         [0044]    This hoop  20  has 8 zones of constant thicknesses separated by 7 zones of variable thicknesses.  
         [0045]    [0045]FIGS. 6 and 7 make it possible to link the various zones of constant and variable thicknesses to the geometry of the rim  30  resulting therefrom. It should be noted that, for illustrative purposes, FIG. 6 has an expanded scale in the direction perpendicular to the axial direction, relative to the scale in the axial direction.  
         [0046]    The two zones Z 1  and Z 8 , of constant thicknesses equal to the thickness E of the original sheet-steel blank, correspond to the two axial ends of the flanges of the hooks  13  and  4 , respectively.  
         [0047]    The zone Z 2  of constant thickness E5 corresponds to the outer seat  12 . Located between the zones Z 1  and Z 2  is the zone T 1 , of variable thickness, of which the thickness varies from E to E5 going from the flange of the hook to the vertical part of the hook.  
         [0048]    Next, there is a transition zone T 2  between the zones Z 2  and Z 3 . The zone Z 3  corresponds substantially to the outer hump  11 . The transition zone T 2  is a connection between the outer seat  12  and the hump  11 . This zone has a slight radius of curvature.  
         [0049]    The zone Z 4  corresponds to the mounting groove  9  extended on both sides of the zones of the inner and outer flanks  8 ,  10  with slight radii of curvature. This zone Z 4  has a thickness E corresponding to that of the original blank. It should be noted that this zone includes the zones of degradation  15 ,  16  and  17  indicated in FIG. 1. Located between Z 4  and Z 3  is the transition zone T 3  which corresponds substantially to the outer flank  10 , this zone also includes the valve hole  14  and the zones of degradation  18 .  
         [0050]    The zone Z 5  corresponds to the connecting zone  7 . The transition zone T 4  is located between Z 4  and Z 5 . This transition zone corresponds substantially to the inner flank  8  of the mounting groove  9 .  
         [0051]    Next, there is the zone Z 6  corresponding substantially to the inner seat  5  and the inner hump  6 . The transition zone T 5  is between Z 5  and Z 6 .  
         [0052]    The zone Z 7  is located substantially in the vertical part (or perpendicular to the axis of rotation of the rim) of the inner hook  4 . This zone is adjacent, on the inside, to T 6  and, on the outside, to T 7 . T 6  corresponds to the axially inner end of the inner seat  5  as far as the vertical part of the hook  4 . T 7  corresponds to the first part of the flange of the inner hook  4 . Finally, there is the zone Z 8  of constant thickness corresponding to the inner edge of the flange of the hook  4 .  
         [0053]    The following table shows the relative thicknesses of the various zones Z 1  to Z 8  in the example of FIGS. 6 and 7, taking as reference at 100 the initial thickness E of the original blank, as well as the recommended ranges of values, taking as reference the thicknesses of the original blank of between 1.9 and 2.75 mm.  
                                                                                                                           Zone                Z1   Z2   Z3   Z4   Z5   Z6   Z7   Z8                        Relative   100   64   80   100   50   73   82   100       thick-       ness       Min.-       50-   70-100       33-   48-80   60-97       max.       80           60       range                  
 
         [0054]    This table shows that the wheel rims of variable thickness according to the invention have a main zone of thickness and weight reduction, the zone Z 5  corresponding to the connecting zone  7 . The thickness reduction may amount to 66% relative to the thickness of the original metal blank.  
         [0055]    The second zone is Z 6 , here the thickness reduction may amount to 50%. It is, however, necessary to retain a thickness greater than 1.2 mm in order to avoid the possibility of, for example, stresses such as those associated with the hooks of the mounting-machine clamps for clamping the wheels leading to damage which may be harmful in fatigue.  
         [0056]    In the case of a wheel of dimensions 6.5 J 15 H2, for a given specification, an ordinary rim has a thickness of the order of 2.0 mm, whereas the thickness E for a rim according to the invention may be 2.2 mm. For such wheels, the saving of weight obtained for a rim according to the invention may amount to 0.7 kg, i.e., on the order of 15 to 20%.  
         [0057]    It should be noted that the thickness reduction and consequently the saving of weight, may, in many cases, be relatively high when the thickness of the original blank increases and also when the diameter of the wheel increases.  
         [0058]    [0058]FIG. 8 illustrates a second example of a hoop obtained by a flow-turning process, corresponding to the profiles of the sections according to the invention.  
         [0059]    This hoop  40  has 6 zones of constant thicknesses separated by 5 zones of variable thicknesses.  
         [0060]    As before, FIGS. 8 and 9 make it possible to link the various zones of constant and variable thicknesses to the geometry of the rim  50  resulting therefrom.  
         [0061]    The two zones Z′ 1  and Z′ 6  correspond to the two axial ends of the hoop, these two zones have the thickness E of the original sheet-metal blank. They correspond here to the whole of the inner hook  4  and the outer hook  13  as far as the ends of the inner seat  5  and the outer seat  12 .  
         [0062]    The zone Z′ 2  of constant thickness E2 corresponds substantially to the outer hump  11 . Located between the zones Z′ 1  and Z′ 2  is the transition zone T′ 1  which corresponds to the outer  12 .  
         [0063]    The zones Z′ 3  and Z′ 4  correspond respectively to the mounting groove  9  extended by the zones, with a slight radius of curvature, of the flanks  10  and  8  of the groove  9  and to the connecting zone  7 . The thickness of the zone Z′ 3  is E, and that of the zone Z′ 4  is E1.  
         [0064]    Located between Z′ 2  and Z′ 3  is the transition zone T′ 2  which corresponds substantially to the valve-hole zone of the flank  10 . Located between Z′ 3  and Z′ 4  is the transition zone T′ 3 , which corresponds substantially to the zone with a slight radius of curvature comprising the axially outer end of the connecting zone  7  and the axially inner end of the inner flank  8  of the mounting groove  9 .  
         [0065]    The zone Z′ 5  corresponds substantially to the inner hump  6 . Its thickness is E3. Located between Z′ 4  and Z′ 5  is the transition zone T′ 4 , of short length.  
         [0066]    Finally, located between the zone Z′ 6  and the zone Z′ 5  is the transition zone T′ 5  which corresponds substantially to the inner seat  5 .  
         [0067]    The example of FIGS. 8 and 9 is a simplified example of the rim profile according to the invention, in which, in fact, the profile between the axially outer end of the inner seat  5  and the axially inner end of the outer seat  12  is identical to that of the rim  30 . The simplification stems from the constant-thickness profile of the two, inner and outer, hooks, which stiffens them. Consequently, the thickness of the original blank of this rim can be compared with that of a standard rim of constant thickness. This profile thus makes it possible to obtain a substantially identical lightening of the weight of the rim.  
         [0068]    The examples of FIGS.  6  to  9  are to be considered merely as nonlimiting examples of the scope of the invention. Indeed, it is possible to facilitate notably the production of the flat profile of the hoop by flow turning by defining a profile with fewer zones of different thicknesses, 4 or 5 for example.