Abstract:
A steering wheel unit with an airbag module and a steering wheel body is connectable to a steering column defining the axial direction. The steering wheel body includes an accommodation for the airbag module in the hub area. The airbag module can be pressed down against the steering wheel body against the force of at least one spring and at least two positioning units act between the airbag module and the steering wheel body. The positioning units are arranged in a center of gravity plane being perpendicular to the axial direction. In order to provide an easy mountability and a high degree of flexibility in respect of the installation space, the positioning units act at least in non-axial direction and the at least one spring is spaced from the positioning units.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention relates to a steering wheel unit according to the preamble of Claim  1 . 
         [0002]    In almost every modern motor vehicle, an airbag module is located in the hub area of the steering wheel for the protection of the driver. The airbag, located in a housing, breaks through a cover which covers the housing when the associated gas generator is triggered and inflates between the steering wheel and the driver. Often the cover also serves as an operating surface for the horn, so that it can be pressed down basically in axial direction (in relation to the steering column) against the steering wheel body. Two basic concepts are known in this respect: 
         [0003]    In the case of the so-called “floating cover” concept, the housing of the airbag module is rigidly connected to the steering wheel body and the cover can be pressed down in the direction of the steering wheel body and therefore also of the housing. 
         [0004]    In the case of the so-called “floating module” concept, cover and housing are rigidly connected with each other and form the airbag module, which can be pressed down against the steering wheel body as a complete unit. In a frequently encountered design, compression springs are located between the floor of the housing and the floor of the recess of the steering wheel body in which the airbag module is located, against whose force the airbag module can be pressed down. Furthermore, positioning units are provided which position the housing in relation to the steering wheel body, whereby the positioning components on the module side are frequently located on the housing floor and the positioning components on the steering wheel side are frequently located on the floor of the recess. The principle of such an arrangement is shown in  FIG. 13 . 
         [0005]    It has become clear that in such an arrangement relatively strong springs must be provided in order to avoid rattling noises during travel. However, the disadvantage of using strong springs is that a correspondingly high degree of force is required in order to operate the horn. 
       PRIOR ART 
       [0006]    Generic patent EP 0823362B1 therefore proposes that both the positioning units and the springs should be provided in the plane of the centre of gravity of the airbag module. In this arrangement, the compression springs are each arranged coaxially around a guide pin, which primarily serves for axial positioning. This results in “positioning and spring units”. The necessary arrangement is rather complicated and correspondingly time-consuming and costly as regards assembly and also requires a relatively large amount of space at one location when mounted. 
       SUMMARY OF THE INVENTION 
       [0007]    Based on this, the present invention sets itself the problem of improving a steering wheel unit of the generic type in such a way that it is simpler to mount and more variable as regards the space which is required. 
         [0008]    This problem is solved by means of a steering wheel unit with the characteristics of Claim  1 . 
         [0009]    With regard to the invention it was recognised that it is sufficient to arrange the positioning units which act in non-axial direction in the plane of the centre of gravity. The position of the springs is not important, so that they are spaced from the positioning units. 
         [0010]    By means of this arrangement, on the one hand flexibility is gained, as the non-axially acting positioning units and the springs can be located on different positions. Furthermore, the concept can be implemented with a lower number of components, which means that the assembly efforts and costs therefore are clearly reduced. 
         [0011]    In a preferred embodiment according to Claim  3 , each positioning means defines the position of the airbag module in relation to steering wheel body in precisely one first non-axial direction, while it does not define the position in a second non-axial direction which stands perpendicular to the first direction. This has, in particular when three positioning units are present in accordance with Claim  2 , the special advantage that a movement of the airbag module in the non-axial plane can be completely eliminated, which very much promotes the desired effect. 
         [0012]    Still further preferred according to Claim  7  the positioning units also serve for the axial positioning. This leads to a reduced number of components needed. 
         [0013]    A particularly simple assembly capability can be achieved by means of a steering wheel unit with the additional features of Claim  9 . 
         [0014]    Further preferred embodiments of the invention result from the further subclaims, and also from the embodiments described in more detail below with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The drawings show: 
           [0016]      FIG. 1  A first embodiment of the invention in a strongly schematised cross sectional view, 
           [0017]      FIG. 2  A second embodiment of the invention in a view corresponding to  FIG. 1 , 
           [0018]      FIG. 3  A third embodiment of the invention in a view corresponding to  FIG. 1 , 
           [0019]      FIG. 4  A fourth embodiment of the invention in a view corresponding to  FIG. 1 , 
           [0020]      FIG. 5  An embodiment of the invention, which basically corresponds to  FIG. 1 , in a view showing a greater wealth of detail, 
           [0021]      FIG. 6  A section along Plane A-A from  FIG. 5 , 
           [0022]      FIG. 7  A section along Plane B-B from  FIG. 5 , 
           [0023]      FIG. 8  A positioning unit similar to a positioning unit which can be seen in  FIGS. 6 and 7  in a perspective view, 
           [0024]      FIG. 9  A variation of the positioning unit shown in  FIG. 8  in a sectional view, 
           [0025]      FIG. 10  A further variation of a positioning unit in a view corresponding to  FIG. 9 , 
           [0026]      FIG. 11A  birds-eye and schematised view of a steering wheel, 
           [0027]      FIG. 12  A securing wire in a perspective view in two positions, 
           [0028]      FIG. 13  Representation of the state of the art in a view corresponding to  FIG. 1 . 
       
    
    
       [0029]    First the state of the art will be described with reference to  FIG. 13 . 
         [0030]      FIG. 13  shows a steering wheel unit  10  which consists of a steering wheel body  12  and an airbag module  20 . The axial direction R a  is defined by the steering column  14 , which can be connected with the steering wheel unit  10 . In the hub area  16  of the steering wheel body  12 , the airbag module  20  is accommodated in an accommodation. The airbag module comprises in addition to the illustrated housing  22  and the illustrated cover  24  a gas generator which is not shown, and an airbag folded into housing  22  (also not shown). Between the housing floor  22   a  and the floor  16   a  of the hub area of the steering wheel body  12 , springs  30  extend so that airbag module  20  can be pressed down against the steering wheel body  12  in axial direction R a . Generally it is also possible to press down the airbag module  20  slightly crossways, in other words not exactly in axial direction. In addition, horn contacts  32  and positioning units  40  are provided on the housing floor  22   a  and on the floor  16   a . These positioning units  40  position the airbag module  20  in non-axial direction, in other words in all directions which are perpendicular to the axial direction R a . Generally, axial positioning units are also present, which position the airbag module  20  in relation to the steering wheel body  12  in axial direction (not shown). The springs  30  are under pretension through these axial positioning units. 
         [0031]    If now, during travel, vibrations are transmitted into the airbag module  20  via steering column  14  and steering wheel body  12 , the airbag module  20  tries to vibrate in non-axial direction, as shown by the arrows in  FIG. 13 , which is not possible because of the non-axial positioning units  40 . However, since the centre of gravity S lies above the positioning units  40 , torques M are exerted on the positioning units, as also indicated in  FIG. 13 . In the case of a given force in non-axial direction, the torque M affecting a positioning unit  40  is proportional to the distance h of this positioning unit M from the centre of gravity plane E S . These torques M then again lead to the unwanted noises. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]    Therefore, according to the invention, and as shown in  FIG. 1 , the positioning units which act in a non-axial direction are moved to centre of gravity plane E S . In this application under the term “centre of gravity plane” the plane is understood in which the overall centre of gravity S of airbag module  20  lies, and which is perpendicular to the axial direction R a . The position of springs  30  and horn contacts  32  can be freely selected, for example a location between the housing floor  22   a  and the floor  16   a  of the hub area is possible, as is shown in  FIG. 1 .  FIGS. 2 to 4  show alternative arrangements of springs  30  and horn contacts  32 . It can be seen that these components can be positioned freely at will, which in particular results in the possibility of adapting the arrangement of the springs and the horn contact to the concrete situation as regards the respective steering wheel in question. The positioning units  40  acting in axial direction however always remain in centre of gravity plane E S  and the springs  30  are at a distance from the positioning units  40  and preferably do not lie in the centre of gravity plane E S . 
         [0033]      FIG. 5  shows a steering wheel unit, as shown in  FIG. 1 , in a greater wealth of detail. In  FIG. 5  one recognises the steering wheel body  12 , which basically consists of a metal skeleton covered in foam, the airbag module  20 , which in addition to the housing  22  and the cover  24  also exhibits a gas generator  26 , springs  30 , horn contacts  32  and positioning units  40 . Two positioning units  40  are shown in  FIG. 5 , in general three positioning units are present, as will be to explained later with reference to  FIG. 11 . The concrete structure of positioning units will be described later in more detail with reference to  FIGS. 6 to 10 . 
         [0034]    In the preferred embodiment shown, the positioning units  40  are formed in such a way that they position the airbag module  20  in both non-axial and axial direction. However, they are not formed in such a stable way that they can securely resist the forces which occur during expansion of the airbag, which is why additional securing elements are provided. These securing elements consist of securing hooks  50 , which in the embodiment shown extend from housing floor  22   a  downwards, and a horseshoe-shaped securing wire  52 , which is held on the floor of the hub area.  FIG. 5  shows the front end sections of the securing wire  52 , whereby the quiescent state  52   a  is shown in solid lines and a position  52   b  swivelled to the inside is shown in broken lines. The position swivelled to the inside only serves so that the airbag module  20  can be disassembled from the steering wheel body  12 . From  FIG. 5  it can be seen that securing hooks  50  and securing wire  52  do not touch each other in normal operation; these components therefore do not make a contribution to the positioning of the airbag module in the steering wheel body  12 . But if during expansion of the airbag, when the positioning units  40  are broken as the case may be, the securing hooks  50  hit the securing wire  52 , the securing hooks come to rest on the securing wire and it is prevented that the airbag module  20  de-attaches from the steering wheel body. 
         [0035]      FIG. 12  shows, in perspective view, a front end area of the securing wire  52  in its outer quiescent position  52   a  and in its position swivelled to the inside  52   b , into which it can be brought with the help of a tool acting from the outside (not shown). The securing wire  52  is secured by means of retaining blocks  19  rigidly connected with the floor  16   a  of the hub area  16 . In its inner position, securing wire  52  is kept in retaining blocks  19  by means of grooves. When the airbag module is assembled on the steering wheel body the securing wire is pressed to the inside by the securing hooks  50 , however not so far that it can slot into the grooves. When the airbag module is completely assembled, the securing wire snaps into its outer position and can then fulfil its securing function. The arrangement in the area of the other end of the securing wire is a mirror image of  FIG. 12 . 
         [0036]    The structure of positioning units  40  is now described with reference to  FIGS. 6 to 8 . The positioning units  40  exhibit a positioning element on the steering wheel side in the form of a receiving element  45  and a positioning element on the module side in the form of a plate-shaped guide element  42 . 
         [0037]    Such a plate-shaped guide element  42  and a receiving element  45  in mounted conditions are shown in  FIGS. 6 and 7  schematically in a section and in  FIG. 8  in a perspective view. The receiving element  45  has a basically U-shaped cross-section, whereby an elastic arm  45   c  extends from an upper end of a first leg  45   a  into the inside of the U. The inner side of the second leg  45   b  lying opposite the first leg  45   a  is of convex shape. The minimum distance between elastic arm  45   c  and second leg  45   b  is, in force-free condition, smaller than the thickness of the plate-shaped guide element  42  in the shape of a plate, so that in assembled state the plate-shaped guide element  42  is pressed onto the second leg, which leads to positional fixing in the non-axial plane, however in only exactly one direction, which is designated with R 1  in  FIG. 7 . 
         [0038]    The positioning units also serve for positioning in axial direction. In order to achieve this, the lower ends of the guide elements  42  are shaped in the form of hooks, so that the hook sections  42   a  formed in this way are pressed against the undersides of elastic arms  45   c  due to the force of the springs. During assembly of the airbag module into the steering wheel, the arms are first pushed to one side by hook sections  42   a  of plate-shaped guide elements  42  and then snap back. The guide elements  42  preferably consist of plastic. 
         [0039]    The receiving elements are preferably separate elements of plastic, which are located with a form fit into recesses in the steering wheel body intended for this purpose. The elements which create the form fit are overhangs  46 . A peg  48  located eccentrically (not shown in  FIG. 6 ) is provided on the lower leg of U-shaped receiving element  45 , in order to ensure that receiving element  45  can only be inserted into the corresponding recess in the correct orientation. 
         [0040]    The steering wheel body is somewhat elastic in the area of the recesses in which the receiving elements  45  are held (for example through the use of a foam material), so that the receiving elements can be clipped out of the steering wheel body for disassembly purposes, by pulling them out in axial direction. They have is to be clipped into the foam again before re-assembly. 
         [0041]      FIGS. 9 and 10  show variants of the elements shown in  FIG. 6 . In the variant of  FIG. 9 , the hook section  42   a  of guide element  42  points in the direction of the second leg  45   b  of receiving element  45 , which exhibits a step. The advantage of this variant is that it is possible to unclip the hook sections  42   a  by means of a combined rotary (see arrow in  FIG. 9 ) and pulling movement (presuming that all pairs of guide elements  42  and receiving elements  45  in the steering unit have the same orientation, which is preferred). 
         [0042]    In the variant shown in  FIG. 10 , the guide element  42  has hook sections  42   a  on both sides. This arrangement makes it easier to pull the receiving elements out of the foam of a steering wheel body, as the tensile force which is exerted on the housing is transferred more symmetrically to the receiving elements. 
         [0043]    If a solution is selected in which the at least one hook section  42   a  of the plate-shaped guide element points in the direction of elastic arm  45   c , it is to be preferred that the upper surface of this hook element  42   a  extends upwards at a slope from the guide element, which can be seen particularly in  FIG. 10 . The underside of elastic arm  45   c  should follow this slope. In this design, the force of the springs  30  help the elastic arm to be pressed against the guide element  42 . In this case, the clamping force eliminates each gap between guide element  42  and the receiving element  45 . This is very helpful in order to further minimise noises in case of vibratory loading. 
         [0044]      FIG. 11  shows a steering wheel from above in schematic form and the position of positioning units  40 . It can be seen that each positioning unit only precisely defines one non-axial position in precisely one direction (R 1  to R 3 ) while it does not define the position in the non-axial direction perpendicular to this (R 4  to R 6 ). By these means on the one hand a static over-determination is avoided and on the other hand compensation of ageing- or temperature-related changes in size of the airbag module is possible, so that the airbag module is held on the steering wheel body free of clearance. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           10  steering wheel unit 
           12  steering wheel body 
           14  steering column 
           16  hub area 
           16   a  floor of hub areas 
           20  airbag module 
           22  housing 
           22   a  housing floor 
           24  cover 
           26  gas generator 
           30  spring 
           32  horn contact 
           40  positioning unit 
           42  plate-shaped guide element 
           42   a  hook section 
           45  receiving element 
           45   a  first leg 
           45   b  second leg 
           45   c  elastic arm 
           46  overhang 
           48  peg 
           50  securing hook 
           52  securing wire 
           52   a  quiescent position 
           52   b  position swivelled to the inside