Abstract:
A steering wheel unit for a motor vehicle. The steering wheel unit has a steering wheel and an airbag module ( 30 ) located in the hub area of the steering wheel. This airbag module forms a housing ( 32 ), an airbag ( 52 ) folded into the housing, and a gas generator ( 54 ). The airbag module can be pushed down against the force of at least one spring element to activate a horn, whereby positioning means are present which define the position of the airbag module in relation to the steering wheel. In order to achieve a high degree of operational reliability and a very exact positioning of the airbag module in the steering wheel, at least one retaining means ( 28;38,68 ) is present which acts between the airbag module and the steering wheel ( 28;38,68 ). This retaining means is ineffective in the absence of outer forces and when the airbag module ( 30 ) is pressed down to activate a horn.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. 10 2008 028 126.3, filed Jun. 13, 2008 and PCT/EP2009/000695, filed Feb. 3, 2009. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The invention relates to a steering wheel unit for motor vehicle having an airbag module incorporated into the steering wheel unit. 
       BACKGROUND OF THE INVENTION 
       [0003]    An airbag module is accommodated within the hub area of the steering wheel of almost every modern motor vehicle. The module has a housing, an inflation device—mostly in the form of a gas generator—and an airbag. The airbag is folded inside the housing and is spanned by a cover element which closes the housing. The cover element often also serves as the operating surface for the horn. In this case, the cover element can be pressed down against the steering wheel against the force of at least one spring element. Two basic designs are known of this type of system. 
         [0004]    On the one hand, it is possible to connect the housing with the steering wheel in a fixed fashion and to arrange the cover element on the housing or the steering wheel so as to be moveable in axial direction. 
         [0005]    Furthermore, it is possible to arrange the entire module, in other words the subassembly consisting of housing, gas generator, airbag and cover element, on the steering wheel so as to be moveable in axial direction. Such a design is known, for example, from patent application DE 196 25 722 A1. In that reference, compression springs are provided between the housing floor and the steering wheel, which push the housing away in the axial direction (related to the steering column). Positioning means which function in the axial direction are present, which limit the movement caused by the springs and define the axial position of the airbag module in the absence of forces from the outside. This means that—with the exception of when the horn is being operated—axial force is permanently transmitted between the airbag module and the steering wheel. 
         [0006]    In addition to the requirement for a very high level of functional reliability, the requirement that the positioning of the airbag module in the steering wheel should be very precise for aesthetic reasons is playing an increasingly important role. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention therefore sets the task of improving a steering wheel unit of the generic type in such a way that in addition to a very high level of operational reliability, very exact positioning of the airbag module in the steering wheel can be achieved. 
         [0008]    According to the invention, in addition to at least one positioning means which defines the axial position of the airbag module in relation to the steering wheel in the absence of an outside force, there is at least one retaining means acting between the airbag module and the steering wheel, which is ineffective in the absence of outside forces and when the airbag module is pressed down, in other words, which does not transfer any forces between the airbag module and the steering wheel when in these states. 
         [0009]    When the gas generator is ignited, very high forces can take effect between the airbag module and the steering wheel. In steering wheel units of the generic prior art type, the positioning means were designed so as to be correspondingly robust, so that they also ensured that the airbag module remained connected to the airbag if the gas generator was ignited. It was frequently necessary to provide strengthening elements for this purpose, in particular made of metal. However, this leads to a tolerance chain, which again has a negative effect on the exact positioning or which considerably increases the time and expense needed for production. 
         [0010]    This problem is solved by means of the additional retaining means. As, in normal operating states, these means do not have any effect, the requirements as regards exact dimensioning are low. On the other hand, because of the presence of the retaining means, requirements regarding the axial positioning means are low as regards their robustness. Because of the distribution of tasks, it is not necessary to make compromises with regard to the design of the retaining means and the positioning means. 
         [0011]    A further advantage of the concept according to the invention is that it permits improved mounting and removal capability, as will be seen later in the descriptions of the preferred embodiments. 
         [0012]    Preferred embodiments of the invention result from the subclaims and also from the embodiments now to be described with reference to the Figures. The Figures are as follows: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a strongly schematised cross-section through the hub area of a steering wheel, in which an airbag module is accommodated, 
           [0014]      FIG. 2  shows the items shown in  FIG. 1  with the airbag module pressed down, 
           [0015]      FIG. 3  shows the items shown in  FIG. 1  during expansion of the airbag, 
           [0016]      FIG. 4  is a birds-eye view of an airbag module in accordance with the present invention with a schematic representation of positioning means, 
           [0017]      FIG. 5  shows a mounting plate, 
           [0018]      FIG. 6  shows area B 1  from  FIG. 4  in a detailed 3-dimensional view, when the housing is held on the mounting plate, 
           [0019]      FIG. 7  shows area B 2  from  FIG. 4  in a view corresponding to  FIG. 6 , 
           [0020]      FIG. 8  shows area B 3  from  FIG. 4  in a view corresponding to  FIG. 6 , 
           [0021]      FIG. 9  is a 3-dimensional view of the housing from below, whereby an omega spring is fixed on the floor of the housing and two positions of the omega spring are shown, 
           [0022]      FIG. 10  shows detail D 1  from  FIG. 9 , 
           [0023]      FIG. 11  shows the housing of  FIG. 9  and the mounting plate of  FIG. 5 , 
           [0024]      FIG. 12  is a birds-eye view of the airbag module according to a second embodiment of the invention in a view corresponding to  FIG. 4  with schematic representation of the positioning means, 
           [0025]      FIG. 13  shows area B 4  of  FIG. 12  in a view corresponding to  FIG. 6 , 
           [0026]      FIG. 14  shows area B 5  in a view corresponding to  FIG. 13 , 
           [0027]      FIG. 14   a  shows a positioning means lying on the outside in assembled state, 
           [0028]      FIG. 14   b  shows the items shown in  FIG. 14   a  in non-assembled state, 
           [0029]      FIG. 15  shows a third embodiment of the invention in a view corresponding to  FIG. 12 , 
           [0030]      FIG. 16  shows area B 6  from  FIG. 15  in a view corresponding to  FIG. 13 , 
           [0031]      FIG. 17  shows area B 7  from  FIG. 15  in a view corresponding to  FIG. 13 , 
           [0032]      FIG. 18  shows a variant of the items shown in  FIG. 17 , 
           [0033]      FIG. 19  shows a further variant of the items shown in  FIG. 17 , and 
           [0034]      FIG. 20  shows the airbag module from  FIG. 15 , which is accommodated in a steering wheel body, whereby the steering wheel body is shown in a form in which it is cut open, 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    The invention will now be explained in more detail with reference to a schematic representation of  FIGS. 1 to 3 , and also with reference to three concrete embodiments. The basic principle, which will now be explained in more detail with reference to  FIGS. 1 to 3 , applies to all three embodiments. 
       Basic Structure 
       [0036]      FIG. 1  shows a longitudinal section through the hub area of a steering wheel unit in schematic form. This steering wheel unit exhibits a steering wheel with a steering wheel body  10 , which has a recess  12  in the hub area. Spokes  14  extend from the hub area. Steering column  16  extends about centrally from the hub. The direction of extension of steering column  16  defines the axial or Z direction for what is to follow. The plane which is perpendicular to this Z direction (this plane is also perpendicular to the drawing plane of  FIG. 1 ) is the XY plane. 
         [0037]    On the floor of recess  12 , a mounting plate  20  is screwed together with steering wheel body  10  by means of screws  22 , in other words is connected to it in rigid fashion. It is also possible that the mounting plate  20  is an integral part of the steering wheel body  10 ; this possibility can help reduce tolerances and manufacturing costs. According to the definitions chosen in this application, mounting plate  20  and all the components which extend from it are considered to belong to the steering wheel. 
         [0038]    An airbag module  30  is accommodated in the recess  12 . This airbag module has a housing  32 , an airbag  52  accommodated in housing  32 , and a gas generator  54 . Housing cover  50 , which also belongs to the airbag module, extends over exit opening  34  of housing  32 , which is opened by the expanding airbag in a known fashion. 
         [0039]    Housing floor  32   a  of housing  32  is connected with mounting plate  20  by means of spiral springs  46 , so that airbag module  30 , and in particular also housing  32 , can be pressed down against the force of these spiral springs  46  in the axial direction against the steering wheel. If airbag module  30  is pressed down far enough, horn contacts  24  and  44  are closed (see  FIG. 2 ). 
         [0040]    Steering wheel, airbag module and the spiral springs  46  together form the steering wheel unit. 
         [0041]    Spiral springs  46  are naturally not able to define the position of airbag module  30  precisely in relation to the steering wheel, so that positioning means have to be provided. In total, the airbag module has 5 degrees of freedom in relation to the steering wheel (three translation degrees of freedom and two rotational degrees of freedom), so that the positioning means have to be designed accordingly. If exactly one positioning means is provided for each possible direction of movement, generally six positioning means are required. Each positioning means consists of two parts, namely a part on the steering wheel side and a part on the module side.  FIG. 1  shows two positioning means, which each serve axial positioning, in other words positioning in the Z direction. Each of these axial positioning means exhibits an axial positioning hook  29  as an axial positioning means on the steering wheel side, and an axial positioning step  39  as axial positioning means on the module side. The sectional drawings only show two axial positioning means, but in practice three of these are usually present. Spiral springs  46  press housing  32  upwards and therefore press axial positioning steps  39  extending from the housing from below against axial positioning hooks  29 , so that the axial position (Z position) of the module is completely defined. For operation of the horn, airbag module  30  can be pressed down exactly in axial direction as shown in  FIG. 2 , or it can be pressed down in tilted fashion. 
         [0042]    The task of the axial positioning means consists solely of axial positioning, they do not have to take any large forces. From this it follows among other things that the respective components can be manufactured of plastic and in particular the axial positioning means on the module side can be manufactured in one piece with the housing. 
         [0043]    However, when operating the gas generator, considerable forces occur, also in axial direction. Therefore retaining means are provided which hold the airbag module on the steering wheel even when, because of the high forces which occur, the axial positioning means fail, for example break ( FIG. 3 ). The retaining means comprise retaining means on the steering wheel side, here namely retaining hooks  28 , and retaining means on the module side, here namely retaining steps  38 . As can be seen from  FIGS. 1 and 2 , the retaining means are without function in normal operating state, i.e. when no force is exercised on the airbag module from the outside ( FIG. 1 ), or when the airbag module is pressed down in order to operate the horn, i.e. the retaining means on the steering wheel side (retaining hooks  28 ) and the retaining means on the module side (retaining steps  38 ) do not touch each other. This means that the retaining means do not enter into competition with the axial positioning means and only low requirements have to be set as regards exact dimensioning. However, it is also in particular possible, to manufacture the retaining means wholly or partially of metal. 
         [0044]    Many advantages of the invention can only be seen with reference to the concrete embodiments, which are explained below in more detail with reference to  FIGS. 4 to 18 .  FIGS. 4 to 11  show a first embodiment. 
       First Embodiment 
       [0045]      FIG. 4  shows a birds-eye view of the airbag in a schematic form and the position and function of the individual positioning means, also in schematic form. In total, five positioning means are present, namely three pure axial positioning means, (designated with Z 1  to Z 3 ), an XY positioning means (designated with X 1 Y 1 ), and also a further X positioning means (designated with X 2 ). The X positioning means and the XY positioning means do not function in the axial direction; they are therefore designated as non-axial positioning means. The axial positioning means are in principle constructed as described above with reference to  FIG. 1 , as will be seen again later. The two further positioning means X 1 Y 1  and X 2  serve to define the position of airbag module  30  in the XY plane and to prevent rotation in this plane. The X 1 Y 1  positioning means contains a first positioning peg  60  as positioning means on the module side, and a circular first positioning receptacle  64  as positioning means on the steering wheel side ( FIG. 8 ). The X 2  positioning means contains a second positioning peg  62  as positioning means on the module side as well as a oblong-shaped second positioning receptacle  66  as positioning means on the steering wheel side. The oblong shape of the second positioning receptacle  66  serves to compensate for differences in length caused by shrinkage or change in temperature. 
         [0046]      FIG. 5  shows mounting plate  20  of the first embodiment. Here, the three axial positioning hooks  29  can be seen, which serve as axial positioning means on the steering wheel side, two retaining hooks  28 , which serve as retaining means on the steering wheel side, as well as the two positioning receptacles  64  and  66 . The axial positioning hooks and the retaining hooks are each formed rigidly.  FIGS. 6-8  show how the positioning means on the steering wheel side work together with the positioning means on the module side. The axial positioning means on the module side are here also axial positioning steps  39 , which are here the underside of U-hoops  74  and  78 . Because of the mirror symmetry of airbag module  30  and mounting plate  20  (the mirror-image plane lies in the XZ plane), the two hoops which belong to the Z 1  or Z 2  axial positioning means are designated as first U-hoops  74 , and the U-hoops belonging to axial positioning means Z 3  are designated as second U-hoop  78 . 
         [0047]    The form of the retaining means on the module side will now be described with reference to  FIGS. 9 and 10 . A spring wire is held on the housing floor  32   a , which because of its form is referred to as omega spring  68 . The end sections  68   a  and  68   b  of this omega spring  68  are each held by two retaining blocks  70   a  to  70   d , whereby each of these retaining blocks  70   a  to  70   d  exhibits a groove  72   a  to  72   d . This groove is on the inner side in each case, so that end sections  68   a ,  68   b  of omega spring  68  can take up two positions, namely an outer position  68   a ′,  68   b ′, which corresponds to the locked state, and an inner position  68   a ″ and  68   b ″, which corresponds to the unlocked state as is shown in  FIG. 9 . 
         [0048]    Immediately behind retaining blocks  70   b  and  70   d  omega spring  68  is on both sides guided through a U-shaped transfer element  76 , each of which extend from a first U-hoop  74 . This means that when an end section of the omega spring  68  is swivelled from the outer to the inner position, the relevant axial positioning step of the corresponding U-hoop  74  is swivelled inwards. Transfer element  76  is formed in one piece with the first U-hoop  74 , which for its own part is formed in one piece with housing  32 . Housing  32  is an injection moulded part of plastic, or has a core made of metal with a plastic overmolding. 
         [0049]    During mounting of the airbag module, the module is introduced into the recess of the steering wheel body from above ( FIG. 11 ), whereby the end sections of omega spring  68  are in their outer position. In this position, these are pressed somewhat inwards by the slanting heads, of retaining hooks  28 , but not so far that they can snap into grooves  72   a  to  72   d  of retaining blocks  70 . This means that after completion of the mounting process, they snap back into their outer position. In the same way, the axial positioning steps  39  of U-hoops  74  and  78  are pressed inwards lightly by the heads of axial positioning hooks  29 , also formed at a slant, and snap back into their force-free initial position following completion of the mounting process. In the mounted state, when the horn is not pressed down, axial positioning hooks  29  lie against axial positioning steps  39  (which are parts of the U-hoops), while the retaining hooks and the end sections of omega spring  68  (which forms the retaining means on the module side) do not touch each other. This means that the axial position (Z position) of the module is only determined by the axial positioning hooks and the axial positioning steps. However, when the gas generator is operated, with resulting forces, the axial positioning hooks and/or hoops  74  and  78  can break. In this case, retaining hooks  28  together with omega spring  68  prevent the airbag from separating from the steering wheel. 
         [0050]    If airbag module  30  is to be removed from the steering wheel, the end sections of omega spring  68  are brought into the inner position with a tool, so that they snap into grooves  72 . In this position, they lie so far to the inside, that they can no longer engage with retaining hooks  28  (unlocked position). During this movement, the end sections of omega spring  68  also swivel axial positioning steps  39  of first U-hoop  74  towards the inside (which is the purpose of transfer elements  76 ). In this position, the corresponding axial positioning steps  39  also lie so far to the inside that they can no longer engage with axial positioning hooks  29  (unlocked state), so that airbag module  30  can be pulled out upwards. As the second U-hoop  78  is not swivelled into a removal position, it is necessary to tilt the airbag module slightly when lifting it out. The oblong shape of the second positioning receptacle  66  makes this tilting possible. Thus, the oblong shape of this second positioning receptacle  66  has two purposes: It serves for the compensation of differences in length due to temperature change or shrinkage as well as for making the tilting possible which is needed during disassembly. 
         [0051]    The one-pieced omega spring is preferred solution. But it has to emphasised that it is also possible to have two separate spring wires, one for each side of the module. 
       Second Embodiment 
       [0052]      FIGS. 12 to 14  show a second embodiment of the invention. The retaining means and the axial positioning means are identically formed to those in the first embodiment and they are therefore not described again here. In contrast to the first embodiment, positioning in the XY plane is achieved by means of outer positioning means  80 ,  82 , and  84 . On the module side, these comprise plate-formed sections  86 ,  88 , and  90  and on the steering wheel side suitable receiving elements  92 ,  94 , and  96 . Such a plate-shaped section  86  and a receiving element  92  in mounted state are shown in  FIG. 14   a  as a section. The receiving element has a basically U-shaped cross-section, whereby from an upper end of a first leg  92   a  an elastic arm  92   c  extends into the inside of the U. The inner side of second leg  92   b  lying opposite first leg  92   a  is convex in form. The minimum distance between the elastic arm and the second leg is, when in force-free condition, smaller than the thickness of the plate-shaped section, so that in mounted condition the plate-shaped section is pressed on the second leg, which leads to positional fixing in the XY plane ( FIG. 14   b ). 
         [0053]    The receiving elements  92 ,  94 , and  96  are preferably separate elements of plastic, which are located into recesses provided for this purpose in the steel wheel body by means of a form fit. The elements serving for the form fit are the side projections  102 . An off-centred peg  100  is provided on the bottom leg of the U-shaped receiving element making sure that the receiving element can only be inserted into the respective recess in its correct orientation. 
       Third Embodiment 
       [0054]      FIGS. 15 to 20  show a further embodiment, which is similar to the second embodiment. In contrast to the second embodiment, here the outer positioning means  80 ,  82 , and  84  also serve for positioning in Z direction (axial direction). In order to achieve this, the lower ends of plate-shaped sections  86 ,  88 , and  90  comprise hook sections  88   a  and  90   a  ( FIGS. 16 and 17 ), so that the hooks sections formed in this way are pressed against the lower faces of the elastic arms  94   c  and  96   c  because of the force of the spiral springs (horn springs). During mounting of the airbag module in the steering wheel, the arms are first pressed to the side by means of the hook-shaped ends of the plate-shaped sections, and then snap back. 
         [0055]    As it is not possible to unlock the Z positioning using the omega spring, the receiving elements  92 ,  94 , and  96  are held in the steering wheel body in such a way that they can be removed from the steering wheel body for example by pushing them to the inside, so that the positioning means are decoupled. It is also possible that the steering wheel body is somewhat elastic (for example by using a foam material) in the area of the recesses in which the receiving elements  92 ,  94 , and  96  are held. In this case the receiving elements can be snapped out of the steering wheel body for disassembly purposes by pushing them in Z-direction. Before a new assembly, they have to be snapped into the foam again. 
         [0056]      FIGS. 18 and 19  show variants of what is shown in  FIG. 17 . In the variant of  FIG. 18  the hook section  90   b  of the plate-shaped section  90  point towards the second leg  96   b  of the receiving element  96  which shows a step. The advantage of this variant is that it is possible to desnap the hook sections by a combined turning (see arrow in  FIG. 18 ) and pulling operation (provided all pairs of plate shaped sections and receiving elements in the steering wheel unit have the same orientation, which is preferred). 
         [0057]    In the variant of  FIG. 19  the plate shaped-section has hook sections  90   a  and  90   b  on both sides. This configuration makes it easier to pull the receiving elements out of a foam core of the steering wheel body since the pulling force which is exerted to the housing is transferred to the receiving elements more symmetrically. 
         [0058]    If a solution is chosen in which at least one hook section  90   a  of the plate-like section points into the direction of the elastic arm  96   c , it is preferred that the upper surface of this hook section  90   a  extends slanting upward from the plate-like section  90  as can be seen in the  FIGS. 16 ,  17  and  19 . The lower face of the elastic arm  96   c  should follow this slant. In this design the force of the horn springs help the elastic arm being pressed against the plate-like section  90 . In this case the clamping force eliminates any gap between the plate-like section and the receiving element. This is very useful to avoid noises during vibration load. 
         [0059]    The  FIGS. 18 and 19  show the third plate-like section  90  and the third receiving element  96 , but it is clear that this is only chosen as an example. The first and the second plate-like section and the first and the second receiving element can be made the same way. A mixing of the described variants in a single steering wheel unit would generally be possible but is usually not preferred. 
         [0060]    The  FIG. 20  shows the arrangement of the receiving elements in the steering wheel body. The retaining means have the same shape as in the first two embodiments, therefore basically consist of an omega spring as retaining means on the module side and retaining hooks as retaining means on the steering wheel side. 
         [0061]    All shown embodiments have the advantage that rattling sounds are widely avoided since the metallic spring wire (the omega spring) is in normal operation conditions not in contact with the steering wheel body. 
         [0062]    While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation, and change without departing from the proper scope and fair meaning of the accompanying claims.