Abstract:
A steering wheel assembly having a fastenerless air bag module assembly is disclosed. This assembly employs a housing with a projection and a steering wheel armature configured to receive the projection. The cover is retained between the housing and the steering wheel armature. Furthermore, the present invention provides a steering wheel cover with a tear seam that permits the air bag to deploy more efficiently with less damage to the occupants.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to steering wheel assemblies having air bags and, more particularly, to a cover for an air bag of a steering wheel assembly. 
     Air bags typically are located beneath a cover of a steering wheel assembly. Historically, the steering wheel was attached to the vehicle and then the air bag module, which included a cover attached over the air bag, was attached. 
     A steering wheel and air bag assembly includes a cover, a steering wheel armature and an air bag. Most covers are formed of a synthetic plastic material that is positioned over the air bag cushion. The cover is generally attached to the steering wheel assembly by rivets. In order for the air bag to deploy properly, it is necessary to provide a thinned portion for the air bag to break through the cover. Typically, the thinned regions included tear seams located in the cover that had generally been either H-shaped, I-shaped, U-shaped or branched shapes as viewed from the driver&#39;s seat. 
     The branched shapes of the tear seams require more energy and time to tear, thus the air bag must be deployed with a very high force for it to deploy and inflate rapidly enough to serve as a crash restraint. In addition, the high force used during deployment can cause the cover to break in high stress regions, resulting in reduced integrity of the air bag module. Additionally, the high deployment forces can lead to cracking or tearing of the module cover. Furthermore, these tear seam designs limit the area of opening in the cover and tend to cause the air bag to deploy straight outward rather than outward and to the sides. By allowing the air bag to expand quickly to the sides, the air bag inflates with less force and more efficiency. 
     The rivets used to couple the cover to the air bag housing require very labor intensive assembly procedures. Further, the rivets produce a high stress concentration interface between the cover and the air bag mounting plate, which can provide crack initiation sites. Air bag modules having an H-shaped or U-shaped cover experience significant stresses on the door hinges caused by inertial forces from the rotating of the door mass. As such, it is desirable to provide a cover design that enables the air bag cover to be retained without rivets with ensured integrity after deployment. It is also desirable to provide a tear seam that ruptures more rapidly and that will deploy in response to a lower inflation force. 
     The present invention provides a steering wheel assembly that receives a fastenerless air bag cover assembly that increases the integrity of the air bag module after deployment. This assembly employs a housing with a projection and a steering wheel armature configured to receive the projection. The cover is retained between the housing and the steering wheel armature. In addition, the present invention provides an air bag cover with a tear seam that permits the air bag to deploy more efficiently with less stress to the module. Specifically, the cover has a circular center from which at least one tear seam radiates. The tear seam ends at an articulated terminus. 
     These and other features and advantages of this invention will become more apparent to those skilled in the art from the following detailed description of the presently preferred embodiment. The drawings that accompany the detailed description can be described as follows. 
    
    
     
       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 rear view of a steering wheel and air bag assembly designed according to the present invention 
         FIG. 2  is a rear view of the air bag cover depicted in  FIG. 1 ; 
         FIG. 3  is a cross sectional view of the steering wheel and air bag assembly along a line  3 — 3  in  FIG. 1 ; 
         FIG. 4  is an alternate air bag cover according to a second embodiment of the invention; 
         FIG. 5  is a side assembly view of the air bag mounted within the steering wheel; and 
         FIG. 6  is a side cross sectional view of a steering wheel and air bag assembly according to the teachings of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     The present invention is generally related to an air bag cover for a steering wheel assembly. However, it is to be understood that the principles embodied herein are equally applicable to other types of applications involving air bags with covers. 
     Referring generally to  FIGS. 1 through 2 , an air bag module  10  is shown. The air bag module  10  includes a housing  12  coupled to a steering wheel armature  14 . A cover  16  is retained by both the housing  12  and the steering wheel armature  14 . The cover  16 , which is shown in phantom for clarity, defines a plurality of retaining apertures  34 , which are used to mount the cover  16  to the housing  12 . The air bag module  10  also includes an inflator or gas generator (not shown) of known type to provide inflation gases to the folded air bag or cushion (not shown). 
     The steering wheel armature  14  includes a pair of support tabs and a pair of bosses (not shown) for supporting and retaining the housing  12 . The steering wheel armature  14  is generally cup shaped, having a bottom  18  and sides  20 . The steering wheel armature  14  is also symmetric to a vertical axis  22 , as shown in FIG.  3 . The shape of the steering wheel armature  14  can be varied, circular, triangular or trapezoidal as needed. The sides  20  of the steering wheel armature  14  each define at least one slot  24 , which slidably accepts a retaining flange  26  formed on the housing  12 . In this embodiment, there are three slots  24  in each side. An aperture for the steering column (not shown) is also located on the steering wheel armature  14 . A mounting hole (not shown) is located in each of the armature bosses (not shown). Each of the retaining apertures  34  on the air bag cover  16  fit over one of the retaining flanges  26 . An air bag inflator and an air bag cushion are located between the air bag cover  16  and the housing  12 . The air bag inflator bracket is secured to the steering wheel armature  14  by means of mounting bolts (not shown) that extend through depending retaining holes. 
     The housing  12  of the air bag module  10  retains the air bag (not shown). As shown in  FIG. 1 , the housing  12  has a plurality of flanges  26 . The housing  12  is preferably made from sheet metal such as stamped steel or aluminum. A body  28  of the housing  12  is generally cup-shaped having a bottom  30  and depending sides  32 . The housing  12  is symmetric to the vertical axis  22 . The shape of the housing  12  can be varied, circular, triangular or trapezoidal as needed. The flanges  26  extend from the sides  32  of the housing body  28 , as shown in  FIGS. 1 and 3 . The flanges  26  are generally rectangular in shape, with a thickness of 1.5 mm and a width of 3 mm. The shape and dimensions may be varied as needed. As mentioned, the flanges  26  are designed to slide into the slots  24  of the steering wheel armature  14  and are also received within the receiving apertures  34  (such as in a snap fit manner) (shown in  FIGS. 3 and 5 ). The snap fit of the flanges  26  into the slots  24  creates a structure that retains the airbag cover before, after and during deployment. In addition, the snap fit design simplifies the assembly of the air bag module  10  by eliminating the need for fasteners, such as rivets. 
     The cover  16  is generally made from thermoplastic olefin or a thermoplastic elastomer. The cover  16  has a plurality of retaining apertures  34 , with the number of retaining apertures  34  equal to the number of the flanges  26  of the housing  12 . The flanges  26  of the housing  12  slide through the retaining apertures  34  (see  FIG. 5 ) and are placed within the support rib  120  to retain the cover  16 . In  FIG. 1 , the cover  16  is shown with a specific geometry. However, the shape of the cover  16  may be a plurality of shapes such as, for example, rectangular or circular. As shown in  FIG. 2 , the cover  16  is also symmetrical with respect to the vertical axis  22 . In addition, air bag cover  16  may also include an air bag scrim. 
     Referring now to  FIG. 1 , in addition to the retaining apertures  34 , the cover  16  is shown with a plurality of tear seams  36 . Although the cover  16  is shown as having six radially projecting tear seams  36 , it can include any number of tear seams  36 . The tear seams  36  originate from a circumference defined by a central radius  38  on the cover  16 . The central radius  38  defines a through hole as shown, which is about four centimeters. However, the central radius  38  may vary for different air bag applications. The tear seams  36  are symmetric with respect to a vertical axis  22  and are evenly spaced with respect to the vertical axis  22 . At an initiation end  40 , the tear seams  36  have a linear section  42  beginning from the circumference defined by the central radius  38  and an articulating section  44 , which occurs before reaching a terminal end  46 . 
     The design of the tear seams  36  allows the air bag to deploy and effectively manage the tear propagation through the cover. The length of the linear section  42  of the tear seams  36  ranges from about two to about four centimeters. The length of the articulating section  44  of the tear seams  36  ranges from about one centimeter to about two centimeters and has a radius of curvature of about 13 mm. The articulating section  44  in the tear seams  36  near the terminal end  46  slows the progression of the tearing of the cover  16 . Upon reaching the terminal end  46 , the tear seams  36  encounter a rip stop  48 . Rip stops  48  are formed by a raised rib to prevent the cover  16  from tearing further. The tear seams  36  are shown with constant thickness. It is envisioned that the thickness of the tear seams  36  can increase from the initiation end  40  to the terminal end  46  to further slow the progression of the cover  16 . 
     As is shown in  FIG. 2 , a central opening  50  is defined in the cover. The central opening  50  is symmetrical with respect to the vertical axis  22 , but can also be offset from the vertical axis  22 . The central opening  50  can be used for inserting instrumentation as disclosed in commonly owned pending application Ser. No. 09/871,037. 
     As shown in  FIG. 4  the cover  16  may include a generally circular central door  52 . This embodiment allows for the advantages with respect to deployments outlined earlier. The central circular door  52  is surrounded by a generally circular tear seam  36 , and coupled to the cover  16  by a hinge  54 . The circular tear seam  36  functions as the initiation end  40  for the radially projecting tear seams  36 . The circular central door  52  and articulating tear seams  36  additionally provide the benefit of significantly masking visual defects in the cover  16  such as read-through of the tear seam  36 . 
     During the manufacturing process, the cover  16  is slid onto the flanges  26  of the housing  12 . Next, the flanges  26  of the housing  12  are received within the slots  24  of the steering wheel armature  14 . When the air bag is deployed, the tear seams  36  on the cover  16  begin separating at the initiation end  40  and slow as they begin to reach the terminal end  46 . When the tear seam separation reaches the terminal end  46 , the tearing of the cover  16  stops due to the rip stop  48 . The sides  20  of the steering wheel armature  14  restrain bell mouthing of the housing  12  and prevent the retaining apertures  34  of the cover  16  from becoming disengaged from the flanges  26 . 
       FIGS. 5 and 6  represent side views of the module  10  coupled to the steering wheel armature  14 . The module is assembled by snapping the cover retaining apertures  34  over the cover retaining flange  26 . At the time of assembly of the module  10  to the steering wheel armature  14 , the module  10  is electrically connected to a crash sensing system (not shown). The module  10  is slid between the side walls  20  of the steering wheel armature  14 . During the insertion, the cover retaining flanges  26  are positioned into the slots  24 . 
     During a deployment, inflation gasses fill the cushion, exerting significant forces on the sides  32  of the housing. As best seen in  FIG. 6 , the cover is retained during the deployment event between the side wall  20  of the armature  14  and the sides  32  of housing  12 . This configuration utilizes the forces on the sides  32  from the deployment to restrain the cover onto the module mounting plate  12 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.