Abstract:
An air bag device includes an air bag, a cover concealing the air bag and including a side wall formed with a groove, and a base plate supporting the air bag and including a peripheral wall that faces the inner periphery of the side wall of the cover and is formed with a slit corresponding to the groove of the cover. A belt is disposed along the outer periphery of the side wall of the cover, and includes a holder cooperating with the peripheral wall of the base plate to hold the side wall of the cover. The belt further includes engagements that are engaged with the groove of the side wall of the cover and the slit of the peripheral wall of the base plate and include each an engaging portion engaged with the inner periphery the peripheral wall of the base plate.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to air bag devices having an air bag and a cover disposed thereon. 
     Air bag devices are known conventionally, which are arranged, for example, at a boss of a steering wheel of a motor vehicle to absorb impact on a driver upon collision. The air bag device comprises an inflator for jetting out gas, an air bag arranged in fold, a cover disposed on the air bag, and a base plate having the inflator, the air bag, and the cover mounted thereon. The air bag device is constructed such that when detecting collision of the motor vehicle, i.e. its sudden deceleration over a predetermined value, the inflator jets out gas instantaneously to distend the air bag. Its pressure serves to break the cover along a predetermined tear line for door-like extension, from which the air bag protrudes to develop in front of the driver. 
     The cover includes a covering portion for covering the folded air bag and a mounting portion that protrudes downwardly from the covering portion, having substantially a box-like form with an opening on the lower side. Likewise, the base plate includes a base portion having the inflator and the air bag mounted thereon and a support portion that protrudes downwardly from the base portion, having substantially a box-like form with an opening on the lower side. With the mounting portion engaged with the outer periphery of the support portion, the mounting portion, the support portion, etc. are fixed to each other by a number of rivets, obtaining the cover fixed to the base plate. 
     However, the structure using a number of rivets for fixing the cover is difficult to reduce a manufacturing cost due to the increased number of component parts and complicated assembling process. 
     JP-A 6-298031 proposes another structure for fixing the cover, wherein the mounting portion of the cover and the support portion of the base plate are formed cylindrically. With the mounting portion engaged with the outer periphery of the support portion, a fastening band is tightly wound on the outer periphery of the mounting portion, obtaining the mounting portion held between the support portion and the band. 
     However, the structure using the fastening band is difficult to control the fastening strength due to the need of applying an appropriate fastening force for assembling the cover to the base plate. Further, due to the fact that no means are provided to position the fastening band, sure fixing of the cover needs to fasten the band in holding it in an accurate position, resulting in complicated fastening work. Furthermore, the use of the fastening band is not available except the cover and the base plate having a rounded section such as a circle, resulting in limited form of the cover and the base plate. 
     JP-A 6-144139 proposes still another structure for fixing the cover, wherein with a pair of substantially C-shaped plates disposed on the outer periphery of the mounting portion of the cover engaged with the support portion of the base plate, the support portion, the mounting portion, and the plates are integrally mounted by a number of rivets. Each plate is formed with claws at the lower end, which are engaged with the support portion of the base plate and the mounting portion of the cover for positioning. 
     However, the structure using the claws has a problem that the claws, arranged for positioning, cannot support the mounting portion against the distending pressure of the air bag, requiring a number of rivets for fixing the air bag, resulting in a difficult reduction in a manufacturing cost due to the increased number of component parts and complicated assembling process. 
     As described above, the structure using the fastening band needs not only to apply an appropriate fastening force for assembling the cover to the base plate, but to fasten the band in holding it in an accurate position, resulting in complicated fastening work and limited form of the cover and the base plate. On the other hand, the structure using the claws has a problem that the claws, arranged for positioning, cannot support the mounting portion against the distending pressure of the air bag, requiring a number of rivets for fixing the air bag, resulting in a difficult reduction in a manufacturing cost due to the increased number of component parts and complicated assembling process. 
     It is, therefore, an object of the present invention to provide air bag devices that enable easy and secure fixing of the cover to the base plate. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention lies in providing an air bag device, comprising: 
     an air bag distended by inflow of gas; 
     a cover concealing said air bag, said cover including a substantially cylindrical mounting portion, said mounting portion being formed with a groove; 
     a support member supporting said air bag, said support member including a support portion facing an inner periphery of said mounting portion of said cover, said support portion being formed with a slit corresponding to said groove of said cover; and 
     a holding member disposed along an outer periphery of said mounting portion of said cover, said holding member including a holder cooperating with said support portion of said support member to hold said mounting portion of said cover, said holding member including a plurality of engagements engaged with said groove of said mounting portion of said cover and said slit of said support portion of said support member, each engagement including an engaging portion engaged with an inner periphery of said support portion of said support member. 
     Another aspect of the present invention lies in providing an air bag device, comprising: 
     an air bag distended by inflow of gas; 
     a cover concealing said air bag, said cover including a substantially cylindrical mounting portion, said mounting portion being formed with a groove; 
     means for supporting said air bag, said supporting means including a support portion facing an inner periphery of said mounting portion of said cover, said support portion being formed with a slit corresponding to said groove of said cover; and 
     means for holding said cover, said holding means being disposed along an outer periphery of said mounting portion of said cover, said holding means including a holder cooperating with said support portion of said supporting means to hold said mounting portion of said cover, said holding means including a plurality of engagements engaged with said groove of said mounting portion of said cover and said slit of said support portion of said support member, each engagement including an engaging portion engaged with an inner periphery of said support portion of said support member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view showing an embodiment of an air bag device according to the present invention; 
     FIG. 2 is a plan view showing the air bag device; 
     FIG. 3 is a sectional view taken along the line III—III in FIG. 2; 
     FIG. 4 is a view similar to FIG. 3, taken along the line IV—IV in FIG. 2; 
     FIG. 5A is a bottom view showing a cover held between a base plate and a holding member or belt; 
     FIG. 5B is an enlarged fragmentary sectional view showing the cover held between the base plate and the holding member; 
     FIG. 6 is a perspective view showing the holding member; 
     FIG. 7 is a view similar to FIG. 6, showing a variant of the holding member; 
     FIGS. 8A-8D are enlarged perspective views, each showing a variant of an engaging portion of an engagement of the holding member; and 
     FIGS. 9A-9C are enlarged front views, each showing a variant of an engaging slit of the support member. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, a description will be made with regard to an air bag device embodying the present invention. Referring to FIG. 4, an air bag device or module  1  is mounted to a steering wheel  2  of a motor vehicle. The steering wheel  2  includes a main body  3  and the air bag device  1  mounted thereto on the driver&#39;s side or the front side. The steering wheel  2  is mounted to a steering shaft, not shown, with an adjustable inclination, and is generally used in the inclined state. In connection with the steering wheel  2 , refer to the driver&#39;s side having the air bag device  1  mounted as the upper or front side, to the vehicular body side as the lower or rear side, to the front glass side or the upper front side of the motor vehicle as the forward side, and the driver&#39;s side or the lower rear side of the motor vehicle as the rearward side. The steering-wheel main body  3  includes an annular rim, not shown, a boss  5  disposed inside the rim, and a plurality of, e.g. four, spokes for connecting the rim and the boss  5 . The boss  5  includes a boss member  5   a  engaged with the steering shaft and a boss plate  5   b  separately or integrally fixed thereto. A pair of mounting brackets of air-bag-device mounting members  8  is fixed to the lower portion of the boss plate  5   b  by screws  8   a  through a horn switch unit if necessary. The air bag device  1  is mounted to the mounting brackets  8  by cap bolts  9 . The lower side portion of the boss  5  is covered with a lower cover  10 . 
     Referring to FIG. 1, the air bag device  1  comprises a base plate or support member  11 , a cover mounted to the base plate  11 , an air bag  15 , an inflator  16 , a retainer  17 , and a belt or holding member  18 . 
     Referring also to FIG. 5A, the base plate  11  is substantially symmetrically shaped by press working of a metal plate. The base plate  11  includes a substantially hexagonal flat base portion  21  and a peripheral wall or support portion  22  obtained by bending downwardly the outer periphery of the base portion  21 , having substantially a box-like form with an opening on the lower side. The base portion  21  has a circular inflator hole  23  substantially in the center, and bolt holes  24  around the inflator hole  23 . The peripheral wall  22  is of a polygonal cylindrical shape, and has inner engagements  25  formed outwardly or radially from the peripheral surface or protruding therefrom through cutting or the like. Moreover, as best seen in FIG. 4, the peripheral wall  22  is formed with brackets  27  on both sides of the base plate  11 . Each bracket  27  has a bolt hole  28 , inside of which a square nut  29  engaged with the cap bolt  9  through caulking or the like is fixedly arranged. 
     Moreover, as shown in FIG. 1, the peripheral wall  22  is formed with an engaging slit  31  in the center of the rear side of the base plate  11 . The engaging slit  31  has a lower end or open end  31   a  with an opening at the edge and an upper end extending to the position to contact the base portion  21 , i.e. it extends over the total width of the peripheral wall  22 . Curved or R-shaped guides  31   b  are formed in the vicinity of the open end  31   a  of the engaging slit  31  to enlarge downwardly. 
     Stoppers  32  are formed outwardly protrusively at the lower end of the peripheral wall  22  at predetermined circumferential intervals. Moreover, recesses  33  are formed at the lower end of the peripheral wall  22  at predetermined positions. Moreover, referring also to FIG. 3, rectangular holes  34  are formed in the peripheral wall  22  in the front and rear positions of the base plate  11 . 
     On the other hand, the cover  12  is integrally formed out of synthetic resins. Specifically, the cover  12  is obtained by injection molding of thermoplastic elastomers such as olefinic elastomer including thermoplastic olefin manufactured by SUMITOMO CHEMICAL CO., LTD. The cover  12  has an external form of a center pad mounted to the steering-wheel main body  3  with four spokes. Specifically, as best seen in FIGS. 2-3, the cover  12  includes a covering portion  41  for covering the boss  5  of the steering-wheel main body  3  and part of the surface of the spokes and a side wall or mounting portion  42  that protrudes downwardly perpendicularly from the lower side of the covering portion  41 . The covering portion  41  and the side wall  42  cooperate to each other to define an air bag compartment or receiver for housing the air bag  15 . A substantially H-shaped weak tear line  44  is concavely formed in the lower side of the covering portion  41  to face the air bag compartment. The side wall  42  of the cover  12  is of a polygonal cylindrical shape, and has an internal form that substantially corresponds to the external form of the base portion  21  of the base plate  11  for engagement. As best seen in FIGS. 5A-5B, a groove  45  is formed in the side wall  42  in the center of the rear side of the cover  12  to extend upwardly from the edge of the lower end. As shown in FIG. 1, the side wall  42  is formed with rectangular inner engaging holes or first holes  46  as engaging receivers and rectangular outer engaging holes or second holes  47  as engaging receivers, the holes  46 ,  47  being arranged radially through the side wall  42 . Moreover, as best seen in FIGS.  1  and  5 A- 5 B, the side wall  42  is formed with protrusions  48  arranged inside and recesses  49  arranged on and through both sides of the cover  12  and having downward openings. 
     As shown in FIG. 1, the air bag  15  is formed, for example, like a flat bag obtained by sewing two pieces of cloth. The air bag  15  has a circular gas inlet  15   a  in the center of the bottom, through which the inflator  16  is arranged, and bolt holes  15   b  around the gas inlet  15   a.    
     The inflator  16  includes a substantially cylindrical main body  16   a  and mounting flanges  16   b  on the outer periphery thereof. Moreover, the inflator  16  has gas jets  16   c  formed above the mounting flanges  16   b  to jet out gas into the main body  16   a.  The mounting flange  16   b  is formed with bolt holes  16   d.  A lead wire is drawn from the lower side of the main body  16   a.  An inflator cover made of synthetic resins may be attached to the main body  16   a  if necessary. 
     The retainer  17  includes a substantially annular main body  17   a  formed with upward flanges on the inner and outer peripheries and having a substantially C-shaped section. Four stud bolts  17   b  are fixed downwardly to the main body  17   a,  with which nuts  50  are engaged from the lower end, respectively. 
     Referring to FIG. 6, the belt  18  is of a substantially annular shape with a discontinuity  52 . Specifically, the belt  18  is obtained by carrying out press working or bending of a steel plate with a substantially fixed width to face both ends without joining. The belt  18  is formed with a holder  53  curved in advance along the outer peripheral surface of the side wall  42  of the cover  12  and engagements  55  obtained by bending both ends with the same width W as that of the holder  53 , i.e. the total width of the steel plate, inwardly by 90- to form substantially an L-shape. Engaging portions  56  are circumferentially distantly formed on the engagements  55  to protrude therefrom substantially semi-cylindrically. 
     Engagements  58  are formed on both sides of the holder  53  to engage with the recesses  49  of the cover  12 , each engagement  58  being dented by the thickness of a mounting portion  8   b  of the mounting bracket  8 . A circular mounting hole  59  is formed in each engagement  58 . The holder  53  has outer engagements  61  formed circumferentially through cutting or the like to protrude inwardly. 
     As shown in FIG. 4, a circular mounting hole  8   c  is formed in the mounting portion  8   b  of the mounting bracket  8  to correspond to the mounting hole  59  of the engagement  58 . 
     Next, a description will be made with regard to manufacturing processes of the air bag device  1 . 
     The base plate  11 , the cover  12 , the belt  18 , etc. are formed in predetermined shapes in advance. 
     Deforming ovally the gas inlet  15   a  of the air bag  15 , the retainer  17  is inserted to the inside of the air bag  15  through the gas inlet  15   a.  The stud bolts  17   b  are drawn outside from the bolt holes  15   b,  and are inserted into the bolt holes  24  of the base plate  11  from the upper side to press the circumference of the gas inlet  15   a  on the base portion  21  of the base plate  11 . 
     In that state, automatic assembling is started with an automatic assembling machine driven by the air pressure or the like. Specifically, with an air bag folder, the air bag  15  is folded in petals around the main body  16   a  of the inflator  16  to be housed in the external form of the base portion  21  of the base plate  11 . 
     Then, the cover  12  is automatically provided from above the base plate  11  on which the air bag  15  is arranged in fold. With a pneumatic press or the like, the cover  12  is pushed in the base plate  11  to house the air bag  15  between the base plate  11  and the cover  12 . The side wall  42  of the cover  12  is engaged with the outer periphery of the base portion  21  of the base plate  11 . The inner engagements  25  of the base plate  11  are engaged with the inner engaging holes  46  of the cover  12 , respectively, and the lower end of the side wall  42  abuts on the stoppers  32  to restrict the engaging amount. Moreover, the protrusions  48  arranged inside the side wall  42  are engaged with the holes  34  to position accurately the cover  12  to the base plate  11  for anti-loose holding. 
     Subsequently, the belt  18  is automatically provided to a predetermined position. With the engagements  55  closely contacted each other by clamp means, the holder  53  is press fitted on the outer periphery of the side wall  42  of the cover  12  by a pneumatic press or the like. At that time, the engagements  55  are inserted not only into the groove  45  of the cover  12  from the lower end, but into the engaging slit  31  of the base plate  11  from the open end  31   a.  Thus, the engaging portions  56  are engaged with the inner surface of the peripheral wall  22 . Moreover, the engagements  55  comes in slide contact with the guides  31   b  of the engaging slit  31 , respectively, to reduce or tighten the belt  18  to a predetermined longitudinal dimension. Thus, the side wall  42  of the cover  12 , compressed and resiliently deformed, is held between the peripheral wall  22  of the base plate  11  and the belt  18 . 
     At the same time, the engagements  58  of the holder  53  are engaged with the recesses  49  of the cover  12 , respectively, for positioning. 
     When pushing the belt  18  in the cover  12 , the outer engagements  61  of the belt  18  abut on the lower end of the side wall  42  of the cover  12  to press it to the inner periphery side. Since the recesses  33  are formed in the base plate  11  to correspond to the outer engagements  61 , the side wall  42  resiliently deformed to the inner periphery side is smoothly received in the recesses  33 . When pushing the belt  18  further, the outer engagements  61  are inserted in the outer engaging holes  47 , and the side wall  42  is resiliently deformed again to engage with the belt  18 , which is thus positioned accurately for anti-loose holding. 
     The side wall  42  of the cover  12  is held between the peripheral wall  22  of the base plate  11  and the belt  18 . And the belt  18  ensures engagement of the inner engagements  25  with the inner engaging holes  46 , whereas the base plate  11  ensures engagement of the outer engagements  61  with the outer engaging holes  47 . Thus, the cover  12  is securely fixed to the base plate  11 . 
     Subsequently, the upper portion of the inflator main body  16   a  is inserted in the inflator hole  23  and the gas inlet  15   a  from the lower side of the base plate  11 . The stud bolts  17   b  of the retainer  17  are inserted in the bolt holes  16   d  of the mounting flange  16   b,  with which the nuts  50  are engaged from the lower end for tightening. Thus, the base plate  11  and the air bag  15  are both held between the retainer main body  17   a  and the mounting flange  16   b,  completing assembling of the air bag device  1 . 
     Then, the air bag device  1  as assembled is mounted to the mounting brackets  8  extending from the boss  5  of the steering-wheel main body  3 . At that time, the mounting portion  8   b  of the mounting bracket  8  is engaged with the engagement  58  of the belt  18 . From the outside of the mounting portion  8   b,  the cap bolt  9  is arranged through the mounting hole  8   c  of the mounting portion  8   b  and the mounting hole  59  of the belt  18  through a washer  9   a.  And the cap bolt  9  is engaged with the square nut  29  through the bolt hole  28  of the bracket  27  of the base plate  11 . Thus, the belt  18  with the inner peripheral surface closely contacting the outer peripheral surface of the side wall  42  is further pressed to the base plate  11  by the cap bolt  9 . 
     If the motor vehicle undergoes impact of collision, etc., a control unit, not shown, actuates the inflator  16  to supply instantaneously inert gas such as nitrogen gas from the gas jets  16   c  to the inside of the air bag  15 . This distends the air bag  15  promptly, the pressure of which breaks the covering portion  41  of the cover  12  along the tear line  44  to protrude the air bag  15  upwardly of the cover  12 . Moreover, the air bag  15  is widely developed in front of the driver to absorb and alleviate impact thereon. 
     In this embodiment, the side wall  42  of the cover  12  is held between the peripheral wall  22  of the metallic base plate  11  and the metallic belt  18 . And the belt  18  ensures engagement of the inner engagements  25  with the inner engaging holes  46 , whereas the base plate  11  ensures engagement of the outer engagements  61  with the outer engaging holes  47 . Thus, the cover  12  can securely be fixed to the base plate  11 . 
     The belt  18  is of an engagement form having the engagements  55  at both ends, on which the engaging portions  56  are formed. Holding of the side wall  42  of the cover  12  is carried out as follows. As soon as the belt  18  is engaged with the outer periphery of the side wall  42  of the cover  12 , the engagements  55  are inserted in the engaging slit  31  of the base plate  11  from the open end  31   a  so that the engaging portions  56  are engaged with the inner surface of the peripheral wall  22  in an anti-loose way. Such only one process enables secure holding of the side wall  42  of the cover  12 , resulting in easy assembling. Moreover, the belt  18  can be formed only by bending a piece of steel plate with substantially fixed width, resulting not only in the effective use of the steel plate as a material, but a reduction in a manufacturing cost due to its simple form requiring no welding process. 
     Though with simple structures, the engagements  55  are formed using the total width of the belt  18 , and the engaging portions  56  are not formed by bending wherein a bent portion is apt to extend, but by embossing wherein a raised or expanded area is created from a surface, resulting in secure holding of the cover  12  against the pressure of development of the air bag  15 . Moreover, since the engaging slit  31  extends from the lower end of the peripheral wall  22  of the base plate  11  to the vicinity of the base portion  21 , the engagements  55  inserted in the engaging slit  31  are disposed in the vicinity of the base portion  21 . Thus, even if the pressure of development of the air bag  15  operates to bend the peripheral wall  22 , the engaging portions  56  of the engagements  55  are supported by the base portion  21 , enabling secure holding of the cover  12 . 
     The guides  31   b  are formed at the engaging slits  31 , which come in slide contact with the engagements  55  inserted from the open end  31   a  to apply longitudinally a force to the belt  18  for tightening, i.e. for compression of the cover  12 , resulting in easy insertion of the engagements  55  in the engaging slit  31 . Moreover, only one process of press fitting the engagements  55  into the engaging slit  31  enables application of a longitudinal force to the belt  18  for an appropriate amount of tightening, resulting in secure holding of the cover  12 . 
     Since the air bag  15  can be held securely, there is no need of separate fixing members such as a rivet, enabling a reduction in the number of component parts and manufacturing processes of the air bag device  1 , facilitating auto-assembling thereof, resulting in a reduction in a manufacturing cost. 
     In this embodiment, the engaging slit  31  and the groove  45  are formed in the rear portions of the base plate  11  and the cover  12 , respectively, where their engagements with the engaging portions  56  of the belt  18  are carried out. Alternatively, the engaging slit  31  and the groove  45  may be formed in other positions. Moreover, in the structure wherein the peripheral wall  22  of the base plate  11  is of a substantially polygonal cylindrical shape with substantially flat supports continuously connected at corners, formation of the engaging slit  31  in the center of the support with the largest longitudinal dimension, e.g. the front support as in this embodiment, enables secure holding of the air bag  15 . Specifically, when undergoing the pressure upon development of the air bag  15 , the holder  53  of the belt  18  is apt to separate from the peripheral wall  22  in the portion with a larger longitudinal dimension. In view of this, the engaging slit  31  is formed in the front peripheral wall  22  with the largest longitudinal dimension. And the engaging portions  56  of the engagements  55  are inserted in the engaging slit  31  to connect the belt  18  and the base plate  11 , enabling secure holding of the cover  12 . 
     Moreover, in this embodiment, the unitary belt  18  serves to cover all circumference of the side wall  42  of the cover  12 . Alternatively, referring to FIG. 7, two or more belts  18  may be adopted, each having the engagements  55  with the engaging portions  56  at both ends. With two separate belts  18  as shown in FIG. 7, the engagements  55  of each belt  18  serve to connect the belt  18  and the base plate  11 , enabling secure holding of the cover  12 . Further, this structure contributes not only to a reduction in the longitudinal dimension of the belts  18 , but to an achievement of easier storage than that of the annular members, resulting in a reduction in a manufacturing cost. Furthermore, the outer engagements  61  are formed in the center of each belt  18  in the cross direction, which enables formation of the two belts  18  in the same shape. Thus, a small form of bending can be adopted, resulting in a reduction in a manufacturing cost. It is noted that the belts  18  as shown in FIG. 7 have no engagement  58 . With the structure having such belts  18 , the mounting portion  8   b  of the mounting bracket  8  with the same thickness as that of the side wall  42  of the cover  12  is held between the belt  18  and the peripheral wall  22  of the base plate  11 , or expansions are formed at the peripheral wall  22  to expand to the belt  18 . 
     Referring to FIGS. 8A-8D, the engaging portion  56  of the engagement  55  may be formed in different shapes according to different methods, such as a cylinder having an end cut on the side of the discontinuity  52  as shown in FIG. 8A, a semi-sphere having an end cut on the side of the discontinuity  52  as shown in FIG. 8B, a cylinder having an end reduced on the side of the discontinuity  52  as shown in FIG. 8C, and a semi-sphere having an end reduced on the side of the discontinuity  52  as shown in FIG.  8 D. In such a way, anti-loose form is achieved at an end of the engaging portion  56  on the side of the discontinuity  52 , which abuts on the peripheral wall  22  of the base plate  11 , enabling secure holding of the cover  12 . Furthermore, the engaging portion  56  may be obtained by fixing a separate member such as a steel block to the engagement  55  by spot welding. Still further, two vertically parallel slits may be formed in the engagement  55  to have therebetween a portion pushed up and expanded in the direction of thickness, which enables increased dimensional accuracy. 
     Further, referring to FIGS. 9A-9C, the engaging slit  31  of the peripheral wall  22  of the base plate  11  may be formed in different shapes such as the slit  31  with curved guides  31   b  at the open end  31   a  as shown in FIG. 9A, the slit  31  with inclined guides  31   b  at the open end  31   a  as shown in FIG. 9B, and the slit  31  with straight and inclined guides  31   b  in the middle position as shown in FIG.  9 C. 
     Finally, it is noted that the present invention can be applied not only to the driver&#39;s air bag device mounted to the steering wheel of the motor vehicle, but the passenger&#39;s one mounted to an instrument panel arranged in front of a passenger seat and that one mounted to a seat or a door. The present invention is also applicable to movers other than the motor vehicle, which require absorption of impact, etc.