Abstract:
A steering wheel assembly ( 10 ) mounts on a vehicle steering shaft ( 14 ) which has a threaded end portion ( 18 ). The steering wheel assembly ( 10 ) comprises a steering wheel armature ( 20 ), an inflatable vehicle occupant protection device ( 80 ) supported on the steering wheel armature ( 20 ), an inflator ( 90 ) for providing inflation fluid for inflating the inflatable vehicle occupant protection device ( 80 ), a hub ( 160 ) fixedly attached to the steering wheel armature ( 20 ) and for non-rotatable attachment to the steering shaft ( 14 ), and a rotatable fastener ( 180 ) rotatably supported on the hub ( 160 ) for axially forcing the hub onto the steering shaft ( 14 ) upon rotation of the fastener in a first direction. The fastener ( 180 ) has a first end portion ( 182 ) including gearing ( 190 ) for rotating the fastener and a second end portion ( 184 ) for threadedly engaging the threaded end portion ( 18 ) of the steering shaft ( 14 ) upon rotation of the fastener. A retainer plate ( 200 ) spaced from the inflator ( 90 ) is fixedly attached with the steering wheel armature ( 20 ). The retainer plate ( 200 ) overlies the first end portion ( 182 ) of the fastener ( 180 ) and retains the fastener on the steering wheel armature ( 20 ).

Description:
TECHNICAL FIELD 
     The present invention is related to a steering wheel assembly which is mounted on a steering shaft of a vehicle and which is rotatable to steer the vehicle. 
     BACKGROUND OF THE INVENTION 
     A vehicle steering wheel is typically attached to a vehicle steering shaft by a splined connection. The splined connection non-rotatably attaches the steering wheel to the steering shaft and transmits torque from the steering wheel, namely, torque generated during turning of the steering wheel, to the steering shaft. The steering wheel is additionally secured to the steering wheel by a nut to prevent relative axial movement between the steering wheel and the steering shaft. The nut is screwed onto a threaded extension located on the end of the steering shaft which extends into the passenger compartment of the vehicle. Usually, the nut is accessed by an installer through the center of the steering wheel once the steering wheel has been placed onto the steering shaft. An air bag module is then typically mounted in the center of the steering wheel and covers the nut. 
     Another type of connection for connecting a vehicle steering wheel to the steering shaft utilizes tapered surfaces on the steering shaft and inside a hub portion of the steering wheel. A gear-headed bolt is used to bring the tapered surfaces into non-rotatable engagement. Threads on the bolt engage an internally threaded surface on the steering shaft to pull the steering wheel onto the steering shaft. The gear head of the bolt is driven by a tool which is inserted into the steering wheel from the side (i.e., perpendicular to the axis of rotation). 
     SUMMARY OF THE INVENTION 
     The present invention is a steering wheel assembly to be mounted on a vehicle steering shaft which rotates about an axis to steer a vehicle and which has a threaded end portion. The steering wheel assembly comprises a steering wheel armature, an inflatable vehicle occupant protection device supported on the steering wheel armature, an inflator for providing inflation fluid for inflating the inflatable vehicle occupant protection device, a hub fixedly attached to the steering wheel armature and for non-rotatable attachment to the steering shaft, and a rotatable fastener rotatably supported on the hub for axially forcing the hub onto the steering shaft upon rotation of the fastener in a first direction. The fastener has a first end portion including gear means for rotating the fastener and a second end portion for threadedly engaging the threaded end portion of the steering shaft upon rotation of the fastener. A retainer plate spaced from the inflator is fixedly attached with the steering wheel armature. The retainer plate overlies the first end portion of the fastener and retains the fastener on the steering wheel armature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other features of the present invention will becomes apparent to one skilled in the art to which the present invention relates upon consideration of the following description of the invention with reference to the accompanying drawings, wherein: 
     FIG. 1 is an exploded perspective view of a steering wheel assembly constructed in accordance with the present invention; 
     FIG. 2 is a sectional view of the steering wheel assembly of FIG. 1 with parts omitted for clarity; 
     FIG. 3 is a view taken along line  3 — 3  in FIG. 2 with parts omitted for clarity; 
     FIG. 4 is a sectional view taken along line  4 — 4  in FIG. 3; 
     FIG. 5 is a perspective view of a component of the steering wheel assembly shown in FIG. 1; 
     FIG. 6 is a view similar to FIG. 3 illustrating a second embodiment of the present invention; 
     FIG. 7 is a schematic sectional view of a steering wheel assembly constructed in accordance with a third embodiment of the present invention; 
     FIG. 8 is a view taken along line  8 — 8  in FIG. 7; and 
     FIG. 9 is a schematic sectional view of a steering wheel assembly constructed in accordance with a fourth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     An exploded perspective view of a steering wheel assembly  10  constructed in accordance with the present invention is shown in FIG.  1 . The steering wheel assembly  10  is to be mounted on an end portion  12  of a vehicle steering shaft  14  (shown in phantom lines in FIG.  2 ). The end portion  12  of the steering shaft  14  is hollow and has a tapered outer surface  16  which includes diametrically opposed flats (not shown). The end portion  12  has an inner surface  18  which is threaded. 
     The steering wheel assembly  10  includes a steering wheel armature  20  (FIG.  1 ). The steering wheel armature  20  is preferably constructed in accordance with the invention described in Applicant&#39;s co-pending application U.S. Ser. No. 09/004,244, entitled “Steering Wheel Armature”, filed Jan. 8, 1998, Attorney Docket No. TRW(AP)3788. The armature  20  is preferably made of cast magnesium. The armature  20  has a circular outer rim portion  22  and an inner ring portion  24  connected by a plurality of radially and axially extending spokes  26 . The ring portion  24  defines a central opening  28  in the armature  20 . The ring portion  28  includes three axially extending apertures  30  spaced about the ring portion. 
     The steering wheel assembly  10  further includes a plastic insert  40  located in the central opening  28  in the armature  20 . The insert  40  is preferably made of a thermoplastic material. The insert  40  has four side walls  42  and front wall  44  which is connected to one of the side walls in a hinge area  46 . The side walls  42  and the front wall  44  together define a cavity  48  in the insert  40 . A generally U-shaped gap  50  separates the front wall  44  from three of the four side walls  42 . The front wall  44  includes a plurality of openings  52 . 
     An integral one-piece cover  54  is molded about the steering wheel armature  20  and the insert  40 . The cover  54  covers the outer rim portion  22 , the inner ring portion  24  and the spokes  26  of the steering wheel armature  20 . Further, the cover  54  has a continuous outer surface  56  which extends uninterruptedly over the insert  40 . The cover  54  is preferably made of a homogenous urethane material. As the cover  54  is molded about the insert  40 , portions of the material of the cover  54  fill the gap  50  defined in the insert to form a generally U-shaped tear seam  58  which is preferably not visible in the outer surface  56 . Additional portions of the cover material fill the openings  52  in the front wall  44  of the insert  40  to form a composite deployment door  60 . The tear seam  58  outlines the boundaries of the deployment door  60 . 
     It is contemplated that the cover  54  could alternatively be made of several cover portions rather than the one-piece cover shown. For example, an alternate cover construction could comprise a first cover portion covering the outer rim portion  22  and the spokes  26  of the armature  20 , while a second cover portion covers the inner ring portion  24  of the armature and the insert  40 . 
     The steering wheel assembly  10  includes a horn switch  70  and an associated backing plate  72  for the horn switch. The horn switch  70  is preferably a membrane type switch known in the art. The backing plate  72  is a semi-rigid part preferably made of a plastic material. The horn switch  70  and backing plate  72  are generally rectangular in shape and are located in the cavity  48  in the insert  40 . The horn switch  70  is located between the deployment door  60  and the backing plate  72 . The horn switch  70  is actuatable by exerting manual pressure on the outer surface  56  of the cover  54  covering the deployment door  60 . Both the horn switch  70  and the backing plate  72  have an elongated slot  74 . A fabric tether  76  having an hourglass shape and a pair of openings  78  at each end extends through the slots  74  in the horn switch  70  and the backing plate  72 , respectively, and is folded over so that the openings  78  at both ends of the tether  76  are aligned. 
     An inflatable vehicle occupant protection device, such as an air bag  80 , is located in the cavity  48  in the insert  40  of the steering wheel assembly  10 . The air bag  80 , shown schematically in the drawings, is folded inside the cavity  48  and abuts the backing plate  72  as well as the four side walls  42  of the insert  40 . The air bag  80  is of known construction and includes a generally ring-shaped air bag retainer  82  (FIGS. 1 and 2) having four equally spaced mounting apertures (not numbered). The air bag retainer  82  is located inside the air bag  80  and encircles a mouth  84  of the air bag. A stud  86  extends through each of the four apertures in the air bag retainer  82  and through corresponding apertures (not numbered) in the air bag  80 . Two of the studs  86  also extend through the openings  78  in the tether  76 . 
     The steering wheel assembly  10  further includes an air bag inflator  90  and a reaction plate  100 . The air bag inflator  90  is electrically actuatable to provide inflation fluid to inflate the air bag  80 . The inflator  90  includes a cylindrical body portion  92  and a flange portion  94  extending radially from the body portion. The body portion  92  of the inflator  90  includes a plurality of circumferentially spaced fluid exit ports  96  and is located inside the air bag  80 . The flange portion  94  of the inflator  90  is circular and extends beyond the mouth  84  of the air bag  80 . 
     The reaction plate  100  is a generally rectangular part preferably made of hardened steel. The reaction plate  100  includes a generally circular first depression  102  (FIG. 1) for receiving the flange portion  94  of the inflator  90 . A generally circular second depression  104  (FIGS. 1 and 2) in the reaction plate  100  is formed radially inward of the first depression  102  and accommodates wiring from the inflator  90 . Four openings  106  (FIG. 1) are spaced around the first depression  102  in the reaction plate  100  and are located so as to align with the studs  86  projecting from the air bag retainer  82 . Nuts  108  are screwed on the studs  86  to secure the air bag  80  and the inflator  90  to the reaction plate  100 . The reaction plate  100  further includes three openings  110  spaced and located identically to the three openings  30  in the ring portion  24  of the steering wheel armature  20 . 
     The steering wheel assembly  10  further comprises a mounting bracket  120  commonly referred to as a spider or spider member. The mounting bracket  120  is preferably made of cast magnesium. The mounting bracket  120  has three leg portions  122  which extend radially and axially from a base portion  124  (FIG.  3 ). Each leg portion  122  includes a foot portion  126  located at a terminal end of the leg portion  122 . The foot portions  126  are axially offset from the base portion  124 . Each foot portion  126  has an opening  128  which aligns with a respective one of the three openings  110  in the reaction plate  100  and a respective one of the three openings  30  in the ring portion  24 . Fasteners, such as self-tapping screws  130  (FIGS.  1  and  2 ), extend through the openings  128  in the foot portions  126 , through the openings  110  in the reaction plate  100 , and into the openings  30  in the ring portion  24  to secure the mounting bracket  120  in the steering wheel assembly  10 . The connection of the mounting bracket  120  to the ring portion  30  also secures the reaction plate  100 , to which the air bag  80  and the inflator  90  are attached, in the steering wheel assembly  10 . The mounting bracket  120  further includes three openings  132  (FIG. 3) for receiving fasteners  134  which secure a back cover  240  to be described later. 
     The base portion  124  of the mounting bracket  120  has a base surface  136  facing the reaction plate  100  and an axially extending surface  138  (FIG. 4) which extends perpendicularly from the base surface. The axially extending surface  138  defines a central passage  140  through the base portion  124  of the mounting bracket  120 . The passage  140  is centered on an axis A. An annular ridge  142  extends from the axially extending surface  138  radially inwardly into the passage  140 . Two spaced apart holes  144  are located in the base portion  124  of the mounting bracket  120  surrounding the passage  140 . Each of the holes  144  is threaded to receive a screw  146 . 
     The base portion  124  of the mounting bracket  120  further includes two radially and axially extending ridges  148 . The ridges  148  define a generally rectangular channel  150  in the base portion  124  extending radially inward from a perimeter edge  152  of the mounting bracket  120  to the passage  140 . 
     A metal hub  160  is partially located in the passage  140  in the base portion  124  of the mounting bracket  120 . The hub  160  is generally tubular in shape and is centered on the axis A. The hub  160  is preferably cast into the mounting bracket  120  and is thus fixed to the bracket. The annular ridge  142  of the mounting bracket  120  extends into the material of the hub  160  to attach the hub to the mounting bracket non-rotatably during casting. Alternatively, it should be understood that the hub  160  could be formed integrally with the mounting bracket  120  as a one-piece cast part. 
     The hub  160  includes radially extending first and second end surfaces  162  and  164 . The first end surface  162  faces toward the reaction plate  100  and is recessed from the base surface  136  of the mounting bracket  120  by a distance S 1 . An axially extending surface  166  defines a bore  168  extending from the first end surface  162  to the second end surface  164  of the hub  160 . The bore  168  is centered on the axis A. The surface  166  defining the bore  168  is tapered to have an interference fit with the tapered outer surface  16  of the steering shaft  14 . The surface  166  inside the hub  160  further includes diametrically opposed flats (not shown) for mating with the corresponding flats on the outer surface  16  of the steering shaft  14 . 
     The steering wheel assembly  10  includes a rotatable fastener  180  centered on the axis A. The fastener  180  has a first end portion  182  and a second end portion  184  which extends from the first end portion. The first and second end portions of the fastener  180  are fixed together and rotate together. Preferably, the fastener  180  is made from one piece of a metallic material. 
     The first end portion  182  of the fastener  180  is disk-shaped and includes parallel, radially extending first and second surfaces  186  and  188 , respectively (FIGS.  4  and  5 ). The first and second surfaces  186  and  188  are separated by a distance S 2  which is the axial thickness of the first end portion  182  of the fastener  180 . The first end portion  182  of the fastener  180  is configured as a spur gear and thus has axially extending gear teeth  190  which are circumferentially disposed about the perimeter of the first end portion  182 . 
     The second end portion  184  of the fastener  190  extends axially from the second surface  188  of the first end portion  182  of the fastener. A cylindrical projection  192  extends axially from the first surface  186  of the first end portion  182  in a direction opposite the direction in which the second end portion  184  extends. The cylindrical projection  192  is centered on the axis A. 
     The second end portion  184  of the fastener  180  includes an externally threaded portion  194  and an anti-cross-threading tip  196  adjacent the terminal end of the fastener. The threaded portion  194  of the second end portion  184  of the fastener  180  is threaded to mate with the threaded inner surface  18  in the end portion  12  of the steering shaft  14 . 
     The first end portion  182  of the fastener  190  rests on and is supported for rotation by the first end surface  162  of the hub  160 . The second end portion  184  of the fastener  190  is located within and rotatable in the bore  168  through the hub  160 . The fastener  190  is rotatable about the axis A which is coaxial with the axis of rotation of the vehicle steering shaft  14 . 
     The steering wheel assembly  10  further includes a retainer plate  200  (FIGS.  3  and  4 ). The retainer plate  200  is preferably made of hardened steel. The retainer plate  200  has a generally square central portion  202  partially encircled by a peripheral portion  204  of the retainer plate. Both the central portion  202  and the peripheral portion  204  of the retainer plate  200  are planar. The peripheral portion  204  of the retainer plate  200  abuts the base surface  136  of the mounting bracket  120 . The central portion  202  of the retainer plate  200  is axially offset from the peripheral portion  204  by a distance S 3 . 
     The perimeter of the retainer plate  200  is defined by three side surfaces. First and second side surfaces  206  and  208  are planar and are perpendicular to one another. A third side surface  210  connects the first and second side surfaces  206  and  208  and is arcuate. Both the first and second side surfaces  206  and  208  extend along the peripheral portion  202  and the central portion  204  of the retainer plate  200 . The third side surface  210  extends only along the peripheral portion  204  of the retainer plate  200 . 
     The central portion  202  of the reaction plate  200  includes a circular opening  212 . The cylindrical projection  192  on the fastener  180  is received in the opening  212  with sufficient clearance around the projection to allow for unrestricted rotation of the fastener in the opening. The central portion  202  of the retainer plate  200  further includes a reaction surface  214  surrounding the opening  212  and facing the fastener  180 . 
     The peripheral portion  204  of the retainer plate  200  includes two spaced apart clearance holes  216 . The clearance holes  216  in the retainer plate  200  align with the holes  144  in the base portion  124  of the mounting bracket  120 . The screws  146  extend through the holes  216  in the retainer plate  200  and into the threaded holes  144  in the base portion  124  of the mounting bracket  120  to attach the retainer plate to the mounting bracket. 
     With the retainer plate  200  attached to the mounting bracket  120 , an axial space  220  is formed between first end surface  162  of the hub  160  and the central portion  202  of the retainer plate  200 . The axial space  220  has a height S 4  which is equal to the sum of the distance S 1  (between the first end surface  162  of the hub  160  and the base surface  136  of the mounting bracket  120 ) and the distance S 3  (the axial offset of the retainer plate  200 ). The height S 4  of the axial space  220  is greater than the axial thickness S 2  of the first end portion  182  of the fastener  180 . The height S 4  of the axial space  220  permits limited axial movement of the first end portion  182  of the fastener  180  in the axial space while retaining the first end portion  182  of the fastener  180  in the space and thereby retaining the second end portion  184  of the fastener in the bore  168  in the hub  160 . 
     The back cover  240  (FIG. 5) for the steering wheel assembly  10  encloses the back side of the steering wheel assembly facing the vehicle steering shaft  14 . The back cover  240  is preferably made of ABS molded plastic but could be made of another plastic material. The back cover  240  has a large opening  242  for encircling a vehicle steering column (not shown). The opening  242  is defined by an axially extending cylindrical inner wall  244  and an outer wall  246  which is connected to the inner wall by a back face  248 . The back cover  240  includes three radially extending flanges spaced about the opening  242  which serve as mount locations  250  for the back cover  240 . Each mount location  250  has a hole  252  for receiving a respective one of the fasteners  134 . The back cover  240  is attached to the mounting bracket  120  by the three fasteners  134  which extend through the holes  252  in the mount locations  250 . 
     The outer wall  246  includes first and second wing portions  254  and  256  (FIGS. 1 and 5) located opposite one another. A lower wall  258  extends between the first and second wing portions  254  and  256  and also connects with the back face  248  of the back cover  240 . Each of the wing portions  254  and  256  has an ear tab  260  for engaging a surface (not shown) in the back side (i.e., facing away from the vehicle occupant) of the molded cover  54  between adjacent spokes  26  of the steering wheel assembly  10 . 
     The lower wall  258  of the back cover  240  includes a hinged trap door  262 . The trap door  262  is generally square in shape and is attached to the lower wall  258  by a living hinge  264  formed in the material of the lower wall. One or more tabs  266  on the trap door  262  hold the trap door flush with the lower wall  258 . A groove  268  is formed in the lower wall  258  immediately above the trap door  262 . The groove  268  allows a tool such as a screwdriver to be inserted underneath the trap door  262  in order to pry the trap door open. The living hinge  264  permits the trap door  262  to fold out away from the steering wheel assembly  10  so that an attachment tool (not shown) can be inserted inside the steering wheel assembly  10  to rotate the fastener  180 . 
     To attach the steering wheel assembly  10  to the steering shaft  14 , the hub  160  in the mounting brackeft  120  is located coaxial with the steering shaft. The hub  160  is placed oaver the end  12  of the steering shaft  14  such that the end of the steering shaft is received in the bore  168  in the hub. The corresponding flats on the outer surface  16  of the steering shaft  14  and in the bore  168  are aligned. By manually pushing the steering wheel assembly  10  toward the steering shaft  14 , the tapered outer surface  16  of the steering shaft is brought into contact with the tapered surface  166  inside the bore  168  in the hub  160 . The anti-cross-threading portion  196  of the second end portion  184  of the fastener  180  extends down inside the steering shaft  14  and the threaded portion  194  of the fastener engages the threaded inner surface  18  of the steering shaft. The trap door  262  in the back cover  240  of the steering wheel assembly  10  is then opened. 
     An attachment tool (not shown) for rotating the fastener  180  is inserted through the trap door  262 . The attachment tool slides into the channel  150  formed in the base portion  124  of the mounting bracket  120  until gear teeth on the attachment tool meshingly engage the gear teeth  190  on the first end portion  182  of the fastener  180 . The gear teeth on the attachment tool are rotated in a direction causing the fastener  180  to rotate in a tightening direction and to begin screwing itself into the end  12  of the steering shaft  14 . The retainer plate  200  overlying the first end portion  182  of the fastener  180  stabilizes the fastener during rotation and prevents the fastener from being tilted due to axial forces exerted on the fastener by the attachment tool or due to torque resulting from the rotation of the gears. 
     The rotation of the fastener  180  in the tightening direction moves the second surface  188  in the first end portion  182  of the fastener into contact with the first end surface  162  of the hub  160  and exerts an axial force on the hub which causes the steering wheel assembly  10  to be pulled downward (to the right as viewed in FIG. 2) onto the steering shaft  14 . As the steering wheel assembly  10  is forced onto steering shaft  14 , the tapered surfaces  166  and  16  in the hub  160  and on the steering shaft  14 , respectively, are pressed together to secure the steering wheel assembly to the steering shaft. 
     The diametrically opposed flats on the surface  16  of the steering shaft  14  and in the bore  168  in the hub  160  provide an alignment of the steering wheel assembly  10  and the steering shaft as well as a means for transmitting torque from the steering wheel assembly to the steering shaft. Torque which is produced by turning the steering wheel assembly  10  (i.e., steering the vehicle) is transmitted through the steering wheel armature  20  and the mounting bracket  120  to the hub  160  which is now non-rotatably attached to the steering shaft  14 . 
     The steering wheel assembly  10  is removed from the steering shaft  14  by opening the trap door  262  and inserting the attachment tool so that its gear teeth are engaged with the gear teeth  190  on the fastener  180 . The fastener  180  is then rotated in an untightening direction opposite the tightening direction. Rotation in the untightening direction causes the fastener  180  to move axially and relatively away from the steering shaft  14 . The second surface  188  in the first end portion  182  of the fastener  180  comes out of contact with the first end surface  162  of the hub  160 . Continued rotation of the fastener  180  in the untightening direction brings the first surface  186  in the first end portion  182  of the fastener  180  into contact with the reaction surface  214  in the central portion  202  of the retainer plate  200 . With the surface  186  of the fastener  180  abutting the reaction surface  214  of the retainer plate  200 , rotation of the fastener applies an axial force on the steering wheel assembly  10  in a direction away from the steering shaft  14  and causes the hub  160  to disengage from the steering shaft. 
     FIG. 6 illustrates a second embodiment of the present invention. In the embodiment of FIG. 6, identical structure to that of the previous embodiment is identified with the same reference numerals. FIG. 6 illustrates a retainer plate  270  having a slightly modified structure from the retainer plate  200  of the previous embodiment. 
     The retainer plate  270  has a larger central portion  272  than the retainer plate  200 . The central portion  272  extends completely over the first end portion of the fastener  180  and covers a portion of the channel  150  formed in the mounting bracket  120  by ridges  148 . A small square-shaped opening  274  is located in the portion of the central portion  272  covering the channel  150 . Further, the central portion  272  of the retainer plate  270  includes an extension  280  which extends in the direction of one of the leg portions  122  of the mounting bracket  120  and which provides the central portion with an L-shaped configuration. The extension  280  of the central portion  272  has an additional opening (not shown) for receiving a third fastener  146  for securing the retainer plate  270  to the mounting bracket  120 . 
     The retainer plate  270  functions identically to the retainer plate  200  described in the previous embodiment, but has additional advantages. By virtue of its larger size and its three mounting locations, the retainer plate  270  is able to withstand larger forces than the retainer plate  200  on its reaction surface  214  (not shown) during untightening of the fastener  180 . Because a portion of the channel  150  which guides the attachment tool to the fastener  180  is covered by the retainer plate  270 , the retainer plate stabilizes the end of the attachment tool during tightening and/or untightening of the fastener  180 . Finally, the square-shaped opening  274  in the central portion  272  of the retainer plate  270  overlying the channel  150  accommodates a retention feature on the attachment tool to further stabilize the tool during tightening and/or untightening of the fastener  180 . 
     FIGS. 7 and 8 illustrate a steering wheel assembly  300  constructed in accordance with a third embodiment of the present invention. The steering wheel assembly  300  includes a steering wheel armature  310  having a rim portion (not shown) and a central mounting plate  312  connected to the rim portion by spokes  314 . The mounting plate  312  includes a hub portion  316  with a bore  318  defined by a tapered surface  320 . The hub portion  316  may be formed in one piece with the mounting plate  312  or may be a separate part which is cast into the mounting plate. A plurality of bosses  322  extend from the mounting plate  312  in a direction away from the hub portion  316 . A clearance hole  324  extends through each of the bosses  322  and through the mounting plate  312 . 
     A rotatable fastener  330  identical to the fastener  180  described in the first embodiment is supported for rotation by the hub portion  316 . A first end portion  332  of the fastener  330  comprises a spur gear with a cylindrical projection  334  extending axially away from the hub portion  316 . A second end portion  336  of the fastener  330  includes a threaded outer surface  338  for engaging the threaded inner surface  18  of the vehicle steering shaft  14 . 
     The steering wheel assembly  300  includes a retainer plate  340  overlying the first end portion  332  of the fastener  330 . The retainer plate  340  is generally rectangular in shape and has a central portion  342  partially encircled by a peripheral portion  344 . The central portion  342  is axially offset from the peripheral portion  344  and has an opening  346  for receiving the projection  334  on the first end portion  332  of the fastener  330 . The peripheral portion  344  has a plurality of openings  348  which align with a plurality of threaded openings  350  in the mounting plate  312  and which receive fasteners  352  to secure the peripheral portion of the retainer plate  340  to the mounting plate. 
     The steering wheel assembly  300  further includes an inflatable vehicle occupant protection device in the form of an air bag module  360  which is secured to the mounting plate  312 . The air bag module  360  includes a folded air bag  362 , an air bag retainer  364 , an air bag inflator  366 , and a reaction plate  368 . The reaction plate  368  is supported by the bosses  322 . A plurality of studs  370  extend through the air bag retainer  364 , the air bag  362 , and the reaction plate  368 . Each of the studs  370  further extends through a respective one of the clearance holes  324  in the bosses  322  and the mounting plate  312 . Nuts  372  are screwed onto the studs  370  to secure the air bag module  360  in the steering wheel assembly  300 . 
     The steering wheel assembly  300  is attached to and removed from the steering shaft  14  by rotating the fastener  330  as described above in the first embodiment regarding the fastener  180 . The reaction plate  340  functions identically to the reaction plate  200  described above. 
     FIG. 9 illustrates a steering wheel assembly  400  constructed in accordance with a fourth embodiment of the present invention. In the embodiment of FIG. 9, identical structure to that of the third embodiment of FIGS. 7 and 8 is identified with same reference numerals. S 1  Similar structure to that of the third embodiment is also identified with the same reference numerals, but with a prime added. 
     The steering wheel assembly  400  includes an air bag module  360 ′ and a mounting bracket  410 . The air bag module  360 ′ includes a reaction plate  430  with a recessed wall portion  432  which is located axially adjacent the first end portion  332  of the fastener  330 . 
     The mounting bracket  410  has a base portion  412  and a flange portion  414  connected by a radially and axially extending connecting portion  416 . The base portion  412  has a centrally located opening  418  for receiving a portion of the rotatable fastener  330 . The fastener  330  is supported for rotation by the base portion  412  of the mounting bracket  410 . The flange portion  414  of the mounting bracket  410  has a plurality of spaced apart openings  420  for receiving bolts  422  for securing the air bag module  360 ′. 
     The steering wheel assembly  400  is attached to and removed from the steering shaft  14  by rotating the fastener  330  substantially as described above in the first embodiment. In the fourth embodiment of FIG. 9, the recessed wall portion  432  of the reaction plate  430  functions as a retainer plate to axially retain the fastener  330 , stabilize the fastener during rotation, and provide a reaction surface during rotation of the fastener in the untightening direction. Further, the steering wheel assembly  400  eliminates bolts which are typically used to attach the air bag module  360 ′ to the mounting plate  312 ′ of the steering wheel armature  310 ′ because the fastener  330  secures the air bag module to the steering wheel armature in addition to securing the steering wheel assembly to the steering shaft  14 . 
     From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.