Abstract:
A seat belt retractor having first and second serially arranged torsion bars and a switching mechanism provides two levels of load limiting. A bridge bolt rotationally fixed to the inboard ends of the torsion bars is threaded into a locking base, and the switching mechanism prevents bridge bolt translation relative to the locking base in a default condition to limit seat belt load with the first torsion bar. Activation of the switching mechanism permits limited translation of the bridge bolt relative to the locking base to limit seat belt load with the second torsion bar. Additional translation of the bridge bolt relative to the locking base is prevented to re-establish load limiting with the first torsion bar. The switching mechanism includes a set of detent wedges, a retaining band and an electrically activated cutting mechanism, all disposed within the retractor frame.

Description:
TECHNICAL FIELD 
     The present invention relates to automotive seat belt retractors, and more particularly to a space-efficient and cost-effective mechanism for achieving dual-level load limiting. 
     BACKGROUND OF THE INVENTION 
     Load limiting seat belt retractors utilize a mechanical energy-absorbing element such as a torsion bar to control the restraining force exerted on an occupant during an actual or anticipated crash event by paying out a relatively small amount of the seat belt for a specified load. Since the desired level of load limiting varies depending on the weight of the occupant and the severity of the crash event, some retractors (referred to herein as dual-level load limiting retractors) incorporate two different energy-absorbing elements and a switching mechanism for selectively changing the load limit by changing the mechanical load path through the retractor. See, for example, the U.S. Patent Publication No. 2006/0022447, and the U.S. Pat. Nos. 6,241,172, 6,616,081 and 6,648,260. However, known dual-level load limiting retractors are relatively expensive to produce, and tend to be difficult to package in a vehicle door pillar due to their increased size. Accordingly, what is needed is a dual-level load limiting retractor that is cost-effective to manufacture and compact in size. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved dual-level load limiting seat belt retractor including a seat belt spool supported in a frame, first and second serially disposed torsion bars, a linearly shiftable bridge bolt, a locking base, and a switching mechanism. The spool is rotationally fixed to the outboard end of the first torsion bar, and the inboard ends of the first and second torsion bars are rotationally fixed to the bridge bolt. The bridge bolt is threaded into the locking base, and in a default condition, the switching mechanism prevents relative rotation and lateral translation of the bridge bolt with respect to the locking base to establish a first level of load limiting due to energy absorption by the first torsion bar. When the switching mechanism is activated during a crash event, the bridge bolt is permitted to rotate with respect to the locking base by a limited amount to establish a second level of load limiting due to energy absorption by the second torsion bar. Following the limited rotation, the further lateral translation of the bridge bolt with respect to the locking base is prevented to re-establish the first level of load limiting. 
     The switching mechanism includes a set of detent wedges, a retaining band and an electrically activated cutting mechanism, all disposed between the seat belt spool and the retractor frame. The retaining band circumscribes the wedges to retain them in openings formed in the locking base, blocking translation of the bridge bolt with respect to the locking base. When the cutting mechanism is activated, it severs the retaining band to allow radial outward movement of the wedges and the limited rotation of the bridge bolt with respect to the locking base. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view of a dual-level load limiting seat belt retractor according to the present invention; 
         FIG. 1B  is an enlarged portion of the sectional view of  FIG. 1A . 
         FIG. 2  is an isometric view of the seat belt retractor of  FIG. 1A  with the seat belt spool removed to show a switching mechanism that is selectively activated to change the mechanical load path through the retractor; 
         FIG. 3A  is simplified cross-sectional diagram of the seat belt retractor of  FIG. 1A  in a default condition providing a first level of load limiting; 
         FIG. 3B  is simplified cross-sectional diagram of the seat belt retractor of  FIG. 1A  in an activated condition providing a second level of load limiting; 
         FIG. 3C  is simplified cross-sectional diagram of the seat belt retractor of  FIG. 1A  in a post-activation condition providing the first level of load limiting; 
         FIG. 4A  is an end-view of the seat belt retractor of  FIG. 1A , with the switching mechanism in a default condition; 
         FIG. 4B  is an end-view of the seat belt retractor of  FIG. 1A , depicting activation of the switching mechanism; 
         FIG. 4C  is an end-view of the seat belt retractor of  FIG. 1A , with the switching mechanism in a post-activation condition; 
         FIG. 5A  is a graph depicting seat belt load as a function of seat belt payout for the default condition depicted in  FIG. 3A ; 
         FIG. 5B  is a graph depicting seat belt load as a function of seat belt payout for the activated condition depicted in  FIG. 3B ; and 
         FIG. 5C  is a graph depicting seat belt load as a function of seat belt payout for the post-activation condition depicted in  FIG. 3C . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIGS. 1A ,  1 B and  2 , the reference numeral  10  generally designates a partially assembled dual-level load limiting seat belt retractor for a motor vehicle according to the present invention. The illustrated assembly includes a metal frame  12  that is mounted in a vehicle door pillar, a spool  14  on which the seat belt (not shown) is wound, and a spool support assembly  16  that couples the spool  14  to the frame  12 . The spool support assembly  16  includes a spool connector  18 , a locking base  20 , first and second torsion bars  22  and  24 , a bridge bolt  26 , and a switching mechanism  28 . The first and second torsion bars  22  and  24  are arranged coaxial with the locking base  20 . The spool  14  and outboard end  22   a  of the first torsion bar  22  are rotationally fixed to the spool connector  18 . The bridge bolt  26  is internally splined, and rotationally fixes the inboard end  22   b  of the first torsion bar  22  to the inboard end  24   a  of the second torsion bar  24 . The outboard end  24   b  of the second torsion bar  24  is rotationally fixed to the locking base  20 . The locking base  20  has an end portion  20   a  that passes through a sidewall  12   a  of frame  12 , and an annulus  20   b  disposed within the frame  12 . The bridge bolt  26  is disposed within the annulus  20   b  of locking base  20 , and is provided with exterior threads that meshingly engage complementary threads formed on the inner periphery of annulus  20   b . The switching mechanism  28  is disposed between the spool  14  and frame  12 , and ordinarily prevents lateral displacement of the bridge bolt  26  toward the end portion  20   a  of locking base  20  due to relative rotation between it and the locking base  20 . 
     A take-up spring (not shown) coupled to the spool connector  18  rotationally biases the spool support assembly  16  in a direction to retract the seat belt. Apart from this spring bias, the components of spool support assembly  16  are free to rotate with respect to the frame  12  during normal usage. In the event of an actual or anticipated crash event, however, a locking mechanism  30  (shown in outline in  FIG. 1 ) responsive to vehicle deceleration and/or rapid payout of the seat belt mechanically engages the locking base  20  to the frame  12 . Although the locking mechanism  30  prevents further rotation of the locking base  20 , the torsion bars  22  and  24  provide a load limiting function when the force applied to the seat belt reaches a predetermined level by absorbing mechanical energy while allowing a limited amount of additional seat belt payout. The torsion bars  22  and  24  have different energy absorption characteristics due to differences in their geometry, and two different levels of load limiting are achieved depending on which torsion bar is absorbing energy. The default energy absorption level is determined by the first torsion bar  22 , which begins absorbing energy at a relatively high load level due to its relatively large size (compared to torsion bar  24 ). A lower energy absorption level determined by the second torsion bar  24  can be selectively activated in the course of an actual or anticipated crash event to allow limited seat belt payout at a lower load level. This may be appropriate, for example, if the occupant is relatively small and/or the crash severity is relatively low. 
     Selective activation of the lower energy absorption level is achieved with the switching mechanism  28 , which includes a set of detent wedges  32 , a retainer band  34 , and a pyrotechnically deployed cutting mechanism  36 . The detent wedges  32  are received in a set of openings  38  formed in the annulus  20   b  of locking base  20  between the spool  14  and retractor frame  12 . The retainer band  34  circumscribes the detent wedges  32 , and retains them within the respective openings  38 . The inboard face  32   a  of each detent wedge  32  is cammed, and when the detent wedges  32  are retained in the openings  38 , their cammed faces  32   a  engage a complementary cammed surface  26   b  formed on the end of bridge bolt  26 . As indicated above, this prevents lateral displacement of the bridge bolt  26  toward the end portion  20   a  of locking base  20  due to relative rotation between it and the locking base  20 . As explained below, activating cutting mechanism  36  severs the retaining band  34 , establishing a period of low energy absorption as the bridge bolt  26  rotates with respect to the locking base  20  and thereby moves laterally toward the end portion  20   a  of locking base  20 . 
     The cutting mechanism  36  includes a generally cylindrical housing  40  captured in a mounting flange  12   b  of retractor frame  12 , a piston  42 , a squib  44 , and an electrical connector  46 . The piston  42  is disposed within an axial bore  48  of housing  40  and includes an integral chisel-point pintle  50  that extends out of bore  48  toward the retaining band  34 . Electrical activation of the squib  44  charges the housing bore  48  with pressurized gas, thereby displacing piston  42  outward and causing the chisel-point  50   a  of pintle  50  to strike and sever the retaining band  34 . 
     The operation of the retractor  10  is now described with respect to  FIGS. 3A-3C ,  4 A- 4 C and  5 A- 5 C. It is assumed for purposes of the description that an actual or anticipated crash event is in progress, and the locking mechanism  30  has mechanically engaged locking base  20  to the retractor frame  12 . 
       FIGS. 3A ,  4 A and  5 A depict an initial or default condition in which the cutting mechanism  36  is not activated, and the retaining band  34  retains the detent wedges  32  in the openings  38  of locking base  20 . In this condition, the bridge bolt  26  is rotationally fixed to the locking base  20 . The force (load) applied to the seat belt rises rapidly once the locking base  20  engages the frame  12  and the occupant moves forward. The corresponding torque applied to the spool  14  is transferred to the outboard end  22   a  of the first torsion bar  22  through the spool connector  18 . The inboard end  22   b  of the first torsion bar  22  attempts to rotate the bridge bolt  26 , but cannot since the bridge bolt  26  is rotationally fixed to the locking base  20  as mentioned above. In other words, the bridge bolt  26  reinforces the torsion bar  24  when rotationally locked, so that the second torsion bar  24  is effectively bypassed. When the load reaches a predetermined limit L 1 , the first torsion bar  22  twists to absorb energy while permitting limited additional seat belt payout as depicted in  FIG. 5A . If the high level load limit condition is deemed to be appropriate given the occupant size and crash severity, the cutting mechanism  36  is not activated, and the load limiting continues at the level L 1  until the occupant energy is expended. 
     If it is determined that a lower level load limit is desired, the cutting mechanism  36  is activated as depicted in  FIG. 4B . This severs the retaining band  34  and allows the bridge bolt  26  to drive the detent wedges  32  radially outward by camming action as depicted in  FIG. 4C . With the detent wedges  32  shifted out of the way, the bridge bolt  26  rotates within the annulus  20   b  of locking base  20  and moves laterally rightward as depicted in  FIG. 3B . The inboard ends  22   b  and  24   a  of the first and second torsion bars  22  and  24  rotate with the bridge bolt  26 , placing the second torsion bar  24  in series with the first torsion bar  22 . Since the second torsion bar  24  has a lower energy absorption characteristic than the first torsion bar  22 , the seat belt load drops to a lower level L 2  as the seat belt continues its load-limited payout; see  FIG. 5B . This condition prevails until rightward lateral movement of the bridge bolt  26  is prevented due to its engagement with the end portion  20   a  of locking base  20  as depicted in  FIG. 3C . During this low-level interval of load limiting, the seat belt may payout an additional 400 mm or so, as indicated in  FIG. 5B . When the cammed end of bridge bolt  26  engages the end portion  20   a  of locking base  20 , the bridge bolt  26  is once again rotationally fixed to the locking base  20 , and the bridge bolt  26  effectively by-passes the second torsion bar  24 . At such point, the first torsion bar  22  is the only energy-absorbing element in the load path, and load limiting can only continue at the upper load limit L 1  as depicted in  FIG. 5C . 
     As best seen in  FIGS. 1A and 2 , the retractor  10  of the present invention achieves a dual-level load limiting functionality without significantly increasing its size. Unlike prior dual-level load limiting retractors, the switching mechanism  28  is disposed within the retractor frame  12 , and the increased functionality is achieved without significantly increasing the retractor width. As a result, the packaging drawbacks associated with prior dual-level load limiting retractors are avoided. At the same time, the additional manufacturing cost required to achieve the increased functionality is relatively low, as the individual components are relatively inexpensive to manufacture and easy to assemble. 
     In summary, the retractor  10  of the present invention presents a compact and low-cost alternative to other dual-level load limiting retractors. While described with respect to the illustrated embodiment, it is recognized that numerous modifications and variations in addition to those mentioned herein will occur to those skilled in the art. For example, the first and second torsion bars  22  and  24  can be formed as a single element instead of two separate elements, and so on. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.