Abstract:
A seat belt retractor has a frame with sidewalls and a rotatable spool extending between said sidewalls. The rotatable spool has seat belt webbing wound thereon. A locking pawl engages ratchet teeth on the outer periphery of the spool for arresting rotation of the spool. A secondary locking mechanism locks the spool against rotation.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a locking mechanism for a seat belt retractor. 
     BACKGROUND OF THE INVENTION 
     A seat belt retractor typically comprises a spool on which seat belt webbing is wound and which rotates about its longitudinal axis to pay out or wind in webbing as required by movement of the vehicle occupant. In the event of a crash, the spool is locked against rotation and thus pay out is prevented and the vehicle occupant is securely restrained against forward motion. 
     The spool is locked by engagement of ratchet teeth at its ends, with toothed pawls mounted on a lockbar extending across the width of the spool. 
     The lockbar is under considerable strain during a crash and must be constructed to be very strong. Typically the teeth on the locking pawls may shear under particularly high crash forces. 
     SUMMARY OF THE INVENTION 
     There is provided in accordance with the present invention a seat belt retractor comprising a retractor frame with side walls and a rotatable spool extending between said side walls, said rotatable spool having seat belt webbing wound thereon, a primary locking mechanism comprises a locking pawl for engaging with ratchet teeth on the outer periphery of the spool for arresting rotation of the spool, and a secondary locking mechanism for locking the spool against rotation. 
     According to a first embodiment of the invention, the secondary locking mechanism comprises a set of teeth formed in a detent that is offset from the plane of the retractor frame side walls and arranged to co-operate with a smooth surfaced spool ratchet flange to lock the spool against rotation. 
     According to a second embodiment of the invention the secondary locking mechanism comprises an inner frame comprising two circular ring portions and a joining portion, and a set of teeth is formed on the inside surface of each ring and arranged to engage the spool ratchet flange to lock the spool against rotation. 
     According to a third embodiment, secondary locking teeth are mounted on a bar fixed to the frame sidewalls across the open side of the retractor frame. This bar may be formed as an extension of the tie plate, which holds the sides of the frame together. 
     According to a fourth embodiment, the secondary locking teeth are mounted on a secondary locking pawl, which is pivotably mounted on a locking pawl support bar. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a seat belt retractor according to a first embodiment of the invention. 
     FIG. 2 is a cross-sectional side view of the seat belt retractor of FIG.  1 . 
     FIG. 3 is a cross-sectional view of the seat belt retractor of FIGS. 1 and 2. 
     FIG. 4 is an isometric view of a seat belt retractor according to a second embodiment of the invention. 
     FIG. 5 is a cross-sectional side view of the seat belt retractor of FIG.  4 . 
     FIG. 6 is a cross-sectional view of the seat belt retractor of FIGS. 4 and 5. 
     FIG. 7 is an isometric view of a part of the seat belt retractor of FIGS. 4 to  6 . 
     FIG. 8 is an isometric view of a seat belt retractor according to a third embodiment of the invention. 
     FIG. 9 is a cross-sectional side view of the seat belt retractor of FIG.  8 . 
     FIG. 10 is an isometric view of another version of a seat belt retractor according to the third embodiment of the invention. 
     FIG. 11 is a cross-sectional side view of the seat belt retractor of FIG.  10 . 
     FIG. 12 is an isometric view of a seat belt retractor according to a fourth embodiment of the invention. 
     FIG. 13 is a side view of the seat belt retractor of FIG.  12 . 
     FIG. 14 is an isometric view of a seat belt retractor according to a first modification of the fourth embodiment of the invention. 
     FIG. 15 is a side view of the seat belt retractor of FIG.  14 . 
     FIG. 16 is an isometric view of a seat belt retractor according to a second modification of the fourth embodiment of the invention. 
     FIG. 17 is a side view of the seat belt retractor of FIG.  16 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the figures like parts are referenced accordingly. FIG. 1 shows the seat belt retractor in perspective, FIG. 2 a cross-sectional side view and FIG. 3 in longitudinal cross-section. A seat belt retractor comprises a U-shaped frame  1  with a spool  2  mounted for rotation between the sidewalls  3  of the frame. The ends of the spool  2  are formed as toothed ratchet wheels  4 . A lockbar  5  has toothed pawls  6  fixed to each end and pivots in the event of a crash to engage the teeth on the pawls  6  with the teeth on the ratchet wheels  4  to lock the spool against further rotation and thus against payout of the webbing to restrain a vehicle occupant. 
     According to a first embodiment of the invention, the secondary locking mechanism comprises a set of teeth formed in a detent that is offset from the plane of the retractor frame side walls and arranged to co-operate with a smooth surfaced spool ratchet flange to lock the spool against rotation. On engagement of the primary locking mechanism, the frame side walls deform and the set of teeth in the detent are pushed closer to, and under high loads against, the ratchet wheel flange. This set of teeth digs into the flange and prevents rotation of the spool. The detent is preferably arranged diametrically opposite the pivot point of the primary locking pawl. 
     In FIGS. 1 to  3 , a secondary locking mechanism is shown comprising teeth  7  projecting towards the spool from detents  8  in the frame  1 , offset from the planes of the side walls  3  of the frame. The teeth  7  are in the plane of a spool ratchet wheel flange  9 . This is a smooth surface adjacent the teeth on the ratchet wheel  4 . 
     In operation, after a crash is detected the primary pawls  6  pivot and engage the respective ratchet wheels  4  to lock the spool  2  against rotation. As the crash force increases, the lockbar  5 , G forces the spool  2  and the flange  9  into engagement with the secondary locking teeth  7  which dig into the flange  9  and thus providing a secondary locking for the spool. 
     According to a second embodiment of the invention the secondary locking mechanism comprises an inner frame comprising two circular ring portions and a joining portion, and a set of teeth is formed on the inside surface of each ring and arranged to engage the spool ratchet flange to lock the spool against rotation. Preferably the joining portion is bolted to the retractor frame. In FIGS. 4 to  7  a second embodiment is shown in which secondary locking teeth  17  project radially inward from rings in an additional frame member  18 . The additional frame member  18  comprises two rings  19  connected by a bar  20  which is bolted to the retractor frame  1 . 
     The support bar may be arranged to respond to movement of the primary locking pawl. Thus the support bar may be mounted to the frame via slots, and a rear extension portion arranged on the primary pawl to engage the support bar when the primary pawl moves into locking position. The rear extension portion moves the bar and thus pivots the secondary locking pawl into engagement with the spool ratchet teeth. The secondary locking teeth  17  engage the smooth outer surface of the spool ratchet wheel flange  9  to perform the secondary locking. The rings  19  of the additional frame piece  18  are connected to the sidewalls  3  of the frame  1  by pegs  21  on the sidewalls  3 , which can slide in slots  22  in the rings  19 . In operation, after the primary pawl  6  has engaged, further crash forces cause distortion of the frame side walls  3  which in turn pushes the secondary locking teeth into engagement with the ratchet wheel flange  9 . The additional frame member is seen more clearly in FIG.  7 . 
     According to a third embodiment, secondary locking teeth are mounted on a bar fixed to the frame sidewalls across the open side of the retractor frame. This bar may be formed as an extension of the tie plate, which holds the sides of the frame together. In FIGS. 8 and 9 the secondary locking mechanism is provided by secondary locking teeth  27  on a bar  28  fixed across the gap between the frame side walls  3 . In FIGS. 10 and 11 the bar  28  is part of tie bars  30 . 
     According to a fourth embodiment, the secondary locking teeth are mounted on a secondary locking pawl, which is pivotably mounted on a locking pawl support bar. This arrangement may be single sided or double sided, i.e. two secondary pawls may be provided, on each side of the spool. The support bar may be arranged such that as the frame deforms under high loads, the secondary pawl pivots into engagement with the ratchet teeth on the spool. 
     In the embodiment of FIGS. 12 to  17 , the secondary locking mechanism is provided by a secondary locking pawl  38  having teeth  37  arranged to engage the teeth on the spool ratchet  4 . The secondary locking pawl  38  is pivotally mounted to a pawl support bar  39  via a pin  40  extending from the pawl  38  and a hole  41  at the end of the support bar  39 . In the embodiment of FIGS. 12 to  15 , the support bar  39  is fixed to the back of the frame by rivets  42 . In operation, under crash conditions the primary locking pawl  6  engages with the ratchet  4  on the spool  2  and prevents the spool rotating. As crash forces increase the frame  1  deforms and the support bar  39 , relative to the frame  1 , moves causing secondary locking pawl  38  to pivot into engagement with the ratchet  4  and more securely lock the spool  2 . 
     In FIGS. 12 and 13, the pawl support bar  39  extends only one side of the retractor frame  1 . A secondary locking pawl  38  is nonetheless provided on both sides and they are connected by means of a rod  50 . Alternatively, a single secondary locking pawl may be used and the rod is omitted. 
     In FIGS. 14 and 15, the support bar  39  extends to both sides of the frame  1  and supports the two secondary pawls  38 , one on each side of the frame  1 . The difference between the single sided support bar modification, and the double-sided version, is clearly seen from a comparison of FIGS. 13 and 15. 
     In the embodiment of FIGS. 16 and 17, the secondary pawl support bar  39  is integral with an actuator part  45 , and instead of being fixed to the frame  1  it is mounted to be slidable relative thereto. To this end slots  43  are provided in the actuator  45  which engage bolts  44  and allows a vertical sliding motion of the actuator  45  relative to the frame  1 . 
     The primary locking pawl  6  has a tail part  46 , which extends through a cutout in the back of the frame  1  and contacts a cam surface  47  of the actuator  45 . 
     In operation of this modification, the primary locking pawl  6  pivots in a crash situation and its leading tooth  48  engages the ratchet wheel  4  and halts rotation of spool  2 . The spacing of the teeth on the ratchet  4  and on the pawl  6  is such that at this stage of operation only the leading tooth  48  engages. 
     As the crash forces increase, the frame  1  distorts and the pawl  6  is pivoted further to bring the other teeth into engagement with the ratchet  4 . As the pawl  6  rotates upwards, its tail part  46  rotates downwards and pushes the actuator  45  downwards. This pulls the support bar  39  downwards and hence causes secondary pawl  38  to pivot down into engagement with the teeth on ratchet  4 . Hence the spool  2  is more securely locked against rotation and the high loads are shared between the pawls. 
     The length of the slots  43  is chosen to limit the extent of movement if the actuator  45  to that needed for full engagement of the secondary locking pawl  38 . 
     One or more stops are preferably provided on the retractor frame to limit the extent of movement of the support bar to the amount needed to engage the secondary pawl with the ratchet teeth. A stop  49  is provided on the frame  1  against which the actuator  45  rests when full engagement of the secondary pawl  38  has occurred to limit the movement of the actuator  45 . 
     The invention is not confined to the particular construction and arrangement of parts herein illustrated and described, but embraces such modified forms thereof as come within the scope of the following claims.