Abstract:
A motor vehicle seat belt retractor having load limiting features for controlling seat belt restraint loads for a retractor having a spool for storing belt webbing and rotatable with respect to a retractor frame, and a locking mechanism for locking the spool to provide vehicle occupant restraint. A load limiting element coupled with the spool limits restraint loading of the seat belt webbing upon locking of the spool. A rotational displacement limiting mechanism limits the displacement of the load limiting element, the limiting mechanism having a cam forming a spiral flight rotatable with the load limiting element and a cam follower engaging the spiral flight. The cam and the cam follower interengage to reach an end position preventing further relative angular displacement beyond a predetermined angular displacement of the load limiting element. Embodiments provide various configurations for the cam and cam follower elements.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 61/788,511, filed Mar. 15, 2013. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a seat belt restraint system for restraining an occupant of a motor vehicle, and more particularly to a seat belt retractor for such a system having a mechanism for limiting torsional deflection of a torsion bar load limiter element. 
     BACKGROUND OF THE INVENTION 
     Seat belt retractors are a standard component of motor vehicle belt restraint systems and have a spool (spindle) for receiving seat belt webbing. The spool is used to wind up and store the webbing. The spool is locked against rotation upon detection at a potential accident situation in order to restrain the occupant via the seat belt. Recently, retractors have been designed having one or more load limiting elements which are structured to allow the spool to rotate and pay out the seat belt webbing upon reaching predetermined webbing load levels between the occupant and seat belt during a restraint event. In this manner, the restraint force imposed on the occupant can be limited in a controlled manner, providing desired load limitation characteristics. 
     More recent enhancements in load limiters have been designed to provide multilevel load limiting capabilities. For example, higher restraining forces may be initially applied, followed by lower restraining forces at a later point during an emergency event, or a low to high load profile can be provided. Torsion bars arranged coaxially within the spool are commonly used as load limiting elements. In an impact condition, one end of the torsion bar is locked to the retractor frame while the other end is coupled with the retractor spool. The bar section between the attachment points undergoes elastic and plastic torsional deflection, enabling torsion controlled relative rotation between the spool and the retractor frame. The resulting controlled extraction of webbing during a restraint event serves to limit belt loading acting on the vehicle occupant. 
     One type of multi-stage load limiting retractor uses a multi-stage torsion bar or a system of torsion bars. The multi-stage torsion bar is essentially two torsion bars that are axially aligned and joined at respective ends. The appropriate stage or portion of the torsion bar may be selectively coupled to provide a secondary load limiting characteristic as desired. 
     Presently available torsion bar type load limiting retractors generally operate satisfactorily. The more sophisticated multilevel load limiting systems also operate in an intended manner. However, there are additional design goals and objectives desired for further improvement. One such design goal is providing a mechanism for limiting the total rotational deflection provided by a torsion bar load limiting element, enabling multiple turns of relative rotation. In addition, in an effort to accurately tailor load limiting characteristics to design criteria, sophisticated digressive and progressive load limiting profile systems have been developed. These systems enable the force load profile of the retractor to be tailored to increase (progressive) or decrease (digressive) over extraction of the webbing. Although retractors having such capabilities are known, the related systems become complex, costly, and can impose packaging size disadvantages. Further design goals include the ability to easily adapt a retractor to provide specific load limiting characteristics to a particular vehicle application along with preferably a low part count, and low cost. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a seat belt retractor that incorporates mechanisms for limiting the rotational displacement of a load limiting element such as a torsion bar. The systems of the present invention further provide progressive and digressive load limiting capabilities. Several embodiments are described, each having a form of a helical or spiral cam which limits relative rotation between components of the retractors. Features may be incorporated into the cam serving to contribute to load limiting force control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a retractor in accordance with the first embodiment of this invention; 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  from  FIG. 1 . 
         FIG. 3  is a cross-sectional view of a second embodiment of the present invention. 
         FIG. 4  is a pictorial view of a portion of a retractor spool in accordance with a third embodiment of the present invention; 
         FIG. 5  is an end view of the spool shown in  FIG. 4 ; 
         FIG. 6  is an exploded view of components of a retractor in accordance with a fourth embodiment of the present invention; and 
         FIG. 7  is an exploded view of the components illustrated in  FIG. 6  from a different perspective, showing different surfaces of the components. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With particular reference to  FIGS. 1 and 2 , spool assembly  12  of a retractor assembly  10  in accordance with a first embodiment of this invention is illustrated. Spool assembly  12  incorporates spool element  14  which forms an outer generally cylindrical surface  16  adapted for engagement with an end of a length of seat belt webbing (not shown), and enables multiple wraps of the webbing to be rolled onto and stored on the spool element. One end of spool element  14  forms bearing stub  18  which is held within suitable bushings or bearing elements carried by a retractor frame (not shown). The opposite end of a spool element  16  abuts rotopretensioner drive pinion  20 , which is provided as part of a pretensioner device incorporating a series of elements such as ball masses which are driven to engage pinion  20  under gas pressure provided by a gas generator. Rotopretensioner devices are well known and do not form a necessary component of the present invention. 
     Tread head  22  is also a conventional retractor component and interacts with an inertia sensitive locking system which restrains rotation of the spool element  14  upon the associated vehicle undergoing inertial loads outside prescribed limits. Upon exposure to such acceleration loads, tread head  22  becomes locked to the retractor frame, which in turn restrains rotation of spool element  14 , in a manner to be described. Tread head  22  further forms a second bearing stub  24 . In normal, non-emergency conditions, spool element  14  is permitted to freely rotate within the retractor frame about bearing stubs  18  and  24 , with a separate torsion rewind spring (not shown) acting on the spool to provide a retracting torsion force. 
     A portion of tread head  22  forms a cylindrical hub  26  fit within a section  28  of central cavity  30  of spool element  14 . Hub  26  is permitted, in certain conditions, to rotate relative to spool element  14 . At one end of central cavity  30 , spool element  14  forms splined bore  32 . Tread head bearing hub  26  forms a similarly shaped splined bore  34 . Torsion bar  36  is installed within spool center cavity  30 , and includes a pair of heads  38  and  40  at opposite ends. Head  38  engages within bore  32  and interacts with splines of the bore to prevent relative rotation between the head and the spindle. 
     A drum or tubular sleeve-shaped load control coupler  42  is positioned within spool cavity  30  and forms a mounted end  48  having keyed or splined inside and outside surfaces  33  and  35 , respectively, best shown in  FIG. 2 . Torsion bar head  40  fits within an internal splined surface  33  of coupler  42 , and coupler end  48  is in turn received by splined bore  34 . Torsion bar end  48 , coupler  42 , and spool  14  are rigidly coupled at the bar end. 
     During normal operation, in which the tread head  22  is not locked, spool assembly  12  is permitted to freely rotate as belt webbing is retracted and protracted from the retractor. Such retractor operation permits movement of the vehicle occupant during normal operating conditions, providing desirable comfort and convenience features. In the event that a collision condition is detected, a pretensioning device such as a rotopretensioner associated with pinion  20  may be activated by sending a firing signal to an associated gas generator. After such activation, the rotopretensioner is typically provided with a mechanism to lock pinion  20  after undergoing pretensioning rotation. In addition to such pretensioner locking, or independent of it, tread head  22  locks in response to inertial loadings acting on the vehicle, as discussed previously. In such locking conditions, tread head  22  is locked to the retractor frame and tension loads acting on the belt webbing produce a torsional load on spool element  14 , which in turn transfers such load to torsion bar  36 . If such restraint loads exceed predetermined levels, torsion bar  36  undergoes elastic (initially) and plastic torsional deformation. This allows controlled payout (protraction) of the belt webbing while limiting belt loads. The force flow in such conditions is illustrated by arrows in  FIG. 1 , which, in a restraint event, is from the seat belt webbing to spool  12 , to torsion bar  36 , and then grounds into the retractor frame. The characteristics of torsion bar  36  are designed to provide predetermined load limiting characteristics. Several turns of relative rotation between torsion bar heads  38  and  40  may occur. Such operation is available with current design load limiting seat belt retractors. 
     Spool assembly  12  provides additional features in accordance with the present invention. Load control coupler element  42  is affixed at one end to torsion bar head  40  and accordingly rotates with torsion bar head  40 . The outside cylindrical surface of coupler  42  forms a helical flight  44  which forms a helical groove, much like a screw thread. Helical flight  44  is formed from free end  46  of the coupler and terminates at near the coupler mounted end  48 . Spool element  14  within center cavity  30  forms an axial groove  50  extending over the axial length of coupler helical flight  44 . Ball element  52  is positioned to fit within the groove  50  and helical flight  44 , and acts essentially as a cam follower type element. Relative rotation between coupler  42  and spool element  14  causes ball element  52  to advance along helical flight  44  from its initial position shown in  FIG. 1  to a terminal position at coupler mounted end  48  where it grounds out and can no longer move axially since it becomes buried at the coupler mounted end. By this relative movement of ball element  52 , the total relative rotation between coupler  42  and spool element  14  is limited to a preset number of turns equal to the number of wraps or angular extent of helical flight  44 . Relative rotation on the order of six revolutions may be provided, for example. Expressed another way, the total rotational deflection between opposing torsion bar heads  38  and  40  is likewise restricted once ball element  52  reaches its terminal position. Once a grounded out, ball element  52  directly couples tread head  22  into connection with spool element  14  and further torsional deflection of torsion bar  36  is prevented. 
     If desired, the force of movement of ball element  52  within helical flight  44  and groove  50  may be controlled through friction forces imposed through appropriate dimensioning of the interactive surfaces. High preloading forces (radial compression of element  5 ) acting on ball element  52  can imposed desirable friction acting on the ball element as it moves. Additional restriction on the movement of ball element  52 , if desired, or an alternative means of imposing restriction can be provided through the addition of a deformable element  54  in the form of a metal strip, which, for example, may be positioned within spool groove  50 . In such a configuration, the interaction between ball element  52 , helical flight  44 , and groove  50  requires deformation of element  54  upon movement of ball element along the helical flight. The interference with the movement of ball element  52  provided by element  54  represents torque acting between coupler  42  and spool element  14 , which adds to the torque acting through torsion bar  36 . Through appropriate design, the deformation of element  54  may be caused to increase over the displacement of ball element  52  to provide progressive load limiting, or decrease with such motion which decreases total load limiting webbing force over deflection, providing digressive load limiting features. 
     Additional refinements of spool assembly  12  may be implemented in connection with this invention. For example, helical flight  44  may trace more than one interlaced track, with a corresponding number of ball elements  52  being provided (i.e. multiple leading threads). The helical shape of flight  40  may feature a changing helix angle, which when interacting with deformable element  54 , may provide additional load/deflection tuning opportunities. A further alternate embodiment could reverse the components forming helical flight  44 , having the helical flight formed on the inside cylindrical surface of the spool cavity  30 , with another groove formed by drum  42 . 
       FIG. 3  illustrates a spool assembly  56  in accordance with a second embodiment. This embodiment differs from the first embodiment in that spiral flight  44  is formed by a tubular extension  58  of tread head  22 . This figure also illustrates the provision of three ball elements  52 , mentioned previously as an alternative design. 
       FIGS. 4 and 5  for illustrate spool assembly  60  in accordance with a third embodiment of the present invention. Spool assembly  60  has features with functional similarities with those discussed previously. In this case, spool assembly  60  utilizes a coupler not disposed within an internal cavity of the spool element, but rather extending from an axial end of the spool as a stub  61 . Stub  61  has three axial grooves  62 . Drum  63  is fixed relative to the retractor frame and forms helical flights  66 . Helical flights  66  may form a single track from its outer end  68  to its inner end  70  or two, three or more tracks may be interlaced (three are shown). An appropriate number of ball elements  72  are used to interact with each of the paths of helical flight  66 . Ball elements  72  interact with the helical flights  66  and grooves  62  to create an interlocking condition when the predetermined relative rotations are completed. The tread head (not shown), when locked, becomes fixed to the retractor frame (not shown). In a manner similar to the first embodiment, rotation of spool element  60  relative to the retractor frame is limited to a predetermined number of turns (full or partial) once deformation of the load limiting device occurs. In another version of spool assembly  60 , stub extension  61  may form the spiral flights with an axial track formed by a fixed component surrounding the stub which both define a movement path for the ball element(s). 
       FIG. 5  is another view of spool assembly  60  further showing the provision of three ball elements  72  each interacting with a separate interlaced tracks of helical flight  66 . 
     Now with reference to  FIGS. 6 and 7 , a fourth embodiment of a spool assembly  80  in accordance with this invention is illustrated. This spool assembly features a spiral flight formation  90  on a plane. Plate element  82  forms radial track  84  extending from the center of rotation of the associated spool element (not shown) to a radially outer position. Baseplate  86  is clamped against plate  82 . Deformable sheet  88 , which is an optional item provided to provide additional friction or restriction to relative load limiting deflection, is sandwiched between the two plates  86  and  82 . 
       FIG. 7  illustrates spiral flight track  90  formed on a face surface of baseplate  86 . A ball element  92  shown in  FIG. 6  is positioned within radial track  84  and engages with spiral flight  90 . In a manner similar to the previous embodiments, ball element  92  is caused to advance along spiral flight  90  upon relative rotation between the associated spool element and its tread head. Such relative movement drives ball element  92  to move along the track of spiral flight  90  and track  84 . Such movement deforms sheet  88  which, in a manner similar to the prior embodiments, imposes an additional restraint torque. Once ball element  92  reaches the terminal end of spiral flight  90 , further relative rotation is prevented. The arrangement of spool assembly  80  shown in  FIGS. 6 and 7  could employ ball element  92  having a starting position within spiral flight  90  at near the center of rotation of the associated spool element. Conversely, through using appropriate handedness of the spiral flight  90 , the ball element  92  could begin at a radially outer position as shown in  FIG. 4 . In configurations of the device shown in  FIGS. 6 and 7  in which deformable sheet  88  is not provided, the interacting elements provide a limited number of rotations (full or partial) during load limiting deflection. 
     This description of the invention refers to in the case of the first two embodiments, a helix shaped flight, and in the case of the last embodiment, a spiral shaped flight. As used herein, “spiral” is intended to encompass both formations with a plane spiral on a plate shaped element, and a helix formed on a cylindrical (or conical) surface being defined as a special case of a spiral. The components forming the spiral (or helix) flight may also be regarded broadly as a cam, with the ball elements interacting with the cam may also be described as a cam follower. Moreover, the embodiments described can be used in connection with load limiting elements beyond torsion bars. The primary features are mechanisms for limiting deflection of load limiting elements, of any type. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.