Abstract:
A seat belt pretensioner having a housing, a belt shaft ( 10 ), a load limiting device ( 70 ) and a pretensioner drive wheel ( 20 ) coupled to the belt shaft ( 10 ). A coupling element ( 60 ) is fixed to the drive wheel ( 20 ) via a detachable connection, and can be locked by a blocking device ( 90 ). An engagement wheel ( 50 ) is provided for connecting the belt shaft ( 10 ) with the pretensioner drive wheel ( 20 ) having engagement portions ( 51 ) engaging into corresponding recesses ( 12 ) on the belt shaft ( 10 ) or on the pretensioner drive wheel ( 20 ) and is fixed by the coupling element ( 60 ). The detachable connection can be disconnected by locking of the coupling element ( 60 ) and rotation of the belt shaft ( 10 ) in the belt webbing extraction direction, thereby the engagement wheel ( 50 ) undergoes relative rotation in relation to the pretensioner drive wheel ( 20 ) or to the belt shaft ( 10 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German patent application number 10 2010 009 155.3, filed Feb. 24, 2010 and PCT/EP2011/000260, filed Jan. 22, 2011. 
     FIELD OF THE INVENTION 
     The invention relates to a seat-belt pretensioner for a motor vehicle belt restraint system. 
     BACKGROUND OF THE INVENTION 
     Seat-belt pretensioners are used in motor vehicles to enhance occupant protection of a belted occupant at the start of an accident by retracting the seat-belt to couple the passenger to the vehicle deceleration as soon as possible, Because of the early coupling of the passenger, the restraint force acting on him or her is reduced, in that it is distributed on the longest possible path of forward displacement. In order that the longest possible path of forward displacement is also utilized in full, the seal-belt retractor with a pretensioner function is furthermore provided with a force limiting device by means of which a force-limited, seat-belt strap extraction is facilitated. The force on the passenger during the force-limited forward displacement is determined by the design of the force limiting device, whereby various force limiting sequences can be adjusted by combining several force limiting elements. 
     The disadvantage with systems mentioned above, however, is that because of the successive opposite rotating movements of the belt shaft during the belt tightening process which produces belt retraction, and the force limitation which occurs during belt extension, the seat-belt strap force limitation can be obstructed by the still applied tensioning force exerted by the seat-belt pretensioner. This disadvantageous force limitation impediment can thereby result in a short-term peak at the start of the force limiting process. 
     A seat-belt pretensioner of the above-referenced type is known from WO 2009/118088 in which a locking element is provided to couple the pretensioner drive wheel to the belt shaft and in which the locking element can be detached by turning of the belt shaft in the direction of the seat-belt strap extraction after the tensioning movement has been completed. The pretensioner drive wheel and the belt shaft are coupled with each other by means of a ramp contour which forces an axial movement of the tension drive wheel with respect to the belt shaft. The locking element is formed, for example, by a spring disk which is moved into a ready position by the activation of the tensioning movement in that it engages with spring arms in a fixed-housing gear. By means of the developing force-limited seat-belt strap extraction movement, the locking element fixed in the fixed-housing gear in the direction of the belt extraction executes a release movement and thereby enables the axial movement of the pretensioner drive wheel with respect to the belt shaft, whereupon the pretensioner drive wheel and the belt shaft disengage. 
     A disadvantage of the solution described above is that the locking element must first be brought to a ready position by the tensioning movement in which it is fixed in the seat-belt strap extraction direction with respect to the housing. Furthermore, the locking element couples the pretensioner drive wheel to the belt shaft by exerting an axial force and must therefore have a certain form stability in order to absorb the impacting forces. 
     The object of the invention is to create a seat-belt pretensioner in which the disadvantages mentioned above are avoided. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is achieved by the seat-belt pretensioner with the characteristics described and claimed herein. The basic idea of the invention is that a coupling element fixed in the circumferential direction via a detachable connection is provided which is blocked in the seat-belt strap extraction direction by a blocking device in a fixed housing, and a engagement wheel is provided to connect the seat-belt shaft with the pretensioner drive wheel which body engages with axially aligned, conical engagement sections into corresponding recesses on the seat-belt shaft or the pretensioner drive wheel and is secured on the pretensioner drive wheel in the circumferential direction by the coupling element. The retractor provides a detachable connection between the coupling element and the pretensioner drive wheel detached in the seat-belt strap extraction direction by the fixed-housing blockage of the coupling element and a rotation of the seat-belt shaft, and thereby enables a rotation movement of the engagement wheel relative to the tension drive wheel and/or to the seat-belt shaft which is superimposed by means of the conical engagement sections with an axial movement. The engagement sections disengage from the recesses and the positive connection between the pretensioner drive wheel and the seat-belt shaft can be terminated. 
     The advantage of the inventive solution can be seen in the fact that the coupling element, which is fixed to the pretensioner drive wheel and prevents the engagement wheel from undergoing relative rotation movement during the tensioning movement, releases independently from the pretensioner drive wheel solely by a fixed-housing blockage and a rotation of the seat-belt shaft in the direction of seat-belt extraction. It is thereby not required to bring the coupling element to a ready position before the release of the connection to the pretensioner drive wheel, as is necessary in the known prior art solution. Furthermore, the coupling element itself is basically less stressed in an axial direction so that the requirements of the strength of the coupling element are lower, and the release movement can be carried out under the influence of smaller external forces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below with reference to a preferred exemplary embodiment. The figures specifically show: 
         FIGS. 1   a  and  1   b  are cross-sectional views of a seat-belt pretensioner in accordance with this invention with a engagement wheel in both an engaged and unengaged position, 
         FIG. 2  is a exploded view of the seat belt shaft with a pretensioner drive wheel and a engagement wheel, 
         FIGS. 3   a  and  3   c  are side views of a seat-belt pretensioner with a blocking device in various positions, 
         FIGS. 4   a  and  4   b  are side views of a seat-belt pretensioner in various positions with a strap and a spring resting thereon. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1   a  and  1   b  show the inventive seat-belt pretensioner with a seat-belt shaft  10 , a lockable profile body  30  fixed to the vehicle opposite a housing, which is not shown, and a force limiting device  70  in the form of a torsion rod tightly tensioned on the seat-belt shaft  10  and on the profile body  30 . Furthermore, a pretensioner drive  40  can be seen which, upon activation, drives the seat-belt shaft  10  in the winding direction for the pretensioning the seat-belt by means of a pretensioner drive wheel  20  and a engagement wheel  50 . The pretensioner drive wheel  20  comprises a seat-belt shaft stud  11  and is secured to it with a lock ring  80 . In  FIG. 1   a , the engagement wheel  50  is in a position in which it engages with its conical engagements  51 , which will be described more precisely later, in corresponding recesses  12  on the seat-belt shaft  10  and thereby creates a positive connection between the pretensioner drive wheel  20  and the seat-belt shaft  10 . 
       FIG. 2  shows an exploded view of the coupling mechanism to transmit the tensioning force exerted by the pretensioner drive  40  to the seat-belt shaft  10 . The coupling mechanism comprises a pretensioner drive wheel  20 , a engagement wheel  50 , and a coupling element  60 . In order to secure the coupling mechanism to the seat-belt shaft stud  11 , a locking ring  80  with an opening  82  and elastic clamping tongues  81  protruding radially inward into the opening  82  is provided, whereby the locking ring  80  locks the coupling mechanism and in particular the pretensioner drive wheel  20  to the seat-belt shaft stud  11  in the extraction direction by wedging the clamping tongues  81 . The pretensioner drive wheel  20  is provided on its radial interior side with a guiding structure  21  in the form of pockets and with a snap-off lug  22  protruding outward. The engagement wheel  50  has several axially aligned, conical engagement sections  51  with narrower fingers  52  connecting thereto, whereby a one-sided undercut  53  is provided at the transition from the engagement sections  51  to the fingers  52 . The coupling element  60  is configured as a toothed coupling ring and has teeth  62  aligned radially outward and several protrusions  64  extending radially inward into the opening  65 , between which corresponding free spaces  61  are formed. In each of the free spaces  61  a groove  63  is provided equidistant to the protrusions  64 . 
     When assembling the coupling mechanism, the coupling element  60  is attached on the front side of the pretensioner drive wheel  20  aligned such that the snap-off lug  22  engages in one of the grooves  63  and the coupling element  60  is fixed in the circumferential direction. The engagement wheel  50  is subsequently inserted with the fingers  52  through the free spaces  61  between the protrusions  64  into the guiding structure  21  of the pretensioner drive wheel  20 . The insertion movement of the engagement wheel  50  is thereby limited in that the engagement wheel  50  with the undercut  53  contacts the protrusions  64  of the coupling element  60 . Since the coupling element  60  and the pretensioner drive wheel  20  are fixed onto each other in the circumferential direction by the snap-off lug  22 , the engagement wheel  50  in this position is also fixed in the circumferential direction by the attachment of the fingers  52  to a side of the guiding structure  21  and the other side to the protrusions  64 . Furthermore, the engagement wheel  50  is also secured in the axial direction by the protrusions  64  adjacent to the undercuts  53  so that the engagement wheel  50  is not unintentionally pressed into the pretensioner drive wheel  20  owing to higher forces. This combination of the pretensioner drive wheel  20 , the engagement wheel  50 , and the coupling element  60  is pushed onto the seat-belt shaft stud  11  and locked by the locking ring  80 . 
       FIGS. 3   a  and  3   c  show the inventive seat-belt pretensioner in various positions before, during, and after the execution of the tensioning movement. Provided on the housing  120  of the seat-belt pretensioner is a blocking device  90  in the form of a bent spring which is shaped into a hook  93  on one end by means of which it is hung into a recess on the housing  120 . The other free end of the spring is formed into a hook-shaped engagement section  91  and, contacts the exterior side of the teeth  62  of the coupling element  60  in the position prior to the activation of the seat-belt pretensioner, as shown in  FIG. 3   a.    
     In the position shown in  FIG. 3   b , the seat-belt pretensioner drives the seat-belt shaft  10  in the winding direction and the engagement section  91  is advanced to the teeth  62 . While the seat-belt shaft  10  is actuated in the winding direction, the seat-belt pretensioner  20  rotates and thus also the coupling element  60  in the clockwise direction so that the engagement section  91  ratchets over the teeth  62  and does not prevent the clockwise rotating movement. After the tensioning movement has been completed, the seat-belt shaft  10  begins to rotate in the seat-belt extraction direction, that is, counterclockwise in this Figure, when exceeding a force limiting level defined by the force limiting device  70 . The rotation movement of the seat-belt shaft  10  is thereby transmitted via the conical engagement sections  51  of the engagement wheel  50  to the seat-belt drive wheel  20  and the coupling element  60 , whereby the counterclockwise rotation movement of the coupling element  60  produced thereby is prevented by the engaging engagement sections  91  of the blocking device  90  so that the snap-off lug  22  on the seat-belt drive wheel  20  drops out, and the fixation of the coupling element  60  with respect to the seat-belt drive wheel  20  is stopped. A stop  110  is provided on the housing  120  of the seat-belt pretensioner at which the blocking device  90  contacts a section adjacent to the engagement section  91 , so that any further counterclockwise rotation of the coupling element  60  is prevented in any case, and the connection between the coupling element  60  and the seat-belt drive wheel  20  is removed. In so far as the blocking device  90  and, in particular the engagement section  91 , has an appropriate inherent stiffness, the stop  110 , can, however, also be eliminated. 
     As can be seen in  FIG. 3   c , the coupling element  60  was twisted slightly in the circumferential direction with respect to the engagement wheel because of the blocking, so that the protrusions  64  are no longer located behind the undercuts  53  of the engagement wheel  50 . Upon another rotation of the seat-belt shaft  10  in the seat-belt strap extraction direction the engagement wheel  50  is pushed out of the corresponding recesses  12  of the seat-belt shaft  10  to the position shown in  FIG. 1   b  because of the conical shape of the engagement sections  51 , so that the connection between the seat-belt shaft  10  and the seat-belt drive wheel  20  is removed. The connection between the seat-belt drive wheel  20  and the seat-belt shaft  10  is thereby immediately released by the force-limited seat-belt strap extraction movement, whereby only tangential forces act to release the coupling element  60 . The conical shape of the engagement sections  91  thereby cause that an axial movement of the engagement wheel  50  is enforced by the rotation movement of the seat-belt shaft  10 , by means of which the connection between the seat-belt drive wheel  20  and the seat-belt shaft  10  is removed. 
     For a better understanding of the mode of operation of the blocking device  90  the seat-belt pretensioner is shown in  FIGS. 4   a  and  4   b  without the seat-belt drive wheel  20 , the coupling  60 , and the engagement wheel  50 . The seat-belt pretensioner comprises a seat-belt pretensioner drive  40  with a piston  41  which rests on a bearing section  101  of a strap  100 . During the further course the strap  100  is wound into a wind-up section  102  of several windings which is fixed in position by fixing devices  130  and  140 . The blocking device  90  rests on the outside of the wind-up section  102  with a support section  102  by actuating a spring tension. 
     In the position shown in  FIG. 4   a  the seat-belt pretensioner has still not been activated so that the strap  100  is fixed on a larger diameter by the fixing devices  130  and  140  and the blocking device  90  does not engage in the teeth  62  because of the shape with the engagement section  91 , as shown in  FIG. 3   a . By activating the seat-belt pretensioner, the piston  41  tensions the strap  100  across the bearing section  101  so that the wind-up section  102  is loosened from the fixing devices  130  and  140  and is pulled together to a smaller diameter. Because of the reduction of the wind-up section  102  the support section  92  of the blocking device  90  resting thereon likewise carries out a movement aimed radially outward by means of which the engagement section  91  engages in the teeth  62 , as is also shown in  FIG. 3   b.    
     The advantage of the inventively designed blocking device  90  consists in that it is held in a mounted position shown in  FIG. 4   a  and in that because of the shape and the contact with the wind-up section  102  it automatically swings into the blocking position upon activation of the seat-belt pretensioner, so that it is guaranteed in every case that the coupling element  60  is blocked at the start of the force-limited seat-belt strap extraction movement following the tensioning movement in the seat-belt extraction direction, and the connection between the seat-belt drive wheel  20  and the seat-belt shaft  10  is then automatically relieved by the rotation movement of the seat-belt shaft  10  in the seat-belt strap extraction direction. 
     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.