Abstract:
A seat belt retractor for motor vehicles with a vehicle-sensitive or belt webbing sensitive controllable locking device, or both, for the belt shaft, having a profiled head that acts as a support for a displaceably mounted locking member that locks the belt shaft in the housing, and a two-stage force limiting device having two force limiting elements that are switchable between a high level and a low level of force limitation via an interposed coupling ( 19, 20 ) that is controlled by means of a control device having a countergear which determines an amount of belt webbing that is extended from the belt shaft.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German patent application number DE 10 2005 061 025.0 filed Dec. 19, 2005, and PCT/EP2006/011315, filed Nov. 25, 2006, the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a seat belt retractor for motor vehicles, comprising a vehicle-sensitive and/or belt webbing-sensitive controllable locking device for the belt shaft, with the retractor comprising a profiled head that acts as a support for a displaceably mounted locking member that locks the belt shaft in the housing, and a two-stage force limiting device that is equipped with two force limiting elements connected in series, the force limiting elements being switchable between a high level and a low level of force limitation via an interposed coupling that is controlled by means of a control device. 
       BACKGROUND OF THE INVENTION 
       [0003]    A seat belt retractor having the above-mentioned characteristics is described in DE 200 15 402 U1. In addition to a torsion bar disposed between the belt shaft and the profiled head thereof, this seat belt retractor has two further force limiting devices connected in series in the form of two friction couplings, which are coupled to one another via an externally and internally toothed ring. The first friction coupling having a high level of force limitation acts between the belt shaft and/or the profiled head thereof and the toothed ring, and the second friction coupling between the toothed ring and the housing. In the initial state, the toothed ring is fixed to the housing via a catch that can be controlled by a control device, wherein, if necessary, the catch is controlled disengaged from the toothed ring by the control device such that the toothed ring can be rotated freely in relation to the belt retractor housing. 
         [0004]    In the known belt retractor, the control of the force limiting elements is configured such that the toothed ring is fixed to the housing via the catch so that the higher level of force limitation acts first in case of an accident. If a signal emitted by the vehicle determines that based on the driver&#39;s height, weight or position, or based on the severity of the accident, the restraining force acting on the occupants is becoming too large, the catch is controlled disengaged from the toothed ring so that the toothed ring can be rotated freely in relation to the housing. Thus, a relative movement occurs between the housing and the toothed ring, wherein the second friction coupling located between the housing and the toothed ring acts as a force limiting device with a lower level of force limitation. Accordingly, the switching operation is accomplished from the high level of force limitation to the lower level of force limitation and is not reversible. Insofar, the known belt retractor has the disadvantage that the high level of force limitation acts first also on lower-weight or buckled-up persons, resulting in an accordingly strong increase in the restraining force. The level of force limitation is only reduced when the restraining force is too high. A further disadvantage is that the known seat belt retractor does permit consideration of the momentary changes in the sitting position by movements of the buckled-up occupants on the seat. 
       SUMMARY OF THE INVENTION 
       [0005]    Therefore, the object of the present invention is to further develop a seat belt retractor of the general type such that the level of force limitation may be more flexibly and better adjusted to the occupants&#39; conditions. 
         [0006]    The solution of this task, including advantageous embodiments and refinements of the present invention, will become apparent from the claims appended to this description. 
         [0007]    The present invention provides a control device for switching the coupling between the high level and low level of force limitation comprising a counter gear, which determines the amount of the respective belt webbing that is extended from the belt shaft, and that the coupling can be reversibly switched back and forth between the high and the low levels of force limitation. 
         [0008]    The present invention has the advantage that, depending on the belt webbing extension at the moment of the accident, the level of force limitation is either adjusted to a high level or to a low level right from the start. Thus, not only is the belt webbing extension contingent upon the body measurements and respective clothing, but also the occupants&#39; sitting position at the moment of the accident may be taken into consideration. If a small and therefore normally light-weight occupant, for whom a low level of force limitation is to be engaged according to prior art, bends forward shortly before the accident, the occupant will possibly be correspondingly near a vehicle part so that the longer belt webbing release path associated with the low level of force limitation may already endanger the occupant. However, in this case the seat belt retractor according to the present invention ensures that the high level of force limitation is immediately engaged via a belt webbing extension request, wherein, while accepting a higher belt webbing force, a potential forward displacement of the occupant is reduced. Therefore, the small occupant is better protected. 
         [0009]    Further, the protection is advantageously improved for such persons, who are buckled-up with a belt webbing extension located shortly before the switching point, and for whom the low level of force limitation is adjusted in this regard. In case of an accident, the activation of the force limiting element having the low level of force limitation will cause a belt webbing extension, at which the switching position may be exceeded and the high level of force limitation be consequently engaged. 
         [0010]    According to one embodiment of the present invention, the coupling comprises a toothed ring, which is rotatably mounted on the housing of the belt extractor and when locked can be engaged by the locking member of the profiled head, and a catch, which can be displaced back and forth between two switching positions by means of the control device, said catch releasing the toothed ring in the one switching position and locking it to the housing in the other switching position. As far as the use of a toothed ring for accomplishing the switching connection between two force limiting devices is known from DE 200 15 402 U1, a movement of the corresponding catch is only provided in one switching direction. 
         [0011]    In a further embodiment, the force limiting elements are disposed with respect to the toothed ring such that the first force limiting stage having the low level of force limitation is implemented by a toothed ring that can rotate in relation to the housing, and the second force limiting stage having the high level of force limitation is implemented by the catch, which is controlled by the control device when engaged in the toothed ring, in the toothed ring that is fixed to the housing. 
         [0012]    In a further embodiment of the present invention, the force limiting element defining the low level of force limitation is a torsion rod, which is disposed in the belt retractor housing outside the belt shaft, one end of the rod being fixed inside the housing and the other end being attached to the toothed ring by means of a connecting means such that the rotation of the toothed ring can be translated into a twisting motion of the torsion rod. EP091330041 describes a torsion rod disposed outside the belt shaft as a second force limiting element, which is connected directly to the belt shaft by means of a gear mechanism and thus rotates together with the belt shaft. In the event of an appropriate load case, the second torsion rod is locked so that a force limiting effect is added to the level of force limitation created by the first torsion rod disposed on the belt shaft. In contrast, according to the present invention, the torsion rod located outside the belt shaft is uncoupled from the second force limiting element, which likewise has the shape of a torsion rod disposed in the belt shaft, for example, by means of the rotatable toothed ring. 
         [0013]    In a further embodiment of the present invention, the connecting means is made of a metal band having a predetermined length, one end of which is attached to the toothed ring and the other end of which is wound onto and attached to a winding spool, which is fixed to the torsion rod and rotatably disposed on the belt retractor housing. This is advantageous in that also at the low level of force limitation, the belt webbing release associated with the twisting motion of the torsion rod is limited to the length of the metal band to be unwound. If the metal band is completely unwound, the force is directly transmitted from the toothed ring to the twisted torsion rod, and consequently to the housing, such that a rigid system is created, which activates the higher level of force limitation of the further force limiting element even with a selected catch and a rotatable toothed ring. In this way, additional safety is provided for persons, for whom the control device only provides the low level of force limitation. 
         [0014]    Regarding the design of the force limiting element defining the lower level of force limitation, an alternative embodiment provides that the force limiting element defining the low level of force limitation is a bending brake that is disposed on the belt retractor housing outside the belt shaft and comprises a pulling element, which is guided through a baffle fixed to the housing, and has one end attached to the toothed ring such that the rotation of the toothed ring can be translated by the baffle into an energy-absorbing pull-through of the pulling element. This has the advantage that the arrangement of a bending brake as an additional force limiting element is cost-effective. In this respect, the effectiveness of the force limitation may be easily adjusted via the length of the pulling element that is to be pulled through the baffle, which is to say the retained reserve thereof. 
         [0015]    In this connection, different embodiments of the present invention provide that the free end, which forms the reserve, of the pulling element is disposed outside around the toothed ring, or that the free end, which forms the reserve, of the pulling element is disposed in a storage housing, which is rotatably mounted on the belt retractor housing, and may be pulled out of the storage housing. 
         [0016]    The force path may advantageously be adjusted during force limitation by the selection of the width of the pulling element in that the pulling element has a uniform width across the length thereof, or in that the width of the pulling element varies across the length thereof as a function of the desired change in the level of force limitation. 
         [0017]    In a further embodiment of the present invention, the counter gear comprises an externally toothed gear wheel that is connected to the belt shaft, a further externally toothed gear wheel that is fixed to the housing, and a third externally toothed gear wheel disposed on a rocker arm, which is pivotably mounted on the housing in two switching positions, wherein all three gear wheels mesh with one another, and switching points for the switching lugs that fix the rocker arm are disposed on the gear wheels, wherein in the predetermined unwinding state the lugs come in contact with one another and thus pivot the rocker arm between the switching positions. A counter gear of this type is known in principle from DE 41 32 876, namely as a retractor switch for the use of a seat belt retractor in a child seat. The counter gear switches the belt webbing-sensitive and/or vehicle-sensitive control system for locking the belt shaft when the retractor is used with a child seat. The known counter gear comprises two gear wheels with corresponding switching lugs that are disposed on a rocker arm, wherein the wheels mesh with the gear wheel that is connected to the belt shaft. As a result of the arrangement of the switching lugs on the individual gear wheels, the belt webbing must first be fully unwound from the belt shaft before a switching operation is possible, so that during a return motion of the belt webbing to an extended position that is suitable for the use of a child seat, the corresponding vehicle-based control system is switched off. The further return motion of the belt webbing up to the storage position then results in a new switching operation by engaging the belt webbing-sensitive and vehicle-sensitive control system. 
         [0018]    A further embodiment of the present invention provides that the gear wheel connected to the belt shaft and the gear wheel mounted on the housing each comprise a switching lug, and that the gear wheel mounted on the rocker arm comprises two switching lugs to restrict the switching hysteresis. Thus, each of the two switching lugs disposed on the gear wheel mounted on the rocker arm interact with the switching lugs of the gear wheel fixed on the shaft and/or with the switching lugs of the gear wheel mounted on the housing such that both switching points may be adjusted via a very small angle of rotation of the belt shaft. In this way, the exact definition of the switching point for controlling the catch in the toothed ring is a function of the extension motion and also the winding motion of the belt webbing. 
         [0019]    To the extent that the rocker arm controlled by the counter gear must control the catch in both of the switching positions, alternative embodiments of the present invention provide that the rocker arm directly controls the catch via a mechanical connection or indirectly via an interposed electrical or pneumatic switch. 
         [0020]    In case of a direct mechanical control, an embodiment of the present invention provides that a control catch is disposed on the catch relatively displaceably thereto and interacting with the rocker arm, the control catch having a control cog that interacts with the outer gearing of the toothed ring. Such an embodiment ensures that via the engagement of the control catch, the catch is engaged in the outer gearing of the rotating toothed ring with precise fit without resulting in tooth-on-tooth locking. This applies not only to the switching operation for the belt extension when buckling up, or when the buckled-up person moves, but also to switching the level of force limitation, for example, when switching occurs due to the engagement of the catch in the outer gearing of the toothed ring as a result of the belt webbing extension related to the forward displacement of an occupant exposed in the first instance to the low level of force limitation. 
         [0021]    A further embodiment of the present invention provides that the catch has a slotted mounting recess for the U-shaped control catch, with which the connecting member of said catch is inserted in the mounting recess allowing clearance for movement. With respect to the reversible configuration of the coupling, it may be provided that with regard to the retraction of the catch, said catch is pretensioned in the release position for the toothed ring by means of a spring. 
         [0022]    A further embodiment of the present invention provides that the control catch is tensioned by a pretensioned spring in the direction of the engagement in the outer gearing of the toothed ring, and that the pretensioned spring is fixed in the release position of the control catch for the toothed ring by means of a separately controlled holding device, which releases the pretensioned spring when the control catch engages in the outer gearing of the toothed ring. This is advantageous in that in case of the dynamic engagement of the control catch in the outer gearing of the toothed ring at the beginning of the rotation of the toothed ring, the switching force of the control catch is increased by releasing the pretensioned spring. As a result of the increased actuating force accomplished in this way, as well as the subsequent meshing force of the control catch in the external gearing of the toothed ring, it is no longer possible to switch off the adjusted higher level of force limitation due to the locking of the toothed ring on the one hand, and on the other hand, as a result of the pretensioning by the spring, the control catch in addition acts as a retraction lock against undesired reversed rotation of the toothed ring. 
         [0023]    In this connection, it may be advantageous that the holding device comprises a gate that may be displaced relative to the pretensioned spring between a holding position and a release position. For this purpose, an embodiment of the present invention provides that the gate is maintained in the holding position by means of a holding lever, which rests positively in a recess configured in the toothed ring and is released from the toothed ring on rotation of said toothed ring. Coupling the holding device to the rotation of the toothed ring ensures that no faulty activation and/or faulty operation occur because the release of the holding device is exclusively activated by the onsetting rotation of the toothed ring, wherein the toothed ring is then fixed by the immediately operative, load absorbing catch. 
         [0024]    Insofar as an embodiment of the present invention provides that the pretensioned spring is disposed and oriented such that, in the release position of the control catch for the toothed ring, the spring force line extends via the pivot bearing of the control catch on the catch and, when the control catch is pivoted, the buckling thereof produces a torque in the engagement direction of the control catch in the toothed ring, this has the advantage that in the resting position of the pretensioned spring, no additional torque is present that would have to be absorbed by the holding device. Only when the control catch is pivoted will the intended torque become active. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The drawings show exemplary embodiments of the present invention, which are described in more detail below: The drawings show: 
           [0026]      FIG. 1  an overall view of a seat belt retractor in accordance with the present invention, 
           [0027]      FIG. 2  the subject matter of  FIG. 1  from a different angle, 
           [0028]      FIG. 3  an exploded view diagram of the subject matter according to  FIGS. 1 and 2 , 
           [0029]      FIG. 4   a  top view of the control device configured as a counter gear in one of the switching positions of the rocker arm, 
           [0030]      FIG. 5  the subject matter of  FIG. 4  in the other switching position of the rocker arm, 
           [0031]      FIG. 6   a  detailed drawing of the toothed ring with the torsion rod having the lower level of force limitation connected thereto, 
           [0032]      FIG. 7  an enlarged cut-out view of the catch with the associated control catch and toothed ring, 
           [0033]      FIG. 8  an overall view of the catch with the control catch mounted thereto, 
           [0034]      FIG. 9  the subject matter of  FIG. 8  with a separate illustration of the catch and control catch, 
           [0035]      FIGS. 10   a - c  the process of engaging the catch into the outer gearing of the toothed ring, 
           [0036]      FIG. 11   a - c  the control catch with a pretensioned spring acting on it and the associated holding, device for the pretensioned spring in the different functional positions of the control catch, 
           [0037]      FIG. 12  a side view of the toothed ring with the torsion rod having the lower level of force limitation connected thereto in the initial position, 
           [0038]      FIG. 13  the subject matter of  FIG. 12  with an unwound metal band, 
           [0039]      FIG. 14  a side view of the toothed ring with a force limiting device configured as a bending brake having the lower level of force limitation, 
           [0040]      FIG. 15  a modified embodiment of the subject matter of  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    The seat belt retractor  10  illustrated in  FIGS. 1 and 2  has a U-shaped housing  11  with lateral housing legs  12  in which a belt shaft  13  is mounted. The so-called spring side of the belt shaft is designated with numeral  14 , wherein a spring housing that is not illustrated may be disposed thereon for the winding up motion of the belt shaft, and also other functional elements, such as a tightening device, for example. Reference numeral  15  designates the end of a torsion rod disposed in the belt shaft  13 , said torsion bar forming the force limiting element having the high level of force limitation. 
         [0042]    A second force limiting element having a level of force limitation that is lower in relation to the torsion rod  15  is provided as a second torsion rod  16  that is disposed outside the belt shaft  13 , the one end of said torsion rod being fixed in a positive locking manner on the associated housing leg  12  of the seat belt retractor housing  11 , and the other end being connected non-rotatably to a winding spool  17  rotatably mounted on the opposite housing leg  12 . A metal band  18  is wound onto the winding spool  17  and attached with the free end thereof to a toothed ring  19  that is mounted rotatably on the respective housing leg  12 . As is apparent from other drawings in the figure, the toothed ring  19  is provided with an outer gearing, which is associated with a force-transmitting catch  20  mounted on the housing leg  12 . 
         [0043]    As is apparent from  FIG. 2 , a bearing plate  28  is disposed on the outside of the housing leg  12  supporting the toothed ring  19 , said bearing plate having a counter gear  22  with a gear wheel  23  that is connected to the associated end of the belt shaft  13 , a gear wheel  24  mounted on the bearing plate  28  and a gear wheel  24  disposed on a rocker arm  25  mounted pivotably on the bearing plate  28 . The rocker arm  21  extends beneath the bearing plate  28  by means of an extension and interacts with the catch  20  in a manner described below. 
         [0044]      FIG. 3  shows the above-described individual components of the seat belt retractor  10  in detail again, wherein it is apparent that a profiled head  26  is disposed at the end of the belt shaft  13 ; which engages in the bearing plate  28 , and is fixed to the torsion rod  15  extending inside the belt shaft  13 , an inertia controlled locking member  27  being disposed on the profiled head  26 , wherein the member may be engagable in the inner gearing  33  configured on the toothed ring  19 . In contrast, the catch  20  is configured for engagement in the outer gearing  34  of the toothed ring  19 . 
         [0045]    As is further apparent from  FIGS. 4 and 5  in an enlarged illustration of the counter gear  22 , the gear wheels  23 ,  24 ,  25  are disposed in relation to one another such that the gear wheel  25  mounted on the rocker arm  21  remains meshed with the shaft gear wheel  23  in each of the two possible switching positions of the rocker arm  21  on the one hand, and with the gear wheel  24  mounted on the bearing plate  28 , on the other hand. Switching lugs  30  and/or  31  are configured on each of the two gear wheels  23  and  24 . In contrast, two switching lugs  32   a, b  are disposed offset from one another at an angle of rotation on the gear wheel  25  mounted on the rocker arm  21 , the switching lug  32   a  meeting with the switching lug  30  of the shaft gear wheel  23  when the counter gear  22  is in the appropriate position ( FIG. 5 ), while the switching lug  32   b  meets with the switching lug  31  of the gear wheel  24  mounted on the bearing plate when the rocker arm  21  is in a different position. When comparing  FIGS. 4 and 5 , it is apparent that upon contact of the two switching lug combinations, the rocker arm  21  is respectively pivoted according to the two switching positions. Based on the two switching lugs  32   a, b  located on the gear wheel  25  disposed on the rocker arm  21 , the corresponding switching point may be defined with a very small clearance of the angle of rotation, so that not only an exactly reproducible switching point in the unwinding direction as well as in the winding direction is created, but also both switching points are closely adjacent to one another in relation to the angle of rotation of the belt shaft  13 . 
         [0046]    The interaction of the toothed ring  19  with the outer torsion rod  16  via the metal band  18  is apparent in detail from  FIG. 6 , and the engagement of the catch  20  mounted on the housing leg  12  in the outer gearing  34  of the toothed ring  19  is apparent in detail from  FIGS. 7 to 9 . In order to ensure the engagement of the catch  20  in the outer gearing  34  of the toothed ring  19  even when the toothed ring  19  is rotating, a separate control catch  29  is provided, which is configured as a U-shaped component with lateral legs  36  and an interposed connecting member  37 , the catch  20  having a slotted mounting recess  38 , in which the control catch  27  is inserted with the connecting member  37  thereof, so that the lateral legs  36  of the control catch  29  enclose the catch  20  between them. In this respect, the control catch  29  as such is mounted directly rotatably on the catch  20  allowing clearance for movement. This mounting is implemented such that a slightly protruding tilt edge  39  meshes with a receiving groove  40  at the bottom of the slotted mounting recess  38  of the catch  20 , so that the control catch  29  may be displaced with minor expenditure of energy in the catch  20 . A control cog  42  that is configured for the engagement into the outer gearing  34  of the toothed ring  19  is provided on the connecting member  37 . A leaf spring  35  engages on the catch  20 , by means of which in the resting position the catch  20  is kept disengaged from the outer gearing  34  of the toothed ring  33 . 
         [0047]      FIGS. 10   a - 10   c  show the implementation of the back and forth switching operation between the two levels of force limitation. As is apparent from  FIG. 10   a  at first, the catch  20  is disengaged from the outer gearing  34  of the toothed ring  19 ; the same also applies to the control catch  29 , which is maintained in the position illustrated in  FIG. 10   a  by means of an extension  41  of the rocker arm  21  that engages on said control catch. In this position, the control catch  27  mounted on the profiled head  26  is also disengaged from the inner gearing  33  of the toothed ring  19 . Thus, in this position of the involved components relative to one another, the belt shaft  13  can rotate freely. If, however, the control catch  27  engaged in the inner gearing of the toothed ring  23  is disengaged, the toothed ring  19  rotates together with the belt shaft in the direction of the belt extension, wherein the toothed ring  19  acts on the outer torsion rod  16  via the metal band  18  attached to the toothed ring, so that the low force limiting element becomes active on further rotation of the belt shaft. 
         [0048]      FIG. 10   b  shows a position of the control catch  29 , as implemented when switching the rocker arm  21  via the counter gear  22 , wherein the extension  41  of the rocker arm  21  now engages the control catch  29  in the motion path of the outer gearing  34  of the toothed ring  19 , so that the control catch  29  is grasped and pivoted when the rotation of the toothed ring  19  starts, wherein the pivoting of the control catch likewise pivots the catch  20  and definitely engages it in the outer gearing  34  of the toothed ring  19  and is entrained accordingly during engagement. 
         [0049]    In the functional position shown in  FIG. 10   b , the control catch  27  has not been engaged in the inner gearing  33  of the toothed ring  19 , so that the position according to  FIG. 10   b  exemplifies the switching position in which, on activation of the locking system and engagement of the control catch  27 , the toothed ring  19  is directly fixed in a force-transmitting manner on the housing leg  12  and/or housing  11  via the catch  20 . Thus, in this position the second torsion rod that is disposed outside the belt shaft cannot become active with the low level of force limitation. When the catch  20  is engaged in the outer gearing  34  of the toothed ring  19 , only the torsion rod  15  having the high level of force limitation disposed in the belt shaft  13  will in fact be engaged. In  FIG. 10   c , this state is shown in a certain intermediate position before the catch  20  is fully engaged in the outer gearing  34  of the toothed ring  19 ; consequently, the catch  20  is “ready for coupling”. 
         [0050]    According to the exemplary embodiment shown in  FIG. 11   a - c , the arrangement of a pretensioned spring  50 , which pretensions the control catch  29  in the direction of the engagement thereof in the outer gearing  34  of the toothed ring  19 , ensures that when the control catch  29  is engaged in the gearing of the toothed ring  19 , an additional switching force and/or additional torque is created. For this purpose, the end of the control catch  29  is tensioned by the pretensioned spring  50 , which is supported oh a spring abutment  51  provided on the belt retractor housing  11 . The pretensioned spring  50  is disposed and oriented with respect to the control catch  29  such that, in the release position of the control catch  29  for the toothed ring  19 , the spring force line extends via the pivot bearing  56  of the control catch  29  on the catch  20  so that in this resting position of the control catch  29 , the pretensioned spring  50  does not create additional torque that would have to be absorbed by the associated holding device. This holding device comprises a gate  52  that is guided displaceably on the spring abutment  51 , wherein the gate fixes the pretensioned spring  50  in the initial position shown in  FIG. 11   a . The gate  52  in turn interacts with a holding lever  53  rotatably mounted about a pivot point  54 , said holding lever being positively supported by means of a lever arm in a recess  55  that is configured on the toothed ring  19 . Hence, in the position shown in  FIG. 11   a , the control catch  29  is held in the non-engaged position thereof with the outer gearing  34  of the toothed ring  19 . 
         [0051]    When an operational demand causes a rotation of the toothed ring  19  is started, this rotation of the toothed ring  19  causes the holding lever  53  to be released from the recess  55  of the toothed ring  19 , pivoting as a result and displacing the gate  52  in the spring abutment  51  such that the pretensioned spring  50  is released, consequently acting upon the control catch  29 . This functional position is shown in  FIG. 11   b.    
         [0052]    When the position of the counter gear  22  is as described in  FIG. 10   b , the control catch  29  is engaged in the motion path of the outer gearing  34  of the toothed ring  19  and is grasped and pivoted with the onsetting rotation of the toothed ring  19 . This engaging movement of the control catch  29 , prompted by the pivoting of the control catch  29 , is supported by the buckling pretensioned spring  50  that consequently exerts additional torque on the control catch  29 . In this way, the switching operation is made irreversible because as a result of the action of the pretensioned spring  50  the control catch is also kept engaged in the outer gearing of the toothed ring, and the control catch  29  at the same time acts as a return stop for the toothed ring  19 . 
         [0053]      FIGS. 12 and 13  again show the connection of the second torsion rod  16 , which is disposed outside the belt shaft  13 , to the low level of force limitation and to the toothed ring  19  via the metal band  18 . From  FIG. 12  it is apparent that upon rotation of the toothed ring  19 , the metal band  18  is unwound from the winding spool  17 , which, due to the torque-proof connection thereof to the torsion rod  16 , twists this torsion rod while absorbing a force. If, according to  FIG. 13 , the metal band  18  is fully unwound from the winding spool  17 , a rigid system is created with respect to the force transmission via the rotatable toothed ring  19  as a result of the force-transmitting attachment of the metal band  18  to the torsion rod  16 . 
         [0054]    In another embodiment for the arrangement of a force limiting element having a lower level of force limitation shown in  FIGS. 14 and 15 , the function of the self-locking seat belt retractor is accomplished as described above. In the embodiment shown in  FIGS. 14 and 15 , the additional torsion rod having the lower level of force limitation is merely replaced by a bending brake. This bending brake comprises a pulling element  60 , preferably in the form of a metal band made of a suitable material, which is attached with one end thereof to the toothed ring  19  and is guided by a baffle  61  that is fixed to the housing. With one free end  62 , which forms a reserve for the pulling movement of the pulling element  60  through the baffle  61 , the pulling element  60  protrudes the baffle  61 . In the embodiment shown in  FIG. 14 , the free end  62  of the pulling element  60  is placed outside around the rotatable toothed ring  19  after the baffle  61  and is thus available as a reserve. In the exemplary embodiment shown in  FIG. 15 , this free end  62  is inserted in a separate reserve housing  63  that is disposed rotatably on the belt retractor housing  11  and may be extracted from there on rotation of the toothed ring  19 . 
         [0055]    The function of the belt retractor according to the present invention will again be explained with reference to the above-mentioned explanations of the functions of the individual components thereof. 
         [0056]    If an occupant fastens the not illustrated seat belt webbing that is wound on the belt shaft  13  of the seat belt retractor  10  and thus extends it from the belt shaft  13 , the counter gear  22  records the extent of the belt webbing extension, wherein the switching lug  30  located on the shaft gear wheel  23  meets with the switching lug  32   a  of the gear wheel  25  mounted on the rocker arm  21  after an appropriate number of revolutions of the belt shaft and switches the rocker arm  21  into a position illustrated in  FIG. 10   b  upon reaching this switching point. Reaching the switching point during belt extension means that an accordingly large occupant has fastened the seat belt and needs the corresponding amount of belt webbing. In case of an accident, only the large force limiting level will apply to such a large occupant so that upon reaching the prescribed switching point, the control catch  29  is placed in the rotation path of the outer gearing  34  of the toothed ring  19 . If during the accident the belt shaft  13  and toothed ring  19  are coupled by the engagement of the control catch  27  mounted on the profiled head  26  in the inner gearing  33  of the toothed ring  19 , the toothed ring  19  actuates the control catch  29 , which consequently engages the catch  20  in the outer gearing  34  of the toothed ring  19 , so that the toothed ring is fixed to the housing  11 . In this position, the second torsion rod  16  and/or the pulling element  60  having the low level of force limitation cannot become active. When the embodiment according to  FIG. 11   a - c  is implemented, the dynamic engagement of the control catch  29  with the catch  20  in the outer gearing  34  of the toothed ring  19  is supported by the pretensioned spring  50 . 
         [0057]    If, on the other hand, a smaller occupant fastens the seat belt, the switching point between the switching lugs  30  and  32   a  activated by the corresponding belt extension will not be reached, so that the rocker arm  21  remains in a position, in which, according to  FIG. 10   a , it keeps the control clutch  29  disengaged from the outer gearing  34  of the toothed ring  19 . In this position, upon coupling to the belt shaft  13 , due to the disengagement of the control clutch  27  from the inner gearing  33  of the toothed ring  19  as a result of an accident, the toothed ring  19  may now rotate relative to the housing, unwinding the metal band  18  from the winding spool  17 ; the rotation of the winding spool  17  produces a force-absorbing twisting of the torsion rod  16 . As the level of force limitation of the torsion rod  16  is lower than the level of force limitation of the torsion rod  15  of the belt shaft  13 , only the low level of force limitation is active when the toothed ring  19  can be rotated, which is appropriate for a smaller occupant. This applies accordingly to the arrangement of the pulling element  60  as a force limiting element. 
         [0058]    If upon buckling up an occupant has required such an amount of belt webbing that the corresponding switching point of the counter gear  22  has not been reached yet, it will first remain in the position described for  FIG. 10   b . However, as an occupant buckled up in such a way moves, bends forward, for example, more belt webbing is extended, and as this belt extension exceeds the correspondingly adjusted switching point between the switching lugs  30  and  32   a , the rocker arm  21  will also be switched for this possibly short-term state, and in this respect a switching readiness in regard to switching only the torsion rod  15  is brought about. If in such a case the occupant straightens up again, the belt retraction associated with a back rotation of the belt shaft  13  causes the switching lug  32   b  of the gear wheel  25  disposed on the rocker arm  21  to meet with the switching lug  31  of the gear wheel  24  disposed on the bearing plate  28 , and this results in a new switching motion of the rocker arm  21  into the position in which, according to  FIG. 10   a , the rocker arm  21  keeps the control catch  29  disengaged from the toothed ring  19  so that the toothed ring is in turn freely rotatable. This back and forth switching is perceivable to the buckled-up occupant; however, it ensures that, depending on the sitting position at the moment of the accident, either the low level of force limitation, or directly the high level of force limitation is also switched with smaller occupants in order to prevent excessive forward displacement of the occupant in the latter case. 
         [0059]    A similar switching state is also possible if, in case of an accident, the belt release related to the second torsion rod  16  becoming active results in the switching point being exceeded, wherein an occupant who in the this case is first subject to the low level of force limitation gets too close to a vehicle part. In such a case as well, in particular as a result of the arrangement of the control catch  29 , the catch  20  may be engaged in the rotating toothed ring  19 , thus activating the high level of force limitation according to  FIG. 10   c.    
         [0060]    The characteristics of the subject matter of the invention disclosed in the specification, claims, abstract and drawings can each be fundamental for the execution of the present invention in the different embodiments as such or in any combination thereof.