Abstract:
A belt retractor for a vehicle safety belt comprises a frame, a belt reel rotatably mounted in the frame, and an energy converter coil having at least one metal strip. The energy converter coil has a plurality of metal strips which are disposed symmetrically with respect to the middle axis of the belt reel.

Description:
RELATED APPLICATION  
       [0001]     This application is a division of U.S. patent application Ser. No. 10/441,699, filed on May 20, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to a belt retractor for a vehicle safety belt.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventional belt retractors comprise a frame, a belt reel rotatably mounted in the frame, and an energy converter coil having at least one metal strip.  
         [0004]     Such belt retractor is known from the German Patent Application 19 13 448. The energy converter coil consists of a metal strip which is accommodated in a drum in a first direction along the inner wall thereof. One end of the metal strip is connected with the drum with which the belt reel is connected as well. The other end is connected with a shaft which has a blocking wheel. In the blocking wheel a blocking pawl can engage, which is actuated by a vehicle-sensitive sensor. In the initial condition, the blocking wheel can rotate freely, so that the unit comprising shaft, metal strip, drum and belt reel can rotate freely. As soon as the blocking pawl engages in the blocking wheel, the shaft is locked in place. If in this condition particularly high forces act in the safety belt, such a high torque will be active between the drum and the shaft that the metal strip accommodated in the drum is wound up onto the shaft in a direction which is opposite to the direction in which the metal strip was accommodated in the drum originally. In this process, the metal strip is bent twice, namely first by about 180°, so that the metal strip extends in the opposite direction, and is then bent back a bit, so that the metal strip conforms to the curvature of the shaft. This bending resistance acts as resistance torque between the drum and the shaft, which in a known manner represents a limitation of the force maximally acting in the safety belt.  
         [0005]     The disadvantage of this known construction consists in that there is an asymmetrical force distribution in the energy converter coil.  
         [0006]     Thus, the object of the invention consists in developing a belt retractor as mentioned above such that more uniform force conditions are obtained.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     According to the invention, a belt retractor for a vehicle safety belt comprises a frame, a belt reel rotatably mounted in the frame, and an energy converter coil having at least one metal strip. The energy converter coil has a plurality of metal strips which are disposed symmetrically with respect to the middle axis of the belt reel. In this way, a symmetrical load distribution is obtained, which leads to a particularly low load on the shaft engaged by the metal strips and on the bearing of the belt reel. It is particularly advantageous when three metal strips are used, which engage the shaft offset by an angle of 120° each.  
         [0008]     In accordance with the preferred embodiment of the invention, a second energy converter coil is provided which can be selectively deactivated. In this way, there is obtained a selectable force limitation characteristic. For instance, both energy converters may initially be connected in parallel, so that in a first phase a particularly high belt webbing force is required to achieve a relative rotation between the drum and the shaft of the energy converter. After the first phase, the second energy converter can be deactivated, for instance time-controlled or controlled by an angle of rotation, so that then a lower belt webbing force is required to obtain a relative rotation between the drum and the shaft.  
         [0009]     Preferably, it is provided that the second energy converter coil has an outer ring, a holding pawl being provided by means of which the holding ring can be supported on the frame. In this way, the second energy converter can be deactivated in a mechanically simple, reliable way. For actuating the holding pawl a pyrotechnical actuator may be provided.  
         [0010]     The characteristic of the energy converters can also be varied in that the thickness of the metal strips or the width of the metal strips changes along their length.  
         [0011]     In accordance with the preferred embodiment of the invention it is provided that mounted to the belt reel is a blocking pawl which can be brought in engagement with blocking teeth of a blocking ring, and that the blocking ring is rotatably accommodated in a housing at which a leaf spring is mounted, which engages in a pocket on the outer periphery of the blocking ring. The leaf springs act as return stop, which prevents that after a force limiting operation the blocking ring is rotated back together with the belt reel under the spring action of the metal strips. If such reverse rotation would be allowed, an undesired, large belt slack would be produced in a secondary crash; a restraining effect would only be ensured again, when the metal strips are biased again by a relative rotation between the shaft and the drum. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a cross-section of a belt retractor in accordance with the invention;  
         [0013]      FIG. 2  shows a segment of  FIG. 1  on an enlarged scale;  
         [0014]      FIG. 3  shows a section along plane III-III of  FIG. 2 ;  
         [0015]      FIG. 4  shows a section along plane IV-IV of  FIG. 2 ;  
         [0016]      FIG. 5  shows a section along plane V-V of  FIG. 2 ;  
         [0017]      FIG. 6  shows a few components of the belt retractor of FIGS.  1  to  5  in a perspective exploded view; and  
         [0018]      FIG. 7  shows the remaining components of the belt retractor in accordance with FIGS.  1  to  5  in a perspective exploded view.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     The belt retractor has a frame  10  in which a belt reel  12  is rotatably mounted. The belt reel can accommodate a safety belt which is not represented. At the left end of the belt reel with respect to  FIG. 1 , a wind-up spring  14  is provided, which acts on the belt reel such that it winds up the safety belt.  
         [0020]     In the interior of the belt reel  12  a torsion-resistant shaft  16  is disposed, which at the left end with respect to  FIG. 1  is non-rotatably connected with the belt reel  12 . At the right end of the belt reel with respect to  FIG. 1 , the shaft  16  is provided with a first bearing collar  18  and a second bearing collar  20 , so that the belt reel is supported at this end.  
         [0021]     The shaft  16  can also be integrally formed with the belt reel.  
         [0022]     At its end facing way from the belt reel  12 , the shaft  16  is non-rotatably provided with a blocking disc  22 , on which a blocking pawl  24  is supported. By means of a conventional blocking mechanism, of which there is merely shown a cam disc  26  covered by a cover  27 , the blocking pawl  24  can be moved from its rest position shown in  FIG. 3  radially outwards and in engagement with blocking teeth  28 , which are formed on the inner periphery of a blocking ring  30 . The blocking ring  30  is surrounded by a holding ring  32 , which is mounted at a housing  34 . The housing  34  in turn is firmly mounted to the frame  10  of the belt retractor. In the holding ring  32 , two leaf springs  36  (see in particular  FIG. 3 ) are mounted, which extend approximately tangentially with respect to the blocking ring  30 , where they each engage in a pocket  38 . The leaf springs  36  are disposed diametrically opposite each other; in combination with the pockets  38  they act as return stop preventing a rotation of the blocking ring  30  in the direction of belt winding.  
         [0023]     A hub  40  is integrally formed with the holding ring  30 , which hub adjoins the blocking ring  30  on its side facing the belt reel  12 . On the hub, two energy converter coils  42 ,  44  are disposed.  
         [0024]     The first energy converter coil  42  (see in particular  FIG. 5 ) has three metal strips  46 , which are disposed in a drum-shaped accommodation space  48  in the interior of the housing  34 . With respect to  FIG. 5 , each metal strip extends from its outer end attached to the housing  34  by means of a fixing pin  50  in clockwise direction along the cylindrical wall of the accommodation space  48 . Each metal strip  46  is guided around the hub  40  over almost four windings and is then connected with the hub against the winding direction. For this purpose a clamping segment  52  is used, which is screwed to the hub  40 . Thus, in the package of metal strips extending along the wall of the accommodation space  48 , the three individual strips alternately follow each other.  
         [0025]     The second energy converter coil  44  is shown in  FIG. 4 . In its structure, it substantially corresponds to the first energy converter coil  42  with the difference that the accommodation space  48  is formed in the interior of an outer ring  54 , which is rotatably mounted in the housing  34 . On the outside of the outer ring  54  a holding recess  56  is formed, in which one end of a holding pawl  58  can engage, which is mounted at the housing. The other end of the holding pawl  58  rests against a piston  60  which is part of a pyrotechnical actuator  62  and can be charged with compressed gas by a gas-generating pyrotechnical charge  64 . In the initial condition, i.e. when the actuator is not actuated, the holding pawl  58  is in its position indicated in  FIG. 4  with continuous lines, in which it engages in the holding recess  56  and holds the outer ring  54  so as to be non-rotatable with respect to the housing  34 .  
         [0026]     In the initial condition of the belt retractor, i.e. when the blocking pawl  24  is in its position shown in  FIG. 3 , the belt reel  12  can rotate freely under the influence of the wind-up spring  14 . As soon as the blocking mechanism is activated, the blocking pawl  24  is moved radially outwards and in engagement with the blocking teeth  28 . As a result, the belt reel  12  is basically non-rotatably blocked, as a torque exerted on the same by the safety belt is supported on the housing  34  and thus on the frame  10  via the shaft  16 , the blocking disc  22 , the blocking pawl  24 , the blocking ring  30 , the hub  40  and the metal strips  46  of the first energy converter coil  42 . The second energy converter coil  44  also serves to support such torque, as in the initial condition the holding pawl  58  engages in the holding recess  56  in the outer ring  54 , so that in the end this ring is also non-rotatably held with respect to the frame  10 .  
         [0027]     As soon as the forces acting in the safety belt exceed a predetermined threshold, the torque acting in the shaft  16  is larger than the resistance torque provided by the energy converter coils  42 ,  44 . Then, the blocking ring  30  rotates in anticlockwise direction with respect to  FIG. 3 , the leaf springs  36  preventing a reverse rotation. Furthermore, the hub  40  rotates in the direction of the arrow P of  FIGS. 4 and 5 . The metal strips  46  are unwound from the outer stack and wound up onto the hub  40  in opposite direction. The resistance torque produced is largely determined by the bending resistance of the metal strips  46 , which are bent twice at the respective bending point. First, they are bent inwards to a larger extent than corresponds to their former curvature, and subsequently they are bent back in the opposite direction, when the metal strip concerned conforms to the curvature of the hub  40 .  
         [0028]     As long as the holding pawl  58  is in its position shown in  FIG. 4 , the two energy converter coils  42 ,  44  are connected in parallel, and the resistance torque provided by the same is added up. If it is desirable to reduce the force acting in the belt webbing, the second energy converter coil can be deactivated. For this purpose, the gas-generating pyrotechnical charge  64  of the actuator  62  is ignited, whereupon the piston  60  swivels the holding pawl  58  into the position indicated in  FIG. 4 . In this position, the outer ring  54  can rotate freely, so that the second energy converter coil  44  no longer provides any resistance torque. Thus, a lower tensile force is required in the safety belt for withdrawing belt webbing from the belt reel.