Abstract:
A belt retractor has a belt spool ( 12 ) rotatably mounted in a frame ( 10 ), a disc adapted to be blocked against rotation on the frame ( 10 ), and a force-limiting arrangement. The force-limiting arrangement includes a cutting body ( 22 ) which is adapted to be guided into a coupling position in which the cutting body ( 22 ) couples the belt spool ( 12 ) to the disc ( 16 ) such that upon a relative rotation between the disc ( 16 ) and the belt spool ( 12 ), the cutting body ( 22 ) cuts material. The force-limiting arrangement has a guiding-in mechanism which guides the cutting body ( 22 ) into the coupling position as a function of accident-related data.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a belt retractor, in particular to a belt retractor comprising a force-limiting arrangement.  
       BACKGROUND OF THE INVENTION  
       [0002]     From DE-A-103 43 534 a belt retractor is known comprising a belt spool rotatably mounted in a frame, a disc adapted to be blocked against rotation on the frame, and a force-limiting arrangement with a cutting body which is adapted to be guided into a coupling position in which the cutting body couples the belt spool to the disc such that upon a relative rotation between the disc and the belt spool, the cutting body cuts material. In this belt retractor, in addition to a first force limitation by means of a torsion rod, at the same time a second force limitation takes place by cutting working of material, which in particular through the parameters of width and depth of cut partially permits a progressive, degressive or constant force level pattern. The force level and its pattern, though, are identical for all occupants.  
         [0003]     It is an object of the invention to provide a belt retractor which allows a more flexible setting of the force level.  
       BRIEF SUMMARY OF THE INVENTION  
       [0004]     The belt retractor according to the invention comprises a belt spool rotatably mounted in a frame, a disc adapted to be blocked against rotation on the frame, and a force-limiting arrangement with a cutting body which is adapted to be guided into a coupling position in which the cutting body couples the belt spool to the disc such that upon a relative rotation between the disc and the belt spool, the cutting body cuts material, the force-limiting arrangement having a guiding-in mechanism which guides the cutting body into the coupling position as a function of accident-related data. Coming into consideration as accident-related data are, in particular, body data of the vehicle occupant, seating position data or data which are representative of the severity of the accident. This data can be determined for example by sensors or in another manner, such as for example by determining the length of the belt webbing withdrawn or the angular acceleration or rotational speed of the belt spool before a blocking. The belt retractor according to the invention therefore allows a force limitation which can be better adapted to the respective circumstances. In particular, the weight and/or the height of the occupant can be taken into account to a sufficient extent for an optimized setting of the force limitation.  
         [0005]     An advantageous development of the belt retractor according to the invention makes provision that the guiding-in mechanism has at least one oblique plane formed on the belt spool or on the disc, the cutting body being adapted to slide on the disc into the coupling position.  
         [0006]     According to a first embodiment, the cutting body is an inertial body, or the cutting body is coupled to an inertial body, which on exceeding a particular angular acceleration or rotational speed and subsequent blocking of the belt spool is moved into the coupling position owing to its inertia or the centrifugal force acting on it. Thereby, it can be achieved that the additional force limitation caused by material cutting takes place only for occupants with a high body weight. In the case of an abrupt deceleration of the vehicle, shortly before the blocking of the belt spool, in the case of a heavy occupant a more rapid belt webbing withdrawal takes place than in the case of a light occupant, who shows a “slower” forward displacement. Only in the first case are the inertia forces onto the cutting body great enough for a guiding-in into the coupling position.  
         [0007]     It is expedient to provide a locking mechanism which holds the cutting body in an initial position. Thereby it is ensured that the cutting body under normal circumstances is not unintentionally guided into the coupling position.  
         [0008]     The locking mechanism can have a pre-stressing spring and/or a mechanical stop.  
         [0009]     According to a second embodiment of the belt retractor according to the invention, the guiding-in mechanism is coupled to a switching mechanism of the belt retractor with which, on rotation of the belt spool, switching functions can be carried out. Such a switching mechanism is known per se and is based on a planetary gear or a control disc coupled to the belt spool via a reduction gear. The switching mechanism is used in particular for a so-called child safety function which is usually activated after a complete withdrawal of the belt webbing. It ensures that the fully withdrawn belt webbing in fact can be wound onto the belt spool again, but can no longer be withdrawn therefrom, in order to thus make possible a reliable fastening of a child&#39;s seat on a vehicle seat by means of a safety belt. The underlying mechanism, which provides for a switching process dependent on the length of the belt webbing withdrawn, can be made use of for the invention. Thus, after withdrawal of a particular belt webbing length, a switching process can be carried out, which leads to a guiding-in of the cutting body into the coupling position. As the length of belt webbing withdrawn is representative of the size of an occupant, therefore automatically a size-dependent setting of the force limitation level takes place.  
         [0010]     A coupling of the guiding-in mechanism to the switching mechanism can take place in that the guiding-in mechanism has a guiding-in element, connected with the cutting body, which is movable by the switching mechanism. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  shows a diagrammatic sectional view of a belt retractor with a force-limiting arrangement;  
         [0012]      FIG. 2  shows a diagrammatic sectional view of a guiding-in mechanism of the force-limiting arrangement according to a first embodiment;  
         [0013]      FIGS. 3, 4  show two variants of the guiding-in mechanism of  FIG. 2 ;  
         [0014]      FIG. 5  shows a diagrammatic sectional view of a guiding-in mechanism of the force-limiting arrangement according to a second embodiment;  
         [0015]      FIG. 6  shows a variant of the guiding-in mechanism of  FIG. 5 ;  
         [0016]      FIG. 7   a  shows a diagrammatic end face view of a guiding-in mechanism of the force-limiting arrangement according to a third embodiment;  
         [0017]      FIG. 7   b  shows a sectional view along the line B-B in  FIG. 7   a;  and  
         [0018]      FIG. 8  shows a diagram of forces of the force-limiting arrangement according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]     The belt retractor shown diagrammatically in  FIG. 1  has a frame  10  and a belt spool  12  rotatably mounted in the frame  10 . A disc  16 , which can rest against the frame  10  so as to be blocked against rotation by a blocking mechanism  18 , is joined to a flange  14  of the belt spool  12 . The disc  16  is connected for joint rotation with the flange  14  of the belt spool  12  up to a certain torque, for example by shear pins. In the hollow interior of the belt spool  12 , a torsion rod  20  is arranged, which is coupled at one axial end for rotation with the disc  16 , and at the opposite end for rotation with the belt spool  12 . A cutting body  22  rests on the disc  16 . The cutting body  22  can be raised by a guiding-in mechanism in the axial direction (in relation to the rotation axis of the belt spool  12  and the disc  16 ) from an initial position into a coupling position, in which it projects axially beyond the end face of the disc  16  facing the flange  14 .  
         [0020]     In  FIG. 2 a  first embodiment of the belt retractor is shown, in which the cutting body  22  rests inter alia on two oblique planes  24  formed in a depression of the end face of the disc  16 . The cutting body  22 , which is illustrated in  FIG. 2  in an initial position, is held by a locking mechanism in the axial direction. The locking mechanism comprises here a mechanical stop in the form of an extension  26 , formed integrally with the cutting body  22 , which engages into an associated mounting  28  on the disc  16 . In addition, the cutting body  22  is secured by a bolt  30  (or a screw) screwed into the cutting body  22 , which penetrates the disc  16  in an oblong hole  32 . A spring  36  arranged between the disc  16  and the head  34  of the bolt  30  braces the cutting body  22  towards the disc  16 . Generally, one of the two measures is sufficient in order to hold the cutting body  22  in the initial position under normal circumstances.  
         [0021]     In a case of blocking, triggered in a vehicle-sensitive or belt webbing-sensitive manner, the disc  16  is blocked by the blocking mechanism  18  against rotation on the frame  10  of the belt retractor. Thereby also the belt spool  12  is coupled non-rotatably to the frame  10  via the torsion rod  20 , so that further rotation of the belt spool  12  in the belt webbing withdrawal direction A is prevented. On exceeding a predetermined belt load, however, the torsion rod  20  twists and permits a belt force limiting rotation of the belt spool  12  relative to the disc  16 .  
         [0022]     In addition to the known belt force limitation by the torsion rod  20 , an additional force limitation can take place through a guiding in of the cutting body  22  into the coupling position. Whether or not such a guiding in takes place depends on the angular acceleration or the rotational speed of the belt spool  12  before its blocking. The belt webbing withdrawal taking place before the blocking of the belt spool  12  brings about a rotation of the belt spool  12 , which is caused by the forward movement of the occupant in the case of an abrupt deceleration of the vehicle, the extent of the angular acceleration or the rotational speed depending in particular on the weight of the occupant.  
         [0023]     The oblique planes  24  and the cutting body  22  are coordinated with each other such that on exceeding a particular rotational speed of the belt spool  12  in the belt webbing withdrawal direction and subsequent blocking of the belt spool  12 , the cutting body  22 , owing to its mass moment of inertia, overcomes the static friction and the pre-stressing force of the spring  36  and slides on the oblique planes  24  into the coupling position. With this movement of the cutting body  22 , the extension  26  comes out of engagement with the mounting  28 , and the bolt  30  is displaced in the oblong hole  32  until the cutting body  22  comes to lie on the elevated support surfaces  38 . A cutting edge  40  of the cutting body  22  projects in this elevated position into a recess  42  in the flange  14  of the belt spool  12 . Upon a rotation of the belt spool  12  relative to the disc  16  in the belt webbing withdrawal direction, accompanied by twisting of the torsion rod  20 , the cutting edge  40  of the cutting body  22  comes into engagement with the material of the flange  14 , a shoulder  44  of the recess  42  pressing the cutting body  22  against a stop  46  of the disc  16 . The bolt  30  is dimensioned such that with this thrust movement it is sheared off. The sheared off part of the bolt  30  is pushed into a free space in which previously a part of the cutting body  22  was received. A further relative rotation between the belt spool  12  and the disc  16  is now only possible by the cutting edge  40  of the cutting body  22  cutting a chip out of the flange  14 .  
         [0024]     Therefore, an energy conversion takes place through material cutting, which is effected parallel to the energy conversion through the twisting of the torsion rod  20 . The profile of the force level of the additional limitation as a function of the rotation angle of the belt spool  12  is determined substantially by the width and depth of cut.  
         [0025]     In  FIGS. 3 and 4  two variants of the guiding-in mechanism are shown, in which the cutting body  22  is held by an inertia spring  48  and respectively  50  arranged between the flange  14  of the belt spool  12  and the cutting body  22  and respectively between the stop  46  of the disc  16  and the cutting body  22 .  
         [0026]     Whereas in the first embodiment shown in FIGS.  2  to  4  the guiding-in mechanism is formed substantially solely by the oblique planes  24 , in the second, similarly constructed embodiment shown in  FIGS. 5 and 6 , the guiding-in of the cutting body  22  takes place by means of a switching element  52  (only illustrated symbolically). The switching element  52  is part of a switching mechanism known per se, with which switching functions can be carried out on rotation of the belt spool  12 . The switching mechanism is designed so that the switching element  52  carries out a switching movement after withdrawal of a particular length of belt webbing from the belt spool  12 . In so doing, the switching element  52  engages the bolt  30  such that the latter raises the cutting body  22  into the coupling position. In this embodiment, the bolt  30  therefore serves as guiding-in element, but just as in the first embodiment is sheared off by the thrust movement of the flange  14  of the belt spool  12 .  
         [0027]     In contrast to the first embodiment, in this embodiment a guiding-in of the cutting body  22  does not take place as a function of the angular acceleration or the rotational speed of the belt spool  12  before its blocking, but rather as a function of the length of the belt webbing withdrawn or the belt webbing remaining on the belt spool  12 , which depends substantially on the size of the occupant.  
         [0028]      FIG. 6  shows a variant of the second embodiment. Here, the pre-stressing spring  36  is arranged between the disc  16  and the cutting body  22 , so that the cutting body  22  is pre-stressed into the coupling position, but is held in the initial position by the switching element  52 . By an actuation of the switching element  52 , the cutting body  22  is guided from the initial position into the coupling position shown in  FIG. 6 .  
         [0029]      FIGS. 7   a  and  7   b  illustrate a third embodiment the guiding-in mechanism of which is constructed similarly to that of the first embodiment. Here, the cutting body  22  is guided into the coupling position as a function of the centrifugal force F Z  acting on the cutting body  22 . Depending on the physical constitution of the vehicle occupant, in fact a significant difference in the speed of belt webbing withdrawal and hence in the rotational speed of the belt spool  12  or the disc  16  can be seen during belt webbing withdrawal. The mechanical implementation of the guiding-in mechanism of this embodiment proceeds from the finding that this speed can only differ when a resistance has arisen at the belt webbing, because it is not until then that the specific kinetic energy of the occupant also has an influence (E=½ mv 2 ).  
         [0030]     Accordingly, provision is made that initially a force limitation takes place on a basic level by means of the torsion rod  20  only. (This level could be designed, for example, as a predefined ideal level for a so-called 5% occupant.) But if the rotational speed of the belt spool  12  exceeds a specific limiting value, the cutting body  22  is additionally connected owing to the higher centrifugal force F Z , so that the level of force limitation is raised. (This level may be designed as a predefined ideal level for 50% or 95% occupants.) If, on the other hand, the rotational speed of the belt spool  12  remains below the predefined limiting value, the switching process does not occur and the force limitation is effected at the lower level.  
         [0031]      FIG. 8  shows the different levels of force limitation in a diagram in which the force limitation is illustrated as a function of the belt webbing withdrawal.  
         [0032]     In all embodiments, the guiding-in of the cutting body  22  into the coupling position can also take place by means of a control element which is coupled to the cutting body  22  and is moved as a function of the angular acceleration/rotational speed of the belt spool or the belt webbing withdrawal length.  
         [0033]     The pre-stressing springs  36 ,  48 ,  50  of the different embodiments/variants can be designed such that they serve not only for securing the cutting body or a control element, but also for setting the inertia, i.e. the necessary angular acceleration of the belt spool for a guiding-in of the cutting body into the coupling position, and hence the moment of guiding in. The pre-stressing springs can be formed in particular by the usual inertia springs in belt retractors.  
         [0034]     It is also possible to combine features of different embodiments with each other. In addition, the cutting body  22  can, as an alternative, be arranged on the belt spool  12  and cut material on the disc  16 .