Abstract:
A force-limiting device ( 4 ) for a seat belt device having at least two parts ( 5, 11, 15 ) controlled with a frequency-dependent, defined movement for energy absorption, wherein the parts ( 5, 11, 15 ) that move the parts ( 5, 11, 15 ) undergo an undulating movement (Q) directed perpendicular to the feed movement (V) in the course of which the gearings ( 8, 10, 18, 19 ) alternatingly engage and disengage. An open spring washer exerts force to move the parts ( 5, 11, 15 ) toward one another and can be automatically increased after a preset belt extraction length. The spring washer ( 6 ) applies a varying spring force to the parts ( 5, 11, 15 ) moving toward one another during the movement.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application No. 10 2013 220 949.5, filed Oct. 16, 2013 and PCT International Application No. PCT/EP2014/071867, filed on Oct. 13, 2014. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a force-limiting device for a safety belt system. 
         [0003]    BACKGROUND 
         [0004]    Known from WO 2006/108451 A1 is, for example, a speed-controlled force-limiting device for a seat belt retractor which has a plurality of masses disposed on the frame of the belt retractor that are made to oscillate. The masses are pivotably supported on the retractor frame and engage alternately, during the force-limiting retracting of the seat belt, with two teeth in an annular gear connected to the belt shaft. On the whole, due to the oscillating masses disposed on the retractor frame, the belt retractor is complex in production and expensive. In addition, due to the oscillating masses the belt retractor requires a relatively large installation space. German patent application DE 10 2008 049 931.5 describes an improved force-limiting device operating on the same principle, wherein two toothed parts are moved in a force-limited manner toward each other, and one of the parts here executes an undulating feed motion during which the toothed parts alternatingly engage and disengage. The oscillating masses are replaced in this embodiment by the part executing the undulating feed movement, so that the force-limiting device described here requires a substantially smaller installation space and has a substantially simpler design. In one exemplary embodiment the part executing the undulating feed movement is realized by a toothed disc which is connectable in a friction-fit manner to the belt shaft. Between the toothed disc and the part against which the toothed disc executes the feed movement, a spring washer can be provided which exerts an axial contact force on the toothed disc, whereby the force-limiting level of the force-limiting characteristic is increased. 
         [0005]    The force-limiting level of the force-limiting device results automatically from the to-be-diminished kinetic energy of the occupant during forward displacement, wherein the basic characteristic of the force-limiting path is strictly determined by the parameters of the toothed disc, such as the number of teeth, the separation of the teeth, the mass of the toothed disc and by the spring properties of the spring washer. 
         [0006]    There are legal requirements, however, such as the US requirements for restraining forces for passengers in the back seats, which mandate an increase of the restraining forces to a higher force-limiting level after a preset length of seat-belt extraction. Such a force-limiting characteristic is not achievable with one of the above-described force-limiting devices. 
         [0007]    In the patent application DE 10 2012 214 521, features are provided to resolve this problem using which the force required to move the parts toward one other can be automatically increased when a preset length of seat-belt extraction is reached. According to one embodiment of the proposed solution these means are formed by an open spring washer which applies a variable spring force to the parts moving toward one other during movement. In addition, during the movement the open ends of the spring washer come into abutment with the stop surfaces of the parts moving toward one another. Using the proposed solution, the spring washer is widened after the abutment of the ends on the stop surfaces, and the undulating shape of the spring washer is enhanced, so that as a result an increased spring force is applied to the two parts that are being moved toward one another, and the force-limiting level increases. 
         [0008]    The object of the invention is to create a force-limiting device of the above-mentioned type which makes possible a progressive force-limiting course of the restraining forces, which can be constructed simply and has greater functional security. 
         [0009]    In order to achieve this object, the invention proposes a force-limiting device having the features described herein by the Figures, and the accompanying description. 
       SUMMARY 
       [0010]    According to the basic idea of the invention, it is proposed that means are provided, using which the force required to move the parts toward one another, from a preset length of belt extraction on, can be automatically increased, and that the means are formed by an open spring washer that applies a varying spring force to the parts that are moving toward one another during movement, wherein one of the parts is displaceably supported on a hub perpendicular to the feed direction, and that on one of the parts a first stop surface, and on the hub a second stop surface is provided, with which the open end of the spring washer comes into contact during the movement. The advantage of the proposed solution is that the force-limiting level of the force-limiting characteristic is automatically increased solely due to the movement from a preset length of belt extraction on. This exploits the advantage that the force-limiting level of the force-limiting device can be varied simply by the force of the parts moving toward one another, because this force is one of the key parameters for the energy dissipation on which the force-limiting device is based. In this way, the parts themselves do not have to be modified in order to change the oscillatory behavior and the force-limiting level. It is particularly important here that the force is variable from a preset length of belt extraction on, so that external actuation is not required. Furthermore, the force is increased by very simple mechanical means, namely an open spring washer whose open ends, from a preset length of belt extraction on, come into contact with stop surfaces and are in this way widened and deformed, so that the spring force of the parts being moved toward one another automatically increases. Because the end assigned to the part that performs the oscillatory movement perpendicular to the feed movement can now be fixed to a hub that serves as a mount for the transversely displaceable part, this end intentionally no longer takes part in performing the perpendicular oscillatory movement. On the contrary, the part that executes the perpendicular oscillatory movement can in this way also carry out a movement relative to the end that is fixed to the hub, so that the spring washer, excluding the middle area, is periodically compressed and decompressed, whereas the ends do not perform any movement perpendicular to the feed movement and are thus not at risk of slipping off one of the stop surfaces. In this way, the functional safety of the force-limiting device can be significantly improved. 
         [0011]    It is further proposed that the hub be formed by an annular gear that is attached to a belt shaft of a belt retractor for conjoint rotation with the belt shaft, or by an annular gear with external teeth that is indirectly or directly blockable for conjoint rotation with a retractor frame of the belt retractor, on which hub one of the two parts that are moved toward one another is supported in a transversely displaceable manner. The annular gear serves the purpose of fixing or carrying along one of the parts that are moved toward one another and thus performs the same relative feed movement in the circumferential direction of the respective other part of the force-limiting device, while the ring, at the same time, serves the purpose of bearing the part that performs the oscillating movement directed perpendicular to the feed direction so that the ring does not also perform this movement. In practical terms, the annular gear forms the shift point at which the feed movement in the circumferential direction and the transversely directed oscillation movement are decoupled from each other. Then in addition, due to the formation of the stop surface on the annular gear, only the feed movement in the circumferential direction is transferred to the end of the annular gear, which leads to the desired increase in spring force, whereas the perpendicular movement that causes the spring end to slip off is intentionally not transferred. 
         [0012]    It is further proposed that the second stop surface is provided in a recess and/or on a projecting tooth of the external teeth of the annular gear. Using the proposed solution the external teeth can also be favorably exploited during the production process to form the stop surface, by some of the teeth being left out and/or one of the teeth being extended outward. 
         [0013]    Furthermore, according to a further preferred embodiment of the invention, a cutout is provided in the part that is transversely displaceable on the hub, the end of the annular gear that comes to rest against the second stop surface of the hub engaging into said cutout. Using the proposed solution, the part that is transversely displaceable on the hub can perform the movement relative to the hub without the movement being hindered by the fixed end. 
         [0014]    It is further proposed that the ends of the spring washer are angled in the direction of the parts that are moving toward one another, and that in one of the parts and in the hub, pockets are provided respectively, wherein the stop surfaces are provided, and that the ends of the spring washer, after a preset length of belt extraction, are automatically stopped by the stop surfaces in the pockets. The advantage of the proposed solution is that the ends of the spring washer, after a preset length of belt extraction, due to their form and the spring properties of the spring washer, automatically slide into the pockets and come to rest against the stop surfaces. The pockets are expediently shaped as identical in form to the angled ends, so that in them, the ends come into contact with largest possible circumferential surface and are supported as well as possible during further movement. The pockets, corresponding to the outwardly angled ends, are designed as slanted recesses, so that the angled ends, corresponding to their form, are accommodated in the pockets, and the spring washer, in its region between the ends, again has a flat course and fits over as large an area as possible against the two parts that are moved toward one another. 
         [0015]    It is further proposed that one end of the spring washer be firmly connected to one of the parts in the direction of movement of the parts that are moving toward one another, and that it have a spiral course starting from one end. Due to the rigidly held end and the spiral course of the spring washer, the spring washer can be coiled during the movement into several windings resting on each other, wherein the spring force exerted to the side, e.g. by the reinforcement of the undulation or by a side bulge of the windings, is increased. 
         [0016]    In addition it is recommended that a spiral guide track be provided into which the spring washer can be inserted. In another preferred embodiment, said guide track is formed by a groove. Using the spiral guide track, the course of the spring washer into which the latter is transported is in practice preset. This ensures that movement in an uncontrolled manner is impeded by a clamping of the spring washer, causing the force-limiting level to be undesirably increased to too high a level. To that end a groove, in particular, is suitable, since the spring washer can be guided therein especially well and continuously. 
         [0017]    The groove here can feature a decreasing depth in the direction of movement of the open end so that the compression force exerted by the spring washer between the two parts and thus the force-limiting level can also be increased with an increasing belt extraction length. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be explained below using a preferred embodiment with reference to the accompanying Figures: 
           [0019]      FIG. 1  shows a belt retractor with an inventive force-limiting device, 
           [0020]      FIG. 2  shows an oblique view of a belt shaft with a cutaway force-limiting device; 
           [0021]      FIG. 3  shows a side view of a cutaway force-limiting device; and 
           [0022]      FIG. 4  shows an annular gear with an oscillating disc deposed on it. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows a belt retractor  1  of a seat belt system with a belt shaft  2  rotatably supported in a retractor frame  3 , on which frame  3  a force-limiting device  4  is disposed. In addition a profile head  13  is provided, which is the carrier of a locking ratchet  9  that is actuated when a preset belt extraction acceleration or a preset vehicle deceleration is exceeded and locks the profile head  13  firmly to the vehicle in a gearing of the retractor frame  3 . 
         [0024]    The force-limiting device  4  includes in its basic construction two fixed parts  5  and  15  with a movable part  11  between the fixed parts  5  and  15 . The part  15  simultaneously forms the housing of the force-limiting device  4  and is connected to the belt shaft  2  for conjoint rotation therewith. On its radial exterior, the part  15  has axially aligned fingers between which the part  5  engages with radially protruding fingers  25  and thus is also connected to part  15  for conjoint rotation therewith and thus also to the belt shaft  2 . The movable part  11  is guided with a radially inward gearing  26  in an axially displaceable manner on an annular gear  7  and is simultaneously connected in the circumferential direction to the annular gear  7  for conjoint rotation therewith. The annular gear  7  is in turn connected to the profile head  13  for conjoint rotation therewith, so that the part  11  with a blocked profile head  13  is to be viewed as also blocked in the circumferential direction. The fixed parts  5  and  15  are respectively provided with annular, axially aligned gearings  10  and  8 , as can be seen in  FIG. 3 . The gearing  10  on part  5  and the gearing  8  on part  15  are each positioned such that they are opposite the ring-shaped, axially aligned gearing  18  or  19  positioned on the movable part  11 . The separation between the movable part  11  and the fixed parts  5  and  15  in the axial direction is selected in each case such that at least one pair of opposite gearings  10  and  19  and/or  18  and  8  are engaged. 
         [0025]    Upon actuation of the blocking device of the belt shaft  2 , the locking ratchet  9  is extended and blocks the profile head  13  and thus the part  11  in the belt extraction direction. The force-limiting device  4  is actuated by the belt extraction force acting in an accident situation during forward displacement of the passenger when the belt extraction force preset by the design of the force-limiting device  4  itself is exceeded. In the process, parts  15  and  5  are forced into a rotary movement with respect to part  11 . Because of the pair-wise engagement of gearings  10  and  19  and  18  and  8  with each other, the part  11  is forced by the teeth flanks gliding past each other into an oscillatory movement perpendicular to the feed movement of parts  15  and  5 , during which it is periodically braked and accelerated. This oscillatory movement is the physical cause for the energy dissipation underlying the force-limiting device  4 , which thus also determines the force-limiting level. The part  11  can also be termed an oscillating disc based on its motion by means of its oscillatory motion it ensures the feed movement of parts  5  and  15  and thus the force-limited rotary motion of the belt shaft  2  in the belt extraction direction. 
         [0026]    Between part  11  and part  5 , an open undulating spring washer  6  is provided, which is supported on part  5  and pushes part  11  against part  15 . Since part  11  must periodically overcome the axial spring force exerted by the spring washer  6  in order to perform the above-described oscillatory motion, the force-limiting level of the force-limiting device  4  is determined in the start phase and during the additional belt extraction movement by the spring force of the spring washer  6 . For example, the belt extraction force in the start phase can be 3-4 kN. 
         [0027]      FIG. 2  shows the belt shaft  2  with a cutaway force-limiting device  4 . The spring washer  6  is open and its free ends  16  and  17  are angled to the side, so that the free ends  16  and  17  exert an additional axial force between parts  5  and  11 , or as the case may be lie spring-tensioned against the parts  5  and  11 . In the initial position before actuation of the force-limiting device  4 , the ends  16  and  17  are located in the direction of rotation behind a pocket  20  and  21  respectively of the annular gear  7  and the part  5 . The pockets  20  and  21  are respectively formed as inclined recesses, which can also be seen in  FIG. 3 . Upon activation of the force-limiting device  4 , the parts  15  and  5  together, with the belt shaft  2 , rotate with respect to part  11 , which is blocked in the circumferential direction, and the profile head  13 . The force-limiting level is determined in this phase by the mass design of the system and the spring force of the spring washer  6 . During the rotary motion of parts  15  and  5 , the spring washer  6  is stationary relative to them, except for the transversely directed oscillating movement. 
         [0028]    After the belt shaft  2  with parts  15  and  5  has been rotated almost one full rotation, in this case a rotation of 340 degrees, with respect to part  11  and thus with respect to the spring washer  6 , the end  17  slides automatically into the pocket  20  because of its outward bend, its front surface thereby coming to rest against a stop surface  22  provided in the pocket  20  of part  5 . During the continuing rotation of part  5  the spring washer  6  is carried along with it via the stop surface  22 , so that the end  16  completes a rotary motion with respect to part  11  and the annular gear  7 . During this movement the force-limiting level of the start phase continues to act. The end  16  then also slides, after completion of almost one complete rotation, in this case also 340 degrees, because of its concave shape, through a cutout  24  in the part  11  into the pocket  21  and is also stopped against a frontal stop surface  23  of the annular gear  7 , as can be seen in the depiction in  FIG. 3 . Starting at this position, the spring washer  6  is fixed in the circumferential direction with respect to both part  5  and to part  11 , so that an additional rotary motion of part  5  with respect to part  11  and thus also of the belt shaft  2  with respect to the profile head  13  is only possible by a deformation of the spring washer  6 . The spring washer  6  thereby rises and increases the axial spring force between the part  5  and the part  11 , so that the force-limiting level abruptly rises, in this case to above 6 kN. Thus with the invention using a force-limiting device  4 , a progressive force-limiting course can be implemented with a shift point defined by the location of the pockets  20  and  21  with respect to the free ends  16  and  17  and the rotational angle consequently to be covered, which shift point in this embodiment corresponds to 680 degrees or about 1.9 revolutions. Depending on the mechanical characteristics, the shape and the arrangement of the spring washer, both the characteristics of the force-limiting increase, as well as the extent of the force-limiting increase, can be regulated. Both continuous and abrupt increases of the force-limiting level are thereby possible. 
         [0029]    Corresponding to the inventive approach, the pocket  21  is intentionally arranged in the annular gear  7  relative to which the part  11  performs the oscillation movement that is directed perpendicular to the feed direction. The end  16  of the spring washer  6  engages into the cutout  24 , so that the part  11 , in this case the oscillating disc, can perform the oscillation movement directed perpendicular to the annular gear  7  without being hindered in doing so by the spring washer  7 . The ends  16  and  17  are thus moved in a purely rotary motion relative to each other, because they also perform the feed movement only in the circumferential direction. In this way the likelihood that the ends  16  and  17  can slip out of the pockets  20  and  21  is diminished. The pocket  21  on the annular gear  17  is formed by leaving out one or more teeth in the external gearing of the annular gear  7  and by outward enlargement of one tooth  28 , the outwardly enlarged tooth  28  here forming a lateral stop surface  23 , against which the end  16  of the spring washer  6  comes to rest on the face side. 
         [0030]      FIG. 4  shows the annular gear  7  with the part  11  and the laterally stopped spring washer  6 . In part  11 , the oscillating disc can be seen in a radial inner section, the cutout  24 , which is formed by punching. In addition, a plurality of the teeth in the external gearing have been left out or shortened, and one tooth  28  has been lengthened. In this case, the omission of the teeth forms the pocket  21 , which is delimited by the enlarged tooth  28  on which the stop surface  23  is also provided. 
         [0031]    The part  11  is supported in a transversely displaceable manner on an external gearing of the annular gear  7  and is rotationally fixed in the circumferential direction so that the part  11 , with a blocking of the profile head  13  and the annular ring  7 , can only oscillate perpendicular to the annular gear  7 . The spring washer  6 , as described above, after the end  17  is fixed to the part  5 , is carried along in the circumferential direction, so that the end  16  of the spring washer  6  is moved in the direction of the pocket  21 . Because the end  16  is angled in the direction of the part  11  and is thus held spring-tensioned against part  11 , the end  16 , when it reaches the cutout  24 , automatically slides through the cutout  24  into the pocket  21  and in that way, during the further movement, is held on its front side against the stop surface  23  or the tooth  28 . Then a further movement of the belt shaft  2  with the parts  5  and  15  relative to the part  11  and the annular ring  7 , or as the case may be the profile head  13 , is only possible with a deformation of the spring washer, whereupon the spring force it exerts between the parts  11  and  5  increases. 
         [0032]    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.