Abstract:
In at least one embodiment of the present invention, a force limiting device for a motor vehicle is provided. The force limiting device is adopted for adjusting the absorption of a force between two parts moving relative to one another during a dangerous situation in the motor vehicle. The force limiting device comprises a kinematic energy absorption device configured such that for a predefined profile of a speed difference between the two parts moving relative to one another different force limiting levels are produced as a function of mass and momentum of the two moving parts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to PCT patent application WO 2006/108451 filed Dec. 15, 2005 and DE patent application 10 2005 016 822.1 filed Apr. 12, 2005. 
     FIELD OF THE INVENTION 
     The present invention relates to a force limiting device for use in a motor vehicle and more particularly to a force limiting device, which is designed for adjusting the absorption of a force between at least two parts moving relative to one another during a dangerous situation or dynamic condition in the motor vehicle. 
     BACKGROUND OF THE INVENTION 
     Force limiting devices are in use, especially in vehicle safety systems, and make it possible to absorb a belt withdrawal force occurring in the vehicle safety system in the event of a crash. The force limiting device enables a limited belt discharge relative to a component holding the belt. These types of force limiting devices are used both in vehicle safety systems that operate rotationally in the form of preferably self-locking seatbelt retractors and also seatbelt retractors combined with tensioners or in the form of pure tensioning devices, such as in the form of end-fitting tensioners, as well as in vehicle safety systems that operate linearly, in which the belt is held on a fitting piece, which moves linearly, by means of a piston/cylinder arrangement for example, either for tensioning or for adjusting a controlled belt discharge. 
     It is desirable, however, to also integrate a force limiting device at other locations in a motor vehicle in order to use relative motions, brought about by a crash, between two parts of the motor vehicle for energy dissipation, for example, between a bumper and the vehicle frame. 
     A force limiting device provided in a seatbelt retractor as part of a vehicle safety system is known from EP 1 222 097 B1 for example. In this case, the force limiting device consists of a torsion bar, one end of which is connected to the belt shaft and the other end of which can be fixed to the housing by means of a suitable locking device. If the seat-belted vehicle occupant is displaced forward in the event of an accident because of the vehicle acceleration or deceleration that occurs, resulting in a corresponding belt withdrawal force acting on the belt shaft, then the belt shaft can rotate by a certain amount, with the torsion bar twisting at the same time, so that a belt discharge will occur over a corresponding rotary travel of the belt shaft. The force that restrains the occupant in his/her forward movement is thereby absorbed. The known belt retractor provides for a second force limiting device in the form of an inertial mass which can be coupled to the belt shaft, the effect of the inertial mass being superimposed on the responding effect of the torsion bar. Depending on the speed at which the seat-belted person moves forward and also depending on the occupant data of the seat-belted person, the inertia of the inertial mass which has been put into rotation by the belt shaft becomes effective such that an additional energy-dissipating and/or force-limiting component becomes effective. 
     However, the known seatbelt retractor and its force limiting device has the disadvantage that the effect of the force limiting device depends on the occupant, particularly on the occupant&#39;s size, weight and seat position in the vehicle, as well as on the severity of the accident and thus, on the momentum conveyed to the occupant in an accident and on the resulting acceleration forces. The effect of the force limiting device therefore results in different force levels, which can be adjusted to the seat-belted occupants and controlled only at great expense in order to prevent the seatbelt system from placing too great a strain on the occupants during an accident. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to design a force limiting device of the type described above in such a manner that its response and effect occur independently of the boundary conditions of an accident. When using this type of force limiting device in a vehicle safety system protecting a vehicle occupant in particular, the force limitation should furthermore be effective independently of the data pertaining to the body and seat position of the occupant. 
     Employing the principles of the present invention is the force limiting device comprises a kinematic energy absorption device which is designed in such a manner that, for a predefined profile of the speed difference between the parts moving relative to one another in the event of danger, different force limiting levels can be realized as a function of the mass of the moving parts and the momentum occurring in the event of danger. 
     At least one embodiment of the present invention frees itself from the basic concept of the known force limiting device, of controlling the force limiting device as a function of the established boundary conditions of an accident, in motor vehicle safety systems in particular, and also as a function of the belt withdrawal force emanating from the occupant. Instead, it makes use of the knowledge that the effects of the acceleration and deceleration forces occurring in an accident last for a limited time, such as for example, on the average of approximately 70 ms. The force limiting system can thus be designed according to the invention such that it will operate during this time period and that a maximum relative motion of the corresponding parts to one another is appropriately defined. The design of the force limiting device makes it possible to establish particular speeds of relative motions, such as for different vehicle designs. Since the possible relative motion of the parts to one another may be defined by the adjusted speed of their motion and the time period in which the force limiting device operates, the relative motion takes place independently of the size of the acting accelerations and forces and may be determined solely by the time period that can be structurally adjusted according to example embodiments of the invention or by the time period corresponding to the acceleration and deceleration forces that occur in the event of an accident. 
     If a force limiting device is used in a vehicle safety system comprising a belt spool that protects a vehicle occupant and rotates relative to a frame fixed to a vehicle, then in the event of an accident there will be a delayed deceleration of the vehicle and the occupant because of the existing elasticities, such as for example, in the seatbelt system and in the seat structures. This delayed deceleration results in a speed difference between the motion of the vehicle and the motion of the occupant. The speed difference, which may be vehicle-specific, can be determined by appropriate tests. In one aspect of the present invention, the belt spool presets a constant withdrawal speed for the belt strap, so that the speed difference between the vehicle and the occupant is thereby constantly adjusted with the result that an additional acceleration starting during the belt withdrawal cannot arise and the absolute deceleration of the occupant will correspond to the absolute deceleration of the vehicle. The force limiting unit can thus be designed accordingly so that the occupant will pass through a predefined forward displacement path in the vehicle during a crash sequence, independently of other external parameters. With the preset withdrawal speed, the impact speed of the occupant, for example onto an airbag, may also be approximately known so that inflation behavior of the airbag can thereby be adjusted. In another example, if the vehicle safety system comprises a collapsible steering column, the plunging of the steering column can be adjusted to correspond to the impact speed of the vehicle occupant, so that the vehicle occupant is strained as little as possible overall. 
     In an application of this type, the discharge speed defined by the frequency of the mass system placed into oscillation may be used in the event of a crash to control a limited discharge of the seat belt holding the vehicle occupant in order to adjust an absorption of the belt withdrawal force. 
     To this end, a first embodiment of the invention provides that a belt strap withdrawal occurring in the event of an accident provides the excitation of the mass system so that it achieves its oscillation frequency and the belt discharge is determined as a function of the time period of the acting belt withdrawal force, wherein it can be provided that mechanically interacting components can convert the belt discharge into the oscillatory excitation of the mass system. The basic principle of this example embodiment is thus based on the fact that the belt strap withdrawal starting at the beginning of the accident places the mass system into oscillation by means of the associated rotation of the belt shaft in the direction of withdrawal. 
     Alternatively, it can be provided for the force limiting device that an external drive, such as for example the drives existing in a clockwork, provides the excitation of the mass system so that it achieves its oscillation frequency and that the drive is triggered in the event of an accident and acts for a predefined time period. To this end, it can be provided according to an example embodiment of the invention that the external drive is configured as a prestressed spring/mass system. In this case, it may be necessary to take care that the toothing of the driven mass system is configured with the driven moving part in such a manner so as to ensure permanent engagement of the respective toothings. 
     If the structural design of the mass system can influence the oscillation frequency, it can be provided according to example embodiments of the invention that a constant oscillation frequency of the mass system is set over the time interval during which the mass system operates. But it may also be provided to set a degressive or a progressive oscillation frequency of the mass system so that a change of the mass of the force limitation occurring during the accident can thereby be preselected. 
     It can be provided that an oscillation cutoff, which takes effect at the end of the time interval and acts to immobilize the mass system, is provided to fix or limit the desired extent of the belt strap withdrawal. 
     In one example of the present invention, it can be provided in a self-locking belt retractor comprising a belt shaft that is mounted in a housing and holds the belt, that the mass system responding to the rotation of the belt shaft in the unwinding direction consists of a plurality of two-armed pendulum masses. The two-armed pendulum masses are distributed over the periphery of the belt shaft and are mounted in a manner that enables them to swing around a center bearing point fixed to the housing. Moreover, in both the final positions of the two-armed pendulum masses swinging movement, a tooth located at each end of the two-armed pendulum masses engages the external toothing of a toothed ring, which in the event of an accident is to be coupled to the belt shaft and rotates therewith. The engagement of the teeth with the toothed ring occurs in such manner that when the toothed ring rotates relative to the pendulum masses, the sliding of the tooth flanks of the teeth configured on the pendulum masses with the tooth flanks of the external toothing of the toothed ring generates the oscillation of the respective pendulum mass. To ensure this sliding, it is possible to choose a flat, but possibly also a round, toothing in the spirit of a sliding toothing. 
     To this end, it can be provided in respective alternative embodiments that two, four or even six pendulum masses are arranged opposite one another in symmetrical arrangement, the invention not being limited to a particular number of pendulum masses. To prevent an imbalance that could possibly occur when the pendulum masses rotate, it can be provided that the pendulum masses are arranged and configured in such a manner that their movements mutually compensate each other. 
     If it is provided according to an example embodiment of the invention that an annular spring engaging the outside periphery of the pendulum masses is provided with projections, which are configured on the annular spring and impinge those regions of the pendulum masses located above the teeth of the pendulum masses, the prestress generated by the annular spring thereby establishing a response threshold at which the pendulum masses become active, so that the mass system consisting of the pendulum masses and annular spring is made to oscillate only when a starting force emanating from the rotation of the belt shaft comprising the toothed ring is exceeded. 
     According to an example embodiment of the invention, it is provided that the pendulum masses are arranged to engage the toothed ring completely and to overlap one another on their outer ends. This may have the advantage that the motion of the individual pendulum masses is transmitted to one another so that the oscillation frequency is better maintained. 
     In other example embodiments of the invention, it is provided that one or a plurality of springs is additionally assigned to the mass system, thereby forming an oscillatory system by means of which a time control can be realized in that the spring or springs absorb the mass system or pendulum masses, respectively. 
     According to a first example embodiment of the invention in this respect, it is thus provided that one end of the pendulum mass is connected to an additional spring controlling the oscillation of the pendulum mass. To this end, it is provided according to one embodiment that the spring is fixed stationary on its other end. 
     The time control can be improved by supplementing the spring system acting on the pendulum mass with additional masses which also act in an absorbing manner. The force limiting device according to the invention can also be adapted to the different deceleration characteristics of different vehicles by means of the influence on the oscillation periods and on the oscillation frequency that is possible in this way. To this end, the spring/mass system is preferably designed in such a manner that the absorbing force acting on the pendulum mass in the case of natural resonance is always larger than the energy supplied by the movement of the moving vehicle part, such as by the rotation of the belt shaft comprising a toothed ring. Since the pendulum masses may swing at a high frequency, approximately up to 2,000 Hertz, the use of a very hard spring may be required. 
     It is thus provided in a first embodiment, that an additional mass is interposed in the spring between its hanging system on the pendulum mass and its stationary fastening. By the interposition of an additional mass, it is possible to adjust a change of oscillation frequency or oscillation time so that the mass system can be adapted to the vehicle deceleration characteristics of different vehicles. 
     In another embodiment, it can be provided that the other end of the spring is fixed to a two-armed, swivel-mounted lever, the other arm of which can be impinged by a switching cam connected to the toothed ring when the toothed ring rotates. 
     Alternatively, it can be provided that the other end of the spring is fixed to an arm attached to the pendulum mass and that an additional inertial mass is interposed in the spring between its hanging system on the pendulum mass and its hanging system on the arm. 
     According to example embodiments, it is provided to let the pendulum mass, as part of the mass system put into oscillation, be impinged by a brake element that slows down its oscillation, wherein an appropriate braking or absorption can be adjusted as a function of the seat position of the vehicle occupant. The spring/mass system is preferably designed in such a manner that, in the case of natural resonance, the braking force on the pendulum masses is larger than the supplied energy. The natural frequency will then also determine the time period of the possible belt strap discharge. 
     According to example embodiments of the invention, the brake element can consist of the brake shoes laterally impinging the pendulum mass or of an absorption element absorbing the oscillation of the pendulum mass, or it can be provided that the oscillation of the pendulum mass can be controlled or slowed down by a control device that operates electromechanically. 
     In an example embodiment of the invention, the mass system responding to the rotation of the belt shaft in the unwinding direction can also be arranged in the interior of the belt shaft, which is hollowly configured and demonstrates a hollow space. In an example embodiment provided to this end, it can be provided that at least one swing-mounted, two-armed pendulum mass, which has one tooth arranged on each of its outer ends, is arranged in the hollow space on the belt shaft, said tooth, in the two final positions of the swinging movement, engaging the external toothing of a bar element, which extends axially into the hollow space of the belt shaft and is connected in nonrotatable fashion to a profile head as part of the locking system on the belt retractor side. The engagement taking place in such a manner that the sliding of the tooth flanks of the teeth configured on the pendulum mass on the tooth flanks of the external toothing of the bar element generates the oscillation of the respective pendulum mass when the belt shaft comprising the pendulum mass rotates relative to the bar element. 
     In an alternative embodiment of the invention, in an application for a self-locking belt retractor comprising a belt shaft which holds the belt and is mounted in a housing, it can be provided that the mass system consists of a pendulum mass. The pendulum mass is mounted and/or fixed to the housing and has a control pin. The control pin engages a continuous control curve in the event of an accident and thereby controls the oscillation of the pendulum mass. The control curve is configured on a control wheel which, in the event of an accident, is to be coupled to the belt shaft and rotate therewith. This configuration is arranged in such a manner that the movement of the control pin in the control curve generates the oscillation of the pendulum mass when the control wheel rotates relative to the control pin and the control pin is carried by the pendulum mass. 
     This may have the advantage that the configuration of the control curve makes it possible to adjust the frequency and time of oscillation. Thus it can be provided according to alternative embodiments of the invention that the control curve demonstrates a uniform course with a constant oscillation frequency of the pendulum mass or an alternating course with a changing oscillation frequency of the pendulum mass. In this example embodiment it can likewise be provided that the end of the control curve demonstrates a retaining recess for the control pin to fix the pendulum mass. 
     It can furthermore be provided that an additional mass is coupled to the pendulum mass by a gearing so that an adaptation to the vehicle deceleration characteristics of different vehicles can again be made. 
     Besides use of the new operating principle for force limitation in a belt shaft as part of a vehicle safety system, the concept of the present invention also extends to applications of the operating principle at other locations in a motor vehicle. Thus, in one example embodiment of the invention, it can be provided that the parts moving relative to one another are a part tightly connected to the motor vehicle and, as a linearly moving component of the motor vehicle, the steering column that can move linearly in the event of a crash. Insofar as the steering wheel with the steering column supporting it can thus be arranged flexible, it is provided that the insertion path of the steering column is absorbed in a part fixed to the vehicle. 
     Alternatively, it can be provided that the parts moving relative to one another are a part tightly connected to the motor vehicle and, as a linearly moving component of the motor vehicle, a part of the vehicle frame that moves in the event of a crash. Thus in a front or rear impact, a part of the vehicle frame can be arranged displaceable relative to another part of the vehicle frame, wherein the operating principle according to the present invention can absorb the displacement movement. 
     Insofar as bumpers are already displaceably arranged relative to the vehicle today in order to intercept lighter impacts, an insertion movement of a bumper of this type can also be used as a moving body part in the event of a crash to perform force limitation. In this respect, it is provided in an example embodiment of the invention, that the parts moving relative to one another are a part tightly connected to the motor vehicle and, as a linearly moving component of the motor vehicle, a body part that moves in the event of a crash. 
     Again, in another example embodiment, the invention provides for use of the new force limiting principle in a linearly operating vehicle safety system comprising a fitting piece, which holds the belt and moves linearly relative to a component fixed to the vehicle and the motion of which enables a limited discharge of the seatbelt. 
     In linearly moving parts of this type, one example embodiment provides, with respect to a structural embodiment of the force limiting device, that the component that is fixed to the vehicle is configured cylindrical and that the component moving relative thereto demonstrates a tube that can move in the cylinder. The inside wall of the cylinder may be provided with a toothing and at least one pendulum mass. The pendulum mass may be mounted so that it can swing in the tube up to the limit stop on the cylinder wall and, has teeth that are configured opposite one another that engage the toothing of the cylinder in its two final positions such that when the tube moves longitudinally relative to the cylinder wall, the sliding of the tooth flanks of the teeth configured on the pendulum mass on the tooth flanks of the inner toothing of the cylinder generates the oscillation of the pendulum mass such that the motion of the tube is controlled by a feed rate defined by the frequency of the swinging movement. 
     To this end, it can be provided that a plurality of pendulum masses is arranged within the tube with swinging movements rotating opposite to one another. 
     To establish a response threshold, it can be provided that the tube in the cylinder is prestressed by a spring when in its starting position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawing reflects example embodiments of the invention which will be described below. The drawing shows: 
         FIG. 1  is a diagram with an illustration of the interrelationship between the vehicle acceleration “a”, the deformation path “s” on the vehicle and the time period of the accident “t”; 
         FIG. 2  is a diagram with an illustration of the interrelationship between the vehicle acceleration “a”, the effective mass of the occupant “m”, and the retaining force “f” appearing on the shoulder strap; 
         FIG. 3  is a top view of the locking side of a seatbelt retractor having a mass system according to at least one embodiment the present invention as force limiting device; 
         FIG. 4  is the object of  FIG. 3  in another embodiment; 
         FIG. 5  is the object of  FIG. 3  in another embodiment; 
         FIG. 6  is a single pendulum mass of the mass system as per  FIG. 3  with an additional spring in a schematic representation; 
         FIG. 7  is the object of  FIG. 6  in another embodiment; 
         FIG. 8  is a single pendulum mass of the mass system as per  FIG. 3  with an additional spring mass system in a schematic representation; 
         FIG. 9  is the object of  FIG. 8  in another embodiment; 
         FIG. 10  is a single pendulum mass with an additional absorbing means in a schematic representation in accordance with at least one embodiment of the present invention; 
         FIG. 11  is the object of  FIG. 10  in another embodiment; 
         FIG. 12  is the object of  FIG. 10  in another embodiment; 
         FIG. 13  is the belt shaft of a seatbelt retractor with the mass system as force limiting device arranged in its interior in accordance with at least one embodiment of the present invention; 
         FIG. 14  is the object of  FIG. 3  and  FIG. 4  in another embodiment containing a control curve; 
         FIG. 15  is a top view of a force limiting device with a mass system, the device arranged in a linearly operating system in accordance with at least one embodiment of the present invention; 
         FIG. 16  is the object of  FIG. 15  in another embodiment; and 
         FIG. 17  is the interaction of the force limiting device with a vehicle safety system for a vehicle occupant in a schematic representation in accordance with at least one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The basic principle considerations of the invention will be explained on the basis of the diagrams depicted. 
     Referring to  FIGS. 1 and 2 , an accident takes place in a narrow time window independently of the boundary conditions of the accident, it being possible to assume the approximate average value of this time window as being within about 70 ms. In  FIG. 1 , the vehicle deceleration “a” is plotted vs. the deformation path “s” of the vehicle parts of interest vs. the time period (t) to an accident. Appropriate crash tests with motor vehicles show that the test results essentially lie within the dotted region, from which there results in particular the narrow time window in which the deformation of the vehicle (deformation path s) takes place as a function of the momentum, that is, the vehicle acceleration that occurs or, to be precise, the vehicle deceleration that occurs. The measuring results illustrated by the dotted area thereby confirm that the kinematics in the crash follow the basic interrelationship t=√{square root over (2s/a)}, i.e. a harder momentum results as the deformation path increases, the crash period essentially remaining constant. 
       FIG. 2  depicts the interrelationship between the vehicle acceleration “a”, the (chest) mass of a vehicle occupant (m), this mass acting in the vicinity of the shoulder strap, and the belt force (f) acting on the shoulder strap. Mathematically, the belt force is the product of mass×acceleration (f=m·a). As shown, this simple mathematical relationship does not apply to belt force limiters known in the prior art because a speed difference between the motion of the motor vehicle and the motion of the vehicle occupant arises due to the delayed deceleration of the vehicle occupant relative to the vehicle, and this speed difference produces a variable additive quantity for the vehicle acceleration “A” in  FIG. 2 . The mathematical relation f=m·a is therefore not valid for this application. 
     Insofar as belt force limiters known in the prior art assume a fixed force value, such as 3,000 N, in the three dimensional coordinate system different combinations of accelerations (a) and masses (m) can settle only into a plane parallel to the base plane. In contrast, the stretched area in the diagram as per  FIG. 2  shows the different retaining forces (f) that apply as a function of combinations of vehicle accelerations (a) and masses (m) when the invention is used. Thus, the center point A applies for a loading case of a mass of 14 kg acting on the diagonal belt and an acceleration of 350 m/s 2 , approximately corresponding to 35 g, which gives rise to a retaining force of approximately 5,000 N. The total mass of the occupant in the loading case is distributed on the two belt strap sections and the three fastening points in the vehicle. The mass assumed in  FIG. 2  represents the mass acting on the top point of the diagonal belt. Insofar as a speed difference between the vehicle and the person is avoided according to the present invention, the vehicle acceleration or deceleration remains the same for other occupants with different occupant data, so that a retaining force of around 3,500 N is established for a smaller person corresponding to point B with a mass of 10 kg at the same vehicle acceleration of 35 g. Accordingly, a correspondingly high retaining force of approximately 6,500 N applies for the occupant as per point C with a (chest) mass of 20 kg. The dotted arrows show the retaining forces that appear if the vehicle acceleration (a) drops while the (chest) mass of the vehicle occupant remains the same, the crash therefore being slighter. In this case, there is a linear decline of the retaining forces (f). 
     In regard to the structural example embodiment of the present invention,  FIG. 3  depicts a self-locking seatbelt retractor. The seatbelt retractor demonstrates a housing  10  having a U-shaped frame  11  within which a belt shaft (not illustrated) is rotatably mounted as carrier of a belt strap wound up thereupon. On the locking side of the seatbelt retractor or housing  10 , which is depicted in a top view in  FIG. 3 , a tooth lock washer  12 , which is mounted on the belt shaft in such a manner that it can swing radially outwards, is surrounded by a toothed ring  13 , which is rotatably mounted on the housing  10  and has an inner toothing  14 . The toothing of the tooth lock washer  12  and the inner toothing  14  of the toothed ring  13  are configured in such a manner that the toothed ring  13  is carried along in the direction of arrow  21  when the tooth lock washer  12  engages the inner toothing  14  of the toothed ring  13  when the belt shaft rotates in the belt withdrawal direction corresponding to arrow  21 . 
     The external side of the toothed ring  13  is provided with an external toothing  15 . Moreover, six symmetrically arranged pendulum masses  16  surround and enclose the periphery of the toothed ring  13 , wherein each pendulum mass  16  is mounted on the housing  10  by a centrally arranged pivot bearing  17  in such a manner that each pendulum mass  16  can carry out the swinging movement indicated by arrow  22  around its respective pivot bearing  17 . To this end, the pendulum mass  16  is configured two-armed with arms  18  extending on both sides of the pivot bearing  17 . A tooth  19  by means of which the respective pendulum mass engages in its two final positions, is arranged on each of the outer ends of the arms  18 . To this end, the toothings of the external toothing  15  and teeth  19  of the pendulum mass  16  are configured in such a manner that, when the toothed ring  13  rotates in the direction indicated by arrow  21 , the associated tooth flanks slide on one another and the rotating toothed ring  13  displaces the pendulum masses  16  into a swinging movement or holds them inside because the tooth  19  on one side of the pendulum mass engages the external toothing  15  of the toothed ring  13 , while the opposite tooth  19  is disengaged from the external toothing. Moreover, when the toothed ring  13  rotates further relative to this pendulum mass, the tooth  19  of the pendulum mass  16  that is presently engaged is pushed out, thereby swinging the pendulum mass  16  in such a manner that its opposite tooth  19  is pushed into engagement with the external toothing  15  of the toothed ring  13 . Upon further rotation of the toothed ring  13  relative to this pendulum mass, the movement proceeds in reverse, so that when the toothed ring  13  rotates, the swinging movement of pendulum mass  16  is maintained by the alternating engagement of its external teeth  19 . At the same time, however, the alternating engagement of the teeth  19  of the pendulum mass  16  does not permit unimpaired rotation of toothed ring  13 . The rotational speed of the toothed ring  13  is instead established as a function of the frequency of the swinging movement. 
     In the event of an accident, a belt-strap-sensor and/or vehicle-sensitive control system (not shown), which in the case of self-locking belt retractors is nevertheless adequately known, deflects the tooth lock washer  12  from engagement with the inner toothing  14  of the toothed ring  13 . If the toothed ring  13  is next fixed by a shear pin (not shown) in order to establish a response threshold for the activation of the mass system, then the toothed ring  13  will rotate in the direction of arrow  21  when a certain force is exceeded. To this end, the external toothing  15  of the toothed ring  13  pushes out the teeth  19  on one side of the pendulum masses  16  arranged on its outside periphery, wherein the pendulum masses  16  again engage the external toothing  15  with the teeth  19  arranged on their other side. During this process, each of the individual pendulum masses  16  is alternately accelerated and decelerated, dissipating the energy. The stronger the acceleration forces acting on the pendulum masses, the greater the deceleration forces appearing at the same time because of their swinging movement. The toothed ring  13  and, because it is coupled by the tooth lock washer  12 , therefore also the belt shaft can rotate only at that speed permitted by the pendulum masses swinging in rhythm, so that the withdrawal on the belt strap caused by the rotation of the belt shaft in the unwinding direction is determined solely by the swinging of the mass system started or initiated by the rotation of the belt shaft, and the withdrawal is therefore independent of the belt withdrawal force acting on the belt strap. 
     Whereas in the prior art, the extent to which the belt strap is withdrawn was essentially determined by the size of the acting and more or less twisting belt force, for example that of a torsion rod used as force limiting device. In the mass system used according to the invention, the extent to which the belt strap can be withdrawn or pulled out depends solely on the time period during which the mass system operates. If, according to experience, an accident occurrence is completed after approximately 70 ms, the mass system can be designed in such a manner that its operation will terminate after approximately this time period so that no further belt withdrawal will occur and the stationary deceleration ring will fix the belt strap to the housing. As not illustrated in detail, an oscillation cutoff, which acts at the end of the set time interval and effects a rest position of the mass system, can be provided. 
     In the illustrated example embodiment, the mass system comprising of the pendulum masses  16  is arranged on the housing  10  of the belt retractor. It is also possible to arrange this mass system on the associated front face of the belt shaft itself. 
     The example embodiment depicted in  FIG. 4  differs from the previously described example embodiment in that the pendulum masses  16  distributed over the periphery of the toothing ring  13  overlap in the direction of rotation as indicated by arrow  21 , in that an overlap projection  23  protruding in the direction of rotation is configured on the one pendulum mass  16  and rests on an overlap recess  24  configured on the pendulum mass  16  that is adjacent in the direction of rotation. In this manner it is possible to avoid the configuration of a tooth  19  under the overlap projection. It is thereby possible to also synchronize the swing movement of the individual pendulum masses  16 . 
     The example embodiment depicted in  FIG. 5  corresponds to the embodiment described in  FIG. 3 . In place of the shear pin mentioned regarding  FIG. 3 , the example embodiment depicted in  FIG. 5  is provided with an annular spring  20 , which externally surrounds the pendulum masses and prestresses all pendulum masses uniformly. This annular spring defines a starting force that must be overcome before the mass system begins to operate. In this respect, the slight loads of the belt shaft caused by a withdrawal force acting on the belt strap underneath the response threshold defined by the annual spring  20  do not result in a rotation of the toothed ring  13  together with a movement of the pendulum masses  16  of the mass system triggered thereby. At the same time, the respective toothings and the annular spring  20  can be configured and fixed in such a manner that, at the end of the force limiting process when the mass system comes to rest, the actual locking toothed system will always come to stop in such a manner that the control elements of the self-locking belt retractor will be synchronized for the belt-strap-sensor and/or the vehicle-sensitive control system. It is thereby possible to use the force limitation function triggered by the mass system many times in succession. 
     Moreover when using a plurality of pendulum masses, it can be provided that these either operate synchronously or also that they move asynchronously. This will affect the extent of the force limitation in the particular case. 
     It can be advantageous to additionally influence the movement of the pendulum masses  16 , especially in view of an intended time control. To this end, it is thus possible for spring systems or spring/mass systems or other brake or absorption elements to engage the pendulum mass  16  or pendulum masses  16  to affect the oscillation period and/or the oscillation frequency of the pendulum masses. To this end, the additional control elements can be configured in such a manner that the belt strap discharge beyond that determined by the rotation of the toothed ring  13  permitted by the pendulum masses can be adjusted variable 
     In the simplest embodiment as per  FIG. 6 , there is provided a spring  25 , one end of which is fixed to an arm  18  of an associated pendulum mass  16  and the other end of which is fixed to a fixed bearing  26 . If it is additionally necessary to overcome the force of the spring  25  during a swinging movement of the pendulum mass  16  in one direction, this will limit the time of the swinging movement. 
     In the example embodiment depicted in  FIG. 7 , the associated end of the spring  25  is fixed to a separated lever  27 , which is rotatably arranged around a fulcrum  28  fixed to the housing. The lever  27  is configured two-armed, comprising a first arm  29  as attachment point for the spring  25  and a second arm  30 , wherein the second arm  30  extends into the turning range of the toothed ring  13  and here fits a lifting cog  31  configured on the toothed ring  13 . When the toothed ring  13  rotates, the force of the spring  25  is changed at a predefined time as a result of the lifting cog  31  hitting the lever  27 , thereby affecting the oscillation behavior of the associated pendulum mass  16 . 
     In the example embodiments depicted in  FIGS. 8 and 9 , the spring system as depicted in  FIG. 6  is supplemented by an additional mass  32 , so that there arises a spring/mass system which likewise affects the oscillation period of the associated pendulum mass  16 . To this end in the example embodiment depicted in  FIG. 8 , an additional mass  32  is interposed in the spring  25 , which is configured in two parts. 
     In the example embodiment depicted in  FIG. 9 , one end of the spring  25 , said spring including mass  32 , is fixed to the pendulum mass  16  and the other end is fixed to a retaining arm  33  located on the pendulum mass  16  itself. 
     Control of the swinging movement is furthermore possible if, as per  FIG. 10 , a brake element  34 , which is indicated by crosshatching and exercises a braking force indicated by arrows  34   a  on the pendulum mass  16 , engages the pendulum mass  16 . 
     In the example embodiment depicted in  FIG. 11 , there is provided a mechanically operating absorption element  35 , which can be configured as a piston/cylinder arrangement for example. 
     In the example embodiment depicted in  FIG. 12 , an electromechanically operating absorption element  36  is assigned to the pendulum mass  16 . 
       FIG. 13  depicts the accommodation of the mass system operating as force limiting device in the interior of the belt shaft of a seatbelt retractor. To this end, the mass system is constructed like the mass system described in  FIG. 3  or in  FIGS. 3 to 5 . Insofar as the front of the associated belt shaft  70  in a seatbelt retractor known from the prior art is connected to a profile head  71 , which is to be locked fixed to the housing when triggered, in such a manner that the belt shaft  70  can rotate further relative to the locked profile head  71  for the purpose of force limitation, the belt shaft  70  is configured as a hollow body comprising an inner hollow space  72 . The corresponding pendulum masses  16  include external teeth  19  which are mounted on the wall of belt shaft  70  and distributed internally over the periphery, as described in regard to  FIG. 3 . The associated external toothing  15  for engaging the teeth  19  of the pendulum mass  16  comprising teeth  19  is configured on a bar element  73 , which is carried by the profile head  71  and extends axially into the hollow space  72  of the belt shaft  70  and is tightly connected to the profile head  71 . If profile head  71  is locked when locking occurs, then the continuous belt tension on the belt strap wound up on the belt shaft  70  causes the belt shaft  70  to rotate further relative to the profile head  71  and relative to the bar element  73  tightly connected thereto, wherein the pendulum masses  16  arranged in the hollow space  72 , together with their teeth  19 , will slide on the external toothing  15  of the bar element  73  because of the further rotation of the belt shaft, thereby generating the swinging movement of the pendulum masses  16  used for force limitation. 
     It is not necessary to use the rotation of the toothed ring caused by the belt force in order to drive the pendulum mass or pendulum masses. Rather it is also possible to provide an external drive that operates like a clockwork, such as in the form of a prestressed spring/mass system, which excites or controls the movement of the pendulum masses for a predefined time period. To this end, a gearing, which can be configured either as self-locking or not self-locking, can be arranged between the pendulum mass and the external drive. 
     The example embodiment depicted in  FIG. 14  essentially corresponds to the example embodiments depicted in  FIGS. 3 and 4  but is based on a different way of initiating the rotation of the belt shaft in the swinging movement of an associated pendulum mass. In this example embodiment, only one pendulum mass  40  is arranged swinging around a fulcrum  41  fixed to a housing, wherein an additional mass  42 , which is connected to the pendulum mass  40  by a gearing  43 , is placed on the free end of the pendulum mass  40 . The mass inertia of the pendulum mass  40  is thereby increased as a whole, taking the interposed gear  43  into consideration. The pendulum mass  40  uses a control pin  44  projecting from it to engage a control wheel  45 , which is rotatably arranged on the housing  10  of the belt retractor, wherein, in like manner as the toothed ring  13 , the control wheel  45  is provided with an inner toothing  46  into which the ejectable tooth lock washer  12  can be injected in such a manner that the rotation of the belt shaft in the direction of arrow  21  can be converted into a corresponding rotation of the control wheel  45 . 
     A spiral-shaped control curve  47 , which is configured in the form of a groove built into the front face of the control wheel  45  and which guides the control pin  44  of the pendulum mass  40 , is provided in the control wheel  45 . The predefined control curve  47  has such a course that it generates corresponding swinging movements of the pendulum mass  40  and its additional mass  42  around the fulcrum  41 . To this end, section  49  of the control curve  47  is shaped noticeably flatter so that in this region the belt strap discharge will be larger over the time unit and the belt force will drop correspondingly. In the end region  50  of control curve  47 , the control curve  47  again demonstrates a steeper course in order to achieve a larger deceleration and to then end in a stop recess  51 , further swinging movement of the pendulum mass  40  and therefore also the possible belt strap discharge being terminated when the control pin  44  reaches it. The control curve  47 , whose design can be modified, thus enables the controlled, predefined belt strap discharge to be adapted to the respective deceleration characteristic of the vehicle of interest. 
     Finally,  FIGS. 15 and 16  depict a linearly operating system in which the relative motion of its components to one other occurring in a crash is converted into a force limitation. This may, for example, relate to a steering column which, as carrier of the steering wheel, can be pushed into a part fixed to the vehicle. In particular, in  FIG. 15  there is provided a part, which is configured as cylinder  55  and fixed to the vehicle and which is provided with an inner toothing  62 . A tubular steering column  56 , which serves as the carrier of a steering wheel  57  and within which, in the illustrated example embodiment, two pendulum masses  59  are arranged so that they can rotate around fixed fulcrums  60 , is guided in the cylinder  55  in a displaceable manner. The steering column  56  demonstrates passages  58 , which are opposite one another and which the teeth  61  arranged on the pendulum masses  59  can pass through during the respective swinging movements of the pendulum mass  59  until they engage the inner toothing  62  of the cylinder  55 . The sequence of motion takes place as described with respect to  FIG. 3 , in that, when the steering column  56  is inserted into the cylinder  55 , the teeth  61  of pendulum masses  59  slide off the inner toothing  62  on one side of the cylinder  55  and become free and, at the same time, come into engagement with the inner toothing  62  of the cylinder  55  on the opposite side because of their swinging movement. Another spring  63 , which defines the starting force to be overcome and simultaneously can also provide for a resetting of the steering column  56  when the force limitation process has terminated, is arranged in the cylinder  55  to support the insertion movement, as basically also described in regard to the example embodiments as per  FIGS. 3 and 5 . 
     The example embodiment depicted in  FIG. 16  basically represents the same relationships wherein, in place of the two pendulum masses  59  depicted in  FIG. 15 , there are now provided four pendulum masses  59 , which extend in the longitudinal direction of steering column  56  and are correspondingly configured two-armed with teeth  61  arranged on each arm. In this respect, the operation described with respect to  FIG. 1  for the pendulum masses  16  corresponds to the example embodiment in  FIG. 16 . 
     Moreover, this type of arrangement can also be applied to a linearly operating seatbelt system in which, for example, a fitting piece as carrier of a seatbelt can be inserted in the cylinder  55  in place of the steering column  56 . In the same manner, it can be provided that the relative motions of a mobile vehicle part, such as a bumper or part of a vehicle side member, can be provided in the cylinder  55  as the part fixed to the vehicle. 
     The interaction of a time-controlled and speed-controlled belt discharge as per the example embodiment explained above can be explained once more on the basis of  FIG. 17 . It depicts a vehicle occupant  71  seated on a seat  70  at the end of the forward displacement enabled by the controlled discharge of the belt strap  72 , in which the head of the occupant  71  strikes an inflating airbag  73 . To this end, the airbag  73  has unfolded itself out of a steering wheel  74 , which is seated on a steering column  76  that is configured as insertable. 
     Since a belt retractor (not illustrated) predefines the speed of the discharge of belt strap  72 , the inflation rate or ventilation rate of the airbag  73  and also the insertion rate of steering column  76  can be appropriately adjusted, wherein the sum of the speed relative to the airbag and the speed relative to the steering column should correspond to the rate of seatbelt discharge. This type of configuration produces the least possible strain on the vehicle occupant  71  as a whole. 
     As a person skilled in the art will appreciate, the above description is meant as an illustration of implementation of the principles of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.