Abstract:
The invention relates to a belt retractor ( 10 ) with a switchable force limiter. Said belt retractor ( 10 ) comprises as the force limiter at least one energy absorbing element ( 13; 14, 15; 61 ) that is linked with the reel shaft ( 12 ) and with the locking device of the belt retractor ( 10 ) via a profiled bead ( 50 ). The belt retractor is further provided with a switch device with which the energy absorption is adjusted. The aim of the invention is to provide a switch device that functions independent of the transmission of power between the reel shaft and the torsion bar. Said switch device (latch  21 ) is designed in such a manner that it can be switched at any time during the already existing energy absorption via the drive ( 26 ) that is independent of the reel shaft ( 12 ) and/or the switch device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a belt retractor provided with a force limiter, whereby the belt retractor has as a force limiting device at least one energy absorbing element that is linked to the spool and via a profiled head to the blocking apparatus of the belt retractor and a switching device is provided that adjusts the energy absorption and that is switchable via an independent drive. Such belt retractors can be designed both as self-blocking belt retractors with a vehicle-sensitive and/or belt-sensitive controllable blocking apparatus and as so-called limit fitting in accordance with EP 0 773 147 A2. 
     WO 97/49583, which forms the generic type, describes a self-blocking belt retractor with the features specified in the foregoing; in the exemplary embodiments individually detailed therein provided as energy absorbing element is a torsion bar with two or three sections switched one after the other, and also described is the arrangement of two energy absorbing elements switched parallel to one another. The associated switching device can only be switched under a load that is, can only be switched given the application of great actuating forces when energy absorption has already been turned on, because an existing force- or load-transferring coupling between the spool and torsion bar has to be overcome, i.e., there must be a switch to such a coupling. The design and operative complexity required for this is correspondingly high. The object of the invention is therefore to provide in a belt retractor having the generic features a reliable switching device that works independently of a force transfer already existing between spool and torsion bar. 
     This object is achieved including advantageous embodiments and further developments of the invention from the contents of the patent claims that follow this specification. 
     SUMMARY OF THE INVENTION 
     The basic idea of the invention is that the switching device, which can be switched at any time via the drive arranged independent from the spool and/or switching device has a pawl borne with a tendency to open that is held in its coupling position by means of a to spool ring that is displaceable in the direction of the spool axis, and in that the spool ring can be axially transferred by means of a pyrotechnically drivable adjusting apparatus into a position in which the spool ring makes it possible to reverse the pawl from its coupling position to its release position. Since the at least one pawl for connecting the torque tube and the spool is borne with a tendency to open, no more actuating forces occur for moving the pawl once the pawl has been released by the displacement of the spool ring. 
     In accordance with one exemplary embodiment of the invention, it is provided that at least two energy absorbing elements are provided arranged in series or parallel to one another, and that the switching device switches from one energy absorbing element to the other energy absorbing element when the drive is triggered. 
     In accordance with another exemplary embodiment of the invention, it is provided that the torsion bar has at least two sections with different cross-sections arranged in series to one another for a establishing a different energy absorption, and a torque tube is provided that connects at least one section of the torsion bar and the spool in a form-fit, as such a design of a belt retractor is known in principle from WO 97/49583. 
     In an alternative embodiment of two energy absorbing elements, it can be provided that a torsion sleeve is arranged connected in switched parallel to the torsion bar, and torsion sleeve or torsion bar can be switched off by means of the switching device allocated to the torsion sleeve and that allocated to the torsion bar. 
     In accordance with one exemplary embodiment of the invention, it can be provided that the spool ring is connected to the spool via form-fit designs and rotates together with it. 
     In particular, it can be provided that the pawl is pivotably borne in a recess in the cross-section of the spool, and the spool ring is arranged spanning the exterior circumference of the spool such that the spool ring in its position maintaining the pawl in its coupling position covers the recess. 
     For effecting the change in position of the spool ring, it can be provided that the control apparatus comprises a set collar that is arranged on the belt retractor rotatable under the effect of a drive across a limited area of rotation for displacing the spool ring and that is supported against a cover of the belt retractor by at least one ramp inclined in the axial direction of the spool and that can be displaced axially due to its twisting, whereby in accordance with one exemplary embodiment of the invention a plurality of ramps are distributed across the circumference of the set collar and the cover. Such a control apparatus is associated with the advantage of an additional servo-effect, since due to the helical movement produced in this manner only a small radial force must be applied in order to produce a much greater axial force for displacing the spool ring and therefore for producing the switch movement under a load. 
     With regard to the design of the drive for rotating the set collar, a linearly displaceable pyrotechnically driven piston can be provided that tangentially impacts a shoulder projecting radially from the set collar. This stationary external arrangement of the pyrotechnic drive substantially simplifies the electrical control, since it is not necessary to transfer electrical signals to the rotating spool and the pyrotechnics attached thereto. 
     The inventive coupling between torque tube and spool makes it possible in accordance with one exemplary embodiment of the invention to arrange a deformable element between spool and torque tube. In accordance with exemplary embodiments of the invention, the deformable element can be designed as shearing bars extending in the circumferential direction of spool and torque tube or as bending bars arranged radially between spool and torque tube. In an additional embodiment of the invention it can be provided that the deformable element is designed as bending bars arranged radially between spool and torque tube. This ensures a gentle transition from the first force level to the other force level. 
     With respect to the cooperation between the belt force limitation created in the belt retractor and the airbag located in the vehicle, the point in time at which the switch takes place from the higher force level to the lower force level is very important. For a control in this regard, in accordance with one exemplary embodiment of the invention it is provided that for igniting the drive for the control device a time switch is provided that is activated by the vehicle sensor and that involves a pre-selected period of time. In this exemplary embodiment of the invention, the ignition occurs at a pre-specified point in time independent of other accident conditions. 
     In an alternative embodiment of the invention it is provided that a device is provided for determining the revolutions of the spool under a load after a pre-selected threshold has been exceeded for determining the belt withdrawal effected by the force limitation, and the ignition of the drive for the set collar occurs when a pre-selected belt withdrawal has been achieved. In this embodiment of the invention, the belt withdrawal occurring during the accident and caused by the effect of the force limiting device of the belt retractor is used as a measure for switching the level of the force limiting device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawings illustrate exemplary embodiments of the invention that are described in the following: In the drawings: 
     FIG. 1 is longitudinal view of a belt retractor with spool and a two-stage torsion bar including the switching device that has a first switch state (release of the pawl); 
     FIG. 1A is the subject of FIG. 1 illustrated with the parts in an exploded view; 
     FIG. 2 is the switching device in accordance with FIG. 1 in a frontal view along the line A—A; 
     FIG. 3 is a schematic representation of the drive for the switching device; 
     FIG. 4 is the switching device in accordance with FIG. 2 after the drive has been actuated; 
     FIG. 5 is the switching device in accordance with FIG. 2 with additional shearing bars for adjusting an additional damping effect; 
     FIGS. 6 a, b  are the switching device in accordance with FIG. 2 with an additional bending bar; 
     FIGS. 7 a, b  are the switching device in accordance with FIG. 2 with a strip-type deforming element; 
     FIG. 8 is the subject of FIGS. 1 and 3 in connection with a control of the switching time of the switching device as a function of the belt withdrawal in a second switching state (blocking the pawls); 
     FIG. 9 is another exemplary embodiment of a belt retractor in accordance with FIG. 1 with two parallel-switched energy absorbing elements. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The belt retractor  10  visible in FIGS. 1 and 1A has a U-shaped frame in the lateral legs  11  of which a belt shaft or spool  12  is rotatably borne. A safety belt  9  is wound on the spool  12 . Located in the interior of the spool as force limiting device is a torsion bar  13  that has a section  14  with a larger cross-section and a section  15  with a smaller cross-section, whereby provided between the sections  14 ,  15  and on the exterior end of the section  15  of the torsion bar  13  are connecting parts  16   a, b  by means of which the spool  12  is attached to the sections  14  and  15  of the torsion bar  13 . On the exterior left side of the illustration in FIG. 1 is the blocking side  17  of the belt retractor, at which the torsion bar  13  is attached to a profiled head  50 . 
     The spool  12  is coupled to the section  14  of the torsion bar  13  having the larger cross-section via a torque tube  18  that covers in a form-fit the associated connecting part  16   a , while for connecting the spool  12  to the section  15  of the torsion bar  13  having the smaller diameter the spool  12  covers directly in a form-fit the connecting part  16   b  arranged on the right-hand end of the section  15 . The switching device for switching the force limiting device from the higher force level to the lower force level is formed via pawls  21 , shown in detail in FIG. 2, that couple the spool  12  and the torque tube  18  in a manner to be described such that when the pawls  21  are in the coupled position the force flow from the spool  12  via the torque tube  18  to the section  14  of the torsion bar  13  occurs such that when there is a corresponding load on the spool  12  from the pull exerted on the safety belt  9  the section  14  twists and makes possible the force limitation at a high level. If the pawls  21  are disengaged from the torque tube  18  by the actuation of the switching device which is yet to be explained, the spool  12  can rotate relative to the torque tube  18 , and now the force flow occurs via the form-fit connection between the spool  12  and the connecting part  16   b  allocated to the section  15  of the torsion bar  13 , so that the section  15  twists and the force can be limited at a lower level. 
     As can be seen in FIG. 2 in conjunction with FIG. 1 a , the pawls  21  effecting the coupling between spool  12  and torque tube  18  in their coupling position are borne in a recess  40  formed in the cross-section of the spool  12  such that they can pivot about a center of rotation  22 , whereby the spool  12  is surrounded on its exterior circumference by a spool ring  45  that spans the recesses  40  in the spool  12  and thereby via supports  24  holds the pawls  21  at their engaging noses  23  engaged with the recesses  51  of the torque tube  18 . If the spool ring  45  is displaced in the axial direction, the pawls  21  are released from the support  24  of the spool ring  45  so that the pawls  21  that are borne with a tendency to open pivot outward into the recess  40  and thereby eliminate the connection between the spool  12  and the torque tube  18 . The tendency to open that the pawls  21  have is adjusted via appropriate designs of the recesses  51  and of the engaging nose  23 . The spool ring  45  is borne on the spool  12  in a form-fit via noses  52  engaging in the profile of the spool  12  and rotates with the spool  12 . 
     The axial displacement of the spool ring  45  is introduced via the drive illustrated in FIG. 3, in which arranged in a cover  20  of the belt retractor  10  that covers the spool end is a drive  26  with a pyrotechnic charge  27  that impacts a piston  28 , causing its linear movement. In its linear movement, the piston  28  acts on a shoulder  29  projecting radially from a set collar  19  such that the set collar  19  is caused to rotate when the piston  28  is actuated. The set collar  19  is borne on the housing  10  or on the cover  20  that is attached to the housing  10  and does not also rotate during the normal function of the spool  12 . However, it can perform a limited rotational movement during its adjusting motion. The set collar  19  is supported against the cover  20  by inclined ramps (not shown in detail) arranged between set collar  19  and cover  20  so that due to the introduced twisting of the cover  20  the set collar  19  is pressed and thus axially displaced. This axial displacement of the set collar  19  is transferred to the spool ring  45  via the adjacent positions of the set collar  19  and spool ring  45  such that the displacement of the spool ring  45  leads to the release of the pawls  21 , as described and illustrated in FIG.  4 . In the illustrated exemplary embodiment, spool ring  45  and set collar  19  rest against one another via overlapping members so that the axial displacement of the set collar  19  is correspondingly transferable to the spool ring  45 . 
     As can be seen in detail in FIGS. 5 through 7, in accordance with exemplary embodiments of the invention it is provided that an additional deformable element is arranged between the torque tube  18  and the spool  12  so that during the switching process from the higher level of force to the lower level of force of the torsion bar there is an additional gradation of the force transfer, that is, there is a more gentle transition; thus, with a two-stage torsion bar it is possible to set an additional third force characteristic. That is, if the pawls  21  pivot so that the spool  12  is uncoupled from the torque tube  18 , during the further process while the section  15  of the torsion bar  13  is being switched to the spool  12  there is a relative rotation of the spool  12  to the torque tube  18 , and at the beginning of this relative rotation a third energy absorbing element is switched on via deformable elements that are additionally arranged between the spool  12  and the torque tube  18  and that at the beginning of the switch to the lower force level first cause an additional dampening that occurs parallel to the section  15  of the torsion bar  13  via the provided deformable elements before, in the further process, there is force limiting only via the section  15  of the torsion bar  12 . 
     In the exemplary embodiment illustrated in FIG. 5, arranged between spool  12  and torque tube  18  are two shearing bars that shear off during a relative rotation of the spool  12  to the torque tube  18 . 
     In the exemplary embodiment illustrated in FIGS. 6 a  and  6   b , two radially arranged bending bars  31  are provided that at their one end are in a recess  32  of the spool  12  and at their other end are adjacent to a member  33  on the torque tube  18  so that during a relative rotation between spool  12  and torque tube  18  the bending bars  31  first are bent into a free space  34  formed on the spool  12  before the section  15  of the torsion bar becomes effective due to the further rotation of the spool  12 . 
     In the exemplary embodiment illustrated in FIGS. 7 a ,  7   a , formed in the spool  12  as deformable element is a strip  35  made of a deformable material that is inserted in a channel  36  and is suspended at its end on the torque tube  18  and thus during a relative rotation between spool  12  and torque tube  18  is pulled out of the channel while deforming before the spool  12  due to its uncoupling from the torque tube  18  while continuing to rotate can engage the section  15  of the torsion bar  13 . 
     For correct switching of the force limitation during an accident and thus for the interplay between the belt retractor equipped with a force limiter and an airbag arranged in the vehicle, the point in time at which the force limiter is switched is very important because the belt withdrawal enabled by the force limiter to a certain degree causes a pre-positioning of the belted person for whom the inflating airbag is intended. 
     Although not illustrated, it is possible to determine the best point in time for the switching process, that is, for igniting the pyrotechnic charge  27 , to trigger this ignition across a specified point in time in that a period of time is pre-selected in an appropriately provided control unit, and after this period of time has elapsed the pyrotechnic charge  27  is ignited. 
     FIG. 8 illustrates an alternative option for the control in which the belt withdrawal caused by the effect of the force limiting device of the belt retractor is used as a measure for igniting the pyrotechnic unit  27 . In this case, a measuring device  37  is provided on the belt retractor  10 , for instance in the form of a potentiometer that determines the speed of the spool rotations or the angular velocity. When the value thus acquired exceeds a threshold value, this is identified as the beginning of the force limitation, and further belt withdrawal is detected via the potentiometer. The angular velocity of the spool  12  during normal, manual belt withdrawal when the safety belt is being put on and during blocking of the belt retractor is substantially lower than when the force limiting device, that is, the energy absorbing elements, have been triggered. The angular velocity during the force limiting process is approximately ten times higher than during normal belt withdrawal. Stored in a control unit  39  provided on the vehicle is a specific measure of belt withdrawal as a prerequisite for igniting the pyrotechnic charge  27 , and when in another provided belt retractor control unit  38  the belt withdrawal speed determined by the potentiometer equals the value pre-specified by the control unit  39  of the vehicle, the control unit  38  initiates ignition of the pyrotechnic charge  27 . At this point in time the force limiting device then switches from the higher force level to the lower force level. 
     Finally, FIG. 9 illustrates one exemplary embodiment of the invention with two energy absorbing elements oriented parallel to one another. In this exemplary embodiment, as well, the spool  12  is rotatably borne in the housing legs  11 . At the blocking-side end  17  of the belt take-up spool  13 , the spool on its front side is allocated a profiled head  50  that rotates in the U-shaped leg  11  of the housing, whereby borne on the profiled head  50  is an anti-blocking member  55  that can be radially deflected under the effect of a control device (indicated but not shown) and that due to the control in the case of a delay is guided into the teeth arranged in the associated through-hole in the housing. 
     The profiled head  50  and the spool  12  are connected to one another by a torsion bar  13  acting as force limiting device in that the one end of the torsion bar  13  is connected in a form-fit to the profiled head  50  and the other end of the torsion bar  13  running in the spool  12  is attached to the spool  12 . The spool  12  widens beyond the lateral leg opposite the blocking side  17  in order to form the recess  40  necessary for bearing the pawls  21 . The first exterior pawl  21  borne on the outermost end of the spool  12  engages a radially projecting flange  60  of the torsion bar  13  so that when the pawl  21  is blocked the spool  12  is connected to the torsion bar  13 . The pawl  21  is kept engaged by an associated exterior spool ring  45 , whereby a first set collar  19  is also allocated to the first spool ring  45 . 
     Provided as additional energy absorbing element is a torsion sleeve  61  that covers the torsion bar  13  and is attached to the profiled head  50  and that also has adjacent to the flange  60  of the torsion bar  13  a radial flange  62 , to which is allocated a second pawl  21  that is borne on the spool  12  and located in its recess  40  and that is kept engaged by a second spool ring  45 , which is itself controllable by a second set collar  19 . With this arrangement, different levels of energy absorption can be controlled by controlling the two pawls  21 . 
     The features of the subject of this document disclosed in the foregoing specification, patent claims, abstract, and drawings can be essential individually and in any combination to realizing the invention in its various embodiments. 
     The specification incorporates by reference the disclosure of German priority document 199 27 427.4 filed Jun. 16, 1999 and PCT/EP00/05474 filed Jun. 14, 2000. 
     The present invention is, of course, in no way restricted to the specific disclosure of the specification and drawings, but also encompasses any modifications within the scope of the appended claims.