Abstract:
The invention relates to a variable-volume airbag module. According to the invention, to limit the air bag volume at least one pull element is provided for which is loosely guided at the level of the air bag and attached at its ends after the airbag has been gathered, and which is released or tears if it is subjected to a certain stress during unfolding of the air bag.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a variable-volume airbag for an airbag module. 
     In order to obtain a certain direction of extension in the initial phase of the unfolding of the airbag or when staged generators are used, it is expedient not to unfold the entire volume of the airbag immediately. When the last-mentioned generators are used, in the event of only a substage being activated, the airbag is filled more slowly and with less gas mass. The inadequate filling of the airbag delays the buildup of membrane forces in the airbag fabric and therefore the restraining force on the occupant. The forward shifting of the occupant would be very great and it would scarcely be possible to avoid contact with the rigid interior contour. 
     It is therefore know to unfold the airbag in this case with a reduced volume. 
     U.S. Pat. No. 5,362,101 thus discloses reducing the volume of the airbag by it being partially laid in folds which are sewn together. In the first phase after ignition of the gas generator, for example after ignition of the first stage of a two-stage gas generator, the airbag is unfolded only up to the reduced volume limited by the seams. When the internal pressure is increased by ignition of the second stage, the abovementioned seams are finally torn open and the airbag can be unfolded to its full volume. 
     The disadvantage of reducing the volume by means of tearing seams resides in the fact that the said seams have to be torn open automatically, but because of tolerances in the fabric and also because of environmental influences (for example, temperature, air humidity), a defined and reproducible tearing open of the seams can scarcely be achieved. Furthermore, the folding of an airbag having taken-in regions and the installation in the airbag module are also problematical because the airbag is distorted because of the darts and, as a result, handling in the production process is made more difficult. 
     Furthermore, airbags for airbag modules are known in which catch bands are arranged in order to obtain certain directions of unfolding and in order to limit the volume in the first unfolding phase, i.e. in an intermediate stage of the unfolding. The catch bands are provided both within the airbag and outside it. They are fastened by both ends to opposite sides of the airbag and are dimensioned in such a manner that they tear off from the airbag or even burst at a predetermined internal pressure. An airbag of this type with an external catch band is disclosed, for example, in the document JP-A-07 069149. 
     These catch band arrangements have the same disadvantages as in the abovementioned arrangements with tearing seams. 
     SUMMARY OF THE INVENTION 
     The invention is based on the object of achieving, with a relatively small outlay, a limitation of the volume of the airbag, the intention being for the increase in size of the airbag to its maximum volume to be possible with improved reproducibility. 
     In the case of a variable-volume airbag for an airbag module, in order to limit the airbag volume in an intermediate stage, at least one pull element is provided which is loosely guided on the airbag and after the airbag is gathered together is secured by its ends and which can be released or torn after being subjected to a certain stress during the unfolding of the airbag. A pull cable or a rigid element can be provided as the pull element. A cord can preferably be used as the pull cable. However, the term pull cable also includes a pull band. 
     In the present case, the pull cable is intended to gather together the airbag in such a manner that during the unfolding in a first phase a smaller volume has to be filled with gas than in a later phase. This is of importance, for example when staged generators are used. In these staged generators, in the event of only a substage being activated, the airbag is filled more slowly and with less gas mass. In the case of an airbag having a single, large volume, this results in it being inadequately filled, which delays the buildup of membrane forces in the airbag fabric and therefore the restraining force on the occupant. The forward shifting of the occupant is very great and it is not possible to avoid contact of the occupant with the rigid interior contour. 
     By adapting the airbag volume or the airbag shape by means of the pull cable to the gas mass provided in each case, and therefore to the filling pressure, this disadvantageous effect of the staged generator can be compensated for. 
     The reduction of the airbag volume by means of a pull cable or a plurality of pull cables also has the advantage that the said cables do not have to be connected to the airbag, as is required, for example in the case of catch bands. Rather, the ends of the pull cable can also be fastened at any desired location. 
     After the airbag is gathered together by means of a pull cable, its ends can be secured by being connected to one another. The pull cable then surrounds the gathered-together region of the airbag without being connected thereto. The reproducibility of the increase in size of the airbag volume is only influenced by the pull cable itself, but not by the airbag fabric and seams provided thereon. The airbag volume can be increased in size at a predetermined stress by the separation of the ends of the pull cable or by the tearing of the pull cable. 
     A further possibility of securing the ends of the pull cable resides in fastening at least one end in the housing of the airbag module. This results in further possibilities of releasing the pull cable by separately arranged mechanisms being provided for the release of at least one end. As a result, the reproducibility of the increase in size of the airbag volume can be increased further, since now even the strength of the pull cable which slightly fluctuates from specimen to specimen has no influence. 
     For the release at least of one end of the pull cable, for example, thermal or mechanical mechanisms can be provided or a detonating cap can also be provided. 
     The airbag can be gathered together by means of the pull cable in such a manner that it is partially constricted or is subdivided into chambers. In the last mentioned case, the pull cable lies over an angular range of at least 360° around the airbag. 
     The pull cable is generally guided such that it bears closely against the airbag in guide elements. For example, tabs, rings or flexible tubes can be used as these guide elements. 
     The pull cable can be provided on the outside and/or inside of the airbag. 
     On the one hand, the pull cable can preferably be provided in regions of the airbag with which the occupant does not have contact after the unfolding of the airbag. This definitely rules out the possibility of the occupant being injured by an external pull cable after the latter has been torn off. On the other hand, a pull cable or a plurality of pull cables can be provided in different regions of the airbag in order to influence both the size and the shape of the airbag in the first phase of the unfolding. 
     Provision is made in one embodiment for a pull cable to extend over a predetermined length of the airbag at a constant distance from the edge of the blow-in mouth of the airbag, and for the two ends to be fastened in the housing of the airbag module. However, it is also possible for the guide elements to be provided alternating at a different distance from the blow-in mouth of the airbag, so that the pull cable runs in a zigzag shape. 
     In a further embodiment, starting from a first circumferential pull cable, at least one second pull cable is provided which runs in the direction of the blow-in mouth essentially perpendicularly with respect to the first pull cable. 
     In another embodiment, at least one eyelet is provided on the airbag for the guiding of the pull element. 
     In this embodiment, at least one pull cable can be provided which is connected fixedly to the airbag housing by one end, runs through the eyelet and is fastened by the other end in a retaining device which opens at a predetermined pressure. 
     In another refinement, at least one pull element is provided which is connected fixedly to the airbag housing by one end and whose other end is provided with a hook which engages in the eyelet and bends up at a certain tensile stress. 
     The invention will be explained in exemplary embodiments with reference to drawings, in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side view of an unfolded airbag which is limited in its extension by means of a cord; 
     FIG. 2 shows the airbag according to FIG. 1 after the release of the cord; 
     FIG. 3 shows a fastening device for the ends of the cord in a side view; 
     FIG. 4 shows the device according to FIG. 3 in a plan view; 
     FIG. 5 shows the device of FIGS. 3 and 4 after release of the ends of the cord; 
     FIG. 6 shows a side view of an unfolded airbag with a second embodiment of a fastening device for the ends of the cord; 
     FIG. 7 shows the airbag according to FIG. 6 after release of the cord end; 
     FIG. 8 shows the fastening device used in the exemplary embodiment of FIG. 6, in a plan view; 
     FIG. 9 shows the fastening device according to FIG. 8 after release of the ends of the cord; 
     FIG. 10 shows the side view of an unfolded airbag which has a cord wrapped around it parallel to the edge of the blow-in mouth; 
     FIG. 11 shows a section through the airbag according to FIG. 10, as seen in the direction of the arrows XI—XI; 
     FIG. 12 shows a section through an extended airbag with a cord provided on the outside of the airbag; 
     FIG. 13 shows the airbag according to FIG. 12 in a folded position; 
     FIG. 14 shows the schematic illustration of an airbag with a cord guided in four tabs which lie on a radius; 
     FIG. 15 shows the schematic illustration of an airbag with a cord guided in eight tabs, with four tabs each lying on different radii; 
     FIG. 16 shows a side view of an unfolded airbag with a circumferential cord; 
     FIG. 17 shows a side view of an unfolded airbag with a cord which is connected fixedly to the airbag module. 
     FIG. 18 shows the airbag according to FIG. 17 after the release of the cord. 
     FIG. 19 shows a side view of an unfolded airbag with a hook at one end of a cord; 
     FIG. 20 shows the airbag according to FIG. 19 after the release of the cord; 
     FIG. 21 shows a side view of an unfolded airbag with a hook as a pull element; 
     FIG. 22 shows the airbag according to FIG. 21 after the release of the hook from the associated eyelet. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates an airbag  1  in which a pull cable is arranged on the outside in the form of a cord  2  which is guided loosely on the airbag  1  in two tabs  3 ,  4 . The airbag is partially gathered together by means of the cord  2  and its ends are fastened in an airbag housing  5  in a retaining device  6 . In the first phase of the unfolding, the cord prevents the airbag from stretching out to its full size. 
     When there is a further rise in the pressure in the airbag, the ends of the cord are released from the retaining device, and the airbag can unfold to its full size, as is illustrated in FIG.  2 . 
     FIGS. 3 to  5  illustrate an exemplary embodiment of a retaining device. The latter consists of two clips  7 ,  8  which are hooked together at one end and are connected to each other at the other end by means of a snap stud  9 . This retaining device enables active volume adaptation, for example in conjunction with a multistage gas generator, since the time of destruction of the snap stud and therefore the release of the cord can be selected as desired. A separately activated mechanism  60 , such as thermal or mechanical mechanisms or a detonating cap may be used to release the pull cable. FIG. 5 shows that the clips  7 ,  8  unfold after destruction of the snap stud and release the ends of the cord  2 . 
     In the exemplary embodiment of FIG. 6, the airbag is likewise partially gathered by a cord  2 , which is guided in tabs  3 ,  4 . However, in this exemplary embodiment, the ends of the cord are fastened in a clamp  10  directly below the tabs  3 ,  4 . The said clamp has a predetermined breaking point  11  (FIG. 8) which breaks open (FIG. 9) at a certain internal pressure of the airbag. In FIG. 6, the airbag has unfolded as far as the secured cord allows it to. After the clamp is broken open, the airbag can be unfolded to its full size, as is illustrated in FIG.  7 . In this exemplary embodiment, only passive volume adaptation as a function of the internal pressure of the airbag is possible. 
     In the exemplary embodiment of FIGS. 10 and 11, two cords  12 ,  13  are guided in a flexible tube  14  in the interior of the airbag. Each of the cords extends over half of the circumference of the airbag and the ends  15 ,  16  of the cords are guided outwards out of the airbag. They can be secured by means of one of the previously illustrated devices after the airbag has been gathered together. 
     By means of the cords the airbag is partially constricted together at  50  in the illustration of FIGS. 10 and 11 and it assumes the illustrated shape in the initial phase of the unfolding forming chambers  51 ,  52 . After the release of the cords it unfolds completely. 
     In FIGS. 12 and 13, a folding device  17  is illustrated schematically. This folding device has a baseplate  18  with a folding tool  19 . In FIG. 12, the said tool is in a starting position, so that the extended airbag  1  can be held with a gas generator  20  already fitted. Tabs are provided which are fastened on the outside of the airbag and of which only one tab  3  can be seen. A cord  21  is guided in the tabs, this cord being wrapped around half of the airbag, similarly to FIG. 11, but it is also possible for it to be wrapped completely around the airbag. One end of the cord is guided outwards through the baseplate  18 . The other end, which is likewise guided outwards, is not illustrated. 
     In the folding of the airbag, as is illustrated in FIG. 13, the cord  21  is pulled downwards out of the baseplate  18  and is fastened after the end of the folding procedure, during the installation of the airbag in the airbag housing, for example in the manner set out in the first exemplary embodiment. 
     FIG. 14 again schematically illustrates the arrangement of a cord  22  guided in four tabs  23 . In this arrangement, the tabs are situated at the same distance from the blow-in mouth  24 . 
     In the exemplary embodiment of FIG. 15, a cord  25  is guided in eight tabs. In this case, four tabs  26  are arranged at a smaller distance from the blow-in mouth  24  than the other four tabs  27 . The cord therefore runs in a zigzag shape in this embodiment. 
     In the exemplary embodiment of FIG. 16, a circumferential cord  28  in a flexible tube-shaped guide  29  is provided, the ends of the said cord being guided to the blow-in mouth  24  of the airbag and being fastened there in the retaining device  6 . The figure shows the airbag in the first phase of the unfolding, in which the airbag volume is limited by the cord  28 . The dashed-dotted line shows the airbag in its full size after the release of the cord. 
     In the exemplary embodiment of FIG. 17, a cord  31  is provided which is connected fixedly to the airbag housing  5  by one end. The cord is guided through an eyelet  32  coupled to the airbag and is fastened in the airbag housing  5  by its other end, by means of the retaining device  6  which has already been described previously. This cord also prevents the airbag from extending to its full size in the first phase of the unfolding. 
     When there is a further rise in the pressure in the airbag, that end of the cord  31  which is fastened in the retaining device is released, and the airbag can be unfolded to its full size, as is illustrated in FIG.  18 . In this case, the cord  31  is pulled out of the eyelet  32  and remains fixedly connected to the airbag housing  5  by one end. In this exemplary embodiment, there are therefore no free cord ends hanging on the airbag, which avoids to an increased extent the occupant coming into contact with the cord. 
     In the exemplary embodiment of FIG. 19, a cord  33  is provided which is connected fixedly to the airbag housing  5  by one end and at the other end has a hook  34 . The latter engages in the eyelet  32 , as can be seen in FIG.  19 . In this manner, the full unfolding of the airbag is likewise prevented. 
     The hook  34  is configured in such a manner that when there is a further increase in the pressure in the airbag, it bends up and slips out of the eyelet, so that the airbag can be unfolded fully, as is illustrated in FIG.  20 . 
     In the exemplary embodiment of FIG. 21, a hook  34  is provided as a rigid pull element that engages in the eyelet  32  and in this manner prevents the complete unfolding of the airbag. As in the preceding exemplary embodiment, at a predetermined internal pressure of the airbag the hook bends up, so that it slips out of the eyelet and releases the airbag, as is illustrated in FIG.  22 .