[0001]    The invention relates to a gas generator for a safety arrangement with a means for holding down propellant granule in a combustion chamber. A preferred field of application are gas generators for motor vehicle safety arrangements such as belt tensioners, airbags, roll bars or current supply cutoff units, which are filled with a corresponding propellant granule, preferably in tablet form, for the production of a defined gas volume in the case of activation.  
         BACKGROUND OF THE INVENTION  
         [0002]    Gas generators are to be used increasingly universally for various applications, e.g. in the driver&#39;s or passenger&#39;s airbag, in which differing gas volumes are made available with differing speed. The propellant granule is used for this in differing concentration, quantity and chemical composition. Fluctuations in the filling level, however, may cause relative movements of the propellant granule in the combustion chamber, a non-uniform distribution of the granule, a nd material abrasion on the propellant granule. Thereby, the required ignition- and pressure propagation is impaired. For these reasons, an arresting of the propellant granule is necessary. In addition, through an arresting, rattling noises of the granulate in the combustion chamber can be avoided.  
           [0003]    As can be seen from the DE 42 27 547 A1, to balance out the filling level and to avoid relative movements, a filling body, designated in the publication as volume equalizing means, is used. This filling body, generally of elastic material, occupies the empty volume of greater or lesser size which is present in most cases and balances out fluctuations in level. The known filling body can, however, only balance out the filling level well within particular limits. In the case of greater level differences, the size or quantity of the filling body must be adapted accordingly. This increases the material costs, but in particular also the manufacturing expenditure, because means for recognizing the filling level and dosing the filling become necessary.  
           [0004]    Furthermore, the use of filling bodies for arresting propellant granule in combustion chambers also proves to be unsuitable because in the case of combustion of the propellant, the chemical and physical characteristics and also the flow-dynamic characteristics at the outlet openings of the produced gas mixture are affected in a negative manner.  
           [0005]    It is therefore an object of the invention to provide gas generators with a means for holding down propellant granule in a combustion chamber, in particular of gas generators for motor vehicle safety arrangements, which means limits the space for the propellant granule irrespective of the required filling level, and reliably arrests it.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a gas generator for a safety arrangement, in particular for a vehicle occupant restraint system, comprising:  
           [0007]    a combustion chamber delimited by a combustion chamber wall and at least one combustion chamber cover;  
           [0008]    propellant granule filled into the combustion chamber to a preselected filling level, and  
           [0009]    a means arranged in the combustion chamber for holding down the propellant granule;  
           [0010]    wherein the means for holding down the propellant granule comprises a disc-shaped holding face, the holding face being pressed into the combustion chamber and connected in a form- and force-fitting manner with the combustion chamber wall, and being pre-stressed for pressing elastically onto the propellant granule, thereby arresting the propellant granule at the preselected filling level and in a variable distance from the combustion chamber cover; and  
           [0011]    wherein the holding face has grid holes having a diameter smaller than the propellant granule.  
           [0012]    The holding face is to be understood here as a flat or corrugated structure, provided with holes, preferably a metallic fabric or a perforated metal sheet, the maximum grid hole diameter of which is smaller than the propellant granule, in order to ensure the arresting of the propellant granule in accordance with the invention. By means of the mesh width of such a grid or the number and size of the holes, in addition the outflow of the combustion gas into the empty space of the combustion chamber can be controlled and pre-filtered. The holding face is constructed such that a form- and force fitting connection to the combustion chamber wall is produced.  
           [0013]    For the production of a form- and force-fitting connection between the holding face and the combustion chamber wall, various known solutions are available, such as for example a corresponding press-fit supported by a chamfering of the holding face edge or by spring rings. The arresting of the propellant granule takes place independently of the filling level by pressing in the disc-shaped holding face into the combustion chamber, the force and speed of pressing in being selected such that the propellant granule is pushed together accordingly and arrested, but a destruction of the propellant granule or abrasion is largely avoided. In addition, the holding face can be used for filtering the outflowing gas in order to avoid an outflow of combustion residues.  
           [0014]    An advantageous further development of the invention consists in that through corresponding dimensioning of the form- and force-fitting connection and of the holding face in relation to the type of material and to the sum of the hole areas, the release of the arresting means is made possible on exceeding a predetermined threshold pressure. Thereby, undesirably high pressure peaks which arise through non-uniform deflagration or faulty propellant granule, can be intercepted. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention is explained in further detail in embodiments with the aid of the drawings, in which:  
         [0016]    [0016]FIG. 1 shows a tubular gas generator according to the prior art, with conventional filling body,  
         [0017]    [0017]FIG. 2 shows a tube gas generator with a grid-shaped holding face, according to the invention, for arresting,  
         [0018]    [0018]FIGS. 3 a  and  3   b  show a grid-shaped holding face according to the invention, for a tubular gas generator and with a chamfered outer edge,  
         [0019]    [0019]FIG. 4 shows a tubular gas generator with a grid-shaped holding face according to the invention, after the permissible pressure threshold value has been exceeded,  
         [0020]    [0020]FIG. 5 shows a partial section through a ring gas generator with a grid-shaped holding face as a hollow ring disc corresponding to the hollow cylindrical combustion chamber,  
         [0021]    [0021]FIGS. 6 a  and  6   b  show a tubular gas generator according to a further embodiment of the invention, and  
         [0022]    [0022]FIG. 7 a  shows a cross-section through the tubular gas generator according to FIG. 6.  
         [0023]    [0023]FIG. 7 b  is a view of a part shown in FIG. 7 a.   
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0024]    [0024]FIG. 1 shows an embodiment of a tubular gas generator in accordance with the DE 42 27 547 A1. Like all gas generators, this embodiment comprises an ignition device  5 , a combustion chamber  4  and a combustion chamber cover or overflow part  6   a  with outflow openings  6   b . Usually a propellant  3  in granulate form is present in the combustion chamber  4 . FIG. 1 additionally shows inside the ignition device  5 : electrical connections  5   a , a primary charge  5   b , a secondary charge Sc and an ignition opening  5   d  in the housing  5   e  of the ignition device  5 . The gas occurring on deflagration of the propellant  3  can escape via the outflow openings  6   b  into the filter chamber  7 , with a corresponding pressure existing in the combustion chamber  4 . The filter chamber  7  is surrounded by a filter tube  7   a  with outflow openings  7   b  and a filter  7   c , which is arrested by an annular metal sheet  7   d . The propellant granule is held down by means of a filling body  2 .  
         [0025]    The gas generator shown in FIG. 2 corresponds in its construction to the gas generator according to the prior art shown in FIG. 1, with the exception of the alterations in accordance with the invention. According to the invention, the propellant granule  3  is arrested in the combustion chamber  4  by the disc-shaped holding face  1  which is provided with grid holes, with a corresponding form- and force-fitting connection existing between the outer edges of the grid-shaped holding face  1  and the combustion chamber wall  4   a . In the embodiment as shown, the holding face  1  is spaced apart from the cover  6   a  thereby defining an empty space  8  free of propellant granule  3 . The holding face  1  presses elastically and with a prestressing onto the propellant granule. This enables the propellant granule  3  to be arrested in a variable distance from the combustion chamber cover  6   a . Relative movements of the propellant granule  3  inside the combustion chamber  4  are thereby prevented, independently of the filling level. Thereby, abrasion and rattling noises are avoided and through the defined position of the propellant granule  3 , a desired ignition- and pressure propagation is ensured. This leads to do away with the filling body, to a simpler manufacture without special filling level measuring means and to the gas generator and the manufacturing installation being able to be used for various quantities of propellant granule and concentrations, as is necessary in the field of airbags. The gas can escape into the filter chamber  7  via the empty space  8  and the outflow openings  6   b  in the cover or overflow part  6   a . The filter chamber  7  is surrounded by a filter tube  7   a  with outflow openings  7   b  and by a filter  7   c  which is arrested by an annular metal sheet  7   d . The ignition device  5  can likewise be seen in FIG. 2 with its components: electrical connections  5   a , primary charge  5   b , secondary charge  5   c  and ignition opening  5   d .  
         [0026]    The additional use of a thin layer of elastic filler  2   a  upstream of the holding face  1  is possible for damping.  
         [0027]    [0027]FIG. 3 a  shows a holding face  1  according to the invention, provided with grid holes and suitable for the tubular gas generator illustrated in FIG. 2. The holding face  1  according to FIG. 3 is constructed as a wire fabric which is constructed so as to be chamfered, i.e. bent to have an angled profile, and sharp-edged at the rim for better arresting on the combustion chamber wall. The size, material, type of weave and the tolerances are selected according to the size of the propellant granule and the desired threshold pressure. If the pressure rises within this threshold value, the opening areas of the grid are sufficient to ensure a pressure equalization within the entire combustion chamber.  
         [0028]    [0028]FIG. 4 shows a tubular gas generator according to FIG. 1, in which the holding face  1 , owing to an exceeding of the pressure threshold value, has been pressed against the end of the combustion chamber and thus the pressure peak has been reduced accordingly.  
         [0029]    [0029]FIG. 5 now shows a partial section through a ring gas generator in which the holding face  21  is constructed as a hollow ring disc, corresponding to the hollow cylindrical combustion chamber  24 . The holding face  21  is arranged here not upstream of the overflow region, consisting of an overflow filter  26   a  and an overflow opening  24   c  in the combustion chamber base  24   b , but rather in the upper region of the combustion chamber  24  lying opposite the ignition device  25 , in order to also show this option. The ignition device  25  is likewise illustrated in FIG. 25 with its components: electrical connections  25   a , primary charge  25   b , secondary charge  25   c  and ignition opening  25   d . The combustion chamber  24  is formed by a combustion chamber cover  24   a  and a combustion chamber base  24   b  screwed therewith, which has lateral overflow openings  24   c  leading to the filter chamber  27 . The filter  27   c  is arranged in the filter chamber  27  upstream of a deflector plate  27   a  which in turn has outflow openings  27   b . The prefiltered gas flows through these to a fine filter  27   d , held by an outer plate  27   e , and then through the outlet opening  27   f . An empty space  28  free of propellant granules  23  is defined between the holding face  21  and the combustion chamber cover  24   a .  
         [0030]    The gas generator shown in FIG. 6 has a tubular outer housing  310  with a combustion chamber  34  formed therein. The combustion chamber  34  is filled with propellant granule  33  in tablet form. The combustion chamber  34  is delimited by a combustion chamber wall  34   a , on the inner face of which longitudinal ribs  312  are formed, extending in axial direction. Between the longitudinal ribs a strip-shaped filter  314  is arranged. An ignition device  35  is integrated into a cover  316  closing the combustion chamber  34  and projects into the combustion chamber  34 .  
         [0031]    In the embodiment shown here, the holding face  31  consists of a perforated metal sheet with grid holes  318 , the diameter of which is smaller than the diameter of the propellant granule  33  and of the propellant tablets (FIG. 7 a ), respectively. In the embodiment shown here, the holding face has two opposed straight edges  320  which engage the longitudinal ribs  312  and thus produce the form- and force-fitting connection. In the region of the straight edges  320 , the holding face  31  is additionally angled, preferably at an angle of approximately 30 degrees, whereby the spreading of the edges  320  into the longitudinal ribs  312  is further favored. For stiffening, the holding face  31  can be additionally provided with crimped portions  322 , preferably in the form of an arc of a circle, which extend substantially transversely to the straight edges.  
         [0032]    After the filling of the combustion chamber  34  with the propellant granule  33 , the holding face  31  is pressed into the combustion chamber  34 . Here, it is under prestressing and presses elastically onto the propellant granule  33 . An empty space  38  is defined between the cover  316  and the holding face  31 . In this way, a simple filling of the gas generator with propellant granule is made possible without costly measurement of the filling height, and at the same time rattling noises by loose propellant granule  33  are avoided, as it is possible to arrest the propellant granule  33  in a variable distance from the cover  316 . For tubular gas generators with reduced power, a reduced quantity of propellant granule  33  can be used without costly changes to the manufacturing process. The position of the holding face in a gas generator with reduced power is likewise illustrated in FIG. 6 b  and is designated by the reference number  31 ′. Between the holding face  31  and the propellant granule  33  in addition a filling body of elastic material (not shown here) can be arranged.  
         [0033]    As an alternative to the embodiment illustrated in FIG. 7 a , the holding face  31  can also be provided on its edge with bulges  324  (FIG. 7 b ) which exert a punctiform stress on the combustion chamber wall and thus bring about the form- and force-fitting connection. In this case, the straight edges and also the longitudinal ribs on the combustion chamber wall can be eliminated.  
         [0034]    Generally, a structural adaptation of the grid-shaped holding face to the shape and dimensions of the combustion chamber, the use of various arresting possibilities and also the adaptation of the grid material, the type and width of the grid for the actual case of application is possible at any time. Also the use of spring sheets with a correspondingly high number of holes is conceivable. Thereby, a prestressing of the grid-shaped holding face can be achieved, which acts in an elastic manner on the propellant granule and thus ensures an equalization of filling volume contractions during the lifespan of the product. In addition, the arresting forces and the threshold pressure value can be set precisely.
The invention relates to a gas generator for a safety arrangement, in particular for a vehicle occupant restraint system, comprising a combustion chamber delimited by a combustion chamber wall and filled with propellant granule, and also a means arranged in the combustion chamber for holding down the propellant granule. The gas generator according to the invention is characterized in that the means for holding down the propellant granule comprises a disc-shaped holding face having grid holes, wherein the holding face is pressed into the combustion chamber and is connected in a form- and force-fitting manner with the combustion chamber wall, and is prestressed and presses elastically onto the propellant granule, the grid holes having a smaller diameter than the propellant granule.