Abstract:
A pretensioning device ( 10 ) for a safety belt, of a motor vehicle, having a gas generator ( 17 ), a piston ( 21 ) guided in a tube ( 16 ), which closes a pressure area ( 20 ) in the tube ( 16 ) that is pressurized by the gas generator ( 17 ) whereupon the piston ( 21 ) is able to move in a pretensioning movement which is able to be conveyed to the safety belt by means of an inertia body ( 19   a ) resting on a front face ( 9 ) of the piston ( 21 ). The piston ( 21 ) forms a pass-through opening ( 1 ) releasable upon exceeding of a predetermined pressure and a recess ( 26 ) is provided in the front face ( 9 ) with an inertia body ( 19   a ) resting thereon, so that a flow connection is created from the pass-through opening ( 1 ) to the area ( 25 ) located behind the piston ( 21 ) in the direction of the pretensioning movement (S).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 201 0 018 512.4, filed Apr. 27, 2010 and PCT/EP2011/001057, filed Mar. 3, 2011. 
     FIELD OF THE INVENTION 
     The invention relates to a pyrotechnic pretensioning device for a safety belt, in particular for use in a motor vehicle. 
     BACKGROUND OF THE INVENTION 
     The basic problem in a pretensioning device is that the pressure relationships in a pretentioner tube produced by gas from a pyrotechnic gas generator change greatly during the pretensioning process. Especially high pressure pressure peaks can lead to the parts of the pretensioning device being damaged or the movement sequence of the pretensioning device being impeded. 
     Already known from DE 195 45 795 C1 is a pyrotechnical driving device with an overpressure safety by means of which an exceeding of a predetermined pressure is prevented in the tube of the pretensioning device. In this solution, an opening is provided in the pretensioner tube in the area of the gas generator and the gas generator is provided with a gas generator casing. Upon exceeding of a predetermined pressure, the wall of the gas generator casing is pressed into the opening of the tube and is thereby so transformed that it tears apart and releases the opening. 
     A disadvantage of the above mentioned solution is that the pressure is released outward outside the tube and thereby a hot jet or even a flash is released whereupon adjacent parts can be damaged. 
     Known from DE 102 12 912 B4 is, for example, a pretensioning device with a piston-cylinder arrangement in which a pressure release valve is provided in the piston by means of which a pressure reduction from the pressure chamber in front of the piston is facilitated. In the pretensioning device described therein, the driving movement of the piston is conveyed by means of a gear rack which engages into a geared wheel connected with the belt shaft. A release of the pressure via the pressure release opening described therein presupposes that a free space is available in the direction of movement of the driving device behind the piston in which the pressure can escape from the pressure chamber. 
     In a pretensioning device known from DE 195 45 795 C1 such a solution is basically not possible, since the drive device to convey the pretensioning movement is formed here by a powered solid body linkage which rests directly on the piston with its first inertia body. The free space required for the solution known from DE 102 12 912 B4 is constricted here at least by the first inertia body. In addition there is also the disadvantage that the first inertia body rests with a very high pressure on the piston during the drive movement of the piston and as a result an opening present in the piston would close. 
     The object of this invention is to create a pretensioning device with an inertia body linkage powered by a piston in which the overpressure from the pressure area located in front of the piston can be reduced abruptly, upon exceeding of a predetermined pressure, without endangering the parts in the vicinity of the pretensioning device. 
     The object is attained in the invention by means of the pretensioning device described by the specification, and the associated figures. 
     To attain the object, the invention proposes that the piston of the pretensioner feature a pass-through opening releasable upon exceeding of a predetermined pressure, and a recess—with an inertia body resting thereon—is provided on the front side through which a gas flow connection is created from the pass-through opening to the space located behind the piston in the direction of pretensioning movement. 
     The advantage of the inventive solution can be seen in that by means of the flow connection so created, the pressure from the pressure area can be reduced, even with an inertia body resting on the front side of the piston, namely during the pretensioning movement or with a blocked pretensioning movement. The pressure is intentionally not released into the surroundings, which is the case with the known prior art, so that parts adjacent to the pretensioning device are not endangered by the released pressure. 
     It is further proposed that at least two recesses be provided in the front area. The advantage from the placement of two or more recesses in the front face is that the pressure change acting on the inertia body because of the flow of the pressurized gas can be made uniform so that the inertia body also does not lose contact on one side with the front face during the pressure release and possibly execute a tipping movement. 
     The smoothest possible change of the pressure forces acting on the inertia bodies can be attained in that the recesses are dimensioned radially symmetrical and/or identical. 
     It has been shown in particular that a smooth and possibly fast pressure reduction can be obtained by the pass-through opening being placed in the middle of the front face of the piston and the recess running radially from the pass-through opening to the outer edge of the piston. 
     It is also proposed that the area of the recess(es) amounts to 15-50% of the surface of the total surface of the piston front face. By means of the proposed portion of the area of the recesses of the front face it is guaranteed that the pressure decreases quickly and the inertia body nonetheless rests on a sufficient bearing surface on the front face of the piston without the danger existing that the part of the front face on which the piston rests is melted or destroyed because of the excessively high stress from the temperatures acting thereon. The portion of the surface of the recesses thereby relates to the area of the front face of the piston on which the first inertial body rests without recesses. 
     It is also proposed that the piston exhibits a higher surface hardness in the area of the front face compared to the other surface. The greater surface hardness of the front face prevents the deformation of the front face in the area of the bearing surface of the inertia body and keeps the recesses from being closed due to deformation of the material. The greater surface hardness can be obtained either by a surface treatment in the area of the front face or also by a specific softening of the surface in the area of the sealing segment resting on the tube. It is thereby important that the piston meets both the requirements regarding sealing with respect to the tube as well as also regarding the force transfer across the front face. 
     Another preferred embodiment of the invention can be seen in the fact that the piston is constructed in at least two parts and exhibits a first part which rests on the interior wall of the tube and exhibits a second part on which the front face is placed, and the second part exhibits a greater strength than the first part. By means of the two-part design of the piston, the piston can be so constructed that it features the needed strength in the area of force transfer and simultaneously the necessary elasticity in the area of its sealing area, which is especially advantageous when the piston is guided in a curved pretensioner tube during the pretensioning process and a slight movement relative to the tube wall can be executed in the course of the curved tube. 
     In this case an especially cost-effective solution can be realized in that the second part of the piston is constructed from a deep-drawn part. The second part can thereby be constructed cost-effectively in mass production, whereby it is especially advantageous that the shaping of the recesses can also occur in the deep-drawing process. 
     It is further proposed that between the first and the second part an insert is provided which closes the pass-through opening. The advantage of the use of the insert can be seen in that with regard to the selection of material and the individual dimensioning it can be so designed that it is destroyed upon exceeding of a previously determined pressure difference between the pressure prevailing in the pressure area and the pressure in the space on the other side of the piston and thus enables a pressure equalization. For the case that the pass-through opening should be released at another pressure difference, only the construction of the insert must be changed or it can be exchanged for an insert of another material without the piston being otherwise changed. 
     It is further proposed that the insert be radially tensioned with a cylindrical ring extension between the first and the second part. By means of the cylindrical ring extension the insert can be fixed in the radial direction. Consequently in addition the insert can be so fixed against a deformation in the axial direction that the insert does not axially bulge in a pre-phase to release the opening, but instead tears at a predetermined pressure difference defined by the design of the insert without a deformation. 
     This abrupt tearing can also be further caused or enhanced in that the first and/or the second part exhibit a radially inward protruding collar narrowing the pass-through opening on which the insert rests. The collar acts with a pressure loading of the insert as a cutting edge which enables the tearing off of the insert by producing appropriate cutting forces in the insert. 
     In this case it is further proposed that the collar be placed on the second part and that the insert rests on the side of the second part facing the pressure area, and cuts the collar upon exceeding of a predetermined pressure in the pressure area. Since the second part exhibits a greater strength than the first part of the piston and the second part with the front face is supported on the inertia body, the pressure force acting on the insert is conveyed across the second part to the inertia body. Since the second part and the inertia body are already designed as appropriately rigid and with a stable shape because of their function to convey the pretensioning drive power, the insert thus is supported on a component assembly that is especially stable in shape and tears off when exceeding a predetermined pressure difference, without the point in time of the tearing off being dependent on a deformation of this component assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following section the invention will be explained in more detail using a preferred embodiment. Specifically recognizable in the figures are: 
         FIG. 1  shows a pretensioning device coupled to a belt roller with a piston guided in a tube; 
         FIG. 2  shows a pretensioner piston with four recesses in the front face in an isometric view; 
         FIG. 3  shows the piston of  FIG. 2  seen in the direction of the front face; 
         FIG. 4  shows a piston with three recesses in the front face in an isometric view; 
         FIG. 5  is a cross-sectional view of the piston from  FIG. 4 ; 
         FIG. 6  shows a two-part piston with adjoining inertia body in a tube of the pretensioning device; and 
         FIG. 7  shows a two-part piston with an insert provided between the parts of the piston. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The belt retractor schematically shown in  FIG. 1  comprises a housing  11  with a side leg  13 , a belt coil shaft  12  placed thereon for a safety belt strap (not illustrated), and a pretensioning device  10  acting after release on the belt spool shaft  12 . The pretensioning device  10  encompasses a drive wheel  14  connected in a rotationally fixed manner with the belt spool shaft  12  which, for example, features an external gearing  15 , a gas generator  17 , in particular pyrotechnic type, to produce a gas pressure, and a tube  16  connecting the gas generator  17  with the belt spool shaft  12  via a drive wheel  14 . The tube  16  is formed by a tube wall  24  which can be part of the housing  11  or alternatively also a separate component part. 
     Provided in the tube  16  is a series of metallic ball-shaped inertia bodies  19  to convey the pretensioning movement caused by the gas pressure produced by the gas generator  17  to the belt spool shaft  12  via the drive wheel  14 . The belt retractor is not limited with regard to the design of the interaction area  18  between the series of inertia bodies  19  and the drive wheel  14 , as well as any possible coupling devices between the drive wheel  14  and the belt spool shaft  12 . For a low friction force transfer the outer diameter of the inertial body  19  is advantageously somewhat smaller than the inner diameter of the tube  16 . 
     Provided in the tube  16  is a piston  21  only shown diagrammatically in  FIG. 1 , which is placed expediently in an area  23  between the gas generator  17  and the series of inertial bodies  19 , meaning directly in front of the first inertial body  19   a  of the chain of inertia bodies  19  in the direction of force conveyance. The piston  21  closes a pressure area  20  in the tube  16  impacted by the gas generator  17  by means of a gas pressure so that the piston  21  upon the pressure impact of the pressure area  20  by the gas generator  17  can be moved to a pretensioning movement. The pretensioning movement of the piston  21  formed by the inertial bodies  19  and the drive wheel  14  is conveyed by means of the force transfer device to the belt spool roller  12  so that the belt strap is tensioned. 
     An embodiment of the inventive piston  21  with four radially symmetrical recesses positioned at a 90 degree angle to each other is shown in  FIGS. 2 and 3 . The recesses  26  extend in a straight line from a pass-through opening  1  placed in the center, to the radial outer edge of the piston  21 . 
     An alternative embodiment of the piston  21  can be seen in  FIGS. 4 and 5  in which the two-part piston  21  is composed of a first part  3  and a second part  4 . For example, the first part  3  is made from an elastic plastic, like POM, and forms the basic body of the piston  21 , while the second part  4  is constructed as a metallic deep-drawn part and forms the front face  9  of the piston  21 . The second part  4  is formed in the deep-drawn process in the middle of a cylindrical section  4   a  and is pressed along with it into the pass-through opening  1 . Furthermore, three recesses  26  are stamped in the front face  9  which are positioned at an angle of 120 degrees to each other and likewise flow into the pass-through opening  1 . 
     An additional embodiment of a piston  21  is presented in  FIG. 6 , and for a better understanding of the invention is illustrated in a tube  16  with an adjoining inertia body  19   a . The piston  21  is also constructed in this embodiment in two pieces with a first part  3  and a second part  4 . The first part  3  is provided with a sealing lip  22  which rests on the interior wall of the tube  16  to seal the pressure area  20 . The second part  4  is connected with it on the side of the first part  3  facing the inertial body  19   a  and forms the front face  9  of the piston  21  on which the inertial body  19   a  rests. Recesses  26  are provided in the front face  9  of the piston  21  which extend from the pass-through opening  1  to the radial outer edge of the piston  21 . The second part  4  exhibits a greater surface hardness and strength than the first part  3  so that the surface of the piston  21  is not deformed in the area of the front face  9  by the forces acting during the tensioning movement, and as a result the recesses  26  can be pressed together. The first part  3  is specifically made of a material of lesser strength and surface hardness, so that the sealing lip  22  is accordingly capable of deformation and during the pretensioning movement rests in a sealing manner on the interior wall of the tube  16 , even in case of slight side movements of the piston  21  or in case of a curved course of the tube. 
     The pass-through opening  1  is formed by a section  1   a  and a section  1   b , as can be seen in  FIGS. 5 and 6 , whereby section  1   b  in the embodiment in  FIG. 5  and section  1   a  in the embodiment shown in  FIG. 6  are each closed by means of a partition wall  7 . Upon exceeding of a pressure in the pressure space  20  determined by the wall strength and the material of the partition wall  7 , said material tears and then releases the pass-through opening  1 . 
     After the tearing of the partition wall  7  the pressure can escape from the pressure area  20  via the pass-through opening  1  and the recesses  26  in the direction of the pretensioning movement S into the area  25  located behind the piston, without it being necessary to have a gap between the inertia body  19   a  and the front face  9 . The over-pressure safety is thereby functionally secured even under a full loading of the pretensioning device. 
       FIG. 7  presents another embodiment of the piston  21  with two parts  3  and  4 , between which there is an insert  30  which is constructed as a pot with a cylindrical ring extension  27  and a tear-off section  29  closing the pass-through opening  1 . The insert  30  is clamped with the ring circular extension  27  in the radial direction between the first part  3  and the second part  4  and in the axial direction between the collars  28  and  31  of the first and second parts  3  and  4 , thereby narrowing the pass-through opening. The insert  30  is thus rigidly clamped both in the axial as well as the radial direction between parts  3  and  4 . Because of its dimensions and the use of plastic as the material, the collar  28  acts as a spring so that the collar  28  can flex somewhat upon the introduction of the insert  30  and the second part  4  without itself being destroyed or damaged. 
     With an increase of the pressure in the pressure area  20  and as a result an exceeding of a predetermined pressure difference of the pressures adjoining on the different sides of the insert  30 , the insert  30  tears apart in the area of the tear-off section  29 , whereby the tear-off begins at the edge of the collar  31  which in this case acts like a cutting edge. Since the insert  30  with the cylindrical ring extension is radially tensioned between parts  3  and  4 , the deformation of the insert  30  and in particular the deformation of the tear-off section  29 , is limited in the axial direction. The tear-off of the insert  30  begins at a spot of the tear-off section  29  lying on the edge of the collar  31  which then subsequently cuts along the edge of the collar  31  in the circumferential direction. 
     The insert  30  thereby is supported on the second part  4  which exhibits a greater strength and is supported with the front face  9  on the inertia body  19   a . By means of the second part  4  the insert  30  finds an appropriate mechanical support, so that the pressure difference on insert  30  specifically results in a tearing off of the insert  30  without the second part  4  being thereby previously deformed and consequently the point in time of the tearing-off can be influenced. Both the second part  4  as well as also the insert  30  can be constructed as metallic deep-drawn parts. In each case the second part  4  should exhibit a greater stability of shape obtained by design or by selection of material than the insert  30 , so that the insert  30  tears off at a predetermined pressure difference. 
     The insert  30  exhibits a slightly smaller outer diameter than the inner diameter of the pass-through opening  1  in the first part  3 , so that the insert  30  is accommodated with free play in the pass-through opening  1  and as a result exerts no radial forces on the first part  3 . Furthermore a support area  32  is constructed on the first part  3 , and when the second part  4  moves in an axial direction of the piston  21 , said second part rests in a form-locking manner in the area of the recesses  26 , so that consequently the expected position of the second part  4  with respect to the first part  3  is determined and the pressure on the insert  30  is limited. 
     Furthermore, a micro-boring  33  with a width of about 0.1 mm is provided in the insert  30  in the area of the tear-off section  29 . The micro-boring  33  enables pressure equalization between the pressure area  20  and the area  25  located behind the piston  21  without the tear-off section  29  having to thereby tear-off. Such pressure equalization then makes sense, for example, when the belt tensioner is activated and the pressure should then escape for a further operation of the belt retractor. By means of this arrangement of the micro-boring  33 , the grooves  34  in the cover area of the piston  21  shown in  FIG. 4  can be omitted. Furthermore the tear-off of the tear-off section  29  can be promoted by the micro-boring  33 . 
     In addition, by means of the micro-boring  29  a movement of the piston  21  against the tensioning movement and against the pressure still existing in the pressure area  20  can be facilitated at the beginning of the force limitation movement. In any event pressure equalization between the pressure area  20  and the space  25  is possible because of the micro-boring  33  without the tear-off section  29  having to tear off, namely independent of exceeding of a predetermined pressure difference between the pressure in the area  25  and the pressure area  20 . 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.