Abstract:
A tensioner for a safety belt has a piston/cylinder unit, which comprises a cylinder and a piston arranged in its interior, a compressed gas source, on the activation of which the piston is displaced, a belt engagement device which engages the safety belt, a traction cable which connects the piston to the belt engagement device, and at least one dampener for the absorption of energy on deceleration of the movement of the belt engagement device. The cylinder has a first end nearer to the belt engagement device and a second end further away therefrom. The piston consists of at least two piston parts, namely of a first, radially outer piston part and a second, radially inner piston part, the second piston part being uncoupled from the first piston part on deceleration of the piston at the end of the tensioning process.

Description:
TECHNICAL FIELD 
     The invention relates to a tensioner for a safety belt. 
     BACKGROUND OF THE INVENTION 
     Hereto known belt tensioners have a piston/cylinder unit, which comprises a cylinder and a piston arranged in its interior, and a compressed gas source, on the activation of which the piston is displaced. A belt engagement means which engages the safety belt is connected by a traction transfer means to the piston. A damping means provides for the absorption of energy on deceleration of the movement of the belt engagement means. 
     Tensioners for safety belts must be constructed as small as possible owing to the small space available in the vehicle. Inside or outside the cylinder, usually a damping means is provided, having a negative influence on the axial length of the tensioner, which damping means brakes the piston at the end of the tensioning way and in so doing absorbs energy, thus reducing load peaks. Owing to the length of the tensioning way and damping way necessary hitherto, known tensioners have such an axial overall length that they cannot be installed in the vehicle without a cable deflector. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a tensioner which, with the same tensioning way compared with known tensioners, is distinctly shorter in axial direction, without this being achieved at the expense of a poorer damping effect. 
     The tensioner according to the invention has a piston/cylinder unit, which comprises a cylinder with a cylinder jacket delimiting an interior of the cylinder and a piston arranged in the interior, and a compressed gas source, on the activation of which a tensioning process is initiated and the piston is displaced in a tensioning direction. Further, it comprises a belt engagement means which engages the safety belt, a traction transfer means which connects the piston to the belt engagement means, and at least one damping means for the absorption of energy on deceleration of movement of the belt engagement means. The cylinder has a first end nearer to the belt engagement means and a second end further away therefrom. The piston consists of at least two piston parts, namely of a first, radially outer piston part and a second, radially inner piston part. The second piston part is uncoupled from the first piston part on deceleration of the piston at the end of the tensioning process. 
     By uncoupling the piston parts on deceleration of the piston, it is possible to decelerate the piston parts independently from each other and to provide for their damping independently from each other. Accordingly, two or more damping means can be provided which, owing to uncoupling the two masses of the piston parts, can be constructed relatively small, because the mass which is to be decelerated or damped is smaller than with a piston having a single part. In addition, the tensioning process can proceed more quickly, because the damping ways for both piston parts which are to be decelerated are shorter than has been the case hitherto. Depending on when the damping of the second piston part occurs, the second piston part can run through the entire tensioning way also undamped and only be subsequently damped and decelerated. The first piston part, having a small mass, can be decelerated immediately before the end of the tensioning way on a short distance, so that the damping ways for the deceleration of the piston parts lie almost exclusively outside the tensioning way, therefore chronologically after the complete tensioning way has been covered by the piston parts. 
     In the preferred embodiment, at least one stop is provided for each of the first and the second piston parts each, the stop for the second piston part decelerating the latter chronologically after the impact of the first piston part. This means that the first piston part is already decelerated whilst the second piston part is still moving in the tensioning direction. When the second piston part is coupled with the traction transfer means and hence also with the belt engagement means, the mass of the first piston part, on braking, cannot exert any stress on the unit consisting of second piston part, traction transfer means and belt engagement means, so that, as a whole, the load peaks occurring in the traction transfer means can be kept small. 
     The stop for the first part can be formed by the end wall of the cylinder at the second end, a projection protruding on the inner side from the cylinder jacket or, according to a further preferred embodiment, by an jacket section of the cylinder, tapering conically, at its second end. 
     The stop for the second part, viewed spatially, can be provided in the tensioning direction after the stop for the first piston part or in front of it. In the latter case, a part connected to the traction transfer means strikes against the stop, which, for example, is the end wall at the first end of the cylinder. By a stop which is constructed in such a way, the second part and, if the traction transfer means and also the belt engagement means are fastened to it, the entire resulting unit can be damped, i.e. not decelerated abruptly. 
     According to one embodiment of the invention, the second piston part, the traction transfer means and the belt engagement means move on, after the deceleration of the first piston part, by a damping way and after the deceleration thereof, move back by at least a portion of the damping way contrary to the tensioning direction. 
     This return movement of the unit comprised of second piston part, traction transfer means and belt engagement means can occur by the forward movement of the vehicle occupant relative to the vehicle after the tensioning process or, when the vehicle occupant is not wearing his belt, by an elastic spring element which can be arranged inside the cylinder and moves the second piston part and thereby the belt engagement means back contrary to the tensioning direction at least by a partial distance after running through the damping way. The spring element has a dual function by having on the one hand a damping effect and, on the other hand, a restoring effect for the second piston part. When the belt engagement means is constructed as a belt buckle, it must be ensured after the tensioning is completed that the vehicle occupant can still actuate the release button of the belt buckle. This can be a problem when the belt buckle for example has dipped too deep between the back part and the seat part of a vehicle, as can be the case with tensioners for rear occupants of vehicles. The elastic spring element moves the belt buckle back by a certain distance, whereby the vehicle occupant can actuate the release button. The spring element offers a desired resistance to the pressing of the release button, so that the belt buckle assumes a stable position and is not displaced. 
     In the preferred embodiment, the end wall on the second end of the cylinder has an opening or it is completely open, so that an actual end wall is no longer present. After the second piston part is uncoupled from the first piston part, the traction transfer means and/or the second piston part extend into the opening or even to outside the piston, so that axial construction space can be saved. As the tensioner is usually arranged a few millimeters or even centimeters in front of a component on the vehicle side, which is usually formed by the floor of the vehicle or a carpet on the floor of the vehicle, in the case of restraint this intermediate space can be further utilized as a whole for extending the tensioning way. The traction transfer means or the second part can partially project into this intermediate space and fill it, so that a portion of the damping way can lie outside the cylinder and the tensioning way to be covered within the cylinder by the piston can be extended. 
     Owing to the saving of space which is able to be achieved by the invention, it is possible to accommodate the belt tensioner in the vehicle without a cable deflector. 
     The tensioner according is further characterized in that the sleeve is made of steel and the cone is made of a light metal or of a non-ferrous metal which is easily formable. 
     Furthermore, separate damping means may be provided for the piston parts. 
    
    
     BRIEF DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of the tensioner according to the invention in longitudinal sectional view in a nonactuated state; 
     FIG. 2 shows the tensioner shown in FIG. 1 after covering the entire tensioning way; 
     FIG. 3 shows the tensioner shown in FIGS. 1 and 2, in which the belt buckle with the traction cable moves beyond the tensioning way in the direction towards the floor of the vehicle; 
     FIG. 4 shows a longitudinal sectional view of a second embodiment of the tensioner according to the invention in a non-actuated state, in which the second piston part can strike against the end wall at the second end of the cylinder; 
     FIG. 5 shows a longitudinal sectional view through the tensioner shown in FIG. 4 after covering the tensioning way, corresponding to the state illustrated in FIG. 2; 
     FIG. 6 shows the tensioner shown in FIGS. 4 and 5 at a moment after covering the tensioning way, when the belt buckle and the traction cable have moved further towards the floor of the vehicle, corresponding to the moment illustrated in FIG. 3; and 
     FIGS. 7 to  9  show a further embodiment of the tensioner according to the invention in various phases which correspond to the phases shown in FIGS. 1,  2  and  3  or  4 ,  5  and  6 , but here an elastic spring element being provided in the interior of the cylinder. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1 a tensioner  1  is illustrated, more precisely a linear buckle tensioner for a safety belt  2 . The tensioner comprises a piston/cylinder unit  3 , consisting of a cylinder  5  and a piston  7  displaceably arranged in the interior of the cylinder, with conventional locking elements  9  on its partially conical external contour. A traction transfer means  11  in the form of a traction cable is fastened to the piston  7 , which traction cable at its opposite end is arranged on B 1  a belt engagement means  13  in the form of a belt buckle by means of a cable press part  15 . A compressed gas source  17  is arranged inside the cylinder at the first end  101  of the cylinder  5  facing the belt engagement means  13 . The compressed gas source  17  has a hollow ring-shaped housing  19 , which is filled with propellant. The traction transfer means extends through a central opening  21  in the housing  19  up to the piston. The housing  19  adjoins an end wall  23  of the cylinder  5 , which serves as a seal between the interior of the cylinder  5  and the exterior. Between the piston  7  and the end wall of the housing  19  nearer to the piston  7 , a working space  25  is formed, into which gas flows after ignition of the propellant inside the compressed gas source  17 , which leads to a displacement of the piston  7  together with the traction transfer means  11  and the belt buckle  13  in tensioning direction R. L 1  designates the maximum tensioning way. 
     The tensioner shown in FIG. 1 is constructed very short in the axial direction. This is achieved on the one hand in that the traction transfer means  11 , compared with the length of the cylinder  5 , is constructed so short that the cable press part  15 , in the case of restraint, penetrates into the interior of the cylinder and in so doing pushes the end wall  23 , configured as a separately constructed part and connected to the cylinder  5  in a form-fitting manner, at the first end  101  of the cylinder into its interior. In addition, damping means of very small construction are provided, which do not lead to a substantial axial increase in construction space. These damping means act on different parts. The piston  7  is composed of several piston parts, namely of a first, radially outer piston part  27  in the form of a steel sleeve and of a second, radially inner piston part  29  in the form of an aluminum cone. The first piston part  27  has a central opening which is exactly adapted to the shape of the cone  29 , so that no gas can flow between the two parts. The second piston part  29  is connected to the traction transfer means  11 , which at the lower end still slightly projects axially over the second piston part. Also between the second piston part  29  and the traction transfer means  11  no leakage gas flows can occur when compressed gas flows into the working space  25  of the cylinder. As the second piston part  29  tapers towards the first end of the cylinder  5 , the two piston parts  27 ,  29  remain connected to each other on flowing of gas into the working space  25 , and the entire piston  7  moves towards the second end  102  of the cylinder  5 . 
     The cylinder  5  has a jacket section  45  at its second end  102  tapering in a conical manner, the transition of which to the circular cylindrical jacket section forms a stop for the first piston part  27  and hence delimits the tensioning way L 1 . Before the piston part  27  strikes against the transition between the circular cylindrical and tapering jacket section  45  of the cylinder  5 , a sleeve  33  formed on the end wall  23  at the first end  101  of the cylinder  5  is plastically deformed over the damping way D 1  by the striking of the cable press part  15  and absorbs energy. This sleeve  33  cooperates with the cable press part as first damping means. In addition, the tapering jacket section  45  can also be slightly deformed plastically by the first piston part  27  striking on it, and can act as a second damping device. Upon deceleration of the first piston part  27  on striking onto the narrowing jacket section  45 , the second piston part  29  is uncoupled from the first owing to the inertial mass of the second piston part  29 , traction transfer means  11  and belt engagement means  13 . The unit thereby materializing moves further downwards (cf. FIG. 3) until the second piston part  29  finally strikes against the jacket section  45 . If the second piston part  29  is uncoupled from the first piston part  27  at the moment shown in FIG. 2 or shortly thereafter, the cable press part  15  tears the end wall  23  out from its seat on the cylinder  5  and pushes it into the interior of the cylinder  5 , as shown in FIG.  3 . Thereby, the gas in the working volume  25  is compressed and is blown into the open air via the resulting opening between the first and second piston parts  27 ,  29 , because the cylinder has an open end wall at the second end  102 . 
     The end wall  23 , which is pushed into the interior of the piston, cooperates with the housing  19  as damping piston and like a pneumatic damping means. When the second piston part  29  strikes against the jacket section  45 , the jacket section  45  or the second piston part  29  can be further deformed slightly and contribute to the damping. As the second piston part  29  is made of aluminum, it can be plastically deformed to a high degree when it strikes onto the jacket section  45  and can thereby absorb a high amount of energy. Apart from light metal, the second piston part  29  can also consist of an easily deformable non-ferrous metal such as brass or copper. The end of the traction transfer means projecting downwards with respect to the second piston part  29  projects into the opening  47  of the lower end wall of the cylinder, which is formed by the jacket section  45 , or even projects out from it and can extend up to the floor of the vehicle. As the damping characterized by the damping way D 2  only occurs after the deceleration of the first piston part  27 , when the two piston parts are uncoupled from each other, these piston parts are decelerated by partially differing damping means. The overall mass of each part which is to be braked is thereby small, so that the damping means can be constructed small. By the uncoupling of the two piston parts from each other, the mass of the first piston part together with the locking elements  9  does not act on the traction transfer means  11  during deceleration, whereby, as is also aimed for, the load on the traction transfer means can be kept low and load peaks are avoided. As the damping way D 2  is, in addition, passed through after the entire tensioning way has been covered, the latter can be covered quickly by the piston  7 . 
     The vehicle occupant, who moves forward after the belt tensioning is completed, presses the belt buckle  13  together with the second piston part back to the position shown in FIG. 2, but not further, because the locking elements  9  prevent the piston  7  from being pushed back further against the tensioning direction R. 
     In the embodiment of the tensioner shown in FIGS. 4 to  6 , the cylinder  5  is shorter than in the embodiments illustrated in FIGS. 1 to  3 , because the second piston part  29 , after the tensioning has been completed (FIG.  5 ), passes partially through the open end wall  51  of the cylinder (cf. FIG.  6 ). The damping way D 2  therefore lies completely outside the cylinder in the space between the cylinder and an interference contour in the vehicle, which remains unused in known belt tensioners. In the embodiment illustrated in FIGS. 4 to  6 , the end wall  51  forms the stop for the first piston part  27 . 
     The embodiment of the tensioner illustrated in FIGS. 7 to  9  differs from that illustrated in FIGS. 4 to  6  in that the cylinder  5  is extended by a section  53  into which the second piston part  29  moves after detaching from the first piston part  27 . An indentation forms the stop  55  for the first piston part  27 . An elastic spring element  57  in the form of a conical spring serves on the one hand to a small extent for damping the traction transfer means  11  when the second piston part  29  detaches itself from the first (see the lower position of the piston  7  in FIG.  7 ). On the other hand, when the second piston part has moved by the damping way D 2  (cf. FIG. 8) with respect to the first damping part  27  in tensioning direction R, the spring element  57  moves back for contact with the first piston part  27 . When the vehicle occupant wishes to remove the safety belt after tensioning is complete, the elastic spring element  53  offers a resistance to a displacement of the belt buckle, so that it is prevented that the unlocking button on the belt buckle  13  can no longer be actuated because of the belt buckle being easily displaceable. This relates in particular to tensioners which are arranged on a rear bench seat and, after the tensioning process, would be situated between the seat- and back upholstery. The elastic spring element  57  moves the second piston part  29 , after tensioning is completed, back into the position shown in FIG.  9 . Compared with the position illustrated in FIG. 8, it can be seen that until the locking elements  9  operate, a slight displacement of the entire piston is still possible contrary to the tensioning position R.