Abstract:
A rotary drive for a belt tensioner includes a housing, a pinion rotatably mounted therein, and at least one driving gear coupled to the pinion, the driving gear being mounted in the housing and being adapted to be driven by pressurized gas introduced into a pressure chamber. Formed in the driving gear is a cavity having a substantially radially extending wall which, in the installed state, is opposite to a protrusion which projects into the cavity and is rigidly connected to the housing, the wall of the cavity and the protrusion forming part of a wall of the pressure chamber.

Description:
TECHNICAL FIELD 
   The invention relates to a rotary drive for a belt tensioner. 
   BACKGROUND OF THE INVENTION 
   A rotary drive of the type concerned is known from EP 0 628 454 A1. In this rotary drive a driving gear configured as a ring gear is coupled via three planetary gears to a sun gear forming a pinion, the planetary gears engaging in the internal toothing of the ring gear and in the external toothing of the sun gear. Formed at the outer periphery of the ring gear is a rotary piston extension which forms, together with the ring gear, a single-vane rotary piston. This single-vane rotary piston moves in an annular space which surrounds the ring gear and may be acted upon with pressurized gas. 
   The invention, in contrast, provides a rotary drive for a belt tensioner which is distinguished by a particularly compact construction. 
   BRIEF SUMMARY OF THE INVENTION 
   This is achieved in a rotary drive for a belt tensioner, including a housing, a pinion rotatably mounted therein, and at least one driving gear coupled to the pinion, the driving gear being mounted in the housing and being adapted to be driven by pressurized gas introduced into a pressure chamber. Formed in the driving gear is a cavity having a substantially radially extending wall which, in the installed state, is opposite to a protrusion which projects into the cavity and is rigidly connected to the housing, the wall of the cavity and the protrusion forming part of a wall of the pressure chamber. The remaining pressure chamber wall is likewise formed by walls of the cavity or the housing. If the pressure chamber is now acted upon with pressurized gas, it will attempt to expand. Since the protrusion is housing-fixed and the wall lying opposite thereto at a small distance is movable relative to the housing (the driving gear is rotatably mounted in the housing), the cavity wall moves relative to the protrusion: The driving gear rotates. Thus, the pressure chamber is configured in the interior of the driving gear itself, whereby a pressure chamber which is disposed outside the gear and which would increase the space required may be dispensed with. Thus, a compact construction having a diameter of approx. 65 mm only and a width of 30 mm may be implemented. 
   Preferably, the wall of the cavity in the non-operated state includes an angle between 10° and 45° with the protrusion. Thus, the intermediate pressure chamber comprises in the installed state a comparatively low volume, which results in the rotary drive taking effect in the case of activation almost without any delay in time. 
   The protrusion preferably comprises a gasket which closes off the pressure chamber gas-tightly to a large extent. 
   For an easy assembly the housing may comprise a cap, at least one extension forming the protrusion being integrally molded thereon. 
   According to a first embodiment of the invention the driving gear is configured as a ring gear whose periphery comprises in a partial region an internal toothing. If in this arrangement the ring gear has the outer diameter of a belt retractor with which the rotary drive is coupled, a maximum possible tensioning efficiency is achieved, the available space being made use of at an optimum. 
   The partial region may extend across more than half of the periphery, whereby a particularly high tensioning efficiency is achieved. 
   Preferably, the cavity substantially has the shape of a circular ring sector whose central angle amounts to less than 180°. By means of the central angle the size of the pressure chamber in the expanded state (after activation of the rotary drive) is determined. At the same time the angle through which the internal toothing extends and the central angle complement each other to approx. 360°. 
   Advantageously, in the housing there is mounted an intermediate gear which meshes both with the ring gear and the pinion. In this manner a planetary gear transmission is obtained, in which the pressure chamber is disposed in the interior of the ring gear. Thus, the tensioning efficiency may variably be adjusted within certain limits by means of different transmission ratios. 
   According to a second embodiment of the invention the driving gear comprises a surrounding external toothing and is in direct engagement with the pinion. Thereby, a rotary drive materializes, which is distinguished by a particularly compact construction and a low (inert) mass. 
   Preferably, the cavity substantially has the shape of a circular ring that is interrupted at one point by the radially extending wall. Thus, a particularly high tensioning efficiency may be achieved. 
   Advantageously, several driving gears are provided which are all able to mesh with the pinion. The driving gears are always in engagement with the pinion, whereby the load is uniformly distributed on a plurality of teeth on the pinion. However, this arrangement does not constitute a planetary gear transmission, but the driving gears are directly and separately driven with pressurized gas. 
   It is possible to provide a common pyrotechnical propellant charge which is connected via branching flow ducts to the pressure chambers of the driving gears. Thereby, it is always ensured that all driving gears are simultaneously acted upon with pressurized gas. 
   Preferably, four driving gears are provided which are uniformly distributed about the pinion. In this manner the pinion is not subjected to any one-sided forces acting laterally thereon. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an exploded view of a rotary drive according to a first embodiment of the invention; 
       FIG. 2  shows a sectional view of the rotary drive of  FIG. 1 ; 
       FIG. 3  shows an exploded view of a rotary drive according to a second embodiment of the invention; 
       FIG. 4  shows a perspective view of a cap of the rotary drive of  FIG. 3 ; 
       FIG. 5  shows a perspective view of a pressure distributing plate which is employed with the rotary drive according to  FIG. 3 ; and 
       FIG. 6  shows a sectional view of the rotary drive of  FIG. 3 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2  show a rotary drive  10  for a belt tensioner according to a first embodiment of the invention. The rotary drive  10  comprises a housing which is formed of a pot-like housing part  12  and a cap  14 . Disposed on the cap  14  is a pyrotechnical propellant charge  16  which includes an ignition means for activating the propellant charge (not shown). 
   Via a hole  18  in the cap  14  the propellant charge  16  is in fluid communication with a pressure chamber  20 . The walls of the pressure chamber  20  are formed by the wall of a cavity  24  configured in a ring gear  22 , the cap  14  itself, and a protrusion in the shape of an extension  26  integrally molded on the cap  14 . The ring gear  22  is mounted in the housing part  12  and in the cap  14  and includes, apart from the cavity  24  which substantially has the shape of a circular ring sector having a central angle &lt;180°, an internal toothing  28 . The latter extends across a partial region of the ring gear  22 , the partial region amounting to somewhat more than half of the periphery, and meshes with an intermediate gear  30  which is likewise mounted in the housing and which in turn is in direct engagement with a pinion  32  that is centrically disposed in the housing. 
   It has to be emphasized that the ring gear  22  comprises a bottom  34  only in the region of the cavity  24  and is designed to be open in the region of the internal toothing  28 . The extension  26  comprises a rubber gasket  36  which seals itself when the pressure chamber  20  is acted upon with pressure. A radially extending wall of the cavity  24  lying opposite to the extension  26  in the installed state is provided with the reference numeral  38  in the Figures. An opening  40  centrally disposed in the bottom of the housing part  12  serves for bearing the pinion  32  and permits to couple the rotary drive  10  to a shaft of a conventional belt retractor (not shown). A further function of the housing is the protection of the interior kinematics of the rotary drive  10  against dirt. 
   In the normal driving operation the rotary drive  10  is initially uncoupled from the shaft of the belt retractor. In a restraint case the pyrotechnical propellant charge  16  is ignited and develops a gas pressure that is directed through the hole  18  in the cap  14  into the pressure chamber  20  of the ring gear  22 , the pressure chamber  20  being sealed by the rubber gasket  36 . The gas pressure quasi “supports” on the housing-fixed and sealed wall formed by the extension  26  and “presses” on the rotationally movable cavity wall  38 . Thereby, the ring gear  22  starts to rotate clockwise, whereby the intermediate wheel  30  is driven anti-clockwise. The intermediate wheel  30  in turn drives the pinion  32  clockwise. In this moment the rotary drive  10 , more precisely the pinion  32 , couples in a manner known per se to the shaft of the belt retractor. 
   On account of the torque transmission from the pinion  32  to the belt retractor shaft obtained in this manner an existing belt slack is retracted, in order to reduce a forward movement of a vehicle occupant during an accident. The rotary drive  10  in accordance with the invention retracts approx. 160 mm of a belt webbing in a period of 8 ms, it being easily possible to implement a variable tensioning efficiency by means of different transmission ratios between the gears involved. In the embodiment shown the transmission between the ring gear  22  and the intermediate gear  30  amounts to approx. 1:3, between the intermediate gear  30  and the pinion  32  it amounts to approx. 1:1.8. Thus, in a 120°-rotation of the ring gear  22  the pinion  32  rotates 1.8 times. 
   A further advantage of the rotary drive  10  in accordance with the invention is that it may be preassembled as a complete module. 
     FIGS. 3 and 6  show a rotary drive  50  for a belt tensioner according to a second embodiment of the invention, which operates according to the same basic principle as the rotary drive  10 . The rotary drive  50 , too, comprises a housing formed of a pot-like housing part  52  and a cap  54 , a pressure distributing plate  56  being, however, additionally disposed thereon. The pressure distributing plate  56  comprises a hole  60  communicating with a pyrotechnical propellant charge  58 , the hole  60  in turn being in fluid communication with four holes  64  in the cap  54  via four branching flow ducts  62 . 
   The cap  54  (see in particular  FIG. 4 ) further comprises four extensions  66 , each of which forms a rigid wall of one of a total of four pressure chambers  68 . The remaining walls of the pressure chambers  68  are formed by the cap  54  and the walls of one cavity  70  each, which has the shape of a circular ring interrupted by a radially extending wall  72  and which is configured in the interior of one of four driving gears  74 . 
   All of the four driving gears  74  comprise a surrounding external toothing  76  that meshes with a pinion  78  which, just as the driving gears  74 , is rotatably mounted in the housing. Depressions  80  provided in the housing part  52  and in the cap  54  serve for bearing the driving gears  74  in the housing. The housing part  52  further comprises an opening  82  in which the pinion  78  is mounted and via which it may be coupled to the shaft of a belt retractor. For sealing the pressure chambers  68  the extensions  66  also include rubber gaskets  84 . 
   As has already been described with reference to the rotary drive  10 , the rotary drive  50  is also uncoupled during the normal retraction of the belt and comes into contact with the belt retractor shaft only in a restraint case. In the latter case the pyrotechnical propellant charge  58  is ignited and develops a gas pressure which is distributed through the flow ducts  62  in the pressure distributing plate  56  on the four sealed pressure chambers  68 . Thereby, the driving gears  74  which are not reciprocally in engagement, but each of which is driven separately for itself, start to rotate anti-clockwise. On account of the rotation of the driving gears  74  the pinion  78  is driven clockwise. In this moment the rotary drive  50  couples to the shaft of the belt retractor and starts to likewise rotate the latter. Thereby, the belt webbing is retracted and the forward movement of the vehicle occupant is reduced. 
   The rotary drive  50  as well is distinguished by a particularly compact construction. On account of different combinations of the driving gears  74  and the pinion  78  regarding the number of teeth and the reference diameter thereof different tensioning efficiencies may be achieved in this arrangement as well. For example, the embodiment shown has a transmission ratio of approx. 2 between the driving gears  74  and the pinion  78 , i.e. almost two rotations of the pinion  78  are possible.