The invention relates to a clutch for a seatbelt tensioner of a seatbelt system in a vehicle.

Reversible seatbelt tensioners are used to increase the safety of the vehicle occupants and their driving comfort. A reversible drive, usually an electric motor, is coupled with a seatbelt spool axis to rotate the seatbelt spool of a seatbelt retractor and to take in loose seatbelt webbing, for instance in situations in which a control unit decides that this could be potentially beneficial. Then, a clutch is closed that couples the reversible drive with the seatbelt spool, and the seatbelt spool is rotated by the reversible drive to draw in seatbelt webbing. In the regular operating mode of the seatbelt system the clutch is open so that the seatbelt spool of the seatbelt retractor can draw in and give out the seatbelt webbing without being hindered by the reversible drive. An example for a clutch for a seatbelt tensioner is shown in DE 10 201 1 1 19 343 A1 . Additionally to the reversible drive, the seatbelt tensioner usually is provided with a single-use pyrotechnic drive that will only be activated in case of an actual crash situation. The seatbelt spool is rotated by the pyrotechnic drive with a far higher rotational velocity than by the reversible drive. So as not to interfere with the pyrotechnic drive and also to prevent an interference with a subsequent load limiting function of the seatbelt system that reduces the force acting on the occupant after the pyrotechnic drive has finished its operation, the clutch usually is decoupled from the seatbelt spool when the pyrotechnic drive is activated. Commonly, the seatbelt spool, the clutch and also the reversible drive and the pyrotechnic drive are mounted on a frame of the seatbelt retractor. The pyrotechnic drive typically is arranged on the side of the seatbelt spool which is opposite the side at which the clutch is mounted. On activation of the pyrotechnic drive, sudden impulses are generated, for instance caused by use of a so-called snake, a flexible, deformable body that is accelerated by a pyrotechnic charge. When the pyrotechnic charge is ignited, the snake is abruptly displaced by the developing gas pressure and interacts with a drive pinion connected to the seatbelt spool axis to rotate the seat belt spool. In case of the pyrotechnic drive of the seat belt tensioner being activated, the seat belt spool should be rotated exclusively by the pyrotechnic drive, and the reversible drive should reliably stay decoupled. Therefore, if the clutch is in the disengaged position when the pyrotechnic drive is initiated, the pawl should safely stay in its disengaged position. It is an object of the invention to provide a clutch fulfilling this requirement.

This object is achieved by a clutch for a seat-belt tensioner in a vehicle, comprising the features of claim 1 . A clutch for a seat-belt tensioner in a vehicle has a driving element adapted for being rotated in a driving direction and in a releasing direction and a driven element having several gear teeth arranged at a regular distance from each other, the driven element being connectable with a seat-belt spool, the driving element and the driven element having a common rotation axis. A pawl is pivotally held at a pivot pin that is arranged on the driving element and forms a pivot axis. The pawl comprises at least one tooth for meshing with the gear teeth of the driven element. The pawl is adapted to assume a disengaged position in which the pawl is not in engagement with the driven element and from which it can be pivoted radially inwards by turning the driving element in driving direction, and an engaged position in which at the least one tooth of the pawl engages with a gear tooth of the driven element to couple the driving element with the driven element. The clutch further comprises a clutch disk ring having an outer essentially circular circumference and a clutch disk arm extending inwards into an opening enclosed by the clutch disk ring, the clutch disk arm interacting with the pawl to guide the pawl into the engaged position and the disengaged position. Also, the pawl is blocked by the clutch disk arm against pivoting radially inwards in the disengaged position. A reinforcement structure is provided in the region of the clutch disk arm, giving an increased rigidity to the clutch disk ring and/or the clutch disk arm, in particular with regard to a deformation radially inwards. The increased rigidity of the clutch disk ring in the region of the clutch disk arm (as compared to the rigidity of the clutch disk ring distanced from the clutch disk arm) prevents a deformation of the clutch disk ring in case that the pyrotechnic drive is activated and a radially inwardly directed force acts on the pawl in consequence. As the pawl is arrested at the clutch disk arm in the disengaged position, inertia forces acting on the pawl will be transferred to the clutch disk arm and to the clutch disk ring. However, according to the invention, the clutch disk ring offers enough resistance to deformation to keep the pawl securely in the disengaged position.

It has proved sufficient to increase the rigidity of the clutch disk ring only in the region of the clutch disk arm so that size and mass of the clutch disk ring do not have to be unduly increased.

The driving element usually is connected to the reversible drive, normally an electric motor that can rotate the driving element in driving direction and in the reversed releasing direction. The driven element is usually fixedly coupled to the pyrotechnic drive so that the driven element is set into rotation when the pyrotechnic drive is activated.

When the pawl is in its disengaged (or neutral) position, the clutch is in its open, disengaged state. The driving element is not coupled with the driven element. Is the pawl in its engaged position, engaged with the driven element, the clutch is in its closed, engaged state. In this application, the terms "disengaged position" and "engaged position" are not only be used to describe pawl positions, but also to denote the open and closed state of the clutch, respectively.

Additionally to the engaged and disengaged position, the pawl can also assume a decoupled position in which the pawl lies radially farther outwards than in the disengaged position. The pawl is transferred from the engaged position to the decoupled position by accelerating the pawl radially outwards beyond the disengaged position. This acceleration can be generated e.g. by a fast rotation of the driven element due to the operation of the pyrotechnic drive. A normal reengagement of the clutch is prevented in the decoupled position, i.e. a rotation of the driving element in driving direction will not lead to the pawl being pivoted radially inwards and will not bring the pawl into the engaged position. The only operating position from which the pawl can be pivoted inwards and the clutch can be closed is the disengaged position.

However, the clutch may be designed so that it is possible to reengage the clutch, by allowing a controlled reengagement process in which the pawl can be transferred from the decoupled position back into the disengaged position, from where the clutch can be closed again.

To normally engage the pawl from the disengaged position, the driving element is rotated in driving direction, moving the pawl into its engaged position in engagement with the driven element.

To this effect, the clutch disk arm has an engagement runway that pushes against the pawl to move the pawl along a predetermined engagement path into the engaged position. Further, the clutch disk arm has a disengagement runway that pushes against the pawl to move the pawl along a predetermined disengagement path into the disengaged position. The runways are arranged at opposite radial faces of the clutch disk arm, for instance. The clutch disk arm usually extends approximately radially inwards into the opening enclosed by the clutch disk ring and lies essentially in the same plane as the clutch disk ring.

Preferably, the clutch disk arm has a rigid section that interacts with the pawl to guide the pawl from the engaged position into the disengaged position. The pawl comprises a first protrusion, while the rigid section has a second protrusion, both protrusions extending along the circumferential direction. To arrest the pawl in the disengaged position and to block the pawl from pivoting radially inwards, the second protrusion on the clutch disk arm is arranged radially directly below the first protrusion on the pawl in the disengaged position. In particular, the second protrusion forms an undercut with regard to the first protrusion. The first and second protrusion are advantageously arranged such that a force acting on the pawl, for instance caused by the activation of the pyrotechnic drive, will not transfer any torque to the clutch disk ring and, therefore, the clutch disk ring will not be rotated.

To achieve this, the only contact between the pawl and the clutch disk arm in the disengaged position may be between radial faces of the first and the second protrusion lying against each other. The radial faces are preferably oriented perpendicular to the radial direction to avoid generating a circumferentially acting force component acting on the clutch disk ring.

The clutch disk arm advantageously is arranged near the middle of the extension of the pawl in circumferential direction. In this region, it is easily possible to design guiding geometries for interaction between the clutch disk arm runways and the pawl for guiding the pawl between its different positions. The guiding geometries on the pawl are preferably realized on two side faces limiting a recess in the surface of pawl in radial direction.

In one preferred example, the clutch disk arm is hook-shaped. The hook is formed by the rigid section extending from the clutch disk ring into the opening enclosed by the clutch disk ring, and a flexible section connected to the end of the rigid section and extending essentially radially outwards and ending in a free end.

To avoid any deformation of the clutch disk ring in the region of the clutch disk arm that might alter the geometric relationship of the contact of the protrusions on the pawl and the clutch disk arm, in a first preferred embodiment the reinforcement structure is an enlarged region at the transition from the clutch disk arm into the clutch disk ring, extending in circumferential direction The reinforcement structure locally increases the rigidity of the clutch disk ring.

The reinforcement structure preferably is realized by a specific geometry of the clutch disk ring and/or the clutch disk arm. For instance, the reinforcement structure may be an enlarged region on the transition from the clutch disk arm into the clutch disk ring, extending in circumferential direction. The increased material thickness in circumferential, radial and/or axial direction automatically increases the rigidity of the clutch disk ring. More specific, the enlarged region may extend in circumferential direction for about 1 .5 to 3 times the circumferential extension of the clutch disk arm adjacent to the reinforcement structure, in particular the extension of the clutch disk arm between the enlarged region and the second protrusion.

Of course, other geometric structures such as a rib structure on the face of the clutch disk ring or webs connecting the clutch disk ring and the clutch disk arm can also be envisioned. It is also possible to provide the clutch disk ring and/or the clutch disk arm with a reinforcement structure in form of a reinforcement layer made from a stiff material that is e.g. embedded in the clutch disk ring and/or the clutch disk arm.

In another preferred embodiment, the reinforcement structure is an axially extending protrusion on the rigid section of the clutch disk arm that in particular increases the rigidity of the rigid section of the clutch disk arm "Axially extending" means that the protrusion extends perpendicularly to the plane of the clutch disk ring parallel to the common rotation axis.

When the clutch disk ring is covered by a plate extending parallel to the clutch disk ring, the protrusion may extend into an opening in the clutch disk ring.

The increased material thickness per se increases the rigidity of the clutch disk arm and reduces any deformation of the clutch disk arm. The opening in the plate allows for the necessary thickness of the protrusion.

So that the pawl is not hindered in its regular motion from the disengaged position into the engaged position and back, the opening preferably is an elongated hole extending along a path the protrusion follows when the pawl moves along its predetermined path from the engaged position into the disengaged position and back.

Additionally, an edge of the opening may act on the protrusion when the clutch disk arm or the clutch disk ring is deformed by a force acting on the pawl. In this case the edge of the opening supports the protrusion and thereby keeps the clutch disk arm and clutch disk ring in their intended position so that the pawl stays arrested in the disengaged position.

The plate is for instance part of the clutch housing so that no additional components have to be used in the clutch. Further, the clutch may comprise a retainer ring surrounding the outer circumference of the clutch disk ring and being coupled by friction with the clutch disk ring, the retainer ring being configured for limited rotation relative to a retaining structure arranged on the clutch housing. In specific situations, in particular during engagement and disengagement of the clutch, the retainer ring provides a limited angular rotation between the driving element and the clutch disk ring which is held back due to the friction coupling with the retainer ring. This limited angular relative rotation of the driving element and the clutch disk ring causes the interaction of the clutch disk arm and the pawl to guide the pawl between its different positions. Generally, the term "radially outwards" in this application is not only used in the true mathematical sense, but also to indicate a general motion of a component away from the rotation axis of the clutch.

Preferred embodiments of the invention are described in detail in the following with reference to the attached drawings, in which: - Figure 1 shows a schematic plan view of a clutch for a seat belt tensioner according to a first embodiment of the invention;

Figure 2 shows an enlarged section of Figure 1 ;

Figure 3 shows a schematic perspective view of a clutch according to a second embodiment of the invention; and - Figure 4 shows an enlarged section of Figure 3.

The figures show a clutch 10, 10' for a seatbelt tensioner in a vehicle in two embodiments. Only the clutch is displayed in the figures. However, as is generally known in the art, the seatbelt tensioner is connected to a seatbelt retractor having a frame in which a seatbelt spool is rotatably arranged that can take up and give out seatbelt webbing. Usually, the clutch 10, 10' is arranged at one side of the seatbelt spool, while a pyrotechnic drive (not shown) of the seatbelt tensioner is positioned at the opposite side of the seatbelt spool. The pyrotechnic drive comprises a mass accelerated by a pyrotechnic charge on activation that interacts with a drive pinion connected to the seatbelt spool, The accelerated mass is for instance a so-called snake which is a cylindrical body of a deformable plastic material that is brought into contact with the drive pinion to set the seatbelt spool into a fast rotation.

The clutch 10, 10' is connected to a reversible drive 12 (only schematically in Fig. 1 ) of the seatbelt tensioner, usually an electric motor, that turns the seatbelt spool in a driving direction D to take up seatbelt webbing under certain conditions. The reversible drive 12 can be operated repeatedly while the pyrotechnic drive is a single-use device.

The clutch 10, 10' serves to couple the reversible drive 12 to the seatbelt spool so that the driving element 14 can turn the seatbelt spool in the driving direction D to wind up seatbelt webbing to tighten the seatbelt around a vehicle occupant. This kind of seatbelt tensioning, also called pretensioning, takes place for instance when an electronic control unit detects an imminent dangerous situation in which the seatbelt should be tensioned as a measure of precaution. If the critical situation passes, the clutch 10, 10' will be disengaged again so that the seatbelt spool again can turn unhindered in both directions.

The pyrotechnic drive is only activated in case of an actual crash and will rotate the seatbelt spool in driving direction D to take up seatbelt webbing. This rotation caused by the pyrotechnic drive is much faster than the rotation caused by the reversible drive 12. Figures 1 and 2 show the clutch 10 according to a first embodiment.

The clutch 10 has a disk-shaped driving element 14 that can be connected with the reversible drive 12 by a driving gear 16 that is part of the driving element 14.

Further, the clutch 10 has a driven element 18 arranged coaxially with the driving element 14 and connected with regard to rotation to the seatbelt spool and, thereby, to the pyrotechnic drive. The seatbelt spool and the driven element 18 always rotate in the same direction and with the same velocity.

The driving element 14 and the driven element 18 have a common rotation axis A, the driven element 18 extending through a center opening 20 of the driving element 14. Generally, when a radial direction r and a circumferential direction U are discussed in this application, these directions always relate to the common rotation axis A. The reversible drive 12 can rotate the driving element 14 in the driving direction D and in the opposite releasing direction R to transfer the clutch 10 into different operating positions. The driving direction D is indicated counterclockwise in the figures while the releasing direction R is indicated clockwise in the figures. When the clutch 10 is closed, the reversible drive 12 is connected to the seatbelt spool (not shown). When the clutch 10 is open in the disengaged position, the reversible drive 12 is disengaged from the seatbelt spool (see Figs. 1 and 2).

The rotational motion of the driving element 14 is transmitted to the driven element 18 by a pawl 22 that is pivotally held on the driving element 14 at a pivot pin that may be formed integrally with the driving element 14. The pivot pin defines a pivot axis 24 for the pawl 22. The pivot axis 24 extends in parallel with the rotation axis A (and thus perpendicular to a plane defined by the driving element 14).

The pawl 22 has a generally curved shape that approximately follows the curvature of the driven element 18. The pawl 22 lies in a plane parallel to that of the driving element 14 and pivots essentially radially inwards towards the rotation axis A. Along its radial inner face between the pivot axis 24 and a free end 26, the pawl 22 has several teeth 28 (in this example four teeth) that can come into engagement with gear teeth 30 of a toothing along the outer circumference of the driven element 18. The pawl 22 can be pivoted around the pivot pin into several positions: a disengaged (or neutral) position in which the pawl 22 is not in engagement with the driven element 18 (shown in the figures), an engaged position (not shown), where the pawl 22 is in engagement with the driven element 18, and a decoupled position (also not shown), where the pawl 22 is located radially further outwards than in the disengaged position and is locked against a regular (re)-engagement.

Engagement and disengagement of the pawl 22 by rotation of the reversible drive 12 in driving direction D and releasing direction R are the regular operations of the clutch. The pawl 22 will reach the decoupled position only e.g. when the pyrotechnic drive is activated. The pawl 22 is moved between the disengaged position and the engaged position by interaction with a clutch disk ring 32. The clutch disk ring 32 encloses an opening 34, is centered around the common rotation axis A and is arranged in a plane above the driving element 14 so that it surrounds the pawl 22.

The clutch disk ring 32 comprises a clutch disk arm 36 extending radially inwards from an inner circumference of the clutch disk ring 32. The clutch disk arm 36 is hook-shaped in this example (see also Fig. 2). The hook is formed by a rigid section 38 connected with the clutch disk ring 32 and extending inwards into the opening 34 enclosed by the clutch disk ring 32, and a flexible, free section 40 connected to the end of the rigid section 38 and extending outwards towards the clutch disk ring 32. The hook sections 38, 40 are connected by a U-shaped connecting portion. In an unstressed state the hook sections 38, 40 are essentially parallel to each other. The flexible section 40 ends in a free end 42 positioned on the far side of the clutch disk arm 36 from the pivot axis 24.

The clutch disk arm 36 is arranged in an axial recess 44 in the body of the pawl 22 that is delimited by a left-hand side face and a right-hand side face, both side faces facing essentially in circumferential direction U. In the recess 44 the thickness of the pawl 22 is reduced in axial direction A with regard to the rest of the pawl 22. The recess 44 and the clutch disk arm 36 are located approximately in the middle of the pawl 22 regarding the extension of pawl 22 in circumferential direction U. On the left and right-hand side faces of the clutch disk arm 36 and of the recess 44, interacting guiding geometries are provided that serve to guide the pawl 22 radially inwards into engagement with the driven element 18 during the engagement process and radially outwards again out of engagement with the driven element 18 back into the disengaged position when the driving element 14 is rotated in releasing direction R.

The clutch disk ring 32 is discontinuous in circumferential direction U. The gap between its two circumferential ends 52, 54 is bridged by an expansion spring 56 that presses the circumferential ends 52, 54 apart in circumferential direction U.

Also, the clutch disk ring 32 is coupled by a return spring 57 with the driving element 14 that allows only a certain relative angular displacement between the driving element 14 and the clutch disk ring 32. The return spring 57 extends parallel to the surface of the driving element 14. The clutch disk ring 32 is surrounded by a retainer ring 58 to which the clutch disk ring 32 is coupled by friction at its inner circumference with the outer circumference of the clutch disk ring 32. The friction is at least partly provided by the expansion spring 56 pressing the circumferential ends 52, 54 of the clutch disk ring 32 apart. The clutch disk ring 32 and the retainer ring 58 are coupled by friction only. The retainer ring 58 has a plurality of radially extending spokes 60 that are arranged at regular distances along the outer periphery of the retainer ring 58. A retaining structure 62, here in form of several retaining pins distributed along the circumference of the retainer ring 58, limits the rotation of the retainer ring 58 to the distance between two adjacent spokes 60, for instance between 10° and 30°.

Instead of the retaining structure 62 described above, any different kind of retaining structure could be provided that allows a limited rotation of the retainer ring 58 in both directions D, R.

The driving element 14 can rotate relative to the clutch disk ring 32 when the clutch disk ring 32 is retained by friction on the retainer ring 58. At a certain threshold of the rotational force, however, the frictional forces will be overcome, and the clutch disk ring 32 will rotate together with the driving element 14.

To transfer the clutch 10 into its closed state (i.e. its engaged position), the pawl 22 has to be brought into engagement with the driven element 18. For this purpose, the reversible drive 12 is activated and the driving element 14 starts to rotate in driving direction D. In the disengaged position of the pawl 22, the spokes 60 of the retainer ring 58 are distanced in driving direction D from the nearest stopping pin.

As the clutch disk ring 32 is coupled by the return spring 57 with the driving element 14, initially the clutch disk ring 32 is entrained by the rotating driving element 14. Due to the frictional force between the clutch disk ring 32 and the retainer ring 58, the retainer ring 58 is also entrained by the clutch disk ring 32 until the spokes 60 come into contact with the stopping pins of the retaining structure 62. Then, the retainer ring 58 stops rotating and, due to the frictional force between the retainer ring 58 and the clutch disk ring 32, the clutch disk ring 32 is also stopped from rotating. This results in the driving element 14 rotating relative to the clutch disk arm 36, and the right-hand face of the recess 44 in the pawl 22 is pushed against an engagement runway 63 formed on the right-hand side (in the figures) of the flexible section 40 of the clutch disk arm 36. Thereby, the pawl 22 is pivoted radially inwards into engagement with the driven element 18. The teeth 28 of the pawl 22 mesh with the gear teeth 30 of the driven element 18 so that the driven element 18 is entrained in driving direction D by the pawl 22.

All directional designation in the application refer to the enclosed drawings. Of course, the clutch 10 could also be realized mirror-inverted. The inward movement of the pawl 22 continues until the free pawl end 26 comes into contact with a pawl stop 64 fixedly arranged on the driving element 14. The pawl stop 64 now also transfers load to the pawl 22 and, therefore, to the driven element 18. The center of the pawl stop 64 lies here on a straight line with the rotation axis A and the pawl pivot axis 24. The relative motion between the driving element 14 and the clutch disk ring 32 continues until a clutch disk stop 68 on the driving element 14 comes into contact with the first circumferential end 52 of the clutch disk ring 32, which is formed here as a projection extending inwards towards the rotation axis A. From this moment on, the driving element 14 pushes the clutch disk ring 32 in driving direction D, and the clutch disk ring 32 rotates together with the driving element 14 and slides inside the retainer ring 58.

When the pyrotechnic drive is not activated, the clutch 10 will be disengaged again and transferred back into the disengaged position by rotating the reversible drive 12 in the releasing direction R. To disengage the clutch 10, the pawl 22 rotates together with the driving element 14 relative to the clutch disk ring 32 which is again held by friction on the retainer ring 58. The left-hand face of the recess 44 (in the figures) comes into contact with a disengagement runway 69 on the left-hand side of the rigid section 38 of the clutch disk arm 36. The contact between the pawl 22 and the clutch disk arm 36 occurs between a first protrusion 70 on the left-hand face of the recess 44 of the pawl 22 and a second protrusion 72 of the left-hand side of the rigid section 38 of the clutch disk arm 36. The disengagement runway 69 is realized at the free circumferential face of the second protrusion 72 face that is inclined against the circumferential direction U. The first protrusion 70 ends in a face with a corresponding inclination so that a force component radial outwards ensues. Entrained by the clutch disk ring 32, the retainer ring 58 rotates until its spokes 60 come into contact with the retaining structure 62. The frictional force between the retainer ring 58 and the clutch disk ring 32 arrests the clutch disk ring 32 and, therefore, also the clutch disk arm 36 with regard to the rotating driving element 14. The first protrusion 70 on the pawl 22 starts sliding radially outwards along the disengagement runway 69 on the rigid section 38 of the clutch disk arm 36. Thereby, the pawl 22 is pivoted radially outwards and loses its engagement with the driven element 18.

As the pawl 22 does not receive an exceedingly high kinetic energy, the motion of the pawl 22 is stopped by a spring element 74 in the disengaged position that act as a pawl end stop and that keeps the pawl 22 from swiveling farther outwards than the disengaged position. The spring element 74 is attached to the face of the driving element 14 and extends essentially parallel to the face of the driving element 14. The spring element 74 is flexible and can be deflected radially outwardly by the free end 26 of the pawl 22, whereby a pretensioning force is generated acting radially inwards on the free end 26.

In the disengaged position, the second protrusion 72 on the clutch disk arm 36 lies directly radially below the first protrusion 70 on the pawl 22. This results in the pawl 22 being locked against involuntary movement radially inwards due to external forces acting on the pawl 22 and, therefore, the pawl 22 is arrested in the disengaged position. External forces may occur e.g. during a crash situation when the pyrotechnic drive is activated and the clutch 10 is not supposed to be engaged.

As is shown in Figure 2, the only contact between the pawl 22 and the clutch disk arm 36 in the disengaged position is here between radial faces 78, 80 of the first and second protrusion 70, 72, respectively. The radial faces 78, 80 have the same inclination and lie against each other. Both radial faces 78, 80 face here in radial direction r so that external forces acting on the pawl 22 will not generate a rotation of the clutch disk ring 32. The pawl 22 is in direct contact with the clutch disk arm 36 in the disengaged position. Therefore, the pawl 22 cannot build up any velocity on occurrence of external forces so that an impulse build-up is also avoided.

To prevent a major deformation of the clutch disk ring 32 in case of external forces, in particular inertial forces, acting on the pawl 22 and being transmitted to the clutch disk ring 32 that might alter the geometric situation between the protrusions 70, 72, a reinforcement structure 82 is provided on the clutch disk ring 32. The reinforcement structure 82 increases the rigidity of the clutch disk ring 32 in the region of the clutch disk arm 36. Therefore, the clutch disk ring 32 and also the clutch disk arm 36 are less prone to be deformed radially inwards.

In the first embodiment, the reinforcement structure 82 is an enlarged region at the transition from the clutch disk arm 36 into the clutch disk ring 32 (see Fig. 2). The enlarged region is provided on the left-hand side of the clutch disk arm 36 facing the first protrusion 70 on the pawl 22 and forms a triangle filling the space between the inner circumference of the clutch disk ring 32 and the circumferential surface of the clutch disk arm 36. In the enlarged region, the thickness of the clutch disk arm 36 (regarded in circumferential direction U) is increased so that it measures 1 .5 to 3 times the circumferential extension d of the clutch disk arm 36 between the enlarged region and the second protrusion 72. The reinforcement structure 82 stiffens the clutch disk ring 32 against deformation radially inwards as well as the clutch disk arm 36 against bending in circumferential direction U. Thus, the pawl 22 is securely arrested in the disengaged position.

A second reinforcement structure 90 is built on the driving element 14. That second reinforcement structure 90 is located between the clutch disc arm 36 and the pivot axis close to the inner surface of the clutch disk ring 32. This second reinforcement structure 90 supports the clutch disc ring 32 in case of deformation.

To reengage the clutch 10, the reversible drive 12 again turns the driving element 14 in driving direction D and the pawl 22 will be once again pivoted inwards into engagement with the driven element 18 as described above. However, should the pyrotechnic drive be activated while the pawl 22 is in its engaged position, the clutch 10 will be decoupled so as not to impede the pyrotechnic drive and a subsequent load limiting function of the seatbelt system.

The pyrotechnic drive rotates the driven element 18 coupled to the seat belt spool. The resulting rotational velocity in driving direction D in this case is far higher than when the driven element 18 is rotated by the reversible drive 12.

As a result, the gear teeth 30 of the driven element 18 act on the pawl 22 and transmit a high kinetic energy to the pawl 22. The pawl 22 is accelerated radially outwards into its decoupled position in which the pawl 22 lies farther radially outwards that in the disengaged position. In the decoupled position, the pawl 22 is unable to engage the driven element 18 again, even if the reversible drive 12 rotates the driving element in driving direction D.

On reaching the decoupled position, a free end 42 of the clutch disk arm 36 snaps below a tangential protrusion 76 on the pawl 22 so that the pawl 22 is locked to the clutch disk ring 32 in a latching connection. In the decoupled position, the pawl 22 is unable to engage the driven element 18 again, even if the reversible drive 12 rotates the driving element in driving direction D.

To return the clutch 10 from the decoupled position into the disengaged position, the driving element 14 is rotated in releasing direction R. Figures 3 and 4 show a clutch 10' according to a second embodiment.

The clutch 10' is in its design and its function identical to the clutch 10 previously described, with the exemption of the configuration of the reinforcement structure 82'.

In this embodiment, the reinforcement structure 82' is an axially extending protrusion that is formed on the rigid section 38 of the clutch disk arm 36. In this example, the protrusion extends in axial direction A away from the disk of the driving element 14.

The protrusion is here part of the second protrusion 72. Therefore, the reinforcement structure 82' enhances the rigidity of the second protrusion 72 and of the clutch disk arm 36 and lessens the deformation of the clutch disk arm 36 in case that the pawl 22 exerts a force thereon. Further, the clutch disk ring 32 is covered by a plate 84 that in this case is part of a clutch housing. The plate 84 comprises an opening 86 formed as an elongated hole extending along the path that the reinforcement structure 82' travels when the pawl 22 pivots from the disengaged position into the engaged position and back. The protrusion forming the reinforcement structure 82' extends into the opening 86. Should the clutch disk ring 32 or the clutch disk arm 36 be deformed radially inwards, the protrusion comes into contact with an edge 88 of the opening 86 and thus is supported by the plate 84, and the clutch disk arm 36 and the clutch disk ring 32 are prevented from further deformation. Therefore, the first protrusion 70 will not lose contact with the second protrusion 72 and the pawl 22 will securely be kept in the disengaged position.