Force-limiting device for a safety belt system

A reversible seat belt tensioner including a rotationally operable plastic driven gear (1), and a drive moved with a belt shaft of the belt tensioner. At least one coupling pawl (4) is provided on the drive gear, engaging a gearing of the drive gear upon activation of the reversible belt tensioner, and in a load-transmitting position produces a linkage between the driven gear (1) and the drive gear. A support ring (3) made of a material having a greater strength than the driven gear (1) is provided and is connected to the driven gear (1) for with a projection (21, 22) which into a recess (15, 16) of the driven gear (1). The recess (15, 16) positioned such that the coupling pawl (4) is braced against the driven gear (1) via the projection (21, 22) which engages in the recess (15, 16).

CROSS REFERENCE TO RELATED APPLICATIONS

FIELD OF THE INVENTION

The invention relates to a gearbox for a reversible seat belt tensioner for a motor vehicle.

BACKGROUND

A reversible seat belt tensioner including a gearbox of the above-mentioned type is known from DE 103 42 230 A1. The gearbox includes, inter alia, a rotary element in the form of an externally-toothed gear, which upon activation of the reversible belt tensioner, is rotationally driven by a worm gear drivable by an electric motor. The rotational motion of the rotary element is transmitted via opposing coupling pawls to a belt shaft of the belt retractor upon which a seat belt is rolled in order to reduce the belt slack. The coupling pawls are each guided under tension into a guide on the rotary element. Furthermore, a friction ring is provided, which couples the two coupling pawls together and which in the event of a movement relative to the rotary element, forces the coupling pawls into a modulating movement. Due to this modulating of the coupling pawls, they are brought into engagement with a gearing of a drive gear, which drive gear is connected to the belt shaft for rotation therewith, so that the coupling pawls subsequently transfer the rotational movement of the driven gear via the drive gear to the belt shaft.

The driven gear is made of plastic, such as POM, for reasons of noise reduction, for weight reasons, and for cost reasons. With modern reversible belt tensioners, maximum tensile forces of 500-600 N can be generated in the upper end of the diagonal belt section abutting against the shoulder, which in this case are transmitted from the electric motor to the safety belt via the gearbox shaft, the externally-toothed driven gear, the coupling pawls, the drive gear, and the belt shaft. A part under particularly high load in the path of force transmission is the driven gear, which is highly loaded, especially in the region of the supporting of the coupling pawls and is thus subject to an increased probability of failure.

The object of the present invention is thus to provide a gearbox for a reversible seat belt tensioner of the above-mentioned type, wherein the probability of failure of the driven gear due to an excessive component loading of the driven gear is reduced.

To achieve this object a gearbox is proposed having the features described herein by the Figures, and the associated description.

SUMMARY AND INTRODUCTORY DESCRIPTION

According to the basic feature of the present invention, it is proposed that a support ring connected to the driven gear for conjoint rotation therewith and made of a material having a greater strength than the driven gear is provided, which protrudes with a projection into a first recess of the driven gear, wherein the recess is positioned such that the at least one coupling pawl in the load-transmitting position is supported on the driven gear via the projection of the support ring which engages in the recess. Due to the projection of the support ring engaging in the recess, a more solid support surface for the at least one coupling pawl is deliberately created, so that the coupling pawl is better supported in the load-transfer position. Due to the improved supporting of the coupling pawl, the probability of a lateral slippage of the coupling pawl and of a resultant interruption in force transfer can be reduced. The forces to be transferred can be introduced into the driven gear, in particular from a smaller contact surface of the projection on which the coupling pawl abuts, via a larger supporting surface by which the projection or the support ring abuts on the driven gear, whereby the maximum local component stresses in the driven gear and thus the probability of damage to the driven gear can be additionally reduced. The support ring here is deliberately made of a higher-strength material, so that it can absorb greater loads without itself being deformed. The support ring serves in practice to moderate the forces introduced into the driven gear when the coupling pawl is under load, and to reduce the local component stresses generated thereby.

Furthermore, it is proposed that a guide including a guide surface is provided on the driven gear, against which guide the coupling pawl abuts when in the load-transmitting position, and the first recess is arranged adjacent to the guide surface, and the projection is shaped such that it extends or supplements the guide surface in a homogeneous profile. Using the proposed solution, the guide surface itself, or an extension thereof is locally reinforced by the projection engaging in the recess, whereas the shape of the guide surface is otherwise not changed, so that the extension movement of the coupling pawls and the coupling process is identical to the proven coupling process known in the prior art.

Since the stress on the driven gear when under load is greatest in the radial outer section of the guide surface, it is particularly expedient when the first recess having the projection engaging therein is disposed in the region of a radial outer section of the guide surface. Therefore, the load is greatest in the radial outer section of the guide surface because this radially outer section is closest to or is located opposite the engagement section of the coupling pawls into the gearing of the drive gear, so that the load of the driven gear is greatest in this region. Thus, a reinforcement of the guide surface in this section or in the extension of the guide surface is especially useful.

It is additionally proposed that the guide is formed by an open groove in the driven gear into which open groove the coupling pawl is inserted, and the support ring covers the guide with the inserted coupling pawl on the open side. The support ring thus secures the coupling pawls additionally against a lateral slippage out from the guide. Thus the support ring serves in an additional function for lateral securing of the coupling pawls, so that in addition the probability of failure of the gearbox due to disengaging of the coupling pawls can be reduced.

Furthermore, the support ring can be axially secured to the driven gear via a latching connection. Due to the proposed axial connection, the support ring including the projection can be held on the driven gear in a particularly simple and assembly-friendly manner by pushing-on and latching.

It is additionally proposed that the support ring is secured from rotating with respect to the driven gear via at least one pin engaging in a recess of the driven gear. The pin forms a torque support, which prevents the support ring from being urged out of its intended position due to occurring centrifugal forces.

In particular, the support ring can be produced in a cost-effective manner as a punched metal part or as a plastic part having a greater strength than the driven gear, wherein the punched metal part has the advantage of a particularly great strength with a simple deformability, and the plastic part has the advantage of very cost-effective manufacture using an injection molding method.

DETAILED DESCRIPTION

InFIG. 1a driven gear1of a gearbox according to the invention including a reversible belt tensioner, is shown, as is known in its fundamental form, e.g., from DE 103 42 230 A1. The operation of the reversible seat belt tensioner is additionally known from WO 03/099619 A2. With respect to an understanding of the operation of the reversible belt tensioner and of the associated gearbox, these publications are incorporated by reference in the disclosure content of this application.

In addition to the driven gear1, a friction ring2, a support ring3and two coupling pawls4can also be seen. The coupling pawls4are inserted in open guides6and7of the driven gear1, subsequently the friction ring2is placed and the composite is axially secured by placement of the support ring3. The coupling pawls4each include a recess11and12into which the friction ring2engages via a tab8and9. The friction ring2is additionally held in a known, friction-fit manner by radially inwardly protruding friction arms10against a part connected to the driven gear1such that the part can freely rotate with respect to the driven gear1, so that during a rotational operating of the driven gear1, the friction ring2lags behind with respect to the driven gear1, and during the rotational movement of the driven gear1, the coupling pawls4are held back with respect to the driven gear1by the engagement of the tabs8and9in the recesses11and12. Due to the retention of the coupling pawls4, the coupling pawls4travel radially outward automatically from the guides6and7during the rotational movement of the driven gear1, and simultaneously engage in a gearing of a drive gear30shown inFIG. 8and described in the publications referenced above, whereby the belt shaft of the reversible belt tensioner is driven in the wind-up direction.FIG. 8further identifies components which are analogues to components described in the described embodiment of the present invention including friction ring2′ and pawls4′. To this extent, the gearbox corresponds in operation to the prior art except for the provided support ring3.

InFIG. 2andFIG. 3, the driven gear1including inserted coupling pawls4can be seen with and without support ring3, whereas the driven gear1can be seen inFIG. 6andFIG. 7in different views without the coupling pawls4. The coupling pawls4in the open guides6and7abut against radially inner guide surfaces25and26, which control the direction and the course of the above-described extension movement of the coupling pawls4. There is a first recess15and16provided on each of the guide surfaces25and26in the radial outer section, that is, adjacent to the extension openings of the guides6and7. Furthermore, two latching hooks13and14protruding toward the viewer on the driven gear1and two additional, circular recesses27and28are provided.

The support ring3is shown in different views inFIG. 4andFIG. 5. The support ring3includes two diametrically opposing projections21and22, projecting on one side in the axial direction and having a trapezoidal cross-section; in the cross-section the protrusions taper radially outward in width. Furthermore, diametrically opposing, axially protruding pins19and20are provided on the support ring3, and two diametrically positioned recesses17and18are provided on the radially outer side. After insertion of the coupling pawls4into the guides6and7, and placing of the friction ring2, the support ring3is placed and specifically in such an alignment that the support ring3engages via the projections21and22into the first recesses15and16and via the pins19and20engages into the second recesses27and28. At the same time, the latching hooks13and14engage in the recesses17and18of the support ring3and latch behind the edge of the support ring3, so that the support ring3is then secured in the axial direction against the driven gear1. Furthermore, the support ring3is secured by the pins19and20engaging in the second recesses27and28and by the latching hooks13and14engaging in the sections17and18to prevent any twisting with respect to the driven gear1. The projections21and22are shaped such that they engage into the first recesses15and16and entirely fill the recesses15and16. Each of the projections21and22includes a lateral flank23and24, which is shaped and positioned such that in the installed position of the support ring3it adjoins the guide surfaces25and26and the guide surfaces25and26supplement or extend in a homogeneous, level, i.e. continuous profile.

The support ring3is made of a material such as metal or a hard plastic, for example, which has a greater strength than the material of the driven gear1. Furthermore, the flanks23and24against which the coupling pawls4abut when under load, that is, in the extended position, have a smaller surface area than the surface via which the projections21and22abut against the driven gear1in the region of the remaining circumferential surface. Due to the proposed design of the support ring3having a greater strength than the driven gear1, the abutment surface of the flanks23and24formed by the projections21and22for the coupling pawls will be deformed at least to a lesser extent, so that the coupling pawls4are better supported when under load. In addition, the reactive force from the stronger projections21and22is transmitted over a greater abutment surface to the driven gear1, so that the local maximum component stresses in the driven gear1are reduced in the region of the abutment of the projections21and22. Furthermore, the coupling pawls4with friction ring2are additionally axially secured by the support ring3held in place by the latching hooks13and14. It is of particular advantage here that the support ring3covers the side of the open guides6and7with coupling pawls4disposed therein, so that the coupling pawls4cannot slip out to the side from the guides6and7, especially when under load.

The first recesses15and16are disposed in a radially outer section of the guide surfaces25and26, or adjacent to the radial outer extension openings of the guides6and7. Thus the guides6and7are reinforced precisely in the region which is under the greatest load via the coupling pawls4. This is because the coupling pawls4, upon extension, engage with the teeth in the radially outer gearing of the driven gear and during the transmission of the rotary movement are placed under load in the circumferential direction, and thus are pressed against the rigid-shape flanks23and24of the projections21and22with a particularly large pressure force. The concept of strength within the meaning of the invention means in particular a greater deformation rigidity and a greater hardness. In this respect the support ring3can be formed, for example, as a shaped metal part or from a hard plastic. In the present exemplary embodiment, the support ring3is formed from POM or zamak, whereby it can be produced at low cost and satisfies the strength requirements placed upon it.