During the normal operation of a vehicle, the seat belt rests loosely on a vehicle occupant's body for reasons of comfort, such that in the event of an crash there is a risk that the vehicle occupant will be forwardly displaced in an excessively vigorous manner, despite the blocking of the winding shaft of the automatic seat belt retractor. The layers of belt webbing on the winding shaft of the automatic seat belt retractor are also wound relatively loosely, so that there is also belt slackness which can lead to excessive forward displacement of the vehicle occupant as a result of a film reeling effect, despite blocking of the winding shaft.

To avoid the risk of excessive forward displacement of the vehicle occupant, it is known, prior to blocking of the winding shaft, to eliminate the belt slackness by means of a seat belt tightener which acts, for example, on the belt buckle or on the winding shaft of the automatic seat belt retractor.

Advantageous embodiments of the present invention are disclosed in the main claim and sub-claims.

EP 0 629 531 A1 teaches a seat belt tightener with a pyrotechnic piston drive with at least one piston, the driving movement of which is transmitted to the winding shaft of an automatic seat belt retractor. The piston is arranged in a guide tube and is moved therein in that a driving gas produced by a

pyrotechnic propellant charge that expands in a pressure chamber in the guide tube and thereby acts on the piston. The piston engages with a pinion coupled to the winding shaft and sets it in rotation. Once the piston has reached its end position and the seat belt is tightened, the automatic seat belt retractor blocks the belt webbing.

DE 42 22 993 A1 teaches a seat belt retractor with a seat belt tightener comprising a pyrotechnic piston/cylinder drive. The cylinder contains a pyrotechnic gas generator that is inserted into its free end and a cup-shaped piston that is held displaceably in the cylinder and is slipped over the cylindrical gas generator, forming two working chambers. One working chamber is formed in the piston head and the other working chamber at the narrow annular end of the piston. Upon activation of the gas generator, the propellant gas acts directly and simultaneously on the acting faces of the two drive chambers for the axial drive of the piston, so that a high initial stress on the components is produced.

Brief Description of the Drawings FIG. 1 is a longitudinal section through a first embodiment of a device according to the invention with a step piston in a starting position.

FIG. 2 is a sectional view along line 2-2 in FIG. 1.

FIG. 3 is a view according to FIG. 1 of the device with an advanced step piston in engagement with a driving pinion.

FIG. 4 is a section along line 4-4 in FIG. 3.

FIG. 5 is a longitudinal section through a second embodiment of a device according to the invention with a step piston in a starting position.

FIG. 6 is a section along line 6-6 in FIG. 5.

FIG. 7 is a view according to FIG. 5 of the device with an advanced step piston in engagement with a driving pinion.

FIG. 8 is a section along line 8-8 in FIG. 7.

FIG. 9 is an enlarged detail from FIG. 1 with half the side of the step piston.

FIG. 10 is a cross section along line 10-10 in FIG. 4. This figure shows the guide tube of the device with the step piston in a schematic view.

FIG. 11 is a cross section corresponding to FIG. 10 through the guide tube with lateral walls deformed by excess pressure.

Detailed Description of the Invention An embodiment of device for tightening a seat belt according to the invention having a piston drive is shown in FIGS. 1 and 2 comprises a pressure and guide tube 1 and a drive member or step piston 2 held displaceably therein. The guide tube is associated with an automatic seat belt retractor (not shown) in such a way that a driven element such as, for example, a gear-wheel 3 which, as part of a movement transmitting system, is connected directly or via a gear to a winding shaft of the automatic seat belt retractor, extends through an orifice 4 in the guide tube 1 into a path of movement of a rack 5 arranged on the step piston 2 (the step piston 2 moves in the direction of the arrow X in FIG. 1).

The movement transmitting system advantageously contains a rack on the step piston and a gear wheel that can be engaged with the rack and is coupled to a winding shaft of an automatic seat belt retractor for the seat belt. However, traction cable connections or lever mechanisms can also be used to connect the step piston to the seat belt, to the belt buckle or to other tensioning components.

If a first tooth of the rack in the direction of movement of the step piston has a reduced tooth height, engagement of the rack in the teeth of the gear wheel is ensured in every position of the gear wheel.

The step piston 2 is located in a starting or standby position according to FIGS. 1 and 2 on a

closed end part 6 of the guide tube 1 and is out of contact with the gear-wheel 3. The step piston 2 and the interior of the guide tube 1 have, in particular, a substantially rectangular or square cross-sectional shape. The step piston 2 is produced, for example, from aluminum. The step piston has, at its side facing the gear-wheel 3, a cavity in which there is fastened a rack 5. The rack comprises, for example, a plurality of adjacent segments such as, for example, precision punched steel parts, and of which the first tooth 8 in the direction of movement is designed as a half tooth with a shortened tip. The rack is fastened on the step piston 2 on the one hand by a projection 9 of the rack engaging in an associated indentation in the step piston, and on the other hand by a holding piece 10 of the rack which is held by a securing projection 11 of the step piston.

The step piston 2 has a piston end face 12 turned toward the closed end part 6 of the guide tube 1 and an elongate indentation or recess 13 which extends from the piston end face 12 axially along a center line 14 into the interior of the step piston 2 and ends in an internal piston head 15. The recess has a substantially rectangular or square cross-sectional shape. An external shoulder 16 is formed on the piston end face 12 at the outer periphery of the step piston.

In an advantageous configuration, the recess on the first pressure chamber has an internal region with a cross section which is constant over the axial length of the first path of movement and an adjoining outer region with an enlarged cross section, the

enlarged cross section forming the discharge orifice from the first to the second pressure chamber. The cross section of the outer region can increase continuously or abruptly.

A piston seal 17 formed as a sealing molding comprises a sealing rim 18 which engages in the shoulder 16 for sealing the step piston 2 against the lateral internal faces 19 of the guide tube 1 and a sealing insert 20 which is adapted to the shape of the piston end face 12 and the recess 13 and forms a continuous sealing layer which covers the entire pressure side or all the pressure faces of the step piston.

The size ratio of the faces, effective for driving the step piston, of the first and second pressure chamber on the step piston is about 1 to 10, but can advantageously be in the range of 1: 5 to 1: 15.

The piston drive contains a tubular casing 21 as charge carrier for a driving means that produces a propellant gas. The tubular casing 21 which is manufactured, for example, of aluminum is covered and closed by a pressed-on closure cap 22 which is inserted in an ignition chamber 23 formed in the closed end part 6 of the guide tube 1 in such a way that the tubular casing 21 extends along the center line 14 into the recess 13 in the step piston 2 almost up to the internal piston head 15. The tubular casing is essentially adapted to the cross-sectional shape of the recess 13 and therefore also has a rectangular or square cross section is divided into three tube portions. A leading first tube portion 24 adjacent to

the piston head 15 is formed with a constant cross section and casing walls parallel to the center line 14 and with a leading end face that is closed by a closure part 25. The closure part is manufactured, for example, integrally with the casing walls and can have notches that simplify opening or breakage of the closure part 25. The cross section of the casing 21 increases in an adjoining central or second tube portion 26, the casing walls being slightly inclined towards the center line 14. A third tube portion 27 of the casing 21 extends to the closure cap 22 with for example a substantially constant cross section.

The sealing insert 20 has an internal portion 28 adapted to the leading first tube portion 24 and resting thereon with internal faces 29 parallel to the center line 14 and a sealing base 30. From a position which is described as changeover point 31 and, in the starting position of the step piston 2 shown in FIGS. 1 and 2, that coincides with the transition between the first and the second tube portions 24,26, the internal faces 29 of the sealing insert 20 expand in the direction of the upper piston end face 12 so the cross section of the internal space defined by the sealing insert 20 increases.

In a bore formed on the closed end part 6 in the guide tube 1 and leading laterally to the ignition chamber 23 there is inserted a percussion igniter 32 which extends into the ignition chamber 23 and rests with its end face 33 on a hollowed flattened region 34 of the closure cap 22.

To activate the piston drive and tighten a seat belt, a striking bolt (not shown) strikes the percussion igniter 32 so that its explosive charge is ignited. A flame enters the closure cap 22 and ignites the pyrotechnic propellant contained in the casing 21 of the charge carrier (the breakage of the percussion igniter 32 and of the closure cap 22 is not shown in FIG. 4). The leading closure part 25 of the casing 21 is ruptued by the pressure of the propellant gas. The propellant gas flows through the ruptured propellant gas discharge orifice 35 (shown in FIG. 3) into a first pressure chamber 36 that is defined by the sealing base 30 and the internal faces 29 of the internal portion 28 of the sealing insert 20 and is sealed from the leading first tube portion 24 of the casing 21 resting on the internal faces 29 of the sealing insert. As a result of the pressure of the propellant gas acting in the first pressure chamber 36 against the sealing base 30 and the piston head 15, the step piston 2 is moved with a first driving force corresponding to the size of the base of the first pressure chamber 36 in the direction of the arrow X in the guide tube 1. The first pressure chamber 36 is thereby axially enlarged but remains sealed by the sealing insert 20 due to the contact of the leading tube portion 24 with the casing 21. The rack 5 engages with the gear-wheel 3 and meshes with its teeth, the correct introduction of the rack 5 into the teeth of the gear-wheel 3 being simplified by the reduced half tooth 8.

As the forward movement of the step piston 2 continues, the changeover point 31 of the sealing

insert 20 passes the opened closure part 25 of the first tube portion 24 on the discharge orifice 35, the receding internal faces 29 causing an overflow or discharge orifice 37 between the outer periphery of the leading tube portion 24 and the internal faces 29 of the sealing insert 20 to begin opening (see FIG. 3) to allow admission of the propellant gas into a second pressure chamber 38 defined by the upper piston end face 12 of the step piston 2 and the internal faces of the guide tube 1. The driving pressure face of the second pressure chamber 38 is formed by the cross- sectional area of the internal chamber of the guide tube 1 minus the effective base or pressure face on the piston head 15 of the first pressure chamber 36.

Since the driving pressure face of the second pressure chamber 38 is considerably larger than that of the first pressure chamber 36, for example by a factor of about 10, a force which is greater by this factor additionally acts on the step piston 2 and accelerates it accordingly, with a much greater propulsive thrust.

A second embodiment of the piston drive according to the invention (see FIGS. 5 to 8) contains a charge carrier for the propellant which is modified in relation to the first example and in which a sleeve- shaped closure cap 40 is pressed onto a casing 21 which is lengthened on the cap side. The sleeve- shaped closure cap 40 has a shape adapted to the ignition chamber 23 and is pressed into the ignition chamber 23. The closure cap 40 and the casing 21 have a lateral orifice 41 through which the percussion igniter 32 penetrates into the interior of the closure

cap 40 and the casing 21. Upon activation of the percussion igniter 32, the flame directly ignites the propellant charge contained in the casing 21, as the closure cap 40 must not be pierced. The design and mode of operation otherwise corresponds to the first embodiment.

Instead of arranging the, for example, powdered propellant charge in the casing 21, the propellant charge, or generally a gas producing unit, can also be mounted outside the casing or the guide tube 1 so that the tubular casing serves merely as an inlet tube for the propellant gas produced externally.

An electric igniter can be used instead of a mechanical percussion igniter to ignite the propellant charge.

The tubular casing 21 can have a circular or oval cross section, the internal faces of the sealing insert 20 being adapted to a casing shaped in this way. With a circular casing, however, it is desirable also to design the recess 13 in the step piston 2 with a circular cross section.

The piston end face 12 defining the second pressure chamber 38 can be substantially plane, beveled towards the recess 13 or provided with additional indentations. The recess and/or the sealing insert 20 are able to open slightly in the form of a funnel toward the second pressure chamber so that the cross section of the overflow or discharge orifice 37 is enlarged during the movement of the step piston 2 and allows greater permeation of propellant gas.

The timing of opening of the overflow or discharge orifice 37 can be determined as a function of the path of movement covered by the step piston 2 by differing design of the length of the discharge portion 24.

FIGS. 10 and 11 show schematically a guide tube 1 produced from aluminum of a piston drive according to the invention in a cross section perpendicular to a longitudinal axis or to the center line 14. The guide tube has an internal, or pressure, chamber 38 with a rectangular cross section contains long and short lateral walls 45,46 defining this internal, or pressure, chamber. The wall thicknesses of the two long lateral walls 45 are so dimensioned that the two lateral walls substantially maintain the rectangular cross section up to a maximum permitted propellant gas pressure in the internal or pressure chamber and the piston seal 17 still seals the pressure chamber. If the propellant gas pressure increases beyond this maximum permitted value, for example if the driving piston is blocked during its forward movement or in the end position for movement of the piston, the two long lateral walls 45 are deformed and bend outwards (see FIG. 11). Located between the two mutually remote external piston faces 47 and the associated deformed lateral walls 45 is an orifice 48 which can no longer be sealed by the piston seal 17 or by the sealing rim as a result of the high propellant gas pressure and therefore forms an excess pressure discharge path and a pressure relief device.