Pyrotechnical inflator and method of making same

An inflator, more particularly for a vehicle occupant restraint system, has a tubular inflator housing which is closed off at its end faces and has radial exit ports via which the gas is able to exit the inflator. The inflator housing has in its interior at least one combustion chamber packed with propellant, at least one ignition unit for igniting the propellant in the combustion chamber and at least one filter which is located upstream of the exit ports. A filter insert featuring an outer housing is provided, the outer housing having a plurality of housing parts which at least partly surround the filter disposed in the filter insert. The outer housing together with its housing parts is radially secured to the inflator housing without the provision of an axial stop. The outer geometry of the outer housing and the inner geometry of the inflator housing permit total insertion of the outer housing or its housing parts in the axial direction into the inflator housing to different insertion depths when the outer housing is still in the non-secured condition.

FIELD OF THE INVENTION
 The invention relates to an inflator, more particularly for a vehicle
 occupant restraint system. The invention further relates to a method of
 making an inflator.
 BACKGROUND OF THE INVENTION
 Known tubular inflators for inflating gas bags in vehicles are usually
 configured single-stage or two-stage. The expense of their fabrication is
 relatively high, this being due to the fact that totally different designs
 are provided for single-stage and two-stage inflators. Tubular inflators
 are mainly provided for restraint systems on the front passenger side
 since their outer geometry is best suited for accommodation in the
 dashboard.
 One highly complicated two-stage tubular inflator configuration is evident
 from U.S. Pat. No. 5,033,390. In this arrangement, at the two axial ends
 of the inflator housing combustion chambers are provided between which two
 expansion spaces are arranged. These two expansion spaces separated from
 each other are formed by two outer disks and an intermediate disk formed
 integrally on an anchor adjoining the outer disks. In addition to this,
 elongated tubes are screwed into place axially from without, which
 comprise a booster charge in their interior and are connected with axial
 covers which seal off the two-part tubular housing at the end faces.
 Moreover, the tubular housing has in its interior axial stops for
 positioning the disks. When an inflator is to be produced which has larger
 or smaller combustion chambers, the housing and anchor parts need to be
 redimensioned. This makes the use of like parts very difficult. It will
 also be readily appreciated that this complicated structure makes
 fabricating the inflator relatively expensive. On top of this the
 individual parts do not lend themselves to be also put to use in
 single-stage inflators. The same drawbacks are exhibited by the inflator
 disclosed in U.S. Pat. No. 5,219,178, in which all parts inserted in the
 tubular housing adjoin each other axially and fix each other in position.
 SUMMARY OF THE INVENTION
 The invention provides an inflator the design principle of which permits
 simple, cost-effective manufacture of both a single-stage and a two-stage
 or multi-stage version. The basic principle of the invention results in a
 kind of modular system in which numerous parts in the single-stage or
 multi-stage version are the same. In addition to this, the inflator in
 accordance with the invention enables the inflator output to be varied by
 simple means without necessitating any basic change in design or a
 different dimensioning of the individual parts. The inflator according to
 the invention for a vehicle occupant restraint system comprises a tubular
 inflator housing including opposite end faces at which it is closed off
 and having radial exit ports via which the gas is able to exit the
 inflator, and having in its interior at least one combustion chamber
 packed with propellant, at least one ignition unit for igniting the
 propellant in the combustion chamber and at least one filter being located
 upstream of the exit ports. The inflator further includes a filter insert
 featuring an outer housing having a plurality of housing parts which at
 least partly surround the filter disposed in the filter insert. In a
 condition in which the outer housing is secured to the inflator housing,
 hereinafter referred to as secured condition, the outer housing, together
 with its housing parts, is radially secured to the inflator housing
 without the provision of an axial stop. The outer geometry of the outer
 housing and the inner geometry of the inflator housing, however, permit
 total insertion of the outer housing or its housing parts in the axial
 direction into the inflator housing to different insertion depths when the
 outer housing is still in a non-secured condition. Whereas in the prior
 art provision is always made for axial stops at the housing parts and for
 parts which are adapted to a specific combustion chamber size, the present
 invention provides for insertion of the outer housing or the housing parts
 of the filter insert into the inflator housing to different insertion
 depths without the provision of an axial stop prior to securing the outer
 housing and the inflator housing. The filter insert can thus be positioned
 within the inflator housing at any required position and subsequently
 secured thereto. When a single-stage inflator is to be manufactured, the
 insert is inserted into the inflator housing configured tubular only to
 such an extent that it is located at one end of the inflator. For a
 multi-stage inflator, practically the same inflator housing can be made
 use of and, where necessary, also the same insert, except for minor
 modifications, which is then simply inserted deeper into the inflator
 housing and secured thereto. Depending on whether the filter insert is
 located in the middle between the ends of the inflator housing or nearer
 to one end thereof, combustion chambers the same or different in size can
 be provided adjoining the filter insert. Changing the volume of the
 combustion chamber in this way necessitates no change to the geometry of
 the parts, this also applying to a single-stage inflator in accordance
 with the invention in which, for downsizing the output, the filter insert
 merely needs to be inserted deeper into the inflator housing otherwise
 dimensioned the same. This results in enhanced variability as regards the
 combustion chamber volumes and it being made easy to adapt the inflator
 output to specific customer requirements. High component correspondence
 and low number of components required result in low costs per item, while
 the preassembling capability of the insert facilitates assembly and helps
 in reducing costs.
 When by contrast the volume of one or both combustion chambers needs to be
 changed in the case of the inflator known from U.S. Pat. No. 5,033,390,
 then both tubes to be assembled to form the inflator housing as well as
 the many axial connecting parts arranged internally need to be totally
 adapted.
 Preferably, the entire filter insert forms a preassembled unit which is
 inserted in the inflator housing. But it is also possible for the filter
 insert to be assembled when it is already in the inflator housing. This
 can be done for example in that the outer housing has two end face walls
 which only partly surround the filter (at the end faces) and clamp the
 filter inbetween and which are inserted from opposite axial openings
 provided in the tubular inflator housing. Preferably, a press fit is
 provided between the inflator housing and the outer housing so as to fix
 the filter insert or its housing parts to some extent during insertion.
 In the preferred embodiment of the invention the insert also has at least
 one expansion space in its interior.
 When the inflator in accordance with the invention is configured as a
 single-stage inflator, the insert is provided, as mentioned, at one axial
 end of the inflator housing, it sealing off the latter from without at
 this end.
 Another possibility of reducing the number of parts employed consists in
 the inflator comprising an insert with an ignition unit extending axially
 through the latter into the combustion chamber. The inserts with and
 without ignition unit hardly differ from each other. Depending on the
 desired installation position or the ambient circumstances, for example,
 the insert with the ignition unit is inserted at the side of the inflator
 having more facilitated access and the insert without ignition unit at the
 other side. As a result of this modular system various configurations can
 be achieved at no additional expense in design or fabrication. If the
 inflator in accordance with the invention is configured multi-stage, then,
 as mentioned, the insert is simply inserted in the inflator housing so far
 that it is located between two combustion chambers. If an insert is
 employed which is open at both end faces then the insert may serve as the
 filter insert for both combustion chambers. Should several filter inserts
 or expansion spaces be needed, then, for example, two inserts may be
 inserted one after the other, one of which is furnished with an end face
 wall which is closed off relative to the other insert, as a result of
 which the combustion chambers are separated from each other.
 In addition, the present invention provides a method of making an inflator
 in accordance with the invention, the method providing a modular system
 for single-stage and multi-stage tubular inflators of different outputs.
 The method according to the invention provides that starting from an axial
 opening in the inflator housing the outer housing of the filter or the
 housing parts is/are inserted to the desired depth, that the outer
 geometry of the outer housing and the inner geometry of the inflator
 housing are so adapted to each other that an even deeper insertion would
 be possible, that the combustion chamber is packed with a propellant and
 that a cover is arranged on the end face of the inflator housing for
 closing the combustion chamber.
 In the method according to the invention there is also the provision that
 the filter insert forms a separate preassembled unit or that the housing
 parts of the outer housing are inserted and thus joined together. This may
 be done, for example, by the provision of two end face walls and a filter,
 the end face walls being inserted from opposite axial openings in the
 inflator housing and clamping the filter or, to put it more generally, a
 shell part having gas exit ports, between them.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The invention will now be described in detail with reference to the
 embodiments shown in the drawings.
 In FIG. 1 a tubular inflator for inflating a front passenger gas bag (not
 shown) is illustrated. This inflator has a tubular inflator housing 3 with
 a combustion chamber 7 packed with propellant 5 in its interior. At its
 end faces the inflator housing 3 is closed off gas-tight by two covers.
 The cover 9 shown on the left in FIG. 1 comprises an elongated ignition
 unit 11 extending into the combustion chamber 7, this ignition unit being
 secured to the cover 9. The ignition unit 11 comprises an igniter 13 and a
 booster charge 15 adjoining the latter. The booster charge 15 is arranged
 in an elongated tube 17. The cover 9 is secured to the housing 3 by
 welding. An elastic disk 19 for pretensioning the propellant 5 is
 positioned in the inflator housing 3 by an indentation 21 on all sides. By
 means of an additional such indentation, for instance, the filter insert
 23 may also be positioned in one direction. A filter insert 23 configured
 as a preassembled unit is inserted in the inflator housing 3 at the axial
 end of the inflator housing opposite the cover.
 The filter insert 23 has an outer housing having two opposite pot-shaped
 end face walls 25, 27. The interior end face wall 25 defines the
 combustion chamber 7 at the right-hand end face of the latter. Ports 29 in
 the end face wall 25 permit passage of the generated gas, as will be
 explained in more detail in the following. Annular shell sections 31 of
 the end face walls facing each other sealingly contact the inside of the
 inflator housing 3. A supporting ring 33 which is gas permeable and is
 made of a perforated plate or a wire mesh is inserted by its radial ends
 into the end face walls 25, 27 and contacts the shell sections 31 at their
 inner faces by a press fit so that this supporting ring connects the end
 face walls 25, 27 to each other. A tubular filter 35 contacts the
 supporting ring 33 again on the inside.
 Between the supporting ring 33 and the inflator housing 3, a first annular
 expansion space 37 materializes. Several gas exit ports 39 distributed
 circumferentially guide the compressed gas into the gas bag in the case of
 restraint.
 The end face wall 27 features no gas exit ports, it instead closing off the
 interior of the inflator at the right-hand axial end. Not shown are
 insulations which close off the ports 29 and/or the gas exit ports 39 to
 prevent the ingress of moisture.
 The interior of the filter insert 22 is hollow so that a second expansion
 space 41 is formed.
 Due to its outer geometry being precisely adapted to the inner geometry of
 the inflator housing 3, the filter insert 23 can be inserted to any
 desired depth in the inflator housing 3, without parts dimensioned
 otherwise being necessary, because no axial stops are provided for
 positioning the filter insert. Inserting the filter insert more or less
 deeply into the inflator housing enables the combustion chamber volume to
 be varied by simple means and the output of the inflator to be adapted to
 the specifications of the vehicle manufacturer. Once the filter insert has
 been inserted and positioned, the outer housing and the inflator housing
 are secured to each other from outside (e.g. by calking and/or welding),
 propellant is introduced, and the cover 9 on the end face which closes the
 combustion chamber 7 is secured to the inflator housing 3. Despite
 differences in the combustion chamber volume corresponding inflators can
 be fabricated in one and the same production line by very simple means.
 When the vehicle is involved in a collision, the igniter 13 is activated
 via a sensor system (not shown). Via the booster charge 15 the propellant
 5 is ignited so that hot gas materializes intermingled with particles and
 condensatable constituents. After opening of the insulation, the hot gas
 flows through the ports 29 into the second expansion space 41, the end
 face wall 25 already acting as a kind of prefilter in this case. In the
 second expansion space 41 cooling of the gas takes place. Part of the
 condensatable constituents of the gas is separated out and retained, like
 the particles, in the filter 35. In the expansion space 41, the gas flow
 is moderated and the gas is distributed evenly. The first expansion space
 37 ensures that the flow passes the filter 35 over practically its full
 surface area in the radial direction and that no axial flow occurs in the
 filter. The cooled and filtered gas re-expands in the expansion space 37
 and gains access to the gas bag through the gas exit ports 39.
 In the expansion spaces 37, 41, to attain subsequent reaction of gas
 constituents and a chemical conversion of some of them, a catalytically
 acting material 43 may be employed, by means of which a reduction of the
 noxious emissions contained in the generated gas, or more precisely the
 generated gas mixture, is achieved. This ensures a reduction of the gases
 generated on combustion of the propellant 5.
 The embodiment depicted in FIG. 2 differs from that shown in FIG. 1 by two
 filter inserts being provided instead of one. The filter insert 23
 provided at the right-hand end corresponds to the filter insert 23 as
 shown in FIG. 1 so that there is no need to detail this. The left-hand
 filter insert 45 has a design configuration similar to that of filter
 insert 23. Accordingly, like components are identified by like reference
 numerals. However, integrated in the filter insert 45 is the ignition unit
 11 which is configured the same as the ignition unit 11 shown in FIG. 1.
 For its locking, the outer end face wall 47 is formed thicker than the end
 face wall 27. The inner end face wall 49 corresponds to the end face wall
 25, the tube 17 of the ignition unit passing through a central opening,
 however, to protrude into the combustion chamber 7. The filter insert 45,
 too, may be inserted to any desired depth in the inflator housing 3. Ports
 39 at the left-hand end of the inflator housing 3 are flowingly connected
 to the two expansion spaces 37, 41 which are defined by the filter insert
 45.
 On activation of the inflator as shown in FIG. 2, the gas generated flows
 through both the left-hand and right-hand filter inserts 45 and 23,
 respectively, as well as via the expansion spaces 41, the filters 35, the
 expansion spaces 37 and the gas exit ports 39 into the gas bag. Due to the
 gas exit ports 39 being arranged symmetrically at both ends of the
 inflator, the gas bag is pressurized evenly so that wobbling of the gas
 bag during inflation as observed in the case of inflators having a
 non-symmetrical gas exit is avoided.
 To improve the filter effect the expansion spaces 37 and/or 41 may even be
 fully packed with filter material.
 The inflator as shown in FIG. 3 is configured two-stage. To form two
 combustion chambers 7, 7' a filter insert 23 common to both combustion
 chambers is shifted roughly up to the middle of the inflator housing 3.
 The filter insert 23 differs from that as shown in FIG. 1 merely by the
 right-hand end face wall 27 also being provided with ports 29, i.e.
 configured identical to the end face wall 25. By inserting the filter
 insert 23 into the inflator housing 3 to differing depths, the ratio of
 the combustion chamber volume can be varied without parts dimensioned
 differingly being necessary for this purpose. At the left-hand end of the
 inflator, a cover 9 with an ignition unit 11 is provided, the
 configuration of the latter corresponding to that of the cover 9 with the
 integrated ignition unit as shown in FIG. 1. At the right-hand end face,
 the inflator is closed off by a cover 51 having an integrated ignition
 unit 53, the booster charge 15 surrounding the igniter 13 and not
 protruding from the cover 51, but flowingly connected to the combustion
 chamber 7 via ports in the cover inner wall.
 If only one stage is to be ignited, an accidental ignition transfer must be
 avoided, i.e. the hot gases generated in the one combustion chamber must
 not flow over into the other combustion chamber. For this purpose burst
 disks 55 are secured, preferably by welding, adjacent to the inner side of
 the end face walls 25, 27, closing off the ports 29 and capable of being
 opened in one direction only. These burst disks 55 simultaneously serve as
 insulants.
 Embodiments of the burst disks 55 are illustrated in FIGS. 4a and 4b.
 Cross-sectionally V-shaped indentations 57 may be oriented in the shape of
 a cross or a circle with radial sections to facilitate opening the burst
 disk 55. The left-hand burst disk 55 is opened only when gas is generated
 in the combustion chamber 7 and the right-hand burst disk 55 only when gas
 is generated in the combustion chamber 7'. After for instance the
 left-hand burst disk 55 has been opened, the gas gains access to the
 expansion space 41. The increase in pressure in the expansion space 41
 results in the right-hand burst disk 55 being pressed against the end face
 wall 27 from which it cannot be released or destroyed.
 The two-stage inflator shown in FIG. 5 comprises one filter insert 45 at
 both of its axial ends, each insert having an integrated ignition unit as
 is evident from FIG. 2. The left-hand and the right-hand filter insert 45
 differ merely by the dimension of the booster charges 15 which are adapted
 to the volume of the assigned combustion chambers 7, 7'. Unlike the
 embodiment as shown in FIG. 3 the two combustion chambers 7, 7' are not
 spaced away from each other by a filter insert but by a partition wall 61,
 this partition wall 61 corresponding to the end face wall 27 as shown in
 FIG. 1. The ports 39 are arranged in the region of the axial ends of the
 inflator housing 3 and not, as in the embodiment shown in FIG. 3, roughly
 in the middle of the inflator housing. The partition wall 61 can be
 inserted into the inflator housing 3 to differing depths to vary the ratio
 of the combustion chamber volumes, similar to the situation as with the
 insert 23 as shown in FIG. 3. The partition wall 61 is connected to the
 inflator housing 3 by welding. So that, irrespective of whether a
 single-stage or a two-stage inflator having one or more filter inserts is
 to be fabricated, always the same inflator housing 3 can be used, the
 ports 39 are not produced until the filter inserts or covers have been
 inserted.
 The inflator as shown in FIG. 6 differs from those as already shown by an
 extra-long filter insert 23 being provided, the end face walls 25, 27 of
 which have a central, large port. The expansion space 41 is packed with
 propellant 5 and thus is no longer acting as an expansion space. By this
 design, the output of the inflator can be increased relative to its
 constructional space. At the left-hand end the inflator is closed off by a
 cover 9 having an ignition unit 11 as already explained above, and at the
 right-hand end a separate cover 63 is provided. The cover 63 is in direct
 contact with the filter insert 23, is inserted into the inflator housing 3
 and is welded thereto.
 Instead of one filter insert 23, two filter inserts 23 may be provided as
 is evident from FIG. 7 representing a two-stage inflator. In this
 arrangement, the left-hand filter insert 23 corresponds to that shown in
 FIG. 6. The right-hand axially shorter filter insert 23 comprises a closed
 end face wall 27 adjoining the left-hand filter insert 23, this wall
 separating the two combustion chambers 7, 7' from each other.
 The embodiment as shown in FIG. 8 largely corresponds to that illustrated
 in FIG. 3, the only difference residing in the configuration of the filter
 insert 23. The filter insert comprises the pot-shaped end face walls 27
 and 25 and is no preassembled unit. Rather, the housing parts are inserted
 into the inflator housing 3 one after the other and are thereby joined
 together. The filter insert 23 further includes no supporting ring 33. The
 filter 35 is clamped axially between the end face walls 25, 27. A further
 filter 135 is provided radially outside the filter 35. A conical
 intermediate filter 137 extends from each end face wall 25, 27 into the
 combustion chambers 7, 7' and divides the latter into two sections. The
 intermediate filters 137 serve to prevent the filter 35 from being clogged
 by slag particles developing on ignition of the propellant.
 The filter insert 23 is assembled as follows: Starting from the right-hand
 axial opening in the inflator housing 3 the end face wall 27 is inserted
 to the desired depth together with the intermediate filter 137, a press
 fit being provided between the end face wall 27 and the inflator housing 3
 which serves to facilitate positioning the end face wall. Starting from
 the left-hand axial opening the left-hand end face wall 25 is then
 inserted together with the filters 35, 135 and the intermediate filter
 137. Subsequently, the end face walls are calked with the inflator housing
 3 from outside by applying a radial force to the latter. It is also
 possible to weld the end face walls 25, 27 to the inflator housing from
 without or from within via the axial openings in the inflator housing, for
 instance by use of laser beams.
 Due to the filter inserts being shiftable, the combustion chamber volume,
 or in the case of two-stage inflators also the volume ratio, can be
 adjusted as desired and very easily. Furthermore, a kind of modular system
 materializes which is characterized by numerous parts being identical. Due
 to this modular system, single-stage or two-stage inflators differing in
 output can be produced in one and the same production line.