GB2328649A - An airbag having a rupturable vent - Google Patents

Abstract

An airbag made from cushion fabric 1 is formed with a vent which has at least one weak surface portion which maintains the surface integrity of the bag when the pressure inside is below a predetermined value, but is adapted to rupture when the pressure exceeds the predetermined value. The vent may take the form of an aperture 2 covered by a cover member 4 which is larger than the aperture 2 and be fixed to the cushion fabric 1 by stitching 5 around at least a portion of its periphery. A reinforcing layer 3 may be fixed by stitching or adhesive around the periphery of the aperture 2 thus forming an envelope with the cushion fabric 1 inside which the cover member 4 is housed. The reinforcing layer 3 may be made of a material which is relatively stiff so that it stays in place until the pressure exceeds the aforementioned predetermined pressure and the vent cover is pushed away from the aperture releasing the gas. The exact nature of the stitching and materials can be chosen to provide specific venting characteristics.

Description

AIR RAG The present invention relates to an airbag for a vehicle safety restraint.

Airbags comprise folded fabric cushions connected to a source of inflation gas which is activated when an emergency situation is detected, for example by a crash sensor. The cushion is inflated within a few milliseconds to protect the vehicle occupant from the adverse effect of impact with interior parts of the vehicle such as the steering wheel, the instrument panel and side door panels.

In order to inflate the cushion quickly enough, the source of inflation gas develops a very high gas pressure and the inflating cushion itself could, under certain circumstances, be a danger to the vehicle occupant. To avoid injury to the occupant caused by the inflating cushion, provision must be made to allow gas to escape from the cushion either by leakage through the fabric itself or through one or more discrete venting holes. When the vehicle occupant impacts against the inflating cushion, the venting acts as a dampener reducing the impact forces felt by the occupant and also preventing the inflated airbag acting like a spring and propelling the occupant backwards.

However the problem with traditional venting is that gas escapes constantly even before the occupant impacts the cushion and this is wasteful, requiring a larger capacity gas generator than might otherwise be needed. In addition the venting capacity of traditional venting takes no account of different sizes and weights of occupant and different crash conditions (eg the velocity of the vehicle, the angle of impact). The cushion will present a different pressure to the occupant depending on the time between deployment and impact.

It is an object of the invention to provide improved venting arrangements for airbag cushions.

According to the present invention there is provided an airbag cushion comprising a fabric bag having a vent comprising at least one weak surface portion which maintains the surface integrity of the bag when the gas pressure in the bag is below a predetermined value and is adapted to rupture when the internal gas pressure is above the predetermined value.

Such a vent has considerable advantages since it allows the cushion to deploy without losing gas to the environment before the occupant starts to interact with the cushion.

Hence an adequate cushion pressure for protection of the occupant is assured regardless of the delay between deployment and interaction. The vent releases gas and provides the appropriate dampening only when the impact of the occupant on the cushion increases the internal pressure to at least the predetermined value.

According to one embodiment of the present invention the vent comprises an aperture in the fabric ofthe cushion, a vent cover member, of a size larger than the aperture, arranged to cover the aperture, and the vent cover member is fixedly secured to the fabric of the cushion around at least a portion of its periphery.

Preferably a reinforcing layer of material, for example, of cushion fabric, is attached to the cushion to form a border around the aperture, and the periphery of the vent cover member is enclosed between the two layers of fabric formed by the circumference of the aperture and the reinforcing layer, The vent cover member may be formed of material with a stiffness slightly higher than that of the airbag cushion fabric so that it has a natural tendency to stay in place, its periphery held between the two layers of fabric until the internal pressure in the cushion is such as to overcome the fiction and the stiffness ofthe material and push the unsecured portion of the periphery of the cover member out of the aperture to allow release of gas and reduce the pressure in the cushion. A suitable material would be a plastic film.

The secured portion of the cover member may be fixed by stitching through one or both layers of fabric. The stitching pattern may be chosen to influence the vent to open at different times and under different pressure conditions. For example a larger portion of the circumference of the member may be stitched, two opposing sections, or any combination of sections, may be stitched, or the stitching may be arranged to attach the vent cover member to only one layer of fabric. The size and strength of the stitching can of course also be adapted to suit the required performance conditions.

The vent cover member may alternatively be formed of a non-woven thin layer of material such as a paper product or a proprietary product such as that known by the trade mark Tyvek. This splits away from the cushion fabric, generally along stitch lines, under predermined pressure conditions without the need for making special lines of weakness or stitched tear lines. In this embodiment the cover member is stitched into the cushion fabric around the whole periphery to cover the aperture. The size, strength and pattern of stitches can be chosen to influence the way in which the vent opens.

An alternative embodiment has the vent cover member enclosing a region of the cushion fabric in which slits have been made, in a particular pattern to determine the required venting characteristics. In one particular embodiment, a generally circular vent cover member is stitched around its circumference onto the airbag cushion fabric. Spaced from and within the circular line of stitching are a series of slits. As the pressure in the cushion builds, the gas escapes through the slits and impinges on the cover member, eventually allowing gas to escape reducing the pressure in the cushion as required.

In yet another embodiment the fabric ofthe cushion is perforated or weakened along lines of weakness which break the predetermined pressure to open the vent. The tear lines would preferably end in small circular holes to avoid the fabric tearing further and the area could be further limited by a line of stitching such as a circular sew line.

For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a plan view of a vent for an airbag cushion in accordance with a first embodiment of the present invention.

Figures 2 and 3 are schematic views illustrating alternative stitch patterns in the embodiment of figure 1.

Figure 4 is a cross-sectional side view of the embodiment of figure 1.

Figure 5 is a top plan view of a vent for an airbag cushion in accordance with a second embodiment of the invention, Figure 6 is a bottom plan view of the vent of figure 5.

Figure 7 is a top plan view of a vent for an airbag cushion in accordance with a third embodiment of the invention, Figure 8 is a bottom plan view of the vent of figure 8.

Figure 9 a) and b) illustrate a further embodiment of the invention.

In the embodiment of figures 1 to 4 airbag cushion fabric 1 is formed with a circular aperture 2 forming a vent hole in the lower panel of the cushion. A reinforcing ring 3 made of fabric extends from an outer edge 3a to an inner edge 3b and is attached to the fabric 1 around the aperture. The reinforcing ring may have the same internal diameter as the aperture or it may be a different size, for example slightly larger as shown in figure 1. It may be attached by adhesive, by ultrasonic or heat welding or by stitching 17 as shown in figure 4, leaving its inner circumference 3b unattached, to form a double layer or envelope. A vent cover member 4, circular and slightly larger than the aperture 2, has its outer periphery 4a received in the envelope between the two layers of fabric 1 and 3.

The cover member 4 is formed of flexible plastic film with a stiffness slightly higher than that of the airbag cushion fabric. It is secured to the cushion fabric 1 by a line of stitching 5 along a small section, to prevent the film from floating and flying out.

Figures 2 and 3 show alternative stitch lines for attaching the vent cover member 4 to the fabric 1 as shown at 5 and 7. Each pattern will give a different performance characteristic.

Figures 5 and 6 illustrate an alternative embodiment in which the aperture takes the form of arcuate slits 8 in the fabric 1. There is no reinforcing ring in this embodiment but instead the vent cover member 4, which is formed of a thin, non-woven material such as paper or Tyvek material (registered trade mark), is stitched to the fabric 1 around its whole circumference as indicated by the line of stitching 5. This could be a sew line of 92/92 but this is only one example.

Figures 7 and 8 illustrate another alternative embodiment. This has a circular aperture 2 in the cushion fabric and a vent cover member 4 with an arcuate side and a straight side generally in the shape of a round top door. Again the cover member 4 is formed of paper or Tyvek material. It is stitched to the cushion fabric 1 with a double line of stitching 9 along the straight side and with a single line of stitching 5 along the other sides. Typically the double line could be sew lines of 138/138 and the single line 5 a sew line at 69/69.

However this is only given as an example and the actual sew lines will be chosen for the particular conditions involved. The double stitch line prevents or at least substantially inhibits tearing of the join along the straight side.

Figure 9a illustrates an embodiment in which crossed lines of weakness 10 are formed either by perforations or by laser. Small circular holes of around 4mm maximum diameter are made at the ends of each line 10 to limit the extent of tearing under pressure, and an additional sew line 12 can be included to further strengthen the outer periphery of the vent. The lines 10 are preferably made parallel to the weft and warp fibres respectively of the fabric (which are schematically indicated at 14 in figure 9a ). At the predermined internal cushion pressure, the lines 10 break and the free flaps 13 ofthe fabric are pushed outwardly by the escaping gas, as shown in figure 9b, to open the venting hole 15.

Claims (22)

CLAIMS:

1. An airbag cushion comprising a bag of fabric material having a vent which comprises at least one weak surface portion, which portion maintains the surface integrity of the bag when gas pressure in the bag is below a predetermined value, and which portion is adapted to rupture when the internal gas pressure is above the predetermined value.

2. An airbag cushion according to claim 1 wherein the vent comprises an aperture in the fabric of the cushion, and a vent cover member, of a size larger than the aperture, arranged to cover the aperture, and wherein the vent cover member is fixedly secured to the fabric of the cushion around at least a portion of its periphery.

3. An airbag cushion according to claim 2 further comprising a reinforcing layer which is attached to the cushion to form a border around the aperture, and wherein the periphery of the vent cover member is enclosed between the two layers of fabric formed by the circumference of the aperture and the reinforcing layer.

4. An airbag cushion according to claim 3 wherein the reinforcing layer is formed of cushion fabric.

5. An airbag cushion according to claim 2, 3 or 4 wherein the vent cover member is formed of a material which has a stiffness higher than the stiffness of the airbag cushion fabric so that it has a natural tendency to stay in place, its periphery held between the two layers of fabric until the pressure in the cushion is above the predetermined value so that the unsecured portion of the periphery of the cover member is pushed by the pressure away from the aperture to allow release of gas and reduce the pressure in the cushion.

6 An airbag cushion according to any one of claims 2 to 5 wherein the vent cover member is formed of a plastics film.

7. An airbag cushion according to any one of claims 2 to 6 wherein the secured portion of the cover member is fixed by stitching through at least one layer of fabric.

8. An airbag cushion according to claim 7 wherein the stitching pattern is adapted and chosen to influence the times and pressure conditions under which the vent opens.

9. An airbag cushion according to claim 8 wherein the vent cover member is stitched into the cushion fabric around the whole periphery to cover the aperture and wherein the cover member splits away from the cushion fabric, along the stitching, under predetermined pressure conditions.

10. An airbag cushion according to claim 8 wherein a majority portion of the circumference of the member is stitched.

11. An airbag cushion according to claim 8 wherein two oppositely sited portions of the circumference are stitched.

12. An airbag cushion according to any one of claims 8 to 11 wherein the stitching is arranged to attach the vent cover member to only one layer of fabric.

13. An airbag cushion according to any one of claims 8 to 12 wherein the size and strength of the stitching is adapted to influence required performance conditions.

14. An airbag cushion according to any one of claims 2 to 13 wherein the vent cover member is formed of a non-woven thin layer of material.

15. An airbag cushion according to any one of claims 2 to 13 wherein the vent cover member is formed of a paper product.

16. An airbag cushion according to claim 15 wherein the vent cover member is formed of a proprietary product known by the Registered Trade Mark TYVEK.

17. An airbag cushion according to any one of claims 2 to 16 wherein the vent cover member encloses a region of the cushion fabric which region comprises at least one opening arranged in a predetermined pattern to determine required venting characteristics.

18. An airbag cushion according to claim 17 wherein the vent cover member is generally circular in shape, is stitched around its circumference onto the airbag cushion fabric and wherein said at least one opening comprises a slit spaced from and enclosed within the circular line of stitching.

19. An airbag cushion according to any one ofthe preceding claims, wherein the fabric ofthe cushion is perforated or weakened along at least one line of weakness which breaks at the predetermined pressure to open the vent.

20. An airbag cushion according to claim 19 wherein generally circular holes are formed at the ends of the or each line of weakness to define end points of tearing.

21. An airbag cushion according to claim 19 or 20 comprising a circular sew line surrounding the or each line of weakness to limit tearing.

22. An airbag cushion substantially as hereinbefore described with reference to Figures 1 and 4 , alternatively as modified by Figures 2 or 3, or with reference to Figures 5 and 6, or Figures 7 and 8 or Figures 9a and 9b, of the accompanying drawings.