This invention relates to vehicle occupant restraint systems and more

particularly to a scat belt prctcnsioncr for taking up slack in a scat belt in the

event of a vehicle collision.

A typical inertia reel scat belt comprises a reel of webbing fitted at one

end with a locking device that only functions if the reel spins too quickly. During

a collision, standard inertia reel scat belts will typically allow 120 to 200 mm of

slack before the locking mechanism functions. Devices which arc more sensitive

than this are impractical when the user unreels the belt to lock it into position in

normal use. The delay in operation can allow the occupant to move out of the

seat before being arrested and whilst this may avoid a fatality, it can lead to

serious whiplash injuries.

Seat belt prctensioncrs are now widely used in vehicles to provide

additional protection to the occupants in the event of a collision. They are

normally connected to collision-sensing devices and use stored energy to wind in

sufficient slack in the belt to lock the belt before the user can begin to move. The

energy can be stored in the form of a spring or in a pyrotechnic gas-generating

composition.

In a typical application, the function of the seat belt prctcnsioncr is to

rotate the scat belt retractor or take-up reel rapidly in a belt retraction direction to

pull the belt against the vehicle occupant and eliminate slack in the belt. Many

such prctcnsioncr devices have been proposed, and exemplary of such prior art

constructions arc those disclosed in US patents numbers 4178016, 4789185 and

5145209; European patent number 0480137; German patent application number

DE3715845 and UK patent application number 2258603. The entire disclosures of these prior art patents arc incorporated herein by reference.

The technical requirements for a scat belt pretensioncr arc very

numerous. Firstly it must not interfere with the normal operation of the inertia

reel scat belt retractor. Secondly it must be capable of activation within a few milliseconds in order to provide adequate protection to the vehicle occupant in the

event of a collision. Thirdly, vehicle manufactures' specifications now require

that the prctcnsioncr should exert a force of 700 to 800 Newtons and retract the

scat belt by a minimum of 100 mm and a maximum of 200 mm. Fourthly, after

activation of the seat belt pretensioner, it should preferably not interfere with the

normal operation of the inertia reel scat belt retractor, in the event that the vehicle

can still be driven after the collision incident.

Further very important practical requirements are that the seat belt

pretensioner should be highly compact, fit on to existing inertia reel seat belt

systems and preferably fit the housing profile of existing equipment.

Scat belt prctensioncrs which have been proposed hitherto either do not

meet the technical requirements outlined above, or arc unacccptably bulky or

impractical.

According to the present invention there is provided a prctcnsioncr, for

example, for an inertia reel scat belt, wherein the motive force is provided by a

projectile impinging upon a drive surface.

In one aspect of the invention, there is provided a pretensioner for a

vehicle scat belt retractor of the type having a rotatable take-up reel on which

seat belt webbing can be wound, which comprises:

0) An elongate pressure chamber;

(ii) a plurality of projectiles located in the pressure chamber;

(iii) gas generating means for generating a high-pressure gas within the

pressure chamber so as to propel the projectiles along the pressure

chamber; and

(iv) a drive surface upon which the projectiles impinge when propelled

along the pressure chamber by the high-pressure gas, the drive surface

being connected to, or comprising a part of, a rotatable take-up reel of

a vehicle seat belt retractor and arranged to be driven by the impinging

projectiles so as to rotate the rotatable take-up reel in a belt retraction

direction when impinged upon by the projectiles, so as, in operation, to

take up slack in the scat belt webbing, and to pull the scat belt webbing against the vehicle occupant.

In a preferred prctcnsioncr according to the present invention, the drive

surface comprises a surface, preferably a circumferential surface, of a wheel

which is connected to, or comprises part of, the rotatable take-up reel of the

vehicle scat belt retractor. The drive surface may be indented, toothed, grooved,

or otherwise shaped, to accept partial intrusion by the projectiles to provide a

drive mechanism. The drive surface may be formed with recesses each of which

is adapted to receive a single projectile.

Alternatively the drive wheel may comprise compliant material

dcformablc by pressure contact between the drive surface and projectile. In one

embodiment the drive wheel may comprise at least a circumferential surface layer of compliant material. In another embodiment the drive wheel may comprise a

circumferential surface layer of rigid material, and radially inward thereof an

annular layer of compliant material.

Suitable compliant materials include resilient material such as rubber, or plastics

materials. In another alternative embodiment the drive wheel comprises a

circumferential channel having resiliency dcformable channel sides, the spacing

of the channel sides being such that a projectile can be received therebetween

only by deforming the channel sides. The channel sides may, for example, be

spring biased towards each other.

Preferably the projectiles impinge tangcntially, or at a low angle of

preferably not more than 10° against the drive surface in order to provide a

maximum acceleration to the surface.

The pressure chamber preferably comprises an elongate, curved tube of

circular cross-section, preferably of steel or other metal capable of withstanding

the pressures involved, having an open end arranged in a tangential direction, or

at a low angle, to the drive surface. The open end of the tube preferably

terminates in an open channel which directs the projectiles against the drive

surface. The open channel is preferably provided with a terminal arrest member,

positioned so as to slow down the projectiles after they have left the drive

surface.

The number of projectiles can be chosen in order to give the required retraction of

the seat belt. Preferably there arc at least 30, and preferably not more than 50

projectiles, and these are preferably sufficient to rotate the central shaft of the

take-up reel by from one and a half to three times, for example, twice, during the

activation of the pretensioner. This will result in a compact device capable of

optimum retraction effect.

In a preferred embodiment, for example, using a drive wheel and

spherical projectiles (balls) which remain in contact one with the next during use,

the number of projectiles will depend on inter alia (i) the diameter of the drive

wheel or pulley, (ii) the number of required revolutions of the drive wheel, and

(iii) the diameter of the projectiles (balls). If the diameter of the drive wheel is

"D" the diameter of the projectiles is "d" and the number of revolutions is "N",

then the number of balls is given by rounding up the result of applying the

formula:

2Vπ(D + d)

In an example, N=2, D=50mm and d=10mm, giving a calculated result of 37.7,

ic. 38 balls.

Preferably the projectiles arc not connected to each other, and most desirably they

arc spherical in form. This leads to a number of advantages. For example

separate unconnected projectiles can be retained in a compact receptacle after

actuation of the prctcnsioncr; if they arc linked together, a larger or longer

receptacle would be required. Projectiles in the form of a row of discrete balls

can also articulate more freely than, for example, a chain. The designer is thus

provided with greater freedom to shape and position the elongate pressure

chamber so as to render the overall design compact, eg by locating parts of the

pressure chamber in positions which would otherwise be inconvenient and

therefore not usable for this or any other purpose.

The projectiles arc preferably spherical in shape. For example, the

projectiles may comprise a scries of metal balls, for example, ball bearings,

which arc arranged in a row in a tubular metal pressure chamber. The diameter

of the metal balls is such as to occupy substantially the entire internal diameter of

the tube, whilst allowing the balls to travel freely therein.

Advantageously, the prctcnsioncr comprises an arcuate portion of the elongate

tube which curves partly or wholly around the drive surface.

Advantageously the axis of the elongate tube is disposed in more than one plane.

This can lead to a prctentsioner of compact design, for example at least part of

the elongate tube may be of helical form, and/or at least part of the elongate tube

may be disposed within the take-up reel. Additionally, the gas generating means

may be at least partly disposed within the take-up reel.

The end of the tube remote from the open end is preferably closed or capped, and

a suitable gas-generating composition can be disposed therein. Such a

gas-generating composition can comprise, for example, sodium azide, or a

gas-generating composition comprising nitroguanidine, as described and claimed

in our co-pending UK Patent Application No.9503066 entitled "Gas Generating

Composition" filed on the same day as this Application. Alternatively, the

gas-generating composition can be provided in a separate chamber connected to

the pressure chamber. The gas-generating composition preferably comprises a

pyrotechnic composition ignited by an electronic ignition system activated by a

deceleration sensor.

After impinging on the drive surface, the projectiles arc preferably

collected in a collection chamber, or an open cavity, which can conveniently be

the housing of the scat belt retractor. The collection chamber or cavity is

preferably provided with a clear panel through which the projectiles can be

viewed, to provide a functional check on the device.

Preferably retaining means arc provided for retaining the projectiles in

the pressure chamber. Such retaining means may comprise, for example, a rigid

foam which is injected into the pressure chamber after the projectiles have been

inserted therein.

The period of activation of the pretensioner is preferably around 20

milliseconds, and the prctcnsioncr is preferably designed to wind in from 120 to

200 mm of scat belt (depending on the amount left on the take-up reel) and cause

the belt to lock.

Preferably when the projectiles are positioned in the collection

chamber they provide no impediment or obstacle to the normal operation of the

seat belt, so that after a collision and subsequent actuation of the pretensioner the

vehicle occupant docs not continue to be held firmly against the scat. The

occupant can thus more readily get out of the vehicle, or continue to drive if

possible.

Embodiments of the invention will now be described with reference to

and as illustrated in the accompanying drawings in which:

Figure 1 shows a scat belt prctcnsioncr drive mechanism according to

the invention in side elevation;

Figure 2 shows the prctcnsioncr of Figure 1 in top elevation;

Figure 3 shows the prctcnsioncr of Figures 1 and 2 in side elevation in

an enlarged scale with parts broken away to illustrate the mode of operation;

Figure 4 shows the pretensioner of Figures 1 to 3 in end elevation again

with parts broken away to show the mode of operation;

Figure 5 shows a scrap view in section of the gas-generating

mechanism.

Figure 6 illustrates an alternative form of drive wheel arrangement for

a pretensioner according to the invention;

Figure 7 illustrates another form of drive wheel arrangement for a

pretensioner according to the invention;

Figure 8 illustrates in end elevation a further form of drive wheel

arrangement for a pretensioner according to the invention;

Figure 9 illustrates in side elevation the drive wheel arrangement of

Figure 8;

Figure 10 is an end elevation illustrating a prctcnsioncr in accordance

with the invention in which the pressure chamber axis is disposed in more than

one plane; and

Figure 11 is a side elevation of the prctcnsioncr illustrated in Figure 10.

Referring firstly to Figure 1, the prctcnsioncr illustrated generally at 1

comprises a curved pressure tube 2 which is coiled helically around a drive

wheel 3 attached to a shaft 4. The pressure tube 2 is mounted eccentrically of the

axis of the drive wheel 3 and has a diameter of curvature slightly greater than the

wheel. The tube 2 has a closed end, at which there is situated a cap enclosing a

gas-gcnerating mechanism 5, and an open end which is cut back, as illustrated by

the broken line 6, to form an open-sided channel 7. At the end of the channel 7

there is provided a projecting arrest member 8. As can be seen from Figure 2, the

curved pressure tube 2 describes approximately IVi turns about the drive wheel 3.

The prctcnsioncr is enclosed in a collection chamber 9, which may comprise the

whole or part of the scat belt retractor housing, the chamber 9 having a clear

panel 10 for viewing the interior thereof.

Referring now to Figure 3, which depicts the pretensioner in operation,

a row of metal balls 11 are illustrated passing along the channel 7 of the pressure

tube 2. There may be, for example, from 30 to 50 of such balls 11 in the pressure

tube 2. The balls 11 have a diameter slightly less than the internal diameter of the

tube 2 and arc freely movcablc therein. The drive wheel 3 is provided with a

series of circumferential indentations 12 about its periphery, which arc spaced

5 apart by a distance such that when adjacent balls 11 are located in the

indentations the surfaces of adjacent balls arc just touching. The arrangement is

shown again in Figure 4 which also shows the drive shaft 4 which is attached to

the take-up reel of a scat belt retractor (not shown).

Figure 5 shows a detail of the gas-gcnerating mechanism 5. A

10 pyrotechnic gas-gcnerating composition 13 is packed into a pressure-resistant

plug 14 firmly mounted at the end 15 of the pressure tube 2. The pyrotechnic

composition 13 is ignited by an electronic igniter (not shown) which is activated

by a deceleration sensor (not shown). A felt plug 16 is positioned at the end 15 of

the pressure tube 2 and serves as a limit member for the travel of the row of balls

15 11.

In the rest, or inactive, position of the pretensioner, the balls 11 are

confined within the pressure tube 2 by a rigid foam 17 which is injected into the

pressure tube 2 after the balls have been inserted and which solidifies to retain the

balls 11 in the pressure tube. The plug 16 prevents the foam from reacting the

20 pyrotechnic composition. ith the balls 11 retained in the tube 2, the drive wheel

3 and shaft 4 are able to rotate freely with the take-up reel of the seat belt

retractor. The ball retaining means may alternatively comprise, for example, a

frangible plug, or a spring means, which is compressed as the balls are ejected

from the pressure tube.

In the event of a collision, the deceleration sensor activates the

electronic igniter which in turn sets off the pyrotechnic gas-gcnerating

composition 13. The volume of gas generated by the composition 13 is sufficient

to displace the felt plug 16. and propel the row of balls 11 at considerable speed

along the pressure tube 2. The foam 17, of course, disintegrates as the balls are

ejected from the pressure tube 2. When the balls reach the open channel 7 they

come into contact with the indentations 12 on the drive wheel 3 and cause the

drive wheel 3 and drive shaft 4 to rotate at high speed. The drive shaft rotates the take-up reel of the scat belt retractor causing retraction of the seat belt by the

desired amount.

The path of the balls is tangential to the circumference of the drive wheel 3 and as they fly off they each come into contact with the arrest member 8

at the end of the open channel 7 and suffer a substantial deceleration. This

prevents the balls from damaging the wall of the collection chamber 9. The

pretensioner is preferably enclosed within the housing of the seat belt retractor,

which comprises the collection chamber 9, and as the balls are ejected from the

open channel 7, they collect in the bottom of the housing from where they can be

viewed through the clear panel 10 to provide a visual check on whether the

pretensioner has been activated. After use, they can be recovered and reused

with a fresh charge of pyrotechnic composition as appropriate.

After the last ball has left the periphery of the drive wheel 3 there is no

further force acting on the drive wheel which is then locked in position by the

5 seat belt retractor lock. When the vehicle has come to a halt, the scat belt

retractor lock is released and the scat belt can again be used normally. The

projectiles 11 arc by this stage clear of the drive surface and thus do not interfere

in any way with such normal use.

Figure 6 illustrates an alternative form of drive which comprising a drive wheel

10 20 which has a circumferential surface layer 21 of a compliant material. In use,

balls 11 are propelled in the direction of the arrow A along a pressure tube 2 by

means of pressure generated by a gas-gencrating mechanism 5, as described with

reference to Figure 1 to 5.

When the balls encounter the surface layer 21 , they are constrained

15 between the wall of the tube 2 and the layer 21, so as to exert a radially inward

pressure thereon as well as a forward pressure tending to rotate the wheel 20 in

the direction of the arrow B.

The compliant material of the surface layer 21 thus deforms as

illustrated so as to create an indentation 22 which receives a ball 11, and a lip 23

which resists free passage of the ball past the wheel. Each successive ball 11

thus drives the wheel 20 to rotate in the direction of the arrow B until rotation of

the wheel enables the ball to escape, past the lip 23, and a subsequent ball 11 then

repeats the cycle. The wheel 20 drives the take-up reel (not shown) via a shaft

29 on which the wheel is mounted.

The compliant layer in this embodiment should be soft enough to

deform under the action of the balls 11 as illustrated, but needs relatively high

shear strength so that the lip 23 docs not shear off under the load imposed by the

balls 11. Suitable materials for the compliant layer are resilient materials such as

polypropylene, polyethylene (especially UHMPE - ultra high modulus

polyethylene), and reinforced rubber ( eg fabric-reinforced).

As illustrated in Figure 7, a drive wheel 24 for use in the invention

comprises a circumferential surface layer 25 of rigid material such as steel or

hard plastics material, and an inner hub zone 26 of a similar material.

Sandwiched between the hub 26 and the circumferential layer 25 is an annular

layer 27 of compliant material, which may be a material similar to that of the

layer 21. In use, when the prctcnsioncr is actuated, the balls 11 are driven in the

direction of the arrow A into the space between the wall of the pressure tube 2

and the layer 25. This space is designed to be somewhat narrower then the

diameter of the balls 11, so that the material 27 must deform in order to admit the

balls into this space. A pressure load is thus created between the balls 11 and the

layer 25 whereby successive balls drive the wheel 24 to rotate in the direction of

the arrow B, until each successive ball escapes from the pinch between the rim 25

and the tube 2.

As illustrated in Figures 8 and 9, a drive wheel 30 comprises a rigid hub 31,

5 mounted on a drive shaft 32 which drives the take-up reel (not shown) to rotate.

Mounted adjacent the hub 31 arc a pair of discs 33 of resilient material such as

spring steel. The discs arc fixed to the hub only near the shaft 32, and they define

between their radially outer portions 33 and the circumference of the hub 31, a

circumferential channel 34. The portion 33 constitute channel sides, and a ball 11

10 can be received into the channel only by deforming the channel sides to move

apart in the direction of the arrows x, as shown in Figure 8. When the

pretensioner is actuated, the balls 11 arc propelled along the tube 2 in the

direction A as previously described. The tube 2 opens adjacent to the wheel 30,

and is so spaced therefrom that the tube wall forces the balls to enter the channel

15 34 to its full depth, so that the channel sides 33 arc sprung apart. By virtue of the

friction thus created between the channel sides 33 and the balls 11, the balls thus

drive the wheel 30 to rotate in the direction of the arrow B. The balls escape

from the pinch between the tube 2 and the channel 34 into a collection area 35,

partly shown in Figure 9. Their removal from the channel 34 is ensured by means

20 of a stripper plate 36. Friction material can be applied to the surfaces of the

channel sides 33, as illustrated at 37.

The embodiments illustrated in Figures 6 to 9 all have a homogeneous

circumference , ie without individual teeth or recesses for receiving individual

balls separately. This has the considerable advantage that the balls 11 can engage

readily with the drive surface of the drive wheel in each case, without the risk that

a ball, especially the first ball, may encounter the drive surface at a location

between two individual teeth or recesses which could cause jamming or damage

to the mechanism.

In addition, a further considerable advantage is that the tension load

which the device can apply to a scat belt is limited by the maximum torque which

can be applied between the balls and the drive wheel. If this maximum is

exceeded, the balls simply escape from the pinch between the drive wheel and

the pressure tube in each of these embodiments. This provides a further safety

feature in that the user is protected against the possibility of excessive tightening

of the belt and the output torque of the wheel is independent of the maximum

pressure produced by the gas generator.

As shown in Figures 10 and 11, a drive wheel 40 is formed of a

compliant material (for example UHMPE) and has a continuous

part-circular-section groove 41 formed in its periphery for receiving balls 11.

The wheel 40 is mounted on a take-up reel 42 of a seat belt retractor.

A gas generator 43 containing pyrotechnic material 44 is housed within

the reel 42, together with the first section 44 of an elongate tube of circular

cross-section 44 of an elongate tube of circular cross-section which constitutes

the pressure chamber 2. The tubular pressure chamber 2 is bent so as to pass

radially outward from the reel and then back on itself so as partially to embrace

the circumference of the drive wheel. Balls 11 (only two are shown for clarity)

are driven, upon actuation of the gas generator 44, in the direction of the arrow A,

so as to pass into the pinch between the rim 41 and the open end 45 of the tube 2,

so as to drive the wheel 40 and the reel 42, to rotate in the direction of the arrow

B, in similar manner to the embodiment of Figure 6. The used balls 11 are collected in a receptacle 46.

The combination of spherical projectiles 11 in a circular-section tube,

the projectiles not being connected to each other, permits that the pressure tube is

strong in resisting the pressure of the gas generator, and can be bent into

three-dimensional shapes while the individual balls have o restriction on their

relative articulation which limits the shape which can be adopted for the tube 2.

Thus the gas generator 43 can be located inside the reel 42, together with part of

the pressure tube 2. This space would otherwise not be used, and the overall bulk

of the pretensioner is thus substantially less than would otherwise be the case.

The reader's attention is directed to all papers and documents which are

filed concurrently with or previous to this specification in connection with this

application and which are open to public inspection with this specification, and

the contents of all such papers and documents are incorporated herein by

reference.

All of the features disclosed in this specification (including any

accompanying claims, abstract and drawings), and/or all of the steps of any

method or process so disclosed, may be combined in any combination, except

combinations where at least some of such features and/or steps are mutually

exclusive.

Each feature disclosed in this specification (including any

accompanying claims, abstract and drawings), may be replaced by alternative

features serving the same, equivalent or similar purpose, unless expressly stated

otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one

example only of a generic scries of equivalent or similar features.

The invention is not restricted to the details of the foregoing

embodiments. This invention extends to any novel one, or any novel

combination, of the features disclosed in this specification (including any

accompanying claims, abstract and drawings), or to any novel one, or any novel

combination, of the steps of any method or process so disclosed.