GB2375329A

GB2375329A - Safety belt pre-tensioner

Info

Publication number: GB2375329A
Application number: GB0114059A
Authority: GB
Inventors: Lennart Haglund
Original assignee: Autoliv Development AB
Current assignee: Autoliv Development AB
Priority date: 2001-05-10
Filing date: 2001-06-08
Publication date: 2002-11-13
Anticipated expiration: 2021-06-08
Also published as: GB0114059D0; GB2375329B; GB0111446D0

Abstract

A pre-tensioner for a safety belt such as a seat belt in a vehicle comprises a sealed cylinder 10 containing a pressurised gas, and a piston 23 which is provided with a pyrotechnic unit 20 adapted to inject gas into the cylinder to move the piston. When the pyrotechnic unit is triggered the piston moves so as to extend out of the cylinder (see figures 2a-2c), and causes tension to be applied to a component 2 connected to or associated with the safety belt 5. The piston may act so as to rupture a membrane 14 before contacting plunger 32, this having a pulley 33 around which a wire 2 connected to safety belt buckle 3 passes. When the membrane 14 is broken by the moving piston both the pressurised gas and the gas generated by the pyrotechnic unit acts upon the moving piston to force it out of the cylinder. The pre-tensioner may act upon two safety belts (figure 4), and the piston 23 may comprise a flexible tubular sleeve (figures 7 and 8).

Description

PATENTS ACT 197 P15379GB-NF/jsd DESCRIPTION OF INVENTION "IMPROVEMENTS IN OR RELATING TO A PRE-TENSIONER" THE PRESENT INVENTION relates to a pre-tensioner, and more particularly relates to a pre-tensioner intended for use with a safety-belt, such as a safety-belt in a motor vehicle.

It has been proposed to provide pre-tensioners associated with safetybelts in motor vehicles, the pre-tensioner being adapted to be actuated, in response to a signal from a sensor responsive to an impact or other accident situation so that, at the very commencement of an accident, the seat-belt is pretensioned so as to embrace the seat occupant tightly within the seat. The minimises the amount of forward movement that the seat occupant can effect, relative to the seat, before the seat occupant is retarded by the safety-belt. If, for example, the seat occupant is wearing loose and bulky clothing, the seatbelt may, in reality, be quite slack and, in the event that an accident should arise, the occupant of the seat will move forward by a substantial distance before the safety-belt retards the seat occupant if the seat-belt is not pretensioned.

There is a need for a pre-tensioner mechanism that is light and relatively inexpensive.

According to this invention there is provided a pre-tensioner to apply tension to a safety-belt, the pre-tensioner comprising a sealed pressure vessel containing pressurised gas, the pressure vessel also containing a piston, and being provided with a pyrotechnic unit adapted to inject gas into the pressure vessel to move the piston, to cause the piston to extend from the pressure vessel, there being means responsive to movement of the piston beyond the pressure vessel to apply tension to at least one element to move a component connected to or associated with a safety-belt to apply tension to the safety-belt.

Preferably the pressure vessel is a pressure cylinder, one end of the pressure cylinder being sealed by a rupturable foil, the other end of the pressure vessel being closed by a closure associated with the pyrotechnic unit, the piston being adapted to rupture the foil on movement thereof.

Conveniently the piston is of hollow tubular form, there being at least one aperture forming a communication between the interior of the piston, and the space between the piston and the pressure vessel.

Advantageously the said aperture formed in the piston is at such a location that, when the piston is fully extended from the pressure vessel, the aperture is on the exterior of the pressure vessel, permitting pressurised gas within the pressure vessel and piston to be vented to the atmosphere.

Conveniently a sealing ring is provided adjacent the foil, the sealing ring having a diameter equal to, or less than the exterior diameter of the piston.

Preferably the pyrotechnic unit is contained within a portion of the closure which projects inwardly into the pressure vessel, that inwardly projecting portion having an aperture initially sealed by a rupturable foil.

Advantageously the piston defines a chamber which communicates with the pressure cylinder.

In one embodiment the piston is formed by a hollow tubular member which initially is re-entrantly folded upon itself within the pressure vessel.

Conveniently the means responsive to movement of the piston comprise a plunger located to be engaged by the piston on movement thereof, and a guide tube to guide the movement of the plunger, the plunger being associated with one said at least one element.

In one embodiment the plunger is connected to one end of a wire or other flexible elongate element which is connected to the said component associated with a safety-belt.

In an alternative embodiment the plunger is connected to a wire or other elongate element having two opposed ends, each said end being connected to a respective component associated with a respective safety-belt.

In a further modified embodiment the plunger is provided with a pulleywheel or the like, the pulley-wheel or the like engaging a wire or other elongate flexible element, one end of which is anchored in position, and the other end of which is connected to the said component associated with a safety-belt.

In order that the invention may be more readily understood, and so that further features thereof may be appreciated, the invention will now be described, by way of example, with reference to the accompanying drawings in which : FIGURE 1 is a part-section and part-diagrammatic view of one embodiment of the invention, FIGURES 2A to 2C are views illustrating a component of Figure 1, showing that component at various stages during deployment of the pretensioner, FIGURES 3A and 3B show a further component of the embodiment of Figure 1 in two alternate conditions, FIGURE 4 is a view corresponding to Figure 3 illustrating a component of a modified embodiment of the invention, FIGURE 5 is a view corresponding to Figure 1 showing an alternative embodiment of the invention, FIGURE 6 is a view illustrating a component of a modified embodiment of the invention, FIGURE 7 is a view illustrating a component of a further modified embodiment of the invention, and FIGURE 8 is a view corresponding to Figure 7 showing the embodiment of Figure 7 after deployment.

Referring initially to Figure 1 of the accompanying drawings, a pretensioner arrangement is provided comprising a unit 1, which will be described hereinafter in greater detail, which is adapted to apply a tension to a wire or cable 2, the wire or cable 2 being directly or indirectly connected to a buckle 3, which buckle is adapted to receive a tongue 4 mounted on a safety-belt 5. It is to be understood that when a tension is applied to the wire 2 by the unit 1, the buckle 3 will be moved in such a way that tension is applied to the safetybelt 5. It is thus envisaged that the described arrangement will operate in a manner similar to prior proposed pre-tensioners.

The unit 1 comprises a pressure cylinder 10 which contains compressed gas. The lower end of the pressure cylinder 10 is provided with an exteriorly threaded neck 11 which terminates with an inwardly directed annular flange 12 defining an opening 13, the opening 13 being sealed by a breakable foil 14, such as a metal foil. A resilient sealing ring 15, for example, of rubber or the like, is provided within the neck 11 immediately adjacent the flange 12, the interior diameter of the sealing ring 15 being slightly smaller than the diameter of the opening 13 defined by the flange 12.

The upper end of the cylinder 10 is closed by a closure 16 which defines a hollow central tubular projection 17 which extends into the cylinder 10, the projection 17 having a lesser diameter than the diameter of the cylinder 10.

The hollow cylindrical projection 17 terminates at a lower opening 18 which is initially sealed by a foil 19 which may be made of metal. Contained within the hollow cylindrical projection 17 is a pyrotechnic unit 20 adapted to receive and be actuated by a signal from a sensor 21 which is responsive to a parameter indicative of an accident, or an accident situation, such as substantial deceleration.

Contained within the cylinder 10 is a hollow piston unit 22. The piston unit 22 comprises an axially extending hollow tubular body 23, the lower end of which is closed by a closure cap 24. The lower surface of the closure cap 24 (in the orientation shown in the accompanying drawings), surface 25, is of a conical form. The cylindrical side surface of the cap 24 may be provided with an axially extending recess or channel. The outer diameter of the closure cap 24 and the outer diameter of the tubular body 23 of the piston 22 is the same as or slightly less than the internal diameter defined by the sealing ring 15. The upper end of the tubular body 23 of the piston 22 is a snug-sliding fit about the hollow cylindrical projection 17 carried by the upper closure 16.

The upper end of the tubular body 23 is provided with a radially outwardly directed flange 26. The flange 26 is provided with a plurality of apertures 27 therein, and the main tubular body 23 of the piston 22 is provided with at least one aperture 28 formed therein. The piston (as will become clearer from the following description) is capable of axial movement relative to the cylinder 10 in a downward direction in the orientation illustrated. An arrangement such as a ratchet (not shown) is provided to prevent the piston subsequently moving in the opposite (e. g. upward) direction.

The pressure cylinder 10 is filled with a gas, such as an inert gas, to a high pressure by introducing the gas through a filling aperture (not shown) in the upper closure 16. The aperture is seated after the cylinder 10 has been filled with gas. Because of the presence of the apertures 27 and the flange 26, the high pressure gas is present in a relatively small space above the flange 26 and beneath the upper closure 16. Because of the presence of the aperture 28, the high pressure gas is present within the hollow interior of the tubular body 23 of the piston 22, and also in the annular space surrounding the piston unit 22. The axial recess or channel in the cylindrical side surface of the

cap 24 ensures that the high pressure gas is in the space between the lower surface 25 of the cap and the foil 14, as the recess or channel bypasses the seal 15. The gas is prevented from escaping from the high pressure tube by the foil 19 which seals the aperture 18 formed in the upper closure 16 and also by the foil 14 that seals the aperture 13 defined by the neck 12. Because the piston 22 is totally within the pressure tube 10, and all surfaces of the piston are exposed to the gas within the pressure tube, the gas does not apply any force on the piston 22 which causes, or tends to cause, the piston to move.

The neck 11 of the pressure tube 10 is threadedly received within a threaded seat 29 formed on a connecting unit 30 which serves to connect the pressure cylinder 10 to a receiving tube 31. The seat defines an aperture coaligned with the aperture 13 of the neck 11. Mounted in one end of the receiving tube 31 is a plunger 32. Mounted on the plunger 32 is a pulleywheel 33. The plunger 32 is also provided with a pressure plate 34, the upper surface of which is provided with a conical depression corresponding with the conical exterior face of the lower closure cap 24 provided on the tubular body 23 of the piston unit 22. The plunger 32 is located at one end of the receiving tube 31 immediately adjacent the aperture 13 which is closed by the foil 14.

A stop 35 is provided on the connector 30 which holds and retains one end of the wire 2. The stop 35 is located adjacent a guide post 36 which may have a recessed outer surface similar to that of a pulley-wheel. The support 30 may also support a rotatably mounted pulley-wheel 37. The wire 2 may thus pass the stop 35, pass the guide post 36, pass partially round the pulley-wheel provided on the plunger 32 and pass the pulley-wheel 37 provided on the connector 30.

It is to be appreciated that when an accident is sensed by the sensor 21, a signal is sent to the pyrotechnic unit 20 and a pyrotechnic material within the pyrotechnic unit 20 is ignited, thus generating hot gas. Referring to Figure 2, it is to be appreciated that Figure 2A shows the components of the high pressure tube 10 before an accident has commenced. The components are thus in the relative positions as shown in Figure 1.

Referring now to Figure 2B, when the pyrotechnic unit 20 generates hot gas, initially the foil 19 sealing the aperture 18 is ruptured, and high pressure gas is injected into the interior of the hollow tubular body 23 of the piston 22.

This causes the piston 22 to start moving. The piston 22 moves axially downwardly (in the orientation shown in Figure 1). The conical lower surface of the lower closure cap 24 ruptures the foil 14 shortly after the piston commences its downward movement. As the cap 24 moves downwardly so the recess or channel in the cap moves past the seal 15. The seal 15 then sealingly engages the exterior of the body 23 of the piston 22. As the piston moves further downwardly, so gas, initially stored in the annular space between the tubular body 22, the piston 22 and the interior of the pressure cylinder 10 flows through the apertures 27 in the flange 26 into the space between the flange 26 and the closure 16. This may tend to retard the movement of the piston unit so that the piston unit does not move over-abruptly. As the piston unit continues to move downwardly, so the flange 26 moves down to a level beneath the lower end of the hollow cylindrical projection 17 carried by the upper closure 16.

The pyrotechnic unit 20 may still be generating gas at this stage.

The piston unit has passed through the sealing ring which forms a substantially gas-tight seat with the exterior of the piston. This prevents any significant quantity of gas escaping from the pressure cylinder along the outside wall of the piston unit.

It is to be appreciated that as soon as the lower closure cap 24 penetrates the foil 14, both the initial charge of compressed gas within the cylinder 10 and the gas generated by the pyrotechnic unit 20 effectively act on the piston tending to drive the piston 22 downwardly. The piston 22 continues to move downwardly until the piston 22 projects more fully from the lower end of the pressure tube 12.

It is to be appreciated that once the piston 22 has reached the position shown in Figure 2C, the aperture 28 is still above the seal 15, thus preventing the escape of substantial quantities of gas to the atmosphere. This, together with the ratchet that prevents upward movement of the piston, helps to ensure that the pretensioning effect is maintained for as long as possible.

Referring now to Figures 3A and 3B, it is to be appreciated that Figure 3A shows the position of the plunger 32 within the receiving tube 31 before the commencement of an accident situation. Figure 3B shows the same components after the piston unit 22 has moved to the fully extended position as shown in Figure 2C. It can be seen that the lower closure cap 24 has engaged with the pressure plate 34 on the plunger 32, and has moved the plunger 32 down the receiving tube 31. The receiving tube ensures that there is always free unobstructed space in front of the plunger. As the plunger 32 moves down the receiving tube 31, the wire 2 is drawn past the pulley 37 and past the pulley 33. The arrangement is such that for each centimetre of movement of the plunger 32, almost two centimetres of cable 2 are drawn into the receiving tube 31. The receiving tube, in this embodiment of the invention, is provided with a cross-section having a circular central portion and two diametrically opposed radially extending projections, each dimensioned to receive part of the wire 2.

It can be seen that as the plunger 32 moves axially along the central portion of the receiving tube 31, the wire of cable 2 is received in the diametrically opposed projecting portions provided for that purpose.

Figure 4 illustrates part of a modified embodiment of the invention in which a single wire or cable 40 is provided, each end of which is adapted to be connected to a respective buckle to receive a respective tongue provided on a respective seat-belt. In this embodiment the connector 30 is provided with the pulley-wheel 37, as described above, and also with a second pulley-wheel 41 in place of the guide post 36. The plunger 32 may be provided with a pulley-wheel 33, but, alternatively, may be provided with a fixed wheel or post. The part of the wire 40 adjacent the fixed wheel or post 33 may be bonded or secured to the post or wheel.

It is to be appreciated that the embodiment as shown in Figure 4 will operate in the same way as the embodiment of Figure 1, save that tension will be applied to both ends of the wire 40, enabling two safety-belts to be pretensioned simultaneously.

Referring now to Figure 5, a modified embodiment of the invention is shown in which a modified form of pressure cylinder is used, and also in which a single wire is pulled into a receiving tube by a plunger in such a way that the length of wire pulled into the receiving tube is equal to the distance travelled by the plunger.

In the embodiment shown in Figure 5, a pressure cylinder 50, which contacts pressurised gas, is provided, at its lower end, with an exteriorly threaded neck 51. The neck 51 terminates at its lower end with a radially

inwardly directed flange 52 which defines an aperture 53, that aperture 53 being sealed by a foil 54 such as a metal foil. A sealing ring 55 is provided located within the interior of the neck 51, resting on top of the inwardly directed flange 52, the sealing ring having an inner diameter slightly less than the diameter of the aperture 53.

The upper end of the pressure cylinder 50 is closed by a hollow closure plug 56 which has a cylindrical projection 57 which extends into the interior of the pressure cylinder 50, the projection 57 having an external diameter substantially equal to the internal diameter of the pressure cylinder 50. The projection 57 defines, at its lower end, an aperture 58 which is sealed by means of a foil, for example a metal foil 59. Received within the hollow closure plug 56 is a pyrotechnic unit 60 adapted to receive a signal from a sensor which is responsive to a parameter, such as an acceleration in excess of a predetermined acceleration, indicative of an accident situation.

Contained within the pressure cylinder 50 is a piston unit 61 comprising a hollow tubular body 62, the lower end of which is closed by a lower closure cap 63, the lower surface of the closure cap 63 being of conical form 64. The external diameter of the closure cap 64 of the tubular body 63 is substantially equal to the interior diameter of the sealing ring 55. The upper end of the tubular body 62 of the piston 61 is closed by a hollow cap 64 which comprises an annular ring 65 resting on the top part of the tubular body 62 which supports a downwardly directed hollow tubular closed-ended projection 66 received as a friction fit within the upper end of the tubular body 62, and which also supports an upwardly directed axially extending flange 67 having an external diameter substantially equal to the internal diameter of the pressure tube 50.

Apertures 68 are formed in the side wall of the tubular body 62 adjacent the hollow cap 64. A further aperture 69 is provided within the lower-most part of the projection 66 that is received in the tubular body 62.

The neck 51 is threadedly received, with a screw-threaded connection, within a threaded seat 70 formed on a connector 71, which serves to connect the pressure cylinder 50 to a receiving tube 72. Received within the upper end of the receiving tube is a plunger 73, the upper surface of which is provided

with a conical depression 74 corresponding to the conical projection 64 provided on the under-surface of the closure cap 63. A pulley-wheel 75 is provided on the connector 71. A cable 76 has one end thereof secured by a fastener 77 to the plunger 73, the wire 76 passing the pulley-wheel 75 to a position where it can be connected to a buckle, equivalent to the buckle 3 as shown in Figure 1.

The pressurised gas cylinder will flow through the described apertures 68 and 69 to fill the piston unit 61 and the space surrounding the piston unit 61.

It is to be appreciated that on deployment of the pyrotechnic unit 60 of the embodiment shown in Figure 5, high pressure gas will initially rupture the foil 59 sealing the aperture 58, and the high pressure gas will be fed to the interior of the hollow cap 64. This will initiate downward movement of the piston 61. The conical lower surface 64 of the closure cap 63 will rupture the foil 54 sealing the aperture 53, and will then engage the conical seat 74 defined on the plunger 73. Once the foil 54 has been ruptured, a force tending to eject the piston unit 61 from the pressure tube 50 will be exerted on the piston unit, not only by the gas from the pyrotechnic unit 60, but also by the compressed gas originally present within the pressure cylinder.

As the piston unit moves downwardly, so gas initially in the annular space between the tubular body 62 of the piston unit 61 and the pressure cylinder 50 will flow through the apertures 68 into the hollow interior of the piston body 62, and will also flow through the aperture 69 into the increasing volume left between the hollow cap 64 and the upper closure plug 56.

As the piston unit moves downwardly, so the plunger 73 will move along the receiving tube 72 applying tension to the wire 76. When the piston is in the fully extended position, gas from within the pressure tube 50 may escape to atmosphere through the apertures 68 which will then be located beyond the sealing ring 55.

Figure 6 shows an embodiment which is effectively a modification of the embodiment of Figure 5.

In the embodiment of Figure 6, the upper closure plug 56 is modified with the upper closure plug being provided with a radially outwardly directed flange 80, which carries a depending skirt 81 which has a lower end portion 82 which tapers inwardly to be sealed against the exterior of the pressure cylinder tube 50. An aperture 83 is provided in the wall of the pressure cylinder tube 50 communicating with the chamber 84 defined by the skirt 81. The remaining components of the embodiment shown in Figure 6 are the same as the corresponding components as shown in Figure 5. It is to be appreciated that the embodiment of Figure 6 will operate in the same way as the embodiment of Figure 5, save that the embodiment contains a greater quantity of prepressurised gas, which will, on deployment of the device, flow from the chamber 84 through the aperture 83. This will exist in the expulsion of the piston unit 61 from the pressure cylinder.

Turning now to Figures 7 and 8, a further modified embodiment of the invention is described. In this embodiment the piston unit is formed from a flexible re-entrantly folded tubular element.

Referring to Figure 7 a pressure cylinder 90 is provided, the lower end of which is provided with a neck 91 provided, at its lower end, with an inwardly directed radial flange 92 defining an aperture 93 which is sealed by a foil, such as a metal foil, 94. A sealing ring 95 is provided which rests on the radially inwardly directed flange 92 and which defines a central aperture having a diameter slightly less than the diameter of the aperture 93. The neck 91 is provided with external screw-threading.

The upper end of the pressure cylinder 90 is closed by a closure 96. The closure 96 has a depending cylindrical projection 97 having an external diameter less than the internal diameter of the pressure tube 90, the projection 97 terminating, at its lower end, with a radially inwardly directed flange 98 defining an aperture 99, that aperture being sealed by a foil, such as a metal foil 100.

A pyrotechnic unit 101 is provided which is mounted on the closure 96 and extends into the depending projection 97.

A piston unit 102 is provided, the piston unit comprising a flexible tubular sleeve 103. One end 104 of the sleeve 103, is securely mounted in position adjacent the junction between the neck 91 and the pressure cylinder 90 by means of a mounting ring 105. The tubular element 103 extends, in the orientation shown in Figure 7, upwardly from the neck 91 to a position adjacent the cylindrical projection 97 carried by the upper closure 97. The tubular

element is then folded back inwardly of itself, extending down within the interior of the first described portion of the tubular element, to terminate with a lower closure cap 106, the lower surface of which is of a conical form. The exterior diameter of the lower closure cap and the portion of the tubular member 103 adjacent thereto is slightly less than the dimension of the internal aperture defined by the sealing ring 95.

In the initial condition, as shown in Figure 7, the piston unit 102 is totally contained within the pressure tube which is sealed by the foil 94, and by the foil 100.

Upon deployment of the device as shown in Figure 7, gas from the pyrotechnic unit 101 will rupture the foil 100 with that gas being introduced to the interior of the lower part of the tubular element 103 adjacent the lower closure cap 106. The closure cap 106 is thus biased downwardly, rupturing the foil 94. The pressurised gas within the pressure tube 90 and the gas from the pyrotechnic unit thus combine to force the piston unit 103 to enter a fully extended state. As the lower closure cap 106 continues to move downwardly, so the entire length of the tubular element 103 will unfold on itself and pass through the sealing ring 95 until the device has the fully extended condition shown in Figure 8.

It is to be appreciated that in the foregoing description, various types of pressure cylinder and piston unit have been described, and also various types of plunger and associated mechanisms for applying tension to one or more cables have been described. It is to be appreciated that each type of pressure cylinder and piston unit as described above may be used with each of the different plunger mechanisms that have been described.

In the described embodiments, the combination of the pyrotechnic unit and the pressure cylinder may be relatively compact and relatively light. The use of a combination of pressurised gas and gas from a pyrotechnic unit provide a good power-to-weight ratio. In the preferred embodiments of the invention, a relatively gentle initial movement of the wire associated with the seat-belt buckle is provided, so that the tension applied to the seat-belt may be applied in a somewhat"gradual"manner, rather than in an abrupt manner which can cause injury.

It is to be appreciated that in an embodiment of the invention various techniques may be utilised to modify the operational characteristic of the device. For example, the sizes of various apertures may be selected so that the gas flow through the apertures is such that a particular operational characteristic is achieved. For example, the apertures 26 in the flange 27 may be appropriately dimensioned, and an appropriate number of apertures may be provided. Equally, the aperture 28 formed in the body of the piston 22 may be appropriately dimensioned, and more than one such aperture may be provided. The interior of the piston may be divided into a plurality of chambers by means of a transverse membrane, the volume of those chambers and the size of an aperture in the membrane being appropriately selected. The chamber located immediately above the foil 29 and beneath the pyrotechnic unit 20 may have a selected volume. Also, the initial volume within the cylinder 10 located above the flange 26 may have a predetermined volume, and also the volume beneath the flange 26 and surrounding the piston body 23 within the cylinder 10 may have a selected volume. In some embodiments there may be a small clearance between the closure cap 24 and the sealing ring 15 providing a flow passage with a predetermined cross-sectional area, and a specific volume may be defined between the lower surface 25 of the cap and the breakable foil 14. All

of these volumes may also be selected to provide an appropriate operational characteristic.

Thus, by selecting the sizes of the apertures and the volumes of the chambers, the speed at which the piston moves may be regulated, and the instant at which the piston moves to the fully extended position can be regulated.

While the invention has been described with reference to embodiments in which a movement is imparted to a seat-belt buckle to pre-tension the seatbelt, in modified embodiments of the invention the movement could be imparted to a retractor, or a terminal anchoring of the seat-belt, on to a device adapted to engage part of a seat-belt to deflect the belt to generate tension.

In the present Specification"comprises"means"includes or consists of and"comprising"means"including or consisting of.

The features disclosed in the foregoing description, or the following Claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS: 1. A pre-tensioner to apply tension to a safety-belt, the pre-tensioner comprising a sealed pressure vessel containing pressurised gas, the pressure vessel also containing a piston, and being provided with a pyrotechnic unit adapted to inject gas into the pressure vessel to move the piston, to cause the piston to extend from the pressure vessel, there being means responsive to movement of the piston beyond the pressure vessel to apply tension to at least one element to move a component connected to or associated with a safety-belt to apply tension to the safety-belt.
2. A pre-tensioner according to Claim 1 wherein the pressure vessel is a pressure cylinder, one end of the pressure cylinder being sealed by a rupturable foil, the other end of the pressure vessel being closed by a closure associated with the pyrotechnic unit, the piston being adapted to rupture the foil on movement thereof.
3. A pre-tensioner according to Claim I or 2 wherein the piston is of hollow tubular form, there being at least one aperture forming a communication between the interior of the piston, and the space between the piston and the pressure vessel.
4. A pre-tensioner according to Claim 3 wherein the said aperture formed in the piston is at such a location that, when the piston is fully extended from the pressure vessel, the aperture is on the exterior of the pressure vessel, permitting pressurised gas within the pressure vessel and piston to be vented to the atmosphere.

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5. A pre-tensioner according to any one of Claims 2 to 4 wherein a sealing ring is provided adjacent the foil, the sealing ring having a diameter equal to, or less than the exterior diameter of the piston.
6. A pre-tensioner according to any one of Claims 2 to 5 wherein the pyrotechnic unit is contained within a portion of the closure which projects inwardly into the pressure vessel, that inwardly projecting portion having an aperture initially sealed by a rupturable foil.
7. A pre-tensioner according to any one of the preceding Claims wherein the piston defines a chamber which communicates with the pressure cylinder.
8. A pre-tensioner according to any one of the preceding Claims wherein the piston is formed by a hollow tubular member which initially is re-entrantly folded upon itself within the pressure vessel.
9. A pre-tensioner according to any one of the preceding Claims wherein the means responsive to movement of the piston comprise a plunger located to be engaged by the piston on movement thereof, and a guide tube to guide the movement of the plunger, the plunger being associated with one said at least one element.
10. A pre-tensioner according to Claim 9 wherein the plunger is connected to one end of a wire or other flexible elongate element which is connected to the said component associated with a safety-belt.
11. A pre-tensioner according to Claim 9 wherein the plunger is connected to a wire or other elongate element having two opposed ends, each said end

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being connected to a respective component associated with a respective safetybelt.
12. A pre-tensioner according to Claim 9 wherein the plunger is provided with a pulley-wheel or the like, the pulley-wheel or the like engaging a wire or other elongate flexible element, one end of which is anchored in position, and the other end of which is connected to the said component associated with a safety-belt.
13. A pre-tensioner for a safety-belt substantially as herein described with reference to and as shown in Figures 1 to 3 of the accompanying drawings.
14. A safety-belt pre-tensioner substantially as herein described with reference to and as shown in Figures I to 3 of the accompanying drawings as modified by Figure 4.
15. A safety-belt pre-tensioner substantially as hereinbefore described with reference to and as shown in Figure 5 of the accompanying drawings.
16. A safety-belt pre-tensioner substantially as herein described with reference to and as shown in Figure 5 as modified by Figure 6 of the accompanying drawings.
17. A safety-belt pre-tensioner substantially as herein described with reference to and as shown in Figures 7 and 8 of the accompanying drawings.
18. Any novel feature or combination of features disclosed herein.