EP2247473A1

EP2247473A1 - Pre-tensioner arrangement

Info

Publication number: EP2247473A1
Application number: EP08724201A
Authority: EP
Inventors: Erik Hjerpe; Dan BRÅSE
Original assignee: Autoliv Development AB
Current assignee: Autoliv Development AB
Priority date: 2008-03-07
Filing date: 2008-03-07
Publication date: 2010-11-10
Also published as: WO2009110823A1; EP2247473A4

Abstract

The present invention relates to a pre-tensioner arrangement (1) for use in a motor vehicle safety device, comprising a piston member (3) dispiaceable in a chamber (2) upon a gas pressure applied to said chamber (2) through an inlet (9) and connectabie to a part of a motor vehicle safety device. The pre-tensioner arrangement (1) has a first mode of operation and a second mode of operation. The present invention aims at providing an improved pre-tensioner arrangement.

Description

PRE-TENSiONER ARRANGEMENT

Technical field

The present invention relates to a pre-tensioner for use in a motor vehicle safety device, comprising a piston member displaceabie in a chamber upon a gas pressure applied to said chamber through an inlet and connectabie to a part of a motor vehicle safety device. The pre-tensioner arrangement has a first mode of operation and a second mode of operation.

Technical background Motor vehicles are today normaϋy provided with a number of safety- devices, such as airbag arrangements and/or seat-belt pre-tensioners. Commonly, such safety devices are arranged to be pyrotechnicaliy operable in order to inflate an airbag or drive a piston of a safety-beit pre-tensioner fast enough in an impact situation. Since they are pyrotechnicaliy operable, such arrangements are not re-settable for subsequent use. This is not generally a problem when the safety device has been actuated in a severe accident situation because it is much more important to prevent injury and possible death to occupants of a motor vehicle than it is to save money and inconvenience in providing a re-settable device, particularly when in such severe impacts, the motor vehicle as a whole is likely to be very severely damaged in any event. However, in low impact accident situations or when it is desirable to actuate (at least partially) a safety-device upon receipt of a signal from a predictive of a crash, i.e. a signal from a pre-crash sensor, it is preferable for a safety device to be operable in a reversible manner. Re-settabSe safety devices of this general type have been proposed previously such as, for example, in US2005/0218632A1 which discloses a motor vehicle provided with a central air supply system in order to operate a number of different pneumatically-operated devices such as air-bags and seat belt pre-tensioners. However relying solely upon reversible pneumatic safety devices of the type disclosed in US2005/0218632A1 can be disadvantageous because pneumatically-operated safety devices cannot easily be actuated sufficiently quickiy in order to provide a proper degree of protection in more severe accident situations occurring at higher speeds.

Therefore, safety devices which can be operated in a number of different operating modes has been proposed. !t has been proposed to so!ve this by providing a safety device that is, depending on the actual situation, operable either pneumatically or pyrotechnically.

With respect to previously known solutions, it is desirable to achieve a reliable pre-tensioner arrangement not being bulky and complex, which is easy to mount in motor vehicle.

Summary of the invention

St is an object of the present invention to overcome the above described drawbacks, and to provide an improved pre-tensioner arrangement.

This and other objects that will be apparent from the following summary and description are achieved by a pre-tensioner arrangement according to the appended claims.

According to an aspect of the present invention, there is provided a pre-tensioner arrangement for use in a motor vehicle safety device, comprising a piston member dispSaceable in a chamber upon a gas pressure applied to said chamber through an inlet and connectabSe to a part of a motor vehicle safety device, said pre-tensioner having a first mode of operation and a second mode of operation, said pre-tensioner further comprising a retaining means arranged to retain, only in said second mode of operation, the piston member in a position to which it is displaced by gas pressure applied to said chamber.

Thus, the pre-tensioner can be reversibly operated in a first mode. That is, it can be re-set for a subsequent re-actuation, for instance if no impact occurs. Easy resetting is enabled since the piston member is not engaging any device that need to be manually re-set. Preferably, such a first mode is activated upon a first signal, such as a signal indicative of a predicted crash, in the first mode of operation gas is preferably supplied to the chamber from a source with compressed gas. Thus, in the first mode of operation the piston- member is retained by gas pressure inside the chamber. Sn a first mode of operation the pre-tensioner may for instance serve to take up an initial degree of slack in the seat be!t passing around a seat occupant in anticipation of an actua! impact or position a safety device prior to an accident situation. Sf no impact eventually occurs, the pre-tensioner can be automatically re-set. However, if an actua! impact is subsequently detected (by a crash sensor) the pre-tensioner may be operated in a second mode of operation. Preferably, such a second mode is activated upon a second signal, such as a sic indicative of an actua! crash. Sn the second mode of operation the member is retained in a position by the retaining element, which enables holding a safety device, connected to the piston memeber, in a pre-tensioned state, in the second mode of operation gas is preferably supplied from a source of gas generated by a pyrotechnica! device, such as a gas generator (pyrotechnic charge).

Preferably, gas is supplied to the chamber through a first chamber iniet in the first mode of operation and through a second chamber iniet in the second mode of operation.

A piston rod is preferably connected to said piston member in order to enable easy connection to a safety device.

Preferably, the pre-tensioner arrangement further comprises a displacement limiting element, which element is arranged to limit displacement of said piston member in the first mode of operation. Thus, the available displacement of the piston member, and thereby a safety device connected to the piston member, can be adapted for conditions in the first mode of operation, as regards for instance occupant comfort. St can thus be taken into consideration how the occupant experiences the pre-tension action in such a pneumatically operated pre-tension action in the first mode of operation.

More preferably, the displacement limiting element is breakable. Thus, a robust and reliable stop element is provided. The stop eSement preferabSy breaks at a pre-determined force level. Hence, the stop element can be adapted to withstand the presseure inside the chamber applied in the first mode of operation. As soon as the pre-tensioner is operated in the second mode of operation, the pressure arises above the pre-determined ieve! causing the stop element to break. Thus, the stop element can be adapted to break as soon as the pre-tensioner is operated in the second mode of operation.

Preferably, the chamber of the pre-tensioner arrangement is formed by a cylinder and the piston member is dispiaceabie axially aiong said cylinder. Hence, a reiiabie pre-tensioner arrangement that is easy to manufacture is provided.

Preferably, the piston member divides the chamber in a first and a second volume, in which second volume an aperture is provided. Hence, the pre-tensioner arrangement can easily be re-set by applying a pressure inside the second cylinder. Thus, when connected to a seat belt system, a pre- tensioner having a presenter function may be provided, which means a space-efficient belt system having few parts can be achieved. Such a pressure may be applied by supplying gas from a source of compressed gas through the aperture. The aperture may also serve as a vent hole. Preferably, the pre-tensioner arrangement further comprises a second piston member engageably arranged to the piston rod. Hence, in the second mode of operation the volume to fill may be a less part of the cylinder volume, i.e. less pyrotechnical charge is required. Furthermore, since the volume to fill always is the same independent of the first mode of operation has been activated or not, shorter pre-tensiong time and/or increased precision is achieved.

Preferably, the pre-tensioner arrangement further comprises a second piston rod member connected to the first piston rod by means of a deformable element, said element being deformed when subjected to a pre-determined force. Hence, a load limiting mode of operation is enabled. More preferably the load limiting element is an energy absorbing element, such as a a winded metal thread.

Brief Description of the drawings The present invention will now be described in more detail with the reference to the accompanying schematic drawings which show preferred embodiments of the invention and in which:

Fig. 1a shows a pre-tensioner arrangement according to a first embodiment of the present in a partial cross-sectional view. Fig. 1b-1d show the pre-tensioner in fig 1a in operation.

Fig, 2a shows a pre-tensioner arrangement according to a second embodiment of the present in a partial cross-sectiona! view.

Fig. 2b-2d show the pre-tensioner in fig 2a in operation. Fig. 3a shows a pre-tensioner arrangement according to a third embodiment in a partial cross-sectional view.

Fig. 3b-3e show the pre-tensioner in fig 3a in operation.

Fig. 4a shows a pre-tensioner arrangement according to a fourth embodiment of in a partial cross-sectional view. Fig. 4b shows the pre-tensioner in fig 4a in operation.

Fig. 5a-b show an example of use of a pre-tensioner arrangement according to an embodiment of the present invention in a perspective view.

Fig. 6 shows a further example of use of a pre-tensioner arrangement according to an embodiment of the present invention. Fig. 7 shows a further example of use of a pre-tensioner arrangement according to an embodiment of the present invention in an exploded view.

Technical description

Figure 1a shows a pre-tensioner arrangement 1 according to a first embodiment of the present invention comprising a cylinder 2, a piston member 3 and a piston rod 4. The piston member 3 is slidably arranged inside the cylinder 2 and the piston rod 4 is connected to the piston member 3 in such a way that the piston rod 4 is displaced upon a displacement of the piston member 3. The piston member 3 is in sealing relationship with the inner cylinder wall 5 by means of a suitable sealing means 6, such as an o- ring, arranged on the piston member 3. Thus, the inner volume of the cylinder 2 is divided into a first 7 and a second 8 cylinder volume, in response to a pressure applied to the cylinder 2 the piston member 3, and thereby the piston rod 4, is axially displaced with respect to the cylinder 2 since the piston member 3 is slidably arranged inside the cylinder 2. The cylinder 2 is further provided with inlets 9, 10 through which gas can be supplied to the first cylinder volume 7. In this case the first inlet 9 is connected to a source of compressed air (not shown) and the second inlet 10 is connected to a pyrotechnic gas generator (not shown) of a kind known in the art. By supplying gas through either of the first 9 or the second 10 inlet the piston member is displaced in a first direction corresponding to pre-tensioning of a safety device (not shown) connected to the pre-tensioner arrangement 1. The cylinder 2 is in this case further provided with a third inlet 11 , through which gas, such as gas from a pneumatic source, can be supplied to the second cylinder volume 8 in order to enable displacement of the piston member 3 in a second direction, opposite to the first direction, corresponding to for instance re-setting of the pre-tensioner arrangement and/or presenting of a seat belt connector. The pre-tensioner arrangement 1 further comprises a retaining arrangement 12, which in this embodiment includes a freewheel arrangement 13 with a bearing wheel 14 fixed to the inner side of the cylinder 2, a lock wheel 15 and lock cylinders 16. The lock cylinders 16 are arranged in the interface between the bearing wheel 14 and the lock wheei 15. Due to the geometry of the lock wheel 15 it is free to rotate in only one direction relative the bearing wheel 14. Sn the other direction the lock cylinders 16 prohibits rotation of the lock wheel 15 relative the bearing wheei 14. The retaining arrangement 12 further comprises means to mechanically engage the lock wheel 15 to the piston rod 4, in this case inner threads 17 of the lock wheel that are complementary to outer threads 18 of the piston rod 4. The threads 17, 18 are engaged in the second mode of operation, thereby forming a "screw joint" between the Sock wheel 15 and the piston rod 4. Thus, the screw joint 15 is formed by inner threads 17 of the Sock wheei 15 that match outer threads 18 in the piston rod. in the second mode of operation the pressure, generated by a gas generator, inside the first volume 7 is relatively high and consequently the screw joint is subjected to significant mechanical stresses. It should be noted that the threads 17, 18 of the screw joint is designed to withstand mechanical stresses in accordance with the actual load case. The pre-tensioner arrangement 1 is further provided with a stop element 19, in this case formed by teeth 20 extending from the inner diameter of the lock wheel 15, see enlarged part of fig 1a. The teeth 20 are dimensioned to withstand a certain load from the piston rod 4 applied due to a pressure working on the piston member 3 inside the first cylinder volume 7. Sn case the pressure inside the first volume 7 exceeds a predetermined level the teeth 20 are deformed, whereby further displacement of the piston member 3 and the piston rod 4 is aϋowed. The piston rod 4 has a number of guiding recesses 21 which are complementary to the teeth 20 of the lock wheel 1. The teeth 20 match with the guiding recesses 21 in the piston rod 4.

The piston rod 4 extends out of the cyiinder 2, The outer end of the piston rod 4 is adapted to be attached to a part of a safety device, by means known in the art. Different ways of attaching the rod to a safety device will be shown. However, the person skiϋed in the art realises that this can be carried out in a number of alternative ways. in this case the outer end of the piston rod 4 is connected to a power transmitting arrangement 23, partly shown in fig 1a, comprising a means 24, such as a wire or a stee! band, capable of transfer a force and a movement from the piston rod 4 to a safety device (not shown). The steel band 24, of which one end 25 is fastened to the cylinder 2, runs partially around a pulley wheel 26 and is in the other end connectable to a part of safety device, such as a connector or a retractor of a of seat belt system. The wheel 26 is rotatably supported to the piston rod 4 by means of a mechanical joint 27 comprising an pin 28 received in apertures 29 in the side portions 30, 31 of an element 32 connected to the piston rod 4. The pulley 26 serves to guide the stee! band 24 upon a displacement of the piston rod 4.

Referring to figures 1 b-1d the function of a pre-tensioner arrangement 1 according to the embodiment in fig 1a is described in the following. In fig 1b the pre-tensioner 1 is operated in a first mode of operation, in this case a pneumatic mode of operation. Upon a triggering signal from a sensor, such as a pre-crash sensor, indicative of an expected crash compressed air is introduced to the first cylinder chamber 7 through a first inlet, as indicated by arrow A. The pressure inside the first volume 7 is then increased and due to a pressure applied on the piston member 3, the piston member 3, and thereby the piston rod 4, is displaced in the direction indicated by the arrow B. The piston displacement enables a safety device connected to the pre-tensioner arrangement 1 to be pre-tensioned. Consequently, a safety device, for instance a seat belt, connected to the pre-tensioner 1 via a power transmitting arrangement 23 wil! be pre-tensioned. in this first mode of operation, the piston member 3 may be displaced until portions of the piston rod 4 contacts the teeth 20 to form an engagement between the teeth 20 and guiding recesses 21 on the piston rod 4, as indicated in the enlarged part of fig 1 b. As mentioned the stop element 19, in this case the teeth 20, is dimensioned to withstand mechanica! stresses corresponding to a pre-determined pressure inside the first chamber volume 7. The teeth 20 thus prevents further displacement of the piston rod 4, in the direction indicated by arrow B, as soon as a contact is established between portions of the piston rod 4 and the teeth 20. Sf no crash or a low impact crash occur the piston member 3 can be re-set by a displacement of the piston member 3 in the direction opposite to B. The re-setting can for instance be carried out by evacuating the compressed air inside the first volume and/or by applying a pressure inside the second cylinder volume 8 by supplying gas from a source of compressed air (not shown) to the second volume 8 through the third inlet 11. As soon as a certain pressure is applied inside the second volume 8 of cylinder the piston member is displaced to be re-set.

Upon receipt of a signal indicative of a crash from a crash sensor, the pre-tensioner is operated in a second mode of operation. Then, a gas generator is activated whereby gas is suppiied to the first chamber 7 through the second inlet 10, as indicated by arrow E in fig 1c. The pressure inside the first cylinder volume 7 is then increased rapidly causing the teeth 20 to be deformed, thereby enabling further displacement of the piston member 3. Upon further displacement of the piston member 3 the lock wheel 15 is rotated due to the screw joint relation between the piston rod 4 and the lock wheel 15, as indicated by the upper enlarged part of fig 1c. Thus, further axial displacement of the piston member causes a rotation of the Sock wheel 15. Due to the free wheel arrangement rotation of the Sock wheel 15 is allowed in only one direction, as indicated in the enlarged part of fig 1c. The piston member is allowed to move in only one direction since it is engaged with the lock wheel by means of the "screw joint". The Sock cylinders 16 prevent rotation of the lock wheel 15, which is illustrated in fig 1d. A safety device connected to the pre-tensioner can thus be held in a pre-tensioned state. In the second mode of operation the piston member is displaceable oniy in the direction corresponding to further pre-tension of a safety device connected to the pre-tensioner due to the retaining element. Thus, in case a force is applied on the piston rod in the opposite direction to B displacement of the piston member is prevented. Such a force is typically applied in case an accident occur, due to forces from the occupant restrained and/or pre- tensioned by the actual safety device.

Figure 2a shows a pre-tensioner arrangement 1 according to a second embodiment of the present invention. Essentially all features disclosed in the first embodiment are also present in the second embodiment with reference numerals identifying similar or same features. Having mentioned this the description will focus on explaining the differing features. The second embodiment differs as regards the retaining arrangement 12. in the second embodiment the retaining arrangement 12 comprises recesses 33 formed in the piston rod 4 and a spring biased latch 34 arranged outside the cylinder 2. in a second mode of operation the breakable element 19 is broken. The latch 34 then interacts with a piston rod recess 33.

The operation of a pre-tensioner arrangement 1 according to embodiment 2 is similar to the one described in relation to the first embodiment and is therefore oniy briefly described for the second embodiment in the following, referring to fig 2b-2d. Sn the first mode of operation the piston member 3 may be displaced, upon a pressure applied inside the cylinder 2, until contact is established between the stop element 19 and the inner cylinder wall 5, see fig 2b. Sn the second mode of operation the stop element 19 is deformed which enables further displacement of the piston member 3, and thereby the piston rod 4 connected to the piston member 3, see fig 2c. Upon further displacement the spring biased latch 34 is engaging one of the recesses 33 formed in the piston rod 4, as indicated by the arrow F in fig 2d. Due to the shape of the latch 34 and the complementary shape of the recesses 33 the piston member 3 is then prevented from being displaced in a direction opposite to the one indicated by arrow D. A safety device, such as a seat-belt system, connected to the pre-tensioner can thus be held in a pre-tensioned state by the pre-tensioner arrangement 1. As mentioned there are a number of recesses 33. Depending on a number of factors the !atch 34 is engaging one of the recesses 33 formed in the piston rod 4. Such factors are for instance the position of the occupant restrained by the actual safety device, the size of the occupant and the kind of clothes the occupant is wearing.

Figure 3a shows a pre-tensioner arrangement 1 according to a third embodiment of the present invention. Essentially ali features disclosed in the first embodiment are also present in the second embodiment with reference numerals identifying similar or same features. Having mentioned this the description will focus on explaining the differing features. The third embodiment differs in that the direction of piston displacement corresponding to pre-tension of a safety device is opposite to the one in embodiment 1 and 2, and in that features are added. in embodiment 3 the pre-tensioner arrangement 1 comprises a second piston member 35, which is movably arranged to the piston rod 4, and a second retaining arrangement 36 capable of engaging the second piston member 35 to the piston rod 4 in the second mode of operation. The second piston member 35 has a flange 55 that contributes to a proper displacement of the piston rod 4 relative the second piston member 35 in the first mode of operation. The second retaining arrangement 36 is formed by a latch 37, which is engageable with further recesses 38 formed in the piston rod 4. The latch 37 is arranged on the second piston member 35 in such a way that movement of the latch 37 in a radial direction relative the second piston member 35 is allowed and recesses 38 in the piston rod. The latch may for instance be guided and/or retained by a groove 57, preferably a groove with complementary shape to the latch, as shown in fig 3a. A recess cover 39 is arranged to prevent the latch 37 from engaging a recess 38 in the first mode of operation. The recess cover 39 is formed by a deformable material and is arranged on the surface of the piston rod 4. Between the piston member 35 and the inner cylinder wall a notch 56 is provided in order to create an initial volume in cylinder volume 7, see fig 3a.

In the first mode of operation the piston rod 4, which is connected to the first piston member 3 is dispSaceabSe relative the second piston member 35. The cylinder 2 has in this embodiment a first section 40 with a certain inner diameter, a second section 41 with a smaller diameter than in section one and a third section 42 between the first and the second section where the diameter transcends from the inner diameter of the first section to the inner diameter of the second section. Upon a piston movement in the second mode of operation the latch 37 is engaging a recess 38 formed in the piston rod due to the transition section 42. Before engaging a piston rod recess 38 the recess cover 39 is deformed by the iatch 37. Thus, the third section 42 is adapted to contribute to the engagement of the second piston member to the piston rod in the second mode of operation. in this embodiment the pre-tensioner arrangement is provided with a similar retaining arrangement 12, including a latch 34 outside the cylinder 2 and piston rod recesses 33, as the one in embodiment 2. in this case a connector 43 of a seat belt system is connected to the piston rod 4 by means of a power transmitting arrangement 23 . Hence, by a displacement of the piston member 3 the seat belt connector 43 is displaced, as indicated by the arrows in fig 3b. Due to the piston member displacement a safety device, in this case a seat belt system, can be pre-tensioned.

Referring to fig 3b-3d the operation of the arrangement shown in fig 3a is described in the following. In the first mode of operation compressed air is supplied to the cylinder through the first chamber inlet, see fig 3b. The piston member 3 may be displaced until contact between the piston member 3 and the stop element 19 is established. Upon activation of the second mode of operation, gas from a gas generator is supplied to the cylinder 2 through the second inlet, see fig 3c. Due to the pressure inside the third cylinder volume the second piston member 35 is displaced. As the second piston member 35 passes the third section 42 the latch 37 is displaced towards the piston rod 4, see fig 3d. Due to the displacement of the latch 37 the recess cover 39 is first deformed and then the latch 37 is engaging one of the recesses 38 formed in the piston rod 4, see enlarged part of fig 3d. Which of the recesses that is engaged is depending on whether the first piston member 3 has been displaced or not when the second mode of operation is activated. After engagement the piston rod is displaced upon further displacement of the piston member 2 due to the pressure suppiied in the third chamber volume. The piston rod is then retained by the retaining arrangement as described above. That is the latch 34 is engaging one of the piston rod recesses 33, see fig 3e. Fig 4a shows a fourth embodiment. Essentially all features disclosed in the third embodiment are also present in the second embodiment with reference numerals identifying similar or same features. Having mentioned this the description will focus on explaining the differing features. Compared to the third embodiment, the fourth embodiment further comprises a second piston rod 44 and a force limiting element 45. In the first and second mode of operation the function is similar to the one of the third embodiment. In the fourth embodiment the pre-tensioner is further operable in a third mode of operation, which corresponds to a force limiting mode. That is, after being retained in the second mode of operation the piston member 3 may be displaced backwards upon a certain force applied to the piston rod 4. Such a force may be applied due to occupant loading of a safety device in a crash situation. Thus, the pre-tensioner serves as load limiter. in the load limiting mode of operation the piston member, and thereby the first piston rod 4 is allowed to move in a direction indicated by arrow H in fig 4b. Consequently, a part of a safety device, in this case a seat belt connector is allowed to be displaced in a direction opposite to the "pre-tension direction", as indicated by arrow I. During such a displacement a load limiting element 45 is deformed, which provides ioad limiting to the occupant restrained by the safety device. The load limiting element 45 is adapted to be deformed when the safety device is subjected to a load corresponding to a certain pre-determined level. Thus, it is secured that the element 45 is not deformed as long as the load do not exceed a pre-determined level.

This ioad limiting function is realised by adding a second piston rod member 44 connected to the first piston rod 4 by means of a deformable element 45, preferably a deformable energy absorbing element. Preferably, as shown in fig 4a, the ioad limiting element is formed by a winded metal thread riveted to the piston ends. Figure 5a-b illustrates a pre-tensioner arrangement according to an embodiment connected to a seat belt connector 43 of a seat-belt system. The piston rod 4 of the pre-tensioner arrangement 1 is connected to the connector 43 by means of a power transmitting arrangement 23, partly described above. in figure 5a it can be seen that the power transmitting arrangement 23 in this case further comprises a wire 46, in one end connected to the steel band 24 and in the other end connected to a connector body 43, and a "force shifting wheel" 47 connected to the cylinder 2. A wireguide 48, having an extended portion 49 and an end portion 50, is connected to the first end of the wire 46 in order to keep it stiff, and for interaction with a guide bracket 58 during connector presenting by Sifting and tilting, see fig 5b. in order to be lifted in the presenter mode of operation the connector 43 may be spring-biased. The tilting movement is a result of the end portion 50 of the wireguide 48 being in contact with the guide bracket 58. Fig 6 illustrates an alternative way of use of a pre-tensioner arrangement according to an embodiment of the present invention connected to a seat belt connector 43 of a seat-belt system. The wire 46 is in this case guided by means of a wire casing 60, which is is connected to the vehicle structure by means of a curved attachment element 61 and an attachment element 62.

Fig 7 shows a further example of use of a pre-tensioner 1 according to an embodiment of the present invention. In this case the pre-tensioner 1 is connected to a retractor 51 of a seat-belt system. The retractor is of a type known in the art where a seat belt 52 is wound on a spool 53 having internal bore provided with inwardly-directed gear teeth, see fig 6.

By pulling the steel band 24 the spool 53 is rotated in a direction effective to tighten the seat-belt.

It will be appreciated that the described embodiment of the invention can be modified and varied by a person skilled in the art without departing from the inventive concept defined in the claims. It is realized by a person skilled in the art that features from various embodiments disclosed herein may be combined with one another in order to provide further alternative embodiments. For instance it is realized that in the first embodiment a stop element 19 may be formed as a portion extending, in the radiai direction, from the piston rod. !n this case recesses, complementary to the teeth, may be formed in the !ock wheel in the first and second embodiment the chamber volume 7 has two chamber inlets 9, 10. Alternatively, the chamber volume 7 may have only one inlet. Then, a valve may be arranged outside the chamber 2 controlling, based on the mode of operation, from which gas source gas is supplied to the chamber 2.

Furthermore, the piston rod displacement can be transferred to a safety device in a number of ways known in the art. Also, the person skilled in the art realises that the pre-tensioner arrangement is connected to the vehicle structure using suitable fastening means (not shown).

Claims

CLAiMS

1. Pre-tensioner arrangement (1) for use in a motor vehicle safety device, comprising a piston member (3) disp!aceable in a chamber (2) upon a gas pressure applied to said chamber (2) through an inlet (9) and connectable to a part of a motor vehicle safety device, said pre-tensioner arrangement (1) having a first mode of operation and a second mode of operation, said pre- tensioner arrangement (1) further comprising a retaining means (12) arranged to retain, only in said second mode of operation, the piston member (3) in a position to which it is displaced by gas pressure applied to said chamber (2).

2. The pre-tensioner arrangement (1) according to claim 1 wherein gas is supplied to said chamber (2) through said chamber inlet (9) in the first mode of operation and through a second chamber inlet (10) in the second mode of operation.

3. The pre-tensioner arrangement (1) according to any one of claims 1- 2, wherein a piston rod (4) is connected to said piston member (3).

4. The pre-tensioner arrangement (1) according any one of claims 1-3, further comprising a displacement limiting element (19), which displacement limiting element (19) is arranged to limit displacement of said piston member (3) in the first mode of operation.

5. The pre-tensioner arrangement (1) according claim 4, wherein said displacement limiting element (19) is breakable,

6. The pre-tensioner arrangement (1) according to any one of claims 1- 5, wherein said chamber (2) is formed by a cylinder (2) and said piston member (3) is displaceable axiaily along said cylinder (2).

7. The pre-tensioner arrangement (1) according to any one of claims 1-

6, wherein said piston member (3) divides the chamber in a first (7) and a second (8) volume, in which second volume (8) an aperture (11) is provided.

8. The pre-tensioner arrangement (1) according to any one of ciaims 1-

7, further comprising a second piston member (35) engageably arranged to said piston rod (4).

9. The pre-tensioner arrangement (1) according to any one of claims 1- 8, further comprising a second piston rod member (35) connected to the first piston rod (4) by means of a deformabie element (45), said element being deformed when subjected to a pre-determined force.

10. The pre-tensioner arrangement (1) according to claim 9, wherein said load limiting element (45) is formed by an energy absorbing element.