IE40276B1 - A tensioning devige - Google Patents

Abstract

1438621 Tightening seat belts SOC NATIONALE DES POUDRES ET EXPLOSIFS 18 Dec 1974 [20 Dec 1973] 54642/74 Heading A3V [Also in Division F2] A seat belt is tightened by a piston moved within a cylinder by release of high pressure gas triggered by an impact detector. At a predetermined pressure a frangible link holding an auxiliary piston fractures and activates a jamming mechanism preventing further advance of the piston. Forward movement of the wearer is damped as gas is compressed by return of the piston. The belt is attached to a rod 2 fixed to piston 6 movable within a cylinder 7. The cylinder is held in casing 30 by a frangible ring 9. Ignitable pyrotechnic charges 27 are. housed in a damping chamber 11, which communicates via ports 13 with expansion chamber 12. A residual damping chamber 19 communicating via small ports 31 is provided in the casing; alternatively a venting valve is provided. An auxiliary piston is housed in piston 6, and is in communication with the expansion chamber via passageways 20. In the embodiment shown the auxiliary piston 17 carries a disc 33 which retains balls 34. It is held by a frangible ring 16. In a modification (Figure 5, not shown) the auxiliary piston (39) is annular. In the embodiment of Figure 6 (not shown) has a central weakened link (41) and a conical head (42) contacting radial rods (43). The rods act against a ring retaining the balls. When the charges are ignited, piston 6 is driven to the right, tightening the belt when the force of the wearer thrown against the belt resists piston advance, the gas pressure rises until the retaining ring or link of the auxiliary piston fractures. Further movement of the piston in that direction is prevented by the jamming action of the balls. Continued forward movement of the wearer against the belt withdraws the piston to its initial position. Fracture of ring 9 releases the cylinder 7 so that damping is continued as gas is compressed in chamber 23. [GB1438621A]

Description

4027 6 This invention relates to a tensioning (lcvic.c particular!y, hut not exclusively foe use ■with safety bolts installed in vehicles capable of travel! i ng nl higit speeds, such as motor cars.

Numerous protection tlnvicos are known for ensuring the safety of people travelling in vehicles, and possibly the most cffcctivo protection is achieved by tbe system;; employing inflatable cushions which are blown up by n very rapid .supply of gas at high pressure with the damping of the person to bo protected being achieved by expelling these gases into the passenger compartment of the vehicle, the cushion undergoing deformation due to (In- pressure' of the said person fxnd thus prov i di ng a large surface are.i of contai: t which prevents any localisation of force on tin- cliest of the person bei hj-. pro tec ted. The (Tl'cctiveiu ss of the protection provided by a simple safety belt is very poor because the person, thrown forwards by bis own inertia, is held back only by two straps fixed rigid.ly to the chassis of the vehicle, and this provides only a restricted surface area of contact with a person and subjects the said person to a very high pressure due to the absence of damping. Some safety belts are equipped with devices which permit some discontinuous damping, for example by forming folds on the belt, the fastenings of the said fold?: rupturi ny. and al lowing, tin-bell to unfold. Another- d i s.ic1vantage of .safety belts resides in the fact that they require an initial, adjusti.it->: to be made evr-ry time a different person user:; the bolt, Mifi that i hey hinder the im>cerients of tile jicrpnn wearing the strap. Me'..'ever, ti ]■ -;i on i i1;; device:., ) i?;v tb.it dc i~>' 4027G in Patent Specification *0. make it )>o.-;.Hiblc for the person to bo protected to move freoly, tin? beJ t coming into close contact with the person only at the instant of a collision irrespective of the initial adjustment of the belt. Since safety belts do not allow for effective damping, the known protection systcias use a combination of such a belt with a mobile sunt mounted on rails firmly fixed to the floor of the vchicle, <1 damping component, fur example a hydraulic damper, making it possible for the seat to shift its position by a limited amount, and absorbing a large amount of energy to r"hieve effective damping.

In accordance with the present invention there is provided a tensioning device such as for use in tensioning a vehicle safety belt comprising a casing, a first piston axially movable in a cylinder defined within tiic casing, which piston is firmly fixed to an axially extending rod having means for attaching an element to be tcnsioiied, means for supplying gas at high pressure to an expansion chamber defined between the first piston and a front end wall of the cylinder, whereby in use, the said gases cause the piston to travel in a first direction along the cylinder and exert a tensioning force 011 an element attached to the rod, an auxiliary piston included in tlio first piston structure and held in an initial position by a frangible element adapted to fracture when the pressure in the expansion chamber reaches a predetermined value, the pressure prevailing in the expansion chamber being communicated to the auxiliary piston v.i .1 at least one permanently open passage way, ami piston stopping i>i«.*nns actuated in response to 4 0 2 7 G displacement of Lin- jiiix i 1 i ar; piston from the initial position fol 1 owi nj; r.'-iietiirr of I hr frangible e 1 einent , I <> prevent movement of the first piston rolntivi- to tin* cylinder in the said first direction, but allow movement 5 of the first piston relative to the cylinder in the opposite direction.

With such a tensioning devicc it is possible to achieve total damping of n person to be protected by dissipating a largo amount of energy comparable to the energy dissipated by an inflatable cushion or by a damping component on rails, and at the same time only put a safety belt in contact with the person to be protected when a collision takes place.

The tensioning device according to the present i nven Lion may serve as a component linking a safety belt to n fixed clement of a chassis of the passenger compartment of a vehicle; for example, in the case of sent belts for protecting people sitting in the front of a motor car, it can be situated between the two seats HO and can form one of the three points of attachment of the l.olt of the strap.

In a preferred embodiment the piston stopping means comprises a jamming mechanism including a support surface, plurality of elements in contact with the support surface and positioned between a portion of the first, piston having a conical bearing surface and the inner surface of the c.yl imler, ;nid an elastic retaining component for holding the element.'; in a posjti.cn which permits free movement of the piston H> relative to the cylinder, the support surface being 4 0271! movab I e I>y the displacement ol I lit' >iux i 1 i aiy |>i;. ton to jam tin.- elemeuIs bolwecn tin- conical bearing surfacc awl tho inner surfncc of the cylinder to prevent movement or the first piston relative to tho cylinder in said one 5 direction.

The auxiliary piston may bo positioned axially inside the first piston, the said auxiliary piston possessing two axially adjacent cylindrical parts of different diameters defininy; a radial shoulder which is ■jO subjected to the pressure of the gases in tho expansion chamber via at least one permanently open orifice.

Alternatively, the auxiliary piston may be annular and positioned at tho periphery of the first piston with tho auxiliary piston being subjected to the 15 pressure of the gases in the expansion chamber via at least one permanently open orifice.

The frangible element may be an integral part of the auxiliary piston which is split in two when the element fractures, with one portion of the fractured 20 auxiliary piston actuating th" piston stopping moans.

The frangible element may alternatively be a separate member with movement of the entire auxiliary piston actuating the piston stopping means when tho member fractures. 25 The means for supplying gas to the expansion chamber preferably comprises at least one annular pyrotechnic charge with a surface area of combustion which increases during combustion, and an ignition system therefore comprising a gas-free powder. 30 In a preferred embodiment the cylinder in which the I'ir.'il pinion I liiVcl.M |ii'n v i >!•■•■ by an inili-r piston itself slidable in the cusing, tin* outer- surfacc of the outer piston partially defining a first damping chamber, the outer piston including means for preventing movement of the outer piston relative to tho casing in one direction of relative axial movement, between the two, which direction corresponds to return movement of the outer piston to an initial position.

Additional pistons could be provided between tho first piston and the casing to allow for additional d.-intp i ng.

Some embodiments of the invention for use with vehicle safety belts are described below by way of » example with reference to the accompanying drawings, in which Figure 1 is a di aj^rairmat.i •: cross-section of an entire tensioning device produccil :i n accordance with a first construction and represented in an initial po.sition; Figure 2 is a longitudinal cross-section through the device of Figure 1, shown during a first damping phase; Figure 3 is a diagrammatic cross-section of an entire tensioning device of an a.l t.ernntivc construction and represented during a second damping phase; Figure 'i is a detailed longitudinal cross-section of *i tensioning device in an initial position; Figure 5 is a detailed longitudinal cross-section illustrating a central piston of alternative construction; Figure 6 is a detailed longitudinal cross-scction illustrating a ccntral piston of another construction; Figure 7 i.s a diagram representing the change in 10 2 7(5 I In- privijsii rr nl' i lit* }*.l?: i--'- .if- .1 film" I i oil of I i iiu»; .mil Figm-i* H i .-i .1 <1 i .!)■;i .iin rrp> i-si-nt. i nj; I In* cli,villain tin- resisting force pxurtcd on the bolt, ns a function of time, for n device constructed According to Figure 6.

The tensioning devicc shown in Figures 1 and 2 comprises a buckle I tor attaching a belt, fixed to an 2 axial rod/and a body 3» a casing 'l of which is, in use, firmly pivotal!y fixed to the chassis of a vehicle by an attachment plate 5* A first central piston 6 is firmly fi x«;<l to the rod 2 and is slidablo in a cylinder defined by ;iii outer piston 7 which is slidahlr wi thin the casing 'l. In tho initial position, ln-forc actuation of the dcvicc, tho ccntral puston 6 is in contact with a front end 8 of the outer piston 7 which is held ir.sidc tho casing 'i by n frangible retaining component 9- A pyrotechnic charge 10 for generating gases at high pressure is loeated in a damping chambor 11 which communicates with on expansion chamber 12 via openings 13« The outer piston is equipped with a component 14 for preventing relative movement between the outer piston and casing in one direction of travel of the piston 7. The component l'l is permanently in working condition and is arranged to prevent any backwards movement of the piston 7 (i.e. to the right as seen in Figure 1 towards its initial position shown in Figure 1, after the outer piston has travelled forwards to the left. The central piston has a component 15 for prcve?iting relative movement between the ccntral and outer pistons in one direction of travel of the central piston. A jamming mechanism of this component 15 is in a non-jmmiiing condition in the initial position of piston 6, as 40371* shown j 11 Figure 1, and during initial displacement of Die central piston backwards into the body which occurs when tho pyrotechnic charge 10 is fired. The jamming mechanism is actuated when the pressure 5 prevailing in the expansion chamber 11 reaches a pre determined vjvluc. At the predetermined pressure a frangible element l6 ruptures to allow an auxiliary piston 17 to shift and actuate the jamming mcchanism to prevent travel of the ccntral piston 6 backwards 10 relative to piston 7> but at the same time allow the piston to move relative to the outer piston towards its initial position. A mechanical pressure-limiting device is provided op the casing h in the form of an external valve l8. 15 The embodiment illustrated in Figuro 3 if of tho oainc construction as that of Figures 1 and 2 exccpt thnt ;» residual compression chamber 19 is provided instead of a valve for limiting the pressure.

In the initial position (Figure l), the UO tensioning devicc has a length determined by the length of the casing h and the length of the attachment rod 2; the prefercntial jamming component 15 of the central piston (t is not: in an operating condition and the ccntral piston can travel freely whilst the outer piston 7 is 25 retained in the position shown l>y the frangible component 0 which may bo, for example, an clastic ring with lugs. When a person to be protected puts on their safety belt, no adjustment is necessary and the belt is not in close contact with the said person, thus leaving him free to 30 move. When a collision occurs, an electrical signal L 40276 generated by ;in impact detector fires the pyrotechnic charge 10 which liberates a large volume of gas within a very short period of time. The gases at high pressure generated by the pyrotechnic charge which has a surface area of combustion which increase* during combustion propel the central piston 6 backwards, and thus reduce tho length of the device (tho casing being firmly fixed to the chassis of the vehicle) to tighten the safety belt and bring it into close contact with the person to be protected. This will normally occur before the piston 6 reaches the end of its path, the position of the central piston 6 at the moment of contact being determined by the initial slack'in the safety belt. The person thrown violently forwards by his own inertia as a rc.sult of the collision, exerts a large resisting force on the central piston and the backwards displacement of the piston is slowed down or stopped. This limits the volume of the expansion chambcr 12 and causes the pressure of the combustion gases to increase. When the pressure in the expansion chamber 12 roaches the predetermined limiting value, the auxiliary piston 17 which is subjected to the full pressure of tho gases via passageways 20, ruptures the frangible element 16 and moves relative to the piston 6 to actuate the component 15 and thus prevent any further movement of the piston 6 backwards. The component 15 allows the piston 6 to travel forwards, to tho left as seen in the drawings, in order to effect a first damping phase (Figure 2) which continues until the central piston 6 has returned to its initial position in contact with the front end 8 of the outer piston 7. A second damp i MR phase commences with tho fracture oT the frangible elastic ring 9 which liberates tho outer piston 7 which can then, under the effect of the forco exertod on the belt by the person to be protected, travel forwards relative to the casing 4, compressing the gases at high pressure present in the darning chamber 11 as it moves. The consequential rise in pressure of the gases is )j.mitcd by tho calibrated valve l8 or in the Figure 3 embodiment, by the residual compression chamber 19* The tensioning device shown in Figure k is of the general construction illustrated diagrammatically in Figure 3. The body comprises a casing JO having an annular cavity forming'the residual compression chamber 19, and onto which there is screwed, firstly, a rear plate 21 equipped with a member 5 for pivotally attaching the device to a vehicle chassis, and secondly, a front, plate 22 equipped with a frangible retaining component 9 which is in the form of «i ring carrying lugs which engage the; outer piston 7 and arc adapted to slionr. Tho outer piston possesses, in its rear portion, a component for preventing rightward movement of the piston relative to the casing and consisting of a conical bearing surface 23 and a number of balls 2'» maintained in contact with the conical bearing surface by means of an elastic ring 2^ which is held compressed by a support ring 26. The outer surface of the outer piston and the inner surface of the casing delimit the damping chambcr 11 which contains, .in its 1'ront. portion, an annular pyrotechnic charge consisting of a main charge 27 produced from powder cylinders containing a multiplicity of perforations, aid two rapid ignition devices 20, the firing - 10 - 40276 wire:: 29 Tor which jmss through the front plate 22 of the ra.'iiug via a .sealed passage. The front end 8 of the outer piston is in sealing contact with the rod 2 which can slide freely therethrough, and with the ccntral piston 5 6. Tho piston 6 includes a component which may be actuated to prevent rightward movement of piston 6 relative to tho outer piston, including an auxiliary piston 17 and n jamming mechanism similar to that of the outer piston. Orifices of small cross-section 31 connect the jO residual compression chambcr 19 to the damping chamber 11, whilst openings of large cross-section 13 connect the latter chwrnher to the expansion chamber 12 and passageways 20 conncct thi.s expansion chamber to an annular surface 32 on the auxiliary piston 17-J5 The casing may, for example, have an external diameter at the rear portion, 75 "an, and overall length, £50 mm. The first value gives an indication of the scale of representation nnd will give a general idea of the size of the device. The main pyrotechnic charge 27 20 posfifisscs an ignition lag of h milliseconds and a duration of combustion of 15 milliseconds, and the pressure of the fi.iscs reaches approximately 80 kg/cm at the end of the combustion phase, the central piston 6 being considered in contact with the rear plate 21 of the outer casing. The 25 active surface areas of the central piston 6 and of the *> outer piston are 8 cm and all the seals arc resistant to high temperatures.

During operation, after the pyrotechnic charge has been fired from an electrical signal transmitted 30 through the wires 29« the pressure increases very rapidly - \\ - 40276 but .smoothly mid the increase is more rapid in the expansion chamber 12 than in the residual compression chambcr by virtue of the relative sizes of upernturcs 13 and 31. This brief local cxcess pressure makes it possible to displuce the piston 6 and the rod 2 backwards more rapidly, the frangible retaining component 9 preventing any premature displacement of the outer piston 7. The central piston, propelled backwards, caiises the safety belt to tighten. When tho straps of tho belt come into contact with the person to bo protected the pressure of the gases increases more rapidly since movement of the piston 6 is opposed, and when this pressure roaches a pre-determined value tho jamming component is actuated. This predetermined pressure can be sot, for example, at 8o kg/cm and as soon as this limit is reached the pressure which is exerted on the annular surface 32 of the auxiliary axial piston 17 via the passageways 20 causes the lugs of the frangible element l6 to fracture and the auxiliary piston 17 to move forwards. Tho piston 17 carries a disc 33, the periphej-.il zone of which pushes a collection of balls 3'j forwards, compressing elastic ring 35, to jam these balls between the conical bearing surface 36 of the piston 6 and the inner surface of the outer piston 7. Operation of this jamming mechanism stops backwards displacement of the piston and only allows the forward displacement when the force exerted on the safety belt by the person to be protected is sufficient to overcome the force exerted on the piston Ct by tho pressure of the combustion gases. This return movement of tho piston towards its initial position lead;: 40276 to compression of the gases owing to a dccrm.sc in the volume of the expansion chamber, <tiid tho dampi ng of the jn'r.'ton to Up protected is thus effected l>y using a bel t retaining force which incrcnucs gradually. The fi outer piston 7 is fti.ll held by the ring 9 and, sincc the initial damping phase takes place within a very short period of time, heat exchanges with the external medium are limited, and the compression of the gases is close to an adiabatic change. During return movement of the JO piston 6, the gases arc forced back through the apertures 13 into tho damping chamber 11 and when the front, facc of the ccntral piston engages with the front, end 8 of, the outer piston, damping becomes discontinuous since the force applied to the attachment j5 buckle by the safety belt must increase to shear off the Jugs of the ring 9 and free the outer piston for forward movement. This forward movement of piston 7 compresses the gases essentially adiabatically driving them back into the residual compression chamber 19 via 20 orifices 31• Any return movement backwards of the outer piston when the damping of the person to be protected is complete, is prevented by the balls 2'l jamming between tho conical surface 25 and the inner surface of the casing. 25 The embodiment of Figure 5 includes an outer piston 7n, through the front end 8i» of which sealingly extends a rod 2a^ to which is screwed a central piston 6a having nil annular chamber 38 which communicates with an expansion 30 chamber 12jj via three passageways ZOa^ (only one shown). - 13 - 40276 Tin' inner cluimltt'i* 'ill iimi t iti iim iiii mix i M nry iiiiiinlar |HmIiiii l'> which in hold in puni l ion liy liigx on n riuh 'll> atul which is in contact with a collcction of balls 3''a which are urged against the annular piston by an clastic ring 35a, which thereby prevents these bnlls from jamming between a conical bearing surfacc 36a of the central piston 6_a and the inner surface of tlie outer piston 7a, as long as the lugs of ring '»0 are not fractured, as will occur when the pressure in the expansion chambcr exceeds a predetermined value. The operation and undcscribed constructional features of this embodiment arc the sumo as in the earlier embodiments described above.

The tensioning device shown in Figure 6 has a construction that will be readily understood from the following description of its operation. In the event of a collision, an electrical signal generated by an impact detector is transmitted to a pyrotechnic charge which generates gas or to a pyrotechnic charge which liberates a gas which is initially compressed, for example in the residual compression chambcr. This gas at high pressure is conveyed into an expansion chambcr 121) via aperaturos 13b and the central piston 6b is propelled backwards and causes tightening of a safety belt attached to rod 2b.

When the straps of the belt come into close contact with a person to bo protected the pressure of the gases increases rapidly, and when this pressure reaches a predetermined value, for example, 80 kg/cm , the force acting on annular surface 321> of an auxiliary piston 17b induces a tensile stress in frangible element 4l, v/hich is sufficient to cause the element to fracture. - l'l 40276 A rrnr pot tion 'il£ of tho piston 6t>, which has a conical end, in displaced backwards by the gas pressure and forccj our control rods '13 out laterally. The rods kj have tapering ends which cooperate with an annular element 44 to move the element forwards when the rods arc moved outwards, to cause a collection of balls 3'lb to jam between a conical bearing surface 3% on the central piston 6b and tho inner surface of the outer piston 7b, and clastic ring 35b being comprcsscd.

In all. the tensioning devices described above, it is to l»e noted that when the central piston returns towards its initial position during the first phase of damping the person to be protected, the damping is due, on tho one hand, to the work involved in compressing tho combustion gases and, on the other hand, to tho work involved in drformi ng the inner surf ace of tlie outer piston, since tlx? balls tend to penetrate into this surface by pressing against the conical bearing surface of the central piston, this penetration being produced by the thrust cither of an auxiliary piston as in Figure 5 or of a mobile clement driven by the auxiliary piston as in Figures h and 6. The damping due to this deformation can represent a large fraction of the total damping when tlie auxiliary piston has a large surface area subjected to the pressure of the gases.

In Figures 7 and 8, which give respectively the pressure and tlie resisting force exerted on the belt, as a function of time, the origins 0 correspond to the instant of firing the ignition powder, the instants T^ correspond to the actuation of the jamming Mechanism of the central 40270 pinion, the i ii.m I,-in In T correspond 1o the ccntral piston arriving nt i In initial position ai'tcr tho first damping phase and tho instants correspond to the end of the damping process. Tho indices of the particular valuos Pn 5 and F^ indicated on the ordinntos correspond to the indices of tho time instants considered. The ordinntes 1' (pressure) and F (force) arc on a linear scalo whilst the scale of time on the abscissae are logarithmic, the correspondence of the instants Tn with the time base being 10 only exemplary and depending essentially on the collision conditions which determined various conditions of operation.

Figure 7 and 8 show the way in which tho pressure and the resisting fortfc change in a device produced in accordancc with the example of Figure 6, all the other components 15 being identical to those of Figure k. In Figure 7t the portion of curve A corresponds to the pressure which prevails in the damping chambcr and the portion of curve I) reprerent:; tlie pressure in tho expansion chambcr, the maximum difference A l'w determining the strengths of the frangible M 20 retaining elements such as the rings with lugs, which hold the outer piston during the initial displacement of the central piston backwards. In Figure 8, the curve in a full line repx-esents the way in which the forcc resisting slackening of belt due to the work done in compressing the 25 gases and to the work done in deformation during damping changes, and the curvc drawn in a broken line illustrates the way in which the resisting force due solely to the work done in compressing the gases changes. The sharp rise occurring nt the instant T^ is due to the additional 30 force required to shear the lugs of the frangitle retaining clciiicnt. - 16 -

Claims (1)

1. 40276 CI.A IMS: 1 . A tensioning devicc such as for use ir. tensioning n vehicle safety belt comprising n casing a first piston axially movable in a cylinder defined within '■ the casing, which piston is firmly fixed to an axially extending rod having means for attaching an element to be tensioned, means for supplying gas at high pressure to an expansion chambcr defined between the first piston and a front end wall of the cylinder, l<) whereby, in use, tho said gases cause the piston to travel in a first direction along tho cylinder and exert a tensioning force on nn element attached to the rod, an auxiliary piston included in the first piston .structure and held in an initial position by 1 ' a frangiblc element adapted to fracture when the pressure in the expansion chamber reaches a predetermined value, the pressure prevailing in the expansion chamber being conmiuniciitcd to the auxiliary piston via at least one permanently open passage way, and piston stopping !0 means actuated in response to displacement of the auxiliary piston away from the initial position following fracture of the frangible c;l en.ent, to prevent movement of the first piston relative to the cylinder in the said first direction, but allow movement of the first piston r> relative to the cylinder in the opposite direction. 2. A devicc according to claim 1, wherein 1ho piston stopping means comprises a jamming mechanism including a support surface, a plurality of elements in contact with the support surface and positioned between a port.ion - 17 - 4027G of the first piston having a conical bearing surface and tlie ituicr surface of the cylinder, and an clastic retaining component for holding the olements in a position which permits free movement of the piston > relative to tho cylinder, the support surface being movable by the displacement of the auxiliary piston to jam the elements between the conical bearing surface and the inner surfaco of tho cylinder to prevent movement of the first piston relative to the > cylinder in said one direction. 3. A dcvice according to claim 1 or 2, wherein the auxiliary piston is positioned axially within the first piston, and the auxiliary piston includes two axially adjacent cylindrical parts of different diumctcrs connected by a radial shoulder whicli is subjected to the pressure prevailing in the expansion chamber which pressure is communicated thereto by at least one permanently open orifice. h. A devicc according to claim 1 or 2, wherein tho auxiliary piston is annular and is positioned at the periphery of the first piston, and the auxiliary piston is subjected to the pressure prevailing in the expansion chambcr which pressure is communicated thereto via at least one permanently open orifice. 5- A device according to any one of clainisl to h, wherein tlie frangible element is an integral part of the auxiliary piston, which part fractures to divide the - 18 - 4027G .-in* i I i iirv piston i ni o two port i , mi<> portion ol tin- I'i-.-ic I.iii-imI pi.slou hciiiy. movahli- i t> iiriilalr ili«; piston mtopping means. C>. A device according to any ono of claims 1 to <; 'l, wherein the auxiliary piston is held in the initial position by separate,frangible retaining elements and displacement of the entire auxiliary pi.ston when the element is fractured actuates the piston stopping means. I" 7. A devicc according to claim 1, wherein the means for supplying gas to the expansion chamber comprises at least one annular pyrotechnic charge located within the casing and means for igniting tho charge. 0. A device according to claim 7» wherein tho IS pyrotechnic charge has a surface area of combustion which i ncreai:o.s during conihnsti on of tho charge. 9. A device according to claim 8 wherein the means for igniting the charge comprises a gas-free powder• ;><) JO. A devicc according to any of claims 1 to 9, wherein the cylinder is consti tuted by an outer piston which is axially movable within the casing, the outer surface of the outer piston partially defining a first damping chambcr, and the outer piston having - 19 - 40276 means for preventing movement of tho outer piston rolatjvc to tho casing in one direction of movement of the outer pi.ston relative to the casing, which direction corresponds to return movement of the outer piston towards an initial position relative to the casing. 11. A tensioning dcvice constructed and arranged to operate substantially as herein described with reference to the accompanying drawings. t Dated this the 2nd day of December, 1974« F. R-. KELLY & CO. BY: - EXECUTIVE 27 Clyde Road^--Bailsbridge, Dublin 4> AGENTS FOR THE APPLICANTS. - 'AO -