WO2005002676A1

WO2005002676A1 - Energy absorber

Info

Publication number: WO2005002676A1
Application number: PCT/GB2004/002885
Authority: WO
Inventors: Julian Elwyn Renton; Peter Thomas Mence Nott
Original assignee: Keyguard Limited
Priority date: 2003-07-03
Filing date: 2004-07-05
Publication date: 2005-01-13
Also published as: GB0315538D0

Abstract

An energy absorber comprises housing and a length of plastically deformable material at least partly contained within the housing. The length of plastically deformable material is secured to the housing at one end and has load-applying means at the opposite end and is folded back on itself to provide a radial fold between the first and second ends. The length of plastically deformable material is constrained by the housing such that when the load applying means is subjected to a load relative to the housing above a threshold value the location of the radial fold moves along the length of plastically deformable material and relative to the housing, thereby absorbing energy by plastic deformation of the material.

Description

ENERGY ABSORBER

This invention relates to an energy absorber and particularly to an energy absorber suitable for use in a fall arrest system.

Energy absorbers are generally intended to control load and absorb energy, typically in the event of a dynamic arrest such as a fall arrest, in order to minimise damaging effects and to assist in protecting machinery and_^preventing loss of life and serious injury. Typically, the energy absorber deploys irreversibly when used. In particular, the invention is intended to provide an energy absorber that has reliable performance characteristics even after relatively long periods of time between initial installation and ultimate deployment and yet, is cost effective to manufacture and compact when installed.

Applications for the invention include a wide variety of situations where people are involved in a crash or a fall from height and where loads need to be controlled to mitigate as far as possible against serious injury or even death. In some instances the energy absorber is used to safeguard a structure that could threaten injury to a person. In other instances it may be connected directly between a person or persons and a structure to limit load, and in other instances a number of energy absorbers could be used to perform a variety of load limiting tasks that ultimately have the effect of controlling and often limiting load on a person or persons in the event of catastrophe. Other applications involved the protection of various forms of equipment against damage that may or may not lead to injury to a person or persons. The invention could be used for the protection of animals.

There are a number of existing irreversible energy absorbers intended to control and typically limit load in the event of a crash or fall but these tend to be relatively complex, expensive to manufacture and are not suitable for applications where space is limited. Many existing absorbers are manufactured from polymer fibre webbing and rely on stitching to fail progressively to deploy the absorber. A problem with this type of absorber is that the polymer webbing and/or stitching is vulnerable to wear, weathering and/or accidental damage and so is only suitable where the absorber can be inspected and replaced regularly.

In a first aspect, this invention provides an energy absorber comprising a housing and a length of plastically deformable material at least partly contained within the housing; the length of plastically deformable material having a first end secured to the housing and a second end having load applying means and being folded back on itself to provide a radial fold between the first and second ends; the length of plastically deformable material being constrained by the housing such that if the load applying means is subjected to a load relative to the housing above a threshold value the location of the radial fold moves along the length of plastically deformable material and relative to the housing, thereby absorbing energy.

According to the present invention there an energy absorber is formed by a length of • substantially plastically deformable material that is folded back on itself typically through an angle of or near to one hundred and eighty degrees about an axis perpendicular to and near to or at the middle of the length of the said length of material and whereby the folded part is formed about an internal radius or a form that approximates to an internal radius, and the length of folded material is partially enclosed within a housing such that movement of the folded material relative to the housing is substantially constrained in the plane that is perpendicular to the length of the folded material and, in particular, in directions perpendicular to the axis of the fold, there is also a means for stopping movement of a first end of the length of folded material relative to the housing in a direction that is typically parallel to the length of the folded material when a tensile load is applied between the second end of the folded material and the housing, the dimensional geometry of the length of folded material and the geometry of the folded portion being such that when the magnitude of the said tensile load is sufficient to cause plastic deformation of the length of folded material such deformation occurs in the region of the fold causing the folded form to move relative to the length of material as a result of yielding and subsequent deformation of the substantially plastic deformable material, towards the said first end of its length such that the axis of the fold also moves and in a direction parallel to the length of the length of folded material and substantially in the line of the direction of the applied tensile load, such movement of the folded axis resulting in movement of the second end of the length of folded material relative to the housing and therefore effectively deploying the energy absorber to absorb energy whereby such energy absorption is provided by movement of the said second end and the load resisting movement of the second end such load being at least partially due to plastic deformation of the folded material.

In some embodiments the invention can absorb energy as a result of compressive loading between the said length of folded material and the said housing, either in addition to or instead of the ability to absorb energy as a result of a tensile loading between the said length of folded material and the said housing, by reversing movement of the folded material relative to the housing such that the movement of the said first end of the length of material is limited by limiting means relative to the housing and the compressive load is applied between the length of folded material and the housing whereby such load is applied to the length of folded material either at its said second end or close to the axis of the fold. Such an embodiment may include a roller to minimise inconsistent frictional effects such that the roller can roll about an axis that is substantially coincident with the axis of the fold and also whereby the axis of the roller can move with the axis of the fold or alternatively, a feature within the housing that may or may not include a roller that could bear on the outside surface of the folded form relative to the substantially internal radius of the folded form, such that when a compressive load is applied between both the length of material and the housing, movement of the said second end of the length of material relative to the housing together with the load resisting such movement largely due to plastic deformation of the folded part of the length of folded material provide energy absorption.

In any embodiment, the length of folded material could have any length and any cross sectional geometry in terms of shape and size. Whilst in typical embodiments where resistance to applied loads is required to be substantially constant throughout deployment of the energy absorber the cross sectional geometry should be substantially constant along the length of the length of material, other embodiments may require the load resisting deployment of the energy absorber to vary during deployment and this may require the length of material to have a cross sectional geometry that varies along to the length of the length of material. Also, the substantially radial form about which the length of material is folded could be arranged to vary during deployment of the energy absorber by varying the respective constraining features within the housing such that if the radial form was allowed to decrease in size during deployment then the load resisting deployment would correspondingly increase in size because of the increase in plastic deformation, the converse being a decrease in the resisting load if the internal diameter of the folded form was constrained by the housing to increase during deployment of the energy absorber.

Some-embodiments-may include more than one length of substantially plastically - deformable folded material whereby each length is partially enclosed in one or more housings. In some such embodiments, typical embodiments with one length of substantially plastically deformable folded material partially enclosed within a housing may be effectively attached together in series and/or parallel whereby the said second ends of folded material may be joined or whereby the housings may be joined or whereby such joined housings may be amalgamated to form one housing or whereby the housing of one absorber is attached to the said second end of another absorber. In any such embodiment, the tensile or compressive load applied to and resisted by the energy absorber is typically attached to suitable locations on either end of the chain of joined absorbers and in some embodiments, this maybe the second end of one absorber at one end of the chain and the second end of another absorber at the other end of the chain particularly where housings are joined or amalgamated.

In some embodiments, the said first end of the length of folded material maybe securely attached to the housing in order to provide a secure end stop in the event that the absorber is completely deployed. This is a requirement for compliance with official Standards in the a number of safety applications where equipment should be able to withstand at least twice the maximum applied loads in practice during deployment of the energy absorber in order to provide a margin for possible error.

In some embodiments, the said length of folded material and the said housing could be amalgamated into one component.

In any embodiment, it may be beneficial to control friction between the mating surfaces of both the length of folded material and housing particularly close to the folded form and along the length towards the said second end where the length of folded material moves relative to the housing when the absorber undergoes deployment. Friction could be controlled in various way including the application of surface coatings on one or both respective surfaces or, alternatively, providing a separate component made of a material with suitable friction properties that could be positioned between such mating- surfaces and constrained such that it does not move relative to the housing when the absorber deploys or whereby, it is free to move relative to the housing when the absorber deploys but also has length suitable to ensure that is remains between the said mating surfaces throughout deployment of the absorber until deployment is complete.

Embodiments of the invention will now be described by way of example only with reference to the accompanying diagrammatic figures, in which:

Figure 1 shows an isometric view of a first embodiment of the invention before deployment;

Figure 2 shows a sectional elevation of the embodiment in Figure 1 ;

Figure 3 shows an isometric view of the embodiment in Figure 1 but with the energy absorber having been deployed;

Figure 4 shows a sectional elevation of the isometric view in Figure 3; Figure 5 shows an isometric view of a second embodiment of the invention where the length of substantially plastically deformable material and the housing are made from one component;

Figure 6 shows a sectional elevation of the embodiment in Figure 5;

Figure 7 shows a sheet of material from which the embodiment in Figures 5 and 6 has been made;

Figure 8 shows an elevation of a third embodiment with two lengths of substantially plastically deformable material in one housing before deployment, the housing alone being shown as a sectional elevation;

Figure 9 shows an elevation of the embodiment in Figure 8 after deployment, the housing alone being shown as a-sectional elevation; -

Figure 10 shows an elevation of a fourth embodiment with two lengths of material in one housing before deployment, the housing alone being shown as a sectional elevation;

Figure 11 shows an elevation of the embodiment in Figure 10 after deployment, the housing alone being shown as a sectional elevation;

Figure 12 shows an elevation of a fifth embodiment where the length of substantially plastically deformable material has two folds, the housing alone being shown as a sectional elevation;

Figure 13 shows an isometric view of a sixth embodiment prior to assembly;

Figure 14 shows an isometric view of the embodiment in Figure 13 after assembly;

Figure 15 shows a sectional elevation of the embodiment in Figures 13 and 14; Figure 16 shows an elevation of a seventh embodiment where the fold geometry varies during deployment and where the housing alone is shown as sectioned.

Figure 17 shows an elevation of an eighth embodiment where the fold geometry varies during deployment and where the housing alone is shown as sectioned.

In Figures 1 and 2 a first embodiment of an energy absorber is shown. The energy absorber is formed by a length of substantially plastically deformable material 1. The length of material 1 comprises first and second substantially flat and straight sections la and lb respectively, each extending in a longitudinal direction, linked by a fold 2. The material 1 is folded back on itself at the fold 2 through approximately one hundred and eighty degrees to provide a radial fold having a substantially internal radial form 3. The fold 2 has a substantially smooth internal curved surface having a substantially constant radius about an axis perpendicular to the surfaces and longitudinal directionsof the-straight sections la-and lb of the length of material 1-. . - -. -

A first end 6 of the length of material 1 is attached to a housing 8. This attachment may be by means such as spot welding applied at a point 7. A second end lc of the length of material 1 opposite the first end has a first attachment means 4 suitable for attachment to a tensile load. The length of material 1 is partially enclosed within the housing 8 such that the second end lc of the material 1 with the first attachment means 4 emerges from one end of the housing 8. At the other end of housing 8 remote from the second end lc of the material 1 with the first attachment means 4 there is provided a second attachment means 10 suitable for attachment to a tensile load.

The length of material 1 is constrained from movement in the housing 8 such that when a tensile load is applied between the first and second attachment means 4 and 10, typically in the directions of arrows 5 and 11, and this applied tensile load is sufficient in magnitude to cause plastic deformation of the length of material 1 at the fold 2, the energy absorber will deploy by movement of the first section la of the length of plastically deformable material 1 relative to the housing in the direction of the applied tensile load. That is, the direction of the arrow 5 in the figures. Thus, the deployment of the energy absorber will occur at a predetermined value of applied tensile load exceeding a threshold value. During the deployment, as a result of this movement of the first section la of the length of material 1, the axis of fold 2 effectively moves in the direction of arrow 5. That is, the material of the second section lb of the length of material 1 is plastically deformed into the fold 2 and material in the fold 2 is plastically deformed to straighten out of the fold 2 and become part of the first section la. As a result of this plastic deformation, energy is absorbed. The fold 2 will move relative to the housing at half the speed of the first section la of the length of material 1.

In this embodiment, housing 8 constrains the movement of the length of material 1 such that the radial form 3 of the fold 2 remains substantially constant during this plastic deformation. As the axis of fold 2 moves, plastic deformation takes place in the length of material 1 progressively along its length towards its first end 6 whereby such deformation resists the applied tensile load. The combination of such resistance due to plastic deformation together with the resulting movement of the first attachment-means- 4 relative to the second attachment means 10 on the housing 8 absorbs energy. Also, because the internal radial form 3 of the fold 2 remains substantially constant, if the cross section of the length of material 1 also remains substantially constant, the resisting load due to plastic deformation remains substantially constant. This feature of a constant deployment load is particularly useful for limiting load in a dynamic situation such as arresting a person's fall from height or securing a person during some other accidental arrest, because the energy absorber extends to absorb energy whilst limiting load to a largely constant predetermined value. Also, energy absorption at a constant resisting force is inherently efficient in terms of minimising the deployment extension of the energy absorber required in order to absorb a desired amount of energy.

Figures 3 and 4 show the embodiment of Figures 1 and 2 after full deployment of the absorber. The axis of the fold 2 has moved towards the first end 6 of the length of material 1 until it has been stopped, and the deployment of the energy absorber arrested, by a rivet 12 that is fixed to the housing 8 and extends between the first and second sections la and lb of the length of material 1. An end stop arrangement such as that provided by rivet 12 and the spot welding at 7 between the first end 6 of the length of material 1 and the housing 8 is typically required for height safety products to comply with official standards which require that an end stop anangement should be provided sufficiently strong to withstand an applied tensile load that is at least twice the magnitude of the highest deployment load during deployment of the absorber. However, in other applications, an end stop with a lower strength may be sufficient. Alternatively, no end stop may be required and it may only be necessary to constrain the first end 6 relative to the housing 8 such that it is unable to move towards the applied load sufficiently for the deployment process described above to be carried out. This may be the case where the extent of the requirement for energy absorption is known and therefore, the energy absorber can be determined always to have sufficient capacity to absorb the required energy.

The first section la of the plastically deformable material 1 in the described embodiments is not plastically deformed during- deployment of the energy absorber. - ^• This first section 1 a acts only as a load path connecting the attachment means 4 and the fold 2. As a result, the relatively long first section la shown in the embodiments could be much shorter with a separate connecting means linking the fold 2 to the attachment means. Such a connecting means could have a different cross section or be formed from different material to the lenngth of material 1. In this case the attachment point of the connecting means to the first section la can be regarded as the attachment means. Although the use of such a separate connecting means is possible, it is expected that the use of a long first section la extending out of the housing will usually be preferred on the grounds of simplicity and ease of manufacture.

Figures 5, 6 and 7 show a second embodiment of the invention. In the second embodiment, a length of substantially plastically deformable material 16 and a housing 15 are made from one single unitary piece of material 18 by folding. A suitable blank for the piece of material 18 is shown in figure 7.

The length of material 16 has first attachment means 20 suitable for attachment to a tensile load and a radial fold 17, folded at point 22 on the piece of material 18 to provide the fold with an internal radial form that undergoes plastic deformation in order for the absorber to deploy as described with reference to Figures 1 to 4. The length of material 16 is also attached to housing 15 in the region of roll form 23. Housing 15 is then completed by folding operations at points 24, 25 and 26 as shown in Figure 7 and a fixing means 19, such as spot welding is used to fix the housing 15. As a result, the radial form of fold 17 is constrained to remain substantially constant during deployment of the absorber by the housing 15. The folding operations at points 24, 25 and 26 are straightforward angular folds to form the desired shape of the housing 15 and not radial folds like the fold 17.

The roll form 23 connecting the length of material 16 to the housing 15 is used to provide an end stop. The use of roll form 23 instead of an angled fold prevents the material being weakened or snapped by excessive folding moments about the connection as the energy absorber reaches the end of it's deployment.

In the first and second embodiments of Figures 1 to 4 and 5 to 7, the loads between the length of material 1 and 16 and the housing 8 and 15 respectively can be particularly high in the region of the respective folds 2 and 17 where the housing is constraining the substantially radial form of the fold to remain subsantially constant during movement of the fold axis during deployment of the energy absorber. In practice, it has been found that such high loading can lead to inconsistent frictional loading and can occasionally cause further problems such as binding together of the mating surfaces due to occasional welding. This can result in the applied tensile load required to start or continue deployment of the energy absorber varying unpredictably.

When this occurs, in order to alleviate the problem, it has been found that it is beneficial in most cases to apply a friction controlling surface coating and/or plating to one or more of the opposed contact surfaces. Such a coating or plating is intended to control the friction such that the frictional loading, and thus the required applied deployment load is constant. A wide variety of types of coating, plating or other surface treatments can be used. In general the energy absorber is only intended to be used once before replacement, so the coatings that may be designed to withstand only one mechanical operation such that they are sacrificially destroyed on deployment of the energy absorber.

This high loading is mainly a problem at the contact point between the first section la of the length of material 1 and the housing 8, and the conesponding point in other embodiments, where there is relative movement of the material 1 along the surface of the housing 8. Accordingly, it may be sufficient to treat only the surface of the housing 8 contacted by the first section la of the length of material 1 during deployment.

Figures 8 and 9 show a third embodiment of the invention. In the third embodiment, there are two similar lengths of substantially plastically deformable material 27 and 28. Both of the lengths of plastically deformable material 27 and 28 are partially enclosed in a housing 29 and are separated along their lengths by a central wall 34 of the housing 29. Alternatively, the unitary housing 29 could be replaced by two separate housings joined -together. - —

The two lengths of material 27 and 28 each have a respective fold 33 and 32. The two lengths of material 27 and 28 are disposed such that a tensile load can be applied between respective second ends 27c and 28c of each the lengths of material 27 and 28, the other, first end 27d and 28d of each of the lengths of material 27 and 28 being attached to the housing 29 at fixings 35 and 36 respectively. The fixings 35 and 36 are shown as being rivetted in the figures, but any other suitable fixing method could be used.

As with the previous embodiments, when the applied tensile load reaches a magnitude sufficient to cause plastic deformation at folds 32 and 33 the axes of the folds 32 and 33 effectively move along the lengths of material 27 and 28 between the said folds and the first ends 27d and 28d fixed to housing 29 at 35 and 36 respectively such that the combination of the resisting force due to plastic deformation and the extension as a result of the movement of the fold axes relative to housing 29 absorb energy. This takes place at a substantially constant resisting force due to the radial form of the said folds 32 and 33 remaining substantially constant as a result of material 27 and 28 being constrained within the housing 29 similarly to the previous embodiments, provided that the cross section of the two lengths of material 27 and 28 is substantially the same and constant throughout their lengths. The operation and deployment of each of the lengths of material 27 and 28 is unaffected by the presence of the other. The main benefit of the third embodiment of Figures 8 and 9 is that large deployment extensions can be achieved from a relatively compact absorber prior to deployment.

In the third embodiment, lengths of consistent friction material 39 and 40 are securely attached to housing 29 on either side of central wall 34, between the central wall 34 and the lengths of material 27 and 28 respectively, to prevent binding between the material 27 and 28 and the mating surfaces either side of wall 34. Such binding prevention could alternatively be provided by a suitable surface treatment as described above. Alternatively, the housing 29 could be made from a material that has suitably consistent friction properties. Such lengths of constant friction material or a housing of constant friction material could also be used in the other embodiments of the - • - invention.

Figure 9 shows the energy absorber of the third embodiment after complete deployment of the absorber. Figure 9 shows operation of an end stop arrangement provided by pins 37 and 38 that are fixed to housing 29 and also by rivets 35 and 36 that secure the respective ends 27d and 28d of the lengths of material 27 and 28 to the housing 29.

Figures 10 and 11 show a fourth embodiment of the invention. The energy absorber according to the fourth embodiment has two lengths of substantially plastically deformable material 42 and 43 partially enclosed within a housing 44. In this arrangement, the two lengths of substantially plastically deformable material 42 and 43 are disposed so that they are in contact and each is arranged as a reflection of the other about their plane of mutual contact. Each length of material 42 and 43 is arranged similarly to the previous embodiments, having respective folds 51 and 52 and being attached at respective first ends 53 and 54 to the housing 44 by attachments 57 and 58. The second ends 45 and 46 respectively of the two lengths of material 42 and 43 are directly adjacent one another and have a shared load attachment means 50 for an applied tensile load in the direction of arrow 47.

When a tensile load is applied between the shared load attachment means 50 of the two ends 43 and 42 and a load attachment means 48 on one end of the housing 44 in the directions of arrows 47 and 49 and such tensile load is sufficiently large to cause plastic deformation at folds 51 and 52 on material lengths 42 and 43 respectively, the axes of folds 51 and 52 move together towards the first ends 53 and 54 such that there is no relative sliding movement between the contact surfaces of the lengths of material 42 and 43 and the housing 44. This lack of relative sliding movement avoids the need to resolve any possible friction problems, which may be encountered in the previous embodiments. The movement of the folds 51 and 52, and thus the deployment of the energy absorber, is stopped by respective rivets 55 and 56 and the attachments 57 and 58 respectively.

Figure 12 shows a fifth embodiment. The energy absorber of the fifth embodiment has one length of substantially plastically deformable material 60 that has three folds 70, 71 and 73. The folds 70 and 71 have an internal radial form with axes that effectively move on deployment of the absorber. The fold 73 does not have to have such an internal radial form, and is not constrained to maintain it's form as the energy absorber is deployed. However, it is prefened for the fold 73 to be a rolled or radial fold and not an angular fold in order to prevent failure of the material 60 due to excessive bending moments at the fold 73. This embodiment has a housing 61 which is effectively divided by a wall 72 with the fold 70 on one side and the fold 71 on the other side. When a tensile load is applied between an attachment means 75 at a second end 64 of the material 60 and an attachment means 66 of the housing 61, and such tensile load is sufficient to cause plastic deformation at fold 70, the axis of the fold 70 effectively moves in a direction substantially parallel to anow 65 until the fold 70 emerges out of the housing 61. The fold 70 then straightens and the fold 73 then also straightens. This enables the tensile load to begin plastic deformation at the fold 71, causing the axis of the fold 71 to move towards a pin 68 that is fixed to the housing 61. A first end 74 of the material 60 is fixed to the housing 61 by a fixing means 69 such that deployment of the material 60 from the housing 61 is stopped when the fold 71 reaches the pin 68. This embodiment provides a compact absorber with a relatively high extension capability as with the embodiment in Figures 8 and 9. However, this arrangement has the disadvantage that the resistance provided against the applied tensile load is significantly reduced for a short proportion of deployment when the fold 70 emerges from the housing 61 before it is restored when the resistance transfers to moving the axis of the fold 71.

In this embodiment, lengths 62 and 63 of consistent friction material are disposed between surfaces of the material 60 and the housing 61 to avoid the effects of inconsistent friction, as already mentioned. Alternatively, friction could instead be controlled by surface coatings or by making the housing 61 out of consistent friction material, as discussed above.

Figures 13, -14 and -15 show a sixth embodiment of the invention in which the energy- absorber is formed by only two components. The first component is a length of substantially plastically deformable material 76 having a fold 81 and the second component is a housing 77, within which the material 76 is partially enclosed. Assembly of the two components is shown in Figure 13. This is carried out by simply inserting the material 76 in the direction of arrow 80 into the housing 77 until a pair of stops 78 and 79 provided on material 76 prevent further insertion relative to housing 77. Figure 14 shows the assembly completed and Figure 15 shows a sectional elevation of Figure 14.

When a tensile load is applied between a load attachment 85 on material 76 and a load attachment 84 on the housing 77, such load being sufficient to cause plastic deformation at a fold 81 on the material 76, the axis of the fold 81 moves substantially in the direction of arrow 83, thereby resisting the tensile load and absorbing energy. A first end 86 of the material 76 is located against the housing 77 to prevent movement relative to the housing 77 when a tensile load is applied by the pair of stops 78 and 79 that abut on the housing 77. This embodiment has the advantage of simplicity, comprising only two components that are easily assembled. Figure 16 shows a seventh embodiment of the invention. In the seventh embodiment, a length of substantially plastically deformable material 90 is folded about a substantially radial form at a fold 93. In this embodiment, the said fold 93 is through an angle greater than one hundred and eighty degrees. The length of material 90 is partially enclosed within a housing 91. The housing 91 has tapered sides 91a and 91b with a taper angle which coincides with the angle of the fold 93. When a tensile load is applied between a load attachment means 99 at a free second end 97 of the material 90 and a load attachment means 98 provided at an end of the housing 91, typically in the direction of arrows 94 and 95, and such tensile load is of a sufficient magnitude to cause plastic deformation at the fold 93, the axis of the fold 93 tends to move substantially in the direction of arrow 95. However, because the sides 91a and 91b of the housing 91 are tapered, as already mentioned, the radius of the substantially radial form of fold 93 is constrained to decrease. As a result of this decrease in radius, the resistance to the tensile load due to plastic deformation, and thus the applied tensile load required to continue the deployment of the energy- absorber,- progressively - increases as the absorber deploys as a result of the greater deformation needed in order to deform the material 90 through a fold about a smaller radius. A length of friction control material 96 may be provided if required.

In this embodiment, the first end 90d of the material 90 that is not attached to a tensile load is arrested against an abutment 92 of the housing 91, which prevents the first end from movement substantially in the direction of anow 95.

This embodiment is useful in applications requiring the resisting load of the absorber to have an increasing profile with respect to its deployment. For example, in many safety applications, most accidents require lower levels of energy absorption than have been generally provided for, so it can be sensible to ensure that loading on a person or persons in such situations is also limited to relatively low levels. However, in more serious accidents that happen rarely, it may be desirable for load levels to be raised slightly in order to increase energy absorption capacity. Figure 17 shows an eighth embodiment that is similar to the seventh embodiment of Figure 16 but reversed to require a reducing deployment load. In this embodiment a length of material 101 with a fold 100 having an angle of less than one hundred and eighty degrees is used, constrained within a housing 102 which has sides l^'02a and 102b which diverge at a corresponding angle. As a result, the radius of the substantially radial form of the fold 100 in the material 101 tends to increase as the fold moves substantially in the direction of arrow 103, subject to the application of sufficient tensile loading. Accordingly, the resistance to the tensile load due to plastic deformation at the fold will tend to decrease as the energy absorber deploys as a result of less deformation being required in order to deform the material about a fold having a greater radius. The length of material 101 has a load attachment means 105 at a second free end 106, and is constrained against movement relative to the housing 102 by having a second end 107 bearing against an abutment 108 of the housing 102. A length of friction controling material 109 may be provided, if required.

In any of the described embodiments, the length of substantially plastically deformable material can be any material provided that it can undergo at least partial plastic deformation under sufficient applied load. Also the cross section geometry relative to the length of material can be any shape and size, although rectangular and circular shapes are likely to be preferred with a view to ease of manufacture.

The cross section is typically constant throughout the length of the material, and this is the case in the disclosed embodiments. However, if desired the cross section geometry could vary along the length of the material in order to vary the resisting load at various stages of deployment of the absorber. It would also be possible to vary the characteristics of the material, but in practice it will usually be simpler to vary the cross section geometry. In general, if the cross sectional shape of the material is constant, increasing cross sectional area in the region of the fold will have the effect of increasing the resisting load due to plastic deformation whilst a decrease in cross sectional area would tend to have the effect of reducing the resisting load. The cross sectional shape could also be varied to change the resisting load. In any of the embodiments, the end of the length of substantially plastically deformable folded material that is constrained not to move relative to the housing on deployment of the absorber may be fixed to the housing to provide a secure end stop. Alternatively, if a secure end stop is not required, the end may be constrained by any means such that is does not move relative to the housing on deployment of the absorber.

In the embodiments the energy absorber operates to absorb energy from an applied tensile load. The energy absorber designs could also be used to absorb energy from an applied compressive load, if desired, by reversing the direction of movement of the fold during deployment. However, is expected that it will usually be prefened to anange the energy absorbers to be subject to applied tensile loads. This is because it is generally more difficult to control and constrain plastically deformable elements when subject to compression loads than when subject to tensile loads.

The embodiments described above are by way of example only and the skilled person will be able to envisage changes to them which remain within the scope of the invention as defined in the appended claims. In general, features disclosed in relation to any of the embodiments may be used with the other embodiments.

Claims

1. An energy absorber comprising a housing and a length of plastically deformable material at least partly contained within the housing; the length of plastically deformable material having a first end secured to the housing and a second end having load applying means and being folded back on itself to provide a radial fold between the first and second ends; the length of plastically deformable material being constrained by the housing such that if the load applying means is subjected to a load relative to the housing above a threshold value the location of the radial fold moves along the length of plastically deformable material and relative to the housing, thereby absorbing energy.

2. The energy absorber according to claim 1, in which the second end of the plastically deformable material also moves relative to the housing.

3. The energy absorber according to claim 1 or claim 2, in which the length of plastically deformable material is constrained to maintain the radius of the radial fold substantially constant as it moves.

4. The energy absorber according to claim 1 or claim 2, in which the length of plastically deformable material is constrained such that the radius of the radial fold reduces as it moves.

5. The energy absorber according to claim 1 or claim 1, in which the length of plastically deformable material is constrained such that the radius of the radial fold increases as it moves.

6. The energy absorber according to any preceding claim, in which the energy absorber comprises multiple lengths of plastically deformable material.

7. The energy absorber according to any preceding claim, in which the or each length of plastically deformable material is constrained by contact with a surface of the housing.

8. The energy absorber according to claim 6, in which each length of plastically deformable material is constrained by contact with a surface of the housing and contact with another one of the lengths of plastically deformable material.

9. The energy absorber according to claim 7 or claim 8, in which at least one of the surface of the housing and the length of plastically deformable material has a friction controlling part where they are in contact.

10. The energy absorber according to any preceding claim and arranged to absorb tensile energy by movement of the radial fold towards the first end of the length of plastically deformable material.

11. The energy absorber according to any one of claims 1 to 9 and arranged to absorb compressive energy by movement of the radial fold towards the second end of the length of plastically deformable material.