GB2319581A

GB2319581A - Vehicle shock-absorber device

Info

Publication number: GB2319581A
Application number: GB9723420A
Authority: GB
Inventors: Claude Dehennau; Sandrine Hellinckx; Jean Schoemans
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1996-11-22
Filing date: 1997-11-05
Publication date: 1998-05-27
Anticipated expiration: 2017-11-05
Also published as: GB2319581B; GB9723420D0; NL1007592A1; FR2756340A1; BE1010760A3; NL1007592C2

Abstract

Device for absorbing frontal impact shocks, comprises a plastic-based hollow element 1 and a piston 2 designed so as to be able to move in the said hollow element, deforming it at least radially. Preferably, the hollow element is reinforced in the circumferential direction by one or more bundles of continuous reinforcing fibres wound on its surface, and the piston is designed so that its movement inside the hollow element causes fracture of the latter.

Description

Shock-absorber device The present invention relates to a shock-absorber device, in particular for motor vehicles. It is important for motor vehicles to have a high level of safety, but to achieve this while avoiding an excessive increase in volume, weight and cost. Moreover, it is desirable for as great as possible a proportion of the materials of a vehicle to be reusable or recyclable.

Reversible - i.e. multiple-usage - shock-absorbing devices are known in many fields. These are, for example, devices comprising pneumatic or hydraulic dampers or springs. However, the latter devices are complex and pose reliability problems (risk of leaks, etc.) . Furthermore, most reversible absorbers of this type are only effective in the case of slight shocks, because of their low energy-absorbing capacity.

Greater effectiveness may be achieved by making use of irreversible (single usage) shock-absorbing devices. Many types of these have already been proposed and applied in the vehicle field; in this case, they are generally interposed between the bumper and the chassis of the vehicle.

Thus, for example, Patent US 5,427,214 describes a device comprising two metal tubes, one able to slide in the other. A shock causes relative sliding of these two tubes so that a narrowed cross-section of the outer tube, strengthened by a ring, radially compresses the inner tube which deforms plastically and thus partially absorbs the energy of the shock. This device is complex to manufacture and it probably operates ineffectively insofar as the energy absorption is based on radial deformation, in compression, of the inner tube, this being less reproducible and making less use of the strength of the material than stressing in tension.

As regards Patent US 4,601,367, this describes a composite tube manufactured by the helical winding of several plies of tapes composed of reinforcing fibres coated with a thermosetting resin, which tapes cross each other at a high angle, approximately perpendicularly to each other. In the event of a shock, this composite tube is compressed axially and the energy of the shock is partially absorbed by the delamination of the fibre tapes at the points where they cross each other, due to the effect of torsional forces. The energy absorption is therefore directly proportional to the total surface area of the intersections of the tapes, which is relatively low, unless a high number of tape plies is used. Furthermore, the fibres are thus stressed only in shear (torsion) and are therefore underexploited compared to stressing in tension. Finally, the use of a thermosetting resin complicates the manufacture and the recycling of this device.

Moreover, we should point out, in a more general manner, that stressing in compression usually creates a risk of buckling, and this results in the device operating with random effectiveness.

The present invention is designed to provide a simple, lightweight, effective, compact and inexpensive shock-absorber device, the use of which makes it possible, in particular, to avoid or reduce the oversizing of other parts of the vehicle, as well as to preserve their integrity in the event of a moderate shock. It aims to reach this objective by making use of a main element having a very simple geometry and based on a plastic, which has the advantage of not corroding and of being able to be recycled. To be sure, plastic bumpers are widely used, but they are capable of only absorbing lowenergy shocks, such as those which may occur during manoeuvres at low speed, typically at most 8 km/h in the case of medium-sized cars. The present invention extends the field of application of plastics to absorbing the energy of more violent shocks such as, for example, those occurring at speeds of about 10 to 20 km/h.

More specifically, the present invention relates to a device for absorbing shocks, which comprises a plastic-based hollow element of elongate shape and a piston designed so as to be able to be forced into the said hollow element, deforming it at least radially.

The hollow element may have any cross-section, for example a polygonal, oval or circular cross-section.

Preferably, the hollow element has a circular crosssection. Its transverse dimensions can be varied or be constant along the axial direction. By way of examples, a hollow element of circular cross-section may have, as required, a cylindrical shape (having a constant diameter) or a frustoconical shape (having a diameter which varies linearly). Preferably, the transverse dimensions of the hollow element are constant, thereby simplifying its manufacture.

The hollow element comprises a plastic. The term plastic is understood to mean any thermosetting or thermoplastic polymer, or blend of such polymers.

Optionally, one or more conventional additives may be added to this polymer or polymers, such as lubricants, plasticizers, stabilizers, antioxidants, pigments, mineral fillers, reinforcing fibres, etc. Preferably, the plastic used for producing the hollow element comprises one or more thermoplastic polymers, which in particular offer advantages with regard to recycling, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc. Very good results have been obtained by using polyolefins. It is preferred to use PP, the heat resistance of which is superior to that of PE.

According to an advantageous variant, the hollow element is reinforced in the circumferential direction.

This circumferential reinforcement may, for example, be obtained by reinforcing fibres which are predominantly oriented circumferentially. These may, in particular, be short fibres which are dispersed within the wall of the hollow element, during its manufacture, and/or bundles of continuous fibres wound on the surface of a hollow mandrel after its manufacture.

Advantageously, the technique of filament winding is used, in which the hollow element is reinforced in the circumferential direction by one or more bundles of continuous reinforcing fibres wound on its surface. The "continuous" fibres used are in this case very long, for example, several tens of metres in length. Any type of fibre can be used: carbon fibre, aramid fibre, glass fibre, etc. It is preferred to use bundles of glass fibre. The said bundle(s) is (are) preferably wound in an almost circumferential manner around the hollow element, making an angle greater than 850 with respect to the axis of the hollow element. Furthermore, it is preferred for the turns which it (they) forms (form) to be touching.

When several bundles are used, it is advantageous to wind them alternately left-handedly and right-handedly. After the reinforcement step, the reinforcing-fibre bundles may advantageously be protected by applying a topcoat of plastic to the outer surface of the hollow element thus reinforced, for example by overextrusion. Such reinforcing-fibre bundles considerably increase the circumferential strength of the hollow element, and consequently its energy absorption capacity. Another advantage of the presence of these reinforcing-fibre bundles is to prevent the hollow element from suddenly cracking over a major part of its length in the event of a shock, or to prevent it from buckling.

It is particularly advantageous to use fibre bundles impregnated with a thermoplastic material (called hereafter "Cofits", standing for "Continuous fibre impregnated by a thermoplastic"). An advantageous manufacturing technique consists in reinforcing a hollow element by means of such a Cofit by heating both the latter and the outer surface of the hollow element, so as to improve their adhesion, and, preferably, by exerting radial pressure on the Cofit at its point of contact with the hollow element, or downstream of this point.

Preferably, the hollow element consists exclusively of plastic (in the above-defined sense, including optional additives) and of optional reinforcing fibres. Furthermore, according to an advantageous variant, the hollow element is, in particular, reinforced by short fibres dispersed within it, these being predominantly oriented parallel to its axis.

The piston, initially placed at one of the ends of the hollow element, is able to move with respect to the latter, into which it may be forced coaxially. The cross-section of the piston generally has the same shape as the internal cross-section of the hollow element. In particular, both may have a circular cross-section.

Furthermore, at least part of the piston must have transverse dimensions which are slightly greater than the internal transverse dimensions of the hollow element so that the piston cannot slide freely inside the hollow element without deforming it radially. It is preferable for the front end of the piston (defined with respect to its direction of insertion in the event of a shock) to be narrower than the hollow element, so as to guide it without any initial radial deformation.

According to an advantageous variant, the hollow element and the piston each have a circular cross-section and the piston is profiled so that its front end has a diameter smaller than the internal diameter of the hollow element (Dc) and so that at least one of its parts has a diameter greater than Dc. The deviation between the transverse dimensions of the front end of the piston and the internal dimensions of the hollow element is preferably from 1 to 10%.

It is advantageous for the piston to have no sharp edges, so as to prevent the piston from lacerating the wall of the hollow element and/or to prevent it jamming in the hollow element, in which case there would be a risk of the effectiveness of the device of the invention being reduced. For this reason, it is preferred for the dimensions of the piston to vary gradually along its axis. In the case of a hollow element of circular cross-section, the piston may, for example, have a frustoconical shape, its small diameter being less than the internal diameter of the hollow element and its large diameter being greater than it.

The principle of operation of the device of the invention is that, in the event of a shock, the piston is forced into the hollow element and that its progression therein is essentially counteracted by the radial forces deforming the hollow element. These radial forces exerted by the piston on the hollow element create circumferential tensions within the wall of the latter. The fact that the wall is thus predominantly stressed in tension significantly reduces the risks of buckling, to which many previously known devices have been exposed.

According to an advantageous variant, the piston is designed so that its insertion into the hollow element does not cause complete fracture of the latter. This is because complete fracture of the hollow element, even if it occurred initially only right at the piston, would run the risk of rapidly propagating into the hollow element, by a notch effect, and of leading to brittle-type fracture, thereby reducing the energy absorbed.

When the hollow element is not reinforced by one or more reinforcing-fibre bundles wound almost circumferentially at its surface, this variant amounts to forcing the hollow element to undergo at the very most merely a plastic deformation. The transverse dimensions of the piston must therefore be such that the circumferential elongation of the hollow element caused by the insertion of the piston is less than the elongation at break of the material making up the hollow element.

When the hollow element consists of a hollow mandrel reinforced by one or more such fibre bundles, this variant amounts to forcing this mandrel to undergo at the very most merely a plastic deformation. In this second case, it is desirable however, for the piston to be designed so that its insertion into the hollow element causes fracture of the reinforcing-fibre bundles. This means that the elongation at break of the fibre bundles used must be less than that of the material making up the mandrel of the hollow element, this being generally the case for fibres such as glass fibres. The energy absorbed by the fracture of the fibre bundles considerably increases the energy absorbed in the event of a shock compared to the unreinforced case, all other things being equal.

In all cases, it is moreover desirable to avoid the use of an impact-sensitive plastic for manufacturing the hollow element.

One advantage which the device of the invention has over the known devices is that its effectiveness, i.e. the energy absorbed, is related to the circumferential strength of the hollow element, that is to say essentially to the thickness of its wall and to its possible reinforcement, and not related to its overall transverse dimensions (to its external diameter in the case of a hollow element of constant circular crosssection). This enables the device of the invention to combine great compactness with high performance.

The piston may consist of any material having a sufficiently high mechanical strength, for example a metal or a suitable plastic. It may be solid or hollow.

The strength of the piston must be such that it undergoes only negligable deformation and, in all cases, does not break before the possible complete fracture of the hollow element when the device of the invention is subjected to a shock. The recyclability of the piston is less critical than that of the hollow element insofar as, unlike the latter, the piston is in principle not damaged by a shock causing the device of the invention to operate. In other words, after such a shock, all that is required in principle is to replace the hollow element in order to make the device of the invention operational again.

With a view to making the device even more effective in cases when the shock is likely to occur at an oblique orientation with respect to the axis of the absorption device, an advantageous variant of the invention consists in the device described above being equipped with one or more guiding elements which force the piston to move coaxially with respect to the hollow element. Specifically, this objective may in particular be achieved by providing a rigid rod, of high mechanical strength, coaxial with the hollow element, and by providing a coaxial hole passing right through the piston, having a diameter slightly greater than that of the said rod, so that the piston can slide on this rod and be guided by it coaxially with respect to the hollow element. Alternatively, the piston could be fixed to a rigid rod which slides through the component to which the hollow element is fixed.

It does not matter whether the hollow element is fixed to the chassis of the vehicle and the piston to a component capable of withstanding shocks (for example a bumper), or whether the opposite is done. Moreover, it may be desirable to combine one or more reversible shock absorbers (elastic blocks, etc.) with the device of the invention so that the latter is not damaged by slight shocks.

According to a particularly simple and costeffective, but non-limiting, variant, the hollow element has a constant cross-section and the piston a variable cross-section, which is suitably profiled. This enables the hollow element to be manufactured by cutting a long initial component obtained, for example, by extrusion (and possible reinforcement) using a continuous process.

According to an advantageous variant of the present invention, the hollow element is increasingly reinforced near that one of its ends which is furthest away from the initial position of the piston. This is because, if the hollow element presents an increasing resistance as the piston is forced into the latter, the energy absorbed per centimetre of insertion increases gradually, something which may be useful in the case of very violent shocks. Such a gradual reinforcement may, for example, be obtained by reinforcing the entire length of the hollow element by winding a ply of fibre filaments, by reinforcing the last two-thirds of its length with a second ply of filaments and by reinforcing the last third of its length with a third ply of filaments.

Gradual reinforcement may also be obtained by varying the pitch of the winding of the fibre bundle or bundles along the longitudinal axis of the hollow element.

Another variant consists in subjecting the hollow element not only to circumferential tensions but also to radial flexural forces. To this end, a piston may be used whose cross-section does not have the same shape as the internal cross-section of the hollow element. For example, if the latter has a circular internal crosssection, the insertion of a piston of oval cross-section into this hollow element will firstly lead to an elastic deformation (ovalization), possibly followed by plastic deformation generating only a small amount of circumferential tension. The total energy absorbed in such a case may be greater than that absorbed in the case of purely circumferential stresses.

'The device described above may serve to absorb shocks in all types of application and is in no way limited to the field of motor vehicles. By way of example, it can also be used in helicopters, by being placed between their body and their skids. Furthermore, it is clear that several of such devices can be used in parallel for the purpose of increasing their total energy absorption capacity.

More generally, the invention also relates to a shock-absorber system for a vehicle, comprising at least one device according to one of the following claims. It also relates to a vehicle comprising at least one such shock-absorber system.

Another aspect of the invention relates to the use of a device as described above for absorbing shocks, preferably in such a way that the piston does not cause complete fracture of the hollow element when it is forced into the hollow element.

The appended figures illustrate in a non-limiting manner the operation of the invention.

Figure 1 shows a device as described above, which comprises a cylindrical hollow element (1) and a piston (2) arranged coaxially with respect to the latter. The hollow element (1) consists of a hollow plastic mandrel reinforced by three bundles (3, 4, 5) of glass-fibre filaments wound helically on its surface, the first (3) extending over its entire length, the second (4) extend ing over its last two-thirds and the third (5) extending over the last third of the hollow element. The piston has a front guiding protuberance (6) which precedes (in the direction of insertion of the piston in the event of a shock) a central part (2) of frustoconical shape, this part itself being extended by a rear cylindrical section (7). The diameter of this cylindrical section (7) is such that its insertion into the hollow element (1) causes the plastic deformation of the mandrel and the gradual fracture of the reinforcing fibres, under the effect of the induced circumferential tensions. For the sake of simplicity, the section of the piston (2) is bounded by straight lines; in practice, the piston has no sharp edge.

The lower end of the hollow element (1) is fixed to the chassis of a vehicle by a fixing device, not shown. The piston (2) is fixed to the bumper of the vehicle by means of a connection piece (8), only part of which is shown. Overall, the longitudinal axis of the device is parallel to the axis of movement of the vehicle. However, it is possible to provide identical auxiliary devices placed in different orientations so as better to absorb non-frontal shocks.

In Figure 1, the device has not undergone any shock. Figure 2 shows the same device in the process of undergoing a shock (the reinforcing fibres have not been shown either, for the sake of simplicity): the piston (2) is forced into the hollow element, gradually causing the plastic deformation of the mandrel and the fracture of the fibres under the effect of the circumferential tensions which it induces therein.

Figure 3 describes a variant of the device of the invention, comprising a rigid rod (9) coaxial with the hollow element (1) and intended to guide the piston (2) coaxially with respect to the latter in the event of a shock happening along an axis which was not exactly parallel to that of the hollow element. The lower end of the rod (9) is fixed to the chassis of the vehicle. Of course, the upper end of the rod (9) should not interfere with the bumper to which the connection piece (8), which can move with respect to this rod, is connected.

The following example illustrates the operatior of the invention in a non-limiting manner.

A hollow mandrel made of HDPE reinforced with 205 by weight of short glass fibres oriented axially, havinc a circular cross-section, with an internal diameter of 93 mm and a wall thickness of 6.7 mm, was manufactured. This mandrel, having a length of 15 cm, was reinforced by the helical winding on its surface of 2 plies of CofitE consisting of continuous glass fibres impregnated wit HDPE (thickness of each Cofit = 0.3 mm; width: 10 mm), at an angle close to 900 with respect to the axis of the hollow element. The strain at break of such a Cofit iE approximately 2%, which corresponds to a circumferential elongation of approximately 7 mm for an external diametez of the hollow element equal to 107.6 mm. A piston of approximately frustoconical shape, the larger diameter of which was 110 mm, was used, thus guaranteeing the fracture of the Cofit without causing that of the mandrel.

This device has made it possible to absorb shocks having an energy of approximately 4.2 kJ, which represents approximately 25% of the energy (from 10 to 2C kJ) dissipated during the frontal impact of a mediumsized car driven at approximately 15 km/h into a stationary obstacle.

Claims

1. Device for absorbing shocks, which comprises a plastic-based hollow element of elongate shape and a piston designed so as to be able to be forced into the said hollow element, deforming it at least radially.

2. Device according to Claim 1, in which the hollow element has a circular cross-section.

3. Device according to one of the preceding claims, in which the hollow element is reinforced in the circumferential direction.

4. Device according to the previous claim, in which the hollow element is reinforced in the circumferential direction by one or more bundles of continuous reinforcing fibres wound on its surface.

5. Device according to one of the preceding claims, in which the hollow element is, in particular, reinforced by short fibres dispersed within it, these being predominantly oriented parallel to its axis.

6. Device according to one of the preceding claims, in which the piston is designed so that its insertion into the hollow element does not cause complete fracture of the latter.

7. Device according to one of the preceding claims, equipped with one or more guiding elements which force the piston to move coaxially with respect to the hollow element.

8. Device according to one of the preceding claims, in which the hollow element is increasingly reinforced near that of its ends which is furthest away from the initial position of the piston.

9. Device according to one of the preceding claims, in which the hollow element and the piston each have a circular cross-section and in which the piston is profiled so that its front end has a diameter smaller than the internal diameter of the hollow element (Dc) and so that at least one of its parts has a diameter greater than Dc.

10. Shock-absorber system for a vehicle, comprising at least one device according to one of the preceding claims.

11. Device for absorbing shocks substantially as hereinbefore described with reference to Figures 1 to 3.