GB2426806A

GB2426806A - A structural element designed to withstand a transverse load tending to bend the element

Info

Publication number: GB2426806A
Application number: GB0508439A
Authority: GB
Inventors: Timothy Scott; Roy Goodrum; Frank Heitplatz; Andreas Scheffler; Jason Carl Lisseman
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2005-04-27
Filing date: 2005-04-27
Publication date: 2006-12-06
Also published as: GB0508439D0; GB2425580B; GB2425580A; DE102006018359B4; GB0605912D0; DE102006018359A1

Abstract

An elongate structural element 10 is designed to withstand a transverse load tending to bend the element. The element 10 has at least one pair of transversely extending surfaces 17 that face towards one another, the outer edges of which surfaces 17 are forced towards and away from one another as the element is bent by the application of a transverse load. The transverse surfaces 17 are coupled for transmission of force from one to the other, the effectives of the coupling being dependent upon the direction and extent of deformation of the element by the applied load.

Description

DESIGN OF STRUCTURAL ELENTS

The present invention relates to a structural element and in particular to a structural element designed to withstand a transverse load acting to bend the element. The structural element may be a cantilever beam (i.e. one loaded between a point of support and an unsupported end) or else the load may be applied between two points of support of the structural element.

Certain structural elements in a motor vehicle need to be designed to yield plastically when the load to which they are subjected during normal use of the vehicle is exceeded.

One such typical structural element is the armature of the steering wheel. The armature comprises a central hub that is secured to the steering column and spokes that connect the hub to the rim of the wheel. The spokes must not bend to any significant extent when the vehicle is being steered or when the steering wheel is used as a support for the driver while getting in and out of the vehicle or when the driver pulls and pushed on the rim of the steering wheel to adjust the position of his seat. However, the armature I required to deform in the event of a collision to absorb energy and reduce injury to the driver.

Another example typical of a structural element to which the present invention may be applied is the seat frame supporting the back of the driver. The frame must be sufficiently rigid to support the weight of the driver and the forces normally applied to the frame, for example when adjusting the position of the seat or the back rest, but it must deform to reduce the risk of whiplash in the event of a rear end collision.

In known designs of structural elements designed to withstand a transverse loading, once the loading limit has been exceeded, their deformation increases substantially linearly as the load increases, in other words their resistance to bending (also termed the yield) and the rate at which they absorb impact energy is substantially constant.

The two typical structural elements mentioned above, namely steering wheel armatures and seat frames, are examples of structural elements where such a constant yield is not ideal. In the case of a seat back, it is desirable in the event of a rear collision for it to give at first to catch the back of the driver but, once the driver's back is firmly supported in the seat, it is preferred to increase the resistance to bending so as to prevent excessive deformation of the seat. Similarly, in the case of a steering wheel armature, a reduced resistance at first helps avoid chest injury but if an air bag housed in the hub of the steering wheel is triggered a greater resistance is to be preferred as the armature acts as a reaction surface for the air bag.

There may also be instances when one may wish the yield of a structural element to vary with the direction in which the load is applied or to have the yield of the element decrease rather than increase after a threshold load is exceeded.

The present invention seeks therefore to provide a structural element that can be designed to allow its yield to be varied as a function of the loading so as to enable the shape of the graph of load versus deflection (herein termed the yield curve) to be optimised to suit the functional requirements of the structural element.

According to the present invention, there is provided an elongate structural element designed to withstand a transverse load tending to bend the element, wherein the element has at least one pair of transversely extending surfaces that face towards one another, the outer edges of the surfaces being forced towards and away from one another as the element is bent by the application of a transverse load, and wherein the transverse surfaces are coupled for transmission of force from one to the other, the effectiveness of the coupling being dependent upon the direction and extent of deformation of the element by the applied load.

In a simple embodiment, the element may have a single transverse notch defining two facing and mutually spaced transverse surfaces. The effect of the transverse notch at first is to reduce the effective width of the structural element thereby reducing its yield. However, once the element has deformed to such an extent that the notch is closed and the two surfaces come into direct contact with one another, then the full width of the element comes into play and the yield is increased.

Instead of a single notch, the element may have several notches defining a set of comb-like teeth which extend transversely to the length of the beam and generally parallel to the direction of the applied load.

If the teeth all have the same length and separation, there will be a relatively abrupt transition from the lower resistance to the higher resistance. Should a more gradual transition be required, then the spacing between the teeth may be graduated so that gaps between the teeth are closed sequentially rather than simultaneously. Alternatively, the shape of the gap between each pair of opposed surfaces may be non-uniform so that the gap closes by working its way up gradually from the roots of the teeth to their tips.

It is possible to strengthen a structural element in one direction only by providing teeth with only a nominal spacing that project from one side of the element. When the force applied tends to deform the side from which the teeth project into a concave curve, the tips of the teeth contact one another and resist the bending. On the other hand, when the force applied tends to bend the side into a convex curve then the tips of the teeth simply move apart and offer no resistance to the bending of the element.

It has so far been assumed that the tips of the teeth are normally unconnected and that they begin to contribute to the resistance to bending of the element only after they have been deformed into direct contact with one another. It is however alternatively possible to provide a weak bridge between two opposed surfaces that ruptures when a load is exceeded. In this case, the weak bridge will at first is resist separation of the surfaces and the resistance will drop when the bridge ruptures.

In this way, the invention allows the yield curve of a structural element to be adapted to achieve optimum resistance at all stages in its deformation.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:- Fig. 1 is a section through a structural element of a first embodiment of the invention, Fig. 2 is a similar section showing an alternative embodiment of the invention, Fig. 3 is a similar section through a further embodiment of the invention.

Fig. 1 shows a structural element 10 in the form of a cantilever beam that is anchored at one end to a support 12.

The beam may for example be a spoke of a steering wheel projecting from a support formed by the central hub of the steering wheel. In an another example, the element may be a side element of a frame surrounding the back of a seat. In both these examples, the element is intended to be rigid during normal use but should yield in the event of a collision to reduce the risk of injury to the driver or a passenger. The invention allows a structural element to yield at different rates depending on the severity and the direction of the impact.

The element 10 in Fig. 1 has teeth 14 projecting from one side and resilient inserts 15 are arranged between some of the teeth 14. If the element 10 is being in the direction of the arrow 16, the teeth 14 play no part in the deformation of the element. It will yield when a predetermined loading is exceeded and the extent of the deformation will then increase substantially linearly with the load.

When the element is bent in the direction of the arrow 18, on the other hand, the deformation is at first resisted by the compression of the resilient inserts 15. When the inserts 15 are fully compressed or when the opposed surfaces 17 at the tips of the teeth 14 not separated by inserts 15 come into contact with one another, the stiffness of the element is substantially increased and the rate of deformation will then drop as the loading is increased.

In the case of a steering wheel spoke, this would mean that at first the spoke will yield to prevent serious injury to the driver's rib cage. When however an air bag in the steering wheel explodes, the spoke will not collapse under the increased loading but will provide a firm a reaction surface for the air bag to allow the driver to be pushed back into this seat by the air bag.

In the case of a seat back, the initial yield will allow the drivers back to be cushioned at the commencement of a rear end collision but once the driver's back is fully supported in the seat, the increased stiffness of the structural element will prevent the seat back from collapsing entirely.

While the embodiment of Figure 1 allows the stiffness of the element to be increased after a certain amount of deformation has taken place, it is possible to design element such that the stiffness will reduce after a threshold has been exceeded.

The embodiment of Figure 2 has two teeth 24 that are connected by a weak bridge 26. When the structural element is loaded in the direction of the arrow 30 it will at first resist bending on account of the bridge 26. However, once the loading has exceeded the threshold needed to rupture the bridge, the stiffness of the element will be reduced significantly.

The embodiment of Fig. 3 shows that the teeth 42 may have a web 42a and a head 42b. The transverse surfaces of the heads 42b may be shaped to slip relative to one another, as in the case of the surfaces 42c, or to lock relative to one another, as in the case of the surfaces 42d.

Furthermore, the webs can be designed to take the transfer load. Once again, the webs may be reinforced by placing compressible inserts (not shown) between them.

The embodiment of Figure 2 also shows how it is possible to have an asymmetrical element that will bend more readily in one direction than in the other. The gaps between the teeth 24 and the teeth 28 are only nominal and will close at soon as the element is bent in the direction of the arrow 32. However, the gaps will merely widen when the element is bent in the direction of the arrow 30 and the teeth 28 will not contribute to the stiffness of the element when bent in that direction.

A seat back is an example of a structural element that benefits from having asymmetrical properties. Under normal loads, the seat back should of course not move at all allowing the driver push against it when getting in and out of the vehicle or while adjusting position of the seat or the steering wheel. In the event of a rear end collision, the seat back is forced backwards by the weight of the driver or passenger and its ideal response would be to yield until the driver is fully supported by the back of the seat and then to become stiffer so that the back of the seat does not collapse. In the event of a forward collision, on the other hand, there is a risk that a heavy load in the back of the vehicle may move push the back of the seat forward tending the crush the driver between the seat and the steering wheel. Therefore, a seat back needs to be stiff at all times when resisting impacts acting to move it forwards.

Such a yield curve can be achieved by providing a seat back with closely spaced forward facing teeth and more widely spaced rearward facing teeth with bridges connecting the tips of the forward facing teeth. Such a design provides the high stiffness required to resist a heavy load intruding into the front passenger area in a severe frontal impact while allowing the seat back to yield in a rear impact. The bridges would provide high initial stiffness to assist the driver during and entry and exit. The bridges would act in tension or in compression depending on the direction in which the load is applied.

Of course the length, width and spacing of the teeth are all parameter that will affect the variation of stiffness with applied load. It will therefore be clear from the above description that the combination of spaced teeth and bridges will allow the yield curve to be varied at will enabling the stiffness of any structural element to be optimised under different degrees of deformation.

Claims

CLIMS

1. An elongate structural element designed to withstand a transverse load tending to bend the element, wherein the element has at least one pair of transversely extending surfaces that face towards one another, the outer edges of the surfaces being forced towards and away from one another as the element is bent by the application of a transverse load, and wherein the transverse surfaces are coupled for transmission of force from one to the other, the effectiveness of the coupling being dependent upon the direction and extent of deformation of the element by the applied load.

2. A structural element as claimed in claim 1, wherein the element has a single transverse notch defining two facing and mutually spaced transverse surfaces.

3. A structural element as claimed in claim 1, wherein the element has several notches defining a set of comb-like teeth which extend transversely to the length of the beam and generally parallel to the direction of the applied load.

4. A structural element as claimed in claim 3, wherein the teeth all have the same length and separation.

5. A structural element as claimed in claim 3, wherein the spacing between the teeth is graduated so that gaps between the teeth are closed sequentially as the element is between.

6. A structural element as claimed in claim 3, wherein the shape of the gap between each pair of opposed surfaces is non-uniform so that the gap closes by working its way up gradually from the roots of the teeth to their tips.

7. A structural element as claimed in any preceding claim, wherein the structural element is strengthened in one direction more than the other by providing teeth with only a nominal spacing that project from one side of the element.

8. A structural element as claimed in any preceding claim, having two opposed transverse surfaces that are coupled to one another by means of a weak bridge designed to ruptures when a predetermined load is exceeded.

9. A structural element constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.