CA1197867A - Laminar strain energy absorber - Google Patents

Abstract

ABSTRACT OF THE INVENTION

An energy absorber (or arrestor) working on the principle of straining an elongated element by pulling it through a tortuous path over rotatable rollers and characterised by the element being of flexible laminar construction.
The flexibility of this element allows it to be formed into a conveniently small coil, the diameter of which being that of an equivalent section rigid element divided by the number of laminae.
The rollers, similarly reduced in diameter, are arranged into two opposed banks which temporarily divide the element into two equal and opposite laminar groups, the angular reactions of which counterbalance each other.
The normally haavily loaded exit rollers are substantially assisted by contact with remote support bearings.
The end of the element is anchored by entrapping two equal groups of laminae, utilising the strain energy principle and retaining the use of the full tensile area.

REFERENCES

(1) Domagala - February 1977 U.K. P.7291 (2) Jackson - April 1968 U.S.A. 3,377,044 Van Zelm - April 1961 U.S.A. 2,980,213 Jackson - October 1965 U.S.A. 3,211,260 Van Zelm et al - April 1961 U.S.A. 2,979,163

Description

r7B~7 LAMLNAR STRAIN ENERGY ABSORB~R

strain energy absorbers operating on the principle of pulling a strain element, such as a metallic strip, through a tortuous path formed by rollers are known to be highly e~fective since the energy expended is irrevocable, the element having become momentarily plasticised by the high bending stresses induced in the process.
Such devices make excellent shock absorbers and, by virtue of their enormous impact attenuation properties~ provide effective means for retarding and arresting high energy system.
A typical example of their use is the arrest of heavy vechicles in emergency runaway conditions.
The rate at which they absorb energy is a function of volumetric displacement and is, therefore, governed by the cross-sectional area of the strained element.
Heavy duty absorbers are consequently equipped with a strip of considerable section, the rigidity of which often imposes certain restrictions upon the choice and implementation of installations~
One such arrestor is described in a previous embodiment by the author.( ) It would frequently be highly desirable to coil a long arrestor strip ~or ease of access to a difficult site, or indeed for its installation aboard a vehicle requiring an arrest distance greater than the length of the conveyance itself.

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Coilable arrestors are knowrl to exist ( ) but the element material stiffness associaked ~ith high absorption rates makes the formation of a coil not only difficult, but, due to its Einal diameter, often renders an installation dimensionally impracticable.
One object of the laminar energy absorber, to which this current invention relates, is to overcome the problem of rigidity and permit the formation of a moderate coil, while retaining the advantages of high absorption rates. This is achieved by the use of a number of laminae in place of ~ and of the same aggregate cross-sectional area as, the single rigid element.
The optimum radius of a coil is governed by the elastic limit of the material if maximum use is to be made of the plastic range above the elastic point, that is to say, if the material is not to be overstrained in the coiling process. For a medium mild steel the coil radius could thus be in the region of 420 times the strip thickness, resulting in dimensional installation difficulties.
Replacing the rigid strip by an appropriate number of laminae reduces the radius in proportion to the number of leaves employed. For example a laminar element consisting of, say, three laminae would need a coil radius of only a third that of the rigid strip.

A further advantage is derived in the formation of a laminar curve where each lamina can be treated separately, reducing the winding torque required in the same proportions.

~`7~i7 Another object of this invention is to reduce the size, weight and inertial efEects of the strainer unit ~or roller box).
In the type of absorber under discussion the deforming roller radius, around which the element is successively bent and straightened, plays a most important role as it sets the value of the bending stress which must be high enough to bring about the onset of plasticity in the worked material, the strain rate being dictated by the ratio mean bend radius thickness of strip. This is true for a whole range of metals from which the element may be manufactured, the choice being dictated by cost, availability and often corrosion resistance.
A laminar absorber made of the same material, the~efore governed by the same radius to thickness ratio, requires much smaller rollers, as the relevant thickness is that oE a lamina, thus satisfying the object of the invention.
Since, for identical capacities, the aggregate roller loads imposed by the layers of laminae are the same as those which would exist had the element been a single rigid strip, great care must be exercised in the design of the unit in view of its much reduced dimensions. Such a design is described on the accompanying drawings.
A further object of this invention is to provide a saEe anchorage for the strain element using the same laminar technique. This is also described in the accompanying drawings in which like numerals refer to like parts.

d~6ir~;9 Figure 1 shows a partly dis-assembled strainer unit (or roller box) consisting, essentially, of two complementary halves, items 12 and 13, housing rollers 1, 3 and 5 and 2, 4 and 6 respectively. These rollers have reduced diameter spigotted journal ends. Fig . 2 shows a section.
The laminar element 9 is entrapped between the rollers when the two halves ~12 and 13) are brought together along faces 18 and 13 and held rigidly on assembly, the entrapping taking a dual path (10 and 11) so that the two box halves share all the loads equally when a tension "T" is applied at the exit end of the element.
Each box half takes half the reaction (T/2) on the front face. As each successive portion of the element enters the box between rollers 1 and 2, dividing itself into two equal portions which travel around rollers 3 and 4 respectively to emerye in a single path again between rollers 5 and 6, the laminae are strained to plasticity, the total work done in straining this ma-terial being e~ual to the product of the tensile force "'r" and the distance travelled, while the rollers are free to rotate on their respective axes.
As a result of this arrangement, the tensile load in that portion of the element situated between rollers 1 and 3 along path 10 (and the comp:lementary portion along path 11) is only approximately one quarter of the reaction on each box half, that is to say, about one eighth of the total force "T", consequently the loads acting on entry rollers 1 and 2 are moderate and these rollers are not subjected to significant bending stresses.

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The tensile load along each respective path between rollers 3 and 5 (and 4 and 6) is much more substantial being of the ~rder of three eighths of the total force ~T~o Consequently the load on roller 3 created by the tensions in both portions and enhanced by the angle of wrap would tend to be excessive were it not for the fact that, with the split path arrangement, this load is exactly counterbalanced by the load roller 4, which is in intimate contact with the former over its whole length.
To this end, slots 14 and 15 are incorporated in the box sides.
This device ensures that the predominant load is taken by the two opposed laminar paths, the rollers being subjected to compression only, while rolling against each other. The only force to be resisted by the spigots and subjecting these rollers to bending is a very modest component acting in the direction of the applied force "T".
Rollers 5 and 6 are also subjected to heavy loads due to the high tension on the tangential portions of the element on each side of the ro]ler. '~his is overcome, in this arrangement, by the provision of substantial remote bearings 7 and 8. Slots 16 and 17 are incorporated in the box sides to ensure that, under load, rollers 5 and 6 are free to take up a position where they make contact, over their lengths, with bearings 7 and 8 respectively. Thus each load is conveniently split into two components, the rnajor one being resisted by a remote bearing of ample proportions, while the minor component, acting in a direction perpendicular to the line of force "T", is acceptably low.

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This preEerred design of roller box offers, therefore, substantial benefits of compactness~ while ensuring that roller stresses and journal loads are kept to a tolerable level by virtue of the counter-balance created by the contrived divergence of two parts of the strain element and the provision of remote main support bearings.
It will be noted that this divergence/convergence creates a natural reaction between the two element halves, which are in perfect balance~ thus ensuring bending conformity over the adjacent for~ing rollers, without any need for any external assistance. The laminar element thus follo~s the path required at both extremities without guide rollers.
The capacity of the device can easily be altered over a limited range by changing the number in the element. Although such a change must be accompanied by a change in the diameter o~
the four end rollers (1, 2, 5 and 6) to maintain conformity, the constructional details of the unit and central rollers 3 and 4, may be retained with economic advantage.
In a variant of this design and depending to a large extent on the width of the laminae Iwhich influence the to-tal force), the remote bearings 7 and 8 can be omitted, particularly if the rollers can be shortened with a beneficial efEect upon the bending stresses in rollers S and 6.
Figs. 3 and 4 show the preferred mode of strain element anchorage. The laminae fit between substantial anchor plates 23 and 24, which are clamped together by screws 21 and 22. The plates 23 and 24 are fashioned to provide a shaped keying slot (shown in scrap view fig. 5~ in which the laminar element is entrapped, again in a diverging/converging manner, by a fashioned key 20. The holding or anchoring principle behind this design is similar to that of the ab~orber itselE in that, in order to break away, the laminae would need to become plasticised before slippage occured~ The apparent difference is that no rollers are involved since motion need not be encouraged.
AS the bending stress required for plastic flow is governed by the bond radius it follows that, by making the entrapment and key fillet radius a fraction of that of the absorber's forming rollers, the factor of safety against slippage withîn the anchorage, will be the inverse of this fraction for the same number of bends and similar angular geometry.
There are many advantages deriving from the use of this principle, the main one being that the tensile area of the laminae is not reduced by the introduction of bolts or pins, which would normally result in a very considerable weakening of the material, in view of their required size.
An additional feature of this form of anchorage is the substantial grip of the clamping screws and the frictional forces derived therefrom.
Yet another enhancement of the safety aspect is the well known logarithmetic friction resistance involving a primary tension which is magnified by ~ where ~ is a coefficient of friction and ~ the total angle of deflection which, in this arrangement, is not inconsiderable. In order to boost the 1~7~ ~

primary tension, scre~s 22 actually penetrate through the whole element, but this does not impair the integrity of the laminae as the reduction in area takes place at a point where, in accordance with the operating principle, no strain energy tension exists as tensile stresses do not begin to build up until the first bend at point of entry.
As in the case of the roller box itself, one Manufactured anchorage set can serve a range of capacities.
Should one require to alter the number of laminae, for example by reduction, an e~ual number of outer layers each side of the entrapment, would be removed and be replaced by short "dummies"
of the same section and shapel serving as packers, all other dimensions and details remaining the same.
Figs. 6 and 7 depict complete installations with single and double symmetrical coils respectively, the anchorage being designated as 25, while 26 denotes the assembled roller box.

Claims (5)

1. An energy absorber (or arrestor) working on the principle o straining a substantial section elongated element beyond its bending elastic limit by pulling it through a tortuous path over rotatable rollers and characterised by a laminar element construction, wherein the improvement in its flexibility permits it to be formed into a coil of diameter smaller than that necessary for a rigid element, the reduction being in a ratio substantially proportional to the number of laminae contained in the flexible element of same width, aggregate thickness and equivalent strain.

2. An absorber of the principle described in claim 1 characterised by a laminar element construction, wherein the improvement in its flexibility permits the use of rollers of diameter smaller than that necessary for a rigid element, the reduction being in a ratio substantially proportional to the number of laminae contained in the flexible element of same width, aggregate thickness and equivalent strain.

3. An absorber as described in claim 2 but characterised by the use of two identical laminar elements, which form two opposed laminae groups being simultaneously strained around two sets of rollers, one set to each element, creating a balance of reactions, which substantially relieves the effect of loads on the opposed inner rollers (with the greater amount of wrap) by maintaining these said rollers in intimate rolling contact with each other.

4. An absorber as described in claim 3 but which is equipped with remote support bearings in rolling contact with the most heavily loaded exit rollers, the said bearings providing substantial relief to these rollers by resisting the major components of the applied loads.

5. An absorber as claimed in any one of claims 1, 2 or 3 characterised by a laminar construction element, the end of which is attached to an anchorage device by means of the entrapment of two equal and opposite groups of laminae deformed into tortuous shapes of small bend radius, one each side of a fashioned key, the plastic bending resistance of the laminae providing the necessary anchorage reaction and preventing relative slip between the various components of the entrapment without the need to weaken the tensile element by any sectional area reduction usually associated with orthodox fasteners.