Cable with impregnated fiber strength member for non-slip clamping

The tendency of modern high strength synthetic fibres, e.g., made of aromatic polyamides, when utilized for tension load-carrying purposes in association with an overhead cable arranged parallel thereto within a common protective covering to slip under load out of the clamping members connected thereto before the load reaches its ultimate tensile strength is overcome by impregnating a bundle of such fibres with an impregnating material capable of breaking down into particles when subjected to sufficient compressive stress which is broken down into such particles by the application of the clamping member, the resultant particles exerting within the clamped region a wedging action between the individual fibres and at the exterior of the bundle as a whole upon the application of the tension load to the bundle. Particularly suitable impregnating materials are natural resins, especially colophonium.

The invention relates to an element for transferring tensile loads, which 
element comprises a bundle of a plurality of synthetic fibres having 
smooth surfaces and a tensile strength in excess of 200 kg/mm.sup.2, a 
modulus of elasticity in excess of 3000 kg/mm.sup.2, and an elongation at 
rupture of less than 10%, the fibres, in order to reduce the risk of slip 
due to their smooth surfaces, being impregnated and bonded, in the area of 
contact with the force-transfer means, at least over a part of their total 
length, with a material which unites them and increases the coefficient of 
friction at the outer surface of the fibres thus bonded. 
An element of this kind is known, for example from page 3, Table II Section 
B of "Kevlar 49, Technical Information, Bulletin No. K-1, June 1974", of 
the Du Pont de Nemours Company. This relates to a type of cable in which 
the fibres are not stranded but are arranged parallel with each other and 
are impregnated with an epoxy resin. After the impregnation, the epoxy 
resin is hardened by heat-treatment at about 180.degree. C. 
However, this known element, which was made purely for experimental 
purposes, namely to measure the tensile strengths attainable with such 
elements, is relatively stiff and cannot be used in this form as a hawser, 
since it breaks relatively easily when bent. The reason for this is that, 
like most hardenable synthetic resins, epoxy resins break, when hardened, 
at relatively low flexural stresses. The notch action arising at such 
breaks leads, within a short time, to consecutive rupture of the fibres 
bridging the break, from the outside of the element towards the inside. 
This element therefore solves the problem of transferring force thereto but 
not the problem of achieving sufficient flexibility to allow the element 
to be used in practice as a hawser. 
There is also no difficulty in solving the problem of flexibility 
independent of the problem of transferring force to the element, since all 
that is necessary to this end is to omit the impregnation of the fibres of 
the element with the material which bonds them and increases the 
coefficient of friction at the outer surface of the fibres thus bonded. 
However, if the impregnation is omitted, transferring force to the element 
becomes an extraordinarily difficult problem, since in this case force 
must be transferred to the individual fibres of the element by static 
friction between the means enclosing the bundle of fibres and the outer 
fibres of the bundle and then between the individual fibres. This means 
that in order to achieve frictional forces corresponding to the high 
tensile strength of the fibres, extraordinarily high pressure would have 
to be applied by the force-transfer means--engaging with the outside of 
the element--to the bundle of fibres, because of the smooth surfaces of 
the fibres and the low-coefficient of friction thereof. If, for example, 
it is desired to form, at the end of such an unimpregnated element, a loop 
around a cable-thimble, by means of a clamping sleeve, a clamping sleeve 
having a length equal to ten times the diameter of the bundle of fibres 
would have to exert a pressure of several tons per square centimeter upon 
the element or bundle of fibres to allow the tensile strength of the 
element to be fully utilized when it is under tension. With clamping 
sleeves, however, it is impossible to apply such high pressures, since 
even a duralumin sleeve, with a wall-thickness equal to half the inside 
diameter of the sleeve would reach its tensile-strength limit at an 
internal pressure of five tons per square centimeter, i.e. it would burst 
when this internal pressure was exceeded, and it should, of course, be 
clear that, in compressing a clamping sleeve, it is impossible to obtain a 
clamping pressure which would force the sleeve open when the compression 
ceases, but that the maximal pressure attainable is far less than the 
internal pressure required to force the sleeve open. Thus since the 
necessary pressure of several tons per square centimeter upon the bundle 
of fibres cannot be achieved with the clamping sleeve, as soon as tension 
is applied the bundle of fibres slides out of the sleeve before the 
tensile strength of the fibres is reached, i.e. the tensile strength of an 
element with unimpregnated fibres is determined, not by the tensile 
strength of the fibres, but by the maximal pressure applicable to the 
bundle of fibres by the force-transfer means engaging with the outside of 
the element, and this is usually far below the tensile strength of the 
fibres, often only one fifth or one tenth thereof. This, however, 
eliminates the advantage offered by these synthetic fibres, since hawsers 
having only one fifth or one tenth of the tensile strength of such fibres 
may also be made from other materials, with less complex equipment and 
without the problems produced by the low coefficient of friction of 
synthetic fibres. 
In spite of the intensive efforts in recent years of those engaged in this 
field, it has hitherto been impossible to produce an element of the type 
in question, which can be used as a hawser, and satisfactorily involves 
both the problem of the transfer of force to the element, and the problem 
of achieving satisfactory flexibility. Although the aforesaid known 
element solves the force-transfer problem, it fails to solve the 
flexibility problem. On the other hand, cables known from the same 
bulletin as this element, and made of synthetic fibres (see page 12, FIG. 
117), solve the flexibility problem but, since there is no impregnation, 
they fail, for the reasons mentioned above, to provide a satisfactory 
solution of the force-transfer problem. A combination of these two 
solutions, for example impregnating the synthetic fibres with a material 
other than that used with the known element, has hitherto not been found. 
It was therefore the purpose of the invention to provide an element of the 
type in question, which may be used as a hawser, which offers satisfactory 
solutions for both the force-transfer and flexibility problems, and which 
thus makes it possible to produce, from synthetic fibers, a hawser in 
which the tensile strength thereof can be fully utilized, thus permitting 
the transfer of tensile forces substantially greater than those obtained 
with a steel cable of the same effective cross-section. 
According to the invention, and in the case of an element of the type in 
question, this purpose is achieved by selecting for the material for 
impregnating the fibres one which breaks down into powder in the area to 
which the stress is applied, when applied compressive or flexural stress 
exceeds the ultimate stress limit of the impregnating material. 
The use of a material of this kind for impregnating the fibres has two 
decisive advantages: in the first place, this material completely 
eliminates any notch-action at locations where it is broken as a result of 
flexural stressing of the element since, under such circumstances, the 
material does not break like glass, but decomposes into a powder, 
particularly in the pressure-area of the bend, thus eliminating the 
lever-action, which in the case of a glass-like break, leads to successive 
rupture of the fibres bridging the break, from the outside of the element 
towards the inside. In the second place, the decomposition of the powder, 
in areas under very high compressive stress, is of decisive importance 
since, as indicated above in the example of a clamping sleeve used as the 
force-transfer means, an extraordinarily high pressure must be applied to 
the bundle of fibres in force-transfer areas, and the said material 
therefore breaks down into a powder in such areas. As seen under the 
microscope, this powder is in the form of small crystals, mainly single 
crystals, which retain their shape even under very high pressures. Since 
the bundle of fibres is also impregnated with this material, the crystals 
produced by disintegration thereof fill the spaces between individual 
fibres of the bundle almost completely, thus transferring, to each 
individual fibre, the pressure acting from the outside upon the bundle of 
fibres. Since the said crystals retain their shape, even under the highest 
pressures, the edges thereof are forced against the individual fibres. 
This, however, results in a considerable increase in the coefficient of 
friction between individual fibres and, since the same naturally applies 
to the outer fibres of the bundle, it also greatly increases the 
coefficient of friction between the outside of the bundle and the means 
enclosing it, the values obtained being substantially higher than would be 
obtainable with fibres impregnated with a pressure-resistant material. The 
main reason for this is that pressure-resistant materials form 
substantially smooth surfaces both on individual fibres and on the outside 
of the bundle of fibres, whereas the crystals, with their edges pressed 
against the individual fibres, wedge, as it were, when the fibres are 
subjected to tension, and the higher the tension, the more strongly are 
the crystals pressed against the fibres between them. 
In the case of the element in question, the said material is preferably a 
resin which breaks down into a powder under compressive and/or flexural 
stressing beyond its ultimate-stress limit. Resins having this particular 
property have hitherto been found only among those consisting completely, 
or at least mainly, of natural resin, but this does not mean that specific 
development could not also lead, under certain circumstances, to a 
synthetic resin possessing this same special pwoperty. However, such 
breaking down into powder, under the action of pressure, should require, 
during the forming of the resin, simultaneous production of a plurality of 
single crystals which subsequently coalesce. This, in turn, requires the 
presence of crystal nuclei, whereas synthetic resin are usually produced 
by polymerization and thus have a totally different formation mechanism. 
Among natural resins, colophonium, in particular, has the ability to break 
down into a powder, under the action of pressure to a pronounced degree. 
In one preferred form of the present element, therefore, the material used 
to impregnate the synthetic fibres is colophonium. 
The fibres in the present element are preferably made of a synthetic 
material, preferably an organic polymer, more particularly an aromatic 
polyamide, as described in the bulletin mentioned hereinbefore, the fibres 
having a tensile strength of at least 250 kg/mm.sup.2, a modulus of 
elasticity of at least 10000 kg/mm.sup.2, and an elongation at rupture of 
less than 3% 
In the present element, the fibres are preferably arranged in the bundle 
parallel with each other. The advantage of this is that unwanted expansion 
of the element is largely eliminated, thus restricting to a minimum any 
sagging, as a result of temperature fluctuations, in the case of 
horizontally mounted elements. Furthermore, this type of arrangement is 
the most satisfactory if the element is to be stressed almost to the 
tensile-strength-limit of the fibres. It also produces the largest 
effective cross-section and the largest number of fibres for a given 
diameter of the element or bundle of fibres, and also the maximal 
load-carrying capacity. Finally, this arrangement of the fibres also 
provides the highest coefficient of static friction in devices such as 
clamping sleeves etc. If, however, the very small elongation of the fibres 
at rupture is too low for a particular application of the element, it is 
better to improve this by stranding the synthetic fibres. 
For the purposes of force-transfer, in the case of at least one of the two 
end-areas of the element, two regions or sections at different distances 
from the ends of the bundle are joined together to form a loop, preferably 
around a circular or thimble-shaped eye, by means of a clamping element, 
and the impregnation of the fibres extends at least to the region most 
remote from the end of the fibres. However, the fibres of the element are 
preferably impregnated with the material over their entire length. 
The clamping elements used to form the loops at the ends of the present 
element preferably comprise at least one clamping sleeve having rounded 
edges at the locations where the fibres emerge therefrom. The advantage of 
rounding these edges is that it prevents them from cutting into the bundle 
of fibres since, within the sleeve, because of the high pressure applied 
thereby to the bundle of fibres, the cross-section of the latter is 
somewhat smaller than outside the sleeve where the bundle is not under 
pressure. The outer fibres of the bundle are therefore bent outwardly 
around the edge of the sleeve as they emerge therefrom. Since the fibres 
are tensed when the element is under tension, a sleeve with a sharp edge 
could cut into the outer fibres. This would cause the outer fibres to 
break. With the element under very high tension, the resulting reduction 
in the load-carrying cross section of the bundle of fibres could cause the 
whole bundle to rupture at this location. This rupturing of outer fibres 
by sleeves with sharp edges is accelerated in practice by the fact that 
wind causes a cable mounted out of doors to swing, the nodal point of this 
swinging being usually located at the transition from one to two cables 
and thus at end-loop formed by a clamping sleeve, where the cable emerges 
therefrom. The cable thus bends constantly back and forth at the nodal 
point. 
If the pressure of the clamping sleeve on the bundle of fibres cannot be 
made high enough to ensure that the end of the bundle will not slip out of 
the sleeve before the tensile strength of the fibres is reached, then the 
tensile force, acting upon the end of the bundle of fibres, which causes 
this to happen when a specific limit-value is exceeded, may be reduced by 
passing several turns of the end-loop, formed by the clamping sleeve, 
around a circular eye. This transfers a not inconsiderable part of the 
overall tension, acting upon the element, directly to the circular eye, 
and the tension acting upon the clamping sleeve is reduced accordingly. In 
this connection, the circular eye may, with advantage, be combined with a 
cable-thimble in such a manner that the parts of the loop between the 
sleeve and the eye pass through the thimble combined with the eye. 
It is desirable to protect the present element against weathering and other 
external influences by enclosing the fibres in a protective covering, 
preferably of polyurethane. Especially if the element has strands running 
parallel with each other, a protective covering this kind is a great 
advantage, since it also holds the bundle of fibres together. The bundle 
is, of course, also held together by the impregnating material, if the 
latter is impregnated over its whole length therewith, but this no longer 
obtains when the material breaks down into powder at the bend-locations 
under repeated flexural loads, as in the case of a swinging cable. Under 
these circumstances, the protective covering still holds the bundle of 
fibres together at such locations and also counteracts unduly sharp 
flexing of the element. It also assists in increasing to a maximum the 
force applied to the bundle at a clamping location, since, if a clamping 
sleeve is applied, not directly to the bundle, but to the protective 
covering, then the coefficient of friction which determines the maximal 
tension that can be transferred, is no longer that between the bundle of 
fibres and the clamping sleeve, but that between the bundle and the 
protective covering and, in the case of the present element, the 
coefficient of friction between the bundle and covering is usually higher 
than that between the bundle and a clamping sleeve applied directly 
thereto, since the edges of the crystals constituting the powder, into 
which the material used to impregnate the fibres breaks down under the 
action of high pressure within the clamping sleeve, obtain a better hold 
on the inner surface of the protective covering, when the element is 
loaded in tension and when, as already explained hereinbefore, the 
crystals interlock, than on the inner metal surface of the clamping 
sleeve. However, this assumes that the material of the protective covering 
is sufficiently strong to withstand the forces transferred by the crystals 
to the inner surface of the covering, even under high tensile loads. This 
may easily be achieved, however, by selecting a suitable material for the 
protective covering. 
The invention also relates to the use of the present element as an 
overhead-cable carrier, in which the element and the cable are enclosed in 
a common protective covering preferably forming two separate channels for 
the fibres of the element and the wire of the cable. In this particular 
application, the present element has decided advantages over steel cables 
used for the same purpose, since the element has a higher tensile strength 
and stretches less than a steel cable of the same diameter, and therefore 
sags less. Furthermore, the danger of the carrier breaking, either due to 
corrosion in the vicinity of the end loop clamping sleeves, in the case of 
steel cables, or due to the fibre-bundle slipping out of the end loop 
clamping sleeves, in the case of unimpregnated cables made of synthetic 
fibres, is completely eliminated by the use of the present element.

In the terminal part, illustrated in FIG. 1, of an element 2 used as a 
carrier for an overhead cable 1, synthetic fibres 3, arranged in strand 
form running parallel with each other, made of an aromatic polyamide, and 
having a tensile strength of 300 kg/mm.sup.2, a modulus of elasticity of 
13400 kg/mm.sup.2, an elongation at rupture of 2.6%, and a specific weight 
of 1.45 g/cm.sup.3, are impregnated with colophonium and are enclosed in a 
protective covering of polyurethane which also encloses wires 5 of the 
overhead-cable and thus unites the cable and element 2. As may be gathered 
from the cross-section in FIG. 2, protective covering 4 forms two channels 
6,7, isolated from each other, one for fibres 3 of element 2 and one for 
wires 5 of cable 1. Part 8 of the protective covering, enclosing synthetic 
fibres 3 is united with part 9, enclosing wires 5 by a bridge 10 integral 
with the covering. In the terminal length illustrated in FIG. 1, bridge 10 
is cut away between element 2 and cable 1 over a length sufficient to 
allow the loop to be formed. At the end 11 of the cut-away, it is 
desirable to fit a clip, or the like, not shown in FIG. 1, enclosing the 
cable and the element, for the purpose of preventing further opening up of 
bridge 10 beyond edge 11 of the cut. The free end of element 2, formed by 
cutting away bridge 10, is formed into a loop 12 for suspending the 
overhead-cable, the loop being secured by clamping sleeve 13. Whereas 
cut-end 11 is usually substantially greater than is shown in the drawing, 
the length of the loop is in proportion to the diameter of the element and 
the cable. 
If desired, the end loop 12 of the element 2 can be formed around a thimble 
or eye shown in FIGS. 4 and 5 and designated 16 and the loop can include 
more than one turn, e.g. two turns, of the end of the element wound around 
the thimble 16. 
The bundle consisting of fibres 3 has a denier of 106500 corresponding to 
an effective fibre cross-section of 8.15 mm.sup.2. The diameter of the 
bundle formed by fibres 3, when fully compressed, is about 3.4 mm. The 
effective cross-section, 8.15 mm.sup.2, and the tensile strength, 300 
kg/mm.sup.2 of the fibres, produce a load limit or ultimate breaking 
stress for the bundle of fibres of 2445 kg. However, repeated application 
to the element of a tensile load of 2500 kg neither ruptured the element 
or the bundle of fibres 3, nor caused end 14 of the said element to slip 
out of clamping sleeve 13. The length of that sleeve is 75 mm, the outside 
diameter, after compression, about 8 mm, the compressive load used being 
30 tons. Part 8 of the protective covering enclosing fibres 3 has a 
wall-thickness of about 1 mm and this is reduced by at least one half 
within the said clamping sleeve. Impregnation of the bundle of fibres is 
achieved by drawing it, before the protective covering is applied, through 
a bath of colophonium dissolved in ether, and by then drying and hardening 
it under heat. Care is taken to ensure that all of the fibres in the 
bundle are wetted by the colophonium over their entire length, and that 
any excess solution is removed from the fibres, for example by drawing the 
bundle out of the bath through a sizing nozzle. Some alcohol may also be 
used as a solvent for the colophonium, but in this case drying and 
hardening take rather longer than when ether is used. It is also possible 
to draw the bundle of fibres through molten colophonium, since the said 
fibres can easily withstand temperatures above the melting point of 
colophonium. In this process, however, some problems arise as regards 
uniform wetting of all fibres in the bundle and removing excess molten 
colophonium. 
Practical tests with the overhead-cable illustrated in FIGS. 1 and 2 have 
shown that suspending the cable from the present element meets all 
existing requirements. This applies to tensile strength, weathering, and 
unusual loads arising when the cable swings in strong wind or ices. In 
these test s, loops 12 were fitted with cable-thimbles. Inspection carried 
out on the cable after the tests showed that the colophonium had broken 
down into powder in the vicinity of cut-end 11, in the areas at each end 
of clamping sleeve 13 and therewithin, and in the vicinity of bend 15 in 
loop 12, indicating high compressive and flexural stresses in these areas. 
However, these areas showed no increase in wear-related phenomena such as 
rupture of the fibres etc. 
FIG. 3 shows, by way of comparison, specific load-carrying capacity as a 
function of the ratio between clamping-sleeve length and fibre-bundle 
diameter in respect of the present element, with natural-resin 
(colophonium) impregnation, synthetic-resin impregnation, and no 
impregnation of the fibres. It may be gathered from this diagram that, in 
the case of natural-resin impregnation, as in the case of the present 
element, and with clamping-sleeve lengths of more than ten times the 
diameter of the bundle of fibres, the specific load-carrying capacity of 
the element is a function only of the tensile strength of the bundle of 
fibres, and that there is no longer any danger of the end of the bundle 
slipping out of the clamping sleeve. In the case of short clamping 
sleeves, the bundle of fibres slips out as soon as the specific load on 
the element exceeds the specific load-carrying capacity indicated by the 
"natural-resin impregnation" curve at the relevant sleeve length. In this 
connection, the specific loading of the element is the ratio between the 
tensile force applied to the loop secured by the clamping sleeve and the 
effective cross-section of the bundle of fibres corresponding to the sum 
of the cross-sections of all of the fibres. 
Comparison of the "natural-resin impregnation", "synthetic-resin 
impregnation", and "no impregnation" curves indicates that the average 
coefficient of friction between the clamping sleeve and the bundle of 
fibres in the given clamping-sleeve length is about three times as high 
with natural-resin impregnation as with no impregnation, and about twice 
as high with synthetic-resin impregnation as with no impregnation of the 
fibres. Where the clamping-sleeve lengths are more than ten times the 
diameter of the bundle of fibres, these relationships no longer apply 
because the curves, as may be seen in FIG. 3, are not linear and, for 
reasons not yet quite clear, tend, at very long sleeve-lengths, towards a 
limit-value which is above the ultimate stress limit of the fibres, 
whereas in the case of synthetic resin impregnation and no impregnation, 
it is below the ultimate stress limit. This hitherto inadequately 
explained effect, however, makes complete utilization of the tensile 
strength of the bundle of fibres impossible with synthetic-resin 
impregnation and no impregnation of the fibres, since the bundle of fibres 
slips out of the clamping sleeve, as the load on the element increases, 
before the tensile strength or ultimate stress limit of the fibres is 
reached. 
The diagram shown in FIG. 3 applies to a constant pressure of the clamping 
sleeve, regardless of its length, on the bundle of fibres amounting to 18 
kg/mm.sup.2. At higher pressure-values, which, however, are scarcely 
attainable with aluminum clamping sleeves, the values appearing in the 
curves increase as the ratio between the higher pressure value and 18 
kg/mm.sup.2. At pressure-values of less than 18.2 kg/mm.sup.2, the values 
appearing in the curves decrease as the ratio between the lower 
pressure-values and 18 kg/mm.sup.2. 
As may be gathered from FIG. 3, the average coefficients of friction 
between the clamping sleeve and the bundle of fibres are 0.435 in the case 
of natural-resin impregnation. 0.28 for synthetic-resin impregnation and 
0.15 for no impregnation of the bundle of fibres. 
In connection with the diagram in FIG. 3, it should also be mentioned that 
clamping sleeves having rounded edges where the bundle of fibres emerges 
therefrom, only the load-carrying length of the sleeve is used in the 
diagram, i.e. width of the rounded edges is subtracted from the length of 
the sleeve. In connection with synthetic-resin impregnation it should also 
be noted that, in spite of the fact that the synthetic-resin impregnation 
curve in this diagram tends towards a limit-value below the ultimate 
stress limit of the fibres, in the loading test the bundle of fibres may 
rupture before slipping out of the clamping sleeve, particularly at the 
bend in the loop and, in the case of sharp-edged sleeves, where the bundle 
emerges therefrom. In such cases, however, the specific load at the moment 
of rupture is below the specific load-carrying capacity or ultimate stress 
limit of the fibres. The reasons for this are the same as those given 
earlier in connection with known epoxy-resin impregnation. 
In conclusion, it should also be pointed out that in the tensile tests for 
establishing the diagram of FIG. 3, use was made of fibre-bundles with a 
denier of 21300, comprising fibres arranged in strands running parallel 
with each other, made of an aromatic polyamide, and having a tensile 
strength of 300 kg/mm.sup.2, a modulus of elasticity of 13400 kg/mm.sup.2, 
and elongation at rupture of 2.6%, and a specific weight of 1.45 
g/cm.sup.3 ; that the diameter of the compressed fibre-bundle was about 
1.5 mm, and the effective cross-section of the bundle was about 1.65 
mm.sup.2 ; and that each of the fibre-bundles used had a loop at each end 
secured by a clamping sleeve, and had no covering.