Field of the invention.
[0001] The present invention relates to a steel cord adapted for the reinforcement of an
elastomer such as a rubber tyre.
Background of the invention.
[0002] Steel cords are widely known to reinforce elastomers. The reinforced elastomers form
a so-called composite material. The steel cords provide for the required strength
while the elastomer provides for the required elasticity. In some applications the
steel cords must be able to follow as much as possible movements of the elastomer,
e.g. in the outer layer of the belt of a radial tyre, the so-called protection layer.
In these applications a high elongation of the steel cord is strongly desired.
This high elongation, i.e. an elongation at break between 5 and 10 %, is achieved
in the so-called high-elongation cords. The high-elongation cords are commonly multi-strand
steel cords (i.e. they comprise a number of strands and each strand comprises a number
of steel filaments) with a high degree of twisting (i.e. very small twisting pitches)
in order to create an elastic cord with the required degree of springy potential.
An example of such a cord is a 3x7x0.22 HE-cord.
These high-elongation cords, although widely used since a long time, present a number
of drawbacks.
First of all, the way of manufacturing high-elongation cords is inefficient and costly
due to the multi-strand character of the cords and to the high degree of twisting
(i.e. the small twisting steps avoid a high output of the twisting process).
Secondly, the high-elongation cords do not enable a complete penetration by the elastomer,
since any available spaces between the filaments have disappeared as a consequence
of the high degree of twisting.
Thirdly, a substantial part of the elongation gets lost during the embedding of the
steel cord in the elastomer. Typically, the elongation at fracture of a high-elongation
cord falls down from about 7.5 % to about 2.5 to 4 % after the vulcanisation in rubber.
Summary of the invention.
[0003] It is an object of the present invention to provide for a steel cord without substantial
loss of the total elongation once it is vulcanized into the elastomer.
It is another object of the present invention to provide for a steel cord with a high
elongation that is largely independent of the constructural features of the steel
cord.
It is a further object of the present invention to provide for a steel cord with a
high elongation and with a full penetration of the elastomer.
It is a further object of the present invention to provide for a steel cord with a
high degree of processability.
[0004] According to the invention, there is provided for a steel cord adapted for the reinforcement
of an elastomer. The steel cord is composed of twisted steel filaments of a pearlitic
structure. The non-embedded steel cord has an elastic and plastic elongation at break
which is of about the same level as the value of the elastic and plastic elongation
of the steel cord once vulcanized in the elastomer.
Suppose the sum of the elastic and plastic elongation at break is x %, and the sum
of elastic and plastic elongation capability in the vulcanized elastomer is y %, both
values of elongation are 'of about the same level' if
For example, if the sum of the elastic and plastic elongation x of the non-embedded
steel cord is 3.5 %, than the sum of the elastic and plastic elongation capability
y of the steel cord in the vulcanized elastomer lies between 3.00 % and 4.00 %.
[0005] Preferably the values x and y fulfill following equation :
[0006] The terms "elastic and plastic elongation" used herein are to be understood as the
total elongation minus the structural elongation.
[0007] The structural elongation, if any, is a result of the cord structure or of the preforming
given to the steel filaments. Structural elongation occurs mainly below a tensile
force of 50 Newton, e.g. below a tensile force of 20 Newton.
The elastic elongation follows Hooke's law ( σ = E x ε) and the plastic elongation
occurs mainly above 85 to 90 % of the breaking force of the cord.
[0008] According to a particular embodiment of the invention the plastic elongation reaches
a high value of about 4 %, which be obtained by a particular mode of stress relieving
the steel cord, as will be explained hereafter. This high value of plastic elongation
is not a consequence of the constructional features (multiple strands, SS-direction,
small twisting steps...) of the cord. As a consequence, the present invention allows
to obtain a high-elongation cord with an elongation that is largely independent -
at least for the elastic and plastic part - of the typical type of steel cord construction.
So it becomes possible to choose a high-elongation steel cord which avoids the disadvantages
of the convenient high-elongation steel cords, i.e. which enables full penetration
of the elastomer between the composing steel filaments and which does not require
a complex and costly way of manufacturing.
[0009] Preferably the total elongation at break, i.e. the sum of the elastic, plastic and
structural elongation, is at least 5 %.
[0010] Preferably the steel cord as a whole is in a stress-relieved state. This stress-relieving
treatment is done after the cord has been twisted to its final form.
A first advantage hereof is a high-elongation steel cord which maintains its degree
of elongation in the elastomer.
A second advantage is a steel cord with a high degree of structural stability, i.e.
no significant residual torsions, a high degree of straigthness and almost no flare.
Such a cord will have no substantial processability problems during the embedding
of the cord in the elastomer and can be used without problems in highly automated
tyre manufacturing processes. This high degree of structural stability of the cord
is obtained without particular and supplemental mechanical post-treatments of the
cord.
The present invention is clearly distinguished from the stress-relieving of individual
steel filaments. Each steel filament that has been stress-relieved individually, also
has a high plastic elongation. Twisting such stress-relieved steel filaments into
a final cord means that every single filament is plastically bent and, dependent upon
the particular way of twisting, that every single filament is twisted around its own
axis. This leads unavoidably to a significant loss of the plastic elongation of the
cord and to the creation of internal tensions in the steel filaments.
[0011] Although the total elongation of the steel cord is largely independent of the particular
type of steel cord construction, the invention steel cord construction is preferably
an open structure. The terms "open structure" refer to a steel cord construction which
enables full penetration of the elastomer into the steel cord. This means that elastomer
may surround every individual steel filament of the steel cord.
The openness may be obtained in two major ways.
A first way for obtaining an openness is to create a structure that is tangentially
open. A tangentially open structure comprises layers of steel filaments that are unsaturated,
which means that spaces exist between the individual steel filaments so that elastomeric
material may penetrate therebetween. Unsaturated layers may be formed by appropriate
choice of the number of filaments in the layer and/or by the diameter of the filaments
in the layer.
A second way for obtaining an openness is to create a structure that is radially open.
In a radially open structure the composing filaments are more remote from an imaginary
axis than they would be in a closed compact form. The radial openness may be obtained
by appropriate preforming of the steel filaments.
Obviously a radial openness may be combined with a tangential openness. An example
is a 3+9-structure, where appropriate preforming of the three core filaments may result
in a radial openness of the core and where the nine layer filaments may form an unsaturated
layer around the core.
[0012] Preferably the steel cord has a tensile strength of at least 2150 MPa.
[0013] The yield strength of the cord at a permanent elongation of 0.2 % is preferably at
least 88 % (e.g. at least 90 % or at least 92 %) of the tensile strength of the cord.
This high yield strength is a direct consequence of the stress-relieving treatment
that is applied on the already twisted cord and of the absence of any supplemental
mechanical post-treatment.
[0014] One example of an invention steel cord may consist of two groups of steel filaments
: a first group of one or more steel filaments and a second group of two or more steel
filaments. If the first group has two steel filaments, these two steel filaments may
be twisted or not. The second group of steel filaments is twisted around the first
group so as to form an unsaturated layer around the first group, which means that
spaces exist in the layer between two or more steel filaments of the second group
and that elastomer may penetrate through the layer to the first group.
Such a type of steel cord construction may comprise following embodiments in a non-limitative
way :
- 2 + n, manufactured according to US-A-4,408,444, the two filaments of the first group
are not twisted and n ranges from 2 to 4 ;
- 1 + m, one filament in the first group functioning as a core and m filaments in the
second group functioning as a layer, where m ranges from 3 to 9 ;
- 2 + m, two twisted filaments in the first group functioning as a core, and m filaments
in the second group functioning as a layer where m ranges from 3 to 9 ;
Due to the unsaturated layer of filaments of the second group and due to the maximum
number of two filaments in the first group, such a steel cord construction enables
a full rubber penetration.
[0015] In addition to a substantial plastic elongation, a steel cord according to the present
invention may also have a substantial structural elongation, e.g. obtained by giving
the individual steel filaments an undulation by appropriate pre-forming or post-forming.
In this way, a high-elongation 1 x n -cord (n ranging from two to five) with full
rubber penetration may be obtained.
Brief description of the drawings.
[0016] The invention will now be described into more detail with reference to the accompanying
drawings wherein
- FIGURE 1 shows the transversal cross-section of a first embodiment of an invention
cord ;
- FIGURE 2 shows the transversal cross-section of a second embodiment of an invention
cord ;
- FIGURE 3 shows the transversal cross-section of a third embodiment of an invention
cord ;
- FIGURE 4 compares the elongation curve of a known high-elongation cord with the elongation
curve of an invention cord ;
- FIGURE 5 shows a general elongation curve of a steel cord.
Description of the preferred embodiments of the invention.
[0017] FIGURE 1 shows the cross-section of a 2+2-invention cord 10. The first group comprises
two non-twisted steel filaments 12, and the second group comprises two steel filaments
14 that are twisted around the first group and around each other thereby creating
an unsaturated layer around the first group. Such a cord can be manufactured in one
single twisting step.
[0018] FIGURE 2 shows the transversal cross-section of a 2 + 6 - steel cord construction
10. The first group consists of two steel filaments 12 that are twisted around each
other. The second group consists of six steel filaments 14 that are twisted around
the first group. As can be seen on FIGURE 2, the layer created by the second group
is unsaturated so that rubber may penetrate. Such a steel cord may be manufactured
in two steps.
[0019] FIGURE 3 shows the transversal cross-section of an alternative embodiment of a invention
steel cord 10. The steel cord consists of four steel filaments 16 where one or more
have been plastically formed into a wave form so that gaps have been created between
the steel filaments 16 even if a tensile force is exerted on the steel cord 10. Such
an open steel cord may be manufactured in one single step. The type of wave applied
to individual steel filaments may vary to a great extent, depending upon the typical
wave form, the amplitude and the pitch. Preferably, however, the pitch of the wave
is substantially smaller than the pitch of the cord in order to create microgaps between
the individual steel filaments. The wave form may be planar or spatial. A typical
example is a wave form that may be obtained by passing the individual filaments between
two toothed wheels, such as disclosed in US-A-5,020,312. Another example is a helicoidal
wave form such as disclosed in EP-A-0 462 716. Still another example is a polygonal
wave form, such as mentioned in WO-A-95/16816.
[0020] FIGURE 4 shows two elongation curves 18 and 20. The abscissa is the elongation ε,
expressed in per cent, and the ordinate is the tensile strength R
m, expressed in MPa or in N/mm
2.
Curve 18 is the elongation curve of a prior art high-elongation cord with a structural
elongation. It shows a relatively large elongation for small initial loads (slope
much smaller than the modulus E of elasticity of steel) and the total elongation at
break is limited once such a cord is embedded in rubber.
Curve 20 is the elongation curve of an invention high-elongation cord with a plastical
elongation. It shows a relatively small elongation for small initial loads (slope
about equal to the modulus of elasticity). The elongation at break is greater than
5 % if not embedded in rubber and it remains that great after vulcanisation in rubber
[0021] The differences between structural, elastic and plastic elongation are illustrated
in FIGURE 5 where an elongation curve 22 is shown. Three main zones can be distinguished.
A first zone 24 is characterized by a relatively large initial elongation in comparison
with small loads (less than 50 Newton). This initial elongation is composed of structural
elongation (major part) and of elastic elongation (minor part). A second zone 26 is
characterized by a linear relationship and forms the purely elastic part. A third
zone 28 starts at the point where the curve leaves the linear relationship and is
characterized by a non-linear saturation-like curve. The third zone is only composed
of the plastic elongation. Summarizing, the structural elongation only occurs in the
first zone, the elastic elongation occurs in both the first and second zone and the
plastic elongation occurs in the third zone. Some steel cord constructions, however,
do not have a substantial structural elongation.
Example
[0022] A high-elongation steel cord 2x0.33 + 6x0.33 with twist directions S/S and twist
pitches 9mm/18mm, according to the invention may be obtained as follows :
- the individual steel filaments receive a last intermediate patenting treatment and
are subsequently coated with a layer of brass ;
- thereafter, the thus coated steel filaments are wet drawn until a final diameter of
0.33 mm and a tensile strength Rm of about 2900 MPa ;
- the wet drawn steel filaments are twisted into the final cord 2x0.33 + 6x0.33, by
means of a double-twisting device in a way that is known as such in the art ;
- the thus twisted cord 2x0.33 + 6x0.33 is subjected to a stress-relieving treatment,
e.g. by passing the cord through a high-frequency or mid-frequency induction coil
of a length that is adapted to the speed of the cord ; indeed it is observed that
a thermal treatment at a specified temperature of about 300 °C and for a certain period
of time brings about a reduction of tensile strength of about 10% without any increase
in plastic elongation at break ; by slightly increasing the temperature, however,
to more than 400 °C, a further decrease of the tensile strength is observed and at
the same time an increase in the plastic elongation at break ; in this way the plastic
elongation can be increased to more than 6%, while the tensile strength decreases
e.g. from 2900 MPa to about 2500 MPa for this particular diameter of 0.33 mm.
[0023] The brass coated steel filaments or steel cords, although this is not strictly necessary,
may be subjected to an acid dip in order to avoid or to take away any zinc oxide layer
that can be created on the brass during the stress-relieving treatment.
A first table summarizes some of the particular properties of a 2x0.33 + 6x0.33 invention
steel cord and compares these properties to the corresponding properties of a convenient
3x7x0.22 HE-cord :
Table 1 :
Properties and features : |
2x0.33+6x0.33 |
3x7x0.22 HE |
direction of twist |
SS |
SS |
lay length (mm) |
9/18 |
4.5/8 |
linear density (g/m) |
5.30 |
6.95 |
optical diameter (mm) |
1.185 |
1.585 |
part load elongation at initial load of 50 Newton (%) |
0.078 |
2.82 |
tensile test on cord not embedded in rubber: |
|
|
breaking load (Newton) |
1652 |
1820 |
tensile strength Rm (MPa) |
2448 |
2280 |
total elongation at break (%) |
5.64 |
6.00 |
yield strength at elongation of 0.2 % (% of Rm) |
91 |
82* |
tensile test on cord embedded in rubber: |
|
|
breaking load (Newton) |
1705 |
1925 |
tensile strength Rm (MPa) |
2527 |
2412 |
total elongation at break (%) |
5.51 |
3.20 |
yield strength at elongation of 0.2 % (% of Rm) |
90 |
83* |
arc height (mm) |
6 |
14 |
3-point bending stiffness (Nmm2) of non-embedded cord |
1010 |
|
3-point bending stiffness (Nmm2) of embedded cord |
1394 |
|
Hunter fatigue test |
|
|
dry not embedded in rubber (MPa) |
900 |
1000 |
dry embedded in rubber (MPa) |
900 |
1000 |
wet embedded in rubber (MPa) |
800 |
450 |
* yield strength has been determined on the elastic part of the tensile-elongation
curve, so leaving away the structural part |
[0024] As may be derived from table 1, the total elongation at break does not decrease significantly
after embedding the invention cord in rubber.
This is a direct consequence of the thermal stress-relieving treatment which has been
applied on the final twisted cord. This thermal treatment occurred at a higher temperature
than the temperature of rubber vulcanisation, so that the vulcanisation process 'was
no longer able' to change the properties of the invention cord significantly.
A further advantage of the invention cord is that the fatigue resistance does not
decrease significantly in wet circumstances, whereas the convenient high elongation
cord sees its fatigue resistance fall to less than 50%. This is a consequence of the
rubber penetration which is complete in the invention cord and incomplete in the prior
art cord.
[0025] A second table compares a 1+5 invention cord to a 1+5 cord where the particular stress-relieving
treatment has not been applied.
Table 2
Properties and features : |
1x0.38+5x0.38 stress-relieved |
1x0.38+5x0.38 prior art |
direction of twist |
S |
S |
lay length (mm) |
20 |
20 |
linear density (g/m) |
5.35 |
5.35 |
optical diameter (mm) |
1.16 |
1.16 |
part load elongation at initial load of 50 Newton (%) |
0.070 |
0.061 |
tensile test on cord not embedded in rubber : |
|
|
breaking load (Newton) |
1703 |
1618 |
tensile strength Rm (MPa) |
2497 |
2382 |
total elongation at break (%) |
6.69 |
3.25 |
yield strength at elongation of 0.2 % (% of Rm ) |
90 |
84 |
tensile test on cord embedded in rubber : |
|
|
breaking load (Newton) |
1755 |
1795 |
tensile strength R m (MPa) |
2574 |
2645 |
total elongation at break (%) |
6.67 |
1.72 |
yield strength at elongation of 0.2 % (% of R m ) |
90 |
84 |
arc height (mm) |
4 |
6 |
3-point bending stiffness (Nmm 2 ) of embedded cord |
1724 |
|
Hunter fatigue test |
|
|
dry not embedded in rubber (MPa) |
1000 |
950 |
dry embedded in rubber (MPa) |
950 |
950 |
wet embedded in rubber (MPa) |
850 |
950 |
[0026] The total elongation at break of a 1+5 prior art cord is only 3.25 % and falls down
to a poor 1.72 % after embedding the steel cord in rubber. The invention 1+5 cord,
in contrast therewith, has a high elongation of 6.69 % and maintains this high level
after embedding the steel cord in rubber.
[0027] With steel filaments of a martensitic structure such as disclosed in GB-A- 1 427
999, instead of steel filaments of a pearlitic structure, the inventors have experienced
that a total elongation at break of at least 5 % is difficult to reach, and that,
even if a high elongation at break is reached for a non-embedded steel cord, this
elongation falls down considerably once the cord has been vulcanized in an elastomer.
[0028] In addition to the above-mentioned characteristics and properties, a steel cord according
to the present invention has following features which make it able for the reinforcement
of elastomers such as rubber:
- the filament diameters range from 0.04 mm to 1.1 mm, more specifically from 0.15 mm
to 0.60 mm, e.g. from 0.20 mm to 0.45 mm ;
- the steel composition generally comprises a minimum carbon content of 0.60 % (e.g.
at least 0.80 %, with a maximum of 1.1 %), a manganese content ranging from 0.20 to
0.90 % and a silicon content ranging from 0.10 to 0.90 % ; the sulphur and phosphorous
contents are preferably kept below 0.03 % ; additional elements such as chromium (up
to 0.2 à 0.4 %), boron, cobalt, nickel, vanadium ... may be added to the composition
;
- the filaments are conveniently covered with a corrosion resistant coating such as
zinc or with a coating that promotes the adhesion to the rubber such as brass, or
a so-called ternary brass such as copper-zinc-nickel (e.g. 64% / 35.5% / 0.5%) and
copper-zinc-cobalt (e.g. 64% / 35.7% / 0.3%), or a copper-free adhesion layer such
as zinc-cobalt or zinc-nickel ; the conventional brass layer may also be provided
with a top flash of nickel, cobalt or copper ; these top flashes, which are known
as such, can be very advantageous in the context of the present invention since they
prevent the zinc in the brass from migrating to the surface and from building zinc
oxide during the stress-relieving treatment ; in the case of a nickel top layer, suitable
amounts of nickel have proved to range from 1 to 4 % weight per cent of the coating
layer, below 1 % the effect of nickel is not pronounced, above 4 % the level of initial
adhesion decreases.
[0029] The invention is suitable for all common and available final tensile strengths from
2150 MPa to about 3500 MPa and more. Due account must, however, been taken of a drop
in tensile strength of about 10 to 15% as a consequence of the thermal stress-relieving
treatment. If for example, a final tensile strength of 3500 MPa is desired, the individual
steel filaments must be drawn to a tensile strength of about 4000 MPa, if a final
tensile strength of 2150 MPa is desired, the individual steel filaments must be drawn
to a tensile strength of about 2400 MPa.
1. A steel cord (10) adapted for the reinforcement of an elastomer, the steel cord comprising
steel filaments (12,14,16) of a pearlitic structure, the steel cord having a plastic
and elastic elongation (26,28) at break of x % and an elastic and plastic elongation
capability in the vulcanized elastomer of y %, the values x and y fulfilling following
equation:
2. A steel cord (10) according to claim 1 wherein
the steel cord has a total elongation at break is of at least 5 %.
3. A steel cord (10) according to claim 1 or claim 2 wherein
the steel cord as a whole is in a stress-relieved state.
4. A steel cord (10) according to claim 3 wherein
the steel cord has a final tensile strength of at least 2150 MPa.
5. A steel cord (10) according to claim 3 or 4 wherein
the yield strength of the cord at a permanent elongation of 0.2 per cent is at least
88 % of the tensile strength of the cord.
6. A steel cord (10) according to any one of the preceding claims wherein
the steel cord has an open structure to enable rubber penetration of the elastomer.
7. A steel cord (10) according to claim 6 wherein
one or more of the steel filaments (12) form a first group and the other steel filaments
(14) form a second group of two or more filaments, the second group being twisted
around the first group so as to form an unsaturated layer around the first group.
8. A steel cord (10) according to claim 7 wherein
the second group consists of three to nine steel filaments.
9. A steel cord (10) according to claim 7 or 8 wherein
the first group consists of one or two steel filaments.
10. A steel cord (10) according to any one of the preceding claims wherein
the steel cord has a structural elongation (24) of at least 0.5 %.
11. A steel cord (10) according to any one of the preceding claims wherein
the filaments have a diameter ranging from 0.04 mm to 1.10 mm.
12. A steel cord (10) according to any one of the preceding claims wherein
the steel filaments have a coating of brass and wherein there is a top flash of nickel,
cobalt or copper on top of the brass coating.
1. Stahlkord (10), welcher für die Verstärkung eines Elastomers ausgelegt ist, wobei
der Stahlkord Stahlfilamente (12, 14, 16) einer perlitischen Struktur umfaßt, wobei
der Stahlkord eine plastische und elastische Bruchdehnung (26, 28) von x % und eine
elastische und plastische Dehnungsfähigkeit im vulkanisierten Elastomer von y % aufweist,
wobei die Werte x und y die folgende Gleichung erfüllen:
2. Stahlkord (10) nach Anspruch 1, wobei der Stahlkord eine Gesamtbruchdehnung von wenigstens
5 % aufweist.
3. Stahlkord (10) nach Anspruch 1 oder Anspruch 2, wobei der Stahlkord insgesamt in einem
Zustand abgebauter Spannung ist.
4. Stahlkord (10) nach Anspruch 3, wobei der Stahlkord eine abschließende Zugfestigkeit
von wenigstens 2150 MPa aufweist.
5. Stahlkord (10) nach Anspruch 3 oder 4, wobei die Streckgrenze des Kords bei einer
permanenten Dehnung von 0,2 % wenigstens 88 % der Zugfestigkeit des Kords ist.
6. Stahlkord (10) nach einem der vorangehenden Ansprüche, wobei der Stahlkord eine offene
Struktur aufweist, um eine Gummidurchdringung des Elastomers zu ermöglichen.
7. Stahlkord (10) nach Anspruch 6, wobei ein oder mehrere der Stahlfilamente (12) eine
erste Gruppe und die anderen Stahlfilamente (14) eine zweite Gruppe von zwei oder
mehr Filamenten bilden, wobei die zweite Gruppe um die erste Gruppe derart verdrillt
ist, daß eine ungesättigte Lage um die erste Gruppe herum gebildet ist.
8. Stahlkord (10) nach Anspruch 7, wobei die zweite Gruppe drei bis neun Stahlfilamente
umfaßt.
9. Stahlkord (10) nach Anspruch 7 oder 8, wobei die erste Gruppe ein oder zwei Stahlfilamente
umfaßt.
10. Stahlkord (10) nach einem der vorangehenden Ansprüche, wobei der Stahlkord eine strukturelle
Dehnung (24) von wenigstens 0,5 % aufweist.
11. Stahlkord (10) nach einem der vorangehenden Ansprüche, wobei die Filamente einen Durchmesser
aus dem Bereich von 0,04 mm bis 1,10 mm aufweisen.
12. Stahlkord (10) nach einem der vorangehenden Ansprüche, wobei die Stahlfilamente eine
Messingbeschichtung aufweisen und wobei eine oberflächliche Plattierung aus Nickel,
Kobalt oder Kupfer an der Oberseite der Messingbeschichtung vorgesehen ist.
1. Câble d'acier (10) conçu pour renforcer un élastomère, le câble d'acier comprenant
des filaments d'acier (12, 14, 16) à structure perlitique, le câble d'acier présentant
un allongement plastique et élastique (26, 28) à la rupture de x % et une capacité
d'allongement élastique et plastique dans l'élastomère vulcanisé de y %, les valeurs
x et y correspondant à l'équation suivante :
2. Câble d'acier (10) selon la revendication 1, caractérisé en ce que le câble d'acier
possède un allongement total à la rupture égal à au moins 5 %.
3. Câble d'acier (10) selon la revendication 1 ou la revendication 2, caractérisé en
ce que le câble d'acier se trouve, dans son ensemble, dans un état de détente.
4. Câble d'acier (10) selon la revendication 3, caractérisé en ce que le câble d'acier
possède une résistance à la traction finale d'au moins 2150 MPa.
5. Câble d'acier (10) selon la revendication 3 ou 4, caractérisé en ce que la limite
d'élasticité du câble avec un allongement permanent de 0,2 % est égale à au moins
88 % de la résistance à la traction du câble.
6. Câble d'acier (10) selon l'une quelconque des revendications précédentes, caractérisé
en ce que le câble d'acier possède une structure ouverte pour permettre la pénétration
de l'élastomère dans le caoutchouc.
7. Câble d'acier (10) selon la revendication 6, caractérisé en ce que l'un des filaments
d'acier (12) ou plusieurs des filaments forment un premier groupe et en ce que les
autres filaments d'acier (14) forment un second groupe de deux filaments ou plus,
le second groupe étant torsadé autour du premier groupe de manière à former une couche
non saturée autour du premier groupe.
8. Câble d'acier (10) selon la revendication 7, caractérisé en ce que le second groupe
est constitué de trois à neuf filaments d'acier.
9. Câble d'acier (10) selon la revendication 7 ou 8, caractérisé en ce que le premier
groupe est constitué de un ou deux filaments d'acier.
10. Câble d'acier (10) selon l'une quelconque des revendications précédentes, caractérisé
en ce que le câble d'acier possède un allongement structurel (24) égal à au moins
0,5 %.
11. Câble d'acier (10) selon l'une quelconque des revendications précédentes, caractérisé
en ce que les filaments ont un diamètre compris entre 0,04 mm et 1,10 mm.
12. Câble d'acier (10) selon l'une quelconque des revendications précédentes, caractérisé
en ce que les filaments d'acier ont un revêtement en laiton et en ce qu'une couche
de finition au nickel, au cobalt ou au cuivre recouvre le revêtement en laiton.