Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
w ~ 90/1214~ 2 2 PCT/EP90/00493
- 1 -
STEEL_CORD WITH IMPROVED FATIGUE STREN~TH
The invention relates to a steel cord for the reinforcement
of elastomers, comprising two strands of at least two fila-
ments each so as to form an m + n -structure, where m is the
number of filaments of the first strand and n the number of
filaments of the second strand, m and n being greater than or
equal to two.
The steel cord according to the invention is particularly
suitable for use as a reinforcement of rubber articles such
---as- tires,-and more particularly for use as a reinforcement of
breaker layers in a tire.
Steel cords for use as a reinforcement of breaker layers in a
tire conveniently comprise steel filaments having a diameter
between 0.05 mm and 0.60mm, preferably between 0.15 and 0.45
mm. A conventional steel composition for such steel cords is
a carbon content above 0.65 %, preferably above 0.80 %, e.g.
0.83 % or 0.85 %, a manganese content between 0.40 and
200.70 %, a silicon content between 0.15 and 0.30 %, and m~xi-
-,mum sulphur and phosphorus contents of 0.03 ~0. However, the
invention is nGt li",i' d to such a steel composition. Other
elements such as chromium, nickel or boron may also be added.
The steel cord usually has a rubber adherable layer such as a
copper, zinc, or brass alloy.
The state of the art of steel cords for reinforcement of elas-
tomers, and more particularly for reinforcement of a breaker
layer of a tire provides several different constructions.
Among these construct,ons the n x 1 -constructions occupy 2
special place. These are constructions with n filaments
twisted together with the same twist pitch and in the same
twist sense, n is an integer number between 3 and 5. The
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problem with these constructions is that they have a central
vnid where rubber cannot penetrate during vulcanisation and
where moisture may easily enter and cause corrosion.
A solution to this problem has been given by the open n x l
-constructions. These are constructions where one or more
filaments are kept apart from each other by giving them a
specified preformation during the twisting process. However,
this preformation must exceed a certain limit in order to
avoid closing the steel cord when this is put under tension
o during the vulcanisation process. The problem is then that
too high a preformation may cause an irregular cord aspect
and instability.
In addition to the n x l -constructions the 2 ~ 2 -construc-
tion which is disclosed in US-A-4,408,444 has been widely
used in the tire manufacturing industry too. This cord has
the advantage of having full rubber penetration whether
brought under tension or not, but has the drawbacks of a poor
fatigue limit and a still too great cord diameter. As a conse-
quence this cord is less suitable when a h;gh fatigue perfor-
mance is required or when a thin rubber ply is a priority.,
It is an object of the present invention to avoid one or more
drawbacks of the prior art.
- 25 It is also an object of the present invention to provide a
cord with a high fatfgue performance whilst still enabling
full rubber penetration.
According to the present invention there is provided a steel
cord for the reinforcement of elastomers, which comprises two
strands of at least two filaments each. These strands are
twisted around each other and form helicoids of a same pitch.
The filaments of the first strand have a pitch differing from
the pitch of said helicoids and have a value of more than
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300 mm. The filaments of the second strand have the same
pitch as the helicoids and are twisted in the same sense as
the helicoids. All the filaments of both strands have a
diameter between 0.0~ and 0.45 mm. The diameter of the
filaments of one of the strands is at least 0.02 mm greater
than the diameter of the filaments of the other of the
strands.
According to a preferable embodiment of the invention the
diameter of the filaments of the second strand is at least
0.02 mm greater than the diameter of the filaments of the
first strand, and preferably up to 0.12 mm greater than the
diameter of the filaments of the first strand.
In this way an alternatiYe m ~ n -construction is provided,
where m is the number of` filaments of the first strand and n
the number of filaments of the second strand.
The filaments conveniently have a circular cross-section, but
this is not necessary. In cases where the Filaments don't
have a circular cross-section, "diameLer" means the diameter
of a circular cross-section with the same surface as the
cross-section of .he fi12meRts.
The filaments within one strand conveniently have the same
diameter, but small differences in the range of 0.01 mm -
0.02 mm may occur.
As will be shown below the inventors have surprisingly foundthat the fatigue limit of the cord according to the invention
is much higher than the fatigue limit of a conventional m ~ n
3o ~construction with the same cross-sectional surface. This is
surprising because the diametcr of tr.e ilaments cf one
strand has been decreased with resp~ct to the conventional
m + n -construction and thé diameter of the filaments of the
other strand has been increased with respect to the convQn-
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WO 90/12145 PCr/EP90/0#493
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tional m ~ n -construction in order to obtain about the same
cross-sectional surface and hence reinforcing effect. It is
hereby understood that, as is generally known in the art,
decreasing the diameter of filaments increases the fatigue
limit and increasing the diameter of filaments decreases the
fatigue limit.
Preferably the number of steel filaments in the first strand
is equal to the number of steel filaments in the second
strand and most preferably this number is equal to two.
The steel-fi-laments-in both strands may have a normal tensile- -
strength, i.e. a tensile strength below the value of
Rm = 2250 - 1130 log d (N/mm2) (I),
- where d is the diameter expressed in mm, or they may have a
high tensile strength, i.e. a tensile strength above the
value of formula (I).
In a special way of carrying out the invention the filaments
of one strand have a normal tensil-e stren~th and the fila-
ments of the other strength have a high tensile strength.
~ If the filaments of the first strand have the smaller
-~ 25 diameter and have a high tensile strength and the filaments
of the second strand have the greater diameter and have a
normal tensile strength then the loss in reinforcing strength
of the first strand with regard to the second strength due to
the smaller diameters may be compensated so that both strands
30 equally contribute to the tensile strength of the whole cord.
However, this is not necessary : the rilaments of the ,irst
strand having the smaller diameter may also have a normal
tensile strength while the filaments of the second strand
having the greater diameter have a high tensile strength.
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WO 90/12145 j~ ~ .3 !~ ~Ji ,,~ ~ PCT/EP~0/00493
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It is further clear that by using filaments with a high ten-
sile strength, the overall diameter of the cord may be
decreased without loss of tensile strength with regard to
m + n-cords with all filaments having a normal tensile
strength.
The invention will now be described in more detail with
reference to the accompanying drawings wherein :
FIGURE l represents a side view and subsequent
cross-sections of a cord according to the present inven-
tion;
FIGURE 2 represents an apparatus for manufacturing a cord
according to the present invention.
FIGURE l represents a cord 1 according to the present inven-
tion. The cord consists of a first strand having two fila-
ments ll and a second strand also having two filaments 12.
The cross-section of the filaments ll of the first strand is
shaded. The filaments ll have a diameter of 0.24 mm and the
filaments l2 have a diameter of 0.28 m~. The two strands zre
twisted around each other with a twist pitch p of 15 mm. The
twist pitch p conver.iently lies bet~e^n 30 and lO0 times the
average diameter of the filaments and preferably between 40
and 80 times the average diameter of the filaments. The fila-
ments 12 of the second strand are twisted in the same sensewith the same twist pitch p while the filaments 11 of the
first strand remain substantially parallel to each other,
i.e. they have an infinite twist pitch.
.
FIGURE 2 represents a double-twisting apparatus 2 for manufac-
turing a cord according to the presell~ invention. The fila-
ments ll of the first strand are drawn from bobbins 21 and
pass through the holes 231 of a guiding plate 23 and come
together at a first guiding pulley 24 of the double-twist2r 2
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W O 9O/1~145 Pcl/EPs~/00~93
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where they are provisionally twisted together. They pass fur-
ther over a flyer 25 and over a reversing pulley 26. Two bob-
bins 27 are stationarily mounted inside the rotor of the
do~ble-twister 2. The filaments 12 of the second strand are
drawn from these bobbins 27 and pass through the holes 281 of
a guiding plate 28 and come together with the provisionally
twisted ~ilaments 11 at the cabling die 29. The filaments 11
and 12 pass over reversing pulley 210, flyer 211 and guiding
pulley 212 to the winding unit 213. Between the cabling die
10 29 and the guiding pulley 212 the filaments 11 are untwisted
so as to form a first strand consisting of substantially
parallel filaments 11, while the filaments 12 are twisted
with the same pitch and in the same direction as the two
strands.
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TEST 1
The fatigue properties of two prior art cords have been com-
20 pared with a cord according to the present invention (NT =
normal tensile, i.e. a tensile strength below the value of
formula (I~; HT = hioh tensile, i.e. a tensile strength above
the value of formula (I)) :
1. prior art cord : 2 x 0.25 NT + 2 x 0.25 NT;
pitch = 14 mm
2. prior art cord : 2 x 0.25 HT + 2 x 0.25 HT;
pitch = 14 mm
3. invention cord : 2 x 0.22 NT ~ 2 x 0.28 HT;
pitch = 14 mm
~o It is understood that in these constructions the first str2nd - -
with substantially parallel filaments is na~,ed first and the
second strand with twisted ~ilaments is named second.
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WO 90/12145 ~ m ~ PCI/EP90/00493
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TABLE 1
¦ cord ¦ cross-section ¦ breaking load I fatigue limit¦
¦ ¦ (mm2) I (N) I(N/mm2)
¦A. 1. ¦ 0.196 ¦ 530 ¦< 600
2. 1 0.196 1 605 1~ 600
3 . I 0. 199 1 604 1 850
¦ B. 1. ¦ 0.196 ¦ 520 ¦ 800
2. 1 0. 196 1 633 1 700
3 . - - I - 0. 199 - I 621 - I 990
3 . I 0. 199 1 ~81 1 900
1,, I .. ~
The fatigue limit has been measured with the well-known
Hunter test.
The second series B. of tests has been made on cords from a
slightly different steel rod type than this of series A.
In both series it may be easily seen that the cord 3.
according to the invention has a much higher fatigue limit
than the cords l. and 2. according to the prior art.
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TEST 2
A second test reveals an additional advantage of the cord
according to the invention, namely a better behaviour under
compression.
The same cords as mentioned under Test 1 have been compared
with each other. The buckling stress, the deformation at the
buckling stress, and the Young's modulus in compression have
been measured for these cords.
The buckling itress is a measul-e 'or the m~ximum com?res;icii
force taken up by the stee~ cord when embedded in rubber. The
greater the buckling stress the greater this maxi~um compres-
- s;on force.
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WO 90/12145 PCI/EP90/00493
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The deformation is the deformation of the cord in rubber when
subjected to this maximum compression.
A high Young's modulus in compression means a cord which does
not allow high deformations under compression whereas a low
Young's modulus in compression allows high deformations under
compression.
Further details about these features and their method of
measurement may be found in the paper by Bourgois L., Survey
of Mechanical Properties of Steel Cord and Related Test
10 Methods, Tire Reinforcement and Tire Performance, ASTM STP
694, R~A. Fleming and D.I. Livingston7 Eds., American Society
for Testing and Materials, 1979, pp. 19-46. - ~
Table 2 mentions the results :
15 TABLE 2
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COMPRESSION BEHAVIOUR
¦ buckling I deformation ¦ compression
I cord I stress I ¦ modulus
20 I ¦ (N/mm2) ¦ (%) ¦ (kN/mm2)
1- 1 433 1 0.40 1 125
2- 1 447 1 0.40 1 125
I 3. 1 475 1 1.12 1 66
25 1 - l _ I L~
TEST 3
A third test has evaluated the influence of the diameter dif-
ference between the two strands on the cord properties.
Following cords hzve been evaluat~d :
1. invention cord : 2 x 0.22 HT + 2 x 0.25 HT
pitch : 14 mm
2. invention cord : 2 x 0.25 NT + 2 x 0.28 HT
pitch : 14 mm
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W O 9Q/12145 PCT/EP90/00493
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3. invention cord : 2 x 0.20 HT ~ 2 x 0.25 HT
: pitch : 14 mm
4. invention cord : 2 x 0.25 HT ~ 2 x 0.30 HT
pitch : 16 ~m
5. invention cord : 2 x 0.22 NT + 2 x 0.28 HT
pitch : 14 mm
6. invention cord : 2 x 0.22 HT + 2 x 0.30 HT
pitch : 14 mm
7. invention cord : 2 x 0.20 HT + 2 x 0.30 HT
pitch : 14 mm
8. invention cord : 2 x 0.22 HT + 2 x 0.35 HT
~ pitch : 16 mm
Table 3 summarizes the results of the P.L.E. values and of`
the fatigue properties of these cords.
P.L.E. means here part load elongation. It is defined as the
increase in length of a gauge length between a tension of 2.S
N and a tension of 50 N and may be expressed as a percentage
- of the original gauge length. It is a measure of the openness
of the steel cord.
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W O 90/1214~ pcT/~p9o/on493
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TABLE 3
I I diameter ¦ P.L.E. ¦ fatigue limit
¦ ¦ difference ¦ 2.5-50 N ¦ Hunter test
5 ¦ cord ¦ (mm) I (%) ¦ (N/mm2)
1. 1 0.03 1 0.16 1 850
. I 0.03 1 0.16 1 850
1 3. 1 0.05 1 0.17 I 8S0
I 4. ¦ O.OS ¦ 0.14 ¦ 900
5. 1 0.06 1 0.14 1 850
- 1- 0.06 1 0.18 I - 90
0.06 I 0.17 1 900
6. I 0.08 I 0.13 I gO0
¦ 7. I 0.10 I 0.14 ¦ 1Q50
1 8. 1 0.13 I 0.40 I 950
~1 . .. _ _ . I _ . I
The fatigue limit remains high with increasing diameter dif-
ference. However, with a diameter difference of 0.13 mm a
P.L.E. value of 0.40 has been measured. This means that the
cord is open : the different filaments do no longer make con-
tact with other filaments over the whole len~th. In contradic-
tion to n x 1 -cords, this is not desired with m + n -cords.
And this is the reason why in a preferred embodiment of the
invention the diameter difference is kept below 0.12 mm (see
claim 3).
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