Steel cord with improved fatigue strength

A steel cord (1) for the reinforcement of elastomers, especially for the reinforcement of breaker layers in a tire, said steel cord comprising two strands of at least two filaments (11, 12) each, said strands being twisted around each other and forming helicoids of a same pitch, the filaments (11) of the first strand having a pitch differing from the pitch of said helicoids and having a value of more than 300 mm, the filaments (12) of the second strand having the same pitch as said helocoids and being twisted in the same sense as said helicoids, all the filaments of both of said strands having a diameter between 0.08 and 0.45 mm, wherein the diameter of the filaments of one of said strands is at least 0.02 mm greater than the diameter of the filaments of the other of said strands. Preferably the diameter of the filaments (12) of said second strand is at least 0.02 mm greater than the diameter of the filaments (11) of said first strand.

BACKGROUND OF THE INVENTION 
The invention relates to a steel cord for the reinforcement of elastomers, 
comprising two strands of at least two filaments 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 0.70 %, a silicon content between 0.15 and 0.30 %, and 
maximum sulphur and phosphorus contents of 0.03 %. However, the invention 
is not limited 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 elastomers, and 
more particularly for reinforcement of a breaker layer of a tire provides 
several different constructions. 
Among these constructions the n.times.1 -constructions occupy a 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 problem with these constructions is that they have a 
central void 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.times.1 
-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 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.times.1 -constructions the 2+2 -construction 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 consequence 
this cord is less suitable when a high fatigue performance is required or 
when a thin rubber ply is a priority. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to avoid one or more drawbacks of 
the prior art. 
It is also an object of the present invention to provide a cord with a high 
fatigue 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 
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 0.08 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 alternative 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, "diameter" means the diameter of a circular cross-section 
with the same surface as the cross-section of the filaments. 
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 found that the 
fatigue limit of the cord according to the invention is much higher than 
the fatigue limit of a conventional m+n -construction with the same 
cross-sectional surface. This is surprising because the diameter of the 
filaments of one strand has been decreased with respect to the 
conventional m+n -construction and the diameter of the filaments of the 
other strand has been increased with respect to the conventional 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 filaments in both strands may have a normal tensile strength, 
i.e. a tensile strength below the value of 
EQU R.sub.m =2250-1130 log d (N/mm.sup.2) (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 tensile strength and the filaments of the other strength 
have a high tensile strength. 
If the filaments of the first strand have the smaller 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 equally contribute to the tensile strength of the whole cord. 
However, this is not necessary: the filaments of the first 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. 
It is also clear that by using filaments with a high tensile 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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 represents a cord 1 according to the present invention. The cord 
consists of a first strand having two filaments 11 and a second strand 
also having two filaments 12. The cross-section of the filaments 11 of the 
first strand is shaded. The filaments 11 have a diameter of 0.24 mm and 
the filaments 12 have a diameter of 0.28 mm. The two strands are twisted 
around each other with a twist pitch p of 15 mm. The twist pitch p 
conveniently lies between 30 and 100 times the average diameter of the 
filaments and preferably between 40 and 80 times the average diameter of 
the filaments. The filaments 12 of the second strand are twisted in the 
same sense with 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. 
FIG. 2 represents a double-twisting apparatus 2 for manufacturing a cord 
according to the present invention. The filaments 11 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-twister 2 where they are provisionally twisted together. They pass 
further over a flyer 25 and over a reversing pulley 26. Two bobbins 27 are 
stationarily mounted inside the rotor of the double-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 filaments 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 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. 
TEST 1 
The fatigue properties of two prior art cords have been compared with a 
cord according to the present invention (NT=normal tensile, i.e. a tensile 
strength below the value of formula (I); HT=high tensile, i.e. a tensile 
strength above the value of formula (I)): 
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1. prior art cord 
2 .times. 0.25 NT + 2 .times. 0.25 NT; 
pitch = 14 mm 
2. prior art cord 
2 .times. 0.25 HT + 2 .times. 0.25 HT; 
pitch = 14 mm 
3. invention cord 
2 .times. 0.22 NT + 2 .times. 0.28 HT; 
pitch = 14 mm 
______________________________________ 
It is understood that in these constructions the first strand with 
substantially parallel filaments is named first and the second strand with 
twisted filaments is named second. 
TABLE 1 
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cross-section 
breaking load 
fatigue limit 
cord (mm.sup.2) (N) (N/mm.sup.2) 
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A. 1. 0.196 530 &lt;600 
2. 0.196 605 &lt;600 
3. 0.199 604 850 
B. 1. 0.196 520 800 
2. 0.196 633 700 
3. 0.199 621 900 
3. 0.199 581 900 
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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 1. and 2. 
according to the prior art. 
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 stress is a measure for the maximum compression force taken up 
by the steel cord when embedded in rubber. The greater the buckling stress 
the greater this maximum compression force. 
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 Methods, Tire Reinforcement and Tire 
Performance, ASTM STP 694, R. A. Fleming and D. I. Livingston, Eds., 
American Society for Testing and Materials, 1979, pp. 19-46. 
Table 2 mentions the results: 
TABLE 2 
______________________________________ 
COMPRESSION BEHAVIOUR 
buckling stress 
deformation 
compression modulus 
cord (N/mm.sup.2) (%) (kN/mm.sup.2) 
______________________________________ 
1. 430 0.40 125 
2. 447 0.40 125 
3. 475 1.12 66 
______________________________________ 
TEST 3 
A third test has evaluated the influence of the diameter difference between 
the two strands on the cord properties. Following cords have been 
evaluated: 
______________________________________ 
1. invention cord 
2 .times. 0.22 HT + 2 .times. 0.25 HT 
pitch: 14 mm 
2. invention cord 
2 .times. 0.25 NT + 2 .times. 0.28 HT 
pitch: 14 mm 
3. invention cord 
2 .times. 0.20 HT + 2 .times. 0.25 HT 
pitch: 14 mm 
4. invention cord 
2 .times. 0.25 HT + 2 .times. 0.30 HT 
pitch: 16 mm 
5. invention cord 
2 .times. 0.22 NT + 2 .times. 0.28 HT 
pitch: 14 mm 
6. invention cord 
2 .times. 0.22 HT + 2 .times. 0.30 HT 
pitch: 14 mm 
7. invention cord 
2 .times. 0.20 HT + 2 .times. 0.30 HT 
pitch: 14 mm 
8. invention cord 
2 .times. 0.22 HT + 2 .times. 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 2.5N and a tension of 50N 
and may be expressed as a percentage of the original gauge length. It is a 
measure of the openness of the steel cord. 
TABLE 3 
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diameter P.L.E. fatigue limit 
difference 2.5-50 N Hunter test 
cord (mm) (%) (N/mm.sup.2) 
______________________________________ 
1. 0.03 0.16 850 
2. 0.03 0.16 850 
3. 0.05 0.17 850 
4. 0.05 0.14 900 
5. 0.06 0.14 850 
0.06 0.18 900 
0.06 0.17 900 
6. 0.08 0.13 900 
7. 0.10 0.14 1050 
8. 0.13 0.40 950 
______________________________________ 
The fatigue limit remains high with increasing diameter difference. 
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 contact with other filaments over the whole length. In 
contradiction to n.times.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).