Steel cord

Abstract

A steel cord for reinforcing rubber or plastic goods having a core strand consisting of a plurality of wire filaments, and a plurality of outer strands cabled together around the core strand and each of which consisting of a plurality of wire filaments. The direction of lay of the outer layer of the core strand filaments and the direction of lay of the outer layer of the outer strands are made opposite to each other, and the former direction is same with that of the cabling of the outer strands. The number of the wire filaments constituting the outer layer of the core strand is the same as the number of the outer strands. The lay length of the core strand, the outer strands and of the cabling are so selected that the filaments in the outer layer of the core strand and the corresponding filaments in the outer strands have linear contact over their whole length. Thanks to the linear contact of the filaments, fretting of the filaments is less likely to appear resulting in improvement in fatigue endurance and breaking strength. Adhesion to the rubber or the plastic material is also improved and the overall diameter of the cord can be reduced for a given effective cross-sectional area ratio.

Description

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The present invention relates to steel cords, and more particularly to steel cords for reinforcing rubber or plastic goods such as tires for vehicles.

2. DESCRIPTION OF THE PRIOR ART

Hitherto, for the purpose of reinforcing radial tires for heavy duty vehicles such as trucks or buses, steel cords of the multi-strand type (or stranded rope structure) have been used which consist of a cord strand in which a plurality of wire filaments are stranded together, and a plurality of outer strands which are cabled to encircle the core strand and each of which consists of a plurality of wire filaments. Such cords of multistrand type thus comprises 28 ( 7.times.4 ), 38 ( 1.times.3+5.times.7 ) wire filaments and so on. These large numbers of wire filaments, as well as the multi-strand type structure is intended for producing high breaking strength and good flexibility so as to resist and bear high internal air pressure and repeating bending stress which the tire case reinforced by those cords must suffer during running of the vehicle.

The cords of conventional structure very often show, after a long distance run of the vehicle, fretting marks in the wire filaments which are caused by mutual abrasion of those wire filaments, and usually some of the frettings are found to have developed to fatigue breaking.

It is considered that those frettings are due to the fact that the wire filaments in the core strand intersect and contact wire filaments in the outer strands forming many point contact therebetween.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome above problem of wire filaments breakage which has been inevitable according to known techniques.

It is another object of the present invention to provide a reinforcing steel cord which is less likely to present fretting marks and is of a higher fatigue endurance, and of an improved breaking strength, as compared to conventional steel cord. It is still another object of the present invention to provide a reinforcing steel cord which is of smaller outer diameter, while maintaining same sectional area, and accordingly maintaining equivalent strength, as compared with conventional steel cords.

It is still another object of the invention to provide a reinforcing steel cord which has good adhesion to the rubber or the plastic for easy penetration into core strands by these goods, as compared with conventional steel cord.

It is still another object of the invention to provide a reinforcing steel cord which is easily manufactured.

According to the invention, there is provided a steel cord for reinforcing rubber or plastic goods, said steel cord comprising a core strand consisting of a plurality of wire filaments stranded together, and a plurality of outer strands cabled together to encircle the core strand and each of which consists of a plurality of wire filaments, characterized in that the lay direction of outer layers of said core strand and of each of said outer strands are opposite to each other, that the lay direction of said cabling of said outer strands is the same as that of the outer layer of said core strand, that the number of wire filaments in the outer layer of said core strand is the same as the number of said outer strands, and that the lay length of said outer layers of said core strand and outer strands and the lay length of said cabling of the outer strands is so selected that the wire filaments in said two outer layers forming linear contact with each other over their entire length.

The wire filaments may be made of iron, steel, or alloy steel such as stainless steel.

The number of wire filaments included in one core strand and one outer strand is preferably three through seven, respectively, but this is not exclusive.

It is also to be noted that the above described cord of multi-strand type may further be wrapped helically by a wire filament and that such modification is within the scope of the invention.

It will be understood that, according to the invention those wire filaments of the outer strand, which are positioned to contact those wire filaments of the core strand are orientated to make linear contact with the later, since the wire filaments in the outer layers of core and outer strands have opposite or reverse lay directions with each other and since the lay direction of the cabling of the outer strands is the same as the direction of lay of outer layer of the core strand.

It is true that such arrangement as described above is similar to that of so called independent wire strand core ( I W S C ) rope or so called independent wire rope core ( I W P C ) rope, both of which are well known to those skilled in the art. In this regards, it is to be appreciated that the present invention provides an improved steel cord in which the mutually contacting wire filaments in the core strand and the outer strands are layed perfectly in parallel with each other, making linear contact, by suitably selecting the number of the wire filaments and lay length.

The above and additional objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawing. It is to be expressly understood, however, that the drawing is for purpose of illustration only and is not intended as a definition of the limits of the invention.

BRIEF EXPLANATION OF THE DRAWINGS

In the drawings:

FIG. 1 shows a cross section of a steel cord embodying the present invention, in which 32 wire filaments ( 1 .times. 7 + 6 .times. 4 + 1) are employed.

FIG. 2 shows a cross section of another steel cord embodying the present invention, in which 27 wire filaments ( 1 + 1 .times. 5 + 5 .times. 4 + 1) are employed.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a steel cord comprises a core strand 1 consisting of wire filaments and six outer strands 2, each of which consist of four wire filaments and are cabled around the core strand.

In this arrangement, the direction of lay of the outer layer of the core strand is Z, while the direction of lay the outer strands is S, and the direction of lay of the cabling of the outer strands is Z. By suitably selecting each length of lay, the convex portions or crests of the core strand 1 are regularly positioned at the concave portions or valleys of outer strands 2, as seen from FIG. 1, or vice versa.

Now, a consideration will be made as to conditions required for obtaining such regularity.

Suppose here that the number of wire filaments stranded in the outer layer of the core strand is m, the number of wire filaments stranded in the outer layer of the outer strand is n, the lay length of the lay cabling is A, the length of the outer layer of the outer strand is B and the lay length of the outer layer of the core strand is C, respectively. Supposing also that both the core strand and each of the outer strands are solid rod, lines of contact of the core strand with the outer strands would be M in number and extend spirally along the periphery of the core strand. It will be understood that the regularity is obtained when the number of crests (or valleys ) of both strands emerging on a spiral line of contact are equal to each other.

Provided that the unit length of the steel cord is represented by l, the outer strand makes 1/A turns around the core strand in the unit length of the steel cord. Since the directions are the same for the lay of the outer layer of the core strand and the lay of group-strand, the number of the crests (or valleys ) of the core strand which intersect a spiral line of contact is given by the following equation. ##EQU1##

At the same time, since the lay direction of the outer layer of the outer strands is opposite to the direction of lay of the cabling of the outer strands, the number of the crests (or valleys) of the outer strands which intersect the spiral line of contact can be given by the following equation, when the angle of lay of the cabling of the outer strands with respect to the axial direction of the cord is represented by .theta.. ##EQU2##

It will be understood, as aforementioned, it is necessary to equalize N to n', in order to obtain a regular arrangement of FIG. 1. Therefore, from the equations (1) and (2), the necessary condition for obtaining the regular arrangement is expressed by the following equation. ##EQU3##

This equation (3) is then converted to the following. ##EQU4##

It will be understood from the equation (4), that when any two out of three lays A, B and C are given, the remaining one of the lays is determined automatically.

The diameters of the core and the outer strands, number of the wire filaments and the diameter of the wire filaments are determined suitably taking the geometric conditions into account, as is the case of conventional stranded ropes.

In designing the steel cord of the present invention, it is preferable to determine the outer diameter of the core strand 0 through 5% larger than the theoretical diameter. Namely, since the crests formed by the wire filaments in the core strand are conveniently and regularly received by the valleys formed between adjacent wire filaments in the outer strand, no gaps, which would otherwise inevitably exist between the core and outer strands, are formed, thereby presenting a compact or close stranding. In other words, the sectional area ratio of the wire filaments is high as compared to conventional steel cords.

For information, the reference numeral 3 in FIG. 1 designates a filament spirally wrapping around said steel cord.

Two embodiments of the invention will be described hereinafter.

First Embodiment:

Five steel cords were manufactured in accordance with the invention in respective combinations of Lays A, B and C as listed in table 1, each combination satisfying the equation (4). Each cord had a dimensional character as specified below.

The above specification means that the steel cord comprises one core strand in which seven filaments of 0.175 mm are stranded, six outer strands in which four filaments of the same dimeter are stranded, and one filament of 0.15 mm wrapped arround the steel cord. It will be understood that in this arrangement m is equal to 6 and n is equal to 4. Two types were manufactured for each one of five steel cords, with and without the wrapping filament 3.

______________________________________ Lays of Lays of No Core Outer Lays of Wrapped or Strand (C) Strand (B) Cord (A) Not ______________________________________ 1 6.1 mm 9.5 mm 18.2 mm Both 2 8.0 mm 9.5 mm 30.5 mm Both 3 9.5 mm 9.5 mm 47.5 mm Both 4 11.1 mm 9.5 mm 83.7 mm Both 5 14.2 mm 9.5 mm .infin. mm Wrapped ______________________________________

As seen from the Table 1, the lay A of the cord is infinite, which means that the six outer strands are extending in the axial direction of the cord without being cabled. In this case it is necessary to spirally wrap the unretained outer strands by the filament 3, as shown in FIG. 1. It will be understood that, as far as the fifth cord is concerned, the equation (4) can be converted to as follows. ##EQU5##

It was observed that all of the wire filaments in the outer layer of the core strand and corresponding wire filaments in the outer strand make linear contact with each other over their whole length.

The breaking strength of the first core was measured to be 202 Kg, which amounts to 95.1% of the sum of the breaking strength of wire elements. This embodiment consisting of 31 wire filaments of the same diameter is free from the problem of mingling of the filaments of the different diameter, and easy to manufacture. In addition, breaking strength and the fatigue endurance are considerably higher than those of the conventional cords.

It will be understood, the equation (4) can be written as follows, as far as this embodiment is concerned. ##EQU6## Second Embodiment:

Steel cords as specified below were manufactured in accordance with the invention.

This means that the cord comprise one core strand having a central filament of 0.12 mm in diameter and five filaments of 0.17 mm stranded around the central filament, five outer strands each of which having four filaments of 0.19 mm in diameter, and a wrapping filament having a diameter of 0.15 mm. The section of these cords of the second embodiment is diagramatically shown in FIG. 2 in which the same numerals designate the same elements as shown in FIG. 1.

Lays A, B and C are chosen to satisfy the equation: ##EQU7##

It will be understood that in the second embodiment, m and n in the equation (4 ) amount to 5 and 4, respectively. The steel cords of the second embodiment thus constructed also showed almost perfect linear contact between the filaments in the outer layer of the cord strand and the corresponding filaments in the outer strands over their entire length.

According to the invention, thanks to the linear contact between the filaments of the core and the outer strands, which is obtained by suitably selecting the number of filaments, directions of lays, and the length of lay, the following advantageous are obtained.

a. The steel cord of the invention suffers less reduction of the breaking strength from the stranding and cabling than the conventional arrangement, and the breaking strength reaches almost to the sum of strengths of all filaments. More strictly, the breaking strength of the cords of the invention amounts to 95 through 98% of the sum of strengths of the filaments, whereas in the conventional arrangement only 90 through 93% of the sum is available for the breaking strength of the cord.

b. Because of the linear contact of the filaments between the core strand and outer strands, fretting can hardly occur. Even if fretting occurs, it causes less reduction in the cross-sectional area of the filaments than that caused by wearing, so that the fatigue endurance is considerably improved.

c. Since the core strand and the outer strands engage each other closely and regularly, both strands would be even and regular in the sheared surface of the cord when the later is sheared as necessitated.

d. For a given diameter of the cord, the larger effective cross-sectional area can be obtained as compared to the conventional arrangement. In other words, the sectional area ratio is considerably higher than that of the conventional arrangement. This feature is particularly advantageous for the use in tires of vehicles where the end numbers per inch is limited.

e. It becomes possible to adopt a larger lay of the cabling, as compared to the conventional construction, without accompanying deformation or loosening of the strand. Even the omission of the cabling becomes allowable provided that the outer strands are suitably wrapped, which omission leads to a reduction in manufacturing cost.

While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims

What is claimed is:

1. In a reinforcing steel cord including a core strand consisting of a plurality of wire filaments, a plurality of outer strands cabled together around said core strand, each outer strand consisting of a plurality of wire filaments, the improvement wherein; said core strand comprises a core filament and an outer layer of filaments, the lay direction of the outer layer of the core strand filaments and the lay direction of the outer strand filaments are opposite to each other, the lay direction of the cabling of the said outer strands is the same as that of the outer layer of said core strand filaments, the number of said filaments constituting said outer layer of said core strand is the same as the number of said outer strands, and the lay length of the core strand, the lay length of the outer strands and the cabling of the outer strands are so selected that the filaments in the outer layer of the core strand and the corresponding filaments in the outer strands contact each other linearly over their whole length.

2. A steel cord as claimed in claim 1, wherein the number of said filaments in the outer layer of the core strand and the number of said outer strands is m and the number of the filaments in the outer layer of the outer strand is n, respectively, the angle of lay of the cabling of the outer strands with respect to the axial direction of the cord is .theta., and wherein the lay length A of said outer layer of said core strand, the lay length B of the outer layer of the outer strand and the lay length C of said cabling satisfies the following equation: ##EQU8##

3. A steel cord as claimed in claim 2, wherein m and n equal 6 and 4, respectively, and the lay length A, B and C are so selected as to satisfy the equation: ##EQU9##