Steel cords for reinforcing rubber articles and pneumatic radial tire using the steel cords

Abstract: The present invention relates to a two-layered twisted steel cord for reinforcing rubber articles, comprising a core (1) including two filaments and a sheath (2) including six or seven filaments wound around the core (1), in which an average twist pitch of the core (1) is set to be at least 30 mm, and when six filaments are used for the sheath (2), a ratio of diameter (ds) of each filament of the sheath (2) to diameter (dc) of each filament of the core, [(ds/dc) x 100], is set in a range of 58.0 % <ds/dc<161.5%, and when seven filaments are used for the sheath (2), the ratio of diameter (ds) to diameter (dc) is set in a range of 47.3% <ds/dc<121.1%. In accordance with the present invention, there are provided two-layered twisted steel cords for reinforcing rubber articles, of which resistance against propagation of corrosion and productivity are improved, and a pneumatic radial tire of which productivity and durability are improved with the steel cords being used as reinforcing materials.

Description: BACKGROUND OF THE INVENTION Field of the Invention: The present invention relates to steel cords for reinforcing rubber articles, which are used as reinforcing materials of rubber articles such as a pneumatic radial tire, a conveyor belt, or the like, and to a pneumatic radial tire in which the steel cords are used as the reinforcing materials. Description of the Related Art: Steel cords are used as reinforcing materials of rubber articles such as a pneumatic tire. For example, as the steel cords used in a tire for heavy-load vehicles, there have been widely used those each having a 3+9+15 structure (numeral 3 indicates the number of core filaments, and numerals 9 and 15 indicate the numbers of intermediate sheath filaments and external sheath filaments, respectively), in which filaments having the same wire diameter are twisted at different twist pitches for each layer. However, since this structure does not have clearances which allow rubber to penetrate into an inner portion of the cord, when the cord contacts water, water penetrates into a hollow portion within the cord into which rubber has not penetrated, thereby resulting in corrosion of the cord. In addition, water is passed along the hollow portion and a corroded region of the cord spreads. In order to solve the above-described problem of corrosion, there has been disclosed, in Japanese Utility Model Application Laid-Open (JP-U) No. 64-30398, a steel cord of two- or three-layered twisted cords having a core formed with two or three filaments being paralleled with each other. However, in the above example shown in JP-U No. 64-30398, when three or more filaments are used for the core, a space which does not in the least allow penetration of rubber is formed in the center of the core. When the steel cord as described above is used for a belt layer of a tire, water penetrates into an inner portion of the cord from a cut portion in the tire caused by traveling on a rough road, thereby resulting in corrosion of the cord. In addition, it is predicted that a drawback arises in that water propagates through the inner portion of the cord so that the corroded region spreads . EP-A-0 399 795 reveals a pneumatic radial tire having a plurality of belt layers each containing two-layered twisted steel cords. A single steel cord comprises a twisted core including two core filaments and a sheath including six to eight sheath filaments wound around the core, thereby forming a 2+6, 2+7 or 2+8 structure. The diameters of the core filaments and the sheath filaments may be different. A twisting direction of the core is the same as in a twisting direction of said sheath. The twisting pitch Ps of the sheath and the twisting pitch Pc of the core satisfy the following relationships: 12 ≤ Ps ≤ 22 mm and 1,0 ≤ Ps/Pc ≤ 4,0 mm. From these relationships it can be learned that Pc ≤ Ps ⇒ Pc ≤ 22 mm Ps/4 ≤ Pc ⇒ with Ps = 22: ⇒ Pc ≥ 5,5 mm ⇒ with Ps = 12: ⇒ Pc ≥ 3 mm i.e. 3 ≤ Pc ≤ 22 mm With these features, the technical problem of separation failure at the ends of the tire belts (so called "BES") of pneumatic radial tires is solved. From DE 29 41 541 A1 there is known a two-layered twisted steel cord for reinforcing rubber articles, said steel cord comprising a core including two or three core filaments and a sheath including eight or ten sheath filaments wound around the core, thereby forming a 2+8 or 3+10 structure. Each of the core and sheath filaments have the same diameter. The core filaments are untwisted and extend linearly and parallel to each other in the longitudinal direction of the steel cord. Thereby, a steel cord with a different section modulus in its horizontal and vertical cross sectional axis is provided. Further, by employing this kind of steel cord as a reinforcing element in a tire, different driving qualities in the radial and lateral direction of the tire can be obtained. SUMMARY OF THE INVENTION A first object of the present invention is to solve the above-described problems, and to provide steel cords for reinforcing rubber articles, which can improve resistance against propagation of corrosion and productivity. A second object of the present invention is to provide a pneumatic radial tire having improved productivity and durability by using the above steel cords as reinforcing materials. The first object of the present invention is achieved by a two-layered twisted steel cord for reinforcing rubber articles, comprising a core including two filaments and a sheath including six or seven filaments wound around the core, wherein an average twist pitch of the core is set to be at least 30 mm, and when six filaments are used for the sheath, a ratio of diameter ds of each filament of the sheath to diameter dc of each filament of the core, [(ds/dc) × 100], is set in a range of 58.0 < ds/dc < 161.5%, and when seven filaments are used for the sheath, the ratio of diameter ds to diameter dc is set in a range of 47.3% < ds/dc < 121.1%. The steel cord of the present invention is preferably constructed in that, when six filaments are used for the sheath, a ratio of a major axis a of the steel cord to a minor axis b, [(a/b) × 100], is set in a range of 100.0% ≧ a/b > 68.4%, and when seven filaments are used for the sheath, the ratio of the major axis a of the steel cord to a minor axis b, [(a/b) × 100], is set in a range of 100.0% ≧ a/b > 66.1%. Further, the steel cord of the present invention is preferably constructed in that the diameter dc of each of filaments which form the core is set in a range of 0.10 mm ≦ dc ≦ 0.40 mm and the diameter ds of each of filaments which form the sheath is set in a range of 0.10 mm ≦ ds ≦ 0.37 mm. Moreover, in the steel cord of the present invention, the filaments which form the core may also be provided in an untwisted manner. The above-described second object of the present invention is achieved by a pneumatic radial tire in which the above-described steel cord is applied to a cord for forming a carcass or a belt layer. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 through 4 are schematic diagrams each showing a cross section of a steel cord for reinforcing rubber articles of the present invention, and Fig. 5 is a schematic diagram showing a cross section of a conventional steel cord for reinforcing rubber articles. DESCRIPTION OF THE PREFERRED EMBODIMENTS In a steel cord according to the present invention, the number of filaments which form the core is limited to two. That is, in a case of a cord having the core formed of one filament, even if rubber penetrates up to the position of the core, a spiral portion into which rubber does not penetrate is formed due to filaments of a sheath being disposed biasedly. In this case, resistance against propagation of corrosion deteriorates largely as compared with the case of the cord having the core of two filaments. On the other hand, in the case of the cord having the core formed of three or more filaments, a hollow portion into which rubber does not penetrate is formed in an interior of the core, and the resistance against propagation of corrosion of the cord also deteriorates. An average twist pitch of the core is set to be 30 mm or greater. When the pitch is less than 30 mm, it is difficult for rubber to penetrate into a clearance between filaments of the core and the productivity of stranded wires deteriorates. Here, the expression "the average pitch is 30 mm or greater" means that two filaments of the core are twisted at an angle of 360° at a distance greater than or equal to 30 mm. For example, one pitch may be formed in such a manner that the filaments are twisted at an angle of 180° at the first distance of 10 mm, are not twisted at the next distance of 10 mm, and are twisted at an angle of 180° at the last distance of 10 mm, or one pitch may be formed in such a manner that the filaments are not twisted at the first distance of 20 mm and are twisted at an angle of 360° at the remaining distance of 10 mm. Meanwhile, in a case in which the core is formed of non-twisted filaments, penetrability of rubber can be further improved and the productivity can also be remarkably enhanced. In addition, even when forming of one or combination of a wave form and a spiral form is made for one or both of filaments of the core, the effects of the present invention are not deteriorated. In the steel cord of the present invention, when there are six filaments used for the sheath, the ratio of the diameter ds of each filament of the sheath to the diameter dc of each filament which forms the core, [(ds/dc) × 100], is set in the range of 58.0 % < ds/dc < 161.5%, and when there are seven filaments used for the sheath, the ratio is set in the range of 47.3%<ds/dc<121.1%. In the above-described respective ranges, a clearance between the adjacent filaments of the sheath, which is sufficient to allow uniform penetration of rubber in the inner portion of the cord, is maintained, and further, even when the filaments of the sheath enter the core portion, the sufficient clearance between the adjacent filaments is maintained. When set out of the ranges, the clearance between the adjacent filaments of the sheath becomes smaller and it is difficult for rubber to penetrate up to the portion of the core. In the steel cord of the present invention, when there are six filaments used for the sheath, the ratio of a minor axis a to a major axis b of the cord, [(a/b) × 100], is preferably set in the range of 100.0% ≧ a/b > 68.4%, and when there are seven filaments, the ratio is preferably set in the range of 100.0% ≧ a/b > 66.1%. In the case of the steel cord which forms a belt layer of a pneumatic radial tire of the present invention, when the ratio of the minor axis a to the major axis b of the cord is set in the above-described respective ranges, even if a cut is formed in a tire cord due to the tire being driven on a rough road, corrosion of the cord is not apt to propagate. Further, when the ratio is set out of the ranges and the cut is formed in the tire cord due to the tire being driven on the rough road, corrosion of the cord is apt to propagate. Further, the preferred range of diameter dc of each filament for forming the core is 0.10 mm ≦ dc ≦ 0.40 mm and the preferred range of diameter ds of each filament for forming the sheath is 0.10 mm ≦ ds ≦ 0.37 mm. When diameters dc, ds are made smaller than the respective lower limit values, manufacturing of the filaments becomes extremely difficult. When they are set to be greater than the respective upper limit values, in a process in which a tire member is manufactured by coating the cord by a rubber sheet, the filament plastically deforms and a warp is thereby formed in the tire member. As a result, the productivity deteriorates. [Examples] Referring now to experimental examples described below, the present invention will be described concretely. In examples 1 through 11, the pneumatic radial tires each having the size of 10.00R20 were manufactured by using, for the belt layer, steel cords each having the structure shown in Table 2-1 and Table 2-2 listed below, and Figs. 1 and 2. In a conventional example, the pneumatic radial tire having the size of 10.00R20 was manufactured by using, for the belt layer, the steel cords each having the structure shown in Table 1 listed below and Fig. 5. Further, in comparative examples, the pneumatic radial tires each having the size of 10.00R20 were manufactured by using, for the belt layer, the steel cords each having the structure shown in Table 1. In examples 12 through 22, the pneumatic radial tires each having the size of 10.00R20 were manufactured by using, for the belt layer, steel cords each having the structure shown in Table 4-1 and Table 4-2 listed below, and Figs. 3 and 4. In a conventional example, the pneumatic radial tire having the size of 10.00R20 was manufactured by using, for the belt layer, the steel cords each having the structure shown in Table 3 below and Fig. 5. Further, in comparative examples, the pneumatic radial tires each having the size of 10.00R20 were manufactured by using, for the belt layer, the steel cords each having the structure shown in Table 3. The tires thus manufactured were evaluated in the following manners. In the drawings, numeral 1 designates a core, numeral 2 designates a sheath (an intermediate sheath), numeral 3 designates an external sheath, and numeral 4 designates a spiral wire. Further, dc indicates the diameter of each filament of the core, ds indicates the diameter of each filament of the sheath, a indicates the minor axis of the steel cord, and b indicates the major axis of the steel cord. Penetrability of rubber in the inner portion of the cord A steel cord was taken out from the belt layer of the tire, an amount of rubber adhering onto the intermediate sheath was measured through the filament of the external sheath of the steel cord, and an amount of rubber adhering onto the core was measured through the filament of the intermediate sheath. The respective amounts of rubber were evaluated and expressed in terms of %, where the state in which rubber does not in the least adhere onto the surfaces of the respective filaments is assumed to be 0%, and the state in which rubber completely adheres onto the surfaces is assumed to be 100%. Resistance against separation The manufactured tires were placed on ten-ton trucks and driven on a rough road so as to be completely worn out. The worn-out tires were dissected and existence of separation having an area of 5cm 2 or greater, which is caused by corrosion of the steel cord due to propagation of water to the inner portion of the steel cord was examined. The number of cuts through the tread The manufactured tires were placed on ten-ton trucks and driven on the rough road so as to be completely worn out. The worn-out tires were dissected and the number of cuts through the tread was measured per unit area. The results obtained from the conventional example and comparative examples 1 through 5 are given in Table 1, and the results obtained from examples 1 through 11 are given in Table 2-1 and Table 2-2. Note that, in Table 1, the first numeral of the cord structure in the conventional example indicates the number of core filaments, the second and third numerals thereof indicate the numbers of intermediate sheath filaments and external sheath filaments, respectively, and the fourth numeral indicates the number of spiral cord filaments. The first numeral of the cord structure in each of the comparative examples 1 through 5 indicates the number of core filaments and the second numeral thereof indicates the number of sheath filaments. Further, in Tables 2-1 and 2-2, the first numeral of the cord structure in each example indicates the number of core filaments, the second numeral indicates the number of sheath filaments, and the third numeral indicates the number of spiral cords. The results obtained from the conventional example and comparative examples 6 through 10 are given in Table 3, and the results obtained from examples 12 through 22 are given in Table 4-1 and Table 4-2. Note that, in Table 3, the first numeral of the cord structure in the conventional example indicates the number of core filaments, the second and third numerals thereof indicate the numbers of intermediate sheath filaments and external sheath filaments, respectively, and the fourth numeral indicates the number of spiral cord filaments. The first numeral of the cord structure in each of the comparative examples 6 through 10 indicates the number of core filaments and the second numeral thereof indicates the number of sheath filaments. Further, in Tables 4-1 and 4-2, the first numeral of the cord structure in each example indicates the number of core filaments, the second numeral indicates the number of sheath filaments, and the third numeral indicates the number of spiral cords. According to the present invention, the steel cords for reinforcing rubber articles, of which resistance against propagation of corrosion and productivity are remarkably improved, can be obtained, and the pneumatic radial tire in which the above steel cords are applied to the belt layer has an excellent penetrability of rubber into the inner portion of each cord, thereby resulting in the durability thereof being improved.

Claims: A two-layered twisted steel cord for reinforcing rubber articles, comprising a core (1) including two filaments and a sheath (2) including six or seven filaments wound around the core (1), wherein an average twist pitch of the core (1) is set to be at least 30 mm, and when six filaments are used for the sheath (2), a ratio of diameter (ds) of each filament of the sheath (2) to diameter (dc) of each filament of the core (1), [(ds/dc) X 100], is set in a range of 58.0 % < ds/dc < 161.5%, and when seven filaments are used for the sheath (2), the ratio of diameter (ds) to diameter (dc) is set in a range of 47.3% < ds/dc < 121.1%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 1, wherein when six filaments are used for the sheath (2), a ratio of a major axis (a) of said steel cord to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 68.4%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 1, wherein when seven filaments are used for the sheath (2), a ratio of a major axis (a) of said steel cord to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 66.1%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 2, wherein the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A two-layered twisted steel cord for reinforcing rubber articles according to claim 3, wherein the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A two-layered twisted steel cord for reinforcing rubber articles, comprising a core (1) including two filaments and a sheath (2) including six or seven filaments wound around the core (1), wherein the filaments which form the core (1) are untwisted, and when six filaments are used for the sheath (2), a ratio of diameter (ds) of each filament of the sheath (2) to diameter (dc) of each filament of the core (1), [(ds/ dc) X 100], is set in a range of 58.0 % < ds/ dc < 161.5%, and when seven filaments are used for the sheath (2), the ratio of diameter (ds) to diameter (dc) is set in a range of 47.3% < ds/dc < 121.1%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 6, wherein when six filaments are used for the sheath (2), a ratio of a major axis (a) of said steel cord to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 68.4%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 6, wherein when seven filaments are used for the sheath (2), a ratio of a major axis (a) of said steel cord to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 66.1%. A two-layered twisted steel cord for reinforcing rubber articles according to claim 7, wherein the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A two-layered twisted steel cord for reinforcing rubber articles according to claim 8, wherein the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A pneumatic radial tire in which steel cords are used as cords for forming a carcass or a belt layer, each of said steel cords being a two-layered twisted steel cord for reinforcing rubber articles and comprising a core (1) including two filaments and a sheath (2) including six or seven filaments wound around the core (1), wherein an average twist pitch of the core (1) is set to be at least 30 mm, and when six filaments are used for the sheath (2), a ratio of diameter (ds) of each filament of the sheath (2) to diameter (dc) of each filament of the core (1), [(ds/dc) X 100], is set in a range of 58.0% < ds/dc < 161.5%, and when seven filaments are used for the sheath (2), the ratio of diameter (ds) to diameter (dc) is set in a range of 47.3% < ds/dc < 121.1%. A pneumatic radial tire according to claim 11, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which when six filaments are used for the sheath (2), a ratio of a major axis (a) of each of said steel cords to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 68.4%. A pneumatic radial tire according to claim 11, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which when seven filaments are used for the sheath (2), a ratio of a major axis (a) of each of said steel cords to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 66.1%. A pneumatic radial tire according to claim 12, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10mm ≤ ds ≤ 0.37 mm. A pneumatic radial tire according to claim 13, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A pneumatic radial tire in which steel cords are used as cords for forming a carcass or a belt layer, each of said steel cords being a two-layered twisted steel cord for reinforcing rubber articles and comprising a core (1) including two filaments and a sheath (2) including six or seven filaments wound around the core (1), wherein the filaments which form the core (1) are untwisted, and when six filaments are used for the sheath (2), a ratio of diameter (ds) of each filament of the sheath (2) to diameter dc of each filament of the core (1), [(ds/dc) x 100], is set in a range of 58.0 % < ds/dc < 161.5%, and when seven filaments are used for the sheath (2), the ratio of diameter (ds) to diameter (dc) is set in a range of 47.3% < ds/dc < 121.1%. A pneumatic radial tire according to claim 16, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which when six filaments are used for the sheath (2), a ratio of a major axis (a) of each of said steel cords to a minor axis (b), [(a/b) X 100], is set in a range of 100.0% ≥ a/b > 68.4%. A pneumatic radial tire according to claim 16, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which when seven filaments are used for the sheath (2), a ratio of a major axis (a) of each of said steel cords to a minor axis (b), [(a/b) x 100], is set in a range of 100.0% ≥ a/b > 66.1%. A pneumatic radial tire according to claim 17, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm. A pneumatic radial tire according to claim 18, comprising the two-layered twisted steel cords for reinforcing rubber articles, in which the diameter (dc) of each of filaments which form the core (1) is set in a range of 0.10 mm ≤ dc ≤ 0.40 mm and the diameter (ds) of each of filaments which form the sheath (2) is set in a range of 0.10 mm ≤ ds ≤ 0.37 mm.

Inventors: YAMANAKA TAKAYA, YAMANAKA, TAKAYA, Yamanaka, Takaya, c/o Bridgestone Corporation
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: D07B1
IPC codes: D07B1