PNEUMATIC TIRE

Abstract

In a pneumatic tire provided with a carcass layer and a belt layer, the carcass layer is formed of an organic fiber cord having an elongation at break of 20% to 30%, an elongation in a sidewall portion under a load of 2.0 cN/dtex of 5.5% to 8.0%, and a product A = D × Ec of a fineness based on corrected mass D (unit: dtex/piece) and a count Ec (unit: piece/50 mm) of 1.8 × 10.sup.5 dtex/50 mm to 3.0 × 10.sup.5 dtex/50 mm, and the belt layer is formed of a steel cord having a stress at break of 3300 MPa or more and a product B = S x Eb of a cross-sectional area S (unit: mm.sup.2/piece), and a count Eb (unit: piece/50 mm) is from 6.0 mm.sup.2/50 mm to 7.5 mm.sup.2/50 mm.

Claims

1. A pneumatic tire, comprising: a tread portion extending in a tire circumferential direction and having an annular shape; a pair of sidewall portions disposed on respective sides of the tread portion; a pair of bead portions disposed in the sidewall portions on an inner side in a tire radial direction; at least one layer of a carcass layer mounted between the pair of bead portions; a plurality of layers of belt layers disposed on an outer circumferential side of the carcass layer in the tread portion; the carcass layer being formed of a carcass cord made of an organic fiber cord, an elongation at break of the carcass cord being from 20% to 30%, an elongation under a load of 2.0 cN/dtex of the carcass cord in the sidewall portions being from 5.5% to 8.0%, a product A = D × Ec of a fineness based on corrected mass D (unit: dtex/piece) per the carcass cord and a count Ec (unit: piece/50 mm) of the carcass cords per 50 mm in a direction orthogonal to an extension direction of the carcass cord being from 1.8 × 10.sup.5 dtex/50 mm to 3.0 × 10.sup.5 dtex/50 mm, the belt layer being constituted of a belt cord formed of a steel cord, stress at break of the belt cord being 3300 MPa or more, a product B = S × Eb of a cross-sectional area S (unit: mm.sup.2/piece) per the belt cord and a count Eb (unit: piece/50 mm) of the belt cord per 50 mm in a direction orthogonal to an extension direction of the belt cord being from 6.0 mm.sup.2/50 mm to 7.5 mm.sup.2/50 mm.

2. The pneumatic tire according to claim 1, wherein the belt cord has an N+M structure in which the number of wire strands of an inner layer N is from two to four and the number of wire strands of an outer layer M is from two to seven.

3. The pneumatic tire according to claim 2, wherein the belt cord has a 2+2 structure in which the number of the wire strands of the inner layer N is two and the number of the wire strands of the outer layer M is two.

4. The pneumatic tire according to claim 2,wherein the wire strand constituting the inner layer is arranged in a non-twisted state.

5. The pneumatic tire according to claim 2, wherein the wire strand constituting the inner layer is arranged in a non-twisted state.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0010] FIG. 1 is a meridian cross-sectional view illustrating a pneumatic tire according to an embodiment of the present technology.

[0011] FIG. 2 is an explanatory diagram schematically illustrating a structure of a belt cord.

DETAILED DESCRIPTION

[0012] Configurations of embodiments of the present technology will be described in detail below with reference to the accompanying drawings.

[0013] As illustrated in FIG. 1, a pneumatic tire of an embodiment of the present technology includes a tread portion 1, a pair of sidewall portions 2 disposed on both sides of the tread portion 1, and a pair of bead portions 3 disposed in the sidewall portions 2 on the inner side in the tire radial direction. Note that “CL” in FIG. 1 denotes a tire equator. Although not illustrated in FIG. 1, which is a meridian cross-sectional view, the tread portion 1, the sidewall portions 2, and the bead portions 3 each extend in a tire circumferential direction to form an annular shape. This forms a toroidal basic structure of the pneumatic tire. Although the description using FIG. 1 is basically based on the illustrated meridian cross-sectional shape, all of the tire components each extend in the tire circumferential direction and form the annular shape.

[0014] A carcass layer 4 including a plurality of reinforcing cords (hereinafter referred to as carcass cords) extending in the tire radial direction is mounted between the pair of bead portions 3 on the right and left. A bead core 5 is embedded within each of the bead portions, and a bead filler 6 having an approximately triangular cross-sectional shape is disposed on an outer periphery of the bead core 5. The carcass layer 4 is folded back around the bead core 5 from an inner side to an outer side in the tire width direction. Accordingly, the bead core 5 and the bead filler 6 are wrapped by a body portion (a portion extending from the tread portion 1 through each of the sidewall portions 2 to each of the bead portions 3) and a folded back portion (a portion folded back around the bead core 5 of each bead portion 3 to extend toward each sidewall portion 2) of the carcass layer 4.

[0015] A plurality (in the illustrated example, two layers) of belt layers 7 are embedded on an outer circumferential side of the carcass layer 4 in the tread portion 1. Each of the belt layers 7 includes a plurality of reinforcing cords (hereinafter referred to as belt cords) inclining with respect to the tire circumferential direction, and the belt cords are disposed so as to intersect with one another between the layers. In the belt layers 7, an inclination angle of the belt cord with respect to the tire circumferential direction is set within a range of, for example, from 10° to 40°.

[0016] To improve the high-speed durability, a belt reinforcing layer 8 is further provided on an outer circumferential side of the belt layers 7. The belt reinforcing layer 8 includes a reinforcing cord (hereinafter referred to as a cover cord) oriented in the tire circumferential direction. As the cover cord, for example, an organic fiber cord can be used. In the belt reinforcing layer 8, the angle of the cover cord with respect to the tire circumferential direction is set to, for example, from 0° to 5°. As the belt reinforcing layer 8, a full cover layer 8a that covers the entire region of the belt layers 7 in the width direction, a pair of edge cover layers 8b that locally cover both end portions of the belt layers 7 in the tire width direction, or a combination thereof can be provided (in the example illustrated, both of the full cover layer 8a and the edge cover layers 8b are provided). The belt reinforcing layer 8 can be formed, for example, by helically winding a strip material made of at least a single cover cord arranged and covered with coating rubber in the tire circumferential direction.

[0017] The present technology mainly relates to the cords (the carcass cord and the belt cords) constituting the respective carcass layer 4 and belt layer 7 described above, and therefore the basic structure of the tire is not limited to that described above.

[0018] In an embodiment of the present technology, the carcass cord included in the carcass layer 4 is formed of a polyester fiber cord obtained by intertwining polyester fiber filament bundles. The elongation at break of the carcass cord (polyester fiber cord) ranges from 20% to 30% and preferably from 22% to 28%. The elongation under a load of 2.0 cN/dtex of the carcass cord in the sidewall portions ranges from 5.5% to 8.0% and preferably from 6.5% to 7.5%. Since the carcass cord (polyester fiber cord) having such physical properties is used for the carcass layer 4, while good steering stability similar to the case of using the conventional rayon fiber cord is ensured, shock burst resistance can be improved. That is, since the carcass cords have the above-described elongation properties, the rigidity of the carcass cords can be appropriately ensured and good steering stability can be exhibited. Further, since the carcass cord has the above-described elongation properties, the carcass cord easily follows local deformation, the deformation during a plunger energy test (when the carcass cord is pressed by a plunger) can be sufficiently tolerated, and breakage energy can be improved. In other words, during travel, breakage durability against protrusion input of the tread portion is improved, so that shock burst resistance can be improved. When the elongation at break of the carcass cord is less than 20%, the effect of improving shock burst resistance cannot be obtained. When the elongation at break of the carcass cord is more than 30%, intermediate elongation tends to be increased, and therefore rigidity is reduced and steering stability possibly deteriorates. When the elongation under a load of 2.0 cN/dtex is less than 5.5%, there is a possibility that cord rigidity becomes high, the compression strain of the turned up end portions of the carcass layer 4 is increased immediately below a ground contact region, and consequently the cord is broken (that is, durability is possibly impaired). When the elongation is more than 8.0% under a load of 2.0 cN/dtex, rigidity is difficult to be ensured, and the effect of improving high-speed steering stability possibly fails to be sufficiently obtained.

[0019] In addition, in the carcass layer 4, the product A = D × Ec of a fineness based on corrected mass D (unit: dtex/piece) per carcass cord and a count Ec (unit: piece/50 mm) per 50 mm of carcass cords in a direction orthogonal to an extension direction of the carcass cord is from 1.8 × 10.sup.5 dtex/50 mm to 3.0 × 10.sup.5 dtex/50 mm, and preferably from 2.2 × 10.sup.5 dtex/50 mm to 2.7 × 10.sup.5 dtex/50 mm. Since the product A described above is the fineness of the carcass cord per unit width in the carcass layer 4, when it satisfies the range described above, durability and braking performance can be improved, and as a result, it is advantageous for improving shock burst resistance and high-speed steering stability. The product A of less than 1.8 × 10.sup.5 dtex/50 mm possibly deteriorates braking performance. The product A in excess of 3.0 ×10.sup.5 dtex/50 mm narrows down the interval of the carcass cord, and thus maintaining durability is difficult. Note that the individual ranges of the fineness based on corrected mass D and the count Ec described above are not particularly limited as long as the product A satisfies the range described above.

[0020] Furthermore, the carcass cord preferably has a heat shrinkage rate of 0.5% to 2.5% and more preferably from 1.0% to 2.0%. Note that “heat shrinkage rate” is a dry heat shrinkage rate (%) of sample cords measured in accordance with JIS L1017 “Test methods for chemical fiber tire cords” with a length of specimen being 500 mm and when heated at 150° C. for 30 minutes. By using cords having such a heat shrinkage rate, the reduction in durability or the deterioration in uniformity due to the occurrence of kinking (such as twisting, breakage, wrinkling, and collapsing in shape) in the cords during vulcanization can be suppressed. In this case, when the heat shrinkage rate of the cord is less than 0.5%, kinking tends to occur during vulcanization, and thus it is difficult to favorably maintain durability. When the heat shrinkage rate of the cord exceeds 2.5%, uniformity may deteriorate.

[0021] Further, a carcass cord twist coefficient K represented by Formula (1) described below is preferably from 2000 to 2500 and is more preferably from 2100 to 2400. Note that the twist coefficient K is a value of the cord after dip treatment. Using a cord having such a twist coefficient K achieves good cord fatigue and can ensure excellent durability. In this case, when the twist coefficient K of the cord is less than 2000, the cord fatigue deteriorates, and thus it is difficult to ensure durability. When the twist coefficient K of the cord exceeds 2500, productivity of the cord deteriorates.

[00001]$\begin{array}{cc}\text{K = T}\times {\text{D}}^{1/2}& \text{­­­(1)}\end{array}$

[0022] (In the formula, T is a cable twist count of cord (count/10 cm), and D is the total fineness of cord (dtex))

[0023] While the carcass cord is made of the polyester fiber as described above, examples of the polyester fibers can include polyethylene terephthalate fibers (PET fibers), polyethylene naphthalate fibers (PEN fibers), polybutylene terephthalate fibers (PBT), and polybutylene naphthalate fibers (PBN), and PET fibers can be suitably used. Whichever fiber is used, the physical properties of the fiber advantageously provide the high-speed durability and the steering stability in a well-balanced and highly compatible manner. In particular, PET fibers, which are inexpensive, allow reduction in the cost of the pneumatic tire. In addition, workability in producing cords can be increased.

[0024] In an embodiment of the present technology, as illustrated in FIG. 2, a steel cord 7C constituting the belt layer 7 includes an N + M structure (in the illustrated example, 2+2 structure) that is formed of: an inner layer 7n (core) made of N pieces of wire strands; and an outer layer 7m (sheath) made of M pieces of wire strands intertwined around the inner layer 7n. The number of wire strands N of the inner layer 7n is 2 to 4, and the number of wire strands M of the outer layer 7m is 2 to 7. In particular, the illustrated 2+2 structure can be suitably employed. Additionally, the twist directions of the inner layer 7n and the outer layer 7m are not identical and are preferably different. In other words, when the inner layer 7n is S-twist, the outer layer 7m is preferably Z-twist, and when the inner layer 7n is Z-twist, the outer layer is preferably S-twist. When the inner layer 7n is non-twisted, the outer layer 7m is preferably S-twist or Z-twist. In particular, in an embodiment of the present technology, it is preferred that the wire of the inner layer 7n is non-twisted (a state in which the wire strands are not intertwined but arranged). When the number of wire strands N of the inner layer of the steel cord 7C constituting the belt layer 7 is less than two, an initial elongation of the cords is degraded. When the number of wire strands N of the inner layer of the steel cord 7C constituting the belt layer 7 exceeds four, the twisted structure is not stable. When the number of wire strands M of the outer layer of the steel cord 7C constituting the belt layer 7 is less than two, cord strength is insufficient. When the number of wire strands M of the outer layer of the steel cord 7C constituting the belt layer 7 exceeds seven, the twisted structure is not stable.

[0025] The belt cord of an embodiment of the present technology has a stress at break of 3300 MPa or more and preferably from 3300 MPa to 3800 MPa. Further, in the belt layer 7, a product B = S × Eb of a cross-sectional area S (unit: mm.sup.2/piece) per belt cord and the count Eb (unit: piece/50 mm) of the belt cord per 50 mm in the direction orthogonal to the extension direction of the belt cord is from 6.0 mm.sup.2/50 mm to 7.5 mm.sup.2/50 mm, and preferably from 6.2 mm.sup.2/50 mm to 7.2 mm.sup.2/50 mm. The belt layer 7 thus configured allows the strength of the entire belt layer 7 to be suppressed, and a disadvantage in the case of using the polyester fiber cord described above for the carcass cord is supplemented and high-speed steering stability can be improved. At this time, in a case where the stress at break of the belt cord is less than 3300 MPa, the count Eb needs to be increased to ensure belt strength, and therefore a tire weight increases and high-speed steering stability is reduced. When the product B = S × Eb is less than 6.0 mm.sup.2/50 mm, the strength of the plunger decreases due to insufficient belt strength. The product B = S × Eb in excess of 7.5 mm.sup.2/50 mm increases the tire weight and decreases high-speed steering stability. Note that the individual ranges of the cross-sectional area S and the count Eb described above are not particularly limited as long as the product B satisfies the range described above.

[0026] An embodiment of the present technology will further be described below by way of Examples, but the scope of an embodiment of the present technology is not limited to Examples.

EXAMPLE

[0027] Pneumatic tires of Conventional Example 1, Comparative Examples 1 to 4, and Examples 1 to 11 that had a tire size of 245/45R20 and the basic structure illustrated in FIG. 1, included the carcass layer with the material of the carcass layer, the cord structure, the fineness based on corrected mass D (unit: dtex/piece) per carcass cord, the count E (unit: piece/50 mm) per 50 mm in a direction orthogonal to an extension direction of the carcass cord, the product of them A = D × E, elongation at break (unit: %), and elongation under a load of 2.0 cN/dtex (unit: %) set as in Table 1, and the belt layer with the structure of the belt cord, the stress at break (unit: MPa), the cross-sectional area S (unit: mm.sup.2/piece) per belt cord, the count Eb (unit: piece/50 mm) of the belt cord per 50 mm in the direction orthogonal to the extension direction of the belt cord, and the product of them B = S × Eb set as in Table 1 were manufactured.

[0028] In Table 1, all of “the elongation at break” and “the elongation under a load of 2.0 cN/dtex” of the carcass cord were measured by conducting a tensile test under the conditions of a length of specimen between grips of 250 mm and a tensile speed of 300 ± 20 mm/min in accordance with JIS L1017 “Test methods for chemical fiber tire cords”. Specifically, “the elongation at break” is the elongation ratio (%) of the sample cord measured at break of the cord, and “the elongation under a load of 2.0 cN/dtex” is the elongation ratio (%) of the sample cord measured under a load of 2.0 cN/dtex. In addition, “the stress at break” of the belt cord was calculated by dividing the strength at break of the cord by the cord cross-sectional area.

[0029] In Table 1, the column of the material of carcass cord is indicated as “rayon” when rayon fiber cords were used and “PET” when polyethylene terephthalate fiber cords were used.

[0030] These test tires were evaluated for shock burst resistance (plunger energy), high-speed steering stability, and tire weight by an evaluation method described below, and the results are also shown in Table 1.

Shock Burst Resistance (Plunger Energy)

[0031] Each of the test tires was assembled on a wheel having a rim size of 20×8J and inflated to an air pressure of 220 kPa. Tire breakage tests (plunger breakage tests) were performed by pressing a plunger having a plunger diameter of 19 mm ± 1.6 mm against the central portion of the tread at a loading speed (plunger pressing speed) of 50.0 mm ± 1.5 m/min in accordance with JIS K6302, and tire strength (tire breakage energy) was measured. Evaluation results are expressed by an index value with the measurement value of Conventional Example 1 as 100. Larger values indicate larger breakage energy (plunger energy) and superior shock burst resistance. Especially, the index value of “130” or more means that good performance is obtained.

High-Speed Steering Stability

[0032] Each of the test tires was assembled on a wheel having a rim size of 20×8J, inflated to an air pressure of 200 kPa, and mounted on a test vehicle having an engine displacement of 2000 cc. Sensory evaluations for high-speed steering stability were performed on a test course including dry road surfaces by test drivers with two occupants riding in the vehicle. The evaluation results were rated by a 5-point method with the result of Conventional Example 1 assigned 3.0 (reference) and expressed as average points by five test drivers excluding the highest point and the lowest point. Larger evaluation values indicate superior high-speed steering stability.

Tire Weight

[0033] The weight of each test tire was measured. Evaluation results are expressed by an index value with the measurement value of Conventional Example 1 as 100. Smaller index values indicate smaller tire weights.

TABLE-US-00001 Conventional Example 1 Comparative Example 1 Comparative Example 2 Carcass layer Material of carcass cord Rayon Rayon PET dtex/50 mm Product A 2.6 × 10.sup.5 2.6 × 10.sup.5 2.1 × 10.sup.5 Elongation at break % 14.0 14.0 17.0 Elongation under 2.0 mm cN/dtex load - - 5.0 Belt layer Cord structure 2+2×0.25 2+2×0.25 2+2×0.25 Stress at break MPa 3,100 3400 3,100 mm.sup.2/50 mm Product B 8.0 7.5 8.0 Evaluation Shock burst Index resistance value (Plunger energy) 100 100 120 High-Speed Steering Stability 3.0 3.2 2.8 Index Tire weight value 100 96 95

TABLE-US-00002 Comparative Example 3 Comparative Example 4 Example 1 Carcass layer Material of carcass cord PET PET PET dtex/50 mm Product A 2.1 × 10.sup.5 2.5 × 10.sup.5 2.5 × 10.sup.5 Elongation at break % 17.0 23.0 23.0 Elongation under 2.0 mm cN/dtex load 5.0 6.5 6.5 Belt layer Cord structure 2+2×0.25 2+2×0.25 2+2×0.25 Stress at break MPa 3400 3,100 3400 mm.sup.2/50 mm Product B 7.5 8.0 7.5 Evaluation Shock burst Index resistance value (Plunger energy) 120 190 190 High-Speed Steering Stability 3.0 2.8 3.0 Index Tire weight value 92 100 96

TABLE-US-00003 Example 2 Example 3 Example 4 Carcass layer Material of carcass cord PET PET PET Product A dtex/50 mm 2.5 × 10.sup.5 2.5 × 10.sup.5 2.5 × 10.sup.5 Elongation at break % 23.0 23.0 23.0 Elongation under 2.0 mm cN/dtex load 6.5 6.5 6.5 Belt layer Cord structure 2+2×0.25 2+2×0.25 2+2×0.25 Stress at break MPa 3400 3700 3700 Product B mm.sup.2/50 mm 7.2 7.2 7.0 Evaluation Shock burst Index value resistance (Plunger energy) 185 190 185 High-Speed Steering Stability 3.2 3.2 3.3 Tire weight Index value 94 93 90

TABLE-US-00004 Example 5 Example 6 Example 7 Example 8 Carcass layer Material of carcass cord PET PET PET PET Product A dtex/50 mm 1.8 × 10.sup.5 3.0 × 10.sup.5 2.5 × 10.sup.5 2.5 × 10.sup.5 Elongation at break % 23.0 23.0 23.0 23.0 Elongation under 2.0 mm cN/dtex load 6.5 6.5 5.5 8.0 Belt layer Cord structure 2+2×0.25 2+2×0.25 2+2×0.25 2+2×0.25 Stress at break MPa 3700 3700 3700 3700 Product B mm.sup.2/50 mm 7.0 7.0 7.0 7.0 Evaluation Shock burst resistance Index value (Plunger energy) 183 187 185 185 High-Speed Steering Stability 3.0 3.3 3.3 3.3 Tire weight Index value 90 90 90 90

TABLE-US-00005 Example 9 Example 10 Example 11 Carcass layer Material of carcass cord PET PET PET Product A dtex/50 mm 2.5 × 10.sup.5 2.5 × 10.sup.5 2.5 × 10.sup.5 Elongation at break % 20.0 30.0 23.0 Elongation under 2.0 cN/dtex load mm 6.5 6.5 6.5 Belt layer Cord structure 2+2×0.25 2+2×0.25 2+2×0.25 Stress at break MPa 3700 3700 4000 Product B mm.sup.2/50 mm 7.0 7.0 6.0 Evaluation Shock burst resistance Index value (Plunger energy) 183 187 180 High-Speed Steering Stability 3.3 3.3 3.5 Tire weight Index value 90 90 88

[0034] As can be seen from Table 1, in comparison to the Conventional Example 1, while the tires of Examples 1 to 11 provided satisfactorily high-speed steering stability, the tires improved shock burst resistance and reduced the tire weight. On the other hand, in Comparative Example 1, since the carcass cord was a rayon fiber cord, even when the belt cord satisfied the requirements of an embodiment of the present technology, shock burst resistance was not able to be improved. In Comparative Example 2, since the elongation at break of the carcass cord and the elongation under a load of 1.5 cN/dtex were small and the stress at break of the belt cords was small and the product B was large, and thus high-speed steering stability was reduced. In Comparative Example 3, the elongation at break and the elongation under a load of 1.5 cN/dtex of the carcass cord were small, and thus the effect of improving high-speed steering stability was not obtained. In Comparative Example 4, since the stress at break of the belt cord was small and the product B was large, high-speed steering stability was deteriorated.