Pneumatic tire

Abstract

To improve steering stability and to maintain or improve high speed durability.SOLUTION: A pneumatic tire comprises a belt ply including steel cords which are so arranged as to be inclined with respect to a tire circumferential direction. A bending hardness of the belt ply, that is calculated by multiplying the number of insertion cords by cord bending hardness per one steel cord, is 10 N / 25.4 mm or more and less than 30 N / 25.4 mm. A circumferential direction strength of the belt ply, that is calculated by multiplying cosθ when an inclination angle of the steel cord with respect to the tire circumferential direction is represented by θ by the product of a cutting load per one steel cord and the number of insertion cords, is 13.5 kN / 25.4 mm or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a pneumatic tire. [Background technology]

[0002] A pneumatic tire generally includes a belt, which is a laminate of multiple belt plies, between the outer surface of the carcass ply and the tread rubber, and the belt plies include steel cords arranged at an angle relative to the tire circumferential direction. The belt tightens the carcass ply to increase the rigidity of the tread, and various proposals have been made to improve durability, steering stability, ride comfort, etc.

[0003] For example, Patent Documents 1 and 2 propose that the cord diameter or filament diameter of the steel cord be specified, and that the cord bending hardness and cord strength per steel cord, as well as the cord strength per belt ply, be within specific ranges in order to reduce tire mass while improving durability and ride comfort.

[0004] Patent Document 3 discloses that, in order to improve road grip while maintaining durability, the filament diameter of the steel cord is reduced to 0.14 to 0.18 mm, and a new belt ply with a larger cord angle is provided between the belt plies.

[0005] Patent Document 4 proposes that the arrangement angle of each steel cord in at least two belt plies be within a specific range in order to improve high-speed durability and abrasion resistance while also achieving excellent handling stability.

[0006] Patent Document 5 discloses that, for the purpose of improving steering stability and vibration ride comfort performance, a tire vertical spring constant is specified, and the stiffness of the steel cord per unit width of the cross belt layer is configured so that the cord tensile stiffness and cord bending stiffness satisfy a specific relational expression. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2020-104772 [Patent Document 2] Japanese Patent Publication No. 2020-104773 [Patent Document 3] Japanese Patent Application Laid-Open No. 2011-051497 [Patent Document 4] Japanese Patent Application Publication No. 8-169207 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-047200 Summary of the Invention [Problem to be solved by the invention]

[0008] Reducing the filament diameter and bending stiffness of the steel cords used in the belt improves road surface conformance and improves handling stability. However, reducing the filament diameter reduces tensile strength and deteriorates high-speed durability. Furthermore, increasing the number of filaments constituting the steel cords to maintain tensile strength increases the bending stiffness of the belt ply, compromising the effect of improving handling stability.

[0009] In view of the above, an object of an embodiment of the present invention is to provide a pneumatic tire that can improve steering stability and maintain or improve high-speed durability.

[0010] Patent Documents 1 and 2 describe the cord strength and bending stiffness of the belt ply, but the bending stiffness exceeds the range specified in the present invention, making it impossible to achieve both high-speed durability and handling stability. Patent Document 3 describes the use of steel cords in which small-diameter filaments are twisted together in the belt ply, but does not describe the circumferential strength and bending stiffness of the belt ply. Patent Document 4 describes the bending stiffness per steel cord, but does not describe the circumferential strength of the belt ply. Patent Document 5 does not describe the breaking load of the steel cord, nor does it describe the circumferential strength taking into account the inclination angle of the steel cord. [Means for solving the problem]

[0011] The present invention includes the embodiments shown below. [1] A pneumatic tire having a belt ply including steel cords arranged at an inclination relative to the tire circumferential direction, wherein the bending hardness of the belt ply, calculated by multiplying the bending hardness of each steel cord by the end count of the cord, is 10 N / 25.4 mm or more and less than 30 N / 25.4 mm, and the circumferential strength of the belt ply, calculated by multiplying the product of the breaking load per steel cord and the end count of the cord, by cosθ, where θ is the angle of inclination of the steel cords relative to the tire circumferential direction, is 13.5 kN / 25.4 mm or more. [2] A pneumatic tire according to [1], wherein the filament diameter of the steel cord is 0.15 mm or more and 0.17 mm or less, the steel cord occupation rate in the width direction of the belt ply is 55% or more and 70% or less, and the inclination angle of the steel cord with respect to the tire circumferential direction is 15° or more and 30° or less. [3] The pneumatic tire according to [1] or [2], wherein the carbon content of the filaments constituting the steel cord is 0.88% by mass or more and 0.95% by mass or less. [Effects of the Invention]

[0012] According to an embodiment of the present invention, by reducing the bending stiffness of the belt ply while maintaining or improving the circumferential strength of the belt ply, it is possible to improve steering stability and maintain or improve high-speed durability. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a half cross-sectional view of a pneumatic tire according to an embodiment. [Figure 2] 1 is a diagram illustrating a method for measuring the cord bending hardness of a steel cord. DETAILED DESCRIPTION OF THE INVENTION

[0014] 1 is a half cross-sectional view showing a pneumatic radial tire for passenger cars as an example of a pneumatic tire. The tire has a tread 1 that forms the contact surface, a pair of left and right bead portions 2, and a pair of left and right sidewalls 3 interposed between the tread 1 and the bead portions 2.

[0015] A toroidally extending carcass ply 4 is provided between a pair of bead portions 2. The carcass ply 4 extends from the tread 1 through both sidewalls 3 and is anchored at the bead portions 2, with both ends of the carcass ply 4 being anchored by being wrapped up from the inside to the outside of a bead core 5 embedded in the bead portion 2. The carcass ply 4 is composed of at least one ply made of organic fiber cords arranged substantially perpendicular to the circumferential direction of the tire.

[0016] A belt 6 consisting of two or more belt plies is provided on the outer periphery of the carcass ply 4 in the tread 1 (i.e., on the outer side in the tire radial direction), and a belt reinforcing layer 7 consisting of organic fiber cords wound spirally at an angle of 0° to 5° with respect to the tire circumferential direction is provided on the outer periphery of the belt ply. A tread rubber 8 is provided on the outer periphery of the belt reinforcing layer 7. The belt reinforcing layer 7 may be a cap ply that covers the entire belt 6 in the width direction, or an edge ply that covers both ends of the belt 6. The belt reinforcing layer 7 may also be omitted.

[0017] In this specification, the tire circumferential direction refers to the circumferential direction centered on the tire rotational axis, the tire radial direction refers to the direction perpendicular to the tire rotational axis, and the tire width direction refers to the direction parallel to the tire rotational axis, also referred to as the tire axial direction.

[0018] The belt 6 is configured by one or more belt plies and is laid over the outer periphery of the crown portion of the carcass ply 4. In this example, the belt 6 is configured by two plies: a first belt ply 6A on the inner side in the tire radial direction and a second belt ply 6B on the outer side.

[0019] These belt plies 6A, 6B include steel cords arranged at an angle relative to the tire circumferential direction. Specifically, the belt plies 6A, 6B are formed by covering steel cords with rubber, and the steel cords are arranged at a constant angle relative to the tire circumferential direction and at predetermined intervals in the tire width direction, and the steel cords are arranged so that they cross each other between the two belt plies 6A, 6B, i.e., so that they are inclined symmetrically relative to the tire circumferential direction.

[0020] In this embodiment, the bending stiffness of each belt ply 6A, 6B (hereinafter referred to as belt ply bending stiffness) is set to 10 N / 25.4 mm or more and less than 30 N / 25.4 mm. When the belt ply bending stiffness is 10 N / 25.4 mm or more, the out-of-plane rigidity of the belt 6 does not become too low, and an excellent ground contact shape can be maintained. When the belt ply bending stiffness is less than 30 N / 25.4 mm, the ability to follow the road surface shape is improved. Therefore, excellent steering stability is obtained.

[0021] The bending hardness of the belt ply is preferably 12 to 28 N / 25.4 mm, more preferably 15 to 27 N / 25.4 mm, and even more preferably 18 to 25 N / 25.4 mm.

[0022] In this specification, the "belt ply bending hardness" refers to the bending hardness (N) per 25.4 mm (= 1 inch) width of the belt ply, and is calculated by multiplying the cord bending hardness per steel cord by the cord end count, as shown in the following formula. Belt ply bending hardness (N / 25.4mm) = cord bending hardness (N / string) x number of cords (strings / 25.4mm)

[0023] In this specification, the "cord bending hardness" is a value defined by the maximum load when a single steel cord is bent at its center with a fulcrum distance of 25.4 mm. The detailed measurement method is as described in the Examples section.

[0024] In this specification, "cord end count" refers to the number of steel cords arranged per 25.4 mm (= 1 inch) width of the belt ply. Here, the width of the belt ply does not refer to the dimension in the tire width direction, but refers to the width of the belt ply in the direction perpendicular to the longitudinal direction of the steel cords within the plane of the belt ply. The longitudinal direction of the steel cords refers to the direction along the linearly extending steel cords.

[0025] In this embodiment, the circumferential strength of each belt ply 6A, 6B (hereinafter referred to as belt ply circumferential strength) is set to 13.5 kN / 25.4 mm or more. By setting the belt ply circumferential strength to 13.5 kN / 25.4 mm or more, an excellent ground contact shape due to the hoop effect can be easily obtained, and high-speed durability can be maintained or improved.

[0026] Although there is no particular upper limit to the circumferential strength of the belt ply, it is preferably 22.0 kN / 25.4 mm. By setting the circumferential strength of the belt ply to 22.0 kN / 25.4 mm or less, an excessive increase in cord end count can be suppressed, preventing the bending hardness of the belt ply from becoming too high. In addition, the distance between adjacent steel cords in the belt ply can be prevented from becoming too close, preventing a decrease in durability due to belt separation.

[0027] The circumferential strength of the belt ply is more preferably 13.8 to 20.0 kN / 25.4 mm, more preferably 14.0 to 19.0 kN / 25.4 mm, and even more preferably 14.2 to 18.0 kN / 25.4 mm.

[0028] In this specification, the "belt ply circumferential strength" refers to the tensile strength of the belt ply in the tire circumferential direction per 25.4 mm in the tire width direction, and is calculated as follows: multiplying the product of the breaking load per steel cord (hereinafter referred to as the cord breaking load) and the number of cords by cos θ, where θ (°) is the inclination angle of the steel cord with respect to the tire circumferential direction. Belt ply circumferential strength (kN / 25.4mm) = Cord breaking load (kN / cord) x Number of cords inserted (cords / 25.4mm) x cosθ

[0029] In this specification, the term "cord breaking load" refers to the maximum load required to break one steel cord, measured in accordance with JIS G3510-1992.

[0030] In this specification, the inclination angle θ of the steel cord is the cord angle of the steel cord when the tire circumferential direction is set to 0°, i.e., the angle between the longitudinal direction of the steel cord and the tire circumferential direction. By multiplying the product of the cord breaking load and the cord count by cos θ, it is possible to directly set the circumferential strength of the belt ply when made into a tire, and it is possible to more reliably maintain or improve high-speed durability.

[0031] The filament diameter of the steel cords constituting the belt plies 6A, 6B is preferably 0.15 mm or more and 0.17 mm or less. When the filament diameter is 0.15 mm or more, it is easy to set the bending hardness of the belt ply to above the above lower limit. When the filament diameter is 0.17 mm or less, it is easy to set the bending hardness of the belt ply to below the above upper limit. The filament diameter is more preferably 0.155 to 0.165 mm.

[0032] The steel cord occupation ratio (hereinafter referred to as cord occupation ratio) in the width direction of the belt plies 6A, 6B is preferably 55% or more and 70% or less. A cord occupation ratio of 55% or more makes it easy to increase the circumferential strength of the belt ply. A cord occupation ratio of 70% or less prevents the distance between adjacent steel cords in the belt ply from becoming too close, thereby suppressing a decrease in durability due to belt separation. The cord occupation ratio is more preferably 56 to 69%, and even more preferably 57 to 65%.

[0033] In this specification, the "cord occupation ratio" is a value calculated by the following formula for a so-called topping roll in which steel cords are aligned and arranged at a predetermined placement density and rubber-coated. Therefore, as in the case of the cord placement density, the width direction of the belt ply in terms of the cord occupation ratio does not refer to the tire width direction, but rather to the width direction of the belt ply in the direction perpendicular to the length direction of the steel cords within the plane of the belt ply. Cord occupancy rate (%) = cord diameter (mm) x number of cords (pieces / 25.4mm) x 100 / 25.4 (mm).

[0034] The inclination angle θ of the steel cords in the belt plies 6A, 6B relative to the tire circumferential direction is preferably 15° or more and 30° or less. An inclination angle of 15° or more makes it easier to maintain excellent steering stability. An inclination angle of 30° or less makes it easier to improve steering stability while maintaining the circumferential strength of the belt ply. The inclination angle θ of the steel cords is more preferably 18° to 28°, and even more preferably 20° to 27°.

[0035] The carbon content of the filaments constituting the steel cord is preferably 0.88% by mass or more and 0.95% by mass or less. That is, it is preferable to use a carbon steel wire rod having a carbon content of 0.88 to 0.95% by mass as the steel material used for the filaments. A higher carbon content results in a stronger and harder steel. As described above, the filament diameter of the steel cord is reduced to reduce the bending stiffness of the belt ply, thereby improving steering stability, while the use of a high-strength material with a high carbon content makes it easier to improve the strength of the steel cord. Therefore, when made into a tire, the circumferential strength of the belt ply is maintained or improved, making it easier to maintain or improve high-speed durability.

[0036] The carbon content of the filament is more preferably 0.90 to 0.95% by mass. Examples of carbon steel wire rods with such a high carbon content include SWRS92A and SWRS87A as specified in JIS G3502:2019.

[0037] The cord structure of the steel cord is not particularly limited, but is preferably an m+n multi-layer twisted structure consisting of a core made of m filaments of the same diameter twisted together and a sheath made of n filaments of the same diameter twisted together around the core, where m is preferably 2 or 3 and n is preferably 6 to 9.

[0038] The bending hardness of each steel cord is not particularly limited, and may be, for example, 50 to 160 cN / cord, 70 to 150 cN / cord, or 90 to 130 cN / cord.

[0039] The breaking load per steel cord is not particularly limited, and may be, for example, 500 to 1000 N / cord, or 600 to 800 N / cord.

[0040] The number of cords to be embedded is not particularly limited, and may be, for example, 18 to 30 cords / 25.4 mm, or 21 to 28 cords / 25.4 mm.

[0041] The type of pneumatic tire according to the embodiment is not particularly limited, and may be a tire for a passenger car or the like. [Example]

[0042] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0043] [Measurement and test methods] The measurement and test methods used in the examples are as follows.

[0044] (Steel cord and belt ply test method) Filament diameter, cord diameter: In accordance with JIS G3510-1992, the diameter of the filament (filament diameter) and the diameter of the steel cord (cord diameter) are measured using a specified thickness gauge.

[0045] Carbon content of filament: Measured by molten steel analysis. General matters for analytical testing and how to take analytical samples are in accordance with Clause 8 (Chemical Composition) of JIS G0404:2014.

[0046] Cord bending hardness: The cord bending hardness per steel cord was measured as follows. At room temperature, as shown in Figure 2, an 80 mm long steel cord 20 was supported by a support 22 in a suspended position with both ends free and the center at 25.4 mm intervals. The midpoint 20M of the support position by the support 22 was placed at a right angle from below against the linear upper edge 21A of a fixed inverted U-shaped jig 21. The upper edge 21A had a rod-like shape with a circular cross section (diameter = 3.0 mm). From this state, the support 22 was pulled upward at a pulling rate of 500 mm / min using a tensile tester (Shimadzu Autograph Corporation). The load applied to the support 22 was measured while the steel cord 20 was bent by the jig 21. The maximum load (cN) at this time was measured, and the average value of n = 5 was taken as the cord bending hardness (cN / cord).

[0047] Belt ply bending hardness (N / 25.4mm): Calculated by multiplying the bending hardness of each steel cord (N / cord) by the number of cords inserted (cords / 25.4mm).

[0048] Cord breaking load: In accordance with JIS G3510-1992, the strength and elongation properties of the steel cord were measured using a tensile testing machine (Shimadzu Autograph, manufactured by Shimadzu Corporation) under tension conditions of a gripping distance of 250 mm and a pulling speed of 25 mm / min, and the maximum load required to break one steel cord was calculated as the cord breaking load (N / cord).

[0049] Belt ply circumferential strength (kN / 25.4mm): Calculated by multiplying the cord breaking load (kN / cord) and the number of cords inserted (cords / 25.4mm) by the cosine of the steel cord inclination angle.

[0050] - Steel cord inclination angle: For an uninflated tire, measure the angle of the steel cord relative to the tire circumferential direction (inclination angle with the tire circumferential direction being 0°) at the tire equator CL (widthwise center) of the tread.

[0051] (Tire Testing Methods) Handling stability: A prototype tire with an internal pressure of 200 kPa was mounted on a test vehicle with an engine displacement of 3,000 cc, and three trained test drivers drove the vehicle on a test course to evaluate the feeling. The tire was scored relative to the tire of Comparative Example 1 on a 10-point scale, with the tire of Comparative Example 1 receiving a score of 6. The average score of the three drivers was expressed as an index, with the evaluation result of the tire of Comparative Example 1 being 100. The higher the number, the better the handling stability.

[0052] High-speed durability: Measured as follows using a drum testing machine equipped with a smooth-surfaced, steel rotating drum with a diameter of 1,700 mm, in accordance with FMVSS 109 (UTQG). The prototype tire was mounted on a standard rim specified by JIS with an internal pressure of 220 kPa, and the load was 88% of the maximum load specified by JATMA. After a 60-minute break-in run at a speed of 80 km / h, the tire was allowed to cool, the air pressure was readjusted, and then the actual run was carried out. The actual run started at 120 km / h, and the speed was increased by 8 km / h every 30 minutes thereafter, until failure occurred. The total distance traveled in the actual run until failure occurred was expressed as an index, with the value for Comparative Example 1 set to 100. The higher the index, the better the high-speed durability.

[0053] [Examples and Comparative Examples] Pneumatic radial tires having a tire size of 235 / 40ZR17 and equipped with a belt 6 consisting of two belt plies 6A and 6B as shown in FIG. 1 were fabricated by vulcanization molding according to a conventional method. In each tire fabrication, the belt ply configuration including the steel cord structure was as shown in Tables 1 to 3, and all other configurations were the same. The twist pitch of the steel cord was 9.7 mm. The inclination angle of the steel cord was symmetrical between the two belt plies 6A and 6B in the tire circumferential direction.

[0054] The obtained prototype tires were used to evaluate the steering stability and high-speed durability, and the results are shown in Tables 1 to 3.

[0055] [Table 1] [Table 2] [Table 3]

[0056] In Comparative Example 1, which served as a control, the carbon content of the filament was 0.85% by mass and the filament diameter of the steel cord was 0.18 mm. The circumferential strength of the belt ply met the specified range, but the bending hardness of the belt ply was greater than the specified range, resulting in poor handling stability.

[0057] In Example 1, the filament diameter was reduced to 0.16 mm compared to Comparative Example 1, thereby reducing the bending hardness of the belt ply, and the carbon content of the filaments was increased to 0.90 mass%, thereby improving the circumferential strength of the belt ply. Therefore, compared to Comparative Example 1, the handling stability was improved and high-speed durability was also improved.

[0058] In Example 2, the belt ply circumferential strength was reduced by reducing the cord end count compared to Example 1. In Example 3, the belt ply circumferential strength was increased by increasing the cord end count compared to Example 1. In Example 4, the belt ply circumferential strength was increased by reducing the inclination angle of the steel cords compared to Example 1. In Example 5, the belt ply circumferential strength was reduced by increasing the inclination angle of the steel cords compared to Example 1. In all of Examples 2 to 5, the belt ply bending hardness and belt ply circumferential strength satisfied the specified ranges, and therefore, compared to Comparative Example 1, handling stability was improved while maintaining or improving high-speed durability.

[0059] In Example 6, the filament diameter was reduced to 0.15 mm compared to Example 1, and the cord end count was increased to match the cord occupancy rate. In Example 7, the filament diameter was increased to 0.17 mm compared to Example 1, and the cord end count was reduced to match the cord occupancy rate. In both Examples 6 and 7, the belt ply bending hardness and belt ply circumferential strength satisfied the specified ranges, and therefore, compared to Comparative Example 1, high-speed durability was maintained or improved, while handling stability was improved.

[0060] In Example 8, the carbon content of the filaments was increased compared to Example 1, thereby increasing the circumferential strength of the belt ply. In Example 9, the carbon content of the filaments was reduced compared to Example 1, thereby decreasing the circumferential strength of the belt ply. In Example 10, the cord end count was increased and the inclination angle of the steel cords was reduced compared to Example 8, thereby further increasing the circumferential strength of the belt ply. In all of Examples 8 to 10, the bending hardness of the belt ply and the circumferential strength of the belt ply satisfied the specified ranges, and therefore, compared to Comparative Example 1, handling stability was improved while maintaining or improving high-speed durability.

[0061] In Comparative Example 2, the inclination angle of the steel cord was increased compared to Example 1, so the circumferential strength of the belt ply was below the specified range. Therefore, high-speed durability was poor. In Comparative Example 3, the filament diameter was reduced compared to Example 1, and the cord count was increased to match the cord occupancy rate, but the bending hardness of the belt ply was below the specified range. Therefore, handling stability was poor.

[0062] In Comparative Example 4, the filament diameter was smaller than in Comparative Example 1, and the bending hardness of the belt ply was reduced and met the specified range, but the circumferential strength of the belt ply was below the specified range, resulting in poor high-speed durability.

[0063] In Comparative Example 5, the cord end count was increased compared to Example 1, resulting in a belt ply bending hardness greater than the specified range, and no improvement in handling stability was observed. Furthermore, belt separation performance deteriorated, and high-speed durability deteriorated. In Comparative Example 6, in order to increase the circumferential strength of the belt ply, the filament diameter was increased, the cord end count was increased, and the inclination angle of the steel cord was reduced. As a result, the belt ply bending hardness exceeded the specified range, and handling stability deteriorated. Although the hoop effect was improved by the increased strength, the high cord end count and cord occupancy rate meant that high-speed durability did not improve and was equivalent to that of Comparative Example 1.

[0064] The various numerical ranges described in this specification can be arbitrarily combined with their upper and lower limits, and all such combinations are considered to be preferred numerical ranges described in this specification. Furthermore, a numerical range described as "X to Y" means from X to Y. [Explanation of symbols]

[0065] 1...tread, 2...bead portion, 3...sidewall, 4...carcass ply, 5...bead core, 6...belt, 6A, 6B...belt ply, 7...belt reinforcing layer, 8...tread rubber

Claims

1. A pneumatic tire having a belt ply including steel cords arranged at an angle with respect to the tire circumferential direction, a bending hardness of the belt ply calculated by multiplying the bending hardness per steel cord by the cord end count is 10 N / 25.4 mm or more and less than 30 N / 25.4 mm; a circumferential strength of the belt ply calculated by multiplying the product of the breaking load per steel cord and the cord end count by cos θ, where θ is the inclination angle of the steel cord with respect to the tire circumferential direction, of 13.5 kN / 25.4 mm or more; Pneumatic tires.

2. 2. The pneumatic tire according to claim 1, wherein a filament diameter of the steel cord is 0.15 mm or more and 0.17 mm or less, an occupation rate of the steel cord in the width direction of the belt ply is 55% or more and 70% or less, and an inclination angle of the steel cord with respect to the tire circumferential direction is 15° or more and 30° or less.

3. The pneumatic tire according to claim 1 or 2, wherein the carbon content of the filaments constituting the steel cord is 0.88% by mass or more and 0.95% by mass or less.

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