Patent Application
20160167435
The present invention relates to a pneumatic tire (to be also simply referred to as a “tire” hereinafter) and, more particularly, to a heavy-duty pneumatic tire used for heavy-duty vehicles such as trucks and buses, especially to a refinement to its carcass ply cords.
In recent years, to reduce the environmental load, it has become an important issue to reduce the weight and the amount of material used, for a tire as well. To tackle this issue, when the amount of material used for a carcass ply serving as a framework member of a tire is assumed to be reduced, it is necessary to maintain a given tire strength in the carcass ply and, in turn, to increase the tensile strength of carcass ply cords, i.e., their tensile fracture strength per unit cross-sectional area.
Conventionally, a so-called multilayer twisted steel cord formed by twisting steel filaments together in two or three layers is commonly employed for the carcass ply of a pneumatic tire used for heavy-duty vehicles such as trucks and buses.
As a technique associated with a refinement to carcass ply cords used for a tire, patent document 1, for example, discloses a steel cord for rubber product reinforcement including a core made of three steel filaments, a first sheath made of nine steel filaments printed with a wave pattern, and a second sheath made of 15 steel filaments printed with a wave pattern. Patent document 2 discloses a pneumatic tire with its carcass reinforced by steel cords having cores each including a plurality of steel filaments and sheaths, each of which is formed by twisting together a plurality of steel filaments having the same diameter as that of the core in the same direction and is disposed around the core, and not having wrap filaments which are looped around the sheaths and constrain the steel filaments of the sheaths. In the steel cord, three steel filaments having a predetermined diameter and tensile strength are used for the core, and eight such steel filaments for the sheath.
Patent Document 1: Japanese Unexamined Patent Application Publication No. H7-292585 (e.g., CLAIMS)
Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-28906 (e.g., CLAIMS)
As described above, to reduce the amount of material used for the carcass ply, it is necessary to increase the tensile strength of the carcass ply cords per unit area. However, as the tensile strength increases, the cords become more vulnerable to shear deformation that occurs in response to input in a direction perpendicular to the longitudinal direction due to embrittlement of steel products. This lowers the cord tensile fracture strength in the contact portions between the bead cores and the carcass ply cords, resulting in a higher rate of tire failure due to cord breakage.
To solve this problem, durability can he improved by interposing a member made of, for example, hard rubber or an organic fiber between the bead cores and the carcass ply. This, however, leads to a heavier weight. Therefore, an established technique has been demanded which achieves a lighter-weight tire by reducing the amount of material used for the carcass ply without degrading the durability of bead portions.
It is an object of the present invention to solve the above-mentioned problem and provide a pneumatic tire having its weight reduced without degrading the durability of bead portions.
To achieve a lighter-weight tire without lowering the strength of the carcass ply in the conventional multilayer twisted steel cord, the use of high-tensile filaments is effective to reduce the amount of steel used. However, because especially filaments having a wire diameter as small as 0.20 mm or less are prone to breakage in the manufacturing process, high-tensile cords are hard to use.
In view of this, it is another object of the present invention to provide a pneumatic tire having both a sufficient carcass strength and a lighter weight while improving the durability of the carcass ply and suppressing breakage in the manufacturing process by refining a multilayer twisted steel cord structure used for the carcass ply.
The inventor of the present invention conducted a close examination and concluded that the above-mentioned problems can be solved by defining the cord diameter of a steel cord used for the carcass ply, based on the relationship with the tensile strength of steel filaments (steel wires) used for the steel cord, and defining the ensile strength of the steel filaments and the number of hits per unit width of the steel cord within predetermined ranges. This inventor thus completed the present invention.
More specifically, the present invention provides a pneumatic tire comprising, as a framework, at least one carcass ply comprising a steel cord covered with rubber, wherein
letting d (mm) be the cord diameter of the steel cord and T (MPa) be the tensile strength of a steel filament forming the steel cord, the following relation is satisfied:
a
1
T−b
1
>d>a
2
T+b
2
(where a1 is 3.65×10−4 mm/MPa, b1 is 0.42 mm, a2 is −8.00×10−5 mm/MPa, and b2 is 0.78 mm), and the tensile strength T of the steel filament is 3000 MPa (exclusive) to 4000 MPa (exclusive) and the number of hits E under a bead core of the steel cord in the carcass ply is 12 hits/25 mm (exclusive) to 38 hits/25 mm (exclusive).
In the present invention, the tensile strength T of the steel filament is preferably 3200 MPa (exclusive) to 3800 MPa (exclusive). Furthermore, in the present invention, the number of hits E of the steel cord is preferably 14 hits/25 mm (exclusive) to 30 hits/25 mm (exclusive).
Moreover, in the present invention, it is also preferable that the steel cord comprises a two-layer twisted cord formed by twisting together steel filaments having a wire diameter of 0.15 to 0.20 mm and a tensile strength T of 3140 to 3630 MPa in one of a 3+8 structure and a 3+9 structure comprising a core and a sheath, and a wrap filament which is looped around the sheath and constrains the sheath should be excluded from the steel cord.
According to the present invention, the aforementioned configuration achieves a pneumatic tire having its weight reduced without degrading the durability of bead portions.
Further, according to the present invention, the aforementioned configuration achieves a pneumatic tire having both a sufficient carcass strength and a lighter weight while improving the durability of the carcass ply and suppressing breakage in the manufacturing process.
(a) and (b) of
Embodiments of the present invention will be described in detail below with reference to the drawings.
In the present invention, it is important that the carcass 5 should be made of at least one carcass ply having steel cords covered with rubber and the steel cords forming the carcass ply should satisfy the following predetermined condition.
Letting d (mm) be the cord diameter of a steel cord used for the above-mentioned carcass ply and T (MPa (N/mm2)) be the tensile strength of a steel filament forming the steel cord, the tire according to the present invention needs to satisfy a relation
a
1
T−b
1
>d>a
2
T+b
2
(where a1 is 3.65×10−4 mm/MPa, b1 is 0.42 mm, a2 is ˜8.00×10−5 mm/MPa, and b2 is 0.78 mm.) As for the upper and lower limits of the cord diameter in the above-described inequality, since too large a cord diameter is disadvantageous in terms of weight because of the thick cord, and the cord diameter needs to be increased to maintain a sufficient tensile fracture strength upon a reduction in tensile strength, the need to increase the cord diameter upon a reduction in tensile strength is specified, depending on the balance between these two limits. Making these two limits fall within the range satisfying the above-described inequality prevents tire failure in the bead portions even when the weight can be reduced by increasing the tensile strength T, thus achieving durability in the bead portions and a lighter weight. This is because reducing the cord diameter makes it possible to reduce input in a direction perpendicular to the longitudinal direction of the cords received from the bead cores when used, thereby maintaining durability in the bead portions. It has conventionally been difficult to manufacture high-tensile cords, and even if such cords can be manufactured, the durability of the bead portions lowers because they are vulnerable to input in a direction perpendicular to the longitudinal direction of the cords. In addition, the number of hits needs to be increased to ensure a given tire strength in small-diameter cords and this degrades the productivity, so a reduction in diameter of the cords has rarely been examined.
Conventionally, steel cords having a cord diameter larger than the range specified by the above-mentioned tensile strength T have been used. In this case, however, since input in a direction perpendicular to the longitudinal direction of the cords from the bead cores is large, attempt to achieve a lighter weight by increasing the tensile strength degrades the durability of the bead portions, and a heavier weight is required to satisfy the durability of the bead portions. Therefore, a sufficient durability in bead portions and a lighter weight cannot be achieved simultaneously. On the other hand, steel cords having a cord diameter smaller than the above-mentioned range are too thin to ensure a sufficient cord tensile fracture strength, thus making it impossible to ensure a strength required by the tire.
Furthermore, in the present invention, the tensile strength T of the above-mentioned steel filaments needs to be 3000 MPa (exclusive) to 4000 MPa (exclusive) and more preferably 3200 MPa (exclusive) to 3800 MPa (exclusive). When the tensile strength T of the steel filaments is 3000 MPa or less, a relatively large amount of steel is required to be used to ensure a given strength, thus making it difficult to achieve a lighter-weight tire. On the other hand, cords including steel filaments having a tensile strength T of 4000 MPa or more are hard to manufacture and are therefore impractical.
Moreover, in the present invention, the number of hits E under the bead core 4 of the steel cord in the carcass ply needs to be 12 hits/25 mm (exclusive) to 38 hits/25 mm (exclusive) and more preferably 14 hits/25 mm (exclusive) to 30 hits/25 mm (exclusive). The number of hits under the bead core 4 of the steel cord in the carcass ply means herein the number of hits in a folding region 5a of the carcass ply illustrated in
In the present invention, since it is important only that steel cords satisfying the above-described predetermined condition should be applied to the carcass ply, specific configurations of the steel cords other than this point of importance, details of the tire construction, the material of each member, and the like are not particularly limited and may be selectively specified from the conventionally known ones as appropriate.
The cord diameter d of the above-mentioned steel cords used in the present invention is preferably, for example, 0.50 to 0.80 mm. Further, examples of the specific structure of the above-mentioned steel cords used in the present invention may include the 3+9 and 3+8 structures.
In particular, in the present invention, a two-layer twisted cord formed by twisting together steel filaments having a wire diameter of 0.15 to 0.20 mm and a tensile strength T of 3140 to 3630 MPa (320 to 370 kgf/mm2) in the 3+8 or 3+9 structure including a core and a sheath is preferably employed.
In this case, reducing the diameter of the steel filaments results in less flexural strain to improve the durability of the carcass ply, and the use of steel filaments having a high tensile strength within an appropriate range makes it possible to ensure a given carcass strength and achieve a lighter weight, with less concerns for breakage in the manufacturing process. Further, the use of steel cords having a two-layer twisted structure: the 3+8 or 3-1-9 structure also allows a small cord diameter and a reduction in amount of rubber used for the carcass ply, and this is preferable in terms of achieving a lighter-weight tire.
When the steel filaments have a wire diameter lamer than 0.20 mm, the filaments are more prone to breakage and the durability of the carcass ply is relatively low, because the flexural strain is relatively high. On the other hand, when the steel filaments have a wire diameter smaller than 0.15 mm, breakage is more likely to occur in the manufacturing process. Note herein that it is crucial to use steel filaments having a small wire diameter of 0.15 to 0.20 mm. Especially when the present invention is applied to a low-profile tire having a tire profile of about 70% at which a high strain occurs in the sidewall portions, filaments having a smaller wire diameter of 0.15 to 0.175 mm are preferably used to suppress filament breakage.
In a cord having the 3+8 or 3+9 structure and formed by using steel filaments having a small wire diameter of 0.15 to 0.20 mm, the ensuring of a given carcass strength and the fabrication of a lighter-weight tire cannot be achieved simultaneously when the tensile strength of the steel filaments is lower than 3140 MPa, while breakage is more likely to occur in the manufacturing process when this tensile strength is higher than 3630 MPa.
(a) and (b) of
As illustrated in
In the preferred embodiment of the present invention, when at least two carcass plies are used, steel cords satisfying the above-described condition according to the present invention for all plies need to be used to obtain an expected effect.
In the present invention, at least one carcass ply needs to be disposed but two or more carcass plies may be arranged, and the carcass plies are typically folded and locked from the tire inside to outside around the bead cores 4, as illustrated in
Additionally, in the tire illustrated in
The present invention will be described in more detail below with reference to Examples.
The number of hits in the carcass ply means hereinafter the number of hits under the bead core.
A heavy-duty tire having a size of 11R22.5 was fabricated by applying steel cords to one carcass ply in accordance with conditions shown in the following table. Four belt layers (material: steel cords) were arranged at angles of +50°, +20°, −20°, and −20° with respect to the tire circumferential direction in turn from inside in the tire radial direction.
The total weights of steel cords contained per unit area in carcass plies used in respective Examples and Comparative Examples were evaluated and represented by index numbers assuming that the total weight of steel cords according to Example 1-1 is 100. The smaller the numerical value, the lighter the weight and the better the result.
The total tensile fracture strengths of steel cords used in respective Examples and Comparative Examples were evaluated and represented by index numbers assuming that the total tensile fracture strength of steel cords according to Example 1-1 is 100. The larger the numerical value, the higher the tensile fracture strength and the better the result.
The durability of the bead portions was evaluated for tires to be tested according to respective Examples and Comparative Examples. More specifically, hydraulic pressure was applied into a tire assembled with a rim until the tire breaks, and it was checked whether filament breakage had occurred in the carcass cords near the bead cores after failure. The result was represented by ◯ for no wire breakage and X for wire or cord breakage.
The productivities of steel cords used in respective Examples and Comparative Examples were evaluated based on the number of breakages that have occurred in manufacturing 100,000-m wires in the final wire drawing process of the wire manufacture. The result was represented by ◯ for less than one breakages/100.000 m and X for one or more breakages/100,000 m.
These results are shown together in the following table.
As shown in the above-described table, in each Example in which steel cords satisfying a predetermined condition according to the present invention were used for the carcass ply, it was confirmed that lightweight properties can be ensured by keeping the cord weight light without deteriorating the total tensile fracture strength of the cords or the durability of the bead portions. Further, the cords used in each Example were excellent in terms of productivity as well.
A truck radial tire having a size of 11R22.5 1.4PR was fabricated by applying steel cords to the carcass ply in accordance with conditions shown in the following table. Only one carcass ply was used. Four belt layers were arranged at angles of +50°, +20°, −20°, and −20° with respect to the tire circumferential direction in turn from inside in the tire radial direction. The carcass strength was represented by index numbers assuming that the carcass strength of a tire according to Comparative Example 2-1 is 100.
The amount of steel cords contained in the carcass ply per unit area was evaluated for tires in respective Examples, Comparative Examples, and Reference Examples and represented by index numbers assuming that the amount of steel cords according to Comparative Example 2-1 is 100. The smaller the numerical value, the lighter the weight and the better the result.
A 100,000-km drum running test was conducted at a speed of 60 km/h, an internal pressure of 8 kgf/cm2, and a load of JIS 100% for tires in respective Examples, Comparative Examples, and Reference Examples, and 10 carcass cords were extracted from each tire after running. The fracture strength was measured for these 10 carcass cords using an Instron tensile tester, the average of measured fracture strengths was divided by the average of fracture strengths similarly obtained for 10 cords extracted from identical portions in new tires in respective Examples and Comparative Examples, and the quotient was defined in terms of percentage as a cord strength holding rate (%). The closer to 100 the numerical value, the higher the holding rate and the better the result.
A 100,000-km drum running test was conducted at a speed of 60 km/h, an internal pressure of 8 kgf/cm2, and a load of JIS 100% for tires in respective Examples, Comparative Examples, and Reference Examples, and one carcass cord was extracted from the carcass of each tire after running. The maximum value d (μm) of the depth of wear of the sheath filaments 12 of the carcass cord due to contact between the bead cores and the steel cords in folding portions of the bead cores was measured (see
A drum running test for running over 20,000 km or until failure occurs was conducted at a speed of 60 km/h, an internal pressure of 1 kgf/cm2, and a load of JIS 40% for tires in respective Examples, Comparative Examples, and Reference Examples, and 10 carcass cords were extracted from the carcass of each tire after running. The number of broken filaments among these 10 carcass cords was counted and divided by the total number of filaments corresponding to the 10 cords, and the quotient was defined in terms of percentage as a filament breakage rate (%). The smaller the numerical value, the smaller the amount of filament breakage and the better the result.
Aside-cut test in which the sidewall portion collides against a squared timber modeled on a curbstone from the shoulder portion of the tire was conducted at a speed of 60 km/h, an internal pressure of 8 kgf/cm2, and a load of JIS 100% for tires in respective Examples, Comparative Examples, and Reference Examples, and 10 carcass cords were extracted from the carcass of each tire after running. The number of broken filaments among these 10 carcass cords was counted and divided by the total number of filaments corresponding to the 10 cords, and the quotient was defined in terms of percentage as a filament breakage rate (%). The smaller the numerical value, the smaller the amount of filament breakage and the better the result.
Breakage properties in the manufacturing process were evaluated for steel filaments used in respective Examples, Comparative Examples, and Reference Examples. The result was represented by {circle around (◯)} for a level at which continuous production is possible with no breakage, ◯ for a level at which continuous production is possible, and X for a level at which continuous production is impossible due to much breakage.
These results are shown together in the following table.
As shown in the above-described table, in a tire to be tested according to each Example which uses a steel cord formed by twisting steel filaments, having a wire diameter of 0.15 to 0.20 mm and a tensile strength of 3140 to 3630 MPa (N/mm2), with the carcass ply in the 3+9 structure, it was confirmed that good results were obtained for all evaluation items.
As shown in the above-described table, in a tire to be tested according to each Example which uses a steel cord formed by twisting steel filaments, having a wire diameter of 0.15 to 0.20 mm and a tensile strength of 3140 to 3630 MPa (N/mm2), with the carcass ply in the 3+8 structure, it was confirmed that good results were obtained for all evaluation items.
1, bead portion; 2, sidewall portion; 3, tread portion; 4, bead core; 5, carcass; 6, belt layer; 11, core filament; 12, sheath filament; 13, wrap filament; 21, core; 22, sheath.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-152009 | Jul 2013 | JP | national |
| 2013-152010 | Jul 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/069295 | 7/22/2014 | WO | 00 |
