JP7648872B2 - Pneumatic radial tires - Google Patents

Description

The present invention relates to a pneumatic radial tire having a steel cord belt layer and a steel cord circumferential reinforcing layer in the tread portion, and more specifically, to a pneumatic radial tire that makes it possible to suppress the occurrence of failures caused by the belt layer and the circumferential reinforcing layer and improve durability.

In recent years, heavy-duty pneumatic radial tires used on trucks, buses, etc. have been converted from dual wheels to single wheels in order to reduce weight and conserve resources. Tires with dual wheels converted to single wheels tend to have a smaller aspect ratio because the tread width is wider without changing the outer diameter. Also, the filling air pressure is set higher so that the capacity of dual wheels can be achieved with a single wheel.

Heavy-duty pneumatic radial tires with wide tread widths and low aspect ratios have a problem in that the load on the belt layer in the tread portion increases significantly during driving, reducing the durability of the belt layer. A conventional measure to reinforce the belt layer of such pneumatic radial tires has been to place a circumferential reinforcing layer containing steel cords oriented in the tire circumferential direction on the outer periphery of the belt layer to increase the circumferential rigidity of the tread portion (see, for example, Patent Documents 1 and 2).

However, because steel cords have a high elastic modulus and are weak against compression, if the steel cords are arranged in the circumferential direction of the tire, as in the circumferential reinforcing layer described above, they are prone to fatigue fracture due to repeated deformation during tire rolling. In particular, the steel cords arranged at the edge portions of the circumferential reinforcing layer suffer fatigue fracture, which is a factor in reducing the durability of the tire.

In addition, if the circumferential reinforcing layer does not tighten sufficiently, it becomes difficult to prevent the belt layer from rising up due to centrifugal force during high-speed driving. This repeated rising of the belt layer reduces the durability of the tire.

Special Publication No. 2001-522748 Special Publication No. 2000-504655

The object of the present invention is to provide a pneumatic radial tire that can suppress the occurrence of failures caused by the belt layer and the circumferential reinforcing layer and improve durability.

In order to achieve the above object, the pneumatic radial tire of the present invention is a pneumatic radial tire having a plurality of belt layers including steel cords inclined with respect to the circumferential direction of the tire and at least one circumferential reinforcing layer including steel cords oriented in the circumferential direction of the tire, disposed on the outer peripheral side of the carcass layer in the tread portion,
a maximum tangent modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer in an initial state and a maximum tangent modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer in a state after being taken out of the tire satisfy the relationship : -2 GPa ≦ Em ₀ - _{Em 1} ≦ 14 GPa ;
a width Wo of the circumferential reinforcing layer and a maximum width Wm of the belt layer satisfy a relationship of 0.5≦Wo/Wm≦0.9,
The elastic modulus Ee of the steel cords arranged in the edge portions of the circumferential reinforcing layer at an elongation of 0.5% to 1.0% and the elastic modulus Ec of the steel cords arranged in the center portion of the circumferential reinforcing layer at an elongation of 0.5% to 1.0% satisfy the relationship 0.8≦Ee/Ec≦1.2.

In the present invention, in a pneumatic radial tire having a steel cord belt layer and a steel cord circumferential reinforcing layer in a tread portion, by defining as above the relationship between the maximum tangent elastic modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer in its initial state and the maximum tangent elastic modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer in its state after being removed from the tire, the relationship between the width Wo of the circumferential reinforcing layer and the maximum width Wm of the belt layer, and the relationship between the elastic modulus Ee of the steel cord arranged in the edge portions of the circumferential reinforcing layer at 0.5% to 1.0% elongation and the elastic modulus Ec of the steel cord arranged in the center portion of the circumferential reinforcing layer at 0.5% to 1.0% elongation, it is possible to suppress the occurrence of failures caused by the belt layer and the circumferential reinforcing layer and improve the durability of the pneumatic radial tire.

In the present invention, it is preferable that the rate of change Rc of the tread circumference at the tire equator and the rate of change Rs of the tread circumference at the shoulders, which change between an air pressure of 100 kPa and a specified air pressure corresponding to the maximum load capacity, satisfy the relationship 0.7≦Rc/Rs≦1.3. This makes it possible to suppress local deformation in the center and shoulders of the tread, and to suppress the occurrence of failures caused by the belt layer and the circumferential reinforcing layer.

The pneumatic radial tire of the present invention preferably has a maximum tire width of 350 mm or more when inflated to the specified air pressure corresponding to the maximum load capacity, and an aspect ratio of 60% or less. Remarkable effects can be obtained in such a wide-width, low-aspect-ratio pneumatic radial tire.

In the present invention, the state of the steel cord taken out of the tire means the state in which the steel cord is taken out of the tire and the rubber covering the steel cord is removed. On the other hand, the initial state of the steel cord means the state of the steel cord in the raw material stage, and it is possible to substantially reproduce the initial state of the steel cord by immersing the steel cord taken out of the tire in toluene for at least 60 minutes and dissolving all the rubber attached to the steel cord. The maximum tangent modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer in the initial state and the maximum tangent modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer in the state taken out of the tire are maximum tangent moduli (GPa) obtained from the tensile stress-elongation curve obtained by performing a tensile test on the steel cord in a predetermined state under the conditions of a test length of 250 mm and a tensile speed of 5 mm/min according to the provisions of JIS-G3510. The maximum tangent moduli Em ₀ and Em ₁ are the average values measured for each of the steel cords taken from the entire width direction of the circumferential reinforcing layer.

The elastic modulus Ee of the steel cord arranged in the edge portion of the circumferential reinforcing layer at 0.5% to 1.0% elongation and the elastic modulus Ec of the steel cord arranged in the center portion of the circumferential reinforcing layer at 0.5% to 1.0% elongation are elastic moduli calculated by the following formula (1) after a tensile test is performed on the steel cord removed from the tire under conditions of a test length of 250 mm and a tensile speed of 5 mm/min in accordance with the provisions of JIS-G3510.
E=0.2×(L _1.0 - L _0.5 )/S _SC ...(1)
Where E: Elastic modulus (GPa)
L _0.5 : Tension at 0.5% elongation (N)
_L1.0 : Tension at 1.0% elongation (N)
S _SC : Cross-sectional area of steel cord [sum of cross-sectional areas of wires] (mm ² )

The elastic modulus Ee of the steel cords arranged at the edge of the circumferential reinforcing layer is the average value of the elastic modulus measured for each of the steel cords included within an area from the edge end of the circumferential reinforcing layer to 5% of the width of the circumferential reinforcing layer. On the other hand, the elastic modulus Ec of the steel cords arranged at the center of the circumferential reinforcing layer is the average value of the elastic modulus measured for each of the steel cords included within an area of 5% of the width of the circumferential reinforcing layer centered on the tire equator.

1 is a meridian cross-sectional view showing a heavy-duty pneumatic radial tire according to an embodiment of the present invention. FIG. 2 is a development view showing a belt layer and a circumferential reinforcing layer of the pneumatic radial tire of FIG. 1 . 1 is a graph showing the relationship between tensile tension and elongation of a steel cord.

The configuration of the present invention will be described in detail below with reference to the attached drawings. Fig. 1 shows a heavy-duty pneumatic radial tire according to an embodiment of the present invention, and Fig. 2 shows the belt layer and circumferential reinforcing layer. This embodiment depicts a pneumatic radial tire with a maximum tire width of 350 mm or more when inflated to the specified air pressure corresponding to the maximum load capacity, and an aspect ratio of 60% or less.

As shown in FIG. 1, the pneumatic radial tire of this embodiment has a tread portion 1 that extends in the circumferential direction of the tire and forms an annular shape, a pair of sidewall portions 2, 2 arranged on both sides of the tread portion 1, and a pair of bead portions 3, 3 arranged on the radially inner side of the sidewall portions 2.

A carcass layer 4 is mounted between a pair of bead portions 3, 3. This carcass layer 4 includes multiple steel cords extending in the tire radial direction, and has a structure in which it is wound from the inside to the outside of the tire around a bead core 5 arranged in each bead portion 3. A bead filler 6 made of a rubber composition with a triangular cross section is arranged on the outer periphery of the bead core 5.

Four belt layers 7 are embedded on the outer diameter side of the carcass layer 4 in the tread portion 1. Each belt layer 7 includes a plurality of steel cords that are inclined with respect to the tire circumferential direction. These belt layers 7 include two central main belt layers 72, 73 in which the steel cords cross each other, and auxiliary belt layers 71, 74 arranged on the inner diameter side and outer diameter side of these main belt layers 72, 73. More specifically, the auxiliary belt layer 71 and the main belt layer 72 are inclined to one side (L-pasted) with respect to the tire circumferential direction, and the main belt layer 73 and the auxiliary belt layer 74 are inclined to the other side (R-pasted) with respect to the tire circumferential direction. The inclination angle of the steel cords constituting the main belt layers 72, 73 with respect to the tire circumferential direction is set, for example, in the range of 15° to 35°, and the inclination angle of the steel cords constituting the auxiliary belt layers 71, 74 with respect to the tire circumferential direction is set, for example, in the range of 15° to 75°.

Between the main belt layers 72 and 73, at least one circumferential reinforcing layer 8 containing steel cords oriented in the tire circumferential direction is disposed. This circumferential reinforcing layer 8 has a jointless structure in which a strip material made of at least one steel cord aligned and rubber-coated is continuously wound at an angle of 5° or less, but essentially 0°, to the tire circumferential direction. The circumferential reinforcing layer 8 may be disposed between the auxiliary belt layer 71 and the main belt layer 72, between the main belt layer 73 and the auxiliary belt layer 74, or may be disposed on the inner side of the auxiliary belt layer 71 or on the outer side of the auxiliary belt layer 74.

In addition, the tread portion 1 has multiple circumferential main grooves 11 that extend in the circumferential direction of the tire, and these circumferential grooves 11 define multiple rows of land portions 12.

In the above pneumatic radial tire, the maximum tangent modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer 8 in its initial state and the maximum tangent modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer 8 in its state after being removed from the tire satisfy the relationship -5 GPa≦Em ₀ -Em ₁ < 15 GPa.

Fig. 3 is a graph showing the relationship between the tensile tension and the elongation of the steel cord. In Fig. 3, the curve _C0 shows the relationship between the tensile tension and the elongation of the steel cord constituting the circumferential reinforcing layer 8 in the initial state, and the curve _C1 shows the relationship between the tensile tension and the elongation of the steel cord constituting the circumferential reinforcing layer 8 in the state taken out of the tire. In the example of Fig. 3, the maximum tangent modulus _Em1 of the steel cord constituting the circumferential reinforcing layer 8 in the state taken out of the tire is smaller than the maximum tangent modulus _Em0 of the steel cord constituting the circumferential reinforcing layer 8 in the initial state due to the penetration of rubber into the steel cord. The steel cord constituting the circumferential reinforcing layer 8 has a cord structure in which the maximum tangent modulus _Em0 in the initial state and the maximum tangent modulus _Em1 in the state taken out of the tire satisfy the above relationship.

The twisted structure of the steel cords used in the circumferential reinforcing layer 8 is not particularly limited, but it is preferable to adopt an N x M structure in which N (N = 3 to 5) strands made of M (M = 5 to 10) wires are twisted together. An example of such an N x M structure is a 3 x 7 structure. The diameter of each wire should be in the range of 0.20 mm to 0.40 mm.

In this way, the maximum tangent elastic modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer 8 in its initial state and the maximum tangent elastic modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer 8 in its state after being removed from the tire satisfy a predetermined relationship, so that in a wide-width and low-aspect ratio pneumatic radial tire, the occurrence of failures caused by the belt layer 7 and the circumferential reinforcing layer 8 can be effectively suppressed and the durability can be improved.

Here, if the value of Em ₀ -Em ₁ is greater than 15 GPa, the reinforcing effect on the belt layer 7 is reduced, making it difficult to suppress the phenomenon in which the edge portion of the belt layer 7 rises due to centrifugal force during high-speed running. Conversely, if the value of Em ₀ -Em ₁ is less than -5 GPa, buckling in the compressive direction is likely to occur in the steel cords located at the edge portion of the circumferential reinforcing layer 8 due to in-plane bending of the belt layer 7 that occurs during cornering. In particular, it is desirable that the maximum tangent elastic modulus Em ₀ is in the range of 30 GPa to 40 GPa, and the maximum tangent elastic modulus Em ₁ is in the range of 20 GPa to 30 GPa.

In addition, in the above pneumatic radial tire, as shown in FIG. 2, the width Wo of the circumferential reinforcing layer 8 and the maximum width Wm of the belt layer 7 satisfy the relationship 0.5≦Wo/Wm≦0.9. As a result, in a wide-width, low-aspect ratio pneumatic radial tire, the occurrence of failures caused by the belt layer 7 and the circumferential reinforcing layer 8 can be effectively suppressed, and the durability can be improved.

Here, if the value of Wo/Wm is less than 0.5, the reinforcing effect on the belt layer 7 is reduced, making it difficult to suppress the phenomenon in which the edge of the belt layer 7 rises due to centrifugal force during high-speed driving. Conversely, if the value of Wo/Wm is greater than 0.9, the in-plane bending of the belt layer 7 that occurs during cornering makes it easier for the steel cord located at the edge of the circumferential reinforcing layer 8 to buckle in the compression direction.

Furthermore, the elastic modulus Ee of the steel cords arranged at the edge portions of the circumferential reinforcing layer 8 at 0.5% to 1.0% elongation and the elastic modulus Ec of the steel cords arranged at the center portion of the circumferential reinforcing layer 8 at 0.5% to 1.0% elongation satisfy the relationship 0.8≦Ee/Ec≦1.2. As a result, in a wide-width, low-aspect ratio pneumatic radial tire, the occurrence of failures caused by the belt layer 7 and the circumferential reinforcing layer 8 can be effectively suppressed, and the durability can be improved.

Here, if the value of Ee/Ec is less than 0.8, the reinforcing effect on the belt layer 7 is reduced, making it difficult to suppress the phenomenon in which the edge of the belt layer 7 rises due to centrifugal force during high-speed driving. Conversely, if the value of Ee/Ec is greater than 1.2, the in-plane bending of the belt layer 7 that occurs during cornering makes it easier for the steel cords located at the edge of the circumferential reinforcing layer 8 to buckle in the compression direction. In particular, it is desirable for the elastic modulus Ee to be in the range of 40 GPa to 65 GPa, and for the elastic modulus Ec to be in the range of 40 GPa to 65 GPa.

In the above pneumatic radial tire, the rate of change Rc of the circumferential length of the tread portion 1 at the tire equator, which changes between an inflated state of 100 kPa air pressure and an inflated state of the specified air pressure corresponding to the maximum load capacity, and the rate of change Rs of the circumferential length at the shoulder portion should satisfy the relationship 0.7≦Rc/Rs≦1.3. This makes it possible to suppress local deformation in the center and shoulder portions of the tread portion 1 and suppress the occurrence of failures caused by the belt layer 7 and the circumferential reinforcing layer 8.

Here, if the value of Rc/Rs is smaller than 0.7, the deformation of the shoulder portion becomes large, and peeling of the edge portion of the belt layer 7 and fatigue fracture of the steel cord at the edge portion of the circumferential reinforcing layer 8 become likely to occur. Conversely, if the value of Rc/Rs is larger than 1.3, the deformation of the center portion becomes large, and peeling between the belt layer 7 and the circumferential reinforcing layer 8 becomes likely to occur.

In a pneumatic radial tire for heavy loads, the outer shape of the tread portion when inflated with an air pressure of 100 kPa corresponds to the outer shape of the tread molding surface of the molding die. Therefore, when the ratio Rc/Rs of the rate of change Rc of the outer circumference of the tread portion at the tire equator to the rate of change Rs of the outer circumference at the shoulder portions is in the above range, it means that the outer shape of the tread portion when inflated with the maximum specified air pressure allowed for the tire changes at a substantially uniform rate over the entire tread width from the tire equator to both shoulder portions relative to the outer shape of the tread molding surface of the molding die. This contributes to improving durability, especially in wide-width, low-aspect ratio pneumatic radial tires.

The tire equator where the rate of change in outer circumference Rc is measured is the center position of the tread portion divided into two equal parts in the tire width direction. The shoulder portion where the rate of change in outer circumference Rs is measured is a position 35% of the total tire width away from the tire equator. However, if circumferential main grooves 11 are present on the tire equator and shoulder portion, the position of the tire equator is the position on the land portion 12 closest to the circumferential main groove 11, and the position of the shoulder portion is the position on the land portion 12 closest to the circumferential main groove 11 and shifted outward in the tire width direction from the circumferential main groove 11.

The peripheral change rates Rc and Rs are calculated by the following equations (2) and (3).
Rc=(Cc ₂ -Cc ₁ )/Cc ₁ ...(2)
Rs=(Cs ₂ -Cs ₁ )/Cs ₁ ...(3)
Where, _Cc1 : Circumference at the tire equator when inflated with an air pressure of 100 kPa _{, Cc2} : Circumference at the tire equator when inflated with the specified air pressure corresponding to the maximum load capacity, _Cs1 : Circumference at the shoulder portion when inflated with an air pressure of 100 kPa, _Cs2 : Circumference at the shoulder portion when inflated with the specified air pressure corresponding to the maximum load capacity. Note that the circumferences at the shoulder portion _Cs1 and _Cs2 are the average values measured at the left and right shoulder portions, respectively.

In the above embodiment, a pneumatic radial tire is described in which the maximum tire width when inflated to the specified air pressure corresponding to the maximum load capacity is 350 mm or more and the aspect ratio is 60% or less, but the present invention is also applicable to pneumatic radial tires that do not satisfy the above requirements. However, the present invention is particularly effective in pneumatic radial tires with wide widths and low aspect ratios as described above.

In a pneumatic radial tire having a tire size of 445/50R22.5, four belt layers including steel cords inclined with respect to the tire circumferential direction and one circumferential reinforcing layer including steel cords oriented in the tire circumferential direction are disposed on the outer peripheral side of the carcass layer in the tread portion. The maximum tangent elastic modulus Em ₀ of the steel cords constituting the circumferential reinforcing layer in the initial state, the maximum tangent elastic modulus Em ₁ of the steel cords constituting the circumferential reinforcing layer in the state after being removed from the tire, and the difference Em ₀ -Em ₁ Tires of the conventional example, comparative examples 1 to 4, and examples 1 to 9 were manufactured with the following settings as shown in Table 1: modulus of elasticity Ec of the steel cord arranged in the center portion of the circumferential reinforcing layer when elongated by 0.5% to 1.0%, modulus of elasticity Ee of the steel cord arranged in the edge portion of the circumferential reinforcing layer when elongated by 0.5% to 1.0%, ratio Ee/Ec, ratio Wo/Wm of width Wo of the circumferential reinforcing layer to maximum width Wm of the belt layer, and ratio Rc/Rs of rate of change Rc of the outer circumferential length of the tread portion at the tire equator and rate of change Rs of the outer circumferential length of the tread portion at the shoulder portion, which changes between a state inflated with an air pressure of 100 kPa and a state inflated with a specified air pressure corresponding to the maximum load capacity.

The durability of these test tires was evaluated using the following test method, and the results are shown in Table 1.

Durability:
Each test tire was mounted on a wheel with a rim size of 22.5 x 17.00 and inflated to 900 kPa. A drum tester equipped with a drum made of steel with a smooth drum surface and a diameter of 1707 mm was used to perform a preliminary run for 60 minutes under the conditions of controlling the ambient temperature at 38±3°C, running speed of 60 km/h, and load of 88% of the JATMA maximum load capacity. After completing the preliminary run, the main run was started at the same speed, with the load set to 120% of the JATMA maximum load capacity, and a slip angle of ±4° was applied with a sine wave of 0.05 Hz, and the running distance until the tire was destroyed was measured. The evaluation results were shown as an index with the conventional example being 100. The higher the index value, the better the durability.

As can be seen from Table 1, the tires of Examples 1 to 9 had improved durability compared to the conventional tire. On the other hand, the tires of Comparative Examples 1, 3, and 4 had too small a value for the ratio Ee/Ec, so the rising of the belt layer could not be sufficiently suppressed, and the durability was worse than that of the conventional tire. The tire of Comparative Example 2 had too large a value for the ratio Ee/Ec, so the steel cords of the circumferential reinforcing layer buckled in the compression direction, and the durability was worse than that of the conventional tire. In addition, the tire of Comparative Example 3 had too small a value for the ratio Ee/Ec, so the rising of the belt layer could not be sufficiently suppressed, and the durability was worse than that of the conventional tire.

In addition, in comparing Examples 1 to 4, it was found that when the value of the ratio Rc/Rs is too large, the deformation of the center portion becomes large, making it easier for delamination to occur between the belt layer and the circumferential reinforcing layer, and when the value of the ratio Rc/Rs is too small, the deformation of the shoulder portion becomes large, making it easier for delamination to occur at the edge of the belt layer and fatigue breakage of the steel cords of the circumferential reinforcing layer.

REFERENCE SIGNS LIST 1 tread portion 2 sidewall portion 3 bead portion 4 carcass layer 5 bead core 6 bead filler 7 belt layer 8 circumferential reinforcing layer 11 circumferential main groove 12 land portion

Claims (3)

A pneumatic radial tire having a plurality of belt layers including steel cords inclined with respect to the tire circumferential direction and at least one circumferential reinforcing layer including steel cords oriented in the tire circumferential direction disposed on an outer peripheral side of a carcass layer in a tread portion,
a maximum tangent modulus Em ₀ of the steel cord constituting the circumferential reinforcing layer in an initial state and a maximum tangent modulus Em ₁ of the steel cord constituting the circumferential reinforcing layer in a state after being taken out of the tire satisfy the relationship : -2 GPa ≦ Em ₀ - _{Em 1} ≦ 14 GPa ;
a width Wo of the circumferential reinforcing layer and a maximum width Wm of the belt layer satisfy a relationship of 0.5≦Wo/Wm≦0.9,
A pneumatic radial tire characterized in that an elastic modulus Ee of a steel cord arranged in an edge portion of the circumferential reinforcing layer at an elongation of 0.5% to 1.0% and an elastic modulus Ec of a steel cord arranged in a center portion of the circumferential reinforcing layer at an elongation of 0.5% to 1.0% satisfy a relationship of 0.8≦Ee/Ec≦1.2. The pneumatic radial tire according to claim 1, characterized in that the rate of change Rc of the tread portion's outer circumference at the tire equator and the rate of change Rs of the tread portion's outer circumference at the shoulder portion, which change between a state inflated with an air pressure of 100 kPa and a state inflated with a specified air pressure corresponding to the maximum load capacity, satisfy the relationship 0.7≦Rc/Rs≦1.3. The pneumatic radial tire according to claim 1 or 2, characterized in that the maximum tire width when inflated to the specified air pressure corresponding to the maximum load capacity is 350 mm or more and the aspect ratio is 60% or less.

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