Pneumatic radial tires

Abstract: A pneumatic radial tire comprises a tread portion (3), a pair of sidewall portions (2), a pair of bead portions (1), a carcass (5) of at least one rubberized ply containing cords arranged in a radial direction and consisting of a main carcass body (5a) and a turnup portion (5b), a belt (6) superimposed about a crown portion of the carcass (5), and a rubber filler (7) arranged between the main carcass body (5a) and the turnup portion (5b) above the bead core (4), wherein the rubber filler (7) is composed of three different rubber stocks (7-1,7-2,7-3), and a hardest rubber stock (7-1) among these rubber stocks is arranged in at least an outer zone in the radial direction of the tire sandwiching a normal line (Lv) drawn from the turnup end (5bE) to the outer surface of the main carcass body (5a) at section of the tire and side the main carcass body (5a), and a rubber stock (7-2) having a middle hardness is arranged between the outer surface of the main carcass body (5a) and the hardest rubber stock (7-1) as a stress-mitigating rubber layer, and a softest rubber stock (7-3) is arranged along an inner surface of the turnup portion (5b).

Description: This invention relates to a pneumatic radial tire, and more particularly to a heavy duty pneumatic radial tire for use on so-called heavy vehicles such as middle-size or larger trucks and buses, industrial vehicles, construction vehicles and the like. Particularly, the invention relates to a pneumatic radial tire having a long service life and excellent bead portion durability without arranging an extra reinforcing member in a bead portion or a region from a bead portion to a sidewall portion and increasing the volume of so-called rubber filler in the bead portion. In the radial tire running under a heavy load, the sidewall portion is largely bent, but also the bead portion existing outward from a flange of a rim in a radial direction of the tire is fallen down toward the outside of the tire (hereinafter referred to simply as tire outside ) or causes so-called bead portion fall-down phenomenon. By such a phenomenon is applied a large compression strain·stress to a turnup end portion of a carcass existing in the bead portion or in the sidewall portion near to the bead portion. Further, a large movement of members is produced in a region ranging from the bead portion to the sidewall portion located in correspondence to a stepping-in zone and a kicking-out zone in a ground contact area of a tread portion of the radial tire along peripheries of these portions, and also a large torsional deformation is created in these portions when a certain slip angle is applied to the tire during the cornering of the vehicle and hence a large shearing strain stress based on such member movement and torsional deformation is applied to the turnup end portion. Since the above large compression strain stress and shearing strain stress are repeatedly input to the turnup end portion during the running of the tire, fatigue of rubber proceeds and finally rubber cracking is created along the turnup end portion, which is liable to result in a separation failure of the turnup end portion. Such a failure is a new problem with respect to the pneumatic radial tire running under a heavy load, and is brought about by increasing requirements of weight reduction and cost reduction and a tendency to adopt a low section profile having, for example, an aspect ratio of not more than 70%. In order to solve the above separation failure, various countermeasures have been proposed. Among them, a typical embodiment of a conventional countermeasure is shown in Fig. 7 of the accompanying drawings, in which a rubber filler 7 taperingly extending from a bead core 4 toward an end of a tread portion (not shown) is arranged between a main body 5a of a carcass and a turnup portion 5b thereof along an outer surface of the main body and divided into a hard rubber stock 7-1 located adjacent the main body 5a and a soft rubber stock 7-3 located adjacent the turnup portion 5b. It has been further attempted to take various countermeasures for the distribution of the hard rubber stock 7-1 and soft rubber stock 7-3. As shown in Fig. 7, for example, the section shape of the hard rubber stock 7-1 is thickened in the vicinity of a position opposite to an end of the turnup portion 5b, or the hard rubber stock 7-1 is selectively arranged in the vicinity of this position. Although it is expected to considerably improve the separation failure of the end portion of the turnup portion 5b to thereby attain improvement of bead portion durability, it has actually been confirmed that satisfactory bead portion durability can not be obtained by these countermeasures. This is because a peeling failure between cord and coating rubber in the main body 5a of the carcass is caused as a negative compensation for the improvement of separation resistance in the end portion of the turnup portion 5b and such a peeling failure concentrates in a region located somewhat outward from a position opposite to the end of the turnup portion 5b in the radial direction of the tire. A tire according to the preamble of claim 1 is known for example from figure 9 of EP-0698513 A. It is, therefore, an object of the present invention to provide a pneumatic radial tire having a considerably improved durability as a whole of a bead portion by effectively preventing the occurrence of peeling failure between cord and coating rubber in the main body of a carcass while maintaining a separation resistance of the turnup portion, even in the case of tires having a low aspect ratio on the assumption that the weight and cost of the tire are reduced. According to the invention, there is provided a pneumatic radial tire comprising a tread portion, a pair of sidewall portions extending from both ends of the tread portion, a pair of bead portions extending from the sidewall portions, a carcass extending between a pair of bead cores embedded in the bead portions to reinforce these portions and comprised of at least one rubberized ply containing cords arranged in a radial direction, in which the carcass consists of a main carcass body and a turnup portion wound around the bead core from the inside toward the outside, a belt superimposed about a crown portion of the carcass to reinforce the tread portion, and a rubber filler taperingly extending from the bead core between the main carcass body and the turnup portion along the outer surface of the main carcass body toward the end of the tread portion, wherein the rubber filler is composed of three rubber stocks having different JIS hardnesses, with a hardest first rubber stock having a maximum hardness among these rubber stocks being arranged in at least an outer zone among both zones in the radial direction of the tire sandwiching a normal line drawn from the turnup end to the outer surface of the main carcass body in section of the tire and adjacent the main carcass body, a second rubber stock having a hardness between maximum hardness and minimum hardness being arranged between the outer surface of the main carcass body and the hardest rubber stock as a stress-mitigating rubber layer, and a softest third rubber stock having a minimum hardness being arranged along the inner surface of the turnup portion, and the hardness of the stress-mitigating second rubber stock being equal to or lower than that of the coating rubber for the cord of the carcass. Since it is assumed that the pneumatic radial tire is used under a heavy load, steel cords or aromatic polyamide fiber cords (Kevlar cords) having a strength close thereto are suitable as the cord used in the carcass. In the case of steel cord, it is sufficient to use one carcass ply, while in the case of aromatic polyamide fiber cord, one or more carcass plies are used. Considering the establishment of weight reduction and cost reduction, the use of steel cord widely used as a cord material in tires is advantageous. In the case of steel cord, it is particularly necessary that the coating rubber has a modulus of elasticity as close to the high modulus of the steel cord as far as possible. That is, it is required that the difference of rigidity between the steel cord and the coating rubber is reduced as far as possible, whereby the low carcass rigidity inherent to the radial structure particularly developed in the sidewall portion is enhanced as far as possible and also the resistance to fatigue fracture in the coating rubber is improved. For this purpose, according to the invention, the hardest rubber stock having the maximum hardness among the three rubber stocks constituting the rubber filler is arranged in at least an outer zone among both outer and inner zones sandwiching the normal line in the radial direction of the tire and adjacent the main carcass body. Of course, the hardest rubber stock may be arranged over a region ranging from the inner zone to the outer zone according to the usual manner. Further, the stress-mitigating rubber layer is arranged between the outer surface of the main carcass body and the hardest rubber stock. In this way, a greater part of a large stress applied by the hardest rubber stock to the coating rubber in the main carcass body under loading is absorbed by the stress-mitigating rubber layer having a hardness equal to or lower than that of the coating rubber to largely decrease shearing stress acting between the coating rubber and the cord, whereby the occurrence of peeling failure therebetween can be prevented. The reason why the hardness of the stress-mitigating rubber layer is higher than that of the softest rubber stock is due to the fact that if the hardness of the stress-mitigating rubber layer is low, deformation excessively concentrates in the stress-mitigating rubber layer under loading and rubber fatigue rapidly proceeds with the advance of the running to finally cause fatigue fracture of the stress-mitigating rubber layer itself. Further, if the hardness of the stress-mitigating rubber layer is higher than that of the coating rubber for the carcass cord, stress of the coating rubber is increased without stress mitigation. Moreover, the softest rubber stock is arranged along the inner surface of the turnup portion, which is effective to attain stress mitigation at the turnup end and is required to maintain separation resistance at this turnup end. In order to sufficiently provide the stress-mitigating action as mentioned above, it is preferable that the JIS hardness of the stress-mitigating rubber layer is within a range of 55-75°, more preferably 62-72°, and that the thickness of the stress-mitigating rubber layer existing between two normal lines drawn from the turnup end of the carcass and the outer end of the hardest rubber stock in the radial direction to the outer surface of the main carcass body is within a range of 0.7-4.5 mm. In order to adapt the function and effect of the stress-mitigating rubber layer to various tires and various use conditions, it is preferable that one outer end of the stress-mitigating rubber layer in the radial direction is located outside the outer end of the hardest rubber stock in the radial direction, and that the inner end of the stress-mitigating rubber layer in the radial direction is located at a position extending inward from the normal line passing through the turnup end of the carcass in the radial direction. In order to sufficiently ensure separation resistance at the turnup end, it is preferable that the ratio of thickness of the hardest rubber stock to total thickness of the rubber filler as measured at the normal line passing through the turnup end is within a range of 0.20-0.55. Further, it is preferable that the thickness of the hardest rubber stock on the normal line passing through the turnup end is thinner than the thickness of the hardest rubber stock on a straight line drawn in parallel to the normal line outward therefrom in the radial direction. The invention will be further described with reference to the accompanying drawings, in which : Fig. 1 is a diagrammatic section view of a first embodiment of a pneumatic radial tire according to the invention; Fig. 2 is a diagrammatic section view of a main part of a second embodiment of a pneumatic radial tire according to the invention; Fig. 3 is a diagrammatic section view of a main part of a third embodiment of a pneumatic radial tire according to the invention; Fig. 4 is a diagrammatic section view of a main part of a fourth embodiment of a pneumatic radial tire according to the invention; Fig. 5 is a diagrammatic section view of a main part of a fifth embodiment of a pneumatic radial tire according to the invention; Fig. 6 is a diagrammatic section view of a main part of a sixth embodiment of a pneumatic radial tire according to the invention; and Fig. 7 is a diagrammatic section view of a main part of a conventional pneumatic radial tire. Various embodiments of pneumatic radial tire according to the invention are shown in Figs. 1-6, in which Fig. 1 is a radial section view in a plane including a rotating axis of the radial tire and Figs. 2-6 are enlarged section views of main parts of the other embodiments of radial tires having various rubber fillers, respectively. The radial tire shown in Fig. 1 (tubeless tire in this embodiment) comprises a pair of bead portions 1, a pair of sidewall portions 2 and a tread portion 3, in which a carcass 5 of one or more plies (one ply in the illustrated embodiment) extends between a pair of bead cores 4 embedded in the bead portions 1 to reinforce the portions 1-3. The carcass 5 is comprised of a rubberized steel cord ply of a radial arrangement (steel cords are arranged at a cord angle of 80-90° with respect to an equatorial plane E). A belt 6 of three or more steel cord layers (four belt layers in the illustrated embodiment) is superimposed about a crown portion of the carcass 5 to reinforce the tread portion 3. Further, the carcass 5 consists of a main carcass body 5a reinforcing the portions 1-3 inside the tire and a turnup portion 5b reinforcing the bead portion 1 by winding about the bead core 4 from the inside toward the outside. Further, the tire is provided with a rubber filler 7 taperingly extending from the bead core 4 between the main carcass body 5a and the turnup portion 5b along the outer surface of the main carcass body 5a toward an end of the tread portion 3. The rubber filler 7 is comprised of three rubber stocks 7-1, 7-2, 7-3 having different JIS hardnesses (hereinafter referred to simply as hardness). Among these rubber stocks, a hardest rubber stock 7-1 having a maximum hardness is arranged in at least an outer zone among both inner and outer zones in the radial direction of the tire sandwiching a normal line Lv drawn from an end 5bE of the turnup portion 5b to the outer surface of the main carcass body 5a (in the illustrated embodiment, a small amount of the stock 7-1 is also arranged in the inner zone). A rubber stock 7-2 has a hardness between maximum hardness and minimum hardness and is layer-like and is arranged between the outer surface of the main carcass body 5a and the hardest rubber stock 7-1 as a stress-mitigating rubber layer. In this case, it is important that the rubber hardness of the stress-mitigating rubber layer 7-2 is lower than that of the coating rubber for the cord (steel cord) of the carcass 5 and is actually within a range of 55-75°, preferably 62-72°. Furthermore, a softest rubber stock 7-3 having a minimum hardness is arranged along an inner surface of the turnup portion 5b. In the embodiment of Fig. 1, the stress-mitigating rubber layer 7-2 is selectively arranged outward from the normal line Lv in the radial direction and an outer end of the layer 7-2 in the radial direction is terminated inside an outer end of the hardest rubber stock 7-1 in the radial direction. Of course, the stress-mitigating rubber layer 7-2 is arranged in a region covering a zone of generating a maximum stress of the coating rubber in the tire of the illustrated embodiment. The embodiments shown in Figs. 2-6 are modification embodiments of three rubber stocks 7-1, 7-2, 7-3 in the rubber filler 7 shown in Fig. 1, so that the following will be described with respect to only portions different from the case of Fig. 1. In the rubber filler 7 shown in Figs. 2 and 3, the outer end of the stress-mitigating rubber layer 7-2 in the radial direction extends beyond the outer end of the hardest rubber stock 7-1 in the radial direction and further the inner end of the stress-mitigating rubber layer 7-2 in the radial direction is located inward from the normal line Lv in the radial direction. Particularly, the length of the inner end of the stress-mitigating rubber layer 7-2 in Fig. 3 beyond the normal line Lv is longer than the length of the case shown in Fig. 2. The arrangement of the stress-mitigating rubber layer 7-2 shown in Fig. 4 is substantially the same as in Fig. 3, but the sectional shape of the layer 7-2 is made thicker and then taperingly extended over the normal line Lv inward in the radial direction, which is different from the case of Fig. 3. In the embodiment of Fig. 4, the thickness of the hardest rubber stock 7-1 located outward from the normal line Lv in the radial direction is gradually increased to effectively improve the separation resistance at the end part of the turnup portion 5b, while the thickness of a portion of the layer 7-2 that is located near to an outer position from the normal line Lv in the radial direction causing a large stress in the coating rubber of the carcass cord is increased to enhance the stress-mitigating effect. In the embodiment of Fig. 5, the thickness of the hardest rubber stock 7-1 located outward from the normal line Lv, specifically the thickness of the hardest rubber stock 7-1 on a straight line (not shown) drawn in parallel to the normal line Lv outward therefrom in the radial direction is thicker than the thickness of the rubber stock 7-1 on the normal line Lv to considerably expand the portion of the rubber stock 7-1 existing outward from the normal line Lv in the radial direction, whereby the prevention of separation at the end part of the turnup portion 5b, which should be importantly considered in accordance with the kind of the tire and the use conditions, can simultaneously be established with the prevention of peeling of the coating rubber from the cords in the main carcass body 5a. In the embodiment of Fig. 6, the sectional shape and arrangement of the hardest rubber stock 7-1 considerably differs from those of the aforementioned embodiments. That is, the hardest rubber stock 7-1 is arranged in the rubber filler 7 as a so-called floating island structure and has a maximum gauge at a position existing on the normal line Lv. This structure attaches importance to the improvement of the separation resistance at the turnup portion 5b, whereby peeling failure of the coating rubber is avoided. According to the invention, the thickness of the stress-mitigating rubber layer 7-2 is desirably within a range of 0.7-4.5 mm in a portion of the layer 7-2 existing between a normal line (not shown) drawn from the outer end of the hardest rubber stock 7-1 in the radial direction to the outer surface of the main carcass body 5a and the above normal line Lv with reference to Figs. 1-6. When the thickness of the layer 7-2 is less than 0.7 mm, the effect of mitigating stress by the coating rubber is small and peeling of the coating rubber is prematurely caused as compared with the end part of the turnup portion 5b, while when it exceeds 4.5 mm, the thickness of the hardest rubber stock 7-1 required for controlling the torsional deformation of the end part of the turnup portion 5b can not be ensured. If the thickness of the stock 7-1 is sufficiently ensured, the thickness of the softest rubber stock 7-3 can not be ensured. In any case, the separation resistance at the end part of the turnup portion 5b is degraded. When a ratio of thickness of the hardest rubber stock 7-1 to total thickness of the rubber filler 7 as measured on the normal line Lv is within a range of 0.20-0.55, it is possible to simultaneously ensure the separation resistance at the end part of the turnup portion 5b and the peeling resistance of the coating rubber to the cords in the main carcass body 5a at higher levels. When the thickness ratio is less than 0.20, the arrangement of the hardest rubber stock 7-1 becomes meaningless, and if it is intended to ensure such an arrangement at this ratio, the total thickness of the rubber filler should be made largely thick, which undesirably increases the weight but also largely increases heat generation of the bead portion 1 to considerably degrade the durability of the bead portion 1. On the other hand, when said ratio exceeds 0.55, the thickness of the softest rubber stock 7-3 is insufficient at the end part of the turnup portion 5b and compression stress created in the end part is not sufficiently mitigated and hence the separation resistance is degraded. The following examples are given in illustration of the invention and are not intended as limitations thereof. There are provided 18 tires of Examples 1-15, Conventional Example and Comparative Examples 1-2 for trucks and buses having a tire size of 11/70R22.5 and a basic structure shown in Fig. 1, in which the carcass 5 is a single rubberized carcass ply of steel cords arranged in the radial direction and the belt 6 is comprised of four belt cross layers each containing steel cords therein. Then, a durability test of the bead portion 1 is carried out with respect to these tires. As regards these tires, the hardness of the coating rubber in the carcass 5, hardnesses of hardest rubber stock 7-1, stress-mitigating rubber layer 7-2 and softest rubber stock 7-3 constituting the rubber filler 7, average thickness of the stress-mitigating rubber layer (i.e. average thickness of stress-mitigating rubber layer existing between normal line Lv and a straight line drawn in parallel to the normal line Lv passing through an outer end of the hardest rubber stock 7-1 in the radial direction), and ratio of thickness of the hardest rubber stock 7-1 to total thickness of the rubber filler 7 as measured on the normal line Lv are shown in Tables 1 and 2. The test for the durability of the bead portion is carried out according to the following method. Each of the above tires subjected to an internal pressure of 8.5 kgf/cm 2 is fitted and run on a drum of an indoor drum testing machine rotating at a speed of 60 km/h under a load of 5000 kg, during which a running distance until the occurrence of trouble in the bead portion is measured. In Tables 1 and 2, the running distance is represented by an index on the basis that the conventional example is 100 as a durability test result. The larger the index value, the better the durability. Further, the trouble form of the bead portion is also shown in Tables 1 and 2. As seen from the results of Tables 1 and 2, the bead portion durability in the tires of Examples 1-15 is considerably improved as compared with the conventional tire and the tires of Comparative Examples 1-2. Moreover, it has been confirmed that the tire weight in these examples is somewhat lighter than that of the conventional example. As mentioned above, according to the invention, the occurrence of peeling failure between the cord and the coating rubber in the main carcass body is advantageously prevented under heavy load conditions while maintaining the weight reduction and cost reduction and hence excellent separation resistance at the end part of the turnup portion can be obtained, whereby there can be provided pneumatic radial tires having a long service life and considerably improved durability as a whole of the bead portion.

Claims: A pneumatic radial tire comprising a tread portion (3), a pair of sidewall portions (2) extending from both ends of the tread portion (3), a pair of bead portions (1) extending from the sidewall portions (2), a carcass (5) extending between a pair of bead cores (4) embedded in the bead portions (1) to reinforce these portions and comprised of at least one rubberized ply containing cords arranged in a radial direction, in which the carcass (5) consists of a main carcass body (5a) and a turnup portion (5b) wound around the bead core (4) from the inside toward the outside, a belt (6) superimposed about a crown portion of the carcass to reinforce the tread portion (3), and a rubber filler (7) taperingly extending from the bead core (4) between the main carcass body (5a) and the turnup portion (5b) along the outer surface of the main carcass body (5) toward the end of the tread portion (3), wherein the rubber filler (7) is composed of three rubber stocks (7-1,7-2,7-3) having different JIS hardnesses, with a first rubber stock (7-1) being arranged in at least an outer zone among both zones in the radial direction of the tire sandwiching a normal line (Lv) drawn from the turnup end (5bE) to the outer surface of the main carcass body (5a) in section of the tire and adjacent the main carcass body (5a), a second rubber stock (7-2) as a stress-mitigating rubber layer, and a softest third rubber stock (7-3) having a minimum hardness being arranged along the inner surface of the turnup portion (5b), characterized in that the hardness of the stress-mitigating second rubber stock (7-2) is equal to or lower than that of the coating rubber for the cord of the carcass (5), the first rubber stock (7-1) is a hardest rubber stock having a maximum hardness among said rubber stocks, and the second rubber stock (7-2) has a hardness between maximum hardness and minimum hardness and is arranged between the outer surface of the main carcass body (5a) and the hardest rubber stock (7-1). A pneumatic radial tire as claimed in claim 1, characterized in that the JIS hardness of the stress-mitigating rubber layer (7-2) is within a range of 55-75°. A pneumatic radial tire as claimed in claim 2, characterized in that the JIS hardness is within a range of 62-72°. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that the thickness of the stress-mitigating rubber layer (7-2) existing between two normal lines drawn from the turnup end (5bE) of the carcass (5) and the outer end of the hardest rubber stock (7-1) in the radial direction to the outer surface of the main carcass body (5a) is within a range of 0.7-4.5 mm. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that the outer end of the stress-mitigating rubber layer (7-2) in the radial direction is located outside the outer end of the hardest rubber stock (7-1) in the radial direction. A pneumatic radial tire as claimed in any of claims 1 to 5, characterized in that the inner end of the stress-mitigating rubber layer (7-2) in the radial direction is located at a position extending inward from the normal line (Lv) passing through the turnup end (5bE) of the carcass in the radial direction. A pneumatic radial tire as claimed in any of claims 1 to 6, characterized in that the ratio of thickness of the hardest rubber stock (7-1) to total thickness of the rubber filler (+7) as measured at the normal line (Lv) passing through the turnup end (5bE) is within a range of 0.20-0.55. A pneumatic radial tire as claimed in any of claims 1 to 7, characterized in that the thickness of the hardest rubber stock (7-1) on the normal line (Lv) passing through the turnup end (5bE) is thinner than the thickness of the hardest rubber stock on a straight line drawn in parallel to the normal line outward therefrom in the radial direction.

Inventors: NAKAMURA HIROSHI, TSURUTA MAKOTO, NAKAMURA, HIROSHI, TSURUTA, MAKOTO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: B60C15
IPC codes: B60C9, B60C15