Heavy duty pneumatic radial tires

Abstract: A heavy duty pneumatic radial tire (1) comprises a tread portion (2), a pair of sidewall portions (4), a pair of bead portions containing a pair of bead cores (3) therein, at least one radial carcass ply (5), and a bead filler comprised of at least two rubber stocks, in which at least a part of an outer surface of a turnup portion (6) of the carcass ply (5) in axial direction of the tire is covered with a deformation-absorbing rubber layer arranged along an upper rubber stock of the bead filler and having a Shore A hardness of not more than 55°.

Description: This invention relates to pneumatic radial tires, and more particularly to a heavy duty pneumatic radial tire having an improved durability in the bead portion thereof. In general, when a pneumatic radial tire is run under loading, a reaction force of such a loading is produced from a road surface at a ground contact region to bend a sidewall portion of the tire, and then the bending deformation is transmitted to a bead portion of the tire. This bending deformation is considerably large in heavy duty pneumatic radial tires for trucks and buses using steel cords for the carcass ply and being run at a high internal pressure under a high loading. In a heavy duty pneumatic radial tire, a carcass ply comprised of inextensible cords having a modulus of elasticity of not less than 2500 kg/mm 2 such as steel cords is wound around a bead core from the inside of the tire toward the outside to form a turnup portion and further a bead portion reinforcing cord layer is arranged outside the turnup portion therealong. Since the turnup portion and the bead portion reinforcing layer are hardly deformed, a large compression strain is generated in the rubber surrounding an outward end of the turnup portion in the radial direction of the tire due to the above bending deformation. Such a compression strain is repeatedly generated every rotation of the tire to create cracking in the rubber in the vicinity of the outward end of the turnup portion or the bead portion reinforcing cord layer, which finally brings about the occurrence of separation failure. In order to prevent the occurrence of separation failure, a combination of hard rubber stock and soft rubber stock is disposed between the carcass ply and its turnup portion above the bead core as a bead filler in such a manner that the hard rubber stock is arranged near to the bead core and the soft rubber stock is arranged near to the outward end of the turnup portion as disclosed in JP-A-4-27606, which comprises the features of the first part of claim 1, and and JP-A-5-16618. In this case, the bending deformation transmitted to the bead portion is absorbed and mitigated by the deformation of easily deformable soft rubber stock to reduce the compression strain generated in the vicinity of the outward turnup end portion, whereby the occurrence of cracking, separation failure or the like is controlled or prevented. Recently, wear resistance of tread rubber has been considerably improved to further prolong the service life, and also the section profile of the tire becomes lower even in heavy duty pneumatic radial tires. Therefore, external force applied to the bead portion of such tires becomes even larger and the conditions required for the bead portion become more severe. As a result, it is strongly demanded to further improve the durability of the bead portion in a heavy duty pneumatic radial tire. It is, therefore, an object of the invention to provide heavy duty pneumatic radial tires provided with a bead filler comprised of at least two rubber stocks having different hardnesses, and having an improved durability of a bead portion for controlling or preventing the occurrence of separation failure in the bead portion. According to the invention, there is provided a heavy duty pneumatic radial tire comprising a tread portion, a pair of sidewall portions extending inward from both side ends of the tread portion in a radial direction of the tire, a pair of bead portions extending inward from the sidewall portions and containing a pair of bead cores therein, at least one radial carcass ply extending between the bead cores and wound around each of the bead cores from the inside of the tire toward the outside thereof to form a turnup portion and containing cords having a modulus of elasticity of not less than 2500 kg/mm 2 therein, and a bead filler extending upward and taperingly from the bead core beyond an outward end of the turnup portion in the radial direction and comprised of at least two rubber stocks in which a lower part near to the bead core is a hard rubber stock and an upper part is a soft rubber stock, characterized in that at least a part of an outer surface of the turnup portion in the axial direction of the tire is covered with a deformation-absorbing rubber layer arranged along the soft rubber stock so as to extend outward beyond the outward end of the turnup portion in the radial direction and having a Shore A hardness of not more than 55°. In preferable embodiments of the invention, the deformation-absorbing rubber layer and the soft rubber stock are made from substantially the same rubber material; and the length of the outer surface of the turnup portion covered with the deformation-absorbing rubber layer is 3 to 30 times the diameter of the cord included in the turnup portion; and the maximum thickness of the deformation-absorbing rubber layer in the axial direction is 1 to 10 times the diameter of the cord included in the turnup portion; and the Shore A hardness of the deformation-absorbing rubber layer is 38-55° and the Shore A hardness of the hard rubber stock is not less than 58°, preferably 58-68°; and a ply end rubber similar to a coating rubber of the carcass ply is arranged on the outward end of the turnup portion so as to extend inward from the outward end along at least one side surface of the turnup portion in the radial direction, and at least one cord chafer having a modulus of elasticity of not less than 2500 kg/mm 2 is arranged outside the turnup portion therealong as a bead portion reinforcing cord layer. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a schematic radial half section view of an embodiment of a heavy duty pneumatic radial tire according to the invention; Fig. 2 is a diagrammatic section view of a first embodiment of the bead portion of a tire according to the invention; Fig. 3 is a diagrammatic section view of a second embodiment of the bead portion of a tire according to the invention; Fig. 4 is a diagrammatic section view of a third embodiment of the bead portion of a tire according to the invention; Fig. 5 is a diagrammatic section view of a fourth embodiment of the bead portion of a tire according to the invention; Fig. 6 is a diagrammatic section view of a fifth embodiment of the bead portion of a tire according to the invention; Fig. 7 is a diagrammatic section view of a sixth embodiment of the bead portion of a tire according to the invention; Fig. 8 is a diagrammatic section view of a seventh embodiment of the bead portion of a tire according to the invention; Figs. 9a and 9b are diagrammatic views illustrating other embodiments of arranging a ply end rubber(s) on an outward turnup end portion of a carcass ply; Fig. 10 is a diagrammatic section view of an eighth embodiment of the bead portion of a tire according to the invention; Fig. 11 is a diagrammatic section view of a ninth embodiment of the bead portion of a tire according to the invention; Fig. 12 is a diagrammatic section view of a tenth embodiment of the bead portion of a tire according to the invention; Fig. 13 is a diagrammatic section view of a eleventh embodiment of the bead portion of a tire according to the invention; Fig. 14 is a diagrammatic section view of a twelfth embodiment of the bead portion of a tire according to the invention; Fig. 15 is a diagrammatic section view of a first conventional bead portion; Fig. 16 is a diagrammatic section view of a second conventional bead portion; and Fig. 17 is a diagrammatic section view of a third conventional bead portion. The present inventors have made various studies on causes of separation failure in heavy duty pneumatic radial tires and have found the following. That is, when a tire is mounted onto a rim and inflated under a high internal pressure, compression deformation is created in rubber sandwiched between the turnup portion of the carcass ply in the bead portion and the rim flange by reaction force of the internal pressure. Since rubber is incompressible, however, the rubber is moved outward from the end of the rim flange in the radial direction by an amount of the compression deformation. On the other hand, since the carcass ply is comprised of cords having a modulus of elasticity of not less than 2500 kg/mm 2 such as steel cord or aromatic polyamide cord, the turnup end portion of the carcass ply hardly deforms and hence shearing strain is generated in rubber located outside the turnup end in the axial direction of the tire. Further, when the tire is run under loading, the sidewall portions located at the ground contact region are largely bent but also the bead portion located outward from the rim flange in the radial direction of the tire is deformed outward in the axial direction of the tire. As a result, a large compression strain is applied to the turnup end portion of the carcass ply existing in the sidewall portion near to the bead portion. Furthermore, the movement of rubber in a substantially circumferential direction is caused in a zone ranging from the bead portion to the sidewall portion located at positions corresponding to stepping-in and kicking-out portions of the ground contact region in the tire tread, whereby a shearing strain is applied to the turnup portion of the carcass ply. The above compression strain and shearing strain are repeatedly applied to the turnup end portion of the carcass ply during the running of the tire, whereby rubber fatigue is promoted to cause rubber cracking along the turnup end portion and finally bring about the occurrence of separation failure. In this type of conventional tire, a bead filler comprised of hard rubber stock and soft rubber stock is disposed between the carcass ply and its turnup portion so as to arrange the hard rubber stock near to the bead core and the soft rubber stock near to the turnup end portion in the radial direction as previously mentioned. In this case, the bending deformation transmitted to the bead portion is absorbed by the soft rubber stock to reduce the compression strain in the outward turnup end in the radial direction. In the conventional tire, however, any countermeasure against the large compression strain and shearing strain produced in the rubber surrounding the turnup end portion of the carcass ply is not arranged outside the turnup end portion in the axial direction of the tire, so that cracking is created in such a rubber to grow into separation failure. In order to solve the aforementioned problem of the conventional tire, according to the invention, at least a part of the outer surface of the turnup portion in the axial direction of the tire is covered with the deformation-absorbing rubber layer arranged along the soft rubber stock so as to extend outward beyond the outward end of the turnup portion in the radial direction and having a Shore A hardness of not more than 55°. Therefore, when the tire according to the invention is mounted onto a rim under a given internal pressure and run under loading, the shearing strain and compression strain resulting from the movement of rubber between the rim flange and the turnup portion are absorbed by the deformation-absorbing rubber layer which is soft and easily deformable as compared with the surrounding rubber, whereby the occurrence of cracking in rubber near to the outward turnup end portion is prevented to further improve the durability of the bead portion. In general, when the radial carcass is comprised of at least one rubberized cord ply containing cords having a modulus of elasticity of not less than 2500 kg/mm 2 , cracking is apt to be caused in rubber located near to the outward turnup end portion in the radial direction outward in the axial direction to finally induce separation failure. In order to prevent the occurrence of cracking, therefore, it is effective to reduce the compression strain in the vicinity of the outward turnup end portion in the radial direction by 20% or more as compared with the conventional tire. To this end, in a preferred embodiment of the invention, the length of the outer surface of the turnup portion covered with the deformation-absorbing rubber layer is preferably within a range of 3 to 30 times the diameter of the cord included in the turnup portion. When the covered length of the outer surface of the turnup portion is less than 3 times the diameter of the cord, the effect of reducing the compression strain in the outward turnup end portion to at least 20% may not be obtained. When said covered length exceeds 30 times the cord diameter, the effect of reducing the compression strain through the deformation-absorbing rubber layer becomes very large, but the bead portion is softened to increase the deformation toward the carcass ply and hence the effect of reducing the compression strain to at least 20% may not be obtained. Further, the maximum thickness of the deformation-absorbing rubber layer in the axial direction is preferably restricted to be within a range of 1 to 10 times the diameter of the cord included in the turnup portion. When the maximum thickness is less than 1 times, the region of absorbing the compression strain becomes narrower and the effect of reducing the compression strain may not be obtained; when said maximum thickness exceeds 10 times, the effect of reducing the compression strain through the deformation-absorbing rubber layer becomes large, but the bead portion is softened to increase the deformation toward the carcass ply and hence the effect of reducing the compression strain to at least 20% may not be obtained. In the tire according to the invention, the deformation-absorbing rubber layer is arranged along the soft rubber stock constituting the bead filler so as to cover the outward turnup end portion of the carcass ply from both sides. From a viewpoint of ease of tire production, it is preferable that the deformation-absorbing rubber layer and the soft rubber stock are united together with each other during the vulcanization after the green tire building step. That is, when a one-piece body of the bead filler and the deformation-absorbing rubber layer is used, the number of tire components is decreased to improve the productivity, but the number of steps in the production of the tire is inversely increased to lower the productivity. This is because, there is added a step that the deformation-absorbing rubber layer portion is first folded outward in the radial direction of the tire and the carcass ply is wound around the bead core from the inside toward the outside and then the above folded rubber layer is returned to the original position to cover the outward turnup end portion from the outside in the axial direction. On the other hand, it is usual to previously form the bead filler comprising an outermost rubber stock, a middle rubber stock and an innermost rubber stock in a separate step and then stick the bead filler onto the carcass ply at the tire building step, but if the deformation-absorbing rubber layer is united with the outermost rubber stock, there is added a complicated step that the innermost and middle rubber stocks are united together in a separate step and stuck onto the carcass ply and thereafter the carcass ply is wound around the bead core and then the one-piece body of the outermost rubber stock and deformation-absorbing rubber layer is stuck thereonto with careful positioning and hence the productivity is lowered. In any case, it is advantageous to use the deformation-absorbing rubber layer and the bead filler separately before the vulcanization step in view of the improvement of the tire productivity. In the invention, the Shore A hardness of the deformation-absorbing rubber layer is not more than 55°, preferably 38-55°, while the Shore A hardness of the soft rubber stock is equal to or somewhat larger than that of the deformation-absorbing rubber layer and the Shore A hardness of the hard rubber stock is preferably not less than 58°, more preferably 58-68° in the case of a bead filler consisting of two rubber stocks. Moreover, when the bead filler is comprised of an outermost soft rubber stock, a middle rubber stock and an innermost hard rubber stock in the radial direction, it is preferable that the Shore A hardness is 38-55° in the outermost soft rubber stock, 58-68° in the middle rubber stock, and not less than 75° in the innermost hard rubber stock, respectively. That is, even if the sidewall portion of the tire is largely deformed at the ground contact region under loading, such a deformation is absorbed by the deformation of the deformation-absorbing rubber layer and the outermost soft rubber stock to reduce the compression strain produced at the outward turnup end of the carcass ply or the bead portion reinforcing layer in the radial direction, whereby the occurrence of separation failure is suppressed or prevented. When the Shore A hardness of the deformation-absorbing rubber layer is more than 55°, the absorption of the above deformation is insufficient and separation failure is apt to be caused. If the Shore A hardness of the deformation-absorbing rubber layer is less than 38°, the absorption of the deformation becomes excessive and hence separation failure is apt to be caused at the boundary between the outermost soft rubber stock in the radial direction and the carcass ply. In a bead filler consisting of two rubber stocks, when the Shore A hardness of the hard rubber stock is less than 58°, the bead filler becomes softer as a whole and the deformation of the sidewall portion becomes larger at the ground contact region and hence the shearing strain is apt to be concentrated in the inside of the outward turnup end portion in the axial direction to cause separation failure, while when the Shore A hardness of the hard rubber stock exceeds 68°, the rigidity of the bead portion is too high and there are apt to be caused heat generation of the bead portion and increase of the rolling resistance requiring separate countermeasures. In the bead filler consisting of three rubber stocks, when the Shore A hardness of the innermost hard rubber stock is less than 75°, the deformation of the carcass ply and the deformation in the bead portion may not be sufficiently be controlled. According to a preferred embodiment of the invention, a ply end rubber similar to a coating rubber of the carcass ply is arranged on the outward end of the turnup portion so as to extend inward from the outward end along at least one side surface of the turnup portion in the radial direction, whereby the outward turnup end is protected by the ply end rubber and advantageously separated from the shearing strain resulting from the movement of rubber between the rim flange and the turnup portion. Furthermore, at least one cord chafer having a modulus of elasticity of not less than 2500 kg/mm 2 such as aromatic polyamide cord, steel cord or the like is preferably arranged outside the turnup portion therealong as a bead portion reinforcing cord layer. According to the invention, the occurrence of separation failure can be suppressed or prevented even when using the above chafer. In Fig. 1 is schematically shown a radial half section of an embodiment of a heavy duty pneumatic radial tire according to the invention, in which numeral 1 is a tubeless tire for a truck or bus having a tire size of 11/70R22.5, numeral 2 a tread portion, numeral 3 a bead core, numeral 4 a sidewall portion, numeral 5 a radial carcass ply, numeral 6 a turnup portion of the carcass ply, symbol B a belt of inextensible cord layers, and symbol O an equatorial plane of the tire. In Fig. 2 is shown a diagrammatically enlarged section of a first embodiment of a bead portion applied to the tire 1 according to the invention, in which a bead filler 7 consisting of an outer soft rubber stock 7A and an inner hard rubber stock 7B is disposed between the carcass ply 5 and its turnup portion 6 so as to extend taperingly from a position near to an outward surface of the bead core 3 beyond an outward end T of the turnup portion 6 in the radial direction of the tire, and outer surfaces of the turnup portion 6 and the soft rubber stock 7A of the bead filler in the axial direction of the tire are covered with a coating rubber 8 for the bead portion. Furthermore, a ply end rubber 9 similar to a coating rubber for the carcass ply is arranged around the outward end T of the turnup portion 6 so as to extend inward along both sides of the turnup portion 6 in the radial direction. Moreover, a deformation-absorbing rubber layer 10 having a Shore A hardness of 38-55°, which is lower than those of the coating rubber 8 and the ply end rubber 9, is arranged between the coating rubber 8 and the soft rubber stock 7A so as to extend outward from a position overlapping with the outward surface of the ply end rubber 9 in the axial direction beyond the outward end T in the radial direction. In the tire of Figs. 1 and 2, the carcass ply 5 is a single rubberized cord ply containing steel cords of 3+9+15x0.175+1 twisting structure arranged at a cord angle perpendicular to the equatorial plane O and the outward end T of the turnup portion 6 is terminated at a position somewhat outward from the top of a rim flange (not shown) in the radial direction when the tire is mounted onto a rim. The bead filler 7 consists of the hard rubber stock 7B extending from above the outer surface of the bead core 3 slantly and outward from a position of the turnup portion 6 near to the bead core 3 in the radial direction and the soft rubber stock 7A disposed thereon and extending taperingly beyond the outward end T of the turnup portion 6 in the radial direction. The ply end rubber 9 has a thickness of about 1 mm and a Shore A hardness of 68-82° substantially equal to that of the coating rubber for the carcass ply. The coating rubber 8 for the bead portion consists of a hard rubber stock 8A arranged in a region contacting with the rim and having an excellent resistance to friction and a relatively soft rubber stock 8B for the sidewall portion 4 extending inward in the form of a wedge to terminate between the hard rubber stock 8A and the turnup portion 6 and having an excellent bending resistance. The deformation-absorbing rubber layer 10 has a Shore A hardness lower than those of the rubber stock 8A and the ply end rubber 9. In this case, it is preferable that a difference in hardness between the deformation-absorbing rubber layer 10 and the rubber stock 8A or ply end rubber 9 is within a range of 8-30°. Moreover, the length of the turnup portion 6 covered with the deformation-absorbing rubber layer 10 corresponds to 14 times the diameter of the cord in the carcass ply, while the maximum thickness of the deformation-absorbing rubber layer 10 in the axial direction corresponds to 3 times the diameter of the cord in the carcass ply. In the embodiment of Fig. 2, a bead portion reinforcing layer 11 containing steel cords of 3x0.24+9x0.255+1 twisting structure therein is arranged from a position contacting with a lower end of the deformation-absorbing rubber layer 10 along the turnup portion 6 toward and along the carcass ply 5. The steel cords in the reinforcing layer 11 are preferably inclined at a cord angle of 50-70° with respect to the arranging direction of the cord in the carcass ply 5. Each length of the ply end rubber 9 extending from the outward end T along both sides thereof may be about 10 mm. In this case, a space produced between the lower end of the ply end rubber 9 and the upper end of the reinforcing layer 11 is filled with an inner end portion of the deformation-absorbing rubber layer 10 in the radial direction. In Fig. 3 is shown a diagrammatically enlarged section of a second embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which one end of the bead portion reinforcing layer 11 is extended outward between the rubber stock 8B and the deformation-absorbing rubber layer 10 along the outer surface of the rubber layer 10 beyond the outward end T of the turnup portion 6 in the radial direction. In this case, the length of the turnup portion 6 covered with the deformation-absorbing rubber layer 10 corresponds to 18 times the diameter of the cord in the carcass ply. In Fig. 4 is shown a diagrammatically enlarged section of a third embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which the deformation-absorbing rubber layer 10 is made from the same rubber material as the rubber stock 7A of the bead filler 7. As a result, the ply end rubber 9 arranged around the outward end T is sandwiched between rubbers having a hardness lower than that of the ply end rubber from both sides in the axial direction. In this case, the number of kinds of rubber used in the tire is decreased to improve the productivity. In Fig. 5 is shown a diagrammatically enlarged section of a fourth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which two fiber cord layers 12 each containing nylon cords of 1260 d/2 are further arranged outside the bead portion reinforcing layer 11. In this case, one of two layers 12 extends from a position covering one end of the reinforcing layer 11 adjacent the carcass ply 5 beyond the other end thereof to the outward end of the deformation-absorbing rubber layer 10, while the other layer 12 extends from a position of a bead base to a position slightly beyond the outward end of the deformation-absorbing rubber layer 10. The cords of the layers 11 and 12 are crossed with each other in opposite directions. Moreover, the cords of the layers 12 are preferably inclined at a cord angle of 40-50° with respect to the arranging direction of the cord in the carcass ply. In Fig. 6 is shown a diagrammatically enlarged section of a fifth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which the rubber stock 8A of the coating rubber 8 is extended outward beyond the outward end T of the turnup portion 6 and the deformation-absorbing rubber layer 10 is sandwiched between the rubber stocks 7A, 8A and 8B. In Fig. 7 is shown a diagrammatically enlarged section of a sixth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which a spacing rubber layer 13 having substantially the same hardness as the ply end rubber 9 is arranged between the rubber stock 7A of the bead filler and the deformation-absorbing rubber layer 10 so that one end of the layer 13 contacts the ply end rubber 9 and the layer 13 substantially extends on an extension line of the turnup portion 6. In this case, the Shore A hardness of the spacing rubber layer 13 is preferably 68-82°. In the second, fourth and sixth embodiments, the effect of absorbing the shearing strain by the deformation-absorbing rubber layer 10 is maintained, while the bending deformation produced under loading causing shearing strain can be controlled by prolonging the length of the bead portion reinforcing layer 11 in the second embodiment, adding the fiber cord reinforcing layers 12 in the fourth embodiment, or adding the spacing rubber layer 13 to improve the rigidity of the bead portion. In order to confirm the effect according to the invention, a test for the durability of the bead portion is made with respect to the above tires of the first to sixth embodiments together with a comparative tire having the same structure as in the first embodiment except that the deformation-absorbing rubber layer 10 is omitted or a portion of the deformation-absorbing rubber layer 10 is replaced with the rubber stock 8B. Details of each of test tires are shown in Table 1. First embodiment Second embodiment Third embodiment Fourth embodiment Fifth embodiment Sixth embodiment Comparative tire Cord angle of the layer 12 - - - 45 - - - Hardness of rubber stock 7B 84 84 84 84 84 84 84 Hardness of rubber stock 7A 63 63 49 63 63 63 63 Hardness of deformation-absorbing rubber layer 10 49 49 49 49 49 49 - Hardness of ply end rubber 9 72 72 72 72 72 72 72 Hardness of spacing rubber layer 13 - - - - - 79 - Note) 1: The hardness means Shore A hardness (°) 2: The hardnesses of rubber stocks 8A and 8B in the coating rubber 8 are 70° and 58°, respectively. 3: The cord angle of the fiber cord reinforcing layer 12 is with respect to the arranging direction of the cord in the carcass ply. 4: The cord angle of the bead portion reinforcing layer 11 is 60° with respect to the arranging direction of the cord in the carcass ply. The test tire is mounted onto a rim of size 22.5x7.50, inflated under an internal pressure of 8.5 kgf/cm 2 , placed on a steel drum having a smooth surface and having a radius of 1.7 m under a load of 5000 kgf, and then run thereon at a speed of 60 km/h until the bead portion is broken. The durability of the bead portion is evaluated by an index on the basis that the running distance of the comparative tire is 100. Moreover, the measurement is carried out at a temperature of 46°C. The test results are shown in Table 2. First embodiment Second embodiment Third embodiment Fourth embodiment Fifth embodiment Sixth embodiment Comparative tire Running distance (index) 125 120 122 135 127 130 100 Note) The larger the index value, the better the durability of the bead portion. The invention will be further described with respect to the heavy duty pneumatic radial tires of seventh to twelfth embodiments according to the invention as shown in Figs. 8-14, in which the deformation-absorbing rubber layer is united with the rubber stock of the bead filler, together with three conventional tires as shown in Figs. 15-17. These tires have substantially the same tire size and structure as in the above first embodiment unless otherwise specified. In Fig. 8 is shown a diagrammatically enlarged section of a seventh embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the first embodiment shown in Fig. 2 in which a bead filler 14 consists of three rubber stocks 14A, 14B and 14C instead of the bead filler 7 and in which a portion 14A-1 of the rubber stock 14A serves as a deformation-absorbing rubber layer. In the bead filler 14, the rubber stock 14A located outward in the radial direction has a Shore A hardness of 49°, and the rubber stock 14B located at a middle position in the radial direction has a Shore A hardness of 63°, and the rubber stock 14C located inward in the radial direction has a Shore A hardness of 84°. Furthermore, the outward surface of the turnup portion 6 in the axial direction is covered with the portion 14A-1 of the rubber stock 14 serving as the deformation-absorbing rubber layer. Moreover, the length of the outward surface of the turnup portion covered with the rubber stock portion 14A-1 is 15 mm as measured inward from the outward end T of the turnup portion 6 in the radial direction, which corresponds to about 14 times the diameter of the cord used in the carcass ply 5, while the maximum thickness of the rubber stock portion 14A-1 in the axial direction covering the outward surface of the turnup portion is 3 mm, which corresponds to about 3 times the diameter of the cord used in the carcass ply. The rubber stock 8B of the coating rubber 8 has a Shore A hardness of 58°. As shown in Fig. 9a, the outward turnup end portion may be covered from both sides with two ply end rubbers 9, or only the outward or inward surface of the outward turnup end portion in the axial direction may be covered with a single ply end rubber 9 as shown in Fig. 9b. In Fig. 10 is shown a diagrammatically enlarged section of an eighth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the seventh embodiment shown in Fig. 8 in which the rubber stock portion 14A-1 is extended inward from an outward end of the bead portion reinforcing layer 11 in the radial direction so as to cover a part of the layer 11. In this case, the total length of the outward surface of the turnup portion and a part of the reinforcing layer 11 covered with the rubber stock portion 14A-1 is 20 mm as measured inward from the outward end T of the turnup portion 6 in the radial direction, which corresponds to about 18 times the diameter of the cord used in the carcass ply 5. In Fig. 11 is shown a diagrammatically enlarged section of a ninth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the seventh embodiment shown in Fig. 8 in which a part of the bead portion reinforcing layer 11 extending inward from its outward end thereof in the radial direction is covered with another ply end rubber 15. In Fig. 12 is shown a diagrammatically enlarged section of a tenth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the seventh embodiment shown in Fig. 8 in which a part of the bead portion reinforcing layer 11 extending inward from its outward end thereof in the radial direction is covered with another ply end rubber 15 and in which the rubber stock portion 14A-1 is further extended beyond the ply end rubber 15 inward in the radial direction. In this case, the total length of the outward surface of the turnup portion and a part of the reinforcing layer 11 covered with the rubber stock portion 14A-1 is 20 mm as measured inward from the outward end T of the turnup portion 6 in the radial direction, which corresponds to about 18 times the diameter of the cord used in the carcass ply 5. In Fig. 13 is shown a diagrammatically enlarged section of an eleventh embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the seventh embodiment shown in Fig. 8 in which the bead filler 14 consists of a rubber stock 14A having a Shore A hardness of 49° and a rubber stock 14C having a Shore A hardness of 84° instead of three rubber stocks 14A, 14B, 14C. In Fig. 14 is shown a diagrammatically enlarged section of a twelfth embodiment of a bead portion applied to the tire 1 according to the invention. This embodiment is a modification of the seventh embodiment shown in Fig. 8 in which the bead filler 14 consists of a rubber stock 14A having a Shore A hardness of 49° and a rubber stock 14B having a Shore A hardness of 63° instead of three rubber stocks 14A, 14B, 14C. In Fig. 15 is shown a diagrammatically enlarged section of a first conventional bead portion applied to the tire 1, which has the same structure as the seventh embodiment of the invention as shown in Fig. 8 except that the rubber stock 14A constituting part of the bead filler 14 does not have a rubber stock portion 14A-1 covering the outward surface of the turnup portion 6 in the axial direction. In Fig. 16 is shown a diagrammatically enlarged section of a second conventional bead portion applied to the tire 1, which has the same structure as the twelfth embodiment of the invention as shown in Fig. 14 except that the rubber stock 14A constituting part of the bead filler 14 does not have a rubber stock portion 14A-1 covering the outward surface of the turnup portion 6 in the axial direction. In Fig. 17 is shown a diagrammatically enlarged section of a third conventional bead portion applied to the tire 1, which has the same structure as the first conventional bead portion shown in Fig. 15 except that the rubber stock 8B of the coating rubber 8 is made from the same rubber material as the rubber stock 14A and has a Shore A hardness of 49°. In order to confirm the effect according to the invention, tests for the durability of the bead portion and weather resistance (resistance to ozone cracking) are made with respect to the above tires of the seventh to twelfth embodiments together with the tires of the first to third conventional embodiments. The durability of the bead portion is evaluated in the same manner as in the above first to sixth embodiments and represented by an index on the basis that the running distance of the conventional embodiment 1 is 100. The larger the index value, the better the durability of the bead portion. In the measurement of the weather resistance, the test tire is inflated under an internal pressure of 8.0 kgf/cm 2 , placed on a steel drum having a radius of 1.7 m under a load of 2725 kgf, and then run thereon at a speed of 45 km/h under outdoor conditions exposed to ultraviolet rays over a distance of 100,000 km. The weather resistance is evaluated by an index on the basis that an average depth of ozone cracking produced in the sidewall portion near to the radially outward turnup end portion in the tire of the conventional embodiment 1 is 100. The larger the index value, the poorer the weather resistance. The test results are shown in Table 3. Running distance (index) Crack depth (index) Seventh embodiment 125 103 Eighth embodiment 128 103 Ninth embodiment 125 103 Tenth embodiment 127 103 Eleventh embodiment 119 105 Twelfth embodiment 112 106 Conventional embodiment 1 100 100 Conventional embodiment 2 90 102 Conventional embodiment 3 103 185 As mentioned above, according to the invention, the deformation-absorbing rubber layer is arranged along the outward surface of the turnup portion in the axial direction near to the outward turnup end portion in the radial direction of the tire, whereby the durability of the bead portion can be further improved as compared with a conventional tire having no deformation-absorbing rubber layer.

Claims: A heavy duty pneumatic radial tire (1) comprising a tread portion (2), a pair of sidewall portions (4) extending inward from both side ends of the tread portion in a radial direction of the tire, a pair of bead portions extending inward from the sidewall portions and containing a pair of bead cores (3) therein, at least one radial carcass ply (5) extending between the bead cores and wound around each of the bead cores from the inside of the tire toward the outside thereof to form a turnup portion (6) and containing cords having a modulus of elasticity of not less than 2500 kg/mm 2 therein, and a bead filler (7;14) extending upward and taperingly from the bead core (3) beyond an outward end (T) of the turnup portion (6) in the radial direction and comprised of at least two rubber stocks (7A,7B;14A,14B,14C) in which a lower part near to the bead core is a hard rubber stock (7B;14C) and an upper part is a soil rubber stock (7A;14A), characterized in that at least a part of an outer surface of the turnup portion (6) in the axial direction of the tire is covered with a deformation-absorbing rubber layer (10;14A-1) arranged along the soil rubber stock (7A;14A) so as to extend outward beyond the outward end (T) of the turnup portion in the radial direction and having a Shore A hardness of not more than 55°. A tire as claimed in claim 1, characterized in that the deformation-absorbing rubber layer (10) and the soft rubber stock (7A) are made from substantially the same rubber material. A tire as claimed in claim 1 or 2, characterized in that the length of the outer surface of the turnup portion (6) covered with the deformation-absorbing rubber layer (10;14A-1) is 3 to 30 times the diameter of the cord included in the turnup portion (6). A tire as claimed in any of claims 1 to 3, characterized in that the maximum thickness of the deformation-absorbing rubber layer (10;14A-1) in the axial direction is 1 to 10 times the diameter of the cord included in the turnup portion (6). A tire as claimed in any of claims 1 to 4, characterized in that the Shore A hardness of the deformation-absorbing rubber layer (10;14A-1) is 38-55° and the Shore A hardness of the hard rubber stock (7B;14C) is not less than 58°. A tire as claimed in any of claims 1 to 5, characterized in that a ply end rubber (9) similar to a coating rubber of the carcass ply (5) is arranged on the outward end (T) of the turnup portion (6) so as to extend inward from the outward end along at least one side surface of the turnup portion in the radial direction. A tire as claimed in any of claims 1 to 6, characterized in that at least one cord chafer having a modulus of elasticity of not less than 2500 kg/mm 2 is arranged outside the turnup portion (6) therealong as a bead portion reinforcing cord layer (11). A tire as claimed in any of claims 1 to 7, characterized in that a deformation-absorbing layer (10) is arranged between the soil rubber stock (7A) of the bead filler (7) and a coating rubber (8) for the bead portion. A tire as claimed in claim 8, characterized in that a spacing rubber layer (13) is arranged between the soil rubber stock (7A) of the bead filler (7) and the deformation-absorbing rubber layer (10). A tire as claimed in any of claims 1 to 7, characterized in that a deformation-absorbing rubber layer (14A-1) comprises a portion of the soil rubber stock (7A) of the bead filler (7).

Inventors: IIDA HIROYUKI, NAKAMURA HIROSHI, TSURUTA MAKOTO, IIDA, HIROYUKI, NAKAMURA, HIROSHI, TSURUTA, MAKOTO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1996
Main classification: B60C15
IPC codes: B60C15