Pneumatic radial tire with high durable bead portions

Abstract: A pneumatic radial tire with highly durable bead portions includes a pair of bead portions (1), a pair of sidewalls (2) and a tread (3) extending between the sidewalls, and further includes as a body reinforcement a carcass (4) extending between ths bead portions and consisting of plies whose cores are radially arranged. At least one ply is turned up about bead cores (6) of the bead portions from inside to outside of the tire to form a turn-up portion. A carcass pass line in radial section of the tire in its self-holding position inflated to 5% of a normal internal pressure under no-load condition includes curvature changing areas (v) in a carcass profile curved toward the turned-up portions in the proximities of positions corresponding to a height (h) of turn-up ends of the ply from a rim diameter line (RL) to cause compressive stresses in the bead portions near to the turn-up ends of the ply by deformation of the carcass when the tire is inflated to normal internal pressure. With the above arrangement, the durability at the bead portions of the tire is improved to an extent required for prolongation of life time obtained by recapping the tire repeatedly.

Description: This invention relates to a pneumatic radial tire having highly durable bead portions. Heavy duty pneumatic radial tires to be used for trucks, buses and the like are particularly required to have high durability of bead portions. Such a requirement results from not only the severe service conditions but also the fact that they are generally used repeatedly by recapping every time tread rubbers have been torn off to realise their longest possible effective life. With such heavy duty pneumatic radial tires, failures at bead portions are often found when they are inspected, whether they are able to be used again by recapping. Such failures at the bead portions are mainly caused by high tensile strains in the proximities of turn-up ends of carcass plies, particularly radially outward thereof. In most cases, the tensile strains give rise to rubber fatigue and separation. In order to improve the durability of the bead portions, it has been proposed to provide reinforcing members or other means to mitigate the above strains and to restrain any growth of cracks due to the rubber fatigue. However, the provision of such means detrimentally affects productivity of tires, and also increases cost and occurrence of heat. In general, the heavy duty pneumatic radial tire to be used for trucks and buses comprises a pair of bead portions, sidewalls and a tread extending between the sidewalls and further comprises as a body reinforcement a carcass extending between the bead portions and having plies whose cords are radially arranged. At least one ply of the carcass is folded from inside to outside about the bead cores to form a turn-up construction. In this case, the durability of the bead portions depends upon the turn-up ends of the carcass. However, direct measures for the turn-up portions to solve these problems have been insufficient to obtain useful effects. The present inventors have investigated and developed a carcass profile of heavy duty pneumatic radial tires capable of improving the durability of bead portions to meet the requirement of renewal of effective life over and over again by recapping without requiring any particular reinforcing members or special means. The invention resides in the discovery that the turn-up ends of the carcass plies governing the durability of the bead portions are subjected to forces in pulling-out directions by filled inner pressure in the tire, while the sidewalls and bead portions are subjected to bending forces so that high tensile strains occur in rubbers facing the turn-up ends resulting in failures at the bead portions, and that compressive stresses introduced into the rubbers are effective in order to reduce the tensile strains (to obtain an effect reducing the tensile strains and hence with residual tensile stresses). Attention is also drawn to the disclosure of EP-A-0273558, which describes a heavy duty radial tire including at least one radial carcass extending from one bead portion to the other bead portion and using inextensible cords and a belt arranged outwardly of the radial carcass for reinforcing a tread of the tire. The tire comprises, during expanding of the tire from 5% to 100% of a normal inner pressure, a first profile portion expanding radially outwardly in a tread zone, a second profile portion depressing axially inwardly of the tire in a radially outer zone of a sidewall from one end of said tread to a tire maximum width position when filled with the normal inner pressure, and a third profile portion expanding axially outwardly of the tire in a radially inner zone of the sidewall from the tire maximum width position when filled with the normal inner pressure to a parting point of the sidewall from the rim, thereby properly distributing strains occurring in the tire in filling with the normal inner pressure. Moreover, a carcass profile in the radial cross-sections of the tire filled with an inner pressure 5% of the normal inner pressure and under no load condition is a particularly defined composite curve, and the carcass profile fulfills certain relations relative to a maximum distance of the carcass line from a bead base line with respect to maximum distance deviated from an equilibrium configuration curve. The present invention aims to provide an improved heavy duty pneumatic radial tire which eliminates all the disadvantages of the prior art and improves the durability at bead portions to an extent required for prolongation of life time obtained by recapping the tire repeatedly. The present invention provides a pneumatic radial tire, comprising a pair of bead portions, a pair of sidewalls and a tread extending between the sidewalls, and further including as a body reinforcement a carcass extending between the bead portions and comprising plies whose cords are radially arranged, at least one ply being turned up about bead cores of the bead portions from inside to outside of the tire, wherein a carcass pass line in radial section of the tire in its self-holding position inflated to 5% of a normal internal pressure under no-load condition comprises an area (v) of continuously changing curvature in a carcass profile curved toward the turned-up portion in the proximity of a position corresponding to a height (h) of the turn-up end of the ply from a rim diameter line (RL), characterized in that said area (v) of continuously changing curvature is positioned between points (M) and (N) spaced apart from each other a distance (ℓ) of 10 mm or less along the carcass pass line and within a carcass pass line area extending radial distances (g) on radially inward and outward sides of the turn-up end of the ply, said distances corresponding to 8% of a maximum radial height (H) of the carcass pass line from the rim diameter line (RL). In one preferred embodiment, each of the areas (v) of continuously changing curvature of the carcass pass line is smoothly continuous with a zone (w) adjacent thereto and having a curvature smaller than that of the area (v) of continuously changing curvature and has a centre of curvature inside the tire. In another preferred embodiment, each of said areas (v) of continuously changing curvature of the carcass pass line is smoothly continuous through an inflection point of the carcass pass line with a zone (w' ) on a side remote from the bead core. Preferably a difference ϑ between angles of tangents (m-m' ) and (n-n' ) at the points (M) and (N) to the carcass line is 5 to 90°. The said zone (w) may be a zone adjacent the area (v) of continuously changing curvature on a side remote from the bead core and has a curvature within 1/500 (mm⁻¹) defined by a point on the carcass pass line radially spaced from the turn-up end of the ply by a distance (i) corresponding to 24% of the maximum radial height (H) and two points (S) and (T) located in a portion of the carcass pass line and spaced along the carcass pass line at a distance corresponding to 12% or more of the maximum radial height (H). The said zone (w' ) adjacent the area (v) of continuously changing curvature on the side remote from the bead core may be a zone located in a portion of the carcass pass line between two points (P) and (Q) radially spaced from the turn-up end of the ply by a distance (i) corresponding to 24% of the maximum radial height (H), preferably a distance (j) corresponding to 12% of the maximum radial height (H) and has a centre of curvature located out of the tire. The tire preferably has wedge rubbers in the form of a convex lens in section along the areas (v) of continuously changing curvature inside the carcass. The tire may further comprise inner liners extending along an inner circumference of the carcass and each of the inner liners comprises a thicker portion corresponding to the area (v) of continuously changing curvature of the carcass pass line. The thickness of the thicker portion of the inner liner is preferably 1. 7 to 8 times, more preferably 2 to 4 times, the average thickness of the entire inner liner. According to the invention, since tensile strains occurring in rubbers in the proximities of turn-up ends of carcass plies having cords radially arranged are eliminated with the aid of a particular carcass profile in the form of a toroidal shape of the carcass pass line extending between bead portions without relying upon the turn-up portions themselves, any reinforcing members or particular means hitherto used as reinforcements are not needed in order to avoid problems arising in connection with the bead portions. In measuring the carcass profiles, the measurement should be effected after keeping a tire filled with a normal inner pressure for about 24 hours, or more preferably after running at a speed of 60 km/h under a load of the order of an empty car for about one hour, in order to remove deformations due to storing for a long time. The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figures 1a and 1b, 2 and 3a-3c are sectional views illustrating carcass pass lines of tires according to the invention; Figures 4-7 are sectional views illustrating sections of embodiments of particular tires according to the invention; and Figure 8 is a sectional view illustrating a section of another embodiment of a tire according to the invention. Figures 1a and 1b, 2 and 3a-3c illustrate one example of a carcass profile of a radial tire according to the invention. In each drawing, the tire comprises bead portions 1, sidewalls 2, a tread 3, a carcass 4, an inner liner 5, bead cores 6, and a belt 7. Reference numeral 8 denotes a rim. Each drawing shows a radial section of one half of a left half of a tire or its principal portion. The carcass 4 is schematically shown in thick solid lines. Bead reinforcing members such as bead chafers and stiffeners are omitted from the drawings and the inner liners 5 are partially omitted. Plies of the carcass 4 having radially arranged cords are turned up about the bead cores 6 from inside to outside of the tire. It is assumed that a radial end of the turn-up end from the bead base or a rim diameter line RL is h, while a maximum radial height of a pass line of the carcass 4 from the line RL is H. An area v of continuously changing curvature of the carcass pass line is located between points M and N spaced a distance ℓ within 10 mm along the pass line in a first pass line area extending over radial or vertical distance g on both sides of a line of the radial distance h from the rim diameter line RL over radial or vertical distances g which are 8% of the maximum height H. In this case, a difference between angles ϑ M and ϑ N is preferably 5°-90° where the angles ϑ M and ϑ N are formed by the rim diameter line RL and tangents m-m' and n-n' to the pass line of the carcass 4 at the points M and N. In a self-retaining or holding position of the tire (refer to solid lines in Fig. 1b) when inner pressure is filled in the tire mounted on the rim to 5% of a standard or normal inner pressure under no-load condition, the area v of the carcass pass line curves toward the turn-up end of the ply (concave as viewed from the inside of the tire) as shown in Figs. 1a and 1b. Under the normal inner pressure as shown in broken lines in Fig. 1b, however, the pass line of the carcass 4 is deformed so as to be smoothly continuous as the tire is inflated owing to the inner pressure, with the result that compressive stresses are caused in the bead portions 1 in the proximities of the turn-up ends of the ply to mitigate the tensile strains. As the compressive stresses serve to reduce the tensile strains under no-load and inner pressure filled condition, effective reduction in tensile strain in the bead portions at the turn-up ends of the ply is accomplished when the tire is running so that failures of the bead portions are effectively avoided. Fig. 2 illustrates a curvature changing area w of a carcass pass line which is adjacent the area v on a side remote from the bead core 6 and radially outwardly extends to a point K spaced a distance j to i (Fig. 1a) from the turn-up end of the carcass corresponding to 12-24% of the maximum radial height H of the pass line of the carcass 4. The carcass pass line has a curvature within 1/500 (mm⁻¹). Compressive stresses are caused in the bead portions to reduce the tensile strains in the same manner as that shown in Fig. 1a. Fig. 3a illustrates a curvature changing area w' of a carcass pass line which is adjacent the area v on the side remote from the bead core 6 and radially outwardly extends to a point P spaced a distance j (Fig. 1a) from the turn-up end of the carcass corresponding to 12% of the maximum radial height H of the pass line of the carcass 4. The pass line of the carcass 4 in the area w' between the points P and Q has a center of the curvature out of the tire. As is clear from Fig. 3c, compressive stresses occur in the proximity of the turn-up ends to mitigate tensile strains in the same manner as in Fig. 1a. The zone w' is smoothly continuous with the curvature changing area v through an inflection point of the carcass pass line. According to the invention, by providing the area v of continuously changing curvature in the carcass pass line the durability of beads can be improved with tire constituting members as they are without changing the construction of the tires and elements of the bead portions. Therefore, the invention is advantageous in terms of quality and productivity of tires. The particular carcass profile as above described can be obtained by controlling distribution of rubber thicknesses from inner walls to the carcass ply on inner sides of axial directions of the tire in forming and vulcanizing the rubber. If it is needed to severely control variations and irregularities in the carcass line due to so-called "rubber flow", the following measures are effective. With tires formed by conventional forming and vulcanizing methods, a carcass ply extending between bead cores is arranged along an inner surface of the tire. In contrast herewith, according to the invention, as shown in Figs. 4, 5 and 7, wedge rubbers 9 in the form of a convex lens in section are previously arranged on an inner surface of the carcass correspondingly to the carcass profile changing areas v or inner liners 5 are used which are formed with portions 5' having partially different thicknesses. Moreover, the above area v can be obtained in the carcass pass line by providing a vulcanizing bladder with continuous ridges at positions corresponding to the area v. Therefore, the variations and irregularities of the carcass pass line in production can be restrained with the aid of these measures in this manner. The tire according to the invention comprises carcass pass line area v of continuously changing curvature in which the carcass profile in radial section is concave as viewed from the inside of the tire in the self-standing position of the tire when 5% of a standard or normal inner pressure is filled in the tire. When the tire is filled with the normal inner pressure, deformations in the carcass and the rubber accompanying therewith are caused concentrically in the concave portions and the proximities thereof so that the tensile strains occurring in the rubber at the bead portions facing the turn-up portions of the carcass 4 are mitigated compensatingly by causing deformations concentrically in the concave portions, thereby accomplishing the improvement of the durability of the bead portions. In order to concentrate the deformations in the concave portions, the concave portions are required to have considerably large curvatures in comparison with the other portions and at the same time have portions such that difference between angles of the tangents m-m' and n-n' to the carcass pass line at the two points M and N spaced a distance less than 10 mm along the carcass relative to the rim diameter line RL is more than 5°. In order to mitigate the tensile strains occurring at ends of the ply with the aid of the effect of the deformations concentrated in the concave portions, moreover, the concave portions and the ends of the ply must be sufficiently adjacent each other. In other words, the concave portions must be arranged on the carcass pass line within the range between the heights h-g and h+g from the rim diameter line RL as shown in Figs. 1-3, where the distance g is 8% of the maximum height H of the carcass line. Although it is required for the concave portions to have curvatures large enough in comparison with those of the other portions of the carcass, if such large curvatures exist in a too wide range of the carcass, the required concentration of deformations cannot be accomplished when the normal inner pressure is filled in the tire. It is therefore needed to provide portions having small curvatures on sides of the tire rotating axis and tire tread with respect to the concave portions. However, since the stiffeners and chafers as reinforcing members are arranged on the side of the rotating axis of the tire, the rigidity of the carcass on this side is very high. Therefore, the carcass profile on this side is not susceptible to deformation of the tire caused by filling the inner air pressure. Accordingly, it is desired to make the curvatures small enough over sufficient length more than 12% of the maximum height H of the carcass line adjacent the concave portions on the side of the tread with radii of curvature more than 500 mm as shown in Figs. 1a and 1b. Instead of the portions of the carcass line having small radii of curvature, the carcass line may have portions which form convex portions as viewed from inside of the tire. With such a feature, the deformations of the tire can be concentrated into the concave portions when filling the normal inner pressure to reduce the strains at the ends of the carcass ply, thereby improving the durability of the bead portions. In other words, as shown in Figs 3a-3c, it is effective that the carcass ply 4 has a convex portion as viewed from the inside of the tire within a range between 24% of the maximum height H of the carcass line from the height h at the end of the carcass ply on the tread side of the line of the height h and 8% of the maximum height H on the side of the tire rotating axis, preferably between 12% of the maximum height H on the tread side and 8% of the maximum height H on the tire rotating axis. In order to bring carcass pass lines of the tires into control of variations and irregularities in production of tires due to "rubber flow" in vulcanization according to the invention, it is effective to arrange wedge rubbers 9 in the proximity of the concave portions on axially inner sides of the carcass ply to make smooth the inner walls of the tire or to form convex portions viewed from the inner side of the tire rotating axis. It is recommendable for the same purpose to partially change the thicknesses of the inner liners 5 in the proximities of the concave portions as shown in Fig. 8, although it is uniform in thickness in the conventional form. In order to sufficiently restrain variation in case line, it is more desirable to have thicknesses of the inner liners in the proximities of the concave portions 1.7 to 8 times, preferably 2 to 4 times, an average thickness of the entire inner liners. Fig. 4 illustrates one embodiment of a invention which is a lug pattern tire (size: 1.00 R20) having two nylon chafers and one wire chafer as shown in the drawing. A maximum height H of the carcass line from a rim diameter line RL is 233 mm. A height h of ply ends is 66.5 mm. Two points M and N on the carcass line are positioned at heights 72 mm (h + 0.024 H) and 65.5 mm (h - 0.004 H) respectively and spaced 7 mm from each other. A difference between angles of tangents at the points M and N to the carcass pass line relative to the rim diameter line RL is 6.0°. Adjacent the concave portions as viewed from inside of the tire, portions of the carcass pass line on the side of the tread are substantially straight line in section over 30 mm (0.129H) length. On the other hand, rubbers in the form of a lens in section having 20 mm width and 2 mm maximum thicknesses are arranged between the carcass ply and the nylon chafer adjacent thereto. Tires as above described were produced by way of trial and durability at bead portions of these tires was tested in a drum running test. In the drum test, a tire to be tested was urged against a drum and the drum was rotatively driven so as to drive the tire. A running distance of these tires increased about 16% in comparison with tires of natural equilibrium carcass profile according to the prior art. Fig. 5 illustrates another embodiment of a tire according to the invention which is a rib pattern tire (size: 10.00 R20) having two nylon chafers and one wire chafer. A maximum height H of the carcass line from a rim diameter line RL is 241 mm and a height h of ply ends is 67 mm. Two points M and N on the carcass pass line are positioned at heights 65.7 mm (h - 0.005H) and 59.2 mm (h - 0.032H) respectively and spaced 7.8 mm from each other. A difference between angles of tangents at the points M and N to the carcass line relative to the rim diameter line is 14°. Adjacent the concave portions as viewed from inside of the tire, portions of the carcass ply on the side of the tread and extending to a height of 90 mm (h + 0.099H) form convex portions as viewed from the inside of the tire. On the other hand, wedge rubbers in the form of a convex lens in section having 20 mm width and 2.7 mm maximum thicknesses are arranged on an axial inside of an inner liner. Tires as above described were produced by way of trial and durability at bead portions of these tires was tested in the drum running test. It was found in the test that a running distance of these tires increased about 15% in comparison with tires of natural equilibrium carcass profile according to the prior art. Fig. 6 illustrates a further embodiment of a tire according to the invention which is a rib pattern tire (size: 10.00 R20) having two nylon chafers and one wire chafer. A maximum height H of the carcass line from a rim diameter line RL is 241 mm and a height of the ply ends is 67 mm. Two points M and N on the carcass pass line are positioned at heights 68.3 mm (h + 0.005H) and 62.8 mm (h - 0.017H) respectively and spaced 6 mm from each other. A difference between angles of tangents at the points M and N to the carcass pass line relative to the rim diameter line RL is 6.0°. Adjacent the concave portions as viewed from inside of the tire, portions of the carcass ply on the side of the tread and extending to a height of 95 mm (h + 0.116H) form convex portions as viewed from the inside of the tire. Tires as above described were produced by way of trial and durability at bead portions of these tires was tested in the drum running test. It was found in the test that a running distance of these tires increased about 18% in comparison with tires of natural equilibrium carcass profile according to the prior at. Fig. 7 illustrates a further embodiment of a tire according to the invention which is a block pattern tire (size: 11/70 R22.5) having two nylon chafers and one wire chafer. A maximum height H of the carcass line from a rim diameter line RL is 165.5 mm and a height h of ply ends is 34 mm. Two points M and N on the carcass path line are positioned at heights 46.3 mm (h + 0.074H) and 42.1 mm (0.049H) respectively and spaced 5.2 mm from each other. A difference between angles of tangents at the points M and N to the carcass line relative to the rim diameter line is 8.5°. Adjacent the concave portions as viewed from inside of the tire, portions of the carcass pass line on the side of the tread are substantially straight line in section over 22 mm (0.133H) length. On the other hand, reinforcing rubbers in the form of a lens in section having 20 mm width and 2.5 mm maximum thicknesses are arranged between the two chafers. Tires as above described were produced by way of trial and durability at bead portions of these tires was tested in the drum running test. A running distance of these tires increased about 12% in comparison with tires of natural equilibrium carcass profile according to the prior art. As can be seen from the above description, according to the invention the durability at bead portions of heavy duty radial tires can be improved to an extent required for prolongation of life time obtained by recapping the tire repeatedly.

Claims: A pneumatic radial tire, comprising a pair of bead portions (1), a pair of sidewalls (2) and a tread (3) extending between the sidewalls, and further including as a body reinforcement a carcass (4) extending between the bead portions and comprising plies whose cords are radially arranged, at least one ply being turned up about bead cores (6) of the bead portions from inside to outside of the tire, wherein a carcass pass line in radial section of the tire in its self-holding position inflated to 5% of a normal internal pressure under no-load condition comprises an area (v) of continuously changing curvature in a carcass profile curved toward the turned-up portion in the proximity of a position corresponding to a height (h) of the turn-up end of the ply from a rim diameter line (RL), characterized in that said area (v) of continuously changing curvature is positioned between points (M) and (N) spaced apart from each other a distance (ℓ) of 10 mm or less along the carcass pass line and within a carcass pass line area extending radial distances (g) on radially inward and outward sides of the turn-up end of the ply, said distances corresponding to 8% of a maximum radial height (H) of the carcass pass line from the rim diameter line (RL). A pneumatic radial tire as claimed in claim 1, characterized in that each of said areas (v) of continuously changing curvature of the carcass pass line is smoothly continuous with a zone (w) adjacent thereto and having a curvature smaller than that of the area (v) of continuously changing curvature and has a centre of curvature inside the tire. A pneumatic radial tire as claimed in claim 1, characterized in that each of said areas (v) of continuously changing curvature of the carcass pass line is smoothly continuous through an inflection point of the carcass pass line with a zone (w' ) on a side remote from the bead core. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that a difference ϑ between angles of tangents (m-m' ) and (n-n' ) at the points (M) and (N) to the carcass line is 5 to 90°. A pneumatic radial tire as claimed in claim 2, characterized in that said zone (w) is a zone adjacent the area (v) of continuously changing curvature on a side remote from the bead core and has a curvature within 1/500 (mm⁻¹) defined by a point on the carcass pass line radially spaced from the turn-up end of the ply by a distance (i) corresponding to 24% of the maximum radial height (H) and two points (S) and (T) located in a portion of the carcass pass line and spaced along the carcass pass line at a distance corresponding to 12% or more of the maximum radial height (H). A pneumatic radial tire as claimed in claim 3, characterized in that said zone (w' ) adjacent the area (v) of continuously changing curvature on the side remote from the bead core is a zone located in a portion of the carcass pass line between two points (P) and (Q) radially spaced from the turn-up end of the ply by a distance (i) corresponding to 24% of the maximum radial height (H), preferably a distance (j) corresponding to 12% of the maximum radial height (H) and has a centre of curvature located out of the tire. A pneumatic radial tire as claimed in any of claims 1 to 6, characterized by further comprising wedge rubbers (9) in the form of a convex lens in section along the areas (v) of continuously changing curvature inside the carcass. A pneumatic radial tire as claimed in any of claims 1 to 7, characterized by further comprising inner liners (5) extending along an inner circumference of the carcass and each of the inner liners comprises a thicker portion corresponding to the area (v) of continuously changing curvature of the carcass pass line. A pneumatic radial tire as claimed in claim 8, characterized in that the thickness of the thicker portion of the inner liner is 1.7 to 8 times, preferably 2 to 4 times, the average thickness of the entire inner liner.

Inventors: IIDA HIROYUKI, KOSEKI HIROYUKI, IIDA, HIROYUKI, KOSEKI, HIROYUKI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1989
Main classification: B60C3
IPC codes: B60C3, B60C15, B60C9