RADIAL TYRE FOR HEAVY DUTY VEHICLES

Abstract: A radial ply tyre for heavy duty vehicles improved in bead durability, which has a bead reinforcing cord layer (5) having an axially inner portion (5A) extending along the inside of the carcass main portion, and an inner liner (6) which is provided with a middle portion (7C) which has a thickness which is gradually decreased radially outwardly, the heights (H7b, H7a) of the lower end (7b) and upper end (7a) of the middle portion (7C) are respectively not smaller than 1.2 and 1.4 times the height (H5a) of the upper edge (5a) of the axially inner portion of the reinforcing layer (5), the difference (Wd) in thickness (W7a) between the upper (7a) and lower ends (7b) of the middle portion (7C) is between 0.5 and 1.0 times the thickness (W7a) at the upper end (7a) of the middle portion (7C) and the thickness (W7c) at 90% of the height (H5a) of the upper edge (5a) of the axially inner portion (5A) of the reinforcing layer (5) is between 0.9 and 1.2 times the thickness (W7a) at the upper end (7a) of the middle portion (7C).

Description: The present invention relates to a radial ply tyre for heavy duty vehicles having improved bead durability. Radial ply tyres have been widely used for heavy duty vehicles such as trucks, buses and the like particularly because of increased running speed due to the recent development and extensions of highway networks and, more especially because of the increase in tyre load for large sized vehicles such as construction vehicles. In comparison with a bias ply tyre, a radial ply tyre has less rigidity in the sidewalls owing to its carcass ply construction, and accordingly the sidewalls have more deformation. As a result, the bead portions are subjected to large repeated deformation when running. In tyres used on heavy dump trucks and in tyres used with refrigerated trucks having a high gravitational centre the strain in the bead portions becomes especially large, and structural failures in the bead portions sometimes occur. In the case where the carcass is composed of steel cords arranged radially of the tyre and turned up at the edges thereof around bead cores from the inside to the outside structural failure is apt to be caused at the upper edge of the turned up portions of the carcass by the stress on the above mentioned upper edges, which stress separates the cords from the surrounding rubber. Such rubber separation at the upper edge of the carcass turned up portion has been identified as a main cause of bead failure. In order to prevent such separation failure, it has been proposed for example as shown in Fig.3, to provide a reinforcing layer C having an inner portion C1 and an outer portion C2 extending radially outwardly from the bead base region A along the axially inner main portion B1 and the axially outer turned up portion B2 of the carcass B, respectively. It is also considered that to prevent rubber separation at the upper edges of the carcass turned up portion, it is more effective to provide reinforcement on the axially inner side of the bead portion to increase the rigidity of the bead portion rather than to dispose a reinforcing layer axially outside the carcass turned up portion to directly cover the upper edge thereof. The tyre shown in Fig.3 which is provided on the axially inner side of the bead portion with the above mentioned axially inner portion C1 extending along the main portion B1 of the carcass B also meets this requirement. Such a tyre, however, sometimes still suffers rubber separation at the upper edge of the axially inner portion C1 and this may be earlier than the above mentioned rubber separation at the upper edge of the carcass turned up portion B2 particularly when the reinforcing layer is composed of only one ply. Incidentally, if the axially inner portion is not formed on the reinforcing layer, the degree of rubber separation at the upper edge of the carcass turned up portion is increased. It is therefore, an object of the present invention to provide a radial ply tyre for heavy duty vehicles, in which rubber separation at the upper edge of the axially inner portion of the bead reinforcing layer is prevented, and the durability of the bead portions is improved. In order to prevent the rubber separation at the upper edge of the axially inner portion of the reinforcing layer, various studies were made by the inventor and he found that, in conventional tyres as shown in Fig.4, which corresponds to the preamble of claim 1 the thickness of the rubber liner D becomes uneven and very thin at the upper edge C1a of the axially inner portion C1, and this is a main cause for the separation. Accordingly the present invention provides a radial tyre for heavy duty vehicles comprising a pair of bead cores disposed one in each bead portion,a radial carcass having radially arranged cords and turned up at the edge portions around the bead cores from the axially inside to the axially outside thereof to form axially outward turnup portions and an axially inward main portion therebetween, a bead reinforcing cord layer comprising an axially inner portion and an axially outer portion the axially inner portion extending radially outwardly from the bead base line L along the inside of the carcass main portion, and the axially outer portion extending radially outwardly from the bead base line L along the outside of the carcass turned up portion and an inner liner disposed on the inside of the carcass main portion and the inner portion of the bead reinforcing layer characterised in that in each bead region the inner liner comprises an upper portion, a middle portion connected at the upper end thereof to the upper portion, and a lower portion connected with the lower end of the middle portion wherein the thickness of the middle portion gradually decreases radially outwardly of the tyre towards the upper portion, the height H76 of the lower end of the middle portion of the inner liner from the bead base line L being not smaller than 1.2 times the height H5a of the upper edge of the axially inner portion of the reinforcing layer from the bead base line L, the height H7a of the upper end of the middle portion of the reinforcing layer from the bead base line L being not smaller than 1.4 times the said height H5a, the difference Wd between the thickness W7a from the inner face of the inner liner at the upper end of the middle portion to the carcass cords and the thickness from the inner face of the inner liner at the lower end of the middle portion to the carcass cords being between 0.5 and 1.0 times the above thickness W7a from the inner face of the inner liner at the upper end of the middle portion to the carcass cords, and the thickness W7c from the inner face of the inner liner at a point in the lower portion, whose height H7c from the bead base line L is 0.9 times the height H5a of the upper edge of the axially inner portion of the reinforcing layer from the bead base line L, to the cords of the reinforcing layer being between 0.9 and 1.2 times the above thickness W7a from the inner face of the inner liner at the upper end of the middle portion to the carcass cords. An embodiment of the present invention will now be described, by way of example only, in conjunction with the diagrammatic drawings in which: Fig.1 is a sectional view showing the bead portion of an embodiment of the present invention. Fig.2 is an enlarged sectional view of part of Fig.1. Fig.3 is a diagrammatical view showing a conventional tyre. Fig.4. is a sectional view showing the upper region of the reinforcing layer of the tyre of Fig.3. In Figs 1 and 2, a radial tyre 1 for heavy duty vehicles has a radial carcass 4 extending between the bead portions of the tyre. The carcass 4 has at least one ply, in this embodiment one ply, of cords 4a which are arranged radially of the tyre and embedded in rubber 4b. The carcass ply is turned up at the edges thereof around bead cores 3 from the axially inside to the outside thereof. For the carcass cord 4a, a high strength cord such as a steel cord or the like, is used. The tyre 1 is provided in each bead portion with a bead reinforcing layer 5. The bead reinforcing layer 5 is disposed around the bead core 3, and extends radially outwardly from the bead base portion under the bead core to form an axially inner portion 5A and an axially outer portion 5B. The height H5a of the upper edge 5a of the axially inner portion 5A from the bead base line L is larger than the height H5b of the upper edge of the axially outer portion 5B from the bead line L. Moreover, in this example, the height H4c of the upper edge 4c of the carcass turnup portion 4B from the bead base line L is larger than the above mentioned height H5b. For the cords of the bead reinforcing layer, high tensile cords such as steel cords, glass fibre cords, carbon fibre cords, aromatic polyamide fibre cords and so on can be used, but steel cords are preferable. The cords are laid at 20 to 70 degrees to the carcass cords. Furthermore, in each bead portion, an organic cord layer 9 composed of plural plies of organic fibre cords is disposed axially outwardly of the outer portion 5B of the bead reinforcing layer 5 and the carcass turnup portion 4B. Still furthermore, a bead apex 10 is disposed radially outwardly of the bead core 3. The tyre 1 is provided with an inner liner 6 extending from one bead portion to the other bead portion along the inner face of the inner portion 5A of the bead reinforcing layer 5 and the inner face of the carcass main portion 4A through the tyre sidewall portions and tread portion in order to prevent air leakage and penetration of moisture into the carcass and to increase the durability of the carcass and the bead. The inner liner 6 is made of a rubber compound having a high airtightness and the compound has a JIS A hardness between 45 and 60 more preferably 50 and 55, and a modulus of elasticity at 200% elongation between 20 and 30 kgf/sq.cm. In each bead portion, the inner liner is provided between its upper portion 7A and lower portion 7B with a portion 7C the thickness of which gradually decreases in the direction radially outwardly of the tyre. The upper portion 7A and the lower portion 7B are smoothly connected to the upper end 7a and the lower end 7b of the above mentioned middle thickness varying portion 7C, respectively, without any step or sudden change in thickness to avoid stress concentration. The lower portion 7B extends radially outwardly from the bead base region beyond the upper edge 5a of the axially inner portion 5A of the bead reinforcing layer 5 to form a thick portion around the upper edge 5a, which thick portion acts to prevent rubber separation in the upper edge region 5a. The height H7b of the lower end 7b of the middle portion 7C from the bead base line L is set to be more than 1.2 times the height H5a of the upper edge 5a of the axially inner portion 5A of the bead reinforcing layer 5 from the bead base line L. Furthermore, the height H7a of the upper end 7a of the middle portion 7C from the bead base line L is set to be more than 1.4 times the above mentioned height H5a. The thickness of the inner liner 6 is arranged so that the thickness from the inner face of the inner liner 6 to the adjacent reinforcement cords including the carcass cords and the bead reinforcing layer cords satisfies the following conditions: The difference Wd between the thickness W7a from the inner face at the upper end 7a to the carcass cords 4a and the thickness from the inner face at the lower end 7b to the carcass cords 4a is s et to be more than 0.5 times and less than 1.9 times the above mentioned thickness W7a at the upper end 7a so as to make the difference a proper small thickness in order to prevent stress concentration. The thickness W7c from the inner face of the inner liner 6 at a point 7c to the cords of the reinforcing layer 5 is more than 0.9 times and less than 1.2 times the above mentioned thickness W7a at the upper end 7a, wherein the point 7c is the point at which height H7c is 0.9 times the height H5a of the upper edge 5a of the inner portion 5A and the thickness of the portion inwards of the inner portion 5A is nearly the same as the thickness of the portion inwards of the carcass main portion 4A. If the difference Wd is made less than 0.5 times the thickness W7a then the difference is too small, and it becomes impossible to keep a proper thickness for the upper portion 7A when the thickness of the lower portion 7B is set in a proper range. That is, the thickness of the upper portion 7A becomes too thick, and thus the tyre weight and cost are undesirably increased. On the other hand, if the difference Wd is more than 1.0 times then the difference is so large that generation of stress can not be prevented, and further the upper part 7A becomes too thin when the lower part 7B is set at a proper thickness. Furthermore, in this embodiment, the thickness W5a from the inner face of the inner liner 6 at the upper edge 5a of the inner portion 5A to the cords of the reinforcing layer 5 is between 2.0 mm and 4.5 mm. By setting the thickness W5a in such a range rubber separation at the upper edge 5a is effectively prevented, and at the same time, airtightness is further improved. If W5a is less than 2.0 mm, the airtightness and reinforcement become poor, and if it is more than 4.5 mm, the rubber thickness becomes so large that the tyre has less flexibility and tyre cost and weight become increased. Incidentally, an easy method to make the tyre according to the present invention is to change the shape of the bladder which is used in the tyre vulcanising process. The bead durability of test tyres was evaluated through a running test using a drum tester under the following test conditions: The test results are indicated in Table 1 using an index based on the assumption that the measured value for the Embodiment tyre 1 is 100. The higher the index then the better the durability. It is apparent from the test results that the Embodiment tyres were improved in bead durability. As described above, in the tyre according to the present invention the rubber separation at the upper edge of the carcass turned up portion is prevented by forming the axially inner portion on the reinforcing layer, and the rubber separation at the upper edge of this axially inner portion of the bead reinforcing layer is effectively prevented by forming the thickness varying portion on the liner at the position radially outward of the upper edge of the axially inner portion of the reinforcing layer.

Claims: 1. A radial tyre for heavy duty vehicles comprising, a pair of bead cores (3) disposed one in each bead portion, a radial carcass (4) having radially arranged cords (4a) and turned up at the edge portions around the bead cores (3) from the axially inside to the axially outside thereof to form axially outward turnup portions (4B) and an axially inward main portion (4A) therebetween, a bead reinforcing cord layer (5) comprising an axially inner portion (5A) and an axially outer portion (5B) the axially inner portion (5A) extending radially outwardly from the bead base line L along the inside of the carcass main portion (4A), and the axially outer portion extending radially outwardly from the bead base line L along the outside of the carcass turned up portion (4B) and an inner liner (6) disposed on the inside of the carcass main portion (4A) and the inner portion (5A) of the bead reinforcing layer (5) characterised in that in each bead region the inner liner (6) comprises an upper portion (7A), a middle portion (7C) connected at the upper end thereof to the upper portion (7A) and a lower portion (7B) connected with the lower end of the middle portion (7C) wherein the thickness of the middle portion (7C) gradually decreases radially outwardly of the tyre towards the upper portion 7A, the height H7b of the lower end (7B) of the middle portion (7C) of the inner liner (6) from the bead base line L is not smaller than 1.2 times the height H5a of the upper edge (5a) of the axially inner portion (5A) of the reinforcing layer (5) from the bead base line L, the height H7a of the upper end (7a) of the middle portion (7C) of the inner liner (6) from the bead base line L is not smaller than 1.4 times the said height H5a, the difference Wd between the thickness W7a from the inner face of the inner liner (6) at the upper end (7a) of the middle portion (7C) to the carcass cords (4a) and the thickness from the inner face of the inner liner (6) at the lower end (7b) of the middle portion (7B) to the carcass cords (4a) is between 0.5 and 1.0 times the above thickness W7a from the inner face of the inner liner (6) at the upper end (7a) of the middle portion to the carcass cords, and the thickness W7c from the inner face of the inner liner (6) at a point (7c) in the lower portion (7B), whose height H7c from the bead base line L is 0.9 times the height (H5a) of the upper edge (5a) of the axially inner portion (5A) of the reinforcing layer (5) from the bead base line L to the cords of the reinforcing layer (5) is between 0.9 and 1.2 times the above thickness W7a from the inner face of the inner liner (6) at the upper end (7a) of the middle portion (7C) to the carcass cords (4a). 2. A tyre according to claim 1 characterised in that the height H4c of the upper edge (4c) of the carcass turnup portion (4B) from the bead base line L is larger than the height H5b of the upper edge (5b) of the axially outer portion (5B) of the bead reinforcing layer (5). 3. A tyre according to claim 1 or 2 characterised in that the thickness W5a from the inner face of the inner liner (6) at the upper edge (5a) of the inner portion (5A) of the bead reinforcing layer (5) to the cords of the reinforcing layer (5) is more than 2.0 mm and less than 4.5 mm.

Inventors: OCHIAI, KIYOSHI
Applicants: SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1989
Main classification: B60C15
IPC codes: B60C5, B60C15