Heavy-duty radial tyre

Abstract: A heavy duty radial tyre comprises a tread portion (2), a pair of sidewall portions (3), a pair of bead portions (4), a carcass (6) extending between the bead portions (4) and composed of a single ply (6A) of steel cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator (C), and a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and composed of two crossed plies (7A,7B) of steel cords laid parallel to each other at an angle of from 0 to 45 degrees with respect to the tyre equator (C), wherein in at least a central part (Y) of the tread portion (2) having 50% of the axial width of the belt, the total of steel amounts (MB1,MB2) of the belt plies being in the range of from 6 to 10 times a steel amount (MC) of the carcass ply, and the thickness (T) of rubber between the radially adjacent belt cords and carcass cords being in the range of from 0.7 to 3.0 mm. Preferably, the two belt plies (7A,7B) have substantially the same steel amount in the central part (Y).

Description: The present invention relates to a radial tyre for a heavy duty vehicle, more particularly to an improved reinforcing structure including a carcass and belt capable of reducing tyre weight without deteriorating other performances such as tyre strength, durability and the like. In general, radial tyres for heavy-duty vehicles such as trucks, buses and the like are provided on the radially outside of the carcass with a steel breaker (A) composed of four plies A1-A4 of steel cords as shown in Fig.6. As to the cord angles of the four plies, the plies are arranged so that the cords form a stiff triangulated construction. Thus, the breaker is very rigid and the tyre is provided with the required strength. On the other hand, nowadays, in connection with environmental issues, especially air pollution control, resource saving, waste disposal and the like, automobile tyres are required to have reduced tyre weight, use less materials and the like. If the number of breaker plies is merely decreased, naturally, the rigidity and strength become insufficient, and the steering stability decreases. For example, if the breaker ply number is decreased to two, in order to obtain the above-mentioned required strength, it is necessary to increase the cord thickness and cord count in each ply, and the resultant breaker renders the tyre weight heavier than ever against expectation. In such a breaker, further, as the rigidity difference between the breaker ply and the adjoining carcass ply is too large, separation failure is liable to occur and the high-speed durability decreases. US-A-3 861 439, on which the pre-characterizing portion of claim 1 is based, discloses a lightweight heavy-duty radial tyre that is formed with at least one carcass ply and at least two tread plies. The tensile strength T, which represents the amount of steel present in a ply, expressed in kilograms, of the plies per centimetre of ply width is reduced to a value between 5 and 15 times the inner radius R, expressed in centimetres, of the tire in the median plane. This tyre is in weight, strength, steering stability, and high-speed durability not yet optimal. It is therefore, an object of the present invention to provide a heavy duty radial tyre, in which the tyre weight is reduced without deteriorating the strength (plunger strength), steering stability, high-speed durability and the like. This object is attained by a tyre of the above mentioned kind which additionally comprises the features set out in the characterizing portion of claim 1. Preferably, the two belt plies comprise substantially the same amount of steel. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings in which: Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is an enlarged cross sectional view showing the tread portion of Fig.1; Fig.3 is a schematic perspective view for explaining the amount of steel in its ply; Fig.4 is a schematic cross sectional view for explaining the rubber thickness between the carcass cords and belt cords; Fig.5 is a diagram showing a relationship between the rubber thickness and the plunger strength; and Fig.6 is a cross sectional view showing a belt structure according to prior art. In Figs.1 and 2, a heavy duty radial tyre 1 according to the present invention comprises a tread portion 2, a pair of sidewall portions 3, a pair of bead portions 4 each with a bead core 5 therein, a toroidal carcass 6 extending between the bead portions 4, and a belt 7 disposed radially outside the carcass 6 in the tread portion 2. In this embodiment, the tyre 1 is a belted radial tyre for trucks and buses. The carcass 6 consists of a single ply 6A extending between the bead portions 4 through the tread portion 2 and sidewall portions 3 and turned up around the bead core 5 in each of the bead portions 4 from the inside to the outside of the tyre. The carcass ply 6A is made of steel cords arranged radially at an angle of from 70 to 90 degrees (usually and in this example 90 degrees) with respect to the tyre equator C. The belt 7 is composed of radially inner and outer plies 7A and 7B. The belt width W, which is defined as the maximum width WA of the wider ply, is set in the range of from 0.80 to 0.95 times (in this example 0.90 times) the tread width TW or the ground contacting width. The difference between the width WB of the narrow belt ply and the width WA of the wide belt ply is set in the range of from 4 to 10 mm in order to avoid stress concentration. In this example, the inner belt ply 7A is wider than the outer belt ply 7B. Each of the belt plies 7A and 7B is made of steel cords laid parallel with each other at an angle of 0 to 45 degrees, preferably 10 to 30 degrees with respect to the tyre equator C. The cord inclination directions of the two plies are reversed with respect to the tyre equator C. In at least a central tread region Y, the total MB (=MB1+MB2) of the amount of steel MB1 of the inner belt ply 7A and the amount of steel MB2 of the outer belt ply 7B is set in the range of from 6 to 10 times the amount of steel MC of the carcass ply 6A. Here, the central tread region Y is defined as a central part of the tread portion having a width equal to 50% of the belt width W. Basically, the "steel amount" of a ply is, as shown in Fig.3, defined as a quotient V/K of the total volume V of the steel of the cords existing in a small unit 10 of the ply having a unit area K (= L1 X L1) which is divided by this unit area K. When the "steel amount" is constant along the tyre axial direction in the above-mentioned central tread region Y, this basic definition based on a small unit 10 can be conveniently used. However, if the "steel amount" varies in the central tread region Y, a part corresponding to the whole region Y is used instead of a small unit 10. Usually, the steel amount MC of the carcass ply 6A is set in the range of from 0.09 to 0.17 mm. In this case, accordingly, the total steel amount MB of the two belt plies is in the range of about 0.54 to 1.7 mm, but preferably it is set in a narrower range of 0.76 to 1.27 mm. Further, it is preferable that the two belt plies 7A and 7B have substantially the same steel amount (i.e MA=MB) and further the same structure in respect of the cord structure, cord count and cord angle(numerically). In the present invention, further, in order to enhance the plunger strength, the thickness T of rubber between the carcass cords 12 of the carcass ply 6A and the belt cords 11 of the radially adjacent inner belt ply 7A is increased to a value in the range of from 0.7 to 3.0 mm in at least the above-mentioned central tread region Y. Thereby, if a large force acts on a relatively narrow spot in the tread portion, the force can be dispersed and mitigated. Further, the carcass less constricts the belt which allows the belt cords 11 to move somewhat with respect to each other. As a result, the plunger strength can be further improved. As shown in Fig.5 which shows a relationship between the rubber thickness T and the plunger strength, when the rubber thickness T is less than 0.7 mm, the plunger strength is not improved. If the thickness T exceeds 3.0 mm, the tyre weight and tyre cost increase. If the steel amount MB is less than 6 times the steel amount MC, the plunger strength greatly decreases. If the steel amount MB is more than 10 times the steel amount MC, the tyre weight increases. In order to provide the above-mentioned rubber thickness T, it is possible to increase the thickness of topping rubber for the carcass ply 6A and/or the inner belt ply 7A. It is however, preferable to dispose a separate rubber layer 15 between the carcass ply 6A and the belt ply 7A. The thickness T1 thereof is usually set in the range of 1.0 mm plus/minus 0.5 mm. The hardness of this rubber layer 15 is adjusted to be substantially the same value as the topping rubber if the carcass topping rubber and the belt topping rubber are the same hardness. If they are different from each other, the mean value can be used. In respect of the tyre manufacturing process, a strip of vulcanised rubber is preferably used as the rubber layer 15 during building a raw tyre in order to maintain an accurate thickness. For example, a strip having a width corresponding to the above-mentioned central tread region Y at least is wound around the carcass crown and the belt is applied thereon. Comparison tests: Test tyres of size 11R22.5 having the structure shown in Fig.1 and the specifications shown in Table 1 were tested for plunger strength, high-speed durability and tyre weight. Plunger strength Test: According to Japanese Industrial Standard D4230, the breaking energy (plunger strength of the tread portion) was measured. The measured breaking energy is indicated in Table 1 by an index based on the conventional belt structure being 100. The larger the index, the higher the strength. Wheel rim: 7.50X22.5 standard rim Inner pressure: 700 kpa High-speed durability Test : Using a tyre test drum, the tyre was run until any visible damage was observed. The running speed was increased every three hours by a step of 10km/h. In Table 1, the total running distance is indicated by an index based on the conventional belt structure being 100. The larger the index, the better the durability. Initial speed: 80 km/h Load: 4000 kgf Wheel rim: 7.50X22.5 standard rim Inner pressure: 850 kpa, Tyre weight: The tyre weight is indicated in Table 1 by an index based on the tyre having the conventional belt structure being 100. The larger the index, the lighter the tyre weight. As described above, the heavy duty radial tyres according to the present invention can be improved in the tyre weight, high-speed durability, plunger strength, steering stability and the like, though the number of belt ply is decreased.

Claims: A heavy duty radial tyre comprising a tread portion (2), a pair of sidewall portions (3), a pair of bead portions (4), a carcass (6) extending between the bead portions (4) and composed of a single ply (6A) of steel cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator (C), and a belt (7) disposed radially outside the carcass (6) in the tread portion (2) and consisting of two crossed plies (7A,7B) of steel cords laid parallel with each other at an angle of from 0 to 45 degrees with respect to the tyre equator (C), characterised in that in at least a central part (Y) of the tread portion (2) having 50% of the axial width of the belt, the total of steel amounts (MB1,MB2) of said two belt plies is in the range of from 6 to 10 times a steel amount (MC) of the carcass ply (6A), wherein "steel amount" of a ply is defined as a quotient (V/K) of the total volume (V) of the steel of the cords existing in a small unit (10) of the ply having a unit area (K) divided by this unit area (K), and the thickness (T) of rubber between the radially adjacent belt cords and carcass cords is in the range of from 0.7 to 3.0 mm. A heavy duty radial tyre according to claim 1, characterised in that the two belt plies (7A,7B) have substantially same steel amount in said central part (Y). A heavy duty radial tyre according to claim 2, characterised in that the two belt plies (7A,7B) are of substantially the same cord count, and the same cord structure and like cords have substantially the same inclination angle numerically.

Inventors: UEYOKO KIYOSHI, UEYOKO, KIYOSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C9
IPC codes: B60C9