Pneumatic radial tyres

Abstract: A heavy duty pneumatic radial tire has an outer profile of a tread portion in radial section of the tire having a given composite arc shape, and reduces interlaminar shearing strain and tensile strain at ends of the belt layers to prevent the occurrence of belt end separation. Specifically, the outer profile of the tread portion in the radial section is a composite arc shape obtained by (1) forming a region between point C and point P₀ with an arc of radius R₀ or forming a region from the point C to an optional point P between the point C and the point P₀ with an arc of the radius R₀ and forming a region between the point P and the point P₀ with a straight line, (2) forming a region between the point P₀ and a point P₁ with an arc of a radius R₁ smaller than the radius R₀ tangent to the arc of the radius R₀ at the point P₀, and (3) forming a region between the point P₁ and the respective side edge of the tread portion with an arc of the radius R₁ and connecting to a buttress line of the tread portion at the side edge or through an arc of a radius smaller than the radius R₁.

Description: This invention relates to pneumatic tires, and more particularly to heavy duty pneumatic radial tires for vehicles such as dump trucks, loaders, graders and so on. When considering the design of a conventional heavy duty pneumatic tire, an outer profile of a tread crown portion is first designed from a viewpoint that distribution of ground contact pressure and distribution of heat build-up over the whole of the tread are made as uniform as possible, and then a mold for the vulcanization of the tire is designed, and thereafter an internal structure of the tire for satisfying different requirements in accordance with applications of the tire is designed assuming the use of such a mold. That is, the conventional heavy duty pneumatic tire is designed from a viewpoint of ground contact pressure distribution and heat build-up distribution as mentioned above. Therefore, as shown in Fig. 1 of the accompanying drawings, the tread crown portion has an outer profile such that the outer profile line extends from a widthwise center point C of a tread portion toward each side end E thereof up to a shoulder portion at a single and large crown radius R and then connects to a buttress line at the side end E to form a square shoulder or in the vicinity of the side end E through a somewhat small shoulder radius r to form a round shoulder. The crown radius R is frequently determined by calculating a ratio to a section width of the tire or a ratio of fall d of the side end E in the tread to an outer diameter D of the tire from commercially available existing tires. Although the conventional heavy duty pneumatic tires are designed based on the above idea and technique, there is no consideration of designing the outer profile of the tread crown portion from a viewpoint that belt durability is previously enhanced by reducing strain produced at an end of a cross belt as far as possible. As a result, there is naturally a limit to enhancing the belt durability even if the structure of the cross belt is variously changed. In the heavy duty pneumatic radial tire, the cross belt is frequently comprised of at least two main cross belt layers each containing substantially inextensible steel cords, which have a function of bearing circumferential tension, and at least one belt protection layer containing high-extensible steel cords having an elongation at break of 4.5-8%. Even in this cross belt, belt separation is apt to be caused at an end portion between the main cross belt layers due to interlaminar shearing strain and at a free end portion of the belt protection layer due to tensile strain in the cord direction. Particularly, as the tire becomes flatter, the strain produced at the end portion of the belt becomes larger, so that there are frequently observed failures of the belt portion due to belt separation and the like before the service life of the tire itself is ended by complete wearing of the tread rubber. To this end, it is strongly demanded to enhance the belt durability by reducing the strain produced at the end portion of the belt as far as possible. However, when the outer profile of the tread crown portion is previously designed, there is a limit for designing the internal structure of the tire so as to satisfy requirements for the enhancement of the belt durability. It is, therefore, an aim of the present invention to provide a pneumatic radial tire having an improved belt durability, particularly a heavy duty pneumatic radial tire comprising at least one carcass ply of radially arranged steel cords and a cross belt of at least two belt layers inclusive of main cross belt layers comprised of steel cords and having an excellent belt durability. That is, the inventions aims to provide a heavy duty pneumatic radial tire having an improved belt durability by adequately designing the outer profile of a tread crown portion having regard to the internal structure of the tire, particularly the belt structure. According to the invention, there is provided a pneumatic radial tire comprising a radial carcass comprised of at least one rubberized cord layer, a belt comprised of at least two main cross belt layers and at least one protection belt layer and a tread portion, in which the belt has a maximum width corresponding to 60-98% of a tread width, and the main cross belt layers contain a plurality of substantially inextensible cords arranged at a cord angle of 15-30° with respect to an equatorial plane of the tire, the cords of which layers being crossed with each other with respect to the equatorial plane, characterized in that: in a radial section of the tread portion inclusive of a rotational axis of the tire, when the widthwise central point of the outer profile of the tread portion is C, a first point of intersection of the outer profile of the tread portion with a normal line drawn from the rotational axis of the tire and passing through an end of a main cross belt layer having a second wide width among the main cross belt layers is P 0 and a second point of intersection of the outer profile of the tread portion with a normal line drawn from the rotational axis of the tire and passing through the end of the belt layer having the maximum belt width is P 1 , then the point P 0 is located at a region such that a first height (h 0 ) obtained by subtracting a radius of the point P 0 from 1/2 of an outer diameter D of the tire is within a range corresponding to 0.45-0.55% of the outer diameter D, and the point P 1 is located at a region such that a second height (h 1 ) obtained by subtracting a radius of the point P 1 from 1/2 of the outer diameter D is within a range corresponding to 135-200% of a third height (h 2 ), said third height (h 2 ) being obtained by subtracting a radius of a third point of intersection of an arc with a radius R 0 with center on the equatorial plane of the tire and passing through the point P 0 and the point C with the normal line passing through the point P 1 ; and the outer profile of the tread portion in the radial section is a composite arc shape obtained by (1) forming a region between the point C and the point P 0 with an arc of the radius R 0 or forming a region from the point C to an optional point P between the point C and the point P 0 with an arc of the radius R 0 and forming a region between the point P and the point P 0 with a straight line, (2) forming a region between the point P 0 and the point P 1 with an arc of a radius R 1 smaller than the radius R 0 tangent to the arc of the radius R 0 at the point P 0 , and (3) forming a region between the point P 1 and the respective side edge of the tread portion with an arc of the radius R 1 and connecting to a buttress line of the tread portion at the side edge or through an arc of a radius smaller than the radius R 1 . In preferred embodiments of the invention, the protection belt layer contains high-extensible steel cords having an elongation at break of 4.5-8%, and the region between the point P and the point P 0 is within a range corresponding to 0-0.4 times the tread width. The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic radial-half section view of a conventional heavy duty pneumatic tire; Fig. 2 is a diagrammatic radial-half section view of an embodiment of a heavy duty pneumatic radial tire according to the invention; and Figs. 3 to 6 are schematic views illustrating various embodiments of belt structures used in the invention, respectively. In heavy duty pneumatic radial tires, breaking failures of the belt by belt end separation and the like are frequently observed at the end portions of the main cross belt layers, which mainly result from interlaminar shearing strain produced at the end portions of the main cross belt layers as previously mentioned. According to the invention, the outer profile of the tread portion is rendered into a composite arc shape, so that the first height h 0 of the outer profile of the tread portion at the first intersection point P 0 corresponding to the end of the main cross belt layer is made smaller than that of the conventional tire, whereby the relative movement of the main cross belt layers can be controlled to reduce the interlaminar shearing strain produced at the end portion of the main cross belt layer. Also, breaking failure of the belt is observed at a free end of the belt layer having a maximum width, which mainly results from tensile strain in the cord direction of the belt layer. According to the invention, the second height h 1 of the outer profile of the tread portion at the second intersection point P 1 corresponding to the free end of the belt layer having the maximum width is made larger than that of the conventional tire, whereby the ground contact pressure can be decreased to reduce the tensile strain. When the first height h 0 at the point P 0 exceeds 0.55% of the outer diameter D of the tire, the effect of reducing the interlaminar shearing strain produced at the end of the main cross belt layer is less, while when it is less than 0.45%, the ground contact pressure in the vicinity of the point P 0 becomes high and other failure such as uneven wear or the like is undesirably caused. When a region L between the optional point P and the point P 0 exceeds 0.4 times the tread width TW, if the arc with the radius R 1 satisfying the second height h 1 is depicted, the arc with the radius R becomes strained. Even when the region L is zero, the first height h 0 can be held at a given level. When the second height h 1 at the point P 1 is less than 135% of the third height h 2 of a phantom extended line of the arc with the radius R 0 , the effect of reducing the tensile strain in the cord direction at the free end of the belt layer is less, while when it exceeds 200%, the region near to each side end E of the tread portion does not contact with the ground. In Fig. 2 is shown an embodiment of a heavy duty pneumatic radial tire according to the invention. This tire has a tire size of 50/65R51 and an outer diameter D of 3060 mm and a tread width TW of 1225 mm. In the illustrated embodiment, a belt is comprised of six belt layers B 1 , B 2 , B 3 , B 4 , B 5 and B 6 , in which the belt layers B 1 , B 2 , B 3 and B 4 are main cross belt layers and the belt layers B 5 and B 6 are protection belt layers, respectively. In each of the main cross belt layers are used steel cords of (7x7)x0.21+1 structure (tensile strength: 400 kgf/cord, elongation at break: 2.5%) at an end count of 20 cords/5 cm, while steel cords of (3x7)x0.23 structure (tensile strength: 170 kgf/cord, elongation at break: 6.8%) are used at an end count of 18 cords/5 cm in each of the protection belt layers. The main cross belt layers B 1 , B 2 , B 3 and B 4 have cord angles of 22° upward to the right, 22° upward to the left, 22° upward to the right and 22° upward to the left with respect to the equatorial plane of the tire, and widths of 830 mm, 870 mm, 1000 mm and 850 mm, respectively. Among these main cross belt layers, a maximum cross cord width is located between the adjoining belt layers B 2 and B 3 in the illustrated embodiment and is 870 mm, which corresponds to 71% of the tread width TW. That is, the belt layer B 2 has a second wide width among the main cross belt layers. The protection belt layers B 5 and B 6 have cord angles of 25° upward to the left and 25° upward to the right with respect to the equatorial plane, and widths of 960 mm and 1150 mm, respectively. Among these belt layers, the belt layer B 6 has a maximum belt width of 1150 mm, which corresponds to 94% of the tread width TW. The first height h 0 of the outer profile of the tread portion at the point P 0 is 16.6 mm, which corresponds to 0.543% of the outer diameter D (= 3060 mm) of the tire, so that the radius R 0 connecting the point C to the point P 0 is 7200 mm. The third height h 2 at the third point of intersection of a phantom extended line of the arc with the radius R 0 (= 7200 mm) with the normal line drawn from the rotating axis of the tire passing through the free end of the belt layer having the maximum width is 22.0 mm, while the second height h 1 of the outer profile of the tread portion at the point P 1 is 31.0 mm, which corresponds to 141% of the third height h 2 , so that the radius R 1 connecting the point P 0 to the point P 1 is 500 mm. A distance L from the point P 0 to a point P constituting the straight line of the outer profile between the point C and the point P 0 is 300 mm, which corresponds to 69% of a half width of the main cross belt layer B 2 . In the illustrated embodiment, the outer profile of the tread portion in the radial section of the tire is a composite arc shape such that the region between the point C and the point P is formed with an arc of the radius R 0 = 7200 mm, the region between the point P and the point P 0 is formed with a straight line, the region between the point P 0 and the respective side end E of the tread portion through the point P 1 is formed with an arc of the radius R 1 = 500 mm and connects at the side end E to a buttress line to form a square shoulder. The main cross belt layers are required to satisfy the following features (1)-(4): (1) They are comprised of at least two rubberized cord layers; (2) The material of the cord is substantially inextensible; (3) The cords of the layers are crossed with each other with respect to the equatorial plane of the tire; and (4) The cord angle inclined with respect to the equatorial plane is 15-30°. In this connection, the belt layers B 5 and B 6 do not satisfy the above requirement (2) though the cords of these layers are crossed with each other, so that they are not the main cross belt layers. The "end of the main cross belt layer having a second wide width" used herein means an end of the main cross belt layer having a maximum width of crossed cord region between adjoining belt layers among the belt layers satisfying all of the above requirements (1)-(4). Such an end of the main cross belt layer is explained with reference to Figs. 3-6. When the main cross belt layers are a pair of belt layers as shown in Fig. 3, the end of the main cross belt layer is not BE 1 but is BE 2 because cords are not crossed at a region between BE 1 and BE 2 . When the main cross belt layers are comprised of three belt layers as shown in Fig. 4, the end of the main cross belt layer is not BE 1 or BE 3 but is BE 2 . When the main cross belt layers are comprised of four belt layers as shown in Fig. 5, the end of the main cross belt layer is not BE 1 , BE 3 or BE 4 but is BE 2 . When the main cross belt layers are comprised of four belt layers as shown in Fig. 6, the end of the main cross belt layer is not BE 1 , BE 2 or BE 4 but is BE 3 . As a comparative example, there is used the conventional heavy duty pneumatic radial tire shown in Fig. 1. In this case, the outer profile of the tread portion is a single arc shape such that a region ranging from the point C toward each side end E of the tread portion is formed with an arc of a radius R = 5000 mm and connects to the buttress line through an arc of a radius r = 1700 mm to form a round shoulder. The tire size and the internal structure of the conventional tire are the same as in the above illustrated embodiment. Then, the interlaminar shearing strain and tensile strain at the end of the belt layer are evaluated with respect to the above invention tire and the conventional tire under the same conditions to obtain results as shown in Table 1. Conventional tire Invention tire First height at end of main cross belt layer (h 0 ) 18.1 mm = D × 0.592% 16.6 mm = D × 0.543% Interlaminar shearing strain at end of main cross belt layer 39.1% (100) 36.5% (93) Second height at end of belt layer having maximum belt width (h 1 ) 32.1 mm = h 2 × 120% 31.0 mm = h 2 × 141% Tensile strain at end of belt layer having maximum belt width +0.541% (stretching) -0.116% (compression) As seen from Table 1, in the heavy duty pneumatic radial tire according to the invention, the interlaminar shearing strain at the end of the main cross belt layer and the tensile strain at the end of the belt layer having the maximum belt width are considerably reduced as compared with those of the conventional heavy duty pneumatic radial tire. In the heavy duty pneumatic radial tire according to the invention, the outer profile of the tread portion has the composite arc shape as defined above, so that the first height h 0 of the outer profile at the end of the main cross belt layer is small as compared with that of the conventional tire and hence the relative movement between the main adjoining cross belt layers B 2 and B 3 can be lessened to reduce the breaking failure of the belt frequently observed at the ends of the main cross belt layer B 2 , B 3 such as belt end separation or the like. Furthermore, the second height h 1 of the outer profile at the free end of the belt layer B 6 having the maximum belt width is large as compared with that of the conventional tire, so that the ground contact pressure in the vicinity of this free end can be decreased to reduce the tensile strain at the free end to thereby reduce the breaking failure of the belt frequently observed at the free end of the belt layer such as belt end separation or the like.

Claims: A pneumatic radial tire comprising a radial carcass comprised of at least one rubberized cord layer, a belt comprised of at least two main cross belt layers (B 1 , B 2 , B 3 , B 4 ) and at least one protection belt layer (B 5 , B 6 ), and a tread portion, in which the belt has a maximum width corresponding to 60-98% of a tread width (TW), and the main cross belt layers contain a plurality of substantially inextensible cords arranged at a cord angle of 15-30° with respect to an equatorial plane of the tire, the cords of which layers being crossed with each other with respect to the equatorial plane, characterized in that: in a radial section of the tread portion inclusive of a rotational axis of the tire, when the widthwise central point of the outer profile of the tread portion is C, a first point of intersection of the outer profile of the tread portion with a normal line drawn from the rotational axis of the tire and passing through an end of a main cross belt layer having a second wide width among the main cross belt layers is P 0 , and a second point of intersection of the outer profile of the tread portion with a normal line drawn. from the rotational axis of the tire and passing through the end of the belt layer having the maximum belt width is P 1 , then the point P 0 is located at a region such that a first height (h 0 ) obtained by subtracting a radius of the point P 0 from 1/2 of an outer diameter D of the tire is within a range corresponding to 0.45-0.55% of the outer diameter D, and the point P 1 is located at a region such that a second height (h 1 ) obtained by subtracting a radius of the point P 1 from 1/2 of the outer diameter D is within a range corresponding to 135-200% of a third height (h 2 ), said third height (h 2 ) being obtained by subtracting a radius of a third point of intersection of an arc with a radius R 0 with center on the equatorial plane of the tire and passing through the point P 0 and the point C with the normal line passing through the point P 1 ; and the outer profile of the tread portion in the radial section is a composite arc shape obtained by (1) forming a region between the point C and the point P 0 with an arc of the radius R 0 or forming a region from the point C to an optional point P between the point C and the point P 0 with an arc of the radius R 0 and forming a region between the point P and the point P 0 with a straight line, (2) forming a region between the point P 0 and the point P 1 with an arc of a radius R 1 smaller than the radius R 0 tangent to the arc of the radius R 0 at the point P 0 , and (3) forming a region between the point P 1 and the respective side edge of the tread portion with an arc of the radius R 1 and connecting to a buttress line of the tread portion at the side edge or through an arc of a radius smaller than the radius R 1 . A pneumatic radial tire as claimed in claim 1, characterized in that the protection belt layer contains high-extensible steel cords having an elongation at break of 4.5-8%. A pneumatic radial tire as claimed in claim 1 or 2, characterized in that the region between the point P and the point P 0 is within a range corresponding to 0-0.4 times the tread width.

Inventors: ANDO SHUJI, HONBO YOICHI, ANDO, SHUJI, HONBO, YOICHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C3
IPC codes: B60C9, B60C11, B60C3