Pneumatic tires

Abstract: A pneumatic tire comprising a belt (3) and a tread (4) disposed radially outside of a toroidal carcass (2) extending between a pair of beads (1), wherein the belt (3) comprises at least one circumferential belt layer (6a, 6b) having rubber coated reinforcing elements extending substantially along the circumferential direction of the tire and at least one slant belt layer (5a, 5b) having rubber coated reinforcing elements extending slantwise with respect to the tire equatorial plane, wherein at least one said slant belt layer (5a, 5b) is protruded radially outwardly at the center portion (C) in the widthwise direction of the belt with respect to its both side portions (S).

Description: This invention relates to pneumatic tires, especially to tires where a carcass is reinforced by a circumferential belt layer comprising reinforcing elements extending in straight, waved, or zigzag manner substantially along the tire circumference. There are known many ways of strengthening circumferential rigidity of a belt in pneumatic tires by adding a circumferential belt layer to the conventional slant belt layer having reinforcing elements extending slantwise with respect to the equatorial plane of the tire. For example, Japanese Patent Laid Open No. 60-22504 discloses a tire having the circumferential belt layer comprising reinforcing elements having a Young's modulus of not less than 2500kgf/mm 2 , whereby the circumferential rigidity is enhanced and separation failure at the belt ends is prevented. Since the elements of the circumferential belt layer are disposed substantially along the circumferential direction of the tire, the elements are tensed and lengthened in the circumferential direction in the ground contacting area. In particular, since the diameter of both tread side portions is less than that of the tread center portion, the diameter of both tread side portions is necessarily expanded, so that the reinforcing elements are considerably lengthened. That is, since the reinforcing elements at the side end portions of the circumferential belt layer are lengthened repeatedly in the circumferential direction during rotation of the tire, the reinforcing elements are apt to be broken at the side end portions of the circumferential belt layer, which is a disadvantage of a belt provided with a circumferential belt layer. An object of the present invention is to prevent the disadvantage caused by providing a circumferential belt layer, in particular breaking of the elements at the edge portions of the circumferential belt layer, and to provide a pneumatic tire having a strengthened circumferential rigidity. The present invention provides a pneumatic tire comprising a belt and a tread disposed radially outside of a toroidal carcass extending between a pair of beads, wherein the belt comprises at least one circumferential belt layer having rubber coated reinforcing elements extending substantially along the circumferential direction of the tire and at least one slant belt layer having rubber coated reinforcing elements extending slantwise with respect to the tire equatorial plane, and wherein at least one said slant belt layer is protruded radially outwardly at the center portion in the widthwise direction of the belt with respect to both its side portions, and the center portion is protruded with respect to the side portions in the radial direction by a distance equal to or larger than the diameter of the reinforcing elements of the slant belt layer. In one preferred embodiment of the invention, the circumferential belt layer(s) are disposed on both side portions of the slant belt layer(s) except for the center portion in the widthwise direction. In another preferred embodiment of the invention, the belt includes at least two circumferential belt layers, and at least one radially inner circumferential belt layer is disposed on both side portions of the slant belt layer(s) except for the center portion in the widthwise direction while at least one radially outer circumferential belt layer is disposed continuously along the widthwise direction. The reinforcing elements of the circumferential belt layer can be wave-shaped or zigzag-shaped cord or monofilament as well as a conventional straight cord or monofilament that is disposed in the circumferential direction, and can be a spirally wound ribbon comprising a plurality of cords or monofilaments coated with rubber. The invention will be further described by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a widthwise cross-section of a pneumatic tire according to this invention; Fig. 2 is an enlarged cross-section of a belt structure according to this invention; Fig. 3 is an enlarged cross-section of another belt structure according to this invention; and Fig. 4 is a cross-section of a conventional belt structure. Fig. 1 is a cross-section of a typical tire according to this invention. This pneumatic tire comprises a carcass 2 extending between a pair of beads 1, a belt 3 comprising at least one slant belt layer 5a, 5b and at least one circumferential belt layer 6a, 6b, and a tread 4. In this particular embodiment, the belt 3 consists of two slant belt layers 5a, 5b disposed outside of the carcass 2 and two circumferential belt layers 6a, 6b disposed on both side portions of the slant belt layers except for the center portion in the widthwise direction. The slant belt layers 5a, 5b are disposed such that the center portion C in the widthwise direction is projected radially outwards with respect to the remaining side portions S thereof. In other words, the diameter of the slant belt layers varies in the widthwise direction such that the diameter of the center portion C is larger than that of an imaginary line connecting both side portions S. The center portion C of the slant belt layers is a region that extends from the mid-equatorial plane of the tire toward each side by 10 - 40% of the maximum width of the slant belt layers, and a part or the whole of this center portion is disposed outside of the side portions S. In this embodiment, two slant belt layers are thus projected radially outwardly at the center portion, but according to the invention only one such slant belt layer can be thus projected. The belt 3 is not limited to the structure as shown in Fig. 1 and may have a structure as shown in Fig. 2 or Fig. 3. Fig. 2 shows a belt consisting of a single circumferential belt layer 6 disposed on the side portions S of the slant belt layers 5a, 5b. Fig. 3 shows a belt consisting of slant belt layers 5a, 5b protruding outwards in the radial direction at the center portion C, and circumferential belt layers 6a, 6b in which the radially outer layer 6b is disposed continuously along the widthwise direction. Since the breaking of the reinforcing elements occurring at the side end portions of the circumferential belt layer is due to the fact that the elements suffer repeated large stretching during tire rotation, it is effective for enhancing the durability to prevent breaking of the reinforcing elements to restrain the stretching concentrated at the side end portions of the circumferential belt layer. Thus, the inventor has studied means for relaxing the tension at the side end portions of the circumferential belt layer because the concentration of the tension at the side end portions of the circumferential belt layer causes stretching of the reinforcing elements at such portions. That is, the inventor carried out investigations based on the idea that, when the belt extending in the circumferential direction is considered as a beam, it is possible to relax the stretch concentrating at the reinforcing elements of the circumferential belt layer at the side end portions by changing the position of the neutral axis where neither stretch nor compression arises in lateral cross-section (cross-section in the widthwise direction of the tire). In particular, when the position of the neutral axis is shifted radially outside toward the circumferential belt layer, the stretching of the circumferential belt layer is made small. Specifically, in order to change the position of the neutral axis in the widthwise cross-section of the belt, the shape of the belt layer in the widthwise cross-section, which was considered as a beam, was studied. On the one hand, it is difficult to change the shape of the circumferential belt layer, because the circumferential belt layer is given a constraint such that the circumferential belt layer has a hooping effect function and a certain spacing between the circumferential belt layer and the bottom of a groove in the tread needs to be retained. On the other hand, it is easier to change the shape of the slant belt because of comparatively less constraint. Further, for truck or bus tires in which steel cords are usually used as reinforcing elements of the circumferential belt layer, it is effective in respect of lightening the tire to place the circumferential belt layer only at the portion necessary for the hooping effect, that is only on both side portions, and therefore the shape of the center portion of the slant belt layer where there is no circumferential belt layer thereon can be easily changed. When the slant belt layer was projected radially outwardly at the center portion and the stretch amount of the reinforcing elements at the end portions of the circumferential belt layer was measured, it was found that the stretching of the elements was much less than that of unchanged slant belt. When at least a part or the whole of the center portion C of the slant belt layer is disposed radially outwardly with respect to the side portions S, the above-mentioned effect of restraining the stretching of the reinforcing elements can be obtained. The center portion C is disposed to be protruded with respect to the side portions S in the radial direction by a distance equal to or larger than the diameter of the reinforcing elements of the slant belt layer. The belt structure of the embodiment shown in Fig. 3 has the advantage of avoiding the breaking of the reinforcing elements because the number of the reinforcing elements of steel cords in the circumferential belt layers is larger than that of the tire shown in Fig. 1 or Fig. 2, and one circumferential belt layer continuously extends in the widthwise direction of the belt, although this structure is disadvantageous in respect of lightening the tire. This embodiment is based on an idea that it is possible to largely transfer the neutral axis of the belt toward the tread by disposing the circumferential belt layer having much larger circumferential rigidity than the slant belt layer radially outside (in the tread) of the projected center portion of the slant belt layer as well as by projecting the slant belt layer. An experiment with respect to measurement of the belt stretching and a running test on a drum showed that this idea is true. On the other hand, the weight of the tire increases, resulting in the disadvantage of degrading rolling resistance and so on. Therefore, the belt structure of this embodiment is useful for enhancing durability of the belt under circumstances wherein increasing the weight of the tire is permitted. Pneumatic radial tires according to this invention (Invention tires 1, 2, 3) having a size of 11/70 R22.5 for trucks and buses and having belt structures according to Figs. 1, 2 and 3 respectively were manufactured. In each tire, the circumferential belt layer comprises wave-shaped (wave length : 36.6mm, amplitude : 1.3mm) steel cords having a structure of 3+9x0.19mm (0.8mm ⊘) at a density of 28 ends/50mm, and the slant belt layer comprises steel cords having a structure of 3+9+15x0.23mm (1.4mm ⊘) and disposed at an angle of 18° with respect to the equatorial plane at a density of 23 ends/50mm. The width of the center portion C of the slant belt layer is 70mm and the width of the side portions S is also 70mm. The whole of the center portion C is projected with respect to both side portions S by a distance in the radial direction of 3.0 mm in the tire of Fig. 1, 1.5 mm in the tire of Fig. 2, and 1.5 mm in the tire of Fig. 3. A comparable tire having a conventional belt structure as shown in Fig. 4 and having the same size was also manufactured. Drum tests on these tires inflated with regulated pressure of 8.0kgf/cm 2 were made over a distance of 50,000 km under 130% load and 50km/h, and then 1/3 of the whole circumference of the tire was dissected and the total number of broken filaments of the cords in the circumferential belt layer was measured. Table 1 shows the results obtained together with the weight of the tires by index in comparison with the comparable tire (Fig. 4) as 100 (control). Tire Number of broken cords Weight of tire Comparable tire Fig. 4 43 100 Invention tire 1 Fig. 1 0 100 Invention tire 2 Fig. 2 2 97 Invention tire 3 Fig. 3 0 104 According to this invention, high performance tires can be obtained because the circumferential rigidity can be enhanced by adding a circumferential belt layer without occurrence of breaking of reinforcing elements in the side end portions of the belt layer.

Claims: A pneumatic tire comprising a belt (3) and a tread (4) disposed radially outside of a toroidal carcass (2) extending between a pair of beads (1), wherein the belt (3) comprises at least one circumferential belt layer (6a,6b) having rubber coated reinforcing elements extending substantially along the circumferential direction of the tire and at least one slant belt layer (5a,5b) having rubber coated reinforcing elements extending slantwise with respect to the tire equatorial plane, wherein at least one said slant belt layer (5a,5b) is protruded radially outward at the center portion (C) in the widthwise direction of the belt with respect to both its side portions (S), and the center portion (C) is protruded with respect to the side portions (S) in the radial direction by a distance equal to or larger than the diameter of the reinforcing elements of the slant belt layer (5a,5b). A pneumatic tire as claimed in claim 1, characterized in that the circumferential belt layer(s) (6a,6b) are disposed on both side portions of the slant belt layer(s) (5a,5b) except for the center portion (C) in the widthwise direction. A pneumatic tire as claimed in claim 1, characterized in that the belt (3) includes at least two circumferential belt layers (6a,6b), and at least one said radially inner circumferential belt layer (6a) is disposed on both side portions of the slant belt layer(s) (5a,5b) except for the center portion (C) in the widthwise direction while at least one said radially outer circumferential belt layer (6b) is disposed continuously along the widthwise direction. A pneumatic tire as claimed in claim 1, characterized in that the belt (3) consists of two slant belt layers (5a,5b) disposed outside of the carcass (2) and two circumferential belt layers (6a,6b) disposed on both side portions (5) of the slant belt layers (5a,5b) except for the center portion (C) in the widthwise direction, and in that both said slant belt layers (5a,5b) are protruded radially outwardly at the center portion (C) in the widthwise direction of the belt with respect to both its side portions (S). A pneumatic tire as claimed in claim 1, characterized in that the belt (3) consists of two slant belt layers (5a,5b) disposed outside of the carcass (2) and a single circumferential belt layer (6) disposed on both side portions of the slant belt layers except for the center portion (C) in the widthwise direction, and in that both said slant belt layers (5a,5b) are protruded radially outwardly at the center portion (C) in the widthwise direction of the belt with respect to both its side portion (S). A pneumatic tire as claimed in claim 1, characterized in that the belt (3) consists of two slant belt layers (5a,5b) disposed outside of the carcass (2) and two circumferential belt layers (6a,6b) disposed on the slant belt layers, in that the radially inner circumferential belt layer (6a) is disposed on both side portions of the slant belt layers except for the center portion (C) in the widthwise direction while the radially outer circumferential belt layer (6b) is disposed continuously along the widthwise direction, and in that both said slant belt layers (5a,5b) are protruded radially outwardly at the center portion (C) in the widthwise direction of the belt with respect to both its side portions (S).

Inventors: NAKANO TATSUO, NAKANO, TATSUO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C9
IPC codes: B60C9, B60C3