Pneumatic radial tires

Abstract: A pneumatic radial tire, in particular a heavy duty pneumatic radial tire, comprises a radial carcass (1) toroidally extending between a pair of bead cores embedded in a pair of bead portions, a tread portion (2), and a belt (3) superimposed about an outer periphery of the carcass to reinforce the tread portion and comprised of at least three rubberized cord layers (3-1, 3-2, 3-3), cords of two adjacent layers (3-2, 3-3) among said layers being crossed with each other at an acute cord angle with respect to an equatorial plane (E) of the tire to form cross cord layers, in which an outer cord layer (3-3) of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer (3-2), and the cross cord layers (3-2, 3-3) are provided with a space cushion rubber (6) separating end portions of said layers and extending from an end of the narrow-width cord layer (3-3) toward the outside of the tire, and the space cushion rubber (6) is a rubber composite comprised of an inner rubber (6i) extending so as to separate the end portions of said layers (3-2, 3-3) from each other and an outer rubber (6u) extending from the inner rubber (6i) toward the outside of the tire and having at least two different rubber compositions, and 100% moduli Mx', M6i and M6u (kgf/cm 2 ) of a coating rubber for the narrow-width outer cord layer (3-3), inner rubber (6i) and the outer rubber (6u) have a relationship of M6u<M6i≦Mx'.

Description: This invention relates to pneumatic radial tires, and more particularly to a heavy duty pneumatic radial tire for trucks and buses in which cracks liable to be created at an end portion of a cord layer in a belt and growth thereof are controlled to suppress the occurrence of separation failure due to the crack rowth. It is well-known that as the running distance of a pneumatic radial tire mounted onto a heavy duty vehicle such as a small or large size truck or bus increases, cracks are created at a cord end of a narrow-width cord layer among cross cord layers having different widths, cords of which layers being crossed with each other at a relatively small cord angle with respect to an equatorial plane of the tire, among plural cord layers constituting the belt. The cracks initially created are minute, like "pecking" of the cord end, but such a minute crack grows to form a large crack along the cord as the running distance of the tire increases. When the crack growth progresses to a certain extent, the front end of the crack grows toward a cord end of the adjoining cord and finally cracks are connected to each other over substantially the full periphery of the cord layer along a side edge of the cord layer. If the crack growth advances to this stage, separation failure is created between the cross cord layers. Accordingly, a period ranging from the occurrence of minute cracks at the cord end to the growth of connected cracks along the side edge of the cord layer on its periphery controls the service life of the tire though belt separation failure. There have therefore been proposed various structures of the tire aiming at the prolongation of tire running time (running distance) though the connection of cracks is unavoidable. JP-A-4-183605 proposes a heavy duty pneumatic radial tire wherein a cushion rubber is arranged between end portions of second and third cross steel cord layers counted from the side of the carcass in a belt comprised of 3 to 4 steel cord layers, and an upper side of an end portion of the third layer is covered with a rubber layer having a thickness of not less than 1.5 mm so as to overlap over a distance of not less than 20 mm from the end of the layer and the rubber layer is made from rubber having a JIS hardness of 65-75 and a modulus at 300% elongation of 130-200 kgf/cm 2 . Furthermore, JP-A-4-252705 discloses a heavy duty pneumatic radial tire wherein a belt edge cushion rubber is interposed between end portions of two cross steel cord layers in a belt comprised of four steel cord layers and such end portion is covered with a rubber sheet having a thickness of 1.0-3.0 mm, and 50% modulus of the rubber sheet is smaller than that of a coating rubber for the steel cord layer but larger that that of a base tread rubber. Moreover, JP-A-6-320906 proposes a heavy duty pneumatic radial tire wherein a belt comprised of four steel cord layers is arranged between a base tread rubber of low modulus and a carcass and a cushion rubber is disposed between end portions of second and third cross cord layers counted from the side of the carcass, and the end portion is covered with a rubber sheet having a thickness of 0.8-3.3 mm likewise the case of JP-A-4-252705, and further the rubber sheet has a JIS hardness smaller than those of rubber for the second and third cord layers and 50% modulus larger than that of the base tread rubber. However, the effect of sufficiently controlling crack growth under severe service conditions of heavy loading, running speed and the like can not be attained even by any of the belt structures described above, so that belt separation failure is naturally liable to be caused even in the tires described. Therefore, even if the tire is recapped, belt separation failure is prematurely caused. Although this type of heavy duty pneumatic radial tire is particularly adaptable to recapping, even when the new tire is completely run, the occurrence of large cracks making the tire unsuitable for recapping is frequently observed at the end portion of the belt. Therefore, it is demanded to further improve the crack resistance and resistance to crack growth at the end portion of the belt and hence to considerably improve the resistance to belt separation. Attention is also drawn to the disclosures of US-A-4062393 and JP-59-075807A which correspond with the preamble of claim 1. It is an object of the present invention to provide a pneumatic radial tire having an improved service life in which the crack resistance and resistance to crack growth at the end portion of the cord layer constituting the belt, particularly a narrow-width cord layer among cross cord layers, are sufficiently enhanced even under severe service conditions of the tire while sufficiently maintaining the rigidity-strengthening function inherent to the belt of the radial tire to thereby improve the resistance to separation in a new tire and also improve the recapping adaptability . According to the invention, there is provided a pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores embedded in a pair of bead portions, a tread portion, and a belt superimposed about an outer periphery of the carcass to reinforce the tread portion and comprised of at least three rubberized cord layers, cords of two adjacent layers among said layers being crossed with each other at an acute cord angle with respect to an equatorial plane of the tire to form cross cord layers, in which an outer cord layer of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer, and the cross cord layers are provided with a space cushion rubber separating end portions of said cross cord layers and extending from an end of the narrow-width cord layer toward the outside of the tire, and the space cushion rubber is a rubber composite comprised of an inner rubber extending so as to separate the end portions of said cross cord layers from each other and an outer rubber extending from the inner rubber toward the outside of the tire and having at least two different rubber compositions, and 100% moduli Mx', M6i and M6u (kgf/cm 2 ) of a coating rubber for the narrow-width outer cord layer, inner rubber (6i) and outer rubber (6u) establish a relationship of M6u<M6i≦Mx'. When the space cushion rubber is comprised of two rubbers, the position of a joint face between the inner rubber 6i and the outer rubber 6u may be located on either an end of the narrow-width cord layer or an end of the end cover rubber provided on the end portion of the layer as a standard point or may be within a region slightly separated from the above standard point toward the inside or outside of the tire. In order to further enhance the effect of the invention, it is desirable that a belt under-cushion rubber is disposed between the carcass and an end portion of the cord layer located adjacent the carcass and has a 100% modulus corresponding to 0.3-0.7 times the 100% modulus of a coating rubber for said cord layer, and/or that either organic fiber cord or steel cord is used in the narrow-width cord layer among the cross cord layers and the distance between the cords in a given end count is unequal. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a left-half section view of a main part of a first embodiment of a heavy duty pneumatic radial tire, which is not however according to the invention of this application; Fig. 2 is a partly enlarged view of the main part shown in Fig. 1; Fig. 3 is a partly developed perspective view illustrating a group of cords arranged in a narrow-width cord layer; Fig. 4 is a left-half section view of a main part of a second embodiment of a heavy duty pneumatic radial tire which is not however according to the invention of this application; Fig. 5 is a left-half section view of a main part of an embodiment of a heavy duty pneumatic radial tire according to the present invention; Fig. 6 is a partly schematic section view perpendicular to a direction of arranging cords in the narrow-width cord layer; Fig. 7 is a schematic view illustrating a shearing strain acting to cord ends; Fig. 8 is a graph showing a relation between crack length, rolling resistance and modulus ratio; and Fig. 9 is a schematic view illustrating cracks created at a cord end portion of a narrow-width cord layer. In the embodiments of Figs. 1, 4, and 5 showing left-half sections of their main parts with respect to an equatorial plane E of the tire, a radial carcass 1 toroidally extends between a pair of bead cores (not shown) embedded in a pair of bead portions (not shown) and is provided on its outer periphery with a belt 3 reinforcing a tread portion 2. Moreover, a tread rubber 2 shown in each of the above embodiments has a two-layer structure of a cap rubber 2tc and a base rubber 2tb. The belt 3 is comprised of at least three rubberized cord layers. In the illustrated embodiments, the belt 3 is comprised of four rubberized cord layers 3-1, 3-2, 3-3 and 3-4 viewed from the side of the radial carcass 1 in order. Among these cord layers 3-1 to 3-4, two adjoining cord layers 3-2 and 3-3 form cross cord layers in which cords of the two layers are crossed with each other at an acute angle of not more than 90° defined between the cords of the layers with respect to the equatorial plane E of the tire. The cross cord angle is preferably within a range of 30-50° in order to satisfactorily develop the function of the belt 3. In other words, it is desirable that the cords in each of the cross cord layers 3-2 and 3-3 are inclined at an angle of 15-25° with respect to the equatorial plane E. Although one set of cross cord layers is existent in each of the above embodiments, the belt 3 may include two or more sets of cross cord layers. As to the belt 3 having the above structure, the width of the cord layer 3-3 located outward in the radial direction of the tire among the cross cord layers 3-2 and 3-3 is narrower than that of the cord layer 3-2, so that ends of the cross cord layers 3-2 and 3-3 form a step difference as sectionally shown in each drawing. Moreover, the width of each of the cord layer constituting the belt is substantially divided into equal parts with respect to the equatorial plane E. The embodiments of Figs. 1-4, show tires which are not however according to the invention of this application. In Figs. 1 and 2, the narrow-width cord layer 3-3 is provided with an end cover rubber 4 covering the end portion of the layer. The end cover rubber 4 shown in Figs. 1 and 2 covers the end portion of the layer 3-3 over a region ranging from an inner surface in the radial direction of the tire through the end to an outer surface thereof. On the other hand, the end cover rubber 4 shown in Fig. 4 covers only the inner surface of the end portion in the radial direction. The end cover rubber 4 has a 100% modulus M 4 (kgf/cm 2 ) larger than a 100% modulus Mx' of a coating rubber for the narrow-width cord layer 3-3. More particularly, it is desirable that the 100% modulus M 4 is within a range of 1.2-4.0 times the 100% modulus Mx' of the coating rubber for the narrow-width cord layer 3-3. In each of the above embodiments, the cross cord layers 3-2 and 3-3 are provided with a space cushion rubber 5 separating end portions of these layers from each other exceeding a thickness t of the end cover rubber 4. The space cushion rubber 5 has a 100% modulus M 5 smaller than a 100% modulus Mx of a coating rubber for the cross cord layers 3-2 and 3-3. More particularly, it is desirable that the 100% modulus M 5 is within a range of not more than 0.95 times, more preferably 0.5-0.9 times the 100% modulus Mx of the coating rubber for the cross cord layers 3-2 and 3-3. Moreover, if the belt includes two or more sets of the cross cord layers, the end cover rubber 4 and the space cushion rubber 5 are applied to a narrow-width cord layer having at least a maximum width among the narrow-width cord layers. Referring to Fig. 2, it is advantageous that the 100% modulus M 5 of the space cushion rubber 5 satisfies a relationship of M 5 ≦Mx - (M 4 - Mx) x (G 4E /G 5E ) with respect to an inner gauge G 4E of the end cover rubber 4 at the end of the narrow-width cord layer 3-3 in the radial direction, a gauge G 3E of the space cushion rubber 5 existing on a vertical line VL drawn from the end of the layer 3-3 to the inner cord layer 3-2 of the cross cord layers, 100% modulus M 4 of the end cover rubber 4 and 100% modulus Mx of the coating rubber for the cross cord layers 3-2 and 3-3. Referring to Fig. 3, both corners of each cord end C 32 in a group of cords C 3 (only three cords are shown in Fig. 3) arranged in the narrow-width cord layer 3-3 form an obtuse angle and an acute angle in its developed plane view, and each cord end C 3E substantially locates on a straight line EL. It is desirable that the width w of the end cover rubber 4 satisfies a relationship of w≧ (50 mm/N) x sin  with respect to an inclination angle  defined between a normal line HL drawn from any one corner of a cord end C 3E of a certain cord C 3 to a center axis line X 3 of another cord C 3 adjacent thereto and the straight line EL and an end count N of cords C 3 per 50 mm as measured in a direction of the normal line HL. However, the end of the end cover rubber 4 having the width w inside the tire is terminated in a region of arranging the space cushion rubber 5. Moreover, symbol Rc is a coating rubber for the cord C 3 in the narrow-width cord layer 3-3. The present invention will be described below with preference to the embodiment of Fig. 5. A space cushion rubber 6 shown in Fig. 5 separates the end portions of the cross cord layers 3-2 and 3-3 from each other and extends from the end of the narrow-width cord layer 3-3 toward the outside of the tire likewise the case of Fig. 1. However, the space cushion rubber 6 is a rubber composite comprised of an inner rubber 6i for separating the end portions of the cross cord layers 3-2 and 3-3 from each other and an outer rubber 6u extending from the inner rubber 6i toward the outside of the tire and having at least two different rubber compositions, which is entirely different from the case of Fig. 1. Even when the end cover rubber is applied to the narrow-width cord layer, the inner rubber 6i and outer rubber 6u always satisfy the above position relation. When 100% moduli of the inner rubber 6i and the outer rubber 6u are M6i and M6u, it is necessary that the 100% moduli M6i and M6u and 100% modulus Mx' of the coating rubber for the narrow-width cord layer 3-3 satisfy a relationship of M6u<M6i≦Mx' . In fact, it is preferable that (M6u/M6i) is within a range of 0.5-0.9 and (M6i/Mx') is within a range of 0.9-1.0. Moreover, a partition face P between the inner rubber 6i and the outer rubber 6u may be inclined inward or outward from the illustrated position. In the tires comprising the cross cord layers 3-2 and 3-3 in Figs. 1-5, it is desirable that a belt under-cushion rubber 9 is arranged between the carcass 1 and the end portion of the cord layer 3-1 in the belt 3 located nearest to the carcass 1. In this case, it is preferable that a 100% modulus M 9 of the rubber 9 is within a range of 0.3-0.7 times the 100% modulus Mx" of a coating rubber for the cord layer 3-1. Moreover, a mutual relation of 100% modulus in coating rubbers for the cord layers 3-1 to 3-3 constituting the belt 3 may be Mx"=Mx'=Mx or Mx"≒Mx'≒Mx. Further, it is preferable to use steel cords or organic fiber cords such as aramide fiber cords or the like as the cord in the narrow-width cord layer. In this case, it is desirable that the cords C 3 are arranged at unequal distances as a section perpendicular to a direction of arranging the cords as shown in Fig. 6. It is well-known that the growing rate of cracks liable to be created at the end portion of the narrow-width cord layer 3-3 in the cross cord layers 3-2 and 3-3 is caused by two kinds of shearing strains produced through deformation at ground contact. Among these strains, a first strain is a shearing strain γ st acting between the cord and the coating rubber along the direction of arranging the cords at a "pecking" stage created at the cord end. As a result, the crack grows in the coating rubber surrounding the cord along the direction of arranging the cords. A second strain is an interlaminar shearing strain γ 23 acting between the end portion of the narrow-width cord layer 3-3 and the cord layer 3-2 in the vicinity of the end C 32 of the cord C 3 . The interlaminar shearing strain γ 23 mainly grows the crack along the cord to cause interlaminar separation and then controls the growing rate of the interlaminar separation. The behaviours of these shearing strains γ st and γ 23 are illustrated in Fig. 7(a) and (b). Fig. 7(a) is a perspective view schematically showing only cords C 2 , C 3 taken out from the end portions of the cross cord layers 3-2 and 3-3, while Fig. 7(b) is a plan view of cords C 2 and C 3 . As shown in Fig. 7(b), the shearing strain γ st is caused by in-plane shearing strain LC of rubber located close to the end of the cord layer 3-3. In order to control the crack growth and prevent the occurrence of separation failure, therefore, it is necessary to simultaneously reduce the interlaminar shearing strain γ 23 and the in-plane shearing strain LC. Now, the reduction of both shearing strains γ 23 and LC will be described below. First, there are mentioned the following conclusions: (1) If the 100% modulus M4 of the end cover rubber 4 is set so as to be larger than the 100% modulus Mx' of the coating rubber for the cord layer 3-3, the shearing strain LC tends to decrease and the interlaminar shearing strain γ 23 tends to increase; (2) If the 100% modulus M 5 of the space cushion rubber 5 at least located outward from the end of the cord layer 3-3 is set so as to be smaller than the 100% modulus Mx' of the coating rubber for the cord layer 3-3, both the shearing strain LC and interlaminar shearing strain γ 23 tend to decrease; (3) If the 100% modulus M6u of the outer rubber 6u is set so as to be smaller than the 100% modulus Mx' of the coating rubber for the cord layer 3-3, the shearing strain LC decreases without increasing or decreasing the interlaminar shearing strain γ 23 ; (4) If the 100% modulus M 9 of the belt under-cushion rubber 9 is within a range of 0.3-0.7 times the 100% modulus Mx" of the coating rubber for the cord layer 3-1, both the shearing strain LC and the interlaminar shearing strain γ 23 tend to decrease. The above items (1)-(4) develop their effects individually, but the tendency of the interlaminar shearing strain γ 23 to increase in the item (1) is undesirable, so that it is required to combine the item (1) with the item (2). In such a combination, if the 100% modulus M 4 is not less than 1.2 x Mx' and the 100% modulus M 5 is not more than 0.95 x Mx', it is possible to control the tendency of the interlaminar shearing strain γ 23 to increase and to further control the shearing strain LC. Further, the combination of the items (1) and (2) contributes to not only the decrease of the shearing strain LC but also the effective control of the interlaminar shearing strain γ 23 if the 100% moduli M 4 , M 5 and Mx and the gauge G 4E of the end cover rubber 4 and the gauge G 5E of the space cushion rubber 5 are adequately set so as to establish a relationship of M 5 ≤ Mx - (M 4 - Mx) x (G 4E /G 5E ). As to the decrease of the in-plane shearing strain LC in the above items (1) - (4), the following considerations apply. In the item (1) the effect of controlling the in-plane deformation of rubber located close to the end of the cord layer 3-3 is obtained by covering the end portion of the cord layer 3-3 with a high-modulus rubber, while as to the strain of rubber between the cord layers 3-2 and 3-3 in the item (2), the shearing strain LC and interlaminar shearing strain γ 23 are decreased by setting the 100% modulus M 5 of the space cushion rubber 5 to a smaller value. Particularly, the item (3) develops the effect of absorbing strain through displacement of step portion of the cord layer 3-2 (step-difference portion to the cord layer 3-3) because the 100% modulus M6u of the outer rubber 6u is lower than the 100% modulus Mx' of the coating rubber for the cord layer 3-3. In the item (4), the 100% modulus M 9 of the belt under-cushion rubber 9 is large, so that the rigidity between the carcass 1 and the cord layer 3-1 increases and hence the tension-bearing of the cross cord layers 3-2 and 3-3 is decreased by such an increment of the rigidity and the relative displacement of each cord layer in the circumferential direction under loading is reduced to decrease both the shearing strain LC and the interlaminar shearing strain γ 23 , while energy loss through the shearing strain between the carcass 1 and the cord layer 3-1 increases, which can not be ignored because the volume occupied by the belt under-cushion rubber 9 is large. This is contrary to recent demand for further reducing fuel consumption, so that it is preferable that the 100% modulus M9 is within a range of 0.3-0.7 times the 100% modulus Mx'' as shown in Fig. 8 in order to at train a balance between strain and low fuel consumption (low rolling resistance). In Fig. 8, the abscissa is a value of modulus ratio M 9 /Mx'', and the left ordinate is a value (index) of crack length K L , and the right ordinate is a value (index) of rolling resistance R.R. As seen from Fig. 8, the crack length K L and the rolling resistance R.R are in a conflicting relation. As to the shearing strain γ 23 , there will mainly be described below the significance on the division of the space cushion rubber 6 participating in the shearing strain γ 23 into the outer rubber 6u and the inner rubber 6i. When the end cover rubber 4 is applied at a larger value of 100% modulus M 4 , even if interlaminar separation is created, the shearing strain γ 23 in the end cover rubber 4 is smaller than the shearing strain γ 23 in the coating rubber for the cord layer 3-3, so that interlaminar separation never arrives at the space cushion rubber 5. However, if it is required to reduce the number of constitutional members, or if it is difficult to use an end cover rubber 4 having a higher modulus in view of the productivity, there is adopted the space cushion rubber 5. Even if the 100% modulus M 5 of the space cushion rubber 5 is a small value, the shearing strain γ 23 " in the space cushion rubber 5 becomes larger while the shearing strain γ 23 created in the end portion of the cord layer 3-3 is reduced. As a result, there is not particularly caused serious trouble while the crack at ''pecking" stage progresses in the coating rubber along the cord, but once such a crack grows to the interlaminar separation, this separation rapidly grows in the space cushion rubber 5 to cause trouble. Therefore, when an end cover rubber 4 having a large 100% modulus M 4 is not used, it is necessary to take the following countermeasure after the occurrence of the interlaminar separation. As a countermeasure, the space cushion rubber 6 is divided into the outer rubber 6u and the inner rubber 6i so that the 100% moduli of these rubbers satisfy the relationship of M6u<M6i≦Mx', whereby the 100% modulus of that portion of the space cushion rubber 6 (inner rubber 6i) in the interlaminar separation is not lowered too much and the 100% modulus of the outer rubber 6u is made smaller than that of the inner rubber to decrease the in-plane shearing strain LC and the shearing strain γ 23 . The control and reduction of the shearing strain γ 23 itself will be described below. Referring to Fig. 9 showing a group of cords C 3 as a developed view similar to Fig. 3, cracks K created in the vicinity of each end C 3E of the cords C 3 through the action of the shearing strain γ st frequently grow to a series of cracks K as the running of the tire proceeds. In this case, the position of the cracks K connecting between the mutual adjoining cords C 3 tends to substantially match with a normal line HL drawn from an obtuse corner of the end C 3E of the cord C 3 in another adjoining cord C 3 even if the above position locates close to the outside of the tire. A distance w m between the straight line EL and a straight line WL (on a developed view) connecting ends of the connected course of the cracks K between the mutual adjoining cords C 3 is a minimum width w of the end cover rubber 4, so that when the width w of the end cover rubber 4 is set to not less than (50 mm/N) x sin  , the growing region of the crack K brought by the shearing strain γ st is rendered into a reducing region of the shearing strain γ st , whereby the growth of the crack K can be controlled. The following examples are given in illustration of the invention and are not intended as limitations thereof. There are provided radial ply tires for trucks and buses having a tire size of 10.00R20 and a structure as shown in Figs. 1-5, in which the carcass 1 is a single rubberized steel cord ply of radial arrangement, and the belt 3 is comprised of four rubberized layers each containing steel cords of 3x0.20+6x0.36 structure. In the belt 3, a first cord layer 3-1 has a width of 160 mm, a second cord layer 3-2 has a width of 185 mm, and a third cord layer 3-3 has a width of 160 mm, and a fourth cord layer 3-4 has a width of 80 mm, while a cord inclination angle of each cord layer with respect to the equatorial plane E of the tire is R52°, R18°, L18° and L18° in the order of the first to fourth layers (R means upward to the right, and L means upward to the left). Therefor, the cross cord layers are the second and third cord layers. Also, the invention is applicable to the third and fourth cord layers in a case that the cord inclining directions of the first to fourth cord layers in the belt 3 are R, R, L and R in this order to form cross cord layers between the second and third cord layers and between the third and fourth cord layers and the widths of the cord layers are made narrower from the second cord layer to the fourth cord layer in order. Moreover, the cord inclining directions R, L may be replaced with each other. The above tires having the common structure are divided into three groups comprising Group 1 to Group 3. The dimension and test results every group are shown in the respective Tables. Moreover, a belt under-cushion rubber 9 having 100% modulus M9 of 23 kgf/cm 2 is applied to Examples, Comparative examples and Conventional Example in each group, respectively. In Group 1, there are manufactured tires of Examples 1-12 according to Figs. 1-3 and Example 13 according to Fig. 6 together with tires of Comparative Examples 1-7 and conventional tire. In the tire of Example 13, the cords of the cord layer 3-3 are divided into many bundles each containing three cords and the mutual bundles are separated apart from each other. The tires of Examples 1-13 are not however according to the invention of this application. In all of the examples, comparative examples and conventional example, rubber having the same rubber composition and 100% modulus Mx is used as a coating rubber for the cord layers 3-1 to 3-4 constituting the belt. In each tire, the end count N per 50 mm of the narrow-width cord layer 3-3 is 24 cords and the inclination angle  between the normal line HL of the cord C 3 and the straight line EL (see Fig. 3) is 72°, so that the value of (50 mm/N) x sin  is 1.98. The values of 100% moduli, modulus ratio, gauges G 4E and G 5E , w and Mx - (M 4 - Mx) x (G 4E /G 5E ) in these examples are shown in Tables 1-3. In Group 2, tires of Examples 14-16 are produced according to Fig. 4 based on the tire of Example 5 together with tires of Comparative Examples 8-12 and a conventional tire. In these tires, the end cover rubber 4 is arranged so as to cover the surface of the end portion at the one side (inside in the radial direction) of the narrow-width cord layer facing the space cushion rubber 5 and extend over the end portion outside the tire, and a rubber having the same rubber composition is used as the rubbers 4 and 5 and the coating rubber for the cord layers 3-1 to 3-4 in the belt. The values of 100% moduli M 4 , M 5 and Mx are shown in Table 4. The tires of Examples 14-16 are not however according to the invention of this application. In Group 3 tires of Examples 17-20 provided with the double cushion rubber 6 comprised of inner rubber 6i and outer rubber 6u are produced according to Fig. 5 based on the tire of Example 5 removing the end cover rubber together with tires of Comparative Examples 13-17 and the conventional tire. The values of 100% moduli M 6i , M 6u and Mx' of the inner rubber 6i, outer rubber 6u and coating rubber for the cord layer 3-3 are shown in Table 5. Each tire in Examples 1-16 and 17-20, Comparative Examples 1-12 and 13-17 and Conventional Examples of Groups 1, 2 and 3 is fitted on a drum of a testing machine under conditions of internal pressure: 7.00 kgf/cm 2 and load: 2600 kgf and run at a speed of 60 km/h over a distance of 100,000 km and thereafter taken out from the testing machine. Then, the tire is cut to measure crack length most grown among cracks created at the end portion of the belt 3. Such a crack is created in the end portion of the third cord layer in all tires. The crack length is an average of values measured at 30 positions at substantially equal intervals in the circumference of the tire and represented by an index on the basis that the conventional example or comparative example is 100. The smaller the index value, the better the property of controlling the occurrence of cracks. The test results are also shown in Tables 1-5 as "trouble form" and crack length (index). In the term "trouble form", the separation means interlaminar separation between the narrow-width cord layer 3-3 and the cord layer 3-2, and the wording "partial separation on circumference (or partial separation)" means that the separation is partially created along the circumference, and the wording "about pecking" means that the trouble at the end of the narrow-width cord layer is controlled to a pecking degree. As seen from Tables 1-5, the tires of Examples 1-20 do not show the occurrence of partial separation and largely decrease the crack length, so that they are considerably superior to the tires of the comparative examples and conventional examples in the resistance to crack growth and the separation resistance. As mentioned above, according to the invention, the occurrence and growth of cracks liable to be created at the end portions of the cross cord layers are advantageously controlled by rationalizing distribution of 100% moduli between coating rubbers for the cross cord layers in the belt and rubber in the vicinity of the end portion of these layers or further by arranging a new rubber member on the end portions of the cross cord layers and slightly widening the width of the coating rubber for the narrow-width cord layer without causing any inconvenience in the structure of the belt itself, whereby it is possible to largely improve the separation resistance of the belt. As a result, the invention can provide heavy duty pneumatic radial tires capable of not only having a long service life as a new product but also sufficiently adaptable to repetitive recapping.

Claims: A pneumatic radial tire comprising a radial carcass (1) toroidally extending between a pair of bead cores embedded in a pair of bead portions, a tread portion (2), and a belt (3) superimposed about an outer periphery of the carcass to reinforce the tread portion and comprised of at least three rubberized cord layers (3-1, 3-2, 3-3), cords of two adjacent layers (3-2, 3-3) among said layers being crossed with each other at an acute cord angle with respect to an equatorial plane (E) of the tire to form cross cord layers, in which an outer cord layer (3-3) of the cross cord layers in a radial direction of the tire has a width narrower than that of an inner cord layer (3-2), and the cross cord layers (3-2, 3-3) are provided with a space cushion rubber (6) separating end portions of said cross cord layers and extending from an end of the narrow-width cord layer (3-3) toward the outside of the tire, characterized in that the space cushion rubber (6) is a rubber composite comprised of an inner rubber (6i) extending so as to separate the end portions of said cross cord layers (3-2, 3-3) from each other and an outer rubber (6u) extending from the inner rubber (6i) toward the outside of the tire and having at least two different rubber compositions, and 100% moduli Mx', M6i and M6u of a coating rubber for the narrow-width outer cord layer (3-3), inner rubber (6i) and the outer rubber (6u) have a relationship of M6u<M6i≦Mx'. A tire as claimed in claim 1, characterized in that a belt under-cushion rubber (9) is disposed between the carcass (1) and an end portion of the cord layer (3-1) located adjacent the carcass and has a 100% modulus corresponding to 0.3-0.7 times the 100% modulus of a coating rubber for said cord layer (3-1). A tire as claimed in claim 1 or 2, characterized in that either organic fiber cord or steel cord is used in the narrow-width cord layer (3-3) among the cross cord layers (3-2, 3-3) and the distance between the cords in a given end count is unequal.

Inventors: OKAMOTO YOICHI, OKAMOTO, YOICHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1997
Main classification: B60C9
IPC codes: B60C9