Pneumatic tires

Abstract: A crown portion of a pneumatic tire comprising at least one radial carcass ply and a belt of at least two cross belt layers (1) is reinforced by arranging between the carcass ply and the belt a crown reinforcing layer (3) containing wavy or zigzag-like cords or filaments arranged substantially parallel with the equatorial plane of the tire.

Description: This invention relates to pneumatic tires, and more particularly to an improvement in an effective reinforcement of a crown portion in the tire. In general, the crown portion of the pneumatic tire has exclusively been reinforced by a belt comprised of at least two belt layers, which can not still avoid problems such as separation failure and the like. In this connection, Japanese Patent laid open No. 54-126306 and No. 55-132307 disclose that a circumferential tension is born by a laminated assembly of cord layers called a limit block or limit assembly to mitigate the circumferential tension acting on the belt, resulting in the prevention of separation failure. In fact, however, there is a fear of causing separation failure between the limit blocks because it is required to laminate cord layers, whose cords are crossed with each other at a very shallow inclination angle with respect to the equator of the tire in order to obtain a high rigidity along the circumference of the tire. On the other hand, separation failure can be avoided by using a cord strip having a cord angle of 0° instead of the limit block because a cut free end of the cord is not formed in the case of using the strip. However, when using such a strip, the enlargement of the belt in the circumferential direction is suppressed in a vulcanizer for a green tire, which causes serious difficulty in the manufacture of tires. Moreover, the twisting of the cord used as a reinforcing element for a strip having a cord angle of 0° may have a structure which easily elongates only at the initial curing stage and provides a given rigidity after the curing. In such a cord, however, the twisting pitch becomes short, which is disadvantageous in view of the productivity, and the cord strength is lowered due to the twisting, so that it is necessary to increase the end count in the strip considering a danger of so-called cut burst in the tire, resulting in an increase of tire weight. Furthermore, it is impossible to use a monofilament as a cord material. In Japanese Patent laid open No. 57-209403 is disclosed a technique for reinforcing a tire with a protect layer comprised of wavy cords, wherein a wavy body composed of fibers or metal cords having an extremely low elongation is arranged as a protect layer in addition to a breaker comprised of at least two layers. However, there is no description on the effect of the protect layer as a crown reinforcing layer in this technique. LU-A-81 371 shows and describes a pneumatic tire comprising at least one radial carcass ply toroidally extending between a pair of bead portions and a belt reinforcing a tread on said radial carcass, said belt being comprised of at least two cross belt layers each containing cords or filaments as a reinforcing element and crossed with each other at an inclination cord angle of 10-45° with respect to the equatorial plane of the tire, and at least one crown reinforcing layer disposed between said carcass ply and said cross belt layers, wherein said crown reinforcing layer is comprised of a strip containing cords or filaments arranged with a relative small inclination angle in parallel with the equatorial plane of the tire as a reinforcing element. GB-A-815 055 teaches to use reinforcing layers containing wavy or zigzag like cords, to avoid in the process of manufacture of the tire,i.e. vulcanisation in a tire mould, undesirable deformations and tensions. It is, therefore, the aim of the present invention to provide a pneumatic tire having a reinforcing means for a tire crown portion in which separation failure at the cross cord belt and crown reinforcing layer serving to reinforce the crown portion of the tire is advantageously prevented by using conventional cords or filaments with simplification of tire manufacture and without increasing the tire weight. This object is achieved by the combination of the features of claim 1. In a preferred embodiment of the invention, the strip has a ratio of amplitude (a) to wavelength (λ) in the wavy or zigzag-like reinforcing element of 0. 015-0. 06. In another preferred embodiment of the invention, the crown reinforcing layer is arranged so that a distance from the outer end in the widthwise direction of the strip to the equatorial plane of the tire is within a range of 25-47.5% of a maximum width of an actually overlapped region in the cross belt layers. The invention will be described with reference to the accompanying drawings, wherein: Figs. 1a to 1c are schematic sectional views of the main part of a tire according to the invention, respectively; Fig. 2 is a schematic view of wavy reinforcing elements used in the invention; Figs. 3a and 3b are schematic sectional views in parallel with the equator of the tire illustrating the deformation behaviour of a tread portion, respectively; Fig. 4 is a graph showing the influence of a position of an end of the crown reinforcing layer upon interlaminar shearing strain at an end of the cross belt layer; Fig. 5 is a view showing a distribution of interlaminar shearing strain accompanied with the deformation of the tread under loading; Fig. 6 is a graph showing a relation between the position of an end of the crown reinforcing layer and the running distance on a drum; and Fig. 7 is a graph showing a comparison of the expansion of tire size between Example 1 and Comparative Example 3. According to the invention, the strip containing wavy or zigzag-like cords or filaments arranged as a reinforcing element along the equator of the tire is used as the crown reinforcing layer. Thus, the elongation is easily obtained in the vulcanization by using such a strip, so that the tire production becomes more convenient. Particularly, it is not required to control the elongation by a twisting means or the like, so that low cost filaments may be used. Furthermore, there is no danger of reducing the strength of cord due to the twisting, so that cord burst is not caused and the tire weight is not increased. In Figs. 1a to 1c are schematically shown a main part, particularly the tread portion of the pneumatic tire according to the invention, respectively, in which the illustration of toroidal carcass ply and bead core is omitted together with sidewall portions and bead portions for convenience' sake. In Fig. 1, numeral 1 is a cross belt layer, numeral 2 a crown reinforcing layer, and numeral 3 a tread rubber. As is well-known, each of the cross belt layers 1 is a so-called tire cord fabric obtained by arranging usual cords or filaments each made from organic fiber, metal or the like, particularly steel wire in parallel with each other as a reinforcing element, coating with rubber, cutting at an angle the resulting rubberized body and rebonding the cut pieces at their cut ends with each other. According to the invention, at least two cross belt layers 1 are laminated one upon the other about the crown portion of the carcass ply so as to cross the cords of these layers with each other at an inclination angle of 10°-40° with respect to the equatorial plane. The crown reinforcing layer 2 is comprised of a strip extending across the equator toward both sides in the widthwise direction (see Figs. 1a and 1b) or a pair of strips arranged only on both sides of the crown portion (see Fig. 1c). Moreover, the number of the strips used is at least one as shown in Figs. 1a and 1b. In any case, the crown reinforcing layer 2 is disposed beneath the cross belt layer 1 and above the crown portion of the carcass ply. In the manufacture of such tires, various parts constituting the tire are combined to form a green tire, which is vulcanized in a vulcanizer. Moreover, the size of the tire during the change from the green tire to the final tire product is expanded by 2-6%. Therefore, the reinforcing element constituting the strip of the crown reinforcing layer 2 should be substantially arranged in parallel to the equator of the tire from the viewpoint of durability, so that it is required to elongate in accordance with the expansion degree of tire size of 2-6% at the vulcanization step. If the reinforcing element is not extensible, the sectional shape of the tire product becomes distorted and the tire can not be put into practical use. For this end, the reinforcing element used in the crown reinforcing layer 2 should have an elongation at break larger than the above expansion degree in the vulcanization. According to the invention, the strip constituting the crown reinforcing layer 2 contains cords or filaments made from organic fiber or metal, particularly steel wire as a reinforcing element. Such a reinforcing element has very small extensibility and is previously formed into a wavy or zigzag-like shape as shown in Fig. 2, wherein a ratio (a/λ) of amplitude a to wavelength λ is within a range of 0.015-0.06. These wavy (or zigzag-like) reinforcing elements are wholly arranged along the equator of the tire in the strip. Moreover, in order to obtain the ratio a/λ of the above range after the vulcanization, the reinforcing elements having a ratio a/λ larger than that after the vulcanization are used to form a strip, which is attached onto the carcass ply in the green tire. The expansion in the size of the green tire inside the vulcanizer may be allowed by such a forming of the reinforcing element, so that the ratio a/λ after the vulcanization is made smaller than that of the green tire and becomes substantially inextensible along the equator of the tire. The crown reinforcing layer 2 is used so that a ratio M defined by the following equation is within a range of 0.25-1.0: wherein m W : total strip number, m B : total cross belt layer number, E Wn : elastic modulus of reinforcing element in nth strip, E Bn : elastic modulus of reinforcing element in nth cross belt layer, A Wn : sectional area of reinforcing element in nth strip, A Bn : sectional area of reinforcing element in nth cross belt layer, N Wn : end count (cords/50 mm) in nth strip, N Bn : end count (cords/50 mm) in nth cross belt layer. Since tension under an inflation of internal pressure after the vulcanization is born by the strip together with the cross belt layers 1 to mitigate the tension of the belt, the crown reinforcing layer 2 prevents the occurrence of separation failure at the cross belt layers. For this purpose, it is preferable that the ratio a/λ is not more than 0.06 and the ratio M is not less than 0.25. Particularly, the ratio a/λ should be preferably within a range of 0.015-0.06 and the ratio M should be within a range of 0.25-1.0. When the tension bearing of the crown reinforcing layer 2 becomes too large, if the ratio M exceeds 1.0, the rigidity of only the strip becomes too large and the expansion of the tire size becomes non-uniform in the widthwise direction of the tire, and consequently uniform wearing can not be expected, so that the ratio M should be limited to 1.0. Furthermore, the lower limit of the ratio a/λ should be preferably 0.015 in order to permit the expansion of the size of the green tire in the vulcanizer. If it is intended to obtain a ratio a/λ of less than 0.015 after the vulcanization, the size expansion of the green tire in the vulcanizer is not allowed and waving is caused in the belt layer in the widthwise direction thereof. In the aforementioned Japanese Patent laid open No. 57-209403 is disclosed the use of a strip containing wavy cords as a protect layer for the belt, wherein the strip is arranged on each side of the belt outward in the radial direction of the tire or between the belt layers. As shown in Fig. 3a, when the strip as a protect layer 2′ is arranged outside the cross belt layers 1 in the radial direction of the tire, the circumferential length of the cross belt layer 1 is shorter than that of the protect layer 2′. In Fig. 3a, a left half portion shows a deformation state of the tire in the ground contacting during running under a load, and a right half portion shows a free state. Further, since the circumferential length of the protect layer 2′ is substantially unchanged even through the deformation in the ground contacting, when the whole of the cross belt layers 1 and the protect layer 2′ is changed into a flat form shown by an arrow F through a bending force shown by an arrow P during the contacting with the ground, the cross belt layers 1 are elongated by the protect layer 2′. On the other hand, when the strip is arranged as a crown reinforcing layer 2 inside the cross belt layers 1 and outside the crown portion of the carcass ply in the radial direction of the tire as shown in Fig. 3b, even if the cross belt layers 1 and the crown reinforcing layer 2 are changed into a flat form F through the bending force P, the cross belt layers 1 located outside the crown reinforcing layer 2 are compressed by the crown reinforcing layer 2. Thus, in the invention, compression force is given to both end portions in the widthwise direction of the cross belt layers 1 located outside the crown reinforcing layer 2, whereby the shearing force is further suppressed at the belt end. Furthermore, the strip constituting the crown reinforcing layer 2 according to the invention contains wavy or zigzag-like cords or filaments as a reinforcing element, so that there is no danger of causing separation failure from the side end of the strip because the cut ends of cords as in the cross belt layer are not existent in the end of the strip and the so-called strain concentration is not caused. In the invention, it is preferable that the distance of the crown reinforcing layer 2 ranging from the outer end in the widthwise direction of the layer 2 to a position of the equator is within a range of 25-47.5% of the maximum width of actually overlapped region between the cross belt layers 1. In Fig. 4 are shown the results on shearing strain measured at the ends of the cross belt layers 1 along the circumference of the tire. Such a shearing strain results in the occurrence of separation failure from the end of the cross belt layer as mentioned above. When the position of the end of the crown reinforcing layer 2 coincides with the position of the outer end of the overlapped region (corresponding to a position of 50% in Fig. 4), the strain at the end of the cross belt layer becomes large. Further, when the end position of the crown reinforcing layer is shifted inward in the widthwise direction, the strain at the end of the cross belt layer is minimum at once and tends to gradually increase. In addition that the separation failure is caused due to the above large shearing strain at the end of the cross belt layer, it is necessary to consider the heat build-up through the crown reinforcing layer 2. Since the crown reinforcing layer 2 is substantially inextensible along the equator of the tire, when the cross belt layers 1 and at least one crown reinforcing layer 2 are used, the shearing strain along the circumference of the tire is caused between these layers at the kicking-out side and stepping-in side as shown in Fig. 5 when the tread is deformed during the contacting with the ground as shown in Fig. 3. Therefore, it is necessary to consider the heat build-up at the wavy crown reinforcing layer due to such a strain. As the width of the crown reinforcing layer 2 becomes wide, the heat build-up becomes large, which is disadvantageous in the prevention of separation failure at the end of the cross belt layer 2. On the other hand, as the width of the crown reinforcing layer becomes narrow, the heat build-up becomes small, which is favourable in respect of the durability of the tire. Furthermore, when the end of the crown reinforcing layer 2 is located away from and inside the end of the cross belt layers 1, the influence of heat build-up at the crown reinforcing layer to the cross belt layer is small and the occurrence of separation failure is hardly caused at the end of the cross belt layers. In order to totally judge the above effect, therefore, a drum test was carried out by varying the position of the end of the crown reinforcing layer in the tire to obtain results as shown in Fig. 6, wherein the abscissa represents the distance from the outer end in the widthwise direction of the crown reinforcing layer to the equator of the tire and the ordinate represents an index of running distance on an indoor drum test. In this case, the heat build-up is considered in addition to the magnitude of strain at the end of the cross belt layer. As seen from these results, the position of the end of the crown reinforcing layer 2 from the equator of the tire is within a range of 25-47.5% of the maximum width of the actually overlapped region between the cross belt layers 1. The following examples are given in illustration of the invention and are not intended as limitations thereof. Examples 1-5, Comparative Examples 1 and 2 Test tires having a tire size of 11/70 R22.5 were manufactured according to the following Table 1. Then, a durability test on a drum was carried out with respect to these test tires to obtain results as shown in the following Table 2, wherein the running distance (km) of the tire was represented by an index on the basis that Comparative Example 1 was 100. As the drum test, the test tire was run on the drum at a speed of 60 km/hr under an internal pressure of 8.0 kgf/cm², to which was applied an initial load of 2500 kg and further a loading of 500 kg was added every 24 hours. Moreover, in Example 1 and Comparative Example 2, the expansion of tire size over a region ranging from crown center to shoulder was measured by increasing the internal pressure from 0.3 kgf/cm² to 8.0 kgf/cm² to obtain results as shown in Fig. 7. Examples 6-10, Comparative Example 3 Test tires having a tire size of 11/70 R22.5 were manufactured according to the following Table 3. Then, the durability test on a drum was carried out with respect to these test tires in the same manner as in Example 1 to obtain results as shown in the following Table 4, wherein the running distance (km) of the tire was represented by an index on the basis that Comparative Example 3 was 100. As mentioned above, according to the invention, reinforcement of the crown portion in the tire effectively suppressing separation failure in the cross belt layer can be realized by arranging a strip containing wavy or zigzag-like cords or filaments as a reinforcing element for the crown reinforcing layer in a proper position without causing separation of the layer itself and causing problems in the tire production.

Claims: A pneumatic tire comprising at least one radial carcass ply toroidally extending between a pair of bead portions and a belt reinforcing a tread (3) on said radial carcass, said belt being comprised of at least two cross belt layers (1) each containing cords or filaments as a reinforcing element and crossed with each other at an inclination cord angle of 10°-40° with respect to an equatorial plane of the tire, and at least one crown reinforcing layer (2) disposed between said carcass ply and said cross belt layers, wherein said crown reinforcing layer is comprised of a strip containing cords or filaments arranged substantially in parallel with the equatorial plane of the tire as a reinforcing element, characterized in that the said cords or filaments of the crown reinforcing layer are of wavy or zigzag-like form, and in that said crown reinforcing layer is arranged so that a ratio M defined by the following equation is within a range of 0.25-1.0: wherein m w : total strip number, m B : total cross belt layer number, E wn : elastic modulus of reinforcing element in nth strip, E Bn : elastic modulus of reinforcing element in nth cross belt layer, A wn : sectional area of reinforcing element in nth strip, A Bn : sectional area of reinforcing element in nth cross belt layer, N Wn : end count (cords/50 mm) in nth strip, N Bn : end count (cords/50 mm) in nth cross belt layer. A pneumatic tire as claimed in claim 1, characterized in that said strip has a ratio of amplitude (a) to wavelength (λ) in the wavy or zigzag-like reinforcing element of 0. 015-0. 06. A pneumatic tire as claimed in claim 1 or 2, characterized in that said crown reinforcing layer is arranged so that a distance from the outer end in the widthwise direction of the strip to the equatorial plane of the tire is within a range of 25-47.5% of a maximum width of an actually overlapped region in the cross belt layers. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that the crown reinforcing layer (2) is comprised of a strip extending across the equatorial plane toward both sides in the widthwise direction. A pneumatic tire as claimed in any of claims 1 to 3, characterized in that the crown reinforcing layer (2) is comprised of a pair of strips arranged on both sides of the crown portion.

Inventors: KOBAYASHI YASUHIKO, KOHNO YOSHIHIDE, KOSEKI HIROYUKI, SATO KIYOSHI, KOBAYASHI, YASUHIKO, KOHNO, YOSHIHIDE, KOSEKI, HIROYUKI, SATO, KIYOSHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1990
Main classification: B60C9
IPC codes: B60C9