Heavy duty pneumatic radial tyres

Abstract: In a heavy duty pneumatic radial tire, when the belt (5) is comprised of not less than three main belt layers (1B, 2B, 3B), a belt layer (Br) other than a belt layer (1B) adjacent to a carcass (4) is a narrow-width rigidity reinforcing layer, or when the belt is comprised of not less than four main belt layers (1B, 2B, 3B, 4B), a belt layer (1Br) adjacent to the carcass (4) and a belt layer (4Br) other than a belt layer (2B) adjacent to the belt layer (1Br) are narrow-width rigidity reinforcing layers. Such a narrow-width rigidity reiforcing layer has a given width (Wr) of 50-60% of the maximum width (Wb) of the main belt layer and an inclination cord angle of 5-15°.

Description: This invention relates to a pneumatic radial tire having an improved belt durability, and more particularly to a heavy duty pneumatic radial tire capable of advantageously attaining resistance to cut separation and resistance to cut penetration in its tread portion. When tires for heavy duty vehicles are run on roads including rough roads, bad roads or roads having scattered protrusions, they are liable to be subjected to deep cut damage in an outer portion of the tread or tread rubber, but are also strongly liable to be subjected to cut separation failure. The term "cut separation failure" used herein means that when the tire is subjected to deep cut damage which reaches the belt located in a central region of the tread portion, such damage leads to separation from the cut damaged portion along the outer peripheral surface of the belt during the running of the tire. The cut separation failure results from the action of shearing strain produced between the outer peripheral surface of the belt and the tread rubber contacting therewith accompanied with the bending deformation behaviour of the belt in the peripheral direction of the tread (hereinafter referred to as the circumferential direction) at the stepping-in side and the kicking-out side of the tread portion during the running of the tire. Since the cut separation failure is created between the tread rubber and the outer peripheral surface of the belt, it is a serious problem because the separation region is remarkably enlarged or tread rubber is sometimes peeled off at the separation region. It has therefore been attempted to enhance the bending rigidity of the belt in the circumferential direction for controlling the shearing strain as far as possible. In fact, it has been attempted to make the inclination angle of cords, particularly steel cords, in the belt with respect to the circumferential direction as small as possible. However, the above measure does not particularly improve the resistance to cut separation. This is because separation failure is also likely to be caused at the end of the belt as the inclination cord angle becomes smaller. Considering this fact, making the inclination cord angle small to improve the bending rigidity of the belt in the circumferential direction is naturally critical and hence it is difficult to develop the desired resistance to cut separation. Furthermore, when the inclination cord angle is made small to enhance the bending rigidity of the belt as a whole, an enveloping property (EP) in the riding of the tread portion over projections such as rocks or stones or EP property against projections is deteriorated, so that the likelihood of subjecting the tread portion to cut damage increases but also the cut depth becomes deeper, which sometimes brings about a critical failure of passing the cut damage through the whole of the tread portion or so-called cut penetration failure. Attention is also drawn to the disclosure of GB-A-2072590, which shows the features of the preamble of claim 1. It is, therefore, an object of the invention to provide a pneumatic radial tire possessing good resistance to cut separation and sufficient resistance to cut penetration while sufficiently maintaining the desired separation resistance at the end of the belt. According to the present invention, there is provided a pneumatic radial tire comprising a radial carcass extending between a pair of bead cores for reinforcing sidewall portions and a tread portion, and a belt reinforcing the tread portion at the outside of the carcass and comprised of a plurality of steel cord layers, in which said belt is comprised of not less than four main cross belt layers, steel cords of which layers being crossed with each other between adjacent belt layers, and a maximum width in the main cross belt layers is within a range of 70-85% of a tread width of the tread portion, and each of a belt layer adjacent to the carcass and a remaining belt layer other than a belt layer facing to the belt layer adjacent to the carcass among the above main cross belt layers is a narrow-width rigidity reinforcing layer having a width corresponding to 50-60% of the maximum width of the main belt layer and an inclination cord angle of 5-15° with respect to an equatorial plane of the tire. The invention will be further described with reference to the accompanying drawings, wherein: Figs. 1a and 1b are schematic sectional views of main parts of embodiments of heavy duty pneumatic radial tires according to the invention, respectively; Figs. 2a and 2b are schematic sectional views of main parts of conventional heavy duty pneumatic radial tires; and Fig. 3 is a graph showing a distribution of circumferential tension in a widthwise direction of a main cross belt layer. In Figs. 1a and 1b are schematically and sectionally shown main parts of embodiments of the heavy duty pneumatic radial tires according to the invention in a left-half section of the tire. Moreover, each of these tires is bisymmetrical with respect to an equatorial plane of the tire. In these figures, numeral 1 is a sidewall portion, numeral 2 a tread portion, numeral 3 a tread rubber, numeral 4 a carcass and numeral 5 a belt. The carcass 4 is a radial rubberized cord ply extending between a pair of bead cores (not shown) for reinforcing the sidewall portion 1 and the tread portion 2, and the belt 5 is a laminate of plural steel cord layers. The belt 5 shown in Fig. 1a is comprised of four main cross belt layers 1B, 2B, 3B and 4B, and the belt 5 shown in Fig. 1b is comprised of four main cross belt layers 1B, 2B, 3B and 4B and a protection belt layer Bp. Moreover, the main cross belt layers are represented by appending symbol B as a belt layer with numerals 1-4 for first to fourth layers gradually arranged outward from a position adjacent to the carcass in the radial direction of the tire (except for the narrow-width rigidity reinforcing layer). The cords of the main cross belt layers 1B-4B are crossed with each other between the adjacent layers, i.e. between 1B and 2B, between 2B and 3B, and between 3B and 4b with respect to the equatorial plane. When the width of the tread portion 2 is Wt and the maximum width of the main cross belt layers is Wb, the maximum width Wb is within a range of 70-85% of the tread width Wt. The narrow-width rigidity reinforcing layer according to the invention is shown by a solid line in each of Figs. 1a and 1b and represented as Br appended with symbol r. According to the invention, the narrow-width rigidity reinforcing layer corresponds to a belt layer 1Br adjacent to the carcass 4 and a remaining belt layer (4Br in the illustrated embodiment) other than a belt layer 2B adjacent to the belt layer 1Br as shown in Figs. 1a and 1b. Of course, the narrow-width rigidity reinforcing layer Br near to the tread in the invention may be applied to any belt layer B located near to the tread. In the figures, the width of the narrow-width rigidity reinforcing layer Br is represented by Wr, which is within a range of 50-60% of the maximum width Wb. The cords in the narrow-width rigidity reinforcing layer Br are arranged at an inclination cord angle of 5-15° with respect to an equatorial plane E of the tire. Moreover, it is desirable that the belt layers B other than the belt layers Br have an inclination cord angle of 18-35° with respect to the equatorial plane E. As a result, the cords of the belt layer Br bear a considerably large tension upon inflation under internal pressure, so that the steel cord used in the belt layer Br preferably has a diameter larger than that of the other belt layers B or a tensile strength higher than that of the belt layers B. Since the arrangement of the protection belt layer Bp is to protect the main cross belt layers from cutting, it is desirable that a rubberized layer of so-called high elongation steel cords as the protection belt layer Bp is arranged on the outer peripheral side of the main cross belt layer. As a result of various studies, it has been found that cut separation failure grows from cut damage generated in the tread portion 2, particularly the central region thereof. Therefore, it has been clarified that the occurrence of the cut separation failure can effectively be controlled by reducing the aforementioned shearing strain in the central region or enhancing the bending rigidity of the belt 5 at the central region in the circumferential direction. On the other hand, increasing the bending rigidity of the belt 5 in the circumferential direction depends on the effect of laminating the belt layers comprising the belt 5 and is considerably dependent upon the degree of the circumferential tension produced in the belt at the time of inflating under internal pressure. From this fact, it has been confirmed that the bending rigidity at the central region of the tread portion 2 may be enhanced by increasing the tension of the belt 5 at the central region at the time of inflating under internal pressure. As a result of further studies, it has been found that the value of the shearing strain produced in the central region of the tread portion 2 is dependent upon the value of circumferential tension acting on the central region of the main cross belt layer having a width corresponding to 50-60% of the maximum width Wb of the main cross belt layer in the belt 5, or the value of the shearing strain can be decreased by increasing the value of the circumferential tension. The distribution of the circumferential tension acting on the belt 5 upon inflation of the tire under internal pressure indicates a parabolic curve in which a maximum value is existent at a widthwise center position E' of the belt 5 (substantially the same as the equatorial plane E of the tire) as shown in Fig. 3. In Fig. 3, a solid line is the distribution of the conventional tire, and dotted lines are the distribution of an embodiment of the tire according to the invention. As a result of examining the sum of circumferential tension (areas A, A' defined between axis of abscissa and curved line), it has been confirmed that the sum of circumferential tension is substantially constant even when the structure of the belt layers comprising the belt 5 is changed, for example the inclination cord angle with respect to the equatorial plane E is changed (i.e. area A ≃ area A'), and that there is not observed a large change in the distribution curve of circumferential tension even when the inclination cord angle in each belt layer is made small. When a belt is comprised of main cross belt layers 1B-4B having a width corresponding to 70-85% of the tread width Wt and a belt layer other than the belt layer 1B among these main cross belt layers is a narrow-width rigidity reinforcing layer Br having a width Wr corresponding to 50-60% of the maximum width Wb of the main cross belt layer and an inclination cord angle of 5-15° with respect to the equatorial plane E, it is possible to change the distribution of circumferential tension in the belt 5 into a curve shown by dotted lines in Fig. 3 wherein the circumferential tension is considerably protruded at a central portion in the widthwise direction. This is due to the fact that the bearing of circumferential tension by the narrow-width rigidity reinforcing layer Br becomes considerably large as compared with those of the main cross belt layers B other than the layer Br. Thus, the circumferential tension acting on the belt 5 can be concentrically borne by the central region of the belt 5, so that the bending deformation at the central region of the tread portion 2 is largely controlled and hence the shearing strain produced between the tread rubber 3 and the belt 5 at the stepping-in side and the kicking-out side of the tread portion 2 is considerably decreased. Even if deep cut damage arriving at the belt 5 is caused in the central region of the tread portion 2, therefore, the growth of separation failure from such cut damage can effectively be prevented to improve the resistance to cut separation. According to the invention, when the belt is comprised of four main cross belt layers 1B-4B having a width corresponding to 70-85% of the tread width Wt and two belt layers among these layers are narrow-width rigidity reinforcing layers Br having a width Wb corresponding to 50-60% of the maximum width Wb of the main cross belt layer and an inclination cord angle of 5-15° with respect to the equatorial plane E and arranged as an innermost layer 1Br and as a belt layer other than the layer 2B adjacent to the layer 1Br (4Br in the illustrated embodiment) as shown in Figs. 1a and 1b, the circumferential tension at the central region of the belt 5 shown by dotted lines in Fig. 3 is further increased and hence the resistance to cut separation is further improved. If the two narrow-width rigidity reinforcing layers Br were adjoined to each other and laminated one upon the other, since the inclination cord angle with the equatorial plane E is small in each layer, a large shearing strain would be generated between the end portions of these layers Br upon inflation under internal pressure and upon contact of the tread portion 2 with the ground and hence separation failure is apt to be caused between these end portions, and therefore it is required to avoid the adjacent lamination of the two narrow-width rigidity reinforcing layers. Moreover, the increase of cord tension at the narrow-width rigidity reinforcing layer upon riding over a projection is largely mitigated by arranging a pair of narrow-width rigidity reinforcing layers 1Br and 4Br because the increase of cord tension is dispersedly borne by both the layers 1Br and 4Br, so that the danger of causing cord breaking-up in the layer 1Br can be removed. By arranging the narrow-width rigidity reinforcing layers Br having a high bending rigidity at the central region of the belt 5, the rigidity at both sides of the central region is largely decreased as compared with the rigidity of the conventional belt 5' (see Figs. 2a and 2b), so that the tread portion 2 provided with the belt 5 including the narrow-width rigidity reinforcing layers Br exhibits a behaviour capable of easily deforming in the widthwise direction of the tread as compared with the tread portion 2 provided with the conventional belt 5' when the tread portion 2 rides over the projections. This largely improves not only the EP property but also the cut resistance and resistance to cut penetration. Moreover, these properties can be more improved by disposing the protection belt layer Bp of high elongation steel cords outside the main cross belt layers. When the inclination cord angle of the narrow-width rigidity reinforcing layers Br with respect to the equatorial plane E exceeds 15°, the improvement of circumferential tension at the central region of the belt 5 is insufficient, while when it is less than 5°, separation failure is apt to be caused at the end portion of the narrow-width rigidity reinforcing layers Br even if the narrow-width rigidity reinforcing layers are not laminated in adjacent relation. Further, the reason why the width Wr of the narrow-width rigidity reinforcing layer Br is restricted to a range of 56-60% of the maximum width Wb of the main cross belt layer in the belt 5 is due to the fact that the shearing strain causing the cut separation failure corresponds to the distribution of circumferential tension at a region corresponding to 50-60% of the maximum width Wb of the main cross belt layer as previously mentioned. Moreover, the separation failure not based on the cut damage is usually caused at the end portion of the wide-width belt layers in the belt 5. In the tires according to the invention, the resistance to such a separation can sufficiently be maintained as compared with the conventional tire even when a narrow-width rigidity reinforcing layer Br is arranged among the main cross belt layers in the belt. The following examples are given in illustration of the invention and are not intended as limitations thereof. (a) Radial tires for trucks and buses (TBR) There is provided an invention tire having a tire size of 275/80R22.5 in which a tire of Example 1 is shown in Fig. 1a. In these tires, the tread width Wt is 210 mm, and the carcass 4 is a radial rubberized ply of steel cords, and steel cord used in each layer of the belt 5 has a structure of (3+9+15) x 0.23 mm and a cord diameter of 1.4 mm. Moreover, there are provided tires of Conventional Example 1 and Comparative Example 1 having the same structure as in the Example except for the structure of the belt 5' shown in Fig. 2a. The dimensions of the belt layers 1B-4B in the belts 5 and 5' are shown in Table 1 in which each of the belt layers 1B and 4B in Example 1 is a narrow-width rigidity reinforcing layer Br. In Table 1, the term "width/angle" means width of the belt layer (mm)/inclination cord angle (with respect to the equatorial plane), and symbols R and L appended before the angle mean arrangements of cord upward to the right and the left, respectively. Furthermore, the term "end count" means the number of cords to be embedded per 25 mm. (b) Radial tires for construction vehicles (ORR) There is provided an invention tire having a tire size of 37.00R57 in which a tire of Example 2 is shown in Fig. 1b. In these tires, the tread width Wt is 840 mm, and the carcass 4 is a radial rubberized ply of steel cords, and steel cord used in each layer of the belt 5 is as follows: Layers 4B, 3B: twisting structure of (3+9+15) x 7 x 0.175 mm cord diameter of 3.2 mm Layers 2B, 1B: twisting structure of (3+9) x 7 x 0.23 mm cord diameter of 2.8 mm Layer Bp: twisting structure of 3 x 7 x 0.23 mm cord diameter of 1.6 mm Moreover, there are provided tires of Conventional Example 2 and Comparative Example 2 having the same structure as in the example except for the structure of the belt 5' shown in Fig. 2b. The dimensions of the belt layers 1B-4B in the belts 5 and 5' are shown in Table 2 in which each of the belt layers 1B and 4B in Example 2 is a narrow-width rigidity reinforcing layer Br. The terms "width/angle" and "end count" in Table 2 have the same meanings as in Table 1. The resistance to cut separation and resistance to cut penetration are evaluated by the following methods with respect to these tires of Examples 1-2, Conventional Examples 1-2 and Comparative Examples 1-2. Resistance to cut separation: A test tire previously provided at its widthwise central region of the tread with cut damage arriving at the belt 5, 5' is run on a drum under an air pressure (kgf/cm2) and a load corresponding to maximum loading ability (kg) for a given time and thereafter a length of separation failure from the cut damage is measured. Resistance to cut penetration: It is evaluated according to a so-called plunger test. A plunger is a steel rod having a diameter of 38 mm for a TBR tire or 90 mm for an ORR tire and a semi-circular shape at its top is fixed vertically and the same test tire as mentioned above is contacted with the semi-circular top and pushed downward in the vertical direction, during which energy required for breaking the tire by the plunger (integral value of force F applied to the plunger and stroke S until breakage) is measured. The measured results are also shown in Tables 1 and 2. Each of both the properties is represented by an index value on the basis that each of Conventional Example 1 for TBR tire and Conventional Example 2 for ORR tire is 100. The larger the index value, the better the property. Conventional Example 1 Comparative Example 1 Example 1 4B width/angle - - 95/R10° end count - - 9.3 3B width/angle 125/R21° 125/R18° 155/L20° end count 9.0 9.1 9.0 2B width/angle 155/L20° 155/L18° 175/R25° end count 9.0 9.1 8.5 1B width/angle 175/R25° 175/R23° 100/L12° end count 8.5 8.7 9.3 Resistance to cut separation 100 108 133 Resistance to cut penetration 100 88 118 Conventional Example 2 Comparative Example 2 Example 2 4B width/angle 480/L22° 480/L20° 340/L9° end count 6.0 6.0 6.3 3B width/angle 550/R22° 550/R18° 550/R22° end count 6.0 6.1 6.0 2B width/angle 620/L22° 620/L18° 620/L22° end count 6.5 6.6 6.5 1B width/angle 560/R22° 560/R20° 360/R12° end count 6.5 6.5 6.7 Resistance to cut separation 100 110 135 Resistance to cut penetration 100 85 115 As seen from Tables 1 and 2, the tires of Examples 1-2 have very good in the resistance to cut separation and resistance to cut penetration as compared with the tires of Conventional Examples 1 and 2. On the contrary, the tires of Comparative Examples 1 and 2 have lower resistance to cut penetration, as compared with the tires of Conventional Examples 1 and 2. Moreover, the resistance to separation at the end of the belt 5 in the tires of Examples 1-2 is substantially equal to that in the tires of the Conventional Examples and Comparative Examples although the evaluation results are not shown in Tables 1 and 2. As mentioned above, according to the invention, there can be provided heavy duty pneumatic radial tires capable of enhancing the resistance to cut separation and improving the resistance to cut penetration while sufficiently maintaining the separation resistance at the end of the belt and having a long service life.

Claims: A pneumatic radial tire comprising a radial carcass (4) extending between a pair of bead cores for reinforcing sidewall portions (1) and a tread portion (2), and a belt (5) reinforcing the tread portion at the outside of the carcass and comprised of a plurality of steel cord layers, in which said belt (5) is comprised of not less than four main cross belt layers (1B, 2B, 3B, 4B), steel cords of which layers being crossed with each other between adjacent belt layers, and each of a belt layer (1Br) adjacent to the carcass (4) and a remaining belt layer (4Br) other than a belt layer (2B) facing to the belt layer (1Br) adjacent to the carcass among the above main cross belt layers is a narrow-width rigidity reinforcing layer having a width (Wr) corresponding to 50-60% of the maximum width (Wb) of the main belt layer, characterized by the maximum width (Wb) in the main cross belt layers (2B, 3B) being within a range of 70-85% of a tread width (Wt) of the tread portion (2), and an inclination cord angle of 5-15° with respect to an equatorial plane (E) of the tire. A pneumatic radial tire as claimed in claim 1, characterized in that the main cross belt layers (B) other than the narrow-width rigidity reinforcing layers (Br) have an inclination cord angle of 18-35° with respect to the equatorial plane (E). A pneumatic radial tire as claimed in claim 1 or 2, characterized in that a protection belt layer (Bp) is arranged on the main cross belt layer (4B) located outward in the radial direction of the tire. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that the main cross belt layers (2B, 3B) are sandwiched between the narrow-width rigidity reinforcing layers (1Br, 4Br).

Inventors: KOBAYASHI YASUHIKO, KOBAYASHI, YASUHIKO
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1995
Main classification: B60C9
IPC codes: B60C9