Pneumatic radial tyres for large-size vehicles

Abstract: The bead portion durability of a pneumatic radial tire for large-size vehicles is improved by disposing a combination of a first protection rubber layer (A) having a 100% modulus of 25-40 kgf/cm 2 and a second protection rubber layer (B) having a 100% modulus of 30-50 kgf/cm 2 in given places of the bead portion (5) so as to locate an inner end (A E ) of the first protection rubber layer (A) in the radial direction of the tire between an outer end (P) of a curved face of the bead heel (8) in the radial direction and an end (Q) of a curved face of a rim flange (7), and to locate an outer end (B S ) of the second protection rubber layer (B) in the radial direction of the tire at such a position outward in the radial direction of the tire that an inclination angle (∝) defined between a line segment (L) connecting said end (B S ) to a curvature center (O) of the curved face of the rim flange (7) and a phantom line (BL) passing through the curvature center (O) and in parallel to a rotating axis of the tire is not less than 20°.

Description: This invention relates to a pneumatic radial tire for large-size vehicles such as trucks, buses, construction vehicles and the like, and more particularly to a pneumatic radial tire for large-size vehicles wherein one bead portion has improved durability. A tire comprising the features of the preamble of claim 1 is known from document US-5033524. A pneumatic radial tire to be mounted onto a large-size vehicle, for example a construction vehicle such as a large-size dump truck used in a large-size engineering work site or ore mining site is subjected to severe service conditions on rough roads under a heavy load, so that there are frequently caused problems in a bead portion contacting with a rim. In this type of tire, a protection rubber layer called a rubber chafer is arranged in a portion contacting with a rim. As performances required of the protection rubber layer, there are mentioned (1) air permeability is less, (2) the bead portion hardly slips against the rim, and (3) cracks hardly occur through friction with the rim flange. In order to satisfy the above performances (1) and (2), it is advantageous to use rubber having a high modulus and less deterioration with lapse of time in the protection rubber layer, while in order to satisfy the above performance (3), it is advantageous to use rubber having a low modulus, so that both rubbers have conflicting material properties. When a protection rubber layer having a high modulus is used for satisfying the performances (1) and (2), therefore, there has hitherto been adopted a structure of disposing a wire chafer on the outside of the carcass as a countermeasure for preventing the occurrence of cracks due to friction with the rim flange. In this case, however, separation failure is easily caused in an end of the wire chafer, particularly an end thereof located at the outside of the carcass as a starting point, so that this countermeasure becomes disadvantageous in view of the durability. On the other hand, when a protection rubber layer having a low modulus is used for satisfying the performance (3), the performances (1) and (2) are ensured by approaching the position of the bead core to the side of the rim to increase the compression ratio of the bead base rubber after the rim assembling onto the rim to thereby strongly push the bead portion toward the rim. In this case, however, the rim assembling operation is obstructed as the compression ratio of the bead base rubber becomes higher and also falling down of a sidewall portion increases due to the low modulus of the protection rubber layer, so that the steering stability is adversely affected. It is, therefore, an object of the invention to provide a pneumatic radial tire for large-size vehicles capable of developing sufficient durability of a bead portion even under severe service conditions subjected to repetitive compression deformation by providing a bead portion structure simultaneously satisfying various performances required in the protection rubber layer. This object is achieved by the features of claim 1. The inventor has made various studies with respect to satisfying various performances required in the protection rubber layer and found that it is effective to use two rubbers in the protection rubber layer and to rationalize the arrangement thereof, and as a result the invention has been accomplished. According to the invention, there is provided a pneumatic radial tire for large-size vehicles comprising a cylindrical crown portion, a pair of sidewall portions extending inward from both ends of the crown portion in a radial direction, a pair of bead portions extending inward from the sidewall portions, and a radial carcass extending between the bead portions through the crown portion, characterized in that the first protection rubber layer A having a 100% modulus of 2·5-4 MPa (25-40 kgf/cm 2 ) is disposed in a region ranging from a vicinity of a bead heel to at least a rim line along an outer surface of the tire when the tire is mounted onto a standard rim and inflated under an internal pressure, and a second protection rubber layer B having a 100% modulus of 3-5 MPa (30-50 kgf/cm 2 ) which is higher than that of the first protection rubber layer A is disposed in a region ranging from the inside of the first protection rubber layer A in a widthwise direction of the tire to a bead base, and an inner end of the first protection rubber layer A in the radial direction of the tire is located between an outer end of a curved face of the bead heel in the radial direction and an end of a curved face of a rim flange, and an outer end of the second protection rubber layer B in the radial direction of the tire is located at such a position outward in the radial direction of the tire that an inclination angle defined between a line segment connecting said end to a curvature center of the curved face of the rim flange and a phantom line passing through the curvature center and in parallel to a rotating axis of the tire is not less than 20°. The term "standard rim" used herein means a rim having an approved size defined in 1996 JATMA YEAR BOOK, while the term "internal pressure" means an air pressure corresponding to a maximum load capacity at an approved size and a ply rating defined in 1996 JATMA YEAR BOOK. Preferably the said inclination angle is not more than 80°. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatic partial section view of a main part of an embodiment of the pneumatic radial tire according to the invention; Fig. 2 is a diagrammatic partial section view of a main part of another embodiment of the pneumatic radial tire according to the invention; Fig. 3 is a diagrammatic partial section view of a main part of a further embodiment of the pneumatic radial tire according to the invention; Fig. 4 is a diagrammatic partial section view illustrating a main part of a bead portion reinforcing structure as a comparative example; Fig. 5 is a schematic view illustrating an evaluation outline of rim slippage; and Fig. 6 is a schematic view illustrating an evaluation outline of a rim assembling operation. In Fig. 1 is shown a left-half section of a pneumatic radial tire for construction vehicles according to the invention in the area of its bead portion, wherein numeral 1 is a bead core, numeral 2 a carcass, numeral 3 a turnup portion of the carcass 2, numerals 4a, 4b reinforcing layers of steel cords mainly disposed inside the carcass 2, numeral 5 a bead portion, numeral 6 a standard rim, and numeral 7 a flange of the rim. The carcass 2 comprises a rubberized steel cord ply of a radial arrangement toroidally extending between a pair of the bead cores 1 and wound around the bead core 1 from the inside of the tire toward the outside thereof so as to form the turnup portion 3. In the bead portion 5 are arranged a first protection rubber layer A having a 100% modulus of 2·5-4 MPa (25-40 kgf/cm 2 ) and a second protection rubber layer B having a 100% modulus of 3-5 MPa (30-50 kgf/cm 2 ), in which the first protection rubber layer A is disposed in a region ranging from a vicinity of a bead heel 8 to at least a rim line 9 outward in the radial direction of the tire along an outer face of the tire, and the second protection rubber layer B is disposed in a region ranging from the inside of the first protection rubber layer A in the widthwise direction of the tire to a bead base 10, whereby various performances required as a protection rubber layer in the bead portion are satisfied. When the 100% modulus of the second protection rubber layer B is less than 3 MPa (30 kgf/cm 2 ), the pressing force of the bead portion to the rim lowers to create slipping of the bead portion on the rim and it is difficult to control falling down of the sidewall portion, while when it exceeds 5 MPa (50 kgf/cm 2 ), cracks are liable to be caused and the rim assembling property is obstructed and becomes unfit for practical use. Moreover, the combined arrangement of protection rubber layers establishing the crack resistance and the resistance to rim slippage at a high level is realized by making the 100% modulus of the second protection rubber layer B higher than that of the first protection rubber layer A. In this case, when the 100% modulus of the first protection rubber layer A is less than 2·5 MPa (25 kgf/cm 2 ), the crack resistance is improved, but the bead portion is slipped from the rim or so-called flatting is caused, while when it exceeds 4 MPa (40 kgf/cm 2 ), the flatting is improved, but the crack resistance is degraded to create diagonal cracks or the like. Further, it is important that an inner end A E of the first protection rubber layer A in the radial direction of the tire is located between an outer end P of a curved face of the bead heel 8 in the radial direction and an end Q of a curved face of the rim flange 7, and an outer end B S of the second protection rubber layer B in the radial direction of the tire is located at such a position outward in the radial direction of the tire that an inclination angle α defined between a line segment L connecting this end B s to a curvature center O of the curved face of the rim flange 7 and a phantom line BL passing through the curvature center O and in parallel to the rotating axis of the tire is not less than 20°. When the inner end A E of the first protection rubber layer A locates inward in the radial direction from the outer end P of the curved face of the bead heel 8 in the radial direction, the pressing force of the bead portion to the rim lowers to create slipping of the bead portion on the rim and also falling down of the sidewall portion is caused. On the other hand, since a zone generating cracks is mainly existent outward from the end Q of the curved surface of the rim flange 7 in the radial direction of the tire, if the inner end A E locates outward from the end Q in the radial direction, the first protection rubber layer A durable to cracking may be not arranged in this zone and the second protection rubber layer B is arranged in such a zone, so that cracks are generated in the second protection rubber layer B. When the end B S of the second protection rubber layer B locates in a position corresponding to the inclination angle α of less than 20°, the volume of the second protection rubber layer B existing in a back region of the first protection rubber layer A is less and hence the pressing force of the bead portion to the rim lowers to create slipping of the bead portion on the rim and also falling down of the sidewall portion becomes conspicuous. On the other hand, if the position of the end B S corresponds to the inclination angle α of more than 80°, there is a danger of locating the end B S into a zone of large sidewall deformation to cause separation failure at an end of the second protection rubber layer B having a high modulus, so that it is preferable to dispose the end B S inward from the position corresponding to the inclination angle α of 80° in the radial direction of the tire. In Figs. 2 and 3 are shown embodiments of disposing the end B s of the second protection rubber layer B outward from the case of Fig. 1 in the radial direction of the tire, respectively. That is, Fig. 2 is an effective case of increasing the pressing force of the bead portion against the rim to prevent slippage of the bead portion on the rim by disposing the end B S between the first protection rubber layer A and the turnup portion 3 of the carcass outward from the case of Fig. 1 in the radial direction of the tire. In this case, the inclination angle α is about 65°. On the other hand, Fig. 3 is an effective case of controlling falling down of the sidewall portion by disposing the end B S along the turnup portion 3 of the carcass outward from the case of Fig. 1 in the radial direction of the tire. In this case, the inclination angle α is about 65°. On the contrary to these cases, Fig. 1 is an effective case of avoiding shifting of the bead portion in the rim assembling by disposing the end B S inward in the radial direction of the tire to thicken the first protection rubber layer A. In this case, the inclination angle α is about 45°. The following examples are given in illustration of the invention and are not intended as limitations thereof. Three pneumatic tires for construction vehicles having a tire size of 26.5R25 (corresponding to PR of 26.5-25) are prepared according to a structure as shown in Figs. 1-3 under a specification shown in Table 1. For comparison, there are prepared two tires having a structure as shown in Figs. 4a and 4b. That is, the tire of Fig. 4a has a structure that only a protection rubber layer B having a high modulus is arranged without a protection rubber layer A and a wire chafer 11 is wound around one bead core 1 along an outer face of the carcass 2, and the tire of Fig. 4b has a structure that only a protection rubber layer A having a low modulus is arranged without a protection rubber layer B and the position of the bead core 1 is located near to the rim as compared with the tire of Fig. 4a to enhance the compression ratio of the bead base rubber to thereby strongly push the bead portion onto the rim. The following properties are evaluated with respect to these tires to obtain results as shown in Table 1. Each of these tires is mounted onto a standard rim and run on a drum of 5 m in diameter at a speed of 10 km/h under an air pressure of 0·5 MPa (5.0 kgf/cm 2 ) and a test load of 22500 kg (150% load) to conduct a durability test. In this case, the separation resistance is evaluated by a running time until separation failure is created in a zone along the turnup portion outward from the tire or at an end portion of the wire chafer (see Fig. 4a), while the crack resistance is evaluated by product of crack number and depth measured on the surface of the bead portion after running for 50 hours. Each of these properties is represented by an index on the basis that Example 1 is 100, in which the larger the index value, the better the property. Furthermore, the rim slippage is evaluated by resistance value applied to a planer 15 when a rim 14 mounted with a test tire 13 is fixed to a rotating shaft 12 and the planer 15 is driven at a state of applying a braking force of 30 t to the rotating shaft 12 through a braking device (not shown) as shown in Fig. 5. The measured resistance value is represented by an index on the basis that a limit force at a ground contact zone of the tire under a vehicle weight is 100, in which the larger the index value, the less the rim slippage. Also, the rim assembling operation is evaluated by a gap a between an inner face of a rim flange 7 and an outer surface of a bead portion which is calculated by assembling a test tire 13 onto a standard rim 6 provided with six holes 16 equally formed in a rim flange 7 near to the rim base over the full circumference and measuring a depth b from an outer surface of the rim flange to an outer surface of a bead portion of the tire through the hole 16 and subtracting the thickness of the rim flange from the measured value. The rim assembling operation is represented by an index on the basis that the gap at a=0 is 100, in which the larger the index value, the better the property. Example 1 (Fig. 1) Example 2 (Fig. 2) Example 3 (Fig. 3) Comparative Example 1 (Fig. 4a) Comparative Example 2 (Fig. 4b) M 100 of protection rubber layer A MPa 3.0 3.0 2.6 - 3.0 M 100 of protection rubber layer B MPa 4·5 4·8 4·8 4·5 - End A E of rubber layer A 23 mm (middle point between P and Q) 30 mm (near to Q) 12 mm (near to P) - - End B s of rubber layer B (inclination angle α°) 45° 65° 65° (adjacent to turnup portion) - - Separation resistance 100 100 120 80 100 Crack resistance 100 100 120 100 100 Rim slippage 100 120 100 100 80 Rim assembling operation 100 100 100 100 100 As mentioned above, the invention can improve the bead portion durability particularly required in pneumatic radial tires for large-size vehicles so as to satisfy the demand for improving the transportation under severe service conditions such as increase of authorized payload accompanied with the increased size of construction vehicles and use on rough roads having many curves.

Claims: A pneumatic radial tire comprising a cylindrical crown portion, a pair of sidewall portions extending inward from both ends of the crown portion in a radial direction, a pair of bead portions (5) extending inward from the sidewall portions, a radial carcass (2) extending between the bead portions through the crown portion, and two protection layers (A,B) located in the bead portions, characterized in that the first protection rubber layer (A) having a 100% modulus of 2.5-4 MPa (25-40 kgf/cm 2 ) is disposed in a region ranging from a vicinity of a bead heel (8) to at least a rim line (9) along an outer surface of the tire when the tire is mounted onto a standard rim (6) and inflated under an internal pressure, and the second protection rubber layer (B) having a 100% modulus of 3-5 MPa (30-50 kgf/cm 2 ) which is higher than that of the first protection rubber layer (A) is disposed in a region ranging from the inside of the first protection rubber layer (A) in a widthwise direction of the tire to a bead base (10), and an inner end (A E ) of the first protection rubber layer (A) in the radial direction of the tire is located between an outer end (P) of a curved face of the bead heel (8) in the radial direction and an end (Q) of a curved face of a rim flange (7), and an outer end (B S ) of the second protection rubber layer (B) in the radial direction of the tire is located at such a position outward in the radial direction of the tire that an inclination angle (∝) defined between a line segment (L) connecting said end (B S ) to a curvature center (O) of the curved face of the rim flange (7) and a phantom line (BL) passing through the curvature center (O) and in parallel to a rotating axis of the tire is not less than 20°. A pneumatic radial tire as claimed in claim 1, characterized in that said inclination angle (∝) is not more than 80°.

Inventors: NAKAMURA YOICHI, NAKAMURA, YOICHI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1998
Main classification: B60C15
IPC codes: B60C15