Heavy duty radial tyre

Abstract: A heavy duty radial tyre comprises a carcass ply (8) turned up around bead cores (5) form the axially inside to the outside of the tyre, a bead apex (8) disposed between the carcass main portion (6A) and each turnup portion (6B), each turnup portion (6B) extending radially outwardly beyond the radially outer end (8t) of bead apex (8) so that the turnup portion (6B) has a parallel part (G) which is placed adjacent to and substantially in parallel with the main portion (6A), and a chafer (9) disposed in each bead portion (4) and extending radially outwardly beyond the radially outer end (8t) of the bead apex (8) along the axially outer side of the turnup portion (6B). The thickness T2 of the chafer is 0.3 to 1.0 times the total rubber thickness T1 between the tyre outer surface and the carcass (6) when measured along a straight line drawn at a right angle to the carcass main portion (6A) through the radially outer end (8t) of the bead apex (8). The length L of the parallel part (G) is in the range of 0.5 to 5.0 times the section width CW of the bead core (5). In the parallel part (G), the rubber thickness N between the carcass cords of the turnup portion (6B) and those of the main portion (6A) is 0.15 to 4.5 times the maximum diameter K of the carcass cords.

Description: The present invention relates to a pneumatic tyre, more particularly to a heavy duty radial tyre having an improved bead and lower sidewall structure capable of improving bead durability. In radial tyres for heavy duty vehicles such as trucks and buses, the bead portion and sidewall lower portion are subjected to a large bending deformation, and accordingly, the outer surface thereof is liable to crack, which sometimes cause further damage such as carcass ply looseness. If the bead or sidewall surface is cracked it is impossible to recycle such tyres by retreading. In order to improve bead durability by increasing the rigidity of the bead portions, hitherto, as shown in Fig.10(A), the bead apex rubber (b) which inserted between the carcass main portion (a1) and turnup portion (a2) has been increased in volume and height. In such a conventional structure, however, under severe service conditions, the increased rubber volume increases heat generation. Further, when the sidewall portions are greatly bent axially outwardly, the carcass ply turnup portion (a2) is subjected to a large compression strain, and this portion is liable to crease. Furthermore, as the radially outer end of the turnup portion approaches the tyre outer surface a relatively large stress is concentrated on the end and carcass ply edge separation, cord looseness and cracks are liable to occur. Thus, bead durability is lost quickly. A radial tyre in accordance with the preamble of claim 1 is known from EP 0 715 977, wherein it is proposed to provide a rubber chafer in each bead portion to define a rim contacting surface and each carcass turnup extends beyond the bead apex so that the turnup has a parallel part adjacent to and parallel to the main carcass portion. The chafer extends radially along the outer side of the turnup beyond the apex. It is therefore, an object of the present invention to provide a heavy duty radial tyre, in which internal temperature rise, creasing of the carcass turnup portion, carcass ply edge separation, cord loose and sidewall cracks are effectively controlled to improve bead durability. Accordingly one aspect of the present invention provides a radial tyre as set out in claim 1. Further features of the invention are in the subclaims. Therefore, the bead apex height is decreased and the rubber volume between the carcass turnup portion and main portion is decreased. On the other hand the rubber volume on the axially outer side of the carcass turnup portion is increased by the chafer. Accordingly, the main heat source shifts from the bead apex to the chafer, and generated heat is easily radiated. Further, by the existence of the parallel part, the turnup portion is prevented from being subjected to compressive strain even if the tyre is greatly deformed. Therefore, internal temperature rise, carcass turnup crease, carcass ply edge separation, cord looseness and sidewall cracks can be prevented, and the bead durability is effectively improved. An embodiment of the present invention will now be described in detail in conjunction with the accompanying drawings:- . Fig.1 is a cross sectional view of a heavy duty radial tyre showing an embodiment of the present invention; Fig.2 is a cross sectional view showing an example of the bead portion; Fig.3 is a cross sectional view showing another example of the bead portion; Fig.4 is an enlarged cross sectional view for explaining the cord spacing in the parallel part; Fig.5 is an enlarged cross sectional view for explained the chafer rubber thickness under the bead core; Fig.6 is a cross sectional view for explaining the contour of the axially outer surface of the tyre; Figs.7 and 8 are diagrams for explaining the radially inward extent of the concave contour; Fig.9 is a cross sectional view showing still another example of the bead structure; Fig.10A is a schematic cross sectional view for explaining a conventional bead structures; Fig.10B is a schematic cross sectional view showing the bead structure of a reference tyre used in comparison tests; , Fig.11 is a graph showing distributions of maximum principal strain; and . Figs.12 and 13 are diagrams for explaining a method of measuring the maximum principal strain. In the drawings, a tyre 1 according to the present invention comprises a tread portion 2, a pair of axially spaced bead portions 4 each with a bead core 5 therein, a pair of sidewall portions 3 extending between the tread edges and the bead portions, a carcass 6 extending between the bead portions 4 and a belt 7 disposed radially outside the carcass 6 in the tread portion 2. Fig.1 shows a meridian section of the tyre under the standard condition in which the tyre is mounted on a standard rim J, inflated to standard pressure and loaded with no tyre load. The standard rim is a rim specified as standard rim or measuring rim in the well known standards, such as JATMA (Japan and Asia), T&RA (North America), ETRTO (Europe), STRO (Scandinavia) and the like, and the standard pressure is a maximum pressure specified in the standard. In this embodiment, the tyre 1 is a tube-less heavy-duty radial tyre for trucks and buses, which is mounted on a 15 deg. tapered rim. The above-mentioned carcass 6 comprises at least one ply 6A of cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator C, and extending between the bead portions 4 through the tread portion 2 and the sidewall portions 3, and turned up around the bead cores 5 from the axially inside to outside of the tyre, so as to form a pair of turnup portions 6B and a main portion 6A therebetween. For the carcass cords, preferably, steel cords are used, but organic fibre cords, e.g. polyester, rayon, nylon, aromatic polyamide and the like can be used. In this example, the carcass 6 is composed of a single ply 6a of steel cords arranged at substantially 90 degrees with respect to the tyre equator C. The belt 7 comprises at least two crossed plies. For the belt cords, steel cords, organic fibre cords, e.g. nylon, aromatic polyamide, rayon, nylon and the like can be used. In this example, the belt 7 is composed of four plies; a radially innermost ply 7A made of parallel steel cords laid at a predetermined angle of 50 to 70 degrees with respect to the tyre equator C; and radially outer plies 7B, 7C and 7D made of parallel steel cords laid at angles of not more than 30 degrees with respect to the tyre equator C. Each bead core 5 is formed by winding a steel wire a predetermined number of times to form a generally hexagonal cross sectional shape and the outside thereof is coated with rubber. Aside from steel wire, high modulus organic cords, e.g. aromatic polyamide and the like may be used. A radially inner side 5i of the hexagon which is the longest side is inclined at 10 to 17 degrees, in this example almost 15 degrees with respect to the axial direction which corresponds to the inclination of the tapered bead seat J1 of the standard rim J. The above-mentioned bead portions 4 are each provided with a bead apex 8 and a chafer 9. The bead apex 8 is disposed between the carcass ply main portion 6A and turnup portion 6B. The bead apex 8 is made of rubber preferably having a JIS A hardness of 60 to 99 degrees, more preferably 70 to 95 degrees. The bead apex 8 extends radially outwardly from the bead core 5 and tapers radially outwardly. Under the above-mentioned standard condition, the bead apex height h1 is in the range of from 6 to 31 %, preferably 8 to 28 %, more preferably 8 to 25% of the carcass section height H. (in Fig.2 about 11%) The bead apex height h1 is measured radially between the radially outer end 8t of the bead apex 8 and the bead base line BL. The carcass height H is measured radially between the thickness centre line of the carcass and the bead base line BL along the tyre equator C. Incidentally, the bead base line BL corresponds to the rim diameter of the standard rim. The above-mentioned carcass turnup portion 6B first extends radially outwardly along the axially outer surface 8o of the bead apex 8 from the axially outer extreme end of the bead core 5 to the radially outer end of the bead apex 8. Then, radially outside of the bead apex 8, the turnup portion 6B extends substantially in parallel with the axially outermost carcass ply main portion 6A (hereinafter, parallel part G) The turnup portion 6B terminates at a position (height h2) which is radially outward of the outer end 8t of the bead apex 8 but radially inward of the maximum width point P1 of the tyre. The height h2 measured from the bead base line BL is preferably in the range of from 15 to 50 %, more preferably 20 to 40 % of the carcass section height H. (in this example 29%) The axially outer surface 8o of the bead apex 8 is defined by a concave line. Thus, the corresponding adjacent part of the carcass turnup portion 6B also has a similar concave profile. On the other hand, the axially inner surface 8i of the bead apex 8 is defined by a substantially straight line. Thus, the adjacent part of the carcass main portion 6A which contacts therewith has a straight profile, whereby deformation of the bead portion due to tyre inflation and application of tyre load can be reduced. In the parallel part G, as shown in Fig.4, the cord spacing N between the axially adjacent carcass main portion and turnup portion is set in the range of from 0.15 to 4.5 times, preferably 1.3 to 3.5 times the maximum diameter K of the carcass cords. That is, a rubber layer whose minimum thickness corresponds to the cord spacing N exists between the carcass main portion 6A and turnup portion 6B, whereby the share therebetween can be effectively mitigated. In this example, a separate rubber layer 12 called a cushion rubber having elastic characteristics similar to the topping rubber for the carcass ply is disposed between the main portion 6A and turnup portion 6B. However, it is also possible to use the topping rubber itself as a substitute for the cushion rubber layer 12 if a required uniform thickness is provided. If the carcass 6 is composed of two or more plies, it is necessary for least one ply to have the above-explained parallel part G. The parallel part G has a length L in the range of from 0.5 to 5.0 times, preferably 1.0 to 4.0 times, more preferably 2.0 to 4.0 times the maximum-section-width CW of the bead core 5 (in this example about 2.6 times). The maximum-section-width CW of the bead core 5 is measured along a direction parallel with the bead bottom line, excluding its coating rubber. If the rubber thickness N is less than 0.15 times the maximum diameter K, the mitigation becomes insufficient, and sometimes the cords of the turnup portion 6B partially come into direct contact with those of the main portion 6A which causes carcass cord looseness. If the thickness N is more than 4.5 times K, even if the main portion 6A and turnup portion 6B are parallel, the turnup portion 6B is liable to be broken when compressed. Further, the effect to reduce the maximum principal strain ε on the outer surface of the region Y has a tendency to decrease, and the heat generation is liable to increase. Here, the region Y is radially inward of the maximum cross-sectional width point P1. If the length L is less than 0.5 times the width CW, the maximum principal strain ε has a remarkable peak in the region Y, and cracks are liable to occur at the position of the peak at a relatively early stage. If more than 5.0 times the width CW, as the outer end of the carcass turnup portion 6B is placed in a region in which the sidewall rubber is relatively thin. Thus, the outer surface of the tyre is liable to undulate, and further the rubber is liable to crack. The above-mentioned chafer 9 is disposed along the axially outer and inner and bottom surface of the bead portion 4 to be exposed at least from the bead toe to a point P3 above the bead heel, defining a surface which direct contacts with the rim. The chafer 9 is made of a rubber having a 100% modulus of from 55 to 75 kgf/sq.cm which is harder than the sidewall rubber 10 which is disposed axially outside the carcass to define the sidewall portion and has a 100% modulus of from 10 to 20 kgf/sq.cm. On the axially outside of the carcass 6 (turnup portion 6B), the chafer 9 extends radially outwardly from the underside of the bead core 5 beyond the radially outer end 8t of the bead apex 8. The radially outer end portion thereof is tapered, and the radially inner end portion of the sidewall rubber 10 is also tapered. Those tapered portions are spliced to each other. In this spliced portion, the chafer 9 is axially inside of the sidewall rubber 10. Thus, the axially outer side thereof abuts the axially inner surface of the sidewall rubber 10, and the axially inner side abuts the axially outer surface of the carcass turnup portion 6B. The height h4 of the above-mentioned point P3 is preferably set in the range of from 1.2 to 2.5 times the height h5 of the rim flange J, each measured from the bead base line BL. The height h3 of the radially outer end 9t of the chafer 9 is preferably more than the middle height hg of the above-mentioned parallel part G. It is preferable that the thickness T2 of the rubber chafer 9 is 0.3 to 1.0 times, preferably 0.6 to 1.0 times the total rubber thickness T1 measured from the outer surface of the tyre to the carcass 6, wherein the thickness is measured along a straight line X passing through the radially outer end 8t of the bead apex 8 at a right angle with respect to the carcass main portion 6A, as shown in Fig.2. If the 100% modulus of the chafer 9 is less than 55 kgf/sq.cm and/or the thickness T2 is less than 0.3 times T1, the bead rigidity becomes insufficient and bead deformation can not be effectively controlled. If the 100% modulus of the chafer 9 is more than 75 kgf/sq.cm, heat generation increases in the bead portion 4 and durability decreases. If the 100% modulus of the sidewall rubber 10 is less than 10 kgf/sq.cm, it becomes difficult to protect the carcass 6 from external injuries. If the 100% modulus of the sidewall rubber 10 is more than 20 kgf/sq.cm, the rubber loses its suppleness and cracks are liable to occur on the tyre outer surface. If the height h4 of the point P3 is less than 1.2 times the rim flange height h5, the sidewall rubber 10 . contacts with the rim flange Jf and chafes when the tyre is deformed. If the height h4 is more than 2.5 times the height h5, heat generation increases in the bead portion 4 and cracks are liable to occur in the surface of the chafer 9. Thus the durability decreases. On the radially inside of the bead core, as shown in Fig.5, the minimum rubber thickness T4 from the bead bottom face 4S to the carcass cords is set in the range of from 1.0 to 6.0 times the maximum diameter K of the carcass cords. If the thickness T4 is less than 1.0 times the diameter K, the chafer rubber is liable to be broken in the bead bottom. If the thickness T4 is more than 6.0 times the diameter K, the engaging force between the tyre bead and rim greatly decreases and bead durability is lost. In Fig.2, the height h3 of the outer end 9t of the chafer 9 is less than the carcass turned up height h2. If the height h3 is too low, it becomes difficult to obtain a necessary bead rigidity. On the other hand, if the height h3 is too high, cracks of the chafer rubber and carcass cord loose become liable to occur at the carcass ply turnup end Be. However, even if h3 > h2 as shown in Fig.3, when the chafer rubber thickness T3 at the turnup end Be is set to be more than 1.7 times the maximum diameter'K of the carcass cord, the cracks and cord loose can be prevented. Further, even when h3 < h2 as shown in Fig.2, if the outer end 9t of the chafer 9 comes near the turnup end Be, cracks and cord loose becomes liable to occur. Therefore, it is preferable that the outer end 9t is radially inwardly spaced apart from the turnup end Be by a certain distance L0 in the range of from 5 to 15 mm. In this embodiment, in order to effectively reduce the tyre weight without deteriorating the improved durability, as shown in Fig.6, the tyre is provided in the region Y with the following mid-lower sidewall contour S1 and lower sidewall contour S2 under the above-mentioned standard condition. The mid-lower sidewall contour S1 extends from the maximum width point P1 to a radially inner point P4 along a circular arc E1 of a single radius R1 of which centre is placed on an axial line passing the maximum width points P1. Thus the mid-lower sidewall contour S1 is convex. The lower sidewall contour S2 is defined as extending on the axially inside of the radially inward extension E1a of the circular arc line E1. The lower sidewall contour S2 is usually composed of a transitional convex upper contour 14 and a concave lower contour 13. The convex upper contour 14 extends along a convex curved line which is preferably a circular arc having a radius R2 in the range of from 0.2 to 0.4 times the radius R1. The convex upper contour 14 is connected to the mid-lower sidewall contour S1 at a point P4 without any inflection point. The concave lower contour 13 extends along a concave curved line which is preferably a circular arc having a single radius R3 in the range of not more than 0.95 times the radius R2. The concave lower contour 13 is connected to the convex upper contour 14 without any inflection point. The concave lower contour 13 extends radially inwardly to a point P2 at least. Figs.7 and 8 show two possible cases of the position (point P2) of the radially inner end of the lower sidewall contour S2. In Fig.7, the lower sidewall contour S2 does not intersect the circular arc line E1a, and contacts with the rim flange Jf at a point Q1 (hereinafter contact point Q1). In this case, the point P2 is this contact point Q1. In Fig.8, the lower sidewall contour S2 intersects the circular arc line E1a at point Q2. In this case, the point P2 is this intersecting point Q2. In Fig.6 in which the contour S2 intersects the line E1a at the contact point Q1, the point P2 is equal to the point Q1 and also the point Q2. In the lower sidewall contour S2, a maximum depression Dm from the circular arc line E1a is occurred at a point Pm of which radial height Hpm is in the range of from 6 to 20 %, preferably 9 to 20 %, more preferably 12 to 18 % of the tyre section height H. And the maximum depression Dm is preferably in the range of from 3 to 6 mm. Further, the thickness from the tyre outer surface to the axially outermost carcass ply main portion (in this example, the main portion 6A of the carcass ply 6a) is gradually increased from the maximum width point P1 (thickness T5) to the above-mentioned point P2 (thickness T6). That is, the thickness increases from P1 (T5) to P4 (T6), and from P4 (T6) to Pm (Tm), and further from Pm (Tm) to P2 (T7). Incidentally, the above-mentioned thickness is measured from the tyre outer surface to the carcass cords, excluding its topping rubber. By providing the above explained profile in the region Y, the rubber volume is decreased, the tyre weight is reduced, and also the heat generation can be improved. Further, the distribution of the maximum principal strain can be further evened. Fig.9 shows another example in which the above-mentioned concave part 13 is not provided. In this case, it is especially preferable that the chafer thickness T2 is in the range of from 0.3 to 1.0 times the total rubber thickness T1. Comparison Test 1 Example tyre A (Fig.9) having the parallel part G but not the concave part 13, Example tyre B (Fig.1). having both the parallel part G and concave part 13, and a conventional tyre (Fig.10A) were measured for the maximum principal strain ε. The results are shown in Fig.11. In the conventional tyre, as shown in Fig.11, a peak Z of about 7 or 8 % occurred in the region Y. However, in the example tyres A and B, a remarkable peak was not observed, and the maximum principal strain was decreased to substantially even value of under 4.0 %. The principal strain ε was obtained as follows: (1) buff the surface of the sidewall portion 3 and bead portion 4; (2) wash the surface with naphtha; (3) apply adhesive agent to the surface; (4) inflate the tyre to a pressure of 50 kpa; (5) draw a radially extending strait line RL and copy a series of circles M from a vinyl tape 15 to the surface along the line RL as shown in Fig.12 wherein the circles are printed in white ink (titanium oxide+DOP+castor oil) using a printing screen; (6) inflate the tyre to the standard pressure; (7) copy the circles from the tyre surface to a new blank tape; (8) measure the circles on the tapes (under the standard pressure and 50 kpa) for the abscissa values and ordinate values shown in Fig.13; and (9) compute the principal strain ε using the following equations. Principal strain ε = ε c + ε r 2 + (ε c + ε r ) 2 + γ 2 2 Circumferential strain ε c = L 1 c - L 0 c L 0 c Radial strain ε r = L 1 r - L 0 r L 0 r Sharing strain γ = ε c + ε r - 2 × ε 135 135 degree direction strain ε 135 = L 1 135 - L 0 135 L 0 135 Under 50 kpa pressure Circumferential length L 0 c = (X10 - X20) 2 + (Y10 - Y20) 2 Radial length L 0 r = (X30 - X40) 2 + (Y30 - Y40) 2 135 degree direction length L 0 135 = (X50 - X60) 2 + (Y50 - Y60) 2 Under the standard pressure Circumferential length L 1 c = (X11 - X21) 2 + (Y11 - Y21) 2 Radial length L 1 r = (X31 - X41) 2 + (Y31 - Y41) 2 135 degree direction length L 1 135 = (X51 - X61) 2 + (Y51 - Y61) 2 Comparison test 2 Heavy duty radial tyres (Example tyres 1 to 17, Reference tyre 1, Conventional tyre 1) were made and tested for bead durability, chafer's cracks and tyre weight. The specifications of the test tyres are shown in Table 1 and below. Carcass: number of ply: 1 cord: steel cord (3X0.20+7X0.23) cord angle: 90 degrees with respect to the tyre equator cord count: 38 /5cm Belt: number of ply: 4 cord: steel cord (3X0.20+6X0.35) cord angle: +67/+18/-18/-18 degrees with respect to the tyre equator (from inner ply to outer ply) cord count: 26 /5cm Tyre size: 11R22.5 Rim size: 8.25X22.5 (standard rim) Bead Durability Test Using a tyre test drum, test tyres were run for 10000 km under the following extraordinary conditions, and the running distance L1 until any visible damage appears was measured. Tyre load: 9000 kg Speed: 20 km/h Inner pressure: 1000 kPa In the table, the ratio of the running distance L1 to 10,000 km is shown as the durability, using an index based on the conventional tyre being 100. The larger the index, the better the durability. Crack Resistance Test After the Bead Durability Test, the bead bottom face was visually inspected for cracks. Tyre Weight Test The tyre weight is indicated by an index based on the conventional tyre being 100. As described above, in the pneumatic tyres according to the present invention, the bead apex height is decreased, and the parallel part is formed and the rubber chafer height is increased. As a result, the bead durability can be effectively improved.

Claims: A radial tyre comprising a tread portion (2), a pair of sidewall portions (3), a pair of bead portions (4) each with a bead core (5) therein, a carcass comprising a ply of cords arranged radially at an angle of from 70 to 90 degrees with respect to the tyre equator (C) extending between the bead portions (4) through the tread portion (2) and sidewall portions (3) and turned up around the bead cores (5) from the axially inside to the outside of the tyre to form a pair of turnup portions (6B) and a main portion therebetween (6A), a bead apex (8) disposed between the main portion (6A) and each turnup portion (6B) of the carcass ply (6) and extending radially outwardly from the bead core (5), a chafer (9) disposed in each bead portion (4) to define a rim contacting surface, each said turnup portion (6B) extending radially outwardly beyond the radially outer end of the bead apex (8) so that the turnup portion has a parallel part (G) which is placed adjacent to and substantially in parallel with the main portion (6A), said chafer (9) extending radially outwardly along the axially outer side of the turnup portion (6B) beyond the radially outer end of the bead apex (8), characterised in that the chafer (9) has a 100% modulus in the range of 55 to 75 kgf/cm 2 and a thickness (T2) which is 0.3 to 1.0 times the total rubber thickness (T1) from the outer surface of the tyre to the outermost carcass cords when measured along a straight line drawn at a right angle to the main portion through the radially outer end (8t) of the bead apex under a normally inflated unloaded condition. A radial tyre according to claim 1, characterised in that the length L of the parallel part (G) is 0.5 to 5.0 times the section width CW of the bead core (5) measured in a direction parallel with the bead bottom line excluding its coating rubber. A radial tyre according to claim 1 or 2, characterised in that in the parallel part (G), the rubber thickness N between the carcass cords of the turnup portion and those of the main portion is 0.15 to 4.5 times the maximum diameter K of the carcass cords.

Inventors: UEYOKO KIYOSHI, UEYOKO, KIYOSHI
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1999
Main classification: B60C15
IPC codes: B60C15, B60C9