Heavy duty radial tyre

Abstract: A heavy duty radial tyre comprises a belt (7) comprising first, second and third plies (11,12,13) disposed in this order from the carcass (6) to the radially outside thereof, each belt ply being made of steel cords laid parallel with each other where with respect to the tyre equator (C), the inclination direction of the cords of the first ply (11) is the same as that of the second ply (12), but reverse to the inclination direction of the third ply (13). The first belt ply cord angle (α1) is 35 to 80 degrees, the second belt ply cord angle (α2) is 15 to 30 degrees, and the third belt ply cord angle (α3) is 15 to 30 degrees. In addition the belt satisfies (A) the total of the second belt ply strength and the third belt ply strength is 3.4 to 10.0 times the first belt ply strength, and/or (B) the second belt ply strength is in the range of from 1.05 to 2.0 times the third belt ply strength, where the strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply.

Description: The present invention relates to a pneumatic tyre, more particularly to a heavy duty radial tyre having an improved belt structure by which the tyre strength (breaking energy) is effectively increased while achieving a belt weight reduction. In a belted radial tyre for heavy duty use such as truck/bus tyres, it is very important to improve the hoop effect of the belt to withstand a high inner pressure and heavy tyre load. Therefore, such a belt layer generally comprises at least three plies of steel cords. In such heavy duty radial tyres, it is also necessary to increase the breaking energy (plunger energy) for the belt in order to prevent a shock-burst (a burst of the tyre caused when its belt plies are cut by a sharp object on the road surface such as stones, rocks or the like.) Indeed, Japanese Industrial Standard (JIS-D4230) and the like in other countries require the breaking energy to excel a regulation value in a tyre breaking test. Hitherto, therefore, the breaking energy is increased by thick steel cords for the belt and/or increasing the cord count for each of the belt plies. For example EP-A-0568870 describes a tyre in accordance with the preamble of present claim 1 having a belt comprising three plies of cords wherein the cord spacing is decreased and the unit strength is increased in moving from the radially inner ply to the outer ply. However, if the cord diameter and cord count are simply increased, the tyre weight and manufacturing cost are greatly increased, and the dynamic tyre performances are deteriorated. Further, the breaking energy sometimes unexpectedly decreases. Therefore, the present inventor made various studies and tests, and discovered the following. (1) The cords of the second and third plies mainly work as a hoop, and the cords of the first ply works to control cord movement of the second and third plies. (2) When a tyre runs over a sharp object, the belt plies are deflected radially inwards of the tyre and at the same time the belt cords angle are changed. If the first ply strength is excessively high, the cord movements of the second and third plies are excessively controlled and their cord angles can not be changed. As a result, the second and third ply cords are liable to be broken. (3) When the belt plies are deflected radially inwards, the ply located further from the sharp object has the largest tensile deformation. Accordingly, the second ply is broken more easily than the third ply. Contrarily, the first ply is difficult to break when the cord angle is large and therefore it is soft against bending. It is therefore, an object of the present invention to provide a heavy duty radial tyre, in which the breaking energy is increased without increasing the tyre weight. According to the present invention, a heavy duty radial tyre comprises a carcass extending between a pair of axially spaced bead portions of the tyre, and a belt disposed radially outward of the carcass in a tread portion of the tyre, the belt comprising at least three plies including first, second and third plies disposed in this order from the carcass to the radially outside thereof, each belt ply made of steel cords laid parallel with each other, wherein with respect to the tyre equator, the inclination direction of the cords of the first belt ply is the same as the inclination direction of the cords of the second belt ply, but reverse to the inclination direction of the cords of the third belt ply, the angle of the cords of the first belt ply to the tyre equator is 35 to 80 degrees, the angle of the cords of the second belt ply to the tyre equator is 15 to 30 degrees, the angle of the cords of the third belt ply to the tyre equator is 15 to 30 degrees, and the total of the ply strength of the second belt ply and the ply strength of the third belt ply is 3.4 to 10.0 times the ply strength of the first belt ply, the ply strength of the second belt ply is 1.05 to 2.0 times the ply strength of the third belt ply, where the ply strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply. Thus the movements of the cords of the second and third belt plies are appropriately controlled, and the cords are prevented from being cut, and the breaking energy for the belt can be increased. Also as the second belt ply is increased in its strength S2, it becomes possible for it to withstand the large tensile stress, and the breaking energy is then increased. Thus overall the breaking energy is remarkably increased. As the cord angles of the second and third belt plies are relatively small, those plies display a tight hoop effect, and the cords of the first belt ply appropriately control the movements of the second and third belt ply cords. Further, the cords of the first-third plies form a truss structure to provide a desirable rigidity for the belt. Embodiments of the present invention will now be described in conjunction with the drawings. Fig.1 is a cross sectional view of an embodiment of the present invention; Fig.2 is a plan view showing an example of the belt thereof; and Fig.3 is a diagram for explaining a crossing state of the cords of the first, second and third belt plies. In the figures, the heavy duty radial tyre 1 comprises a tread portion 2 with tread edges, a pair of axially spaced bead portions with a bead core 5 therein, a pair of sidewall portions 3 extending radially inwardly from the tread edges to the bead portions, a toroidal carcass 6 extending between the bead portions 4 through the tread portion 2 and sidewall portions 3, and a belt 7 disposed on the radially outside of the carcass 6 in the tread portion 2. The carcass 6 comprises at least one ply of cords extending between the bead portions 4 and turned up around the bead cores 5 from the inside to outside of the tyre to form a pair of turnup portions 6a and a main portion 6b therebetween. Each turnup portion 6a terminates above the bead core 5 and below the maximum tyre width position. The carcass ply cords are arranged radially at an angle of 75 to 90 degrees with respect to the tyre equator C. Preferably and in this embodiment, the carcass 6 comprises only one ply of steel cords arranged radially at 90 degrees. However, the carcass 6 may be comprised of a plurality of organic fibre cords, e.g. aromatic polyamide, nylon, rayon, polyester or the like. In order to increase the lateral stiffness of the tyre, each bead portion 4 is provided with a bead apex 8 between the turnup portion 6a and main portion 6b. The bead apex 8 is made of a hard rubber compound extending radially outwardly from the bead core 5 so as to reinforce the bead portion 4 and the lower sidewall portion. In this embodiment, the belt 7 consists of four plies; a first ply 11, a second ply 12, a third ply 13 and a fourth ply 14, which are disposed in this order from the carcass 6 towards the tread surface 2A. Each of the belt plies 11, 12, 13 and 14 is made of steel cords 20 laid parallel each other. Each belt cord 20 is made of steel filaments twisted together. The axial width W2 of the second belt ply 12 is larger than the axial width W1 of the first belt ply 11. The axial width W3 of the third belt ply 13 is substantially the same as, but slightly larger than the width W1. The second belt ply 12 is the widest of the four belt plies, and its width W2 has a maximum value WM, which is set in the range of from 0.80 to 0.95 times the tread width TW, whereby a substantially overall width of the tread portion 5 is reinforced. The fourth belt ply 14 is the narrowest, and its width W4 has a minimum value. This ply 14 works mainly to protect the inner belt plies 11, 12 and 13 and thereby to increase the cut resistance of the tread portion. In each tread edge portion, cushion rubber layers 15 and 16 are disposed so as to cover the edges of the first-third belt plies 11-13 to mitigate stress concentration. Since the inner cushion rubber layer 15 has a tapered axial inner edge region, the space between the carcass and the belt 7 or the first belt ply 11 is gradually increased towards the axially outside. The whole width of the first belt ply 11 contacts with the second belt ply 12, but the edge of the third belt ply 13 is spaced therefrom and the space is filled with a rubber layer 16. As shown in Fig.2, the belt cords 20A and 20B of the first and second belt plies 11 and 12, respectively, are inclined with respect to the tyre equator C in a certain direction. (for example towards the right side so as to have different right-side upward inclinations). The belt cords 20C and 20D of the third and fourth belt plies 13 and 14, respectively, are inclined in the reverse direction to that of the belt cords 20A and 20B. (In Fig.2, therefore, towards the left side so as to have different left-side-upward inclinations). The angles α1, α2 and α3 to the tyre equator C, of the belt cords 20A, 20B and 20C respectively are: 35 degrees ≤ α1 ≤ 80 degrees 15 degrees ≤ α2 ≤ 30 degrees 15 degrees ≤ α3 ≤ 30 degrees whereby, as shown in Fig.3, the belt cords 20A, 20B and 20C form a stiff truss structure, and a desired belt rigidity is provided to maintain steering stability. According to the present invention, the ply strength S1 of the first belt ply 11 is set to be smaller than either of the ply strength S2 of the second belt ply 12 and the ply strength S3 of the third belt ply 13. S1 < S2 S1 < S3 and the sum total S2+S3 of the ply strengths S2 and S3 is set in the range of 3.4 to 10.0 times the ply strength S1. 3.4 S1 ≤ (S2+S3) ≤ 10.0 S1 Here, the ply strength S is the tensile rupture strength of the belt ply per unit width. In this embodiment, the ply strength Si is defined as the product Ei x Ni of the tensile strength Ei of one belt cord 20 in a ply and the cord count Ni of the ply per a unit width, for example 5 cm width, in the direction at a right angle to the belt cords. (i = 1, 2 and 3). S1 = E1 x N1 S2 = E2 x N2 S3 = E3 x N3 As a result, in the above-mentioned truss structure, movement of the belt cords 20B and 20C is effectively reduced, but the angles of the belt cords 20B, 20C can be changed relatively easily when the tyre treads upon a sharp object and the tread portion is deflected radially inwardly. Therefore, the breaking energy for the belt is increased, and cord breakage in the second and third belt plies 12 and 13 is effectively prevented. If the sum total (S1+S2) is less than 3.4 times S1, the movement of the belt cords 20B and 20C is excessively controlled, which results in cord breakage in the second and/or third belt plies 12 and 13. When the sum total (S1+S2) is more than 10.0 times S1, the truss structure loses its balance, and the belt rigidity greatly decreases, which results in uneven wear. Further, the cornering force decreases to deteriorate steering stability. The lower limit for the total (S1+S2) is more preferably 4.0 times S1. The upper limit is more preferably 7.0 times S1, still more preferably 4.8 times S1. Further, in the belt 7 in this example, the ply strength S2 of the second belt ply 12 is set to be larger than the ply strength S3 of the third belt ply 13 to meet the following equation (5). 1.05 S3 ≤ S2 ≤ 2.0 S3 When the tyre runs over a rock and the like, as the belt plies deflect radially inside, the second belt ply 12 is subjected to a tensile deformation which is larger than the third belt ply 13. Therefore, by increasing the ply strength S2 of the second belt ply 12 and by limiting it within the above-mentioned range, the belt cords 20B are prevented from being broken. In this embodiment, therefore, this effect and the above-mentioned effect from the equation (4) are combined to effectively increase the breaking energy for the belt 7. The cord angle α1 of the first belt ply 11 is not less than 54 degrees, whereby the first belt ply 11 is provided with appropriate flexibility against the bending deformation and the cords thereof are prevented form being broken. The cord angle α4 of the fourth belt ply 14 is preferably set at the substantially same value as the cord angle α3, that is, within plus or minus 5 degrees, and the ply strength S4 is less than the ply strength S3, so that the fourth belt ply 14 does not increase the belt rigidity and does not hinder the above-explained effect. TEST 1: Tyres of size 11R22.5 having the construction shown in Fig.1 and specifications given in Table 1 were prepared and tested for breaking energy. In the test, the breaking energy for each test tyre was measured according to the tyre strength test specified in Japanese Industrial Standard JIS-D4230. The tyre inner pressure was 7.00 kgf/sq.cm, and the rim size was 7.5x22.5. The results are indicated in Table 1 by an index based on that the reference tyre 1 is 100. The larger index is better. The belt structures used in the test tyres are shown in Table 2. Tyre Ex.1 Ex.2 Ref.1 Ref.2 First belt ply -Construction B B A A -Cord angle α1 (deg) 67 67 67 67 -Width W1 (mm) 145 145 145 145 -Strength S1 (kgf/5cm) 2110 2110 3200 3200 -Cord total weight G1(g/m) 65.8 65.8 106.6 106.6 Second belt ply -Construction C D C D -Cord angle α2 (deg) 18 18 18 18 -Width W2 (mm) 160 160 160 160 -Strength S2 (kgf/5cm) 4160 4980 4160 4980 -Cord total weight G2(g/m) 147.2 159.4 147.2 159.4 Third belt ply -Construction C D C D -Cord angle α3 (deg) 18 18 18 18 -Width W3 (mm) 140 140 140 140 -Strength S3 (kgf/5cm) 4160 4980 4160 4980 -Cord total weight G3(g/m) 147.2 159.4 147.2 159.4 Fourth belt ply -Construction A A A A -Cord angle α4 (deg) 18 18 18 18 -Width W4 (mm) 75 75 75 75 -Strength S4 (kgf/5cm) 3200 3200 3200 3200 -Cord total weight G4(g/m) 106.6 106.6 106.6 106.6 Test Results S2+S3)/S1 4.0 4.7 2.6 3.1 S2/S3 1.0 1.0 1.0 1.0 GO(=G1+G2+G3) 360.2 384.6 401.0 425.4 Breaking energy P (index) 110 125 100 115 P/GO 0.305 0.325 0.249 0.270 Belt ply A B C D E Cord structure 3/0.20 +6/0.35 2+7/0.22 3/0.20 +6/0.35 3+8+ 13/0.23 3+9+ 15/0.22 Breaking strength E (kgf) of a cord 160 88 160 249 265 Weight of a cord (g/m) 5.33 2.74 5.33 7.97 8.50 Cord count N (/5cm width) 20 24 26 20 20 Ply strength S(=EXN) per 5cm 3200 2112 4160 4980 5300 Cord total weight (G=gXN) per 5cm width 106.6 65.8 147.2 159.4 170 As apparent from Table 1 if Ex.1 is compared with Ref.1 and Ex.2 with Ref.2, by decreasing the ply strength S1 of the first belt ply to a value in the range expressed by the above-mentioned equation (4), the breaking energy was increased and at the same time the cord weight of the first-third belt plies was decreased. According to the other aspect of the present invention, the ply strength S2 of the second belt ply 12 is set in the range of from 1.05 to 2.0 times the ply strength S3 of the third belt ply 13. This limitation is the same as the equation (5). In other words, the limitation by the above-explained equation (5) is effective either alone or in combination with the limitation by the equation (4). When the ply strength S2 is less than 1.05 times the ply strength S3, the prevention of breaking of the second belt ply cords becomes insufficient. When S2 is more than 2.0 times S3, the third belt ply 13 becomes weak and causes a cord breakage. TEST 2: Test tyres of size 12.00R24 mounted on a rim of size 8.50Vx24 and inflated to an inner pressure of 7.75kgf/sq.cm were measured for the breaking energy in the same way as in the above-mentioned test. The tyres had the construction shown in Fig.1 and specifications given in Table 3. The belt structures used in the test tyres are also given in Table 2. The results are indicated in Table 3 by an index based on that the reference tyre 3 is 100. The larger index is better. As apparent form Table 3 if Ex.3 is compared with Ref.4, Ex.4 with Ref.5 and Ex.5 with Ref.7, by increasing the ply strength S2 than the ply strength S3 to meet the equation (5), the breaking energy was increased without increasing the cord weight of the first-third belt plies. Further, as apparent if compared Ref.4 with Ref.5, Ref.6 with Ref.7, and Ex.5 with Ref.8, by specifically decreasing the ply strength S3 than the ply strength S2, the cord weight could be decreased without decreasing the breaking energy. Incidentally, the ply strength S is substantially in proportion to the total weight of the belt cords embedded in a unit width of the belt ply. Accordingly, by adjusting the ply strength S1,S2 and S3 to meet the above-mentioned equation (4) and/or equation (5), the breaking energy for the belt plies can be increased without increasing the tyre weight. As explained above, in the heavy duty radial tyres according to the present invention, as the cord directions, cord angles and ply-strength of the first-third belt plies are set in the specific ranges, the tyre strength (breaking energy) can be increased without increasing the belt weight.

Claims: A heavy duty radial tyre comprising a carcass (6) extending between a pair of axially spaced bead portions of the tyre, a belt (7) disposed radially outwards of the carcass (6) in a tread portion (2) of the tyre, the belt (7) comprising at least three plies including first (11), second (12) and third (13) plies disposed in this order from the carcass (6) to the radially outside thereof, each belt ply being made of steel cords laid parallel with each other, wherein with respect to the tyre equator (C), the inclination direction of the cords (20A) of the first belt ply (11) is the same as the inclination direction of the cords (20B) of the second belt ply (12), but reverse to the inclination direction of the cords (20C) of the third belt ply (13), characterised in that the angle (α1) of the cords (20A) of the first belt ply (11) to the tyre equator (C) is 35 to 80 degrees, the angle (α2) of the cords (20B) of the second belt ply (12) to the tyre equator (C) is 15 to 30 degrees, the angle (α3) of the cords (20C) of the third belt ply (13) to the tyre equator (C) is 15 to 30 degrees, said belt satisfying both of the following conditions (A) and (B): (A) the total of the ply strength of the second belt ply (12) and the ply strength of the third belt ply (13) is 3.4 to 10.0 times the ply strength of the first belt ply (11), where the ply strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply; (B) the ply strength of the second belt ply (12) is in the range of from 1.05 to 2.0 times the ply strength of the third belt ply (13). A heavy duty radial tyre according to claim 1, characterised in that the total of the ply strength of the second belt ply (12) and the ply strength of the third belt ply (13) is 4.0 to 7.0 times the ply strength of the first belt ply (11). A heavy duty radial tyre according to claim 1, characterised in that the total of the ply strength of the second belt ply (12) and the ply strength of the third belt ply (13) is 4.0 to 4.8 times the ply strength of the first belt ply (11). A heavy duty radial tyre according to any of claims 1 to 3, characterised in that the belt further comprises a fourth belt ply (14) disposed radially outwardly of the third belt ply (13), the ply strength of the fourth belt ply (14) is smaller than the ply strength of the third belt ply (13). A heavy duty tyre according to claim 4, characterised in that the angle (α4) of the cords (20D) of fourth belt ply (14) is within ±5° of the angle (α3) of the cords (20C) of the third belt ply (13). A heavy duty tyre according to either of claims 4 or 5, characterised in that the fourth belt ply (14) is the narrowest belt ply.

Inventors: MIYAZAKI TATSUYA, MIYAZAKI, TATSUYA
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LIMITED
Application year: 1995
Main classification: B60C9
IPC codes: B60C9