Tire with a steel aramid belt package

Abstract: The mass of a tire (10) can be reduced by the use of a belt package (12) having a combination of a steel reinforced inner (first) belt ply (14) and an aromatic polyamide reinforced outer (second) belt ply (16). The belt plies are to be made without the cost of having folded belts or any other non-standard construction features. The improved endurance of the tire (10) of this invention is assured by the physical features of the individual belt plies as well as required physical relationships between belt plies. The breaking strength t, the spacing P and the angle A with respect to the tire's mid-plane M for the reinforcing members in each belt ply are defined. In particular, the strength T2 per unit width of the aromatic polyamide reinforced second belt ply (16) is made to be within zero to 30 percent greater than the strength T1 per unit width of the steel reinforced first belt ply (14) to improve tire endurance. The tire (10) of this invention uses angles A2 of the aromatic polyamide reinforcing members (17) from the mid-plane M greater than the angles A1 of the steel reinforcing members (15) from the same mid-plane M, preferably by 1 to 8 degrees.

Description: The subject matter of this invention relates to radial pneumatic tires for vehicles and particularly to the design and configuration of an improved belt package. The modern day tire must be durable, perform well and be resistant to wear, as well as light in weight and economical to construct. The ability of vehicles to perform at high rates of speed and to have excellent cornering stability remains a desirable feature to the consumer. The tire must produce the higher forces to achieve this performance and at the same time have a smooth ride and absorb shock loads from bumps and holes in the roadway. Another desirable feature of the tire is to resist the penetration of water into the belts which produces corrosion of the reinforcing materials. Conventional radial tires use multiple belt plies with steel reinforcing materials to achieve vehicle performance. For most applications the tire is sufficiently strong using two steel belt plies. However, these steel reinforced belt plies have a higher mass, do not absorb shock loads very well and are subjected to higher corrosion. One solution to reduce the tire's weight, improve its shock absorbing ability and to make it more resistance to corrosion is to use a high modulus textile cord for the reinforcing members in one or more belt plies. An aromatic polyamide cord material has been used to replace steel reinforcing members in some tires with limited success. Improvements in rigidity (stiffness), riding comfort, low cost of manufacture, tire weight, corrosion resistance and belt edge separation have been noted with different designs. However, the need remains to make these improvements to tires even greater, especially to increase their high speed and impact endurance, to increase their ability to resist corrosion, to help them maintain good wear rates and to maintain their relatively low weight. Various designs are discussed below. Tires having multiple belt plies with at least one ply having aromatic polyamide reinforcing members are described in U.S. Patent Nos. 3,851,693; 4,184,529; 4,184,530; and 4,407,347 as well as French Patent No. 2,536,018 and Japanese Patent Application No. 90/136,304. More than two belt plies are described in these patents. Such constructions are of interest in the art as showing the use of an aromatic polyamide material for a belt ply reinforcing structure. U.S. Patent 4,184,529 teaches a belt package for an off-road tire used on heavy vehicles. This belt package has three belt plies reinforced with steel cables and the possibility to use synthetic textile reinforcing members is mentioned. The reinforced belt package of U.S. 4,184,530 includes a single inner belt ply having aromatic polyamide reinforcing members and a single outer belt ply having metallic reinforcing members, plus two belt edge strips having textile reinforcing members. The angle of the aromatic polyamide belt ply reinforcing members, measured from the mid-plane, is less than the angle of the steel belt ply reinforcing members. For future discussions in this specification the angles are assumed to be measured relative to the mid-plane (mid-circumferential plane) unless otherwise specified. Other patents also show the inner belt having aromatic polyamide reinforcing materials. In U.S. Patent 4,407,347 two inner belt plies preferably have aromatic polyamide reinforcing members disposed at 15 to 30 degrees plus an outer belt ply with steel reinforcing members disposed at 60 to 90 degrees. The belt package further includes two lateral strips that adjoin the steel reinforced belt ply at both edges. Two additional patents which also include a belt package with auxiliary layers or cover plies are French Patent 2,536,018 and Japanese Patent Application 90/136,304. These patents use an inner belt ply having steel reinforcing members, a middle belt ply having aromatic polyamide reinforcing members and an outer cap ply of nylon or polyester reinforcing material at 0 to 10 degrees. A wide range of angles is given for the reinforcing materials in each of the two inner plies, being from 0 to 35 degrees, and the angles in respective plies are not the same. In particular, according to the examples given in JP 90/136 304, the angles used are 22 degrees for the steel reinforcing members, and 30 degrees for the aromatic polyamide reinforcing members. A number of patents describe the use of a belt ply comprising aromatic polyamide reinforcing members where one or more of the belt plies are folded to give improved endurance of the belt package. These include U.S. Patent Nos. 3,949,797 and 4,854,360 and French Patent Nos. 2,235,810 and 2,472,484. The use of folded belt plies allows certain performance improvements, however, the cost of making tires with folded belt packages is increased by their relatively complex construction requirements. Also, these tires have greater mass as a result of the folded belt plies. A number of patents illustrate only two unfolded belt plies in the belt package with one belt ply having aromatic polyamide as a reinforcing material. French Patent No. 2,248,161 describes a belt package with a single outer belt ply having steel reinforcing members and a single inner belt ply having aromatic polyamide (aramid) reinforcing members. The angles of the reinforcing members are briefly mentioned as being "normal" values, without further definition, and the aramid belt ply is wider than the steel belt ply. In U.S. Patent 4,936,366, corresponding to the preamble of claim 1, the single unfolded aromatic polyamide reinforced belt ply can be the inner belt ply or the outer belt ply, and the same is true with the other unfolded steel reinforced belt ply. This patent requires the angles of the steel reinforcing members to be greater than the angles of the aromatic polyamide reinforcing members by 1 to 6 degrees. This angular relationship is specified to substantially equalize the bending stiffness between the two belt plies. This stiffness relationship should allow the two belt plies to be interchanged. Angles of the steel reinforcing members are in a range of 18 to 25 degrees whereas the angles of the aromatic polyamide reinforcing members are in a range of 17 to 24 degrees. The width of the inner belt ply may be equal to the width of the outer belt ply with both equal to or slightly greater than the tread width. The breaking strength of the 2x0.30 millimeter high strength steel reinforcing members is much lower than the breaking strength of the 1670x3 aromatic polyamide reinforcing members. Also, the pace (spacing) of the steel reinforcing members, in this U.S. Patent 4,936,366, is less than the pace of the aromatic polyamide reinforcing members. In U.S. Patent 3,851,693 all of the examples described in the specification concern three belt plies consisting of two conventional steel reinforced plies and one textile reinforced ply (rayon or aromatic polyamide). The steel reinforcing members are at 15 degrees from the mid-plane and the textile reinforcing members are at 32 degrees from this same plane. Whereas the art defines reinforcing member angles over a very wide range of values, no specific information is provided on the density of the belt reinforcing members (pace) to achieve specific belt ply strength and stiffness values and parametric relationships between the two plies. The main object of the present invention is to define a radial pneumatic tire having a low mass belt package, a low cost, continuing to perform as well as or better than a standard tire with two steel reinforced belt plies, said light weight tire having an improved speed rating without degradation of the wear life, while maintaining its endurance at a same or even greater level without having to use folded plies or cap plies. This definition of the tire of the invention includes defining the physical properties of the belt reinforcing and rubber matrix materials as well as the density of reinforcing members in each ply, their angles from the mid-plane of the tire and stiffness values for the two belt plies and the belt package. In particular, according to one embodiment, the invention is a radial pneumatic tire comprising: (a) - a toroidal carcass having at least one reinforced ply extending from a first bead portion to a second bead portion; (b) - a circumferentially extending ground engaging tread portion disposed radially outward of a crown portion of the carcass; (c) - a sidewall portion extending radially inward from each axial edge of the tread portion to a respective bead portion; and (d) - a belt package between the tread portion and the crown portion of the carcass having a predetermined width and consisting of a single unfolded first belt ply having essentially parallel metallic reinforcing members at a first angle (A1) from the mid-plane (M), and a single unfolded second belt ply having essentially parallel aromatic polyamide reinforcing members at a second angle (A2) from the mid-plane (M), said tire being characterized in that: (e) - the first belt ply is positioned radially outward of the carcass and the second belt ply is positioned radially outward of the first belt ply; (f) - the first angle A1 is less than the second angle A2 by at least one degree: (g) - the breaking strength (t1) of each of the individual metallic reinforcing members is greater than the breaking strength (t2) of each of the individual aromatic polyamide reinforcing members; and (h) - the strength (T2) per unit width of the second belt ply in the circumferential direction is within 0 to 30% greater than the strength (T1) per unit width of the first belt ply in the circumferential direction. In another embodiment of this invention the pneumatic tire comprises, in place of the feature (h), the following feature: (i) - the bending stiffness (BY1) of the first belt ply about a lateral y-axis is greater than the bending stiffness (BY2) of the second belt ply about the lateral y-axis said lateral y-axis being parallel to the rotational axis of the tire. Values of bending stiffness BY1 in the range of about 5 percent to about 150 percent greater than the bending stiffness BY2 have been determined as optimum for various tire designs according to the invention. Further features of the present invention will become apparent to those skilled in the art to which the present invention relates from reading the following specification with reference to the accompanying drawings, in which : Fig. 1 is a cross-sectional view of the tire showing a belt package having two belt plies in accordance with this invention; Fig. 2 is a plan view of the belt package of this invention showing the reinforcing members in each ply; Fig. 3 is a graph showing the relative strength per unit width of the two plies of a belt package of one embodiment of this invention; Fig. 4 is a graph showing the relative strength per unit width of the two plies of the preferred belt package of this invention; Fig. 5 is a graph of actual vs. predicted high impact endurance values for six test tires; and Fig. 6 is a bar graph showing the bending stiffness of each belt ply for four possible belt package configurations. The tire of this invention is a radial pneumatic tire suitable for use on a passenger car, sports car, light truck or like vehicle. The objective of a low cost and light weight tire is achieved by using relatively simple construction methods. No cap plies are used with the tire of this invention, and the carcass plies as well as the bead construction are standard in the industry. A cross-sectional view of the tire of this invention is illustrated in Fig. 1. The tire 10 of this invention as illustrated in Fig. 1 has a toroidal cross-section formed by a carcass, it maintains its air pressure by having an innerliner portion 50. The toroidal carcass 30 has at least one reinforced ply extending from a first bead portion 20 to a second bead portion 22. Each end 32 of the carcass 30 wraps around a respective bead 20 or 22 and extends radially outward from the bead. The radial direction is that associated with the z-axis direction extending outward from the bead toward a tread 60 of the tire. The tread 60 has a tread width TW being a widthwise portion of the tire in the lateral y-axis direction that contacts a surface supporting the tire. A sidewall portion 40 extends radially inward from each lateral edge of the tread portion 60 to a respective bead 20 or 22. A belt package 12 is located between the crown of the carcass 30 and the tread 60 as illustrated in Fig. 1. The belt package 12 has a lateral width being a predetermined value consistent with the tread width TW. The belt package consists of a first belt ply 14 located radially outward of the crown of the carcass 30 and radially inward of the tread 60. The first belt ply 14 preferably has a lateral width value in the range of about 100 to about 105 percent of the tread width TW. The belt package 12 also consists of a second belt ply 16 being radially outward of the first belt ply 14 and also radially inward of the tread 60. The second belt ply 16 preferably has a lateral width value of about 90 percent to about 100 percent of the tread width TW. The first belt ply 14 is preferably made to have a lateral width at least 5 percent greater than the lateral width of the second belt ply 16. An edge distance E between the inner belt ply 14 and the outer belt ply 16 is shown in Fig. 2. The belt plies are symmetrical about the mid-plane M and the edge distance E exists at both edges. The construction of the belt package 12 is illustrated in Fig. 2, as viewed radially inward from the tread side of the tire. The first belt ply 14 has metallic reinforcing members 15 disposed at a first angle A1 from the mid-plane M of the tire. A first spacing or pace P1 between adjacent essentially parallel metallic reinforcing members 15 is exaggerated in this illustration to clearly show their orientation. The breaking strength of each individual reinforcing member 15 is illustrated by vector t1. The metallic reinforcing members 15 are preferably made of steel wires having two to four wires. Preferably, each wire can have a 0.23 to 0.32 millimeter diameter and is made with carbon steel having about 0.70 percent to about 0.80 percent of carbon by weight. The more preferred diameter is about 0.28 millimeter. The steel reinforcing members are commonly designated in the industry as 2x0.28, 3x0.28 or 4x0.28 normal strength steel reinforcing members. A twist rate standard in the industry is also used. The more preferred metallic reinforcing member for this invention is the 4x0.28 member 15. Preferably, the breaking strength t1 of the steel reinforcing members 15 has a value in the range of about 30 deca Newtons (daN) to about 65 daN, the tensile modulus of elasticity for these steel reinforcing members being in a range of about 120 giga Pascals (GPa) to about 185 GPa. The more preferred tensile modulus of the metallic reinforcing member is approximately 170 GPa. Other cables can be used within the scope of this invention including designations 2x0.30 and 4x0.23. The choice of the pace P1 of the steel reinforcing members 15 permits to achieve predetermined strength and stiffness values for the first belt ply 14 in relation to the second belt ply 16. The pace P1 preferably has a value in the range of about 1.00 millimeter to about 1.70 millimeters, more preferably a value of about 1.60 millimeters. The second belt 16 ply has aromatic polyamide reinforcing members 17 disposed at a second angle A2 from the mid-plane M, in a direction opposite to the first angle A1. The second spacing or pace P2 between the essentially parallel aromatic polyamide reinforcing members 17 is amplified to illustrate their relative orientation. The breaking strength of the aromatic polyamide reinforcing members 17 is illustrated by the second vector t2. The aromatic polyamide members 17 preferably have a two ply construction (assembly of 2 single threads twisted together) with a diameter in the preferential range of about 0.55 millimeter to about 0.70 millimeter. The corresponding aromatic polyamide yarns preferably have a mass per length of 1500 grams/9000 meters (i.e. a linear density of 1500 denier), the reinforcing members 17 being then commonly referred to as 1500x2 (denier) cords. The breaking strength t2 of the aromatic polyamide reinforcing members 17 preferably has a value in the range of about 30 daN to about 50 daN. The pace P2 preferably has a value in the range of about 0.90 millimeter to about 1.45 millimeters, more preferably a value of about 0.95 millimeter. A three ply cord construction (for instance 1500x3) is also within the scope of this invention. The tensile modulus of elasticity of the aromatic polyamide members, being a secant modulus at 1-2 percent unit strain, is preferably in the range of about 20 GPa to about 23 Gpa. The ratio of the breaking strengths t1/t2, for the preferred belt package reinforcing members 15 and 17 of this invention, is preferably in a range of values of 1.05 to 1.75. That is, 1.05 < t1/t2 < 1.75. To balance the load taken by the respective belt plies 14, 16 of the belt package 12 and produce a belt package having greater endurance, a "ply strength" or strength per unit width in the circumferential direction x (Fig. 2) is specified for the tire 10 of this invention. The strength per unit width (T) of a belt ply can be expressed by the equation: T = t P Cos 2 A where t is the breaking strength of the respective reinforcing members, A is the respective angle between a longitudinal extent of the reinforcing members and the mid-plane M, and P is the respective spacing of the reinforcing members. The strength per unit width is defined for both the first ply 14 and the second ply 16, and a system of equations can be written as follows: T1 = (t1/P1) Cos 2 A1 (first ply 14) T2 = (t2/P2) Cos 2 A2 (second ply 16) There are three independent variables t, P and A for each dependent variable T. Any one or all of the independent variables can be changed to affect the strength per unit width T. Alternately, two or more independent variables can be changed without changing the dependent variable T (i.e., the angle A can increase along with a decrease in spacing P to give the same T). The ability to change T1 and T2 by changing either respective angles A1 and A2, respective breaking strengths t1 and t2 or respective spacings P1 and P2, is essential to be able to adjust the relative value of T1 with respect to T2 for the different reinforcing members of this invention. The independent variables are illustrated in Fig. 2. The improved belt package is characterized by a relative strength per unit width between each respective belt ply to yield a balanced design. That is, defining the relationship of the dependent variable T1 with respect to dependent variable T2 is essential to achieve the objectives of this invention. In one embodiment of this invention, for a balanced design, the relative strength per unit width values (T1 vs. T2) are selected such that the second strength per unit width value T2 of the second belt ply 16 in a circumferential direction is within zero to 30 percent greater than the first strength per unit width value T1 of the first belt ply 14 in the same circumferential direction. That is to say the relative strengths per unit width are such that: T1 ≤ T2 ≤ 1.3xT1 within the range of angles A1, A2 given herein for each ply. By definition, these values of T1 and T2 are also a function of the breaking strengths t1,t2 and the reinforcing member spacings P1,P2 as defined herein. The angles A1 and A2 of the reinforcing members from the mid-plane M are selected to help give relative strength per unit width values within defined limits. The angles can be easily changed to vary strength per unit width values of each belt ply to achieve a balanced strength per unit width relative value between the two belt plies. A balanced design based on relative strength per unit width values between the two belt plies (T1 vs. T2) is achieved by having a predetermined value of strength per unit width known to give improved tire endurance. It has been determined that a T1 value of approximately 350 deca Newtons per centimeter gives a well balanced design. Having selected the reinforcing members with breaking strengths in the range of values disclosed herein, the angle A of the reinforcing members is selected for each belt ply 14,16 to best conform to low cost manufacturing, and to be within a reasonable range of values to keep the members in tension. Angular differences (A2-A1) are preferably in a range of values from about 1 degree to about 8 degrees. A large angle range, as taught by the art, is not practicable. The steel reinforcing members 15 are positioned so that the first angle A1 with respect to the mid-plane M is preferably in a range of values of about 20 degrees to about 28 degrees. The aromatic polyamide reinforcing members 17 are positioned such that the second angle A2 from the mid-plane M is preferably in a range of values of about 21 degrees to about 29 degrees, said first angle A1 being opposite to said second angle A2 with respect to the mid-plane M. The pace P of reinforcing members in each of the belt plies is adjusted to substantially yield a preferred strength per unit width value. Preferred values for the first strength per unit width T1 are in a range of values from about 275 deca Newtons per centimeter (daN/cm) to about 400 daN/cm. The preferred second strength per unit width T2 is in a range of values from about 390 daN/cm to about 450 daN/cm. For example, the graphs of Figs. 3 and 4 show relative strength per unit width values within zero to 30 percent of each other for a belt package. In this example, the values of breaking strength t are determined according to the chosen reinforcing members of the invention and the values of pace P have been selected to make T2 within zero to 30 percent greater than the value of T1, over a range of preferred angular values A1 and A2. The graphs of Figs. 3 and 4 further show what the effect of changing the angle A of the reinforcing members has on the strength per unit width T values. The strength per unit T width is simply called "strength" in the graphs. The curve 71 of Fig. 3 shows the value of the first strength per unit width T1 of the first belt ply 14 in the circumferential direction as a function of the angle A1 using 3x0.28 steel reinforcing members 15. The curve 81 of Fig. 4 shows the corresponding first strength per unit width T1 as a function of the angle A1 using the preferred 4x0.28 reinforcing members 15. The curve 72 of both Fig. 3 and Fig. 4 shows the value of the second strength per unit width T2 of the second belt ply 16 in the circumferential direction as a function of the angle A2 of the preferred 1500x2 aromatic polyamide reinforcing members 17. To make the strength per unit width equal in both belt plies in Fig. 4, select for example an angle A1 of about 21 degrees for the steel reinforcing members 15 and project the vertical line 84 to the curve 81. A horizontal line 86 corresponding to a value of 350 deca Newtons per centimeter (daN/cm) is projected to curve 72 for the aromatic polyamide reinforcing members 17. An intersection with curve 72 is projected vertically by line 88 to give an angle A2 of about 27 degrees for the aromatic polyamide reinforcing members. Therefore, for this example, an angle difference of six degrees will give a completely balanced design by having equal belt ply stiffness per unit width values for the two belt plies 14,16. In another embodiment the tire 10 of the invention is made to have a high impact endurance. This type of endurance is associated with a tire that is resistant to shock loading from road hazards, bumps, pot holes and the like in a roadway. These loading conditions greatly reduce the overall endurance of the tire and have been found to be associated with certain stiffness values of the individual belts, as well as the composite belt package. There are five different stiffness values that characterize each belt plus the belt package of the tire. These values can be calculated for individual belt plies as well as for the total belt package. Two values are extensional stiffness values, two are bending stiffness values and one is a shear stiffness value. Methods to calculate these stiffness values for composite materials are well known in the art. The mathematics is outlined by the matrix equations in "Mechanics of Composite Materials" by Robert M. Jones, which was published by Hemisphere Publishing Corp. in 1975. This material is included herein by reference. High impact endurance HIE was found to have a direct correlation with the relative bending stiffness about the lateral y-axis (Fig. 2) between individual belt plies, as well as the composite shear stiffness of the belt package in the x-y plane. The relationship for high impact endurance (HIE) as a function of stiffness values, was determined to be: HIE = FUN[(BY1-BY2),CGZ] where the independent variables are (BY1-BY2) and CGZ. The parameter BY1 is the bending stiffness of the first belt ply about the lateral y-axis, parameter BY2 is the bending stiffness of the second belt ply about the lateral y-axis and parameter CGZ is the composite shear stiffness of the belt package in the x-y plane (about the z-axis). The value of HIE is the distance traveled by the tire during a high impact endurance test before it fails. The HIE is directly proportional to the difference in bending stiffness values (BY1-BY2) as well as the composite shear stiffness CGZ. Proportionality coefficients can be determined from a correlation analysis for each tire size and construction using high impact endurance test values. For example, Fig. 5 shows a 1:1 curve 90 for an exact correlation of predicted vs. actual HIE values, with the results of six test tires 3A, 3B, 3C, 4B, 5A and 5B plotted relative to the exact correlation curve. The high impact endurance magnitudes shown are relative to tire 4B having a value of 100. Proportionality coefficients were obtained for this test series of six tires. The standard RxR confidence factor was 0.97 for this high impact test example, being an excellent correlation of actual test data with that predicted by the functional relationship for HIE. The relative magnitude of the high impact endurance values are accurately predicted by the HIE functional relationship. From the high impact endurance functional relationship HIE=f(BY1,BY2,CGZ) above, it is noted that improved endurance exists when the bending stiffness difference (BY2-BY1) is negative. That is, according to one embodiment of this invention, the bending stiffness BY1 of the first belt ply about the lateral y-axis is made greater than the bending stiffness BY2 of the second belt ply about the same lateral y-axis. Optimum values for (BY2-BY1) with respect to CGZ can be achieved to maximize the high impact endurance HIE value. Values of bending stiffness BY1 in the range of about 5 percent to about 150 percent greater than the bending stiffness BY2 have been determined as optimum for various tire designs according to the invention. Preferably the bending stiffness BY1 of the first belt ply is at least 45 percent greater than the bending stiffness BY2 of the second belt ply about respective lateral y-axes. The relative magnitude of the bending stiffness (BY2-BY1) is an essential factor for improved high impact endurance, while maintaining a relatively high composite shear stiffness CGZ value. For the tires of the invention, the preferred composite shear stiffness value is greater than about 2500 mega Pascal millimeters. The bar graph of Fig. 6 illustrates the relative bending stiffness (BY2-BY1) of belt plies in four test tires 3C, 3A, TT1 and TT2 having different belt package constructions, only the tire TT2 being made according to the invention. The same steel reinforcing members (4x0.28) having the same pace (1.60 millimeters) are used in each first belt ply of each tire and the same aromatic polyamide reinforcing members (1500x2) having the same pace (0.94 millimeter) are used in each second belt ply of each tire. The left vertical bar of each set of bars is for the first belt ply and the vertical bar on the right is for the second belt ply of the belt package. The tires 3C and 3A are the same tires as illustrated in Fig. 5. For instance, for the tire TT2 of the invention, the bar designated S24 is for a steel reinforced first belt ply 14 with reinforcing members 15 at 24 degrees (A1 = 24) from the mid-plane M of the tire, whereas the bar designated A28 is for an aromatic polyamide reinforced second belt ply 16 with reinforcing members 17 at 28 degrees (A2 = 28) from the mid-plane M. Relative values of the bending stiffness, called "stiffness" in the graph, are given based on a range of values from 0 to 100. Note that a much greater relative bending stiffness (BY2-BY1) is possible using the test tire TT2 of this invention. The high impact endurance HIE functional relationship also shows a correlation with composite shear stiffness CGZ about the z-axis. A tire having a higher composite shear stiffness will yield a larger high impact endurance when all other factors are the same. Tire 3A has the same relative bending stiffness (BY2-BY1) as tire 3C in Fig. 6, however, the composite shear stiffness of tire 3A is greater than that of tire 3C such that the overall high impact endurance HIE of tire 3A is greater than that of tire 3C (see Fig. 5). The parameters that yield a balanced strength per unit width relationship also result in a higher impact endurance. According to the invention, it is essential to have the angle A1 of the steel reinforcing members less than the angle A2 of the aromatic polyamide reinforcing members, angular differences (A2-A1) being preferably in a range of values from about 1 degree to about 8 degrees. The more preferred difference (A2-A1) is approximately 3 degrees. The stiffness of the belt plies are not only a function of the tensile modulus of the reinforcing members, the spacing or pace P of the reinforcing members and the angle A of reinforcing members from the mid-plane M, but also the thickness and material properties of the rubber matrix material within each belt ply. The modulus of elasticity of the rubber matrix material 18,19 (Fig. 2), at ten percent unit strain, is preferably in a range of values of about 5 mega Pascals (MPa) to about 25 MPa. The more preferred rubber modulus of elasticity, at ten percent unit strain, is about 11 Mpa. Preferably, the thickness of the green rubber skim on each side of each of the reinforcing members of the belt package has a value in the range of about 0.35 millimeter to about 0.90 millimeter. More preferably, the green rubber skim thickness on each side of the steel reinforcing members 15 is about 0.65 millimeter and the green rubber skim thickness on each side of the aromatic polyamide reinforcing members 17 is about 0.40 millimeter. Other skim thickness values are also possible within the scope of this invention. Rubber material properties and skim thickness values are selected to give the low cost light weight durable tire of this invention. The tire of this invention has a balanced strength per unit width relationship between each belt ply in the circumferential direction, a large difference in the bending stiffness about a lateral y-axis between the two belt plies and a relatively large composite shear stiffness of the belt package in the plane of the belt plies. Tire tests have shown that this tire 10 of the invention has improved running endurance as well as high impact endurance. This tire 10 has a relatively low mass by using a second belt ply 16 with aromatic polyamide reinforcing members 17. In addition, the reduced pace P2 of the aromatic polyamide reinforcing members 17 is such that corrosion of the first steel reinforced belt ply 14 is reduced. A passenger car tire having a first ply with steel reinforcing members (steel cables) and a second ply with aromatic polyamide reinforcing members (aramid cords) was compared with a witness tire having both plies with steel reinforcing members. Both tires were of the same P185/65 R14 size and construction. The steel reinforced belt plies were all reinforced with 4x0.28 steel cables at an angle A1 of 20 degrees, and the aromatic polyamide reinforced belt ply was reinforced with 1500x2 aramid cords at an angle A2 of 23 degrees. The pace P1 of the steel reinforcing members was 1.60 millimeters and the pace P2 of the aromatic polyamide reinforcing members was 0.95 millimeter. The following Table shows a comparison of physical parameters. PARAMETER WITNESS TIRE STEEL-ARAMID TIRE MASS (kilograms) 7.1 6.8 HIGH SPEED LIMIT (relative) 100 (ref.) 114 "UTQG" TEMPERATURE (temperature resistance - relative) 100 (ref.) 108 WEAR RATE most worn rib (rel.) 100 (ref.) 75 Other performance parameters were essentially the same in this test comparison. In particular, handling and comfort of the tires were approximately equal. More generally, in comparison with the results of the above Table, similar or very close improvements were obtained when the aromatic polyamide reinforcing members 17 of the second belt ply 16 were disposed at an angle A2 from the mid-plane between 1 to 8 degrees larger than the disposed angle A1 of the steel reinforcing members 15 of the first belt ply 14. An improvement in the high impact endurance HIE of tire TT2 of Fig. 6 over the above witness tire is anticipated to be at least 10 percent. Consider the two tires TT1 and TT2 of Fig. 6 as having the same construction except for the angle A1 of the steel reinforcing members relative to the angle A2 of the aromatic polyamide reinforcing members. The tire TT1 with A1>A2 has a composite shear modulus CGZ 4 percent greater than tire TT2 with A1<A2. However, tire TT2 has a calculated high impact endurance HIE which is 6.2 percent greater than the HIE for tire TT1.

Claims: A radial pneumatic tire (10) comprising: (a) - a toroidal carcass (30) having at least one reinforced ply extending from a first bead portion (20) to a second bead portion (22); (b) - a circumferentially extending ground engaging tread portion (60) disposed radially outward of a crown portion of the carcass (30); (c) - a sidewall portion (40) extending radially inward from each axial edge of the tread portion (60) to a respective bead portion; and (d) - a belt package (12) between the tread portion (60) and the crown portion of the carcass (30) having a predetermined width and consisting of a single unfolded first belt ply (14) having essentially parallel metallic reinforcing members (15) at a first angle A1 from the mid-plane M, and a single unfolded second belt ply (16) having essentially parallel aromatic polyamide reinforcing members (17) at a second angle A2 from the mid-plane M. characterized in that: (e) - the first belt ply (14) is positioned radially outward of the carcass (30) and the second belt ply (16) is positioned radially outward of the first belt ply (14): (f) - the first angle A1 is less than the second angle A2 by at least one degree; (g) - the breaking strength tl of each of the individual metallic reinforcing members (15) is greater than the breaking strength t2 of each of the individual aromatic polyamide reinforcing members (17); and (h) - the strength T2 per unit width of the second belt ply (16) in the circumferential direction is within 0 to 30% greater than the strength T1 per unit width of the first belt ply (14) in the circumferential direction. A tire (10) according to claim 1, wherein the feature (h) is replaced by: (i) - the bending stiffness BY1 of the first belt ply (14) about a lateral y-axis is greater than the bending stiffness BY2 of the second belt ply (16) about the lateral y-axis, said lateral y-axis being parallel to the rotational axis of the tire (10). A tire (10) according to claim 1, wherein the following feature is added: (i) - the bending stiffness BY1 of the first belt ply (14) about a lateral y-axis is greater than the bending stiffness BY2 of the second belt ply (16) about the lateral y-axis, said lateral y-axis being parallel to the rotational axis of the tire (10). A tire (10) according to claim 2 or 3, wherein BY1 is in a range of about 5% to about 150% greater than BY2. A tire (10) according to any one of claims 1 to 4, wherein the ratio (t1 / t2) is in the range of about 1.05 to about 1.75. A tire (10) according to any one of claims 1 to 5, wherein t1 is in the range of about 30 daN (deca Newtons) to about 65 daN, and t2 is in the range of about 30 daN to about 50 daN. A tire (10) according to any one of claims 1 to 6, wherein the predetermined width of the belt package (12) is characterized by a first width of said first belt ply (14) being in a range of about 100 to about 105 percent of the tread width TW of the tire (10) and a second width of said second belt ply (16) being in a range of about 90 percent to about 100 percent of the tread width TW, said first width being greater than said second width by at least 5 percent. A tire (10) according to any one of claims 1 to 7, wherein the metallic reinforcing members (15) are made of multiple strands of steel wire having a carbon content in a range of about 0.7 to about 0.8 percent by weight to give a tensile modulus of elasticity of the metallic reinforcing member in a range of about 120 giga Pascals (GPa) to about 185 GPa. A tire (10) according to any one of claims 1 to 8, wherein the aromatic polyamide reinforcing members (17) have a two ply construction, a diameter in the range of about 0.55 millimeter to about 0.70 millimeter and a secant modulus at 1 to 2 percent strain in the range of about 20 giga Pascals (Gpa) to about 23 Gpa. A tire (10) according to any one of claims 1 to 9, wherein the angular difference (A2-A1) is in a range of values from about 1 degree to about 8 degrees. A tire (10) according to claim 10, wherein said metallic reinforcing members (15) are positioned such that the first angle A1 is in a range of about 20 degrees to about 28 degrees and said aromatic polyamide reinforcing members (17) are positioned such that the second angle A2 is in a range of about 21 degrees to about 29 degrees. A tire (10) according to any one of claim 1 or claims 3 to 11, wherein T1 is in the range of about 275 deca Newtons per centimeter (daN/cm) to about 400 daN/cm, and T2 is in the range of about 390 daN/cm to about 450 daN/cm. A tire (10) according to any one of claims 1 to 12, wherein the belt package (12) is further characterized by a rubber matrix material (18,19) having a tensile modulus at ten percent unit strain in a range of values from about 5 mega Pascals (MPa) to about 25 MPa and green rubber skim thicknesses in the range of values from about 0.35 millimeter to about 0.90 millimeter. A tire (10) according to any one of claims 1 to 13, wherein the pace P1 of the steel reinforcing members (15) has a value in the range of about 1.00 to about 1.70 millimeters, and the pace P2 of the aromatic polyamide reinforcing members (17) has a value in the range of about 0.90 to about 1.45 millimeters. A tire (10) according to any one of claims 1 to 14, wherein said belt package (12) is further characterized by a composite shear stiffness greater than about 2500 mega Pascal millimeters.

Inventors: CREECH DEAN B, CREECH, DEAN B.
Applicants: MICHELIN RECH TECH, MICHELIN RECHERCHE ET TECHNIQUE S.A.
Application year: 1995
Main classification: B60C9
IPC codes: B60C9