Pneumatic radial tire for a passenger car having ultra fine steel cords for a carcass ply

Abstract

Disclosed is a pneumatic radial tire for a passenger car having ultra fine steel cords for a carcass ply in which adhesive force between a carcass ply layer and a side wall or a rim flange rubber adjacent to a turn up portion on an outside and a stiffness of the rim flange rubber are larger than a bending stiffness of the carcass ply layer so as to improve durability of a bead part while reducing poor air inletting generated at a carcass turn up portion.

Claims

1. A pneumatic radial tire for a passenger car which uses ultra fine steel cords having a wire diameter of 0.14 mm or less as a reinforcing material of a carcass ply, wherein a bending force of the carcass ply (F.sub.P; kgf) satisfies a relation equation below between a turn up height of the carcass ply (h, mm), a tensile strength of a rim flange rubber (M.sub.RF; kgf/cm.sup.2) at the time of 50% elongation and a unit cross sectional area (At; 1×t cm.sup.2) depending on a thickness of rim flange rubber (t):
0<F.sub.P<(0.025 kgf/mm×h)+(M.sub.RF×At).

2. The pneumatic radial tire according to claim 1, wherein the bending force of the carcass ply F.sub.P is larger than a bending force of carcass ply F.sub.pf made of an organic fiber.

3. The pneumatic radial tire according to claim 1, wherein the ultra fine steel cord has a wire diameter of 0.04 to 0.14 mm, and a breaking force of 25 to 35 kgf.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

(2) FIG. 1 is a partial perspective view illustrating an ultra fine steel cord according to an example of the present invention;

(3) FIG. 2 is a cross-sectional view illustrating the ultra fine steel cord of FIG. 1;

(4) FIG. 3 is a partial perspective view illustrating an ultra fine steel cord of a comparative example described in the present invention; and

(5) FIG. 4 is a cross-sectional view illustrating the ultra fine steel cord of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

(6) Hereinafter, preferable embodiments of the present invention will be described with reference to the accompanying drawings. Referring to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views. In the embodiments of the present invention, a detailed description of publicly known functions and configurations that are judged to be able to make the purport of the present invention unnecessarily obscure will be omitted.

(7) According to the present invention, in order to solve problems such as detaching of the carcass ply after a carcass ply turning up process of the tire forming, or decreasing of the bead durability, a method of designing a product so as to have a higher adhesive force of the ultra fine steel cord for a carcass ply and a higher strength of the rim flange rubber than the bending force of the carcass ply with ultra fine steel cords is proposed. Accordingly, a relation as represented in Equation 1 below will be established.
F.sub.P<F.sub.A+F.sub.R  [Equation 1]

(8) Wherein, F.sub.P denotes the bending force of the carcass ply with ultra fine steel cords, F.sub.A denotes the adhesive force of the carcass ply with ultra fine steel cords, and F.sub.R denotes the strength of the rim flange rubber.

(9) The bending force of the carcass ply with ultra fine steel cords F.sub.P is proportional to the bending force of the ultra fine steel cord F.sub.S and the number of the cords N, and a bending force of the steel cord F.sub.S is determined by the number of the wires n and a wire diameter d as represented in Equation 2 below.
F.sub.P=N×F.sub.S=N×2400(n×d.sup.4)  [Equation 2]

(10) Herein, the strength of the steel cord is proportional to Young's modulus, a twisted angle of the wire, and the number of the wire n, and proportional to the fourth power of the wire diameter d (proportional to a sectional moment of the wire). Equation 2 is an equation in which all parameters other than the number of the wire and the wire diameter are simplified to a constant.

(11) An adhesive force of the carcass ply with ultra fine steel cords F.sub.A and the strength of the rim flange rubber F.sub.R have a relation with a turn up height of the carcass ply h, a tensile strength of the rim flange rubber M.sub.RF at the time of 50% elongation, and a thickness of the rim flange rubber t as represented in Equations 3 and 4 below.
F.sub.A≧0.025 kg.sub.f/mm×h  [Equation 3]

(12) Equation 3 is an equation obtained based on experience in tire design, and represents the relation between the adhesive force and the turn up height of the carcass ply as a proportional expression.
F.sub.R≧M.sub.RF×At  [Equation 4]

(13) Accordingly, the bending force of the carcass ply with ultra fine steel cords F.sub.P may be represented as Equation 5 below.
F.sub.P<(0.025 kg.sub.f/mm×h)+(M.sub.RF×At)  [Equation 5]

(14) If the F.sub.P is zero (0), back tension of the carcass ply becomes zero (0), and it is an ideal state. Of course, since the steel ply is made of a material having a basic inherent stiffness and thickness, in practice, the zero (0) state is not possible. Therefore, Equation 5 may be represented as Equation 6 below.
0<F.sub.P<(0.025 kgf/mm×h)+(M.sub.RF×At)  [Equation 6]

(15) Herein, since F.sub.P=0 means that, when turning up the carcass ply, it is maintained in the state without repulsive force, this is the most suitable state. However, since the bending force of the carcass ply with ultra fine steel cords F.sub.P is larger than a bending force of carcass ply F.sub.pf made of a conventional organic fiber, the present invention is designed based thereon, and the carcass ply made of the organic fiber may be defined to F.sub.P or more. Accordingly, Equation 6 may be modified as Equation 7 below.
F.sub.pf<F.sub.P<(0.025 kg.sub.f/mm×h)+(M.sub.RF×At)  [Equation 7]

(16) Hereinafter, a technique of the present invention for using the ultra fine steel cord for a carcass ply in a pneumatic radial tire for a passenger car having a low aspect ratio will be described in detail.

(17) An ultra fine steel cord 100 used in an example according to the present invention as a reinforcing material of the carcass ply is formed as illustrated in FIGS. 1 and 2. The ultra fine steel cord 100 includes two core wires 102 and 104 arranged in a central portion thereof, and seven to nine side wires 106 which have a wire diameter of 0.04 to 0.14 mm, and are twisted outside of an imaginary circle formed by the core wires 102 and 104 about the same to form a 2+n structure. Conventionally, since the wire diameter forming the ultra fine steel cord 100 is 0.15 mm or more, an upper value of the range of the wire diameter used in the ultra fine steel cord is limited to 0.14 mm or less, and drawing the steel cord of 0.04 mm or less is impossible by a current technique using a high carbon steel wire, such that a lower value thereof is limited to 0.04 mm or more.

(18) The diameter of the prepared ultra fine steel cord 100 is 0.3 to 0.5 mm, and a breaking force is 25 to 35 kgf. Herein, the breaking force is a strength of the steel cord required for supporting a load in the pneumatic radial tire for a passenger car. The minimum strength of the ultra fine steel cord required for supporting the load should be 25 kgf or more. If the strength thereof exceeds 35 kgf, the strength of the carcass ply is higher than necessary, and it may be an excessive design.

(19) However, the ultra fine steel cord 100 may have, for example, a structure of 1+n, m+n, l+m+n, etc., without particular limitation thereof. That is, various types of ultra fine steel cord structure may be used within a range of the strength and bending stress of the carcass ply, without particular limitation thereof.

(20) Table 1 illustrates a comparison of physical properties and durability of a prototype tire between 2+7×0.095 (two core wires and seven side wires having a wire diameter of 0.095 mm, respectively) which is the ultra fine steel cord 100 according to the example of the present invention and a conventional ultra fine steel cord 200 which is a comparative example.

(21) The prototype product using the carcass ply with ultra fine steel cord was evaluated with a tire size of 245/40 R18, and a prototype tire having a turn up height of the carcass ply and a thickness of the rim flange was manufactured according to the present invention.

(22) In case of the bending force of the carcass ply with ultra fine steel cord, both ends of the carcass ply were mounted on a support, and a central portion thereof was pressed downward, and then a force applied to the carcass ply was measured. In case of the adhesive force of the carcass ply, after the adjacent carcass ply and a rubber sheet were adhered to each other by applying a constant force, a force necessary to detach the same was measured. Then, the performances of the product were evaluated in terms of belt and bead durabilities, and vehicle driving emotion (ride comfort and steering stability).

(23) As a result of evaluation, it can be seen that the example using the 2+7×0.095 ultra fine steel cord having a small wire diameter as the carcass ply in order to reduce the bending force of the ultra fine steel cord 100 may sufficiently secure the durability and other performances of the product even when having a reduced turn up height of the carcass ply and decreased thickness of the rim flange rubber.

(24) However, it can be seen from the evaluated result that, in case of the comparative example using a tire in which the ultra fine steel cord 200 having a structure of 1×0.13+6×0.12 (which uses one core wire 202 with a wire thickness of 0.13 mm and six side wires 206 with a wire thickness of 0.12 mm) as the carcass ply, if the thickness of the rim flange rubber is decreased, the bead durability is decreased by 8%. If the thickness of the rim flange rubber is increased to 2 mm in order to improve the bead durability, the durability as well as the entire driving performances are improved, and thereby it is possible to design the product even if the bending force of the carcass ply is increased due to the improving of the entire driving performances.

(25) TABLE-US-00001 TABLE 1 Comparative Section Example Example Properties Steel cord (carcass) 2 + 7 × 0.095 1 × 0.13 + 6 × 0.12 of cord Cord diameter 0.37 0.33 Twisted length (direction), mm (S or Z) 5S/9S 8S breaking force kgf 25.8 25.2 Bending force of carcass ply (F.sub.P), kgf 2.1 3.5 Tire evaluation (245/40 R18) Turn up height of carcass ply (h), mm 30 60 60 60 Thickness of rim flange rubber (t), cm 0.5 0.3 0.5 0.7 Adhesive force of carcass ply (F.sub.A), kgf 0.8 1.5 1.5 1.5 Strength of rim flange rubber (F.sub.R), kgf 1.5 0.9 1.5 2.1 I/D Belt durability (%) 100 100 100 100 Bead durability (%) 100 128 92 108 F/D Ride comfort 100 100 110 110 Steering stability 100 110 110 120

(26) While the present invention has been described with reference to the preferred embodiments, the present invention is not limited to the above-described embodiments, and it will be understood by those skilled in the related art that various modifications and variations may be made therein without departing from the scope of the present invention as defined by the appended claims.