Metal cords for rubber reinforcement and tires using the same

Abstract: A metal cord usable for the reinforcement of rubber articles is comprised of a helically formed single wire filament having a twisting angle of 75-84.5° or a bundle of two twisted metal filaments each having a twisting angle of 75-84.5°. Such metal cords are preferably used as a rubber reinforcement in a pneumatic tire.

Description: This invention relates to a metal cord having a particular twisting structure, to be used as a rubber reinforcing material in tires for vehicles, conveyor belt and the like, as well as to a pneumatic tire using such metal cords as a reinforcing material. Steel cords of so-called 1x4 or 1x5 twisting structure comprised of 4-5 filaments are widely used as a rubber reinforcement for tires or the like. Recently, the use of steel cords having 1x2 twisting structure has been adopted for attaining weight reduction, low fuel consumption and low cost of tires for vehicles, and also the use of single wire cord comprised of only one wire filament has been proposed. However, when the cord of a twisting structure simplified by merely reducing the number of filaments, such as 1x2 twisting structure, single wire cord or the like, is used in the belt of a tire, if excessive force is applied during cornering, a so-called buckling deformation is caused in the ground contacting portion outside the belt and hence cord breakage is caused due to the deformation input. Therefore, there is adopted a procedure of adding a new reinforcing material to the edge portion of the belt in the building of the tire. Since the use of such a new reinforcing material is contrary to the purpose of attaining weight reduction, low fuel consumption and the like of the tire, it largely decreases the merit of using steel cord of 1x2 twisting structure or single wire cord as a rubber reinforcement. A steel cord of 1x2 twisting structure is proposed in Japanese Patent laid open No. 62-117893 and No. 62-234921. However, such steel cords have very poor breaking property because the angle of the steel filament itself also increases with respect to the longitudinal direction of the cord, and therefore such steel cords are not used in the belt portion of a tire as they are. Also, Japanese Patent laid open No. 2-229286 (EP-A-0385666) proposes steel cords of 1x2 twisting structure, in which forming ratios of two filaments are changed to periodically and alternately create portions where the filaments contact each other and portions wherein there is no filament contact (portion surrounded by rubber) in the longitudinal direction of the cord. It is described that when the steel cord formed by changing the forming ratios of the filaments is applied to the belt portion of a tire, there is prevented penetration of water resulting from cut failure on the tread surface into the steel cord of the belt layer in the longitudinal direction of the cord, whereby cord breakage due to the occurrence of rust can be prevented. In the steel cord of 1x2 twisting structure, however, it is not necessarily required to alternately create filament contacting portions and non-contact portions as described in JP-A-2,229 286 because the steel cord of 1x2 twisting structure does not produce cord breakage and the like due to the occurrence of rust through penetration of water. Moreover, it is substantially impossible to embed such steel cords in rubber for a tire while maintaining alternate contacting portions and noncontact portions in the cord. Even if these cords are embedded in the rubber such that the contacting portions and the noncontact portions alternately exist in the longitudinal direction of the cord, the angle of the filament itself with respect to the longitudinal direction of the cord at the contacting portion is too large, so that cord breakage may be caused. On the other hand, a single wire cord is described in Japanese Patent laid open No. 57-198101 and No. 1-229704, which is a simple straight wire cord, so that the resistance to cord breakage is very poor unless a reinforcing member such as layer material or the like is used. It is, therefore, an aim of the present invention to improve the resistance to cord breakage in a metal cord of 1x2 twisting structure or single wire without using another reinforcing member for attaining weight reduction, low fuel consumption and low cost of the rubber articles. The present inventors have made various studies in order to solve the problems of the conventional metal cord of 1x2 twisting structure or single wire and found that the resistance to cord breakage is improved by adopting the following structure. That is, in a first aspect, the invention provides a metal cord formed by twisting two metal filaments at the same pitch, characterized in that a twisting angle α 1 is represented by an equation (1) of 75°≤α 1 ≤84.5° and said filaments are contacted with each other at two positions per one twisting pitch. In this case, each of these filaments preferably has a filament diameter of 0.15-0.8 mm, more preferably 0.20-0.6 mm. In a second aspect, the invention provides a metal cord comprised of a helically formed single wire, in which a twisting angle α M is represented by an equation (2) of 75°≤α M ≤84.5°. In this case, the single wire preferably has a filament diameter of 0.20-1.2 mm, more preferably 0.25-1.0 mm. According to a third aspect of the invention, there is provided a pneumatic tire comprising a carcass, a belt and optionally a chafer as a tire skeleton member, characterized in that a metal cord defined in the above first and second aspects of the invention is used in at least one of these skeleton members. According to the invention, it has been found that the resistance to cord breakage is improved by using a metal cord defined in the first and second aspects as a rubber reinforcement, and that when these cords are applied to a tire, weight reduction, low fuel consumption and low cost of the tire can be attained. The term "twisting angle" used herein means a complement of an angle of a metal filament with respect to the longitudinal direction of the cord. Considering the case that the cord is a true circle, the twisting angle is shown in Fig. 1, which can be considered as an extended cord as shown in Fig. 2. In these drawings, numeral 1 is a metal filament, letter P a twisting pitch, letter R a cord diameter, letter d a filament diameter, and letter R 0 a cord radius ( R 0 =R/2-d/2 ). The twisting angle is shown by a letter α and has the following relation: α = tan -1 P/π(R-d) which is determined by a twisting pitch or forming pitch P, filament diameter d and cord diameter R from the following relation: tan α = P/2πR 0 = P/π(R-d). Thus, the twisting angle is theoretically determined by the cord diameter, pitch (twisting pitch in case of 1x2 structure, forming pitch in case of single wire) and filament diameter, and can actually be measured from the shape of the filament 1 itself as shown in Fig. 3. In case that the shape of the cord is ellipsoidal, it is particularly preferable to actually measure the twisting angle, in which an average value is adopted by measuring at 10 positions in the longitudinal direction of the cord. According to the invention, the section of the cord develops the desired effect even in the circular shape and the ellipsoidal shape. When ellipsoidal cords are embedded in the belt layer of a tire, it is desirable to measure an angle of the filament with respect to the longitudinal direction of the cord from the tread surface. Moreover, in the ellipsoidal cord, the ratio of major axis a to minor axis b is not particularly limited, but the ratio a/b is preferably within a range of 1≤a/b≤2 in view of the prevention of cord breakage. The invention will be further described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a schematic view showing a part of a metal filament in the longitudinal direction of the metal cord; Fig. 2 is a schematic view showing an extended state of the metal filament; Fig. 3 is a schematic view of a part of the metal cord; Fig. 4 is a schematic view showing a part of a conventional steel cord of 1x2 twisting structure and sections thereof at various positions in the longitudinal direction; Fig. 5 is a schematic view showing a part of a steel cord of 1x2 twisting structure according to the first aspect of the invention and sections thereof at various positions in the longitudinal direction; and Figs. 6 and 7 are schematic views of embodiments showing a part of a single wire cord according to the second aspect of the invention and sections at various positions in the longitudinal direction, respectively. In the metal cord according to the first aspect of the invention, as shown in Fig. 5, the twisting angle of the filament with respect to the longitudinal direction of the cord is considerably large as compared with that of the conventional metal cord of 1x2 twisting structure shown in Fig. 4, so that when the metal cords according to the invention are used, for example, in a belt layer of a tire, if bending input such as buckling deformation or the like is applied to the belt layer, these cords act to mitigate strain at the surface of the filament, whereby the cord breakage can be avoided. In the metal cord according to the second aspect of the invention, an adequate angle of the filament with respect to the longitudinal direction of the cord is held as shown in Fig. 6, so that the cord breakage can be prevented. Moreover, when the metal cord according to the second aspect of the invention is subjected to a forming as shown in Fig. 7, the resistance to cord breakage is considerably improved as compared with the conventional single straight cord. In Figs. 4 to 7, numerals 2 and 3 are metal filaments, and sections of the cord (A 1 , B 1 , .....) in the longitudinal direction thereof are shown beneath arrow positions (A 0 , B 0 , .....,), respectively. According to the invention, it has been found that in the metal cord of 1x2 twisting structure or single wire, when the angle of the twisting filament or single wire filament with respect to the longitudinal direction of the cord is not less than 5.5°, the resistance of cord breakage is largely improved as mentioned above. However, when the angle is too large, the lowering of the strength of the metal cord itself becomes large and the volume of one metal cord occupied in the belt layer increases to decrease the end count, so that the angle of the filament to the longitudinal direction of the cord is preferably not more than 15° for maintaining the strength of the belt itself in the tire. Particularly, when the metal cord is used in the belt layer of a tire, it is preferable that the angle of the filament with respect to the longitudinal direction of the cord is within a range of 7.5°-10° from a viewpoint of the resistance to cord breakage and the strength of the belt itself. The metal cords of 1x2 twisting structure according to the first aspect of the invention are produced by forming each of metal filaments at a given forming ratio through a preformer before the twisting and then twisting two of these formed filaments, or by continuously twisting two filaments with each other in the longitudinal direction and then forming a bundle of these twisted filaments at a given forming ratio through a preformer. Similarly, the metal cords of single wire filament according to the second aspect of the invention are easily produced by forming the filament at a given forming ratio through a preformer. Moreover, these metal cords may be shaped into an ellipsoidal cord by subjecting the cord to flat working through pushing rolls or the like. The following examples are given in illustration of the invention and are not intended as limitations thereof. In this example, the metal (steel) cords according to the invention are applied to a belt of a pneumatic tire for a passenger car. The tire to be tested has a tire size of 175/70 R13 and comprises two steel belt layers having a cord angle of 68° with respect to an equatorial plane of the tire, in which a first belt layer has a width of 140 mm and a second belt layer has a width of 130 mm. After the test tire was inflated under an internal pressure of 1.7 kgf/cm 2 and run on general road over a distance of 40,000 km, it was cut to measure the number of broken positions in the steel cord at the belt layer. The measured results are shown in Table 1. As seen from Table 1, the occurrence of steel cord breakage in the steel cords according to the invention (Examples 1-4) is reduced to a half or less of that in the conventional steel cord of 1x4 twisting structure (Comparative Example 1). Even in the steel cords of 1x2 twisting structure, when the twisting angle is more than 84.5° (Comparative Example 2) or less than 75° (Comparative Example 3), the breakage of the steel cord is substantially equal to or more than that of Comparative Example 1. When the twisting angle is restricted according to the invention (Examples 1-4), the occurrence of steel cord breakage is considerably reduced, so that the restriction of the twisting angle according to the invention develops a good effect. Furthermore, the occurrence of cord breakage is considerably reduced even in the metal cords of single wire filament (Examples 3 and 4). As mentioned above, according to the invention, the twisting angle of the filament in the metal cord is restricted to a particular angle range, whereby the resistance to cord breakage can considerably be improved. Further, when such metal cords are used, for example, in each constituting part of a tire, weight reduction and low fuel consumption and low cost of the tire can be attained. Therefore, the metal cords according to the invention may be widely used as cords for rubber articles.

Claims: A metal cord formed by twisting two metal filaments (2,3) at the same pitch, characterized in that a twisting angle α 1 is represented by an equation (1) of 75°≤α 1 ≤84.5° and said filaments are contacted with each other at two positions per one twisting pitch. A metal cord as claimed in claim 1, characterized in that said filament has a filament diameter of 0.15-0.8 mm. A metal cord as claimed in claim 2, characterized in that said filament diameter is 0.20-0.6 mm. A metal cord comprised of a helically formed single wire (2), characterized in that a twisting angle α M is represented by an equation (2) of 75°≤α M ≤84.5°. A metal cord as claimed in claim 4, characterized in that said wire has a filament diameter of 0.20-1.2 mm. A metal cord as claimed in claim 5, characterized in that said filament diameter is 0.25-1.0 mm. A pneumatic tire comprising a carcass, a belt and optionally a chafer as a tire skeleton member, characterized in that a metal cord claimed in any of claims 1 to 6 is used in at least one of said skeleton members.

Inventors: BUNDO MOTONORI, MOROTOMI KAZUMA, BUNDO, MOTONORI, MOROTOMI, KAZUMA
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1992
Main classification: B60C9
IPC codes: B60C9, D07B1