EP0155287A1

EP0155287A1 - Steel cord for rubber articles

Info

Publication number: EP0155287A1
Application number: EP19840903272
Authority: EP
Inventors: Yoshiaki Yamada; Michio Nishikawa
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1983-09-02
Filing date: 1984-09-03
Publication date: 1985-09-25
Also published as: JPS6059188A; WO1985001074A1; EP0143767A1; JPH0367155B2; BR8407046A

Abstract

Un câble d'acier de renforcement d'un article en caoutchouc tel qu'un pneumatique, une bande transporteuse, un tuyau pour haute pression, consiste en une pluralité de filaments d'acier torsadés dans la même direction de torsion et avec le même pas de torsion. Selon l'invention, les filaments sont déformés en spirale de manière à ne pas introduire de contraintes de torsion ou de composantes de déformation. Ceci est obtenu à l'aide de dispositifs de préformage combinés à une machine de torsion tubulaire, comprenant des bobines de filaments montées dans la machine et dont le nombre correspond à celui des filaments nécessaires pour les différentes couches respectives, y compris toute âme du câble d'acier multicouche. En éliminant les composantes de torsion agissant sur les filaments, la résistance à la fatigue, la stabilité d'adhésion et la productivité de fabrication sont améliorées.A steel cable for reinforcing a rubber article such as a tire, a conveyor belt, a hose for high pressure, consists of a plurality of steel filaments twisted in the same direction of twist and with the same pitch of torsion. According to the invention, the filaments are deformed in a spiral so as not to introduce torsional stresses or deformation components. This is obtained using preforming devices combined with a tubular twisting machine, comprising spools of filaments mounted in the machine and the number of which corresponds to that of the filaments required for the respective respective layers, including any core of the cable. multilayer steel. By eliminating the twisting components acting on the filaments, fatigue resistance, adhesion stability and manufacturing productivity are improved.

Description

STEEL CORD FOR RUBBER ARTICLES

This invention relates to an improvement of steel cords to be embedded in a molded body, made of rubber or rubber-like elastomeric material to reinforce the rubber or elastomeric article.

Recently, one kind of such reinforcing steel cord has been highlighted, of which all filaments are twisted in the same twisting direction with the same twisting pitch. This is called "compact cord", because such twisted filaments are completely in line contact with each other to avoid so-called "fretting wear" which occurs in conventional steel cord because of point contact between the filaments. For twisting filaments into steel cord of this kind, a so-called "buncher type twisting machine" has mainly been used because of its favourable productivity. With the twisting mechanism of this machine, however, the individual filaments are unavoidably subjected to torsion when twisted into cord.

The inventors have developed steel cord of this kind and made efforts to clarify various performances of these cords when used in composite bodies with rubber. In these investigation, particularly actual performance tests of steel cord radial pneumatic rubber tires for trucks and buses used under particularly severe conditions, the inventors have found that the prior art compact cords have some disadvantages, when used in composite bodies, with respect to the fatigue resistance and adhesion stability.

In more detail, when such tires are used under the severe conditions, tire burst and separation often occur, the former being due to the fatigue destraction of the steel cord used in the belt of plie in the tires, and the latter being due to the decrease in adhesion between the steel cord and the rubber of the tires. In other words they are caused by the loss of the steel cord performance in the composite body. This can of course be observed not only in the rubber tires but also in other rubber articles reinforced by steel cord such as conveyor belts, high pressure rubber hoses and the like.

The inventors have further found that the above described buncher type twisting machine, used in factories for manufacturing the above mentioned steel cord, sometimes tends in actual use to cause filament rupture so that the rate of operation is unexpectedly much reduced, which provides a problem on the productivity which was considered to be comparatively high up to now.

As the result of the inventor's investigation to eliminate the above disadvantages of the prior art, it appeared that these disadvantages are caused by the fact that the steel cords twisted by the buncher type twisting machine contain unavoidable torsional stress components in their filaments.

In other words, the fatigue resistance of the steel cord is decreased by the reduction in toughness of the individual filaments due to the torsional stress components in these filaments, which promotes fatigue rupture under repeated bendings in loaded conditions of the rubber articles, particularly, rubber tires. Moreover, the adhesion between the cord and the rubber is affected, because of a decrease in cohesion between the brass plating layer and the iron basis material, due to torsional stresses acting upon the filaments, and by decrease in adhesion between the fllmanents and rubber of the tire so as to cause the premature adhesion destruction.

It is furthermore found that the decrease in productivity due to rupture of the filaments in production processes is also caused by the torsional stress components acting on the filaments. It is a principal object of the invention to provide a steel cord for reinforcing rubber articles, which eliminates the above disadvantages of the prior art so as to improve the fatigue resistance, the adhesion stability and the productivity of the twisters making such cord.

In order to achieve the above object, according to the invention the filaments are helicordally deformed, so as substantially not to include any torsional stress component. This obtained by means of preformers combined with a tubular twisting machine housing therein filament bobbins whose number corresponds to that of the filaments required for respective layers including a core of the multiple layer steel cord.

When helicoidally deforming the filaments with preformers included in a tubular twisting machine, the filaments are subjected only to plastic bending deformation and not to torsional deformation. For that reason, the toughness of the filaments is not reduced by the twisting operation. This fact serves to improve the fatigue resistance. Moreover, as the quality of the brass plating layer at the filament surface is not affected by any torsional deformation of the filaments, the adhesion between the plating layer and the rubber of the tires is not affected either. In the. twisting process, furthermore, filament rupture occurs very rarely so that the productivity is improved. Particularly, all the filaments, including the core filaments for multiple layer steel cords, are twisted in one step to remarkably improve the productivity in conjunction with few ruptures of the filaments.

In order that the invention may be more clearly understood, preferred embodiments will be described, by way of example, with reference to the accompanying drawings. Figs. 1a-1f are schematic sectional views of steel cords to which the invention is applied.

Fig. 2a is a schematic perspective view of the steel cord shown in Fig. 1b. Fig. 2b is a schematic perspective view of one filament of a steel cord of the prior art subjected to torsional stress.

Fig. 2c is a schematic perspective view of one filament of the steel cord shown in Fig. 2a not subjected to torsional stress.

Fig. 3 explanatorily shows an arrangement of a tubular twisting machine combined with preformers for manufacturing the steel cords according to the invention.

Figs. 4a and 4b illustrate important parts of the tubular twisting machine and preformers shown in Fig.3; and

Fig. 5 is a schematic perspective view for explaining the spiral deformation of filaments of steel cords by the preformer according to the invention.

Figs. 1a-1f illustrate various cross-sections of steel cords for rubber articles. The cord shown in Fig. 1a consists of a core of two filaments and an outer layer of eight filaments surrounding the core. The other cords respectively consist of a three-filament core and a nine-filament outer layer (Fig. 1b), a two-filament core, a first eight-filament outer layer and a second fourteen-filament outer layer (Fig. 1c), a three-filament core, a first nine-filament outer layer and a second fifteen-filament outer layer (Fig. 1d), a four-filament core and a ten-filament outer layer (Fig. 1e) and a six-filament core and a twelve-filament outer layer (Fig. 1f). A perspective view of the cord shown in Fig. 1b, is further given in Fig. 2a. As can be seen from Fig. 2a, the respective filaments are twisted in the same direction and with the same pitch. The twisting direction of the filaments shown in the drawing is the Z-direction. However, it may of course be and S-direction.

Figs. 2b and 2c illustrate the distinction between a filament subjected to torsional stress (Fig. 2b) and a filament of a cord not subjected to torsional stress shown in Fig. 2a.

Each filament of the hitherto used compact type steel cord unavoidably contains a torsional stress and deformation component as shown in a dot-and-dash line in Fig. 2b because filaments are twisted by a buncher type twisting machine. On the other hand, each filament of the cord according to the invention substantially does not contain any torsional stress or deformation component, as shown in two dots-and-line in Fig. 2c.

The steel cord for a rubber article according to the invention is formed by filaments passing through an assembly as shown in Fig. 3. The assembly comprises a tubular twisting machine 1 combined with preformers 2, a pair of drive pulleys 3 by which the filaments are subjected to tensile force, and a guide roller 4 arranged between the pair of drive pulleys 3 and a winding up spool 5.

The tubular twisting machine 1 houses therein a plurality of filament bobbins 1a, 1b, ... 1n, whose number corresponds to the number of filaments required for respective filament layers, including a core for forming the steel cord. Figs. 4a and 4b illustrate the tubular twisting machine of which the principal part is given in a sectional view. Referring to Fig. 4a, the tubular twisting machine 1 comprises cradles 6 for the filament bobbins, each cradle being pivotally supported in the machine and rotatable around an axis of the machine so as to have the filament unwound from the bobbin. The machine 1 further comprises a number of guide rollers 7, each supported by the respective cradle 6 and guide rollers 8 , 9 and 10 directly fixed to the machine. The preformers 2 are arranged between an guide plate 11 and a twisting-die 12.

Referring to Fig. 5: this illustrates a principle of the single preformer for deforming or transforming a filament. The preformer 2 comprises at least three collars 17, 18 and 19 arranged on one end 16 of a shaft extending from the tubular machine 1, and transforming pins, each extending from the collar. The collars are adjustably fixed on the shaft so as to adjust the relative angular positions of the pins so that the filament 20 delivered from the filament bobbin is helicoidally deformed or transformed into a filament 20' shown in Fig. 5, which does not contain any torsional stress or deformation and is twisted together with other similar filaments to form a cord 21 in Figs. 4a and 4b.

The deformation or transformation of each the filament at the preformer 2 is freely adjustable in the range of 20-200%, preferably 50-150%, assuming that the degree of helicoidal deformation of a filament in a completed steel cord is 100%.

It is of course understood that with the deformation of less than 100%, the obtained steel cords 21 are of the abovementioned compact type. In such "compact" type, the cross-sectional configuration of the cord is constituted by a number of filament cross-section circles of which the adjacent ones are in principle tangent to each other, in such a way that when connecting by a line the centre-point of each circle with the centre-point of the adjacent ones, a network of equilateral triangles is obtained.

If the deformation is more than 100%, various open type steel cords are obtained without disturbing the alignment of the filaments, so as to ensure the penetrability of rubber between the filaments, thereby effectively avoiding the corrosion of the cord due to water penetration through cuts in a tire, in conjunction with a suitable improvement of adhesion of the rubber to the filaments.

In this manner and according to the invention, filaments required for respective layers, including a core, are twisted so as to form a multiple layer steel cord, by the tubular twisting machine 1 enclosing the filament bobbins corresponding to the respective filaments, and in combination with the preformers 2 for helicoidally deforming the filaments. The deformation of the filament, imparted in this manner according to the invention, consists only of bending components without causing any torsional deformation, so that the deforming action does not reduce the toughness of the filaments themselves and does not cause any residual shearing stress due to torsion and therefore completely eliminates disadvantages such as decreases in strength efficiency and fatigue resistance properties of the steel cord and adhesion with rubber, and there is no risk of decrease in productivity due to ruptures of the filaments during the twisting thereof.

The inventors carried out the following experiments on the steel cords for reinforcing rubber articles to obtain the results described hereinafter. The tests were on one hand rotating beam tests and three-roller tension bending tests as fatigue tests on steel cords, and on the other hand peeling tests as adhesion tests on test pieces of steel cords embedded in rubber pieces and on test pieces cut from tires after used for trucks and buses.

In the rotating beam test , steel cords were bent or curved in radii 10-20 mm and then rotated about their incurved axes by driving their ends into rotation, so that the steel cords were repeatedly subjected to bending actions until they broke down and the fatigue resistance was then estimated by the number of rotation before breakdown. In the three-rooler tension bending tests, the steel cord was fixed at one end and subjected to a tensile force by means of weights acting on the cords through a guide pulley. Three rollers were arranged in parallel with each other but in staggering manner so that their axes form a flat isosceles triangle when seen in the direction of these axes. One of the steel cords extended alternately over the three rollers. The rollers were reciprocatively moved in the direction of the cord with a stroke sufficient to exert positive and negative bending actions on the cord until the cord was broken down and the fatigue resistance estimated by the number of reciprocative movements.

Frequency of rupture

The frequency of rupture of the filaments, caused in the process to produce 1 ton of steel crds by means of the conventional buncher type twisting machine was determined as and index 100. In this case, the relative frequency of rupture of steel cords according to the invention was at an index 20.

The typical embodiments of the invention have been explained. However, the same effect can be expected on steel cords according to the invention whose numbers of filaments are 19, 30, 37, 44, 48 and 52. In any cases, the steel cord can be provided with a helicoidal wrapping wire if required.

As can be seen from the above description, the deformation of filaments for forming steel cords according to the invention consists only of bending components without any torsional component, so that it does not reduce the toughness of the filaments themselves and for that reason increases the fatigue resistance of the filaments. There is less possibility of reduction of adhesion between the iron and the plating metal and of variation in properties of the plating metal which would be due to shearing deformation of the plating layer surfaces caused by torsional deformation of the filaments, so that a high adhesion performance is insured for a long period of time. Moreover, the overtwisting for compensating the elastic return of the twisted bundle as in the buncher type machine is not necessary, so that rupture of filaments is rare, if any, so that the productivity is increased. Furthermore, all the filaments, even if used for forming multiple layer cords, can be simultaneously twisted together in one step to further improve the productivity. The same pitches and twisting directions are simply controlled so as to completely eliminate irregularity in pitch in a longitudinal direction to decrease irregularity of fatigue of the cord. Moreover, the deformation of the filaments can easily be controlled so that the helicoidal deformation of all the filaments of the respective layers can be controlled, depending upon the objects to ensure the high penetrative performance of rubber into between filaments of the steel cord.

The invention is preferably applied to steel cord in which the filaments are of the high-tensile type. These are filaments of which the tensile strength exceeds 2250 - 1130 d Newton/mm² , in which d is the diameter, in millimeter, of the filament.

It is further understood by those skilled in the art that the foregoing description is that of preferred embodiments of the disclosed steel cords and that various changes and modifications may be made in the invention without departing from the spirit and scope therof.

Claims

CLAIMS :

1. A steel cord for reinforcing a rubber article consisting of a plurality of steel filaments twisted in the same twisting direction with the same twisting pitch, characterized by the fact that said filaments are helicoidally deformed and free of any substantial torsional stress component.

2. A steel cord according to claim 1, in which the filaments lie in a compact configuration.

3. A steel cord according to any one of claims 1 and 2, in which the filaments are of a high-tensile strength type.

4. A process for twisting the steel cord according to any one of claims 1 to 3, the process comprising helicoidally deforming, and free of any torsional deformation, the individual filaments, and twisting these filaments by means of a tubular stranding machine, in which the unwinding bobbins of all the filaments of the cord are mounted, including of the filaments of the core of a multiple layer steel cord.