Steel cord for reinforcing rubber product

Abstract: A steel cord (7) comprises a plurality (from 3 to 27) of component steel wires tightly twisted at a regular pitch P from 5 to 20 mm in the same direction. Each wire has a diameter d from 0.1 to 0.4 mm. At least one of the wires (1) includes a spiral portion having a spiral pitch P₁ from 0.1P to 0.7P and a spiral diameter d ₁ from d + 0.02 mm to d + 0.2 mm. At least one other wire (2) includes a straight portion. At any cross section of the cord, 25 to 66% of the wires are spiral and the other wire(s) is (are) straight. The stretch of the cord under a load of 5 kg is from 0.10 to 0.40%. A method of an apparatus for producing such steel cord are described and claimed.

Description: The present invention relates to steel cords used as reinforcements for rubber products such as automobile tires and conveyor belts. Generally, a steel cord of this type is formed by twisting a number of element wires or strands, and a reinforcement includes a number of such cords extending in parallel and coated with rubber. The cord is required to be mechanically strong and chemically adherent to the rubber. The cord should also enable the fluid rubber compound to flow between the wires during manufacture and fill the cavities or spaces among them, so that the wires and rubber form a solid cord. Figs. 10 and 11 of the accompanying drawings show two types of conventional steel cords 27 and 28 of "1 x n" closed strands, "n" being the number of wires. The cord 27 has a single ply of twisted wires 29 (n = 3 through 6) for pneumatic radial tires for automobiles. The cord 28 has a bundle of twisted wires 29 (n = 7 through 27) for pneumatic radial tires for heavy loads such as for trucks and buses. The wires 29 are tightly or closely twisted and form a center cavity H (Fig. 10) or cavities H (Fig. 11) among them. When a complex sheet is formed of steel cords and two rubber sheets, the rubber compound is prevented from flowing into the cavity or cavities H, and merely surrounds the outside of the cord. This prevents a complete complex of steel cords and rubber from being formed. Accordingly, in an automobile tire, for example, containing such cords 27 or 28, there is poor adhesion between the cords and rubber, and when the car is driven, the cords and the rubber may separate. This degrades the tire function, and allows water contained in the rubber or penetrating through cracks in the tire to enter the cavity/cavities. H. The water may then flow along the cords and corrode them, with the result that they may become mechanically weakened. In the cord 28 (Fig. 11), the outer and inner wires are twisted in different spirals. When repeated bending stresses are applied on the cord, the inner wires may project between the outer wires, so that the cord may be fatigued. Fig. 12 shows a conventional open-strand steel cord 30, which is formed by twisting three wires 29 loosely with clearances I between them (see Japanese patent early publication S.55-90692). In order for the rubber compound to sufficiently flow into the inside of the cord between the wires, the clearances I are required to be 0.01 mm or larger. However, the larger clearances result in the following problems; The free space where the wires 29 can move is enlarged, so that they may deflect and the strand may not be uniform longitudinally. This may cause the cord to buckle when it is loaded with repeated bending stresses. Because the cord stretches or elongates greatly under very low load, its handling workability is low. In addition, the tension under very low load applied to the cord when a complex sheet is formed reduces the clearances I, so that the rubber compound may not flow sufficiently into the inside of the cord. It is an object of the invention to provide an improved steel reinforcing cord for reinforcing a rubber product with the stretch of the cord being small under very low load, the cord having a stable strand, not buckling easily being less fatigued under repeated bending stressed, and being suitable for forming a complete complex with rubber. It is another object of the invention to provide an apparatus for producing such a cord in one process at a low cost. EP-A-0143767 discloses a reinforcing cord comprising between three and twenty seven steel wire elements tightly twisted in the same direction at a regular pitch P in the range 5mm through 20mm, each of said wire elements having a diameter d in the range 0.1mm through 0.4mm and at least one of said wire elements being a straight wire element as defined hereinbelow. The invention provides a reinforcing cord comprising between three and twenty seven steel wire elements tightly twisted in the same direction at a regular pitch P in the range 5mm through 20mm, each of said wire elements having a diameter d in the range 0.1mm through 0.4mm and at least one of said wire elements being a straight wire element as defined hereinbelow, characterised in that at least one of said wire elements is a spiral wire element comprising at least a spiral portion having a spiral pitch P₁ in the range 0.1P through 0.7P and a spiral diameter d ₁ in the range d + 0.02mm through d + 0.2mm, in that at any transverse cross-section of the cord, the number of said spiral portions is 1/4 through 2/3 the total number of said wire elements and that the stretch of the cord under a load of 49N (5 kg) is between 0.10% and 0.40%. A straight wire element of the cord, as will be described in more detail in the description of specific embodiments of the invention which follows, is twisted into a spiral having a pitch P. Each spiral element is also twisted to form a spiral having a pitch P. The spiral wire elements are additionally twisted independently of the straight wire elements by the spiral forming means into a spiral having a spiral pitch P₁ not possessed by the straight wire elements. If the twist pitch P is too small, the cord is weak and the productivity is low. If P is excessive, the strand is not stable and the cord is inferior in its handling workability and resistance to fatigue. If the spiral pitch P₁ is smaller than 0.1P, the cord stretch under very low load is large and, because the formation of a wire to form a spiral requires an excessive plastic defamation, the wire may be easily broken. If P₁ exceeds 0.7p, the tension due to the flow of rubber compound when forming a rubber product, or the ironing force on the cord surface reduces the clearances between the wires, so that the compound does not sufficiently penetrate into the cord. If the wire diameter d is smaller than 0.1 mm, the cord is mechanically weak. If d exceeds 0.4 mm, the cord diameter is too large and the cord is not flexible. If the spiral diameter d1 is smaller than d + 0.02 mm, the rubber compound, even if highly flowable, does not sufficiently penetrate when pressurizing and vulcanizing a rubber product. If d1 exceeds d + 0.2 mm, the strand is not stable, the resistance to fatigue is low, the cord stretch under very low load is large, and the workability is low. If the number of spiral portions at any cross section of the cord is fewer than 1/4 of the total number of wires, the rubber compound does not sufficiently penetrate. If they exceed 2/3, the cord stretches under very low load and the cord diameter is large. If the cord stretch under 49N (5kg) load is excessive, the workability is low when forming a cord and a rubber product. In addition, the tension and bending stress on the cord reduce the clearances between the wires. This stretch should be a minimum, but is rarely less than 0.10 % by the nature of a steel cord. The invention also includes apparatus for producing a reinforcing cord, the apparatus comprising frame means, a plurality of feeders each for feeding a steel wire element, tensioning means for tensioning a said wire element from at least one of said feeders, spiral forming means comprising a plurality of spaced apart pins arranged such that, in use, at least one wire element from said tensioning means passes successively over and under adjacent pins, and strand means for twisting the wire elements to form a cord of the wire elements from the feeders, said feeders, tensioning means and strand means being supported by said frame means, characterised in that said pins extend from a surface which is parallel to and rotatable around an axis along which a respective wire element from said tensioning means is fed during said use, said pins being arranged relative to said axis such that the wire element continues to move along said axis on leaving the spiral forming means. In order that the invention may be well understood some embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: Figs. 1a and 1b are respectively a schematic view in cross-section and a schematic side view of an embodiment a spiral wire element; Figs. 2, 3 and 4a-4d are schematic views in cross-section of embodiments of a reinforcing cord according to the invention; Fig. 5 is a schematic side view of the cord of Fig. 4c; Figs. 6a-6f are schematic views in cross section of further embodiments of reinforcing cords according to the invention; Figs. 7a and 7b are views similar to Figs. 1a and 1b showing another embodiment of a spiral wire element; Figs. 8a, 8b, 8c and 9 are schematic views in cross-section of further embodiments of reinforcing cords according to the invention; Figs. 10-12 are schematic views in cross-section of prior art reinforcing cords; Figs. 13 and 14 are respectively schematic views of two embodiments of apparatus for producing a reinforcing cord according to the invention; Fig. 15 is an enlarged perspective view of a spiral former of the apparatus shown in Figs. 13-14; and Figs. 16, 17 and 18 show three tables which further explain the invention. Figs. 1a-1b show a spiral wire element 1 made, in this specific example, of brass-plated hard steel, and having: a wire diameter d in the range 0.1 mm through 0.4 mm; a spiral or apparent diameter d₁ (when viewed from the end) in the range d + 0.02 mm to d + 0.2 mm; and a spiral pitch P₁ in the range 0.1P through 0.7P where P is the pitch of the wire elements of a reinforcing cord as described in more detail below. Figs. 2-5 show reinforcing, or steel, cords 3-8 formed of one or more spiral element wire/s 1 and one or more straight element wire/s 2 tightly twisted or intertwined in the same direction at a regular pitch P. Each straight wire 2 is also made of brass-plated hard steel and has the same diameter d. It will be appreciated that a so-called straight wire element in the cord (as best seen in Figure 5) is twisted to form a spiral having a pitch P and that each spiral wire element in the cord is also twisted to form a spiral having a pitch P. The spiral wire element as described above is further twisted to have a spiral pitch P₁ not possessed by a straight wire element. Table 1 (Fig. 16) attached hereto shows types, experimental results etc. of cord samples according to the invention and comparative samples, as shown in Figs. 2-5, and prior art samples. The following is a discussion of the symbols and abbreviations contained in Table 1. The comparative samples differ from the samples according to the invention in that at least one of the following parameters falls outside the requirements of the invention: the number Ns of spiral wires 1; the spiral pitch P₁ ; or the spiral diameter d ₁. The diameter R (Figs. 2, 3) is an average of the outer diameters of each sample measured by a JIS B7502 micrometer at three longitudinal points at intervals about 100 mm. S (Figs. 2, 3) represents the outer diameter, measured by the same micrometer, of a cord formed by tightly twisting the same number of straight wires only, each having the same diameter, as each sample according to the invention: RCPR: Rubber Compound Penetration Ratio Each cord under 49N (5 kg) of tensile load was embedded in rubber compound having 3.43 N/mm² (35 kg/cm²) of 100 % modulus. After vulcanizing the rubber, the cord was taken out and disassembled into wires. A certain length L₁ of the disassembled cord was observed, and the length L₂ which bore traces of contact with the rubber was measured. The percentage of L₂ to L₁ represents the rubber compound penetration ratio. This value should be 60% or higher. SL: Stretch under 49N (5 kg) Load The value SL is a percentage of stretch of each sample under 49N (5 kg) of load, and should be 0.40 % or lower in view of cord workability and strand stability. RF: Resistance to Fatigue Steel cords for each sample were embedded in a sheet of rubber compound having 3.43 N/mm² (35 kg/cm²) of 100 % modulus. The resultant sheet was tested by a three-point pulley bending fatigue tester, which determined the frequency of cycles of fretting wear, buckling etc. until the cord broke. When the indexes for the frequencies for the prior art samples of closed strand were 100, those for the other samples having the same number of wires represented the resistance to fatigue. HW: Handling Workability HW represents the difficulty in handling and working the cords when they are produced and the complex sheets are formed. In comparison with the prior art samples of closed strand, the symbols X, Δ and 0 mean much more difficulty, more difficulty and substantially the same, respectively. As shown in Table 1, prior art cord Nos. 1, 6, 12 and 18 of closed strands lack RCPR. Prior art cord Nos. 2, 7, 13 and 19 of open strands are high in RCPR and RF, but large in SL and low in HW. Comparative cord Nos. 3, 8, 9, 14-16 and 20-22 are inferior in terms of at least one of RCPR, SL, RF and HW. Cord Nos. 4, 5, 10, 11, 17 and 23 according to this invention satisfy all the four terms and are optimum as reinforcements for rubber products. Figs. 6a-6f show reinforcing, or steel, cords 9-11 and 9a-11a formed by tightly twisting some spiral wires 1 and some straight wires 2, in such a manner that at least one spiral wire 1 contacts each straight wire 2. Table 2 (Fig. 17) is similar to Table 1, but shows data relating to samples of the invention and comparative samples as shown in Figs. 6a-6f and prior art samples. In the case of samples according to the invention, not only did the rubber compound sufficiently penetrate into the cords, as shown in Table 2, but it was able to completely surround the wires. This considerably reduces the fretting wear caused by the contact between the wires, and improves the RF. Prior art cord Nos. 1, 6 and 11 in Table 2 of closed bundle strand lack RCPR. Comparative cord Nos. 2, 3, 7, 8, 12 and 13 in Table 2 are inferior in at least one of the four terms (RCPR, SL, RF and HW). Cord Nos. 4, 5, 9, 10, 14 and 15 of the invention in Table 2 satisfy all the terms. Figs. 7a and 7b show another spiral wire element 14 of brass-plated hard steel having a diameter d. The wire 14 comprises alternate spiral 12 and straight 13 portions, each having a length L3. The spiral portion 12 has a spiral pitch P₁. Figs. 8a-8c show steel cords 15, 15a, 16 and 17 comprising of two or more such wires 14 and one or more straight wire/s 2 tightly intertwined at a regular pitch P. In Figs. 8a-8c, the spiral portions 12 of each wire 14 are displaced longitudinally from the spiral portions 12 of the other wire or wires 14, so that any transverse cross-section of the cord contains at least one spiral portion 12. Table 3 (Fig. 18) is similar to Tables 1 and 2, but shows data relating to samples of the invention and comparative samples, as shown in Figs. 8a-c, and prior art samples. N3 is the number of spiral portions 12 appearing in any cross-section of the cord. Prior art cord Nos. 1 and 2 in Table 3 are substantially the same as cord Nos. 1 and 2 in Table 1. Comparative cord Nos. 3-6 in Table 3 are inferior in at least one of RCPR, RF and HW. Cord Nos. 7-9 of the invention in Table 3 satisfy all the four terms. With respect to samples according to Fig. 8c, not shown in Table 3, similar results were obtained. It is possible to form a multi-strand steel cord, as a stronger reinforcement, by twisting some strands each being one of the cords of the embodiments of the invention. In this case, the number of strands should be 3-7 and each strand should have 3-7 wires to maintain the strand stability and prevent the cord diameter from increasing extremely. In each embodiment, the cord diameter R should range from 1.O1S to 1.5OS, and more preferably to 1.30S. If R is too small, rubber compound does not sufficiently penetrate into the cord. If R is excessive, it is necessary to thicken the compound. This is not economical and increases the weight of rubber product. Fig. 9 shows an elliptical steel cord having a minimum diameter D1 and a maximum diameter D2, and formed of three straight wires 2 and two spiral wires 1 or wires 14. The ratio D1/D2 should range from 0.6 to 0.9. By orienting the minimum diameter D1 across a rubber sheet, the sheet can be thinned and the resistance to fatigue can be improved. The reinforcing cords of the above-described embodiments have passages along their entire length for rubber compound to penetrate into the cords, have stable strands, do not greatly stretch under very low load, and are very workable when forming a complex sheet. When the cords are coated with rubber compound, the rubber substantially completely surrounds each wire to prevent the cords from corroding and the cords and compound from separating. The cords do not easily buckle under repeated bonding stresses, and their resistance to fatigue is improved. When the cords are reeled, the reels avoid puncture, which might occur if the cords stretched greatly under very low load. For example, a tire having steel cords of the invention contained in at least one of a belt, a carcass and a chafer, has a high performance and a long life. The tire improves the safety of the car and can reduce the fuel consumption. Fig. 13 shows an apparatus of the double-twist type for producing a reinforcing cord as shown in Fig. 2. A conventional wire feeder 50 comprising a plurality of feeders 50 and tension means is fixed to a frame, in the form of dancer rollers 52. Specifically, the feeder includes three sets of reels 52 and dancing rollers 54. Each, reel 52 is braked and each roller 54 is biased upwardly to apply tension to a steel wire element 2, which has a diameter of 0.28 mm. A spiral forming means, o spiral former, 56 for forming a spiral wire element 1 is supported on an axis 0, and connected through a gear system to an electric motor 62, so as to be rotatable in one direction at 15,000 rpm. As best shown in Fig. 15, the former 56 includes a rectangular plate 64 which (as viewed in Fig.15) is displaced from the axis 0 and has a surface 66 extend parallel to the axis which surface 66 is rotatable around the axis 0. Three sintered hard alloy pins 68 are fixed to the surface 66 at a regular interval, and extend across the axis 0. The outer, or end, pins are aligned parallel to the axis 0 such that the axis 0 forms a tangent to a respective point on the surface of each pin as best seen in Fig.13 or 14. The center pin is displaced from the other two pins a distance which is substantially less than the 1 mm diameter of the three pins. The wire element 2 from the center reel 52 is fed along the axis 0 and passes through the rotating former 56 along the axis 0 passing successively over and under adjacent pins 68, whereby it is twisted to form a spiral wire element 1 as shown in Fig. 1b. It will be noted that the wire from the center reel which is fed along the axis 0 passes successively over and under adjacent pins 68 and continuses to move along the axis 0 on leaving the spiral former 56, deviating from said axis only to pass under the center pin as best seen in Fig.15. A gatherer 70 fixed on the axis 0 gathers the spiral wire and the two straight wires from the other reels 52. A conventional twister 72 is supported on the axis 0 to be rotated at 12,000 rpm in the direction opposite to the spiral former 56. The twister 72 includes two overtwist rollers 76 and 78 supported on parallel axes, which extend perpendicularly to and diametrically of the axis 0. The smaller roller 76 has one peripheral groove and the larger roller 78 has two peripheral grooves. The gathered wires pass first around one groove of the larger roller 78 then around the groove of the smaller roller 76, and finally around the other groove of the larger roller. The rotation of twister 72 provisionally twists the wires at the inlet of gatherer 70 and forms a primary strand of the wires. A conventional buncher type strander 80 includes two circular end flyers 82 supported on the axis 0 by the frame. Each flyer 82 has two turn rollers 84. To the downstream flyer 82 is fixed an inner gear 86 on the axis 0. A cradle 81 is supported at its top by the flyers 82 which rotatable relative to it. The cradle supports rollers 88, a capstan 90 and a bobbin 92. The capstan 90 has a gear 94 connected to the gear 86. The bobbin 92 is connected to the capstan 90 by a belt 96. The flyers 82 are rotated at 5,000 rpm in the same direction as the twister 72, but the cradle 81 does not rotate by gravity, so as to form a final strand as a reinforcing cord from the primary strand. The cord is taken up at a regular speed by the capstan 90 and wound up by the bobbin reel 92. The spiral diameter d₁ and pitch P₁ of the spiral wire element 1 can be changed by selecting the rotational speed of former 56 and the interval between the former pins 68. The strand twist pitch P likewise changes with the rotational speeds of strander 80 and capstan 90. Fig. 14 shows another apparatus for forming a reinforcing cord as shown in Fig. 2 This apparatus includes a braked reel 152, a dancer roller 154 and a spiral former 156, which are similar to the components 52, 54 and 56 in Figs. 13 and 15. A conventional strander 180 includes a tubular body 182 supported by a frame coaxially with the former 156 on an axis 0. To the body 182 are fixed three hollow shafts 184, 186 and 188 coaxially. The rear shaft 184 is connected to a motor 162 to be rotated in the direction opposite to the former 156. The body 182 has six turn rollers 190. The hollow shafts 184, 186 and 188 each rotatably support a coaxial pin 192 thereinside. The three pins 192 are fixed to two cradles 194 within the body 182. Each cradle 194 supports a braked feeder reel 196 and a dancer roller 198. To the front hollow shaft 188 is fixed, adjacent its front end, a guide disc 200 having three peripheral recesses to guide the wires from the former 156 and reels 196. The front end of shaft 188 has a conical head 202. Three rows of cone pins 204 extend perpendicularly to the conical head surface. The pins 204 at each row are arranged similarly to the former pins 68 of Fig. 15. When the tubular body 182 rotates, the cradles 196 do not move by gravity. The wires are twisted at a gatherer 206 to form a strand, which is taken up by a capstan 208 and a bobbin 210. The twister 72 (Fig. 13) and cone pins 204 (Fig. 14) are provided to obtain a secure strand twist and to prevent the strand from flaring back, and to give the wire a precisely spiral form depending on the twist pitch. The twister 72 and the pins 204 may be omitted from the apparatus of Fig.13 and 14 respectively. The twister 72 may otherwise be provided immediately after the spiral former 56 to give the wire a more precisely spiral form. The formers 56 and 156 may be adapted to rotate periodically to form a wire as shown in Fig. 7. Immediately before the bobbin 92 (Fig. 13) or 210 (Fig. 14), may be provided rollers or a mold having an elliptical hole for shaping the cord to be elliptical as shown in Fig. 9.

Claims: A reinforcing cord comprising between three and twenty seven steel wire elements (1,2;14,2) tightly twisted in the same direction at a regular pitch P in the range 5mm through 20mm, each of said wire elements having a diameter d in the range 0.1mm through 0.4mm and at least one of said wire elements being a straight wire element (2) as hereinbefore defined, characterised in that at least one of said wire elements is a spiral wire element (1;14) comprising at least a spiral portion having a spiral pitch P₁ in the range 0.1P through 0.7P and a spiral diameter d ₁ in the range d + 0.02mm through d + 0.2mm, in that at any transverse cross-section of the cord, the number of said spiral portions is 1/4 through 2/3 the total number of said wire elements (1,2;14,2) and that the stretch of the cord under a load of 49N (5 kg) is between 0.10% and 0.40%. A cord as claimed in claim 1, wherein at any cross-section of the cord each said straight wire element (2) contacts at least one said spiral portion. A cord as claimed in claim 1 or 2, wherein at least two of said spiral wire elements (14) each comprises alternate spiral (12) and straight (13) portions. A cord as claimed in claim 1, 2 or 3, having a diameter between 1.01 S and 1.50 S , S representing the diameter of a reinforcing cord (A) formed by tightly twisting the same number of the wire elements having the same diameter, the wire elements of said cord (A) being straight wire elements (2) only. A cord as claimed in any one of the preceding claims, wherein each said wire element (1,2;14,2) is a brass-plated hard steel wire element. A reinforcing cord comprising three through seven twisted strands, each said strand consisting of a cord as claimed in any one of the preceding claims, each said cord comprising a number of wire elements (1,2;14,2) in the range 3 through 7. Apparatus for producing a reinforcing cord, the apparatus comprising frame means, a plurality of feeders (52;152,196) each for feeding a steel wire element (2), tensioning means (54;154) for tensioning a said wire element from at least one of said feeders, spiral forming means (56;156) comprising a plurality of spaced apart pins (68) arranged such that, in use, at least one wire element (2) from said tensioning means passes successively over and under adjacent pins (68), and strand means (80;180) for twisting the wire elements (1,2;14,2) to form a cord of the wire elements, said feeders, tensioning means and strand means being supported by said frame means, characterised in that said pins (68) extend from a surface (66) which is parallel to and rotatable around an axis (0) along which a respective wire element from said tensioning means (54;154) is fed during said use, said pins (68) being arranged relative to said axis such that the wire element (1;14) continues to move along said axis (0) on leaving the spiral forming means. Apparatus as claimed in claim 7, wherein said tensioning means comprises a dancing roller (54;154).

Inventors: HINOURA KOICHI, KOBAYASHI TAKANORI, KOBAYASHI YOSHIRO, NAGAMOTO AKIHIRO, SHIMIZU TOSHIAKI, HINOURA, KOICHI, KOBAYASHI, TAKANORI, KOBAYASHI, YOSHIRO, NAGAMOTO, AKIHIRO, SHIMIZU, TOSHIAKI
Applicants: TOKUSEN KOGYO COMPANY LIMITED
Application year: 1991
Main classification: D07B1
IPC codes: D07B3, D07B1, B60C9, D07B7