Steel cord for elastomer reinforcement

Abstract

A steel cord for the reinforcement of elastomer products such as elevator belts, conveyor belts, synchronous or timing belts or hoses or tyres is presented. The steel cord comprises strands and monofilaments made of steel filaments. The strands themselves are also made of steel filaments twisted together. The strands form the outer layer of the steel cord. The monofilaments are twisted into the cord with the same lay length and direction as the strands and are positioned in the valleys between the strands on the radial outer side of the steel cord. The steel cord has the advantage that it has a better fill factor and a rounder aspect. Furthermore the monofilaments may act as an early wear indicator of the elastomer product.

Claims

1. A steel cord comprising strands and monofilaments made of steel, wherein said strands comprise strand filaments made of steel twisted together with a strand lay length and direction, wherein said strands are twisted together with a cord lay and direction, said strands forming the outer layer of said steel cord, wherein said monofilaments are twisted with the cord lay and direction and fill valleys between adjacent strands on the radial outer side of said outer layer of said steel cord, and wherein said monofilaments have a monofilament tensile strength, said monofilament strength being lower than the tensile strength of the strand filaments closest to said monofilaments.

2. The steel cord according to claim 1 wherein said monofilaments have a diameter, said diameter of said monofilaments is larger than a gap between said adjacent strands.

3. The steel cord according to claim 1 wherein said monofilaments remain within a circumscribed circle to said strands of said steel cord.

4. The steel cord according to claim 1 wherein said monofilaments have a diameter, said monofilament diameter being smaller than the diameter of said strands closest to said steel monofilaments.

5. The steel cord according to claim 4 wherein said monofilaments have a diameter, said monofilament diameter being larger than any of the diameters of the strand filaments.

6. The steel cord according to claim 1 wherein said monofilaments have a total monofilament breaking load, said total monofilament breaking load being lower than 20% of the breaking load of said steel cord.

7. The steel cord according to claim 1 wherein the cross sectional area of one of said monofilaments is between 2% and 5% of the total metallic cross sectional area of said steel cord.

8. The steel cord according to claim 1 wherein at least one of said monofilaments is coated with an electrically insulating layer.

9. The steel cord according to claim 1 wherein at least one of the monofilaments is locally weakened at intervals.

10. The steel cord according to claim 1 wherein said steel cord further comprises a core, said strands being twisted around said core.

11. The steel cord according to claim 10 wherein said core comprises synthetic or natural organic fibres.

12. The steel cord according to claim 10 wherein said core comprises steel filaments, forming a core strand.

13. The steel cord according to claim 10 wherein said core has a core diameter, said strands have a strand diameter, wherein said core diameter is smaller than said strand diameter.

14. The steel cord according to claim 13 wherein the number of outer strands is three, four or five.

15. The steel cord according to claim 1 wherein at least said monofilaments are larger than 0.25 mm.

16. An elastomer product comprising steel cords according to claim 1.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

(1) FIG. 1 is a cross section of a first preferred embodiment of the inventive steel cord.

(2) FIG. 2 is a cross section of a second preferred embodiment of the inventive steel cord.

(3) FIG. 3 describes a possible method of manufacture of the inventive steel cord;

(4) FIG. 4 shows a monofilament having regular pinches acting as a local weakening of the filament viewed on top.

(5) In the FIGS. 1, 2 and 4, reference numbers having equal unit and tens numbers indicate corresponding items across figures. FIG. 3 follows its own numbering.

MODE(S) FOR CARRYING OUT THE INVENTION

(6) According a first preferred embodiment a cord of the following construction is presented:
[(3×0.22).sub.10 z+5×(0.17+5×0.23).sub.12 z|5×0.25].sub.16.3 S

(7) In the mirror image of the steel cord every ‘z’ is replaced with ‘s’ and vice versa.

(8) The formula must be read as follows: The decimal numbers indicate the diameter of the filament, integers indicate the number of filaments or strands; The brackets contain filaments and/or strands that are laid together in one step; The sub-indexes indicate the lay length in mm and direction; A plus sign indicates that the items on both sides of the ‘+’ are laid together have a different lay-length and/or direction; A stroke indicates that the items on both sides of the ‘|’ are laid together with the same lay-length and/or direction.

(9) A cross section of this cord 100 is represented in FIG. 1. Outer strands 102 are made of a central steel filament 110 of size 0.17 mm around which five steel filaments 106 of size 0.23 mm are twisted at lay length 12 mm in ‘z’ direction. The core 108 is in this case a steel filament core wherein three filaments 109 of size 0.22 mm are twisted around each other with lay length 10 mm in the ‘z’ direction. Around the core 108, five outer strands 102 are twisted together with five monofilaments 104, 104′, 104″, 104′″, 104″″ at lay length 16.3 in ‘S’ direction wherein the strands alternate with the monofilaments. The strands 102 form the outer layer of the steel cord 100. The monofilaments 104 to 104″″ are nested in the valleys between the strands at the radial outer side of the outer layer.

(10) The lay direction of the strand ‘z’ is opposite to the lay direction of the cord ‘S’. The monofilaments 104 to 104″″ all remain within the circumscribed circle 112 that is tangent to the strands 102. The monofilament 104 is closest to the outer filament of the strands 106. The diameter of the monofilament 104 is 0.25 mm and this is larger than the diameter 0.23 mm of the strand filament 106 closest to the monofilament 104. Indeed the diameter of the monofilament is 8.7% larger that of the closest outer filament. Even more: the monofilaments are the largest filaments in the steel cord.

(11) The comparative Table 1 below shows the features of the cord when using 0.725% carbon steel and 0.825% carbon steel compared to a 0.725 wt % carbon prior-art cord (‘Prior art’) without monofilaments.

(12) TABLE-US-00001 TABLE 1 Property 0.725 wt % C 0.825 wt % C Prior art Tensile strength (MPa) 0.22 mm 2960 3150 2960 0.17 mm 2960 3150 2960 0.23 mm 2880 3060 2880 (*) 0.25 mm 2750 2900 — Diameter (mm) 1.73 1.73 1.73 Metallic cross section (mm.sup.2) 1.51 1.51 1.27 Metallic fill factor (%) 64 64 54 Mean Breaking Load (N) 3970 4200 3340

(13) The monofilament (*) of 0.25 mm shows a lower tensile strength than the closest filaments of the strand 0.23 mm for both 0.725 wt % C and 0.825 wt % C. However, the difference between the tensile strength is less than 200 MPa (130 MPa and 160 MPa respectively) so they are still very well comparable to one another. Each one of the monofilaments accounts for 3.25% of the total cross sectional area of the cord.

(14) The contribution of the monofilaments to the breaking load can easily be assessed by the following procedure: First the breaking load of the inventive cord is determined. The result is ‘A’ newton; From the inventive cord, the monofilaments are removed. This can easily be done, as the monofilaments are at the outer side of the steel cord; The breaking load of the remaining cord is measured: the result is ‘13’ newton.

(15) The contribution of the monofilaments to the total breaking load is then 100×(A−B)/A in percent. In the above case of 0.725 wt % C the contribution of the monofilaments to the breaking load is 16%. Hence, if all monofilaments would break at the same spot during use, there will still remain 84% of the original breaking load. It is to be noted that whatever the breaking load of the monofilaments is, they will always contribute to the breaking load of the steel cord.

(16) According a second embodiment a cord of the following make is suggested of which the cross section is shown in FIG. 2:
[(3×0.15).sub.9 z+4×(0.19+5×0.265).sub.14 z|4×0.28].sub.16.3 S

(17) The mirror image has all lay directions reversed.

(18) In this case the monofilaments of diameter 0.28 mm have been indented to locally reduce the tensile strength in order to obtain controlled fraction spots. To this end the monofilaments are lead in between two gears that run synchronized to one another. The phase between the gears is so adjusted that the teeth face one another (there is no gear meshing). The gap between the gear teeth is adjusted between 0.70 to 0.95 the diameter of the monofilament. When now the wire is led between the two gears two flats form diametrically to one another. This is depicted in FIG. 4 wherein the wire 204 shows cross sections 224 that are round in between the flats 220. At the flats—that are less than two times the diameter of the wire long—the cross section 226 is flattened. An apparatus to make such flats on a wire is illustrated in WO 2015/054820 wherein the procedure to make the flats is described in [33], [46] and FIGS. 5a and 5b. The disclosure is herewith specifically and/or entirely incorporated into this application.

(19) The flats 220 result in a 10% lower breaking load of the monofilaments resulting in an overall decrease of the breaking load of the steel cord of 2% which is low. The flats result in controlled fracture places. If all monofilaments would be broken at the same spot, this would only result in a decreased of 14.3% in breaking load i.e. still 85.7% of the original breaking load is maintained.

(20) As the monofilament is locally flattened the flats will maintain a gap between the monofilament and the outer strands. Such gaps are expected to improve the elastomer penetration into the core of the steel cord.

(21) On this second embodiment adhesion tests with thermoplastic polyurethane were performed both with and without an adhesive. As an adhesive an organo functional silane was used as known from WO 2004/076327. To this end steel cords were embedded into small injection molded cylinders of length 25 mm and diameter 12.5 mm and pulled out along the axis after cooling for 24 hours.

(22) TABLE-US-00002 TABLE 2 Pull-out force (N) Second embodiment Prior art Without adhesive 1200 1250 With adhesive 2500 2300

(23) The prior art cord is the cord of the second embodiment without monofilaments.

(24) Much to their surprise the inventors did not find a significant difference between the inventive cord and prior art cords when no adhesive is used. As in that case the major part of the adhesion is due to mechanical anchorage, it appears that the mechanical anchorage is not affected by the relatively smoother outer surface. A further advantage is that as the outer metal surface of the inventive cord increases by the introduction of the monofilaments, the adhesion after application with an adhesive also greatly improves.

(25) A third not shown embodiment has the formula:
[(3×0.15).sub.9z+4×(0.244+6×0.238).sub.14z|4×0.28].sub.16.3 S

(26) A fourth not shown embodiment can be build a follows:
[(0.21+6×0.20).sub.9z+6×(0.19+6×0.18).sub.14z|6×0.21].sub.16.3 S

(27) The latter example is somewhat less preferred as the diameter of the monofilament is not substantially different from the other diameters.

(28) FIG. 3 illustrates how the cord can be made. In a bunching process 300 known per se, the core 308, strands 302 and monofilaments 304 are assembled at cabling die 318. The strands are drawn from a rotating pay-off stand 320 whereby their lay length is shortened during pay off. Because the lay direction of the cord is opposite to that of the strand, the lay length of the strand will increase during travel in the bow 310. The rotating pay-off stand exactly compensates for this. The monofilaments 304 can be statically paid off as they do not have a lay length. Device 322, described in WO 2015/05482, induces flats into the wire. While in this case only one monofilament is deformed, it is equally well possible to deform the other monofilaments. The flat sections introduce local preferred fracturing spots where the monofilaments are more likely to break. Two guiding pulleys 316 and 316′ situated at either end of the bow 310 guide the steel cord 301 to the spool 314. On the path of the steel cord 301 a torsion elimination device 312 is introduced.