STEEL CORD WITH ADAPTED ELONGATION PROPERTIES

Abstract

In a tire the strength of steel cord and the resilience of rubber are a successful combination. However, in some specific areas of a tire, more elongation is expected from the steel cord, while still a sufficient degree of stiffness is expected. A steel cord is presented that has these properties. The steel cord comprises two or more steel elements that are twisted together. The steel elements comprise one or more steel filaments. In total the steel cord comprises N filaments, each with a cross sectional area A. When the steel elements are individualised out of the steel cord they show a helix pitch length of L.sub.o, while a single pitch has a centre line length of S. The inventive steel cord shows a P value of at least 50 newton, wherein P=NE (A/S).sup.2. Further methods are presented to produce this steel cord.

Claims

1-19. (canceled)

20. A steel cord for the reinforcement of a rubber product comprising two or more steel elements twisted together, said steel elements comprising one or more steel filaments, said steel cord comprising in total N steel filaments, each of said steel filaments having a cross sectional area A expressed in square millimeters said steel elements have, after individualization and under a tension of half a newton per filament in said steel element, a center line, said center line having a helix shape with a helix pitch length L.sub.o in millimeter, wherein the length of the center line of the steel element over one pitch is S millimeter, wherein the quantity P expressed newton: $P={\mathrm{NE}\left(\frac{A}{S}\right)}^2$ is larger than 50 newton and wherein E is the modulus of steel.

21. The steel cord according to claim 20, said filaments have an equivalent diameter d defined by A=d.sup.2/4, wherein S/d is smaller than 30.

22. The steel cord according to claim 20, wherein the ratio L.sub.o/S is smaller than 0.95.

23. The steel cord according to claim 20, wherein the filaments of said steel cord, when in closed condition, have a pitch length of L.sub.o in millimeter, wherein the ratio L.sub.o/S is larger than 0.98.

24. The steel cord according to claim 23, wherein the structural elongation .sub.0 defined as (L.sub.oL.sub.0)/L.sub.o is larger than 3.5 per cent and smaller than 10 per cent.

25. The steel cord according to claim 20, wherein the force at the structural elongation .sub.0 is larger than 50 N and smaller than 120 N.

26. The steel cord according to claim 20, wherein the number of steel elements is two, three or four and wherein the number of steel filaments within one steel element is one, two or three.

27. The steel cord according to claim 20, wherein the number of filaments N is from and including 2 to 8 included.

28. A method to produce a steel cord comprising the following steps: (a) unwinding a number of steel elements with diameter de from spools; (b) providing a mandrel wire of diameter D; (c) twisting said steel elements around said mandrel wire with a cord number of twists N.sub.e per unit length in a cord twist direction thereby forming an intermediate cord; (d) removing the mandrel wire from said intermediate cord by turning said mandrel cord out of said intermediate cord, resulting in the steel cord; (e) winding the steel cord on a take-up spool.

29. The method according to claim 28, wherein the intermediate cord is wound on an intermediate spool after step (c) and unwound from said intermediate spool for performing step (d).

30. The method according to claim 28, wherein the intermediate cord is directly led from step (c) to step (d).

31. The method according to claim 28, wherein the step of removing the mandrel wire from said intermediate cord by turning said mandrel wire out of said intermediate cord is performed by moving said intermediate cord linearly; turning said mandrel wire out of said intermediate cord through a flyer relatively rotating around said intermediate cord thereby leaving the steel elements as a steel cord; winding the mandrel wire on a driven mandrel spool; winding said steel cord on a driven take-up spool.

32. The method of claim 31, wherein said driven take-up spool is inside said flyer or wherein said driven mandrel spool is inside said flyer.

33. The method according to claim 28, wherein the steel elements are steel filaments and the ratio of D/d.sub.e is larger than or equal to 0.8 and smaller than or equal to 2.

34. The method according to claim 28, wherein the steel element is a plurality of steel filaments and the ratio of D/d.sub.e is larger than or equal to 0.5 and smaller than or equal to 1.2.

35. The method according to claim 28, wherein the steel filaments in said steel elements of said intermediate cord are twisted to a steel element twist number N.sub.e in the cord twist direction, said element twist number being larger or equal than said cord twist number Ne and wherein said cord twist number Ne is larger than 150 twists per meter.

36. The method according to claim 28, wherein the filaments in said elements of said steel cord are twisted to an element twist number N.sub.e in the cord direction, said element twist number being smaller than said cord twist number.

37. The method according to claim 28, wherein said mandrel wire is one out of the group comprising: a metal wire, a steel wire, a steel cord, an organic yarn, an organic cord, an organic filament.

38. The method according to claim 28, wherein the combination of the total number N of steel filaments, wherein each of said steel filaments has a cross sectional area A expressed in square millimeters said steel elements having, after individualisation and under tension of half a newton per filament in said steel element, a center line, said center line having a helix shape, wherein the length of the center line of the steel element over one pitch is S in millimeter, is such that the quantity P expressed in newton: $P={\mathrm{NE}\left(\frac{A}{S}\right)}^2$ is larger than 50 newton and wherein E is the modulus of steel.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0062] FIG. 1 shows the geometrical elements of a helix of importance for understanding the invention;

[0063] FIG. 2 shows a typical load-elongation diagram of a steel cord and the individualized steel element indicating the different thereof;

[0064] FIG. 3a and FIG. 3b illustrate a first method of manufacturing the inventive steel cord;

[0065] FIG. 4 shows a second method of manufacturing the inventive steel cord;

[0066] FIG. 5 shows the load-elongation diagrams of the samples in TABLE I.

[0067] Figures are provided with reference signs of which the unit and tens digit refers to similar items across figures and the hundred digit refers to the number of the figure.

MODE(S) FOR CARRYING OUT THE INVENTION

[0068] A first method of manufacturing is presented in FIGS. 3a and 3b. In a first step, illustrated in FIG. 3a, the intermediate cord 304 is produced by means of an external buncher that is a buncher that has the take-up spool outside the buncher and the pay-off spools on the stationary cradle 326. A number of steel elementsin this case fourare unwound from spools 330 mounted on the cradle 326. The mandrel wire 312 is fed from spool 314, from the flyer entrance, over a first flyer 320, to the flyer exit. The mandrel wire may e.g. be a metallic steel wire of diameter 0.30 to 0.40 mm. On the cradle 326 the steel elements are joined with the mandrel wire 312. After entering the second flyer of the buncher 322, the steel elements are twisted together with half the cord number of twists N.sub.c/2. At the exit of the second flyer 322, the steel elements obtain their final number of twists N.sub.c. Typical values for N.sub.c are between 50 and 250.

[0069] In order to remove the residual torsions from the steel elements twisted around the mandrel wire, the assembly is fed through a false twister 324 for plastically overtwisting the steel elements. The resulting intermediate cord 304 is free or practically free of residual torsions and is wound on an intermediate spool 302.

[0070] The steel elements comprise two, or three steel filaments that are twisted together with a lay length in a lay direction resulting in n.sub.e twists per meter. A typical value is from 25 to 150 twists per meter for the steel elements. The mandrel wire acts as a moving deformation pin, thorn, mandrel . . . around which the steel elements are plastically formed. It allows to give much higher degrees of plastic deformation of the steel elements than would be possible with conventional preforming systems. Moreover, by the use of a mandrel wire it becomes possible to deform also steel elements that are in the form of a strands. Strands cannot be deformed with e.g. preforming pins.

[0071] The final number of plastic twists N.sub.e that after the bunching steps are present in the steel elements is thus n.sub.e+N.sub.c. Typically this number will be smaller than 300 twists per meter. It will be nearing zero if n.sub.e=N.sub.c that is the steel elements is made with the same number of twists as that cord number twists but are twisted in the opposite direction.

[0072] In a further step of the method, as illustrated in FIG. 3b, the intermediate steel cord 304 is unwound from intermediate spool 302 on cradle 309 of an external bunching machine. At the entrance of the flyer 306, the mandrel wire 312 is separated from the intermediate cord 304 by turning it out of the cord. The mandrel wire unwinding speed (twists per second) is equal to the linear speed (meter per second) times N.sub.c (twists per meter) The mandrel wire is guided over the flyer 306 and wound on the driven take-up spool 314. The steel cord 308 that is freed from the mandrel wire is wound on the take-up spool 310 mounted on the cradle. Precautions have to be taken to align the turning out of the mandrel with the speed of exit.

[0073] In an alternative embodiment of the method depicted in FIG. 4, the use of an intermediate spool is made superfluous by feeding the intermediate cord 404 made on an external buncher for forming the intermediate cord directly to the internal buncher for taking up the steel cord 410. The intermediate cord 404 is formed by unwinding the mandrel wire 412 from spool 414, through first flyer 420, adding the four steel elements from spools 430 to the mandrel wire 412 thereby forming the intermediate cord after guiding it through second flyer 422 and false twister 424. The intermediate cord is fed into the internal buncher whereby the mandrel wire 412, is wound out of the intermediate cord 404 at the entrance of the flyer 406. The mandrel wire is subsequently wound onto mandrel take-up spool 414. The mandrel wire can be reused in a next production cycle.

[0074] Turning now to the product properties, FIG. 1 illustrates the basic geometrical elements of the main product claim. FIG. 1 shows an individualised steel element 100 that has a helix shape with the Z-axis as the helix axis. The helix has a helix pitch length indicated L.sub.0. The steel element has a cross sectional area indicated A. The steel element has a centreline (indicated dashed) that has a centreline length S

[0075] The sectional area of the steel filament can be easily established by measuring the diameter of the steel filaments and calculating the surface. The number of filaments N can be found by counting them.

[0076] The length of the centreline S can be determined by an axial scanning apparatus as described in WO 95/16816 or similar apparatus IM6000 obtainable from KEYENCE. The apparatus comprises two axially aligned chucks, 100 to 500 mm apart, for holding the individualised steel element at its ends during the test. A controlled tension is applied to the steel element of half a newton per filament in the steel element by means of weight. A linear scanning apparatus such as a KEYENCE LS 3034 laser scan system in combination with a KEYENCE LS 3100 processing unit is made to travel parallel to the axis of the steel element by means of an encoding high precision linear drive (accuracy is better than 10 m at a step size of 50 m). The measurement plane of the laser scan system is perpendicular to the Z-axis. The laser scan system can scan the outer edges of the steel element up to a precision of 0.5 m.

[0077] In a first scan at the equidistant discrete measuring positions z.sub.j, z apart, the lower and upper edges of the steel element are determined and the average of both is used as the position of the centreline along the axis perpendicular to the Z-axis, i.e. the X-axis. In this way the positions x(z.sub.j) are measured and stored in a computer. The index j is the sequential number of the sampled.

[0078] Then the chucks are turned 90 and the scan is repeated. Now, the values y(z.sub.j) along the Y-axis, perpendicular to X and Z-axis are measured and stored. In this way the triplets (x(z.sub.j), y(z.sub.j), z.sub.j) are obtained that determine the shape of the centreline of the steel element. As this shape is substantially a helix the curves (z.sub.j, x(z.sub.j)) and (z.sub.j, y(z.sub.j)) are similar to a cosine and sine as a function of z.sub.j. The start of the first turn and the end of the last turn can be determined and this is the axial length l over which n helix pitches are counted. This axial length covers m measuring points

[0079] Now the total length s of the centreline over the axial length l can be calculated by adding the m1 measured sections:

[00003]$s={.Math.}_{l=1}^{m-1}\sqrt{{\left(x_{l+1}-x_l\right)}^2+{\left(y_{l+1}-y_l\right)}^2+{\left(z_{l+1}-z_l\right)}^2}$

[0080] The length of the center line of the steel element over one pitch S is thus equal to s/n. In the same manner the helix pitch length L.sub.o is equal to l/n. As the number n of helix turns measured is readily larger than 50 or even 100, the numbers S and L.sub.o are averages over a large number of helix turns.

[0081] The relation between the geometrical parameters L.sub.o, L.sub.c, S and the load-elongation diagram is illustrated in FIG. 2. There the load-elongation curve of a steel cord according the invention is depicted as 202. Parallel to the elongation axis 206, the axial length of one helix pitch L is shown on the axis 208 as it varies with the load F applied (ordinate axis, 210). When the steel cord is at the very low measuring tension, the axial length of a single helix turn is L, and the elongation e is zero. relates to L through:

[00004]$=\left(L-L_o\right)/L_o$

[0082] When a tangent line (shown dashed) is drawn to the straight part of the curve, this can be extended towards the elongation axis 206. The crossing corresponds to the structural elongation .sub.0 as at this point the steel elements are closed and reach the corresponding helix pitch L.sub.c. That this point indeed corresponds to the closing of the steel elements can be demonstrated by imagining that the steel elements have an increasing modulus: all the corresponding tangent curves will go through the point (0, .sub.0) as the slope of the tangent line rises to vertical. When the steel cord is further stretched over 60, the ratio (L/S) remains constant while both L and S further increase due the elongation of the steel.

[0083] For the purpose of this application, the ratio (L.sub.c/S) is calculated as:

[00005]$\left(L_c/S\right)=\sqrt{1-{\left(\frac{d}{S\sin \left(/N\right)}\right)}^2}$

wherein d is the equivalent diameter of the steel filament.

[0084] When now considering an individualized steel element, a curve akin to 204 is obtained. But here the crossing with the L axis corresponds to a fully stretched helix that is a helix with length S. The elongation is then (SL.sub.o)/L.sub.0.

[0085] In a series of experiments, samples were prepared according the method depicted in FIGS. 3a and 3b. The results on these samples are summarised in TABLE I. [0086] (a) The first column is a reference number to the curves in FIG. 5 showing the load-elongation curve of the mentioned constructions; [0087] (b) The Construction column is a representation of the intermediate cord, wherein the first number indicates the diameter of the mandrel wire, followed by the arrangement of the steel elements. If for the intermediate cord one departed from a strand this is indicated by the parenthesis ( . . . ). E.g. 0.4+2(20.225) indicates 2 steel elements each comprising two filaments of diameter 0.225 that have been twisted around a mandrel wire of diameter 0.4 mm. [0088] (c) The N.sub.e column indicates the number of twists per meter (t/m) the steel filaments have obtained in the intermediate cord. [0089] (d) The N.sub.c column indicates the number of twists per meter (t/m) the steel elements have obtained in the intermediate cord. [0090] (e) The ratio D/d.sub.e is the ratio between the diameter of the mandrel wire and the steel element diameter;

[0091] Note: the lay direction of filaments in the steel element and the lay direction of the steel element in the steel cord where identical and in the S direction.

[0092] The different geometrical and mechanical properties following have all been obtained on the final steel cord, that is the steel cord wherefrom the mandrel wire has been removed: [0093] (a) N is simply the number of steel filaments in the steel cord; [0094] (b) A1000 is the cross sectional area of a single steel filament expressed in mm.sup.2 times 1000. [0095] (c) S is the length of the centre line in one helix pitch according the described measurement procedure; [0096] (d) L.sub.o is the axial length of one helix pitch according the described measurement procedure; [0097] (e) P is the quantity calculated according the definition of the claim; [0098] (f) S/d is the ratio of S divided by the equivalent diameter of the steel filament; [0099] (g) L.sub.o/S and L.sub.c/S are the ratios of the indicated quantities; [0100] (h) .sub.0 is the structural elongation as determined by the procedure of FIG. 2. [0101] (i) F(.sub.0) is the force at the structural elongation as derived from the load elongation diagram.

[0102] FIG. 5 shows the different load elongation of the samples made. In the use of the steel cord, the samples 522, 523, 503 show the most preferred behaviour. Less preferred, but still very useful load elongation curves are exhibited by 507, 521. The other samples 524, 514 and 509 are not preferred.

TABLE-US-00001 TABLE I N.sub.e N.sub.c A 1000 S L.sub.o P .sub.o F (.sub.o) Ref. Construction (t/m) (t/m) D/d.sub.e N (in mm.sup.2) (in mm) (in mm) (in N) S/d L.sub.o/S L.sub.c/S (in %) (in N) 522 0.4 + 2 (2 0.225) 300 200 0.89 4 39.761 5.731 5.382 121 25.5 0.939 0.985 5.79 78.86 507 0.4 + 4 (2 0.225) 300 200 0.89 8 39.761 5.790 5.259 237 25.7 0.908 0.948 4.77 101.67 523 0.4 + 2 0.225 200 200 1.78 2 39.761 5.961 5.589 56 26.5 0.938 0.993 5.19 52.47 503 0.4 + 4 0.225 200 200 1.78 4 39.761 5.977 5.669 111 26.6 0.949 0.986 3.96 74.04 521 0.3 + 4 0.225 200 200 1.33 4 39.761 5.528 5.302 130 24.6 0.959 0.984 2.00 52.29 524 0.3 + 2 0.15 200 200 2.00 2 17.671 5.926 5.793 11 39.5 0.978 0.997 1.14 12.02 514 0.35 + 3 0.15 133 133 2.33 3 17.671 9.284 9.202 7 61.9 0.991 0.998 0.37 7.21 509 0.35 + 3 (2 0.15) 200 133 1.17 6 17.671 8.940 8.812 15 59.6 0.986 0.994 0.77 17.03