BELT COMPRISING STEEL CORDS ADAPTED FOR WEAR DETECTION

Abstract

A belt containing steel cords, the steel cords containing strands made of steel filaments wherein the largest diameter filaments are at least intermittently positioned at the radially outer side of the steel cord. Such a configuration can be obtained by using steel cord constructions wherein the thickest filaments are positioned outside of the steel cord which is contrary to the current practice. In a further embodiment the largest diameter filaments fill up some or all of the valleys of the strands at their radially outer side. These monofilaments thus have the same lay length and direction as the strands in the steel cord. The advantage of putting the largest filaments at the outside is that they will break first and thus will be readily detectable by electrical, magnetic or visual means. In this way a belt is provided that can be monitored easier and more conveniently than prior art belts.

Claims

1. A belt comprising steel cords held in parallel arrangement to one another by an elastomer jacket, said steel cords comprising strands twisted together with a cord lay direction and cord lay length, wherein said strands comprise steel filaments twisted together, each of said steel filaments having a filament diameter, wherein in said steel cord a group of largest diameter filaments have a filament diameter that is strictly larger than the remainder of the filaments, and wherein each of said largest diameter filaments is at least intermittently present at the radial outer side of said steel cord.

2. The belt according to claim 1 wherein the group of largest diameter filaments are made of steel having a relative magnetic permeability larger than 50 and wherein at least said largest diameter filaments have a remanent magnetisation.

3. The belt according to claim 1 wherein the largest diameter filaments differ in diameter by at least 1% and at most 40% with relative to the next smaller diameter steel filaments.

4. The belt according to claim 1 wherein the cross sectional area of each one of the filaments out of the group of largest diameter steel filaments is between 2% and 10% of the total cross sectional area of said steel cord.

5. The belt according to claim 1 wherein said steel cords further comprise monofilaments made of metal, said monofilaments belonging to the group of largest diameter filaments, said monofilaments being twisted with said cord lay length and direction, said monofilaments filling some or all of the valleys between adjacent strands on the radial outer side of said steel cord, wherein the diameter of said monofilaments is larger than the gap between said adjacent strands.

6. The belt according to claim 5 wherein said monofilaments remain within the circumscribed circle to the strands of said steel cords.

7. The belt according to claim 5 wherein said group of largest diameter filaments of said steel cord consists of said monofilaments.

8. The belt according to claim 5 wherein said monofilaments have a monofilament tensile strength, said monofilament tensile strength being lower than the tensile strength of any other of said steel filaments in said steel cord.

9. The belt according to claim 5 wherein said monofilaments of said steel cord have a total monofilament breaking load, said total monofilament breaking load being lower than 20% of the breaking load of said steel cord.

10. The belt according to claim 5 wherein at least one of said monofilaments of at least one of said steel cords is coated with an electrically insulating layer.

11. The belt according to claim 5 wherein at least one of said monofilaments of at least one of said steel cords is locally weakened at intervals.

12. The belt according to claim 1 wherein said steel cords further comprise a core, said strands being twisted around said core.

13. The belt according to claim 12 wherein said core of said steel cords comprises steel filaments forming a core strand.

14. The belt according to claim 13 wherein said core strand has a core strand diameter, said strands have a strand diameter, wherein said core strand diameter is smaller than said strand diameter.

15. The belt according to claim 12 wherein the number of strands in said steel cords is three, four or five.

16. The belt according to claim 1 wherein the closest distance between the surface of said belt and any one of said steel cords is larger than half the diameter of said largest diameter filament and smaller than ten times the diameter of said largest diameter filament.

Description

BRIEF DESCRIPTION OF FIGURES IN THE DRAWINGS

[0075] FIG. 1a: is a cross section of a belt embodying the inventive principles;

[0076] FIG. 1b: is an enlargement of the cross section of the steel cord used in the belt of FIG. 1a.

[0077] FIG. 2: is a steel cord with monofilaments for use in an inventive belt

[0078] FIG. 3: is another steel cord with monofilaments for in an inventive belt

[0079] FIG. 4: shows a monofilament with local flats.

MODE(S) FOR CARRYING OUT THE INVENTION

[0080] In FIG. 1a the inventive belt with its cross section is depicted. The belt has a length dimension indicated L, a width indicated W and a thickness indicated t. The steel cords 101 have a diameter indicated D. They are encased in a polymer 104 that in this case is an ester polyol based thermoplastic polyurethane as is known in the art. The closest distance between the surface of the belt and any one of the steel cords is indicated with . For exampleto put things in proportionthe length of the belt can be hundreds of meter while the width of the belt is 20 mm and the thickness 2.83 mm. The diameter D of the steel cord is 1.77 mm. The shortest or closest distance between the surface of the belt and any one of the steel cords is 0.53 mm.

[0081] The structure of the steel cord 101 is shown in an enlarged view in FIG. 1b. The steel cord 101 comprises five strands 102 that are wound around the core 108 with a lay length of 17 mm in S direction. The strands 102 are composed of a centre steel filament 110 around which five outer steel filaments 106 are twisted with a lay of 12 mm in Z direction. The diameter of the outer filaments is 0.23 mm. The diameter of the centre filament 110 is 0.18 mm. The structure of the core is on itself immaterial to the invention as long as it does not contain filaments that are larger than the outer filaments of the strand. In this case a core consisting of steel filaments has been chosen made of a central filament of diameter 0.15 mm around which five filaments of diameter 0.18 mm are wound with a lay of 10 mm in Z direction. The gap between strands is 0.028 mm. It is advantageous for the torsion behaviour of the belt to mirror the lay directions of the steel cords between neighbouring steel cords.

[0082] The group of largest diameter filaments is thus formed by the outer steel filaments 106 of size 0.23 mm that is larger than the remainder of the filaments of sizes 0.18 mm and 0.15 mm. These largest diameter filaments describe a helix around the centre filament of 0.18 mm. And the strand itself is twisted around the core in a helical shape. Hence, each one of the largest diameter filaments willover a certain length of the steel cordcome to the surface of the steel cord. In other words: the largest diameter filaments are intermittently present at the radial outer side of the steel cord.

[0083] The largest diameter filament of 0.23 mm is in this example 27.8% larger relative to the next smaller diameter filaments of size 0.18 mm. The total cross sectional area of the cord is 1.31 mm.sup.2 while the cross sectional area of a single largest diameter filament is 0.0415 mm.sup.2 i.e. 3.2% of the total cross sectional area of the cord. Note that the closest distance between the surface of the belt and any one of the steel cords is 2.3 times the diameter of the largest diameter filament.

[0084] The largest diameter filaments are made of carbon steel with a carbon content of about 0.725 wt % carbon. The steel has a relative magnetic permeability of about 100. As the carbon steel is ferromagnetic and can easily be magnetised either before use or even during use. During use the belt can be led through a constant magnetic field for example generated by a DC electromagnet. The DC magnet does not have to be permanently on: now and thenfor example prior to a belt inspectionrestoring the loss in remanent magnetism is sufficient to allow any fractured largest diameter filament to be detected by a magnetic detector.

[0085] FIG. 2 describes an alternative embodiment for the steel cord in the belt of FIG. 1a. The steel cord 200 comprises five strands 202 that are twisted around a core 208 with a lay length of 16.3 mm in S direction. The strands 202 are made of five outer steel filaments 206 with diameter 0.23 mm that are twisted around a centre steel filament 210 of 0.17 mm diameter at lay 12 mm in Z direction. The core 208 is made of three steel filaments 209 of diameter 0.22 twisted at lay 10 Z. Particular about the construction is that it comprises five monofilaments 204, 204, 204, 204, and 204 that fill up all the valleys between the strands at the radial outer side of the steel cords and have the same lay and direction as the strands in the steel cord. The monofilaments have a diameter of 0.25 mm and as they are the largest filaments, the monofilaments are the group of largest diameter filaments.

[0086] The configuration of the cord can be conveniently expressed in a cord formula:

[(30.22).sub.10 Z+5(0.17+50.23).sub.12 Z|50.25].sub.16.3 S

[0087] In the mirror image of the steel cord every z is replaced with s and vice versa.

[0088] The formula must be read as follows: [0089] The decimal numbers indicate the diameter of the filament, integers indicate the number of filaments or strands; [0090] The brackets contain filaments and/or strands that are laid together in one step; [0091] The sub-indexes indicate the lay length in mm and direction; [0092] A plus sign indicates that the items on both sides of the + are laid together have a different lay-length and/or direction; [0093] A stroke indicates that the items on both sides of the | are laid together with the same lay-length and/or direction.

[0094] The lay direction of the strand z is opposite to the lay direction of the cord S. The monofilaments 204 to 204 all remain within the circumscribed circle 212 that is tangent to the strands 202 resulting in steel cord with a smooth outer surface. The diameter of the monofilament 204 is 0.25 mm and this is larger than the next smaller diameter 0.23 mm of the strand filament 206. Indeed the diameter of the monofilament is 8.7% larger relative to the next smaller diameter steel filament. The diameter of the monofilament is also larger than the gap between strands that in this case is 0.008 mm.

[0095] The monofilaments are made of AISI 304 stainless steel (SS) that has a fivefold higher electrical resistance than carbon steel. They also have a markedly lower tensile strength of 1750 MPa, lower than that of the other carbon steel filaments. They are therefore a good wear indicator for the belt. However, a fracture cannot easily be detected by magnetic means as the magnetic permeability of AISI 304 austenitic stainless steel remains below 10 even after cold working through wire drawing.

[0096] A brief calculation will show that the rupture of one single monofilament in the whole length of the steel cord in the elevator belt will only lead to a marginal change in electrical resistance over that steel cord. In order to improve the detection by electrical means, it is more preferred to coat the stainless steel with an electrically insulating plastic. As the monofilament then becomes electrically isolated from the remainder of the steel cord, their fracture can easily be detected by measuring the resistance of the individual monofilaments even when taken in parallel.

[0097] The comparative Table 1 below shows the features of the cord when using 0.725% carbon steel compared to a 0.725 wt % carbon prior-art cord (Prior art) without monofilaments.

TABLE-US-00001 TABLE 1 Property 0.725 wt % C Prior art Tensile strength (MPa) 0.22 mm 2960 2960 0.17 mm 2960 2960 0.23 mm 2880 2880 (SS) 0.25 mm 1750 Diameter (mm) 1.73 1.73 Metallic cross section (mm.sup.2) 1.51 1.27 Metallic fill factor (%) 64 54 Mean Breaking Load (N) 3558 3340

[0098] Each one of the monofilaments accounts for 3.25% of the total cross sectional area of the cord.

[0099] The contribution of the monofilaments to the breaking load can easily be assessed by the following procedure: [0100] First the breaking load of the steel cord is determined. The result is A newton; [0101] From the steel cord, the monofilaments are removed. This can easily be done, as the monofilaments are at the outer side of the steel cord; [0102] The breaking load of the remaining cord is measured: the result is B newton.

[0103] The contribution of the monofilaments to the total breaking load is then 100(AB)/A in percent. In the above case of 0.725 wt % C the contribution of the monofilaments to the breaking load is 8.5%. Hence, if all monofilaments would break at the same spot during use, there will still remain 91.5% 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.

[0104] According a second embodiment a cord 300 of the following make is suggested of which the cross section is shown in FIG. 3:

[(30.15).sub.9 z+4(0.19+50.265).sub.14 z|40.28].sub.16.3 S

[0105] The mirror image has all lay directions reversed. The gap between strands is 0.009 mm.

[0106] In this case the monofilaments 304, 304, 304, 304 of diameter 0.28 mm have been indented to locally reduce the tensile strength in order to obtain controlled weak 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 304 shows cross sections 324 that are round in between the flats 320. At the flatsthat are less than two times the diameter of the wire longthe cross section 326 is flattened.

[0107] The flats 320 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.

[0108] 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 which is further advantage.