Description Technical Field

[0001 ] The present invention relates to a wire rope, in particular to a wire rope with double extruded layers and the method of manufacturing thereof.

Background Art

[0002] Thermoplastic or elastomer coated wire rope, such as disclosed in US 3,824,777, has been demonstrated to have properties such as fatigue life, wear resistance and corrosion resistance superior to those of equal size bare wire rope. Such improved properties are derived from the separation of the central core from the outer strands and the outer strands from each other by the thermoplastic or elastomer coatings. Suitable coating materials for wire rope include polypropylene, polyurethane, polyethylene, nylon, tetrafluoroethylene or polyvinylchloride. Such a coating reduces or eliminates the central core to outer strand and strand to strand contact and abrasion when the rope is in service. Further, the coating traps any desired lubricant, such as disclosed in US 3,979,896 within the strands and resists the ingress of abrasive or corrosive elements into the rope.

[0003] Regarding the manufacture of such wire rope, the wire strands or wires of the wire rope can be individually coated with thermoplastic or elastomer. As an alternative solution, the wire strands or wires in the rope can also be jointly covered by a sheath. In order to realize specific objects, a second sheath may also be applied on top of the first sheath as shown in Fig. 1 . As disclosed in US 5,573,852, when applying the covering material, i.e. synthetic resin, the strands or wires often lie in such a way that the synthetic resin can also penetrate between the strands or wires whereupon the rope is compressed in order to attain the desired small diameter. Therefore the covering does not adopt the exact desired surface outlines. In order to avoid this risk, a second outer covering having the same composition of the first inner sheath can be applied. It is further disclosed that a lubricant may be included between the two coverings so that make it possible, during tensing of the rope, the rope with the inner covering can move relative to the outer covering.

[0004] Two different functions are desired to be reached in wire rope, such as fatigue resistance and wear resistance, and this is difficult to be realized by one single polymer coating. Hence one tries to fulfill these two different functions by applying two different polymer coatings. However, it can cause problems when applying two sheaths having different materials on wire rope since the two types of materials in the two sheaths of wire rope are not endurable with each other, i.e. lack of affinity.

Disclosure of Invention

[0005] It is an object of the present invention to provide a wire rope with at least two coatings having affinity with each other and the method to manufacture such a wire rope.

[0006] It is a further object of the present invention to provide a wire rope having combined good fatigue and wear resistance.

[0007] According to a first aspect of the invention, there is provided a wire rope comprising a central core, a plurality of outer elongated elements surrounding said central core, a polyamide or a polyether block amide (PEBA) coating applied on said central core and outer elongated elements, and a thermoplastic polyurethane (TPU) coating covering on the polyamide or the PEBA coating.

[0008] The elongated elements herewith may be strips, straps or single wires, but preferably are wire strands. Said polyamide or PEBA coating may be applied individually on each of said central core and outer elongated elements. Alternatively, said polyamide or PEBA coating extends from substantially the outer surface of the outer elongated elements radially down to and into the central core and coats the wire rope as a whole.

[0009] The wire strand of the wire rope may be formed by twisting metallic wires and said metallic wires have interstitial spaces between each other. The interstitial spaces are preferably all filled with the polyamide or the PEBA. Preferably, the wire strands of wire rope comprise plain steel wires and/or galvanized steel wires. As an example, the steel wires of the present inventions may have the following steel composition: a carbon content ranging between 0.2 wt % and 0.8 wt %, a manganese content from 0.3 wt % to 0.80 wt%, a silicon content ranging from 0.10 wt % to 0.50 wt %, a maximum sulfur content of 0.05 wt %, a maximum phosphorus content of 0.05 wt %, the remainder being iron and possible traces of copper, chromium, nickel, vanadium, molybdenum or boron. The corrosion resistant coating may be any coatings having corrosion resistant function. Preferably, the corrosion resistant coating is a galvanized layer. More preferably, the coating is a hot dipped zinc and/or zinc alloy.

[0010] In order to have good fatigue resistance, as a normal practice, a lubricating film may be put around said individual metallic wires. A standard lubricated wire can be used, which is normally lubricated with petroleum or asphalt base lubricants. It should be noted, however, that rope preheating places some constraints on the lubricant which must not oxidize or otherwise significantly deteriorate at the required preheat temperature.

[001 1 ] According to the present invention, the first coating around wire stands is polyamide or PEBA. It is found that these materials can be applied on the steel wire strands, by such as extrusion, to give good fatigue resistance.

[0012] The polyamide may be carboxylic acid polyamide, and can be commercially available PA-6, PA-1 1 or PA-12.

[0013] Thermoplastic polyether block amide (PEBA) elastomers are plasticizer- free thermoplastic copolyester elastomers. The soft segment is the polyether and the hard segment is the polyamide. They are easy to process by injection molding and profile or extrusion. PEBA is a high performance elastomer with high strength and very good dynamic properties (flexibility, impact resistance and fatigue resistance). PEBA is commercially available, such as the product under trade name of PEBAX®. It is a block copolymer obtained by polycondensation of a carboxylic acid polyamide (PA6, PA1 1 , PA12) with an alcohol termination polyether (e.g. Polytetramethylene glycol PTMG). [0014] According to the present invention, thermoplastic polyurethane (TPU) coating is applied on top of polyamide or PEBA. TPU due to its superior properties is a widely used thermoplastic. TPU, in particular polyester- based TPU, is used in present invention due to its excellent abrasion or wear resistance. The TPU coating herein refer to a layer of TPU spreading over the polyamide or PEBA surface. The TPU layer or TPU coating has a conformable shape of the wire rope and rather uniform thickness. In particular, the wire rope can remain its circular cross-section after it is coated with TPU. The thickness of the TPU coating may be between 1 to 50 mm, preferably between 3 to 30 mm depending on the application. When the outer elongated elements are individually coated with polyamide or PEBA, the applied TPU may penetrate in-between the gaps of adjacent outer elongated elements. Herein, the thickness of the TPU coating refers to the thickness at locations where the distance between the periphery of the coated wire rope and one of the outer elongated elements is the shortest (as indicated by L1 ,L2 in Fig. 1 and L3,L4 in Fig. 2). Preferably, the deviation of the thickness within a TPU layer or coating (e.g. L1 -L2 in Fig. 1 , or L3-L4 in Fig. 2) is less than 20 mm, more preferably less than 10 mm and most preferably less than 5 mm.

[0015] Importantly, it is realized by the present invention that polyamide or PEBA and TPU have good affinity and therefore excellent fatigue and wear resistance can be obtained at the same time when they are applied on the same wire rope.

[0016] The wire rope can have varied configuration. For instance, the central core and the outer elongated elements are in the form of seven metallic wires having a central metallic wire and six outer metallic wires. Moreover, the wire rope may further comprise a plurality of intermediate elongated elements surrounding the central core, and the plurality of outer strands surround the intermediate elongated elements. The wire rope can have an oval cross-section. Preferably, the wire rope has a circular cross-section. The diameter of the wire rope can vary between 10 to 400 mm, e.g. between 20 to 200 mm or between 50 to 100 mm, depending on the applications. [0017] According to a second aspect of the invention, there is provided a method of manufacturing a wire rope comprising the steps of a) providing a central core, b) winding a plurality of outer elongated elements around said central core so as to said outer elongated elements intimately contacting each other and said central core to thereby form a plurality of interstitial spaces among said outer elongated elements and between said outer elongated elements and said central core, c) extruding a polyamide or a polyether block amide (PEBA) coating extending from substantially the outer diameter of the outer elongated elements radially down to and into said central core so as to substantially fill in said plurality of interstitial spaces, and d) extruding a thermoplastic polyurethane (TPU) coating covering on the polyamide or the PEBA coating.

[0018] The injection of the molten polymer material into the wire rope is carried out by a conventional extrusion process. The rope is preheated to a temperature in the range of 100 °C to 240 °C. The molten polymer is supplied by an extruder to an extrusion head and flows into the rope and usually takes the easiest path to the atmosphere.

[0019] Herewith, steps c) and d) can be carried out by co-extrusion. Co-extrusion is the extrusion of multiple layers of material simultaneously. This type of extrusion utilizes two or more extruders to melt and deliver a steady volumetric throughput of different viscous plastics to a single extrusion head (die) which will extrude the materials in the desired form. The layer thicknesses are controlled by the relative speeds, toolings and sizes of the individual extruders delivering the materials.

[0020] Alternatively, steps c) and d) can take place in two separate extruders and extrusion heads. The polyamide or PEBA coating is first extruded via the first extruder and the first extrusion head. Thereafter, the TPU coating is extruded via the second extruder and the second extrusion head. The advantage of this two steps extrusion is that the temperature profile for both materials can be set apart from each other to avoid mixing of both materials due to uniform temperature at the crosshead. This double extrusion carried out in two steps results in smooth layer and better adhesion between two extruded layers. [0021 ] Importantly, step d) should take place immediately, i.e. the line speed should be high enough, after step c) so that the polyamide or PEBA coating has not completely cooled down.

Brief Description of Figures in the Drawings

[0022] The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:

[0023] Fig. 1 is a cross-section of a wire rope according to the first embodiment of the present invention.

[0024] Fig. 2 is a cross-section of a wire rope according to the second embodiment of the present invention.

[0025] Fig. 3 is a schematic view of co-extrusion with one extrusion head and two extruders.

[0026] Fig. 4 is a schematic view of two-step extrusion with two extruders and two extrusion heads.

[0027] Fig. 5 shows how to measure the distance between two neighboring strands in the wire rope.

[0028] Fig. 6 shows a test configuration for carrying out a RPK dynamic test on wire ropes.

Mode(s) for Carrying Out the Invention

[0029] Embodiment 1

[0030] Figure 1 shows a cross-section of a wire rope according to the first embodiment of the present invention. As shown in Figure 1 , the strand 10,12 is in the form of one central steel wire of e.g. 5 mm in diameter twisted with six outer steel wires 14 with the same diameter. Preferably, the wires are galvanized steel wire and coated with a lubricant film. Strand as such taken as a central core is twisted together with another six strands in the same configuration. The twisted steel strands are individually separated by polyamide coating 16, such as commercially available PA- 12, or PEBA such as PEBAX®.

[0031 ] The coated strands are bundled together, preferably by twisting. This twisted wire rope is further covered with TPU coating 18. The thickness of the outer TPU coating is in the range of 1 to 50 mm, preferably 10 mm, more preferably 3 mm.

In this embodiment, each strand was first coated by either PA-12 or PEBAX® individually, and thereafter the coated strands are twisted together. This twisted wire rope was further extruded as a whole to have a second or outer TPU coating 18. In extrusion process, the wire strands are preheated. The preheated temperature and the extrusion temperature for the coatings are summarized in table 1 .

8/15 13935-LZH

[0033] Table 1.

[0034] Embodiment 2

[0035] Alternatively, as shown in Fig. 2 the wire rope 20 having configuration 7x7 was coated as a whole by a first inner coating 22, e.g. PA-12 or PEBAX® and a successive second outer coating 24, e.g. TPU (type IROGRAN A92E). The strand is preferably made of galvanized steel wire, e.g. having a diameter of 5 mm. In addition, the wires may be coated with a lubricant.

[0036] The double coatings 22,24 can be formed by extrusion process. The wire rope 20 was first preheated to certain temperature. Then the preheated wire rope 20 was coated with the first material 22, i.e. either PA-12 or PEBAX®. Preferably, the PA-12 or PEBAX® is intimately and directly contacting substantially all exposed exterior surfaces of the outer strands 26 and extending down to and into the central core strand 27 of the wire rope 20 and the interstitial spaces 28 between the wire strands 26,27. More preferably, the PA-12 or PEBAX® also fills in the interstitial spaces 29 between the wires of each strand 26,27. This is implemented by injection of the molten material into the wire rope 20. It is important to control the distance between the two extrusion heads (dies) with respect to the rope diameter so that essentially complete filling of the wire rope can take place. The distance between the two extrusion crossheads depends on the properties of the two polymers, especially the melting point of the polymers. Moreover, the pressure of injection should be sufficient. As an example, a distance of 1 .90 m and a pressure of 15 to120 bar at cross-head are used in this invention. Afterward the second coating i.e. outer coating TPU was extruded thereon.

[0037] According to the present invention, the first inner coating 22 and the second outer coating 24 may be formed by co-extrusion process. As shown in Fig. 3, PA-12 or PEBAX® was melted in extruder A and TPU was melted in extruder B. The extrusion head C is developed for co- extrusion with two extruders. The wire rope was preheated before passing through the extrusion head C and cooled down after passing through the extrusion head C when running along the line direction D. Table 1 shows the preheating temperature and the extrusion temperatures for the inner material, i.e. either PA-12 or PEBAX® and the outer material TPU in several examples. [0038] Alternatively, the two coated materials can be extruded in two separate extrusion steps. As shown in Fig. 4, in the first extrusion step, PA-12 or PEBAX® was melted in the first extruder A. The wire rope passed through the extrusion head C and was coated as a whole. In the subsequent second extrusion step, the wire rope passed through the second extrusion head D which is connected to extruder B filled with TPU. Thus, the temperature profile for both materials is set apart from each other. The possible mixing of two materials is avoided. Preferably, the extruder A and extruder B are not too far along the line direction E and the second extrusion takes place immediately after the first extrusion. The line speed must be high enough to avoid too high cooling down of the first coated material after passing through extrusion head C. As an example, the distance between extruder A and B is 1 .90 m and the line speed of 5 m/min is used in the present embodiment.

[0039] The strands of wire rope coated as a whole are in good position within the wire rope. The distance between the different strands within the wire rope are measured. Herein, the distance of two different strand are measured under microscopy: first, a cross-section of the wire rope is made; second, the central of the core wire of two neighboring strands are connected by straight line as shown in Fig. 5; then, the distance between two closest wires which are individually belonging to two neighboring strands 50,52 is taken as the distance between the strands. The minimum, maximum, average and standard deviation of the distance between neighboring strands in a wire rope are listed in table 1 . As shown in table 1 , the distance between neighboring strands are quite comparable. As an indication, the strands are substantially symmetrically distributed and in good position.

[0040] The wire ropes extruded in two steps are subjected to a test which is called the dynamic RPK test and which is illustrated in Fig. 6. The RPK test is applied herewith to evaluate the fatigue resistance of the wire rope according to the present invention. The coated wire rope 60 forms a closed circle around clamp wire 61 , driving pulley 62 e.g. having a diameter of 200 mm, two test pulleys 63 and Idler pulley 64. The driving pulley 62 continuously changes its direction of rotation with a frequency of 120 changes per minute. An axial weight 66 of 4500 N is attached to the Idler pulley 64. The test length is 30 cm. The number of cycles before fracture is measured.

[0041 ] The results of RPK test of wire ropes coated under different conditions are compared in table 1 . The number of cycles in table 1 is the average value of three times measurements. As shown in table 1 , the preheating temperature and extrusion temperatures both play a role regarding the endurance in RPK test. However, for any wire rope manufactured according to the present application the number of cycles before fracture is above 76 000, which is much higher than the test result, i.e. around 45 000, of only Polypropylene (PP) coated conventional wire rope with a same configuration. This shows that the two coating materials have good affinity. The outer coating TPU provide good wear resistance and the inner coating PA-12 or PEBAX® considerably reduces the degree of fretting between the steel wires and results in an increased resistance against fatigue. Therefore, the coated wire rope according to the invention has good fatigue and wear resistance.

[0042] In addition, as can be seen from table 1 , the samples with the inner PEBA coating exhibit high values of fatigue resistance i.e. the double extruded wire rope with PEBAX® and TPU shows much better fatigue resistance compared with double extruded wire rope with PA-12 and TPU. This on the one hand can be attributed to the better properties of PEBAX®, and on the other hand can be attributed to the better affinity between PEBAX® and TPU.

[0043] It should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention. List of references

10 central wire strand

12 outer wire strand

14 wire

16 polyamide coating

18 TPU coating

20 wire rope

22 PA-12 or PEBAX®

24 TPU

26 strand

27 central core

28 interstitial spaces between wire strands

29 interstitial spaces between wires 50 strand

52 strand

60 wire rope

62 driving pulley

64 Idler pulley

66 axial weight