Steel strand and belt reinforced with said steel strand

Abstract

The invention relates to a steel strand and a belt comprising the steel strands. The belt is particularly designed for use as an elevator belt. Such belts are very long and must sustain many cycles. The invention solves the problem that when incorporating the steel strands in belts, the belt does not track well on a crowned pulley and is wavy after removal of the load. This problem is solved with a steel strands having a core, a first layer of steel filaments and a second layer of filaments wherein the total of the angles spanned by the filaments of the second layer with vertex at the centre of the strands is less than 270°. In this manner the contact surface between polymer and steel strand is greatly increased compared to prior art steel strands leading to a better mechanical anchorage and – when an adhesive is used – greatly improved adhesion force;

Description

Title: Steel strand and belt reinforced with said steel strand.DescriptionTechnical Field

[0001] The invention relates to a steel strand and a belt that is reinforced with such steel strands. The steel strands are designed to reinforce a belt for use in an elevator.Background Art

[0002] As of the turn of the century the use of polymer belts in elevators for transportation of persons or goods has only increased. These ‘elevator belts’ offer many advantages over the traditional steel ropes used to drive lifts up and down in the elevator shaft. Elevator belts have a predictable lifetime that greatly exceeds that of steel ropes. For the same strength they are much lighter than steel ropes. Moreover, they offer excellent traction to the pulleys they run thanks to the polymer. One of the main advantages of using a belt is that they allow for a lower diameter drive pulley than conventional steel ropes. Low diameter drive pulleys allow for compact direct drive units, that can be mounted at the top of an elevator shaft thus eliminating the need for a roof top machine room.

[0003] Many of the aforementioned advantages - notably the strength, the predictable fatigue life, the low diameter drive pulleys - are made possible by the use of cords made of high strength, fine diameter steel filaments as strength members to the polymer. Other advantages of using steel cords compared to organic fibres such as polyaromatic, carbon or high density polyethylene fibres is their fire resistance and resistance to creep. In particular steel cords made from multiple strands twisted together, the strands by themselves consisting of steel filaments twisted together are highly preferred for their favourable elongation properties and good anchorage into the jacket polymer. A typica configuration is 7x7 constructions comprising a king strand around which six outer strands are twisted. The king strand and the outer strands consist of a king wire around which six sheath wires are twisted. Another configuration is 19+8x7 where the king strand comprises a king wire around which first sixfilaments are twisted, whereon subsequently twelve filaments are twisted in a subsequent operation. It will be clear that the production of such steel cords, having many different steel filament diameters is complex and labour intensive.

[0004] Quite many attemps - reference is made to WO 2019 / 002162 A1 of current applicant for an overview - have already been made to replace steel cords with steel strands. Such steel strands will obviously somewhat fewer filaments hence thicker steel filaments in order to reach the same strength but from fatigue point of view this is acceptable. While in WO 201 / 002162 A1 the problem of core migration is adequately solved, other problems have arisen during implementation. More in particular the higher modulus and Hookian behaviour of steel strands makes the use of strands in a belt challenging. In particular the elongation at low loads is smaller than in the case of steel cords, reflecting the higher modulus of strands. Now when using crowned pulleys the tracking of the belt is due to the elongation of the steel cords in the belt: when forced off the crown, the belt bends in the plane, and thereby climbs back to the top of the crown pulley. So elongation behaviour is crucial to make steel strands in belt work.

[0005] Due to the low elongation, the belt behaviour becomes more sensitive to minor differences in strand properties. If some of the strands, or certain lengths in a single strand are stretched during use, the elongation may become permanent. And this acquired elongation results in an uneven, irregularly waved, sometimes curled belt when the load is removed from the belt: the wrinkled belt problem. When taking the belt from the elevator, the used belt - although remaining generally straight - cannot be laid flat on a surface anymore. After many endeavours, the inventors propose the solution that will now be described in detail to solve this wrinkled belt problem.Disclosure of Invention

[0006] The main object of the invention is to solve the ‘wrinkled belt’ problem. As the origin of the problem is related to the tension members in the belt, also a steel cord construction is proposed that solves this problem.

[0007] According a first aspect of the invention a steel strand is presented.

[0008] The steel strand comprises a core and a number ‘N’ of first layer steel filaments twisted around the core with a strand lay length and a strand lay direction. The filaments of the first layer have a diameter designated ‘dT, the core has a diameter designated ‘d0’ hereinafter.

[0009] Around the first layer, a second layer of steel filaments is provided at the same lay direction and lay length. The strand is thus an ‘equal lay’ strand, having one single lay length and direction. The second layer filaments have a second diameter, called ‘d2’ hereinafter. The second diameter d2 is equal to or larger than d1 (d2>d1 ).

[0010] When considering a perpendicular cross section of the steel strand, each of the second layer filaments will subtend, span an angle when considered from the centre of the steel strand, that is also the centre of the core, with the sides of the angle tangent to the cross section of the filament. The total sum of those angles of all filaments of the second layer is less than or equal to 270°.

[0011] The sum of all angles spanned by the filaments of the second layer with vertex at the centre of the steel strand can also be less than 260°, or even less than 250°. In any case the total sum of angles must be larger 200° as otherwise the second layer barely adds to the breaking load of the strand. The breaking load of the strand is that tensile force (in N) at which the strand catastrophically fails.

[0012] It is to be noted that it is not a prerequisite of the invention that the centres of the second layer filaments are all at the same distance from the centre of the steel strand. In other words, it not a necessity of the invention that the centres of the second layer are situated on a circle. Some second layer filaments can be somewhat closer to the centre and others somewhat further away from the centre. The subtending angle may differ slightly from filament to filament in the second layer, but the sum of angels can still be determined.

[0013] Further - depending on the lay length of the filaments in the second layer - the cross section of a round filament will show slightly elliptical. It remains perfectly possible to draw tangent lines out of the centre of the strand touching the sides of the ellipse to determine a spanning angle.

[0014] As the second layer steel filaments cover less than 270° of the circle, 90° of the second layer open up and therein the first layer filaments are present. The advantages of this are manyfold:- As the first layer filaments can now by contacted by the polymer of the belt (see further), the first layer filaments are also held by the polymer.- Due the extreme gaps that open in between the second layer filaments, the steel strand is much better mechanically anchored in the polymer.- The contact surface per unit of length between steel filaments and polymer also increases, compared for example to the semiWarrington cords as presented in WO 2019 / 002162 A1 .- In case an adhesive is used, this results in more adherent surface, hence a higher adhesive force between steel strand and polymer. As in a belt all forces are carried by the steel strands while the force moments of the drive sheave or pulley interact with the polymer, a good force transfer between steel strand and polymer is needed. The invention increases both the mechanical anchorage and increases chemical adhesion.- Due the very open second steel filament layer, contact pressure when the belt runs over the sheave of the elevator is also transmitted to the first layer steel filaments. In this manner the core is better held.

[0015] The total angle spanned by the second layer filaments is proportional to the number of filaments in the second layer, proportional to the diameter d2 of the second layer filaments and inversely proportional to the distance of the second layer filaments to the centre of the core. Also the strand lay length has an influence on this as with shortening lay length, the filaments make an increasing angle to the axis of the strand. In a perpendicular cross section, the circular strands will therefore appear as ellipses. The sides of the angle spanned by the filament must thus be taken tangent to this ellipse.

[0016] The number of second layer filaments is equal to or more than the number ‘N’ of first layer filaments but is not more than ‘2N-1’. When ‘2N’ filamentswould be present the second layer is considered ‘saturated’ meaning that no more filaments can be added to the layer. In that case one obtains a Compact Cord type of cord which is a single lay cord, with all filaments taken equal. The second layer can therefore be called ‘unsaturated’.

[0017] However, to allow all second layer filaments to be accommodated as good as possible in the configuration - meaning that configuration that is stable, of lowest potential energy - it is most preferred that filaments of the second filament layer nest in the crevices, the recesses, the valleys formed by the twisted first layer filaments. That implies that the number of second layer filaments is preferably ‘N’. A special case of such configuration is called a Seale configuration, wherein the number of second layer filaments is equal to the number of first layer filaments and the steel filaments of the second layer have a diameter such that the second layer is completely closed, or in other words: the total spanning angle of the second layer steel filaments is 360° or at least very close to 360°.

[0018] The number ‘N’ may be equal to 5, 6, 7, 8 or 9. Particularly preferred numbers are 6, 7 and 9 as they result in sufficiently small filament diameters while the number of filaments in the strand remain limited. The diameter of a filament has a large impact on its fatigue life expectation. Smaller diameters suffer from less bending stresses for the same bending diameter of the strand on itself or the strand in the belt.

[0019] The second layer filaments may have a diameter that is equal to the diameter of the first layer filaments. Alternatively the second layer filaments have a diameter that is larger than the first layer steel filaments, for example larger than 5% or larger than 10% or even larger than 20% of the diameter of the first layer steel filament. Thus, d2 is larger than 1.05xd1 , 1.10xd1 or 1.20xd1. Preferably the diameter d2 remains smaller than 2.00xd1 , or less than 1 ,75xd1 , even more preferred less than 1 ,50xd1 . This again to have improve fatigue life.

[0020] The diameter of the first layer steel filaments is chosen in combination with the strand lay length to at least have gaps in between the first layer steel filaments. The total angle of all gap angles measured from the centre of the core i.e. the total angular gap span is at least 20 degrees and at most50 degrees. The angular gaps between adjacent first layer steel filaments does not need to be equal for all gaps, but it is more preferred if they are. The presence of the gaps between the first layer steel filaments prevents the blockage of the first layer filaments against one another if the core extends. The total angular gap should not be larger than 50 degrees as then the manufacturing of the cords becomes difficult.

[0021] The core can be a single steel wire or can even be an organic fibre such as poly aromatic chain aramid, nylon, polyethylene, or even high molecular weight polyethylene. However, these are all less preferred as they give rise to an unacceptable increase in elongation and creep.

[0022] More preferred is that the core is made of steel filaments that are twisted together that is: the core is a steel strand on its own. Even more preferred for the core are equal lay constructions that have none zero order helix deformation filament i.e. have no straight, undeformed filament throughout the core. Instead the core comprises all steel filaments that have the shape of a helix (‘first order helix deformation’). Zero order helix, or straight filaments can not absorb compression. Consequently, they wick out of the steel strand under the repeated loading and unloading of the strand during use, hence should be avoided.

[0023] At least the core lay length is different than the strand lay length. The lay directions of core and strand may be equal or opposite. By preference the core lay length is smaller than the strand lay length. Preferably, the lay length of the core is smaller than one third, a fourth, fifth or even a tenth of the strand lay length.

[0024] Alternatively, the core lay length is smaller than ten times, less than eight times, less than five times or even less than three times the diameter ‘d0’ of the core. A short lay length of the core is important in that it must be able to stretch, extend, elastically as it has the shortest length in the strand compared to the helically deformed first and second layer steel filaments.

[0025] The core can simply be two, three, four or five steel filaments that are twisted together. Most preferred is three as this forms a stable configuration.

[0026] Alternatively a 12 wire semi-Warrington construction can be envisioned comprising a core-core existing of 3 filaments twisted together. A ‘corecore’ should be interpreted as ‘the core of a core strand’. In the recesses formed by the filaments 3 larger outer filaments are nested. In between each pair of the 3 larger outer filaments a pair of smaller filaments is positioned. An example is given in US 4829760 herewith incorporated in its entirety by reference. Another equally well preferred embodiment is a 9 wire semi-Warrington construction comprising a core-core of 3 fine wires and a jacket of six wires of alternating medium and large size. Such a cord is described in US 3358435.

[0027] In order to impart even more elongation to the core, the use of crimped steel filaments helps. A crimped steel wire shows bends with straight segments in between. With ‘straight’ is meant that the bends have a radius of curvature larger than 100 times the wire diameter. Already one crimped steel filaments will have an influence on the extension behaviour of the core. More preferred is that all core steel filaments are crimped.

[0028] In a further preferred embodiment the core is pre-coated with a polymer. Preferably this polymer is the same as that of the jacket of the belt. Moreover, during production of the steel strand, the core polymer is made to enter the gaps between the first layer steel filaments. The polymer forms a resilient cushion in between the first layer steel filaments. The polymer of the core also helps to prevent motion of the first layer steel filaments: it fixes in the steel filaments in position.

[0029] In a further preferred embodiment the steels strand is coated with an organic primer that promotes adhesion between the steel filaments and the polymer.

[0030] The primer is chosen to improve adhesion to the polymer wherein the reinforcement strand is intended to be used. Typical organic primers are phenolic resin, epoxy, cyanoacrylate, or acrylic based such as for example those marketed under the brand name Loctite®.

[0031] However, these coatings are relatively thick (more than one micrometer) and may require quite some processing time. Therefore a nanoscopic organic coating taken out of the group comprising or consisting of organofunctional silanes, organo functional zirconates and organo functional titanates are preferred. Preferably, but not exclusively, the organo functional silane primers are selected from the compounds of the following formula:Y-(CH2)n -SiX3wherein :Y represents an organo functional group selected from -NH2, CH2=CH-, CH2=C(CH3)COO-, 2,3-epoxypropoxy, HS- and, Cl-X represents a silicon functional group selected from -OR, -OC(=O)R’, -Cl wherein R and R’ are independently selected from C1 to C4 alkyl, preferably -CH3, and -C2H5; and n is an integer between 0 and 10, preferably from 0 to 10 and most preferably from 0 to 3

[0032] The organo functional silanes described above are commercially available products. These primers are particularly suited to obtain adhesion with polyurethanes. The organic coating has a thickness of less than 1 micrometer, preferably less than 500 nanometer, such as between 5 and 200 nm. Thin coatings of this size are preferred as they follow the outer surface of the reinforcement strand in a conformal way and do not obstruct the filling of the polymer in the valleys between the outer layer filaments thanks to their thinness. In the case of the inventive strand this is important as the outer surface of the steel strand shows a very curvy surface.

[0033] For all the embodiments of the invention strand or belt, whether preferred or not, whether alternatives or additions the following is valid:

[0034] Whenever reference is made to ‘steel filaments’ in this application, substantially round steel wires with a diameter of between 0.02 to 0.40 mm, more preferred between 0.04 and 0.35 mm or between 0.10 and 0.30 are meant. These filaments have a high tensile strength - that is the breaking load (in N) of the filament divided by its cross-sectional area (in mm2) - that is above 2000 N / mm2, preferable above 2350 N / mm2, for example above 2700 N / mm2. An upper limit of tensile strength is currently 4000 N / mm2and that for plain carbon steel (see further).

[0035] With ‘steel’ any type of steel is meant. Plain carbon steel is preferably used. Such a steel generally comprises a minimum carbon content of 0.40 wt% C or at least 0.70 wt% C but most preferably at least 0.80 wt% C with a maximum of 1.1 wt% C, a manganese content ranging from 0.10 to 0.90 wt% Mn, the sulfur and phosphorous contents are each preferably kept below 0.03 wt%; additional micro-alloying elements such as chromium (up to 0.2 to 0.4 wt%), boron, cobalt, nickel, vanadium - a non-exhaustive enumeration- may also be added. Such carbon steel filaments can be produced at strengths in excess of 2000 MPa, preferably above 2700 MPa, while now strengths above 3000 MPa are becoming current and inroads are being made for strengths over 3500 MPa. Also preferred are stainless steels. Stainless steels contain a minimum of 12 wt% Cr and a substantial amount of nickel. More preferred are austenitic stainless steels, which lend themselves more to cold forming. The most preferred compositions are known in the art as AISI (American Iron and Steel Institute) 302, AISI 301 , AISI 304 and AISI 316 or duplex stainless steels known under EN 1 .4462.

[0036] Preferably the steel filaments are provided with a metallic coating or metallic alloy coating. Such alloy can be used to impart corrosion protection to the steel or to make the filaments adhere to a polymer or to combine both: corrosion protection and adhesion. Corrosion resistant coatings are e.g. zinc or a zinc aluminum alloy. Most preferred is a low zinc, hot dip coating as described in EP 1280958. Such zinc coating has a thickness lower than two micrometer, preferably lower than one micrometer, e.g. 0.5 pm. An alloy layer zinc-iron is present between the zinc coating and the steel.

[0037] Other preferred metallic adhesion coatings are for example brass coatings - copper-zinc alloys - when the steel strand is for reinforcing rubber. So called ‘ternary brass’ such as copper-zinc-nickel (e.g. 64 % by weight / 35.5 wt. % / 0.5 wt.%) and copper-zinc-cobalt (e.g. 64 wt.% / 35.7 wt.% / 0.3 wt.%), or a copper free adhesion system such as zinc-nickel or zinc-cobalt can also be used.

[0038] According a second aspect of the invention a belt is presented.

[0039] The belt comprises a plurality of steel strands and a polymer jacket, said steel strands being oriented along the length dimension of the belt and held in parallel relationship by the polymer jacket. For the purpose of this application with ‘in parallel relationship’ it is meant that the steel strands are organised in a single surface, for example a plane surface.

[0040] The belt differs from prior art belts in that the steel strands are those according the first aspect of the invention. By using such steel strands the polymer fills the openings between the second layer filaments, down to the first layer filaments.

[0041] In order to quantify the degree of contact surface per unit length of the steel strands and the polymer, one can compare this to the surface of an imaginary cylinder circumscribing the steel strand. In a perpendicular cross section, a smallest circumscribing circle can be identified, with diameter ‘D’. This circle thus has a perimeter ‘TTD’, IT being Archimedes’ constant. The surface of the imaginary cylinder is thus ‘TTD’ times the length of the cylinder.

[0042] In the same perpendicular cross section, a contact contour, or contact curve, where the polymer contacts the steel filaments of the first or second layer steel filaments. The total length along this curved interface is called ‘C’. In the inventive belt the length of the contact contour ‘C’ is larger than 1 .5 times the perimeter of the circumscribing circle ‘TTD’. The ratio C / TTD can even be larger than 1 .6 or 1 .7 or even 1 .8. For calculating ‘C’, only the length along the contour wherein the polymer contacts the steel filaments of first or second layer must be taken into account. Patches along the contour ‘C’ wherein the polymer does not contact the filaments must not be taken into account.

[0043] This means that the contact area between the polymer of the belt and the steel strand is at least 50%, or 60% or 70%, even 80% larger than that of smooth cylindrical surface of the same diameter. It follows that the adhesion surface is thus also much larger, hence the overall adhesion of the steel strands to the polymer greatly increases.

[0044] The polymer jacket is encasing, surrounding, holding the steel strands in position. Practical usable polymers are thermohardening polymers likerubber and thermoplastic polymers, the latter being preferred for their ease of processing and the possibility to easily alter the mechanical properties of the polymer. Most preferred thermoplastic materials are thermoplastic polyurethane (TPU) and thermoplastic polyolefins (TPO).

[0045] TPUs derived from a poly ether polyol resist hydrolysis well but have lower mechanical properties. TPUs derived from a poly ester polyol have better mechanical properties but are less resistant to hydrolysis. The resistance to hydrolysis and the mechanical properties of TPU derived from poly carbonates are in between both other types. Most preferred are poly ether polyol based TPU and poly carbonate polyol based TPUBrief Description of the Figures in the Drawings:

[0046] FIG 1 shows a first embodiment of a belt according the invention.

[0047] FIG 2 shows a first embodiment of the steel strand according the invention.

[0048] FIG 3 shows a second embodiment of a belt according the invention.

[0049] In the reference numbering, the hundred digit refers to the number of the figure, while the unit and tens digit refers to equivalent features across figures.Mode(s) for Carrying Out the Invention

[0050] Figure 1 shows a first embodiment of a belt 100 according the invention.The belt comprises steel strands 102 arranged in parallel. Typically from 5 to 15 steel cords can arranged like this in a single belt, but normally 8 to 10 suffice to impart sufficient strength to the belt. The jacket 110 is a poly carbonate polyol based polyurethane. Also indicated in the steel strand 102 is the core 108, the first layer filaments 106 and the second layer filaments 104.

[0051] Figure 2 shows a detailed view of the steel strand 102 of Figure 1 . The steel strand 202 comprises a core 208 made of three filaments of diameter 0.28 mm that are twisted together at a lay of 5.1 mm in S direction. The lay length of the core is just shorter than 10 times the diameter of the core that is 0.60 mm. This is the core diameter ‘d0’. The core was coated with PU 216 up to a diameter of 0.70 mm. Subsequently 9 first layer steel filaments206 and 9 second layer steel filament 204 were added to the core in a single operation, at a lay of 14 mm in S direction. The first layer filament 206 has a diameter d1 of 0.31 mm and the second layer filament 204 has a diameter d2 of 0.33. The total angular gap angle in the first layer is then 38.3°. In a subsequent operation, an organofunctional silane adhesive was applied and during drying the polyurethane of the core is molten into the gaps indicated 214. The polyurethane thus fills the gap in between the first layer filaments.

[0052] The angle indicated ‘a’ has its vertex at the centre of core and the sides tangent to the second layer filament. By adding the angles of all 9 filaments a total angle of 243° is obtained, that is smaller than 270°.

[0053] In practise, the sum of the angles spanned by the filaments in the second layer of the strand can be determined by making a perpendicular cross section of the steel strand - e.g. by casting in epoxy, followed by perpendicular sawing and polishing - determining the centre of the core that is also the centre of the strand - and then analysing the cross section with an optical microscope - e.g. Zeiss Axio Imager.Alm - preferably completed with an image processing program such as Imaged (available from University of Wisconsin, Laboratory for Optical and Computational Instrumentation). Angles spanned by each of the second layer filaments can be easily extracted from the cross section in this way and summed. In the same manner the total angular gap span of the first layer can be determined, by measuring the angular gap of the first layer filaments gap.

[0054] Subsequently a belt was produced with twelve of the cords of Figure 2 organised parallel to one another. The belt is made by the techniques known in the art such as extrusion of parallel arranged steel strands through a single extrusion head or by laminating parallel unwound steel strands in between two sheets, the former method being more preferred over the latter method.

[0055] On a perpendicular cross section of the belt the contour length and the diameter ‘D’ of the cord can also be measured. As the gaps between the second layer steel filaments is wide the polyurethane easily flows in between the gaps. The total contour length ‘C’ measured is about 10.2mm while the diameter ‘D’ of the steel strand is 1 .75 mm. The ratio of the contour length over the perimeter ‘TTD’ of 5.50 mm is thus 1.86. To be noted that for a Seale strand with N=9 this ratio is 1 .40 as in a Seale strand the second layer filaments are closed to one another.

[0056] The belt of Figure 1 was tested in a test elevator installation for 40600 cycles. The tracking on a crowned pulley was observed and was found to be in line with belts using 7x7 multistrand belts. After the test the load was removed from the belt and no wrinkling was observed. After removal of the belt and rolling out in the length on a clean floor, the belt remained perfectly straight without any curbing effect, as well as flat on the ground.

[0057] Figure 3 shows another embodiment of a belt 300 with strands 302 wherein the core 308 is made from a semi-Warrington with three core-core filaments of 0.18 mm, surrounded with three groups of filaments(0.15|0.22|015) resulting in an overall core diameter of 0.70 mm. The core was not extruded. The first layer filament 306 diameter and the second layer filament 304 diameter was set equal to 0.31 mm. The total angle spanned by second layer filaments was 243° that is below 270°. The circumscribed circle is indicated 312 and has diameter 1.77 mm and thus a perimeter of 5.56 mm. The contacting contour is indicated 314 and has a length of 9.74 mm. The ratio C / TTD is 1 .75.

[0058] The belt of Figure 3 is currently under test.

Claims

Claims1 . A steel strand comprising a core and a number ‘N’ of first layer steel filaments twisted around said core with a strand lay length and a strand lay direction in a first layer, said first layer steel filaments having a first diameter ‘dT, said core having a core diameter ‘d0’ characterized in that around said first layer a second layer of steel filaments is provided at the same lay direction and lay length, said second layer filaments having a second diameter ‘d2’, said second diameter ‘d2’ being larger than or equal to ‘dT, said second layer filaments spanning a total angle of less than or equal to 270 degrees as measured from the centre of the steel strand in a perpendicular cross section of said steel strand.

2. The steel strand of claim 1 wherein the number of second layer steel filaments is equal to or more than the number ‘N’ and less than ‘2N-T.

3. The steel strand according to any one of claims 1 to 2 wherein the number N is equal to 5, 6, 7, 8 or 9.

4. The steel strand according to any one of claims 1 to 3 wherein in between the first layer steel filaments gaps are present, said gaps having a total angular gap span of at least 20 degrees and at most 50 degrees.

5. The steel strand according to any one of claims 1 to 4 wherein said core is an equal lay strand comprising core steel filaments that are free from zero order helical deformations and are twisted together with a core lay length that is different from said strand lay length.

6. The steel strand according to claim 5 wherein said core has a core lay length that is shorter than one third of the strand lay length.

7. The steel strand according to any one of claims 1 to 6 wherein said core comprises two, three, four or five core steel filaments twisted together.

8. The steel strand according to any one of claims 1 to 6 wherein said core comprises from nine to twelve core steel filaments, the core steel filaments are arranged in a semi-Warrington arrangement.

9. The steel strand according to any one of claims 5 to 8 where at least one core steel filament is crimped, said crimped steel filament comprising bends with segments in between.

10. The steel strand according to claim 4, said core is coated with a polymer, said polymer being present in the gaps between the first layer steel filaments.11 . The steel strand according to any one of claims 1 to 10 wherein said steel strand is coated with an organic primer that promotes adhesion to a polymer.

12. A belt comprising a plurality of steel strands and a polymer jacket, said steel strands being oriented along the length dimension of said belt and held in parallel relationship by said polymer jacket, characterised in that said steel strands are according any one of claims 1 to 11 , and said polymer fills the openings between the second layer steel filaments, down to first layer steel filaments.

13. The belt according to claim 12 wherein said steel strands in a perpendicular cross section have a smallest circumscribing circle with a diameter D and a perimeter TTD, said steel strands have a contact contour where the polymer contacts the filaments of said first or said second steel filament layer, said contact contour having a length ‘C’, said length ‘C’ being larger than 1.5 times said perimeter TTD.

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