Description

Technical Field

[0001 ] The invention relates to a steel cord for the reinforcement of an elastomer product such as a belt incorporating said steel cord, for example an elevator belt, toothed belt - also known as a timing belt - or a synchronous belt. Alternatively the steel cord can be used for the reinforcement of a tire.

Background Art

[0002] Belts exist in all strengths, widths and lengths: for driving machinery such as V-belts, for conveying goods such as long haul heavy conveyor belts for conveying ore, lignite or coal, or tiny timing belts for use in automation machinery where precision displacement is desired. The focus of this application is on belts that are used for lifting elevators, for synchronous belts and their reinforcing steel cords. Typically these belts are between 1 to 10 cm wide, less than 10 mm thick and extend anywhere from 0.10 to 100 meter.

[0003] These belts are made by encasing steel cords in a polymer, by preference a thermoplastic polymer, such as polyurethane. As these belts run over sometimes very small pulleys, the diameter of those pulleys dictate the diameter of the filaments that still can be used. That is: the smaller the pulleys are, the smaller the diameter of the filaments must be in order to survive the repeated bending of the filament. Moreover, the strength of the steel cords - their breaking load - is just only marginally smaller than the sum of the breaking loads of the individual filaments. Therefore the filaments in the steel cord of a timing belt are generally very thin, thinner than or equal to 0.15 mm, and have a high tensile strength, typically in excess of 2 300 MPa. The tensile strength of a filament is the load in newton at which the wires breaks divided by the cross sectional area of the wire in square millimetre and is expressed in N/mm ² or MPa.

[0004] Simply embedding a bunch of filaments - as suggested in EP 2 697 147 B1 - together and holding them together by means of a glue does not work. Due to the bending the filaments will change position and the steel cord will flatten. Also reverse bending - that is when the direction of bending changes alternatingly - will make the filaments wriggle over one another leading to premature belt failure. The problem is called the ‘filament migration’ problem. In particular when there is a central, non deformed core filament, this core filament is most vulnerable to migration, and ‘core migration’ results.

[0005] Steel cords for use in the types of belts (elevator belts, timing belts or synchronous belts) envisaged for this invention are therefore always made of steel filaments that are twisted together into strands and subsequently these strands are twisted into a cord. Typical combinations are for example 3x3 constructions that consist of three strands twisted together in a cord lay direction, each of the strands being made up of 3 filaments twisted together in a lay direction opposite to the cord lay direction. Alternatives are 7x3, 7x7, 3+5x7, 2x3... that must be interpreted in a similar way. See e.g. CN110172849.

[0006] However, the use of the fine (less than or equal to 0.15 mm) and strong steel filaments make the costs of those steel cords excessive. Further these cords are made in a two step process that is: first strands must be made e.g. 3x0.15, and those strands must be assembled into cords e.g. 3x3x0.15.

[0007] In order to reduce those manufacturing costs, and to prevent the filament migration, the inventors propose the following idea.

Disclosure of Invention

[0008] It is an object of the invention to provide a steel cord that is cheaper to make and that is suitable for reinforcing belts. It is a further object of the invention to make a cord that is ‘fit for use’ that is: it fulfils the intended purpose, without surviving the overall lifetime of the belt.

[0009] According a first aspect of the invention a steel cord is presented as defined in claim 1 . The steel cord comprises from two strands onward up to and including seven strands i.e. 2, 3, 4... or 7 strands. The strands are twisted together with a cord lay length in a cord lay direction. The cord lay direction may be ‘S’ i.e. strands turning in the counter clockwise direction when travelling along said strands or ‘71 that are strands turning in the clockwise direction while travelling along the cord.

[0010] Each of said strands comprises two or three filaments that are twisted together with a strand lay length in a strand lay direction. Strand lay directions will be indicated by small letter ‘s’ or ‘z’. For clarity, the cord may comprise strands of both: of two filaments and three filaments, or may comprise strands that only have two filaments or may comprise strands that only comprise three filaments.

[0011 ] Specific of the steel cord is that the cord lay direction and the strand lay direction are equal and wherein in each one of said strands the strand lay length is equal to or smaller than the cord lay length.

[0012] The advantage of this arrangement is that the filaments of different strands will contact each other along a line, rather than in a point as is the case in prior art steel cords such as 3x3 Zs (or Sz). Line contact implies that wires of different strands do not have crossed contacts. The disadvantage of those line contacts is that a single filament may be wicked out of the steel cord by repeated bending. This is an unfavourable situation.

[0013] In order to mitigate this unfavourable situation the inventors propose in a first preferred embodiment a steel cord wherein the filaments have been twisted together with the strand lay length and direction and the cord lay length and direction, but wherein one filament is locally not helically deformed that is: remains locally straight. However, outside this straight section that same filament is again helically deformed, while another filament takes its place and is locally straight. With ‘locally’ a distance that is not larger than fifty cord lay lengths is meant.

[0014] Hence intermittently one filament is straight while the remaining filaments show a helical deformation, said one filament being straight over a distance not larger than fifty cord lay lengths, said one filament being helically deformed outside said distance.

[0015] For the avoidance of doubt: it is not because a certain filament has a straight section over a certain distance, where outside of this straight section it has obtained a helical deformation, that it cannot longer become straight in another part of the steel cord away from the first straight section.

[0016] For further clarification: at any section of the steel cord there is just one filament that is straight, all of the other filaments having obtained a helical deformation. However, this just one filament does not remain the same filament over distance. Over at the most fifty cord lay lengths one other filament takes over this straightness intermittently, and then another one and this in an undetermined, random order.

[0017] In a second preferred embodiment, the distance whereover the one filament is straight is at least five cord lay lengths. Outside that distance, the filament again becomes helically deformed.

[0018] In a third preferred embodiment, all filaments of the steel cord are equal.

[0019] In a fourth preferred embodiment, the strand lay length of at least one of the two up to and including seven strands is 2 to 20% shorter. It suffices that at least one of the strands shows this shorter lay length. The remaining strands may have the same lay length.

[0020] Introducing one or more strands with a slightly shorter lay length forces filaments to exchange position within the cross section. In this way filaments alternatingly will take a central position without deformation and an off-centre position where the filament is helically preformed. This prevents that one straight filament over the complete length starts to wick out of the cord.

[0021] In a fifth preferred embodiment, the strand lay length of all of the from two to up to and including seven strands have mutually different strand lay lengths. That is: there are no two strands that have an exactly equal lay length.

[0022] While in principle any number between 2x2 i.e. 4 up to 7x3 i.e. 21 filaments can be present in the steel cord, more favourable numbers of filaments are 6, 8, 11 or 18. A sixth preferred embodiment. This is just one number below the number of filaments that form a compact construction (7, 9, 12, or 19). In a compact construction, no spaces are present that would allow the exchange of filament positions. [0023] For all above listed embodiments, the filaments typically - but without being limited thereby - may have a diameter of 0.15 mm or larger. The filaments have a tensile strength of at least 1 500 MPa, or more than 2 000, or higher than 2 500, or even higher than 2750 MPa. Typically the filaments are made of plain, high carbon (more than 0.60 wt% carbon) that are cold drawn to final diameter.

[0024] According a second aspect of the invention different types of belts using the above mentioned steel cords are used.

[0025] In a first preferred embodiment a belt is presented wherein only one steel cord is present. This one steel cord is wound in a flat ribbon with a helix twist opposite to that of the one steel cord lay direction. For example if the cord lay length is in Z direction, the ribbon is wound in S direction. This ribbon is covered, encased in a polymer jacket thereby forming a belt, that in this case is an endless, loop belt. The belt can be e.g. a synchronous or flat belt.

[0026] In a second preferred embodiment a belt is presented that is reinforced with two steel cords of opposite lay direction, wherein each one of said steel cords is wound in a helical winding, the helical winding starting from the centre of the belt towards the edges of the belt. It follows that the helical windings have an opposite winding direction. Further, the helix winding direction of each one of those windings is opposite of that of the steel cord in the winding. Again this is loop belt.

[0027] In a third preferred embodiment two steel cords of opposite cord lay direction are wound in a side-by-side relationship in a helical winding within said belt. Here the helix direction of the helical winding is immaterial. Again this is an endless belt, that is: no connection is made in it.

[0028] In a fourth preferred embodiment three or more steel cords are arranged in a side by side relationship. Adjacent steel cords have opposite lay directions. The steel cords remain parallel to the edges of the belt meaning that is there is a zero winding angle of the steel cords in the belt. Typically these belts are made by unwinding a set of steel cords, guiding them through a wire guide into an extrusion head subsequently embed them in a polymer jacket by means of extrusion. It follows that no closed loop forms and the belt is provided in the form of one length with a two ends. This belt is particularly suited for long applications e.g. as in an elevator belt.

[0029] In third aspect of the invention a tire is claimed. The tire comprises beads a carcass reinforcement folded over said beads and a belt reinforcement covering the carcass reinforcement. The belt and/or the carcass reinforcement comprises the above described steel cords. As customary in a tire all reinforcement of carcass and belts are held in rubber.

Brief Description of Figures in the Drawings

[0030] Figure 1a, 1 b, 1c, 1 d shows cross sections of an example of the inventive cord at different positions along the length of the cord;

[0031] Figure 2 shows the same filaments of Figure 1 but now in an unravelled state.

Mode(s) for Carrying Out the Invention

[0032] In what follows the invention will be explained by means of an example.

Three spools were made comprising a strand 2x0.185 that is: two filaments of diameter 0.185 mm are twisted together. The lay was 276 mm in ‘s’ direction corresponding to 1000/276 i.e. 3.62 twists per meter. In a subsequent operation, the three strands were bunched together to a 3x2x0.185 Ss, that is the strands were twisted together at a lay of 12 mm, also in ‘S’ direction or 83.33 twists per meter. Due to the bunching process the lay of the strand shortens to 11 .5 mm in ‘s’. There is therefore a minute difference between the strand lay length and the cord lay length of 4% of the cord lay length. The cord had an overall breaking load of 460 N.

[0033] As the cord and strand lay are very close to one another, the filament predominantly align themselves such that line contacts between filaments form. That is: the cord takes a compact configuration. Due to the fact that strand and lay direction are identical and the strand lay length and cord lay length are slightly different, at some point filaments of one strand will be compelled to exchange positions, thereby leaving another filament in the core of the configuration. Hence, occasionally, due to the small twist difference of the strands compared to the twist of the cord filaments of the strands have to switch position. Another filament will then take the central position and remain straight for a brief length.

[0034] This is illustrated in the Figures 1 and 2. Figure 1a, 100 shows the perpendicular cross section of the above example at a first position ‘a’. The different pairs of filaments are indicated: 102, 102’ form first strand, 104, 104’ a second strand and 106, 106’ a third strand. Filaments of the same strand show the same hatching direction. In the position ‘a’ the filament 102’ occupies the central position.

[0035] Figure 2 is a representation of the individual filaments that have been unravelled and laid in one plane. The dotted lines correspond to the position along the length of the steel cord of cross sections 1a, 1 b, 1c and 1 d, respectively. As the filament 102’ occupies the central position in Figure 1a, it keeps that position over a number of lay lengths. As a result the filament 102’ remains straight over a length of about 6 cord lays.

[0036] In a later position ‘b’ the filaments 102 and 102’ of the same strand have exchanged position. Now 102 remains straight over a certain length. Likewise in position c, the filament 104’ becomes central and thus is straight. The central position is overtaken by filament 106 at the position ‘d’ and hence that filament is straight at that position.

[0037] Notice that over all filaments there is each time one filament that is straight over a length shorter than 50 times the lay length. Also notice that some filaments do not become central over the length shown in Figure 2: filament 106’ never enters into the centre. It follows that another filament must be central twice over this length and this is indeed filament 102’.

[0038] In a similar manner other constructions can be produced like for example a first strand 3x0.185 at a lay length of 278 mm in ‘z’ can be combined together with a second strand 3x0.185 with infinite lay length. After bunching both strands together at a lay length of 12 mm in Z’ direction, the first strand will have a lay length of 11 .5 mm ‘z’ while the second will have a lay length of 12.0 mm ‘z’. The filaments of the first strand will intermittently take the central position, while the filaments of the second strand never take the central position, that is remain undulated over the complete length.