Reinforcement strand for reinforcing a polymer article
A reinforcement strand ( 400 ) comprises a core ( 403 ) around which steel filaments ( 404 ) are twisted all with the same final lay length and direction. The steel filaments are arranged in an intermediate layer comprising N first steel filaments and an outer layer of 2N steel filaments circumferentially arranged around the intermediate layer. In the intermediate layer filaments will contact one another at a closing lay length that is determined by the number of steel filaments N in the intermediate layer, the diameter of the core and the diameter of the first steel filaments. By choosing the final lay length and direction equal to the between two and six times the closing lay length gaps will form between the intermediate layer filaments. The 2N outer layer filaments are further divided into a group of smaller ( 408 ) and a group of larger ( 406 ) diameter steel filaments.
1. A method of manufacturing a reinforcement strand comprising the steps of:
providing a core having a core diameter;
providing N first steel filaments having a first diameter;
providing N second steel filaments having a second diameter, larger than said first diameter;
providing N third steel filaments having a third diameter;
twisting said core, said N first steel filaments, said N second steel filaments and said N third steel filaments together in one lay direction to final lay length, wherein said N first steel filaments form an intermediate layer, said N second and N third steel filaments form an outer layer, said N second and N third layer steel filaments occupying alternating positions in said outer layer;
wherein
during said twisting, the intermediate lay length of first, second and third steel filaments is shortened to between the closing lay length and 83% of the final lay length and thereafter the first, second and third steel filaments are untwisted to said final lay length, whereby said intermediate lay length remains larger than or equal to the closing lay length, said closing lay length being that lay length at which the first steel filaments in said intermediate layer touch one another.
2. The method of claim 1 wherein at the intermediate lay length the second and third steel filaments of the outer layer are radially compressed towards said intermediate layer.
3. The method of claim 2 wherein the radial compression is performed by one or more radial compression means chosen from the group comprising: torsion restriction pulleys, straightener and cabling die.
4. The reinforcement strand produced according the method of claim 1 wherein, upon cutting the reinforcement strand thereby forming a reinforcement strand end, at least said first steel filaments of said intermediate layer retract into reinforcement strand end.
5. The method according to claim 1 wherein said final lay length is larger than two times and smaller than six times said closing lay length.
6. The method according to claim 1 wherein the number ‘N’ is one out of the group consisting of 5, 6, 7, 8 and 9.
7. The method according claim 1 wherein said core is one out of the group comprising:
a straight single wire,
a single wire with bends comprising straight segments in between said bends,
a strand with 2, 3 or 4 steel filaments, and
a strand wherein the number of core steel filaments is nine or twelve and wherein the core steel filaments are arranged in a semi-Warrington construction.
8. The method according claim 1 wherein the twisting of the core, the first, second and third steel filaments can be performed in one of the following ways:
by means of cabling;
by means of bunching.