section of the central wire 11 is not smaller than 0 8 thus the inner layer includes just six second wires 13 the middle layer includes several first wires 12 and several third wires 14 the first wires 12 and the third wires 14 are annularly arranged the third wires 14 are interspersed between any two adjacent first wires 12 therefore the third wires 14 and the first wires 12 are alternatively arranged quantities of the first wires 12 and the third wires 13 are equal to quantity of the second wires 12 of the inner layer respectively in the present embodiment first second and third wires are all six wires and are eighteen wires in total preferably sizes of cross section of the third wires 14 are equal to size of cross section of the central wire 11 respectively each third wire 14 is adjacent to two second wires 13 and each first wire 12 is adjacent to one second wires 13 thus the third wires 14 which are larger than the first wires 12 are held in the recesses between any two adjacent second wires 13 the outer layer includes several fourth wires 15 which are annularly arranged quantity of the fourth wires 15 is equal to total quantity of the first wires 12 and the third wires 14 in the present embodiment the middle layer includes six first wires 12 and six third wires 14 thus there are twelve fourth wires 15 in the outer layer preferably each fourth wire 15 is located adjacent to one first wire 12 and one third wire 14 thus the fourth.