by the tangential belt 2 and three sockets 4 5 and 6 also mounted on the base 1 parallel to the drive pulley 3 and respectively supporting three rotationally driven axles 7 8 and 9 on which are mounted at right angles friction discs 10 in conventional manner the axles 7 8 and 9 are located at the apices of a triangle so that the friction discs mutually overlap so as to define a zig zag path for the yarn as can be seen in fig2 4 and 5 the friction discs 10 each have a convex peripheral surface 11 with which the yarn 12 during its travel is in contact and an upper face 13 and a lower face 14 which are generally planar according to this invention each disc possesses on at least one and preferably both of its upper and lower faces 13 14 a recess 16 of which the bottom is joined to the convex peripheral surface 11 by a surface which forms a sharp edge 15 thus in the embodiment illustrated in fig2 and 3 each recess 16 is of annular shape and has a bottom 20 joined to the convex peripheral surface 11 by a cylindrical surface 21 the intersection between the two surfaces 11 and 21 being a sharp edge 15 fig4 illustrates another embodiment in.