be reduced in that the elastic yielding of the braking body is less rapid and marked due to the lower intensity of the traction t to which said low count threads are subjected this problem is eliminated by tapering the truncated cone braking body i e imparting thereto an inclination of the generatrices with respect to the axis of the cone the taper being greater than the angle u03b1 which the thread forms with the axis of the drum in the portion comprised between the output circumference c1 and the thread guide g in the embodiment of fig8 the braking body 120 has a taper which is greater than the angle u03b1 which the thread would form in the absence of the body 120 with the axis 34 a 34 of the drum ta in the portion comprised between the output circumference c1 and the terminal thread guide g the thread is therefore redirected by the terminal or smaller section s of the truncated cone body 120 which is provided with a metallic ring 121 accordingly as shown by the schematic view of fig9 the thread discharges onto said ring 121 of the truncated cone braking body 120 a second axial component where t is the traction applied to the thread after the truncated cone braking body 120 f is the friction coefficient between the thread and the ring 121 and u03b2 is the angle of winding of the thread on said ring the component h 39 a is added to the component ha which said thread.