roll 36 as long as the thread tension is constant the forces acting on the sliding lever 27 are in equilibrium on the other hand the tension of the unwinding thread f 34 sensed by the tension feeler roll 36 acts on lever 27 via the lever arm 29b while on the other hand the spring 30 acts on the sliding lever 27 via the lever arm 29a if the tension falls the sliding lever 27 pivots clockwise due to the spring return force acting on lever arm 29a thereby sliding the funnel 23 together with the brake ring 21 to the right in the drawing so that the brake ring moves in an area of the support surface 22 with a larger diameter this increases the pressure force thus leading to a corresponding increase in thread tension thereby compensating for the drop in thread tension in the reverse case when the tension of the unwinding thread f 34 increases this tension acting on roller 36 causes a counterclockwise pivoting of lever 27 so that the brake ring 21 is pushed to the left in the drawing thereby reducing the braking effect and lowering the thread tension in this way fluctuations in the thread tension can be continuously and automatically compensated and the unwinding tension is automatically maintained at a substantially constant value the desired theoretical tension can be preselected by setting the tension of the spring 30 the.