News Side partOn classic spool holders, there is a risk that the spool unrolls too much when the 3D printer pulls the filament.With my spool holder, the spool will unroll only the length needed.On the side view, the filament is showed in black. First, the filament rolls freely between the green and pink rolls (upper ones).As the filament is pulled to the top, it exerts a force on the lower pink roll.This force is pivoting counterclockwise both the 2 pink rolls as well as the 2 orange levers.The filament is then caught between the green and pink rolls.By friction, the filament carries the green roll along which in turn unrolls the spool by using the other gears - purple, blue, red, and then yellow.When unrolling, the spool loosens the filament, therefore the force exerted on the lower pink roll. As a consequence, the filament is not caught anymore. It does not carry along the green roll, which does not unroll the spool anymore.The process goes again and repeats itself endlessly.Gears multiply the force. As a consequence, only a small force is needed on the green roll for rolling the spool.The lower legs of the 2 orange levers are thin. Therefore they are insuring a spring and regulator effect for absorbing the filament violent jolts.This also insures that the system does not work backwards. Roll and gear dimensions are calculated for a spool with a wound filament diameter less than 200mm.The spring effect of the two levers can be modified by changing their leg thickness.The green roll adhesion can be improved by covering it with rubber.All components can be assembled with 4mm diameter axis (in red on some pictures) – M4 screw, threaded shaft, etc.