US Patent 7931069 Active tension device for a window covering

A tension device that prevents a window covering from operating properly until the tension device is installed. The tension device incorporates a drive wheel, casing, anchoring arm and drive stud. The drive stud has two sections, one keyed and one cylindrical. Before the tension device is installed, the keyed section of the drive stud engages the drive wheel and the casing, thereby preventing them from rotating relative to each other. When the tension device is installed, the drive stud must be inserted into the anchoring arm causing the keyed section of the drive stud to disengage the drive wheel and thereby allow the drive wheel to rotate relative to the casing.