In one embodiment, a rotor is coupled to the support such that the rotor can rotate about an axis of rotation relative to the support. The rotor has a first surface for supporting a piece of flexible material at a diameter D about the axis of rotation. The carriage is coupled to the support such that the carriage is constrained to slide parallel to the axis of rotation of the rotor. A second plurality of pulleys comprises a third pulley, a fourth pulley, and a fifth pulley. The piece of flexible material is coupled to the rotor, then helically wound around the first surface of the rotor with a lead l, then wrapped around the third pulley, then wrapped around the first pulley, then wrapped around the fourth pulley, then wrapped around the second pulley, then wrapped around the fifth pulley, then coupled to the support.