Abstract: This disclosure relates to an apparatus for converting rotational movement into linear movement. The apparatus includes a roller unit and a rotatable screw. The roller unit is adapted to be journaled on a fixed body, and has peripheral flanges axially spaced so as to have a substantially constant pitch. The screw is adapted to be journaled on a movable body in parallel with the roller unit, and to be rotatably driven. The screw includes an outer thread having a pitch for engagement with the flanges, and rotation of the screw causes the movable body to travel along the roller unit.