A rotatable drive mechanism for a drug delivery device may include a lead screw having a distal end, a proximal end, and external threads, a bearing, and a ball screw driver having a threaded aperture wherein the threaded aperture is configured to rotatably engage with the external threads of the lead screw. The mechanism may further include a biasing device disposed between the bearing and the ball screw driver generating an axial drive force, and an activation device configured to release the biasing device and allowing the biasing device to expand from a compressed position to an extended position through which the axial drive force of the biasing device causes the ball screw driver to axially move toward the distal end of the lead screw and rotate the lead screw such that a threaded engagement between the lead screw and the ball screw driver bears the axial drive force.
