A drug delivery device (10) comprises a shaft (20) a drive system (30) operably connected to the shaft and configured to only rotate the shaft in a first direction a drug reservoir (12) having a plunger (16) used to adjust a volume of the drug reservoir a piston rod (14) operably connected at a first end to the plunger and providing threads (70) on an exterior surface thereof, the piston rod being operably connected to the shaft, wherein said drug delivery device provides a secured position which permits the rotation of the shaft to drive the piston rod and move the plunger to adjust the volume of the drug reservoir, and an unsecured position which permits the drug reservoir to be removed unobstructed from the drug delivery device and replaced without having to rotate the shaft in a direction opposite to the first direction. Said drug delivery device further comprises a partially threaded nut (74) that is spring (84) biased, which allows engagement with the threads of the piston rod in a first position to permit plunger (16) movement via rotation of the shaft, and which disengages with the threads of the piston rod in a second position, thereby preventing plunger movement with or without shaft rotation.
