A drive mechanism for a delivery device is presented containing, a rotator having an outer surface and an inner surface, the rotator configured to rotate from a first position to a second position. A needle cover having an inner surface that engages the outer surface of the rotator. A rotatable plunger rod having an outer surface configured to engage the inner surface of the rotator. When the rotator rotates from the first position to the second position, the rotatable plunger rod is rotated by the rotator and a plunger rod spring is released from a pre-tensioned state to exert a force on the rotatable plunger rod.