A drive mechanism for a delivery device or a positioning mechanism comprising a rod (5), a rotatable cam member (9), and a follower member (6) being coupled to the cam member (9) in such a manner that a rotation of the cam member (9) is transferred to a reciprocating movement of the follower member (6) or a part thereof (7), the follower member (6) being coupled to the rod (5) via a ratchet mechanism (62, 64) in such a manner that the reciprocating movement of the follower member (6) or the part thereof (7) drives the rod (5).
