A spindle for driving a bung of a cartridge. The spindle includes a generally circular shaft having an outer surface. The generally circular shaft extends from a distal end to a proximal end of said circular shaft. A first helical groove is provided along a first portion of the outer surface. The first helical groove having a first pitch. A second helical groove provided along a second portion of the outer surface of the generally circular shaft. The second helical groove overlapping the first helical groove. The second helical groove having a second pitch.

An actuator comprising a drive, at least one rotatable threaded element and at least one output element driven in translation thereby is described, characterized in that the threaded element has at least two regions of differing pitch and a rotational axis of the threaded element is inclined relative to a translational axis of the output element.