A spindle (242； 414； 542； 642) for driving a bung of a cartridge is provided. The spindle includes a generally circular shaft having an outer surface (560). The generally circular shaft extends from a distal end to a proximal end of said circular shaft. A first helical groove (219； 519； 619) is provided along a first portion of the outer surface. The first helical groove having a first pitch. A second helical groove (221； 521； 621) 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.本發明提出一種用於驅動藥筒之塞子的心軸(242；414；542；642)。該心軸包含一具有一外表面(560)的大致圓形軸桿。該大致圓形軸桿從該圓形軸桿之一遠端延伸至一近端。一第一螺旋溝槽(219；519；619)沿該外表面之一第一部分提供。該第一螺旋溝槽具有一第一節距。一第二螺旋溝槽(221；521；621)沿該大致圓形軸桿外表面之一第二部分提供。該第二螺旋溝槽重疊於該第一螺旋溝槽。該第二螺旋溝槽具有一第二節距。
