POWER UNIT FOR USE IN AN AUTOINJECTOR AND METHOD OF ASSEMBLING SUCH POWER UNIT

A power unit (15) for use in an autoinjector (10) for driving a plunger (500) distally along an axis in an expelling movement so as to expel a drug from a held drug container, the power unit (15) comprising: a) a plunger (500) comprising retaining geometries (515) and defining radially protruding click protrusions (525), b) a drive spring (550) arranged in a tensed state with a force biasing the plunger (500) distally, and c) a power base (400) coupled to the drive spring (550) and the plunger (500), the power base (400) comprising a base part grounding the drive spring (550), and retaining elements (410) releasably engaging retaining geometries (515) of the plunger (500), the power base (400) further defining a resilient click arm (410) configured to cooperate with the click protrusions (525) of the plunger (500), the click arm (410) being moved radially to provide a click noise for each click protrusion that passes the click arm during the expelling movement, wherein the click arm (410) comprises a straining surface geometry (415d) arranged to cooperate, in a preparing step, with a tool (20) arranged distally relative to the click arm for moving the click arm (410) radially into a tensed state enabling the plunger (500) to be moved proximally. A method of assembling the power unit (15) is further disclosed.