The invention relates to a portable liquid drug delivery device (102) comprising a driving part (104) comprising a driving mechanism (212) and a reservoir part (106) comprising- a reservoir (200) adapted to retain a liquid drug,- an outlet port (208) in fluid communication with the reservoir (200),- a threaded piston rod (202) with a piston (204),- an engagement member (400) having an inner thread (500) configured to engage with the thread of the piston rod (202) in an engagement position and to disengage from the thread of the piston rod (202) in a disengagement position, the engagement member (400) being prestressed in the disengagement position, the prestress being adapted to force the engagement member (400) to move from the disengagement position to the engagement position, the engagement member (400) being adapted for only converting in the engagement position a torque of the driving mechanism (212) into a linear movement of the piston rod (202), the piston (204) being adapted upon the linear movement of the piston rod (202) to deliver the liquid drug to the outlet port (208).
