A torsion spring driven drug delivery device for delivering set doses of a liquid drug, comprising: - A housing (1, 101) storing a cartridge containing the liquid drug and a torsion spring (34, 134) for exerting a torque - A piston rod (15, 115, 215) having a threaded outer surface and a not circular cross-section, - A rotatable drive member (20, 120) mating the not circular cross-section (17, 117) of the piston rod (15,115) or having an inner thread (9, 109) mating the outer thread (16, 116) of the piston rod, - A nut member (5, 105) non-rotatable engaging the housing (1, 101) and having an inner thread mating the outer thread (16, 116) of the piston rod (15, 115) or having a cross-section mating the not circular cross-section (17, 117) of the piston rod (15, 115), such that the piston rod (15, 115) is moved axially forward in the housing when the drive member (20,120) is rotated relatively to the nut member, - A spring drive assembly (30, 130) which is rotatable by the torsion spring (34, 134), positioned between the housing (1, 101) and the spring drive assembly (30, 130), to rotate the rotatable drive member (20, 120) to move the piston rod (15,15) 25 forward in the housing (1, 101), and wherein, the rotatable drive member (20,120) is coupled to the spring drive assembly (30, 130) by an axial movable clutch element (40, 140) which can move axially between a first position and a second position, characterized in that i) in said first position, when no injection is being performed, the rotatable drive member (20, 120) is disconnected from the spring drive assembly (30, 130) such that the drive member (20, 120) is rotatable independently of the spring drive assembly (30, 130), thereby allowing proximal movement of the piston rod (15, 115), and ii) in said second position, when an injection is being performed, the rotatable drive member (20, 120) is connected to the spring drive assembly (30, 130) such that the spring drive assembly (30, 130) rotates the drive member (20, 120) to move the piston rod (15, 115) distally. - A housing (1, 101) storing a cartridge containing the liquid drug and a torsion spring (34, 134) for exerting a torque - A piston rod (15, 115, 215) having a threaded outer surface and a not circular cross-section, - A rotatable drive member (20, 120) mating the not circular cross-section (17, 117) of the piston rod (15,115) or having an inner thread (9, 109) mating the outer thread (16, 116) of the piston rod, - A nut member (5, 105) non-rotatable engaging the housing (1, 101) and having an inner thread mating the outer thread (16, 116) of the piston rod (15, 115) or having a cross-section mating the not circular cross-section (17, 117) of the piston rod (15, 115), such that the piston rod (15, 115) is moved axially forward in the housing when the drive member (20,120) is rotated relatively to the nut member, - A spring drive assembly (30, 130) which is rotatable by the torsion spring (34, 134), positioned between the housing (1, 101) and the spring drive assembly (30, 130), to rotate the rotatable drive member (20, 120) to move the piston rod (15,15) 25 forward in the housing (1, 101), and wherein, i) in said first position, when no injection is being performed, the rotatable drive member (20, 120) is disconnected from the spring drive assembly (30, 130) such that the drive member (20, 120) is rotatable independently of the spring drive assembly (30, 130), thereby allowing proximal movement of the piston rod (15, 115), and ii) in said second position, when an injection is being performed, the rotatable drive member (20, 120) is connected to the spring drive assembly (30, 130) such that the spring drive assembly (30, 130) rotates the drive member (20, 120) to move the piston rod (15, 115) distally.