Patent ID: 8608709

Claim:
An injection device for apportioning set doses of a drug from a reservoir comprising a housing ( 30 ), a piston rod ( 40 ) having a not circular cross-section ( 42 ) and an outer thread ( 41 ) a piston rod drive ( 50 , 60 ) comprising; a first drive element ( 50 , 60 ) mating the not-circular cross section ( 42 ) of the piston rod ( 40 ), a second drive element ( 50 , 60 ), which has an inner thread ( 53 ) mating the thread ( 41 ) of the piston rod ( 40 ), a dose setting and injection mechanism comprising a dose setting member ( 20 , 70 ) coupled to the piston rod drive ( 50 , 60 ) through a thread connection ( 63 , 73 ) along which the dose setting member ( 20 , 70 ) by rotation is screwed out from the proximal end of the housing ( 30 ) a distance determined by the angle of rotation during the setting of a dose and wherein the thread connection ( 63 , 73 ) transforms an axial movement of the dose setting member during expulsion of a dose into a rotation of one of the first or second drive elements ( 50 , 60 ) relative to the other, and wherein a scale drum is axially supported by the dose setting member ( 20 , 70 ), characterized in that, the scale drum element ( 80 ) further is rotatably connected to the dose setting member ( 20 , 70 ) through a releasable coupling ( 23 , 83 ) such that the scale drum ( 80 ) in one position of a releasable coupling ( 23 , 83 ) follows rotation of the dose setting member ( 20 , 70 ) and in a different position is rotatable released from the dose setting member ( 20 , 70 ), and wherein during setting of a dose the scale drum ( 80 ) is coupled to the dose setting member and wherein during dose administration the scale drum is decoupled.