Patent ID: 8257318

Claim:
An injection device for apportioning set doses of a drug from a reservoir comprising: a housing, a piston rod that has a non-circular cross section and an outer thread and rotates during expelling of the drug from the reservoir, and a piston rod drive comprising: a first drive element mating the non-circular cross section of the piston rod to allow axial displacement but not rotation of the piston rod in relation to first drive element, a second drive element which has an inner thread mating the thread of the piston rod, a dose setting and injection mechanism comprising a dose setting member associated with the piston rod drive through a thread connection along which the dose setting member is structured to be screwed away from the proximal end of the housing a distance determined by the angle of rotation and which thread connection by axial returning of the dose setting member transforms this axial movement to a rotation of one of the first or second drive elements relative to the other, wherein a scale drum is releaseably coupled to the dose setting member to follow rotation of the dose setting member when a dose is set and which scale drum is released from the dose setting member and coupled to the piston rod drive when the set dose is expelled.