Control of plunger position in an injection device

An injection device comprising:

• a housing;
• a receptacle for receiving a medicine cartridge and configured for attachment to a proximal end of the housing;
• a leadscrew having an end for engaging with a bung of a cartridge, the leadscrew having a first thread extending axially along an outer surface thereof and having at least one axially extending guide track;
• a drive gear having a second thread on an inner surface thereof, the leadscrew and the drive gear being rotatably coupled by means of said first and second threads;
• a drive mechanism for causing relative rotation of said leadscrew and said drive gear in order to deliver a dose from the cartridge;
• a locking bush located proximal to said drive gear and being engaged with the or each guide track of the leadscrew to allow relative axial movement of the leadscrew and the locking bush while preventing relative rotational movement, the locking bush being configured to allow rotation of the leadscrew when the receptacle is disconnected from the housing and to prevent rotation when the receptacle is connected to the housing;
• and
• a stop member located proximal to said locking bush and being engaged with the or each guide track of said leadscrew to allow relative axial movement of the leadscrew and the stop member while allowing the two components to rotate together, said stop collar and a profile of the or each guide track being configured to define an end stop for the leadscrew in the proximal direction.