Drive mechanism for an injection device

An injection device for expelling of a number of preset or user-selectable doses of a medicament, the injection device comprising:

• a housing extending along an axial direction and configured to accommodate a cartridge containing the medicament and having a bung sealing a proximal end of the cartridge,
• a windup expelling mechanism comprising a piston rod, a mechanical energy reservoir, and a trigger, wherein one end of the mechanical energy reservoir is connected to the housing, wherein the trigger is movable between an idle position and a dose expelling position relative to the housing and configured to, when moved into the dose expelling position, release energy from the mechanical energy reservoir to the piston rod thereby axially driving the piston rod relative to the housing to urge against the bung,
• a dose setting mechanism comprising a handle for rotationally selecting a dose and/or for arming the windup expelling mechanism, a dose tracking member, a ratchet sleeve, and a driver, wherein the dose tracking member is rotatable relative to the housing within a range of positional states and operatively connectable to the handle for tracking a rotation thereof, wherein the ratchet sleeve is rotationally locked to the dose tracking member and wherein another end of the mechanical energy reservoir is connected to the ratchet sleeve, and
• wherein the driver is axially displaceable between a dose setting position and a dose dispensing position, wherein when in the dose dispensing position, the driver is rotationally locked to the piston rod and is rotationally disengaged from the handle, and wherein when in the dose setting position, the driver is rotationally disengaged from the piston rod and is rotationally locked to the handle.