Drive mechanism for an injection device

An injection device for expelling a number of doses of a medicament, the injection device comprising:

• an elongated housing extending along a longitudinal axis and configured to accommodate a cartridge, the cartridge containing the medicament and having a bung sealing a proximal end of the cartridge;
• an expelling mechanism comprising a trigger and a piston rod, wherein when induced by the trigger the piston rod is configured to urge against the bung along the longitudinal axis in a distal direction relative to the housing;
• and
• a dose setting mechanism comprising a rotatable member and a counterpart member, wherein the counterpart member comprises a first mechanical pattern, and wherein the rotatable member comprises a second mechanical pattern complementary shaped to the first mechanical pattern, wherein the first mechanical pattern comprises at least one of a protrusion or a recess, and the second mechanical pattern comprises at least one of a complementary-shaped recess or protrusion,
• wherein for setting a dose the rotatable member is rotatable relative to the housing and relative to the counterpart member within a range of numerous rotational states, and wherein during setting of the dose, the rotatable member is constrained along the longitudinal axis relative to the housing or relative to the counterpart member,
• wherein for expelling the dose, one of the rotatable member or the counterpart member is displaceable along the longitudinal axis into an expelling position relative to the other one of the rotatable member or the counterpart member only when the first mechanical pattern is aligned with the second mechanical pattern.