A drug delivery device for dispensing a dose of a liquid medicament with a rotationally driveable expelling mechanism adapted to expel the dose of the liquid medicament from a medicament container, a round torsional drive spring adapted to apply a driving torque to the rotationally driveable expelling mechanism when energized by an external strain, wherein the round torsional drive spring encloses an interior space of varying shape and/or extension according to the level of external strain applied to the round torsional drive spring, and at least one restraining element defining a number of rest surfaces to internally support the round torsional drive spring increasingly with the level of external strain applied thereto, can be improved by providing the rest surfaces with an outer shape as to essentially maintain the centre of the interior space throughout the entire variation in external strain applicable to the round torsional drive spring.
