Power Unit for Drug Delivery Device

A method of pre-straining a torsion spring for a drug delivery device, comprising:

• (i) prior to insertion in a housing of the drug delivery device, attaching a first end portion of the torsion spring to an interior surface of a hollow body of a first interface member extending along a reference axis, the first interface member comprising a first engagement structure,
• (ii) attaching a second end portion of the torsion spring to a second interface member comprising a tubular structure adapted to extend at least partially through the hollow body, a skirt on the tubular structure, and a second engagement structure,
• (iii) inducing a relative rotational motion about the reference axis between the first interface member and the second interface member, thereby bringing the torsion spring to a twisted state, and
• (iv) bringing the first interface member and the second interface member to a relative axial position in which the first engagement structure and the second engagement structure are engaged, thereby securing the torsion spring in the twisted state,
• wherein in the relative axial position of the first interface member and the second interface member the torsion spring is accommodated in a user inaccessible space defined by the hollow body and the skirt.

A method of mounting a power unit in a drug delivery device housing, the power unit comprising a first interface member comprising a first engagement structure, a second interface member comprising a second engagement structure, and a torsion spring comprising a first end portion (23) being attached to the first interface member and a second end portion being attached to the second interface member, the torsion spring being pre-strained to apply a torque for inducing a relative rotational motion between the first interface member and the second interface member, the method comprising:

• (i) inserting the power unit, while in a first state in which the first engagement structure and the second engagement structure are engaged to prevent the relative rotational motion between the first interface member and the second interface member, into the drug delivery device housing along a reference axis until the first interface member meets an axial stop,
• (ii) introducing a relative motion along the reference axis between the first interface member and the second interface member by urging the second interface member further into the drug delivery device housing along the reference axis, thereby bringing the power unit to a second state in which the first engagement structure and the second engagement structure are disengaged, and
• (iii) prior to the power unit reaching the second state rotationally releasably locking or restricting the second interface structure relative to the drug delivery device housing.