Piston rod assembly for a drug delivery device

A drug delivery device for dispensing medicament, comprising:

• a first housing component configured to house a medicament cartridge;
• a medicament cartridge housed in the first housing component, wherein the medicament cartridge comprises an axially moveable piston;
• a second housing component interlocked with the first housing component, wherein the second housing component is configured to house a drive mechanism;
• and
• a drive mechanism housed in the second housing component, wherein the drive mechanism comprises a piston rod assembly configured to engage the piston of the medicament cartridge, and wherein the piston rod assembly comprises
• (i) a piston rod having a longitudinal axis and terminating at a distal end,
• (ii) an adjusting member displaceably engaged on the distal end of the piston rod, wherein the adjusting member is displaceable along the longitudinal axis of the piston rod while engaged on the distal end, and
• (iii) at least one interlock member configured to mutually lock the adjusting member and the piston rod together in an arbitrary relative axial position,
• wherein the interlock member is integral to the piston rod and comprises at least one resiliently biased tongue member, where the resiliently biased tongue member is configured to engage with a portion of a corrugated section of an outer surface of the adjusting member.

A method of assembling a drug delivery device, comprising:

• positioning a medicament cartridge in a first housing component, wherein the medicament cartridge comprises an axially moveable piston having a proximal end;
• positioning a drive mechanism in a second housing component, wherein the drive mechanism comprises a piston rod assembly having a distal end configured to engage the proximal end of the piston of the medicament cartridge, where the piston rod assembly comprises a piston rod having a distal end and an adjusting member, where the adjusting member is engaged and longitudinally displaceable on the distal end of the piston rod;
• determining an axial position of the piston with respect to the first housing component;
• determining an axial position of the piston rod assembly with respect to the second housing component;
• adjusting the axial position of the piston rod assembly by displacing the adjusting member relative to the piston rod to an axial distance between the distal end of the piston rod assembly and the proximal end of the piston;
• engaging a resiliently biased tongue member of the piston rod assembly with a portion of a corrugated section of an outer surface of the adjusting member;
• interconnecting the first housing component and the second housing component.