Drive mechanism for a drug delivery device and a method for assembling a drug delivery device

1. A drive mechanism for a drug delivery device, the drive mechanism comprising:

a piston rod comprising a first piston rod member and a second piston rod member; and

an adjustment member, wherein the drive mechanism has a first state in which the first and the second piston rod member are moveable with respect to each other by operating the adjustment member, thereby adjusting a length of the piston rod, and

wherein the adjustment member is arranged at least partially inside the second piston rod member and wherein the adjustment member is configured to be moved completely into the second piston rod member.

2. The drive mechanism according to claim 1 , wherein the drive mechanism has a second state in which the first piston rod member is fixed to the second piston rod member to define a fixed length of the piston rod.

3. The drive mechanism according to claim 1 ,

wherein the adjustment member comprises a first connection member,

wherein the first piston rod member comprises a second connection member, and wherein the first connection member and the second connection member are configured to prevent a rotational movement of the adjustment member and the first piston rod member relative to each other and to permit a relative translational movement of the adjustment member and the first piston rod member when the first connection member and the second connection member are connected to each other.

4. The drive mechanism according to claim 1 ,

wherein the adjustment member comprises a third connection member, and

wherein the second piston rod member comprises a fourth connection member, and wherein the third connection member and the fourth connection member are configured to prevent a rotational movement of the adjustment member and the second piston rod member relative to each other and to permit a relative axial movement of the adjustment member and the second piston rod member when the third connection member and the fourth connection member are connected to each other.

5. The drive mechanism according to claim 4 , wherein the first piston rod member is moveable relative to the second piston rod member when the third connection member and the fourth connection member are not connected to each other.

6. The drive mechanism according to claim 4 , wherein a relative movement of the first piston rod member and the second piston rod member is prevented when the third connection member and the fourth connection member are connected to each other.

7. The drive mechanism according to claim 1 , wherein the first piston rod member comprises a fifth connection member and the second piston rod member comprises a sixth connection member, and

wherein when the fifth connection member and the sixth connection member are connected to each other such that a movement between the first and the second piston rod member is restricted to a concurrent rotational and translational movement.

8. The drive mechanism according to claim 1 ,

wherein the first piston rod member is arranged at least partly inside the second piston rod member, and

wherein the first piston rod member is arranged at least partly inside the adjustment member.

9. The drive mechanism according to claim 1 , wherein the adjustment member is arranged such that a rotational movement of the adjustment member relative to the second piston rod member advances the first piston rod member relative to the second piston rod member.

10. The drive mechanism according to claim 1 , further comprising a spring member that is adapted to move the piston rod after adjustment of the length of the piston rod in a proximal direction.

11. A method for assembling a drug delivery device, the method comprising:

providing a first piston rod member, a second piston rod member and an adjustment member;

arranging the adjustment member at least partly inside the second piston rod member;

operating the adjustment member to move the first piston rod member with respect to the second piston rod member; and

coupling the first piston rod member with the second piston rod member such that movement between the first piston rod member and the second piston rod member is prevented,

wherein the drive mechanism has a first state in which the first and the second piston rod member are moveable with respect to each other by operating the adjustment member, thereby adjusting a length of the piston rod, wherein coupling the first piston rod member with the second piston rod member comprises moving the adjustment member completely into the second piston rod member such that the adjustment member is arranged inside the second piston rod member to a greater extent than in the first state of the drive mechanism.

12. The method of claim 11 , further comprising:

providing a cartridge comprising a bung, wherein the movement of the first piston rod member in a distal direction is stopped when the first piston rod member abuts the bung.

13. The method of claim 11 ,

wherein the drug delivery device further comprises a spring member, and wherein the method comprises:

applying a force on the spring member during the movement of the first piston rod member in the distal direction; and

after coupling the first piston rod member and the second piston rod member to each other, releasing the spring member whereby the piston rod is moved in a proximal direction by a predefined distance.

14. A drive mechanism for a drug delivery device, the drive mechanism comprising:

a piston rod comprising a first piston rod member and a second piston rod member; and

an adjustment member, wherein the drive mechanism has a first state in which the first and the second piston rod member are moveable with respect to each other by operating the adjustment member, thereby adjusting a length of the piston rod,

wherein the adjustment member is arranged at least partially inside the second piston rod member,

wherein the adjustment member comprises a third connection member,

wherein the second piston rod member comprises a fourth connection member, and wherein the third connection member and the fourth connection member are configured to prevent a rotational movement of the adjustment member and the second piston rod member relative to each other and to permit a relative axial movement of the adjustment member and the second piston rod member when the third connection member and the fourth connection member are connected to each other, and

wherein the first piston rod member is moveable relative to the second piston rod member when the third connection member and the fourth connection member are not connected to each other.

15. A drive mechanism for a drug delivery device, the drive mechanism comprising:

a piston rod comprising a first piston rod member and a second piston rod member; and

an adjustment member, wherein the drive mechanism has a first state in which the first and the second piston rod member are moveable with respect to each other by operating the adjustment member, thereby adjusting a length of the piston rod,

wherein the adjustment member is arranged at least partially inside the second piston rod member,

wherein the first piston rod member comprises a fifth connection member and the second piston rod member comprises a sixth connection member, and

wherein when the fifth connection member and the sixth connection member are connected to each other such that a movement between the first and the second piston rod member is restricted to a concurrent rotational and translational movement.