Drive mechanism

1. A drive mechanism for a medicament delivery device, comprising

a plunger rod arranged to act on a stopper arranged in a medicament container;

a compression drive spring operably connected to said plunger rod and configured to apply a first force on said plunger rod; and

a spring guide arranged coaxial with, and radially outside, said compression drive spring,

wherein the spring guide is configured to prevent buckling of said drive spring,

wherein said spring guide comprises a tubular body and a resilient force element configured to apply an expansive second force between said medicament container and the tubular body.

2. The drive mechanism according to claim 1 , the tubular body extending along a length of said compression drive spring.

3. The drive mechanism according to claim 1 , wherein said compression drive spring is arranged coaxially with and radially outside said plunger rod.

4. The drive mechanism according to claim 1 , wherein said resilient force element comprises a spring-guide spring arranged between a proximally facing surface of said tubular body and a distally facing surface of said medicament container.

5. The drive mechanism according to claim 4 , wherein said spring-guide spring is arranged coaxial with, and radially outside said compression drive spring.

6. The drive mechanism according to claim 5 , wherein said spring-guide spring has a winding direction that is opposite a winding direction of said compression drive spring.

7. The drive mechanism according to claim 1 , wherein said resilient force element is integrated with the tubular body of the spring guide.

8. A medicament delivery device, comprising:

a drive mechanism comprising:

a plunger rod arranged to act on a stopper in a medicament container;

a compression drive spring operably connected to said plunger rod and configured to apply a first force on said plunger rod, and

a spring guide arranged coaxial with, and radially outside, said compression drive spring, wherein the spring guide is configured to prevent buckling of said compression drive spring, wherein said spring guide comprises a tubular body and a resilient force element configured to apply an expansive second force between said medicament container and the tubular body; and

a housing arranged to accommodate said drive mechanism and said medicament container.

9. The medicament delivery device according to claim 8 , the tubular body extending along a length of said compression drive spring.

10. The medicament delivery device according to claim 8 , wherein said compression drive spring is arranged coaxially with and radially outside said plunger rod.

11. The medicament delivery device according to claim 8 , wherein said resilient force element comprises a spring-guide spring arranged between a proximally facing surface of said tubular body and a distally facing surface of said medicament container.

12. The medicament delivery device according to claim 11 , wherein said spring-guide spring is arranged coaxial with, and radially outside said compression drive spring.

13. The medicament delivery device according to claim 12 , wherein said spring-guide spring has a winding direction that is opposite a winding direction of said compression drive spring.

14. The medicament delivery device according to claim 11 , wherein the resilient force element is integrated with the tubular body of the spring guide.

15. The medicament delivery device according to claim 8 , further comprising:

a member guard in a central passage of the housing and moveable along a longitudinal axis; and

a rotator coupled to the member guard, wherein the rotator axially retains the plunger rod in a distal position in the housing,

wherein distal movement, along the longitudinal axis, of the member guard relative to the housing causes the rotator to rotate about the longitudinal axis, thereby releasing the plunger rod from the rotator such that the plunger rod moves proximally, relative to the housing, due to the first force applied to the plunger rod by the compression drive spring.

16. The medicament delivery device according to claim 15 , wherein the rotator is coaxial with and radially outside the spring guide.

17. The medicament delivery device according to claim 15 , wherein the rotator comprises a passage having a cross-sectional shape defined by one or more cutout sectors and one or more land sectors, and

wherein a distal end of the plunger rod comprises one or more wings configured to: (i) engage the one or more land sectors when the plunger rod is in the distal position of the housing prior to rotation of the rotator, and (ii) pass through the one or more cutout sectors responsive to rotation of the rotator.

18. The medicament delivery device according to claim 17 , further comprising a cap configured to prevent rotation of the plunger rod while the rotator rotates.

19. The medicament delivery device according to claim 15 , wherein the spring guide is coupled to and rotationally fixed with the rotator.

20. The medicament delivery device according to claim 15 , wherein a proximal end of the compression drive spring engages a portion of the plunger rod configured to engage the stopper of the medicament container, and

wherein a distal end of the compression drive spring engages an end wall of the rotator.