Injection device comprising a locking nut

1. An injection device for injecting a dose of drug, the injection device comprising:

a housing,

a dose setting mechanism being operable to set a desired dose, the dose setting mechanism comprising a rotatable dose knob, operation of the dose setting mechanism causing energy to be stored in a spring member,

an injection mechanism comprising a piston rod adapted to cooperate with a piston positioned in a cartridge containing a drug to be delivered in order to cause a set dose to be delivered from the cartridge via the injection device, the injection mechanism being driven by releasing energy previously stored in the spring member during dose setting,

a dosage tube being axially movable in a proximal direction relatively to the housing during dose setting and being axially movable in a distal direction relatively to the housing during injection of a set dose, and

retaining means is a locking nut being axially fixed relatively to the housing, said locking nut being adapted to be rotationally locked relatively to the housing during dose setting, and adapted to be able to perform rotational movement relatively to the housing during injection of a set dose, the retaining means arranged to prevent axial movement of the dosage tube in a distal direction relatively to the housing during dose setting.

2. An injection device according to claim 1 , wherein the locking nut and the dosage tube are connected via mating threads formed on the dosage tube and the locking nut, respectively.

3. An injection device according to claim 1 , further comprising a locking item being movable between a locking position in which it prevents the locking nut from rotating relatively to the housing, and an unlocking position in which the locking nut is allowed to rotate relatively to the housing.

4. An injection device according to claim 3 , wherein mating teeth formed on the locking nut and the locking item engage when the locking item is in the locking position.

5. An injection device according to claim 3 , wherein the locking item is moved from the locking position to the unlocking position in response to operation of the injection mechanism.

6. An injection device according to claim 1 , wherein the dosage tube is prevented from performing rotational movements relatively to the housing during injection of a set dose.

7. An injection device according to claim 1 , wherein the dosage tube and the piston rod are connected via mating threads formed on the dosage tube and the piston rod, respectively.

8. An injection device according to claim 1 , further comprising means for preventing rotational movement of the piston rod during dose setting.

9. An injection device according to claim 8 , wherein the means for preventing rotational movement of the piston rod comprises a key and groove connection between the piston rod and a member being fixed relatively to the housing.

10. An injection device according to any of claim 8 , wherein the means for preventing rotational movement of the piston rod comprises a third thread connection provided between the piston rod and a member being fixed relatively to the housing.

11. An injection device according to claim 10 , wherein the third thread connection has a pitch being directed in a direction which is opposite to the direction of the outer thread of the piston rod engaging the inner thread of the dosage tube.

12. An injection device according to claim 1 , wherein the spring member is or comprises a compressible spring.

13. An injection device according to claim 1 , further comprising a release mechanism for releasing energy stored in the spring member, thereby causing a set dose to be injected.

14. An injection device according to claim 13 , wherein the release mechanism is axially movable.