ZERO-POINT ADJUSTMENT OF PREFILLED DRUG DELIVERY DEVICE

Abstract

The present invention provides a solution for zero-point adjustment of drug delivery devices, including twin chamber drug delivery devices with a single dose engine.

Claims

1. A drug delivery device comprising: a cartridge unit comprising a cartridge holder carrying a drug cartridge having a cartridge body and a piston structure, and a dose expelling unit comprising: a housing extending along a longitudinal axis, a piston rod for pressurising the drug cartridge, the piston rod comprising a contact surface adapted to contact the piston structure, and a non-self-locking thread, a nut member rotationally fixed with respect to the housing and engaged with the non-self-locking thread, the nut member carrying a first coupling part of a unidirectional ratchet coupling, and a piston rod drive member axially fixed with respect to the housing and rotatable about the longitudinal axis, the piston rod drive member being operatively coupled with a second coupling part of the unidirectional ratchet coupling, wherein the piston rod drive member and the piston rod are rotationally interlocked, such that a rotation of the piston rod drive member in a first direction is associated with distal movement of the piston rod through the nut member, and a rotation of the piston rod drive member in a second direction is associated with proximal movement of the piston rod through the nut member, wherein in a pre-assembled state the nut member assumes a first nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are disengaged and in an assembled state the nut member assumes a second nut member position relative to the housing and the piston rod drive member in which the first coupling part and the second coupling part are engaged to render the piston rod drive member unidirectionally rotatable in the first direction, and wherein the nut member is configured to undergo irreversible displacement from the first nut member position to the second nut member position during assembly of the dose expelling unit and the cartridge unit in response to a converging relative axial motion between the housing and the cartridge holder, after contact between the piston structure and the contact surface has been established.

2. The drug delivery device according to claim 1, wherein the cartridge holder comprises a proximal rim portion which applies a proximally directed force to the nut member during the converging relative axial motion between the housing and the cartridge holder, the proximally directed force bringing the nut member from the first nut member position to the second nut member position.

3. The drug delivery device according to claim 1, wherein the cartridge unit further comprises: a second drug cartridge having a second cartridge body and a second piston structure, the drug cartridge and the second drug cartridge being held in parallel by the cartridge holder, and the dose expelling unit further comprises: a second piston rod for pressurising the second drug cartridge, the second piston rod comprising a second contact surface adapted to contact the second piston structure, and a second piston rod thread in engagement with a second thread of the nut member, and a second piston rod drive member axially fixed with respect to the housing and rotatable about a second axis parallel with the longitudinal axis, the second piston rod drive member and the second piston rod being rotationally interlocked, and the second piston rod drive member being rotationally coupled with the piston rod drive member, and wherein the second piston rod has an axially variable dimension which is set during assembly of the dose expelling unit and the cartridge unit, after contact between the second piston structure and the second piston rod has been established.

4. The drug delivery device according to claim 3, wherein the second piston rod comprises a front piston rod portion and a rear piston rod portion capable of undergoing relative sliding motion along one another, and wherein the axially variable dimension is set by axially interlocking the front piston rod portion and the rear piston rod portion.

5. The drug delivery device according to claim 4, wherein the piston rod drive member and the second piston rod drive member are arranged in a casing which is fixed to the housing, wherein the casing is provided with a distal slot and a proximal slot axially spaced apart from the distal slot, and wherein the nut member comprises a protrusion adapted to move from a position in the distal slot to a position in the proximal slot during movement of the nut member from the first nut member position to the second nut member position and structured to prevent subsequent movement of the nut member towards the first nut member position.

6. The drug delivery device according to claim 4, further comprising a central gearwheel, wherein the piston rod drive member comprises a first gearwheel toothing and the second piston rod drive member comprises a second gearwheel toothing, and wherein the piston rod drive member and the second piston rod drive member are rotationally coupled via the central gearwheel.

7. The drug delivery device according to claim 6, wherein the second coupling part forms part of the central gearwheel.

8. The drug delivery device according to claim 4, wherein the second piston rod thread is self-locking.

9. The drug delivery device according to claim 4, wherein the front piston rod portion and the rear piston rod portion are interlocked by laser welding through a bore in the cartridge holder.

10. The drug delivery device according to claim 4, wherein the drug cartridge further has a drug outlet sealed by a penetrable septum, and the second drug cartridge further has a second drug outlet sealed by a second penetrable septum, and wherein when the penetrable septum and the second penetrable septum are axially aligned relative to the cartridge holder the second piston structure is positioned distally of the piston structure.

11. A method of performing zero-point adjustment of a drug delivery device comprising A) a cartridge unit comprising a cartridge holder carrying a drug cartridge having a cartridge body and a piston structure, and B) a dose expelling unit comprising a housing extending along a longitudinal axis, a piston rod for pressurising the drug cartridge, the piston rod comprising a contact surface adapted to contact the piston structure, and a non-self-locking thread, a nut member rotationally fixed with respect to the housing and engaged with the non-self-locking thread, the nut member carrying a first coupling part of a unidirectional ratchet coupling, and a piston rod drive member axially fixed with respect to the housing and rotatable about the longitudinal axis, the piston rod drive member being operatively coupled with a second coupling part of the unidirectional ratchet coupling, wherein the piston rod drive member and the piston rod are rotationally interlocked, such that a rotation of the piston rod drive member in a first direction is associated with distal helical movement of the piston rod in the nut member, and a rotation of the piston rod drive member in a second direction is associated with proximal helical movement of the piston rod in the nut member, the method comprising: (i) arranging the piston rod in the nut member such that the contact surface is positioned distally of a final contact surface assembly position, (ii) bringing the piston structure and the contact surface into mutual abutment, (iii) inducing a converging relative axial motion between the cartridge holder and the housing, thereby firstly forcing the piston rod proximally in the nut member by structure of the piston structure and secondly moving the nut member axially relative to the housing from a first nut member position in which the first coupling part and the second coupling part are disengaged to a second nut member position in which the first coupling part and the second coupling part are engaged, rendering the piston rod drive member unidirectionally rotatable in the first direction, and (iv) axially interlocking the cartridge holder and the housing.

12. The method according to claim 11, wherein bringing the piston structure and the contact surface into mutual abutment involves inducing a converging relative axial motion between the cartridge holder and the housing.

13. A method of performing zero-point adjustment of a drug delivery device comprising: a cartridge unit comprising a cartridge holder carrying; a first drug cartridge having a first cartridge body and a first piston structure, and a second drug cartridge having a second cartridge body and a second piston structure, and a dose expelling unit comprising a housing extending along a longitudinal central axis, a first piston rod for pressurising the first drug cartridge, the first piston rod comprising a first contact surface adapted to contact the first piston structure, and a non-self-locking thread, a second piston rod for pressurising the second drug cartridge, the second piston rod comprising a front piston rod portion having a second contact surface adapted to contact the second piston structure, and a rear rod portion having a second piston rod thread, the front piston rod portion and the rear piston rod portion being capable of undergoing relative sliding motion along one another, a nut member rotationally fixed with respect to the housing and respectively engaged with the non-self-locking thread and the second piston rod thread, the nut member carrying a first coupling part of a unidirectional ratchet coupling, a first piston rod drive member axially fixed with respect to the housing and rotatable about a first axis parallel with the longitudinal central axis, the first piston rod drive member being operatively coupled with a second coupling part of the unidirectional ratchet coupling, and the first piston rod drive member and the first piston rod being rotationally interlocked such that a rotation of the first piston rod drive member in a first direction about the first axis is associated with distal helical movement of the first piston rod in the nut member, and a rotation of the first piston rod drive member in a second direction about the first axis is associated with proximal helical movement of the first piston rod in the nut member, and a second piston rod drive member axially fixed with respect to the housing and rotatable about a second axis parallel with the longitudinal central axis, the second piston rod drive member being rotationally coupled with the first piston rod drive member, and the second piston rod drive member and the rear piston rod portion being rotationally interlocked such that a rotation of the second piston rod drive member in a first direction about the second axis is associated with distal helical movement of the rear piston rod portion in the nut member, and a rotation of the second piston rod drive member in a second direction about the second axis is associated with proximal helical movement of the rear piston rod portion in the nut member, the method comprising: (i) arranging the first piston rod and the second piston rod in the nut member such that the first contact surface is positioned distally of a final first contact surface assembly position and the second contact surface is positioned distally of a final second contact surface assembly position, (ii) aligning the cartridge holder and the housing along the longitudinal central axis such that the first piston structure is aligned with the first axis and the second piston structure is aligned with the second axis, (iii) inducing a converging relative axial motion between the cartridge holder and the housing, said motion comprising a first part motion which brings the cartridge holder and the housing to an intermediate assembly position and in the course of which the first contact surface is moved proximally relative to the nut member by structure of the first piston structure and the second contact surface is moved proximally relative to the rear piston rod portion, the second piston rod thereby obtaining an intermediate second piston rod assembly configuration, and a second part motion which brings the cartridge holder and the housing to a final assembly position and during which the first piston rod, the second piston rod, and the nut member are moved jointly relative to the first piston rod drive member, the first coupling part of the unidirectional ratchet coupling thereby entering into engagement with the second coupling part of the unidirectional ratchet coupling, rendering the first piston rod drive member unidirectionally rotatable in the first direction about the first axis and the second piston rod drive member unidirectionally rotatable in the first direction about the second axis, (iv) axially interlocking the front piston rod portion and the rear piston rod portion in the intermediate second piston rod assembly configuration, and (v) axially interlocking the cartridge holder and the housing in the final assembly position.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In the following the invention will be further described with references to the drawings, wherein

[0061] FIG. 1 is an exploded view of a reservoir sub-assembly and a piston rod sub-assembly of a drug delivery device according to an embodiment of the invention,

[0062] FIG. 2 is a longitudinal section view of the reservoir sub-assembly and the piston rod sub-assembly,

[0063] FIGS. 3-6 are longitudinal section views of the reservoir sub-assembly and the piston rod sub-assembly in respective states during a zero-point adjustment procedure according to an embodiment of the invention,

[0064] FIGS. 7a and 7b are partially sectioned, perspective views of a nut member and a transmission case in different assembly states,

[0065] FIGS. 8a and 8b are side-views of the different assembly states of FIGS. 7a and 7b, and

[0066] FIG. 9 is a longitudinal section view of an assembled drug delivery device.

[0067] In the figures like structures are mainly identified by like reference numerals.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0068] When/If relative expressions, such as “upper” and “lower”, “left” and “right”, “horizontal” and “vertical”, “clockwise” and “counter-clockwise”, etc., are used in the following, these refer to the appended figures and not necessarily to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as their relative dimensions are intended to serve illustrative purposes only.

[0069] FIG. 1 is an exploded view of a reservoir sub-assembly 10 (FIG. 2) and a piston rod sub-assembly 40 (FIG. 2) of a drug delivery device 1 (FIG. 5) according to an embodiment of the invention. The reservoir sub-assembly 10 comprises a cartridge holder 11 of a generally oval shape, which enables accommodation of a first cartridge 20 holding a first medium and a second cartridge 30 holding a second medium. The first cartridge 20 has a generally cylindrical first cartridge body 21 and is closed by a first cartridge septum 22, respectively a first piston 25 (FIG. 2). The second cartridge 30 has a generally cylindrical second cartridge body 31 and is closed by a second cartridge septum 32, respectively a second piston 35 (FIG. 2). In this exemplary embodiment of the invention the second cartridge 30 is shorter than the first cartridge 20, and the first piston 25 is placed closer to the proximal end of the cartridge holder 11 than the second piston 35 is, the latter being clear e.g. from FIG. 2.

[0070] The cartridge holder 11 has a distal end portion 14 with a transversal distal end face 15, in which a first opening 16 and a second opening 18 are formed, and a side wall protrusion 17 for reception of a manifold needle unit (not shown). The first cartridge 20 and the second cartridge 30 are arranged in parallel within the cartridge holder 11 such that the first cartridge septum 22 is positioned adjacent the first opening 16 and the second cartridge septum 32 is positioned adjacent the second opening 18 to thereby allow for easy penetration of the respective cartridge septa by dedicated rear needles (not shown) in the manifold needle unit. At its proximal end portion the cartridge holder 11 is provided with a number of protrusions 12 for snap-fit engagement with a housing 2 (FIG. 5) of the drug delivery device 1. A first piston washer 29 is arranged in abutment with the first piston 25, and a second piston washer 39 is arranged in abutment with the second piston 35.

[0071] The piston rod sub-assembly 40 comprises a first piston rod structure 44, a second piston rod structure 50 (FIG. 2), a nut member 60, and a transmission case 70 having a bearing 71 which supports a first gearwheel 67, a second gearwheel 69, and a central gearwheel 68.

[0072] The first piston rod structure 44 comprises a first piston rod 45 which is provided with a non-self-locking thread 46 and has a first contact surface 48 adapted to abut the first piston washer 29 and apply a driving force to the first piston 25 therethrough.

[0073] The second piston rod structure 50 is a two-part structure. One part comprises a second piston rod 51 having a section with a self-locking thread 52 and a projecting portion 54. Another part comprises an extension 55 having a hollow rear portion 56 adapted to slidably receive the projecting portion 54 and a solid front portion 57 ending in a second contact surface 58 which is adapted to abut the second piston washer 39 and apply a driving force to the second piston 35 therethrough. In a pre-assembled state, before final assembly of the drug delivery device 1, the second piston rod 51 and the extension 55 are capable of sliding relative motion along a longitudinal axis of the second piston rod 51, and the axial extension of the second piston rod structure 50 is thereby variable.

[0074] In the present embodiment the first piston washer 29 and the second piston washer 39 form part of the reservoir sub-assembly 10. It is noted, however, that they may alternatively form part of the piston rod sub-assembly, the first piston washer 29 being attached to, or integral with, the first piston rod 45 at the first contact surface 48, and the second piston washer 39 being attached to, or integral with, the extension 55 at the second contact surface 58.

[0075] The nut member 60 comprises a first nut 61a, which is in threaded engagement with the non-self-locking thread 46 and thereby supports the first piston rod 45, and a second nut 61b which is in threaded engagement with the self-locking thread 52 and thereby supports the second piston rod 51. Respective nut walls 63 extend proximally from the first nut 61a and the second nut 61b. Each nut wall 63 carries a protrusion 64 for coupling the nut member 60 to the transmission case 70, in a manner that will become clear from the below. The nut member 60 also has four legs 62 extending distally from the first nut 61a and the second nut 61b, arranged to interface with a proximal rim portion of the cartridge holder 11 during assembly of the drug delivery device 1.

[0076] In the bearing 71 the first gearwheel 67 has a first gearwheel toothing 67t, the second gearwheel 69 has a second gearwheel toothing 69t and the central gearwheel 68 has a central gearwheel toothing 68t which mesh with both the first gearwheel toothing 67t and the second gearwheel toothing 69t, thereby rotationally coupling the first gearwheel 67 and the second gearwheel 69. The central gearwheel 68 further has a ratchet toothing 68r arranged distally of the central gearwheel toothing 68t.

[0077] The transmission case 70 further comprises four guide rods 74 and a pair of unlocked state slots 72, respectively locked state slots 73 adapted for reception of the respective protrusions 64 on the nut walls 63 to define two distinct coupling states of the nut member 60 and the transmission case 70.

[0078] The first gearwheel 67 is arranged about and rotationally locked with respect to the first piston rod 45, and the second gearwheel 69 is arranged about and rotationally locked with respect to the second piston rod 51. A rotation of the first gearwheel 67 thus causes a corresponding rotation of the first piston rod 45, and vice versa, and a rotation of the second gearwheel 69 causes a corresponding rotation of the second piston rod 51, and vice versa.

[0079] FIG. 2 is a longitudinal section view of the reservoir sub-assembly 10 and the piston rod sub-assembly 40 in respective assembled states. The figure shows the parallel arrangement of the first cartridge 20, with a first chamber 26 holding first contents, and the second cartridge 30, with a second chamber 36 holding second contents, and reveals a bore 19 in the cartridge holder 11, the purpose of which will be described in the below.

[0080] The first piston rod 45 extends through the first nut 61a past the legs 62, and the first contact surface 48 is deliberately positioned more distally of the nut member 60 than it will be in the final assembly position of the first piston rod structure 44. Similarly, the second piston rod 51 extends through the second nut 61b, and the second contact surface 58 is deliberately positioned more distally of the nut member 60 than it will be in the final assembly position of the second piston rod structure 50. A portion of the projecting portion 54 resides within the hollow rear portion 56 of the extension 55 and is friction fitted therewith.

[0081] To ensure precision of the dose administration system it is essential that there is no air gap between the first piston washer 29 and the first piston rod structure 44, respectively between the second piston washer 39 and the second piston rod structure 50 in the final assembly state of the drug delivery device 1. A specific axial displacement of either piston rod structure as a consequence of an activation of a dose expelling mechanism (illustrated in FIG. 5) in the drug delivery device 1 is thereby transferred directly to the associated piston.

[0082] Accordingly, to eliminate the risk of potential air gaps a zero-point adjustment is performed by the manufacturer during assembly of the drug delivery device 1. The construction of the piston rod sub-assembly 40 allows for a particularly simple zero-point adjustment procedure, as will be described hereafter with reference to FIGS. 3-6.

[0083] FIG. 3 is a longitudinal section view of the reservoir sub-assembly 10 and the piston rod sub-assembly 40 at the onset of the zero-point adjustment procedure where the first piston washer 29 has been brought in abutment with the first contact surface 48, by converging relative axial motion between the cartridge holder 11 and the transmission case 70, and there is a clearance, X, between the proximal rim of the cartridge holder 11 and the distal end of the respective legs 62. The transmission case 70 is axially and rotationally fixed in a housing 2 (FIG. 5) of the drug delivery device 1 at this stage. However, for the sake of clarity the housing 2 has been omitted from FIGS. 3-6.

[0084] The initial respective positions of the first contact surface 48 and the second contact surface 58 relative to the nut member 60 are in principle correlated with the respective positions of the first piston 25 in the first cartridge body 21 and the second piston 35 in the second cartridge body 31, and FIG. 3 does in fact also show that the second piston washer 39 abuts the second contact surface 58. However, due to various manufacturing tolerances the second piston washer 39 may not be brought in abutment with the second contact surface 58 at the same time as the first piston washer 29 touches the first contact surface 48. Regardless of which contact is established first though, the design of the piston rod sub-assembly 40 ensures that the second contact is established subsequently by mere converging relative axial motion between the cartridge holder 11 and the transmission case 70.

[0085] In the shown state of the piston rod sub-assembly 40, which is further illustrated in FIGS. 7a and 8a showing a perspective view, respectively a side view of the nut member 60 and the transmission case 70 in a first of the aforementioned two distinct coupling states (the transmission case being partially sectioned in the perspective view for the sake of clarity), the central gearwheel 68 is capable of rotation in both directions about its own axis of extension. This is because the protrusion 64 occupies the unlocked state slot 72 (FIG. 8a) and the ratchet arm 65 is axially spaced apart from the ratchet toothing 68r.

[0086] Hence, if the first piston washer 29 reaches the first contact surface 48 before the second piston washer 39 reaches the second contact surface 58 a continued converging relative axial motion between the cartridge holder 11 and the transmission case 70 will cause the piston washer 29 to press the first piston rod 45 backwards due to the first chamber 26 being filled with an incompressible liquid. Since the first piston rod 45 is engaged with the first nut 61a via the non-self-locking thread 46 the axial force from the first piston washer 29 will cause a helical proximal motion of the first piston rod 45 as it rotates through the first nut 61a.

[0087] The rotation of the first piston rod 45 will cause a rotation of the first gearwheel 67, which, due to the meshing of the first gearwheel toothing 67t and the central gearwheel toothing 68t, will cause a rotation of the central gearwheel 68, which in turn, due to the meshing of the central gearwheel toothing 68t and the second gearwheel toothing 69t, will cause a rotation of the second gearwheel 69 and thereby a helical proximal motion of the second piston rod 51. Notably, the axial displacement of the second piston rod 51 will be smaller than that of the first piston rod 45 because the self-locking thread 52 has a smaller pitch than the non-self-locking thread 46. Hence, eventually, during the pressing back of the first piston rod 45 the second piston washer 39 will be brought in abutment with the second contact surface 58.

[0088] Alternatively, if the second piston washer 39 reaches the second contact surface 58 before the first piston washer 29 reaches the first contact surface 48 a continued converging relative axial motion between the cartridge holder 11 and the transmission case 70 will simply cause the extension 55 to be pushed axially backwards over the projecting portion 54 until the first piston washer 29 is brought in abutment with the first contact surface 48.

[0089] After establishment of the initial contact between the first piston washer 29 and the first piston rod 45, shown in FIG. 3, the cartridge holder 11 and the transmission case 70 undergo further converging relative axial motion, eliminating the clearance, X. In FIG. 4 the proximal rim of the cartridge holder 11 has thereby been brought in abutment with the respective legs 62, and at this point the first piston washer 29 abuts the first piston rod 45, while the second piston washer 39 abuts the extension 55. In other words, both parts of the dose administration system have been zero-point adjusted.

[0090] During the converging relative axial motion between the cartridge holder 11 and the transmission case 70 which eliminates the clearance, X, the first piston rod structure 44 undergoes helical proximal motion due to the non-self-locking thread 46 of the first piston rod 45, whereas the second piston rod structure 50 undergoes a combined helical proximal motion of the second piston rod 51 and a pure axial proximal motion of the extension 55, the former driven by the intermeshing gearwheels in the transmission case 70 and the latter by the axial force from the second piston washer 39. Since the axial component of the helical proximal motion of the second piston rod 51 is smaller than the axial proximal motion of the extension 55 the second piston rod structure 50 changes axial dimension until the cartridge holder 11 and the respective legs 62 interact, at which point the projecting portion 54 has reached its final position in the hollow rear portion 56.

[0091] Further converging relative axial motion between the cartridge holder 11 and the transmission case 70 after contact is established between the proximal rim of the cartridge holder 11 and the legs 62 will force the nut member 60 towards the transmission case 70 and cause the protrusion 64 to snap out of the unlocked state slot 72 and into the locked state slot 73, thereby interlocking the nut member 60 and the transmission case 70 in a second of the aforementioned two distinct coupling states. In this second coupling state, which is illustrated by a perspective, partially sectioned, view in FIG. 7b and a side view in FIG. 8b, the ratchet arm 65 is axially aligned, and in engagement, with the ratchet toothing 68r, ensuring unidirectional rotational motion of the central gearwheel 68.

[0092] The central gearwheel 68 is thereby allowed to rotate in a direction which causes a direction of rotation of the first gearwheel 67 and the second gearwheel 69 that corresponds to respective advancements of the first piston rod 45 through the first nut 61a and the second piston rod 51 through the second nut 61b, but not allowed to rotate in the opposite direction. The first gearwheel 67 and the second gearwheel 69 are consequently also restricted to unidirectional rotational motion. This means that the first piston rod 45 and the second piston rod 51 can no longer undergo proximal displacement relative to the nut member 60, and the established contact between the first contact surface 48 and the first piston washer 29, respectively the second contact surface 58 and the second piston washer 39, is accordingly secured.

[0093] Notably, the joint motion of the cartridge holder 11 and the nut member 60 relative to the transmission case 70 does not cause any rotation of the first piston rod structure 44 and the second piston rod structure 50, since the first piston rod 45 and the second piston rod 51 are shifted proximally together with the first nut 61a and the second nut 61b. Furthermore, since the second piston rod 51 is shifted the same distance as the second piston washer 39 the second piston rod structure 50 does not undergo a further reduction in axial dimension. The resulting interrelated positions of the various components of the reservoir sub-assembly 10 and the piston rod sub-assembly 40, constituting the final assembly state of these sub-assemblies, are depicted in FIG. 5.

[0094] FIG. 6 is a v-section view of the reservoir sub-assembly 10 and the piston rod sub-assembly 40 in their final assembly state, showing that the extension 55 after having assumed the above described position relative to the second piston rod 51 is subsequently fixed thereto by laser welding a wall portion of the hollow rear portion 56 to the projecting portion 54 through the bore 19. Thereby, a second piston rod structure 50 with a non-adjustable axial dimension, which is capable of transferring an axial force to the second piston washer 39 to drive the second piston 35, is provided.

[0095] FIG. 9 is a longitudinal section view of the assembled drug delivery device 1, where the cartridge holder 11 is connected to the housing 2. Due to the above described zero-point adjustment the first piston rod structure 44 is in contact with the first piston washer 29 which is in contact with the first piston 25 in the first cartridge 20, and the, now rigid, second piston rod structure 50 is in contact with the second piston washer 39 which is in contact with the second piston 35 in the second cartridge 30. The dosing system is thus pre-primed and an activation of the two piston rod structures via the dose expelling mechanism will accordingly lead to an immediate axial displacement of the two pistons. The dose expelling mechanism is powered by a tensioned torsion spring 5 which is releasable to cause a rotation of the central gearwheel 68 in the transmission case 70. Details of the release mechanism for the torsion spring 5 as well as further details of the drive mechanism for the piston rod structures are not provided herein, since these are irrelevant to the present invention.

[0096] It is noted that whereas the described exemplary embodiment of the invention concerns a twin chamber type of drug delivery device the overarching principle of zero-point adjustment disclosed in the above applies equally to a drug delivery device with a single drug reservoir, such as a fountain pen shaped injection device. In that case the cartridge holder 11 is adapted to carry a single cartridge with a piston, the housing 2 accommodates a single piston rod (having a non-self-locking thread) and, instead of rotationally coupled gearwheels, a single piston rod drive member, e.g. itself carrying a ratchet toothing, and the nut member 60 comprises a single nut and a ratchet arm for engagement with the ratchet toothing after contact between the piston and the piston rod is established. The mechanism for rotating the piston rod drive member during a dose expelling event may in principle be any of a number of known drive mechanisms, a specific example being the one sketched in WO 2017/072233. The nut member may be retained in the first nut member position and/or the second nut member position by respective snap fittings or friction fittings with e.g. the housing.