Assembly of a reset mechanism for an injector pen

DESCRIPTION

Field of the invention

An injector pen provides a mechanism for setting a desired dose of a drug and delivering that dose to a patient from a drug cartridge. The drug cartridge typically contains multiple doses of the drug in a chamber and the injector pen comprises a piston rod that is progressively extended from the pen to expel doses from the chamber, the piston rod moving through a distance that determines the volume of the dose delivered.

The present invention relates to pens that accommodate disposable drug cartridges, whereby the mechanism of each pen can be re-used when the drug cartridge is replaced. This requires the pen to be reset by moving the piston rod from the final position it adopted in relation to a used drug cartridge back to its initial position for use with a new drug cartridge. Reverse movement of the piston rod is prevented during normal operation of an injector pen but a reset mechanism is provided to permit reverse movement when the used drug cartridge has been removed. The invention relates to the structure and method of assembly of such reset mechanisms.

Background of the invention

Many designs of injector pen are known. Typically the pen comprises a cylindrical pen housing and a dose selector. The pen further comprises means for attaching a drug cartridge to the pen housing; and a piston rod, which moves along the axis of the pen to act on a piston in the drug cartridge. In a dose setting mode, the user moves the dose selector axially or helically relative to the pen housing to a position that determines the dose for delivery. In this mode, movement of the dose selector does not cause movement of the piston rod. Subsequently, in a drug delivery mode, the user pushes the dose selector back to its original position in the pen housing to deliver the dose. A button on the end of the pen is pressed to engage the drug delivery mode and push the dose selector into the pen housing. During the drug delivery mode, a drive mechanism within the pen housing converts the axial movement of the dose selector into axial movement of the piston rod through a distance suitable to deliver the required dose from the cartridge. The distance moved by the piston rod is thus determined by the distance moved by the dose selector but the two distances need not be equal.

The piston rod comprises a helical thread on its outer surface. The piston rod also comprises a number of axially extending flats or slots that interrupt the helical thread and provide the piston rod with a non-circular profile. The piston rod is surrounded by and engaged by a piston rod guide, which is mounted within the pen housing to be capable of rotation but not axial movement. The piston rod is also engaged by a bearing nut or other bearing element that is mounted within the pen housing to be incapable of axial movement and locked against rotation during normal use of the pen. The movement of the piston rod is thus driven and constrained by its engagement with the rotating piston rod guide and with the fixed bearing element. This gives rise to two possibilities for driving the piston rod:

• In one design of injector pen, the fixed bearing element engages the piston rod via a complementary non-circular profile, thereby preventing the piston rod from rotating but permitting it to slide axially through the bearing element. In this design, the piston rod guide engages the piston rod via its helical thread so that, as the piston rod guide rotates about the non-rotating piston rod, a screw action drives the piston rod to slide forwards along the axis, guided by the bearing element.

• In an alternative design of injector pen, the bearing element engages the piston rod via a complementary helical thread, thereby preventing the piston rod from moving forwards unless it simultaneously rotates. In this design, the piston rod guide engages the piston rod via its non-circular profile so that, as the piston rod guide rotates about the axis, the piston rod is forced to rotate with it, thereby screwing the piston rod forwards along the axis, guided by the bearing element.

Many different pen mechanisms are known for selectively converting the movement of the dose selector into rotation of the piston rod guide. However, the present invention is concerned only with the operation of the pen “downstream” of the piston rod guide and is not limited to any particular “upstream” mechanism.

While a given cartridge is attached to the pen housing, each use of the pen causes the piston rod to advance further along the axis until the cartridge is exhausted or an insufficient dose remains. During that time, a one-way element within the pen mechanism prevents the piston rod being moved in the reverse direction along the axis. The one-way element is typically a circular ratchet that acts between the housing and the piston rod guide, permitting the piston rod guide to rotate only in the direction that causes the piston rod to advance. If any attempt is made to move the piston rod in the reverse direction, then the piston rod guide becomes rotationally fixed, like the bearing element. One of the piston rod guide and the bearing element has a threaded engagement with the piston rod, which prevents it from sliding axially. The other of them has a non-circular engagement with the piston rod, which prevents it from rotating. Therefore, the piston rod is locked against all forms of reverse movement.

When a used cartridge is replaced, the pen needs to be reset by returning the piston rod to its initial position so it can be used to displace the drug from the new cartridge. Thus a reset mechanism is required, which changes the normal operation and permits the piston rod to move in the reverse direction. The reset mechanism typically achieves this by releasing the bearing element from being rotationally locked with respect to the housing. This removes one of the two constraints on the piston rod, which can therefore be moved in the reverse axial direction, while the bearing element rotates and the piston rod guide remains fixed. If the piston rod has a threaded engagement with the fixed piston rod guide, then the piston rod is screwed back along the axis. In that case, the piston rod has a non-circular engagement with the bearing element, which co-rotates with it. In the alternative design, the piston rod has a non-circular engagement with the fixed piston rod guide so the piston rod slides back along the axis. In that case, the piston rod has a threaded engagement with the bearing element so the bearing element must rotate as piston rod slides axially through it. Fig. 55 of published patent US 5505704 A discloses an example of this type of reset mechanism for an injector pen. A piston rod passes through a transverse wall of the pen housing and is rotationally engaged with a pullback nut. The pullback nut is able to slide axially along the piston rod in order to bring facing teeth on the nut and the wall into or out of engagement. This acts as a clutch to permit or prevent rotation of the pullback nut relative to the wall. Inserting a drug cartridge into the housing pushes the pullback nut against the force of a compression spring to engage the teeth and lock the nut against rotation, thereby preventing reverse axial movement of the piston rod during use of the drug cartridge. When the drug cartridge is removed, the spring urges the nut away from the wall. It becomes free to rotate and the position of the piston rod can be reset by screwing it back along the axis through the threaded bore of a piston rod guide (not seen in Fig. 55).

Patent EP 2274030 Bl discloses an injector pen with a similar reset mechanism. In this case, a nut means is in threaded engagement with a piston rod. When a cartridge holder is inserted into the pen, it pushes a locking means into a guiding position, in which teeth on the locking means engage teeth on the nut means. This prevents the nut means rotating and thereby prevents the piston rod being moved in the proximal direction. When the cartridge holder is removed, a biasing means pushes the nut means and the locking means apart and frees the nut means to rotate as the piston rod is moved through it in the proximal direction to reset the pen.

In injector pens, the drug cartridge is mounted in a cartridge housing, which needs to be inserted along the axis into an open end of the pen housing. To ensure that the cartridge housing remains firmly seated and cannot be withdrawn unintentionally by sliding it in the axial direction, it is common to attach the cartridge housing to the pen by a screw thread coupling. This would preferably be achieved by moulding the pen housing to have an integrally formed screw thread around the interior wall of its open end. To form such a feature requires the mould to have a rotational core, which can be withdrawn after moulding by rotating it along the line of the screw thread. However, the angular orientation of the reset mechanism about the axis relative to the other parts of the pen mechanism is critical if it is to be aligned correctly with the arms of the circular ratchet and operate correctly. In known devices, this requires the pen housing to be provided with an alignment feature such as an axial rib at a predetermined position on the internal circumference of the housing. By its nature, the alignment feature destroys the rotational symmetry of the interior of the housing and thereby prevents the use of a rotational core. For this reason, known injector pens with reset mechanisms require an additional part fitted into the mouth of the open end, which provides the screw thread or bayonet required to attach the cartridge housing. This additional part is undesirable in terms of stock control and the assembly process and it would be advantageous if it could be avoided.

In the manufacture of known injector pens, the provision of a reset mechanism requires the parts of the mechanism to be added individually, in series, through the open end of the pen housing, which adds to the already complex assembly process. It would be advantageous if the assembly process could be simplified.

Summary of the invention

The invention provides a reset mechanism for an injector pen as defined in claim 1.

The invention further provides a combination of a jig and a reset mechanism subassembly as defined in claim 10. The combination optionally further comprises an injector pen housing.

The invention also provides a method of forming an injector pen reset mechanism subassembly on a jig, as defined in claim 16.

The invention further provides a method of using a jig to assemble an injector pen housing and the reset mechanism sub-assembly, as defined in claim 21. The same jig may optionally be used to form the reset mechanism sub-assembly before assembling it with the injector pen housing. Alternatively, the method of assembling the injector pen housing and the reset mechanism sub-assembly may use a sub-assembly that was pre-formed without the use of a jig according to the invention. The dependent claims define features and method steps that are preferred but are not essential to the invention.

In accordance with the invention, the injector pen reset mechanism is formed as a discrete sub-assembly, i.e. the parts of the reset mechanism are assembled to each other before they are assembled into the injector pen housing. This is possible because the ratchet housing is formed as a component separate from the pen housing, which forms part of the reset mechanism sub-assembly. Features of the reset mechanism such as snap-fit connections preferably hold the components of the sub-assembly together so that the sub-assembly can be handled as a single unit. This enables the manufacture of the sub-assembly in a separate process - optionally at a different time or location - from the final assembly of the injector pen.

Using a jig to assemble the injector pen reset mechanism sub-assembly with an injector pen housing enables the alignment of the sub-assembly and the pen housing respectively to be determined relative to the jig, rather than directly relative to each other, therefore it is not necessary for the pen housing to have alignment features that are capable of engaging the sub-assembly. It follows that the pen housing may be provided with an external alignment feature, e.g. formed on the rim of the open end of the housing, which can engage the jig but which does not interfere with the use of a rotational core to form an internal screw thread in the pen housing.

As a preliminary step, either a pre-formed sub-assembly may be placed on the jig or a sub-assembly may be built in situ on the jig. Either way, components of the subassembly engage respective seats on the jig to determine the alignment of each component relative to the jig. It should be noted that the seats on the jig are not necessarily formed on discrete parts: one part of the jig may be shaped to provide seats for more than one component of the sub-assembly. While some components of the subassembly need to be assembled with predetermined alignments, those alignments are not necessarily unique. For example, if a component or the feature with which it is to interact has 2-fold or 3 -fold rotational symmetry, it may be acceptable to align it in any of 2 or 3 angular orientations. On the other hand, in some embodiments of the invention, a unique angular alignment of at least the ratchet housing relative to the pen housing may be necessary to ensure that a window in the pen housing correctly aligns with a scale on the dose dial to indicate the dose of drug that has been set.

Injector pens are typically used in a medical context so for convenience the present specification uses the term “drug” to refer to the substance that is stored in the injector pen for delivery to a “patient”. However, the invention does not depend on the nature of the drug or the patient. Accordingly, the “drug” may be any liquid that it is desired to deliver in measured quantities for medical, cosmetic or other purposes; and the “patient” may be any human, animal or other subject into which it is desired to deliver that liquid.

Where the term “cut-out” is used in this specification, it does not imply that any process of cutting or removal has taken place to form the feature in question. A cut-out may thus be formed by moulding it as an integral feature of the shape of the part in which the cut-out is defined.

Drawings

Figure 1 is a longitudinal cross-section through an injector pen that incorporates a reset mechanism sub-assembly according to the present invention.

Figure 2 is a detail of the injector pen of Fig. 1 showing, in perspective view, the engagement between a cartridge housing and an injector pen housing.

Figure 3 is a transverse cross-section through the ratchet of the injector pen of Fig. 1. Figure 4 is a perspective view, sectioned along the axis, of the reset mechanism subassembly of Fig. 1.

Figure 5 is a perspective view, sectioned along the axis, of part of the injector pen of Fig. 1, showing the reset mechanism in an injection state.

Figure 6 is a perspective view, similar to Fig. 5, showing the reset mechanism in a resetting state.

Figure 7 is a perspective view of a jig for assembly of the reset mechanism subassembly of Fig. 1 Figure 8 is a perspective view, partially in section, showing assembly of the reset mechanism sub-assembly on the jig of Fig. 7.

Figure 9 is a side view showing a first step in the assembly of a pen housing with the reset mechanism sub-assembly of Fig. 8.

Figure 10 is an enlarged side view, partially in section, showing a second step in the assembly of the pen housing with the reset mechanism sub-assembly of Fig. 8.

Detailed description of a preferred embodiment

The injector pen shown in Figure 1 comprises a generally cylindrical pen housing 2, which is open at one end (the distal end) to receive a cartridge housing 4. The cartridge housing 4 is coupled to the pen housing 2 via an internal screw thread 5 formed inside the open end of the pen housing 2.

Figure 2 shows a detail of the coupling between the cartridge housing 4 and the pen housing 2. The rim of the open end of the pen housing 2 is moulded with a small notch or cut-out 7, while the cartridge housing 4 includes a small pip 6 formed at the base of its screw thread. As the cartridge housing 4 is screwed along the threaded coupling one further full turn from the position shown in Figure 2, the pip 6 will first interfere with the rim of the open end, offering resistance to the final part of the rotation, and will then click into position in the cut-out 7. This firstly provides haptic feedback to the user that the cartridge housing 4 has been fully screwed into position and secondly resists accidental unscrewing of the cartridge housing 4 from the pen housing.

The cartridge housing 4 contains a drug cartridge 8. The proximal end of the cartridge 8 is sealed by a piston 10, which can be moved along the axis of the cartridge 8 to displace a quantity of a drug 9 from the distal end of the cartridge 8. The distal end of the cartridge 8 comprises coupling means such as a second screw thread 12, by which a disposable needle (not shown) can be attached to the cartridge housing 4 for delivery of the displaced drug into the skin of a patient. When the injector pen is not in use, the cartridge housing may be protected by a cap 14. A piston rod 16 lies on the axis of the injector pen, with the distal end of the piston rod 16 engaging the piston 10 of the cartridge 8. The pen housing 2 contains a drive mechanism for driving the piston rod 16 forwards (i.e. in the distal direction) to displace the piston 10 through a measured distance and deliver a corresponding measured dose of the drug 9 from the cartridge 8. Protruding from the proximal end of the pen housing 2 are a dose dial grip 18 for setting the desired dose and an injection button 20 for operating the drive mechanism to deliver the dose that has been set. The mechanisms for setting and delivering the dose will be described only briefly because the details of them are not relevant to the present invention.

The dose dial grip 18 is attached to a cylindrical dose dial 22, which is nested concentrically within the pen housing 2 and is coupled to the pen housing 2 via a helical thread 24. Rotating the dose dial grip 18 causes the dose dial 22 to follow the helical thread 24 and extend from the proximal end of the pen housing by a distance that determines the dose to be set. This distance is proportional to the distance that the piston rod 16 will need to move to deliver that dose. A scale printed along a helical track on an external surface of the dose dial 22 is visible through a window 25 in the pen housing 2 to indicate the selected dose.

A generally cylindrical piston rod drive 26 is nested concentrically inside the dose dial 22 and is configured to track the axial movement of the dose dial 22. However, as long as the injection button 20 is not pressed, the piston rod drive 26 does not follow the rotation of the dose dial 22. Therefore, during dose setting, movement of the dose dial 22 does not drive movement of the piston rod 16.

When the desired dose has been set and the injection button 20 is pressed, axial teeth on a flange 28 of the piston rod drive 26 engage complementary teeth on a shoulder of the dose dial 22, locking the two components rotationally together. As the injection button 20 is pushed in the distal direction along the axis, it urges the dose dial 22 to follow the helical thread and return to its initial position. (The injection button 20 is mounted on a central bearing so that the dose dial 22 can rotate freely relative to the button.) In this injection mode, the piston rod drive 26 is both rotationally and axially locked to the dose dial 22 so it follows the same helical path.

A generally cylindrical piston rod guide 30 is nested concentrically within the piston rod drive 26 and immediately surrounding the piston rod 16. The piston rod guide 30 is mounted to be capable of rotation relative to the pen housing 2 but is prevented from moving axially. The piston rod guide 30 has a pair of axial channels that receive a pair of tabs 34 on the piston rod drive 26 so that piston rod guide 30 and the piston rod drive 26 are rotationally locked together but can slide axially relative to one another. Thus, during injection of the drug, the helical movement of the piston rod drive 26 is converted to pure rotation of the piston rod guide 30.

The piston rod 16 comprises a helical thread 36 on its outer surface, which typically has a smaller pitch than the helical thread 24 of the dose dial. Because of the ratio between these two pitches, the small axial movement of the piston rod 16 that is required for delivery of a dose can be determined by a proportionately larger axial movement of the dose dial grip 18, thereby enabling the dose to be set precisely and reducing the force that needs to be applied to the injection button 20. The piston rod 16 also comprises a pair of axially extending flats 38 that interrupt the helical thread and provide the piston rod 16 with a non-circular outer profile. In the illustrated embodiment, the piston rod guide 30 has a central bore 39 with a complementary non-circular profile, as seen in Figure 3. The piston rod guide 30 is thereby rotationally locked to the piston rod 16 but permits the piston rod 16 to slide axially through the bore 39.

To recap, during dose delivery, axial movement of the injection button 20 drives helical movement of both the dose dial 22 and the piston rod drive 26. The helical movement of the piston rod drive 26 drives pure rotation of the piston rod guide 30, which in turn drives rotation of the piston rod 16. The piston rod 16 remains free to slide axially relative to the piston rod guide 30 at the same time as it rotates. During dose setting, the dose dial 22 moves helically but the rotational component of that movement is not transmitted to the piston rod drive 26, therefore the piston rod drive 26 does not cause the piston rod guide 30 or the piston rod 16 to rotate. Moreover, the piston rod guide 30 is provided with a circular ratchet 40, comprising resilient arms which engage the teeth 43 of a fixed ratchet housing to prevent the piston rod guide 30 rotating in a direction that would cause the piston rod 16 to be withdrawn in the proximal direction along the axis. The ratchet 40 also provides sufficient resistance to rotation of the piston rod guide 30 in the other direction to prevent unintentional movement of the piston rod 16 in the distal direction along the axis when such movement is not being positively driven.

The injector pen reset mechanism, to be described in more detail below, comprises a bearing element 44 that is locked both axially and rotationally relative to the pen housing 2 during dose setting and dose delivery. A bore in the bearing element 44 comprises an internal thread 46 that engages the external thread 36 of the piston rod 16. Thereby, during dose delivery, when the piston rod 16 is rotated within the fixed bearing element 44, the threaded coupling between them causes the piston rod 16 to advance along the axis.

The ratchet 40 permits the piston rod 16 to move only forwards along the axis, i.e. in the direction that displaces the drug 9 from the cartridge 8. Eventually, the drug 9 will be exhausted or an insufficient dose of the drug 9 will remain and the cartridge 8 will need to be replaced. In order to operate with the new cartridge, the piston rod 16 must be reset by moving it back along the axis to its initial position. However, the resetting process cannot be a simple reversal of the forward movement of the piston rod 16 during dose delivery because that would require rotation of the piston rod guide 30 in the direction that is blocked by the ratchet 40. Accordingly, a reset mechanism is provided, which frees the bearing element 44 to rotate when the cartridge 8 and/or cartridge housing 4 has been removed. As a result, the piston rod 16 can now be pushed back along the axis, sliding without rotation through the bore of the non-rotating piston rod guide 30, while the bearing element 44 remains axially fixed and spins around the piston rod 16 as a result of the threaded coupling 46 between them.

In accordance with the present invention, the reset mechanism is formed as a subassembly, seen in Figure 4. The reset mechanism comprises the aforementioned ratchet housing 42, bearing element 44 and piston rod 16. It further comprises a locking sleeve 48 and a compression spring 50.

The ratchet housing 42 surrounds the piston rod 16 without engaging it. The ratchet housing 42 is designed to be force-fitted into the pen housing 2 such that, after assembly of the pen, the ratchet housing 42 cannot move axially or rotate relative to the pen housing 2. To this end, an outer cylindrical surface of the ratchet housing 42 is provided with crush ribs 51, which will be crushed against an internal wall of the pen housing 2 when the injector pen is assembled. Other ways could be provided of assembling the ratchet housing 42 and the pen housing 2 together, by a purely axial movement, such that they thereby become fixed against relative axial or rotation movement. For example, they might have co-operating non-circular cross sections and/or a snap fit connection. However, the crush ribs 51 are easy to mould on the ratchet housing 42 and do not interfere with the use of a rotational core to mould the pen housing 2 or require the insertion of an additional component to carry the screw thread.

The bearing element 44 engages the piston rod 16 via the coupling between the respective internal and external threads 36,46. The bearing element 44 is capable of rotation about the axis, relative to the ratchet housing 42, when not locked by the locking sleeve 48 as described below. The bearing element 44 comprises an annular bearing surface 52, which bears against the ratchet housing 42 and has a small contact diameter to minimize the resistance to rotation of the bearing element 44. A number of arms 54 extend axially from the ratchet housing 42 and terminate in hooks 55 that snap- fit behind an annular flange 56 of the bearing element 44. This snap-fit coupling retains the bearing element 44 on the ratchet housing 42 while permitting the bearing element 44 to rotate relative to the ratchet housing 42.

The locking sleeve 48 engages the ratchet housing 42 such that relative rotation between the locking sleeve 48 and the ratchet housing 42 is prevented, while some movement of the locking sleeve 48 parallel to the axis is permitted. In the illustrated embodiment, this relationship is achieved by a number of arms 58 that extend in the proximal direction from the locking sleeve 48 and slide in complementary axial channels 60 of the ratchet housing 42. The arms 58 terminate in hooks 59, which snap- fit behind shoulders at the distal ends of the channels 60. This snap-fit coupling retains the locking sleeve 48 on the ratchet housing 42, while limiting movement of the locking sleeve 48 in the distal direction.

The compression spring 50 is mounted between the ratchet housing 42 and the locking sleeve 48 to bias the locking sleeve 48 to move in the distal direction. Alternative biasing means, such as a ring of resiliently compressible material, could perform the same function in other embodiments of the invention (not illustrated).

The main components of the reset mechanism sub-assembly are held together by the snap-fit couplings between the ratchet housing 42, the bearing element 44 and the locking sleeve 48. The compression spring 50 is securely located between the ratchet housing 42 and the locking sleeve 48, while the piston rod 16 is securely threaded through the bearing element 44. Thus the sub-assembly has sufficient integrity to be manufactured separately, stored and transported to a different location if required, then later assembled as a combined unit into an injector pen.

Figures 5 and 6 show the reset mechanism after it has been assembled into an injector pen.

In Figure 5, the piston rod 16 is in its initial position, fully retracted along the axis and ready to displace the piston 10 of a full cartridge 8. The reset mechanism is also in an injection state, which it adopts whenever a cartridge housing 4 is coupled to the pen housing 2. As the cartridge housing 4 is screwed into the pen housing 2, the proximal end of the cartridge 8 or of the cartridge housing 4 pushes the locking sleeve 48 in the proximal direction, against the biasing force of the compression spring 50, to an axial position indicated by dimension x. In this position, axially facing teeth 62 on the locking sleeve 48 mate with complementary teeth 64 on the bearing element 44. The locking sleeve 48 is prevented from rotating by its arms 58 being located in the channels 60 of the stationary ratchet housing 42, therefore in the injection state the engagement of the respective teeth 62,64 also prevents the bearing element 44 from rotating. Because the bearing element 44 is locked against rotation in the injection state, it can perform its function of constraining the piston rod 16 to advance along the axis when the piston rod 16 is rotated within the threaded bore 46.

In Figure 6, the piston rod is in an advanced axial position, having displaced substantially all of the drug 9 from a cartridge 8. The reset mechanism is also in a resetting state, which it adopts whenever a cartridge housing 4 is not coupled to the pen housing 2. Upon removal of the cartridge 8 or the cartridge housing 4, the compression spring 50 biases the locking sleeve 48 in the distal direction, to an axial position indicated by dimension y, which is determined by the hooks 59 of locking sleeve arms 58 engaging the ends of the ratchet housing channels 60. Dimension y is greater than dimension x in Figure 5, the difference being sufficient to disengage the teeth 62 of the locking sleeve 48 from the teeth 64 of the bearing element 44, which thereby becomes free to rotate about the axis. It is now possible to reset the piston rod 16 by pushing it in the proximal direction along the axis, back to its initial position. The piston rod guide 30 remains locked against rotation by the ratchet 40 and the piston rod 16 slides through the non-circular bore 39 of the piston rod guide, also without rotating. The bearing element 44 cannot move axially so the movement of the piston rod 16 through its threaded bore 46 causes the bearing element 44 to rotate. The pitch and shape of the piston rod thread 36 and the bearing element thread 46 should be designed that they do not lock when driven in this way; and the contact between the bearing surface 52 and the ratchet housing 42 should also be designed to minimize the resistance to rotation of the bearing element 44.

The reset mechanism may be assembled on a jig 70, which is shown in Figures 7 and 8. The jig 70 is generally cylindrical but comprises a single, axial alignment rib 72 on its surface, which breaks the rotational symmetry and defines a unique angular orientation. An end face 74 of the jig 70 serves as a first seat for receiving the locking sleeve 48 of the sub-assembly (shown in section in Figure 8). There are three projections 76 that project axially from the end face 74 of the jig 70 and engage with internal cut-outs 78 of the locking sleeve 48 to determine the angular alignment of the locking sleeve 48 on the jig 70. After the locking sleeve 48 has been seated on the jig 70, the compression spring 50 (shown in section in Figure 8) is located on it.

The ratchet housing 42 and the bearing element 44 are snap-fitted together, separately from the jig 70, as previously described and the piston rod 16 is threaded through the bore 46 in the bearing element 44. Now the ratchet housing 42, bearing element 44 and piston rod 16 together are brought up to the jig 70 and the piston rod 16 is inserted through a complementary non-circular hole 80 in the centre of the end face 74. The ratchet housing 42 is rotated about the piston rod 16 until it achieves the desired angular alignment, in which cut-outs 82 in the ratchet housing 42 are aligned with the respective projections 76 of the jig 70. One of the projections 76 - in this example, the one adjacent to the alignment rib 72 - is wider than the other two, whereby, following poka- yoke principles, the ratchet housing 42 can be placed on the jig 70 in only one predetermined angular alignment. In this alignment, the channels 60 of the ratchet housing 42 are correctly aligned with the arms 58 of the locking sleeve 48. It should be noted that in an alternative sequence of steps, the ratchet housing 42 may be placed on the jig 70 before the bearing element 44 is snap-fitted to it.

In the illustrated embodiment of the invention, the bearing element 44 also comprises cut-outs 84. While the alignment of the bearing element 44 is not critical during the operation of the injector pen, in this embodiment the cut-outs 84 of the bearing element 44 need to be aligned with the cut-outs 82 of the ratchet housing 42 to allow the projections 76 to engage the cut-outs 82. Thus the projections 76 of the jig 70 form a second seat 86 that receives the ratchet housing 42 in the predetermined angular alignment and, in this embodiment, they also form a third seat 88 that receives the bearing element 44 in the predetermined angular alignment. As illustrated, the second and third seats 86,88 may be delimited by small shoulders on the projections 76.

Once the ratchet housing 42 and bearing element 44 are correctly aligned, they can be pushed onto the jig 70, thereby compressing the spring 50, until the hooks 55 on the arms 54 of the locking sleeve 48 snap-fit behind the shoulders at the ends of the channels 60 of the ratchet housing 42 to hold the various components of the reset sub- mechanism assembly together. If desired, the reset mechanism sub-assembly can then be removed from the jig 70, to be assembled into an injector pen at a later time. If the jig 70 is only to be used in this way, then the illustrated alignment ring 90 and spring 93 (described below) serve no purpose and may be omitted. Alternatively, the reset mechanism sub-assembly may be left on the jig 70 for immediate assembly with other components of the injector pen, as described below.

Figures 9 and 10 show selected steps in the assembly of an injector pen, using the same jig 70 as in Figures 7 and 8. If the reset mechanism sub-assembly is not already mounted on the jig 70 as a result of having been assembled there, it should be brought up to the jig 70 with its components arranged in the predetermined orientation so that the first seat 74 receives the locking sleeve 48 and the second seat 86 receives the ratchet housing 42. The piston rod guide 30 is then slid along the piston rod 16 until the ratchet arms 41 at its distal end engage the teeth 43 of the ratchet housing 42.

As shown in Figure 9, the pen housing 2 is then placed on the jig 70, being moved along the axis to surround the piston rod guide 30. The pen housing 2 initially comes to rest in the position shown in Figure 10, where a small internal shoulder 92 abuts the crush ribs 51 of the ratchet housing 42. The jig 70 comprises an alignment ring 90, which is rotationally fixed but is capable of sliding axially along the alignment rib 72. The ring 90 is urged into engagement with the housing 2 by a compression spring 93 that acts between the ring 90 and a spring seat 91 on the jig 70. (In an alternative embodiment of the invention, the compression spring 93 may be omitted and the ring 90 is moved into engagement with the housing 2 by hand.) The housing 2 must be rotated about the axis until the cut-out 7 on the rim of the pen housing 2 is positioned to receive a pip 94 on the ring 90, as shown in Figure 10. The movable ring 90 thus provides a seat for the pen housing 2 to determine its angular alignment relative to the jig 70 and, indirectly, relative to the reset mechanism sub-assembly that is already mounted on the jig 70. It is not necessary for the pen housing 2 to have any internal alignment feature that directly engages the reset mechanism sub-assembly. The pen housing 2 is then pushed along the axis towards the jig 70 to force-fit the pen housing 2 over the crush ribs 51 of the ratchet housing 42. While that happens, the ring 90 slides back along the alignment rib 72, against the force of the compression spring 93, from an initial position where the internal shoulder 92 of the pen housing 2 rests against the crush ribs 51 to a final position where the ring 90 abuts a shoulder 95 on the jig 70. The ring 90 meanwhile remains in engagement with the housing 2 to ensure that the housing 2 maintains the predetermined angular alignment as it is force- fitted onto the ratchet housing 42.

It should be noted that the method steps described with reference to Figures 9 and 10 need not be carried out in the illustrated orientation. In particular, it may be convenient to invert the jig 70 in order to apply pressure to the pen housing 2 as it is force-fitted onto the ratchet housing 42.

The remaining steps for assembly of the injector pen involve inserting the drive mechanism and other components into the proximal end of the pen housing 2. They are conventional and will not be described or illustrated here.

The illustrated embodiment of the invention and the associated assembly method may be varied in numerous ways within the scope of the claimed invention. In particular, and as mentioned above in relation to the prior art, in alternative embodiments of the invention (not illustrated) the movement of the piston rod 16 could be constrained in a different way between the piston rod guide 30 and the bearing element 44. In such embodiments, the external thread 36 of the piston rod 16 would engage an internal thread of the piston rod guide 30, while the non-circular cross-section of the piston rod 16 would engage a non-circular bore in the bearing element 44 to prevent the piston rod 16 from rotating. During dose delivery, the piston rod guide 30 is driven in the same way as in the illustrated embodiment and, as the piston rod guide 30 rotates about the non-rotating piston rod 16, the threaded coupling between them urges the piston rod 16 forwards along the axis, while sliding through the bearing element 44. It will be apparent to the reader that these embodiments require only minor differences in the steps of assembling the piston rod 16 with the piston rod guide 30 and with the bearing element 44, which nevertheless fall within the scope of the claimed invention.