A Piston Rod Drive for an Injection Device

Abstract

The invention relates to a piston rod drive arrangement for an injection device used for self-administration of a plurality of individually set doses. The drive arrangement is made from a first element (10) mating the non-circular cross-section (3) of a piston rod (1) and a second element (20) having an inner thread (21) mating the outer thread of the piston rod. Whenever the first element (10) and the second element (20) are rotated in relation to each other the piston rod (1) is moved in an axial direction. In order to minimise the play in the threaded connection a resilient element (23) is proved to apply a force to the piston rod (1) in a longitudinal direction.

Claims

1. A piston rod drive arrangement for an injection device, comprising; a piston rod having an outer surface with an outer thread which outer thread extend helically in a longitudinal direction along a centre line (X) and which outer surface further is provided with a longitudinal extending engagement surface such that the outer surface of the piston rod has a non-circular cross-section, a first element engaging the longitudinal extending engagement surface of the non-circular cross-section of the piston rod, a second element having an inner thread mating the outer thread of the piston rod, such that the piston rod is moved in the longitudinal direction when the first element and the second element are rotated in relation to each other, and wherein, a resilient element apply a force to the piston rod in the longitudinal direction.

2. A piston rod drive arrangement according to claim 1, wherein the resilient element is formed integral with the second element.

3. A piston rod drive arrangement according to claim 1, wherein the resilient element is an integral part of the inner thread of the second element.

4. A piston rod drive arrangement according to claim 1, wherein the inner thread is formed from one or more thread segments, wherein at least one thread segment terminates in a resilient arm which abuts the outer thread on the piston rod in the longitudinal direction.

5. A piston rod drive arrangement according to claim 4, wherein the resilient arm define a helix angle different from the helix angle of the thread segment carrying the resilient arm.

6. A piston rod drive arrangement according to claim 1, wherein the outer thread of the piston rod is unbroken.

7. A piston rod drive arrangement according to claim 6, wherein the piston rod carries a series of knobs extending radially from the unbroken outer thread.

8. A piston rod drive arrangement according to claim 7, wherein each knob in the series extend from the top of the outer thread in a radial direction perpendicular to the longitudinal direction of the piston rod.

9. A piston rod drive arrangement according to claim 7, wherein the knobs in the same series are provided following a generally straight line in parallel with the longitudinal direction of the piston rod thereby together forming the longitudinal extending engagement surface.

10. A piston rod drive arrangement according to claim 7, wherein the first element engages the longitudinal extending engagement surface made up by the knobs.

11. A piston rod drive arrangement according to claim 10, wherein the first element is provided with a rigid flange generally parallel with the longitudinal direction.

12. A piston rod drive arrangement according to claim 11, wherein the first element further is provided with a resiliency urging the knobs rotationally against the rigid flange.

13. A piston rod drive arrangement according to claim 11, wherein the resiliency is a resilient arm which urges the longitudinal extending engagement surface made up by the knobs against the rigid flange.

14. An injection device comprising: a housing securing a drug container, and a piston rod drive arrangement according to claim 1, wherein the first element is non-rotatable secured in the housing and the second element is rotatable.

15. An injection device comprising: a housing securing a drug container, and a piston rod drive arrangement according to claim 1, wherein the second element is non-rotatable secured in the housing and the first element is rotatable.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention will be explained more fully below in connection with a preferred embodiment and with reference to the drawings in which:

[0050] FIG. 1 show a cross sectional view of the threaded connection between the piston rod and the second element.

[0051] FIG. 2 show the cross sectional view of FIG. 1 with the first element engaging the piston rod.

[0052] FIG. 3 show a perspective view of the second element.

[0053] FIG. 4 show a perspective view of the second element cut into half.

[0054] FIG. 5 show a perspective view of the engagement between the thread segment of the second element and the outer thread of the piston rod.

[0055] FIG. 6 show the perspective view of FIG. 5, viewed from a different angle.

[0056] FIG. 7 show a perspective view of the engagement between the first element and the piston rod.

[0057] The figures are schematic and simplified for clarity, and they just show details, which are essential to the understanding of the invention, while other details are left out. Throughout, the same reference numerals are used for identical or corresponding parts.

DETAILED DESCRIPTION OF EMBODIMENT

[0058] When in the following terms as “upper” and “lower”, “right” and “left”, “horizontal” and “vertical”, “clockwise” and “counter clockwise” or similar relative expressions are used, these only refer to the appended figures and not to an actual situation of use. The shown figures are schematic representations for which reason the configuration of the different structures as well as there relative dimensions are intended to serve illustrative purposes only.

[0059] In that context it may be convenient to define that the term “distal end” in the appended figures is meant to refer to the end of the piston rod pointing toward the user during injection, and which end carries a piston rod foot whereas the term “proximal end” is meant to refer to the opposite end pointing away from the user during injection. Distal and proximal is meant to be along an axial orientation of the piston rod along a virtual centre line marked “X” in FIG. 1 which discloses the piston rod drive arrangement according to a first embodiment.

[0060] The piston rod 1 is a rod-shaped element carrying a helical outer thread 2 and a non-circular cross section 3 as best seen in FIGS. 5 and 6.

[0061] The piston rod 1 has both an axial or longitudinal direction and a radial direction extending perpendicular to the axial or longitudinal direction. By being rod-shaped is meant that the piston rod has a generally circular outer surface and extends much longer in the longitudinal direction than in the radial direction. The longitudinal direction follows along the centre line which in FIGS. 1 and 2 is indicated by a broken line (X) and the helical outer thread (2) extend helically in the longitudinal direction. The outer thread 2 need not be formed in the entire length of the piston rod 1 as disclosed but can be present only in a limited longitudinal part of the piston rod 1. Further, the piston rod 1 can be provided with more than one helical outer thread 2 which can even be of different pitches. The pitch of a thread is meant to be the distance between the thread crests. The crest is the helically extending top of the thread, which is separated by the root in the longitudinal direction.

[0062] The non-circular cross section of the piston rod 1 can be provided in many different ways. In one example it can be one or more longitudinal extending track(s) carved into the surface of the piston rod 1 or it can be that the outer circular surface of the piston rod is flatten on one or more side(s). In a preferred embodiment the non-circular cross section is provided as a radial extending engagement surface or flange provided on top of an unbroken outer thread 2 as will be explained.

[0063] The second element 20 is provided with an inner thread 21 which mates the outer thread 2 of the piston rod 1 such that the outer thread 2 of the piston rod 1 is able to rotate in the inner thread 21 like a nut and bolt engagement

[0064] In an injection device, the liquid drug is usually stored in a container such as e.g. a glass cartridge, which is proximally closed by a movable rubber plunger. During injection the piston rod 1 moves towards the left (i.e. in the distal direction as seen in FIG. 1 and FIG. 2) such that the conical tip 6 of the piston rod 1 moves the rubber plunger further into the cartridge. The conical tip 6 of the piston rod 1 usually carries a not-shown piston rod foot to properly distribute the force onto the rubber plunger of the cartridge. In the FIGS. 1 and 2, the centre line (X) of the piston rod 1 defines the axial or longitudinal direction.

[0065] As disclosed in FIG. 1, the longitudinal extending piston rod 1 can be moved forward in the distal direction in two different ways; [0066] a) The second element 20 can be secured rotational i.e. such that the second element 20 is prevented from rotating, while the piston rod 1 is rotated. This will screw the piston rod 1 rotational forward in a helical movement. [0067] b) Alternatively, the second element 20 can be rotated while the piston rod 1 is maintained non rotational. This will move the piston rod 1 strictly axially forward without rotating the piston rod 1.

[0068] In order to either rotate the piston rod 1 or to secure it rotational, a first element 10 is provided as disclosed in FIG. 2. This first element 10 has an internal shape 11 mating with the non-circular cross section 3 of the piston rod 1. This internal shape 11 can be any kind of shape that engages with the non-circular cross section 3 of the piston rod 1.

[0069] In the above solution a); the first element 10 engages and rotates the piston rod 1, whereas in the above example b); the first element 10 engaging the piston rod 1 is kept stationary while the second element 20 is rotated.

[0070] The non-circular cross section 3 of the piston rod 1 is formed as a longitudinal extending engagement surface provided in or on the piston rod 1 such that the first element 10 is able to engage this longitudinal extending engagement surface.

[0071] In a preferred embodiment the non-circular cross section 3 of the piston rod 1 is made as a series of radial knobs 7, 9 provided on the top of the helical crest 5 making up the outer thread 2 of the piston rod 1, and the internal shape 11 of the first element 10 is made as a rigid flange 12 abutting a series of knobs 7, 9 as will be explained.

[0072] In one embodiment, the second element 20 is non-rotational secured in the housing of the injection device, e.g. by having click-arms 25 which are press fitted into similar grooves in the housing, or by the housing having a protruding part engaging the area surrounding the click-arms 25. Alternatively, the second element 20 can be formed integral with the housing, whereas the first element 10 can be rotated e.g. by a spring engine. The rotation of the first element 10 is thus transformed to a similar rotation of the piston rod 1 which is thus brought forward in the distal direction in a combined rotational and axial movement i.e. a helical movement.

[0073] In the embodiment depicted in FIG. 1 and in FIG. 2, the inner thread 21 of the second element 20, is actually made up from two thread segments 22 having a certain pitch, whereas the outer thread 2 of the piston rod 1, is provided as a helical root 4 surrounded by a helical crest 5 which is unbroken as will be explained later.

[0074] However, all though FIG. 1 uses the reference number “21” to illustrate the inner thread, all the figures actually depicts the inner thread as two thread segments indicated by the reference number “22”.

[0075] If the thread 21 of the second element 20 is an ordinary and well-known thread having a constant pitch, a small axial (and non-rotational) movement of the piston rod 1 is possible since there is a small axial clearance between the thread segments 22 thread 21 and the root 4 defining the thread connection between the second element 20 and the piston rod 1. The size of this axial clearance is the result of the moulding tolerances, but since the thread connection should not operate to tight, these tolerances are usually chosen such that a little axial clearance is present.

[0076] In order to eliminate the clearance an axial force is, according to the present invention, applied to the piston rod 1 such that the crest 5 of the helical thread 2 is pressed against the thread segments 22.

[0077] As disclosed in FIGS. 3 and 4, each thread segment 22 of the second element 20 is peripherally provided with a peripheral and resilient arm 23 which makes up an integral part of the thread segment 22. As best seen in FIG. 3, two such segments 22 each having a peripheral arm 23 are preferred.

[0078] As best seen in FIG. 5-6, the resilient arm 23 of each thread segment 22 is moulded such that the arm 23 each has a pitch angle that slightly deviates from the pitch angle of the thread segment 22 carrying the resilient arm 23. This is e.g. indicated with a “P” in FIG. 5. The axial distance between the peripheral end of the resilient arm 23 and the remaining part of the thread segment 22 is thus smaller than the actual pitch of the thread as indicated in FIG. 5. The pitch of the outer thread 2 of the piston rod 1 and the inner thread of the thread segment 22 are preferably the same with the resilient arm 23 narrowing the gap there between.

[0079] The resilient arm 23 thus applies an axial force to the crest 5 and urges the piston rod 1 in the proximal direction such that the opposite side surface of the crest 5 of the helical outer thread 2 is pressed against a distal side surface 24 of the thread segment 22. Since the force (indicated with “F” in FIG. 5 and in FIG. 6) always apply a light pressure on the piston rod 1 in the proximal direction any play in the threaded connection can be avoided. However, the axial direction of the force F can be in either of the axial directions.

[0080] Since the force F is applied onto the helical crest 5 of the outer thread 2, this helical crest 5 needs to be unbroken. Otherwise the force F of the resilient arm 23 cannot be delivered onto the crest 5 during a full rotation of 360 degrees. The non-circular cross-section of the piston rod 1 thus has to be of a kind that leaves the helical crest 5 unbroken.

[0081] In the example given in the figures and best seen in FIGS. 5 and 6, the non-circular cross-section 3 is applied as a plurality of knobs 7 extending radially from the helical crest 5 of the outer thread 2 of the piston rod 1. These knobs 7 are provided in parallel series. As further seen in FIG. 5 and in FIG. 6 a further protrusion 9 can be applied between the knobs 7. These protrusions 9 can, as disclosed, have a smaller height.

[0082] Each knob 7 has a side surface 8 as seen in FIG. 6. Together these side surfaces 8 form a longitudinal extending engagement surface making up the non-circular cross-section 3 of the piston rod 1. The further protrusion 9 also has a side surface.

[0083] As shown in FIG. 7, the first element 10 preferably has a rigid flange 12 in parallel with the longitudinal direction along the centre axis X, which rigid flange 12 abuts the further protrusions 9 and a resilient arm 13 which presses the further protrusions 9 against the rigid flange 12. Alternatively, the resilient arm 13 could press the side surface of the knobs 7 against the rigid flange 12. Any number of such rigid flanges 12 and resilient arms 13 can be provided.

[0084] The principle of the resilient arms 13 is to push the piston rod 1 rotorical against the rigid flanges 12 to avoid any play between the piston rod 1 and the first element 10.

[0085] Further, the first element 10 is externally provided with a number of external protrusions 14 which again is driven by a not shown drive mechanism such as e.g. an electric motor or a spring motor during expelling of the set dose. The injection device into which the piston rod drive arrangement is implemented is preferably a torsion spring driven automatic pre-filled injection device e.g. of the type disclosed in WO 2014/060369.

[0086] The three element making up the piston drive arrangement of the present invention; the piston rod, the first element and the second element could be implemented in a large variety of different injection devices. However, the shape of the three elements as illustrated in the appending figures indicates that the preferred injection device is a pen-shaped injection device.

[0087] Some preferred embodiments have been shown in the foregoing, but it should be stressed that the invention is not limited to these, but may be embodied in other ways within the subject matter defined in the following claims. In one example, the outer thread 2 of the piston rod 1 could be asymmetrical broken by the longitudinal extending engagement surface 8 in such way the at least one of the resilient arms 23 in turn apply an axial pressure onto the piston rod 1.