fig1 shows a drug delivery device in the form of an injection pen . the device has a distal end ( left end in fig1 ) and a proximal end ( right end in fig1 ). the component parts of the drug delivery device are shown in fig2 . the drug delivery device comprises a body or housing 10 , a cartridge holder 20 , a lead screw ( piston rod ) 30 , a drive sleeve 40 , a nut 50 , a number sleeve 60 , a button 70 , a dial grip or dose selector 80 , a torsion spring 90 , a cartridge 100 , a gauge element 110 , a ratchet spring 120 , a button spring 130 and a bearing 140 . a needle arrangement ( not shown ) with a needle hub and a needle cover may be provided as additional components , which can be exchanged as explained above . all components are located concentrically about a common principal axis i of the mechanism which is shown in fig3 . the housing 10 or body is a generally tubular element having a proximal end with an enlarged diameter . the housing 10 provides location for the liquid medication cartridge 100 and cartridge holder 20 , windows 11 a , 11 b for viewing the dose number on the number sleeve 60 and the gauge element 110 , and a feature on its external surface , e . g . a circumferential groove , to axially retain the dose selector 80 . a flange - like or cylindrical inner wall 12 comprises an inner thread engaging the piston rod 30 . the housing 10 further has at least one internal , axially orientated slot or the like for axially guiding the gauge element 110 . in the embodiment shown in the figures , the distal end is provided with an axially extending strip 13 partly overlapping cartridge holder 20 and the gauge element 110 . the figures depict the housing 10 as a single housing component . however , the housing 10 could comprise two or more housing components which may be permanently attached to each other during assembly of the device . the cartridge holder 20 is located at the distal side of housing 10 and permanently attached thereto . the cartridge holder may be a transparent or translucent component which is tubular to receive cartridge 100 . the distal end of cartridge holder 20 may be provided with means for attaching a needle arrangement . a removable cap ( not shown ) may be provided to fit over the cartridge holder 20 and may be retained via clip features on the housing 10 . the piston rod 30 is rotationally constrained to the drive sleeve 40 via a splined interface . when rotated , the piston rod 30 is forced to move axially relative to the drive sleeve 40 , through its threaded interface with the inner wall 12 of housing 10 . the lead screw 30 is an elongate member with an outer thread 31 ( fig4 a ) engaging the corresponding thread of the inner wall 12 of housing 10 . the thread 31 may have a large lead - in , for example a wedge shape form , at its distal end to engage a corresponding housing thread form on the first rotation . the interface comprises at least one longitudinal groove or track and a corresponding protrusion or spline of the driver 40 . at its distal end , the lead screw 30 is provided with an interface for clip attachment of the bearing 140 . in the present embodiment , this interface comprises two clip arms extending in the distal direction defining an insertion space between them for insertion of a bearing 140 interface . as an alternative , the interface may comprise only one single clip arm extending more than 180 ° about the longitudinal axis , or may comprise one or several clip arms . the clip arm ( s ) may have a bended form with a recessed clip portion . preferably , the clip arm ( s ) form a cylindrical outer face having a diameter equal to or smaller than the outer diameter of the lead screw 30 at the base of the groove ( flute base ) of the outer thread 31 . a concave contact surface is provided between the clip arms for abutment of a corresponding portion of bearing 140 . the drive sleeve 40 is a hollow member surrounding the lead screw 30 and arranged within number sleeve 60 . it extends from an interface with the number sleeve 60 to the contact with the button spring 130 . the drive sleeve 40 is axially movable relative to the housing 10 , the piston rod 30 and the number sleeve 60 in the distal direction against the bias of button spring 130 and in the opposite proximal direction under the bias of button spring 130 . a splined tooth interface with the housing 10 prevents rotation of the drive sleeve 40 during dose setting . this interface comprises a ring of radially extending outer teeth 41 at the distal end of drive sleeve 40 and corresponding radially extending inner teeth 14 of the housing component 10 . when the button 70 is pressed , these drive sleeve 40 to housing 10 spline teeth 14 , 41 are disengaged allowing the drive sleeve 40 to rotate relative to housing 10 . a further splined tooth interface with the number sleeve 60 is not engaged during dialling , but engages when the button 70 is pressed , preventing relative rotation between the drive sleeve 40 and number sleeve 60 during dispense . in the embodiment shown in fig4 a and 4 b this interface comprises inwardly directed splines 61 on a flange 62 on the inner surface of the number sleeve 60 and a ring of radially extending outer splines 42 of drive sleeve 40 . the corresponding splines 61 , 42 are located on the number sleeve 60 and the drive sleeve 40 , respectively , such that axial movement of the drive sleeve 40 relative to the ( axially fixed ) number sleeve 60 engages or disengages the splines to rotationally couple or decouple the drive sleeve 40 and the number sleeve 60 . preferably , the splines 61 , 42 are arranged such that they are decoupled when teeth 41 of drive sleeve 40 and inner teeth 14 of housing component 10 mesh and engage when teeth 41 and inner teeth 14 disengage . in a preferred embodiment the splines 61 , 42 are longer in the axial direction compared with teeth 41 , 14 . this allows engagement of the splines 61 , 42 shortly before disengagement of teeth 41 , 14 . in other words , the splines 61 , 42 and the teeth 41 , 14 are designed and arranged such that actuation of the button 70 rotationally constrains the drive sleeve 40 to the number sleeve 60 before the drive sleeve 40 is allowed to rotate relative to housing 10 . similarly , as the button 70 is released after dose dispensing axial movement of the drive sleeve 40 first rotationally constrains the drive sleeve 40 to the housing and thereafter decouples splines 61 , 42 . as an alternative to the corresponding splines 61 , 42 teeth may be provided . a fourth interface of the drive sleeve 40 which is shown in fig7 a to 8 in more detail comprises two clicker arms 43 located at the proximal end of drive sleeve 40 and a ring of ratchet teeth 64 of number sleeve 60 . the driver 40 has a threaded section 44 providing a helical track for the nut 50 . in addition , a last dose abutment or stop is provided which may be the end of the thread 44 track or preferably a rotational hard stop for interaction with a corresponding last dose stop of nut 50 , thus limiting movement of the nut 50 on the thread 44 . at least one longitudinal spline engages a corresponding track of the lead screw 30 . as shown in fig7 a in more detail , the proximal end of drive sleeve 40 is provided with a web 45 . the clicker arms 43 extend circumferentially in opposite directions of web 45 as cantilevers . the free end of each clicker arm 43 is provided with an outwardly directed tip with different ramp angles in the clockwise and the counter clockwise direction . a groove 46 extends on the inner side of drive sleeve 40 from one clicker arm 43 over the web 45 to the other clicker arm 43 . this groove 46 holds ratchet spring 120 which biases the clicker arms 43 radially outwards if they are deflected radially inwards . further , the drive sleeve is provided with a bearing face 47 in the form of an inner wall located distally of the clicker arms 43 . the last dose nut 50 is located between the number sleeve 60 and the drive sleeve 40 . it is rotationally constrained to the number sleeve 60 , via a splined interface . it moves along a helical path relative to the drive sleeve 40 , via a threaded interface ( thread 44 ), when relative rotation occurs between the number sleeve 60 and drive sleeve 40 which is during dialling only . as an alternative , the nut 50 may be splined to the driver 40 and threaded to the number sleeve 60 . in the embodiment shown in the figures , the nut 50 is a full nut , but in alternative embodiments it may be a half nut , i . e . a component extending approximately 180 ° around the center axis of the device . as a further alternative , if the driver 40 was formed from two separate components that became rigidly engaged during assembly then the nut 50 could also be a complete nut . a last dose stop is provided engaging stop of drive sleeve 40 when a dose is set corresponding to the remaining dispensable amount of medicament in the cartridge 100 . the dose indicator or number sleeve 60 is a tubular element as shown in fig2 and 3 . the number sleeve 60 is rotated during dose setting ( via dose selector 80 ) and dose correction and during dose dispensing by torsion spring 90 . together with gauge element 110 the number sleeve 60 defines a zero position (‘ at rest ’) and a maximum dose position . thus , the number sleeve 60 may also be seen as a dose setting member . for manufacturing reasons the number sleeve 60 of the embodiment shown in the figures comprises a number sleeve lower 60 a which is rigidly fixed to a number sleeve upper 60 b during assembly to form the number sleeve 60 . number sleeve lower 60 a and number sleeve upper 60 b are separate components only to simplify number sleeve 60 mould tooling and assembly . as an alternative , the number sleeve 60 may be a unitary component . the number sleeve 60 is constrained to the housing 10 by features towards the distal end to allow rotation but not translation . the number sleeve lower 60 a is marked with a sequence of numbers , which are visible through the gauge element 110 and the window 11 b in the housing 10 , to denote the dialled dose of medicament . further , the number sleeve lower 60 a has a portion with an outer thread 63 engaging the gauge element 110 . end stops are provided at the opposite ends of thread 63 to limit relative movement with respect to the gauge element 110 . an inner surface of the number sleeve upper 60 b is provided with a ring of ratchet teeth 64 . as shown in fig5 b and 7 b ratchet teeth 64 have different ramp angles in the clockwise and the counter clockwise direction corresponding to the ramp angles of the tip of clicker arm 43 . this ratchet provides a detented position between the number sleeve and drive sleeve corresponding to each dose unit , and engages the different ramped tooth angles during clockwise and anti - clockwise relative rotation . in addition , clutch features which have the form of a ring of splines 65 in the embodiment of fig5 b are provided inwardly directed on number sleeve upper 60 b for engagement with splines 71 of the button 70 during dose setting and dose correction . an interface for attachment of the torsion spring 90 to the number sleeve lower 60 a comprises large lead - ins and a groove feature with a pocket or anchor point for receiving a first coil or hook portion of the spring . the groove has an end feature in the form of a ramp that is in interference with the hook portion of the spring . the design of the groove is such that the spring 90 may be received within the pocket without interfering with the gauge element 110 . the button 70 which forms the proximal end of the device is permanently splined to the dose selector 80 . a central stem 72 extends distally from the proximal actuation face of the button 70 . the stem 72 is provided with the splines 71 for engagement with splines 65 of the number sleeve upper 60 b ( fig5 a ). button 70 is rotationally constrained via splines 65 , 71 to the number sleeve upper 60 b when the button 70 is not pressed , but this spline interface is disconnected when the button 70 is pressed . the button 70 has a discontinuous annular skirt with further splines 73 . when the button 70 is pressed , splines 73 on the button 70 engage with splines on the housing 10 , preventing rotation of the button 70 ( and hence the dose selector 80 ) during dispense . these splines 73 disengage when the button 70 is released , allowing a dose to be dialled . the dose selector 80 is axially constrained to the housing 10 . it is rotationally constrained , via the splined interface , to the button 70 . this splined interface which includes grooves interacting with spline features formed by the annular skirt of button 70 remains engaged irrespective of the dose button 70 axial positions . the dose selector 80 or dose dial grip is a sleeve - like component with a serrated outer skirt . the drive spring 90 is a torsion spring attached at its distal end to the housing 10 and at the other end to the number sleeve 60 . the torsion spring 90 is located inside the number sleeve 60 and surrounds a distal portion of the drive sleeve 40 . the spring has a hook at one end for attachment on the number sleeve 60 . a similar hook end is provided at the opposite end for attachment on the housing 10 . the torsion spring 90 is pre - wound upon assembly , such that it applies a torque to the number sleeve 60 when the mechanism is at zero units dialled . the action of rotating the dose selector 80 , to set a dose , rotates the number sleeve 60 relative to the housing 10 , and charges the torsion spring 90 further . the torsion spring 90 is formed from a helical wire with at least two different pitches . in fig4 a and 4 b both ends are formed from ‘ closed ’ coils , i . e . the pitch equals the wire diameter and each coil contacts the adjacent coil . the central portion has ‘ open ’ coils , i . e . the coils do not contact each other . the cartridge 100 is received in cartridge holder 20 ( fig3 ). the cartridge 100 may be a glass ampoule having a moveable rubber bung 101 at its proximal end . the distal end of cartridge 100 is provided with a pierceable rubber seal which is held in place by a crimped annular metal band . in the embodiment depicted in the figures , the cartridge 100 is a standard 1 . 5 ml cartridge . the device is designed to be disposable in that the cartridge 100 cannot be replaced by the user or health care professional . however , a reusable variant of the device could be provided by making the cartridge holder 20 removable and allowing backwinding of the lead screw 30 and the resetting of nut 50 . the gauge element 110 is constrained to prevent rotation but allow translation relative to the housing 10 via a splined interface . the gauge element 110 has a helical feature on its inner surface which engages with the helical thread cut in the number sleeve 60 such that rotation of the number sleeve 60 causes axial translation of the gauge element 110 . this helical feature on the gauge element 110 also creates stop abutments against the end of the helical cut in the number sleeve 60 to limit the minimum and maximum dose that can be set . the gauge element 110 is a window element which has a generally plate or band like component having a central aperture 111 or window and two flanges extending on either side of the aperture . the flanges are preferably not transparent and thus shield or cover the number sleeve 60 , whereas the aperture 111 or window allows viewing a portion of the number sleeve lower 60 a . further , gauge element 110 may have a cam and a recess interacting with a clicker arm ( not shown ) of the number sleeve 60 at the end of dose dispensing . as can be seen in fig6 the ratchet spring 120 has the form of a ring segment . it may be curved metal plate having a higher stiffness compared with clicker arms 43 . the button spring 130 is a compression spring . the axial position of the drive sleeve 40 , ratchet spring 120 and button 70 is defined by the action of the button spring 130 , which applies a force on the drive sleeve 40 in the proximal direction . this spring force is reacted via the drive sleeve 40 and button 70 , and when ‘ at rest ’ it is further reacted through the dose selector 80 to the housing 10 . in the ‘ at rest ’ position , it ensures that the button splines 71 are engaged with the number sleeve splines 65 , and the drive sleeve teeth 41 are engaged with teeth 14 of the housing 10 . the bearing 140 is axially constrained to the piston rod 30 and acts on the bung 101 within the liquid medicament cartridge . it is axially clipped to the lead screw 30 , but free to rotate . the bearing 140 comprises a disc having a stem extending in the proximal direction . the stem has at its proximal end a convex contact surface . in addition , a recessed portion may be provided on the stem . the curvature of the convex contact surface and the concave contact surface of the piston rod is chosen such that the contact diameter between the bearing 140 and lead screw 30 is small to minimize the frictional losses at this interface . the design of the clip interface between bearing 140 and lead screw 30 permits the lead screw 30 to be assembled axially , from the proximal end and through the thread engagement to the housing 10 , which simplifies assembly . in addition , this design allows a simple “ open and shut ” mould tooling for both components . with the device in the ‘ at rest ’ condition as shown in fig4 a , the number sleeve 60 is positioned against its zero dose abutment with the gauge element 110 and the button 70 is not depressed . dose marking ‘ 0 ’ on the number sleeve 60 is visible through the windows 11 b and 111 of the housing 10 and gauge element 110 , respectively . the torsion spring 90 , which has a number of pre - wound turns applied to it during assembly of the device , applies a torque to the number sleeve 60 and is prevented from rotating by the zero dose abutment . it may also be possible to ‘ back - wind ’ the mechanism slightly due to an offset between the zero dose stop and the angular offset of the drive sleeve spline teeth . this has the effect of preventing possible weepage when a dose is dialled and the zero dose abutment is disengaged . the automated assembly of the torsion spring 90 into the number sleeve 60 can be achieved by incorporating large lead - ins and a groove feature to the number sleeve 60 . as the torsion spring 90 is rotated during assembly , the hook end form locates in the groove feature before engaging the anchor point in the number sleeve 60 . to help to prevent the torsion spring 90 disengaging the anchor point during subsequent assembly steps it is possible to create an interference between the torsion spring 90 and the number sleeve 60 , or a one - way clip feature . the user selects a variable dose of liquid medicament by rotating the dial grip or dose selector 80 clockwise , which generates an identical rotation in the number sleeve 60 via the splined interfaces between the dial grip 80 and the button 70 and between the button 70 and number sleeve upper 60 b . the window 11 b in the housing 10 and the window 111 of the gauge component 110 allow the user to view the set dose number on the number sleeve lower 60 a . the drive sleeve 40 is prevented from rotating as the dose is set and the number sleeve 60 rotated , due to the engagement of its splined teeth 41 with the teeth 14 of housing 10 . relative rotation therefore occurs between the drive sleeve 40 and the number sleeve 60 , causing the ratchet 43 , 64 to slip . the user torque required to rotate the dial grip 80 is a sum of the torque required to wind up the drive spring 90 , and the torque required to overhaul the ratchet feature 43 , 64 . the clicker arms 43 on the drive sleeve 40 are forced radially outwards by the ratchet spring 120 and into engagement with the teeth 64 on the number sleeve upper 60 b . the torque required to overhaul the ratchet in the dose set direction is a function of the radial force supplied by the ratchet spring 120 , the clockwise ramp angle of the ratchet 43 , 64 , the friction coefficient between the mating surfaces and the mean radius of the ratchet features 43 , 64 . as the user rotates the dial grip 80 sufficiently to increment the mechanism by 1 increment , the number sleeve 60 rotates relative to the drive sleeve 40 by 1 ratchet tooth 64 . at this point the ratchet teeth 64 re - engage into the next detented position . an audible click is generated by the ratchet re - engagement , and tactile feedback is given by the change in torque input required . with no user torque applied to the dial grip , the number sleeve 60 is now prevented from rotating back under the torque applied by the drive spring 90 , solely by the ratchet engagement 43 , 64 between the drive sleeve and the number sleeve upper . the torque necessary to overhaul the ratchet 43 , 64 in the anti - clockwise direction is a function of the radial force applied by the ratchet spring 120 , the anti - clockwise ramp angle of the ratchet 43 , 64 , the friction coefficient between the mating surfaces and the mean radius of the ratchet features . the torque necessary to overhaul the ratchet 43 , 64 must be greater than the torque applied to the number sleeve 60 by the drive spring 90 . the ratchet ramp angle is therefore increased in the anti - clockwise direction to ensure this is the case whilst ensuring the dial - up torque is as low as possible . the user may now choose to increase the selected dose by continuing to rotate the dial grip 80 in the clockwise direction . the process of overhauling the ratchet interfaces 43 , 64 between the number sleeve 60 and drive sleeve 40 is repeated for each dose increment . additional energy is stored within the drive spring 90 for each dose increment and audible and tactile feedback is provided for each increment dialled by the re - engagement of the ratchet teeth 64 with clicker arms 43 . the torque required to rotate the dial grip 80 increases as the torque required to wind up the drive spring 90 increases . the torque required to overhaul the ratchet in the anti - clockwise direction must therefore be greater than the torque applied to the number sleeve by the torsion spring when the maximum dose has been reached . with the mechanism in a state in which a dose has been selected , the user is able to deselect ( correct ) any number of increments from this dose . deselecting a dose is achieved by the user rotating the dial grip 80 anti - clockwise . the torque applied to the dial grip 80 by the user is sufficient , when combined with the torque applied by the drive spring 90 , to overhaul the ratchet 43 , 64 between the drive sleeve 40 and number sleeve upper 60 b in the anti - clockwise direction . when the ratchet 43 , 64 is overhauled , anti - clockwise rotation occurs in the number sleeve 60 returning it towards the zero dose position , and unwinding the drive spring 90 . with the mechanism in a state in which a dose has been selected , the user is able to activate the mechanism to commence delivery of a dose . delivery of a dose is initiated by the user depressing the button 70 axially as shown in fig4 b . the drive sleeve 40 travels axially with the button 70 . the splined tooth interface 42 , 61 rotationally constrains the drive sleeve 40 to the number sleeve lower 60 a . the splined tooth interface 14 , 41 between the drive sleeve and the housing 10 disengages , so the drive sleeve 40 can now rotate and is driven by the drive spring 90 via the number sleeve 60 , the ratchet 43 , 64 between the number sleeve upper and the drive sleeve and via splines 42 , 61 between the number sleeve lower and the drive sleeve . rotation of the drive sleeve 40 causes the piston rod 30 to rotate due to their splined engagement , and the piston rod 30 then advances due to its threaded engagement to the housing 10 . delivery of a dose continues via the mechanical interactions described above while the user continues to depress the button 70 . if the user releases the button 70 , the button spring 130 returns the drive sleeve to its ‘ at rest ’ position ( together with the button ), engaging the splines 14 , 41 between the drive sleeve and housing , preventing further rotation and stopping dose delivery . once the delivery of a dose is stopped , by the number sleeve 60 returning to its zero dose position , the user may release the button 70 , which will re - engage the spline teeth 14 , 41 between the drive sleeve 40 and housing 10 . the mechanism is now returned to the ‘ at rest ’ condition . although two clicker arms 43 are shown on the drive sleeve 40 , there could be one or many . the clicker arms 43 are shown as cantilevers , but could be replaced by beams that are built - in at both ends . any number of axial fingers at the end of the drive sleeve 40 could be used instead of the circumferential clicker arms 43 . the ratchet spring 120 may not be necessary , if any compliant parts of the ratchet system provide sufficient torque and do not suffer from creep . the clicker arms 43 on the drive sleeve 40 could be replaced by a metal pressing that is rotationally constrained relative to the drive sleeve and that incorporates flexible arms to interact with the teeth on the number sleeve upper 60 b . the drive sleeve 40 could be split into two parts with the part at the proximal end incorporating the ratchet features 43 and / or the bearing face 47 for the button 70 . although there is no tactile or audible feedback during dose dispense in the depicted embodiment , it could be added between any component that rotates during dose dispense and any component that does not , e . g . between the number sleeve upper 60 b and the button 70 .
the present disclosure is generally directed to a drug delivery device for selecting and dispensing a number of user variable doses of a medicament . the device comprises a housing , a dose selector operable to set a dose by rotation relative to the housing , a number sleeve arranged within the housing , a drive sleeve which is rotationally constrained to the housing during dose setting and dose correcting and which is rotatable relative to the housing during dose dispensing , a piston rod coupled to the housing and to the drive sleeve a drive spring arranged between the housing and the number sleeve , and a ratchet operatively arranged between the drive sleeve and the number sleeve . according to the disclosure the ratchet comprises at least one clicker arm of one of the number sleeve and the drive sleeve and ratchet teeth of the other of the number sleeve and the drive sleeve .