fig1 and 2 show views of the drug delivery device . fig1 illustrates the component parts incorporated into the injection device which are a body 10 , a cartridge holder 20 , a trigger 40 , a dial member 50 comprising a dial 51 and a dial cover 52 a medicament cartridge 30 , a last dose nut 60 , a dial gear 70 , a trigger spring 80 , a prism 90 , a number wheel 100 , a release gear 110 , a belt assembly 120 , a belt gear 130 , a main spring 140 and a chassis 150 . the body 10 , the cartridge holder 20 and chassis 150 form a housing which has a distal end at the side receiving the cartridge 30 ( right hand side in fig2 ) and an opposite proximal end . the cartridge holder defines a longitudinal axis of the housing . a rotational axis is provided perpendicular to this longitudinal axis with the trigger 40 , the dial member 50 , the last dose nut 60 , the dial gear 70 , the trigger spring 80 , the number wheel 100 and the release gear 110 are arranged concentrically about this rotational axis . the medicament cartridge 30 is housed within the cartridge holder 20 . the cartridge holder 20 is rigidly constrained in the body 10 . the cartridge holder 20 provides location and containment of the medicament cartridge and prism 90 . the belt assembly 120 comprises a belt 121 and a plunger 122 . the belt 121 is a flexible element with high tensile modulus and strength . suitable materials include glass or aramid fibre reinforced poly - urethane . features at each end of the belt 121 provide axial constraint and allow it to carry a tensile load . the distal end of the belt 121 is connected to the plunger 122 via spline features as shown in fig3 . the opposite end of the belt 121 is restrained by the belt gear 130 and partially wound onto it . fig3 shows the belt 121 assembled to the belt gear 130 in the “ as delivered ” condition ( prior to any doses being delivered ). the distal face of the plunger 122 abuts a bung of the medicament cartridge 30 and the main spring 140 acts directly on the proximal surface of the plunger 122 . it is the main spring 140 acting on the plunger 122 that drives the bung axially in order to deliver medicament . tension in the belt 121 prevents the main spring 140 releasing and , therefore , by controlling the release of the belt 121 , accurate control of the medicament delivery can be achieved . fig4 shows the main spring 140 in its fully compressed state , i . e . the state prior to dispensing the first dose , interposed between the plunger 122 and a bearing face of the chassis 150 . the belt 121 is held in tension by the main spring 140 and follows a curved path in the device defined by a belt guide feature 151 on the chassis 150 . the main spring 140 is supplied to the user in the fully charged state ( near “ coil bound ”). it acts between the proximal face of the plunger 122 and an abutment on the chassis 150 . tension in the belt 121 prevents the energy stored in the main spring 140 from being released until a dose is dispensed . the belt gear 130 controls release of the belt 121 through a geared interface with a pinion 111 of the release gear 110 . it is radially constrained by the chassis 150 via a combination of abutments . the combined effect of these abutments ensure that the resultant force acting on the belt gear 130 from the belt 121 biases the geared interface with pinion 111 of the release gear 110 into engagement as shown in fig5 . this acts to minimize backlash between the gears and also reduce the risk of disengagement in the event of shock loading . the chassis 150 locates the mechanism within the body 10 and is rigidly fixed into the body 10 via spline and spring clip features . it provides location for the belt gear 130 and belt 121 . flexible features within the chassis 150 ( chassis locking arms 152 ) fix the release gear 110 rotationally during dialing ( fig6 a ) but disengage to allow rotation during triggering ( fig6 b ). abutments adjacent to these chassis locking arms 152 provide tangential support and prevent excessive deflection when loaded by the release gear 110 . the number wheel 100 incorporates stop features 101 , 102 which engage with abutments 153 , 154 on the chassis 150 and correspond with the minimum ( fig7 a ) and maximum ( fig7 b ) dose set . this restricts the maximum dose that may be set and creates the end of dose stop when the mechanism returns to the zero unit position . the number wheel 100 is printed with a series of numbers on the external surface which create the dose display when viewed through the prism 90 . the number wheel 100 is rotationally coupled to the dial gear 70 as shown in fig8 . further , the number wheel is axially located between the chassis 150 and the body 10 and radially constrained by the body 10 . the dose set is displayed on the outer surface of the device to provide feedback to the user . in this embodiment , the prism 90 reflects the display from the number wheel 100 so that the dose is displayed on the front face of the device ( upper side in fig2 ). the prism 90 is retained within the cartridge holder 20 and body 10 once assembled as shown in fig1 . the prism 90 uses the phenomenon of “ total internal reflection ” to achieve reflection of the number without any special treatment to the surfaces ( such as metal coating ). the nature of this prism is that the display is mirrored . to account for this , the printing on the number wheel 100 is reversed so the net effect provides a conventional dose number display . an additional function of the prism 90 is that the surfaces can be designed to also provide magnification , in addition to the primary function of reflection . alternative prism arrangements ( for example a penta - prism ) could perform the same function without mirroring the display if required . an alternative embodiment negates the requirement for the prism 90 component and displays the dose on the side of the device . the number wheel 100 is then printed with conventional , non - mirrored , text and a small window is formed in the side of the body 10 . the dial gear 70 is rotationally coupled to the dial member 50 during dialing ( fig9 ) and rotationally coupled to the release gear 110 during dispense ( fig1 ). the dial gear 70 may translate axially between abutments provided by the release gear 110 and the dial member 50 and is biased into contact with the dial member 50 via the trigger spring 80 when the trigger 40 is not depressed . the trigger spring 80 acts between the dial gear 70 and release gear 110 . the chassis locking arms 152 are axially coupled to the dial gear 70 with snap clips which permit relative rotation . the dial member 50 comprises the dial 51 and the dial cover 52 which are permanently and rigidly fixed together . the dial member 50 is axially and radially located in the body 10 via snap clips and the rotational position is detented via a flexible cantilever arm 53 locating in radial ratchet teeth 11 within the body 10 ( fig1 ). these detent features provide positive feedback to the user during dialing and align the dial member 50 and number wheel 100 with the body 10 so the units of the dose display accurately align with the prism 90 . the trigger 40 is snap fitted into the dial member 50 and axially constrained between abutments on the dial member 50 and dial gear 70 . the user may axially translate the trigger 40 between these abutments by overcoming the force of the trigger spring 80 which is transferred through the dial gear 70 ( fig1 ). during dose setting , the release gear 110 is in toothed engagement with the belt gear 130 and rotationally fixed by the chassis locking arms 152 . when the trigger 40 is depressed , the release gear 110 is rotationally coupled to the dial gear 70 and is released from the chassis locking arms 152 . it is axially constrained between the dial gear 70 and chassis 150 and is biased toward the chassis 150 abutment by the trigger spring 80 . the mechanism incorporates a last dose nut 60 to prevent setting of a dose greater than that which remains within the medicament cartridge . this is positioned between the dial gear 70 and release gear 110 since the dial gear 70 rotates relative to the release gear 110 during dose set and not during dispense . the last dose nut 60 is splined to the inner surface of the dial gear 70 and threaded to the release gear 110 such that clockwise rotation of the dial member 50 rotates the last dose nut 60 and translates it towards the last dose stop on the release gear 110 . the last dose nut 60 is successively translated towards the stop as doses are set until the cartridge dose limit is reached . at this point the last dose nut 60 contacts the abutment 112 on the release gear 110 which prevents further clockwise rotation of the last dose nut 60 and , therefore , rotation of the dial gear 70 and dial member 50 . fig1 and 14 show the device shortly before the nut contacts abutment 112 . the number of permissible rotations of the last dose nut 60 is determined by the capacity of the cartridge 30 . the dial member 50 is rotated by the user in a clockwise direction to set a dose starting from the position shown in fig1 and 16 . the dose can be cancelled by rotating the dial member 50 in a counter - clockwise direction either before any dispense or , alternatively , if the trigger 40 is released mid - dispense , the remaining dose may be cancelled . the selected dose is displayed through the body 10 via the number wheel 100 and prism 90 as described previously . irrespective of whether the dial member 50 is rotated clockwise or counter - clockwise the dose displayed will always indicate the dose to be dispensed . in addition , the dose display also decrements as the dose is dispensed and thus displays the dose remaining to be dispensed . as the dose is dialed up the number wheel 100 is driven away from the zero unit stop 153 on the chassis 150 and towards the maximum unit stop 154 . the dial member 50 can be rotated by the user in both clockwise and counter - clockwise directions when the number wheel 100 is not in contact with the zero dose stop abutment 153 or maximum dose stop abutment 154 of the chassis 150 . the zero unit abutment 153 prevents counter - clockwise rotation of the dial member 50 below the zero unit position . the maximum dose abutment 154 prevents setting of a dose greater than the mechanism maximum which is depicted in fig1 and 18 . the detent features 11 , 53 between dial member 50 and body 10 controls the position of the dial member 50 to ensure that discrete units are selected and that the spline features between dial member 50 and release gear 110 are correctly aligned to permit spline meshing when the device is triggered . during dose setting , the release gear 110 is biased by the trigger spring 80 into engagement with the locking arms 152 , which then couple the release gear 110 to the chassis 150 . the release gear 110 is therefore fixed rotationally during dose set . this in turn prevents rotation of the belt gear 130 and , therefore , release of the belt 121 . the device may be triggered by the user through application of an axial force on the trigger 40 ( fig1 a ). the trigger 40 acts on the dial gear 70 , translating the dial gear 70 and chassis locking arms 152 , compressing the trigger spring 80 . as the dial gear 70 translates it first decouples from the dial member 50 as the face teeth disengage . at this stage ( mid trigger position , fig1 b ) the dial member 50 can no longer be rotated in either direction since the dial member 50 detent arm 53 is prevented from deflecting by an annular abutment on the dial gear 70 . further translation of the trigger 40 couples the dial gear 70 to release gear 110 via splines and finally decouples the release gear 110 from the chassis 150 ( fig1 c ). on triggering , the release gear 110 rotates , controlled by the dial gear 70 and number wheel 100 . the belt gear 130 rotates , due to the torque generated by the main spring 140 acting through the belt 121 . as the main spring 140 extends , the plunger 122 is driven against the bung , creating a distal translation and causing medicament to be dispensed . since the release gear 110 , dial gear 70 and number wheel 100 are rotationally coupled , the number wheel 100 also rotates during dispense in a counter - clockwise direction , returning towards the zero unit stop 101 , 153 . at the zero unit position the number wheel 100 contacts the abutment 153 on the chassis 150 , preventing further rotation of the dial gear 70 , release gear 110 and belt gear 130 , stopping release of the belt 121 and any further dispense of medicament . the trigger 40 is subsequently released , re - engaging the chassis locking arms 152 to lock the rotational position of the release gear 110 , belt gear 130 , belt 121 , plunger 122 and main spring 140 independently from the zero unit stop between chassis 150 and number wheel 100 . this allows the next dose to be set without immediate release of the main spring 140 . aside from the last dose nut 60 , release gear 110 , belt gear 130 , belt 121 , plunger 122 and main spring 140 all other components in the device return to their original positions once the entire dose has completed dispense . the release gear 110 splines that engage with the chassis locking arms 152 are angled so the release gear 110 is turned against the torque induced by the main spring 140 as they re - engage when the trigger 40 is released ( fig2 a , 20 b ). back - winding the release gear 110 ensures that the chassis locking arms 152 react the main spring 140 force in place of the zero unit stop as the trigger 40 is released . this prevents the release gear 110 rotating to take up clearance at this interface when the subsequent dose is dialed ( and the zero unit stop is disengaged ), which could lead to the dispense of some fluid . feedback is provided to the user during dose setting by the interaction between the dial member 50 detent arm 53 and the body 10 ratchet features 11 . dispense feedback is created through interaction between the chassis 150 and ratchet features on the release gear 110 . a cantilever arm on the chassis 150 rides over the ratchet features on the release gear 110 . a single , distinctive click is created as the device returns to the zero unit stop . this provides clear feedback to the user that the dose has been completed in addition to the dispense clicker ceasing . a cantilever arm 155 in the chassis 150 engages with the dial gear 70 when in the dispense condition . this arm is deflected as the dial gear 70 approaches the zero unit stop and rapidly released as the dial gear 70 engages the zero unit stop ( fig2 a to 21 c ). it is possible to incorporate a mechanism that allows the user to control the speed of dispense by the distance that they move the trigger 40 . in this second embodiment the features and functions are identical to the first embodiment as described above . however , an additional system 160 is included as shown in fig2 a to 22 c . the embodiment shows a multiplate clutch system 160 integrated into the device acting between the dial member 50 ( which is locked during dispense ) and dial gear 70 . the system comprises a carrier 161 which is splined to the dial member 50 , a clutch spring 162 and a clutch pack comprising rotating plates 163 which are splined to the dial gear 70 and static plates 164 which are splined to the dial member 50 via carrier 161 . for the embodiment shown in fig2 a to 22 c , force applied to the clutch pack 163 , 164 from the clutch spring 162 reduces as the trigger 40 is depressed . multiple clutch plates 163 , 164 increase the torque capacity of the clutch for a given clutch spring force . in this embodiment , the overall trigger 40 travel is increased by the addition of the user variable speed control . the facility for removing the need for a user to prime the device when first used can also be incorporated . this involves removing the variable distance ( dependent on component and cartridge tolerances ) between the cartridge bung 31 and the plunger 122 during manufacture such that the plunger 122 is in contact ( and applies a small force ) to the bung 31 when assembled . this “ prime elimination ” is achieved using the following method : the cartridge holder 20 must be divided into two separate components ( cartridge holder 20 and rear body ) and the device assembled omitting the rear body . a small dose of approximately 10 units is dialed by rotating the dial member 50 as the user would . the belt gear 130 is rotationally coupled to an assembly tool with torque measurement capability . the trigger 40 is depressed to release the mechanism and the torque generated in the belt gear 130 is measured as it is rotated clockwise via the assembly tool , thus releasing belt 121 . as the belt 121 is released , the plunger 122 approaches the bung 31 under the main spring 140 force . when the plunger 122 contacts the bung 31 , the bung will begin to react a proportion of the main spring 140 force , thus reducing the belt gear 130 torque . measurement of this change in torque as the belt 121 is released allows a specific force to be applied to the bung 31 by the main spring 140 . release of the trigger 40 subsequently locks the mechanism and any set doses remaining are then dialed to zero . finally the rear body is clipped into position to complete the assembly .
the present disclosure relates to a drive mechanism which is suitable for an injection device , especially a drug delivery device . the mechanism comprises a housing having a longitudinal axis defined by a compartment for receiving a cartridge , a plunger suitable for acting on a bung of a cartridge retained in the housing , a strained pressure spring arranged between the housing and the plunger , a retaining member coupled to the plunger , and a release member operable between a first state , in which the release member constrains the retaining member to the housing , and a second state , in which the release member is movable relative to the housing .