text: fig1 shows a pressurised metered dose inhaler 1 which comprises an actuator housing 2 that contains a pressurised dispensing container 3 . the actuator housing 2 comprises a generally tubular body 7 and a depending mouthpiece 6 at one end covered by a dust cap 106 . the tubular body 7 may have a generally circular cross - sectional shape . however , in the illustrated example the tubular body 7 comprises a squarer cross - sectional shape with a front wall 7 a , side walls 7 b and rear wall 7 c . a stem block 8 is provided at a basal end of the tubular body 7 nearest the mouthpiece 6 . the actuator housing 2 may be formed from two mouldings comprising a front case 17 a and a rear case 17 b . the front case 17 a comprises the mouthpiece 6 and a portion of the tubular body 7 comprising the front wall 7 a and half of each side wall 7 b . in addition ; the front case 17 a comprises a base 7 d of the actuator housing 2 from which project the stem block 8 , a c - section channel 8 and a tubular extension 10 defining a bore . the c - section channel 80 comprises an aperture 81 . the rear case 17 b comprises the rear wall 7 c and the remainder of the side walls 7 b . a rim 71 provided on an inner face of the rear case 17 b , as shown in fig4 , defines a cavity 71 a the function of which will be described below . the rim 71 is generally circular except for a flattened section 71 a . the front and rear cases 17 a , 17 b may be snap - fit together by means of formations 7 e on each casing . a viewing window 62 in the form of a cut out is provided in the rear wall 7 c of the actuator housing 2 . a lock - out aperture 61 is provided in the front wall 7 a just above the mouthpiece 6 . the dust cap 106 is provided with a body that encloses the mouthpiece 6 and a tang 161 that in use can protrude through the lock - out aperture 61 when the dust cap 106 is in position on the mouthpiece 6 . the pressurised dispensing container 3 typically comprises a canister 4 and a valve ( not shown ). to assemble the pressurised metered dose inhaler , the dispensing container 3 is inserted into an open end 7 f of the tubular body 7 of the housing 2 such that a valve stem of the valve is received in the stem block 8 . according to the present disclosure , the pressurised metered dose inhaler 1 includes a dose indicator device marked generally by reference 9 . the dose indicator device is located towards the mouthpiece end of the tubular body 7 situated between the stem block 8 and the rear wall 7 c . in general the dose indicator device 9 comprises an inner wheel 11 , an annular outer wheel 12 , an actuator member 13 , a cup - shaped housing 70 and a spring 15 . the cup - shaped housing 70 of the dose indicator device 9 defines , together with the rear wall 7 c , a housing enclosure of the dose indicator device 9 which locates and aligns the inner wheel 11 and annular outer wheel 12 . in the following description , unless the context otherwise requires , the term “ inwardly - facing ” refers to a direction which is inwards towards the pressurised dispensing container 3 within the tubular body 7 . the term “ outwardly - facing ” refers to a direction which is outwards away from the pressurised dispensing container 3 within the tubular body 7 . further , unless the context otherwise requires , terms such as “ upwards ” refer to the direction towards the open end 7 f of the tubular body 7 while terms such as “ downwards ” refer to the opposite direction . the inner wheel 11 comprises a generally solid cylindrical body 20 that is mounted in use to rotate about a first rotational axis 21 as marked on fig5 . the body 20 is provided with three layers which are positioned adjacent to one another in a direction along the first rotational axis 21 . the first layer comprises a plurality of primary indexing teeth 22 . in the example shown there are 10 primary indexing teeth 22 . adjacent the primary indexing teeth 22 is the second layer which comprises a plurality of indentations 24 . each indentation 24 takes the form of a part circular cut out in the body 20 of the inner wheel 11 . in the example shown there are 10 indentations 24 , 1 for each of the primary indexing teeth 22 . adjacent the indentations 24 is the third layer which comprises a flexible drive arm 23 . the flexible drive arm 23 comprises an arm portion 23 a that depends from the body 20 and terminates at a distal end with a circular pin 23 b . as shown most clearly in fig9 , an end face 20 a of the body 20 of the inner wheel 11 comprises a peripheral rebate 25 . the annular outer wheel 12 , as shown in fig8 , has a generally annular body 30 which in use is rotatable about the longitudinal axis 21 . the body 30 comprises an annular portion 38 a dependent flange 36 . the annular body 30 defines a central aperture 31 which accommodates on assembly as described below a portion of the inner wheel 11 . the flange 36 extends both radially outwards of the annular portion 38 and also for a distance radially inwardly of the annular portion 38 to define an annular shoulder 39 as shown in fig8 . a plurality of indexing teeth 33 are provided around an outer face of the annular portion 38 . in the example shown 21 indexing teeth 33 are provided . as well as the indexing teeth 35 , the outer face of the annular portion 38 is provided with a land 33 a the function of which will be described below . an outwardly - facing face 36 a of the flange 36 and / or the end face 20 a of the inner wheel 11 are provided with one or more indicia to provide information to a user of the pressurised metered dose inhaler 1 regarding the number of doses dispensed from the inhaler or remaining in the inhaler . for example , the indicia may comprise a set of increasing or decreasing numbers , a series of pictograms , a series of words or a band of changing colour — e . g . a band which changes from green to red around the circumference of the annular outer wheel 12 . as shown in fig2 to 4 , the actuator member 13 comprises a yoke 41 at its upper end from which depend two legs 42 defining a channel 44 therebetween . an actuator drive arm 45 in the form of a flexible extension is provided on one of the legs 42 facing , and extending into , the channel 44 . in use , as described below the actuator drive arm 45 drives the rotation of the inner wheel 11 . also depending from the yoke 41 is a plunger rod 46 . the cup - shaped housing 70 is shown most clearly in fig8 and 9 . the cup - shaped housing 70 comprises a body 50 which has a generally disc - shaped planar portion 51 and a dependent rim 52 which is generally circular except for a flattened section 52 a . a projection 74 located at one point of the periphery of the body 50 projects perpendicularly to the planar portion 51 . the disc - shaped portion 51 of the body 50 comprises a centrally - located aperture 54 bounded by a rim 54 a which , as described below , in use accommodates a portion of the inner wheel 11 . a portion of the rim 54 a is interrupted and in the gap is provided a first flexible restraining arm 57 in the form of a flexible arm portion which has a circular pin at its distal end . a second flexible restraining arm 55 that again has a circular formation at its distal end is provided on the inner face of the rim 52 . opposite the restraining arm 55 is provided on the inner face of the rim 52 a deflector 59 the function of which will be described below . to assemble the pressurised metered dose inhaler 1 , the annular outer wheel 12 , inner wheel 11 and cup - shaped housing 70 are first nested together . when nested the primary indexing teeth 22 of the inner wheel 11 project through the aperture 54 with the indentations 24 being aligned with and bearing against the inner face of the rim 54 a . also the peripheral rebate 25 of the inner wheel 11 engages against the shoulder 39 of the annular outer wheel 12 . with the inner wheel 11 and annular outer wheel 12 nested together the pin 23 b of the flexible drive arm 23 lies radially just outwards of the location of the indexing teeth 33 of the annular outer wheel 12 but extends in the direction of the longitudinal axis 21 to be in line with the plane of the indexing teeth 33 as shown in fig6 . as such , the arm portion 23 a of the flexible drive arm 23 spans the annular portion 38 . at the same time , the restraining arm 55 of the cup - shaped housing 70 is engaged with one of the indexing teeth 33 . the flange 36 is fully received within the cavity of the cup - shaped housing 70 . as such , a very compact arrangement is obtained as shown in fig5 where the depth of the inner wheel 11 and annular outer wheel 12 are fully contained within the depth of the cup - shaped housing 70 . the nested annular outer wheel 12 , inner wheel 11 and cup - shaped housing 70 are then inserted into the cavity 71 a defined by the rim 71 of the rear case 17 b . the rim 71 serves to locate the other components and prevent rotation of the cup - shaped housing 70 relative to the rear case 17 b by virtue of the inter - engagement of the flattened sections 52 a and 71 b . next , the spring 15 is inserted into the bore of the tubular extension 10 . the actuator member 13 is then inserted with the two legs 42 being slidingly received in the c - section channel 80 . at the same time the plunger rod 46 is engaged in the open end of the bore of the tubular extension 10 so as to contact and rest on the upper end of the spring 15 . the front and rear cases 17 a and 17 b are then fastened together . once fastened the cup - shaped housing 70 is abutted against the rear face of the c - section channel 80 such that the indexing teeth 22 of the inner wheel 1 project through the aperture 81 in the c - section channel 80 into alignment with the legs 42 and flexible drive arm 45 of the actuator member 13 . the abutment prevents any of the components becoming disengaged from one another . in addition , when abutted the projection 74 of the cup - shaped housing spans across the top of the c - section channel 80 preventing the actuator member 13 from becoming detached from the c - section channel is the actuator housing is inverted . a pressurised dispensing container 3 can now be inserted through the open end 7 f of the tubular body 7 such that a leading face of a ferrule of the dispensing container 3 contacts the yoke 41 of the actuator member 13 . the aperture in the middle of the yoke 41 accommodates the valve stem to enable it to project beyond the yoke 41 into engagement with the stem block 8 . the action of the spring 15 on the base of the plunger rod 46 ensures that in the rest position , the yoke 41 is held in face to face contact with the ferrule of the dispensing container 3 . fig6 and 7 illustrate the relative positions of the inner wheel 11 , annular outer wheel 12 and cup - shaped housing 70 at the rest position . at rest , the distal end of the actuator drive arm 45 is out of contact with the primary indexing teeth 22 of the inner wheel 11 . also , in this rest position , the first flexible restraining arm 57 is engaged with one of the indentations 24 of the inner wheel 11 and the second flexible restraining arm 55 is engaged with one of the indentations 33 of the annular outer wheel 12 . the purpose of the first and second flexible restraining arms is to restrain inadvertent rotation in either direction of either the inner wheel 11 or the annular outer wheel 12 other than when the components are being actively driven on actuation of the dispensing container 3 as will be described below . in other words , the action of the flexible restraining arms helps to prevent actuation of the dose indicator device 9 if the device is dropped , shaken or knocked . in operation , as is normal for a pressurised metered dose inhaler , the dispensing container 3 is depressed relative to the housing 2 such that the canister 4 moves downwardly within tubular body 7 towards the stem block 8 to actuate the valve . on actuation of dispensing container 3 , downward movement of the canister 4 and valve 5 within the tubular body 7 causes the actuator member 13 to be moved downwardly within the tubular body 7 so as to compress spring 15 due to contact between the yoke 41 and the ferrule . at the same time , the legs 42 and thus the actuator drive arm 45 are moved downwardly relative to the c - shaped channel 80 and the inner wheel 11 . this brings the distal end of the actuator drive arm 45 into contact with one of the primary indexing teeth 22 and onward movement of the actuator arm 45 causes the inner wheel 11 to be rotated by one increment . rotation of the inner wheel 11 is accommodated by radially - outward flexing of the first flexible restraining arm 57 such that the circular formation at the distal end of the first flexible restraining arm 57 is displaced from its initial indentation 24 and then re - engages into a neighbouring indentation 24 associated with a neighbouring primary indexing tooth 22 to that being engaged by the actuator drive arm 45 . during rotation of the inner wheel 11 the pin 23 b moves around the annular gap between the teeth 33 of the annular outer wheel 12 and the rim 52 of the cup - shaped housing 70 . this movement of the pin 23 b does not interact with the teeth 33 until contact with the deflector 59 as described below . on release of the dispensing container 3 , the dispensing container 3 moves back upwardly within the tubular body 7 under the internal spring bias of the valve 5 . this upward movement allows the actuator member 13 to move back upwardly within tubular body 7 under action of spring 15 . this causes in turn the legs 42 and yoke 41 to move back upwardly relative to the inner wheel 11 back into the at rest position . depending on the number of primary indexing teeth 22 and their relative spacing , this upward movement may be accommodated by the actuator drive arm 45 flexing and riding back over the neighbouring primary indexing tooth 22 . in such a situation , the engagement of the first flexible restraining arm 57 in the indentation 24 prevents any back rotation of the inner wheel 11 . thus , on each actuation of the dispensing container 3 , the inner wheel 11 is rotated by one increment . successive actuations of the dispensing container 3 continue to rotate the inner wheel 11 until the point that the pin 23 b moves round into engagement with the deflector 59 . at this point , on actuation of the dispensing container 3 , the inner wheel 11 is rotated as described above and at the same time the pin 23 b is deflected radially inwardly into engagement with one of the teeth 33 of the annular outer ring such that the annular outer wheel 12 is rotated by one increment . advantageously , no transmission cog is required to transfer the motive force from the inner wheel 11 to the annular outer wheel 12 . as with the movement of the inner wheel 11 , the rotation of the annular outer wheel 12 is accommodated by radially - outward flexing of the second flexible restraining arm 55 to move the circular formation at the distal end of the second flexible restraining arm from its tooth 33 on to a neighbouring tooth 33 . both the inner wheel 11 and the annular outer wheel 12 rotate in the same sense , which may be designed to be either clockwise or anti - clockwise . in this way , reciprocal longitudinal movement of the dispensing container 3 can be used to create rotational movement of the inner wheel 11 and the annular outer wheel 12 to change display of dosage indicia provided on the inner wheel 11 and or annular outer wheel 12 . at the end of life of the pack the rotation of the inner wheel 11 will bring the pin 23 b into contact with the land 33 a of the annular outer wheel 12 . at this point further rotation of the annular outer wheel 12 is prevented since the pin 23 b is unable to engage with a tooth 33 to rotate the annular outer wheel 12 . when not in use the dust cap 106 can be placed on the mouthpiece 6 . the tang 161 projects through the lock - out aperture 61 to prevent actuation of the counter by contacting and blocking downward movement of the yoke 41 . fig1 to 12 show another embodiment of pressurised metered dose inhaler 1 according to the present disclosure . the structure and function of this embodiment is similar to that of the embodiment of fig1 to 9 and in the following only the differences will be described in detail . equivalent features have been referenced using equivalent reference numerals . the tubular body 7 of the actuator housing 2 of this embodiment is formed from two mouldings comprising a top case 117 a and a bottom case 117 b . the bottom case 117 b fully defines the mouthpiece 6 , the stem block 8 and the base 7 d of the actuator housing . in addition , the c - section channel 80 and the tubular extension 10 form part of the bottom case 117 b . as can be seen in fig1 , the bottom case 117 b fully houses the dose indicator device 9 . the top case 117 a in this embodiment shields the majority of the body of the pressurised dispensing container 3 . the top case 117 a may be formed from a transparent or translucent material to allow markings or writing on the pressurised dispensing container 3 to be read without the need to remove the container from the actuator housing 2 . the top and bottom cases 117 a , 117 b may be snap - fit together by means of formations on each casing . as before , the pressurised dispensing container 3 is inserted into the actuator housing 2 through the open upper end 7 f — in this embodiment fully defined by the top case 117 a . operation of the pressurised metered dose inhaler 1 , including actuation of the dose indicator device 9 is the same as in the previous embodiment .