referring to fig1 there is shown a preferred embodiment of a battery powered hand held vacuum cleaner ( 1 ) in accordance with the present invention . the vacuum cleaner ( 1 ) generally comprises a main housing portion ( 2 ) and a nose cone ( 4 ) and can be releasably mounted on a charging unit ( 6 ) in a known manner . the charging unit ( 6 ) can optionally be mounted on a wall for storage and re - charging of the vacuum cleaner . the main housing portion ( 2 ) comprises a handle ( 8 ) and houses a motor ( 10 ) which is powered by a re - chargeable battery unit ( 12 ) also housed within the main housing portion . when the vacuum cleaner is mounted on the charging unit ( 6 ), the battery unit ( 12 ) is electrically connected to the charging unit to charge the battery unit . for this purpose the charging unit ( 6 ) can be connected to a mains electricity supply via an electrical cable ( 14 ). the motor ( 10 ) is switched on and off by movement of an actuator ( 16 ) which is slidably mounted on the main housing portion ( 2 ). the actuator ( 16 ) operates a switch unit ( 18 ) which switches the motor ( 10 ) on and off . the motor ( 10 ) rotatingly drives a fan ( 20 ). when the fan ( 20 ) is rotated by the motor ( 10 ) it draws air into it axially from the front of the fan through an inlet ( 84 ) provided in the front of the main housing portion ( 2 ), as indicated by arrows ( a ). the fan ( 20 ) expels air radially , as indicated by arrows ( b ). the air expelled by the fan ( 20 ) exits the main housing portion ( 2 ) via vents ( not shown ) provided in the housing portion . a dual filter system ( 34 ) which comprises a pre - filter ( 36 ) and a primary filter ( 38 ) is releasably latched onto the front of the main housing portion ( 2 ) in front of the inlet ( 84 ) to the fan ( 20 ) by a pair of pegs ( 40 ) which extend from the front of the main housing portion . the pegs ( 40 ) have enlarged heads and can be releasably latched within a corresponding pair of keyhole shaped apertures provided in the pre - filter ( 36 ) and in the primary filter ( 38 ) in a manner described in more detail below . the nose cone ( 4 ) can be releasably fitted onto the main housing portion ( 2 ) by the engagement of detent members ( 22 , 24 ) provided on the main housing portion with co - operating recesses ( 26 , 28 ) provided in the nose cone . detent member ( 24 ) can be retracted from recess ( 28 ) by depressing a release lever ( 30 ) against the force of a biasing spring ( 32 ) in order to remove the nose cone ( 4 ) from the main housing portion ( 2 ). the nose cone ( 4 ) comprises an inlet tube ( 42 ) for the ingress of dust and debris to be collected by the vacuum cleaner . at the rear end of the inlet tube ( 42 ) is provided a deflector ( 44 ) for deflecting dust and debris downwardly into a collecting chamber ( 46 ) provided in the nose cone ( 4 ). in use when the vacuum cleaner ( 1 ) is detached from the charging unit ( 6 ) and the motor ( 10 ) is switched on , the impeller ( 12 ) rotates to draw an airflow into the front end ( 48 ) of the inlet tube ( 42 ), along the inlet tube ( 42 ) through the dual filter system ( 34 ) and axially into the fan ( 20 ). the front end ( 48 ) of the inlet tube is directed by a user towards a surface from which dust and / or other debris is to be collected . the airflow generated by the fan ( 20 ) entrains dust and debris within it which is pulled up the inlet tube ( 42 ), and a portion thereof is deflected downwardly with the airflow by the deflector ( 44 ) towards collecting chamber ( 46 ). in this way some of the dust and debris entrained in the airflow will be removed from the airflow and collected within the collecting chamber ( 46 ). the remainder of the dust and debris entrained within the airflow will be separated from the airflow by the dual filter system ( 34 ). the proportion of dust and debris separated from the airflow by the pre - filter ( 36 ) will drop downwardly , due to gravity , and will be collected in the collecting chamber ( 46 ) of the nose cone ( 4 ). the remaining , relatively small , proportion of the dust and debris separated from the airflow by the primary filter ( 38 ) will be collected in the space between the pre - filter ( 36 ) and the primary filter ( 38 ). when a user has collected a pre - determined amount of dust and debris or has completed a cleaning task , the motor ( 10 ) will be switched off and the nose cone ( 4 ) removed from the main housing portion ( 2 ) by depressing the release lever ( 30 ). the dust and debris contained in the collecting chamber ( 46 ) can then be disposed of . also , the dual filter system ( 34 ) can be removed as required by the operator from the main housing portion ( 2 ). any dust or debris collected on or between the pre - filter ( 36 ) and the primary filter ( 38 ) can then be removed . the dual filter system ( 34 ) and nose cone ( 4 ) can then be re - fitted onto the main housing portion ( 2 ) and the vacuum cleaner ( 1 ) is ready for further use or can be stored away for subsequent use . one of the benefits of the arrangement utilizing a dual filter attached to the main housing portion ( 2 ) is that this design provides for relative ease of emptying and cleaning the vacuum cleaner . in particular , the user may hold the housing unit in one hand while removing the collecting chamber ( 26 ) with an opposed hand , the dust and debris being primarily retained within the collecting chamber ( 46 ) and thus can be emptied at the operator &# 39 ; s convenience . no additional step is required to remove the prefilter or primary filter from the main housing or from the nose cone ( 4 ). any debris collected between the primary filter and the pre - filter can simply be removed by vibrating or shaking the housing unit with the filters attached thereto . this enables a continuous operation to empty the vacuum cleaner . it also eliminates the need for the user to handle the dual filter system ( 34 ) when emptying the nose cone ( 4 ), and the possibility of further contaminating the filter system during emptying of the nose cone . still another advantage of the vacuum cleaner ( 1 ) of the present invention is that by mounting the dual filter system ( 34 ) on the main housing portion ( 2 ), rather than within the nose cone ( 4 ), the capacity of the nose cone for containing dust and dirt debris is increased . in contrast , previously developed systems have incorporated the filter element within the removable nozzle or like element , which significantly reduces the capacity of the nozzle for holding contaminants . it will be appreciated , however , that the dual filter assembly could be mounted within the nozzle cone ( 4 ) with little modification to the nozzle cone . the dual filter system ( 34 ) will now be described in more detail with reference to fig1 to 13 . the primary filter ( 38 ) of the dual filter system ( 34 ) shown in fig1 comprises a molded plastic frame within which is supported an accordian - like section or “ concertina ” of woven polyester filter material ( 50 ). the molded plastic frame comprises a rim ( 52 ) from which extend two opposing side support frames ( 54 , 56 ). each side support frame ( 54 , 56 ) comprises an outer frame ( 60 a , 60 b , 60 c ) within which are formed an array of lower forked support arms ( 58 a ) and an array of upper forked support arms ( 58 b ) which co - operate in a zig - zag formation . the concertina of filter material ( 50 ) is supported between the zig - zag formation of opposing support arms ( 58 a , 58 b ) and by the end support arms ( 60 a , 60 c ) of the outer frame . in addition , triangular shaped sections of woven polyester filter material ( 62 ) are supported between adjacent support arms ( 58 a ), adjacent support arms ( 58 b ), adjacent support arms ( 60 a , 58 a ) and adjacent support arms ( 60 c , 58 a ). the triangular sections of filter material ( 62 ) provide a side filter surface across the area contained within the outer frame ( 60 a , 60 b , 60 c ) of each support frame ( 54 , 56 ). the primary filter ( 38 ) is manufactured by folding a concertina of filter material ( 50 ) within and locating triangular sections of filter material ( 62 ) appropriately within a molding die and then injecting plastic material into the molding die to form the molded plastic frame comprising rim ( 52 ) and opposing support frames ( 54 , 56 ). the opposing support frames support the woven filter material ( 50 , 62 ) securely in position . the folding of the filter material ( 50 ) in the concertina arrangement and the additional triangular sections of filter material ( 62 ) provides a primary filter ( 38 ) with a relatively large cross - sectional area . the rim ( 52 ) of the primary filter ( 38 ) has formed within it on opposing sides keyhole shaped apertures ( 64 , 66 ). the keyhole apertures are used to releasably latch the primary filter ( 38 ) to co - operating pegs ( 40 ) provided on the front of the main housing portion ( 2 ) of the vacuum cleaner of fig1 . as shown in fig1 in use the primary filter ( 38 ) is surrounded by a pre - filter ( 36 ) shown in fig5 to 9 . the pre - filter ( 36 ) comprises a molded plastic open box - shaped casing having front surface ( 66 ) and four side surfaces ( 68 a to 68 d ), and a rim ( 70 ). a resilient seal element ( 72 ) extends around the periphery of the rim ( 70 ). the front surface ( 66 ) and the four side surfaces each have formed through them an array ( 74 ) of circular filter holes , for example , the array of filter holes ( 74 a ) formed in the front surface ( 66 ) as shown in fig7 and the array of filter holes ( 74 b ) formed in the side surface ( 68 b ) as shown in fig9 . each of the holes in the arrays of holes ( 74 ) has a diameter of approximately 1 mm although the size may vary , if required , between 0 . 75 mm and 1 . 75 mm . the arrays of holes in the pre - filter ( 36 ) provide a first “ coarse ” filtering stage in front of the primary filter which provides a second “ fine ” filtering stage . to aid in the injection molding of the pre - filter ( 36 ), the front face ( 66 ) of the pre - filter comprises a solid circular region ( 76 ) from which extend radially outwardly six solid arms ( 78 ). the solid circular region ( 76 ) and arms ( 78 ) assist in the flow of molten plastic material during the injection molding process , which flow would otherwise be impaired by narrow channels between the holes forming the arrays of holes ( 74 ). the solid circular region ( 76 ) also provides a region of the pre - filter ( 36 ) that can prominently display information such as trademarks . it has been found in practice that the 1 mm diameter size of the holes in the pre - filter ( 36 ) is generally greater than the majority of the particles of dust and debris entering the nose cone ( 4 ) in use of the vacuum cleaner shown in fig1 . however , the pre - filter ( 36 ) still deflects the majority of particles of dust and debris , irrespective of their size , out of the airflow drawn through the pre - filter ( 36 ) by the fan ( 20 ). these deflected particles of dust and debris are collected in the collecting chamber ( 46 ) of the nose cone ( 4 ). up to 90 % of particles of debris and dust normally picked up during household vacuuming and entrained in the airflow in front of the pre - filter ( 36 ) can be removed from that airflow by the pre - filter ( 36 ). however , the amount of particulate removed from the airflow is dependent on the size of that particulate . the 1 mm diameter circular hole size used in the pre - filter ( 36 ) would be expected not to deflect the generally much smaller sized particles of dust and debris entrained in an airflow through it . however , when the particles of dust and debris entrained within the airflow hit the front surface of the pre - filter ( 36 ), the momentum of the particles is reduced or eliminated . the airflow into the pre - filter ( 36 ) is often insufficient to then recapture such particles and these particles fall into the collecting chamber ( 46 ). this significantly reduces the volume of particles of dust and debris which reach the primary filter ( 38 ). consequently , there is a significant reduction in the amount of blocking of the pores of the filter material used therein by conglomerations of such particles . furthermore , as the particles that reach the primary filter ( 38 ) will have passed through the 1 mm diameter holes in the pre - filter ( 36 ), they will be of more uniform diameter than if the pre - filter was not used . it has been found that this increased level of uniformity of diameter of particles incident on the filter material ( 50 , 62 ) of the primary filter ( 36 ) is more resistant to binding and thus tends to reduce further the blocking up of the pores in the primary filter by conglomerations of particles . this is because particles of similar diameter are less likely to bind together to form a conglomerated mass that could block the filter material than particles of dissimilar diameters . the more uniform diameter particles that have passed through the pre - filter ( 36 ) tend not to bind and instead fall away from the filter material ( 50 , 62 ) into the space between the pre - filter ( 36 ) and the primary filter ( 38 ). this reduction of blocking of the pores of the primary filter improves the suction of the vacuum cleaner ( 1 ) during use as the airflow to the fan ( 20 ) is not impeded by a blocked primary filter ( 38 ). in a similar way to the primary filter ( 38 ), the rim ( 70 ) of the pre - filter ( 36 ) has formed within it on opposing sides keyhole shaped apertures ( 80 , 82 ). these apertures ( 80 , 82 ) are used to releasably latch the pre - filter ( 38 ) to co - operating pegs ( 40 ) provided on the front of the main housing portion ( 2 ) of the vacuum cleaner ( 1 ) of fig1 . when the primary filter ( 38 ) is received within the pre - filter ( 36 ), the keyhole shaped apertures ( 64 , 80 ) and ( 66 , 82 ) line up so that the dual filter system ( 34 ) comprising the pre - filter ( 36 ) and the primary filter ( 38 ) can be releasably latched to the main housing portion ( 2 ) as a single assembly . to fit the dual filter system ( 34 ) to the main housing portion ( 2 ), the widened portions of the keyhole shaped apertures ( 64 , 80 ) and ( 66 , 82 ) are fitted over the enlarged head of a corresponding one of the pair of pegs ( 40 ). the dual filter system ( 34 ) is then rotated slightly to slide the stem of the pegs ( 40 ) into the narrow portion of the respective keyhole shaped apertures ( 64 , 80 ) and ( 66 , 82 ) in order to securely latch the dual filter system ( 34 ) to the main housing portion ( 2 ) over the inlet ( 84 ) to the fan ( 20 ). one of the pegs ( 40 ) comprising an enlarged head ( 86 ) and a stem ( 88 ) is shown clearly in dotted lines in fig1 . fig1 shows in more detail the resilient seal element ( 72 ) which surrounds the rim ( 70 ) of the pre - filter ( 36 ). the seal element ( 72 ) is made of a resilient material such as rubber , silicone , neoprene or any other suitable elastomer . the seal element ( 72 ) is intended to form an air tight seal between the pre - filter ( 36 ) and the primary filter ( 48 ), between the dual filter system ( 34 ) and the nose cone ( 4 ), and between the dual filter system and the main housing portion ( 2 ). the rim ( 70 ) has a stepped portion ( 90 ) which extends around its periphery and the resilient seal element ( 72 ) is molded over this stepped portion . the seal element ( 72 ) comprises a radially outwardly extending portion ( 92 ) which , as seen in fig1 engages the inner surface of the wall of the nose cone ( 4 ), when the nose cone is fitted to the main housing portion ( 2 ). this forms an airtight seal between the dual filter system ( 34 ) and the nose cone . the seal element also comprises a first axially rearwardly extending portion ( 94 ) ( fig1 ) which extends around the periphery of the rim ( 52 ) of the primary filter ( 38 ) to engage a forward surface of the main housing portion ( 2 ) to form an airtight seal between the dual filter system ( 34 ) and the main housing portion . as a consequence , an airtight seal is also formed between the pre - filter ( 36 ) and the primary filter ( 38 ). the seal element ( 72 ) further comprises a second axially rearwardly extending portion ( 96 ) against which the rim ( 52 ) of the primary filter ( 38 ) abuts when the pre - filter ( 36 ) and primary filter ( 38 ) are together fitted onto the main housing portion . this provides further air tight sealing between the pre - filter ( 36 ) and primary filter ( 38 ). this sealing is further enhanced by extending the seal element ( 72 ) at the keyhole shaped apertures ( 80 , 82 ) so that an extension ( 98 ) ( see also fig6 ) of the seal element extends around each keyhole shaped aperture ( 80 , 82 ) partly within a recess ( 100 ) formed in the lower side of the rim ( 70 ) around the apertures . thus , when the dual filter system ( 34 ) is assembled and fitted onto the main housing portion ( 2 ), the primary filter ( 38 ) is inserted into the pre - filter ( 38 ) such that the concertina of filter material ( 50 ) is received within the box - shaped cavity of the pre - filter , the keyhole shaped apertures ( 64 , 80 ) and ( 66 , 82 ) are aligned , the second axially extending portion ( 96 ) of the seal ( 72 ) abuts the rim ( 52 ) of the primary filter ( 38 ), and the first axially extending portion ( 94 ) of the seal ( 72 ) extends around the periphery of the rim ( 52 ) of the primary filter ( 38 ). the dual filter system ( 34 ) is then latched onto the pegs ( 40 ) provided on the main housing portion ( 2 ). this latching of the dual filter system ( 34 ) to the pegs ( 40 ) presses the rims ( 52 , 70 ) of the filters together and so reinforces the seal between the pre - filter ( 36 ) and primary filter ( 38 ) provided by the portion ( 96 ) of the seal element ( 72 ). it also presses the rim ( 70 ) of the pre - filter ( 36 ) towards the front surface of the main housing portion ( 2 ) and thus reinforces the seal between the pre - filter and the main housing portion provided by the portion ( 94 ) of the seal element ( 72 ). an alternative preferred embodiment of pre - filter ( 38 ) which provides an improved releasable latch connection between the dual filter system ( 34 ) and the main housing unit ( 2 ) of the vacuum cleaner of fig1 is shown in fig1 . in the arrangement of fig1 , a raised cam surface ( 102 ) is provided on the upper surface of the rim ( 70 ) of the pre - filter ( 38 ) bordering the narrow portion and part of the widened portion of each keyhole shaped aperture ( 80 , 82 ). each cam surface ( 102 ) comprises a pair of ramps ( 104 ) facing towards the widened end of the aperture it borders and an opposing pair of ramps ( 106 ) facing towards the narrow end of the aperture it borders . each cam surface ( 102 ) co - operates with the underside of the enlarged head portion ( 86 ) of a respective peg ( 40 ). when the dual filter system ( 34 ) is fitted over the pair of pegs ( 40 ) provided on the main housing portion ( 2 ), the enlarged head portion ( 86 ) passes through the widened portion of the respective key shaped apertures ( 64 , 80 ) and ( 66 , 82 ). then the dual filter system ( 34 ) is rotated by a small amount so that the neck portion of each peg ( 40 ) slides into the narrow portion of the respective apertures and the enlarged head portion ( 86 ) of each peg latches the dual filter system onto the main housing portion ( 2 ). with the improved arrangement shown in fig1 , rotation of the dual filter system ( 34 ) in this way causes the underside of the enlarged head ( 86 ) of each peg to ride up the pair of ramps ( 104 ) on the respective cam surface ( 102 ). this urges the rim ( 70 ) of the pre - filter ( 36 ) towards the main housing portion ( 2 ) and thus towards the rim ( 52 ) of the primary filter ( 38 ) against the biasing force of the axially extending portions ( 94 , 96 ) of the seal element ( 72 ). further rotation of the dual filter system ( 34 ) causes the underside of the enlarged head ( 84 ) of each peg to move down the opposing ramps ( 106 ) and come to rest in its final latched position against the flat raised portion of the cam surface ( 102 ) surrounding the narrowed portion of each aperture ( 80 , 82 ). this slightly releases the compression of the axially extending portions ( 94 , 96 ) of the seal element ( 72 ). however , with the dual filter system ( 34 ) in its latched position , the rim ( 70 ) of the pre - filter system is urged sufficiently towards the main housing portion ( 2 ) and towards the rim ( 52 ) of the primary filter ( 36 ) for the axially extending portions ( 94 , 96 ) to form an air tight seal against the main housing portion and the rim ( 52 ) respectively . this ensures that in use of the vacuum cleaner ( 1 ) all airflow to the fan ( 20 ) passes through the pre - filter ( 36 ) and through the primary filter ( 38 ). furthermore , the two ridges ( 108 ) on each cam surface ( 102 ) between the opposing ramps ( 104 , 106 ) serve as a retaining means to retain the pegs ( 40 ) in their latched position against the resilience of the axially extending portions ( 94 , 96 ) of the sealing element ( 72 ). in this way the dual filter system ( 34 ) can only be unlatched from the pegs ( 40 ) by rotation in a direction opposite to that required to latch the dual filter system to the pegs , which rotation can only occur if a force is applied which is sufficient to compress the axially extending portions ( 94 , 96 ) so that the underside of the enlarged heads ( 86 ) of the pegs ( 40 ) can ride over the ridges ( 108 ) in the cam surfaces ( 102 ). alternatively , or in conjunction with the use of the cam surfaces ( 102 ), internal ribs may be provided around the interior surface of the collecting chamber ( 46 ). as the collecting chamber ( 46 ) is moved into engagement with the main housing portion ( 2 ), these ribs engage with the rim ( 70 ) of the pre - filter system and operate to compress the axially extending portions ( 94 , 96 ) of the sealed element ( 72 ) into respective engagement with the main housing portion ( 2 ) and primary filter ( 36 ) accordingly . the use of ribs in this manner will provide a uniformly distributed compression force about the entire periphery of the seal of the pre - filter . fig1 shows a cross - section of the seal element ( 72 ) shown in fig1 , with like parts identified by like numerals . fig1 also shows a modified portion of the outer wall of the nose cone ( 4 ) adjacent the seal element ( 72 ) from that shown in fig1 . in the fig1 embodiment , an internal raised wall ( 110 ) extends around the portion of the internal surface of the nose cone ( 4 ) which surrounds the seal element ( 72 ). this creates an improved air tight seal between the dual filter system ( 34 ) and the inner surface of the nose cone ( 4 ). the radially extending portion ( 92 ) of the resilient seal element ( 72 ) abuts the rear surface of the wall ( 110 ). in use of the vacuum cleaner the underpressure created in the forward part ( 112 ) of the nose cone ( 4 ) relative to the ambient air pressure in the region ( 114 ) in front of the main housing portion ( 2 ) reinforces a sealing engagement between the portion ( 92 ) of the sealing element ( 72 ) and the wall ( 110 ). the pressure differential urges the front surface of the resilient , radially extending portion ( 92 ) of the seal element ( 72 ) into sealing engagement with the rear surface of the wall ( 110 ). again , this ensures that all the airflow through the nose cone ( 4 ) passes through the dual filter arrangement ( 34 ) into the fan ( 20 ). fig1 shows an alternative preferred embodiment to that shown in fig1 , with like parts identified by like numerals . in this embodiment the wall ( 110 ) is not required on the internal surface of the nose cone ( 4 ), and the radially extending portion ( 92 ) of the seal element ( 72 ) is bent rearwardly to form and third axially rearwardly extending portion ( 116 ). in fig1 , the radially extending portion ( 92 ) is bent rearwardly through approximately 90 °. thus , as shown in solid lines in fig1 , when the nose cone ( 4 ) is fitted on the main housing portion ( 2 ) with the vacuum cleaner switched off , there is no sealing engagement between the seal element ( 72 ) and the nose cone ( 4 ). however , when the vacuum cleaner is switched on , the pressure differential between the underpressure created in the region ( 112 ) of the nose cone ( 4 ) and the ambient pressure in the region ( 114 ) causes the third resilient axially extending portion ( 116 ) to move outwardly into the position showed in fig1 in dotted lines . in this position , the third resilient , axially extending portion ( 116 ) is urged into sealing engagement with the internal surface of the wall of the nose cone ( 4 ). thus , when the vacuum is in use a seal is provided between the dual filter system ( 34 ) and the nose cone ( 4 ) to ensure that all the airflow through the nose cone passes through the dual filter system ( 34 ). however , when the vacuum cleaner ( 1 ) is not in use , the lack of engagement between the seal element ( 72 ) and the nose cone ( 4 ) can help to prevent wear of the portion ( 116 ) of the seal element due to the repeated removal and replacement of the nose cone ( 4 ) during the lifetime of the vacuum cleaner . those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms . therefore , while this invention has been described in connection with particular examples thereof , the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings , specification and following claims .
a dual filter arrangement for a vacuum cleaner for filtering particles of dust and debris from an airflow passing through the vacuum cleaner . the dual filter comprises a coarse pre - filter having an array of pores formed therein of substantially uniform diameter and a fine primary filter which is located rearwardly of the pre - filter in the direction of flow of the airflow . the dual filter is releasably latched to a main housing portion of the vacuum cleaner housing . a nose cone is removably secured to the housing and circumscribes the dual filter assembly . the dual filter enables even more efficient filtering of contaminants from dust and dirt entrained air ingested by the vacuum cleaner . attachment of the dual filter to the main housing portion provides the additional advantage of increasing the capacity of the nose cone and eliminating the need for the user to handle the dual filter assembly when emptying contaminants collected within the nose cone . a unique seal arrangement provides a substantially airtight seal between the dual filter system and the main housing to which it is secured .