A known type of filter assembly comprises a tubular filter medium with two ends and two end caps that are fixable in sealing contact with respective ones of the ends so as to close the ends. The end caps are made of plastics material and this allows each end cap to be sealed to the corresponding end of the filter medium by welding techniques in which an appropriate region of the end cap is softened by heating and the end of the filter medium is then inserted into the softened region. On cooling, the end cap is sealed to the end of the filter medium. One of the end caps (or optionally both) is provided with a port that allows fluid to pass across the end cap to or from the interior of the tubular filter medium. Fluid is filtered by passing the fluid across the filter medium-either from the outside to the inside of the filter medium, or from the inside to the outside of the filter medium. Hence, each end cap directs fluid through the filter medium.

In accordance with a first aspect of the invention, there is provided a filter assembly comprising a filter medium, a member and a snap-fit mechanism engaged or engagable to fix the member in sealing contact with the filter medium so that the member directs flow of fluid through the filter medium during filtration, the member having a wall with a free edge, the wall penetrating into the filter medium so as to form said seal.

Many known filter assemblies include a housing containing two or more filter media.

The filter media are cylindrical and may, for example, be formed of depth filter material. For stress resistance purposes, the housing is often of circular cross-section, although other cross-sections are sometimes used. In order to obtain efficient filtration and fluid flow, there is usually a minimum diameter for a particular cylindrical filter medium. Likewise, manufacturing constraints provide a limit on the maximum diameter of the filter media. Accordingly, for a particular fluid flow, it may be necessary to insert two or more cylindrical filter media into a housing.

The insertion of cylindrical filter media into a cylindrical housing is inefficient since the cross-sectional area of the filter media compared to the cross-sectional area of the housing needed to accommodate the filter media is comparatively low. This means that for a particular flow volume and filtration performance, a comparatively large volume housing must be used with a comparatively large number of filter media.

According to a second aspect of the invention, there is provided a filter assembly comprising a housing and at least two filter elements within the housing, each filter element comprising a non-circular tube of depth filter media having at least one side, the at least two filter elements being arranged side-by-side in the housing with said sides adjacent.

According to a third aspect of the invention, there is provided a filter assembly comprising a housing having a curved cross-sectional shape and at least two filter media within the housing, each filter medium having first and second sides, the first side of each filter medium having a curved shape similar to the shape of a portion of the cross-section of the housing and being adjacent said portion and the second side of each filter medium extends away from the housing and being adjacent the second side of another filter medium.

According to a fourth aspect of the invention, there is provided a filter assembly comprising a housing and a plurality of filter media arranged side-by-side within the housing, each filter medium being formed of depth filter material and having a non- circular cross-section so as to achieve a packing density within the housing greater than a corresponding plurality of filter media of circular cross-section.

By using filter elements that are non-circular, higher packing densities can be achieved.

The following is a more detailed description of embodiments of the invention, by means of example, reference being made to the appended schematic drawings in which: Figure 1 is a cross-sectional representation of an assembled filter assembly in accordance with the invention ; Figure 2 is a perspective view of a first end cap of the filter assembly of Figure 1; Figure 3 is a perspective view of a second end cap of the filter assembly of Figure 1 ; Figure 4 is a perspective view of the first and second end caps assembled together (the filter medium being omitted for clarity); Figure 5 is an enlarged view showing a part of the filter assembly of Figure 1 at a first stage during the assembly process; Figure 6 is a similar view to Figure 5 showing the filter assembly of Figure 1 at a second, subsequent, stage of the assembly process; Figure 7 is a cross-sectional view of a second filter assembly; Figure 8 is a cross-sectional view of a third filter assembly; Figure 9 is a representation of a fourth filter assembly; Figure 10 is a cross-sectional view of the fourth filter assembly; Figure 11 is a plan view of an inner face of a closed end cap of the fourth filter assembly; Figures 12 to 14 are cross-sectional views, respectively on lines XII, XIII and XIV of Figure 11, of the closed end cap; Figure 15 is a plan view of an inner face of an open end cap of the fourth filter assembly; Figure 16 is a side view of a tie rod of the fourth filter assembly; Figure 17 is a cross-section taken on the line XVII of Figure 16; and Figure 18 is an enlarged view of an end of the tie rod of Figures 16 and 17.

As shown in Figure 1, the filter assembly comprises a tubular filter medium 10 having first and second ends, and first and second end caps 8,9, which, when the filter assembly is assembled, close respective ones of the first and second ends.

As best seen in Figure 2, the first end cap 8 has a planar wall 12 that has a generally triangular outer perimeter. A tubular wall 13, that is generally triangular in cross- section, extends from the centre of the planar wall 12 in a direction normal to the planar wall 12. As seen in Figures 1 and 2, the tubular wall 13 has a plurality of holes 14 extending therethrough. As seen in Figure 2, the tubular wall 13 is provided with four thickened regions 15 that extend in the direction of the length of the tubular wall 13, and that are spaced from one another around the tubular wall 13. The thickening is achieved by material that extends to the inside of the tubular wall 13. Each thickened region 15 has two of the holes 14 passing thereacross.

The tubular wall 13 is also provided with four inwardly directed projections 16-each projection 16 being located axially in line with a respective one of the thickened regions 15 and towards the free end 17 of the tubular wall 13. Each projection 16 is spaced from the corresponding thickened region 15 so as to provide a respective groove 18 therebetween. Each projection 16 has a surface 19 that is closest to the free end 17 of the tubular wall 13 and that slopes inwardly from the tubular wall 13 towards the planar wall 12.

As seen in Figures 1 and 2, the perimeter of the first end cap 8 is provided with a flange 20 that extends normally to the planar wall 12 in the same direction as the tubular wall 13.

The planar wall 12 of the first end cap 8 is also provided with inner and outer sealing walls 21,22-each of which extends outwardly of and around the tubular wall 13 and within the flange 20. The outer sealing wall 22 helps to hold the filter medium in the required shape (triangular in cross-section).

The inner sealing wall 21 is seen best in Figures 5 and 6 (from which the outer sealing wall 22 is omitted for clarity). In these Figures, it can be seen that the inner sealing wall 21 has an outer surface 23 that extends normally to the planar wall 12 and an inner surface 24 that extends obliquely to the planar wall 12 so that the inner and outer surfaces 23,24 meet at an edge 25 referred to as the knife edge.

The outer sealing wall 22 can have the same configuration.

As seen best in Figure 1, the first end cap 8 is provided with a port 26 that extends across the planar wall 12 opening within the tubular wall 13.

Referring now to Figures 1 and 3, the second end cap 9 has a planar wall 27 that also has a generally triangular outer perimeter.

An inner flange 28, generally triangular in cross-section, extends normally to the planar wall 27 from the centre of the planar wall 27. Four arms 29, spaced from one another around the inner flange 28, extend from the inner flange 28 in a direction normal to the planar wall 27. Each arm 29 is resilient and is provided at its free end with a respective, outwardly directed projection 30. Each projection 30 has a sloping surface 31 that extends outwardly and towards the planar wall 27.

An outer flange 32 extends around the outer perimeter of the planar wall 27, in a direction normal to the planar wall 27, and to the same side of the planar wall 27 as the inner flange 28.

As best seen in Figure 3, three resilient fingers 33 are spaced around the outer flange 32 and extend away from the planar wall 27 in a direction normal to the planar wall 27.

Located between the inner and outer flanges 28,32 are inner and outer sealing walls 34,35 that are of a similar configuration to the inner and outer sealing walls 21,22 of the first end cap 8.

The second end cap 9 is also provided with a port 36 (which is optional) that extends across the planar wall 27 and that opens within the inner flange 28.

The filter medium 10 is a coreless filter medium of known type. The filter medium is composed of a plurality of fibres with spaces between the fibres through which fluid flows during filtration. The filter medium 10 has an inner region 37 in which the fibres are more tightly packed and an outer region 38 in which the fibres are more loosely packed. (A filter medium having a constant packing throughout may be used.) In this way, when fluid is being filtered from the outside of the filter medium 10 to the inside of the filter medium 10, larger particles are trapped in the outer region 38 whereas smaller particles are trapped within the inner region 37.

The filter medium 10 is also generally triangular in cross-section.

In order to assemble the filter assembly, the first end cap 8 is laid on a working surface with the tubular wall 13 extending upwardly. The filter medium 10 is then slid onto the tubular wall 13 so that the inner surface of the filter medium 10 contacts the outer surface of the tubular wall 13. At this stage, the knife edges 25 of the inner and outer sealing walls 21,22 contact the lower end of the filter medium 10.

The second end cap 9 is then fitted to the assembly. During fitting of the second end cap 9, the resilient arms 29 are inserted within the tubular wall 13 and the resilient fingers 33 are held outward of the outer surface of the filter medium 10. Each arm 29 lies axially in line with a respective one of the projections 16 provided on the inside of the tubular wall 13. As the second end cap 9 is fitted to the first end cap 8 each sloping surface 31 of the arms 29 contacts the inclined surface 19 of the corresponding projection 16 on the inner side of the tubular wall 13. As the first and second end caps are pushed together, the contact between the surfaces 19,31 causes the arms 29 to flex inwardly and ride over the projections 16.

The second end cap 9 is then pushed hard against the first end cap 8 so that the free end 17 of the tubular wall 13 comes to rest against the planar wall 27 of the second end cap 12 and so that the projections 30 on the arms 29 snap into the grooves 18.

This interlocking of the projections 30 with the grooves 18 fixes the first and second end caps 8,9 together (as seen in Figure 1). Hence, the tubular wall 13 and the arms 29 form a snap-fit mechanism for fixing the first and second end caps 8,9 together.

The contact between the free end 17 of the tubular wall 13 and the planar wall 27 of the second end cap 9 (and the lengths of the arms 29 together with the positions of the grooves 18) ensures that the first and second end caps 8,9 are fixed in a predetermined spatial relationship relative to one another. This relationship is such that the distance between the planar walls 12,27 corresponds to the length of the filter medium 10.

As the end caps 8,9 are pushed together, as described above, the inner and outer sealing walls 21,22 on the first end cap penetrate into the lower end of the filter medium 10 so as to seal the lower end against the first end cap, and the inner and outer sealing walls 34,35 on the second end cap 9 penetrate into the upper end of the filter medium 10, so as to seal the upper end against the second end cap 9.

This sealing process is shown for the inner sealing wall 21 of the first end cap 8 in Figures 5 and 6.

As shown in these Figures, the position of the outer surface 23 of the inner sealing wall 21 corresponds approximately to the interface between the inner and outer regions 37,38 of the filter medium 10. As seen in Figure 6, as the inner sealing wall 21 penetrates into the lower end of the filter medium 10, the inclination of the inner surface 24 of the inner sealing wall 21 causes the fibres of the inner region 37 of the filter medium 10 to be compressed together between the inner sealing wall 21 and the tubular wall 13. This compression reduces the volume of the spaces between the fibres and this, in turn, prevents or resists fluid flow between the fibres during filtration. This helps to ensure a good seal between the end cap and the filter medium.

Finally, the fingers 33 are released so that they bear against the outer surface of the filter medium 10 and push the filter medium 10 against the tubular wall 13. These fingers 33 hold the filter medium 10 in the required shape-triangular in cross- section).

During filtration using the filter assembly, the fluid to be filtered is presented to the outer surface of the filter medium 10. The fluid passes through the filter medium to the tubular wall 13 and through the holes 14 in the tubular wall 13. Filtered fluid in the space within the tubular wall 13 then passes out of the filter assembly through the ports 26 and 36 (or through one of these ports if the other one is closed). The sealing between the end caps 8,9 and the filter medium 10 prevents fluid by-passing filter medium 10 by passing between the filter medium 10 and the end caps 8,9.

The tubular wall 13 acts as a"core"supporting the filter medium 10.

It will be appreciated that a large number of modifications may be made to the filter assembly described above.

The filter medium 10, and the end caps 8,9, need not have the shapes described above. For example, the filter medium 10, the tubular wall 13, and the perimeters of the planar walls 12 and 27 may be generally circular, rather than triangular.

In the filter assembly described above, the tubular wall 13 and the arms 29 act as a snap-fit mechanism allowing the end caps to be fixed to one another. However, other snap-fit mechanisms may be provided.

In another embodiment, the tubular wall 13 is omitted from the first end cap 8.

Instead, a separate tubular member, similar to the tubular wall 13 is provided. The separate member can be fixed to the inner surface of the filter medium thereby carrying the medium. The first end cap 8 is provided with arms similar to the arms 29 of the second end cap 9. In this embodiment, the arms on the first end cap snap-fit with holes in the tubular member. Similarly, the arms on the second end cap 9 snap-fit with holes in the tubular member. The sealing walls 21,22,34,35 seal with the ends of the medium as described above.

A"double-sided"end cap might be used. Thus, for example, the first end cap 8 may be provided with a further tubular wall, further sealing walls and a further flange that extend from the other side of the planar wall 27. In this way, a second filter medium 10 could be fitted over the further tubular wall, as described above, and a further second end cap 9 could be fixed to the adapted first end cap 8 so as to hold the second filter medium between the adapted first end cap and the further second end cap 9. In this way, two filter media 10 are connected together with the internal spaces of the filter media being in fluid communication by way of the port 26.

A similar"double-sided"end cap may be made by providing the second end cap 9 with arms, sealing walls, an outer flange and fingers on the other side of the planar wall 27.

The end caps may be provided with grooves and'0'rings for sealing the assembly within suitable housings.

The end caps may be made of acetal resin for compatibility with fuel. Other materials, e. g. plastics materials may be used. Suitable plastics are nylon, polyproplyene, polyester.

In a second filter assembly shown in Figure 7, the assembly comprises first and second end caps 39, 40, a filter medium 10 identical to the filter medium 10 described above, and a separate tubular core 41.

The first end cap 39 has a circular planar wall 42 provided with a cylindrical flange 43 extending from the perimeter of the planar wall 42. The cylindrical flange 43 is provided with a plurality of holes 44 passing across the flange 43.

The planar wall 42 is also provided with inner and outer annular sealing walls 45,46 that extend to the same side of the planar wall 42 as the cylindrical flange 43 and are similar to the sealing walls 21,22,34,35. A port 47 is provided in the planar wall 42.

The second end cap 40 has a circular, planar wall 48. A plurality of resilient arms 49 extend from the perimeter of the planar wall 48 and are spaced from one another around the planar wall 48. Each arm 49 is provided with a respective inwardly directed projection 50.

The second end cap 40 also has inner and outer sealing walls 51,52 similar to the sealing walls 21,22,34, 35.

The tubular core 41 is cylindrical and is provided with a plurality of holes 53 through its wall.

The filter assembly is assembled as follows. The filter medium 10 is placed within the cylindrical flange 43 of the first end cap 39 so that an end of the filter medium 10 lies against the knife edges of the sealing walls 45,46.

The tubular core 41 may be bound to the inner surface of the filter medium 10 (thereby carrying the filter medium) or it may be unconnected and inserted separately into the inner space of the filter medium 10. The second end cap 40 is then positioned with the arms 49 outside the cylindrical flange 43 and the second end cap 40 is pushed towards the first end cap 39 until the cylindrical flange 43 contacts the planar wall 48 of the second end cap 40. At this stage, the projections 50 snap into respective ones of the holes 44 in the cylindrical flange 43.

Thus, the cylindrical flange 43 and the arms 49 act as a snap-fit mechanism.

As the first end cap 39 and the second end cap 40 are pushed together as described above, the sealing walls 45,46,51,52 penetrate into the ends of the filter medium 10 so as to seal the ends of the filter medium 10 against the end caps 39, 40, as described above.

In operation, fluid to be filtered is presented to the outside of the filter assembly and passes through the holes 44 in the cylindrical flange 43. The fluid passes through the filter medium 10 and through the holes 53 in the tubular core 41 to the inside of the tubular core. The fluid then passes out of the filter assembly through the port 47.

By choosing an appropriate filter medium, inside to outside flow can also be used.

A third embodiment of a filter assembly is shown in Figure 8. The filter assembly of Figure 8 comprises first and second parts of a housing 54,55, an end cap 56, a filter medium 10 that is identical to the filter medium 10 described above and a tubular core 41 that is identical to the tubular core 41 described above.

The first part 54 of the housing has a circular, generally planar end region 57 and a generally cylindrical flange 58 that extends around an outer perimeter of the end region 57.

At the free end of the flange 58 is provided a lip 59 that extends radially outwardly from the flange 58.

The end region 57 is provided with a central port 61 that passes across the end region 57. An annular sealing wall 60 extends around the port 61 to the same side of the end region 57 as the flange 58.

The second part 55 of the housing has a circular, generally planar end region 63 and a generally cylindrical flange 64 that extends around an outer perimeter of the end region 63.

An annular locking formation 65 extends around the free end of the flange 64.

Starting from the end of the flange 64, the locking formation 65 has a first portion 66 that extends radially outwardly from the flange 64 to a second portion 67 that extends in an axial direction away from the end region 63. The free end of the second portion 67 of the locking formation 65 is provided with a lip 68 that projects radially inwardly.

The lip 68 has an inclined surface 69 that extends radially inwardly and towards the end region 63 of the second part 55.

The end region 63 of the second part 55 is provided with a central port 70 that extends across the end region 63. Additionally, the end region 63 is provided with raised "pips"or projections that extend to the same side of the end region 63 as the flange 64.

The end cap 56 consists of a circular, planar wall 71 and an annular sealing wall 72 that extends around the planar wall 71 adjacent an outer perimeter of the planar wall 71.

The sealing walls 60,72 have similar configurations to those of the sealing walls 21, 22,34,35 described above.

In order to assemble the third filter assembly, the filter medium 10 together with the tubular core 41 located within and contacting the inner surface of the filter medium 10 are placed so that an end of the filter medium 10 lies against the sealing wall 60 of the first part 54 of the housing. The end cap 56 is then placed so that the sealing wall 72 of the end cap 56 lies against the other end of the filter medium 10. The second part 55 of the housing is then placed over the end cap 56 and the associated end of the filter medium 10 so that the inclined surface 69 of the locking formation 65 contacts the lip 59 of the first part 54. The first and second parts 54,55 of the housing are then pushed together so that the locking formation 65 flexes outwardly and rides over the lip 59 until the lip 68 snaps into position adjacent the lip 59 as shown in Figure 8.

The projections or pips bear against the end cap 56.

During this assembly, the sealing walls 60,72 penetrate into the respective ends of the filter medium 10, as described above, so as to seal the first part 54 against one end of the filter medium 10 and the end cap 56 against the other end of the filter medium 10.

Suitable means (not shown) are provided to give a fluid tight seal between the two parts 54,55 of the housing.

In operation, fluid is passed into the housing via the port 70 and passes around the outside of the filter medium 10. The fluid then passes through the filter medium 10 and through the holes 53 in the core 41 to the interior of the core 41. Fluid in the interior of the core 41 passes out from the housing via the port 61.

The components of the third filter assembly can be assembled in a different manner, as follows. Firstly, the end cap 56 is sealed against an end of the filter medium 10 by pushing the sealing wall 72 into the end of the filter medium 10. The planar end region 57 of the first housing part 57 is then sealed against the other end of the filter medium by pushing the sealing wall 60 into the other end. Finally, in order to fix the end cap 56 and the first housing part 54 in their respective sealing positions relative to the filter medium 10, the second housing part 55 is snapped onto the first housing part 54 so that the pips bear against the end cap 56.

A fourth filter assembly is shown in Figures 9 to 18. As best seen in Figures 9 and 10, the fourth filter assembly comprises four separate filter elements 80a, 80b, 80c, 80d, an open generally disc-shaped end cap 81, a closed generally disc-shaped end cap 82 and four tie rods 83a, 83b, 83c, 83d.

The four filter elements 80a to 80d are identical and only one of these, 80a, will be described in detail. The same reference numerals will be used to indicate corresponding features of the four filter elements 80a, 80b, 80c, 80d.

Referring to Figures 9 and 10, the filter element 80a comprises a tubular filter medium 84 (not shown in Figure 10), a tubular inner core 85 and a tubular outer cage 86. The filter medium 84 has an outer surface which, in cross-section, has a shape corresponding generally to a quadrant of a circle. That is to say the outer surface of the filter medium has two mutually perpendicular planar regions or sides (corresponding to radii of a circle) and an arcuate region or side (corresponding to the circumference of the circle). Each region (or side) is connected to the other regions (or sides) by a curved corner. The filter medium has an inner surface that has a similar shape to the outer surface but that is, of course, smaller. Accordingly, the terms "tube"and"tubular"as used in this specification include tubes having cross-sections other than circular cross-sections.

The inner core 85 is a relatively rigid tubular wall that lies against, and has the same shape as, the inner surface of the filter medium 84. The outer cage 86 is a thin, relatively rigid tubular wall that lies against, and has the same shape as, the outer surface of the filter medium 84. Hence the inner core 85 has first and second planar regions 87,88 and an arcuate region 89. The first and second planar regions 87,88 are joined by a first rounded corner 90. The first planar region 87 is joined to the arcuate region 89 by a second rounded corner 91, and the second planar region 88 is joined to the arcuate region 89 by a third rounded corner 92.

The filter medium 84 may be formed from a fibrous, depth filter material, such as that sold by Pall Corporation under the Trade Mark PROFILE. Each of the inner core 85 and the outer cage 86 is provided with a plurality of openings so as to allow fluids to pass freely through. The inner core 85 and the outer cage 86 serve to maintain the filter medium 84 in the quadrant-like shape described above and prevent compression of the filter medium 84 in an axial direction. If desired one of the inner core 85 and the outer cage 86 can be omitted.

The closed end cap 82 is shown in Figures 11 to 14. As seen in these Figures, the closed end cap 82 comprises a planar wall 93 having an inner face 94 and an outer face 95.

As best seen in Figure 11, four filter element receiving formations 96a, 96b, 96c, 96d are provided on the inner face 94 of the planar wall 93. The filter element receiving formations 96a to 96d are identical and only one of these, 96a, will be described in detail. The same reference numerals are used to indicate corresponding features of the filter element receiving formations 96a, 96b, 96c, 96d.

As best seen in Figures 11,13 and 14, the filter element receiving formation 96a has an outer tubular wall 97 that extends from the inner face 94 of the planar wall 93 in a direction normal to the planar wall 93. The outer tubular wall 97 has, in cross- section, the generally quadrant-like shape described above-the inner surface of the outer tubular wall 97 corresponding generally in shape and size to the outer surface of the outer cage 86.

The filter element receiving formation 96a also comprises four posts 98, 99,100,101, each of which extends from the inner face 94 of the planar wall 93 in a direction normal to the planar wall 93. The posts 98, 99,100,101 lie within the outer tubular wall 97 and are positioned around a notional quadrant, the notional sides of which lying parallel to the corresponding sides of the outer tubular wall 97. The notional quadrant on which the four posts 98 to 101 lie is slightly smaller than the cross- sectional shape of the inner core 85.

Additionally, the filter element receiving formation 96a comprises a tubular sealing wall 102 that extends from the inner face 94 of the planar wall 93 in a direction normal to the planar wall 93. As shown in Figure 11, the sealing wall 102 has the generally quadrant-like shape described above. As shown in Figures 13 and 14, the sealing wall 102 has an outer surface 103 that extends normally to the inner face 94 and an inner surface 104 that is inclined so that it extends outwardly as it extends away from the inner face 94. The outer and inner surfaces 103,104 meet at an edge 105 that is known in the art as a knife-edge.

As seen in Figure 11, the four filter element receiving formations 96a, 96b, 96c, 96d are angularly spaced around the inner face 94 of the closed end cap 82 such that the arcuate regions of the outer tubular walls 97 lie at the outer perimeter of the planar wall 93.

As best seen in Figures 11 and 12, the closed end cap 82 also has four tie rod receiving sockets 106a, 106b, 106c, 106d. Each of the four sockets 106a to 106d serves to receive, in a snap-fit manner, an end of a respective one of the four tie rods 83 a to 83d. The sockets 106a to 106d are annularly spaced around the planar wall 93 and lie near the outer perimeter of the planar wall 93. The sockets 106a to 106d are identical and only one of the sockets, 106a, is described in detail. The same reference numerals are used to describe corresponding features of the four sockets 106a to 106d.

The socket 106a comprises a generally cylindrical formation 107 that extends from the inner face 94 of the planar wall 93 in a direction normal to the planar wall 93. The cylindrical formation 107 is continuous with an aperture 108 that extends through the planar wall 93. As best seen in Figure 12, an annular ridge 109 extends from the planar wall 93 around and into the aperture 108. The annular ridge 109 comprises a rear surface 111 that extends from the outer face 95 of the planar wall 93, co-planar with the outer face 95, to an annular edge 112 of the annular ridge 109. An inclined surface 110 extends from the annular edge 112 outwardly and towards the inner face 94 of the planar wall 93.

As seen in Figures 11 to 13, a hole 113 extends across the planar wall 93 at the centre of the planar wall 93.

As seen in Figure 15, the open end cap 81 is similar to the closed end cap 82. Features of the open end cap 81 that are common to the closed end cap 82 are given the same reference numerals and are not described in detail. The open end cap 81 differs from the closed end cap 82 in two aspects. Firstly, the open end cap 81 has four polygonal holes 116a, 116b, 116c, 116d that extend across the planar wall 93. Each polygonal hole 116a to 116d is positioned within a respective one of the filter element receiving formations 96a to 96d-so that each polygonal hole 116a to 116d lies within the notional quadrant formed by the four posts 98 to 101 of the corresponding filter element receiving formation 96a to 96d. Secondly, the open end cap 81 has no central hole 113.

The four tie rods 83 a to 83d are identical and only one of the tie rods, 83 a, will be described in detail. The same reference numerals are used to indicate the corresponding features of the tie rods 83a to 83d.

As seen from Figures 16 to 18, the tie rod 83a has two end portions 118,119 that are connected by a central portion 117. The central portion 117 is cross-shaped in cross- section. The two end portions 118,119 are identical and only one of the end portions, 118, is described in detail. The same reference numerals are used to describe the corresponding features of the end portions 118,119. Starting from the central portion 117, the end portion 118 has a first cylindrical surface 120 which extends to a first frustro-conical surface 121. The first frustro-conical surface 121 narrows to a second cylindrical surface 122 which is very short in the axial direction. From an outer end of the second cylindrical surface 122 a radially extending surface 123 extends outwardly to a third cylindrical surface 124 which, in turn, extends to a second frustro- conical surface 125 which narrows to an end surface 126.

The fourth filter assembly is assembled as follows.

The closed end cap 82 is placed on a working surface with the inner face 94 upwards. Each of the filter elements 80a to 80d is then located into a respective one of the filter element receiving formations 96a to 96d. Each filter element 80a to 80d is received into the corresponding filter element receiving formation 96a to 96d in an identical manner and the receipt of one filter element, 80a, into the corresponding filter element receiving formation 96a, is described in detail.

The filter element 80a is located such that the outer surface of the outer cage 86 fits closely within the inner surface of the outer tubular wall 97. The posts 98 to 101 lie against the inner surface of the inner core 85. The post 98 is located at the first rounded corner 90. The post 99 is located at the second rounded corner 91. The post 100 is located at the third rounded corner 92. Finally, the post 101 is located mid-way along the arcuate region 89.

The filter elements 80a to 80d are located in the corresponding filter element receiving formations 96a to 96d in this manner, but they are not yet pushed fully into the filter element receiving formations 96a to 96d. Instead, they are located such that the edges 105 of the sealing walls 102 contact lower ends of the respective filter media 84.

Each one of the tie rods 83a to 83d is then inserted into a respective one of the sockets 106a to 106d of the closed end cap 82 so that, for each tie rod 83a to 83d, the second frustro-conical surface 125 of one of the end portions 118,119 lies against the inclined surface 110 of the corresponding socket 106a to 106d.

The open end cap 81 is then positioned so that each one of the filter elements 80a to 80d locates, in the manner described above, into a respective one of the filter element receiving formations 96a to 96d of the open end cap 81. Simultaneously, the upper end portions 118,119 of the four tie rods 83 a to 83 d locate, in the manner described above, in respective ones of the sockets 106a to 106d of the open end cap 81.

The open end cap 81 is then pushed towards the closed end cap 82. This performs two functions.

Firstly, each end of each filter element 80a to 80d is pushed fully into the corresponding filter element receiving formation 96a to 96d. During this process, each sealing wall 102 penetrates into the corresponding filter medium end so as to seal the filter medium end to the planar wall 93 of the adjacent end cap 81,82. The inclined inner surfaces 104 of the sealing walls 102 act to compress fibres of the filter media 84 against the corresponding inner cores 85, in a similar way to that described above in respect of the first filter assembly and shown in Figures 5 and 6. Secondly, and simultaneously, each end portion 118,119 of each tie rod 83 a to 83d locates fully into the corresponding socket 106a to 106d. This involves each annular ridge 109 riding over the second frustro-conical surface 125 and the third cylindrical surface 124 of the corresponding tie rod end portion 118,119. Each annular ridge 109 (which is resilient) then snaps into the groove formed by the first frustro-conical surface 121, the second cylindrical surface 122 and the radially extending surface 123 of the corresponding tie rod end portion 118,119. Hence the tie rods 83a to 83d serve to lock the open and closed end caps 81,82 in a predetermined spacing relative to one another, by a snap-fit mechanism. The spacing is such that the ends of the inner and outer cages 85,86 and the ends of the filter media 84 contact the corresponding inner faces 94 of the open and closed end caps 81,82.

The inner cores 85 and the outer cages 86 prevent the filter media 84 from bending, or compressing in an axial direction, and this in turn ensures that the sealing walls 102 penetrate fully into the filter media 84, thereby forming effective seals between the media 84 and the end caps 81,82.

In use, the fourth filter assembly is inserted into a housing (not shown). The housing, which may be of metal, is cylindrical with one end open and the other end closed. The housing has a circular cross-sectional shape that has an axis co-axial with the axis of the filter assembly, when it is inserted into the housing. The arcuate sides of each filter element 80a, 80b, 80c, 80d have a curvature similar to the curvature of the housing and the planar sides extend generally radially away from the housing. The arcuate sides are closely adjacent the housing. Fluid to be filtered is passed to the outsides of the filter elements 80a to 80d (and as shown in Figure 9 can pass between adjacent sides of the filter elements). Fluid becomes filtered as it passes across the filter elements 80a to 80d to the insides of the filter elements 80a to 80d. Filtered fluid lying inside the filter elements 80a to 80d drains through the polygonal holes 116. It will be appreciated that the filter media 84 of the fourth filter assembly, as a result of being provided with curved sides adjacent the housing and adjacent planar sides, achieve a packing density of filter material within the volume defined between the end caps 81, 82 that is greater than would be occupied by cylindrical filter media having the same wall thickness as the wall thickness of the filter media 84. Thus, within that volume, the filter media 84 can handle a greater throughput of fluid to be filtered and also have a greater dirt capacity and thus a greater life.

The use of cylindrical filter media of the same wall thickness as the filter media 84, and whose perimeter tangentially intersects the three sides of that media will produce a circle (in cross-section) whose perimeter is 13% smaller than the perimeter of one of the filter media 84. Put another way, each filter media 84 has a surface area that is 15% greater than a circular cross-section medium occupying the same position.

Thus, the four filter media 84 described above achieve a 60% increase in surface area over such circular media.

It will be appreciated that this is only one example of a number of configurations that achieve greater packing densities than cylindrical filter media. For example, instead of four generally quadrant shaped media 84, there could be two media of generally D- shaped cross-section or three similar media each subtending an angle of about 120°, or five or more media.

In addition, the filter unit described above with reference to Figures 9 to 18 is, as described above, intended to be mounted in a housing of circular cross-section.

However, housings of other cross-sections may be used with suitably shaped filter media having adjacent sides achieving packing densities within those housings greater than can be achieved with filter media of circular cross section. For example, in an elongate rectangular housing, a plurality of filter media 84 of the kind described above with reference to Figures 9 to 18 could be located side-by-side with the media 84 arranged in alternately oppositely facing directions and with the corners between the planar sides directed in respective opposite directions. In this case, the arcuate sides could be replaced by straight sides. Plainly different end caps would be required.

The filter media need not all be the same shape. They could be of different shapes.

In these alternative arrangements of the fourth filter assembly, a filter assembly is provided with a plurality of filter media arranged side-by-side in the housing with each filter medium having a non-circular cross-section so as to achieve a packing density within the housing greater than a corresponding plurality of filter media of circular cross-section. In a housing having a cross-sectional shape that is curved, the provision of filter media with a correspondingly curved outer side adjacent the housing and a second side extending away from the housing adjacent the associated side of a second filter medium, provides increased packing densities. The filter assemblies described above can be assembled more quickly and with the expenditure of less energy than conventional filter assemblies in which the end caps are sealed to the medium by welding. Moreover welding apparatus is not necessary.

It will be appreciated that the snap-fit mechanisms used in the filter assemblies of the first aspect of the invention may either be irreversible, or they may be of a type in which the fixing achieved is reversible, for example to allow a filter element or filter medium to be changed.

It will be appreciated that the various aspects of the invention are independent of one another. Thus, a filter assembly according to the first aspect of the invention may have only a single filter medium or it may have a plurality of filter media. Where a plurality of media are provided, the media may or may not be shaped to maximise packing density as described above. Additionally filter assemblies that are in accordance with one or more of the second, third or fourth aspects of the invention may or may not be provided with snap-fit mechanisms and/or sealing walls as described above.