Patent Description:
Filter candles and filter arrangements for filtering liquids or gases are used in very different industrial fields and for different purposes. Many machines or motors are provided with lubricating oil filters with replaceable filter elements. Fuel filters with replaceable filter elements may also be used in internal combustion engines. Other fields of use are constituted, for example, by water filtration or the filtration of gases. The fluid filters in turn may be provided with an individual filter element or a plurality of filter elements in order to increase the filtering surface. A cleaning device such as a backwashing device may be arranged in the filter element in order to clean the filter during operation and/or to backwash the filter counter to the filtering direction.

In most known fluid filters, the replaceable filter elements are locked in position at a through-opening in a retaining device on the filter side, such as for example a perforated plate, via a connection which is able to be plugged together, wherein a cylindrical connecting socket is provided on the filter element side, said connecting socket being inserted into the associated through-hole in the retaining device on the filter side. The perforated plate at the same time generally undertakes the separation between clean space for the unfiltered filtrate and the inflow side for unfiltered fluid. A release of the filter elements from the retaining device is frequently prevented by means of separate securing elements.

<CIT> discloses a filter arrangement having a plurality of filter candles as filter elements, wherein each filter candle comprises a cylindrical top piece as a connecting socket, wherein the top piece is provided with a receiving groove for an O-ring as a seal and is also provided with a peripheral annular groove, an oval through-opening of a securing plate which is displaceable transversely to the perforated plate being able to be inserted therein. The perforated plate and the securing plate are provided with a plurality of through-openings in order to secure a plurality of filter candles simultaneously. The securing plate acts transversely to the longitudinal axis of the filter elements.

<CIT> discloses a filter arrangement in which the individual filter elements are provided on the connecting socket with a plurality of flattened portions which are offset over the periphery and with latching recesses between the flattened portions. The filter elements are fastened to a perforated plate, wherein a specific receiving socket is inserted into each through-opening, the connecting socket on the filter element being able to be inserted into said receiving socket. The receiving socket in turn is provided with a number of recesses corresponding to the number of flattened portions, a clamping element engaging therein. For locking the receiving socket and the connecting socket, the filter element together with the connecting socket is inserted into the receiving socket and then rotated in order to secure the filter element axially in the latching position. The clamping element in this case acts counter to a rotation of the filter element relative to the retaining device. The seal is implemented by an O-ring which acts between the front face of the receiving socket and the front face of the connecting socket. The connecting socket and the receiving socket consist of metal, in particular sheet metal parts, the production thereof being relatively costly, in particular due to the recesses required. A seal is only ensured when the retaining forces acting in the axial direction of the filter elements are sufficiently high. The solution, however, provides the advantage of anti-twist protection between the filter element, on the one hand, and the retaining device, on the other hand.

<CIT> discloses a generic filter element according to the preamble of claim <NUM> for a filter for hydraulic fluids. A container is forming a housing inside which a single filter cartridge is arranged. The filter cartridge and the housing comprise cooperating parts for their connection, namely a collar rooting from the bottom of the housing and having an outer coupling lateral surface as well as a respective collar on the filter cartridge having an inner coupling lateral surface for engaging outside the outer coupling surface on the bottom of the filter housing; the respective co-operating surfaces, do have a polygonal cross section with flat portions and curved transitions. The filter cartridge is placed on the outer periphery of the collar fixed to the bottom of the housing. A special formed seal having a cooperating polygonal cross section is needed to ensure a leakage free seal between the cartridge and the housing.

It is the object of the invention to provide a filter arrangement and filter elements therefor which are simpler to mount and to produce, which ensure a secure seal and which at the same time provide anti-twist protection between the filter element and the retaining device.

This object is achieved for a filter element with the invention as defined in claim <NUM>. Preferred embodiments of the filter element are claimed in claims <NUM> to <NUM>, and advantageous embodiments of a filter arrangement comprising a fluid filter with a fastened filter element according to the invention are claimed in claims <NUM> to <NUM>.

According to the invention, a receiving groove is provided for the sealing ring on the connecting socket, namely on the outer periphery of the connecting socket, and the groove base of said receiving groove is having the polygonal cross section with flat portions and curved transitions alternating with one another, and the polygonal groove base serves as an inner radial bearing surface for the sealing ring. The polygonal shape ensures a secure anti-twist protection, both relative to vibrations and also relative to significant variations in load due to flow or pressure, wherein the sealing ring acting in the polygonal through-opening ensures the best possible seal.

Compared to solutions known from the prior art with a spring element, the production is simplified at the same time. The polygonal cross-sectional shape on the groove base, on the connecting socket and on the internal wall of the retaining device is located in the sealing region and by alternating the flat portions and curved transitions in the sealing region it is possible to prevent filter elements with circular connecting sockets from being able to be fitted and/or having to have a small connecting socket, such that it is already obvious to the fitter by visual inspection that a seal would not be ensured.

In the filter element according to the invention, the lateral surface also has a polygonal cross section with flat portions and curved transitions alternating with one another.

In a filter arrangement using a filter element according to the invention, the connecting socket comprises the receiving groove for the sealing ring, the groove base thereof forming the radial bearing surface for the sealing ring, wherein the groove base and the internal wall in each case have a polygonal cross section with flat portions and curved transitions alternating with one another. It is particularly advantageous if the groove base and the lateral surface and the internal wall in each case have an even number of flat portions of similar type to one another.

In principle, different types of polygonal cross sections could be provided on the groove base of the receiving groove, on the connecting socket of each filter element and/or on the through-opening in the retaining devices. In order to obtain advantageous conditions for the mounting and the pre-compression of the sealing ring, however, the particularly preferred embodiment for the polygonal cross sections of the groove base and the internal wall, and the lateral surface on the connecting socket, in each case provide a hexagonal cross section with <NUM> flat portions and <NUM> curved transitions. It is even more advantageous if in this embodiment a symmetrical hexagonal cross section is provided in which the flat portions on the groove base, on the lateral surface and on the internal wall of the through-opening oppose one another in pairs and in parallel at a specific spacing (A) and the curved transitions on the groove base, on the internal wall and on the lateral surface in each case have a uniform radius of curvature (Ri) relative to one another, wherein for the ratio of the spacing relative to the radius of curvature, the following applies:<MAT> and preferably the following applies: <MAT>.

It goes without saying that due to the required adaptation of the clearance or transition between interlockable parts, namely the connecting socket and the through-bore, the radius of curvature RKM of the lateral surface is different from the radius of curvature of the internal wall RKW and the radius of curvature of the groove base RN, wherein preferably the radii of curvature in each case have a uniform difference relative to one another. Accordingly, in the particularly preferred embodiment the radius of curvature Ri of a curved transition on the lateral surface and the radius of curvature Ri of a curved transition on the groove base are in each case concentric relative to one another and have a common central point.

The flat portions may be partially planar; however, the flat portions may also have a crowned surface, partially and/or along their entire extension, the curvature thereof being smaller than the curvature of the curved transitions and/or the radius of curvature thereof being significantly larger than the radius of curvature of the curved transitions. The radius of curvature of the crowned surface could be larger by at least a factor of <NUM> than the radius of curvature of the curved transitions. The radius of curvature may be varied along the crowned surface and/or along the curved transitions, since circular crowning would not be required.

In filter elements or filter arrangements according to the present invention, the connecting socket may consist of plastics, in particular an injection-moulded part (injection part) made of plastics, wherein preferably in connecting sockets made of plastics the spacing of the receiving groove from the front face of the connecting socket is greater than the groove width of the receiving groove. Alternatively, the connecting socket may consist of metal. In particular in a connecting socket made of metal, the spacing of the receiving groove from the front face may be less than the groove width of the receivers. In particular with a metal design of the connecting socket, it is advantageous if this connecting socket is of multi-part configuration and has at least two bent parts sealingly joined together at a joining point, wherein preferably the joining point is located in the region of the receiving groove and/or in the extension of one of the side walls of the receiving groove.

In order to simplify the plugging-in or insertion of the filter element into the associated through-opening in the retaining device, the connecting socket on the lateral surface may have a guide surface on the front face and/or an insertion bevel is provided on the internal wall of the through-opening in the retaining device, wherein it is particularly expedient if both are present and are accordingly able to cooperate; in particular in the case of a metal connecting socket it is advantageous if the front face of the connecting socket is provided with a region protruding axially over the guide surface. The region protruding on the front face may, in particular, be of annular configuration and/or the region protruding on the front face may form a contact surface for a vertical mounting of unmounted filter elements. The protruding region not only forms a contact surface but at the same time also a protective region for the initial insertion zone on the connecting socket and/or the guide surface.

Further preferably, the fluid filter may be a multi-element filter and the retaining device accordingly then has a plurality of through-openings for receiving a plurality of filter elements. The filter elements may, in particular, be filter candles, basket filter elements, multimantle filter elements or ring filter elements, and the element body itself may form the filter material or is provided with a corresponding filter material such as filter fabric, filter cloth, or the like.

The internal wall of each through-opening of the retaining device may also be provided in each case with an insertion bevel for cooperating with the front face of the connecting socket of the filter element to be fastened, in order to assist the plugging together and/or interlocking of the connecting socket and the through-opening in the retaining device.

The retaining device may consist of a perforated plate with through-openings which are configured integrally in the perforated plate and which in each case have a polygonal cross section, or the retaining device may consist of separate inserts which comprise the through-openings having a polygonal cross section and which are fastened to the perforated plate.

Each filter element may also be configured, in particular, as a filter candle with ends which are open on both sides, wherein the connecting socket with the polygonal, in particular hexagonal, cross section of the receiving groove and/or the lateral surface is arranged at one candle end, and a round socket having a circular cylindrical peripheral wall is arranged at the other candle end. In filter candles which are open on both sides, preferably the round socket may have a diameter which is smaller than the spacing between two flat portions on the connecting socket at the other end of the filter candle.

Further advantages and embodiments of filter elements or filter arrangements according to the invention are disclosed in the following description of exemplary embodiments shown schematically in the drawings, wherein the invention is not limited to the exemplary embodiments. In the drawings:.

In <FIG> a fluid filter, in particular for filtering liquids, is denoted as a whole by the reference numeral <NUM>, wherein a housing of the fluid filter <NUM> is indicated schematically by reference numeral <NUM>. The fluid filter <NUM> during operation is provided with a plurality of replaceable filter elements <NUM>, in <FIG> only a total of three thereof being shown, liquid flowing through said filter elements either from outside to inside or from inside to outside during the filtering operation of the liquid to be cleaned. To this end, on the housing <NUM> the fluid filter <NUM> has an inlet for the fluid to be filtered, in particular a liquid such as lubricating oil, liquid fuel, emulsion or water, an outlet for filtrate filtered in the fluid filter <NUM>, and optionally an outlet for filtered-out residue, if the fluid filter <NUM> is provided with a backwashing device for backwashing all filter elements <NUM> in the filter <NUM>. Since the fluid filter corresponding to the principal construction is known to the person skilled in the art, and the practical embodiment of the fluid filter is not significant for the invention, no further description thereof will be made here.

In the multi-element fluid filter <NUM> shown in <FIG>, for the operation of the filter a plurality of individual filter elements <NUM> are used in a replaceable manner, said filter elements all having the same construction relative to one another and in the exemplary embodiment according to <FIG> being open at both filter element ends. The fluid filter <NUM> shown is designed to receive a total of sixty filter elements <NUM>. For fastening the filter elements <NUM> two perforated plates are arranged spaced apart from one another in the fluid filter <NUM>, namely an upper perforated plate <NUM> through which the individual filter elements <NUM> are inserted, as well as a lower perforated plate <NUM> into which the filter elements are inserted. In this case, the upper perforated plate <NUM> and the lower perforated plate <NUM> have a corresponding number of more than fifty through-bores <NUM> in the lower perforated plate <NUM> and through-holes <NUM> in the upper perforated plate <NUM>, and each filter element <NUM> has a lower connecting socket <NUM> for the through-bores <NUM> in the lower perforated plate <NUM> and an upper connecting socket <NUM> for the through-holes <NUM> in the upper perforated plate <NUM>. All of the through-holes <NUM> in the perforated plate <NUM> in this case have the same dimensions as one another and are arranged on a plurality of pitch circles, in order to be able to secure the largest possible number of individual filter elements <NUM> to the perforated plate <NUM> and thereby in the multi-element filter. In the exemplary embodiment shown, each disc-shaped circular perforated plate <NUM> has sixty through-holes <NUM>, distributed over <NUM> pitch circles and each perforated plate <NUM> correspondingly has sixty through-holes <NUM> and additionally a plurality of central through-recesses <NUM> via which a fluid to be filtered, which flows into the housing <NUM> of the filter <NUM>, may be supplied to both open candle ends of the filter candle <NUM> arranged between the perforated plates <NUM>, <NUM>. Moreover, in each case the perforated plates <NUM>, <NUM> have a central opening <NUM>, a backwashing device, not shown, for backwashing the filter elements <NUM> being rotatably mounted thereby and being able to be connected to a backwashing outlet in order to backwash the individual filter elements separately or in groups, as is known per se to the person skilled in the art relative to filters, which is why no further details are provided thereof here. Preferably in each case an insertion bevel 6A is provided on both sides, both on the upper face and on the lower face of the perforated plate <NUM> for each through-hole <NUM>, said insertion bevel preferably consisting of a round depression on the external periphery.

In all filter elements <NUM> a filter element body 10A extends between the two connecting sockets <NUM>, <NUM>, said filter element body, for example, being able to support a filter fabric, being able to consist of said fabric, or being able to form the filter material itself, as for example is the case in slotted filter candles. In the exemplary embodiment shown in <FIG>, the connecting sockets <NUM>, <NUM> in each case consist of an injection-moulded part made of plastics, wherein the lower connecting sockets <NUM> are configured as round sockets and have a circular cylindrical lateral surface <NUM> with a receiving groove <NUM> for a sealing ring <NUM> which in the mounted state of the filter candles <NUM> is sealingly located in the through-bore <NUM> in the lower perforated plate <NUM>. The through-bores <NUM>, therefore, have a circular cylindrical cross section with a circular internal wall <NUM> correspondingly adapted to the diameter of the round sockets. The upper connecting sockets <NUM> and the through-holes <NUM> in the upper perforated plate <NUM>, however, in each case have a polygonal cross section according to the present invention. The construction of the connecting sockets <NUM> and the associated through-holes <NUM> is now explained with reference to <FIG>.

<FIG> shows a detailed view of the connecting socket <NUM> at the upper end of the element body 10A. The connecting socket <NUM> on the lateral surface <NUM>, spaced apart from the upper front face 21A of the connecting socket <NUM>, has a receiving groove <NUM> for an O-ring <NUM> as a sealing ring. The connecting socket <NUM> has on the outer periphery a hexagonal cross section with a total of six flat portions <NUM> arranged so as to be distributed uniformly over the periphery, wherein in each case a curved transition <NUM> is formed between two adjacent flat portions <NUM>, said curved transition preferably having a uniform radius of curvature which remains the same over the length of the curved transition <NUM>. <FIG> shows in a particularly clear manner the alternate configuration of flat portions <NUM> and curved transitions <NUM> on the lateral surface <NUM> of the connecting socket <NUM>. <FIG> also makes it possible to identify by means of the dashed line that the groove base 33A of the receiving groove <NUM> also has a hexagonal cross section with flat portions <NUM> and curved transitions <NUM> alternating with one another, wherein in each case a flat portion <NUM> on the lateral surface <NUM> extends parallel and at the same distance from a flat portion <NUM> on the groove base <NUM>, and a curved portion <NUM> on the lateral surface <NUM> extends parallel and at the same distance from a curved portion <NUM> on the groove base 33A. The internal opening <NUM> of the connecting socket <NUM>, however, is circular with a radius of curvature RR about the central longitudinal axis M of the filter element. Each of the curved transitions <NUM> of the groove base 33A has a radius of curvature RN which is smaller than half the spacing AN between two flat portions <NUM> of the groove base 33A opposing one another, and each curved transition <NUM> of the lateral surface <NUM> has a radius of curvature RM which is smaller than half the spacing AM between two opposing flat portions <NUM> of the lateral surface <NUM>. The radii of curvature RN for the curved portions <NUM> of the groove base 33A and RM for the curved portions <NUM> of the lateral surface <NUM> in each case have a common origin which is located at a corresponding distance from the central longitudinal axis M of the filter element. The respective curved portions <NUM> of the lateral surface located adjacent to one another on a partial curve and the curved portions <NUM> of the groove base 33A thus extend parallel to one another and at a uniform distance from one another.

The embodiment according to the invention with a hexagonal cross section is also manifested by the different thickness ratios of the wall thickness of the connecting socket <NUM> in the region of the curved portion <NUM> of the lateral surface <NUM> as shown in the upper half of <FIG>, and in the region of the flat portions <NUM> as shown in the lower half of <FIG>. The receiving groove <NUM> configured in the lateral surface <NUM>, however, has a uniform groove depth and groove width over the periphery. In the connecting socket <NUM> according to <FIG>, the internal opening <NUM> widens toward the rear end (right-hand end in <FIG>) via a step <NUM> so that here and in the wider region 28A of the internal opening <NUM>, the element body of the filter element may optionally be attached and secured to a separate casing made of filter fabric.

Correspondingly adapted to the dimensions of the lateral surface <NUM> and/or the groove base 33A of the connecting socket <NUM>, each through-hole <NUM> in the perforated plate <NUM>, which is provided for receiving the connecting socket, has a hexagonal cross section on the internal wall <NUM>, differing from a circular cylindrical design, with in each case six flat portions <NUM> and six curved transitions <NUM> which alternate with one another in the peripheral direction and in each case have the same dimensions as one another, so that the symmetry of the internal wall <NUM> is provided in the peripheral direction as is clear, in particular, from <FIG>. In order to permit the mounting of the filter candles according to the invention, the curved transitions <NUM> of the internal wall of the through-holes <NUM> have a radius of curvature RW which is smaller than half the spacing Aw of two opposing flat portions <NUM>, wherein in each case the spacing Aw and radius of curvature RW of the internal wall are slightly larger than the spacing (AM, <FIG>) and radius of curvature (RM, <FIG>) of the lateral surface (<NUM>, <FIG>).

<FIG> shows the mounting position of two adjacent upper connecting sockets <NUM> of correspondingly adjacent filter elements <NUM> in the adjacent through-holes <NUM> of the perforated plate <NUM>. As may be clearly identified, in each case the sealing ring, which is formed by a round O-ring <NUM> and is arranged inside the respective groove <NUM> of the connecting socket <NUM>, is spaced apart from the respective front faces of the perforated plate <NUM> by several millimetres and is located entirely inside the respective through-opening <NUM> and, therefore, is correspondingly located in the gap sealing region between the outer periphery of the lateral surface <NUM> of the connecting socket <NUM> and the internal wall <NUM> of the through-holes <NUM>.

<FIG> show a second exemplary embodiment of a connecting socket <NUM> preferably consisting of plastics for filter elements, not otherwise shown in more detail. In the case of the connecting socket <NUM> the lateral surface <NUM> also has a hexagonal cross section with six flat portions <NUM> and curved portions <NUM> respectively alternating with one another, and the groove base 133A of the receiving groove <NUM> in the lateral surface <NUM> also has a hexagonal cross section with in each case flat portions <NUM> and curved portions <NUM> respectively alternating with one another. The curved portions <NUM> of the groove <NUM> have in each case the radius of curvature RN and the curved portion <NUM> of the lateral surface <NUM> has the radius of curvature RM and two flat portions <NUM> opposing one another of the lateral surface <NUM> have the spacing AM, and two opposing flat portions <NUM> of the groove base 133A have the spacing AN. The ratios are in turn such that the respective radius of curvature RN and/or RM in each case is preferably considerably less than half the spacing AN and/or AM, wherein the ratio Ai/ Ri (where i=N, M) is preferably between approximately <NUM> and <NUM>. In contrast to the previous exemplary embodiment, the connecting socket <NUM> also has on the rear face a clamping or retaining structure <NUM> for the element body. The connecting socket <NUM> is particularly well suited to an element body having a preferably separate support body for a filter fabric, wherein the support body in this case may have a plurality of helical and/or spiral-shaped struts extending between both connecting sockets of a filter candle, for example, said struts forming intersections and apertures being formed therebetween in order to support a filter fabric securely on the outer periphery of the support body, also relative to high compressive loads. In each case only the associated ends of the struts of the support body are thus secured in the clamping or retaining structure <NUM>. A corresponding support body is disclosed, for example, in <CIT> of the applicant, the disclosure thereof being taken as a further reference. The retaining structure <NUM> in this case has a clamping gap <NUM> between an external wall projection <NUM>, the external surface thereof forming the lateral surface <NUM> with the hexagonal cross section, and said external wall projection being circular on the internal periphery, and an internal wall projection <NUM> located spaced apart therefrom with a number of recesses <NUM> corresponding to the number of spiral struts. The support body may in this case have six spirals/struts and in each case one of the six recesses <NUM> opposes a flat portion <NUM> on the lateral surface <NUM>. In the exemplary embodiment shown, the clamping gap has a smaller depth than the spacing of the groove <NUM> from the rear end; but in the case of a correspondingly larger thickness of the connecting socket and/or the wall projection, the groove could also be configured in the radial extension of the clamping gap in the lateral surface.

<FIG> show a further exemplary embodiment of a multi-element fluid filter <NUM> with filter elements <NUM> which are preferably open on only one side. The fluid filter <NUM>, therefore, requires only one perforated plate <NUM> which in the exemplary embodiment shown is provided for receiving a total of seven individual filter elements <NUM>, wherein six filter elements <NUM> are arranged on an outer pitch circle and the seventh filter element <NUM> is arranged centrally therebetween. The individual filter elements <NUM> overall have a substantially larger diameter and a longer length and thus also a considerably larger filtering surface than in the previous exemplary embodiment, and they have in the exemplary embodiment shown a metal support body such as for example a perforated plate (not shown), a pleated filter means <NUM> being supported on the outer periphery thereof and optionally being surrounded by a protective sleeve <NUM>. In the filter elements <NUM> a connecting socket <NUM> which preferably consists of a metal is used for each filter element <NUM>, and none of the connecting sockets <NUM> cooperates directly with a through-hole in the perforated plate <NUM> forming the retaining device on the filter side, but seven stepped through-openings <NUM> are provided in the perforated plate <NUM>, in each case an insert <NUM> being sealingly inserted therein in a fluid-tight manner and said insert in each case cooperating with the connecting socket <NUM> of a filter element <NUM>.

Firstly, the construction of the connecting sockets <NUM> is now explained with reference to <FIG>. The connecting sockets <NUM> in each case consist of metal and namely in this case of bent parts <NUM>, <NUM> which, for example, in each case are produced from sheet metal rings which are shaped by deep-drawing and which are sealingly connected together at a joining point <NUM>. Both bent parts <NUM>, <NUM> are configured in an annular manner with a central median axis M, wherein the bent part <NUM> on the rear face and remote from the joining point <NUM> has an annular surface <NUM> which extends perpendicular to the median axis M, in the mounted state as shown in <FIG> pleated filter means <NUM> being supported thereagainst and being able to bear against the front face. A circular cylindrical annular collar <NUM> adjoins the annular surface <NUM>, on the mounted filter element said annular collar being able to bear externally against the pleated filter cloth, or a protective and supporting element surrounding this filter cloth, and preferably extending slightly conically relative to the median axis M. Inwardly a U-shaped bent-back structure <NUM> adjoins the annular surface <NUM>, said bent-back structure having a radial outer wall projection <NUM>, an intermediate projection <NUM> extending at right-angles thereto, and an internal wall projection <NUM> extending again at right-angles thereto. The internal wall projection <NUM> forms on the connecting socket <NUM> a partial portion of the peripheral wall of the internal opening <NUM>, which is configured to be circular cylindrical in the case of the connecting socket <NUM>.

A second bent part <NUM> is connected to the bent part <NUM>, said second bent part in turn being configured to be annular and having a circular cylindrical internal wall projection <NUM> which merges with an intermediate projection <NUM> extending angled back at right-angles thereto, said intermediate projection extending so as to be aligned perpendicular to a central axis M of the connecting socket <NUM>. The intermediate projection <NUM> in turn merges with a first outer wall projection <NUM>, extending angled back at right-angles thereto, which merges with a transition projection <NUM> outwardly angled back at right-angles thereto and an outer edge projection <NUM> in turn extending angled back at right-angles thereto. Viewed parallel to the central axis M, the internal wall projection <NUM> axially projects over the outer edge projection <NUM>.

In the mounted state of the two bent parts <NUM>, <NUM>, the two intermediate projections <NUM>, <NUM> are located flat against one another and the two intermediate projections <NUM>, <NUM> form the joining surface on which the two bent parts <NUM>, <NUM> are sealingly connected together, in particular are welded together by means of laser welding in a fluid-tight manner. The two internal wall projections <NUM>, <NUM> in this case extend aligned and plane-parallel to one another and thus form the peripheral wall of the internal opening <NUM> of the connecting socket <NUM>. In the mounted state of the two sheet metal parts <NUM>, <NUM> the groove <NUM> of the connecting socket is produced between the transition projection <NUM>, the first outer wall projection <NUM> of the plate part <NUM>, and the partial portion of the intermediate projection <NUM> on the plate part <NUM> which protrudes radially over the joining surface between the two plate parts <NUM>, <NUM> and/or protrudes over the outer wall projection <NUM> of the plate part <NUM>. The wall projection <NUM> on the bent parts <NUM> is correspondingly shorter than the wall projection <NUM> on the bent part <NUM>. The lateral surface of the first outer wall projection <NUM> forms at the same time the groove base 233A of the groove <NUM>. The wall projection <NUM>, partially forming the sealing surface, at the same time forms one of the side walls of the receiving groove <NUM> for the sealing ring in the portion which protrudes radially over the wall projection <NUM>. According to the invention, the first outer wall projection <NUM> forming the groove base 233A, the edge projection <NUM> and the outer wall projection <NUM> aligned in the extension thereof, do not have a circular cross section but a hexagonal cross section, with in each case alternating curved portions <NUM> and flat portions <NUM> arranged so as to be offset relative to one another over the periphery, on the external lateral surface <NUM> of the connecting socket <NUM> formed by the projections <NUM>, <NUM>, and accordingly also curved portions <NUM> and flat portions <NUM> on the wall projection <NUM> forming the groove base 233A as, in particular, is clear in <FIG>. In each case, two opposing flat portions <NUM> of the external lateral surface <NUM> accordingly have the spacing AM, and in each case two opposing flat portions <NUM> of the groove base 233A accordingly have the spacing AN. The curved portions <NUM> of the groove base 233A have in each case the radius of curvature RN and the curved portions <NUM> of the external lateral surface have the radius of curvature RM. The origin of both radii of curvature coincides but is not located on the longitudinal axis M of the connecting socket <NUM>, being located at a radial spacing therefrom. The wall projections <NUM> on the rear bent part <NUM> in the insertion direction preferably extend parallel to the median axis M, but the front edge projection <NUM> may extend slightly angled back inwardly relative to the median axis M in order to simplify the insertion of the connecting sockets <NUM> in the correspondingly designed through-openings on the retaining device on the filter side.

<FIG> show suitable through-openings <NUM>, wherein the through-openings <NUM> co-operating with the connecting sockets configured according to the invention on the filter elements, in the exemplary embodiment in <FIG> are not directly configured in a perforated plate but are configured on separate inserts <NUM>. The inserts <NUM> are circular on their external periphery and have two portions 272A and 272B defined from one another by a step <NUM>. The portion 272B is adapted in order to be inserted into corresponding through-bores or through-passages in the retaining plate (<FIG>, <NUM>), wherein the step <NUM> then bears against the upper face of the stepped through-passage (<NUM>, <FIG>). The internal periphery <NUM> of the smaller portion 272B of the inserts <NUM> is also circular and merges via a transition bevel <NUM> with that portion 272A, the internal wall <NUM> thereof being configured for receiving with anti-twist protection the connecting sockets (<NUM>, <FIG>) arranged on the filter elements and shaped according to the invention. The portion 272A thus forms the actual receiving region for the connecting socket and serves as a sealing surface and/or sealing mating surface for the sealing ring (O-ring) in the receiving groove on the associated connecting socket. Corresponding to the embodiment according to the invention, the internal wall <NUM> in the portion 272A accordingly has a hexagonal cross section with in each case curved portions <NUM> and flat portions <NUM> respectively alternating with one another in the peripheral direction. In each case two opposing flat portions <NUM> of the internal wall <NUM> have the spacing Aw and the curved portions <NUM> in each case have the radius of curvature Rw. The front face of the larger portion 272A is provided at the transition with the internal wall <NUM> with an insertion bevel <NUM> in order to provide additional assistance for the insertion of the front face of the connecting socket (<NUM>, <FIG>). Accordingly, the insertion bevel <NUM> and the edge projection (<NUM>, <FIG>), which is also bevelled and extends inwardly, are guided during the mounting process, whereby even with relatively heavy or bulky filter elements the associated mating surfaces may be located in a simple and secure manner. In the mounted state, as may be primarily identified in <FIG>, the sealing ring <NUM> is located in the associated receiving groove <NUM> in the connecting socket <NUM> and is pressed all around with a uniform contact pressure against the internal wall <NUM> of the insert <NUM> and correspondingly ensures a fluid-tight seal between the connecting socket <NUM> and the retaining device <NUM>.

The following table advantageously shows dimensions (in mm) for the different spacings and radii without limiting the invention thereto:.

Claim 1:
Filter element for fluid filters, having an element body comprising a longitudinal axis, containing a filter means through which fluid is able to flow, and having at least one connecting socket (<NUM>; <NUM>; <NUM>) which is open on the front face at one end of the element body for the releasable fastening of the filter element to a through-opening in a retaining device in the fluid filter, wherein the connecting socket (<NUM>; <NUM>; <NUM>) comprises a lateral surface (<NUM>; <NUM>; <NUM>) having a polygonal cross section with flat portions (<NUM>; <NUM>; <NUM>) and curved transitions (<NUM>; <NUM>; <NUM>) alternating with one another and a bearing surface for a sealing ring, which bearing surface is having a polygonal cross section with flat portions (<NUM>; <NUM>; <NUM>) and curved transitions (<NUM>; <NUM>; <NUM>) alternating with one another, characterized in that a receiving groove (<NUM>; <NUM>; <NUM>) is provided for the sealing ring on the outer periphery of the lateral surface of the connecting socket (<NUM>; <NUM>; <NUM>), the groove base (33A; 133A; 233A) of said receiving groove provides an inner radial bearing surface for the sealing ring and is having the polygonal cross section with the flat portions (<NUM>; <NUM>; <NUM>) and the curved transitions (<NUM>; <NUM>; <NUM>) alternating with one another, wherein the sealing ring in the groove base serves for cooperating with an internal wall of the through-opening in the retaining device and is an O-Ring.