Abstract:
A filtering device for filtering fluids includes a filter housing, a filter element and an attaching device for attaching the filter element to the housing. The filter element is attachable to the filter housing using a bayonet lock.

Description:
FIELD OF INVENTION 
   The present invention relates to a filter device for filtering fluids, comprising a filter housing, a filter element and an attaching device for attaching the filter element to the housing. The present invention also relates to an application device having such a filter device. 
   BACKGROUND 
   Filter devices of the named type are known and are used in a large number of technical applications, especially in order to filter particles above a certain size out of a stream of liquid or gas. Filter devices of this sort are normally constructed in such a way that a filter element is removably attached to a housing using an attaching device. It is thus possible to release the filter element from the housing and remove it therefrom, in order to check it, clean it, replace it or the like. 
   In many applications, such removal of the filter element must be performed at short time intervals. 
   In the related art, attaching devices are known which provide such attachment and removal of the filter element by using locking screws or screw-mounted covers. Attachment of that type is complicated to release and causes long down times of the system or device with which the filter device operates, when the filter element is to be released from the housing or removed from it. 
   There is thus a need for an attaching device that allows simple and quick attachment and release of a filter element from a filter housing, which overcomes drawbacks of the prior art, such as those discussed above. 
   SUMMARY OF THE INVENTION 
   The present invention provides a device for attaching a filter element to a filter housing using a bayonet-type configuration. The manner of attachment according to the present invention achieves secure, positive-lock attachment of the filter element to the filter housing. The attachment may be releasable in a short amount of time by a simple motion, such as a small rotation. That reduces the time which is needed to release the filter element from the filter housing or to remove it therefrom. The filter device according to the present invention, when used in conjunction with a device which further processes the fluid being filtered, permits a short down time of the device when filter replacement or filter removal is necessary. 
   According to one aspect of the invention, the attaching device is movable between a closed position, in which the filter element is attached in a cavity of the filter housing and this cavity is sealed off from the surroundings, and an open position, in which the filter element may be removed from the filter housing. The cavity is connected with at least one fluid inlet and one fluid outlet. The filter element is positioned in the cavity in such a way that fluid which flows from the fluid inlet to the fluid outlet must pass through the filter element, and in so doing is filtered. The fluid to be filtered is normally under elevated pressure compared to the ambient pressure, but could also be at a lower pressure than the ambient pressure. In order to prevent fluid from escaping from the cavity into the surroundings, or environmental media, such as air, from entering the cavity from the surroundings, it is therefore necessary to seal the cavity off from the surroundings. 
   Advantageously, the attaching device may also be moved to a relief position located between the open and closed positions. In this relief position, the filter element is attached to the filter housing and the cavity is not sealed off from the surroundings. This embodiment is especially advantageous in order ensure safe removal of the filter element from the cavity. Since the fluid in the cavity is normally at a different pressure than the environment surrounding the filter device, as stated earlier, the resulting pressure difference may make removal of the filter element from the cavity more difficult and/or may lead to a risk of injury to the operating personnel of the filter device when performing the removal. It is conceivable, for example, that when the attaching device is moved from the closed to the open position, an overpressure in the cavity may cause parts of the attaching device to separate from the attaching device at high speed and injure the operator. It is also conceivable that the fluid which is under pressure may escape from the cavity at the moment the cavity is opened. Since health-endangering fluids or fluids at high temperature may be involved, uncontrolled escape of the fluid under high pressure is undesirable. It is also conceivable that the removal of the filter element or parts of the attaching device may be made more difficult by underpressure in the cavity. The relief position provided by the present invention makes it possible to reduce the pressure difference between the cavity and the surroundings in a controlled manner, without the filter element becoming detached from the housing prematurely. 
   According to another aspect of the invention, the attaching device is configured so that, in the relief position, all elements of the attaching device are secured to the filter housing, so that no elements of the attaching device can become separated from the housing prematurely. In the preferred embodiment, as described above, after the pressure difference between the cavity and the surroundings has been equalized for a certain length of time under controlled conditions in the relief position, the attaching device is moved to the open position and the filter element is then removed in a simple and safe manner. 
   Preferably, the attaching device is designed so that it may be moved between the open, closed, and relief positions by rotating a first attaching element around an axis of rotation. The axis of rotation may advantageously be parallel to the direction in which the filter element is removed from the filter housing or inserted into it, or may coincide with this direction. In order to achieve the various positions of the attaching element through rotation, means may be provided for applying torque to the first attaching element. For example, inside or outside hexagonal surfaces may be provided. In addition, there may be provision for an axial displacement of the attaching element in a direction parallel to the axis of rotation to be superimposed on the rotational movement around an axis of rotation, in order to move the attaching device between the positions. 
   The angle of rotation between the open and closed positions is preferably about 180°. A relief position, if provided, may be provided for example at a rotational angle of about 90°. This small rotational angle makes especially fast removal of the filter element and especially fast arrival at the relief position possible. 
   According to another aspect of the invention, the attaching device may be constructed in such a way that it includes a first attaching element having an elevation that interacts with a recess in a second attaching element. In this case one of the two attaching elements may be connected with the filter housing, or implemented as an attaching element connected with the filter housing in a single piece. The other attaching element is attachable through the interaction of the elevation with the recess on the forenamed attaching element. In this embodiment there may be provision, for example, for a first attaching element to have a surface on which an elevation is formed in a single piece. The elevation interacts with a recess in a surface of a second attaching element. The second attaching element is connected separably or in a single piece with the filter housing. The two attaching elements may be connected with each other through interaction of the elevation with the recess, so that the first attaching element may be removably attached to the filter housing. 
   In one exemplary embodiment, the first attaching element may be connected with the filter element in such a way that when the first attaching element is attached to the second attaching element or to the filter housing, the filter element is also attached to the filter housing. At the same time, the cavity in which the filter element is located my also be sealed from the surroundings, for example, by achieving a seal between the first and second attaching elements and between the second attaching element and the filter housing using elastic sealing elements. 
   In another exemplary embodiment, the elevation is formed on a surface of an attaching element which is connected removably or in a single piece with the filter housing, and the recess is formed in the surface of an attaching element, which may be removably attached to the forenamed attaching element. 
   Advantageously, the recess in the second attaching element includes an insertion section which extends in the direction of insertion from a reference surface lying perpendicular to the direction of insertion, a blocking section adjacent to the insertion section, which runs in a radial or radial-axial direction, and a closing section adjacent to the blocking section, which runs in a radial direction, the closing section extending further in the direction of the reference surface than the blocking section. 
   The reference surface here may be, for example, the surface in which the opening cross section of the recess in the second attaching element lies, through which the filter element is inserted into the cavity. In particular, the reference surface may be the outer surface of the second attaching element, which lies perpendicular to the direction in which the filter element is inserted into the cavity, or perpendicular to the axial direction in which a first attaching element that is attachable by a rotary movement is attached to a second attaching element. 
   The insertion section is constructed so that it may receive the elevation of the first attaching element and guide it in the direction of insertion. The closing section is arranged so that movement of the elevation of the first attaching element contrary to the axial direction of insertion is not possible, and the first attaching element is thus immovably fixed in this axial direction. The blocking section is designed so that it prevents the elevation from sliding into the insertion section without the application of force. Thus the blocking section can for example prevent the elevation from being able to be moved into the insertion section by a pure rotary movement, because the blocking section is designed so as to make a combined radial-axial motion necessary in order to move the elevation from the closing section into the insertion section. 
   In another exemplary embodiment, the recess includes an insertion section which extends in the direction of insertion from a reference surface lying perpendicular to the direction of insertion; a blocking section, adjacent to the insertion section, which runs in a radial or radial-axial direction; and a pressure relief section, adjacent to the blocking section, which runs in a radial direction. The pressure relief section extends further in the direction of the reference surface than the blocking section. The recess further includes a second blocking section, adjacent to the pressure relief section, which runs in a radial or radial-axial direction, and a locking section, adjacent to the second blocking section, which runs in a radial direction, the locking section extending further in the direction of the reference surface than the second blocking section. 
   With the aforementioned arrangement of the individual sections of the recess, it is possible to move the attaching device of the filter device according to the present invention from the closed position to a pressure relief position, in which case a blocking effect is achieved by the second blocking section, with the effect that both a radial (rotary) motion and an axial (shifting) motion are necessary for the movement from the closed to the pressure relief position. This prevents the attaching device from being moved to the pressure relief position unintentionally, or from moving to the pressure release position by itself, for example due to vibrations that occur during operation. 
   It is also possible to move the filter device according to the present invention from the pressure release position to the open position. In the same way as described earlier, when this is done a blocking effect is achieved by the first blocking section, with the result that a combined radial-axial motion, i.e. a superimposed rotary and shifting motion, is necessary in order to get from the pressure release position to the open position. This again prevents the attaching device from being moved unintentionally from the pressure release position to the open position, or from moving by itself due to forces that arise during operation. This also prevents unintentional movement of the attaching device further into the open position when it is being moved from the closed position to the pressure relief position. Such operation is particularly advantageous when the pressure relief stage is run through briefly or not at all. 
   The previously described embodiments having one or two blocking sections between the closed/pressure release and open positions are especially advantageous, when an elastic element is provided at the same time which exerts a force operating in an axial direction on one of the attaching elements. This force is advantageously oriented so that it acts contrary to the axial direction of movement which is necessary to overcome a blocking section lying between two positions. This accomplishes further securing of the attaching device in one of the two or three possible positions. A simple exemplary embodiment having such an elastic element may, for example, contain a pressure spring which is positioned in the cavity that receives the filter element, and which is pre-tensioned so that it is directed against a first attaching element contrary to its direction of insertion into the cavity, this first attaching element working together with a second attaching element that is attached to the housing containing the cavity. 
   In another exemplary embodiment, the first attaching element has a cylindrical surface on which the elevation is formed in a single piece, and which is arranged around the insertion direction in rotational symmetry. According to this embodiment, especially simple production of the attaching device is possible. The cavity that receives the filter element is implemented advantageously as a borehole open to one side (blind hole). The direction of insertion of the first attaching element is then directed from the opening of the cavity in the direction of the bottom of its hole. As explained earlier, this embodiment advantageously provides for a pressure spring to be positioned in the cavity, which exerts a pressure on the first attaching element that is directed contrary to the direction of insertion. The pressure spring may be braced for example against the floor of the cavity, or it may be braced against a projection formed in the cavity. 
   The elevation may be designed, for example, as a cylindrical pin, which extends in a radial direction and protrudes beyond the cylindrical surface of the first attaching element. In this case the cross sectional area of the pin must be matched to the loads which occur in operation. These loads are due to the pressures which arise in the cavity due to operation, and to the spring force which is exerted on the first attaching element by a pressure spring which may be provided in the cavity. The design of the elevation may depart from a round cross sectional area, especially if these forces are great, and oval or rectangular cross section geometries, or cross section geometries that extend in the radial or axial direction in some other way, may be provided in order to prevent bending, shearing or other failure of the elevation, and consequently of the attaching device, due to the operating forces. 
   In addition, the second attaching element in the embodiments described above may designed advantageously as a cover plate which is removably connected with the housing, and in which a plurality of sections are formed. The sections may in particular represent the sections designated in claims 7 and 8 in the form of a plurality of steps. The sections may also be incorporated into the cover plate in the form of indentations. Furthermore, the side of the cover plate facing the housing may have stepped recesses, which form indentations when the cover plate is attached to the housing, which indentations are limited in the axial direction on both sides, namely on one side by a surface of the housing and on the other side by a surface of the cover plate. 
   An additional aspect of the present invention is an application device for delivery of fluids, which includes at least one filter device in one of the embodiments described earlier. 
   Such application devices normally include a supply channel which is connectable to a source of fluid, a valve system which is movable between an open and a closed position, and a nozzle system. The filter device according to the present invention may be positioned functionally here between the supply channel and the valve system. It is also conceivable to place the filter device between the valve system and the nozzle. It is also customary to provide a plurality of valve and nozzle systems, which are supplied in common from one source of fluid. Provision may be made here, for example for economic reasons, for only one filter device, which effects filtration of the fluid which is supplied to the plurality of valve systems. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the present invention will be described on the basis of the figures. The figures show the following: 
     FIG.  1 : A perspective view of a first attaching element of an attaching device of the filter device according to the present invention; 
     FIG.  2 : A perspective view of the top of a second attaching element of the attaching device of the filter device according to the present invention; 
     FIG.  3 : A perspective bottom view of the attaching element of  FIG. 2 ; 
     FIG.  4 : A top view of the attaching element of  FIG. 2 ; 
     FIG.  5 : A sectional front view at cutting line A—A of  FIG. 4 ; 
     FIG.  6 : A sectional front view of the attaching element of  FIG. 2  at cutting line D—D of  FIG. 4 ; 
     FIG.  7 : A side view of the attaching element of  FIG. 4 ; 
     FIG.  8 : A bottom view of the attaching element of  FIG. 2 ; 
     FIG.  9 : A sectional oblique view of the attaching element of  FIG. 2  at cutting line C—C in  FIG. 8 ; 
     FIG.  10 : A front view of the attaching element of  FIG. 1 ; 
     FIG.  11 : A top view of the attaching element of  FIG. 1 ; 
     FIG.  12 : A side view of the attaching element of  FIG. 1 ; 
     FIG.  13 : A sectional front view of the attaching element of  FIG. 1  at cutting line A—A in  FIG. 12 ; 
     FIG.  14 : A schematic execution of the attaching element of  FIG. 2 ; 
     FIG.  15 : A partial sectional front view of the filter element with spring element; and 
     FIG.  16 : A front view of the filter element with spring element and one attaching element. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a first attaching element  1  of an exemplary attaching device of the filter device according to the present invention has a cylindrical surface  10 , on which an elevation  20  is formed. Cylindrical surface  10  is arranged in rotational symmetry around an axis  30 . Elevation  20  extends in a radial direction from axis  30 . In addition, there are hexagonal surfaces  40  formed around rotational axis  30  of the first attaching element  1 , to facilitate engagement of first attaching element  1  with a tool, for example, to permit applying a torque to the first attaching element  1 . 
   Referring to  FIGS. 2-9 , a second attaching element of the attaching device of the filter device according to the present invention, constructed as a cover plate  2 , has an outer surface  60  to which there two boreholes  61   a,    61   b  in perpendicular orientation. Boreholes  61   a,    61   b  have cylindrical counter-bores  62   a,    62   b,  so that conventional commercial socket-head cap screws may be inserted through boreholes  61   a,    61   b  and screwed down, after which the screw head is countersunk into the second attaching element in such a way that it does not protrude beyond the outer surface  60 . The second attaching element is constructed as a cover plate  2 , which is made from a rectangular starting piece. Centered in the middle of outer surface  60  there is a through borehole  80  which extends to a lower surface  65  of the cover plate (FIG.  3 ). Borehole  80  has a cylindrically shaped surface  81  extending over a reference circle, which corresponds to cylindrical surface  10  of the first attaching element  1 . The diameter of surface  81  is slightly greater than the diameter of cylindrical surface  10 , so that it is possible to shift and twist the first attaching element  1  easily when the latter is inserted into borehole  80  of the second attaching element  1 . In the exemplary embodiment shown, cylindrical borehole  80  has a radial opening  85  open over a circumferential angle of about 120° in a radial direction, since a corner area of the rectangular starting piece from which the second attaching element is made is removed. The cover plate  2  therefore has an approximately L-shaped form in the top view, as may be recognized clearly in FIG.  4 . 
   Referring to  FIG. 1 , elevation  20  extends over a circumferential angle which is smaller than the circumferential angle of the opening of borehole  80 . This makes it possible to insert the first attaching element into borehole  80  in the axial direction, with elevation  20  projecting into the radial opening  85  of borehole  80 . 
   Referring again to  FIGS. 2-9 , cover plate  2  has a first lateral surface  61  that extends over the entire length of cover plate  2 , and a first face  62  that extends over the entire width of cover plate  2 . Opposite first lateral surface  61 , a second lateral surface  63  is formed, which lies parallel to first lateral surface  61 . Second lateral surface  63  extends from a corner, at which it abuts on first face  62 , in the direction of the radial opening  85  of borehole  80 , and is interrupted by the latter, causing it to not extend over the entire length of cover plate  2 . Located parallel to first face  62  is a second face  64 , which extends from a corner, at which it abuts on first lateral surface  61 , in the direction of the radial opening  85  of borehole  80 . Second face  64  therefore does not extend over the entire width of cover plate  2 . The radial opening  85  of borehole  80  is limited by an opening lateral surface  82  positioned parallel to first lateral surface  61 , and by an opening face  83  positioned parallel to first face  62 . Opening lateral surface  82  extends from a corner, at which it abuts on second face  64 , approximately in the direction of the center point of borehole  80 . Opening face  83  extends from a corner, at which it abuts on second lateral face  63 , in the direction of borehole  80 . 
   Referring to  FIG. 3 , cover plate  2  has a lower surface  65 , through which boreholes  61   a,    61   b  extend. Lower surface  65  has a plurality of recesses  90 ,  93 ,  95 ,  99  in the area around borehole  80 , which extend from lower surface  65  in the direction of outer surface  60  and are open toward borehole  80 . Extending in the area between the center point of borehole  80  and first face  62  is a closing recess  90 , which is limited by a surface  91  positioned parallel to outer surface  60  and by a surface  92  positioned parallel to first face  62 . Closing recess  90  extends in a direction parallel to first face  62 , far enough that it is able to receive elevation  20 . That is, closing recess  90 , in particular surface  91  of recess  90 , extends over a circumferential angle that corresponds at least to the circumferential angle over which elevation  20  of the first attaching element  1  extends. When the first attaching element  1  is inserted into borehole  80 , and elevation  20  of the first attaching element  1  engages closing recess  90  of cover plate  2 , the attaching device of the filter device according to the present invention is in the closed position. 
   With continued reference to  FIGS. 2-3 , extending adjacent to closing recess  90  is a first blocking recess  93 , which is also open in the direction of the center point of the borehole  80  and in the direction of lower surface  65  of cover plate  2 . First blocking recess  93  is limited by a surface  94  positioned parallel to outer surface  60 , and by surface  92  positioned parallel to face  62 . 
   Surface  94  of first blocking recess  93  is at a greater distance from outer surface  60  than surface  91  of locking recess  90 . To get from the closed position in closing recess  90  through the blocking position of first blocking recess  93 , the first attaching element must therefore be pressed further in the axial direction toward lower surface  65 , and at the same time be turned counterclockwise, with respect to  FIG. 2 , for example using hexagonal surfaces  40 . 
   Adjacent to first blocking recess  93  is a relief recess  95 , which is limited by a surface  96  positioned parallel to outer surface  60  and by a surface  97  positioned parallel to second face  64 . Surface  96  of relief recess  95  is connected with surface  94  of blocking  93  via an oblique surface  98 . Relief recess  95  extends over a circumferential angle which is greater than the circumferential angle over which elevation  20  of the first attaching element  1  extends. Relief recess  95  is therefore able to receive elevation  20  in its entirety. Surface  96  of relief recess  95  is positioned closer to outer surface  60  than surfaces  91  and  94  of locking recess  90  or blocking recess  93 . Oblique surface  98  is therefore oriented obliquely in the direction of outer surface  60 , starting from surface  94  of blocking recess  93 . 
   Elevation  20  of the first attaching element is moved from the blocking position in blocking recess  93  to the relief position in relief recess  95  by a rotational motion in the counterclockwise direction, with respect to  FIG. 2 , and simultaneous axial motion directed from lower surface  65  toward outer surface  60 . 
   Adjacent to relief recess  95  is a second blocking recess  99 , which is limited by a surface  100  positioned parallel to outer surface  60  and by a surface  101  positioned parallel to second face  64 . Surface  100  of second blocking recess  99  is at a greater distance from outer surface  60  than surface  96  of relief recess  95 . Surface  100  is connected with surface  96  via a curved surface  102 , which is oriented perpendicular to outer surface  60 . 
   Elevation  20  of first attaching element is moved from relief recess  95  into blocking recess  99  by an axial motion in the direction from outer surface  60  toward lower surface  65 , and a subsequent rotational motion in the counterclockwise direction of FIG.  2 . Through an additional motion in the counterclockwise direction of  FIG. 2 , recess  20  is rotated into the radial opening  85  of borehole  80 . Since this radial opening  85  receives elevation  20  completely, the first attaching element  1  may accordingly be removed from cover plate  2  through an axial motion in the direction from lower surface  65  toward upper surface  60 . 
   Referring to  FIG. 5 , cylindrical surface  81  of borehole  80  is connected with outer surface  60  via a conical surface  84 . This makes it easer to insert the first attaching element  1  into borehole  80 . 
   Referring to  FIGS. 10-13 , the first attaching element  1  has hexagonal surfaces  40  which are positioned around center axis  30  at angles of approximately 60° to each other. Hexagonal surfaces  40  are designed so that torque may be exerted on the first attaching element using a conventional tool (not shown), such as an open end wrench, a box wrench or a socket wrench. 
   Adjacent to hexagonal surfaces  40  in the axial direction is a cylindrical lateral surface  10  from which elevation  20  protrudes in a radial direction. Elevation  20  extends over an angle of about 90°. It is limited by an outer circumferential surface  21 , a first contact surface  22  lying radially with respect to lateral surface  10  and a second contact surface  23  lying tangential to lateral surface  10 , and two opposing surfaces  24 ,  25  that face in the axial direction. The surfaces  24 ,  25  facing in the axial direction have the shape of a ring section. Elevation  20  is located between the two axial ends of the first attaching element  1 , and is at a greater distance from the first axial end  17  on which hexagonal surfaces  40  are formed than from the opposite, second axial end  12 . 
   In the area of the second axial end  12  of the first attaching element  1 , a ring groove  11  having a rectangular cross section is incorporated into lateral surface  10 . The function of ring groove  11  is to receive a sealing element. 
   The second axial end  12  of the first attaching element  1  has a 45° chamfer  13 . Chamfer  13  makes it easier to insert the first attaching element into borehole  80 . Chamfer  13  works together with conical surface  84  to facilitate insertion of the first attaching element  1  into borehole  80 . The first attaching element  1  is inserted with its second end  12  in front into borehole  80 , from outer surface  60  in the direction of lower surface  65 . 
   Referring in particular to  FIG. 13 , a cylindrical borehole  14  extends in the axial direction from the second axial end  12  of the first attaching element  1 . Borehole  14  is formed as a blind bore, and has a floor surface  15  at its end opposite the second end  12 . Borehole  14  has a countersink  16  in the area of its opening. The function of borehole  14  is to receive a pressure spring. At the first axial end  17 , having hexagonal surfaces  40 , there is likewise a blind bore  18  extending in the axial direction, which may be used, for example, to guide a tool. 
   Referring to  FIG. 14 , cover plate  2  is shown schematically by a segment A which has surfaces  91 ,  94 ,  98 ,  96  and  100  of recesses  90 ,  93 ,  95  and  99  on its underside. Opposite segment A is a segment B, which is represented for example by the housing to which the adapter plate is attached. Elevation  20  (shown schematically in FIG.  14 ), is movable from an open position  200  to first blocking position  202  by means of an axial-radial motion  201 . From first blocking position  202 , elevation  20  is movable to a relief position  204  by means of a radial-axial motion  203 , the axial component of motion  203  being contrary to the direction of the axial component of motion  201 . From this relief position  204 , elevation  20  is again movable to a second blocking position  206  by an axial-radial motion  205 . The axial motion direction of motion  205  is contrary to the axial motion direction of motion  203 , and thus in the same direction as the axial motion direction of motion  201 . From the second blocking position  206 , elevation  20  is movable to a closed position  208  by means of a radial-axial motion  207 . The axial motion direction of motion  207  is contrary to the axial motion direction of motion  205 . From the closed position  208 , elevation  20  is again movable by means of motions of correspondingly opposite direction to second blocking position  206 , relief position  204 , first blocking position  202  and open position  200 . 
   Referring to FIG.  15  and  FIG. 16 , there is an O-ring seal  19  positioned in rectangular-cross-sectioned ring groove  11  of the first attaching element  1 . O-ring seal  19  projects slightly beyond lateral surface  10  of the first attaching element  1 , and is thus able to provide a seal against cylindrical surface  81  of borehole  80  and a cylindrical surface formed coaxially to that cylindrical surface  81 , which is formed in a cavity of the housing (not shown) to which the second attaching element is attached by screws inserted through boreholes  61   a,    61   b.  In the forenamed cavity there is a filter element  110 , which may be made, for example, from a cylindrical screen  111 . Filter element  110  has bracing elements  112 ,  113  at its two axial ends. Bracing element  112  braces the filter element against the floor of the cavity. Bracing element  113  acts against a pressure spring  114 , and is irremovably connected thereto, for example, by welding. Pressure spring  114  is inserted into borehole  14  of the first attaching element  1 , and is braced against the floor surface  15  of that borehole, as depicted in FIG.  16 . Pressure spring  114  exerts a pressure on the first attaching element  1 , which acts upon it contrary to the direction of insertion, which is directed from outer surface  60  in the axial direction toward lower surface  65 . When filter element  110  is received in the housing cavity and the first and second attaching elements  1 ,  2  are engaged, with elevation  20  in the closed position, elevation  20  is biased by spring  114  against cover plate  2 , with its ring section surface  24 , which faces the hexagonal surfaces  40 , against surfaces  91 ,  94 ,  98 ,  96  and  100  of sections  90 ,  93 ,  95  and  99 . As a result of the bias force created by spring  114 , elevation  20  is not able to move inadvertently from the closed position to the relief position, since this requires a motion contrary to the bias force in order to raise elevation  20  over surface  94  of blocking recess  93 . In the same way, the bias force prevents elevation  20  from being rotated inadvertently from the relief position to the open position, since this requires a motion contrary to the bias force in order to raise elevation  20  over surface  100  of second blocking section  99 . 
   In order to be able to execute the forenamed axial motion directions or to apply the force that overcomes the force of the pressure spring, at the transition from hexagonal surface  40  to lateral surface  10  there is an axial ring surface  41  against which a tool applied to hexagonal surfaces  40  may be braced, so that a force can be exerted through this tool against the spring force of spring  114 .