Patent Publication Number: US-11376528-B2

Title: Filter system with non-return valve and filter element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of international application No. PCT/EP2018/081045 having an international filing date of 13 Nov. 2018 and designating the United States, the international application claiming a priority date of 22 Dec. 2017 based on prior filed German patent application No. 10 2017 012 018.8, the entire contents of the aforesaid international application and the aforesaid German patent application being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention concerns a filter system with a non-return valve, in particular as an oil filter of an internal combustion engine, as well as a non-return valve and a filter element for such a filter system. 
     DE 10 2015 103 662 A1 discloses a filter with a filter housing, with a housing cover that closes the filter housing in operation and is removable from the filter housing, with an inlet for liquid to be filtered and with an outlet for filtered liquid. Furthermore, DE 10 2015 103 662 A1 discloses an exchangeable filter insert, separating a raw side and a clean side of the filter from each other, with a filter bypass valve that is comprised of a valve seat and a valve body that is movably guided relative thereto and prestressed by a force acting in valve closure direction. The valve seat is arranged in or at the housing cover and fixed at the cover, wherein the valve body is guided in or at the housing cover and fixed at the cover, and wherein a spring loading the valve body in the valve closure direction is guided in or at the housing cover and fixed at the cover. The filter insert comprises a spring support. In the state of the filter insert mounted in the filter and with the housing cover attached to the filter housing, the spring is supported and pretensioned by means of the spring support at its end which is facing away from the valve body. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to create a filter system with a non-return valve, in particular an oil filter of an internal combustion engine, wherein the non-return valve reduces a possible drop of the oil level in an oil circuit at low oil pressure or when the internal combustion engine is not operating and wherein the non-return valve can be inexpensively mounted. 
     Further objects of the invention are to provide a non-return valve for such a filter system as well as a filter element for such a filter system with a non-return valve. 
     The aforementioned object is solved according to an aspect of the invention by a filter system for filtering a fluid, comprising a filter housing with a housing top part and a housing bottom part which extend along a housing axis, an outlet socket arranged at the housing top part for discharging the filtered fluid, a non-return valve which is arranged in the interior of the outlet socket, a filter element that separates a raw fluid side from a clean fluid side, wherein the filter element comprises a valve seat for the non-return valve, and wherein the non-return valve closes in a closed position a fluid path through the outlet socket and opens in an open position the fluid path through the outlet socket. 
     A further object is solved according to an aspect of the invention by a non-return valve for use in a filter system with a filter element, wherein the non-return valve, which is arranged in the filter system in the interior of an outlet socket of the filter system, acts with a closure element that is supported by a spring element against a valve seat upon intended installation of a matching filter element, wherein the valve seat is formed at or in the filter element. Only when using a matching filter element, in particular matching in length, the filter element with the valve seat formed thereat can interact with the non-return valve so that a drop of the fluid pressure, for example, when turning off the internal combustion engine, is prevented. Upon removal of the filter element, the non-return valve is leaking so that fluid drainage is possible even from the clean region when servicing the filter element. 
     A further object according to an aspect of the invention is solved by a filter element for use in a filter system, with a longitudinal axis, comprising a filter bellows arranged about the longitudinal axis, wherein the filter element comprises a socket with an opening for draining the filtered fluid from the interior of the filter bellows, wherein the socket is embodied as a valve seat of a non-return valve. 
     Beneficial configurations and advantages of the invention result from the further claims, the description, and the drawing. 
     A filter system for filtering a fluid, in particular as an oil filter of an internal combustion engine, is proposed, comprising a filter housing with a housing top part and a housing bottom part which extend along a housing axis, an outlet socket arranged at the housing top part for discharging the filtered fluid, a non-return valve which is arranged in the interior of the outlet socket, and a filter element that separates a raw fluid side from a clean fluid side. The filter element comprises a valve seat for the non-return valve. In this context, the non-return valve closes in a closed position a fluid path through the outlet socket and opens in an open position the fluid path through the outlet socket. 
     At the clean side of the filter system according to the invention, a non-return valve is arranged which closes in a closed position the fluid path through the outlet socket and opens in an open position the fluid path through the outlet socket. This is in particular advantageous for a suspended filter system. In a suspended filter system, the housing top part is fixedly mounted, for example, and the filter element can be separated together with the housing bottom part from the housing top part. When such a filter system is used, for example, as an oil filter system in an internal combustion engine, the oil level does not drop when the motor is standing still because the non-return valve prevents the return of the oil into the filter element. Furthermore, the oil pressure is built up faster when restarting the engine. Advantageously, in a service situation in which the filter element is to be removed or exchanged, the non-return valve is not completely seal-tight so that, despite the presence of non-return valve, the oil column can drain through the non-return valve. In this way, more used oil can be removed and replaced with new oil when exchanging the filter element. 
     When the filter element is inserted into the filter housing, the closure element of the non-return valve is pressed seal-tightly by a spring element against the opening in the end disc, for example, the socket of the central tube in the filter element. In this way, the non-return valve is in its closed position and the fluid path through the outlet socket of the filter housing is closed. With increasing oil pressure buildup, the closure element of the non-return valve can be lifted off its valve seat by the fluid pressure and open the opening in the outlet socket in this way. The outlet socket can advantageously be arranged concentrically to the housing axis in order to enable simple mounting of the filter element without precise rotatory positioning. 
     When the filter element is removed, the closure element, e.g. a disc, is resting captively on the valve holder which is embodied, for example, as a retaining ring. By means of spacer segments and penetrations arranged therebetween, for example, leakage openings, the closure element is not resting completely seal-tightly on the valve holder. In this context, the valve seat, on which the closure element is resting when the filter element is inserted, is arranged in the filter element, for example, at the socket of the central tube. In this way, the actual functionality of a non-return valve can be realized. In the installed state, the socket of the filter element projects into the valve holder and past it. In this way, the closure element can contact seal-tightly the socket without or with minimal fluid pressure. A seal between the socket of the filter element and the outlet socket prevents leakage of the fluid when the filter element is installed. 
     The non-return valve enables in this way advantageously a retaining function of the oil circuit for fast lubrication of the internal combustion engine when restarting it. An optimal oil discharge with beneficial discharged oil volume, i.e., as large as possible, contributes further to the advantages of the filter system with non-return valve according to the invention. 
     According to an advantageous embodiment, the filter element can comprise a filter bellows arranged about its longitudinal axis. The filter element can comprise furthermore a socket with an opening in a longitudinal axis of the filter element for discharging the filtered fluid from the interior of the filter bellows. In this context, the socket can advantageously be embodied as a valve seat, in particular, the socket can be provided additionally also as an actuating element. Upon insertion of the filter element into the filter housing top part, wherein the filter element can be connected to the filter housing bottom part, for example, detachably, for example, locked, the socket of the filter element can lift in this context the closure element of the non-return valve out of its initial position. Since the socket is embodied as a valve seat and the closure element is pushed by the spring element against the valve seat, the non-return valve is thus in its closed position. Increasing fluid pressure counteracts the spring element so that the closure element can be lifted off its valve seat and open the opening of the outlet socket at least partially. 
     The socket of the filter element can have a central opening as a valve seat which can comprise a circular cross section for manufacture-technological reasons and for a better sealing action. 
     According to an advantageous embodiment, the non-return valve can comprise a closure element and a spring element, wherein the spring element is supported at the housing top part. In particular, the spring element can be clamped between closure element and housing top part in this context. The spring element pushes in this way the closure element against the sealing valve seat so that the non-return valve is in its closed position and the fluid path through the outlet socket is blocked. The spring element in this context can expediently rest and be supported with one end against the housing top part. When the spring force is overcome, the closure element can lift off its valve seat and in this way open the fluid path through the outlet socket. 
     According to an advantageous embodiment, the filter bellows can comprise at least a first end disc at a first end face, which end disc comprises an opening for receiving a central tube in the interior of the filter bellows, wherein the socket with the valve seat is embodied at an end of the central tube which is facing the end disc. When being filtered, the fluid flows from a radial outward position through the filter bellows into the interior of the filter bellows. The central tube in the interior of the filter bellows serves in this context for radially supporting the filter bellows against the hydraulic pressure of the fluid during filtering. The central tube, which can be constructed, for example, as a plastic tube with penetrations and stays, can be embodied as a socket in the direction of the outlet socket. This socket can advantageously serve as a valve seat for the non-return valve against which the closure element in the closed state of the non-return valve is resting seal-tightly. The socket, upon insertion of the filter element into the filter housing, can lift the closure element out of its initial position in which the closure element is resting against the valve holder. 
     According to an advantageous embodiment, the non-return valve, in the removed state of the filter element, can be designed to allow for leakage. Expediently, the non-return valve, in the removed state of the filter element in which the non-return valve is in its initial position, can exhibit leakage so that upon exchange of the filter element as much fluid as possible can drain from the fluid circuit. 
     According to an advantageous embodiment, in the interior of the outlet socket a valve holder can be arranged on which the closure element is resting in the removed state of the filter element. In particular, in the installed state of the filter element, the socket of the filter element can project through the valve holder. The valve holder serves as support of the closure element of the non-return valve which is pretensioned by the spring element. In this way, the non-return valve can be held in its initial position until the filter element is inserted into the filter housing and the closure element is lifted off the valve holder by the socket of the central tube, which projects through the valve holder, and is pushed against the valve seat formed on the socket. The valve holder serves also as an attachment of the non-return valve in the outlet socket in that it is supported against the wall of the outlet socket, for example. 
     According to an advantageous embodiment, leakage openings can be formed between closure element and valve holder. In particular, in this context the valve holder can have radially inwardly projecting stays on which the closure element is resting in the removed state of the filter element. The leakage openings can be embodied in this context between the stays, for example. Upon exchange of the filter element, when the closure element is again resting on the valve holder, the fluid can drain through the leakage openings so that the fluid circuit can be emptied therethrough. For this purpose, expediently the diameter of the socket is selected such that the leakage openings are located outside of the diameter of the socket. In the installed state at low fluid pressure, the non-return valve is closed and is open only when sufficient fluid pressure is present. With the filter element removed, the non-return valve is embodied to allow for leakage in a targeted fashion due to the leakage openings. 
     According to an advantageous embodiment, the valve holder can be embodied as a bent sheet metal part which can be clamped in the outlet socket in radial direction. A bent sheet metal part can be inexpensively produced with suitable tolerance dimensions. Also, in this way, claws can be provided beneficially at the radially outer side of the valve holder with which the valve holder upon insertion into the outlet socket can dig into the wall of the outlet socket and can be safely secured in this way by radial bracing. 
     According to an advantageous embodiment, the non-return valve can comprise a valve housing which is arranged in the interior of the outlet socket and/or at the valve holder and at which the spring element is supported. The spring element can thus be supported at an end of the valve housing in a suitable way whereby the force can be transmitted via the valve housing to the outlet socket and thus to the filter housing top part. Alternatively, the valve housing can also be connected to the valve holder. When this valve holder is fixed at the filter housing, the spring force can thus be transmitted to the filter housing so that the spring element is supported at the filter housing. 
     According to an advantageous embodiment, the valve housing and the valve holder are connectable to each other, in particular embodied together as one piece. Due to the embodiment of a joined unit of valve housing and valve holder, mounting of the non-return valve in the outlet socket can be simplified as a whole. Also, with a one-piece configuration of valve housing and valve holder, a particularly inexpensive unit can be created. Advantageously, the non-return valve can be embodied in this way as a preassembled unit which allows for beneficial organization of assembly times and costs of the filter system. 
     According to an advantageous embodiment, the filter element can be embodied to be insertable with its central tube at least partially into the outlet socket wherein the outlet socket is configured to be sealed relative to the opening of the end disc and/or relative to the central tube, in particular wherein the socket of the central tube is sealed at its outer side against the outlet socket. By sealing the outlet socket relative to the opening of the end disc and/or relative to the central tube, it can be achieved that the filtered fluid can flow completely through the outlet socket out of the filter element. In this way, the advantageous function of the non-return valve can also be utilized beneficially because no auxiliary fluid paths between filter element and outlet socket are possible. 
     According to an advantageous embodiment, the central tube at the end that is oppositely positioned to the central opening can comprise a closure element which is configured to be connectable to the housing bottom part. The central tube can advantageously be seal-tightly closed off at the other end so that the filtered fluid drains completely through the outlet socket. In this context, the closure element can be utilized expediently for connecting to the housing bottom part, for example, by locking. In this way, a beneficial releasable connection between filter element and housing bottom part can be realized also. 
     According to a further aspect, the invention concerns a non-return valve for use in a filter system with a filter element. The non-return valve, when mounted as intended in the filter system, is arranged in the interior of an outlet socket of the filter system, with a closure element supported against a valve seat by a spring element. In this context, the valve seat is embodied at or in the filter element. 
     The non-return valve enables thus advantageously a retaining function of the oil circuit for quick lubrication of the internal combustion engine when restarting. An optimal oil drainage with beneficial draining oil volume, namely as large as possible, contributes further to the advantages of the filter system with non-return valve according to the invention. 
     According to a further aspect, the invention concerns a filter element for use in a filter system, with a longitudinal axis, comprising a filter bellows arranged about the longitudinal axis. In this context, the filter element comprises a socket with an opening for draining the filtered fluid from the interior of the filter bellows, wherein the socket is configured as a valve seat of a non-return valve. 
     When the filter element is inserted into the filter housing, the closure element of the non-return valve is pushed seal-tightly against the opening in the end disc, for example, a socket of the central tube in the filter element, by means of the spring element. In this way, the non-return valve is in its closed position and the fluid path through the outlet socket of the filter housing is closed. With increasing oil pressure buildup, the closure element of the non-return valve can be lifted by the fluid pressure off its valve seat and in this way open the opening in the outlet socket. 
     According to an advantageous embodiment, the filter bellows can comprise at least a first end disc at a first end face, which end disc comprises an opening for receiving a central tube in the interior of the filter element. In this context, the socket with the valve seat is formed at an end of the central tube facing the end disc. 
     When filtering, the fluid flows from a radially outward position through the filter bellows into the interior of the filter bellows. The central tube in the interior of the filter bellows serves in this context for reinforcement of the filter bellows against the hydraulic pressure of the fluid during filtering. The central tube, that can be configured, for example, as a plastic tube with penetrations and stays, can be embodied as a socket in the direction of the outlet socket. This socket can advantageously serve as a valve seat for the non-return valve on which the closure element of the non-return valve is seal-tightly resting in the closed state of the non-return valve. The socket of the filter element, upon insertion of the filter element into the filter housing, can lift the closure element of the non-return valve out of its initial position in which the closure element is resting on a valve holder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages result from the following drawing description. In the drawings, embodiments of the invention are illustrated. The drawings, description, and the claims contain numerous features in combination. A person of skill in the art will consider the features also expediently individually and combine them to expedient further combinations. 
         FIG. 1  shows a longitudinal section through a filter system according to an embodiment of the invention with a non-return valve in a closed position. 
         FIG. 2  shows a longitudinal section of the filter housing top part of the filter system in  FIG. 1  with the non-return valve in an initial position. 
         FIG. 3  shows a longitudinal section of the filter element of the filter system in  FIG. 1 . 
         FIG. 4  shows an enlarged detail of the filter system according to  FIG. 1  with focus on the non-return valve in a closed position. 
         FIG. 5  shows an enlarged detail of the filter system according to  FIG. 1  with focus on the non-return valve in an open position. 
         FIG. 6  shows a plan view of the valve holder. 
         FIG. 7  shows a longitudinal section of the non-return valve in an initial position. 
         FIG. 8  shows a longitudinal section of a filter system according to a further embodiment of the invention with a non-return valve in a closed position. 
         FIG. 9  shows an enlarged detail of the filter system according to  FIG. 8  with focus on the non-return valve in a closed position. 
         FIG. 10  shows an enlarged detail of the filter system according to  FIG. 8  with focus on the non-return valve in an open position. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the Figures, same or same-type components are identified with same reference characters. The Figures show only examples and are not to be understood as limiting. 
       FIG. 1  shows a longitudinal section of a filter system  100  for filtering a fluid, in particular as an oil filter of an internal combustion engine, according to an embodiment of the invention with a non-return valve  30  in a closed position P 1 . The filter system  100  comprises a filter housing  110  with a housing top part  114  and a housing bottom part  112  that extend along a housing axis M. At the housing top part  114 , an outlet socket  108  for discharging the filtered fluid is arranged which can be connected, for example, to an oil circuit of an internal combustion engine. 
     In the interior of the outlet socket  108 , a non-return valve  30  is arranged. The non-return valve  30  comprises a closure element  32  and a spring element  36 . The spring element  30  is supported at the housing top part  114 , in particular, the spring element  30  is clamped between closure element  32  and housing part  114 . The non-return valve  30  closes in a closed position P 1  a fluid path  56  through the outlet socket  108  and in an open position P 2  opens the fluid path  56  through the outlet socket  108 . 
     A filter element  10  that is arranged in the interior of the filter housing  110  separates a raw fluid side  50  from a clean fluid side  52 . The filter element  10  comprises a filter bellows  12  which is arranged about its longitudinal axis L. The filter bellows  12  is flowed through by the fluid in a radial direction from the exterior to the interior. The fluid path  56  is illustrated by arrows. The filter bellows  12  comprises a first end disc  14  at a first end face  15  which comprises an opening  22  for receiving a central tube  18  in the interior  24  of the filter bellows  12 . The socket  28  is formed at the end of the central tube  18  which is facing the end disc  14 . The socket  28  with an opening  26  is arranged in the longitudinal axis L of the filter element  10  and serves for discharging the filtered fluid from the interior  24  of the filter bellows  12 . The socket  28  is embodied as a valve seat  34  and is provided furthermore as an actuating element  33  for lifting the non-return valve  30 . In this context, the non-return valve  30  is closed until a sufficient fluid pressure is built up which can lift the closure element  32  off the valve seat. When inserting the filter element  10  into the filter housing top part  114 , the socket  28  which projects through the valve holder  40 , lifts the closure element  32  of the non-return valve  30  out of its initial position on the valve holder  40 . In this way, the non-return valve  30  is initially in its closed position P 1 . 
     The filter element  10  is embodied to be insertable with its central tube  18  at least partially into the outlet socket  108 . The outlet socket  108  is embodied to be sealable relative to the opening  22  of the end disc  14  and relative to the central tube  18 . In particular, the socket  28  is sealed at its outer side by the circumferential seal  19 , for example, an O-ring seal, against the outlet socket  108 . 
     Housing top part  114  and housing bottom part  112  of the filter housing  110  are connected to each other by a screw thread  118 . Both housing parts  112 ,  114  are sealed by a circumferential seal  120 , for example, an O-ring seal, relative to the environment. At the end which is positioned opposite the central opening  26 , the central tube  18  of the filter element  10  comprises a closure element  20  which closes a central opening of the end disk  16 . An axially outer face of the closure element  20  having an annular groove (see  FIG. 3 ) which is embodied to engage an axially projecting centering part  122  forming a locking arrangement  116  (see  FIG. 1 ) locking the central tube  18  to the housing bottom part  112 . The filter element  10  is seal-tightly closed by the closure element  20  at the lower end. The filter element  10  can be placed with the closure element  20  onto a centering part  122  arranged in the housing bottom part  112  and can be connected, for example, by a locking arrangement  116 , to the centering part  122 . In this way, the filter element  10  can be inserted with the housing bottom part  112  into the housing top part  114  and the filter housing  110  can be seal-tightly closed by screwing the housing bottom part  112  to the housing top part  114 . 
     By inserting the filter element  10  into the housing top part  114 , the closure element  32  of the non-return valve  30  will contact the socket  28  of the central tube  18  and thus the valve seat  34  whereby the fluid path  56  through the outlet socket  108  is closed and can be opened only by building up a corresponding hydraulic pressure of the fluid. 
     By releasing the screw connection (screw threads  118 ), the filter element  10  with the housing bottom part  112  can be pulled out of the filter housing  110  again. In this way, the socket  28  is removed from the closure element  32 . The closure element  32  thus rests again against the valve holder  40 . Fluid can flow through the leakage openings  42  in the valve holder  40 , as illustrated in  FIG. 6 , past the closure element  32  and the fluid circuit can thus be emptied via the outlet socket  108 . 
     The filter element  10  can be released with an appropriate force from the locking action with the centering part  122 . A new filter element  10  can then be inserted into the housing bottom part  112 . 
     In this regard, a longitudinal section through the filter housing top part  114  of the filter system  100  in  FIG. 1  is illustrated in  FIG. 2 , with the non-return valve  30  in an initial position without filter element  10 . The non-return valve  30  is arranged in the interior of the outlet socket  108 . The non-return valve  30  comprises a valve housing  38  with a spring element  36  and a closure element  32 . At the open end of the valve housing  38 , a valve holder  40  is arranged on which the closure element  32  is resting in the initial position in the state when the filter element  10  is removed from the housing top part  114 . The closure element  32  is forced for this purpose by the spring element  36 , which is supported at the valve housing  38 , against the valve holder  40 . The valve housing  38  is connected to the valve holder  40 . Since the valve holder  40  is connected to the outlet socket  108 , the spring element  36  as a whole is supported at the housing top part  114 . 
     The housing top part  114  comprises a screw thread  118  for closing the filter housing  110  with the housing bottom part  112 . 
       FIG. 3  shows a longitudinal section through the filter element  10  of the filter system  100  in  FIG. 1 . The filter element  10  comprises a filter bellows  12  which is arranged about a longitudinal axis L. The filter bellows  12  is seal-tightly closed at its two end faces  15 ,  17  by end discs  14 ,  16 . The end disc  14  comprises an opening  22  for receiving a central tube  18  in the interior  24  of the filter element as a reinforcement of the filter bellows  12  against the hydraulic pressure of the fluid. The central tube  18  is seal-tightly closed at the lower end face  17  with a closure element  20 . With this closure element  20 , the filter element  10  can be connected by a centering part  122  to the housing bottom part  112 . The central tube  18  tapers at its upper end, which is facing the end disc  14 , into a socket  28  with an opening  26  which serves for discharging the filtered fluid out of the interior  24  of the filter bellows  12 . At the outer side of the socket  28 , a circumferential seal  19  is arranged for sealing against the outlet socket  108  upon insertion of the filter element  10  into the housing top part  114 . The socket  28  serves with its tapering end, which is embodied so as to point away from the central tube  18 , as an actuating element  33  and as a valve seat  34  of the non-return valve  30 . 
     In  FIGS. 4 and 5 , in an enlarged detail of a part of the filter system  100  according to  FIG. 1 , the non-return valve  30  is illustrated in a closed position P 1  ( FIG. 4 ) as well as in an open position P 2  ( FIG. 5 ). The non-return valve  30  is arranged in the interior of the outlet socket  108 . The non-return valve  30  comprises the valve housing  38  with a spring element  36  and a closure element  32 . The closure element  32 , which is embodied, for example, as a rubber plate, is reinforced by a reinforcement element  46  that is arranged between closure element  32  and spring element  36 . The reinforcement element  46  can be embodied, for example, as an embossed sheet metal part. The spring element  36  is secured at one end by the valve housing  38  by means of crimped tabs  48  and can thus be supported at the valve housing  38 . The other end of the spring element  36  pushes via the reinforcement element  46  on the closure element  32 . 
     At the open end of the valve housing  38 , a valve holder  40  is arranged on which the closure element  32  is resting in an initial position when the filter element  10  is in the state removed from the housing top part  114 . The valve housing  38  is connected to the valve holder  40 . Since the valve holder  40  is connected to the housing top part  114 , in particular dug into it, the spring element  36  can be supported as a whole on the housing top part  114 . 
     The filter element  10  is inserted into the housing top part  114  with the housing bottom part  112 , which is not illustrated in the illustrated detail view, so that the socket  28  of the central tube  18  of the filter element  10  engages the outlet socket  108  of the housing top part  114 . The socket  28  of the central tube  18  projects for this purpose through the valve holder  40 . The socket  28  is sealed against the outlet socket  108  by means of the circumferential seal  19 . The socket  28  pushes as actuating element  33  with the valve seat  34  against the closure element  32  of the non-return valve  30 . In this context, the non-return valve  30  is closed until a satisfactory fluid pressure lifts the closure element  32  off its valve seat  34  at the socket  28 . 
     In  FIG. 4 , the non-return valve  30  is illustrated in the closed position P 1  in which the closure element  32  is resting seal-tightly on the valve seat  34 . In the installed state, the socket  28  of the filter element  10  projects into the valve holder  40  and past it. In this way, the socket  28  can rest seal-tightly against the closure element  32 . The seal  19  prevents leakage of the fluid when the filter element  10  is installed. 
     In  FIG. 5 , the non-return valve  30  is illustrated in the open position P 2 . In the open position P 2 , the closure element  32  is lifted off the valve seat  34  because the force of the spring element  36 , which is embodied as a soft spring, is overcome by the hydraulic pressure of the filtered fluid. The filtered fluid can thus flow about the closure element  32  into the outlet socket  108  and therefore out of the filter system  100 . 
       FIG. 6  shows a plan view of the valve holder  40 . The valve holder  40  can be embodied, for example, as a cropped bent sheet metal part which can be braced radially in the outlet socket  108 . The valve holder  40  in the illustrated embodiment is of an annular configuration. The valve holder  40  can thus be pushed into the outlet socket  108  of the housing top part  114  and then digs with the cropped rim  41 , which can be seen in  FIG. 7  in the longitudinal section through the non-return valve  30 , into the inner side of the outlet socket  108 . In this way, the valve holder  40  can be fixedly secured in the outlet socket  108 . 
     A plurality of webs  44  project from the outer rim  41  in radial direction inwardly. The closure element  32  is resting on these webs  44  in the initial position when the filter element  10  is not arranged in the housing top part  114 . The closure element  32  is pushed by the spring element  38  against these webs  44 . Leakage openings  42  are formed between the webs  44 , respectively. These leakage openings  42  are thus formed between the closure element  32  and the valve holder  40  so that through them the fluid can drain again from the outlet socket  108  when the filter element  10  is removed and in this way the closure element  32  no longer rests seal-tightly against the valve seat  34  of the filter element  10 . For this purpose, the diameter of the socket  28  is expediently selected such that the leakage openings  42  are located outside of the diameter of the socket  28 . In this way, in the installed state at low fluid pressure, the non-return valve  30  is closed and is opened only at sufficient fluid pressure. When the filter element  10  is removed, the non-return valve  30  is embodied to allow for leakage due to of the leakage openings  42 , and the oil circuit of an internal combustion engine can be emptied from the clean side. 
     In  FIG. 7 , a longitudinal section through the non-return valve  30  in an initial position is illustrated. The non-return valve  30  comprises the valve housing  38  with a spring element  36  and a closure element  32 . The closure element  32  that is embodied, for example, as a rubber plate is reinforced by a reinforcement element  46  that is arranged between closure element  32  and a spring element  36 . The reinforcement element  46  can be embodied, for example, as an embossed sheet metal part. 
     The spring element  36  is held at one end by the valve housing  38  by means of crimped tabs  48  and can thus be supported at the valve housing  38 . The other end of the spring element  36  presses via the reinforcement element  46  on the closure element  32 . 
     At the open end of the valve housing  38 , a valve holder  40  is arranged on which the closure element  32  is resting as initial position in the state when the filter element  10  is removed from the housing top part  114 . The valve housing  38  is connected to the valve holder  40 . Valve housing  38  and valve holder  40  can be embodied to be locked with each other or welded to each other, for example. In particular, valve housing  38  and valve holder  40  can also be embodied together as one piece. 
     Since the valve holder  40  is connected to the housing top part  114 , in particular locked therein, the spring element  36  can be supported as a whole at the housing top part  114 . 
       FIG. 8  shows a longitudinal section through a filter system  100  according to a further embodiment of the invention with a return valve  30  in a closed position P 1 . For avoiding unnecessary repetitions, only the differences to the embodiment illustrated in  FIG. 1  will be discussed. 
     The non-return valve  30  in the embodiment illustrated in  FIG. 8  is arranged without its own valve housing in the interior of the outlet socket  108 . This can be seen clearly in the two enlarged details in  FIGS. 9 and 10 . The non-return valve  30  is thus not embodied as a preassembled unit but is inserted with its individual components into the outlet socket  108 . The spring element  36  is arranged directly in a step  109  of the outlet socket  108  and is supported at ribs, extending radially inwardly in the direction of the spring element, at this step  109  and thus at the housing top part  114 . The fluid flows between the ribs past the spring element. A valve holder  40  is arranged in a similar manner as in the embodiment illustrated in  FIG. 1  in the outlet socket  108 . In this way, the closure element  32  can be resting also in the initial position without filter element  10  in a similar way as above at this valve holder  40 . When the filter element  10  is pushed into the housing top part  114 , the closure element  32  is lifted off the valve holder  40  by means of the socket  28  as actuating element  33 . Thus, the non-return valve  30  is in the closed position P 1 . 
     In the enlarged detail of  FIG. 9  of the filter system  100  according to  FIG. 8 , the non-return valve  30  is illustrated in the closed position P 1 . It can be clearly seen that the spring element  36  is arranged in the step  109  of the outlet socket  108  and is supported at the step  109 . The valve holder  40  is arranged at the beginning of the step  109  of the outlet socket, wherein the webs  44  project into the free opening of the step  109  so that the closure element  32  can rest on them when the filter element  10  is removed. Leakage openings  42  (not visible) are embodied (as can be seen in  FIG. 6 ) between the webs  44  so that fluid from the outlet socket  108  can flow past the closure element  32  and the fluid circuit can be emptied in this way when the filter element  10  is removed. 
     In the enlarged detail of Fig. of the filter system  100 , the non-return valve  30  is illustrated in the open position P 2 . In the open position P 2 , the closure element  32  is lifted off the valve seat  34  because the force of the spring element  36 , which is embodied as a soft spring, is overcome by the hydraulic pressure of the filtered fluid. The filtered fluid can thus flow about the closure element  32  into the outlet socket  108  and thus out of the filter system  100 .