Patent Publication Number: US-11033842-B2

Title: Filter insert for a filter having a filter bypass valve, and a method of using the filter insert

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 14/917,902, filed Mar. 9, 2016, which is a 371 of PCT/EP2014/002400 filed Sep. 6, 2014 and which claims the benefit of the U.S. Provisional Application No. 61/875,834, filed on Sep. 10, 2013, U.S. Provisional Application No. 61/975,121, filed on Apr. 4, 2014, and the priority of the German patent application No. 202014104029.3 filed on Aug. 28, 2014, the entire disclosures of which are incorporated herein by way of reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a filter having a filter housing having an inlet for liquid to be filtered and having an outlet for filtered liquid, having an exchangeable filter insert that separates an unfiltered side and a filtered side of the filter from one another, and having a filter bypass valve that is made up of a valve seat and a valve body that is guided so as to be movable relative to the valve seat and is preloaded in the closing direction. Moreover, the present invention relates to a filter insert that works together with the above-named filter. 
     A first filter of the type named above is known from U.S. Pat. No. 6,579,448 B2. This document discloses a filter having an exchangeable filter insert and having a filter bypass valve. A valve seat of the filter bypass valve is situated on the filter insert, specifically on its upper end plate. A valve body, a spring that preloads the valve body in the closing direction, and a spring support are fashioned so as to be fixed to the housing. Specifically, the spring support is provided on a central pipe socket or supporting element fixed to the housing. 
     Another filter is known from U.S. Pat. No. 6,685,829 B1. This document discloses a filter having an exchangeable filter insert and having a filter bypass valve. A valve seat is provided on the filter insert, here specifically on its lower end plate. A valve body, a spring that preloads the valve body in the closing direction, and a spring support are here realized so as to be fixed to the housing. The valve body is fashioned on a central pipe socket that is fixed to the housing but is capable of displacement in an axially limited manner. The spring is situated as a pressure spring between the pipe socket and the housing. 
     DE 10 2009 021 973 A1 discloses a further filter having an exchangeable filter insert and having a filter bypass valve. The valve seat, the valve body, the spring, and a first spring support of the filter bypass valve are here parts of the filter insert. On a central pipe socket fixed to the filter, a second spring support is provided that, when the filter insert is placed onto the pipe socket during installation of the filter insert in the filter housing, takes over the supporting of the spring and increases its prestressing. 
     From U.S. Pat. No. 8,123,937 B2, additional filters are known having an exchangeable filter insert and having a filter bypass valve. The valve seat and the valve body of the filter bypass valve are here each provided on the filter insert, specifically on its upper end plate. A pressure spring that preloads the valve body in the closing direction, and an additional actuating element that, when the filter insert is in place, transmits the spring force to the valve body, are configured so as to be fixed to the housing, specifically in a central supporting element fixed to the housing. 
     WO2006/112 853 A1 also discloses a filter having an exchangeable filter insert and having a filter bypass valve. Here, the valve seat is provided on the filter insert, specifically on its upper end plate. A valve body, a spring that preloads the valve body in the closing direction, and a spring support are realized so as to be fixed to the housing. Specifically, the valve body and the spring are guided in an upper end region of a central supporting element that is fixed to the housing. The spring support is also fashioned on the supporting element. 
     In filters of the type described above, in which the valve seat on the exchangeable filter insert and the valve body are realized so as to be fixed to the filter, after each change of filter insert there is a valve mating with interacting valve components that is not checked beforehand for tightness and is also not capable of being checked ahead of time for tightness. In practice, this results relatively frequently in filter bypass valves that are not tight in their closing position, through which, undesirably, there is a constant flow of liquid that is unfiltered and that impairs the purity necessary for the actually intended use of the filtered liquid. The solutions known from the existing art as indicated in the three last-named documents above, in which in addition to the valve seat the valve body is also provided on the exchangeable filter insert, require an additional actuating element, and in part even an additional spring, which disadvantageously increases the number of components and thus the production and assembly costs. Also, in this way the filter inserts require significantly more outlay for their construction, and are therefore more expensive, which has noticeable effects every time the filter insert is exchanged. Finally, the known filters have the disadvantage that the opening pressure of the filter bypass valve can be modified only with a relatively high outlay. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is therefore to provide a filter of the type named above that avoids the disadvantages of the existing art and that in particular ensures a good and long-lasting tightness of the filter bypass valve in its closing position, requiring as few components as possible for its functioning and capable of being manufactured at low cost, and with which it is possible to modify the opening pressure of the filter bypass valve with a low technical outlay. Moreover, an object of the present invention includes the provision of a filter insert that works together with the filter, the filter insert also having for its part as simple a design as possible and being capable of being manufactured at low cost, and with which the filter can be operated reliably and with a high degree of effectiveness. 
     The solution of the first part of the object, relating to the filter, is achieved according to the present invention with a filter of the type named above that is characterized in that the valve seat is situated in the filter so as to be fixed to the filter, that the valve body is guided in the filter so as to be fixed to the filter, that a spring that preloads the valve body in the closing direction is guided in the filter so as to be fixed to the filter, and that the filter insert has a spring support that, when the filter insert is in place in the filter housing, supports and prestresses the spring at its end facing away from the valve body. 
     The present invention advantageously achieves the effect that the immediately interacting valve parts can be checked for tightness during the manufacture of the filter, because the valve seat and the valve body of the filter bypass valve are both components fixed to the filter, and these valve parts are therefore already present in the filter when the filter insert is not in place. It is additionally advantageous that the valve seat and the valve body of the filter bypass valve always remain in the filter when its filter insert is changed, thereby excluding unchecked and possibly untight valve matings. 
     A significant further advantage is that it requires only a low outlay to modify the opening pressure of the filter bypass valve, by simply modifying the filter insert with regard to the position or length of the spring support that supports and prestresses the springs. In this way, the springs can be more or less strongly prestressed during installation of the filter insert, which permits a problem-free setting of the desired opening pressure of the filter bypass valve, even during a running series production. 
     It is also advantageous that the filter bypass valve makes do with only one spring, and, besides the valve body, requires no additional movable actuating elements. Apart from the spring support, the filter insert does not have to accommodate or have any additional parts of the filter bypass valve, so that, as a consumable part, the filter insert remains technically simple and low-cost. 
     An advantageous embodiment of the filter provides that the filter housing has in its interior a central pipe socket, that the valve seat is fashioned on or attached to the pipe socket, and that the valve body and the spring are guided on the pipe socket. In this embodiment of the filter, the central pipe socket is used for an integrated housing of the filter bypass valve, which advantageously avoids additional required constructive space. 
     The present invention further proposes that the valve seat is formed by an annular body that is axially fixed on the outer circumference of the pipe socket, and is tightly situated thereon, having at least one through-opening running in its axial direction. In this way, the interior of the pipe socket remains free for the flowing through of filtered liquid. 
     So that on the one hand an adequately large flow cross-section can be provided for the bypassing of the filter as needed, and on the other hand the annular body can be fashioned so as to be adequately and permanently stable, it is proposed that a plurality of through-openings are made in the annular body, situated at a distance from one another in the circumferential direction thereof. 
     Preferably, for reasons related to flow and production, the through-openings are made so as to have a cross-section that is round or is shaped as a segment of a ring. 
     For the reliable and long-lasting ensuring of the desired tightness of the filter bypass valve in its closed position, it is provided that the valve seat formed by the annular body is made of an elastomeric material, or is provided with an elastomeric coating at least on its end face forming the valve seat, or that the valve body is made of an elastomeric material, or is provided with an elastomeric coating at least on its side facing the valve seat. 
     Corresponding to the above-described realization of the valve seat, the valve body is usefully formed by an annular valve body that is guided on the outer circumference of the pipe socket and is axially displaceable. In this way, the valve body is guided so as to be displaceable in the axial direction in a technically simple yet reliable fashion, and in this way can move between its closing position, situated on the valve seat, and an open position at a distance from the valve seat. 
     In order to avoid losses of tightness of the filter bypass valve in its closing position that may possibly occur over the time of use of the filter due to a relative rotation of the valve body and the valve seat, a further embodiment of the present invention provides that a guide that guides the valve body on the outer circumference of the pipe socket in axially displaceable fashion forms a lock against a rotation of the valve body in the circumferential direction relative to the pipe socket. Such a lock can for example be realized in a technically simple manner through one or more grooves running in the axial direction on one of the two components, and one or more ribs running therein on the other of the two components. 
     In order to ensure a tilt-free and clamping-free guiding of the valve body on the pipe socket, a secure sealing of the filter bypass valve in its closing position, and a reliable and precise seating of the valve on the valve body, it is preferably provided that the valve body is made L-shaped in cross-section, having a longer limb of the L running in the axial direction and a shorter limb of the L running in the radial direction pointing outward or inward, an end face, facing the valve seat, of the shorter L limb forming a surface of the valve body that works together with the valve seat, an end face, facing away from the valve seat, of the shorter L limb forming a seating surface and the outer circumference or inner circumference of the longer L limb forming a centering surface for the spring, and the inner circumference of the valve body forming a guide surface for guiding the valve body on the outer circumference of the pipe socket. 
     The spring support for the spring that preloads the valve body of the filter bypass valve in the closing direction is, according to the present invention, provided on the filter insert, and, in a concrete development, the spring support is preferably formed by a, or on a, supporting element that forms a part of the filter insert, or by an, or on an, end plate of the filter insert. Thus, for the spring support, elements that are already present on or in the filter insert are used, which avoids an additional component outlay. 
     In order to realize the interaction of the spring and spring support in as secure and reliable a fashion as possible, the present invention proposes that between the free end of the spring support and the end of the spring facing this free end there is situated an intermediate ring that is guided in axially displaceable fashion on the outer circumference of the pipe socket, such that when the filter insert is in place the spring abuts the side of the intermediate ring facing away from the spring support and the spring support abuts the side of the intermediate ring facing away from the spring. Advantageously, the intermediate ring can be realized and shaped differently at its two axial sides, and in particular so as to be adapted optimally to the spring on the one side and to the spring support on the other side. 
     So that the spring, in its provided position, is held securely even when no filter insert is situated in the filter, a stop is usefully situated on the side of the spring facing away from the valve body, on the pipe socket, such that when the filter insert is in place in the filter housing the end of the spring remote from the valve body has an axial distance from the stop, and when the filter insert is removed from the filter housing the end of the spring remote from the valve body abuts the stop immediately or indirectly. 
     A first development relating to this provides that the stop is formed by a stop collar situated fixedly on the pipe socket in the axial and circumferential direction, having a collar part that protrudes radially outward having one or more open intermediate spaces distributed around its circumference. 
     An alternative embodiment provides that the stop is formed by stop tongues situated fixedly on the pipe socket, or made in one piece therewith, in the axial and circumferential direction, each having a radially outward-protruding stop tab, open intermediate spaces existing between each two stop tabs over the circumference of the pipe socket. 
     In order to prevent installation of unsuitable foreign filter inserts in the filter according to the present invention, it is proposed that the spring support has one or more support arms that are shaped and configured corresponding to the shape and configuration of the open intermediate spaces, and that, when the filter insert is placed into the filter housing, run through the open intermediate spaces and form a key-lock coding with the stop. This achieves the effect that only those filter inserts can be placed into the filter that have the fitting coding, in the form of a particular shape and configuration of the arms. Foreign filter inserts that do not have the required coding either cannot be put in place, because they collide with the collar part, or cannot support the springs, because they do not have the spring support. 
     In addition, it is proposed that the stop has, on its side facing away from the spring, between each two adjacent intermediate spaces, a respective individual introductory bevel, or two roof-shaped introductory bevels, by which, during installation of the filter insert, the support arms situated thereon of the spring support can automatically be guided into the intermediate spaces. In this way, it is ensured that a mutual finding of support arms and intermediate spaces takes place even without particular attention or operating steps on the part of the installation or maintenance personnel. 
     In a particularly advantageous embodiment, the filter according to the present invention is realized as a screw-on exchangeable filter, an end face of the filter being fashioned as a connecting flange for connecting the filter to a coupling flange of a device having a circuit for liquid to be filtered, such as an internal combustion engine of a motor vehicle, and an end region of the pipe socket having, in the region of the connecting flange, a screw threading that can be screwed together with a counter-threading in or on the coupling flange. In this embodiment, the filter according to the present invention can be used as a replacement for a conventional screw-on filter cartridge that is only exchangeable as a whole, without having to make any modifications or adaptations at the side of the coupling flange. As determined by the coupling flange, the connecting flange of the filter is shaped so as to fit, for example with a central pipe socket protruding axially from the filter housing. For a coupling flange, as is also known in practice, having an outwardly protruding threaded connector, the filter according to the present invention can of course also be realized having a central pipe socket that is flush with the plane of the connecting flange or is recessed relative thereto. 
     For example, the screw threading on the end region of the pipe socket is an external threading, and the counter-threading is an inner threading in the coupling flange. 
     Alternatively, the screw threading on the end region of the pipe socket can be an inner threading, and the counter-threading can be an external threading on a threaded nipple situated on the coupling flange. 
     At the filter, the pipe socket can be connected fixedly and non-detachably to the filter housing, and the filter housing can, in particular for the purpose of filter maintenance, be capable of being unscrewed from the coupling flange together with the pipe socket as a unit. 
     Alternatively, the pipe socket can be screwed to the filter housing in detachable fashion, and, after a first screwing of the filter onto the coupling flange, the pipe socket can remain permanently connected to the coupling flange, so that subsequently the filter housing can be screwed off from the coupling flange without the pipe socket. 
     In an embodiment of the filter, the pipe socket is made in one part or in one piece. 
     Alternatively, the pipe socket can be formed from two pipe socket segments that can be detachably connected to one another, in particular screwed together. 
     For the two-part realization of the pipe socket, it is preferably further provided that a first, flange-side pipe socket segment has or bears the valve seat, the valve body, and the spring, and, after a first screwing of the filter onto the coupling flange, remains permanently connected thereto, and that a second pipe socket segment remote from the flange is connected fixedly and non-detachably to the filter housing. 
     In order to prevent the filter from running empty when there is a standstill of the liquid circuit, it is proposed that the first pipe socket segment at the flange side has or bears a return check valve covering the inlet of the filter. 
     Here, in a further embodiment, the return check valve can have a flexible valve membrane and a rigid membrane bearer, each surrounding the flange-side pipe socket segment, and the membrane bearer can be fashioned in one piece with the first pipe socket segment, or can be connected thereto in positionally fixed fashion, preferably locked. Alternatively, the membrane bearer can be a flange-type expansion of the valve seat, i.e., a part of the filter bypass valve. In this way, in each of the named embodiments a space-saving housing and technically simple integration of the return check valve is achieved. 
     In addition, when the filter insert is in place in the filter the valve membrane is usefully clamped at its radially inner edge region against the membrane bearer by the filter insert, in particular by a sealing ring situated radially inwardly thereon. In this way, particular holding means for the valve membrane are advantageously not required. 
     Many filters of the type considered here have a screw cover as a part of a filter housing, or have a screw-on housing. For safe operation that does not damage the environment, it is essential that the screw cover or the screw-on housing not become detached automatically during operation of the filter. For filters according to the present invention in which the filter housing is fashioned as a screw-on housing or has a screw cover, it is therefore provided that the filter is provided with an anti-rotational lock, an anti-rotational lock ring being guided in axially displaceable fashion in the filter housing on the pipe socket, such that when the filter insert is in place in the filter housing, the spring presses the anti-rotation lock ring, with its end face facing away from the spring, against a counter-surface of the filter housing or of the screw-on housing or screw cover. In this way, an additional very advantageous use of the spring is achieved, because the spring not only preloads the valve body of the filter bypass valve in its closing direction, but now additionally also loads an anti-rotational lock ring with a force required for its securing effect. 
     In order not to have to install a separate component in the filter for the anti-rotation lock ring, the anti-rotation lock ring is preferably connected to a supporting element forming a part of the filter insert, or is made in one piece therewith. 
     A development of the filter provides that the end face, facing away from the spring, of the anti-rotation lock ring and the counter-surface of the filter housing each have a contour that varies in its axial height, regarded in the circumferential direction, and that interlock. This has the effect that when the filter housing or screw cover or screwed-on housing is unscrewed, a certain specifiable resistance has to be overcome in order to rotate the interlocking contours relative to one another. By choosing a sufficiently high resistance that still however permits an intentional unscrewing, an undesired automatic unscrewing is prevented. 
     In addition, here it is preferably provided that an upward incline running in the unscrewing direction is steeper than a downward incline running in the screw-on direction of the interacting contours. In this way, advantageously the torque that has to be applied for loosening is greater than the torque required for tightening. Here, the degree of this difference can be influenced and determined by the difference between the mentioned upward incline and the mentioned downward incline. The interacting contours can for example be realized so as to be wave-shaped, or as a sequence of bevels, also with flat segments situated between them. 
     Because the spring support that prestresses the spring of the filter bypass valve is a part of the filter insert, it has to be ensured that during operation of the filter the filter insert assumes a defined position in the filter housing, seen in the axial direction, because otherwise an incorrect prestressing of the spring, and thus a wrong opening pressure of the filter bypass valve, would occur. Therefore, in this regard the present invention proposes that the filter housing has, in its circumferential wall, an inner diameter step against which the filter insert can abut with its flange-side end plate during its introduction into the filter housing, for axial positioning. 
     Filters of the type considered here as a rule require maintenance, because the useful life of the filter insert is limited, and the insert has to be replaced from time to time. In order to ensure that, during each filter maintenance including a change of filter insert, the seal that seals the filter against the coupling flange is also replaced, it is preferably provided that a sealing ring bearer is radially outwardly connected or made in one piece with a flange-side end plate of the filter insert, on which there is attached, or there can be attached, a sealing ring that seals the filter against the coupling flange of the device having a circuit having liquid to be filtered. 
     So that, when the filter is attached on the coupling flange, the sealing ring can move into its sealing position without being forced, it is provided that the sealing ring bearer has a cylindrical outer circumferential surface on which the sealing ring is attached or can be attached in an axially floating fashion. 
     A first configuration which is advantageous functionally and from the point of view of manufacturing results if the filter bypass valve is situated at the height of a first axial end of the filter insert. 
     A second configuration which is advantageous functionally and from the point of view of manufacturing results if the filter bypass valve is situated at the height of a second axial end of the filter insert. 
     Particularly preferably, the filter bypass valve is situated in the region of the end of the filter insert that is the upper end during operation of the filter, because there the least possible amount of disturbing deposition of dirt particles on the parts of the filter bypass valve is to be expected. 
     In a further preferred embodiment of the filter according to the present invention, it is provided that it is realized as a screw-on exchangeable filter, the filter being fashioned for connection with a coupling flange of a device having a circuit having liquid to be filtered, the filter having a flange-side base plate having a screw threading that can be screwed together with a counter-threading in or on the coupling flange, and the filter bypass valve being situated in a valve holder that forms a part of the base plate or is connected to the base plate. In this embodiment as well, a compact construction and advantageous manufacturability are achieved. 
     In a further embodiment, here it is provided that the filter housing is realized as a screw housing having a screw threading on its flange-side end region, by which the filter housing can be screwed onto a screw threading situated radially outwardly on the base plate. 
     It is further proposed that after a first assembly of the filter on the coupling flange the filter housing can be unscrewed from the base plate together with the filter insert for filter maintenance, and that the base plate with the valve holder and the filter bypass valve on the coupling flange are remaining parts of the filter. During filter maintenance, only the filter insert is replaced, while the filter bypass valve is fixed to the filter, i.e., is a part of the filter that is permanent over the lifespan of the filter. 
     A development provides that the valve holder is made closed at its circumference, and has on its end face remote from the flange at least one stop for the spring in its state relieved of stress by the spring support of the filter insert, and at least one passage for filtered liquid. In this way, the spring is secured against loss, and at the same time permits a flow of filtered liquid through the valve holder. 
     In order to prevent a disturbing or damaging entrance of dirt particles into the outlet for filtered liquid during filter maintenance with exchange of the filter insert, it is proposed that there be situated between the spring and the passage a covering element loaded by the spring in the direction toward the passage, such that in the assembled state of the filter the spring support situated on the filter insert supports the spring via the covering element, and the covering element closing the passage when the filter insert is removed. 
     In order to prevent an emptying of the filter when the liquid circuit is at a standstill in the filter embodiments having a base plate as well, at least one eccentric through-opening forming the inlet of the filter is usefully made in the base plate, and on the side of the base plate remote from the flange there is situated a return check valve that covers the inlet, the return check valve having a circular valve membrane that covers the inlet and that is clamped, at its radially inner edge region, between the base plate and the valve holder connected to the base plate. In addition, here, advantageously, no particular fastening and holding means for the valve membrane are required. 
     So that the filter can be manufactured at low cost in large piece counts and has as low a weight as possible, preferably both the base plate and the filter housing of the filter are parts made of plastic. Usefully, these parts are manufactured as specifically shaped injection-molded parts requiring no further cutting processing. Alternatively, the base plate and/or the filter housing can also be parts made of metal, e.g., shaped plate parts made of steel plate, or die-cast parts, preferably made of light metal. 
     In order to make it possible to prestress the spring of the filter bypass valve reliably and in a technically simple manner, it is preferably provided that the spring support is formed by a plurality of support arms connected to a central supporting element of the filter insert or made in one piece therewith, extending into the valve holder when the filter insert is in place in the filter. Alternatively, the spring support can also be made in annular fashion in order to provide as large a contact surface as possible to the spring of the filter bypass valve. 
     For the purpose of the greatest possible prevention of the use of unsuitable or lower-quality foreign filter inserts in the filter according to the present invention, the present invention proposes that the valve holder has, for each support arm, an introduction opening that is matched in its position and shape, and that the support arms and the introduction openings form with one another a key-lock coding that allows installation only of the filter insert having support arms positioned and shaped with the correct fit to the introduction openings. The support arms and the associated introduction openings can have the same design as one another. Alternatively, the support arms can be made differently from one another and the associated introduction openings can correspondingly also be made different from one another in order to achieve a key-lock coding that presents a still greater obstacle to the use of foreign filter inserts. 
     In order to protect the support arms of the spring support from forces that occur during the positioning of the support arms relative to the introduction openings and that could cause damage, the present invention proposes that in addition to the support arms forming the spring support the filter insert has first positioning elements, that the valve holder has second positioning elements in addition to the introduction openings, and that, when placed into the filter, the filter insert can be guided by the first and second positioning elements into a position in the circumferential direction that is ready for engagement of its support arms relative to the introduction openings of the valve holder. 
     According to a development in this regard, the positioning elements are preferably formed by at least one introductory bevel, having a subsequent axial introduction groove on the one hand, and at least one introduction projection or axial introduction rib on the other hand. 
     In order to make it possible to manufacture the valve holder advantageously with regard to technology and cost, and to secure the parts housed therein of the filter bypass valve in the valve holder, it is provided that the valve holder is fashioned in two parts, having a first holder part connected to the base plate and having the valve seat, and having a second holder part connected to the first holder part and situated remote from the valve seat. The first holder part is preferably connected fixedly to the base plate, e.g., by welding. The holder parts are preferably locked to one another, because such a locking connection requires little outlay during manufacture, and can easily be produced during assembly. 
     For the above-described embodiment of the filter, it is also preferably provided that the counter-threading in or on the coupling flange is situated as an outer threading on a threaded nipple that protrudes from the plane of the coupling flange and forms a part of the coupling flange. 
     In a different embodiment of the filter, it is provided that it is realized as a screw-on exchangeable filter, the filter being fashioned for connection to a coupling flange of a device having a circuit having liquid to be filtered, the filter housing having at the flange side a screw threading that can be screwed together with a counter-threading in or on the coupling flange, the filter bypass valve being situated in a valve holder having a hollow cylindrical end piece at the flange side, and the end piece being capable of being pressed into a filtered liquid channel of the coupling flange before or during a first assembly of the filter on the coupling flange. In this way, a very simple and rapid connection of the valve holder containing the filter bypass valve to the coupling flange is achieved, the valve holder remaining permanently on the coupling flange after this connection is produced. Only the filter housing can be screwed off from the coupling flange, together with the filter insert situated therein which here as well has the spring support for prestressing the spring of the filter bypass valve. 
     So that when the filter is open, i.e., the filter housing is disassembled and the filter insert is removed, the spring of the filter bypass valve cannot be lost, but at the same time filtered liquid can flow through the valve holder, it is provided that the valve holder is made closed at the circumference and has on its end face remote from the flange at least one stop for the spring, in its state in which it is not loaded by the spring support of the filter insert, and has at least one passage for filtered liquid. 
     In order, in this embodiment of the filter as well, to prevent dirt particles from falling into the outlet for filtered liquid when the filter insert is exchanged, a covering element, loaded by the spring in the direction toward the passage, is usefully situated between the spring and the passage, such that in the assembled state of the filter the spring support situated on the filter insert supports the spring via the covering element, and such that the covering element closes the passage when the filter insert is removed. When the filter insert is in place in the filter, its spring support holds the covering element in a position removed from the passage, so that a flow of liquid through the passage to the outlet of the filter is then enabled. 
     In order to achieve the second part of the object, a filter insert that works together with the filter of the type described above is proposed that is characterized in that it has a spring support, and that when the filter insert is in place in the filter housing the spring of the filter bypass valve can be supported and can be prestressed, at its end facing away from the valve body, by the spring support of the filter insert. 
     With such a filter insert, the filter according to the present invention can be operated in a manner corresponding to its purpose and with the advantages explained above, such that, by means of the spring support on the filter insert, the spring of the filter bypass valve in the filter can be prestressed, and thus, by means of corresponding realization of the filter insert, the desired opening pressure of the filter bypass valve can be defined without having to modify or exchange other parts of the filter for this purpose. 
     On the filter insert, the spring support is preferably fashioned by a, or on a, supporting element that forms a part of the filter insert. As a rule, the filter insert already has a supporting element that supports the filter material body during operation of the filter against collapsing as a result of pressure differences between its two sides. Thus, here the supporting element is beneficially additionally used for the integration of the spring support. 
     Alternatively, the spring support can be formed by an end plate of the filter insert, or can be provided on an end plate of the filter insert. 
     The filter insert can advantageously be manufactured in various embodiments, in particular having different axial length or position of its spring support, whereby a prestressing of the spring, and thus the opening pressure of the filter bypass valve of the filter equipped with the filter insert, can be set to a desired value. It is particularly advantageous that, as needed, the opening pressure of the filter bypass valve can also still be modified later by exchanging the filter insert for one having a different spring support length or spring support position, without having to make any modifications to the filter itself. 
     In order to prevent an installation of foreign filter inserts in the filter according to the present invention, for the filter insert it is further proposed according to the present invention that the spring support has a plurality of support arms that are shaped and configured corresponding to the shape and configuration of the open intermediate spaces in the stop or of the introduction openings in the valve holder of the associated filter, such that when the filter insert is placed into the filter housing the support arms can be guided through the open intermediate spaces or introduction openings, and the support arms forming a key-lock coding with the open intermediate spaces or with the introduction openings. In this way, only one particular matched filter insert can be put in place in a particular filter, which prevents the use of non-fitting, and often qualitatively lower-value, foreign filter inserts. 
     Finally, for the filter insert according to the present invention it is proposed that it has, in addition to the support arms forming the spring support, first positioning elements, and that by means of the first positioning elements and by means of second positioning elements cooperating therewith and provided on the valve holder in addition to the introduction openings, when the filter insert is placed into the filter it can be guided into a position in the circumferential direction of its support arms ready for engagement relative to the introduction openings of the valve holder. In this way, the support arms can be made relatively delicate, because they have to exert only axial forces on the spring or on the covering element. In contrast, the support arms are completely relieved of the stress of forces acting in the circumferential direction, such as those that occur during the positioning of the filter insert relative to the filter bypass valve or to its valve holder, because these forces are accepted only by the positioning elements. The support arms on the one hand and the positioning elements on the other hand can therefore each be optimally contoured and dimensioned for their respective purpose, which promotes durability and operational reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, exemplary embodiments of the present invention are explained on the basis of a drawing. 
         FIG. 1  shows a first filter having an exchangeable filter insert and having a filter bypass valve, in an angled longitudinal section, 
         FIG. 2  shows the filter of  FIG. 1  in a top view, with the sectional line I-I for  FIG. 1 . 
         FIG. 3  shows the filter of  FIG. 1  in cross-section according to the sectional line III-III in  FIG. 1 , 
         FIG. 4  shows an enlarged detail of the filter of  FIG. 1  with the filter bypass valve in the closed position, 
         FIG. 5  shows the same detail as  FIG. 4 , here with the filter bypass valve in the open position. 
         FIG. 6  shows the filter of  FIG. 1  without filter insert, in longitudinal section, 
         FIG. 7  shows the filter in a second embodiment, in the delivered state, in longitudinal section, 
         FIG. 8  shows the filter of  FIG. 7  in the installed state, with closed filter bypass valve, in longitudinal section, 
         FIG. 9  shows the filter of  FIG. 8  without filter insert, in longitudinal section, 
         FIG. 10  shows the filter of  FIGS. 8 and 9  with removed filter housing and removed filter insert, in longitudinal section, 
         FIG. 11  shows the filter insert of the filter according to  FIGS. 7 and 8  as an individual part, in longitudinal section, 
         FIG. 12  shows the filter housing of the filter of  FIGS. 7 through 9 , in longitudinal section, 
         FIG. 13  shows the filter of  FIG. 8 , here with open filter bypass valve, in longitudinal section, 
         FIG. 14  shows the filter of  FIG. 7  in cross-section according to the sectional line XIV-XIV in  FIG. 7 , 
         FIG. 15  shows a valve seat of the filter according to  FIGS. 7 through 14 , in a first embodiment, in a top view, 
         FIG. 16  shows the valve seat of the filter according to  FIGS. 7 through 14 , in a second embodiment, in a top view, 
         FIG. 17  shows die filter in a third embodiment, in the installed state with closed filter bypass valve and with an anti-rotation lock, in longitudinal section, 
         FIG. 18  shows the filter of  FIG. 17  in cross-section according to the sectional line B-B in  FIG. 17 , 
         FIG. 19  shows the filter of  FIG. 17  in cross-section according to the sectional line C-C in  FIG. 17 , 
         FIG. 20  shows the filter of  FIGS. 17 through 19  in longitudinal section according to the sectional line D-D in  FIG. 19 , 
         FIG. 21  shows the filter of  FIGS. 17 through 20  in longitudinal section according to the sectional line E-E in  FIG. 20 , 
         FIG. 22  shows the filter of  FIGS. 17 through 21  in longitudinal section according to the sectional line F-F in  FIG. 20 , 
         FIG. 23  shows a central pipe socket as a part of the filter according to  FIG. 17 through 22 , as an individual part, in an oblique view from above, 
         FIG. 24  shows a supporting element as a part of the filter according to  FIG. 17 through 22 , as an individual part, in an oblique view from above, 
         FIG. 25  shows a filter housing as a part of the filter according to  FIG. 17 through 22 , as an individual part, in an oblique view from below, 
         FIG. 26  shows a constructive unit of the filter housing according to  FIG. 25  and of the pipe socket according to  FIG. 23 , in an oblique view from below, 
         FIG. 27  shows an enlarged detail from  FIG. 21  with the anti-rotation lock in a first embodiment, 
         FIG. 28  shows the detail of  FIG. 21  with the anti-rotation lock in a second embodiment, 
         FIG. 29  shows the filter in a fourth embodiment, in the installed state with closed filter bypass valve and with a first filter insert shown in two different embodiments in the left and right halves of the Figure, in longitudinal section, 
         FIG. 30  shows the filter in an embodiment modified relative to  FIG. 29 , in the installed state with closed filter bypass valve and with a second filter insert shown in two different embodiments in the left and right halves of the Figure, in longitudinal section, 
         FIG. 31  shows the filter in a further embodiment modified relative to  FIG. 29 , in the installed state with closed filter bypass valve, and with a third filter insert shown in two different embodiments in the left and right halves of the Figure, in longitudinal section, 
         FIG. 32  shows a valve body and a modified embodiment, in longitudinal section. 
         FIG. 33  shows a valve seat in a modified embodiment, in longitudinal section, 
         FIG. 34  shows a filter in a further embodiment, in longitudinal section, 
         FIG. 35  shows the filter of  FIG. 17  in a modified embodiment, having an anti-rotation lock for the valve body of the filter bypass valve, in longitudinal section, 
         FIG. 36  shows the filter of  FIG. 35  in cross-section according to the sectional line H-H in  FIG. 35 , 
         FIG. 37  shows the detail circled in  FIG. 36  in an enlarged representation, 
         FIG. 38  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 39  shows the filter of  FIG. 38  in cross-section according to the sectional line G-G in  FIG. 38 , 
         FIG. 40  shows the filter of  FIG. 38  in cross-section according to the sectional line H-H in  FIG. 38 , 
         FIG. 41  shows the filter housing and the filter insert of the filter situated therein of  FIG. 38 , in longitudinal section, 
         FIG. 42  shows the filter housing having the filter insert of  FIG. 41 , in an oblique view from below, 
         FIG. 43  shows the filter insert of the filter of  FIG. 38  in an oblique view from below, 
         FIG. 44  shows the filter insert of  FIG. 43  in longitudinal section, 
         FIG. 45  shows the filter insert of  FIG. 44  in cross-section according to the sectional line F′-F′ in  FIG. 44 , 
         FIG. 46  shows the filter housing of the filter of  FIG. 38  in an oblique view from below, 
         FIG. 47  shows a coupling flange of the filter with filter parts remaining thereon after removal of filter housing and filter insert, 
         FIG. 48  shows the subject matter of  FIG. 47  in longitudinal section, 
         FIG. 49  shows detail Y of  FIG. 39  in an enlarged representation, 
         FIG. 50  shows detail X of  FIG. 40  in an enlarged representation, 
         FIG. 51  shows a constructive unit including a filter bypass valve and a return check valve as parts of the filter of  FIG. 38 , in an oblique view from below, 
         FIG. 52  shows the constructive unit of  FIG. 51  together with a coupling flange and a filter housing with filter insert, in an exploded view, obliquely from below, 
         FIG. 53  shows detail Z of  FIG. 48  in an enlarged representation, 
         FIG. 54  shows detail W of  FIG. 38  in an enlarged representation, 
         FIG. 55  shows the filter insert of the filter in a modified embodiment for a filter without return check valve, in an oblique view from below, 
         FIG. 56  shows the filter insert of  FIG. 55  in longitudinal section, 
         FIG. 57  shows the filter in a further embodiment in longitudinal section, 
         FIG. 58  shows a constructive unit with filter bypass valve as a part of the filter of  FIG. 57 , in a front view, 
         FIG. 59  shows the constructive unit of  FIG. 58  in a state assembled to a coupling flange, in an oblique view from above, 
         FIG. 60  shows the filter housing with filter insert of the filter situated therein of  FIG. 57 , in an oblique view from below, 
         FIG. 61  shows the filter housing with filter insert of  FIG. 60 , in longitudinal section, 
         FIG. 62  shows the constructive unit of  FIG. 58  together with a coupling flange and a filter housing with filter insert situated therein, in an exploded view, 
         FIG. 63  shows the subject matter of  FIG. 59  in longitudinal section, 
         FIG. 64  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 65  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 66  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 67  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 68  shows the filter of  FIG. 67  in cross-section according to the sectional line B-B in  FIG. 67 , 
         FIG. 69  shows detail V of  FIG. 68  in an enlarged representation, 
         FIG. 70  shows the filter of  FIG. 67  in a cross-section according to the sectional line D-D in  FIG. 67 , 
         FIG. 71  shows detail U of  FIG. 70  in an enlarged representation, 
         FIG. 72  shows the filter of  FIG. 67  in longitudinal section according to the sectional line C-C in  FIG. 68 , 
         FIG. 73  shows detail X in  FIG. 72  in an enlarged representation, 
         FIG. 74  shows the filter of  FIG. 72  in cross-section according to the sectional line I-I in  FIG. 72 , 
         FIG. 75  shows detail W in  FIG. 74  in an enlarged representation, 
         FIG. 76  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 77  shows detail Y of  FIG. 76 , in an enlarged representation, 
         FIG. 78  shows the filter according to  FIG. 76  without filter housing, with filter insert shown only partly, in a first phase of the assembly on the coupling flange, in a front view, partially in longitudinal section, 
         FIG. 79  shows the filter of  FIG. 78  in the same representation, in a second phase of the assembly on the coupling flange, 
         FIG. 80  shows the filter of  FIG. 78  in the same representation, in a third phase of the assembly on the coupling flange, 
         FIG. 81  shows the filter of  FIG. 78  in the same representation, in a fourth phase of the assembly on the coupling flange, 
         FIG. 82  shows the coupling flange together with a constructive unit, including a filter bypass valve and a return check valve, of the filter of  FIG. 76 , after the removal of filter housing and filter insert, in a front view, 
         FIG. 83  shows the constructive unit of  FIG. 82 , without spring and without covering element of the return check valve, in a longitudinally sectioned view, 
         FIG. 84  shows the complete constructive unit of  FIG. 82  in longitudinal section, 
         FIG. 85  shows the filter housing and the filter insert situated therein, in a longitudinally sectioned view, 
         FIG. 86  shows a filter bypass valve in a modified embodiment, together with the coupling flange, in longitudinal section, 
         FIG. 87  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 88  shows a constructive unit, including a filter bypass valve and a return check valve, of the filter of  FIG. 87 , in the state connected to a coupling flange, in an oblique view from above, 
         FIG. 89  shows the filter in a further embodiment, in longitudinal section, 
         FIG. 90  shows the filter of  FIG. 89  in a state with filter housing and filter insert removed from the coupling flange, in longitudinal section, 
         FIG. 91  shows the filter in a further embodiment, in a state in place on a coupling flange, in longitudinal section, and 
         FIG. 92  shows the filter of  FIG. 91  in a state disassembled from the coupling flange, in longitudinal section. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description of the Figures, identical parts in the various Figures of the drawing have always been provided with the same reference characters, so that not every reference character has to be explained again in every Figure. 
       FIG. 1  of the drawing shows an exemplary embodiment of a filter  1 , realized as a screw-on exchangeable filter, and shown here in longitudinal section. Filter  1  has an essentially cup-shaped filter housing  2  having a circumferential wall  20 , having an open end face  21 , here pointing downward, and a closed end face  22 , here pointing upward. In the center of upper end face  22  a tool mount projection  23 , such as a hexagon, is integrally formed, on which a screw tool can be placed. 
     Through the interior of filter  1  there runs a central pipe socket  4  whose outer, here lower, end region  40  protrudes from filter housing  2  in the axial direction, and is provided there with an outer screw threading  40 ′. On its inner, here upper, end region  41 , pipe socket  4  abuts the underside of upper end face  22  of filter housing  2 . Here, in the depicted exemplary embodiment pipe socket  4  is fixedly connected to filter housing  2 , for example by welding along a weld seam  25 ′. For this purpose, filter housing  2  and pipe socket  4  are for example made of steel plate. In its region situated inside filter housing  2 , pipe socket  4  has a plurality of perforations  42  distributed in the circumferential and longitudinal direction. 
     In addition, in filter housing  2  of filter  1  there is situated an exchangeable filter insert  3  that is made up of a hollow cylindrical filter material body  30  and two end plates  31  and  32  enclosing said body at the ends. Each end plate  31 ,  32  has a central perforation  31 ′.  32 ′, so that filter insert  3  can be placed onto pipe socket  4  from below, and in this way can be introduced into the interior of filter housing  2 . Moreover, on the inner circumference of filter material body  30  there is situated an inner grid-type support body  37  for the radial supporting of filter material body  30  when liquid to be filtered flows through it in the radial direction from the outside to the inside. 
     In addition, filter  1  has an integrated filter bypass valve  5 , which here is situated at the height of upper end plate  32  of filter insert  3 . Filter bypass valve  5  has a valve seat  50  that is fashioned as an annular element, and is attached tightly on the outer circumference of inner and region  41  of pipe socket  4 . A plurality of axial through-openings  50 ′, of which only one is visible in  FIG. 1 , run through valve seat  50  fashioned as annular element. 
     A likewise annular valve body  51  works together with valve seat  50 , said valve body being guided in axially displaceable fashion underneath valve seat  50 , on the outer circumference of pipe socket  4 . Valve body  51  is preloaded in the direction toward valve seat  50 , and thus in the closing direction of filter bypass valve  5 , by a spring  52  realized as a helical pressure spring. At its end oriented away from valve body  51 , spring  52  is supported by filter insert  3 . For this purpose, filter insert  3  has, in the depicted exemplary embodiment, a spring support  53  that here has the form of four protruding support arms  53 ′ that run upward in the axial direction and that are fashioned in one piece with inner supporting element  37 . This spring support  53  supports spring  52  at its lower end via an intermediate ring  54  that is guided in axially displaceable fashion on the outer circumference of pipe socket  4 . 
     The force with which spring  52  acts on valve body  51  in the closing direction is thus determined, inter alia, by spring support  53 , here specifically by its axial length or axial position. This offers the advantageous possibility of easily modifying the opening pressure of filter bypass valve  5 , by installing into filter  1  a filter insert  3  having a correspondingly modified spring support  53 . A larger axial length of spring support  53  results in a higher prestressing of spring  52 , and thus to a higher opening pressure of filter bypass valve  5 . Conversely, a shorter axial length of spring support  53  results in a reduced prestressing of spring  52 , and thus to a lower opening pressure of filter bypass valve  5 . 
     Finally, on the outer circumference of pipe socket  4 , underneath spring  52 , a stop  43  is also situated in axially non-displaceable fashion, and so as to be incapable of rotation in the circumferential direction, the stop here being formed by a sheet metal part and for example pressed onto pipe socket  4 . Distributed around its circumference, stop  43  has a plurality of openings or open intermediate spaces (not visible in  FIG. 1 )  44 , through each of which there runs a respective support arm  53 ′ of spring support  53 . Here, intermediate spaces  44  and support arms  53 ′ of spring support  53  form with one another a key-lock coding that permits only the placement of a fitting filter insert  3 , matched to the coding, into filter  1 . 
     Filter insert  3  separates an unfiltered side  13  of filter  1 , situated outside filter insert  3 , from a filtered side  14  of filter  1 , situated in the interior of filter insert  3  and of pipe socket  4 . For the sealing of filter insert  3  against pipe socket  4  onto which filter insert  3  is placed, filter insert  3  has on its lower end plate  31  a sealing lip  35  that radially outwardly encloses perforation  31 ′, and likewise has on its upper end plate  32  a sealing lip  36  that radially outward encloses perforation  32 ′. 
     Lower end face  21  of filter  1  is fashioned as connecting flange  10 , which can be brought into a detachable connection with a coupling flange on a device having a circuit having liquid such as lubricant oil that is to be filtered, such as an internal combustion engine, using threading  40 ′ of pipe socket  4 . Radially external to outer end region  40  of pipe socket  4  there is situated an inlet  11  through which the liquid to be filtered can flow from below to above into unfiltered side  13  of filter  1 . The interior of pipe socket  4  forms an outlet  12  for filtered liquid. 
     In order to seal the flange connection between filter  1  and the associated device not shown in  FIG. 1 , such as an internal combustion engine, a sealing ring  39  is used that is situated on lower end face  21  of filter  1 . Here, sealing ring  39  is held by a sealing ring bearer  34  that is situated circumferentially radially inward from sealing ring  39 . Here, sealing ring bearer  34  is made in one piece with lower end plate  31  of filter insert  3 , and is connected to end plate  31  via connecting webs  33  that are situated at distances from one another, seen in the circumferential direction. Inlet  11  for liquid to be filtered runs through the intermediate spaces between connecting webs  33 . 
     In normal operation of filter  1 , the liquid to be cleaned, such as lubricant oil of an internal combustion engine, runs through inlet  11  fashioned on lower end face  21  to unfiltered side  13  of filter  1 , and from there runs through filter material body  30  to filtered side  14  of filter  1 , situated radially inwardly from the filter material body. From there, the filtered liquid moves through perforations  42  in pipe socket  4  into its interior, and flows out downward through the interior, through outlet  12 . Here, filter bypass valve is in its closed position, as shown in  FIG. 1 . 
     The spatial orientation of filter  1  does not play a role in its operation; filter  1  can be used in any spatial position. The orientation shown in the drawing is given only as an example. Preferably, however, the filter realization according to  FIG. 1  is a standing configuration in which filter bypass valve  5  is situated at the top in the filter. In this way, a deposition of dirt particles on the parts of filter bypass valve  5 , which could possibly disturb the functioning, is avoided to the greatest possible extent. In this way, dirt particles that have previously settled in the lowest region of the filter on its unfiltered side  13  are also prevented from being carried along to filtered side  14  when filter bypass valve  5  is open. 
     If an impermissibly high pressure difference, above a specifiable boundary value, occurs between unfiltered side  13  and filtered side  14  of filter  1 , this has the result that the liquid at unfiltered side  13  acts, through through-openings  50 ′, on valve body  51  so strongly that the valve body is moved away from valve seat  50 , against the force of spring  52 , thus opening filter bypass valve  5 . In this state, a flow path for the liquid is released that bypasses filter material body  30 , going from unfiltered side  13  through through-openings  50 ′ to filtered side  14 , and through perforations  42  into the interior of pipe socket  4 , i.e., to outlet  12  of filter  1 . If the pressure difference decreases below the boundary value, the force of spring  52  again predominates, and filter bypass valve  5  closes. 
       FIG. 2  of the drawing shows filter  1  of  FIG. 1  in a top view, and sectional line I-I of the longitudinal section shown in  FIG. 1  is also shown in  FIG. 2 . Visible here is only filter housing  2 . Facing the viewer is the upper end face  22 , remote from the flange, of filter housing  2  having central tool attachment projection  23 . Circumferential wall  20  of filter housing  2  runs circumferentially. 
     In  FIG. 3 , filter  1  of  FIG. 1  is shown in cross-section. Circumferential wall  20  of filter housing  2  runs radially externally. In the interior of housing  2  there is situated filter insert  3  with filter material body  30 , formed by a filter material strand arranged in folds. Underneath filter material body  30  is situated lower end plate  31  of filter insert  3 , to which plate sealing ring bearer  34  is connected in one piece via connecting webs  33 . 
     Radially inwardly from filter material body  30 , inner supporting element  37  is situated, having spring support  53  in the form of the four support arms  53 ′ running in the axial direction. Between support arms  53 ′ of spring support  53 , stop  43  is visible, with the open intermediate spaces  44  provided therein for support arms  53 ′ of spring support  53 . The radially innermost part is formed by central pipe socket  4 , at whose lower inner end outlet  12  is situated. 
       FIGS. 4 and 5  show an enlarged detail of filter  1  with filter bypass valve  5  according to  FIG. 1 , shown in  FIG. 4  in the closed position and in  FIG. 5  in the open position. 
     At the top in  FIGS. 4 and 5 , a part of upper end face  22  of filter housing  2  having tool attachment projection  23  can be seen. Central pipe socket  4  is welded, with its inner end region  41 , to the underside of end face  22 , and runs downward from there. 
     In the left part of  FIGS. 4 and 5 , a small part of filter insert  3  can be seen having filter material body  30  and upper end plate  32  with its sealing lip  36 , which abuts outer circumference of valve seat  50  in sealing fashion and here separates unfiltered side  13  from filtered side  14  of filter  1 . 
     Valve seat  50 , having the shape of an annular element, is fixedly and tightly attached on the outer circumference of inner end region  41  of pipe socket  4 . Through-openings  50 ′ run through valve seat  50  in the axial direction. 
     Axially underneath valve seat  50  is situated valve body  51 , also realized in the form of an annular element, guided displaceably in the axial direction on the outer circumference of pipe socket  4 . Valve body  51  is preloaded with a force acting in the closing direction by spring  52 , situated underneath valve body  51 . As long as the pressure difference between unfiltered side  13  and filtered side  14  does not exceed a specifiable boundary value, spring  52  holds valve body  51  in liquid-tight seating on valve seat  50 , and a flow of liquid from unfiltered side  13  to filtered side  14  is possible only by flowing through filter material body  30  of filter insert  3 . 
       FIG. 5  shows the open state of filter bypass valve  5 , which occurs when the pressure difference between unfiltered side  13  and filtered side  14  exceeds the specified boundary value. In this case, the force exerted in the opening direction through through-openings  50 ′ on valve body  51  by the liquid situated at unfiltered side  13  is greater than the force exerted in the closing direction on valve body  51  by spring  52 . Valve body  51  is now removed from valve seat  50  through displacement downward in the axial direction, thus releasing a flow path for the liquid from unfiltered side  13  to filtered side  14  through through-openings  50 ′ and perforations  42  of central pipe socket  4 , bypassing filter material body  30 . In this way, impermissibly large pressure differences between unfiltered side  13  and filtered side  14  can be avoided, which occur for example when the liquid to be filtered is viscous as a result of low temperature, and/or when filter material body  30  is clogged with dirt particles. 
       FIG. 6  shows filter  1  in a state with filter insert  3  removed, again in longitudinal section as in  FIG. 1 . The outer cladding of filter  1  forms filter housing  2  with its circumferential wall  20  and its closed upper end face  22  on which tool attachment projection  23  is integrally formed. 
     In the interior of housing  2  there is situated central pipe socket  4 , which is connected with its upper end region  41  fixedly and tightly to the underside of upper end face  22  of filter housing  2 . 
     All parts of filter bypass valve  5 , with the exception of spring support  53 , which forms a part of filter insert  3 , are situated on central pipe socket  4 . At the very top on pipe socket  4 , annular valve seat  50 , having through-openings  50 ′, is externally seated in positionally fixed and liquid-tight fashion. Under it is situated axially displaceable valve body  51 , also annular in shape, which here is seated in sealing position on the underside of valve seat  50  and seals through-openings  50 ′. Valve body  51  is brought into this closing position by spring  52 , whose lower end abuts axially displaceable intermediate ring  54 . Here, because intermediate ring  54  is now not loaded by spring support  53  of filter insert  3 , intermediate ring  54  is supported on stop  43 , which is attached non-displaceably and non-rotatably on the outer circumference of pipe socket  4 , for example pushed on with a press fit. In this way, all axially displaceable parts of filter bypass valve  5 , namely valve body  51 , spring  52 , and intermediate ring  54 , are held captive on pipe socket  4 . 
     In order to install filter insert  3  into filter  1 , according to  FIG. 6  filter insert  3  is pushed into filter housing  2  from the open end face  21 . By rotating filter insert  3  in the circumferential direction, support arms  53 ′ of spring support  53  are brought into coincidence with open intermediate spaces  44  of stop  43 , so that support arms  53 ′ then pass through intermediate spaces  44 , and, upon further pushing in of filter insert  3 , move into abutment on intermediate ring  54 . Filter  1  completed in this way can then be screwed, with its connecting flange  10 , onto a coupling flange of an associated device. Here, during the screwing on filter insert  3  is pressed still further into filter housing  2 , thereby simultaneously tensioning spring  52 . At the end of this screw-on process, sealing ring  39  seals the flange connection, and spring  52  has reached its target prestressing. 
       FIG. 7  shows filter  1  in a second embodiment, in the delivered state, in longitudinal section. Filter  1  here as well has a cup-shaped filter housing  2  having a circumferential wall  20  having an open (here lower) end face  21  and a closed (here upper) end face  22  in whose center a tool attachment projection  23  is integrally formed. A threaded connector  25 , situated concentric to circumferential wall  20  and protruding into the interior of filter housing  2 , is fixedly connected to upper end face  22  of filter housing  2 , here welded by a weld seam  25 ′. For this purpose, filter housing  2  and threaded connector  25  are made of steel plate, housing  2  preferably being a deep-drawn part. 
     In addition, a central pipe socket  4  is situated concentric to circumferential wall  20  in the interior of filter housing  2 , the pipe socket being provided in its inner (here upper) end region  41  with an outer threading  41 ′ by which pipe socket  4  is screwed into threaded connector  25 . 
     In its outer (here lower) end region  40 , pipe socket  4  has an outer threading  40 ′ by which filter  1  can be screwed onto a coupling flange of an associated device (not shown here) such as an internal combustion engine of a motor vehicle. In its region between outer end region  40  and inner end region  41 , pipe socket  4  has a plurality of perforations  42  distributed in the circumferential and axial direction. Moreover, pipe socket  4  here has three stops  43 , distributed approximately in its longitudinal center around its circumference. Here, each stop  43  is formed by a stop tongue  43 . 2  pressed in one piece from the material of pipe socket  4 , having a stop tab  43 . 3  pointing outward in the radial direction. 
     A filter insert  3  is pushed from below onto the outer circumference of pipe socket  4  and threaded connector  25 . Filter insert  3  is made up of a hollow cylindrical filter material body that is enclosed at the ends by a lower end plate  31  and an upper end plate  32 . Each end plate  31 ,  32  has a central perforation  31 ′,  32 ′, each having an associated sealing lip  35 ,  36 . A grid-type supporting element  37  is situated radially inwardly from filter material body  30 . 
     A sealing ring bearer  34  is connected in one piece to lower end plate  31  via connecting webs  33 , the bearer bearing a circumferential sealing ring  39 . In the unloaded state, sealing ring  39  protrudes outwardly, here downward, in the axial direction, past lower end face  21  of filter housing  2 , and, in the assembled state of filter  1 , seals the filter against an associated coupling flange. 
     Finally, filter  1  according to  FIG. 7  includes another filter bypass valve  5  that here is situated in the lower region of filter housing  2  at the height of lower end plate  31  of filter insert  3  on pipe socket  4 . A valve seat  50  is formed by an annular body situated tightly on pipe socket  4 , having through-openings  50 ′ adjacent to one another in the circumferential direction and running in the axial direction. In the assembled state of filter  1 , valve seat  50  is fixed in the axial direction and in the circumferential direction. An annular valve body  51  that can be displaced on pipe socket  4  works together with valve seat  50 , the valve body being preloaded in the closing direction of filter bypass valve by a spring  52  in the form of a helical pressure spring  5 . The end of spring  52  facing away from valve body  51  is supported by a spring support  53  that is fashioned on supporting element  37 , more precisely on the lower end of its axial struts  38 . In  FIG. 7 , filter bypass valve  5  is shown in its closing position. Here, valve body  51  abuts valve seat  50  in sealing fashion, and seals through-openings  50 ′ present therein. 
     The side of filter  1  pointing downward in  FIG. 7  forms a connecting flange  10  for connecting filter  1  to an associated coupling flange. Radially outwardly from protruding lower end region  40  of central pipe socket  4 , an inlet  11  for liquid that is to be cleaned such as lubricant oil runs into the interior of filter  1  at its unfiltered side  13 . Radially inward from filter material body  30  there is situated a filtered region  14  of the filter that is connected via perforations  42  to the interior of pipe socket  4 , which forms a downward-leading outlet  12  for filtered liquid. 
     In the state shown in  FIG. 7 , filter  1  can be supplied as a pre-manufactured unit e.g., to an engine or vehicle manufacturer, who can then quickly and easily install filter  1 . Here, for this filter embodiment a suspended configuration is preferred in which filter bypass valve  5  is then situated at the top in filter  1 , in its cleanest region. In general, however, this filter  1  can also be used in any spatial position. 
       FIG. 8  shows filter  1  of  FIG. 7  in the installed state with closed filter bypass valve  7 , in longitudinal section. In the lower part of  FIG. 8 , a part is shown of a device  6  such as crank housing  6  of an internal combustion engine, having at the top a coupling flange  60  on which filter  1  is in place with its connecting flange  10 . For this purpose, using a screw tool placed on tool attachment projection  23  filter  1  is screwed into lower end region  40 , provided with threading  40 ′, of pipe socket  4 , into a counter-threading  61 ′ of coupling flange  60 , until lower end face  21  of filter housing  2  abuts the surface of coupling flange  60 , or until a specifiable tightening torque is reached. Outwardly, the flange connection is sealed in liquid-tight fashion by the now-compressed sealing ring  39 ; in the drawing, for reasons of graphic representation sealing ring  39  is shown only in its relaxed, non-compressed state. Valve seat  50  now abuts, with its lower end face, a stop  65  fashioned in coupling flange  60 , defining and securing the axial position of the valve seat  50 . 
     In normal operation of this filter  1 , liquid to be cleaned such as lubricant oil flows through an unfiltered liquid duct  62  in device  6  to inlet  11  of filter  1 , and to its unfiltered side  13 . The liquid then flows through filter material body  30  of filter insert  3  in the radial direction from the outside toward the inside, and moves to the filtered side  14  of filter  1 , leaving behind dirt particles in filter material body  30 . The filtered liquid flows downward through perforations  42 , and, via outlet  12 , into filtered liquid duct  61  of device  6 . 
     In this normal state, in which a pressure difference between unfiltered side  13  and filtered side  14  remains below a specifiable boundary value, filter bypass valve  5  is closed, as shown in  FIG. 8 . 
     In its further parts, filter  1  in  FIG. 8  corresponds to filter  1  in  FIG. 7 , to whose description reference is made. 
       FIG. 9  shows filter  1  of  FIG. 8  without filter insert  3 , in longitudinal section. Here, it is clear in particular that when filter insert  3  is not present spring  52  is supported with its end remote from valve body  51  on stop  43 , specifically stop tab  43 . 3 . In this way, it is ensured that spring  52  and valve body  51  are held captive on central pipe socket  4  when no filter insert  3  is present. 
     For the placement of a filter insert  3  into filter  1 , filter housing  2  is removed, here by unscrewing from central pipe socket  4 , as shown in  FIG. 10 , also in longitudinal section. For this purpose, the screw connection of lower end region  40  of pipe socket  4  to outer threading  40 ′ in threading  61 ′ of coupling flange  60  is realized with a larger unscrewing resistance than is the screw connection between threaded connector  25  on filter housing  2  and threading  41 ′ on upper end region  41  of pipe socket  4 . Thus, when filter housing  2  is removed by unscrewing, pipe socket  4  remains connected to coupling flange  60 . The larger unscrewing resistance of the screw connection between pipe socket  4  and coupling flange  60  can for example be achieved in that this screw connection is provided with a micro-encapsulated screw securing that deploys its securing effect when the screw connection is first produced. 
     When filter housing  2  is removed, here a filter insert  3  can be placed onto pipe socket  4  from above, and filter housing  2  can then be screwed on again. As soon as filter insert  3 , together with filter housing  2 , has been moved downward a certain distance on pipe socket  4 , spring support  53  on supporting element  37  of filter insert  3  (compare  FIG. 8 ) comes into engagement with the upper end, facing away from valve body  51 , of spring  52 , and prestresses this spring to a desired degree during the further movement downward. Thus, here as well filter insert  3  provides the setting of a desired prestressing of spring  52 , and thus a desired opening pressure of filter bypass valve  5 . 
       FIG. 11  shows filter insert  3  of filter  1  according to  FIGS. 7 and 8  as an individual part, in longitudinal section. Hollow cylindrical filter material body  30  is tightly enclosed at its two ends by end plates  31  and  32 . Lower end plate  31  has central perforation  31 ′ having circumferential sealing lip  35 . Moreover, sealing ring bearer  34 , with sealing ring  39  held thereon, is connected in one piece to lower end plate  31  via connecting webs  33 . Upper end plate  32  has a central perforation  32 ′ that is surrounded by sealing lip  36 . 
     Grid-type supporting element  37  is situated inside filter material body  30 , and this supporting element has a plurality of axial struts  38  situated at a distance from one another in the circumferential direction. Here, its lower end forms spring support  53  for spring  52  of filter bypass valve  5 . 
     During a filter maintenance, a used filter insert  3  is exchanged for a fresh filter insert  3 ; all further parts of filter  1  continue to be used. Filter insert  3  is usefully made completely of combustible materials, so that consumed filter inserts can be easily and completely disposed of thermally. 
       FIG. 12  shows filter housing  2  of filter  1  from  FIGS. 7 through 9  as an individual part in longitudinal section. Rotationally symmetrical hollow cylindrical circumferential wall  20  runs radially externally. Open end face  21  is situated at the bottom. At the top, closed end face  22  is situated, having tool attachment projection  23 . Threaded connector  25 , fixedly connected to the rest of filter housing  2  by weld seam  25 ′, is situated inside filter housing  2 , concentric to circumferential wall  20 . 
       FIG. 13  shows filter  1  of  FIG. 8 , here in an operating state with open filter bypass valve  5 , in longitudinal section. This operating state occurs when a pressure difference between unfiltered side  13  and filtered side  14  of filter  1  exceeds a specifiable boundary value, for example as a result of a high viscosity of the liquid at low temperature and/or when filter material body  30  is clogged with dirt particles. This has the result that the force exerted in the opening direction on valve body  51  by the liquid exceeds the force exerted on valve body  51  in the closing direction by spring  52 . In this case, the force of the liquid ensures that valve body  51  is displaced, against the force of spring  52 , in the axial direction on the outer circumference of pipe socket  4 , and is lifted off from valve seat  50 . In this way, an immediate flow connection is released from unfiltered side  13  through through-openings  50 ′ of valve seat  50  to filtered side  14 , and through the interior of pipe socket  4  to outlet  12  of filter  1 , bypassing filter material body  30 . 
     With regard to the further parts shown in  FIG. 13 , reference is made to the description of  FIG. 8 . 
       FIG. 14  shows filter  1  of  FIG. 7  in cross-section along the sectional line XIV-XIV in  FIG. 7 . Radially externally, circumferential wall  20  of filter housing  2  is visible. Radially inwardly, there follows sealing ring bearer  34 , which is connected to lower end plate  31  of filter insert  3 , situated in the background, by a plurality (here nine) of connecting webs  33  situated at a distance from one another in the circumferential direction. Further radially inward there follows hollow cylindrical filter material body  30 , formed by a filter material strand arranged in folds. Radially inward from filter material body  30  there is situated supporting element  37  with spring support  53  fashioned thereon. The radially innermost part is formed by central pipe socket  4  with its stop  43  fashioned in one piece therewith and protruding radially outward. 
       FIG. 15  shows a valve seat  50  of filter  1  according to  FIGS. 7 through 14  in a first embodiment, in a top view. Here it is particularly clear that valve seat  50  has the form of an annular body. Through-openings  50 ′ are here realized in the shape of annular segments in order to provide as large a through-flow cross-section as possible in the open state of filter bypass valve  5 . Radially inward in valve seat  50 , here an inner threading  56  can be seen that includes only one thread pitch, which is used to screw valve seat  50  onto threading  40 ′ on outer end region  40  of central pipe socket  4 , as is shown for example in  FIGS. 7 through 10 and 13 . 
       FIG. 16  shows valve seat  50  of filter  1  according to  FIGS. 7 through 14  in a second embodiment, in a top view. Here as well, valve seat  50  has the shape of an annular element, but here through-openings  50 ′ are realized in the form of round openings closely adjacent to one another. Inner threading  56  is provided radially inwardly here as well. 
       FIG. 17  of the drawing shows Filter  1  in a third embodiment, in the installed state with closed filter bypass valve  5 , and in addition having an anti-rotation lock  47 , in longitudinal section. For reliable functioning of the liquid filter  1  having a screw-on housing, such as filter housing  2  in filter  1  according to  FIG. 17 , it is essential for an automatic detachment by rotation to be reliably prevented. In the example of filter  1  according to  FIG. 17 , this is achieved by an integrated anti-rotation lock  47 . Anti-rotation lock  47  is here fashioned between supporting element  37  of filter insert  3  on the one hand and threaded connector  25  on the inner side of filter housing  2  on the other hand. Supporting element  37  cannot be rotated in the circumferential direction relative to central pipe socket  4 , or can be so rotated only to a very limited extent, but is made displaceable in the axial direction. Because, as is also the case in the exemplary embodiments described above, here spring support  53  fashioned on supporting element  37  supports spring  52  of filter bypass valve  5  at its end remote from valve body  51 , spring  53  exerts a force acting in the axial direction on axially movable supporting element  37  in the direction toward threaded connector  25 . In this way, an upper end face of supporting body  37  and a lower end face of threaded connector  25  are pressed against one another with a force determined by the spring force of spring  52 . In addition, supporting element  37  and threaded connector  25  are fashioned with interlocking contours on their surfaces that contact one another, as is further explained below. These contours provide the desired securing of filter housing  2 , fashioned as a screw housing, against undesirable automatic rotating loose from central pipe socket  4 . 
     In this embodiment of filter  1  as well, central pipe socket  4  has in its center region on its circumference stop  43 , here fashioned in the form of tabs that protrude outward in the radial direction fashioned in one piece with pipe socket  4 . 
     In the example according to  FIG. 17 , it is preferably provided that filter housing  2  is fashioned in one piece with threaded connector  25 , and is an injection-molded part made of plastic. Preferably, it is further provided that central pipe socket  4  is here an injection-molded part made of plastic. A suitable plastic having the required mechanical, thermal, and chemical stability is for example polyamide (PA), which can contain a certain portion of glass fibers. 
     As is also the case in the exemplary embodiments described above, this exemplary embodiment of filter  1  according to  FIG. 17  is equipped with a filter bypass valve  5 . For this purpose, here as well valve seat  50  with its through-openings  50 ′ is screwed onto outer threading  40 ′ of lower end region  40  of pipe socket  4 . Valve element  51  is situated above valve seat  50  and here as well can be displaced in the axial direction on outer circumference of pipe socket  4 . Spring  52  loads valve body  51  with a force acting in the closing direction. 
     With regard to the further parts in  FIG. 17  and their functioning, reference is made to the above description, in particular of  FIG. 8 . 
       FIG. 18  shows filter  1  of  FIG. 17  in cross-section along the sectional line B-B in  FIG. 17 . Radially externally, filter housing  2  is visible with its circumferential wall  20 . Radially inwardly therefrom, in the background there is situated lower end plate  31  with connecting webs  33  for sealing ring bearer  34 . Filter material body  30  is situated on lower end plate  31  in the form of the folded filter material-strand. Grid-type supporting element  37  is situated radially inward from filter material body  30 . Here again, the radially innermost part of filter  1  is formed by central pipe socket  4 , over whose outer circumference there protrude a total of four protruding tabs as stop  43 .  FIG. 18  illustrates that supporting element  37  and the tabs forming stop  43  overlap, seen in the radial direction, so that a rotation of supporting element relative to central pipe socket  4  is not possible, or is possible only to a very limited extent. 
       FIG. 19  shows filter  1  of  FIG. 17 , also in cross-section, here according to sectional line C-C in  FIG. 17 . The radially outer part of filter  1 , up to and including filter material body  30 , corresponds to that of  FIG. 18 , to whose description reference is made. 
     Radially inward from filter material body  30 , here threaded connector  25  of filter housing  2  is sectioned close to its lower end face, wherein a total of eight recesses  28 , regularly spaced from one another in the circumferential direction, are formed as end-face contour. Conversely, on the end face, oriented toward threaded connector  25 , of anti-rotation lock ring  47 ′, which forms a part of supporting element  37 , cams  48  are integrally formed in one piece, which enter into engagement with recesses  28 . Recesses  28  and cams  48  form together with spring  52  anti-rotation lock  47 . Because supporting element  37 , as described above, is preloaded by spring  52  in the direction toward threaded connector  25 , recesses  28  and cams  48  form a kind of locking engagement that prevents undesired automatic rotation of filter housing  2  against supporting element  37 . Because for its part supporting element  37  is secured against rotation relative to pipe socket  4 , filter housing  2  cannot automatically rotate loose from central pipe socket  4 , even when there are vibrations or pressure pulsations that occur during practical operation of filter  1 . 
       FIG. 20  shows filter  1  of  FIGS. 17 through 19  in longitudinal section according to sectional line D-D in  FIG. 19 . Underneath threaded connector  25  of filter housing  2 , supporting element  37  of filter insert  3  is situated in axially displaceable fashion on the outer circumference of central pipe socket  4 . The upward-oriented end face of supporting element  37 , and the downward-oriented end face of threaded connector  25 , together form the above-described anti-rotation lock  47 . 
     The downward-oriented end face of supporting element  37  here again forms spring support  53 , which supports the end of spring  52  facing away from valve body  51 . The tabs of stop  43  integrally formed in one piece on central pipe socket  4  provide, on the one hand, the desired rotational securing of supporting element  37  relative to pipe socket  4 , and on the other hand form the stop for spring  52  when no filter insert  3  is present in filter  1 . 
     With regard to the further parts in  FIG. 20  and their functioning, reference is made to the above description. 
       FIG. 21  shows filter  1  of  FIGS. 17 through 20  in longitudinal section along sectional line E-E in  FIG. 20 , anti-rotation lock  47  being here particularly illustrated. In the upper region of filter housing  2 , threaded connector  25  is sectioned in its edge region. In the end face of threaded connector  25 , here one of the recesses  28  can be seen. In recess  28  there engages one of the cams  48 , which is integrally formed on the upper end face of supporting element  37 , here in continuation of one of its axial struts  38 . 
     With regard to the further details and  FIG. 21 , reference is made again to the above description. 
       FIG. 22  shows filter  1  of  FIGS. 17 through 21 , again in longitudinal section, here according to sectional line F-F in  FIG. 20 . In this section, central pipe socket  4 , supporting element  37 , valve seat  50 , valve body  51 , and spring  52  are visible in a side view. With its lower end, spring  52  loads valve body  51  with a force acting in the closing direction of filter bypass valve  5 , i.e., with a force acting in the direction toward valve seat  50 . 
     With its upper end, the same spring  52  loads supporting element  37  with a force acting upward in the direction toward threaded connector  25 . The upper end of supporting element  37  is fashioned as anti-rotation lock ring  47 ′ of anti-rotation lock  47 . Cams  48  protrude past the upper side of anti-rotation lock ring  47 ′, which cams here again stand in engagement with end-face recesses  28  of threaded connector  25 , thus together forming anti-rotation lock  47 . 
     With regard to the further parts in  FIG. 22 , reference is made to the preceding description. 
       FIG. 23  shows central pipe socket  4  as part of filter  1  according to  FIGS. 17 through 22  as an individual part, in an oblique view from above. In its upper end region  41 , pipe socket  4  has outer threading  41 ′, and in its lower end region  40  outer threading  40 ′ is provided. In the region of pipe socket  4  situated between threadings  40 ′.  41 ′, perforations  42  are made therein. 
     At an axial distance from the lower end of upper threading  41 ′, distributed over the circumference the four tabs are integrally formed as stop  43 , which protrude radially outward. In the axial direction, underneath each tab of stop  43  a rib-shaped longitudinal guide  49  extends downward, each terminating at a distance from lower threading  40 ′. Together with the tabs of stop  43 , longitudinal guides  49  bring about the locking of supporting element  37  against rotation relative to pipe socket  4  when filter insert  3  is situated on pipe socket  4 . Moreover, the tabs of stop  43  support spring  52  when filter insert  3  is removed from filter  1 , as described above. 
       FIG. 24  shows supporting element  37  as part of filter  1  according to  FIGS. 17 through 22 , as an individual part, in an oblique view from above. In particular, here it is clear that supporting element  37  has the shape of a hollow cylindrical annular grid that has a number of axial struts  38 , as well as a respective annular strut above and below. Here, the upper annular strut forms the anti-rotation lock ring  47 ′ with cams  48  of anti-rotational lock  47 . Supporting element  37  is preferably an injection-molded part made of plastic. 
       FIG. 25  shows filter housing  2  as part of filter  1  according to  FIG. 17 through 22 , as an individual part, in an oblique view from below, i.e., in a view towards its open end face  21 . The radially outer part of filter housing  2  is formed by its circumferential wall  20 . Inside, in the background, there is situated closed end face  22 , from which threaded connector  25  protrudes into the interior of filter housing  2 . The free end face, facing the observer, of threaded connector  25  is provided with recesses  28  configured at regular spacings from one another in the circumferential direction. 
       FIG. 26  shows a constructive unit made up of filter housing  2  according to  FIG. 25  and pipe socket  4  according to  FIG. 23 , in an oblique view from below. In order to form this constructive unit, pipe socket  4  is screwed into threaded connector  25  with its upper end region  41  and outer threading  41 ′ made there. On pipe socket  4 , perforations  42 , stop  43 , and longitudinal guides  49  are visible. Screw threading  40 ′ is attached on outer end region  40 , facing the observer, of pipe socket  4 . 
       FIG. 27  shows an enlarged detail of  FIG. 21  with anti-rotation lock  47 , in a first embodiment. At the top in  FIG. 27 , an edge region of threaded connector  25 , with one of recesses  28  in its end face, is visible. Under this, a part can be seen of supporting element  37  with one of its axial struts  38  and one of the cams  48 . Here, recess  28  and cams  48  stand in engagement with one another, in order in this way to form anti-rotation lock  47 . As described above, supporting element  37 , movable in the axial direction, is loaded by spring  52  (not visible here) with a force oriented in the direction toward threaded connector  25 . As  FIG. 27  illustrates, recess  28  here has, in both directions of rotation, i.e., to the left and to the right in  FIG. 27 , symmetrical bevels each having the same upward incline or the same downward incline. 
       FIG. 28  shows the detail of  FIG. 21  with anti-rotation lock  47  in a second embodiment, for which it is characteristic that recess  28  in the end face of threaded connector  25  has, in the direction of rotation, i.e., to the left and to the right in  FIG. 28 , two different upward or downward inclines, i.e., is asymmetrical. In  FIG. 28 , the upward incline of recess  28  to the right is stronger than to the left. This brings about different rotational resistances depending on the direction of rotation of filter housing  2  relative to supporting element  37 , usefully in such a way that in the tightening direction of rotation a lower resistance of anti-rotation lock  47  is produced and in the loosening direction of rotation a larger resistance thereof is produced. 
       FIG. 29  of the drawing shows filter  1  in the installed state with closed filter bypass valve  5 , in longitudinal section, two different embodiments of a filter insert  3  fitting filter  1  according to  FIG. 29  being shown in the left and right half of  FIG. 29 . Here, in both embodiments, supporting element  37  of filter insert  3  is situated as a separate individual part on the inner circumference of filter material body  30 , and is supported with its upper end against the lower side of upper end face  32 . As described above, the lower end of supporting element  37  forms spring support  53  for spring  52  of filter bypass valve  5 . 
     The two filter inserts  3  in  FIG. 29  differ in the position of the spring support  53  fashioned thereon in each case; spring  52  is the same in both halves of the Figure. In the case of filter insert  3  shown in the left half of  FIG. 29 , spring support  53  is situated at a greater axial distance from valve body  51 , while in the case of filter insert  3  shown in the right half of  FIG. 29 , its spring support  53  is situated at a smaller distance from valve body  51 . Filter insert  3  in the left half of  FIG. 29  thus produces a lower prestressing of spring  52  than does filter insert  3  in the right half of  FIG. 29 . In this way, correspondingly different opening pressures of filter bypass valve  5  are also set. In this way it is therefore advantageously possible to set the opening pressure of filter bypass valve  5  to a desired value, or to easily modify it as needed, solely by changing the position of spring support  53  inside exchangeable filter insert  3 , without having to modify any other parts of filter  1 . 
     With regard to the further parts in  FIG. 29  and their function, reference is made to the above description, in particular of  FIGS. 7 and 8 . 
       FIG. 30  shows filter  1  in an embodiment modified relative to that of  FIG. 29 , again in the installed state with closed filter bypass valve  5  and with a second filter insert  3  shown in two different embodiments in the left and right halves of the Figure, in longitudinal section. 
     Differing from  FIG. 29 , in the example according to  FIG. 30  supporting element  37  of filter insert  3  is realized in one piece with upper end plate  32 , so that here upper end plate  32 , together with supporting element  37  formed in one piece therewith, is used to set the prestressing of spring  52 . Here as well, the different prestressing of spring  52  takes place through different axial positioning of spring support  53  on filter insert  3 , as a comparison of the left and right halves of  FIG. 30  shows. Here as well, therefore, through simple exchange of filter insert  3  with different axial positioning of spring support  53 , the opening pressure of filter bypass valve  5  can be set, and as needed can be set to a different value, without having to carry out modifications on the rest of filter  1 . 
       FIG. 31  shows filter  1  in an embodiment further modified relative to that of  FIG. 29 , again in the installed state with closed filter bypass valve  5  and having a third filter insert  3  shown in two different embodiments in the left and right halves of the Figure, in longitudinal section. In this example it is essential that in each case filter insert  3  is realized having a spring support  53  that is fashioned in one piece with lower end plate  31 . For this purpose, lower end plate  31  has radially inwardly a hollow cylindrical projection that runs axially upward, ending in a collar that is oriented radially inward and that forms spring support  53 . In filter insert  3  shown in the left half of  FIG. 31 , the projection running axially upward is fashioned with a greater length than in the case of filter insert  3  shown in the right half of  FIG. 31 , thereby producing two different prestressings of spring  52 , and thus also two different opening pressures of filter bypass valve  5 . 
       FIG. 32  shows, in longitudinal section, a valve body  51  in an embodiment modified relative to the examples described above. Characteristic of this valve body  51  is that its end face that works together with valve seat  50  is provided with an elastomeric coating  57  in order to improve the sealing effect in interaction with valve seat  50 . 
       FIG. 33  shows a valve seat  50  in an embodiment modified relative to the above-described examples, in longitudinal section. Characteristic for this embodiment of valve seat  50  is that its end face that works together with valve body  51  is provided with an elastomeric coating  57 , which likewise improves the sealing effect in interaction with valve body  51 . 
     Moreover, in the right part of  FIG. 33  one of the through-openings  50 ′ is visible, through which the liquid flow runs when filter bypass valve  5  is open. Finally, on the inner circumference of annular valve seat  50 , inner threading  56  situated there is visible, having a thread pitch with which valve seat  50  can be screwed onto threading  40 ′ on outer end region  40  of central pipe socket  4 . 
     Inside a filter  1 , both valve seat  50  and valve body  51  can be provided with elastomeric coating  57 ; alternatively, it can also suffice to equip only valve seat  50  or only valve body  51  with elastomeric coating  57 . 
     In the above-described exemplary embodiments of filter  1 , this filter is a screw-on exchangeable filter having an exchangeable filter insert  3  that can be attached on coupling flange  60  of the associated device  6 , such as an internal combustion engine, instead of a screw-on exchangeable filter cartridge that can be exchanged only as a whole. Differing from this,  FIG. 34  shows a filter  1  in an angled longitudinal section, having a separate base  6 ′ with which it can be connected to an associated device  6  such as an internal combustion engine or to a functional module forming a part of the device. Moreover, filter  1  according to  FIG. 34  is a so-called suspended filter  1  in which a screw cover  20 ′, forming a detachable part of filter housing  2 , can be screwed off downward and removed. Correspondingly, here filter insert  3  is also put in place from below and taken off downwardly. 
     Here, the non-detachable part of filter housing  2  is realized in one piece with base  6 ′, and is for example a pressure die-cast part made of light metal such as aluminum. Screw cover  20 ′ can also be a pressure die-cast part made of light metal, or alternatively can also be an injection-molded part made of plastic. By means of a sealing ring  39 , screw cover  20 ′, in its screwed-on state, seals against the rest of filter housing  2  in liquid-tight fashion. On its outer side, here pointing downward, screw cover  20 ′ has a tool attachment projection  23 , for example a hexagon. Inside tool attachment projection  23 , here a standard known screw valve is situated as a drain valve, in order to make it possible to completely drain the interior of filter  1  of liquid before opening filter housing  2 . 
     In the interior of filter housing  2 , here as well there is situated a filter insert  3  that is made up of a hollow cylindrical filter material body  30  tightly enclosed at its two ends by end plates  31  and  32 . In the interior of filter material body  30 , here as well there is situated a grid-type supporting element  37  whose basic shape is hollow and cylindrical. As is known, here filter insert  3  is detachably locked to screw cover  20 ′ using locking arms situated on its lower end plate  31 . 
     In addition, in the interior of filter housing  2  there is situated a central pipe socket  4  that has in its upper end region  40  an external screw threading  40 ′ with which it is screwed into a counter-threading  61 ′ in base  6 ′. With its end plate  32 , which is at the top in  FIG. 34  and has a central perforation, filter insert  3  is placed from below onto central pipe socket  4 . 
     A filter bypass valve  5  is situated on the outer circumference of pipe socket  4 , at the height of upper end plate  32 . For this purpose, immediately above upper end plate  32  an annular valve seat  50  is attached non-displaceably in the axial direction on the outer circumference of pipe socket  4 , here by screwing on. Through-openings  50 ′ run through valve seat  50 , in its axial direction. 
     Immediately below valve seat  50  a valve body  51  is displaceably guided in the axial direction on the outer circumference of pipe socket  4 . Seen in longitudinal section, valve body  51  has the shape of an upside-down L, a longer L limb forming the guide on pipe socket  4 , and a shorter L limb protruding radially outward. An upper end face of the shorter L limb works together with a lower end face of the valve seat  50 ; a lower end face of the shorter L limb forms a seating surface for a spring  52  that loads valve body  51  with a preloading force in the closing direction. 
     The lower end of spring  52 , facing away from valve body  51 , is here supported on a spring support  53 , sectioned and visible at left in  FIG. 34 , which is an integral and one-piece part of supporting element  37  inside filter insert  3 . 
     It can be seen that in this embodiment of filter  1  as well, the prestressing of spring  52 , and thus the opening pressure of filter bypass valve  5 , is determined and defined, or can be modified as needed, through the axial position of spring support  53  on filter insert  3 . 
     During operation of filter  1  according to  FIG. 34 , a liquid to be cleaned such as lubricant oil of an internal combustion engine flows through an unfiltered liquid duct  62  in base  6 ′ to inlet  11  of filter  1 , and to its unfiltered side  13 . Flowing through filter material body  30  of filter insert  3 , the liquid moves to filtered side  14  of filter  1 , leaving behind dirt particles, and flows through supporting element  37  into central pipe socket  4 , and flows through this socket to outlet  12 , which goes into a filtered liquid duct  61  in base  6 ′. 
     As long as a pressure difference between unfiltered side  13  and filtered side  14  remains below a specifiable boundary value, filter bypass valve  5  remains closed. If the pressure difference exceeds the specifiable boundary value, the force of the liquid acting through through-openings  50 ′ on valve body  51  opens filter bypass valve  5 , and a flow path is released from unfiltered side  13  through through-openings  50 ′ and perforations  42  in pipe socket  4 , going immediately to filtered side  14 , bypassing filter material body  30 . 
     Here, valve body  51  is displaced against the force of spring  52  on the outer circumference of central pipe socket  4 , in the axial direction thereof. For the purpose of establishing a good, tilt-free guiding of valve body  51  on central pipe socket  4 , the axial length of valve body  51  should usefully be at least as large as its inner diameter. 
     For the maintenance of filter  1  according to  FIG. 34 , the filter is first drained of fluid by opening the drain valve. Subsequently, screw cover  20 ′ is unscrewed, whereby simultaneously filter insert  3  locked thereto is moved out from filter housing  2 , moving downward, and is pulled off from central pipe socket  4 . In this way, spring support  53 , as part of filter insert  3 , is also removed from the downward-pointing end of spring  52 . To prevent spring  52  from falling out of filter housing  2  and becoming lost, on the inner end (here pointing downward) of pipe socket  4 , distributed around its circumference, a plurality of radially outward-protruding tabs are integrally formed as stop  43 , on which spring  52  abuts when filter insert  3  is removed. In this way, spring  52  and valve body  51  are secured in captive fashion on central pipe socket  4 . 
     During the manufacture and first installation of filter  1  according to  FIG. 34 , central pipe socket  4 , with valve seat  50  and a valve body  51  situated thereon, as well as spring  52 , are screwed into threading  61 ′ of filtered liquid duct  61 , using threading  40 ′ on upper end region  40 , and then remain there. During filter maintenance, only screw cover  20 ′ then has to be unscrewed, and filter insert  3  is exchanged together with spring support  53  integrated therein. 
     As mentioned above, filter  1  is capable of operation in any spatial position; thus, instead of as a suspended filter as in  FIG. 34  filter  1  can also be used as a lying or standing filter, or in any intermediate or oblique position. Here, filter bypass valve  5  is usefully situated in a position that during operation is situated as high up as possible, in order to keep contamination of the valve seat  50  and valve body  51  by dirt particles deposited from the liquid as low as possible. 
       FIG. 35  of the drawing shows filter  1  of  FIG. 17  in a modified embodiment, namely having a rotational securing for valve body  51  of filter bypass  5 , in longitudinal section. Like the exemplary embodiments described above, this exemplary embodiment of filter  1  according to  FIG. 5  is also, equipped with a filter bypass valve  5 . For this purpose, here as well valve seat  50  with its through-openings  50 ′ is screwed onto outer screw threading  40 ′ of lower end region  40  of pipe socket  4 . Valve body  51  is situated above valve seat  50 , and here as well is displaceable in the axial direction on the outer circumference of pipe socket  4 . Spring  52  loads valve body  51  with a force acting in the closing direction. 
     Differing from the exemplary embodiment according to  FIG. 17 , in the exemplary embodiment according to  FIG. 35  valve body  51  is secured against rotation relative to pipe socket  4  and relative to valve seat  50 . For this purpose, on the outer circumference of pipe socket  4 , on its side pointing to the right in  FIG. 35 , there is situated a protruding longitudinal rib  45  that runs in the longitudinal direction of pipe socket  4 , and is for example integrally formed in one piece. Fitting thereto, on the inner circumference of valve body  50  a longitudinal groove  55  running in the axial direction of the valve body is formed which accommodates longitudinal rib  45  with a necessary degree of movement play. Together, longitudinal rib  45  and longitudinal groove  55  form a valve body guide that permits axial displacement of valve body  51  on pipe socket  4 , and at the same time form a lock against a rotation of valve body  51  in the circumferential direction relative to pipe socket  4  and relative to valve seat  50 . In this way, during the time of use of filter  1 , leaks of filter bypass valve  5  in its closed position that may occur due to a relative rotation of valve body.  51  and valve seat  50  are reliably prevented. 
     With regard to the further parts in  FIG. 35  and their function, reference is made to the above description, in particular of  FIGS. 8 and 17 . 
       FIG. 36  shows filter  1  of  FIG. 35  in cross-section along the sectional line H-H in  FIG. 35 . Radially outwardly, filter housing  2  with its circumferential wall  20  is visible. Radially inward therefrom, in the background there is lower end plate  31  with a sealing ring bearer  34 . The filter material body  30 , in the form of the folded filter material strand, is situated on lower end plate  31 . Here, annular valve body  51  is situated radially inwardly from filter material body  30 . Between the inner circumference of lower end plate  31  and the outer circumference of valve body  51 , a small radially external part of valve seat  50 , otherwise covered by valve body  51 , can be seen. 
     Here again, the radially innermost part of filter  1  is formed by central pipe socket  4  with perforations  42 , of which two are visible here in section. On the outer circumference of pipe socket  4 , on its side pointing here to the right, longitudinal rib  45  protruding in the radial direction and running in the axial direction of pipe socket  4  can be seen. Fitting longitudinal rib  45 , on the inner circumference of annular valve body  51  longitudinal groove  55  is formed, which accommodates longitudinal rib  45 .  FIG. 36  illustrates that a rotation of valve body  51  relative to central pipe socket  4  and relative to the valve seat  50  attached non-rotatably on pipe socket  4  is not possible. 
       FIG. 37  shows the detail circled in  FIG. 36  in an enlarged representation. At left in  FIG. 37 , a part of central pipe socket  4  can be seen with its longitudinal rib  45  situated on the outer circumference. Radially outwardly, i.e., here to the right, there follows valve body  51  having longitudinal groove  55 , which accommodates longitudinal rib  45 , in its inner circumference. In order not to hinder the axial mobility of valve body  51  necessary for the valve function, longitudinal rib  45  and longitudinal groove  55  have adequate movement play relative to one another. 
     Radially outwardly from valve body  51 , a small part of lower end plate  31  with filter material body  30  can also be seen. 
       FIG. 38  shows filter  1  in a further embodiment in longitudinal section, in a state in which it is installed on a coupling flange  60  of a device  6 . Filter  1  again has a cup-shaped filter housing  2  that is open at its flange-side end face  21 . Inside filter housing  2  there is situated a central pipe socket  4  that is made up of a first flange-side pipe socket segment  4 . 1  and a second pipe socket segment  4 . 2  that is remote from the flange and is tightly connected to end face  22  of filter housing  2  remote from the flange, the two pipe socket segments  4 . 1  and  4 . 2  being screwed together. 
     On flange-side pipe socket segment  4 . 1 , there are situated the parts of filter bypass valve  5 , namely, at bottom, first valve seat  50 , above this the axially displaceable valve body  51 , and above this spring  52 , which preloads the valve body  51  in the closing direction of filter bypass valve  5 . On its end facing away from valve body  51 , in the operation-ready state shown in  FIG. 38  spring  52  is supported and prestressed by spring support  53 . Here as well, spring support  53  is part of filter insert  3 , more precisely of its central supporting element  37 . 
     On the upper end of flange-side pipe socket segment  4 . 1  there is situated a stop  43  on which the upper end of spring  52  abuts when filter housing  2 , and together with it filter insert  3 , are separated from the rest of filter  1 . In this way, spring  52  and valve body  51  are secured against loss. At the upper side, stop  43  is realized with roof-shaped introductory bevels  46  over which support arms  53 ′ are guided when filter  1  is assembled. 
     In addition to filter bypass valve  5 , filter  1  according to  FIG. 38  also has a return check valve  15  that covers inlet  11  and that prevents filter  1  from running empty when there is a standstill of the liquid circuit. 
       FIG. 39  shows filter  1  of  FIG. 38  in a cross section along the sectional line G-G in  FIG. 38 . Circumferential wall  20  of filter housing  2  runs radially externally. Radially inward therefrom, return check valve  15  with spring tongue plate  18  is visible. Next, further radially inward there follows sealing ring  39 ′, then followed by valve seat  50 . Still further radially inward, flange-side pipe socket segment  4 . 1  is sectioned, followed still further radially inwardly by threaded nipple  63  of associated coupling flange  60 . Outlet  12  of filter  1  is situated in the center of  FIG. 39 . 
       FIG. 40  shows filter  1  of  FIG. 38  in cross-section along the sectional line H-H in  FIG. 38 . Here as well, radially externally first circumferential wall  20  of filter housing  2  is sectioned. Radially inward therefrom there is situated filter material body  30  of filter insert  3  with central supporting element  37 . At four points on supporting element  37 , uniformly spaced from one another in the circumferential direction, spring supports  53  are integrally formed on supporting element  37 , and protrude radially inward. Radially inward from supporting element  37  there then follow stop  43  and pipe socket segments  4 . 1  and  4 . 2 . 
       FIG. 41  shows filter housing  2  and filter insert  3  situated therein of filter  1  of  FIG. 38  in longitudinal section, together forming the constructive unit that is removed during maintenance of filter  1 . For this purpose, by rotating filter housing  2  the flange-remote pipe socket segment  4 . 2 , with its lower outer threading  41 ″, is screwed out of flange-side pipe socket segment  4 . 1 , filter insert  3  being carried along due to its frictional connection to filter housing  2 . When filter housing  2  has been completely unscrewed, filter insert  3  can be pulled out through the open underside of filter housing  2 , and can be replaced by a fresh filter insert  3 , including supporting element  37  with spring support  53 . 
     Seal  39  is here also part of filter insert  3 , and is borne by a sealing ring bearer  34  that is connected to lower flange-side end plate  31  of filter insert  3  via connecting webs  33 , or is realized in one piece therewith. 
       FIG. 42  shows filter housing  2  with filter insert  3  of  FIG. 41 , in an oblique view from below. Of filter insert  3 , here its lower, flange-side end plate  31 , connecting webs  33 , sealing ring bearer  34 , and radially outer sealing ring  39  are visible. Radially inner sealing ring  39 ′ runs around central perforation  31 ′ of end plate  31 . Through perforation  31 ′, a part of inner supporting element  37  of filter insert  3  is visible. 
       FIG. 43  shows filter insert  3  of filter  1  of  FIG. 38 , in an oblique view from below. The essential part of filter insert  3  is formed by its hollow cylindrical filter material body  30 , covered at the underside by end plate  31  and at the upper side by end plate  32 . From the radially external edge of lower end plate  31 , a plurality (here five) of connecting webs  33 , distributed uniformly around the circumference, extend downward in the axial direction, which bear sealing ring bearer  34  on which outer sealing ring  39  is externally situated. Going around central perforation  31 ′ of lower end plate  31 , radially inner seal  39 ′ is again visible. Through perforation  31 ′, a small part of grid-shaped inner supporting element  37  is visible. Here, it is particularly clear that the two sealing rings  39  and  39 ′ are parts of filter insert  3  that are each replaced, without special measures, with filter insert  3  during each filter maintenance. 
       FIG. 44  shows filter insert  3  of  FIG. 43  in an angled longitudinal section. In the interior of filter material body  30 , there is situated supporting element  37  with spring supports  53 , distributed around the inner circumference of supporting element  37 . On the lower side of filter insert  3 , radially inner sealing ring  39 ′ runs around perforation  31 ′ of lower end plate  31 . Radially outer sealing ring  39  is situated on the outer circumference of sealing ring bearer  34 . 
       FIG. 45  shows filter insert  3  of  FIG. 44  in cross-section according to the sectional line F′-F′ in  FIG. 44 . Sealing ring  39  is radially outwardly visible, borne by sealing ring bearer  34  running radially inward therefrom. Connecting webs  33  extend upward from sealing ring bearer  34 . Further radially inward, second sealing ring  39 ′ is sectioned. 
       FIG. 46  shows filter housing  2  of filter  1  of  FIG. 38  in an oblique view from below. In the interior of filter housing  2 , flange-remote pipe socket segment  4 . 2  runs concentric to circumferential wall  20 , with its threading  41 ″ for connection to second, flange-side pipe socket segment  4 . 1 , which remains on coupling flange  60  after a first assembly of filter  1  when filter housing  2  is unscrewed, as described above. In pipe socket segment  4 . 2 , one of the perforations  42  provided therein is visible, through which during operation of filter  1  filtered liquid moves into the interior of central pipe socket  4 . 
       FIG. 47  shows a coupling flange  60  of filter  1  having filter parts remaining thereon after removal of filter housing  2  and filter insert  3 . The filter parts remaining on coupling flange  60  are flange-side pipe socket segment  4 . 1 , with filter bypass valve  5  situated thereon, and return check valve  15  also situated thereon. 
     At the top in  FIG. 47 , stop  43 , situated on the upper end of pipe socket segment  4 . 1 , is visible with top-side introductory bevels  46 , and in the interior of the upper end of pipe socket segment  4 . 1  its inner threading  40 ″ is visible with which threading  41 ″ of flange-remote pipe socket segment  4 . 2  (not present here) can be screwed together. Under stop  43 , spring  52  of filter bypass valve  5  is visible, which preloads the valve body  51  in the closing direction and presses it against valve seat  50 . 
     Return check valve  15  includes a circular flexible valve membrane  15 ′ and a spring tongue plate  18  situated above it, connected to the outer circumference of valve seat  50  via a plurality of locking connections  17  distributed in the circumferential direction. Going around return check valve  15 , coupling flange  60  of device  6  can be seen. 
       FIG. 48  shows the subject matter of  FIG. 47  in longitudinal section; here it is particularly clear that flange-side pipe socket segment  4 . 1  is screwed, with screw threading  40 ′ made on its outer region, i.e., here its lower end region  40 , onto a threaded nipple  63  situated centrally in coupling flange  60  and having a counter-threading  61 ′. Threaded nipple  63  is in turn permanently screwed to device  6  in the center of coupling flange  60  via a further screwed connection. The screw connection of pipe socket segment  4 . 1  to threaded nipple  63  takes place during a first assembly of filter  1  and is secured against detachment by an anti-rotation lock  67  when filter housing  2  (not shown in  FIG. 48 ) is unscrewed, together with flange-remote pipe socket segment  4 . 2 , from flange-side pipe socket segment  4 . 1 . 
     On the outer circumference of flange-side pipe socket segment  4 . 1 , the parts of filter bypass valve  5  are provided in the configuration described above. At the top on pipe socket segment  4 . 1  there sits stop  43  with top-side introductory bevels  46 , which, during assembly of filter  1 , guide spring support  53 , or support arms  53 ′, situated in filter insert  3  into intermediate spaces  44  at the lower end of the introductory bevels  46 . In this way, only one filter insert  3  can be used having spring supports  53 , or support arms  53 ′, in the number, shape, and distribution matching intermediate spaces  44 . 
     Radially outward from valve seat  50 , there is situated return check valve  15  which covers inlet  11 , and which here has a separate membrane bearer  15 ″. 
       FIG. 49  shows detail Y of  FIG. 39  in an enlarged representation. At the far left, a small part of threaded nipple  63  is visible, surrounded radially outwardly, i.e., to the right, by flange-side pipe socket segment  4 . 1 . Further outward, i.e., here to the right, follow valve seat  50  and radially inner sealing ring  39 ′. At the right, i.e., radially outward therefrom, a part of return check valve  15  with spring tongue plate  18  is still visible. The parts of return check valve  15  are connected to valve seat  50  via locking connections  17 ;  FIG. 49  shows one of these locking connections  17 .  FIG. 49  further illustrates that sealing ring  39 ′ is adapted in its course to the configuration and position of locking connections  17 , and here deviates radially outwardly in each case. This curve of sealing ling  39 ′ adapted to the position of locking connections  17  is also visible in  FIGS. 42, 43 , and  45 . 
       FIG. 50  shows detail X of  FIG. 40  in an enlarged representation, filter material body  30  being visible here in the right part of  FIG. 50 , which body is supported radially inwardly, i.e., at left in the Figure, by inner supporting element  37 . Made in one piece with supporting element  37 , here one of the spring supports  53  is visible, which engages in one of the intermediate spaces  44  in stop  43 . Radially inward from stop  43 , i.e., to the left thereof in this Figure, small parts of pipe socket segments  4 . 1  and  4 . 2  can be seen. 
       FIG. 51  shows a constructive unit including filter bypass valve  5  and return check valve  15  as parts of filter  1  of  FIG. 38 , in an oblique view from below. At the upper end of flange-side pipe socket segment  4 . 1 , stop  43  is visible with introductory bevels  46  and intermediate spaces  44  between them. Under these is situated spring  52  of filter bypass valve  5 , which preloads valve body  51  thereof in the closing direction against valve seat  50 . 
     In the lower part of  FIG. 51 , return check valve  15  is visible; here the view from below is directed to membrane bearer  15 ″ having perforations for inflowing liquid to be filtered. At the upper side, flexible valve membrane  15 ′ is situated on membrane bearer  15 ″. Underneath membrane bearer  15 ″, anti-rotation lock  67  is visible. 
       FIG. 52  shows the constructive unit of  FIG. 51  together with coupling flange  60  and filter housing  2  having a filter insert  3 , in an exploded view from below. At top in  FIG. 52 , filter housing  2  is visible, with filter insert  3  situated therein. In the center part of the Figure, the constructive unit of pipe socket segment  4 . 1  with filter bypass valve  5  and return check valve  15  is visible. At the bottom in  FIG. 52 , coupling flange  60  of device  6  is visible, having threaded nipple  63  situated in its center having counter-threading  61 ′ for the threading (not visible here) provided in the lower end region of pipe socket segment  4 . 1 . 
       FIG. 53  shows detail Z of  FIG. 48  in an enlarged representation, in order to illustrate the embodiment of return check valve  15  used here. At bottom in  FIG. 53 , a part of coupling flange  60  of device  6  is visible, having unfiltered liquid duct  62  that leads to inlet  11  of filter  1 . Here, inlet  11  is fashioned in the form of a plurality of perforations, distributed in the circumferential direction, in membrane bearer  15 ″. On the upper side of membrane bearer  15 ″ there is situated flexible valve membrane  15 ′, loaded at the top by spring tongue plate  18  which exerts a resetting force, acting in the closing direction, on valve membrane  15 ′. The unit made up of membrane bearer  15 ″, valve membrane  15 ′, and spring tongue plate  18  is connected to valve seat  50  of filter bypass valve  5  by locking connections  17  mentioned above. 
       FIG. 54  shows detail W of  FIG. 38  in an enlarged representation, here with installed filter insert  3  and filter housing  2  put into place. As this Figure illustrates, in this assembled state radially inner sealing ring  39 ′ presses valve membrane  15 ′ and the associated spring tongue plate  18  against membrane bearer  15 ″, and in this way provides a secure holding together and liquid-tight termination in this region. Outwardly, filter housing  2  is sealed in liquid-tight fashion by radially external sealing ring  39 , which is situated between filter housing  2  and coupling flange  60 . 
       FIG. 55  shows filter insert  3  of filter  1  in a modified embodiment for a filter  1  without return check valve  15 , in an oblique view from below. Differing from filter insert  3  for the previously described filter  1  having return check valve  15 , in filter insert  3  according to  FIG. 55  radially inner sealing ring  39 ′ is made circular, because here it does not require the particular shaping in order to take into account the position of locking connections  17 . In its remaining parts, filter insert  3  according to  FIG. 55  corresponds to the previously explained exemplary embodiment. 
       FIG. 56  shows filter insert  3  of  FIG. 55  in longitudinal section; here as well the continuous circular curve of radially inner sealing ring  39 ′ is visible. With regard to the further elements of filter insert  3  according to  FIG. 56 , reference is made to the preceding description. 
       FIG. 57  shows filter  1  in a further embodiment in longitudinal section; for this embodiment, differing from the example according to  FIG. 38 , it is characteristic that no return check valve  15  is present. Inlet  11 , which conducts liquid to be filtered from unfiltered liquid duct  62  to unfiltered side  13  of filter  1 , is here always open. In other respects, filter  1  corresponds to the exemplary embodiment described on the basis of  FIG. 38 . Filter  1  according to  FIG. 57  is suitable in particular for applications in which it is not disturbing if the liquid flows out of filter  1  when there is a standstill of the liquid circuit. 
       FIG. 58  shows a constructive unit having filter bypass valve  5  as part of filter  1  of  FIG. 57 . Here, filter bypass valve  5  is again borne by flange-side pipe socket segment  4 . 1 , on whose outer circumference there are situated, going from bottom to top, valve seat  50 , valve body  51 , spring  52 , and stop  43  having introductory bevels  46 , and intermediate spaces  44  situated between them. Screw threading  40 ′ is attached inwardly in the lower end region of pipe socket segment  4 . 1 . In the lower part of  FIG. 58 , anti-rotation lock  67  is visible. Due to the absence of the return check valve, in  FIG. 58 , compared to  FIG. 51 , no membrane bearer having the other parts of the return check valve is present. 
       FIG. 59  shows the constructive unit of  FIG. 58  in a state assembled to coupling flange  60 , in an oblique view from above. The state visible here results when filter housing  2  is removed, together with filter insert  3 , from coupling flange  60  for filter maintenance. When filter housing  2  is unscrewed, the two pipe socket segments  4 . 1  and  4 . 2  are separated from one another, because, due to anti-rotation lock  67 , flange-side pipe socket segment  4 . 1  is secured against unscrewing from coupling flange  60 . Running around anti-rotation lock  67  is annular unfiltered liquid duct  62 , which leads to inlet  11  of filter  1 . At bottom left in  FIG. 59 , filtered liquid duct  61  is visible, through which the filtered liquid is led off during operation of filter  1 . In the example according to  FIG. 59 , coupling flange  60  is fashioned on a base  6 ′ that can be connected, via lines that lead further, to the rest of device  6  having the liquid circuit with filter  1 . 
       FIG. 60  shows filter housing  2  with filter insert  3  of filter  1  situated therein, of  FIG. 57 , in an oblique view from below. Radially inward from downward-pointing open end face  21  of circumferential wall  20  of filter housing  2 , there runs sealing ring  39 , borne by sealing ring bearer  34 . Sealing ring bearer  34  is connected to lower end plate  31  of filter insert  3  by connecting webs  33 . Circular inner sealing ring  39 ′ runs around central perforation  31 ′ in lower end plate  31 . Through perforation  31 ′, a part of inner supporting element  37 , with a short segment of one of the spring support  53 , is visible. Filter material body  30  is situated radially outward from supporting element  37 . 
       FIG. 61  shows filter housing  2  with filter insert  3  of  FIG. 60 , in longitudinal section. Pipe socket segment  4 . 2  remote from the flange is placed in sealing fashion into upper end face  22 ; here both filter housing  2  and also pipe socket segment  4 . 2  are parts made of metal such as steel plate. For mutual fixing and sealing, usefully filter housing  2  and pipe socket segment  4 . 2  are welded to one another in the region in which they abut one another. At the lower end region of pipe socket segment  4 . 2 , threading  41 ″ is visible, used for screw connection to flange-side pipe socket segment  4 . 1 . 
     At the height of lower end plate  31  of filter insert  3 , circumferential wall  20  has an inner diameter step  24  on which lower end plate  31  abuts with its upper side, whereby filter insert  3  is given a defined position relative to filter housing  2  in the axial direction. Moreover, filter insert  3  is centered in the radial direction relative to filter housing  2  by connecting webs present on lower end plate  31  on the one hand and by seal  36  on the inner circumference of upper end plate  32  on the other hand. Radially outwardly situated sealing ring  39  is supported on an axially downward-oriented circumferential surface of circumferential wall  20 . Radial inner sealing ring  39 ′ runs around central perforation  31 ′ in lower end plate  31 . 
     In the interior of filter material body  30 , grid-type supporting element  37  is situated, with which, here, spring supports  53  are realized in one piece as radially inward-protuding ribs. Due to the angled course of the section, here only one of the spring supports  53  is visible in the left half of  FIG. 61 . Here, supporting element  37  is connected both to lower end plate  31  and to upper end plate  32 , for example by gluing or welding. 
       FIG. 62  shows the constructive unit of  FIG. 58 , together with a coupling flange  60  and a filter housing  2  with filter insert  3  situated therein, in an exploded view. Filter insert  3  is pushed into filter housing  2  from the open flange-side end face  21  of filter housing  2 , and is held in filter housing  2  by mutual friction or by a locking connection. The constructive unit visible below it includes the parts explained above on the basis of  FIG. 58 , in particular filter bypass valve  5 . At bottom in  FIG. 62 , coupling flange  60  is visible, here as part of a base  6 ′ that is part of a device  6  that has a liquid circuit having liquid to be filtered. Threaded nipple  63  with its threading  61 ′ protrudes centrally from coupling flange  60 . 
       FIG. 63  shows the subject matter of  FIG. 59  in longitudinal section. In coupling flange  60 , threaded nipple  63  is permanently and non-detachably screwed together with a lower threaded segment. Flange-side pipe socket segment  4 . 1  is screwed to upper threading segment  61 ′ by means of its lower threading  40 ′, and is secured against detachment by anti-rotation lock  67 . The parts of filter bypass valve  5 , as explained above, are situated on pipe socket segment  4 . 1 . Because in the state shown in  FIG. 63 , in which filter housing  2  and filter insert  3  are removed, spring supports  53  do not act on spring  52 , here the spring is now expanded to its maximum length and abuts stop  43  with its upper end. In this way, loss of spring  52  and of valve body  5 . 1  is prevented. 
       FIG. 64  shows filter  1  in a further embodiment, in longitudinal section. This filter  1  is again a screw-on filter that is screwed onto a coupling flange  60  of a device  6  having a liquid circuit. Here, coupling flange  60  again has a central threaded nipple  63  having a threading  61 ′ that protrudes from the plane of coupling flange  60 . In addition to its filter housing  2  and filter insert  3 , filter  1  has a base plate  16  that has, radially inwardly, an inner threading  16 ′, and has radially outwardly an outer threading  16 ″, and that has, in its region between the two named threadings  16 ′,  16 ″, a plurality of perforations distributed in the circumferential direction as inlet  11  for liquid to be filtered. Base plate  16  is screwed to threaded nipple  63  by threading  16 ′. Filter housing  2  is screwed to outer threading  16 ″, which has for this purpose screw threading  26  on the inside of the lower end region of its circumferential wall  20 . 
     In addition, a valve holder  58  is attached on threaded nipple  63  over base plate  16 , on or in which holder the parts of filter bypass valve  5  are situated. Here, valve holder  58  is realized in two parts, having a lower holder part  58 . 1  and an upper holder part  58 . 2  fixedly connected therewith. Here, valve seat  50  is fashioned in a base of lower holder part  58 . 1 . Valve body  51  is realized as a flat circular plate, and, in its closing position shown in  FIG. 64 , lies in sealing fashion on valve seat  50 . By means of a helical pressure spring  52  situated in the valve holder  58 , valve body  51  is preloaded with a force acting in the closing direction. Here as well, spring  52  is again prestressed by a filter insert  3 , in that a spring support  53 , realized as a ring-shaped element in one piece with central supporting element  37  in filter material body  30 , presses from above on spring  52  with an annular support collar that is oriented radially inward and then axially downward. Here, supporting element  37  is connected only to lower end plate  31 . The upper end of supporting element  37  is situated at a distance from the underside of upper end plate  32 . 
     Here, filter insert  3  has a lower end plate  31  that has on its underside a circumferential seal  35  with which lower end plate  31  is placed in sealing fashion onto the outer circumference of valve holder  58 . From the radially outer edge of lower end plate  31 , connecting webs  33  again extend downward in the axial direction, which webs bear a circumferential sealing ring bearer  34  on whose outer circumference there is situated a radially external sealing ring  39  that seals filter housing  2  against coupling flange  60 . At the upper side, filter insert  3  has a closed end plate  32 . 
       FIG. 65  shows filter  1  in a further embodiment, in longitudinal section. In many parts, filter  1  corresponds to the example according to  FIG. 64 . Differing here is that filter bypass valve  5 , in the example according to  FIG. 65 , has a covering body  59  assigned to it that ensures that during a filter maintenance with removal of filter housing  2  and of filter insert  3 , filtered liquid outlet  12  is closed in order to prevent dirt particles from moving from filter insert  3  or from filter housing  2  into outlet  12  and from there into filtered liquid duct  61  of device  6 . For this purpose, covering body  59  is situated on the upper side of spring  52 , and is preloaded by spring  52  with a force oriented upward, i.e., oriented away from valve body  51 . In the assembled state of filter  1  shown in  FIG. 65 , support arms  53 ′ of spring support  53  press, through introduction openings  53 ″ provided in the upper side of valve holder  58 , onto the upper side of covering body  59 , which in turn presses with its lower side on the upper end of spring  52 , thus prestressing spring  52 . 
     When filter housing  2  is removed from coupling flange  60  together with filter insert  3 , base plate  16  and a valve holder  58  with filter bypass valve  5  remain on coupling flange  60 . Here, support arms  53 ′ are removed upwardly from covering body  59 , causing the covering body, under the action of the force of spring  52 , to come to lie against the underside of the upper end wall of valve holder  58 , with passages  58 ″ provided there for filtered liquid. In this way, passages  58 ″ are closed, and no dirt particles can move into filtered liquid outlet  12 . 
     So that support arms  53 ′ of spring supports  53  situated on supporting element  37  can exert the required force on covering body  59 , and, via this, on spring  52  of filter bypass valve  5 , here supporting element  37  is supported with its upper end face, remote from the flange, on the inside of the upper end plate  32 , remote from the flange, of filter insert  3 . Filter insert  3  is in turn supported on the inside of filter housing  2  with its upper end plate  32  remote from the flange. 
     In addition, in this filter  1  supporting element  37  is situated in the interior of filter material body  30  so as to be capable of rotation relative to the rest of filter insert  3 . For this purpose, supporting element  37  lies with its upper end against the underside of upper end plate  32  of filter insert  3 , but is not connected to this end plate  32 . Supporting element  37  here does not have immediate contact with flange-side lower end plate  31  of filter insert  3 . This capacity for rotation of supporting element  37  is required here because supporting arms  53 ′ of spring support  53  prevent supporting element  37  from rotating as soon as supporting arms  53 ′ have entered into the associated introduction openings  53 ″ of valve holder  58 , because valve holder  58  is seated in rotationally fixed fashion on threaded nipple  63 . However, for the further screwing of filter housing  2  onto base plate  16 , filter housing  2  must be further rotated together with filter insert  3  situated therein, which is enabled by the configuration of supporting element  37  in filter insert  3  so as to be capable of rotation relative to the rest of filter insert  3 . 
       FIG. 66  shows filter  1  in a further embodiment, in longitudinal section. Filter  1  according to  FIG. 66  corresponds in most of its parts to the example according to  FIG. 64 . Different here is that in the example according to  FIG. 66  a return check valve  15  is allocated to inlet  11 . Return check valve  15  lies on the upper side of base plate  16  in the form of a flexible valve membrane  15 ′, and in this way covers the openings in base plate  16  forming inlet  11 . Return check valve  15  is automatically opened by liquid to be filtered coming from unfiltered liquid duct  62 . When there is a standstill of the liquid circuit, return check valve  15  ensures that filter  1  does not run empty. 
       FIG. 67  shows filter  1  in a further embodiment, in a first, angled longitudinal section along the sectional line A-A in  FIG. 68 ; here filter  1  is shown in a state assembled to a coupling flange  60  of device  6  such as an internal combustion engine. A threaded nipple  63  having an outer threading  61 ′ situated above the flange plane is part of coupling flange  60 , threaded nipple  63  here being realized as a double nipple and being permanently screwed into device  6  with a lower outer threading. 
     Via an inner threading  16 ′, the base plate  16  of filter  1  is screwed together with upper outer threading  61 ′ of threaded nipple  63 , this screw connection taking place during a first assembly of filter  1  on coupling flange  60 . A later detachment of this screw connection is prevented by an anti-rotation lock  67 . 
     In addition, filter  1  has a filter housing  2 , which here has the shape of a downwardly open cup. In the lower edge part of its circumferential wall  20 , housing  2  has an inner threading  26 , which here stands in screwed engagement with outer threading  16 ″ of base plate  16 . Both base plate  16  and housing  2  are here parts made of plastic. The plastic parts are usefully manufactured as injection-molded parts, permitting manufacture with a specific shape without subsequent cutting processing. Alternatively, base plate  16  and housing  2  can be parts made of metal, in particular pressure die-cast parts made of light metal such as aluminum. 
     In the interior of housing  2  there is situated a filter insert  3  that is made up of a hollow cylindrical filter material body  30  having two end plates  31  and  32  enclosing it at its ends. In filter material body  30 , there is situated a grid-type supporting element  37  that supports this body during operation. Via a central opening in lower end plate  32 , filter insert  3  is placed into filter  1  with a sealing by a sealing collar. 
     From the radially outer edge of lower end plate  32 , distributed around its circumference, a plurality of connecting webs  33  extends downward in the axial direction. Due to the angled course of the sectional plane of  FIG. 67 , here only one of connecting webs  33  is visible in the left half of  FIG. 67 . 
     At their lower ends, connecting webs  33  go into a circumferential sealing ring bearer  34  having a cylindrical outer surface, on which a circumferential seal  39  is situated in the form of an elastic sealing ring that is approximately square in its cross-section in the unloaded state. 
     Filter insert  4  is set in a defined desired position relative to filter housing  2  via its connecting webs  33 . 
     In its radially lower outer edge region, base plate  2  extends radially outward with an outer edge projection, under sealing ring bearer  34 . 
     In the lower edge part of filter housing  2 , there is fashioned a support surface that points downward in the axial direction, on which seal  39  abuts with its upper side. When filter  1  is in place on coupling flange  60 , as shown in  FIG. 67 , seal  39  is pressed in the axial direction far enough that it expands radially outward and inward, and seals both axially and radially. 
     A plurality of openings, distributed in the circumferential direction of base plate  16 , run through the radially outer part of base plate  16  between its inner threading  16 ′ and its outer threading  16 ″, as inlet  11  for liquid to be filtered. At the upper side, inlet  11  is covered by a return check valve  15 . Outlet  12  for filtered liquid runs centrally through base plate  16  and through threaded nipple  63 . 
     In the center on the upper side of base plate  16 , there is situated a filter bypass valve  5  that extends into supporting element  37 . A valve holder  58  that forms a part of filter bypass valve  5  and whose basic shape is hollow-cylindrical, is fixedly connected, e.g., friction-welded, at its lower end to base plate  16 . Valve holder  58  is here made up of two parts, namely a first, lower holder part  58 . 1 , which is fixedly connected to base plate  16 , and a second, upper holder part  58 . 2 , connected to first holder part  58 . 1 , here by a locking connection. Here, holder parts  58 . 1  and  58 . 2  are made of plastic. 
     In valve holder  58 , at the bottom a valve seat  50  is fashioned that works together with a valve body  51  guided above it in axially movable fashion in valve holder  58 . Above valve body  51 , there is situated in valve holder  58  a spring  52  that exerts a force acting on valve body  51  in its closing direction when filter insert  3  is placed into filter  1 . 
     Above spring  52 , here there is further situated in the valve holder  58  an axially movable covering body  59  on whose underside the upper end of spring  52  abuts and on whose upper side, as long as filter insert  3  is in place in the filter, spring support  53  abuts with its support arms  53 ′, which hold covering body  59  in a position at a distance from the underside of the upper end of the valve holder. In this position, covering body  59  releases a liquid passage  58 ″ in an upper end wall of the valve holder  58  for filtered liquid in the direction toward outlet  12 . In the depicted example, liquid passage  58 ″ is formed by a plurality of smaller openings distributed in the circumferential direction, which together provide the required flow cross-section. 
     When filter insert  3  is removed from filter  1 , spring  52  presses covering body  59  against the end wall of valve holder  58  and against liquid passage  58 ″ situated there, and closes this passage. This prevents dirt particles from being able to fall into filtered liquid outlet  12  during a filter maintenance. 
     Moreover, covering body  59  ensures an advantageous distribution of the forces acting between valve spring  52  and spring support  53 . 
     Covering body  59  can also be omitted if, for the provided use of filter  1 , the protective function against dirt falling in and the function of distribution of forces can be done without. 
     Finally, at the upper end region of valve holder  58  another radially inward-pointing stop  58 ′ is integrally formed, which ensures a limitation of the movement upward, i.e., away from valve seat  50 , of covering body  59  and of the upper end of spring  52 . 
     Supporting element  37  again has spring support  53 , here in the form of a plurality of supporting arms  53 ′ pointing downward in the axial direction and distributed around the inner circumference of supporting element  37 . Due to the angled course of the sectional plane, in  FIG. 67  only one of supporting arms  53 ′ is visible in the left half of  FIG. 67 . Guided by positioning means described below, when filter  1  is assembled each support arm  53 ′ extends from above through a fittingly positioned and dimensioned introduction opening  53 ″ in the upper side of valve holder  58 , into the valve holder, thus coming to abut covering body  59 , or, if this covering body is not present, immediately on the upper end of spring  52 . In this way, support arms  53 ′ and introduction openings  53 ″ form a key-lock coding that permits only the installation of a fitting filter insert  3  in filter  1 . 
     When filter housing  2 , in which filter insert  3  has previously been placed, is screwed onto base plate  16 , filter insert  3  moves, together with housing  2 , in the direction toward base plate  16 , causing support arms  53 ′ of spring support  53  to gradually prestress spring  52  more and more strongly, until housing  2  has reached its end position, and spring  52  has thereby reached its final prestressing. Spring  52  now presses valve body  51  in the closing direction against valve seat  50  with a specifiable force. All parts of filter bypass valve  5  are configured so as to be fixed to the filter, i.e., are not parts of exchangeable filter insert  3 . Only spring support  53  is part of exchangeable filter insert  3 . 
     The valve membrane of return check valve  15  is clamped at its radially inner edge region between base plate  16  and lower holder part  58 . 1  of valve holder  58 , which makes separate holding means for return check valve  15  dispensable. 
     During operation of filter  1 , liquid to be filtered flows through an unfiltered liquid duct  62  in the radially outer part of the coupling flange  60 , and through inlet  11 , as well as through return check valve  15 , to an unfiltered side  13 , external to filter insert  3 , of filter  1 . From there, the liquid flows, while depositing solid particles, through filter material body  30  of filter insert  3 , in the radial direction inward to filtered side  14  of filter  1 . From there, the filtered liquid flows through the interior of supporting element  37 , through liquid passage  58 ″ in valve holder  58 , and then through central outlet  12  and through the interior of threaded nipple  63 , into a filtered liquid duct  61  in coupling flange  60 . 
     If the pressure difference between unfiltered side  13  and filtered side  14  of filter  1  exceeds a specifiable boundary value, valve body  51  is lifted off from its valve seat  50 , against the force of valve spring  52 , and an immediate flow path is released for the liquid from unfiltered side  13  to filtered side  14 , bypassing filter material body  30  of filter insert  3 . 
     When there is a standstill of the liquid circuit, return check valve  15  ensures that no liquid flows out from the interior of filter  1 , so that, when there is a restart of the liquid circuit, an immediate supply of filtered liquid to downstream consumers is ensured. 
       FIG. 68  shows filter  1  of  FIG. 67  in cross-section along sectional line B-B in  FIG. 67 . Externally, in  FIG. 68  filter housing  2  is visible, whose circumferential wall  20  is here sectioned. Radially inward therefrom is situated filter insert  3 , through whose filter material body  30  the section runs. Distributed around the circumference of filter insert  3 , the four connecting webs  33  are visible from above, extending further downward in the axial direction. In the interior of filter material body  30 , there is situated grid-type supporting element  37 , which supports the filter material body during operation. Further radially inward therefrom is situated valve holder  58 , in which the parts of filter bypass valve  5  are situated, of which here only a part of valve spring  52  and, in the center, a part of valve body  51  are visible. In  FIG. 68 , sectional line A-A illustrates how the longitudinal section shown in  FIG. 67  runs through filter  1 . 
       FIG. 69  shows detail V of filter  1  of  FIG. 68  in an enlarged representation. At the upper right in  FIG. 69 , a part of filter material body  30  of filter insert  3  can be seen. Under it, a segment of supporting element  37  is visible. Further downward, a segment of valve holder  58  is then visible; here it can be seen that the valve holder  58  is made up of a first holder part  58 . 1  and a second holder part  58 . 2  which are plugged into one another over a part of their height. The two holder parts  58 . 1  and  58 . 2  are connected to one another by locking connections  58 . 3 , of which one is visible in section in  FIG. 69 . Further downward, finally, a part of valve body  51  is visible. 
       FIG. 70  shows filter  1  of  FIG. 67  in cross-section according to sectional line D-D in  FIG. 67 . Here as well, radially outwardly filter housing  2  is again visible, whose circumferential wall  20  is sectioned. Radially inwardly therefrom there is again situated filter insert  3  with filter material body  30 , and connecting webs  33  extending axially downward. Radially inward from filter material body  30  there is situated the associated supporting element  37 . Radially inwardly therefrom in turn there is situated valve holder  58 . As  FIG. 7  illustrates, valve holder  58  has, on its upper side visible here, a number of introduction openings  53 ″ through which support arms  53 ′ of spring support  53  of filter insert  3  extend into the interior of valve holder  58 . Moreover, in addition to introduction openings  53 ″ in the upper side of valve holder  58  there is present a liquid passage  58 ″ in the form of a plurality of openings distributed in the circumferential direction, through which filtered liquid flows during operation of filter  1 . 
       FIG. 71  shows detail U of filter  1  of  FIG. 70 , in an enlarged representation. At bottom right, a part of filter material body  30  can be seen. Radially inwardly, i.e., in  FIG. 71  above this at the left, a part of supporting element  37  is visible. Radially inwardly therefrom, upper part  58 . 2  of the valve holder  58  is visible, in whose upper side introduction openings  53 ″ for support arms  53 ′ and the openings forming liquid passage  58 ″ can be seen. Here, introduction openings  53 ″ and support arms  53 ′ form a key-lock coding that permits only the installation of a particular filter insert  3 , having support arms  53 ′ in a fitting configuration, number, and shape, into filter  1 . 
       FIG. 72  shows filter  1  of  FIG. 67  in a second longitudinal section, rotated relative to  FIG. 67 , along sectional line C-C in  FIG. 68 . In the interior of the valve holder  58 , here valve body  51 , valve spring  52 , and covering body  59  can be seen. Support arms  53 ′ of spring support  53  of filter insert  3  abut the upper side of covering body  59 , and via this upper side compress spring  52 . As a result, spring  52  exerts a specifiable force acting in the closing direction on valve body  51  of filter bypass valve  5 . At the upper right on valve holder  58 , an opening of liquid passage  58 ″ is visible through which filtered liquid flows during operation of filter  1 . 
     With regard to the further individual parts and reference characters appearing in  FIG. 72 , reference is made to the preceding description. 
       FIG. 73  shows detail X of filter  1  of  FIG. 72  in an enlarged representation. At the bottom, a small part of base plate  16  can be seen, to whose upper side first holder part  58 . 1  of valve holder  58  is fixedly connected, e.g., welded. At lower left in  FIG. 73 , valve seat  50  is visible, fashioned as part of first holder part  58 . 1 . Valve body  51 , in its closed position, here abuts valve seat  50 , valve body  51  having on its surface facing the valve seat  50  an elastomeric coating  57  in order to improve the sealing effect in the closed position. Spring  52 , in the form of a helical pressure spring, runs around an upper part of valve body  51 . Radially externally, i.e., here to the right of first holder part  58 . 1 , a part of second holder part  58 . 2  can be seen, holder parts  58 . 1  and  58 . 2  being connected to one another by locking connections  58 . 3 , of which one is visible in  FIG. 73 . 
     Radially outwardly from valve body  53 , i.e., to the right thereof in  FIG. 73 , a small part of filter insert  3  is visible with filter material body, lower end plate  31 , sealing collar  35 , and grid-type supporting element  37 . Opening  31 ′, by which filter insert  3  is placed in sealing fashion onto second holder part  58 . 2  of valve holder  58 , is situated centrally in lower end plate  31 . 
     At lower right in  FIG. 73 , finally, a part of return check valve  15  is visible, whose valve membrane  15 ′ is clamped, at its radially inner edge region, in sealing fashion between base plate  16  and holder part  58 . 1  of valve holder  58 . 
       FIG. 74  shows filter  1  of  FIG. 72  in cross-section along sectional line I-I in  FIG. 72 . Radially outwardly, filter housing  2  of filter  1  is situated, circumferential wall  20  of housing  3  here again being sectioned. Further radially inward, filter insert  3  is visible with filter material body  30 , here sectioned, supported radially inwardly by supporting element  37 . Radially inward from supporting element  37 , valve holder  58  is visible, in which covering body  59  is situated. In the exact center of  FIG. 74 , finally, valve body  51  is partially visible from above. 
       FIG. 75  shows detail W of filter  1  of  FIG. 74  in an enlarged representation. At left, a part of filter material body  30  of filter insert  3  is visible. To the right thereof runs supporting element  37 . Further to the right, there then follow segments of valve holder  58  and of covering body  59 , which is displaceably guided in the valve holder  58  axially, i.e., perpendicular to the plane of the drawing of  FIG. 75 , but is secured against rotation in the circumferential direction by interlocking contours. 
       FIG. 76  shows filter  1  in the same representation as in  FIG. 67 , but here with filter bypass valve  5  in the open position. The state of filter bypass valve  5  shown in  FIG. 76  results when in filter  1  a pressure difference prevails between the unfiltered side  13  and filtered side  14  that exceeds a specifiable boundary value, for example when filter material body  30  of filter insert  3  is clogged with previously filtered-out dirt particles. In this case, the liquid at the unfiltered side  13  of filter  1  exerts on valve body  51  of filter bypass valve  5  a force acting in the opening direction that is greater than the force exerted in the closing direction by valve spring  52 . The force acting in the opening direction displaces valve body  51  in the direction away from its valve seat  50 , thus releasing an immediate flow connection from unfiltered side  13  to filtered side  14  of filter  1 , through a valve through-opening  50 ′, bypassing filter material body  30 . As soon as the pressure difference between unfiltered side  13  and filtered side  14  falls below the specifiable threshold value, the force of valve spring  52  again predominates, so that valve body  51  is then moved back into its closing position, in which it is seated in sealing fashion on valve seat  50 . 
     With regard to the further individual parts and reference characters in  FIG. 76 , reference is made to the preceding description. 
       FIG. 77  shows detail Y of filter  1  of  FIG. 76  in an enlarged representation. At the right in  FIG. 77 , valve body  51  with elastomeric coating  57  is partly visible, having a distance from its valve seat  50 , so that valve through-opening  50 ′ of filter bypass valve  5  is released. Thus, here there is an immediate flow connection from unfiltered side  13  to filtered side  14  of filter  1 . At the top in  FIG. 77 , a part of valve spring  52  is visible, here compressed by the force produced by the pressure difference between unfiltered side  13  and filtered side  14 . To the left of valve body  51  and spring  52 , holder parts  58 . 1  and  58 . 2  of valve holder  58  are visible. Further to the left thereof, a small part of filter insert  3  is also visible. Finally, at bottom in  FIG. 77  a small part of base plate  16  is visible, bearing at its upper side valve holder  58  with filter bypass valve  5  and return check valve  15 , here visible only in small part. 
       FIGS. 78 through 81  show filter coupling flange  60  with base plate  16  attached thereon together with valve holder  58 , and having a filter insert  3 , shown only partially for reasons of clarity, in various phases of the installation of filter insert  3 . Here, of filter insert  3 , only its lower end plate  31 , with connecting webs  33 , sealing bearer  34 , and seal  39 ; are shown in a front view, as well as grid-type supporting element  37  in longitudinal section. 
       FIG. 78  shows filter coupling flange  60  with base plate  16  of filter  1  screwed thereon in a front view, and with filter insert  3  in a first phase of the installation of filter insert  3  during a filter maintenance. As described above, filter insert  3  has a spring support  53  having a plurality of support arms  53 ′ that enter through introduction openings  53 ″ in valve holder  58  into the interior thereof. So that support arms  53 ′ reliably reach the fitting position for this purpose, seen in the circumferential direction, on filter insert  3  there are provided, in addition to support arms  53 ′, first positioning elements  37 ′ that work together with second positioning elements  37 ″ on valve holder  58 . Here, first positioning elements  37 ′ are situated on the inner circumference of supporting element  37 , between support arms  53 ′, and have the form of oblong ribs or webs running in the axial direction and protruding radially inward. Second positioning elements  37 ″ are formed by a respective declining bevel running in the circumferential direction of the valve holder  58  on the upper end of its outer circumference, going over into an axial groove on the outer circumference of valve holder  58 . In  FIG. 78 , filter insert  3  is just being placed, by means of central opening  31 ′ of its lower end plate  31 , into an arbitrary position, regarded in the circumferential direction, from above onto the valve holder  58 ; here support arms  53 ′ and first positioning elements  37 ′ have not yet come into contact with the valve holder  58 . 
       FIG. 79  shows the subject matter of  FIG. 78  in the same representation, here in a second phase of the installation of filter insert  3 . Here, filter insert  3  is moved further downward relative to valve holder  58 , causing first positioning elements  37 ′ to come into contact with second positioning elements  37 ″. Here, first positioning elements  37 ′ abut, with their downward-pointing end surface, seen in the circumferential direction, on some point of the bevels forming a part of second positioning elements  37 ″, at the top on valve holder  58 . 
       FIG. 80  shows the subject matter of  FIGS. 78 and 79  in the same representation, here in a third phase of the installation of filter insert  3 . Here, as a result of a rotation in the circumferential direction usefully carried out together with the rotational movement of the filter housing  2  (not shown here) when it is screwed onto base plate  16 , filter insert  3  has now reached a position in which first positioning elements  37 ′ have arrived at the lower end of the bevels of second positioning elements  37 ″, and now, seen in the axial direction, are positioned exactly over the vertical groove as second part of second positioning elements  37 ″. A further rotation of supporting element  37  with first positioning elements  37 ′ relative to valve holder  58  is now no longer possible. For this reason, supporting element  37  is here situated so as to be capable of rotation in filter insert  3  relative to lower end plate  31  of filter insert  3 . 
       FIG. 81  shows the subject matter of  FIGS. 78 through 80  in the same representation, here in a fourth phase of the installation of filter insert  3 . First positioning elements  37 ′ now enter into the axial grooves, forming part of second positioning elements  37 ″, on the outer circumference of valve holder  58 . At the same time, support arms  53 ′ of spring support  53  enter, through introduction openings  53 ″ provided for them, into the interior of valve holder  58 , and move covering body  59  situated there and explained above downward, thus prestressing valve spring  52  of filter bypass valve  5 . The mutually fitting finding of supporting element  37  and valve holder  58  is thus brought about automatically, without requiring particular attention on the part of maintenance personnel with regard to the mutual positioning of the named parts. 
     Due to the fact that separate positioning elements  37 ′ are provided on filter insert  3 , support arms  53 ′ of spring support  53  are relieved of all forces connected with the positioning, in particular forces acting in the circumferential direction. 
       FIG. 82  shows filter coupling flange  60  with base plate  16  of filter  1  screwed thereon, and with valve holder  58 , in a front view, without filter insert  3  and without filter housing  2 . At bottom in  FIG. 82 , a part of device  6  having filter  1  is shown with coupling flange  60 , to which base plate  16  of filter  1  is connected. Base plate  16  has on its outer circumference outer threading  16 ′, used to screw on filter housing  2  (not shown here). Return check valve  15  is situated radially externally on the upper side of base plate  16 . Valve holder  58 , with filter bypass valve  5  housed therein, is attached centrally on base plate  16 . Valve holder  58  is made up of the two holder parts  58 . 1  and  58 . 2 , connected to one another via locking connections  58 . 3 . On the upper side of valve holder  58  there can be seen introduction openings  53 ″ for support arms  53 ′ of spring support  53 , as well as liquid passage  58 ″ for filtered liquid. The above-described second positioning elements  37 ″ are situated in the upper region of the outer circumference of valve holder  58 . 
     In the example shown, four support arms  53 ′ and four first and second positioning elements  37 ′ and  37 ″ are provided; however, a smaller or larger number is also possible. 
       FIG. 83  shows base plate  16  of filter  1  with valve holder  58  and valve body  51  and having return check valve  15 , but without valve spring  52  and covering body  59 , in a sectioned view. Base plate  16 , visible at bottom in  FIG. 83 , has on its inner circumference inner threading  16 ′, which is used to screw base plate  16  onto coupling flange  60  (not shown here). 
     On its outer circumference, base plate  16  has outer threading  16 ″, which is used to screw on filter housing  2 , also not shown here. Underneath outer threading  16 ″, there runs a radially outward-protruding outer edge projection of base plate  16 . 
     Openings for the supply of liquid to be filtered run through the region of base plate  16  situated between inner threading  16 ′ and outer threading  16 ″. At the upper side, inlet  11  is covered by return check valve  15 . Valve membrane  15 ′ of return check valve  15  is mounted at its inner edge region on base plate  16 , in that a lower edge region of the valve holder  58  clamps the edge region from above. Outlet  12  runs centrally through base plate  16 . 
     Valve holder  58  is fixedly connected to the upper side of base plate  16 . Valve holder  58  is made up of the two holder parts  58 . 1  and  58 . 2 , which are connected to one another via locking connections  58 . 3 . In the interior of valve holder  58 , valve body  51  of filter bypass valve  5  is guided movably in the axial direction. Valve body  51  works together with valve seat  50  fashioned at the bottom on valve holder  58 . 
     In the upper end face of valve holder  58 , introduction openings  53 ″ for support arms  53 ′ and liquid passage  58 ″ for filtered liquid can be seen. On the axially upper end region of the outer circumference of valve holder  58 , at right in  FIG. 83  one of the two positioning elements  37 ″ is visible. 
       FIG. 84  shows base plate  16  of filter  1  with the parts according to  FIG. 83 , and in addition having valve spring  52  and covering body  59 , in longitudinal section. Here a state is shown that arises when filter insert  3  is removed. In this state, valve spring  52  is not prestressed by spring support  53  with its support arms  53 ′, so that valve spring  52  can extend to its maximum length provided by valve holder  58 . This has the result that now covering body  59  has moved into an upward-displaced position in which it abuts a stop  58 ′ at the upper end of valve holder  58 . In this position, covering body  59  closes all the openings forming liquid passage  58 ″ in the upper end face of the valve holder  58 . In this way, it is ensured that in this state a disturbing or damaging falling in of dirt particles, through liquid passage  58 ″ into outlet  12  for filtered liquid, during a filter maintenance is not possible. 
     Outlet  12 , and inner threading  16 ′ situated therein, are again visible in the center of base plate  16 . In the lower region of outlet  12 , elements of anti-rotation lock  67  are visible, which ensures that base plate  16 , after being screwed onto threaded nipple  63  of coupling flange  60  during first assembly of filter  1 , is fixed thereto and can no longer be screwed off. 
     Return check valve  15  is again visible on the radially outer part of the upper side of base plate  16 . 
       FIG. 85  shows filter housing  2  with filter insert  3  placed therein, in a longitudinal section. Radially outwardly, screw housing  2  of filter  1 , with circumferential wall  20 , can be seen. In the interior of filter  1 , filter insert  3  is situated with its above-described individual parts. In the interior of filter material body  30  of filter insert  3 , there is situated supporting element  37 , having on its inner circumference spring support  53  having support arms  53 ′ distributed in the circumferential direction and having first positioning elements  37 ′ integrally formed thereon in one piece. Here, supporting element  37  is introduced from below, through central opening  31 ′ in lower end plate  31 , into the interior of filter insert  3  and of filter material body  30 , and, during operation of filter  1 , supports filter material body  30  radially inwardly against collapse. Here, supporting element  37  terminates at a distance from upper end plate  32 ; alternatively, however, it could also be realized extending up to this end plate  32 . 
     The four connecting webs  33 , spaced uniformly from one another in the circumferential direction, extend downward in the axial direction from the radially outer edge of lower end plate  31  of filter insert  3 . Inner threading  26  is provided on the inner circumference of circumferential wall  20  of filter housing  2 , close to its lower end face  21 . On the radially outward-oriented surface of connecting webs  33 , a respective bulge or locking cam  33 ′ is integrally formed, somewhat above circumferential sealing bearer  34 , which bulge or cam provides an easily detachable locking of filter insert  3  in filter housing  2  in interaction with a locking recess  28 ′ on the inner circumference of circumferential wall  20 . In this way, filter insert  3  is determined in a defined manner in its position relative to screw housing  3 , by means of its connecting webs  33 . 
     At their axially lower end, connecting webs  33  go over into circumferential sealing bearer  34 , here made in one piece with the webs, on which bearer sealing ring  39  is situated. 
     The unit shown in  FIG. 85  of filter housing  2  and filter insert  3  is screwed off from coupling flange  60 , or from base plate  16  permanently attached thereon, when there is a filter maintenance. All parts of filter bypass valve  5 , with the exception of spring support  53 , remain on base plate  16 , and are thus elements fixed to the filter. Subsequently, used filter insert  3  can be withdrawn from housing  2  and replaced by a fresh filter insert  3 . Because seal  39  is part of filter insert  3 , seal  39  is here automatically also replaced. After screwing the unit made up of housing  2  and fresh filter insert  3  onto base plate  16 , filter  1  is again ready for operation. 
     With regard to the further individual parts and reference characters shown in  FIG. 85 , reference is made to the preceding description. 
       FIG. 86  shows a filter bypass valve  5  of filter  1  in a modified embodiment, together with a coupling flange  60 , in longitudinal section. This filter bypass valve  5  is provided for filter  1  having a central pipe socket  4  made up of two pipe socket segments  4 . 1  and  4 . 2 , as described above. In  FIG. 86 , only flange-side pipe socket segment  4 . 1  is visible, here provided in its lower end region  40  with an outer screw threading  40 ′ with which pipe socket segment  4 . 1  is permanently screwed into an inner threading  61 ′ centrally in coupling flange  60 . A threaded nipple, as provided in some exemplary embodiments described above, is thus not present here. 
     On a center region, seen in the longitudinal direction, of pipe socket segment  4 . 1 , annular valve seat  50  is seated so as to be non-displaceable in the axial direction, the valve seat having through-openings  50 ′ that run parallel to the longitudinal direction of pipe socket segment  4 . 1 . On the upper side of valve seat  50  there is situated the also annular valve body  51 , guided in displaceable fashion on pipe socket segment  4 . 1 , in the axial direction thereof. On its side facing the valve seat  50 , valve body  51  has an elastomeric coating  57  for improving the sealing effect in the closed state of filter bypass valve  5 . Above valve body  51  is situated spring  52  surrounding pipe socket segment  4 . 1 , which spring here abuts stop  43  with its upper end, in the absence of filter insert  3 . At the upper side, stop  43  has a plurality of roof-shaped introductory bevels  46  situated in the circumferential direction of pipe socket segment  4 . 1 , between which there are respectively situated intermediate spaces  44  for guiding through support arms  53 ′ of spring support  53 . 
     As  FIG. 86  further illustrates, in its depicted closed position valve body  51  covers perforations  42  made in pipe socket segment  4 . 1 . When valve body  51  is in its open position, i.e., is displaced upward relative to valve seat  50 , perforations  42  are at least partly released, whereby a very short immediate flow path from inlet  11  to outlet  12  is released, bypassing filter insert  3 . 
       FIG. 87  shows filter  1  in a further embodiment, in longitudinal section. In large part, filter  1  corresponds to the exemplary embodiment according to  FIG. 66 ; in particular, filter housing  2  and filter insert  3  are identical in the two examples. Different in these two exemplary embodiments is that filter  1  according to  FIG. 87  does not have a base plate, and is designed for a coupling flange  60  not having a central threaded nipple. Instead of central threaded nipple, coupling flange  60  in  FIG. 87  has a collar having an outer threading  61 ′ onto which an inner threading  26  on filter housing  2  can be screwed. This screw connection is sealed by sealing ring  39 . 
     As do all other exemplary embodiments, filter  1  according to  FIG. 87  also has a filter bypass valve  5 , here again situated in a valve holder  58 . Valve holder  58  is here again realized in two parts, having a first, lower holder part  58 . 1 , and a second, upper holder part  58 . 2 , which here are for example sheet metal parts. Lower holder part  58 . 1  has a downward-protruding hollow cylindrical segment that forms outlet  12  and is pressed into a fittingly shaped and dimensioned opening in the center of coupling flange  60 . After first assembly, valve holder  58  is thus seated in a press-fit seating in coupling flange  60 , from which it is then no longer detached in normal operation of filter  1 . In its remaining parts and functions, filter bypass valve  5  in  FIG. 87  corresponds to filter bypass valve  5  as already described in  FIG. 64 . With regard to the other parts and reference characters in  FIG. 87 , reference is made to the preceding description. 
     A return check valve  15  is here attached immediately on coupling flange  60 , and is thus here not an integral component of filter  1 . 
       FIG. 88  shows filter bypass valve  5  and return check valve  15  of  FIG. 87  in the state connected to coupling flange  60 , in an oblique view from above; here filter housing  2  and filter insert  3  are unscrewed from coupling flange  60 . On the upper side of coupling flange  60 , the collar thereof, with an outer threading  61 ′, is radially outwardly visible. Radially inwardly therefrom, return check valve  15 , here attached immediately on coupling flange  60 , is visible. In the center of coupling flange  60 , valve holder  58  is attached, with filter bypass valve  5  situated therein, of which only spring  52  is immediately visible. 
       FIG. 89  shows filter  1  in a further embodiment in longitudinal section, this filter  1  corresponding in large part to the exemplary embodiment according to  FIG. 87 . Differing from the example according to  FIG. 87 , in the example according to  FIG. 89  a covering body  59  is additionally situated in valve holder  58 . Covering body  59  is situated between the upper end of spring  52  of filter bypass valve  5  and the underside of upper end face of the valve holder  58 . In the assembled state ready for operation of filter  1 , as shown in  FIG. 89 , support arms  53 ′ of spring support  53  of filter insert  3  engage from above through introduction openings  53 ″ of valve holder  58  into this valve holder, and press onto spring  52  via covering body  59 . In this way, spring  52  is prestressed, and exerts a specifiable preloading force on valve body  51  of filter bypass valve  5 . At the same time, in this way covering body  59  is held at an axial distance from passages  58 ″ in the upper side of valve holder  58 , making possible a flow of filtered liquid from the interior of filter insert  3  through outlet  12  to filtered liquid duct  61  in coupling flange  60  of device  6 . 
     If, as shown in  FIG. 90 , filter housing  2  is screwed off from coupling flange  60  together with filter insert  3 , then spring  52  presses covering body  59  upward against the underside of the upper end face of the valve holder  58 , and then closes passages  58 ″ situated there. In this way, dirt particles are reliably prevented from falling into outlet  12  and into filtered liquid duct  61 . 
     With regard to the further individual parts and functions of filter  1  according to  FIGS. 89 and 90 , reference is made to the preceding description, in particular of  FIG. 87 . 
       FIG. 91  shows filter  1  in a further embodiment, in a state assembled on a coupling flange  60 , in longitudinal section. Filter  1  has a one-piece central pipe socket  4  that is fixedly connected at its upper end region  41  to flange-remote end face  22  of filter housing  2 , for example by welding. Characteristic for this exemplary embodiment is that central pipe socket  4  has in its lower flange-side end region  40  an inner threading  40 ′ by which filter housing  2  is screwed onto a fitting outer threading  61 ′ on a threaded nipple  63  attached centrally in coupling flange  60 . 
     Filter bypass valve  5  of filter  1  according to  FIG. 91  is situated in the upper region of central pipe socket  4 , and corresponds to the embodiment as described above in  FIGS. 1 and 4 through 6 , to which reference is made in this regard. 
     Finally,  FIG. 92  of the drawing shows filter  1  of  FIG. 91  in a state removed from coupling flange  60 , in longitudinal section. Filter insert  3  is seated with a frictional fit on central pipe socket  4  in the interior of filter housing  2 , so that when filter housing  2  is unscrewed from threaded nipple  63  filter insert  3  is carried along with it. In the state shown in  FIG. 92 , filter insert  3  can be withdrawn downward from filter housing  2 , and replaced by a new, fresh filter insert  3 . Spring support  53 , which prestresses spring  52  of filter bypass valve  5 , is here as well provided on filter insert  3 , specifically on its central supporting element  37 . When filter insert  3  is withdrawn from filter housing  2 , spring  52  is relieved of tension until it abuts stop  43  with its lower end. Stop  43  is here realized in the form of a sheet metal sleeve that is attached in axially non-displaceable fashion on the outer circumference of pipe socket  4 , and that is axially supported at its lower end in some of the perforations  42  made in pipe socket  4 . Thus, spring  52  and valve body  51  of filter bypass valve  5  are here as well secured against loss. 
     Here as well, seal  39  is again part of filter insert  3 , and is situated on a circumferential sealing ring bearer  34 , which is again connected to lower, flange-side end plate  31  of filter insert  3  via connecting webs  33 , or is realized in one piece therewith. 
     As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art. 
     LIST OF REFERENCE CHARACTERS 
     
         
           1  filter 
           10  connecting flange 
           11  inlet 
           12  outlet 
           13  unfiltered side 
           14  filtered side 
           15  return check valve 
           15 ′ valve membrane 
           15 ″ membrane bearer 
           16  base plate 
           16 ′ screw threading internal on  16   
           16 ″ screw threading external on  16   
           17  locking connection between  15 ″ and  50   
           18  spring tongue plate 
           2  filter housing 
           20  circumferential wall 
           20 ′ screw cover 
           21  flange-side end face 
           22  flange-remote end face 
           23  tool attachment projection 
           24  inner diameter step 
           25  threaded connector on  22   
           25 ′ weld seam 
           26  screw threading 
           28  recesses in  25   
           28 ′ locking recesses internal in  20   
           3  filter insert 
           30  filter material body 
           31  lower, flange-side end plate 
           31 ′ central perforation in  31   
           32  upper, flange-remote end plate 
           32 ′ central perforation in  32   
           33  connecting webs 
           33 ′ locking cams on  33   
           34  sealing ring bearer 
           35  seal/sealing lip on  31   
           36  sealed/sealing lip on  32   
           37  inner support body 
           37 ′ first positioning means on  3   
           37 ″ second positioning means on  58   
           38  axial struts of  37   
           39  first sealing ring (outer) 
           39 ′ second sealing ring (inner) 
           4  central pipe socket 
           4 . 1  first, flange-side pipe socket segment 
           4 . 2  second, flange-remote pipe socket segment 
           40  outer end region 
           40 ′ screw threading on  40   
           40 ″ threading on  4 . 1  for  4 . 2   
           41  inner end region 
           41 ′ threading on  41   
           41 ″ threading on  4 . 2  for  4 . 1   
           42  perforations 
           43  stop 
           43 . 1  stop collar 
           43 . 2  stop tongues 
           43 . 3  stop tab 
           44  intermediate spaces 
           45  longitudinal rib on  4   
           46  introductory bevels 
           47  anti-rotation lock 
           47 ′ anti-rotation lock ring 
           48  cam on  47 ′ 
           49  longitudinal guide for  37   
           5  filter bypass valve 
           50  valve seat 
           50 ′ through-opening(s) 
           51  valve body 
           52  spring 
           53  spring support 
           53 ′ support arms 
           53 ″ introduction opening(s) for  53 ′ in  58   
           54  intermediate ring 
           55  longitudinal groove in  51   
           56  inner threading in  50   
           57  elastomeric coating on  50 ,  51   
           58  valve holder 
           58 . 1  first, lower holder part 
           58 . 2  second, upper holder part 
           58 . 3  locking connection between  58 . 1  and  58 . 2   
           58 ′ stop for  52  in  58   
           58 ″ passage 
           59  covering body 
           6  device having liquid circuit 
           6 ′ base 
           60  coupling flange 
           61  filtered liquid duct 
           61 ′ counter-threading in/on  61   
           62  unfiltered liquid duct 
           63  threaded nipple 
           65  stop for  50   
           67  anti-rotation lock