BACKGROUND OF THE INVENTION 
     The invention relates to a liquid filter which is combined with a cooler for the liquid being filtered, and which contains a valve stem with a gasket for closing openings which are intended to facilitate draining the filter housing during an oil change. The invention furthermore relates to a cooler which is suitable for installation in this filter-cooler combination. 
     A filter with valve stem for closing a drain opening is disclosed, for example, in EP 612 549 B1. If one consults FIG. 2, for example, of this document, the principle of the operation of the valve stem will be apparent from the example of its installation. A filter element  16  is installed upright in a pot-like or cylindrical housing  1 . The cylindrical housing is closed by a screw cover  2 , a valve stem  3  being integrated into the cover and sealing at its end an opening for draining any oil remaining in the filter when the filter cartridge is removed. This opening is released whenever the screw cover  2  is opened, since the valve stem is positively guided axially by the screw cover  2 . In this manner a thorough draining of the filter housing can be achieved, and lubricating oil is prevented from passing into the environment when the filter is changed. Also, the total draining permits a complete oil change, which is advantageous since residues would bring contaminants into the fresh oil from the outset. 
     If a filter system of the kind described is combined with a cooler for the liquid being filtered, it is desirable also to drain the oil cooler. Unlike the filter element, however, the oil cooler has a complex geometry, so that a certain amount of residual oil always remains therein. It cannot in this case be removed by using the valve stem to uncover a drain hole in the bottom of the housing, because residual oil will still remain in the cooler element itself. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an improved liquid filter with a cooler. 
     It is also an object of the invention to provide a liquid filter with a cooler which assures a thorough drainage of the old oil before an oil change. 
     Yet another object of the invention is to provide an improved cooler for use with a liquid filter. 
     These and other objects are achieved in accordance with the present invention by providing a liquid filter comprising a cylindrical housing having an upwardly oriented opening, relative to gravity, and a threaded cover screwed onto the housing to close the opening, the housing having a filter inlet and a filter outlet for a liquid to be filtered, a filter cartridge sealingly disposed in the housing between the inlet and outlet so that liquid from the inlet must pass through the filter cartridge to reach the filter outlet, a cooler disposed underneath the filter cartridge in the housing such that liquid which has passed through the filter cartridge can flow through the cooler to the filter outlet, the cooler having an outlet opening, a valve stem extending along the axis of rotation of the threaded cover, the valve stem having a range of axial play relative to the threaded cover which is limited by the threaded cover, and at least one seal on the valve stem which seals the cooler outlet opening, the outlet opening being opened by unscrewing the threaded cover from the housing. 
     In accordance with a further aspect of the invention, the objects are achieved by providing a cooler for installation in a liquid filter as describe above in which the cooler has a drain outlet opening which is closed when the cooler is installed in the liquid filter and the threaded cover is screwed onto the filter housing. 
     The liquid container according to the invention has a pot-like or cylindrical housing whose opening points upward with respect to the force of gravity. The cylinder does not need to be oriented precisely vertical; instead it may be inclined at an angle to the vertical. A filter cartridge is installed in the cylindrical housing through which the liquid to be filtered can flow from a filter inlet to a filter outlet. A cooler is disposed beneath the filter cartridge. It has an entry for the filtered liquid, which communicates with the clean side of the filter cartridge. This can be achieved by also installing the cooler in the cylindrical housing of the filter and having it communicate through gaskets with the appropriate connections. Another possibility is to arrange the cooler under the filter housing, in which case the housing as well as the cooler must have connections communicating one with the other. These connections must assure the circulation of the liquid through the filter cartridge and cooler, and provide for the circulation of a coolant in the cooler. In both situations, however, the cooler has a drain opening which is closed by a gasket. This gasket is applied to a valve stem, which is connected to the screw cap of the housing such that its axial play is limited by this connection. A defined axial play can be provided, which also includes the case where the axial free play equals zero. To attach the valve stem to the screw cover, a catch or snap fastener, for example, can be provided, which permits a certain axial free play. Another possibility is to attach the valve stem permanently to the screw cover, for example by adhesively bonding the valve stem to the cover, or even to manufacture the valve stem in one piece with the screw cover. 
     In any case, the result of the axial fastening of the valve stem to the screw cover is that, when the cover is unscrewed, the gasket is moved away from the drain opening in the cooler. The drain opening is located on the cooler such as to promote its drainage. If this cooler is used, for example, for the lubricating oil circuit of an internal combustion engine, when the oil is changed, the cooler can be completely or at least largely emptied by means of the drain opening, so that the fresh oil is not immediately contaminated by residues of the used oil. In this way longer engine service life can be achieved, as well as longer intervals between oil changes, so that the reliability of operation of the internal combustion engine increases, and greater economy in the operation of the internal combustion engine is achieved. 
     During the operation of the cooler, the seal or gasket effectively prevents any bypassing of the cooler passages, which would reduce its cooling efficiency. Since the cooler has no filtering function, however, perfect sealing is not necessary, and a slight residual leakage may be technically acceptable. Therefore the gasket does not need to satisfy any stringent requirements. The gasket can be made from an elastomer, but it is also possible to create it by an appropriate geometrical configuration of the valve stem. 
     In accordance with another embodiment of the invention, the valve stem has an additional gasket which sealingly closes a drain opening in the housing. Thus the drain opening is likewise opened by the removal of the screw cover and can promote the drainage of the entire filter housing. At the same time the drain opening can also be used to carry out of the housing the liquid issuing from the cooler. 
     One particular preferred embodiment envisions a center tube provided in the cooler and the drain opening is provided in this central tube. The valve stem can then extend into this central tube and seal the drain opening there. If the cooler a plate type cooler, the center tube has additional advantages during manufacturing. It passes through the individual plates and is soldered to them in order to create spaces in the cooler for the liquid being cooled as well as for the cooling liquid. 
     The central tube as well as the valve stem must be arranged in the axis of rotation of the screw cover. This assures that the rotation of the screw cover will not cause any collision of the valve stem with parts in the interior of the filter housing. By rotating the screw cover, the necessary axial displacement of the valve stem is achieved, which results in the opening of the drain. 
     Another possibility is to provide the drain opening in the bottom plate of the cooler. In this case, however, the cooler must be inserted as a component into the housing, while at the same time an intermediate passage is formed which is constituted partly by the bottom plate of the cooler and partly by the wall of the housing. The drain opening of the cooler leads into this passage, so that opening it permits the liquid residue to be removed from the cooler. The intermediate passage must have a connection either to the outlet or to the drainage opening in the filter housing. With this arrangement an especially fast removal of the liquid from the cooler can be achieved. 
     In accordance with another embodiment of the invention, the valve stem is fastened to a support tube which accommodates the filter cartridge. The support tube must then be fastened in turn to the screw cover in order thereby to limit the axial play of the valve stem. The filter cartridge is fastened on this support tube. This is preferably accomplished by placing the filter cartridge with at least one end plate onto the support tube. The valve stem can be fastened on the support tube with an appropriate means, such as a catch or snap fastener or by adhesive bonding. The support tube and valve stem, however, can also be manufactured in one piece. The advantage of this arrangement consists in a material saving, since the valve stem does not have to be guided all the way to the screw cover. The support tube is a component which has to be provided anyway due to the design. 
     An additional advantageous embodiment is obtained by providing a coupling device between the filter cartridge and the cooler to join these two components sealingly together. The liquid can thus be conveyed from the clean side of the filter cartridge directly to an inlet in the cooler. Advantageously, this coupling device is designed so that the filter cartridge can be placed with its end plate in sealing engagement with the coupling device, e.g. with a tubular stub on the sealing device. In this way a seal is simultaneously assured, since the end plate yields elastically. The other end of the coupling device preferably is disposed on the top plate of the cooler, which simultaneously contains the inlet for the liquid to be cooled. In this manner the different filter cartridges can be fitted to the cooler by modifying only a single component. This has an advantageous influence on the cost of production of a series of filters. 
     A cooler according to the invention is suitable for cooperating with the liquid filter described above. This can be done either by installing it in the filter housing or by building it onto the latter. In the second mode of installation it is necessary to assure that the valve stem can reach from the cylindrical housing into the attached oil cooler. Also, it it is necessary to assure that the liquid running out of the cooler can either be collected or can be fed to the liquid circuit. To be able to be built into the system according to the invention, the cooler requires the outlet opening already described. 
     These and other features of preferred embodiments of the invention, in addition to being set forth in the claims, are also disclosed in the specification and/or the drawings, and the individual features each may be implemented in embodiments of the invention either individually or in the form of subcombinations of two or more features and can be applied to other fields of use and may constitute advantageous, separately protectable constructions for which protection is also claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in further detail hereinafter with reference to illustrative preferred embodiments shown in the accompanying drawings in which: 
     FIG. 1 shows a filter with a pot-like housing and a built-in oil cooler in longitudinal section, and 
     FIGS. 2 through 4 show variants of the detail X of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The filter according to FIG. 1 comprises a casing with a cylindrical housing  10  and a threaded cover  11  which is screwed sealingly onto the housing. A filter cartridge  12  and a cooler  13  are disposed inside the housing  10  such that the oil that is to be filtered and cooled travels the course through the filter indicated by the arrows shown in FIG.  1 . 
     The oil passes through a filter inlet  14  to a raw (i.e. unfiltered) liquid side  15  which is formed by the housing. From there it passes through a filter medium  16  which is mounted by end plates  17  sealingly on a support tube  18  and on a coupling device  19 . After pasing through the filter medium the filtered liquid enters a clean (i.e., filtered) liquid side  20  in the interior of the filter cartridge  12 . The filtered oil is conveyed through the coupling device  19  to an inlet  21  which is contained in a top plate  22  of the cooler  13 . It flows through the cooler in a manner not specifically shown and thus reaches a cooler outlet  23  in the bottom plate  14  of cooler  13 . The cooler outlet  23  communicates with a filter outlet  25  in the housing. The cooler furthermore has conventional connections, not shown, for the cooling water, and these connections can be joined in front of and in back of the plane of drawing to corresponding coolant connections in the base  26  of the housing  10 . Their structural configuration can correspond to that of the filter outlet  25 . 
     To assure the liquid circuit described in normal operation, the various components must be sealed relative to one another. The filter cartridge  12  is sealed by the end plates  17 , which have inherent elasticity, over the support tube  18  and coupling devices  19 . The coupling device  19  has a resilient margin  27  which rests against the top plate  22  of the oil cooler and thus creates the connection between the clean side  20  of the filter cartridge and the cooler inlet  21 . The resilience or elasticity of the margin of the coupling device also provides the sealing action at this location. Alternatively, the coupling device  19  can also be made part of the oil cooler and be soldered to the cooler. This simultaneously solves the sealing problem. The outlet  25  and an intermediate passage  28  are sealed against the bottom plate  14  of cooler  13  by a molded gasket  29 . This is necessary since the interior space formed by the housing is part of the raw side  15  of the filter. 
     An additional function of the filter is provided by the schematically represented bypass valve  30 . If the pressure drop across the filter medium  16  increases to an excessive level, the bypass valve opens in order to assure a supply of lubricating oil to the internal combustion engine. The oil flows through openings  31  in a catch or snap fastener  32  which fastens the support tube  18  to the screw cover  11 , and passes through the bypass valve  30  directly to the clean side  20  of the filter. 
     Due to its limited axial play a, the catch or snap fastener  32  assures that the valve stem  33 , which is attached to or integrated with the support tube  18 , is moved along with the threaded cover  11  when the cover is opened. The valve stem  33  is disposed in a central tube  34  in the oil cooler. From there it extends into a drain opening  35  in the base  26  of the housing. The valve stem  33  has O-rings  36  and  36   z  in the area of the central tube  34  and in the area of the drain opening  35 , respectively, which bear against the walls around them and thus produce a seal. The intermediate passage  28  is completely separated from the liquid circuit and the openings communicating with the housing. The molded gasket  29  additionally contributes to this. As a result, during normal operation of the filter, no oil can pass through an outlet opening  37   a  which is contained in the bottom plate  24  of the cooler, since this opening leads into the sealed intermediate passage  28 . Of course, the intermediate passage  28  will first fill with oil. Thereafter no further flow of oil through the outlet opening  37   a  is possible so long as the drain opening  35  remains sealed off by the valve stem  33  and seal  36   z.    
     The valve stem  33  is located precisely in the axis of rotation  38  established by the thread of the screw cover  11 , so that the valve stem, which is coupled to the screw cover  11  via the center tube  18 , can be removed together with the screw cover  11  from the housing  10 . To do this, the drain opening  35  is first uncovered so that the cooler  13  can empty through the outlet opening  37   a . When the valve stem is withdrawn, the central tube  34  can additionally be used in order to drain the oil contained in the housing, which thereby reaches the drain opening  35 . 
     Another variant embodiment for draining the filter is illustrated in FIG.  2 . An outlet opening  37   b  is contained in the central tube  34  of the cooler  13 , and is sealed from the rest of the housing by an O-ring  36  as well as a disk or washer  39 . If the valve stem is retracted from the central bore by unscrewing the cover, not shown in this figure, the oil in the cooler can flow out through the outlet opening  37   b , which is preferably in the lowermost trough of the oil cooler, not shown, into the housing interior. 
     An additional variant of the cooler drainage is illustrated in FIG.  3 . The outlet opening  37   b  is likewise contained in the central tube  34 . The valve stem  33  carries a first O-ring  36  and an additional O-ring  36   z . The outlet opening  37   b  is arranged between the two O-rings such that O-ring  36  provides a seal relative to the interior of the filter housing, and O-ring  36   z  provides a seal relative to the drain opening  35 . When the valve stem  33  is lifted by unscrewing the cover—not shown—the oil can flow out of the oil cooler  13  through the drain opening  35 . 
     An additional variant embodiment for draining the oil from the filter housing is depicted is FIG.  4 . In this variant, the outlet opening  37   b  again is provided in the central tube  34 , and is closed by a disk or washer  39 . In this embodiment, disk  39  completely covers the outlet opening  37   b . The drain opening  35  is closed by an O-ring  36   z  at the end of the valve stem  33 . In this variant the drain opening  35  can additionally be used in order to assure the drainage of the filter housing. For this purpose a supplemental passage  40  is provided which permits the entire filter housing to be drained through the path provided for the oil in the cooler, even when the cooler  13  is installed. In this manner the drainage speed can be increased, thereby facilitating the maintenance procedure. 
     Of course, it is also possible to provide a plurality of outlet openings  37   a  and  b  on the oil cooler. In this manner too the speed of the drainage of residual oil can be increased. In particular, it is possible to combine outlet openings  37   b  in the central tube with outlet openings  37   a  in the bottom plate of the cooler  13 . 
     The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the described embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equivalents thereof.
A liquid filter, especially including an oil filter cartridge ( 12) which is contained in a pot-shaped or cylindrical housing ( 10) with threaded cover ( 11), into which a cooler ( 13) is preferably installed. An outlet opening ( 37 a) is contained in the oil cooler ( 13) which facilitates complete emptying of the cooler when the filter is changed. In this way, a complete replacement of the oil can be performed, so that the replaced oil is not contaminated from the outset by residues of the old oil. Thus, service intervals can be lengthened. To prevent a short circuit of the outlet opening ( 37 a) during operation, the opening is closed by a valve stem ( 33) coupled to the threaded cover ( 11) and opened by unscrewing the cover.