Abstract:
A filter device may include a filter housing having at least one filter element configured to separate the filter housing into a raw side and a clean side. A bypass valve may control a bypass for circumventing the at least one filter element, wherein the bypass valve may have a valve member for controlling a bypass opening connecting the clean side of a coupling chamber of the bypass valve with the raw side. The valve member may be configured to close the bypass opening via a pretension generated by a closing spring. The bypass valve may be an actuating device for changing the pretension acting on the valve member and the actuating device may be coupled with at least one control chamber and configured to set the pretension acting on the valve member based at least in part on a control pressure of the control chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to German Patent Application 10 2009 038 402.2 filed on Aug. 24, 2009 and PCT/EP2010061685 filed on Aug. 11, 2010, which are hereby incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a filter device, in particular a fluid filter device, preferably an oil filter device, with the features of the introductory clause of Claim  1 . 
       BACKGROUND 
       [0003]    Filter devices of this type can be arranged for example in industrial facilities in a hydraulic circuit, in order to filter impurities out from the hydraulic medium. The hydraulic medium can be a lubricating oil or a coolant. Preferably it is a coolant/lubricant. Lubricants which serve at the same time for cooling can be used for example in processing machines. In such applications, this may concern an open hydraulic circuit in which at least a portion of the coolant/lubricant is introduced directly into the processing zone. In the case of a processing by machining, an intensive contamination of the coolant/lubricant occurs. Such impurities are removed from the coolant/lubricant by means of filter devices which are presented here. The purified coolant/lubricant can then be used again. 
         [0004]    By the filtering operation, filter elements which separate a raw side from a clean side within the respective filter device can gradually become clogged by the separated impurities. A so-called filter cake forms on the raw side on the filter element. Through these accretions, the through-flow resistance of the filter element increases. At the same time also a significant improvement to the filtration effect occurs. 
         [0005]    In order to avoid damage to the filter element or respectively the filter device in the case of too high a pressure rise on the filter element, it is basically possible to provide a bypass for circumventing the filter element, which can be controlled by means of a bypass valve. The bypass valve then opens in the case of an unacceptably high differential pressure between the raw side and the clean side. 
         [0006]    A filter device is known from DE 101 05 612 A1, in which a filter element separates a raw side and a clean side in a filter housing, and in which a bypass valve controls a bypass circumventing the filter element. For this, the bypass valve has a valve member for controlling a bypass opening, which connects fluidically with the clean side a coupling chamber of the bypass valve which is fluidically connected with the raw side. The valve member is driven into a closed position, closing the bypass opening, by a pretension produced by a closing spring. The pretension acting on the valve member can be changed by means of an actuating device. In the known filter device, the pretension can be set in a stationary manner to a desired opening pressure by means of the actuating device, depending on the respective purpose of use of the filter device, before the installation of the filter device. 
         [0007]    Further filter devices in which the opening pressure, at which a bypass valve opens, can be set in a stationary manner for the respective case of application, are known from DE 20 2007 013 216 U1 and from DE 14 36 289 A. 
         [0008]    From FR 15 75 877 A1 a filter device is known which comprises a valve arrangement with a congestion indicator device. The congestion indicator device here has an indicator element which is connected with a membrane mounted between the raw side and the clean side. A congestion of the filter element leads to a pressure rise on the raw side, which leads to a corresponding adjustment movement of the membrane and hence to a corresponding adjustment movement of the indicator element. 
         [0009]    In hydraulic systems, in particular in a coolant/lubricant circuit, different system pressures can come into use. For example, a load or working operation and an idle operation can be differentiated from each other. In an idle operation, for example, a minimum volume flow can be necessary, in order to be able to maintain a lubrication of the machines which are connected to the hydraulic circuit. In order to consume as little energy as possible here, a comparatively low pressure level prevails in the hydraulic system during such an idle operation. In order to be able to guarantee the low pressure of the idle operation also in the case of a contaminated filter element, the bypass valve must be able to open already at a relative low pressure. During a working operation or load operation, the hydraulic system must also be able to realize the desired cooling in addition to the lubrication. Accordingly, a greater volume flow is necessary. In addition, in the working operation, the contamination of the coolant/lubricant occurs, so that the full filtration effect of the filter element is to be exhausted. Accordingly, it is desirable to use the system pressure which is available for the filtration of the entire volume flow. Consequently, the bypass valve must only open for the working operation at a relatively high opening pressure. Therefore, contradictory requirements arise for the bypass valve. In addition to this there are particular operating conditions, such as for example a cold start of the hydraulic system, in which the respectively used hydraulic medium, in particular an oil, has a higher viscosity than at its operating temperature. 
       SUMMARY 
       [0010]    The present invention is concerned with the problem of providing for a filter device of the type named in the introduction an improved embodiment, which is distinguished in particular in that it provides different opening pressures for the bypass valve at varying system pressures. 
         [0011]    This problem is solved according to the invention by the subject matter of the independent claim. Advantageous embodiments are the subject matter of the dependent claims. 
         [0012]    The invention is based in particular on the general idea of equipping the bypass valve with an actuating device, by means of which in the installed state or respectively during the operation of the filter device the pretension can be changed, by which the valve member is driven into its closed position. By means of the actuating device, in particular forces can be produced dependent on at least one control factor, which for example increase or reduce the closing force of a closing spring, in order to thus be able to vary the opening pressure or respectively the opening pressure difference, starting from which the bypass valve opens the bypass. By means of such an actuating device, therefore the opening pressure difference can be varied for different operating conditions. The pretension can be adapted dynamically to varying operating conditions. In particular it is thereby possible, at a low system pressure to also provide a low opening pressure, whilst at the same time a correspondingly increased opening pressure can be provided for a high system pressure. Therefore, for the operating conditions mentioned, namely idle operation and load operation, distinctly different opening pressures can be realized for the bypass valve, in order to make possible the desired functionality for the respective operating condition. 
         [0013]    To realize this actuating device, which operates dynamically, i.e. during the operation of the filter device, the actuating device can be coupled with a control chamber and can change the pretension acting on the valve member as a function of a control pressure prevailing in the control chamber. 
         [0014]    An embodiment is particularly advantageous here in which the actuating device has an actuating member which separates the control chamber fluidically from the coupling chamber and which is adjustable as a function of a pressure difference between the coupling chamber and the control chamber for changing the pretension acting on the valve member. Therefore, the pressure difference between the coupling chamber and the control chamber can be used as a drive factor or control parameter, in order to adjust the actuating member and hence the pretension. 
         [0015]    For example, in accordance with preferred alternatives, the control chamber can communicate with an environment of the filter housing, so that the control pressure depends on the environmental pressure and varies with the environmental pressure. Alternatively, the control chamber can be coupled with a control device for setting the control pressure, whereby quasi arbitrary control parameters can be drawn upon for changing or adapting the control pressure. In particular then also external parameters can be taken into consideration, which correlate for example from an operating condition of a machine equipped with the filter device. Alternatively, the control chamber can also be closed hermetically, whereby the control pressure can be set statically to a predetermined absolute pressure. In this case, no dynamic adaptation of the control pressure is present. Rather, this concerns an alternative solution, by means of which the control pressure in the control chamber can be specified fixedly or respectively absolutely, depending on the case of application. 
         [0016]    In accordance with an advantageous embodiment, the closing spring can be supported indirectly via a coupling rod or directly on the actuating member. By changing the position of the actuating member, which can be for example stroke-adjustable, the closing spring can be compressed or decompressed, which leads to a corresponding change to the pretension. Such an actuating member can be configured and arranged particularly simply so that it enables the desired change to the spring pretension. 
         [0017]    The actuating member can, for example, be a membrane which separates the coupling chamber from a control chamber. Such a membrane is arranged on the one hand fixedly relative to the filter housing, whilst on the other hand it is movable relative to the filter housing and thereby can adjust a support site of the closing spring indirectly via the coupling rod, or directly. The adjusting of the membrane takes place here as a function of a pressure difference between the coupling chamber and the control chamber. As the coupling chamber communicates with the raw side, the pressure prevailing in the control chamber provides the opening pressure difference. In this respect, the pretension is controlled by the pressure on the raw side. 
         [0018]    According to an advantageous further development, the control chamber can be constructed in a pressure cell, on which likewise the membrane is arranged. With such a type of construction, the control chamber can be, in particular, a completely closed chamber, so that an absolute pressure is fixedly provided, against which the pressure in the coupling chamber works, in order to adjust the membrane. 
         [0019]    In an alternative embodiment, the actuating member can be configured as a piston which is arranged in a stroke-adjustable manner in a cylinder. Through the adjustability of the piston relative to the filter housing, therefore also the support site of the closing spring can be adjusted, in order to vary its pretension. The piston can also separate a coupling chamber from a control chamber in the cylinder. The said control chamber can—as previously in the case of the pressure cell—be sealed hermetically with respect to the exterior. Likewise, it is possible to connect the control chamber via at least one connection opening with an, in particular atmospheric, environment of the filter housing in a communicating manner. In this case, the pressure prevailing in the coupling chamber works against the environmental pressure. Furthermore, in an alternative embodiment, it is also possible to act on the control chamber with a control pressure via a control pressure connection. Such a control pressure can then be specified in a quasi arbitrary manner. For example, a control device can be provided, which is connected by means of a corresponding control pressure pump, in order to generate control pressures of different extent, which can then be supplied to the control chamber. The control device can be connected for example via at least one pressure sensor with the hydraulic system, in particular with the raw side of the filter housing or with the clean side of the filter housing, in order to be able to set the control pressure as a function of the measured pressures. 
         [0020]    Further important features and advantages of the invention will emerge from the subclaims, from the drawings and from the associated description of the figures with the aid of the drawings. 
         [0021]    It shall be understood that the features mentioned above and to be further explained below are able to be used not only in the respectively indicated combination, but also in other combinations or in isolation, without departing from the scope of the present invention. 
         [0022]    Preferred example embodiments of the invention are illustrated in the drawings and are explained in further detail in the following description, wherein the same reference numbers refer to identical or similar or functionally identical components. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    There are shown, diagrammatically in each case: 
           [0024]      FIG. 1-7  respectively a highly simplified longitudinal section through a filter device in the region of a bypass valve in different embodiments, in which within the respective figure, a closed state (a) is illustrated on the left, and an open state (b) on the right. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In accordance with  FIG. 1-7 , a filter device  1 , which is preferably a fluid filter device and in particular an oil filter device, comprises at least one filter element  2  which is arranged in a filter housing  3  and separates there a raw side  4  from a clean side  5 . In the example, the filter element  2  is configured as a ring filter element, which extends cylindrically with respect to a longitudinal central axis  6 . In accordance with arrows  7 , the filter element  2  is flowed through radially from the exterior towards the interior, so that the raw side  4  is arranged externally and the clean side  5  is arranged internally. Furthermore, the filter device  1  is equipped with a bypass valve  8 , which controls a bypass, indicated by arrows  9 , which circumvents the filter element  2 , i.e. with an opened bypass valve  8  connects the raw side  4  with the clean side  5 . The bypass valve  8  has a valve member  10 , by means of which a bypass opening  11  is able to be controlled. Here, the valve member  10  closes the bypass opening  11  in the closed position, illustrated on the left, whereas in the open position, illustrated on the right, it more or less frees the bypass opening  11 . To realize the desired barrier effect, the valve member  10  can have a comparatively rigid valve plate  12 , which on its side facing the bypass opening  11  carries a seal  13  which is configured in the example as a sealing plate. The bypass opening  11  can also have a bead-like opening edge  14 , which forms a valve seat with which the seal  13  of the valve member  10  cooperates in the closed position. With an opened valve member  10 , the bypass opening  11  connects a coupling chamber  15  of the bypass valve  8  with the clean side  5 , i.e. here with the interior of the filter element  2 . The coupling chamber  15  communicates in turn via connection openings  16  with the raw side  4 . 
         [0026]    The bypass valve  8  has in addition a closing spring  17 . The closing spring  17  serves to prestress or respectively drive the valve member  10  with an elastic- or closing force into the closed position. In  FIG. 1  the elastic force is designated by F D . In the embodiments which are shown here, the closing spring  17  is respectively configured as a helical spring. Basically, however, other spring forms are also conceivable, such as for example a plate spring. In the embodiments of  FIG. 1-3 , the closing spring  17  is configured as a tension spring, so that it introduces a tensile force, acting in the closing direction, onto the valve member  10 . In contrast thereto, in the embodiments of  FIG. 4-7 , the closing spring  17  is configured as a compression spring, which introduces a compressive force, acting in the closing direction, onto the valve member. 
         [0027]    The bypass valve  8  is, in addition, equipped with an actuating device  18 , by means of which a pretension, driving the valve member  10  into the closed position and which is produced in whole or in part by the elastic force of the closing spring  17 , can be altered. For this, the actuating device  18  preferably has an actuating member  19 , which is adjustable as a whole or only in an actuating region, in particular is stroke-adjustable. The closing spring  17  is supported on this actuating member  19 . The closing spring  17  can be supported directly on the actuating member  19  here, as in the embodiments of  FIG. 1-3 . Alternatively, the closing spring  17  can also be supported indirectly on this actuating member  19  via a coupling rod  20 , as in the embodiments of  FIG. 4-7 . 
         [0028]      FIG. 1-3  show in addition an embodiment in which the closing spring  17  is supported on the one hand on the actuating member  19  and on the other hand on an abutment  21 , which is configured fixedly relative to the bypass opening  11 . In these embodiments, the coupling rod  20  serves to connect the actuating member  19  securely with the valve member  10 . Consequently, an adjustment of the actuating member  19  leads directly to an adjustment of the valve member  10 . The forces acting on the actuating member  19  are also transferred here directly to the valve member  19  via the coupling rod  20 , whereby accordingly the pretension acting on the valve member  10  changes. 
         [0029]    In contrast to this, in the embodiments of  FIG. 4-7  the closing spring  17  is supported on the one hand on the valve member  10  and on the other hand on the coupling rod  20 . In addition, in these embodiments the coupling rod  20  is connected securely with the actuating member  19 , so that an adjustment of the actuating member  19  leads to an adjustment of the coupling rod  20 , which changes the position of the support site of the closing spring  17  on the coupling rod side and thereby directly changes the pretension of the closing spring  17 . In the embodiments shown here, the valve member  10  is arranged so as to be stroke-adjustable along the coupling rod  20 . For this, the coupling rod  20  penetrates the respective valve member  10 . In addition, in these embodiments, the closing spring  17  is arranged on a side of the valve member  10  facing away from the actuating member  19 . In contrast to this, in the previously explained embodiments of  FIG. 1-3  the closing spring  17  is arranged on the same side of the valve member  10  on which the actuating member  19  is also situated. 
         [0030]    In the embodiments of  FIGS. 1 and 4 , the actuating member  19  is configured as membrane  22 . The membrane  22  is connected on the one hand securely with the coupling rod  20  and on the other hand is fixedly arranged with respect to the filter housing  3 . It is flexible in so far as that the central actuating region of membrane  22 , which is securely connected with the coupling rod  20 , is adjustable with respect to the filter housing  3 , in particular is stroke-adjustable. For example, the membrane  22  is produced from a metal sheet. Likewise, the membrane  22  can be produced from plastic. The membrane  22  separates the coupling chamber  15  from a control chamber  23  in a pressure-tight manner, so that the pressure difference between the coupling chamber  15  and the control chamber  23  acts on the membrane  22  and brings about its adjustment. 
         [0031]    In the embodiments of  FIGS. 1 and 4 , the said control chamber  23  is constructed in a pressure cell  24 , on which the membrane  22  is also arranged. The pressure cell  14 , together with the membrane  22 , completely encloses the control chamber  23 , so that the latter is sealed hermetically with respect to the exterior. In accordance with the preferred embodiments shown here, the pressure cell  24  has, in addition, a dividing wall  25 , which contains the bypass opening  11 . In addition, the coupling chamber  15  is formed in the interior of the pressure cell  24 . In this respect, the pressure cell  24  contains the complete bypass valve  8 . In other words, the bypass valve  8  is integrated into the pressure cell  24 . The pressure cell  24  and the bypass valve  8  thereby form a valve assembly  26 , which is a separate component with respect to the filter housing  3 , and in particular is able to be mounted independently of the filter housing  3 . In the embodiments of  FIGS. 1 and 4  shown here, this valve assembly  26  is arranged on the filter element  2 . The filter element  2  and the valve assembly  26  form separate components here, which are installed adjacent to each other. For example, the filter element  2  can have an inner edge  27 , on which a filter material  28  rests radially internally. The filter material  28  is arranged axially on an end disc  20 , which is radially open and against which the valve assembly  26  is installed. For example, the valve assembly  26  is inserted axially with a cylindrical section of the pressure cell  24 , having the dividing wall  25 , into the opening of the end disc  29  or respectively into the inner edge  27 . Likewise it is possible to form the valve assembly  26  integrally on the filter element  2 . For example, the end disc  29  can be formed integrally on the pressure cell  24 . 
         [0032]    Irrespective of whether the valve assembly  26  is installed onto the filter element  2  or is formed integrally thereon, the valve assembly  26  and the filter element  2  can form a valve/filter unit  30 , which is able to be completely pre-assembled and can be incorporated as a unit into the filter housing  3 , and is also exchangeable as a unit. This simplifies the mounting of the valve device  1 . 
         [0033]    In the embodiments of  FIG. 2 ,  3 ,  5 - 7 , the actuating member  19  is configured as piston  31 . This piston  31  is arranged here so as to be stroke-adjustable in a cylinder  32 . In the examples presented here, the cylinder  32  is formed integrally in the filter housing  3 . Likewise, a constructed embodiment is also basically conceivable, in which the cylinder  32  is formed in a separate housing, which is incorporated into the filter housing  3 . 
         [0034]    In accordance with the embodiments of  FIGS. 2 ,  5  and  6  in the cylinder  32  the piston  31  can, as above the membrane  22 , separate the coupling chamber  15  from a control chamber, which is likewise designated by  23 . Depending on the pressure difference between the control chamber  23  and the coupling chamber  15 , a stroke or respectively the actuating force for the piston  31  is produced, which has a direct effect on the position of the valve member  10  in the embodiments of  FIGS. 2 and 3 , and which in the embodiments of  FIGS. 5 and 6  affects the pretension of the closing spring  17 . 
         [0035]    In the embodiment shown in  FIG. 2 , the control chamber  23  is connected in a communicating manner via at least one connection opening  33  with an environment  23  of the filter housing  3 . The said environment  23  is generally atmospheric, so that approximately 1 bar environmental pressure prevails. 
         [0036]    In the embodiments of  FIGS. 3 ,  5  and  6  on the other hand, at least one control pressure connection  35  is provided, via which the control chamber  23  can be acted upon with a control pressure. For this, in accordance with  FIG. 5 , a control pressure pump  36  can be provided, which is connected in a suitable manner to the control pressure connection  35  and which is connected on the suction side for example again to the environment  34 . To actuate the pump  36 , a control device  37  can be provided, by means of which the control pressure P 3  is able to be set. For this, the control device  37  receives input signals on the input side via corresponding control lines, depending on which input signals it actuates the pump  36 . For example, the control device  37  can be connected to a pressure sensor  38  which detects the pressure on the raw side. In this way, the control pressure, i.e. the pressure in the control chamber  23 , can be set as a function of the pressure on the raw side. The control pressure can be produced here pneumatically or hydraulically. 
         [0037]    In the embodiments of  FIGS. 2 ,  3  and  5 - 7 , in which the cylinder  32  is formed integrally in the filter housing  3 , the cylinder  32  is closed on a side facing away from or at a distance from the valve member  10  by means of a closure  39 , which for this can for example be screwed into the filter housing  3 . A seal is designated here by  40 , which can be arranged axially between the closure  39  and the filter housing  3 . 
         [0038]    In  FIG. 2  the connection opening  33  is guided through a wall of the filter housing  3 . In the embodiments of  FIGS. 5 and 6 , the control pressure connection  35  is guided through the wall of the filter housing  3 . In the embodiment shown in  FIG. 3 , the control pressure connection  35  is guided through the closure  39 , whereby the closure  39  has a dual function, because on the one hand it closes the cylinder  32  and on the other hand it has the pressure connection  35 . 
         [0039]    The embodiment shown in  FIG. 6  differs from the embodiment shown in  FIG. 5  in particular by an actuating member spring  41 . It serves to pretension the actuating member  19 , here the piston  31 , in a direction which intensifies the pretension which is produced by means of the closing spring  17 . For this, the actuating member spring  41  is arranged in the control pressure chamber  23  and thereby increases the force acting from the control pressure chamber  23  onto the piston  31 . 
         [0040]      FIG. 7  shows an embodiment in which the piston  31  in the cylinder  32  separates a first control chamber  42  from a second control chamber  43 . Here, the two control chambers  42 ,  43  are fluidically uncoupled from the coupling chamber  15 . This means that both control chambers  42 ,  43  have no communicating connection to the coupling chamber  16 . For this, the coupling rod  20  penetrates a guide bore  44  which is introduced in a base of the filter housing  3 . In the region of this guide bore  44 , in addition a seal  45  can be provided, in order to seal the first control chamber  42  with respect to the coupling chamber  15 . The first control chamber  42  is able to be acted upon with a first control pressure via a first control pressure connection  51 . The second control chamber  43  is able to be acted upon with a second control pressure via a second control pressure connection  52 . Through the difference of the two control pressures, the resulting force acting on the piston  31  can be set, in order to be able to increase or reduce the pretension of the closing spring  17  in a targeted manner. Basically, it is likewise possible to leave one of the two control pressure connections  51 ,  52  open with respect to the environment  34 , so that the respective control pressure connection  51 ,  52  then serves as a connection opening to the, in particular atmospheric, environment  34 . 
         [0041]    In the embodiments of  FIG. 4-6 , the actuating device  18  is equipped in addition with an end stop  46 , which is also designated below as first end stop  46 . The first end stop  46  serves to delimit the adjustment path of the actuating member  19  in a direction which intensifies the pretension driving the valve member  10  into the closing position. In  FIG. 4 , this first end stop  46  is formed by a base of the pressure cell  24  lying opposite the membrane  22 . The coupling rod  20  comes to lie on the front face against this first end stop  46 . In the embodiment shown in  FIGS. 5 and 6 , the first end stop  46  is respectively formed on the closure  39  against which the respective piston  31  comes to rest via a stop element  47 . In addition, the embodiments of  FIG. 4-6  show a further or second end stop  48 , which delimits the adjustment path of the actuating member  19  in the opposite direction, i.e. in a direction which reduces the pretension which drives the valve member  10  in its closed position. In the embodiment shown in  FIG. 4 , this second end stop  48  is formed on a sleeve  49  which is arranged coaxially to the coupling rod  20  and which is fixedly arranged with respect to the filter housing  3 . In the example, the sleeve  49  is supported by the dividing wall  25 , for example on the abutment  21 . In the embodiments of  FIGS. 5 and 6 , the second end stop  48  is formed by the abutment  21  which is fixed to the housing, wherein the coupling rod  20  has an annular step  50 , which cooperates with the abutment  21  or respectively with the second end stop  48 . 
         [0042]    The filter devices  1  which are presented here operate as follows: 
         [0043]    In accordance with  FIG. 1 , the filter element  2  is equipped with a fixedly arranged bypass valve  8 . The bypass valve  8  is surrounded by a valve cage which surrounds the coupling chamber  15  and which has the connection openings  16 . A base of the valve cage, facing away from the filter element  2 , is formed by the pressure cell  24 . The membrane  22  is securely connected with the coupling rod  20 , which can also be designated as the shaft of the valve  8 . The coupling rod  20  draws the valve member  10 , configured as a plate, by the force F D  of the closing spring  17  onto the respective valve seat, which is formed here by the opening edge  14  of the bypass opening  11 . The valve member  10  keeps the valve  8  or respectively the bypass opening  11  closed. The pressure cell  24  is acted upon in its control chamber  23  with a control pressure P 3 , which can be 1 bar for example. In normal operation, the filter housing  3  is acted upon on the raw side with an internal pressure P 1 , which can be 10 bar for example. This internal pressure P 1  also acts on the outer side of the membrane  22 , facing the coupling chamber  15 , in the pressure cell  24 . This now produces a force F M , which forms from the difference of the forces F P1 −F P3 . These are the forces resulting from the pressure difference P 1 −P 3 . This resulting force F M  onto the membrane  22  acts in addition to the closing force F D  of the closing spring  17  onto the valve member  10 . If the system pressure now changes, i.e. the pressure on the raw side P 1 , for example in the idle state of the arrangement, for example to 2.5 bar, then the membrane force F M  onto the coupling rod  20  reduces and hence also the opening pressure of the bypass valve  8 . The increase or respectively reduction of the closing force of the valve  8  is calculated from the ratio of the membrane surface to the surface of the valve member  10 . The bypass valve  8  opens when the pressure from P 2  on the clean side  5  of the filter element  2  drops and hence the force F P4  on the valve member  10  exceeds the sum of the forces F P2 , F D  and F M . 
         [0044]    In the embodiment shown in  FIG. 2 , the bypass valve  8  is integrated into the filter housing  3  and is accordingly not changed with the filter element  2 . The function of the membrane  22  in the pressure cell  24  is undertaken here by the piston  31  in the cylinder  32 . 
         [0045]    In the embodiment shown in  FIG. 3 , the control pressure P 3  can be adapted to the operating conditions in a variable manner. The increase of the control pressure P 3  brings about a lowering of the valve opening pressure. This means that the valve member  10  then already opens in the case of a smaller differential pressure between the raw side and the clean side. The control pressure P 3  can be applied hydraulically or pneumatically. 
         [0046]    In the embodiment shown in  FIG. 4 , the spring travel of the closing spring  17  is delimited or respectively defined respectively by a mechanical stop  46  or respectively  48 . In the state of rest or respectively in idle mode, the membrane  22  is drawn by the closing spring  17  against the upper or second stop  48 . A spring travel S 1  is long and thereby the elastic force F 1  is low, so that also the valve opening pressure is low in this case. This situation is illustrated in  FIG. 4   b . The system pressure on the raw side is then P 1.1 . The pressure on the clean side if then P 2.1 . 
         [0047]    When the system pressure now increases on the raw side to an increased value P 1.2 , the membrane  22  is pressed into the pressure cell  24 . Starting from a defined pressure, the end position at the lower or first stop  46  is then reached. On the clean side, the pressure P 2.2  then prevails. A spring travel S 2  is now short and accordingly the elastic force F 2  is high. Therefore, the valve opening pressure is also set high. 
         [0048]    Through the delimited spring travel between the paths S 1  and S 2 , the valve opening pressure can be set to a value associated with the lower system pressure. When the system pressure increases, the closing spring  17  is pretensioned up to the specified position of the membrane  22  at the lower or first stop  46 . Thereby, the higher valve opening pressure is effective. A further increase of the system pressure no longer changes the valve opening pressure. 
         [0049]    The embodiment shown in  FIG. 5  largely corresponds to the embodiment shown in  FIG. 4 , wherein here, however, the pressure control does not take place via the membrane  22 , but rather via the piston  31 . At the same time, provision is made in this embodiment to specify the control pressure P 3  via the control pressure connection  35 . 
         [0050]    In the embodiment shown in  FIG. 6 , the piston  31  is driven with the additionally provided actuating member spring  41  into the upper end position associated with the second end stop  48 , and is held therein until the piston force, applied by the system pressure, exceeds the elastic forces of the actuating member spring  41  and those of the closing spring  17 , and drives the piston  31  in the direction of the first end stop  46 , i.e. here into the lower position. The changeover pressure of the valve  8  is determined here by the piston surface and by the elastic force of the actuating member spring  41 . The transition phase from low to increased valve opening pressure hereby becomes smaller. 
         [0051]    In the embodiment shown in  FIG. 7 , the pretension of the closing spring  17  is changed through the pressure difference between the two control chambers  42 ,  43 . By varying the pressure difference, the position of the piston  31  or respectively its force and hence the pretension acting on the valve member  10  can be set. 
         [0052]    In the example, this takes place by tensioning or respectively relieving of tension of the closing spring  17 , i.e. by changing the elastic force. Alternatively, it is basically also possible in this type of construction to dispense with the closing spring  17  and to produce the pretension, acting and able to be set at the valve member  10 , exclusively via the actuating device  18 . 
         [0053]    In a simple embodiment for both control chambers  42 ,  43  a predetermined control pressure is provided, which is supplied to the respective control chamber  42 ,  43  alternately, whilst the respective other control chamber  42 ,  43  is relieved towards the environment  34 . Thereby, the valve opening pressure can also be varied here between two settings.