Abstract:
A linear valve drive for connection to a valve body having a valve seat and at least one flow passage with at least one inlet and at least one outlet, comprises a drive housing with a drive unit accommodated therein, an actuating element axially shiftable by the drive unit and coupled with a valve closing element to be pressed against the valve seat, and optionally closing the flow passage, and a supporting unit surrounding the actuating element, which is designed to be attached to the valve body. A spring system loaded by actuating the linear valve drive is provided at the supporting unit, which in dependence on the compression path has different spring rates. Furthermore, a correspondingly equipped valve is described.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a linear valve drive for connection to a valve body and a valve with a linear valve drive and a valve body. 
       BACKGROUND 
       [0002]    Prior art valves for controlling or regulating fluids are known, which include a valve body and a linear valve drive formed separate from the valve body, to which the valve body can be connected, in order to form the valve. The linear valve drive typically includes a drive unit which can shift an axially shiftable actuating element on which a valve closing element typically is provided or integrally molded. The drive unit can move the actuating element together with the valve closing element into a closed position, in which the valve closing element rests on a valve seat formed in the valve body and seals the same, so that no fluid can flow through the valve. 
         [0003]    One problem consists in that the tight closing force of the valve can be reduced when the linear valve drive is switched off or is switched powerless. This problem also can occur when the components of the linear valve drive expand for example due to thermal changes, which causes a drive system that was exactly matched previously to have larger tolerances, which can lead to a reduced tight closing force. 
         [0004]    Another problem consists in that the linear valve drive or its drive unit gets jammed or blocked, when the actuating element is shifted into its closed position. This occurs in particular when the drive unit is overloaded. This can be the case, for example, when the actuating element is moved against a stop such as the valve seat or an internal stop in the drive system. 
         [0005]    From EP 2 222 524 B1 a solenoid valve is known, which includes a setting spring for setting a pretensioning force and a regulating spring, which act against each other, in order to provide a cumulative spring force of the spring drive, with which a valve tappet of the solenoid valve is shifted. Both springs have a constant spring rate, so that the cumulative spring force also has a constant spring rate along the compression path. 
         [0006]    It is the object of the invention to provide a linear valve drive and a valve, respectively, with a simple, reliable and inexpensive construction which prevents blocking of the linear valve drive and with which the required tight closing force can be maintained when the linear valve drive is switched off. 
       SUMMARY 
       [0007]    The present invention provides a linear valve drive for connection to a valve body including a valve seat, which also has at least one flow passage with at least one inlet and at least one outlet, wherein the linear valve drive comprises a drive housing with a drive unit accommodated therein, an actuating element axially shiftable by the drive unit, which is coupled with a valve closing element to be pressed against the valve seat and optionally closing the flow passage, and a supporting unit surrounding the actuating element, which is designed to be attached to the valve seat, wherein a spring system loaded by actuating the linear valve drive is provided on the supporting unit, which in dependence on the compression path has different spring rates. 
         [0008]    The idea underlying the invention is to provide a linear valve drive which has a spring system which due to the path-dependent spring rate can provide several safety functions at the same time. The spring system for example can comprise a blocking or overload protection and at the same time maintain a required tight closing force. Due to the changing spring stiffness of the spring system, these different functions can be provided by a single spring system. The construction of the linear valve drive thereby is compact and simple, as the one spring system includes the plurality of functions and provides the same at one common place. The maintenance of the linear valve drive thereby is facilitated in addition. 
         [0009]    In general, the spring rate corresponds to the slope of a function in a force-path diagram. The changing spring rate or spring stiffness of the spring system in dependence on the compression path means that the function of the spring force in a force-path diagram is no straight line, but includes a curve or a discontinuity, i.e. is not linear. 
         [0010]    The linear valve drive is equipped such that the spring system maintains the required tight closing force. Even if the linear valve drive is switched off or switched powerless or thermal changes are present, the spring system maintains the tight closing force. Occurring tolerances are compensated by the spring system. 
         [0011]    In addition, the linear valve drive is equipped such that the spring system provides an overload protection for the linear valve drive. The overload protection comes into effect when the linear valve drive is in its closed position and would further actuate the actuating element into the closed position. 
         [0012]    Furthermore, the linear valve drive is equipped such that the spring system provides a blocking protection for the linear valve drive. Jamming or blocking of the linear valve drive thus is prevented. This applies in particular also when the linear valve drive is detached, i.e. no valve body is connected. 
         [0013]    Accordingly, the spring system has three safety-relevant functions at the same time for the operation of the linear valve drive. 
         [0014]    One aspect provides that along the power flow path extending via the supporting unit from the drive unit to a coupling point of the linear valve drive a linear bearing is provided at the valve body, which allows an axial relative displacement of two portions of the linear valve drive relative to each other, wherein the spring system attempts to press two portions into a starting position in a first direction, and wherein the spring system is arranged such that it transmits axial forces in a maximally extended position of the actuating element. A compact construction of the linear valve drive thus can be ensured. The spring system acts in axial direction and can actively be compressed when an axial relative displacement occurs. Via the spring system, forces acting in axial direction thereby can be absorbed at least in part and spring forces acting in axial direction also can be released. Furthermore, the spring system also is effective when no valve body is connected and the actuating element is extended maximally, so that at least a safety function also is given in the linear valve drive without valve body connected thereto. 
         [0015]    According to one embodiment the supporting unit is a component separate from the drive housing, wherein between the supporting unit and the drive housing the linear bearing and the spring system are arranged. The supporting unit and the drive housing thereby can axially be shifted relative to each other. The valve body can firmly be connected to the supporting unit, so that the same is axially shiftable with the supporting unit relative to the drive housing. This results in an axial relative shiftability between the drive housing and the valve body, in case the same is connected to the linear valve drive. 
         [0016]    Another aspect provides that the supporting unit is a tube enclosing the actuating element, which extends into an opening in an end wall of the drive housing, in order to be mounted therein, in particular wherein at the housing-side end of the tube the spring system engages, which is supported on the inside of the housing. The spring system thus is arranged directly at the interface between the supporting unit and the drive housing, whereby an axial relative displacement of the supporting unit with respect to the drive housing is possible via the spring system. Furthermore, it thereby is ensured that the supporting unit is at least partly captively mounted in the drive housing. The supporting unit can be formed rotationally symmetrical, for example, due to the fact that the tube has a circular cylindrical cross-section. 
         [0017]    According to a further embodiment the supporting unit has at least two portions axially movable relative to each other and connected via the linear bearing, between which the spring system acts. According to this embodiment, the axial relative displacement is provided between the two portions of the supporting unit. In this embodiment it is possible in particular that the portion of the supporting unit associated to the drive housing is firmly connected with the drive housing, whereas the valve body is connectable to the other portion. 
         [0018]    In particular, the two portions axially movable relative to each other can telescopically be pushed into each other. In this way a linear valve drive can be provided, which has a large maximum adjustment path and yet has a compact construction. 
         [0019]    The spring system can comprise at least one first spring subsystem and one second spring subsystem, which have different spring rates. A particularly simple spring system thus is created, which has at least two different spring rates. The spring system thus can have two functions at a single place. Furthermore, one of the two spring subsystems can easily be exchanged or be replaced by another system, in order to subsequently vary the characteristic of the spring system. The maintenance thereby is simplified as well. 
         [0020]    In particular, the spring subsystems can be connected in series. This results in a particularly simple and compact construction of the spring system. Furthermore, the spring rate of the spring system easily can be formed dependent on the compression path, as initially the spring subsystem with the smaller spring rate is active chiefly or almost exclusively. 
         [0021]    The spring system can be constructed by several spring subsystems. The spring subsystems, in particular in a starting position, preferably directly rest against each other, i.e. without interposed elements of a stiffer material. 
         [0022]    According to a further aspect, the first spring subsystem is a closing system which with a non-activated drive unit urges the actuating element in direction of the closed position, and the second spring subsystem is an overload protection which is compressed when the linear valve drive not coupled with a valve body moves the actuating element maximally to the outside and against an internal stop. The two spring subsystems thus each cover a different function of the spring system. The spring subsystems are arranged such that initially almost only the first spring subsystem chiefly is active with a first, shorter compression path and exerts an additional force on the valve closing element. The second spring subsystem on the other hand only is noticeably compressed when a predetermined adjustment path is exceeded. The second spring subsystem in particular serves to absorb forces which proceed from the drive unit. Due to the design of the second spring subsystem it is ensured in addition that the same also is active when no valve body is connected to the linear valve drive. The overload protection function thus already is fully operational in the separately formed linear valve drive. 
         [0023]    In particular, the second spring subsystem has a higher spring rate than the first spring subsystem. This is connected with the different functions covered by the two spring subsystems. The second spring subsystem protects the linear valve drive, in particular the drive unit accommodated therein, from a mechanical overload, which is why it has a higher spring rate. On the other hand, the first spring subsystem maintains an additional tight closing force, when the valve closing element is in a closed position and the drive unit is switched off. Due to the lower spring rate of the first spring subsystem, the drive unit hardly is loaded when the actuating element is shifted against the spring force of the first spring subsystem. 
         [0024]    Another aspect provides that the second spring subsystem has a spring rate between 400 N/mm and 16000 N/mm, in particular at about 500 N/mm, and/or the first spring subsystem has a spring rate between 0.1 N/mm and 600 N/mm, in particular at about 200 N/mm. These are typical spring rates which are suitable to ensure the different functions of the spring subsystems. In particular, the chosen spring rates depend on the size of the linear valve drive and the strength of the drive unit. 
         [0025]    The first and/or the second spring subsystem can be formed of several spring elements which in particular are connected in series. Via the number of spring elements the spring rate can be set correspondingly. Furthermore, a spring subsystem with a higher spring rate thus can be provided in a simple way. A subsequent modification or adaptation of the spring rate of a spring subsystem or of both spring subsystems likewise is possible in that a spring element is exchanged or an additional one is added. The spring subsystems generally can be formed as spring packs. 
         [0026]    According to an exemplary embodiment, the first spring subsystem and the second spring subsystem each can be formed of at least one disk spring surrounding the actuating element, wherein the disk springs of the spring subsystems are stacked on top of each other. A rotationally symmetrical and compact construction of the spring system thereby is created, as the two spring subsystems are directly stacked on top of each other. Furthermore, the power transmission thereby is optimized, as a homogeneously distributed power transmission exists. 
         [0027]    Another aspect provides that at least one spring travel limiter present between the spring elements is provided, which limits the spring travel of the spring system. The spring travel limiter ensures that the spring system only is provided with a limited spring travel. The spring travel limiter therefor preferably is arranged in the first spring subsystem, so that from a particular spring travel of the entire spring system the spring travel of the first spring subsystem is limited by the spring travel limiter. The consequence is that from this particular spring travel onwards only the second spring subsystem is active and protects the linear valve drive from an overload. The spring travel limiter arranged in the first spring subsystem thus ensures a targeted transition from the first to the second spring subsystem. 
         [0028]    In particular, the actuating element can be a valve tappet at whose end the valve closing element is provided. A valve drive thereby can be formed, in which the actuating element and the valve closing element in particular are formed in one part. 
         [0029]    One aspect provides that the linear bearing additionally forms a pivot bearing which rotatably supports two portions of the linear valve drive relative to each other, preferably by more than 360°. This function among other things is made possible by the spring system, as the spring system is formed rotationally symmetrical. The linear valve drive thereby can be used in various positions, whereby a higher flexibility is present for example for aligning the ports of the valve drive. 
         [0030]    The object of the invention furthermore is solved by a valve with a valve body and a linear valve drive as described above, wherein the valve body includes at least one inlet, at least one outlet, at least one flow passage and at least one valve seat. The aforementioned advantages of the linear valve drive analogously can be transferred to the valve. 
         [0031]    According to one embodiment the supporting unit is attached to the valve body, wherein the linear bearing and the spring system are provided between the supporting unit and the valve body. The valve formed in this way is characterized in that the axial shiftability is provided between the valve body and the entire linear valve drive, in particular between the valve body and the supporting unit, which is part of the linear valve drive. The linear valve drive as such can be formed in one part, so that no axial relative displacement of the components is possible. 
         [0032]    According to a further aspect the spring system is formed such that the linear valve drive is rotatable relative to the valve body about the longitudinal axis, preferably by more than 360°, i.e. in particular endlessly, wherein in particular the linear bearing also forms a pivot bearing. The relative rotatability of the linear valve drive, in particular of the drive housing, with respect to the valve body likewise is given with connected valve body. The entire linear valve drive accordingly can be rotated relative to the valve body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]      FIG. 1  shows a perspective view of a valve according to the invention, 
           [0034]      FIG. 2  shows a sectional view of the valve according to the invention from  FIG. 1  in the region of the supporting unit, 
           [0035]      FIG. 3  shows a view of the valve shown in  FIG. 2  in an open position, wherein the sectional plane is chosen slightly different from the one in  FIG. 2 , 
           [0036]      FIG. 4  shows the detail view of the valve shown in  FIG. 2  in a closed position, and 
           [0037]      FIG. 5  shows a spring characteristic of the spring system used. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]      FIG. 1  shows a valve  10  which comprises a linear valve drive  12  with a drive housing  14  and a valve body  16 . 
         [0039]    The valve body  16  includes an inlet  18  and an outlet  20 . Between the inlet  18  and the outlet  20  a flow passage  22  is formed, through which a fluid can flow whose flow rate can be set, in particular be regulated or controlled, by the valve  10 . 
         [0040]    The valve body  16  is connected to the separately formed linear valve drive  12  via a coupling point  23  which is provided on a supporting unit  24  of the linear valve drive  12 . In the embodiment shown, the supporting unit  24  is formed separate from the drive housing  14  and separate from the valve body  16 , as can be taken for example from  FIG. 2  in which the valve  10  is shown in a sectional representation in the region of the supporting unit  24 . 
         [0041]      FIG. 2  shows that the linear valve drive  12  includes a drive unit  26  which in  FIG. 2  is represented in broken lines. An axially shiftable actuating element  28  is coupled with the drive unit  26  and can be driven and hence shifted by the drive unit  26 . The drive unit  26  can be a pneumatically, hydraulically or electrically actuatable drive unit. Alternatively, the drive unit  26  also can be a drive unit to be actuated manually. 
         [0042]    The drive unit  26  or generally the linear valve drive  12  can include a step-up gear unit  27  which in the illustrated embodiment is formed as spindle-nut assembly which converts a rotatory movement of an electric motor into a linear movement of the actuating element  28 . 
         [0043]    In the embodiment shown, the actuating element  28  is formed as valve spindle which extends from the step-up gear unit  27  through the tubular supporting unit  24  into the valve body  16 . The supporting unit  24  therefore can also be referred to as spindle tube. The actuating element  28  has an axial end  30  which is associated to the valve body  16 . At this axial end  30  a valve closing element  32  is arranged on the actuating element  28  which can be formed separate from the same. 
         [0044]    The valve closing element  32  can cooperate with a valve seat  34  formed in the valve body  16 , in order to close the valve  10 . For this purpose, the actuating element  28  is shifted into its closed position by the drive unit  26 , so that the valve closing element  32  sealingly rests on the valve seat  34 . Then, the fluid no longer can flow through the flow passage  22 , as the same is blocked. 
         [0045]    At the valve seat  34  a seal  35   a  can be provided, which cooperates with the valve closing element  32 , in order to improve the sealing effect when the valve closing element  32  rests on the valve seat  34 . Alternatively or in addition a seal  35   b  can be provided at the valve closing element  32 . 
         [0046]    In general, the valve closing element  32  serves to vary the cross-section through the flow passage  22 , whereby a flow rate of the fluid can be set. The valve closing element  32  thus can take various intermediate positions between the open position ( FIG. 3 ) and the closed position ( FIG. 4 ). 
         [0047]    The actuating element  28  in particular can be formed as valve tappet at whose end the valve closing element  32  is formed in one part. 
         [0048]    Furthermore,  FIG. 2  reveals that the drive housing  14  consists of several parts, as it comprises an end-face lid element  14   a  with an end wall  36  as well as a shell-shaped housing part  14   b  which is coupled with the lid element  14   a . Furthermore, the housing  14  includes a housing lid  14   c  opposite the lid element  14   a  (see  FIG. 1 ), which likewise is coupled with the shell-shaped housing part  14   b.    
         [0049]    The drive housing  14 , in particular the lid element  14   a,  has an opening  38  in the region of the end wall  36 , into which the supporting unit  24  is inserted. The supporting unit  24  partly extends through the opening  38  into the drive housing  14  and is axially shiftably guided there. The axial end  40  of the supporting unit  24 , which protrudes into the drive housing  14 , is shaped like a collar, wherein the end  40  for example can be manufactured or subsequently be formed, in particular be bent as such. 
         [0050]    Against the formed end  40  of the supporting unit  24  a disk- or ring-shaped supporting element  42  rests, on which a spring system  44  furthermore is supported. The supporting element  42  serves for the improved contact of the spring system  44  and for pretensioning the spring system  44 , as will yet be explained below. 
         [0051]    In general, the spring system  44  is provided in the power flow path between the drive unit  26  and the coupling point  23  of the linear valve drive  12  at the valve body  16 , whereby among other things the axial relative displacement between two portions of the linear valve drive  12  is possible. The drive unit  26  is firmly coupled with the drive housing  14 , in particular with the housing lid  14   c  or the shell-shaped housing part  14   b.    
         [0052]    In the embodiment shown, the spring system  44  is arranged such that with its first end it supports on the supporting unit  24  formed separate from the drive housing  14  via the supporting element  42  and with its second end it supports on an inside of the drive housing  14 , in particular on the inside of the lid element  14   a.  The spring system  44  thereby resiliently lies in the axial power flow path between the valve body  16  and the drive housing  14 , so that axial compressive forces can be transmitted. Due to the arrangement of the spring system  44  between the drive housing  14  and the supporting unit  24 , the drive housing  14  can shift in axial direction relative to the supporting unit  24  and the valve body  16  firmly connected thereto. 
         [0053]    In the region of the spring system  44  a linear bearing  45  accordingly is formed, which provides for the axial relative displacement of the supporting unit  24  with respect to the drive housing  14 . 
         [0054]    In the embodiment shown, the spring system  44  comprises a first spring subsystem  46  and a second spring subsystem  48 , which are arranged in series and are arranged directly on top of each other. The two spring subsystems  46 ,  48  in addition have a different spring rate, as can be taken from  FIG. 5 , to which reference will be made later. Due to the different spring rates of the spring subsystems  46 ,  48  arranged in series, the entire spring system  44  has a spring-travel-dependent spring rate or a spring rate dependent on the compression path of the spring system  44 . 
         [0055]    Alternatively, the spring system  44  also can include more than two spring subsystems, whereby a correspondingly finer adjustment of the travel-dependent spring rate of the spring system  44  is possible. 
         [0056]    In a further alternative aspect, the spring system  44  can be formed by a single spring which for example has several steps, whereby the travel-dependent spring rate is realized. 
         [0057]    In the embodiment shown, the two spring subsystems  46 ,  48  each are formed of disk springs which are stacked on top of each other and surround the actuating element  28 . The disk springs accordingly are formed substantially disk- or ring-shaped and in a homogeneous way act on the drive housing  14  as well as the supporting element  42 . 
         [0058]    For example due to the rotationally symmetrical design of the spring system  44  the linear bearing  45  at the same time can form a pivot bearing, whereby the portions of the linear valve drive  12  axially shiftable relative to each other also are rotatably mounted relative to each other. This means that in the embodiment shown the drive housing  14  and the supporting unit  24  with the valve body  16  connected thereto can be rotated relative to each other by more than 360°. As a result, the linear valve drive  12  and the valve  10  can be used in various installation positions. 
         [0059]    In the embodiment shown, the first spring subsystem  46  is formed by two spring elements which are connected in series and thus form a spring pack. The second spring subsystem  48  on the other hand merely includes a single spring element. The second spring subsystem  48  like the first spring subsystem  46  can be formed by several spring elements or the first spring subsystem  46  like the second spring subsystem  48  merely by one spring element. 
         [0060]    In general, instead of or in addition to the illustrated disk springs coil springs, leaf springs, elastomer springs or torsion springs can be used, which can also be combined with each other, in order to achieve the desired spring rates and properties. 
         [0061]      FIG. 2  furthermore reveals that a spring travel limiter  50  is provided in the spring system  44 . In the embodiment shown, the spring travel limiter  50  is a ring arranged between the two spring elements of the first spring subsystem  46  and thereby limits the spring travel of the first spring subsystem  46 , as will yet be explained below. 
         [0062]    Alternatively or in addition a spring travel limiter also can be provided in the second spring subsystem  48 . 
         [0063]      FIG. 3  shows that the valve  10  is in its open position. In this position the drive unit  26  has completely retracted the actuating element  28 , so that the valve closing element  32  is not in contact with the valve seat  34 . The free flow cross-section in the flow passage  22  is at a maximum in the illustrated open position. 
         [0064]    In the open position, the supporting unit  24  rests against a stop  54  which is formed at the housing  14  via a stop surface  52 . At the stop  54  there is also provided a seal  56  which is formed ring-shaped. The stop  54  thus limits the axial relative movement of the supporting unit  24  to the housing  14 . 
         [0065]    A comparison of  FIGS. 2 and 3  shows that in the position shown in  FIG. 2  the actuating element  28  and the valve closing element  32  arranged thereon already have almost been in the open position. This can be recognized particularly well by the position of the supporting element  42 . 
         [0066]      FIG. 4  shows the same section as it is shown in  FIG. 3 , but the actuating element  28  as well as the valve closing element  32  arranged thereon have been shifted into the closed position by the drive unit  26 . 
         [0067]    In this position the counterforce to the axial closing force is transmitted to the drive housing  14 , to which the drive unit  26  is attached, via the supporting unit  24 , the supporting element  42  resting against the same and the spring system  44  supporting on the supporting element  42 . Correspondingly, the spring system  44  is located in the axial power flow path. 
         [0068]    Furthermore, in the closed position the supporting unit  24  no longer rests against the stop  54  via its stop surface  52 , so that a gap is formed between the housing  14  and the supporting unit  24  in the region of the stop  54 . 
         [0069]    With respect to  FIG. 5 , in which the spring characteristic of the spring system  44  is shown in a normalized representation, the mode of operation of the spring system  44  will be explained. 
         [0070]    In the open position shown in  FIG. 3 , the linear valve drive  12  is in its starting position in which the actuating element  28  is retracted completely. In this position, the spring system  44  can be pretensioned depending on the design and arrangement of the supporting element  42 . This starting position corresponds to an adjustment path of +10 in the diagram shown in  FIG. 5 . 
         [0071]    When the actuating element  28  is transferred from the open position shown in  FIG. 3  into the closed position shown in  FIG. 4  by means of the valve closing element  32 , a force must be applied in the region of the path from +10 to 0 against the force A as shown in  FIG. 5 . 
         [0072]    As soon as the valve closing element  32  reaches the valve seat  34 , which corresponds to the adjustment path at 0, the force required to further shift the actuating element  28  increases. The minimum tight closing force B existing at this position is at least 100% of the force acting on the valve closing element  32  through the fluid. This guarantees that the valve closing element  32  cannot be shifted and opened due to the fluid force acting on the valve closing element  32 . Preferably, the minimum tight closing force B is at least 105% of the fluid force. 
         [0073]    From this position, the drive unit  26  shifts the actuating element  28  further axially in direction of the valve seat  34 , so that the seal  35   a  and/or  35   b  provided between the valve closing element  32  and the valve seat  34  is/are compressed more strongly. 
         [0074]    When the actuating element  28  at 0 is shifted further axially along the adjustment path, the first spring subsystem  46  chiefly, i.e. almost only is compressed, wherein the linear valve drive  12  must exert a correspondingly higher force acting against the tight closing force C along a path S, as can be taken from the diagram in  FIG. 5 . 
         [0075]    The further the actuating element  28  is shifted along the adjustment path S, the more strongly is the first spring subsystem  46  compressed. At the same time, the drive housing  14  moves in axial direction relative to the supporting unit  24 . This can be clearly recognized when  FIGS. 3 and 4  are compared in the region of the opening  38  in the end wall  36  or the position of the supporting elements  42  is compared. 
         [0076]    The first spring subsystem  46  is formed such that the tight closing spring force C is as constant as possible along the path S. In practice, however, minimum deviations from this theoretical ideal case are obtained. The spring force of the first spring subsystem  46  is chosen such that a tight closing spring force C is obtained, which is high enough to maintain the sealing function and at the same time is not too high, so that the drive unit  26  and the seal are not loaded unnecessarily when it shifts the actuating element  28  against the tight closing spring force C. 
         [0077]    The path S typically is chosen so long that all possible changes in length due to thermal influences, the setting of the seals  35   a,    35   b,  and a possible clearance in the linear valve drive  12  are taken into account, in order to ensure that the same can be compensated. Usually, the path S approximately is 2% to 15% of the entire adjustment path of the valve  10 , in particular between 5% and 10%. The path S or the adjustment path along the path S guarantees that the required tight closing force can be applied durably, even if the drive unit  26  is switched off. 
         [0078]    In normal operation, the drive unit  26  is switched off at approximately half of the path S, so that the valve closing element  32  remains in this position and a sufficient tight closing force is guaranteed via the first spring subsystem  46 . This position of the valve closing element  32  furthermore can be maintained due to self-locking of the components of the linear valve drive  12 . 
         [0079]    Should the drive unit  26  shift the actuating element  28  with the valve closing element  32  further than to the point provided in normal operation, the first spring subsystem  46  can be compressed maximally or the spring travel limiter  50  provided in the first spring subsystem  46  is used. This is shown in  FIG. 4  and in the diagram of  FIG. 5 . 
         [0080]    From point D (see  FIG. 5 ) the force no longer is transmitted via the first spring subsystem  46 , as the two spring elements of the first spring subsystem  46  contact the interposed spring travel limiter  50 . The first spring subsystem  46  thus no longer is compressible, which leads to the fact that the second spring subsystem  48  chiefly becomes active. The second spring subsystem  48  accordingly is compressed only when a certain spring travel of the spring system  44  or adjustment path of the actuating element  28  is reached. Via the spring travel limiter  50  a targeted transition can be set, from which the second spring subsystem  48  is compressed. 
         [0081]    As mentioned already, the two spring subsystems  46 ,  48  have different spring rates, whereby the spring system  44  has a spring-travel-dependent spring rate. This is also illustrated in  FIG. 5 . 
         [0082]    The second spring subsystem  48  has a distinctly higher spring characteristic than the first spring subsystem  46 . For example, the second spring subsystem  48  can have a spring rate between 400 N/mm and 16000 N/mm, in particular the spring rate of the second spring subsystem  48  is about 500 N/mm. On the other hand, the first spring subsystem  46  can have a spring rate between 0.1 N/mm and 600 N/mm. In particular, the spring rate of the first spring subsystem  46  is about 200 N/mm. 
         [0083]    Due to the higher spring rate of the second spring subsystem  48  it is ensured that the drive unit  26  is protected from a mechanical overload. This safety function is important in particular during fast shifting of the valve  10  into its closed position. Due to the second spring subsystem  48 , the drive unit  26  must work against its higher spring rate and does not hit a stop directly, like the valve seat  34  or an internal stop. When hitting the stop unbraked, the drive unit  26  otherwise might get jammed or blocked mechanically. 
         [0084]    The first spring subsystem  46  on the other hand merely represents a closing system which with non-activated drive unit  26  urges the actuating element  28  and the closing element  32  arranged thereon in direction of the closed position, so that the valve  10  remains in its closed position, even if the drive unit  26  is switched off. 
         [0085]    Due to the spring-travel-dependent spring rate, the spring system  44  accordingly provides a uniform spring system which includes two functions, namely the overload protection and the maintenance of the required tight closing force. Furthermore, a rotatability of the drive housing  14  with respect to the valve body  16  or the supporting unit  24  is achieved via the spring system  44 , as has been mentioned already. The spring system  44  illustrated in the embodiment thus even has three functions. 
         [0086]    The spring system  44  also is effective when no valve body  16  is connected to the linear valve drive  12 . The actuating element  28  then would be shifted downwards, until a stop surface of the nut of the nut-spindle assembly  27  gets in contact with the end  40  of the supporting unit  24  (see  FIG. 2 ). The spring system  44  would then be activated in a way analogous to  FIG. 4  and effectively prevent blocking of the drive unit  26 . 
         [0087]    Furthermore, there can be provided a controller which records the current consumption of the linear valve drive  12 , in particular of the drive unit  26 . Due to the different spring characteristics of the two subsystems  46 ,  48  a different current consumption is obtained. By means of the recorded and subsequently evaluated data an optimization of the operation of the linear valve drive  12  can be performed. 
         [0088]    In a further embodiment not illustrated here the supporting unit  24  can be formed in two parts, wherein the spring system  44  is arranged between two portions of the supporting unit  24 . The linear bearing then is effected between the two portions of the supporting unit  24 , so that the two portions are axially movable relative to each other. The portion of the supporting unit  24 , which is associated to the drive housing  14 , can integrally be connected with the drive housing  14 . 
         [0089]    Advantageously, the two portions of the supporting unit  24  can telescopically be pushed into each other, so that a compact construction and yet large adjustment path of the linear valve drive  12  is possible. 
         [0090]    In another embodiment not illustrated here the spring system  44  is provided in the region of the coupling point  23  of the linear valve drive  12  at the valve body  16 , so that the linear bearing also is present there. In this embodiment, the supporting unit  24  can be formed in one part with the drive housing  14 , in particular with the lid element  14   a.    
         [0091]    The supporting unit  24  and its portions, the spring system  44  as well as the coupling point  23  in particular can be formed such that the corresponding linear bearing at the same time forms a pivot bearing. 
         [0092]    According to the invention a linear valve drive  12  and a valve  10  thus is created, which has a compact construction in which a spring system  44  is provided, which combines several functions at one place.