Abstract:
A switching valve includes a stator and a rotor. The stator includes multiple connection ports. The rotor has predetermined switching positions and interacts with the stator to form a fluidic connection or a fluidic disconnection of predetermined connection ports. The rotor can be mounted rotatably via a bearing and pressing device loaded with a predefined pressing force in a direction of the stator. The bearing and pressing device includes a compensation element to load the rotor and transmit the pressing force. The compensation element is configured to make an elastic flexural deformation so that the compensation element loads the rotor when the rotor is wobbling with respect to an axis of rotation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority benefit under 35 U.S.C. §119 to German Patent Application No. 10 2012 107 377.5, filed on Aug. 10, 2012; German Patent Application No. 10 2012 107 378.3, filed on Aug. 10, 2012; and German Patent Application No. 10 2012 107 379.1, filed on Aug. 10, 2012, which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The invention relates to a switching valve for liquid chromatography, in particular a high-pressure switching valve for high-performance liquid chromatography (HPLC). 
       BACKGROUND 
       [0003]    In HPLC, high-pressure switching valves are used for an entire series of different tasks, for example in order to remove a sample to be examined from a sample container, to feed it into a sample loop and, from there, to introduce it into a high-pressure liquid flow in the direction of a chromatography column, or in order to flush various components or in order to switch over between a plurality of columns. 
         [0004]    Switching valves of this type are installed as a rule in automatic samplers for HPLC, in column ovens or in fraction collectors. 
         [0005]    When switching valves are used, it is almost always advantageous if only short capillary paths, that is to say short lengths of capillaries which guide the medium to be switched, are required between the relevant components. For example, when samples are guided in an eluent flow, it is advantageous if only short capillary paths have to be overcome, since the dispersion of the sample, that is to say the mixing of the sample with the eluent which is situated upstream and downstream of the sample in the flow path, is then low. In addition, the pressure loss in the relevant system is minimized by short capillary paths. 
         [0006]    In order to make short capillary paths possible, it is advantageous or indispensable in many cases if the switching valve is constructed so as to be as compact as possible. As a result, the switching valve can be used in a space-saving and variable manner. 
         [0007]    Despite a compact construction, it goes without saying that a switching valve of this type also has to ensure precise and reproducible positioning of the moving parts which guide the medium to be switched. 
         [0008]    Switching valves, as used for introducing a sample into the fluid stream, normally have a stator in which there are provided multiple connection ports for the supply and discharge of the fluid to and from the switching valve. The ports are connected via ducts to opening cross sections which are formed on a switching surface of the stator, for example in the face side of a substantially cylindrical stator element. The rotor likewise has a switching surface which interacts with the switching surface of the stator, wherein in the switching surface of the rotor there are formed grooves which serve to connect certain opening cross sections and/or ports of the stator to one another as a function of two or more switching positions. Here, the rotor and the stator must be pressed against one another with an adequately high pressing force in order to attain a sealing action in the plane of the switching surfaces even in the case of high pressures such as arise in liquid chromatography, in particular HPLC. 
         [0009]    Such switching valves are described for example in WO 2009/101695 A1 or US 2010/0281959 A1. 
       SUMMARY 
       [0010]    In recent times, in HPLC, as a consequence of the high pressure used here for the medium to be switched, the stator and the rotor are configured in such a way that the two interacting faces are composed in each case of a hard material or are coated with a hard material. It is necessary in this case that the rotor is mounted such that it can wobble by sufficient angular amounts with respect to the stator, in order to compensate for production and assembly tolerances and to produce as uniform a pressure distribution as possible in the area of contact in every angular position. To this end, it is known to mount the rotor such that it can wobble by means of a flexible cushion (for example, DE 10 2011 000 104, not a prior publication; similar to WO 2011/008657 A2). This functionality is also to be capable of being realized in the valve according to the present invention. 
         [0011]    On the basis of the prior art mentioned at the outset, the invention is therefore based on the object of providing a switching valve for liquid chromatography, in particular a high-pressure switching valve for high-performance liquid chromatography, which switching valve exhibits as homogenous a pressure distribution as possible in the contact surface between the rotor and the stator and simultaneously ensures exact guidance and positioning of the rotor and is of a compact structural size. 
         [0012]    The invention is based on the realization that it is more favourable to produce a compensation element for making wobbling movements for the rotor possible not in the form of a cushion from a relatively soft, elastic material, such as a plastic, or to use complicated mountings, but rather to manufacture the compensation element from a relatively rigid, pressure-resistant and temperature-resistant material, in particular from a metal or ceramic, and to make the wobbling movements possible by way of a suitable geometry. This results in considerably improved temperature resistance and long-term stability. 
         [0013]    The longitudinal axis of the compensation element ( 21 ,  21 ′) is preferably aligned substantially with the rotational axis of the rotor ( 23 ). 
         [0014]    According to the invention, the compensation element has a head region, which loads the rotor with a loading face, and a base region, by way of which the compensation element is supported against a unit of the bearing and pressing device, which unit generates the pressing force, or against an element of the bearing and pressing device, which element transmits the pressing force, and a bending region which is provided between the head region and the base region and is configured in such a way that it makes an elastic flexural deformation possible such that the loading face of the head region loads the full area of the rotor even in the case of wobbling movements of the rotor in every angular position of the rotor and in the process a substantially homogeneous pressure distribution is generated in the contact plane between the rotor and the stator. 
         [0015]    In one embodiment of the invention, the head region can be of substantially rigid and flexurally strong configuration. To this end, the head region can have a cross section which is enlarged in comparison with the bending region, with the result that practically no elastic deformations take place in the head region. 
         [0016]    A compensation element of this type can be produced in a simple way and inexpensively, for example as a simple turned part made from metal. In addition, a relatively hard, pressure-resistant material ensures excellent long-term stability. 
         [0017]    According to one embodiment of the invention which is simple to manufacture, the compensation element is configured as a rod-shaped element or wobble bar. In particular, every hard, sufficiently pressure-resistant, sufficiently flexurally elastic and temperature-resistant material, such as steel or ceramic, is suitable as material. It goes without saying that the geometry of the compensation element also has to be selected with consideration of the material properties, in such a way that, at least in the bending region, the desired elastic deformation is ensured with maintenance of the pressure force which is to be transmitted in the axial direction. 
         [0018]    Here, the bending region or the bending region and the base region can be configured as a preferably cylindrical region, the geometry and the material of the bending region being selected in such a way that the required wobbling movements are made possible. 
         [0019]    According to another embodiment, the bending region can have two solid body joints or thin places which are sufficiently pressure-resistant in the axial direction in order to transmit the pressing force, the thin places or solid body joints being flexurally elastic in each case transversely with respect to the longitudinal axis of the compensation element, and the longitudinal extent planes of the two thin places, from which the bending movement takes place, or the bending axes of the solid body joints enclosing an angle which is not equal to zero, preferably lying perpendicularly on one another. 
         [0020]    The stator of high-pressure switching valves of this type usually has a stator end face, in which a plurality of opening cross sections are formed by a plurality of ports which are formed by channels in the stator. The rotor has a rotor end face which loads the stator end face and in which one or more grooves are formed which in each case connect defined port openings in a fluidic and pressure-tight manner depending on the rotary position of the rotor with respect to the stator. In addition or instead of one or more grooves, opening cross sections of one or more connecting channels which are provided in the rotor can also be provided in the rotor end face. If the cross-sectional openings of the connecting channels are brought into congruence with the opening cross sections of predefined ports in a defined rotary position of the rotor, a connection of said ports is achieved. 
         [0021]    The (central) region of the stator end face, in which region the opening cross sections of the ports lie, and which region has to be sealed with respect to the rotor end face (which is usually planar overall), is usually of slightly elevated configuration with respect to the surrounding region of the stator end face. The sealing area between the rotor and the stator is therefore stipulated by the size of the elevated region of the stator end face. According to one refinement of the invention, the radial extent of the loading face of the head region of the compensation element is selected to be at least as great as the radial extent of the region of the rotor end face, in which the grooves are provided. As a result, a homogeneous pressure distribution can be ensured and maintained in the contact area between the stator and the rotor. The loading face can also be selected to be as great as the entire sealing area between the rotor and the stator, which sealing area, in the case of an elevated configuration of the relevant region of the stator end face, in which region the cross-sectional openings of the ports are provided, is stipulated by said configuration. 
         [0022]    According to one refinement of the invention, the base region of the compensation element can be supported against a coupling element of preferably cup-shaped configuration of the bearing and pressing device, the coupling element being connected to the rotor in a rotationally fixed but axially displaceable manner and so as to make required wobbling movements of the rotor possible. The coupling unit which comprises the compensation element and the coupling element rotates together with the rotor and transmits the pressing force to the rotor. 
         [0023]    To this end, the bearing and pressing device can have a spring unit which is preferably of annular configuration and loads the coupling element with the pressing force. The coupling element can be of pot-shaped configuration, the compensation element being supported with the base region on the bottom of the coupling element and protruding beyond the end side of the coupling element with its head region, at least with the upper end face thereof which faces the rotor. 
         [0024]    According to one refinement of the invention, the bearing and pressing device can have a receiving part which is mounted such that it can be rotated about the rotational axis of the rotor and is fixed axially, and against which the spring unit is supported, the receiving part being connected, directly or via the spring unit, fixedly to the coupling element so as to rotate with it, and the receiving part, the spring unit, the coupling element and the compensation element forming, together with the rotor, a unit which can be rotated about the rotational axis of the rotor. 
         [0025]    Here, the receiving part can be mounted by means of a single radial bearing, preferably a radial anti-friction bearing, which has such a high axial load-bearing capability that it can absorb the axial pressing force which is necessary for pressing the rotor onto the stator. To this end, the bearing can be configured, for example, as an angular-contact ball bearing. 
         [0026]    According to one embodiment of the invention, the receiving part can be of hollow-cylindrical or pot-shaped configuration and can have a preferably circumferential, radial flange for support with respect to the bearing, the spring unit being supported against the end side of the flange of the receiving part, which end side faces the rotor. The receiving part can therefore receive the coupling element and, if required, mount it axially displaceably and therefore centre it coaxially at the same time. However, the centering can also take place only via the spring unit which is of annular configuration and can be configured in such a way that it loads the coupling element with a radially inwardly directed centering force when the spring unit is prestressed axially (and the diameter of the annular opening is reduced as a result) from its relieved starting position (during the assembly of the valve). 
         [0027]    On its end side (which faces the rotor) or on the flange, the receiving part can have, for example, a circumferential, axial projection which interacts, for radial positioning and fixing of the spring unit, with that region of the spring unit which is supported against the end side of the receiving part or of the flange. The spring unit can therefore be supported radially against the axial projection of the receiving part, as a result of which the spring unit itself is centered and the abovementioned centering of the coupling element can also be brought about. Since the coupling element is connected fixedly to the rotor so as to rotate with it (and it goes without saying also fixedly in the radial direction), the positioning of the rotor with respect to the stator is also brought about as a result. 
         [0028]    According to one refinement of the invention, the receiving part has a drive region which faces away from the stator and can be coupled to the output of the drive device. As a consequence of the axial displaceability of the coupling element with respect to the receiving part and the axially fixed mounting of the receiving part, simple coupling of a drive device can therefore take place, for example in the form of an actuable motor drive or an actuating element which can be actuated manually. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    The invention will be explained in more detail below on the basis of exemplary embodiments illustrated in the drawing, in which: 
           [0030]      FIG. 1  is a perspective, cut-away illustration of a first embodiment of a switching valve according to the invention; 
           [0031]      FIG. 2  is a perspective, cut-away illustration of the switching valve itself (without drive device) of a second embodiment of a switching valve according to the invention; and 
           [0032]      FIG. 3  is a perspective, cut-away illustration of a further embodiment of a switching valve according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0033]    The switching valve  1  illustrated in  FIG. 1  is composed of a housing  3  which has a first housing part  5  and a second housing part  7 . In the second, pot-shaped, housing part  7  there is accommodated a drive unit  45 , for example in the form of an electric motor. In the first housing part  5 , the switching valve itself is accommodated in the upper region and a gearing unit  37  is accommodated in the lower region, which gearing unit is coupled to the drive unit  45 . The first housing part  5  is closed off by means of a first cover part  17  and a second cover part  33 . 
         [0034]    The switching valve itself is composed of a stator  29 , a rotor  23 , and a bearing and pressing device  8  for the rotatable mounting of the rotor  23  in the housing  3  and the generation of a pressing force with which the rotor is acted on in the direction of the stator. In a known way, the stator has port opening cross sections, formed in the stator face surface, of connection ports  31  for the supply and discharge of the medium to be controlled. The stator face surface interacts with a rotor face surface of the rotor  23 , in which grooves are formed. Depending on the angular position of the rotor  23  relative to the stator  29 , the grooves provided in the stator end surface connect in each case predetermined port opening cross sections, such that in each case the relevant connection ports  31  are fluidically connected. 
         [0035]    In the embodiment illustrated in  FIG. 1 , the bearing and pressing device  8  is composed of a bearing  11 , a receiving part  13 , a spring unit  27  and a coupling unit  16 . 
         [0036]    The first housing part  5  has, in its interior, a shoulder  9  on which the annular bearing  11  is supported axially. The bearing  11  is in the form of a radial bearing with adequate load-bearing capacity in the axial direction, for example an angular-contact ball bearing. The substantially hollow-cylindrical or pot-shaped receiving part  13  is supported, by way of a flange which extends radially into the housing interior almost as far as the inner wall of the housing  3  or of the housing part  5 , on the bearing  11 , such that the receiving part  13  is mounted so as to be fixed in the axial direction and rotationally movable. Here, the flange or the bearing  11  must self-evidently be designed such that the parts which are movable relative to one another do not come into contact, so as to permit an unhindered rotational movement. In the example illustrated, a small axial annular gap is thus provided between the outer ring of the bearing  11  and the flange of the receiving part  13 . 
         [0037]    The coupling unit  16 , which is composed of a coupling element  15  and a compensation element  21 , is of substantially hollow cylindrical or pot-shaped form, and extends by way of its lower region into the receiving part  13 . The coupling element  15  has an outer contour which substantially corresponds to the contour of the interior of the receiving part  13 . The coupling part  15  is thus guided displaceably in the axial direction by the receiving part  13 . 
         [0038]    In the embodiment of a switching valve  1  illustrated in  FIG. 1 , the inner diameter of the interior of the receiving part  13  is selected so as to substantially correspond to the outer diameter of the coupling element  15 , such that adequately precise centering of the coupling element  15  in the radial direction relative to the receiving part  13  and thus relative to the valve axis A is attained. This is because both the stator  29 , by way of its longitudinal axis (which runs perpendicular to the stator face surface and coaxially with respect to the port opening cross sections), and also the rotor  23 , by way of its axis of rotation, must be aligned as precisely as possible with respect to one another such that the stator axis and the rotor axis are in alignment (and form the valve axis). At the same time, through mutually corresponding selection of the outer diameter of the coupling element  15  and of the inner diameter of the receiving part  13 , precise axial displaceability of the coupling element  15  in the valve axis is attained. This however entails correspondingly low manufacturing tolerances. 
         [0039]    In the embodiment of a switching valve  1  illustrated in  FIG. 1 , the coupling element  15  is rotationally conjointly connected to the receiving part  13  merely by frictional locking, and is mounted so as to be rotationally movable together therewith by means of the bearing  13 . The pressing force required for the frictional locking corresponds to the pressing and/or sealing force which acts at the boundary between the rotor  23  and the stator  29  and which is generated by the spring unit  27 . In the embodiment illustrated, the spring unit is realized as a single annular spring element. It is self-evidently possible for a stack of annular spring elements to be used instead of a single annular spring element. 
         [0040]    To realize the frictional locking, the coupling element  15  has, in its upper region, a flange which extends radially outward over the circumference, which flange rests by way of its underside on the annular spring unit  27  in the form of the annular spring element  27 . The spring element  27  is arranged in the annular region between the outer wall of the coupling element  15  and the inner wall of the first housing part  5 , and is supported against the annular face surface of the receiving part  13  or the face surface of a radially outwardly running flange of the receiving part  13 . 
         [0041]    The rotationally conjoint connection between the coupling element  15  and the receiving part  13  may also be realized by virtue of positive locking being provided between the two parts, in particular through the formation of projections or grooves in the inner wall of the receiving part  13  and correspondingly complementary interacting grooves or projections on the outer circumference of the coupling element  15 . The positive locking must however be realized such that an axial movement of the coupling element is permitted. 
         [0042]    The rotationally conjoint connection between the coupling element  15  and the receiving part  13  may also be realized indirectly by means of positive locking between the receiving part  13  and the spring unit  27  and between the spring unit  27  and the coupling element  15 . 
         [0043]    The axial securing of the bearing and pressing device  8  in the substantially hollow cylindrical first housing part  5  is realized by means of a first cover part  17  screwed into the upper opening of the first housing part  5 . The first cover part  17  is of substantially annular form and engages by way of a shoulder  19  over the upper face side of the coupling element  15 , which is likewise of substantially hollow cylindrical or pot-shaped form, wherein the interior of the coupling element  15  has a tapering diameter in its lower region. The compensation element  21  in the form of a wobble bar is provided in the interior of the coupling element  15 . The compensation element  21  has a substantially rigid, flexurally stiff head region, a foot region which, in the illustrated exemplary embodiment, is likewise of substantially rigid and flexurally stiff form, and a bending region provided between the foot region and head region. The compensation element  21  is supported by way of its lower end or the foot region in the interior of the coupling element  15  and projects by way of the upper end side of the head region slightly beyond the upper annular face side of the compensation element  21 . As can be seen from  FIG. 1 , the compensation element  21  or the wobble bar is received coaxially in the coupling element  15 , which in turn is received coaxially in the receiving part  13 . The compensation element  21  may also be pressed by way of its foot region into the coupling part  15 . In this way, precise machining of that face surface of the compensation element  21  which faces toward the stator  23  is possible in the pressed-in state. In particular, the projecting length of the face surface of the compensation element  21  in relation to the annular face surface of the coupling element  15  can be set in a precise manner by means of retroactive machining in the pressed-in state. 
         [0044]    The rotor  23  is received in the central opening of the annular first cover part  17 , wherein the outer diameter of the cylindrical rotor  23  substantially corresponds to the inner diameter of the annular cover part  17 . The receiving opening however serves not for guiding the rotor  23  but rather for sealing off the interior of the housing  3  or of the housing part  5  with respect to dust, moisture and other environmental influences. For this purpose, a sealing ring may be provided in a groove formed in the inner wall of the recess, which sealing ring acts on the circumferential wall of the rotor  23  and thus provides the desired sealing action. Instead of the bores for the pins  25 , the rotor  23  may also have corresponding blind holes. This yields the advantage that the bearing and pressing device  8  is sealed off with respect to the rotor face surface and, for example, no lubricant can escape to the outside from the interior of the bearing and pressing device  8 . 
         [0045]    The rotor has three axial bores which serve for receiving in each case one connecting bolt  25 . The connecting bolts  25  engage by way of an upper region into the respective bore in the rotor  23  and by way of a lower end region into a corresponding bore in the face surface of the coupling element  15 . In this way, the rotor is coupled rotationally conjointly to the coupling element  15 . At the same time, the bores in the rotor are formed such that the rotor  23  and thus the rotor face surface are held such that they can perform a tumbling movement through a small but sufficient angular range. 
         [0046]    The cover part  17  has, in its upper region, a receiving region for the stator  29 , which is likewise of substantially cylindrical form and has multiple radially obliquely inwardly running ducts into which in each case the front end of a connection port  31  that can be screwed into a second cover part  33  extends. Of the connection ports  31 , only a single connection port  31  is illustrated in  FIG. 1  because the two other connection ports are situated in the respectively cut-away part of the illustration. In the same way, only a single one of the connecting bolts  25  is visible in  FIG. 1 . The second cover part  33  engages over the stator  29  and presses the latter by way of its stator face surface against the rotor face surface when the second cover part  33  is connected by means of screws  35  to the first cover part. The bearing and pressing device  8  and the first housing part  5  and the first and second cover parts  17 ,  33  are coordinated with one another here such that an adequate pressing force is generated. For the assembly of the switching valve  1 , it is self-evidently also possible for the first and second cover parts  17 ,  33  to firstly be connected to one another, and for the cover part as a whole to then be screwed, together with the stator held therein, into the housing  3  or the first housing part  5 . 
         [0047]    For the mounting of the valve head, the bearing  11  is firstly introduced into the interior of the first housing part  5 . Subsequently, the receiving part  13 , the spring unit  27  and the coupling element  15  with the compensation element  21  pressed therein are inserted into the first housing part. Subsequently, the first cover part  17  is screwed on such that the above-mentioned components are fixed in the interior of the first housing part  5 . Subsequently, the rotor  23  can be inserted. The cover part  17  is formed, with regard to the axial thickness of its inner region, such that the rotor  23 , after the mounting onto the connecting bolt  25 , still projects slightly by way of its upper face surface, in which the grooves (not illustrated in any more detail) are provided, beyond the face-side surface of the cover part  17 , which supports the stator  29 . Subsequently, the stator  29  is placed onto the rotor  23  such that the lower face side of the stator, in which the opening cross sections of the ducts connected to the connection ports  31  are provided, rests on the upper face surface of the stator  23 . It must be taken into consideration here that the central region of the stator face surface in which the opening cross sections of the ports are situated, and which must be sealed off with respect to the rotor face surface (which is normally, on the whole, planar), is normally formed so as to be raised slightly in relation to the surrounding region of the stator face surface. The sealing surface between the rotor and stator is thus defined by the size of the raised region of the stator face surface. 
         [0048]    The second cover part  33  is subsequently placed on, which second cover part is designed so as to act on the upper side of the stator  29 . The second cover part  33  is connected to the first cover part  17  by means of the screws  35 , whereby the stator  29  is acted on with an axial force such that an axially acting sealing force is generated which is sufficient to seal off the grooves provided in the rotor  23  with respect to the stator face surface or the central, raised region of the stator face surface even at the high pressures prevailing in the HPLC. The pressure force is generated here by the spring unit  27  in the form of the annular spring element and/or by the screwing-on of the second cover part  33 . 
         [0049]    As already described above, the compensation element  21  in the illustrated embodiment has an adequately rigid head region and an adequately rigid foot region which are not deformed, or are at most scarcely (elastically) deformed, under the pressing force to be transmitted from the coupling element  15  to the rotor  23 . By contrast, the interposed cylindrical bending region permits an elastic bending deformation such that the upper face surface, by which the compensation element  21  acts on that surface of the rotor  23  which faces towards it, can jointly perform possible wobbling movements of the rotor, and here, the contact pressure at the contact surface between the upper face surface of the compensation element  21  and the rotor  23  is substantially uniformly distributed. Furthermore, the compensation element  21  acts on the rotor  23  coaxially, such that, in the likewise coaxial pressing surface between the rotor  23  and the stator  29 , a substantially uniform pressure distribution is attained even if the rotor  23  performs a wobbling movement about its axis during a rotation because the stator face surface and/or the rotor face surface do not run exactly perpendicular to the axis of rotation of the rotor  23 . 
         [0050]    Here, the bending region of the compensation element  21  must self-evidently also be sufficiently pressure-resistant that the desired pressing force can be transmitted to the rotor  23 . Said region must thus be designed to be at least as rigid as the spring unit  27  in the axial direction. It is only adequate bending elasticity that is desired. 
         [0051]    It is pointed out at this juncture that the foot region of the compensation element  21  need not imperatively be of flexurally stiff form. Said foot region may be in the form of an elongation of the bending region, such that the bending region and foot region are combined to form one region with identical or very similar characteristics. A rigid foot region however facilitates the coaxial support with respect to the element which transmits the pressing force, in this case the coupling element  15 . Furthermore, a rigid, flexurally stiff foot region can facilitate the pressing of the compensation element  21  into the coupling element  15 . 
         [0052]    In another embodiment which is not illustrated, it is also possible to dispense with a specially designed head region, wherein the bending region and head region may have the same cross section. 
         [0053]    In the embodiment illustrated in  FIG. 2  of a valve  1 ′ according to the invention, substantially only the upper part is illustrated, that is to say the valve itself which is accommodated in the first housing part  5 . 
         [0054]    Said embodiment substantially equates to the embodiment in  FIG. 1  and differs substantially only in two crucial aspects. 
         [0055]    Firstly, the compensation element  21 ′ is designed differently. Secondly, the centering of the coupling element  16 ′ is realized no longer by means of an axially displaceable and radially fixed mounting of the coupling element  15 ′ in the receiving part  13 . 
         [0056]    Instead of a cylindrical bending region with a relatively small diameter, the compensation element  21 ′ has two bending regions, offset with respect to one another by 90° about the longitudinal axis, in the form of constrictions  63 . In the embodiment illustrated in  FIG. 2 , the constrictions have a constant thickness and run parallel and symmetrically with respect to the longitudinal axis of the compensation element  21 ′. The constrictions  63  may however also be of any other form suitable for ensuring respectively adequate bending elasticity in the direction perpendicular to the surface of the constriction or to the longitudinal plane of extent thereof (in the case of a symmetrical embodiment of the plane of symmetry). It is also possible for the axial length of the constrictions  63  to be reduced to such an extent that they are practically in the form of solid joints with corresponding pivot axes running perpendicular to one another. 
         [0057]    The compensation element  21 ′ designed in this way thus also permits a wobbling movement of the rotor  23  and simultaneously transmits the required axial pressing force to the rotor  23 . 
         [0058]    For the centering of the coupling unit  16 ′, the outer diameter of the coupling element  15 ′ is selected to be slightly smaller than the inner diameter of the interior of the receiving part  13 , such that at least pre-centering of the coupling element  15 ′ and thus also of the coupling element  16 ′ and of the rotor  23  in the radial direction relative to the receiving part  13  is generated. 
         [0059]    The annular spring element  27 , which again is in the form of a plate spring, is supported by way of its outer circumference in the radial direction against an encircling axial edge  65  of the radially extending flange of the receiving part  13 ′. By way of its inner surface of the annular chamber, the spring unit  27  acts on the outer circumference of the coupling element  15 ′, wherein the diameter of the respective region of the coupling element  15 ′ even in the unloaded state substantially corresponds to the inner diameter of the spring unit  27 . During the assembly of the switching valve  1 ′, the spring unit  27  is compressed axially such that the inner diameter is reduced and the spring unit  27  acts on the coupling element  15 ′ with a radially inwardly directed force and in this way effects the centering of the coupling unit  16 ′. 
         [0060]    The mode of operation of the switching valve  1 ′ otherwise corresponds to the mode of operation of the switching valve  1  illustrated in  FIG. 1 , such that in this regard, reference is made to the statements made above. 
         [0061]    The following description applies to both variants of switching valves  1 ,  1 ′ as per  FIGS. 1 and 2 , wherein the description is given only with reference to  FIG. 1 , and can be transferred analogously to the embodiment as per  FIG. 2 . 
         [0062]    In the embodiment illustrated in  FIG. 1 , in the lower region of the first housing part  5 , there is provided a planetary gear set which, in the exemplary embodiment illustrated, has two transmission ratio stages. The gearing unit in the form of the planetary gear set therefore has two sun gears  39 ,  41  which have in each case a hollow axle. A hollow cylindrical drive output shaft  43  of a drive unit  45  is inserted into the hollow axle of the sun gear  39 . The sun gear  39  is connected rotationally conjointly to the drive output shaft  43 . The drive unit  45  and the gearing unit  37  together form the drive device for the rotational movement of the rotor  23  of the switching valve  1 . 
         [0063]    Of three planet gears or planet wheels  47  of the second stage of the planetary gear set, only one planet wheel  47  is visible in  FIG. 1 . Each planet wheel  47  has a coaxial pen  49  which engages into a corresponding receiving bore in the lower face side of the wall of the receiving part  13 . In this way, each of the planet wheels  47  and thus the entire drive output of the planetary gear set, is connected to the receiving part  13 , such that the receiving part  13  can hereby be driven in rotation. 
         [0064]    A transmission element  51  in the form of a bar-shaped element  53  is guided through the hollow cylindrical drive output shaft  43  of the drive unit  45  and through the sun gear  41 . The bar-shaped element engages by way of its upper end into the lower receiving opening of the coupling element  15  and is connected rotationally conjointly to the coupling element  15 , for example by adhesive bonding, welding or the like. 
         [0065]    As can be seen from  FIG. 1 , the bar-shaped element  53  engages through the drive device formed by the gearing unit  37  and the drive unit  45 , and is supported by way of its lower end in a receptacle in the base of the second housing part  7 . 
         [0066]    Here, the bar-shaped element  53  jointly performs every rotational movement of the rotor  23 , wherein the rotational movement is loaded at most by frictional forces and otherwise takes place in a load-free manner. As a result, there are practically no torsional forces whatsoever within the bar-shaped element, such that the rotational position of the lower end of the bar-shaped element  53  constitutes an exact reproduction of the rotational position of the rotor  23 . 
         [0067]    It is pointed out at this juncture that said exact reproduction is subject at most to a degree of play that arises as a result of the connection of the rotor  23  to the coupling element  15  via the connecting bolts  25 . This is because the receiving bores in the rotor  23  must be slightly larger than the outer diameter of the connecting bolts  25  in order to permit a slight wobbling movement of the rotor  23 . This is necessary in order to compensate manufacturing and/or assembly tolerances which result in the face surface of the rotor  23  and the face surface of the stator  29  not being positioned exactly in alignment. Said wobbling movements are made possible by the fact that the rotor  23  rests by way of its lower face surface on the face surface of the compensation element or the wobble bar  21 . The latter is dimensioned such that it can be deformed within the required narrow limits by the high axial forces that must be transmitted via the compensation element  21  from the coupling element  15  to the rotor  23 . 
         [0068]    By contrast, as already stated, the rotational movement of the bar-shaped element  53  takes place in a substantially load-free manner. 
         [0069]    In the lower region, that is to say in the rear region of the drive unit  45 , there is provided a device  55  for detecting the rotational position of the rotor. Said device comprises a marker element  57  which is connected, in the lower region, to the bar-shaped element  53 . For this purpose, the marker element  57  may have a central bore through which the bar-shaped element  53  extends. The fixing may be realized for example by means of adhesive bonding or by means of a knurled screw. Furthermore, the device for detecting the rotational position of the rotor comprises a sensor element  59  which is arranged so as to be situated opposite the circumferential surface of the substantially cylindrical marker element  57 . The marker element may for example have magnetic or optical marks or markings along its circumferential surface, the movement or position of which marks or markings is detected by the sensor element  59 . The signal of the sensor element  59  may be supplied to an evaluation and control unit (not illustrated in any more detail). The latter can actuate the drive unit  45 , as a function of the signal from the device for detecting the rotational position of the rotor  23 , such that the rotor  23  is moved in a controlled manner into a predefined rotational position. 
         [0070]    The marker element  57  and the sensor element  59  may for example be designed such that magnetic or optical position detection takes place. A suitable sensor element  59  is for example a Hall sensor for detecting magnetic marks of a corresponding magnetic marker element  57 . As an optical detector or optical sensor element  59 , use may be made, for example, of a photodiode which detects light reflected by the optical marks of the respective optical marker element  57 . For this purpose, the optical marks of the marker element  57  may for example also be illuminated by an LED. 
         [0071]    In principle, however, any device for detecting the rotational position of the rotor is suitable which is capable of detecting the circumferential position or circumferential movement of the transmission element  51  in the form of the bar-shaped element  53  and generating a corresponding signal. 
         [0072]    As a result of the provision of corresponding marks on the marker element  57 , it is possible here to determine both the absolute rotational position and also the relative rotational position in relation to a predefined initial value. 
         [0073]    In an embodiment which is not illustrated, the device  55  for detecting the rotational position of the rotor may also be designed such that the transmission element  51  or the bar-shaped element  53  actuate a potentiometer, preferably the sliding contact of a potentiometer. Even though, in this embodiment, the position detection does not take place in a contactless manner, it can be realized in an extremely simple and inexpensive form. 
         [0074]    The embodiment of a switching valve  1  illustrated in  FIG. 3  differs from the embodiment as per  FIG. 1  substantially merely in that the device for detecting the rotational position of the rotor is arranged not radially at the lower end of the bar-shaped element  53  but rather axially. For this purpose, the device  53  comprises a marker element  57 ′ which, in turn, has a coaxial bore into which the lower end of the bar-shaped element  53  engages. In the case of this marker element  57 ′, however, the marker surface itself or the markings are provided not on the radially outwardly pointing circumferential surface, as is the case in the embodiment as per  FIG. 1 , but rather on the downwardly directed face surface  57 ′a thereof. 
         [0075]    As illustrated in  FIG. 2 , the marker element  57 ′ may also be formed in two parts and have a receiving part  57   1  which, on the face side, has a pot-shaped recess in which the marker element itself, for example a radially magnetized permanent magnet  57 ′ 2 , is arranged and held. 
         [0076]    The retention and guidance of the bar-shaped element  53  is realized in the exemplary embodiment illustrated in  FIG. 3  merely by the fastening to the coupling element  15  and by means of the engagement through the drive output shaft  43  and through the sun gears  39  and  41 . The mounting and guidance of the bar-shaped element  52  may however also be realized additionally or exclusively in the housing part  7 . 
         [0077]    Opposite the face surface  57 ′a of the marker element  57 ′ there is arranged a sensor element  59 ′ in the form of a Hall sensor chip which is capable of detecting the magnetic marker (radially running) of the radially magnetized permanent magnet  57   2 . The sensor element  59 ′ is provided on a printed circuit board  61  on which are also provided the electronics for the evaluation and generation of a signal which represents the absolute or relative position of the rotor  23 . 
         [0078]    Since the device  55  or the evaluation electronics is situated at the axially rear end region of the switching valve  1 , it is possible in all of the embodiments as per  FIGS. 1 to 3  for the switching valve to be inserted by way of its front head region (for example including the entire first housing part  5 ) into a wall of a column oven. Owing to the position of the device  55 , the sensor device and/or corresponding evaluation electronics are prevented from being exposed to inadmissibly high temperatures. 
         [0079]    It is pointed out here that it is self-evidently also possible in the embodiment as per  FIG. 1  for evaluation electronics or detection electronics for generating a suitable analogue or digital signal, which represents the position of the rotor in the form of a digital or analogue signal, to be provided in the lower base region of the second housing part  7 . 
         [0080]    Furthermore, said structural form has the advantage that the device for detecting the rotational position of the rotor is provided axially in the rear region, and a highly compact structural form can be attained overall. In particular, the structural form is not increased with regard to its radial extent by the provision of a corresponding device  55 . This makes it possible for multiple such switching valves  1  to be positioned, for example inserted into the wall of a column oven, with a very small spacing between their axes.