Abstract:
An electromagnetically actuated valve with a valve body which is assigned to a valve seat, is mechanically prestressed in a first direction toward a first switching position and can be adjusted by means of a magnetic actuator in a second, opposite direction into a second switching position is disclosed. The valve body has a planar contact surface which can be brought into sealing contact with a surface of the valve seat, which surface is planar at least in sections.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to German patent application no. 10 2010 005 168.3, filed Jan. 20, 2010, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The disclosure relates to a preferably electro-magnetically actuated valve and in particular to a low-pressure valve, as used, for example, in a hydraulic machine. 
         [0003]    Valve-controlled hydraulic machines of this type are known, for example, from EP 1 537 333 B1. The European patent document shows a hydraulic machine of axial or radial piston construction which can be operated in principle as a motor or as a pump, with the volumetric delivery or capacity being adjustable via the valve timing gear. In an exemplary embodiment which is described, the hydraulic machine is embodied in the form of an axial piston machine, wherein a multiplicity of pistons arranged in a cylinder is supported on a rotatably mounted swash plate. Each piston together with the associated cylinder space delimits a working space which can be connected via a valve on the low-pressure side and a valve on the high-pressure side to a pressure medium inlet or to a pressure medium outlet. 
         [0004]    In the known solution, the two valves are embodied in the form of electrically releasable or lockable nonreturn valves which are actuable via the pump timing gear in order to operate the particular working space in “full mode”, in “partial mode” or in “idle mode”. As a result, the volumetric delivery or capacity can be adjusted in an infinitely variable manner from a maximum value to 0. The hydraulic machine is operated in accordance with a regulating algorithm via a control unit in order to obtain a total volumetric delivery flow (pump) or total capacity flow (motor) with as few pulsations as possible. The volumetric flow is frequently adjusted by a phase-gating control, but may also be adjusted by a phase-chopping control. 
         [0005]    Hydraulic machines with a capacity/volumetric delivery which can be changed via the valve timing gear are also referred to as digital displacement units (DDU). All positive displacement principles are basically applicable in this case. However, piston machines, in particular of radial piston construction, are advantageous, since said piston machines make it possible to separately form and therefore actively control the input and output for each positive displacer. In this case, it may be highly expedient to differentiate between pump and motor operation such that then the control element may differ in appearance for the low-pressure and high-pressure connections. 
         [0006]    A prerequisite for the above-described type of control (DDU) is that the valves on the low-pressure and high-pressure sides can be switched in highly dynamic fashion such that the above-described pressure medium flow paths can be very rapidly blocked off or opened up for flow. The control elements on the low-pressure side or high-pressure side can be embodied in the form of, for example, switching valves, preferably of seat-type construction, which are preferably actuable by a magnetic actuator. Various and sometimes contradictory requirements are imposed on a valve of this type. A hydraulic machine which is minimized in respect of construction space requires a valve, the overall dimensions of which are small and which has a flow cross section with is as large as possible and is therefore low in resistance. However, such a large flow cross section requires a greater valve lift, this contradicting the requirement for high valve dynamics with as little electrical power consumption as possible. Furthermore, the requirements imposed on the valve vary at different operating points of the hydraulic machine. For example, large volumetric flows and requirements for low switching times arise from high rotational speeds, and low rotational speeds are associated with long switching-on periods for the magnet coils. The mechanical loads and requirements imposed on the sealing system also change via the pressure prevailing at the particular connection. 
         [0007]    U.S. Pat. No. 7,077,378 B2 discloses a valve on the low-pressure side for a hydraulic machine of this type, wherein an annular throughflow opening is closed by an approximately cup-shaped valve element. Said annular throughflow opening is bounded by an inner and an outer sealing seat, and therefore the specific surface pressure on the sealing edge is comparatively low compared to a conventional valve cone. In the known solution, the platelike or cup-shaped valve element is prestressed into an open position via a spring and can be adjusted by means of a magnetic actuator into its closed position in which the valve element rests on the above-described sealing seats and the throughflow opening is blocked. During flow through said valve, the platelike valve element, which is prestressed in the opening direction thereof, can be acted upon by flow forces effective in the closing direction, and therefore the throughflow cross section is reduced and, correspondingly, the pressure loss is increased. Although said undesirable closing movement could be countered by a more powerful opening spring, the valve dynamics would deteriorate as a result or a more powerful magnetic coil together with associated power electronics would be necessary. To avoid this drawback, use is made, according to U.S. Pat. No. 7,077,378 B2, of a permanent magnet which acts upon the valve element in the open position thereof with a magnetic force. When the magnetic actuator is energized, the field of the permanent magnet is neutralized and, in addition, a magnetic force which is effective in the closing direction is generated. With a solution of this type, a correspondingly more efficient magnetic actuator is therefore required. In addition, this solution does not exhibit the desired dynamics either, since the field of the permanent magnet has to be weakened first before the valve element can be moved in the direction of the closed position thereof. Furthermore, a relatively high contact pressure force is required between the valve element and valve seat in order to ensure adequate tightness of the valve in the closed state. 
       SUMMARY 
       [0008]    By contrast, the disclosure is based on the object of providing a low-pressure valve of this generic type, the valve having improved functionality. Furthermore, a hydraulic machine equipped therewith is to be provided. 
         [0009]    This object is achieved by means of a low-pressure valve with the features of the present disclosure and a hydraulic machine according to the present disclosure. 
         [0010]    According to the disclosure, the electromagnetically actuated valve of the low-pressure-valve type is formed with a valve body which is assigned to a valve seat, is prestressed in a first direction, preferably the opening direction, and can be adjusted by means of a magnetic actuator in a second direction, preferably the closing direction. The valve is provided with a flat, preferably two-edge sealing system. This means that a flat or planar (plane) annular contact surface is formed on the valve body and can be brought into sealing contact with the likewise planar (plane) valve seat. The planar contact surfaces can be produced in a simple manner, ensure a fluidtight contact connection between the valve body and valve seat, and reduce the surface pressure. 
         [0011]    It is advantageous to divide the contact surface of the valve body radially into an outer and inner sealing edge by an encircling (annular) groove. Said groove (weakening in the material) brings about easier axial movability of the inner sealing edge (sealing surface) with respect to the outer sealing edge (sealing surface) such that the two sealing edges can easily move relative to each other in the axial direction. The tightness of the valve can thereby be improved. 
         [0012]    A preferred development of the disclosure makes provision for a relative tilting movement to be possible between a valve tappet or magnet armature and the valve body mounted thereon. This is structurally achieved by a bore in the valve body having a radial excess size in relation to the valve tappet or magnet armature such that the valve body is held on/at the valve tappet with radial play. The valve body or the planar contact surfaces thereof can thereby be placed in a sealing manner on the (planar) valve seat even if the valve seat is aligned with respect to the valve tappet with relatively great tolerances. The outlay on production can be further reduced as a result. 
         [0013]    According to a particular aspect of the disclosure, the valve tappet is formed or provided with only one magnet armature, wherein the valve seat bushing itself has a guide bore for the axially displaceable mounting of the valve tappet. Said guide bore forms the sole mounting of the valve tappet, and therefore tolerance chains caused by components which are used for the mounting of the valve tappet and are constructed next to one another are not produced. By this means, the functional capability of the valve is improved and the outlay on manufacturing reduced. 
         [0014]    According to a further particular aspect of the disclosure, the low-pressure or outlet valve equipped with a flat, preferably two-edge sealing system is formed merely with the valve seat bushing and without an outer valve housing, wherein the valve seat bushing is screwed directly into the housing of the hydraulic machine. That is to say, in this case, all of the fluid flow passages which are assigned to the low-pressure valve and to date were formed in the valve housing or the outer valve bushing are now formed in the housing of the hydraulic machine. It is thereby possible to increase the flow cross sections in the valve because of the omission of the outer valve bushing and therefore to realize an increased volumetric flow. In this case, the valve may have the multiple magnet-armature construction or the simplified individual magnet-armature construction. 
         [0015]    Finally, according to the disclosure, a hydraulic machine with preferably actively controllable valves on the low-pressure side and with valves on the high-pressure side is proposed. At least one of the valves on the low-pressure side is designed as a low-pressure valve according to one of the preceding aspects. 
         [0016]    At this juncture, it should also be pointed out that the active controllability of the valves on the high-pressure side is not required if only pump operation is to be provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The disclosure is explained in more detail below using preferred exemplary embodiments and with reference to the accompanying drawings, in which: 
           [0018]      FIG. 1  shows a longitudinal section through a valve on the low-pressure side of a hydraulic machine (for example of a swash plate compressor or radial piston compressor/motor) in the open state in the form of a reference valve, 
           [0019]      FIG. 2  shows a partial longitudinal section of a valve according to a first exemplary embodiment in which the structural features differing from the reference valve are illustrated, 
           [0020]      FIG. 3  shows an enlargement of the connecting section between the closing element and armature tappet of the valve according to the disclosure from  FIG. 2 , 
           [0021]      FIG. 4  shows a longitudinal section through a valve on the low-pressure side according to a second exemplary embodiment of the disclosure, and 
           [0022]      FIG. 5  shows a longitudinal section through a modified valve according to the second preferred exemplary embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]      FIG. 1  shows the basic construction of a nonreturn valve on the low-pressure side, in the form of a reference valve for the subject matter of the disclosure, said valve corresponding to the outlet valve in a hydraulic motor. In a pump, the valve would be the inlet valve or suction valve. 
         [0024]    According to  FIG. 1 , the low-pressure valve or outlet valve  22  has a valve body  40  which is prestressed into an open position by a spring  38  and can be adjusted by means of a magnetic actuator  30  into a closed position against a valve seat  42  such that an annular throughflow cross section  44  is blocked. The outlet valve  22  is embodied in the form of a “disk valve”, the valve element  40  having a tappet  46  which bears an approximately mushroom-shaped closing disk  48  on the lower end section thereof in  FIG. 1 . When the throughflow cross section  44  is open (view according to  FIG. 1 ), a pressure medium connection between a connecting passage A on the inlet side and a passage B opening in a working space  8  of the hydraulic machine is opened, and therefore pressure medium can flow from the inlet passage A into the working space  8  or in the opposite direction, from the working space  8  to the passage A. During passage of the flow from A to B, virtually no flow forces effective in the closing direction occur, and therefore the valve disk  48  could in principle be held in the open position thereof solely by the force of the spring  38 . Upon flow from B to A, the forces resulting from the pressure medium flow act in the closing direction, and therefore the spring  38  is no longer sufficient by itself in order to hold the valve disk  48  in the open position thereof. In order to fix the open position, which is illustrated in  FIG. 1 , of the valve disk  48 , the actuating magnet  30  is formed, according to the disclosure, with a main coil  50  and a secondary coil  52  to which a main armature  54  and a secondary armature  56  are respectively assigned. According to the disclosure, the valve disk  48  is held in the open position thereof by the secondary coil  52  being energized. The latter, upon being energized, generates a magnetic field by means of which—as explained in more detail below—the valve body  48  is held in the open position thereof. 
         [0025]    According to the disclosure, in the illustrated basic position of the low-pressure valve or outlet valve  22 , the main coil  50  can also be energized. The magnetic field generated in the process acts in the closing direction on the valve disk  48 —but in the relative position illustrated, the force acting on the valve disk  48  via the coil  50  is smaller than the force generated via the secondary coil  52 , and therefore the valve disk  48  is held in the illustrated open position when the coils  50 ,  52  are energized simultaneously. 
         [0026]    The valve  22  has a valve bushing/collet  58  which can be screwed into a corresponding bore in a hydraulic machine housing  2 . The valve bushing  58  opens axially via the passage B in the working space  8  and furthermore has a star-shaped radial bore  60  opening in the passage B. A seat bushing  62 , in the circumferential wall of which an outlet passage  64  opening at one end toward the passage A and at the other end toward the valve seat  42  is formed, is screwed into the valve bushing  58 . A multiplicity of connecting webs  66  dividing the annular outlet passage  64  into circular ring segments are formed in that end section of the outlet passage  64  which is on the valve-seat side. The mouth regions of said circular ring segments form an encircling inner sealing edge  68  and an outer sealing edge  70  which are each positioned obliquely with respect to the valve axis and on which, when the outlet valve  22  is closed, encircling sealing surfaces  72 ,  74  of the valve disk  48  rest in a sealing manner such that the annular throughflow cross section  44  is blocked. 
         [0027]    According to the illustration in  FIG. 1 , the valve disk  48  is of mushroom-shaped design, with the two sealing surfaces  72 ,  74  being formed on the rear side which faces away from the working space  8 . In the exemplary embodiment illustrated the valve tappet  46  passes through a rearwardly projecting hub projection  76  of the valve disk  48  where it is held in a rotationally fixed manner. In the exemplary embodiment illustrated, the tappet  46  and the valve disk  48  are axially connected via a tension spring  78  which is held in a spring holder  80  of the spring plate  48  and is supported on the base of the spring holder  80  via a spring plate. That end section of the tension spring  78  which is remote from said base acts on a spring plate  82  which is fastened to the lower end section, in  FIG. 2 , of the tappet  46  and runs somewhat spaced apart axially from the adjacent end surface  84  of the spring plate  48  such that the latter is mounted in a spring-elastic manner on the tappet  46  by means of the force of the tension spring  78 . 
         [0028]    That part of the tappet  46  which upwardly adjoins the hub projection  76  in  FIG. 1  is held in an axially displaceable manner in a guide bushing  86  of a multi-part coil former  88  which dips by means of radial projections into corresponding recesses of the seat bushing  62  and is secured there via a spring ring  90 . The guide bushing  86  has an axial guide bore  92  for the tappet  46 . In the closed position, the hub projection  76  dips into an end recess  94  of the guide bushing  86 . 
         [0029]    Radially outside the guide bore  92 , an annular recess  96 , into which sections of the secondary coil  52  are inserted, is provided on the guide bushing  86 . The secondary coil  52  is covered toward the top ( FIG. 1 ) by a magnetizable pole ring  98  which is adjoined in the axial direction by a separating ring  100 . Said separating ring  100  is supported on the base of a casing section  102  of the guide bushing  86 , which dips into a corresponding inner recess  104  of the seat bushing  62 . A coil holder  106  which extends downward toward the base of the guide bushing  86  with an axial projection  108  is inserted into said region of the guide bushing  86  which is engaged around by the casing section  102 . Said axial projection  108  is engaged around by the main coil  50  which is therefore inserted into the annular space between the coil holder  106  and the casing section  102  of the guide bushing  86 . That end surface of the main coil  50  which is located at the bottom in  FIG. 2  is supported axially on the separating ring  100  via a further pole ring  110 . 
         [0030]    Level with the separating ring  100 , the guide bushing  86  has a disk-shaped section  87  made of a para-magnetic material, for example a stainless steel, which section is connected, for example by welding, to the two other parts of the guide bushing  86  which conduct the magnetic field, and at the same time contributes to the magnetic fields of the two coils  50  and  52  being clearly separated from each other. When the coil is energized, there is therefore no dynamic effect on the valve body  40  in an undesired direction. 
         [0031]    That end section of the tappet  46  which is located at the top in  FIG. 1  passes through an inner bore  112  in the coil holder  106 . Said inner bore is widened in the central region thereof to form a spring space  114  for the spring  38 . The latter is supported at one end on an annular shoulder  116  of the spring space  114  and acts at the other end on the main armature  54  which, together with the adjacent end surface of the axial projection  108  of the coil holder  106 , delimits a main working air gap  118  of a main stage of the magnetic actuator  30 . To optimize the characteristic of the magnet, the main armature  54  dips with an armature projection  120  into an end recess  122  of the axial projection  108 . 
         [0032]    That end surface of the main armature  54  which is located at the bottom in  FIG. 1  and is remote from the main working air gap  118  bears against a distance washer  124  via which the secondary armature  56  is spaced apart in the axial direction from the main armature  54 . The secondary armature  52  acts by means of its end surface, which is formed with an annular projection  126 , on a radially projecting supporting shoulder  128  of the tappet  46  such that the force of the spring  38  is transmitted via the main armature  54 , the spacer ring  124  and the secondary armature  56  to the tappet  46  and prestresses the latter downward ( FIG. 1 ) such that the spring plate  48  is held by the spring force in its illustrated open position. A secondary working air gap  130  which is minimal in the open position of the outlet valve  22  is formed between that end surface of the secondary armature  52  which is provided with the annular projection  126  and the correspondingly configured end surface of the guide bushing  86 . 
         [0033]    The two coils  50 ,  52  are energized via the control electronics  132  fitted on the multi-part coil former  86 . 
         [0034]    When the secondary coil  52  is energized, the valve disk  48  is magnetically locked in the open position thereof. In order to close the valve disk  48 , as already explained above, first of all the two coils  50 ,  52  are energized in the illustrated open position of the outlet valve  22 , with the abovementioned secondary working air gap  130  being minimal. In this position, the main working air gap  118  is at maximum, and therefore the magnetic force acting on the main armature  54  is correspondingly small and the valve disk  48  therefore continues to be locked in its open position by the force of the spring  38  and the magnetic field generated by the secondary coil  52 , even when the current strength is low. As can be gathered from the illustration according to  FIG. 2 , the separating ring  100  which is produced from para-magnetic material causes the magnetic fields of the two coils  52 ,  54  to be separated. Without such a separation of the magnetic fields, the magnetic field generated by the main coil  50  could flow through the secondary working air gap  130  or the magnetic field generated by the secondary coil  52  could flow through the main working air gap  118 , and therefore a magnetic force could be generated in the undesired direction. Accordingly, the components are selected in respect of the material such that there need not be any concern that the two magnetic fields will combine when the coils  50 ,  52  are energized simultaneously. For this reason, the seat bushing  62  is also produced from a para-magnetic material. The components of the coil former  88  (guide bushing  86 , coil holder  106 , pole rings  98 ,  124  and separating ring  98 ) are preferably connected frictionally to one another and behave as a single component within the valve structure. 
         [0035]    Owing to the already minimal secondary working air gap  130 , even at a low current level, the secondary armature  56  develops a sufficient force in order to keep the outlet valve  22  open. The locking force can be matched to the use conditions by varying the current level. This may be required, for example at higher rotational speeds of the hydraulic machine, if the pistons  6  press high volumetric flows through the outlet valve  22 . 
         [0036]    As soon as the magnetic field of the main coil  50  has been built up, the secondary coil  52  is switched currentlessly to close, and therefore the magnetic force component which is effective in the opening direction is dispensed with, and the main armature  54  carries out a stroke upward counter to the force of the spring  58  and, in the process, closes the main working air gap  118 . The main armature  54  carries along the tappet  46  at the same time here, and therefore the secondary armature  52  is moved upward in the axial direction and the secondary working air gap  130  is enlarged. The valve disk  48  executes a corresponding stroke until the two sealing surfaces  72 ,  74  rest on the sealing edges  78  and  70 , respectively, and the throughflow cross section  44  is closed. Said closing force can likewise be adjusted, again by varying the current level in order to energize the main coil  50 . 
         [0037]    An advantage of this concept is that the magnetic field of the main coil  50  is already completely built up before the outlet valve  22  is closed and therefore exerts the maximum possible force on the valve disk  48 . The valve disk  48  is then virtually prestressed. The comparatively small secondary coil  52  converts the activating signal (independently on/off) substantially more rapidly owing to its significantly smaller time constant in comparison to the main coil  50 , thus increasing the dynamics of the valve. 
         [0038]    In order to open the valve, the main coil  50  is switched currentlessly such that the valve disk  48  is moved back into the basic position thereof by the force of the spring  38 . Said opening movement can be assisted by the secondary coil  52  being energized, and therefore the resetting movement of the valve disk  48  is accelerated. This permits higher valve dynamics, both in the closing direction and in the opening direction of the outlet valve  22 , than in conventional solutions. 
         [0039]    A valve according to a first exemplary embodiment of the disclosure is described below with reference to  FIGS. 2 and 3 . Only the features which differ from the reference valve according to  FIG. 1  are dealt with here, with all of the other structural features and functions being the same as in the reference valve. To this extent, reference is made at this juncture to the description above in respect of the features which are not expressly illustrated and/or described in  FIGS. 2 and 3 . 
         [0040]      FIG. 2  is a partial longitudinal sectional view of the low-pressure or outlet valve  22  according to the disclosure, the view showing the valve seat section  42  together with the associated valve body  40 . 
         [0041]    In the first preferred exemplary embodiment of the disclosure according to  FIG. 2  too, the valve body  40  is formed as a mushroom-shaped valve disk consisting of an outer sealing ring  1 , an inner valve body hub  2  and a number of radially extending webs/spokes  4  which connect the valve body hub  2  to the outer sealing ring  1  with throughflow openings  6  being formed. The radially outer sealing ring  1  has a planar (flat) contact surface  10  into which an encircling groove  12  is incorporated (milled). By this means, the contact surface  10  is divided into two sealing lips or sealing edges  14 ,  16  which are spaced apart radially by the groove  12 . 
         [0042]    On the side of the seat bushing  62 , the valve seat  42  is likewise formed by a planar end surface  18  of the seat bushing  62 , in which end surface the throughflow cross sections  44  which are separated from one another by the webs  66  open out in an encircling axial groove  45 . The sealing edges  14 ,  16  which are spaced apart radially from each other are oriented here in such a manner that, when the valve  22  is closed as per  FIG. 2 , said sealing edges sit in a sealing manner on the planar end surface  18  of the seat bushing  62  and therefore close the axial groove  45  in the end surface  18  of the seat bushing  62 . Owing to the groove  12 , it is reliably ensured that both the sealing edge  14  and the sealing edge  16  bear against the seat body  62 . 
         [0043]    The inner valve body hub  2  is configured in a similar manner to the valve body hub according to  FIG. 1 , i.e. with the hub projection  72 , in which the valve tappet  46  is guided in a sliding manner, and with the spring holder  80 , into which the tension spring  78  is inserted, the tension spring pressing the valve body  40  against the valve seat  42 . However, in contrast to the reference valve, the valve body hub  2  does not protrude axially over the sealing edges  14 ,  16  but rather is set back axially behind the sealing edges  14 ,  16 . For this reason, the planar end surface  18  of the seat bushing  62  does not have to be formed with any end recess, as in the reference valve, or the end recess  94  can be formed as a flat and optionally planar turned groove such that the guide bore  92  in the seat bushing  62  obtains a maximum length. 
         [0044]    As can furthermore be gathered from  FIG. 2 , in the closed state of the valve  22 , the two sealing edges  14 ,  16  protrude radially in the region of the valve seat  42  into the mouth openings of the passages  64  such that only a radially outer or inner part of each sealing edge  14 ,  16  is in contact with the end surface  18  of the seat bushing  62 . By this means, the mouth opening edges press slightly into the flat sides of each sealing edge  14 ,  16 , thus resulting in a fluidtight closure of the mouth openings. 
         [0045]      FIG. 3  shows the connecting region between the valve tappet  46  and valve body hub  2  on an enlarged scale. 
         [0046]    According thereto, the valve tappet  46  penetrates the valve body hub  2  with an annular gap  20  (illustrated exaggerated in  FIG. 3 ) of preferably approx. 0.1 mm width being formed. By this means, the mushroom-shaped valve body  40  can not only be displaced axially in relation to the valve tappet  46  within the scope of the maximum adjustment distance of the tension spring  78  in order to compensate for an excessive stroke of the valve tappet  46  and in order to achieve a sufficiently high contact pressure of the valve body  40  against the valve seat  42  but can also tilt slightly with respect to the valve tappet  46 . By means of this tilting movement which is permitted to a limited extent, the valve body  40 , upon coming into contact with the valve seat  42 , is matched virtually automatically to deviations in the orientation of the end surface  18  of the seat bushing  62  and therefore ensures a secure sealing seat on the seat bushing  62 . This is also assisted by the relatively short guidance of the valve body  40  on the valve tappet  46  (owing to the axially retracted hub projection  76 ). Therefore, tolerances, for example involving perpendicularity to the valve seat  42  and valve body  40 , do not have to be selected to be as exacting as in the reference valve according to  FIG. 1 , which simplifies the manufacturing of the components and therefore also improves the functionality. 
         [0047]    Furthermore, the inner section of the spring holder  80  is formed with a radial constriction  24  which serves to guide the tension spring  78  and therefore replaces or renders superfluous the additional spring plate (shown in  FIG. 1 ) on the inner spring seat. 
         [0048]    The manner of operation of the valve  22  according to the first preferred exemplary embodiment of the disclosure is the same as for the reference valve according to  FIG. 1 , and therefore reference can be made at this juncture to the corresponding passages in the description. A further crucial factor in the first exemplary embodiment according to the disclosure is the double flow around the valve body  40  in the region of the outer sealing ring or closing member  1  which, in the open state of the valve  22 , opens up two throughflow cross sections (circulating flow profiles) between itself and the valve housing or the valve bushing  58 . 
         [0049]      FIG. 4  shows a second exemplary embodiment of a valve  22  according to the disclosure. In this case too, only those technical features which differ from the first preferred exemplary embodiment of the disclosure will be described in more detail below. Otherwise, reference is made to the above passages in the description which also apply to the second exemplary embodiment. 
         [0050]    In the valve according to the disclosure according to  FIG. 2 , as in the reference valve according to  FIG. 1 , the magnet armature consists of a total of four individual components, namely
       the guide rod or valve tappet  46 ,   the main armature  54 ,   the secondary armature  56  and   the distance washer or spacer ring  124 .       
 
         [0055]    The valve tappet  46  here is guided in two further components, namely
       the coil former  88  and   the cover or coil holder  106 .       
 
         [0058]    This arrangement is disadvantageous in so far as, firstly, the outlay on manufacturing to produce the four abovementioned components with extremely exacting tolerances is very high and, secondly, the installation has to be carried out with great care in order to achieve exact axial orientation of the components for clamping-free movement of the valve tappet. In addition, the guides in the coil former  88  and in the coil holder  106  have to be manufactured with a relatively large amount of play in order to avoid jamming of the magnet armature or valve tappet  46 . 
         [0059]    In the valve  22  according to  FIG. 4 , the magnet armature  134  is manufactured from a single component. Put in other words, the valve  22  according to  FIG. 4  has the valve seat bushing  62 , on the planar seat bushing end surface  18  of which the valve seat  42  is formed (as in  FIGS. 2 and 3 ). A guide bore  36  (in contrast to  FIGS. 2 and 3 ) is formed in the valve seat bushing  62  for the direct, displaceable mounting of the valve tappet  46  in the valve seat bushing  62 . Main and secondary armatures according to the first exemplary embodiment of the disclosure and also the reference valve are replaced by the single/individual magnet armature  134  which is either placed onto the valve tappet  46  or is formed integrally therewith. 
         [0060]    Furthermore, the valve tappet  46  ends below the coil holder  106  which, in the present exemplary embodiment, is screwed onto the end sides of the webs  66 . In contrast to the previous exemplary embodiment according to  FIG. 2 , the coil holder  106  therefore does not have any guiding function. The fit between the valve tappet  46  and valve seat bushing  62  is selected here to be highly exacting (substantially free from play) in order, inter alia, to keep tilting of the valve tappet  46  to a minimum. Should slight tilting nevertheless occur, the further pole ring  110  takes on the function of a second guide. 
         [0061]    Another advantage of this arrangement according to the second preferred exemplary embodiment of the disclosure resides in simpler manufacturing and a reduction in the tolerance chains. In addition, the friction of the magnet armature/valve tappet  46  is reduced, which further increases the valve dynamics. 
         [0062]    Finally,  FIG. 5  illustrates a modification of the second exemplary embodiment of the disclosure. 
         [0063]    This involves an individual magnet armature construction, as has been previously described with reference to  FIG. 4  but, in contrast to the exemplary embodiment according to  FIG. 4 , a valve bushing/valve housing is not provided. That is to say, the valve  22  consists exclusively of the valve seat bushing  62  which is screwed directly into the housing of the hydraulic machine. In this case, the fluid passages formed in the valve seat bushing  62  in the first and second exemplary embodiments are formed directly in the housing of the hydraulic machine. The working space  8  receiving the valve body  40  is also located in the housing of the hydraulic machine. 
         [0064]    All of the further structural features are identical to the second exemplary embodiment of the disclosure according to  FIG. 4 . As an alternative thereto, however, the valve according to the first preferred exemplary embodiment of the disclosure could also be formed without the valve bushing/valve housing in order to be screwed directly into the housing of the hydraulic machine. 
         [0065]    The concept according to the disclosure can be used in valves both on the low-pressure side and high-pressure side, which valves may be closed or open when not energized. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           1  Outer sealing ring 
           2  Inner valve body hub 
           4  Radial webs 
           6  Throughflow openings 
           8  Working space 
           10  Planar contact surface 
           12  Encircling groove 
           14  Radially outer sealing edge 
           16  Radially inner sealing edge 
           18  Seat bushing end surface 
           20  Annular gap 
           22  Outlet valve 
           24  Constriction in the spring holder 
           26  Magnetic actuator 
           30  Magnetic actuator 
           34  Control unit 
           36  Guide bore 
           38  Spring 
           40  Valve body 
           42  Valve seat 
           44  Throughflow cross section 
           45  Axial groove 
           46  Tappet 
           48  Valve disk 
           50  Main coil 
           52  Secondary coil 
           54  Main armature 
           56  Secondary armature 
           58  Valve bushing 
           60  Star-shaped radial bore 
           62  Seat bushing 
           64  Outlet passage 
           66  Web 
           68  Sealing edge 
           70  Sealing edge 
           72  Sealing surface 
           74  Sealing surface 
           76  Hub projection 
           78  Tension spring 
           80  Spring holder 
           82  Spring plate 
           84  End surface 
           86  Guide bushing 
           87  Disk-shaped section 
           88  Coil former 
           90  Spring ring 
           92  Guide bore 
           94  End recess 
           96  Recess 
           98  Pole ring 
           100  Separating ring 
           102  Casing section 
           104  Inner recess 
           106  Coil holder 
           108  Axial projection 
           110  Further pole ring 
           112  Inner bore 
           114  Spring space 
           116  Annular shoulder 
           118  Main working air gap 
           120  Armature projection 
           122  End recess 
           124  Spacer ring 
           126  Annular projection 
           128  Radial shoulder 
           130  Secondary working air gap 
           132  Control electronics 
           134  Individual magnet armature