REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/261,220 filed Mar. 16, 2012, the entire subject matter of which is incorporated herewith by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to a solenoid valve for controlling a fluid having a first housing part, with an axial guide oriented toward the interior for an armature that, under the action of a solenoid at least partially surrounding the first housing part to the outside, can be moved in the axial guide. A reset spring and a closing element acts on the valve closing member and has a second housing part located coaxially to the first housing part. 
     BACKGROUND OF THE INVENTION 
     DE-U-85 22 724 describes a solenoid valve with a first cylindrical housing part formed from a ferromagnetic material and a second, sleeve-shaped housing part that likewise is of a ferromagnetic material for accommodating a magnet armature controlling a valve seat body of the valve. For this purpose, the valve seat body has several hydraulic medium ports that, in the base position of the solenoid valve, are hydraulically separated from one another by a plunger attached to the magnet armature. The two housing parts themselves are magnetically separated from one another, but are physically connected by a ring-shaped, further housing part made of a nonmagnetic material in this respect. The magnetic separation by the further housing part serves among other things to oppose a magnetic short circuit in any case and to ensure effective feed of the magnetic lines of force into the magnetic armature if the actuating coil of the known solution is energized. 
     Due to the further housing part being of a nonmagnetic material, the known solution is, however, relatively expensive to implement for magnetic separation. Especially when the known solenoid valve is used at high pressures, a failure site is known to be formed by the other ring-shaped housing part for the magnetic separation. 
     To correct these disadvantages, DE 100 38 139 B4 has already proposed in a guide element of a magnetizable base material with at least one region of reduced magnetizability being implemented as an integral component of the base material. In one configuration of the known teaching, a circumferential groove is made in a pressure pipe suitable for solenoid valves and is provided with an additive material of reduced magnetizability by a laser application method or laser welding method, where austenitic materials are indicated as especially suitable, such as in particular nickel, chromium, and manganese. These laser treatment methods are also associated with a certain cost. 
     EP 0 951 412 B2 discloses a generic solenoid valve for the actuation of liquid and gaseous working media. This solenoid valve is usable especially for hydraulic brake systems for motor vehicles. The known solenoid valve has a first, preferably cylindrical, housing part surrounded by a solenoid and forming a receiver for an armature. The receiver forms an axial guide for the armature. An energy storage mechanism in the form of a reset spring acts on the armature together with a valve stem that acts on it and that, designed as a kind of valve closing member for a closing solenoid valve, presses on a valve seat interacting with corresponding fluid ports within the valve body. Coaxially to the first housing part, a second housing part has a cylindrical recess that extends in the longitudinal extension of the solenoid valve and that is connected to the valve ports and is located in the valve seat. 
     The first and second housing parts are each produced in one piece from a ferromagnetic material that surrounds the armature in the form of a thin-walled sleeve serving as a pole tube. The sleeve wall thickness remains the same over its length for reducing a magnetic short circuit and should be no greater than is necessary for reliable accommodation of the mechanical stress. The integral execution of the first housing part with the second housing part necessary in this respect is done preferably via metal cutting steps and can presuppose correspondingly high production and mounting accuracies. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved solenoid valve that is simple to produce and mount and that ensures reliable, effective magnetic separation to prevent short circuits. 
     This object is basically achieved by a solenoid valve where the first housing part is provided with a wall reduction that is oriented toward the interior in the direction of the armature and of the second housing part and that at least partially effects a magnetic separation of the two housing parts from one another. The wall reduction can be easily obtained using production technology, for example, using a rolling treatment tool or pressing tool. In particular, the solution according to the invention does not require the introduction of additive materials to help prevent a magnetic short circuit and to be able to effect magnetic separation to increase the efficiency of the magnetic system. As a result of the depression made from the interior and forming the wall reduction of the first housing part, the material of the wall of the housing is compressed in this respect so that even at higher compressive stresses, reliable steadying by the wall compression is achieved. Alternatively, the depression can be made by cutting so that the material is not compressed in this region. 
     For the purposes of this invention, the reference to a wall reduction expresses the fact that, viewed geometrically, a recess is on the inside of the wall of the housing part; but no weakening of the material in terms of a lack of compressive stiffness need be assumed. For one with average skill in the art in the field of solenoid valve technology, it is surprising that reliable magnetic separation is achieved with a wall reduction and still produces effective compressive steadying by the compression of the remaining wall thickness material. Even in the event of failure of the solenoid valve, high pressures can then still be reliably accommodated by parts of the magnetic housing relative to the fluid flow or flow of media. 
     The first housing part is preferably formed in the manner of a thin-walled sleeve as a pole tube in the region of the axial guide for the (magnet) armature, with the sleeve projecting into or beyond the second housing part. Because the first housing part in the overlapping region with the second housing part is constituted as a thin-walled sleeve, the first housing part can be fixed on the second housing part by a forming process, for example, by flanging, without the compressive stiffness of the overall device being adversely affected in the process. 
     Advantageously, a groove is around the outer circumference on the second housing part, with the edge of the first housing part be crimped into it. In addition to a compressively very strong connection of the first housing part to the second housing part, a sealing contact connection is also created in this way. 
     An especially reliable arrangement for the solenoid valve according to the invention is achieved if the wall reduction is provided in an overlapping region of the armature with the second housing part. A ring-shaped end of the armature overlaps a step-shaped shoulder on a side of the second housing part that is adjacent to the armature. Especially preferably, in a position spaced away from the armature to the housing part, a middle section of the groove in the sleeve overlaps an empty space in the overlapping region so that in this way, in addition to a magnetic decoupling, a reliable, power-guiding introduction of the magnetic lines of force into the armature can take place with the valve actuating part. 
     The individual components of the solenoid valve can also be more easily mounted owing to the simplified handling of two housing parts that are kept shorter in their axial length. 
     In one structurally advantageous version of the solenoid valve, in the region where the groove is located, the armature is formed as a cylinder that, in a traveling motion, partially crosses the second housing part formed as a piston in this region. Thus, regardless of its position, the piston is continuously in centered engagement with the first housing part. 
     One option of manually opening the solenoid valve in an emergency operation for a malfunction, for example, dictated by a failure of the solenoid, is enabled by a rod-shaped stem supported in the second housing part to be axially displaceable. It is able to be actuated from the outside by a set screw guided to be able to turn in the second housing part. The extension itself is provided with a radial widening on its one free end and, in this way, is secured to be axially immovable in the set screw. However, when the set screw or actuating screw is turned by hand, the stem can move the armature, and in this respect the tip, connected to the armature as a closing element, is lifted by the valve closing member. The closing element with its tip is inserted into the armature on its free end side and is held by a steel ring and a flange. Even for a tight armature, releasing the valve closing member is thus easily possible as soon as the rod-shaped stem with its widened end takes hold of the armature after an idle stroke. In the armature, the closing element itself can have a certain radial play so that even in the event of production-dictated alignment errors, the cone of the closing element with its free tip strikes the pilot seat on the valve closing member in the middle without difficulty. 
     In the valve member, a nonreturn valve is preferably installed that, together with the tip of the closing element, clears or closes a bypass bore in the valve closing member. The reset spring or another type of energy storage mechanism is preferably to be located in a recess in the armature around the stem. The assignment of the valve ports and the shape of the valve closing member can be configured such that flow through the solenoid valve in both directions is possible. The reset spring can be supported on the armature and the second housing part preferably to bring about a closed position of the valve closing member. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a side elevational view of a solenoid valve according to an exemplary embodiment of the invention, not to scale. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a longitudinal section through a solenoid valve  1  for controlling a fluid, such as a liquid working medium for a hydraulic consumer of a hydraulic system, which consumer is not detailed. The solenoid valve  1  is made as a piloted valve that can be activated electromagnetically. In its essential parts, the solenoid valve  1  comprises a cylindrical first housing part  2  with dimensions that change incrementally along its outer and inner diameter. 
     With its connecting branch  32 , shown on the right edge in  FIG. 1 , having an O-ring  34  inserted into a circumferential groove  33 , the first housing part  2  forms a fluidic connection to a hydraulic system (not shown), especially to a hydraulic consumer. The connecting branch  32  is part of a valve body  35  in whose axial region the first housing part  2  has a large wall thickness compared to its other wall regions. In this respect, the connecting branch  32  forms a type of cartridge valve. A valve port  29  extending radially in the valve body  35  from the two sides can be connected to its valve port  30  routed centrally and axially out of the connecting branch  32 , where a valve closing member  7  made as a valve spool controls this fluid-carrying connection. The valve closing member  7  is made as a stepped spool and is axially actuated by a closing element with tip  27 . The press fit  24  is created by a seal  25  in the manner of an O-ring fitting into corresponding receivers resting on the closing element and on the armature  4 . The valve closing member  7 , with its right edge shown in  FIG. 1 , forms a valve seat together with the bore constituting the valve port  30 . 
     The conical closing element with its tip  27  rests in a first bypass bore  28  in the valve closing member  7  with its right free end. A second bypass bore  28 ′ extending through the valve closing member  7  is spaced radially by a distance relative to the first bypass bore  28  projecting centrally through the valve closing member  7  and is arranged parallel to the first bypass bore  28 . The second bypass bore  28 ′ is connected to the valve port  29  to carry fluid via a corresponding throttle site. Likewise, the bypass bore  28  is connected via a throttle site to a rear pressure space  36 , where the tip of the closing element in the operating position shown in the FIGURE mates with this throttle site. In this respect, the first bypass bore  28 , with the tip  27  of the closing element removed, forms a fluid-carrying connection with respect to the rear pressure space  36  extending between magnet armature  4  and the facing end side of the valve closing member  7 . 
     A nonreturn valve  26  is located on the end of the valve closing member  7  facing the valve port  30 . In the direction of the FIGURE, on the left the first housing part  2  undergoes transition from the valve body  35  into a thin-walled sleeve  14  forming the pole tube within the solenoid valve. Furthermore, the sleeve  14  forms an axial guide  3  for the armature  4  over a length corresponding to about half its total length. A lubricating medium can be placed between the magnet armature  4  and the sleeve  14 , or parts of the inner housing wall of the sleeve  14  are widened in their outside diameters relative to the other wall parts somewhat in diameter so that offset lubricating pockets form in which the fluid then forms a kind of sliding seal for the magnet armature  4 . 
     The rod-shaped stem  8  extends through the piston-like armature  4  that can move depending on the energizing of a solenoid  5  surrounding the sleeve  14  in its essential length. The stem  8  is guided with an extension  11  through a recess  10  of a second housing part  9  whose free end region is adjoined by a set screw  23 . Set screw  23  is rotationally guided by a corresponding inner screw connection in the second housing part  9 . When a set screw head  37  with a knurl is turned, the stem  8  can be moved and can clear the valve closing member  7  via the armature  4  and the closing element that has been inserted into the armature  4  with tip  27  in the sense of an opening position. External cover caps can be removed from the other magnet housing parts for the corresponding emergency operation. 
     Like the first housing part  2 , the second housing part  9  is essentially a cylindrical, one-piece body, with corresponding diameter adjustments along its outer side. The second housing part  9  is surrounded by the sleeve  14  of the first housing part  2  to approximately to half of the axial extension, forming a plug connection  12 , with the second housing part  9  having a circumferential groove  16  into which an edge  15  of the sleeve  14  is crimped. The forming region  13  of the sleeve  14  in the longitudinal section is constituted as right-angle bend; i.e., after forming, the edge  15  comes to rest offset in parallel in the circumferential groove  16 . This molding process can be carried out with a pressing tool or rolling treatment tool. An O-ring  38  between the sleeve  14  and the second housing part  9  in a groove of the second housing part  9  seals the first housing part  2  against the second housing part  9 . 
     For magnetic separation of the two housing parts  2 ,  9 , in the axial middle of the solenoid  5 , the sleeve  14  is provided with a wall reduction  17  to approximately half of the other wall thickness of the sleeve  14 . The wall reduction  17  is formed by a groove  18  with flanks  20  that extend flat on the inner circumference  19  of the sleeve  14 . This wall reduction  17  encompasses the inner region of the sleeve  14  in the manner of a ring. The wall reduction  17  facilitates the magnetic separation, especially between the two housing parts  2 ,  9 . The armature  4  is located in its forward position shown in the FIGURE. Between an annular end  39  of the armature  4  and an adjacent, assigned step-shaped shoulder on a side of the second housing part  9  adjacent to the armature  4 , an empty space is formed in that forward position that further promotes the magnetic separation and allows a defined transition of the lines of force from the armature  4  to the housing part  9 . With this type of production of the magnetic separation of the two housing parts  2 ,  9  from one another, no thermal loading of the components is necessary at all, such as, for example, in the known hard-facing of a nonmagnetic material. When the components are joined, stresses and material distortion that adversely affect the accuracy cannot then occur. The empty space can also be filled with fluid that can be accordingly displaced again in the travelling motion of the armature  4 , for example, via the sliding seal of the armature  4 . 
     A small radial play of the armature  4  is in the region of the wall reduction  17 . In all other respects, the armature  4  with its ring-shaped end  22  is guided on the second housing part  9 . In this way, in the overlapping region  21  a type of piston-cylinder arrangement is implemented and acts on the armature  4  to center it so that it is always supported in a guided manner on the second housing part  9  at its left and right free ends. 
     As an energy storage mechanism, a reset spring  6  is placed around the stem  8  in a cylindrical bore of the recess  31  that extends from the end  39  of the armature  4  that faces the second housing part  9 . The reset spring  6  is supported on a seal arrangement  40  around the stem extension  11 . The seal arrangement  40  is placed around the extension  11  in the manner of a gland packing and seals the stem  8  in turn to prevent loss of a hydraulic medium relative to the second housing part  9 . In the intermediate space between the armature  4  and the second housing part  9 , hydraulic media can appear that, via a longitudinal bore  42  in the armature, can traverse the armature  4  from its rear end side to the front end side in the direction of the pressure space  36  and vice versa. In this way, balancing of the hydraulic medium is achieved so that in the region of the armature  4  neither an overpressure nor an underpressure can build up that could otherwise lead to problems in the operation of the armature  4 . The solenoid valve  1  furthermore has a shielding housing, especially around its solenoid  5 , with appropriate plastic and elastomer material being used here. This magnet structure is conventional for solenoid valves and is not further detailed. To energize the solenoid  5 , a plug  41  is used. The plug  41  is attached to the upper side wall of the valve housing for linking to a control and current supply unit, not detailed. 
     Flow can take place through the valve body  35  of the solenoid valve  1  both from the valve port  29  to the valve port  30  and also vice versa. In the energized state of the solenoid  5 , the armature  4  and the closing element with tip  27 , as well as the valve closing member  7 , viewed in the direction of the FIGURE, are moved to the left against the reset force of the reset spring  6 , with flow through the valve body  35  in both directions between the valve ports  29  and  30  then being possible. 
     In the unenergized state of the solenoid valve, however, flow of the hydraulic medium from the valve port  29  to the valve port  30  is prevented; but conversely there is the possibility that the fluid connection may exist between the valve port  30  and the valve port  29 . For this flow direction from port  30  to port  29 , however, the valve spool  7  must be pushed against the force of the reset spring  6 . This pushing takes place, for example, at a pressure difference of about 1.5 bar (check function). As described, the solenoid  5  is not energized. With the solution according to the invention, a type of 2/2-way seat valve is then implemented that, magnetically actuated and piloted as a cartridge valve, can control pressures of even 350 bar and that is closed in the normal state and that, as shown, allows a return or reverse flow function. 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.
A solenoid valve for controlling a fluid has a first housing part ( 2) and an axial guide ( 3) facing inwards for an armature ( 4) that can be displaced in the axial guide ( 3) under the effect of a solenoid ( 5) that at least partially surrounds the exterior of the first housing part ( 2). A return spring ( 6) and a closing element ( 27) impact a valve closing element ( 7). A second housing part ( 9) is arranged coaxially to the first housing part ( 2). The first housing part ( 2) has a reduced wall portion ( 17) that faces inwards towards the armature ( 4) and the second housing part ( 9) and that effects a magnetic separation of the two housing parts ( 2, 9) to at least some extent.