Abstract:
A solenoid valve, particularly for a hydraulic braking system of a motor vehicle, includes a valve sleeve in which a pole core is fixed and an armature is disposed in an axially displaceable manner. The armature includes at least one longitudinal groove in the outer shell surface thereof. A spring washer is configured to be braced between the armature and the pole core. The end face of the armature facing the pole core is configured to be at least substantially convex, and the end face of the pole core facing the armature is configured to be at least substantially concave. A braking system includes the solenoid valve.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/064900, filed on Aug. 30, 2011, which claims the benefit of priority to Serial No. DE 10 2010 042 676.8, filed on Oct. 20, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The disclosure relates to a solenoid valve, particularly for an hydraulic braking system of a motor vehicle, having a valve sleeve, in which a pole core is fixed and an armature is arranged so that it is axially displaceable, wherein the armature comprises at least one longitudinal groove in the outer shell surface thereof, and wherein a disk spring is/can be braced between the armature and the pole core. 
     The disclosure further relates to a braking system, particularly for a motor vehicle, having at least one solenoid valve for controlling and/or regulating a pressure and/or a volumetric flow of a hydraulic medium of the braking system. 
     Solenoid valves and braking systems of the aforementioned type are known from the state of the art. Solenoid valves, which are designed as normally closed solenoid valves, are used, in particular, for safety-relevant brake applications, such as, for example, ABS or ESP systems (ABS=antilock braking system; ESP=electronic stability program). Such valves are capable of assuming at least two different hydraulic settings, for example open or closed, or they are also further adjustable through partial lifting between open and closed as so-called control valves. The closed position of the solenoid valve in the non-energized, i.e. unactuated state is usually ensured by way of a compression spring. A magnetic or electromagnetic actuator, which comprises an electromagnetic coil and magnetically active components in the solenoid valve, serves for switching or actuation of the solenoid valve. The components provided here in the valve are a pole core and an axially displaceable armature. On actuation, a pole core enclosed by the electromagnetic coil exerts a tensile force on the armature, in order to displace the latter axially, thereby exposing a valve opening. Such solenoid valves usually have a magnetic force characteristic such that the magnetic force increases very sharply (exponentially) as the working air gap diminishes, that is to say as the distance between the armature and the pole core diminishes. A known way of optimizing the magnetic force characteristic is to arrange a disk spring, which acts in parallel with the compression spring and which may be magnetizable, between the pole core and the armature. As the working air gap diminishes here, a radially running secondary flow occurs, the radial component of which no longer contributes to the axial force and allows only a shallow rise in the latter—despite the diminishing working air gap. It is thereby possible to exert a beneficial influence on the magnetic force, so that it approximates to the ideal state. 
     Providing the armature with at least one longitudinal groove, which serves to equalize the hydraulic pressure between the working air gap and the pressure chamber at the valve tip assigned to the armature, is also known. In hitherto known solenoid valves, however, the longitudinal groove means that at its outer circumference the spring disk does not bear fully on the end face of the armature facing the pole core. Particularly in the case of a disk spring having one or more radial recesses, the spring characteristic would depend overall on the rotational position of the disk spring relative to the armature. Furthermore, the longitudinal groove reduces the fatigue strength of the disk spring, since the material stress increases significantly due to the deformation introduced via the longitudinal groove. 
     SUMMARY 
     A distinctive feature of the solenoid valve according to the disclosure is that the end face of the armature facing the pole core is of at least substantially convex design and the end face of the pole core facing the armature is of at least substantially concave design. This serves to ensure that at its outer circumference the disk spring bears against the pole core and centrally against the armature. The longitudinal groove formed in the armature thereby does not exert any effect on the working of the disk spring. The pole core is, as usual, enclosed over its full circumference by the valve sleeve and also does not have any longitudinal grooves, since the pole core preferably tightly seals off the valve sleeve axially. The convex design of the end face of the armature and the concave design of the end face of the pole core allow the armature, when it is magnetically attracted by the pole core, to penetrate into the pole core in certain areas, the spring arranged between them being braced between the pole core and the armature. Centrally, the end face of the armature preferably has a plane bearing surface for the disk spring, the diameter of the bearing surface having a further influence on the spring characteristic of the disk spring. 
     The disk spring is preferably of annular design, so that it has an outside diameter and an inside diameter. The outside diameter preferably corresponds at least substantially to the outside diameter of the pole core and/or of the armature. The inside diameter is preferably selected according to the desired spring force of the disk spring. 
     The disk spring more preferably comprises at least one radial recess on its outer circumference. The recess serves to optimize the spring characteristic of the disk spring for the respective application. The disk spring is more preferably made from a magnetizable material. 
     The disk spring preferably comprises a plurality of radial recesses distributed, in particular uniformly, over its outer circumference. The radial recesses are more preferably designed and arranged in such a way that the disk spring, at its outer circumference, is of star-shaped design with narrowly tapering spring tongues. Since the disk spring, at its outer circumference, bears on the pole core, all spring tongues of the disk spring are in contact with the pole core and exert a corresponding spring force, irrespective of the rotational position of the disk spring. The advantageous solenoid valve is therefore a simple way of ensuring a desired spring characteristic of the disk spring at all times. 
     According to an advantageous development of the disclosure the end face of the pole core comprises at least one axial projection as an axial stop for the armature. In order to define the minimum working air gap between the pole core and the armature, and/or to determine a maximum deformation of the disk spring, the pole core is provided with the axial projection on its end face. When the solenoid valve is actuated, the armature encounters the axial projection, which serves as axial stop. 
     The axial projection is preferably arranged centrally. Particularly if the disk spring is of annular design, the axial projection is arranged centrally in such a way that it reaches through the recess in the disk spring, and so that the annular disk spring is deformed past the axial projection. 
     For this purpose the axial projection preferably has an outside diameter which is smaller than the inside diameter of the annular disk spring. The axial projection is preferably of a truncated cone shape, so that the inclined outer shell surface thereof serves or can serve as guide for the disk spring. 
     According to a development of the disclosure the armature, in the end face facing the pole core, comprises an opening, through which in certain areas a thrust piece, supported in the armature, is displaceable or through which it reaches. The thrust piece is preferably subjected to a pre-stressing force, which is generated by a compression spring arranged in the armature, so that the thrust piece is pushed in the direction of the opening. Here an axial stop of the armature and an axial stop of the thrust piece each prevent the thrust piece being pushed out of the armature. The thrust piece is designed in such a way that the free end thereof projects through the opening in the armature and bears against the end face of the pole core. The compression spring in the armature therefore generates a pre-stressing force, which pushes the armature away from the pole core in the direction of the valve seat provided on the opposite side, in order to close the valve in the non-energized state. 
     The diameter of the opening is preferably smaller than the outside diameter of the axial projection of the pole core. This ensures that in the actuated or energized state of the solenoid valve the axial projection of the pole core or the axial stop of the pole core bears on the end face of the armature. In particular, the diameters are suitably selected in such a way as to prevent the axial projection from penetrating into the opening and any tilting or jamming of the solenoid valve possibly resulting from this. 
     A distinctive feature of the braking system according to the disclosure is the design of the solenoid valve, as has been described above. The braking system is preferably embodied as an ABS or ESP system of a motor vehicle. The advantageous design of the solenoid valve affords a solenoid valve permanently endowed with the desired spring characteristic, which is easy to produce and to assemble, making the braking system reliable and cost-effective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be explained in more detail below with reference to the drawing, in which: 
         FIG. 1  shows a representation in longitudinal section of a normally closed solenoid valve in the state of the art, 
         FIG. 2  shows an advantageous embodiment of a disk spring of the solenoid valve, 
         FIG. 3  shows an enlarged representation in longitudinal section of an advantageous embodiment of the solenoid valve, 
         FIGS. 4A and 4B  show bearing contacts of the disk spring and 
         FIGS. 5A and 5B  show the advantageous solenoid valve in different operating states. 
     
    
    
     DETAILED DESCRIPTION  
       FIG. 1  shows a simplified representation in longitudinal section of a normally closed solenoid valve  1  in the state of the art. The solenoid valve  1  comprises a valve sleeve  2 , in which a pole core  3  is fixed and an armature  4  is arranged so that it is axially displaceable. The pole core  3  here closes one end of the valve sleeve  2 . A valve body  5 , which comprises a valve seat  6 , into which a valve tip  7  is pushed, is arranged on the opposite side to the pole core  3 . The valve tip  7  is integrally formed with a pin  8 , which is pressed in in an axial recess  9  of the armature  4 . The axial recess  9  extends through the entire armature  4  a thrust piece  10  being arranged at the opposite end to the pin  8 . Viewed in cross section, the thrust piece  10  has a T-shape, the cross member forming an axial stop  11  arranged in the armature  4  and the longitudinal member extending through a tapered portion  12  of the axial recess  9  beyond the end face  13  of the armature  4  facing the pole core  3 . The thrust piece  10  bears on the end face  14  of the pole core  3  facing the armature  4  and therefore bridges the so-called working air gap  15  between the pole core  3  and the armature  4 . Here the end face  14  of the pole core  3  is of convex design and the end face  13  of the armature  4  is of concave design. 
     A compression spring  16  in the form of a helical spring, which pushes the thrust piece towards the pole core  3  and thereby pushes the armature  4  with the valve tip  7  into the valve seat  6 , is braced between the thrust piece  10  and the pin  8 . 
     In its outer shell surface  17  the armature  4  comprises two opposing longitudinal grooves  18  extending over the entire length of the armature  4 . The longitudinal grooves  18  produce a fluid-tight connection between the working air gap  15  and a pressure chamber in the valve body  5  that can be closed by the valve tip  7 , in order to ensure an equalization of pressure between these. 
     If a disk spring, not represented in  FIG. 1  is arranged between the armature  4  and the pole core  3  for influencing and optimizing the spring force characteristic of the solenoid valve  1 , the longitudinal grooves  18  mean that the disk spring is not uniformly stressed and/or does not bear on the end face  13  of the armature  4  over the full circumference. 
     The longitudinal grooves  18  may prevent the disk spring from bearing on the armature  4  in certain areas, particularly if the disk spring, as represented in a top view of the armature  4  in  FIG. 2 , is embodied as an annular disk spring  19 , which at its outer circumference comprises multiple radial recesses  20  (only some of which are provided with reference numerals here) distributed or formed over the circumference. Here, depending on the rotational position of the disk spring  19 , the radial tongues  21  remaining between the radial recesses  20  may lie in the area of the longitudinal grooves  18  and thereby not contribute to the spring force, or may have a negative influence on the spring characteristic actually expected. 
     The solenoid valve  1  is therefore preferably designed in such a way that the end face  13  of the armature  4  facing the pole core  3  is of convex design and the end face  14  of the pole core  3  facing the armature  4  is of concave design, as shown in an enlarged representation in longitudinal section of the solenoid valve  1  in  FIG. 3 .  FIG. 3  here shows only the pole core  3 , the armature  4  and the disk spring  19  arranged between them. It can clearly be seen in  FIG. 3  that owing to the advantageous design of the solenoid valve  1  the disk spring  19  bears at its outer circumference on the pole core  3  and centrally on the armature  4 , so that irrespective of the rotational position of the disk spring  19  the longitudinal grooves  18  of the armature  4  cannot influence the spring stiffness of the disk spring  19 . As shown in  FIG. 2 , the annular disk spring  19  has a central opening  22 . The inside diameter thereof is suitably chosen so that it is larger than the outside diameter of the thrust piece  10  projecting over the end face  13 , so that said thrust piece can pass through the disk spring  19  into bearing contact with the pole core  3 . 
       FIGS. 4A and 4B  show a perspective representation of the armature  4  ( FIG. 4A ) and of the pole core  3  ( FIG. 4B ), the line contact  25  or  26  formed between the disk spring  19  and the armature  4  or the pole core respectively being drawn in according to the advantageous embodiment of the solenoid valve  1  featured here. 
     As shown in  FIG. 3 , the pole core  3  further comprises an axial projection  23  on its end face  14 . The axial projection  23  is arranged or aligned centrally or coaxially and forms an axial stop  24  for the armature  4 . Centrally the end face  13  of the armature  4  comprises a plane bearing surface  27  departing from the convex shape, which defines the maximum bearing area of the disk spring  19  against the armature  4 . Here the chosen inside diameter of the opening  22  of the disk spring  19  is larger than the outside diameter of the axial projection  23 , so that when the armature  4  is drawn against the pole core  3  the axial projection  23  penetrates through the disk spring  19 , and the armature  4  with its end face  13  strikes against the axial stop  24 , as represented in  FIG. 5B . 
       FIGS. 5A and 5B  show the detail in  FIG. 3A  in different operating positions. According to  FIG. 5A  the solenoid valve  1  is on its normally closed state, whilst  FIG. 5B  shows the solenoid valve  1  in the actuated, that is to say energized state, in which the armature  4  is displaced in the direction of the pole core  3 , so that the armature  4  bears with its end face  13  on the axial projection  23  of the pole core  3 . 
     The advantageous geometry of the pole core  3 , of the armature  4  and of the disk spring  19  ensures a line contact  25 ,  26  in the form of a circle at all times, as represented in  FIGS. 4A and 4B . This allows the use of disk springs  19  having radial recesses, as are represented in  FIG. 2 , since the spring characteristic is retained irrespective of the rotational position of the disk spring. The fatigue strength of the disk springs used moreover increases due to the continuous bearing surface or the continuous line contact, the line contact in the case of the disk spring  19  with radial recesses  20  obviously not being continuous but being regularly (as seen in the circumferential direction) and reliably present, irrespective of the rotational position of the disk spring  19 . The spring characteristic can be adjusted via the diameter d 1  of the pole core  3  and the diameter d 2  of the bearing surface  27  of the armature  4 . Varying these dimensions serves to vary the lever arm and therefore the spring characteristic curve of the disk spring  19 . The axial stop  24  for the armature  4  allows a precise adjustment and checking of the pre-stressing force of the compression spring  16  and the disk spring  19 , especially when assembling the solenoid valve  1 .