[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2016 207 340.0, filed on Apr. 29, 2016 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
       FIELD OF INVENTION 
       [0002]    The disclosure relates to a guide ring of a piston pump for a vehicle brake system, in which a piston of the piston pump for a vehicle brake system is supported movably in a cylinder liner of the piston pump for a vehicle brake system by means of the guide ring. 
       BACKGROUND 
       [0003]    A vehicle, in particular a passenger vehicle, is generally equipped with a vehicle brake system that has at least one wheel brake for braking a wheel of said vehicle. In this context, the wheel brake is often of hydraulic design and receives a flow of a fluid or brake fluid, which is contained within a fluid line. A hydraulic unit inserted in the fluid line pressurizes the fluid by means of a pump, usually by means of a piston pump for a vehicle brake system having a piston capable of rectilinear movement. During the movement, the piston is guided in a cylinder liner by means of a guide ring. During the movement, the piston performs a guided backward and forward movement within the guide ring. 
         [0004]    During the backward and forward movement, the piston enters and exits from a pressure piston, in which the fluid is contained. At the same time, there is air acting on the piston outside said pressure chamber, in particular in a drive chamber for driving the movement of the piston. In this case, the fluid is at least temporarily under high pressure, while the air is under the usual air pressure of the environment. 
       SUMMARY 
       [0005]    According to the disclosure, a guide ring of a piston pump for a vehicle brake system is provided in which a piston of the piston pump for a vehicle brake system is supported movably in a cylinder liner of the piston pump for a vehicle brake system by means of the guide ring. In this arrangement, a seal, by means of which the passage of air between the piston and the guide ring is prevented during movement of the piston, is held on the guide ring. 
         [0006]    In previous piston pumps for vehicle brake systems, a sealing ring is formed between the cylinder liner and the piston. This sealing ring is arranged as a separate component in addition to the guide ring by means of which the piston is guided. This sealing ring forms a seal between the piston and the cylinder liner against the passage of liquid and, at the same time, against the passage of air. The effect of these two functions of this known sealing ring against the passage of liquid and against the passage of air is that the sealing ring cannot be matched in an optimum manner to the respective individual function and that compromises have to be made as regards the requirement for sealing against the passage of liquid and sealing against the passage of air. 
         [0007]    According to the disclosure, on the other hand, a seal is deliberately attached to or held on a guide ring. This seal specifically prevents air from flowing through between the piston and the guide ring, which would otherwise be possible. Thus, the seal spans or closes an interspace between the guide ring and the piston against the passage of air. For this purpose, the seal on the guide ring according to the disclosure rests against the piston, and the piston slides past the seal during its movement. By virtue of the fact that the seal of this kind forms a seal between the guide ring and the piston, a tendency for the passage of air even to arise at a sealing ring arranged adjacent thereto is prevented, in particular. More particularly, it is thereby possible to match and optimize said sealing ring specifically to a sealing function as a liquid seal. Thus, the solution according to the disclosure not only separates the two sealing functions of sealing against the passage of liquid and against the passage of air but furthermore transfers the process of sealing against the passage of air to the guide ring. The guide ring simultaneously assumes an additional function. Thus, this sealing against air by means of the guide ring advantageously takes place on the side of the piston on which the air-filled drive chamber is also situated. 
         [0008]    The seal according to the disclosure is advantageously formed by means of a flexible plastic, in particular an elastomer. The seal is preferably joined to the guide ring by means of an injection molding process. In an injection molding process, plastic is liquefied and injected under pressure into a mold. Particularly in the case of a guide ring, which is already composed of plastic, it is advantageously possible, by means of an injection molding process, to mold on a seal composed of a different plastic. It is thereby possible to produce a material connection between the guide ring and the seal. Here, the guide ring is preferably produced from a harder plastic than the seal, which is then embodied from a softer flexible plastic. With the seal, the guide ring thus forms a unit which can be installed in a single installation step. 
         [0009]    As an alternative, the seal is advantageously press-fitted on the guide ring. Press-fitting the seal on the guide ring imposes on the seal a compressive force which deforms said seal. The deformation counteracts a restoring force of the seal, by means of which the seal is pressed and nonpositively held against the guide ring. The guide ring furthermore advantageously has an undercut, in which the seal can be fixed positively in addition or as an alternative to a nonpositive joint. Another advantage of a press-fitted seal is that such a seal can be preinstalled on the associated guide ring and is exchangeable. 
         [0010]    An annular recess, in which the seal is inserted, is furthermore preferably formed in the guide ring, wherein, in particular, the recess is designed as a step-shaped chamfer. During preassembly, the seal can be inserted and, in particular, press-fitted easily into the annular recess. In particular, the recess is embodied as a partial negative form of the inserted seal. In the case of a seal inserted in this way, a combination of nonpositive engagement and positive engagement is then preferably employed. 
         [0011]    Preferably, the piston can furthermore be moved into a pressure chamber for a pressure buildup, and the seal is arranged on that side of the guide ring which faces away from the pressure chamber. A pressure buildup in the pressure chamber is then performed by means of the piston. As it moves into the pressure chamber, the piston displaces a fluid contained therein out of the pressure chamber. During this process, the pressure in the pressure chamber increases. As the piston is retracted from the pressure chamber, a vacuum furthermore forms there. The vacuum has the tendency to make air flow along the piston into the pressure chamber. Since the seal according to the disclosure is arranged on that side of the guide ring which faces away from the pressure chamber, the seal is forced against the piston by the air owing to this tendency for air to flow in along the piston. Improved sealing is thereby achieved. This also prevents air from already penetrating into the gap, situated behind it, between the piston and the guide ring. The assembly is thus advantageously sealed by means of the seal on its side of the guide ring facing away from the pressure chamber. 
         [0012]    As a preferred option according to the disclosure, the seal is furthermore embodied with at least one sealing lip, which rests against the piston and extends substantially in the axial direction thereof in the installed state. In the installed state, the sealing lip hugs the piston by means of a longitudinal side facing the piston. In this case, the sealing lip extends in an axial direction virtually parallel to the piston. During the backward and forward movement of the piston, the sealing lip slides by means of the longitudinal side facing the piston along the surface of the piston. Advantageously according to the disclosure, the contact of the sealing lip prevents the sealing lip from tilting during the backward and forward movement of the piston and thus reduces mechanical stress on the sealing lip. Since, advantageously according to the disclosure, the sealing lip rests by means of its longitudinal side on the piston, the wear on the associated tip of the sealing lip is also low. In addition, the sealing lip advantageously compensates, as a kind of spring, for a possible movement of the piston in a radial direction. 
         [0013]    The seal is furthermore preferably embodied with an annular sealing element, and the sealing lip is arranged on that side of the sealing element which faces away from the pressure chamber. A seal in the form of a ring surrounds the piston, which is cylindrical in this case, and seals it over its entire circumference. As explained above, arranging the sealing lip on the side facing away from the pressure chamber has the effect that a contact pressure acts on the sealing lip by means of air pressure. This contact pressure forces the sealing lip against the surface of the piston. Advantageously according to the disclosure, the sealing effect of the sealing lip is increased by means of the contact pressure. 
         [0014]    The seal is furthermore advantageously embodied with a circular sealing element, and the inside diameter of the sealing element is smaller in the uninstalled state of the piston than the outside diameter of the piston. A smaller inside diameter of the sealing element than the outside diameter of the piston has the effect that, during the installation of the piston, the sealing element, which is flexible in this case, is forced outward by the piston at the inside. Since the flexible sealing element tends to retain its original shape, the sealing element presses against the piston on the inside. Advantageously according to the disclosure, a sealing surface is formed between the piston and the sealing element. As a particularly advantageous characteristic, the sealing element is embodied with a convex surface segment toward the piston. By means of the convex surface segment curved toward the piston, the contact pressure in the sealing element is lower at both axial ends of the surface segment than in the center. In this way, sliding of the piston on the sealing element during a backward and forward movement of the piston is made easier in a particularly advantageous manner. As an alternative to the convex surface segment, the sealing element is embodied with a concave surface segment there. Advantageously according to the disclosure, the ends of the surface segment are then forced axially outward in the installed state of the piston and thus enlarge the sealing surface of the sealing element. 
         [0015]    The seal according to the disclosure is preferably embodied by means of an elastomer which has an elasticity modulus of from 0.30 to 30.00 N/mm 2 , preferably of from 5.00 to 25.00 N/mm 2 , particularly preferably of from 10.00 to 15.00 N/mm 2 . It has been found that, in the present case a seal with this elasticity modulus has a particularly advantageous sealing effect with respect to air. The abovementioned elasticity modulus has the advantage that the seal hugs the piston particularly well. At the same time, it does not excessively hinder a backward and forward movement of the piston owing to friction between the seal and the piston. In particular, the seal is preferably embodied from the material EPDM. 
         [0016]    The disclosure is furthermore also directed to a use of a guide ring according to the disclosure in a piston pump for a vehicle brake system. In the case of such a use, the abovementioned advantages are obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Illustrative embodiments of the solution according to the disclosure are explained in greater detail below with reference to the attached schematic drawings, in which: 
           [0018]    There are shown in: 
           [0019]      FIG. 1  shows a partial longitudinal section through a piston pump for a vehicle brake system, having a first illustrative embodiment of a guide ring according to the disclosure with a seal; 
           [0020]      FIG. 2  shows a detail of a first variant of the seal shown in  FIG. 1 ; 
           [0021]      FIG. 3  shows a detail of a second variant of the seal shown in  FIG. 1 ; 
           [0022]      FIG. 4  shows a detail of a third variant of the seal shown in  FIG. 1 ; 
           [0023]      FIG. 5  shows a detail of a fourth variant of the seal shown in  FIG. 1 ; 
           [0024]      FIG. 6  shows a partial longitudinal section through a piston pump for a vehicle brake system, having a second illustrative embodiment of a guide ring according to the disclosure with a seal; 
           [0025]      FIG. 7  shows a detail of the seal shown in  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION 
       [0026]      FIG. 1  shows part of a piston pump  10  for a vehicle brake system, said pump being used to produce hydraulic pressure at wheel brakes of a vehicle. The pressure is produced by means of the piston pump  10  for a vehicle brake system in that a piston  12  moves into a pressure chamber  14  and, in the process, subjects a liquid  16  to excess pressure relative to the environment. The movement of the piston  12  is produced by means of an eccentric drive in an eccentric chamber  18 . In this case, the eccentric drive converts a rotary motion of a drive shaft into a translational motion of the piston  12 . After this driven movement into the pressure chamber  14 , the piston  12  is forced back out of the pressure chamber  14  by means of a spring. During this return movement of the piston  12 , a vacuum forms in the pressure chamber  14  relative to the environment and relative to the pressure in the eccentric chamber  18 . 
         [0027]    During its backward and forward movement, the piston  12  is guided in a cylinder liner  22  of the piston pump  10  for a vehicle brake system by means of a guide ring  20  of L-shaped cross section. The guide ring  20  is inserted with an accurate fit at the outside into the cylinder liner  22  and, on the inside, surrounds the piston  12  in a ring shape. At the same time, a narrow gap in the form of an annular interspace  28  remains between the outside of the piston  12  and the inside of the guide ring  20 . 
         [0028]    A seal  24  is arranged on the guide ring  20  on the side facing the eccentric chamber  18 . The seal  24  is used to prevent air  26  from flowing from the eccentric chamber  18  into the interspace  28 . Thus, the seal  24  spans or closes the interspace  28  between the guide ring  20  and the piston  12  with respect to the eccentric chamber  18 . The seal  24  of this kind is simultaneously used to prevent air from reaching the pressure chamber  14  from the eccentric chamber  18 , especially when the piston  12  is moved out of the pressure chamber  14  and a vacuum arises in said chamber. 
         [0029]    In the axial direction of the piston  12 , a flexible sealing ring  30  of substantially H-shaped cross section is arranged between the cylinder liner  22  and the piston  12  on that side of the guide ring  20  which faces the pressure chamber  14 . On this sealing ring  30 , each of the edge regions is rounded. 
         [0030]    In this arrangement, there is also a disk-shaped, radially oriented backing ring  32  between the guide ring  20  and the sealing ring  30 . On the outside, the backing ring  32  extends as far as the cylinder liner  22  and, on the inside, it extends as far as the piston  12  and thereby prevents wear on the sealing ring  30  in the edge regions thereof facing the guide ring  20 . 
         [0031]    The sealing ring  30  separates the liquid  16  contained in the pressure chamber  14  from the guide ring  20  and from the eccentric chamber  18  in a manner known per se, especially when the piston  12  is moved into the pressure chamber  14  and a vacuum arises in said chamber. 
         [0032]    Thus, the interspace  28  is delimited with respect to the eccentric chamber  18  by means of the seal  24  and, at the same time, with respect to the pressure chamber  14  by means of the sealing ring  30 . In this way, the tasks of sealing against air passing out of the eccentric chamber  18  into the pressure chamber  14  and of sealing against liquid passing out of the pressure chamber  14  into the eccentric chamber  18  are separated from one another at the guide ring  20 . Overall, a particularly advantageous sealing assembly is thereby created. 
         [0033]    According to  FIG. 1 , the guide ring  20  has a recess  34  of rectangular cross section on the side facing away from the pressure chamber  14  and facing the eccentric chamber  18 . The recess  34  faces the piston  12  and forms a step-shaped chamfer, into which the seal  24  is press-fitted as an annular sealing element that can be installed separately. Like the sealing ring  30 , the seal  24  here rests on an outside  36  of the piston  12 . As an alternative, the seal  24  is molded into said recess  34  and onto the guide ring  20  so as to be fixed thereon by means of an injection molding process. 
         [0034]      FIGS. 2 to 5  show variants of the seal  24 , in each case in cross section. The seals  24  shown can each be press-fitted into the recess  34  of the guide ring  20  or molded on by means of an injection molding process. In  FIGS. 2 to 5 , the seals  24  are in each case shown before installation in the associated assembly. In particular, therefore, the seals  24  are shown without deformation relative to the piston  12 , which is then adjacent thereto. The outside  36  of the piston  12  is in each case illustrated by a dashed line  38  in  FIGS. 2 to 5  for the installed state of the seals  24 . 
         [0035]      FIG. 2  shows a seal  24  of substantially rectangular cross section with straight lateral edges. The seal  24  has a seal surface  40  in the direction of the piston  12 . The seal surface  40  is embodied as a surface which is curved convexly outward in cross section. According to  FIG. 3 , the seal  24  shown there is embodied at the seal surface  40  as a surface which is curved concavely inward in cross section. During installation on the piston  12 , the seal is compressed in the region of the seal surface  40 . In the installed state of the piston  12 , the compressed seal  24  then hugs the piston  12 , preferably completely, by means of its seal surface  40 , as shown in  FIG. 1 . 
         [0036]      FIG. 4  shows a seal  24  on which a sealing lip  42  is integrally molded. The sealing lip  42  is formed on the seal  24  on the edge facing the eccentric chamber  18  and the piston  12  and, in cross section, extends substantially axially into the eccentric chamber  18 . During installation of the seal  24 , the sealing lip  42  is placed against the piston  12  and, during this process, is aligned in a fully axial orientation. Owing to its elastic restoring force  44 , the sealing lip  42  then hugs the outside  36  of the piston  12  by means of its lateral surface, which is at the bottom in  FIG. 4 . 
         [0037]    In addition, the seal  24  can be designed in such a way that, as shown in  FIGS. 2 and 3 , it also hugs the outside  36  of the piston  12  by means of its seal surface  40 .  FIG. 5  shows a variant thereof, in which the seal surface  40  is embodied as an undulating surface in cross section. 
         [0038]      FIG. 6  shows another illustrative embodiment of a piston pump  10  for a vehicle brake system. 
         [0039]    There, the associated guide ring  20  does not have a recess  34 . On the contrary, the associated seal  24  is of L-shaped cross section and one leg of its L shape, the radially oriented leg, is molded directly, in a fixed manner, onto the guide ring  20  by means of an injection molding process at that end of the guide ring  20  which faces the eccentric chamber  18 . The second, axially oriented leg of the L shape forms a sealing lip  42  which, like the sealing lip shown in  FIGS. 4 and 5 , comes to rest against the outside  36  of the piston  12 . 
         [0040]      FIG. 7  illustrates how the seal  24  of this kind appears in the uninstalled state. In cross section, the sealing lip  42  of the seal  24  then extends slightly obliquely inward in a radial direction, with the result that, after installation, it likewise gives rise to a restoring force  44  acting against the outside  36  of the piston  12  owing to its elasticity.
A guide ring for a piston pump of a vehicle brake system includes a body and a seal. The body is configured to movably support a piston of the piston pump in a cylinder liner. The seal is held on the guide ring and configured to inhibit passage of air between the piston and the guide ring during movement of the piston.