Abstract:
A piston, e.g., for a control valve in a vehicle compressed air system, is housed in a cylinder in an axially displaceable manner along a longitudinal center axis separating a first cylinder chamber and a second cylinder chamber. The piston has a peripheral annular groove of approximately rectangular cross-sectional in which a sealing ring is inserted, the free end of which is disposed on the inner cover surface of the cylinder. The sealing ring defines, together with at least one groove wall, at least one pressure support chamber The piston also comprises, in the region of the annular groove, at least one axial opening, which joins the at least one pressure support chamber to the associated first or second cylinder chamber. A pneumatic or hydraulic connection independent of any geometric deformations of the sealing ring is enabled between at least one cylinder chamber and the associated pressure support chamber.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a piston, for example for a control valve in a compressed air system of a vehicle. 
       BACKGROUND OF THE INVENTION 
       [0002]    The state of the art discloses a wide variety of pressurized-support sealing elements, such as so-called “Airzet” sealing rings (Airzet PR and Airzet PK), available from Freudenberg Simrit GmbH &amp; Co. KG and elsewhere, for example, groove rings or the like for sealing a piston against a cylinder enclosing it. Thus, DE 296 10 628 U1, for example, discloses a sealing arrangement having an elastic sealing ring, which comprises a retaining part and a sealing part together with an approximately Z-shaped connecting part situated between these two parts. The end faces of the sealing part are formed by projecting strips, between which there are passage cross sections in order to allow the media on both sides of the sealing ring access to the grooves of the Z-shaped connecting part, thereby ensuring the pressurized support effect. For sealing a piston against a cylinder, this sealing arrangement is inserted into a circumferential piston groove. One disadvantage of this sealing arrangement is that, in the event of a deformation of the sealing ring during operation, for example, especially in the case of a radial compression of the latter, the passage cross sections of the apertures situated between the strips may be at least partially closed, so that the full pressurized-support seating effect no longer prevails, which can occur particularly in pistons that are used for control valves of vehicle compressed air systems. Such control valves are disclosed, for example, in DE 10 2009 040 759 A1, DE 10 2009 029 968 A1, DE 195 10 492 A1, DE 10 2006 017 503 A1, or DE 102 45 916 A1. 
       SUMMARY OF THE INVENTION 
       [0003]    Generally speaking, it is an object of the present invention to provide a piston that, through pressurized-support scaling of simple design, allows two cylinder chambers to be sealed off from one another. 
         [0004]    In known pistons, these are sealed in relation to the inner circumferential surface of an assigned cylinder by means of a pressurized-support sealing ring, in which the pressurized-support occurs with the aid of openings formed on the sealing ring, through which the media on both sides of the sealing ring have access to pressurized-support chambers on the scaling ring primarily from a radial direction. Access to such pressurized-support chambers irrespective of such a sealing ring can be created by forming relevant apertures in the piston, which connect each of the assigned cylinder chambers to the respective pressurized-support chambers, irrespective of any deformations of the sealing ring. 
         [0005]    The present invention accordingly relates to embodiments of a piston, for example for a control valve in a compressed air system of a vehicle, which can be accommodated in a cylinder so that it is axially displaceable along a longitudinal center axis, such that it separates a first cylinder chamber and a second cylinder chamber from one another. The piston has a circumferential annular groove having an approximately rectangular cross sectional geometry, in which it is possible to insert a sealing ring, which can be applied with its free end to the inner circumferential surface of the cylinder and which, after insertion into the annular groove of the piston, together with at least one groove wall, defines at least one pressurized-support chamber. In the area of its annular groove, the piston comprises at least one axial aperture, which connects at least one pressurized-support chamber to the assigned first or second cylinder chamber. 
         [0006]    This affords a reliable connection between the (preferably two) pressurized-support chambers and the two cylinder chambers, and, at the same time, a piston of simple design. This connection serves to allow the passage of a fluid and is advantageously maintained irrespective of any geometrical deformation of the sealing ring that might occur, for example in the event of accidental crushing or compression of the ring inside the annular groove of the piston. 
         [0007]    In the context of this description, fluid is taken to mean any gas or a liquid. The piston is preferably used in a pneumatically operated control valve and may have the function of a relay piston. 
         [0008]    Although a piston according to embodiments of the present invention may be equipped with a sealing ring, which together with the groove walls of the annular groove of the piston forms just one pressurized-support chamber, and in which this single, annularly circumferential pressurized-support chamber is connected to its assigned pressure chamber by at least one axial aperture in the groove wall, it is preferable to use a sealing ring-piston combination having two pressurized-support chambers. 
         [0009]    According to an embodiment of the present invention, therefore, the piston, in the area of its annular groove in each of its two groove walls, comprises at least one axial aperture, which connects to pressurized-support chambers to the first or second cylinder chamber respectively assigned to them. The two pressurized-support chambers are formed by the sealing ring and by each of the groove walls respectively assigned to the sealing ring. 
         [0010]    In another embodiment, the first groove wall and the second groove wall each have at least three apertures, which are arranged along the annular groove, circumferentially spaced at uniform intervals from one another. In this way, it is possible to achieve uniform and reliable ventilation of the pressurized-support chambers on the sealing ring by means of the fluid. 
         [0011]    The axial apertures in the first groove wall and in the second groove wall may also, in each case, be arranged circumferentially and radially one above the other. This allows a significant simplification of the production process, since the apertures in the two grooved wails arranged parallel one above the other can be produced in one operation and also by means of one and the same tool. 
         [0012]    With regards to the actual formation of the apertures in the groove wails, according to a first embodiment these may be formed as slit-shaped cutouts running radially and each having an approximately rectangular cross sectional geometry. The apertures in each of the groove walls thereby have an especially large flow cross section for delivering the fluid into the pressurized-support chambers. 
         [0013]    In this context, the cutouts in the first groove wall each preferably run from their radial outer edge radially inwards at least into the area of the first pressurized-support chamber situated below. Despite a minimal structural weakening of the associated groove wall of the piston, this affords an especially effective delivery of the fluid from the first cylinder chamber into the assigned first pressurized-support chamber of the sealing ring. 
         [0014]    The cutouts in the second groove wall may likewise each run from their radial outer edge radially inwards at least into the area of the second pressurized-support chamber situated above. This results in fluid dynamics affording an effective delivery of the fluid from the second cylinder chamber into the second pressurized-support chamber largely irrespective of deformations of the sealing ring, while at the same time minimizing the weakening of the mechanical structure of the piston. 
         [0015]    In order to maximize the utilizable flow cross section, at least one of the cutouts in the first groove wall and/or in the second groove wall in each case may extend from their radial outer edge radially inwards to the groove base. 
         [0016]    In a further embodiment, radially inner end portions of the slit-shaped cutouts each have a rounding. This serves to prevent the generation of mechanical notch stresses and the associated risk of cracking inside the piston. 
         [0017]    According to another embodiment the apertures are formed as axially parallel straight bores in at least one groove wall; these bores are each arranged in the radial area of at least one pressurized-support chamber, and the bores are each introduced perpendicularly to the normal to the surface in the groove wall assigned to the pressurized-support chamber. This minimizes the mechanical weakening of the piston compared to the slit-shaped cutouts and reduces the manufacturing cost of producing the apertures on the piston groove. 
         [0018]    Although the axial apertures in the groove walls, which are formed by slit-shaped and radially running cutouts or axially parallel bores, carry a very slight risk of blockage, the piston, in addition to these axial apertures, may have at least one radial channel in at least one groove wall, which also has a flow connection to an assigned pressure chamber. In a further embodiment, therefore, the piston, in the area of its annular groove in a groove wall, may have at least one axial aperture, which connects a first pressurized-support chamber to a first cylinder chamber, and at least one radial channel, which connects the second pressurized.-support chamber to the second cylinder chamber, may be formed in the other groove wall on the scaling ring side. Although this design is less than ideal, with cleaner fluids and qualitatively high-grade, abrasion-resistant sealing ring material it can be used successfully and highly economically, since the formation of such a radial channel is much more cost-effective than producing the axial apertures in the groove walls. 
         [0019]    The sealing ring is advantageously formed from an elastomer affording high elasticity and great abrasion resistance. This results in efficient mutual sealing of the cylinder chambers while, at the same time, affording great resistance to wear. For example, a hydrogenated acrylonitrile/butadiene rubber (HNBR) may be used, resulting in an excellent sealing effect at the same time as providing a long service life of the pressurized-support sealing between the piston and the cylinder. It is also possible, particularly where a metal piston is used, to produce the axial and radial ventilation apertures by machining. 
         [0020]    In a preferred embodiment of the present invention, the piston is embodied as a control piston of a control valve of a compressed air system of a vehicle. 
         [0021]    This embodiment may be further enhanced in that the control valve comprises at least one relay valve having a control piston. 
         [0022]    Finally, the control valve may additionally be embodied as an axle modulator, which comprises at least one relay valve, preferably two relay valves, each having a control piston. 
         [0023]    Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
         [0024]    The present invention accordingly comprises the features of construction, combination of elements, and arrangement of parts, all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention is described in more detail below with reference to the accompanying drawing figures, in which: 
           [0026]      FIG. 1  shows a partial cross section through a conventional piston having a pressurized-support sealing ring; 
           [0027]      FIG. 2  shows a diagrammatic and perspective partial view of a piston according to an embodiment of the present invention; 
           [0028]      FIG. 3  shows a partial cross section through a piston according to  FIG. 2 ; 
           [0029]      FIG. 4  shows a piston as in  FIG. 2 , but having a sealing ring that forms only one pressurized-support chamber; 
           [0030]      FIG. 5  shows a piston as in  FIG. 2 , in which a radial channel is formed on the lower annular groove wall; and 
           [0031]      FIG. 6  is a diagrammatic, sectional representation of a control valve having a piston according to an exemplary embodiment of the present invention. 
       
    
    
     LIST OF REFERENCE CHARACTERS 
       [0032]      10  conventional piston 
         [0033]      12  annular groove 
         [0034]      14  sealing ring (“Airzet” sealing ring) 
         [0035]      16  retaining portion on the sealing ring  14   
         [0036]      18  sealing portion on the sealing ring  14   
         [0037]      20  intermediate portion on the sealing ring  14   
         [0038]      22  first pressurized-support chamber on the sealing ring  14   
         [0039]      24  second pressurized-support chamber on the sealing ring  14   
         [0040]      26  arrow, fluid flow 
         [0041]      28  arrow, fluid flow 
         [0042]      30  first cylinder chamber 
         [0043]      32  second cylinder chamber 
         [0044]      34  first strip on the sealing ring  14   
         [0045]      36  second strip on the sealing ring  14   
         [0046]      38  bore, aperture in the first groove wail 
         [0047]      39  bore, aperture in the second groove wall 
         [0048]      40  piston (for example, relay piston) 
         [0049]      42  annular groove 
         [0050]      44  first groove wall 
         [0051]      46  second groove wall 
         [0052]      48  groove base 
         [0053]      50  sealing ring 
         [0054]      51  sealing ring 
         [0055]      52  first pressurized-support chamber 
         [0056]      54  second pressurized-support chamber 
         [0057]      56  retaining portion on the sealing ring  50   
         [0058]      58  sealing portion on the sealing ring  50   
         [0059]      60  seal projection on the sealing ring  50   
         [0060]      62  intermediate portion on the sealing ring  50   
         [0061]      64  first cylinder chamber 
         [0062]      66  second cylinder chamber 
         [0063]      68  longitudinal center axis 
         [0064]      70  cutout, aperture in groove wall  44   
         [0065]      72  cutout, aperture in groove wall  46   
         [0066]      74  end portion of the cutout  70   
         [0067]      76  end portion of the cutout  76   
         [0068]      78  rounding of the cutout  70   
         [0069]      80  rounding of the cutout  72   
         [0070]      82  length of the cutout  70   
         [0071]      84  length of the cutout  72   
         [0072]      86  outer edge of the first groove wall 
         [0073]      88  outer edge of the first groove wall 
         [0074]      89  channel 
         [0075]      90  inner circumferential surface of the cylinder 
         [0076]      92  cylinder 
         [0077]      94  material thickness of the first groove wall 
         [0078]      96  material thickness the second groove wall 
         [0079]      98  arrow, fluid flow 
         [0080]      100  arrow, fluid flow 
         [0081]      102  groove depth 
         [0082]      104  piston diameter in the annular groove area 
         [0083]      106  piston height in the annular groove area 
         [0084]      110  control valve 
         [0085]      111  axle modulator 
         [0086]      112  solenoid valve 
         [0087]      113  solenoid valve 
         [0088]      114  solenoid valve 
         [0089]      115  solenoid valve 
         [0090]      116  brake pressure sensor 
         [0091]      117  brake pressure sensor 
         [0092]      118  relay valve 
         [0093]      119  relay valve 
         [0094]      120  control piston 
         [0095]      121  control piston 
         [0096]      122  control electronics 
         [0097]      123  piston head 
         [0098]      124  piston head 
         [0099]      125  plate valve 
         [0100]      126  plate valve 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0101]      FIG. 1  shows a simplified partial cross section through a known embodiment of a piston  10  having a pressurized-support sealing ring  14 . The piston  10  has a circumferential annular groove  12  with a rectangular cross sectional geometry, into which a sealing ring  14  with bilateral pressurized support is inserted. The sealing ring  14  here is designed, merely by way of example, as a so-called Airzet sealing ring having a Z-shaped cross sectional geometry. Any other embodiment of a pressurized-support sealing element may be used instead of this Airzet sealing ring. 
         [0102]    The sealing ring  14  comprises a retaining portion  16  bearing against the groove base, together with a sealing portion  18  having a free end, which portions are connected together by a slanting intermediate portion  20 . The sealing portion  18  preferably bears with slight radial, mechanical pre-stressing against an inner circumferential surface of a cylinder. The intermediate portion  20  together with the inside of the annular groove  12  forms a first pressurized-support chamber  22  and a second pressurized-support chamber  24 , which each have an approximately V-shaped cross sectional geometry. 
         [0103]    As is indicated by the two flow arrows  26  and  28 , to support the sealing effect of the sealing ring the two pressurized-support chambers  22 ,  24   14  are pneumatically or hydraulically connected to the two cylinder chambers  30 ,  32  separated by the piston  10 . This allows a fluid, such as compressed air, for example, contained in the two cylinder chambers  30 ,  32  to pass into the two pressurized-support chambers  22 ,  24 , each assigned to a cylinder chamber  30 ,  32 . The ducts necessary for the free, unimpeded passage of the fluid are created on the sealing ring  14  by integral molded-on strips  34 ,  36  or projections, which, for this purpose, have multiple, radially oriented openings or cutouts, and which each come to rest with a light contact pressure on opposing, parallel inside faces of the annular groove  12 . 
         [0104]    A disadvantage with this known construction, as indicated by the flow arrows  26 ,  28 , is that the fluid is only able to flow from the cylinder chambers  30 ,  32  into the two pressurized-support chambers  22 ,  24  radially from outside, so that, even in the event of a slight axial deformation of the sealing ring  14 , the passages to the pressurized-support chambers  22 ,  24 , kept open by the two strips  34 ,  36 , are at least partially closed, and the pressurized-support of the sealing ring  14  by the pressurized fluid is no longer fully assured. The further technical details of the sealing ring  14  can be gleaned from DE 296 10 628 U1, for example, which is incorporated herein by reference. 
         [0105]      FIG. 2  shows a simplified perspective partial view of a piston  40  designed according to an embodiment of the present invention. The cylindrical piston  40  has a peripheral, circumferential annular groove  42  having a first and a second groove wall  44 ,  46 , which run parallel and at an axial interval from one another, and a groove base  48  connecting the two groove walls  44 ,  46 . The piston  40  can be used as a relay valve in which compressed air is used as fluid. 
         [0106]    The two parallel groove walls  44 ,  46  and the groove base  48  together with a groove aperture form a rectangular cross sectional geometry. A sealing ring  50 , which in this exemplary embodiment together with the two groove walls  44 ,  46  define a first pressurized-support chamber  52  and a second pressurized-support chamber  54 , is seated with a light contact pressure in the annular groove  42 . The sealing ring  50  further comprises a radially inner retaining portion  56  for secure locating inside the annular groove  42 , and a sealing portion  58  facing radially outwards and having a seal projection  60 , the two pressurized-support chambers  52 ,  54  of approximately V-shaped cross section being formed more or less opposite on a slanting intermediate portion  62  of the sealing ring  50 . The piston  40  axially separates a first cylinder chamber  64  and a second cylinder chamber  66  from one another, the chambers each being filled with a suitable fluid, such as compressed air, for example, an alternative gas mixture or a liquid. 
         [0107]    According to an embodiment of the present invention, at least one slit-shaped, cutout facing radially inwards in the direction of a longitudinal center is  68  of the piston  40  is, in each case, made as a through-aperture in the first groove wall  44  and in the second groove wall  46 , of which cutouts two standing for all further unrepresented cutouts or apertures bear the reference numerals  70 ,  72  and have an approximately cuboid geometry. 
         [0108]    At least three cutouts  70 , arranged axially above one another and spaced at uniform intervals from one another over the circumference of the piston  40 , are preferably provided in each of the two groove walls  44 ,  46 , in order to achieve the most efficient connection possible, in terms of fluid dynamics, of the two cylinder chambers  64 ,  66  with the two pressurized-support chambers  52 ,  54  respectively assigned to them. The cutouts  70 ,  72  pass axially through the groove walls  44 ,  46  and perpendicularly to the latter or parallel to the longitudinal center axis  68 , in order to allow fluid to pass with the lowest possible flow resistance. The cutout  70  in the upper groove wall  44  extends radially inwards up to the groove base  48  of the annular groove  42 . 
         [0109]    The fluid from the opposing cylinder chambers  64 ,  66  separated by the piston  40  passes through the two cutouts  70 ,  72  with virtually no resistance into the pressurized-support chambers  52 ,  54  intended to optimize the sealing effect on the sealing ring  50 . The sealing effect of the piston  40  is substantially supported by a slight radial expansion of the sealing ring  50  due to the pressurized fluid flowing into the pressurized-support chambers  52 ,  54  on both sides. As a result, the seal projection  60  is brought to bear firmly with a radial force against an inner circumferential surface  92  of a cylinder  92  coaxially enclosing the piston  40  (cf.  FIG. 3 ). 
         [0110]    In order to reduce the tendency to cracking inside the piston  40  due to notch stresses, end portions  74 ,  76  of the cutouts  70 ,  72 , facing radially inwards in the direction of the longitudinal center axis  68 , each have a rounding  78 ,  80  with a suitable radius of curvature. 
         [0111]    The radial length  82 ,  84  of each of the cutouts  70 ,  72 , starting from outer edges  86 ,  88  of the first and the second groove wall  44 ,  46 , is designed so that ideally the cutouts  70 ,  72  completely cover the pressurized-support chambers  52 ,  54  radially, in order to obtain the freest possible fluid pressure equalization between the pressurized-support chambers  52 ,  54  and the cylinder chambers  64 ,  66 . Since the geometry and/or the fitting direction of an Airzet sealing ring or a similar sealing ring is not fi prescribed, the length of the slit-shaped cutouts  70 ,  72  may be equal or unequal to one another. In order to cater for different sealing ring variants and fitting variants, the slit-shaped cutouts  70 ,  72  nevertheless preferably extend radially up to the groove base  48 . 
         [0112]    Apart from the strips to form a cavity for the passage of fluid, the sealing ring  50  may be formed substantially according to the bilaterally pressurized-support sealing ring discussed above in detail in connection with  FIG. 1 , (“Airzet” sealing ring). It should be appreciated that other pressurized-support sealing elements having a modified geometrical design may also be used. 
         [0113]    As  FIG. 2  shows in the area of the left-hand sectional face, in addition to or instead of the slit-shaped cutouts  70 ,  72 , multiple bores  38 ,  39 , axially parallel to the longitudinal center axis  68 , may be introduced into the two groove walls  44 ,  46  as apertures. These bores  38 ,  39  are likewise introduced into the groove walls  44 ,  46  in the area of the pressurized-support chambers  52 ,  54  and pass axially right through these groove walls  44 ,  46 , in order to allow the freest possible passage of the fluid. 
         [0114]      FIG. 3  shows a diagrammatic partial cross section through a piston  40 ′, as is also represented in  FIG. 2 . The sealing ring  50  is again seated in the annular groove  42  and by means of its seal projection  60  seals the piston.  40 ′ against the inner circumferential surface  90  of the cylinder  92 . Both the piston  40 ′ and the cylinder  92  are arranged coaxially with the longitudinal center axis  68 . A first slit-shaped cutout  70  is formed in the first groove will  44 , while a second cutout  72  is made in the second groove wall  46 , axially below the former. The first cutout  70  extends axially over the full material thickness  94  of the first groove wall  44 , and the second cutout  72  extends over the full material thickness  96  of the second groove wall  46 . A fluid contained in the first cylinder chamber  64  passes through the first cutout  70  in the first groove wall  44  into the first pressurized-support chamber  52 , according to the first flow arrow  98 , while the fluid from the second cylinder chamber  66  is able to flow largely unimpeded through the second cutout  72  situated in the second groove wall  46  into the second pressurized-support chamber  54 , in the direction according to the second flow arrow  100 . 
         [0115]    The material thickness  94 ,  96  of both groove walls  44 ,  46  is, in each case, preferably less than or equal to the groove depth  102  of the annular groove  42 . In addition, the piston diameter  104  in the area the annular groove  42  is preferably greater than a piston height  106  in the area. 
         [0116]    As  FIG. 4  shows, a sealing ring  51 , which together with a groove wall  44  of the annular groove  42  forms just one pressurized-support chamber  52 , can be inserted into the annular groove  42  of the piston  40 ″. In this case, it suffices if the fluid pressure passing through the slit-shaped cutout  70  in the first groove wail  44  into the single pressurized-support chamber  52  presses the upper sealing ring portion, rendered thinner and hence more flexible by this cutout  70 , with an additional force radially against the inner circumferential surface  90  of the cylinder  92 . 
         [0117]    In the exemplary embodiment shown in  FIG. 5 , two pressurized-support chambers  52 ,  54  are formed by the sealing ring  50  and the groove wails  44 ,  46  of the annular groove  42  of the piston  40 ′″. The first, upper pressurized-support chamber  52  is connected to the first, upper pressure chamber  64  via at least one slit-shaped cutout  70 , while the second, lower pressurized-support chamber  54  has a fluid connection to the second, lower pressure chamber  66  via a radial channel  89 , which is formed on the side of the second, lower groove wall  46  close to the sealing ring. Such radial channels  89  may also additionally be arranged on the respective groove walls  44 ,  46  provided with the axial apertures  38 ,  39 ,  70 ,  72 , in order to assist the ventilation of at least one pressurized-support chamber  52 ,  54 . 
         [0118]    The slit-shaped cutouts  70 ,  72 , made in at least one of the two groove walls  44 ,  46 , each allow a straightforward fluid pressure equalization between the two cylinder chambers  64 ,  66  and the two pressurized-support chambers  52 ,  54 , even if operation gives rise to a geometrical deformation of the sealing ring  50 , for example. An optimum sealing effect between the cylinder chambers  64 ,  66  can thereby be achieved under all service conditions of the piston  40 ,  40 ′,  40 ″,  40 ′″. The piston  40 ,  40 ′,  40 ″,  40 ′″ may be a pneumatically operated relay piston of a pneumatic system, for example. 
         [0119]      FIG. 6  is a diagrammatic sectional representation of an exemplary embodiment of a control valve  110  of a compressed air system of a vehicle in which the piston  40 , or the piston  40 ′,  40 ″ or  40 ′″ is fitted. The control valve  110  is an axle modulator  111  of an electronically controlled braking system (EBS) of a commercial vehicle having a pneumatically operated braking system. The construction of the axle modulator  111  is known in the art; an axle modulator having the basic construction represented in  FIG. 6  has been commercially available from the applicant since at least 2004 as “2 nd  generation EBS axle modulator” under part number 480 104 104 0. 
         [0120]    The axle modulator  111  regulates the brake cylinder pressure on both sides of one or two axles of the commercial vehicle. It comprises two pneumatically independent pressure control circuits each having two solenoid valves  112 - 115 , a brake pressure sensor  116 ,  117  and a relay valve  118 ,  119 , and having common control electronics (ECU)  120 . The axle modulator  111  uses rotational speed sensors to register the wheel speeds, evaluates these and relays them to a central module of the vehicle, which then determines the desired pressures. The axle modulator  111  independently undertakes anti-lock brake controls (ABS). If wheels of the vehicle show a tendency to lock or spin, the axle modulator  111  adjusts the desired pressure setting. 
         [0121]    The relay valves  118 ,  119  each comprise one of the pistons  40 ,  40 ′,  40 ″ or  40 ′″ previously described as control piston  120 ,  121 ; in the exemplary embodiment represented, the relay valve  118  arranged on the left-hand side of the figure comprises the piston  40  having a sealing ring  50  according to  FIG. 2  as control piston  120 , while the relay valve  119  on the right-hand side of the figure comprises the piston  40 ″ having a sealing ring  5 l according to  FIG. 4  as control piston  121 . It is also possible, however, to use pistons of identical design, that is, for example, two pistons  40  each having sealing rings  50 . 
         [0122]    The pistons  40 ,  40 ″ of the relay valves  118 ,  119  each bear on the cylinder  92  with their sealing rings  50 ,  51 . The pistons  40 ,  40 ″ each have a piston head  123 ,  124 , each of which actuates an assigned plate valve  125 ,  126 . 
         [0123]    The possible uses of the pistons  40 ,  40 ′,  40 ″ and  40 ′″ are not limited to an axle modulator  111 , even if the pistons  40 ,  40 ′,  40 ″ and  40 ′″ are preferably used in an axle modulator  111  of the type shown in  FIG. 6 , or in another axle modulator, for example in a third-generation axle modulator (for example WABCO unit number 480 105 001 0). Thus, it is feasible to use the pistons  40 ,  40 ′,  40 ″ and  40 ′″ in control valves of similar construction in vehicle compressed-air systems, particularly in control valves having at least one relay valve, for example in pilot-operated control valves of air drier systems, in relay valves of automobile air suspension systems, in proportional relay valves, in redundancy valves, in brake power sensors, in footbrake valves, trailer modulators, or in trailer control valves. It is likewise feasible to provide a single-Channel axle modulator having just one relay valve, as is available from the applicant under part number 480 106 511 0, with a relay valve having a control piston, which comprises a piston  40 ,  40 ′,  40 ″,  40 ′″ of the design type previously described. 
         [0124]    It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
         [0125]    It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.