Abstract:
A valve system for activating a piston of a piston-cylinder arrangement for a hydraulic or fluid device includes a pilot control valve including 3/2-way valve and a main valve arrangement having a first and a second main valve. The first and second main valves include 2/2-way valves, wherein in a first position the pilot-control valve is configured to move the first main valve into an open position so as to direct a path for a high pressure fluid to a space above the piston, and wherein in a second position the pilot-control valve is configured to connect the space to a low-pressure tank so as to relieve a pressure in the space above the piston via the second main valve, and wherein the pilot-control valve is configured to open the second main valve and configured to close the first main valve.

Description:
Priority is claimed to German Patent Application No. DE 10 2009 014 421.8, filed Mar. 26, 2009, the entire disclosure of which is incorporated by reference herein. 
     The invention relates to a valve arrangement. 
     BACKGROUND 
     Such valve arrangements are used to actuate piston-cylinder arrangements. The piston is located at one end of a piston rod, with the result that the cross-sectional area of the space above the piston is larger than the cross-sectional area below the piston since the cross-sectional area of the piston rod is subtracted from this cross-sectional area. If high-pressure fluid is then fed to the spaces above and below the piston, the piston moves in a first direction because the force applied to the upper side of the piston by the high-pressure fluid is larger owing to the larger cross-sectional area than the force applied to the underside of the piston. If the space above the piston is relieved of pressure while this space and the fluid contained therein are connected to a reservoir vessel, also referred to as a low-pressure tank, which is at low pressure the piston moves in a direction opposed to the first direction. The piston rod is therefore extended out of the cylinder when the space above the piston is acted upon, and is retracted again when the pressure is relieved. 
     Any fluid may be used as the medium. Hydraulic oil is generally used but also compressed air in specific cases. The hydraulic oil can be made available here by specific high-pressure tanks whose design is insignificant for the present invention. 
     Such piston-cylinder arrangements are used, in particular, for activating the movable contact piece of high-voltage power switches, and can, of course, also be used in other applications in which components such as, for example, crane arms, shovels of shovel excavators and the like are to be moved. 
     The connection of the space above and below the piston to the high-pressure tank and the connection of the space above the piston to the low-pressure tank or to other connections is brought about by means of mostly electrically actuated valves, using a 3/2-way valve or two 2/2-way valves, the latter operating independently of one another. 
     Depending on the application case, the intention is to be able to achieve, for example, switching over which is without switching losses and during which a volume flow from the pressure connection to the low-pressure tank via both control edges is to be avoided during the switching process, and also to be able to achieve a flow resistance or volume flow of different magnitudes depending on the switched position, a short switching time or activation with a small pilot-control volume. 
     However, when a 3/2-way valve is used these requirements can frequently only be met inadequately or with a high level of expenditure on manufacturing and high manufacturing costs. If two 2/2-way valves are used, during switching over the open valve must firstly be closed before the closed valve is opened if a switching loss is to be avoided. However, in the case of pilot-controlled valves this requires at least two pilot-control valves with a suitable electrical actuation system with, for example, delayed or sensor-controlled triggering of the second valve. This entails further high costs and an unnecessarily long delay of the opening of the second 2/2-way valve after the first closes. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to provide a valve system of the type mentioned at the beginning in which the above-mentioned requirements can be met with a low level of expenditure on manufacture and with low switching losses. 
     In this context, the invention is characterized in that the 3/2-way valve serves as a pilot-control valve for a valve arrangement having two main valves which are embodied as 2/2-way valves, wherein the pilot-control valve moves the first of the main valves into the open position in order to direct the high-pressure fluid to the piston-cylinder arrangement, wherein the second main valve, which clears a connection from the piston-cylinder arrangement to a low-pressure tank is closed, and said pilot-control valve actuates the second main valve to open and at the same time moves the first main valve into the closed position. 
     One advantageous embodiment of the invention with two main valves which each have a slide which is displaceably arranged within a valve body and has control faces to which pressurized fluid can be applied can be characterized in that each main valve respectively has three control faces, a first and a second control face of which respectively act on the slide in one direction, and the other third control face of which respectively acts on the slide in the other direction, wherein the sum of the two identically acting control faces is equal to the other control face acting in the opposite direction. 
     In this context, the control faces of each main valve each can correspond to an actuation element, wherein the surface area ratio of the third control face (third actuation element) to the second control face (second actuation element) of the second main valve is always greater than the surface area ratio of the third control face (third actuation element) to the first control face (first actuation element) of the first main valve. 
     It is particularly advantageous that the control faces can be formed by radially extending annular faces and/or radially extending end faces on the slides. 
     In particular, the valve system can be characterized in that the third control face of the first main valve is formed by the end face of the slide and is connected to the pilot-control valve. 
     Furthermore, the first and second control faces of the first main valve can be formed by annular faces, which are formed on the slide, and by an end face of the slide. 
     The second control face of the second main valve is formed here in a particularly advantageous way by an annular face which is arranged on the slide of the second main valve and is connected to the pilot-control valve. 
     The end faces of the slide of the second main valve are connected here as a third control face to the low-pressure tank. 
     High-pressure fluid can particularly advantageously be applied alternately to the first and third control face of each main valve via the pilot-control valve. 
     According to a further embodiment of the invention, the first control face of the first main valve is continuously connected to high pressure via a high-pressure feed line, and the first control face of the second main valve is continuously connected to low pressure. 
     In this context, the valve system can be characterized in that high pressure is applied to the second control faces of the first and second main valves when the first main valve is opened and the second main valve is closed, and low pressure is applied thereto when the first main valve is closed and the second main valve is opened. 
     Each main valve can respectively contain a helical compression spring which acts on the associated slide iii the closing direction. However, said helical compression springs are not necessary. 
     A further embodiment of the valve system can be characterized in that the slide of the second main valve has a longitudinal bore which passes completely through the slide, with the result that the space which accommodates the helical spring is connected to the end face and therefore to the low-pressure tank. 
     In a similar way, the valve system can be characterized in that the slide of the first main valve has a longitudinal bore which passes partially through the slide and which connects the space for accommodating the helical compression spring to a duct in the interior of the first main valve, which duct is connected to the piston-cylinder arrangement. 
     In this context, the control faces of each main valve each can correspond to an actuation element, wherein the surface area ratio of the third control face (third actuation element) to the second control face (second actuation element) of the second main valve is always greater than the surface area ratio of the third control face (third actuation element) to the first control face (first actuation element) of the first main valve. 
     Therefore, the surface area ratios of the control faces on the slides of the main valves are configured in such a way that a significantly higher pilot-control pressure is required to open the first main valve than to close the second main control valve. A sufficiently large flow resistance in the region of the pilot-control valve in relation to the flow resistances in the line sections leading from the pilot-control valve to the main valves ensures that when the pilot-control valve switches the pilot-control volume flow is always firstly implemented through the still open main valve, while the latter is closing, and the pilot-control pressure does not change significantly in the process. Only after the possibly still open main valve has closed does said main valve no longer implement any volume flow, with the result that the pilot-control pressure increases further, or in a different case decreases until the other main valve opens. 
     In this context, as a result of the rising pilot-control pressure, the second main valve is firstly closed, and the first main valve then opened, whereas when the pilot-control pressure is dropping the first main valve firstly closes and then the second main valve opens. As a result, the desired switching behaviour is achieved by means of actuation by a single common pilot-control valve without a need for separate, chronologically offset actuation of the main valves. 
     While the control face sums and ratios according to the invention are complied with at each slide of the main valves, diameters of each main valve and further parameters can be selected freely within wide limits independently of the other main valve. 
     In the case of leakage or if a volume flow occurs from the consumer, that is to say a piston-cylinder arrangement, the main valves can open automatically. Furthermore, automatic closing occurs if no further volume flow is required by the consumer, for example because a connected working piston has moved into its end position. 
     If the consumer docs not implement any volume flow, an immediate opening of the main valve is made possible when the pilot-control valve switches over without a delay due to the closing of the other main valve. 
     Owing to the ratios of the control faces, the hydraulic forces on the first main valve cancel one another out as soon as the same pressure prevails at the connection directed towards the consumer as at the connection directed to the pressure supply. As soon as the consumer-side pressure drops, the first main valve is opened again when the slide is in the closed position, for example due to a compression spring. In a corresponding way this also applies to the second main valve. 
     The invention, further advantageous embodiments of the invention and further advantages will be explained and described in more detail on the basis of the drawing in which a valve arrangement according to the invention is illustrated schematically. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In said drawing: 
         FIG. 1  is a switching diagram of a valve arrangement, 
         FIG. 2  is a sectional view of the first main valve of the arrangement according to  FIG. 1  in a schematic illustration, and 
         FIG. 3  is a sectional view of the second main control valve of the arrangement in  FIG. 1 , likewise in a schematic illustration. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic switching diagram of a valve arrangement  10  with two main valves  11  and  12  and a pilot-control valve  13 . The two main valves  11  and  12 , also referred to below for short as first and second valves  11 ,  12 , are 2/2-way valves with different designs, as will be explained in more detail further below. The one outlet  14  of the first valve  11  is connected via a connecting line  21  with a piston-cylinder arrangement  15  which has, in a cylinder housing  16 , a piston  17  on which a piston rod  18  is integrally formed. The outlet  14  is connected here to the space  19  above the piston  17 . The space  20  below the piston  17  is connected to a high-pressure supply  27  via a line  55 , but this is not significant for the functionality since the restoring force of the piston  17  can also be applied differently, for example, by means of a spring. Owing to the different cross sections of the spaces below and above the piston  17 , if high-pressure fluid is applied to both spaces  19  and  20  a force acts on the piston  17  and drives it out of the cylinder  16  in the direction P 1  of the arrow. In the process, the movable switching contact piece  50  of a high-pressure power switch  51  can be connected to the piston rod  18 , with the result that the switch can be switched on and off by actuating the two valves  11  and  12 . In the position illustrated here, the switch  51  which is opened here would be closed if high-pressure fluid is present in the spaces  19  and  20 ; for the switching-off process, the space  19  above the piston  17  would be relieved of pressure, with the result that the fluid located in the space  20  below the piston  17  pulls the piston  17  counter to the direction P 1  of the arrow, and therefore pulls the piston rod  18  into the cylinder  16 . The application in a switch is merely exemplary. 
     However, a further connecting line  22  is connected to the connecting line  21  at a node point  23 , said further connecting line  22  being coupled to an output opening  24 , referred to for short as opening or outlet  24 , which is also closed in the position shown here. The outlet  24  is located on the second valve  12 . 
     The two valves  11  and  12  each have a further opening or outlet  25  and  26 , of which the opening  25  of the valve  11  is connected to a high-pressure supply  27 , which can be a high-pressure accumulator or a pump, and of which the opening  26  of the valve  12  is connected to a low-pressure tank  28 , which is represented only symbolically here. The opening  25  is connected via a return line  29  to a first actuation element  30  of the valve  11 , and the opening  14  is connected via a return line  33  to a second actuation element  35  of the valve  11 . The opening  26  is connected via a return line  31  to a first actuation element  32  of the valve  12 , and the opening  24  is connected via a return line  34  to a second actuation element  36  of the valve  12 . 
     The two valves  11  and  12  which are embodied as 2/2-way valves are assigned the pilot-control valve  13  which is embodied here as a 3/2-way valve. It has openings  37 ,  38  and  39 . The opening  38  is connected here to the high-pressure supply  27 , and the opening  37  to the low-pressure tank  28 . The opening  39  can be connected either to the high-pressure supply  27  or to the low-pressure tank  28  by activating an electromagnetic controller  40  and  41  or by some other kind of external application of force. The outlet opening  39  is connected via a node point  54  to lines  52  and  53 , each with a third actuation element  42  and  43  of the valves  11  and  12 . The third actuation element  42  of the valve  11  serves to move the valve  11  into its opened position when the connection  39  of the pilot-control valve  13  is connected to the high-pressure supply  27 . The third actuation element  43  of the valve  12  serves to close the second valve  12  when the connection opening  39 , or else for short the connection  39 , of the pilot-control valve  13  is connected to the high-pressure supply  27 . It is to be noted here that the term “connection opening” in the text which follows is also referred to for short as “connection”. In this way, the space  19  below the piston  17  is connected to the high-pressure supply  27 , and the piston  17  moves out of the cylinder housing  16 . If the connection  39  of the pilot-control valve  13  is connected to the low-pressure tank  28 , the pressure also drops at the third actuation elements  42  and  43 . As a result, the first actuation element  30  can close the first valve  11 , and the second actuation element  36  can open the second valve  12 . As a result, the space  19  above the piston  17  is connected to the low-pressure tank  28 , and the piston  17  moves into the cylinder housing  16 . 
     The actuator elements  30 ,  35 ,  32 ,  36  and the actuator elements  42  and  43  are described further below in terms of design and method of operation in conjunction with  FIGS. 2 and 3 , where the term “actuation element” is also explained. 
     Reference will now be made to  FIG. 2 . 
     The valve  11  has a valve body  200  which surrounds an interior space  201  in which a slide  202  can move in a sliding fashion. The interior space  201  has a first interior space section  203  and a second interior space section  204  which has an internal diameter which is enlarged compared to that of the interior space section  203 . The two interior space sections  203  and  204  are connected to one another via a radial annular face  205  which forms a step. The second interior space section  204 , also referred to for short as second section, is closed off by a base  206  which has a depression  207 , see below. 
     A region  229 , which acts as a sealing face and is represented in this case as a bevelled chamfer is located between the first interior space section  203  and the annular face  205 . 
     The valve body  200  has, approximately in the central region, a bore  212  which engages radially through the valve body  200  and opens into the space  201 . A further bore  220 , which extends perpendicularly with respect to the longitudinal extent of the valve body  200 , opens into the region of the section  204  of the valve body  200 . 
     The slide  202  is mounted in a slideable fashion within the valve body  200 . Said slide  202  has a first slide section  221 , the external diameter of which corresponds to the internal diameter of the section  203 , a second slide section  222 , the external diameter of which is smaller than the external diameter of the first section  203  and is dimensioned in such a way that fluid can flow through, a third slide section  223 , which is slightly larger than the internal diameter of the section  203  with the result that a seal can be produced at the sealing face  229  when the slide  202  is pressed entirely to the left (in the drawing), and a fourth slide section  224 , the external diameter of which corresponds to the internal diameter of the depression  207  and is smaller than the external diameter of the slide section  221  but larger than the external diameter of the slide section  222 . The fourth slide section  224  engages continuously in the depression  207 , i.e. in each position of the slide  202 , and the slide section  221  also engages continuously in the slide section  203 . In the depression  207  between the end face  225 , located in the depression  207 , of the slide  202  and the base  226  of the depression  207 , a helical compression spring  227  is arranged in a spring-receptacle space  231  formed between said end face  225  and said base  226 , which helical compression spring  227  is supported by one of its ends against the end face  225  and by its other end against the base  226  of the depression  207  and presses the slide  202  to the left (in the drawing), with the result that the slide section  223  bears or is pressed with its sealing edge  228  facing the chamfer  229  against the chamfer  229  which acts as a sealing face. It is illustrated here that the inner edge of the annular face  205  has a chamfer with the result that the sealing edge  228  of the slide  202  is pressed against the chamfer  229 , for example here by the force of the helical compression spring  227 , and therefore forms a seal. Of course, the sealing edge  228  could also have a chamfer and come to bear on an inner edge, which does not have a chamfer or has a chamfer at a different angle, between the section  203  and the annular face  205 , which could be a variant. Any other way of embodying a sealing contact would also be conceivable. 
     Taking the second slide section  222  up to the end face  225  as a basis, an inner bore  230  extends within the slide  202  with the result that the space  235  is connected, in the region of the second slide section  222 , to the spring-receptacle space  231  in which the spring  227  is located. If high-pressure fluid is located in the space  235  in the region of the second slide section  222 , the pressure will also be present in the spring-receptacle space  231  with the spring  227 , and, owing to the dimensions, will support the force of the helical compression spring  227  and press the slide  202  with the third slide section  223  with respect to the annular face  205  or the chamfer  229 . 
     The element denoted as the third triggering element  42  acts as a third control face which is formed by the free end face  232  of the slide  202 . 
     The second triggering element  35  acts as a second control face, which is formed by the annular faces  233 ,  236  and the end face  225  on the slide  202 , which annular faces  233  and  236  are located between the slide sections  221  and  222 , and respectively the slide sections  222  and  223 . The first triggering element  30  acts as a first control face which is formed via the annular face  234  between the slide sections  223  and  224 . Here, the end face  232  is of equal size to the sum of the annular faces  233 ,  234  and of the end faces  225  minus the annular face  236 , with the result that if the main valve  11  is under high pressure like the line  52 , the slide  202  is pressed against the sealing face or chamfer  229  exclusively by the force of the spring  227 . The helical compression spring  227  would not be necessary for the function here and could therefore also be omitted; it merely supports the switching process, see further below; the slide  202  would be freely movable in the valve body because the forces are all in equilibrium. 
     The pilot-control valve  13  is connected to the third triggering element  42  of the first valve  11  via the connecting line  52 , wherein the pressurized fluid which is present in the connecting line  52  acts on the free end face  232  of the slide  202 . 
     The following is also to be noted: the first actuation element  30  therefore corresponds to the first control face, the second actuation element  35  corresponds to the second control face, and the third actuation element  42  corresponds to the third control face, in each case of the first valve  11 . 
     Reference will now be made to  FIG. 3 . 
       FIG. 3  shows a schematic longitudinal sectional diagram of the main valve  12 . The latter has a valve body  300 , the interior  301  of which has a plurality of sections with different internal diameters, and the end which is on the left in the drawing is adjoined by a first section  302 , which, via a conical stage or chamfer  303  which opens at the other end of the valve body  300 , merges with a second section  304  with a slightly larger diameter with an intermediate internal duct  317 . The section  304  is adjoined by a base section  306  in which a depression  308  is formed, said depression  308  closing off the valve body  300  at this end. 
     The valve body  300  has two bores  315  and  316  which extend transversely with respect to its longitudinal axis, the first bore  315  of which opens into the internal duct  317  between the first and second sections  302  and  304 . The second bore  316  opens into the part of the section  304  which faces the base section. The first bore  315  is therefore located in the region of the plane of transition from the first section  302  to the second section  304  of the valve body  300 , with the interior space  317  adjoining the conical stage  303 . The bore  215 , which corresponds to the opening  24 , is assigned to the line  22 , and the second bore  316  is assigned to the third actuation element  43 . 
     A slide  314  is accommodated within the valve body  300 , which slide  314  has a first section  318 , the external diameter of which is slightly larger than the internal diameter of the first section  302  of the valve body  300 , with the result that the slide  314  can abut with its end edge or sealing edge  314  against the conical stage  303  when the slide  314  is in the position shown in  FIG. 3 . The external diameter of the section  318  is to be dimensioned in such a way that when the sealing edge is opened sufficient fluid can flow through. As a result, the slide  314  seals the annular space  317  against the region  321 , lying in front of the end face  320  adjoining the sealing edge  319 , within the first section  302  of the valve body  300 , to which region  321  the low-pressure tank  28  is connected. The sealing contact, composed of the chamfer  303  and  319 , can also have a different geometric design, which is insignificant for the functionality of the system. 
     The first section  318  of the slide  314  is adjoined by a second section  322  with a larger external diameter, as a result of which a step  323  which points to the end face  320  is formed, and to which step  323  the pressurized fluid which is present in the annular space  317  applies a force which presses the slide  314  against the base face  309  of the depression  308  of the valve body  300 . 
     The second section  322  of the slide  314  is adjoined by a third section  324  with which the slide  314  engages in the interior of the depression  308 . In the space  325 , also referred to as the spring-receptacle space, between the slide  314  and the end face  331  thereof and the base  309 , a helical compression spring  326  is located, which helical compression spring  326  presses the slide  314  with its sealing edge  319  against the conical face or conical step  303 . The external diameter of the third section  324  is smaller than that of the first section  318 . 
     The slide  314  has a longitudinal bore  327  which extends in its longitudinal direction and which opens into the end faces  320  and  331 , and therefore into the space  325 , and therefore connects the spaces  321  and  325  to one another. Low pressure is present continuously at the space  321  since the latter is connected to the low-pressure accumulator  28 . Accordingly, the connection  26  is equal to the space  321 . 
     The junction between the sections  322  and  324  is formed by an annular face  330 . 
     The second control face, formed by the annular face  323 , corresponds, in the switching diagram in  FIG. 1 , to the second actuation element  36 , and the third control face, formed by the annular face  330 , corresponds to the third actuation element  43 ; the first control face, corresponding to the first actuation element  32 , is formed by the difference between the end faces  320  and  331 . 
     The method of functioning of the valve arrangement is as follows: 
     It is assumed that pressurized fluid is to be applied to the space  19  above the piston  17  in order to move the piston rod  18  out of the cylinder  16 . For this purpose, the pilot-control valve  13  is actuated in such a way that fluid under high pressure is fed via the line  53  to the actuation element  43  and therefore to the annular face  330  of the slide  314 . The slide  314  is therefore moved to the left, as a result of which the connections  24  and  26  are disconnected by pressing the edge  319  onto the chamfer  303 , which is assisted by the compression spring  227 . At the same time, the pressurized fluid passes via the line  52  to the third actuation element  42  of the first valve  11 , which corresponds to the end face  232  of the slide  202 , and pushes the slide  202  counter to the pressing force which acts on the slide  202  by means of the first actuation element  30  of the first valve  11 , and the force of the compression spring  227  to the right (in the drawing) with the result that the sealing edge  228  lifts off from the sealing face  229  and the connection  25  is connected to the connection  14 , with the result that fluid under high pressure passes to the space  19  above the piston  17  and the piston  17  moves out of the cylinder  16 . 
     If the pressure in the space above the piston  17  is to be relieved, the pilot-control valve  13  is switched over with the result that fluid at low pressure is present at the connection  39 , with the result that the force acting on the first actuation element  30  from the fluid under high pressure pushes the slide  202  to the left, and therefore disconnects the connections  25  and  14  as a result of the contact of the sealing edge  228  and the sealing face  229 . At the same time, the third actuation element  43  of the second valve  12  is connected to the low-pressure accumulator  28  via the line  53 , with the result that the slide  314  is pushed to the right counter to the force of the compression spring  326  by the force with which acts from the pressure in the line  22  and  34  on the second actuation element  36  in the form of the annular face  323  of the second valve  12 , and as a result the connections  24  and  26  are connected. The fluid can therefore flow out of the space  19  above the piston  17  to the low-pressure accumulator  28  via the connections  24 ,  26 , and the force which acts on the piston  17  to the right, for example as a result of the pressure from the high-pressure supply being applied to the space  20 , moves the piston  17  into the cylinder  16 . 
     The inventive configuration of the surface area ratios ensures that one main valve  11  or  12  is always closed before the respective other valve can be opened, without chronologically offset actuation of the two main control valves  11 ,  12  becoming necessary. In order to achieve this, it is necessary to ensure that the surface area ratio of the third actuation element  43  of the second valve  12  and (with respect to) of the second actuation element  36  of the second valve  12  is always larger than the surface area ratio of the third actuation element  42  of the first valve  11  and (with respect to) of the first actuation element  30  of the first valve  11 . 
     The slides each have a longitudinal bore, as mentioned above, wherein the longitudinal bore  230  on the slide  202  of the first main valve  11  is connected to the space of the first main valve  11  which, in terms of flow is located downstream of the control edge, i.e. downstream of the sealing edge  228 / 229 , with respect to the face turned towards the piston-cylinder arrangement  15 . This ensures that the pressure which drops behind the sealing edge  228 / 229  also drops at the end face  225  acting as a compensation face, as a result of which an opposing force is generated, which acts in the opening direction and partially compensates the flow force acting in the closing direction. The same also occurs in the second main control valve  12  insofar as the pressure in the space upstream of the end face  320  is equal to the pressure at the end face  331 . 
     The inventive disconnection of the sealing points or control edges into the control edges located in the first main valve and those located in the second main valve permits the two control edges to be configured in terms of diameter, flow behaviour and further features in a way which is appropriate for demand. As a result, while the respective suitable control face ratios are complied with at each slide, the diameter and various further parameters can be freely selected within wide limits independently of the other main valve. 
     A particular advantage of the invention is that, when the two compression springs  326  and  227  are used, the two main valves  11  and  12  close again after the ending of the movement of the piston  17  by virtue of the forces which are applied. This permits immediate opening of the necessary main valve during the subsequent switching of the pilot-control valve without a delay as a result of the previous closing of the other main valve if the switching over takes place at a time at which there is no volume flow implemented at the consumer. This is achieved by virtue of the fact that the two control faces on one side of a main valve are each precisely of the same magnitude as the individual control face acting in the opposite direction. As a result, the hydraulic forces at the main valve cancel one another out as soon as the same pressure is present at all the connections. If leakages were to have occurred in the stationary state which, depending on the position of the piston  17 , leads to a drop in pressure or increase in pressure in the piston space  19 , the main valves can open automatically and compensate these leakages. As a result, the piston always remains in the desired position when the pilot-control valve  13  is not activated. 
     The inner faces of the valve bodies in which outer faces of the slide can be dimensioned as a duct seal, and there is of course also the possibility of using annular seals here. 
     List of Reference Numerals
       10  Valve arrangement     11  First main valve, first valve     12  Second main valve, second valve     13  Pilot-control valve     14  First outlet     15  Piston-cylinder arrangement     16  Cylinder housing     17  Piston     18  Piston rod     19  Space above the piston     20  Space below the piston     21  Connecting line     22  Further connecting line     23  Node point     24  Outlet opening, opening, outlet, connection to the second main valve     25  Further opening, outlet, at the first main valve     26  Further opening, outlet, at the second main valve     27  High-pressure supply     28  Low-pressure tank     29  Return line     30  First actuation element of the valve  11       31  Return line     32  First actuation element of the valve  12       33  Return line     34  Return line     35  Second actuation element of the valve  11       36  Second actuation element of the valve  12       37  Opening     38  Opening     39  Opening, each on the pilot-control valve  13       40  Electromagnetic controller     42  Third actuation element of the valve  11       43  Third actuation element of the valve  12       50  Movable contact element     51  High-voltage power switch     52  Line     53  Line     54  Node point     55  Connecting line     200  Valve body     201  Interior space     202  Slide     203  First interior space section     204  Second interior space section     205  Annular face     206  Base     207  Depression     221  First slide section     222  Second slide section     223  Third slide section     224  Fourth slide section     225  End face     220  Base     227  Helical compression spring     228  Sealing edge     229  Sealing face     230  Internal bore     231  Spring-receptacle space     232  End face     233  Annular face     234  Annular face     235  Space in the region of the second slide section  222       236  Annular face     300  Valve body     301  Interior space     302  First section     303  Conical stage, chamfer     304  Second section     306  Base section     308  Depression     314  Slide     315  First bore     316  Second bore     317  Internal duct, annular space     318  First section of the slide  314       319  End edge or sealing edge     320  End face     321  Region upstream of the end face  320       322  Second section     323  Annular face     324  Third section     325  Spring-receptacle space     326  Helical compression spring     327  Longitudinal bore