Abstract:
The invention relates to a control valve ( 10 ) for controlling the pressure and flow of hydraulic oil from or to the working connections (A, B) of a consumer. The pressure and the pressure flow are controlled by means of at least one sliding piston which can be actuated by at least one drive mechanism and which can be displaced in a slider hole and in co-operating annular channels. The invention is characterized in that the annular channels are symmetrically arranged, whereby a tank connection annular channel is arranged in the symmetrical centre point on a symmetrical axis. Annular channels associated with the working connections (A, B), pump-pressure annular channels, load-sensing annular channels and end-chamber annular channels are arranged behind each other on both sides. According to the invention, the latter can be connected to other channels. The aim of the invention is to create equivalent hydraulic ratios for both working connections A and B and to keep the number of control edges in the control valve to a minimum.

Description:
PRIORITY CLAIM 
   This is a U.S. national stage of application No. PCT/IB02/00661, filed on 4 Mar. 2002. Priority is claimed on that application and on the following application: Country: Switzerland, Application No.: 0522/01, filed: 21 Mar. 2001. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a directional valve for controlling the pressure and flow of hydraulic oil to an from working connections of a consumer, wherein the pressure and flow can be controlled by a slide piston which is actuable by a drive and displaceable in a slide bore and by annular ducts connected to the piston. 
   2. Description of the Related Art 
   Such directional valves are suitable, for example, for the activation of hydraulic drives which move implements or tools on working appliances, such as harvesting machines and loaders. The hydraulic drives may in this case be, for example, single-acting plunger cylinders, double-acting synchronous or differential cylinders or oil motors for one or two directions of rotation. Directional valves for such applications are known in large numbers and in the most diverse possible versions. 
   A directional valve of this type is known from DE 32 25 003. The pressure medium quantity is controlled by means of clocked switching magnets in proportion to an analog input signal. In this directional valve, there are no load recording lines. The usefulness of such a directional valve is therefore greatly restricted. In one of the exemplary embodiments, three of the annular ducts of the directional valve are connected to the tank. This solution is therefore not particularly advantageous because, with a view to the dynamic behavior of the direction valve and also to the manufacturing costs, the aim must be to keep the number of annular ducts as low as possible. 
   A directional valve is also known from DE-A1-196 46 445 which shows a valve arrangement containing two directional valves. Each of these directional valves serves for activating a double-acting consumer. Each of the directional valves is assigned a pressure balance in each case. The common pressure balance is placed in the valve slide of each directional valve, said valve slide being designed as a hollow slide. As a result of the axial movement of the valve slide into one of the working positions A and B, this pressure balance can be assigned to one or other of the working connections A, B. Directional valves and lifting cylinders of such a type are used in mobile hydraulics, for example in agricultural appliances. 
   DE-A1-196 46 426 discloses an arrangement which contains two directional valves and which likewise contains the pressure balance in the valve slide designed as a hollow slide. In one of the directional valves, there is an additional control magnet, as a result of the activation of which the control piston of this directional valve is brought into a position in which the two working connections of the cylinder to be controlled are connected to one another, this being known as a floating position. It is specified, in this respect, that separate ducts for control lines and pilot valves are not necessary in order to achieve the floating position. 
   DE-A1-197 07 722 discloses an arrangement, by means of which the inflow and return to and from a double-acting consumer can be controlled independently of one another. This is achieved by means of a continuously controllable directional valve for the inflow and continuously controllable throttle devices from the working connection A or B to the return. Here, too, the pressure balance is arranged in the valve slide of the directional valve. 
   Details of a valve, to be precise its stop valves, are known from DE-A1-199 19 014. The pistons of these stop valves bear with their end faces on one another, but these possess recesses forming a pressure space. When this pressure space is acted upon by a pressure of defined magnitude, the pistons can be moved axially apart from one another. The floating position for the valve is thereby assumed. 
   An arrangement with a directional valve which manages without a pressure balance is known from GB-A-2 298 291. The function of the latter is achieved by alternative means, to be precise by pressure sensors in the inflow and outflow of the hydraulic consumer, a pressure sensor for the pump pressure, travel sensors in each case on a valve slide acting independently for the working connection A or B, and an electronic control, using the signals from these sensors. This also shows that a plurality of identical valve units can be assembled to form a block controlling a plurality of consumers. In the event of the failure of sensor signals, a direct control of the volume flow or of the pressure in the working connection is no longer possible. Additional means are therefore necessary in order to compensate this disadvantage to an extent such that it is at least still possible to move the working appliance into a safe position. 
   The above description of the known prior art shows that there is a series of solutions for achieving a comprehensive functionality of such directional valves. The aim in this context is always to provide valves which are as simply constructed as possible and can be produced cost-effectively. The functionality and, particularly, the dynamic behavior are determined by the configuration of the valve slide and of the annular ducts and connecting lines cooperating with it. This also governs the number of control edges. In this case, the behavior is also decisively influenced by dimensional tolerances. It is also always necessary to take into account that the inflow metering and the return throttling for the hydraulic consumer are to be independent of one another. 
   SUMMARY OF THE INVENTION 
   The object on which the invention is based is to provide a directional valve which, along with a simple construction allowing cost-effective manufacture, has a dynamic behavior which is improved, as compared with the prior art. What is also achieved thereby is that the directional valve is suitable for different applications and by virtue of special refinements possesses a wide functional scope. 
   According to the invention, a directional valve for controlling the pressure and low of hydraulic oil to and from a consumer includes first and second working connections for connecting to said consumer; a slide bore having an axis of symmetry; a slide piston movable in the slide bore; and a plurality of annular ducts surrounding the slide bore and communicating with the slide bore. The annular ducts include an annular tank connection duct on the axis of symmetry, and symmetrically arranged pairs of ducts including first and second annular working ducts connected to respective first and second working connections; first and second annular pump pressure ducts outside of said working ducts and connected to each other by a pump pressure duct connection; first and second annular load sensing ducts outside of said pump pressure ducts and connected to each other by a load-sensing connecting line; and first and second annular end space ducts outside of said annular load sensing ducts and connectable to other annular ducts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a hydraulic diagram of a first embodiment, 
       FIG. 2  shows a diagram of the arrangement of pressure ducts in the directional valve, 
       FIGS. 3 and 4  show this diagram with variants of connections, 
       FIG. 5  shows this diagram with an inserted slide piston and with two drives actuating the latter, in a neutral position, 
       FIG. 6  shows the same diagram in one of the working positions, 
       FIG. 7  shows a diagrammatic section through a slide piston with two inner pressure balances, 
       FIG. 8  shows a hydraulic diagram of a further exemplary embodiment, 
       FIG. 9  shows a diagrammatic section through a further exemplary embodiment, 
       FIG. 10  shows a diagram of a further exemplary embodiment, 
       FIG. 11  shows this diagram in the floating position, 
       FIG. 12  shows a further exemplary embodiment, 
       FIG. 13  shows a hydraulic diagram of this, and 
       FIG. 14  shows a further hydraulic diagram of this. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIG. 1 , a differential cylinder  1  has a first pressure space  2  and a second pressure space  3  which are separated from one another by a piston  4 . Fastened to the piston  4  is a tappet  5  which transmits the movement of the piston  4  to an implement, not illustrated. The differential cylinder  1  is in this case only one possible example of use. Instead, for example, an oil motor may also be used. 
   The differential cylinder  4  is activated by a directional valve  10  which is designed according to the invention. In a known way, the directional valve  10  has working connections A and B, the first working connection A being connected to the first pressure space  2  and the second working connection B to the second pressure space  3  of the differential cylinder  1 . 
   The directional valve  10  consists of a number of components and its construction is outlined below. A releasable nonreturn valve  11 A lies at the working connection A and a releasable non-return valve  11 B lies at the working connection B. Depending on the application, the releasable nonreturn valves  11 A,  11 B may even be dispensed with. Secondary pressure-limiting and feed valves  12 A and  12 B are arranged between a tank connection T and the respective working connections A and B. These secondary pressure-limiting and feed valves  12 A,  12 B act, for example, as suction follow-up valves. They are necessary, depending on the application, when external forces, the magnitude and direction of which may change, act on the tappet  5 . They are mentioned here only for the sake of completeness, belong to the known prior art and are therefore unrelated to the implementation of the idea of the invention. 
   A slide piston  13  determines the functioning of the directional valve  10 . This slide piston  13  is activatable, as will also be shown later. Pressure balances  14 A and  14 B are arranged between the slide piston  13  and the respective releasable nonreturn valves  11 A and  11 B or the working connections A and B. Since each of the working connections A and B is therefore assigned a separate pressure balance  14 A and  14 B, these are also designated as individual pressure balances. The pressure balances  14 A and  14 B therefore follow the slide piston  13  here. This is a principle often employed in the known prior art. 
     FIG. 1  shows, in addition, a pump connection P, from which the directional valve  10  is fed with hydraulic oil. An annular duct, described later, of the directional valve  10  is connected in a known way to the slide piston  13  by means of this pump connection P. Also shown is a load-sensing connection LS max  which, with regard to valves of such a type, belongs to the prior art and therefore is not described any further here. The pump connection P, tank connection T and load-sensing connection LS max  are present at the right and at the left margin of the diagram in  FIG. 1 , this being intended to express the fact that the directional valve  10  is constructed in such a way that a plurality of such directional valves  10  can be lined up to form a block, so that a plurality of consumers can be controlled. For the sake of clarity, the pressures are not depicted. A pressure p LSmax  prevails at the load-sensing connection LS max . 
   The slide piston  13  is axially displaceable by means of a drive. The drive is to be capable of displacing the slide piston  13  out of a neutral position corresponding to a position of rest in two directions. It is therefore state of the art to provide two such drives, to be precise a first drive  15 . 1 , which presses the slide piston  13  to the right, and a second drive  15 . 2 , which presses the slide piston  13  to the left. The drives  15 . 1 ,  15 . 2  are electrically controllable proportional magnets which act on the slide piston  13 . In specific simple applications, the drives  15 . 1 ,  15 . 2  may also be switching magnets which have only the two positions “ON” and “OFF”. In this case, they press the slide piston  13  in each case counter to a control spring  16 . By way of one of the drives  15 . 1  or  15 . 2  being activated, therefore, the slide piston  13  is displaced out of its position of rest, on account of the control springs  16 , the displacement of the slide piston  13  being proportional to the activation of the drives  15 . 1  and  15 . 2  when the drives  15 . 1  and  15 . 2  are proportional magnets. 
   The action of the pressure balances  14 A,  14 B is not discussed at this juncture because it is known from the known prior art. Pressure sensors or a differential-pressure sensor are likewise not depicted, which are present in any case and which are required in order to measure the pressure at the working connections A and B, this being a precondition for the movement of the piston  4  of the differential cylinder  1  to remain controllable in the event of changing directions of force. 
     FIG. 2  shows a diagram of the arrangement of pressure ducts in the directional valve  10 . This relates to that part of the directional valve  10  in which the slide piston  13  (FIG.  1 ), not illustrated here, is axially displaceable in a slide bore  18 . This diagram shows the arrangement of pressure ducts, which according to the invention is symmetric to an axis of symmetry S, and their line-up likewise according to the invention. An annular tank-connection duct  19  is located in the middle, that is to say on the axis of symmetry, that is to say at the symmetry center point. Connected to this annular tank-connection duct  19  is a tank-connection duct connection  20  which leads to the two end faces of the housing of the directional valve  10 . In the diagrammatic sectional drawing, the duct connection  20  is depicted by broken lines, because it lies in a different plane. It will also be shown that it is thereby possible, according to the invention, to connect the annular tank-connection duct  19  to other spaces by means of this tank-connection duct connection  20 . 
   Annular spaces open toward the end faces are located at the two ends of the housing of the directional valve  10 , to be precise, at one end, a first annular end-space duct  21  and, at the other end, a second annular end-space duct  22 . It will also be shown that the tank-connection duct connection  20  can be connected to these two annular end-space ducts  21 ,  22 , which likewise belongs to the essence of the invention. 
   The tank-connection duct connection  20  then consequently has the effect that the two annular end-space ducts  21 ,  22  have the same pressure, so that the same pressure acts on the end faces of the slide piston  13  ( FIG. 1 ) axially displaceable in the slide bore  18 . The slide piston  13  is thus pressure-relieved. 
   This tank-connection duct connection  20  does not necessarily have to be connected to other spaces. There are applications in which, for example, there is not to be this connection of the two annular end-space ducts  21 ,  22  to the annular tank-connection duct  19 . There is therefore provision, according to the invention, for the two annular end-space ducts  21 ,  22  to be connectable to the annular tank-connection duct  19  by means of the tank-connection duct connection  20 . This is dealt with again in more detail, as is the fact that there are or may be other connection possibilities. It will already be mentioned here, however, that, owing to the possibility that the annular tank-connection duct  19  may or may not be connected to the two annular end-space ducts  21 ,  22  by means of the tank-connection duct connection  20 , the directional valve  10  is designed, according to the invention, in many different variants, thus making it possible, on the basis of a universal directional valve  10 , to provide a multiplicity of variants for different applications. According to the invention, therefore, there is provision for the annular end-space ducts  21 ,  22  to be connectable to other annular ducts or other lines. 
   On both sides of the centrally arranged first annular tank-connection duct  19 , annular ducts for the working connections A and B follow, to be precise an annular A-duct  23  on one side and an annular B-duct  24  on the other side. Located behind them on both sides, as seen from the middle, are annular pump-pressure ducts, on one side a first annular pump-pressure duct  25  and on the other side a second annular pump-pressure duct  26 . These two annular pump-pressure ducts  25 ,  26  are connected to one another, according to the invention, by means of a pump-pressure duct connection  27  and are connected to the pump connection P (FIG.  1 ). The annular pump-pressure ducts  25 ,  26  are followed, as the next pair of annular ducts, by a first annular load-sensing duct  28  on one side and a second annular load-sensing duct  29  on the other side. Such annular load-sensing ducts are known per se, but are not present in the prior art according to DE-A1-32 25 003. The fact that, according to the invention, these annular load-sensing ducts  28 ,  29  are present broadens the possibilities of use of the directional valve according to the invention in a very significant way. According to the invention, the two annular load-sensing ducts  28 ,  29  are connected by means of a load-sensing connecting line  30 . The load-sensing connecting line  30  is led, in the same way as the tank-connection duct connection  20 , to the two end faces of the housing of the directional valve  10 . This serves for the possible provision of further advantageous design alternatives for various uses of the directional valve  10 , as is also described. 
   Moreover, a pilot-pressure connecting line  31  is also shown, which is generally present, but is used only for specific applications. The pilot-pressure connecting line  31  is led to the two end faces of the housing of the directional valve  10  in the same way as the tank-connection duct connection  20  and the load-sensing connecting line  30 . This, too, serves for the provision of variants of the directional valve  10 , to be precise those which are controlled by pilot pressure. 
   What is achieved by the arrangement according to the invention of the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22  in terms of symmetry and succession is, on the one hand, that equivalent hydraulic conditions prevail for both working connections A and B, and, on the other hand, that the number of control edges in the directional valve  10  is minimized. It is noteworthy that the directional valve  10  according to the invention, like that according to one of the exemplary embodiments of DE-A1-32 25 003, has seven annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29 , but at the same time, as mentioned, contains the annular load-sensing ducts  28 ,  29  which are absent in DE-A1-32 25 003. This was achieved, according to the invention, in that, according to the invention, the single annular tank-connection duct  19  lies on the axis of symmetry S and further annular tank-connection ducts are dispensed with. What is meant by “equivalent hydraulic conditions”, in this context, is that the static and dynamic forces on the slide piston  13  which occur due to the flow of hydraulic oil from and to the differential cylinder  1  ( FIG. 1 ) are very similar in the two symmetric regions of the directional valve  10  and constitute virtually no disturbance variables which would make the flow regulation from and to the working connections A, B unsymmetrical. 
   The symmetry and arrangement according to the invention of the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22  has the appreciable benefit that the directional valve  10  can be used for very different applications, for example different hydraulic drives, such as, for example, single-acting plunger cylinders, double-acting synchronous or differential cylinders or oil motors. As a result, the directional valve  10  can be equipped differently with a view to different applications, as will also be shown. 
     FIG. 3  shows the same diagram, but in this case with drives  15 . 1  and  15 . 2  mounted on both sides on the end faces of the housing of the directional valve  10 . In this case, in each of the two drives  15 . 1  and  15 . 2 , there is a clearance  32  which serves, in the drive  15 . 1 , to connect the annular tank-connection duct  19  to the first annular end-space duct  21  via the tank-connection duct connection  20  and, in the drive  15 . 2 , correspondingly to connect the annular tank-connection duct  19  to the second annular end-space duct  22  via the tank-connection duct connection  20 . It has already been mentioned that the intention of this is to operate, in a hydraulically pressure-relieved manner, the slide piston  13  ( FIG. 1 ) which is axially movable in the slide bore  18 . 
   The branches of the load-sensing connecting line  30  and the pilot-pressure connecting line  31  terminate, blind, at the drives  15 . 1  and  15 . 2 , because they are closed off by the housings of the drives  15 . 1  and  15 . 2 . 
   A variant is shown in FIG.  4 . Here, in the drives  15 . 1  and  15 . 2 , there are likewise clearances  32 , but these are placed differently, so that they perform a different function. In this design variant, the clearances  32  make a different connection, to be precise, in the drive  15 . 1 , the connection between the first annular end-space duct  21  and the load-sensing connecting line  30  and, in the drive  15 . 2 , the connection between the second annular end-space duct  22  and the load-sensing connecting line  30 . In this connection, too, the same pressure prevails on the two end faces of the slide piston  13 , so that the latter is therefore pressure-relieved. 
   In order to achieve the condition of pressure relief for the slide piston  13 , it is likewise possible that the annular end-space ducts  21 ,  22  are connectable to the annular pump-pressure ducts  25  or  26 , this being by means of the pump-pressure duct connection  27 , and this not being depicted in  FIGS. 2  to  4  for the sake of clarity. 
   So that the various possibilities of the advantageous connection of the annular end-space ducts  21 ,  22  to other annular ducts or other lines can be implemented in a simple way, all these connecting lines are also led onto the end faces of the housing of the directional valve  10 . By means of the drives  15 . 1 ,  15 . 2  and the position of the clearance  32 , it is then determined which connection is actually implemented. Without any modifications having to be carried out on the housing of the directional valve  10 , various advantageous embodiments are thus possible. 
   However, since there may also be the wish to use standardized drives  15 . 1 ,  15 . 2 , it is also possible, within the scope of the invention, to implement the respectively desired connections between the annular end-space ducts  21 ,  22  and other annular ducts or other lines by means of connections within the housing of the directional valve  10 , for example by means of corresponding bores. 
     FIG. 5  shows the same diagram, but in this case with the slide piston  13  arranged in it and, again, with drives  15 . 1  and  15 . 2  mounted at both lateral ends. 
   The slide piston  13  has a first annular groove  33  lying exactly centrally and two further annular grooves  34  which lie symmetrically to the center and which cooperate with the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22  and which thus make it possible for the hydraulic oil to flow, as is also outlined. 
   In the position of the slide piston  13 , as shown in  FIG. 5 , there is the above-mentioned “neutral” position, in which no throughflow is possible from the annular tank-connection duct  20  to the annular A-duct  23  or to the annular B-duct  24 . When the slide piston  13  is displaced to the left, then, on the one hand, a connection is made between the annular A-duct  23  and the first annular pump-pressure duct  25  via the left annular groove of the annular grooves  34  and, on the other hand, a connection is made between the annular tank-connection duct  19  and the annular B-duct  24  via the right annular groove of the annular grooves  34 . This is one of the working positions. The other working position is obtained by means of a similar displacement of the slide piston  13  to the right. 
   As already mentioned, the movement of the slide piston  13  takes place by means of the drive  15 . 1  or  15 . 2 , with the participation of the respective control spring  16 . It is important that the two end faces of the slide piston  13  be exposed to the same pressure, as has already been mentioned. 
   The drive  15 . 1  is depicted diagrammatically on one side. It has a magnet armature  40  which is movable by a coil, not illustrated. When the coil is excited, the magnet armature  40  acts via a tappet  41  on one end face of the slide piston  13 . Between the end face of the slide piston  13  and the drive  15 . 1  is clamped the control spring  16  which is supported, for example, against a ring  42  on the housing of the drive  15 . 1 . On the opposite side, the second drive  15 . 2  is shown, which contains the same elements as the drive  15 . 1 . For this exemplary embodiment with magnetic drives  15 . 1 ,  15 . 2 , the tank-connection duct connection  20  connects the two annular end-space ducts  21 ,  22  to the annular tank-connection duct  19  in the way illustrated in FIG.  3 . This purpose is served, here again, by the two clearances  32  on the end faces, facing the slide piston  13 , of the drives  15 . 1  and  15 . 2 . In this exemplary embodiment, therefore, the slide piston  13  is pressure-relieved.  FIG. 5  shows the slide piston in its neutral position, in which the two drives  15 . 1 ,  15 . 2  are not activated, so that the slide piston  13  is centered in the middle under the action of the two control springs  16 . In this position, both the annular tank-connection duct  19 , the two annular working-connection ducts, to be precise the annular A-duct  23  and the annular B-duct  24 , and the annular pump-pressure ducts  25 ,  26  are shut off, because none of the annular grooves  33 ,  34  makes a connection between the annular ducts. No hydraulic oil can therefore flow from and to the differential cylinder  4 . The differential cylinder  4  is consequently stationary. 
     FIG. 6  repeats the illustration of  FIG. 5 , but, here, in a position of the slide piston  13  in which the slide piston  13  is displaced to the right due to the excitation of the first drive  15 . 1 . The right control spring of the control springs  16  is compressed under the action of the drive  15 . 1 . In this position of the slide piston  13 , then, there is a connection from the annular A-duct  23  to the annular tank-connection duct  19  via the left annular groove of the two annular grooves  34  and, at the same time, a connection from the annular pump-pressure duct  26  to the annular B-duct  24  via the right annular groove of the two annular grooves  34 . The result of this is that, during this activation of the first drive  15 . 1 , hydraulic oil can flow from the pump line P ( FIG. 1 ) via the annular pump-pressure duct  26  to the annular B-duct  24  and from there via the working connection B ( FIG. 1 ) into the second pressure space  3  of the differential cylinder  1  (FIG.  1 ), while at the same time hydraulic oil can flow out of the first pressure space  2  of the differential cylinder  1  via the working connection A and the annular A-duct  23  to the annular tank-connection duct  19  and from there to the tank connection T. This corresponds to the “lowering” function for the differential cylinder  1 . 
   In a similar way to this, the activation of the second drive  15 . 2  leads to the “raising” function, in which, as is not shown additionally in a figure, hydraulic oil can flow from the pump connection P via the annular pump-pressure duct  25  to the annular A-duct  23  and further on via the working connection A to the first pressure space  2  of the differential cylinder  1 , while at the same time hydraulic oil can flow out of the second pressure space  3  of the differential cylinder  1  via the working connection B and the annular B-duct  24  to the annular tank-connection duct  19  and from there to the tank connection T. 
     FIG. 7  shows a diagrammatic sectional drawing of a slide piston  13  with two inner pressure balances  14  (FIG.  1 ), such as are known in principle from the prior art. Each of these pressure balances  14  has a pressure-balance piston  50  which is axially displaceable within an axial bore  51  of the slide piston  13 . The position of the pressure-balance pistons  50  is determined in a known way by the prevailing pressures and a pressure-balance control spring  52  which is supported, on the one hand, on the pressure-balance piston  50  and, on the other hand, on a closing cap  53 . These closing caps  53  are screwed on both sides into the slide piston  13  and at the same time form the end faces of the slide piston  13 . 
     FIG. 8  shows a hydraulic diagram of a further exemplary embodiment. Here, in contrast to  FIG. 1 , the drive of the slide piston  13  does not take place by means of two magnetic drives  15 . 1 ,  15 . 2 , but by means of a single hydraulic drive  60 . Parts with the same reference numerals correspond to the elements shown in FIG.  1 . 
   A first quick-action switching valve  61 A and a second quick-action switching valve  61 B lie one behind the other between the tank connection T and a pilot-pressure connection P pilot . These quick-action switching valves  61 A,  61 B act as controllable hydraulic resistances, the size of the respective resistance being determined by the clocking ratio of activation, for example by means of pulse-width-modulated signals, that is to say by the ratio “OPEN to SHUT” or “OPEN to (OPEN+SHUT)”. The result of this is that, in the connecting line, given the reference numeral  62 , between the two quick-action switching valves  61 A,  61 B, a pressure p St  can be controlled which can be set or varied, as desired, within the limits of the pressure prevailing at the tank connection T and at the pilot-pressure connection P pilot . This variable pressure p St  serves for controlling the slide piston  13 , because it is supplied to the drive  60  for the slide piston  13 . Moreover, the slide piston  13  is influenced by a control spring  16  which belongs to the drive  60  and which corresponds in functional terms to the control springs  16  of the first exemplary embodiment (FIG.  1 ). In this advantageous design variant, therefore, the directional valve  10  is a pilot-controllable directional valve. 
     FIG. 8  again shows, in addition, the pump connection P from which the directional valve  10  is fed with hydraulic oil. By means of this pump connection P, the annular tank-connection duct  19  ( FIG. 2 ) of the directional valve  10  is connected to the control piston  13  in a similar way to the previous exemplary embodiment. Also shown is the load-sensing connection LS max  which, as already mentioned, belongs to the prior art in the case of valves of such a type and is therefore not described any further here. The pump connection P, tank connection T, load-sensing connection LS max  and pilot-pressure connection P pilot  are also present at the right and at the left margin of the diagram in  FIG. 8 , which again is intended to express the fact that the directional valve  10  is constructed in such a way that a plurality of such directional valves  10  can be lined up to form a block, so that a plurality of consumers can be controlled. In this case, it is advantageously possible that, with the basic form of construction being the same, the individual directional valves  10  have different embodiments according to the general idea of the invention, so that, for example, one is according to FIG.  1  and another is according to FIG.  8 . The pressures are again not depicted for the sake of clarity. A pilot pressure P pilot  prevails at the pilot-pressure connection P pilot , a pressure p LSmax  prevails at the load-sensing connection LS max  and a control pressure p St  prevails in the connecting line  62 . 
   It will be assumed here, for the description of functioning, that the drive  60  is a drive with a differential cylinder, as will also be shown later. On the one hand, the pilot pressure p pilot  and, on the other hand, the control pressure p St  act on this drive  60 . By the control pressure p St  being varied as a result of the activation of the quick-action switching valves  61 A,  61 B, the piston of the drive  60  can be moved and this movement is transmitted to the slide piston  13 . 
     FIG. 9  shows a diagrammatic sectional drawing of the directional valve  10  with the drive  60  mounted on it. The two quick-action switching valves  61 A,  61 B are installed in the drive  60 . The drive  60  consists essentially of a drive piston  70  which is directly connected, for example by means of a screw connection, to the slide piston  13  on one side via a piston rod  71 . The rigid connection of the drive piston  70  and the slide piston  13  makes it possible that the drive  60  can move the slide piston  13  out of the middle neutral position in both directions, so that it is possible to manage with a single drive  60 . A control-pressure space  72  is adjacent to one side of the drive piston  70 , while a pilot-pressure space  73  is arranged, surrounding the piston rod  71 , on that side of the drive piston  70  which faces the slide piston  13 . The control pressure p St  capable of being influenced by the quick-action switching valves  61 A,  61 B prevails in the control-pressure space  72 , while the pilot pressure P pilot  prevails in the pilot-pressure space  73 . The pilot-pressure line  31 , which is present in the directional valve  10  and was already shown in  FIGS. 2  to  4  and the connection P pilot  of which is also shown in  FIG. 8 , is thus continued into the housing of the drive  60  and, as can likewise already be seen from  FIG. 8 , is connected to the quick-action switching valve  61 A and, moreover, to the pilot-pressure space  73 . It is also possible to see from  FIG. 9  the run of the connecting line  62 , already shown in  FIG. 8 , in the housing of the drive  60 , said connecting line making a connection between the quick-action switching valves  61 A,  61 B and the control-pressure space  72 . The tank-connection duct connection  20  here connects the annular tank-connection duct  19  to the first annular end-space duct  21 . The possibility, provided in the directional valve  10 , that the annular tank-connection duct  19  is connectable to the second annular end-space duct  22  by means of the tank-connection duct connection  20  is not utilized here. Since the slide piston  13  is actuated hydraulically in the case of a hydraulic drive  60 , in that a hydraulic pressure acts on one end face of the slide piston  13 , this connection therefore does not need to be present here. Instead, the tank-connection duct connection  20  leads into the drive  60 , specifically to the quick-action switching valve  61 B, as can likewise already be seen from  FIG. 8 , since it is shown there that the quick-action switching valve  61 B has a connection to the tank connection T. 
   The piston rod  71  is surrounded by the control spring  16 , already shown in FIG.  8 . This control spring  16  is supported, on one side, against the piston  70  or a step  76  via a first ring  75 . It is supported, on the other side, on part of the end face of the slide piston  13  via a second ring  77 . It is therefore a restrained spring. In this ring  77 , there is an orifice  78 , by means of which the pilot-pressure space  73  is connected to the second annular end-space duct  22 . The movement of the drive piston  70  and therefore of the slide piston  13  is thus influenced by the pressures in the control-pressure space  72  and in the pilot-pressure space  73  and also by the control spring  16 . By virtue of the arrangement of the control spring  16 , as shown and described, the latter holds the slide piston  13  in the neutral position, shown in  FIG. 9 , which is equivalent to the neutral position in the first exemplary embodiment (FIG.  5 ). 
   On the side located opposite the drive  60 , the first annular end-space duct  21  is closed by means of a plate  80 . The control-pressure space  72  is closed off by means of an insert  81 . The plate  80  may have a similar or identical shape to the insert  81 . The clearance  32  already mentioned is arranged in this plate  80  in such a way that said clearance connects the first annular end-space duct  21  to the annular tank-connection duct  20 . 
   It will be assumed here, for the description of functioning, that the drive  60  is an example in which the effective cross section of the piston  70  in the control-pressure space  72  is twice as large as the effective cross section in the pilot-pressure space  73 . When the two quick-action switching valves  61 A,  61 B are activated in such a way that the pressure in the control-pressure space  72 , which corresponds to the pressure in the connecting line  62 , amounts to half the pressure in the pilot-pressure space  73 , which corresponds to the pressure at the pilot-pressure connection P pilot , the same force acts on both sides of the piston  70  of the drive  60 , so that the piston  70  and consequently the control slide  13  are stationary and are held in the neutral position by the control spring  16 . 
   When the pressure p St  in the connecting line  62  and therefore in the control-pressure space  72  is reduced as a result of the corresponding activation of the quick-action switching valves  61 A,  61 B, the drive  60  moves the slide piston  13  to the right counter to the force of the control spring  16 . When the pressure p St  in the connecting line  62  and therefore in the control-pressure space  72  is increased, this again being achieved by means of the corresponding activation of the quick-action switching valves  61 A,  61 B, the drive  60  moves the slide piston  13  to the left. 
   When the drive  60  is in equilibrium in terms of the pressure forces, the prestressed control spring  16  retains the slide piston  13  between stops in the middle position shown in FIG.  9 . The stops are in this case, on the one hand, the first ring  75  which is supported against the piston  70  or the step  76  and, on the other hand, the second ring  77  which is supported on part of the end face of the slide piston  13 . The rings  75  and  77  form, together with the prestressed control spring  16 , a virtually rigid part which, in the neutral position shown here, can move with a play of only a few tenths of a millimeter between the stops which are provided by the slide piston  13 , on the one hand, and by the piston  70  or the step  76 , on the other hand. In this position, the slide piston  13  shuts off the connection from the pump connection P to the working connections A and B. This position of the slide piston  13  is the “neutral” position. 
   In the working positions, therefore, the slide piston  13  can be displaced proportionally by means of the drive  60  and assume any desired positions within the limits of the maximum possible stroke. In this case, owing to the symmetry of the annular ducts  28 ,  25 ,  23 ,  19 ,  24 ,  26  and  29 , the behavior is identical in terms of its action for the working connections A and B. 
   It is highly advantageous, as a direct consequence of the invention, that the directional valve  10  can be used, with the same symmetric arrangement of the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22 , both for equipping with magnetic drives  15 . 1  and  15 . 2  ( FIGS. 5 and 6 ) and for equipping with a single hydraulic drive  60 . The only necessary variation in the arrangement of the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22  is that, in the one case, there is a connection from the second annular end-space duct  22  to the annular tank-connection duct  19  by means of the tank-connection duct connection  20 , but not in the other case. The manufacture of different variants of directional valves  10  for different applications consequently becomes highly efficient. 
     FIG. 10  shows a diagram of a further exemplary embodiment. This corresponds to the greatest possible extent to that of  FIG. 5 , but has the two following essential differences. The slide piston  13  ( FIG. 5 ) is divided, here, into two individual slide pistons, to be precise a first slide piston  13 . 1  and a second slide piston  13 . 2 . The second difference from  FIG. 3  is that the drives  15 . 1  and  15 . 2  ( FIG. 5 ) belonging to the slide pistons  13 . 1 ,  13 . 2  do not have a pressing action on the slide pistons  13 . 1 ,  13 . 2 , but a pulling action. In view of this significant difference, the drives in  FIG. 10  are designated by the reference numerals  15 . 1 ′ and  15 . 2 ′.  FIG. 10  shows the neutral position in which neither of the two drives  15 . 1 ′ and  15 . 2 ′ is excited. 
   When the drive  15 . 1 ′ is excited, its magnet armature  40  pulls the slide piston  13 . 1  to the left, so that the throughflow of hydraulic oil between the annular tank-connection duct  19  and the annular A-duct  23  is enabled. During this excitation of the drive  15 . 1 ′, however, the other slide piston  13 . 2  does not remain in its position corresponding to the neutral position, but is likewise pressed to the left under the action of the control spring  16  assigned to it, so that the two slide pistons  13 . 1  and  13 . 2  continue to rest against one another, that is to say continue to act in the same way as a one-part slide piston  13  (FIG.  5 ). The throughflow of hydraulic oil from the annular pump-pressure duct  26  to the annular B-duct  24  is correspondingly also possible. 
   The situation is reversed correspondingly when the drive  15 . 2 ′ is excited. Then, its magnet armature  40  pulls the slide piston  13 . 2  to the right, so that the throughflow from the annular B-duct  24  to the annular tank-connection duct  19  is possible. During this excitation of the drive  15 . 2 ′, the other slide piston  13 . 1  likewise does not remain in its position corresponding to the neutral position, but is likewise pressed to the right under the action of the control spring  60  assigned to it, so that the two slide pistons  13 . 1  and  13 . 2  continue to rest against one another, that is to say continue to act in the same way as a one-part slide piston  13  (FIG.  5 ). The throughflow of hydraulic oil from the annular pump-pressure duct  25  to the annular A-duct  23  is correspondingly also possible. This solution therefore exactly corresponds in functional terms to that which is shown in  FIGS. 5 and 6  and described with reference to these figures. 
   By the slide piston  13  ( FIG. 5 ) being divided into the two slide pistons  13 . 1  and  13 . 2  and by these being actuated by the two drives  15 . 1 ′ and  15 . 2 ′, however, there is in this case the possibility that the two slide pistons  13 . 1  and  13 . 2  can be moved independently of one another. When the drives  15 . 1 ′ and  15 . 2 ′ are excited simultaneously, the magnet armature  40  of the drive  15 . 1 ′ pulls the slide piston  13 . 1  to the left and the magnet armature  40  of the drive  15 . 2 ′ pulls the slide piston  13 . 2  to the right, in each case counter to the action of the associated control spring  16 . This situation is illustrated in FIG.  11 . When the slide pistons  13 . 1  and  13 . 2  are in this position during the excitation of the two drives  15 . 1 ′ and  15 . 2 ′, it is possible for hydraulic oil to flow from the annular A-duct  23  to the annular tank-connection duct  19  and at the same time also from the annular B-duct  24  to the annular tank-connection duct  19 . In this position, the hydraulic oil can flow almost without any resistance from the annular A-duct  23  to the annular B-duct  24 , and vice versa. This results in a very simple way in the floating position. This solution is therefore extremely advantageous. Additional means which are necessary in the known prior art are therefore not required here. 
   For this embodiment with two pulling drives  15 . 1 ′ and  15 . 2 ′ and the two slide pistons  13 . 1  and  13 . 2 , it is mandatory that the two annular end-space ducts  21 ,  22  be connected to the annular tank-connection duct  19  via the tank-connection duct connection  20 , in order to ensure that the same pressure prevails at the end face on the two slide pistons  13 . 1  and  13 . 2 , in order to operate them with pressure relief. 
     FIG. 12  shows a further exemplary embodiment which corresponds as closely as possible to that of  FIGS. 10 and 11 , but, instead of the magnetic drives  15 . 1 ′ and  15 . 2 ′, has hydraulic drives  60 . 1  and  60 . 2  which correspond to the drive  60  already shown in FIG.  9 . In the exemplary embodiment of  FIG. 12 , there is the divided slide piston  13  which is divided into the slide pistons  13 . 1  and  13 . 2 . This version corresponds in functional terms to that of  FIGS. 10 and 11 , only with the difference that, in the example of  FIG. 12 , the movement of the two slide pistons  13 . 1  and  13 . 2  takes place as a result of the activation of the quick-action switching valves  61 A,  61 B, as is described above. In  FIG. 12 , the slide piston  13 . 2  illustrated on the right is displaced to the right as a result of the corresponding activation of the quick-action switching valves  61 A,  61 B, with the result that the control spring  16  contained in the drive  60 . 2  is compressed, that is to say its spring force codetermines the position of the slide piston  13 . 2 . As already described with regard to the exemplary embodiment of  FIG. 11 , in this exemplary embodiment it is possible, without additional means being required, to achieve the floating position in a highly advantageous way by means of the corresponding activation of the quick-action switching valves  61 A,  61 B of the two drives  60 . 1 ,  60 . 2 . 
   A hydraulic circuit for this exemplary embodiment is shown in FIG.  13 .  FIG. 13  corresponds in principle to  FIG. 8 , but, instead of the single slide piston  13  with the hydraulic drive  60  actuating the latter, has the two separate slide pistons  13 . 1  and  13 . 2  with the associated drives  60 . 1  and  60 . 2 . There is correspondingly also double the number of quick-action switching valves  61 , to be precise, on the one hand, the quick-action switching valves  61 . 1 A and  61 . 1 B, which are assigned to the drive  60 . 1 , and the quick-action switching valves  61 . 2 A and  61 . 2 B, which belong to the drive  60 . 2 . 
   Instead of the single connecting line  62  of  FIG. 8 , in which the control pressure p St  for controlling the drive  60  prevails, two connecting lines  62 . 1  and  62 . 2  are shown in  FIG. 13. A  control pressure p St1 , which has the effect of controlling the drive  60 . 1 , prevails in the connecting line  62 . 1 . In a similar way to this, a control pressure p St2 , which has the effect of controlling the drive  60 . 2 , prevails in the connecting line  62 . 2 . 
   A further hydraulic diagram is shown in FIG.  14 . This largely corresponds to  FIG. 13 , but, instead of the differential cylinder  1 , there are two hydraulic consumers independent of one another, to be precise a first consumer  100 A and a second consumer  100 B. The first consumer  100 A is connected to the first working connection A of the directional valve  10 , and the second consumer  100 B is connected to the second working connection B. Since, as mentioned above, in the case of a divided slide piston  13 , the two slide pistons  13 . 1  and  13 . 2  can be controlled independently of one another, it is therefore possible, by means of one directional valve  10  in the form of construction shown in  FIG. 12 , to operate two hydraulic consumers  100 A,  100 B independently of one another. 
   The possibilities afforded by the directional valve  10  according to the invention become obvious. Moreover, automatic control systems are possible with the aid of pressure sensors and electrical control members. Instead of quick-action switching valves, the directional valve  10  according to the invention may also use other means, for example electrically controllable pressure-reducing valves. 
   By virtue of the symmetry and arrangement according to the invention of the annular ducts  21 ,  28 ,  25 ,  23 ,  19 ,  24 ,  26 ,  29  and  22  and of the equivalent hydraulic conditions thereby achieved for the two working connections A and B, it is possible, in addition to the exemplary embodiments shown above, also to implement further variants for different applications. Thus, for example, it is possible to dispense with the pressure balances  14 . Also, as shown above, it is possible to use two hydraulic drives  60  instead of two magnetically actuable drives  15 , in which case it is possible to provide such a variant with the slide piston  13  divided into two slide pistons  13 . 1 ,  13 . 2 , in order in a simple way to implement the function of the floating position even in the case of a directional valve  10  controlled by pilot pressure. It thus becomes possible, by means of the invention, to implement a large number of variants in the manner of a modular construction system without directional valves  10  constructed in a fundamentally different way being necessary.