valve

Abstract

A valve, in particular a proportional valve, with a main piston (10) for controlling a main volume flow and a pilot piston (12) for controlling a pilot volume flow, the main piston (10) and the pilot piston (12) being guided longitudinally movably in a valve housing (14), the pilot piston (12) being controllable by a solenoid device (16), in which the position of the main piston (10) is adjustable by the pilot piston (12) and the fluid pressure present at the main piston (10) is transferred to the pilot piston (12) via a fluid communication (18). The solenoid device (16) is operated, the pilot piston (12) in its operating position holds the main piston (10) in a closed position blocking the main volume flow, and when the solenoid device (16) is not operated, the pilot piston (12) is in its initial position and the fluid pressure in the pilot chamber (20) drops towards the tank side (T) until the main piston (10) reaches an open position controlling the main volume flow, disconnected from the pressure supply via the main piston (10).

Description

[Technical field]

[0001] The invention relates to a valve, in particular a proportional valve, which has a main piston for controlling a main volume flow and a pilot piston for controlling a pilot volume flow, the main piston and the pilot piston being guided longitudinally movably in a valve housing, the pilot piston being controllable by a solenoid device. [Background technology]

[0002] From German patent application DE 10323595 A1 a valve, in particular in the form of a proportional seat valve or piston valve, is known which has a valve housing, at least three fluid ports passing through the valve housing, a main piston guided in the valve housing and a pilot piston which is controllable by an energizable solenoid device and which operates a pilot control, whereby when the pilot control is opened, fluid from one of both ports which can be controlled by the main piston passes through a cross-sectional narrowing in the main piston and the pilot control to a third port which can be controlled by the pilot piston, and on the basis of the resulting pressure drop the main piston reaches a respective control position in which it can control both fluid ports with a fluid quantity.

[0003] From European patent application EP 2 880 315 B1 a valve, in particular a pilot-controlled proportional directional valve, is known which has a valve housing with a fluid inlet and a fluid outlet, the flow of fluid between which can be adjusted by a main piston, a pilot valve chamber with a pilot valve closing member which can be moved by an operating device is provided on the rear side of the main piston, the pilot valve closing member allowing the flow of fluid between the pilot valve chamber and the fluid outlet to be adjusted by means of this pilot valve closing member, an inlet orifice is arranged between the fluid inlet and the pilot valve chamber, the opening cross section of which can be reduced by a control element, the inlet orifice has a plurality of orifice holes, the opening cross section of at least one of which can be reduced by the control element, and by dividing the inlet orifice into a plurality of orifice holes it is possible to switch the inlet orifice into a plurality of opening stages.

[0004] Such pilot-controlled electrohydraulically operated directional valves control the start, stop and direction of the volume flow as well as the speed, acceleration and deceleration of a hydraulic consumer, for example in the form of a hydraulic working cylinder, connected to the valve. Pilot-controlled directional valves are primarily used for controlling large volume flows with low flow losses. However, large volume flows with low flow losses mean a large opening cross section and thus a large stroke of the main piston. The obstacles that impair the function of the valve are the volume flow and the pressure difference, i.e. the flow and friction forces which interfere with the switching action depending on the valve performance. In particular in the case of mechanical actuation by a spring, usually in the form of a return spring, the design value of the spring force may be too small compared to the friction forces to return the piston valve. In order to meet the stroke and force requirements for such a solenoid system, it would be necessary to design the solenoid system with correspondingly large dimensions or to pilot the valve hydraulically. Valves with hydraulic pilot control are therefore necessarily known in the prior art for such applications, as mentioned above. Summary of the Invention [Problem to be solved by the invention]

[0005] The object of the present invention is to proceed from this prior art and further improve the known valve with regard to its functional behavior. This object is achieved by a valve which has as a whole the features of patent claim 1. [Means for solving the problem]

[0006] According to the characterizing feature of claim 1, the position of the main piston is adjustable by means of a pilot piston, the fluid pressure present in the main piston is transmitted via fluid communication to a pilot chamber having the pilot piston, when the solenoid device is operated in the operating position, the pilot piston holds the main piston in a closed position blocking the main volume flow, and when the solenoid device is not operated or energized, the pilot piston is in an initial position and the fluid pressure in the pilot chamber drops towards the tank side, disconnected from the pressure supply via the main piston, until the main piston reaches an open position controlling the main volume flow, thereby providing an improved control quality with low pressure losses while avoiding leakage during operation of the valve according to the invention overall.

[0007] In a preferred embodiment of the valve according to the invention, the solenoid arrangement comprises a pull-type solenoid, which when energized preferably relies on the action of an energy storage means (F 磁気バネ) from the initial position to the operating position, where the main piston is held in the closed position. When de-energized, the pilot piston is in the initial position opposite the main piston, whereby the pilot chamber is isolated from the pressure supply and opens towards the tank. If the pressure then increases on the inlet side of the main piston, this fluid pressure pushes the main piston to the open position and opens the fluid communication for directing the main volume flow between two fluid ports in the valve housing, preferably one of which enters the valve housing axially and the other radially. To obtain the thus presented "de-energized open", a pull-type solenoid is used instead of a push-type solenoid. In this way, the concept of stroke control proportional to the magnetic force (current) is realized by the pull-type solenoid, resulting in a de-energized open valve. As a result, a linearly descending valve characteristic curve is obtained, compensating for the flow forces and the sealing friction at the main piston, for the desired linear adjustment of the position of the main piston depending on the magnetic force.

[0008] In a preferred embodiment of the valve according to the invention, the pilot piston is actuated by the force of the energy storage means (F バネ ) and the force of the solenoid device acting in the same direction (F アクチュエータ ) and the pilot piston can be brought to the operating position under the combined action of friction force (F 摩擦 ) and F アクチュエータ -F バネ -F 摩擦 = 0, so the pressure is essentially in equilibrium.

[0009] A spring is connected to the pull-type solenoid, and this spring is designed as a magnetic spring, which generates a magnetic force (F 磁石 ) and the same force (F 磁気バネ ), and the total force resulting from this is (F アクチュエータ ) is called the formula F アクチュエータ =F バネ -F 摩擦 It is expressed as:

[0010] During the opening movement of the main piston to control the main volume flow between two possible fluid ports in the valve housing, the pilot piston is force balanced so that the stroke of the main piston can be adjusted by the pilot control.

[0011] Further advantageous embodiments of the valve according to the invention are the subject of the dependent claims. It is particularly advantageous if the main piston is connected to a displacement measuring device in order to detect its position. Due to the advantageous control of the main piston via the pilot piston, the valve according to the invention requires a solenoid device of correspondingly small dimensions with a reduced opening cross section in the region of the controlled volume flow, so that only a small stroke of the main piston is required. In this way, a highly dynamic proportional throttle valve is obtained.

[0012] In the following, the valve according to the invention will be explained in more detail on the basis of an embodiment shown in the drawings, which are principle longitudinal sections and are not drawn to scale. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a longitudinal cross-sectional view of the main components of the entire valve. [Diagram 2] FIG. 2 is a longitudinal cross-sectional view of the front of the valve shown in FIG. 1 in a closed position. [Diagram 3] FIG. 3 is a longitudinal section corresponding to FIG. 2, of the valve in an open position. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] The valve shown is a so-called proportional valve and has a main piston 10 for controlling the main volume flow. Furthermore, a pilot piston 12 is present for controlling the pilot volume flow, both main piston 10 and pilot piston 12 being guided so as to be longitudinally displaceable in a valve housing 14. As can be seen in particular in Figure 1, the pilot piston 12 is controlled so as to be displaceable in the valve housing 14 by a solenoid device 16.

[0015] The position of the main piston 10 can be adjusted by a pilot piston 12, the fluid pressure present in the main piston 10 being transmitted via a fluid communication 18 to a pilot chamber 20 with the pilot piston 12, which holds the main piston 10 in a closed position blocking the main volume flow, as shown in FIG. 2, when the solenoid device 16 is operated or energized. When the solenoid device 16 is not operated or energized, the pilot piston 12, starting from the energized operating position shown in FIG. 2, moves to the left as viewed in the direction of view of the figure, to an initial position as shown in FIG. 3, in which the fluid pressure in the pilot chamber 20 drops until it reaches an open position in which the main piston 10 controls the main volume flow, substantially disconnected from the pressure supply via the main piston 10. This is illustrated in FIG. 3 for one of the possible open positions. When the directions "right" and "left" are mentioned, this means as viewed in the direction of view of the figure.

[0016] The position of the main piston 10 is adjusted with the aid of the pilot piston 12. In this way, the balance of forces at the pilot piston 12 is taken into account, which is approximately pressure balanced, apart from any possible manufacturing tolerances, and therefore no compressive forces are present. In this case, the relationship according to the following formula arises: F アクチュエータ -F バネ -F 摩擦 =F アクチュエータ -(F1 バネ +x*c)-F 摩擦 =0 however,

[0017]

number

[0018] An additional compression spring 24 can act on the solenoid 17 in the direction opposite to the direction of its suction operation. This compression spring 24 is used to provide a so-called dead current until the solenoid 17 as a whole overcomes the spring force and starts to move. In this way, possible disturbances in the valve operation are prevented.

[0019] In the following, the individual forces that occur, particularly the spring forces, are shown by force arrows in FIG. 1 together with their directions of action. The spring force F acting on the end face side of the pilot piston 12 is バネ is indicated by arrow 26, the magnetic force on the solenoid 17 is indicated by arrow 28, and the spring force F acting on the solenoid is indicated by arrow 29. 磁気バネ is indicated by arrow 29.

[0020] As can further be seen from the figure, the pilot piston 12 is designed as a hollow piston and has a first recess 30 in the form of an annular step on its outer periphery, which in a defined operating position establishes a fluid communication with the main piston 10 when the solenoid device 16 is actuated or energized, according to the representation of Fig. 2, and reaches an initial position under the action of both compression springs 23, 24 and blocks such a fluid communication when the solenoid device 16 is not actuated or energized, according to the representation of Fig. 3. The pilot piston 12, designed as a hollow piston, has a fluid communication passage 32 along its inner periphery. The fluid communication passage 32 opens with one free end 34 into the pilot chamber 20 and leads via a radially extending passage section 36 of this axially extending passage 32 to a second annular recess 38 on the outer periphery of the hollow or pilot piston 12. When the solenoid device 16 is not operated, the pilot piston 12, according to the representation according to Fig. 3, is disconnected from the fluid communication with the main piston 10 by the first recess 30 and enters into fluid communication with the tank connection 40. All the above mentioned fluid connections, as well as the flow passage portion 36 and the tank connection 40, can be multiple as shown, but for the sake of simplicity and clarity, usually only one fluid connection is given one reference number.

[0021] A seat-tight changeover valve 42 is connected in the fluid communication between the main piston 10 and the pilot chamber 20. When the valve is "energized" and closed, this changeover valve 42 transmits the higher pressure of the two fluid ports 44, 46 in the valve housing 14, through which the main volume flow to be controlled is guided, as a signal pressure to the pilot chamber 20. If the fluid pressure at the axial fluid port 44 on the end side is higher than the fluid pressure at the radial fluid port 46 arranged transversely thereto, the fluid with the higher pressure reaches the changeover valve 42 with a closing ball 48 via the fluid port 44 in the front region of the main piston 10, which is designed seat-tight in this respect, and via a longitudinal passage 50 with a transverse hole 51. Such a closing ball 48 moves to the right as viewed in the direction of view of the figure and closes the radial passage 52 arranged in the main piston 10 and opening into the fluid port 46 in the valve housing 14. The valve chamber 54 of the changeover valve 42 is also connected to a further longitudinal passage 56, which is also provided in the main piston 10, and whose free end face opens into a fluid chamber 58 in the main piston 10. The fluid chamber 58 is connected to a further fluid chamber 62, which is formed as an annular chamber between the valve housing 14 and the main piston 10, via two communicating passages 60 arranged in the main piston 10 at an inclination angle of 45° to the longitudinal axis of the valve. The further fluid chamber 62 then passes into a radially enlarged annular chamber 64 arranged in the valve housing 14, from which a longitudinal passage section 66 leads opposite each other into a further annular chamber 67, which passes into a radially arranged transverse passage section 68 in the valve housing 14. This transverse passage section 68 widens in cross section in the direction of the pilot piston 12 at its inner free end in the shape of a groove and partially overlaps in fluid communication with the first annular recess 30 on the outer periphery of the pilot piston 12, according to the representation in FIG. 2. Such fluid communication is established as soon as the solenoid device 16 is energized and the pilot piston 12 moves from left to right starting from the initial unenergized position shown in FIG. 3 to reach the right-hand operating position shown in FIG. 2.The valve is then in a closed position blocking the fluid path between the fluid ports 44,46.

[0022] As previously explained, it is understood that each of the illustrated flow passages and other connections can be multiplexed in the valve as necessary to ensure reliable fluid communication. In any case, the components 50, 52, 54, 56, 58, 60, 62, 64, 66, 67, 68 and 30 form a possible fluid communication 18 between the fluid ports 44, 46 in the main piston 10 and the pilot chamber 20 between the main piston 10 and the pilot piston 12 as long as the solenoid device 16 is energized. Thus, in the closed valve position of the main piston 10, the higher pressure in the fluid port 44 or 46, respectively, is transmitted to the pilot chamber 20 as long as the pilot piston 12 assumes the energized operating position shown on the right in FIG. 2. Otherwise, in the open position where the solenoid device 16 is not operated and is not energized, the fluid communication 18 between the first recess 30 and the transverse flow passage portion 68 is blocked by the pilot piston 12 assuming the initial position shown on the left in FIG. 3. In this way, the pressure in the pilot chamber 20 is switched to the substantially pressure-free tank side T (see Figure 1) via the tank connection 40, allowing the main piston 10 to assume one of the open positions shown in Figure 3, in which position a main fluid communication is established between the fluid ports 44 and 46 in both possible fluid directions.

[0023] If the pressure in the further fluid port in the valve housing 14, the radial fluid port 46, is greater than the pressure in the fluid port 44, the changeover valve 42 is switched by the closing ball 48 taking up the closed position in which the left-hand flow passage 50 is closed as viewed in the direction of view of the figure. In this way, fluid with a settable pressure reaches the further longitudinal flow passage 56 via the fluid port 46 and the radial flow passage 52 as well as the valve chamber 54. If the pilot piston 12 then takes up the energized right-hand operating position shown in FIG. 2, the remaining fluid communication 18 is opened through this path towards the pilot chamber 20. Normally, the fluid pressure from a pressure supply device, such as a hydraulic pump, is present in the fluid port 44, and a hydraulic consumer, such as a hydraulic cylinder, is connected to the fluid port 46. Within the scope of the use of such proportional valves, other assignments of the fluid ports 44, 46 are conceivable. This valve is particularly suitable as a so-called load valve, which can be reduced in a controlled manner on the basis of its seat sealing, in particular if the actuating solenoid device 16 cooperates with an electronic control or electronic adjustment, not shown in detail.

[0024] As can be further seen from the figure, the main piston 10 is composed of two components 70, 72, which define a fluid chamber 58 at their free end faces facing each other as seen in the axial movement direction of the valve pistons 10, 12. A second compression spring 74 acts on the free end face of the second component 72, which faces the solenoid device 16, as another energy storage means. This second compression spring 74 is supported at its other free end on the inner part of the valve housing 14 and is designed as a compression spring, biasing the main piston 10 in its closing direction. Arranged coaxially on the inner periphery of the second compression spring 74, the compression spring 22 extends therefrom and is supported at one free end on an inner stepped shoulder of the second component 72 and at the other free end on an annular projection 76 on the pilot piston 12. In this way, the main piston 10 is biased by the compression spring 74, and the pilot piston 12 is biased by the spring 22, respectively, with their respective spring forces.

[0025] In the following, it will be explained how the movement of the main piston 10 is controlled. The movement of the main piston 10 to the left in the observation direction of Fig. 2, i.e. to the state shown in Fig. 3, is carried out as follows. The pressure present in the respective fluid port 44 or 46 upstream of the main piston 10 is transmitted to the pilot piston 12 via the changeover valve 42 and the fluid communication 18, and when the solenoid device 16 is energized, the pressure in the pilot chamber 20 first holds the main piston 10 in the closed state due to the area difference formed by the opposing end faces of the main piston 10, according to the representation shown in Fig. 2. At that time, according to the representation shown in Fig. 2, the pilot piston 12 is in tight-seat contact with the valve housing 14 via the annular valve seat 77, so that leakage flow cannot unintentionally flow out of the pilot chamber 20 towards the tank connection 40 when the valve is closed.

[0026] When the force generated by the solenoid device 16 is no longer generated (de-energized open), the pilot piston 12 moves to an initial position shown in FIG. 3, to the left as viewed in the direction of view of the figure, taking into account the compression spring 23 acting at least between the pilot piston 12 and the solenoid 17 of the solenoid device 16. In such an initial left position, the recess 30 on the outer periphery of the pilot piston 12 is closed by the corresponding housing part of the valve housing 14. Similarly, the respective lateral passages 68, which are connected in fluid communication with the fluid chamber 58 and thus with the respective pressure inlet side of the main piston 10, are closed, preferably in a fluid-tight manner, by the adjacent outer periphery of the pilot piston 12. Furthermore, the main piston 10 is biased towards the closed position by an outer second compression spring 74.

[0027] The pilot chamber 20, thus cut off from the pressure supply from the side of the main piston 10, can then be kept unpressurized via the fluid communication passage 32 in the pilot piston 12 towards the tank connection 40, due to the inevitable drop in pressure in the pilot chamber 20. In this way, fluid communication is established from the pilot chamber 20 via the fluid communication passage 32 to the respective transverse passage section 36 of this fluid communication passage 32. In addition to the end face inlets of the pilot piston 12, there is another permanent fluid communication to the pilot chamber 20 via the transverse passage guides 75, each of which opens at one free end into the passage 32 of the pilot piston 12. Since the pilot piston 20 is in its initial open position when de-energized, the second recess 38 in the valve housing 14 is operated to the left by the compression spring 23, from which the fluid can flow via the annular shoulder recess 79 in the valve housing 14 into the relief chamber 80 formed by the enlarged annular recess in the valve housing 14. Then, when the valve seat 77 is opened, the pressurized fluid from the pilot chamber 20 side flows out of this relief chamber 80 into the annular passage 78. The annular passage 78 is provided in the pilot piston 12 with a settable length so that in all possible positions of movement of the pilot piston 12, the annular passage 78 overlaps and is in fluid communication with the tank connection 40. The fluid present in the annular passage 78 is then sent to the tank side T of the valve arrangement via the respective tank connection 40, so that the pilot chamber 20 is completely depressurized. Depending on the fluid pressure present in the respective main piston 10, the main piston 10 is operated to open against the spring action of the compression springs 22 and 74, and a new equilibrium is created with the valve open.

[0028] When the solenoid 17 of the solenoid arrangement 16 is energized, the corresponding solenoid 17 or solenoid armature is moved against the action of the additional compression spring 24 and the magnetic spring 23 into a moving or resting position to the right as viewed in the direction of view in Fig. 1. For this purpose, the solenoid arrangement 16 has an energizable coil 81 which, when correspondingly energized, moves the solenoid 17 or solenoid armature from the initial position shown in Fig. 1 to the right into an operating position. When the coil 81 is de-energized via a conventional plug connection 82 on the solenoid arrangement 16, the two compression springs 23, 24 allow an unhindered return to the initial position shown in Fig. 1, i.e. "de-energized open", which corresponds to the position of the pilot piston 12 shown in Fig. 3.

[0029] In this initial position of the solenoid device 16, the pilot piston 12 is also in the initial position on the left according to the representation according to FIG. 3, and as already explained, the pilot chamber 20 is no longer supplied with fluid at a settable pressure from the side of the main piston 10. The fluid pressure present in the pilot chamber 20 also acts essentially on the rear surface of the main piston 10, so that as long as the pilot piston 12 takes the position shown in FIG. 2, the main piston 10 is kept securely closed due to the area ratio. The operating or energized position therefore corresponds to the position in which the main piston 10 is securely closed. Furthermore, the fluid reaches the rear surface of the pilot piston 12 via the fluid communication passage 32 of the pilot piston 12 and establishes a pressure balance for the moving components. A seat-tight interruption of the fluid path from the pilot chamber 20 to the tank connection 40 is then achieved by the valve seat 77 according to the representation only of FIG. 2. Such an interruption of the fluid path is leak-tight.

[0030] Furthermore, the main piston 10 has, at its left free end face, a ball-shaped end face 98 of a settable radius, which serves to establish a linear relationship between the stroke of the main piston 10 and the opening area at the fluid port 44 or 46 formed by this stroke. Otherwise, the main piston 10 is guided longitudinally movably on the inner periphery of the valve housing 14 via a stepped annular surface. Such an inclined surface forms a further valve seat 100, with which the main piston 10 in the closed position according to Figs. 1 and 2 provides a seat-tight barrier between the fluid ports 44 and 46. The individual sealing ring systems, which will not be described in detail, usually take the form of conventional O-sealing rings, serve to fluid-tightly separate the individual fluid chambers and fluid sections from one another as described above.

[0031] As shown in particular in Fig. 1, the valve body according to Fig. 2 and Fig. 3 can be inserted in its entirety in the usual way into a valve block 102 with the fluid ports 44, 46. There is a permanent fluid communication between the tank connection 40 and the tank side T of the entire valve arrangement via a fluid communication 104 which is partially guided in the valve block 102. It can further be seen from Fig. 1 that the communication passage 60 is surrounded towards the outside by an inner circumferential part of the valve housing 14. As can further be seen from Fig. 1, a flange part 106 is placed on the valve block 102 from the outside and is rigidly connected to the valve block 102 in the usual way. Furthermore, a part of the valve housing 14 is firmly screwed to the flange part 106 on the inner circumferential side of the flange part 106 via a threaded part 108.

Claims

1. A valve, in particular a proportional valve, comprising a main piston (10) for controlling a main volume flow and a pilot piston (12) for controlling a pilot volume flow, said main piston (10) and said pilot piston (12) being guided in a valve housing (14) so ​​as to be longitudinally movable, said pilot piston (12) being controllable by a solenoid device (16), in which the position of said main piston (10) is adjustable by said pilot piston (12) and in which the fluid pressure present at said main piston (10) is transferred to said pilot piston (12) via a fluid communication (18). a solenoid device (16) for controlling the main volume flow, the solenoid device (16) being operated so that when the pilot piston (12) is in an operating position, it holds the main piston (10) in an open position blocking the main volume flow, and when the solenoid device (16) is not operated, the pilot piston (12) is in an initial position and the fluid pressure in the pilot chamber (20) drops towards the tank side (T) until the main piston (10) reaches an open position controlling the main volume flow, disconnected from the pressure supply via the main piston (10).

2. The solenoid device (16) has a pull-type solenoid (17), and when the solenoid (17) is energized, it preferably operates by an energy storage means (F 磁気バネ 2. The valve according to claim 1, characterized in that the pilot piston (12) is moved from an initial position to an operating position against a force (F) at which the main piston (10) is held in a closed position.

3. The pilot piston (12) is actuated by the force of the energy storage means (F バネ ) and the force (F 磁気バネ -F 磁石 ) and the pilot piston (12) can be brought to the operating position by the combined action of a friction force (F 摩擦 ) Considering F アクチュエータ -F バネ -F 摩擦 3. The valve according to claim 1, wherein the pressure is substantially balanced, i.e., =0.

4. 4. The valve according to claim 1, wherein the pilot piston (12) is designed as a hollow piston, which has on its outer periphery a first recess (30) which establishes fluid communication with the main piston (10) when the solenoid device (16) is actuated, and which forms on its inner periphery a fluid communication passage (32) which opens at one free end (34) into the pilot chamber (20) and opens via a passage portion (36) of the fluid communication passage (32) into a second recess (38) in the outer periphery of the hollow piston, which is in fluid communication with a tank connection (40) in the valve housing (14) when the solenoid device (16) is not actuated, and thus the first recess (30) is isolated from fluid communication with the main piston (10).

5. 5. The valve according to claim 1, further comprising a changeover valve (42) connected in fluid communication between the main piston (10) and the pilot chamber (20), the changeover valve (42) transmitting the higher pressure of each of two fluid ports (44, 46) in the valve housing (14) through which the main volume flow is conducted as a signal pressure to the pilot chamber (20).

6. 6. The valve according to claim 1, wherein as part of the fluid communication (18), a fluid chamber (58) is provided in the main piston (10), the fluid chamber (58) opening via at least one communicating passage (60) in the main piston (10) to another fluid chamber (62) between the valve housing (14) and the main piston (10), the fluid chamber (62) opening via at least one further communicating passage (64, 66, 67, 68) in the valve housing (14) at least partially overlapping with a first recess (30) in the pilot piston (12).

7. 7. The valve according to claim 1, characterized in that at least one further communication passage (78, 80) is arranged in the pilot piston (12) and in the valve housing (14), by means of which a second recess (38) in the pilot piston (12) can be connected with a tank connection (40).

8. 8. A valve according to any one of the preceding claims, characterized in that the main piston (10) is held towards the closed position by further energy storage means (74).

9. 9. The valve according to claim 1, wherein each energy storage means is formed of a compression spring (22, 74), the compression spring (74) for the main piston (10) has a higher spring stiffness than the compression spring (22) for the pilot piston (12), and the compression spring (74) for the main piston (10) encompasses the compression spring (22) for the pilot piston (12).

10. 10. The valve according to claim 1, wherein the main piston (10) has a free end face (98) which is at least partially formed in a ball shape and has a radius which establishes a linear relationship between the stroke of the main piston (10) and the opening area of ​​one of the fluid ports (44, 46) in the valve housing (14) achievable by said stroke.

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