Abstract:
A pilot-operated pressure shut-off valve having a main control piston which, when an upper system limit pressure in the hydraulic system is reached, connects an inlet, via which pressure medium can be fed to a hydraulic system, to an outlet by taking up a first switching position and, when a lower system limit pressure is reached, separates from the outlet by taking up a second switching position. The pressure shut-off valve has a pilot valve arrangement by which the fluidic connection of a control space adjacent to the main control piston can be changed in order to control the main control piston, and which has a valve housing, with a first pilot piston and a second pilot piston accommodated within the first pilot piston for compact construction and capability to adjust the two pilot pistons mechanically completely independently of each other.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
   The invention relates to a pilot-operated pressure shut-off valve which, when an upper system limit pressure is reached in a hydraulic system having a hydraulic accumulator, connects an inlet feeding the hydraulic system to an outlet to a tank and separates this connection when removal of hydraulic fluid from the hydraulic accumulator has caused the system pressure to drop to a lower system limit pressure and which has a main control piston and, in order to control the main control piston, a pilot valve arrangement having two pilot pistons and two pilot springs, the adjustment of which makes it possible for the upper system limit pressure and the lower system limit pressure to be set independently of each other. 
   A pilot-operated pressure shut-off valve of this type is disclosed, for example, in DE 41 12 065 A1 or in DE 36 08 100 C2. In the case of the pressure shut-off valve according to DE 41 12 065 A1, the pilot valve arrangement comprises two complete pilot valves having a respective valve housing, having a pilot piston in a bore of the valve housing and having a pilot spring which is situated in a spring space and the prestress of which can be changed with the aid of a setting screw. The two pilot valves are placed one above the other onto the housing of the main stage. This pressure shut-off valve has a fairly large construction and is relatively expensive. 
   In the case of the pressure shut-off valve according to DE 36 08 100 C2, the pilot valve arrangement only has one valve housing. In the latter, two valve bores, each of which accommodates one of the two pilot pistons, run at a distance from and parallel to each other. The two pilot springs are accommodated next to each other in an extension of the valve bores in a cover fitted onto the valve housing of the pilot valve arrangement. The pilot valve arrangement in this pressure shut-off valve still has a fairly complex construction. 
   There are also pilot-operated pressure shut-off valves in which the pilot valve arrangement has just one pilot piston and one pilot spring and an adjustment of the one system limit pressure always also involves an adjustment of the other system unit pressure. The difference between the two limit pressures is a percentage of the upper system limit pressure, this percentage depending on the size of a surface difference on the pilot piston and on the prestress of the pilot spring. A pilot-operated pressure shut-off valve of this type, in which the upper system limit pressure and the lower system limit pressure cannot be set independently of each other, is disclosed, for example, in the applicant&#39;s specification sheet RD 26 411/03.98. 
   SUMMARY OF THE INVENTION 
   The invention is based on the object of developing a pilot-operated pressure shut-off valve having the introductory-mentioned features in such a manner that the pilot valve arrangement is constructed compactly and simply, can be produced cost-effectively and can be interchanged for a pilot valve arrangement of a pressure shut-off valve, in which the upper system limit pressure and the lower system limit pressure cannot be set independently of each other. 
   The objective which is sought is achieved according to the invention wherein, the two pilot pistons, the two pilot springs and the two setting screws are arranged lying concentrically inside one another, with it being possible to adjust the two pilot pistons mechanically completely independently of each other. In this manner, the pilot valve arrangement is constructed very compactly with a low height. Only one valve housing is needed for the pilot valve arrangement. Instead of a conventional pilot valve arrangement which does not permit any independent setting of the two limit pressures from each other, said pilot valve arrangement can easily be constructed on a main stage. In comparison with a pilot valve arrangement having two valve bores for the two pilot pistons, the machining of the valve housing of a pilot valve arrangement according to the invention is substantially simplified, since only one valve bore is needed for the two pilot pistons. The compact, concentric arrangement of the pilot pistons, the pilot springs and the setting screws also makes possible a cartridge-type construction which it has hitherto not been possible to realize. 
   The complete mechanical independence of the two pilot pistons in respect of their movement possibilities can thus be achieved in a simple manner wherein the outer pilot piston is situated with an outer collar between two stops fixed on the housing, wherein the one stop is formed on a bushing inserted into the valve housing, and wherein the inner pilot piston penetrates the bushing and is situated with an outer collar between the bushing and a further insert placed in the valve housing. 
   In order for the first pilot piston to be reliably switched over when the upper system limit pressure is reached and to remain in the one switching position until the system pressure drops to the lower system limit pressure, wherein said pilot piston is a stepped piston along with other features of the invention. A pressure space is formed upstream of the stepped surface with pump pressure being produced in it with the main stage closed and which is relieved of pressure by or for switching over the first pilot piston when the upper system limit pressure is reached. The first pilot piston is acted upon on the stepped surface by the pressure prevailing in the pressure space in the same direction as by the first pilot spring. The first pilot piston is acted upon on a large, first active surface by the system pressure counter to the direction of action of the first pilot spring. The minimal, lower system limit pressure is determined, with an established, upper system limit pressure, by the surface difference between the first active surface, on which the system pressure produces a force, and the second active surface, on which the pump pressure produces a force. According to another feature of the invention the stepped surface of the first pilot piston or, more generally, the active surface on the first pilot piston, at which the pump pressure produces a force directed in the same direction as the spring force, is at least one third of the size of the large active surface, at which the system pressure produces a counterforce. The first pilot piston already reliably switches over at such a ratio of sizes. Preferably, according to another feature of the invention the stepped surface is approximately two thirds of the size of the large active surface. In principle, the stepped surface can also be made even larger in comparison with the large active surface. However, this no longer lies within the sense of a compact construction. In addition, a ratio of sizes of two thirds is sufficient in order also to make possible the greatest desired difference between the upper system limit pressure and the lower system limit pressure. 
   According to yet other features of the invention the second pilot piston is formed as a stepped piston and is acted upon by pump pressure on the stepped surface in the direction of action of the second pilot spring while it is acted upon on a large, first active surface by the system pressure counter to the direction of action of the second pilot spring. This ensures that the second pilot piston, over the entire pressure and quantity range of the pressure shut-off valve, passes reliably into the switching position determined by the second pilot spring. According to another feature of the invention the size of the stepped surface of the second pilot piston preferably lies in the region of 5% of the large active surface on the end of the second pilot piston. 
   The objective sought by the invention can be achieved irrespective of which of the two pilot pistons is the outer pilot piston which accommodates the other pilot piston in it. However, with regard to a compact construction, it has proven particularly favorable if, according to another feature of the invention the first pilot piston is the hollow piston in which the second pilot piston is guided. 
   It is particularly advantageous for controlling the main control piston if, according to another feature of the invention the two pilot pistons of the pilot valve arrangement can be used to control two throughflow cross sections which are arranged in series between the control space of the main control piston and a tank connection. For controlling the throughflow cross sections, the pilot pistons are acted upon, according to other features of the invention by the different pressures and by the pilot springs. It should be pointed out here that the construction according to these features affords advantages over previously known pilot connections even with a detached construction of the pilot valve arrangement, i.e. even if the two pilot pistons are not arranged one inside the other or if there are even two separate pilot valves. However, the control arrangement according to these features is particularly favorable if the two pilot pistons are arranged lying one inside the other, since then, according to features of the invention the relieving of the control space on the main control piston, i.e. the opening of the two throughflow cross sections lying in series, is possible with little structural outlay. If the first pilot piston is the outer pilot piston, then in the case of a construction according to further features of the invention the fluid path across the two throughflow cross sections is formed in a particularly simple manner. 
   The reliability of switching over to the inlet connection with the system is also increased by a construction according to still other features of the invention. This is because the second pilot piston covers a greater distance in the closing direction owing to the further aperture in the first pilot piston. If then, owing to the rise in pressure in the outer annular space, the first pilot piston moves more rapidly than the second pilot piston in the closing direction, its first apertures are already covered on the outside by the control edge fixed on the housing in the event that said apertures are opened once again on the inside. The small clearance between the pilot pistons and between the first pilot piston and the housing is used in a specific manner for a small leakage flow from the further aperture into the relief space, the leakage flow also being able to include the first apertures and constituting part of the entire leakage flow. Due to the additional leakage flow which is caused by the further aperture and is to be reduced in order to raise the system pressure, the second pilot piston covers the greater distance. In particular, it has turned out that, owing to the further aperture, the opening cross section between the first apertures and the control edge fixed on the housing no longer has to have such precise tolerances, and the pressure shut-off valve nevertheless reliably switches. 
   Finally, according to still other features of the invention the invention is also already implemented solely by the pilot valve arrangement having the corresponding features from the introductory-mentioned paragraph and having other features of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Two exemplary embodiments of a pilot-operated pressure shut-off valve according to the invention are illustrated in the drawings. The invention will now be explained in greater detail with reference to the figures of these drawings, in which 
       FIG. 1  shows the first exemplary embodiment in a longitudinal section through the pilot valve arrangement, 
       FIG. 2  shows a connection diagram of the exemplary embodiments shown, 
       FIG. 3  shows the second exemplary embodiment which differs from the first exemplary embodiment by a further bore in the wall of the first pilot piston, which bore can be influenced by the inner, second pilot piston, and 
       FIG. 4  shows a developed view of the outer pilot piston from  FIG. 3  in the region of its bores interacting with a control edge fixed on the housing and with a control edge on the second pilot piston. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   According to the connection diagram of  FIG. 2 , the pilot-operated pressure shut-off valve which is shown comprises a main stage  10  and a pilot valve arrangement  11 , which are respectively indicated by a dash-dotted rectangle. The main stage  10  has an inlet connection  12 , an outlet connection  13  and a system connection  14 . Leading off from the latter is a system line  15  to which a hydraulic accumulator  16  and directional control valves (not illustrated specifically) for controlling hydraulic consumers are connected. The inlet connection  12  and the system connection  14  are connected to each other via a nonreturn valve  17  which opens from the inlet connection toward the system connection. The main stage  10  includes a main control piston  20  with which a throughflow cross section between the inlet connection  12  and the outlet connection  13  can be opened and closed. The main control piston is guided on a first diameter in a bore  21  in the housing  22  of the main stage and is able to sit with a frustoconical surface  23  on a seat edge  24 , the diameter of which is slightly smaller than the guiding diameter. The bore  21  is closed on the one side by the valve housing  25  of the pilot valve arrangement  11 , which valve housing is placed onto the valve housing  22 . A control space  26  is formed in the bore between this valve housing  25  and the main control piston  20  and is used to accommodate a weak helical compression spring  27  which is supported on the housing  25  and the main control piston  20  and loads the latter in the direction of the seat edge  24 . That end surface  28  of the main control piston  20  which lies within the seat edge  24  delimits a space which is open toward the inlet connection  12 . This space is fluidically connected to the control space  26  via a nozzle  29  formed in the main control piston  20 . 
   A hydraulic pump  30  which is driven by an electric motor  31  is connected to the inlet connection  12 . 
   When the main control piston  20  takes up its closed position, as is shown in  FIGS. 1 to 3 , the hydraulic fluid conveyed by the hydraulic pump  30  flows through the nonreturn valve  17  of the system line  15  and therefore to the hydraulic accumulator  16 . If the quantity of pressure medium removed from the system line  15  is less than the quantity flowing into it, the pressure in it and in the hydraulic accumulator  16  rises. The main stage  10  of the pressure shut-off valve shown is controlled in such a manner that the main control piston  20  opens when the pressure in the hydraulic accumulator  16  has reached an upper system limit pressure. The hydraulic pump  30  subsequently conveys the hydraulic fluid, which is sucked up from the tank  32 , in a circulating manner via the inlet connection  12 , via the throughflow cross section between the seat edge  24  of the housing  22  and the frustoconical surface  23  of the main control piston  20  and via the outlet connection  13  back to the tank  32 . Removal of hydraulic fluid from the hydraulic accumulator  16  causes the pressure in the latter to consequently drop. If, finally, the lower system limit pressure which has been set is reached, the main control piston  20  closes the throughflow cross section between the inlet connection  12  and the outlet connection  13 , with the result that the hydraulic pump  30  conveys again into the system line  15 . During circulating conveying, the pressure in the inlet connection  12  is low and is determined essentially by the force of the helical compression spring  27 . The nonreturn valve  17  prevents hydraulic fluid from flowing out of the system line  15  into the inlet connection and via the main control piston  20  and the outlet connection  13  to the tank  32 . 
   The main control piston  20  is controlled by the pilot valve arrangement  11  which, as seen in connection terms, has two pilot valves  40  and  41  which are constructed as 2/2-way directional control valves and which lie in series between the control space  26  on the main control piston  20  and the outlet connection  13 . The series connection provides for fluid connection, via both of the pilot valves  40  and  41  when both of the valves are open, between the control space  26  and the outlet connection  13 , as shown in  FIG. 2 . A relief line  42  leads from the pilot valve  40  through the pilot valve housing  25  and the valve housing  22  of the main stage  10  to the outlet connection  13 . The control space  26  is connected to the pilot valve  41  via a damping nozzle  43 . The first pilot valve  40  has a first pilot piston  44  which is acted upon in the direction of the closed position by a first helical compression spring  45 , the prestress of which can be changed in order to set the upper system limit pressure. In the closing direction, the pilot piston  44  is acted upon on a large active surface  46  by the pressure from the accumulator, i.e. by the system pressure. The active surface  46  provides approximately the total force on the piston  44  (as shown in  FIG. 1 ) with other relatively small contributions to the force being provided by undulations/steps in the side surface of the piston. A similar comment applies to an active surface  50  of the piston  48 , to be described below. Moreover, with the helical compression spring  45  in the closing direction, the pressure which prevails between the damping nozzle  43  and the pilot valve  41  acts on an active surface  147  (shown also in  FIG. 1 ), the size of which is approximately two thirds of the size of the active surface  46 . In the static state of the main control piston  20 , this pressure is approximately equal to the pressure in the control space  26 . 
   The pilot valve  41  has a second pilot piston  48  which is acted upon in the closing direction by a second helical compression spring  49 , the prestress of which can be changed in order to set the lower system limit pressure. In the closing direction, the pilot piston  48  is acted upon by the system pressure in precisely the same manner as the pilot piston  44 , specifically on an active surface  50  (shown also in  FIG. 1 ). With the helical compression spring  49  in the opening direction, the pressure produced between the damping nozzle  43  and the pilot valve  41  again acts on the pilot piston  48 . The size of the active surface  151  for this pressure is approximately only 5% of the size of the active surface  50 . 
   The space in which the helical compression springs  45  and  49  are situated is located on the relief line  42 . 
   If, during operation, the main control piston  20  takes up its closed position and the hydraulic fluid conveyed by the hydraulic pump  30  passes via the nonreturn valve  17  to the hydraulic accumulator  16 , the pilot valve  40  is in its closed position and the pilot valve  41  is in its open position. The control space  26  is thus blocked off toward the relief line  42 . The pressure in it is equal to the pressure in the inlet connection  12 . Under the action of this pressure and under the action of the helical compression spring  27  the main control piston  20  maintains its closed position. Virtually the same pressure is produced on the active surfaces  46  and  147  of the pilot valve  40  and on the active surfaces  50  and  151  of the pilot valve  41 . The drop in pressure via the nonreturn valve  17  is negligible. The pressure in the hydraulic accumulator  16  rises with the inflow of pressure medium and is finally of such magnitude that the differential surface between the two active surfaces  46  and  147  as the pressure application surface is sufficient for a throughflow cross section to be opened in the pilot valve  40 . The pressure arising on the active surface  147  instantly starts to fall, with the result that the pilot valve  40  reliably switches into its open position. Hydraulic fluid can now flow from the control space  26  via the two pilot valves  40  and  41  and the relief line  42  to the tank  32 . The main control piston  20  is relieved of pressure on the spring side and opens. The pressure in the inlet connection  12  drops to a low value determined by the prestressing of the helical compression spring  27 . The nonreturn valve  17  closes. The hydraulic fluid conveyed by the hydraulic pump  30  consequently flows via the throughflow cross section between the seat edge  24  of the housing  22  and the frustoconical surface of the main control piston  20  back to the tank  32 . Only a small quantity of control oil that is determined by the hydraulic resistance of the nozzle  29  and the pressure equivalent to the force of the helical compression spring  27  flows via the pilot valve arrangement to the tank. The pressure by which the pilot valve  40  can be brought into its open position is equal to the upper system limit pressure. Its magnitude is determined by the prestress of the helical compression spring  45  and can be changed by changing this prestress. 
   While the hydraulic pump  30  is conveying in circulation, the pressure in the hydraulic accumulator  16  gradually decreases by removal of hydraulic fluid for hydraulic consumers. Finally, the pressure is so low that the force which it produces on the active surface  50  of the second pilot valve  41  becomes smaller than the force of the helical compression spring  49 . The latter now moves the pilot piston  48  in the closing direction, as a result of which the throughflow cross section is closed by the valve  41  and pressure builds up in the control space  26 , into which pressure medium continues to flow via the nozzle  29 , and therefore also on the active surfaces  147  and  151  of the pilot valves  40  and  41 . The build up of pressure on the active surface  151  of the pilot piston  48  brings about reliable closing of the pilot valve  41 . The pressure in the control space  26  is equal to the pressure in the inlet connection  12 , with the result that the main control piston  20  closes under the action of the helical compression spring  27  and the pump pressure acting on a surface remainder outside the seat edge  24 . The pressure in the inlet connection  12  and in the control space  26  and on the active surfaces  147  and  151  therefore rises to the system pressure which at this instant is identical to the lower system limit pressure. Even before this lower system limit pressure is reached on the active surface  147 , the pilot valve  40  also passes into its closed position. The inflow of hydraulic fluid to the hydraulic accumulator  16  causes the system pressure to rise, with, because of the very small active surface  151  in comparison with the active surface  50 , a slight rise above the lower system limit pressure being sufficient in order to bring the pilot valve into its open position again. This does not have any effect on the main control piston, since the pilot valve  40  is already in its closed position and prevents relief of the control space  26 . Only when the system pressure is again as high as the upper system limit pressure does the pilot valve  40  switch again into its open position. 
   For reliable and rapid switching of the pilot valve  40  from its open position into its closed position, the active surface  147  is to be at least one third of the size of the active surface  46 . If, on the other hand, an upper system limit pressure is set by adjustment of the helical compression spring  45 , then, from the ratio of the size of the surface  147  to the size of the surface  46 , a pressure is produced which acts on the active surface  46  and against which the helical compression spring  45  could bring the pilot valve  40 , when the active surface  147  is relieved, into the closed position even without a switching operation of the pilot valve  41 . This pressure is therefore the minimum lower system limit pressure which can be maintained at a given upper system limit pressure. If the ratio between the surface  147  and the surface  46  is, for example, one third, then at a set, upper system pressure of 210 bar, the minimum lower system limit pressure would be 140 bar. If the ratio of the surface  147  to the surface  46  is two thirds, as is preferred, then at a set, upper system limit pressure of 210 bar, the minimum lower system limit pressure is 70 bar. Within this range, the lower system limit pressure can be set by adjustment of the helical compression spring  49 . However, the presence of the active surface  51  also provides a limitation for the minimum interval between the upper system limit pressure and the lower system limit pressure. 
   In a structural respect, the two pilot valves  40  and  41  are integrated one inside the other in a very compact manner, so that they, as is apparent in particular from the section according to  FIG. 1 , appear as a single valve. Otherwise, in  FIG. 1  the components of the main stage, the nonreturn valve  17 , a hydraulic accumulator  16  and a hydraulic pump and an electric motor  31  are shown in a similar manner as in  FIG. 3  and are provided with the same reference numbers as in  FIG. 3 . The pilot valve arrangement according to  FIG. 1  has a plate-like valve housing  25  in which a blind bore  55  of large volume is made from one side surface. A multiply stepped valve bore  56  which has its largest diameter on the opposite side surface and is closed there by a closure screw  57  opens centrally into the blind bore  55 . The valve bore  56  has the smallest diameter directly adjoining the base  58  of the blind bore  55 . A stepped hollow piston, as first pilot piston  44 , is guided directly in the valve bore  56  and protrudes out of the valve bore  56  into the blind bore  55 . An annular space  61  is formed between a stepped surface  59  of the pilot piston  44 , which surface is directed away from the closure screw  57 , and an axially opposite stepped surface  47  of the valve bore  56  and a channel  62  leading through the valve housing  25  opens radially into it. The annular space  61  is fluidically connected via this channel to the control space  26  on the main control piston, with the damping nozzle  43  being screwed into the channel  62 . The pilot piston  44  is acted upon on a resulting active surface, which is identical to the stepped surface  47  of the valve bore  56 , in the direction of the closure screw  57  by the pressure prevailing in the annular space  61 . 
   Toward the closure screw  57 , the section of the pilot piston  44  having the outside diameter of the stepped surface  59  is adjoined by an outer collar  63  with which the pilot piston  44  can strike, on the one hand, in the direction toward the closure screw  57  against a bushing  64 , which is inserted into the bore  59  and is held in a fixed position, and, in the opposite direction, can strike against a further step  65  of the valve bore  56 . The path of displacement of the pilot piston  44  is defined by the two axial stops and the axial extent of the outer collar  63 . A further bushing  66  is situated between the bushing  64  and the closure screw  57 . Said further bushing is pressed by the closure screw  57  against the bushing  64  and the latter is pressed in turn against a step of the valve bore  56 . 
   Centrally, the first pilot piston  44  has a continuous axial bore  69  in which the second pilot piston  48  can be displaced axially. The axial bore  69  is a stepped bore having a bore section of larger diameter which opens outward on that end side of the pilot piston  44  which faces the bushing  64 , and having a bore section of smaller diameter which is open toward the blind bore  55  of the housing  25 . The cross sections of the two bore sections of the bore  69 , which merge into each other in the stepped surface  51  on the pilot piston  44 , differ from each other only by approximately 5%. Within the bore section having the smaller diameter, the axial bore  69  is connected to the outside of the pilot piston  44  via a plurality of apertures  70  situated axially at the same height. If, as shown in  FIG. 1 , the pilot piston  44  bears against the bushing  64 , the apertures are covered on the outside by that wall section of the valve bore  56  which is situated between the stepped surface  47  of the valve bore  56  and the base  58  of the blind bore  55 . The edge between the base  58  of the blind bore  55  and the valve bore  56  forms a control edge  71  which is fixed on the housing and interacts with the apertures  70 . It is passed over by the apertures  70  and hence a throughflow cross section from the apertures  70  into the blind bore  55  is produced when the valve slide  44  is displaced away from the bushing  64  onto the step  65  of the housing  25 . 
   The pilot piston  48  is stepped corresponding to the stepped axial bore  69  and has a guide section in the region of the bore section of smaller diameter and a guide section which is slightly larger in diameter in the bore section of larger diameter. The two guide sections are spaced far apart, with the diameter of the piston section between the two guide sections being reduced once again relative to the diameter of the smaller guide section. As a result and by the step  51 , an annular space  72  has been produced radially between the outer pilot piston  44  and the inner pilot piston  48  and axially between the two guide sections thereof. Said annular space is permanently connected via a radial bore  73  in the pilot piston  44  to the annular space  61  and is therefore fluidically connected to the control space  26  on the main control piston  20 . The pressure arising in the annular space  72  produces, on an annular surface of the pilot piston  48 , which annular surface corresponds to the size of the stepped surface  51  of the pilot piston  44 , a force which acts in the direction of the closure screw  57 . The outer edge  74  on that end side of the guide section of smaller diameter of the pilot piston  48  which faces the annular space  72  forms a control edge which interacts with the apertures  70  on the pilot piston  44 , which is situated, in the switching position of the pilot piston  48  that is shown in  FIG. 1 , between the stepped surface  51  on the pilot piston  44  and the apertures  70  and is displaced in the other switching position of the pilot piston  48  to such an extent that there is an open, fluidic connection between the annular space  72  and the apertures  70  irrespective of the current switching position of the pilot piston  44 . 
   The second pilot piston  48  protrudes beyond the pilot piston  44  in the direction of the closure screw  57 , passes through an inner collar of the bushing  64  and is caught with a head  75  between this inner collar and the bushing  66 . 
   The bushing  66  is provided on the outside with a turned groove  76  which is open toward a bore  77  of the housing  25 , said bore being fluidically connected to the system line  15  and therefore to the hydraulic accumulator  16 . Those end surfaces of the pilot pistons  44  and  48  which face the closure screw  57  are exposed via radial and axial bores in the bushings  54  and  56  to the pressure arising in the turned groove  76 , i.e. to the system pressure. At the pilot pistons, this pressure produces a force which acts upon them such that they move away from the bushings  64  and  66  in the direction into the blind bore  55 . The active surface on the pilot piston  44  is identical to an annular surface having an inside diameter, which is identical to the diameter of the larger section of the axial bore  69 , and having an outside diameter, which is identical to the outside diameter of the stepped surface  47  of the housing  25 . The active surface on the pilot piston  48  is identical to the cross-sectional surface of the larger guide section of this piston. 
   The two pilot springs  45  and  59  which, like the pilot pistons  44  and  48 , are arranged concentrically one inside the other, are situated in the blind bore  55 . The outer pilot spring  45  is supported via a spring plate  77  on the first pilot piston  44 , loading the latter in the direction of the closure spring  57 . On the other hand, it is supported on a setting screw  78  which is screwed into the blind bore  56 . The inner pilot spring  49  is supported via a spring plate  78  on the pilot piston  48  protruding beyond the pilot piston  44  and likewise loads said pilot piston in the direction of the closure screw  57 . In addition, the pilot spring  49  is supported on a setting screw  80  which is screwed centrally into the setting screw  78  and can be adjusted axially with respect to the setting screw  78  by rotation. 
   The blind bore  55  is part of the relief channel  42  which also includes a transverse bore  81  in the housing  25 , via which bore the relief fluid path leads to the tank  32 . 
   In  FIG. 1 , the pilot pistons  44  and  48  take up the switching positions illustrated in the connection diagram in  FIG. 2 . The apertures  70  in the pilot piston  44  are covered on the inside by the pilot piston  48  and on the outside by the housing  25 . Pump pressure is produced in the control space  26  on the main control piston  20  and in the annular spaces  61  and  72 . Said pump pressure acts upon the pilot piston  48  on a surface with the size of the surface  51  in the same direction as the pilot spring  49 . The resulting active surface, on which the pump pressure acts on the pilot piston  44 , does not correspond precisely to the size of the active surface  147 , but rather is reduced by the surface  51  in comparison with the active surface  147 . However, for the sake of simplicity, the corresponding step of the valve bore  56  is provided with the reference number  147  from  FIG. 2 . This is because the surface  51  is very small in comparison to the surface  147  and can be disregarded for the qualitative understanding of the valve. The pilot pistons  44  and  48  are acted upon in the opposite direction on the active surfaces which have already been explained by the system pressure. If the hydraulic accumulator  16  is charged, this pressure is virtually identical to the pump pressure. 
   During charging for the first time the pilot piston  48  therefore switches over from the switching position shown in  FIG. 1  into the other switching position, if the system pressure is of such a magnitude that it produces a force identical to the force of the pilot spring  49  on a surface, which is as large as the surface  50  reduced by the surface  51 . The apertures  70  in the pilot piston  44  are therefore opened on the inside toward the annular space  72  and therefore toward the control space  26  on the main control piston  20 . If the system pressure has risen to such an extent that it produces a force which is equal to the force of the pilot spring  45  on a surface  46  of the pilot piston  44  reduced by the active surface  147 , the pilot piston  44  is moved from the switching position shown in  FIG. 1  in the direction of its second switching position. In this process, the apertures  70  are also opened on the outside, so that hydraulic fluid can flow out of the annular space  61  via the radial bore  73 , the annular space  72  and the apertures  70  into the blind bore  55  and from there into the tank  32 . The drop in pressure which is caused as a result in the annular space  61  leads to a rapid switching through of the pilot piston  44 . Tank pressure now prevails in the annular spaces  61  and  62 . The active surface, at which the system pressure acts on the pilot piston  48 , is now identical to the cross section of the larger guide section of the pilot piston  48 . Correspondingly, the compressive force acting against the pilot spring  49  is also larger than when the hydraulic accumulator  16  is charged for the first time. The pilot piston  48  is therefore brought back into the switching position shown in  FIG. 1  at a pressure which is somewhat lower than the pressure which was sufficient during charging of the hydraulic accumulator  16  for the first time in order to bring the pilot piston  48  into the switching position (not shown in  FIG. 1 ) against the pilot spring  49 . When the pilot piston  48  is reset when the lower system limit pressure is reached, the apertures  70  in the pilot piston  44  are closed on the inside, with the result that the pressure in the annular spaces  61  and  62  is again identical to the pressure in the inlet of the pump to the main control piston  20 . The main control piston  20  therefore closes the connection between the inlet and the tank  32 . The pressure in the inlet and in the annular spaces  61  and  72  rises to the system pressure, as a result of which the pilot piston  44  is also brought back again to the switching position shown in  FIG. 1 . This takes place before the pilot piston  48  is displaced again against the spring  49  into the other switching position, in which the apertures  70  are again opened on the inside. 
   The exemplary embodiment according to  FIGS. 3 and 4  is largely identical to the exemplary embodiment according to  FIG. 1 . Accordingly, the same reference numbers as in  FIG. 1  are used for the pilot pistons and the various bores and spaces. Also, only the differences will be discussed below. Otherwise, reference is made to the description for  FIG. 1 . 
   A first difference resides in the fact that the first pilot piston  44  does not, as in the exemplary embodiment according to  FIG. 1 , run directly into a plate-like housing, but rather the pilot valve is formed in a cartridge-type construction and has a valve sleeve  85  which accommodates the pistons and springs and which is screwed into a valve plate  86 . 
   Furthermore, the first pilot piston  44  has, in the region of bores  84  which correspond to the apertures  70  according to  FIG. 1  and of which four of identical size are distributed uniformly over the circumference in the same radial plane, a further bore  87  which is smaller in diameter and which, as seen in the circumferential direction, is situated centrally between two bores  84 , but is offset in the axial direction relative to the bores  84  in the direction of the step in the axial bore  69  of the pilot piston  44 . As a result, the control edge  74  still leaves the bore  87  partially open on the inside when the bores  84  have already been covered on the inside. The diameter of the bores  84  in the present case is 1.2 mm and the diameter of the bore  87  is 0.7 mm. 
   When the upper system limit pressure has been reached and the valve is in the state in which the inlet is connected to the outlet, the two pilot pistons  44  and  48  are displaced to the right as far as a stop. In the switching-back phase after the lower system limit pressure is reached, the second pilot piston  48  first of all migrates to the left and closes the bores  84  on the inside, so that the pressure rises in the annular space  61 . In this annular space, a certain pressure which is dependent on the upper system limit pressure and on the lower system limit pressure has to be reached so that the pilot piston  44  switches back. Owing to the leakage which is enlarged in comparison with the first exemplary embodiment because of the bore  87 , the second pilot piston  48  continues to remove and also closes the bore  87 . The pressure in the annular space  61  rises, with then, if the certain pressure is reached, the first pilot piston  44  moving to the left and the bores  84  also being closed on the outside. The main control piston then closes and the pressure in the accumulator rises. 
   After a certain rise in the pressure in the accumulator, the second pilot piston  48  switches again into its right end position, in which the head  75  bears against the bushing  64 . It has been shown that the bore  87  reduces the sensitivity of the closing operation of the main control piston to tolerances in the size and in the position of the bores  84  in comparison to a solution without a bore  87 .