Abstract:
A control arrangement to supply-pressure medium to at least two hydraulic consumers having a variable-displacement pump controlled according to required flow and the setting of which can be changed by a, pump controller as a function of highest load pressure of the actuated hydraulic consumers; two adjustable meter-in variable restrictors, a first of which is arranged between a feed line, which leads from the variable-displacement pump, and a first hydraulic consumer, and the second of which is arranged between the feed line and a second hydraulic consumer; and two pressure compensators, a first of which is connected downstream of the first meter-in variable restrictor and the second of which is connected downstream of the second meter-in variable restrictor, and the control piston of which can be acted on in opening direction by the pressure downstream of the associated meter-in variable restrictor. To ensure in this type control arrangement that a brief excess quantity from the variable-displacement pump is not passed on to the hydraulic consumers, the control pistons of the pressure compensators can be acted on in closing direction by a control pressure which is present in a rear control space, is derived from the feed pressure prevailing in the feed line with aid of a valve device and changes with the feed pressure. There is also a pressure differential valve which, with small structure, allows a pressure at its outlet to follow, with a fixed pressure difference, a rising pressure at its inlet. Together with a restricted pressure relief of the outlet to the tank, this type pressure differential valve ensures in each case a fixed pressure difference between the outlet pressure and the inlet pressure. This valve is particularly suitable for use in the control arrangement.

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to a control arrangement which is used to supply at least two hydraulic consumers with pressure medium. The invention also relates to a pressure differential valve which is used in particular in said control arrangement. 
     A hydraulic control arrangement of this type is known, for example from EP 0 566 449 A1. This document relates to a hydraulic control arrangement using the load-sensing principle, in which a variable-displacement pump is set, as a function of the highest load pressure of the actuated hydraulic consumers, in each case in such a way that the feed pressure is higher than the highest load pressure by a defined pressure difference. The pressure medium flows to the two hydraulic consumers via two adjustable meter-in variable restrictors, a first of which is arranged between a pump line leading from the variable-adjustment pump and a first hydraulic consumer, and the second of which is arranged between the pump line and the second hydraulic consumer. The pressure compensators connected downstream of the meter-in variable restrictors mean that, if a sufficient quantity of pressure medium is supplied, there is a defined pressure difference across the meter-in variable restrictors irrespective of the load pressures of the hydraulic consumers, so that the quantity of pressure medium flowing to one hydraulic consumer is only dependent on the opening cross section of the meter-in variable restrictor in question. If a meter-in variable restrictor is opened further, it is inevitable that a greater quantity of pressure medium will flow through it, in order to generate the defined pressure difference. 
     The variable-displacement pump is in each case adjusted in such a way that it supplies the quantity of pressure medium which is required. This is therefore known as control based on the required flow. 
     The pressure compensators which follow the meter-in variable restrictors are acted upon in the opening direction by the pressure downstream of the respective meter-in variable restrictor and in the closing direction by a control pressure which prevails in a rear control space and which usually corresponds to the highest load pressure of all the hydraulic consumers supplied by the same hydraulic pump. If, in the event of simultaneous actuation of a plurality of hydraulic consumers, the meter-in variable restrictors are opened so wide that the quantity of pressure medium supplied by the hydraulic pump, which has been moved all the way to its stop, is lower than the total quantity of pressure medium required, the quantities of pressure medium flowing to the individual hydraulic consumers are reduced in equal proportions irrespective of the prevailing load pressure of the hydraulic consumers. This is therefore referred to as control with load-independent flow distribution (LIFD control). Hydraulic consumers which are controlled in this way are known as LIFD consumers for short. Since, with LIFD control, the highest load pressure is also sensed and a feed pressure which lies above the highest load pressure by a defined pressure difference is generated by the pressure medium source, LIFD control is a special case of a load-sensing control (LS control). 
     There is no load-independent flow distribution in the case of a plurality of hydraulic consumers to which pressure medium flows in each case via a meter-in variable restrictor with upstream pressure compensator which is acted upon in the closing direction only by the pressure upstream of the meter-in variable restrictor and in the opening direction only by the load pressure of the corresponding hydraulic consumer and by a compression spring. This is a case of a simple LS control and an LS consumer. A control set-up of this type is known, for example, from DE 197 14 141 A1. In the event of simultaneous actuation of a plurality of hydraulic consumers and of insufficient quantities of pressure medium being supplied by the variable-displacement pump, in this case only the quantity of pressure medium flowing to the hydraulic consumer with the highest load pressure is reduced. 
     An advantage of LS control with pressure compensators connected upstream of the meter-in variable restrictors compared to LS control with pressure compensators connected downstream of the meter-in variable restrictors, however, is that, in the event of an excess quantity being supplied for a brief time by the variable-adjustment pump and an associated rise in the feed pressure, the upstream pressure compensators, by reducing their opening cross section, do not allow any increase in the pressure difference across the meter-in variable restrictors, so that no further pressure medium flows across the meter-in variable restrictors and the speed of the hydraulic consumers is not changed. The excess quantity flows back to a tank via a pressure-limiting valve. In the case of a control set-up with pressure compensators connected downstream of the meter-in variable restrictors, by contrast, the excess quantity is passed through to the hydraulic consumers. 
     Depending on whether the user attaches more importance to a load-independent flow distribution or to preventing excess quantities from flowing to the hydraulic consumers, he will select an LIFD control or an LS control. This has hitherto been a drawback for the manufacturers of hydraulic components, since they have to offer control blocks for both LIFD control set-ups and for LS control set-ups. These differ considerably, since very divergent structures are required depending on whether a pressure compensator is connected upstream or downstream of the corresponding meter-in variable restrictor. 
     SUMMARY OF THE INVENTION 
     By contrast, the invention is based on the objective of providing a hydraulic control arrangement which has the features of the introductory-mentioned type, i.e. in which in particular pressure compensators are connected downstream of meter-in variable restrictors, in such a way that the flow of excess quantities to the hydraulic consumers is prevented. 
     The desired object is achieved, according to the invention, in a hydraulic control arrangement of the generic type, wherein the control pistons of the pressure compensators can be acted on in the closing direction by a control pressure which is present in a rear control space, is derived from the feed pressure prevailing in the feed line with the aid of a valve device and changes with the feed pressure. While in the known hydraulic control arrangement with pressure compensators connected downstream of the meter-in variable restrictors these compensators are acted on in the rear control space by the highest load pressure, on which the delivery quantity of the variable-displacement pump has no influence, in a control arrangement according to the invention the control pressure which is present in the rear control space is derived from the feed pressure and changes with the latter. Therefore, if the feed pressure rises on account of a delivery quantity from the variable-displacement pump rising beyond demand, the control pressure also rises. The control pistons of the pressure compensators are moved accordingly in the closing direction, so that the pressure downstream of the meter-in variable restrictors also rises and the pressure difference across the meter-in variable restrictors does not change. However, a constant pressure difference across a meter-in variable restrictor also means a constant quantity of pressure medium flowing across the meter-in variable restrictor. Therefore, while maintaining the basic arrangement of meter-in variable restrictor and downstream pressure compensator, and therefore without fundamental changes to a control block, the same control performance as in the case of a control set-up with pressure compensators connected upstream of the meter-in variable restrictors, i.e. control blocks of altogether different construction, is achieved with minor modifications. 
     Therefore, the difference between the feed pressure and the control pressure, when the variable-displacement pump has not been displaced as far as its stop, i.e. when there is a sufficient quantity of pressure medium, is preferably no greater than between the feed pressure and the highest load pressure. This is because if the pressure difference were greater, the quantity of pressure medium flowing to one hydraulic consumer would depend on whether the load pressure of this hydraulic consumer is higher or lower than the control pressure. The control pressure is preferably slightly higher than the highest load pressure, so that on the one hand there are no unnecessary throttling losses at the pressure compensators, but on the other hand in each case the pressure compensator assigned to the hydraulic consumer with the highest load pressure is still within the control range. 
     In principle, it is conceivable for the pressure difference between the feed line and a rear control space at a pressure compensator to be produced by connecting a nozzle between the feed line and the control space and by connecting a flow-regulating valve between the control space and a tank. In each case a defined quantity of control fluid would flow out of the control space to the tank via the flow-regulating valve. This quantity of control fluid would flow to the control space via the nozzle. Therefore, there would be a constant pressure gradient across the nozzle. However, the quantity of pressure medium flowing via a nozzle is highly dependent on the viscosity of the pressure medium. It therefore appears more appropriate to use a pressure differential valve, an inlet of which is connected to the feed line and an outlet of which is connected to the rear control space of a pressure compensator, instead of a nozzle. According to a feature of the invention, the pressure differential valve is preferably set to a fixed pressure difference and has a movable valve member which is acted on by the feed pressure for the purpose of opening fluid communication between the feed line and the control space at the pressure compensator and is acted on by the control pressure and by a spring for the purpose of closing this communication. 
     The invention provides a particularly preferred configuration, according to which the rear control spaces of a plurality of pressure compensators are directly connected to one another, so that the same control pressure prevails in these control spaces. Therefore, only one valve device for deriving the control pressure from the feed pressure is required for these pressure compensators. In a particularly advantageous configuration, the control arrangement has a load signaling line, to which the highest load pressure of the hydraulic consumers actuated in each case is input via selection valves, and a valve which opens up fluid communication from the load signaling line to the rear control space of at least one pressure compensator when the difference between the feed pressure and the highest load pressure falls below a defined level. In this way, in the case of undersaturation, i.e. in the event of insufficient pressure medium being delivered by the variable-displacement pump, the result is a load-independent flow distribution between the hydraulic consumers, the control space of the pressure compensators of which is connected to the load signaling line. 
     If the supply of pressure medium to one hydraulic consumer is to be prioritized over the supply to another hydraulic consumer in the event of undersaturation, this is advantageously achieved by a configuration, wherein the rear control space at the pressure compensator of the hydraulic consumer which is to be supplied with pressure medium as a priority is then separate from the control spaces at the pressure compensators of the other hydraulic consumers. The control pressure in this priority consumer is derived from the feed pressure via a further valve device. Moreover, there is a priority valve, by means of which, in order to maintain a desired pressure difference across the meter-in variable restrictor arranged upstream of the pressure compensator of the prioritized hydraulic consumer, and therefore to maintain a sufficient supply of pressure medium to the prioritized hydraulic consumer, in the event of a quantity of medium delivered by the variable-displacement pump not meeting demand, the control pressure in the rear control space of the other hydraulic consumers is raised to above the control pressure in the case of saturation. The priority valve preferably has a first port, which is connected to the feed line, and a second port, which is connected to the rear control spaces of the pressure compensators assigned to the hydraulic consumers which are not prioritized, and has a valve member, which, in the direction of opening the connection between the first port and the second port, can be acted on by the pressure prevailing in a line section downstream of the meter-in variable restrictor assigned to the prioritized hydraulic consumer and by an additional force, and, in the direction of closing the connection between the first port and the second port, can be acted on by the feed pressure. Downstream of the meter-in variable restrictor, a control space of the priority valve may be connected to the line section upstream or downstream of the pressure compensator, since the priority valve comes into action when the pressure compensator is completely open and because the same pressure, namely the load pressure of the prioritized hydraulic consumer, then prevails upstream and downstream of the pressure compensator. 
     A further object of the invention is to provide a pressure differential valve which is used in particular to derive a control pressure for a pressure compensator from the feed pressure in a control arrangement, which is of particularly small structure, so that it can readily be inserted into a control block. 
     A pressure differential valve of this type is obtained by features of the invention. 
     Advantageous configurations of a pressure differential valve of this type are also provided by the invention. 
     A further object of the invention is to provide a pressure differential valve which is used in particular to derive a control pressure for a pressure compensator from the feed pressure in a control arrangement in accordance with one of patent claims 1 to 9 and which is of particularly small structure, so that it can readily be inserted into a control block. 
     A pressure differential valve of this type is obtained by means of the features given in the defining part of patent claim 10. 
     Advantageous configurations of a pressure differential valve of this type are given in patent claims 11 to 13. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In each case one exemplary embodiment of a control arrangement according to the invention and of a pressure differential valve used therein are illustrated in the drawings. The invention will now be explained in more detail with reference to the figures shown in the drawings, in which: 
     FIG. 1 shows a circuit diagram of the exemplary embodiment of the control arrangement which in the event of undersaturation has an LIFD performance and which preferably includes a prioritized hydraulic consumer, 
     FIG. 1 a  shows an alternative for activation of the priority valve shown in FIG. 1, 
     FIG. 2 shows the circuit diagram of a variable-displacement pump used in the exemplary embodiment, and 
     FIG. 3 shows a longitudinal section through the pressure differential valve used in the exemplary embodiment shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with FIG. 1, a variable-adjustment pump  10  with a variable-displacement means  11  sucks pressure medium out of a tank and discharges it into a system of feed lines  13 . In this exemplary embodiment, three hydraulic consumers  14 ,  15  and  16 , which are all constructed as differential cylinders, are supplied with pressure medium via the feed lines. To control the speed and direction of movement, each differential cylinder  14 ,  15  and  16  is assigned a meter-in variable restrictor  17 ,  18  and  19 , respectively, and a {fraction (4/3)}-way valve  20 ,  21  and  22 , respectively. In practice, a meter-in variable restrictor and a directional control valve are in each case integrated in one another in such a manner that, through the actuation of a valve slide which is spring-centered in a center position, in a specific direction out of the center position, the direction of movement of the differential cylinder is preset, and the opening cross section of the meter-in variable restrictor is determined by the displacement executed during the movement of the valve slide. For a specific design solution, reference is made at this point to EP 0 566 449 A1, which has already been mentioned above. The meter-in variable restrictors  17 ,  18  and  19  are connected to the system of feed lines  13 . Between a meter-in variable restrictor  17 ,  18  and  19  and a directional control valve  20 ,  21  and  22 , respectively, there is in each case a pressure compensator  23 ,  24  and  25 , respectively, of which the control piston (not shown in more detail) is acted on in the opening-direction by the pressure downstream of the respective meter-in variable restrictor, and in the closing direction is acted on by a control pressure prevailing in a rear control space  26 . The directional control valves  20 ,  21  and  22  each have two consumer ports  30 ,  31  which are connected to pressure spaces of the corresponding differential cylinder, a feed port  32 , which is connected to the outlet of the respective pressure compensator, and a return port  33 , from which a return line leads to the tank  12 . In the center position of a directional control valve, the two consumer ports are blocked and the feed port is connected to the tank port. Therefore, the line section between the outlet of the pressure compensator and the feed port is freed of pressure load. In a lateral working position of a directional control valve, pressure medium flows to one pressure space of a hydraulic cylinder, while pressure medium can flow out of the other pressure space to the tank. 
     In the closing direction, the control pistons of the pressure compensators  23 ,  24  and  25  are acted on not only by a control pressure but also by a weak compression spring  34 , which is equivalent to a pressure of, for example, only 0.5 bar. Moreover, the control spaces  28  and  27  of the two pressure compensators  23  and  24  are connected to one another via a passage  35 , so that the same control pressure is always present in both control spaces  26  and  27 . 
     Shuttle valves  36 , which are linked to one another in such a manner that in a load signaling line  37 , which leads to the variable-displacement means  11  of the pump  10 , in each case the highest load pressure of all actuated differential cylinders is present, are connected to the outlets of the pressure compensators  23 ,  24  and  25  and to the feed ports  32  of the directional control valves. The result of this, as can be seen in particular from FIG. 2, is that the load signaling line  37  is connected to a control valve  39  by means of three ports, one of which is connected to an adjustment cylinder  40  of the variable-displacement pump  10 . A further port of the control valve  39  is connected to a supply line  13 , and the third port is connected to tank  12 . In the direction of a connection between the first port and the second port, the control piston of the control valve  39  is acted on by the pressure in the supply line  13 , and in the direction of a connection between the first port and the third port, the control piston of the control valve  39  is acted on by the pressure in the load signaling line  37  and by a control spring  41 . Variable-displacement pumps and control valves as shown in the circuit diagram of FIG. 2 are generally known and are commercially available without problems. It is therefore unnecessary to provide any further details of these components. It should merely be pointed out that the load-sensing pump control shown has the effect of establishing a pressure in the supply line  13  which is higher than the pressure in the load signaling line  37  by a pressure difference which is equivalent to the force of the control spring  41 . 
     A pressure differential valve  45  is arranged between the system of feed lines  13  and the passage  35  between the two control spaces  26  of the pressure compensators  23  and  24 . An inlet opening  46  of this valve is connected to the feed lines  13 , and an outlet opening  47  is connected to the passage  35 . Depending on the position of a piston slide  48 , which cannot be seen in FIG. 1 but is visible in FIG. 3, of the pressure differential valve  45 , the inlet opening  46  and the outlet opening  47  are blocked with respect to one another or are in fluid communication with one another via a more or less large opening cross section. In the direction of reducing the opening cross section between the inlet opening and the outlet opening, the piston slide  48  is acted on by the pressure prevailing in the passage  35  and in the control spaces  26  of the pressure compensators and by a compression spring  49 , and in the direction of increasing the opening cross section is acted upon by the feed pressure prevailing in the supply lines  13 . The active surfaces on the piston slide for the control pressure and the feed pressure to engage on are of equal size, so that the pressure differential valve  45  ensures that the control pressure which is present in the passage  35  follows a rising feed pressure in each case with an interval of a differential pressure which is equivalent to the force of the compression spring  49 . By way of example, the pressure differential valve  45  is set in such a way that the control pressure is 20 bar lower than the feed pressure. The passage  35  is connected to tank  12  via a low-flow regulator  50 , so that the control pressure in the passage  35  is also able to follow a decreasing feed pressure as a result of pressure medium flowing out via the low-flow regulator so. 
     Between the load signaling line  37  and the passage  35  there is a nonreturn valve  51 , which opens from the load signaling line  37  toward the passage  35  when the pressure in the passage  35  becomes equal to the pressure in the load signaling line  37 . The control pressure which is present in the control spaces  26  of the pressure compensators  23  and  24  therefore cannot drop below the highest load pressure which is present in the load signaling line  37 . 
     There is a second pressure differential valve  52 , which is constructed identically to the pressure differential valve  45  and the inlet opening  46  of which is likewise connected to a supply line  13 . The outlet opening  47  of the pressure differential valve  52  is connected to the control space  26  of the pressure compensator  25 . The piston slide of the pressure differential valve  52  is controlled in exactly the same way as the piston slide of the pressure differential valve  45 . Both valves are set to the same pressure difference of, for example, 20 bar. If a sufficient quantity of medium is delivered by the variable-displacement pump  10 , therefore, the control pressure in the control spacers  26  is 20 bar lower than the feed pressure and, since, by way of example, the latter is supposed to be 25 bar higher than the highest load pressure, the control pressure is 5 bar higher than the highest load pressure. Therefore, all the pressure compensators  23 ,  24  and  25 , including the one which is assigned to the consumer with-the highest load pressure, are in the control position. Furthermore, the control space  26  of the pressure compensator  25  is connected to tank  12  via a second low-flow regulator  50 . 
     If the variable-displacement pump  10  is providing its maximum delivered quantity and this quantity does not meet demand, the differential cylinder  16  is to be supplied with pressure medium on a priority basis ahead of the other two hydraulic cylinders  14  and  15 . For this purpose, there is a priority valve  55 , which is constructed as a proportional variable restrictor with an inlet  56  and an outlet  57 . The latter is in fluid communication with the passage  35 . The inlet  56  is connected to a supply line  13  upstream of the meter-in variable restrictor  19 . The movable valve member, which is not shown in more detail, of the priority valve, in the direction of closing the connection between the inlet and the outlet, is acted on by the pressure in the inlet, i.e. by the feed pressure, and, in the direction of opening the connection, is acted on by the pressure downstream of the meter-in variable restrictor  19  and by the force of a control spring  58 . The control spring  58  is constructed, for example, in such a way that there is an equilibrium of forces at the valve member of the priority valve if the pressure difference between the feed pressure and the pressure downstream of the meter-in variable restrictor  19  is 19 bar. This value is slightly lower than the value of the pressure difference across the pressure differential valve  52  minus a pressure value of 0.5 bar which is equivalent to the force of the compression spring  34 . Therefore, while in normal operation there is a pressure difference of 19.5 bar across the meter-in variable restrictor  19 , the priority valve  55  does not respond. If, as a result of a reduction in the feed pressure, the pressure difference across the meter-in variable restrictor  19  drops to below 19.5 bar, the pressure compensator  25  opens completely, so that the pressure downstream of the meter-in variable restrictor  19  is equal to the load pressure of the prioritized hydraulic consumer  16 . On the spring side, the load pressure of the consumer is now present at the priority valve  55 . This pressure is able to open the priority valve  55  against the feed pressure, with the result that the pressure in the passage  35  and therefore in the control spaces  26  of the pressure compensator  23  and  24  is raised to above the highest load pressure. Therefore, the pressure compensators  23  and  24  are adjusted in the closing direction until, as a result of a rise in the pressure downstream of the meter-in variable restrictors  17  and  18 , an equilibrium of forces is once again reached at the control pistons. Now, however, the pressure difference across the meter-in variable restrictors  17  and  18  has been reduced. The flows of pressure medium flowing to the consumers  14  and  15  have been reduced. Ultimately, by raising the pressure in the control spaces  26  of the pressure compensators  23  and  24 , the priority valve  55  ensures that, as a result of a rise in the control pressure in the passage  35  the pressure difference across the meter-in variable restrictors  17  and  18 , and therefore the flows of pressure medium flowing to the hydraulic consumers  14  and  15 , are in each case reduced to such an extent that a quantity of pressure medium which generates a pressure difference which is approximately equal to the pressure difference in normal operation is flowing across the meter-in variable restrictor  19 . 
     As mentioned above, in the case of undersaturation, i.e. when the priority valve  55  is intended to respond, load pressure prevails downstream of the meter-in variable restrictor  19 . Alternatively, therefore, the spring-side control space  55  of the priority valve  55  may be connected not to the connection between the meter-in variable restrictor  19  and the pressure compensator  25 , but rather to the outlet of the pressure compensator  25 , as shown in FIG. 1 a . The valve member of the priority valve  55  is then always acted on by the load pressure of the priority hydraulic consumer  16  in the direction of opening the connection between the inlet  56  and the outlet  57 . The priority valve can then be set to the same pressure difference which prevails across the meter-in variable restrictor  19  in normal operation, since in normal operation the pressure difference between the load pressure of the priority hydraulic consumer  16  and the feed pressure is higher than the pressure difference across the meter-in variable restrictor  19 , and therefore the priority valve  55  definitely does not respond. 
     If the situation of undersaturation occurs with only one of the hydraulic consumers  14  and  15  actuated, the control pressure in the passage  35  becomes equal to the highest load pressure, prevailing in the load signaling line  37 , of the two hydraulic consumers  14  and  15  as a result of the feed pressure being lowered. Therefore, the highest load pressure is also signaled to the passage  35  via the nonreturn valve  51 . Consequently, a further drop in the feed pressure no longer leads to a further fall in the control pressure in the passage  35  and in the control spaces  26  of the pressure compensators  23  and  24 . These ensure that, irrespective of the level of the feed pressure, a pressure which is higher than the highest load pressure by the pressure equivalent of the springs  34  prevails between them and the meter-in variable restrictors  17  and  18 . This pressure, which is slightly above the highest load pressure, is present downstream of both meter-in variable restrictors  17  and  18 . Feed pressure prevails upstream of both meter-in variable restrictors  17  and  18 . Therefore, the pressure difference across the meter-in variable restrictor  17  is equal to the pressure difference across the meter-in variable restrictor  18 . Therefore, in the event of undersaturation, the flows of pressure medium to the hydraulic consumers  14  and  15  are in relative terms reduced irrespective of whether the prioritized consumer  16  is also actuated. The consumers  14  and  15  are therefore LIDF consumers. 
     If the demand for pressure medium from all hydraulic consumers which are actuated simultaneously is covered by the variable-displacement pump  10 , the pressure differential valves  45  and  52 , together with the flow regulators  50 , ensure that the control pressures in the control spaces  26  of the pressure compensators follow the feed pressure with a fixed difference. If the variable-adjustment pump  10  then briefly produces a quantity which exceeds demand, for example because a wide-open meter-in variable restrictor is closed altogether, the feed pressure rises strongly for a brief period. The control pressures follow this rise, so that the control pistons of the pressure compensators are acted on by an increased control pressure in the closing direction, move in the closing direction of the pressure compensators and as a result raise the pressure downstream of the meter-in variable restrictors, so that the pressure difference across the meter-in variable restrictors  17 ,  18  and  19  remains constant or only increases slightly. Consequently, the speed of a hydraulic consumer also does not change. The excess quantity flows away to the tank via a pressure-limiting valve.  60 . 
     As has already been indicated, the pressure differential valves  45  and  52  used in the control arrangement shown in FIG. 1 are identical and, as can be seen from FIG. 3, are constructed as insertion cartridges. They have a cartridge casing  70 , through which a stepped valve bore  71  passes in the axial direction. An adjustment screw  72 , which is used to close the valve bore  71  and to support the control spring  49 , is screwed into the valve bore  71  from one end. This control spring is situated in the section of the valve bore  71  which has the larger diameter, into which the adjustment screw  72  has also been screwed. By means of its end facing away from the adjustment screw  72 , the control spring  49  is supported on the piston slide  48 , which is guided in an axially movable fashion in the valve bore  71 . The free space in the valve bore between the adjustment screw  72  and the piston slide  48  can be referred to as a spring space  75 . A star-shaped arrangement of radial bores  76 , which form the outlet  47  of the pressure differential valve, opens freely into this spring space. At an axial distance from the radial bores  76  and separated in fluid terms from the radial bores  76  by a sealing arrangement  77  after installation in a block, further radial bores  78 , which form the inlet of the pressure differential valve, pass through the cartridge housing  70 . Also after installation in a block, there is free fluid communication between the radial bores  78  and the end side  79  of the cartridge housing  70 , at which that section of the valve bore  71  which has the smaller diameter passes to the outside, along the cartridge housing  70 . 
     The piston slide  48  is guided axially in the latter section of the valve bore  71 , where on the outside it has an annular groove  80 , creating an annular space between it and the wall of the valve bore  71 . An axial blind bore  81 , which extends as far as the region of the annular groove  80 , where it is connected to the annular groove  80  via individual radial bores  82 , is formed in the piston slide  48  from the end side which faces the adjustment screw  72 . Further radial bores  83  provide open fluid communication between the bore  81  and the spring space  75  and therefore the outlet  47  even when an end side of the piston slide  48  is bearing against a stop of the adjustment screw  72 . The piston slide  48  has an outer shoulder  84 , by means of which it can be pressed against the inner shoulder of the valve bore  71  by the control spring  49 . When the piston slide  48  is bearing against the inner shoulder, the annular groove  80  is situated between the star-shaped arrangement of radial bores  78  and the end side of the cartridge housing  70 . There is no opening cross section between the radial bores  78  and the annular groove  80 . On both sides of the annular groove  80 , the piston slide  48  is guided as a sliding seal in the valve bore  71 , so that the radial bores  78  are separated in fluid terms from the spring space  75 , and the annular groove  80  is separated in fluid terms from the space in front of the end side  79  of the valve housing. Therefore, there is no fluid communication between the inlet  46  and the outlet  47  of the valve. In operation, the piston slide  48  is acted on by the inlet pressure from the end side  79  of the valve housing  70 . This inlet pressure is counteracted by the compression spring  49  and, on a surface which is the same size as that exposed to the inlet pressure, the outlet pressure which is present at the outlet  47 . Equilibrium prevails at the piston slide  48  if the outlet pressure is lower than the inlet pressure by a pressure difference which is equivalent to the force of the compression spring  49 . Through rotation of the adjustment screw  72 , it is possible to change the prestress of the compression spring  49  and therefore the pressure difference between the inlet pressure and the outlet pressure.