Abstract:
The invention relates to a hydraulic control device for a priority, first hydraulic consumer and a subordinate, second hydraulic consumer, pressure medium being deliverable to the first or the second consumer via a first or a second metering diaphragm. A pressure scale which allows a constant pressure difference to be adjusted above the first metering diaphragm is mounted upstream from the first metering diaphragm. For this purpose, said pressure scale is provided with a valve piston encompassing a first control edge, by means of which a first flow area between a feeding duct and the first metering diaphragm can be controlled. A second control edge which allows a second flow area to be controlled between the feeding duct and a load signaling line is provided on the valve piston of the first pressure scale.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a hydraulic control device for a primary hydraulic consumer and a secondary hydraulic consumer. 
     A hydraulic control circuit of this type is known, e.g., from DE 197 03 997 A1. The pressure medium flows to the two hydraulic consumers via metering orifices. A pressure scale is located upstream of the first metering orifice, which is assigned to the primary, first hydraulic consumer, and a pressure scale is located downstream of the second metering orifice, which is assigned to the secondary, second hydraulic consumer. The pressure scales serve to maintain constant pressure differences via the metering orifices when the quantity of pressure medium delivered is sufficient, independently of the load pressures. As a result, the quantity of pressure medium flowing to a hydraulic consumer depends only on the opening area of the particular metering orifice. The pressure medium source is typically an adjustable hydropump that is controllable as a function of the highest load pressure such that the pressure in a supply line is greater than the highest load pressure, by a certain pressure difference. 
     With regard for the first consumer, the control circuit corresponds to a load-sensing control (LS control). LS control or LS consumers are typically referred to when hydraulic consumers are controlled to which pressure medium flows via a meter orifice and an upstream pressure scale, and when the pressure scale registers the falling pressure via the particular metering orifice and holds it constant. The pressure scale is acted upon in the closing direction only by the pressure in front of the metering orifice, and it is acted upon in the opening direction only by the load pressure of the particular hydraulic consumer and by a compression spring. 
     With regard for the second consumer, the control circuit corresponds to an LUDV control. In this case, the pressure scale located downstream of the second metering orifice is acted upon in the opening direction by the pressure after the second metering orifice, and it is acted upon in the closing direction by a control pressure that is present in a rear control space, the control pressure typically corresponding to the highest load pressure of all hydraulic consumers supplied by the same hydropump. If several hydraulic consumers controlled in this manner are actuated simultaneously, the quantities of pressure medium flowing to them are reduced by the same ratio when the quantity of pressure medium delivered by the hydropump is less than the partial quantities of pressure medium demanded. This case is referred to as a control with load-independent flow distribution (LUDV control). The hydraulic consumers controlled in this manner are referred to as LUDV consumers. LUDV control is a special case of load-sensing control (LS control). In that case as well, the highest load pressure is also sensed, and the pressure medium source generates an inlet pressure that is greater than the highest load pressure by a certain amount Δp. 
     Publication DE 197 03 997 A1 mentioned above discloses a priority-based switching between the LS consumer and one or more LUDV consumers, in which priority is given to supplying the LS consumers with pressure medium. In addition to the pressure scale of the LS consumer, a priority valve is provided that includes a first connection, which is connected with a line section upstream of the first metering orifice and a second connection connected with the load-sensing line, and the valve element of which is capable of being acted upon—in the direction in which the connection between the first connection and the second connection is opened—by the load pressure of the primary hydraulic consumer, i.e., the LS consumer, and by an additional force. In the closing direction, the valve element is acted upon by pressure upstream of the metering orifice of the LS consumer—in a supply line or between the pressure scale and the first metering orifice. In this manner, it is ensured that priority is given to supplying the LS consumer with pressure medium. In particular, the pressure upstream of the first metering orifice is regulated to a value that is higher than the load pressure of the primary consumer at least by an amount that corresponds to the additional force that acts on the valve element of the primary valve. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide—based on the described state of the art—a hydraulic control device that is simpler and more cost-effective to manufacture. 
     This object is attained by a hydraulic control device according to the present invention. 
     The present invention relates to a hydraulic control device for a primary hydraulic consumer and one more secondary hydraulic consumers. The primary consumer is controlled by a first metering orifice, upstream of which a (LS) pressure scale is located. The secondary consumer is supplied by a second metering orifice, which is located downstream a pressure scale, in the manner of LUDV control. 
     The present invention is characterized by the fact that a further control edge is provided on the valve piston of the pressure scale of the primary consumer, which controls the supply of pressure medium from a supply line into a load-sending line. Two control edges are therefore provided on the valve piston of this pressure scale. The first control edge controls the flow of pressure medium supplied to the first metering orifice in the sense of an individual pressure scale for the primary consumer. The second control edge controls a flow area between the inlet and the load-sensing line. As a result, the pressure in the load-sensing line can be increased if the falling pressure difference at the metering orifice of the primary consumer falls below a certain value. This results in an increase in the pressure level upstream of the second metering orifice and, therefore, a reduction in the flow of pressure medium supplied to the secondary consumers. As a result, a sufficient quantity of pressure medium is available to the primary consumer. 
     The present invention makes clever use of the knowledge that the pressure scale of the primary consumer and the control mechanism of a pressure increase in the load-sending line are controllable using the same pressure signals, in order to realize these two functionalities in a single valve having a simple design. Compared with the conventional means of attaining the object of the present invention, a separate primary valve is not needed, thereby saving material, installation space, and costs. In addition, the inventive control device requires little maintenance, given the smaller number of movable components. 
     The pressure difference that results at the metering orifice of the primary consumer may be held nearly constant, independently of the operating state, since this pressure difference is determined by the control spring of the pressure scale in every operating state. When the quantity delivered by the pump is adequate and the secondary consumers are load-guiding, the pressure scale of the primary consumer behaves in the manner of an individual pressure scale and throttles the supply of pressure medium in such a manner that a pressure difference determined by the control spring is produced via the metering orifice of the primary consumer. If undersaturation exists, the pressure in the load-sensing line is regulated by the second control edge such that, in turn, the pressure difference at the metering orifice of the primary consumer corresponds to the pressure equivalence of this control spring. In comparison, with traditional control, various springs are provided in the pressure scale and in a primary valve that controls the load-sensing line. To ensure system behavior that may be unambiguously determined, considering the production tolerances, these springs are adjusted to different pressure equivalence values. Under certain circumstances with the conventional system, therefore, a noticeable reduction in pressure occurs at the metering orifice of the primary consumer, e.g., during the transition to undersaturation. 
     For example, the control edges are located such that a moving direction to open the first flow area corresponds to the moving direction to open the second flow area. This means that the control edges are formed on surfaces of the valve piston that are oriented in the same axial direction. The fact that the two control mechanisms of the pressure scale are actuated in the same direction makes it easier to realize them using a single valve piston. 
     According to a particularly preferred embodiment, the second flow area is not opened—i.e., pressure medium is not supplied to the load-sensing line—until the hydraulic resistance at the first flow area is nearly minimal. This means that the control mechanism of a pressure increase in the load-sensing line does not engage until the regulation of the flow rate across the pressure scale—that is, through the first area—has reached the upper flow limit of its control range. As a result, an unnecessary increase in the pressure level of the variable-displacement pump and a throttling of the secondary LUDV consumers is prevented for as long as the variable-displacement pump continues to deliver a sufficient quantity of pressure medium. When the control regions of these two control mechanisms adjoin each other in this manner and do not overlap—or they overlap only slightly—it is also possible to always ensure a stable operating state of the inventive hydraulic control device that may be unambiguously assigned to the particular load conditions. 
     A simple design of the pressure scale of the primary consumer results when it is designed as a gate valve with a valve bore and includes an inlet chamber and two outlet chambers—a first outlet chamber connected with the metering orifice, and a second outlet chamber connected with the load-sensing line. 
     The complexity of the pressure scale of the primary consumer may be reduced when an end face of the valve piston abuts the first outlet chamber, which is connected with the metering orifice. As a result, the pressure in the outlet chamber acts simultaneously as control pressure on the valve piston, in order to act upon it in the closing direction of the two flow areas. 
     According to a second, preferred embodiment, a fluid path is formed in the valve piston, which connects a control pressure space formed on an end face of the valve piston with the first outlet chamber. A fluid path of this type is easy to manufacture and is a space-saving way to apply pressure to a control pressure space of the pressure scale upstream of the metering orifice. 
     The fluid path preferably includes a bore that leads into the circumferential surface of the valve piston and is capable of being moved to overlap with the second outlet chamber. A particularly advantageous design of the pressure scale is obtained, since the fluid path serves simultaneously as a pressure line to the control pressure space and as a flow-through path into the second outlet chamber. Only a small flow area is still required between the inlet line and the load-sensing line, and a quantity of pressure medium supplied to and removed from the control pressure chamber is also small. A fluid path with a small diameter may therefore be used. Since fluid may not be supplied to the load-sensing line until the flow area between the inlet chamber and the first outlet chamber of the pressure scale is already largely open, the fluid pressure in the first outlet chamber largely corresponds to the fluid pressure in the inlet chamber. Fluid may therefore be easily supplied to the second outlet chamber from a region in the first outlet chamber, instead of directly from the inlet chamber, and the simple valve design described may be attained. 
     As an alternative, a recess is provided in the valve piston that may be moved to overlap simultaneously with the inlet chamber and the second outlet chamber. As a result, the first flow area and the second flow area may be designed independently of each other, if necessary. 
    
    
     
       The present invention and its advantages are described in greater detail below with reference to the exemplary embodiment presented in the figures. 
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a circuit diagram of a hydraulic control device with a primary consumer and a control valve that controls the flow of pressure medium to the primary consumer and the supply of pressure medium to a load-sensing line. 
         FIG. 2  is a sectional image of the control valve shown in  FIG. 1 , 
         FIG. 3A  is a sectional image of the control valve shown in  FIG. 1 , in an alternative design, 
         FIG. 3B  is a symbolic depiction of the control valve shown in  FIG. 3A , 
         FIG. 4  is a schematic diagram of a further design of the control valve shown in  FIG. 1 , and 
         FIG. 5  is a circuit diagram of a hydraulic control device, comparable to  FIG. 1 , with a by-pass line for indicating a load pressure of the primary consumer to the load-sensing line. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to  FIG. 1 , a variable-displacement pump  10  with a displacement control  11  suctions pressure medium out of a tank  12  and supplies it to a system of supply lines. Via the supply lines, a first hydraulic consumer  14 , which is designed as a synchronous cylinder, and at least one second hydraulic consumer  15 , which is a differential cylinder, are supplied with pressure medium. The direction and speed of the synchronous cylinder  14  are determined via actuation of a 4/3-proportional directional control valve  16 , the valve spool of which is centered via a spring in a central position, in which the four working connections and one control connection  18  of directional control valve  16  are blocked. When the valve spool is displaced from its central position in one direction or the other, a metering orifice  17  is opened to an extent that depends on the displacement of the valve spool. Downstream of the metering orifice, control connection  18  is connected with the approach to synchronous cylinder  14 . 
     A control valve  45  with the function of a 2-way pressure scale is installed between a supply line  13  and a supply connection  19  of directional control valve  16 . Accordingly, control valve  45  controls the flow area of a fluid connection  20  between its inlet  46  and one of its outlets  23 , i.e., between supply line  13  and supply connection  19  of directional control valve  16 . Valve piston  48  of control valve  45  is acted upon, in the direction of closing connection  20 , by pressure upstream of a metering orifice  17  and, in the direction of closing, via a control line  61  by pressure in control connection  18  of directional control valve  16 , i.e, by the load pressure of synchronous cylinder  14 , and by a control spring  21 . The force of control spring  21  is designed such that it is equivalent to a pressure difference of, e.g., 15 bar above metering orifice  17 . 
     While control valve  45  assigned to first hydraulic consumer  14  is therefore located upstream of first metering orifice  17 , second pressure scale  30  assigned to second hydraulic consumer  15  is located downstream of a second metering orifice  31 . To control the direction of differential cylinder  15 , a directional control valve  32  is located between second pressure scale  30  and the differential cylinder, via which pressure does not drop noticeably when differential cylinder  15  is actuated, compared with the drop in pressure at metering orifice  31 . Metering orifice  31  and the control grooves required to control direction are designed on the same valve spool in a known manner, so that direction and speed are automatically controlled jointly. Control piston  33  of pressure scale  30  is acted upon—in the direction of opening the connection between metering orifice  31  and directional control valve  32 —by the pressure after metering orifice, and, in the direction of closing the connection, by a control pressure that exists in a rear control pressure space  34 , and by a weak compression spring  35 , which is equivalent to a pressure of, e.g., only 0.5 bar. The front side of control piston  33  is connected via a channel  36  extending in the control piston with control pressure space  34 . A non-return valve  37  that is open toward the control pressure space is located in channel  36 . 
     In parallel with metering orifice  31 , pressure scale  30 , and directional control valve  32  for second hydraulic consumer  15 , further metering orifices, pressure scales, and directional control valves for further hydraulic consumers may be connected to the system of supply lines  13 . Control pressure spaces  34  of all pressure scales  30  are connected with each other, so that the same pressure forms in these control pressure spaces. When a second hydraulic consumer is actuated, control pistons  33  of the pressure scales attempt to move into a position in which a pressure occurs on their front side that is higher than the pressure in control pressure spaces  34  only by the pressure difference equivalent to the force of compression spring  35 . 
     Disregarding first hydraulic consumer  14  entirely, the highest load pressure of all actuated, second hydraulic consumers  15  is transferred to control pressure spaces  34  via channels  36  and non-return valves  37 . Control pressure spaces  34  are connected to a load-sensing line  38  that leads to displacement control  11  of pump  10 . Load-sensing line  38  is also connected with tank  12 , via a current control  55 . These current controls relieve the pressure on load-sensing line  38  when none of the hydraulic consumers is actuated. 
     Variable displacement pumps and related controllers are known in general and are readily available on the market. It is therefore not necessary to discuss them in greater detail. It should merely be noted that the pump control serves to adjust a pressure in supply line  13  that is higher than the pressure in load-sensing line  38  by a pressure difference Δp equivalent to the force of a control spring. Pressure difference Δp is, e.g., 20 bar, and is therefore higher than pressure difference of 15 bar, which is equivalent to the force of control spring  21  of control valve  45 . 
     First hydraulic consumer  14  should be supplied with pressure medium with priority over second hydraulic consumer  15 . A second controllable connection  22  is provided in control valve  45  for this purpose. Connection  22  is designed as an orifice with a proportionally controllable flow area between inlet  46  and an outlet  47 . Outlet  47  is connected with load-sensing line  38 . 
     For the second fluid connection  22  controlled by it, valve piston  48  of control valve  45  is acted upon, in the direction of closing, by a pressure upstream of metering orifice  17  and, in the direction of opening, by the load pressure of primary consumer  14  applied via control line  61 , and by control spring  21 . 
     Control valve  45  is shown in greater detail in  FIG. 2 . A valve bore  71  is provided in valve housing  70 . Valve piston  48  is displaceably supported in this bore. The valve bore is abutted by an inlet chamber  72  and two outlet chambers  73  and  74 . The inlet chamber is connected with connection  46 , which is designed as a bore, and, therefore, with supply line  13 . Outlet chamber  73  is connected with outlet  23 , i.e., with metering orifice  17 . Outlet chamber  74  leads into load-sensing line  38 , via connection  47 . 
     Controllable fluid connection  20  is established via a radially recessed section  76  of valve piston  48 . Control edge  77  is formed on valve piston  48 , on a step located in the direction of inlet chamber  72 . Control edge  77  bounds a first flow section between itself and a housing segment  78 , which is formed between inlet chamber  72  and outlet chamber  73 . 
     Fluid connection  22  is formed by a recess  78  in the circumferential surface of valve piston  48 . Recess  78  may be, e.g., an axial groove or a radial step of the valve piston. A control edge  79 , which bounds recess  78  in the direction of outlet chamber  74 , forms a second controllable and closable flow area with outlet chamber  74 . 
     A control pressure space  50  is connected to control line  61 , which directs the load pressure of primary consumer  14 . The pressure in control pressure space  50  acts on valve piston  48  in the direction of opening of fluid connections  20  and  22 . In addition, the force of control spring  21  on valve piston  48  acts in the direction of opening. The pressure present in control pressure space  49  acts in the closing direction. Control pressure space  49  is fluidly connected via a fluid channel  75  formed in valve piston  48  with radially recessed section  76  and, therefore, with outlet chamber  73 . 
     The mode of operation of the inventive control device will now be explained with reference to  FIGS. 1 and 2 . An equilibrium of forces involving the following force and pressure components sets in at valve piston  48  of control valve  45 :
 
 p   LS   +p   21   =p   38   +Δp Δp   DW   (Equation 1),
 
in which p LS  is the load pressure of primary consumer  14 , p 21  is the pressure equivalent of the force of control spring  21 , p 38  is the load pressure in load-sensing line  38 , Δp is the control pressure difference of pump displacement control  11 , and Δp DW  is the pressure that is falling at control edge  77  of connection  22 .
 
     Control edge  79  is positioned such that connection  22  does not open until the flow area at control edge  77  is nearly at a maximum, i.e., when pressure drop Δp DW  at control edge  77  has reached a value Δp DW*  that is nearly a minimum. Value Δp DW*  depends on the flow rate at control edge  77 , however. 
     If the flow of pressure medium conveyed by the pump is sufficient to supply all of the consumers, control pressure difference Δp remains constant at the value set by the control spring of pump displacement control  11 , e.g, 20 bar. 
     As long as the secondary consumers are load-guiding, that is, as long as the pressure in supply line  13  is greater than the sum of the load pressure of primary consumer  14  and the pressure equivalence of control spring  21 , a pressure drop Δp DW  is generated via control edge  77  to regulate the supply to the primary consumer. The pressure drop Δp DW  results in a throttling of excess pressure present in supply line  13  with respect to primary consumer  14 . Pressure P 38  in the load-sensing line corresponds to the highest load pressure of the secondary consumer, which is referred to below as p LUDV . 
     With a load pressure of the primary consumer of
 
 p   LS &gt;( p   LUDV   +Δp )− p   21   −−Δp   DW*   (Equation 2),
 
in which (p LUDV +Δp ) is the supply pressure that may be generated by secondary consumer  15 , the control mechanism of a throttling at first control edge  77  is exhausted, and the associated flow area is completely open.
 
     Therefore, when the supply pressure (p 38 +Δp ) falls above or below the value p LS +P 21 +Δp DW* , second control edge  79  opens the flow area of connection  22 . As a result, pressure p 38  in load-sensing line  38  increases to values greater than P LUDV . If the pressure (p 38 +Δp ) present in supply line  13  was previously dependent only on load pressure P LUDV  of the secondary consumers, the supply line pressure (p 38 +Δp ) is now determined by load pressure p LS  of primary consumer  14 . Supply line pressure (P 38 +Δp ) is controlled using control edge  79  and the feedback via displacement control  11 . Equation 1 directly results in the dependency
 
( p   38   +Δp )= p   LS   +p   21   ′+Δp   DW*   (Equation 3),
 
when one considers that control spring  21  is loaded less when regulation is carried out at control edge  79  than when regulation is carried out at control edge  77 , i.e., it has a slightly less pressure equivalence p 21 ′ than p 21 , and when Δp DW*  is assumed to be a slight pressure drop at the flow area, which is nearly completely open and is bounded by control edge  77 . Essentially, the pressure in supply line  13  is regulated to a value that is higher than the load pressure of the primary consumer  14  by pressure equivalence p 21 ′ of control spring  21 .
 
     When the flow of pressure medium conveyed by pump  10  does not suffice to supply all consumers, Δp may no longer be regarded as constant. The control capacity of pump  10  and its displacement control  11  are exhausted, and the pressure in supply line  13  drops. As before, connection  22  opens when the supply line pressure (p 38 +Δp ) drops to p LS +p 21 +Δp DW* . This results in an increase of the pressure present in load-sensing line  38 . As a result, the pressure between metering orifice  31  and pressure scale  30  of secondary consumer also increases. The pressure difference that is present at metering orifice  31  is reduced and, therefore, the flow of pressure medium that may be supplied to the secondary consumer also decreases. If necessary, when control pressure difference Δp has dropped accordingly, the pressure in load-sensing line  38  may increase to the supply pressure (p 38 +Δp ) and completely halt the supply to secondary consumer  15 , via pressure scale  30 . It is also possible to limit several secondary consumers  15  in this manner. Via this mechanism of throttling secondary consumer  15 , the supply pressure (p 38 +Δp ) is regulated per equation 3, to a value that is essentially higher than the load pressure of the primary consumer  14  by pressure equivalence p 21 ′ of control spring  21 . 
     In every case, a reliable supply of primary consumer  14  is therefore ensured such that a pressure difference that corresponds to pressure equivalence p 21  or p 21 ′ of control spring  21  is present above metering orifice  17 . 
       FIG. 3A  shows a control valve  85 , which is a modified design of control valve  45 . A symbolic depiction of control valve  85  is shown in  FIG. 3B . The only difference between control valve  85  and control valve  45  is that control valve  85  has valve piston  88 . Similar to valve piston  48 , valve piston  88  includes a radially recessed piston section  76 . A fluid channel  75  extends out of this piston section and leads into control pressure space  49  located on an end face of valve piston  88 . In contrast to valve piston  48 , there is no recess in the circumferential surface of the piston with which inlet chamber  72  may be connected directly with outlet chamber  74 . Instead, a bore  86  is formed perpendicularly to the axis of valve piston  88 . Bore  86  leads into fluid channel  75 . Together with a fine control groove  87 , bore  86  forms a control edge  89  for controlling a flow area at outlet chamber  74 . It should be noted that this opening area formed between control edge  89  and valve housing  70  does not open until the hydraulic resistance or pressure drop Δp DW  at control edge  77  has already reached a value Δp DW*  close to the minimum value. The pressure of the pressure medium, which is supplied by radially recessed piston section  76  via fluid path  75  when control edge  89  is opened therefore approximately corresponds to the pressure in inlet connection  46 . As a result the pressure in load-sensing line  38  may be increased nearly to the supply fine pressure that is present at inlet connection  46 . 
     A further design of a control valve  95  that may be used in place of control valve  45  or  85  is shown in  FIG. 4 . The symbolic depiction of control valve  95  corresponds to that shown in  FIG. 3B . A valve bore  91  is provided in valve housing  90  of control valve  95 . An inlet chamber  92  and two outlet chambers  93  and  94  are located at valve bore  91 . Chambers  92 ,  93  and  94  are fluidly connected with related connections  46 ,  47  and  23 , as shown in  FIG. 4 . A cylindrical valve piston  96  is movably guided in valve bore  91 . Valve piston  96  includes an axially extending blind hole  97  that is open in the direction of outlet chamber  93 . From circumferential surface of valve piston  96 , two radially extending bores  98  and  99  extend toward blind hole  97 . 
     Bore  98  may be moved to overlap with inlet chamber  92 . As a result, a fluid connection is created from inlet connection  46  via bore  98 , blind hole  97 , outlet chamber  93 , and outlet connection  23 . Control edge  100 , which plays a decisive role in the control of the flow area of this connection, is the edge of bore  98  on the circumferential side. A fluid connection from inlet connection  46  to outlet connection  47  is created via bore  98 , blind hole  97 , bore  99 , and outlet chamber  94 . Control edge  101 , which is decisive for this, is the edge of bore  99  on the circumferential side. Bore  99  is located such that it does not overlap with outlet chamber  94  until the flow area controlled at bore  99  results in a slight hydraulic resistance/pressure drop Δp DW* . As a result, the pressure in load-sensing line  38  may be increased nearly to the inlet pressure present at inlet connection  46 . 
     On an end face of valve piston  96  facing away from blind hole  97 , valve piston  96  bounds a control pressure space  50  formed in valve housing  90 . It is connected to control line  61 , which guides the load pressure of primary consumer  14 . The pressure in control pressure space  50  acts in the direction of opening of the connections controlled by bores  98  and  99 . In addition, control spring  21  located in control pressure space  50  acts in the opening direction. In the closing direction, valve piston  96  is acted upon directly by the pressure in outlet chamber  93 , since valve piston  96  abuts outlet chamber  93  with its end face that leads into blind hole  97 . With this embodiment of control valve  95 , a very low pressure drop Δp DW*  at bore  98  may be attained, and by locating outlet chamber  93  on the end-face end of valve piston  96 , it is not necessary design a separate control chamber or a control line that leads thereto, internally or externally. 
       FIG. 5  shows a further embodiment of the inventive hydraulic control device. The embodiment shown in  FIG. 5  is largely equivalent to the design shown in  FIG. 1 . The difference from the embodiment shown in  FIG. 1  is that control line  61  that leads from control connection  18  of directional control valve  16  to control valve  45  is also connected with load-sensing line  38 , via a non-return valve  63  located in a by-pass line  62 . Non-return valve  63  blocks from load-sensing line  38  toward channel  61 , i.e., toward control connection  18  of directional control valve  16 . In addition, a non-return valve  64  is also located between second connection  47  of control valve  45  and load-sensing line  38 . Non-return valve  64  blocks toward connection  47 . 
     With the embodiment shown in  FIG. 1 , as described above, even when a sufficient quantity of pressure medium is conveyed, a change takes place in the control mechanism of control valve  45  when load pressure p LS  of primary consumer  14  exceeds the supply pressure (P LUDV +Δp ) specified by the secondary consumers, minus pressure equivalent p 21  of control spring  21  (pressure drop Δp DW*  at control edge  77  is negligibly small). When the primary consumer becomes load-guiding in this sense, the control valve loses its functionality as an LS pressure scale. This is replaced by the mechanism of controlling the pressure in load-sensing line  38 . 
     With the embodiment shown in  FIG. 5 , when a sufficient quantity of pressure medium is pumped, and given a load-guiding, primary hydraulic consumer  14 , the load pressure of this hydraulic consumer is directed via non-return valve  63  into load-sensing line  38 . The pressure in supply line  13  is therefore higher than the load pressure of hydraulic consumer  14  by control pressure difference Δp of variable-displacement pump  10 . In this case, control valve  45  has the function of an LS pressure scale and throttles the flow of pressure medium directed to metering orifice via first control edge  77 . The pressure difference present above pressure scale  17  therefore corresponds to pressure equivalent p 21  of control spring  21 . 
     Pressure medium is not directed to load-sensing line  38  via connection  22  until—when undersaturation occurs—the pressure (p 38 +Δp) in supply line  13  has dropped to the sum of load pressure P LS  of hydraulic consumer  14 , pressure equivalent P 21  of control spring  21 , and a slight pressure drop Δp DW*  at control edge  77 . The pressure drop via metering orifice  17  is basically not reduced, because, as undersaturation continues, pressure p 38  in load-sensing line  38  via control valve  45  increases and, as a result, pressure scales  30  of LUDV consumers  15  are displaced in the closing direction. 
     Non-return valve  64  prevents pressure medium from flowing from hydraulic consumer  14  via non-return valve  63  into the system of supply lines, provided that the pressure in the supply lines is not yet above the load pressure, e.g., at the beginning of an actuation. 
     Non-return valve  64  may be eliminated when connection  47  of control valve  45  is connected with non-return valve  63  in such a manner that non-return valve  63  blocks toward connection  47 . 
     List of Reference Numerals 
     
         
           10  Variable-displacement pump 
           11  Displacement control 
           12  Tank 
           13  Supply line 
           14  Synchronous cylinder 
           15  Differential cylinder 
           16  4/3-way proportional directional control valve 
           17  Metering orifice 
           18  Control connection 
           19  Supply connection 
           20  Fluid connection 
           21  Control spring 
           20  Fluid connection 
           23  Outlet 
           30  Pressure scale 
           31  Metering orifice 
           32  Directional control valve 
           33  Regulating piston 
           34  Control pressure space 
           35  Compression spring 
           36  Channel 
           37  Non-return valve 
           38  Load-signalling line 
           45  Control valve 
           46  Inlet 
           47  Outlet 
           48  Valve piston 
           49  Control pressure space 
           50  Control pressure space 
           55  Current control 
           61  Control line 
           62  Bypass line 
           63  Non-return valve 
           64  Non-return valve 
           70  Valve housing 
           71  Valve bore 
           72  Inlet chamber 
           73  Outlet chamber 
           74  Outlet chamber 
           75  Fluid channel 
           76  Recessed piston section 
           77  Control edge 
           78  Recess 
           79  Control edge 
           85  Control valve 
           86  Bore 
           87  Fine-control groove 
           88  Valve piston 
           89  Control edge 
           90  Housing 
           91  Valve bore 
           92  Inlet chamber 
           93  Outlet chamber 
           94  Outlet chamber 
           95  Control valve 
           96  Valve piston 
           97  Blind hole 
           98  Radial bore 
           99  Radial bore 
           100  Control edge 
           101  Control edge