Abstract:
A control device for controlling a hydraulic consumer, is equipped with a distributing valve having a control pressure chamber and a control slide that can be displaced against the force of a spring by the build-up of a control pressure in the control pressure chamber. A pilot control valve controls the supply and discharge of control fluid into and out of the control pressure chamber. A release device is used to drive the control fluid out of the control pressure chamber, bypassing the pilot control valve.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a control device for the control of a hydraulic consumer having a hydraulically actuable directional valve in accordance with the description herein. The invention furthermore relates to a hydraulic pilot control device in accordance with the description herein. 
     Hydraulic control devices having hydraulically actuable directional valves are used inter alia in hydraulic systems of vehicles. A plurality of directional values are expediently interlinked in the form of valve disks in a so-called control block. Using such a hydraulic control device, lifting devices of a lift truck or of an agricultural tractor, loading cranes, the bucket of a wheeled loader or also driving and steering functions of a vehicle are operated hydraulically; in the event of a control regulated by flow requirement (load sensing), individual valve disks have a pressure balance for the control of the hydraulic pressurizing medium flow flowing across the valve. 
     Mineral oil is usually used as the hydraulic pressure fluid or pressurized medium in industrial hydraulics and mobile hydraulics. However, a water-based pressurized medium is also used for certain areas of application. The term fluid will be used in the following for hydraulic fluids. 
     A hydraulic control device is known, for example, from DE 197 15 020 A1 Directional valves are arranged in different valve disks for the control of hydraulic consumers. The directional valves have valve pistons for the control of pressurized medium connections and in each case two spring chambers. A control pressure is built up in a spring chamber to actuate the valve piston against the spring bias. The respective control pressure is generated by an electrically actuated pressure control valve. Two respective pressure control valves are provided as pilot control valves for a valve disk. The valve piston can thus be deflected in two mutually oppositely set directions. The electric control of the pressure control valves as a rule takes place by means of an operating element. 
     In rare cases, electrically controlled pressure control valves fail in that the control piston of the pressure control valve jams and can no longer be electrically actuated. One reason for this can be dirt particles carried along in the fluid flow. If the control piston is just located in the control position at which the output of the pressure control valve is blocked against the control fluid supply port and the tank port, no more control fluid can be displaced out of the corresponding spring chamber of the directional valve. The directional valve is thus blocked in the controlled position and the movement carried out by the hydraulic consumer cannot be stopped. Such a block can also not be released by a counter-control (counteraction) at the operating element whereby the oppositely disposed spring chamber is subjected to control pressure via the corresponding control pressure valve since, as said, not fluid can be displaced from the blocked spring chamber. 
     DE 103 08 910 A1 deals with a safety valve which is integrated into the supply line of pilot control valves. The described, electromagnetically actuated 3/2 way valve can connect the supply line of the pilot control valves either to a pressurized medium source or to a container. A relief passage from the output port to the spring chamber is provided in the valve disk of the 3/2 way valve. When the magnet is actuated, the relief passage is blocked by the magnetic plunger. When the magnet is not actuated, the relief passage to the spring chamber, and thus to the container, is open, provided that the valve disk does not follow the magnetic plunger. The complex and/or expensive construction is disadvantageous in this valve. Nor can the construction be transmitted easily to pilot control valves formed as pressure reducing valves. In addition, the supply line cannot be relieved if a continuous actuation occurs due to an error in the control electronics of the 3/2 way valve. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide an improved control device for the control of a hydraulic consumer which reliably enables the valve piston of a pilot controlled directional valve to be guided back from an actuation position into a neutral position and which is in particular characterized by a simple, cost-effective design. 
     This object is satisfied in accordance with the invention by a control device and hydraulic pilot control device having the features herein. 
     The control device in accordance with the invention for the control of a hydraulic consumer is equipped with a directional valve which has a control pressure chamber and a control slider which is adjustable against the force of a spring by building up a control pressure in the control pressure chamber. A pilot control valve controls the inflow and outflow of control fluid into and out of the control pressure chamber. It is the special feature of the present invention that a relief device is present by which the control fluid can be displaced out of the control pressure chamber while bypassing the pilot control valve. 
     In this way, the control device in accordance with the invention enables the return of the control slider from the actuated position in a reliable manner. In normal operation, the directional valve can be controlled like a conventional directional valve. On a malfunction of the pilot control valve in which the outflow of control fluid from the control pressure chamber via the pilot control valve is blocked, the control slider can nevertheless be displaced from the actuated position. Such a control device thus has a high operational security. The control slider of the directional valve can be guided back into the neutral position or even actuated into a counter-direction not only on a jamming of the control piston of the pilot control valve, but even on a long-term actuation of the pilot control valve due to an error in its electric control circuit. The control device in accordance with the invention can moreover be realized simply and cost favorably. The bypassing of the pilot control valve can in particular be realized with the help of favorable standard components such as check valves or pressure relief valves. 
     In accordance with a further aspect of the present invention, a hydraulic pilot control device has a control fluid supply port and at least one pressure control valve which generates a regulated control pressure at a control pressure outlet. A check valve opening toward the control fluid supply port is provided between the control pressure outlet and the control fluid supply port. 
     Such a pilot control device enables the reliable displacement of control fluid from a control pressure chamber while bypassing the pressure control valve. The operating security of a hydraulic control device can thereby be increased. Such a pilot control device moreover has a particularly simple structure and only requires a few additional components in comparison with a conventional pilot control device. 
     Further advantageous aspects are set forth herein. 
     In accordance with a particularly preferred embodiment of the present invention, the relief device includes a relief line and a check valve via which control fluid can be displaced from the control pressure chamber into the relief line. A relief device with a particularly uncomplicated structure is thereby set forth. The behavior of the relief device can be controlled in a simple manner with the aid of a pressure prevailing in the relief line. 
     The relief line can preferably be connected to a tank via a pressure relief valve. A pressure required for the bypassing of the pilot control valve can thus be set simply at the pressure relief valve. This pressure can furthermore be set independently of the pressure of the control fluid supply line. When the pressure relief valve is actuable manually, a bleeding of the control pressure chambers can be carried out in a simple manner. 
     In accordance with a further preferred embodiment, the relief line is in fluid communication with a control fluid supply line of the pilot control valve. A control device designed in this manner enables a particularly simple and efficient securing of a control pressure chamber against a blocking of the fluid outflow. In addition, the pressure required for the bypassing of the pilot control valve always corresponds to the supply pressure of the control fluid supply line so that it does not have to be set separately. 
     The pressure in the relief line can preferably be limited to a value which is as high or higher than the maximum control pressure, i.e. the pressure the pilot control valve can set at a maximum at its outlet. It is thereby ensured that no displacement of control fluid into the relief line takes place on a normal control of the directional valve. 
     In accordance with a further preferred embodiment, the pressure in the relief line can be restricted to a value which is less than the sum of the maximum control pressure and a pressure corresponding to a biasing force of the spring. The force required for the restoration of the control slider from the actuated position can be applied in this manner by hydraulic action on the control slider, e.g. by pressurizing a control pressure chamber arranged oppositely. If the same pressure is present in both control pressure chambers, the control slider returns to a neutral position with the aid of the springs counteracting its deflection. 
     If the relief line can be relieved by a switching valve to a tank, the control slider returns to its neutral position without any further measures and very rapidly. It can even be deflected in an opposite direction by pressurizing of a control pressure chamber arranged oppositely. 
     The directional valve preferably has two control pressure chambers through which the control slider can be acted on in mutually opposite directions. Furthermore, control fluid can be displaced into two different branches of the relief line from the two control pressure chambers via a separate check valve, the two different branches of the relief line are fluidly separately from one another, and two switching valves are present via which the branches of the relief line can be relieved to a tank separately from one another. The different branches of the relief line and thus the connected control pressure chambers can thereby be relieved independently of one another in the event of a defect of the pilot control valve. This is an important requirement for the realization of a safe drive which allows a hydraulic motor not only to be stopped on a defect of the pilot control valve, but also to carry out a withdrawal movement. A defective pilot control valve can in particular be bypassed or the one control pressure chamber can be relieved and the oppositely arranged control pressure chamber can be pressurized by a further pilot control valve so that a hydraulic motor controlled by the directional valve carries out a withdrawal movement. 
     Control fluid from two different branches of the relief line can preferably be supplied to a pressure relief valve via a respective check valve which opens toward the pressure relief valve. Oppositely arranged control pressure chambers of the directional valve can thus be secured against a blocking of the pilot control valves by a simple, efficient design of the control device. In addition, control fluid can be displaced from a control pressure chamber whose pilot control valve fails by pressurizing an oppositely disposed control pressure chamber. It is thus possible to stop a hydraulic consumer by an opposite control at the operating element on a failure of a pilot control valve. The branches of the relief line can moreover additionally be relieved separately from one another, e.g. by a switching valve, so that a withdrawal movement of a hydraulic motor can be carried out despite a defect of the pilot control valve. 
     A plurality of directional valves are preferably provided, with control fluid being able to be displaced from each control pressure chamber of the different directional valves via a respective separate check valve into a relief line or a branch of the relief line. The control pressure chambers can thus also be secured against a failure of the pilot control valves for a plurality of directional valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention and its advantages will be explained in more detail in the following with reference to the embodiments shown in the Figures. 
       There are shown: 
         FIG. 1  a side view of a directional valve disk of a hydraulic control block—partly as a cross-section—with an additional fluid line via which control fluid can be displaced from the control pressure chambers; 
         FIG. 2  a diagram of a hydraulic control device having two directional valves which are secured against a blocking of the fluid outflow from the control pressure chambers in the manner shown in  FIG. 1  and which additionally have a manually actuable bleeding function; 
         FIG. 3  a diagram of a hydraulic control device having two directional valves and two branches of a relief line which can be relieved by switching valves independently of one another and which can moreover drain off control fluid to the tank via a pressure relief valve; 
         FIG. 4  a diagram of a hydraulic control device in which control fluid can be displaced from the control pressure chambers into the control fluid supply line; and 
         FIG. 5  a side view of a directional valve disk of a hydraulic control block—partly as a cross-section—in an embodiment corresponding to the diagram of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with respect to  FIG. 1  with reference to a directional valve disk such as is used in a hydraulic control block. The invention is, however, not restricted to this specific construction of a hydraulic control device, but can rather be used in hydraulic control devices of almost any construction. 
     The valve disk  1  shown in  FIG. 1  has a base body  3  with a valve bore  25  within which a control slider  26  is movably guided. Different control edges are formed by the valve bore  25  and the control slider  26  via which fluid connections between a fluid supply port  10  and the ports  22 ,  23  for a hydraulic consumer can be controlled. Connections between the consumer ports  22 ,  23  and tank ports  12 ,  13  can equally be controlled. 
     The valve disk shown is made in the load sensing technique. The load pressure applied at the consumer ports  22  and  23  is thus detected and supplied to a load pressure report line  16 . The details of the load sensing technique are not relevant to the present invention and will therefore not be described in more detail. The load sensing technique is, however, familiar to the person skilled in the art. 
     The valve  25  is covered to the right side and to the left side of the base body  3  by control covers  30 ,  31 . Spring chambers  32  and  33  are formed in the control covers  30 ,  31  and a respective biased spring  34  and  35  are located in them. The springs  34 ,  35  are supported at the base body  3  via spring plates  28 ,  29 . The control slider  26  is centered in a middle position by the effect of the biased springs  34 ,  35  and of the spring plates  28 ,  29 . 
     The spring chambers  32 ,  33  moreover form pressure control chambers which can be acted on by a control pressure. Due to the control pressure acting in a spring chamber—e.g.  32 —the control slider  26  experiences a force in the direction of the other spring chamber—e.g.  33 —against the bias of the spring  35  arranged therein. If the force exerted onto the control slider  26  by the control pressure overcomes the bias of the spring  35 , the control slider  26  moves out of its centered position. 
     Pressure control valves  38  and  40  are moreover inserted into the control cover  30  attached to the valve disk  1  at the left hand side. The pressure control valves  38 ,  40  are both connected to a control fluid supply line  18  via the fluid passage  42 . A further fluid passage  43  connects the pressure control valves  38 ,  40  to a control fluid return line  20 . 
     The pressure control valve  38  can be actuated via an electromagnet (not shown) and generates a control pressure proportional to the magnetic force at its outlet. The control pressure generated by the pressure control valve  38  is propagated into the spring chamber  33  via a fluid passage  39 . This control pressure effects a force directed to the left at the control slider  26 . The pressure control valve  40  likewise equipped with an electromagnet is in communication with the spring chamber  32  via the fluid passage  41 . The control pressure generated by the pressure control valve  40  is thus applied in the spring chamber  32  and effects a force directed to the right at the control slider. 
     Furthermore, the spring chamber  32  is connected to a check valve  46  which opens toward a fluid line  48 . A check valve  47  is likewise connected to the spring chamber  33  and opens toward the fluid line  48 . The fluid line  48  leads to a fluid tank via a pressure relief valve  50 . In the directional valve disk shown, this is expediently brought about by connection to the control fluid return line  20 . The outlet of the pressure relief valve  50  can, however, equally be connected to a leakage oil port or to another fluid return line. The pressure relief valve  50  is set to a pressure which corresponds at least to the control pressure which can be generated at a maximum by the pressure control valves  38 ,  40 . 
     The pressure control valves  38 ,  40  each have a control piston which allows control fluid to flow out of the control fluid supply line  18  into the respective spring chamber  32  and  33  respectively until the pressure preset by the magnetic force is reached. If the pressure in the spring chamber is higher than this preset pressure, the control piston allows control fluid to flow out into the control fluid return line  20  via the respective pressure control valve  38 ,  40 . 
     The control piston has a positive excess coverage with respect to the valve housing of the pressure control valve  38  or  40 . This means that once the preset pressure in the spring chamber is reached, the spring chamber is blocked both with respect to the control fluid supply line  18  and with respect to the control fluid return line  20 . If the control piston acts as a block in such a control position, no more control fluid can flow out of the corresponding spring chamber via the pressure control valve. 
     The case will be looked at as an example that the control slider  26  is deflected to the right out of the centered position due to the pressure prevailing in the spring chamber  32 . If the pressure control valve  40  now acts as a block so that no more control fluid can flow out of the spring chamber  32 , the control slider  26  first maintains the deflected position. As soon as the pressure in the spring chamber  32  is increased by an actuation of the control slider  26  directed to the left to a pressure which corresponds at least to the pressure set at the pressure relief valve  50 , control fluid flows via the check valve  46  while bypassing the pressure control valve  40  into the fluid line  48  and via the pressure relief valve  50  to the tank. It is thus possible to guide the control slider  26  back into a centered position despite the blocked pressure control valve  40 . Since the pressure set at the pressure relief valve  50  is above the highest control pressure which the pressure control valves  38 ,  40  can generate, no impairment of normal operation is present. 
     The actuation of the control slider  26  directed to the left, for the bypassing of the blocked pressure control valve  40 , can in particular take place by pressurizing the spring chamber  33 . A machine operator who notices that the pressure control valve  40  is acting as a block—for instance because the hydraulic consumer has not stopped despite a terminated operating procedure—can counteract this at the operating element. The pressure control valve  38  thereby generates a control pressure in the spring chamber  33  and effects a force onto the control slider  26  directed to the left. In addition, a force acts on the part of the spring  35  which corresponds to the control pressure generated in the spring chamber  32  before the blocking onto the control slider  26  deflected to the right. The force exerted by the spring  35  corresponds, however, at least to the spring bias. 
     If the pressure in the spring chamber  32  reaches a value—due to the stress of the spring  35  and the control pressure in the spring chamber  33 —which corresponds at least to the pressure set at the pressure relief valve  50 , the control fluid in the spring chamber  32  flows out via the return valve  46  and the pressure relief valve  50 . The control slider  26  thus returns to the centered position. 
     So that the displacement of the control fluid from the spring chamber  32  is possible by pressurizing the spring chamber  33 , the pressure set at the pressure relief valve  50  may at most correspond to the sum of the pressure equivalent of the spring bias and the maximum control pressure which can be generated. A slightly deflected control slider  26  can then also be guided back via the check valve  46  while displacing control fluid and the pressure relief valve  50  into the centered position. 
     Common pressure control valves are able to generate a control pressure of 30 bar. The bias of the springs  34  and  35  which center the control slider  26  in each case corresponds to a pressure of 5 bar which acts on the side surface of the control slider  26 . The pressure relief valve  50  is accordingly preferably set to a pressure between 32 bar and 35 bar. In this manner, the control slider  36  can be reliably guided back to the centered position, even when the valves  38 ,  40  generating the control pressure act in a blocking manner. The return of the control slider  26  is possible without any mechanical intervention into the valve disk  1 —solely by hydraulic actuation. 
     The mechanisms described for the displacement of control fluid from the left spring chamber  32  naturally also apply in an analog manner to the right hand spring chamber  33 , in particular with respect to a displacement of control fluid via the check valve  47  and the pressure relief valve  50 . 
     In the example described, an unwanted deflection of the control slider  26  is corrected by means of an opposite actuation by the machine operator. A return of the control slider while bypassing a pilot control valve can, however, also be carried out by an automatic electronic control. The position of the control slider  26  is first detected for this purpose. If the control slider  26  does not return to the centered position, although no desired pressure is applied at any of the pressure control valves, the electronic control acts on the control slider  26  in a direction opposed to its deflection by actuation of a pressure control valve. A blocked pressure control valve can be bypassed by the fluid line  48  in this context. 
     Instead of detecting the position of the control slider  26 , a conclusion can also be drawn on the position of the control slider  26  by detection of the operating state such as a rotational speed. 
     A pilot controlled directional valve can also be hydraulically actuated via a pilot control valve designed as a directional valve instead of via two pressure control valves  38 ,  40 . If, in accordance with the invention, a fluid line is provided via which control fluid can be displaced from the control pressure chambers while bypassing the pilot control valve, the control slider of the pilot controlled valve can also be guided back from an actuated position on a displacement of the pilot control valve. The pressure required for this can be built up e.g. by manual actuation. Alternatively, a hydraulic emergency actuation of the control slider can be provided. 
       FIG. 2  shows the diagram of a hydraulic control device  52  which is equipped with two constantly adjustable pilot controlled directional valves  54  and  55  for the control of hydraulic consumers. The directional valves  54  and  55  can have an analog construction to the directional valve disk shown in  FIG. 1 . The respective control sliders of the directional valves  54  and  55  are centered by springs. An electrically actuated pressure control valve  60 ,  61 ,  62  and  63  is connected in each case to the control pressure chambers (not shown) of the directional valves  54  and  55  for the generation of a preset control pressure. The pressure control valves  60 ,  61 ,  62  and  63  are supplied with a control fluid via a control fluid supply line  18 . The control fluid supply pressure is built up by a pump  56  and is fixed by the pressure relief valve  57 . Furthermore, a control fluid return line  20  is connected to each pressure control valve  60 ,  61 ,  62 ,  63  to guide control fluid back to a tank  58 . 
     The control pressure chambers of each directional valve are connected to a fluid line  68  via a respective return valve  64 ,  65 ,  66  and  67 . The return valves  64 ,  65 ,  66  and  67  open in the direction of the fluid line  68 . The fluid line  68  leads to the tank via a pressure relief valve  70 . The pressure relief valve  70  can be opened by manual actuation. An anticavitation valve  71  is connected parallel to the pressure relief valve  70  and opens toward the fluid line  68 . The anticavitation valve  71  can also be integrated into the pressure relief valve  70 . 
     The functional principle of the control device shown in  FIG. 2  substantially corresponds to the functional principle of the control device shown in  FIG. 1  which was expanded to two directional valves. 
     Control fluid can be displaced from each control pressure chamber of the two directional valves  54  and  55  at a pressure, which corresponds to the response pressure of the pressure control valves  70 , while bypassing the pressure control valves  60 ,  61 ,  62 ,  63 . In this case, the control fluid flows to the tank  58  via the corresponding return valve  64 ,  65 ,  66 ,  67 , via the fluid line  68  and the pressure relief valve  70 . The response pressure of the pressure relief valve  70  is above the maximum control pressure which can be generated by the pressure control valves  60 ,  61 ,  62 ,  63 . The response pressure is moreover not above a pressure which corresponds to the spring bias plus the highest control pressure which can be generated by the pressure control valves  60 ,  61 ,  62 ,  63 . 
     The control slider of each directional valve  54  and  55  can thus be reliably guided back into the spring-centered position even when one of the pressure control valves fails. The return of the control slider can in particular take place by hydraulic actuation. 
     It is particularly advantageous in the control device shown in  FIG. 2  that control fluid can be displaced from every control pressure chamber of the directional valves  54 ,  55  into a single common fluid line  68 . In addition, only one single pressure relief valve  70  is required to secure the control pressure chambers. The control device shown in  FIG. 2  can be expanded to further directional valves in a simple manner. Its control pressure chambers are connected to the fluid line  68  via a check valve opening toward the fluid line  68  for security. 
     The response pressure of the pressure relief valve  70  can be set independently of the supply pressure of the control fluid supply line  18 . The control fluid supply line  18  can be set to a higher pressure than the pressure relief valve  70  or to a higher pressure than the highest control pressure which can be generated by the pressure control valves  60 ,  61 ,  62 ,  63  to supply further control fluid consumers or to ensure shorter regulation times. 
     The control device  52  shown in  FIG. 2  additionally makes it possible to bleed the control pressure chambers of the directional valves  54 ,  55  or the control fluid system in a simple manner. For this purpose, the pressure relief valve  70  can be opened by manual actuation. Control fluid flowing into the control pressure chambers can flow out to the tank  58  without hindrance via the check valves  64 ,  65 ,  66 ,  67  and the open pressure relief valve  70 . Trapped air is drained off to the tank  58  together with the control fluid. 
       FIG. 3  represents a diagram of a further hydraulic control device  72 . The control device  72  differs from the control device  52  shown in  FIG. 2  as shown in the following. In this connection, the same reference numerals are associated with the same components. 
     The control pressure chambers of the directional valves  54  and  55  are connected via check valves  64 ,  66  and  65 ,  67  to two separate branches  68   a  and  68   b  of a fluid line. The fluid line  68   a  and  68   b  serves as a relief line in the case of a defect of one of the pilot control valves  60 ,  61 ,  62  and  63 . The control pressure chambers of the directional valves  54  and  55  arranged to the left in  FIG. 3  are connected to the line branch  68   a  via the check valves  64  and  66 . The line branch  68   a  leads, on the one hand, to the pressure relief valve  74  via a further check valve  78 . On the other hand, the branch  68   a  can be connected directly to a tank via a switching valve  76 . The control pressure chambers arranged at the right in  FIG. 3  are connected to the line branch  68   b  via the check valves  65  and  67 . Said line branch leads to the pressure relief valve  74  via the check valve  77 . A switching valve  75  is moreover present by which the line branch  68   b  can be connected to a tank. The switching valves  75  and  76  are each configured such that they connect the respective line branch  68   a  or  68   b  to the tank in a non-actuated position and interrupt a connection between the line branch  68   a  and  68   b  and the tank in an actuated position. 
     As in the control device  52  shown in  FIG. 2 , control fluid can be displaced out of the controlled control pressure chamber of the directional valve  54  via the return vale  64 , the line branch  68   a , the check valve  78  and the pressure relief valve  74  to the tank  58  in the control device  72  in the event of a blockage of a pilot control valve—in the following the pilot control valve  60  as an example. Control fluid can thus be displaced out of the left hand control pressure chamber via the check valve  64  until the control slider has returned to its neutral position by controlling the pilot control valve  61  and by the effect of the restoring springs on the control slider of the directional valve  54 . 
     In addition, the line branches  68   a  and  68   b  can be relieved to the tank independently of one another by the switching valve  75  and  76  respectively. In the normal operating state, the switching valves  75  and  76  are actuated, i.e. they interrupt the connection between the line branches  68   a  and  68   b  and the tank. On a blockage of e.g. the pilot control valve  60 , the switching valve  76  can be switched into the non-actuated position so that the line branch  68   a  is pressure relieved. Control fluid can then flow out of the left hand control pressure chamber of the directional valve  54  via the check valve  64  toward the tank. The control slider of the directional valve  54  can thereupon return to its neutral position. When a control pressure is generated by actuation of the pilot control valve in the right hand chamber of the directional valve  54 , the control slider can even be deflected beyond the neutral position in the direction of a restriction of the left hand control pressure chamber. This makes it possible not only to stop a hydraulic consumer/motor controlled by the directional valve  54 , but also to cause it to carry out a withdrawal or return movement. Important safety demands decisive e.g. for hydraulic driving drives are thereby satisfied. 
     The fluid separation of the line branches  68   a  and  68   b  by the return valves  77  ad  78  enables the line branches to be relieved independently of one another by the switching valves  75  and  76  respectively. Only in this manner can an actuation of a directional valve  54  or  55  take place for the execution of a withdrawal movement, whereas one of the line branches  68   a  or  68   b  is relieved. In addition, the hydraulic consumer controlled by the directional valve can be stopped by a counter-control (counteraction) at the operating element in every case, even when the switching valves  75  and  76  are left in the actuated position. A changeover valve can also be used for the supply of control fluid from the line branches  68   a  and  68   b  to the pressure relief valve  74  instead of the shown check valves  77  and  78 . 
     On a failure of the control electronics, the switching valves  75  and  76  return into a non-actuated position in which the line branches  68   a  and  68   b  are relieved. The hydraulic consumers controlled by the directional valves  54  and  55  are thereby stopped. 
     In  FIG. 4 , a diagram of a further hydraulic control device  80  is shown. The control device  80  is equipped with a pilot controlled, constantly adjustable directional valve  82 . The control slider of the directional valve  82  is spring-centered. The hydraulic control of the directional valve  82  takes place by two pressure control valves  38  and  40  which are each connected to a spring chamber of the directional valve  82 . A pump  56  ensures the supply of the pressure control valves  38  and  40  with control fluid via the control fluid supply line  18 . The pressure in the control fluid supply line  18  is preset by a pressure relief valve  84 . The pressure control valves  38  and  40  are connected to the tank  58  via control fluid return lines  20 . 
     A check valve  85  opening toward the control fluid supply line  18  is connected parallel to the pressure control valve  38  between the outlet of the pressure control valve  38  and the control fluid supply line  18 . A further check valve  86  is connected parallel to the pressure control valve  40  between its outlet and the control fluid supply line  18 . The check valve  86  also opens in the direction of the control fluid supply line  18 . 
     Control fluid can thus be displaced via the check valve  85  into the control pressure supply line  18  from the control pressure chamber connected to the pressure control valve  38 . Control fluid can equally be displaced from the control pressure chamber connected to the control pressure valve  40  via the check valve  86  into the control pressure supply line  18 . 
     The pressure required to displace fluid from a control pressure chamber via the check valve  85  or  86  into the control fluid supply line  18  corresponds to the supply pressure of the control fluid supply line  18 . The supply pressure is set to the highest control pressure which can be generated by the pressure control valves  38  and  40  or slightly higher. So that the displacement of the fluid from a control pressure chamber by hydraulic action on the control slider is possible on the side of the oppositely disposed control chamber, the supply pressure in the control fluid supply line  18  may not be higher than the sum of a pressure corresponding to the spring bias of the centering springs and the highest control pressure which can be generated by the pressure control valves  38  and  40 . 
     If e.g. the pressure control valve  40  jams and blocks the left hand control chamber of the directional valve  82  while the control slider is deflected to the right, a control pressure can be generated in the right hand control pressure chamber by means of the pressure control valve  38 . A pressure which enables the displacement of the control fluid via the check valve  86  into the control fluid supply line  18  arises in the left hand control pressure chamber by the effect of the control pressure generated in the right hand control pressure chamber and by the force of the spring in the right hand spring chamber onto the control slider. The control fluid displaced out of the left hand control pressure chamber either flows via the check valve  84  to the tank  58  or via the pressure control valve  38  into the right hand control pressure chamber. 
     The control slider of the directional valve  82  can thus also be reliably returned to the centered position on a failure of a pressure control valve. The control device in accordance with  FIG. 4  achieves the securing of the control pressure chambers against a blocking of the outflow with a very low effort of additional components. Only check valves  85  and  86  are connected parallel to the pressure control valves  38 ,  40 . 
       FIG. 5  represents a valve disk  90  of a control block which has the structure in accordance with the diagram shown in  FIG. 4 . The structure of the valve disk  90  corresponds in substantial parts to the structure of the valve disk  1  shown in  FIG. 1 . The same components are provided with the same reference numerals and are not described again in the following. 
     The base body  3  of the valve disk  90  with its components and ports as well as the right hand control cover  31  in particular correspond to the respective components shown in  FIG. 1 . The left hand control cover  93  has a spring chamber  32  as a left hand control pressure chamber. The biased spring  34  and the spring plate  28  are located therein. The pressure control valves  38  and  40  are furthermore inserted in the left hand control cover  93 . The pressure control valve  40  generates the control pressure in the control pressure chamber  32 . The pressure control valve  38  generates the control pressure applied in the control pressure chamber  33 . The pressure control valves  38  and  40  are connected to the control fluid supply line  18  or to the control fluid return line  20  via the fluid passages  42  and  43 . 
     The check valves  85  and  86  are additionally arranged in the control cover  93 . The check valve  85  leads from the fluid passage  39  connected to the outlet of the pressure control valve  38  to the fluid passage  42  connected to the control fluid supply line  18 . It opens in the direction of the control fluid supply line  18 . The check valve  86  leads from the outlet of the pressure control valve  40 —the fluid passage  41 —likewise to the fluid passage  42 . The check valve  86  also opens in the direction of the control fluid supply line  18 . 
     A valve disk corresponding to the circuit shown in  FIG. 4  can thus be provided in a particularly simple manner. Only the left hand control cover is expanded by two check valves with respect to a conventional valve disk. Although the valve disk  90  has a security against a blocking of the pressure control valves  38  and  40 , it only has a slightly more complex structure than a conventional valve disk. 
     REFERENCE NUMERAL LIST 
     
         
           1  valve disk 
           3  base body 
           10  fluid supply port 
           12  tank port 
           13  tank port 
           16  load pressure report line 
           18  control fluid supply line 
           20  control fluid return line 
           22  consumer port 
           23  consumer port 
           25  valve bore 
           26  control slider 
           28  spring plate 
           29  spring plate 
           30  control cover 
           31  control cover 
           32  left hand spring chamber/control pressure chamber 
           33  right hand spring chamber/control pressure chamber 
           34  spring 
           35  spring 
           38  pressure control valve 
           39  fluid passage 
           40  pressure control valve 
           41  fluid passage 
           42  fluid passage 
           43  fluid passage 
           46  check valve 
           47  check valve 
           48  fluid line 
           50  pressure relief valve 
           52  hydraulic control device 
           54  constantly adjustable directional valve 
           55  constantly adjustable directional valve 
           56  pump 
           57  pressure relief valve 
           60  pressure control valve 
           61  pressure control valve 
           62  pressure control valve 
           63  pressure control valve 
           64  check valve 
           65  check valve 
           66  check valve 
           67  check valve 
           68  fluid line 
           68   a  fluid line branch 
           68   b  fluid line branch 
           70  pressure relief valve with manual actuation 
           71  anticavitation valve 
           72  hydraulic control device 
           74  pressure relief valve 
           75  switching valve 
           76  switching valve 
           77  check valve 
           78  check valve 
           80  hydraulic control device 
           82  directional valve 
           84  pressure relief valve 
           85  check valve 
           86  check valve 
           90  valve disk 
           93  control cover