VALVE ARRANGEMENT AND METHOD FOR CONTROLLING A LIFTING MECHANISM OR ATTACHMENT

Description

[0001] The invention relates to a valve arrangement for controlling a lifting gear or attachment according to the preamble of patent claim 1 and a method for controlling such a lifting gear or attachment.

[0002] Such valve arrangements are used, for example, to control the working function of mobile work equipment, in particular agricultural vehicles such as tractors with power lifts, combine harvesters with a mower table control, forage harvesters with a bit control or municipal machines with power lifts.

[0003] For example, modern tractors are designed with a rear linkage and a front linkage, with the pressure medium supply often being controlled by a LS control, i.e., dependent on the highest load pressure of the consumers. The rear or front linkage can each be double-acting, whereby the pressure chambers of a lifting cylinder of the linkage, which act, for example, in the "lifting" / "supporting" or "lowering" / "pressing" directions, can be connected to a pressure medium source or a return line via a continuously adjustable directional control valve to implement the respective functions (lowering, pressing, lifting, supporting, neutral position, or floating position).

[0004] The continuously adjustable directional control valve is controlled via an electrical control unit of the working machine, with the setpoint values ​​being set, for example, via a front control unit or a rear control unit of the working device.

[0005] For various operations, such as lowering with cold oil or loosening jammed catch hooks, and for operating various attachments such as cultivators, packer rollers, etc., it is necessary to apply a defined hydraulic flow rate and limited pressure to the pressure chamber of the lifting cylinder, which acts in the "pressing" direction. This can be achieved, for example, by appropriately controlling the continuously adjustable directional control valve and / or via separate pressure control valves (pressure relief or pressure reducing valves).

[0006] The publications DE 10 2004 033 315 A1 and DE 10 2006 004 423 B4 show valve arrangements of this type in which the pressure in the pressure chamber of the lifting cylinder acting in the "pushing" / "lowering" direction and the pressure medium flow paths connected to it is limited by a pressure relief valve.

[0007] The two publications DE 10 2017 219 942 A1 and EP 2 884 118 A1 each show valve arrangements of the type in which the pressure is limited by means of a pressure reducing valve. A similar solution is disclosed in US 10321621 B2.

[0008] Document DE 101 38 389 A1 describes a valve arrangement with a continuously adjustable directional control valve equipped with electronic pressure control. Document DE 195 10 331 A1 discloses a control system for a directional control valve in a hydraulic drive system that maintains a constant operating pressure at the consumer by changing the (volume flow) inlet and outlet cross-section.

[0009] A continuously adjustable (double-acting) directional control valve suitable for controlling such lifting gear is shown in DE 10 2013 207 299 A1, wherein the directional control valve has intermediate positions for passive movements, i.e. movements due to gravity or the like.

[0010] When using hydromechanical valves for pressure control, such as the pressure relief or pressure reducing valves mentioned above, the additional pressure control valves increase the system costs.

[0011] This disadvantage is eliminated in the solutions described below, which regulate the pressure via the directional control valve, using a pressure sensor value, for example, as a feedback variable. However, one disadvantage is that the control performance is highly dependent on the signal processing chain: pressure sensor - processing in the control algorithm - valve control - valve response, and possibly also the dynamics of the pressure supply. In addition, a valve spool of the continuously adjustable directional control valve must pass through a neutral position during control, which has a disruptive effect on the control system.

[0012] In contrast, the object of the invention is to provide a valve arrangement and a method for controlling a mobile work device in which the pressure limitation is simplified compared to the solutions described above.

[0013] This object is achieved with regard to the valve arrangement by the combination of features of patent claim 1 and with regard to the method by the features of the independent patent claim 11.

[0014] Advantageous further developments of the invention are the subject of the subclaims.

[0015] The valve arrangement according to the invention and the method according to the invention are designed to control a lifting cylinder of a lifting gear or attachment of a mobile working device, in particular an agricultural utility vehicle, and have a proportional / continuously adjustable directional control valve. This has a first working connection, which is in pressure medium connection with a first

[0016] Actuation direction (e.g. lifting) effective pressure chamber of the lifting cylinder.

[0017] A further working connection can be brought into pressure medium connection with a second pressure chamber of the lifting cylinder which is effective in a second actuating device (e.g. lowering).

[0018] The directional control valve further has a return / tank connection and a pressure connection. By adjusting a slide of the directional control valve, opening cross-sections (metering orifices) between the connections can be opened or closed in order to control the pressure medium supply to or from the lifting cylinder. According to the invention, the slide is designed with two active-lower control edges, via which, when the slide is adjusted, an inlet cross-section can be opened in the direction of the pressure chamber acting in the second actuation direction and, at the same time, preferably in the opposite direction, an outlet cross-section in the pressure medium flow path can be opened or closed from the pressure chamber acting in the second actuation direction in the direction of the return connection (tank).Accordingly, when the directional control valve is moved into the active-lowering range, the spool assumes its own pressure control position, in which a dynamic pressure equilibrium is established, which depends on the inlet and outlet cross-sections controlled by the active-lowering control edges. This results in a defined pressure drop across the outlet edge, which determines the outlet cross-section, in a specific spool position, without any lifting mechanism movement and the associated "flow disturbance," which is referred to as "block pressure" in the inventive concept. By varying the spool deflection and thus adjusting the aforementioned opening cross-sections, the "block pressure" can then be directly adjusted without any lifting mechanism movement.

[0019] In simple terms, when the slide valve is moved into the active-lowering range, the cylinder's pressure chamber, which acts in the "pressing" / "lowering" direction, is subjected to an incoming pressure fluid volume flow. At the same time, a defined outlet cross-section is also present, resulting in a predictable "block pressure" while the consumer volume flow induced by the lifting cylinder is zero. The "block pressure" is increased by reducing the volume flow from the pressure chamber (closing the outlet cross-section). Increasing the pressure fluid volume flow to the consumer (increasing the inlet cross-section) increases the "block pressure" accordingly.

[0020] According to the invention, the valve arrangement or the method further comprises at least one releasable check valve arranged in a pressure medium flow path to the pressure chamber effective in the first actuation direction from the work connection connectable thereto, which is mechanically coupled to the slide, so that the check valve can be opened at least when the slide is adjusted into the active-lowering range.

[0021] Supporting the lifting cylinder in a predetermined target position is improved if a pilot-operated check valve is assigned to the pressure chamber acting in the support direction. This valve is mechanically opened to allow the pressure medium to flow out. The stroke of the slide is transmitted to the check valve via a coupling mechanism, thus opening it.

[0022] According to an advantageous development of the invention, the directional control valve is designed with adjustment ranges of "floating position" and / or "neutral" and / or "passive lowering" and / or "raising" in addition to the above-described "active lowering" slide adjustment range.

[0023] The "active lowering" slide adjustment range can be located adjacent to the "passive lowering" slide adjustment range. This allows for a continuous, overlapping transition between the two adjustment ranges.

[0024] A load pressure-independent volume flow can be achieved by designing the valve arrangement as an LS valve arrangement with an LS pressure compensator upstream of the directional control valve and an LS pressure regulator of the pump.

[0025] To further improve operational reliability, a pressure relief valve can be provided in a pressure medium flow path between one or both working connections and the respective pressure chamber(s) connected to them.

[0026] This pressure relief valve (secondary pressure relief valve) can absorb overpressure situations induced by faults, so that in such a situation pressure medium can be directed towards the return line.

[0027] This may be necessary, for example, if the lifting gear has to move upwards due to a ground contour.

[0028] The functionality of the method and valve arrangement according to the invention can be further improved by providing a position sensor that detects the position of the lifting gear / attachment. This position sensor can, for example, detect the stroke of the lifting cylinder, a lifting gear angle, or the like. In principle, the signal from this position sensor can be used to calculate the theoretical volume flow or, alternatively, the lifting gear speed, so that the control system can be intervened in the event of deviations from the target values. This will be discussed later.

[0029] Another or additional option for making necessary control interventions due to malfunctions is to use a pressure sensor to measure the pressure in the respective pressure chamber of the lifting cylinder or in the pressure medium flow path leading to it. Depending on the signal from this pressure sensor, the spool position can then be corrected according to the control deviation using a control algorithm in a control unit. This concept will also be explained in more detail in the specific description.

[0030] The control of the directional control valve is particularly simple if it is designed to be electro-hydraulically controlled.

[0031] Preferred embodiments of the invention are explained in more detail below with reference to schematic drawings. They show: Figure 1 a simplified hydraulic circuit diagram of a valve arrangement for controlling a lifting gear; Figure 2a detailed view of a directional control valve of the valve arrangement according to Figure 1 ; Figure 3 a qualitative representation of the adjustment of the directional control valve according to Figure 2 adjusting cross-sectional profiles; Figure 4 a part of the representation according to Figure 3 , showing the course of a "block print" which is established according to the inventive concept; Figure 5 a representation according to Figure 3 , in which the "block pressure" increases comparatively moderately by a suitable control of the directional control valve and Figure 6 one also on Figure 3 based representation in which the cross-sectional profiles are modified with regard to a variance of the return characteristics.

[0032] The invention is explained below using a tractor's hitch assembly as an example. In principle, however, the valve assembly according to the invention can also be used with other double-acting consumers on mobile implements.

[0033] As explained at the beginning, modern tractors are designed with a lifting mechanism 2 which has a double-acting lifting cylinder 4 (also called lifting mechanism cylinder) which is controlled via a valve arrangement 6 according to the invention. In the illustrated embodiment, the lifting cylinder 4 is designed as a differential cylinder with a bottom-side pressure chamber 8 and an annular chamber 10, which can be connected via the valve arrangement 6 to a variable displacement pump 12 or a pressure medium sink, in this case a tank T. In the illustrated embodiment, the lifting mechanism 2 is designed as a rear lifting mechanism, with a piston rod 14 of the lifting cylinder 4 actuating an arm 18 pivotably mounted on a lifting shaft 16, as well as other coupling elements to which, for example, an attachment such as a packer roller 20, a seed drill or a plough is attached.

[0034] The illustrated valve arrangement 6 for controlling the lifting gear 2 is designed as a LS control system with a continuously adjustable directional valve 22 forming metering orifices and an LS pressure compensator 24 connected upstream of the valve. In a conventional manner, such an LS control system ensures that the pressure upstream of the metering orifice being controlled is only a certain pressure difference above the individual load pressure, regardless of the pump pressure. However, the invention in no way requires such an LS control system.

[0035] According to the hydraulic circuit diagram in Figure 1The pressure connection of the variable displacement pump 12, which is designed, for example, as an axial piston pump, is connected via an inlet channel 26 to a pressure connection P' of the pressure compensator 24. This is preloaded into a basic position by a spring 28, in which an output connection D of the pressure compensator 24 is blocked. The pressure in the inlet channel 26 is tapped via a control channel 30 and guided to a control surface of the pressure compensator 24, which control surface acts in the opening direction against the force of the spring 18. As mentioned, the spring 28 and the pressure in an LS channel 32 act in the opposite direction. This LS channel is pressurized with the pressure downstream of the metering orifice controlled by the directional control valve 22.Depending on this metering orifice opening, the pressure compensator 24 adjusts itself to a control position in which the pressure drop across the metering orifice is kept constant and corresponds approximately to the pressure equivalent of the force of the spring 28, so that a constant pressure medium volume flow across the metering orifice (directional control valve 22) is ensured.

[0036] The delivery pressure of the variable displacement pump 4 present in the inlet channel 26 is adjusted in an LS control system depending on the highest load pressure applied to the tractor's consumers. This load pressure—in the illustrated embodiment, the load pressure at the lifting cylinder 4—is reported to an LS pressure regulator 34, via which the variable displacement pump is adjusted in a conventional manner. However, as mentioned above, such an LS control system, in which the pump's delivery flow is adjusted depending on the highest load pressure, is not a prerequisite for the system according to the invention.

[0037] An output port D of the pressure compensator 24 is connected via an inlet line 36 to a pressure port P of the directional control valve 22. This also has a return port T, an LS relief port LSE, and another pressure port P', which and the LS relief port LSE are in pressure medium connection with the LS line 32. The LS relief port LSE is assigned an LS output port LSR, which in turn is connected to the tank T via an LS return line 43. As will be explained in more detail below, LS pressure relief can be carried out by opening the LS return line.

[0038] The directional control valve 22 also has an output port P‴, which is connected via a channel 38 to the pressure port P' and thus also to the LS line 32. A working port A is connected via a working line 40 to the bottom-side pressure chamber 8 of the lifting cylinder 4. Another working port B is connected via a working line 42 to the annular chamber 10 of the lifting cylinder 4, through which the piston rod 14 passes.

[0039] In the representation according to Figure 1 a slide 44 of the directional control valve 22 is pre-tensioned into a neutral position via centering springs 46, 48, which is described below with reference to Figure 2 is explained in more detail.

[0040] The adjustment of the directional control valve 22 is carried out via a pilot valve arrangement 50, which is shown in the illustration according to Figure 1formed by two electrohydraulic pilot control elements 52, 54. The pilot valve arrangement 54 is controlled by a control unit 56, which is implemented with suitable control software. The control oil required to control the pilot valve arrangement 50 is provided via a control oil supply 58, which is connected to the two pilot control elements 52, 54 via a pilot line 60. The control oil required to actuate the pilot valve arrangement 50 can be discharged to tank T via the LS return line 43.

[0041] The position of the slide 44 is detected by a position sensor 62 and reported to the control unit 56, so that the control of the pilot valve arrangement 50 also takes place in dependence on the signal of the position sensor 62.

[0042] Further details of the directional control valve 22 are shown in the enlarged illustration in Figure 2 explained.

[0043] As explained above, the spool 44 of the directional control valve 22 is preloaded into a neutral position N by the two centering springs 46, 48, in which the ports P, P", P‴ and A are blocked. The working port B is connected to the tank port T, so that the pressure medium can flow from the working line 42 to the tank T via a tank channel 64. Furthermore, the LS relief ports LSE and LSR are connected to each other, so that the LS line to the tank T is also relieved.

[0044] When adjusting the valve slide 44 to the left (view from Figure 2) in the "lifting" area, a metering orifice 65 connecting the two connections P and P‴ is opened so that pressure medium can flow from the pressure connection P via the connection P‴ and the channel 38 to the pressure connection P" and then from there via the working connection A and the working line 40 into the bottom-side pressure chamber 8 of the lifting cylinder 4. The annular space 10 of the lifting cylinder 4 is connected to the tank T via the working line 42, the working connection B, the tank connection T and the tank channel 64 so that the pressure medium can flow out of the decreasing annular space 10. The piston of the lifting cylinder 4 extends and the working device is raised or at least supported accordingly.

[0045] When the valve slide 44 is moved from its Figure 2shown basic position to the right, it is adjusted into a "lowering passive (S passive) area" in which the connections P, P‴ and P" are blocked off. The two working connections A, B are both connected to the tank connection T. The LS pressure relief is also open towards the LS return line 43, so that the attachment, in this case the packer roller 20, is subjected to the lifting cylinder 4 in the retraction direction due to its own weight, so that the pressure medium is displaced from the shrinking pressure chamber 8 towards the tank T. From this, pressure medium can flow into the enlarging annular space 10.

[0046] Upon further adjustment of the slide valve 44, the "lowering active (S active) range" is reached, in which, similar to the lifting range, the two connections P and P‴ are connected to each other, so that pressure medium is guided from the inlet line 36 via the metering orifice 65 into the channel 38. In the S active range, an inlet cross-section 66 between the connections P" and B is opened by a control edge of the slide valve 44. Simultaneously, or at least in an overlapping manner, an outlet cross-section 68 is opened via a further control edge, through which the working connection B and thus the annular space 10 acting in the "lowering / pressurizing" direction is connected to the tank T. The adjustment of the cross-sections 66, 68 preferably takes place in opposite directions.

[0047] As explained at the beginning, without any lifting mechanism movement, a defined pressure drop results across this outlet cross-section 68 at a specific slide deflection, whereby this defined pressure drop ("block pressure") can be varied by varying the slide deflection, i.e. by opening or closing the inlet cross-section 66 or the outlet cross-section 68. The inlet cross-section 66 and the outlet cross-section 68 preferably change in opposite directions, so that when the volume flow from the consumer decreases, the "block pressure" is increased, while when the volume flow to the consumer decreases, the "block pressure" is reduced. Depending on the slide deflection, a dynamic pressure equilibrium is thus established, enabling fine pressure adjustment on the lifting cylinder 4 in the "lowering" direction.

[0048] If the slider 44 is further adjusted to the right (view from Figure 2) a floating position ("Float") is set, in which the two working ports A, B are both connected to the tank T. Furthermore, the LS pressure relief is effective.

[0049] In this floating position, the lifting gear 2 is not subjected to any force, so that the working device rests on the ground solely due to its own weight.

[0050] In order to prevent an undesired lowering of the lifting cylinder 4, the Figure 1In the illustrated embodiment of the invention, a pilot-operated check valve 70 is provided in the working line 40, which, in its closed position, prevents pressure medium from flowing out of the pressure chamber 8. The check valve 70 is mechanically opened when the slide 44 is adjusted into the "float" and "active / passive lowering" ranges. This is achieved by mechanically coupling the slide 44 to the check valve 70, so that the latter is opened when the slide 44 is adjusted into the aforementioned ranges.

[0051] Instead of the check valve 70, corresponding check valves can also be integrated into the directional control valve 22. These serve, in particular, to prevent the attachment / power lift from sinking due to a leak.

[0052] In one embodiment of the invention, according to Figure 1Optionally, a pressure sensor 72 can be provided, via which the pressure in the annular space 10 or in the working line 42 is recorded and reported to the control unit 56. If the signal from the pressure sensor 72 deviates from the setpoint, for example due to an inaccurate deflection of the slide valve 44 or other disturbances, the slide valve position can then be corrected according to the control deviation via the control algorithm of the control unit 56. In order to efficiently reduce excessive pressure due to pressure medium volume flows, the "passive lowering" range can be included in this control in order to only use one control edge opening in the "tank / return" direction without any incoming pressure medium. This is necessary, for example, if the attachment has to deviate due to a rising bottom contour at a constant pressure / bottom immersion depth.

[0053] In order to absorb overpressure situations induced by disturbances, a secondary pressure relief valve 74, 76 can be provided according to the invention, via which the pressure in the working line 42 or the working line 40 is limited, so that when the pressure set at the pressure relief valve 74, 76 is exceeded, the pressure medium releases pressure medium in the direction of the tank T. This can be necessary, for example, if the lifting gear 2 has to give way due to a ground contour, so that the pressure relief valve 74 in the working line 42 then opens.

[0054] Another option is to detect the deflection of the lifting cylinder 4 by a position sensor 78. In the embodiment according to Figure 1This is designed as an angle sensor that detects the angle of attack of arm 18. This makes it possible, for example, to calculate the theoretical volume flow on the "pushing side" (alternatively, the cylinder speed / hoist speed, which are approximately proportional over large ranges). Knowing this theoretical volume flow can, for example, make the following system improvements.

[0055] This allows the pump to actively refill the tank with the flow rate up to a theoretical flow rate during lowering without causing pressure to build up. Due to tolerances, this theoretical flow rate is not fully utilized.

[0056] The resulting advantage is better filling of the lifting cylinder 4, allowing for a continuous transition to pressure control. A further advantage is that during lowering, the slide 44 can be deflected further without building up pressure. This may potentially maximize the single-acting lowering speed, since the control edge determining the discharge cross-section 68 is then fully open.

[0057] A further improvement is that when evading a floor contour, the tank drain on the "pressure side" can be opened in a controlled manner, depending on the theoretical pressure medium flow rate and the target pressure. The advantage of this is that when evading, even without closed-loop pressure control, the pressure only increases moderately, or in a system with pressure control, the regulator is supported by the pilot control.

[0058] Furthermore, it is possible to limit the flow rate during lowering in "push pressure control" by reducing the "push pressure." If the "push pressure" is already 0 bar and lifting gear 2 is still moving too fast, the slide position can be further adjusted to further close the "carry" of the discharge edge toward tank T. This then corresponds to switching to the "lowering range" without pressure control.

[0059] When "lowering" without pressure control, the system can automatically switch to "pressing pressure control" if the volume flow falls below a minimum speed.

[0060] For further clarification, the Figure 3Qualitative cross-sectional profiles are shown that occur during the previously described adjustment of the slider 44. The "Lift", "Neutral", and "Free travel / Float" ranges, in particular their exact beginning and the gradients, are shown only as examples in this figure.

[0061] The Figure 3 The upper cross-sectional profile shows the opening cross-section in the previously described adjustment ranges of the slide 44 between the pressure chamber 8 ("carry") and the return (T). Accordingly, this cross-section is controlled open in the two "lower" ranges and is then open in the free position ("float").

[0062] The opening cross-section from the pump 12 in the direction of the pressure chamber 8 ("carrying") is only controlled in the "lifting" range and decreases with increasing adjustment of the slide 44 in the "neutral" direction.

[0063] According to the inventive concept, at the end of the "lowering passive" adjustment range, a defined flow cross-section 68 is directed from the annular space 10 ("press") to the return flow (T). This cross-section is then enlarged again during the transition from the "lowering active" range to the "float" range.

[0064] In the adjustment range "Lowering active", an inlet cross-section is then opened according to the above explanations, which enables the pressure medium connection between the pump 12 and the annular space 10 and which is then closed again in the free passage ("Float") range.

[0065] In this area, both pressure chambers 8 ("carrying") and 10 ("pressing") are connected to the tank T so that pressure medium can flow around.

[0066] As shown in the detailed illustration in Figure 4As shown, without any movement of the lifting gear 2 (volume flow induced by the lifting gear 2 equals 0), a predictable pressure is established in the "lowering active" range, which is referred to in this application as "block pressure." As mentioned above, reducing the pressure medium flow from the consumer (lifting cylinder 4) to tank T increases the "block pressure," while reducing the pressure medium flow to the consumer decreases the "block pressure."

[0067] By appropriately adjusting the discharge cross-section towards tank T, the gradient of the "block pressure" can be influenced with increasing deflection and thus increasing pump flow. This is quite clearly shown in Figure 5 which shows that with increasing inflow quantity and simultaneous enlargement of the outlet cross-section 68 towards the tank T, the increase in the "block pressure" compared to the increase according to Figure 4can be flattened so that the "block pressure" only increases moderately.

[0068] As in Figure 6 As indicated, in the "Lowering active" area, the cross-sectional profile when opening the pressure medium connection from the pressure chamber 8 ("carry") to the return (T) as well as the profile of the opening cross-section from the annular chamber 10 ("press") to the return (T) can also be varied. Thus, as in Figure 6 indicated by dashed lines, the full effective discharge cross-section 68 in the direction of "lowering", ie the cross-section determining the pressure medium connection from the pressure chamber 8 to the tank T, must already be set to a maximum before the Figure 6 The pressure control range indicated by the dashed line is reached. As shown in Figure 6 As indicated by the term "variance" in the opening cross-sectional profiles, these areas can also overlap each other to a greater or lesser extent.

[0069] Disclosed are a valve arrangement and a method for controlling a lifting gear or attachment of a mobile work device, wherein a double-acting lifting cylinder is controlled via a directional control valve. This valve is designed with two control edges that simultaneously change an inlet cross-section and an outlet cross-section, allowing, for example, a pressure adjustment on the lifting cylinder in the "lowering" direction. A releasable check valve is mechanically coupled to the spool of the directional control valve, so that the check valve can be opened at least when the spool is adjusted to the active lowering range. List of reference symbols:

[0070] .2Lifting gear 4Lifting cylinder 4 6Valve arrangement 8Pressure chamber 10Annular chamber 12Variable displacement pump 14Piston rod 16Lifting shaft 18Arm 20Packer roller 22Directional valve 24Pressure compensator 26Inlet channel 28Spring 30Control channel 32LS line 34LS pressure regulator 36Inlet line 38Channel 40Working line 42Working line 43LS return line 44Slide 46Centering spring 48Centering spring 50Pilot valve arrangement 52Pilot control element 54Pilot control element 56Control unit 58Control oil supply 60Pilot line 62Position sensor 64Tank channel 65Measuring orifice 66Inlet cross-section 68Outlet cross-section 70Check valve 72Pressure sensor 74Pressure relief valve 76Pressure relief valve 78Position sensor

Claims

1. Valve assembly for controlling a lifting mechanism (2) or a mounted implement of a mobile working implement, in particular of an agricultural utility vehicle, wherein the lifting mechanism or the mounted implement has a lifting cylinder (4), wherein the valve assembly comprises a continuously adjustable directional control valve (22), which has a working connection (A), which can be brought into pressure medium connection with a pressure chamber (8) of the lifting cylinder (4) effective in a first actuating direction and which has a further working connection (B), which can be brought into pressure medium connection with a second pressure chamber (10) of the lifting cylinder (4) effective in a second actuating direction, wherein the directional control valve (22) has a tank connection (T) and a pressure connection (P), wherein opening cross sections between the connections can be opened up or shut by adjusting a slide (44) of the directional control valve (22), the slide (44) has two active-lowering control edges via which, when the slide (44) is adjusted into an active-lowering region (loweringactive), an inlet cross section (66) can be opened up or shut in the direction of the pressure chamber (10) effective in the second actuating direction and at the same time a discharge cross section (68) can be opened up or shut, preferably in the opposite direction, from the pressure chamber (10) effective in the second actuating direction in the direction of the tank connection (T), characterized by at least one unlockable nonreturn valve (70) which is arranged in a pressure medium flow path to the pressure chamber (8) effective in the first actuating direction from the working connection connectable thereto and which is mechanically coupled to the slide (44) so that the nonreturn valve (70) can be opened up at least when the slide (44) is adjusted into the active-lowering region (loweringactive).

2. Valve assembly according to Claim 1, wherein the directional control valve (22) has a slide adjustment range of floating position and / or neutral and / or passive-lowering (loweringpassive) and / or lifting.

3. Valve assembly according to Claim 2, wherein the slide adjustment range of active-lowering (loweringactive) is adjacent to the slide adjustment range pf passive-lowering (loweringpassive).

4. Valve assembly according to any one of the preceding claims, wherein an LS pressure governor (24) is provided in an inlet to the directional control valve (22).

5. Valve assembly according to any one of the preceding claims, having a pressure control valve (74, 76) in a pressure medium flow path between a working connection (A, B) and the pressure chamber (8, 10) connectable thereto.

6. Valve assembly according to any one of the preceding claims, having a position sensor (78) for detecting a position of the lifting mechanism / mounted implement.

7. Valve assembly according to any one of the preceding claims, having a pressure sensor (72) in a pressure medium flow path to one of the pressure chambers (8, 10) from the working connection (A, B) connectable thereto.

8. Valve assembly according to Claim 6 or 7, having a control unit (56), which is configured, preferably in the manner of a closed-loop control, to adjust / correct the slide position depending on the pressure signal of the pressure sensor (72) and / or depending on the signal of the position sensor (76).

9. Valve assembly according to any one of the preceding claims, wherein the directional control valve (22) is electro-hydraulically activated.

10. Method for controlling a lifting mechanism or a mounted implement of a mobile working implement, in particular of an agricultural vehicle, wherein the lifting mechanism or the mounted implement has a lifting cylinder (4), wherein the method has a continuously adjustable directional control valve (22), which has a working connection (A), which can be brought into pressure medium connection with a pressure chamber (8) of the lifting cylinder (4) effective in a first actuating direction and which has a further working connection (B), which can be brought into pressure medium connection with a second pressure chamber (10) of the lifting cylinder (4) effective in a second actuating direction, wherein the directional control valve (22) also has a tank connection (T) and a pressure connection (P), wherein opening cross sections between the connections can be opened up or shut by adjusting a slide (44) of the directional control valve (22), and wherein the method comprises the following steps: - adjusting the slide (44) into an active-lowering adjustment region (loweringac-tive) in which - via a first active-lowering control edge, an inlet cross section (66) to the pressure chamber (10) effective in the second actuating direction is opened up, and at the same time or in overlap, via a second active-lowering control edge, a discharge cross section (68) is opened up or shut from the pressure chamber (10) effective in the second actuating direction to the tank (T) so that a dynamic pressure equilibrium arises, and - opening up at least one unlockable nonreturn valve (70) which is arranged in a pressure medium flow path to the pressure chamber (8) effective in the first actuating direction from the working connection connectable thereto and which is mechanically coupled to the slide (44) when the slide (44) is adjusted into the active-lowering adjustment region (loweringactive).