DE29622693U1 - Check valve arrangement - Google Patents

Description

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ifflf' [File:ANM\MA7211B1.doc] Description, 18.02.97 &iacgr; II ' ·
Utility model application
Mannesmann Rexroth GmbH Description

CHECK VALVE ARRANGEMENT

The invention relates to a check valve arrangement according to the preamble of patent claim 1 and a valve arrangement provided with such a check valve arrangement.

In mobile hydraulics, such valve arrangements are used, among other things, to control double-acting pressure cylinders, such as those used as front or rear power lifts on agricultural tractors. These power lifts can be used to operate various peripheral devices, such as mowers, packers, plows, cultivators, rollers, etc.

Under certain operating conditions, it may be necessary to keep a load in a raised position, and this position must be maintained even when the hydraulic supply is switched off. For example, a mower must be secured in a raised position when the tractor is approaching the meadow to be mown or when it is parked in the yard.

Usually, the power lifts are controlled by proportional valves that have a zero position in which the working lines leading to the power lift and a pump line are shut off. However, such proportional valves are usually designed as slide valves, so that the hydraulic fluid in the working lines cannot be shut off without leakage. Therefore, the working lines are provided with pilot-operated check valves, which can be used to lock the hydraulic fluid between the consumer and the check valve without leakage.

[Fi!e:ANM\MA7211B1 .doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

Unlocking the check valve allows the hydraulic fluid to flow back to the proportional valve.

DE 27 35 559 C2 shows a valve arrangement in which a main cone can be opened with the help of a push-on piston for unlocking. The main piston is designed with a pre-opening that is closed by a closing body. This can be brought into its open position by controlling the push-on piston via a tappet, so that the spring side of the main cone is relieved and the main cone is opened by the load pressure applied to its differential surfaces. At the same time, the push-on piston opens a tank connection so that the hydraulic fluid from the consumer can flow past the raised main cone to tank T without the need for a return to a control directional valve that is connected upstream of the check valve arrangement. The known combination of control valve and check valve arrangement can only be used to connect a single-acting lifting cylinder, the cylinder chamber of which can be connected either via the control valve to a pump or via the push-on piston to the tank T.

When the closing body is lifted, the main cone is suddenly opened due to its surface difference, so that the hydraulic fluid is released relatively quickly towards the tank without it being possible to influence the return flow velocity.

DE-44 36 548 A1 shows a valve arrangement for controlling double-acting lifting cylinders, in which a check valve arrangement is connected in each working line. This in turn contains a main cone, which is designed with a surface difference (i.e. the valve seat diameter is smaller than the effective outside diameter of the main cone), which can be opened by a push-on piston. This valve arrangement can indeed

[File:ANM\MA7211B1.doc] Description, 18.02.97 J.*.,·*·..

Utility model application Mannesmann Rexroth GmbH

double-acting consumer can be controlled, but since the main cone is again designed with a difference in area, the main cone is immediately opened into its open position after lifting off its valve seat, so that a maximum flow cross-section is immediately made available. Even with this known valve arrangement, it is not possible to influence the return flow.

In contrast, the invention is based on the object of creating a check valve arrangement and a valve arrangement with such a check valve arrangement, with which the return flow of the hydraulic fluid from the consumer can be controlled with minimal circuit effort.

This object is achieved with regard to the check valve arrangement by the features of patent claim 1 and with regard to the valve arrangement by the features of patent claim 8.

By designing the main cone without a surface difference and determining the opening process by applying the push-on piston, a sudden tearing of the main cone can be prevented. The opening stroke of the main cone can then be influenced by appropriately controlling the push-on piston, so that the volume flow of the returning hydraulic fluid can be regulated (flow control).

The spring side of the main cone can be connected to the tank through a connecting hole in the push-on piston. This is opened during the axial movement of the push-on piston, so that the spring chamber of the main cone is relieved towards the tank and can therefore be lifted off its valve seat more easily.

[Fi!e:ANM\MA7211B1.doc] Description, 18.02.97 · j j

Utility model application ··· *'

Mannesmann Rexroth GmbH

The overall length can be minimized if the main cone is designed with a hub-shaped projection into which a tappet of the push-on piston dips in order to lift the closing body of the pre-opening. The annular face of the hub-shaped projection serves as a contact surface for the adjacent face of the push-on piston. These contact surfaces are machined in such a way that they lie against one another in a sealing manner, so that the interior of the hub-shaped projection and the outer circumference of the main cone are sealed against one another, so that when the connecting bore is open, no pressure equalization can take place between the pressure prevailing at the valve seat and the pressure prevailing at the pre-opening.

In addition to the connecting bore, the tank connection is controlled via recesses through which a space downstream (seen in the return flow direction) of the main cone seat can be connected to the tank.

The discharge control can be varied particularly finely by piloting the ram via a pilot valve.

In the initial position, a gap is formed between the opposing faces of the main cone and the push-on piston, which is larger than the distance between the push-on piston's tappet and the closing body. This measure ensures that the closing body is opened first before the push-on piston runs into the main cone.

Since the check valve arrangement according to the invention can establish a connection from the consumer to the tank, the control valves connected upstream for controlling the consumer can be designed more simply, since no connection to the tank has to be established via the latter.

[File:ANM\MA7211S1 .doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

The valve arrangements provided with a check valve arrangement according to the invention are used particularly advantageously in the control of double-acting cylinders, with each working line to the cylinder being assigned its own check valve arrangement. These are each pre-controlled by a pilot valve, via which a control side of the valve slide of the upstream directional control valve is also controlled in order to direct the hydraulic fluid flow to one of the cylinder chambers.

By applying current to one of the pilot valves, the valve spool is brought into a working position in which the hydraulic fluid is guided through one check valve arrangement to the consumer, while the displaced hydraulic fluid flows back from the consumer to the other check valve arrangement, which is unlocked by the control pressure of the pilot valve, so that the hydraulic fluid can flow to the tank via the check valve arrangement.

The pilot valves used are preferably pressure reducing valves in cartridge design.

Other advantageous developments of the invention are the subject of the further subclaims.
25

A preferred embodiment of the invention is explained in more detail below using schematic drawings. They show:

Fig. 1 a valve arrangement with a directional valve and a check valve arrangement, which can be controlled via a pilot valve,

Fig. 2 is a detailed view of the directional control valve and a check valve arrangement from Fig. 1. and

[File:ANM\MA7211B1.doc] Description, 18.02.97 J . . . .

Utility model application ··· ·· ·»

Mannesmann Rexroth GmbH

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Fig. 3 a schematic circuit diagram of the valve arrangement from Fig. 1

Fig. 1 shows a sectional view through a valve arrangement which is designed in disc construction. The valve disc or plate forms a valve housing 2 in which receiving bores for a continuously adjustable directional control valve 4 and two check valve arrangements 6, 8 are formed.
10

The two check valve arrangements 6, 8 are arranged along a common axis which runs parallel to the axis of the directional control valve 4.

Two pilot valves 10, 12 in cartridge design are also screwed into the valve housing 2, via which the directional control valve 4 and the two check valve arrangements 6, 8 can be controlled. These pilot valves 10, 12 are, for example, electrically operated pressure reducing valves, via which the pressure at a pump connection P, which is led via a pump channel 14 to radial connections 16 of the pilot valves 10, 12, can be reduced to a system pressure at the axial output connection 18 of the respective pilot valve 10, 12. Each pilot valve 10, 12 also has a radial connection 19, which opens into a tank channel 20, via which excess hydraulic fluid can be returned to a tank connection T.

The output connection 18 of each pilot valve 10, 12 opens into a control channel 22 or 24, which open into a valve bore 26 in which a valve slide 28 of the directional control valve 4 is guided. The two control channels 22, 24 open on both sides of the valve slide faces, so that they are pressurized with the pressure in the respective control channel 22, 24, i.e. with the output pressure of the pilot valve 10 or 12.

[File:ANM\MA7211B1.doc] Description, 18.02.97

Utility model application

Mannesmann Rexroth GmbH

The valve slide 28 is also pre-tensioned into its illustrated zero position by two compression springs ₃₀₇₃₂ acting on the front sides. The valve springs are supported on the inner bore by closure caps 34 which are screwed into the valve housing 2 and thus form the axial closure of the valve bore 26. The other end of the compression springs 32, 34 rests on a spring plate 36 which, in the illustrated zero position, is supported with a peripheral section on a housing shoulder.

The detailed structure of the directional control valve 4" and a check valve arrangement 6 is explained below using Fig. 2, which shows these components in an enlarged view.

In the left end section of the valve slide 28 in Fig. 2, a blind hole-shaped inner bore 38 is made, in which a pressure compensator piston 40 is guided so as to be axially displaceable. In the zero position shown in Fig. 2, the pressure compensator piston 40 - hereinafter referred to as piston 40 - rests with a radial collar 42 on a contact shoulder 46 of the inner bore 38. The piston 40 is preloaded in the direction of this contact position by a control spring 44. The control spring 44 is in turn supported on a locking screw 46, which is screwed into the annular jacket of the valve slide 28 and which, together with the annular jacket end face, forms the contact surface for the spring plate 36 (left in Fig. 2). The piston 40 has a connecting bore with an axial blind hole and a radial throttle bore, which opens at the outer circumference of the piston 40.

In the area of the radial collar 42, a compensating bore 52 is formed in the annular casing of the valve slide 28, which opens into a control annular chamber 54, which is connected to a control connection LS via a line indicated by dashed lines, so that a control pressure corresponding to the load pressure can be applied in the spring chamber.

[Fi!e:ANM\MA7211B1.doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

In the middle area of the valve slide 28, an output bore star 56 is formed, to which two annular spaces 58 and 60 are assigned, which are connected to connecting channels 62 and 64, respectively. These connecting channels 62, 64 lead to the inlet connections of the check valve arrangements 6 and 8, respectively. The annular spaces 58, 60 are provided with chamfers 66, which enable fine control of the output bore star 56 when the valve slide 28 is axially displaced. The axial length of the piston 40 is selected such that in the zero position (Fig. 2) the output bore star 56 is closed by the right end section of the piston 40. The radial throttle bore 50 is then closed by the inner circumferential wall of the inner bore 38.

In the area of the inner end section of the inner bore 38, an inlet bore star 68 opens, which in the zero position is closed by a web 70 that is formed between the pump channel 14 and a pump branch channel 14a. The two pump channels 14, 14a run approximately in the radial direction to the valve bore 26. The inner bore 38 is in turn provided with chamfers 66 for fine control in the area of the web 70.

The inlet bore star 68 and the pump channels 14, 14a with the corresponding annular spaces form an adjustable measuring orifice, while the piston 40 and the outlet bore star 56 form a measuring throttle, via which the system pressure downstream of the measuring orifice is throttled to the load pressure in the connecting channels 62 and 64. By connecting the measuring orifice and the measuring throttle in series, it is ensured that the pressure drop across the measuring orifice (inlet bore star 68) remains constant regardless of the pressure in the connecting channels 62, 64.

[File:ANM\MA7211B1.doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

As already mentioned at the beginning, the spring chambers of the compression springs 30, 32 are connected to the control channels 22 and 24, respectively, so that the control pressure prevails at the front sides.

The two check valve arrangements 6, 8 have an identical structure, so that for the sake of simplicity only the check valve arrangement 6 shown in Fig. 2 is described. This has a main cone 72, which is pressed against a valve seat 76 via a check spring 74 with a conical section, so that in this closed position the connection from the connecting channel 62 to a working channel 78 is interrupted.

In the embodiment shown, the main cone 72 is designed with a pre-opening 80, which is closed with a ball 82, which is pressed against a pre-opening seat via the check spring 74 and a spring plate. A throttle bore 8_4 opens into the spring chamber of the check spring 74, which in turn opens into the working channel 78. A hub-shaped projection 86 is formed on the right-hand end section in Fig. 2, on the outer circumference of which an annular groove 88 is provided. The inner bore of the hub-shaped projection 86 tapers conically towards the pre-opening 80.

The annular face 90 of the projection 86 serves as a contact surface for a push-on piston 92, which is guided coaxially to the main cone 72 in a receiving bore 94. At its end section adjacent to the main cone 72, the push-on piston 92 has a tappet 96, which plunges into the inner bore of the hub-shaped projection 86 and whose end section has a smaller diameter than the pre-opening 80, so that the tappet 96 can also plunge into this pre-opening 80. The push-on piston 92 is pre-tensioned by a spring 98 against the front side of an annular space 100 of the receiving bore 94. In this stop position, there is a predetermined gap between the annular face 90 and the

fFi!e:ANM\MA7211B1.doc] Description, 18.02.97

Utility model application

Mannesmann Rexroth GmbH

adjacent end face of the push-on piston 92, which is greater than the distance of the end section of the plunger 96 to the outer circumference of the ball 82.

On the outer circumference of the push-on piston 92, in the area of a tank channel 102 opening into the receiving bore 94, a plurality of recesses 104 distributed over the circumference are formed, via which, when the push-on piston 92 is axially displaced, the tank channel 102 can be connected to the working channel 62, which opens into the receiving bore 94 via an annular space 109.

Furthermore, a relief bore is formed in the push-on piston 92, the axial section 108 of which opens into the left-hand end face as shown in Fig. 2 and which opens via a radial section 110 between the annular space 106 of the tank channel 102 and the annular space 100 (basic position). The annular space 106 of the tank channel 102 is in turn provided with fine control phases 107. The axial section 108 opens radially within the annular end face into the inner bore of the projection 86.

As can also be seen in Fig. 2, the left part of the main cone 72 is guided in the inner bore of a closure plug 112, which is screwed into the left end section of the receiving bore for the check valve. The control channel 22 is extended from the left end face of the valve slide 28 to the area of the closure plug 112 and from there - as indicated by dashed lines - to the annular space 100, so that the control pressure applied by the pilot valve 10 can be adjusted in this, which acts on the push-on piston 92 against the force of the spring 98.

As can be seen from Fig. 1, the tank channel 102 is led to the annular space 106 of the (right) check valve arrangement 8 and the annular space 100 is connected to the annular space 106 via the dashed line.

[Flle:ANM\MA7211B1.doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

th channel is connected to the control channel 24, so that the push-on piston 92 of the check valve arrangement 8 is acted upon by the control pressure applied by the pilot valve 12 in the direction of the main cone 72, while the push-on piston 92 of the check valve arrangement 6 is controlled by the pilot valve 10.

When the main cone 72 of the check valve arrangement 8 is lifted, a connection to a working channel 114 is opened, which is led to a working connection B of the valve arrangement. The working channel 78 of the check valve arrangement 6 is led to a working connection A. The two connections A, B can be connected to a consumer, for example to the cylinder chamber or the annular chamber of a lifting cylinder 116. The connection can be made, for example, via hose couplings.

A connecting channel is provided between the tank channel 102 and the working channel 114, into which a conventional check valve 118 is connected, which allows a flow from the tank channel 102 to the working channel 114 in the event of an excessive pressure build-up in the tank T, but prevents a reverse flow.

Fig. 3 shows a schematic hydraulic circuit diagram of the valve arrangement according to Fig. 1. Accordingly, the control sides of the directional control valve 4 are controlled via the two pilot valves 10, 12, each of which is also assigned a check valve arrangement 6 or 8, on the control side of which the control pressure corresponding to the load pressure acts. Downstream of the measuring orifice of the directional control valve 4 is the pressure compensator with the pressure compensator piston 40, via which the pressure drop across the measuring orifice is kept constant regardless of the load pressure. The pressure compensator is controlled in the closing direction by the control spring 44 and the pressure at the control connection LS and in the opening direction by the pressure

[FiIeANM \MA7211B1.doc] Description, 18.02.97 Utility model application Mannesmann Rexroih GmbH

downstream of the metering orifice (directional control valve 4). By appropriately controlling the pilot valves 10, 12 and the resulting displacement of the valve slide of the directional control valve 4 and the check valve arrangements 6, 8, one of the working connections is supplied with hydraulic fluid - as explained in more detail below - while the other working connection is connected to the tank T via the valve arrangement. If the load pressure downstream of the directional control valve 4 is greater than the load pressure at the control connection LS, the pressure compensator is moved to the upper position in Fig. 3 so that this higher load pressure is fed into the load signaling line.

The function of the valve arrangement shown in Figures 1 to 3 will be briefly explained below.

To extend the lifting cylinder 116, the cylinder chamber must be supplied with hydraulic fluid via the working connection A. For this purpose, the pilot valve 12 is energized so that a control pressure that corresponds to the output pressure of the pilot valve 12 acts on the right-hand end of the valve slide 28 in Fig. 1. This control pressure moves the valve slide 28 to the left against the tension of the compression spring 30 so that the inlet bore star 68 is opened and hydraulic fluid can enter the inner bore 38 from the pump channel 14. The pump pressure lifts the piston 40 from its stop position so that the outlet bore star 5 6 is opened against the force of the control spring 44 and the load pressure until an equilibrium is established between the piston spring side and the front (right in Fig. 1). The hydraulic fluid can now enter from the inner bore 38 through the opened outlet bore star 56 into the connecting channel 62, so that the main cone 72 opens in the direction ¹ - ie

against the force of the return spring 74. If the pump pressure is sufficient - or more precisely, the output pressure at the pressure compensator - the main cone 72 is

[File:ANM\MA7211B1 .doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

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its valve seat so that the hydraulic fluid can flow through the working channel 78 to the connection A and from there into the cylinder chamber of the lifting cylinder 116.

Due to the pressure build-up in the cylinder chamber, hydraulic fluid is displaced from the annular space of the lifting cylinder 116, which is led via the connection B and the working channel 114 to the check valve arrangement 8.

The control pressure that is generated when the pilot valve 12 is actuated also prevails in the annular space 100 and thus at the rear of the push-on piston 92, so that the latter is moved to the right in the illustration according to Fig. 1 against the force of the spring 98 (Fig. 2). After a predetermined stroke, the plunger 96 of the push-on piston 92 comes into contact with the ball 82, so that the latter is lifted from its seat against the preload of the return spring 74. The main cone 72 is still in contact with its seat. The axial movement of the push-on piston 92 opens the radial section 110 of the compensating bore by the control edge of the annular space 106, so that the tank pressure prevails in the compensating bore and in the bore of the projection 86.

Since the throttle bore 84 has a significantly smaller diameter than the pre-opening 80, the pressure in the spring chamber of the check spring 74 is reduced because hydraulic fluid cannot flow quickly enough from the working channel 114 through the small throttle bore 84. This relieves the spring side of the main cone 72. Due to the effect of the control pressure, the push-on piston 92 is moved to its stop position against the projection of the main cone 72 when the ball 82 is open, so that the latter is carried along by the push-on piston 92 and lifted off its valve seat. In this stop position, the front side of the push-on piston 92 and the annular front surface 90 of the main cone 72 lie against one another in a sealing manner, so that the inner bore of the projection 86 is sealed against the outer circumference.

[Fiie:ANM\MA7211B1 .doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

Due to the axial displacement of the push-on piston 92 and the main cone 72 adjacent to it, the working channel 114 is connected via the annular groove 88 to the annular space 109, which in turn is connected via the recesses 104 to the tank channel 102, the latter connection being opened by the recesses 104. The hydraulic fluid can now flow back from the working channel 114 into the tank channel 102 and thus to the tank connection T.

The extension of the lifting cylinder 116 is terminated by de-energizing the pilot valve 12, so that both main cones 72 of the check valve arrangement 6, 8 are moved back into their closed position and the hydraulic fluid is clamped between the check valve arrangements 6, 8 and the lifting cylinder 116 without leaking oil.

To retract the lifting cylinder 116, the pilot valve 10 is energized in the opposite way, so that the valve slide 28 moves to the right and the annular space of the lifting cylinder 116 is supplied with hydraulic fluid via the connection B, while the hydraulic fluid in the cylinder space is returned to the tank T via the working connection A and the unlocked check valve arrangement 6.

The valve arrangement according to the invention enables the setting of a floating setting without the directional control valve having to be designed with a so-called fourth switching position, i.e. a limit switching position, which must be provided in addition to the zero position and the large number of working positions in the prior art.

In the valve arrangement according to the invention, both pilot valves 10, 12 are energized to set the floating position, i.e. to connect the working connections A and B to the tank T, so that the same control pressure is applied to both control sides of the valve slide 28 and this is pre-tensioned to its zero position. No hydraulic

[File: ANMWA7211B1 .doc] Description, 18.02.97

Utility model application

Mannesmann Rexroth GmbH

fluid is conveyed from the pump connection P to the check valve arrangements 6, 8. The control pressure in the control channels 22 and 24 actuates both push-on pistons 92 in their opening direction, so that the main piston 72 is lifted from its valve seat in the manner described above and the connection between the working channels 78 and 114 and the tank channel 102 is opened. The lifting cylinder can now be moved "by hand" since the hydraulic fluid can circulate with practically no resistance.

The inventive design of the valve arrangement makes it possible to implement a large number of variants with the same valve housing 2, since the essential functional elements are all designed as built-in elements without any changes to the channels, connections, etc. being necessary.

A check valve arrangement and a valve arrangement in which such a check valve arrangement is used are disclosed, in which a push-on piston can be brought into contact with a main cone in order to lift it off its valve seat. The main cone is designed without a surface difference and the push-on piston is controlled via a pilot valve, so that the lifting process of the main cone from the valve seat can be controlled.

The inventive design of the check valve arrangement enables the control of the inlet and outlet throttling for the consumer. The inlet and outlet throttling can be set differently for engine and cylinder operation. By avoiding (switching off) the outlet throttling, the throttling loss at the valve can be roughly halved. To do this, a control pressure difference is first generated at the valve spool of the directional control valve and then both control pressures are increased by appropriately controlling the pilot valves while maintaining the pressure difference (at the valve spool). The valve spool remains in its preset position and the outlet

[File:ANM\MA7211B1 .doc] Description, 18.02.97 Utility model application Mannesmann Rexroth GmbH

The stop valve (check valve arrangement 6 or 8) is fully opened. The generation of the pressure difference and the parallel increase in the control pressures is advantageously initiated by operating a lever or switch and controlled by an associated electrical control unit.

Claims (1)

Protection claims Check valve arrangement with a main cone (72) prestressed against a valve seat (76) and designed with a pre-opening (80), the pre-opening closing body (82) of which can be brought into its open position by means of a push-on piston (92), characterized in that the push-on piston (92) can be brought into contact with the main cone (72) to unlock the check valve arrangement, so that its opening stroke is determined by the stroke of the push-on piston (92), and that the diameter of the main cone seat is approximately equal to the main cone diameter on the seat side. 35 5. Check valve arrangement according to claim 1, characterized by a connecting bore (108, 110) of the push-on piston (92), which can be opened to connect the pre-opening (80) to a tank connection (T) during the stroke of the push-on piston (92). Check valve arrangement according to claim 2, characterized in that the main cone (72) has a hub-shaped projection (86) on the end section on the push-on piston side, into the inner end face of which the pre-opening (80) opens and the annular end face (90) of which is a sealing contact surface for the push-on piston (92), which dips into the hub bore with a tappet (96). Check valve arrangement according to claim 3, characterized in that the connecting bore (108, 110) opens into the hub bore of the projection (86). Check valve arrangement according to one of the preceding claims, characterized in that the push-on piston (92) has on the outer circumference several circumferentially distributed [File:ANM\MA7211A1.doc] Claims, 18.02.97 Utility model application Mannesmann Rexroth GmbH Has recesses (104) via which a space downstream of the main cone seat can be connected to a tank channel (102) when the push-on piston (92) is activated. 6. Check valve arrangement according to one of the preceding claims, characterized in that the push-on piston (92) is hydraulically controlled. 7. Check valve arrangement according to one of claims 3 to 6, characterized in that the push-on piston (92) is prestressed by a spring (98) against a stop such that a gap between the contact surface (90) and the adjacent end face of the opening piston (92) is larger than the distance between the tappet (80) and the outer circumference of the closure body (82). 8. Valve arrangement with a directional valve, via which a pump connection (P) can be connected to a working line (78, 114) leading to a consumer (116), into which a check valve arrangement (6, 8) with a main cone (72) and a push-on piston (92) according to one of the preceding claims is connected, via which the working line (78, 114) can be connected to a tank connection T. 9. Valve arrangement according to claim 8, characterized in that the push-on piston is pre-controlled by an electrically operated pilot valve (10, 12). 10. Valve arrangement according to claim 8 or 9 for controlling a double-acting consumer (116), with a directional valve (4) which has two output connections (62, 64), each of which has a check valve arrangement (6, 8) according to one of claims 1 to 7, wherein each push-up piston (92) can be controlled via a pilot valve (10, 12) whose output pressure is controlled via a [Fi!e:ANM\MA7211A1.doc] Claims, 18.02.97 Utility model application Mannesmann Rexroth GmbH Control channel (22, 24) is guided to a front side of the directional valve slide (28), so that the directional valve (4) can be brought into a position in which one working connection (A, B) can be connected to the pump connection P and the other working connection (A, B) can be connected to the tank T via the check valve arrangement (6, 8) which is subjected to the same control pressure as the directional valve slide (28). 11. Valve arrangement according to one of claims 8 to 10, characterized in that the pilot valve is a pressure reducing valve (10, 12) in cartridge design. 12. Valve arrangement according to one of claims 8 to 11, characterized by a check valve (118) which is connected in a connecting line between the tank connection T and the working connection B. 13. Valve arrangement according to one of claims 8 to 12, characterized in that the directional control valve is a continuously adjustable directional control valve (4) which has a variable measuring orifice (68, 66) downstream of which an individual pressure compensator (40, 56) is connected.