WO1999024720A1

WO1999024720A1 - Hydraulic circuit

Info

Publication number: WO1999024720A1
Application number: PCT/DE1998/003050
Authority: WO
Inventors: Winfried RÜB
Original assignee: Mannesmann Rexroth Ag
Priority date: 1997-11-10
Filing date: 1998-10-19
Publication date: 1999-05-20
Also published as: DE19749639A1

Abstract

A hydraulic circuit for driving a consumer and a process for driving such a circuit are disclosed. A comparison valve (16) is associated to the slide of a load-adapted proportional valve (14). When a predetermined maximum load pressure on the proportional valve slide is reached, a differential pressure can be applied to the comparison valve, moving it into a position in which the load pressure is limited to a predetermined maximum value.

Description

description

HYDRAULIC CIRCUIT

The invention relates to a hydraulic circuit for controlling a consumer according to the preamble of claim 1 and a method for controlling such a circuit.

1, to which reference is already made at this point, shows a so-called load sensing system (LS system) in which two consumers 2, 4 can be supplied with hydraulic fluid by means of a hydraulic pump, which is designed as a constant pump 6 in the exemplary embodiment shown. The latter is assigned an inlet pressure compensator 8, which is acted upon on the one hand by the pressure at the pump outlet (pl) and on the other hand by the highest load pressure of the two consumers 2,4. This highest load pressure is fed to the face of the inlet pressure compensator 8 via a shuttle valve 10. An individual pressure compensator 12 and a proportionally adjustable directional control valve 14 are connected in series in the feed lines to the consumers 2, 4. The individual pressure compensator 12 can be arranged in the direction of pressure build-up in front of the proportional valve 14 (see FIG. 1) or else after the proportional valve 14.

The pressure at the input of the proportional valve 14 also acts on one end face of the individual pressure compensator 12. The other end face is acted upon by the load pressure, which is also present on an end face of the inlet pressure compensator 8. The individual pressure compensator and the proportional valve 14 thus form a continuously adjustable flow control valve. The proportional valve has the function of an adjustable measuring orifice on which the pressure drop is kept constant by the individual pressure compensator 12, so that a load pressure independent but adjustable flow volume flow is adjustable.

Since the pressure difference at all proportional valves is kept constant in such an LS system, the volume flow to each consumer is proportional to the respective opening cross section of the proportional valve.

With a downstream individual pressure compensator, the volume flows to the consumers are always in the ratio of the opening cross-sections of the orifice plates. In the case of an upstream individual pressure compensator (Fig. 1), the most heavily loaded consumer can stop if the sum of the requested volume flows of all consumers is greater than the maximum delivery flow of the pump.

A disadvantage of the circuit described above is that the system is essentially influenced by influencing the delivery flow supplied by the pump 6. If one of the consumers 2, 4 is moved back under the influence of external loads, such as in agricultural machinery, forklift trucks, snow plows, mowers etc., the load pressure which increases as a result cannot be limited by influencing the pump 6 but only by additional secondary pressure relief valves.

A disadvantage of these systems is that the hydraulic oil volume flow to or from the consumer has to flow through the secondary pressure relief valve, so that corresponding energy losses occur.

In contrast, the invention is based on the object of a hydraulic circuit for controlling a consumer and a method for controlling such a device

To create procedures in which the load pressure of the user can be limited with minimal equipment and energy expenditure.

This object is achieved with regard to the hydraulic circuit by the features of patent claim 1 and with regard to the method by the features of patent claim 8.

The measure of applying a control pressure difference to the two end faces of the valve slide, by means of which the valve slide is moved into such a control position when a predetermined maximum pressure is reached, since the load pressure is limited to the predetermined value, no additional valves for load pressure Limitation in work lines required. The pressure is thus limited only by adjusting the proportional valve as a function of an applied control pressure, so that no unnecessary energy losses occur when the pressure is limited by a pressure relief valve connected to the supply line or the return line.

To operate the proportional directional control valve, control lines are routed to the end faces of the valve spool, which are operatively connected to one another via a comparison or pressure valve, which acts on the end faces of the valve spool with the corresponding control pressure difference when the predetermined maximum pressure is reached.

Below the preset maximum load pressure, that is, when the comparison valve has practically no effect, both valve faces are subjected to the same pressures. This does not necessarily have to be the tank pressure, but much higher pressures can also be used to act on the end faces of the slide. For example, the highest load pressure or any other system pressure can be Slider end faces are guided. The designer thus has more leeway to design the circuit

In principle, the hydraulic circuit described above and the method for controlling it can also be used if the proportional valve is not assigned an individual pressure compensator. If the system works with an individual pressure compensator, this can be connected upstream or downstream of the proportional valve.

A circuit of particularly simple construction is obtained if the two control lines carry the highest load pressure and are connected to one another via a connecting line with throttles, a pressure relief valve being provided in order to connect one of the control lines to a tank when the maximum pressure is reached, so that the Directional valve spool is moved into its control position by the pressure difference. This pressure limiting valve is preferably electrically pilot-operated so that the maximum load pressure setting can be carried out in a particularly simple manner.

As an alternative to this, the comparison valve can be a hydraulically actuated proportional valve, in which one side of the control valve spool is subjected to the load pressure, while the other side of the control valve spool is acted upon by the force of a control spring and the pressure in the return line. When the maximum pressure is reached, the control slide is moved due to the pressure difference applied to its end faces into a deflection position in which an end face of the directional valve spool is acted upon by the pump pressure, so that it is brought into its control position.

To set the maximum load pressure, the control spring is preferably subjected to an adjustable preload. In a further alternative, the pressure valve is designed as a 2/2-way valve, one end face of which the load pressure can be applied to and that when the maximum load pressure is reached, the 2/2-way valve is moved from its spring-loaded blocking position into a through position in which the proportional valve spool is moved into its control position in order to limit the load pressure to the maximum value.

The latter alternative is particularly suitable for movements of pliers or other movements in which an object is held with limited pressure.

In the control position of the proportional valve spool, both the inflow to the consumer and the return flow from the consumer to the tank can be influenced.

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

Preferred exemplary embodiments of the invention are explained in more detail below with the aid of schematic drawings. 1 shows the structure of a conventional hydraulic circuit for controlling several consumers;

2 shows the basic concept of a circuit according to the invention;

3 shows an exemplary embodiment of the circuit from FIG. 2 with a mechanically controlled proportional directional control valve; 4 shows an exemplary embodiment with an electrically controlled proportional directional control valve;

Fig. 5 shows an embodiment in which the comparison valve is designed as a 2/2-way valve; Fig. 6 shows another embodiment of a circuit according to the invention, in which the comparison valve is designed as a pressure relief valve and Fig. 7 is a plate valve which is constructed after the circuit of FIG. 6.

FIG. 2 shows the essential part of the invention of a hydraulic circuit for supplying several consumers, which in principle can be constructed, for example, similar to that as shown in FIG. 1.

This means that each consumer 2, 4 is assigned a proportional valve 14 and the highest acting load pressure is fed via a shuttle valve circuit to a constant pump with inlet pressure compensator (6,8 in Fig. 1) or to the pump regulator of a control pump. The proportional valve 14 is generally associated with an individual pressure compensator 12, which can be connected downstream or upstream of the proportional valve 14. In certain cases, however, this individual pressure compensator 12 can also be dispensed with, so that in the case of such circuits without an individual pressure compensator, the volume flows to those consumers with a lower load pressure are dependent on the pump pressure and on their own load pressure if no other suitable measures are taken. However, it does not matter for the invention, that is, the function essential to the invention is independent of whether individual pressure compensators are provided or not.

The proportional valve 14 can be an electrically, hydraulically or mechanically controlled valve and, in the spring-loaded neutral position shown, shuts off two working connections A, B connected to one of the consumers 2 or 4, opposite a pump connection P, and a tank connection T.

When the proportional valve spool is moved into the positions indicated by a, the connection from the pump connection P to the working connection B is opened, so that the consumer 2 in FIG. 1 is supplied with hydraulic fluid becomes. This consumer can be a hydraulic cylinder, for example, which is then extended. The hydraulic fluid flowing back from the consumer is led via the connection A to the tank connection T. In the circuit shown in FIG. 1, the cylinder space of the hydraulic cylinder would be supplied with hydraulic fluid, while the annular space would be relieved towards the tank.

When moving the proportional valve spool in the positions labeled b, the working connection B is connected in the opposite way to the pump connection P and the working connection A to the tank connection T, so that the annular space (FIG. 1) of the consumer 2 is supplied with hydraulic fluid and that Hydraulic fluid located in the cylinder space to the tank T is expanded.

The proportional valve is activated on the basis of a certain mechanical, hydraulic or electrical control pulse, so that the proportional valve slide is brought into a position in which the consumer is supplied with the predetermined fluid volume flow.

Since the individual load pressure is limited to a predetermined value for this consumer, comparatively complicated circuits for pressure limitation of the pressure in the working line between the connections A, B and the consumer must be provided in the previously known load sensing systems. Such a pressure limitation, which can be set individually for each consumer, is required, for example, in the case of broom rollers, snow plow blades and mowers, the weight of which is borne by the hydraulic cylinders to an adjustable extent. The higher the pressure in the hydraulic cylinders is set, the less weight the unit rests on the ground. For this maximum load pressure setting, the proportional valve 14 is assigned a comparison valve arrangement 16 which, depending on the load pressure on the consumer, applies a control difference to the end faces of the proportional valve slide, so that when a maximum load pressure is reached it is shifted into a position in which the load pressure on the consumer cannot continue to rise. Of course, the reverse flow can also be controlled in the reverse manner. This means that the comparison valve arrangement 16 has the effect that both end faces are acted upon by the same control pressure below the maximum load pressure, while when the limit value is reached a control pressure difference is built up by which the proportional valve spool is displaced such that the inflow of hydraulic fluid to the consumer is throttled and hydraulic fluid flowing back can be drained from the consumer to the tank.

In order to implement this control, the comparison valve arrangement 16 is connected to the working lines connected to the connections A and B via load pressure lines 18, 20. Depending on the control pressure difference in the load pressure lines 18, 20, a control pressure difference is applied to the end faces of the proportional valve spool via control connections X, Y, so that it can be moved into the desired position to limit the load pressure at the associated consumer.

3 shows an exemplary embodiment in which the proportional valve 14 is operated by hand in order to supply the consumer with the desired hydraulic volume flow.

The comparison valve 16 is designed as a 4-way proportional valve, which in its basic position 0 is the two

End faces of the proportional valve spool with the tank Final T connects. The pump connection is blocked in the neutral position 0.

When the proportional valve spool is moved to the positions marked d, the control connection Y is connected to the pump connection P, while the control pressure at the control connection X is released towards the tank via the tank connection T.

When the valve spool is displaced into the positions labeled e, the connection from the pump port P to the control port X and the connection between the control port Y and the tank port T are opened, so that the proportional valve spool is acted upon in the opposite direction with a control pressure.

The valve spool of the comparison valve 16 is biased into its neutral position 0 by means of compression springs 22, 24, the compression springs 22, 24 acting on the valve spool of the comparison valve 16 with an adjustable preload. This means that the valve slide can only be moved from its neutral position when the spring preload of one of the compression springs 22, 24 is overcome.

As in the exemplary embodiment according to FIG. 2, the load pressure in the working lines is guided via the load pressure lines 18, 20 to the end faces of the valve slide of the comparison valve 16. The spring preload of the two compression springs 22, 24 is set such that the valve slide of the comparison valve 16 is only shifted from its neutral position when the load pressure difference in the load pressure lines 18, 20 exceeds a predetermined maximum value.

To control the consumer, the proportional valve 14 is actuated, for example, in such a way that the proportional valve slide into the ones marked here with a Positions are brought so that the connection from the pump connection P to the working connection B is opened and the connection from the working connection A to the tank connection T is opened. The consumer is supplied with the predetermined hydraulic fluid volume flow. In this phase, the comparison valve 16 remains in its neutral position 0, in which both end faces of the proportional valve spool are subjected to the same control pressures. In the present case, these control pressures are equal to the pressure acting on the tank connection T.

When the maximum load pressure in the load pressure line 20 is reached, a control pressure difference acts on the two end faces of the control slide of the comparison valve, which is sufficient to bring the control slide against the bias of the compression spring 24 from its neutral position into the positions designated e, in which the pump connection P are connected to the control connection X and the tank connection T to the control connection Y. That is, the right-hand end of the proportional directional valve spool in FIG. 3 is acted upon by the pump pressure, so that the proportional valve spool is brought from its position labeled a to the right through the neutral position N to the positions labeled b in which the working port B is connected the tank connection T and the working connection A is connected to the pump connection P, so that no further pressure build-up can take place in the working line connected to the working connection B or the inflow is throttled.

When the consumer is actuated in the opposite direction, that is to say when the proportional directional valve slide is moved manually into the positions designated by b, the load pressure limitation takes place in the reverse manner. In this embodiment, the load pressure can be adjusted by changing the preload of the compression springs 22, 24.

4 shows an embodiment in which the proportional valve 14 is designed as an electrically operated valve. This proportional valve is provided with a further control connection Z, via which the pressure at the pump connection P is guided by means of a control line 28 to a control connection Z 'on the comparison valve 16. This comparison valve 16 has practically the same structure as that in FIG. 3, but the switch positions d and e are interchanged.

In the neutral position N shown of the proportional directional control valve spool, the control connection Z is connected to the tank connection T. The valve spool of the comparison valve 16 is also in its neutral position 0, in which the control connections X, Y are connected to the control connection Z 'and thus via the control line 28 and the control connection Z to the tank connection T. This means that the tank pressure is present on both ends of the proportional valve spool. The working connections A, B and the pump connection P are shut off.

When energizing the electromagnet arranged on the left in FIG. 4, the proportional valve slide is brought into its positions indicated by a, in which the pump connection P is connected to the working connection A and the working connection B is connected to the tank connection T. In addition, the pressure is via the control connection Z. at connection P or at connection A via control line 28 to control connection Z 'and from there via control connections X, Y to the end faces of the proportional valve spool. Since these two pressures are the same, they compensate each other so that the proportional valve spool eventually shifted by the action of the electromagnet.

The consumer is now supplied with hydraulic fluid via the working line connected to the working connection A, so that the load pressure in this working line rises and is led via the load pressure line 18 to the left end face of the valve slide of the comparison valve 16 in FIG. 4. The load pressure in the working line connected to the working connection B is led via the load pressure line 20 to the other end face. After reaching the predetermined maximum load pressure difference, the valve spool of the comparison valve 16 is brought against the action of the compression spring 22 marked positions in which the control connection Z 'are connected to the control line X and the control line Y to the tank connection T, since at the control connection Z' 4, the right end face of the proportional valve spool in FIG. 4 is acted upon by the pump pressure via the control port X, while the other end face is connected to the tank T via the control port Y. This control pressure difference brings the proportional valve spool into a position in which the inflow of hydraulic fluid to the consumer is throttled or even reversed until the load pressure on the consumer has dropped below the predetermined maximum load pressure again.

The load pressure in the other working line can of course be limited in the opposite manner by actuating the proportional valve 14.

5 shows an exemplary embodiment in which the load pressure only has to be limited in one direction. Such tasks can be found, in particular, in forceps movements and in devices in which holding is carried out with limited pressure. The comparison valve arrangement 16 must therefore only be used in such applications. work on a work line and can therefore be carried out more easily.

In the embodiment shown in Fig. 5, a pressure limitation is only required in the extension direction of the consumer (hydraulic cylinder) 2, so that only the load pressure in the working line 30 must be limited, which connects the working port A with the cylinder space of the consumer 2.

In principle, the proportional valve 14 has the same structure as that from FIG. 4, so that a repeated description can be dispensed with.

In the exemplary embodiment shown, the comparison valve 16 is designed as a 2-way proportional valve, which is biased into its neutral position 0 by an adjustable compression spring 32. The end face of the valve spool of the comparison valve 16 removed from the compression spring 32 is connected to the working line 30 via a load pressure line 34, so that this end face is acted upon by the load pressure. When a predetermined load pressure in the load pressure line 34 is reached, the valve slide of the comparison valve 16 can be brought from its blocking position into the flow positions in which an outflow line 36 is connected to the tank connection T. The outflow line 36 branches off to the proportional valve 14 into two control lines 38, 40, which are each guided to an end face of the proportional valve spool. A throttle 42 is arranged in the right control line 40 in FIG. 5. In the area between this and the end face of the proportional valve spool, a connecting line 44 leads to the control connection Z of the proportional valve 14. That is, in the neutral position N of the proportional valve 14 shown in FIG. 5, the two end faces of the proportional valve spool are above the control eranschluß Z, the connecting line 44 and the two control lines 38, 40 connected to the tank port T.

To extend the hydraulic cylinder 2, the left-hand electromagnet of the proportional valve 14 is energized in FIG. 5, so that the proportional valve spool is moved to the positions marked with a, in which the connection from the pump connection P to the work connection A and the connection from the work connection B to the tank connection T can be controlled. As a result, the hydraulic cylinder 2 is extended and, for example, a forceps gripping movement is carried out. The pressure at the pump connection P is routed via the control connection Z and the control lines 44, 40 and 38 to the two end faces of the proportional valve spool, so that the control pressure is the same on both sides.

When the maximum load pressure that can be set by the compression spring 32 is reached, the 2-way proportional valve (comparison valve) 16 is brought into its flow position, so that the outflow line 36 and the control line 38 are connected to the tank connection T. As a result, the left end face of the proportional valve spool in FIG. 5 is relieved towards the tank T, while due to the throttle 42 on the right end face a control pressure corresponding to the pressure at the working port A is still present. As a result, the proportional valve slide is brought into a position in which the hydraulic fluid flow to the consumer 2 is throttled or interrupted, so that the load pressure in the working line 30 drops again below the predetermined maximum value. In this exemplary embodiment, the comparison valve 16 is not in operative connection with the other working line 46, which is connected to the working connection B, since when the hydraulic cylinder 2 is loosened, that is to say when the hydraulic cylinder 2 is moved back, no increase in load pressure can take place.

6 shows a further exemplary embodiment of the circuit according to the invention, with the comparison valve 16 is designed as an electrically actuated pressure relief valve. In this exemplary embodiment, the proportional valve 14 is designed as a 3-way proportional valve and is used in a circuit for controlling a double-acting hydraulic cylinder with inlet throttling. In such circuits, the proportional valve 14 is followed by a switching valve via which the cylinder space or the annular space of the hydraulic cylinder can optionally be supplied with hydraulic fluid. This means that switching from, for example, lifting to lowering is carried out by actuating the switching valve, while the proportional valve can remain in its working position. The proportional valve 14 shown in FIG. 6 is again an electrically actuable valve, with a pump connection P and the only working connection A being shut off in the spring-loaded neutral position N shown, while a control connection Xi is also shut off and a control connection X2 with the tank connection T is connected.

To initiate the lifting / lowering of the hydraulic cylinder, the electromagnet on the right in FIG. 6 is energized, so that the proportional valve slide is brought into the working positions marked with a. In these working positions, the pump connection P is connected to the work connection A and the tank connection T is shut off. The return of the hydraulic fluid from the hydraulic cylinder takes place through its own tank line without return flow restriction, which is not shown in FIG. 6. The two control connections Xi and X2 are also connected to each other in the working positions.

When the other electromagnet is energized, it will

Proportional valve 14 is brought into a floating position S, in which the pump connection P is shut off and the working connection A and the control connection X2 are connected to the tank connection. The load pressure of the consumer or another suitable system pressure is tapped via a load pressure line 46 and led to the end faces of the proportional valve spool via two control lines 48, 50. The two control connections X _] _, X2 are also connected to the control line 50. A throttle 52 is also connected in this. A load pressure line 54 branches off from the control line 50 and is led to the inlet connection D of the pressure relief valve. An output port E of the pressure relief valve is connected to the tank T. The end face of the pressure valve valve slide, which is remote from the electromagnet, is likewise acted upon by the control pressure prevailing in the load pressure line 54 via a branch line 56.

When the consumer is actuated, the proportional valve is brought into the positions marked with a and the switching valve (not shown) is actuated in such a way that the consumer is raised or lowered. As long as the load pressure at the consumer is below the desired maximum value, the same control pressure is present on both ends of the proportional valve spool, so that it remains in its position specified by the force of the electromagnet. When the maximum load pressure is reached, the pressure relief valve, which is normally in a closed position, is opened by the control pressure acting via the control line 56, so that the pressure in the load pressure line 54 and thus also in the region between the throttle 52 and the right end face of the proportional valve spool is reduced towards the tank T, while the pressure on the left-hand end of FIG. 6 still corresponds approximately to the maximum load pressure due to the action of the throttle 52. As a result, the proportional valve spool in the illustration according to FIG. 6 is shifted to the right, so that the inflow to the consumer is throttled and the load pressure thus drops again below the preset value. The maximum load pressure is set by Control of the solenoid of the pressure relief valve 16.

7 finally shows a section through a valve housing 2, in which the valve arrangement described above for controlling a hydraulic cylinder with inlet throttling is accommodated. The valve housing 2 is of disk or plate construction, so that several valve disks of this type can be joined together to form a control block. The hydraulic cylinder is connected to two working ports A and B; Furthermore, the valve housing has a tank connection T, a pump connection P and a control connection X. The proportionally adjustable directional control valve 14 from FIG. 6, a switching valve 60 and a check valve 62 are also accommodated in the valve housing 2, the axes of which are parallel to one another in the plate are arranged. In the illustrated embodiment, the proportional valve 14, the switching valve 60 and the check valve 62 are actuated by electromagnets.

The exact structure of the proportional valve 14, the switching valve 60 and the pressure relief valve 62 are described in the applicant's prior application 196 46 447, so that for the sake of simplicity reference is made to these statements which are to be included in the disclosure of the present application.

Accordingly, the proportional valve 14 has a proportional valve slide 64, the two end faces of which are acted upon by a load pressure LS via the control lines or control channels 48 or 50. The control channel 50 is provided with a nozzle bore which forms the throttle 52 in FIG. 6. The electrically operable pressure relief valve 16 is screwed into a connection bore of the valve housing 2, which is arranged at the bottom right in FIG. 7 and via which, when a maximum load pressure in the area between the nozzle bore (throttle 52) and the one in FIG. A connection to a tank line 55 is opened on the end face of the proportional valve spool 64, so that the right end face of the proportional valve spool 64 is relieved and the volume flow can be throttled.

The invention thus makes it possible in a simple manner to limit the maximum load pressure individually for each consumer and to connect the individual proportional valves in an LS or a LUDV system. The valve arrangements according to the invention can also be used in mixed systems with load signaling valves and non-load-adapted valves, so that an inexpensive assembly of the valve assemblies in the modular system is made possible.

The circuit according to the invention does not result in unnecessary energy losses in the pressure limitation, since hydraulic fluid not conveyed unnecessarily by the pump must be drained from the refueling via a secondary valve.

The tank pressure is usually applied to the end faces of the proportional valve spool. However, since other pressure levels are allowed in the system used, the constructive freedom is increased, especially when designing series and modular systems.

Disclosed is a hydraulic circuit for controlling a consumer and a method for controlling such a circuit, in which a valve slide of a load-adjusted proportional valve is assigned a comparison valve, via which a pressure difference can be applied to the valve slide of the proportional valve when a predetermined maximum load pressure is reached, so that this is moved to a position in which the load pressure is limited to the predetermined maximum value.

Claims

claims

1. Hydraulic circuit for controlling a plurality of pressure-limited consumers, each with an electrically, mechanically or hydraulically operated proportional valve (14) assigned to a consumer (2), via which the assigned consumer (2) via a working line carrying the load pressure with a pump and via a return line can be connected to a tank, the pump delivery flow being set as a function of the heavily loaded consumer (2), characterized by a comparison valve (16) which is in the two front ends of the valve slide (64) of the proportional valve (14) guided control lines (X, Y; 38, 40; 48, 50) and via which a pressure difference can be applied to the valve spool (64) in order to keep a predetermined maximum pressure in the working line to the assigned consumer (2) constant.

2. Hydraulic circuit according to claim 1, characterized in that the proportional valve (14) is connected in series to an individual pressure compensator (12).

3. Hydraulic circuit according to claim 1 or 2, characterized in that the control lines (48, 50) carry the load pressure and are connected to a throttle (52), and the comparison valve is a pressure relief valve (16) via which the control line (50) is connected to a tank connection T when the maximum load pressure is reached.

4. Hydraulic circuit according to claim 3, characterized in that the pressure relief valve (16) is electrically piloted.

5. Hydraulic circuit according to claim 1 or 2, characterized in that the comparison valve is a hy- draulically operated proportional valve (16), in which a valve spool end face is acted upon by a control line (18, 20) with the load pressure and the other side with the force of a control spring (22, 24) and through the control pressure connected to the return line Control line (18, 20) is acted on, so that when the maximum load pressure in the control line (18, 20) is reached, the valve spool of the proportional valve (16) is shifted, which connects the two control lines (X, Y) to one another in their basic position and in its deflection positions one of the end faces of the valve slide of the proportional valve (14) is subjected to the pump pressure.

6. Hydraulic circuit according to claim 5, characterized in that the control spring (22, 24) can be acted upon by an adjustable preload.

7. Hydraulic circuit according to claim 1 or 2, characterized in that a control line (40) with the load pressure leading working line (30) and via a throttle (42) with the other control line (38) is connected, which in turn via an outflow line (36) and a 2/2-way valve (16) is connected to the tank connection T, one end face of the 2/2-way valve (16) being acted upon by the load pressure, so that when the maximum load pressure is reached, the 2/2 - Directional control valve (16) can be brought from a spring-loaded blocking position into a through position.

8. A method for controlling a hydraulic circuit with a plurality of hydraulic consumers, each of which is connected via a proportional directional control valve to a common pump or to a tank, with the steps: - mechanical, hydraulic or electrical actuation of the proportional valve (14), so that Hydraulic fluid is pumped from the pump to the consumer (2); - Detecting the load pressure in a working line (30) to the consumer (2);

- Acting on the end faces of a proportional valve spool (64) of the proportional valve (14) with a control pressure difference, so that the proportional valve spool (64) is moved into a control position to hold a predetermined maximum load pressure.