DE102020210441A1 - Proportional spool valve with a pressure relief valve, pressure relief valve and hydraulic system - Google Patents

Abstract

The present invention relates to a proportional spool valve 1 with a spool 4 that can be switched from a neutral position N via a pilot control 3 that generates a pilot control force into at least one first switching position S1, a pressure port 5, a first pressure outlet A, a return line 6 and one with a load pressure outlet 7 connected first load pressure line 8. The pressure port 5 is connected in the first switching position S1 with the first pressure outlet A and the first load pressure line 8. A second load pressure line 9 branches off from the first load pressure line 8 , the second load pressure line 9 being connected to the return line 6 and a pressure relief valve 2 being arranged in the second load pressure line 9 . The pilot control force acts in the opening direction of the pressure relief valve 2. The invention also relates to a pressure relief valve 2 and a hydraulic system 100.

Description

The present invention relates to a proportional slide valve with a pressure relief valve for load pressure pressure relief, such a pressure relief valve and a hydraulic system with a proportional slide valve according to the invention.

In mobile hydraulics, for example when used in a work vehicle such as a forestry harvester, pressure relief valves are regularly used to limit the maximum load pressure (also known as LS pressure limitation) in the load pressure signal circuit. The pressure limitation to a maximum load pressure is necessary to protect the steel construction of the working vehicle. For example, the weld seams on a forestry harvester mast are designed for a service life that depends in particular on the maximum load (i.e. the maximum expected weight of the tree trunk to be lifted) and the speed of movement of the mast with the load raised.

A mobile hydraulic system for such an application is offered, for example, by HAWE Hydraulics SE under the name PSL, cf. the publication D 7700-2 Prop. directional spool valve type PSL, PSV (status: August 2011). The proportional spool valve shown there has a spool that can be switched from a neutral position into at least one first switching position via a pilot control that generates a pilot control force. Furthermore, the proportional slide valve has a pressure or P port, a first pressure outlet or A or B port, a return line and a first load pressure line connected to a load pressure outlet. In the first switching position, the pressure connection is connected to the first pressure outlet and the first load pressure line. A second load pressure line branches off from the first load pressure line, the second load pressure line being connected to the return line and a pressure relief valve being arranged in the second load pressure line. The pressure-limiting valve limits the maximum load pressure to be reported to the pump or the pump regulator to the set value, for example to 300 bar.

The disadvantage of this solution is that there is no possibility of increasing the maximum load under certain conditions in which the higher maximum load has no negative effects on the steel construction of the working vehicle.

Against this background, it is the object of the present invention to show a proportional slide valve for use in the mobile hydraulic system of a work vehicle, in which, under certain circumstances, higher loads than the loads actually permissible at maximum deflection of the slide piston can be moved.

The object is achieved with a proportional slide valve according to claim 1. Advantageous developments are described in the dependent claims.

The proportional slide valve according to the invention is distinguished from the proportional slide valves known from the prior art in that the pilot control force acts in the open control direction of the pressure-limiting valve. An additional force component thus acts in the opening direction of the pressure-limiting valve, which reduces the force required to open the pressure-limiting valve against the actuating device of the pressure-limiting valve. The actuating device can be, for example, an adjustable spring or a magnet.

In other words, an additional force component that is dependent on the deflection of the slide piston is reported to the pressure-limiting valve in the open-control direction. If the maximum load pressure is limited in such a way that the maximum load is reached with a maximum deflection of the spool, a load lying slightly above the actual maximum permissible load can be moved with a smaller deflection of the spool. Since only a relatively low speed can be driven with a relatively small deflection of the slide piston, no disadvantage to the service life of the steel structure is to be feared. The total force acting on the steel structure remains largely constant, since a higher load is acting, but at the same time no maximum speed is possible.

It is advantageous here if the pilot control is a hydraulic pilot control and the pilot control force is a pilot control pressure. For this purpose, a signaling line arrangement preferably connects the pilot control to the pressure-limiting valve, so that the pilot-control pressure is reported to the pressure-limiting valve via the signaling line arrangement. In this way, the additional pilot control force acting in the opening direction of the pressure-limiting valve can be transmitted in a simple manner.

The proportional slide valve expediently has a second pressure outlet. The proportional slide valve can preferably be switched from the neutral position into a second switching position via the pilot control, the pressure connection being connected to the second pressure outlet and the first load pressure line in the second switching position. Consequently, two different hyd raulic consumers are controlled via the proportional slide valve, with the limitation of the maximum load pressure, which is dependent on the deflection of the slide piston, also being active when the second pressure output is controlled.

It is advantageous if a third load pressure line branches off downstream of the slide piston and upstream of the first pressure outlet in the direction of flow to the pressure outlet, with the third load pressure line opening into the second load pressure line upstream of the pressure relief valve and a first check valve being arranged in the third load pressure line.

It has been shown here that the pressure-limiting valve can also be used alone as a shock valve. Thus, according to the invention, a proportional slide valve is also shown which has a slide piston that can be switched from a neutral position into at least one first switching position, a pressure connection, a first pressure outlet, a return line and a first load pressure line connected to a load pressure outlet. In the first switching position, the pressure connection is connected to the first pressure outlet and the first load pressure line. A second load pressure line branches off from the first load pressure line, the second load pressure line being connected to the return line and a pressure relief valve being arranged in the second load pressure line. A third load pressure line branches off upstream of the slide piston and downstream of the first pressure outlet, the third load pressure line opening into the second load pressure line upstream of the pressure relief valve and a first check valve being arranged in the third load pressure line.

Since a large amount of hydraulic fluid can drain off quickly via the pressure relief valve (i.e. high flow rate in l/min), the pressure relief valve is also suitable for use as a shock valve. The check valve is arranged in such a way that the pressure in the second load pressure line acts in the closing direction. In addition, a spring or prestressing device or the like can also be provided on the check valve in order to ensure that the check valve does not open unintentionally, particularly in the case of low volume flows. As soon as a pressure peak at the first pressure connection opens the non-return valve, hydraulic fluid can flow out to the tank via the pressure relief valve, which is then also open, and the return line.

It is advantageous if a second check valve is arranged in the second load pressure line upstream of the third load pressure line. The second check valve prevents the hydraulic fluid from flowing out to the spool when the first check valve opens.

Furthermore, the first check valve can be a spring-loaded check valve. This has the advantage that, on the one hand, the position of the closing element is defined in the depressurized position. Furthermore, it can also be ensured in this way that the first check valve does not open unintentionally if the pressure in the second load pressure line is possibly insufficient.

It is also conceivable that a fourth load pressure line branches off downstream of the slide piston and upstream of the second pressure outlet in the flow direction to the second pressure outlet, with the fourth load pressure line opening into the second load pressure line upstream of the pressure-limiting valve and a third check valve being arranged in the fourth load pressure line. The pressure relief valve thus also acts as a shock valve for pressure peaks at the second pressure connection.

Furthermore, the third check valve can be a spring-loaded check valve. This has the advantage that, on the one hand, the position of the closing element is defined in the depressurized position. Furthermore, it can also be ensured in this way that the third check valve does not open unintentionally if the pressure in the second load pressure line is possibly insufficient.

Furthermore, the object is achieved with a pressure-limiting valve according to claim 11. Advantageous developments are described in the dependent claims.

The pressure-limiting valve according to the invention for a proportional slide valve, as described above, has a valve housing, an actuating device and a valve piston that can be moved axially in a piston chamber of the valve housing. The valve housing has at least one inlet port and at least one first connection port, with the valve piston blocking or proportionally releasing a flow path between the inlet port and the first connection port, in that the valve piston moves between a blocked position and an open position in the piston chamber against an actuating force generated by the actuating device emotional.

The pressure-limiting valve according to the invention is particularly distinguished from the pressure-limiting valves known from the prior art in that the valve housing has a second connection port, with a pressure at the second connection port acts against the actuating force of the actuating device on the valve piston in the open direction of the pressure-limiting valve. This results in the advantages already described above, that the reported pilot control force or the reported pilot control pressure "helps" when opening the pressure relief valve.

The second connection port preferably has a bore that passes through the valve housing in the axial direction. Consequently, the bore is designed coaxially with the axis of movement of the valve piston. Such a bore is easy to produce.

An axially movable movement element is expediently arranged in the bore, with the movement element transmitting a force acting in the direction of the open position to the valve piston when pressure is applied to the second connection port. This is particularly advantageous when the pilot force is transmitted hydraulically and is therefore a pilot pressure. In this case, the force is a product of the pilot pressure and the cross-sectional area of the moving element and “helps” open the pressure relief valve.

It is advantageous if the movement element has a conical surface and the second connection port has a corresponding contact surface facing the valve piston. In particular, it is advantageous if the contact surface is a seat. On the one hand, this prevents the movement element from falling out of the bore or the connection connection during assembly. On the other hand, damping can also be integrated at least in the closing direction of the pressure-limiting valve, since the hydraulic fluid cannot flow off via the second connection port due to the seat-tight contact of the moving element.

Furthermore, the object is achieved with a hydraulic system having a proportional slide valve according to the invention. The pressure-limiting valve of the hydraulic system is preferably a pressure-limiting valve according to the invention.

• 1 einen Hydraulikschaltplan eines erfindungsgemäßen Hydrauliksystems;
• 2 einen Hydraulikschaltplan einer zweiten Ausführungsform eines erfindungsgemäßen Hydrauliksystems;
• 3 einen Hydraulikschaltplan einer dritten Ausführungsform eines erfindungsgemäßen Hydrauliksystems;
• 4 einen Hydraulikschaltplan einer vierten Ausführungsform eines erfindungsgemäßen Hydrauliksystems;
• 5 einen Schnitt durch ein erfindungsgemäßes Druckbegrenzungsventil in der Sperrstellung;
• 6 das in 5 gezeigte Druckbegrenzungsventil in der Offenstellung; und
• 7 eine vergrößerte Darstellung eines zweiten Verbindungsanschlusses des in 6 gezeigten Druckbegrenzungsventils.

The invention is explained in more detail below with reference to exemplary embodiments shown in the figures. Here show schematically:

• 1 a hydraulic circuit diagram of a hydraulic system according to the invention;
• 2 a hydraulic circuit diagram of a second embodiment of a hydraulic system according to the invention;
• 3 a hydraulic circuit diagram of a third embodiment of a hydraulic system according to the invention;
• 4 a hydraulic circuit diagram of a fourth embodiment of a hydraulic system according to the invention;
• 5 a section through a pressure relief valve according to the invention in the blocking position;
• 6 this in 5 pressure relief valve shown in the open position; and
• 7 an enlarged view of a second connection terminal of the in 6 shown pressure relief valve.

In 1 a hydraulic circuit diagram of a hydraulic system 100 according to the invention is shown according to a first embodiment. The hydraulic system 100 is a mobile hydraulic system for a working vehicle, for example a forestry harvester, and includes a proportional slide valve 1 with a pressure relief valve 2. The pressure relief valve 2 is shown here as part of a semi-mounted block, although an integral design is also possible. The proportional slide valve 1 also has a hydraulic pilot control 3, via which a slide piston 4 can be switched from a neutral position N into a first switching position S1 and a second switching position S2. The proportional slide valve 1 has a pressure port 5 for connecting an input block (not shown), a return line 6 and two pressure outlets A, B for connecting hydraulic consumers.

Furthermore, the proportional slide valve 1 has a load pressure signal circuit with a load pressure outlet 7 , a first load pressure line 8 , a second load pressure line 9 and a load pressure inlet 10 . The load pressure input 10 is connected to the first load pressure line 8 via a shuttle valve 11 . A load pressure signal from a further proportional slide valve (not shown) can be fed into the load pressure signal circuit via the load pressure input 10 . Depending on whether the higher load pressure is then present at the load pressure input 10 or in the first load pressure line 8 , either the load pressure input 10 or the first load pressure line 8 is connected to the load pressure output 7 via the shuttle valve 11 . The load pressure signal present at the load pressure outlet 7 is reported via the input block to a hydraulic pump (not shown) or to a pump controller.

If the spool 4 is in the first switching position S1 or the second switching position S2, the pressure input 5 is a Inflow regulator 12 connected both to the first load pressure line 8 and to one of the two pressure outputs A, B, namely with a first pressure output A in the first switching position S1 and a second pressure output B in the second switching position S2. For example, a mast of the forest harvester can be controlled via the first pressure output A. In the neutral position N of the slide piston 4, the first load pressure line 8 and the second load pressure line 9 are connected to the return line 6, so that they are completely relieved towards a tank.

The second load pressure line 9 branches off from the first load pressure line 8 upstream of the shuttle valve 11 and is connected to the return line 6 . The pressure relief valve 2 is arranged in the second load pressure line 9 . As shown, a nozzle 13 is arranged in the branch of the second load pressure line 9 . Furthermore, the pressure in the second load pressure line 9 is reported to the inflow controller 12 .

In this exemplary embodiment, the pilot control 3 is a hydraulic pilot control 3 which generates a pilot control force for deflecting the slide piston 4 . The pilot control force is therefore a pilot control pressure, which is reported to the pressure-limiting valve 2 via a signaling line arrangement 14 . As shown, the pilot pressure in the signaling line arrangement 14 acts on the pressure-limiting valve 2 on the open-control side.

The signaling line arrangement 14 comprises a first signaling line 14a, a second signaling line 14b and a shuttle valve 14c. Depending on whether the spool 4 is deflected from the neutral position N via the pilot control 3 in the direction of the first switching position S1 or in the direction of the second switching position S2, the pilot control pressure is reported to the pressure relief valve 2 via the first signaling line 14a or via the second signaling line 14b.

The pilot pressure therefore acts against the closing force of an actuating device 15 of the pressure relief valve 2 and "helps" when opening the pressure relief valve 2. In this exemplary embodiment, the actuating device 15 is a spring, although other actuating devices (such as a magnet, for example) are also conceivable. Depending on the deflection of the spool 4, a pilot force proportional to the deflection also acts in the open direction of the pressure relief valve 2. If the spool is deflected, a pressure of up to 20 bar, for example, can be reported to the pressure relief valve 2 via the signaling line arrangement 14.

The pressure relief valve 2 is designed so that it opens at maximum deflection of the spool 4 at a maximum load pressure to be defined in the second load pressure line 9 and thus relieves the load pressure signal circuit to the tank via the return line 6 . If the maximum load pressure is limited in such a way that the maximum load is reached with a maximum deflection of the slide piston 4, a load slightly above the actual maximum load can be moved with a smaller deflection of the slide piston 4. Since only a relatively low speed can be driven with a relatively small deflection of the slide piston 4, no disadvantage for the steel construction of the forestry harvester is to be feared. It is then possible to move a load in excess of the actual maximum load at a slow speed.

In 2 A hydraulic circuit diagram of a second embodiment of a hydraulic system 100' according to the invention is shown. The hydraulic system 100′ according to the second embodiment differs from the hydraulic system 100 according to the first embodiment in that a third load pressure line 16 branches off upstream of the slide block 4 and downstream of the first pressure outlet A. The third load pressure line 16 opens into the second load pressure line 9 upstream of the pressure limiting valve 2 . A first check valve 17 is arranged in the third load pressure line 16 . In this exemplary embodiment, the first check valve 17 is a spring-loaded check valve. Furthermore, a second check valve 18 is arranged in the second load pressure line 9 downstream of the junction of the third load pressure line 16 .

On the other hand, a fourth load pressure line 19 branches off upstream of the spool 4 and downstream of the second pressure outlet B. The fourth load pressure line 19 opens into the second load pressure line 9 upstream of the pressure relief valve 2 and upstream of the second check valve 18. A third check valve 20 is arranged in the fourth load pressure line 19, which is a spring-loaded check valve in this exemplary embodiment.

Since a large volume flow can flow off over a relatively short time via the pressure-limiting valve 2, the pressure-limiting valve 2 is also suitable as a shock valve. As soon as a pressure surge occurs via the first pressure outlet A, for example if the mast of the forestry harvester gets stuck, the first non-return valve 17 opens when the pressure in the third load pressure line 16 upstream of the first non-return valve 17 is greater than the sum of the pressure in the second load pressure line device 9 and the spring preload of the first check valve 17. The pressure relief valve 2 also opens and thus relieves the second load pressure line 9 and the third load pressure line 16 via the return line 6 to the tank. The second check valve 18 prevents hydraulic fluid from flowing to the spool 4 . Accordingly, a pressure surge at the second pressure outlet B is relieved via the fourth load pressure line 19, the third check valve 20 and the pressure-limiting valve 2 to the tank.

In 3 a hydraulic circuit diagram of a third embodiment of the hydraulic system 100" according to the invention is shown. The hydraulic system 100" according to the third embodiment differs from that in FIG 2 Hydraulic system 100' shown is that the third load pressure line and thus the first check valve are not provided. Consequently, the pressure-limiting valve 2 only acts as a shock valve for the pressure outlet B.

Furthermore, the signaling line arrangement 14 in this exemplary embodiment includes only the second signaling line 14b, which reports the pilot control pressure of the pilot control 3 directly to the pressure-limiting valve 2 . A shuttle valve is not provided. Thus, in the first switching position S1, the load pressure limited at the pressure-limiting valve 2 actually acts, it being possible for a load that is above the maximum load pressure to be moved in the second switching position S2.

In 4 a hydraulic circuit diagram of a fourth embodiment of a hydraulic system 100''' according to the invention is shown. The hydraulic system 100''' according to the fourth embodiment differs from the hydraulic system 100" according to the second embodiment in that the signaling line arrangement 14 is completely omitted. Consequently, in this fourth embodiment, even with a small slide deflection, no load can exceed the maximum defined load Nevertheless, the pressure relief valve 2 ′ can also work as a shock valve here if a pressure surge occurs at the first pressure outlet A and via the third load pressure line 16 or at the pressure outlet B and via the fourth load pressure line 19 .

A pressure-limiting valve 2 will now be described below, as is used in a hydraulic system 100, 100', 100'' according to the first, second and third embodiment.

The pressure-limiting valve 2 is shown in accordance with FIG 5 and 6 screwed by way of example into a block 21 of the proportional slide valve 1 which is only partially shown. For this purpose, the pressure-limiting valve 2 has a valve housing 22 with an external thread, which can be screwed into a corresponding bore in the block 21 . The pressure-limiting valve 2 has a valve piston 24 that can be moved axially against the force of the actuating device 15 in a piston chamber 23 of the valve housing 22 . The valve housing 22 has a multiplicity of input connections 25 arranged radially on the circumference of the valve housing 22 . As shown, the input connections 25 open into a corresponding first chamber 26 formed in the block 21. A first connection 27, to which the second load pressure line 9 is connected, opens into the first chamber 26.

Furthermore, the valve housing 22 has a multiplicity of first connection terminals 28 which are arranged radially on the circumference of the valve housing 22 . As shown, the first connecting connections 28 open into a second chamber 29 formed in the block 21. A second connection 30, to which the return line 6 is connected, opens into the second chamber 29.

Furthermore, the valve housing 22 has a second connection port 31 . The second connection port 31 opens into a third chamber 36 formed in the block 21. A third port 37, to which the signaling line arrangement 14 is connected, opens into the third chamber 36.

The second connection port 31 has a bore 32 that penetrates the valve housing 22 axially and is coaxial with the movement axis of the valve piston 24 . An axially movable movement element 33 is arranged in the bore 32 . In the exemplary embodiment shown, the movement element 33 is a needle with a conical surface 34. The second connection port 31 has a corresponding contact surface 35 facing the valve piston 24 in the form of a seat surface. Consequently, the movement element 33 forms a seat valve with its conical surface 34 together with the contact surface 35, cf 7 . Furthermore, this arrangement has the advantage that the moving element 33 is held securely in the bore 32, for example when the pressure-limiting valve 2 is being installed.

The valve piston 23 is caused by the force generated by the actuating device 15 in the 5 held locked position shown. In this position, no hydraulic fluid can flow from the input ports 25 to the first connection ports 28 . The flow path between the input ports 25 and the first connection ports 28 is blocked.

As soon as pressure in the second load pressure line 9 acts on the valve piston 24 via the first connection 27, this becomes axial in proportion to the pressure moves against the actuating force of the actuating device 15 in the piston chamber 23. Thus, a flow path between the input ports 25 and the connection ports 28 is opened. In addition to the pressure at the first connection 27, the pilot control pressure prevailing in the signaling line arrangement 14 acts on the valve piston 24 in the open control direction or in the direction of the open position. This moves the movement element 33 in the axial direction so that it transmits a force to the valve piston 24. The pilot pressure thus also acts on the valve piston 24 in the opening direction of the pressure limiting valve 2. The force "helps" to open the pressure limiting valve 2 and is a product of Pilot pressure and cross-sectional area of moving element 33.

When the pressure relief valve 2 closes, the valve piston 24 moves axially in the piston chamber 23 in the direction of the second connection port 31 in the 5 shown blocking position. Here, the pressure in the piston chamber 23 increases and the moving element 33 is also moved axially until the conical surface 34 rests tightly on the contact surface 35. As a result, the gap leakage between bore 32 and moving element 33 is reduced. This makes it possible for the movement of the valve piston 24 to be damped when the pressure-limiting valve 2 closes.

Reference List

1

Proportional spool valve

2, 2'

pressure relief valve

3

pre-control

4

slide piston

5

pressure connection

6

return line

7

load pressure output

8th

first load pressure line

9

second load pressure line

10

load pressure input

11

shuttle valve

12

inflow controller

13

jet

14

alarm line arrangement

14a

first reporting line

14b

second reporting line

14c

shuttle valve

15

actuating device

16

third load pressure line

17

first check valve

18

second check valve

19

fourth load pressure line

20

third check valve

21

block

22

valve body

23

piston chamber

24

valve piston

25

input port

26

first chamber

27

first connection

28

first connection port

29

second chamber

30

second connection

31

second connection port

32

drilling

33

movement element

34

conical surface

35

contact surface

36

third chamber

37

third connection

100

Hydraulic system according to the first embodiment

100'

Hydraulic system according to the second embodiment

100''

Hydraulic system according to the third embodiment

100'''

Hydraulic system according to the fourth embodiment

A

first print output

B

second pressure outlet

N

neutral position

S1

first switch position

S2

second switch position

Claims (17)

Proportional slide valve (1) with a slide piston (4) that can be switched from a neutral position (N) via a pilot control (3) that generates a pilot control force into at least one first switching position (S1), a pressure connection (5), a first pressure outlet (A), a return line (6) and one connected to a load pressure outlet (7). first load pressure line (8), the pressure port (5) being connected to the first pressure outlet (A) and the first load pressure line (8) in the first switch position (S1), with the first load pressure line (8) forming a second load pressure line (9) branches off, the second load pressure line (9) being connected to the return line (6) and a pressure limiting valve (2) being arranged in the second load pressure line (9), characterized in that the pilot control force acts in the opening direction of the pressure limiting valve (2). Proportional slide valve (1) after claim 1 , characterized in that the pilot (3) is a hydraulic pilot (3) and the pilot force is a pilot pressure. Proportional slide valve (1) after claim 2 , characterized in that a signaling line arrangement (14) connects the pilot control (3) to the pressure-limiting valve (2), so that the pilot control pressure is reported to the pressure-limiting valve (2) via the signaling line arrangement (14). Proportional slide valve (1) according to one of the preceding claims, characterized in that the proportional slide valve (1) has a second pressure outlet (B) and via the pilot control (3) from the neutral position (N) into a second switching position (S2) can be switched, the pressure connection (5) being connected to the second pressure outlet (B) and the first load pressure line (8) in the second switching position (S2). Proportional slide valve (1) according to one of the preceding claims, characterized in that a third load pressure line (16) branches off downstream of the slide piston (4) and upstream of the first pressure outlet (A) in the direction of flow to the first pressure outlet (A), the third load pressure line (16) upstream of the pressure relief valve (2) in the second load pressure line (9) opens and a first check valve (17) is arranged in the third load pressure line. Proportional spool valve (1) with a spool (4) that can be switched from a neutral position (N) into at least one first switching position (S1), a pressure connection (5), a first pressure outlet (A), a return line (6) and a with a first load pressure line (8) connected to the load pressure outlet (7), with the pressure port (5) being connected to the first pressure outlet (A) and the first load pressure line (8) in the first switching position (S1), with the first load pressure line (8) forming a second load pressure line (9) branches off, the second load pressure line (9) being connected to the return line (6) and a pressure relief valve (2') being arranged in the second load pressure line (9), characterized in that a third load pressure line (16) is upstream of the slide piston (4) and branches off downstream of the first pressure outlet (A), the third load pressure line (16) upstream of the pressure relief valve (2') opening into the second load pressure line (9) and nd a first check valve (17) is arranged in the third load pressure line (16). Proportional slide valve (1) after claim 5 or 6 , characterized in that upstream of the third load pressure line (16) a second check valve (18) is arranged in the second load pressure line (9). Proportional spool valve (1) according to any of the previous ones Claims 5 until 7 , characterized in that the first check valve (17) is a spring-loaded check valve. Proportional spool valve (1) according to any of the previous ones Claims 5 until 8th , characterized in that a fourth load pressure line (19) branches off downstream of the spool (4) and upstream of the second pressure outlet (B) in the direction of flow to the second pressure outlet (B), the fourth load pressure line (19) upstream of the pressure relief valve (2') in the second load pressure line (9) opens out and a third check valve (20) is arranged in the fourth load pressure line (19). Proportional spool valve (1) according to any of the previous ones Claims 5 until 9 , characterized in that the third check valve (20) is a spring-loaded check valve. Pressure-limiting valve (2) for a proportional slide valve (1) according to one of the preceding claims, having a valve housing (22), an actuating device (15), and a valve piston (24) which can be moved axially in a piston chamber (23) of the valve housing (22), wherein the valve housing (22) has at least one input port (25) and at least one first connection port (28), and the valve piston (24) blocks or proportionally opens a flow path between the input port (25) and the first connection port (28) by the valve piston (24) moves between a blocked position and an open position in the piston chamber (23) against an actuating force generated by the actuating device (15), characterized in that the valve housing (22) has a second connection port (31), wherein a Pressure at the second connection port (31) acts against the actuating force of the actuating device (15) on the valve piston (24) in the opening direction of the pressure relief valve (2). Pressure relief valve (2) after claim 11 , characterized in that the second connection port (31) has a bore (32) passing through the valve housing (22) in the axial direction. Pressure relief valve (2) after claim 12 , characterized in that an axially movable movement element (33) is arranged in the bore (32), the movement element (33) transmitting a force acting in the direction of the open position to the valve piston (24) when pressure is applied to the second connection port (31). Pressure relief valve (2) after Claim 13 , characterized in that the movement element (33) has a conical surface (34) and that the second connection port (31) has a corresponding, the valve piston (24) facing contact surface (35). pressure relief valve Claim 14 , characterized in that the contact surface (35) is a seat surface. Hydraulic system (100, 100', 100'', 100''') with a proportional slide valve (1) according to one of Claims 1 until 10 . Hydraulic system (100, 100', 100''. 100''') according to Claim 16 , Wherein the pressure-limiting valve (2) is a pressure-limiting valve (2) according to one of Claims 11 until 15 is.

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