EP0968334B1 - Hydraulic control system for a mobile work machine, especially a wheel loader - Google Patents

Description

The invention is based on a hydraulic control arrangement, for a mobile machine, especially for one Wheel loader is used and the features from the generic term of claim 1.

From DE 39 09 205 C1 it is known the pitch vibrations of wheel loaders, especially when the loading shovel and higher driving speed occur with a damping system to dampen the component of the hydraulic control assembly of the wheel loader. For vibration damping are in general two hydraulic lift cylinders for lifting and lowering the loading shovel via a check valve to a hydraulic accumulator connectable from a hydraulic pump via a filling line, the one before the directional control valve block from the pump line branches, can be charged. That between the hydraulic accumulator and the shut-off valve arranged on the lift cylinders is closed, as long as the loading shovel is used, and can be opened by the driver or automatically as soon as the Driving pitch vibrations occur or as soon as the driving speed above a certain value, e.g. over 6 km / h lies.

The branch of the filling line in front of the directional control valve block brings with it that the hydraulic accumulator is not only one Actuation of the directional valve assigned to the lift cylinders, but when actuating any directional valve that leads to a Pressure build-up in the pump line leads, is charged. For example, can also actuate the hydraulic steering of the Steering valve belonging to the working machine to an inflow of Lead pressure medium to the hydraulic accumulator. Is that still then? Check valve open, so an uncontrolled movement of the Lift cylinders take place.

From US 5 195 864 A and DE 9 208 242 U are hydraulic Control arrangements with damping devices are known.

Another damping system against pitching vibrations, that too Part of the hydraulic control arrangement of a work machine is known from DE 41 29 509 C2. It branches the filling line from a work line, which between the Lift cylinders and the directional valve assigned to them runs. The check valve in the filling line is pressure controlled and can be of that in the work management Load pressure of the lift cylinder against the storage pressure from which one rear control chamber can be acted on the valve member of the check valve and against the force of a weak compression spring be opened. The memory pressure is therefore only slight in each case smaller than that occurring during a work cycle highest load pressure of the lift cylinder. To dampen pitching vibrations becomes the rear control chamber of the check valve Relieved via a pilot valve to the tank, so that the check valve opens and pressure medium free between the hydraulic accumulator and Lift cylinders can be pushed back and forth.

It has now been shown that due to special kinematic Conditions on a wheel loader when piercing the bucket in the earth build up a higher pressure in the lift cylinders can, when it is in the hydraulic accumulator, and then pressure medium is displaced from the lift cylinders into the hydraulic accumulator, even if the assigned directional valve is not operated. The Wheel loader is raised at the front so that the front wheels in the Turn the air and dig the rear wheels. This is for the charging process disadvantageous.

The invention has for its object a hydraulic Control arrangement with the features from the preamble of the claim 1 evolve so that uncontrolled movements to dampen pitch vibrations of the machine the hydraulic cylinder that can be connected to the hydraulic accumulator is largely avoided become.

This goal is achieved with a hydraulic control arrangement the features from the preamble of claim 1 according to the invention achieved in that the hydraulic accumulator can only be refilled, when the directional valve assigned to the hydraulic cylinder is actuated is. Thus, when another directional control valve is actuated no pressure medium flow to the hydraulic accumulator, so that the actuation of another directional valve does not lead to a movement of the hydraulic cylinder. It also prevents that when the directional control valve is not actuated from the hydraulic cylinder Pressure fluid is displaced into the hydraulic accumulator. The included Pressure medium therefore prevents movement of the cylinder piston and thus a change in the relative position between the boom and the body of the working machine, so that a lifting of the front axle is prevented.

Advantageous configurations of a hydraulic control arrangement can be found in the subclaims.

The hydraulic cylinder is usually a double-acting and controllable with the help of a directional valve, which consists of a medium Rest out in two opposite directions Working positions is adjustable. According to claim 2 it is provided that the hydraulic accumulator only when the directional valve is actuated in which one direction can be filled. The tax arrangement this simplifies. With a wheel loader e.g. is it are sufficient if the hydraulic accumulator only when the Flow pressure medium flow in the direction of lifting the lift cylinder can.

According to claim 3 is used to control the fillability of the hydraulic accumulator advantageously uses a control valve that by actuating the directional valve from a first switching position, in which it prevents the hydraulic accumulator from being filled, in which the filling line is so blocked that even then no pressure medium flows to the hydraulic accumulator when the pump pressure or the load pressure is higher than the storage pressure, in a second switching position is switchable, in which the hydraulic accumulator can be filled.

The control valve that can be switched by actuating the directional control valve is advantageously a seat valve, so that a largely Leak-free shut-off of the filling line guaranteed is.

Basically, it is conceivable that by actuating the directional valve switchable control valve mechanically with this too couple. Directional control valves used in mobile hydraulics are increasingly no longer directly mechanical, but via pre-control signals that can be specified using a pilot control device driven. According to claim 5, a pilot signal is now for the directional valve also used to generate a control signal to switch the control valve from the first Bring switching position into the second switching position.

An embodiment according to claim 6 is particularly preferred, after which the filling line from one between the directional valve and branches off the hydraulic cylinder running working line and itself a pressure-controlled shut-off valve with a Valve member is that of the prevailing in the working line Load pressure in the opening direction and from the pressure in a rear Control room can be acted upon in the closing direction. To the The rear control room is filled with the hydraulic accumulator connected to the hydraulic accumulator. To dampen pitching vibrations The working machine must have an open connection between the hydraulic accumulator and the hydraulic cylinder. For this the Relieved control room to the tank. To fill the Prevent hydraulic accumulator when the directional valve is not actuated is the rear control room against the outflow of Lockable pressure medium. The check valve can then from the load pressure cannot be opened.

Claims 7 to 10 contain advantageous developments a hydraulic control arrangement according to claim 6.

Several embodiments of a hydraulic according to the invention Control arrangement are shown in the drawings. Based The present invention will now become closer to these exemplary embodiments explained.

Figur 1

das erste Ausführungsbeispiel, bei dem die Fülleitung an die zu zwei Hydrozylindern führende Arbeitsleitung angeschlossen ist und sich in der Fülleitung ein Sperrventil befindet, das von zwei elektromagnetisch betätigbaren Pilotventilen gesteuert wird,

Figur 2

die zweite Ausführung, die weitgehend gleich der ersten Ausführung aufgebaut ist und bei der zwei einzelne Pilotventile der ersten Ausführung zu einem einzigen Pilotventil zusammengefaßt sind, und

Figur 3

die dritte Ausführung, bei der die Fülleitung vor dem Wegeventil von der Pumpenleitung abzweigt und sich in der Fülleitung ein Steuerventil befindet, mit dem bei nicht betätigtem Wegeventil die Fülleitung absperrbar ist.

Show it

Figure 1

the first embodiment in which the filling line is connected to the working line leading to two hydraulic cylinders and in the filling line there is a check valve which is controlled by two electromagnetically actuated pilot valves,

Figure 2

the second embodiment, which is largely identical to the first embodiment and in which two individual pilot valves of the first embodiment are combined to form a single pilot valve, and

Figure 3

the third embodiment, in which the filling line branches off from the pump line in front of the directional control valve and a control valve is located in the filling line, with which the filling line can be shut off when the directional control valve is not actuated.

The hydraulic control arrangements shown are each for Wheel loaders, tractors, telescopic handlers or other machines provided and comprise a control block 10 with a plurality of directional valves, in particular with a directional valve 11, the one can take spring-centered position and with the two Hydraulic cylinder 12 designed as a differential cylinder can be controlled with which the boom of a wheel loader is lifted and can be lowered. The directional control valve 11 has a first one Working connection 13, from which a first working line 14 to the bottom pressure chambers 15 of the hydraulic cylinder 12 leads. A second working line 16 runs between a second working connection 17 of the directional valve 11 and the piston rod side Pressure chambers 18 of the hydraulic cylinders 12.

In the embodiment according to Figure 1 branches off from the work management 14 from a filling line 20 leading to a hydraulic accumulator 21. In the filling line, a check valve 22 is arranged, which is a 2-way cartridge valve is formed and a movable valve member 23 has. This is a stepped piston with the Face of the piston section smaller in diameter Kind of a seat valve can sit on a seat cone. On the said end face becomes the valve member 23 from in the working line 14 prevailing pressure, i.e. the load pressure of the two Hydraulic cylinder 12 acted upon in the opening direction. At the Annular surface between the two piston sections of the valve member 23 acts the accumulator pressure in the opening direction. In Closing direction, the valve member 23 is one in one rear control chamber 24 prevailing pressure and one weak compression spring 25 is applied.

On the plate 26 with the 2-way cartridge valve 22 are three more Valves built. The first valve 30 is a 3/2-way valve with a first input 31, which with the between the working line 14 and the check valve 22 section the filling line 20 is connected, and with a second Input 32, which is connected to the hydraulic accumulator 21. On Output 33 of the directional control valve 30 is dependent on the load pressure in the working line 14 either with the input 31 or with the input 32 connectable. It works in the sense of a connection of the output 33 with the input 32 an adjustable Compression spring on the valve member not shown Valve 30. In the sense of a connection of the output 33 with the Input 31, the valve member from the pressure in input 31, So acted upon by the load pressure of the hydraulic cylinder 12.

A pilot valve 40 is placed on the directional control valve 30 is designed as a 2/2-way seat valve and as an unlockable Check valve can be considered. Through the housing of the Pilot valve 40 leads through from outlet 33 of the directional valve 30 outgoing control channel 41 to a connection P one another pilot valve 50, which is a 4/2-way valve. Whose Valve member takes a under the action of a compression spring 51 Rest position on, in the passage between the port P and there is a connection A with a connection of the pilot valve 40 is connected. A tank connection T and another connection B of the pilot valve 50 are blocked in its rest position. The tank connection is over through the housing of the various Channels leading through valves with a leakage connection Y of the plate 26 connected. Port B of the pilot valve 50 is with the rear control room of a second yourself connected in the plate 26 2-way cartridge valve 60, via which the piston rod-side pressure chambers 18 of the hydraulic cylinders 12 connected to a tank port T of the plate 26 can be. The valve member of the pilot valve 50 can be of one Electromagnets 52 are brought into a switching position, in which the port P is shut off and the two ports A and B are connected to the terminal T.

The pilot valve 40 is in a line 53 between the connection A of the pilot valve 50 and the rear control chamber 24 of the check valve 22 arranged. Its valve member is from one Compression spring 42 and of that in the rear control chamber of the check valve 22 prevailing pressure towards a rest position pressurized in which there is an outflow of pressure medium from the rear control chamber 24 for connection A of the pilot valve 50 prevents an inflow of pressure medium from port A of the pilot valve 50 to the rear control room 24 but allows. The The valve member can be moved from an electromagnet 43 into a switching position in which there is a free passage between the port A of the pilot valve 50 and the rear control room 24 of the check valve 22.

The directional control valve 11 of the control block 10 is hydraulically proportional operable, the pilot pressures using a hydraulic Pilot control unit 55 is generated and via control lines 56 are given to the directional valve 11. To the pilot line, into which a pilot pressure is set when the Hydraulic cylinder 12 are operated in the direction of lifting the boom and the bottom-side pressure chambers 15 with pressure medium is an electrical pressure switch 57 connected, which switches at a certain pilot pressure. This particular pilot pressure is preferably that Pilot pressure at which the spring-centered valve spool of the directional valve 11 begins to move. By switching the Pressure switch 57 becomes the electromagnet 43 of the pilot valve 40 excited and put this pilot valve in the switch position, in which there is an open connection between port A of the pilot valve 50 and the rear control chamber 24 of the check valve 22 prevails.

The solenoid 52 of the pilot valve 50 is arbitrarily by the driver if pitching vibrations occur, or automatically at a certain speed of the mobile machine, e.g. at a speed of 6 km / h on Suspense.

In the rest position of the directional valve 11 is in the pilot line 56, to which the pressure switch 57 is connected, none Pilot pressure on, so that the pressure switch 57 in its Rest position and the solenoid 43 of the pilot valve 40 is de-excited. The pilot valve 40 therefore takes the one in the figure 1 shown switching position in which an outflow of pressure medium from the rear control chamber 24 of the check valve 22 is prevented. Should now when inserting the loading shovel in the ground or other bulk material in the pressure rooms 15 the hydraulic cylinder 12 build up a pressure, so this can Pressure shut-off valve 22 cannot be opened because of an opening of the check valve would only be possible if pressure medium from the Control room 24 would be displaced. The hydraulic accumulator 21 is therefore in its state of charge due to the pressure build-up in the pressure rooms 15 not affected. The same applies if the directional valve in the direction of lowering the hydraulic cylinder 12 is actuated. Also then the pilot valve 40 remains in the switching position shown, so that the check valve 22 does not depend on the pressure in the pressure chambers 15 the hydraulic cylinder 12 can be opened.

In contrast, when the directional valve 11 is actuated in the direction of lifting the pressure switch 57 switches, causing the electromagnet 43 is actuated and the valve member of the pilot valve 40 in the second switching position. As long as the pressure in the pressure rooms 15 below that on the compression spring 34 of the directional control valve 30 set pressure remains, this switches the memory pressure via the pilot valve 50 and the pilot valve 40 to the rear Control chamber 24 of the check valve 22 through. The load pressure in the pressure chambers 15 of the hydraulic cylinder 12 this opens Check valve 22 whenever it is at least about the force the compression spring 25 equivalent small pressure difference above that Memory pressure is. There can then be pressure fluid in the hydraulic accumulator get, so that this one can see from the force the weak compression spring 25, always on the largest during a work cycle occurring load pressure in the pressure rooms 15 is charged. Only if the load pressure on valve 30 die Power to overcome the compression spring 34 remains the check valve 22 closed. Because after switching the valve 30 is in the rear control chamber 24 of the check valve 22 Load pressure on, so that in conjunction with the compression spring 25, the check valve 22 is held securely. The pressure in the hydraulic accumulator 21 can therefore the set on the compression spring 34 of the valve 30 Do not exceed value. For security reasons, however additionally a pressure relief valve 58 is provided, the Input is connected to the hydraulic accumulator 21.

It is now assumed that the loading shovel is loaded and with the Wheel loader is driven to an unloading point. As soon as one certain speed is exceeded, both magnets 43 and 52 of the pilot valves 40 and 50 energized so that both pilot valves from the rest positions shown in Figure 1 switch to the other switch position. Now the back one Control chamber 24 of the shutoff valve 22 via the pilot valve 40 and through pilot valve 50 to port Y of plate 26 connected and thus relieved to tank 27. The valve member 23 the check valve 22 is of its load and accumulator pressure Seat lifted off, leaving an open connection between the hydraulic accumulator 21 and the pressure chambers 15 of the hydraulic cylinders 12. Since the memory pressure 21 the during the work cycle corresponds to the maximum pressure reached when opening the Check valve 22 no sagging, but at most a light one Raise the loading shovel. It may be that during the working game Load pressures occur that cause valve 30 to switch bring and therefore the state of charge of the hydraulic accumulator 21 does not follow. However, these load pressures occur only in special Situations, e.g. when tearing one off in the ground anchored object or when driving the loading shovel against a stop on, but are not by the weight of the Loading shovel and the cargo due to driving alone of the wheel loader works. The state of charge of the hydraulic accumulator 21 is therefore always sufficient to keep the bucket level keep that it takes when opening the check valve 22.

About the one that is also opened by switching the pilot valve 50 Valve 60 can pressure fluid from the piston rod side Pressure chambers 18 of the hydraulic cylinder 12 displaced into the tank or fed from the tank to make volume changes to equalize the pressure spaces 18 during the open connection the pressure chambers 15 occur with the hydraulic accumulator 21.

The embodiment of Figure 2 differs only in With regard to the pilot valves 40 and 50 from that of Figure 1. It is therefore with regard to the control block 10 that Hydraulic cylinder 12, the hydraulic accumulator 21, the check valves 22 and 60, the directional control valve 30 and the pilot control unit 55 to the corresponding Description of Figure 1 referenced. Differently to the embodiment of Figure 1 is that now the two pilot valves 40 and 50 combined into a single pilot valve 60 are, which is placed on the directional control valve 30. The pilot valve 60 has a spring-centered central position in which the connections T and B are blocked off and between the connections P and A only a flow from P to A is possible. The valve member of the pilot valve 60 can be activated by driving a Electromagnet 52 from the central position into a side Switch position in which the connections A and B are connected to the terminal T. From the middle position in the opposite direction, the valve member can be acted upon with the pressure prevailing in the pilot line 56 be adjusted in the for the supply of pressure medium in the bottom-side pressure chambers 15 of the hydraulic cylinder 12 a pilot pressure is built up. In this other switching position of the directional valve 60, port A is freely connected to port P. Port A is now directly connected to the rear control room 24 of the check valve 22 connected.

In the middle position or in the lower position of the directional valve 11, the pilot valve 60 assumes its central position, if the speed of the work machine a certain one Value does not exceed or the electromagnet 52 is not arbitrary has been switched on. In the raised position of the directional valve 11 becomes the valve member of the directional valve 60 with pressure acts and reaches the switch position in which the connection P is connected to port A. In the back Control chamber 24 of the check valve 22 is then depending on the position of the valve 30 the accumulator pressure or the load pressure. The hydraulic accumulator is up to the maximum load pressure reached or to the pressure set on the compression spring 34 of the valve 30 charged. To dampen pitching vibrations during faster The solenoid 52 is energized so that the valve member of the directional control valve 60 reaches the switching position in which the Ports A and B are connected to port T. The back Control rooms of the check valves 22 and 60 are pressurized relieved, so that the two check valves 22 and 60 open and an open connection between the hydraulic accumulator 21 and the Pressure rooms 15 of the hydraulic cylinder 12 and consists of the pressure rooms 18 pressure medium displaced and in the pressure chambers 18 pressure medium can be fed. On this occasion, be express noted that the pressure at port T of a Valve blocks are usually a few bar above the tank pressure there is at the leakage oil connection Y, so that the check valve 60th of the pressure at the tank connection T against the tank pressure in the rear Control room can be opened.

It is particularly advantageous that after slowing down and De-excitation of the electromagnets 43 and 52 after execution Figure 1 and the electromagnet 52 alone in the execution according to Figure 2 because of the operation of the pilot valves 40 and 60 as check valves immediately store pressure in the rear Control chamber 24 of the check valve 22 is controlled and therefore the shut-off valve closes. Influencing the hydraulic accumulator 21 or an influence by the hydraulic accumulator 21 the control is then at a subsequent lowering Hydraulic cylinder 12 or unloading the bucket, meanwhile the directional valve 11 is in its central position, locked out.

In the embodiment according to FIG. 3, the filling line 20 branches out the valve control block 10 from a pump line 65. In the filling line 20 leading to several hydraulic accumulators 21, in Flow direction of the pressure medium from the pump line 65 to seen the hydraulic cylinders 21, first a 2/2-way valve 70, a pressure control valve 71 and a check valve 72 are arranged. In the rest position brought about by a compression spring Directional control valve 70 whose two connections are shut off from each other. The directional control valve can be controlled by an electromagnet 73 can be switched into a through position. And the electromagnet 73 is energized whenever that happens Directional valve 11 located within the control block 10 for actuation the hydraulic cylinder 12 is actuated in the direction of lifting. In the middle position of the directional valve 11 and when it is actuated the directional control valve 70 is in its direction Starting position.

A maximum pressure is set on the pressure control valve 71 to to which the hydraulic accumulator 21 can be charged. As long as this pressure is not reached, the pressure control valve switches the outlet of the directional valve 70 to the check valve 72 through. When the pressure is reached, the valve with the outlet of the directional valve 70 connected input of the valve 71 blocked and the with the connection connected to the recoil is connected to the tank.

The hydraulic accumulator 21 can via a line 74 with the between the working connection 13 of the directional control valve and the pressure chambers 15 of the hydraulic cylinder 12 extending working line 14 connected become. In this line 74 is a 4/2-way valve 75 installed, the rest position under the action of a compression spring 76 occupies in the two sections of line 74 against each other are cordoned off and therefore no connection between the hydraulic accumulators 21 and the working line 14. Except the two necessary for opening and closing line 74 The directional valve 75 has two further connections for two sections of a line 77 by the work line 15 between the directional control valve 11 and the pressure chambers 18 the hydraulic cylinder 12 leads to the tank 27. At rest of the directional control valve 75 are also the two sections of the line 77 cordoned off from each other. The directional control valve 75 can be of one Electromagnets 78 are brought into a switching position in each of the two sections of line 74 and the two Sections of line 77 are interconnected. The electromagnet 78 is energized when the hydraulic with the shown Control arrangement equipped work machine a certain Driving speed exceeds. Then there are the hydraulic accumulators 21 connected to the pressure chambers 15 of the hydraulic cylinders 12, so that pitching vibrations can be damped. volume changes the pressure spaces 18 can be compensated via the line 77 become.

Since the electromagnet 73 of the directional control valve 70 is only excited, when the directional control valve 11 in the direction of lifting the hydraulic cylinder 12 is actuated, that is, when pressure medium 15 is supplied to the pressure chambers only then is the hydraulic accumulator 21 charged possible. For example, not happen to one during faster travel of the working machine in line 65 Pressure over the filling line 20 and that when driving fast switched valve 75 pressure medium in the pressure chambers 15 of the hydraulic cylinders 12 arrives, even if the directional valve 11 in Middle position and an extension of the piston rods Hydraulic cylinder 12 is not desired.

Claims (10)

1. A hydraulic control arrangement for a mobile working machine, in particular for a wheel loader, having at least one hydraulic cylinder (12) with the aid of which a working tool can be moved, having a directional control valve (11) for controlling the pressure-fluid channels between the hydraulic cylinder (12), a pressure-fluid source and a tank (27), having a hydraulic accumulator (21) which can be connected to the pressure-fluid source via a filling line (20), and having a control valve (50, 60, 75) with the aid of which a connection between the hydraulic accumulator (21) and the hydraulic cylinder (12) can be controlled to open and close, characterized in that the hydraulic accumulator (21) can be filled up only when the directional control valve (11) is actuated.
2. A hydraulic control arrangement according to Claim 1, characterized in that the directional control valve (11) can be adjusted into working positions in two opposite directions starting from a middle neutral position, and that the hydraulic accumulator (21) can be filled up only when the directional control valve (11) is actuated in the one direction.
3. A hydraulic control arrangement according to Claim 1 or 2, characterized by a control valve (40, 60, 70) which can be switched over by actuating the directional control valve (11) from a first control position, in which it prevents filling of the hydraulic accumulator, into a second control position, in which the hydraulic accumulator (21) can be filled.
4. A hydraulic control arrangement according to Claim 3, characterized in that the control valve (40, 60) which can be switched over by actuating the directional control valve (11) is a seat valve.
5. A hydraulic control arrangement according to Claim 3 or 4, characterized in that the directional control valve (11) can be driven via a pilot control signal which can be selected with the aid of a pilot control device (55), and that, given the presence of a pilot control signal for the directional control valve (11), a control signal for switching over from the first control position into the second control position can be fed to the control valve (40, 60, 70) that can be switched over by actuating the directional control valve (11).
6. A hydraulic control arrangement according to any of the preceding claims, characterized in that the filling line (20) branches off from a working line (14) running between the directional control valve (11) and the hydraulic cylinder (12), that arranged in the filling line (20) is a pressure-controlled shut-off valve (22) with a valve element (23) to which the load pressure prevailing in the working line (14) can be applied in the opening direction and the pressure in a rear control chamber (24) can be applied in the closing direction, and that the rear control chamber (24) can be connected to the hydraulic accumulator (21) in order to fill the hydraulic accumulator (21), and can be connected to the tank (27) in order to open the connection between the hydraulic accumulator (21) and the hydraulic cylinder (12), and can be blocked off against the outflow of pressure fluid when the directional control valve (11) is not actuated.
7. A hydraulic control arrangement according to Claim 6, characterized in that, when the directional control valve (11) is not actuated, pressure fluid can be applied from the hydraulic accumulator (21) to the rear control chamber (24) of the shut-off valve (22) by a check valve (40, 60) which opens towards the control chamber (24).
8. A hydraulic control arrangement according to Claim 6 or 7, characterized by a pilot valve (60) by means of which in a middle position the rear control chamber (24) of the shut-off valve (22) is blocked off against the outflow of pressure fluid, which can be adjusted from the middle position into a first lateral control position in which the rear control chamber (24) is relieved to the tank (27), and which can be adjusted by actuation of the directional control valve (11) from the middle position into a second lateral control position, in which accumulator pressure can be applied to the rear control chamber (24).
9. A hydraulic control arrangement according to Claim 6 or 7, characterized by a first pilot valve (50) having a control position in which the rear control chamber (24) of the shut-off valve (22) can be relieved to the tank (27), and having a control position in which accumulator pressure (21) can be applied to the rear control chamber (24), and by a second pilot valve (40), which is arranged in the pressure-fluid channel (53) running from the rear control chamber (24) to the first pilot valve (50), and has a control position in which the pressure-fluid channel (53) is blocked off against outflow of pressure fluid from the rear control chamber (24), and a control position in which the pressure-fluid channel (53) is open.
10. A hydraulic control arrangement according to Claim 9, characterized in that the two pilot valves (40, 50) can in each case be actuated by an electromagnet (43, 52).

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