WO2020173833A1

WO2020173833A1 - Refilling unit for refilling hydraulic oil into a hydraulic system pre-pressurized by a low-pressure accumulator and pre-pressurized hydraulic system having such a refilling unit

Info

Publication number: WO2020173833A1
Application number: PCT/EP2020/054661
Authority: WO
Inventors: Eric SCHUERR; Stefan Heitzenroeder; Stanislas Trappe
Original assignee: Robert Bosch Gmbh
Priority date: 2019-02-28
Filing date: 2020-02-21
Publication date: 2020-09-03
Also published as: DE102019202683A1

Abstract

The invention relates to a refilling unit for refilling hydraulic oil into a hydraulic system pre-pressurized by a low-pressure accumulator, characterized by a reservoir, in which a reservoir chamber having hydraulic oil is disposed and which has a refilling connection, and by a movable refilling piston, which delimits the reservoir chamber, the movement of which displaces hydraulic oil through the refilling connection. The invention also relates to a pre-pressurized hydraulic system having such a refilling unit.

Description

Refill unit for refilling hydraulic oil in a hydraulic system which is pressurized with a low pressure accumulator and a hydraulic system which is pressurized

System with such a refill unit

description

The invention relates to a refilling unit for refilling hydraulic oil in a hydraulic system which is pressurized with a low-pressure accumulator.

The invention also relates to a hydraulic system comprising a closed hydraulic system with a hydrostatic displacement machine, with an electric machine that is mechanically speed-coupled to the displacement machine, with a hydraulic cylinder that is fluidically connected to the displacement machine via a first working line and a second working line is and is actuated by reversing the fluid flow through the Ver displacement machine in opposite directions, and with a biased to a low pressure low-pressure hydraulic accumulator, which is fluidly connectable via a valve arrangement each Weil with the working line in which the lower pressure prevails.

A servohydraulic drive representing such a pressure-pretensioned closed hydraulic system is known from DE 10 2012 020 581 A1. The abbreviation SHA, equal to servohydraulic axis, is used for such a drive. Such a SHA usually comprises a hydrostatic displacement machine, the stroke volume of which can optionally be adjusted by a hydraulic adjustment device, an electric machine that is mechanically speed-coupled to the displacement machine, a hydro-cylinder that is fluidically connected to the displacement machine via a first working line and a second working line is and is actuated by reversing the fluid flow through the Ver displacement machine in opposite directions, and a low-pressure hydraulic accumulator preloaded to a Nie derdruck, which is fluidically connectable via a valve assembly to the working line in which the lower pressure prevails. The nie The pressure accumulator has the function of clamping the hydraulic oil in the system at a low pressure, which can be between 2 bar and 4 bar, for example, and also serves as a compensating tank to compensate for the different amounts of pressure medium that are in the working lines, in the hydraulic cylinder and in the displacement machine including a possibly existing adjustment device for the displacement of the displacement machine.

Ideally, a SHA has no leakage, so that the storage pressure does not drop if the temperature and work cycles remain the same. In practice, however, a slight leak and thus a decrease in the accumulator pressure over a longer operating period cannot be completely avoided. Oil samples are also taken at an SHA at regular intervals to check the quality of the oil. These oil withdrawals also cause the accumulator pressure to drop.

The aim of the invention is to make it possible in a simple manner to replace the hydraulic oil that has been lost in a pressurized closed hydraulic system due to leakage or removal.

This possibility is created by a refill unit which has a storage container in which there is a storage space with hydraulic oil and which has a refill connection, and a displaceable refill piston which delimits the storage space and, by moving it, hydraulic oil can be displaced through the refill connection. A hydraulic system according to the invention comprising a closed hydraulic system with the features indicated at the beginning is characterized by a refill unit as indicated above.

A refill unit according to the invention can advantageously be further developed. A hydraulic system according to the invention can likewise be further developed in an advantageous manner.

It is therefore advantageous if the amount of hydraulic oil displaced from the storage space per unit of time is adjustable. You can then set the amount per unit of time depending on the total amount of hydraulic oil to be topped up and in this way by limiting By reducing the amount of refill per unit of time, among other things, ensure that the pressure in the storage room does not become too high.

The amount that is refilled per unit of time can be set directly by specifying the speed at which the refill piston is moved.

If a constant force is applied to the refill piston during a refill process, the amount per unit of time can be specified by displacing the hydraulic oil from the reservoir via a throttle valve whose flow cross-section is adjustable. Due to the constant force acting on the refill piston, a certain pressure is built up in the reservoir of the refill unit due to the throttle valve. The amount flowing through the throttle valve then results from the flow cross section of the throttle valve and from the difference between the pressure in the storage space and the pressure in the low-pressure hydraulic accumulator. Small pressure drops in the lines and in any valves or filters present between the throttle valve and the low-pressure accumulator are not taken into account here.

The flow cross-section of the throttle valve can advantageously be reduced to zero. Thus, a refilling process can be ended by adjusting the throttle valve to zero.

There may be a non-return valve on the refill connection, which blocks in the direction of the storage area. This prevents the storage container from being subjected to a high pressure from the pressurized hydraulic system because the refill unit has not been connected to the hydraulic system at the correct point. The check valve thus fulfills a safety function and protects the storage container from bursting if it is not designed to withstand the high pressures occurring in the hydraulic system.

It is advantageous if there is a coupling part with a non-return valve which blocks away from the refill connection and the storage space and which can be mechanically unlocked when the coupling part is connected to a counterpart. Thus, when the refill unit is uncoupled from the hydraulic system, the storage space is automatically shut off. A measuring connection for connecting a manometer or a pressure sensor can be located at the refill connection. A filter can also be connected downstream of the refill connection, which retains dirt particles that could migrate from the refill unit into the hydraulic system. The check valve, which blocks in the direction of the refill connection and the storage space, the measuring connection, the coupling part and the filter can be present individually or in any combination with one another.

In a special embodiment, the refill piston can be displaced by compressed air. In this case, the refill piston is acted upon by a constant force as long as the pressure of the compressed air is constant.

A pressure reducing valve can be used to set the pressure applied to the refill piston to be lower than the pressure of the compressed air network. At the same time, this setting also specifies the pressure to which the low-pressure hydraulic accumulator is charged. If the pressure in the low-pressure hydraulic accumulator and thus in the storage space is so high that there is an equilibrium of forces on the refill piston, the refill process ends.

Preferably, the storage container has a housing, the interior of which is divided by the refilling piston into the storage space and an air-filled space to which compressed air can be applied. The refill piston can have a piston rod on one side, which crosses the air-filled space and is guided out of the housing, with the outer end of the piston rod dipping into a protective housing. The refill piston is well guided with the aid of the piston rod. Due to the piston rod, the effective area of the refill piston on the air side is smaller than on the oil side, so that the pressure to which the low-pressure hydraulic accumulator is charged is slightly lower than the pressure with which the refill piston is applied. For example, an air pressure of 2.9 bar can result in a storage pressure of 2.7 bar.

There can be several storage containers which are combined to form a unit and whose individual refill connections are fluidically connected to one another and merged to form a common refill connection. Refill unit according to any preceding claim, wherein it comprises a drive device which acts mechanically on the refill piston, and wherein the drive device and the storage container are releasably connected to one another. The drive device preferably comprises an electric motor, of which the refill piston can be displaced in a straight line via a helical gear. The speed at which the refill piston moves is directly specified by the speed of the electric motor.

The electric motor can preferably be switched off automatically as a function of the low pressure of the closed hydraulic system detected by a pressure sensor.

The refill unit can also be connected to the closed hydraulic system in a stationary manner, the electric motor being able to be switched on automatically as a function of the low pressure of the closed hydraulic system detected by a pressure sensor.

If, in a hydraulic system according to the invention, the low-pressure hydraulic accumulator can be connected via a releasable check valve that locks towards it and a filter with the Arbeitslei device in which the lower pressure prevails, the refill unit is advantageously between the releasable check valve and the filter connected to the closed hydraulic system. The filter may retain existing oil in the refilled oil

In a hydraulic system according to the invention, a pressure sensor can be present from which the pressure in the low-pressure hydraulic accumulator can be detected directly. An electric motor, of which the refill piston can be displaced in a straight line via a helical gear, is advantageously controllable as a function of the accumulator pressure detected by the pressure sensor. This can be done like a two-point control. When the accumulator pressure falls below a minimum value, the electric motor is switched on; when the accumulator pressure reaches a maximum value, the electric motor is switched off. The condition of the system must also be taken into account, especially if the hydraulic cylinder is a differential cylinder. The further the piston rod of the differential cylinder is extended, the greater the amount of hydraulic oil that is in the hydraulic cylinder, and the smaller the amount of hydraulic oil that is in the low-pressure hydraulic accumulator, and the lower the accumulator pressure. A pressure signal is therefore advantageously used for Control of the electric motor used, which the pressure sensor emits rozylinders in a very specific state of the system, for example when the piston rod of the hydraulic cylinder is completely retracted. A pressure-dependent electrical switch is also considered as a pressure sensor.

An embodiment of a hydraulic system according to the invention and two Ausfüh approximately examples of a refill unit according to the invention are shown in the drawings Darge. The invention will now be explained in more detail using the figures of these drawings.

Show it

Figure 1 as a circuit diagram, the embodiment of a hydraulic system according to the invention,

Figure 2 shows the first embodiment of a refill unit according to the invention as a circuit diagram, with two storage containers and two refill pistons are present and the refill pistons are displaceable by compressed air,

FIG. 3 shows the refill unit from FIG. 2 in a perspective view and FIG

FIG. 4 shows the second exemplary embodiment of a refill unit according to the invention, with only one storage container and only one refill piston being present and with the refill piston being displaceable by an electric motor via a helical gear.

The pressure-preloaded hydraulic system 9 according to FIG. 1 comprises a hydrostatic displacement machine 10 which can be operated both as a pump and as a motor and whose stroke volume is constant. The displacement machine has a first working connection 12 and a second working connection 13 and is, for example, an axial piston machine with a swash plate design. With the displacement machine is mechanically without interconnection of a transmission a speed controllable electric machine 14 gekop pelt, with this coupling the electric machine in operation as a motor drive the displacement machine in operation as a pump in both directions of rotation and in egg operation as a generator can be driven in both directions of rotation by the displacement machine in its operation as a motor. The speed of the machine combination consisting of the displacement machine and electric machine can be detected by a speed sensor, not shown. The hydraulic system also comprises a hydraulic cylinder 16, which is designed as a differential cylinder with a piston 17 and a piston rod 18 on one side and thus with an annular cylinder chamber 19 on the piston rod side and a cylinder chamber 20 with a circular disk-shaped cross section. The cylinder chamber 20 is connected directly to the working port 12 of the displacement machine 10 via a working line 21. The cylinder chamber 19 is connected directly to the working connection 13 of the displacement machine 10 via a working line 22.

The hydraulic system also includes a hydropneumatic accumulator 29, which is referred to below as a hydraulic accumulator for short, which is designed, for example, as a bladder accumulator or diaphragm accumulator and which is preloaded to a low pressure, for example between 2 and 4 bar. The hydraulic accumulator 29 is thus a low-pressure hydraulic accumulator and as such is connected to a low-pressure manifold 30. This is fluidically connected to the working line 21 via a check valve 31 that opens towards the latter and to the working line 22 via a check valve 32 that opens towards the latter. Between the low pressure manifold 30 and the working line 21 are also a pressure relief valve 33, through which the pressure in the working line 21 and thus in the cylinder chamber 20 is limited to a maximum value, and a releasable check valve 34, which opens to the working line 21 and can be unlocked by a pressure in the working line 22. After unlocking the releasable check valve 34, pressure fluid can flow from the working line 21 to the Niederdrucksammellei device 30 and thus to the hydraulic accumulator 29. Between the low pressure manifold 30 and the working line 22 are also a pressure relief valve 35, through which the pressure in the working line 22 and thus in the cylinder chamber 19 is limited to a maximum value, and a releasable check valve 36 that opens to the working line 21 and can be unlocked by a pressure in the working line 21.

After unlocking the unlockable check valve 36, pressure fluid can flow from the working line 21 to the low-pressure collecting line 30 and thus to the hydraulic accumulator 29.

In the low pressure manifold 30, a filter 37 is looped between the two releasable Rückschlagven valves 34 and 36 and the two pressure relief valves 33 and 35, the two check valves 31 and 32 and the connection point of the hydraulic accumulator 29, so that one of the two releasable from a working line Return impact valves the hydraulic accumulator 29 flowing hydraulic oil flows through the filter 37. In the bypass to the filter 37 there is a check valve 38 which opens towards the hydraulic accumulator 29 when the pressure drop across the filter exceeds a maximum value. This can happen if the filter is very dirty. The check valve 38 protects the filter from excessively high pressures.

In series with the combination of filter 37 and check valve 38, a check valve 39 is arranged between this com bination and the releasable check valves 34 and 36, which locks towards the filter 37. The check valve 39 prevents a flow of hydraulic oil from the hydraulic accumulator 29 and the pressure relief valves 33 and 35 through the filter 37 via one of the releasable check valves 34, 36 in a Arbeitslei device. This prevents the entry of dirt particles, which have been captured by the filter in the reverse flow direction, into a working line.

The check valve 31 and the pressure limiting valve 33 as well as the check valve 32 and the pressure limiting valve 35 are usually combined to form what is known as a pressure suction valve.

Finally, a leakage connection 40 of the displacement machine 10 is fluidically connected to the hydraulic accumulator 29.

Various sensors are used to monitor and control the pressurized hydraulic system. As already mentioned, the rotational speed of the hydrostatic displacement machine 10 and the electric machine 14 can be detected with the rotational speed sensor. The temperature of the leakage fluid flowing from the displacement machine 10 to the hydraulic accumulator 29 is detected with a temperature sensor 41. The pressure in the hydraulic accumulator 29 is detected with a pressure sensor 42. The pressure in the cylinder chamber 20 is recorded with a pressure sensor 43. The pressure in the cylinder chamber 19 is detected with a pressure sensor 44. The position of the piston 17 of the hydraulic cylinder 16 is detected with a displacement sensor 45. From the position, the speed can be calculated by differentiation and the acceleration of the piston 17 by further differentiation.

The pressurized hydraulic system according to FIG. 1 can have a slight leak. In addition, oil samples are taken regularly to check the oil quality check. Leakage and oil sampling cause the preload pressure in the system to drop over time. Provision is now made for the amount of oil that has escaped and removed from the system to be refilled when the accumulator pressure drops below a certain value, for example when the piston rod 18 is fully inserted. For refilling, a hydraulic block 50, in and on which the above-described valves and the filter are installed and attached, has a connection coupling 51 which has a built-in check valve 52 and from which a line 53 extends, which runs between the check valve 39 and the filter 37 opens into the low-pressure manifold 30. A refill unit 60 is connected to the connection coupling 51 via a further coupling 54, which is also equipped with a check valve 52, and optionally a hose 55. When the clutches 51 and 54 are put together, the two check valves 52 are unlocked. The refill unit 60 comprises a storage container with a storage space in which there is hydraulic oil, and a refill piston, which is displaced for refilling, and hydraulic oil from the storage space via the line 55, the couplings 54 and 51, the line 53 and the filter 37 to Hydraulic accumulator 29 displaced.

According to FIGS. 2 and 3, the refill unit 60 is a hydropneumatic unit. According to the exemplary embodiment according to FIGS. 2 and 3, it then has two hydraulic cylinders as a storage container 61. The interior of each storage container is divided into a storage space 63 for hydraulic oil and a compressed air space 64 by a refill piston 62. A piston rod 65 is arranged on one side of the refill piston 62 and crosses the compressed air space 64 and emerges from the storage container. The outer end of the piston rod 65 is surrounded by a protective housing 66 which is open to the atmosphere via a silencer 67. The two compressed air chambers 64 are jointly connected to a compressed air network via a pressure control valve 68. The pressure in them can be read off via a manometer 69. A pressure limiting valve 70 connected to them protects the reservoirs 61 from excessive compressed air pressure.

The two reservoirs 63 for hydraulic oil are jointly fluidly connected to a refill connection 73 via a check valve 71, which blocks towards them, and a throttle valve 72, the flow cross section of which is adjustable, which is provided with an outwardly blocking, pushable check valve 74. The pressure in the storage spaces 63 can be read on a manometer 75. The hydraulic oil is transferred from the refill connection to the pressure-preloaded, closed hydraulic system 9 via hose 55. wear. When refilling, the throttle valve 72 should be opened slowly from its closed position so that too much hydraulic oil does not flow into the system 9. The storage spaces 63 can be filled again via a connection 77 when they have become empty.

It can be seen from FIG. 3 that the two storage containers 61 are held together as a unit by two plates 78 in front of their end faces. On one plate 78 are the outlets from the storage spaces, the pressure gauge 75, the check valve 71, the throttle valve 72 and the connection coupling 73. On the other plate, the pressure control valve 68, the pressure gauge 69 and the pressure relief valve 70 are mounted.

The refill unit 60 according to FIG. 4 has a storage container 80 made of plastic and a drive unit 81 which can be separated therefrom, which houses a geared electric motor 82 with built-up housing 83 for electronic components for controlling the electric motor and for displaying operating states. The electronic components and the electric motor are supplied with electrical energy from a voltage source 84. In the reservoir 80, a refill piston 85 adjoins a reservoir 86 which is filled with hydraulic oil. The refill piston is secured against twisting and is coupled to a threaded spindle 87 via a section designed as a threaded nut so that it moves in the longitudinal direction of the threaded spindle when the threaded spindle is rotated. The coupling between the threaded nut and the threaded spindle is largely tight. When the threaded spindle is rotated in one direction of rotation, the refill piston 85 moves in the direction of a refilling connection 73 and thereby reduces the size of the reservoir 86, so that hydraulic oil is displaced from this.

When the storage container 80 and drive unit 81 are combined, the threaded spindle is non-rotatably coupled to an output shaft 88 of the electric motor 82 via a coupling 87.

The outlet 89 of the storage container 80 or also a coupling with a non-return valve 74 via which the refill unit is connected to the system 9 can be regarded as the refill connection 73. As in the exemplary embodiment according to FIGS. 2 and 3, in the exemplary embodiment according to FIG. 4, a check valve 71 blocking the reservoir 80 is arranged between the direct outlet 89 from the reservoir and the coupling 73, which is seen here as a refill connection , that can also withstand high pressures. Between the outlet 89 and the check valve 71, a manometer 75, which indicates the pressure in the storage space 86, is connected to the fluid path that leads from the outlet 89 to the system 9.

The fluid path between the refill connection 73 and the system 9 also has a hose 90, a filter 91 and further hoses 92 and 93. The filter 91 can also sit between the outlet 89 and the refill connection 73. It catches any abrasion between the storage container 80 and the refill piston 85 and prevents premature clogging of the filter 37 of the system 9.

According to FIG. 4, the pressure sensor 42 from FIG. 1 is drawn separately. This pressure sensor sends a signal corresponding to the accumulator pressure to the electronics in the housing 83. A pressure switch can also be provided which reacts to the pressure in the hydraulic accumulator 29 and whose signal is used to control the electric motor 82.

The refill unit 60 according to FIG. 4 can be permanently connected to the system 9.

If the electric motor 82 is not rotating, the refill unit has no influence on the system. If the recorded accumulator pressure falls to a lower threshold value, the electric motor is switched on and rotates in a direction which leads to a displacement of the refill piston 85 and a reduction in the size of the storage space 86. As a result, hydraulic oil is refilled from the storage space 86 into the hydraulic accumulator 29. This can be done continuously or dis continuously with intermediate stops of the electric motor 82. If the accumulator pressure reaches an upper threshold value, the electric motor is switched off. If the storage pressure falls back to the lower threshold value, the electric motor is switched on again and the refill piston 85 reduces the storage space 86 further.

If the storage space 86 is emptied, the storage container 80 is exchanged for a filled Vorratsbe container. List of reference symbols

10 hydrostatic displacement machine

12 working connection of 10

13 working connection of 10

14 electric machine

16 hydraulic cylinders

17 pistons of 16

18 piston rod from 16

19 cylinder chamber

20 cylinder chamber

21 Working management

22 Working management

29 hydraulic accumulators

30 low pressure manifold

31 check valve

32 check valve

33 Pressure relief valve

34 pilot operated check valve

35 Pressure relief valve

36 pilot operated check valve

37 filters

38 check valve

39 check valve

40 Leakage connection

41 temperature sensor

42 pressure sensor

43 pressure sensor

44 pressure sensor

45 displacement sensor

50 hydraulic block

51 Connection coupling

52 check valve in 51

53 management 54 clutch

55 hose

60 refill unit

61 Reservoir

62 refill piston

63 pantry

64 Compressed air space

65 piston rod

66 Protective housing

67 silencer

68 pressure control valve

69 manometer

70 Pressure relief valve

71 check valve

72 throttle valve

73 Refill connection

74 check valve

75 manometer

77 connection

80 storage container

81 drive unit

82 geared electric motor

83 housing

84 voltage source

85 refill piston

86 pantry

87 lead screw

88 output shaft of 82

89 outlet to 80

90 hose

91 filters

92 hose

93 hose

Claims

1. Refill unit (60) for refilling hydraulic oil in a hydraulic system (9) pressurized with a low pressure accumulator, characterized by a reservoir (61, 80) in which there is a reservoir (63, 86) with hydraulic oil and a refill connection (73), and by a displaceable refill piston (62, 85) which delimits the storage space (63, 86) and, by moving it, hydraulic oil can be displaced through the refill connection (73).

2. Refill unit (60) according to claim 1, wherein the amount of hydraulic oil displaced per Zeitein unit from the storage space (63, 86) is adjustable.

3. Refill unit (60) according to claim 1, wherein the speed with which the refill piston (62, 85) is displaced directly, is adjustable.

4. Refill unit (60) according to claim 1, wherein the refill piston (62) can be acted upon with a constant force and the hydraulic oil can be displaced via a throttle valve (72) whose flow cross section is adjustable.

5. Refill unit (60) according to claim 1, wherein the flow cross-section of the throttle valve (72) can be reduced to zero.

6. refill unit (60) according to any preceding claim, wherein there is a check valve (71) on the refill connection (73), which blocks in the direction of the storage space before.

7. refill unit (60) according to any preceding claim, wherein it has a coupling part (73) with one of the storage space (63, 86) blocking away non-return valve (74) that mechanically when connecting the coupling part (73) with a counterpart is unlockable.

8. refill unit (60) according to any preceding claim, wherein one of the refill connection (73) has a measuring connection for connecting a manometer (75) or a pressure sensor.

9. refill unit (60) according to any preceding claim, wherein the refill connection (73) is followed by a filter (91).

10. refill unit (60) according to any preceding claim, wherein the refill piston (62) is displaceable by compressed air.

11. refill unit (60) according to claim 10, wherein a pressure reducing valve (68) is provided, with which the pressure with which the refill piston (62) is acted upon can be adjusted.

12. refill unit (60) according to claim 10 or 11, wherein the Vorratsbehäl ter (61) has a housing, the interior of which is divided by the refill piston (62) in the storage space (63) and an air-filled space (64), which is divided with Compressed air can be acted upon.

13. The refill unit (60) according to claim 1, wherein the refill piston (62) has a piston rod (65) on one side which crosses the air-filled space (64) and is guided out of the housing, and wherein the outer end of the piston rod (65 ) is immersed in a protective housing (66).

14. refill unit (60) according to any preceding claim, wherein meh er storage containers (61) are present, the individual refill connections are fluidically connected to each other and lead together to a common refill connection (73).

15. Refill unit (60) according to any preceding claim, wherein it comprises a drive device (81) which acts mechanically on the refill piston (85), and wherein the drive device (81) and the storage container (80) are releasably connected to one another.

16. Refill unit (60) according to claim 15, wherein the drive device comprises an electric motor (82), of which the refill piston (85) is linearly displaceable via a screw gear with a spindle (87).

17. Refill unit (60) according to claim 16, wherein the electric motor (82) can be switched off automatically as a function of the low pressure of the closed hydraulic system (9) detected by a pressure sensor (42).

18. Refill unit (60) according to claim 16 or 17, wherein it is stationary connected to the closed hydraulic system (9) and wherein the electric motor (82) as a function of the low pressure of the closed hydraulic system detected by a pressure sensor (42) ( 9) can be switched on automatically.

19. A hydraulic system comprising a closed hydraulic system (9) with a hydrostatic displacement machine (10), with an electric machine (14) which is mechanically speed-coupled to the displacement machine (10), with a hydraulic cylinder (16) which is connected to the displacement machine ( 10) is fluidically connected via a first working line (21) and a second working line (22) and can be actuated in opposite directions by reversing the fluid flow through the displacement machine (10), and with a low-pressure hydraulic accumulator (29) preloaded to a low pressure, which Can be fluidically connected via a valve arrangement (31, 32, 34, 36) to the working line (21, 22) in which the lower pressure prevails, and a refill unit (60) according to one of the claims 1 to 18.

20. Hydraulic system according to claim 19, wherein the Niederdruckhydro memory (29) via a releasable check valve (34, 36) which blocks towards it, and a filter (37) with the working line (21, 22) in which the lower Pressure prevails, is connectable and the refill unit (60) is connected to the closed hydraulic system between the pilot operated check valve (34, 36) and the filter (37).

21. Hydraulic system according to claim 19 or 20, wherein a pressure sensor (42) is present, of which the pressure in the low-pressure hydraulic accumulator (29) is directly detected bar and wherein the electric motor (82), from which the refill piston (85) via a screw gear (87) is linearly displaceable, can be controlled as a function of the accumulator pressure detected by the pressure sensor (42).