DE102019202683A1 - Refill unit for refilling hydraulic oil in a hydraulic system which is pressurized with a low-pressure accumulator and a pressurized hydraulic system with such a refill unit - Google Patents

Abstract

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 with such a refill unit. Disclosed is a refill unit for refilling hydraulic oil in a hydraulic system pressurized with a low-pressure accumulator, characterized by a reservoir in which there is a storage space with hydraulic oil and which has a refill connection, and by a displaceable refill piston which delimits the storage space and, by moving it, hydraulic oil can be displaced through the refill connection. Also disclosed is a pressurized hydraulic 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 and can be actuated by reversing the fluid flow through the displacement machine in opposite directions, and with a low-pressure hydraulic accumulator preloaded to a low pressure, which can be fluidically connected via a valve arrangement to the working line in which the lower pressure prevails.

A servohydraulic drive representing such a pressure-biased closed hydraulic system is known from FIG DE 10 2012 020 581 A1 known. 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 hydraulic cylinder that is fluidically connected to the displacement machine via a first working line and a second working line and is actuated by reversing the fluid flow through the displacement machine in opposite directions, and a low-pressure hydraulic accumulator preloaded to a low pressure, which is fluidically connectable via a valve arrangement to the working line in which the lower pressure prevails. The function of the low-pressure accumulator is to clamp in 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 compensation tank to compensate for the different amounts of pressure medium that are in the working lines, in the hydraulic cylinder and located in the displacement machine including an adjustment device for the stroke volume of the displacement machine that may be present.

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 period of operation 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 outset 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 refilled and in this way, by limiting the amount of refill per unit of time, ensure that the pressure in the storage space 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. Low pressure drops in the lines and, if necessary, between the throttle valve and the low-pressure accumulator Existing valves or filters are not taken into account here.

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

There can be a non-return valve on the refill connection, which blocks in the direction of the storage space. 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 tank 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. This means that 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.

The storage container preferably has a housing, the interior of which is divided by the refill 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 subjected. 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 combined 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 detachably 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 to the working line in which the lower pressure prevails via a releasable check valve that blocks it, and a filter, the refill unit is advantageously connected to the closed one between the releasable check valve and the filter hydraulic system connected. 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, can advantageously be controlled 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. Advantageously, a pressure signal is used to control the electric motor, which the pressure sensor emits 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 a pressure sensor here.

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

• 1 als Schaltbild das Ausführungsbeispiel einer erfindungsgemäßen hydraulischen Anlage,
• 2 das erste Ausführungsbeispiel einer erfindungsgemäßen Nachfülleinheit als Schaltbild, wobei zwei Vorratsbehälter und zwei Nachfüllkolben vorhanden sind und die Nachfüllkolben durch Druckluft verschiebbar sind,
• 3 die Nachfülleinheit aus 2 in einer perspektivischen Ansicht und
• 4 das zweite Ausführungsbeispiel einer erfindungsgemäßen Nachfülleinheit, wobei nur ein Vorratsbehälter und nur ein Nachfüllkolben vorhanden sind und wobei der Nachfüllkolben von einem Elektromotor über ein Schraubgetriebe verschiebbar ist.

Show it

• 1 as a circuit diagram the embodiment of a hydraulic system according to the invention,
• 2 the first embodiment of a refill unit according to the invention as a circuit diagram, with two storage containers and two refill pistons being present and the refill pistons being displaceable by compressed air,
• 3 the refill unit off 2 in a perspective view and
• 4th the second 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 pressurized hydraulic system 9 to 1 includes 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, an electric machine whose speed can be regulated is mechanically without the interposition of a gear 14th coupled, whereby the electric machine in operation as a motor can drive the displacement machine in its operation as a pump in both directions of rotation and in 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 includes a hydraulic cylinder 16 acting as a differential cylinder with a piston 17th and a one-sided piston rod 18th and thus with an annular cylinder chamber on the piston rod side 19th and with a cylinder chamber that is remote from the piston rod and is circular in cross section 20th is trained. The cylinder chamber 20th is about a working line 21st directly to the working connection 12 the displacement machine 10 connected. The cylinder chamber 19th is about a working line 22nd directly to the working connection 13 the displacement machine 10 connected.

The hydraulic system also includes a hydropneumatic accumulator, referred to below for short as a hydraulic accumulator 29 , 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 to a low-pressure manifold 30th connected. This is with the work management 21st via a check valve that opens towards this 31 and with the work management 22nd via a check valve that opens towards this 32 fluidically connected. Between the low pressure manifold 30th and the work management 21st are also a pressure relief valve 33 through which the pressure in the working line 21st and thus in the cylinder chamber 20th is limited to a maximum value, and a pilot operated check valve 34 arranged to the working line 21st opens towards and through a pressure in the working line 22nd is unlockable. After unlocking the unlockable check valve 34 can pressurized fluid from the working line 21st to the low pressure manifold 30th and thus to the hydraulic accumulator 29 flow. Between the low pressure manifold 30th and the work management 22nd are also a pressure relief valve 35 through which the pressure in the working line 22nd and thus in the cylinder chamber 19th is limited to a maximum value, and a pilot operated check valve 36 arranged to the working line 21st opens towards and through a pressure in the working line 21st is unlockable. After unlocking the unlockable check valve 36 can pressurized fluid from the working line 21st to the low pressure manifold 30th and thus to the hydraulic accumulator 29 flow.

In the low pressure manifold 30th is between the two pilot operated check valves 34 and 36 and the two pressure relief valves 33 and 35 , the two check valves 31 and 32 as well as the connection point of the hydraulic accumulator 29 a filter 37 looped in, so that the hydraulic accumulator can be reached from a working line via one of the two pilot operated check valves 29 Hydraulic oil flowing in via the filter 37 flows. In bypass to the filter 37 is a check valve 38 arranged to the hydraulic accumulator 29 opens 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 excessive pressures.

In series with the combination of filters 37 and check valve 38 is between this combination and the pilot operated check valves 34 and 36 a check valve 39 arranged that to the filter 37 locks out. The check valve 39 prevents hydraulic oil from flowing from the hydraulic accumulator 29 and the pressure relief valves 33 and 35 through the filter 37 via one of the pilot operated check valves 34 , 36 in a work line. This prevents dirt particles, which have been retained by the filter in the reverse flow direction, from entering a working line.

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

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

Various sensors are used to monitor and control the pressurized hydraulic system. As already mentioned, the speed sensor can be used to measure the speed of the hydrostatic displacement machine 10 and the electric machine 14th are recorded. With a temperature sensor 41 the temperature of the displacement machine 10 to the hydraulic accumulator 29 flowing case drain detected. With a pressure sensor 42 the pressure in the hydraulic accumulator 29 detected. With a pressure sensor 43 becomes the pressure in the cylinder chamber 20th detected. With a pressure sensor 44 becomes the pressure in the cylinder chamber 19th detected. With a displacement sensor 45 becomes the position of the piston 17th of the hydraulic cylinder 16 detected. The position can be used to differentiate the speed and a further differentiation to determine the acceleration of the piston 17th can be calculated.

The pressurized hydraulic system according to 1 can have a slight leak. In addition, oil samples are taken regularly to check the oil quality. 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 withdrawn from the system to be topped up when the accumulator pressure, for example, when the piston rod is fully retracted 18th drops below a certain value. There is a hydraulic block for refilling 50 , in and on which the valves and the filter described above are installed and attached, a connection coupling 51 that have a built-in check valve 52 has and one of the lines 53 goes out between the check valve 39 and the filter 37 in the low pressure manifold 30th flows out. With the connection coupling 51 is about another clutch 54 also with a check valve 52 equipped, and if necessary a hose 55 a refill unit 60 connected. When putting the couplings together 51 and 54 are the two check valves 52 been 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 thereby hydraulic oil from the storage space via the line 55 who have favourited clutches 54 and 51 , The administration 53 and the filter 37 to the hydraulic accumulator 29 repressed.

According to the 2 and 3 is the refill unit 60 a hydropneumatic unit. It then has according to the embodiment according to 2 and 3 as a storage container 61 two hydraulic cylinders. The inside of each reservoir is through a refill piston 62 in a pantry 63 for hydraulic oil and in a compressed air space 64 is divided. On the refill piston 62 is a piston rod on one side 65 arranged that the compressed air space 64 crosses and exits the reservoir. The outer end of the piston rod 65 is of a protective housing 66 surrounded by a silencer 67 is open to the atmosphere. The two compressed air rooms 64 are shared via a pressure control valve 68 connected to a compressed air network. The pressure in them is via a pressure gauge 69 readable. Through a pressure relief valve connected to it 70 are the storage containers 61 protected against excessive compressed air pressure.

The two storage rooms 63 for hydraulic oil are common via a check valve 71 that blocks them, as well as a throttle valve 72 , whose Flow area is adjustable, with a refill connection 73 fluidically connected, the one with an outwardly blocking, pushable check valve 74 is provided. On a manometer 75 can be the pressure in the pantries 63 can be read. The hydraulic oil is drawn from the refill connection via a hose 55 to the pressurized closed hydraulic system 9 transfer. When refilling, the throttle valve should 72 from its closed position, open slowly so that not too much hydraulic oil enters the system 9 flows. Via a connection 77 can use the pantries 63 refilled when they are empty.

Out 3 it can be seen that the two reservoirs 61 through two panels 78 are held together as a unit in front of their front sides. On one plate 78 are the outlets from the storage rooms, the pressure gauge 75 , the check valve 71 , the throttle valve 72 and the connection coupling 73 . On the other plate are the pressure control valve 68 , the manometer 69 and the pressure relief valve 70 assembled.

The refill unit 60 according to 4th has a storage container 80 made of plastic and a drive unit that can be separated from it 81 who have favourited a geared electric motor 82 with built-on housing 83 for electronic components for controlling the electric motor and for displaying operating states. The electronic components and the electric motor are powered by a voltage source 84 supplied with electrical energy. In the storage container 80 adjoins a refill piston 85 to a pantry 86 which is filled with hydraulic oil. The refill piston is secured against rotation and has a threaded spindle via a section designed as a threaded nut 87 coupled in such a way 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 turned in one direction of rotation, the refill piston moves 85 towards a refill port 73 and reduces the storage space 86 so that hydraulic oil is displaced from this.

The threaded spindle is with the unified reservoir 80 and drive unit 81 via a clutch 87 non-rotatably with an output shaft 88 of the electric motor 82 coupled.

As a refill connection 73 can directly the outlet 89 of the storage container 80 or a coupling with a check valve 74 can be viewed via the refill unit to the system 9 is connected. As in the embodiment according to 2 and 3 is also in the embodiment according to 4th between the immediate outlet 89 from the storage container and the coupling 73 , which is regarded here as a refill connection, to the storage container 80 non-return valve blocking out 71 arranged that can withstand high pressures. Between the outlet 89 and the check valve 71 is a pressure gauge 75 that the pressure in the pantry 86 indicates connected to the fluid path leading from the outlet 89 to the system 9 leads.

The fluid path between the refill port 73 and the system 9 still has a hose 90 , a filter 91 and other hoses 92 and 93 on. The filter 91 can also be between the outlet 89 and the refill connection 73 sit. 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 4th is the pressure sensor 42 out 1 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 responds to the pressure in the hydraulic accumulator 29 reacts and its signal in the control of the electric motor 82 comes in.

The refill unit 60 according to 4th can be permanently with the system 9 be connected. When the electric motor 82 does not turn, the refill unit has no effect 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 that moves the refill piston 85 and to a reduction of the storage space 86 leads. This removes hydraulic oil from the reservoir 86 in the hydraulic accumulator 29 refilled. This can be continuous or discontinuous with intermediate stops of the electric motor 82 happen. If the accumulator pressure reaches an upper threshold value, the electric motor is switched off. If the accumulator 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.

Is the pantry 86 the storage container is emptied 80 exchanged for a filled storage container.

List of reference symbols

10

hydrostatic displacement machine

12

Working connection from 10

13

Working connection from 10

14th

Electric machine

16

Hydraulic cylinder

17th

Piston of 16

18th

Piston rod from 16

19th

Cylinder chamber

20th

Cylinder chamber

21st

Work management

22nd

Work management

29

Hydraulic accumulator

30th

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

filter

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

coupling

55

tube

60

Refill unit

61

Storage container

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

casing

84

Voltage source

85

Refill piston

86

Pantry

87

Threaded spindle

88

Output shaft of 82

89

Outlet on 80

90

tube

91

filter

92

tube

93

tube

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102012020581 A1 [0003]

Claims (21)

Refill unit (60) for refilling hydraulic oil into a hydraulic system (9) which is pressurized with a low-pressure accumulator, characterized by a storage container (61, 80) in which there is a storage space (63, 86) with hydraulic oil and which has 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). Refill unit (60) Claim 1 , wherein the amount of hydraulic oil displaced from the storage space (63, 86) per unit of time is adjustable. Refill unit (60) Claim 1 , the speed at which the refill piston (62, 85) is displaced can be set directly. Refill unit (60) 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), the flow cross section of which is adjustable. Refill unit (60) Claim 1 , wherein the flow cross section of the throttle valve (72) can be reduced to zero. Refill unit (60) according to one of the preceding claims, a check valve (71) which blocks in the direction of the storage space on the refill connection (73). Refill unit (60) according to any preceding claim, wherein it has a coupling part (73) with a non-return valve (74) which blocks away from the storage space (63, 86) and which can be mechanically unlocked when the coupling part (73) is connected to a counterpart. Refill unit (60) according to one of the preceding claims, wherein one of the refill connection (73) has a measuring connection for connecting a manometer (75) or a pressure sensor. Refill unit (60) according to one of the preceding claims, wherein a filter (91) is connected downstream of the refill connection (73). Refill unit (60) according to any preceding claim, wherein the refill piston (62) is displaceable by compressed air. Refill unit (60) Claim 10 , wherein a pressure reducing valve (68) is present with which the pressure with which the refill piston (62) is acted upon can be adjusted. Refill unit (60) Claim 10 or 11 wherein the storage container (61) has a housing, the interior of which is divided by the refill piston (62) into the storage space (63) and an air-filled space (64) to which compressed air can be applied. Refill unit (60) Claim 1 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 the outer end of the piston rod (65) dips into a protective housing (66). Refill unit (60) according to one of the preceding claims, wherein there are several storage containers (61), the individual refill connections of which are fluidically connected to one another and are brought together to form a common refill connection (73). 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 detachably connected to one another. Refill unit (60) Claim 15 , wherein the drive device comprises an electric motor (82), of which the refill piston (85) can be displaced in a straight line via a screw gear with a spindle (87). Refill unit (60) 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). Refill unit (60) Claim 16 or 17th , wherein it is connected in a stationary manner to the closed hydraulic system (9) and wherein the electric motor (82) can be switched on automatically as a function of the low pressure of the closed hydraulic system (9) detected by a pressure sensor (42). 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 via a second working line (22) and by reversing the fluid flow through the The displacement machine (10) can be actuated in opposite directions and has a low-pressure hydraulic accumulator (29) which is preloaded to a low pressure and which can be fluidically connected to the working line (21, 22) via a valve arrangement (31, 32, 34, 36) in which the lower pressure prevails, and a refill unit (60) according to one of the Claims 1 to 18th . Hydraulic system according to Claim 19 , wherein the low-pressure hydraulic accumulator (29) can be connected to the working line (21, 22), in which the lower pressure prevails, via a releasable check valve (34, 36) that blocks it, and wherein the Refill unit (60) is connected to the closed hydraulic system between the pilot operated check valve (34, 36) and the filter (37). Hydraulic system according to Claim 19 or 20th , wherein a pressure sensor (42) is present, from which the pressure in the low-pressure hydraulic accumulator (29) can be detected directly and wherein the electric motor (82), from which the refill piston (85) can be shifted in a straight line via a helical gear (87), as a function of the accumulator pressure detected by the pressure sensor (42) can be controlled.

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