DE102018201484A1 - Hydrostatic drive - Google Patents

Abstract

The invention relates to a hydrostatic drive with a hydraulic cylinder with a cylinder housing and with a movable within the cylinder housing piston which is assembled with at least one piston rod and optionally with the inclusion of the piston rod two cylinder chambers separated from each other, with a hydraulic unit having two ports, of which a first working port is fluidically connected to a first cylinder space of the two cylinder chambers and a second working port is fluidically connected to a second cylinder chamber of the two cylinder chambers. From the hydraulic unit pressure medium between the two cylinder chambers is displaceable in both directions. Each fluid branch between a working port of the hydraulic unit and the corresponding cylinder chamber of the hydraulic cylinder is protected by a maximum pressure limiting device.
The invention has for its object to further develop the known hydrostatic drive so that the hydraulic unit is protected from excessive squeezed.
This is achieved in that the pressure in a first fluid branch, to which pressure medium from the second fluid branch is supplied, is limited as a function of the pressure in the second fluid branch to a pressure value which is smaller than the pressure value of the pressure to which the maximum pressure limiting device is set.

Description

The invention relates to a hydrostatic drive with a hydraulic cylinder with a cylinder housing and with a movable within the cylinder housing piston which is assembled with at least one piston rod and optionally with the inclusion of the piston rod two cylinder chambers separated from each other, with a hydraulic unit having two ports, of which a first working port is fluidically connected to a first cylinder space of the two cylinder chambers and a second working port is fluidically connected to a second cylinder chamber of the two cylinder chambers. From the hydraulic unit pressure medium between the two cylinder chambers is displaceable in both directions. Each fluid branch between a working port of the hydraulic unit and the corresponding cylinder chamber of the hydraulic cylinder is protected by a maximum pressure limiting device.

Such a hydrostatic drive is for example from the DE 10 2011 056 894 B4 , which shows a hydrostatic drive with a differential cylinder, or from the DE 10 2010 036 204 A1 or the DE 10 2012 020 581 A1 known, which show a hydrostatic drive with a synchronous cylinder. The special feature of such a hydrostatic drive is that with a shift of a certain amount of pressure medium from one cylinder chamber into the other cylinder chamber, the pressure changes in the cylinder chambers are not symmetrical, taking into account the area conditions on the piston, but depend on the volumes of the cylinder chambers. For example, if the piston is in a position where the volume of one cylinder space is minimum and the pressure in that cylinder space is small and the volume of the other cylinder space is maximum and the pressure in that cylinder space is high, then under the pressure in the cylinder space Cylinder space with minimal volume increase much more than the pressure in the cylinder chamber with maximum volume drops when a certain amount of pressure medium is moved from the cylinder chamber with large volume in the cylinder chamber with a small volume. This phenomenon is related to the compressibility of the hydraulic pressure medium. On the phenomenon is in the DE 10 2010 036 204 A1 in connection with a hydrostatic drive in which the hydraulic cylinder is a synchronous cylinder has been received. However, the phenomenon also occurs in hydrostatic drives with a differential cylinder.

The invention has for its object to further develop the known hydrostatic drive so that the hydraulic unit is protected from excessive squeezed. The total pressure is generally understood to be the sum of the pressures at the working ports of the hydraulic unit.

The protection of the hydraulic unit from too high a total pressure is achieved in a hydrostatic drive with the features specified above, that the pressure in a first fluid branch, the pressure medium from the second fluid branch is supplied, in response to the pressure in the second fluid branch a pressure value is limited, which is smaller than the pressure value of the pressure to which the maximum pressure limiting device is set.

While in the known hydrostatic drives of the total pressure can be in principle twice as high as the pressure to which the maximum pressure limiting device is set, in a hydrostatic drive according to the invention, the total pressure is limited to a value uncritical for the hydraulic unit. Only when the pressure in the second fluid branch, the pressure medium is removed, has dropped to a value that does not lead to an inadmissibly high total pressure together with a maximum pressure in the first fluid branch, the pressure limit is set to the small value in the first fluid branch and the maximum pressure limiting device again determines the maximum possible pressure. It is conceivable that the lower pressure value can be continuously increased with the fall in the higher pressure in the second fluid branch. However, this can only be achieved in a complex manner.

In a preferred embodiment, therefore, the smaller pressure value is a fixed value. As such, the lower pressure may be a tank pressure or a biasing pressure that prevails at least in the fluid branches due to a biasing device. This would mean, however, that only after a drop in the pressure in the second fluid branch to a value that does not result in an excessively high total pressure in the first fluid branch due to a malfunction, a pressure in the first fluid branch could be constructed ,

It therefore seems more favorable if the smaller pressure value is selected such that the sum of this pressure and the pressure to which the maximum pressure limiting device is set is equal to the total pressure according to the specification of the axial piston unit. In the first fluid branch, therefore, when the pressure limitation with the lower value is released, a pressure is already applied so that the piston of the hydraulic cylinder is moved shortly thereafter.

Preferably, the pressure in the first fluid branch is limited to the fixed smaller pressure value only until the pressure in the second fluid branch has dropped to the lower pressure value. This also helps on the one hand, that the piston soon sets in motion. On the other hand, no inadmissibly high total pressure can occur more.

In terms of circuitry, a solution is preferred in which a pressure relief valve is provided, which is set to the smaller pressure value and between the pressure relief valve and the first fluid branch, a valve is arranged, which is open when the pressure in the second fluid branch is higher than in the first Fluid branch, and which is closed when the pressure in the second fluid branch is less than in the first fluid branch.

The valve between the pressure limiting valve and the fluid branch is advantageously a pilot-operated check valve which blocks towards the pressure limiting valve and whose control connection is fluidically connected to the second fluid branch.

Preferably, only one of the smaller pressure value set pressure relief valve is present, which is connected via a valve assembly in each case with the fluid branch, in which there is the lower pressure compared to the other fluid branch.

To connect the one pressure-limiting valve to one or the other fluid branch, a first unlockable check valve is interposed between the pressure-limiting valve and the first fluid branch and a second unlockable check valve is interposed between the pressure-limiting valve and the second fluid branch in an advantageous development of the hydrostatic drive.

For maximum pressure protection, a first pressure limiting valve is connected to the fluidic connection between the first working connection of the axial piston unit and the first cylinder space, and a second pressure limiting valve is connected to the fluidic connection between the second working connection of the axial piston unit and the second cylinder space.

The pressure limitation according to the invention to a smaller value is provided in particular when the hydraulic unit is designed as an axial piston unit with a piston drum which is pressed against a control surface provided with control slots. If the total pressure in such an axial piston unit is too high, this can lead to a lifting of the piston drum from the control surface, to the inflow of pressure medium into the housing and to a destruction of the unit.

Three embodiments of a hydrostatic drive according to the invention, each comprising a synchronous cylinder, are shown as a hydraulic circuit in the drawings. Based on these drawings, the invention will now be explained in more detail.

• 1 das erste Ausführungsbeispiel, das einen Gleichgangzylinder mit genau zwei Zylinderräume umfasst,
• 2 das zweite Ausführungsbeispiel, das einen Gleichgangzylinder mit drei Zylinderräumen umfasst, wobei einer der Zylinderräume mit dem einen oder dem anderen der weiteren Zylinderräume fluidisch verbindbar ist, und
• 3 das dritte Ausführungsbeispiel, das ebenfalls einen Gleichgangzylinder mit drei Zylinderräume umfasst, wobei jedoch die konstruktive Gestaltung des Gleichgangzylinders gegenüber 2 geändert ist.

Show it

• 1 the first embodiment, which comprises a synchronous cylinder with exactly two cylinder chambers,
• 2 the second embodiment, which comprises a synchronous cylinder with three cylinder chambers, wherein one of the cylinder chambers with one or the other of the other cylinder chambers is fluidly connectable, and
• 3 the third embodiment, which also includes a synchronous cylinder with three cylinder chambers, but with respect to the structural design of the Gleichgangzylinders opposite 2 is changed.

The in 1 hydrostatic drive shown comprises a synchronous cylinder 10 , in its cylinder housing 11 a simple piston 12 is guided longitudinally movable. On the piston 12 are two piston rods 13 fastened, one of which on one end face and the other on the other end face of the cylinder housing 11 protrudes from this. The two piston rods have the same diameter. Overall, so are through the cylinder housing 11 , the piston 12 and the piston rods 13 a first cylindrical in cross-section cylinder space 20 on one side of the piston 12 and a second annular cylinder space 21 on the other side of the piston 12 educated.

The hydrostatic drive according to 1 further comprises a hydraulic unit 25 , which may be designed as an axial piston unit with a constant displacement and is driven by an electric motor, not shown, whose speed is adjustable. The permissible total pressure for the axial piston unit 25 be 400 bar. The axial piston unit 25 has two working connections 26 and 27 on, of which the first work connection 26 over a line 28 with the first cylinder space 20 and the second work connection 27 over a line 29 with the second cylinder space 21 is fluidically connected. The first work connection 26 , The administration 28 and the first cylinder room 20 be the first fluid branch 30 and the second work connection 27 , The administration 29 and the second cylinder space 21 be as a second fluid branch 31 designated.

The first fluid branch 30 is about the combination of a pressure relief valve 32 and a Nachsaugventils 33 with a hydraulic accumulator 34 connected. The second fluid branch 31 is about the combination of a pressure relief valve 35 and a Nachsaugventils 36 also with the hydraulic accumulator 34 connected. In practice, the hydraulic accumulator is at a low pressure of about 2 to 3 bar charged, so that at any time a complete filling of the fluid branches is ensured. For the later following considerations of the pressures in the fluid branches is the pressure in the hydraulic accumulator 34 neglected. The pressure relief valves 32 and 35 For example, they are set to a pressure of 300 bar and thus leave the pressure in the fluid branches 30 and 31 not higher than 300 bar. Would you have a pressure of 3 bar in the hydraulic accumulator 34 consider, so the pressure relief valves 32 and 35 Of course, to be set to a pressure of 297 bar, of course, to let the pressure in the fluid branches not higher than 300 bar.

To the first fluid path 30 is also a first check valve 40 connected to the fluid path 30 locks, but by the pressure in the fluid path 31 is unlockable and with a control port 41 to this fluid path 31 connected. The area ratio of the thus unlockable check valve 40 is 1: 1. That means the check valve 40 closed when the pressure in the first fluid path 30 is higher than in the second fluid path 31 and that the check valve 40 is open when the pressure in the fluid path 31 is higher than in the fluid path 30 , The force acting in the closing direction of a weak spring of the check valve 40 be neglected.

To the second fluid path 31 is also a second check valve 45 connected to the fluid path 31 locks, but by the pressure in the fluid path 30 is unlockable and with a control port 46 to this fluid path 30 connected. The area ratio at the thus also unlockable check valve 45 is also 1: 1. That means the check valve 45 closed when the pressure in the second fluid path 31 is higher than in the first fluid path 30 and that the check valve 45 is open when the pressure in the fluid path 30 is higher than in the fluid path 31 , In this consideration, also the force acting in the closing direction of a weak spring of the check valve 45 neglected.

The working connections, from which the two pilot-operated check valves 40 and 45 would be open together with the input of a pressure relief valve 50 connected, whose output as the outputs of the two pressure relief valves 32 and 35 to the hydraulic accumulator 34 connected. The pressure relief valve 50 is for example set to a pressure of 100 bar.

It is now assumed that the piston 12 together with the piston rods 13 , according to 1 considered to be moved to the right. The axial piston unit 25 is driven to supply a major amount of pressure fluid to the cylinder space 21 and via the suction valve 36 a shortage of pressure medium the hydraulic accumulator 34 takes and into the cylinder room 20 promotes. In the cylinder room 20 There is a pressure caused by the resistance against which the piston 12 to move, and by the pressure in the cylinder chamber 21 is determined. By the pressure in the cylinder space 20 becomes the pilot operated check valve 45 opened so that the pressure in the cylinder chamber 21 prevails, at the entrance of the pressure relief valve 50 pending. However, this does not respond because the pressure in the cylinder chamber 21 lower than 100 bar. After the piston 12 or the one piston rod 13 or one of these moving load has encountered a stop, the pressure in the cylinder chamber increases 20 on the pressure relief valve 32 set value, namely to 300 bar. The cylinder space 20 is now very large, the cylinder chamber 21 is very small.

To the piston 12 and the piston rods 13 Moving away from said stop in the opposite direction to the left to another stop becomes the axial piston unit 25 now driven in the opposite direction. Via the axial piston unit 25 now flows pressure medium from the cylinder chamber 20 to the cylinder room 21 , At the beginning of the pressure medium flow is the pressure in the cylinder chamber 20 still 300 bar. The removal of a certain small amount of pressure medium from the cylinder chamber 20 , which still has a very large volume and is still under high pressure, only leads to a slight reduction in the pressure in the cylinder chamber 20 , for example to 270 bar. Would now the same small amount of pressure medium cylinder space 21 supplied, so the pressure could there because of the small volume of the cylinder space 21 strong, for example, rise to 230 bar. The total pressure at the axial piston unit would then be 500 bar and thus considerably higher than the admissible total pressure of 400 bar, so that the axial piston unit could be damaged. However, by the first still in the cylinder room 20 pending high pressure the check valve 45 kept open, so by the pressure relief valve 50 prevents the pressure in the cylinder chamber 21 higher than 100 bar. Because the pressure in the cylinder chamber 20 is not higher than 300 bar, so can the total pressure at the axial piston unit 25 not higher than 400 bar.

By removing pressure medium, the pressure falls in the cylinder chamber 20 finally at 100 bar and below. When reaching 100 bar, the check valve closes 45 and the check valve 40 opens. The task of limiting the pressure for the cylinder chamber 21 now takes over the pressure relief valve 35 , which is set to 300 bar. Depending on the load to be moved, the piston start 12 and the piston rods 13 to join one certain pressure difference between the two cylinder chambers 21 and 20 to move. Because during this movement the pressure in the cylinder space 20 is not more than 100 bar, the permissible total pressure of 400 bar can not be exceeded.

The change from the movement to the left to a stop to a movement to the right takes place accordingly, so it need not be discussed in detail.

The in 2 hydrostatic drive shown comprises a synchronous cylinder 60 in which in a cylinder housing 11 an annular piston 61 and a one-sided attached to the annular piston and an end wall 62 of the cylinder housing 11 penetrating piston rod 63 are guided longitudinally movable. From the front wall 62 opposite side is inside the cylinder housing 11 a central pillar 64 high, the stationary with respect to the cylinder housing 11 is arranged and sealed by the annular piston 61 into the hollow piston rod 63 protrudes. In the hollow piston rod 63 the column carries a ring collar 65 whose diameter is greater than the diameter of the column 64 is and the sealing on the inner wall of the hollow piston rod 63 is applied.

Due to the described training are in the hydraulic cylinder 60 a total of four cylinder rooms available. A cross-sectionally annular, first cylinder space 66 is located axially between the collar 65 the column 64 and the ring piston 61 and is by an effective in the retraction of the piston rod effective area A1 on the ring piston 61 limited. A cross-section circular disk-shaped, second cylinder space 67 is located axially between the collar 65 and the closed end of the piston rod 63 and is by an active in the extension direction of the piston rod effective surface A2 on the piston rod 63 limited. A cross-sectionally also annular, third cylinder space 68 is located axially between the front wall 62 of the cylinder housing 11 and the ring piston 61 and is by an effective in the retraction of the piston rod effective area A3 on the ring piston 61 limited. The mentioned active surfaces are now selected in size such that A2 = A1 + A3.

On the piston rod 63 remote side of the ring piston 61 there is a fourth cylinder space 69 that has a ventilation device 70 is associated with the atmosphere and in which there is accordingly atmospheric pressure.

The hydrostatic drive according to 2 includes as the hydrostatic drives according to 1 furthermore a hydraulic unit 25 , which may be designed as axial piston unit with a constant displacement and is driven by an electric motor whose speed is adjustable. The permissible total pressure for the axial piston unit 25 be 400 bar. The axial piston unit 25 has two working connections 26 and 27 on, of which the first work connection 26 over a line 28 with the first cylinder space 66 and the second work connection 27 over a line 29 with the second cylinder space 67 is fluidically connected. Based on the terms used for the first embodiment are the first working port 26 , The administration 28 and the first cylinder room 66 in turn, as the first fluid branch 30 and the second work connection 27 , The administration 29 and the second cylinder space 67 as a second fluid branch 31 designated.

Also in the embodiment according to 2 is the first fluid branch 30 via the combination of a pressure relief valve 32 and a Nachsaugventils 33 with a hydraulic accumulator 34 connected. The second fluid branch 31 is about the combination of a pressure relief valve 35 and a Nachsaugventils 36 also with the hydraulic accumulator 34 connected.

Also in the embodiment according to 2 is at the first fluid path 30 Furthermore, a first check valve 40 connected to the fluid path 30 locks, but by the pressure in the fluid path 31 is unlockable and with a control port 41 to this fluid path 31 connected.

To the second fluid path 31 is also a second check valve 45 connected to the fluid path 31 locks, but by the pressure in the fluid path 30 is unlockable and with a control port 46 to this fluid path 30 connected.

The working connections, from which the two pilot-operated check valves 40 and 45 would be open together with the input of a pressure relief valve 50 connected, whose output as the outputs of the two pressure relief valves 32 and 35 to the hydraulic accumulator 34 connected.

The design of the pressure relief valves 32 . 35 and 50 , the suction valves 33 and 36 and the pilot operated check valves 40 and 45 is the same as in the embodiment according to 1 , Likewise, the pressure relief valves to the same values as in the embodiment according to 1 set. The hydraulic accumulator 34 is on the same pressure as in the embodiment according to 1 biased.

The third cylinder room 68 is via an electromagnetically actuated 2/2-way seat valve 75 , which locks in a rest position, with the first fluid path 30 and another electromagnetically operated 2/2-way seat valve 76 , which also locks in its rest position, connectable to the second fluid path. This is the cylinder chamber 68 over a line 77 to the two directional seat valves 75 and 76 connected.

If the directional seat valve 75 open and the way seat valve 76 is closed, so are the two cylinder chambers 66 and 68 together fluidly with the work connection 26 the hydro unit 25 connected. The active surfaces A1 and A3 add up to a total effective area acting in the retraction direction, which is the same size as the effective area acting in the extension direction A2 is. If the directional seat valve 76 open and the way seat valve 75 is closed, so are the two cylinder chambers 67 and 68 together fluidly with the work connection 27 the hydro unit 25 connected. Because the active surfaces A2 and A3 acting in opposite directions, the effective effective in the extension direction effective area A2 around the effective area A3 diminished, so that there is an effective total effective area in the extension direction, which is the same size as the active surface in the retraction direction A1 is. If the directional seat valve 75 open and the directional valve 76 is closed, so you have a Gleichgangzylinder with large effective areas, and if the Wegesitzventil 76 open and the directional valve 75 is closed, you have a Gleichzugzylinder with smaller effective areas.

The pressure relief valves 32 . 35 and 50 as well as the valves 33 . 36 . 40 and 45 work regardless of the cylinder space 68 now with the cylinder chamber 66 or with the cylinder space 67 is connected in the same manner as in the embodiment according to 1 ,

In 3 is from a hydrostatic drive only the hydraulic cylinder 80 shown. This has a multi-part cylinder housing 11 in which two cavities are formed, each by a partial piston 81 or partial piston 82 a composite piston 83 in each case two cylinder chambers 66 and 69 or cylinder spaces 67 and 68 are divided. The two part pistons 81 and 82 are by a pole 84 extending from the one cavity tightly through the cylinder housing 11 extends through the other cavity. From the pole 84 opposite side protrudes from the sub-piston 81 a piston rod 63 outward, which has a larger diameter than the rod 84 and a smaller diameter than the partial piston 81 Has. The diameter of the part piston 82 is equal to the diameter of the piston rod 63 , All three cylinder chambers 66 . 67 and 68 are thus annular spaces, the sub-piston 82 with the retraction direction of the piston rod 63 effective active surface A1 the cylinder space 66 and the partial piston 81 with the extension direction of the piston rod 63 effective active surface A2 the cylinder space 67 and with the direction of retraction of the piston rod 63 effective active surface A3 the cylinder space 68 limited. Because the diameter of the part piston 82 equal to the diameter of the piston rod 63 is the same as in the embodiment according to 2 for the active surfaces the following relationship: A2 = A1 + A3.

The cylinder chambers 66 . 67 and 68 of the hydraulic cylinder 80 out 3 are over lines 28 . 29 and 77 as well as the cylinder chambers 66 . 67 and 68 of the hydraulic cylinder 60 out 2 with the in the 2 interconnected shown other components of a hydrostatic drive. The cylinder space 69 of the hydraulic cylinder 80 is in turn by a ventilation device 70 vented to the atmosphere. The hydraulic cylinder 80 So it can be just like the hydraulic cylinder 60 be operated as a synchronous cylinder with large effective surfaces and as a synchronous cylinder with respect to the large effective surfaces smaller effective surfaces.

LIST OF REFERENCE NUMBERS

10

Rod cylinders

11

cylinder housing

12

Piston of 10

13

piston rod

20

annular cylinder space

21

annular cylinder space

25

axial piston pump

26

Work connection from 25

27

Work connection from 25

28

management

29

management

30

fluid branch

31

fluid branch

32

Pressure relief valve

33

cavitation valve

34

hydraulic accumulator

35

Pressure relief valve

36

cavitation valve

40

unlockable check valve

41

Control terminal of 40

45

unlockable check valve

46

Control terminal of 45

50

Pressure relief valve

60

Rod cylinders

61

Ring piston of 60

62

Front wall on 11 from 60

63

piston rod

64

pillar

65

Ring collar on 64

66

Cylinder room in 60

67

Cylinder room in 60

68

Cylinder room in 60

69

Cylinder room in 60

70

breather

75

2/2-way seat valve

76

2/2-way seat valve

80

hydraulic cylinders

81

partial piston

82

partial piston

83

piston

84

pole

A1

Effective area in 66

A2

Effective area in 67

A3

Effective area in 68

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been 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.

Cited patent literature

• DE 102011056894 B4 [0002]
• DE 102010036204 A1 [0002]
• DE 102012020581 A1 [0002]

Claims (10)

Hydrostatic drive having a hydraulic cylinder (10, 60, 80) with a cylinder housing (11) and with a within the cylinder housing (11) movable piston (12, 61, 83), which is assembled with at least one piston rod (13, 63) and optionally with the inclusion of the piston rod (63) two cylinder chambers (20, 21; 66, 67) separated from each other, with a hydraulic unit (25) having two working ports (26, 27), of which a first working port (26) fluidly with a first cylinder space (20; 66) of the two cylinder chambers (20, 21; 66, 67) and a second working port (27) is fluidically connected to a second cylinder chamber (21; 67) of the two cylinder chambers (20, 21; 66, 67) , and by the pressure means between the two cylinder chambers (20, 21; 66, 67) is displaceable in both directions, and with a maximum pressure limiting device (32, 35) for each fluid branch between a working port (26, 27) of the hydraulic unit (25) and the corresponding cylinder around (20, 21; 66, 67) of the hydraulic cylinder (10), characterized in that the pressure in a first fluid branch (30), the pressure medium from the second fluid branch (31) is supplied, depending on the pressure in the second fluid branch is limited to a pressure value which is smaller than the pressure value of the pressure to which the maximum pressure limiting device (32, 35) is set. Hydrostatic drive to Claim 1 , where the smaller pressure value is a fixed value. Hydrostatic drive to Claim 2 , wherein the smaller pressure value is selected so that the sum of this pressure and the pressure to which the maximum pressure limiting device (32, 35) is set equal to the sum pressure according to the specification of the hydraulic unit (25). Hydrostatic drive to Claim 2 or 3 , wherein the pressure in the first fluid branch (30) is limited to the fixed smaller pressure value only until the pressure in the second fluid branch (31) has dropped to the lower pressure value. Hydrostatic drive to Claim 4 wherein a pressure limiting valve (50) is provided, which is set to the smaller pressure value and wherein between the pressure limiting valve (50) and the first fluid branch (30), a valve (40) is arranged, which is open when the pressure in the second fluid branch (31) is higher than in the first fluid branch (30) and closed when the pressure in the second fluid branch (31) is less than in the first fluid branch (30). Hydrostatic drive to Claim 5 wherein the valve is a pilot operated check valve (40) which blocks the pressure relief valve (50) and whose control port (41) is fluidically connected to the second fluid branch (31). Hydrostatic drive to Claim 5 or 6 , wherein a pressure limiting valve (50), which is set to the smaller pressure value, can be connected to the fluid branch (30, 31) via a valve arrangement (40, 45) in which the lower pressure prevails compared to the other fluid branch. Hydrostatic drive after the Claims 6 and 7 wherein between the pressure relief valve (50) and the first fluid branch (30) a first pilot-operated check valve (40) and between the pressure relief valve (50) and the second fluid branch (31) a second pilot-operated check valve (45) is arranged. Hydrostatic drive according to any preceding claim, wherein a first pressure limiting valve (32) is connected to the first fluid path (30) for maximum pressure protection and a second pressure limiting valve (35) is connected to the second fluid path (31). A hydrostatic drive as claimed in any preceding claim, wherein the hydraulic unit is an axial piston unit (25) having a piston drum movable in the displacer piston and urged against a control surface provided with control slots.

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