Working cylinder with pressure medium overflow

Abstract

The invention relates to an actuating cylinder with a pressurized medium overflow, in which the piston comprises a first inner annular groove with a seal and a second inner annular groove with a piston ring bearing, which bears elastically against the cylinder tube inner wall and has a piston ring gap, the seal in the first inner annular groove has axial movement play, the cylinder tube has a recess in the cylinder tube inner wall, which is axially arranged in such a way that when the piston is in an end position towards a first closing part, the recess lies opposite the first inner annular groove, the actuating cylinder is an o-ring. The overflow operating state is designed for an overflow operating state and for a lifting operating state, in which in the overflow operating state the piston is located in an end position towards the first closure and the seal is located, by application of pressure, in an end play position of its axial movement distal in the direction of the first closure, the overflow cross section between the radial limit of the seal and the recess is opened and the overflow flow path is opened, and in the lifting operating state the seal is located, by application of pressure, in a proximal end play position opposite the distal end play position and the overflow cross section and the overflow flow path are closed.

Description

[Technical field]

[0001] The invention relates to a double-acting hydraulic working cylinder with a pressure medium overflow between the working chambers, preferably at both ends of the piston. [Background technology]

[0002] According to the state of the art, solutions are known which allow the overflow of pressure medium in the working cylinder at one or both end positions of the piston. These solutions are based on the requirement to transfer pressure medium from one pressure chamber to another in certain applications, e.g. in master-slave systems or systems with hydraulic accumulators, in order to compensate for leakage flow losses or to provide pressure equalization.

[0003] For example, valve systems are known in the prior art that are equipped with a spring-loaded valve body including a tappet, which is arranged in the piston of a working cylinder. In the stroke position of the piston, the valve body is not actuated and is closed. When the piston reaches an end position, the tappet, which presses against the inside of the closing part of the cylinder tube and actuates the valve body, opens the corresponding valve, allowing the pressure medium to flow from the pressurized working chamber to the other working chamber.

[0004] Publication DE 3013381 C2 describes one such solution, where two valves operated by tappets are arranged in the working piston, each of which opens just before the piston's respective end stop in the closure of the cylinder tube is actuated, opening to connect the working chamber with the backflow connection in order to prevent damage to the components.

[0005] DE 8324442 U1 describes the arrangement of a check valve in the piston, an internal connecting line in the piston and the piston rod of a double-acting piston-cylinder unit for pressure equalization with a hydraulic accumulator. When pressure is applied by the pressure medium via the main connection of the working cylinder, the hydraulic accumulator filling and the equalization of pressure medium losses between the piston chamber and the piston rod chamber take place in the retracted end position of the piston.

[0006] Publication DE1279908A discloses a double-acting hydraulic lift cylinder with valve stems protruding from both sides of the piston, the valve stems having different lengths such that two stages of pressure equalization between the working chambers are achieved when the respective end positions of the piston are reached.

[0007] In publication DE 2245129 A a further arrangement of a tappet control valve in the piston of the working cylinder is disclosed: near the end positions of the piston, both working chambers of the working cylinder are connected to one another in such a way that a corresponding tappet in contact with the inner wall of the closing part opens the valve and pressure equalization takes place by overflow of the pressure medium.

[0008] The subject of publication DE 102005008070 A1 is a hydraulic cylinder whose piston has a valve arrangement for pressure equalization between the working chambers, the valve also being opened near the end positions of the piston in order to allow the overflow of pressure medium between the working chambers of the hydraulic cylinder with the aid of a tappet.

[0009] All of the valve arrangements in the pistons of the working cylinders mentioned above have symmetrically designed valve arrangements which use tappets to lift a spring-loaded valve element from its seat just before the piston reaches the respective end position so that a connection between the working chambers is established and pressure medium is allowed to overflow for pressure equalization.

[0010] The design-related complexities and the relatively small cross-section for the overflow of the pressure medium as well as the installation space available for the valve components due to the space conditions within the piston limit the possible range of applications of the described solution.

[0011] Publication EP 3610160 B1 describes a new solution devised by Bumach Engineering International BV, which is provided with a double-acting overflow valve allowing precise adjustment of the overflow cross section. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] DE3013381C2 [Patent Document 2] DE8324442U1 [Patent Document 3] DE1279908A [Patent Document 4] DE2245129A [Patent Document 5] DE102005008070A1 [Patent Document 6] EP3610160B1 Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention is to develop an advantageous solution for a structure which allows a definable overflow of pressure medium between the working chambers of a double-acting hydraulic working cylinder in definable positions, in particular in the end positions of the piston, and which is characterized by high operational reliability and low wear. [Means for solving the problem]

[0014] The problem is solved by the features stated in claim 1. Preferred embodiments arise from the subclaims.

[0015] The working cylinder with pressure medium overflow according to the invention has a cylinder tube, a first and a second closure, an interior, a first and a second working chamber, a first and a second pressure medium connection and a piston unit.

[0016] It is the actuating cylinder that is driven by the pressure flow and brings about the linear movement of the piston unit. Preferably, the actuating cylinder is designed as a hydraulic cylinder. In particular, it may be a differential actuating cylinder, but also a synchronous cylinder or a cylinder of another design.

[0017] The interior of the working cylinder is formed by the cylinder tube and the two closures. Depending on the operating state, the pressure medium is guided into and out of the cylinder via two pressure medium connections. The first pressure medium connection is assigned to the first working chamber and the second pressure medium connection is assigned to the second working chamber. The pressure medium connections give a choice of external connections, especially for hydraulic hoses or hydraulic lines.

[0018] The piston unit consists of a piston and a piston rod, which are connected to each other at the longitudinal axis in a snap-fit ​​or press-fit manner. The piston unit may have a monolithic design. The piston is arranged in the interior and moves along the longitudinal axis of the cylinder. The longitudinal axis divides the interior into a first working chamber and a second working chamber. The piston rod of the piston unit passes through a closure designed as a guide closure, which provides the possibility of force transmission to parts located outside the interior.

[0019] The piston has a circumferential first inner ring groove in its outer surface that slides against the inner surface of the cylinder tube. A seal disposed in the first inner ring groove and against the inner cylinder wall of the cylinder tube separates the first working chamber from the second working chamber and seals them from each other.

[0020] The piston unit performs a stroking motion as a result of the alternating flow and flow of pressure medium into and out of the working chambers, and a seal also keeps the working chambers separated during the stroking motion by sliding against the inner wall of the cylinder tube.

[0021] According to the invention, the working cylinder with pressure medium overflow is characterised in particular by the features described below.

[0022] According to the invention, the seal in the first inner ring groove has axial movement play, which means that it can assume different axial positions in the first inner ring groove, defined by the width of the groove.

[0023] The piston further has a circumferential second inner ring groove and a piston ring disposed therein. The piston ring elastically contacts the inner wall of the cylinder tube. According to the present invention, the piston ring has a piston ring gap, and the piston ring gap forms an open cross section.

[0024] According to the invention, the cylinder tube also has a recess in the inner wall of the cylinder tube, axially arranged to lie opposite the first inner ring groove at the end position of the piston in the first closed part. Preferably, the recess may be a simple drilling with a shallow depth. Other designs are also possible, such as keyways, semicircular grooves, spiral grooves or other recessed profiles. The recess is always designed and positioned in such a way that, when the first inner ring groove lies exactly opposite the recess, the axial position of the seal and thus the movement play results in an opening cross section for the pressure medium, hereafter called the overflow cross section.

[0025] Furthermore, the working cylinder is designed for an overflow operating condition and for a stroke operating condition.

[0026] In the overflow operating condition, the piston is axially located in an end position in the first closure, which is the stroke end position of the stroke actuated by the second working chamber.

[0027] In the overflow operating state, the seal is located in an end position of its axial movement play in the direction of the first closure by pressurization of the second working chamber. As a result, an overflow cross section is created between the radial limit of the seal and the recess so as to be open. Furthermore, an overflow flow passage is formed from the second pressure medium connection, via the second working chamber, via the overflow cross section and the piston ring gap, along the first working chamber to the first pressure medium connection. The invention encompasses any sequential arrangement of the overflow cross section and the piston ring gap, regardless of whether the pressure medium in the overflow passage flows first through the overflow cross section and then through the piston ring gap or vice versa.

[0028] According to an essential feature of the invention, the piston ring gap forms a throttling cross section for the pressure medium in the overflow cross section and thus relieves the seal in the overflow cross section.

[0029] In the stroke operating state, the piston is out of the stroke end position. In the stroke operating state, pressure is applied to the first working chamber. The seal is located in a proximal end play position opposite the distal end play position due to the pressurization of the first working chamber. The overflow cross section and therefore the overflow flow path are closed. As a result, a complete seal is already achieved at the start of the stroke, allowing movement out of the start of the stroke position without loss of pressure flow.

[0030] The overflow of pressure medium at the stroke end positions of the piston, in combination with the defined ring gap of the piston rings, results in a limited overflow of pressure medium.

[0031] The advantage of this is that when the end position is reached, the pressure in the entire hydraulic system does not suddenly increase to maximum hydraulic pressure so that the entire system is relieved.

[0032] Another advantage is given by the fact that the flow conditions in the overflow section are relieved by the piston ring gap which then acts as a throttle. Firstly, the relatively delicate seal which forms part of the wall of the overflow section is directly protected so that it undergoes significantly less wear than if the overflow section itself had to provide the throttle effect. Secondly, the pressure fluctuations and shear stresses of the pressure medium are reduced as it overflows the overflow section.

[0033] This design has the specific advantage of avoiding the diesel effect, which can occur in the current state of the art, when the air and oil aerosol formed in the foam is compressed by a high pressure peak and often interacts with cavitation in the pressure medium, causing the oil particles in the foam to autoignite and burn due to overheating. Negative effects include possible damage to the hydraulic system itself, apart from premature deterioration of the pressure medium.

[0034] In the present invention, such effects are eliminated thanks to the overflow possibility which relieves the pressure medium pressure. Furthermore, wear on all system components through which the pressure medium flows is minimized.

[0035] Advantageously, even for an arrangement of several parallel working cylinders of the same size, it can be guaranteed that all cylinders reach their end positions: a slightly reduced speed of movement of the parallel connected working cylinders caused by damping, friction, manufacturing tolerances or other disturbance effects can be compensated by the overflow so that all cylinders reach their end positions.

[0036] Furthermore, the actuating cylinder with pressure medium overflow according to the invention can also be used as a master cylinder in a master-slave configuration, thereby ensuring the correct operation of the master and slave cylinders.

[0037] According to an advantageous further development of the working cylinder with pressure medium overflow according to claim 1, this working cylinder is characterized in that the cylinder tube has a further recess in the inner wall of the cylinder tube. This recess is arranged axially in such a way that the recess in the second closing part is located opposite the first inner ring groove in the end position of the piston. The working cylinder is therefore designed for a further overflow operating state and for a further stroke operating state. In the further overflow operating state, the piston is located in an end position in the second closing part. The seal is located in an end position of its axial movement play, which is distal in the direction of the second closing part, due to the pressurization of the first working chamber. A further overflow cross section is opened between the radial limit of the seal and the further recess. As a result of this, a further overflow flow path is opened from the first pressure medium connection via the first working chamber, via the further overflow cross section and the piston ring gap, via the second working chamber to the second pressure medium connection.

[0038] In a further stroke operating condition, the seal is located at a further proximal end of play position opposite the further distal end of play position due to pressurization of the second working chamber, in the process a further overflow cross section and a further overflow flow path are closed.

[0039] In this application, the position designations distal and proximal are used depending on the operating condition: the term distal refers to the axially outward direction as viewed from the respective end position.

[0040] According to this further development, the working cylinder has the advantage that overflow is possible in both end positions due to further recesses in the inner wall of the cylinder tube in the opposite end positions. Correspondingly, the statements made regarding the overflow operating state apply to the further overflow operating state and the statements made regarding the stroke operating state apply to the further stroke operating state.

[0041] In this way, the advantages of the overflow of pressure medium as specified in claim 1 can be utilized in both the forward and the retracted piston positions. In particular, parallel forward and reverse movement of several parallel cylinders or of a master-slave arrangement is thus ensured until the end positions of all cylinders are reached.

[0042] According to another advantageous further development of the working cylinder with pressure medium overflow according to one of the previous claims, this is characterized in that the cross section of the piston-ring gap is smaller than the overflow cross section or the further overflow cross section.

[0043] Thus, the loads on the seal during use caused by the pressure of the pressure medium can be minimized. When overflowing the seal, the pressure medium is only slightly sheared, so that the pressure restriction achieved by the defined ring gap of the piston ring releases the seal. Due to the low shear, friction and temperature changes when flowing past the seal are minimized. This in turn leads to an increased seal life and ensures a consistent overflow behavior. As the seal is a robust component, higher loads due to the throttling effect of the piston ring in the piston ring gap are harmless. Advantageously, the flow rate can be adjusted in a very precise and structurally simple manner by the piston ring gap.

[0044] In particular, it is possible to provide the low flow rates required for switching valves, which may be necessary to operate the switching process for hydraulically actuated switching valves, in this way ensuring a reliable switching process of the valve thanks to the controlled pressure medium overflow.

[0045] According to another advantageous further development of the actuating cylinder with pressure medium overflow according to one of the previous claims, this is characterized in that the recess or the further recess is designed as a cold-formed embossed part.

[0046] In order to optimize the machining effort of the cylinder tube during production and make it more cost-effective, the recesses may be provided as cold-formed embossments. This solution also has the advantage that no chips or production burrs remain in the cylinder tube, which could cause damage to the seal during operation. Furthermore, with this further development, the service life of the seal is increased. With the additional compression of the material structure at the reduced cross section in the recess, a constant strength of the cylinder tube can be maintained. Advantageously, a correspondingly thinner outer surface or skin of the cylinder tube can be used. With this further development, advantageously, any tolerances that would otherwise have to be given due to weakening by the machined recesses can be omitted.

[0047] The invention is explained in more detail by way of an exemplary embodiment with the aid of the accompanying drawings, in which: FIG. [Brief description of the drawings]

[0048] [Figure 1] FIG. 2 is a schematic cross-sectional view of an actuating cylinder with a pressure medium overflow. [Diagram 2] FIG. [Diagram 3] FIG. 2 is a schematic cross-sectional view of the overflow operating state, detail A.1. [Figure 4] FIG. 13 is a schematic cross-sectional view of the stroke operating state, detail A.2. [Diagram 5] FIG. 2 is a schematic cross-sectional view of a further overflow operating state, detail B.1. [Figure 6] FIG. 2 is a schematic cross-sectional view of a further stroke operating state, detail B.2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Here, like reference numbers in the various figures refer to like features or components, and reference numbers are also used in the description even when they are not shown in the associated figure.

[0050] 1 shows an exemplary actuating cylinder with a pressure medium overflow in a schematic cross-section. This exemplary embodiment is an advantageous further development of an actuating cylinder in which an overflow is possible in both end positions.

[0051] The working cylinder is shown with a cylinder tube 1, which is axially limited by two closures 3, 4 on either side forming an interior 2. In an exemplary embodiment, the first closure 3 is designed as a base closure and the second closure 4 is designed as a guide closure.

[0052] The piston unit 9 consists of a piston rod 9.2 and a piston 9.1. The piston 9.1 is supported so that it can move linearly in the interior 2. The piston 9.1 divides the interior 2 of the working cylinder into two working chambers 5, 6 into which the pressure medium flows or is discharged depending on the stroke direction. The two working chambers 5, 6 are separated from each other by a seal 11. The pressure medium flows into or out of the respective working chambers 5, 6 through corresponding pressure medium connections 7, 8 assigned to the working chambers 5, 6. The first pressure medium connection 7 is assigned to the first working chamber 5, which in this case is the piston chamber, and the second pressure medium connection 8 is assigned to the second working chamber, which in this case is the piston rod chamber.

[0053] The seal 11 is arranged in a first inner ring groove 10 of the piston 9.1 and is designed here as an O-ring made from an elastomer. The axial width of the first inner ring groove 10 is greater than the width of the seal 11 so that the seal 11 is axially movable.

[0054] Furthermore, the piston 9.1 has a second inner ring groove 12. A piston ring 13 is mounted in this groove. The piston ring 13 has a piston ring gap 13.1 which provides an overflow possibility for the pressure medium.

[0055] At the end part of the cylinder tube 1 facing the first closure part 3, a recess 14 is arranged in the inner wall 1.1 of the cylinder tube. Another recess 15 is provided at the opposite end position of the cylinder tube assigned to the second closure part 4. Depending on the direction of movement of the piston 9.1, the seal 11 is located opposite the recess 14 or the further recess 15, providing an overflow possibility for the pressure medium at the end of the stroke.

[0056] 2 shows in a schematic image sequence the various operating states of the working cylinder, the allocation of the enlarged parts labelled A and B being shown simultaneously.

[0057] When the piston is moved to an end position, the seal 11 moves over the recesses 14, 15 in the inner wall 1.1 of the cylinder tube. As a result, the pressure medium is given the opportunity to flow between the two working chambers 5, 6 via the recesses 14, 15. If the inflow direction of the pressure medium changes so that the stroke movement of the working cylinder is reversed, the seal 11 is repositioned in the first inner ring groove 10. As a result, the outer radial limit of the seal 11 again contacts the inner wall 1.1 of the cylinder tube in a circumferential manner and the working chambers 5, 6 are again sealed against each other. The stroke movement of the piston 9.1 is continued to the next end position without pressure medium overflow.

[0058] This process is illustrated in detail by figures 3 to 6 in schematic cross-sections of a portion of the piston 9.1 and the inner wall 1.1 of the cylinder tube in the region of the piston ring 13 and the seal 11 on the respective recesses 14, 15. Figures 3 and 4 refer to a stroke end position at the first closure part 3, figures 5 and 6 refer to a further stroke end position at the second closure part 4. Figures 3 and 5 show two overflow operating states. Figures 4 and 6 show the two stroke operating states at their respective start.

[0059] FIG. 3 shows the working cylinder in the overflow operating state of detail A.1. The piston 9.1 is in an end position in the first closure part 3. The seal 11 is supported so as to be axially movable in the first inner ring groove 10. Due to the excess pressure in the second working chamber 6, the seal 11 abuts on the distal side wall of the first inner ring groove 10 in the direction of the first closure part 3 in the distal end position. In FIGS. 3-6 the excess pressure is illustrated by three parallel arrows in the free movement space of the first inner ring groove 10.

[0060] The pressure medium can flow into the first working chamber 5 via the piston ring gap 13.1 of the piston ring 13 and subsequently via the overflow cross section 14.1 formed radially between the seal 11 and the recess 14. This represents the overflow flow path. The pressure medium is throttled by the defined cross section of the piston ring gap 13.1 and the seal 11 is released.

[0061] FIG. 4 shows the stroke operating state of detail A.2. Here, the stroke operating state is at its beginning. The piston 9.1 is still in its end position in the first closure part 3. The pressure medium flows into the first working chamber 5 through the first pressure medium connection 7. The excess pressure moves the seal 11 axially in the direction of the second closure part 4 and presses it against the proximal side wall of the first inner ring groove 10. As a result, the outer radial limit of the seal 11 is no longer directly axially located opposite the recess 14, but again at least partially abuts against the inner wall 1.1 of the cylinder tube. The two working chambers 5, 6 are thus again sealingly separated from each other and the stroke can be performed without overflow.

[0062] Figure 5 shows the working cylinder in the further overflow operating state of detail B.1. The piston 9.1 is in an end position at the second closure 4. Due to the excess pressure in the first working chamber 5, the seal 11 in the distal play end position abuts against the distal wall of the first inner ring groove 10 in the direction of the second closure 4. The seal faces a further recess 15 in the axial direction, so that the outer radial limit no longer reaches the cylinder inner wall as circumferentially thereafter, and a further overflow cross section 15.1 is formed.

[0063] The pressure medium can flow from the first working chamber 5 into the second working chamber 6 via a further overflow cross section 15.1 formed radially between the seal 11 and the further recess 15 and subsequently via the piston ring gap 13.1 of the piston ring 13. This represents a further overflow flow path. In this reverse overflow direction too, the pressure medium is throttled by the defined cross section of the piston ring gap 13.1 and the seal 11 is released.

[0064] FIG. 6 shows the further stroke operating state of detail B.2. Here, the further stroke operating state is at its beginning. The piston 9.1 is still in its end position in the second closure 4. The pressure medium flows through the second pressure medium connection 8 into the first working chamber 6. Due to the excess pressure, the seal 11 is moved axially towards the first closure 3 and is pressed against the proximal side wall of the first inner ring groove 10. As a result, the outer radial limit of the seal 11 no longer directly axially against the further recess 15, but again at least partially against the inner wall 1.1 of the cylinder tube. As a result, the two working chambers 5, 6 are again sealingly separated from each other and the stroke can be performed without overflow. [Explanation of symbols]

[0065] 1 Cylinder tube 1.1 Inner wall of cylinder tube 2 inside 3 First Closure 4 Second Closure 5. First working chamber 6 Second working chamber 7 First pressure medium connection 8 Second pressure medium connection 9 Piston Unit 9.1 Piston 9.2 Piston rod 10 First inner ring groove 11 Seal 12 Second inner ring groove 13 Piston rings 13.1 Piston Ring Clearance 14 Recess 14.1 Overflow Section 15 Further recesses 15.1 Further overflow sections

Claims

1. A working cylinder with a pressure medium overflow, comprising a cylinder tube (1), first and second closure parts (3, 4), an interior (2), first and second working chambers (5, 6), first and second pressure medium connections (7, 8) and a piston unit (9), said interior (2) being formed by said cylinder tube (1) and said closure parts (3, 4); said first pressure medium connection (7) being assigned to said first working chamber (5) and said second pressure medium connection (8) being assigned to said second working chamber (6), the piston unit (9) having a piston (9.1) and a piston rod (9.2) arranged therein, the piston (9.1) being arranged in the interior and forming the first and second working chambers (5, 6) therein, a circumferential first inner ring groove (10) and a seal (11) arranged in the first inner ring groove (10) and in contact with an inner cylinder wall (1.1) of the cylinder tube (1) and separating the first and second working chambers (5, 6) from each other, In the working cylinder with pressure medium overflow, the seal (11) in the first inner ring groove (10) has axial movement play; the piston (9.1) has a circumferential second inner ring groove (12) and a piston ring (13) disposed in the second inner ring groove (12) and elastically contacting the inner wall (1.1) of the cylinder tube and having a piston ring gap (13.1); the cylinder tube (1) has a recess (14) in the inner wall (1.1) of the cylinder tube, which is axially arranged opposite the first inner ring groove (10) at an end position of the piston (9.1) in the first closing part (3), The working cylinder is designed for an overflow operating state and for a stroke operating state, In the overflow operating state, the piston (9.1) is in an end position in the first closure part (3), the seal (11) is in a distal play end position of its axial movement in the direction of the first closure part (3) due to pressurization of the second working chamber (6), an overflow cross section (14.1) is opened between the radial limit of the seal (11) and the recess (14), an overflow flow path is opened from the second pressure medium connection (8) via the second working chamber (6), via the overflow cross section (14.1) and the piston-ring gap (13.1), via the first working chamber (5) to the first pressure medium connection (7), In the stroke operation state, The seal (11) is located at a proximal end position opposite the distal end position by pressurization of the first working chamber (5), and the overflow cross section and the overflow flow path are closed.

4. The actuating cylinder having a pressure medium overflow,

2. the cylinder tube (1) has a further recess (15) in the inner wall (1.1) of the cylinder tube, axially arranged opposite the first inner ring groove (10) at the end position of the piston (9.1) in the second closing part (4), the working cylinder is designed for a further overflow operating condition and for a further stroke operating condition; In the further overflow operating state, the piston (9.1) is located in an end position in the second closure part (4), the seal (11) is located in a distal end position of its axial movement play in the direction of the second closure part (4) due to pressurization of the first working chamber (5), a further overflow cross section (15.1) is opened between the radial limit of the seal (11) and the further recess (15), and a further overflow flow path is opened from the first pressure medium connection (7) via the first working chamber (5), via the further overflow cross section (15.1) and the piston-ring gap (13.1), via the second working chamber (6) to the second pressure medium connection (8), In the further stroke operating state, The seal (11) is located in a further proximal end position opposite the further distal end position by pressurization of the second working chamber (6), and the further overflow cross section (15.1) and the further overflow flow path are closed.

2. The working cylinder with pressure medium overflow according to claim 1 .

3. The cross section of the piston ring gap (13.1) is smaller than the overflow cross section (14.1) or the further overflow cross section (15.1).

3. The working cylinder with pressure medium overflow according to claim 1 or 2.

4. The recess (14) or the further recess (15) is designed as a cold-formed embossed part.

4. An actuating cylinder with a pressure medium overflow according to claim 1 ,

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