WO2009015865A1

WO2009015865A1 - Lifting cylinder and lifting device

Info

Publication number: WO2009015865A1
Application number: PCT/EP2008/006242
Authority: WO
Inventors: Helma Dannhoff
Original assignee: Helma Dannhoff
Priority date: 2007-07-31
Filing date: 2008-07-29
Publication date: 2009-02-05
Also published as: DE102007036274A1; EP2171290A1; DE102007036274B4

Abstract

The invention relates to a lifting cylinder (10, 10', 10') having a cylinder cavity (12) and having a lifting piston (13) which is arranged in the cylinder cavity (12) and which comprises a piston section (13a) and a rod section (13b) and which divides the cylinder cavity (12) into a piston head chamber (12a) and a piston ring chamber (12b). The lifting cylinder (10, 10', 10') has a valve arrangement (25) which, in the open state, opens a connection which can be traversed by a fluid between the piston head chamber (12a) and the piston annular chamber (12b), and which, in the closed state, closes off said connection. The valve arrangement (25) is designed so as to be open when the lifting piston (11) is in a retracted position and the fluid pressure in the piston head chamber (12a) is below a predefined limit pressure, and so as to be closed when the fluid pressure lies above the limit pressure or the lifting piston (13) is in a partially deployed position. Also described are lifting devices (1) having lifting cylinders (10, 10', 10') of said type.

Description

Lifting cylinder and lifting device

The invention relates to a lifting cylinder having a cylinder cavity and a piston arranged in the cylinder cavity, which comprises a piston portion and a rod portion and which divides the cylinder cavity into a piston head space and a piston annulus. Moreover, the invention relates to a lifting device with such a lifting cylinder.

Such lift cylinders are used in many applications when it comes to lifting loads. The extension of the lifting piston from the lifting cylinder takes place by pumping a fluid, for example a hydraulic oil, into the piston bottom space situated between the bottom of the lifting cylinder and the side of the piston section facing away from the rod section, thereby lifting the lifting piston. At the same time, a fluid is compressed and / or displaced in the piston ring space located above the piston section around the rod section. In this case, the piston bottom space is increased and the piston ring space is reduced. The retraction is carried out in the reverse manner.

A special purpose is the lifting of so-called lifts. Lifting platforms, in particular, when they are to carry heavy loads, are often lifted with a relatively large, rigid cylinder, which is located centrally below the lift. As an alternative, lifting platforms are also constructed with a relatively stiff, heavy scissor mechanism. Since these mechanisms block the space under the lift, these designs do not lend themselves when free space is required under the lifted lift, for example in facilities where loads are to be stored at different levels. Thus, there are applications in which first loads are to be placed on the lift, then the lift should be raised and then in the room under the lift more items to be stored. In this way as many as possible Items are stored on a kieinen base. Such systems are therefore often in vehicles such. As hearse, car transporters, etc. used. For such constructions several, correspondingly smaller lifting cylinder must be installed in the edge region or the corners of the lift. This raises the problem that all lifting cylinders evenly off and must be retracted, otherwise it can lead to jamming and blocking the lift. In extreme cases, this can also lead to accidents.

In order to ensure a uniform extension and retraction of a plurality of lifting cylinders, the lifting heights are currently measured with position sensors and there is a control via electronic proportional valves or proportional flow divider. The exact measurement of the lifting height and the precise control are relatively complex.

Another way to realize several concurrent lifting cylinder is a series connection or series connection of the lifting cylinder. In this series connection, in each case the piston ring space of a first lifting cylinder in the row is connected to the piston bottom space of the lifting cylinder following in the row. In this case, any number of lifting cylinders can be connected in series. If now a fluid is pumped into the piston head space of the first lifting cylinder, the lifting piston lifts and displaces the fluid in the piston ring space of the first lifting cylinder, which is pressed into the piston head space of the second lifting cylinder, etc. It is important to ensure that the piston ring space of a the preceding row arranged lifting cylinder is adapted to the volume of the piston head space of the subsequent lifting cylinder, which can be defined by the thickness of the rod portion.

One problem here is in particular the venting and filling of the lifting cylinder and the connecting lines between two lifting cylinders. Although Hydrauiiköi is theoretically not compressible. However, this does not apply to the air bubbles that are inevitably located in the oil. In particular, after a first start-up, but also after a certain number of operating hours, therefore, a maintenance is required in which it is ensured that the lines or the interiors of the cylinder are vented to eliminate the influence of air. Otherwise, there will be a non-synchronism of the lifting cylinder, which leads to the above-mentioned problems. Such venting and refilling with oil is relatively expensive. There must be appropriate ventilation valves and oil filler valves at various points throughout the lifting device, in particular between two lifting cylinders, installed. The entire maintenance process must be carried out very carefully and therefore time consuming.

DE 2 245 129 A shows a working cylinder which has, within a piston, a valve arrangement with two check valves located opposite one another in a bore in the piston. In each of the two end positions of the piston, one of the non-return valves is mechanically opened. By the other check valve then a pressure equalization between the piston head space and piston annulus is possible, but fluid can flow only in one direction. Similarly, DE 34 17 447 A1 shows a piston in a hydraulic cylinder with two serving as a pressure relief valves, counteracting check valves. The check valves are each held by a compression spring in a closed position, wherein in the end positions by plunger, the bias of each of the springs is mechanically increased, so as to increase the response pressure of the relevant pressure relief valve. DE D 20723 la / 60 shows a hydraulic cylinder with several telescopically telescoping pistons. The piston ring chambers are each connected to the piston head space via the piston ring space to the piston bottom space blocking check valves, which are mechanically unlocked to each other in certain positions of the lifting cylinder. By this arrangement, the lifting force increases with increasing cylinder stroke. All these Constructions thus offer no solution to the above problem.

It is an object of the present invention to provide a lifting cylinder of the type mentioned and a lifting device, which are virtually maintenance-free with respect to this venting problem.

This object is achieved by a lifting cylinder according to claim 1 and by a lifting device according to claim 18.

A lifting cylinder according to the invention is characterized in that it has a valve arrangement which, in the opened state, opens a connection through which a fluid can flow between the floor space of the piston and the piston ring space and closes this connection in the closed state. The valve assembly is designed so that it is open in both directions between the piston head space and the piston annulus when the reciprocating piston is in a retracted position and also the fluid pressure in the piston head space is below a predetermined limit pressure, and that they are at a fluid pressure above the Limit pressure or in a partially extended position of the reciprocating piston (ie, in an intermediate position) is closed. Under a retracted position here is the end position or a position in a defined short distance before the end position of the reciprocating piston to understand the cylinder bottom.

By inventively provided valve arrangement, it is thus possible to press the fluid in a certain position of the reciprocating piston, namely, when it is in the retracted position, through the piston head space to the piston ring space and thus the reciprocating piston completely from a port on the piston head space, in which Typically, the fluid is pumped to extend the reciprocating piston to fill. At the same time, the fluid can be compressed in the complete lifting cylinder. In particular, the complete cylinder in both directions, ie Both front piston bottom space to Koibenringraum and in the opposite direction, vented and flushed. Only above the limit pressure is ensured that the valve assembly is closed, so that is extended by a further pumping of fluid into the piston head space of the reciprocating piston.

This also applies if not only an inventive lifting cylinder is used in a lifting device according to the invention, but a plurality of such lifting cylinders are connected in series one behind the other. Even then filling and flushing the entire system is possible as long as the pressure in the piston head space of the cylinder is below the limit pressure. Likewise, the fluid in the entire fluid system can be compressed.

Furthermore, this arrangement has in a series connection of several lifting cylinders according to the invention has the advantage that all lifting cylinder drive in the retracted position, even if one of the lifting cylinder gets a little too early in the retracted position, since yes in this lifting the valve device is open and the lifting cylinder becomes completely permeable within the overall cycle.

The dependent claims relate to advantageous embodiments and further developments of the invention.

Particularly preferably, the valve arrangement is designed such that it is also open at least in a flow direction from the piston head space to the piston ring space when the piston is in an extended position, ie when the piston in the upper end position with fully extended rod or in a defined short Distance before this end position of the reciprocating piston is located on the end wall of the cylinder guide bushing. In this preferred construction, therefore, the valve assembly is completely closed only in the intermediate positions of the reciprocating piston between the two end positions. By such opening of the valve assembly in the extended position can be avoided that in a series connection of several lifting cylinder, the lifting cylinder remains up, if the external load on the lifting cylinder is smaller than the force exerted by the residual oil pressure in the cylinder on the rod of the lifting cylinder force. Since the valve assembly is opened, a very small external force on the rod is sufficient to push the piston out of the extended position. The reciprocating piston can therefore not be "caught" in the extended position and thus lead to a blockage, but in this position the valve arrangement can not be flowed through in the direction of flow from the piston ring space to the piston bottom space Piston ring space of the reciprocating piston is retracted safely.

In certain applications, it may be advantageous to incorporate such a valve arrangement in a parallel to the lifting cylinder, the piston bottom space and the piston annulus interconnecting bypass, are controlled in the appropriate valves using suitable limit switches and pressure sensors, which are arranged on or in the lifting cylinder and Detect the position of the piston in the cylinder and the pressure in the piston head space. Particularly preferably, however, the entire valve arrangement is integrated in the lifting cylinder. Such a design is particularly compact, and it can be dispensed with elaborate sensors and switches.

In a particularly preferred variant, in this case the piston head space and the piston ring space are connected to one another via two paths running in the reciprocating piston. In this case, the paths are each closed by arranged in the reciprocating piston valve units, wherein one of the valve units is automatically opened only when the reciprocating piston is in the retracted position and the fluid pressure in the piston head space is below the limit pressure and another of the valve units is automatically opened only when the reciprocating piston is in the extended position. Such a structure can be implemented relatively easily and inexpensively.

The valve arrangement preferably has a first valve unit arranged in the reciprocating piston with a first valve piston which is designed such that it is pressure-balanced with respect to the fluid pressure in the piston bottom space and is opened by a specific mechanical pressure. "Pressure compensated" means in this case that the valve unit can not be opened by a fluid pressure, but only by the mechanical pressure exerted on the valve piston, for example, from a direction from the piston head space is formed and / or stored so that it is at a fluid pressure in the piston head space below the limit pressure in a stop position and in the retracted position of the reciprocating piston on the first valve piston exerts a mechanical pressure to open the first valve unit and that at a fluid pressure in Piston bottom space is pressed above the limit pressure in an empty position, so that it can no longer exert sufficient mechanical pressure on the first valve piston in the retracted position of the reciprocating piston In this position, it is thus ensured that the valve piston of the first Ventileinhei t does not go against the stop, so that it is not pressed above the desired limit pressure in the open position accordingly.

Such a stop element is so realizable that it is arranged in a recess in a bottom of the lifting cylinder relative to the first piston mounted in the piston and by a spring element, for example a mechanical spring or a Luftdruckbzw. Gas pressure or hydraulic spring is held in the stop position. In this case, the spring element must be designed so that the force on the Stop element must be so large that the stop element can be pressed by a (fluid) pressure in the piston head space above the limit pressure in the said empty position.

The reciprocating piston is preferably mounted in a similar manner in the reciprocating piston so that it is pressed by a spring element toward the piston bottom space in a closed position in which the first valve piston with a sealing surface against a first valve seat arranged in the piston and in which the first valve piston in the Protrudes piston bottom space. That is, the valve piston protrudes in this position out of the bottom surface of the piston portion and thus can press against the stop member in the bottom of the cylinder when it is in the stop position and not in the idle position.

One possibility, the valve piston or the first valve unit in such a way that it is pressure-balanced, is that the first valve piston in the reciprocating piston in a first valve bore, which may be located, for example, within a inserted into a suitable recess in the piston valve, between a first cavity in the reciprocating piston and the piston head space is stored. In this case, the first cavity and the piston bottom space are connected to one another such that a fluid can flow unhindered from the piston head space into the first cavity. The cross-sectional area of the first valve piston toward the piston bottom space and the cross-sectional area of the first valve piston toward the first cavity are of substantially equal size, so that the fluid in the piston head space acts on the valve piston in both directions along the axis of movement of the valve piston with the same force. As a result, the pressure balance with respect to the pressure in the piston head space is ensured.

In the reciprocating piston can also be preferably realized a connection in the form of a bore or the like between the first cavity of the reciprocating piston and the piston ring space, wherein the valve piston for Realization of the valve unit is designed so that it closes exactly this connection in the closed position.

Preferably, the valve assembly further comprises a second valve unit arranged in the reciprocating piston with a second valve piston which is designed such that it is pressure-balanced with respect to the fluid pressure in the piston annulus and is opened by a mechanical pressure when the reciprocating piston is in the extended position. This allows the

Opening the valve assembly can be realized in the extended state of the reciprocating piston.

Also, this second valve piston is preferably mounted in the reciprocating piston so that it is pressed by a spring member toward the piston annulus back into a closed position in which the second valve piston with a sealing surface against a second piston seat arranged in the piston presses and in the second valve piston into the piston annulus protrudes. That is, here protrudes the second valve piston out of the rod portion surrounding the annular surface of the piston portion and automatically pushes in the extended position of the reciprocating against the upper end wall of the lift cylinder, such as the usually located there guide bush, so that it is against the force of the spring element is pressed from the closed position to an open position.

The spring elements of the first and second valve piston can each z. B. mechanical springs or air pressure or gas pressure or hydraulic springs.

Also, the second valve piston is - similar to the first valve piston - preferably stored in the reciprocating piston in a second valve bore between a second cavity in the reciprocating piston and the piston annulus. Again, the second cavity and the piston annulus are interconnected so that a fluid between the piston annulus and unhindered the second cavity can flow and the cross-sectional area of the second valve piston to the piston ring space and the cross-sectional area of the second valve piston are substantially equal to the second cavity. It is thus made here in the same manner as in the first valve unit, the pressure balance.

Particularly preferably, the first cavity and the second cavity in the reciprocating piston are connected to one another via two paths. One way is closed in the closed position of the first valve piston by the first valve piston and the other way is closed in the closed position of the second valve piston by the second valve piston.

The connection path from the first cavity to the piston bottom space and preferably also the connection path from the second cavity to the piston ring space preferably extend in each case through the corresponding first and second valve pistons. This is easily possible by a continuous, longitudinal bore is introduced into the first valve piston and / or in the second valve piston.

In order to ensure that in the extended position of the reciprocating piston, the valve assembly in one direction only, namely from the piston head space to the piston ring space, can be flowed through, the valve assembly preferably has a check valve. This can be z. B. in or on the communication path between the first cavity of the reciprocating piston and the piston head space and close this connection path.

Preferably, the valve assembly also has an actuating element which automatically opens the non-return valve in the retracted state of the reciprocating piston, so that the fluid can then flow unhindered from the piston ring space in the piston crown space.

In a particularly preferred, simple implementation, the check valve closes the bore in the first valve piston between the first Cavity in the piston and the Koibenbodenraum. The actuator comprises a plunger extending through the bore in the first valve piston which is automatically actuated when the reciprocating piston moves to the retracted position. For example, the check valve may have a ball which is resiliently mounted on the first cavity facing side of the bore and is pressed by the ram extending through the bore.

As already explained above, a lifting device according to the invention can be constructed with a single such lifting cylinder. Such a lifting device is particularly easy to fill and vent.

Special advantages of the lifting cylinder according to the invention, however, when the lifting device has a plurality of series-connected lifting cylinder, wherein preferably the rods of each lifting cylinder are connected end to a common load. Basically, a variety of structures are possible.

In a preferred variant, the first piston head space of a first cylinder in the row of lifting cylinders is connected to a pump. These

Pump for pumping out the first lift cylinder from a reservoir, a fluid in the piston head space. In addition, the piston head space of the first lifting cylinder is connected via a lowering valve to the reservoir, wherein the fluid is pushed back when retracting the piston into the cylinder into the reservoir when the lowering valve is open. In this case, the last lifting cylinder in the series may be formed as a plunger cylinder, d. H. it is a simple cylinder with a reciprocating piston without a valve device according to the invention. But particularly preferred is also the last lifting cylinder in the series a lifting cylinder of the type according to the invention. Then preferably the piston ring space of the last lifting cylinder via a pressure relief valve in turn with the

Reservoir connected. The invention will be explained in more detail below with reference to the attached drawings (not to scale) with reference to exemplary embodiments. They show:

1a shows a longitudinal section through an embodiment of a lift cylinder constructed according to the invention in a central position;

Figure 1 b is an enlarged view of the valve unit in the piston section of the lifting cylinder according to Figure 1a;

Figure 2 shows a longitudinal section through a lifting cylinder according to Figure 1a in a retracted position of the reciprocating piston with a stop element in a AnschMlagsteMüng.

FIG. 3 shows a longitudinal section through a lifting cylinder according to FIGS. 1a and 2 in the retracted position of the lifting piston with a stop element in an idle position;

Figure 4 shows a longitudinal section through a lifting cylinder according to Figures 1a, 2 and 3 in a fully extended position of the reciprocating piston;

Figure 5 is a schematic representation of a lifting device with a plurality of series-connected lifting cylinders according to the invention according to a first embodiment.

Figure 6 is a schematic representation of a lifting device with a plurality of series-connected lifting cylinders according to the invention according to a second embodiment.

Identical or identical elements are each provided with identical reference numerals in the various figures and are therefore not named again in connection with each figure. Figures 1a to 4 show an embodiment of a lifting cylinder, in which according to the invention a special valve assembly 25 is installed. The lifting cylinder 10 has - like each lifting cylinder - a cylinder housing 11, which includes a cylinder cavity 12 in which a reciprocating piston 13 is movably mounted.

In the bottom 19 of the cylinder 10, a stop element 60 is installed here, the concrete structure and function will be explained later with reference to Figures 2 and 3.

On the bottom 19 opposite end face, the cylinder housing 11 a guide sleeve 11 b with a central guide hole 11c and a bore through which a rod portion 13b of the reciprocating piston 13 protrudes. In this case, the rod portion 13b is sealed relative to the guide sleeve 11b by means of an O-ring seal 11d.

At its end facing the bottom 19 of the lifting cylinder 10, the lifting piston 13 has a piston portion 13 a, the diameter of which is adapted to the inner diameter of the cylinder cavity 12, so that it slides along the inside of the jacket wall 11 a of the cylinder housing 11. The piston portion 13a is sealed by seals 14, 15, 16 with respect to the inner wall of the cylinder housing 11, so that through the piston portion 13a of the piston 13 of the cylinder interior 12 in a piston head space 12a, which extends between the piston bottom surface 17 facing away from the rod portion 13b and the base surface 19f the cylinder bottom 19 is located, and a piston annulus 12 b, which is located on the side facing the rod portion 13 b of the piston portion 13 a side is divided. The volume of the piston bottom space 12a and the piston ring space 12b depends on the position of the piston portion 13a within the cylinder cavity 12, respectively. Through a first connection 22 in the bottom end region of the cylinder cavity 12, a fluid can reach the piston head space 12a or from the piston head space 12a. Via a second connection 23 in the rod-side end region of the cylinder housing 11, an inflow or outflow of the fluid from the piston ring space 12b is possible. It is assumed below that the fluid is a hydraulic oil.

In the piston portion 13a of the reciprocating piston 13, a special valve assembly 25 is inserted, which consists of two via two connecting channels 30, 31 interconnected valve units 26, 27. The exact structure of these valve units 26, 27 will be explained below with reference to the enlarged illustration of the detail area X in FIG. 1 b.

In principle, both valve units 26, 27 - apart from a check valve 36 on the first valve unit 26 - have a similar structure, wherein the first valve unit 26 centrally in the piston portion 13a from the piston bottom side and the second valve unit 27 radially outwardly offset from the piston ring side is inserted into the piston portion 13a.

Both the first and the second valve unit 26, 27 essentially consist of a valve piston 54, 55, which is movably mounted in a suitable valve bore 56, 57. For constructional or assembly reasons, this valve bore 56, 57 is not introduced directly into the material of the piston portion 13a, but in each case in a valve sleeve 48, 49, which screwed into correspondingly larger blind holes in the piston portion 13a and with respect to the respective blind bore in each case with a seal 50, 51 are sealed. The valve pistons 54, 55 are in each case sealed by means of a seal 52, 53 in relation to the valve bore 56, 57 located in the valve sleeve 48, 49. Through the valve discs 54, 55 each have a longitudinally extending through bore 45, 46. Through this bore 45, 46, the hydraulic oil from the piston head space and the piston annulus in a located at the end of the valve bore 56, 57 first and second cavity 28th , 29 stream. These cavities 28, 29 are each introduced thereby in a simple manner in the piston portion 13a by the blind holes for inserting the valve sleeves 48, 49 are made slightly longer.

In the first cavity 28 of the first valve unit 26 is also a check valve 36 in the form of a ball, which is supported by a coil spring 35, which is at the end of the blind bore into which the valve sleeve 48 is inserted, against the upper end of the first Valve piston 54 extending bore 45 is pressed. The check valve is mechanically by means of an actuating element in the form of a plunger 37, which extends longitudinally through the bore 45 through the first valve piston 54, aufdrückbar. This plunger 37 has at one point along its longitudinal extent an annular projection 38 or one or more projections, which are held in a recess 47 in the longitudinal bore 45 of the valve piston 54. The plunger 37 can not slip out of the valve piston 54 but only a very specific way in the longitudinal direction of the valve piston 54 are moved, but which is sufficient to push the check valve 36. The function of this check valve 36 will be explained later in more detail.

The first valve piston 54 and the second valve piston 55 each have in their outer circumferential surface an annular recess, so that in each case between valve sleeve 48, 49 and valve piston 54, 55, a valve inner chamber 58, 59 is formed. The valve inner chamber 58 of the first valve unit 26 is connected via a first connecting bore 30 with the second cavity 29 of the second valve unit 27. In addition, the valve inner chamber 59 of the second valve unit 27 via a second Connecting bore 31 is connected to the first cavity 28 of the first valve unit 26.

In the position of the first valve piston 54 shown in FIGS. 1 a and 1 b, the connection of the first cavity 28 to the valve inner chamber 58 of the first valve unit 26 and thus to the first connection bore 30 is closed by the first valve piston 54 itself, since the latter has a sealing surface 41 against a first cavity 28 facing sealing seat 42 presses on the valve sleeve 48. This is accomplished by a helical spring 33, which is supported between the upper end face 39 of the first valve piston 54 and a step in the blind bore in the piston section 13a, into which the valve sleeve 48 has been inserted. Only when the valve piston 54 is slightly displaced toward the end of the blind bore relative to the spring force caused by this spring 33, the valve unit 26 is open and the first cavity 28 is connected to the valve inner chamber 58 in the first valve unit 26 and thus with the first connection bore 30th and further connected to the second cavity 29 of the second valve unit 27 and consequently to the piston annulus 12b.

Similarly, in the intermediate position shown in Figures 1a and 1b in the second valve unit 27, the path from the second cavity 29 to the valve inner chamber 59 of the second valve unit 27 by the second valve piston 55 is closed by a corresponding spring 34 which extends between a Stage in the blind bore for the second valve unit and an end face 40 of the second valve piston 55 is supported, with a sealing surface 43 against a sealing seat 44 to the second cavity 29 facing end face of the valve sleeve 49 is pressed. If the second valve piston 55 is displaced against the spring force in the direction of the end of the blind bore for the second valve unit 27, the second valve unit 27 is opened and the second cavity 29 is connected to the valve inner chamber 59 of the second valve unit 27 and thus to the second connection bore 31 first cavity 28 of the first valve unit 26 connected. Both valve units 26, 27 are each designed so that they are pressure balanced. As already explained, the hydraulic oil can pass from the piston head space through the central bore 45 in the first valve piston 54 into the first cavity 28 of the first valve unit 26. It is further ensured that pointing to the first cavity 28 total cross-sectional area on which the hydraulic oil exerts the pressure is the same as the piston bottom space 12a facing total cross-sectional area of the first valve piston 54th Since due to the permeability through the central bore 45 in the first Cavity 28 is the same pressure as in the piston head space 12a, is consequently - regardless of the pressure in the piston head space 12a -the force on the first Ventiikoibeπ 54 from both directions always the same, so that opening of the valve piston 54 by a fluid pressure in the piston head space 12a not possible is.

The same applies to the second valve piston 55. This is also constructed so that the second cavity 29 facing cross-sectional area is the same size as the piston ring space facing cross-sectional area of the valve piston 55th

The first valve unit 26 is therefore only open when a mechanical pressure is exerted on the first valve piston 54 from the piston bottom side and this is shifted relative to the force applied by the spring 33 a piece in the direction of the first cavity 28. In the same way, the second valve unit 27 can only be opened by applying a mechanical pressure from the piston ring chamber side to the second valve piston 55 and displacing it slightly in the direction of the second cavity 29 in relation to the force applied by the spring 34.

The first valve piston 54 is therefore, as will be explained in detail later, the mechanical opening in a retracted end position of the Reciprocating piston 13 so iang formed that it protrudes with its piston bottom space side end of a piece of the piston portion 13a. Likewise, the second valve piston 55 is so long that it protrudes a piece from the piston portion 13a into the piston ring space 12b.

For assembly, the first valve piston 54 can first be inserted from the later side facing the first cavity 28 into the valve sleeve 48. The springs 35, 33 and the ball of the check valve 36 are then inserted into the blind bore in the piston section 13a, and finally the valve sleeve 48, together with the inserted valve piston 54, is screwed into the blind bore in the piston section 13a. In the same way, the second valve piston 55 is inserted into the second valve sleeve 49 and inserted after inserting the spring 34 in the second azenthian blind bore in the piston portion 13a from the piston ring chamber side 12b in the piston portion 13a.

In the intermediate position of the piston between the retracted position and the extended position shown in FIGS. 1a and 1b, a mechanical pressure is exerted neither on the first valve piston 54 nor on the second valve piston 55. Therefore, both valve units 26, 27 are closed. There is consequently no open connection through which the hydraulic oil could pass from the piston head space 12a into the piston ring space 12b or vice versa. If more hydraulic oil is pumped into the piston head space 12a and at the same time the possibility is given that hydraulic oil is displaced from the piston ring space 12b through the opening 23 on the piston ring space 12b, then the entire lifting piston 13 moves in an extension direction. Conversely, hydraulic oil is pumped through the port 23 in the piston annulus 12b and given the possibility that hydraulic oil is forced out of the piston head space 12a through the port 22 on the piston head space 12a, the piston 13 moves to the retracted position. The mode of operation of the first valve unit 26 in conjunction with the stop element 60 can be seen with reference to FIGS. 2 and 3.

For this purpose, the concrete construction of the stop element 60 will be explained first. This stop element 60 is a piston-like element introduced in a central blind bore 20 in the bottom 19 of the lifting cylinder 10. In this case, the diameter of the cylindrical stop element 60 is selected so that it is adapted to the inner diameter of the blind bore 20. About a seal 61, the stopper member 60 is sealed from the inner wall of the bore 20. Opposite the bottom of this bore 20, the stop element 60 is supported by a coil spring 63. To guide is located on the side facing away from the Koibenbodenraum 12b side in the stop member 60 has a cylindrical recess 62 and at the bottom of the bore 20 is a kind of holding and guide pin 65 with a step on which the spring 63 is supported. Via a radial bore 21, the blind bore 20 is connected to the outer space, d. H. it is located within the blind bore 20 of the outside air pressure.

In the upper area to the piston head space 12a of the lifting cylinder 10 toward the blind bore 20 is extended and provided with a thread. In addition, the stop element 60 has a smaller diameter section in the end region facing the piston head space 12a. In this extended portion of the blind bore 20, a retaining ring 64 is screwed, whose inner diameter is slightly larger than the outer diameter of the front portion of the stop member 60 so that it can extend through the retaining ring 64 therethrough. About this retaining ring 64, a stop is formed to the piston head space 12a, at which the step in the abutment member 60, which is formed between the smaller diameter portion and the larger diameter portion supported. The stop element 60 is thus pressed by the coil spring 63 against the retaining ring 64. This position is here as a "stop position" of the stop element 60th designated. By a pressure from the piston head space 12a on the stopper member 60, this can be further pressed against the force of the spring 63 in the blind bore 20 in a so-called "idle position." The spring 63 is selected or adjusted so that when the fluid pressure ( ie, the pressure of the hydraulic oil) in the piston head space 12a above a limit pressure, for example, 10 bar, the force exerted by this pressure on the end face 6Of of the stopper 60 is sufficient to urge the stopper 60 from the stop position to the idle position.

To mount the stop element, the spring 63 can be inserted into the blind bore 20 from the upper side of the lifting cylinder 10 when the guide bush 11 is open and then the stop element 60 is inserted into the blind bore 20. Subsequently, the retaining ring 64 can be screwed into the extended front portion of the blind bore 20.

Figure 2 shows a situation in which the lifting piston 13 is fully retracted and the stop element 60 is in the stop position. The pressure within the piston bottom space 12a, whose volume is very low here, is below the limit pressure. By forming a raised portion 17a in the piston bottom surface 17 is ensured that always a certain minimum volume remains in the piston head space 12a, which can be filled via the port 22 with hydraulic oil or from which the oil can be pushed out. Since, in the state according to FIG. 2, the pressure in the piston bottom space 12a is still below the limit pressure, the spring 63 holds the stop element 60 securely in the stop position. By the stopper 60, the first valve piston 54 of the first valve unit 26 is pressed against the pressing force of the spring 33 in the open position. The spring 33 is just set so strong that it pushes the first valve piston 54, when no mechanical pressure is exerted on the piston bottom chamber side of him, against the seal friction back safely into the closed position. At the same time the plunger 37 is pressed into the valve and thus the spherical Retreatment valve 36 opened. The spring 35 for the check valve 36 is set even weaker than the spring 33, which presses the first valve piston 54 in the unloaded state in the closed position.

In this position, therefore, the first valve unit 26 is opened and the hydraulic oil can freely from the piston head space 12 a through the central bore of the first valve piston 54 in the first cavity 28, from there into the valve inner chamber 58 of the first valve unit 26, further through the first connecting hole 30 in FIG reach the second cavity 29 of the second valve unit 27 and from there continue to flow through the central bore 46 in the second valve piston 55 into the piston ring space. That is, the connection between the piston head space 12a and the piston ring space 12b is permeable, it being ensured by the operation of the plunger 37 that the check valve 36 does not close, so that there is a permeability in both directions.

In this situation, consequently, the lifting cylinder 10 can be flushed and vented in both directions. In particular, in this way a complete initial filling of both the piston head space 12a and the piston annulus 12b through the port 22 is possible simply by pumping via the port 22 as long as hydraulic oil until it is displaced from the port 23 on the piston annulus 12b again.

For example, for this purpose, hydraulic oil can be pumped via the connection 22 into the lifting cylinder 10 with a pump from a reservoir. Is then ensured that the terminal 23 a

Backflow to the reservoir is ensured, taking a first

Pressure relief valve is interposed, which opens only above a certain maximum pressure, the pressure will increase in the cylinder cavity 12, which in turn is ensured by the pressure relief valve, that the pressure is not too high. If the pressure in the cylinder cavity 12 (and thus also in the piston head chamber 12a) reaches the set limit pressure of, for example, 10 bar (which is below the maximum pressure of the pressure limiting valve), then the stop element 60 is moved into the idle position against the spring force exerted by the spring 63 pressed. This is shown in FIG. In this position, the stop element 60 can no longer press from the piston head space against the plunger 37 and the valve piston 54 of the first valve unit 26. Due to the springs 35 and 33, therefore, both the check valve 36 is closed and the second valve unit 27 is pressed in total into the closed position. Thus, the connection between the piston head space 12a and piston annulus 12b is interrupted and further flushing of the lifting cylinder 10 is no longer possible. With increasing pressure in the piston head space 12a of the lifting piston moves in the direction of the extended position, as already explained in Figure 1a. In this case, the hydraulic oil in the piston annulus 12b is further compressed. However, since an outflow is provided via a pressure relief valve, in this case the maximum pressure predetermined by the pressure limiting valve is established in the piston ring space 12b so that the lifting piston 13 continues to expand as the pressure in the piston head space 12a continues to increase due to further pumping-in of hydraulic oil.

FIG. 4 shows the situation in the fully extended end position. In this position, the second valve piston 55 abuts against the inner surface 11 f of the guide bush 11 b of the lifting cylinder 10 and is pressed into the open position, that the second valve unit 27 is opened. The spring 34, which holds the second valve piston 55 in the closed position, is set relatively weak, so that it is just sufficient without external mechanical pressure to spend the valve piston 55 against the sealing friction force in the closed position.

In this position, therefore, the second valve unit 27 is opened. It can thus from the piston head space 12a of hydraulic oil through the central bore 45 in the first valve piston 54 into the first cavity 28 and from there via the second connection bore 31 into the inner valve chamber 59 of the second valve unit 27, from there through the open valve unit 27 into the second cavity 29 of the second valve unit 27 and from there through the central bore 46 of the second valve piston 55 into the piston ring space 12b. However, a reverse flow from the piston annulus 12b in the piston head space 12a is not possible, since in this case the check valve 36 in the first cavity 28 closes the connection. In this position, therefore, only a one-sided flow from the piston head space 12a in the piston annulus 12b is possible.

The piston annulus has in this position, as can be seen in Figure 4, a very small volume and is essentially formed by a recessed segment 18a in the Kolbenringfiäche i δ, which is formed around the second valve unit 27 in the piston ring surface 18. The remaining piston annulus 12 b is always connected to the port 23.

Both the first valve piston 54 and the second valve piston 55 are provided at their outside ends with a slot 52, 53. Through this slot 52, 52 hydraulic oil can pass through the respective central bore 45, 46 and back even with a stop of the respective valve piston 54, 55 to the stop element 60 and to the inner wall 11f of the guide sleeve 11b.

Figure 5 shows a first embodiment, as with lifting cylinders according to the invention 10, 10 ', a lifting device 1 can be constructed. The lifting device 1 in this case consists of a total of three lifting cylinders 10, 10 ', 7. All three lifting cylinders 10, 10', 7 have the same length and also have reciprocating piston of the same length. The lifting cylinders 10, 10 ', 7 are arranged on a base 3 and the top side with a common load, here for example a lift 2, coupled, which exerts a force F on the lifting piston of the lifting cylinder 10, 10', 7. The lifting device 1 should work so that all lifting cylinders 10, 10 ', 7 are evenly extended or lowered and so the lift 2 can be raised and lowered evenly. The three lifting cylinders 10, 10 ', 7 are for this purpose connected in series, ie the piston ring space 12b of the first cylinder 10 is connected to the piston head space 12a' of the second cylinder 10 'and the piston ring space 12b' in turn with the piston head space of the third lifting cylinder 7. Die first two cylinders 10, 10 'in the row are constructed in the manner according to the invention, for example as shown in FIGS. 1a to 4. The last cylinder 7 is a simple plunger cylinder without a valve arrangement according to the invention in the piston section of the reciprocating piston.

The lifting device 1 is controlled by means of a hydraulic control 70, which is connected via a line 8 to the connection 22 to the piston head space 12a of the first lifting cylinder 10. The hydraulic controller 70 is shown here purely schematically with the aid of conventional switching symbols.

By means of a pump 71 is supplied from a reservoir 72 via a to the

Line 8 connected pumping line 73 a hydraulic oil 72 for filling the lifting cylinder 10, 10 ', 7 in the piston head space 12 a of the first

Lifting cylinder 10 pressed. This flows in the manner described above

Hydraulic oil then through the valve assembly in the first lifting cylinder 10 in the

Piston annular space 12b of the first lifting cylinder, from there via a line 4 in the piston head space 12a 'of the second lifting cylinder 10', there again by a corresponding valve assembly 25 in the piston annulus 12b 'of the second lifting cylinder 10' and from there via a line 5 in the

Piston bottom space of the last lifting cylinder 7.

From a certain pressure in the piston head space of the last plunger cylinder 7, which is given by the (external load influenced) external pressure on the reciprocating piston of the plunger cylinder 7, the reciprocating piston is raised. This pressure also acts in the second lifting cylinder 10 'and in the first lifting cylinder 10. The springs in the stop elements of first and second Hubzyünders 10, 10 'are set so that the limit pressure in the piston head space of the first Hubzyünders 10 and the second Hubzyünders 10' is selected so that it is below the minimum external pressure on the respective lifting cylinders 10, 10 ', 7. For example, the limiting pressure could be around 10 bar.

Thus, if so much hydraulic oil is pumped into the line system of the lifting device 1 by means of the pump 71, that this limit pressure of 10 bar is reached in the piston head space 12a of the first lifting cylinder 10, the lifting piston 13 lifts there and further oil is removed from the piston ring space 12b first Hubzyünders 10 displaced and pumped through the line 4 in the piston head space 12a 'of the second Hubzyünders 10'. This leads to a simultaneous lifting of the lifting piston 13 'in the second lifting cylinder 10' and to a displacement of the hydraulic oil from the piston ring chamber 12b 'of the second lifting cylinder 10', which in turn reaches the piston head space of the plunger cylinder 7 and raises the existing there lifting piston. If the lifting cylinders 10, 10 ', 7 are selected such that the piston ring surface of a respective preceding lifting cylinder 10, 10' corresponds to the piston bottom surface of the following lifting cylinder 10 ', 7, it is automatically ensured that the lifting pistons are extended completely uniformly, since in the previous lifting cylinder in the piston annulus exactly the amount of hydraulic oil is displaced, which is needed to raise the reciprocating piston in the subsequent lifting cylinder by the same distance. If the load is to be held in a certain position, the pump 71 is simply turned off. A check valve 75 in the pumping line 73 ensures that the hydraulic oil can not flow back into the reservoir 72.

In the initial filling can also be ensured that with the help of an (optional) bleed screw 6 in the line 5 just before the last cylinder 7, the hydraulic oil when it is under sufficient pressure, is vented. For lowering, a return line 74 to the reservoir 72 is connected to the connecting line 8 parallel to the pumping line 73, on which the pump 71 is arranged. In this return line 74 is a lowering valve 76, which is provided for lowering to passage. The hydraulic oil may then continue to pass back into the reservoir 72 through a counterbore 77 (in the form of a flow restrictor).

To avoid overloading the pump 71, the pumping line 73 behind the pump 71 and the return line 74 via a pressure relief valve 78, for example, with a maximum pressure of 250 bar connected to each other.

FIG. 6 shows an example of a further lifting device 1 'which has a very similar structure to the lifting device 1 in FIG. 5. A difference is that the last cylinder 10' in the row of cylinders is not a simple plunger cylinder as in the exemplary embodiment according to FIG. In this case, the connection 23 of the piston ring space 12b is connected via a line 9 to the hydraulic control 70 ', which accordingly has a slightly different construction than the hydraulic control 70 in FIG.

This extended hydraulic control 70 'now has a line piece 81 which is coupled to the line 9 and into which a pressure relief valve 80 is installed. Via this pressure relief valve 80, the line 9 is connected to the return line 74 to the reservoir 72, wherein the connection point is behind the lowering valve 76. The hydraulic oil can consequently flow back from the piston ring space 12b "of the last cylinder 10" via the pressure limiting valve 80 into the reservoir 72, when the maximum pressure predetermined by the pressure limiting valve 80 in the piston ring space 12b 'of the last cylinder is exceeded. Via a bypass line 82 with a check valve 79, the pressure limiting valve 80 can also be bridged with respect to a flow in the direction of the lifting cylinder 10 ". The pressure relief valve 80 is here preferably set to 50 bar. Above 50 bar, oil or the air in the system can no longer be compressed. Accordingly, no oil can be added in the system. In addition, the reciprocating pistons are dimensioned so that precisely when the pressure of 50 bar rests in the piston ring space 12b ^{1 of} the last lifting cylinder 10, a limiting pressure of, for example, 10 bar is reached in the piston head space 12a of the first lifting cylinder 10. In the piston head space 12a The second lifting cylinder 10 'then has a pressure of, for example, 20 bar and a pressure of, for example, 40 bar in the piston bottom space 12a "of the third lifting cylinder 10". are each set so that they are pressed at exactly these limit pressures in the empty position and thus close the first valve units.

With this construction, it can be ensured that by simply pumping in the hydraulic oil during initial startup or after an oil change with the pump 71, just as much oil is pumped into the system until the air is completely compressed or through an optional bleeder screw 6 the system has been vented. The system is then filled in the ideal condition. Further measures for venting are not necessary.

Automatically, all the reciprocating pistons 13, 13 ', 13 "are uniformly raised when the pressure of 50 bar behind the last lifting cylinder 10' is reached when hydraulic oil is pumped into the first reciprocating piston 10. In a simple manner, it is likewise possible to actuate the lowering valve 76 behind the piston first lifting cylinder 10 all lifting cylinders 10, 10 ', 10 "are retracted evenly.

By the opening of the valve assemblies in the reciprocating piston 13, 13 ', 13 "in the fully extended state is thereby avoided that one of the lifting cylinder 13, 13', 13" remains above when the external load F on the lifting device on this lifting cylinder 10, 10 ', 10 "is smaller than that by the residual oil pressure in the piston head space on the relevant reciprocating piston 13, 13 ', 13 "force, thus aiso ensures that even with a very uneven loading of the lift 2, all the lifting cylinders 10, 10', 10" always retract evenly.

Thus, with the lifting cylinders according to the invention, a lifting device with ideally coupled lifting cylinders can be constructed, which achieve a nearly perfect synchronism with only minimal errors without a mechanical connection, without a displacement transducer, etc. and load-independent. A maintenance of this system is at best in case of material failure, for example, to change sealing rings, necessary. On the other hand, it is not necessary to perform regular maintenance for ventilation, as in other systems. At each extension and retraction of Hubzyiinder inevitably a balance between the cylinders is carried out in the lowest position and also made a short rinse with hydraulic oil.

It is finally pointed out once again that the lifting cylinders and lifting devices illustrated in the figures and the description are merely exemplary embodiments which can be varied to a large extent by the person skilled in the art without departing from the scope of the invention. In particular, it should again be noted that certain features shown in the embodiments are also optional, such as the use of a bleed screw in the lifting device or the use of a second valve unit in the valve assembly, which opens the valve assembly in the extended state, for example in the system it is ensured that the load on all lifting cylinders is fairly even. In this case, for example, can be dispensed with the check valve, which is indeed required in the first place, so that the valve assembly in the extended state can only be flowed through in one direction. In particular, depending on the intended use and the specific structure of the lifting device, the lifting cylinders can also be designed for limit pressures other than those specified.

Claims

claims

A lifting cylinder (10, 10 ', 10 ") having a cylinder cavity (12) and a piston (13) arranged in the cylinder cavity (12), which comprises a piston section (13a) and a rod section (13b) and which holds the cylinder cavity (12 ) in a piston bottom space (12a) and a piston annulus (12b), characterized by a valve assembly (25) which opens in the open state of a fluid between the piston head space (12a) and the piston annulus (12b) through which connection and in the closed state closes this connection and which is designed so that the valve assembly (25) in both directions between the piston head space (12a) and the piston annulus (12b) is opened through-flow when the reciprocating piston (11) is in a retracted position and the fluid pressure in the piston head space (12a) is below a predetermined limit pressure, and that the valve assembly (25) is closed when the fluid pressure is above the limit pressure or when the reciprocating piston (11) is in a partially extended position.

2. Lifting cylinder according to claim 1, characterized in that the valve arrangement (25) is formed so that the valve arrangement (25) is at least in a flow direction from the piston head space to the piston ring space also open when the reciprocating piston (13) is in an extended position.

3. Lifting cylinder according to claim 1 or 2, characterized in that the valve arrangement (25) in the lifting cylinder (13) is integrated.

4. Lifting cylinder according to claim 3, characterized in that the piston head space (12a) and the piston ring space (12b) via two in the reciprocating piston (13) extending paths are interconnected, wherein the paths in each case by a in the reciprocating piston (13) arranged valve unit (26 . 27) are closed and a valve unit (26) is automatically opened only when the reciprocating piston (13) is in the retracted position and the fluid pressure in the piston head space (12a) is below the limit pressure and another valve unit (27) is automatically opened, when the reciprocating piston (13) is in the extended position.

5. Lifting cylinder according to claim 3 or 4, characterized in that the valve arrangement (25) has a in the reciprocating piston (13) arranged first valve unit (16) with a first valve piston (54) which is formed so that they are with respect to the fluid pressure in Piston bottom space (12a) is pressure-balanced and is opened by a certain mechanical pressure, and that in the piston head space (12a) a stop element (60) is arranged, which is designed and / or stored so that it is at a fluid pressure in the piston head space (12a) is below the limit pressure in a stop position and in the retracted position of the reciprocating piston (13) on the first valve piston (54) exerts a mechanical pressure to open the first valve unit (26) and that at a fluid pressure in the piston head space (12a) above the limit pressure is pressed into an empty position, so that in the retracted position of the reciprocating piston (13) no mechanical pressure on the first Venti Lkolben (54) exercises.

6. Lifting cylinder according to claim 5, characterized in that the stop element (60) in a recess (20) in a bottom (19) of the lifting cylinder (10) relative to the piston (13) mounted in the first valve piston (54) is arranged and by a spring element (63) is held in the stop position, wherein the spring element (63) is formed so that the stop element (60) is pressed by a pressure in the piston head space (12a) above the limit pressure in the empty position.

7. Lifting cylinder according to claim 5 or 6, characterized in that the first valve piston (54) in the reciprocating piston (13) is mounted so that it by means of a spring element (33) to the piston head space (12 a) out in a Schiießsteliung is pressed, in which the first valve piston (54) with a sealing surface (41) against a piston arranged in the piston (13) first valve seat (42) presses, and in which the first valve piston (54) from the reciprocating piston (13) in the Piston floor space (12 a) protrudes.

8. Lifting cylinder according to one of claims 5 to 7, characterized in that the first valve piston (54) in the reciprocating piston (13) in a first valve bore (56) between a first cavity (28) in the reciprocating piston (13) and the piston head space (12a ), wherein the first cavity (28) and the piston head space (12a) are interconnected so that a fluid from the piston head space (12a) in the first cavity (28) can flow, and wherein the cross-sectional area of the first valve piston (54) towards the piston bottom space (12a) and the cross-sectional area of the first valve piston (54) are substantially equal to the first cavity (28).

9. Lifting cylinder according to claim 8, characterized by a connection between the first cavity (28) of the lifting piston (13) and the piston ring space (12b), wherein the first valve piston (54) closes the connection in the closed position.

10. Lifting cylinder according to one of claims 3 to 9, characterized in that the valve arrangement (25) has a in the reciprocating piston (13) arranged second valve unit (27) with a second valve piston (55) which is formed so that they are in terms of Fluid pressure in the piston annulus (12b) is pressure balanced and is opened by a mechanical pressure when the reciprocating piston (13) is in the extended position.

11. Lifting cylinder according to claim 10, characterized in that the second valve piston (55) in the reciprocating piston (13) is mounted so that it is pressed by means of a spring element (34) to the piston ring space (12b) out in a closed position in which the second Valve piston (55) with a Sealing surface (43) against a Hubkoiben (13) arranged second valve seat (44) presses and in which the second valve piston (55) protrudes from the reciprocating piston (13) in the piston annulus (12b).

12. Lifting cylinder according to claim 10 or 11, characterized in that the second valve piston (55) in the reciprocating piston (13) in a second valve bore (57) between a second cavity (29) in the reciprocating piston (13) and the piston annulus (12b) stored is, wherein the second cavity (29) and the piston annulus (12b) are interconnected so that a fluid between the piston annulus (12b) and the second cavity (29) can flow, and wherein the cross-sectional area of the second valve piston (55) for Koibenringraurn (12b) and the cross-sectional area of the second valve piston (55) to the second cavity (29) are substantially equal in size.

13. Lifting cylinder according to claim 12, characterized in that the first

Cavity (28) and the second cavity (29) in the reciprocating piston (13) via two

Ways (30, 31) are interconnected and the one path (30) in the closed position of the first valve piston (54) through the first valve piston

(54) is closed and the other way (31) in the closed position of the second valve piston (55) through the second valve piston (55) is closed.

14. Lifting cylinder according to one of claims 8 to 13, characterized in that the first valve piston (54) has a bore (45) through which the first cavity (28) and the piston head space (12 a) are interconnected, and / or the second valve piston

(55) has a bore (47) through which the second cavity (29) and the piston annulus (12b) are interconnected.

15. Lifting cylinder according to one of the preceding claims, characterized in that the valve arrangement (25) has a check valve (36) has, which prevents a backflow of the fluid from the piston ring space (12b) in the piston head space (12a).

16. Lifting cylinder according to claim 15, characterized in that the valve arrangement (25) has an actuating element (37), which in the retracted position of the lifting piston (13) opens the check valve (36), so that the fluid from the piston ring space (12 b) in the piston bottom space (12a) can flow.

17. Lifting cylinder according to claim 14 and claim 16, characterized in that the check valve (36) closes the bore (45) in the first valve piston (54) between the first cavity (28) in the reciprocating piston (13) and the Koibenbodenraum (12b) and the adjusting element (37) comprises a plunger (37) extending through the bore (45).

18 lifting device (1) with a lifting cylinder (10, 10 ', 10 ") according to one of claims 1 to 17.

19. Lifting device (1) according to claim 18, characterized by a plurality of series-connected in series lifting cylinder (10, 10 ', 10 ") according to one of

Claims 1 to 17.

20. Lifting device (1) according to claim 19, characterized in that the piston head space (12a) of a first lifting cylinder (10) is connected in series with a pump (71), which for extending the first lifting cylinder (10) from a reservoir (10). 72) pumps a fluid into the piston head space (12a), and the piston head space (12a) of the first lift cylinder is also connected via a lowering valve (76) to the reservoir (72), wherein with the lowering valve (76) open the fluid upon retraction of the reciprocating piston (13) is pushed back into the lifting cylinder (10) in the reservoir (72).

21. Lifting device according to claim 19 or 20, characterized in that a last lifting cylinder (7) in the row is a plunger cylinder (7).

22. Lifting device according to claim 19 or 20, characterized in that a last lifting cylinder (10 ") in the row is a lifting cylinder (10") according to one of claims 1 to 18 and the piston ring space (12b ") of the last lifting cylinder (10"). ) is connected via a pressure relief valve (80) to the reservoir (72).