WO1993012343A1

WO1993012343A1 - Pneumatically operable lifting cylinder

Info

Publication number: WO1993012343A1
Application number: PCT/DE1992/001022
Authority: WO
Inventors: Klaus Flessa
Original assignee: Itronic Fuchs Gmbh
Priority date: 1991-12-07
Filing date: 1992-12-04
Publication date: 1993-06-24
Also published as: DE4140376A1

Abstract

A pneumatically operable lifting cylinder for linearly moving a piston rod (58) along a guide tube (2) has a first piston end section (12) firmly secured to the guide tube (2) and a second piston end section (32) movable against the force of an axial return component (54), to which the piston rod (58) is secured. Between these two sections (12, 32) there are N(N/>=2) lifting chambers (40, 41, 42) to which pressure can be applied from external fluid sources. These are separated from one another with seals by N-1 pistons (20, 31) arranged between the lifting chambers (40, 41, 42). At both end sections (15), each of the N-1 pistons (20, 31) has a hooked end section (18) which engages with the corresponding hooked end section (18) of the adjacent piston (20, 31) or piston section (12, 32) so that each of the N lifting chambers (40, 41, 42) can be individually moved in its travel (50, 51, 52) between two extreme positions. The latter are defined on the one hand by the minimum distance between adjacent pistons (12, 20, 31, 32) predetermined via the axial length of each of the two hook sections (15) and on the other by the maximum distance between adjacent pistons (12, 20, 31, 32) predetermined via the sum of the axial lengths of the two hook sections (15) minus their axial thickness. It is thus possible in a simple manner to obtain eight different settings of the piston rod (58) with three chambers (40, 41, 42) with a geometrically increasing travel (50, 51, 52).

Description

Pneumatically to ^* -eibbarer Hubzyl Direction

The invention relates to a pneumatically driving lifting cylinder for linearly adjusting a piston. , ^~ J & along a guide tube with an adjustable stroke, Move with a firmly connected to the guide tube first Endkolbenabschnitt and a counter to the force of an axial restoring element! Piston portion cozy second terminally, to which the piston rod is attached, wherein between the two sections, a first Stroke chamber is provided which can be acted upon by pressure from an external fluid source such that the stroke of the piston rod can be changed between two extreme positions which can be predetermined by stops of the stroke cylinder.

Such a lifting cylinder is described in patent application DE 40 14 760 and is e.g. used in dispenser devices in order to apply a predetermined amount of adhesive or solder to a corresponding location on a printed circuit board for subsequent assembly. In each work step, the piston volume is pressurized with compressed air, the movable piston section moving the piston rod with a valve needle attached to it, and the latter itself applying the adhesive by displacing the corresponding volume.

Such a known device has the disadvantage that it only allows a single preset stroke and thus the delivery of a single predetermined amount of adhesive. The prior art teaches only the variation of the outflow quantity with a manual adjustment of the stop of the movable piston part, so that the displacement of the Stop surface of the possible stroke of the piston is changed and in particular limited. With such a lifting cylinder, it is then possible to change the amount of adhesive or solder to be dispensed between two working cycles. However, the quantity of adhesive to be applied cannot be changed during processing, for example of a printed circuit board, without the processing process being interrupted. This means that a high processing speed of, for example, printed circuit boards with different amounts of adhesive to be applied is not possible.

On the basis of this prior art, the object of the invention is to create a lifting cylinder of the type mentioned at the beginning, with which it is possible to start different predetermined piston strokes in the operating mode at high processing speeds, for example in order to to provide different quantities of adhesive for the PCB assembly at successive processing points.

This object is achieved according to the invention in that N lifting chambers which can be acted upon individually by pressure from external fluid sources are provided, N being a natural number greater than or equal to two, which are separated from one another by N-1 pistons arranged between the lifting chambers, and that each of the N-1 pistons has at its two end sections a hook end section which engages with the corresponding hook end section of the adjacent piston or piston end section, so that each of the N lifting chambers can be moved individually in its stroke between two extreme positions is, on the one hand, due to the possibly longer over the entire axial length of the two adjacent hook sections predetermined minimum distance of the adjacent pistons from one another and on the other hand predetermined by the maximum distance of the adjacent pistons from each other by the axial length of the possibly shorter of the two adjacent distance cylinder sections of the hook sections minus the axial thickness of the other hook end section are.

Because a successive arrangement of several lifting cylinders is provided, which preferably have respective lifting heights that follow the binary powers, it is possible to apply a large number of different piston strokes in stop operation with a small number of components as well as control elements and valves receive.

«

Further expedient refinements of the invention are characterized in the subclaims.

An exemplary embodiment of the invention is explained in more detail below by way of example with reference to the drawings. Show it:

1 is a schematic cross-sectional view of a dispenser device with a multi-stroke cylinder according to an embodiment of the invention in the idle state,

2 is an enlarged sectional view of the multi-stroke cylinder from FIG. 1,

3 shows a piston of a multi-stroke cylinder according to FIG. 1 or FIG. 2, 4 is an enlarged sectional view of the dispenser device according to FIG. 1,

5 shows a cross-sectional view of the device according to FIG. 1 with a pressurized multi-stroke cylinder, and

Fig. ' 6 is an enlarged sectional view of the multi-stroke cylinder from FIG. 5.

1 shows a cross-sectional view of a dispenser device for dispensing adhesive for an SMD placement device, which is constructed from a pneumatically operated lifting cylinder 1 for driving a piston rod and a dispenser unit 91.

The lifting cylinder 1 has a cylindrical tube 2 which e.g. in the assembly production of printed circuit boards with an inner diameter of e.g. 8 mm is designed. The tube 2 is advantageously made of a light metal, the tube 2 preferably being anodized on its inner surface.

The tube 2 has a plurality of lateral connections 3, 4, 5. Instead of the three connections 3, 4 and 5 provided in the exemplary embodiment shown, only two or more than three connections could also be provided. These connections 3, 4, 5 consist of an extension 6, preferably made in one piece with the tube 2, on each of which a hose connection piece 7 is arranged. Hoses (not shown in the figures) which can be pressurized with fluid or pressure are provided on the hose connectors 7. hen, which are controlled by control means provided individually for each hose. These can be connected to a single pressure source, for example via solenoid valves, so that each of the connections 3, 4 or 5 can be individually pressurized in a simple manner.

The connections 3, 4 and 5 have inner bores 8 which open into the interior 9 of the tube 2. The diameter of the bores 8 advantageously corresponds to the inside diameter of the hoses to be connected and that of the respective connector 7.

In the exemplary embodiment shown, the connections 3, 4 and 5 are arranged essentially at equal axial distances parallel to one another, with a ventilation hole 10 being provided in the jacket of the tube 2 and additionally one below the movable end piston section 32 between two connections. The distances between the connections 3, 4 and 5 and possibly further connections can, however, be of different sizes depending on the design of the lifting cylinder 1. In particular, the distances between the connections 3, 4 and 5 from one another can follow the extreme values do adjustable piston strokes.

For the further description of the lifting cylinder, reference is made to FIG. 2, which shows the lifting cylinder in an enlarged view in a section of FIG. 1. The same features in FIG. 2 and the subsequent figures have the reference symbols used in connection with FIG. 1. In the tube 2, a sealing internal thread 11 is provided at the upper end, into which a closing piston piece 12 can be screwed. The end piston piece 12 thus firmly connected to the cylindrical tube 2 has a flattened gripping area 13, an external thread 14 which can be screwed into the internal thread 11 and a hook section 15 which is symmetrical with respect to the drawing and cross-sectional plane.

The hook section 15 consists in particular of a spacer cylinder section 17, which is preferably milled out of the cylindrical material of the end section 12, this milling extending beyond the central axis 16 of the tube 2 of the piston, and a claw-like hook end section 18, which likewise consists of a There is a cylinder section which is obtained from a milling of the end section 12 machined from the same side but not reaching as far as the central axis 16.

In the illustrated embodiment of a stroke cylinder 1 for use in SMD assembly, the tube 2 has a length of 7 to 10 cm. The cylindrical outer diameter of the hook section 15 is 7.8 mm. The milling 29 of the spacer cylinder section 17 which extends beyond the central axis 16 of the tube 2 is here 1.5 mm. The hook end section 18 extending beyond the central axis 16 of the tube 2 extends 1 mm beyond this central axis 16.

The claw-like hook end section 18 of the end section 12 is arranged in the tube 2 essentially at the level of and relative to the first compressed air connection 3. A first movable piston 20 engages in the end section 12, which is shown in a further enlarged perspective view in FIG. 3. The piston 20, which is axially movable in the tube 2, is an element which is essentially symmetrical with respect to the axis 16 and, moreover, is symmetrical with respect to a plane 21 shown in FIG. 3, which corresponds to the axis 16 including the plane of the drawing in the other figures .

The piston 20 has a cylindrical spacer section 22 with an outer diameter of 7.8 mm, to which a groove 23 and 24 is connected on both sides. The e.g. Grooves 23 and 24 machined by recesses, as shown in FIGS. 1 and 2, are provided for receiving sealing rings 25 and 26 which are simply unbuttoned. The sealing sleeves or sealing rings 25 and 26 each seal the adjacent end sections of the piston 20 from one another via the central section 22. Hook-shaped end sections 27 and 28 adjoin the grooves 23 and 24 at the two opposite ends of the piston 20. The radial shape of the hook sections 27 and 28 is identical to that of the hook section 15 of the end section 12.

This means that they have a milling 29 provided normal to the plane 21, which leaves a cylinder sector section 30, the radial height of which is 3.9-1.5 = 2.4 mm less than the radius of the cylinder 20 . These milled sections 30, the length 33 of which advantageously varies along the axis 16, are followed by the NEN hook end sections 18, each having a cut parallel to the plane 21, which leaves a cylinder sector section, the height of 3.9 + 1 = 4.9 mm is greater than half the diameter of the tube 20.

The piston 20, with the sealing O-rings or sealing sleeves 25 and 26 inserted in the grooves 23 and 24 and with the end section 12 and with the second piston 31 adjoining the first piston 20, is in engagement with the cylinder jacket of the tube 2 1 and 2 introduced. This is followed by a piston rod section 32 which engages with the hook section 15 of the piston 31. The pistons 20, 31 and 32 are preferably made with a radius that is approximately 1/10 mm smaller than the inner radius of the tube 2. This allows a fluid medium to spread between the tube 2 and the pistons 20, 31 and 32, the sealing sleeves 24 and 25 each sealing the lifting chambers from one another and forming the guide for the pistons 20, 31 and 32 in the tube 2.

The height 33, 33 'or 33' 'of each cutout 17 or 30 of a piston 20 is selected such that it corresponds to the height of the corresponding cutout of the adjacent piston 31 or piston section 12.

Thus, two adjacent pistons 12 and 20, 20 and ^'31, or 31 and 32 against each other in each case by the height of the cut-out 33 or 33' or 33 '' minus the axial thickness of the associated adjacent klauenarti¬ gen hook end portion 18 against each other axially ver push! i. Associated hook end sections 18 are preferably of the same thickness.

In the embodiment of the invention shown in the figures, with the three pressure connections 3, 4 and 5 standing at right angles to the lifting cylinder axis 16, two lifting pistons 20 and 31 are provided in addition to the end sections 12 and 32, so that the position shown in FIG. 2 form three piston chambers 40, 41 and 42 shown.

10 When the pipe 2 is pressurized only via the pressure connection 5, the pressure in the pipe 2 increases between the two sealing rings 43 and 44 of the second movable piston 31 and the piston rod section 32. The compressed air flowing in via the connection 5

15 is distributed between the two sealing rings 43 and 44, since between the inner wall of the tube 2 and the cylinder jackets of the pistons 31 and 32 a play of e.g. 0.1 mm. As a result, in particular the pistons 31 and 32 are pressed apart by air between

20 flows in these sections on the surfaces 45 and 46. The pistons 31 and 32 are thus pressed apart until the respective hook end sections 18 of the pistons 31 and 32 touch and the cavity originally located between these claws or hooks

25 42 has disappeared in favor of the cavities 92 and 93 on the surfaces 45 and 46, as shown in FIG. 6.

The axial length of the three stroke-determining cutouts 33, 33 'and 33''is preferably predetermined in such a way that the strokes 50, 51 and 52 shown in FIG. 6 correspond to a strictly monotonously increasing row and all differ are. In particular, the strokes can be proportionate to the smallest Stroke 50 correspond to the geometric series 1, 2, 4, etc., but any other suitable ratio, such as 1, 2, 5, 10 etc., can also be selected. It is also possible to provide strokes of the same type, so that a pressurization of M from a total of N lifting chambers causes a movement of M times an individual lifting chamber path.

In the exemplary embodiment shown in the figures, the thickness of each claw-like hook end section 18 is uniformly approximately 2 mm. The lengths of the cutouts 33, 33 ', 33' 'are 2.5 mm for the corresponding distance cylinder sections 17 for the pistons 12 and 20, 3 mm for the pistons 20 and 31 and 4 mm for the pistons 31 and 32. Since the possible stroke distance results from the difference between the length of the cut-out 33, 33 'and 33' 'and the thickness of the respective hook end section 18, the three stroke lengths 50, 51 and 52 are respectively 0.5 and 1 or 2 mm.

If one designates the stroke 50 with the relative size 1 and the above-mentioned ratio of the stroke paths is available according to a geometric series, so that the strokes 51 and 52 can be identified with the numerals 2 and 4 with respect to the stroke 50, the control lies accordingly pressurizing solenoid valves before an adjustable stroke of the piston rod 58 between the relative sizes 0 to 7 in eight different equal increments.

The arrangement of the piston volume na increasing from the end section 12 by increasing axial strokes 50, 51 and 52 can also be reversed. This essentially depends on which stroke distances require the most Strokes correspond. In particular, an alternating arrangement of the lifting chambers 40, 41 and 42 in terms of their stroke size is also possible by appropriate design of the cutouts 33, 33 'and 33''.

In order to enable pressure equalization for the areas between the various adjacent pressure pistons 20, 31 and 32, the cylinder jacket bores 10 are provided between two sealing rings 23 and 24 in the central area 22 of the respective piston 20, 31 and 32.

The end section 32 has a single sealing ring 23 which seals the lifting cylinder 1 against the outside space. The piston rod 58 is fastened in the end section 32 and advantageously screwed in. The piston rod 58 is carried out centrally by a guide 53 which is screwed into the lower end of the tube 2. Arranged between the guide 53 and the lower movable piston section 32 is a compression spring 54 which forms a restoring element on correspondingly provided centering shoulders 55 and 56 which, without pressurizing the connections 3, 4 and 5 with the external fluid, the pistons 30, 31 and 32 moves into the rest position shown in FIG. 1 and thus reduces the strokes 50, 51 and 52 to 0.

When pressure is applied to one or more of the connecting pieces 7, the associated piston, which is arranged further in the direction of the piston rod 58, moves away from the end section 12, which is firmly anchored in the tube 2. The movement of the pistons and piston sections 20, 31 and 32 ends at the moment when the one aligned in the direction of the end section 12 claw-like hook section 18 with its hook surface comes into engagement with the hook section 18 of the correspondingly adjacent piston 30 or 31 or piston section 12 arranged in the direction of the piston rod 58. Then the maximum extension of the respective adjacent pistons and piston sections 20, 31 and 32 is given, which essentially corresponds to the sum of the length of the respective two hook sections 18 minus the axial extension of the hook sections 18 themselves.

The size of the maximum achievable stroke 50 and 51 and 52 then results from the difference to the length of the lifting cylinder when the pressurizing medium is released. Then said adjacent pistons and piston sections 12, 20, 31 and 32 are pushed together by spring 54, the smallest distance between the pistons being reached at the moment when the hook section 18 of one piston 12, 20, 31 or 32 comes into contact with the piston surface of the correspondingly adjacent piston 12, 20, 31 and 32 opposite it. For a given piston chamber, therefore, the maximum achievable stroke 50, 51 or 52 is given by mutually assigned identical hook sections 18 which, when the pistons 12, 20, 31 and 32 are pushed together by the spring 54, essentially stop simultaneously with the said one Kick the piston wall.

The piston rod 58 protrudes into the dispenser device 91 shown in FIGS. 1 and 4. FIG. 4 shows the dispenser device 91 on an enlarged scale. This has a frame 60 which is rigidly connected to the production machine. On The frame 60 has two flanges 61 and 62 which enclose a volume 64 which can be acted upon by compressed air via the connection 63. The volume 64 is sealed off from the environment via the sealing rings 65 and 66. The flanges 61 and 62 have passages in which a guide tube 67 is arranged. In the guide tube 67, the piston rod 58 is fastened via a lock nut 68 to a receptacle 76 on which a needle 70 is adjustably arranged via an internal hex 69. The guide 67 is designed in its upper section as a slotted tube 72, into which the lower guide tube 53 can be inserted and fixed with the aid of a clamp 73.

Thus, the tube 2 is rigidly connected to the guide 67 via the guide tube 53. Since this connection can be released quickly, maintenance work can be carried out quickly and a lifting cylinder can be exchanged quickly. It is particularly advantageous that the elements are fixed without screwing, so that twisting of the supply and control lines is avoided.

The guide 67 is axially displaceable with respect to the frame 60 with pressure that can be supplied via the pressure connection 63 and an associated return spring 74. A stop damping ring 75 can also be provided.

The unit tube 2 / guide 67 can thus be pneumatically axially adjusted with respect to the frame 60 in the direction of the arrow 75 via the pressure supplied through the compressed air connection 63. A dispenser head 80 is screwed onto the guide 67 and has a lateral pressure connection 81 through which adhesive or solder can be shot into the storage cavity 83, in which the needle 70 is guided, through a bore 82. With the corresponding retracted position of the needle 70, the adhesive fills the storage cavity 83 and can be dispensed via the bore 84 in the head piece 85. In order to avoid damage to the discharge opening 84, the head 85 has a stop or spacer 86 which is placed on the circuit board to be contacted or the like when the adhesive is dispensed, so that the tip of the opening 84 is at a distance of a few inches. 10 millimeters above this circuit board is arranged so that it can neither bump open nor that it is immersed in the dispensed adhesive or closes the opening.

The mode of operation of the dispenser head 91 is explained in the exemplary embodiment presented with reference to FIGS. 5 and 6 and to FIGS. 1 and 2.

In the rest position, the connections 3, 4 and 5 are all pressurized with compressed air, so that the needle 70 seals the outflow in the sealing seat according to FIG. 5. When a predetermined one of the three pressure connections 3, 4 or 5 is opened, the sealing needle 70 partially retracts into the guide 67 in accordance with the sum of the strokes 50, 51 and / or 52. When the control valves are actuated in the case of FIG. 1, all the strokes 50, 51 and 52 return to zero. Then the adhesive or the solder can flow through the opening 82 into the storage cavity 83 and into the dispensing opening 84 in such an amount resulting from the preset stroke of the pistons 20, 31 and 32. This amount is pressed out of the discharge opening 84 when pressure is applied again, which essentially corresponds to the cross-sectional volume of the cross-section displaced by the needle 70 times the height of the lifting sections 50 and / or 51 and / or 52.

In another advantageous exemplary embodiment, the needle 70 is located in the sealing seat in the dispenser head 80 when there is no pressurization via the connections 3, 4 and 5.

The multi-stroke cylinder described here can be used in the described embodiment to populate printed circuit boards, but it can also be used in any size, for example the SMD component, and with any number and sequence of piston strokes in all cases. in which a predetermined, sometimes rapidly changing piston stroke must be achieved in stop operation. A larger number of the multi-stroke pistons 12, 20, 31, 32 used increases the flexibility of the application, ie the desired number and the achievable accuracy of the differently adjustable strokes. In the case of a pneumatically operated lifting cylinder with N piston chambers, there are 2 ^N setting options which must be adjustable by a corresponding regulating device of the pressure regulating valves. Such a device requires N-1 movable pistons plus the fixed and the displaceable end piston section.

In addition to the piston 12, 20, 31, 32 in the exemplary embodiment with the mentioned cutouts, it is also possible in a simpler embodiment of the invention to use only curved hooks instead of the to provide milled hook sections 15, the hook part of which has a surface with which the hook can be supported in each case on the piston surface of the adjacent piston. It is only necessary that two adjacent piston sections 12, 20, 31, 32 are in engagement with one another in such a way that they have a spacer for a maximum distance and a spacer for a minimum distance.

It would therefore also be possible to provide a flexible thread or wire between two fully cylindrical piston sections 20, 31, 32 which, when pistons 20, 31, 32 are pushed together, in a central recess near one axis of one or both pistons 20, 31, 32 to be included. The greatest length and thus the stroke 50, 51, 52 of this multi-stroke cylinder result from the thread length provided minus the height of the central recesses mentioned.

Claims

1. Pneumatically driven lifting cylinder for linearly adjusting a piston rod (58) along a guide tube (2) with an adjustable stroke, with a first end piston section (12) firmly connected to the guide tube (2) and with one against the force of a Axial return element (54) displaceable second end piston section (32) to which the piston rod (58) is fastened, a first lifting chamber (40) being provided between the two sections (12 and 32), which pressurizes from an external fluid source can be acted upon in such a way that the stroke (50) of the piston rod (58) can be changed between two extreme positions which can be predetermined by stops of the lifting cylinder, characterized in that N stroke chambers (40, 41, 42) which can be acted upon by pressure from external fluid sources are provided, where N is a natural number greater than or equal to two, the pistons (20, 31) arranged between the lifting chambers (40, 41, 42) being sealed (25, 26) by N-1 ) and that each of the N-1 pistons (20, 31) has at its two end sections (15) a hook end section (18) which is in contact with the corresponding hook end section (18) of the adjacent piston (20, 31) or piston end section (12, 32) is in engagement, so that each of the N lifting chambers (40, 41, 42) can be moved individually in its stroke (50, 51, 52) between two extreme positions, which on the one hand are controlled by the the total axial length (17 + 18) of the possibly longer of the two adjacent hook sections (15) predetermined minimum distance of the adjacent pistons (12, 20, 31, 32) from each other and on the other hand by the over the axial length (33) of the possibly shorter the the two adjacent spacer cylinder sections (17) of the hook sections (15) minus the axial thickness (18) of the respective other hook end section are predetermined maximum distance between the adjacent pistons (12, 20, 31, 32) from one another.

2. Lifting cylinder according to claim 1, characterized gekennzeich¬ net that the length of the hook portions (15) of the i-th lifting chamber (40, 41, 42) adjacent pistons (12, 20, 31, 32) are designed such that the i-th of the N lifting chambers (40, 41, 42) has a stroke (50, 51, 52) of 2 ^i_1 times with respect to the stroke (50) of the first lifting chamber (40).

3. Lifting cylinder according to claim 1 or claim 2, characterized in that the spacing cylinder sections (17) of two opposite end sections (15) of adjacent pistons are of the same length in the axial direction.

4. Lifting cylinder according to one of the preceding claims, characterized in that the guide tube (2) is made from an internally anodized metallic material.

5. Lifting cylinder according to one of the preceding claims, characterized in that the pistons (20, 31, 32) in the region of the end sections (15) have a smaller diameter than the tube (2).

6. Lift cylinder according to claim 5, characterized gekennzeich¬ net that each piston (20, 31) in its central region has two spaced-apart sealing sleeves (24, .25, 43, 44).

7. Lifting cylinder according to claim 6, characterized in that in the area between the sealing sleeves (24, 25, 43, 44) in the guide tube (2) a pressure-compensated

5 c end ventilation hole (10) is provided.

8. Lifting cylinder according to one of the preceding claims, characterized in that the axial Rückstell¬ element (54) is designed as a compression spring.

10

9. Pneumatically driven lifting cylinder for linear adjustment of a piston rod (58) along a guide tube (2) with an adjustable stroke, with a first end piston fixed to the guide tube (2).

15 section (12) and with a second end piston section (32) which is displaceable against the force of an axial restoring element (54) and to which the piston rod (58) is attached, a first lifting chamber () between the two sections (12 and 32) 40) is provided

20 which can be acted upon by pressure from an external fluid source in such a way that the stroke (50) of the piston rod (58) can be changed between two extreme positions which can be predetermined by the stroke of the lifting cylinder, i.e. that n

25 lifting chambers (40, 41, 42) which can be pressurized individually from external fluid sources are provided, N being a natural number greater than or equal to two, the pistons (N - 1 arranged between the lifting chambers (40, 41, 42) 20, 31) sealing from each other (25,

30 26) are separated, and that each of the N-1 pistons (20, 31) has at its two end sections (15) a fastening element which is connected to the corresponding fastening element of the adjacent piston (20, 31) or piston end sections (12, 32) about a flexible or foldable connecting element is connected, which can be received in a central recess near the axis of one or both pistons (12, 20, 31, 32) when pistons (12, 20, 31, 32) are pushed together, so that each of the N lifting chambers ( 40, 41, 42) can be moved individually in their stroke (50, 51, 52) between two extreme positions, which, on the one hand, due to the height of the central recess close to the axis of one or both pistons (12, 20, 31, 32 ) predetermined minimum distance of the adjacent pistons (12, 20, 31, 32) from each other and on the other hand by the maximum distance of the adjacent pistons (12, 20, 31, 32) predetermined over the length of the connecting element minus the height of the said central recesses are predetermined by each other.

10. Lift cylinder according to claim 9, characterized gekenn¬ characterized in that the connecting element is made of a biegsa¬ men thread or wire.