CA1040065A - Cylinder-piston combination, particularly for high-pressure application - Google Patents

Abstract

"CYLINDER-PISTON COMBINATION, PARTICULARLY
FOR HIGH-PRESSURE APPLICATION"

ABSTRACT OF THE DISCLOSURE.

A sealing tube which is at least partly elastic is secured at its ends to a piston part and a cylinder part of the combination, respectively, to form a leakproof, sealed connection for introduction of a working pressure fluid between the cylinder and the head of the piston, the sealing tube having the end which is relatively movable with respect to one of the parts connected to the other part by means of a transition zone which extends in the direction of movement, the sealing tube and at least a portion of the transition zone having a low-friction surface which is preferably durable and, additionally, may be regenerated, for example by the intro-duction of pressurized lubricant to form a hydrostatic bearing and spacing the transition zone from the stationary part. The pressure in the hydrostatic bearing is balanced against the pressure of the working fluid by a self-adjusting throttling choke formed by the gap between the deformed sealing tube, when under pressure of the working fluid, and the pressure of the lubricant, so that a pressure balance at the transition zone will arise supporting the sealing tube within the stationary part with the interposition of the pressurized fluid. If only low friction surfaces are used, then polytetrafluoroethylene or similar surfaces on the tube and/or the relatively movable part may be applied thereto.

Description

~0~65 The present invention relates a cylinder-piston combination, and more particularly to a cylinder-piston structural arrangement for high-pressure, hydraulic application to control positioning of heavy loads, remote - control positioning, remote control locating, servo positioning, and the like. Combination of dual piston-or double-acting cylinder arrangements can provide push-pull/effects.
Specifically, the present invention is directed to the structural arrangement which provides for sealing of the fluid, preferably hydraulic fluid within a working space, that is, within a c~inder chamber by means of a flexible elastic sealing tube, p~erably made of an elastomer which is attached with its respective ends to the piston,and cylinder, respectively, to form a leakproof, sealed, tight connection for the working fluid which may be under very high pressure, for example in the order of 100 at gauge.
Piston-cylinder arrangements for high-pressure application provided with leakproof seals are difficult to use in continuous high-power applications in which high operating frequencies, high operating speeds, and high pressures arise. The low mechanical strength of the sealing tubes has heretofore inhibited such applications because, to provide a leakproof, tight seal, the sealing tube must be locally clamped and has been terminated by an abrupt transition on the respective support element, that is, the piston or the cyllnder, depending on the end of the tube ~: ' -Qk65 under consideration. These termination resulted in high-stress gradients in radial direction at localized points or zones of the sealing tube. These hlgh stresses greatly reduced the durability of the sealing tubes. If sliding or movable seals are used between a relatively movable piston and cylinder, using elastic substance, then high coefficients -of friction inhibit rapid and efficient operation; the high frictional forces can be reduced only by special arrangements. Heat due to friction, as well as the frictional force itself, cause rapid deterioration of the elastic portions of the seal, typically of elastomer material.
It is an object of the present invention to provide a cylinder-piston positioning or drive arrangement which is so constructed that it permits high power or force transfer with high efficiency and at high speed in continuous operation without losing its leakprooF seal.
~ S~bject matter of the present invention: Briefly, a sealing tube is provided which is slidable with respect to one part. A transition zone is connected to the sealing tube and to the other part extending in the dlrection of movement, the sealing tube and at least the transition zone being supported in or retained on a durable or regenerating sliding bearing surf~ce.
In accordance with a preferred embodiment of the invention, the transition zone is formed with an elastically deformable surface extending over at least a portion of its length and is supported on the sliding bearing surface, loaded by the pressure of the workïng medium, by an arrangement which provides for decreasing application pressure from the side of the working medium, which may be uniformly decreasing S pressure or a decrease occurring in steps.
The sliding, bearing surface may be a fixed coating or cover on the support walls for the sealing tube or part of the surface of the support walls. Tlle sliding surface on the sealing tube is extendable or stretchable and bonded or secured to or is part of the surface thereof. The sliding surface may, however, also be a fluid surface such as a lubricating filling which is sealed towards the outside. The low-friction sliding surface may also be a lubricant introduced between the flexible, resilient sealing tube and the inner surface of the cylinder, whereby the slippery, sliding bearing surface is formed as a hydrostatic bearing. The lubricant flow is conducted over a choke after passing the gap between the sealing tube and the cylinder wall. The choke maintains the pressure balance between the luhricant and the working medium before the lubricant can be relieved of pressure and escape, for recycling and re-use.
The choke may be formed by a portion oE the transition zone, the surface of which is so constructed that in unloaded state it has some radial play; upon loading, a choke-type gap is formed, the ` size of which depends on the pressure of the working medium as well as the pressure of the lubricant and lubricant flow, so that the gap s will be self-adjusting. The lubricant may be branched off from the working pressure fluid,if this fluid is a liquid with lubricating properties.
The transition zone is preferably formed of a plurality of parts, particularly at least two parts which are constructed of mater~ls of different stiffness, adhered together at a junction surface. The separate elements in the transition zone can be arranged in various ways; it is preferred,however, that at least one of the elements of lower stiffness than another, or others,forms at ieast a portion of the surface of the transition zone. The transition zone is usually loaded by the working medium at working medium pressure, most commonly at the end or side of the sealing medium. ~ deformation will result in the transition zone which depends on the pressure of the working medium but which changes over the length of the transition zone, decreasing at the median thereof, the deformation depending, in general, on the particular shape or form of the separate elements as well as on the shape and form of the junction . surfaces between the various elements.
In a preferred form7 the lubricant flow for the hydrostatic bearing is independent of lubricant pressure.
The invention will be described by way of example with reference to the accompanying drawings, wherein:
Fig. 1 is a longitudinal sectional view of a piston-cylinder apparatus in which a resilient sealing tube is -loaded internally by the working fluid;

~34~(~ti 5 - Fig. 2 is a transverse section along line II-II,with the interior elements of the arrangement omitted;
Fig. 3 is an enlarged Eragmentary view of the region circled in Fig. 1 and identified at III;
Fig. 4 is a longitudinal sectional schematic view of a piston-cylinder arrangement in which the working pressure fluid is applied to the outside of the sealing tube;
Fig. 5 is a fragmentary cross-sectiopal area of one form of the sealing tube at the fixed, non-movable end thereof;
Fig. 6 is a fragmentary transverse section of a portion of the cylinder-piston arrangement illustrating a portion of the piston and the transition zone of the sealing tube with two elements;
Fig. 7 is a view similar to Fig. 6, having a transition zone with three elements;
Fig. 8 is a transverse section similar to Fig. 6, showing a transition zone having four pa~ialelements with junction surfaces below the top surface of ~ e transition zone;
Figs. 10 and 11 are transverse sections of a transition zone having six partial elements;
Fig~ 11 is a transverse section of a transition zone having seven partial elements, in which one partial element surrounds the other like a sleeve;

~U4~ S

Fig. 12 is a fragmentary sectional view of a transition zone having three partial elements joined to an elastic, flexible metal tube; and Fig. 13 is a transverse sectional view of a transition zone having two partial elements.
The cylinder-piston combination - see Fig. l - has - a cylinder element l, of approximately square or rectangular outer cross section (see Fig. 2) with a bore 2. The cylinder is closed off at one end by a cylinder end cover 3 secured thereto , for example, by means of bolts (not shown).
The junction surface between the cylinder body l itself and the cover 3 is sealed by an O-ring 5 located in a groove 4 o the cylinder l.
A piston 6, which is essentially cylindrical or slightly conical, is located in bore 2 and guided therein by means of a piston rod 7. The piston rod 7 is slidable in a sleeve 8 which, in turn, is secured to a holder or clamp 9. The holder 9 extends through a bore lo formed in the cover 3 of the cylinder and is secured therein by means of a nut 3. The holder 9 is formed with a longitudinal duct l2, extending lengthwise thereof, and terminating in mouths l3. The pressure working medium which may, for example, be a pressurized gas, hydraulic fluid or the like, may enter and leave through the duct 9 and the openings l3 thereof. The holder 9 is formed with a thickened region l4 in the vicinity of the mouths l3. The thickened region 14 has ridges 15 thereon. The thickened 14 is used to clamp the flxed end 16 of an elastic sealing tube 17 between the inner wall of the bore 2 of the cylinder and the outer surface of the holder 9. The tube is clamped tight to be leakproof. The other end 18 of tube 17 is connected to a transition zone A extending in the direction of movement of the piston 6. The transition zone mer~es into the piston 6 itself. This transition zone A is formed of two partial elements 60, 70. The partial elements 60, 70 have different stiffness; the partial element 60 may be made of the same material as the sealing tube 17 and has a lower sti.ffness than the partial element 70 which merges into the piston 6. As ~ result, the deormation of the transition zone A is greatest at the side thereof where the sealing tube 17 is located, and decreases in axial direction towards the piston. The two partial elements 60, 70 are secured together at theirjunction surface 62. The attachment at the junction surface may be by means of adhesive or chemical bonding. The junction surface 62 does not pass through the surface of the transition zone A and,therefore,deformation of the transition zone decreases continuously even if the thickness of material of the partial element 60 is constant over its entire length. At the side of the sealing . tube, a thickened region 61 is provided in the transition zone to influence the deformation of the transition zone A
and, further, to increase the junction surface 62.

l~oa6~ ' The outside of the sealing tube 17 has a durable sliding or low-friction surface 21 applied thereto.
Surface 21, which may be in form of a thin layer, is securely adhered to the tube 17. It is important -that the surfaces between the tube 17 and the inner wall of the cylinder 1 be of low friction; thus, the sliding surface may also be applied to the interior wall of the cylinder 1, and, ifdesired, sliding low-friction s~rface layers can be applied to both the tube 17 and the inner wall of the bore 2 of the cylinder 1.
~ ring groove 19 is located at the transition or junction between the cylinder 1 and the cover 3. The groove 19 i9 connected by means of a gap 20 with the bore 2 of the cylinder. The groove 19 communicates with a duct 22 (Fig. 2) formed in the wall of the cylinder 1. Duct 22 is in communication with a connecting line 23 which is connected to the cylinder 1 by means of a pipe thread coupling. A
stream of lubricant is applied through duct 23, preferably independent o~ pressure. ~s a result, the pressure in the groove 19 and the duct 20 will adjust itself to correspond to the pressure of the working fluid. The lubricant escapes through the groove 19 and the gap 20 between the sealing tube 17 and the wall of the bore 2 of the cylinder.
It will spreadat the outer circumference o~ the sealing tube 17 and can escape at the piston end of the cylinder-piston combination. The deformation of the transition zone ~)40~65 A depends on the pressure of the working fluid and on the pressure of the lubricant stream. At the surface of the transition zone A, the~fore, due to the deformation, a self-adjusting throttling region will be formed acting as a hydraulic choke wlth respect to the lubricant which will achieve a pressure balance with the pressure of the working fluid or medium. Asfar as the sealing tube 17 is concerned, therefore, the forces acting thereon will be in balance: The sealing tube, the inside of which is loaded by the pressure of the working fluid, is supported throughout its outer circumference by a thin hydrostatic film of lubricant in pressure-balance with the pressure of the working 1uid. ThUs, the sealing tube is supported from the inner wall of the bore 2 of the cylinder without, however, touching the wall o the cylinder, so that the tube can slide freely with respect thereto. The resulting coefficient of friction is extremely low.
The lubricant which escapes at the piston-end of the combination is collected and re-cycled. A collecting bellows 25 is located at the piston-end of the cyllnder 1.
Bellows 25 is secured in the groove 26 and may be held at the outside by a clamp ring, if needed. The bellows is not stressed by fluid pressures and thus no specially devised or constructed holding arrangement is needed. The bellows 25 is centrally secured to piston 6 by means of a bolt 27 and clamped between a cover 28 and an internal shield 29. Shield 29 itself is held by a sleeve 30 supported on a shoulder 31 formed in the piston 6.
The lubricant collected with1n the bellows 25 is removed by means of ducts 32, 33 (Fig. 2) formed in the cylinder 1 and connected to a removal line 34. The ducts 32, 33 can be arranged in any suitable configuration and it is only necessary to so locate them that lubricant can be removed from the bellows 25. They may, for example, be secured in fluid-tight connection through a small opening formed in the bellows 25 itself and, since the fluid therefrom will not be under pressure, can be removed by a flexible, for example plastic tubing.
Fig. 4 illustrates an arrangement in which the sealing tube 35 is loaded by pressure fluid at the outside thereo~. The sealing tube 35 is engaged by a smooth piston 36, the end of which is rounded. The end portion 16 of the sealing tube 35 is clamped by means of a clamp element 37 which is secured to piston 36 by bolt 38. The cylindrical portion 37' of the clamp 37 is extended to form a piston guide portion for the piston 36. The clamp element 37 is formed with through-bores 39 which conduct the working pressure fluid to the outside of the sealing tube 35. The other end 418 of the sealing tube 35 merges into the transition zone A which, similar to the arrangement of Fig. l, includes the partial elements 460, 470, secured together at their junction surface 462. In the description ~:~4V065 that follows, similar parts have been given similar reference numerals, incremented by hundred numeral corresponding to the respective drawing. The transition zone ~ facing the sealing tube is formed with a thickened portion 461 which, however, in contrast to the zone 61 of Fig. 1, is located at the outside of sealing tube 35. The - part-element 470 is, actually, a portion of the end 40 of the cylinder itself. In this embodiment as well, the thickness of the partial element 460 decreases towards the outside within the transition zone A so that, as in the embodiment of Fig. 1, the localized stiffness and form stability of the transition zone increase looked at from the side oE the working pressure fluid.
A bore 43 in piston 36 provides working pressure fluid which can be applied to the piston by a suitable connection screwed into the coupling bore of a coupling bolt 42. Bolt 42 also secures the holding end 44 to the piston 36. The holding end 44 has threaded bores 45, 46 for connection of supply,and drain lines, respectively, for lubricant; and a bore 47 with a ring groove 48 sealed by means of 0-rings 50 located in grooves 49. A
longitudinal bore 51 communicates with groove 48 and conducts lubricant ~ the end 416 of tube 35. A small ring groove or gap 54 is formed between a head portion 53 and the piston 36 itself, sealed by an 0-ring 52. The ring groove is similar to ring groove 19 (Fig. 3) to permit 1~40(~65 escape of lubricant between the piston 36 and the sealing tube 35 and allow spreading of the lubricant to escape at the other end 418 of the tube 35. The region beneath the end 418, that is, in the transition zone A, forms a hydraulic choke of-variable cross section, controlled by .
the pressure of the working pressùre fluid. A sealing bellows 425 is provlded to collect lubricant for removal through duct 55.
The bolt 38 has a bore extending therethrough to permit working pressure fluid to enter the cylinder chamber and to cause relative movement between the piston and the cylinder. The working pressure fluid is applied to the outside of the sealing tube 35 through the bores 39. The clamping bolt 38 holds the clamp element 37 which, in turn, is secured to the end 416 of the sealing tube 35 and further holds the head portion 53 to the piston 36 itself.
The cylinder chamber is closed off by an outer cover 56 sealed by an O-ring 57. Lubrication is effected similar to that explained in connection with Fig. l, and the same low coefficients of friction will obtain herein.
The structure of Fig. 1 may require a larger passage 12 than that shown and described in connection therewith. If a larger duct is required, the end 516 of the sealing tube 517 (Fig. 5) can be constructed to have an externally extending lip, as seen in Fig. 5. The other 104~ 65 reference numerals in Fig. 5 correspond to those of Fig. l. The end portion 516 is held in the wall of the bore 2 of the cylinder and by a preformed collar portion 3' of the bottom cover plate 503 of the cylinder for secure and sealed connection.
The transition zone A may have various shapes and arrangements with respect to the partial elements thereof, as shown, for example, in Figs. 6 to 13.
Fig. 6: The transition zone A has two partial elements 660, 670 of different stiffness. The elements are attached or secured together for example by adhesives, or by ch~?mical bonding. The partial element 660 is an extension of the end 618 of the sealing tube which is formed with a thickened region 661. It has lesser stiffness ~han the partial el~m ent 670 which is formed as a portion of the piston 666. Again, deformation of the surface of the transition zone A is obtained, in decreasing direction looked at from the side of -the working pressure fluid medium towards the piston.
Flg. 7: The transition zone A of the end 718 of the sealing tube includes three partial elements 760, 763, 770 having two junction surfaces 762, 762'. The junction surface between partial elements 760, 753 extends at an inclination from the side of the working pressure fluid and passes through the surface of the transition zone A.

~i~4~ui~i The piston is shown at 766, forming one of the partial elements.
Fig. 8 shows the end 818 of the tube in the transition zone A,and four partial elements 860, 864, 865, 870. The two partial elements 864, 865 are formed as closed tubes or sleeves and are located between the partial elements 860, 870. The relative stiffness of all the partial elements is different, decreasing in the direction towards partial element 860 from the stiffest element 870.
All junction surfaces 862 are within the transition zone A. The piston 866 forms one of the elements 870. The end 818 of the tube is formed with a thickened region 861.
Fig.9:The end 918 of the tube is located in a transition zone A. :Eormed of six partial elements. A thickened region 961 has a partial element 960 of lowest stiffness applied thereto; four disk-shaped partial elements 965 of increasing stiffness are joined to the partial element 960 and the last one is the piston 966, having a disk end, and forming the partial element 970 of highest stiffness.
The intermediate junction surfaces 962 include at least one secured bonded connection. Due to the larger number of partial elem.ents, with increasing stiffness, the pressure with respect to the support wall does not increase in excessively great steps even if, as in this embodiment, the junction surfaces of the disk-shaped partial elements penetrate the surface of the transition zone A. Not all ~)4~ S

.
the junction surfaces have to be adhered together if the piston 966 is constantly loaded by a counteracting force, for example by a spring, holding the elements together.
Fig. lO: The arrangement is similar to Fig. 9;
the -transition zone A has seven partial elements. The partial element 1065 is joined to a sleeve-like partial element 1063.
The junction surfaces are shown at 1062; the piston 1066 forms one of the partial elements, namely the stiffest partial element 1070.
Fig. ll: The end 111 8 of the sealing tube is within the transition zone A and merges into partial element 1160 which has a decreasing thickness looked at from the side of the working pressure fluid. Five partial elements 1165 are located within the partial element 1160, as is the partial element l170 of hi~hest stiffness which, itself, may be the piston or part thereof. A thickened portion of the end 111 8 of the tube is shown at 1161. If the number of partial elements of increasing stiffness is increased and the thickness of the partial elements is decreased then, in a limiting condition, a transition zone in which the stiffness changes continuously with respect to lengt~' will result.
Such a transition zone can be made of suitable plastic, for examply polyurethane, and can be so constructed that the piston forms an integral part thereof.
Fig. 12 A metal tube 1217 has an end 1218 which is joined to the transition zone A consisting of the partial ~ .. . . ... . .. ~ . . , ............. .... . . . . ~1 ~ elements 1267, 1268, 1270. Although the partial elements may all consist of the same material, deformation of the transition zone is non-uniform. The partial element 1268 has a wall thickness which increases towards the piston 1266.
Preferably, the working pressure fluid is applied from the inside against the partial element 1268. The partial element 1268 forms the throttling region or choke for the hydrostatic pressure lubr-ication, and provides/self-adjusting choking passage therefor. Partial elem~ ts 1267, 1270 are provided for guidance. They may be formed with grooves at the outside thereof to permit passage of lubricant, as schematically shown at 1269, 1269'.
Fig. 13: The choke or throttling of the lubricant, as controlled by the pressure of the working pressure fluid, is located within the transition zone A. The transition zone, joined to the end 1318 of the flexible tube, includes the partial elemenk 1360 with the thickening 1361 and a stiff, non-deformable partial element 1370 merging into the piston 1366. The partial element 1370 has a partial zone 1371 at the side of the sealing tube, which zone 1371 is relative formed of reduced diameter / to the adjacent zone 1372.
The zone 1372 has a diameter which has just slight play with re~pect to the wall of the cylinder.
The choke is formed by a piston 1376 held by means of a spring 1374 against a stop or abutment 1375. A bore 1373 is in communication with the working pressure fluid.

- - -The lubricant passes along the zone 1371 and in the gap formed by the reduced diameter thereof and then through a passage 1377 towards the face of the piston 1376 which is opposed to the face against which the working pressure fluid is applied.
The lubricant forces the piston towards the left (Fig. 13) and is relieved of pressure and drains off through drain line 1378 which terminates in the ring duct selectively opened by leftward movement of the choke piston 1376.
A minor portion of the lubricant flows through the 10 gap formed by the enlarged zone 1372 of the partial element i370 and the inner wall of the cylinder. This gap may, if desired, by sealed, for example by an O-ring. The lubricant pressure is held in balance with the working fluid pressure also in the arrangement of Fig. 13. Movement of 15 ~e choke piston 1376 can be damped by suitable damping arrangements, not shown, and well known.
Parl~ial elements inform of thin metallic disks located between othex partial elements of lower stiffness may be used. Also, partial elements of substantial stiffness 20 which have surface coatings, or surface layers of materials of lesser stiffness, can be used.
The transition zone A should be so arranged that in the first partial region thereof it deforms, corresponding to the pressure of the working pressure fluid. It is 25 supported at its support wall with in the mean or average decreasing pressure; the first partial zone merges into a V~65 second partial zone in which the deformation decreases and the transition region no lon~er is supported by the support wall, that is, is self-supporting. The transition zones may include level or bowed disk-shaped partial elements of differential stiffness and with junction surfaces passing through the outside surface thereof. Such partial elements have stepped characteristics which are effective up to the outer surface of the tra~sition zone regarding locallzed deformation. Thus,~the decrease of pressure on the support wall occurs in steps within the transition zone A or within a partial region thereof. Continously decreasing pressure agai.nst the support wall can be obtained by a transition zone which has either a continuously variable stiffness or a finite number of partial elements which are so arranged that the junction surfaces are located beneath the surface of the transition zone and are continuous. The least stiff (or most flexible) partial element then covers the entire surface of the transition zone, as shown in Figs. 1, 4, 8 and 11 for example. Lookec~ at from the working pressure fluid, the stability of shape of the transition zone is progressively increasing.

The length of the transition zone A is indicated by the respective arrows in the respective Figures, and extends from an end 18 (and 418, 518...1318) of the sealing tube, that is, from that point at which the thickness of material of the -- lg ~4~5 ~ sealing tube begins to vary, up to the partial element formed by a portion of the piston 6, 36, 666...1366. The embodiments descnbed in connection with Figs.j may be applied to the arrangement shown in Fig. 4 or in Fig. 5, and are not restricted to the specific embodiment shown, which is illustrated in connection with the structure specifically described in connection with Fig. 1.
The working pressure of the piston-cylinder combination may be high, for example 100 at-gauge, and higher.
The sealing tube 17,/ may have good characteristics regarding elasticity, due to its support on the wall of the cylinder, so that the force necessary to merely move the piston, that is, to overcome friction only, is low. The coefficient of friction can be so low that, when utilizing pressure lubrication, the coeficient of friction may be less than 0.001. This particularly low friction can be obtained.by the hydrostatic bearing obtained between the sealing tube and the adjacent inner surface of the support wall, since the sealing tube surface and the inner surface of the support wall are not in physical engagement with each other, due to the interposed film of lubricant.
_ . If the working fluid is practically incompressible under high pressure operating conditions the displacement of the piston-cylinder combination is effectively directly proportional to the -quantity of working .luid supplied. The combination ~4V()6S

~ may be used for leakproof piston pumps, for servo positioning systems and the like. Placing tWo piston-cylinder combinations in paired arrangement and connecting the two pistons(or cylinders respectively) together results in the double-acting combination.
The pressure lubrication results in extremely low -coefficients or friction. Low friction i5 also obtainable without pressure lubrication, however, if slippery sliding surfaces 21 are applied to the sealing tube, and/or to the engaging movable surface. Such sliding surface may be elastic or flexible mesh or net systems made, for exa~ple, of smooth or textured man-made fibers or yarns, such as nylon, ~o o /,y 7~e f rq ~ oro e 7~y/c~
A ~ or the like. Threads, yarn, or woven or knit fabric macle of such material may be applied to the sealing tubes, if necessary, with an intermediate layer of other adhesive or adhering yarn or thread made, for example, of elastic material, various types of elastomers, nylon, cotton, or the like.
Various changes and modifications may be made and features described in connection with any one of the embodi-ments may be used with any of -the others, within the scope of the inventive concept.

Claims (31)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Cylinder-piston combination, particularly for high-pressure, hydraulic application, having two relatively movable parts, one part forming a piston, the other forming a cylinder in which the piston is relatively movable, and a sealing tube which is at least partly elastic and secured with its ends to the piston part and to the cylinder part, respectively, to form a leakproof sealed connection with respect to pressure fluid introduced into the chamber formed by the cylinder and the head of the piston, wherein the sealing tube which is relatively movable with respect to one of said parts is connected, at the end thereof with a transition zone extending in the direction of movement, which is connected to the other part, and the sealing tube and at least a portion of the transition zone is retained and supported on a low-friction surface.

2. Combination according to claim 1, wherein the low-friction surface is a durable surface.

3. Combination according to claim 1, wherein the low-friction surface is a regenerating lubricating film.

4. Combination according to claim 1, wherein the low-friction surface is a durable surface having low-friction characteristics and a regenerating lubricant film.

5. Combination according to claim 1, wherein the transition zone is formed with an elastically deformable surface extending over at least a portion of its length, the transition zone loaded by the pressure of the working fluid, and being supported with average decreasing pressure applied over the low-friction surface on said one part.

6. Combination according to claim 1, wherein the low-friction surface includes a film of lubricant supplied under pressure to the surface of the sealing tube facing said one part.

7. Combination according to claim 1, wherein the low-friction surface, formed by a hydrostatic bearing, comprises a stream of lubricant introduced in the zone between the sealing tube and said one part, and means forming a hydraulic choke throttling the stream of lubricant and balancing its pressure with respect to the pressure of the working pressure fluid, the stream of lubricant draining, after loss of pressure, and after passage through the choke.

8. Combination according to claim 7, wherein the choke is formed by a gap between said one part and the transition zone the surface of the transition zone, when unloaded, having radial play with respect to said one part and, upon loading by the working pressure fluid, and by the pressure of the lubricant forming a self-balancing throttling gap and thus forming said hydraulic choke.

9. Combination according to claim 7, wherein the hydraulic choke comprises a choke piston located within the trans-ition zone and pre-loaded by the pressure of the pressure fluid;
the transition zone including a rigid, non-deformable partial element having means retaining said choke piston and applying the lubricant to one face thereof while applying the working pressure fluid to another face, and ducts communicating with said piston and selectively uncovered by movement of said piston upon application of pressure fluids to opposed faces thereof to balance the pressure of said fluids at the two faces of said piston by selective drainage of fluids at opposite faces thereof.

10. Combination according to claim 9, wherein the rigid element of the transition zone, comprises two partial elements, one of said partial elements having a smaller diameter than the other; the larger one of said elements being fitted in said one part and having a dimension to prevent substantial escape of pressurized lubricant, the smaller one of said elements having said duct extending therethrough to apply pressurized lubricant collected in the zone between said one part and said one partial element to a face of the choke piston to provide for balancing of the pressures of the working pressure fluid and the lubricant.

11. Combination according to claim 1, wherein the transition zone comprises at least two partial elements securely connected together along a junction surface and formed of materials of differential stiffness.

12. Combination according to claim 11, wherein at least one of the partial elements, of lesser stiffness than the other partial element forms a partial zone of the surface of the transition zone.

13. Combination according to claim 11, wherein one partial element is a sleeve-shaped end portion of the sealing tube, and forming a junction surface with the other partial element having greater stiffness and being securely attached thereto along said junction surface, said junction surface extend-ing in the direction of movement of the other part and beneath the surface of the transition zone.

14. Combination according to claim 13, wherein the end portion of the sealing tube is formed with decreasing material thickness.

15. Combination according to claim 13, wherein the end portion of the sealing tube is formed with uniform material thickness.

16. Combination according to claim 11, wherein the transition zone includes a thickening in the material of the sealing tube.

17. Combination according to claim 11, wherein the transition zone comprises at least three partial elements of differential stiffness separated by junction surfaces, at least one junction surface passing through the surface of the transition zone.

18. Combination according to claim 11, wherein the partial elements form a transition zone of continuously increas-ing stiffness in a direction away from application of the working pressure fluid.

19. Combination according to claim 1, wherein the end of the sealing tube is securely connected to said one of said parts to form a non-movable end thereof; said end of the sealing tube being formed with a thickened region and clamped at three sides thereof.

20. Combination according to claim 19, wherein the sealing tube has decreasing wall thickness in the direction towards its clamped end.

21. Combination according to claim 1, further compris-ing a piston guide means connected to the piston guiding the piston concentrically with respect to the sealing tube.

22. Combination according to claim 21, wherein the guide means comprises a piston rod located interiorly of the sealing tube and guiding the piston interiorly of the sealing tube.

23. Combination according to claim 21, wherein the guide means comprises a guide sleeve secured to the piston and guiding the piston within the inner wall of the cylinder and exteriorly of the sealing tube.

24. Combination according to claim 19, comprising a clamping means clamping the fixed end of the non-moving end of the sealing tube; and duct means are formed in the combination having an outlet leading working pressure fluid to one surface of the sealing tube.

25. Combination according to claim 1, wherein the low-friction surface is applied to a wall of the sealing tube and comprises at least one low-friction plastic material.

26. Combination according to claim 1, wherein the low-friction surface is applied to the sealing tube and comprises an elastic fabric formed of knit, net or woven material comprising yarn or thread of plastic material.

27. Combination according to claim 26, wherein the fabric is formed at its back side with smooth or textured elastic or adhesive yarn of threads of at least one of the materials, an elastomer, a polyamide and cotton.

28. Combination according to claim 11, wherein the partial elements comprise closed tubular sleeves concentric with the end portion of said sealing tube and having differential stiffness, increasing in stiffness towards the center thereof.

29. Combination according to claim 11, wherein the partial elements comprise axially stacked disk-like elements of differentially increasing stiffness, the stiffness of said disk-shaped elements increasing as the elements are further away from the side facing the working pressure fluid.

30. Combination according to claim 29, wherein the end portion of the sealing tube surrounds at least in part said stacked elements.

31. Combination according to claim 11, wherein the resilient sealing tube comprises a metal tube and the end portion is attached to the transition zone, said transition zone com-prising the same material as said resilient tube and having increasing wall thickness in the direction towards said other part.