FR2593249A1 - CURTAIN COMPRISING A CYLINDER AND A ROD - Google Patents

Abstract

Jack comprising a cylindrical body 6 provided with a cover 7 at each of its two axial ends and in which is placed a piston 8 movable axially, this piston being linked to a jack rod 9 which passes through it in a sealed manner and by axial sliding at the at least one of the covers 7. A tight closing device 22 is attached externally to the end 7, 21 of the cylinder which is crossed by the jack rod 9, this device comprising a peripheral wall 23 gas-tight. This wall forms an annular accumulation chamber 29 which is completely sealed with respect to the ambient environment 20 adjacent to the jack, this accumulation chamber 29 being provided with an opening 30 connected to a channel 32 for venting. (CF DRAWING IN BOPI)

Description

! "Cylinder with a cylinder and a rod" The invention relates to a

  cylinder comprising a cylindrical body provided with a cover at each of its two axial ends and in which is placed an axially displaceable piston, this piston being connected to a cylinder rod which passes through at least one of the two

  covers by sliding in the axial direction.

  In testing, development, or development laboratories, or the like, for example biochemical laboratories or premises in which chips for the computer industry are developed or produced, referred to as "sterile chambers", it is It is necessary to have a sterile atmosphere, free from dust and dirt, so that the work done is not affected or disturbed by pollution of the ambient air. It is in this respect hardly possible to use in such premises cylinders of current model to achieve desired positions, because these cylinders can not be made perfectly tight to the pressurized fluid at the crossing of the cylinder cover by the rod ferin, despite the use of sealing rings. If the pressurized fluid is for example a hydraulic fluid, particles thereof are deposited on the cylinder rod and integrate with the environment during the deployment of this rod outside the cylinder, so that the ambient air takes care of these particles. This phenomenon is favored by the aging of the device, because the sealing rings wear out gradually. The danger of pollution of the surrounding air is even greater when the device is moved pneumatically, because the pneumatic fluid leaves the cylinder, in case of leakage, forming violent eddies, so that the surrounding air is constantly animated by disruptive movements. As, on the other hand, these pneumatic fluids commonly comprise oil particles for the purpose of lubrication, they are transmitted to the surrounding atmosphere of the cylinder by the vortices created, this atmosphere being consequently highly polluted, so that the requirements in rooms

  sterile can not be filled.

  The invention relates to a jack according to the prior art mentioned above, but the operation of which does not involve any pollution, fouling, or other similar harmful effect, of the immediate environment, of

  so that it can be installed in sterile rooms.

  This result is obtained by the fact that a sealing device is externally attached to the end of the cylinder which is

  crossed by the cylinder rod, this device comprising a peripheral wall

  gastight wall, this wall surrounding with clearance and over its entire periphery at least a portion of length of the portion of the cylinder rod which protrudes from the cylinder and which immediately follows, and the same wall being on the one hand fixed with cylinder sealing by its axial end located on the side of this cylinder and being further sealingly connected to the cylinder rod by its axially opposite end and located on the side of this cylinder rod, so as to form a chamber annular accumulation device which is completely sealed with respect to the ambient environment adjacent to the jack, this accumulation chamber being provided with a

  opening that can be connected to a vent channel.

  It is thus added, adjacent to the outlet point of the jack rod of the cylinder, or more precisely of its lid, a sealed accumulation chamber which captures the impurities contained in the pressurized fluid leaving the internal space of the cylinder. cylinder because of

  leaks, so that these impurities can not be integrated into the environ-

  immediately from the cylinder. In addition, these leaks of pressurized fluid, particularly compressed air, can be blown off, through the opening of the accumulation chamber and its venting channel, to a space located at the outside of the chamber in which the jack is

  installed. Advantageously, no suction device is then necessary.

  This is because the leakage current is normally at a pressure level higher than that at the point where the channel opens, so that it follows the path of weaker resistance through this channel. outdoors. It is therefore possible to install the cylinder of the invention in

  rooms where maximum sterility of the air is required.

  According to an additional characteristic, this jack is characterized in that the peripheral wall is fixed to the jack rod by its axial end situated on the side of this jack rod, and in that this peripheral wall comprises one or more portions of length which extend over the entire periphery of the wall and which, in the longitudinal direction of the cylinder rod, are extensible, respectively elastic and flexible, and / or have elastic rubber properties, and that both ends of the peripheral wall, which are non-releasably connected to the cylinder and the other to the cylinder rod in a completely sealed manner, move relative to each other during deployment and retraction of the rod cylinder out of, and into, the cylinder. The peripheral wall of the accumulation chamber is then completely closed at its two axial ends, because it is fixed on one side to the cylinder and on the other side to the cylinder rod. There is no relative movement, both between the peripheral wall and the cylinder and between the peripheral wall and the piston rod, so that a real seal can be guaranteed during a possible sliding game

  between the peripheral wall and one of the two parts of the cylinder cinnamon.

  The sealing quality remains good throughout the service life of the device, because there is no wear at the fixing points of the device.

  peripheral wall on the one hand to the cylinder and on the other hand to the cylinder rod.

  When the cylinder rod is actuated, the end of the peripheral wall attached thereto is driven with this rod, which is made possible by

  the fact that length portions of the peripheral wall are exten-

  sibles, respectively elastic or flexible. According to a form of

  the entire peripheral wall is in the form of a bellows. We then obtain an accumulation chamber whose volume, at the

  of a pump, enlarges and shrinks alternately, the movement of

  contraction of this chamber of accumulation leading to the evaluation of

  cuation accumulated leakage volume, through the opening and vent channel. It is thus guaranteed a constant evacuation of the accumulation chamber, even when the cylinder is actuated using a

relatively low pressure.

  According to one embodiment, these portions of length of the peripheral wall, which are extensible, respectively elastic or flexible,

  are made in the form of a kind of bellows.

  Advantageously, the entire peripheral wall is made in the form of a bellows. For example, the length portions of the peripheral wall, which are in the form of a bellows, are composed of a material with elastic rubber properties. It is thus possible to produce peripheral walls that can be installed at lower cost, and whose flexible length portions are subject to low wear. A particularly strong bellows is obtained by: - the length portions of the peripheral wall which are formed as bellows, are composed of annular elements placed one behind the other in the axial direction, in particular elastically deformable and preferably metallic, which, in order to achieve a structure in

  bellows form, are connected to each other in an alternately

  by their zones of large and small diameter, for example by brazing, welding, gluing or the like, and also - that the annular elements are curved in the form of a plate, two adjacent elements being connected to each other in a manner symmetrical with respect to their contact plane. According to one embodiment of this jack, the peripheral wall connects to the side of the cylinder immediately and in axial extension to

  a cover which is traversed by the cylinder rod, and consequently has

  an outer diameter substantially corresponding to the diameter of the lid, respectively to the diameter of the cylinder body. The device of the invention then forms a compact construction, which is advantageously

  compatible with the original transverse dimensions of this cylinder.

  According to another embodiment, the peripheral wall comprises

  carries a portion of length which is extensible, respectively flexible or elastic, in the longitudinal direction and which is in particular constructed under

  bellows, this portion of length being placed against the former

  the end of the peripheral wall which is located on the side of the cylinder rod, and being on one side sealingly attached to the cylinder rod, while on the other side it is fixed in a sealed manner, by its axial end; which is turned towards the cylinder, to a portion of separate length, rigid and sleeve-shaped, which surrounds with play the cylinder rod and whose side

  opposite is fixed, also with sealing, to the cylinder and preferably

  its cover, which is crossed by the cylinder rod. Thus is obtained an elegant solution for the construction of the peripheral wall, which

  ensures for this one a simple and safe assembly.

  According to an embodiment of this latter variant, the portion of bellows-shaped length is releasably tightened on the parts to which it is connected in the axial direction, and has for this purpose, at one or both ends thereof. axial, an annular bead of

  maintenance, which is introduced into a corresponding annular groove for holding

  of the sleeve-shaped part, respectively of the cylinder rod. It is thus realized a particularly secure waterproof connection between the wall

  peripheral and cylinder, respectively the body of this cylinder.

  According to a last embodiment of this jack, the opening of

  venting is performed in a rigid part of the peripheral wall.

  and in particular, where appropriate, in its sleeve-shaped length portion, preferably in the form of a tapped radial bore. It is thus possible to equip without creep also more model cylinders

  former with the sealing circuit of the invention.

  The invention will now be described in more detail with the aid of the accompanying drawings in which: Figure I is a longitudinal sectional view of a jack, with an end portion provided with a sealing device according to the invention ; FIG. 2 is an enlarged view of the holding zone between the peripheral wall and the cylinder rod, in an advantageous embodiment, and

  Figure 3 is an advantageous embodiment of a bellows.

  Reference is first made to the embodiment of the jack according to the invention which is shown in FIG.

  axial of a jack 5, whose body 6 is only partially represented.

  This is closed at both ends by a cylinder cover 7, only one of which is shown in the drawing. Inside this body is a piston 8 displaceable in the axial direction and shown in dashed lines, a cylinder rod 9 being placed coaxially at its axial end. It passes through the cylinder cover 7 by a bushing 10, in which it is axially displaceable in a coaxial guide sleeve 14 and fixed in this bushing 10. Furthermore, an annular seal 15 is placed in the bushing 10. the side of the sleeve 14 located towards the piston 8, while on the other side of this sleeve is placed a scraper ring 16. The two rings 15,16 closely enclose the cylinder rod 9. The annular seal 15 ensures sealing against the outside of the chamber 17 between the piston 8 and the lid 7, while the ring of scraping prevents dirt from outside and collected by the

  cylinder rod 9 can not penetrate inside the jack.

  There is further provided a feed opening 18 which passes through the cover 7 to terminate in the chamber 17, and to which a fluid line can be connected for feeding and evacuating a

fluid under pressure.

  At the other end, not shown, the cylinder is also

  a feed opening for the second chamber of the jack, so that the piston 8 and the rod 9 can be driven by a movement of

  back and forth in the longitudinal direction 19, by means of injection or

  corresponding cuation of the pressurized fluid.

  In this embodiment, it is a cylinder which comprises only a rod which passes through the cylinder cover. The invention is however not limited to such an embodiment, but it can also relate to an embodiment which comprises a cylinder rod which is animated

  back and forth through the two lids of the

  tremité of the cylinder. In this case, we find at the other end, not shown, the cylinder, a device similar to that already partially

  described, and which will be described more fully hereinafter.

  The jack described here is intended to be installed in a sterile chamber, in which are performed tests or other work that

  require a surrounding air and the cleanest possible atmosphere.

  The sealing ring 15 and the scraper ring 16 guarantee complete sealing of the chamber 17 with respect to the surrounding environment 20 adjacent to the jack, but there still appears, even in small quantities, a leakage current between the seals. and the jack rod, which is brought outwardly by the jack rod by adhesion thereto. If the pressurized fluid used for the actuation of the cylinder is a pneumatic fluid, these outward leakage currents are particularly harmful, since a lubricant, such as oil, is commonly mixed with this fluid, this lubricant. being driven by the outgoing air and coming swirling integrate into the ambient environment, so that it then comes to be deposited inconveniently in the surrounding area to the cylinder. For these reasons, a sealing device 22 is arranged outwardly at the end of the cylinder 21, where there is the cover 7 which is traversed by the cylinder rod 9, this sealing device 22 sensing the current leakage that passes through the passage 10, so that the environment 20 remains totally

  sterile. This sealing device 22 will now be described in more detail.

  The sealing device 22 comprises a peripheral wall of

  closure 23, gas-tight, which encloses with play, over all its circumference

  the portion 24 of the piston rod protruding from the cylinder 5 protruding from its lid 7. It is furthermore fixed by means of an axial end piece 25 in a leaktight manner to the lid 7, and is sealingly connected at its other axial end 26 to the cylinder rod. It is thus created, in axial extension to the cover 7 and between the latter and the immediate environment 20, an annular accumulation chamber 29 and completely sealed, which is capable of collecting the leakage currents of fluid

  pressurized crossing the passage 10.

  The accumulation chamber 29 is connected via a vent opening 30 to a conduit-free duct 32 or hose 31, the free end of which is shifted so that its mouth opens into a chamber 33 which is separated from the environment 20 which constitutes the chamber containing the jack 5, that is to say the sterile chamber. In the case of Figure 1, the pipe

  31 is attached to a partition 34 shown schematically.

  The gases and impurities that are captured by the charging chamber

  mulation 29 can now be evacuated through the venting channel 32, without they enter the sterile chamber 20. There is thus no need for a particular suction device or pump, the evacuation leakage currents being only realized by the pressure difference

between rooms 29 and 33.

  It is also possible, according to an alternative embodiment not shown, to provide a completely rigid wall 23 and to close it sealingly with respect to the jack rod 9 by means of a sealing ring which surrounds the rod portion 24, which is then movable relative thereto. In fact, a seal of this kind is subject to wear over time, so that a seal can not be guaranteed

  absolute that by changing often the sealing ring in question.

  In order to achieve an improvement of the seal, which guarantees, each time, even after a long period of use, an absolute seal between the peripheral wall 23 and the cylinder and between this wall and the cylinder rod it is provided, as illustrated in FIG. 1, that the peripheral wall is also fixed, by its end 26 located on the side of that of the cylinder rod, to the rod portion 24 in a non-translatable manner with respect to that this. Such a static tight closure is more satisfactory, and as in this case there is no relative movement at the location of the sealing surfaces, an absolutely tight separation is obtained between the accumulation chamber 29 and

the surrounding area 20.

  This embodiment has the consequence that the end 26 is driven, with the piston rod 9, of a back and forth movement in

  axial direction when actuating the cylinder. As a result, the peripheral wall

  23 is made in an extensible, flexible or flexible form, in the longitudinal direction 19, forming a portion of length 35 which extends the periphery of the body and consequently surrounds the cylinder rod 9, this piece 35 being otherwise able, although not necessarily, be made of material

  with elastic gum properties.

  This means that the ends 25 and 26 of the peripheral wall 23 move away and come closer to each other during the movement of

  back and forth from the rod in the cylinder 5, which results in either an enlargement

  or a narrowing of the volume of the accumulation chamber. The peripheral wall 23 is then driven by a pump movement, through which

  it contracts and unfolds alternately.

  This pump operation has the consequence that each time, even when the fluid entering the accumulation chamber 29 due to leakage is at a very low pressure relative to the external atmosphere 33, this fluid is definitely pumped outwardly through opening 30 and channel 32, without the need for a pump

separate suction.

  It is understood that the peripheral wall 23 may have, in

  longitudinal direction, several extensible sections forming a

ble with the same properties.

  In the exemplary embodiment shown in FIG. 1, the compressible and extensible length portion 35 is made in the form of a bellows 36. This bellows tightly grips the piston rod 9 by its end 26 which is attached to this end. rod 19, and is plugged coaxially on the rod 9 with a predetermined radial clearance. This bellows-shaped piece ends before the cover 7 and is connected, at the end of its circular end 37, to the cylinder body 6 by means of a rigid and elongated piece 38 in the form of a sleeve. This elongated sleeve-shaped piece 38 is a separate piece - the peripheral wall 23 then being composed in this example of two parts - to which the bellows 36 is attached.

waterproof way.

  The substantially hollow cylinder-shaped bushing 39 is coaxially placed on the piston rod 9 and is positioned relative thereto at a pre-measured radial distance. It is moreover fixed in a sealed manner, by its side opposite the end 37 of the bellows, to the cover 7, in that it is fitted on a cylindrical projection formed on the cover 7 and extending longitudinally in the direction of the outer end 44 of the cylinder rod while surrounding it. This extension 40 also carries the rings 15, 16 described previously,

as well as the guide sleeve.

  The sleeve 39 is advantageously coaxially tightened on the extension 40, so as to provide a gastight connection without placing a sealing device between these two parts. It is understood, however, that the sleeve 39 can be installed in other ways; by

  example, it may be provided with an annular extension spaced radially

  which can be assembled by screwing on the cover 7 after

sition of a sealing ring.

  The bellows 36 shown in Figure 1 is entirely composed of an elastic material and is preformed so that it has a pot shape. Its peripheral walls 45 form, in longitudinal section, corrugations for producing a bellows, and its bottom 46, which is positioned in a radial plane with respect to the axis 19, is equipped with a central opening 47 allowing the passage of the portion 24 of the cylinder rod. At this end is formed, in the same room with the bellows, a sleeve-shaped end projection 26, which is also in

  elastic material and which is advantageously surrounded by a metal band-

  The bellows 36 is thus fixed to the rod 9 in such a way that

  Removable and non-releasably relative to this rod 5.

  The other end 37, located on the side of the cylinder body 6, also has a socket shape and is equipped with a peripheral bead 49 on the inner side looking towards the cylinder rod 9, which is engaged in a peripheral outer groove 50 provided in a protruding part of the sleeve 39. This ensures that the bellows 36 does not detach from the sleeve 39, even in the case of a large deployment of this bellows, the end portion 37 of the bellows being for this also surrounded by a

  annular collar 51, the latter ensuring a tight seat.

  It is easily understood that the end 26 located on the side of the cylinder rod can also be further anchored to the cylinder rod 9 by

  complementarity of shapes, as shown in section in Figure 2.

  In this case the rod 9 is provided with a peripheral groove 52 in which a peripheral bead 53 is complementaryly engaged.

  provided at the end of the bellows 36.

  The aforementioned vent opening 30 for the accumulation chamber 29 may be made through the cover 7, so that it opens on one side on the outer face of the cover 7 and the other side in the accumulation chamber 29 (this variant is not shown). Advantageously, the vent opening 30 is placed in a rigid part of the peripheral wall 30, which is constituted in the embodiment of FIG. 1 by the longitudinal sleeve-shaped part 38. protrudes part of its length by extension 40, in the axial direction towards the end 44 of the cylinder rod, and the opening 30 is made in this area. This opening 30 is then advantageously made in the form of a tapped radial bore, into which a connecting nut can be screwed for fixing the flexible pipe 31. The embodiment shown has the advantage of making it possible to equip old-model cylinders, without having to modify these; the sealing device 22 must only be placed and tightened on a cover of

cylinder.

  According to a not shown embodiment of the jack according to the invention, the entire peripheral shell 23 is constituted by a bellows which is placed directly, with sealing, as well on the rod

  cylinder only on the cylinder cover.

  Advantageously, the peripheral shell 23 is connected in all embodiments to a cylindrical axial extension of the cylinder cover, which is traversed by the cylinder rod 9, and its outer section substantially corresponds to the external section of the lid. of cylinder 7 and the cylinder body 6. This provides a compact embodiment, which does not substantially increase the width of the cylinder relative to the original. Advantageously, it is possible without difficulty to replace an old cylinder, not equipped with a sealing device, without

  raise congestion problems.

  FIG. 3 represents, schematically and in longitudinal section

  nale, another embodiment of a bellows 36 '. This bellows 36 'consists of a series of separate annular elements 54, 54', in particular elastically deformable, which are placed one behind the other in the axial direction 19, these annular elements being connected to each other. another, to form a bellows-shaped structure, alternately and sealingly by their largest 55 and their smaller 56 sides. This connection is for example made by stirring, welding, gluing or the like, and the annular elements are preferably metallic. FIG. 3 shows the welding beads 55, 56 which are made in particular by

  electric welding or laser welding.

  To give the bellows 36 'good elasticity in the axial direction, the separated annular elements advantageously have a trim shape, adjacent annular elements 54, 54' being connected to each other.

  The other symmetrically with respect to their contact plane 57,57 '.

  The bellows 36 'of FIG. 3 is thus particularly advantageous when the gases entering the accumulation chamber 29 are under high pressure, which can be the case when large leaks occur at the crossing 10 and the channel venting 32 does not allow, due to its small section, to quickly evacuate the volume

  leak. In this case, the bellows 36 ', which is composed of metal elements,

  liquefaction, opposes self-damage, can not round or burst. It should also be noted that the jack 5 shown in FIG. 1 is equipped with a damping device 58 for damping the impact.

  piston 8, this device does not, however, part of the invention.

Claims (14)

- CLAIMS -   1.- jack comprising a cylindrical body with a cover at each of its two axial ends and in which is placed an axially displaceable piston, said piston being connected to a cylinder rod which passes through at least one of two covers by sliding in the axial direction, characterized in that a sealing device (22) is externally attached to the end (7,21) of the cylinder which is traversed by the cylinder rod (9), this device comprising a gastight peripheral wall (23), this wall surrounding with clearance and over its entire periphery at least a portion of length of the part (24) of the cylinder rod which protrudes from the cylinder (5) and which makes it immediately following, and this same wall being on the one hand fixed sealingly to the cylinder (5) by its axial end (25) located on the side of this cylinder and being on the other hand sealingly connected to the cylinder rod (9). ) p its axially opposite end and located on the side of this cylinder rod, so as to form an annular accumulation chamber (29) which is totally sealed with respect to the ambient medium (20) adjacent to the jack, this accumulation chamber ( 29) being provided with an opening (30) which can be connected to   a channel (32) for venting.   2. Cylinder according to claim 1, characterized in that the vent channel (32) which is on one side connected to the accumulation chamber (29), opens on the other side in a chamber (33) which is separate from the chamber (20) containing the jack (5), or in a receptacle, or as the case may be in the outside atmosphere.   3. Cylinder according to claim 1 or claim 2, characterized   in that the peripheral wall (23) is fixed to the cylinder rod (9) by its axial end (26) situated on the side of this cylinder rod, and in that this peripheral wall (23) comprises one or more portions of length (35) which extend over the entire periphery of the wall and which, in the   longitudinal direction (19) of the cylinder rod (9), are extensible,   elastic and flexible, and / or have elastic rubber properties, and that both ends (25,26) of the peripheral wall (23), which are connected to the cylinder (5) in a non-translatable manner and the other to the cylinder rod (9) in a completely sealed manner, move relative to each other during deployment and retraction of the rod   cylinder (9) out of, and into, the cylinder (5).   4. Cylinder according to claim 3, characterized in that the volume of the accumulation chamber (29) increases and then decreases during the deployment and the retraction of the cylinder rod (9), which produces a pumping effect.   5. Cylinder according to claim 3 or claim 4, characterized   in that these length portions (35) of the peripheral wall (23), which are extensible, respectively elastic or flexible, are made   in the form of a kind of bellows (36,36 ').   6. Cylinder according to one of claims 3 to 5, characterized in that   that the entire peripheral wall (23) is made in the form of a bellows.   7. Cylinder according to claim 5 or claim 6, characterized   characterized in that the length portions (35) of the peripheral wall (23), which are in the form of a bellows (36), are composed of a   material with elastic rubber properties.   8. Cylinder according to claim 5 or claim 6, characterized   characterized in that the length portions (35) of the peripheral wall (23) which are formed as bellows (36 ') are composed of annular elements (54, 54') placed one behind the other in the axial direction (19), in particular elastically deformable and preferably metallic, which, in order to produce a bellows-shaped structure, are connected to each other in a sealed manner alternatively by their zones of large (55) and   small (56) diameter, for example by brazing. welding, gluing or the like.   9. Cylinder according to claim 8, characterized in that the annular elements (54,54 ') are curved plate-shaped, two adjacent elements (54,54') being connected to each other so that symmetrical by   compared to their contact plan (57,57 ').   10. Cylinder according to one of claims I to 9, characterized in that   that the peripheral wall (23) connects to the side of the immediate cylinder.   and in axial extension to a cover (7) which is traversed by the cylinder rod, and consequently has an outer diameter substantially corresponding to the diameter of the cover (7), respectively   to the diameter of the cylinder body (6).   Il.- Cylinder according to one of claims 3 to 10, characterized in that   that the peripheral wall (23) has a portion of length (35) which is extensible, respectively flexible or elastic, in the longitudinal direction and which is in particular constructed in the form of a bellows (36,36 '), this portion of length being placed against the end (26) of the peripheral wall (23) which is located on the side of the cylinder rod, and being on one side sealingly attached to the cylinder rod, while on the other side it is fixed in a sealed manner, by its axial end (37) which is turned towards the cylinder (5), to a portion of separate length (38), rigid and in the form of a sleeve, which loosely surrounds the cylinder rod ( 9) and whose side   opposite is fixed, also with sealing, to the cylinder (5) and preferably   its cover (7) which is traversed by the cylinder rod (9).   12. Cylinder according to claim 11, characterized in that the portion of bellows-shaped length (35) is releasably tightened on the parts to which it is connected in the axial direction, and for this purpose includes one or at its two axial ends an annular holding bead (49, 53), which is introduced into a corresponding annular groove (50, 52) of the sleeve-shaped part (39),   respectively of the cylinder rod (9).   13. A cylinder according to claim 11 or claim 12, characterized in that the sleeve-shaped length portion (38,39) is plugged, and in particular pressed, on a cylindrical extension of the   cover (7) coaxially surrounding the cylinder rod (9).   14. Cylinder according to one of claims 1 to 13, characterized in that   the vent opening (30) is formed in the lid (7)   crossed by the cylinder rod (9).   15.- A cylinder according to one of claims 3 to 13, characterized in that the vent opening (30) is formed in a rigid portion of the peripheral wall (23), and in particular the case in its sleeve-shaped length portion (38), preferentially under the   form of a tapped radial bore.