GB2185545A - Sealing device for piston rod of piston and cylinder unit - Google Patents

Abstract

A piston and cylinder unit comprises a cylinder barrel (6) closed at both its ends by end caps (7) and has a piston (8) sliding axially within it connected with a piston rod (9) extending through at least one of the end caps (7) so that the piston rod (9) may move through a packing in the cap in the longitudinal direction (19). The cylinder end (7 and 21) having the piston rod (9) extending through it, has a sealing device (22), which comprises a gas-tight peripheral wall (23) which surrounds at least part of the length of the piston rod portion (24) extending out of the cylinder barrel (6) with play. The wall defines a header space (29) since it is connected at one of its ends (25) hermetically with cylinder (5) and furthermore at the other end (26) it is joined hermetically to the piston rod (9). The header space (29) has a port (30) to be connected with a venting duct (32). <IMAGE>

Description

SPECIFICATION A piston and cylinder unit The invention relates to a piston and cylinder unit of the type more particularly comprising a cylinder barrel shut off at both its axial ends by end caps, a piston sliding in the axial direction within the barrel, and a piston rod connected with the piston and extending through a rod packing in at least one ofthe end caps so that it may be slid in the longitudinal direction.

In experimental, developmental and manufacturing laboratories, as for instance in biochemical laboratories or in industrial clean rooms for the dev elopmentormanufacture of microchipsforthe computer industry, there is generally a requirement for a dust-free, uncontaminated and sterile atmos pheresothatthe operations to be performed therein are not interfered with by the air-borne polluants.It is for this reason that it has so far hardly proved practical for piston and cylinder units of the initially mentioned type to be used in such premises for positioning or the like operations since it has not been possible to make sure that there is absolutely no chance ofthe fluid driving medium leaking past the sealing rings used in the cap where the piston rod extends out of the cylinder. If for instance the medium is a hydraulic one, athin film of it may be drawn on the piston rod past the rings and so come into contact with the outside atmosphere with the resultthatthe airwill be contaminated with the medium. This effect tends to become more pronounced with advancing age of the equipment, since the sealing rings are subject to gradual wear.However there will be an even greaterdangerofcontamina- tion ofthe air if the piston and cylinder unit is airpowered, for if there is a leak it will escape from the cylinder in a jet so that a swirling motion of the surrounding air is produced and maintained. Since furthermore this escaping airwill normally be admixed with particles of the oil used for lubrication, such oil will be discharged into the atmosphere in the form of a suspension. The air will then be so contaminated that the industrial clean room standard will no longer be fulfilled.

Accordingly one object of the present invention is to devise a piston and cylinder unit of the initially described type which precludes contamination, pollution or other undesired effects on the immediate sur roundingssothat it may be employed in industrial clean rooms.

In order to achieve these or other object of the invention appearing in the course ofthe presentspecification and claims, the unit comprises a cylinder barrel and first and second end caps closing two axial ends thereof, a piston arranged in said barrel and able to slide in the axial direction thereof, a piston rod attached to said piston and adapted to ex tendfora given portion of its length out through said first end cap with sealing engagementtherebetween and, a sealing arrangement peripherally enclosing said piston rod outside said barrel, said sealing arrangement being made up, of a gas-tight peripheral wall surrounding with its entire periphery at least a portion of the piston rod outside said barrel with a radial clearance therebetween, said peripheral wail having one axial end thereof hermetically secured to said barrel and an opposite axial end thereof hermetically engaging said piston rod so as to define an annular header space between it and said piston rod, and of duct means for the discharge offluid from said header space into a receiving space.

In this construction the position at which the piston rod projects from the cylinder or the cylinder cap is now directly connected with a sealed header space which receives any escaping driving fluid and substances entrained therein, owing to there being a leak, and such fluid and entrained substances are no longer ableto find theirway into the immediate sur- roundings ofthe piston and cylinder unit. In fact, such leaked fluid (more especially air) and contaminants are conveyed through the port of the header space and the venting duct leading therefrom to a position where they are able to be blown off into the atmosphere which is outside the space in which the piston and cylinder unit is installed.It is an advantage in this respect that no separate exhausting means is requiredforthissincethe leaking airin the outlet end of the venting duct will normally be at a higher pressure and, following the path of least re sistance, will escape through the venting or discharge duct. The piston and cylinder unit in accordancewith the invention may thus be used in-spaces in whichtheair isto have a superior degree of purity, meeting maximum standards.

Further useful developments of the invention are described in the claims.

In keeping with one such further development of the invention, the end of the peripheral wall adjacent to the piston rod is fixedly secured to the latter and the peripheral wall has one or more sections ofthe length, extending completely over the periphery, and which are ableto be stretched in the length direction of the piston rod or are resilient and flexible and/or have elastomeric properties so that the two end parts of the peripheral wall fixedly secured to the cylinder on the one hand and to the piston rod on the otherwith a full sealing action, are ableto movetowardsand awayfrom each other as the piston rod is extended and retracted.

In this furtherfeature of the invention the sealing peripheral wall of the header space is completely sealed off at the two axial end parts, since on the one hand it is secured to the piston cylinder and on the other hand it is secured to the piston rod. There is no relative motion either between a rigid peripheral wall andthe cylinder or between the peripheral wall and the piston rod so that it is possible to guarantee an essentially more reliable seal means than in a design in which there would be a gland seal between the peripheral wall and one ofthetwo said cylindercomponents.Furthermore, the quality of the seal maintained is at a consistently high level of efficiencyduring thefull working life of the arrangement inasfaras the points of attachment between the peripheral wall and the cylinder, and between it and the piston rod are not subect to any wear. On operation ofthe piston rod the terminal part of the peripheral wall will be entrained with it as is rendered possible by the said sections of the length of an extensible or resilientlyflexible nature running in the peripheral direction of the peripheral wall.

In accordance with a still further development of the invention the unit is so contrived that on extension and retraction ofthe piston rod from and into the cylinder the volume ofthe header space is alternately increased and diminished so that there is a pumping effect.

The result of this action with the alternately increasing and decreasing volume ofthe header space like a pump is thatthe contracting movement of the peripheral wall expelsthe portions of leaked fluid, which have coliected in the header space, out through the venting opening and the venting duct.

This ensures a permanent scavenging of the header space even if the piston and cylinder unit is operated with a relatively low pressure.

The resilient or flexible sections of the length ofthe peripheral wall able to stretch and retract in the axial direction may take the form of a bellows, that is to say of a tube with circumferentially aligned folds therein. It is possible for the entire peripheral wall to be in the form of a bellows. Furthermore, sections of the length of the peripheral wall in the form of bellows may be constructed of a material with elastomeric properties, that is to say one having the elastic properties of rubber.

Thesefeaturesenablethe peripheral wall to be manufactured cheaply, and the flexible sections thereof are only subjectto a slight degree of wear.

It is furthermore possible for the bellows-like longitudinal sections of the peripheral wall to be composed of annular elements which are arranged in axial succession and are deformable and more especially metallic and so designed that the elements are hermetically joined together alternately attheir inner and outer diameters, for example by soldering, brazing, welding or bonding or any other suitable method to form a bellows. The annular elements may have the form of bells or cups which are joined together so asto be bilaterally symmetrical about planes extending through their hermeticjoins.

This form ofthe bellows is particularly sturdy.

As afurther possible development of the invention, the peripheral wall is directly joined to the end cap having the piston rod running through it as an axial extension, the external diameter of the peripheral wall being preferably generally equal to the diameter of the cap or of the cylinder barrel.

This leads to a compact design of the arrangement in accordance with the invention which has the advantage of not involving any departure from the original size (i.e. the size of an otherwise similar prior art unit) in the longitudinal direction.

The peripheral wall constructed in the form of a flexible or resilient longitudinal portion, more es pecially in the form of a bellows, which is adjacentto the end partofthe peripheral wall adjacent to the piston rod and which is securely joined to the piston rod hermetically, furthermore has its axial end part adjacent to the cylinder hermetically secured to a separate rigid sleeve-like longitudinal section ofthe peripheral wall surrounding the piston rod with a clearance. This sleeve-like extension is fixedly secured hermetically to the cylinder at its end cap which has the piston rod extending through it.

This feature represents a particularly neat design of the peripheral wall while at the sametimeensur- ing simple and reliable assembly thereof.

The bellows-like section of the length may be detachably clamped to the components which are joined to it axially, and preferably the said section of the length has an annular retaining bead on one or both of its axial endstofitintoasuitableannularre- tainer groove on the sleeve-like componentorsec- tion.

This guarantees a particularly simple form of hermetic connection between the peripheral wall and the cylinder or the cylinder barrel.

The venting port may be provided in a rigid part of the peripheral wall, more especially in its sleeve-like section.

This further development of the invention ensures that even less modern designs of pre-existing piston and cylinder units may be modified with little trouble by fitting the sealing device in accordance with the invention.

Preferred forms of the invention will now be described with reference to the accompanying drawings.

Figure 1 shows the one portion of a piston and cylinder unit, as fitted with the sealing device in accordance with the invention, in longitudinal section.

Figure2 shows the join between the peripheral wall and the piston rod on a larger scale and in accordance with a preferred form of the invention; Figure 3 shows an advantageous form of the bellows.

Reference is firstly had to the embodiment of a piston and cylinder unit in accordance with the invention to be seen in figure 1. The readerwill seethe right hand axial end portion of a piston and cylinder unit generally referenced 5 whose cylinder barrel 6 is only shown in part. The cylinder barrel 6 is shut off at both ends by cylinder end caps 7 (of which only one is shown). Inside the barrel there is an axially sliding piston 8 indicated in broken lines which is joined to the left hand one axial end of a coaxially arranged piston rod 9. The piston rod extends through the cylinder end cap 7 at an opening 10, it running axially in a support bushing 14 secured in the opening 10.

Furthermore there is a rod packing ring 15 mounted in the opening 10 so that, as seen from the piston,the packing is in front of the bushing 14, while on the other, opposite side of the bushing 1 4there is a wiper ring 16. The two rings 15 and 1 6 encompass the piston rod 9 and engage it. The rod seal ring 15 serves for sealing off the cylinder space 17 between the piston Band the cylinder end cap 7 from the out side, whereas the wiper ring 16 serves to-prevent dirt on the piston rod 9 from being drawn into the interior of the cylinder.

There is furthermore a feed port 18 extending through the cylinder end cap 7 into the interior ofthe cylinder. This port may be connected with a pressure line forthe supply and discharge of driving fluid under pressure.

At the opposite left hand end ofthe cylinderthere is a supply port leading into-the second cylinder space so thatthe piston 8 and the piston rod 9 may be reciprocated in the longitudinal direction of the cylin- der barrel by the supply and discharge of fluid.

In the present working example of the invention it is a question of a piston and cylinder unitwhose piston rod only extends through one of the cylinder end caps. However the invention is not limited to such an instance and is fully applicable to systems with a piston rod extending through both the cylinder end caps. In such a case there will be a simiiar arrangement as described, and to be further described, at the opposite end ofthe cylinderwhich is not shown.

The piston and cylinder unit described in the present instance is intended for use in an industrial clean room in which experiments or other operations are performed which require a fastidiously pure atmosphere. The rod sealing ring 15 and the wiper ring 16 do not as such however guarantee a 100% sealing offof the cylinder 17 from the im- mediate surroundings or space 20 ofthe piston and cylinder unit. Every time even only a slight leak occurs between the seals and piston rod there will be a leakage into the room and contamination will be caused by matter sticking to the piston rod.If the fluid usedforoperation of the piston and cylinder unit is air, leaks to the outside will be particularly ob jectionablesincetheairwill normallycontainlubri- cant such an oil which will be squirted into the space 20 and will be a source oftrouble owing to its being deposited in the immediate vicinity of the piston and cylinder unit. It is forthis reason that the cylinder end 21 and the cylinder end cap 7 having the piston rod extending through them are provided with an external sealing device 22, which traps and collects the leaking air passing through the opening 10 so that the space 20 around the piston and cylinder unit is completely free of dirt. The sealing device 22 will now be explained in some detail.

The sealing device 22 possesses a gas-tight peripheral wall 23 which surrounds with play the full length of a longitudinal section of the piston rod part 24 projecting out through the piston and cylinder unit 5 past the end cap 7. Furthermore the peripheral wall 23 is connected to its one axial end part 25, adjacent to the cylinder, to the cylinder end cap 7 while its opposite axial end part 26 adjacent to the piston rod it is in sealing engagement with the piston rod. The result is then an annular header space 29 forming an axial extension from the cylinder end cap 7 and being completely sealed off from the space 20 around the piston and cylinder unit. This header space is able to receive the fluid leaking through the opening 10.

The header space 29 is connected via a venting part 30 with venting duct 31 in the form of a pipe or of flexible tubing 31 and the free end 32 of the venting duct 31 is so fitted that it opens in space 33, that is to say the outer atmosphere separate from the space 20 in which the cylinder unit 5 is installed, that is to say the clean room. For this pu rpose the flexible tu bing 31 will be seen from figure 1 to be suitably mounted on a support bulkhead 34 which is diagrammatically shown.

The air and dirt trapped in the header space 29 are now able to leave the arrangement through the venting duct 32 without leaking into the clear room. For this it is not necessaryto have any special exhausting device or pump and the disposal of the leaking air is ensured simply by there being a pressure gradient between the spaces 29 and 33.

It is also possible, although there is no embodimentillustratingthis,to makethe peripheral wall complete rigid and to have a seal between it and the rod in the end part 26 by means of a gland surrounding the part 24 of the piston rod part so that the rod part 24 is able to slide therein. However such a seal will be subjectto searduring the course oftime so that a full sealing action will be only guaranteed if the seal ring forming the gland is frequently changed.

In order, therefore, to cause an improvement effect so that there is full a sealing effect however long the arrangement has been in use, between the peripheral wall 23, the cylinder and the piston rod there is, as illustrated in figure 1, an arrangement in which the end part 26 adjacent to the piston rod of the peripheral wall is also secured to the piston rod part 24 so that no relative motion between the parts is possible. Such a static seal is substantially easier to provide, and since there is no relative motion at the seal interface it is possible for the header space 29to be completely sealed off here from the immediate surroundings or space 20.

This design of the invention meansthatthe end part 26 together with the piston rod 9 is moved backwards and forwards in the axial direction 19 when the piston and cylinder unit 5 is operated. Accordinglythe peripheral wall 23 is able to be extended along a section of the length running along the wall and therefore surrounding the piston rod 9 in the length direction 19, that is to saythe peripheral wall 23 is flexible. This section is best also made of material with elastomeric properties although this feature is not obligatory.

The result of this is that while maintaining the sealed condition of the header space 29 the two end parts 25 and 26 of the peripheral wall 23 are moved towards and away from each other when the piston rod is moved out ofthe cylinder unit 5 and back into it so that consequently there is an increase and decrease in the volume of the header space. The peripheral wall 23 accordingly performs a pumping movement inasfar as it contracts and expands in the section 35 of the length.

Theeffectofthispumping action isthateven if driving air has leaked into the header space 29 and is under a moderate pressure in comparison with the outer surroundings or space 33, such air will nevertheless be pumped out through the port 30 and the duct 32 to the outside without it being necessaryto employ a separate scavenging pump for this purpose.

It will be clearthatthe peripheral wall 23 as seen in the length direction may comprise a numberofflex- ible sections or may be made throughout with flexible properties.

In the working example shown in figure 1 the extensible and collapsible section 35 of the length is in the form af a bellows 36. This bellows directly adjoins the right hand end part 26 fixedly secured to the piston rod 9 and is such that there is radial play between it and the piston rod 9. This bellows section ends short ofthecylinder end cap 7 and its circular bellows end part 37 adjoins a rigid section 38 ofthe length of the peripheral wall in the form of a sleeve 39. This section 38 ofthe length in the form of a sleeve is a separate component -the peripheral wall 23 thus comprises two parts in this form ofthe invention -to which the bellows 36 is hermeticallyjoined.

The generally tubular sleeve 39 is coaxially arran- ged in relation to the piston rod 9 and has a radial ciearance between it and the rod 9. Itfurthermore has its end remote from the bellows end part 37 secured to the cylinder end cap 7 hermetically by being clamped on the cylindrical extension 40 formed on the right hand side ofthe cylinder end cap 7. This extension 40 runs in the longitudinal direction 19towards the outer end 44 of the piston rod and surrounds it coaxially. The extension also bearsthetwo rings 15 and 16alreadyalludedto.

The sleeve 39 is preferably coaxiallyfitted on the extension 40 with a press fit so that even without a seal ring between the two components there is a gastight connection. It will however be clearthatthere are otherways of mounting the sleeve 39, asforinst- ance by providing it with a radially projecting an nular flange which is screwed on the cylinder end cap 7 with a sealing ring therebetween.

The bellows 36 shown in figure 1 consists entirely of elastomeric material and is so premolded that it has bell-like shape. Its wall parts 45will be seen to be folded or meandering in longitudinal section and, in an end part 46 aligned with a radial planewith re spectto the direction 19, it has a central opening 47 to accept the piston rod part 24. The sleeve-like end part 26 is formed atthe edge-ofthe opening 47. There is a band 48, preferably made of metal, crimped on the end part 26 to clamp it onto the piston rod 9. The bellows 36 is thus detachably and firmly secured to the piston rod 9 so that it will notslidethereon.

The end part 37 of the bellows nearest the cylinder barrel 6 is also fashioned in the form of a sleeve and on its inner edge turned towards the piston rod 9 it has a peripheral bead 49 which fits into an external peripheral groove 50 in a part of the sleeve 39 of re duced diameter. This ensures that even at maximum extension the bellows 36 will not slip off the sleeve 39. Furthermore, the end part 37 ofthe bellows is en circled by a crimped ring 51 which guarantees a se cure sealing fit and engagement.

It will be furthermore be clearthatthe end part 26 adjacent to the piston rod may as well be positively fitted to the piston rod 9 as is shown as a detailed view in figure 2. In this case the piston rod 9 pos sesses a peripheral groove 52 into which a cor responding peripheral bead 53 atthe end ofthe bellows 36 is received with a positive interlocking en gagement.

The venting port 30 already previously mentioned leading outofthe headerspace 29 may be arranged to extend fully through the cylinder end cap 7so that it opens at one end on the outside ofthe cylinder end cap and atthe other end in the header space 29 (this construction not being shown). However it is an advantage if the venting port 30 is arranged in a rigid part ofthe peripheral wall 23, and in the case ofthe design of figure 1 this will be the sleeve-like section 38 of the length. The section 38 has some of its length extending pastthe extension 40 in the axial direction towards the end 44 ofthe piston rod, and here the port 30 is arranged, which is preferably in the form of a radially aligned threaded hole into which a connecting screw nipple may be inserted to provide a screw connection with the flexible tubing 31.

The construction illustrated offers the advantage that it may also be fitted to pre-existing piston and cylinder units without having to make changes to them; the sealing device 22 only has to be mounted on a cylinder end cap and clamped in place thereon.

In the case of a further possible working example of the piston and cylinder unit that is not illustrated, the entire peripheral wall 23 is in the form of a bellows which is directly secured and sealed not only on the piston rod but also one the cylinder end cap through which it extends.

It is an advantage in all embodiments ofthe invention if the peripheral wall 23 is joined directly and by way of an axial extension to the cylinder cap 7 having the piston rod 9 running through it, the external diameter of the peripheral wall being essentially identical to the external diameter ofthe cylinder end cap 7 and of the cylinder barrel 6. This results in a com pactly constructed arrangement, which at the most only causes a slight increase in the breadthofthe piston and cylinder unit. It is an advantage in this re spectthat it may befitted to less modern piston and cylinder units not having a sealing arrangement, without taking up an unnecessary amount of space.

Figure 3 shows a partial view of a further possible form of the bellows 36' used in the invention. This bellows 36' consists of separate annular elements 54 and 54', which are more particularly elastic and are arranged in succession in the axial direction 19, They are alternatelyjoined at their outer and inner edges 55 and 56, respectively, hermetically. The connections may be by soldering, brazing, welding or bonding or the like and the annular elements are preferably made of metal. At the edges 55 and 56 it is possible to see the sectioned weldments in figure 3.

Such welds may be more especially resistance or laser welds.

In order to ensure that the bellows 36' has a satisfactory degree of elasticity in the axial direction, the separate annular elements are preferably given a cu p-like form and so arranged that each pair of elements 54 and 54' are symmetrically arranged about their contact planes 57 and 57'.

- The bellows 36' as shown in figure 3 is more especiallyadvantageous if the gases escaping into the header space 29 are under a high pressure, as may be possible if there is heavy leakage through the opening 10 and the venting duct 32 has such a small borethatthe leaked gas is notableto escape quickly through it at a low pressure. In this case the bellows 36' consisting of metallic elements will be sufficiently robust notto be damaged and twill notbulge outwards or burst.

It is to be added thatthe cylinder unit 5 shown in figure 1 is fitted with a snubbing device 58 (not illustrated in detail) to retard the piston 8. The invention is however not limited to a piston and cylinder unit with such a device.

Claims (22)

1. A piston and cylinder assembly comprising a cylinder, first and second end caps closing two axial ends thereof, a piston arranged in said cylinder and able to slide in the axial direction thereof, a piston rod attached to said piston and adapted to extend for a given portion of its length out through said first end cap with sealing engagementtherebetween and a sealing arrangement peripherally enclosing said piston rod outside said cylinder, said sealing arrangement comprising a gas-tight peripheral wall surrounding with the entire peripherythereof at least a portion ofthe piston rod outside said cylinder with a radial clearancetherebetween, said peripheral wall having one axial end thereof hermetically secured relative to said cylinder and an opposite axial end thereof hermetically engaging said piston rod so as to define an annular header space between said peripheral wall and said piston rod, and duct means for the discharge of fluid from said header space into a receiving space.

2. A piston and cylinder assembly as claimed in claim 1 wherein said receiving space is selected from the group comprising: a space outside an enclosure in which said peripheral wall is arranged, a receiving vessel, and the outside atmosphere.

3. A piston and cylinder assembly as claimed in claim 1 or2 wherein one end of said peripheral wall is anchored to said piston rod and said peripheral wall has at least one portion of its length able to be extended and contracted in the axial direction of the wall in the direction of motion of the piston rod so that the two ends of the peripheral wall, of which one is secured to the cylinder and the other is secured to the piston rod, may be moved towards and away from each otherwhen the piston rod is extended from the cylinder and retracted back into it.

4. A piston and cylinder assembly as claimed in claim 3wherein said portion ofthe length ofthewall is flexible and has elastomeric properties.

5. A piston and cylinder assembly as claimed in claim 3 or4wherein on moving said piston rod into and out of said cylinder the volume of said header space is decreased and increased so as to exert a pumping action.

6. A piston and cylinder assembly as claimed in any of claims 3 to 5 wherein said at least one portion ofthe length of said wall allowing axial extension and contraction of the peripheral wall partasthe piston rod is moved into and out of the cylinder is in the form of a bellows.

7. A piston and cylinder assembly as claimed in any of claims 3 to 6wherein the entire peripheral wall is in the form of a bellows.

8. A piston and cylinder assembly as claimed in claim 7 wherein said bellows is made of a material with elastic properties resembling those of rubber.

9. A piston and cylinder assembly as claimed in claim 7 or 8 wherein said bellows is made up of annular elements which are sealingly joined together alternately at inner and outer edges thereof by a joining method selected from the group comprising: soldering, brazing, welding, adhesive bonding.

10. A piston and cylinder assembly as claimed in claim 9 wherein said annular elements joined to form said bellows are of an elastic metallic material.

11. A piston and cylinder assembly as claimed in claim 9 or 10 wherein said annular elements have a bell-like configuration and are arranged in pairs so thatthe members of each pair are symmetrical about a plane passing through a join connecting them together.

12. A piston and cylinder assembly as claimed in any of the preceding claims wherein said peripheral wall adjoins said first end cap directly as an axial extension thereof.

13. A piston and cylinder assembly as claimed in claim 12wherein said peripheral wall has a diameter the same as the diameter of the said first end cap of the cylinder.

14. A piston and cylinder assembly as claimed in any of claims 1 to 1 1,furthercomprising a rigid sleeve surrounding said piston rod with play adjacent to said first end cap and sealingly connected with said cylinder and with one end of said wall.

15. A piston and cylinder assembly as claimed in claim 14wherein said sleeve is connected with said cylinder byway of said first end cap.

16. A piston and cylinder assembly as claimed in claim 1 4wherein joints between the ends of the peripheral wall on the one hand and the piston rod and the cylinder on the other hand are disconnectable.

17. A piston and cylinder assembly as claimed in claim 16 wherein at least one of such joints comprises a retainer bead on said wall and means defining an annulargrooveto receive such bead.

18. A piston and cylinder assembly as claimed in claim 14wherein the sleeve is mounted with a press fit on a male part ofthefirst end cap extending into such sleeve.

19. A piston and cylinder assembly as claimed in any of the preceding claims wherein said duct means comprises a port in said first end cap.

20. A piston and cylinder assembly as claimed in claim 14wherein said duct means includes a radial port extending through said sleeve.

21. A piston and cylinder assembly substantially as described herein before with reference to the accompanying drawings.

22. Any novel subject matter or combination including novel subject matter herein disclosed-in the foregoing Specification or Claims and/or shown in the drawings, whether or not within the scope of or relating to the same invention as any of the preceding claims.