GB2188698A

GB2188698A - Gas spring unit

Info

Publication number: GB2188698A
Application number: GB08707746A
Authority: GB
Inventors: Rudolf Muller
Original assignee: Profil Verbindungstechnik GmbH and Co KG
Current assignee: Profil Verbindungstechnik GmbH and Co KG
Priority date: 1986-04-02
Filing date: 1987-04-01
Publication date: 1987-10-07
Also published as: GB2188698B; IT8747799A0; ES2002647A6; DE3611011A1; FR2596835B1; DE3611011C2; IT1205996B; GB8707746D0; FR2596835A1

Abstract

In order to create a cylinder- piston gas spring unit with a high load bearing capacity and useful in higher frequency applications, a rear scavenging of the entering piston 8 with external atmosphere is achieved, the air filling the chamber created by the rear face 28 of the piston head 7 and the part of the piston rod 9 penetrating into the cylinder 1. The external atmosphere enters the gas spring and is expelled therefrom through air ducts 32,34 and 35, the entering air being filtered at 33. The working gas chamber, which includes spaces 5, 12 as well by the atmospheric air, particularly as the main cylinder space, is cooled through the thin wall of an internal sleeve 4. <IMAGE>

Description

SPECIFICATION Cylinder-piston unit for compressible pressure medium This invention relates to a cylinder-piston unit for a compressible pressure medium for use in, for example, the construction of tools, moulds and equipment, for generating large forces on stripper, holding-down and crossslide systems, where gas spring cylinders are now increasingly used instead of helical springs and plastics springs.

Known gas spring cylinders consist of a cylinder casing with a freely movable piston and a free, open end, which is situated opposite the end through which the piston rod of the freely movable piston passes. These gas spring cylinders are equipped, at their free, open end, with an internal or external thread respectively and are screwed either individually to a baseplate, or in groups to a larger baseplate or storage plate. In either case these plates serve for supplying the screwed-in cylinder casings with an inert gas, preferably with nitrogen, in such a way that this gas acts upon the face of the piston opposite to the piston rod.

A thrust force comprising the product of piston area and gas pressure thus acts upon the piston. This force, or the sum of the forces from a plurality of gas spring cylinders, acts upon movable plate, beam or lever systems, in order to execute, for example, holding, clamping and/or restoring functions. Gas spring cylinders of this class possess a virtually uniform spring constant (pressure rise divided by spring travel). This advantage is contrasted with a property of these gas spring cylinders which is disadvantageous in many applications, namely that they must be connected to a gas storage system, for example via hose or pipe connections leading to a tank, or must be screwed into tank plates drilled with holes. These storage tanks or plates are usually components of the tools, moulds or equipment in which the gas springs are to act.

These known gas spring cylinders therefore have only limited mobility, because they are "tied" via hose or pipe connections to a tank system or are "captured" in position in a plate system. This makes it quite difficult or even impossible to carry out modifications to the relevant machine or tool if the modifications necessitate a position change of the cylinders.

Examples of application for gas spring cylinders are, however, increasingly encountered, in which "unimpeded mobility" is a pre-requisite for their use and/or an external storage system is not possible for the cylinders due to reasons of space and safety. For overcoming these disadvantages, so-called "autonomous" gas spring cylinders have been developed, which constitute a closed, independent gas spring system which can be utilised in any spatial installed position. The disadvantages of these known "autonomous" gas spring cylinders lie especially in the fact that they possess only a relatively low basis loadbearing capacity, apart from heat problems associated with the transporting of gas, especially in their guide wall regions, during use.

In accordance with the present invention, however, a cylinder-piston unit for a compressible pressure medium comprises a piston reciprocatingly guided within an outer cylinder which is closed at one end, and is characterised by provision for a rear scavenging with external atmosphere as the piston progressively penetrates into the outer cylinder.

It is thus possible for a cylinder-piston unit, in accordance with the present invention, to possess a larger basic load-bearing capacity than the known autonomous systems, without changes in dimensions; at the same time the operational possibilities can be widened to the extent that even applications involving higher frequencies are made possible, without leading to the failure, hitherto usual in such cases, of the sealing elements due to overheating. In particular, as will be explained below in more detail, not only can the basic load-bearing capacity be increased, but also the pressure medium, which becomes strongly heated in the alternating strokes due to compression and/or frictional heat of the relatively moving parts, can be cooled in an advantageous manner.

Preferably, the piston includes an at least partly hollow piston rod opening through a piston head into the outer cylinder.

The heat exchange producing the abovementioned cooling is especially favoured by a double or second wall extending at least partly along the length of the outer cylinder and separated therefrom by a hollow space for the compressible pressure medium. Thus, the pressure medium can flow externally of the double wall, with the external atmosphere flowing in internally of the double wall, as the piston penetrates into the outer cylinder. The desired increase in basic load-bearing capacity is assured by the achieved utilisation and association of the hollow space. The double wall can extend through the length of the piston stroke to serve internally as a piston guide.

An embodiment which is especially favourable in regard to manufacturing, and also for the desired properties, includes a preferably cylindrical cooling and guide sleeve which creates, between itself and the outer cylinder, at least one hollow space. This cooling and guide sleeve, which wouid correspond to the aforementioned double wall, fulfills a number of advantageous functions. Thus the cavity created between it and the outer cylinder serves firstly as a buffer or receptacle for gas compressed by the entering piston, and flow ing into this cavity. It also serves as a heatexchanging wall because, according to this invention, external atmosphere flows behind the entering piston.The atmosphere flows along the inner surface of the cooling guide sleeve, and thereby cools the sleeve heated by the compressed gas flowing along the outer face of the cooling and guide sleeve and/or heated by the piston head movement. Thus a selfcooling effect as the piston ascends and descends is obtained for the cylinder-piston unit of this invention, with the result that a working frequency increased by approximately fifty percent is possible for the cylinder-piston unit of this invention.

The basic load-bearing capacity of the cylinder-piston unit of this invention is considerably higher than that of known forms of construction, because it permits the compression of the entire volume defined by the piston head front face and the stroke. In the state of the art, only the penetrating piston rod volume is compressed, because the piston head is flowed around by compressed medium and thus acts in pressure upon the rear face of the piston head. That is to say, the annular area of the piston head is in pressure equilibrium during each phase of operation.

Furthermore, by the construction according to this invention of the piston-cylinder unit, a desirably large internal volume can be created inside the cylinder components so that the unit, containing so to speak its own gas reservoir, obtains by these special design measures a spring characteristic in which the pressure rise adopts a desirably small value over the spring travel. It will be remembered that the piston rod is preferably at least partly hollow and opens through the piston head.By the aforementioned removal of the heat resulting from the compression operation, the result is furthermore achieved that a temperature below the limit set for the stability, that is to say wear resistance of the sealing and guide elements, can be established in the cylinderpiston unit so that, in particular, excellent sealing of the piston head with respect to the internal wall of the cooling and guide sleeve during the entire life of the cylinder-piston unit remains assured.

The cooling effect enhanced by the cooling and guide sleeve becomes especially large if this sleeve is relatively thin-walled compared with the other components. The ratio of the wall thickness of the cooling and guide sleeve to that of the outer cylinder may be up to 1:5, preferably 1:3 to 1:4. By the thinness of the cooling and guide sleeve wall, not only is the cooling effect increased and heat transfer accelerated, but there is also a positive action upon the sealing performance of the piston head. As the piston head advances the pressure behind the cooling and guide sleeve is increased by the compressed gas flowing into the hollow space and thus the sleeve is slightly deformed towards the piston head producing a stronger and tighter bearing.In contrast, in the state of the art, the gas pressure increasing as the piston advances acts in exactly the opposite direction, since this pressure is exerted upon the inner wall of the outer cylinder, that is in a direction away from the piston head and its sealing surface.

An end of the cooling and guide sleeve further from the closed end of the outer cylinder can serve internally as a stop for the return stroke of the piston and as a guide neck for the piston.

Preferably, said above-noted end of the cooling and guide sleeve is mounted at the end of the outer cylinder further from the closed end thereof. The other end of the cooling and guide sleeve can possess a peripheral, continuous apertured flange bearing with its outer wall against the inner wall of the outer cylinder. A mounting at both ends of the sleeve is thus obtained, providing the further advantage that a co-axial loading of the cylinder-piston unit and of the piston by the external load is not absolutely necessary. The construction of the flange ensures that the gas, during the upward and downward movement of the piston, can sweep around that edge of the sleeve situated inside the outer cylinder.

The desired gas exchange between the hollow space and the remaining interior of the outer cylinder is thus possible.

The rear scavenging required by this invention is facilitated by a connection between the rear of the piston and the external atmosphere to provide for scavenging at a rear face of a piston head and at a part of a piston rod penetrating into the outer cylinder. There may be one or more air ducts communicating with the external atmosphere through the outer cylinder and/or cooling and guide sleeve. The air duct or ducts may pass through the outer cylinder and the cooling and guide sleeve at or near those ends thereof further from the closed end of the outer cylinder. Preferably the air duct or ducts terminate just beyond an end of the piston stroke along the inner wall of the cooling and guide sleeve. The air duct or ducts passing through the outer cylinder may lead into an annular groove in the outer wall of the cooling and guide sleeve, and the air ducts passing through the cooling and guide sleeve may lead out from said annular groove in star pattern, preferably uniformly distributed around the circumference.

The number of the air ducts is determined especially according to the stroke of the particular model, and the volume of the external atmosphere or air to be sucked in for the desired cooling, in such a manner that turbulence is avoided as far as possible, which could lead to temperature increases during sucking in, and also during the expulsion of the external air in the return movement of the piston. As a difference from the state of the art, therefore, the piston of the cylinder-piston unit of this invention continually pumps relatively cool environmental air during the individual working cycles, drawing in the air as the piston advances into the annular space between the piston rod and the inner wall of the cooling and guide sleeve, this air then being discharged to the environment in the return stroke of the piston.Thus the already mentioned self-cooling effect is obtained, which does not exist in the state of the art where the rear face of the piston is completely swept by the compressed and therefore heated medium as the piston enters. The environmental air occupies the entire cavity between the piston rod and the inner wall of the outer cylinder on the rear face of the piston head.

In order to prevent dirt particles from the surrounding atmosphere from entering the annular space above the rear face of the piston head during sucking in of air, at least one filter is provided in each air duct, preferably in the region of the outer cylinder wall.

The tightness of the cylinder-piston unit which, as explained above, is autonomous in respect of the pressure medium, i.e. contains the "working gas" in the entire volume itself, can be still further increased by a base being connected as one piece with the outer cylinder at the closed end thereof. Moreover, a preferably graphite-equipped bursting safety device can be provided in the base, which ensures that if a maximum admissible pressure for operating safety is exceeded a rapid pressure release takes place without risk to the environment. Such bursting safety devices are themselves known. For filling the cylinder-piston unit according to this invention, a charging valve can be used, especially a high pressure filling valve with a screw plug fitted in the base laterally on the outer cylinder.

A virtually self-securing and self-sealing connection between the cooling and guide sleeve and the outer cylinder can be achieved by a form-fitting locking. The locking may be located at or near those ends of the outer cylinder and the cooling and guide sleeve remote from the closed end of the outer sleeve. The locking may comprise a ring having holding and securing functions, which is retained on the one hand in a circumferential groove formed in the inner wall of the outer cylinder and on the other hand in a circumferential groove formed in the outer wall of the cooling and guide sleeve.Since this securing ring provides a very important holding function for the cylinder-piston unit of this invention, and must on no account be damaged during assembly, the circumferential groove formed in the outer cylinder may be equipped, towards the closed end of the outer cylinder, with a bevel, the lower edge of the groove leading into the outer cylinder inner wall at a more acute angle than the upper. Damage-free assembly can be still further facilitated if the circumferential groove formed in the cooling and guide sleeve possesses no groove wall away from the closed end of the outer cylinder.Thus the sleeve in the installed, that is charged and pressurised state of the cylinder-piston unit, is pressed against the upper edge of the circumferential groove formed in the outer cylinder by means of the securing ring in a virtually self-securing manner, since it is pre-stressed by pressure.

Finally, a damage-free insertion of the sleeve and securing ring is still further improved by a chamfer being provided on the inner wall of the outer cylinder at its end furthest from its closed end.

A cylinder-piston unit, in accordance with the present invention, will now be described in more detail, by way of example only, with reference to the accompanying drawing, which is a vertical longitudinal section through the cylinder-piston unit.

An outer cylinder 1 constitutes basically the housing of the cylinder-piston unit according to this invention. The outer cylinder 1 is closed at one end, by a base 2 connected as one piece with it, into which base a graphiteequipped bursting safety device 3 is inserted, which ensures that the operating pressure does not rise above a maximum admissible value.

Inside the outer cylinder 1 there is a further cylinder 4 which, as a cylindrical cooling and guide sleeve (hereinafter termed simply "sleeve"), constitutes between itself and the outer cylinder 1 an annular space 5, which is closed towards the open end of the outer cylinder 1 (at the top in the illustration and hereinafter termed "nearest the piston"). This closure is obtained by a mounting for the sleeve 4 provided by a flange-like construction 6 of the sleeve 4. The flange 6 bears sealingly with its external surface against the internal surface of the outer cylinder 1, as will be explained in more detail later. Inwardly, this flange 6 penetrates into the sleeve q in such a manner that it engages behind the piston head 7 of a piston 8, and sealingly surrounds its piston rod 9.The piston rod 9, as indicated by the broken line 11, is at least partly hollow, this hollow space 12 being open towards the cylinder interior through the piston head 7, and thus forming a part of the total volume available for the compression functions of this cylinder-piston unit.

At the inner end, remote from the end nearest the piston, the sleeve 4 possesses a peripheral flange 13, which bears with its outer face against the inner face of the outer cylinder 1 and thus constitutes a second mounting for the sleeve 4 inside the outer cylinder 1.

The flange 13 is equipped, at its outer periphery, with through openings 14, which ensure the already mentioned connection between the annular space 5 and the remaining interior of the cylinder, to provide the possibility for gas movements to be explained below.

The gas tightness of the cylinder-piston unit of this invention is achieved, amongst other things, by a high performance sealing ring 15 having inserted O-ring, by stripper and sliding rings 16,17 respectively on the piston 7 and by appropriate seals 18 and 12 on the piston rod guide, while a sealing element 21 at the outer face of the upper sleeve flange 6 ensures gas tightness between the outer cylinder 1 and the sleeve 4.

In the region of the lower sleeve flange 13, an internal snap ring 22 is provided, which constitutes a stop and limits the downward movement of the piston 8, the head of which is guided by the sleeve 4.

A very important component of the cylinderpiston unit, which fulfills the holding and retaining functions, is a ring 23, provided in the region of the flange 6 of the sleeve 4, between the flange 6 and the upper, open end of the outer cylinder 1. As can be seen especially from the enlarged illustration of detail A, the holding and securing ring 23 lies simultaneously in two grooves, namely on the one hand in a circumferential groove 24 formed in the inner wall of the outer cylinder 1, and on the other hand in a circumferential groove 25 formed in the outer wall of the flange 6. The circumferential or annular groove 24 has a bevel 26 pointing towards the interior of the cylinder in order to protect the safety ring 23 during installation, while the circumferential or annular groove 25 is open upwards.The securing ring 23 can be pushed without any great resistance from above past chamfer 36 into the groove 24, the sleeve 4 having previously been inserted into the outer cylinder 1 so that the sleeve 4 can then be pulled upwardly relative to the securing ring 23 for installation. As detail A shows with especial clarity, the above-explained and illustrated construction of the annular grooves 24 and 25 and their mutal association ensures locking, as soon as excess pressure above that of the outer atmosphere exists in the cylinder interior, because then this pressure acts first upon the piston head, the active force being \ obtained from the product of the piston head area 27 and the gas pressure obtaining in the cylinder interior.In the assembled state, with internal pressure, the upper face of the piston head 7, that is the piston head rear face 28 constituting an annular area, bears against the annular area formed by the inwardly projecting sleeve flange 6. The force acting upon the piston and attempting to press it outwards is transmitted in the same direction onto the sleeve 4, and thus a pressure action is exerted upon the securing ring 23 by the sleeve 4, but the securing ring 23 is prevented by the circumferential groove 24 from escaping upwards. The sleeve 4 is also acted upon by another force, which is given by the product of the internal gas pressure and the area from the difference of the internal and external dia meters of the sleeve and which acts in the same direction as the force transmitted by the piston 8.The sleeve 4 and thus also the piston 8 are therefore self-securingly and sealin gly held and locked to the outer cylinder 1 due to the internal pressure of the cylinder piston unit, and cannot be dismantled so long as an above-atmospheric pressure is present inside the cylinder. Additional measures for this locking are basically not required.

The foregoing description of the construc tion of the cylinder-piston unit of this invention shows that it is a fully closed, "autono mous" gas spring system, in which the sleeve 4 together with the installed piston 8 is intro duced from the upper end, from which the free end of the piston rod 9 projects, and is then locked by the holding and securing ring 23.

For filling the cylinder-piston unit with pre ferably nitrogen gas, and for monitoring the gas pressure in the cylinder interior, a high pressure filling valve 29 with screw plug 31 recessed into the base 2 in such a manner that it is accessible from the side.

The aforementioned communication between the piston head rear face 28 and the' atmos phere, for the purpose of sucking external air during advancing of the piston 8 into the cyl inder, is obtained in the example shown by an air duct 32, which passes through the wall of the outer cylinder 1 in the region of the upper mounting of the sleeve 4 and possesses, in this region, a filter insert 33, which ensures as the environmental air is drawn in that no dirt particles are sucked into the interior.Inside, this air duct 32 leads into an annular groove 34 in the outer face of the sleeve flange 6, from which a plurality of air ducts 35, distri buted in a star pattern around the circumfer ence, lead out, these air ducts terminating in the interior of the sleeve 4 in a region in which the rear face of the piston head bears against the upper sleeve flange 6 in the fully extended position.

The foregoing description shows that an autonomous system has been created, which not only fulfills the important requirement for satisfactory functioning in respect of the pres ence of a sufficiently large storage volume in the cylinder interior, but which also possesses other important properties which ensure long working life and especially a high working fre quency.

The required storage volume is obtained by the optimum creating of hollow spaces in the cylinder interior, including particularly the annu lar space 5 and the piston rod cavity 12. By the association of these hollow spaces with each other and with further spaces or dis placements of volume resulting from the movement of the piston 8, assurance is fur thermore provided that, in contrast to the state of the art, the heat of compression produced in operation of the gas spring can be rapidly dissipated. This is achieved firstly especially by the relatively small wall thicknesses of the sleeve 4, which may be up to five times thinner than the outer cylinder wall.

When the piston 8 is pushed into the outer cylinder, an annular space is produced above the piston head rear face 28, into which space external atmosphere, that is fresh air, flows via the air duct system 32, 34, 35. This air is rapidly heated through the relatively thin wall of the sleeve 4, the heat of friction created between the piston seals 15, 16 and 17 and the inner face of the sleeve 4 being very rapidly absorbed by the incoming air.

Heat produced by the compression and the resultant gas transportation can also be rapidly removed in this region, because the pressure rise propagates into the annular space 5 via the passages 14 in the lower sleeve flange 13, so that heat produced in this region can be dissipated outwardly via the outer cylinder 1 and also via the sleeve 4 into the annular space created above the piston rear face 28 as the piston enters. In the return stroke of the piston, the then heated external air is discharged via the duct system 32, 34 and 35 to atmosphere. In addition, heat exchange also takes place in the region of the piston rod chamber 12, because filling gas compressed and therefore heated in there gives up its heat via the piston rod wall into the annular space created as the piston advances and filled with surrounding air.

So long as the piston 8 is in the upper limiting position shown, the sleeve 4 is surrounded below the sealing element 21 on all sides by the filler gas, that is subjected to the same pressure on both sides of its wall, so that the sleeve wall in this situation is hardly subjected to any load because the forces exerted by the gas pressure on the inner and outer faces of the sleeve largely cancel out. If, however, the piston is in the lower limiting position, a force resulting from the gas pressure then existing on the external surface of the sleeve acts upon the outer face of the sleeve wall and loads this wall in pressure, so that the sealing effect between the piston head 7 and the inner face of the sleeve 4 guiding it is at least not reduced but indeed is increased.The cross-section of the sleeve wall is so designed and the material chosen is such that this pressure difference existing in the advanced position of the piston can be accepted without adverse deformations.

Not only does the cylinder-piston unit of this invention thus constitute an autonomous system, but by it the result is achieved that adverse heat effects due to rapid upward and downward movements of the piston are advantageously and reliably avoided, in that suction and expulsion of environmental air lead to a self-cooling effect in especially critical regions of the interior of the cylinder-piston unit.

The optimum utilisation of volume and at the same time favourable spatial arrangement of these hollow spaces furthermore leads to an especially high basic load-bearing capacity.

The stability of the assembled unit and tightness are guaranteed especially by the already explained manner of fitting the safety ring 23.

Finally, by the filter unit 33 assurance is provided that even in very dusty surroundings no dust particles can penetrate into the interior of the cylinder-piston unit. Since after approximately one million working strokes in highly dust laden surrounding air, a check of the cylinders for dirt is usually in any case carried out for reasons of safety, the filter unit 33 and if necessary seals, such as the rings 15, 16 and 17 provided on the piston head 7, can be changed at the same time.

Claims

1. A cylinder-piston unit for a compressible pressure medium comprising a piston reciprocatingly guided within an outer cylinder which is closed at one end, characterised by provision for a rear scavenging with external atmosphere as the piston progressively penetrates into the outer cylinder.

2. A cylinder-piston unit according to claim 1, in which the piston includes an at least partly hollow piston rod opening through a piston head into the outer cylinder.

3. A cylinder-piston unit according to claim 1 or claim 2, in which a double wall extends at least partly along the length of the outer cylinder and is separated therefrom by a hollow space for the compressible pressure medium.

4. A cylinder-piston unit according to claim 3, in which the double wall extends through the length of the piston stroke to serve internally as a piston guide.

5. A cylinder-piston unit according to any preceding claim, in which there is a cooling and guide sleeve creating at least one hollow space between itself and the outer cylinder.

6. A cylinder-piston unit according to claim 5, in which the cooling and guide sleeve is relatively thin-walled.

7. A cylinder-piston unit according to claim 6, in which the ratio of the wall thickness of the cooling and guide sleeve to that of the outer cylinder has a value up to 1:5.

8. A cylinder-piston unit according to claim 7, in which said wall thickness ratio is in the range 1:3 to 1:4.

9. A cylinder-piston unit according to any one of claims 5 to 8, in which an end of the cooling and guide sleeve further from the closed end of the outer cylinder serves internally as a stop for the return stroke of the piston and as a guide neck for the piston.

10. A cylinder-piston unit according to any one of claims 5 to 9, in which an end of the cooling and guide sleeve further from the closed end of the outer cylinder is mounted at the end of the outer cylinder further from the closed end thereof.

11. A cylinder-piston unit according to any one of claims 5 to 10, in which there is a form-fitting locking of the cooling and guide sleeve and the outer cylinder.

12. A cylinder-piston unit according to claim 11, in which the locking comprises a ring having holding and securing functions, which is retained on the one hand in a circumferential groove formed in the inner wall of the outer cylinder and on the other hand in a circumferential groove formed in the outer wall of the cooling and guide sleeve.

13. A cylinder-piston unit according to claim 12, in which the circumferential groove formed in the outer cylinder is furnished with a bevel towards the closed end of the outer cylinder.

14. A cylinder-piston unit according to claim 12 or claim 13, in which the circumferential groove formed in the cooling and guide sleeve has no groove wall away from the closed end of the outer cylinder.

15. A cylinder-piston unit according to any one of claims 5 to 14, in which an end of the cooling and guide sleeve nearer the closed end of the outer cylinder has a peripheral apertured flange whose outer wall bears against the inner wall of the outer cylinder.

16. A cylinder-piston unit according to any preceding claim, in which there is a connection between the rear of the piston and the external atmosphere to provide for scavenging at a rear face of a piston head and at a part of a piston rod penetrating into the outer cylinder.

17. A cylinder-piston unit according to claim 16, in which there is at least one air duct in communication with the external atmosphere through the outer cylinder and/or the cooling and guide sleeve if present.

18. A cylinder-piston unit according to claim 17 and any one of claims 5 to 15, in which the air duct or ducts pass through the outer cylinder and the cooling and guide sleeve at or near those ends thereof further from the closed end of the outer cylinder.

19. A cylinder-piston- unit according to claim 18, in which the air duct or ducts passing through the outer cylinder lead into an annular groove in the outer wall of the cooling and guide sleeve, and the air ducts passing through the cooling and guide sleeve lead out from said annular groove in a star pattern.

20. A cylinder-piston unit according to claim 19, in which the star pattern is uniformly distributed circumferentially around the cooling and guide sleeve.

21. A cylinder-piston unit according to any one of claims 17 to 20, in which there is at least one filter in the or each of the air ducts.

22. A cylinder-piston unit according to claim 21, in which the or each of the filters is carried by the outer cylinder.

23. A cylinder-piston unit according to any preceding claim, in which the inner wall of the outer cylinder at that end thereof furthest from the closed end of the outer cylinder is chamfered.

24. A cylinder-piston unit according to any preceding claim, in which the closed end of the outer cylinder includes a base integrally connected with the outer cylinder.

25. A cylinder-piston unit according to any preceding claim, in which the closed end of the outer cylinder includes a pressure relief safety device.

26. A cylinder-piston unit according to any preceding claim, in which the closed end of the outer cylinder includes a charging valve.

27. A cylinder-piston unit according to claim 26, in which the charging valve is a high pressure filling valve with a screw plug.

28. A cylinder-piston unit substantially as hereinbefore described with reference to the accompanying drawing.