GB1592306A

GB1592306A - Hydraulic control cylinders

Info

Publication number: GB1592306A
Application number: GB45604/77A
Authority: GB
Original assignee: Francaise du Ferodo SA
Current assignee: Valeo SE
Priority date: 1976-11-03
Filing date: 1977-11-02
Publication date: 1981-07-01
Also published as: ES463767A1; DE2749268C2; IT1117683B; SE7712390L; BR7707308A; SE432812B; DE2749268A1; JPS5357344A

Description

(54) IMPROVEMENTS IN HYDRAULIC CONTROL CYLINDERS (71) We, SOCIETE ANONYME FRANCAISE DU FERODO, a French Body Corporate, of 64 Avenue de la Grande Armee, 75017 Paris, France, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates generally to hydraulic control cylinders, and concerns more particularly cylinders of this kind which participate in a hydraulic clutch control system of the type suitable for equipping an automobile vehicle.

As is known, a hydraulic clutch control system usually comprises a first cylinder, known as the master cylinder, in which the piston is subjected to the action of an appropriate control means, such as a clutch pedal, a second cylinder, known as the slave or operating cylinder, in which the piston is adapted to control a suitable operating element, such as a clutch release fork, and a pipe circuit connecting the master cylinder to the operating cylinder.

As compared with conventional cable type mechanical clutch control systems, hydraulic clutch control systems offer undeniable advantages of better efficiency due to a reduction of the friction involved, and of easier installation because of the ease with which the piping connecting the master cylinder to the operating cylinder can be placed in position.

In practice, however, their use has been restricted by the difficulty of achieving fluidtightness of the hydraulic system of which they form part.

In the conventional manner this fluidtightness is usually achieved with the aid of seals.

Nevertheless, despite the care taken in their construction, in practice these seals often give rise to leaks which entail frequent overhaul of the hydraulic control system in which they are fitted.

Moreover, in order to function correctly these seals require to have accurate guidance of the piston with which they are associated, and this, apart from the friction resulting therefrom, leads to relatively expensive systems.

Finally, these seals themselves give rise to friction, which is all the more substantial because in order to be able to withstand a high pressure without leaking they are necessarily clamped relatively firmly on the piston on which they are fitted, and this friction, which is added to that due to the accurate guidance of the piston, is harmful to the efficiency of the whole system.

The present invention has the general object of reducing or eliminating these disadvantages.

It is based on the use-which is known per se-of a deformable wall to effect the sealing of the hydraulic chamber of a cylinder.

According to the present invention, there is provided a hydraulic cylinder comprising a hollow body having a bore, a piston mounted for movement in the bore and defining a hydraulic pressure chamber in the hollow body, a deformable wall connected between the piston and the hollow body and sealingly closing the hydraulic chamber, and at least one radially extending annular guide bearing surface on the piston disposed inside the hydraulic chamber, a substantial annular clearance being provided between the guide bearing surface and the bore of said hollow body permitting the piston to move with clearance in the bore.

Thus, the piston guide surfaces capable of giving rise to friction are immersed in the fluid, which in practice is oil, present in the hydraulic chamber of the cylinder, this arrangement being such as to minimise this friction and therefore to improve substantially the efficiency of the entire system, the efficiency having already been increased by the use of a deformable wall to replace the use of seals.

Furthermore, the substantial annular clearance provided between the guide bearing surface of the piston and the bore of the hollow body in which this guide bearing surface slides makes use of the ability to incline the piston relative to the axis of the bore thanks to the deformable wall used, since the latter does not require accurate guiding of the piston.

By "substantial clearance" is here understood clearance between for example 0.5 and 2% of the corresponding average transverse dimension of the piston.

The originality of this clearance will be better realised if it is emphasised that, other conditions being the same, the value of this clearance is usually lower than 0.1% of the diameter of the corresponding piston.

The substantial clearance according to the invention advantageously permits an additional reduction of friction which may occur between the guide bearing surface of the piston and the bore of the hollow body in which the bearing surface is mounted for sliding.

In addition, it advantageously makes it possible to accept a relatively rudimentary and therefore economical construction of the piston, which may for example be a casting, without any finish machining being subsequently required.

Finally, the cylinder according to the invention is advantageously utilised to form at least either the master cylinder of the operating cylinder of a hydraulic clutch control system.

The deformable wall in the cylinder according to the invention may be composed of a simple diaphragm, for example a diaphragm made of a flexible material, such as rubber, enabling it to assume a U-shaped configuration in cross-section.

In the development of cylinders of this kind it has been found that for certain particular applications, especially for clutch control systems in which the operating pressures may exceed 25 bars, the elongation undergone because of such pressures by a diaphragm of flexible material could attain values which are incompatible with the other requirements with which these cylinders must comply, particularly with regard to the volume of fluid which it is necessary to displace in order to effect operation.

According to a development of the invention, provision is made in this case for the defor.mable wall to comprise a bellows made of a material having a low elongation rate, for example metal, or woven or nonwoven wire gauze rendered fluid-tight in suitable manner, for example by means of synthetic material.

Apart from the fact that a bellows of this kind is able to withstand a high pressure with minimum elongation of its wall, it has in itself a significant axial elasticity, whereby the advantage is gained that it is consequently possible to dispense with the elastic means usually employed to return the piston in question to a determined position of rest.

Other conditions being equal, the advantage is thereby gained that the number of parts necessary to form the cylinder of the invention is reduced.

In order that the bellows used in this cylinder may thus always be able to act as return means for the piston to which it is connected, provision is preferably made for this bellows to be subjected when at rest to compressive prestressing sufficient to ensure that during operation it remains under compression for at least part of its development.

Such compressive prestressing is found to correspond advantageously to working conditions which are particularly favourable for the bellows to which it is applied, although slight tension is acceptable for it.

The characteristics and advantages of the invention will moreover be clear from the description given below by way of example and with reference to the accompanying diagrammatical drawings, in which: Figure 1 is a view in axial section of a master cylinder for a hydraulic clutch control system; Figure 2 is an end view of this master cylinder in the direction of the arrow II in Figure 1; Figure 3 is another partial view thereof in axial section on the line Ill-Ill in Figure 2; Figure 4 shows by itself the piston installed in a master cylinder of this kind, in a cross-section taken on the line IV-IV in Figure 1; Figure 5 shows on a larger scale a detail of Figure 1, namely the inset V in that Figure; Figure 6 is a view in axial section of another operating cylinder for a hydraulic clutch control system, taken on the line VI-VI in Figure 7; Figure 7 is a view in cross-section of this operating cylinder, taken on the line VIl-VIl in Figure 6; Figure 8 is a partial view thereof in axial section on the line VIlI-VIlI in Figure 7; Figure 9 is a similar view to that in Figure 6 and relates to a modified embodiment; Figure 10 is a similar view to that shown in Figure 1 and relates to a modified embodiment; Figure 11 is a view in axial section of another master cylinder for a hydraulic clutch control system; Figures 12 and 13 show on a larger scale details of construction constituting respectively the insets XII and XIII in Figure 11; Figure 14 is a similar view to that in Figure 13, relating to a modified embodiment; Figure 15 is a view in axial section of another operating cylinder for a hydraulic clutch control system; Figure 16 shows on a larger scale a detail of construction corresponding to an inset XVI in Figure 15; Figure 17 is a similar view to that shown in Figure 15 and relates to a modified embodiment; Figure 18 shows on a larger scale a detail of this modified embodiment corresponding to an inset XVIII in Figure 17; Figure 19 is a similar view to that of Figure 18 and relates to a modified embodiment.

In the embodiment illustrated in Figures 1 to 5 the cylinder according to the invention, which is more particularly intended to constitute a master cylinder for a hydraulic clutch control system, comprises a hollow body 10 of synthetic material moulded over a length of metallic tubing 11 which forms therein an internal bore 12.

At one of its ends, which is closed by an end member 13, this hollow body 10 is provided externally with a connector 14 adapted to bring its internal bore 12 into communication with a suitable pipe.

In irs median zone the hollow body is provided with a compensation chamber 16 adapted to permit communication between its internal bore 12 and a source (not shown) of hydraulic liquid, which in practice is oil, under the control of a valve 17 whose stem 18 projects slightly into the interior of the said internal bore 12, after the style of the arrangements usually adopted for the master cylinders of hydraulic brake control systems.

These arrangements being well known per se, and not forming part of the present invention, they will not be described in detail here.

At its opposite end, which is open, to that where the connnector 14 is disposed the hollow body 10 is provided externally with two radial legs 20 forming lugs 21 adapted to fasten it to a suitable support.

In the internal bore 12 of the hollow body 10 a piston 23 is movably mounted, and this piston, in conjunction with on the one hand the end member 13 and on the other hand a deformable wall 24 connecting it annularly to the hollow body 10 in accordance with the arrangements described in detail hereinbelow, forms therein a hydraulic pressure chamber 25 which by means of the connector 14 can be connected to appropriate piping and which the deformable wall 24 closes sealingly.

In the embodiment illustrated in Figures 1 to 5, the piston 23 is a generally tubular part of synthetic material and has in its interior a transverse wall 27 which closes it.

This transverse wall 27 is generally curved and, with its concavity turned towards the outside, it forms by itself a pivotable support point for an operating rod 28 diagrammatically shown in broken lines in Figure 1, this rod being intended to permit the operation of the master cylinder in question by the associated operating means, which in practice is a clutch pedal; this operating rod 28 is thus adapted to act at the centre of thrust of the piston 23.

Inside the hydraulic chamber 25 the transverse wall 27 of the piston 23 serves as support for a return spring 29 which at the other end bears against the end member 13 of the body 10 and thus permanently urges the piston 23 in the opposite direction to the said end member 13.

In the embodiment illustrated in Figures 1 to 5 the deformable wall 24 is composed of a flexible diaphragm whose section has the general shape of a recumbent U, whose concavity is turned towards the end member 13 and which on its internal periphery has a bead 30 by which it is engaged in a groove 31 in the piston 23.

A metal hoop 32 disposed annularly around the piston 23 in a position corresponding to its groove 31 ensures the effectiveness of the connection of the deformable wall 24 to this piston 23.

On its outer periphery the diaphragm constituting this deformable wall 24 has a bead 34 by which it is gripped between two parts, one of which is the hollow body 10, which for this purpose is provided, some distance from its outlet 35 leading to the outside, with a transverse shoulder 36 adapted to support the said bead 34.

In the embodiment shown in Figures 1 to 5, the other part effecting the gripping of the outer periphery of the diaphragm constituting the deformable wall 24 is a protective cover 37 engaged in the open end of the hollow body 10 and suitably fastened to the latter.

In the embodiment shown in Figures 1 to 5, the cover 37 is fastened to the hollow body 10 by a ~ detest action; the cover 37, which is of synthetic material, has on its outer periphery an annular rib 38 (Figures 1 and 5) of generally trapezoidal section, this rib being intended to engage in a groove 39 of complementary section provided for the purpose in the inner periphery of the open end of the hollow body 10.

In order to facilitate the engagement of the cover 37 in the hollow body 10 the outlet 35 of the hollow body 10 leading to the outside is internally bevelled, and similarly the leading edge of the cover 37 is bevelled externally.

When the cover 37 is clipped into the body 10, the two parts involved, namely the cover 37 and the hollow body 10, are necessarily elastically deformed to permit the passage of the rib 38 of the cover 37, until this rib reaches the groove 39 in the hollow body 10.

The end member 40 of the cover 37 has an opening 41 for the passage of the operating rod 28 and, around this opening, it serves by itself as rest abutment for the piston 23, against which abutment the piston 23 is urged into contact by the return spring 29 associated with it.

In the usual manner the piston 23 is in addition equipped transversely with at least one annular guide bearing surface.

In accordance with a characteristic arrangement of the invention this guide bearing surface is disposed inside the hydraulic chamber 25 and is therefore immersed in the liquid contained in that chamber.

In the embodiment illustrated in Figures 1 to 5 the piston 23 is in practice equipped with two cylindrical guide bearing surfaces disposed some distance from one another, namely a first bearing surface 43 which is disposed at the free end of the piston 23 which is nearer the interior, and a second bearing surface 44 which is disposed substantially in a position corresponding to the transverse wall 27 with which this piston is provided, this bearing surface in the example illustrated being divided into two unitary bearing surface elements 44A, 44B close to one another.

From place to place the guide bearing surfaces 43, 44 of the piston 23 are provided circularly with longitudinal grooves 45 suitable for the axial circulation of fluid in the hydraulic chamber 25 on each side of the said bearing surfaces.

A substantial annular clearance J is provided between each guide bearing surface 43, 44 of the piston 23 and the bore 12 of the hollow body 10 (Figure 4).

As explained above, this clearance is in practice between 0.5% and 2% of the corresponding mean transverse dimension of the piston 23, that is to say between 0.5% and 2% of the diameter of the piston in the case of a cylindrical piston of circular section of the type illustrated.

The clearance J advantageously assists a reduction of the friction between the guide bearing surfaces 43, 44 of the piston 23 and the internal bore 12 of the hollow body 10, which friction is already low because the guide bearing surfaces in question move in a liquid, which in practice is oil.

In addition, the clearance J permits a relatively rudimentary and therefore economical construction of the piston 23, for example by casting.

This clearance is however made sufficiently small to ensure that, also taking into account the distance between the guide bearing sur- faces 43, 44 with which it is associated, the inclination of the piston 23 relative to the axis of the bore permitted by this clearance will not be of such a nature as to bring about any pinching of the diaphragm constituting the deformable wall 24 which would be prejudicial to the life of the latter, for example in the event of accidental misalignment of the piston 23, particularly when the arrangement is stored before being installed.

In practice, and as illustrated in Figure 1, the stem 18 of the valve 17 extends radially between the bearing surfaces 43 and 44 of the piston 23, and is therefore interposed on the path followed by the bearing surface 43 during the return of the piston 23 to its rest abutment, so that in its position of rest, as illustrated in Figure 1, the piston 23 brings about the rocking of the valve 17 into the open position.

In this position of rest the hydraulic chamber 25 and the utilisation circuit which it serves are therefore systematically filled with fluid.

When under the action of the operating rod 28 the piston 23 moves away from its rest abutment, in the direction of the arrow F in Figure 1, the valve 17 freed by the bearing surface 43 of the piston 23 rocks into the closed position, and the piston 23 forces the liquid present in the hydraulic chamber 25 through the connector 14 in the direction of the utilisation circuit served.

Through its pivotable support on the piston 23, in conjunction with the ability of this piston to be inclined on its axis, the operating rod 28 can freely follow the pivoting movement of the operating element, which in practice is a clutch pedal, to which it is connected.

Taking into account the reduced internal friction, which has been reduced not only because of the utilisation of a deformable wall 24 to effect the sealing of the hydraulic chamber 25 but also because of the arrangements which according to the invention.en- able the friction of the piston 23 to be reduced, and taking into account the fact that the pressure in the hydraulic chamber 25 essentially applies its action to the deformable wall 24, thereby further reducing the consequences of any friction of the piston 23, the cylinder according to the invention has excellent efficiency, that is to say a substantial proportion, in practice almost all, the energy applied to its piston 23 is available at the outlet of its connector 14.

In practice, for example in the case of a hydraulic clutch control system, a master cylinder of this kind is associated with a "slave", or operating, cylinder adapted to operate any actuating element, for example a clutch release fork.

An operating cylinder of this kind is shown in Figures 6 to 9.

Its composition is generally similar to that of the previously described master cylinder.

It therefore comprises a hollow body 50 equipped on the one hand with radial feet forming lugs 51 for fastening it, and on the other hand a connector 52 for its connection to the corresponding master cylinder.

In addition, in the example illustrated the hollow body 50 of the operating cylinder according to the invention is equipped with a drain connection 53.

In the hollow body 50 a hydraulic chamber 55 is formed, which is in communication with the connector 52 and is sealingly closed by a deformable wall 56 disposed annularly between the said hollow body and a piston 57 mounted for movement axially of the body.

As previously, this piston 57 is equipped transversely with an annular guide bearing surface.

In the example shown this bearing surface is a generally spherical guide bearing surface disposed at the inner end of the piston 57, and this generally spherical guide bearing surface is therefore, as previously, disposed inside the hydraulic chamber 55.

In practice, and as illustrated, this spherical guide bearing surface 58 is engaged in a socket 60 of the hollow body, projecting from the end member of the latter, that is to say on the transverse wall 62 of the hydraulic chamber 55 at the opposite end to the deformable wall 56.

As previously, the latter is composed of a diaphragm.

On its inner periphery this diaphragm is gripped between two annular cups 63, 64 which are disposed back to back, that is to say with their concavities turned in opposite directions on each side of the diaphragm, and which are simply engaged by force on bearing surfaces 66, 67 of the piston 57, preferably having the same diameter.

The cup 63 which is at the greater distance from the end member 62 of the hydraulic chamber 55 bears against a transverse shoulder 68 of the piston 57 which delimits the corresponding bearing surface 68 of the latter, while the cup 64 which is nearer the end member 62 serves as support for the return spring 69 which, as previously, is associated with the piston 57 and thus participates in the gripping of the diaphragm constituting the deformable wall 56.

The bearing surfaces 66, 67 of the piston 57, on which surfaces the cupls 63, 64 are engaged, have formed therebetween a groove 70 serving to receive a bead 72 forming the inner periphery of the said diaphragm.

The cup 64 which is situated on that side of the diaphragm constituting the deformable wall 56 which corresponds to the hydraulic chamber 55 has generally a larger diameter than that of the cup 63 which is situated on the other side of the said diaphragm, thereby limiting the ability of the piston 57 to incline relative to its axis in the aligned position and therefore opposing any gripping of this diaphragm.

In an arrangement similar to that described above, the diaphragm constituting the deformable wall 56 is gripped on its outer periphery between the hollow body 50 and another member which in the example illustrated in Figures 6 to 8 is a sleeve 74, made for example of metal or synthetic material, which is fastened by a detent action to the hollow body 50, as described above, and for this purpose is engaged in the open end of the said hollow body.

At its outer end the sleeve 74 has a radial collar 75 which projects towards the outside and which permits the fastening to it of a dust guard 76, which is also engaged in a groove 77 in the piston 57.

In dependence on the liquid injected into the hydraulic chamber 55, the piston 57 is driven outside the hollow body 50, and by its free end it accordingly controls the operating element, which in practice is a clutch release fork, to which it is connected, its pivotal mounting in the hollow body 50 enabling it to follow by itself the displacement of this operating element.

In the modified embodiment illustrated in Figure 9 the hollow body 50 is of metal and the sleeve 74 associated with it for the gripping of the outer periphery of the diaphragm constituting the deformable wall 56 is fastened to the said hollow body by crimping.

For example, as illustrated, the hollow body 50 has at its open end a lip 78 which is folded by crimping over a collar 79 forming the corresponding end of the sleeve 74.

Figure 10 illustrates a modified embodiment of a master cylinder according to the invention which is more particularly suitable for cases where the piston 23 of this cylinder is rigidly coupled, for example by screwing, as illustrated, to the operating rod 28 with which it is associated.

In this case, as illustrated, this piston 23 is provided inside the hydraulic chamber 25 with only one annular guide bearing surface 43, which is generally spherical in order to facilitate a limited pivotable movement of the piston 23 in the hollow body 10.

As previously - and moreover this also applies to the generally spherical guide bearing surface 58 of the piston 57 of the operating cylinders previously described - substantial annular clearance is provided between the generally spherical guide bearing surface and the bore in which it is slidingly mounted.

In the example shown in Figure 10 the hollow body 10 is made in one piece, for ex ample of metal, and it is crimped on the protective cover 37 engaged in its open end for the purpose of gripping the outer periphery of the deformable diaphragm 24.

The other constructional arrangements for this modified embodiment are otherwise similar to those described with reference to Figures 1 to 5.

As will have been noted, when che guide bearing surface of the piston is generally spherical, as illustrated in Figures 6, 9, and 10, it is advantageously disposed at the inner free end of this piston, and therefore at the greatest possible distance from the other end of this piston by which the latter cooperates with the operating or actuating element with which it is associated, thereby advantageously minimising the inclination which this piston has to assume in order to follow the movement of the element in question.

Figure 11 shows a master cylinder com prising a hollow body 10 of synthetic material moulded over a length of metal tubing 11 which forms therein an internal bore 12 with which a connector 14 and a chamber 16 are in communication.

A piston 23, which in the example illustrated is of metal, is mounted for movement in this internal bore 12.

Conjointly with on the one hand the hollow body 10 and on the other hand a deformable wall 24 connecting it annularly to the said hollow body, the piston 23 defines in this hollow body 10 a hydraulic pressure chamber 23.

As described albove, the piston has on its outer periphery two guide bearing surfaces 43, 44, spaced apart from one another, in the interior of the hydraulic chamber 25.

In the example illustrated the deformable wall 24 is composed of a bellows made of a material having a low elongation ratio, for example a metal.

A bellows of this kind is well known per se; it may have a single wall or multiple walls and/or be in one piece or composed of separate turns suitably connected successively two by two, for example by welding.

In the embodiment illustrated the bellows constituting the deformable wall 24 has at one end a radial collar 80 directed away from its axis, by means of which it is gripped between two elements, namely on the one hand the hollow body 10 and on the other hand a protective cover 37 which is attached to the hollow body 10, by a detent action in the example illustrated, as described above, and which is therefore axially secured to the said hollow body 10.

At its other end this bellows ends in a generally cylindrical bearing surface 81 by which it is secured to the piston 23, for ex ample by brazing, welding, adhesive bonding, screwing, crimping, or other means.

As illustrated in Figure 11 when at rest the bellows constituting the deformable wall 24 is preferably under sufficient initial compressive stress for it to hold the piston 23 in contact with the base of the protective cover 37.

Consequently, during operation the bellows always remains in compression.

In the modified embodiment illustrated in Figure 14 the piston 23 is of synthetic material and at the end by which the bellows constituting the deformable wall 24 has to be secured to this piston, the cylindrical bearing surface 81 of the bellows is gripped between the said piston 23 on the one hand and a holding ring 83 attached to the said piston on the other hand.

In the example illustrated this ring 83 is provided annularly with a slot 84 by which it is engaged on the corresponding end face of the piston 23 and it is fastened to the latter by a detent action, in accordance with the methods already described above; either the piston 23 or the ring 83 is provided annularly with a groove in which is engaged a rib provided for this purpose in complementary manner, projecting from either the cover 83 or the piston 23; as illustrated, the cylindrical bearing surface 81 of the bellows constituting the deformable wall 24 is held between this projection and this detent groove, but this arrangement is not indispensable.

In Figure 15 an operating cylinder can be seen.

As described above, the base of the hollow body 50 of this operating cylinder carries a projecting socket 60 adapted to guide a piston 57, which for this purpose is provided at its end with a generally spherical guide bearing surface 58.

As previously, the deformable wall 56 sealingly closing the hydraulic chamber 55 formed in the hollow body 50, which hydraulic chamber is in addition in communication with a connector 52, is a bellows made of a material having a low elongation ratio, preferably of metal.

In the example illustrated the hollow body 50 is composed 88 in Figure 16; as an alternative a simple sealing material, such as resin, may also be used. At its other end the bellows constituting the deformable wall 56 has a radial collar 89 which is directed towards its axis and by which it is gripped between a shoulder 90 on the piston 57 on the one hand and, on the other hand, a washer 91 suitably attached to the said piston 57, for example by a force-fit or by crimping.

As previously, this bellows is when at rest under initial compressive stress, the latter being made sufficient to ensure that during operation it always remains in compression.

In the modified embodiment illustrated in Figures 17 and 18 a bellows of this kind is lined internally by a sleeve 92, towards which it is urged by the pressure of the hydraulic chamber 55 which it closes.

This sleeve 92, which is for example of metal, is fastened to the piston 57 at one end.

For example, as illustrated, the sleeve 92 has for this purpose a portion 93 of reduced diameter, by which it is engaged by force on the piston 57 to bear against a rounded shoulder 94 on the latter.

At its other end the sleeve is secured to the bellows with which it is associated.

For example, and as illustrated in Figures 17 and 18, this bellows has for this purpose a projecting radial collar 95 which is directed towards its axis and by which it is secured to the corresponding end face of the sleeve 92, for example by brazing or adhesive bonding.

At its other end the bellows constituting the deformable wall 56, which extends generally between the sleeve 92 and the hollow body 50, around the said sleeve 92, is secured to the said hollow body 50.

For example, and as illustrated, it is provided for this purpose at its corresponding end with a radial collar 96 which projects away from its axis and by which it is gripped between the hollow body 50 on the one hand and, on the other hand, a tubular extension 97 axially fastened to the latter, for example by a detent action, as described above.

Obviously the sleeve 92 has in its normal portion a diameter sufficient to enable it to be engaged without friction over the guide socket 60 with which the hollow body 50 is provided for guiding the piston 57.

In the modified embodiment illustrated in Figure 19 the bellows constituting the deformable wall 56 is fastened, for example by brazing or adhesive bonding, by means of an annular bearing surface 98 to the corresponding end of the sleeve 92.

The present invention is not limited to the embodiments described and illustrated, but includes any alternative embodiment and/or combination of their various elements within the scope of the appended claims.

In particular, when the deformable wall is in the form of a bellows, it is of little importance whether the elements between which the bellows is optionally gripped at one of its ends belong to the hollow body or to the piston of the corresponding cylinder.

Furthermore, any material having a low elongation ratio other than a metal, for ex ample a synthetic material, may also be used to form a bellows of this kind; a woven or nonwoven metal gauze, suitably sealed for example by means of synthetic material, may also be suitable Finally, although it is preferable for the bellows always to work in compression, it is also accepltable for it to work in slight tension, for example with an elongation lower than 5% in relation to its configuration at rest.

WHAT WE CLAIM IS:- 1. A hydraulic cylinder comprising a hollow body having a bore, a piston mounted for movement in the bore and defining a hydraulic pressure chamber in the hollow body, a deformable wall connected between the piston and the hollow body and sealingly closing the hydraulic chamber, and at least one radially extending annular guide bearing surface on the piston disposed inside the hydraulic chamber, a substantial annular clearance being provided between the guide bearing surface and the bore of said hollow body permitting the piston to move with clearance in the bore.

2. A hydraulic cylinder according to Claim 1, wherein said annular clearance is between 0.5% and 2% of the corresponding average transverse dimension of the piston.

3. A hydraulic cylinder according to Claim 1 or Claim 2, wherein the guide bearing surface of the piston is cylindrical.

4. A hydraulic cylinder according to Claim 3, wherein the piston has two cylindrical guide bearing surfaces spaced apart from one another.

5. A hydraulic cylinder according to Claim 4, wherein one of the cylindrical guide bearing surfaces of the piston is divided into at least two unitary bearing surface elements.

6. A hydraulic cylinder according to Claim 1 or Claim 2, wherein the piston has only one guide bearing surface and the latter is generally spherical.

7. A hydraulic cylinder according to Claim 6, wherein the bore of said hollow body is formed in a portion of the hollow body which projects from the transverse wall of the hydraulic chamber opposite to that where the deformable wall is located.

8. A hydraulic cylinder according to Claim 6 or Claim 7, wherein the generally spherical guide bearing surface of the piston is disposed at the inner end of the piston.

9. A hydraulic cylinder according to any of Claims 1 to 8, wherein the deformable wall comprises a diaphragm the outer periphery of which is gripped between two parts one of which consists of the hollow body, the two parts being suitably secured to one another,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

88 in Figure 16; as an alternative a simple sealing material, such as resin, may also be used. At its other end the bellows constituting the deformable wall 56 has a radial collar 89 which is directed towards its axis and by which it is gripped between a shoulder 90 on the piston 57 on the one hand and, on the other hand, a washer 91 suitably attached to the said piston 57, for example by a force-fit or by crimping.

As previously, this bellows is when at rest under initial compressive stress, the latter being made sufficient to ensure that during operation it always remains in compression.

In the modified embodiment illustrated in Figures 17 and 18 a bellows of this kind is lined internally by a sleeve 92, towards which it is urged by the pressure of the hydraulic chamber 55 which it closes.

This sleeve 92, which is for example of metal, is fastened to the piston 57 at one end.

For example, as illustrated, the sleeve 92 has for this purpose a portion 93 of reduced diameter, by which it is engaged by force on the piston 57 to bear against a rounded shoulder 94 on the latter.

At its other end the sleeve is secured to the bellows with which it is associated.

For example, and as illustrated in Figures 17 and 18, this bellows has for this purpose a projecting radial collar 95 which is directed towards its axis and by which it is secured to the corresponding end face of the sleeve 92, for example by brazing or adhesive bonding.

At its other end the bellows constituting the deformable wall 56, which extends generally between the sleeve 92 and the hollow body 50, around the said sleeve 92, is secured to the said hollow body 50.

For example, and as illustrated, it is provided for this purpose at its corresponding end with a radial collar 96 which projects away from its axis and by which it is gripped between the hollow body 50 on the one hand and, on the other hand, a tubular extension 97 axially fastened to the latter, for example by a detent action, as described above.

Obviously the sleeve 92 has in its normal portion a diameter sufficient to enable it to be engaged without friction over the guide socket 60 with which the hollow body 50 is provided for guiding the piston 57.

In the modified embodiment illustrated in Figure 19 the bellows constituting the deformable wall 56 is fastened, for example by brazing or adhesive bonding, by means of an annular bearing surface 98 to the corresponding end of the sleeve 92.

The present invention is not limited to the embodiments described and illustrated, but includes any alternative embodiment and/or combination of their various elements within the scope of the appended claims.

In particular, when the deformable wall is in the form of a bellows, it is of little importance whether the elements between which the bellows is optionally gripped at one of its ends belong to the hollow body or to the piston of the corresponding cylinder.

Furthermore, any material having a low elongation ratio other than a metal, for ex ample a synthetic material, may also be used to form a bellows of this kind; a woven or nonwoven metal gauze, suitably sealed for example by means of synthetic material, may also be suitable Finally, although it is preferable for the bellows always to work in compression, it is also accepltable for it to work in slight tension, for example with an elongation lower than 5% in relation to its configuration at rest.

WHAT WE CLAIM IS:- 1. A hydraulic cylinder comprising a hollow body having a bore, a piston mounted for movement in the bore and defining a hydraulic pressure chamber in the hollow body, a deformable wall connected between the piston and the hollow body and sealingly closing the hydraulic chamber, and at least one radially extending annular guide bearing surface on the piston disposed inside the hydraulic chamber, a substantial annular clearance being provided between the guide bearing surface and the bore of said hollow body permitting the piston to move with clearance in the bore.
2. A hydraulic cylinder according to Claim 1, wherein said annular clearance is between 0.5% and 2% of the corresponding average transverse dimension of the piston.
3. A hydraulic cylinder according to Claim

1 or Claim 2, wherein the guide bearing surface of the piston is cylindrical.
4. A hydraulic cylinder according to Claim 3, wherein the piston has two cylindrical guide bearing surfaces spaced apart from one another.
5. A hydraulic cylinder according to Claim 4, wherein one of the cylindrical guide bearing surfaces of the piston is divided into at least two unitary bearing surface elements.
6. A hydraulic cylinder according to Claim 1 or Claim 2, wherein the piston has only one guide bearing surface and the latter is generally spherical.
7. A hydraulic cylinder according to Claim 6, wherein the bore of said hollow body is formed in a portion of the hollow body which projects from the transverse wall of the hydraulic chamber opposite to that where the deformable wall is located.
8. A hydraulic cylinder according to Claim 6 or Claim 7, wherein the generally spherical guide bearing surface of the piston is disposed at the inner end of the piston.
9. A hydraulic cylinder according to any of Claims 1 to 8, wherein the deformable wall comprises a diaphragm the outer periphery of which is gripped between two parts one of which consists of the hollow body, the two parts being suitably secured to one another,

for example a detent action or by crimping.
10. A hydraulic cylinder according to Claim 9, wherein the other of the parts effecting the gripping of the deformable diaphragm comprises a protective cover which itself serves as an abutment for the piston.
11. A hydraulic cylinder according to Claim 9, wherein the other of the parts effecting the gripping of the deformable diaphragm is a sleeve.
12. A hydraulic cylinder according to any of Claims 1 to 11, wherein on its inner periphery the deformable wall is gripped between two annular cups which are disposed back to back with their concavities turned in opposite directions, and which are force-fitted onto bearing surfaces of the piston, one of these cups bearing on a transverse shoulder on the piston which limits the corresponding bearing surface thereof, while the other serves as support to a return spring associated with the piston.
13. A hydraulic cylinder according to Claim 12, wherein the bearing surfaces of the piston on which are force-fitted the cups gripping the deformable wall frame a groove serving to receive a bead forming the inner periphery of the deformable wall.
14. A hydraulic cylinder according to Claim 12 or Claim 13, wherein the cup which is situated on that side of the deformable wall which corresponds to the hydraulic chamber has a diameter greater than that of the cup which is situated on the other side of the deformable wall.
15. A hydraulic cylinder according to any of Claims 1 to 8, wherein the deformable wall comprises a bellows made of a material having a low elongation ratio, for example metal.
16. A hydraulic cylinder according to Claim 15, wherein when at rest, the bellows is under sufficient initial compressive stress to ensure that during operation it remains in compression for at least part of its development.
17. A hydraulic cylinder according to Claim 16, wherein any tensile elongation of the bellows is limited to at most 5% of its configuration when at rest.
18. A hydraulic cylinder according to any of Claims 15 to 17, wherein at one end the bellows is gripped between two elements which belong respectively to either the hollow body or to the piston and which are axially secured to one another.
19. A hydraulic cylinder according to any of Claims 15 to 18, wherein at one end the bellows is secured to the hollow body or to the piston by adhesive bonding, welding, screwing, brazing, crimping, or other means.
20. A hydraulic cylinder according to any of Claims 15 to 19, wherein the bellows is lined internally with a sleeve towards which it is urged by the pressure of the hydraulic chamber which it closes.
21. A hydraulic cylinder according to Claim 20, wherein at one end the sleeve is fastened to the piston and at its other end it is fastened to one end of the bellows, the other end of which is secured to the hollow body.
22. A hydraulic cylinder according to any of Claims 1 to 21, wherein the hollow body is of synthetic material moulded over a length of metallic tubing which forms its bore therein.
23. A hydraulic cylinder according to any of Claims 1 to 22, comprising either the master cylinder or the operating cylinder of a hydraulic clutch control system.
24. A hydraulic cylinder assembly substantially as hereinbefore described with reference to Figs. 1 to 5, Figs. 6 to 8, Fig. 9, Fig. 10, Figs. 11 to 13, Fig. 14, Figs. 15 and 16, Figs. 17 and 18, or Fig. 19 of the accompanying drawings.