US3731596A

US3731596A - Two-circuit wheel cylinder

Info

Publication number: US3731596A
Application number: US00131195A
Authority: US
Inventors: J Burgdorf
Original assignee: Deutsche ITT Industries GmbH
Current assignee: TDK Micronas GmbH; ITT Inc
Priority date: 1970-04-30
Filing date: 1971-04-05
Publication date: 1973-05-08
Anticipated expiration: 1990-05-08
Also published as: GB1286423A; JPS519857B1; FR2090851A5; DE2021208A1

Abstract

There is disclosed various embodiments of a two-circuit wheel cylinder including two concentric cylinders and two pistons associated therewith. The cylinders and pistons cooperating to form two pressure chambers. The appropriate surfaces of the two pistons are separately sealed to cooperating surfaces of the two cylinders so that a leak in either seal arrangement may be readily detected externally.

Description

[ 1 May 8, 1973 United States Patent Burgdori 2 8 2 mumu mmm l 2 222 95WMU999 2/ 19 i 922 "N n m i I. m M mSa wwwn cyw rho m r APKHTMB 0690 786 66677664 99999999 11111111 ll/l/l/l 7 67402 1 1 11 2 023327 72280 08 09672233 657 9 0 1 23333332 in 0 Y R M 3 mm W N x N I L O c Y n C H .l L 0 E d w 8 .l E r. f 1 H M M 7 u 9 w y 1 n n a T 3 I 5 I Mm U cu TX m. C 0 T R JG IN A I r. e m o e t n O m m. d e W M n 11 1 1 4 5 3 2 5 7 7 2 rt r1 rt Isidore Togut, Edward Goldberg and Menotti J. Lombardi, .lr.

ABSTRACT There is disclosed various embodiments of a two-cir- 14 Claims, 6 Drawing Figures agar/67 ig...

Primary Examiner-Charles J. Myhre Assistant Examiner-R. H. Lazarus AttorneyC. Cornell Remsen, J'r., Walter J. Baum, Paul W. Hemminger, Charles L. Johnson, Jr., Philip M. Bolton,

cuit wheel cylinder including two concentric cylinders and two pistons associated therewith. The cylinders and pistons cooperating to form two pressure chambers. The appropriate surfaces of the two pistons are separately sealed to cooperating surfaces of the two cylinders so that a leak in either seal arrangement may be readily detected externally.

Data

.92/52, 92/61 Int. 1/02 .92/107,' 108, 51.52, 107, 108

[21] Appl. No.: 131,195

[30] Foreign Application Priority Apr. 30, 1970 Germany....................

[52] [58] Field of Search....................

[56] References Cited UNITED STATES PATENTS 3,065,733 11/1962 2,236,266 3/1941 Hewitt............ 2,533,959 12/1950 Rothschild..... 2,604,076 7/1952 Trevaskis 2,766,591 10/1956 PATENIEnHAYmm SHEET 2 BF 3 TWO-CIRCUIT WHEEL CYLINDER BACKGROUND OF THE INVENTION The invention relates to a two-circuit wheel cylinder, in particular for vehicle brakes, with two cylinders concentrically disposed with respect to each other and connected with each other in a manner to form two pressure chambers and two pistons each associated with one of the two pressure chambers. I

It is required that such two-circuit wheel cylinders are to work faultlessly even upon failure of one of the two circuits.

In order to comply with this requirement, it is known to arrange a piston between two cylinders such that the piston is alternatively actuated by two independent pressure chambers (published German patent application No. 1,430,356; French Pat. No. 1,555,573; and British Pat. No. 1,161,701). It is further known (US. Pat. No. 2,766,591) to arrange two pistons disposed within each other in two cylinders disposed in cascade. Because of the basic requirement that as small an axial length as possible be provided with respect to wheel brake cylinder, the two-circuit cylinder as described in the US. Patent has additional guideways. Thus, it consists of six cylindrical rings disposed within one another. Two guide surfaces at the minimum are subject to corrosion due to the bad sealing arrangement.

A further disadvantage of the above cited US. Patent, the published German patent application and the French patent consists in the fact that these arrangements lack the feature of a genuine two-circuit system, i.e., if the sealing arrangement fails which is separating the two pressure chambers an alarm system registers such a failure. Only the British Pat. No.

1,161,701 shows in its FIGS. 4 and 5 afwo circuit wheel cylinder with the characteristic alarm feature of a genuine two-circuit system.

The two-circuit wheel cylinder of the cited British patent, however, has the disadvantage that the manufacture and the assembly of a cup-shaped piston with honed interior and exterior surfaces is quite complicated and expensive. The assembly and the operation are further complicated by the fact that a compen sation of the possible eccentricities of the bores cannot take place. Here, like in the cited published German patent application and in the cited US. patent there is the danger of tilting or jamming, respectively, on the part of the cup-shaped piston.

Finally, the optimal ventilation of the two-circuit system is not safeguarded due to the arrangement of the feeder lines asassessed in FFGS. 4' aria same cited British patent. Foreign matter or air having collected in the pressure chamber near the seals 33 or 32, respectively, can be seized only with difficulties by the pressure medium current in the center of the cylinder because of gravitation or because of buoyancy, respectively.

SUMMARY OF THE INVENTION The present invention has an object to provide a twocircuit wheel cylinder where leakage of one or both circuits can bedetected with ease externally and a simple manufacture and assembly of the individual parts are safeguarded; In addition, upon failure of one of the two circuits, the pressure in the operating chamber of the non-failed circuit is maintained, the force exerted on the brake linings is not changed, i.e., the effective surfaces of the pistons of the two circuits are equivalent.

Further objects of the present invention are to provide good ventilation, to prevent guide surface corrosion or to reduce the number of guide surfaces subject to corrosion and to prevent jamming or tilting of the piston during operation.

These objects are achieved according to the present invention by the fact that the inner pressure chamber is formed by the inner cylinder and a piston, and the outer pressure chamber is formed by the inner and outer cylinders together with an annular or cup-shaped piston lying between the opposite surfaces of the two cylinders.

A feature of the present invention is the provision of a two-circuit wheel'cylinder for vehicle brakes comprising: a first cylinder; a second cylinder disposed con centrically with respect to the first cylinder; a first second cylinders piston disposed concentrically with respect to both of the cylinders; a second piston disposed concentrically with respect to both of the cylinders; a first pressure medium sealing arrangement disposed in association with given surfaces of the first and second, cylinders and the second piston to form a first pressure chamber formed by cooperating surfaces of the first and second cylinders and the second piston; and a second pressure medium sealing arrangement disposed in association with given surfaces of the first piston and at least the second cylinder to form a second pressure chamber formed by cooperating surfaces of the first piston and the second cylinder.

In order to provide simple assembly and faultless operation the inner cylinder is held and guided in a groove on a wall of the front the outer cylinder disposed transversly of the longitudinal axis thereof and i a sleeve with radial play, said sleeve being connected centrally at said wall, said groove being provided with a sealing ring.

In order to recognize the failure of the above mentioned sealing ring of a pressure chamber rapidly a radial outer sealing ring as well as a radial inner sealing ring are arranged in the groove of the wall of the outer cylinder, the chamber or passage way between the sealing rings being in communication with the atmosphere. The genuine feature of a two-circuit system is achieved by the fact that a piston actuated by the outer pressure chamber is formed like a cylinder or an annular member, said piston being sealed at its inner and outer surfaces.

The advantages of the invention especially lie in the fact that by means of the disposition of an outer piston there exists a connection between the pressure medium scaling arrangement of the outer piston and the pressure medium sealing arrangement of the inner piston with the atmosphere. Thus, a leakage of one of these sealing arrangements can be noticed easily from the outside. The assembly is simplified since the arrange ment of the inner cylinder with play in the groove of the transverse wall of the outer cylinder allows for compensation of the eccentricities. Jamming during operation is prevented by means of the Cardanic suspension of the inner cylinder. The maximum of the jamming forces occurring is equivalent to the resilient action of the sealing ring or the sealing rings, respectively. Due to the arrangement shown in FIG. 2 hereof the effective surfaces of the two pressure medium circuits are equivalent and, hence, also the force acting on the brake lining, even if one of the two pressure medium circuits fails.

The manufacture of cylinders with ground exterior surfaces results in considerably easier manufacturing than the manufacturing of the cup-shaped pistons with honed interior surfaces disclosed in the above cited published German patent application, British patent and US. patent. The sleeve in the interior of the inner cylinder at the same time brings about holding and resilient actions as well as the maximum of possible forced circulation for the ventilation of the pressure medium. Inner and outer steps in the vertical wall of the inner cylinder have the effect that the inner cylinder, even upon failure of one of the two pressure medium circuits always abuts against the seal in the groove of the transverse wall of the outer cylinder.

BRIEF DESCRIPTION OF THE DRAWING Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is an axial cross sectional view ofa two-circuit wheel cylinder in accordance with the principles of the present invention having a cylindrical outer piston;

FIG. 2 is an axial cross sectional view of a two-circuit wheel cylinder in accordance with the principles of the present invention having an annular outer piston;

FIG. 3 is a cross sectional view of an enlarged section of the embodiments of FIGS. 1 and 2;

FIG. 3a is a cross sectional view of an enlarged section of the embodiments of FIGS. 1 and 2;

FIG. 4 is an axial cross sectional view of a two-circuit wheel cylinder in accordance with the principles of the present invention having an inner cylinder provided with the inlet and outlet ducts; and

FIG. 5 is a cross sectional view of a section of FIG. 4 illustrating an alternative way of securing the inner and outer cylinders to each other.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 wheel cylinder housing (first cylinder) includes a wall disposed transversely of the longitudinal axis of house 1. Inner (second) cylinder 2is fastened with radial play in groove 3 disposed in the transverse wall of housing I. By means of sleeve 4 and hollow bolt 5 inner cylinder 2 is held and guided axially against seal 6 disposed in groove 3 of the transverse wall of housing I. Seal 6 can also be in the form of

seals

36 and 37 shown in FIGS. 3 and 3a. The inner surface of cupshaped (first) piston 8 is guided along outer surface 7 of cylinder 2. Also inner surface 10 of cylindrical (second) piston 9 is guided along outer surface 7 while outer surface 11 is guided along the inner surface of housing 1. Housing 1 includes in its transverse wall inlet and

outlet nozzles

13 and 12, respectively, for outer or first pressure chamber 16 and inlet and outlet nozzles 14 and 15, respectively, for inner or second pressure chamber 19. Inlet nozzle 13 leads to pressure chamber 16 which is formed by cylinder 2, housing 1 and piston 9. The first pressure medium sealing arrangement includes dynamic seal or sealing member 17 disposed in inner surface 10 of piston 9 and dynamic seal or sealing member 18 disposed in the inner surface at the open end of housing 1 to prevent the pressure medium escaping into the atmosphere. The second pressure medium sealing arrangement for pressure chamber 19 is provided by means of seal or sealing member 20 lying in the inner surface at the open end of the cup-shaped piston 8 to prevent the pressure medium escaping into the atmosphere. Between seals 20 and 17 there is a cavity filled with grease or the like, said cavity leading into the atmosphere. Upon actuation of cup-shaped piston 8, the pressure medium passes through inlet nozzle 14 via the pocket hole bore 21 bored from inside and hollow bolt 5 into pressure chamber 19. Upon ventilation of pressure chamber 19, the pressure medium inevitably is passed through duct 22 along sleeve 4 and inner cylinder 2 into the ventilation or outlet nozzle 15. The open end of housing 1 and the end of cylindrical piston 9 which is opposite outer pressure chamber 16 are suitably connected to each other by means of protecting cap 23. Protecting cap 23 is fixed at groove 24 disposed in the inner surface of cylindrical piston 9 and at groove 25 disposed in the outer surface of housing 1. Cavity 26 formed by protecting cap 23 is preferably filled with grease or the like.

The operation of the two-circuit wheel cylinder of this invention is as follows:

If inner pressure chamber 19 fails, cylindrical piston 9 is displaced via outer pressure chamber 16. At the same time, cylindrical piston 9 presses the cup-shaped piston 8 axially against the brake shoe. If one of dynamic seals 17 or 20 starts leaking, the pressure medium from chambers 19 or 16 enters the cavity between cylindrical piston 9 and cup-shaped piston 8. Hence, the pressure medium escapes into the atmosphere, the defect or failure, thus, being easily noticed or detected from the outside by a failure indicating system (not shown).

FIG. 2 illustrates a two-circuit wheel cylinder similar to the one illustrated in FIG. 1. The difference between the wheel cylinders of FIGS. 1 and 2 is that in FIG. 2 piston 27 (the second piston) is annular instead of cylindrical. In the even and/or bale-like guide surfaces 28 and 29 of annular piston ,27 are disposed dynamic seals or sealingmembers 30 and 30a. The outer surface of cup-shaped piston 31 (the first piston) is designed such that wiping device 33 can be provided attached to the inner surface of cylinder 34 (the first cylinder). Protecting cap 32 (cap 23 of FIG. 1) connects cylinder 34 with cup-shaped piston 31. The cavity between cupshaped piston 31 and cylinder 34 as well as the cavity formed by protecting cap 32 are preferably filled with grease or the like. The operation of this two-circuit wheel cylinder is essentially the same as the operation described with regard to FIG. 1.

F IG. 3 shows an enlarged section of groove 3, FIGS. 1 and 2 and an alternative to seal 6. Inner cylinder 35 (cylinder 2 of FIG. 1) divides groove 3 disposed in the transverse wall of outer cylinder 34 into two chambers. These two chambers are connected with each other. These chambers are sealed to the atmosphere by two

radial seals

36 and 37. A third chamber is provided between

seals

36 and 37 and is connected with the atmosphere via bore 38. Upon failure of one of

seals

36 or 37, the pressure medium escapes into the atmosphere via bore 38, the leakage being indicated by the alarm indication system (not shown).

FIG. 3a shows an enlarged section of the FIGS. 1 and 2 adjacent groove 3 with an alternative configuration for inner cylinder 35 (cylinder 2 of FIG. 1). Inner cylinder 35 has a stepped configuration adjacent groove 3. Thus, inner cylinder 35 is a stepped cylinder. This has the advantage that even upon failure of one of the two pressure medium circuits inner cylinder 35 is always pressed against

seals

36 and 37 provided in groove 3 in the transverse wall of outer cylinder 34 (cylinder 1 of FIG. 1).

The main advantage of the two-circuit wheel cylinder of this invention is the compact axial length.

Referring to FIG. 4 there is illustrated therein a first piston 45 (piston 8 of FIG. 1) having an inverted cuplike configuration and a second or inner cylinder 41 encircling piston 45. In this embodiment the first and second pressure medium sealing arrangement are different than in FIG. 1 and 2. In contrast to the embodiments of dynamic seals 17 and 20 (first and second sealing arrangement) provided at the outer surface of the inner cylinder 2 in accordance with FIGS. 1 and 2, dynamic seal 55 (the second sealing arrangement) for pressure medium chamber 49 (chamber 19 of FIG. 1) is arranged at the inner surface of inner cylinder 41 and dynamic seal 53 (the first sealing arrangement) for pressure medium chamber 51 is arranged at the outer surface ofinner cylinder 41.

In the case ofa wheel cylinder according to FIG. 1 or FIG. 2, the total pressure surface is equivalent to the circular surface of the outer cylinder or housing 1. In the case of a wheel cylinder according to FIG. 4, the total pressure surface is equivalent to the circular surface of the outer cylinder or housing 40 less the annular surface of inner cylinder 41. Thus, it is possible to achieve different pressure surfaces by altering the configuration of cylinder 41, the configuration of housing 40 being unchanged.

. Inner cylinder 41 is fixed axially with respect to cylinder 40 by an inwardly extending projection 40a from cylinder 40 being received in abutment or outward extending ring-like member 42 and Seeger ring 43 carried by cylinder 41. Piston 45 is guided by inner surface 44 of inner cylinder 41. Cylindrical piston 46 is disposed between inner cylinder 41 and outer cylinder 40 and has honed or ground inner and outer surfaces 47 and 48. Upon failure in inner pressure'chamber 49, the pressure medium is pressed into outer pressure chamber 51 via inlet duct 50. Outer pressure chamber 51 is formed by outer cylinder 40, inner cylinder 41 and cylindrical piston 46. The first pressure medium sealing arrangement for first pressure chamber 51 is provided by static seal or sealing member 52 and dynamic seal or sealing member 53 lying in inner cylinder 41 as well as dynamic sealer or sealing member 54 lying in outer cylinder 40. The annular surface or end of piston 46 actuated by the pressure medium of pressure chamber 51 presses the other end of piston 46 against the brake linings. Inner pressure chamber 49 is provided by a wall of cylinder 41 disposed transversely of cylinder 41 and the bottom of the inverted cup-shaped piston 45. The second pressure medium sealing arrangement for chamber 49 is provided by dynamic seal or sealing member 55 in the inner surface of cylinder 41. In the event of leakage in one of the

dynamic seals

53 or 55, the pressure medium of chambers 49 or 51 passes into cavity 56 filled with grease or the like and escapes beyond wiper 57 into the atmosphere. In the event of leakage of the dynamic sealing 54, the pressure medium of chamber 51 enters cavity 58 formed by protecting cap 59. Protecting cap 59 is fastened in outer cylinder 40 by means of ring 61 provided in groove 60 and is also fixed in groove 62 provided in the outer surface of piston 46. Due to the high pressure in outer pressure chamber 51 the leaking pressure medium can easily escape from the cavity provided by protecting cap 59 into the atmosphere, such that the non-illustrated pressure difference alarm system is activated.

FIG. 5 illustrates an enlarged cross-section of an alternative manner in which

cylinders

40 and 41 of FIG. 4 may be anchored together to prevent relative axial movement. For assembling reasons Seeger ring 63 is mounted on the outside of inner cylinder 65 (cylinder 41 of FIG. 4) and the inwardly extending projection 67 of outer cylinder 40 is received in the groove formed by ring 63 and abutment 66 carried by cylinder 65.

While I have described above the principles of my invention in connection with specific apparatus it is to be more clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

Iclaim:

1. A two-circuit wheel cylinder for vehicle brakes comprising:

a first hollow cylinder having a longitudinal axis;

a second hollow cylinder disposed concentrically with respect to said first cylinder and said axis;

a first hollow piston having a hollow circular cross section transverse to said axis, said first piston being disposed concentrically with respect to both of said cylinders and said axis;

a second annular piston having a hollow circular cross section transverse to said axis, said second piston being disposed concentrically with respect to both of said cylinders and said axis;

at least one of said first and second pistons having a circular wall thereof parallel to said axis disposed between the inner surface of said first cylinder and the outer surface of said second cylinder;

first pressure medium sealing arrangement disposed-in association with given surfaces of said first and second cylinders and said second piston to form a first pressure chamber formed by cooperating surfaces of said first and second cylinders and said second piston; and

a second pressure medium sealing arrangement disposed in association with given surfaces of said first piston and at least said second cylinder to form a second pressure chamber formed by cooperating surfaces of said first piston and said second cylinder.

2. A cylinder according to claim 1 wherein said first cylinder includes a wall disposed tranversely of said longitudinal axis,

a groove formed in the inner surface of said wall to receive one end of said second cylinder, and

a seal disposed in said groove associated with said one end of said second cylinder; and

further including a sleeve secured to said inner surface of said wall and connected to the other end of said second cylinder to hold and freely guide said second cylinder in said groove. 3. A cylinder according to claim 2, wherein said seal includes a first radial seal disposed between the outer sur face of said second cylinder and the outer surface of said groove, and

a second radial seal disposed between the inner surface of said second cylinder and the inner surface of said groove; and

said wall includes a passage way disposed between said first and second radial seal in communication with the atmosphere.

4. A cylinder according to claim 3, wherein the wall of said second cylinder includes a stepped portion adjacent said groove such that upon failure of one of said two circuits said stepped portion is pressed against said wall of said first cylinder by pressure medium moving said first and second pistons in the opposite direction.

5. A cylinder according to claim 1, wherein said first piston is a cup-shaped piston.

6. A cylinder according to claim 1, wherein said first piston is disposed to encircle said second cylinder; and

said second sealing arrangement includes at least a first sealing member disposed between the inner surface of said first piston and the outer surface of said second cylinder.

7. A cylinder according to claim 6, wherein said first piston is disposed to encircle said second cylinder and within said first cylinder; and

said second sealing arrangement further includes a second sealing member disposed between the inner surface of said first cylinder and the outer surface of said first piston.

8. A cylinder according to claim 1, wherein said first piston is disposed within said second cylinder; and

said second sealing arrangement includes a sealing member disposed between the outer surface of said first piston and the inner surface of said second cylinder. 9. A cylinder according to claim 1, wherein said second piston is cylindrical in form; and said first sealing arrangement includes a first sealing member disposed at one end of said second piston between the inner surface of said second piston and the outer surface of said second cylinder, and

a second sealing member disposed at the other end of said second piston between the outer surface of said second piston and the inner surface of said first cylinder.

10. A cylinder according to claim 1, wherein said second piston is annular in form; and

said first sealing arran ement includes a first sealing mem er disposed between the inner surface of said second piston and the outer surface of said second cylinder, and a second sealing member disposed between the outer surface of said second piston and the inner surface of said first cylinder. 1 l. A cylinder according to claim 1, wherein said second cylinder includes a wall disposed transversely of the longitudinal axis thereof; said wall including a pair of longitudinally spaced ring-like members extending from the outer surface thereof; and said first cylinder includes a inwardly extending annular projection; said ring-like members receiving said projection to anchor said first and second cylinders against axial movement relative to each other; said wall containing inlet and outlet bores for said second pressure chamber. 12. A cylinder according to claim 11, wherein said first piston is disposed within said second cylinder; said second piston is disposed to encircle said second cylinder and engage the inner surface of said first cylinder; said first piston has an inverted cup-like configuration; said second pressure chamber is founded by said wall, the vertical inner wall of said second cylinder and the outer surface of the bottom of said inverted cup-like first piston; said second cylinder is longitudinally shorter than said first and second pistons; and further including a wiper carried on the outer surface of said first piston opposite said bottom thereof and extending to the inner surface of said second piston to confine a grease filling between said first and second pistons and an end of said second cylinder. 13. A cylinder according to claim 1, further including ing

at protecting cap interconnecting the end of said first cylinder opposite said first pressure chamber and said first piston; said protecting cap being filled with grease.

Claims

1. A two-circuit wheel cylinder for vehicle brakes comprising: a first hollow cylinder having a longitudinal axis; a second hollow cylinder disposed concentrically with respect to said first cylinder and said axIs; a first hollow piston having a hollow circular cross section transverse to said axis, said first piston being disposed concentrically with respect to both of said cylinders and said axis; a second annular piston having a hollow circular cross section transverse to said axis, said second piston being disposed concentrically with respect to both of said cylinders and said axis; at least one of said first and second pistons having a circular wall thereof parallel to said axis disposed between the inner surface of said first cylinder and the outer surface of said second cylinder; a first pressure medium sealing arrangement disposed in association with given surfaces of said first and second cylinders and said second piston to form a first pressure chamber formed by cooperating surfaces of said first and second cylinders and said second piston; and a second pressure medium sealing arrangement disposed in association with given surfaces of said first piston and at least said second cylinder to form a second pressure chamber formed by cooperating surfaces of said first piston and said second cylinder.

2. A cylinder according to claim 1, wherein said first cylinder includes a wall disposed tranversely of said longitudinal axis, a groove formed in the inner surface of said wall to receive one end of said second cylinder, and a seal disposed in said groove associated with said one end of said second cylinder; and further including a sleeve secured to said inner surface of said wall and connected to the other end of said second cylinder to hold and freely guide said second cylinder in said groove.

3. A cylinder according to claim 2, wherein said seal includes a first radial seal disposed between the outer surface of said second cylinder and the outer surface of said groove, and a second radial seal disposed between the inner surface of said second cylinder and the inner surface of said groove; and said wall includes a passage way disposed between said first and second radial seal in communication with the atmosphere.

6. A cylinder according to claim 1, wherein said first piston is disposed to encircle said second cylinder; and said second sealing arrangement includes at least a first sealing member disposed between the inner surface of said first piston and the outer surface of said second cylinder.

7. A cylinder according to claim 6, wherein said first piston is disposed to encircle said second cylinder and within said first cylinder; and said second sealing arrangement further includes a second sealing member disposed between the inner surface of said first cylinder and the outer surface of said first piston.

8. A cylinder according to claim 1, wherein said first piston is disposed within said second cylinder; and said second sealing arrangement includes a sealing member disposed between the outer surface of said first piston and the inner surface of said second cylinder.

9. A cylinder according to claim 1, wherein said second piston is cylindrical in form; and said first sealing arrangement includes a first sealing member disposed at one end of said second piston between the inner surface of said second piston and the outer surface of said second cylinder, and a second sealing member disposed at the other end of said second piston between the outer surface of said second piston and the inner surface of said first cylinder.

10. A cylinder according to claim 1, wherein said second pisTon is annular in form; and said first sealing arrangement includes a first sealing member disposed between the inner surface of said second piston and the outer surface of said second cylinder, and a second sealing member disposed between the outer surface of said second piston and the inner surface of said first cylinder.

11. A cylinder according to claim 1, wherein said second cylinder includes a wall disposed transversely of the longitudinal axis thereof; said wall including a pair of longitudinally spaced ring-like members extending from the outer surface thereof; and said first cylinder includes a inwardly extending annular projection; said ring-like members receiving said projection to anchor said first and second cylinders against axial movement relative to each other; said wall containing inlet and outlet bores for said second pressure chamber.

12. A cylinder according to claim 11, wherein said first piston is disposed within said second cylinder; said second piston is disposed to encircle said second cylinder and engage the inner surface of said first cylinder; said first piston has an inverted cup-like configuration; said second pressure chamber is founded by said wall, the vertical inner wall of said second cylinder and the outer surface of the bottom of said inverted cup-like first piston; said second cylinder is longitudinally shorter than said first and second pistons; and further including a wiper carried on the outer surface of said first piston opposite said bottom thereof and extending to the inner surface of said second piston to confine a grease filling between said first and second pistons and an end of said second cylinder.

13. A cylinder according to claim 1, further including a protecting cap interconnecting the end of said first cylinder opposite said first pressure chamber and the end of said second piston opposite said first pressure chamber; said protecting cap being filled with grease.

14. A cylinder according to claim 1, further including a protecting cap interconnecting the end of said first cylinder opposite said first pressure chamber and said first piston; said protecting cap being filled with grease.