US3747477A

US3747477A - Variable volume hydraulic apparatus

Info

Publication number: US3747477A
Application number: US00039225A
Authority: US
Inventors: U Aldinger
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1969-05-28
Filing date: 1970-05-21
Publication date: 1973-07-24
Anticipated expiration: 1990-07-24
Also published as: FR2047337A5; GB1313682A; DE1927074A1; CH505987A

Abstract

A hydraulic pump or motor of the radial piston type has a control ring for the pistons whose eccentricity to the axis of the rotor determines the transported volume of fluid. Two pistons are diametrically disposed in an axial plane and abut the control ring. The pistons are respectively connected with a source of fluid whose pressure is variable, and with the high pressure side of the hydraulic apparatus so as to vary the eccentricity of the control ring while the same is guided by two supporting means which are diametrically disposed in an axial plane perpendicular to the first-mentioned axial plane.

Description

[451 July 24,1973

1 1 VARIABLE VOLUME HYDRAULIC APPARATUS [75] Inventor: Ulrich Aldinger, Stuttgart, Germany [73] Assignee: Robert Bosch GmbH, Stuttgart,

Germany 221 Filed: May 21,1970 211 Appl.No.:39,225

[30] Foreign Application Priority Data May 28, 1969 Germany P 19 27 074.7

[52] 0.8. CI. 91/497 [51] Int. Cl. .L F011: 13/06 [58] Field of Search 91/497, 484, 485, 91/487; 418/25, 26

[5 6] References Cited UNITED STATES PATENTS 2,009,851 7/1935 Link 417/217 2,509,256 5/1950 Sorensen 417/219 3,028,814 4/1962 Rumsey 417/219 2,433,484 12/1947 Roth 418/26 3,155,047 11/1964 Reel A 91/485 2,630,681 3/1953 Ferris 418/26 2,164,888 6/1939 Sassen et a1. 91/487 3,087,437 4/1963 l-lenrichsen..... 91/484 2,716,945 9/1955 Presnell 91/485 FOREIGN PATENTS OR APPLICATIONS 109,849 10/1917 Great Britain 91/497 Primary Examiner-William L. Freeh Attorney-Michael S. Striker [57] ABSTRACT A hydraulic pump or motor of the radial piston type has a control ring for the pistons whose eccentricity to the axis of the rotor determines the transported volume of fluid. Two pistons are diametrically disposed in an axial plane and abut the control ring. The pistons are respectively connected with a source of fluid whose pressure is variable, and with the high pressure side of the hydraulic apparatus so as to vary the eccentricity of the control ring while the same is guided by two supporting means which are diametrically disposed in an axial plane perpendicular to the first-mentioned axial plane.

5 Claims, 4 Drawing Figures PAIENTED JUL 24 3. 747. 477

sum 1 or 2 Fly. 7

77 w 48 I 8 7 55 103 J I 7 47 uvmvron.

Ulrich AL D/NGE R 1 VARIABLE VOLUME HYDRAULIC APPARATUS BACKGROUND OF THE INVENTION Radial piston pumps and vane pumps and hydraulic motors are known whose piston chambers are alternately connected with the high pressure side and low pressure side during each revolution of the pump or motor, and which are provided with a control ring on which the outer ends of the pistons or vanes abut. The control ring is freely movable in a cavity of the housing, and assumes a neutral position in which its center coincides with the axis of the rotor, or operative eccentric positions in which the pump or motor operates.

In accordance with the prior art, rather complicated mechanical apparatus is provided for displacing the control ring relative to the axis of the rotor.

It is one object of the invention to provide a hydraulic apparatus, either a pump or a motor, with simple and reliable means for mounting and displacing the control ring.

Another object of the invention is to provide operating means for displacing the control ring while compensating deformations of the same caused by the action of the operating means and by the supporting means of the ring. 7

Another object of the invention is to provide hydraulic operating meansfor displacing the control ring for varying the volume of fluid transported by the apparatus.

Another object of the invention is to provide operating means including at least one operating piston abutting the control ring and controlled by pressure fluid whose pressure is selectable and adjustable.

SUMMARY OF THE INVENTION In accordance with the invention, the control ring means is supported on a plurality of supporting means which inwardly project from the housing, and at least some of which are movable radially with respect to the control ring.

A preferred embodiment of the invention comprises a rotor having an axis, peripheral chambers, and piston elements mounted in the chambers for substantially radial movement; a control ring surrounding the rotor and having an inner control surface engaged by the pis ton elements so that the latter move in the respective chambers when the center of the control ring is located eccentric to the axis of the rotor; housing means having an inner surface for ming an inner cavity in which the control ring and rotor are located, and including fluid supply means and fluid delivery means communicating with the chambers; a pair of supporting means mounted in the housing diametrically disposed in an axial control plane inwardly projecting from the inner surface of the housing means, and having free inner ends abutting the outer surface of the control ring for guiding the same in a controlled direction with the center of the control ring moving in an other axial plane perpendicular to the axial control plane between a neutral position in which the center of the control ring coincides with the axis and a plurality of operative positions in which the center is eccentric to the axis; and operating means including at least one pair of cylinders diametrically disposed in the other axial plane and mounted on the housing means, a pair of operating pistons movable in the cylinders, respectively, and abutting the outer surface of the control ring, first conduit means for supplying to one of the cylinders pressure fluid at selected pressures, and second conduit means for supplying to the respective other cylinder high pressure fluid.

In this manner, the movement of the control ring by the operating pistons and the volume displaced by the piston elements, depends on the selected pressure of the fluid in the first conduit means, and on the pressure in the second conduit means.

It is advantageous to provide additional cylinders and pistons angularly spaced 45 from the operating pistons and supporting means. Preferably, slide shoes are used to which pressure fluid is supplied so that a hydrostatic pressure area forms on the inner face of the shoe which is in contact with the outer surface of the control ring.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in-connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic cross sectional view illustrating a first embodiment of a pump according to the invention;

FIG. 2 is a cross-sectional view illustrating a second embodiment of the invention;

FIG. 3 is a cross-sectional view illustrating a third embodiment of the invention; and

FIG. 4 is a cross-sectional view illustrating a fourth embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It will be assumed that the hydraulic apparatus shown in FIGS. 1 to 4 operate as pumps, but the apparatus can also be used as a hydraulic motor. The general constructions of the embodiment of FIGS. 1 4 is-the same, and corresponding parts will be designated by like reference numerals.

In all illustrated embodiments, a rotor 4 is driven from a prime mover to rotate about an axis at which the axial planes l3 and 14 intersect at right angles. A valve means 5 is stationarily mounted in a cylindrical bore of rotor 4, and is located in the axial plane 14 to separate the high pressure conduit 7 and the low pressure conduit 6 from each other.

Conduits

6 and 7 communicate with supply means and delivery means for a fluid, depending on whether the apparatus is operated as a pump or motor.

The cylindrical rotor body has peripheral radially extending bores 8, forming cylinder chambers in which pistons 9 are mounted for radial movement. FIGS. 1 to 4 show only one cylinder 8 and piston 9, but it will be understood that a plurality of angularlyspaced cylinder chambers and pistons therein is provided angularly spaced about the periphery of the cylindrical, rotor 4, only one piston being shown for the sake of simplicity in view of the conventional nature of this arrangement.

The cylinder chambers 8 on one side of the axial plane 14 are connected by openings 10 either with the low pressure supply conduit 6 or with the high pressure delivery conduit 7 during each revolution of rotor 4.

Each piston 9 has a spherical seat engaged by a spherical head on a piston rod 11 to which a slide shoe 12 is secured. The outer surface of slide shoe 12 is partcircular, so that its contour matches the circular inner surface of a control ring 3, located in a cavity 2 of a housing means 1 which is large enough to permit translatory movement of control ring 3 in the direction of the axial plane 14.

A pair of diametrically arranged supporting means and 16 is located in the axial control plane 13 which extends at right angles to axial plane 14. The inner end faces of abutment means 15 and 16 engage diametrically located portions of control ring 3, guiding the same for translatory movement with the center of control ring 3 moving in the direction of axial plane 14 between a position coinciding with the axis of rotor 4, and a plurality of eccentric positions, one eccentric end position being shown in the drawing.

Due to the fact that supporting means 15 and 16 project from the inner surface of housing 1, they cause only little wear of the control ring. The length of the inner end faces of supporting

means

15 and 16 is preferably three to four times the maximum eccentricity of the center of control ring 3 in relation to the axis of rotor 4. Supporting means 15, 16 may be inserts consisting of a wear resistant material, or be integral parts of the housing 1 and cast with the same. 4

Two diametrically disposed

operating pistons

17 and 18, which also constitute supporting means for control ring 3, are located in the-axial plane 14. Operating pistons l7 and 18 are mounted for radial movement in corresponding cylinders formed on housing 1, and are urged by springs and 19, respectively, into engagement with control ring 3, even if no pressure fluid is supplied to the cylinders of

pistons

17 and 18. Piston 18 has a greater effective surface and a larger spring 19 than piston 17 and spring 20 so that control ring 3 is biased toward the illustrated eccentric end position.

In the embodiment of FIG. 1,

other pistons

21 and 22 are provided in corresponding cylinders of housing 1, and are biassed by

springs

23 and 24 toward engagement with control ring 3. Pistons 21 and 22 are located in the two quadrants on the high pressure side of the pump, and of axial plane 14, and spaced angles of 45 from

planes

13 and 14.

The cylinders of

pistons

17, 22 and 21 are connected by high pressure conduit means 101 with a radial high pressure conduit 7a communicating with the high pressure delivery conduit 7'.

Another conduit means 102 communicates with the cylinder of operating piston 18, and contains valve means 103 for selectively varying and adjusting the pressure of the pressure fluid supplied to piston 18. When pressure fluidis supplied through conduit 102, piston 18 advances inward, and control ring 3 is pressed against the end face of piston 17, and braked in this eccentric end position. The amount of eccentricity of the center of control ring 3 in relation to the rotor axis, depends on the height of the pressure supplied throughconduit means 102 to piston 18.

Control ring 3 is not pressed against the inner surface 1 of easing 1, but against the end face of piston 17. The

pressure of control ring 3 is distributed to a greater number of supported points. I

When the control pressure applied to piston 18 is relieved, piston 17 moves control ring 3 against the action of spring 19 to a neutral position in which the center of control ring 3 coincides with the rotor axis, and the pump idles and delivers no fluid.

The embodiment of FIG. 2 corresponds to the embodiment of FIG. 1, the

pistons

21 and 22 being omitted, while

pistons

47 and 48 are provided with

slide shoes

55 and 56, respectively. It will be understood that

pistons

21 and 22 could also be provided in the embodiment of FIG. 2.

The control ring 33 is supported and guided by supporting

means

45 and 46, and is operated by

pistons

48, 56 and 47, 55 to move between positions of different eccentricity, as explained with reference to FIG. 1. A selectively variable control pressure is supplied to the cylinder of piston 48, and high pressure fluid is supplied by conduit 101 to the cylinder of piston 47. Springs 49 and 50 urge the

pistons

48 and 47, respectively, toward control ring 33. Piston 48 has a cylindrical or spherical seat engaged by a corresponding cylindrical spherical head 56' of slide shoe 56, and piston 47 has a spherical or cylindrical seat engaged by aspherical or cylindrical head 55' of slide shoe 55. A duct connects each cylinder with a pressure area at the end face of each slide shoe so that the pressure of conduit 102 prevails under slide shoe 56, and the pressure of conduit 101 prevails on the slide face of slide shoe 55. Due to the produced hydrostatic pressure, the friction during the displacement of control ring 33 is reduced, and a lesser adjusting force required. During the pumping operation, control ring 33 cannot rotate since it is braked by

piston

47, 55.

The axis or center of the cylindrical or spherical head 55, 56' is located on the outer circular surface of control ring 33. Due to this fact, the slide shoes cannot be tilted by control ring 33 which rotates during adjustment of the apparatus.

FIG. 3 shows an embodiment of the invention in which the operating

pistons

77 and 78 are provided with

slide shoes

85 and 86, as explained with reference to FIG. 2. Instead of rigid supporting means 15, 16 or 45, 46, slide shoes 75, 76 are provided in the axial plane 13 in diametrical position in relation to control ring 63. Slide shoes and 76 have spherical or cylindrical heads mounted in corresponding seats in the inner surface 67 of the housing. The duct in slide shoe 75 on the high pressure side of the pump is connected with high pressure conduit 101 which communicates with the delivery conduit 7 of the pump through radial conduit 7a. The duct in slide shoe 76 is connected by a conduit 119 with a radial conduit 60 which communicates with the low pressure supply conduit 6 of the pump.

Conduits

101 and 119 are connected over

check valves

87 and 88 with the cylinder in which piston 77 is mounted. When the apparatus is operated as a pump, high pressure fluid from conduit 101 enters the cylinder of piston 77 through the opening check valve 87, but cannot enter the low pressure conduit 119 due to the provision of check valve 88.

When the direction of rotation of rotor 4 is reversed, conduit 119 becomes the high pressure conduit, and conduit 101 becomes the low pressure conduit, and piston 77 is operated by the high pressure produced by the pump since check valve 88 opens and checkvalve 87 closes.

In the embodiment of FIG. 3, the control ring 63 is completely hydrostatically supported, so that very small friction forces occur, permitting rotation of control ring 63 even during operation of the pump. Control ring 63 rotates at an intermediate speed, lower than the rotary speed of rotor 4, so that slide shoe 1?. slides at a comparatively low speed on the inner annular surface of control ring 63.

If the supporting slide shoe 56 is connected with the low pressure side of the pump, pressure fluid can be discharged from the pressure area in the curved face of slide shoe 76 abutting the outer surface of control ring 63, so that the same is lubricated and cooled along its periphery.

As in the embodiments of FIGS. 1 and 2, pressure fluid at a selectable control pressure is supplied to the cylinder in which piston 78 is mounted.

The embodiment of FIG. 4 combines features of the embodiments of FIGS. 1, 2 and 3. Supporting

slide shoes

105 and 106 are arranged and constructed as the slide shoes 75 and 76 in FIG. 3. Operating

pistons

107 and 108 correspond to operating

pistons

77 and 78 of the embodiment in FIG. 3, and have corresponding slide shoes.

Pistons

111 and 112 are also provided with slide shoes, and correspond to

piston

21 and 22 in the embodiment of FIG. 1, preventing deformation of control ring 93 which is located in the cavity of the housing having the inner surface 92. Pressure conduit means 101 connects the delivery conduit 7 with the cylinders of

pistons

111 and 112, and over a check valve 117 with the cylinder of piston 107. A low pressure conduit 119 connects the cylinders of

pistons

113 and 114 with the low pressure supply conduit 6, and over a check valve 118 with a cylinder of operating piston 107.

High pressure conduit 101 is also connected with a duct of supporting slide shoe 105, and low pressure conduit 119 is also connected with the duct of supporting slide shoe 106. The cylinder of piston 108 is connected with a, source of pressure fluid at a selectable control pressure.

Pistons

111, 112, l13-and 114 prevent a deformation of control ring 93 by the pressure between

pistons

107 and 108. The slide shoes around the periphery of control ring 93 support control ring 93 for translatory movement from the eccentric end position shown in FIG. 4 to a position in which the center of control ring 93 coincides with the axis of rotor 5. When the direction of rotation is reversed, high pressure fluid is supplied to

pistons

113, 114, and 107, and also to the supporting slide shoe 106, while low pressure fluid is supplied to pistons 11], 112 and supporting slide shoe 105.

The effective surface of

pistons

111, 112, 113 and 114 is selected in accordance with the occurring deformation of control ring 93- so that the same remains exactly circular.

The embodiments of FIGS. 3 and 4, which permit rotation of control rings 63 and 93, respectively, are particularly suited for high pressure and high rotary speed. The illustrated apparatus has been described for operation as a pump, but it is evident that the apparatus can also serve as a hydraulic motor.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of variable volume hydraulic apparatus having a control ring slidably en- 6 gaged by piston elements differing from the types described above.

While the invention has been illustrated and described as embodied in a pump or hydraulic motor in which a control ring for piston elements is operated by fluid pressure to vary the transported volume of fluid, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

I claim:

1. Variable volume hydraulic apparatus, comprising a rotor having an axis, chambers, and piston elements mounted in said chambers for substantially radial movement; a control ring surrounding said rotor and having an inner control surface engaged by said piston elements so that the latter move in said chambers when the center of said control ring is located eccentric to said axis; housing means having an inner surface forming an inner cavity in which said control ring and said rotor are located, and including a fluid supply conduit and a fluid delivery conduit communicating with said chambers; at least two supporting means mounted in said housing means disposed symmetrically to an axial control plane inwardly projecting from the inner surface of said housing means, and having free inner ends abutting the other surface of said control ring for guiding the same in one direction with the center of said control ring moving in an other axial plane perpendicular to said axial control plane between a neutral position, in which the center of said control ring coincides with said axis, and a plurality of operative positions in which said center is eccentric to said axis; and operating means including at least one pair of cylinders diametrically disposed in said other axial plane and mounted in said housing means, a pair of operating pistons movable in said cylinders, respectively, and abutting the outer surface of. said control ring, first conduit means including a pressure adjusting valve means connected with a source of pressure fluid and being operable for supplying to one of said cylinders pressure fluid at selected control pressures, and second conduit means connected with said supply and delivery conduits in said housing for supplying to the respective other cylinder high pressure fluid from the one of said supply and delivery conduits at which high pressure prevails so that the displacement of said control ring by said operating pistons in said other axial plane, and the volume displaced by said piston elements depends on the adjusting of said pressure adjust-ing valve and on the selected control pressure of the fluid in said first conduit means overcoming the pressure in said second conduit means; each supporting means and each operating piston including a movable slide shoe having a curved face on said inner end matching the curvature of said outer surface of said control ring and slidingly engaging the same, each slide shoe being formed with a duct for receiving pressure fluid, said duct ending at said curved face forming on the same a hydrostatic pressure area.

2. l-lydraylic apparatus as claimed in claim 1 wherein each slide shoe has a cylindrical or spherical mounting portion having the center of curvature thereof located on said outer surface of said control ring.

3. Hydraulic apparatus as claimed in claim 1 comprising two diametrically disposed supporting means in said axial control plane; wherein ducts in said slide shoes of said two supporting means respectively communicate with said supply and delivery conduits so that the ducts in slide shoes on the high pressure side of said other axial plane receive high pressure fluid and the ducts on the low pressure side of said other axial plane receive low pressure fluid.

4. Hydraulic apparatus as claimed in claim 1 wherein said second conduit means include two conduits connecting said supply and delivery conduits, respectively, with said ducts, and two check valves in said conduits, respectively, so that high pressure fluid is supplied to said ducts irrespective of which of said conduits contains high pressure fluid during operation of the apparatus as pump or motor.

5. Variable volume hydraulic apparatus, comprising a rotor having an axis, chambers, and piston elements mounted in said chambers for substantially radial movement; an integral control ring surrounding said rotor and having an inner control surface engaged by said piston elements so that the latter move in said chambers when the center of said control ring is located eccentric to said axis; housing means having an inner surface forming an inner cavity in which said control ring and said rotor are located, and includ-ing a fluid supply conduit and a fluid delivery conduit communicating with said chambers; at least two supporting means mounted in said housing means disposed symmetrically to an axial control plane inwardly projecting from the inner surface of said housing means, and having free inner ends abutting the outer surface of said control ring for guiding the same in one direction with the center of said control ring moving in an other axial plane perpendicular to said axial control plane between a neutral position, in which the center of said control ring coincides with said axis, and a plurality of operative positions in which said center is eccentric to said axis; said operating means including at least one pair of cylinders diametrically disposed in said other axial plane and mounted in said housing means, a pair of operating pistons movable in said cylinders, respectively, and slidingly abutting the outer surface of said control ring, each supporting means and each operating piston including a movable slide shoe having a curved face on said inner end matching the curvature of said outer sur face of said control ring and slidingly engaging the same, each slide shoe being formed with a duct for receiving pressure fluid, said duct ending at said curved face forming on the same a hydrostatic pressure area, first conduit means including a pressure adjusting valve means connected with a source of pressure fluid and being operable for supplying to one of said cylinders pressure fluid at selected control pressures, and second conduit means connected with said supply and delivery conduits in said housing for supplying to the respective other cylinder high pressure fluid from the one of said supply and delivery conduits at which high pressure prevails so that the displacement of said control ring by said operating pistons in said other axial plane, and the volume displaced by said piston elements depend on the adjusting of said pressure adjusting valve and on the selected control pressure of the fluid in said first conduit means overcoming the pressure in said second conduit means, said second conduit means including two conduits connecting said supply and delivery conduits, respectively, with said other cylinder, and two check valves in said two conduits, respectively, operable in opposite flow directions so that high pressure fluid is supplied to said other cylinder irrespective of which of said two conduits contains high pressure fluid during the operation of the apparatus as a pump or motor, so that the high pressure applied by said other piston on said control ring is overcome by the adjusted greater pressure acting in said one of said cylinders on

Claims

1. Variable volume hydraulic apparatus, comprising a rotor having an axis, chambers, and piston elements mounted in said chambers for substantially radial movement; a control ring surrounding said rotor and having an inner control surface engaged by said piston elements so that the latter move in said chambers when the center of said control ring is located eccentric to said axis; housing means having an inner surface forming an inner cavity in which said control ring and said rotor are located, and including a fluid supply conduit and a fluid delivery conduit communicating with said chambers; at least two supporting means mounted in said housing means disposed symmetrically to an axial control plaNe inwardly projecting from the inner surface of said housing means, and having free inner ends abutting the other surface of said control ring for guiding the same in one direction with the center of said control ring moving in an other axial plane perpendicular to said axial control plane between a neutral position, in which the center of said control ring coincides with said axis, and a plurality of operative positions in which said center is eccentric to said axis; and operating means including at least one pair of cylinders diametrically disposed in said other axial plane and mounted in said housing means, a pair of operating pistons movable in said cylinders, respectively, and abutting the outer surface of said control ring, first conduit means including a pressure adjusting valve means connected with a source of pressure fluid and being operable for supplying to one of said cylinders pressure fluid at selected control pressures, and second conduit means connected with said supply and delivery conduits in said housing for supplying to the respective other cylinder high pressure fluid from the one of said supply and delivery conduits at which high pressure prevails so that the displacement of said control ring by said operating pistons in said other axial plane, and the volume displaced by said piston elements depends on the adjusting of said pressure adjust-ing valve and on the selected control pressure of the fluid in said first conduit means overcoming the pressure in said second conduit means; each supporting means and each operating piston including a movable slide shoe having a curved face on said inner end matching the curvature of said outer surface of said control ring and slidingly engaging the same, each slide shoe being formed with a duct for receiving pressure fluid, said duct ending at said curved face forming on the same a hydrostatic pressure area.

2. Hydraylic apparatus as claimed in claim 1 wherein each slide shoe has a cylindrical or spherical mounting portion having the center of curvature thereof located on said outer surface of said control ring.

5. Variable volume hydraulic apparatus, comprising a rotor having an axis, chambers, and piston elements mounted in said chambers for substantially radial movement; an integral control ring surrounding said rotor and having an inner control surface engaged by said piston elements so that the latter move in said chambers when the center of said control ring is located eccentric to said axis; housing means having an inner surface forming an inner cavity in which said control ring and said rotor are located, and includ-ing a fluid supply conduit and a fluid delivery conduit communicating with said chambers; at least two supporting means mounted in said housing means disposed symmetrically to an axial control plane inwardly projecting from the inner surface of said housing means, and having free inner ends abutting the outer surface of said control ring for guiding the same in one direction with the center of said control ring moving in an other axial plane perpendicular to said axial control plane between a neutral position, in which The center of said control ring coincides with said axis, and a plurality of operative positions in which said center is eccentric to said axis; said operating means including at least one pair of cylinders diametrically disposed in said other axial plane and mounted in said housing means, a pair of operating pistons movable in said cylinders, respectively, and slidingly abutting the outer surface of said control ring, each supporting means and each operating piston including a movable slide shoe having a curved face on said inner end matching the curvature of said outer surface of said control ring and slidingly engaging the same, each slide shoe being formed with a duct for receiving pressure fluid, said duct ending at said curved face forming on the same a hydrostatic pressure area, first conduit means including a pressure adjusting valve means connected with a source of pressure fluid and being operable for supplying to one of said cylinders pressure fluid at selected control pressures, and second conduit means connected with said supply and delivery conduits in said housing for supplying to the respective other cylinder high pressure fluid from the one of said supply and delivery conduits at which high pressure prevails so that the displacement of said control ring by said operating pistons in said other axial plane, and the volume displaced by said piston elements depend on the adjusting of said pressure adjusting valve and on the selected control pressure of the fluid in said first conduit means overcoming the pressure in said second conduit means, said second conduit means including two conduits connecting said supply and delivery conduits, respectively, with said other cylinder, and two check valves in said two conduits, respectively, operable in opposite flow directions so that high pressure fluid is supplied to said other cylinder irrespective of which of said two conduits contains high pressure fluid during the operation of the apparatus as a pump or motor, so that the high pressure applied by said other piston on said control ring is overcome by the adjusted greater pressure acting in said one of said cylinders on the respective operating piston.