US3792643A

US3792643A - Fluid system

Info

Publication number: US3792643A
Application number: US00227057A
Authority: US
Inventors: R Scheafer
Original assignee: Individual
Current assignee: Individual
Priority date: 1972-02-17
Filing date: 1972-02-17
Publication date: 1974-02-19
Anticipated expiration: 1991-02-19

Abstract

A fluid system having a variable displacement pump fluidly connected through valving means to a fluid linear actuator in such a manner that fluid pressure directed to one portion of the linear actuator causes expansion thereof at a selected rate, while fluid pressure directed to another portion of the linear actuator causes contraction thereof at the same rate. The fluid linear actuator comprises a main fluid cylinder having a piston reciprocably mounted therein, with a piston rod extending from one side of the piston and externally of the main fluid cylinder. The piston divides the main fluid cylinder into expansible pressure chambers, one of which is exposed to the rod carrying side of the piston to generate a force thereagainst when pressurized to move the piston in one direction, while the opposite side of the piston is exposed to the pressure of the fluid in the other pressure chamber which exerts a force to move the piston in an opposite direction. A secondary fluid cylinder is carried by the main fluid cylinder with the longitudinal axes of the secondary and main fluid cylinders being radially spaced and parallel to one another. The secondary cylinder has a piston reciprocably mounted therein forming one pressure chamber on the side of the piston in which a second piston rod is mounted and which piston rod extends from the secondary cylinder for connection with the piston rod of the main fluid cylinder, such that the main cylinder piston and the secondary piston reciprocate together as a unit. The effective pressure responsive area of the side of the piston in the main cylinder opposite the rod carrying side thereof is substantially equal to the sum of the effective pressure responsive area of the rod side of the piston in the main cylinder and the effective pressure responsive area of the rod side of the piston within the secondary cylinder, such that the force generated against the two pistons for moving the pistons in one direction is equal to the force exerted against the pistons for moving the same in the opposite direction. Valving means are provided for interconnecting the pressure chambers associated with both the main and secondary fluid cylinders so that the rate of expansion and contraction of the cylinders may be selectively increased as the force exerted on the pistons is decreased and vice versa to provide a fluid linear actuator having a wide speed-force range.

Description

United States Patent 1 Sclreafer [451 Feb. 19,1197l FLUID SYSTEM [76] Inventor: Robert E.- Scheafer, 3325 Newgate Rd., Troy, Mich. 48084 [22] Filed: Feb. 17, 1972 [21] Appl. No.: 227,057

[52] U.S. Cl. 91/411 B, 60/97 H, 60/425 [51] Int. Cl. FlSb 11/16 [58] Field of Search. 60/52 R, 97 H, 425; 91/411 B, 91/411 R [56] References Cited UNITED STATES PATENTS 1,843,082 1/1932 Ferris et al 60/97 H 1,866,348 7/1932 Ferris 60/97 H 2,497,608 2/1950 Herrstrum et al 60/52 R 2,935,852 5/1960 Russell 91/172 Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or FirmHauke, Gifford, Patalidis & Dumont [57] ABSTRACT A fluid system having a variable displacement pump fluidly connected through valving means to a fluid linear actuator in such a manner that fluid pressure directed to one portion of the linear actuator causes expansion thereof at a selected rate, while fluid pressure directed to another portion of the linear actuator causes contraction thereof at the same rate. The fluid linear actuator comprises a main fluid cylinder having a piston reciprocably mounted therein, with a piston rod extending from one side of the piston and externally of the main fluid cylinder. The piston divides the main fluid cylinder into expansible pressure chambers, one of which is exposed to the rod carrying side of the piston to generate a force thereagainst when pressurized to move the piston in one direction, while the opposite side of the piston is exposed to the pressure of thefluid in the other pressure chamber which exerts a force to move the piston in an opposite direction. A secondary fluid cylinder is carried by the main fluid cylinder with the longitudinal axes of the secondary and main fluid cylinders being radially spaced and parallel to one another. The secondary cylinder has a piston reciprocably mounted therein forming one pressure chamber on the side of the piston in which a sec- 0nd piston rod is mounted and which piston rod extends from the secondary cylinder for connection with the piston rod of the main fluid cylinder, such that the main cylinder piston and the secondary piston reciprocate together as a unit. The effective pressure responsive area of the side of the piston in the main cylinder opposite the rod carrying side thereof is substantially equal to the sum of the effective pressure responsive area of the rod side of the piston in the main cylinder and the effective pressure responsive area of the rod side of the piston within the secondary cylinder, such that the force generated against the two pistons for moving the pistons in one direction is equal to the force exerted against the pistons for moving the same in the opposite direction. Valving means are provided for interconnecting the pressure chambers associated with both the main and secondary fluid cylinders so that the rate of expansion and contraction of the cylinders may be selectively increased as the force exerted on the pistons is decreased and vice versa to provide a fluid linear actuator having a wide speed-force range.

10 Claims, 4 Drawing Figures FLUID SYSTEM BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fluid systems and, in particular, to a fluid system employing a fluid linear actuator so constructed that the forces generated for expanding and contracting the actuator are equal.

2. Description of the Prior Art In fluid systems utilizing a variable displacement pump and a fluid cylinder for operation over a wide range of speeds, the variable force from the fluid cylinder is always constant and at a value that is determined by a relief valve setting and the area of the piston within the fluid cylinder. Since horsepower is a function of speed, force, and a constant, the horsepower characteristics of such a system will increase with an increasing speed, whereas, in most applications the load horsepower requirements generally remain constant even with increases and decreases in speed. Since it is a conventional practice to base the design of a system upon the horsepower needed to drive the system pump at maximum speed and pressure ratings rather than the load power requirements, the system may be overdesigned as the load power requirements may never approach the value of the maximum pump speed and pressure rating. For example, in a milling machine the force requirement decreases as the cutting speed increases for a given material, and thus, the load horsepower requirements for such an application may be almost constant. This characteristic can of course be easily matched by a constant displacement pump and a variable displacement rotary motor, however, since the linear operation of a fluid cylinder is required in many applications and there is no such thing as a continuously variable area cylinder, the system must be overdesigned to provide the necessary characteristics. If a continuously variable area cylinder were available, the system employing such a cylinder would require a pump of considerably less output than a fixed area cylinder for the same speed range.

It would therefore be desirable to provide a linear actuator in which the rate of expansion and/or contraction thereof may be varied with a corresponding increase or decrease in the force characteristics of the actuator, and which actuator has speed-force characteristics approaching the heretofore unavailable continuously variable area fluid cylinder.

SUMMARY OF THE PRESENT INVENTION The present invention, which will be described subsequently in greater detail, comprises a linear actuator with a main fluid cylinder having a main piston reciprocably mounted therein dividing the main cylinder into two pressure chambers. The main piston carries a rod extending through one of the pressure chambers and externally of the fluid cylinder to reciprocally drive an external load. A secondary cylinder, fixedly attached to the main cylinder, has a secondary piston reciprocably mounted therein and forming a pressure chamber. A piston rod, extending from the secondary piston through the last mentioned pressure chamber, is operably coupled to the main cylinder piston rod, such that the main and secondary pistons move as a unit.

The effective pressure responsive area of the side of the main piston opposite the rod side thereof is equal to the sum of the effective pressure responsive area of the rod side of the main piston and the effective pressure responsive area of the rod side of the secondary piston, such that the force for expanding the linear ac tuator is equal to the force for contracting the linear actuator. The secondary and main cylinders are connected to one another such that their longitudinal axes are radially spaced from and are parallel to one another. Suitable valving means are provided for interconnecting the pressure chambers of the main and secondary cylinders so as to provide, in effect, a variable area linear actuator.

It is therefore an object of the present invention to provide a fluid system having a new and improved linear actuator which has a wide speed-force range.

Other objects, advantages, and applications of the present invention will become apparent to those skilled in the art of fluid systems and linear actuators when the accompanying description of some examples of the best modes contemplated for practicing the invention is read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF Til-IE DRAWING The description herein makes reference to the accompanying drawing wherein like reference numerals refer to like parts throughout the several views, and in which:

FIG. 1 is a schematic circuit diagram of a fluid system illustrating a preferred embodiment of the present invention;

FIG. 2 is a schematic circuit diagram illustrating another example of the present invention;

FIG. 3 is a schematic circuit diagram illustrating yet another example of the present invention; and

FIG. 4 is a fragmentary view of the circuit illustrated in FIG. 3 incorporating a modification of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing and, in particular, to FIG. ll, there is illustrated a schematic circuit diagram of a fluid system Ill) incorporating the principles of the present invention and comprising a reversible, variable displacement pump 12 which may be preferably of the axial piston type having

port connections

14 and 16, either of which may be an inlet port or an outlet port depending upon the direction of flow from the reversible, variable displacement pump 12. The

port connections

14 and 16 of the pump 12 are, respectively, fluidly connected to a linear actuator 18 by

main conduits

20 and 22.

The linear actuator 18 comprises: a main fluid cylinder 24 formed from an enclosed cylindrically shaped tubular member 26, the interior of which forms a longitudinal bore 28 within which is reciprocably mounted a piston 30. Piston 30 divides the bore 28 into two

expansible pressure chambers

32 and 34. The side 36 of the piston 30 exposed to the pressure of the fluid within the expansible pressure chamber 34 has a piston rod 38 which extends therefrom and externally of the main fluid cylinder 24 through an aperture 40 in one of the closed ends of the cylinder 24. When the pressure chamber 34 is communicated to a source of fluid pressure, such as the pump 12, the pressure of the fluid within the chamber 34 exerts a force against the annular surface of piston side 36, which moves the piston 30 leftwardly as viewed in FIG. 1, so as to contract the main fluid cylinder 24. When the pressure chamber 32 is communicated to the high pressure side of pump 12, the pressure of the fluid within the chamber 32 exerts a force against the piston side 42 to move the piston 30 to the right as viewed in FIG. ll, so as to expand the main fluid cylinder 24.

The linear actuator 18 further comprises a secondary fluid cylinder 44 formed from an enclosed tubular member 46 having an internal bore 48, within which is reciprocably mounted a piston 50 which, in turn, divides the interior bore 48 into two

expansible chambers

52 and 54. One side 56 of the piston 50 has a piston rod 58 extending therefrom and externally of the fluid cylinder 44 through an aperture 60 in one of the closed ends of the member 46. The external ends of the

piston rods

38 and 58 are coupled to one another by a connecting member 62, such that both

pistons

30 and 50 reciprocate together as a unit. The side 56 of the piston 50 is adapted to be communicated to fluid pressure to exert a force on the piston 50 so as to move the same leftwardly as viewed in FIG. 1, while the chamber 52 on the opposite side of the piston 50 is normally vented to the atmosphere through vent aperture 64 or optionally the chamber 52 may be connected to a reservoir (not shown).

The pump port connection 14 is connected to the pressure chamber 32 of the main fluid cylinder 24 by the conduit 20. The port connection 14 may also be connected to the pressure chamber 34 via conduits 66 and 22 when on-off valves 70 and 72 are opened. A branch conduit 74, connected to the conduit 66 intermediate the on-off valves 70 and 72, permits fluid communication between the pump port connection 14 and the secondary cylinder chamber 54 when the on-off valve 70 and an on-off valve 76 in conduit 74 are both opened.

Conduit 22 communicates the port connection 16 of the pump 12 to the main cylinder pressure chamber 34 through another on-off valve 78,'while a branch con duit 80 communicates the secondary cylinder chamber 54 to the port connection 16 via an on-off valve 82. As will be explained in greater detail hereinafter, the

pump port connections

14 and 16 may be fluidly connected to any of the

pressure chambers

32, 34 and/or 54 by proper manipulation of the on-off

valves

70, 72, 76, 78 and 82.

A shuttle relief valve 84 of a conventional construction is provided between

conduits

20 and 22 so that irrespective of which

pump port connection

14 or 16 high pressure fluid is being delivered from, any excessive fluid in the system will be exhausted back to the low pressure side of the pump 12 so as to prevent over pressurization of the system 10.

The effective pressure responsive area of the piston side 42 exposed to the fluid pressure communicated to the main pressure chamber 32 is equal to the sum of the pressure responsive area of piston side 36 exposed to the pressure of the fluid within the main pressure chamber 34 and the effective pressure responsive area of the piston side 56 exposed to the fluid in the secondary pressure chamber 54, such that when fluid under pressure is communicated to the pressure chamber 32 a force of a predetermined magnitude will be exerted against the piston 30 to shift the piston rightwardly as viewed in FIG. 1 and expand the linear actuator 18. When fluid at the same pressure is communicated to both

pressure chambers

34 and 54, a combined force is generated against the

pistons

30 and 50, which force tends to move the pistons leftwardly and contract the linear actuator 18. That is, the force exerted on the surface 42 of the piston 30 to expand the linear actuator 18 is substantially equal to the sum of the forces exerted against the

piston sides

36 and 56 to contract the linear actuator 18.

By properly controlling the amount of fluid communicated from the pump 12 to the

pressure chambers

32, 34 and 54, the rate of movement of the

pistons

30 and 50 rightwardly or leftwardly, that is the expansion and contraction of the pistons in the

fluid cylinders

24 and 44 as well as the net force acting on the

pistons

30 and 50, can be varied over a wide speed-force range.

For example, when the on-off valves and 82 are both closed and the

onoff valves

72, 76 and 78 are opened, fluid pressure is delivered from the pump 12 through the port connection 14 to the pressure chamber 32 of the main fluid cylinder 24, while the

pressure chambers

34 and 54 are exhausted to the port connection 16, thereby forming a closed loop circuit, that is the amount of fluid delivered from pump 12 to chamber 32 equals the amount of fluid exhausted from

chambers

34 and 54 back to pump 12. It can thus be seen that during expansion of the linear actuator 18 the full pressure responsive area of the piston side 30 is exposed to fluid pressure, generating a maximum force and requiring a maximum flow of fluid to move the same, and thus the linear actuator 18 will expand at a relatively low rate while generating a maximum output force. When the variable displacement pump is reversed so that high pressure fluid is delivered from the port connection 16 to the

pressure chambers

34 and 54, a maximum force is exerted on the

pistons

30 and 50 to cause contraction of the linear actuator 18.

When, for example, on-off

valves

72 and 82 are both closed while on-off valves 70, 76 and 78 are opened, fluid pressure delivered from the connection port 14 of pump 12 will be communicated to both

pressure chambers

32 and 54, while the chamber 34 is exhausted back to the pump 12 via port connection 16. It can thus be seen that the net force differential acting on the two

pistons

30 and 50 to cause expansion of the device will be equal to the difference between the effective area of the main piston side 42 and the area of the secondary piston side 56. Since the area of piston side 56 is equal to the cross-sectional area of the main piston rod 38, the net effective responsive area tending to expand the linear actuator 18 will be equal to the area of the piston side 42 minus the area of the main piston rod 38. Thus, an intermediate force for expanding the actuator 18 may be obtained. Since

chambers

32 and 54 are in communication and chamber 54 is being contracted as chamber 32 is expanded, the fluid in chamber 54 will flow into chamber 32, which results in a faster rate of expansion of the actuator 18. When the flow from the pump 12 is reversed, fluid pressure will be communicated to chamber 34 to contract the actuator 18 at the same rate and at the same force as during expansion, while the pressure chamber 32 is exhausted to the pump 12 and chamber 54. It should be noted that as the

pistons

30 and 50 are being stroked to expand the actuator 18, the fluid within the pressure chamber 54 is being compressed and cycled into chamber 32, and when the actuator 18 is being contracted and the pressure chamber 32 is being contracted, a portion of the fluid is returned to pressure chamber 54 as the same is being expanded, whereby a smaller volume of fluid is required from the pump 12, thereby increasing both the speed of contraction and expansion of the actuator, while the output force is decreased.

When the on-off valves 76 and 78 are closed and the on-off

valves

70, 72 and 82 are opened, fluid underpressure will be delivered from port connection 14 to both

chambers

32 and 34, while pressure chamber 54 is being exhausted. Since the net pressure responsive area tending to expand the actuator 18 will be equal to the cross-sectional area of the piston rod 38, a relatively small force is obtained for expanding the actuator 18. When the piston 30 is moved rightwardly to expand chamber 32, fluid from the pressure chamber 34 is cycled back to the chamber 32, and when the piston 30 is moved leftwardly to contract chamber 32, fluid from chamber 32 will be directed to both the chamber 34 and the pump 12, such that the amount of fluid being directed to and from the pump 12 is equal during both the expansion and the contraction phases of the actuator. Since a small volume of fluid is required from the pump 12, the actuator 18 will expand and contract at a substantially greater velocity than in the heretofore described examples, while the force imparted by the actuator will be reduced to a minimum.

Referring now to FIG. 2 there is illustrated another example of the present invention in the form of a fluid system 89 comprising a fluid linear actuator 90 having the same components as the actuator 18 and which components are designated by the same reference numerals as the actuator 18 illustrated in FlG. 1. The actuator 90 is illustrated as comprising the main cylinder 24 and two secondary cylinders 44 and 44', the piston rods 58 and 58' of which are coupled to the main piston rod 38 by connecting member 62, such that the pistons 50 and 50' of the two secondary cylinders move with main piston 30 as a unit. The effective pressure responsive area of the piston side 42 is substantially equal to the sum of the pressure responsive areas of the main piston side 36 and the piston sides 56 and 56 of the two

secondary pistons

50 and 50, respectively. In the embodiment illustrated in FIG. 2, the pressure chamber 34 is in constant fluid communication with the two secondary pressure chambers 54 and 54' via apertures 92 and 93 extending through the walls of the

tubular members

26, 46' and 46.

The

port connections

14 and 16 of the pump 12 are respectively connected by

conduits

93 and 95 to the port connections of a directional control valve 94 which is adapted to selectively communicate fluid from the

port connections

14 and 16 to

conduits

96 and 98 which, in turn, are respectively communicated to the main fluid chamber 32 and the

fluid chambers

34, 54 and 54' of the actuator 90. In the embodiment illustrated, high pressure fluid is delivered from the unidirectional pump 12 through port connection 16.

Movement of the directional control valve 94 is controlled by a pilot valve 100 having an inlet port in constant communication with the high pressure delivery port connection 16 of the pump 12. When the pilot valve 100 is shifted to one position, high pressure fluid from the conduit 95 is communicated via a line 101 to one side of the directional control valve 94 to shift the same, whereby fluid pressure is communicated to conduit 96, while the other conduit 98 is communicated to the pump port connection 14. When the pilot valve 100 is shifted in the opposite direction, fluid pressure is communicated via line 103 to the opposite side of the directional control valve 94 to shift the same in an opposite direction, thereby reversing the flow of fluid between the pump 12 and the actuator 90, that is, fluid pressure from pump port connection 16 is communicated to the conduit 98 while fluid is returned to pump port connection from the conduit 96. A shuttle valve 97, similar to the hereinbefore described shuttle valve 84, is provided in the system 89 between

conduits

96 and 98 and functions to prevent over pressurization of the actuator 90, and at the same time relieves any excess fluid in the event the aforementioned area differentials are not equal due to variations in manufacturing tolerances.

The pilot valve and the directional control valve 94 are conventional in their construction and operation and thus a further detailed description of their construction and operation is not deemed to be necessary.

The

conduits

96 and 98, respectively, have pilot operated

check valves

102 and 104 and the conduit 96 is provided with a sequence valve intermediate the check valve 102 and the chamber 32. The check valve 102 and the sequence valve 105 permit unrestricted flow from the pump 12 through directional control valve 94 and conduit 96 to the pressure chamber 32, wherein the pressure exerts a force on the piston 30 to move the same rightwardly (as viewed in H6. 2) to expand the fluid cylinders, while the check valve 104 permits unrestricted flow from the pump 12 through directional control valve 94 and conduit 98 to the

pressure chambers

34, 54 and 54', wherein the pressure exerts a force on the

piston

30, 50 and 50 to move them leftwardly (as viewed in FIG. 2) to retract the pistons in the fluid cylinders. However, the check valves normally function to prevent the flow of fluid through

conduits

96 and 98 from the pressure chambers back to the pump 12. In order to permit a backflow from the actuator to the pump 12 through-either of the

check valves

102 or 104, the pilot portions thereof are respectively communicated via

lines

106 and 108 to the high

pressure outlet lines

103 and 101 of the pilot valve 100, such that when the valve 100 is shifted in a manner to communicate pressure via line 101 to one side of the directional control valve 94 to actuate the same so as to communicate high pressure fluid from the pump 12 to the conduit 96, the pilot check valve 104 is opened by the pressure fluid from line 101 and permits the fluid in conduit 98 to flow from the

pressure chambers

34, 54 and 54 back to the pump port connection 14; whereas when the pilot valve 106 is shifted in the opposite direction so as to communicate pressure fluid via line 103 to the directional control valve 94 to actuate the same so as to communicate-pressure fluid from pump 12 to

chambers

34, 54 and 54 via conduit 98, the check valve 102 is opened by pressure from line 103 and permits the fluid in conduit 96 to flow from pressure chamber 32 back to pump 12. When the system 89 is not being operated but the actuator 90 is required to hold a load in a fixed, for example a vertical or raised position, the

check valves

102 and 104 will remain closed and thus function to provide a positive hold of the position of the actuator 90 since the fluid within the

chambers

32, 34, 54 and 54' is trapped therewithin and the pistons are restrained from reciprocal movement, whereby the position of the load held by the actuator 90 is fixed.

Referring now to FIG. 3 wherein there is illustrated a schematic circuit diagram of a fluid system 110 embodying a third example of the present invention and comprising a linear actuator 112, which is the same as the linear actuator 18 disclosed in FIG. 1, and the corresponding elements of both actuators 18 and 112 are identified by the same numeral designations. The pump 12 is of the reversible, variable displacement type having the

outlet connection ports

14 and 16 respectively in fluid communication with the

pressure chambers

32 and 34 of the main cylinder 24 via conduits 114 and 116. Conduit 114 also communicates with a variable displacement motor 118 via a branch conduit 120, while the conduit 116 communicates with a second variable displacement motor 122 via a second branch conduit 124. The outlet ports of the

variable displacement motors

118 and 122 communicate with each other and to the pressure chamber 54 of the secondary cylinder 44 via conduit 126.

The

variable displacement motors

118 and 122 are coupled to each other by a suitable mechanical coupling means 128 in such a manner that as the displacement of the variable displacement motor 118 is increased the displacement of the variable displacement motor 122 is decreased, and vice versa.

An example of constructing the

variable displacement motors

118 and 122 to achieve a synchronized operation is illustrated in FIG. 4 as comprising a unitary housing construction 140 having a longitudinal bore 142 enclosed at its opposite ends by valve plates 144 and 146 that define the inlets and outlets of the

motors

118 and 122, respectively, and which communicate in the conventional manner with a plurality of arcuately spaced, parallel cylinder bores 148 and 149 respectively formed in a pair of axially spaced cylinder barrels 150 and 152 respectively. The cylinder barrels 150 and 152 are rotatably mounted on a common shaft 154 which, in turn, is rotatably supported at its opposite ends by bearings 156 and 158 carried in the valve plates 144 and 146, respectively. The cylinder barrel 150 has a plurality of pistons 160 reciprocably mounted within each of the cylinder barrel bores 148, with the outer ends of each of the pistons slidably engaging a thrust bearing surface 162 formed on one side of a triangularly shaped thrust block 163 which is adapted to rotate about an axis 164 that is transversely disposed with respect to the axis of rotation of the cylinder barrels 150 and 152. Similarly the cylinder barrel 152 has a plurality of pistons 166 reciprocably mounted within each of the cylinder barrel bores 149 with each piston 166 having outer ends that slidably engage a thrust bearing surface 168 formed on another of the triangular sides of the thrust block 163. As can be seen with reference to FIG. 3, as the thrust block 163 is rotated about the axis 164 to incline the thrust bearing surface 168 with respect to the axis of rotation of the cylinder barrel 152, the displacement of the motor 122 is increased, while at the same time the thrust bearing surface 162 is rotated toward a position wherein it lies in a plane that is perpendicular to the longitudinal axis of the cylinder barrel 150 and thus decreases the stroke of the pistons 160 therewithin and thus decreases the displacement of the motor 118. Similarly when the thrust block 163 is rotated in an opposite direction the displacement of the motor 118 is increased while the displacement of the motor 122 is decreased. If the swash block is made symmetrical, the displacement of one of the motors will be decreased in proportion to the increase of the displacement of the other motor and thus the synchronized manner of operation hereinbefore described can be easily achieved.

In operation, the displacements of the variable displacement motors 1 18 and 122 are preset to allow fluid to flow through at a predetermined rate. If, for example, the volume of the pressure chamber 32 is adapted to receive 14 cubic inches of fluid, while the volume of the chamber 34 is adapted to receive 4 cubic inches, the effective rate of expansion and contraction of the chambers may be selectively varied over a wide range in the following manner. For example, when the pump 12 is delivering 10 cubic inches per second and the variable displacement motor 118 is permitting 4 cubic inches of flow therethrough as the chamber 54 is compressed, that is, chamber 32 is expanding, a total of 14 cubic inches per second would be delivered to the chamber 32 and the pistons would advance 1 inch. However, if the variable displacement motor 118 is preset to permit only 1 cubic inch of flow between the

chambers

54 and 32, the pistons will advance only a distance of 0.786 inch. In this condition, the variable displacement motor 122 must deliver 2.14 cubic inches, which in combination with the 7.86 cubic inches of fluid exhausted from chamber 34 provides a total of 10 cubic inches per second of fluid to the pump and thus the amount of fluid delivered to the pump 12 is equal to the output.

It can be seen that by decreasing the displacement of the motor 118 to values less than 4 cubic inches, while increasing the displacement of the variable displacement motor 122 up from a zero displacement to a total of 4 cubic inches, the rate of expansion or contraction, depending upon the direction of flow of pressure fluid from pump 12, may be selectively varied over the entire flow range of the actuator 112.

It can thus be seen that the present invention has provided a fluid system which utilizes a main fluid cylinder in conjunction with one or more secondary fluid cylinders and in which the effective pressure responsive area for expansion of the cylinders is equal to the effective pressure responsive areas for contraction of the cylinders and in which the rate of flow of fluid and force exerted by the fluid cylinders may be varied over a wide range of operation.

Although several embodiments of the present invention have been disclosed, it is to be understood by those skilled in the art of fluid systems that other embodiments may be had and modifications may be made to the present systems without departing from the spirit of the invention or from the scope of the appended claims.

What is claimed is as follows:

1. A fluid system comprising:

a first tubular member having an internal bore closed at its opposite ends;

a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers;

a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends;

first valve means for communicating fluid pressure to the pressure chamber associated with the rod side of said piston to generate a force against said piston to move same in one direction;

second valve means for communicating fluid pressure to the pressure chamber on the side of said piston opposite said rod to generate a force against said piston to move said piston in an opposite direction;

a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from andparallel to the longitudinal axis of said first tubular member;

a second piston reciprocally mounted within said second tubular member bore and defining an expansible pressure chamber;

a piston rod carried on one side of said second piston and extending through the pressure chamber of said second tubular member and externally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods such that said pistons reciprocate together;

third valve means for communicating a pressure fluid to the pressure chamber of said second tubular member for exerting a force on the rod side of said second piston for moving said second piston in one direction;

the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod sides of said first mentioned piston and said second piston;

a fluid pump having an outlet for delivering fluid under pressure at a selected rate of flow and an inlet for receiving fluid at the same selected rate of flow;

said first and second valve means being adapted to, first, communicate said pump outlet to the rod side pressure chambers of said first tubular member and said second tubular member, while communicating said pump inlet to the pressure chamber of said first tubular member opposite said rod for moving said first and second pistons in said one direction at a predetermined velocity and with a predetermined force;

said first and second valve means being adapted to,

second, communicate said pump outlet to the pressure chamber of said first tubular member opposite said rod and to communicate said pump inlet to the rod side pressure chambers of said first tubular member and said second tubular member for moving said first and second pistons in said opposite direction at said predetermined velocity and with a said predetermined force.

2. A fluid system comprising:

a first tubular member having an internal bore closed at its opposite ends;

second valve means for communicating fluid pressure to the pressure chamber on the side of said piston opposite said rod to generate a force against said piston to move said piston in one direction;

a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member;

means for connecting the extended external ends of said piston rods such that said. pistons reciprocate together;

the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod sides of said first mentioned piston and. said second piston;

the rod side pressure chamber of said first tubular member being in constant fluid communication with said second tubular member pressure chamher;

.a first conduit means connecting said second valve means to the pressure chamber of said first tubular member opposite said rod;

second conduit means connecting the rod side pressure chamber of said first tubular member and the pressure chamber of said second tubular member to said first valve means;

said first and second valve means being adapted to selectively direct fluid under pressure to one of said conduit means while exhausting fluid pressure from the other of said conduit means;

check valve means disposed in each of said conduit means for normally preventing flow from said pressure chambers to said first and second valve means, said check valve means being responsive to open and permit fluid to flow from said pressure chambers through their associated conduits when said first and second valve means are actuated to direct fluid pressure to the other of said conduits. r 3. The fluid system defined in claim 1 wherein said first and second valve means are further adapted to communicate said pump outlet to the rod side pressure chambers of said first tubular member and said second tubular member while the pressure chamber opposite said rod of said first tubular member is communicated to said pump inlet to exert a second force on said piston the outlet of said fluid pump to said pressure chamber of said first tubular member opposite said rod, while communicating the inlet of said pump to the rod side pressure chambers of said first tubular member and said second tubular member to move said piston in said other direction at the said second velocity and with said second force.

4. The fluid system defined in claim 1 wherein said first and second valve means are further adapted to communicate said pump outlet to both fluid pressure chambers of said first tubular member and communicate said inlet of said pump to said second tubular member pressure chamber to move said piston in said one direction at a second velocity greater than said pre determined velocity and a second force less than said predetermined force, and said first and second valve means being adapted to communicate the outlet of said pump to said second tubular member pressure chamber while communicating the inlet of said pump to both pressure chambers of said first tubular member to move said pistons in said other direction at said second speed and second force.

5. A fluid system comprising:

a first tubular member having an internal bore closed at its opposite ends;

a second tubular member having an internal bore closed at oneend, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member;

the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the cffective pressure responsive areas of the rod sides of said first mentioned piston and said second piston;

a plurality of said second tubular members, each having a second piston connected to said first piston, the effective pressure responsive area of the side of said rod being substantially equal to the sum of the effective pressure responsive area of the rod side of said first piston and the effective pressure responsive areas of the sides of said second pistons associated with said second tubular members.

6. The system defined in claim 2 wherein said check valves are pilot operated valves and are operable to open in response to a control pressure communicated to said first and second valve means.

7. A linear actuator comprising:

a first tubular member having an internal bore enclosed at its opposite ends;

a piston rod carried on one side of said piston and extending through one of said pressure chambers and externally of said tubular member to one of said closed ends;

said one pressure chamber being adapted to be communicated to a source of fluid pressure to generate a force against the rod side of said piston to move same in one direction;

the pressure chamber opposite said rod being adapted to be communicated to said source of fluid pressure to generate a force against the other side of said piston to move same in an opposite direction;

a second tubular member having an internal bore at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member;

a second piston reciprocably mounted within said second tubular member bore and defining an expansible pressure chamber therein;

a piston rod carried on one side of said second piston and extending through said last mentioned pressure chamber and externally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods such that said first and second pistons reciprocate together;

said second tubular member pressure chamber being adapted to be communicated to a source of pressure fluid to exert a force on the side of said second piston carrying said piston rod to move said second piston in said one direction; and

the effective pressure responsive area of the side of said first piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod side of said first piston and the side of said second piston associated with said second tubular member pressure chamber,

said linear actuator further comprising a plurality of said second tubular members, each having a second piston connected to said first piston, the effective pressure responsive area of the side of said first piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod side of said first piston and of the rod sides of said second pistons.

8. A linear actuator comprising:

a first tubular member having an internal bore closed at its opposite ends;

a piston reciprocally mounted within said bore and dividing said bore into two expansible chambers; a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends;

a second tubular member having an'internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member,

a piston rod carried on one side of said second piston and extending through said second tubular member pressure chamber and externally of said second tubular member through said closed end;

means for variably supplying fluid pressure to said chambers to thereby vary the force generated by said linear actuator said means comprising a first and second variable displacement motoring means connected with said chambers and control means coupling said first and second variable displacement means such that the displacement of one of said motoring means is increased as the other of saidmotoring means is decreased.

9. A fluid system comprising:

a first tubular member having an internal bore enclosed at its opposite ends;

a source of fluid pressure;

said pressure chamber on the rod side of said piston being adapted to be communicated to said source of fluid pressure to generate a force against said piston to move same in one direction;

the pressure chamber on the side of said piston opposite said rod being adapted to be communicated to said source of fluid pressure to generate a force against said piston to move same in an opposite direction;

a second piston reciprocably mounted within said second tubular member bore and defining a second expansible pressure chamber therein;

a piston rod carried on one side of said second piston and extending through said second pressure chamber and externally of said second tubular member through said closed end;

means for connecting the extended external ends of said piston rods such that said first and second pislld tons reciprocate together;

first and second variable displacement motoring means, said first motoring means having an inlet connected to the pressure chamber on the rod side of said first piston and an outlet connected to said second pressure chamber, said second motoring means having an inlet connected to pressure chamber opposite said rod and an outlet connected to said second pressure chamber, displacement control means coupling said first and second motoring means such that the displacement of one of said motoring means is increased as the displacement of the other is decreased;

said second tubular member pressure chamber being adapted to be selectively communicated to said source of pressure fluid through said first and second motoring means to exert a force on the rod side of said second piston to move said second piston in said one direction; and

the effective pressure responsive area of the side of said first piston opposite said. rod being substantially equal to the sum of the effective pressure responsive area of the rod side of said first piston and the effective pressure responsive area of said side of said second piston associated with said second tubular member pressure chamber.

10. The fluid system defined in claim 9 wherein said first and second motoring means comprise:

a housing having a longitudinally disposed bore;

a pair of axially spaced cylinder barrels rotatably mounted about a common axis in said housing bore, each of said cylinder barrels having a plurality of arcuately spaced, parallel bores opening to the opposite faces of each of said cylinder barrels;

first valving means at one end of said housing bore communicating one face of one of said cylinder barrels with high and low pressure passages defining said first motoring means inlet and outlet;

second valving means at the other end of said housing bore communicating one face of the other of said cylinder barrels with high and low pressure passages defining said second motoring means inlet and outlet;

first piston means reciprocably mounted in said plurality of bores in said one cylinder barrel and having outer ends extending therefrom for engagement with a thrust surface, the amount of inclination of which controls the amount of displacement of said one cylinder barrel and thus the amount of displacement of said first motoring means;

a second piston means reciprocably mounted in said plurality of bores in said other cylinder barrel and having outer ends extending therefrom for engagement with a second thrust surface, the amount of inclination of which controls the displacement of said other cylinder barrel and] thus the displacement of said second motoring means, said first and second thrust surfaces being formed on the opposite side of a common swash plate adapted to be pivoted in one direction to increase the displacement of one of said motoring means while simultaneously and in synchronism decreasing the dis placement of the other motoring means.

:I: :9: 1|: m at;

Patent No. 3 I 792 l 43 February 19, 1974 Dated Inventor(s) (SEAL) Atteat:

EDWARD .FLETCHER, JR. Attesting Dfficer FORM PO-IOSO (10-69) Robert E. Soheafer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 60, delete "main".

Col. 4, line l3, after "pistons", insert -30 and 5 O-.

Col. 5, line 49, delete "93" and insert 91--. N

Col. 6, line 34, after "pistons", insert --30, 50 and 50--;

line 35, after "valves", insert l02 and l04-; line 52, delete "106", and insert lOO-.

Col. 7, line 43, after "pistons", insert -l60-.

Col. 9, delete lines 66-67.

Col. 10, delete lines 1-2. 7

Col. 11, delete lines 36-39.

Signed and sealed this 11th day of June 197A.

0. 'MARSHALL DANN Commissioner of Patents USCOMlM-DC 603 76'P69 u.s. GOVERNMENT PRINTlNG OFFICE: um c ase-3:5

Claims

1. A fluid system comprising: a first tubular member having an internal bore closed at its opposite ends; a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers; a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends; first valve means for communicating fluid pressure to the pressure chamber associated with the rod side of said piston to generate a force against said piston to move same in one direction; second valve means for communicating fluid pressure to the pressure chamber on the side of said piston opposite said rod to generate a force against said piston to move said piston in an opposite direction; a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member; a second piston reciprocally mounted within said second tubular member bore and defining an expansible pressure chamber; a piston rod carried on one side of said second piston and extending through the pressure chamber of said second tubular member and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said pistons reciprocate together; third valve means for communicating a pressure fluid to the pressure chamber of said second tubular member for exerting a force on the rod side of said second piston for moving said second piston in one direction; the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod sides of said first mentioned piston and said second piston; a fluid pump having an outlet for delivering fluid under pressure at a selected rate of flow and an inlet for receiving fluid at the same selected rate of flow; said first and second valve means being adapted to, first, communicate said pump outlet to the rod side pressure chambers of said first tubular member and said second tubular member, while commUnicating said pump inlet to the pressure chamber of said first tubular member opposite said rod for moving said first and second pistons in said one direction at a predetermined velocity and with a predetermined force; said first and second valve means being adapted to, second, communicate said pump outlet to the pressure chamber of said first tubular member opposite said rod and to communicate said pump inlet to the rod side pressure chambers of said first tubular member and said second tubular member for moving said first and second pistons in said opposite direction at said predetermined velocity and with a said predetermined force.

2. A fluid system comprising: a first tubular member having an internal bore closed at its opposite ends; a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers; a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends; first valve means for communicating fluid pressure to the pressure chamber associated with the rod side of said piston to generate a force against said piston to move same in one direction; second valve means for communicating fluid pressure to the pressure chamber on the side of said piston opposite said rod to generate a force against said piston to move said piston in an opposite direction; a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member; a second piston reciprocally mounted within said second tubular member bore and defining an expansible pressure chamber; a piston rod carried on one side of said second piston and extending through the pressure chamber of said second tubular member and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said pistons reciprocate together; third valve means for communicating a pressure fluid to the pressure chamber of said second tubular member for exerting a force on the rod side of said second piston for moving said second piston in one direction; the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod sides of said first mentioned piston and said second piston; the rod side pressure chamber of said first tubular member being in constant fluid communication with said second tubular member pressure chamber; a first conduit means connecting said second valve means to the pressure chamber of said first tubular member opposite said rod; second conduit means connecting the rod side pressure chamber of said first tubular member and the pressure chamber of said second tubular member to said first valve means; said first and second valve means being adapted to selectively direct fluid under pressure to one of said conduit means while exhausting fluid pressure from the other of said conduit means; check valve means disposed in each of said conduit means for normally preventing flow from said pressure chambers to said first and second valve means, said check valve means being responsive to open and permit fluid to flow from said pressure chambers through their associated conduits when said first and second valve means are actuated to direct fluid pressure to the other of said conduits.

3. The fluid system defined in claim 1 wherein said first and second valve means are further adapted to communicate said pump outlet to the rod side pressure chambers of said first tubular member and said second tubular member while the pressure chamber opposite said rod of said first tubular member is communicated to said pump inlet to exert a second force on said piston of a lesser value than said first predetermined value aNd to move said pistons in said one direction at a second velocity greater than said predetermined velocity but with the same volume of fluid from said pump; and said first and second valve means being adapted to connect the outlet of said fluid pump to said pressure chamber of said first tubular member opposite said rod, while communicating the inlet of said pump to the rod side pressure chambers of said first tubular member and said second tubular member to move said piston in said other direction at the said second velocity and with said second force.

4. The fluid system defined in claim 1 wherein said first and second valve means are further adapted to communicate said pump outlet to both fluid pressure chambers of said first tubular member and communicate said inlet of said pump to said second tubular member pressure chamber to move said piston in said one direction at a second velocity greater than said predetermined velocity and a second force less than said predetermined force, and said first and second valve means being adapted to communicate the outlet of said pump to said second tubular member pressure chamber while communicating the inlet of said pump to both pressure chambers of said first tubular member to move said pistons in said other direction at said second speed and second force.

5. A fluid system comprising: a first tubular member having an internal bore closed at its opposite ends; a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers; a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends; first valve means for communicating fluid pressure to the pressure chamber associated with the rod side of said piston to generate a force against said piston to move same in one direction; second valve means for communicating fluid pressure to the pressure chamber on the side of said piston opposite said rod to generate a force against said piston to move said piston in an opposite direction; a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member; a second piston reciprocally mounted within said second tubular member bore and defining an expansible pressure chamber; a piston rod carried on one side of said second piston and extending through the pressure chamber of said second tubular member and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said pistons reciprocate together; third valve means for communicating a pressure fluid to the pressure chamber of said second tubular member for exerting a force on the rod side of said second piston for moving said second piston in one direction; the effective pressure responsive areas of the rod sides of said first mentioned piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod sides of said first mentioned piston and said second piston; a plurality of said second tubular members, each having a second piston connected to said first piston, the effective pressure responsive area of the side of said rod being substantially equal to the sum of the effective pressure responsive area of the rod side of said first piston and the effective pressure responsive areas of the sides of said second pistons associated with said second tubular members.

7. A linear actuator comprising: a first tubular member having an internal bore enclosed at its opposite ends; a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers; a piston rod carried on one side of said piston and extending through one of said pressure chambers and externally of said tubular member to one of said closed ends; said one pressure chamber being adapted to be communicated to a source of fluid pressure to generate a force against the rod side of said piston to move same in one direction; the pressure chamber opposite said rod being adapted to be communicated to said source of fluid pressure to generate a force against the other side of said piston to move same in an opposite direction; a second tubular member having an internal bore at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member; a second piston reciprocably mounted within said second tubular member bore and defining an expansible pressure chamber therein; a piston rod carried on one side of said second piston and extending through said last mentioned pressure chamber and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said first and second pistons reciprocate together; said second tubular member pressure chamber being adapted to be communicated to a source of pressure fluid to exert a force on the side of said second piston carrying said piston rod to move said second piston in said one direction; and the effective pressure responsive area of the side of said first piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod side of said first piston and the side of said second piston associated with said second tubular member pressure chamber, said linear actuator further comprising a plurality of said second tubular members, each having a second piston connected to said first piston, the effective pressure responsive area of the side of said first piston opposite said rod being substantially equal to the sum of the effective pressure responsive areas of the rod side of said first piston and of the rod sides of said second pistons.

8. A linear actuator comprising: a first tubular member having an internal bore closed at its opposite ends; a piston reciprocally mounted within said bore and dividing said bore into two expansible chambers; a piston rod carried on one side of said piston and extending externally of said tubular member through one of said closed ends; a second tubular member having an internal bore closed at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member, a second piston reciprocally mounted within said second tubular member bore and defining an expansible pressure chamber; a piston rod carried on one side of said second piston and extending through said second tubular member pressure chamber and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said pistons reciprocate together; means for variably supplying fluid pressure to said chambers to thereby vary the force generated by said linear actuator said means comprising a first and second variable displacement motoring means connected with said chambers and control means coupling said first and second variable displacement means such that the displacement of one of said motoring means is increased as the other of said motoring means is decreased.

9. A fluid system comprising: a first tubular member having an internal bore enclosed at its opposite ends; a source of fluid pressure; a piston reciprocally mounted within said bore and dividing said bore into two expansible pressure chambers; a piston rod carried on one side of said piston and extending through one of said pressure chambers and externally of said tUbular member to one of said closed ends; said pressure chamber on the rod side of said piston being adapted to be communicated to said source of fluid pressure to generate a force against said piston to move same in one direction; the pressure chamber on the side of said piston opposite said rod being adapted to be communicated to said source of fluid pressure to generate a force against said piston to move same in an opposite direction; a second tubular member having an internal bore at one end, said second tubular member having a longitudinal axis radially spaced from and parallel to the longitudinal axis of said first tubular member; a second piston reciprocably mounted within said second tubular member bore and defining a second expansible pressure chamber therein; a piston rod carried on one side of said second piston and extending through said second pressure chamber and externally of said second tubular member through said closed end; means for connecting the extended external ends of said piston rods such that said first and second pistons reciprocate together; first and second variable displacement motoring means, said first motoring means having an inlet connected to the pressure chamber on the rod side of said first piston and an outlet connected to said second pressure chamber, said second motoring means having an inlet connected to pressure chamber opposite said rod and an outlet connected to said second pressure chamber, displacement control means coupling said first and second motoring means such that the displacement of one of said motoring means is increased as the displacement of the other is decreased; said second tubular member pressure chamber being adapted to be selectively communicated to said source of pressure fluid through said first and second motoring means to exert a force on the rod side of said second piston to move said second piston in said one direction; and the effective pressure responsive area of the side of said first piston opposite said rod being substantially equal to the sum of the effective pressure responsive area of the rod side of said first piston and the effective pressure responsive area of said side of said second piston associated with said second tubular member pressure chamber.

10. The fluid system defined in claim 9 wherein said first and second motoring means comprise: a housing having a longitudinally disposed bore; a pair of axially spaced cylinder barrels rotatably mounted about a common axis in said housing bore, each of said cylinder barrels having a plurality of arcuately spaced, parallel bores opening to the opposite faces of each of said cylinder barrels; first valving means at one end of said housing bore communicating one face of one of said cylinder barrels with high and low pressure passages defining said first motoring means inlet and outlet; second valving means at the other end of said housing bore communicating one face of the other of said cylinder barrels with high and low pressure passages defining said second motoring means inlet and outlet; first piston means reciprocably mounted in said plurality of bores in said one cylinder barrel and having outer ends extending therefrom for engagement with a thrust surface, the amount of inclination of which controls the amount of displacement of said one cylinder barrel and thus the amount of displacement of said first motoring means; a second piston means reciprocably mounted in said plurality of bores in said other cylinder barrel and having outer ends extending therefrom for engagement with a second thrust surface, the amount of inclination of which controls the displacement of said other cylinder barrel and thus the displacement of said second motoring means, said first and second thrust surfaces being formed on the opposite side of a common swash plate adapted to be pivoted in one direction to increase the displacement of one of said motoring means while simultaneously and in syNchronism decreasing the displacement of the other motoring means.