WO1994000691A1

WO1994000691A1 - Radial piston pump

Info

Publication number: WO1994000691A1
Application number: PCT/CA1993/000264
Authority: WO
Inventors: George Kadlicko
Original assignee: Microhydraulics Inc.
Priority date: 1992-06-22
Filing date: 1993-06-22
Publication date: 1994-01-06
Also published as: AU4414593A; GB9213216D0

Abstract

A variable capacity radial piston machine (10) comprising a housing (12), a rotor (24) rotatably supported in said housing (12) and having at least one radially extending cylinder (40) formed therein, a piston (46) movable in said cylinder (40) in a radial direction, a stator (56) radially spaced from said rotor (24) and having a cam surface (54) engageable by said piston (46) as said rotor (24) rotates, said stator (56) being ajustable relative to said rotor (24) in a radial plane to vary the radial spacing between said cam surface (54) and rotor (24) and thereby the stroke of said piston (46) in said cylinder (40), said cam surface (54) being rotatable relative to said stator (56), an electric control device to control the capacity of the machine (10).

Description

RADIAL PISTON PUMP

The present invention relates to hydraulic machines commonly known as hydraulic pumps or motors. Hydraulic machines are well known and are in use in a variety of applications. Fixed capacity machines provide a constant displacement of fluid for each cycle of the machine and frequently take the form of gear pumps. Variable capacity machines provide a mechanism for regulating the displacement for each cycle of the machine so that the flow rate delivered may be adjusted or the mechanical advantage and hence torque may be adjusted. Variable capacity machines have typically been the axial piston type of machine in which pistons are displaced axially in a direction parallel to the axis of rotation and the stroke of the piston is adjusted by a swashplate or a tilting head. The swashplate is angularly adjustable in a direction transverse to the axis of rotation and acts to transfer fluid energy into mechanical rotation or vice versa. Such pumps are, however, relatively complicated, particularly in the design of the swashplate where relatively high axial forces are resolved to produce a transverse orbiting load. These high loads tend to lead to friction forces in the swashplate mechanism which makes adjustment difficult.

Radial piston machines utilize a stator and rotor that are eccentric to one another so that as the rotor rotates, the piston reciprocates within the cylinder. By adjusting the eccentricity of the rotor and stator, the capacity of the machine may be adjusted. With such arrangements there is the disadvantage that slip must be acco odated between the pistons and the stator. Such machines, however, are relatively compact and can be used with relatively high pressures.

Variable capacity machines may be controlled in a variety of ways. The machine may be pressure compensated so that its capacity is adjusted to maintain a constant delivery pressure. Alternatively, the machine may be horsepower-controlled so that flow and pressure are monitored and the capacity is adjusted to maintain the hydraulic horsepower substantially constant.

Adjustment of the displacement is usually performed by a servo motor whose movement is controlled by a servo valve. The nature of the servo valve and motor will depend upon the manner in which the machine is to be controlled, so that a pressure-controlled machine will utilize a different servo valve than a power- controlled machine. As a result, it is necessary to have a number of different machine configurations to cover all possibilities. It is therefore an object of the present invention to provide an improved controller for a hydraulic machine.

It is a further object of the invention to provide an improved radial piston hydraulic machine. According to a first aspect of the present invention, there is provided a variable capacity radial piston machine comprising a housing, a rotor rotatably supported in said housing and having at least one radially extending cylinder formed therein, a piston movable in said cylinder in a radial direction, a stator radially spaced from said rotor and adjustable relative to said rotor in a radial plane to vary the stroke of said piston in said cylinder, said stator having a cam surface engageable by said piston as said rotor rotates, said cam surface being rotatable relative to the stator. As a preference, the cam surface is formed with an inclined face that is engaged by the pistons to one side of the longitudinal axis of the pistons. In this way, slippage between the piston and cam ring tends to induce rotation of the piston in its cylinder.

As a further preference, alternate pistons are staggered axially and the cam surface is formed with a pair of oppositely directed inclined faces, each engaged by an alternate one of said pistons.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which

Figure 1 is an end view of a radial piston machine having a portion thereof shown in section for clarity;

Figure 2 is a view on the line 2-2 of Figure 1; Figure 3 is a view on the line 3-3 of Figure 1;

Figure 4 is a view on the line 4-4 of Figure 2 on an enlarged scale;

Figure 5 is a schematic representation of a controller for the pump shown in Figure 1; Figure 6 is a sectional view of an alternative embodiment of a radial piston machine;

Figure 7 is a view on the line 7-7 of Figure 6; and

Figure 8 is a sectional view of a transmission using the radial piston machines shown in Figures 6 and 7.

Referring therefore to the drawings and in particular Figures 1 and 2, a radial piston hydraulic machine 10 has a housing 12 comprising a body 14 and a front cover 16. The body 14 and front cover 16 are secured to one another by bolts 20 to provide an internal cavity 22.

A rotor assembly 24 is rotatably supported in the housing 12 by means of a pair of roller bearings 26,28. Rotor assembly 24 includes a rotor body 25 and a forwardly projecting shaft 27. The roller bearing 26 is secured on a shoulder 30 formed on the rear of the rotor body 25. The bearing 28 is carried by the front cover 16 and engages the shaft 27 as it projects forwardly through the front cover 16. A seal 34 seals between the front cover 16 and the shaft 27. The rotor assembly 24 is thus free to rotate about an axis indicated at AA within the housing 12. Shaft 27 may be connected to a pulley or gear to transmit torque either to or from the rotor assembly 24.

As can best be seen in Figure 2, a plurality of axial ducts 36 are formed in the body 25. The ducts 36 extend axially from rear face 38 of the rotor body 25 arid are uniformly spaced about the axis AA. At the opposite end to the face 38, each of the ducts 36 communicates with a pair of radially extending cylinders 40,42. The cylinders 40,42 extend radially from a respective axial duct 36 to the peripheral surface 44 of rotor body 25. As can be seen from Figure 2, the cylinders 40,42 are axially offset from one another so as to intersect the respective axial duct 36 at axially spaced locations. Rotor assembly 24 thus has alternate cylinders 40,42 axially spaced or staggered relative to the circumferentially adjacent cylinder.

Each of the cylinders 40,42 receives a piston 46 which is slidable in a radial direction but maintains sealing contact between the cylinder 40,42 and the piston 46. The radially outer end 48 of the piston 46 is conical and bears against a respective one of a pair of inclined surfaces 50,52 of a cam ring 54. Cam ring 54 is part of a stator 56 which includes a carrier 57 and a bearing 58. Bearing 58 permits rotation of the cam ring 54 relative to the carrier 57. Carrier 57 is pivotally connected to the housing 12 by a pin 59. Pin 59 is supported in bearings 60,62 located in the front cover 16 and body 14 respectively. Diametrically opposite the pin 59, the carrier

57 is formed with a radially extending ear 66 that carries an axially extending pin 68. The pin 68 engages with a slot 70 formed in a piston 72 of a servo motor 74, the details of which will be described below. As can best be seen from Figure 3, the pivotal mounting of the stator 56 to the housing 12 permits adjustment of the radial spacing between the peripheral surface 44 of the rotor body 25 and the inclined surfaces 50,52 of the cam ring. The cam ring 54 is circular and by adjusting its eccentricity relative to the rotor assembly 24 - that is, by adjusting the radial spacing between the ring 54 and the surface 44 - the stroke of the piston 46 within the cylinders 40,42 can be adjusted. It will also be noted that the stator 56 can be moved to a position in which the cam ring 54 is concentric with the axis of rotation of rotor assembly 24 so that zero displacement is obtained. By moving the stator 56 through the concentric or null position, a hydraulic reversal of the machine can also be obtained.

The flow of fluid through the machine 12 is best seen from Figures 2 and 3. Rear face 38 of rotor body 25 bears against a stationary port plate 104 carried in the end cap 18. The port plate 104 includes a pair of kidney-shaped ports 106 that are connected through axial conduits 108,110 to corresponding bores 112,114 located in the rear of body 12. Each pair of bores 112,114 is connected to a respective one of transversely extending conduits 116,118 to supply and discharge fluid to and from the machine 10. Each conduit includes a pressure transducer 119 to provide a control signal to a pump controller 91 described in further detail below. The bores 112,114 each include a sleeve 120 which is biased against the port plate 104 by a spring 124 and sealed within the bore by an O-ring 126. The sleeve 120 accomodates a small amount of misalignment between the port plate 104 and the body 12 while maintaining a seal to prevent leakage between the bores 112,114.

The kidney-shaped ports 106 therefore alternately connect the axial ducts 36 with either the conduit 116 or conduit 118 over one revolution of the rotor 25 within the housing 12. This force is compensated in part by the pressure forces acting on the sleeve 120. Further compensation is provided by a pair of equalizing pistons 128 located between the kidney ports 106. Each of the equalizing pistons 128 is located in a bore 130 and communicates with the end face 38 of the rotor through a small passageway 132. Pressurized fluid in the axial bores 36 is thus introduced into the cylinder 130 as the rotor rotates, so that the piston 130 provides a balancing force on the plate 104 to hold it sealed against the rear face 38.

The movement of the stator 56 is controlled by a servo motor 74 including the piston 72. As shown in Figure 1, the piston 72 is slidable within a bore 76 and has operating chambers 78,80 located at opposite ends. Fluid flow to the chambers 78,80 is controlled by a spool valve 82 (Figure 2) through supply ducts 84,86. A displacement transducer 90 monitors the position of a magnetic insert 88 carried by the piston 72 and provides a position feedback signal to a controller 91 described below. The feedback signal is indicative of the displacement of the stator 56 and thus the fluid flow delivered by the machine 10. The spool valve 82 is shown in Figure 4 and includes a spool 92 that controls flow from a pressure supply line 94 to either of the ducts 84,86. In a neutral position, flow through both ducts 84,86 is inhibited but movement to either side of neutral connects one port to the supply line 94. The other of the ports is then connected to sump to allow movement of the piston 72 within the bore 76.

Displacement of the spool 92 is controlled by a pressure control assembly 96. The pressure control assembly 96 receives fluid from the pressure supply 94 through a filter assembly 95 to respective ones of control ports 98,100. The pressure in the ports 98,100 is modulated by a pair of variable orifices 99 that are adjusted by a common operating member 101. Movement of the operating member 101 is controlled by a coil assembly 103 comprising a pair of coils 103a,103b. Adjustment of the current to the coil assembly 103 moves the operating member 101 and effectively increases the flow through one of the orifices 99 and decreases the flow through the other, thereby establishing a pressure differential in the control ports 98,100. The control ports 98,100 are connected to chambers 104 formed at opposite ends of the spool 92 so that a differential pressure in the ports 98,100 tends to displace the spool 92 from the neutral position and supply one of the ducts 84,86. A position transducer 102 provides a valve feedback signal for the spool 92 to the controller 91 shown in Figure 5.

Displacement of the rotor is therefore controlled by the current supplied to coil 103 so that controller 91 may modulate the current flow to implement a chosen control strategy and control displacement of the stator 56 accordingly.

As exemplified in Figure 5, the controller 91 receives inputs from the displacement transducer 30 indicative of the pump capacity and from the fluid pressure transducers 119 in the ports 116,118. It also receives a feedback signal from transducer 102 indicative of spool position. If the machine 10 is to be controlled to maintain a constant horsepower, the product of the pressure and displacement signals is computed in the controller 91 and compared with a preset value. An error signal is generated which is used to control the current to the coil 103.

Referring therefore to Figure 5, the displacement transducer 90 provides an input to a differential amplifier 140 whose other input is a reference voltage.142 obtained from a five-volt regulated supply and adjusted through a potentiometer 144. The output of the differential amplifier 140 is thus indicative of the position of the piston 72 and accordingly of the capacity or flow rate of the machine 10. Output 146 is applied as one input of a multiplier integrated circuit 148 whose other input is derived from the pressure transducer 119. The output of the multiplier 148 is therefore indicative of the horsepower absorbed by the machine 10.

The horsepower signal 150 is provided as an input to a differential amplifier 152 whose other input is a reference voltage indicating the desired horsepower to which the machine 10 is to be controlled. The output 154 from amplifier 152 negates transistor 154 to provide an error signal indicating the difference between the desired horsepower and the horsepower absorbed. Error signal 156 is applied to op amp 158 whose other input is the pump displacement signal 146. The difference between the error signal 156 and the pump displacement signal 146 thus indicates the direction in which the pump is to be adjusted to maintain the controlled horsepower level. The output 160 from amplifier 158 therefore provides the error signal used to control the current flow to the coils 103a or 103b. Output 160 is fed as one input to a differential amplifier 162 whose other input is derived from the position transducer 102 associated with the spool 92. The output 164 of amplifier 162 is therefore an error signal indicating the displacement required from the spool 92 to adjust the capacity of the machine 10. The error signal 164 is applied as one input of a current controlling power amplifier 166 whose other input is derived from a 12-volt regulated supply 168. Output 168 of op amp 166 and the error signal 164 from op amp 162 are applied to opposite sides of the coils 103a, 103b that are connected in parallel and isolated by current limiting diodes 170,172. If the error signal 164 decreases indicating movement of the spool 92 in a particular direction, the difference between the reference voltage from supply 168 and the error signal 164 will increase to increase the output voltage 168. Current will thus flow through the diode 170 and coil 103b to move the operating member 101 in the appropriate direction. One of the orifices 99 closes and the other one opens to introduce the differential pressure in the control ports 98,100 and thus moves the spool 92 to supply fluid to the appropriate chamber of servo motor 70. The current supplied to the coil 103b will be dependent upon the magnitude of the difference in the error signal 164 and the reference voltage applied to the amplifier 166 so that the differential pressure in the control ports 98, 100 will likewise be proportional to the error signal.

Upon movement of the spool 92, the position transducer 102 will provide a control signal that operates to reduce the error signal 164 and thus reduce the voltage difference across the coil 214. The current flowing and hence the pressure differential in the ports 98,100 will be reduced until the spool 92 returns to a neutral position. At this time, the error signal from the amplifier 158 should also have reduced to zero as the new pump displacement will maintain the course power at the controlled level.

It will be apparent that the control of the current to the coils 103 is dependent upon the input to the amplifier 162. This input has been described as an error signal derived from a control horsepower setting but it will be apparent that the input could be derived from other controls. For example, the machine 10 may be pressure controlled by providing the error signal 160 to be indicative of the variation from a desired pressure setting. Similarly, the control is conducive to intervention through other control parameters such as by ensuring that the capacity is held at zero during start of the machine 10 and thereby ensuring that additional power requirements are not placed upon the system until such time as the prime mover, either an electric motor or an internal combustion engine, has achieved an operating condition. The pressure control assembly 96 and the controller 91 therefore provide a flexible control system permitting different control strategies to be implemented by the machine 10.

In operation, therefore, assuming that the machine 10 is operating as a pump, power is supplied to the shaft 32 to rotate rotor assembly 24 within the housing 12. Assuming that the stator 56 is disposed to one side of the null position, the radial spacing of the cam ring 54 relative to the peripheral surface 44 will vary as the rotor rotates. The pistons 40,42 follow the cam ring 54 and thus reciprocate within the respective cylinders 40,42. It will be noted from Figure 3 that the crossover between the kidney-shaped ports 106 is circumferentially displaced by 90" from the pivotal axis of the stator 56 so that crossover occurs when the distance between the rotor assembly 24 and cam ring 54 is either a maximum or minimum. Thus, during the period in which the bore 36 is connected with one of the conduits - for example, 116 - the pistons 46 are moving radially outwardly in their cylinders 40,42 causing fluid to flow from the conduit 116 into the cylinders. Thereafter, the connection between the duct 36 and conduit 116 is terminated and the duct 36 is connected through the other kidney port 106 to the conduit 118. During its period of connection to that port, the cam ring 54 causes the piston to move radially inwardly and discharge fluid from the cylinder 40,42 through its associated duct 36 and into the conduit 118. Thus the conduit 116 operates as a supply duct and the conduit 118 operates as a discharge duct. During rotation, slippage between the cam ring

54 and the outer end 48 of the pistons 46 is mitigated by the rotation of the cam ring 54 relative to the stator 56. Thus, the frictional forces acting between the cam ring 54 and the pistons 46 tend to rotate the cam ring 54 relative to the stator 56. However, due to the eccentricity of the cam ring 54 relative to the rotor 24, some differential motion will inevitably occur between the piston 46 and cam ring 54. The axial offset of the cylinders 40,42 and the inclined faces 50,52 of the cam ring 54 cause the engagement of the pistons 46 to be offset from their longitudinal axis. Thus, as slippage occurs between the cam ring 54 and the piston 46, the piston 46 tends to rotate within the cylinder 40,42 in a direction depending upon the relative movement between the cam ring 54 and the piston 46. Rotation between the piston 46 and cylinder 40,42 will be in one direction as the pistons 46 move radially outwardly and in the opposite direction as they move radially inwardly. This rotation reduces wear between the surfaces of the cam ring 54 and the pistons 46 and distributes wear evenly between the piston and cylinder walls. To adjust the capacity of the machine 10, the stator 56 may be moved to vary the eccentricity of the cam surface 54 relative to the axis of rotation of the rotor 24 and thereby the stroke of the pistons 46 within the cylinders 40,42. When operating as a pump, the direction of flow may be reversed by moving the stator 56 across the null point (i.e. so that the stator 56 and rotor 24 are concentric) so that the port 118 is now connected to the cylinders 40,42 as the pistons 46 move radially outwardly and the port 116 connected to the pistons 46 as they move radially inwardly. The conduit 116 thus becomes the pressure conduit and the conduit 118 the supply conduit.

The disposition of the kidney ports relative to the pivotal axis of the stator 56 facilitates the control of the stator by providing that the net pressure forces acting on the pistons 42 are solved in a direction normal to the direction in which the stator is adjusted. The pressure forces will act radially through the pivot point whereas the servo motor 74 moves the stator circumferentially about the pivot point. The forces required to adjust the stator are therefore not influenced by the pressure delivered by the machine allowing accurate control of the machine by the motor 72. This is particularly important where it is desirable to operate the machine 10 at low capacities or at the null position. The machine 10 may also operate as a motor in which pressure is supplied through one of the ports and causes the rotor to rotate within the housing. By varying the capacity of the machine by adjustment of the stator 56, the speed of rotation of the rotor 24 may be adjusted. Reversal of direction can also be obtained by moving the stator to the opposite side of the null point.

In each mode of operation, however, the capacity of the machine 10 is controlled by the valve 82 in response to modulation of the pressure in ports 98,100. The modulation is achieved through adjustment of the current to the coil 103 and so it is possible to utilize different controllers 91 with the same machine to achieve different control strategies.

The arrangement of hydraulic machine shown in Figures 1-3 lends itself to the production of machines of different capacities due to its modular design. Thus, in Figures 6 and 7, there is shown a hydraulic machine utilizing the components shown in Figures 1-3 to provide a machine of different configuration. In the description of Figures 6 and 7, like reference numerals will be used to describe like components, with a suffix "a" added for clarity.

Referring therefore to Figures 6 and 7, the rotor assembly 24a is supported within a housing 12a by bearings 26a and 28a. The rotor body 25a includes axial ducts 36a uniformly spaced around the circumference of the body 25a. The ducts 36 communicate through a port plate 104a similar in construction to that described above and which therefore will not be described in further detail.

It will be seen from Figure 7 that the machine 10a is provided with four separate cam rings 54a. Each of the cam rings 54a is rotatably supported by bearings 58a within a carrier 57a. The carrier 57a is supported on a pin 59a so that the carrier 57a is movable within the housing 12a to adjust the eccentricity of the cam rings 54a relative to the rotor body 25a.

Each of the cam rings 54a is associated with a respective set of pistons 46a. Two sets of pistons 46a are associated with cylinders 40a communicating with a common axial duct 36a. Similarly, the remaining sets of pistons 46a are associated with cylinders 42a which also communicate with a common axial duct 36a. As can be seen from Figure 7, the ducts 36a communicating with the cylinders 40a alternate, with the ducts 38a communicating with the cylinders 42a. Thus, alternate sets of pistons 46a are staggered circumferentially relative to their adjacent set of pistons.

The carrier 57a is adjusted by a servo motor 72a controlled in a manner described above. However, with the arrangements shown in Figures 6 and 7, the eccentricity of each of the carriers 54a is adjusted by the servo 72a so that a multiplying effect is obtained due to the extra sets of pistons 46a.

It will be appreciated that additional sets of pistons 46a can be added and a common adjustment of the pistons achieved through linking the cam rings 54a to a common carrier 57a. In this way, the total capacity of the machine 10a can be chosen to suit a particular application and full adjustment within the range can be achieved. The modular arrangement of the machine 10 also lends itself to use in a hydraulic transmission as shown in Figure 8. In the arrangement shown in Figure 8, like reference numerals to those used in Figures 6 and 7 will be used to denote like parts, with a suffix "b" added for clarity.

In the arrangement of Figure 8, a pair of hydraulic machines, indicated at 10b for the pump and lib for the motor for clarity of description, are hydraulically interconnected through internal ports 116b and 118b provided through a central portion of the housing 12b. Each machine includes a rotor assembly 24b having a rotor body 25b with axial ducts 36b communicating through port plate 104b with the ducts 116b, 118b respectively. The arrangement of pistons 46b and the cylinders 40b and 42b is similar to that described above with respect to Figures 6 and 7. In this embodiment, however, the carrier 57b is split so that each adjacent pair of pistons 46b is adjustable independently of the next pair of pistons. Each of the carriers 57b is adjustable through a servo motor 72b so that the eccentricity of the cam rings 54b relative to the rotor body 25b can be individually adjusted. It will, however, be noted that in the embodiment shown in Figure 8, the cylinders 40a,42a are defined by sleeves inserted into bores in the rotor body 25b so that fitting with their pistons 46b may be accomplished prior to insertion into the rotor body 25b.

The control for the transmission shown in Figure 8 is accomplished through servo valves similar to those shown in Figure 4 for each of the carriers controlled by a common control module as shown in Figure 5. In operation, however, the eccentricity of the cam rings 54b in the motor lib is initially a maximum, i.e. the motor has its maximum capacity, whereas the capacity of the pump 10b is maintained at a minimum.

The eccentricity of the carriers 57b in the pump 10b is adjusted sequentially so that the displacement of the pump 10b progressively increases. During this operation, the eccentricity of the carriers 57b in the pump unit lib remain at a maximum. Once the carriers in the pump 10b have attained their maximum eccentricity, the eccentricity of the carriers 57b in the pump lib are progressively reduced. This again is done sequentially so that the speed of the output shaft associated with the motor lib progressively speeds up. The control of the servo motor 72b operates so that the eccentricity of the last of the carriers 57b to be adjusted cannot be reduced to less than 50% of the capacity of the motor so that a velocity ratio between the input and output shafts is maintained.

It will be apparent that the sequential operation of the pump and motor carriers permits the controller for the pump and motor to maintain a constant horsepower or follow a predetermined acceleration curve over an extended range of output speeds.

Claims

We claim:

1. A variable capacity radial piston machine comprising a housing, a rotor rotatably supported in said housing and having at least one radially extending cylinder formed therein, a piston movable in said cylinder in a radial direction, a stator radially spaced from said rotor and having a cam surface engageable by said piston as said rotor rotates, said stator being adjustable relative to said rotor in a radial plane to vary the radial spacing between said cam surface and rotor and thereby the stroke of said piston in said cylinder, said cam surface being rotatable relative to said stator.

2. A machine according to claim 1 wherein said stator is disposed about said rotor.

3. A machine according to claim 2 wherein said stator is pivotally connected to said housing for movement about an axis spaced from and parallel to the axis of rotation of said rotor.

4. A machine according to claim 3 wherein said cam surface is circular and said stator is movable to a position in which the centre of curvature of said cam track is coincident with the axis of rotation of said rotor.

5. A machine according to claim 4 wherein said stator is movable to either side of said position.

6. A machine according to claim 1 wherein each of said cylinders is connected to a respective fluid duct in said rotor to permit transfer of fluid between said duct and cylinder as said piston moves radially within said cylinder.

7. A machine according to claim 6 wherein said ducts extend axially in said rotor.

8. A machine according to claim 7 wherein said ducts traverse a non-rotatable porting plate as said rotor rotates, said porting plate connecting said duct alternately to a supply conduit and discharge conduit.

9. A machine according to claim 7 wherein alternate cylinders are staggered in an axial direction.

10. A machine according to claim 9 wherein pairs of adjacent cylinders intersect a common duct at axially spaced locations.

11. A machine according to claim 1 wherein said cam surface is circular and is disposed about said rotor.

12. A machine according to claim 11 wherein alternative cylinders are staggered in an axial direction.

13. A machine according to claim 12 wherein said cam surface includes a pair of oppositely directed inclined tracks, each of which is engageable by respective ones of said pistons.

14. A machine according to claim 13 wherein said tracks are disposed to one side of the axis of said cylinders whereby a difference in the angular velocity of said pistons and said track induces rotation of said piston within said cylinder.

15. A machine according to claim 14 wherein pairs of adjacent cylinders intersect a common duct at axially spaced locations, said duct extending axially in said rotor to transfer fluid between said cylinder and supply and discharge conduits.

16. A machine according to claim 15 wherein said ducts traverse a non-rotatable porting plate as said rotor rotates, said porting plate connecting said duct alternately to one of said supply and discharge conduits.

17. A control for a variable capacity hydraulic machine comprising a hydraulic actuator operable on the machine to control the capacity thereof, a valve operable to control flow to said actuator to cause movement thereof, said valve being movable by fluid pilot pressure to a first position in which fluid is directed to move said actuator in a first direction, a pressure modulator to modulate said pilot pressure and thereby adjust said valve, said modulator being responsive to changes in an electrical control signal to modulate said pilot pressure, and a comparator to receive signals indicative of a controlling parameter and adjust said control signal upon departure of said controlling parameter from a reference valve, whereby the capacity of said machine is adjusted to return said controlling parameter to said reference valve.

18. A control according to claim 17 wherein said modulator includes a variable orifice to regulate said pilot pressure and changes in said control signal vary the flow through said orifice and thereby said pilot pressure.

19. A control according to claim 18 wherein said orifice includes an operating member movable to a fixed orifice to adjust fluid flow therethrough.

20. A control according to claim 19 wherein said operating member is movable by a coil and said control signal adjusts current flowing in said coil.