GB2340551A

GB2340551A - Control of a pair of pumps or motors using a differential

Info

Publication number: GB2340551A
Application number: GB9817864A
Authority: GB
Inventors: Mike Dark
Original assignee: SU AUTOMOTIVE Ltd
Current assignee: SU AUTOMOTIVE Ltd
Priority date: 1998-08-18
Filing date: 1998-08-18
Publication date: 2000-02-23
Anticipated expiration: 2018-08-18
Also published as: GB2340551B; GB9817864D0

Abstract

A variable capacity hydraulic pump assembly 10 comprises a differential device 40 having a power input, a first power output to which a first pump 20 is drivingly connected, and a second power output to which a second pump 30 is drivingly connected. The first pump 20 of axial length L1 has a higher specific output capacity than the second pump 30 of axial length L2. The specific flow capacity of the overall pump assembly 10 may be varied by controlling the back pressure of the outlet of one of the first or second pumps 20,30 so that the speed of the other of the first or second pumps 20,30 is increased through the differential device 40. The invention is equally applicable to hydraulic motors assemblies.

Description

2340551 OIL PUMP The present invention relates to variable capacity

hydraulic power converter assemblies and in particular variable capacity oil pump assemblies used to supply the lubricant for internal combustion engines and winch motors. The term hydraulic power converter means either a hydraulic pump, i.e. a device which converts mechanical energ into hydraulic energy or a hydraulic tly motor, i.e. a device which converts hydraulic energy into mechanical energy.

0 Fixed capacity oil pumps used for supplying lubricating fluid to internal 0 0 combustion engines are known. They are driven directly off the engine, and the quantity of oil supplied per engine revolution is constant. Thus by doubling the engine speed from say 1,000 RPM to 2,000 RPM will cause a doubling in the amount of oil supplied by the pump.

Note that the above example assumes a volumetric efficiency of the hydraulic pump of 100% and for ease of explanation all following examples in the specification relating to the volumetric efficiency of hydraulic pumps or hydraulic motors assumes that such efficiency is 100%.

2 The problem with such known oil pumps is that an internal combustion engine oil requirement does not have such a linear relationship with engine speed. By way of example a 1.6 litre saloon car engine may require approximately 25 litres per minute at an engine speed of 1,OOORPM but only requires 40 litres per minute at an engine speed of 6,OOORPM. Thus a constant capacity pump arranged to give 25, litres per minute at 1,00ORPM would give 150 litres per minute at 6,OOORPM, an over-supply of 110 litres per minute. This over-supply is vented to the low pressure side of the oil system, eg to the engine sump or oil pump inlet port. Such venting represents wasted energy.

Thus according to the present invention there is provided a variable capacity hydraulic power converter assembly including, a differential motion device having a power input, a first power output and a second power output, a first hydraulic power converter drivingly connected to the first power output, and a second hydraulic power converter drivingly connected to the second power output, the differential motion device allowing variation of speed of the first power output relative to the second power output such that the specific flow capacity of the assembly is variable.

The specific flow capacity of the hydraulic power converter assembly may be varied by altering the back pressure on at least one of the outlets of 3 the first or second hydraulic power converters.

According to the present invention there is also provided a variable capacity hydraulic power converter assembly including, a differential motion device having a power output, a first power input and a second power input, a first hydraulic power converter drivingly connected to the first power input, 0 a second hydraulic power converter drivingly connected to the second power input, the differential motion device allowing variation of speed of the first power input relative to the second power input such that the speed of operation of the power output of the assembly is variable.

The speed of operation of the power output of the hydraulic power converter may be varied by altering the supply flow to at least one of the first or second hydraulic power converters.

The differential motion device may be a bevel gear type differential unit or an epicyclic unit.

The specific flow capacity of the first hydraulic power converter may be the same or different from that of the second hydraulic power converter.

The invention shall now be described by way of example only with reference to the accompanying drawings in which:- 4 FIG. 1 is a variable capacity oil pump assembly according to the present invention shown without the pump assembly housing for clarity; FIG.2 is an exploded view of FIGA; FIG.3 is a schematic view of the oil pump assembly of figure 1 shown with the second pump hydraulically locked; FIG.4 is a graph of oil flow requirement versus engine speed for a typical 1.6 litre internal combustion engine; 0 FIG.5 is a valve assembly positioned so as to hydraulically lock the second pump of figure 3; FIGS.6, 7 and 8 are equivalent views to FIGS.3, 4 and 5 but with the first pump being hydraulically locked; FIG.9 shows a second embodiment of an oil pump assembly according to the present invention with the front half of the pump body removed for clarity; FIG.10 is a sectional view of FIG.9 showing the valve gallery; and FIG.11 is a schematic diagram of a variable capacity motor.

With reference to FIGS.1 and 2 there is shown a variable capacity oil pump assembly 10 including a first pump 20, a second pump 30 and a differential motion device in the form of a bevel gear type differential unit 40. First pump 20 is a gerotor pump and comprises a housing (not shown), a female annulus 21 and a male rotor 22.

Operation of gerotor pumps is known, but in summary the male rotor sits inside the female annulus and has a number (N) of radially outwardly facing lobes. The female annulus has a larger number (generally N + 1) of radially inwardly facing lobes. The male rotor rotates about an axis which is eccentric from the axis about which the female annulus rotates. By driving say the male rotor, the meshing of the lobes causes the female annulus to also rotate, causing chambers formed between the male rotor lobes and the female annulus lobes to progressively increase and decrease in volume. By arranging an inlet port to feed oil to appropriate chambers whilst they are increasing in

C' size and by arranging for the pump to discharge fluid into an outlet port whilst 0 the chambers are decreasing in size, pumping of fluid can be achieved.

In the present case the axis of rotation A of the male rotor 22 is coincident with the axis of rotation of the drive shaft I I but note that the male rotor 22 is not rotationally fast with the drive shaft 11.

On one axial side of the male rotor 22 there is a bevel gear 23 (not shown, but identical to bevel gear 33 see below). In this case the bevel gear is integrally formed with the male rotor.

Second pump 30 is also a gerotor pump with a female annulus 31, and a male rotor 32 having a bevel gear 33. Male rotor 32 also rotates about axis 6 A and also is not rotational fast with drive shaft 11.

Note that the axial length LI of the first pump 20 is greater than the axial length L2 of the second pump 30 resulting in the first pump having a higher specific output capacity than the second pump (see below).

Differential unit 40 comprises a differential gear carrier 41 formed in two halves 42 and 43, which are secured together by a fastening means (not 0 shown). The differential gear carrier 41 has a power input in the form of a splined bore (not shown) which co-operates with and is rotationally fast with a corresponding splined portion (not shown) of the drive shaft 11.

The differential gear carrier has, in this case, four apertures 44, with each aperture containing a differential -Dear pinion 45.

All apertures 44 are identical and all differential gear pinions 45 are identical thus only one combination of an aperture and differential gear pinion will be described in detail.

Differential gear pinion 45 comprises a differential bevel gear 46 mounted partway along a shaft 47. The aperture 44 is shaped so as to 1h 7 accommodate the differential gear pinion 45 and allows rotation of the differential gear pinion 45 about the axis B of the shaft 47. Shaft 47 is arranged radially relative to axis A of drive shaft 11. The differential bevel gear is dimensioned such that it projects axially (having regard to axis A) on each side of the differential gear carrier 41 allowing it to mesh with rotor bevel gear 23 and rotor bevel gear 33.

At any particular instance those teeth of the differential -ear pinions 45 which mesh with the rotor bevel gear 23 can be regarded as a first power output of the differential unit, and those teeth of the differential gear pinions 45 which mesh with the rotor bevel gear 33 can be regarded as a second power output of the differential unit. Thus a particular tooth of a particular gear pinion 45 may act at one instance as part of the first power output and at a later instance, when rotation of the pinion 45 about corresponding axis B through 180 degrees has occurred, as part of the second power output.

Operation of the differential unit is as follows.

Rotation of the drive shaft 11 causes the differential gear carrier 41 to 0 rotate the differential gear pinions 45 about -axis A. The differential gear pinions 45 then cause the male rotor 22 and male rotor 32 to also rotate about axis A. Under certain conditions the drive shaft 11, male rotor 22 and male 8 rotor 32 can all rotate at the same speed.

However, if male rotor 22 is blocked from turning, the differential gear 0 pinions 45 will, in addition to rotating about axis A, rotate about their respective axes B causing male rotor 32 to rotate twice as fast as drive shaft 11. Similarly if male rotor 32 is blocked from turning, male rotor 22 will rotate at twice the speed of the drive shaft 11.

Furthermore, under other conditions it is possible to have one of the male rotors 22 or 32 rotating at any speed between ORPM and the speed at which the drive shaft 11 is rotating, which results in the other of the male rotors 22 or 32 rotating at a correspondingly faster speed of between the speed at which the drive shaft 11 is rotating and twice that speed.

Thus the differential unit always ensures that the sum of the speeds at which the male rotor 22 and the male rotor 32 are rotating is always twice the speed at which the drive shaft 11 is rotating.

The specific output capacity of a pump is defined by the volume flow rate of pumped fluid per 1,000 RPM of the power input to the pump. Thus whilst the specific output capacity of the first pump is fixed and the specific output capacity of the second pump is also fixed, the specific output capacity 9 of the oil pump assembly (i.e. the assembly of the first pump, second pump and differential unit) can be infinitely varied between a maximum and a minimum level as follows.

The maximum specific output capacity of the oil pump assembly achieved by blocking the outlet from the second pump 30. This causes the female annulus 31 and male rotor 32 to become hydraulically locked and unable to rotate, which in turn causes the male rotor 22 of the first pump 20 to rotate at twice the speed of the drive shaft 11.

The minimum specific output capacity of the oil pump assembly is achieved when the outlet from the first pump 20 is blocked thus causing the male rotor 22 and female annulus 21 to become hydraulically locked. This results in the male rotor 33 of the second pump rotating at twice the speed of the drive shaft 11.

Because the axial length L2 of the second pump is smaller than the axial length L1 of the first pump, the specific output capacity of the second pump is consequently smaller than the specific output capacity of the first pump. Thus, the maximum specific output capacity of the oil pump assembly, that is to say the maximum capacity of the combined outputs from the first and second pump, is achieved when the second pump is hydraulically locked and 1 the first pump rotates at twice the speed of the drive shaft. The minimum specific output capacity of the oil pump assembly is achieved with the first pump hydraulically locked and the second pump rotating at twice the speed of the drive shaft 11.

The typical total engine oil requirements of an internal combustion engine of 1.6 litre are shown in FIG.4 and varies from 25 litres per minute at an engine speed of 1,00ORPM to 40 litres per minute at an engine speed of 6,OOORPM. The specific capacity requirement drops from 25 litres per minute per 1,000 engine RPM when the engine is running at 1, OOORPM to 6.66 litres per minute per 1,000 engine RPM when the engine is running at 6,OOORPM.

Thus in this example by installing a variable capacity pump assembly according to the present invention and by arranging for the pump capacity of the first pump 20 to produce 25 litres per minute at 2,OOORPM (ie a specific output capacity of 12.5Umin/1,000 pump input RPM, and by hydraulically locking the second pump when the engine is running at 1, OOORPM, the correct flow will be achieved at an engine speed of 1,OOORPM (see FIGS.3 and 4).

0 In a similar manner by arranging for the pump capacity of the second pump to be 40 litres per n-dnute when running at 12,OOORPM (ie a specific output capacity of 3.33Umin/1,000 pump input RPM), and arranging for the first pump 20 to be hydraulically locked when the engine speed is 6, OOORPM the correct flow rate can be achieved when the engine is running at 6,OOORPM (see figures 6 and 7).

Between these two extremes it is also possible for the oil pump assembly to produce the correct oil flow at appropriate engine speed by varying the relative rotational speeds of the first and second pumps.

One method of varying the specific output of the oil pump assembly is by the use of a spool valve 50.

Spool valve 50 comprises a body 51 with a first inlet port 52 receiving oil from the first pump and a second inlet port 53 receiving oil from the second pump. The spool valve 50 also has a spool bore 54 one end of which acts as an outlet 55 to supply oil to the engine. Mounted in the spool bore 54 there is a spool 56 with an inlet 57 and an outlet 58. Spool 56 also has a over-pressured bleed hole 59. The spool is biased towards the right when viewing FIG.5 by a spring 60, movement of the spool being limited by a stop (not shown) on body 51.

With the engine running at 1,OOORPM the engine oil pressure is relatively low and the spool 56 adopts the position as shown in FIG.5 in which the first inlet port 52 is connected to the spool bore 54 via the inlet 57. The 12 second inlet port 53 is closed by the spool 56. Under these conditions the oil pump assembly produces a total flow of 25 litres per minute since the second pump is hydraulically locked.

With the engine running at 6,OOORPM the engine oil pressure increases to such an extent that the spool 56 is moved to the left when viewing FIG.5 by compressing the spring 60 and then adopts the position as shown in figure 8. Under these conditions the second inlet port 53 is connected to the spool bore 54 via the inlet 57. The first inlet port 52 is closed by the spool 56. Thus the oil pump assembly will produce a flow of 40 litres per minute since the first pump is hydraulically locked.

Should there be a further increase in the engine oil pressure the spool 56 is moved further to the left when viewing FIG.8 such that the bleed hole 59 aligns with the bleed port 61 (which is connected to the low pressure side of the oil system eg the engine sump), and thus excess oil flow is vented to the low pressure side of the system.

FIGS.9 and 10 show a second embodiment of a variable capacity oil pump assembly 70 according to the present invention. Oil pump assembly 70 includes a first pump 71, a second pump 76 and an epicyclic unit 80.

b 13 First pump 71 is a gerotor type pump with a female annulus 72 and a male rotor (not shown). The female annulus 72 has an outer toothed gear 73 which is rotationally fast with the female annulus 72.

The second pump 76 is also a gerotor pump with the same specific output capacity as the first pump. The second pump includes a female annulus 77 and a male rotor (not shown). The female annulus 77 is rotationally fast with an outer gear 78.

The epicyclic unit 80 includes a sun gear 81, three planet gears 84, 85, 0 86 and an annulus wheel 90.

The sun gear 81 is rotationally fast with a sun wheel 82 which meshes with and drives the first pump gear 73.

The annulus 90 is rotationally fast with an annulus wheel 91 which meshes with and drives the second pump gear 78.

The planet gears 84, 85 and 86 are all mounted on a carrier (not shown) which is engine driven. Each planet gear 84, 85 and 86 meshes with 0 the sun gear 81 and the annulus 90.

14 The epicyclic unit 80 is arranged such that when the annulus 90 is held stationary the sun gear 81 rotates at 4.75 times the planet carrier speed, and if the sun gear 81 is held stationary then the annulus 90 rotates at 1.2667 times the planet carrier speed. Thus the maximum specific output capacity of the pump assembly 70 is produced when the second pump 76 is hydraulically locked and the first pump 71 (being driven by the sun wheel) is running at 4.75 times the planet carrier speed. The minimum specific output capacity of the pump assembly 70 is produced when the first pump 71 is hydraulically locked and the second pump 76 is being driven via the annulus at 1.2667 times the speed of the planet carrier.

The speed at which the first pump 71 rotates relative to the speed of the second pump 76 can be infinitely varied between the two extremes of maximum and minimum specific output capacity of the pump assembly 70 by varying the rotational speed of the sun gear 81 relative to the annulus 90. One method of achieving this is to use spool valve 92 (in a manner similar to that used when varying the specific output capacity of the oil pump assembly 10) as follows:

FIG.10 shows a spool valve 92 which includes a body 93 and a spool 94.

In the position shown in FIG. 10 the spool 94 is biased towards the left is as shown in FIG. 10 by a spring (not shown). In this position the spool closes the outlet port 79 of the second pump 76 resulting in the hydraulic locking of the second pump resulting the variable capacity pump assembly 70 producing its maximum specific flow capacity via the first pump 71 running at 4.75 times the planet carrier speed.

Engine oil pressure is applied to the left-hand end of spool 94 and as the engine oil pressure increases the spool is progressively moved toward the right when viewing FIG.10 resulting in the progressive closing of outlet port 74 from first pump 7 1, and the progressive opening of outlet port 79 of second pump 76. With sufficient engine oil pressure the outlet port 74 will be fully closed and the outlet port 79 will be fully open resulting in the variable capacity oil pump assembly producing its minimum specific output capacity since the first pump will be hydraulically locked and the second pump will be running at 1.2667 times the planet carrier speed.

Assuming that the planet carrier of the variable capacity oil pump assembly 70 is driven at engine speed then with the engine running at 1, 000 RPM and the output from the second pump assembly hydraulically locked, the first pump will be running at 4,750 RPM and can be sized such that it produces 25 litres per minute at this speed (ie a specific output capacity of 5.263LImin/1,000 pump input RPM).

1 16 With the engine running at 6,000 RPM and with the first pump hydraulically locked the second pump will be running at 1.2667 x 6,000= 7, 600 RPM, and since the specific output capacities of the first and second pumps are the same, second pump will produce 7,600 x 5.263 - 1,000 litres per minute = 40 litres per minute. Thus the oil flow requirement of the 1. 6 litre engine above can be met by the variable capacity oil pump assembly 70.

The invention is applicable to gerotor oil pumps being driven by their 0 male rotors (see figure 2) and also by gerotor oil pumps being driven by their female annulus (see figure 9).

Furthermore the invention is not limited to the use of gerotor oil pumps since either of the first andlor second oil pumps shown in the figures could be replaced by another form of pump such as a gear pump or a piston pump.

Also the method of varying the specific flow capacity of pump assemblies according to the present invention is not restricted to varying the back pressure on the outlet side of the first or second pump, and in particular is not restricted to hydraulically locking one or other of the first or second pumps. For example mechanical or electro-mechanical devices could be used to slow the speed of rotation of one or other of the first and second pumps.

17 The invention is not limited to variable capacity hydraulic pumps. Thus the invention can be used to provide a hydraulic motor assembly with a variable output capacity (since the same basic hydraulic power converter can be used as a pump or as a motor, depending on how the power is supplied to the hydraulic power converter).

For example constant pressure or constant power motors are often used in winches. For lowering, or letting out a rope a high speed a low capacity 0 41> motor is desirable, but to lift or tow heavy loads an increase in torque is required, which can be achieved using a high capacity low speed motor.

Figure 11 shows a schematic diagram of the variable capacity hydraulic motor assembly 110 in which hydraulic fluid is supplied via inlet port 103 from a hydraulic power pump (not shown) the hydraulic fluid then flows to first (high capacity) motor 120 and/or second (low capacity) motor 130 dependent upon the position of spool valve 150. The hydraulic fluid then passes to outlet 104 where it is returned to the power pump. First motor 120 is connected to differential motion device 140 by first input shaft 101, and second motor 130 is connected to differential motion device 140 by second input shaft 102. Differential motion device 140 has an output shaft 111.

One manner in which variable capacity can be achieved is by sensing 18 the motor inlet pressure, for example at inlet 103 and applying this pressure to end 150 A of spool valve 150 to act against the spring pressure created by spring 150 B. Axial movement of spool valve 150 then controls the inlet restriction to each motor element. Power is output through the differential motion device 140.

It is also possible to manually control the capacity of the motor for example by the provision of a screw adjustment for spool valve position.

Note that when the hydraulic power converter is used as a motor the operation of the spool valve is reversed, when compared with a pump assembly i.e. an increase in inlet pressure to the motor (which would follow an increase in motor load) results in the inlet of the high capacity motor 120 opening.

The following example shows how a variable capacity hydraulic power converter can be used for winching.

Example

Consider a 30 k W system, with a power pump output of 180 llmin at 100 Bar.

I 19 180 m' x 100 x 10' Pa = 3000OW = 30kW 60000 This drives a constant power hydraulic motor assembly which is used for a crane winch. The motor assembly consists of two motor elements (Motor A and Motor B), a differential motion device, and a 10: 1 planetary' reduction gear head on the power output shaft from the differential motion device. The winch diameter is 0.5m. (Torque radius - 0.25m).

Motor B capacity - 0.5 1/Rev Motor A capacity - 0.1 I/Rev MOTOR A MOTOR B DEFF.OUT 10:1 REDUCTION % RPM % RPM RPM Torque RPM Torque power power 1,800 0 0 900 318 90 3,180 0 0 100 360 180 1,592 18 15,920 900 so 180 540 531 54 5,310 The maximum mass the crane can lift is (i.e. using high capacity motor 1 B only) is:

Mass = torque 15920 6491kg radius x 9.81 0.25 x 9.81 with a lift rate of:

rpm x diameter x pi = (_L8) x 0.57r = 0.47 ms ( 60 sec onds) (60) The maximum rate of lift is (i.e. using, low capacity motor A only) is:

0 1PM x diameter x pi = (92) x 0.5 n = 2.36 ms (60 sec onds) (60) with a load of:

Mass = torque 3180 1297kg radius x 9.81 0.25 x 9.81 Clearly in a further embodiment motor 120 could be of the same capacity as motor 130 and the differential motion 140 could be an epicycle unit arranged such that the gear ratio between power input 101 and power 0 21 output I I I was different to the gear ratio between power input 102 and power 0 output 11 Clearly further embodiments of a variable capacity hydraulic motor assembly could use any sort of hydraulic motor such as a hydraulic gear motor or a hydraulic gerotor motor or hydraulic piston motor.

Also the method of varying the speed or torque of output is not restricted to a spool arrangement arranged on the hydraulic input to the hydraulic motors, for example a spool valve could be ranged on the hydraulic output from the motor or mechanical or electromechanical devices could be used to slow the speed of rotation of one or other of the first and second motors.

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Claims

1. A variable capacity hydraulic power converter assembly including a differential motion device having a power input, a first power output and a second power output, a first hydraulic power converter drivingly connected to the first power output, and a second hydraulic power converter drivingly connected to the second power output, the differential motion device allowing variation of speed of the first power output relative to the second power output such that the specific flow capacity of the assembly is variable.

2. A variable capacity hydraulic power converter assembly as defined in claim 1, in which the first and second hydraulic power converters include first and second fluid outlets and wherein the assembly includes means which in use vary the back pressure on at least one of the outlets in order to vary the specific flow capacity of the assembly.

3. A variable capacity hydraulic power converter assembly including a differential motion device having a power output, a first power input and a second power input, a first hydraulic -power converter drivingly connected to the first power input, a second hydraulic power converter drivingly connected to the second power input, the differential motion device allowing variation of speed of the first power input relative to the second power input such that the speed of operation of the power output of the assembly is variable.

4. A variable capacity hydraulic power converter assembly as defined in claim 3 in which the first and second hydraulic power converters receive first and second supply flows via first and second inlets in use and wherein the assembly includes means for varying the first or second supply flow so as to vary the speed of operation of the power output of the assembly.

5. A variable capacity hydraulic power converter assembly as defined in any preceding claim in which the specific flow capacity of the first hydraulic power converter is the same as that of the second hydraulic power converter.

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6. A variable capacity hydraulic power converter assembly as defined in any one of claims 1 to 4 in which the specific flow capacity of the first hydraulic power converter is different from that of the second hydraulic power converter.

7. A variable capacity hydraulic power converter assembly as defined in any preceding claim in which at least one of the first and second hydraulic power converters is a gerotor type device.

8. A variable capacity hydraulic power converter assembly as defined in any preceding claim in which the differential motion device is a bevel gear type differential device having a gear carrier rotatable about an axis, the gear carrier supporting at least one differential gear pinion, the or each differential gear pinion being in engagement with first and second bevel gears.

9. A variable capacity hydraulic power converter assembly as defined in claim 8 in which the first and second hydraulic power converters are gerotor type devices, each having a male rotor in driving engagement with a female rotor.

10. A variable capacity hydraulic power converter assembly as defined in claim 9 in which at least one of the first and second bevel gears is integral with a male rotor.

11. A variable capacity hydraulic power converter assembly as defined in claim 9 or 10 in which the first and second gerotor devices are of different axial length.

12. A variable capacity hydraulic power converter assembly as defined in any one of claims 9 to 11 in which the first and second gerotor pumps and the bevel gear type differential device all rotate about a common axis.

13. A variable capacity hydraulic power converter assembly as defined in any one of claims 9 to 12 in which the gear carrier is rotationally fast with a shaft.

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14. A variable capacity hydraulic power converter assembly as defined in claim - 13 in which the shaft supports at least one male rotor.

15. A variable capacity hydraulic power converter assembly as defined in any one of claims 9 to 14 in which the bevel gear type differential device is situated between the first and second gerotor devices.

16. A variable capacity hydraulic power converter assembly as defined in any one of claims 1 to 7 in which the differential motion device is an epicyclic differential device having a sun gear in engagement with at least one planet gear, the or each planet gear being supported by a planet carrier and being in engagement with an annulus gear.

17. A variable capacity hydraulic power converter assembly as defined in claim 16 in which the first and second hydraulic power converters are gerotor type devices, each having a male rotor in driving engagement with a female rotor.

18. A variable capacity hydraulic power converter assembly as defined in claim 17 in which the sun gear is in driving engagement with the first gerotor device.

19. A variable capacity hydraulic power converter assembly as defined in claim 18 in which the sun gear is in driving engagement with the female rotor of the first gerotor device.

20. A variable capacity hydraulic power converter assembly as defined in any one of claim 17 to 19 in which the annulus gear is in driving engagement with the second gerotor device.

21. A variable capacity hydraulic power converter assembly as defined in claim 20 in which the annulus gear is in driving engagement with the female rotor of the second gerotor device.

22. A variable capacity hydraulic power converter assembly as defined in any preceding claim when dependent upon claim 2 in which the means for varying the back pressure is a spool valve having a bore with at least one of the first and second outlets being connected to the bore, and a spool moveable within the bore to vary the opening size of the at least one of the outlets.

23. A variable capacity hydraulic power converter assembly as defined in claim 22 in which the spool is moveable such that at least one of the outlets can be isolated from the bore, hydraulically locking the associated first or second hydraulic power converter.

24. A variable capacity hydraulic power converter assembly as defined in claim 22 or 23 in which the spool is biased in a first direction by a bias means.

25. A variable capacity hydraulic power converter assembly as defined in any one of claims 22 to 24 in which the spool is biased in a second direction, in use, by outlet system pressure of the assembly.

26. A variable capacity hydraulic power converter assembly as defined in any preceding claim when dependent upon claim 4 in which the means for varying the supply flow is a spool valve having a bore with at least one of the first and second inlets being connected to the bore, and a spool moveable within the bore to vary the opening size of the at least one of the inlets.

27. A variable capacity hydraulic power converter assembly as defined in claim 26 in which the spool is moveable such that at least one of the inlets can be isolated from the bore, hydraulically locking the associated first or second hydraulic power converter.

28. A variable capacity hydraulic power converter assembly as defined in claim 26 or 27 in which the spool is biased in a first direction by a bias means.

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29. A variable capacity hydraulic power converter assembly as defined in any one of claims 26 to 28 in which the spool is biased in a second direction, in use, by inlet system pressure to the assembly.

30. A variable capacity hydraulic power converter assembly as herein before described with reference to or as shown in figures 1 to 8, or 9 and 10, or 11 of the accompanying drawings.