AU749866B2

AU749866B2 - A power unit for use as a pressure-fluid-operated motor and/or a pressure fluid pump

Info

Publication number: AU749866B2
Application number: AU17619/99A
Authority: AU
Inventors: Esko Raikamo
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-12-22
Filing date: 1998-12-22
Publication date: 2002-07-04
Anticipated expiration: 2018-12-22
Also published as: CA2315752A1; FI102916B; DE69827527T2; FI974588A0; WO1999032760A1; EP1042590B1; ATE282138T1; EP1042590A1; FI102916B1; DE69827527D1; AU1761999A; FI974588A; CN1283252A

Abstract

A power unit for use as a pressure-fluid-operated motor and/or a pressure fluid pump, the power unit comprising a cylinder space, a piston moving in the cylinder space and channels for pressure fluid. The cylinder is annular and it comprises pistons which are of the same shape as the cross section of the cylinder space, and the pistons are arranged to rotate around the axis of the cylinder space. The power unit further comprises a transmission shaft and locking members for locking the piston so that it cannot rotate with respect to the cylinder space.

Description

WO 99/32760 PCT/FI98/01012 1 A POWER UNIT FOR USE AS A PRESSURE-FLUID-OPERATED MOTOR ANDIOR A PRESSURE FLUID PUMP The invention relates to a power unit for use as a pressure-fluidoperated motor and/or a pressure fluid pump, the power unit comprising an annular cylinder space and at least two pistons moving with respect to each other, the pistons being substantially of the same shape and size as the crosssection of the circumference of the cylinder space, and in the power unit at least one of the pistons is arranged to rotate with respect to the cylinder space around its axis so that the piston can move in the cylinder space in the direction of its circumference, and a transmission shaft arranged to rotate around the axis of the cylinder space with said piston, and channels for leading pressure fluid into and out of the spaces between the pistons.

Various power units intended to be used as pressure-fluid operated arrangements are widely known, such as pressure-fluid-operated motors and pressure fluid pumps. There are various kinds of pressure-fluid-operated motors, such as piston motors, screw motors, gear motors and vane motors.

Different kinds of pressure fluid pumps are also known, e.g. piston pumps, screw pumps, gear pumps and vane pumps. The same power unit often functions both as the motor and the pump, whereby e.g. a hydraulic pump and a hydraulic motor connected to it may be identical in the same device.

Both the structure and production of power units comprising pistons are usually complicated, and thus they have gaskets and several parts prone to wear. It is rather expensive to produce them, and their service increases the operating costs considerably. In screw-type arrangements, on the other hand, the screw mechanism is expensive and difficult to manufacture. Vane motors and gear motors as well as vane pumps and gear pumps are relatively cheap to produce, but the efficiency of vane-type power units is poor in all respects, and their operation is very inaccurate.

A further problem of the prior art solutions is that when they are used for operating an actuator, it is rather difficult to guide the actuator to a pre-determined position accurately, and controlling of a certain alternating working motion, for example, requires additional bulbs and control systems, which makes the use of prior art solutions difficult and expensive.

An object of the present invention is to provide a power unit for use as a pressure-fluid-operated arrangement, which is simple and easy to WO 99/32760 PCT/FI98/01012 2 implement, and which can produce an accurate motion in the same way as a stepper motor, if so desired. The power unit of the invention is characterized in that it comprises locking members for locking at least one of the pistons so that it cannot rotate or for slowing down its motion with respect to the axis of the cylinder space so that at least one piston and transmission shaft rotating with it can simultaneously rotate with respect to the cylinder space around its axis.

The basic idea of the invention is that there is an annular, closed cylinder space around the rotational axis and at least two pistons which are arranged to rotate around the axis and are of the shape of the cross section of the annular space, the pistons being arranged to rotate around said axis in such a manner that at least one piston at a time can be arranged to be immobile with respect to said axis or its motion can be slowed down. Another essential idea of the invention is that it comprises channels through which pressure fluid can flow into or out of the part of the cylinder space between the two pistons, as required. When pressure fluid is supplied into one of the spaces between the pistons, one of the pistons being arranged to be immobile or its motion being slowed down, the pressure fluid causes the other piston to move, and the piston makes the shaft connected directly or indirectly to it rotate with respect to the cylinder space. Correspondingly, when one shaft is rotated, it moves the piston connected to it, and when one piston is arranged to be immobile or its motion is slowed down, and the shaft forces the pressure fluid to flow out of the space between the pistons.

An advantage of the invention is that by using pistons which, if need be, can be arranged to be immobile or to rotate with respect to the axis of the cylinder space, the pistons being arranged one after another in the annular cylinder space, it is possible to produce a substantially continuous rotating motion which is parallel to the cylinder space. If one or more pistons are used in such a manner that they are arranged to be immobile at the same time, the arrangement can be used for providing a deflection angle of the desired degree, whereby the arrangement functions like a stepper motor. A further advantage of the invention is that by using clearances that are close-fitting enough between the pistons and the other surfaces of the cylinder space, substantially no gaskets are needed in the arrangement, which will function efficiently, comprising substantially no wearing parts. Thus it is possible, to WO 99/32760 PCT/FI98/01012 3 implement a versatile arrangement, which is rather economical to produce and operate.

The invention will be described in greater detail in the accompanying drawings, in which Figures la to 1c schematically illustrate an embodiment implemented according to the operating principles of a power unit of the invention, Figure 2 is a schematic exploded view of an embodiment according to the power unit illustrated in Figure 1, Figure 3 is a schematic, partially sectional view of the embodiment of the invention shown in Figure 2 in the direction of the shaft, Figures 4a to 4c schematically illustrate another embodiment of the power unit of the invention in principle, Figure 5 is a schematic exploded view of another embodiment of the power unit illustrated in Figure 4, Figure 6 is a schematic, partially sectional view of the embodiment of Figure 5 in the direction of the shaft, and Figures 7a and 7b are schematic general views of the embodiments of the invention.

Figures la to 1c illustrate schematically the basic structure of the power unit of the invention. The figures show a power unit comprising an annular, closed cylinder space 1. In the middle of the cylinder space 1 there is a shaft co-axial with the cylinder space, the shaft being formed by two coaxially rotating transmission shafts 2 and 3. The cylinder space further comprises two pistons 4 and 5 of the shape of the cross-section of the cylinder space, which both are irrotatably connected on their respective transmission shafts 2 and 3. Thus the transmission shaft 2 and piston 4 can rotate with respect to the cylinder space 1 independently of the transmission shaft 3 and piston 5 and vice versa, excluding the situation in which the pistons meet each other during the rotating motion. In principle, the transmission shafts 2 and 3 extend outside the cylinder space 1, and in practice through the end flanges needed for forming the cylinder space, so that power can be transmitted through the shafts to the power unit, i.e. to the piston mounted on the shaft in question, or the power generated by the pressure of the pressure fluid acting on the piston can be transmitted out of the power unit. For the sake of simplicity the end flanges are not illustrated in Figures la to 1c.

WO 99/32760 PCTIF198/01012 4 A channel 6 goes through the transmission shaft 2. The opening 6a of the channel is shown on the right side of the piston 4 in the figure, i.e. it is in the part of the cylinder space 1 marked with V1. Similarly, at the end of the transmission shaft 3 (not shown) there is a channel leading to the surface of the piston 5 corresponding to the surface of the piston 4 so that there is a connection from the surface to the part of the annular cylinder space 1 marked with V2 in the figures.

When the power unit is used as a hydraulic motor, the piston 4, for example, is first locked so that it cannot rotate, whereafter pressure fluid is fed into the part V1 of the cylinder space between the pistons 4 and 5 via the channel 6. When the channel via the piston 5 through the transmission shaft 3 is open at the same time, the pressure of the pressure fluid in the space V1 makes the piston 5 to move in the direction indicated with arrow A, and at the same time the pressure fluid flows out of the space V2 via the channel of the piston 5. When the piston 5 has moved into the position shown in Figure lb or is even in contact with the piston 4, the piston 5 is locked so that it cannot rotate and the feed of the pressure fluid is connected the other way round. In the situation of Figure 1c pressure fluid has been fed into the space V2 between the pistons 5 and 4 through the transmission shaft 3 and via the opening 7a of the piston 5. In this situation the pressure fluid pushes the piston 4 forwards in the annular cylinder space, thereby simultaneously rotating the transmission shaft 2 as the pressure fluid discharges into the pressure fluid channel 6 at the opening 6a in the transmission shaft 2 of the piston 4 and flows out along it. By alternating feeding of the pressure fluid with locking of the pistons it is possible to make the pistons rotate in the cylinder space 1. This rotating motion can be recovered at the ends of the transmission shafts 2 and 3 and transmitted through them to the device to be operated.

Correspondingly, by connecting the shafts appropriately to locking members in a separate body, the casing 8 around the cylinder space 1 can be made to rotate and thus the generated power can be recovered from its rotation. It is also possible to produce a rotating motion, even though one of the pistons were not completely locked so that it cannot rotate, but its rotating motion were slowed down e.g. with a brake or by other means which slow down its motion.

The function described above can also be reversed, whereby the transmission shaft 2 or 3 is rotated mechanically, which rotates the piston WO 99/32760 PCT/FI98/01012 mounted on it. In that case the piston pushes the pressure fluid out of the pressure fluid channel while fluid without pressure flows into the other space from the other pressure fluid channel, the power unit of the invention thus functioning as a pump.

Figure 2 is an exploded view of an embodiment of the power unit according to the invention. The figure shows that the transmission shafts 2 and 3 are connected to the pistons 4 and 5 in such a manner that the pistons 4 and 5 can position themselves by the shaft of one of the pistons. For the transmission shafts to keep their direction and position regardless of the acting powers, there is a supporting shaft 9 arranged between them, the shaft being mounted at the ends of the transmission shafts 2 and 3 in a suitable way. The mounting can be implemented with slide bearings or with other known bearings. The figure further shows end flanges 10, by means of which an annular cylinder space 1 can be formed around the transmission shafts 2 and 3 inside the casing 8. The figure also shows one-way clutches 11 and 12 mounted at the ends of the transmission shafts 2 and 3 outside the end flanges 10. Such one-way clutches comprise an inner circumference, outer circumference and locking members between them. A one-way clutch functions in such a way that the inner circumference and the outer circumference can freely rotate in one direction with respect to each other, but they are prevented from rotating in the reverse direction by the locking members. One-way clutches of this kind and their structure are widely known per se, and the clutches are freely available, wherefore their structure and details need not be dealt with in greater detail in this context.

In this embodiment of the invention the one-way clutches 11 and 12 are to be mounted on the transmission shafts 2 and 3 in such a manner that the inner circumferences of the of the one-way clutches are, with respect to the transmission shafts 2 and 3, arranged to be irrotatable e.g. with the key slots 13 shown in the figure and keys 14 to be pushed into them. Furthermore, the one-way clutches 11 and 12 are mounted on the transmission shafts 2 and 3 in such a manner that the free rotation directions of the one-way clutches on the same shaft are reverse. They are further mounted on the shafts in such a manner that on both transmission shafts 2 and 3 the free rotation directions of the one-way clutches 11 situated next to the end flanges 10 are parallel as shown with broken arrows by the one-way clutches in the figure. The outer circumference of the one-way clutches 11 is in turn arranged to be irrotatable WO 99/32760 PCT/F198/01012 6 with respect to the end flanges 10, and the outer circumferences of the outer one-way clutches 12 are arranged to be irrotatable with respect to separate fasteners 15. In principle, the fasteners 15 can be part of a uniform body or they can be fastened to the same body or bed so that they are irrotatable with respect to each other. In some embodiments the transmission shafts can be connected to rotate a device or a shaft alternately by means of suitable gearing or the like. The figure also shows pressure fluid couplers 16, through which pressure fluid can be fed into and out of the cylinder space 1 via the channels 6 and 7. The figure further shows a section of the piston and shaft along the line A to A to illustrate how the channel 6 and the opening 6a are interconnected to feed pressure fluid into and out of the cylinder space 1.

Figure 3 is a schematic, partially sectional side view of the embodiment of the invention according to Figure 2, illustrating the arrangement as assembled. As can be seen in the figure, the casing 8 and the end pieces 10 form a closed cylinder, in which an annular cylinder space is formed around the transmission shafts 2 and 3. The one-way clutches 11 and 12 on the transmission shafts 2 and 3 are arranged in such a manner that the one-way clutches 12 are fastened to the couplers 15 and the one-way clutches 11 are fastened to the end flanges 10, as shown in the figure. There may be separate spacing rings 17 between the one-way clutches so as to keep them at an appropriate distance from each other, even though the construction can also be implemented otherwise. The figure also shows a key 14 which connects the transmission shaft 3 to the one-way clutches 11 and 12 at that end where the key is situated. There is also a corresponding key at the end of the transmission shaft 2, although it is not illustrated in the figure.

The figure shows that the piston 5 is of the same shape and size as the cylinder space 1, thus closing the whole cylinder space 1. In this embodiment the piston 5 is fastened to the transmission shaft 3 with fastening bolts 18, which go through the piston 5 surface next to the flange 8 and extend to the transmission shaft 3. There is a channel 7 coming through the transmission shaft 3, and another one coming through the opening 7a of the piston 5, the channel extending to the channel 7 in the radial direction, thereby forming a uniform channel for pressure fluid. Since the fastening bolts 18 are in the middle of the piston 5, the outer surface on both sides of the bolt holes of the piston 5 seals the piston with respect to the casing 8. The piston 4 (not shown) and the shaft 2 are interconnected similarly and arranged to function in WO 99/32760 PCTIFI98/01012 7 the same way. In addition to bolt fastening, the pistons can be fastened to their shafts in several other fastening ways known per se, provided that the joint between the pistons and the shafts is firm, and the clearances between the different surfaces are small enough or can be sealed with a suitable gasket.

In this embodiment feeding pressure fluid into the cylinder space 1 between the pistons 4 and 5 causes one of the pistons to lock so that it cannot rotate by means of the one-way clutch 11 with respect to the end flange and the other to lock with respect to the coupler 15. As a consequence, the whole construction, i.e. the casing, end flanges and one of the pistons, rotates with respect to the coupler 15, whereby the power of the rotating motion can be transmitted to an appropriate actuator through the casing 8 and end flanges 10. Correspondingly, when pressure fluid is fed into the other space between the pistons, the pistons connect the other way round, i.e. the piston that connected irrotatably to the end flange in the previous stage connects irrotatably to the fastener at its side and the other piston connects to the end flange instead of the fastener. As a result of the feed of pressure fluid the casing 8, end flanges 10 and one of the pistons again rotate in the same direction with respect to the fasteners 15. In this embodiment the one-way clutches 11 and 12 function as locking members by means of which the shafts, depending on their use, can be locked so that they cannot rotate with respect to the casing and end flanges forming the cylinder space, so as to produce a continuous rotating motion.

Figures 4a to 4c illustrate another embodiment of the invention. In this embodiment the piston 4 is fixedly mounted on the casing 8, and only the piston 5 is arranged to rotate around the shaft. In the figure, the same numbers have the same significance as in the previous figures to avoid confusion.

In this embodiment the piston 5 is mounted fixedly on the casing 8, whereby they form a uniform part, and only the piston 4 rotates with the shaft 2. When pressure fluid is fed into the space V1 via the channel 6, the piston 4 rotates forwards around the shaft while the rotating motion is transmitted forwards through the one-way clutches in the same way as in Figures 1 to 3.

When pressure fluid is fed into the space V2 via the channel in the shaft 3, the piston 5 moves away from the piston 4 rotating around the shaft, simultaneously rotating the casing 8. The shaft 3 connected to the casing and WO 99/32760 PCT/F198/01012 8 its end flange then transmits the rotating motion forwards according to the principle described above.

Figure 5 is an exploded view of a practical embodiment of the embodiment shown in Figure 4. In this embodiment the power unit comprises an auxiliary shaft 19, around which the entire power unit is arranged to rotate.

The auxiliary shaft 19 goes through the shafts 2 and 3 so that they can rotate around the auxiliary shaft 19. At the ends of the auxiliary shaft 19 there are channels 6 and 7 extending inside the shaft, but only the channel 7 is shown in the figure. For the pressure fluid, there are openings 6a and 7a on the both sides of the piston 4 and channels extending through the shaft 2, the channels being almost parallel with the radius. At the ends of the channels there are pressure fluid grooves 2a and 3a in the auxiliary shaft 19. These grooves are intended to correspond to the channels in the shaft 2 of the piston 4, so that pressure fluid can be optionally fed into either side of the piston 4 via the channels 7 and 6.

The figure further shows auxiliary flanges 20 and an auxiliary casing 21 which form a uniform housing around the casing 8 so as to provide power transmission. In this arrangement the one-way clutches 11 are connected to the auxiliary flanges 20 of the casing 8 of the cylinder space instead of the end flanges 10, whereby they function as was explained in connection with Figures 1 to 3, except that the power is transmitted from the shaft 2 and 3 to the auxiliary flanges 20 in such a manner that the arrangement formed by the auxiliary flanges 20 and the auxiliary casing 21 rotates around the piston 4 or the piston 5 while the casing 8 rotates around the auxiliary shaft 19 with respect to the fasteners Figure 6 is a schematic, partially sectional side view of the power unit of Figure 5 in the direction of the shaft. As is seen in the figure, the auxiliary casing 21 and the auxiliary flanges 20 form a housing around the casing 8 and the end flanges 10. The piston 4 is mounted on the shaft 2, which rotates around the auxiliary shaft 19. The channel 6 extends through the pressure fluid groove 2a in the auxiliary shaft 19 to the axial channel leading to the channel opening 6a, whereby pressure fluid can flow into the groove 2a along the channel 6 and from the opening 6a to the part V1 of the cylinder space 1. Correspondingly, on the other side of the piston 4 there is an opening 7a, which is connected to the pressure fluid groove 3a so that pressure fluid can be fed via the channel 7 at the other end of the auxiliary shaft 19 through WO 99/32760 PCTIFI98/01012 9 the opening 7a to the part V2 of the cylinder space 1. Thus the pistons rotating alternately around the auxiliary shaft 19 make the cylinder formed by the auxiliary flanges 20 and the auxiliary casing 21 rotate in the desired direction.

Instead of a separate auxiliary casing 21 and auxiliary flanges 20 it is possible to use an arrangement in which one of the auxiliary flanges and the auxiliary casing 21 are formed as an integral part. It is also possible to use two cylinder halves which both comprise one auxiliary flange 20 and a casing-like part separating cylindrically from it, the casing-like parts of two such pieces being joined together so that they form a uniform cylinder. Furthermore, instead of a closed auxiliary casing 21 it is possible to use one or more fasteners spaced from one another on the cylinder circumference, the fasteners interconnecting the auxiliary flanges Figures 7a and 7b are schematic general views of some embodiments of the power unit of the invention. These show how pistons can be arranged in such a manner that the same power unit comprises several pistons which are mounted symmetrically e.g. with respect to the rotating axis.

Thus there are two pairs of pistons in both figures. The pistons of each pair are mounted symmetrically with respect to the rotational axis so that they are in balance. Figure 7a illustrates application of the embodiment of the invention according to Figures 1 to 3, where all pistons rotate with respect to the casing of the cylinder space, whereas Figure 7b illustrates application of the embodiment of the invention according to Figures 4 to 6, where half of the pistons rotate around a separate shaft and half of the pistons are arranged irrotatably with respect to the casing 8 of the cylinder space.

According to this principle, pistons may be arranged into groups containing several pistons. In that case the most obvious embodiment is such in which the pistons of both the groups are arranged symmetrically with respect to the rotational axis according to the principle shown in Figures 7a and 7b. When several pistons are used, it is possible to provide a motor or a pump which is powerful with respect to its size, functions accurately and is easy and simple to use as a stepper motor or a feeding pump. In these cases, feeding of pressure fluids into the spaces between the pistons can also be implemented as disclosed above or in another way known per se.

The invention can be implemented in various ways. It is not necessary to use two separate end flanges in the device, but one of the end flanges and the casing may be formed as an integral part. The axial cross- WO 99/32760 PCTIFI98/01012 section of the pistons is preferably such that their sides are parallel with the radii of the rotational axis, even though cross-sections of other kinds can also be used. The size of the pistons may also vary.

Instead of two pistons, three or more pistons can also be used, if so desired. In these embodiments it is, however, sometimes necessary to use shafts arranged within each other for transmitting the rotational motion.

Correspondingly, if there are several pistons fastened onto the same shaft, it is possible to generate power multiplied by the number of the pistons. If the number of pistons is even, they are preferably arranged into two groups with respect to the rotational axis and the groups are arranged symmetrically.

Instead of the one-way clutches, it is possible to use locking members of other kinds, such as different clutches, brakes or latching mechanisms. Similarly, different timers can be used on feeding the pressure fluid so as to regulate the feeding, which produces a rather smooth motion and accurate stepping. In these embodiments it may be necessary to use separate controlling to operate the locking members so that the power unit operates as a motor or a pump in the desired way.

When one-way clutches are used, they simultaneously function as the bearings of the pistons, but when locking members of other kinds are used, the mounting may have to be implemented differently. Even though mounting implemented with slide bearings may be sufficient in some cases, conventional bearings of other kinds can also be mounted on the shafts of the pistons.

The arrangement with auxiliary shafts shown in Figures 4 to 6 can also be applied to the embodiments shown in Figures 1 to 3.

It is possible to use different gases or gas mixtures, such as air, or different hydraulic fluids, such as oil, water, etc, as the pressure fluid in the power unit of the invention.

The channels for leading pressure fluids into and out of the spaces between the pistons can be arranged to go through the shafts, through the shafts and the pistons, through the end flanges forming the walls of the cylinder space, or through the casing, etc in the ways known per se.

The power unit of the invention can function as a feeding pump or as a stepper motor, since its motion from one position to another can be restricted. To produce a motion of the desired degree, the rotational motion of the shaft can be avoided by using different transmission mechanisms, or the WO 99/32760 PCT/F198/01012 11 motion can be restricted by using several pistons, by means of which it is possible to provide a deflection angle of the desired degree, and thus the quantity of motion or the amount of the pressure fluid can be adjusted.

Instead of the external cylinder shown in Figures 5 to 6 it is naturally possible to transmit the power out of the power unit with a separate secondary shaft or by using other known power transmission solutions.

Claims

1. A power unit for use as a pressure-fluid-operated motor and/or a pressure fluid pump, the power unit including an annular cylinder space and at least two pistons moving with respect to each other, the pistons being substantially of the same shape and size in cross-section as the cross-section of the circumference of the cylinder space, at least one of the pistons is arranged to be immobile or selectively immobile with respect to the casing of the cylinder space, and at least one of the pistons is arranged to rotate with respect to the cylinder space around its axis so that the piston can move in the cylinder space in the direction of its circumference, and a transmission shaft arranged to rotate around the axis of the cylinder space with said piston, and channels for leading pressure fluid into and out of the cylinder space, the power unit also including locking members for locking at least one of the pistons so that it cannot rotate or for slowing down its motion with respect to the axis of the cylinder space so that at least one piston and transmission shaft rotating with it can simultaneously rotate with respect to the cylinder space around its axis, characteri z ed in that one or more channels are arranged to lead pressure into and out of the cylinder space on one side of the pistons and another or other channels are arranged to lead pressure into and out of the cylinder space on the other side of the pistons, and that the locking members are one-way clutches which allow the transmission shaft to rotate around the axis of the cylinder space in one direction, but lock the transmission shaft and the piston connected to it so that they cannot move around the axis of the cylinder space in the other direction.

2. A power unit as claimed in claim 1, characteri z ed in that the at least S one piston arranged to be immobile or selectively immobile with the casing and the casing are mounted on another transmission shaft rotating co-axially with the axis of the cylinder space.

A power unit as claimed in claim 2, characteri z ed in that a channel leads through one transmission shaft to one surface of the piston rotating with respect to the cylinder space, and there is an opening of the channel on one side of the piston in the direction of the circumference of the cylinder space.

4. A power unit as claimed in any one of the preceding claims, e r i z e d in that it includes an even number of pistons with alternate RaI,02100// 14 April 2002 -13- pistons being arranged to act on one transmission shaft and the other pistons being arranged to act on the other transmission shaft, and the pistons mounted on one transmission shaft can move with respect to the other transmission shaft.

A power unit as claimed in claim 4, c h a r a c t e r i z e d in that the pistons arranged to act on each transmission shaft are substantially symmetrically arranged with respect to the rotational axis of the cylinder space.

6. A power unit as claimed in any one of the preceding claims, c h a r a c t e r i z e d in that the cylinder space is formed of a separate casing and at least of one end flange mounted on the side of the casing.

7. A power unit as claimed in any one of the preceding claims, c h a r a c t e r I z e d in that it includes a separate auxiliary shaft which is arranged to go through the transmission shaft so that the transmission shaft can rotate around it.

8. A power unit as claimed in claim 7, c h a r a c t e r i z e d in that the channels for leading pressure fluid into and out of the spaces between the pistons are arranged to go through the auxiliary shaft.

9. A power unit as hereinbefore described with reference to Figs. la to ic.

10. A power unit as hereinbefore described with reference to Figs. 2 and 3. o

11. A power unit as hereinbefore described with reference to Figs. 4a to 4c.

12. A power unit as hereinbefore described with reference to Figs. 5 and 6.

13. A power unit as hereinbefore described with reference to Figs. 7a and 7b. I oo00o. 7 RAI0o2 O/ 19 April 2002