GB1593731A

GB1593731A - Axial piston hydraulic machines

Info

Publication number: GB1593731A
Application number: GB3112876A
Authority: GB
Original assignee: UK Secretary of State for Industry
Current assignee: UK Secretary of State for Industry
Priority date: 1976-07-26
Filing date: 1976-07-26
Publication date: 1981-07-22
Also published as: FR2359997A1; JPS5316901A; DE2733687A1

Description

(54) IMPROVEMENTS IN OR RELATING TO AXIAL PISTON HYDRAULIC MACHINES (71) I, THE SECRETARY OF STATE FOR INDUSTRY, London do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to axial piston hydraulic machines and more particularly it is concerned with hydraulic pumps and motors for effecting transfers of energy between a rotating shaft and a working fluid.

It is known to provide a hydraulic pump having a plurality of pistons arranged parallel to an axis of rotation of a shaft of the pump. Such pumps have been called "axial piston" hydraulic pumps. These pumps have a plate, which may be a slipper plate or swash plate, against which the pistons react. The plate is inclined at an angle to the shaft during pumping. Each piston will generally carry a slipper which has a spherical portion received by the piston, and a flat slipper face which slides over the inclined plate. The inclined plate can be a simple slipper plate of fixed orientation or it can be a swash plate, whose orientation can be varied between limits. The swash plate can be of simple construction so that the slippers bear directly on the swash plate, or it can be more complex, incorporating a slipper plate in the face nearest the cylinder block.In use, the block in which the pistons are housed is rotated by the shaft and the pistons are driven in and out of their cylinders by their reaction against the inclined plate. This movement of the pistons can be used to provide pulses of hydraulic fluid under pressure to an output conduit.

It is known to provide hydraulic fluid to the cylinders and to receive the pulses of fluid from them by means of a port plate having first and second kidney ports flanked by lands and situated adjacent a ported surface of the cylinder block. This ported surface has a number of "block" ports corresponding to the number of pistons, and each block port communicates with one of the cylinders. If the shaft of the pump is rotated and fluid is supplied to the appropriate one of the two kidney ports, a succession of pulses of fluid can be delivered to the other of the kidney ports.

In the pumps described above, the cylinder block is in use subject to a number of forces.

Firstly, the forces of reaction between the pistons and the inclined plate have a component normal to the rotational axis of the block. Secondly as the block ports are normally of smaller cross-section than the cylinders, the hydrostatic pressure of fluid within the cylinders of the block will act on the ends of the cylinders remote from the pistons to urge the block towards the port plate. The resultant of this second force, summed over all the cylinders, is not co-incident with the rotational axis of the block when one group of cylinders is at higher pressure than another group.

Thirdly, the hydrostatic pressure of fluid in the kidney ports acting on the block and in any fluid which is leaking across the lands acts on the block to force it away from the port plate. Again, if the pressure in one kidney port is larger than that in the other, the resultant of the third force will not be co-incident with the rotational axis of the pump.

In addition to the three forces described above, the block can be acted upon by hydrodynamic forces between it and the port plate. For example, if the block runs tilted with respect to the port plate the gap between them will be wedge-shaped and there can be hydrodynamic lubrication between then.

Furthermore, if the surface of either the port plate or the block is formed with some portion inclined at a small angle to the opposed surface there can be some hydrodynamic lubrication even with the port plate and the ported surface running generally parallel.

In order that the transfer of hydraulic fluid across the interface should take place efficiently, any gap between the lands and the ported face must not be too large. It is known to control the size of the gap by arranging that the hydrostatic forces on the cylinder block urging it towards the port plate are somewhat greater than the hydrostatic forces urging the block away from the port plate, so that there is an overall "clamping force" urging the block towards the port plate. In many known designs this clamping force is then resisted by, for example, a thrust bearing race or imposed hydrodynamic forces to maintain a gap of predetermined size, without imposing an undue load between the mating faces of the cylinder block and port plate during normal running.

An incomplete appreciation of all the afore mentioned factors, as well as a widely held belief in the need to provide a separate means to limit loading between these mating forces has resulted in known designs of axial piston hydraulic machines being bulkier than would otherwise have been necessary.

It will be obvious to those skilled in the art that much of the above discussion of hydraulic pumps is applicable to axial piston hydraulic motors, in which a supply of pressurised hydraulic fluid is supplied to a plurality of pistons to urge a shaft to rotate.

This invention seeks to provide an axial piston hydraulic machine which is more compact (for a given output) than the known machines.

According to the present invention there is provided an axial piston hydraulic machine comprising a) a cylinder block having a ported surface, a plurality of block ports in the ported surface, and a plurality of cylinders formed in the cylinder block, with each of the block ports communicating with a respective one of the cylinders; and b) a port plate at an interface with the ported surface and having a first and a second kidney-shaped port so disposed that relative rotation of the cylinder block and the port plate effects alternate communication across the interface between each one of the cylinders and the first and second kidney-shaped ports, c) each cylinder having a piston slideable axially therein, each piston being provided with a slipper having a spherical seating in the end of its respective piston remote from the ported surface; and d) a slipper plate providing a bearing surface on which the slippers can run; the machine being constructed such that in use the resultant of hydrostatic pressure forces within the cylinders acting on the cylinder block is substantially co-linear with the resultant of hydrostatic pressure forces of fluid acting to force apart the cylinder block and port plate.

Preferably the machine is constructed such that in use the ported surface of the cylinder block and a surface of the port plate remain in contact with one another during relative rotation thereof so as to resist substantially the whole of the net axial loading between the cylinder block and the port plate.

It is to be understood that the expression "in contact" as used above comprises "rubbing" contact and "lubricated" contact in which there is boundary layer lubrication by, for example, the hydraulic fluid between the two surfaces.

This is to be contrasted with situations where the ported surface and the port plate are not "in contact" for example where they have over all their adjacent surfaces a comparatively thick film of lubricant which exerts a hydrodynamic or hydrostatic pressure on the adjacent surfaces. It will be appreciated that pumps formed such that the adjacent surfaces run in contact are distinguished from those which are formed to cope with momentary running contact but which could not withstand longer periods of contact, which latter pumps are not according to this invention. It will also be appreciated by those skilled in the art that when two surfaces are running "in contact" as above there will generally be some small local degree of hydrodynamic lubrication and lift provided specifically by micro-asperities on the surfaces.

An embodiment of pump according to a preferred aspect of the invention having one of the two contacting surfaces on the block and the port plate formed of a thermoplastic material identified by the trade name "GLAMAT 58" (trade mark) of the Glacier Metal Co Ltd of Wembley, Middlesex England has been constructed and operated. The properties of this material resemble in some ways those of thermoplastic materials and in other ways those of thermo-setting materials. It is resistant to the conditions in which it finds itself during operation of the pump, that is higher than ambient temperature and the presence of a rotating cylinder block pressing against it in the presence of hydraulic fluid.

The other of the two contacting surfaces is a steel surface in this embodiment.

Another embodiment of hydraulic machine according to a preferred aspect of the present invention which has performed satisfactorily employs the material "XYLAN 1010" (trade mark) made by Plastic Coatings Ltd of Leicester, England instead of Glamat 58. Another possible alternative is "DRAYLOY" (trade mark) made by Drayloy Ltd of Sheffield, England but this material has not yet been tested by the present Applicant.

It is envisaged that "GLAMAT 58" or any other satisfactory material will find use on the slipper faces of the slippers, or on the slipper plate itself, for the design considerations of the slippers have similarities with those of the port plate and block. Thus, it is proposed for example, that the slippers will each have only a single land ie that the slipper faces will be formed without separate raised lands and that these lands will move on an annulus of Glamat 58 provided on the slipper plate, or alternatively the single land of each slipper face will be of Glamat 58.

Preferably, the shaft and cylinder block will be supported by a cylindrical bush having its centre (ie the centre of its mid-plane) situated at the point where the plane of the resultant of the components of the force of reaction between the pistons and the inclined plate normal to the longitudinal axis of the shaft intersects the longitudinal axis of the shaft. With the centre of the bush in this position, there is no tendency for the cylinder block to tilt under the action of this resultant.

The port plate and ported surface are constructed and arranged so that, when the machine is in use, the resultant of the hydrostatic forces between the cylinder block and the port plate is substantially co-linear with the resultant of the hydrostatic pressure forces acting on the ends of the cylinders remote from the pistons.

If the resultants are opposed in this way, there is a reduced tendency for the block to tilt relative to the port plate. Generally the two opposed resultants will not be equal and this is necessary in use to ensure that there will be an overall clamping force urging the block into contact with the port plate. A preferred aspect of the present invention enables this clamping force to be borne wholly by the contact between port plate and the ported surface without the need for any external thrust bearing or for maintenance of hydrodynamic support forces. A more compact design is thus made possible.

The ported surface and the port plate can be generally planar with the port plate having raised lands flanking the kidney ports, the lands being adjacent the flat surface of the ported face of the block.Alternatively they can be formed as portions of spherical surfaces with coincident (or nearly coincident) centres. The notional centres of the spheres can be on the cylinder block side of the interface or on the port plate side of the interface. The use of such spherical surfaces means that the machine can be made more tolerant of inaccurate dimensions in its components and it is capable of tolerating, for example, some angular discrepancy between the shaft and the port plate. This tolerance can help to reduce the cost of the machines by reducing manufacturing costs.With the notional centres on the cylinder block side of the interface, the arrangement is more tolerant of misalignment between the block and the port plate. With the centres on the port plate side the arrangement is better able.to withstand the effects of any possible forces tending to tilt the block relative to the port plate.

The use of slippers having only a single land makes possible the provision of a larger number of pistons than would be possible in a known machine of similar output and dimensions.

Thus, for example, a 21-piston pump is feasible.

The provision of such a large number of pistons can be useful in that it produces a smoother output from the pump and this can contribute towards the reducing of the noise produced by the pump during operation.

It is envisaged that machines according to the invention may be provided with a facing of Glamat 58 (trade mark) or other suitable material such as XYLAN 1010 (trade mark) or DRAYLOY (trade mark) also on other of their surfaces between which there is relative movement. Thus, for example, it is envisaged that there may be benefits in providing the surfaces of the spherical portions of the slippers, and the cylindrical surfaces of the pistons, with coatings of the material. The coatings can be applied to the components of the pump by, for example, stamping, or pressing, as appropriate for the material concerned. They may then in many cases be further shaped by electrochemical machining or spark erosion.Alternatively they can in some cases be provided as separate components (for example, an entirely plastics annulus could be inserted between the block and an end plate to serve as a port plate).

For a better understanding of the invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the drawings filed with the provisional specification, in which; Figure 1 is an axial section of a hydraulic machine according to the invention, Figure 2 is a section taken on the line AA' of Figure 1, Figure 3 is an axial section of a hydraulic motor according to the invention, and Figure 4 is an axial section of a hydraulic pump according to the invention.

Figures 1 and 2 show the construction of a hydraulic machine purely schematically. A casing 1 and a port plate 2 forming an end cover to the casing enclose a cylinder block 3.

The cylinder block 3 is carried on a shaft 4 and cannot rotate relative to the shaft but only with the shaft. The shaft 4 and block 3 can rotate in the casing 1 on a support bush 9 and a steady bush 11, the latter being provided to assist in maintaining alignment of the shaft but without carrying any appreciable load.

The cylinder block has a plurality of cylindrical cavities 40. These cavities are distributed regularly around the circumference of the cylinder block and all at the same radial distance from the axis of rotation of the block 3. Each cavity 40 contains a piston 8 which is urged outwardly of the cavity by a piston spring 14.

Each piston 8 carries a slipper 7. The slipper and piston are ball-jointed to permit the orientation of the slipper relative to the piston to vary between wide limits. The slipper 7 has a relatively large, generally planar surface (the slipper face) which contacts a slipper plate 5 to resist the action of the piston spring 14. The slipper plate 5 does not rotate relative to the casing 1.

The arrangement is such that when there is relative rotational movement between the shaft 4 and the casing 1 the slippers 7 slide over the surface of the inclined slipper plate 5 and the pistons 8 move in and out of their respective cavities 40 in sequence. The extent of movement of the pistons 8 ie their stroke can be changed by choosing an alternative slipper plate 5 with a different angle of inclination relative to the longitudinal axis of the shaft 4.

The machine described above with regard to Figure 1 can be adapted for use as a hydraulic motor or as a hydraulic pump. As a pump, energy is fed to the machine as rotational energy in the shaft and is used to pressurize hydraulic fluid contained in the cavities 40 and expel the fluid from the cavities 40 in a sequence of pulses as the inclined plate forces the pistons 8 into the cavities 40 in sequence. In the remaiming part of the pump cycle, hydraulic fluid at lower pressure is fed to the cavities 40 as the pistons 8 withdraw from the cavities under the action of the piston springs 14.

Thus, in Figure 1, the pistons 8 withdraw from the cavities 40 when they move from the bottom to the top of the slipper plate 5, and when they move from the top to the bottom of the slipper plate 5, they compress fluid behind the pistons 8 in the cavities 40. As a motor, of course, supplies of high pressure fluid to the cavities 40 urge the pistons 8 outwardly of the cavities 40 and if the supply is fed to the cavities in sequence it can generate rotation of the shaft.

Fluid is fed to and from the cavities 40 along bores 31 which extend into block ports arranged on an annulus in a steel ported surface 32 of the cylinder block 3 (which may be made of, for example, steel or cast iron). The port plate 2 is provided with a land 33 which is of annular shape and faces the block ports in the ported surface 32.

Figure 2 shows the construction of the port plate 2 in greater detail. The annular land 33 has two kidney ports 34 and 35 of kidney shape. Each kidney port communicates with conduits 36 and 37 respectively. The land 33 is of the said Glamat 58 synthetic polymeric material bonded to the steel port plate 2.

When the machine of Figures 1 and 2 is in use as a pump, the casing 1 is fixed to prevent rotation and the shaft 4 is driven to rotate. Hydraulic fluid is supplied along one of the conduits 36 and 37 so that fluid can be supplied to the cavities 40 in sequence across the interface between the port plate 2 and the ported surface 32 as the pistons 8 withdraw from the cavities 40 in sequence. As the pistons then begin to extend into the cavities 40 in sequence the fluid is expelled from them across the interface once more and into the other of the two conduits 36 and 37. Use of the machine as a motor is somewhat different in that high pressure fluid is supplied to those cavities 40 into which the piston 8 is fully extended, so that the high pressure can expel the pistons in sequence and cause the cylinder block 3 to rotate.

Operation of the machine gives rise to reaction forces between the slipper plate 5 and the pistons 8, acting normally to the shaft axis as described above. These reaction forces can be considered to act through points lying in the plane which includes the centres of all the ball joints between the slippers 7 and the pistons 8.

It is a feature of the illustrated machine that the centre of the support bush 9 is located at the point where this plane intersects the shaft axis. This means that there is no tendency for these reaction forces to twist the cylinder block 3 in the plane of Figure 1. It is to be noted also that the cylinder block is supported on a single support bush 9 and there is no support bush for the cylinder block adjacent its ported surface 32.

The forces of reaction between the ported surface 32 and the land 33 result from the fluid pressure in the kidney ports acting on the ported surface. As explained above, the pressures are not uniform around the circumference of the port plate 2 but their variation corresponds to that of the reaction forces at the slipper end of the cylinder block 3. The position and shape of the block ports and the kidney ports 34 and 35 in the land 33 is selected having regard to an analysis of reaction forces across the interface to achieve a "balanced" cylinder block 3.

As noted above, the resultant of the hydrostatic forces on the cylinder block 3 (ie those acting on the cylinder ends) urging it towards the port plate is resisted by the resultant of hydrostatic pressure forces of fluid acting on the block in the kidney ports and leaking across the lands. The difference between these resultants constitutes the clamping force urging the block towards the port plate 2. These resultants are substantially colinear and are therefore such as to "balance" the cylinder block, ie not to produce twisting forces on the cylinder block in the plane normal to Figure 1 and including the longitudinal axis of the shaft 4. With machines as shown in Figure 2 it is thought that a clamping pressure between the ported surface 32 and the land 33 which is relatively larger than those hitherto employed can be tolerated.For the sake of completeness, it should also be noted that the forces exerted by springs 14 will also contribute to the total clamping force.

Figure 3 shows a motor having a casing 1 with an end plate forming a port plate 2. The port plate is retained in the casing by a circlip 15 and is sealed by an O-ring 16. A shaft 4 rotates within a steady bush 11 and drives cylinder block 3 which is carried on splines on the shaft 4 and is supported on a support bush 9. The cylinder block is rotatable with the shaft 4 between the port plate 2 and an inclined swash plate 5 which is prevented from rotating relative to the casing 1 by a dowel 21 retained by a grub screw 20.

The cylinder block 3 has nine cylinders 40, in each of which is a piston 8. Each piston receives the spherical head of a slipper 7 which has a flat face including an annular raised portion ie a land (not shown) which is slidable over the face of the swash plate 5. The slippers 7 are retained in position by a retaining ring 6.

Each slipper 7 has an axial bore 30 through which fluid from the cylinder behind the piston 8 can flow to the interface between the slipper 7 and the swash plate 5. Each piston 8 is urged towards the swash plate by a compression spring 14 in the cylinder behind the piston.

Each cylinder 40 has a bore 31 which communicates with the end face of the cylinder block 3. This end face thus has a plurality of circular block ports, and is termed the ported surface 33 of the cylinder block. The port plate 2 is provided with two kidney-shaped plate ports (not shown) in a raised annular portion 32 called a land. The land 32 is constructed of the above described thermoplastic synthetic polymeric material known as Glamat 58. The angular position of the port plate relative to the swash plate 5 is controlled by a timing screw 19.

The support bush 9 and steady bush 11 are lubricated by fluid from within the casing 1 by oilways 34 respectively. The centre of the support bush 9 lies in the plane in which the centres of the spherical heads of all the slippers 7 lie. The swash plate 5 is inclined to the shaft 4 at an angle of 16".

The shaft is supported against endwise movement by thrust washers 12 and 13 between which is retained a collar 42 formed on the shaft. The thrust washers 12 and 13 are themselves held in position by a seal housing 10. This housing 10 is secured to the casing 1 by screws 22. Leakage of fluid from the housing is prevented by an O-ring 17 and a shaft lip seal 18.

When the motor is in use, hydraulic fluid under high pressure is fed to one of the two kidney shaped plate ports in the port plate 2.

The plate port selected is that which communicates with the pistons 8 which have been urged far into their respective cylinders by the swash plate 5. The fluid urges the pistons against the swash plate 5 causing the cylinder block, and hence the shaft 4 to rotate with the slippers 7 sliding over the swash plate 5. The slippers are believed to be supported by a film of fluid between them and the swash plate, fed to the slipper face through the bore 30 at the pressure under which the fluid is present in the cylinder.

The fluid is of course at high pressure when it passes from the port plate 2 into the bores 31 ie the block ports on the ported surface 33 of the cylinder block 3. In general, the cylinder block 3 will be urged away from the port plate 2 by the action of the fluid present in the kidney ports and acting on the ported surface of the block. The dimensions of and location of the ports are such that the opposed axial forces acting on the cylinder block 3 are substantially co-linear so that there are substantially no undesired turning moments on the block, although some imbalance may be caused by the forces exerted by the springs 14.

Once the fluid has served to act on the pistons it is expelled from the motor through the other of the kidney-shaped plate ports.

Fluid which builds up in the casing 1 around the cylinder block is released from the casing by means not shown. Accumulation of fluid in the casing is necessary to feed the oilways 34 and 35.

The machine can be used as a pump by supplying a torque to the shaft 4 and providing a supply of hydraulic fluid to those pistons which are moving outwardly of their cylinders.

Some adjustment of the angular position of the port plate 2 by means of the timing screw 19 may be appropriate.

The machine of Figure 3 has a swept capacity in the cylinders of 20 cu inches. It has been operated as a pump or motor at speeds of rotation of up to 2,200 rpm. Fluid pressures of around 4,000 psi and an output of up to around 400 HP are typical of its operation.

Figure 4 shows an axial piston pump having many components corresponding with those of the embodiment of Figure 3. Items corres ponging to 15, 19, 20 and 21 of Figure 3 are not shown, but the provision of suitable items will be obvious to those skilled in the art. The pump of Figure 4 has a ported surface 33 and a corresponding surface of a port plate 2 formed as portions of a sphere. Furthermore, the angle of inclination of the swash plate 5 relative to the shaft can be controlled by the piston and push rod arrangement shown generally as 36.

When the embodiment of Figure 4 is in use, the shaft 4 is rotated and fluid is fed to the kidney port which communicates with the cylinders of expanding volume. The rotation of the shaft then drives the pistons into these cylinders to force the fluid out through the other of the kidney ports. The capacity and performance figures of this pump are similar to the pump of Figure 3. Another unillustrated embodiment of pump according to the invention and similar in construction to that of Figure 1 has seventeen cylinders with a swept volume in total of 10 cu inches. It has been run at speeds of up to 1,800 rpm and, when delivering fluid at a pressure of 2,000 psi, it has a power output of around 100 HP.

WHAT I CLAIM IS: 1. An axial piston hydraulic machine comprising a) a cylinder block having a ported surface, a plurality of block ports in the ported surface, and a plurality of cylinders formed in the cylinder block, with each of the block ports communicating with a respective one of the cylinders; and b) a port plate at an interface with the ported surface and having a first and a second kidney-shaped port so disposed that relative rotation of the cylinder block and the port plate effects alternate communication across the interface between each one of the cylinders and the first and second kidney-shaped ports; c) each cylinder having a piston slideable axially therein, each piston being provided with a slipper having a spherical seating in the end of its respective piston remote from the ported surface; and d) a slipper plate providing a bearing surface on which the slipper can run; the machine being constructed such that in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Each cylinder 40 has a bore 31 which communicates with the end face of the cylinder block 3. This end face thus has a plurality of circular block ports, and is termed the ported surface 33 of the cylinder block. The port plate 2 is provided with two kidney-shaped plate ports (not shown) in a raised annular portion 32 called a land. The land 32 is constructed of the above described thermoplastic synthetic polymeric material known as Glamat 58. The angular position of the port plate relative to the swash plate 5 is controlled by a timing screw 19. The support bush 9 and steady bush 11 are lubricated by fluid from within the casing 1 by oilways 34 respectively. The centre of the support bush 9 lies in the plane in which the centres of the spherical heads of all the slippers 7 lie. The swash plate 5 is inclined to the shaft 4 at an angle of 16". The shaft is supported against endwise movement by thrust washers 12 and 13 between which is retained a collar 42 formed on the shaft. The thrust washers 12 and 13 are themselves held in position by a seal housing 10. This housing 10 is secured to the casing 1 by screws 22. Leakage of fluid from the housing is prevented by an O-ring 17 and a shaft lip seal 18. When the motor is in use, hydraulic fluid under high pressure is fed to one of the two kidney shaped plate ports in the port plate 2. The plate port selected is that which communicates with the pistons 8 which have been urged far into their respective cylinders by the swash plate 5. The fluid urges the pistons against the swash plate 5 causing the cylinder block, and hence the shaft 4 to rotate with the slippers 7 sliding over the swash plate 5. The slippers are believed to be supported by a film of fluid between them and the swash plate, fed to the slipper face through the bore 30 at the pressure under which the fluid is present in the cylinder. The fluid is of course at high pressure when it passes from the port plate 2 into the bores 31 ie the block ports on the ported surface 33 of the cylinder block 3. In general, the cylinder block 3 will be urged away from the port plate 2 by the action of the fluid present in the kidney ports and acting on the ported surface of the block. The dimensions of and location of the ports are such that the opposed axial forces acting on the cylinder block 3 are substantially co-linear so that there are substantially no undesired turning moments on the block, although some imbalance may be caused by the forces exerted by the springs 14. Once the fluid has served to act on the pistons it is expelled from the motor through the other of the kidney-shaped plate ports. Fluid which builds up in the casing 1 around the cylinder block is released from the casing by means not shown. Accumulation of fluid in the casing is necessary to feed the oilways 34 and 35. The machine can be used as a pump by supplying a torque to the shaft 4 and providing a supply of hydraulic fluid to those pistons which are moving outwardly of their cylinders. Some adjustment of the angular position of the port plate 2 by means of the timing screw 19 may be appropriate. The machine of Figure 3 has a swept capacity in the cylinders of 20 cu inches. It has been operated as a pump or motor at speeds of rotation of up to 2,200 rpm. Fluid pressures of around 4,000 psi and an output of up to around 400 HP are typical of its operation. Figure 4 shows an axial piston pump having many components corresponding with those of the embodiment of Figure 3. Items corres ponging to 15, 19, 20 and 21 of Figure 3 are not shown, but the provision of suitable items will be obvious to those skilled in the art. The pump of Figure 4 has a ported surface 33 and a corresponding surface of a port plate 2 formed as portions of a sphere. Furthermore, the angle of inclination of the swash plate 5 relative to the shaft can be controlled by the piston and push rod arrangement shown generally as 36. When the embodiment of Figure 4 is in use, the shaft 4 is rotated and fluid is fed to the kidney port which communicates with the cylinders of expanding volume. The rotation of the shaft then drives the pistons into these cylinders to force the fluid out through the other of the kidney ports. The capacity and performance figures of this pump are similar to the pump of Figure 3. Another unillustrated embodiment of pump according to the invention and similar in construction to that of Figure 1 has seventeen cylinders with a swept volume in total of 10 cu inches. It has been run at speeds of up to 1,800 rpm and, when delivering fluid at a pressure of 2,000 psi, it has a power output of around 100 HP. WHAT I CLAIM IS:

1. An axial piston hydraulic machine comprising a) a cylinder block having a ported surface, a plurality of block ports in the ported surface, and a plurality of cylinders formed in the cylinder block, with each of the block ports communicating with a respective one of the cylinders; and b) a port plate at an interface with the ported surface and having a first and a second kidney-shaped port so disposed that relative rotation of the cylinder block and the port plate effects alternate communication across the interface between each one of the cylinders and the first and second kidney-shaped ports; c) each cylinder having a piston slideable axially therein, each piston being provided with a slipper having a spherical seating in the end of its respective piston remote from the ported surface; and d) a slipper plate providing a bearing surface on which the slipper can run; the machine being constructed such that in

use the resultant of hydrostatic pressure forces within the cylinders acting on the cylinder block is substantially co-linear with the resultant of hydrostatic pressure forces of fluid acting to force apart the cylinder block and port plate.

2. A machine according to claim 1 constructed such that in use the ported surface of the cylinder block and a surface of the port plate remain in contact with one another during relative rotation thereof so as to resist substantially the whole of the net axial loading between the cylinder block and the port plate.

3. A machine according to claim 2 wherein one of the said surfaces in contact is formed of a low-friction plastics material.

4. A machine according to claim 3 comprising a distinct annular member of the said plastics material located at the said interface a surface of the annular member constituting one of the said surfaces in contact.

5. A machine according to any one preceding claim in which the cylinder block is supported on a support bush whose centre lies in the plane containing the centres of the said spherical seatings when the slippers are in bearing contact with the slipper plate.

6. A machine according to any one preceding claim wherein one of the surfaces of the slippers or of the slipper plate is formed of a low friction plastics material.

7. A machine as claimed in any one of the preceding claims wherein the interface between the port plate and the ported surface of the cylinder block is planar.

8. A machine as claimed in any one of claims 1 to 6 wherein the interface between the port plate and the ported surface is of partspherical form.

9. A machine as claimed in claim 8 wherein the centre of the notional sphere defining the said interface is on the same side of the interface as the cylinder block.

10. A machine substantially as hereinbefore described with reference to Figures 1 and 2, Figure 3 or Figure 4 of the drawings filed with the provisional specification.