GB2502824A

GB2502824A - Axial piston variable stroke hydraulic machine

Info

Publication number: GB2502824A
Application number: GB201210159A
Authority: GB
Inventors: Tony Markham
Original assignee: WATER HYDRAULICS Co Ltd
Current assignee: WATER HYDRAULICS Co Ltd
Priority date: 2012-06-08
Filing date: 2012-06-08
Publication date: 2013-12-11
Also published as: GB201210159D0

Abstract

An axial piston variable stroke hydraulic machine includes pistons 2 acting in cylinders which are rotatable around an axis. A swash plate 3 has a planar face in contact with the pistons and opposed to a curved surface which is mounted rotatably in a correspondingly-shaped bearing surface 8 so that the angle between the face of the swash plate and the axis can be varied. An actuation mechanism 13-16 is provided for selectively rotating the swash plate to a desired angle in the bearing surface and a valve arrangement controls flow of fluid into and out of the cylinders. The bearing surface is formed with recesses 11, 12 which is in fluid communication with the output of the cylinders, where the machine is a pump, or the input to the cylinders, where the machine is a motor.

Description

AXIAL PISTON VARIABLE STROKE HYDRAULIC MACHINE

Field of the Invention

This invention relates to a variable stroke axial piston pump/motor, par-ticularly for use with aqueous fluids.

Background to the Invention

Variable stroke oil hydraulic axial piston pumps are well-known, and offer advantages in precise control of flow and pressure. Axially-arranged pistons are rotated relative a swash plate whose angle can be varied to vary the stroke of the pistons. However, these types of pump have not been usable where it is desired to pump water or other water mixtures, because the lack of lubrication and the corrosive nature of the fluid would result in very high friction in the mechanism, particularly the adjustment of the angle of the swash plate, that would cause a very slow and laboured response that would render the design useless. Equally, in motor use for certain applications, oil-based hydraulic fluids cannot readily be used. For example, in food preparation factories, there is a risk of fluid leakage contaminating the food, while in areas of high fire risk flam- mable hydraulic fluids must be avoided. Water is therefore preferred as the hy-draulic fluid for these applications.

There are many applications where there is a need for a substantially constant-pressure output with varying demand. For example, multiple water lances may be installed, and a varying number of them will be in use at the same time. This is typically provided for by installing a bank of individual pumps, which are switched in and out according to the demand. However, as each pump is switched in or out, there is a step in the output pressure.

Summary of the Invention

According to one aspect of the invention, an axial piston variable stroke hydraulic machine comprises a plurality of pistons acting in respective cylinders arranged to be rotatable around an axis, a swash plate having a planar face in contact with the pistons, the swash plate having a curved surface opposing the planar face, said curved surface being mounted so as to be rotatable in a corre-spondingly-shaped bearing surface whereby the angle between the face of the swash plate and said axis can be varied, means for selectively rotating the swash plate to a desired angle in the bearing surface, and a valve arrangement for controlling flow of fluid into and out of the cylinders, characterised in that the bearing surface is formed with at least one recess therein in fluid communica-tion with the output of the cylinders, where the machine is a pump, or the input to the cylinders, where the machine is a motor.

The recess or recesses provide a hydrostatic balancing force to the very high axial force produced by the pistons against the swash plate, thereby reduc-ing the force required to rotate the swash plate.

The means for selectively rotating the swash plate may comprise a first piston biasing the swash plate towards a position in which the angle between the face of the swash plate and said axis is a maximum, and a second piston configured to exert an opposed and greater rotational force on the swash plate than that of the first piston so as to reduce the angle from the maximum to the desired angle.

Another aspect of the invention provides an axial piston variable stroke hydraulic machine comprises a plurality of pistons acting in respective cylinders arranged to be rotatable around an axis, a swash plate having a planar face in contact with the pistons, the swash plate having a curved surface opposing the planar face, said curved surface being mounted so as to be rotatable in a corre-spondingly-shaped bearing surface whereby the angle between the face of the swash plate and said axis can be varied, means for selectively rotating the swash plate to a desired angle in the bearing surface, and a valve arrangement for controlling flow of fluid into and out of the cylinders, characterised in that the means for selectively rotating the swash plate comprise a first piston biasing the swash plate towards a position in which the angle between the face of the swash plate and said axis is a maximum, and a second piston configured to ex-ert an opposed and greater rotational force on the swash plate than that of the first piston so as to reduce the angle from the maximum to the desired angle.

Preferably, the first and second pistons are controlled by a slide valve ar-rangement comprising a valve stem which rotates with rotation of the swash plate and has a port arranged to supply fluid to or vent fluid from the second pis-ton, and a control plate which co-operates with the valve stem such that sliding of the control plate relative to the valve stem selectively exposes the port either to a source of pressurised fluid or to a fluid outlet, and rotation of the stem with the swash plate relative to the control plate causes the control plate to close the port, thereby achieving an equilibrium position of the valve.

Brief Description of the Drawings

In the drawings, which illustrate exemplary embodiments of the invention: Figure 1 is a partially-cut away perspective view of the hydraulic ma-chine, showing the swash plate rotated to its full extent; Figure 2 is a perspective view of the machine of Figure 1, with the cylin-der block removed; Figure 3 is a view corresponding to Figure 2, with the swash plate re-moved; Figure 4 is a view from the opposile end of Ihe swash plale, and with Ihe control piston and valve assembly partially exploded; Figure 5 is an enlarged view of the valve assembly illustrated in Figure 4; Figure 6 is a view of the control piston and valve assembly with parts of the valve assembly removed for clarity, and parts rendered transparent; and Figures 7 to 9 are diagrammatic views of the two main components of the valve arrangement illustrating successive stages in the operation of the valve.

Detailed Description of the Illustrated Embodiment

Referring first to Figures 1 to 3, the machine comprises a cylinder block 1 in which a plurality of parallel cylinders (not shown) are mounted in a ring, each cylinder containing a respective piston 2 contacting a swash plate S contained wilhin a swash plale assembly 4. The cylinders are mounted for rolation wilhiri the cylinder block 1 around a shaft 5, which serves either as a drive shaft, in the case of use as a pump, or an output shaft, in the case of use as a motor. The shaft 5 extends through an aperture in the centre 6 (Figure 2) of the swash plate 3.

The swash plate 3 is essentially in the form of a cylindrical body divided approximately along its longitudinal axis to present a planar swash surface 3a surrounding the aperture 6, the hemi-cylindrical surface being received in a cor-respondingly-shaped recess 8 (Figure 3) in the swash plate assembly 4. The recess 8 has an opening 9 through the centre thereof, through which the shaft 5 extends, to be carried by a bearing assembly 10. The recess 8 is also provided with grooves or pockets 11 and 12 extending circumferentially over a portion of the surface on opposed sides of the opening 9, the larger pocket 11 being nearer to a control piston and valve assembly 13 for the swash plate 3 than the other pocket 12. Each pocket 11 and 12 is connected to the output from the pistons, when the machine serves as a pump, or the inlet supply to the pistons when the machine serves as a motor to apply a hydrostatic force to the under-side of the swash plate serving to balance the force exerted by the pistons, thereby reducing the friction between the swash plate 3 and the bearing surfac-es in the recess 8, in turn reducing the force needed to rotate the swash plate.

The larger pocket 11 serves to compensate for any turning moment longitudi-nally of the swash plate caused by the application of the control forces applied by the control pistons, as hereinafter described.

A pivot block 14, in the form of a cylinder 14a extending from a rectangu-lar base, is secured to one end of the swash plate 3 so as to extend parallel to the axis of rotation of the swash plate. The cylinder 14a is engaged by opposed minor and major control pistons 15 and 16 movable in respective bores in the assembly 13. Movement of the pistons 15 and 16 displaces the pivot block 14, in turn causing rotation of the swash plate 3 in its recess 8. The minor piston 15 is connected as hereinafter described to the outlet/inlet to the pistons so that the pump or motor pressure is delivered thereto at all times when the machine is operative, thereby urging the swash plate towards the full deflection thereof, typically around 15-16°. A spring is also provided to bias the minor piston to- wards the full deflection position of the swash plate, even when there is no wa- ter pressure. When it is desired to rotate the swash plate away from the full de-flection position, the major piston 16 is supplied with the same water pressure by the valve part of the assembly 13 as hereinafter described, and because it has a larger cross-sectional area than the minor piston 15, it exerts a force on the pivot block which opposes and exceeds the force exerted by the minor pis-ton, thereby causing the swash plate to rotate.

Referring now to Figures 3, 5 and 6, the valve part of the assembly com-prises a valve stem 17 extending through a valve block 18 into a bore in the end of the swash plate which extends along the axis of rotation of the plate 3. The stem 17 has a pair of drillings 20 and 21 extending longitudinally therein, each opening in a planar end face 22 of the stem. A first of the drillings 20 opens into the bore in the swash plate 3, from which opening a drilling extends radially through the swash plate to open into the larger pocket 11. A passageway (not shown) in the body containing the swash plate assembly allows fluid communi- cation between the larger and smaller pockets 11 and 12, so that when pressur- ised water is supplied the larger pocket 11, the smaller pocket 12 is also pres- surised. It will be noted that the pressurised water in the first drilling 20 is sup-plied to the face of the stem within the bore 19 in the swash plate. The stem 17 acts as a piston as the pressure biases the stem outwardly of the bore.

Pressurised water is supplied to the face of the minor piston 15 through a series of drillings (not shown) in the casing of the machine connected to the same supply as at the input to the valve arrangement, so that the minor piston is permanently biased when the machine is operating towards the full deflection position for the swash plate 3. A spring maintains the bias when the machine is not operating.

The second drilling 21 opens in a face pocket 21a in the face 22 of the stem and communicates within the stem with a radial drilling which in turn communicates with a drilling in the valve body leading to a second longitudinal drilling within the pivot block 14. The major piston 16 has a pad 27 on the end thereof shaped to conform with the cylindrical surface of the pivot block and having a recess within its shaped surface having a drilling extending longitudi-nally through it and the major piston 16 to communicate with the opposed face of the piston. A radial drilling extends in the pivot block from the second drilling and opens in the cylindrical surface of the pivot block 14 within the recess in the pad 27.

The control valve arrangement forming part of the assembly 13 compris-es a control plate 28 mounted in a carrier 29 which is in turn slidably mounted in a slot 30 extending through the valve block 18. The carrier 29 has an arm 32 extending out of the block 18 and driven by a variable force solenoid or the like to permit electrical control of the machine, for example, or through another form of driver. The control plate 28 has a drilling 33 therethrough which on its upper side is exposed to a supply of pressurised water from the pump output/motor supply. On its underside, the plate 28 has a generally [-shaped recess 34 communicating with the drilling 33 so that the recess 34 is at substantially the same water pressure as the upper side of the plate, thereby ensuring that there is a hydrostatic balance across the plate. In effect, therefore, the plate 28 floats freely and can therefore be moved laterally with the minimum of force. The un-derside of the plate 28 also has a free area 35 separate from the recess and in communication with the casing pressure and, separating these, a surrounding land 36. Sliding the plate 28 in the slot 30 relative to the openings of the drill-ings 20 and 21 in the lace of the valve stem 17 can achieve three alternative positions: 1. An Equilibrium position in which the drilling 20 is exposed to the recess on the underside of the plate, and thereby to water at sys-tem maximum pressure, the drilling 21 being closed and the major and minor pistons thus being balanced; 2. A Forward position in which the drilling 21 is also exposed to the recess, thereby supplying system pressure water to the major pis- ton and causing a reduction in the angle of the swash plate rela-tive to its maximum deflection; and 3. A Reverse position in which the drilling 21 is exposed to the free area and thus to the casing pressure which, being lower than the system pressure, releases the pressure on the major piston and permits the minor piston to move the swash plate so as to in-crease its angle of deflection.

When the controller for the system needs to make an adjustment of the machine output, the carrier 29 is slid in the slot 30 causing the valve to change from its Equilibrium position to either the Forward or Reverse position, depend- ing on which direction it is moved in. In either case, the major and minor pis-tons are changed from their balanced position to one in which one of the pistons overcomes the other and causes rotation of the swash plate. Since the stem rotates with the swash plate and relative to the valve block, the positions of the drillings 20 and 21 relative to the valve plate 28 will change as the swash plate turns, until the Equilibrium position is again achieved. The valve thus acts as a servo valve with effectively a mechanical feedback link.

Figures 7 to 9 are simplified diagrams illustrating the operation of the valve. They illustrate the relationship between the underside configuration of the control plate 28 and the face 22 of the valve stem 17. All other components have been excluded for the sake of clarity. Figure 7 illustrates the Equilibrium position, in which the whole of the face pocket 21a is closed by the raised land 36, while the drilling 20 communicates with the recess 34, and is thus connect-ed to the source of pressurised water. When the system needs to decrease the swash plate angle, a command signal to the solenoid displaces the plate 28 suf-ficiently to expose the face pocket 21a to the recess 34 (Figure 8), and thus admit pressurised fluid to the drilling 21, which supplies the major piston 16.

The major piston 16 in turn displaces the pivot block 14, rotating the swash plate. Since the stem 17 is caused to rotate with the swash plate, this rotation continues until the face pocket 21a is again closed by the raised land 36, to achieve the Equilibrium position. When the systems needs to increase the swash plate angle, a command signal to the solenoid moves the plate in the opposite direction until the face pocket 21a is opened to the free area 35, as il-lustrated in Figure 9. This permits fluid from the major piston to be released or vented, thus permitting the minor piston 15 to move the pivot block in the oppo-site direction. This, in turn causes rotation of the valve stem 17 in the opposite direction, until a position is reached in which the face pocket 21a is fully closed by the raised land 36, again achieving Equilibrium.

Claims

CLAIMS1. An axial piston variable stroke hydraulic machine, comprising a plurality of pistons acting in respective cylinders arranged to be rotatable around an axis, a swash plate having a planar face in contact with the pistons, the swash plate having a curved surface opposing the planar face, said curved sur-face being mounted so as to be rotatable in a correspondingly-shaped bearing surface whereby the angle between the face of the swash plate and said axis can be varied, means for selectively rotating the swash plate to a desired angle in the bearing surface, and a valve arrangement for controlling flow of fluid into and out of the cylinders, characterised in that the bearing surface is formed with at least one recess therein in fluid communication with the output of the cylin- ders, where the machine is a pump, or the input to the cylinders, where the ma-chine is a motor.
2. A machine according to Claim 1, wherein the bearing surface is formed with two or more of said recesses, the recesses being dimensioned and arranged to compensate for any turning moment in a plane through the said ax-is due to operation of the means for rotating the swash plate.
3. A machine according to Claim 1 or 2, wherein the means for se-lectively rotating the swash plate comprise a first piston biasing the swash plate towards a position in which the angle between the face of the swash plate and said axis is a maximum, and a second piston configured to exert an opposed and greater rotational force on the swash plate than that of the first piston so as to reduce the angle from the maximum to the desired angle.
4. An axial piston variable stroke hydraulic machine, comprising a plurality of pistons acting in respective cylinders arranged to be rotatable around an axis, a swash plate having a planar face in contact with the pistons, the swash plate having a curved surface opposing the planar face, said curved sur-face being mounted so as to be rotatable in a correspondingly-shaped bearing surface whereby the angle between the face of the swash plate and said axis can be varied, means for selectively rotating the swash plate to a desired angle in the bearing surface, and a valve arrangement for controlling flow of fluid into and out of the cylinders, characterised in that the means for selectively rotating the swash plate comprise a first piston biasing the swash plate towards a posi-tion in which the angle between the face of the swash plate and said axis is a maximum, and a second piston configured to exert an opposed and greater ro-tational force on the swash plate than that of the first piston so as to reduce the angle from the maximum to the desired angle.
5. A machine according to Claim 3 or 4, wherein the first and second pistons are controlled by a slide valve arrangement comprising a valve stem which rotates with rotation of the swash plate and has a port arranged to supply fluid to or vent fluid from the second piston, and a control plate which co-operates with the valve stem such that sliding of the control plate relative to the valve stem selectively exposes the port either to a source of pressurised fluid or to a fluid outlet, and rotation of the stem with the swash plate relative to the con- trol plate causes the control plate to close the port, thereby achieving an equilib-rium position of the valve.
6. A machine according to Claim 5, wherein each end of the valve stem is exposed to substantially the same fluid pressure.
7. A machine according to Claim 5 or 6, wherein each face of the control plate is exposed to substantially the same fluid pressure.
8. A machine according to Claim 5, 6 or 7, comprising an electric ac-tuator connected to the control plate for selectively sliding the control plate.
9. A machine according to Claim 8, wherein the electric actuator is a variable force solenoid.
10. An axial piston variable stroke hydraulic machine, substantially as described with reference to, and or as shown in, the drawings.