WO2014135883A2

WO2014135883A2 - A pump

Info

Publication number: WO2014135883A2
Application number: PCT/GB2014/050666
Authority: WO
Inventors: Andrew Middleton; Peter Chalk
Original assignee: Marshalsea Hydraulics Limited
Priority date: 2013-03-07
Filing date: 2014-03-06
Publication date: 2014-09-12
Also published as: WO2014135883A3; GB201304139D0

Abstract

An axial piston pump having a swash plate arranged for rotation about a rotational axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising at least one piston that is reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing, the piston housing having a fluid inlet and a fluid outlet in communication with the bore, a valve disposed in the bore between the fluid inlet and the fluid outlet, an inlet seal disposed in the bore of the piston housing between the fluid inlet and the fluid outlet, the inlet seal configured for sealing engagement with the piston, a chamber being defined between the inlet seal and the valve, wherein as the at least one piston moves outwardly it generates suction in the chamber until such time as it passes a certain position relative to the inlet seal whereupon fluid is permitted to enter the chamber from the fluid inlet and, as the at least one piston moves inwardly, once it passes inwardly of said certain position relative to the inlet seal it forces the fluid in the chamber through the valve to the fluid outlet. The relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate may be selectively adjustable.

Description

A Pump

The present invention relates to a pump and more particularly to an axial displacement pump in which one or more pistons are axially displaced by a rotary member such as, for example, a swash plate.

Conventional axial piston pumps have a plurality of pistons each mounted in a corresponding cylinder, the piston and cylinder pairs being disposed in a circular array within a housing often referred to as the cylinder block. At one end the pistons bear against a cam surface of a swash plate (also sometimes referred to as a wobble plate) that is inclined to the longitudinal axes of the pistons. Relative rotation of the cylinder block and the swash plate has the effect of translating the rotation into reciprocal movement of the pistons within their respective cylinders. As each piston is forced into its respective cylinder it pumps fluid through an outlet valve and as it retracts it creates a suction pressure that serves to draw fluid in through an inlet valve.

Providing such an inlet valve increases the cost and complexity of such an axial piston pump. In addition, the suction pressure created may not be sufficiently high, resulting in stall of the fluid being sucked into the cylinder It is one object of the present invention to obviate or mitigate one or more of the aforesaid disadvantages. It is also an object of the present invention to provide for an improved or alternative axial piston pump.

According to a first aspect of the invention there is provided an axial piston pump comprising a swash plate arranged for rotation about a rotational axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising at least one piston that is reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing, the piston housing having a fluid inlet and a fluid outlet in communication with the bore, a valve disposed in the bore between the fluid inlet and the fluid outlet, an inlet seal disposed in the bore of the piston housing between the fluid inlet and the fluid outlet, the inlet seal configured for sealing engagement with the piston, a chamber being defined between the inlet seal and the valve, wherein as the at least one piston moves outwardly it generates suction in the chamber until such time as it passes a certain position relative to the inlet seal whereupon fluid is permitted to enter the chamber from the fluid inlet and, as the at least one piston moves inwardly, once it passes inwardly of said certain position relative to the inlet seal it forces the fluid in the chamber through the valve to the fluid outlet.

This is advantageous in that it removes the need for an inlet valve. In this regard, the flow of fluid from the fluid inlet is only permitted for a certain position of the piston relative to the inlet seal. Accordingly, this arrangement acts in the manner of an inlet valve.

In addition, due to the suction created in the chamber when the piston is moving outwardly, but while it is inward of said certain position relative to the inlet seal, a relatively high level of suction is created in the chamber which, when the piston moves outward of said certain relative position, produces a rapid in-flow of fluid into the chamber. This prevents the flow of fluid from stalling. The inlet seal may be fixed relative to the piston housing. As the at least one piston reciprocates in the bore of the respective piston housing it may move relative to the inlet seal. The inlet seal may be mounted in a groove in the respective piston housing. The inlet seal may be substantially annular. The inlet seal may extend substantially around the at least one piston. The inlet seal may be a piston ring.

When the at least one piston is inward of said certain position relative to the inlet seal, the fluid inlet may not be fluidly connected to the chamber and when the at least one piston is outward of said certain position relative to the inlet seal, the fluid inlet may be fluidly connected to the chamber.

When the at least one piston is inward of said certain position relative to the inlet seal, the fluid inlet may not be fluidly connected to the chamber due to the sealing engagement of the inlet seal with the at least one piston and when the at least one piston is outward of said certain position relative to the inlet seal, the inlet seal may not be in sealing engagement with the at least one piston such that the fluid inlet is fluidly connected to the chamber.

When the at least one piston is outward of said certain position relative to the inlet seal, the fluid inlet may be fluidly connected to the chamber by a clearance between the piston and a surface of the respective piston housing that defines the bore. Said certain position of the at least one piston relative to the inlet seal may be a position in which an end of the piston passes the inlet seal. Alternatively, the certain relative position may be a different relative position. For example, the at least one piston may be provided with an end section of reduced diameter, compared to the remainder of the piston, and when the end section reaches the inlet seal the fluid inlet is connected to the chamber by a clearance between the section of reduced diameter and the inlet seal. In this case, the end of the piston will still be within the chamber.

Preferably, as the at least one piston moves inwardly but is located outwardly of said certain position relative to the inlet seal, fluid is not forced from the chamber through the valve to the fluid outlet by the movement of the at least one piston.

Preferably as the at least one piston moves inwardly but is located outwardly of said certain position relative to the inlet seal, the movement of the at least one piston forces fluid in the chamber back to the fluid inlet.

Preferably, as the at least one piston moves outwardly but is located outwardly of said certain position relative to the inlet seal the inlet seal, suction is not generated in the chamber by the movement of the at least one piston.

The chamber may be defined by a substantially cylindrical hollow insert disposed in the bore.

The relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate may be selectively adjustable.

The adjustability of the relative axial positions of the swash plate and the piston housing enables the effective pumping stroke (that part of the stroke of the piston that allows fluid to be exhausted through the outlet or be drawn in through the inlet) to be varied. It will be understood that the reference to the axial position of the swash plate relative to the piston housing is intended to exclude the variation in axial position that occurs between the cam surface and the piston housing as a result of rotation of the swash plate during normal operation of the pump. It will be appreciated that the relative axial distance between a given axial location on the piston housing and a given axial location on the swash plate on the rotational axis of the swash plate is selectively adjustable. In this regard as the swash plate rotates a given axial location, on the swash plate, on the rotational axis of the swash plate does not move in the axial direction.

The relative axial distance may be adjusted by axial movement of the piston housing, by axial movement of the swash plate or a combination of the two.

The relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate may be selectively adjustable such that the axial position of the piston housing relative to the at least one piston, for a certain axial position of the at least one piston, is selectively adjustable.

In this regard, the certain axial position of the at least one piston is a certain absolute position of the at least one piston. This is the position of the at least one piston for a certain rotational position of the swash plate.

The relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate may be selectively adjustable such that the relative axial distance between the inlet seal and a given axial location on the swash plate on the rotational axis of the swash plate is selectively adjustable.

The relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate may be selectively adjustable such that the axial position of the inlet seal relative to the at least one piston, for a certain axial position of the at least one piston, is selectively adjustable.

The piston housing may be received in a respective housing support.

The housing support and the piston housing may be interconnected by a coupling by which the relative axial distance may be adjusted. The coupling may convert rotational movement of the piston housing into axial movement thereof.

The coupling may comprise a screw thread engagement. The screw thread engagement may be defined by a screw thread defined on an internal surface of the housing support and an external screw thread defined on the piston housing. Alternatively they may be interconnected by another type of coupling that permits axial adjustment. In one exemplary embodiment the relative axial distance may be adjusted by any kind of coupling that permits rotational movement of the piston housing in the housing support being translated into axial movement thereof.

The piston housing may have a tool engagement feature with which a tool may be engaged to effect rotation of the piston housing relative to the housing support.

The housing support may comprise a first support member that is penetrated by a fluid supply passage that is in fluid communication with the inlet of the piston housing. The first support member may be in the form of a plate which may be disc-shaped.

The piston housing may be supported by the first support member.

The housing support may also comprise a second support member connected to the first support member. The second support member may extend in a direction away from the swash plate.

The inlet of the piston housing may comprise a plurality of inlet passages that may be disposed at angular intervals around the bore. The outlet of the piston housing may comprise a plurality of outlet passages that may be disposed at angular intervals around the bore.

The valve is preferably a check valve, which may be of any suitable design.

The fluid outlet of the piston housing may be in fluid communication with an outlet port defined in the housing support. The outlet port may be defined in the second member of the housing support.

There may be a substantially annular passage defined between the piston housing and the housing support, the annular passage interconnecting the fluid supply passage and inlet of the piston housing. The coupling between the piston housing and the housing support may be provided in the annular passage. In the embodiment where the coupling is in the form of a screw thread engagement a wall of the annular passage defined by the piston housing or the housing support may have a screw thread for engagement with a complementary screw thread on the other of the piston housing and the housing support. Thus the annular passage may serve to accommodate the relative axial movement. The pump may have a plurality of such pistons and piston housings and the first support member may have a plurality of supply passages, one for each piston housing.

The pump may have a plurality of such pistons and piston housings and the supply passage may have a plurality of branch passages for supplying fluid to the inlet of each piston housing.

The swash plate and therefore the piston may be translated in the axial direction towards the piston housing so as to adjust the relative axial distance.

The pumping mechanism may further comprise a biasing member for urging the piston against the cam surface. The rotation of the swash plate forces the piston into the bore of the piston housing against the force of the biasing member and then allows it to extend out of the housing under the action of the biasing member.

The biasing member may be disposed between the piston and the housing support so as to urge the piston in a direction out of the housing. By acting against the housing support rather than the piston housing, any adjustment of the relative axial distance does not affect the biasing member.

The piston may have an enlarged head or a shoulder against which the biasing member acts. The biasing member may be disposed substantially concentrically with the piston. One such biasing member may be provided for each piston. The pump may have a plurality of such pistons and piston housings, which may be arranged in a circumferential array.

In an embodiment where there is a plurality of pistons and piston housings there may be a respective supply passage for delivering fluid to each fluid inlet and a respective exhaust passage for exhausting fluid from the fluid outlet, wherein the supply passages are independent of one another and the exhaust passages are independent of one another so as to prevent mixing of fluids that pass through each piston housing bore. Alternatively some or all of the supply passages may be in fluid communication and/or some or all of the exhaust passages are in fluid communication

The axis or axes of the bore(s) may be substantially parallel to the axis of rotation of the swash plate. The piston housing may be, for example, a cylinder with a cylinder bore for receipt of the piston. The piston housing may be received in a respective housing support that may comprise, for example, a cylinder block. In an embodiment where there is a plurality of piston housings, each may be received in a respective housing support.

In an alternative arrangement the swash plate and therefore the pistons may be translated in the axial direction towards the piston housings so as to adjust the relative axial distance. This may be achieved in a variety of ways. For example, the swash plate or a shaft that forms part of the swash plate has a thread for engagement with a complementary thread on a housing. Rotation of the housing causes axial displacement of the swash plate and therefore the pistons. The housing may, for example, comprise a cylinder with an internal thread. It will be appreciated that other forms of adjustable connection may be adopted. For example a bayonet style coupling may be used. In another alternative the swash plate or at least a shaft of the swash plate is housed in a hydraulic cylinder such that axial movement of the swash plate relative to the cylinder is achieved under hydraulic pressure.

In some applications such axial piston pumps do not work well at low speeds of operation. The volumetric flow rate of the pump is typically adjusted by altering the speed at which the pump is driven (by for example a motor) or by adjusting the angle of the swash plate cam surface. The latter involves relatively complex mechanical arrangements. According to a second aspect of the present invention there is provided an axial piston pump comprising a swash plate arranged for rotation about a rotational axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising at least one piston that is reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing, the piston housing having a fluid inlet and a fluid outlet in communication with the bore, a valve disposed in the bore between the fluid inlet and the fluid outlet, the piston being moveable inwardly of the piston housing so as to force fluid under pressure from the bore through the valve to the outlet, wherein the relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate is selectively adjustable. The adjustability of the relative axial positions of the swash plate and the piston housing enables the effective pumping stroke (that part of the stroke of the piston that allows fluid to be exhausted through the outlet or be drawn in through the inlet) to be varied. It will be understood that the reference to the axial position of the swash plate relative to the piston housing is intended to exclude the variation in axial position that occurs between the cam surface and the piston housing as a result of rotation of the swash plate during normal operation of the pump.

The relative axial distance between the piston housing and the given axial location on the rotational axis may be adjusted by axial movement of the piston housing, by axial movement of the swash plate or a combination of the two.

The pump may have plurality of such pistons and piston housings, which may be arranged in a circumferential array.

The axis or axes of the bore(s) may be substantially parallel to the axis of rotation of the swash plate.

The piston housing may be, for example, a cylinder with a cylinder bore for receipt of the piston. The piston housing may be received in a respective housing support that may comprise, for example, a cylinder block. In an embodiment where there is a plurality of piston housings, each may be received in a respective housing support.

The housing support and the piston housing may be interconnected by a screw thread engagement by which the relative axial distance may be adjusted. Alternatively they may be interconnected by another type of coupling that permits axial adjustment. In one exemplary embodiment the relative axial distance may be adjusted by any kind of coupling that permits rotational movement of the piston housing in the housing support being translated into axial movement thereof.

The screw thread engagement may be defined by a screw thread defined on an internal surface of the housing support and an external screw thread defined on the piston housing. The piston housing may have a tool engagement feature with which a tool may be engaged to effect rotation of the piston housing relative to the housing support. In an alternative arrangement the swash plate and therefore the pistons may be translated in the axial direction towards the piston housings so as to adjust the relative axial distance. This may be achieved in a variety of ways. For example, the swash plate or a shaft that forms part of the swash plate has a thread for engagement with a complementary thread on a housing. Rotation of the housing causes axial displacement of the swash plate and therefore the pistons. The housing may, for example, comprise a cylinder with an internal thread. It will be appreciated that other forms of adjustable connection may be adopted. For example a bayonet style coupling may be used. In another alternative the swash plate or at least a shaft of the swash plate is housed in a hydraulic cylinder such that axial movement of the swash plate relative to the cylinder is achieved under hydraulic pressure

The piston housing may have an inlet seal disposed in the bore of the piston housing between the fluid inlet and the fluid outlet, the inlet seal configured for sealing engagement with the piston. A chamber may be defined between the inlet seal and the valve in which fluid is pressurised by the reciprocation of the piston in the bore.

The housing support may comprise a first support member that is penetrated by a fluid supply passage that is in fluid communication with the inlet of the piston housing. The first support member may be in the form of a plate which may be disc-shaped. In an embodiment where there is a plurality of pistons and piston housings, the piston housings are supported around the first support member and there may be a plurality of supply passages, one for each piston housing. Alternatively, the supply passage may have a plurality of branch passages for supplying fluid to the inlet of each piston housing. The housing support may also comprise a second support member that is connected integrally or otherwise to the first support member. The second support member may extend in a direction away from the swash plate and may be in the form of a cylinder block. In an embodiment where there is a plurality of piston housings there may be a corresponding plurality of second support members.

The valve is preferably a check valve, which may be of any suitable design. The fluid outlet of the piston housing may be in fluid communication with an outlet port defined in the housing support. The outlet port may be defined in the second member of the housing support. There may be a chamber defined between the inlet seal and the valve in which fluid is pressurised by the reciprocation of the piston in the bore. The chamber may be defined by a substantially cylindrical hollow insert disposed in the bore.

There may be a substantially annular passage defined between the piston housing and the housing support, the annular passage interconnecting the fluid supply passage and inlet of the piston housing. The coupling between the piston housing and the housing support may be provided in the annular passage. In the embodiment where the coupling is in the form of a screw thread engagement a wall of the annular passage defined by the piston housing or the housing support may have a screw thread for engagement with a complementary screw thread on the other of the piston housing and the housing support. Thus the annular passage may serve to accommodate the relative axial movement.

The pumping mechanism may further comprise a biasing member for urging the piston against the cam surface. The rotation of the swash plate forces the piston into the bore of the piston housing against the force of the biasing member and then allows it to extend out of the housing under the action of the biasing member. The biasing member may be disposed between the piston and the housing support so as to urge the piston in a direction out of the housing. By acting against the housing support rather than the piston housing, any adjustment of the relative axial distance does not affect the biasing member.

The piston may have an enlarged head or a shoulder against which the biasing member acts. The biasing member may be disposed substantially concentrically with the piston. One such biasing member may be provided for each piston.

According to a third aspect of the present invention there is provided an axial piston pump comprising a rotary swash plate arranged for rotation about an axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising a plurality of pistons that are each reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing by a biasing member, each piston housing having a fluid inlet and a fluid outlet, a valve disposed between the fluid inlet and the fluid outlet of each piston housing, wherein fluid is deliverable to each fluid inlet via a respective supply passage and fluid is discharged from each outlet via a respective exhaust passage, the supply passages being independent of one another and the exhaust passages being independent of one another so as to prevent mixing of fluids that pass through each piston housing bore.

The plurality of pistons and piston housings may be arranged in a circumferentially array. The axes of the bores may be substantially parallel to the axis of rotation of the swash plate.

The axial piston pump preferably comprises an outer housing in which the swash plate and pumping mechanism are received.

Each piston housing may be, for example, a cylinder, with a cylinder bore for receipt of the piston. Each piston housing may be received in a respective housing support that may comprise, for example, a cylinder block. Each piston housing may have an inlet seal disposed in the bore of the piston housing between the fluid inlet and the fluid outlet, the inlet seal configured for sealing engagement with the piston. A chamber may be defined between the inlet seal and the valve in which fluid is pressurised by the reciprocation of the piston in the bore. There may be a common support member for supporting the plurality of piston housings, the common support member being penetrated by the separate supply passages. The common support member may be in the form of a plate which may be disc-shaped. The piston housings may be supported around the common support member in, for example, a circumferential array. There may be separate housing supports, each of which is associated with the piston housing. Each separate housing support may be connected integrally or otherwise to the common support member. The separate housing supports may extend in a direction away from the swash plate and may be in the form of a cylinder block. Each separate housing support may define at least part of the exhaust passage.

The valve is preferably a check valve, which may be of any design.

The rotation of the swash plate forces the piston into the bore of the piston housing against the force of the biasing member and then allows it to extend out of the housing under the action of the biasing member. The biasing member may be disposed between the piston and the housing support so as to urge the piston in a direction out of the housing. By acting against the housing support rather than the piston housing, any adjustment of the relative axial distance does not affect the biasing member.

Any feature of any of the above aspects of the invention may be combined with any features of the other aspects, in any combination. A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a longitudinal sectioned view of an axial piston pump embodying the present invention;

Figure 2 is an end view of an embodiment of a swash plate and pumping mechanism of the pump of figure 1 ;

Figure 3 is a longitudinal sectioned view along line C-C of figure 2, showing one of the piston and piston housings of the pumping mechanism in section, the piston housing being disposed in a minimum pump displacement position;

Figure 4 corresponds to the view of figure 3 but with the piston housing shown in a maximum pump displacement position;

Figure 5 shows the sectioned piston and cylinder block of figure 3 in more detail, with the piston housing shown in the minimum pump displacement position; and Figure 6 corresponds to the view of figure 5 but with the piston housing shown in a maximum pump displacement position.

Referring now to the figure 1 of drawings, the exemplary pump comprises a housing generally indicated by reference numeral 1 , a swash plate 2 supported for rotation in the housing by bearings 3, a rotary drive shaft 4 extending through the housing and connected to the swash plate, and a pumping mechanism generally indicated by reference numeral 5. The drive shaft 4 is typically connected to a motor in order to effect rotation of the swash plate 2.

The pumping mechanism 5 is driven by the swash plate 2 and comprises three generally cylindrical pistons 6 that extend along respective longitudinal axes. The pistons 6 are arranged in a circular array and each guided for reciprocation in a respective cylinder 7. Reciprocation of the pistons 6 in the cylinders 7 serves to pump fluid to an outlet as will be described in more detail below.

Referring now to figures 2 to 4, the swash plate 2 is of a slightly different embodiment compared to that of figure 1 but it functions in the same manner. It has an annular cam surface 10 that is inclined such that a normal to the plane occupied by the cam surface 10 is at an oblique angle to the axis of rotation X of the shaft and plate. Each piston 6 and cylinder 7 is supported in a disc-shaped support plate 1 1. On one side of the support plate 1 1 each piston 6 extends out of its respective cylinder 7 towards the swash plate 2 and terminates in a convex head 12 for abutment with the cam surface 10 of the swash plate 2. The piston has an end 302 opposite to the end of its head 12. The end 302 is substantially planar and extends in a plane that is substantially perpendicular to the longitudinal axis of the piston 6.

On the opposite side of the support plate 1 1 to the swash plate 2 there are three cylinder blocks 13 fixed thereto, each block 13 being designed to receive a respective cylinder 7. The cylinder blocks 13 each have an internal screw thread 14 at one end for engagement with a complementary screw thread 15 defined on an external surface of the cylinder 7. The precise axial location of the cylinder 7 relative to the cylinder block 13 and the support plate 1 1 is thus adjustable by means of the connection of the screw thread connection.

A piston (compression) spring 16, supported over each cylinder, acts between the underside of the piston head 12 and the support plate 1 1 and serves to bias the respective piston 6 towards the swash plate 2. As the drive shaft 4 rotates the swash plate 2 is rotated such that a given point on the cam surface 10 moves not only such that it describes a circular locus but also in a direction parallel to the axis of rotation, thereby imparting axial forces to the pistons 6 against the biasing force of the piston springs 16. The magnitude of the force depends on the axial position of the point of contact between the swash plate 2 and the piston head 12. For a given piston 6 this axial position and thus the force applied by the swash plate varies continuously as the plate rotates. The pistons 6 thus reciprocate within the cylinders 7, the swash plate 2 moving each piston 6 in turn from an extended position to which it is urged by the piston spring 16 out of the cylinder 7 and a retracted position in which it is forced into the cylinder 7 by the swash plate 2. This movement, which can be discerned by comparing the views of figures 3 and 4, provides the pumping action for the fluid as is described in more detail below. The support plate 1 1 defines an inlet port 20 at its outer periphery through which fluid enters the pumping mechanism 5. The fluid is transported along a substantially radial passage 21 towards the centre of the support plate where it communicates with three branch passages 22, one for each piston and cylinder assembly. In an alternative embodiment (not shown) a separate inlet port and fluid passage is provided for each piston and cylinder assembly.

The pistons 6, cylinders 7 and cylinder blocks 13 are shown in more detail in figures 5 and 6 in which the piston springs 16 have been removed for clarity. Each cylinder 7 comprises an outer body 23 that is penetrated along its length by a central bore 24. Fluid enters the bore through radial inlet ports 25 disposed towards a first end of the cylinder 7 and exits the bore 24 through radial exhaust ports 26 defined towards a second end of the cylinder. The inlet ports 25 are in fluid communication with the respective branch passage 22 of the support plate 1 1 and the exhaust ports 26 are in fluid communication with an outlet port 27 defined in the cylinder block 13. An outlet check valve 28 is disposed in the bore 24 at an axial location between the inlet and exhaust ports 25, 26 and serves to control the egression of fluid to the outlet port 27.

At a first end, the cylinder bore 24 is open and receives the piston 6. The internal diameter of the bore 24 of this end portion is very slightly greater than the outer diameter of the piston 6 and has two annular grooves 29 for sealing rings (not shown) that seal against the piston 6 in use. Further along the bore 24, at the inlet ports 25, the diameter increases to an inlet cavity 30 at the end of which is a narrow annular projection 31 of reduced diameter.

The second end the cylinder bore 24 is closed by means of a cylindrical retaining plug 32 that is secured in the cylinder by means of a screw thread connection 33. The end of the retaining plug 32 that faces the piston 6 has a blind bore 34 for receipt of a compression spring 35 that forms part of the outlet check valve 28 and the opposite end has a socket 36 for receipt of a suitable tool by which the plug 32 may be screwed or unscrewed into or out of engagement with the cylinder block 13. When the retaining plug 32 is fully inserted (as shown in the figures) it is wholly received within the bore 24 and leaves a small axial clearance. At this location the wall of the cylinder 7 is penetrated by a pair of opposed apertures 37 designed for receipt of an adjusting bar (not shown) for rotating the cylinder 7 relative to the cylinder block 13 thereby adjusting their relative axial positions.

Each cylinder 7 is sealed to the support plate 1 1 by a ring seal (not shown) disposed in an annular groove 40 and to the respective cylinder block 13 at two axially spaced locations in each of which a sealing ring (not shown) is received in corresponding first and second annular recess 41 , 42 in the block 13. In the axial space between the seal in groove 40 and sealing ring in the first annular recess 41 there is a radial clearance between the external surface of the cylinder 7 and the internal surface of the cylinder block 13, the clearance defining an annular inlet passage 43 that affords fluid communication between the branch passage 22 of the support plate 1 1 and the radially directed inlet ports 25 in the cylinder 7. Similarly there is a corresponding annular outlet passage 44 defined in the axial space between the sealing rings in the first and second recesses 41 , 42 that provides fluid communication between the radial exhaust ports 26 and the outlet port 27 of the cylinder block 13.

The outlet check valve 28 is disposed between inlet cavity 30 and the retaining plug 32 and comprises a cylindrical valve seat member 45 that is disposed in outer body 23 of the cylinder 7 and defines a central aperture 46 that is selectively blocked and unblocked by a spherical valve member 47 in the manner of a check valve. The valve member 47 is biased towards the closed position (in which the central aperture 46 in the valve seat member 45 is blocked) by the compression spring 35 that acts between a bottom of the blind bore 34 and a spring seat 48 that bears against the valve member 47. When the fluid in the inlet cavity 30 is at a sufficient pressure the valve member 47 is forced clear of the valve seat 45 against the action of the compression spring 35 such that the fluid can egress through the central aperture 46 to the exhaust ports 26. An axial clearance between the valve seat 45 and the retaining plug 32 accommodates the movement of the valve member 47 between the open and closed positions and allows fluid to flow to the exhaust ports 26.

Immediately adjacent to the annular projection 31 , on the valve side, is a dynamic inlet seal 50 for sealing against the external surface of the piston 6. The inlet seal 50 is fixed relative to the cylinder 7. As the piston 6 reciprocates in the bore 24, the piston 6 moves relative to the inlet seal 50. The inlet seal 50 is substantially annular piston ring and is mounted in a substantially annular groove 305 in the outer body 23 of the cylinder 7. The inlet seal 50 extends substantially around the piston 6 and forms a sealing engagement with the piston 6 when the piston 6 is received within the seal 50.

A substantially cylindrical chamber 301 is defined between the inlet seal 50 and the valve 28. The chamber 301 is defined by a radially inner surface of the outer body 23 of the cylinder 7.

A spacer insert 51 of hollow cylindrical form is provided in the chamber 301 , between the inlet seal 50 and the valve seat 45, the volume defined by the hollow affording fluid communication between the inlet cavity 30 and the check valve 28. In an alternative embodiment (not shown) the spacer insert may be replaced by an integral part of the outer body 23 of the cylinder.

As will now be described, in operation as the swash plate 2 rotates fluid is drawn in through the radial inlet ports 25 of each cylinder 7 in sequence and then pumped out through the radial exhaust ports 26 and the outlet port 27 in the respective cylinder block 13.

Specifically, as the piston 6 moves axially outwardly of the cylinder 7 (the direction in which the piston head 12 moves away from the cylinder 7) suction is generated in the chamber 301 , due to the vacuum created by the removal of the piston 6 from the chamber 301 . The vacuum is created throughout the chamber 301.

In this regard, while the end 302 of the piston 6 is disposed axially inwardly (the inward direction being opposite to the outward direction) of the inlet seal 50, the inlet seal 50 forms a sealing engagement with the piston 6. This maintains the vacuum created within the chamber 301 as the piston moves outwardly. Furthermore, due to the sealing engagement of the inlet seal 50 with the piston 6, the radial inlet ports 25 are not fluidly connected to the chamber 301 . In figure 4 the piston 6 is shown in a position in which the end 302 of the piston 6 is disposed inwardly of the inlet seal 50. As the piston 6 moves outwardly of the cylinder 7, under the influence of the piston spring 16, the suction is generated in the chamber 301 until such time as the end 302 of the piston 6 passes outwardly of the inlet seal 50, whereupon fluid is permitted to enter the chamber 301 from the radial inlet ports 25, via the cavity 30. In this respect, an annular clearance is provided between an outer surface of the piston 6 and the inner surface of the body 23 that defines the cavity 30. When the end 302 of the piston 6 is outward of the inlet seal 50, there is an axial clearance between the piston 6 and the inlet seal 50 such that the inlet seal 50 is not in sealing engagement with the piston 6 such that the radial inlet ports 25 are fluidly connected to the chamber 301 via the inlet cavity 30. Figure 6 shows the position of the piston 6 in which the end 302 of the piston 6 is just outward of the inlet seal 50.

Due to the suction created in the chamber 301 , fluid is then sucked from an appropriate source through the inlet port 20 of the support plate 1 1 , through the radial inlet ports 25, through the inlet cavity 30, through the inlet seal 50 and into the chamber 301 , i.e. into the volume defined by the spacer insert 51 .

This arrangement is advantageous in that it removes the need for an inlet valve. In this regard, the flow of fluid from the radial inlet ports 25 is only permitted when the end 302 of the piston 6 is outward of the inlet seal 50. Accordingly, this arrangement acts in the manner of an inlet valve.

In addition, due to the vacuum created in the chamber 301 , before the radial inlet ports 25 are fluidly connected to the chamber 301 , a relatively high level of suction is created in the chamber 301 which produces a rapid in flow of fluid into the chamber 301 . This prevents the flow of fluid from stalling.

When the end 302 of the piston 6 is located outwardly of the inlet seal 50, as the piston 6 moves outwardly, suction is not be generated in the chamber 301 by the movement of the piston 6. As the swash plate 2 continues to rotate, the piston 6 is subsequently forced back, inwardly, into the cylinder 7. When the end 302 of the piston 6 is disposed outwardly of the inlet seal 50, the movement of the piston 6 inwardly does not act to force the fluid in the chamber 301 out through the check valve 28. In this regard, due to the fluid communication between the radial inlet ports 25 and the chamber 301 , fluid in the chamber 301 is forced back out through the radial inlet ports 25.

Once the end 302 of the piston 6 passes inwardly of the inlet seal 50, it acts to force the fluid in the chamber 301 through the check valve 28 to the outlet port 27. As stated above, when the piston 6 is in such a position, the inlet seal 50 forms a sealing engagement with the piston 6. Accordingly, as the piston 6 moves inwardly through the chamber 301 fluid in the chamber 301 cannot pass back from the chamber 301 to the radial inlet ports 25 and is forced through the check valve 28 to the outlet port 27. In this regard, the force exerted on the fluid in the chamber 301 pressurises the fluid and the force of the fluid on the check valve 28 causes it to open against the force of the valve spring 35, whereupon the fluid flows out through the exhaust ports 26 to the outlet port 27 in the cylinder block 13. It will be appreciated from the foregoing that the pumping action of intake and exhaust occurs only during part of the extension and retraction stroke of the piston 6. As the piston 6 extends out of the cylinder 7 (under the influence of the piston spring 16) it generates suction until such time as the end 302 of the piston 6 passes the inlet seal 50 whereupon fluid is permitted to enter the cylinder 7 (intake). In the opposite retraction stroke the piston 6 is forced into the cylinder 7 by the swash plate 2 and once the end 302 of the piston 6 passes the inlet seal 50 it applies pressurise to the fluid whereupon the check valve 28 is forced open and the fluid is discharged from the cylinder (exhaust). The effective pumping stroke (in which either intake or exhaust is effected) occurs when the end 302 of the piston 6 passes the inlet seal 50 in each direction of travel.

The provision of the screw thread engagement 14, 15 between each cylinder 6 and cylinder block 13 allows the effective pumping stroke of the pistons 6 to be adjusted. Relocating the axial position of the cylinder 7 relative to the cylinder block 13 by screwing has the effect of moving the axial position of the inlet seal 50 with respect to the piston 6, the annular inlet passage 43 providing an axial clearance that accommodates such adjustment. Although the stroke length of the piston 6 relative to the cylinder 7 remains unchanged the effective pumping (intake and exhaust) strokes are altered since they are dependent on the relative positions of the piston 6 and the inlet seal 50. A stop ring 52 is provided at the end of the axial clearance and serves to limit the extent of travel of the cylinder relative to the cylinder, by virtue of abutment of the external screw thread 15 with the stop ring 52.

In figures 2, 3 and 5, the piston 6, cylinder 7 and cylinder block 13 that are shown in section are disposed relative to each other such that minimum pump displacement occurs i.e. the effective pumping stroke is at a minimum. It can be seen that the threads 15 of the cylinder 7 are located at the right hand end of the annular inlet passage 43 and abut a stop ring 52 at the end of the cylinder block 13, the stop ring limiting the travel of the cylinder. The axial position of the cylinder 7 has been adjusted (by virtue of the screw thread connection) relative to the cylinder block 13 such that the inlet seal 50 is located at a first axial position relative to the cylinder block 13. This position of the seal 50 is distal from the end of the piston 6 so that when the piston 6 is forced into the cylinder 7 much of the piston stroke is taken up in reaching the inlet seal 50 at the expense of the exhaust part of the stroke. Similarly during extension of the piston 6 out of the cylinder 7 the suction part of the stroke is relatively short. In figures 4 and 6, it can be seen that the axial position of the cylinder 7 has been adjusted such that screw thread 15 is at the opposite end of the annular inlet passage 43 and the inlet seal 50 is at a second axial position, which is closer to the support plate 1 1 than the first axial position. In this position maximum pump displacement is permitted i.e. the effective pumping stroke is at a maximum as more of the piston stroke is used in the pumping action. It will be understood for a given speed of rotation of the swash plate 2, a greater volume of fluid is pumped when a cylinder is adjusted to the maximum pump displacement position compared to that when the cylinder is set to the minimum pump displacement position.

In the exemplary embodiment the pump may be used to pump a single fluid into three separate outlet lines or the outlet port associated with each cylinder block may be joined to a common outlet line. Alternatively the pump may be used to pump three different fluids into separate outlet lines, one fluid passing through each piston and cylinder pair. It will be appreciated that other combinations are possible and the present invention may be embodied in a pump with any practical number of piston and cylinder pairs. It is to be appreciated that whilst the embodiment described above allows the effective pumping stroke to be adjusted by moving the cylinders relative to the cylinder blocks, the same effect may be achieved by keeping the cylinders stationary relative to the cylinder blocks and adjusting the position of the pistons and the swash plate relative to the cylinders and cylinder blocks.

The adjustable nature of the pump makes it particularly suitable for accurate metering at slow flow rates such as in chemical injection applications. It removes the need for hydraulic adjustment and enables the use of a smaller drive motor. The replacement of the traditional inlet check valve with an inlet seal 51 that is inset into the cylinder outer body 23 and performs in the manner of a valve reduces the back flow losses associated with check valves at slow speed improves priming. The inlet seal may be of any design that contacts the piston during the working stroke and is not limited to an O-ring. The seal may or may not be energised by a spring.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims.

For example, in the embodiment above the position of the inlet seal 50 relative to a given location on the swash plate 2, on the rotational axis of the swash plate 2 is adjustable by virtue of the adjustable relative axial position of the cylinder and the cylinder block 13. However, it will be appreciated that, alternatively, the cylinder 7 may be fixed relative to the cylinder block 13. In this regard, the inlet seal 50 may be fixed relative to the piston 6 for a certain axial position of the piston 6. Such an arrangement would not provide a variable flow pump, but would provide the above described advantages of removing the need for an inlet valve and producing a high level of suction that prevents stall.

Furthermore, the piston(s) may be provided with an end section of reduced diameter, compared to the remainder of the piston, such that when the end section reaches the inlet seal 50, the radial inlet the ports 25 are connected to the chamber 301 by a clearance between the section of reduced diameter and the inlet seal 50. In this case, the end 302 of the piston will still be within the chamber 301. Accordingly, it is envisaged that, in alternative arrangements, it does not have to be the case that the radial inlet ports 25 are selectively fluidly connected and disconnected to the radial inlet ports 25 when the end 302 of the piston 6 passes the inlet seal 50 and that this fluid connection and disconnection may occur at different positions of the piston 6 relative to the inlet seal 50.

Furthermore, although in the embodiment above each of the cylinders is adjustable separately, an alternative structure is envisaged in which two or more cylinders are adjusted together by means of a suitable actuator. The described and illustrated embodiments are to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the scope of the inventions as defined in the claims are desired to be protected. It should be understood that while the use of words such as "preferable", "preferably", "preferred" or "more preferred" in the description suggest that a feature so described may be desirable, it may nevertheless not be necessary and embodiments lacking such a feature may be contemplated as within the scope of the invention as defined in the appended claims. In relation to the claims, it is intended that when words such as "a," "an," "at least one," or "at least one portion" are used to preface a feature there is no intention to limit the claim to only one such feature unless specifically stated to the contrary in the claim. When the language "at least a portion" and/or "a portion" is used the item can include a portion and/or the entire item unless specifically stated to the contrary.

Claims

An axial piston pump comprising a swash plate arranged for rotation about a rotational axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising at least one piston that is reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing, the piston housing having a fluid inlet and a fluid outlet in communication with the bore, a valve disposed in the bore between the fluid inlet and the fluid outlet, an inlet seal disposed in the bore of the piston housing between the fluid inlet and the fluid outlet, the inlet seal configured for sealing engagement with the piston, a chamber being defined between the inlet seal and the valve, wherein as the at least one piston moves outwardly it generates suction in the chamber until such time as it passes a certain position relative to the inlet seal whereupon fluid is permitted to enter the chamber from the fluid inlet and, as the at least one piston moves inwardly, once it passes inwardly of said certain position relative to the inlet seal it forces the fluid in the chamber through the valve to the fluid outlet.

An axial piston pump according to claim 1 wherein when the at least one piston is inward of said certain position relative to the inlet seal, the fluid inlet is not fluidly connected to the chamber and when the at least one piston is outward of said certain position relative to the inlet seal, the fluid inlet is fluidly connected to the chamber.

An axial piston pump according to claim 2 wherein when the at least one piston is inward of said certain position relative to the inlet seal, the fluid inlet is not fluidly connected to the chamber due to the sealing engagement of the inlet seal with the at least one piston and when the at least one piston is outward of said certain position relative to the inlet seal, the inlet seal is not in sealing engagement with the at least one piston such that the fluid inlet is fluidly connected to the chamber.

An axial piston pump according to either of claims 2 or 3 wherein when the at least one piston is outward of said certain position relative to the inlet seal, the fluid inlet is fluidly connected to the chamber by a clearance between the piston and a surface of the respective piston housing that defines the bore.

An axial piston pump according to any preceding claim wherein said certain position of the at least one piston relative to the inlet seal is a position in which an end of the piston passes the inlet seal.

An axial piston pump according to any preceding claim wherein as the at least one piston moves inwardly but is located outwardly of said certain position relative to the inlet seal, fluid is not forced from the chamber through the valve to the fluid outlet by the movement of the at least one piston.

An axial piston pump according to claim 6 wherein as the at least one piston moves inwardly but is located outwardly of said certain position relative to the inlet seal, the movement of the at least one piston forces fluid in the chamber back to the fluid inlet.

An axial piston pump according to any preceding claim wherein as the at least one piston moves outwardly but is located outwardly of said certain position relative to the inlet seal, suction is not generated in the chamber by the movement of the at least one piston.

An axial piston pump according to any preceding claim wherein the relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate is selectively adjustable.

An axial piston pump according to claim 9, wherein the relative axial distance is adjusted by axial movement of the piston housing, by axial movement of the swash plate or a combination of the two.

An axial piston pump according to either of claims 9 or 10 wherein the relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate is selectively adjustable such that the axial position of the piston housing relative to the at least one piston, for a certain axial position of the at least one piston, is selectively adjustable.

An axial piston pump according to any of claims 9 to 11 wherein the relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate is selectively adjustable such that the relative axial distance between the inlet seal and a given axial location on the swash plate on the rotational axis of the swash plate is selectively adjustable.

An axial piston pump according to any of claims 9 to 12 wherein the relative axial distance between the piston housing and a given axial location on the rotational axis of the swash plate is selectively adjustable such that the axial position of the inlet seal relative to the at least one piston, for a certain axial position of the at least one piston, is selectively adjustable.

An axial piston pump according to any preceding claim wherein the piston housing is received in a respective housing support.

An axial piston pump according to claim 14, wherein the housing support and the piston housing are interconnected by a coupling by which the relative axial distance may be adjusted.

An axial piston pump according to claim 15, wherein the coupling converts rotational movement of the piston housing into axial movement thereof.

An axial piston pump according to claim 16, wherein the coupling comprises a screw thread engagement.

An axial piston pump according to claims 16 or 17, wherein the piston housing has a tool engagement feature with which a tool may be engaged to effect rotation of the piston housing relative to the housing support.

An axial piston pump according to any one of claims 14 to 18, wherein the housing support comprises a first support member that is penetrated by a fluid supply passage that is in fluid communication with the inlet of the piston housing.

An axial piston pump according to claim 19, wherein the piston housing is supported by the first support member.

An axial piston pump according to claim 19 or 20, wherein the housing support also comprises a second support member connected to the first support member.

An axial piston pump according to claim 21 , wherein the second support member extends in a direction away from the swash plate.

An axial piston pump according to any preceding claim, wherein the pump has plurality of such pistons and piston housings.

An axial piston pump according to any one of claims 19 to 23, wherein the pump has a plurality of such pistons and piston housings and the first support member has a plurality of supply passages, one for each piston housing.

An axial piston pump according to any one of claims 19 to 24, wherein the pump has a plurality of such pistons and piston housings and the supply passage has a plurality of branch passages for supplying fluid to the inlet of each piston housing.

26. An axial piston pump according to any preceding claim, wherein the swash plate and therefore the piston are translated in the axial direction towards the piston housing so as to adjust the relative axial distance.

27. An axial piston pump according to any preceding claim, wherein the pumping mechanism further comprises a biasing member for urging the piston against the cam surface.

An axial piston pump according to claim 27, wherein the biasing member is disposed between the piston and the housing support so as to urge the piston in a direction out of the housing.

An axial piston pump according to any preceding claim, wherein there is a plurality of pistons and piston housings, a respective supply passage for delivering fluid to each fluid inlet and a respective exhaust passage for exhausting fluid from the fluid outlet, wherein the supply passages are independent of one another and the exhaust passages are so as to prevent mixing of fluids that pass through each piston housing bore.

An axial piston pump comprising a rotary swash plate arranged for rotation about an axis and having a cam surface, and a pumping mechanism driven by rotation of the cam surface, the pumping mechanism comprising a plurality of pistons that are each reciprocally disposed in a bore of a respective piston housing and biased outwardly of the piston housing by a biasing member, each piston housing having a fluid inlet and a fluid outlet, a valve disposed between the fluid inlet and the fluid outlet of each piston housing, wherein fluid is deliverable to each fluid inlet via a respective supply passage and fluid is discharged from each outlet via a respective exhaust passage, the supply passages being independent of one another and the exhaust passages being independent of one another so as to prevent mixing of fluids that pass through each piston housing bore.

An axial piston pump according to claim 30, wherein there is a common support member for supporting the plurality of piston housings, the common support member being penetrated by the separate supply passages.

An axial piston pump according to claim 31 , further comprising separate housing supports, each of which is associated with the piston housing.