GB2166499A - Pump - Google Patents

Abstract

A pump comprises a plurality of piston-cylinder units (16, 17) which are arranged concentrically about a pump shaft (13) and actuated by a swash plate (28) rotatable relative to the units. An annular control disc (36) is arranged eccentrically of the pump shaft (13) and serves to bring inlet and outlet openings (18, 37) into compound action. The piston (17) of each unit is connected with a ball (20) of a ball joint, the ball being firmly connected with a gudgeon plate (21) which slides on the swash plate (28) and stands in constant concentric position relative to the pump shaft. The ball is received in a shoe (19) which displaceably bears against the piston (17) by way of a hydrostatic bearing, provided by pressure fluid moving between piston (17) and the rod (24) which holds the ball (20) in the shoe (19). The valve phasing may be altered by axially displacing shaft (14) whereupon groove (38) and its peg follower cause it to rotate and turn control disc (36) with it. <IMAGE>

Description

SPECIFICATION Pump The present invention relates to a pump and has particular reference to an axial piston pump with a plurality of piston-cylinder units arranged concentrically around a pump shaft and actuable by a swash plate on relative rotation of the piston-cylinder units and the swash plate.

In the case of known pumps of this kind (US 3 074 345), the support of each piston relative to the swash plate is effected by a ball joint fixedly connected to the piston, the ball of the ball joint being received in a piston shoe which in turn is supported at the swash plate. DE-AS 1 278 251 discloses a pump in which the ball joint is fixedly mounted in a gudgeon plate and engages a rod with a ball in the piston. A disadvantage of this known construction is, in particular, that the resulting forces on the piston negatively influence the service life of the pump, as the sum of the bearing reaction of the piston in the cylinder grows and at the same time the canting pressure increases. This has the consequence that relative long piston guides are necessary in order to achieve a reasonable service life.

It would thus be desirable to avoid the disadvantages of the known pump and to increase the pump service life.

According to the present invention there is provided a pump comprising a shaft carrying a swash plate, a slide plate so slidably engaging the swash plate as to undergo tilting movement about a point on the shaft axis on relative rotation of the plates, a plurality of pistons arranged concentrically around the shaft axis in respective cylinders and so coupled to the slide plate as to be reciprocatingly displaced in the cylinders during movement of the slide plate, and an annular valve element arranged eccentrically of the shaft axis and displaceable relative to the cylinders to place each cylinder alternately in communication with inlet means and outlet means of the pump during relative rotation of the plates, each piston being coupled to the slide plate by coupling means comprising a ball fixed to the slide plate and a ball cup which receives the ball and in use movably bears against the piston by way of a hydrostatic bearing.

In a preferred embodiment, the pump is an axial piston pump with a plurality of pistoncylinder units which are arranged concentrically around the pump shaft and actuated by a swash plate, wherein a relative rotation is present between the piston-cylinder units and the swash plate. An annular rotatable control disc is arranged eccentrically of the pump shaft, in a given case fixed relative to the swash plate, and brings inlet openings and outlet openings into compound action. Each piston is connected with a ball joint comprising a separate ball, and the ball on the one hand is firmly connected with a slide or gudgeon plate, which slides on the swash plate and stands in constant concentric position relative to a piston housing, and on the other hand lies in a cup displaceably bearing against the piston, a hydrostatic bearing being provided between the cup and the piston.

Preferably, the piston is constructed as a piston open at both sides and a piston rod is resiliently retained generally concentrically in its interior, for example by a spring, the piston rod being pivotably anchored by its other end in a sleeve or hollow bolt which carries the ball and is firmly connected with the slide plate.

Expediently, a hydrostatic bearing is provided between the slide plate and the swash plate and/or between the swash plate and a support surface, such as an axial support plate, opposite to the slide plate.

An embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation of a pump embodying the invention; and Figure 2 is a detail view, to an enlarged scale, of a piston and parts of a swash plate and a slide plate of the pump of Fig. 1.

Referring now to the drawings, there is shown an axial piston pump comprising a pump housing 11 which is closed by a lid 12 and in which is mounted a rotatable pump shaft 13 connected to a control shaft 14. For the purpose of adjustment, as will be later described, the shaft 14 is slidable and rotatable relative to the shaft 13.

Sealingly mounted in the housing 11 is a piston housing 15, in which are arranged a plurality of piston-cylinder units parallel to and concentrically around the shaft 13, each unit comprising a piston 1 7 slidable in a cylinder 16. Provided at the lower end of each cylinder 16 is a bore 18 acting as an inlet opening and outlet opening. Each piston 17 is open at both ends and a respective piston shoe 19 displaceably bears on the trailing end of the piston. The shoe 19 in turn receives a respective ball 20 which is mounted on a respective hollow bolt 22 anchored in a gudgeon plate 21. The anchoring of the bolt 22 is effected by the collar 23 of the bolt.

As shown in Fig. 2, the ball 20 and shoe 19--together forming a ball joint-are held together by a piston rod 24 with an enlarged end 25 pivotably suspended in the bolt 22, the rod 24 extending as far as into the interior of the associated piston 1 7 and being held therein by a spring 26 under compression.

The spring 26 bears against a centring disc 27 on the rod 24, which disc at the same time effects centring of the rod 24 in the piston 17.

The plate 21 slides on the swash plate 28 and is on the one hand held in axial direction against the swash plate by a bearing plate 29 and on the other hand centred in the swash plate by a radial bearing 30. The swash plate 28 is fixedly keyed to the pump shaft 13 or in some other way non-rotatably connected thereto and is supported in axially parallel direction by an axial support disc 31.

Present between each piston 17 and shoe 19 is a hydrostatic bearing, which is provided by pressure fluid from the space 33 between the piston 17 and the rod 24. Equally, a respective hydrostatic bearing is provided between each ball 20 and the plate 21, between the plate 21 and the swash plate 28, between the swash plate 28 and the support plate 31 and radially between the plate 21 and the swash plate 28 at the bearing 30, these hydrostatic bearings being provided by pressure fluid from channels 34 and nozzles 35.

Mounted on the control shaft 14 is an annular control disc 36 which is eccentric but nonrotatable relative to the shaft 14, the control disc 36 being radially displaceable by eccentric bearing means on the shaft 14 to effect cyclic opening and closing of the bores 18 relative to inlet and outlet openings 37 of the pump when the shaft 14 rotates. In addition, through axial displacement of the shaft 14, this shaft is rotated in relation the shaft 13 by a cam groove 38 and peg follower and the disc 36 correspondingly displaced in phase position relative to the shaft 13, whereby the bores 18 are opened and ciosed to a greater or lesser extent in each pump shaft revolution and the conveying volume of the pump thereby regulated.

The plate 21 is secured against rotation relative to the piston housing 15 by, for example, bevel gears.

On operation of the pump, initially the disc 36 is so rotated that the pressure of th pressure fluid builds up as quickly as possible in the cylinders 16. As soon as sufficient pressure is present, a small part of the pressure fluid is ducted through the spaces 33 between the pistons 17 and rods 24 to the positions of the hydrostatic bearings, wherein initially the hydrostatic bearings 32 between the pistons 17 and shoes 19 are created so that the shoes 19 can slide on and relative to the pistons 17 with the lowest possible friction. By means of these bearings 32 the pistons 17 are kept free of nearly the entire transverse forces, so that the guide paths of the pistons 17 in the cylinders 16 can be kept short in view of the very small canting pressure.At the same time, the pressure fluid flows up to the nozzles 35 and forms the hydrostatic bearings between the balls 20 and plate 21.

The further bearings are then formed in the same way, wherein the formation of each bearing with respect to size, shape of pocket, etc., can be easily calculated by the expert.

It will be appreciated that it is quite possible for one or other of the bearings to be replaced by a ball bearing, a roller bearing or a normal slide bearing, as it is only necessary to ensure that the bearing between each piston 17 and shoe 19 is a hydrostatic bearing.

Moreover, the pump can be constructed in such a way that the swash plate 28 remains stationary and the piston housing 15 together with the gudgeon plate 21 rotates about or with the pump shaft.

A pump embodying the present invention as hereinbefore described may have the advantage that a significantly longer service life is achieved and may possess a better overall efficiency than the known pumps. Moreover, a better length/diameter ratio of the piston may be possible, which can lead to a more compact pump format.

Claims (9)

1. A pump comprising a shaft carrying a swash plate, a slide plate so slidably engaging the swash plate as to undergo tilting movement about a point on the shaft axis on relative rotation of the plates, a plurality of pistons arranged concentrically around the shaft axis in respective cylinders and so coupled to the slide plate as to be reciprocatingly displaced in the cylinders during movement of the slide plate, and an annular valve element arranged eccentrically of the shaft axis and displaceable relative to the cylinders to place each cylinder alternately in communication with inlet means and outlet means of the pump during relative rotation of the plates, each piston being coupled to the slide plate by coupling means comprising a ball fixed to the slide plate and a ball cup which receives the ball and in use movably bears against the piston by way of a hydrostatic bearing.

2. A pump as claimed in claim 1, wherein each piston is open at both ends thereof and the coupling means coupling that piston to the slide plate further comprises a piston rod having a portion extending substantially axially in and resiliently connected to the piston and a portion projecting out of the piston and pivotably retained in the ball of that coupling means.

3. A pump as claimed in claim 2, wherein each of the balls is mounted on a sleeve secured to the slide plate and the projecting portion of the associated piston rod is pivotably mounted in the bore of the sleeve.

4. A pump as claimed in any one of the preceding claims, comprising means to resist separating movement of the plates in the direction of the shaft axis.

5. A pump as claimed in any one of the preceding claims, comprising means to cause the swash plate to, in use, movably bear by way of a hydrostatic bearing against at least one of the slide plate and a support surface opposite to the slide plate.

6. A pump as claimed in any one of the preceding claims, comprising means to cause the slide plate to, in use, be centred in the swash plate by way of a hydrostatic bearing.

7. A pump as claimed in any one of the preceding claims, comprising duct means to conduct pressure fluid from the cylinders to form the hydrostatic bearings.

8. A pump as claimed in any one of the preceding claims, wherein the shape and size of the region accommodating each hydrostatic bearing is predetermined in dependence on the constituent fluid of the bearing.

9. A pump substantially as hereinbefore described with reference to the accompanying drawings.