WO1995022001A2

WO1995022001A2 - Water pressure pumps and motors

Info

Publication number: WO1995022001A2
Application number: PCT/GB1995/000292
Authority: WO
Inventors: Tony Markham
Original assignee: J.H. Fenner & Co. Limited
Priority date: 1994-02-12
Filing date: 1995-02-13
Publication date: 1995-08-17
Also published as: WO1995022001A3; AU1587895A

Abstract

A water pump or motor comprises an outer casing (1, 2, 3), and a cylinder block (5) which is rotatably mounted within the outer casing (1, 2, 3). A drive/output shaft (4) is connected to the cylinder block (5), and a plurality of piston assemblies (13), each comprising a piston body and a slipper are mounted within respective piston bores in the cylinder block (5). An angled swash plate (8) is fixed relative to the rotatable cylinder block (5) against which the slippers are pre-loaded, and a port plate controls the flow of water to and from each of the piston bores. The cylinder block (5) is connected to the drive/output shaft (4) by de-coupling means (27) so as to enable the cylinder block (5) to float within the outer casing (1, 2, 3) relative to the drive/output shaft (4). A gap or pocket (53) is provided between the outer casing or the cylinder bearing (17) supporting the cylinder block (5) and the cylinder block (5), which pocket (53) is adapted to receive water under pressure. The rubbing surfaces of at least the principal relatively moving components of the pump/motor are comprised, respectively, of hard and soft materials, the soft materials being polymeric materials.

Description

DESCRIPTION

WATER PRESSURE PUMPS AND MOTORS

The present invention relates to water pressure pumps and motors of the piston type such as are generally disclosed in PCT/GB92/02160 and more especially to improvements therein.

A typical water pressure pump/motor of the piston type generally comprises an outer casing, a cylindrical cylinder block keyed to a drive/output shaft and a plurality of piston assemblies each of which is mounted within a respective bore in the cylinder block. Each piston assembly comprises a piston which is slideable axially back and forth within its bore and a slipper which is connected to the end of the piston by means of a ball joint. An angled swash plate is located opposite the slippers and against this the slippers are supported whilst being able to slide over the surface thereof. At the opposite end of the cylinder block from the swash plate there is located a port plate in which are situated two ports. Each port is continuous with a respective bore or passage in the outer casing and serves either as a water inlet or outlet port depending on whether the device is operating as a pump or a motor. One of the ports aligns with each bore in the cylinder block as it passes the point on the swash plate inclined towards the cylinder block, and the other port aligns with each bore as it passes the point on the swash plate inclined away from the cylinder block. In order to facilitate change in the operating mode of the device from pump to motor a changeover valve is usually provided within the external water circuit connecting to the device.

When in use as a pump low pressure water is supplied via the low pressure port and the cylinder block is rotatably driven within the outer casing by means of a prime mover connected to the drive shaft. As the cylinder block rotates water is drawn into each piston bore in turn as it passes the low pressure port and is expelled therefrom under pressure as each piston bore passes the high pressure port.

When in use as motor high pressure water is supplied via the high pressure port which has the effect of driving each piston in turn down its respective piston bore against the incline of the swash plate. This, in turn, causes the cylinder block, and thence the output shaft, to rotate. Low pressure water is exhausted via the low pressure port as each piston bore in turn passes by it. As a direct consequence of the low viscosity of water, water pressure pumps/motors suffer from several problems which simply do not manifest themselves in oil pressure pumps/motors. When operating as a motor, friction between the moving parts of the device, arising from the absence of a lubricant, is a major problem. This problem does not manifest itself in pumps as there will always be an external prime mover which drives the pump and which can be relied upon to overcome this friction. To a greater or lesser extent the friction effect diminishes once the motor has started as hydrodynamic films are developed between the moving parts which effectively allow them to aquaplane relative to one another. Nevertheless, in order to improve operating efficiency and ensure a long working life it is desirable to build features into the motor which compensate for this friction.

Another problem which arises whether the device is operating as a pump or as a motor is leakage between the port plate and the cylinder block. In a water pump/motor journal or bush bearings must be used to support the cylinder block and the drive shaft. Oil or grease filled bearings simply cannot be used as these are vulnerable to water ingress which would result in bearing failure. Journal bearings require a clearance to be provided to allow a cooling film of water to form with the moving part. For the drive shaft this clear¬ ance is typically of the order of 0.5 mm and for the cylinder block it is typically of the order of 2.5 mm.

It will be understood that these clearances cause eccentricities which would cause angular mis-match between the port plate and the cylinder block, and hence leakage. This problem is, of course, made worse by the low viscosity of water. Even careful and accurate machining of the port plate and the cylinder block cannot serve to overcome the problem completely.

Lastly, there is the problem of contaminates within the water entering the gaps between moving components and causing wear and even jamming of the pump/motor mechanism. In this regard, it must be born in mind that whilst water can be filtered before use, calcium deposits and corrosion particles in particular soon build up within the pump/motor.

It is an object of the present invention to provide a water pressure pump/motor of the piston type in which the problems associated with conventional water pressure pumps/motors and referred to hereinabove are obviated or at least substantially mitigated. According to a first aspect of the present invention there is provided a water pump or motor of the above kind characterised in that the cylinder block is connected to the drive/output shaft by de-coupling means so as to enable the cylinder block to float within the outer casing relative to the drive/output shaft. To this end the drive/output shaft is connected to the cylinder block via a dummy shaft, which dummy shaft is loosely connected to both the end of the drive/output shaft and the cylinder block.

It is envisaged that various mechanical systems may be used to de-couple the cylinder block from the drive shaft. The essential design criteria of any such system is that it should allow a degree of freedom of movement of the cylinder block within the casing relative to the inner end of the drive shaft in any direction, whilst still ensuring efficient torque transmission.

In a preferred embodiment of the invention the drive/output shaft is connected to the cylinder block via a dummy shaft, which dummy shaft is connected to the drive shaft and to the cylinder block by loosely interconnecting splines. The splines are machined so as to allow longitudinal movement and angular movement of the dummy shaft to the drive/output shaft and of the cylinder block to the dummy shaft. In an alternative embodiment of the present invention the drive/output shaft is connected to the cylinder block by means of a resiliently deformable intermediate member. This too will allow the cylinder block to move freely within the outer casing whilst providing an efficient mechanism for the transmission of torque.

By allowing the cylinder block to float relative to the drive/output shaft the cylinder block is able to adjust its position relative to the port plate and accommodate any misalignment between these two components which might give rise to water leakage.

According to a second aspect of the present invention there is provided a water motor of the above kind characterised in that a gap or pocket is provided between the outer casing or the cylinder bearing supporting the cylinder block and the cylinder block, which pocket is adapted to receive water under pressure. Preferably the water under pressure is tapped from the high pressure side of the motor hydraulic circuit.

Not only does the gap provide a flow of cooling water and reduce the adhesive forces inherent between the outer casing/cylinder bearing and the cylinder block present upon start up of the motor, but being at the same pressure as the water in the high pressure side of the motor, this water pressure thus equals and opposes the cylinder block load generated by the piston reaction on the swash plate. This has the effect of reducing the bearing load at start up prior to the surfaces going hydro-dynamic or partially hydro- dynamic. The water under pressure also has the effect of creating a hydrostatic pocket between the outer casing/cylinder bearing and the cylinder block which facilitates rotation of the cylinder block within the housing.

On start up static friction is approximately one hundred times that of dynamic friction, thereby greatly reducing starting capability of the motor. The force from the pocket is therefore only required initially.

The cylinder block pressed against the cylinder bearing area seals the pocket on start due to the radial load generated from the slipper/swash reaction. The balance force increases by function of system pressure until the cylinder block rotates. The optimum size of the pocket is calculated by area required to oppose the radial load applied by the swash to slipper interface. Preferably a throttle is supplied in the supply line of high pressure water to the pocket. The throttle ensures that the flow of water to the pocket is minimal once the motor has started. Therefore, it does not greatly affect volumetric efficiency. The throttle may be passive or it may be self regulating. Where the motor is of the type which is capable of operating in both directions (clockwise and anti¬ clockwise) and has two high pressure ports, one for each direction, it is convenient to provide a switching valve which enables switching between the two high pressure ports to tap the water under pressure therein.

According to a third aspect of the present invention there is provided a water pressure/pump motor in which the rubbing surfaces of relatively moving components are comprised respectively of hard and soft materials, the soft materials comprising polymeric materials.

This has the advantage of allowing the rubbing surfaces to move relative to each other without high friction whilst being partially lubricated and cooled by water in the pump/motor. The soft material also allows the pump/motor to withstand ingress of contaminates. In this regard contaminates are allowed to bed into the soft material thereby preventing them from causing wear and possible failure of the pump/ motor. Lastly, the soft/hard interfaces allow adjustment of the moving parts to compensate for any inaccuracies of machining and misalignment which exist therebetween. Examples of the soft/hard interfaces in a pump/ motor are the cylinder block (metal/ceramic) and the cylinder block bush (polymer) ; the cylinder block (metal/ceramic) and the port plate (polymer); each of the pistons (metal/ceramic) and the piston sleeves (polymer); the swash plate (metal/ceramic) and the slippers (polymer); and the shaft (metal/ceramic) and the shaft bush polymer.

The soft interface will generally comprise a high performance, high temperature, semi-crystalline, engineering thermoplastic which may be fibre rein¬ forced. The hard interface will generally comprise corrosion resistant materials suitable for use in water, notably stainless steel, modified stainless steel, corrosion resistant alloys, ceramics and ceramic composites.

In a preferred embodiment of the present invention the port plate is comprised of metal reinforced polymer. This allows a soft/hard interface to be provided between the port plate and the cylinder block, whilst ensuring that the port plate has sufficient rigidity to resist distorting under the pressure imbalance which exists across it in use. An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 shows a section through a water pressure motor embodying all three aspects of the present invention; and.

Fig. 2 shows an exploded view of the shaft, dummy shaft and cylinder block employed in the water pressure motor of Fig. 1; Referring to Fig. 1 of the drawings there is shown a water pressure motor of the piston type which is shown having a cylindrical outer casing 1 which is closed at one end by a port casting 2 and at the outer end by a snout end casting 3. A shaft 4 extends into the outer casing through an aperture in the snout end casting 3 and is maintained in place by means of a thrust bush 30 which is held in place by bolts 31. The shaft 4 is supported within the aperture in the snout end casting 3 by a bush 16. A rotary shaft seal 33 is provided between the shaft 4 and the outer end of the thrust bush 30 to prevent any fluid leakage therebetwee .

A cylinder block 5 is connected to the inner end of the shaft 4 via dummy shaft 27 and as with aconventional water pressure motor carries a plurality of piston assemblies, generally designated 13, in axial¬ ly extending bores machined therein. A port plate 6 is situated between the port casting 2 and the adjacent end of the cylinder block 5 and an angled swash plate 8 is provided between the snout end casting 3 and the other end of the cylinder block 5.

As so far described the water pressure motor is of conventional design and operation and therefore it is not proposed to give a detailed description herein. For this the reader may refer to PCT/GB92/02160.

In conventional water pressure pumps/motors the shaft is keyed to the dummy shaft which in turn is keyed to the cylinder block, thereby providing a rigid connection between all three components.

In the water pressure pump/motor of the present invention a similar arrangement to that of a conventional water pump/motor is adopted except that the connection between the dummy shaft 27 and the cylinder block 5 allows the cylinder block 5 to float within the space defined between the outer casing 1, the port plate 6 and the swash plate 8. This arrange¬ ment is shown in detail in Fig. 2 of the accompanying drawings. As can be seen the shaft 4 extends through a central bore 40 in the cylinder block 5 and the inner end thereof is received in a closed bore in the inner end of the dummy shaft 27. Both the inner end of the shaft 4 and the inside of the closed bore in the dummy shaft 27 comprise a plurality of axially extending splines 41 and 42, respectively. These splines 41 and

42 loosely intermesh with each other to provide effective torque transmission, whilst still allowing the dummy shaft 27 a degree of freedom of movement relative to the inner end of the drive/output shaft 4.

A radially outwardly extending flange 43 is carried towards the outer end of the dummy shaft 27 and the perimeter of this flange 43 is machined to provide a plurality of semi-circular teeth or splines 44. It should be noted that these teeth are of relatively short axial length. A recess or counterbore is machined in the end face of the cylinder block 5 around the central bore 40 therein, which counterbore is so shaped as to loosely receive the flange 43 when the dummy shaft 27 is inserted in the end of the central bore 40. The profile of the perimeter of the recess matches that of the flange 43 in that it comprises a plurality of teeth or splines 45 which are adapted to mate with and intermesh with the splines 44. The splines 44 and 45 act in the same way as splines 41 and 42 in that they provide effective torque transmission whilst allowing the cylinder block 5 a degree of freedom of movement within the casing. The freedom of movement afforded to the cylinder block 5 allows it to adjust its position relative to the port plate 6 and accommodate for any misalignment therewith which might give rise to water leakage. In this regard it is able to move longitudinally and angularly relative to the end of the drive/output shaft 4.

As shown in Fig 1 a narrow bore 50 in the port casing 2 and in the outer casing 1 connects the high pressure port 51 in the port casing 2 to a gap or pocket 53 formed in the inner surface of the cylinder block bearing 17 to interface with the cylinder block 5 itself. (The other port 52 visible in Fig. 1 is the low pressure return port) . The motor shown in Fig. 1 is a uni-directional one, that is to say it rotates in only one direction, and therefore it has only one high pressure port 51. However, in a bi-directional motor, that is to say one which can rotate clockwise and anti¬ clockwise two high pressure ports 51 are provided each of which is connected to the narrow bore 50 via a poppet or switching valve which allows switching between the two high pressure ports 51 depending on the direction in which the motor is to be run.

The high pressure water in the gap 53 applies a force to the cylinder block 5 which is equal and opposite to the cylinder block load generated by the piston reaction on the swash plate 8. This develops a hydrostatic film between the cylinder block 5 and the cylinder block bush 17 and facilitates starting of the motor. In other words the cylinder block 5 aquaplanes as it rotates within the bush 17.

The water which is fed to the gap 53 is controlled by a small restrictor 54 in the narrow bore

50. This restrictor 54 is visible in the enlarged detail of Fig. 1. The restrictor 54 ensures that when the cylinder block 5 is in contact with the bush 17 pressure will build up and move/break the interface contact. However, once the motor is up and running it is no longer necessary to supply water under pressure to the pocket. With the motor running, friction is no longer a problem and water leakage from the piston assemblies 13 and port plate 6 provide adequate cooling. Flow to the gap 53 is, therefore, throttled by the restrictor so that volumetric efficiency of the motor is maintained as high as possible. If the forces on the cylinder block cause it to move against the bush 17 the gap 53 will repressurise due to leakage of water from it being restricted i.e. it self compensates.

In the water pressure motor shown in Fig. 1 each of the interfaces between relatively moving components are defined by hard and soft materials, the soft materials comprising polymeric materials. This has the advantage of allowing the rubbing surfaces to move relative to each other without high friction whilst being partially lubricated and cooled by water in the pump/motor. The soft material also allows the pump/motor to withstand ingress of contaminates. In this regard contaminates are allowed to bed into the soft material thereby preventing them from causing wear and possible failure of the pump/ motor. Lastly, the soft/hard interfaces allow adjustment of the moving parts to compensate for any inaccuracies of machining and misalignment which exist therebetween.

The soft/hard interfaces in the pump/motor exist between the cylinder block 5 (metal/ceramic) and the cylinder block bush 17 (polymer); the cylinder block 5 (metal/ceramic) and the port plate 6 (polymer); each of the pistons 11 (metal/ceramic) and the piston sleeves 13 (polymer); the swash plate 8 (metal/ceramic) and the slippers 12 (polymer); and the shaft 4 (metal/ceramic) and the shaft bush 60 (polymer) .

As shown the port plate 6 is comprised of metal reinforced polymer. This allows a soft/hard interface to be provided between the port plate 6 and the cylinder block 5, whilst ensuring that the port plate 6 has sufficient rigidity to resist distorting under the pressure imbalance which exists across it in use. Selecting which surface is hard and which is soft is firstly a function of the ease of manufacture. Secondly, different surface conditions and wear regimes are present on each of the moving interfaces within the pump/motor, this second condition can also effect which way round the combination is selected. Thirdly, the loading profile of the hard to soft interface is important. If the soft interface is highly loaded, it must be contained within a member which can stop it extruding from the interface, i.e. a metal ring or housing.

The polymer surface distributes the load over a larger area by deforming and reducing point contact and surface stresses and this can also contribute to hydrodynamic support being generated due to the formation of flatter surfaces.

Hard to hard surfaces which are unable to deform do not form optimum surface interfaces and point contact can remain.

The interfaces may be comprised of PEEK against stainless steel, or PEEK against a ceramic. Other polymers may be used instead of PEEK, such as PES.

The three aspects of the present invention referred to hereinabove all find application in water pressure motors. However, it will be understood that in water pressure pumps only the first and third aspects are applicable - since start up friction is not a problem in water pressure pumps it is not necessary to provide a hydrostatic pocket in accordance with the second aspect of the present invention.

Claims

1. A water pump or motor comprising an outer casing, a cylinder block which is rotatably mounted within the outer casing, a drive/output shaft connected to the cylinder block, a plurality of piston assemblies each of which is mounted within a respective piston bore in the cylinder block and each of which comprises a piston body and a slipper, an angled swash plate which is fixed relative to the rotatable cylinder block and against which the slippers are pre-loaded, and a port plate for controlling the flow of water to and from each of the piston bores in turn, characterised in that the cylinder block is connected to the drive/output shaft by de-coupling means so as to enable the cylinder block to float within the outer casing relative to the drive/output shaft.

2. A water pump or motor according to claim 1, characterised in that the drive/output shaft is connected to the cylinder block via a dummy shaft, which dummy shaft is connected to the drive/output shaft and to the cylinder block by intermeshing but loosely fitting teeth or splines.

3. A water pump or motor according to claim 2, characterised in that the cylinder block has a long¬ itudinally extending central through bore which is adapted to receive the drive/output shaft in one end thereof and the dummy shaft in the other end thereof in engagement with the inner end of the drive/output shaft, wherein the said other end of the through bore has an enlarged counterbore formed therein, which counterbore has a plurality of teeth or splines around the perimeter thereof and the dummy shaft comprises a radially outwardly extending flange towards the outer end thereof, which flange has a plurality of teeth or splines around the perimeter thereof and is adapted to be received in the said counterbore.

4. A water pump or motor according to claim 3, characterised in that the radially outwardly extending flange is chamfered on its inwardly facing surface to facilitate angular movement of the cylinder block relative thereto.

5. A water pump or motor according to claim 1, characterised in that the drive/output shaft is connected to the cylinder block via a resiliently deformable coupling member which allows the cylinder block a degree of freedom relative to the inner end of the drive/output shaft, whilst providing efficient torque transmission.

6. A water motor comprising an outer casing, a cylinder block which is rotatably mounted within the outer casing, a drive/output shaft connected to the cylinder block and each of which comprises a piston body and a slipper, an angled swash plate which is fixed relative to the rotatable cylinder block and against which the slippers are pre-loaded, and a port plate for controlling the flow of water to and from each of the piston bores in turn, characterised in that a gap or pocket is provided between the outer casing or the cylinder bearing supporting the cylinder block and the cylinder block, which pocket is adapted to receive water under pressure.

7. A water motor according to claim 6 character¬ ised in that water under pressure is tapped from the high pressure side of the motor hydraulic circuit.

8. A water motor according to claim 6 or 7, characterised in that a throttle is provided within the supply line of high pressure water to the pocket.

9. A water motor according to claim 8, character¬ ised in that the throttle is a passive restrictor.

10. A water motor according to claim 8, character- ised in that the throttle is a self regulating valve.

11. A water motor according to any one of claims 6 to 10, characterised in that the pocket is formed in the inner surface of the cylinder bearing to interface with the cylinder block.

12. A water motor according to any one of claims 6 to 11, characterised in that the motor has two high pressure ports for bi-directional operation and switching means for switching between the two ports to tap water under pressure therefrom for the said pocket.

13. A water pump or motor comprising an outer casing, a cylinder block which is rotatably mounted within the outer casing, a drive/output shaft connected to the cylinder block, a plurality of piston assemblies each of which is mounted within a respective piston bore in the cylinder block and each of which comprises a piston body and a slipper, an angled swash plate which is fixed relative to the rotatable cylinder block and against which the slippers are pre-loaded, and a port plate for controlling the flow of water to and from each of the piston bores in turn, characterised in that the rubbing surfaces of at least the principal relatively moving components of the pump/motor are comprised, respectively, of hard and soft materials, the soft materials being polymeric materials.