IMPROVEMENTS RELATING TO VARIABLE DELIVERY PUMPS

Abstract: A variable delivery pump comprises an outer casing 25, a cam ring 24 mounted in the casing for movement about a pivot 27 to vary the throw of the cam ring relative to the rotor 15 and vary the pump delivery. Two roller abutments 28 in recesses in the casing co-operative with arcuate surfaces 29 on the cam ring and pivot 27 to guide and locate the cam ring in its pivotal movement and to define chambers 34, 35, 36 betweem the cam ring and the casing. A spring 32 urges the cam ring into its maximum delivery position, and differential pressures obtained e.g. from the upstream and downstream sides of a control orifice in the pump outlet passage are app­ lied in chamber 34, 35 respectively to produce a force acting in opposition to spring 32. These pressures in chambers 34, 35 act in concert to produce a resultant force acting to load the cam ring into tighter sealing engagement with the roller abut­ ments 28 in opposition to the force exerted on the ring inter­ nally by the pump delivery pressure.

Description: IMPROVEMENTS RELATING TO VARIABLE DELIVERY PUMPS This invention relates to variable delivery pumps and is more particularly concerned with a variable delivery pump of the known kind comprising an outer casing, a cam ring mounted for pivotal movement within the outer casing, and a rotor mounted within the cam ring for rotation about a fixed axis and having in its periphery angularly spaced radial slots in which respective piston elements are mounted for radial movement, said piston elements sweeping the internal surface of the cam ring as the rotor rotates, thereby to pump a fluid from an inlet port to an outlet port, means for adjusting the position of the cam ring about its pivot mounting so as to vary the extent of radial movement of the piston elements and the delivery of the pump, and spring means urging the cam ring towards its position corresponding to maximum delivery. According to the invention, the cam ring is mounted in the casing by its pivot and by two sealing abutments which are disposed at the opposite side of the cam ring from the pivot and at opposite sides of the cam ring to each other, the pivot and the two abutments serving to divide the space between the cam ring and the outer casing into three chambers whereby fluid pressures can be applied in the chambers at opposite sides of the pivot for producing a central force on the cam ring In preferred arrang'einenttordi to the invention fluid pressures are applied in the two chambers at opposite sides of the pivot which produce a resultant central force in excess of the force exacted by the delivery pressure acting internally of the cam ring and urging the cam ring in the opposite direction to said resultant central force. Additionally or alternatively the fluid pressures applied in these two chambers may produce on the cam ring a central force acting in opposition to the spring means. In preferred arrangements according to the invention said abutments also serve in conjunction with co-operating surfaces in the pump to limit the range of movement of the ring in each direction about said pivot. In one such arrangement, said abutments comprise part-cylindrical abutment surfaces on the casing and complementary arcuate external surfaces centred on the pivot are provided on the ring and have each at one end a portion adapted to come into abutment with the complementary part-cylindrical abutment thereby to limit said range of movement. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is an axial section of a pump according to the invention, Figure 2 is an end view of the pump in the direction of the arrow 2 in Figure 1, Figure 3 is a sectional view on the line 3-3 of Figure 2, and Figure 4 is a view on the section line 4-4 of Figure 1. Referring first to Figures 1 and II, the pump is variable-delivery positive-displacement pump and comprises two end members 10, 11 providing respectively plain and roller bearings 12, 13 for a drive shaft 14 on to which is keyed a rotor 15 having radial slots 16 in its periphery. Rollers 17 are disposed in the slots and serve in rotation of the rotor to pump liquid from an inlet port 18 to an outlet port 19. These ports are kidney-shaped in the usual manner and are formed in the end member 10. The outlet port leads to a delivery passage 20 (see Figure 3) and a restricting orifice 21 is provided in the delivery passage the purpose of which will be explained presently. The rollers 16 engage a cam surface formed internally of a cam ring 24 which is surrounded by the fixed casing part 25 clamped between the two end members 10, 11. The cam ring has an external semi cylindrical projection 27 which rests in a complementary recess in the casing part 25 to form a 'pivot about which the ring can move, sliding relative to the adjoining faces of the two end members 10, 11. Two bearing rollers 28 forming abutments are disposed in respective recesses in the casing part at opposite sides of the cam ring to each other with respect to a first line containing the axis of pivot 27 and the general centre of the profile of the cam, and at the opposite side to the pivot 27 of a second line at right angles to the first line and containing the general centre of the profile of the cam. Arcuate surfaces centred on the pivot axis are formed on the ring to engage the rollers 28. At their ends nearer each other the arcuate surfaces have out-turned portions 30 which serve as stops to limit the pivotal movement of the cam ring 24 in each direction. In a minor modification rollers 28 are located in pockets in the outer surface of cam ring 24 and the co-operating arcuate surfaces and stops are formed on the casing part 25. A compression spring 32 disposed in a recess in the casing part acts against the ring, urging it into its position of maximum delivery by the pump, and the delivery of the pump can be controlled by means of a threaded plug 33 against which the spring is seated. The abutments 28 and the pivot 27 define three chambers 34, 35, 36 between the ring and the casing part. The chamber 36 remote from the pivot is in permanently-open communication with the inlet passage of the pump so as to be vented. The chamber 34 remote from the spring 32 is in communication with the outlet port 19 and the chamber 35 nearer the spring is in communication with the delivery passage 20 at the downstream side of the restricting orifice 21 by way of interconnecting passages 37, 38 and 39 leading to the recess for spring 32, see Figure 3. Since there is a pressure drop across the orifice 21, there is a pressure differential between the two chambers 34, 35 acting in opposition to the spring 32 and thus to reduce the throw of the ring as the delivery of the pump increases, so that the pump delivers a constant amount of liquid for a given setting of the spring force. The pressures in the chambers 34, 35 also act jointly in opposition to the force applied by the pressure of the pumped liquid on the internal surface of the cam ring 24 over the peripheral extent defined between the two rollers 17 which are instantaneously nearest to but beyond, opposite ends of the outlet port 19. Since the pressures in the two chambers 34, 35 jointly act on a larger area than that acted on by the delivery pressure internally of the ring, the resulting differential force can be, and is, arranged by appropriate selection of the positions of the two abutments 28, i.e. their distance apart, to press the ring into close engagement with the abutments 28. Also, by selection of the distances of the two abutments 28 from the line of action of the force acting internally of the ring due to the delivery pressure, compensation can be provided for the imbalance of the components of the forces acting in the two chambers acting parallel to the said line of force. A relief valve (not shown) may be provided communicating with the chamber 35 and a further restrictor (not shown) may be provided in one of the passages 37, 38, 39 between the chamber 35 and the downstream side of the restrictor orifice 21. In normal operation the pressure drop across the restrictor orifice is quite small and there is no flow into or out of the chamber 35 but if the relief valve opens then the pressure drop in the chamber 35 becomes significant and causes the cam ring 24 to move against the spring 32 into its central or minimum delivery position, and the further restricted serves to allow 'the cam ring to move rapidly to respond to the opening of the relief valve and to damp the flow of fluid which would tend to restore the cam ring to its former position, while the relief valve remains open. A device as described in our European Patent Specification No.171183 may be provided acting on the cam ring to apply to the movement of the ring a damping force which varies in dependence on the instantaneous position of the cam ring. The cam face of the cam ring is preferably shaped as described in our European Patent Specification No.171182 If desired, the restrictor orifice 21 may be omitted and the pressure differential in the chambers 34, 35 may be obtained by an aiternative means, for example from the response of some apparatus externally of the pump. The coil spring 32 may if desired by replaced by a leaf spring. It will be understood that the abutment rollers 28 may be replaced by simple abutment surfaces.

Claims: CLAIMS 1. A variable delivery pump comprising an outer casing (25), a cam ring (24) mounted for pivotal movement within the cuter casing, and a rotor (15) mounted within the cam ring for rotation about a fixed axis and having in its periphery angularly spaced radial slots in which respective piston elements (17) are mounted for radial movement, said piston elements sweeping the internal surface of the cam ring as the rotor rotates, thereby to pump a fluid from an inlet port to an outlet port, means for adjusting the position of the cam ring about its pivot mounting so as to vary the extent of radial movement of the piston elements and the delivery of the pump, and spring means (32) urging the cam ring towards its position corresponding to maximum delivery, characterised in that the cam ring is mounted in the casing by its pivot (27) and by two sealing abutments (28) which are disposed at the opposite side of the cam ring from the pivot and at opposite sides of the cam ring to each other, the pivot and the two abutments serving to divide the space between the cam ring and the outer casing into three chambers (34, 35, 36) whereby fluid pressures can be applied in the chambers (3h, 35) at opposite sides of the pivot (27) for producing a central force on the cam ring. 2. A pump as claimed in claim 1, characterised by means for applying in the two chambers (34, 35) at opposite sides of the pivot (27) fluid pressures which produce on the cam ring a resultant control force uring the abutments into tighter engagement between the cam ring and the outer casing and in excess of the force exerted by the delivery pressure acting internally of the cam ring and urging the cam ring int he opposite direction to said resultant control force. 3. A pump as claimed in claim 1, characterised by means for applying in the two chambers (342 35) at opposite sides of the pivot (27) fluid-prssures producing on the cam ring a control force acting in opposition to the spring means (32). 4. A pump as claimed in claim 3, characterised in that said means for applying fluid pressures in said two chambers comprises a restrictor (21) in the output passage of the pump, the fluid pressures upstream and downstream of the restrictor being applied in said two chambers respectively in opposition to and assisting the spring means (32). 5. A pump as claimed in claim 4, characterised in that a relief is provided for limiting the pressure applied in one of said two chambers (34, 35). 6. A pump as claimed in claims 4 and 5, characterised in that the relief valve operates on the pressure applied in the chamber communicating with the outlet passage at 14 upstream side of the restricted and in that a second restrictor is provided in a passage extending between such chamber and the upstream side of the first said restrictor. 7. A pump as claimed in any one of claims 1 to 6, characterised in that the third chamber (36) is vented. 8. A pump as claimed in any one of claims 1 to 7, characterised in that said sealing abutments comprise respective rollers (28) mounted in recesses in either the cam ring (24) or the outer casing (25) and in sliding sealing engagement casing (25) and in sliding sealing engagement with arcuate surfaces on the other of these components (24 or 25) which surfaces are centred on the axis of the pivot (27).

Inventors: BRISTOW, IAN TREVOR
Applicants: HOBOURN ENGINEERING LIMITED
Application year: 1986
Main classification: F04C15
IPC codes: F04C2, F04C14