Gerotor pumps

Abstract: A method of setting up the required running tolerances in a gerotor pump having annulus and rotor meshed together and located in a body comprises pressing a cover plate (Fig.1) or equivalent as an interference fit into the body and then allowing it to relax and recover elastically to set up the required tolerances at axial ends of the gerotor set.

Description: This invention relates to gerotor pumps, which have a male lobed rotor with n teeth located in a female lobed rotor with n+1 teeth so as to provide a set of pumping chambers between the lobes. The rotors and chambers rotate and in so doing the chambers increase and decrease in size to perform induction and ejection of pumped fluid The chambers are each bounded in the circumferential direction by two lines of contact between rotor and annulus. At axial ends the chambers are bounded by the pump body, which most usually includes a cylindrical cavity housing the gerotor set, so that the base of the cavity provides one axial end wall for the chambers, and a cover plate for the body provides the opposite end wall. Pumping efficiency depends primarily upon chamber sealing, and one of the most critical areas available for control is that of axial clearance between the parts, i.e. the running tolerance, which is the axial distance between one end of the gerotor set and the chamber in which it runs The manufacturing methods used for the gerotor set frequently involve compaction and sintering of powdered metals for reasons which will be understood by the production engineer, and this is followed by machining of the axial faces of the gerotor parts in order to provide the required smoothness of finish to those end faces which are to rub on the body and cover plate. There is a dilemma for the pump manufacturer in that particularly small manufacturing tolerances are needed for the body cavity axial length and for the gerotor parts axial length in order to achieve high efficiency. This is done, in convention production engineering, by setting acceptable standards, gauging all components and rejecting those outside the preset limits. This is inevitably expensive because of the unavoidable rejection rate, and particularly so with powdered metal compacts which sometimes require to be machined more or less because of variations in the material. The object of the present invention is to enable small assembled tolerances to be achieved in a less expensive way. A method of manufacturing of a pump according to the preamble of claim 1 is known from EP-A-361716. According to the invention the pump body cavity is closed by a part which is axially fixed in position by being an interference fit with another pump component, and the axial location of said part and hence the internal running tolerance is arrived at by displacing said part relative to said other component under a predetermined load until all clearances are removed, i.e. all parts are in contact, and then removing the displacing force to allow natural elastic recovery of the parts to their unloaded length so as to establish the internal clearance or running tolerance. Various possibilities are within the scope of the invention including those of using a stationary shaft, or of using a rotatable shaft, for the gerotor set. Where there is a stationary shaft, the cover plate can be an interference fit with the shaft for the purposes of the invention, but where there is a rotatable shaft which is to transmit drive, the axial tolerance control member may be fixed relative to the pump body rather than the shaft. Four embodiments of the invention are now more particularly described with reference to the accompanying drawings wherein each of the figures is a sectional elevation of a different one of the four. Turning now first to Figure 1, the pump body 10 has a cylindrical cavity journalling the annulus 12 which is meshed with the rotor 14. The body is formed with inlet and outlet ports 16 opening to the chambers defined between the gerotor parts 12, 14. In this instance, a drive shaft 18 is employed which is journalled in a bush 20 and driven for example by means of a tang 22 on the shaft. The shaft is an interference fit with the rotor at 24. Cover plate 26 is employed, and the axial tolerance between the gerotor parts and the body and cover plate is determined by pressing the cover plate in the direction of arrow A into the body in which it is an interference fit at 28, under a predetermined load, and then allowing the cover plate to recover elastically so as to set up the internal tolerances. It will be appreciated that with this design, the gerotor set can vary in axial dimension quite substantially but a standard tolerance can be set up. In the arrangement in Figure 2 the shaft 32 is an interference fit at 34 in a cavity in the body 30. In this instance the shaft has a head 36. Bush 38 journals the rotor 40 which is meshed with the annulus 42. Rotor 40 is integral with a radial flange 44 which forms both a cover plate and a drive transmission device for example by provision of gear teeth 46 at its periphery. Again it will be appreciated that axial running clearances are provided by pressing the head 36 to drive the shaft further into the body under the predetermined load and then relaxing it. A seal member may be provided peripherally at 48 between the rotatable component 44 and the stationary body 30: no bearing is necessary at that point because of the running tolerance/clearance. Figure 3 can be considered as a version of Figure 2 in which shaft 50 is first pressed into interference fit at 52 into the body 54, and then, instead of using a head on the shaft, a separate location component 56 which is an interference fit at 58 is pressed onto the projecting end of the shaft 50 to set up the required running tolerances in the same way as previously described. In Figure 4 the annulus 60 is driven by means of gear teeth 62 provided at an axial outer end of the annulus, the axially inner end of the same being journalled in the body as before. Male rotor 66 is meshed with the annulus and runs on shaft 68 with an interposed bush 70. Shaft 68 is an interference fit at 72 in the body, and cover plate 74 is an interference fit at 76 with the same shaft 68. Again, running tolerances are set up in the same way. All of the illustrated versions use journal bushes at one point or another for rotary components, but the presence or absence of these journal bushes depends upon the nature of the materials employed and in some cases components may be journalled directly on the shaft or directly in the body as the case may be.

Claims: A method of manufacturing a pump comprising a body cavity, a gerotor set of annulus (12) and rotor (14) located in the cavity, and a part (26) closing said cavity, the base of the cavity and said part being spaced axially by a distance equal to the axial length of the gerotor set plus a required axial clearance as running tolerance, characterised in that said part (26) is axially fixed in position by being an interference fit with another pump component (10), and the axial location of said part and hence the value of the said axial clearance is arrived at by displacing said part (26) relative to said other component under a predetermined load until said part abuts the gerotor set at one axial end of the same and the gerotor set abuts the cavity base at the opposite end of the gerotor set so that no running tolerance remains, and then removing the displacing force from the displaced part to establish running tolerance. A method as claimed in Claim 1 wherein said part closing the cavity is a cover plate (26) and said other component is a pump body (10) affording said cavity, said cover plate being disposed in said cavity. A method as claimed in Claim 1 wherein the said part 26) closing the cavity is a stationary part. A method as claimed in Claim 1 wherein the said part (44) closing the cavity is axially fixed but rotatably free. A method as claimed in Claim 1 wherein the said part (44) closing the cavity is a driven member for the gerotor set. A method as claimed in Claim 1 wherein the said part (26) closing the cavity is a cover plate.

Inventors: FREEMAN RICHARD ROBERT, FREEMAN, RICHARD ROBERT
Applicants: CONCENTRIC PUMPS LTD, CONCENTRIC PUMPS LIMITED
Application year: 1991
Main classification: F04C2
IPC codes: F04C15, F04C2, F01C21