Gerotor pumps

Abstract: A gerotor pump, Figure 1, has the usual male lobed rotor 16 with n lobes meshed in the female lobed annulus 12 with n + 1 lobes, the two being both but relatively rotatable on parallel axes 14,18. The improvement consists of making the lobes helically extending which smooths pressure peaks and reduces noise.

Description: This invention relates to gerotor devices which comprise a rotor having n lobes and which is located eccentrically and internally of a lobed annulus having a larger number of lobes (e.g. n + 1 ). These parts form a series of chambers each bounded by lines of contact between the respective parts, and of different volumes: adjacent to a position where one rotor lobe is fully meshed between two annulus lobes the chambers are minimal, and at an approximately diametric position, (according to whether the rotor has an odd or even number of lobes) the chamber is maximal. When the rotor is rotated relative to the annulus the individual chambers vary in volume. It is known from U.S. patent 4863357 to make both stator (a stationary annulus) and rotor conical, with variable pitch spiralling lobes which result in constant axial length chambers which however reduce in volume due to the reduced lobe height along the cones. This is to be used as a fluid compressor with flow essentially along the axis from the large end to the small end of the cone. Such a compressor requires a substantial axial length. It is also known to make down-hole motors, that is axial flow pumps using the tubular stator with a spirally multiple-start internal thread, meshed with a generally cylindrical rotor having a multiple-start male thread differing in start number. Again the axial length is an important factor in pump output and flow is essentially along the axis from one end to the other. Many gerotor designs are used as i.c. engine lubrication pumps in situations where compact axial dimensions are important and sometimes the inlet and outlet are to be at the same axial end. The spiralling arrangements with axial flow through the pump are then unsuitable for both reasons and the conventional arrangement for such pumps is to use a rotor and annulus which are both prismatic and moreover mount the annulus for rotation at a different speed to that of the rotor so that it is no longer a stator, which avoids the need for a wobble stick type drive. In such an arrangement there is a pulsating pressure output which can be part-smoothed by providing an outlet port extending over a substantial arcuate segment so that a series of such chambers is exposed to the outlet port. There is a necessary clearance between the parts in a radial direction for example but not exclusively as so-called dirt clearance, but this is as small as possible to avoid leakage from high pressure chambers to lower pressure chambers, and ideally every chamber is always bounded by two lines of contact between the annulus and rotor. There are two specific problems with pumps of this kind namely noise and pressure fluctuation or ripple. The former, noise. is due to clearance being taken up especially as the chambers go from the inlet side to the outlet side and vice versa, so that in practice as each rotor lobe moves to the lowest pressure position it tends to hammer on the annulus. The second problem is due to the succession of chambers moving into register with the ports and the pressure ripple is of greatest amplitude and lowest frequency with smaller values of n (and vice versa) GB 2022708 A describes apparatus primarily intended as a gas compressor or expander. Very many different possibilities are discussed in this published application and in particular providing a gerotor set with spiralling lobes, estimated from Fig. 2 of the drawings of the application to result in a helix angle of about 60 deg. that is to say the peak of one lobe at one end will be 60 deg. out of phase with the peak of the same lobe at the other end. Somewhere in excess of 200 deg. or arc of the rotor and annulus are exposed at one end by cutaways in sideplates to provide an inlet port, and a narrow exhaust port is provided at a zone asymmetrically relate to the inlet port. Flow is or may be axial. US-A 2830542 shows a fluid pump which may be a gerotor pump having both the annulus and rotor made of two axially adjacent parts which are slightly out of phase with one another in order to take up manufacturing tolerances. Only one of the two axially adjacent parts is shaft driven and the other is driven by drive transferred via the other parts. The object of the invention is to provide improvements. According to the invention, a gerotor pump comprises a lobed annulus meshed internally with a lobed rotor having fewer lobes, both of which are rotatable on parallel axes in a cylindrical cavity in a pump body. said body having fixed inlet and outlet ports provided axially of the annulus and rotor to admit fluid to and discharge fluid from the working chambers of the pump which lie between the rotor and annulus, the lobes of both rotor and annulus spiralling helically along their length, and is characterised in that the inlet and outlet ports are symmetrically disposed about a plane containing the axis of both rotor and annulus, both ports being located at one and the same axial end of the rotor and annulus, and the helix angle being such that one end of each lobe is in the range 5-15 deg. out of phase with the opposite end of each lobe The angle of inclination of the helix will depend upon other parameters including axial length of the gerotor set, and in general the helix will be such as to locate one axial end of each inter-lobe chamber no more than a small fraction of one lobe out of phase with the opposite end. Most significantly, the invention is believed to reduce noise because, instead of the full width of one lobe hammering on the other the highest pressure point will roll helically and travel axially, thus spreading the time value in like fashion. The invention is now more particularly described with reference to the accompanying drawings wherein:- Figure 1 is a sectional elevation of a typical gerotor pump; Figure 2 is a perspective view of the annulus of said pump; Figure 3 is a plan view of said annulus; Figures 4 and 5 are views similar to Figures 2 and 3 showing the rotor of said pump, and Figure 6 is a side elevation of said rotor. Turning first to Figure 1 the pump comprises a body 10 having a cylindrical cavity which journals annulus 12 for rotation about axis 14. Rotor 16 is mounted to turn on axis 18. In the illustrative example the rotor has five lobes and the annulus six. One or other of the rotor and annulus is driven by means not shown for example a shaft projecting axially, and the rotation is transferred to the other of the rotor set but at a different speed. The end wall of the body has inlet and outlet ports formed therein as shown by dotted lines. Assuming the direction of rotation to be in that of the arrow A, port 20 is the inlet and port 22 is the outlet. A series of chambers 30,32, 34, 36 and 38 is formed between the parts. The number of chambers is equal to the number of rotor lobes. As the gerotor turns, chamber 30 expands in volume as it goes through the positions of 32,34 and then decreases in volume as it goes through the positions 36 and 38. During the expansion time, fluid is induced into the pump, and during the contraction time, fluid is expressed out of the pump, through the respective ports. The outlet pressure expressed graphically against the rotation cycle will be seen to be maximal when the chamber 38 sweeps over the final portion (in the direction of rotation) of the outlet port 22, becoming minimal as the line of contact 42 between the rotor and the annulus, at the trailing end of the chamber 38, passes that point. Pressure increases again to a maximum and then falls to the minimum when the next contact line passes the point. This is the source of the ripple effect mentioned earlier herein. The pump as described so far in connection with Figure 1 is entirely conventional and typical of the prior art. Both rotor and annulus are prismatic, having identical shape and dimension for their opposite axial ends, and with points on the periphery at each end connected by straight lines lying in planes essentially containing the axis of rotation of the part. According to the invention both annulus and rotor are non-prismatic and whilst having like end faces are spiralled at one and the same helix angle for both components. Thus Figure 1 could be a cross section taken on any point along the axis 18. The selected helix angle in relation to axial length of the components is such as to provide a small fraction of the helix angle necessary if a complete phase change were required. Thus for a six lobed annulus, a 60 deg. helix turn would bring about one phase change. In the illustration about 10 deg. is employed as indicated on Figure 3. Values of this order and within the range 5-15 deg. are preferred by other angles are possible. The effect of the helicity may be considered thus: the highest pressure chamber 38 in effect extends circumferentially for (in the case of the illustrative example) 10 deg. So that during passage of this chamber through the zone before the following sealing line cuts off delivery, high pressure fluid can be delivered over a more widely distributed portion of the revolution cycle than in the prior art. This beings about the smoothing effect. The noise reduction phenomena needs a more complex explanation, but simply expressed is due to the pressure smoothing. Each pressure peak in the fluid applies an equal and opposite reaction to the rotating parts, so that if the pressure peak is distributed over 10 deg. of arc instead of near instantaneously, the minimised mechanical reaction of the pump components avoids or reduces noise generation correspondingly.

Claims: A gerotor pump comprising a lobed annulus (12) meshed internally with a lobed rotor (16) having fewer lobes, both of which are rotatable on parallel axes (14.18) in a cylindrical cavity in a pump body (10), said body having fixed inlet and outlet ports (20,22) provided axially of the annulus and rotor to admit fluid to and discharge fluid from the working chambers (30-38) of the pump which lie between the rotor and annulus, the lobes of both rotor and annulus spiralling helically along their length, characterised in that the inlet and outlet ports are symmetrically disposed about a plane containing the axis of both rotor and annulus, both ports being located at one and the same axial end of the rotor and annulus, and the helix angle being such that one end of each lobe is in the range 5-15 deg. out of phase with the opposite end of each lobe A gerotor pump as claimed in Claim 1 wherein the out of phase angle is of the order of 10 deg

Inventors: FREEMAN RICHARD ROBERT, FREEMAN, RICHARD ROBERT
Applicants: CONCENTRIC PUMPS LTD, CONCENTRIC PUMPS LIMITED
Application year: 1991
Main classification: F04C2
IPC codes: F04C14, F04C2, F04C29, F04C28