This invention relates to a reversible unidirectional flow gerotor pump _* Such pumps are used in apparatus where unidirectional pump output is required even if the direction of rotation of the pump is reversed. One example of such a pump is disclosed in our British prior patent No. 2,029,905.

Such pumps generally employ a rotatable reversing ring which, in response to reversal of the direction of rotation of the pump, may automatically allow the rotational axis of the pump annulus to orbit through an angle of 180 ^* about the axis of the inner rotor so as to reposition the annulus and thereby maintain unidirectional flow.

One drawback with such pumps results from the need to positively couple the annulus and reversing ring together during such reversals so that the annulus can rotate the reversing ring between diametrically opposite stop positions. Friction alone has not proved entirely satisfactory in practice and this has led to the use of spring loaded couplings between the annulus and the reversing ring as in Patent No. 2,029,905 for instance. Experience shows however that such couplings are not wholly satisfactory because they can give rise to problems with wear and they are, in any event, more cumbersome to manufacture and assemble.

According to the present invention we provide a reversible unidirectional flow gerotor pump comprising an inner toothed rotor and a toothed annulus which meshes with the inner rotor and rotates about an axis which is eccentrically related to the rotor axis, the axis of the annulus being movable between a pair of operative positions

in one of which liquid is pumped in a predetermined direction during rotation of the rotor and annulus in one direction and in the second of which liquid is pumped in the same direction during rotation of the rotor and annulus in the opposite direction, characterised in that the annulus is supported externally by carrier means providing two seating positions for the annulus corresponding respectively to said pair of opposite positions and in that the annulus- and/or the carrier means has sufficient radial freedom relative to the rotor axis to enable the temporary pressure fluctuation which occurs in response to rotation reversal to initiate transfer of the annulus between said seating positions.

In some embodiments of the invention, the carrier means is movable either translationally or about a pivot so as to afford one degree of radial freedom relative to the rotor" axis. The annulus may be free for radial movement relative to the carrier in a second direction or alternatively the annulus may be constrained by the carrier to rotate with only normal running clearance and the carrier itself may be movable in a second direction to afford a further degree of radial freedom relative to the rotor axis.

In a modification, the carrier may be fixed against movement and the annulus may be permitted radial freedom into mutually orthogonal directions _*

Examples of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a sectional view of a gerotor pump in accordance with the invention, the pump gυ__-_ 7 Q PΓ _ WHO

being shown in its normal operative condition with the rotor and annulus rotating clockwise;

Figure 2 is a view similar to Figure 1 but showing the initial stages of transfer of the annulus from one position to another as a result of rotation reversal;

Figure 3 is a schematic view illustrating a modified form of carrier to that shown in Figures 1 and 2; and Figure 4 is a view of a further form of carrier.

Referring first to Figures 1 and 2, the pump comprises, an inner toothed rotor 10, an annulus 12. having one extra tooth, a carrier 14 which supports- the annulus 12 and an outer housing 15. The axis 16 of the rotor is fixed and is substantially co-axial with an input drive shaft (not shown) coupled to the inner rotor. The axis 18 of the annulus is eccentrically related to- the- axis 16- and in the condition shown in Figure 1 the axis _. 18 is effectively fixed as long as the rotor and annulus rotate clockwise.

The inner periphery of the carrier 20 comprises a lower arc 22 centred on a centre of curvature which substantially coincides with the- axis 18 when the annulus is seated within the lower half. The upper part of the carrier is centred on a centre of curvature which is vertically offset from that of the lower half and the two halves are joined by planar intermediate sections 25 so that the bore of the carrier is slightly elongated in a vertical direction to afford the annulus a certain degree of radial freedom in that direction relative to the. axis 16. This radial freedom is of no significance in normal clockwise rotation

of the annulus since the annulus bears (substantially frictionlessly because of hydrodynamic pressure) against the lower half 22 of the'carrier. The pump creates a unidirectional liquid flow from inlet port 21 to outlet port 23.

The carrier 20 is movable about a fulcrum 24 between a first slightly tilted position as seen in Figure 1 and a second position in substantially mirror image relation to that of Figure 1 wherein the centre of curvature of the lower half 22 is disposed on the opposite side of the axis 16. The carrier outer periphery is also non-circular and comprises two substantially semi-cylindrical halves 26, 28 which meet at the plane 30 and are centered on different centres of curvature so that, in each tilted position, one half 26, 28 bears against, and is substantially complementary to, the cylindrical inner periphery 31 of the outer housing 16. Because the carrier 20 is tiltable in this manner, it will be seen that, with respect to the axis 16, the annulus 12 is afforded a second degree of radial freedom substantially orthogonal to the first.

In normal clockwise operation as seen in Figure 1, the axis 18 of the annulus will be substantially fixed despite the radial freedom available. If however, reversal of drive occurs so that the rotor and hence annulus turn counter-clockwise, there will be a tendency for pressure to develop in the region of inlet port 21 and a suction effect in the region of the outlet port 23. This temporary pressure fluctuation, in conjunction with reverse rotation of the annulus, will initiate shifting of the annulus away from its normal seated position

in the lower half 22 of the carrier with consequent tilting of the carrie 20 towards the- mirror image position. Figure 2 illustrates an intermediate point during such shifting of the annulus and the carrier.

When the carrier completes its tilting motion, the annulus can reseat in the lower half 22 an its axis 18 will then be located on the opposite side of the axis'1.6 thereby allowing unidirectional pumping (from inlet port 21 to outlet port 23) to be maintained despite the drive reversal.. It will be noted, that th repositioning of the annulus in this manner does; not rely upon rotational coupling between the annulus and carrier. If a subsequent drive reversal occurs, the above sequence will take place in reverse to bring the annulus; bac to the- position shown in Figur 1-

In the embodiments of Figures 1 and 2, one of the degrees of radial freedom arises from the tiltable mounting of the carrier. However, the carrier need not be tiltably mounted for movement between the two positions described; it may fo instance be slidably mounted for translational movement between those positions.

In another modification, the two degrees of radial, freedom may both be afforded by the carrier. Thus, for example, as shown schematicall in Figure 3, the carrier 20 ma be mounted within the outer housing 15 with radial freedom in directions A and B, i.e. so that the carrier may tilt about the- fulcru 2 and slide substantially vertically as. viewed in Figure 3. In such an embodiment, the annulus may be mounted within the carrier without any radial play (apart from normal running clearance).

Figure 4 illustrates a further modification in which the carrier 20 may be fixed against movement and may, if desired, be secured to or be formed by the outer housing. In this case, the carrier presents two seating surfaces 50,52 each of about 90 ^* angular extent and having the same radii of curvature (generally complementary to the annulus) with different centres of curvature 54, 56 respectively spaced apart by the distance C, corresponding to substantially' twice the eccentricity of axis

18 relative to axis 16. The two seating surfaces 50', 52 are separated by a cusp 58 which assists in proper location of the annulus at one seating surface or the other.

Th carrier is completed by an upper semi-cylindrical surface 60 of larger radius of curvature than the surfaces 50, 52 and centred on the axis 16. In this way, the annulus is afforded- two degrees of radial freedom in the directions A and B to allow it to shift from one seating surface to the other in response to drive reversal. Thus-, when the annulus is seated by surface 50, it will remain in operative stable condition at that position as long as the drive rotation takes place in a clockwise direction. If however drive is reversed, the resulting temporary pressure fluctuations will de-stabilise the annulus and, in conjunction with reverse motion of the annulus, cause it to shift away from the surface 50 until the annulus reseats itself at surface 52 whereupon normal unidirectional flow will continue.