The present invention relates to a variable output pump.

Variable output gerotor pumps are well known. The reader is referred to European Patent No. 565 340 B l and European Patent No. 81 1 787 Bl in the name of the Applicant. As shown in those patents, known variable output pumps are typically driven by an external drive gear 52 press fixed onto a drive shaft 50, which drive shaft is powered by a prime mover. In use, the drive shaft 50 turns an integral first rotor 10, which drives an annulus 16, which in turn drives a second rotor 12.

An aim of the invention is to provide a different means of driving a variable output

pump.

According to a first aspect of the invention there is provided a variable output rotary

internal gear pump having an outer annulus and two inner rotors arranged inside a

housing, the two inner rotors being relatively rotationally adjustable, wherein the

annulus is directly driven from a part other than one of the rotors.

In one preferred embodiment of the invention, the pump is a gerotor pump.

According to a second aspect of the invention there is provided a variable output gerotor pump having an outer annulus and two inner rotors, the two inner rotors being

relatively rotationally adjustable, wherein the annulus is directly driven from a part

other than one of the rotors.

Preferably, the annulus is driven by an input gear.

The annulus is . preferably driven on a surface other than its inner surface, more preferably on its outer facing surface, most preferably the annulus is driven at a radially outermost extent of its outer facing surface. The annulus preferably has gear teeth on its outer facing surface.

The annulus preferably has integral gear teeth.

Preferably a single annulus drives the two inner rotors.

A stationary shaft preferably supports the first rotor. The stationary shaft may be pressed into the pump housing or the shaft may be integral with the pump housing.

Preferably an inner bushing supports the first rotor.

A bush or bearing is preferably provided in the housing to support the drive loads from

the input gear. Most preferably, the bush or bearing is provided between part of the annulus and part of the housing.

In another preferred embodiment of the invention, the pump is a crescent pump.

A gerotor pump in accordance with the invention will now be described, by way of

example only, and with reference to the accompanying Figures, in which:-

Figure 1 is a cross section of a gerotor pump,

Figure 2 is a first axial view of a gerotor pump, showing gear teeth on a periphery of an

annulus,

Figure 3 is another axial view of a gerotor pump, taken from an opposite side to Figure

2,

Figure 4 is a side view of part of a gerotor pump, and

Figure 5 is a perspective view of part of a gerotor pump.

Referring to Figure 1 , a pump sub assembly 4 comprises a first housing part 6 and a

second housing part 8, which cooperate to define an enclosure. First and second

externally lobed rotors 10, 12 are arranged end to end within an outer internally lobed

rotor (or annulus) 16. Lobes 17 are shown in Figures 3 and 5. Through - filling bores 19 which extend through each lobe 17 along axis parallel to the rotational axis of annulus,

will be noted. The first and second housing parts 6, 8 provide a housing for the first and second rotors 10, 12 and the annulus 16. First and second rotors also have through -

filling bores 1 1 , 13 which extend through each of the lobes, along axis parallel to the

rotational axis A, C, as shown in Figure 2. It should be noted that the through - filling

bores 19, 1 1 , 13 in the annulus 16, and rotors 10, 12 may not always be needed. An

inlet port 18 is formed in the first and second housing parts 6, 8. An outlet port 22 is

also formed in the first and second housing parts 6, 8.

The second rotor 12 is fast with (an optional) bush 56 and both rotate on an eccentric

58 which is fast with shaft 60, journalled in the pump housing 6,8, and which shaft is arranged to be angularly turned. The shaft 60 is keyed to a pinion 64 and the pinion 64 is arranged to be turned by rack 66. Such an arrangement allows the second rotor 12 to be turned (or indexed) with respect to the first rotor 10. Indexing can alternatively be carried out by hydraulic, pneumatic, mechanical (e.g. a worm gear) or electronic means

or a combination of any of the aforementioned.

The conventional drive gear employed in European Patent No. 565 340 Bl is not required. Instead, in accordance with the invention, drive means act directly on the

annulus 16. In particular, gear teeth 23 are integral to the annulus 16 as shown in Figure

1. Teeth 23 are driven by a suitable input gear 25.

The gear teeth 23 can be one piece with the material of the annulus 16 (as in a sintered part for example) or the gear teeth 23 can be provided on a separate ring gear which is

made fast with the annulus 16 using a press fit, adhesive or similar, as shown in Figure 1. The gear teeth 23 may be spur or helical in form and will be of width/length suitable

to the application (typically 1/4 to 1/3 of rotor length) but may be longer or shorter.

The teeth 23 could also be suitable for use with drive chains as used in some interna]

combustion engines.

To support the non-indexing first rotor 10 one can employ a stationary shaft 40 which

may be pressed into the pump housing 6, 8. Alternatively, the first rotor 10 can be supported on a journal diameter 42 which is an integral part of the pump housing 6, 8. These two options are shown in the upper and lower part of Figure 1. A bushing 70 can be provided in some applications to support the non-indexing rotor 10.

A bush/or bearing support 72 can be provided in the pump housing 6, 8 to support the drive loads from the input gear 25.

Referring to Figure 2, centre 'A' is the axis of the stationary shaft 40 which is pressed (or integrated) into the housing (indicated by 6, 8 in Figure 1 ). The longer, non indexing

rotor 10 rotates on this shaft.

Centre ¹ B' is the axis of the outer annulus 16 and therefore also the axis of the external gear teeth 23.

Rotor indexing (i.e. providing relative rotational adjustment between the two rotors) has

been discussed at length in EP 0 565 340 and in EP 0 81 1 787 (in the name of the Applicant) and so for the sake of conciseness it will not be described in detail herein. In the embodiment of the invention shown, Centre ¹ C is the axis of the, shorter, indexing rotor 12, and is indexed 120 degrees counter clockwise. It will be appreciated that in

the full output mode centre ¹ C will be coincidental with axis 'A' and in the lowest

output mode centre 'C will be indexed 180 degree to axis 'A', i.e. wholly out of phase.

Figure 3, taken from the other side of the rotor set, simply shows the full extent of rotor

10.

Figure 4 shows the gear teeth 23 length with respect to the annulus overall length (i.e. about two-fifths) and the relative positions of the axis 'A' and 'B ¹ .

Figure 5 still shows the indexing rotor 12 has been indexed 120 degrees counter

clockwise.

The gear teeth 23 could also be teeth for co-operating with and being driven by a chain or similar drive mechanism.

In the embodiment shown in Figure 5, there is no bush 70 in the bore of the non-indexing rotor 10, this being an optional feature.

In use, both rotors remain constantly in mesh with the -annulus internal form at all

times. The input drive is via the external gear teeth 23 on the annulus outside diameter.

This annulus 16 drives both rotors 10, 12.

It has been noticed by the Applicant in private tests that, owing to the elimination of the

conventional drive gear, there can be a reduction in the axial length of the pump 4. This

can have installation advantages.

The effective displacement of the pump 4 is increased by a ratio equal to the rotor set

combination. For example, using a 6/7 set (6 outer lobes on inner rotor 12 and 7 inner lobes on aimulus 16), for one revolution of the outer rotor or annulus 16, the inner rotor has rotated 7/6 revolutions. That equates to an additional 16.7% flow above a conventionally driven pump. This increases the potential pump displacement for a given rotor length, and again has potential installation advantages. A 5/6 set (as shown) gives an additional 20% displacement.

It also appears that the assembly method disclosed herein gives rise to a reduction in manufacturing costs.