This invention relates to pumps of the kind comprising a male rotor with n _^ lobes which is located internally of and meshed with a female annulus having n+1 lobes. These two form a gerotor set which is driven either from the annulus or the rotor and the two turn relative to one another and about parallel axes. Λ series of chambers is formed between the lobes and each chamber extends between two lines of contact be ween the rotor and annulus. These lines lie generally on the peaks, or maximum radius portions of the rotor lobes, and move along the annulus as the parts rotate at different speed. Hence the chambers increase in size as they proceed from a position adjacent a plane containing both axes and adjacent the point of full mesh between a male lobe and a female recess between lobes (or vice versa) towards a diametrically opposite position at a place where only the crests (maximum radius portions) of the lobes of both rotor and annulus meet. This travel is the induction stroke and fluid is sucked into the chambers ns they follow this path from an inlet port at an axial end of the chambers.

Similarly, as the chambers continue in their travel on the opposite side of said plane returning to the start point, they diminish and expel fluid through a second port or outlet.

Λs stated, pumps of the kind mentioned in the foregoing two paragraphs are well known and exist in many variations.

With internal combustion engines the direction of rotation of the main shaft (e.g. the crank shaft of the engine) is usually unidirectional because of valve timing and ignition timing requirements, and hence a pump of this kind e.g. used as the lubrication oil pump and driven from such a crankshaft is also unidirectional. But with certain rotary machines for example some kind of

compressors, the direction of rotation is unimportant and may vary from one cycle of operation to another. Tf a gerotor pump is used with such a machine, the effect on the pump of changing the direction of rotation is to expel fluid through the inlet and suck through the outlet: usually this is unacceptable.

Tt is therefore known in the prior art to provide means for shifting the eccentricity of one axis of I:lie gerotor relative to the other, according to the direction in which the annulus or rotor is driven. Usually the shift is through 180 degrees in said reversal that is from one side of the stationary axis to the other. This enables the inlet and outlet to remain unchanged and give uni irectional flow through the pump irrespective of reversible drive direction.

Many different schemes have been put forward to cause the automatic shift. Thus it is known to mount the annulus in an eccentric ring which is itself angularly movable in a pump body cavity, and to dispose a blade spring between the annulus and the eccentric so as to create a frictional drag between the two. When the annulus turns in one direction, this drags the eccentric ring to one position against the stop and hence fixes the position of the axes. When the drive direction is reversed, the spring drags the eccentric in the opposite direction and hence changes the axis positions. Difficulties with this design are power loss caused by the frictional drag, which is effective during the whole of the operation although only needed at the start-up point, and the additional space required to accommodate the additional component, i.e. the eccentric ring.

Another approach has located the annulus in a carrier ring which is freely pivoted, and use the carrier ring to shift the position of the parts with respect to a drive shaft so as to bring about the required result, but again extra components and additional volume arp required and the operation is not found reliable.

The object of the present invention is to solve the

problem and provide improvements and particularly re u e both the number of components needed and the volume req ire .

According to the invention a pump comprises a male rotor with r^ lobes l cated in and meshed with a female rotor having n+1 lobes so as to form a series of chambers between the lobes each bounded by lines of 'contact bet een the rotor lobes and the annulus, charac erised in that said rotor is journal led on a boss which is cylindrical about a main axis and which is mounted for pivotal movement about an axis eccentric to the main axis and in that limit means are rovided to control he extent of pivotal movement so that at the limits of the movement the said main axis is displaced 180° about the axis of eccentricity.

Preferably drive is transmitted by the annulus because this simplifies matters, but it is possible t arrange for drive to the rotor at the alternative positions occupied according to the direction of drive.

The invention is now more particularly described with reference to the accompanying drawings wherein:

Figure 1 is an end elevation of a pump bo y to house a gerotor pump set;

Figure 2 is a sectional elevation of the same but with parts removed for clarity;

Figure 3 is an alternative embodiment ; and

Figure A is a perspective view of an eccentric used in the various embodiments.

Turning first to Figure 1, this shows the inlet and outlet ports 10, 12 relative to the circular chamber bounded by the line 14 which in use contains the annulus (not shown) of the gerotor set. These ports are communicated to flov/ passages which may lead for example to an inlet port 16 and an outlet port 18. Also indicated is central axis 20 which is concentric to the surface 14, and a cut-away 22 extending arcuately over about 180° about the centre 20.

In Figure 2, the pump set annulus 30 is shown, which is internally lobed with n+1 lobes and is connected for drive by means of co-axial projection 32 which may for example be engaged with the end of a crankshaft 34 by means of flats or a key and eyway. The rotor, not shown, having n_ lobes is located internally, of the ann lus and has a concentric bore journall d on boss 36.

The boss is cylindrical and has a main axis. Hence the rotor turns about that axis when the annulus is driven .

The bos 36 (see also Fi ure 4) is, in Fi ure 2, journalled on the fulcrum pin 38 which is eccentric of the boss main axis, and this pin may be fast, for example a drive fit, in a bore in the end wall of the annulus and/or in the parallel face of the cover component 40.

The limit pin 42 is carried by the boss 36.

In operation, the annulus is driven, and this transmits drive to the rotor albeit at a different speed, so that the rotor turns on the boss 36. The pressure difference between one side of the pump and the other due to the direction of turning causes the boss 36 to pivot on the fulcrum 38 until the limit pin 42 reaches one or other end of the recess 22 according to the direction of the pressure difference. When the direction of rotation of the annulus changes, the boss 36 automatically moves around to re-position the rotor and take the limit pin 42 from one end to the other of the recess.

The arrangement in Figure 3 differs only in that the boss 36 is journalled on pivot pin 48 which has n head 50 and in that the annulus has drive means 52 engaging with the crankshaft or like.

It will be appreciated by those skilled in the art that the pin 38 could be made integral with the boss 36 for example by a powder moulding technique. So could the pin 42. Alternative annulus drive means may be used, for

example by providing the annulus wi h extern l gea tee h and transmitting drive from a pinion train. Alternatively, the rotor can b dri n, For example b providing the rotor with a portion projec ing through the u body .