GEROTOR-TYPE PUMP

Abstract: The pump has an inner rotor bounded by an outer peripheral shape (20) which is generated by moving a first circle around a first trochoid. The inner rotor is mounted for rotation about a first axis. The pump also has an outer rotor mounted for rotation about a second axis which is off-set from the first axis. The outer rotor is bounded by an inner peripheral shape (22) which is generated by moving a second circle around a second trochoid.

Description: GEROTOR-TYPE PUMP This invention is concerned with a Gerotor-type pump which may be used, for example, as an oil pump. Gerotor-type pumps are well known and comprise an inner rotor provided with external teeth which is located within a hollow outer rotor which is provided internally with teeth meshing with the external teeth of the inner rotor. The outer rotor has one more tooth than the inner rotor and the inner rotor has an axis of rotation which is offset or eccentric with respect to an axis of rotation of the outer rotor. By this arrangement, rotation of one rotor causes the other rotor to rotate as it is driven by the inter eshing teeth. During rotation, due to the eccentricity of the axis of rotation, the intermeshing relationship of the teeth changes progressively forming chambers between the teeth which change in volume to create a pumping action. The inner rotor of a Gerotor-type pump is designed according to a well-established method. In this method, starting with a circle of diameter A (the base circle) , a circle of diameter B (the rolling circle) is rolled around the outside of the base circle while tracing the track of a point at a distance e (the eccentricity) from the centre of the rolling circle. This gives a curve called a trochoid. It is necessary that the rolling circle rolls around the base circle an exact number of times. The ratio of the diameters A to B is the number of "teeth" (n) on the inner rotor. Next, in designing the inner rotor, a circle of diameter C (the locus or track circle) is moved around the aforementioned trochoid with the centre of the circle on the trochoid. The track of the radially innermost point on the locus circle is the shape of the inner rotor. Hitherto, the outer rotor of a pump of the Gerotor-type has been designed by drawing a circle of radius R. R is defined by (A + B) divided by 2 plus an adjustment for clearance. Next, n plus 1 centres are defined equally distributed around the circle of radius R. Each of these centres represents the centre of a tooth of the outer rotor. About these centres, circular arcs of radius r are drawn facing towards the centre of the circle of radius R. The radius of the arcs r is defined by C divided by 2 minus an adjustment for clearance. The design of a rotor of conventional type is shown in Figure 1. In this case, the inner rotor has 5 teeth and the outer rotor has 6 arcuate teeth. As can be seen from Figure 1, the teeth of the outer rotor are joined by arcs S of a circle, (centred at the centre of the circle of radius R) . The invention provides a pump of the gerotor type comprising an inner rotor and an outer rotor, the inner rotor being located within the outer rotor and being mounted for rotation about a first axis and the outer rotor being mounted for rotation about a second axis which is off-set from said first axis by an eccentricity of the pump, the inner rotor having an outer surface which has a toothed shape and is meshed with an inner surface of the outer rotor which has a toothed shape which has one more tooth than the inner rotor, said toothed shape of the inner rotor being a shape which is generated by moving a first circle around a first trochoid with the centre of the circle on the trochoid, characterised in that said toothed- shape of the outer rotor has a shape which is generated by moving a second circle around a second trochoid with the centre of the circle on the trochoid. A pump according to the invention operates more smoothly than conventional pumps giving quieter operation and longer life. The pump also has a more efficient pumping action. Preferably, in a pump according to the invention, said first and second circles have diameters which differ by a predetermined operating clearance between the rotors. There now follows a detailed description, to be read with reference to the accompanying drawings, of a pump which is illustrative of the invention and of an illustrative method by which shapes of the rotors of the illustrative pump are generated. In the drawings: Figure 1 is a diagrammatic representation of the outer peripheral shape of an inner rotor and the inner peripheral shape of an outer rotor of a conventional pump of the gerotor type, showing the rotors meshed; and Figure 2 is similar to Figure 1 but shows the illustrative pump on a larger scale. The conventional pump shown in Figure 1 comprises an inner rotor bounded by an outer peripheral shape 10 and an outer rotor bounded by an inner peripheral shape 12. The illustrative pump shown in Figure 2 comprises an inner rotor bounded by an outer peripheral shape 20 and an outer rotor bounded by an inner peripheral shape 22. The shapes 10 and 20 are identical but the shapes 12 and 22 are different. The shapes 10 and 20 are generated by the above- mentioned well-established method using a base circle of diameter A, a rolling circle of diameter B, an eccentricity of e, and a track circle of diameter C. In this case, the number of "teeth" (n) was five, ie the ratio of A to B was five but n can be any whole number above two. The coordinates of a point on the trochoid generated by a point distance e from the centre of the rolling circle as it is rolled around the base circle are given by Equations 1 and 2 where θ is the angle subtended at the origin. Equation 1 x = - (n + 1) cos (θ) - ecos ( (n + 1) θ) Equation 2 y = - (n + 1) sin (θ) - esin ( (n + 1) θ) The coordinates of the shapes 10 and 20 are then given by X and Y in Equation 3. Equation 3 (X-x) 2 + ( Y-y) 2 = (* ) 2 Differentiating Equation 3 with respect to θ gives Equation 4. Equation 4 by (X-x) bx 0 δθ + ( Y-y) = δθ Rearranging Equation 4 and substituting in K as defined in Equation 6 gives Equation 5. Equation 5 ( Y-y) = -K (X-x) Equation 6 bx r - δθ δθ Differentiation of Equations 1 and 2 and substitution into Equation 5 gives Equation 7. Equation 7 --fsin(θ) + esin ( (n + 1) θ) K = 2 - cos (θ) - ecos ( (n + 1) θ) Substituting Equation 5 into Equation 3 and solving gives Equations 8 and 9 whose innermost points define the coordinates of points on the shapes 10 and 20. Equation 8 Equation 9 From Figures 1 and 2, it can be seen that the shapes 10 and 20 are toothed with five generally-arcuate teeth joined by convex arcs. The shape 12 of the outer rotor of the conventional pump shown in Figure 1 is generated by drawing six (n+1) arcs on a circle of radius R (equal to A+B divided by 2) , each arc having a radius of C divided by 2 minus a clearance. The shape 22 of the inner peripheral surface of the outer rotor of the illustrative pump is, however, generated from the trochoid whose coordinates are defined as the envelope of the rotated inner rotor trochoid and by Equations 10 and 11 in which R is defined by Equation 12, z is defined by Equation 13 and the angle v takes the values defined by Equation 14. Equation 10 x - Rcos (2v) - - - - sin (2zv) sin (2v) ± 2e Ai 1 -(--•--•—) sin 2 ( zv) cos (2v) cos ( zv) Equation 11 y = Rsin (2 v) + ^— sin ( 2zv) cos (2v) Equation 12 R = - (A +B) Equation 13 z = (n+1) Equation 14 _ _π _π _5π_ 7_π _Jπ llπ V 6 " ' 2 ' 6 ' ' 6 ' 2 6 In order to generate the shape 22 from the trochoid defined by Equations 10 and 11, Equations 6, 8 and 9 are used with θ replaced by v and with C equal to the diameter of the track circle minus a small clearance. K is determined by differentiating Equations 10 and 11 with respect to v to give Equations 15 and 16 which define K according to Equation 17. The coordinates of the shape 22 are then given by the innermost points of Equations 8 and 9, where x, y and K are given by Equations 10, 11 and 15 to 17. Equation 15 K A = - [2i?sin(2v) + 2 (z ^ )2 cos (2zv) sin (2v) R + 2 -= z-≡e- 2 sin (2zv) cos (2v) + 2 sin (2v) cos (zv) + zcos (2v) sin (zv) ] ] Equation 16 K B = 2Rcos (2v) + 2 (z ) cos (2zv) cos (2v) 2 - 2 -^ - sin (2zv) sin (2v) JR. + 2cos (2v) cos (zv) - zsin (2v) sin (zv) ] Equation 17 K = -±

Claims: CLAIMS A pump of the gerotor type comprising an inner rotor (20) and an outer rotor (22) , the inner rotor being located within the outer rotor and being mounted for rotation about a first axis and the outer rotor being mounted for rotation about a second axis which is off¬ set from said first axis by an eccentricity of the pump, the inner rotor having an outer surface which has a toothed shape and is meshed with an inner surface of the outer rotor which has a toothed shape which has one more tooth than the inner rotor, said toothed shape of the inner rotor being a shape which is generated by moving a first circle around a first trochoid with the centre of the circle on the trochoid, characterised in that said toothed shape of the outer rotor (22) has a shape which is generated by moving a second circle around a second trochoid with the centre of the circle on the trochoid. A pump according to claim 1, characterised in that said first and second circles have diameters which differ by a predetermined operating clearance between the rotors (20, 22). A pump according to either one of claims 1 and 2, characterised in that said second trochoid has a shape given by equations 10 and 11 given in the illustrative example. A pump according to claim 3, characterised in that the toothed shape of the outer rotor (22) is given by equations 8 to 11 and 15 to 17 given in the illustrative example.

Inventors: WHITHAM GAVIN PETER, WHITHAM, GAVIN, PETER
Applicants: T & N TECHNOLOGY LTD, WHITHAM GAVIN PETER, T & N TECHNOLOGY LIMITED, WHITHAM, GAVIN, PETER
Application year: 1995
Main classification: F04C2
IPC codes: F04C2