(57) In an N, N+1 lobe type pump, dirt clearances are provided at the bases of the interlobe
roots 22 of the annulus 14 and the shaping and dimensioning of the inner rotor and
annulus are such that elsewhere there is a normal running clearance at the zones of
drive contact between the two rotors. To prevent the creation of excessive clearances
in the positions of full mesh especially at position D, the inner rotor 10 and annulus
14 are conjugate (as defined) such that at positions B and D normal running clearances
are maintained.
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