Centrifuges of the type wherein a helical conveying scroll is mounted within a frustoconical centrifuge bowl to separate solids and liquids from liquid-solid mixtures are well known. In devices of this type, a rotary speed differential is established between the rotating bowl and the internal conveying scroll so that rotation of the helical scroll relative to the bowl can drive the solid components of the mixture to one end of the bowl where they are extracted. Typically, such devices establish the rotary speed differential by means of a gear box. In addition to being a high maintenance item, the gear boxes employed do not permit any variation in differential speed between the scroll and bowl and this lack of capability of varying the differential speed produces a reduction in separating efficiency in the face of variations in the consistency of material being fed into the centrifuge.
While various attempts have been made to provide a speed differential adjustment capability, -- see for example U.S. Pat. Nos. 3,734,399 and 3,923,241 -- these attempts have involved the addition of fluid pumps and variable speed regulating mechanisms which are not only expensive, but do not provide any substantial advantage insofar as maintenance costs are concerned over the conventional gear box.