(under subclass 233) Sifters of spiral of involute shape.
(1) Note. The sifting surface itself has the form specified and may be plane, curved, or tubular in cross section, and may have movement or be stationary.
(2) Note. This subclass does not include cylindrical or drum sifters which have spiral guide walls inside the same, which guide walls define spiral passages along the sifting surface; but it will include cylindrical sifting surfaces where the cylinders are bent or twisted into spirals.
