The Mechanical Handling and Storing of Material

WORM OR ARCHIMEDEAN SCREW CONVEYORS 45 The spirals which form the principal part are rolled by special machines. The illustra- tion shows the mode of fixing the spiral to the spindle. Fig. 47 shows another method for this. The spirals are made of various sections, from a round bar about | in. in diameter to L or T section. The best form, however, is the flat bar, as shown in the illustration. In all worms the ratio of the pitch to the diameter must depend upon the kind of material to be conveyed. It ranges from one-quarter to a pitch equal to the diameter of the worm, and even sometimes more. The greater the pitch the greater the capacity and consequently driving power required. It is therefore usual to employ worm conveyors for specifically heavy materials, such as cement, with a small pitch, and those for grain and other light materials with a larger pitch. The space between the screw and the trough should be either as small as possible Fig. 46. Open Spiral or Anti-friction Worm Conveyor. or just a little larger than the size of the largest piece contained in the material to be conveyed, as if fractions of the material are larger than the space, they become wedged between the screw and the trough, and may thereby cause trouble. Well-made worm conveyors, with well-fitting troughs—i.e., not too tight — of sufficient rigidity not to require too many intermediate bearings, with the latter of a design not to obstruct the flow of the material, are undoubtedly good and ser- viceable conveyors for all flour-mill pro- ducts, meal, seeds, cereals, cattle food, crushed seed cakes, material used in the manufacture of linoleum, such as cork powder and sawdust, and as a matter of fact for all fine materials which are not gritty or cutting, always provided the Fig. 47. Method of Fixing Spiral of Conveyor worms are not too long and are not re- to Spindle. quired for too large a capacity. Worm conveyors of the continuous paddle or spiral types are made with diameters of from 4 in. to 18 in. and even more, but those of 12 in. and over, except for short distances of, say, not exceeding 40 to 50 ft., and for the materials just mentioned, can hardly be called satisfactory. As to the worms of the smaller diameters and for the same materials, lengths not exceeding 150 ft. for 4-in. to 8-in. worms, and 100 ft. for 9-in. to 10-in., should be the limit of their economical utility. It is true that there are installations to be found in which these limits are exceeded, but they cannot be called economical. In the end it comes to this, that before the