The Mechanical Handling and Storing of Material

54 THE MECHANICAL HANDLING OF MATERIAL of this chapter, as the trough though stationary laterally, is revolving, but it is essentially a worm conveyor. The advantages are the absence of internal bearings, which are the principal cause of trouble when using ordinary worm conveyors for material of a cutting nature. The spiral is best chosen of a width equal to one-third of the diameter of the cylinder, i.e., for a 12-in. worm the spiral strip would be 4 in. wide, and thus leave a clear space of 4 in. in the centre of the tube. The best pitch for the screw is two-fifths of the diameter. The supporting rollers need not be so close together as the internal bearings of ordinary worm conveyors, as the cylindrical body of the tubular conveyor is so much more rigid. The distance apart of the supporting rollers depends upon the strength of the construction, but as a general rule they may be about twice as far apart as the bearings in ordinary worm conveyors. Intermediate outlets are not impossible, but they are not advisable owing to the complication and expense. This form of conveyor is more expensive in initial cost, and the fixing is also more costly than that of ordinary worms, but its life is longer. The introduction of this Fig. 58. Tubular Worm Conveyor. conveyor may be considered to be the first step in the direction of providing a more economical conveyor for fine material. One of the essential features is that the speed must be chosen within the correct limits. If too high the capacity diminishes, as the centrifugal force counteracts the propulsion ; for it must be remembered that with this appliance the forward movement of the material is effected by gravity, inasmuch as through the revolution of the tube the material contained in it is carried up the side in the direction of the revolution until the angle of repose is „exceeded, when it is compelled to fall, and is then guided in a slightly forward direction by the spiral attached to the inside of the tube. The capacity of this tubular conveyor is not very large, as the pitch of the screw must be smaller than with most ordinary worms, say about 0-33 to 0'4 of the inner diameter of the tube, and the number of revolutions is also limited, as already mentioned. It is thus obvious that the greater the pitch of the screw the greater the tendency of the material to cling to the inside of the tube, which tendency is still further encouraged when the centrifugal force begins to exert itself, so that a point is reached where the material is carried too far up the side of the tube, and there is then a tendency for it not to slide forward in its appointed channel but to fall, more than slide, back from its higher position, and so some of the mateiial may fall back into the previous thread of the screw, which reduces the capacity more and more as the speed is increased, and eventually the centrifugal force being greater than the gravity, the material revolves round and round on the inner periphery of the tube and conveying ceases altogether.