The Mechanical Handling and Storing of Material

44 THE MECHANICAL HANDLING OF MATERIAL four of the ordinary paddles, so that only half the number of holes are required on the spindle. The chief advantages of the paddle-bladed and crescent-bladed worms are that, if for any reason it may be desired to change the direction in which the material is being conveyed, it is only necessary to slacken the nuts on the shanks and turn the blades to the reverse angle. The crescent blades in this case still form a continuous screw, but the paddle blades on being reversed form a broken non-continuous worm, and though this will still convey the material in the right direction, it will only convey it the width of a paddle at each revolution ; that is, only at a quarter of the proper speed. It is therefore necessary to remove every other blade and replace it on the other side of the spindle, when the worm will again be practically continuous. 'The continuous or closed spiral worm is illustrated in Fig. 44. The only difference Fig, 44. Continuous Worm Conveyor, or Closed Spiral. Fig. 45. Showing Con- struction of Closed Spiral Worm. between this and the last-mentioned conveyor consists in this, that the screw is not composed of single blades, but is one continuous sheet-iron spiral, which is secured to the spindle at intervals more or less frequent according to the size and capacity of the worm. The most approved method of construction is to cut a narrow spiral groove in the spindle, and to secure the sheet-iron spiral into this groove. The usual method employed for producing the close spiral of a continuous worm conveyor has been to cut segments from sheet iron and rivet them together. A later and better method is to run a strip of hoop iron through special rollers in such a way as to thin and therefore stretch it at the outer edge, tapering to the inner edge, whereby a spiral is pro- duced. This is probably the best method for worm conveyors intended for heavy and cutting materials, as the iron strip or hoop may be chosen of any gauge. One of the latest methods of producing the spiral is also from a strip of light hoop iron, but instead of rolling it down to a tapering section the inner periphery is reduced, thus forming a spiral by a series of triangular pleats, as shown in Fig. 45. This does not only produce a very rigid spiral from comparatively light material, but the projecting pleats also aid the conveying action by offering more resistance to the material to be handled than a smooth blade. Of course the worm must be so arranged that the pleats are on the conveying side of the worm, i.e., point in the direction in which the material is to be conveyed. Such a worm would be most suitable for grain and similar light materials. Ther^ is a fourth form of worm which ought to come under this heading, i.e., the open spiral, sometimes called an anti-friction conveyor. This was introduced about the year 1887, and is a very simple yet efficient conveyor. Fig. 46 illustrates this appliance, which has the advantage of being inexpensive.