The Mechanical Handling and Storing of Material

396 THE MECHANICAL HANDLING OE MATERIAL imagine a flat, very thin, and perfectly smooth spade dug into sand in the direction of its plane. In that case the only resistance caused will be to the progress of the cutting edge of the spade, that is to say, to its cutting action, because the grains of sand lying directly in the path of the blade are pushed to one side. But suppose the blade of the spade to be thicker and somewhat curved, like the jaws of a grab, then it will displace a certain amount of sand which must be pushed aside in proportion to the depth of the curve. In such a case the sand will either be compressed, or, if the blade does not enter a great depth, the sand displaced will be pushed outwards on the oblique side of the spade in the direction of the line ab (see A, Tig. 555). According to the resistance of the sand the line ab forms an angle Y with the horizontal. The angle y = 45°, assuming 5 to be the natural angle of repose of the material. 1 he resistance thus set up increases very considerably should the surface of the spade or jaw be rough, and, as shown in A, Fig. 555, the sand is heaped up as the spade enters the ground, being displaced by the body, of the spade. A similar resistance is felt on the other side of the blade, to which must be added the friction between the blade and the sand. So far it has been assumed that the sand was level on either side of the blade, and that the blade entered vertically, as shown in A, Fig. 555 : but if B, Fig. 555, is accepted as a truer picture of what actually takes place, namely, that the sand is heaped up much higher on the right or interior side of the grab, then it is obvious that the resistance is greater on the right side. According to Mi Salomon, the total amount of resistance can be considerably lessened by a judicious shaping of the digging blade. A grab with cylindrically shaped jaws was therefore chosen, the axis of rotation coinciding with its radius (see C, Fig. 555). IiAen with this there will be d considerable amount of resistance on the outer walls of the blade, because the material lying below cannot escape, but must be compressed. Salomon found that when the axis of rotation was placed slightly above the centre of the radius of the blade, as in D, Fig. 555, the outer resistance was diminished, and the internal digging power of the jaws increased. Ihc sum total of these experiments, although of small practical value, point to the advisability of slightly raising the axis of rotation of the jaws. It was, however, judiciously pointed out by jBaron Hanffstengsl that in these experi- ments the point of rotation was fixed, and that on that account a fairly great pressure between the sand and the grab jaws could be set up, whereas in practice the total vertical pressure cannot at the most exceed the weight of the grab with its contents, and will probably be less, as a portion of the weight is held by a chain. Mi Salomon says that the higher the point of rotation is removed from the centre,