The Mechanical Handling and Storing of Material

CHAPTER XVII CONVEYING BY GRAVITY Trough-shaped inclined shoots are largely used to convey material by gravity to and from mechanical devices for handling such materials. The angle at which such shoots are fixed depends on a variety of circumstances. If the material is not friable or likely to be injured or deteriorated in value by quickly descend- ing the shoot on a steep gradient, and coming more or less abruptly to rest at the lower end, there is no par- ticular need to take precautions, and the shoot can be erected to connect the receiving point with that of de- livery, as long as the gradient is sufficient to ensure the descent of the material. When the material is friable and deteriorates by breakage, the incline of the conveying shoot must be so chosen that the material will only just slide down, and that in so doing it cannot attain a velocity sufficient to injure it. These remarks refer particularly to coal,1 and it may therefore be of interest to give a few inclines at which coal will descend a shoot in safety. The inclines are given in inches per yard. Mine coal (large and small as delivered in colliery tubs) - 15| in. Cobbles 141 „ Nuts - - - - - 15 ,, Slack or smut (dust coal) - - - 19 ,, Cannel coal ----- 141 „ The incline of a shoot depends generally upon the coefficient of friction between the material and the shoot and also on the form of the pieces. The incline varies from 30° to 60°, the lower figure being for such materials as washed gravel, coarse dry sand, and cement clinker from the revolving retorts, and the other extremes would be for fine cement, moist sugar, salt, washed coal, etc. It will be seen from the above that a wooden shoot, Fig. 262. Gravity Sack Shoot. for instance, would have to be steeper than an iron one. The safest plan in doubtful cases is always to rig up a short length of experimental shoot, especially if not much fall is available. Gravity Sack Shoots.2—If sacks have to be lowered from one floor to another, and the distance between the two terminals of the shoot is sufficiently great for a straight 1 For shoots from elevators, see page 26. 2 From The Miller of 4th December 1911. ’95