The Mechanical Handling and Storing of Material

i4 THE MECHANICAL HANDLING OF MATERIAL pulley plus about 1 ft., in order to clear the preceding buckets and to reach the discharge spout, except in cases where special buckets—referred to later—are used. The elevator buckets generally begin to discharge when in the highest position on top of the pulley. To effect perfect discharge from a perpendicular elevator the centrifugal force must be sufficient to overcome the gravity of the material, so that for a specifically heavy material it is necessary to have a greater centrifugal force, i.e., higher speed of elevator than for a specifically lighter material, as no matter with what velocity a body may be thrown off tangentially from the elevator pulley, the moment it is free from its support the attraction of the earth asserts itself and causes the body to eventually fall. If the direction o the material is horizontal, as is the case in the delivery from a vertical elevator, the attraction of the earth causes the body to move in a parabolic curve, and the heavier the body, the more will its path deviate from the horizontal at every instance (see Fig. 7). A light body at a given linear velocity will proceed in an approximately horizontal line for a longer distance than a heavy body, as the earth’s attraction is more or less counteracted by the friction of the atmosphere. The centrifugal force of pulleys revolving at the same speed is in direct proportion to the diameter of the pulleys. For example, in a 2-ft. pulley it will be twice what it is in a pulley 1 ft. in diameter running at the same speed when the speed of an elevator pulley of a given diameter is increased, the centrifugal force increases in proportion to the square of its velocity; consequently the centrifugal force of a pulley 2 ft. in diameter, running at 50 revs, per minute, will be four times the centrifugal force of a pulley of the same diameter running at only 25 revs, per minute. This indicates clearly that in the case of grain elevators it is quite inaccu- rate to estimate the number of revolutions of the elevator pulley by a fixed belt speed for pulleys of all sizes, which is a mode of calculation too fre- Fig. 7. Diagram showing Discharge of quently adopted. Elevator Bucket. In addition to what has been mentioned con- cerning clean delivery at the top terminal of elevators, the shape of the elevator buckets—correctly chosen for each material— determines also to some extent the useful effect of the theoretical capacity of eleva- tors; for this depends upon the amount spilled on the up-going strand, and that spilled at the top terminal between the buckets and the delivery shoot. If a shallow bucket, intended for soft and clinging materials, is employed for sharp or lively materials, it will fill itself with more than it can carry, and some will fall back at the point where the elevator band begins to turn over the top terminal pulley. A soft and clinging material will fill such buckets equally full, or perhaps even more so, but owing to its nature it will not spill out. The efficiency of elevators finally depends upon. the regularity of the feed, so that the buckets are not sometimes overloaded and sometimes only partly full. This can be regulated by a feeding device, as described on page 202. A grain elevator which has come under the special notice of the authoi is one in a large Cardiff flour mill. The pulleys are 24 in. diameter by 18 in. on face, and the speed is 60 revs. The buckets are 15 in. wide, project 6| in. from the band, and are 17 in. pitch. The belt speed is 375 ft. per minute, and 265 buckets passing