The Mechanical Handling and Storing of Material

AERIAL CABLEWAYS OR CABLE-CRANES 3°9 advisable to connect the guys to anchorages separate from the main cable, so that failure of the one need not affect the other. The towers would thus be safe even though the main cable should break away. The supporting towers are usually of timber in temporary installations, whilst steel structures are used for permanent work, or where timber is not obtainable. A frame timber tower is commonly used for trenching apparatus, with one back and one or two front guys on each tower j for ta.ll towers the main timbers must be spliced and tiussed. For more substantial work four posts are used, and one guy at the rear to take the strain on the cable. Steel frame towers are designed like other structures, dispatched in sections, and put together at the site. Installations with movable towers must have the main cable held taut by counter- weights, or the guys must be shifted from time to time. The moving of the towers is performed by tackle-blocks and line, the main hoisting engine furnishing the diiving power. If convenient, the tail of the fall used for moving the tower may, be hitched to the hoisting block, and the hoisting drum operated to move the tower. For permanent installations endless chains and separate engines are generally used for moving the towers. The cableway with travelling towers can perform all the functions of the travelling crane without limitations of reach. The travelling carriage of the cableway is a simple frame of steel bars carrying two sheaves running on the main cable, and other sheaves over which the hoisting rope runs. Telpherage systems require carriages with framing on one side of the rope only, so that the carriage may pass the towers supporting the main cable at intervals. The sheaves in the lower part of the carriage perform the same functions as the upper block of a tackle, and the hoisting line or lines may be single and directly attached to the bucket, or make any number of turns about blocks to reduce the strain on the lines. As lubrication is likely to be neglected, the carriage should be provided with hard steel axles and good anti-friction bushings for its sheaves. A few explanatory remarks on the method by which this aerial dumping of loads is effected may not be out of place. The carriage is drawn along the cable by an endless or hauling rope which passes from the carriage over the head tower, and several times round the winch drum on the engine, to secure frictional hold, then back over the head tower to the tail tower, returning to the. rear end of the carriage. I he hoisting rope passes from the engine over the carriage to the large fall block foi laising the load. An auxiliary rope, the dump line, comes from the other side of the same drum of the engine and passes to a smaller block attached to the rear end of the skip. The hoisting rope carries the whole weight of the skip, while the dump line comes in slack but at the same rate of speed. When the bank or other discharging destination is reached, the dump line is shifted to that section of the drum having an incieased diameter, and being thus drawn in at a higher rate of speed, the load is discharged. The most difficult problem of the cableway is the judicious selection of the hoisting rope. The strain upon it, and therefore the sag, varies with the loading or unloading of the bucket. For cableways of short span and for light work a single hoisting line may be led from the drum of the engine around a sheave in the top of the adjacent tower over to the carriage, and down over a sheave to the bucket. Enough permanent weight on the bucket, or bucket tackle-block, is supplied to keep the hoisting rope up off the ground. With this arrangement the hoisting rope must be wound or unwound with the traversing rope to maintain the height of the bucket. 1 he out-haul end of the traversing rope must also support a strain equal to that in the hoisting line in addition to its own initial tension. This strain is generally taken off the traversing rope by