Page:Popular Science Monthly Volume 62.djvu/75

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full height of the tramway or by an automatic deflection of their motion and with no appreciable loss of speed the buckets may be caused to descend and discharge their contents at any point below the tramway.

The bridge tramways are usually built in plants or groups of three or four bridges which may be supported on either single or double piers. The tramways are, as a rule, provided with hinged aprons designed to extend over the vessels and very frequently cantilever extensions are provided at the opposite end, so that the buckets are enabled to serve a space of 300 to 350 feet in width. The operation of the tub or bucket is effected by means of a wire cable connected with a drum in the engine room, and the engine is usually of the double cylinder type. The piers supporting a bridge tramway are on wheels, and it is thus possible to skew or move sideways the entire structure, in order to bring the bridges in line with the hatches of the vessel being unloaded. This type of machine hoists the bucket of ore from the hold of the vessel, conveys it to any desirable point on the tramway and automatically dumps the material on the dock or into waiting railroad cars.

As indicating the capacity of the bridge tramway, it may be cited that a plant of three bridges will readily handle 1,200 tons of ore in a day of ten working hours, hoisting the material, conveying it a distance of 100 to 150 feet and dumping automatically. In the case of the bridges of exceptional length a round trip can be made from the PSM V62 D075 Clamshell bucket of automatic unloader.pngClam-shell Bucket of Automatic Unloader. hold of the vessel to the extreme end of the cantilever and back again, a distance of 600 feet in one minute, and in actual operations a rate of 45 seconds per trip has been maintained for hours at a time. The buckets or tubs for conveying ore are usually of one ton or a ton and a half capacity. There are several modifications of the bridge tramway system, notably cable tramways in which wire cables are substituted for the bridges and what are technically known as 'fast plants' wherein instead of the long bridges there are extremely short ones with tramway projections over the vessel and cantilever extensions over the railroad tracks on the dock, the effect of this short haul being to reduce tremen-