The Mechanical Handling and Storing of Material

312 THE MECHANICAL HANDLING OF MATERIAL tig. 447. Electrically Driven -Bleichert Cable-Crane with Man-Trolley for Lignite Workings. (The operator has the skip always in view, even when lowering it into a pocket.) of either letting down the skip in dumping it, or leaving sufficient height above to allow raising one end enough to dump the contents. To reduce this lost head room as much as possible, the two hoisting lines may be connected directly to the skip instead of carrying them round fall-blocks, and thence to the tail tower■ the speed of hoisting is then the same as that of the hoisting rope, and no head room is lost by tackle-blocks; but the two lines must be wound in or out with the traversing line of the carriage. The result of the combination is that dumping by clamping the dumping line and lowering the front of the skip must be done while the carriage is stationary, and does not work well for dumping into cars, for either the man in charge of the dumping cannot reach the skip to guide it, or the skip is brought down within reach, and fouls the dumped material in the car before clearing itself. The other method of winding in the dumping line at a higher speed can be applied only while the car- riage is running towards the engine, since the dumping line can only be shifted while winding up. In order to dump near the tail tower it would be necessary to stop the carriage, and, if required, lower the load sufficiently to leave head room for the dumping block. If spoil banks are to be raised at both ends of the cableway, both systems must be applied together with all the attendant complica- tions. These may, however, all be avoided, and aerial dumping easily effected at any point on the line with the carriage stationary or moving in either direction by using similar but separate drums for the hoisting and dumping lines. A standard three-drum hoisting engine thus actuates the complete cableway, the drums being oper- ated together in either direction, or one or more as desired may be lowered under the control of the brake. This form of cableway engine is necessary for operating self-closing grabs of all types. As such buckets may be found necessary or convenient to deal with all materials encountered in excavating, the use of the three-drum engine is preferable to the method of holding or winding in one line as described. But little extra care is necessary on the part of the engineman in keeping the skip level when using independent lines, as it is only necessary to keep the dumping line well slacked off, the skip taking its own level as it hangs by its chains from the hoisting rope. A little trouble may be experienced on long spans and with heavy loads from the varying sag of the main cable as the carrier takes up or drops the load. The sagging of