The Mechanical Handling and Storing of Material

428 THE MECHANICAL HANDLING OF MATERIAL that the velocity with which the load can be moved is greater than when a travelling cabin is used and a heavier load has to be handled; tbe speed being from 12 to 14 ft. per second, as against 5 to 6 ft. per second in the other type. American transporters generally run at higher speeds than those usual in Europe : - _____ •=_____ - ■ jgr- - --- j---- too great a speed is, however, not advisable, on account of the strain caused to the structure by the stopping and starting of the running loads. The principal difficulty with bridge cranes with fixed winches is the complicated leading of the rope or ropes, and the conse- quent great wear and tear on the ropes caused either by the ropes ... being bent in two directions, or by their being led over small sheaves. The use of chains would obviate this difficulty; but these are unsuitable, not only on ac- count of their weight, but also because they lack the elasticity of wire ropes, and are therefore more likely to break under the strain caused by the sudden stopping and starting, and also by the shocks occasioned by the use of grabs at the loading end of the apparatus. It is probably on this account that some firms prefer to fit their transporters with ordinary skips instead of with mechanical grabs. With the latter the wear and tear on the ropes is certainly greater. Instances have been known where wire ropes have had to be replaced after working only a few weeks. If grabs are used it is best to use a one-rope grab, because a two-rope grab will complicate the handling of the plant. Another difficulty in the use of transporters with stationary winches arises from the fact that the man manipulating the winch cannot control the operations so well as if accompanying the load. It is, therefore, sometimes necessary to employ a signalman ..