The Mechanical Handling and Storing of Material

43° THE MECHANICAL HANDLING OF MATERIAL of wheels, of friction clutches, and of a third wheel c, which can be coupled between the two larger spur wheels. If both drums revolve in the same direction, they both wind up the rope, and thus elevate the load; whereas, in the other case, one drum winds up as much rope as the other pays out, and the load is therefore conveyed in a horizontal direction. In a scheme such as is illustrated in Fig. 603, the load is supported on an endless rope which is wound round the conveying drum b, and by means of this drum can be drawn either backward or forward. The drum a is for lifting and lowering the load by shortening or lengthening the endless rope which supports it, this being accomplished by a pair of jockey pulleys, as shown in the illustration. The demand for transporters with less complicated winding gear led to the intro- duction of those worked by one rope only, and their action is illustrated in Fig. 604. One end of the rope is attached to the winding drum of the winch and the other to the traveller. Supposing the winch to be standing, the load would descend by its own gravity Fig. 605. and move the running head or traveller in an upward direction; but in order to prevent this, the traveller is secured to the girder upon which it runs by a catch and notches cut in the girder. As soon as the load has been raised to its topmost position, and the fall-block has entered the traveller as shown, it is retained there by a hook, which automatically releases the catches that locked the running head to the girder. On a further movement of the winding drum, the load proceeds in an upward direction, and when empty, the running head returns automatically down its own support. The crane is manipulated as follows : The traveller is pulled by the winch past one of the notches near which the unloading is to take place. The rope is then somewhat slackened to