The Mechanical Handling and Storing of Material

270 THE MECHANICAL HANDLING OF MATERIAL Fig. 391. Obach Coupling. to tighten the eccentric against the rope, Lastly, the operation of coupling and uncoupling should be performed automatically at predetermined points. Under the second heading it should be mentioned that sufficient compression of the rope to obtain the necessary grip is in itself by no means advantageous to the rope,, so that a tendency to bending it would aggravate the evil very considerably. Under heading three comes the consideration of sufficient latitude in the inability of adjustment to allow for the variation in thickness which is often noticed after the rope has been at work for some time; this also includes splices, and the slippery condition of the rope from wet and hoar frost. Appliances which satisfy these con- ditions are in use at the present day, and we will now pursue their development in chronological order. Fig. 391. This coupling, designed by Obach, dates from 1870 and is prob- ably the first device of this kind ever made. It is an application of the prin- ciple of the eccentric and is exceedingly simple. The rope rests on roller b and the eccentric a is turned until it grips the rope which travels in the direction of the arrow (see drawing), and thereby helps nd as the grip on the carrier depends solely on the friction between rope and eccentric it is not very great. The method of coupling or uncoupling is exceedingly simple, lifting lever c. The weak points of this early appliance are that in the event of a temporary slackening of the speed of the rope, the carrier, by reason of its momentum, would proceed faster than the rope and would thereby create a tendency to loosen the coupling, so that if any of the carriers were on an inclined portion of the rope they might run back. To remedy this defect was the first move towards an improvement, and the outcome was a new appliance brought out in 1874 by Bleichert & Otto; this is shown in Fig. 392. The essential feature of this appliance is a reversal of the action of that described previously, that is to say, the eccentric a, or rather an eccentric segment, has its smallest radius towards the rope instead of vice versa, so that in whichever direction the rope or the load may pull, it has a tendency to grip and hold fast. As, however, this arrangement by itself does not give a very large latitude, the turning point b of the eccentric has been made movable at one end of the bracket c, so that by the additional movement of the angle bracket c the relative position of the eccentric to the rope can be changed within larger limits. The grip of this improved coupling is, however, not sufficient for heavy duty, as it being effected by either dropping or Fig. 392. Bleichert-Otto Coupling.