The Mechanical Handling and Storing of Material

472 THE MECHANICAL HANDLING OE MATERIAL in the enormous top weight which necessitates a wide pontoon to ensure stability. This will be better realised when it is considered that in order to unload large ships with side beam the jib must be very long in order to reach to the side remote from the pontoon with the elevator leg; this necessitates very considerable balance weight. Such a machine also lacks facilities for adjustment; if, for instance, the elevator has to work on the near side of the pontoon it becomes necessary to warp the pontoon off the ship, and these not infrequent readjustments involve a considerable waste of time. The first-mentioned effect of the top weight has been overcome by the construction shown in diagram in Fig. 652. In this case the elevator leg and its jib are balanced by a weight carried comparatively low down on levers pivoted at the centre of the post, these levers being of equal length to the back end of the jib, and the connecting link of such a length that jib and levers are always parallel; by such a parallel motion the apparatus is in balance in any position. As the balance weight is now low down, a narrower pontoon may be employed and still give sufficient stability. The second defect is overcome in a construction shown in the diagram, Fig. 653, which represents the final development of the present marine leg. It will be seen from the diagram that in addition to the first improvement the post itself is pivoted and balanced by a counterweight at its lower end. It will be evident that in such a combination the centre of gravity of the jib and the balance weight, if properly balanced, must coincide with some point on the centre line of the crane post, and if the post be extended downwards the balance weight at the bottom of the post can be made to balance the other two weights and bring the centre of gravity of the whole to the centre of the fulcrum upon which the post rotates. This combined balancing is true, and can be maintained with a fixed load on the elevator. It is, however, upset by the addition of grain, but not to any serious extent. In practice the elevator is allowed to slightly overbalance to allow it to sink into the grain, whilst the balance at the bottom of the post is rather overweighted to counteract any capsizing tendency. The result of this combination of levers is to give an extraordinary range of motion to the elevator leg, and at the same time to allow practically no movement of the centre of gravity of the whole. The elevator can, therefore, be placed upon a turntable and worked either side on or end on with impunity, and when required for towing can be turned right round and the elevator stowed upon the deck or rested upon the weigh-house, as shown in the drawings. I he removal of all capsizing movement also enables a very much smaller barge to be used, thereby reducing the capital cost and taking up less room at the ship’s side. 1 he line drawings Fig. 654,1 illustrates one of two floating elevators built upon this system to the order of Messrs C. J. King & Sons, Bristol, by Messrs Spencer & Co., of Melksham, Wilts. The maximum capacity of these elevators is 120 tons per hour. I he leg, which is telescopic, has a total length extending to 44 ft. and 27 ft. when closed. The endless chain carrying the buckets is so arranged that when the leg is lowered, as much chain is let off in front as is taken up at the back, so that it remains at constant length and even tension. Each elevator is driven by a direct coupled electro-motor of 16 B.H.P. at the head of the elevator, thus doing away with all driving chains. It is carried on the end of a cantilever jib of 30 ft. centres, which is in turn carried on the top of a cantilever post 1 The illustrations are reproduced from The Engineer of 9th July 1909, while the description is from information kindly furnished by Mr A. H. Mitchell.