The Mechanical Handling and Storing of Material

ELEVATORS FOR LARGE OBJECTS 33 work. Guides are also desirable, though sometimes omitted. The packages are generally loaded on the ascending side; but they can be removed from either the ascending or descending trays as may be most convenient. If the packages are not removed at all no harm is done to them, as they simply go round the circuit again. This is a feature of some importance. Obviously, if the carriers were rigidly attached to the chains the package would be thrown off in passing over the top shaft, but as they are freely pivoted to the chains and the centre of gravity is kept well below the point of suspension, the constant force of gravity preserves the stability of the suspended carriers when passing between the top wheels. The carrying capacity of a swing tray elevator is very high. Thus, if the trays are placed 5 ft. apart, and the chain speed is only 40 ft. per minute, the rate of working will be 8 packages per minute, or 480 per hour, which can sometimes be increased to twice that number by placing two packages on each tray. Should this rate of working be too great for the particular service required, then the distance apart of the carriers can be increased or the speed of the chain reduced. 'Fhe theoretical carrying capacity can be multiplied indefinitely by placing the swing trays closer together and running the chain faster; but the practical limit is fixed by the quickness of the persons feeding and unloading the elevator. This limit is reached when the trays are about 4 ft. apart and the chain speed is about 60 ft. per minute, thus giving a time interval of 4 seconds and a capacity of 15 trays per minute, or 900 per hour. Naturally the actual rate of working depends a good deal on the size and weight of the packages, the best results being obtained when handling small packages of nearly uniform size and weight. Skips full of cotton cops or bobbins weighing 90 lb., however, can be handled quite successfully at the rate of three a minute, even by young lads, when using suitable loading tables ; from which the skips are slid on to the slowly ascending swing trays without being lifted bodily, and slid off the trays at the various unloading points. The power required to drive a swing tray elevator is surprisingly small. A motor of 1 H.P. is often sufficient, and a large elevator serving seven floors is easily driven by a 5 H.P. motor when handling a steady stream of packages. This economy of power results from the elevator being perfectly balanced, coupled with the slow speed of the main shafts, and the continuous uniform motion of the chains and carriers. Swing tray elevators are preferably driven at the top and the tension screws placed at the bottom. Then, if the chains are allowed to become slack, their own weight will maintain contact with the top wheels, and the load on the shaft bearings is a minimum. Sometimes these elevators are driven at the bottom, when power is not available on the top floor; but the load on the shaft bearings is thereby increased, and the chain must be kept taut all round the circuit. It is specially important that the chains should be of ample strength and bearing surface, and provided with suitable attachments for the reception of the pivoted carriers or swing trays. For heavy elevators the Gray pin chain is suitable, while for light duty the Ewart orpinless type of chain is most commonly employed. Where floor space is restricted and existing machinery is to be cleared, it is some- times necessary to crank an elevator to get it in the space available, as in the example shown in Fig. 32. This machine lifts and lowers tall cans full of hemp sliver at the rope factory of Messrs Hawkins & Tipson, Blackwall, and was built by the Chain Belt Engineering Co., Derby. It effectively solved a rather difficult problem. Evidently a cage hoist was inapplicable. 3