The Mechanical Handling and Storing of Material

576 THE MECHANICAL HANDLING OF MATERIAL stores and offices, was erected in a convenient central position on the north quay of the inner basin. It is well known that the intermittent working of dock machinery causes excessive peak demands. If generating plant is designed of sufficient capacity to meet these demands it is necessarily large in proportion to the average load, and as a result, in addition to being expensive in first cost, it does not run under the most economical conditions. In order, therefore, to keep the plant within reasonable dimensions and to ensure economical running conditions, it has hitherto been the invariable practice to install a battery of accumulators with a reversible booster plant, in connection with dock installations. The possibility of adopting such a system, which has many good points, was carefully considered for adoption at the Rothesay Dock, but while it was recognised that it serves a very useful purpose under certain conditions of varying demands, such an installation would not adequately meet the more stringent requirements of the case involving as it did the special demands on the coal tips. It was therefore decided to investigate the possibilities of further developing the system, where excessive loads have to be dealt with, by coupling a prime mover, either electrically or steam driven, to a generator in conjunction with a flywheel. It was apparent that if these special coal hoist generators, and a further generator for supplying power for ordinary dock purposes, could be practically constructed in combination with a flywheel it would be very economical, and would dispense with the usual battery of accumulators, the flywheel taking care not only of the larger peaks due to the coal tip, but also of the smaller and more frequent demands for the other appliances. The combination eventually designed consists of a high speed engine of the triple- expansion type, capable of developing about 450 B.H.P. as a normal load, when running at any speed between 320 and 375 revs, per minute. Direct-coupled to the crank- shaft of this engine is a continuous-current main generator, capable of producing current at a constant voltage irrespective of speed and load. This generator, which has a normal output of 340 E.H.P. with a considerable overload capacity, is used for supplying power to the ordinary dock appliances. The end of the main generator shaft remote from the engine is extended to take a special flexible coupling, by means of which it is connected to the flywheel shaft on which is mounted the flywheel and two coal hoist generators. In designing the-plant it was intended that the storage of power in the flywheel should be such that with both tips working with full load and to their full lift, the average demand from the engine should not exceed its normal rated power. By trial it was ascertained that these conditions were fulfilled, and that, even with the main generator loaded to about 63 per cent, of its normal output in addition to the tips working as above, the demand only slightly exceeded the normal output of the engine with a drop in speed of the combination of about 20 per cent. The flywheel is entirely enclosed in a cast-iron case, the object of providing this being that, should the power required to maintain the flywheel and two hoist generators at normal speed prove excessive, the former could be run in a vacuum, but as the power required was only about 20 E.H.P., when running at the maximum speed of 375 revs, per minute, the installation of the necessary gear for producing the vacuum was considered quite necessary. Of course the above power is only consumed when the coal tips are in operation, and under ordinary working con- ditions considerable economy is effected by disconnecting the flywheel by means of the special coupling referred to above, when the engine and main generator run as an ordinary generating plant. The Tips.—The two tips first erected, each of the high level and fixed type, are so arranged that the loaded wagons are run on the tipping platform at the quay level, raised to the required height and tipped, after which the empty wagon is lowered to the high