The Mechanical Handling and Storing of Material

512 THE MECHANICAL HANDLING OF MATERIAL progress being entirely automatic he does not accompany it. It runs down the track, discharges its load at any predetermined point, and then returns empty to the receiving- terminal. The time taken in making a trip of 300 ft., dumping the load, and returning to the starting point, is about fifty seconds. As the loaded truck descends it gains in speed, and as it approaches the end of its trip it raises a weight by means of a wire cable and tripper with which it comes in contact. At this point the load is automatically discharged, when the inertia stored in the aforesaid weight is given up again to the truck to afford it a sufficient start, so that its own momentum will carry it back again to the starting point. It is important that the raising of the weight should form a gradual movement, in order to minimise to the utmost any stress on the various parts of the mechanism. Fig. 719 shows the details of such a plant at work at the Avonbank Electricity Works, Bristol, and erected by Messrs Babcock & Wilcox, Ltd. A general drawing of this installation will be found facing page 640. The ordinary method of discharging these trucks is by opening the sides by means Fig. 717. Western Dump Car (Back View). of a tripping block placed on the track, to let the load out on both sides.1 The floor of the truck is A-shaped in order to run the material'clean out (see Figs. 720 and 721). The sides are hinged, but the lower ends are fastened, not to the truck, but to each other; thus if one is unfastened, both are open. The load is always evenly discharged, and there is practically no risk of the truck overturning, though the gauge is a very narrow one, being only 21 in. from outside to outside of rail heads. These cars are built of wood, and lined with sheet steel, and are provided with self-lubricating bearings, rubber springs, and steel axles. The bearings are of a peculiar construction, and are so arranged that the car will run round a curve of 30 ft. radius with practically the same ease as on a straight line. The steel wire rope which raises the weight is detached from the truck, except during the time that the car is raising the weight, and receiving the impulse to return. Curved tracks are admissible, but should be as near the feeding terminal as possible ; they require special rails to suit the flexible running gear used on these cars. All material received over the railway can be weighed by placing platform scales on the track at the loading end. The attendant who loads the trucks can also, at the same time, weigh the loads, and enter the weight in the tally book as the trucks are running down 1 For similar self-unloaders see Chapter XXIII. on Rope Haulage, pages 255 and 258.