The Mechanical Handling and Storing of Material

198 THE MECHANICAL HANDLING OF MATERIAL on either hardened pivot bearings or on ball bearings. In this way the gradient of a roller shoot or runway can be re- duced to 2-| to 5 per cent., according to the nature of the packages. The work done by gravity on a box weighing 40 lb. in falling 100 ft. along the incline is the product of the weight, and the total vertical displacement of say 5 ft. If ten boxes per minute are handled, then the power expended will be 10 x 40 lb. x 5 ft. = 2,000 ft.-lb. per minute, or about Tlg- H.P. This little calculation gives an idea of the amount of physical labour to be done by the lads or girls feeding a gravity conveyor in a rather ex- treme case. Though apparently not un- duly great for a short period, it would be too exhausting if kept up continuously for. some hours. For this reason the combination of a roller runway with a power-driven in- clined chain elevator finds frequent appli- cation in bottling factories, an example being given in Fig. 268. This shows a double strand elevator with round cross- bars, which push up the boxes on a series of light steel rollers, the elevator being driven by a small electric motor. After arriving at the top, the boxes complete their journey of 110 ft. by gravity. In the various London establishments of the Direct Supply Company for soda- water siphons, the cases containing the full or empty siphons are handled on roller runways, both on the level and on the incline. Those on the level have, of course, to be Fig. 265. Spiral Gravity Shoot of Large Capacity. UJ