The Mechanical Handling and Storing of Material

76 THE MECHANICAL HANDLING OF MATERIAL that grain could be conveyed in all security on a perfectly level band provided it were properly delivered thereon, z.6., at the same speed as that at which the band was travelling- Band conveyors for grain often run perfectly flat, except at the point or points where grain is fed to them ; but at these points two separate cylindrical curving rolls are applied, which slightly hollow the band for a few feet, and then allow it to lie flat again on its supporting rollers. These endless bands run over terminal pulleys, and are in addition supported on their way by a series of rollers, which are more frequent on the loaded than on the empty strand. Such conveyors are usually fitted with some kind of tightening gear to keep the belt in tension. Supporting rollers are fitted at intervals of 6 ft. to the upper or working strand of the band, and at every 12 ft. on the lower or réturn strand. Sometimes it is desirable to use both strands of the band for conveying purposes, thus making it a double service machine, in which case the supporting rolls for the lower strand must be as close together as for the top. In such a case the terminal pulleys should be larger than usual in order to give the two strands a greater distance from each other, to allow sufficient Fig. 101. Band Conveyor suitable for Conveying on Both Strands. space between the strands to conduct the grain to the lower band, and discharge it again before reaching the other terminal. The two strands can be run any distance apart by using two additional guide pulleys for the terminals, as shown in Big. 101. In such a case one band might run along the top floor of the granary whilst the other travelled along the bottom. Throw-off carriages are indicated on both top and bottom bands. The tightening of a band conveyor is effected in a similar manner to that of tightening elevator bands,1 by means of two screws which push or pull the two pedestals of one terminal pulley further away from the other terminal; but as such an appliance does not admit of great latitude, it is insufficient for very long conveyors. In such cases the tightening gear would consist of a pulley, held in tension by weights, over which the belt passes. This tightening pulley can be placed at any point on the return strand of the belt, but is generally situated at the end opposité to the driving end of the conveyor. Fig. 102 shows a tightening pulley as used at the terminal. In Fig. 103 is illustrated a form of tightening pulley which can be applied to any part of the returning band. It is undoubtedly preferable to use tension screws for tightening the band if it be possible, and in cases where throw-off carriages with fixed pulleys are used, and where 1 See Tightening Gears, page 191.