The Mechanical Handling and Storing of Material

BAND CONVEYORS 77 the conveyor is not too long, there is no objection to this course; but if throw-off carriages of the swivelling type are used, as in the case where the material has to flow on the band past the throw-off carriage, it becomes necessary to have tightening gears with weighted pulleys. As to the allowance to be made for tightening belts, that must greatly depend on the quality of the band. A good conveying band, with a breaking strain of say 400 lb. per ply per inch width# requires an allowance for tightening of 4 per cent. This Fig. 102. Mode of Tightening Band Conveyor at Terminal. would indicate that with a band of good quality even of great length the screw tightening gear can be employed if otherwise suitable. Where permissible, belt conveyors should always be erected sufficiently high above the floor, so that any accumulation of material which may be spilled, or, in the case of fine powders, that may be carried back by the return strand, cannot interfere with the rollers Fig. 103. Mode of Tightening Band Conveyor on any Part of Return Strand. by coming into contact with them. A sufficient space under the return half of the belt also makes it more convenient to clear up any such accumulations at intervals. To withdraw the feed from a band conveyor at any intermediate point, a throw-off carriage is employed, and the credit for the invention of this device is largely due to the Fig. 104. Diagrammatic View of Throw-off Carriage. experiments of Mr Lyster. To remove the grain he made trials with several contrivances, including air blast and brush devices driven from the band itself. Both methods, however, gave but indifferent success. In both cases a most objectionable amount of dust was raised. Moreover, the friction of the brushes on the band proved in time injurious to the latter. The idea then occurred of diverting the stream of grain by means of an upward deflection of the carrying band, thus casting the grain clear of the band and into the air for a short distance, so that it could in falling be caught and led off through a spout on either side as required. The device consisted in principle of two guide pulleys which in most