6.7 SCREW CONVEYORS 6.7.1 Definition, Characteristics and Use A screw conveyor consists of a continuous or interrupted helical screw fastened to a shaft which is rotated in a U-shaped tough to push fine grained bulk material through the trough. The bulk material slides along the trough by the same principle a nut prevented from rotating would move in a rotating screw. The load is prevented from rotating with screw by the weight of the material and by the friction of the material against the wa of the trough is suitable for any pulverized or granular non viscous material, and even at high A temperature: The conveyor is particular Su for mixing or blending more than one materials dur- ing transportation. and also for controlling feed rate of materials in a processing plant Abrasien and consequently certain amount of degradation of the material is unavoidable, hence it is Do suitable for brittle and high abrasive materials. It is also not able for largelumped, packing or sticking material 6.7.2 Descriptive Specifications A typical screw conveyor is shown in Fig. 6.7.1. The screw shaft, if short (up to 5 meters). is supported at two ends. But for longer shafts (upto 40 to 50 m), they are supported by bearing hangers, at intermediate points. The shaft may be solid or hollow. Hollow shafts are lighter and can be easily joined to make a long shaft. The screw shaft is driven at one end, and the design may permit discharge of material from the bottom or one end. Opposite handed screw at two sides will cause the center dis- charge. The U-shaped fabricated trough are generally covered at the top to avoid particulate pollution. The bottom portion of the trough is of circular cross section matching the diameter of the screw. Generally a radial gap of 10 mm to 20 mm is kept between the screw and the trough. depending on size of the Screw