Screw conveyor is a mechanism that uses a rotating helical screw blade, called a "flighting", usually within a tube, to move bulk materials. They are used in many bulk handling industries. Screw conveyors in modern industry are often used horizontally or at a slight incline as an efficient way to move semi-solid materials. They usually consist of a trough or tube containing either a spiral blade coiled around a shaft, driven at one end and held at the other, or a shaft less spiral", driven at one end and free at the other. Screw conveyors can be operated with the flow of material inclined upward. When space allows, this is a very economical method of elevating and conveying. As the angle of inclination increases, the capacity of a given unit rapidly decreases.