The slope beyond which the material on a conveyor tends to roll downhill. The maximum slope on which a conveyor can operate depends on (1) the material carried, (2) the loading or feeding efficiency, (3) the size and type of belt, and (4) the environment. In general, in the case of run-of-mine coal and ore, belt conveyors can operate up to about 18 degrees . If the material conveyed contains large lumps, spillage may result if the belt is too narrow.