Most conveyor roller guides are written for warehouse lines, where the roller only has to turn. A quarry, cement or port belt is a different problem: the roller must survive load, dust, water and misalignment at once. This guide works through the three decisions that set service life. First, how trough angle and roller type interact, since a steeper trough raises the load share on the centre roller and changes edge strain, spillage and tracking. Second, how roller diameter and spacing combine with belt tension and material weight to hold sag inside limits, with a spacing criterion you can recompute and two worked examples. Third, how seal grade, not bearing brand, decides whether a roller survives a wet or dusty environment. A later section explains L10 bearing life and the load conversion from belt weight, material load and roller spacing. It closes with a field diagnosis table for seized rollers, flat-spotted shells, uneven wear, water ingress and abnormal noise.