ANGLE OF REPOSE

The angle of repose is a fundamental property of a bulk material that quantifies its internal friction. It is the maximum stable slope angle of a heap of that material, measured from horizontal, when the heap is undisturbed. A pile of dry sand sits at roughly 34°; crushed limestone at 38°; coal at 27–45° depending on moisture; dry wheat grain at 25–28°. Cohesive materials like wet clay or moist fines can support angles of 50° or more; very free-flowing materials like polished plastic pellets settle below 20°.

In conveyor design, the angle of repose is the starting reference for two other angles. The surcharge angle on a moving belt is typically 5–15° lower than the angle of repose because the belt's vibration agitates the material; this surcharge angle is what determines belt capacity. The maximum conveyable incline is also derived from the angle of repose: a smooth-cover belt cannot carry material up an incline steeper than about (repose − 5°) before significant rollback occurs; chevron belts extend this limit, but the underlying material property is still the angle of repose.

Engineers should distinguish the angle of repose (static) from the wall friction angle (material on a chute or skirtboard surface) and the internal friction angle (used in silo and hopper design). The Bulk Material Properties reference on this site lists angle-of-repose values for 40+ commonly conveyed materials, along with densities and abrasiveness ratings.