Angle of Repose and Handling Dry Bulk Solids

Dry bulk solids present special challenges for transporting. Granulated and pelleted materials have attributes similar to liquids in that they flow at a certain rate. Flowability of a bulk material depends on many factors including the density of the material, size of granules and pellets, and the surface texture of the material itself. When choosing a conveyor belt for your material, these things will be important factors.

Angle of repose is the angle above a horizontal plane at which a material will naturally rest. When agitated by a rumbling belt conveyor, for example, this angle is significantly but predictably reduced. The dynamic angle of repose, or angle of surcharge, plays a large part in the transporting of dry bulk materials. The size and structure of conveyor belt needed to move the product efficiently is largely based on this angle. Free flowing materials require different conveyor belt specifications than sluggish ones. Knowing the characteristics of the material will go a long way to identifying the conveyor belt required to comply with the angle.

Size of Individual Chunks
The size of the individual pieces of the material and whether they are uniform has a great effect on the angle of repose. If a product is small, it will move more freely and have a smaller angle of repose. It will tend to flatten itself out and fill any vessel it's placed in, including sections of a conveyor belt. Larger and irregularly shaped pieces such as bituminous coal or wood chips will have a much higher angle of repose. These materials will not take the shape of a pocket as readily due to decreased flowability, leading to piles stacking at higher angles. Materials like this will require special considerations for belting structure. On a flat surface, a trough belt may be ideal. But on an incline, a CamFlex belt with flexible sidewall and extra tall nubs may be needed.

Speed and Angle of the Conveyor
When transporting materials on incline belt conveyors, speed will be an important factor. If the product is moving slowly up a 10 percent incline, it will be much more stable than one moving quickly up a 30 percent rise. Higher speeds create vibration and greater acceleration at certain points and bring the angle of surcharge into play, meaning the angle at which the product piles is greatly reduced. This, in turn, has a profound effect on the load capacity of the material to be transported. If the applicable angle of repose is less that the angle of the conveyor, you'll need a belt with sufficient cleats, nubs, and sidewall to hold the material in place. Knowing the difference between static and dynamic angle of repose could potentially save both time and money.

Texture of Material (friction between pieces)
The type of material being conveyed goes hand in hand with the speed it will be transported at. If a material is slick or polished, such as sand, it will flow easily, whereas rough objects like rough stone will stay put. If slick objects are stacked in a large pile, they quickly spread out to a flatter state. This has implications for the conveyor belt you choose. At shallower angles, nubs will hold the material in place just fine. But for higher angles, sidewalls and wide horizontal cleats or buckets of the CamWall belt will be needed to prevent the material from flowing back down.

Density of the Material
Density directly affects the angle of repose. Heavier pieces create more surface friction with each other, leading to a more stable pile of material. Even large objects, such as polished stones, can move easily with less friction to hold them in place. Identifying the density of any material to be moved is an important step in knowing how to best transport the product.

Angles of repose and surcharge must be calculated correctly to design a system to efficiently move bulk materials and achieve the highest level of productivity. Cambelt has worked with hundreds of different materials and has a large collection of data on the characteristics of bulk materials to help you design your belt conveyor system correctly. Getting the conveyor belt right for the bulk solids you are moving will prevent inefficiency and increase profit.

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