You mentioned that the sole job of a plow is to prevent large debris from entering between the belt and pulley. I agree that this is an important duty of the plow, but I also suggest that the plow should be used to prevent fines (wet or dry) from entering between the pulley and belt. Fines can create two problems -- that of uneven build-up on the pulley, which creates a belt training problem; and that of gradual build-up and hardening which acts the same as a large stone between the pulley and belt, which can do damage to the belt carcass.
This problem exists with materials such as gypsum, cement, salt, fertilizer, and others. I've also seen problems of this nature with belts carrying limestone.
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