A trommel, also called a rotary screen, is used to separate materials. As the barrel spins, smaller materials are sifted through the screen while the bigger stuff stays in the middle.
This fellow on YouTube needed a way to separate rocks from dirt. Using “a 1/4 HP motor [purchased] on Craigslist for $25, some rusty bike rims, cheap 2×4’s and automotive belts,” he built this in a single day:
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