It's the same mass static motion that keeps the grains from falling down if the screw is what's moving. Where was the confusion in that? The mechanics remain the same, you've just got a flimsier larger mass as the crank. It'd probably work just as well for gravel or course wood chips, but should fail compared to the screw for fine particles and would wear out much faster. There's a lot of odd inventions, or somewhat obvious ones, that don't see widespread adoption because they're simply not <b>quite</b> as good and are thus butchered by the Pareto Principal.