I don't really understand how this is counter-intuitive, to me it makes perfect sense : from the point of reference of the dust, moving the screw or moving the casing are one and the same : when the casing move, if you were on it, you'll see the screw moving, the same way we see the Sun moving because we're on Earth even though in reality it's the Earth that rotates and not the Sun. It's the same action, the same force, the only difference is that in one case you're pushing them from the screw to the casing and in the other you're pushing them from the casing to the screw, but because they're going to follow the shape of the screw these are equivalent.<br><br><br>Obviously if it were open on the difference, it would be just turning and not doing anything, but that's also true for the screw.,(reply>), I'd add that the main advantage of having the casing rotate is means the moving part at the intake is lot bigger than it would be with the screw rotating - this alone may be what results in its greater efficiency.,(reply>), There is a difference, the particles have to slide across the screw, so the friction layers are not identical.