People forget Newton's  Third Law, actions and reactions and all that...<br><br><br>Friction between the stuff and the screw, and the stuff and the casing is what makes a grain elevator work.  If the friction "breaks down" (at any one interface) the stuff just goes round and round.<br><br><br>With low viscosity liquids an (low velocity) Archimedes screw starts becoming ineffective once the angle is such that the liquid is able to "flow" back down the "flights" (&gt;45 degrees or so) .