I still don't understand why it's better to rotate the outside pipe than rotate the inside screw...,(reply>), You'd achieve the same effect as without the cylinder, i.e. it would look the same as if the cylinder weren't there.<br>The cylinder is pulling material in from the base of a chamber at a massive rate, whereas the screw can only affect the particles it touches at the base of the spiral.<br>By keeping the screw still, the rotation of the cylinder is encouraged inside, scooping the sand up and around the interior of the cylinder inverse to its outside rotation (sorry, it's hard to explain elegantly). <br>It also prevents two tides of sand from either entrance being smashed together into a massive cloud up the centre of the cylinder.