Serviced one of their machines at Boral, (making compressed fibre sheet).One point they don"t say about what"s the advantage of turning the outer tube VS turning an inner screw,is you don't need the fixed screw to be the full length of the tube,once material has travelled up a couple of metres of "screw",it keeps pushing up ,by the new material been fed in through the base.Where as a turning screw system needs the screw to be full length to carry the material up and out.,(reply>), Wow, I didn't realize that - makes sense!  Thank you for replying.,(reply>), Thank you, I was wondering about that!,(reply>), @Shadowmech88 Because the "relative motion" explanation is every so slightly incomplete.  Sure, in physics it doesn't matter whether you turn the screw or the cylinder, but that is only true if you're only considering the screw and cylinder in your system.  There is one other component, the grain, specifically the grain outside the cylinder that isn't being lifted yet.  If you turn the screw the outside grain is rotating relative to the screw and stationary relative to the cylinder, if you turn the cylinder the opposite is true.  While you turn the cylinder, grain is being shoved into openings at the bottom of the cylinder creating "pressure" that pushes on the grain being lifted inside the shaft(this is a case of granulated materials acting kinda like a liquid and kinda like a solid), this same pressure generating action doesn't occur if you turnt he screw.  If the screw were fixed to the bottom plate however, and both turned in unison, it should theoretically do the same, but that would take alot more energy to rotate.,(reply>), Other than where the screw could be mounted. This was my thinking as well. A big advantage and where the biggest difference comes from is how the material is sucked/grabbed from container.<br><br>Technically, we might be able to close the gap by having the immobile tube in a turning inner screw system have the same scoops but to also have flared paddles where the inner tube emerges from the outer tube to push material against the scoop.,(reply>), A partial length screw would be like an impeller in a pump. They work, at least with liquids, and gases. Many real world examples. <br><br>My theory is that a partial screw will work as long as the portion without a screw isn’t too long. I think it’s a matter of friction of the material being moved and the inside of the cylinder, like others above has pointed out.<br><br>If it was frictionless (theoretical ideal case) it would work for any length. If it has friction (real world case), at some point it will become too long, too much friction to overcome , and not work.