"How did no-one spot this earlier"     In short: They probably did, but there was no necessity for it or its benefits do not outweigh the low costs of a conventional system. <br><br>I think I can answer this question from my perspective, working in mechanical engineering.<br>Rotating the outside cylinder is mechanically more complex than rotating the screw (e.g. it requires less common large bearings) and if you want a sealed system it needs large rotational seals, which is problematic. Since conventional designs work fine for almost all cases, and are readily available, no-one bothers to innovate unless the job requires its unique properties (e.g. they claim it produces less dust)<br>I see that my colleagues also understand the theory of relative motion fairly well, so I doubt it was never thought of before.<br>Don't get me wrong, I love more variety in technical solutions, it is very interesting and educational.,(reply>), If you go to the Olds USA site you will see a sample of the successful applications that have been found for the Olds Elevator.  The central screw is kept straight by placing it under tension by the way.  It has zero or at least immeasurable product degradation because of the wide edge gap. 1/2" 1" no prob.  No grinding.  Of course a rotating casing was "thought of" before, but it is the simple feed principle that wasn't.,(reply>), Another thing to admire for a second: The simplicity and cost-effectiveness of the modern screw hopper!