The volume of rotating sand would then engage with the stationary screw, stretched under tension from the top to the bottom of the machine, thus keeping it straight and clear of the inside of the tube. Only that sand engaging with the screw would be elevated, leaving a boundary layer rotating with the tube, the thickness of which would be equivalent to the clearance between the outside diameter of the screw and the inside diameter of the tube. This boundary layer would drive the inner core of rising, rotating sand like a long continuous nut up a long thread.