The vortex motion described in a steeply inclined screw elevator is formed by the circular continuity of the material being transported. The motion results from a dynamic reaction to the inclination of the flight contact surface and, whereas the frictional resistance of the casing influences the mechanical efficiency of the operation, it does not influence the direction of motion. In contrast to a horizontal screw conveyor where there is normally a dead residue layer in the clearance between the underside of the screw and the casing, a steep elevarot has a boundary layer that is prone to back leakage.

In the case of a hrizontal screw conveyor the cross sectional loading is normally not such as to have much spilling over the centre shaft and conveying is advanced for the bulk as one pitch length per revolution. There is a helical movement of the screw contents in a screw feeder according to the combined angle of flight helix and surface contact friction angle. Iwould not normally describe this motion as a vortex because it applies to a relatively dense condition of the bulk material, whereas a vortex is usually thought of a a 'dilate swirling'.

On the point about friction, it is possible for internal friction to exceed unity because this term relates to the 'effective' resistance of the material to shear; think of interlocking particles that cannot reorient easily or laminated flakes. I believe that racing tyres can have an effective contact friction coefficient greater than one, perhaps on a rough surface where the rubber itself has to shear. ■