Are inclined screw blades for vertical grain augers advantageous?

Abstract

Due to modern technology, screw blades are often manufactured by rolling them out of one single strip of steel. When simultaneously some blade inclination is applied, less residual stresses and/or larger possible ratios between outer and shaft diameter are claimed by some manufacturers, which seems plausible. However, it is sometimes also claimed that the efficiency of the conveyors is increased by such an inclined blade. The underlying idea is that the blade will cause extra pressure between the material to be conveyed and the tube of the conveyor. This in turn would delay the angular speed of the material, thus resulting in a steeper upward motion. In order to investigate this phenomenon, previous investigations based upon the conveyance of a single point mass have been modified for an inclined screw blade. It appeared that such a blade has no significant advantages over a normal one, which was confirmed by a series of tests. For completeness, tests were also carried out with a leading screw, resulting in worse results. In the case that the pitch equals the diameter and for friction coefficients of 0.3 at both screw blade and tube wall, an increase of no more than 5% in capacity will occur, even for blade inclination angles up to 30°. At the same time, however, the efficiency can drop by 15%.