Go off vertical to 45 degrees or less and you will see a large increase in efficiency.

Vertical screws are like black magic, only a select few know how to make them work. I have only ever seen them used on dry free flowing materials.

If the material is not sticking to the flights & pipe, sometimes welding or bolting longitudinal keys inside the trough in the gap to the screw flighting stops the material from rotating and lets the flighting push it along.

Let us know if it works. ■

No dimensions or constructional details are given, so it is difficult to comment on the specific equipment. However, the material to be handled has to flow by gravity to the inlet of a screw elevator and exert a pressure to overcome the rejection force induced by rotation, even at a low speed. Both the mix you describe and the final mass to be transported are not particularly free flowing, so I would expect it to perform poorly at a steep inclination. JD is right to say that it has far better prospects at 45 degrees, for various reasons.

So far as the material that is captured by the screw and is rotating to slip as a coherent plug, a behaviour usually referred to as 'logging' in a full horizontal screw, this is (obviously) because the restraining forces are inadequate to 'grip' the surface against the forces holding the product to the screw surface. At relatively low speed of rotation there is little radial force induced by centripetal acceleration and, with poor flow materials there is little transverse force exerted by virtue of the principle stress ratio. At a low fill ratio, determined by the efficiency of the infeed that is expected to be poor, the surface area of casing contact will also be low, whilst the coverage of the flight surface is virtuall constant, so the balance of forces depends more sensitively on the relative surface friction between flight and casing surface, with the central shaft and corner between the flight and shaft offering additional drag. Your reference to using sand on the casing did notmake clear whether the sand was stuck on the casing surface, or just introduced to the material being conveyed, but a rough casing and smooth screw surface should help. One should also make sure that the flight surface is free from weld irregularities, disjointed butt connections between flights and that it is as smooth as practical. Surragate materials for testing should be compared on surface friction values, as well as bulk strength.

Another thing to bear in mind in dealing with damp products is that surface tension can be a significant factor, but that a surface film tends to act as a Bingham layer and offer very low resistance to slip, once relative movement is initiated.

I would also summise that the construction utilises 'standard' screw fllights with pitch equal to the flight diameter, which offers a coarse blade angle to the elevating requirement. As this is manually operated test equipment, nother probability is that the screw is relatively small in diameter, say no more than 100 mm diameter, which also mitigates against a decent size of infeed for flow and has a low ratio of volume to surface area, so gravity forces are low, whilst molecular and surface tension forces are constant. Send a drawing of the construction to lyn@ajax.co.uk, and I will see if I can make any further suggestions. ■