You will never get 100% filling as the screw is constantly moving along its length and the screws volume itself will always have a reduced capacity because it is always moving, except in use of a water screw used to elevate water from one point to another.

the auger used in a TBM or even a micro tunneler is usually fully open except at the delivery point and the material is pulled ang the bottom of the trough created by the auger.

Another good example are the dual augers used in an undercutter used to provide stress relief in a mine face during mining in a room and pillar mining sequence.

The bottom auger is referred to as the horizontal bug duster and the auger used to elevate the cuttings is the stacking bug duster auger.

The horizontal bug duster is installed in a tube where the auger tube is entirely open for almost its entire length to facillitate cuttings removal this auger pushes the material to the stacking bugduster where it is elevated to a higher point and dropped to the the side and rear of the cutter bar of the undercutter to allow for stacking and to avoid pulling the cuttings back into the existing kerf/cut.

Another excellent example is a single stage snow blower. The auger is designed with narrow flighting to move material quickly and blow it out of the snow blower spout at the same time.

Any auger is not going to fill to it capacity due to its trying to pull and push the material down as it is moving it forward at the same time while normal traveling along it length which is why it is perfect for moving materials.

lzaharis ■

The reason that a limit is quoted for the cross sectional loading of conveying screws is that, above about 45% cross sectional fill on a horizontal screw, the material will spill over the shaft to fall back into the prior pitch space. In a ‘U’ trough conveyor material at a high sectional loading will also spill on back above the screw rim on the vertical side of the casing wall. This back-spill increases the degree of loading and alters the mechanics of conveying from a 'Gravity' mode to a ‘Flood’ mode form. In ‘Gravity’ mode conveying, the material is moved forward by sliding down the surface of the screw flight blade and it advances an axial distance equal to one flight pitch for each rotation of the shaft. In ‘Flood’ mode, the machine behaves like a feeder and the material is promoted to axial motion by sliding around the inclined face. In this case, the direction of helical movement is related to the angle of contact friction between the material and the blade surface in combination with the relevant helix angle of the flight. As the flight helix angle varies along the flight face, from the rim of the flight to the shaft, the behaviour is more complicated than a conveying screw. The design of feeder screws to provide progressive extraction along the length of a hopper outlet slot should therefore take account of the wall friction of the bulk material. For a more detailed analysis, see ‘Entrainment patterns of screw hopper dischargers’ by L.Bates, ASME Jrnl. of Eng. For Ind. May 1969, pp 295 – 302. The reader will also find that literature on feeder screws is relatively sparse. They should not be designed on the basis of data relating to screw conveyors. For further information see ‘Guide to the Design, Selection and Application of Screw Feeders’ by Lyn Bates, published by The British Materials Handling Board. Contact Lyn at email given below for details.

With an inclined screw, the conveying capacity falls off progressively with shaft inclination, because to surface repose angle remains constant with the horizontal while the helix angle of the flight tilts back with the shaft inclination to become more shallow. This is where flight friction starts to become important as the bulk material has to slip on the flight face and the portion near the shaft rapidly becomes too shallow for slip to occur. A crude rule of thumb is that a standard pitch flight at 30 degrees inclination will only move about a third of its horizontal capacity. Shorter pitch flights can be used, but these move less anyway and offer only limited redress. However, in a flooded mode the screw will work at any angle, but not self-clear of product when the in-feed stops, even if the screw continues turning.

This can partly be countered by running the machine above a critical speed the amount conveyed exceeds the fall-back down the casing. Screw Elevators work in this dynamic mode, but their design is usually in the domain of specialists because of the added complexity of securing a reliable in-feed against the centrifugal force exerted by the prior contents at the inlet. A new innovation is the patented ‘Static Screw Elevator’, made by Olds Engineering and Ajax Equipment. With this the screw is stationary and the casing rotates, a combination that has many operating advantages, one of which is that the boundary layer in the casing is held static, so the capacity is far greater than a normal elevator because there is no leakage. The material is also moved much more gently, as it is driven up the spiral face by frictional drag, rather than whirled around as a highly agitated vortex.

Ribbon screws are another animal altogether and actually move more than full bladed screws as feeders. There proportions and shape, with varients such as 'Lynflow' ribbons, are uved for andling sticky and damp products, but expert advice is very desirable on their specification. Likewise, paddle blades and varients have useful, special features that can be exploited by experienced designers.

I hope that this brings a little clarity to the subject of handling by screws, which are quite often undertaken on a crude basis, some of which work well but more than a few are expensive failures. The moral is, buy cheap – take risks.

Various publications are available from lyn@ajax.co.uk on this and related subjects. ■