Can I assume since you describe it as an "auger" you have a tubular casing rather than a U-trough? ■

Many manufaturers of screw conveyors publish capacity charts for different sizes and speed of screws, but on the information available this application and its criteria are not fully defined. I would make a few points. First to establish whether the machine is to act as a feeder, controlling the rate of handling, or is it a conveyor to transport a feed rate determined by prior equipment. This will determine whether the inlet section should have a reduced pitch construction to regulate the loading along the length of the conveying section. The density figure quoted presumably relates to a loose poured condition, whereas in transit along a screw the cement is likely to be more dilate, according to the speed of rotation of the machine, so the conveyor will have to allow for the volume increase. Then, a 35 ft. long 4" screw with a 1" diameter shaft will require 2 or three intermediate bearings to prevent it deflecting to run on the casing. This inhibits the handling capacity and can be a perennial maintenance problem when handling abrasive products.

Assuming a cross sectional loading of 25% and operating speed of 50 rpm, the machine would handle about 166 cub.ft/hr, which at, say, 75 lbs/cub.ft, gives a capacity of around 5.5 tons/hr, so the target is not unreasonable. The density in transit would be slightly higher at 1 rpm, so the handling rate is not directly proportional to rotational speed, but near enough for rough caculations.

The key questions are therefore - How is the feed rate to be controlled and what detailed design of intermediate bearing and auger joint is to be adopted? The later will influence the performance and reliability of the unit. ■

The 'loading' factor is the percentage of the 'swept volume by the rotaation of the flight that is occupied by the material being conveyed. Material will spill over the centre shaft as the loading approaches 45%, so this is the maximum value recommended for conveying by gravity mode, that is the movement of material forward by sliding down the face of the screw flight. If the loading is increased by a higher infeed rate, considerably more overspill takes place and an increasing proportion of the material tends to rotate along the axis of the screw. When the infeed is flooded, the screw moves the product along the axis in a spiral direction dependent on the combination of the screw helix angle and the angle of contact friction of the material on the face of the screw flight. As the helix angle varies from root to tip of the flight, the efficiency of transport is sensitive to the screw geometry and the material/flight friction.

The discharge of a screw that is lightly loaded tends to be cyclic with the screw rotation, as material pushed forward by the flight face falls away and there is little to follow until the next flight content reaches the outlet. Cohesive material will also tend to fall away in 'chunks', particualrly with slow turning screws, as the bulk will hold together to overhang the outlet intil the internal stresses exceed the cohesive strength. A third type of irregular discharge is that of avalanching by a free flowing product as the movement forward of the base of the repose pile over the outlet releases a layer of surface repose as the lower particles fall away. Specialised suppliers have developed techniques for dealing with these irregularities, which distinguishes the performance of custom built units from machines built form stock items of screw equipment. It is quite easy to build screw type equipment and many applications can be served by standard forms of equipment. However, where performance is important, the cost and value of expertise is usually well justified in seeking the services of a specialist. The cost of equipment not working well can easily exceed the total equipment cost, making the cost margin between a do-it-yourself job and a custom-built design rather insignificant.

My book, 'Guide to the Design, Application and Selection of Screw Feeders' sets the background to the technology cannot transfer the many years of collective experience that is held by specialised equipment providers, but can provide a warning to inexperienced persons of the sensitivity of the equipment to the physical properties of the bulk material that is being handled and the difference in handling characteristes at differing loading levels, etc. ■