You might get a copy of this screw conveyor document as a start point.

http://www.cemanet.org/publications/...350-2003pv.pdf

In truth, the only people who really know are those who make their living designing, manufacturing and selling screw conveyors. ■

According to the Siwertell Ship Unloader, 90 degrees at least. ■

SIt is true that screws can work at any inclinationup to 90 degrees, but the conveying mechanism changes as the angle of slope approaches the position at which material will not slide down the face of the screw blade by gravity. The conveying capacity falls off increasingly quickly well before this angle is reached so, as a general rule, you would expect a screw conveyor to handle about a third of its horizontal capacity if inclined at 30 degrees and the rate of coveying diminish very rapidly as the angle increased above this for a number of reasons. It is therefore unusual for 'U' trough type screw conveyors to operated at angles steeper than 30 degrees to the horizontal.

Screws in circular casings will operate at steeper angles in one of three modes. One is in a 'flood' mode, where the casing is virtually full, like a screw feeder, so the contents cannot fall back down the casing. This is not a common method because the equipment will cease delivering almost as soon as the infeed stops and the process is mechanically inefficient. The more usual method of steep screw conveying is as a screw elevator, where motion of the bulk is generated in the 'dynamic' mode, the screw rotating at a speed to develop sufficient frictional restraint within the casing to inhibit rotation of the mass and drive material up the face of the screw flight. This well established technique suffers fallback in the flight tip clearance and significant agitation of the bulk being conveyed in the rotational vortex of the bulk moving up the screw. This mechanism also creates a resistance to the infeed of material, so tends to require a very well designed supply hopper and infeed chute system or an assisted feed by virtue of a separate screw feeder offering direct injection to the elevator.

The third, and most recently exploited technique, is the static screw elevator, developed by Olds Engineering in Australia, where the casing rotates and a boundary layer of material rotating with the casing drive the material resting on the screw flight up the spiral helix with no fallback, in a more coherent mode and at higher bulk densities and cross sectional loading, consequently handling higher capacities in more gentle conditions.

A word of caution is merited. Whereas scrrew conveyors will generally work well with most bulk materials and design data is readily available for CEMA and many equipment manufacturers, the design of steep screw elevators is best left to specialists, as there are many trips and traps for the unwary that can prove to be very expensive. ■

Dear Sir,

We have manufactured screw conveyor of 37° inclination 8000mm length for handling DAP powder with a capacity of 25 TPH.

And it is working satisfactorily for last 3 years.

Regards

Rajarshi ■

Individual examples are fine, but when it comes to new applications it is the principles and details that matter. Sucesses tend to be promoted, but everyone keeps quiet about failures and I would reiterate that steeply inclined elevating screws can be made to work at any angle up to 90 degrees, but operate on a different mechanical principle and are not as easy to design as horizontal screw conveyors, the nature of the bulk material being more important and many extra factors to be taken into account. ■

Listen and learn from the wise words of Lyn. ■

A great deal depends on the application.

High speed screw conveyors, often known as augers are frequently used in agricultural applications and can be used vertically. The screw rotates at around 750 rpm and its diameter is usually not more than 20 cm. BUT - they tend to be quite inefficient, and have a rapid rate of wear, particularly of the flight and the thrust bearings. Because of this rapid wear rate, they are normally designed for seasonal use only, not year round industrial use.

A proper industrial screw conveyor is generally restricted to about 35 degrees from the horizontal, varying a little with the moisture content of the grain. ■