Origins of the Screw Conveyor
In order to understand the screw conveyor we must first look at its origins. The screw pump was invented by Archimedes around 250bc and it remains the oldest pump design in history. The screw pump is often referred to as the 'Archimedes pump'. The screw conveyor has slightly different design properties, but the overall concept remains the same.
Operating Principle of a Screw Conveyor
Materials (liquid or solid state) are loaded into the conveyor and pushed along by a helical screw blade ('flighting') mounted onto a rotating shaft. Materials remain entrained between the screw blade and casing until the point of discharge is reached. The point of discharge is often (but not always!) a hole into which the material is dropped (storage area or silo etc.), or, onto another conveyor. Most screw conveyors are driven by electric motors although the speed does not have to be constant in order for the conveyor to work.
A typical screw conveyor will consist of a helical screw blade, bearings for support at either end and maybe at various places along the shaft depending upon the total shaft length, gaskets for sealing the case, inspection covers, a drive motor and a drive gearbox. The design is relatively simple and easy to reproduce. The technological advances have now allowed for motor speeds to be regulated which sometimes negates the need for a gearbox.
The applications of screw conveyors are numerous due to their versatility. Screw conveyors can be installed either horizontally or inclined and can be used for solids or liquids. Typical applications include grain, soy bean, wood chippings, water, sewage sludge and many many more. The screw conveyor is robust as it does not require small clearances in order to operate. Damage to one blade would increase the loading on the next blade and possibly cause an overflow, but a total failure is unlikely. Construction materials are chosen in order to reduce the risk of erosion and corrosion, but repairing of screw conveyors is relatively easy and is often achieved by welding additional material onto the screw blades rather than replacing the helical shaft entirely.
It is possible to regulate the discharge rate from the conveyor by adjusting the frequency the drive motor receives (variable speed drive (VSD) motors). The easy regulation of speed and robust design make this conveyor very attractive to many industry processes.
Capacity Limiting Factors
The angle of inclination is a capacity limiting factor; the capacity falls-off sharply as the angle is increased. Other design factors that limit the screw conveyor capacity include the rotational speed, clearance between the blades and casing, screw pitch, overall dimensions of the unit and type of material conveyed. The particle size of the material conveyed is important as it dictates how much volumetric space between the blades is utilized for conveying e.g. sand will fill the whole possible volumetric space whilst large lumps of coal will not.