A screw conveyor is made of helical flight pieces that are fixed to a pipe or a solid shaft and then placed inside a U shaped or tubular trough. As the screw rotates, material piles up in front of the flats and then the material is pushed through the trough. Remaining particles of the material are carried forward with the next flat. Particles in the heap, adjacent to the flight surface, are then carried part way up the flight surface and then flow down on the forward-moving side of the heap. The screw is rotated by a motor and may be cooled by water if the material that is being handled is hot.
Screw conveyors are generally used to move bulk materials and a wide variety of solid particles such as sand and gravel, lumpy materials, cement, wood chips, sludge, food grains, etc. However, sticky, lumpy and fibrous items may sometimes create problems in a screw conveyor. The motor should be able to handle the extra load when the material jams up. The trough should have openings so that any jammed material can be easily cleaned and periodic maintenance can be performed. Shaft less screw conveyors with replaceable synthetic trough liners are also available for damp and sticky materials such as sludge cake etc.
Screw conveyors are popular conveyor systems and are used in various types of industries such as food processing, forestry, power generation, pulp and paper, recycling and waste treatment.
Most screw conveyors are made of stainless steel, mild steel or a combination of the two. It usually depends on the type of material the conveyor will be transporting. Screw conveyors can also be equipped with bearings with mechanical seals for transporting wet or fatty items. The seals will ensure there are no leaks and will also cut down on costly maintenance.
Screw conveyors are available in different lengths and widths. While selecting the screw, the following factors should be considered:
- bulk density of the material
- speed of the conveyor
- the maximum load supported by the trough
- load capacity
- the maximum load per foot of the conveyor
- volume of material being transported per minute
- mass flow of material per minute