How screw conveyor capacity is calculated
Each turn of the screw pushes the material forward by one pitch. The capacity is the free cross-section of the trough times the pitch, times the speed, reduced by how full the trough runs and by the incline:
Q = 60 · π ÷ 4 · (D² − d²) · S · n · φ · C
- D, d
- screw and shaft diameter in metres
- S
- pitch in metres
- n
- speed in rpm
- φ
- trough loading (fill factor)
- C
- incline factor, 1 for a horizontal screw
Worked example
A 300 mm screw with a 76 mm shaft and full pitch turns at 60 rpm with 30 % trough loading. It moves 21.4 m³/h, which is 25.7 t/h of cement at 1.2 t/m³.
Trough loading by material
| Material type | Examples | Loading |
|---|---|---|
| Free-flowing, non-abrasive | Grain, flour, plastic pellets | 45 % |
| Average, mildly abrasive | Cement, lime, soda ash, coal | 30 % |
| Abrasive or sluggish | Sand, ash, ore, clinker | 15 % |
Incline factor
| Incline | 0° | 5° | 10° | 15° | 20° |
|---|---|---|---|---|---|
| Factor C | 1.0 | 0.9 | 0.8 | 0.7 | 0.65 |
Typical design values. Manufacturers publish their own capacity tables, which take precedence.
Speed limits
Faster screws convey more but wear faster and can damage fragile material. A flight tip speed of about 1 m/s is a sensible upper limit for abrasive materials; free-flowing grain tolerates more. The calculator shows the tip speed so you can check this.