An inclined screw must deliver material uphill while limiting fallback. Its useful capacity therefore cannot be assumed equal to the horizontal rating of an otherwise similar machine. Select the actual angle, screw geometry and operating speed together, and check both steady flow and restart after material has settled toward the lower end.
For cement, fly ash or dry-mortar ingredients, a tubular screw conveyor can fit an inclined transfer route. The tube does not eliminate fallback or make every powder behave alike. Ignoring the operating configuration can produce a delivery shortfall, excessive recirculation or a loaded-start condition that the drive cannot handle. Conversely, increasing motor size alone does not create additional conveying volume.
The mechanics of inclined conveying and efficiency loss
In a horizontal screw, gravity influences where material lies in the cross-section while contact with the rotating flight and housing produces transport. Raising the discharge introduces a component of gravity opposing upward movement. Some material may move back relative to the flights, so geometric displacement per revolution is not the same as delivered volume. Material already slips or circulates in horizontal service; zero inclination is not proof of perfect volumetric efficiency.
The KWS guide discusses angle bands up to 10°, 10–20°, 20–30° and 30–45°, with increasing attention to fallback, geometry and power. These are that guide’s design categories, not a universal correction curve for every conveyor. A 15° or 30° installation still needs data for its powder and configuration. Two machines at the same angle can differ because of pitch, clearance, inlet condition, speed, moisture and retained air.
KWS treats inclines above 45° in its vertical-screw design guidance. This classification does not mean every screw physically stops conveying at 45°, nor does it establish VOGO’s permitted angle. Steeper and vertical systems need an appropriate feed arrangement and a specific design basis. Choose the lowest practical incline when comparing layouts, then confirm whether the space saved by a steeper route justifies the associated equipment and operating demands.
Capacity calculations and compensation strategies
A preliminary geometric model for a shafted screw is:
Qv = 60 × (π/4) × (D² − d²) × P × n × φ
Qm = ρ × Qv
D and d are screw and centre-pipe outside diameters in m, P is pitch in m, n is screw speed in rpm, and φ is assumed occupancy of the available annular area. Qv is m³/h, ρ is operating bulk density in t/m³ and Qm is t/h. This model alone does not calculate inclined delivery. Use a supplier’s consistent, validated method for actual slip, geometry and inclination, without counting a correction twice. See the capacity calculation guide.
For a purely hypothetical delivered-capacity factor C = 0.75, a 20 m³/h requirement would need a base selection capacity of 20/0.75 ≈ 26.7 m³/h. This is an arithmetic example, not a recommended factor at any angle. Capacity loss must not automatically be represented as a lower physical fill: a poorly conveying section can contain more material while delivering less.
Compare the following options against verified performance:
- Speed: a change can increase delivery within a validated range, but can also increase agitation, wear or air entrainment. Check current, temperature and product behaviour.
- Pitch: shorter pitch may improve retention on an incline while reducing geometric volume per revolution. Evaluate the combined result; do not assume shortening always increases output.
- Diameter: more available volume may permit the required rate at an acceptable speed. Check lump passage, shaft geometry, supports and the meaning of a catalogue diameter.
- Drive: lifting, friction and recirculation affect running power; settled material and head load affect restart torque. Verify the complete drive train, not just motor kilowatts.
If no suitable configuration meets the duty, reconsider the route or conveying method. A steeper installation should not be created by tilting a horizontal selection without further verification.
Material characteristics and project configuration
Do not use 0.5–1.0 t/m³ as a common density range for cement, fly ash and every dry-mortar powder. Obtain the actual operating range, including aerated and settled states. Moisture and storage can alter cohesion; a material described as free flowing can still flush when aerated or consolidate during a stop. Test conditions should represent the inlet state, not just a convenient laboratory sample.
An enclosed tubular housing can help control the path and contain dust when its connections and seals are suitable. It does not inherently stop powder bypass or preserve a pressure head that guarantees uphill movement. Any operating pressure needs a separately verified limit for the tube, flanges, covers and shaft seals. Air management and downstream pressure are part of the system assessment.
Confirm required mass and volume flow, centreline length, elevation, inclination, inlet and outlet geometry, speed range and start/stop sequence. Direct withdrawal from a flooded silo outlet is feeder duty and requires the inlet extraction profile and head load to be considered. Hanger and end bearings, shaft deflection, axial loads and supports must suit the selected layout. A rigid-looking tube does not prove that working clearances remain adequate under load and temperature.
During commissioning, verify the agreed operating cases: stable feed, expected density variation, downstream acceptance and the specified loaded restart. Record delivered mass over time, screw speed, current and temperature, and inspect for leakage. Conduct restart tests only within the approved procedure and design limits; do not deliberately create an uncontrolled blockage to test torque.
Conclusion
An inclination selection must demonstrate delivery at the actual angle and material state, with a drive and structure suitable for normal operation and restart. Keep capacity, physical filling and power as separate checks rather than assuming one correction settles all three.
VOGO lists selection diameters from Ø89 to Ø407 mm and reference capacities from 1.3 to 170 t/h across its range. These are not ratings for every inclined duty. Review the product configuration and datasheet, then confirm the material, angle and layout for the project.
