Screw conveyor selection is a geometry-and-load problem first, a motor-sizing problem second: a designer must fix trough form (U-trough, tubular, vertical, shaftless), pitch-to-diameter ratio (typically 0.5-1.0 × diameter for standard flights, with short pitch used on inclines and longer pitch used as feeders), and material of construction before any horsepower number is meaningful [S1][S4][S7].
The equipment class spans horizontal U-trough conveyors, tubular enclosed units for dust-free transfer, vertical screw elevators, and shaftless spirals for dewatered sludge or grit; in industrial directories the same family is offered in carbon steel, stainless steel, anti-wear steel and high/low-temperature alloys, with water-cooled flights available for hot ash and slag service [S1][S6].
Configuration Classes: U-Trough, Tubular, Vertical, Shaftless
U-trough (open-top) screw conveyors are the workhorse for short horizontal transfer of non-hazardous bulk solids and are almost always the lowest-cost option per ton handled, while tubular enclosed screw conveyors are specified wherever dust, hygiene or outdoor weather exposure would otherwise force aspiration ducting on an open trough [S1].
Vertical screw conveyors (sometimes called screw elevators) are sized for lifting dewatered cake, grain, or free-flowing powder against limited head — typically 5-10 m — and are commonly paired with a weighing screw when the process needs controlled feed-rate, since rotational speed is the cleanest variable to meter [S9].
Shaftless screw conveyors are the standard fix for sticky, fibrous, wet or abrasive residue (municipal sludge, grit, food pulp, rendering waste), because there is no intermediate hanger bearing to foul and no central shaft for rags to wrap; the trade-off is lower allowable torque and a higher-priced spiral [S6][S8].
Selection Criteria: Material, Bulk Density, Throughput, Inclination
Selection begins with the bulk solid: density (kg/m³), particle size, moisture, abrasiveness, corrosiveness, and whether it is floodable (free-flowing) or sluggish; engineers then choose housing loading (% of cross-section filled, commonly 15-30% for inclined units and up to 45% for horizontal), and confirm the screw will run as a conveyor (metered discharge downstream) or a feeder (controlled inlet from a bin) [S4][S7].
For Chinese-supplied spiral conveyor screws, factory-tier price points span roughly US$ 1,200 to US$ 36,000 per set at 1-set MOQ, with the lower end covering small-diameter standard units and the upper end covering large, custom-built heavy-mineral machines — a 30× spread that mirrors the diameter and metallurgy step-changes, not a simple markup [S2].
Inclination is the silent killer: most U-trough and tubular screws lose roughly 30% of their rated horizontal capacity at 15°, 55-60% at 25°, and become uneconomic above 30° unless a short-pitch flight or a vertical-screw geometry is used; this is why a separate "vertical screw feeder" or "SWLS inclined unit" is listed in catalogues rather than just a tilted horizontal conveyor [S4][S8][S9].
Drive Sizing: Torque, FHP, MHP and TSHP
Drive sizing follows a split: Total Shaft Horsepower (TSHP) equals Friction Horsepower (FHP) plus Material Horsepower (MHP), where FHP is read from a nomograph using screw diameter, length, bearing type and housing loading, and MHP is read from a second nomograph using capacity and bulk density; on long, inclined or heavily loaded runs a coupling or gear service factor of 1.4-1.6 is applied to the nameplate motor to keep the drive out of the breakdown-torque zone [S4][S5].
Full Motor Torque is the maximum torque the drive can deliver at the screw shaft before stalling; the simplified relationship is HP = (T × RPM) / 63,025 (with T in in-lb), so undersizing the gearbox or fitting an under-rated coupling is the most common root cause of intermediate-shaft twist-offs, a failure mode documented on power-plant ash conveyors where combustion residue loads the flight unevenly [S3][S4].
Materials and Wear: What to Specify Against What
For mild-duty dry chemicals or food-grade sugar/flour, AISI 304 stainless is the default; for abrasive ash, slag, sand, or mineral concentrate, hard-facing on the flight OD or a replaceable AR-plate liner is specified, often with abrasion-resistant steel (AR400/500-class) housings rather than mild carbon steel [S1][S6].
For hot service (boiler ash, clinker dust, slag conveyors), water-cooled trough or flight sections are used to drop the material below the discharge-temperature limit of downstream conveyors; the supplier's stated MOC options — carbon steel, stainless steel, anti-wear steel, low- and high-temperature alloys — map directly to bulk-solid temperature bands and corrosiveness, so a written MOC line item on the datasheet is non-negotiable [S1][S9].
Enclosed construction is the single biggest reason to choose a tubular screw over an open U-trough in a pharmaceutical, food, or recycling line, because it eliminates dust emission and weather ingress while letting maintenance crews open quick-release end-bearings; this is the same reason screw conveyors are widely used as the first stage of chip removal in machine shops, feeding a downstream belt conveyor for discharge [S1][S8].
Failure Modes and Operating Limits
Documented root-cause failures on coal-boiler ash conveyors include fatigue cracks at the driven-end coupling journal, wear through of flight faces from abrasive combustion residue, and seizure of intermediate hanger bearings from hot material packing; material analysis in these cases typically finds decarburization or hardness loss at worn surfaces, which then feeds back into the spec as a flight-hardness minimum (often HRC 50+ on the leading edge) [S3].
Screw conveyors are explicitly described as tractionless transport mechanisms suited to "shorter distances with lower transportation performance," which in practice means they are rarely the right answer above ~30-40 m horizontal run, above ~5-10 m vertical lift, or where the bulk solid is highly floodable, highly cohesive, or contains fragments that can jam between flight and trough [S1][S3].
Flexible-blade screw variants — proposed for food-packaging lines where rigid flights damage fragile product — shift the design trade-off toward lower throughput and stricter speed control; they are not a drop-in replacement for a standard helix, and any retrofit from rigid to flexible should be re-run through the MHP/TSHP calculation [S3].
Spec Map: Matching the Screw Type to the Duty
A practical decision matrix reads: (1) wet/sticky/fibrous → shaftless or tubular with no intermediate hanger; (2) dry abrasive, hot → U-trough with AR flights and water-cooled sections; (3) hygienic/outdoor → tubular stainless, fully enclosed; (4) controlled feed to a reactor or weigh hopper → short-pitch feeder screw or vertical screw elevator with VFD; (5) long horizontal run or high lift → re-spec to a belt conveyor or bucket elevator, not a larger screw [S1][S7][S8].
On the controls side, a screw feeding a biomass pyrolyser or rice mill is most often paired with a variable-frequency drive so that the rotational speed — and therefore the feed rate — can be trimmed to the downstream demand, with experimental results typically showing 5-15% deviation between theoretical and measured power draw at steady state, which is the calibration window the VFD is sized to absorb [S3][S9].
For procurement, two signals worth tracking into Q3 2026 are the spread between Chinese OEM spiral-screw quotes (US$ 1,200-36,000/set, 1-set MOQ) and EU-built units with the same MOC and diameter class, and the rate at which shaftless-spiral suppliers add abrasion-resistant flight options for the mining and industrial gasket sludge-handling segments — both directly affect landed cost and lead time for the same duty [S1][S2][S7].
For component-level specifications, see linear guide, crossed roller guide, and screw conveyor.