Screw conveyor sizing collapses to four measurable inputs — trough diameter (mm), shaft RPM, bulk density (kg/m³), and incline angle (°) — and the rest of the geometry, including pitch ratio, flight thickness, and hanger-bearing spacing, follows from those [S1][S2].
Selection is rarely "pick the biggest"; the practical decision is whether a standard CEMA 300/352 trough/diameter/hp combination handles the duty, or whether a custom screw conveyor is justified by abrasiveness, temperature, or sanitary constraints [S1]. For solids-handling plants, this gate is the one that controls capex and downtime risk more than any later detail.
Trough Diameter, RPM and the Capacity Equation
The standard capacity formula, used by most North American builders, is Capacity (ft³/hr) = (D² − d²) / 4 × π × RPM × 60 × 0.7854 × (loading %), where D = outside flight diameter and d = pipe/shaft diameter; the same shape holds in metric with m³/h and D in metres [S1]. Most standard troughs ship in 6, 9, 12, 14, 16, 18, 20, and 24 in (150–600 mm) sizes, with 9 in (230 mm) and 12 in (300 mm) the high-runner general-purpose diameters for non-abrasive granulars.
Typical shaft speeds fall between 30 and 180 RPM for 6–12 in units, dropping to 10–60 RPM on 18–24 in troughs, because the goal is to keep the outer edge of the flight below 5–7 m/s for non-abrasive material and well under that for abrasive, fibrous, or high-temperature duty [S1]. The vendor feed for the Definitivecology STS/PM transport screw (a sludge-duty unit) confirms that non-free-flowing, sticky materials drive the trough down and the pitch up, rather than the reverse [S2].
Bulk Density, Inclination, and the Loading-Factor Gate
Bulk density is the single largest multiplier on motor power: the CEMA-style standard table assigns groups from Class I (~0.4–0.8 t/m³, light grains) up to Class V (~1.6–2.4 t/m³, heavy ore), and required horsepower scales near-linearly with that group [S1]. Inclination cuts capacity sharply — a horizontal baseline is 1.0, a 15° lift is roughly 0.80, a 20° lift 0.65, and 30° typically 0.50 of horizontal capacity for a standard screw; above 30° a special steep-incline or vertical screw pump configuration is normally selected instead [S1].
Loading factor (the % of the trough cross-section the material occupies) sits at 15–30% for abrasive or heavy lumps, 30–45% for general granular, and 45%+ for light/fine free-flowing material — exceeding the band is a fast route to flight wear and motor overload [S1]. For inclined runs, vendors in the 2026 feed also cut the RPM, not just the capacity table, to keep the flight-edge velocity inside the wear envelope [S1].
Flight Type, Pitch and Liner Choices by Material
Flight geometry is material-specific: standard pitch (pitch = 1.0 × diameter) is the default for most horizontal duty; short pitch (2/3 D) is used for incline and sticky material; long pitch (1.5–2.0 D) for free-flowing granular at high throughput; and ribbon flights for cohesive or stringy bulk where the screw cannot be allowed to "carry" material around the shaft [S1]. For the higher-temperature or abrasive classes, hard-facing (e.g., chromium-carbide overlay) or replaceable liner plates in AR400/AR500 are common, and pitch can be increased only after the abrasion and temperature class is locked.
Stainless 304/316 troughs and FDA/USDA-approved finishes are specified for food, dairy, and pharmaceutical duty; carbon steel with epoxy or rubber liner covers most mining, aggregate, and cement work. The Definitivecology STS/PM heavy-duty transport screw, a steel-bodied sludge/recycling unit, illustrates the abrasiveness-driven extreme of this branch: heavier flights, thicker shaft, and a construction tuned for irregular, contamination-tolerant waste streams rather than food-grade finish [S2]. Hanger bearings — the intermediate shafts that support long runs — are spaced on roughly 8–10 ft (2.4–3.0 m) centres, and that number tightens as material becomes more abrasive or fine [S1].
Horsepower, Torsion and the Motor-Gate
Required horsepower comes from a dual sum: material hp (lift + horizontal push) and frictional hp (bearings, seals, hanger bearings, flight-to-trough drag), with a service factor of 1.0–1.5 typically applied for clean, free-flowing material and 2.0–3.0 for heavy, abrasive, sticky, or high-temperature duty [S1]. Shaft torsional rating is a parallel gate: oversized motors that blow past the shaft torque rating stall the flight, and a 1.5–2.0× design margin on shaft diameter vs computed torque is the standard defensive move.
For variable-feed applications, VFDs on the drive motor are now the default in the 2026 vendor feed, and gear-reducer sizing is sized to the worst-case (lowest-RPM, highest-density) point, not the average [S1]. When the bulk is heavy, abrasive, hot, or inclined, the conservative path is a shafted screw, not a shaftless/shaftless spiral — a different equipment class used for dewatered sludges, filter cake, and difficult bulk that would wrap around a centre pipe [S1][S2].
When a Standard CEMA Build Fits, and When It Doesn't
A standard CEMA 300/352 trough/diameter/hp combination fits: bulk density below ~1.0 t/m³, particle size under 1/3 of the flight diameter, temperature under 150 °C, no more than 30 % of trough loading on inclines, and run lengths under ~30 m without intermediate hanger redesign [S1]. It is the wrong choice — call it a custom screw conveyor requirement — when the duty hits any of: bulk density above ~1.6 t/m³, particle size above 1/3 of flight diameter, temperatures above 200 °C, severe abrasion (silica content typically above 5–6 %), corrosive chemicals, sanitary washdown, ATEX/IECEx dust zones, or inclined lifts above 30° [S1].
The same gate applies to TIV/ATEX-classified atmospheres (gas Group IIA/IIB or dust Group IIIC, per IEC 60079 series) and to hygienic duties (3-A / EHEDG) — both are non-standard by CEMA 300/352 definitions and shift the build into a custom geometry, special bearing seals, and often a different drive-train layout. As one engineering reference puts it in plain terms: "the rule of thumb is that screw conveyors work best for horizontal or slightly inclined, free- to semi-free-flowing material; outside those, the geometry needs to be re-thought" [S1].
Specification Cut: Picking Among the Main Options
Lining the four main screw-conveyor classes against four spec levers gives a workable comparison for a 2026 spec-first buyer. The table below is a decision aid, not a marketing matrix, and each cell maps to a measurable input: [S1]
- U-trough, standard pitch, CEMA 300 carbon steel, dry bulk (grains, fly ash, light aggregate): cost band LOW, max temperature ~150 °C, abrasion tolerance LOW–MED, sanitary rating NONE. Best fit: free-flowing granular, horizontal runs, low-to-moderate tonnage.
- Tubular (pipe) screw, standard pitch, CEMA 352 carbon or SS, enclosed bulk (cement, lime, plastic pellets): cost band MED, max temperature ~200 °C, abrasion tolerance MED, sanitary rating LOW. Best fit: dusty or fume-controlled transfer, sealed transfer points.
- Shaftless spiral, heavy-gauge SS or abrasion-resistant steel, sludge/filter-cake duty: cost band HIGH, max temperature ~80–120 °C in SS, abrasion tolerance HIGH, sanitary rating MED (depending on finish). Best fit: dewatered sludge, screenings, wet/sticky bulk, plants where a centre pipe would bind.
- Ribbon or paddle flight, special pitch, custom-built, high-temperature or cohesive bulk: cost band HIGH, max temperature ~400–500 °C in alloy builds, abrasion tolerance MED–HIGH, sanitary rating LOW–MED. Best fit: hot ash, clinkers, cohesive chemicals, batch/reactor discharge. For deeper procurement cross-checks against bulk-bag feed into the same line, the spec-first FIBC and belt-conveyor selection pieces complement this guide — see FIBC bulk bag spec cut and belt conveyor selection for upstream/downstream integration [S1].
Cross-referencing the 7-spec-lever selection piece, the practical decision is: lock the four hard numbers (D, RPM, density, angle) first, then walk the flight/pitch/hanger/motor gates in that order — skipping the first four leads to oversizing and overspend, skipping the second four leads to wear, jamming, and unplanned downtime [S1].
Real Failure Modes and What They Tell the Buyer
The published failure-analysis literature on screw conveyors in coal-fired boiler service names the dominant failure modes directly: flight wear at the outer edge, shaft deflection, hanger-bearing seizure, and fatigue cracking at the flight-to-shaft weld [S5]. Each one traces back to a spec decision — flight-edge velocity was set too high, hanger spacing was set too generous, abrasion class was under-rated, or weld geometry was chosen without a fatigue review.
The same paper notes that bulk density variability (coal being a wide-ranging class, ~0.6–1.0 t/m³) is routinely under-estimated at spec stage, and the conservative design path — drive sized to the 90th percentile of expected density, not the nameplate — is what separates a 5-year conveyor from a 15-year one [S5]. For buyers comparing Chinese OEM trough-and-flight builds against EU/US integrators, the failure modes that show up in service (weld quality at the flight root, shaft straightness, hanger-bearing alignment) are the levers that justify the premium on the EU/US side; the Chinese side wins on lead time and on standard-dimension CEMA frames, but loses when custom metallurgy or hygienic finish is on the spec.
Trackable signals for the next 6–12 months: (1) wider adoption of VFD drives on standard CEMA 300/352 units rather than fixed-speed gear motors; (2) growth in shaftless spiral quotes for sludge and waste-recycling duty, with the Definitivecology STS/PM-style heavy transport screw as a representative product line [S2]; (3) tighter dust-zone compliance (ATEX/IECEx) on enclosed tubular units, which tends to push buyers toward stainless or fully sealed troughs even where the chemistry does not strictly require it [S1].
For component-level specifications, see linear guide.