Screw Conveyor

What is a Screw Conveyor

A screw conveyor is a mechanism that uses a rotating helical screw blade, called the flight or flighting, usually turning within a trough or tube, to move liquid or granular material. The flight is fixed to a central pipe (the screw or auger), and as the assembly rotates, each turn of the helix pushes the captured material forward along the trough. The volumetric transfer rate is directly proportional to rotational speed, which makes the screw conveyor naturally suited to variable-rate metering simply by changing rpm. This linear speed-to-rate relationship is the single most useful property of the machine and the reason it is the default choice for controlled discharge from hoppers and silos.

The principle is ancient. The Archimedes screw, a helical surface inside a cylinder used to lift water for irrigation, is the direct ancestor and has been in use since classical antiquity. The modern bulk-handling screw conveyor is an industrialisation of that idea in a fixed trough rather than a rotating cylinder. The portable grain auger, the version most farmers recognise, was invented by Peter Pakosh in 1945 in Toronto, Canada, and turned the screw conveyor into an everyday agricultural tool. Today screw conveyors appear in grain elevators, cement and fly-ash plants, food and rendering lines, snowblowers and combine harvesters, and throughout municipal wastewater treatment.

A complete screw conveyor has four functional groups: the screw (pipe plus flighting), the trough or tube that contains it and the material, the drive end (gear reducer, motor, and a thrust-rated end bearing that absorbs the axial reaction of conveying), and, on longer shafted units, intermediate hanger bearings that support the screw at each coupling joint. Inlet and discharge spouts, covers, and trough seals complete the enclosure. The hanger bearing is the wear-critical part: it sits in the material stream, so its material (babbitt, bronze, hard iron, UHMW, or graphite-impregnated) is chosen to match abrasiveness and temperature, and on sticky or stringy materials it is eliminated entirely by going shaftless.

Screw conveyors excel over short to medium distances, typically up to roughly 30 to 40 m (100 to 130 ft) per drive for a shafted unit, and at modest capacities. They are not the right tool for long-distance, high-tonnage transport, where belt conveyors win on energy and wear, nor for very abrasive high-velocity duty, where the flight and trough erode quickly. Their advantages are total enclosure (dust and odour containment), a small cross-section, the ability to load and discharge at multiple points along the length, and the ease of inclining or sealing the path. Within that envelope they are among the lowest cost-per-tonne handling machines made.

Four engineering quantities determine whether a screw conveyor is correctly applied: the bulk material class (which fixes trough loading), the required capacity, the conveyor length and incline, and the resulting drive horsepower and torque. Get the material class wrong and every downstream number is wrong, because trough loading, material factor, and the recommended component series all flow from it. The chapters below build up from that classification.

Configurations and Flight Construction

Screw conveyors are categorised first by orientation and trough type, then by how the flight itself is made. Orientation runs from horizontal, the simplest and highest-capacity arrangement, through inclined, to vertical, where capacity is sacrificed for lift in a minimum footprint. The housing follows: a U-shaped (open or covered) trough for horizontal and gently inclined duty, and a close-fitting tubular housing for inclined and vertical conveyors where material would otherwise slip backward over the open trough edge. A third major branch is the shaftless design, which removes the centre pipe and all hanger bearings so sticky, stringy, or dewatered materials cannot wrap or clog. The table below summarises the main families.

Flight construction divides into two manufacturing methods, and the choice affects both cost and capability. Helicoid flighting is cold-rolled from a flat steel strip into a continuous helix, then welded to the pipe. The rolling process work-hardens and slightly tapers the outer edge, giving a smooth, continuous surface that is economical for thinner sections and smaller to mid diameters. Sectional flighting is cut as individual flat washers (single turns), formed into one-pitch segments, and welded edge to edge onto the pipe. Sectional flights can be made from thicker plate, in larger diameters, and in special edge or face profiles, so they cover heavy-duty, abrasive, and modified-flight work that helicoid cannot.

Beyond the basic single helix, the flight surface itself can be modified to change how material is handled. Ribbon flighting is an open helix standing off the pipe on legs, leaving a gap between flight and shaft so sticky, gummy, or viscous materials cannot pack against the pipe. Cut flighting notches the outer edge to lift, turn, and gently mix material as it advances. Cut-and-folded flighting cuts the edge and folds the segments forward to lift and aggressively agitate, useful where the screw must also aerate or blend. Paddles, up to four per pitch and individually adjustable in angle, replace or supplement the flight where mixing, not conveying, is the dominant requirement; reversing paddle angle even lets the screw resist forward flow to extend residence time.

Two more details belong to construction. The coupling shaft joins screw sections and carries the full conveying torque through each joint; CEMA 300 standardises 2-bolt and 3-bolt coupling shafts with defined diameters (for example 1, 1 1/2, 2, 2 7/16, 3, 3 7/16, 3 15/16, 4 7/16, and 4 15/16 in), and the coupling torque rating is a hard limit that the drive must not exceed. The component series (sometimes written 2D, 3D, and heavier) prescribes minimum trough, pipe, and flight thicknesses for a given material code; cement, for instance, is assigned a 2D heavy-duty series in the CEMA 350 material table. Specifying a lighter series than the material code calls for is a common cause of premature wear-through.

Flight and Pitch Types

Pitch is the axial distance the flight advances in one full turn, and it is the single most important geometric variable after diameter. Standard nomenclature expresses pitch as a fraction of the screw diameter: a standard-pitch screw has pitch equal to diameter (a 1:1 ratio). Reducing the pitch reduces the axial speed of the material at a given rpm but improves the screw's ability to hold material on inclines and to meter precisely. Increasing the pitch moves free-flowing or fluid material faster. The table below lists the CEMA pitch types, their ratio, and the standard application.

Standard pitch (1:1) is the workhorse and the basis of every published capacity table. It is the most common single-flight screw and is recommended for horizontal runs and for inclines up to about 10 degrees, above which material slip begins to erode capacity sharply. When in doubt for a horizontal conveyor, this is the default.

Short pitch (2:3) and half pitch (1:2) shorten the helix to fight gravity on inclined and vertical conveyors and to give finer metering in feeders. The closer flight spacing keeps material from cascading back over each turn, but it also reduces the volume advanced per revolution, so the capacity factor drops to roughly 0.67 and 0.50 of standard respectively, which is why inclined screws are usually run faster and in a tubular housing to recover throughput.

Variable and tapered pitch are feeder-specific. A screw feeder that ran a constant pitch under a long hopper slot would draw all its material from the back of the opening and leave a stagnant zone at the front (ratholing). Increasing the pitch progressively along the inlet, or tapering the flight outside diameter up from roughly half to full, forces uniform withdrawal across the entire opening so the bin empties evenly. This is the defining feature that separates a properly designed feeder from a conveyor pressed into feeder duty.

Two flight-count and surface variants round out the catalogue. Double-flight screws carry two parallel helices on one pipe for smoother, more even discharge and gentler handling of free-flowing materials at the same pitch. The modified surfaces from Chapter 2 (ribbon, cut, cut-and-folded, paddles) each carry their own capacity factor below 1.0, because every notch, fold, or paddle that promotes mixing also lets some material slip rather than advance. Always apply both the pitch capacity factor and the modified-flight capacity factor when they are combined; ignoring either undersizes the conveyor.

Materials of Construction

The material of the flight, pipe, trough, and hanger bearings is chosen against three properties of the bulk solid: abrasiveness, corrosiveness, and sanitary or temperature demands. The CEMA 350 bulk material table encodes the first of these directly: each material carries an abrasiveness rating that maps to a recommended component series and, for the worst cases, to hardfacing. A mismatch shows up as flight-edge rounding, trough wear-through, or, with corrosive media, pitting and perforation. The wrong hanger-bearing material is the most frequent maintenance complaint, because it sits in the flowing stream.

Carbon steel (mild steel, typically ASTM A36 trough and pipe, with formed flighting) is the default for non-corrosive, non-sanitary bulk solids: grain, aggregates, sand, cement, fly ash, and wood chips. It is the lowest cost and the basis of standard catalogue components. For abrasive duty the flight outer edge is protected by hardfacing (weld overlay) or the whole screw is built from abrasion-resistant plate such as AR400, AR500, or Hardox-grade steel, which trade ductility for surface hardness and dramatically extend life against sand, glass cullet, frac sand, and slag.

Stainless steel covers corrosion and hygiene. Type 304 handles mildly corrosive chemicals and general food contact; 316L, with 2 to 3 percent molybdenum and low carbon, is specified where chlorides, acids, or aggressive cleaning chemicals are present, and is the standard for dairy, pharmaceutical, and clean-in-place (CIP) duty where the screw and tube are electropolished to a low surface roughness. Stainless conveyors cost several times a carbon-steel equivalent, so they are reserved for duties that genuinely require them rather than applied by default.

The table below is a first-pass material guide. It is for initial selection only; before purchase, obtain the manufacturer corrosion and wear data for the actual concentration, temperature, particle size, and velocity, because abrasion and corrosion both accelerate non-linearly with those variables.

Hanger-bearing material deserves its own note. For free-flowing non-abrasive solids a bronze or oil-impregnated bearing suffices; for abrasive materials a hard-iron or ceramic bearing is used; for food and pharma a UHMW or sealed bearing keeps the contact surface clean; and for sticky or fibrous materials the correct answer is usually to delete the hanger bearing by selecting a shaftless conveyor and accepting its shorter span and lower fill.

Key Specification Parameters

A screw conveyor specification is shorter than an instrument spec but every line drives a sizing calculation. The eight parameters below decide capacity, power, and service life. The dominant figure is trough loading, the cross-sectional fill fraction, because it links the material class directly to throughput.

Diameter and speed. CEMA standardises diameters from 4 to 36 in (100 to 900 mm), with 6, 9, 10, 12, 14, 16, 18, 20, and 24 in (150 to 600 mm) the common sizes. Maximum recommended rpm falls as diameter rises: a 6 in screw runs up to about 120 rpm at 30 percent loading, while a 24 in screw is held to roughly 65 rpm at the same loading, because peripheral speed and material agitation grow with diameter. Larger, slower screws move more per revolution but cannot be spun as fast.

Trough loading and capacity. Trough loading is set by the material class in the CEMA 350 table, not chosen freely: free-flowing, non-abrasive light materials run at 45 percent; average flowability, mildly abrasive materials at 30 percent; and sluggish, abrasive, or degradable materials at 15 percent. Capacity follows from a published volume per revolution. A 16 in (400 mm) screw at 30 percent (30A) loading carries about 31.2 ft3/hr per rpm, so at its 80 rpm maximum it delivers roughly 2,496 ft3/hr; multiply by bulk density for mass flow, then apply the pitch and modified-flight capacity factors. The comparison table below shows the relationship across sizes.

Length and incline. Length sets the friction horsepower and, on shafted units, the number of hanger bearings; practical single-drive shafted runs are usually held to about 30 to 40 m (100 to 130 ft). Incline reduces capacity because material slips backward; standard pitch is good only to about 10 degrees, and beyond roughly 20 degrees the conveyor is treated as a special application needing short or half pitch, a tubular housing, and vendor calculation.

Horsepower and torque. CEMA computes total horsepower as friction plus material power. Friction horsepower is HPf = L x N x Fd x Fb / 1,000,000 (length in feet, N in rpm, Fd the diameter factor, Fb the hanger-bearing factor). Material horsepower is HPm = C x L x W x Ff x Fm x Fp / 1,000,000 (capacity in ft3/hr, bulk density W, and the flight, material, and paddle factors). The sum is multiplied by an overload factor when below 5 HP and divided by drive efficiency (about 0.88) for motor power. The result must then be checked against the coupling-shaft torque rating, which is an independent mechanical limit.

Remaining parameters. Inlet and discharge spout sizes and positions, trough end and seal type (packing gland, lip, or purged seal for dusty or hazardous duty), drive arrangement (shaft-mount reducer, end drive, or distribution drive), and enclosure rating round out the specification. For dust-hazard materials, an ATEX or NFPA-compliant sealed and grounded design replaces the open cover.

Selection Decision Factors

To turn the preceding chapters into a model, follow the sequence below. The order matters: most sizing errors come not from one wrong number but from deciding capacity or power before the material class is fixed. These eight steps work as a fixed RFQ template.

1. Identify the bulk material and its CEMA code: bulk density, lump size, flowability, abrasiveness, and any corrosive, sticky, hot, or hazardous traits. This single step fixes trough loading, material factor, and the minimum component series for everything that follows.
2. Set the required capacity and convert to volume: divide the design mass rate by bulk density to get ft3/hr or m3/hr, and decide whether the duty is conveying (15 to 45 percent fill) or flooded feeding (metering from a hopper).
3. Choose orientation and pitch: horizontal standard pitch for flat runs and inclines to 10 degrees; short or half pitch in a tubular housing above that; vertical for high lift in a tight footprint; variable or tapered pitch for feeders.
4. Size diameter and speed: pick the smallest diameter whose maximum recommended rpm meets the volume at the required trough loading, then back off speed for abrasive or degradable materials to extend life.
5. Apply capacity factors: multiply by the pitch factor and any modified-flight factor (cut, cut-and-folded, ribbon, paddle), and re-check that the chosen diameter and speed still deliver the rate.
6. Select materials of construction: trough, pipe, flight, and hanger-bearing material per the Chapter 4 guide, with hardfacing or AR plate for abrasion and 316L for corrosion or sanitary CIP; go shaftless for sticky or fibrous materials.
7. Calculate horsepower and verify torque: compute friction plus material horsepower, apply the overload and efficiency factors, then confirm the coupling-shaft and screw torque ratings are not exceeded at full-load and startup conditions.
8. Specify the enclosure, seals, and drive: spout sizes and positions, trough-end seal type, dust or ATEX rating, gear reducer and motor, and the inlet/discharge interface to upstream and downstream equipment.

One dimension that buyers routinely underweight is serviceability and interchangeability. Because CEMA 300 dimensional standards are open, troughs, screws, hanger bearings, and coupling shafts from different manufacturers are largely interchangeable, so wear parts can be sourced competitively rather than from a single vendor; confirm the supplier builds to CEMA dimensions before assuming this. Local spare-part stock, field replacement of hanger bearings and wear liners, and clear access for screw removal determine downtime over a 10 to 20 year service life. KWS Manufacturing, Martin Sprocket and Gear (screw conveyors since 1969), Thomas Conveyor, Orthman, Hapman, and WAM Group (Europe) all build to these standards and maintain spare-part networks, which makes them dependable choices for projects where uptime, not unit price, governs total cost.