A Chain Scraper Conveyor is a bulk-material handling machine that drags loose solids along an enclosed trough using transverse flight bars fixed to one or two endless chains. The basic configuration traces back to Arthur Redler's 1920 patent for the en-masse conveyor in England, which proved you could move grain, coal and ash without lifting individual particles. The chain pulls the flights, the flights shear and push the material as a slug, and the trough contains the load. Modern longwall armored face conveyors built by companies like Caterpillar and JOY move 3,000+ tonnes per hour of run-of-mine coal under loads no belt could survive.
Chain Scraper Conveyor Interactive Calculator
Vary trough area, chain speed, bulk density, and fill factor to see conveyor capacity and animated material movement.
Equation Used
The article capacity relation multiplies the filled trough area by chain speed to get volumetric flow, then multiplies by bulk density to get mass throughput. Because density is entered in t/m3 and velocity is in m/s, the calculator multiplies by 3600 to report tonnes per hour.
- Steady uniform material slug in the carrying trough.
- Bulk density is entered in tonnes per cubic meter.
- Fill factor represents the usable filled fraction of the trough cross-section.
- No allowance is included for slipback, recirculation, startup surges, or spillage.
Operating Principle of the Chain Scraper Conveyer
The mechanism is brutally simple. An electric drive turns a sprocket at one end of a steel trough. That sprocket pulls one or two endless roller chains the length of the trough and back. Bolted across the chain at fixed pitch — usually 400 mm to 1,000 mm — are flight bars, the scrapers. Material drops into the trough through an inlet, the flights drag it along the bottom, and it discharges at the far end either through a chute or over the head sprocket. The return run travels above or below the carrying run depending on whether you're running a single-strand drag chain conveyor or a double-strand armored face conveyor.
The reason it's designed this way is containment and abrasion tolerance. A belt conveyor handling hot bottom ash, sticky filter cake, or 200 mm lump coal would either burn, plug, or get sliced to ribbons. A scraper flight in an enclosed trough doesn't care — the chain takes the tension, the flights take the wear, and the trough liner takes the abrasion. You replace those three items on a wear schedule, not the whole machine.
Tolerances matter more than people think. Chain pre-tension must sit inside a narrow window — usually 1.5% to 2.5% elongation over installed length. Run it too slack and the chain jumps the sprocket teeth at startup, which shears flight bolts and bends flights. Run it too tight and you cube the bearing load on the head and tail shafts, which kills the gearbox in months. If you notice the chain slapping the trough top on the return run, that's elongation past 3% — time to remove links. If you hear a rhythmic bang at the head sprocket, a flight is bent or a chain link has seized, and you need to stop before it climbs the sprocket and tears the housing.
Key Components
- Drive sprocket and head shaft: Transmits torque from the gearmotor into the chain. Sprocket tooth profile is matched to the chain pitch within ±0.5 mm — get the pitch wrong and the chain rides high on the teeth and skips. Typical head shaft sizes range from 80 mm to 200 mm depending on installed power, which on a longwall AFC can hit 3 × 1,200 kW.
- Flight bars (scrapers): Steel or hardened-steel cross members bolted to the chain at fixed pitch. They do the actual material shearing and pushing. Flight-to-trough clearance is typically 5 to 10 mm — tighter and the flight gouges the liner, looser and fines slip back under the flight and recirculate.
- Roller or forged-link chain: Carries all the tension. Mining-duty scraper chain is typically 26 × 92 mm to 42 × 146 mm round-link chain, rated for 1,200 kN to 2,200 kN minimum break load. The chain elongates 1-3% over its life as link bearing surfaces wear.
- Trough and wear liner: Encloses the load and absorbs abrasion. Bottom liner is usually Hardox 450 or chromium-carbide overlay, 8-20 mm thick. When liner thickness drops below 4 mm in coal service, you replace it before the structural pan plate gets cut through.
- Tail tensioner: Hydraulic or screw take-up that holds the chain at correct tension as it elongates. On AFCs this is automatic and pressure-monitored; on small drag chain conveyors it's a manual screw checked monthly.
- Drive gearmotor: Sets chain speed. Most scraper conveyors run 0.3 to 1.2 m/s. Slower for sticky or hot material, faster for free-flowing grain or wood chips. Above 1.5 m/s you start lifting fines into the return run, which throws material out of the inlet.
Real-World Applications of the Chain Scraper Conveyer
You find Chain Scraper Conveyors anywhere belts can't survive — hot, abrasive, lumpy, sticky, or fully enclosed material flows. They dominate underground coal in the form of armored face conveyors, they handle bottom ash under coal-fired boilers, and they move filter cake and tailings in mineral processing plants. The same mechanism, scaled down, runs grain through every silo complex in North America under the name en-masse conveyor.
- Underground coal mining: Caterpillar PF6 and JOY AFC armored face conveyors running behind longwall shearers, moving 2,500-3,500 t/h of run-of-mine coal at face widths up to 400 m.
- Coal-fired power generation: Submerged scraper conveyors (SSCs) under boiler ash hoppers — Clyde Bergemann and United Conveyor Corporation supply these for stations like Drax in the UK, handling 60-150 t/h of bottom ash quenched in a water-filled trough.
- Cement plants: Drag chain conveyors moving clinker and hot kiln dust at temperatures up to 400 °C, where rubber belts simply cannot operate. FLSmidth and AUMUND build dedicated hot-material flight conveyors for this duty.
- Grain and feed handling: En-masse conveyors at country elevators and feed mills, moving 50-500 t/h of corn, wheat, or soybean meal through enclosed dust-tight troughs. Schlagel and Sweet Manufacturing are common North American suppliers.
- Mineral processing: Filter cake and concentrate transfer at copper and iron ore concentrators — the sticky 8-12% moisture cake plugs belts and chutes, so plants like Escondida use trough chain conveyors between filter presses and stockpiles.
- Biomass and waste-to-energy: Bottom ash and fuel feed conveyors at biomass plants. Babcock & Wilcox and Valmet specify scraper flight conveyors for woody biomass below the grate, where temperatures reach 250 °C and the material is mixed with burning embers.
The Formula Behind the Chain Scraper Conveyer
The core sizing question is mass throughput — how many tonnes per hour will this conveyor actually deliver at a given chain speed and trough fill? At the low end of typical operating range, around 0.3 m/s, the conveyor handles sticky, hot, or angular material gently with high fill factor but limited tonnage. At the nominal 0.6-0.8 m/s sweet spot, you get the best ratio of throughput to chain wear. Push above 1.2 m/s and the flights start aerating the material, fill factor collapses, and predicted tonnage no longer matches actual tonnage — you're moving air, not coal. The formula below assumes en-masse flow, which is what a properly designed scraper conveyor produces.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Mass throughput | t/h | tons/h (short) |
| A | Trough cross-sectional area available for material | m² | ft² |
| v | Chain (flight) speed | m/s | ft/s |
| ρ | Bulk density of conveyed material | t/m³ | lb/ft³ |
| φ | Fill factor (fraction of trough cross-section actually occupied by material) | dimensionless | dimensionless |
Worked Example: Chain Scraper Conveyer in a wood-pellet plant ash conveyor
A wood-pellet manufacturing plant in Georgia is sizing a single-strand drag chain conveyor to remove 25 t/h of bottom ash and char from below a 50 MW biomass boiler. The trough is 600 mm wide × 400 mm deep, the available material cross-section A is 0.18 m², the ash bulk density ρ is 0.55 t/m³, and the plant wants to verify the conveyor delivers the rated tonnage across the operating speed range of 0.3 to 1.0 m/s with a fill factor φ of 0.55.
Given
- A = 0.18 m²
- ρ = 0.55 t/m³
- φ = 0.55 dimensionless
- vnom = 0.6 m/s
- vlow = 0.3 m/s
- vhigh = 1.0 m/s
Solution
Step 1 — at nominal chain speed 0.6 m/s, plug straight into the throughput equation:
That's the theoretical ceiling at full fill — well above the 25 t/h target, which means at nominal speed the conveyor is operating at roughly 21% utilisation. That's healthy headroom for ash-rate spikes when the boiler de-ashes a clinker.
Step 2 — at the low end of the typical operating range, 0.3 m/s:
At 0.3 m/s the conveyor still hits more than twice the duty rate. This is where you'd run it if the ash is unusually hot or contains glowing embers — slow speed gives the water-quench bath more dwell time and reduces the chance of flight bars warping. The chain creeps so slowly you can watch individual flights pass.
Step 3 — at the high end, 1.0 m/s:
In theory. In practice, above about 0.9 m/s the fine ash starts behaving like a fluid — flights aerate it, φ drops from 0.55 toward 0.35, and real tonnage flattens out around 130-140 t/h instead of climbing further. You also start throwing fines back into the boiler hopper through the inlet because the flights are moving faster than the material can settle. The sweet spot for this duty sits at 0.5-0.7 m/s.
Result
Nominal throughput Qnom is approximately 117. 6 t/h, which gives the 25 t/h ash duty a comfortable 4.7× design margin. At 0.3 m/s the conveyor still delivers ~59 t/h with gentle handling, while at 1.0 m/s the calculated 196 t/h falls apart in practice because aeration drops the fill factor and the inlet starts spitting fines back — the real sweet spot is 0.5-0.7 m/s. If you measure significantly less than predicted on commissioning, check three things in order: (1) inlet feeder rate is below conveyor capacity, so the trough never reaches design fill, (2) flight-to-trough clearance has opened past 12 mm from a worn liner, letting material slip back under each flight, or (3) chain elongation past 2.5% has pulled the flights out of pitch with the discharge geometry, dumping material short of the chute.
When to Use a Chain Scraper Conveyer and When Not To
Choosing a Chain Scraper Conveyor over a belt conveyor or a screw conveyor is almost always about material properties and containment, not about cost or efficiency. Scrapers cost more to run, wear faster, and draw more power per tonne — but they survive duties that destroy the alternatives. Here's how the three stack up on the dimensions that actually drive the decision.
| Property | Chain Scraper Conveyor | Belt Conveyor | Screw Conveyor |
|---|---|---|---|
| Typical speed | 0.3-1.2 m/s | 1.5-6 m/s | 0.5-1.0 m/s tip speed |
| Capacity range | Up to 3,500 t/h (AFC) | Up to 40,000 t/h (overland) | Up to 300 t/h |
| Max material temperature | 400-600 °C with steel trough | 80 °C standard, 150 °C heat-resistant belt | 400 °C with cast housing |
| Lump size handled | Up to 300 mm | Up to 400 mm with impact bed | Limited to ~⅓ screw diameter |
| Power per tonne (relative) | High (1.0×) | Low (0.3-0.5×) | Very high (1.5-2.5×) |
| Wear-part replacement interval | 6-24 months (flights, chain, liner) | 3-7 years (belt) | 12-36 months (flights, trough) |
| Containment / dust control | Fully enclosed, dust-tight | Open by default, covers add cost | Fully enclosed |
| Capital cost (relative, per metre) | Medium-high (1.5×) | Low (1.0×) | Low (0.8×) |
| Best fit | Hot, abrasive, lumpy, sticky bulk solids | High tonnage free-flowing material over distance | Short runs, powders, mixing duty |
Frequently Asked Questions About Chain Scraper Conveyer
You almost certainly bolted at least one flight in upside-down or with the wrong leading-edge orientation. A scraper flight has a rake angle — usually 5-10° back from vertical — that lets it shear into the material without trying to plough the trough liner. Install it backwards and the flight presents its flat face to the material, which doubles the cutting force per flight.
Quick diagnostic: shut down, walk the carrying run, and confirm every flight leans the same direction relative to chain travel. If you find even two flights reversed in a 60 m conveyor, that's enough to add 25-30% to motor amps.
The most common culprit is sluice water carrying fines back upstream. In an SSC, the trough is flooded to quench ash. If the water inflow is too high or the discharge weir is set too low, you get a return current that picks up fines from each flight and drifts them back toward the inlet. Real fill factor drops from your assumed 0.55 to maybe 0.30-0.35.
Check the discharge weir elevation and the makeup-water flow. Rule of thumb: water inflow should be no more than 1.5× the evaporation plus carry-out rate. If you're dumping 3-4× that volume in to keep the trough cool, you're hydraulically fighting your own conveyor.
Two questions decide it: installed power and lump size. Below about 75 kW per drive and lump sizes under 150 mm, single-strand center-chain is cheaper, simpler, and easier to retension. The chain runs down the centerline of the trough with flights cantilevered off both sides.
Above 75 kW or with run-of-mine lumps over 200 mm, you need double-strand outboard chains — one on each side of the trough — because a single center chain can't transmit the torque without elongating prematurely and the flights twist under one-sided lump loading. Every longwall AFC is double-strand for exactly this reason. The capital cost step-up is roughly 40%, but you buy 5-10× the chain life.
This is almost always material packing under the tail sprocket, not a sizing problem. Fines that escape past the inlet skirts collect in the tail box, and when feed starts, the chain tries to drag a compacted plug of material through the bottom of the tail. Starting torque on the drive doubles or triples until the breaker trips.
Pull the tail box inspection cover and look for compacted material below the tail shaft. Fix is to add or repair the tail-box scavenger — a small auger or hand-cleaning port — and to tighten the inlet skirt-rubber clearance to under 5 mm. If the conveyor has run for years without this issue and just started, your skirt rubbers are worn out.
Yes, but the angle limit depends entirely on material angle of repose, not on the conveyor itself. For free-flowing materials like dry ash or grain, you'll lose tonnage above about 25° because material starts cascading back over each flight as it lifts. For sticky or interlocking materials like wet filter cake or wood chips, you can run up to 45° before throughput collapses.
Above 45°, you need close-pitched flights (300-400 mm instead of 600-1,000 mm) and you should expect throughput to derate by 50% versus horizontal. Vertical scraper conveyors do exist but they're really a different machine — closer to a continuous bucket elevator with paddle flights.
Industry rule is 2% elongation over installed length is the action point and 3% is the absolute limit. At 2%, take up the slack with the tail tensioner. When the tensioner runs out of travel, remove a pair of links and reset.
Run past 3% and two things happen — both expensive. First, chain pitch no longer matches sprocket pitch, so the chain rides up on the sprocket teeth and the bottom of each tooth wears into a hooked profile. Once the sprocket is hooked, you can't just fit a new chain; the new chain will skip on the worn teeth and shock-load the gearbox at every revolution. Second, the flights fall out of register with the discharge chute and you start dumping material short of the chute, which jams the return run.
For a single-strand center-chain conveyor, uneven liner wear means the chain is tracking off-center, which is almost always caused by either a tail shaft that's out of square with the trough centerline, or unequal feed distribution at the inlet skirts. Material loaded heavier on one side pushes the flights and chain sideways under load.
Measure tail shaft squareness with a tape from each end of the shaft to the tail-end frame corners — you want the two diagonals within 3 mm of each other. If squareness is good, watch the inlet during a feed event; you'll usually see material dropping off-center because of an upstream chute deflector wearing through. Fix the feed pattern and the wear evens out within a month.
References & Further Reading
- Wikipedia contributors. Chain conveyor. Wikipedia
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