13.2 Screw, Bucket, Roller, Chain & Pneumatic Conveyors
Key Takeaways
- Screw conveyor capacity falls sharply with incline, dropping roughly 30% at 15 degrees and about 55% at 25 degrees, so inclined screws must be re-rated.
- Hanger bearings are the highest-wear item in a screw conveyor and must be selected for the material — hardened, wood, or self-lubricating — never standard rolling-element bearings.
- Every inclined bucket elevator requires an anti-runback backstop so a loaded leg cannot reverse and dump its load into the boot on power loss.
- Centrifugal-discharge bucket elevators run at high speed with spaced buckets; continuous-discharge elevators run slowly with buckets nested to protect fragile material.
- Dilute-phase pneumatic conveying uses high air velocity at low pressure and suspends the material, while dense-phase uses low velocity at high pressure and moves the material in slugs.
Belt conveyors are only one part of Task D-20. Process plants move product by screw, bucket, chain, roller and air, and each has its own failure signature.
Screw Conveyors (Augers)
A screw conveyor is a helical flight welded to a pipe or shaft, rotating in a U-shaped or tubular trough, pushing material axially.
| Part | Notes |
|---|---|
| Flighting | Sectional or helicoid; hard-surfaced for abrasives; may be ribbon (sticky materials) or cut-and-folded (mixing) |
| Coupling shafts and bolts | Join screw sections; bolts are a wear item and shear if the conveyor is jammed |
| Hanger bearings | Support intermediate joints inside the material stream — the highest-wear component |
| End bearings and seals | Carry thrust; thrust bearing is placed at the discharge end so the screw is in tension |
| Trough and cover | Bolted covers must be interlocked or bolted with tools-required fasteners |
Hanger bearing selection is a favourite exam point. These bearings run submerged in the conveyed product, so a sealed ball bearing would be destroyed immediately. Choices are hard-iron or hardened-steel bushings for abrasives, oil-impregnated bronze or hardwood (rock maple) for grain and food service, and UHMW or nylon for corrosive or wash-down duty. Hanger bearings must be inspected on a scheduled interval because a failed hanger lets the screw drop and grind the trough through.
Incline de-rating: capacity is calculated for horizontal service and falls quickly as the screw is inclined, because material rolls back over the flight. Approximate loss is about 10% at 5 degrees, 30% at 15 degrees, and 55% at 25 degrees; above roughly 30 degrees a tubular housing with close-clearance flighting is required.
Trough loading is limited by material: about 45% for light free-flowing product, 30% for average, and 15% for abrasive or sluggish material. Overloading a screw stalls it, shears coupling bolts and destroys hanger bearings.
Safety: an open screw conveyor is one of the most lethal machines in a plant. Covers must be secured, guarded and interlocked, and lock-out is mandatory before removing any cover.
Bucket Elevators
A bucket elevator raises bulk material vertically using buckets on a belt or chain running between a head section and a boot section.
| Type | Speed | Discharge mechanism | Suits |
|---|---|---|---|
| Centrifugal | High (roughly 1.2–2.0 m/s belt speed) | Material thrown out by centrifugal force at the head pulley; buckets spaced apart | Free-flowing granular material — grain, sand, cement |
| Continuous | Low | Material slides over the back of the preceding bucket; buckets nested | Fragile, abrasive or aerated material |
| Positive discharge | Low | Buckets snubbed back and inverted over the head | Sticky or slow-flowing material |
Critical service points:
- Backstop (anti-runback): mandatory. Without it, a loss of power on a loaded leg lets the whole column run backward and pile into the boot, wrecking the machine and creating a serious hazard on restart.
- Boot take-up: tensions the belt or chain; must be adjusted evenly on both sides.
- Boot cleanout and inspection door: buildup in the boot causes belt slip and heat.
- Explosion protection: grain, sugar, flour, coal and many other dusts are combustible. Elevator legs commonly carry explosion relief panels, bearing temperature sensors, belt-alignment and belt-slip switches. Bearing temperature monitoring and belt-slip detection are dust-explosion prevention devices, not just maintenance conveniences.
Roller and Chain Conveyors
Roller Conveyors
| Type | Drive | Notes |
|---|---|---|
| Gravity roller | None — slope | Typical pitch 1.5–3% depending on load; roller centres must give at least three rollers under the shortest package |
| Belt-driven live roller (BDLR) | Flat belt pressed against the roller undersides | Quiet, gentle; belt tension via pressure rollers |
| Chain-driven live roller (CDLR) | Roll-to-roll chain loops | Heavy loads, pallets, hot or dirty environments |
| Line-shaft | Spools and urethane bands off a common shaft | Low cost, light duty, slip-band overload protection |
| Motorized drive roller (MDR) | 24 V DC motor inside a roller, zoned control | Zero-pressure accumulation, energy efficient |
Roller conveyor faults a millwright fixes: seized bearings (a single locked roller scuffs product and stalls light loads), out-of-square frames causing product skew, worn drive bands, and incorrect roller pitch after a product change.
Chain and Drag Conveyors
- Apron / pan conveyors — overlapping steel pans on two strands of heavy chain; used for hot, sharp or very heavy material such as forgings or foundry sand.
- Slat conveyors — slats on chain, used in assembly and bottling.
- Drag / en-masse conveyors — flights on a chain pull material through an enclosed trough; compact and dust-tight, common in grain and biomass.
- Overhead trolley conveyors — enclosed-track or I-beam trolleys on chain for paint lines and meat processing.
Chain conveyor service centres on chain elongation (measure over a defined number of pitches; replace at roughly 2–3% elongation), sprocket tooth hooking, correct catenary sag, and lubrication compatible with the product. Both strands on a two-strand conveyor must be tensioned and phased identically or the flights will run crooked and gouge the trough.
Pneumatic Conveying
Pneumatic systems move dry bulk powder and granules through pipe using air.
| Mode | Air velocity | Pressure | Behaviour | Suits |
|---|---|---|---|---|
| Dilute phase | High (roughly 15–35 m/s) | Low (under about 100 kPa) | Particles fully suspended in the air stream | Non-abrasive, non-fragile powders; long routing flexibility |
| Dense phase | Low (roughly 2–10 m/s) | High (up to about 600 kPa) | Material moves as slugs or plugs along the pipe bottom | Abrasive or friable products; lower product degradation and pipe wear |
System types are pressure (push) from a blower or compressor, vacuum (pull) using a vacuum blower and receiver, or combined. Key components include the blower or compressor, the rotary airlock (rotary valve) that meters material into a pressurized line while sealing against air blow-back, diverter valves, the receiving cyclone or filter receiver, and the bin vent filter.
Rotary airlock service is core millwright work: vane-tip-to-housing clearance is typically 0.1–0.2 mm and must be measured and restored, because worn clearance lets conveying air blow back through the valve, killing throughput. Airlocks are also a severe amputation hazard and must be locked out and the inlet chute cleared before any hand enters the housing.
Pneumatic system troubleshooting: a line plug is usually caused by velocity dropping below the material's saltation velocity — too little air, a leaking airlock, a partially closed diverter, or a wet product. Excessive pipe elbow wear indicates dilute-phase velocity that is too high for an abrasive product; long-radius or ceramic-lined elbows are the fix.
A horizontal screw conveyor handling wood chips is re-installed at a 20 degree incline to feed a new hopper, keeping the same screw, drive and speed. Downstream production drops well below expectation. What is the technical explanation?
Why must hanger bearings in a screw conveyor use hardened bushings, hardwood, bronze or UHMW rather than sealed rolling-element bearings?
A dilute-phase pneumatic conveying line moving abrasive alumina is wearing through its elbows every few weeks. What change most directly addresses the cause?