Screw conveyors for sticky and fibrous materials

A screw conveyor is a simple machine right up to the day the material changes. Wet fines, bark, shredded waste, filter cake, wood chips with the long splinters left in: each turns a screw that ran for years into one that trips its overload twice a shift. These are our notes on what we look at when a screw jams, and on the choices that decide whether it will.

Why do screws jam on sticky or fibrous material?

A screw moves material only because the trough and gravity stop the material turning with the flight; the flight then slides under it and pushes it along. Anything that lets the load rotate with the screw, or fills the free space around the shaft, stops the conveying.

• Build-up on the shaft and flight — sticky material packs into the corner between shaft and flight, shrinking the free area and the pitch until the screw carries a solid core round with it.
• Fibres wrapping — long fibres wind onto the shaft and, above all, onto the hanger bearings where the flight is interrupted, forming a plug later material cannot pass.
• Bridging at the inlet — the material arches over the screw in the hopper above; the screw runs empty, then the arch collapses and floods it.
• Trough loading too high — a screw filled well above its design fraction lifts sticky material over the shaft, where it rides round instead of moving forward.
• Compaction at the discharge — if the outlet cannot pass material as fast as the last flight delivers it, a plug forms against the end plate and the screw stalls.

Which flight for which material?

Standard pitch

Pitch roughly equal to the diameter. General conveying of free-flowing material, horizontal or on a slight incline; the reference for the others.

Short pitch

Pitch around two thirds of the diameter or less. Better rate control for metering, less fall-back on inclines, and the usual last section of a feeder screw.

Variable pitch

Pitch increasing along the inlet so that capacity grows along the length and the whole hopper outlet draws, not just the back end — why a screw under a hopper is a feeder, as the silo flow note explains.

Ribbon flight

The flight held off the shaft on lugs, leaving an open core. Sticky material has no corner to pack into and is turned over as it moves; capacity is lower for the diameter.

Paddle and cut flight

Flights cut away, folded or replaced by paddles to mix, break lumps or aerate. Capacity falls and power rises; for conditioning, not distance.

Shaftless

A heavy spiral with no central shaft, riding on a replaceable trough liner. No hanger bearings, nothing for fibres to wrap; the liner is a consumable and speed is limited, but for fibrous and sticky duties it is often the first choice.

How full and how fast?

The two settings that decide most jams are trough loading — the fraction of the cross-section the material occupies — and speed. Free-flowing material can run fairly full; sticky, fibrous or abrasive material is run at a small fraction of the cross-section, commonly a third or less. The same tonnage then needs a larger diameter, which also lets the screw turn slower, and slow and large beats small and fast on every difficult material: less lifting over the shaft, less heat, less wear. Inclination cuts capacity sharply and sticky material falls back on itself, so difficult screws stay close to horizontal.

What do we look at when a screw jams?

1. The inlet

   Flood-fed from a hopper, or fed at a controlled rate? A flood-fed screw needs a feeder section — short pitch, reduced diameter or variable pitch — or it takes everything the hopper gives; an extended inlet spreads the draw.

2. The hangers

   Every hanger bearing is a place where the flight stops and the material must be pushed through a gap. On fibrous material we count them and try to remove them: shorter screws, larger shafts, or a shaftless spiral.

3. The discharge

   Is the outlet at the very end of the trough, or is there a dead pocket beyond it where material packs? Can the chute below block? A blocked discharge stops the screw, so it gets a level probe and the drive an overload limit.

4. The trough and the liner

   Sticky material releases from a smooth polymer liner far better than from mill-finish steel; abrasive material wants hard plate and a hard-faced flight edge. A drop-bottom trough makes cleaning a job instead of a shift.

5. The material on its worst day

   Moisture, fibre length and temperature at the wettest, coldest point of the year. The material properties note sets out what to measure; the biomass storage note shows how much a fuel changes between delivery and use.

When is a screw the wrong answer?

Over long distances, where hangers multiply and torque climbs; on very abrasive material, which grinds flight and trough until the clearance passes fines back; on large lumps, which wedge; and at high rates, where power per tonne is poor. For those the comparison of conveyor types points to a belt, a drag chain or an elevator. Screws remain unbeatable for short, enclosed, metered duties, which is where our conveying systems work puts them, alongside the other moist or sticky duties in the Flow Finder.