Coffee Screw Conveyor

Will a screw conveyor break green coffee beans?

At correct operating speeds for coffee — 30 to 60 RPM — breakage is low and acceptable for standard dry-mill applications. The key factors are: do not overfill the trough beyond 40% cross-sectional fill; keep RPM at the lower end for delicate Arabica or specialty peaberry lots; ensure the inlet chute is not creating a drop height that impacts beans against the flight. Compared with a bucket elevator (which has impact forces at the boot and head), a slow-running screw conveyor is generally gentler. For extremely fragile micro-lot specialty coffee, a belt conveyor is the lower-risk choice for the final transfer before bagging.

What is the maximum incline angle for a coffee screw conveyor?

For green coffee, 15–20° is the practical maximum with short-pitch flighting. At 10° incline, expect approximately 15–20% capacity reduction compared with the horizontal rating. At 15–20°, expect 30–50% capacity reduction and a tendency for beans to slide back during stops — include a discharge gate if the conveyor will be stopped under load. Beyond 20°, a bucket elevator is the correct machine: it maintains rated capacity on any steep incline and is gentler than forcing a screw at steep angles where beans are compressed against the trough floor under the weight of material above.

What is the difference between a U-trough and a tubular screw conveyor for coffee?

A U-trough has an open top covered by a removable bolted cover — the standard configuration for coffee dry mills. The cover is easy to remove for cleaning, inspection, and clearing blockages. A tubular (pipe) housing completely encloses the screw in a round cross-section tube; it is more dust-tight than a U-trough but harder to access for cleaning and inspection. For coffee, the U-trough is almost always the correct choice: coffee is not toxic and the ability to quickly inspect and clean the trough between lots matters more than the marginal improvement in dust sealing that a tubular conveyor provides.

Can I customise the trough length to fit my mill layout?

Yes. VMAC fabricates screw conveyors to custom lengths to match your plant layout. Standard modular trough sections are available in 1.5 m, 2 m, 3 m, 4 m, and 6 m lengths, flanged and bolted together. When ordering, provide the required centre-to-centre distance between inlet and discharge, the inlet and discharge heights, and whether any intermediate discharge openings are required. Custom inlet chute angles and discharge spout positions are fabricated as part of the order at no additional fabrication premium for standard custom lengths.

Should I use mild steel or stainless steel for a coffee screw conveyor?

Mild steel is the standard material and is appropriate for all conventional dry-mill applications transferring dry green coffee at normal ambient moisture. It is cost-effective, readily weldable for repairs, and surfaces in contact with coffee have no health or contamination concern for standard commercial grades. Stainless steel (SS 304) trough and flighting is recommended for specialty-grade and export-certified operations that require food-contact compliance documentation, for installations in humid coastal mills where condensation causes surface rust on mild steel, and for any application where the conveyor will also handle washed parchment or wet coffee from a honey-process station.

How do I size the screw conveyor for my throughput?

Match the screw diameter to your peak hourly throughput — not your average. A 200 mm auger conveyor rated at 3,500 kg/hr handles a mill running at 2,000 kg/hr comfortably while providing headroom for surge loads. Running a screw at 60–70% of its rated capacity produces better flow and lower breakage than running at 95% capacity. For the main transfer after the huller in a medium curing works processing 4 TPH, a 250 mm or 300 mm screw conveyor is appropriate. For small estate transfers under 1 TPH, a 150 mm or 200 mm unit is standard.

How do I clean a screw conveyor between coffee lots or at season end?

For lot-to-lot changeovers, run the conveyor empty for 30–60 seconds after the last of the previous lot has cleared — this purges residual beans from the trough. For season-end cleaning, remove the trough cover panels, brush or vacuum the trough floor and flight surfaces, and inspect the hanger bearings for accumulation of chaff and bean debris around the bearing housing. Do not use water to wash a mild-steel trough — dry cleaning only. Stainless-steel troughs can be wiped down with a damp cloth after dry brushing. Reassemble the cover panels and run the empty screw briefly to verify free rotation before the next season.

What maintenance does a coffee screw conveyor require?

Routine maintenance is minimal: lubricate the end bearings and hanger (intermediate) bearings per the schedule — typically every 200–300 operating hours for grease-packed bearings. Inspect the helical flighting for wear at the outer diameter — the flight tip is the highest-wear surface. Check the trough floor for grooves worn by the flight tip; a clearance of 3–6 mm between flight tip and trough is normal; if it exceeds 10 mm, separation efficiency drops and bean fines accumulate under the flight. Check the shaft-to-gearbox coupling alignment annually. Replace the drive belt or chain (if applicable) at the first sign of stretching or cracking.

Is a screw conveyor or a belt conveyor better for transferring coffee in my dry mill?

It depends on your priorities. Choose a screw conveyor (auger conveyor) when dust and chaff containment is critical — for example, directly under a huller or screen grader where parchment dust is heavy — or when your transfer run is short (1–6 m) and floor space is limited. Choose a belt conveyor when bean gentleness is paramount (specialty peaberry or micro-lots), when the run is long (more than 8–10 m where belt costs become competitive), when the incline exceeds 20°, or when the material may contain residual stones or grit that would rapidly abrade a screw flight. In practice, most Indian curing works and East African mills use both types in the same building — screws for enclosed dusty transfers, belts for long open-hall runs.