Food-grade screw conveyors are specified with 304 or 316 stainless steel product-contact surfaces, sanitary sealed hanger bearings, and fully open trough ends to support wash-down and clean-in-place (CIP) regimes [S1][S2].
For bakeries, dairy, confectionery, beverage, and pet-food plants, the differentiating variables are moisture content, fat/oil load, particle geometry, and the required sanitation class — not raw conveyor capacity [S1].
Why food-grade differs from industrial screw conveyors
A standard industrial screw conveyor built to CEMA dimensions uses carbon-steel flights, mild-steel troughs, and greased iron hanger bearings — a configuration that fails FDA and EHEDG expectations in any wet or allergenic food line [S2]. Inotek's cement-duty SCC series, for example, ships in Q114–Q323 mm diameters with standard reducer ratios of 140, 200, and 240 rpm — useful for comparison, but with no sanitary finishing on the flight or trough [S1].
Food-grade units use continuously welded 304 or 316 stainless flights polished to a defined surface roughness (commonly Ra ≤ 0.8 µm on product zones) and fully enclosed or shaftless screws to eliminate product entrapment, with bearing housings sealed to IP65/IP66/IP69K depending on wash-down intensity [S1][S2].
Selection criteria that drive the spec
The first mechanical decision is shafted versus shaftless: shaftless screws (crimped or ribbon flights on a single wire or rod-less helix) carry wet, sticky, or fibrous materials — wet grain mash, cocoa liquor, spent yeast — that would wrap a conventional shaft; shafted screws remain standard for dry, free-flowing powders like flour, sugar, salt, and milk replacer [S1][S2].
Capacity is then set by the CEMA equation modified for the food product's bulk density, angle of repose, and moisture, with conveyor diameter typically in the 100–400 mm range for inline batching and 150–600 mm for bulk intake from silos or dumper bags. Inclined runs above 15° require paddled or cut-flight screws to prevent rollback of granular product [S1].
Sanitation class dictates the rest: USDA Dairy, 3-A Sanitary Standards, and EHEDG Type EL Class I/II each impose different trough-end seal, gasket, and bearing-lubricant rules; food-grade grease (NSF H1 registered) is the practical baseline for any bearing within the product zone [S2].
Materials, finishes, and components
Product-contact flights are commonly 304 stainless for general dry goods and 316 stainless where chloride exposure is high — salted snacks, brines, brewing, and certain dairy cleaning chemistries. Helicoid flights cold-rolled from special steel in 3–10 mm thickness and 15–500 mm outside diameter are the stock geometry that food OEMs re-cut to spec [S1].
UHMW or PTFE liner strips on the trough inside diameter reduce friction and prevent metal-to-metal abrasion on stainless flights, extending service life when conveying abrasive sugar or coarse grain. KWS maintains an inventory of CEMA stock components manufactured to CEMA Standards and available for immediate shipment [S2].
Comparison of the main food-duty screw types
Four configurations cover almost every food and beverage application: (1) shafted U-trough with hinged covers, used for dry bulk flour, sugar, and grain — lowest cost, hardest to clean; (2) shafted tubular/pipe screw, fully enclosed tube for dust-free transfer of flour or starch, cleanable but limited length per intermediate bearing; (3) shaftless screw, the workhorse for wet, sticky, or fibrous streams such as wet grain, fruit pulp, and confectionery waste; (4) paddle or cut-flight screw for inclined lifts and dewatered cake [S1][S2].
On four decision criteria the comparison is: cleaning access — U-trough (best manual), tubular (best CIP), shaftless (good but open trough end), paddle (poorest); moisture tolerance — shaftless wins, tubular and U-trough fail in long wet runs; capacity at 250 mm diameter — shafted designs lead, shaftless trails by roughly 25–40% in the same envelope; capital cost — U-trough is lowest, tubular and shaftless run 1.5–2.5× depending on metallurgy and length [S1][S2].
Use cases by sub-industry
Grain, flour, and dry-blend operations typically specify shafted U-trough in 304 stainless with FDA-acceptable gaskets and removable cover clamps for daily wash-down; milk-replacer and whey-powder plants push further toward tubular enclosed screws to manage dust and allergen control [S1][S2].
Brewery spent-grain and wet-mash lines, as well as distillery draff handling, are the dominant shaftless application: the lack of a central shaft prevents binding and wrap, and the open trough end allows direct discharge into a compactor or truck. In pet-food extrusion lines, inclined screw conveyors at 30–45° lifts use paddled flights to move kibble and bits without crushing, often paired with belt conveyor or chain conveyor sections downstream for finished-product handling [S1][S2].
Limitations, failure modes, and what to avoid
Screw conveyors are not gentle — they shear and compress product, so fragile items (whole berries, soft pasta, expanded snack shapes) suffer damage above 200 rpm. Where breakage losses exceed 2–3% of throughput, vibratory or belt options should be considered instead [S1].
Common failure modes in food service are: hanger-bearing grease wash-out in CIP (replace with NSF H1 food-grade grease or sealed polymer bearings); flight wear at the intake end when abrasive sugar or salt is fed without a UHMW liner; trough-end seal failure on U-trough designs where gaskets were underspecified; and product buildup at the shaft-to-flight weld on shafted designs — a 3-A and EHEDG audit finding on older lines [S2]. The 48.3% early-shipment rate that KWS publishes on its industrial line is irrelevant to food-grade lead time, which is dominated by sanitary welding capacity and surface-finish scheduling [S2].
Sourcing, standards, and build verification
Specify to the CEMA Standard 300/350 dimensions for interchangeability of stock components, then overlay the food-specific requirements: 3-A Sanitary Standards for dairy and egg processing, USDA-FSIS construction guidance for meat and poultry, and EHEDG Doc. 2 for hygienic design in wet processing [S1][S2].
Ask the vendor for documented material certificates on every product-contact surface (304 vs 316 traceable to heat), surface-roughness measurement records (Ra values on welds and flights), and a weld procedure specification that calls for continuous welds ground flush — not stitch welds that trap product. For long B2B procurement, additional conveyor spec context for distribution centers and package-handling conveyor options is worth a side read when the upstream or downstream line is also being scoped [S2].
Final signal: a 316-stainless tubular screw for high-moisture wet grain and a 304-stainless shafted U-trough for dry flour/sugar remain the two stock configurations to quote first; track whether the OEM offers NSF H1 grease as standard and whether 3-A or EHEDG certificates are bundled with the build sheet on the next RFQ round [S1][S2].