A helicoid screw is a positive-displacement, friction-dependent device: with one moving part in contact with the bulk, the rotating flight both pushes and shears the material, so any particle caught between the flight edge and the trough wall experiences a defined crushing load on every revolution [S5].
Damage is therefore a function of geometry, fill level, and speed rather than an unavoidable property of the equipment, and engineers who control those three variables can keep a screw conveyor inside the breakage budget of moderately fragile bulk solids [S1][S5].
Where the breakage actually happens inside an auger
The literature is consistent on the failure mechanism: kernels or granules are pinched between the flight edge and the tube or trough wall, and additional surface damage is produced by continuous sliding friction along the bottom of the casing [S5]. A 15 cm (6 in.) auger handling dry shelled corn at high rotational speed and partial fill can produce "considerable kernel damage," while the same auger operating full and slow preserves the product [S5]. A 25 cm (10 in.) diameter unit running 24 m (80 ft) long draws roughly 30 kW (40 HP) and illustrates the torque and pressure environment inside a working screw [S5].
Hanger bearings, when fitted so the screw is not supported on the casing, reduce kernel pinching but complicate cleanout, a direct trade-off between breakage rate and sanitary design [S5]. Worn hanger bearings and misaligned coupling shafts amplify local crushing loads at intermediate supports, so maintenance condition is itself a damage variable [S8].
Friable materials the screw conveyor should not be asked to move
The class of products flagged as incompatible with conventional augers is well defined in the industry: cereal flakes, chewing gum and confectionery, coffee beans, cookies, fruits and vegetables, ice cubes, pasta, pet food, snack foods, vitamins, ammonium nitrate, coated seeds, fertilizer prills, friable pellets, pastilles, propellant, metal crystals, sulphur flakes, and TNT flakes [S2]. A lead screw operates on the same helix-on-cylinder principle at much lower clearance and even tighter tolerances, which is why lead screws are never used for friable bulk and only as motion-transmission hardware.
Manufacturers of gentle-handling equipment treat auger rotation as a known degradation source rather than a tunable parameter: the rotation of the helix within the screw conveyor can cause damage to fragile materials such as nuts and coffee, and cable/disc or tubular systems are recommended in its place [S3]. Spiroflow's flexible screw conveyor is positioned as suitable for blends rather than single delicate pieces, because the helical motion continually re-mixes the batch and tolerates some breakage that a pure transfer would not [S4]. For materials requiring gentle handling or where friability is a concern, a belt conveyor is generally specified over an auger on cost-of-damage grounds [S6].
Selection rule: use a screw conveyor only inside a defined breakage envelope
A practical decision matrix lines up the four bulk-handling options against the three decision variables that actually drive breakage: contact mechanics, transfer points, and containment of fines. [S4]
First, contact mechanics. A standard screw conveyor uses a metal flight dragging against a metal trough, which is the highest local contact-stress option. A flexible screw conveyor replaces the rigid flight with a spiral in a polymer tube, dropping contact stress but retaining helical re-mixing [S4]. A tubular cable and disc conveyor replaces the flight with a sealed nylon-jacketed stainless cable and solid discs that do not contact the product, giving the lowest local stress [S3]. A ball screw recirculates balls between nut and screw for low-friction motion, but is a positioning element, not a bulk conveyor, and illustrates how contact stress is engineered out by replacing sliding with rolling.
Second, transfer points. Bucket, cable, and tubular conveyors can run horizontal-to-vertical-to-horizontal from a single drive, eliminating transfers between machines, and each transfer is identified as a major cause of product damage [S2]. A screw conveyor is typically a single horizontal or shallow-incline run, so it is competitive on transfer count but loses on contact mechanics.
Third, containment. Aero mechanical and enclosed cable/disc systems are airtight, which matters for combustible-dust snack products because static-bearing dust stays inside the conveyor rather than feeding an explosion [S4]. A chain conveyor is a closer mechanical relative of a screw, using an endless chain rather than a helix, and offers higher allowable particle size and lower rotation-induced crushing for the same throughput range, but still drags product along a casing and is not classed as a gentle-handling device for friables.
The resulting rule is: a standard screw conveyor is acceptable for robust granules, pellets, and free-flowing bulk where up to a few percent of fines generation is tolerable; a flexible screw is acceptable for blends and mildly fragile flakes; a tubular cable/disc, aero mechanical, or interlocking bucket system is required for true friables, coated products, and any dusty combustible stream.
Operating variables that cap breakage in a screw
When the process forces an auger, four levers reduce damage to a defensible level, and each is documented in the research. [S5]
Run the screw full, not partially filled. A partially filled, rapidly rotating auger maximises kernel damage, while a large-diameter screw operating full and rotating relatively slowly achieves the rated capacity with minimal breakage [S5].
Drop rotational speed. UniTrak's TipTrak documentation treats motor speed as a primary degradation control, with operators slowing the conveyor for more friable products and raising it for robust bulk [S2].
Hold the slope near horizontal. At slopes up to roughly 30 degrees from horizontal, product slides on the advancing face of the flight, distributing load over the face; above that angle the material rides on the flight tip, increasing point loads on individual particles [S5].
Specify proper hanger-bearing geometry. Bearings arranged so the screw is not supported by the tube reduce kernel pinching, at the cost of more demanding cleanout that must be managed by slope or by wash-down design [S5][S8].
These four controls do not turn a screw conveyor into a gentle-handling device; they only narrow the gap between acceptable and unacceptable breakage for moderately fragile bulk.
Where a screw conveyor is the wrong default
Three application classes fail the breakage budget even with the levers above, and should be redirected at the specification stage rather than retrofitted after commissioning. [S4]
Food and snack flakes, including cereal, crisps, pretzels, and coated nuts, where the public-facing defect rate is a marketing problem and a few percent breakage is a recall driver [S2][S3][S4].
Industrial friables and energetic materials, including ammonium nitrate, sulphur flakes, propellant, and TNT flakes, where breakage creates dust, and dust plus static plus an auger-fed air path is an explosion hazard, not just a yield loss [S2][S4].
Coated and structured products, including coated seeds, catalyst carriers, and metal crystals, where the coating or crystal face is the saleable attribute and a screw flight will strip it [S2][S3].
For each of these, a cable-drag, tubular, or interlocking-bucket conveyor is the primary recommendation in the cited vendor literature, with a flexible screw retained only for blends where re-mixing is desired and minor breakage is acceptable [S2][S3][S4].
Standards, codes, and verification that actually apply
There is no ISO or CEMA standard that defines a maximum allowable breakage percentage for a screw conveyor handling a given bulk solid; CEMA Standard No. 350 and the related screw-conveyor dimensional standards govern capacity, pitch, and power, not product integrity. Verification is therefore an application test, not a standard-driven check: run a trial of the candidate screw at the planned diameter, speed, and fill level, measure the percent of fines or broken pieces at discharge, and compare against the product specification. [S5]
For sanitary applications the conveyor must additionally meet the cleanability requirements of the food or pharma code in force at the site, and hanger-bearing geometry should be reviewed against those requirements at the same time [S5].
The breakable-product question, in other words, is answered by an engineered test on the actual bulk solid at the actual speed and fill level, not by reference to a published standard [S1][S5].
The next signal worth tracking is CEMA's ongoing work on screw-conveyor power and capacity, since any tightening of the recommended full-fill operating point would shift the speed-and-fill envelope for friable applications; the other is vendor disclosure of measured breakage rates for flexible screws handling coated and flavoured snack products, which is the dataset most often missing in current cut sheets [S4][S5].
This topic is covered further in Directional vs Proportional Hydraulic Valve: Spec Decision Map.