Start with the Material: Defining Conveyor Duty
Reading Time: 7 minutes
Conveyors are often specified with two figures: length and throughput. Both are necessary. Together, they still leave most of the duty undefined.
A belt carrying dry aggregate at 50 tonnes per hour faces a very different job from one carrying the same mass of mixed aluminium fraction. The material may occupy more space, arrive with greater impact or contain pieces that bridge across an opening. Its effect on the belt can change again when moisture increases.
Those differences decide far more than capacity. They influence the conveyor type, loading arrangement, route and conditions at discharge. Starting with a familiar machine and adapting it around the material reverses the engineering process.
A useful conveyor duty describes what will enter the system, how it will arrive and what the downstream process needs to receive. It also covers the operating pattern and credible upset conditions. Without that basis, a detailed calculation can be perfectly accurate for the wrong problem.
Define a material envelope
Throughput is usually stated as mass per hour. The belt carries the volume that mass occupies.
Mass flow = bulk density × loaded cross-sectional area × belt speed
Bulk density is therefore a design input, and it is different from the density of the solid material. A stream of shredded aluminium contains voids between individual pieces, giving it a much lower bulk density than solid aluminium. That value may change as the particle mix becomes finer or the material settles in storage.
The lowest credible bulk density is normally used to check whether the required mass flow will fit within the available belt cross-section. Light material occupies more space for the same tonnes per hour. The highest credible density matters when checking a fully loaded belt, power demand and loads carried by the supporting structure.
Using one convenient average can conceal both problems. The conveyor may run out of carrying volume with light material, then see higher mechanical loads when a denser batch arrives.
ISO 5048 provides methods for calculating the capacity and loaded cross-section of smooth belt conveyors, alongside operating power and belt tensile forces. The calculations still depend on the duty being defined correctly. A formula cannot decide which material condition should govern the design.
The material will vary a lot
A commercial material name is only a starting point. “Recycled aluminium” could describe light sheet, dense castings, extruded profiles or sections of window frame. The base material may be the same, yet those forms load and move through equipment differently.
The specification should establish a material envelope: the normal feed together with the credible extremes the conveyor must handle. Useful limits include the bulk-density range, expected moisture, contamination and maximum item dimensions. Particle distribution matters as well, particularly when occasional oversize pieces can control the belt width or chute opening.
ANSI/CEMA 550-2020 sets out a systematic method for describing bulk materials through their physical characteristics and handling properties. That is a better starting point than relying on a commercial name that tells the designer very little about behaviour.
One sample may still give a false sense of certainty. Feedstock can vary between suppliers, while seasonal conditions change moisture and upstream wear alters the product leaving a shredder or screen. The range should reflect production rather than the cleanest sample available during design.
Shape can be as important as nominal particle size. Long pieces may lie across a belt and contact both skirts. Flat items can become airborne at a transfer, while flexible material may wrap around exposed shafts. Fine particles find gaps that retain larger pieces, and sharp edges introduce a cutting mechanism that a thick belt cover may not resist.
Flow behaviour changes with condition. Free-flowing material can roll back on an incline; cohesive feed may stay on the carrying surface and then resist clean discharge. Moisture can turn a predictable hopper outlet into a source of surges or blockage. The static angle of repose also differs from the surcharge angle formed by material on a moving belt.
Testing becomes valuable when published values do not represent the duty. ASTM D6393/D6393M-25 covers Carr-index measurements for suitable powders and granular materials, including loose and packed bulk density. For cohesive bulk solids, ASTM D6128-22 covers shear testing used to determine properties such as cohesive strength and wall friction for bin and hopper design. The test method must suit the material and the decision being made.
Consider how the material will arrive
The same average throughput can create very different conditions at the loading zone. A metered stream from an upstream feeder produces a relatively stable bed. Batch loading by shovel or grab introduces a short peak, followed by a much lower average. Designing around the hourly figure can hide the few seconds that overload the transfer.
Define the normal rate, credible peak and duration of the surge. Drop height affects the impact energy entering the belt, while feed direction controls how much the stream must turn and accelerate after landing. Material arriving across the belt can push the load towards one skirt and disturb tracking beyond the transfer point.
Where practical, the chute should introduce material close to the belt centreline and in its direction of travel. The belt also needs suitable support beneath the complete loading zone. These decisions have a direct effect on spillage, wear and the stability of the load. Our article on controlling conveyor spillage examines that transfer-point behaviour in more detail.
The distinction between a feeder and a conveyor matters here. A feeder controls the rate leaving storage. A conveyor transports the rate it receives. Placing a belt directly beneath a hopper can expose it to pressure from the material head and require it to start while loaded. That is a different duty from receiving a free-falling stream at a controlled rate.
The duty will dictate the machine
Conveyor selection becomes easier once the material and loading conditions are understood. Each arrangement accepts a different balance of impact, containment, route and maintenance demand.
| Conveyor Type | Application | Consider |
|---|---|---|
| Troughed belt | Continuous transport over longer distances and at higher capacities | Impact, belt damage, stable loading and cleaning |
| Steel hinged belt or apron | Sharp scrap, heavy pieces and severe loading conditions | Moving mass, drive requirements, structural support and maintenance access |
| Screw conveyor | Enclosed transport over shorter routes, with some ability to meter | Sliding wear and material build-up on the flights |
| Vibratory conveyor or feeder | Spreading material and providing controlled feed to process equipment | Material response to vibration and transmitted structural loads |
| Pneumatic conveying | Fine powders that suit enclosed transport through pipework | Product degradation, conveying velocity, dust risk and energy demand |
The route and process interfaces still influence the choice. Inclination can reduce the stable cross-section carried by a smooth belt, especially with free-flowing material. Cleats, textured covers or sidewalls may help, though they change cleaning and discharge behaviour.
The discharge has a process job to do. Belt speed and pulley geometry set the trajectory, and a mixed stream may separate as it leaves the belt. A screen or magnetic separator may need a thin, evenly distributed bed rather than a concentrated stream. Carrying the required tonnes per hour means little if the next machine receives the material in an unusable form.
Wear protection should follow the actual mechanism. Reducing an unnecessary drop or aligning the incoming stream can improve service life before thicker components are added. Liners should protect the main structure where abrasion or impact is expected, and every wear part needs enough access for inspection and replacement.
Plan for surge conditions
Normal throughput describes only part of the operating day. The specification should state expected hours, number of starts and whether the conveyor may start under load. Reversing duty changes the requirements at both ends. Outdoor equipment also sees changing weather and material moisture.
Credible upset conditions belong in the duty. A downstream stoppage may fill the discharge chute. An oversize object can jam at a transfer, while a mistracking belt may contact the structure. An emergency stop can leave every conveyor in the line carrying material.
Controls should prevent upstream equipment from feeding a stopped conveyor. The restart sequence needs to clear the downstream route before more material is released. Sensors and interlocks must be chosen around the way the process can actually fail, with safe access retained for inspection and recovery.
These conditions affect drive selection, belt tension and the supporting structure. They also influence how much material can remain in the system during a shutdown. Leaving them until commissioning turns operating reality into a late design change.
Test with real material samples
Performance testing should use feedstock that represents production. A short run with clean, uniform material may reach the target rate without revealing carryback, unstable loading or progressive build-up. It may also miss the peak created by the normal loading method.
Acceptance should cover the way material moves through the complete system. Check sustained throughput at the credible peak and confirm that the belt remains stable. Observe transfer points for loss and dust, record drive loading and inspect the discharge into the next process. The test needs enough duration for gradual build-up to become visible.
A practical duty specification should leave the test team with clear answers:
Which material conditions represent normal production?
What extremes must the conveyor handle without intervention?
How is the material introduced, and what peak does that create?
What must the downstream process receive?
Under which conditions may the conveyor start or stop?
What evidence will demonstrate acceptable performance?
A well-defined duty gives every later decision a common reference. Belt width, drive selection, chute geometry and wear protection all return to the same material data. When that data reflects production, the design has a defensible basis and the acceptance test has a clear purpose.
ATLUS develops conveyors and transfer equipment around the material being handled and the process it must support. Learn more about our work in bulk-material processing and design engineering, or discuss a conveyor duty with ATLUS.
