Apron feeders: how they work, the capacity calculation, design points at the hopper and discharge, and the wear and maintenance of each part
Apron feeders for mining and bulk handling: the pans, chain, rollers, sprockets, drive and skirts and what each does, the capacity calculation from bed width, depth, speed and density with a worked example, the design points at the hopper outlet and discharge, drive power from bed shear, and the wear and maintenance of each part, including trending chain pitch. With a side elevation drawing.
Key takeaways
- An apron feeder is a slow, heavy chain conveyor of overlapping steel pans that draws material from under a hopper or dump pocket at a controlled rate.
- It suits large, heavy, abrasive or sticky material and high impact from trucks and loaders, which is why it sits under ROM bins and ahead of primary crushers.
- Capacity is skirt width times bed depth times pan speed times bulk density, with a fill factor. Speed is kept low, commonly well under a third of a metre a second, to limit wear.
- Most of the drive power goes into shearing the bed against the skirts and the material above, so the hopper and skirt geometry matter as much as the throughput.
- Chain and sprockets are the big wear items. Measuring chain pitch and trending it plans their change, and keeping a bed of material on the pans protects them from impact.
A primary crusher is only as steady as the feeder in front of it. Haul trucks dump into a ROM bin in surges of a hundred tonnes or more, with rocks the size of a fridge. Something has to take that impact, hold the load of the bin, and deliver an even stream to the crusher hour after hour. On most mine sites that is an apron feeder.
This guide covers apron feeders: the parts and what each does, the capacity calculation, the design points at the hopper and the discharge, and the wear and maintenance of each part. It sits beside the guides to fixed plant maintenance in mining, crusher maintenance and liners and the bulk materials handling design brief. The figures are illustrative.
How an apron feeder works
Overlapping steel pans are bolted to two strands of heavy chain, which run over carrying rollers on the loaded side, round a driven head sprocket at the discharge and back over return rollers to the tail. Material in the hopper sits on the pans, and as they move it is drawn out under the hopper wall and between the skirts as a bed of set depth. At the head it falls onto the next conveyor, crusher or screen.
The feed rate is set by the speed of the pans, through a variable speed drive, and by the bed depth, fixed by the skirts or a gate. Apron feeders run slowly, which keeps wear down and lets them handle material that would tear a belt: large lumps, heavy impact, abrasive ore, hot clinker or sticky clay.
| Part | What it is | Notes |
|---|---|---|
| Pans (flights) | Overlapping cast or fabricated steel pans bolted to the chain, carrying the material | Sized for impact and abrasion, overlapped so fines do not drop through |
| Chain | Two strands of crawler track type chain, or strand chain on lighter feeders | Pitch matched to the pans, with sealed and lubricated joints on heavy duty |
| Carrying rollers | Rollers under the chain on the loaded run, closest under the hopper | Take the weight of the bed and the impact of dumping |
| Return rollers | Support the chain on the empty return run | Lighter duty, but sit where spillage collects |
| Head shaft and sprockets | Drive the chain at the discharge end | Sprocket teeth wear with the chain, and are changed with it |
| Tail shaft and take-up | Idler wheels at the loading end, with screw or hydraulic take-up for chain tension | Sets the chain sag on the return run |
| Drive | Motor, variable speed drive and a planetary or shaft mounted gearbox, or a hydraulic drive | High torque at low speed, with the speed varied to set the feed rate |
| Skirts and liners | Side plates that hold the bed on the pans, lined with wear plate | Set the bed width, and usually widen toward the discharge |
Capacity
The capacity of an apron feeder is the volume of the bed moving past a point each second, times the bulk density. The bed is the skirt width by the bed depth, and a fill factor allows for the bed surface not being flat. The worked example below is illustrative. A design would also check the capacity at the minimum and maximum drive speed, and against the rate the next machine can accept.
| Input | Value | Notes |
|---|---|---|
| Skirt width, b | 1.8 m | Inside the skirt liners at the discharge |
| Bed depth, h | 0.9 m | Set by the skirt height or a gate |
| Pan speed, v | 0.12 m/s | Apron feeders run slowly to limit wear |
| Bulk density, ρ | 1.8 t/m³ | Loose, as fed |
| Fill factor, k | 0.85 | Allows for the surface profile of the bed |
| Capacity, Q = 3600 × b × h × v × ρ × k | about 1,070 t/h | The rate at the design speed, adjustable with the drive |
Speed sets the wear. Make the feeder wide and deep enough to run slowly.
Design points at the hopper and the discharge
Most apron feeder problems start at the interface with the hopper. If the feeder draws only from the front of the outlet, the back of the hopper stagnates, material segregates and the load on the pans is uneven. Skirts that widen toward the discharge, or an outlet that tapers, give an even draw along the whole length. The drive power, likewise, comes mostly from shearing the bed against the skirts and the material above it, so it depends on the hopper and skirt geometry, not just on the tonnes per hour.
These are the questions to settle in the design brief and check at the design review, with access for chain, pan and roller changes designed in from the start.
| Point | The issue | What helps |
|---|---|---|
| Hopper outlet | Material should draw across the whole outlet length, not just the front | Skirts that widen toward the discharge, or an outlet that tapers, give an even draw |
| Load on the pans | The weight of the material in the hopper sits on the feeder | A sound bed and the right outlet geometry keep the load on the pans close to the bed weight rather than the full head |
| Impact | Material dumped from a truck or loader lands on the pans | Keep a bed of material on the feeder so rock lands on rock, not on steel |
| Discharge | Material falls off the head end onto the next conveyor or crusher | A chute and spill plate that centre the load, and a dribble conveyor for fines |
| Drive power | Mostly the shear of the bed against the skirts and the material above, plus any lift on an incline | Calculated from the hopper and skirt geometry, not from the throughput alone |
Wear and maintenance
The chain and sprockets are the largest wear cost, and they are changed together, because a new chain on worn sprockets, or the reverse, wears quickly. Chain pitch measured over a set number of links, trended over time and projected to the wear limit, plans the change into a shutdown rather than leaving it to a failure, which is condition based maintenance applied to the biggest item on the feeder.
Pans, rollers and skirt liners are inspected on a regular walk around, with the carrying rollers under the hopper given the most attention because they carry the most load. Spillage is the quiet problem: fines that escape the skirts or drop at the head build up under the feeder, bury the return rollers and the tail, and turn a small leak into a cleanup job every shift.
| Part | Wear and failure | Practice |
|---|---|---|
| Pans | Cracking, wear through, bent edges, loose or missing bolts | Inspect each pan on a walk around, change before material leaks into the chain |
| Chain and sprockets | Pin and bush wear, chain stretch, tooth wear, link cracking | Measure pitch over a set number of links and trend it, change chain and sprockets together |
| Carrying rollers | Seized or flat rollers, failed seals, worn treads | Check rotation and seals under the hopper, where rollers work hardest |
| Skirts and liners | Liner wear, gaps that let material escape and jam | Set and maintain the gap to the pans, change liners on wear limit |
| Drive | Gearbox oil degradation, torque arm wear, coupling wear | Oil analysis and condition checks on the gearbox, torque arm and coupling inspection |
| Spillage | Fines under the feeder burying return rollers and the tail | Dribble conveyor or regular cleanup, with the source traced and fixed |
Working on an apron feeder
An apron feeder holds energy in ways that are easy to miss: the weight of material in the hopper above it, chain tension at the take-up, and the drive. Work on the pans or chain with material in the hopper needs the hopper emptied or the load secured, and every source of energy isolated under the site's permit to work and isolation process. AS/NZS 4024.3610 sets out the general safety requirements for conveyor systems in Australia.
Common mistakes
- Running the feeder fast to make up for a narrow or shallow design, so it wears out quickly.
- Letting the hopper run empty, so dumped rock lands directly on the pans.
- Changing the chain without the sprockets, or the sprockets without the chain.
- Never measuring chain pitch, so the change is set by a failure.
- Skirt liners set with large gaps to the pans, so material jams and escapes.
- Spillage under the feeder accepted as normal.
- Sizing the drive from throughput alone, without the shear load from the hopper.
Common questions
What is an apron feeder?
An apron feeder is a heavy duty feeder made of overlapping steel pans bolted to two strands of chain, running slowly under a hopper, bin or dump pocket. It draws material out at a controlled rate and delivers it to a conveyor, crusher or screen. It is used for large lump sizes, abrasive or sticky material and heavy impact loading, typically in mining and quarrying.
How do you calculate apron feeder capacity?
Capacity in tonnes per hour is 3600 times the skirt width in metres, times the bed depth in metres, times the pan speed in metres per second, times the bulk density in tonnes per cubic metre, times a fill factor for the bed profile. A 1.8 m wide feeder with a 0.9 m bed at 0.12 m/s carrying material of 1.8 t/m³ with a fill factor of 0.85 delivers about 1,070 t/h.
What is the difference between an apron feeder and a belt feeder?
A belt feeder uses a conveyor belt and suits finer, less abrasive material with little impact. An apron feeder uses steel pans on chain and suits large lumps, heavy impact from dumping, hot, abrasive or sticky material. Apron feeders cost more and run slower, but survive duty that would destroy a belt.
What speed does an apron feeder run at?
Slowly. Apron feeders commonly run at a few hundredths to a few tenths of a metre per second, much slower than a belt conveyor, because wear on the pans, chain, rollers and sprockets rises steeply with speed. The feed rate is usually set by a variable speed drive, with the bed depth fixed by the skirts or a gate.
What wears out on an apron feeder?
The chain and sprockets, from pin and bush wear and tooth wear. The pans, from impact and abrasion. The carrying rollers under the hopper, from load and failed seals. The skirt liners, and the return rollers and tail section where spillage builds up. Chain pitch measured over a set number of links and trended is the usual way to plan a chain and sprocket change.
Key terms
Plain-language definitions from our glossary for the concepts this article leans on.
Standards and further reading
- CEMA publications on bulk handling equipment (Conveyor Equipment Manufacturers Association)
- AS/NZS 4024.3610:2015 Safety of machinery, conveyors, general requirements (Standards Australia)
- TUNRA Bulk Solids, research and testing for bulk materials handling (University of Newcastle)
Related case studies and tools
- Conveyor Capacity Calculator (tool)
- Grain flow paths and chute upgrades guided by DEM (case study)
Related reading
Running plant like this?
Enco helps the teams that run and maintain it, with maintenance strategy, asset data and planning, condition monitoring and reporting software, and engineering studies. See the work we have delivered, or tell us what you are dealing with.