Material Handling Equipment: Belt Conveyors, Screw Conveyors and Bucket Elevators

Quick Answer

Bulk material handling equipment is selected on material properties first, geometry second. Belt conveyors are the most economical for long horizontal and gently inclined runs, with maximum incline limited by the material's angle of repose — typically 18-20° for most ores and aggregates, up to 22° for crushed stone, and only 12-15° for rounded or fine free-flowing material — beyond which cleated, sidewall or pocket belts are needed. Screw conveyors suit short runs up to about 30-40 m with capacities to roughly 100 m³/h, handle inclines to 20° with falling efficiency, and are unsuitable for abrasive and sticky materials. Drag and en-masse chain conveyors move material in a fully enclosed trough, with high capacity in a small cross-section and the ability to turn corners. Bucket elevators handle vertical lifts to 60 m and beyond — centrifugal discharge at belt speeds of 1.2-2.0 m/s for free-flowing materials, and continuous discharge at 0.5-1.0 m/s for fragile or abrasive materials where impact must be avoided. Belt conveyor power is calculated from CEMA or DIN 22101 methods; the one figure to get right is that transfer chutes and skirting cause more operating problems than every other conveyor component combined — spillage, belt damage, blockage and dust all originate there.

A cement plant in West Africa commissioned a new clinker transport line: a 240 m belt conveyor at 14° incline feeding a bucket elevator, feeding a silo.

Within eight weeks the belt was spilling material along the first 40 m of its length, the bucket elevator was returning about 15% of its load down the boot, and the plant had three men on near-continuous clean-up duty.

The conveyor was correctly sized. The belt, drive, idlers and structure were all fit for purpose. Two things were wrong, both at transfer points. The loading chute discharged material onto the belt at an angle and velocity that did not match the belt's direction and speed, so every load impacted and bounced. And the bucket elevator inlet was fed directly into the up-leg instead of into the boot, so buckets were digging into a moving stream and throwing material back.

The remedial work — a redesigned loading chute with a hood-and-spoon transfer, longer skirtboards with correctly adjusted seal rubber, and a reconfigured elevator inlet — cost about 46,000 USD. The clean-up labour alone had been running at roughly 70,000 USD a year, before counting belt wear and the production constraint.

Conveyor systems are rarely let down by the conveyor. They are let down by what happens at the ends.

Belt Conveyors

The workhorse of bulk handling: lowest cost per tonne-kilometre of any mechanical conveyor for long horizontal runs.

Components

Belt — the carrying and tension member.

• Fabric (ply) belts — EP (polyester warp, nylon weft) is standard; tensile ratings from 250 to 2,000 N/mm. Most industrial conveyors.
• Steel cord belts — for high tension and long single flights, to 7,500 N/mm and beyond. Overland conveyors.
• Covers: Grade M/N for general abrasion, Grade X/H for severe abrasion and cutting, heat-resistant grades for clinker and hot sinter (to 150-200°C continuous, more for short exposure), oil-resistant for grain and oily materials, flame-retardant for underground and enclosed plants, food-grade where required.
• Cover thickness: top cover sized against impact and abrasion — typically 4-10 mm on carrying side, 2-3 mm on pulley side. Under-specifying the top cover is false economy on abrasive material.

Idlers — the rollers supporting the belt.

• Carrying idlers: three-roll troughing at 20°, 30°, 35° or 45°. Deeper troughs carry more for a given belt width but stress the belt edges more. 35° is the common industrial compromise.
• Impact idlers at loading points, with rubber rings or discs to absorb load impact
• Return idlers: single flat roll, or V-return
• Spacing: carrying idlers typically 1.0-1.5 m, closer at loading points; return idlers 2.4-3.0 m. Spacing is calculated from belt sag — generally limited to 2-3% of idler spacing — because excessive sag causes spillage and increases power.
• Bearing life and sealing determine maintenance load. A conveyor with 800 idler rolls and poor seals becomes a permanent maintenance task.

Pulleys — drive, tail, snub, bend and take-up.

• Drive pulley lagging (rubber, ceramic or diamond-pattern) raises the friction coefficient, allowing the required drive torque without slip
• Pulley diameter is set by belt construction and tension — too small and the belt's carcass is overstressed in bending, which causes ply separation
• Crowning or correct alignment for tracking

Drive — motor, gearbox and coupling or gearmotor, with backstop on inclined conveyors to prevent reverse running under load.

• Soft start matters on long conveyors: direct starting applies high torque to a loaded belt and produces a tension wave that can damage the belt and splices. VFD, fluid coupling or soft starter is standard above moderate lengths.

Take-up — maintains belt tension and accommodates belt stretch and thermal change. Screw take-up for short conveyors; gravity (counterweight) take-up for anything long, because it maintains constant tension automatically.

Structure — stringers, trestles, gantries, walkways and covers.

Cleaning and sealing — primary and secondary belt scrapers, V-plough on the return before the tail pulley, skirtboards with seal rubber at loading points. These are not accessories; they are the difference between a clean plant and a permanent clean-up crew.

Safety — pull cords, belt misalignment switches, zero-speed switches, blocked chute detectors, guards at all nip points, emergency stops accessible along the full length.

Capacity and sizing

Belt conveyor capacity depends on belt width, trough angle, belt speed, material surcharge angle and bulk density.

Q (t/h) = 3.6 × A × v × ρ

Where A is cross-sectional area of the load (m²), v is belt speed (m/s) and ρ is bulk density (t/m³).

Belt speed must respect the material: fine dusty materials are limited to about 1.5-2.0 m/s to control dust generation, general aggregates run 2.0-3.5 m/s, and coal and overland conveyors reach 4-6 m/s. Higher speed means a narrower belt for the same capacity — cheaper — but more dust, more wear, more degradation of friable material and worse spillage at transfer points.

Minimum belt width versus lump size: a rule of thumb is belt width at least 3 times the maximum lump size for unsized material and 4-5 times for sized material with a high proportion of large lumps. Running large lumps on a narrow belt causes edge spillage and belt damage.

Incline limits

The practical maximum incline is governed by the material sliding back on the belt, which relates to its angle of repose:

Beyond these limits the options are:

• Cleated belts — moulded or attached cleats, to roughly 45°
• Sidewall (corrugated sidewall) belts — flexible sidewalls and cleats, to 90° vertical, widely used in cement, fertiliser and bulk terminals
• Pocket and pouch belts — fully enclosed, vertical and around corners
• Pipe conveyors — the belt is formed into a tube, allowing steeper inclines, tight curves and complete dust containment
• Change to a bucket elevator for the vertical component

Power and tension

Methods: CEMA (Conveyor Equipment Manufacturers Association) in North America, DIN 22101 and ISO 5048 elsewhere. All compute:

• Effective tension (Te) from the sum of frictional resistance of empty belt and load, lift or lowering force, and secondary resistances at pulleys, scrapers and skirting
• Power = Te × v at the drive pulley, divided by drive efficiency
• Maximum belt tension including starting conditions, from which belt rating, pulley diameters and shaft sizing follow

Points that cause underpowered conveyors:

• Secondary resistances at scrapers, skirtboards, ploughs and transfer points can add 10-25% to the calculation and are frequently omitted
• Low temperature raises belt and idler drag substantially
• Starting torque on a fully loaded inclined conveyor can be well above running torque
• Material build-up on idlers and pulleys increases drag over time, so some margin is appropriate

Screw Conveyors

A helical flight rotating in a trough or tube.

• Capacity: up to about 100 m³/h for standard designs, more for large shaftless designs
• Length: economical to 30-40 m; longer runs need intermediate bearings, which obstruct flow and wear
• Incline: possible to 20-25°, but capacity falls sharply — roughly 30% loss at 15° and 50% at 25° — because material flows back over the flight
• Enclosed, so dust containment and odour control are inherent
• Can meter — output is proportional to speed, making screw feeders a standard dosing device

Variants:

• Shafted screw — standard, with hanger bearings on longer runs
• Shaftless screw — no centre shaft, no hanger bearings, so it handles sticky, stringy and fibrous material (sludge, screenings, wet waste) that would wrap a shafted screw. A liner in the trough takes the wear.
• Ribbon flight — open helix on arms, for sticky and viscous material
• Cut-and-folded flight — for mixing while conveying
• Variable pitch — for controlled drawdown from a hopper along the full inlet length rather than only at the back
• Live bottom / multiple screw — hopper discharge across a wide opening

Not suitable for: highly abrasive materials (the flight and trough wear rapidly), very friable materials (degradation), large lumps, or anything that packs hard under pressure.

Trough loading is typically 15-45% of cross-section depending on material flowability — overfilling causes torque spikes, flight damage and blockage.

Chain Conveyors

Drag chain / en-masse conveyors

A chain with flights moves through an enclosed trough, conveying material as a mass rather than in individual pockets. The material layer moves with the chain through internal friction, so the effective cross-section is far greater than the flight area.

• High capacity in a small cross-section
• Fully enclosed — excellent dust and odour containment
• Can convey horizontally, on incline, and around corners in some designs
• Gentle on friable material compared with screws
• Lower power than a screw for the same duty over longer distances
• Standard in grain handling, cement, biomass, ash handling and bulk terminals

Apron and pan conveyors

Overlapping steel pans on chains. For hot, heavy, large-lump and abrasive materials that would destroy a belt — hot clinker, sinter, slag, foundry scrap, crusher discharge. High capital cost, high strength, long life.

Scraper and submerged scraper conveyors

For ash, slag and wet material, often running submerged in a water trough for cooling and sealing — standard on boiler bottom ash handling.

Bucket Elevators

Buckets on a belt or chain, lifting material vertically.

Discharge types

Centrifugal discharge

• Belt speed 1.2-2.0 m/s
• Material is thrown out of the bucket by centrifugal force at the head pulley
• Buckets spaced apart, digging material from the boot
• High capacity, compact head
• Suits free-flowing, non-fragile, low-abrasion material: grain, cement, sand, fertiliser, aggregate
• Not for fragile or very abrasive material — the digging action and the throw both degrade and wear

Continuous discharge

• Speed 0.5-1.0 m/s
• Buckets closely spaced; material is discharged gently over the back of the preceding bucket
• Fed directly into the buckets rather than dug from the boot
• Suits fragile, abrasive, large-lump and aerated materials: clinker, coke, crushed stone, lime, friable pellets
• Larger head section, lower capacity per bucket size

Positive discharge

• Snubbed to invert the buckets fully over the discharge
• For sticky and poorly flowing materials that will not leave a bucket by gravity or throw alone

Design points

Lift height: standard designs to 60 m; special designs considerably higher. Belt elevators generally suit higher speeds and lighter duty; chain elevators suit higher temperature, larger lumps and heavier duty.

Belt versus chain:

• Belt — quieter, higher speed, cheaper, but limited temperature (to about 150-200°C with heat-resistant belt) and belt tension limits lift and capacity
• Chain — higher temperature, heavier loads, large lumps, more robust; noisier, needs lubrication and chain wear monitoring, and chain elongation needs periodic take-up adjustment

Boot design is the most common source of problems. The inlet should feed into the up-leg side at a point and angle that loads buckets without the buckets having to dig through a moving stream, and the boot must have an accessible clean-out. In the West African example, feeding into the up-leg incorrectly caused 15% recirculation.

Backstop is mandatory — a loaded elevator running backwards on power loss empties its entire contents into the boot and jams or destroys it.

Overload and speed monitoring — zero-speed and slip detection so a broken belt or chain is detected before the full load dumps.

Explosion protection where handling combustible dust — grain, flour, sugar, coal, biomass, aluminium. Bucket elevators are classic dust explosion locations: an enclosed volume with a dust cloud and potential ignition sources. NFPA 61, NFPA 660 and the ATEX framework cover the requirements: explosion vents, isolation, ignition source control, and dust control at inlets and outlets. This is not optional and the consequences of ignoring it are catastrophic.

Transfer Points — Where the Problems Are

The single highest-value area of conveyor engineering, and the most neglected.

The physics: material arriving at a transfer has a velocity and a direction. The receiving belt has a different velocity and direction. Every mismatch becomes impact, turbulence, dust and spillage, plus accelerated belt and liner wear.

Good transfer design:

• Match velocity and direction. Hood-and-spoon chutes curve the material stream so it lands on the receiving belt at close to belt speed and in the belt's direction. The improvement over a plain box chute is substantial.
• Centre the load. Off-centre loading makes the belt run off, which causes spillage, edge damage and structure wear.
• Control the drop height. The less free fall, the less dust and degradation.
• Skirtboards extending far enough past the loading point for the material to settle — typically 2-3 m, with adjustable seal rubber set correctly. Over-tightened seal rubber grooves the belt; too loose and it spills.
• Impact idlers under the loading zone, at closer spacing.
• Chute liners matched to the material: ceramic tile, chromium carbide overlay plate, UHMWPE for sticky and non-abrasive, AR400/AR500 steel for general abrasion.
• Dust control: enclosure, dust curtains, settling chamber volume, and extraction only where enclosure cannot do the job. Enclosing and settling costs far less to run than extraction and filtration, and most transfer-point dust is generated by air displacement that good enclosure design prevents in the first place.
• Clean-out access designed in. A chute that cannot be cleaned will be cleaned by hitting it with a hammer, repeatedly, until it leaks.

Belt cleaning: a primary scraper at the head pulley, a secondary scraper further along the return, and a V-plough before the tail pulley. Carryback is what makes the area under a conveyor dirty and what causes material to be trapped between belt and tail pulley.

Material Properties That Drive Selection

The material data that must be on the enquiry:

• Bulk density, loose and compacted
• Particle size distribution and maximum lump size
• Angle of repose and surcharge angle
• Abrasiveness — CEMA class or an index
• Moisture content and whether it varies
• Stickiness and cohesion — does it adhere to steel, bridge in a hopper, build up on belts
• Temperature — and whether there are hot excursions
• Friability — will it degrade in handling, and does that matter to the product
• Corrosivity — chlorides, acids, alkalis
• Combustibility and explosibility — Kst value, minimum ignition energy, minimum explosible concentration for dust
• Flowability — a flow function from shear cell testing where hopper design matters
• Aeration behaviour — fine powders can fluidise and flood through equipment

Of these, stickiness and moisture variability cause the most unpleasant surprises. A material that handles perfectly at 6% moisture may bridge, build up and block at 11%, and the plant will see 11% in the rainy season whatever the design basis said.

Common Specification Mistakes

1. Transfer chutes designed as boxes rather than engineered. The West African example: 46,000 USD of remedial work against roughly 70,000 USD a year of clean-up.

   Prevention: Engineer each transfer with velocity and direction matching, centred loading, correct skirting length and adjustable seal rubber, impact idlers and liners matched to the material.

2. Incline exceeding the material's limit. The belt carries material up and it slides back, so the conveyor runs at a fraction of its rated capacity and spills continuously.

   Prevention: Check the maximum incline against the specific material, and use cleated, sidewall or pocket belts, or a bucket elevator, where the geometry demands more.

3. Belt speed too high for a dusty material. Dust generation rises sharply with speed and with drop height.

   Prevention: Limit fine and dusty materials to 1.5-2.0 m/s, accept a wider belt, and enclose transfer points.

4. Secondary resistances omitted from the power calculation. Scrapers, skirting and ploughs can add 10-25%, and the drive is then undersized.

   Prevention: Include all secondary resistances explicitly in the CEMA or DIN 22101 calculation, and verify starting torque on a fully loaded incline.

5. Screw conveyor specified for abrasive material. Flight and trough wear rapidly and the unit needs constant repair.

   Prevention: Use belt, drag chain or apron conveyors for abrasive materials, or accept hardfaced flights and replaceable liners with a defined wear allowance.

6. Screw conveyor on an incline without capacity derating. Capacity falls around 30% at 15° and 50% at 25°.

   Prevention: Apply the incline derating factor, or use a different conveyor type for inclined duty.

7. Centrifugal discharge bucket elevator on fragile or abrasive material. The digging action in the boot and the throw at the head both degrade product and wear buckets.

   Prevention: Use continuous discharge with direct bucket feed at 0.5-1.0 m/s for fragile, abrasive and large-lump materials.

8. Bucket elevator inlet fed into the up-leg incorrectly. Buckets dig into a moving stream and throw material back down.

   Prevention: Design the inlet to load buckets without digging, with the feed point and angle set by the elevator supplier, and provide boot clean-out access.

9. No backstop on an inclined conveyor or bucket elevator. On power loss the loaded equipment runs backwards and dumps its contents.

   Prevention: Specify a backstop or holdback on every inclined conveyor and every bucket elevator.

10. No explosion protection on combustible dust handling. Bucket elevators and enclosed conveyors are classic dust explosion locations.

    Prevention: Characterise the dust (Kst, MIE, MEC), apply NFPA 61 and NFPA 660 or the ATEX framework, and specify venting, isolation and ignition source control.

11. Belt top cover thickness under-specified for abrasive duty. The belt is replaced in two years instead of eight.

    Prevention: Specify top cover thickness and cover grade against the material's abrasiveness and the drop height at loading, and compare belt life over ten years rather than purchase price.

12. Screw take-up on a long conveyor. Tension drifts as the belt stretches, and nobody adjusts it.

    Prevention: Specify gravity counterweight take-up on all but short conveyors, sized for the full range of belt stretch and thermal movement.

13. Design based on nominal material moisture. The material behaves differently in the wet season and the plant blocks.

    Prevention: State the full moisture range and design the chutes, hoppers and liner selection for the worst case, not the average.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies bulk material handling equipment and components:

Belt conveyors

• Complete belt conveyor systems, design, supply and erection support
• Conveyor belting: EP fabric 250 to 2,000 N/mm, steel cord to 7,500 N/mm
• Belt covers: general abrasion, severe abrasion, heat resistant, oil resistant, flame retardant, food grade
• Cleated, corrugated sidewall and pocket belts for steep incline and vertical duty
• Pipe conveyor belts
• Idlers: 20°, 30°, 35° and 45° troughing sets, impact idlers, return idlers, V-return, self-aligning and garland sets
• Idler rolls in steel, rubber-lagged, HDPE and ceramic-coated
• Pulleys: drive, tail, snub, bend and take-up, with rubber, ceramic and diamond lagging
• Screw and gravity take-up assemblies with counterweights
• Belt scrapers, primary and secondary, V-ploughs, skirtboard and seal rubber
• Belt splicing materials, cold and hot vulcanising kits, mechanical fasteners
• Conveyor structure, stringers, trestles, gantries, walkways and covers

Screw and chain conveyors

• Shafted screw conveyors and feeders, trough and tube type
• Shaftless screw conveyors with replaceable trough liners
• Ribbon flight, cut-and-folded and variable pitch screws
• Live bottom and multiple screw hopper dischargers
• Drag chain and en-masse conveyors, fully enclosed
• Apron and pan conveyors for hot and large-lump material
• Scraper and submerged scraper conveyors for ash and slag
• Conveyor chain, sprockets, flights and wear bars

Bucket elevators

• Centrifugal, continuous and positive discharge elevators
• Belt and chain types, lifts to 60 m and above
• Buckets in pressed steel, stainless, nylon and polyurethane
• Boots with clean-out access, heads with engineered discharge geometry
• Casings, inspection doors, explosion vents and isolation devices
• Backstops, zero-speed, slip, misalignment and overload detection

Feeders and ancillary

• Apron feeders, belt feeders, vibrating feeders and rotary valves
• Weigh feeders and belt weighers
• Hoppers, bins, silos and chutes with engineered flow design
• Chute liners: ceramic tile, chromium carbide overlay, UHMWPE, AR400 and AR500 plate
• Dust control: enclosures, curtains, settling chambers, extraction hoods, bag filters and cyclones
• Magnetic separators and metal detectors
• Diverter gates, slide gates and flop gates

Drives and controls

• Shaft-mounted reducers, helical and bevel-helical gearboxes, gearmotors
• Electric motors in IE3, IE4 and IE5, including Ex-rated
• Fluid couplings, soft starters and variable frequency drives
• Backstops and holdbacks
• Safety switches: pull cord, belt misalignment, zero speed, blocked chute
• Guards and emergency stop systems

Engineering support

Send material properties (bulk density, size distribution, maximum lump, abrasiveness, moisture range, temperature, stickiness, friability, combustibility), required capacity, and the layout with lengths, lifts and inclines. We will return equipment selection with belt width and speed, idler and pulley specification, calculated power and belt tension to CEMA or DIN 22101, transfer chute concept, and liner and dust control recommendations. For combustible dusts we will state the explosion protection basis.

Certification

Material certificates to EN 10204 3.1 for structural and wear components, conveyor belt test certificates for tensile strength, cover abrasion and fire resistance where applicable, welding procedure and welder qualification documentation, gear unit rating calculations, ATEX documentation for hazardous-area equipment, dust explosion protection documentation with Kst-based venting calculations, and factory acceptance test records for drive assemblies and complete units.

Logistics

Idlers, pulleys, scrapers and standard components generally ship in 3-6 weeks. Conveyor belting 4-10 weeks depending on specification and length. Screw conveyors, chain conveyors and bucket elevators typically 10-18 weeks. Complete conveyor systems with structure 14-24 weeks. Shipping from Istanbul by road to Europe, the Caucasus and Iraq, and by sea to Gulf, African and Asian destinations.

Send your material properties, capacity and layout and we will return equipment selection with power calculations within five working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Plant Equipment Series

• Industrial Gearboxes and Couplings
• Industrial Electric Motors: IE Classes and Enclosures
• Plant Construction Equipment: Mixers, Torque Tools and Lifting
• Carbon Steel Plate: A516 and A36 Grades

Frequently Asked Questions

Q: What is the maximum incline for a belt conveyor?
A: The limit depends on the material. Crushed sized stone reaches 20 to 22°, run-of-mine ore and coal 18 to 20°, damp sand 20 to 22°, dry free-flowing sand 15 to 16°, fine cement powder 12 to 15°, and rounded gravel or pellets only 10 to 12°. Beyond those limits, cleated belts reach about 45°, corrugated sidewall belts handle vertical lifts, and pocket belts and pipe conveyors handle steep inclines with full containment.

Q: What is the difference between centrifugal and continuous discharge bucket elevators?
A: Centrifugal discharge runs at 1.2 to 2.0 m/s with spaced buckets that dig material from the boot and throw it out at the head, giving high capacity for free-flowing, non-fragile materials such as grain, cement and sand. Continuous discharge runs at 0.5 to 1.0 m/s with closely spaced buckets fed directly rather than digging, and discharges gently over the back of the preceding bucket, which suits fragile, abrasive and large-lump materials such as clinker, coke and crushed stone.

Q: When should a shaftless screw conveyor be used?
A: Shaftless screws have no centre shaft and therefore no hanger bearings, so material cannot wrap around a shaft or snag on bearings. That makes them the right choice for sticky, stringy and fibrous materials — dewatered sludge, wastewater screenings, wet waste, pulp — which would quickly block a shafted screw. A replaceable trough liner takes the wear, and inclines are possible with reduced capacity.

Q: Why do belt conveyors spill material at transfer points?
A: Material arrives at a transfer with a velocity and direction that rarely match the receiving belt. Every mismatch produces impact, bounce, turbulence, dust and spillage. The fixes are an engineered hood-and-spoon chute that curves the stream to land near belt speed in the belt's direction, centred loading, limited drop height, skirtboards extending 2 to 3 metres with correctly adjusted seal rubber, and impact idlers under the loading zone.

Q: How do you calculate belt conveyor capacity?
A: Capacity in tonnes per hour equals 3.6 multiplied by the load cross-sectional area in square metres, belt speed in metres per second and bulk density in tonnes per cubic metre. The cross-sectional area depends on belt width, trough angle and the material's surcharge angle. A 1,000 mm belt at 2.5 m/s with a 35° trough carrying material at 1.6 t/m³ handles roughly 750 t/h.

Q: Why do bucket elevators need explosion protection?
A: A bucket elevator handling combustible dust such as grain, flour, sugar, coal, biomass or aluminium is an enclosed volume containing a dust cloud with potential ignition sources from friction, misalignment and static. That is the classic configuration for a dust explosion, and elevator explosions have historically propagated into connected equipment. NFPA 61, NFPA 660 and the ATEX framework require dust characterisation, explosion venting, isolation and ignition source control.

Q: What belt cover grade should be specified for abrasive material?
A: General-duty Grade M or N covers suit moderate abrasion, while Grade X or H covers are needed for severe abrasion and cutting from sharp-edged material. Top cover thickness matters as much as grade — typically 4 to 10 mm on the carrying side depending on abrasiveness and drop height at loading. Under-specifying the cover often reduces belt life from eight years to two, which far outweighs the purchase saving.