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Belt conveyor construction, pulleys, idlers, take-ups and belt training, plus the bucket-elevator, screw, pneumatic, roller, apron, chain and food-handling conveyor families a millwright installs and troubleshoots.
What the exam tests here
Belt construction and carcass types; splice methods; pulley types (crowned, head, drive, snub, bend) and lagging; the idler family and belt-training rules; take-up types; loading, chutes and skirting; the conveyor taxonomy (bucket elevators, screw, pneumatic, roller, apron, chain); and screw-flight/pitch types plus designation. Belt training, idlers, pulleys and the conveyor-type distinctions are the highest-yield — and this is day-job material.
A conveyor belt has three sections:
| Section | Role |
|---|---|
| Top cover | Thicker — resists gouging and abrasion on the load side |
| Bottom cover | Underside of the belt |
| Carcass | The strength/tension element — supports the load, impact, and splice fasteners |
| Carcass type | Construction | Use |
|---|---|---|
| Solid woven | Single interlocking ply | No ply separation |
| Reduced-ply | Fewer plies / special weaves | — |
| Multiple-ply | More than 3 layers, usually ≤8 | — |
| Steel cord | Embedded cables | Extreme constant tension, or limited take-up travel |
Splicing joins the belt into a loop:
Vulcanized splice angle
45° for belts ≤24″ wide; 60° for wider belts
Splice ends must be square regardless of the cut angle
Watch out
The splice must not lead into the load direction — a splice that runs against the load peels open.
Choosing the belt and splice: pick the belt to suit its speed, the load, the material carried, the temperature and surroundings, and whether it rides on idler rolls or slides on a flat bed. A mechanical splice's type and size are set by the product carried, the belt type, the roller (pulley) size, the belt width and the speed.
Pulley diameter is set by the belt, its speed, and the number of plies.
| Pulley | Purpose / notes |
|---|---|
| Crowned | Larger diameter at the centre. Crown the tail pulley, never the drive/head. Needs a ~3 ft unsupported approach (no idlers ahead of it) so the belt can climb to the crown |
| Head | Largest pulley; reduces belt flexing (load travels toward the head end). Usually the drive — not crowned (high-tension zone) |
| Drive | Transmits power; lagged, smooth, or self-cleaning — not crowned (high-tension zone) |
| Smooth steel | Fabric-ply belts; steel-cord belts use special engineered smooth drums |
| Self-cleaning wing | Debris falls between the wings |
| Tail | Smaller — returns the belt; the low-tension pulley you crown for tracking |
| Snub | Increases the drive's arc of contact |
| Bend | Changes belt direction |
Lagging (rubber or ceramic) covers the face of the drive pulley, and of pulleys that run on the dirty (carrying) side of the belt:
Watch out
Crown the tail pulley, not the drive/head — crowning a high-tension drive pulley overstresses the belt centre and lagging. Troughed idlers do most of the tracking; crowning is only a secondary aid. Never crown high-tension steel-cable belts or multiple/snubbed-drive pulleys (low-tension belts ≤ ~35 kN/m / 200 PIW may be crowned throughout).
Two families: carrying idlers support the loaded run (troughing or flat); return idlers support the empty run.
| Idler | Where / notes |
|---|---|
| Troughing | Bulk/spill; carries more than flat. Tilt forward 2° in the travel direction. Frame-mounted = 3 rolls, outer rolls at 20°/35°/45°, centre horizontal |
| Deep troughing (35° / 45°) | Holds more and spills less than a 20° trough: the fix for fine or free-flowing loads that spill, but only on a belt rated to trough that deep, with the transition distances reset to suit |
| Off-set centre roll | Low headroom |
| Suspended | Cable-linked, quick-release, 3- or 5-roll; favoured in mining — the swing absorbs misalignment |
| Transition | Gradually troughs the flat belt onto the troughing idlers and flattens it back toward the head pulley; bad spacing over-stretches the edges |
| Flat-belt | Manufacturing; no spill |
| Rigid side-guide | Near pulleys — keeps the belt off the frame |
| Self-aligning / training | Minor correction only; works only if the bed is true. The belt riding a side-guide roll pivots the idler back to centre |
| Impact | At loading points — resilient rolls, spaced close, 20°/35°/45° |
Belt training:
Exam tip
Training idlers give minor correction only and work only if the bed is true — train the return idlers first, one person only.
Take-ups provide drive tension and absorb belt stretch.
| Take-up | Notes |
|---|---|
| Gravity | Near the drive; spans over 30 m (100 ft); floating pulley |
| Horizontal / carriage | Limited space — counterweight or winch |
| Screw | Hand-set, compact; small light belts, at the tail |
| Pneumatic / hydraulic | Automatic, pressure-adjusted |
Loading:
| Feeder | Suits |
|---|---|
| Screw | Not lumpy material |
| Belt | Fine material |
| Drag-scraper | Small lumps |
| Apron | Lumpy / abrasive |
| Reciprocating-plate | — |
| Control-gate | — |
| Vibrating | Anti-caking; not for wet or sticky material |
| Rotary vane / drum / table | — |
Trippers discharge at intervals along the belt to fill bins or silos.
Accessories:
Drives transmit by friction — the arc of contact (wrap factor) sets the capacity:
| Drive | Arc of contact / notes |
|---|---|
| Single | 180°, or 190–240° with a snub pulley |
| Tandem | One motor, both pulleys — 360–420° |
| Dual | Two motors — DC, or AC with fluid couplings |
| Multiple | Long high-volume conveyors — reduces belt tension |
Belt speed
Belt speed = π × drive-roller Ø × roller rpm
Chain the drive ratios first. Example: 1750 rpm ÷ 35 reducer = 50 rpm; × 8″⁄16″ pulleys = 25 rpm; π × 24″ × 25 = 1 885 in/min ÷ 12 ≈ 157 ft/min
Exam tip
Most belt wear is at the loading point — load in the direction of travel, at belt speed, and centred.
Buckets on chain or belt lift pulverized, granular or lumpy material from the boot to the discharge chute.
| Type | Speed | Notes |
|---|---|---|
| Centrifugal discharge | 56–90 m/min (high) | Material thrown off by centrifugal force; cement, grain; speed is critical |
| Continuous bucket | ≤45 m/min (low) | Loaded through a leg; protects delicate or dusty material |
| Super-capacity | 23–37 m/min | Heavy double-strand steel roller chain; high lifts, large capacity |
| Positive-discharge | ~37 m/min | Buckets between two chains; a snub sprocket inverts them |
| Component | Contains |
|---|---|
| Casing | Ribbed angle iron |
| Head | Head pulley, bearings, discharge, drive |
| Boot | Tail pulley, take-up, loading leg |
| Belt or chain | SBS / S / Class C chain |
A hunting-tooth sprocket (odd tooth count + double-pitch chain) contacts different teeth each revolution for longer life.
Exam tip
Centrifugal-discharge elevators run fast (56–90 m/min) and speed is critical; continuous-bucket elevators run slow (≤45 m/min) to protect the material.
| Flight design | Use |
|---|---|
| Sectional | Removable |
| Double flight | Two flights at standard pitch — smoother, even discharge (scales) |
| Helicoid | Thick, smooth, hardened — grain |
| Ribbon | Sticky materials |
| Cut flight | Mixing / chopping |
| Paddle | Maximum mixing |
| Cut-and-folded | Agitation, cooling, drying |
| Pitch | Use |
|---|---|
| Standard | Pitch = flight diameter |
| Short | Feeders, steep inclines |
| Long | Free-flowing material, liquids |
| Variable | Feeder under bins |
| Stepped | Smooth flow |
| Tapered diameter | — |
Screw designation — e.g. 12H512 RH
12 = diameter (in) · H = helicoid flight · 5 = twice the coupling diameter (2-7/16″ couplings) · 12 = flight-tip thickness (64ths) · RH = right-hand lay
Exam tip
A screw conveyor has both radial and thrust bearings — the thrust bearing sits at the discharge end, where it takes the axial push of the moving material.
| Pressure range | Pressure | Equipment / notes |
|---|---|---|
| Low | ≤12 psig | Lobe compressor; gate-lock feeders |
| Medium | 15–45 psig | Rotary blower or sliding-vane + fluid-solids pump; needs 60 % through a 200-mesh screen; ranges to ~650 m; cement plants |
| High | 45–125 psig | Oil-free air with chemical dryers/aftercoolers; blow tanks; inert gas if fire/explosion risk |
Exam tip
Vacuum: many sources → one bin. Pressure: one source → many destinations.
Roller conveyors handle from a few grams to 25 t.
| Roller conveyor | Notes | Max incline |
|---|---|---|
| Gravity | Any horizontal direction; sloped runs use gravity; needs flat rigid package bottoms | — |
| Live — chain-drive | All, every-other, or continuous chain; heavy pallets | ≤10° |
| Live — belt-drive | Belt between the rolls; adjustable pressure rollers | ≤10° |
| Live — roller-bed | Belt on a roller bed; most versatile | ≤30° |
| Roller | Use |
|---|---|
| Straight | — |
| Bottle | Tapered end — positioning |
| Eccentric | Right-angle moves |
| Tapered | Curves |
Apron conveyors carry heavy or hot bulk (rock, clinker) on overlapping metal pans:
Chain conveyors use the chain as the tension element:
Exam tip
Roller-bed live conveyors are the most versatile and climb to 30°; chain- and belt-driven live rollers are limited to 10°.
Slurry systems (watery or corrosive mixes) = pump + non-collapsible, wire-reinforced hose or hardened pipe.
Food handling must meet health regulations across heat/cold and wet/dry — rubber is being replaced.
Wire mesh belts (stainless, plated, or Teflon):
| Synthetic | Temperature range | Notes |
|---|---|---|
| Polypropylene | ≤104 °C; brittle below 7 °C | Chemical resistance |
| Acetal | −46 °C → 93 °C | Stronger; containers |
| Polyethylene | −73 °C → 66 °C | Flexible, high-impact; meat/poultry approved |
| Nylon | — | Bends rather than breaks — jam-prone lines |
Exam tip
Fix only ONE sprocket on the shaft so the chain can track; the others float.
Exam tip
Tracking rule of thumb: the belt steers away from the idler end you advance. Adjust, run a full revolution, then adjust again — small moves.
The full study guide covers every block of the 433A Industrial Millwright exam, with practice questions and a timed mock exam.
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