ANATOMY / CONVEYING
Conveyor idlers: troughing angle, spacing and selection
Anatomy of a conveyor idler set, how troughing angle changes the load a belt carries, how close idlers should be, where impact, transition and training idlers go, and what a seized roll does.
Part of the guide: How to specify a bulk material conveying system
Idlers are the most numerous component on a belt conveyor. They set how much the belt can carry for its width, how well the load stays on it, how much power the drive spends turning them and — when one seizes — whether the belt is merely worn or on fire. This note takes a set apart, then puts it back on the conveyor in the right places.
What is in an idler set?
- Centre roll
- The horizontal roll under the belt centreline; it carries most of the load and takes most of the wear.
- Wing rolls
- The inclined rolls that form the trough; their angle to the horizontal is the troughing angle. Two-roll V sets and five-roll sets exist for narrow and very wide belts.
- Frame
- The pressed or fabricated bracket that holds the rolls at their angle and bolts to the stringer. Its squareness decides tracking; its stiffness, whether the rolls stay in line under a lump.
- Shaft
- A fixed shaft through each roll, flatted at the ends to sit in the frame slots. The roll turns on it.
- Bearings
- Deep-groove ball bearings pressed into each end of the roll, rated for load and speed. In most environments the bearing life is the life of the roll.
- Seals and labyrinths
- The labyrinth, contact seal and grease reservoir that keep dust and water out of the bearing — what separates a roll that lasts years from one that lasts months.
- Roll shell
- The steel, polymer or aluminium tube the belt runs on. Wall thickness and finish set wear life and, with the roll mass, the effort needed to turn it.
How does the troughing angle change what the belt carries?
The load cross-section on a troughed belt is the area between the belt and the surface of the material, which stands at the surcharge angle — lower than the angle of repose, because a moving belt shakes the load flat; the material properties note explains the difference. A deeper trough carries a larger cross-section for the same belt width; common three-roll angles run from about 20° to 45°. The limit is the belt, which must be flexible enough across its width to sit into the trough and touch the centre roll when empty; a stiff belt in a deep trough rides on the wing rolls and wanders.
How close should idlers be?
Close enough that the belt does not sag much between them: sag lets the load shift at every idler, spills material, lifts the skirt seal in the loading zone and costs drive power. The usual limit is a small percentage of the spacing — commonly in the region of one to three per cent — and it depends on the belt tension at that point, lowest near the tail of a head-driven belt, so the take-up setting matters to spillage. Carry-side spacing is commonly in the region of a metre or so, closer for wide belts, dense materials and under the loading zone; return idlers can be two or three times further apart.
Which special idlers go where?
| Idler | Where it goes | What it does | Watch for |
|---|---|---|---|
| Impact | Under the loading zone, close pitch | Rubber discs absorb impact and protect the carcass | Discs worn to the hub; sag under the skirt |
| Transition | Between the last full-trough set and the pulley | Flattens the belt gradually so the edges are not overstretched | Too short for the trough depth: edge stress, unsettled tracking |
| Training or self-aligning | Sparingly, carry and return | A pivoting frame that steers the belt back when it drifts | Constant working; a stuck pivot; see the tracking checklist |
| Return | Under the return strand, wide pitch | Supports the empty belt; flat, or a two-roll V on wide belts | Carry-back build-up; seized rolls hidden below the walkway |
| Rubber-disc return | Return strand on sticky duties | Spaced discs limit the area for carry-back to build on | No substitute for cleaners; uneven disc wear |
| Garland or suspended | Loading zones; heavy or mobile duties | Rolls on a chain hung from the stringers, conforming to the load | Tracking less firmly held; link wear |
| Offset or inline | A frame choice | Offset staggers the centre roll so the belt is not pinched between roll ends; inline is compact | Roll gap against belt thickness |
What wears idlers out, and what does a seized roll do?
A roll dies through its bearings: dust and water get past the seal, the grease degrades, the bearing roughens and locks. Once a roll stops turning the belt drags across it; the shell wears a flat, then a knife edge that cuts the cover, and the friction heats the belt — on conveyors carrying coal, biomass or any combustible dust, a seized idler is one of the classic sources of fire. A seized wing roll also steers the belt. Seized rolls are found by walking the belt and listening, and on long belts by thermal or acoustic monitoring.
Idlers are the first thing to fix in a conveyor specification after the belt width, and among the equipment, components and spares we stock — with the seal chosen for the environment, which is where the money in an idler goes.
QUESTIONS WE HEAR
What troughing angle should I use for a belt conveyor?
Common three-roll sets run from about 20° to 45°. A deeper trough carries more for the same belt width and keeps the load away from the edges, but the belt must be flexible enough to sit into the trough when empty and the transitions at the pulleys must be longer. Ask the belt supplier what trough its belt accepts.
How far apart should conveyor idlers be?
Carry-side idlers are commonly in the region of a metre apart for typical widths, closer for wide belts and dense material, and closer still under the loading zone; return idlers can be two to three times wider. The governing check is belt sag, commonly kept to a few per cent of the spacing, which depends on belt tension.
Why do idlers seize?
Almost always through the bearings: dust or water gets past the seal, the grease breaks down, and the bearing roughens and locks. Impact overload and corrosion shorten the path. Seal design matters more than any other feature of the roll, so specify the seal for the environment rather than the cheapest roll on the list.