The one principle that explains almost every case
A belt steers toward the side of a roller or pulley it touches first. That is the whole mechanism. Anything that makes one side of the belt arrive earlier — a pulley face that is not perpendicular to the belt path, an idler rotated slightly in plan, a lump of carry-back building up on one end of a roller, a load dumped off-centre — will walk the belt in that direction, continuously, until something stops it.
Two consequences follow, and they are the reason so much tracking work fails. First, the cause is upstream of where you see the drift, sometimes many metres upstream. Second, the belt is obeying, not misbehaving: if you correct the symptom without removing the instruction, the belt will simply go on trying, now fighting your correction, usually against its own edge.
The five real causes
| Cause | How it presents | Tell-tale |
|---|---|---|
| Off-centre loading | Belt tracks true empty, drifts as soon as it is loaded — always toward the light side. | The single most useful test on any conveyor: run it empty, then loaded. If drift only appears loaded, stop looking at idlers and go to the chute. |
| Material build-up | Carry-back caking on return idlers or the tail pulley creates a bigger effective diameter on one side. | Drift that gets worse over days and improves after a clean-up. Usually a scraper or cleaner problem in disguise. |
| Frame not square or level | Persistent drift in one region of the conveyor that no amount of idler adjustment fixes for long. | Measure diagonals and check level across the stringers. Common after a structure has been bumped, resited or built in a hurry. |
| Idler / pulley misalignment | Drift begins at a specific point along the belt and continues from there. | Walk the belt backwards from where the drift starts; the offending idler is usually within a few frames upstream. |
| Belt condition | Drift that travels with the belt — the same section wanders each time it passes a fixed point. | Camber, a stretched edge, or a splice not square to the belt centreline. Mark the belt and watch: if the wander follows the mark, it is the belt. |
The diagnostic sequence that actually finds it
- Watch it empty, then loaded. This single comparison splits the problem space in half — loading and chute issues on one side, geometry on the other.
- Establish whether the drift is fixed to the structure or to the belt. Chalk-mark a belt section. If the wander travels with the mark, the belt or the splice is the cause; if it happens at the same physical location regardless, it is the structure.
- Walk backwards from where drift begins. The instruction always comes before the symptom. Look for build-up, a rotated idler, a shifted bracket, a bent frame.
- Fix in this order: clean, square, align, then adjust. Cleaning and squaring are free and remove most causes. Alignment of idlers and pulleys comes next. Tracking rollers are last, and they are for holding a properly aligned belt in place — not for compensating a fault you have not found.
- Change one thing at a time and let the belt make several full passes. A conveyor responds slowly; adjusting three idlers at once produces a result nobody can attribute.
Why the tracking-roller shortcut is expensive: forcing an incorrectly loaded or misaligned belt back into position with training idlers keeps the belt on the structure — while the edge grinds against whatever is holding it there. Belt edges are not repairable in the way a splice is; by the time the fraying is obvious, you are pricing a belt rather than an afternoon's alignment. The shortcut does not avoid the cost, it converts a maintenance job into a capital one.
What monitoring adds — and what it doesn't
Mistracking is progressive and mostly invisible between inspections. A belt that has begun to wander does not announce it; it simply moves a little further each week until an operator notices spillage, a rubbing noise, or a torn edge. That is precisely the shape of problem continuous monitoring is good at: drift sensing at the points where the belt is most likely to move gives you a trend rather than a discovery, and the trend can be acted on during planned time.
Being honest about the limits: a drift sensor tells you the belt has moved and when it started. It cannot tell you whether the cause is a chute, a caked idler or a splice — that is still a walk along the belt with the sequence above. What it changes is when that walk happens, and whether it happens before or after the edge is damaged. On a plant where a conveyor stoppage halts everything behind it, that timing is the whole value — the wider scope of what to watch on a conveyor system (idler and pulley bearings, motor and gearbox condition, pull-cord status) is covered in conveyor belt monitoring, and the failure-detection physics behind the bearing side in bearing fault detection.
The South African footnote
Two local realities change the calculus. Long overland and underground conveyors mean the inspection walk is expensive in time and, on some sites, in access permits — which raises the value of knowing which section to walk. And repeated stop-start cycles from load shedding are hard on tracking: a belt that starts under an uneven load behaves differently from one already running, so plants that shed frequently see tracking problems appear faster than their inspection intervals were designed for. Both argue for trending drift continuously rather than sampling it on a route.