Belt conveyors
Why a belt runs off, and the order to fix it in
Quick answer
The diagnostic rules for belt tracking and a safe order of work, from pulleys and idlers to loading.
Two diagnostic rules
- If the belt runs off in one place on the conveyor, the cause is probably in the alignment or level of the structure, idlers or pulleys there.
- If one part of the belt runs off wherever it goes, the cause is probably in the belt, its splice or the way it is loaded.
- When the belt is loaded off centre, the centre of gravity of the load seeks the middle of the trough and leads the belt away from the lightly loaded edge.
What each part does to the belt
| Part | Effect on tracking | Practice |
|---|---|---|
| Pulleys | The belt walks toward the edge that touches first when a pulley is out of square. A pulley that is not level makes the belt run to the low side. | Keep all pulleys level and at 90 degrees to the belt path. Only a snub pulley may be shifted for training, and only after other methods. |
| Crowned pulley | Little steering. Effective only with a long unsupported belt span, about four belt widths, leading in. | Little use on the head pulley or at high tension. Fabric belts only; steel-cord belts need flat pulleys. |
| Troughing idlers, knocked | Shifting one end of an idler forward on the side the belt runs to centres the belt in that direction. | Spread the shift over a length of belt before the trouble spot, keep it to a minimum and undo it in the same idlers if you overcorrect. Not for reversing belts. |
| Troughing idlers, tilted | Tilting forward in the travel direction acts on both sides and helps erratic belts. | No more than 2 degrees. It wears the pulley covers faster, so use it sparingly. |
| Return idlers | Flat rolls have no self-aligning action, but shifting the axis gives a constant correction. | Use centre-pivoted self-aligning return rolls near the tail. V-return idlers help wide fabric belts, but only if the belt centre can touch. |
| Self-aligning carrying idlers | Skewed by a drifting belt and steer it back. | Place a pair about 12 and 24 m before the head pulley as insurance. Do not put them on a convex curve. |
| Side guide rolls | Not a way to make a belt run straight. | Use only to protect the belt as an emergency measure and keep them clear of the belt edge when running. |
The belt itself
A belt that is very stiff across its width has trouble touching the centre roll of the trough, so it is harder to train. New belts sometimes wander over parts of their length because tension is not yet even; it usually evens out under running tension.
Order of work
- Start with square and level pulleys, idlers and structure. Move the belt slowly and intermittently the first time, with people placed along the line and a way to stop it.
- Correct the places where the belt could be damaged first.
- Train the return run first, working back to the tail pulley, so the belt reaches the loading point centred.
- If the belt is flexible, train it empty. If it is stiff and erratic, put some load on once the return run is centred.
- Guide the take-up carriage so the shaft stays level. A belt does not centre itself on a tilted pulley.
- Make small changes, wait for a full belt revolution and start before the place where the belt runs off.
- Check the loaded belt. Off-centre loading and dirt on the snub pulley and return rolls are the usual causes of new trouble.
Takeaway. Square and level first, clean next, load on the centre next. Use training idlers last, and make small changes.
Put it to work in the tool