Pulleys, lagging, transition distance and turnovers
Pulley types, the lagging that grips and protects them, and the geometry near the pulleys that decides how long the belt and splices last.
Pulley types
| Type | What it is | Typical use |
|---|---|---|
| Standard steel | Rim and two end discs on compression hubs | Most common |
| Wing | Open face with angled recesses | Tail pulley; lets fine material fall out instead of being trapped |
| Crowned | Largest at the centre | Takes up slight misalignment; fabric belts only |
| Negative crown | Raised areas set in from each end | Tail pulley tracking aid |
| Snub | Small pulley near the drive | Adds wrap and friction |
| Bend | Redirects the belt | Gravity take-up and return run |
Lagging
- Lagging raises friction, lets the belt run on a lower slack-side tension, reduces slip when wet (if grooved) and protects the pulley and belt cover. Lag drive pulleys always.
- Bolted lagging uses fabric-reinforced rubber with countersunk bolts. It needs about 3 mm of top cover without grooves and 6 mm with grooves, with bolts no more than about 250 mm apart in either direction.
- Vulcanized lagging is about 12 mm of rubber bonded to the shell, without bolts. It lasts longer and removes the risk of a loose bolt cutting the belt. It can be applied in a spiral wrap for lower tension or in sheets.
- Grooved lagging breaks the film of water on wet drive pulleys. A herringbone groove about 5 mm deep on 25 mm spacing is typical.
- Replaceable rubber pads in metal guides are another bolted option.
Transition distance
Transition distance is the distance from the centre of the head or tail pulley to the first regular troughing idler. The belt edge has to climb from the flat pulley to the full trough over this distance, so the edges are stretched more than the centre. A short transition puts high tension on the edges, up to about 115% of the rated tension, and low or even zero tension at the centre, which can cause the belt to buckle. This leads to splice failure and cracks in the carcass at the idler junction. Transition idlers step the trough angle down in stages. A half-trough arrangement raises the pulley by half the idler height and cuts the distance the edge has to travel.
Turnovers
A belt turnover twists the belt by 180 degrees after the discharge so the clean side runs on the return rolls. It is useful for long conveyors handling wet or sticky material. It needs terminal pulleys at both ends so the belt lies flat on the first and last bend pulley. Its length depends on belt width, construction and tension. Vertical rolls near the midpoint and 45-degree rolls at quarter points help hold the belt, spaced about 600 mm apart and offset about 25 mm inward for full belt contact.