TRANSITION LENGTH

Transition length is the axial distance between the last full-trough carrying idler and the centreline of the terminal pulley (head or tail). Over this distance the belt cross-section must change from the design trough angle (commonly 35°) to fully flat at the pulley face. Because the belt edges must travel a longer path than the belt centre during this geometry change, the edges go into elevated tension. If the transition is too short, the edge tension exceeds the carcass elastic limit and damages the splice, the carcass, or the belt edge itself.

CEMA and DIN both provide minimum transition length tables indexed against belt rating, trough angle and percentage of rated tension. A common rule of thumb: for a 35° trough at full rated tension, fabric belts need transition length of about 1.5× belt width; steel cord belts of about 2.0× belt width; for 45° troughs add another 30 %. For lightly loaded belts running well below rated tension, transition length can be reduced. Where physical space is tight (transfer towers, tail boxes against a wall), a half-trough pulley (the terminal pulley raised so its top sits roughly halfway between flat and trough) can halve the required transition length without exceeding edge stress limits.

The transition zone is normally fitted with [transition idlers](/glossary/transition-idler) whose wing angles step progressively from the design trough to flat, easing the geometry change. Designers should verify transition length on every new conveyor; it is one of the most common omissions in retrofits and capacity upgrades where a higher trough angle is adopted without lengthening the transition.