BELT SAG

Belt sag is the catenary-like dip a conveyor belt forms between two carrying idlers under its own self-weight plus the weight of any bulk material on it. Some sag is unavoidable and even desirable — a small downward curve seats the belt firmly into the trough — but excessive sag drains energy through repeated flexing, causes material to surge backward over each idler, accelerates cover wear and can pull the belt off centre.

Industry practice limits sag percentage to roughly 1.5–2 % of the idler pitch on the carrying side under maximum load and 2–3 % on the return side. Sag percentage is computed as sag depth ÷ idler spacing × 100. The minimum belt tension required to keep sag within limits is back-calculated from the loaded mass per metre and the idler spacing; this minimum tension drives the take-up sizing on the conveyor. On long belts with light loads, sag tension can actually exceed the tension needed to transmit Te — making it the governing design case for the take-up.

Common causes of out-of-spec sag are wide idler spacing (often inherited from a previous belt that was lower-rated), undersized take-up travel, oversized belt selection (heavier belt sags more), and dust build-up on return idlers that effectively shortens the belt and removes take-up reserve. The belt sag calculator on this site reports sag depth and percentage for a given tension, belt weight, material load and idler spacing.