Conveyor belt sag, also known as catenary sag, refers to the tensionless section of the belt that hangs down on the return side (or underside) of the conveyor. It is an intended section of the belt that is required to keep your conveyor running smoothly.
The catenary sag is a natural way to tension the belt. It allows gravity to give the belt the exact amount of tension instead of relying on a mechanical take-up or tension system. Mechanical take-up or tension systems are required when catenary conveyor belt sag is not included in the design of the conveyor system.