These must be estimated so that an adequate allowance for a safety margin can be made.

Two aspects of failure should be considered: its likelihood, and its consequences. The reliability of the belt drive, its duty and required service life will help to determine the former. The manner of failure and the belt type will be concerned with the latter.

Belts may fail catastrophically by fracture (especially if initial tension is too high or they are overloaded) or gradually (if tension is too low and the belt slips on the pulley).

In the latter case there may be a danger of fire since great heat can be generated.

All belt drives should be guarded so that access is prevented and the consequences of failure are contained.