Questions often arise surrounding the difference between the minimum breaking strength and the safe working load of a rope. The minimum breaking strength of a rope is usually the optimum strength achieved under laboratory conditions, in accordance with prescribed test procedures, on new rope, pulling at a slow, steady rate in a straight line. Real life applications normally involve many different types of forces and factors which will cause the rope to fail at much reduced loads. As a result, the safe working load of any rope is much lower than its minimum breaking strength.
The safe working load varies from rope to rope and across applications. However, it is always a fraction of the quoted minimum breaking strength. In life critical applications, or applications where there is a risk of serious damage in the event of a rope failure, the safe working load is generally 1/12 – 1/15 of the rated minimum breaking strength. In applications with minimal risk, the safe working load of the rope may be 1/5 of the rated minimum breaking strength.
Other things to be considered when determining the safe working load of a rope include applications where:
* smaller ropes are used, because the rope fibers can be more easily and severely damaged by cutting, abrasion and sunlight;
* loads cannot be accurately predicted;
* operations and use procedures are not well defined and/or controlled;
* extreme dynamic loading (shock loads) are possible;
* knots are used as they can significantly reduce the minimum breaking strength.
The safe working load of a rope must always be determined by an engineer or safety professional familiar with the application.
