15.5.1 Working Load Limit/SWL
Consideration must be given to the WLL of the sling and the SWL with which it is then attributed for the particular service conditions so as to ensure that no part of the sling is overloaded by the method of use. In the case of some older standards, the WLL of a wire rope sling was based on the minimum breaking load of the rope from which the sling is made, divided by a factor of safety of 5:1. For slings manufactured to modern standards, the minimum breaking load of the rope is first multiplied by a factor which allows for the efficiency of the termination before the factor of safety is applied. For ferrule secured eyes the efficiency factor is 0.9 and for hand spliced eyes the efficiency factor is 0.8. The terminal fittings of a wire rope sling may well be rated at a different factor of safety e.g. 4:1. Ensure that the rope and fittings are matched based on their working load limits, not their safety factors. The working load limit of a wire rope sling depends upon the following factors:
The size and tensile strength of the rope. It will be noted that different but similar constructions of rope of equal tensile strength have the same minimum breaking loads.
The core of the rope. A rope with a steel core will have a marginally higher breaking load than a similar rope with a fibre core. It will at the same time be slightly less flexible and more resistant to crushing.
The number of parts of wire rope (single or double) per sling leg. For some applications double part legs are preferable as they give more flexibility than the equivalent capacity single part leg. They are however more costly and therefore normally only used for large capacities in order to utilize smaller diameter wire rope whilst offering a greater bearing area to the load.
The geometry of the sling, i.e. the number of legs and, in the case of multi-leg slings, the angles between the legs and the vertical and their disposition in plan.
The method of rating, this may be either the uniform load method or the trigonometric method, dependent on the application. Whilst slings may be found in service rated by either method, the uniform load method is the preferred method for rating multipurpose slings. For an explanation of these two methods, refer to section 1 1.A5.4.3 Methods of Rating Lifting Slings.
The maximum angle of inclination at which the sling may be rated is 60° (120° included angle) but it may only be rated for use at 45° (90° included angle). The SWL to be marked on the sling should be assessed by a Competent Person and will be the same as the WLL in normal conditions or less than the WLL under special conditions. The SWL of the sling selected must be at least equal to the maximum load to be lifted. The maximum load that can be lifted by a sling may also vary from the marked SWL depending upon the method of use. The most commonly used methods are dealt with in section 15.5.2 Method of Use.