Over the past few years, the use of high performance synthetic slings has become increasingly popular. One of the key properties of synthetic slings is their very low elasticity. Do the design factors developed for steel rigging hold true for the new generation of lifting material? In this review, we will zoom in on these issues, determine the problems and make recommendations to designers and engineers, to help them avoid potentially unsafe situations.
At present, sling tension calculations typically take into consideration precise sling lengths. However, due to manufacturing tolerances, in real life situations, a certain degree of mismatch in sling length must be expected. In particular when lifting with four slings attached to a single lifting point, different sling lengths will result in a different load on each sling. Naturally, the shortest sling will be subjected to a greater load than the longest sling. Elasticity of the material to some extent compensates for this effect, as the shortest sling stretches to distribute more of its load across the other three slings in the rigging; however, with materials with low elasticity, the level of compensation is restricted.
To tackle this uncertainty, suppliers are called upon to manufacture their slings in what is known as matched pairs. This term has a number of possible definitions, but generally speaking it refers to an item-to-tem length discrepancy of max. 0.5 times the sling diameter. As a consequence, to some extent tolerance can be controlled, but with bigger loads (and hence larger sling diameters), the allowable difference in length also grows.
To compensate for this length difference, engineers tend to multiply the minimum required breaking load by a safety factor, typically of 1.25. This is known as the Skew Load Factor. In truth this factor is totally arbitrary, and today is applied identically for all types of material, irrespective of whether the sling is produced from polyester, HMPE or steel.
Let us assume we are lifting a box at 4 equally distributed lifting points. We will assume the centre of gravity precisely at the centre of the box. In the ideal situation, all 4 slings will support an equal load. In this imaginary situation, let us assume 4 slings from the same production batch, in matched pairs, in other words all within 0.5x the diameter of each other. In the worst case scenario, the two opposite corners will be suspended from slings with the shorter length at the lower end of the tolerance scale. The other two slings (at the other corners) feature a tolerance at the upper end of the tolerance limit.
In this situation, the Skew Load Factor of 1.25 must now compensate for a maximum load difference of 125% per sling. This means that in an ideal situation, both pairs support 50% of the load. In the worst case, however, the Skew Load Factor means that a load distribution of 62.5%/37.5% would be permitted.
Let us now place this theory in a real case situation. In this situation we use 4 HMPE soft slings with a length of 10,000 mm and a diameter of 200 mm. The maximum permissible length tolerance, according to the matched pair definition is 100mm. Without becoming too technical, a sling can be expected to stretch by 0.5% during normal use and 1% at 2x the Working Load Limit (WLL). If in our situation the two opposite slings are both short (9900mm) and the other two are 10,000mm, two slings will bear almost the entire load, while the other two barely touch the hardware. Rather than 1.25, the Skew Load Factor factor is in fact 2.
Simply increasing the safety factors is effectively treating the symptoms while leaving the underlying problem untouched. Solving the underlying problem, however, would be far more effective. Rather than simply decreasing the permissible length differences, the factor calculation should at the very least be disconnected from the diameter of the sling. The sling length is in fact a far more efficient parameter to work with. Moreover, it is useful to be aware that it is far more meaningful to determine Skew Load Factor factors for each individual project/rigging situation, rather than applying the standard, regulated factor of 1.25, and taking account of the length of the actual hardware used.