If one is fortunate, the load will have a lifting eye big enough either to engage directly with the sling hook or to accept a shackle which in turn can engage with the sling hook. If the sling leg is also in a straight line, then, so far as the geometry of the lift is concerned, the application is as simple as it can get. The complications begin when the sling is wrapped around the load or passes through it, or when the sling leg contacts the load and cannot take the load in a straight line.
For many years, there have been standard allowances to be made for the ‘mode of use’. For example, in choke hitch, it is normal to apply a mode factor of 0.8 to reduce the safe working load to 80% of its value in straight pull.
Most of the data which lay behind these mode factors came from tests conducted by manufacturers on their own products and was not in the public domain. However, as part of the work done in the 1990s leading to the harmonised European Standards, the UK’s Health and Safety Executive (HSE) carried out a series of research programmes into this subject.
The tests were carried out on webbing slings, roundslings, man-made fibre rope slings, chain slings, wire rope slings with ferrule secured turn back eyes and wire rope slings with ferrule secured flemish eyes. All the textile slings and wire rope slings had soft eyes although the webbing slings had leather reinforced eyes. They were all supplied as complying with the relevant standards current at the time. Although not covering every variation, the slings used were nevertheless representative of the slings in regular use.
The first tests looked at their properties in straight pull and all exceeded the required minimum factor of safety, some by a considerable margin. They were then tested in choke hitch around a large diameter round tube. Again they all met the minimum requirements, taking account of the mode factor and, at failure, they all broke at the point of choke. At that point, the sling is being bent around its own diameter which indicates the effect of bending around a small radius.
The next test was again in choke, but around a universal beam and here a significant difference was found between the metallic and non-metallic slings. The small radius at the tip of the beam flanges caused the non-metallic slings ultimately to fail at that point, and at a load considerably less than 80% of the minimum failure load in straight pull. The metallic slings still failed at the choke and within the minimum requirements.
However, with the wire rope slings they observed that, as the load was applied, the slings deformed at the corners and effectively hooked onto the flange tips resulting in uneven tension around the beam.
The research also looked at the energy absorbed by the slings in these three tests. Energy absorbed is a measure of the sling’s resistance to shock loading such as might occur if the load slips in the sling as a choke hitch tightens. The energy absorbed in choke around the pipe was approximately half that in straight pull. However, when choked around the universal beam it was half that again, for example, only about a quarter of that in straight pull – a very significant reduction.
What these results serve to illustrate is that the mode factor of 0.8 for choke hitch is appropriate for both metallic and non-metallic slings provided the sling is loaded correctly. Bending a sling around a small radius not only reduces its ultimate strength but also its ability to withstand shock loading.
In fact, this particular conclusion represents a very simplified summary of a small part of the research, which went on to look at many other aspects of sling safety, one of which was packing. Significantly, the correct use of packing will eliminate or control to an acceptable level the effects described above.
Packing is used for two main reasons, to protect the load from damage by the slings and to protect the slings from damage by the load. The former is perhaps fairly obvious and it is the latter I want to address here.
The packing that is appropriate depends upon the load being lifted and the type of sling being used. Non-metallic slings are susceptible to being cut if loaded over a sharp edge and, as already illustrated, it does not have to be a knife edge. A knife edge will slice through a textile sling but the small radius corner of a rolled steel section or a concrete panel is sharp enough to significantly reduce the ultimate failure load.
If chain is used over a square corner, it generally forms a natural bridge with one link across the corner, held clear of the edge by the link either side. However, if the angle is more acute, the link across the corner will make contact and be subject to a high bending load. If the edge is sharp, notching of the link may occur. Wire rope needs an absolute minimum radius at least equal to that of its own diameter – but that will still result in permanent deformation of the rope. Ideally the radius should be at least four times its own diameter.
So, whatever the type of sling, the first requirement for the packing is that it should provide sufficient radius to prevent cutting or notching of the sling and not reduce the failure load below 0.8 of the minimum failure load in straight pull.
Webbing slings and other types of flat sling, such as those made of steel mesh or plaited from small diameter wire ropes, must have the load evenly distributed across their width. This is not usually a problem if the sling is around a straight edge and square to it. However if the edge is uneven or the sling is at an angle, one side of the sling will take most of the load and it will start to tear from that side. Once a tear is initiated, sudden failure is likely. The second requirement for the packing is therefore to ensure the load is correctly distributed onto the sling.
When forming a choke hitch, particularly with a springy medium such as wire rope, there is usually quite a lot of movement as the tension increases and the choke tightens. As was observed during the HSE research, if the rope is bent around a corner it can permanently deform and hook itself onto the corner, preventing a uniform tension around the load.
A similar effect was observed when testing the polypropylene rope sling, causing an imbalance of tension between the strands to the extent that premature failure occurred. Therefore the size and shape of the packing and the means by which it is held in place must allow for such movement without it being displaced.
Choke hitch is often used as a means of gripping the load. Consequently, although the packing should allow movement to set the choke as the tension increases, once the sling is under tension, it should not prevent the load being gripped.
When lifting bundles of materials such as reinforcing bar, the rigger is all too often seen battering down the choke to tighten the bundle. This is a very bad practice which increases still further the tension at the point of choke. In addition, it fails to secure the material immediately under the choke. It is far better to use a longer sling and wrap fully around the load before hooking back in choke.
Materials used for packing are invariably subjected to high crushing loads and, if bridging an uneven surface, bending. Unsuitable materials can make matters worse and actually become a hazard rather than removing one. Materials which crush or collapse suddenly can give rise to shock loading and suddenly fly out of position.
So what makes suitable packing? Several purpose-made standard products are available, along with simple designs for fabricating items to suit the job in question. In particular, there are corner protectors for webbing slings where the sling is reeved through the protector allowing some movement but holding it in place.
Similar steel corners providing a suitable radius can be fabricated for wire rope and fibre rope slings. For repetitive lifting operations, their ease of use and greater efficiency more than cover the additional cost.
If special fittings are not justified then more general packing material can be used. Timber has been widely used but caution is required. The sling should be across the grain and the timber should be of adequate section to avoid it splitting or crushing. Also it may be necessary to secure it in place by some other means until it is held by the tension in the sling. It is potentially dangerous for the rigger to hold it and that should be avoided.
Another suitable material is rubber from sections of old vehicle tyres or conveyor belt. It has the merit of being flexible and less likely to split or fly out than timber.
Understanding how the various types of sling behave in their various modes of use and how the risks arising from such use can be eliminated or reduced to an acceptable level by suitable packing is one of the key skills of the rigger. Inadequate packing has been the cause of many sling failures, and this is therefore an issue which demands particular attention from those with responsibility for the safety of lifting operations.
