The materials they are made from are usually typical of their era. Wire rope slings of relatively low grade rope with hand spliced terminations, mild steel shackles, grade 40 chain slings with welded joining links etc. The factors of safety were usually higher than required by present day standards and the overall result was that, relative to its capacity, the lifting gear was very heavy to handle. Designers therefore tended to utilise it to the maximum and calculate the loading at the particular angle at which the sling leg operated. Even for general purpose slings, the trigonometric method of rating was the norm. For those not familiar with this method, it allowed the SWL to vary with the leg angle, the maximum being with all legs vertical. The SWL was usually marked as the value at 90º included angle and the values for other angles obtained from tables or wall charts. The slinger had to watch the angles carefully and ensure that the load was shared equally between the legs.
Today things are rather different. Considerably higher grades of chain and wire rope are employed and various kinds of textile sling are widely available, all of which are lighter and easier to handle than previously. The normal practice now for general purpose multi-leg slings is to rate them for leg angles in the range of 0-45º to the vertical and, if required, for an extended range of 45º-60º. The slinger’s ability to judge the angle is therefore less critical. Moreover, the rating allows for several foreseeable deviations from the ideal geometry. This is in line with the modern approach of eliminating or designing out hazards wherever possible, rather than relying solely on the skill of the user.
The ideal geometry is that which is symmetrical, giving each leg an equal share of the load. To achieve that, all the legs must be in use and at the same angle to the vertical. Three leg slings must be equally disposed about the load’s centre of gravity when viewed in plan, ie from above. Four leg slings must also be equally disposed in plan but, when lifting a rigid load, must also have precisely the same leg length to achieve an equal share of the load; otherwise most of the load will be shared by two legs across one of the diagonals. Clearly this is a tall order, so some latitude is required to achieve a workable system.
The four leg situation is addressed to some extent in the initial rating, which is the same as that of a three leg sling. Therefore provided that two of the legs take no more than two thirds of the load, the other two will share the remaining third and no leg will be overloaded. This covers the majority of situations where the load has a degree of flexibility. Thus it is only when lifting a rigid load that any further reduction is required to take account of leg length tolerances.
For the other requirements of symmetry, as a general guide, the loading can be assumed to be adequately symmetrical if all the following conditions are met:
(a) all sling legs are in use
(b) the load is less than 80% of the marked SWL or WLL*
(c) none of the sling legs exceeds the maximum permitted angle b to the vertical for the marked SWL
(d) sling leg angles ß to the vertical are all at least 15º
(e) sling leg angles ß to the vertical are all within 15º to each other
(f) in the case of three and four leg slings, the plan angles d are all within 15º of each other
• For general purpose slings and lifting accessories, the terms SWL and WLL are synonymous.
If the slinging geometry does not comply with the above, then the load will not usually be sufficiently evenly distributed amongst the legs. The amount of load that will be imposed on an individual leg depends upon the following:
(a) the number of legs in the sling, or in use
(b) the angle between each of the legs and the vertical
(c) the distribution of the legs in plan view
(d) the total load being lifted
The relationship between these factors is a little complex, especially for three and four leg slings. What happens as these factors vary can be identified in general terms, although to quantify the effect requires some calculation.
If not all the legs are in use, then the safe working load of the sling must be reduced. The amount by which it should be reduced can be calculated exactly, but an easy way of ensuring that the sling is never overloaded is to reduce the safe working load from that marked on the sling according to the number of legs in use. This inevitably means that in some cases the sling will be under-utilised but has the advantage of simplicity. For example, a four leg sling with only two legs in use, reduced SWL = 2/4 or 1/2 x SWL marked. A three leg sling with only two legs in use, REDUCED SWL = 2/3 x SWL marked.
If the angle ß between each of the legs and the vertical is not the same, the loading in the leg with the smaller angle to the vertical increases. Ultimately if one leg is vertical it will take the entire load. Unequal angles to the vertical may occur because of the position of attachment points on the load, particularly with an irregularly shaped load. However it often happens because the load tilts when lifted due to the position of the centre of gravity being misjudged. The effect is significant, and becomes greater as the included angle between the legs decreases. There is some reserve of strength to counter this but, as a guide, a multi-leg sling should not be used with an included angle of less than 30º (ß less than 15º to the vertical) or a difference in angles to the vertical of more than 12º (ie equal to 6º of tilt) without an allowance being made.
For three and four leg slings the problem becomes three dimensional in that the distribution of the legs, when viewed in plan, also affects the share of the load imposed on each leg. The sling geometry of three and four leg slings is as follows:
Three leg slings: With a three leg sling, it is assumed that, viewed in plan, the legs are at 120º to each other. If two of the legs are closer than that, the third leg will receive a greater share of the load. Ultimately, if two of the legs are side by side, ie at zero angle to each other, then they will receive only half the load between them leaving the third leg to take the other half on its own and thus be overloaded.
Four leg slings: With a four leg sling, it is assumed that, viewed in plan, the legs are symmetrically disposed, the lower attachment points making the corners of a rectangle. Ideally, the nearer the rectangle is to a square the better, but this is by no means essential. However the smaller the angle between the legs the greater the effect of unequal angles. On a four leg sling, the unequal effect can occur across either or both of the horizontal axes, ie along the length of the rectangle and/or across the width of the rectangle.
As previously mentioned, the four leg sling is also affected by the rigidity of the load. Even if all the legs have the same angle to the vertical and are symmetrically disposed in plan, small differences in the leg lengths due to manufacturing tolerances or the positions of attachment points may prevent the load being equally distributed. The standard method of rating takes some account of this by rating a four leg sling the same as a three leg sling. However if the load is very rigid, the majority of it may be on only two diagonally opposite legs with the other two providing balance. In such cases, the sling should be de-rated to two-thirds of its standard rating.
The problem I mentioned at the outset concerned a 40 year old design lifting a very rigid load on four legs, each intended to be loaded to the maximum. Whilst the slings and other lifting accessories could be replaced with higher capacity modern equipment, the lifting points on the load were still the originals and each was rated at only a quarter of the total load. Moreover, when viewed in plan, the lifting points formed a narrow rectangle – far from ideal. As always there is more than one way to control a risk and, with careful planning and some additional load monitors, a controlled test may well demonstrate that the lift can be achieved safely. But this particular incident certainly serves to underline the greater degree of simplicity offered by the modern approach to sling geometry.
