1.A5.5 SAFE USE OF SLINGS

If the slinging geometry does not comply with the assumptions given in section 1.A5.5.2 Rating Assumptions, then the load will not usually be evenly distributed amongst the legs. The amount of load that will be imposed on an individual leg depends upon the following:

The relationship between these factors is a complex one 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 calculation.

For a two leg sling, if each leg has the same angle to the vertical, then the load will be shared equally between them. If, however, one leg has a smaller angle to the vertical than the other, that leg will have a greater share of the load imposed upon it. This will result in the load tilting. The sling may also not be symmetrical due to the position of the attachment points

The effect of unequal angles increases as the difference between the angles increases. Additionally, the effect is more significant as the angle between the legs decreases although with a uniform load rated sling this is offset to a certain extent because at such an angle there is a degree of redundancy available. This redundancy is however insufficient to fully counter the effect.

As a guide, at an included angle of 30° a difference of angles to the vertical of 12°, i.e. equal to 6° of tilt, will load the downhill leg, i.e. the leg with the smaller angle to the vertical, to its maximum rating if the sling is lifting to its maximum rated capacity. This effect becomes even more significant for three and four leg slings.

For three and four leg slings, a difference between the angle each leg has to the vertical has a similar effect to a different degree, but in addition 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:

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 uniform load method of rating takes some account of this by assuming that only three of the legs are bearing the load. However, in extreme cases the load may be carried on only two diagonally opposite legs with the other two providing balance only. In such cases, the sling should be de-rated to two-thirds of its standard rating.

If any of the assumed conditions are not met, then it is possible that a large portion of the load will be imposed on only one or, at best, two legs of the sling. In such circumstances, it should be assumed that all of the load is being carried by one leg, so the sling should be de-rated accordingly as described in the following paragraph by assuming that only one leg is in use.

If a multi leg sling is used with less than its actual number of legs attached to the load, then obviously the SWL of the sling must be reduced. The amount by which it should be reduced can be calculated exactly, but it is rather complex, as a number of factors need to be taken into account including the method of rating. An easy way of ensuring that the sling is never overloaded is to reduce the SWL from that marked on the sling according to the number of legs in use.

e.g. a 4 leg sling with only 2 legs in use, REDUCED SWL = 2/4 i.e. ½ x WLL MARKED

a 3 leg sling with only 2 legs in use, REDUCED SWL = ⅔ x WLL MARKED

This inevitably means that in some cases the sling will be under-utilised. If maximum utilisation is required, then reference should be made to a person who understands the factors involved and can therefore perform the necessary calculations.

A single or multi-leg sling may be used with the legs straight if, for example, the legs are terminated in a hook(s) which can be attached directly to a suitable lifting point on the load as shown in Figure 1.A5.4.2.1-1 , Figure 1.A5.4.2.1-2 , Figure 1.A5.4.2.2-1 , Figure 1.A5.4.2.3-1  and Figure 1.A5.4.2.4-1 . There is no particular problem in this method of use provided, of course, that normal precautions are observed and the angle factor, in the case of multi-leg slings, is taken into account.

Single leg or multi-leg slings may both be used in choke hitch, which is illustrated in figures Figure 1.A5.4.2.1-2 , Figure 1.A5.4.2.1-2 , Figure 1.A5.4.2.2-2  and Figure 1.A5.4.2.3-1 . The basic advantages of a choke hitch are firstly that a sling may be attached to a load which has no suitable lug, eyebolt, etc., and secondly that the sling tends to bind the load together.

In forming a choke hitch, the sling is bent round a small diameter, which may be the eye of the sling itself or the saddle of a hook, link or other fitting. In these circumstances, the load in the sling will be increased at the point of choke and for this reason some de-rating is necessary in order to prevent the sling being locally overloaded. For the convenience of the operative, the same factor is used for all types of slings. The SWL should be reduced to 0.8 of the SWL of the straight leg.

Care should be taken when applying choke hitch to select a sling of sufficient length to ensure that the angle at the choke does not exceed 120° and that the sling positions itself naturally. Slings should never be ‘battered’ down to achieve an angle greater than 120°, unless further de-rated in accordance with national standards

Double wrap choke hitch is a variation on choke hitch where the sling is passed one complete turn around the load before being choked. (See figure Figure 1.A5.5.4.3-1 ) This increases the binding effect and should be used on loose loads such as bundles of tubes. The sling should be de-rated by the same amount as for ordinary choke hitch.

The basket hitch is normally used with slings in pairs (1A5.12) for handling loads such as a large cylindrical object but it is not suitable for cradling loose bundles. If only one sling is used, the sling should be passed through the load at a point above the centre of gravity to ensure it is safely secured. (See Figure 1.A5.5.4.4-1 )

If a sling in basket hitch is used with both legs parallel, i.e. with an included angle of 0° between the legs of the basket, then twice the WLL may be lifted. With the terminations of both ends of the sling on the hook (see Figure 1.A5.5.4.4-1 ) the load lifted may be increased to not more than 1.4 x the WLL provided the angle to the vertical does not exceed 45°.

If two slings are used in basket hitch in the same manner (see Figure 1.A5.5.4.4-2 ) the load may be increased to 2.1 x the WLL, again provided that no angle to the vertical between adjacent or diagonally opposite legs exceed 45°. The above factors for basket hitch assume that all sharp edges are adequately packed.

A double wrap basket hitch is when the sling is passed completely around the load as shown in Figure 1.A5.5.4.5-1 . This will help to ensure the security of loose bundles. If security of the load is the prime consideration, then double wrap choke hitch is recommended. The factors are the same as for basket hitch.

Double choke hitch is a variation of choke hitch where the load is carried on two parts and for this reason the SWL in choke hitch may be varied in accordance with the manufacturer’s or supplier’s advice. Where this is not available, the single choke hitch rating should be used. (See Figure 1.A5.5.4.6-1 ) This is sometimes known as ‘halshing’.

Endless slings are generally used in choke hitch and may need de-rating as recommended by relevant standards or the manufacturer or supplier. For instance, fibre rope slings should be de-rated to 0.8 x WLL. Endless chain slings require no de-rating when used in choke hitch as the sling is designed for this method of use. It is assumed that at the points of attachment to both the lifting appliance and the load, the radii around which the sling passes are large enough to avoid damage to the sling.

The weight of the load should be ascertained before lifting. (See section 1.A2 APPENDIX 2 - LOAD ESTIMATION - WEIGHT AND CENTRE OF GRAVITY) The lifting method selected should be suitable for the load. The sling should be strong and long enough for the load, both in terms of its WLL and its actual condition. The sling should be carefully inspected for obvious defects before use.

The load should be secure, stable and balanced when lifted so an assessment of the position of its centre of gravity will be necessary (see section 1.A2 APPENDIX 2 - LOAD ESTIMATION - WEIGHT AND CENTRE OF GRAVITY) to ensure that the lifting point is approximately over it. Failure to do this is likely to cause the load to swing wildly on being lifted, or even to fall out of the sling.

Any loose parts of the load should be adequately secured either by the lifting method or by other means.

The sling must be firmly secured to the load, e.g. by means of hooks on to purpose designed lifting points, eyebolts, etc or by a suitable method of slinging. The sling must not be twisted, knotted or kinked in any way, nor should the lifting points be overloaded by the slinging method.

It is also essential that there is adequate packing between sling and load and guidance on this subject is given in section 1.A5.5.7 Control of Lifting Equipment, Storage, Handling and Inspection.

The rated angle (e.g. 45° to the vertical or 90° included angle) must not be exceeded and the angle at any choke must not exceed 120°, unless in accordance with the de-rating in applicable national standards, or at any basket should not exceed 90°.

When using three or four leg slings with out of balance loads or with unequally spaced legs, two legs may support the majority of the weight while the other leg or legs merely act as a balancer. If the lifting points on the load are not in the same horizontal plane, the load, if it is flexible enough, will distort to accommodate the equal leg lengths of the sling. Alternatively, if the load is rigid, two legs will be likely to support the majority of the weight and may be overloaded while the remainder provide the balancing load.

Before commencing a lift, a recognised code of signals should be used between the slinger and the crane driver. Ensure that the load is free to be lifted, e.g. all holding down bolts and/or dowels have been released and any seals broken. Check for overhead obstacles such as power lines, pipe work etc.

The slinger should always ensure that everyone is clear before giving the signal to lift. Unless unavoidable, no one should be allowed under a suspended load and as far as possible, all people should be kept clear of the area of operations. If unavoidable, additional measures to mitigate the associated risk and consequence of failure should be included, i.e. secondary positive holding devices to hold the load should the primary holding device fail. A trial lift should be made. People should not ride on loads except in very exceptional circumstances and only when authorised by a Competent Person and additional risk reduction measures have been provided.

Traditionally tag lines have been used to help to control long or bulky loads (See figure Figure 1.A5.5.5.4-1 ). The slinger should always ensure that everyone is clear before commencing the lift. It must be noted however that their use must be carefully considered at the planning stage as there are potentially hazards associated with their use such as, when using polypropylene or fibre ropes

Which may lead to common injuries such as trapped hands or fingers; therefore consideration should also be given to other devices that are available for load manipulation –

A suitable setting down area should be selected before lifting. When lowering, a trial set down should be made before the slings are released. Make sure that the load is placed on battens, dunnage or packing so that the slings can be readily withdrawn. Trapped slings should never be dragged out from under a load, nor should slings be used to drag a load. (See Figure 1.A5.5.5.4-2 )

The slinger should always be careful not to set the load down on his own toes, another common accident. Having set the load down correctly, the empty sling legs should be manually withdrawn by the slinger and hooked back on to the crane hook or upper terminal fitting to prevent accidental ‘hook-up’ to surrounding objects or striking an individual. (See Figure 1.A5.5.5.4-3 )