You also need to know what has to be done with the load but, for the sake of simplicity, let us assume that it has to be lifted level, travelled and landed level.
Knowing the weight of the load is a relatively obvious requirement. It can be obtained in a variety of ways, including from drawings, technical specifications and safety instructions or by calculation or estimation.
Knowing the position of the centre of gravity is equally essential so as to position the slings accurately. To lift a load level, the hook of the lifting machine must be directly over the centre of gravity of the load
Knowing the position of the centre of gravity is equally essential so as to position the slings accurately. To lift a load level, the hook of the lifting machine must be directly over the centre of gravity of the load. If it is not, the load will tilt until the centre of gravity is directly below the hook. If the position of the centre of gravity is not documented, it is possible to estimate it with reasonable accuracy. However, a little trial and adjustment may be necessary to get it right. The need to allow for adjustment may well influence your choice of sling.
The next question is whether there are any lifting points. If you are lucky and the item is designed to be lifted, then there may be purpose designed lifting points. However, all too frequently, engineering components or items of plant and machinery which need to be lifted for installation or maintenance purposes do not have such lifting points. Equally, materials such as steel, timber and pre-cast concrete elements are unlikely to have them.
If there are what appear to be lifting points, it is essential to check that they are for the whole load and not just part of it. For example, the drive motor of a machine may have an eyebolt that is only intended to facilitate lifting the motor, not the whole machine. Also check whether the lifting points are distributed uniformly about the centre of gravity when viewed in plan.
First you need to know something about the load. For example, is it delicate?
If there are no lifting points, then the lifting sling will have to be attached in some other way. For example, there may be suitable threaded holes into which a conventional eyebolt, or its modern equivalent, the swivel link bolt, can be screwed. Such threaded holes may be there primarily for another purpose such as for a bolt or stud. If so, check that the material around the holes is strong enough to bear the share of the load which will be imposed, particularly if it is to be applied at an angle. Also check that the eyebolt or swivel link bolt seats fully down and is supported over the whole of its base.
A frequent solution to the absence of lifting points is to pass the sling through an aperture in the load or wrap it around the load. Passing through an aperture has the advantage of making the load captive. Wrapping around the load can provide a very secure means of connection but it requires care to ensure that the sling cannot slip.
In selecting lifting points or making connections, the strength of the load itself must be considered. A flexible or fragile load may easily collapse or be damaged by its own self weight if not adequately supported. Remember that, due to the angle between the legs and the vertical, a multi-leg sling will always exert an inward force on the attachment points. If the load is vulnerable in this way, you need to consider whether a simple sling is the best method of lifting the load or if it would be better to use a lifting or spreader beam. This can support the load at several points and eliminate the inward force.
When making connections by passing the sling through the load or wrapping it around the load, you have to complete the connection by either choking the sling back onto itself or forming a basket by connecting back to the master link or crane hook
Assuming that the load is strong enough, the last consideration before selecting your sling is whether the load or sling might be damaged by contact with one another.
Armed with all the necessary information about the load and the fundamental characteristics of the various sling types, let us look at several typical loads and determine what slings will be suitable.
The simplest is a load with a single lifting point directly above the centre of gravity such as an electric motor with an eyebolt. Any type of single leg sling is all that is required. Depending on the lower termination, it can be hooked directly into the eyebolt or connected via an intermediate shackle. As a general point, always ensure that hooks, shackles and similar items seat properly in their mating components so as to transmit the forces in the manner intended. Whilst only a single leg sling is required, it is an accepted practice to use one leg of a multi-leg sling provided it has adequate capacity when employed in that way.
If there are two lifting points, then, when viewed in plan, the line between them must pass through the centre of gravity. When viewed in elevation, if the position of the lifting points is such that both legs are the same length, then any type of two leg sling will do the job. However, if unequal leg lengths are required, a two leg chain sling fitted with clutches provides the simplest method of adjusting the leg length to the nearest link. If the legs are not both at the same angle to the vertical, the one which makes the smaller angle will take a larger share of the load and this must be taken account of when deciding on the capacity required. As for the single leg, connections may be direct or through intermediate components such as shackles provided everything seats properly. Two single legs can be used instead but take care to avoid crowding the lifting machine hook or loading it incorrectly. It is good practice to join the slings with a bow shackle, the pin of which should engage with the hook.
With three lifting points, the centre of gravity must lie within the triangle formed by the points. With a three leg lift, the share of the load which each leg takes depends partly upon its angle to the vertical and partly on its disposition in plan. If the legs are not equally disposed, the one making the largest angles to the others will take a larger share of the load. Again the choice will be influenced by any need for adjustability.
With four lifting points, a new problem may arise: ensuring that the load is shared by all the legs. Again the centre of gravity must lie within the rectangle formed by the lifting points. If the load is rigid, it is likely that most (if not all) the load will be imposed on two diagonally opposite legs with the other two taking little or nothing. If the lifting points, sling legs and load are all strong enough to be loaded like this, it is not a problem. However, if a more equal loading is required, then a fine adjustment of at least one leg length is required. This is finer than can be achieved with a chain clutch so a rigging screw is usually the best option. Some chain sling systems include a rigging screw component and standard rigging screws are easily incorporated into wire rope slings.
If the load is slightly flexible, then load distribution is usually not a problem provided the leg lengths are reasonably accurate. For example, to suspend a rectangular spreader beam, a simple four leg wire rope sling with a leg to each corner is both cheap and efficient.
When a connection to the load is made by passing the sling through the load or wrapping it around the load, the potential problem arises of whether the load might be damaged by contact with the sling or equally whether the load might damage the sling. To avoid damage to the finished surface of a load you either need to insert packing to protect it or use a solution with inherently protective properties such as a textile sling. Webbing slings in particular can spread the load over a wider area but must be applied so that the webbing is flat to the surface. If only the edge is loaded it will both defeat the purpose of using a wide sling and overload the edge of the sling. Roundslings will flatten and spread the load to some degree and naturally avoid the problem of edge loading.
To avoid the load damaging the sling, you need to avoid sharp edges or ensure that adequate packing is in place. An edge doesn’t have to be razor sharp to be a problem. Even the corner of a concrete construction panel can be sharp enough to cut a textile sling. Wire rope is less likely to cut but can suffer local wire breaks and will certainly be permanently kinked. Chain is the most tolerant of sharp edges although even then a little packing is good practice and avoids the risk of causing a notch in a link.
When making connections by passing the sling through the load or wrapping it around the load, you have to complete the connection by either choking the sling back onto itself or forming a basket by connecting back to the master link or crane hook. A basket hitch should never be used unless the load is captive by passing the sling through the load or there is more than one connection and the shape of the load prevents slippage. Choke hitch is commonly used but often abused. Left to itself a choke hitch will settle to a natural angle of about 120 degrees under the choke. If used for lifting loose bundles of items such as reinforcing bars or tubes, those at the top are not gripped so the temptation is to ‘batten down’ the choke to tighten it. This is very bad practice and leads to local overload. Far better to wrap the sling completely around the load before choking so that every item is gripped.
When wrapping or choking there is often a conflict of objectives. For example, when a textile sling is necessary to protect the load but adjustment is still required. This is where intelligent combination of sling types can easily solve the problem. Use the textile sling for the connection and hook it onto a chain sling for the adjustment. Combining textile and metallic slings in this way is fine but a word of caution is necessary. Because textile slings in particular stretch more under load than metallic slings, you should avoid having legs of different materials for multi-leg applications. As the tension increases the load will tilt and, in extreme circumstances, may become unstable.
There is one other common method of connecting to a load and that is by hooking directly into one of its natural features. For example, a two leg sling is used to lift a length of pipe by hooking over the ends. Care is essential to ensure that the hooks are correctly loaded through their seat and in this example it is likely that the hook will be tip loaded. A hook loaded on or near its tip can open out at well below its safe working load. Many sling systems include special hooks for such applications and they are shaped so as to be loaded correctly and avoid the risk. There are special terminal fittings for many applications such as for lifting barrels, bales and crates or engaging with trunnions.
In conclusion, I trust this article illustrates how the relative merits of the various types of sling can be brought into play. Where the sling is for a particular application, the optimum type can be specified. For general purpose applications, it should also illustrate the benefits of having available a good selection of types and sizes, together with other accessories such as shackles, eyebolts or swivel link bolts and rigging screws.
Knowing the position of the centre of gravity is equally essential so as to position the slings accurately. To lift a load level, the hook of the lifting machine must be directly over the centre of gravity of the load LEEA 1 First you need to know something about the load. For example, is it delicate? LEEA 2 Derrick Bailes, chief executive, Lifting Equipment Engineers Association LEEA 3 When making connections by passing the sling through the load or wrapping it around the load, you have to complete the connection by either choking the sling back onto itself or forming a basket by connecting back to the master link or crane hook LEEA 4
