There are some obvious decisions to be made such as the lifting capacity, the number of legs required and how the sling is to be attached to the load. However before doing so it is important to have an understanding of the fundamental characteristics of the various types of sling generally available.
You need to be aware that there is a considerable choice of lifting medium used in slings. They all fall into one of three basic types, chain, wire rope and textile, each of which has its advantages and limitations. One important point to remember is that the materials the sling is made from must be suitable for the purpose. There are chains, wire ropes and textiles manufactured for a whole variety of applications but only some of them have the properties necessary for lifting slings. Therefore I strongly recommend that the user always ensures that the slings they use comply with recognised lifting sling standards.
Chain has the advantage of being durable, very flexible and its length can be made adjustable. It will tolerate a wide range of temperature from -40 degrees C to at least 200 degrees C without de-rating and higher still with appropriate temporary de-rating. However, compared to other types, it is relatively expensive and, depending upon the grade of chain used, relatively heavy.
Grade in this context refers to the maximum stress level in the chain rather than its quality. Over time, higher strength materials have allowed chain makers to increase the stress levels effectively allowing a smaller and therefore lighter chain for a given load. Grade 8 is the most popular in use today although grade 10 is steadily increasing market share. A limited amount of grade 4, 5 and 6 is still in use.
The vast majority of modern chain slings are mechanically assembled from a system comprising a range of standard components. From these, any configuration of single leg, multiple leg or endless slings can be assembled by the sling manufacturer. Most systems include a considerable choice of hooks and other terminal fittings to match particular types of load.
Wire rope is generally not quite so durable and its upper temperature limit is normally about 100 degrees C. It isn’t as flexible as chain but its stiff yet springy characteristic can be an advantage if it has to be pushed through an aperture or under a load. However, if wrapped around a load, it will tend to deform permanently to the shape of the load. Wire rope suitable for slings also comes in a variety of grades and constructions. Size for size, ropes with a steel core are stronger but stiffer than those with a fibre core and, like chain, the higher the maximum stress level, the lighter the rope for a given load. Grades 1770 and 1960 are commonly used.
In most applications, the rope is terminated by looping it back and securing the loop with a metal ferrule which is pressed onto the rope by a hydraulic press. There are one or two variations of the process but all produce a loop or soft eye which is the simplest form of sling termination. With the addition of a thimble to support and protect the eye, various terminal fittings similar to those used for chain slings can be captured within the eye facilitating the manufacture of single and multiple leg slings.
Wire rope is generally cheaper and lighter than chain which can be advantageous, particularly for long slings in straight pull such as might be required to suspend a spreader beam. The leg length cannot easily be adjusted although small adjustments can be made by incorporating a rigging screw. For outdoor use, particularly in a marine environment, galvanised wire rope can be used with advantage. Broken wires can occur for various reasons. From a strength point of view, a small number will not affect the safety of the sling. However, they present a serious risk of injuring the slinger’s hands and may therefore necessitate scrapping an otherwise serviceable sling.
The great advantage of textile slings is their light weight in comparison to chain and wire rope, something any slinger will appreciate if they have to carry them far. They are also very flexible and relatively soft and therefore kind to the surface finish of a load. There are three designs of textile sling in general use: flat webbing slings, roundslings and fibre rope slings.
When wrapped around a load, the flat webbing sling can spread the force over a relatively wide area thereby reducing the surface pressure and the risk of damage to a delicate finish. However it is essential that it is wrapped square to the corner so that the load is uniformly distributed. If used at an angle, the outer edge only will be loaded and can easily start to tear. Once that starts, catastrophic failure can easily occur. Webbing slings can be made endless or with an eye at each end and, similar to a wire rope sling, various terminal fittings can be captured within the eye to facilitate the manufacture of other configurations.
Roundslings are manufactured from hanks of parallel yarns encased in a woven tubular sheath so their configuration is always endless. The yarns are free to flatten against the load and, whilst the load is not spread as widely as is possible with a webbing sling, the yarns remain uniformly loaded. Although terminal fittings can be captured within the sling, this is relatively rare and, if a multiple leg sling is required, it is usually assembled by the slinger using suitable shackles etc. Roundslings tend to have greater elastic stretch under load than other types of sling.
The earliest form of textile sling, fibre rope slings, were originally made from natural fibre rope. Today most are made from synthetic fibre rope which is more durable and relatively low cost. In many countries their use has declined as webbing slings and roundslings have gained popularity. The rope is made endless or terminated with an eye by means of a hand made splice. Again, similar to a wire rope sling, various terminal fittings can be captured within the eye to facilitate the manufacture of other configurations.
All textile slings are susceptible to being cut if taken over a sharp edge without adequate protection. Furthermore, it doesn’t have to be razor sharp; even the corner of a concrete construction panel can be sufficient to cause serious problems. Fortunately there are several easy ways of protecting the sling including some which can be incorporated into the sling during manufacture. Textile slings are not generally suitable for use outside the temperature range of 0 degrees C to 80 degrees C.
A note of caution is necessary about exposing any type of sling to chemicals. Advice should always be sought from the sling manufacturer before first use. To illustrate the point, in the case of chain slings, the higher grades in particular can be susceptible to hydrogen embrittlement if exposed to a strong acidic environment. Wire ropes can lose their lubricant and start to corrode from inside. Textiles have varying resistance to chemicals. Polyester is resistant to moderate strength acids but is damaged by alkalis. Polyamide (Nylon) is virtually immune to the effect of alkalis but is attacked by moderate strength acids. Polypropylene is little affected by either acids or alkalis but is damaged by some solvents, tars, paints etc. Remember that moderate strength solutions will become stronger as the water or solvent evaporates.
Having provided an overview of the main characteristics of the various types of sling, next month’s article will explain how to match those characteristics to particular applications.
Lifting slings are without doubt the most versatile means of connecting a load to the lifting machine slings 1 Part of the skill is in selecting the right sling or slings for the job slings 2 Chain has the advantage of being durable, very flexible and its length can be made adjustable slings 3 You need to be aware that there is a considerable choice of lifting medium used in slings, says Derrick Bailes slings 4 Roundslings are manufactured from hanks of parallel yarns encased in a woven tubular sheath so their configuration is always endless slings 5
