Lifting steel coils with synthetic slings

Port operators at the Port of Antwerp, Belgium, requested a new type of synthetic sling for repetitive lifting of steel coils, whilst maintaining low weight and ease of use. DSM Dyneema is the inventor of the HMPE gel spinning process and manufacturer of Dyneema fibre.

Market challenge

For repetitive handling of heavy goods, such as loading and unloading of bulk cargo (in which case the term stevedoring is frequently used), wire rope slings, steel hoisting mats or chain slings are still commonly used. The use of steel-based slings, however, presents some serious disadvantages. First of all, their high mass hampers ergonomic handling and often requires two workmen to handle them. Additionally, broken steel wires may protrude from the sling, and such 'meat hooks' pose a high risk for hand and other body injuries. The use of steel-based slings can also damage the goods that are being handled.

Heavy-duty roundslings based on synthetic fibres can replace wire rope slings in some cases, but problems are still encountered upon handling of, for example, heavy goods that are highly abrasive or have sharp edges, such as unpacked steel coils.

In such cases, the synthetic roundslings generally have a short service lifetime: after only a limited number of lifting jobs damage, such as tears or rips, or even cuts in the cover of the roundsling are frequently observed.

One way of increasing the service lifetime of a roundsling is to use additional protective pads between the roundsling and the goods.

Such pads, however, need to be manually placed at critical spots, thereby increasing labour and handling time, and consequently reducing the average number of lifts per time unit significantly. In addition, such pads may not be placed at the right spot, or may shift during use; resulting in inadequate, or even unsafe lifting.

Design considerations

Roundslings made with synthetic fibre consist of a core and a cover. Typically, an endless roundsling consists of a load bearing core that is fully enclosed within a protective cover. The cover is typically made of webbing with the ends overlapped and sewn.

The two most relevant norms that were available and could be used as a reference are EN-1492-2 (2) and WSTDA RS1 (3). Since the concept was developed in Europe, the EN-1492-2 norm was used as a reference and guidance norm.

After consulting with the distributor and end-user in the Port of Antwerp a vertical rated load of 20t and a sling length of 4m (13.1ft) were chosen. All trial slings were tested with these design values, and only the cover quality was changed over time to accomplish a large number of lifts with each sling. With a safety factor (SF) of 7 the minimum break load of the slings should be 140t.

Cut and abrasion resistant cover

The Dyneema fibre is cut and abrasion resistant. It has been used in cut-resistant gloves for many years. The same yarn properties that provide the cut resistance in gloves were used to design a highly cut resistant protection cover.

The cover is a hollow tubular 3D woven, consisting of four woven plies constructed into a hollow tubular format with two layers forming the wall, which are made by spirally interweaving a single multi-stranded and twisted weft yarn within a multiplicity of warp yarns.

The cover was tested on a laboratory scale against a range of covers currently commercially available and proved to be the best performing cover. Tests included sawing resistance against a steel wire cable, abrasion resistance and cutting resistance.

Behaviour of the core under load

As the cover is based on Dyneema fibre it does not stretch much when under load. The core is also manufactured from high modulus fibre. Samples of a complete sling were tested to determine behavior under load.

The stretch of a sling containing a core is in linear relation with the load applied. Only when loaded for the first time does the sling show a non-linear 'construction stretch', which is removed from the sling during the first load. Every consecutive loading will show a linear behavior between zero load and full load applied.

A sling was preloaded three times and then broken. The sling was a 20t (196kN) work load sling with a length of 3.8m, designed with a safety factor 7. It was preloaded three times to 50% of the minimum required break strength of 140t (1,372kN). The first load including the ‘construction stretch’ started at the 40mm mark and ended at 123mm, showing a stretch of 83mm at 50% break load. The second and all subsequent loads started at the 85mm mark and showed a stretch up to the 165mm mark, showing a stretch of 80mm up to break load, recalculated to 2.1% (safety factor 7).

The overall result is a sling construction which will stretch only 0.3% at the rated working load, showing a linear elongation performance that is similar to wire rope based lifting slings.

Tests in the Port of Antwerp

A 13kg roundsling, made completely with Dyneema fibre, was evaluated against equally long standard steel hoisting mats with a mass range 70-100kg, in lifting steel coils of mass 15-35t per coil. In practice about half of the coils are packaged, the remainder is transported in non-packaged form, which means that the slings are in direct contact with the sharp edges of coils during lifting operations.

The steel hoisting mats have such a high mass that handling needs to be performed by two workers. Steel hoisting mats were found to have a typical service lifetime of 150 to 200 lift jobs (on packaged and unpackaged coils).

Steel hoisting mat

The roundsling made from Dyneema fibres could be handled by one worker during stevedoring, and showed hardly any visible damage after 521 lifting jobs (of which about 50% were on unprotected steel coils). This is a much longer service life than standard steel-based products. The roundsling was further inspected by removing the cover, to reveal no visible damage to the core fibres.

The average residual strength of the core was subsequently measured to be more than 70% of its initial strength, which is more than double of what is generally accepted as a minimum level (SF 2-28%) of residual strength for a sling in use. The roundsling being tested could have safely performed many more lifting jobs. Earlier comparative tests had already revealed that roundslings with a core based on HMPE fibres, and with various covers made from polyamide 66 or polyester fibres had to be taken out of service after only a few lifting jobs.

Cover damage versus core strength values

When looking at roundslings the discard criterion that is generally accepted is damage to the sleeve. In this design it was decided to incorporate a special design feature in the sleeve. The sleeve contains red indicator yarns on the inside of the weft.

The user instructions need to clearly state that the sling should be discarded when the red yarns become visible. The port operator counted the number of lifts that each of the slings had performed, and slings were taken out of service after a specified number of lifts. The slings were break-tested to establish the retention strength of the core in relation to the number of lifts.

The retention strength of the sling decreases as the number of lifts increases. Cover damage normally occurs at around 1,000 lifts. It can be concluded that the retention strength of the core is above the generally accepted value of 28% (SF 2) under conditions similar to these tests.

DSM Dyneema is the inventor of the HMPE gel spinning process and manufacturer of Dyneema fibre slings 10 Preparing for the test in the port of Antwerp, Belgium slings 9 Abrasion tests slings 8 High Modulus Polyethylene (HMPE) fibres have been introduced in lifting slings as an alternative to steel wire, chain or polyester slings 7 Tests included sawing resistance against a steel wire cable slings 6 Stevedores in several ports in Europe had been looking for solutions, trying everything from polyester webbings protected by extra sleeves or even solid polyurethane coatings on polyester webbing slings slings 5 The cover was tested on a laboratory scale against a range of covers currently commercially available and proved to be the best performing cover. Tests included sawing resistance against a steel wire cable, abrasion resistance and cutting resistance slings 4 In practice about half of the coils are packaged, the remainder is transported in non-packaged form, which means that the slings are in direct contact with the sharp edges of coils during lifting operations slings 3 The roundsling made from Dyneema fibres could be handled by one worker during stevedoring, and showed hardly any visible damage after 521 lifting jobs (of which about 50% were on unprotected steel coils) slings 2 Port operators in the port of Antwerp, Belgium, requested a new type of synthetic sling to be used for repetitive lifting of steel coils, whilst maintaining low weight and ease of use slings 1