Polyethylene fiber slings raise performance and lower costs

Print Page Email

Faced with challenges such as a shrinking labor force to do this physically demanding work, concerns about protecting crews and valuable or vulnerable items, requests to lift larger and more complex loads, and the perennial need to control costs, lifting companies are upgrading from traditional lifting slings to those made with new synthetic materials. At first glance, replacing steel wire rope and chain, and even conventional synthetics such as nylon and polyester, with high-performance polyethylene (HPPE) does not appear to be a radical change. But in reality, this ultra-strong yet soft and lightweight material can have a major, positive impact across the board – improving safety, reducing potential for damage, speeding up the lifting process, requiring fewer workers, consolidating sling inventories, and increasing the flexibility of operations.

Traditional Sling Materials Add to Lifting Burden

Steel Wire and Chain Slings

Although wire and chain are still widely used for heavy lifting slings they have many drawbacks. First, steel wire is very heavy, chain even heavier. In fact, large steel slings may require a separate, small crane just to hoist them into position. And because the slings add so much weight to the total load, companies typically must stock a large array of slings so they can select the sling with the precise work load required and no more.

Further, steel poses a number of safety and liability issues. Carrying, attaching and manipulating these heavy slings can cause back injuries. “Fishhooks” of cut strands can cut workers’ hands. Because steel wire has a rough, unyielding surface, this material can easily damage delicate objects or scratch painted and coated surfaces, leading to claims against the crane operator. Similarly, chains can damage items being hoisted.

Other issues with steel slings include corrosion, permanent kinking (dog-leg) from being bent over sharp edges in the load, and handling problems from twists induced in steel wire.

Polyester and Polyamide Slings

To mitigate steel’s disadvantages, some suppliers offer polyester and polyamide (Nylon) round slings and web slings. These slings are significantly lighter than steel wire and chain; however, to provide sufficient strength for hoisting very heavy loads, they must be made very thick and bulky. In fact, these slings may become so bulky they cannot be used with standard shackles and eyes.

Although they are soft and smooth, unlike steel, the covers of polyamide and polyester slings are also surprisingly delicate. Easily torn or abraded, the slings typically must be replaced frequently – sometimes after only a few uses.

While polyester or polyamide add less weight to the load than steel, they introduce a new challenge: elongation. The core material in these slings stretches significantly under load, causing difficulty in achieving exact placement of the load. Elongation can also cause the sling to slip along the load’s surface when the lift commences, which can abrade the sling cover and possibly damage the item being hoisted.

High-Performance Polyethylene Slings Address Diverse Lifting Challenges

The migration from steel and conventional synthetics to high-performance polyethylene in lifting slings is gaining momentum. Dyneema®, manufactured by DSM Dyneema, is a leading HPPE widely used in fiber for rope and netting – and now, heavy lifting slings.

HPPE offers many advantages over steel wire and conventional synthetics for heavy lifting operations.

Exceptional Strength with Minimal Weight

HPPE solves the problem of delivering enough strength to hoist the heaviest loads without increasing the weight and mass of the sling to an unacceptable level. In fact, a sling made of HPPE fiber weighs only one-seventh as much as a wire based sling of similar diameter! In comparison with polyester, an HPPE sling is about 75 percent of the diameter and less than half the weight.

The benefits to crane operators include the ability to lift heavier net loads, fewer crew members needed to handle the sling, and sometimes even the flexibility to transport a sling in a car instead of requiring a truck.

Low Elongation

Minimizing elongation of the sling is important for precise lifting, maneuvering and placement of the load – and for safety. While steel chain and wire slings do not stretch, they have many other drawbacks. The ideal sling combines low elongation with light weight and high strength. Dyneema® HPPE fiber provides a very low elongation, plus weight and strength advantages as described above.

Durability

The work life of a sling can be affected by many environmental and usage factors: abrasion, bending fatigue,tension fatigue and degradation from exposure to sunlight, chemicals or sea water. Slings made with HPPE exhibit excellent resistance to these wear factors. The material is not affected by salt, and is resistant to most chemicals and UV light. It has superior tension fatigue performance, surpassing polyester, nylon and steel wire rope by several degrees of magnitude.

Safety and Protection

The lighter weight and smooth surface of HPPE slings help protect crews from hand or back injury. These slings are easier to handle than steel wire slings, and do not pose a risk of cuts from frayed wires. They also avoid surface scratches, abrasion and damaging of the load that can occur with steel chain or wire slings.

A Compelling Case for System Cost Reduction

With all these benefits, one might wonder why the heavy lifting industry has not made a wholesale conversion to polyethylene slings. For insight, it is interesting to look at the ship mooring industry, which is much farther along in the adoption of this engineered material. About 15 years ago, only steel wire and low-performance synthetics were used for mooring lines. Initial acceptance of lines made with HPPE fiber was slow, as the initial cost of these ropes is higher than steel wire or polyester lines. However, the industry has seen how lifetime costs of a mooring system can be significantly reduced through the use of HPPE – primarily from the ability to complete jobs faster and use the lines longer. As an example, today, mooring lines made with Dyneema® fiber are the standard for new LNG (Liquefied Natural Gas) tankers.

Like mooring ropes, low-weight HPPE slings can dramatically reduce the overall cost of heavy lifting operations, more than offsetting their initial purchase price. The following are key areas in which crane operators can achieve cost savings.

• Faster Operations: Because HPPE slings are easier to attach and maneuver, and are less prone to stretching and slipping than conventional slings, lifting operations can be completed faster and more precisely. The soft, flexible slings form better and tightly around sharp curves and unusual shapes, unlike stiff steel wire. It is less bulky than other synthetics, allowing the use of small attach points.

Recently 90 stainless steel coils were unloaded at Dundalk marine terminal (port of Baltimore USA) using HPPE slings, manufactured by Slingmax Inc, resulting in 1.5 hours time saving versus braided chain slings.

• Fewer Crew Members and Less Equipment: With the increasing difficulty of finding workers for physical labor such as lifting operations, streamlining crew numbers becomes more important. Replacing steel wire or chain slings with HPPE slings can reduce the number of workers needed for rigging. For example, a steel sling that requires three people to handle it might require only two with a sling from Dyneema® fiber.

In addition, these lightweight, compact slings can be rigged by hand instead of having to bring in an additional crane to hoist the slings, and can be transported in the back of a car instead of requiring a truck.

• Lower Risk of Injury and Damage: Another area of cost reduction is insurance payments. With a lower likelihood of worker injury and damage to the load, operators can reduce liability and worker’s compensation insurance rates.

For the installation of offshore wind farms in the Irish Sea, Mammoet Van Oord, an international marine contractor, selected heavy lifting slings made with super-strong Dyneema® polyethylene fiber. The special round slings, manufactured by the Unitex Group, are designed to lift heavy loads weighing up to several hundreds of tons.

Lighter weight makes handling the slings easier, especially in rough seas, and helps prevent back injuries that are caused most frequently by maneuvering the heavier steel-wire slings. Further, wire carries a high risk of worker injury because loose threads can act like fishhooks and inflict serious cuts to the hand or arms – a situation that is avoided with slings made from smooth Dyneema® fiber.

• Smaller Sling Inventory: Slings made with HPPE fiber help cut inventory in two ways. Steel wire and chain slings add so much weight to the load that riggers need to use a sling with the exact work load for the particular job – resulting in a large inventory, for example comprising 5-ton, 10-ton, 20-ton, 30-ton and 40-ton slings. In contrast, a 40 ton HPPE sling could easily be used to hoist loads of 15 tons, while still retaining weight and handling advantages. So a company might only need to stock a few slings such as a 10-ton and a 40-ton sling to cover all their lifting needs.

Machinefabriek Amersfoort, a worldwide specialist in machining, heavy milling and turning of large metal parts, has begun using lifting slings made with ultra-strong Dyneema® fiber. The exceptional strength-to-weight ratio of Dyneema® HPPE has enabled the company to replace multiple polyester slings with fewer, lighter weight slings, simplifying part handling and reducing storage.

Future Trends: replacing running rigging

Another potential application for HPPE fiber in heavy lifting is replacing steel wire running rigging for cranes to increase the net load of a crane. Crane hoisting lines could potentially be replaced by (non-rotating) braided rope made from Dyneema®, saving more than 80 percent of the weight of the line.

?This would pay off quickly with tall tower cranes constructing the high-rise buildings in Asia and the Middle East, as the weight of the steel wire hoisting line starts decreasing the net pay load of the crane when these reach a few hundred meters height. Replacing steel wire with rope made from Dyneema® could more than double the payload on the higher cranes.

Dyneema slings at work offshore Dyneema slings at work offshore Dyneema slings dockside Dyneema slings dockside