Lifting Slings Technical Specifications
Types of Lifting Slings
Webbing Slings
Webbing slings are manufactured from synthetic fibers, primarily polyester or nylon, offering high strength-to-weight ratios and flexibility. Polyester slings exhibit low stretch, making them suitable for lifts where minimal elongation is desired, and are resistant to most acids but susceptible to strong alkalis. Nylon slings offer higher stretch, making them good for shock loading, and are resistant to alkalis but damaged by acids. These slings are lightweight, non-marring to delicate loads, and available in various ply configurations (single, double, triple, four-ply) and widths. Their Working Load Limit WLL is clearly marked, and color coding often indicates capacity in certain regional standards. Webbing slings require protection from sharp edges, abrasion, and extreme temperatures to prevent damage and maintain their WLL.
Chain Slings
Chain slings are constructed from heat-treated alloy steel chain, typically Grade 80 or Grade 100, providing superior strength, durability, and resistance to high temperatures and abrasive environments. They are highly adaptable, with components like master links, connecting links, and hooks allowing for adjustable leg lengths and configurations (single, double, triple, quadruple legs). Grade 100 chain offers approximately 25% higher WLL than Grade 80 for the same nominal size. Chain slings are robust and repairable, making them suitable for demanding industrial applications where heavy, abrasive, or hot loads are encountered. However, they are heavier than synthetic slings and can potentially damage sensitive surfaces if not properly protected.
Wire Rope Slings
Wire rope slings are fabricated from multiple strands of high-carbon steel wire helically wound around a central core. Common constructions include 6x19 and 6x37, referring to the number of strands and wires per strand, respectively, with 6x37 offering greater flexibility. These slings provide excellent strength, good abrasion resistance, and moderate temperature resistance. They are available in various configurations such as single leg, eye-and-eye, endless, and multi-leg assemblies, often terminated with thimbles, Flemish eyes, or swaged sleeves. While strong and durable, wire rope slings can develop kinks or broken wires, which necessitate removal from service. Their rigidity compared to synthetic slings can be a factor in certain lifting scenarios.
Round Slings
Round slings consist of an endless loop of synthetic fiber yarns (typically polyester) encased in a woven protective jacket. The load-bearing core fibers are not exposed, offering enhanced protection against cuts, abrasion, and UV degradation. This construction eliminates the need for splices, providing a continuous loop with uniform strength throughout. Round slings are highly flexible, lightweight, and conform well to the shape of the load, reducing the risk of damage. They are often color-coded to indicate WLL, similar to webbing slings. Inspection for internal damage can be challenging due to the protective cover, emphasizing the importance of visual inspection for external wear, cuts, or heat damage to the jacket, which can indicate potential core fiber compromise.
Key Technical Considerations
Working Load Limit WLL
The Working Load Limit WLL is the maximum mass or force which a lifting accessory is authorized to sustain in general service. This value is determined by applying a safety factor to the sling's Minimum Breaking Load MBL. For multi-leg slings, the WLL is calculated based on the maximum permissible angle between the sling legs. It is critical never to exceed the WLL, as this can lead to catastrophic failure. The WLL is also affected by the type of hitch used (vertical, choker, basket), with choker hitches typically reducing the sling's effective WLL.
Sling Angle
The angle at which sling legs are deployed significantly impacts the tension exerted on each leg and thus the effective WLL. As the angle between the sling legs increases (i.e., the legs become more horizontal), the tension on each leg increases for a given load. A common guideline is to maintain sling angles between 30 and 60 degrees from the horizontal plane to prevent excessive tension and potential overloading of the sling components. Angles less than 30 degrees should be avoided unless the sling is specifically rated for such low angles.
Inspection and Maintenance
Regular inspection is paramount for the safe operation of all lifting slings. This includes pre-use visual checks by the operator and periodic thorough inspections by a competent person at intervals determined by use frequency and severity, typically every one to twelve months. Criteria for removal from service include, but are not limited to, cuts, nicks, snags, melt marks, or chemical burns on synthetic slings; cracked, bent, or deformed links/hooks on chain slings; and broken wires, severe abrasion, or kinking on wire rope slings. Proper storage away from environmental extremes and corrosive substances also extends sling lifespan and maintains safety.