8.1 Sling Types: Synthetic, Wire Rope, and Alloy Steel Chain Slings

Quick Answer: Lifting slings serve as the critical mechanical connection between the crane hook and the load. Under ASME B30.9 and OSHA 29 CFR 1926.251 / 1910.184, riggers select between synthetic web/roundslings, wire rope slings, and alloy steel chain slings based on weight, environmental exposure, and edge geometry. Synthetic slings offer flexibility but must not be used above 180°F under OSHA 1910.184 (194°F/90°C under ASME B30.9) and require edge protection against cuts; nylon resists alkalis but fails in acid, while polyester resists acid but fails in alkalis. Wire rope sling eyes may not be formed with wire rope clips or knots (OSHA 1926.251(c)(4)(iii)) and suffer severe capacity loss if bent around objects with a D/d ratio below 25. Chain slings used for overhead lifting must be alloy steel such as Grade 80 or Grade 100; carbon steel chain is not for overhead lifting. Any sling missing an authentic manufacturer capacity tag must be removed from service immediately.

In crane hoisting operations, the crane's hoisting drum, wire rope, and boom structure only deliver lifting force to the hook bowl. The safe transfer of that force into the load depends entirely on the rigging assembly. 230 CMR 6.01(3) adopts ASME B30.9-2018 for slings, and OSHA 1926.251 governs rigging on construction sites, so hoisting operators and riggers must understand the mechanical properties, material vulnerabilities, and inspection rejection criteria of every sling type deployed on the jobsite.

1. Synthetic Web Slings and Roundslings: Material Science & Environmental Vulnerabilities

Synthetic slings are widely utilized across construction and industrial facilities because of their light weight, flexibility, non-marring surface contact, and ease of handling. However, synthetic polymers possess distinct chemical, thermal, and mechanical limitations that govern their safe deployment.

Material Science: Nylon vs. Polyester

Synthetic web slings and endless roundslings are manufactured primarily from two synthetic polymers with contrasting chemical and physical behaviors:

1. Nylon (Polyamide):
   • Elongation: Nylon is an elastic polymer that stretches approximately 6% to 10% of its length when loaded to its rated Working Load Limit (WLL), with ultimate elongation reaching 15% before tensile rupture. This elasticity makes nylon ideal for absorbing dynamic shock loads encountered during crane starts and stops.
   • Chemical Resistance: Nylon is highly resistant to hydrocarbons, grease, oils, and dilute alkaline (basic) environments (such as ammonium hydroxide and caustic cleaners). However, nylon is severely degraded and destroyed by acids. Contact with dilute inorganic mineral acids (hydrochloric acid, sulfuric acid, nitric acid) or battery fluids causes rapid chemical depolymerization, dissolving the load-bearing yarn structure.
2. Polyester (Polyethylene Terephthalate / Dacron):
   • Elongation: Polyester is a low-stretch polymer that exhibits only approximately 3% elongation at rated WLL. This minimal stretch provides superior vertical load positioning control and is mandatory when lifting loads with minimal overhead clearance or where height differentials must be tightly restricted.
   • Chemical Resistance: Polyester exhibits exceptional resistance to common acids and bleaching agents. Conversely, polyester is severely attacked and degraded by strong alkalis and caustic compounds (such as sodium hydroxide, potassium hydroxide, and concentrated lime solutions).

Core Yarn Construction & Internal Inspection Aids

• Synthetic Flat Web Slings: Heavy-duty web slings are manufactured from continuous-filament synthetic yarns woven into heavy multi-ply straps. High-quality web slings incorporate red core warning yarns woven directly into the interior load-bearing core. When abrasive surface wear, severe chafing or cutting penetrates the outer yarns, these bright red yarns become visible. Manufacturers treat exposed warning yarns as a removal signal, alongside the general removal criteria for cuts, abrasion, melting and other damage.
• Synthetic Roundslings: Roundslings consist of a continuous, endless loop of unspun polyester yarn fibers protected by a non-load-bearing, woven tubular synthetic jacket. The jacket functions solely to protect the internal core yarns from dirt, ultraviolet exposure, and abrasion. If the outer jacket suffers tears, cuts, punctures, or heat fusion that exposes the internal core yarns, or if knots or hard spots indicate broken load-bearing fibers, the roundsling must be removed from service.

Thermal Thresholds & Photodegradation

• Thermal Limits: ASME B30.9 limits nylon and polyester web slings and roundslings to 194°F (90°C), and OSHA 1910.184(i)(7) limits nylon and polyester web slings to 180°F; follow the lower figure. Exposure to temperatures above this limit, including proximity to engine exhaust manifolds, steam lines, or direct welding spatter, causes polymer softening, tensile degradation, and thermal melting.
• Ultraviolet (UV) Photodegradation: Both nylon and polyester suffer accelerated degradation from prolonged exposure to solar ultraviolet radiation. UV rays break the covalent molecular bonds of synthetic polymers, leading to surface fading, fiber brittleness, and loss of tensile strength. Slings left in outdoor sunlight can lose significant strength over time without obvious fraying. Synthetic slings must be stored in dark, cool, dry enclosures when not in active service.

Edge Softening & Cut Protection

Synthetic slings have virtually zero cut resistance when tensioned across unrounded edges. ASME B30.9 requires synthetic slings to be protected from sharp edges and abrasive surfaces. Wherever a sling bears on an edge that could cut it, use engineered cut protection rated for that use. Corner protectors, heavy polyurethane wear pads, engineered synthetic sleeves, or split sections of heavy-duty rubber hose must be installed to eliminate direct point contact between the tensioned web and the sharp steel or concrete interface.

2. Wire Rope Slings: Metallurgy, Eye Splices & D/d Bend Ratios

Wire rope slings provide rugged durability, abrasion resistance, and high structural strength across harsh industrial environments. They consist of multiple steel wires twisted into strands, which are helically closed around a central steel or fiber core.

Steel Grades and Core Mechanics

• Extra Improved Plow Steel (EIPS & EEIPS): Modern lifting slings are manufactured from high-carbon steel drawn into wire filaments classified as Extra Improved Plow Steel (EIPS, nominal tensile strength approximately 1,960 N/mm²) or Extra Extra Improved Plow Steel (EEIPS, nominal tensile strength approximately 2,160 N/mm²). EEIPS delivers approximately 10% higher breaking strength than EIPS for an identical diameter.
• Independent Wire Rope Core (IWRC): For hoisting slings, an IWRC is the standard choice in heavy rigging applications. An IWRC consists of a complete miniature wire rope positioned at the center of the outer strands. It provides superior transverse resistance to crushing under heavy contact loads, prevents lateral core collapse, and maintains rope diameter. Fiber cores (FC), made of natural sisal or synthetic fibers, are unsuitable for heavy crane lifting and high-temperature work because they crush easily and dry out; OSHA 1910.184 removes fiber-core wire rope slings from service after exposure above 200°F.

Eye Splice Integrity: Flemish Eye vs. Prohibited Clips

• Flemish Eye Mechanical Splice: The gold standard for wire rope slings is the mechanical Flemish eye splice. In this fabrication method, the wire rope strands are split into two groups, looped back around in opposite directions to form the eye, and married back together in a mechanical interlock. A carbon steel sleeve is then placed over the tail ends and hydraulically swaged under extreme pressure to achieve cold-metal flow around the wires. This configuration typically achieves about 90% to 95% of the wire rope's nominal catalog breaking strength. A cracked or damaged sleeve is still a reason to remove the sling from service.
• Hand-Tucked Eye Splices: In hand-tucked splices, strand tails are tucked directly beneath adjacent strands without a pressed sleeve. While acceptable in specialized marine applications, hand-tucked splices achieve only 70% to 90% efficiency and have a dangerous propensity to untuck and un-lay if the sling is subjected to axial rotation during a lift.
• No Clip or Knot Eyes: Under OSHA 29 CFR 1926.251(c)(4)(iii), eyes in wire rope bridles, slings or bull wires shall not be formed by wire rope clips or knots. Clips deform the rope strands, reduce efficiency and can slip under shock loading. OSHA 1926.251(c)(4)(iv) also removes wire rope from service if, in any length of eight diameters, visible broken wires exceed 10% of the total wires.

The D/d Ratio (Bend Radius Efficiency)

When a wire rope sling is bent around a curved surface—such as a shackle pin, a crane hook bowl, or a cylindrical load trunnion—the outer wires of the rope are forced into extreme tensile elongation while the inner wires compress against the core. This localized bending stress significantly reduces the rope's ultimate tensile capacity.

The severity of this stress is defined by the D/d ratio, where:

• $D$ is the diameter of the curved contact surface (e.g., shackle pin diameter, hook saddle diameter, or pipe diameter).
• $d$ is the nominal diameter of the wire rope sling.

Sling manufacturers base the published basket ratings of single-part, mechanically spliced wire rope slings on a D/d ratio of 25:1 ($D/d \ge 25$). As the bend diameter decreases relative to rope diameter, strength drops rapidly:

These are the industry efficiency figures published by sling manufacturers for single-part, mechanically spliced wire rope slings. Other constructions, such as hand-spliced or braided slings, use different reference D/d ratios, so check the manufacturer's chart.

3. Alloy Steel Chain Slings: Grade Classifications, Elongation & Thermal Derating

Alloy steel chain slings are rugged, flexible, and exceptionally resistant to abrasive construction environments, sharp edges, and elevated operating temperatures. However, hoisting regulations strictly govern the specific metallurgical grades permitted for overhead crane lifting.

Alloy vs. Carbon Steel Chain

OSHA 1910.184 and 1926.251 cover alloy steel chain slings, which must carry permanent identification stating size, grade, rated capacity and reach (1926.251(b)(1) also requires the sling manufacturer). Common alloy grades for overhead lifting are Grade 80 and Grade 100 (Grade 120 is also made):

• Grade 80 Alloy Steel Chain: Manufactured from heat-treated, quenched, and tempered alloy steel containing nickel, chromium, and molybdenum. It has a nominal tensile strength of 800 N/mm², and the links are embossed at regular intervals with the manufacturer's grade marking (e.g., "8", "80" or "G800").
• Grade 100 Alloy Steel Chain: A premium alloy steel formulation delivering approximately 1,000 N/mm² nominal tensile strength. Grade 100 provides approximately 25% higher rated WLL than Grade 80 for the exact same link diameter, reducing overall rigging weight for heavy picks.

Safety Factor & Ductile Elongation Warning

Alloy chain slings are rated with a design factor of 4 (ASME B30.9), compared with 5 for wire rope and synthetic slings. Alloy chain is also ductile: links stretch, bind and lose articulation before rupture, which gives a visual warning of overload. OSHA 1926.251(b)(6) requires a thorough periodic inspection of alloy chain slings at least every 12 months, with a record of the most recent month each sling was inspected.

High-Temperature Limits

Unlike synthetic slings, alloy steel chain slings can work in hot environments such as foundries and steel mills, within limits:

• OSHA 29 CFR 1910.184(e)(6): an alloy steel chain sling heated above 1,000°F must be permanently removed from service, and at service temperatures above 600°F the working load limit must be reduced per the chain or sling manufacturer's recommendations.
• Many chain manufacturers' tables begin reducing the working load limit at lower temperatures, around 400°F, and list permanent strength losses after high-temperature exposure. Use the manufacturer's table for the grade in use.
• For comparison, OSHA limits nylon and polyester web slings to 180°F (1910.184(i)(7)), and ASME B30.9 uses 194°F (90°C). Fiber-core wire rope slings are removed from service after exposure above 200°F, and IWRC wire rope slings used above 400°F or below −60°F follow the sling manufacturer's recommendations (1910.184(f)(3)).

4. Hitch Configurations, Choke Angles & Mechanical Capacity Reductions

The manner in which a sling is connected to a load dictates the actual force transmitted into the sling material. The three fundamental rigging hitches are vertical, choker, and basket hitches.

1. Vertical Hitch ($1.0 \times \text{WLL}$)

A vertical hitch connects the crane hook directly to a single lifting attachment point on the load. The entire load weight is carried along the longitudinal axis of the sling. A vertical hitch provides 100% of the sling's base rated capacity ($1.0 \times \text{WLL}$).

2. Choker Hitch ($0.75\text{--}0.80 \times \text{WLL}$ and Choke Angle Derating)

A choker hitch wraps the sling body completely around the load, passing one eye through the opposite eye or through a sliding choker hook. The standard rated capacity of a choker hitch is 75% to 80% of its vertical rating when the choke angle is $120^\circ$ or greater. The capacity reduction occurs because the sling body is bent sharply around its own fitting while simultaneously experiencing radial compressive pinch forces.

When choking narrow, square, or rectangular loads (such as structural wide-flange I-beams or small-diameter pipe bundles), the angle of choke formed at the sliding eye decreases below $120^\circ$. As this choke angle tightens, the localized shearing forces intensify dramatically, requiring severe capacity deratings under ASME B30.9:

3. Basket Hitch ($2.0 \times \text{WLL}$ Conditional)

A basket hitch wraps the sling body beneath the load, connecting both eyes back up to the crane hook. A basket hitch delivers 200% of the base vertical rated capacity ($2.0 \times \text{WLL}$) under one strict physical condition: both sling legs must hang completely vertical ($90^\circ$ to the horizontal). If the legs angle inward toward the hook bowl, the hitch is transformed into an inclined bridle basket, and its capacity must be multiplied by the horizontal sling angle factor ($\sin\theta$).

5. Mandatory Sling Identification Tags & Removal Criteria

OSHA 1926.251(a)(2) requires rigging equipment to have permanently affixed, legible identification markings, as prescribed by the manufacturer, showing the recommended safe working load, and it must not be used without them. ASME B30.9 specifies the tag contents.

Identification Tag Contents

ASME B30.9 identification requirements vary by sling type, but a sling tag generally shows:

1. Manufacturer's name or trademark.
2. Rated load for at least one hitch type and the angle on which it is based (many tags list vertical, choker and basket ratings).
3. Size: rope diameter, chain size or web width.
4. Number of legs, if more than one.
5. Type-specific items: for example chain grade and reach, the synthetic material type, or the manufacturer's code, stock number or individual sling identification.

OSHA adds its own minimums, such as size, grade, rated capacity and sling manufacturer for welded alloy steel chain slings (1926.251(b)(1)).

Absolute Removal Rule: Missing or Illegible Identification Tags

If a sling's identification tag is missing, detached, defaced, worn smooth, or illegible, the sling must be removed from service immediately. Never "field stamp" a sling or attach a makeshift tag based on guesswork. Under OSHA 1926.251(a)(2)(iii), rigging without affixed, legible identification markings must not be used, so the sling is removed from service until the manufacturer re-tags it or it is destroyed.