Short answer: by combining high-strength steel wire arranged in multiple strands around a core, wire rope slings spread and resist tensile forces, flex repeatedly without breaking, and—when used with correct hitch type, angle, fittings, and safety factors—keep working stresses well below the rope’s breaking strength.

How it works (key points)

• Construction and strength

  • A wire rope is many individual steel wires twisted into strands, with the strands twisted around a core. This geometry gives very high tensile strength and redundancy (one broken wire doesn’t mean immediate failure).
  • The rated breaking strength (nominal) of the rope is the maximum static load to rupture it; slings are used at a fraction of that (the Working Load Limit, WLL) determined by a safety factor.

• Safety factor and Working Load Limit (WLL)

  • Manufacturers/standards apply a design (safety) factor to get WLL = breaking strength / factor. Common factors:
    • General lifting: 5:1 minimum (often used).
    • Lifting people: 10:1 (or as required by regulation).
    • Specific equipment or regulations may require different factors—always use the manufacturer’s WLL or applicable standard.
  • Always use the WLL rather than raw breaking strength.

• How loads are carried

  • Tensile load in the rope: When a sling is loaded the rope fibers/strands carry tensile force along the length.
  • Load distribution: In a multi-leg sling the load is shared by legs; load in each leg depends on geometry and angles.
  • Sling angle effect: Leg tension increases as the angle from vertical increases. For two identical legs symmetrically arranged:
    • Tension in each leg T = W / (2 · cos θ)
      • W = vertical load
      • θ = angle between the leg and vertical
    • Example: at 30° from vertical (θ = 30°), cos30° = 0.866 → T ≈ W / (1.732) ≈ 0.577W per leg, so combined = W (but each leg sees ~58% of W). At 60° from vertical (cos60° = 0.5) T = W.
    • Small basket/choker angles (closer to horizontal) dramatically increase leg tension—avoid very shallow angles.

• Hitch types and their effect

  • Vertical (single-leg): WLL = WLL of the sling in straight tension.
  • Basket: sling under the load in a cradle; generally doubles the capacity compared to a single leg (but check tag/standards).
  • Choker: tightens around the load and reduces rated capacity (usually slight reduction relative to vertical or basket—refer to sling tag).
  • Multi-leg hitches: distribute load across legs; calculate leg tensions from angles to ensure none is overloaded.

• End fittings and attachments

  • Thimbles, eyes, sleeves, hooks, shackles, master links, and swaged/fitted ferrules transfer the load to the lifting hardware. Properly made eyes/thimbles prevent sharp bending and localized wear.
  • Attachments and hardware must have equal or greater WLL than the sling.

• Flexing, bending, and fatigue

  • Wire rope handles repeated bending around sheaves, hooks, edges, and eyes. However, repeated flexing and small bending radii cause fatigue — follow minimum D/d (sheave diameter to rope diameter) recommendations to reduce fatigue.
  • Avoid sharp edges or use protective sleeves to prevent cutting and abrasion.

• Corrosion and lubrication

  • Corrosion reduces cross-section and strength. Proper lubrication and inspection reduce corrosion and internal wear.

Common failure modes to guard against

• Overload (exceeding WLL, including due to improper angle)
• Abrasion and cutting from sharp edges
• Localized crushing or kinking (permanently weakens rope)
• Corrosion and internal wire breakage
• Fatigue from repeated flexing over too-small sheaves
• Improperly made terminations (poor swages, bad splices, damaged thimbles)
• Heat damage or welding splatter

Inspection and safe-use checklist (practical)

• Verify sling tag shows WLL and construction; confirm WLL suits the load and hitch/angle.
• Calculate leg tensions if using multi-leg or basket hitches and ensure no leg exceeds its WLL.
• Check for kinks, birdcaging, broken wires (visible broken wires limit use), corrosion, abraded areas, crushed sections, stretched/damaged end fittings, and missing lubrication.
• Protect slings from sharp edges (edge protectors, sleeves).
• Use proper D/d ratios and sheave sizes to reduce fatigue.
• Don’t shock-load; lift smoothly.
• Follow manufacturer instructions and applicable standards (OSHA, ASME B30.9, or local regulations).

Summary A wire rope sling carries loads safely by using a redundant, high-strength steel construction sized by a conservative safety factor (WLL), by transferring load through proper hitches and fittings, and by avoiding damaging conditions (overload, sharp edges, small sheaves, corrosion, kinks). Correct calculation of leg tensions (accounting for sling angle), regular inspection, and using protective hardware are the practical steps that keep the sling operating safely.