Lifting sling and wire rope load

Introduction

A sling leg carries more than its share of the load as soon as it leans: at 30° from the vertical each of two legs carries 58 % of the load, at 60° the full load. Most sling failures trace back to that angle factor, to assuming four legs share equally, or to a choke hitch used at its straight-pull rating. This calculator computes the tension in each leg from the load, the number of legs assumed to carry it, the angle (entered directly or from the spread and leg length), a dynamic factor and the hitch, then compares it with the working load limit of a Grade 8 chain sling, a 6×36 wire rope sling or any tag-rated sling, and gives the maximum load the arrangement may lift.

It is for riggers, deck and engine officers planning lifts with ship's cranes and chain blocks, superintendents checking a lift plan, and students learning the sling-angle rule.

Formulas

Leg angle from geometry (two legs, or the diagonal of four):

sin β = (spread / 2) / leg length

Loads:

Hook load   F = W g × DAF
Leg tension T = F / (n_eff cos β)
Compression on the load per leg = T sin β
Angle factor = 1 / cos β

Working load limits (single leg, straight pull):

Grade 8 chain:  WLL ≈ 0.0314 d² tonnes (d in mm)                           EN 818-4
Wire rope 6×36 IWRC 1770 MPa: MBL = 0.356 d² × 1770 / 1000 kN;  WLL = MBL × 0.9 / 5   EN 12385-4, EN 13414-1
Rated sling: tag value

Capacity and utilisation:

Leg capacity = WLL × hitch factor   (straight 1.0, choke 0.8, basket with vertical legs 2.0)
Utilisation  = T / (leg capacity × g)
Maximum load W_max = leg capacity × g × n_eff cos β / (g × DAF)

• W — load mass; DAF — dynamic amplification factor; n_eff — legs assumed to carry the load (all for 1–2 legs, 3 for 3–4 legs by default); β — angle of each leg to the vertical

Assumptions and limitations

• Equal sharing between the effective legs and equal angles. Four-leg slings are rated on three legs unless the legs are adjustable or the load is rigid with exact geometry.
• The centre of gravity is under the hook. An offset CoG overloads the near legs; use a moment balance for the actual leg loads.
• β ≤ 60° by EN 13414-1 and ASME B30.9, 45° or less recommended; the calculator warns above both limits.
• WLL formulas are for Grade 8 chain and 6×36 IWRC 1770 MPa rope with ferrule-secured eyes; other grades, constructions and terminations differ. The sling tag governs.
• Hitch factors are the standard ones; bending over small diameters (D/d below about 20 for wire rope), sharp edges and knots reduce capacity further and are not covered.
• Shackles, hooks, master links, lifting lugs and the crane hook block need their own checks against the leg tension or total load.
• The dynamic factor is an input; the calculator does not derive it from crane speed or sea state.

Worked example

A 10 t engine part is lifted with a two-leg Grade 8 chain sling of 16 mm chain, leg length 4 m, lift points 4 m apart, straight hitch, in the engine room with the overhead crane (DAF 1.1).

1. Angle: sin β = 2 / 4 → β = 30°, included angle 60°.
2. Hook load: 10 000 × 9.81 × 1.1 = 107.9 kN.
3. Leg tension: 107.9 / (2 × cos 30°) = 62.3 kN = 6.35 t per leg (angle factor 1.155). Each leg also pushes 31.1 kN horizontally into the part.
4. Chain WLL: 0.0314 × 16² = 8.04 t per leg (EN 818-4 table 8.0 t); straight hitch factor 1.0.
5. Utilisation: 6.35 / 8.04 = 79 % — acceptable. Maximum load for this arrangement: 8.04 × 2 × cos 30° / 1.1 = 12.7 t.

Shortening the legs to 2.5 m would make β = 53°, tension 8.9 t per leg and the lift not permitted with this sling; a choke hitch at 30° would reduce the capacity to 6.4 t per leg and also fail.

FAQ

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