8.3 Hitch Mode Factors: Choked, Basket & Wrap Hitches

The lifting capacity of any sling is not determined solely by its material strength; it depends heavily on how the sling is attached to the load. The method of attachment is called the hitch configuration. To account for the mechanical stresses, localized bending radii, and friction forces introduced by different hitches, standards apply a dimensionless multiplier called the Hitch Mode Factor ($M$) (sometimes denoted as $F$).

The effective Working Load Limit of a sling assembly in a specific hitch is calculated as:

$WLL_{effective} = WLL_{inline} \times M$

Where $WLL_{inline}$ is the rated capacity of the sling in a direct, straight-line pull.

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1. Straight Inline Pull ($M = 1.0$)

The baseline reference configuration is the straight inline hitch ($M = 1.0$). In this setup, one end of the sling attaches to the crane hook and the other attaches directly to a load bearing point (such as a rated eye bolt or shackle) in a straight vertical line. The sling experiences pure tensile stress without localized bending or choke friction.

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2. Choked Hitch Mechanics ($M = 0.8$)

In a choked hitch, the sling passes around or through the load and reeves through its own end fitting (or chokes around itself) before connecting to the crane hook.

Physical Mechanisms of Strength Reduction

A choked hitch reduces sling capacity by 20% ($M = 0.8$) compared to straight inline rating due to two distinct mechanical factors:

1. Severe Localized Bending Radius: At the choke point, the sling body undergoes sharp curvature around the fitting or reeved eye, introducing non-uniform outer-fiber bending stresses.
2. Choke Frictional Vector Forces: As tension is applied, the choke point squeezes inward against the load, creating a severe force concentration at the contact nip point.

The Standard Angle of Choke Rule

The standard mode factor of $0.8$ applies ONLY if the angle of choke is $120^\circ$ or greater. The angle of choke ($\theta_{choke}$) is the internal angle formed by the sling body as it emerges from the reeved eye.

If the load geometry or rigging practice forces the choke angle below $120^\circ$, additional reduction factors must be applied according to LEEA recommendations:

BEST PRACTICE: Never force a choke tight by hammering the reeved eye down the sling body. Forcing the choke creates an artificially acute angle of choke ($<90^\circ$), crushing the sling fibers and drastically reducing capacity. Allow the choke to seat naturally at its natural equilibrium point ($\ge 120^\circ$).

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3. Basket Hitch Mechanics ($M = 2.0$ Vertical)

In a basket hitch, the sling cradles the load by passing under it, with both ends (eyes) running upward to connect to the crane hook or master link.

Parallel Vertical Legs ($M = 2.0$)

If both legs of the basket hitch are strictly vertical ($0^\circ$ to the vertical plumb line), the load weight is split equally between two vertical legs. Consequently, a parallel vertical basket hitch doubles the sling's straight-line capacity:

$WLL_{basket\_vertical} = WLL_{inline} \times 2.0$

Bending Radius Limits ($D/d$ Ratio)

The basket factor of $2.0$ is contingent upon maintaining an adequate curvature ratio ($D/d$ ratio), where $D$ is the diameter of the load around which the sling is wrapped, and $d$ is the diameter of the sling body:

• Wire Rope Slings: Maintain $D/d \ge 2$ (the load or fitting diameter at least twice the rope diameter—see Section 5.3); tighter bends cause permanent kinking and core crushing. Drum and sheave diameters are a separate, much larger ratio (typically $16\times$ to $25\times$ rope diameter).
• Chain Slings: Chain links must fit smoothly over the curve without edge loading individual links.
• Web Slings: Width of web sling must not be bunched or creased over small pin diameters.

Basket Hitches at an Angle

If the legs of a basket hitch spread outward to form an angle with the vertical ($\theta$), the effective capacity decreases due to vector geometry. The combined mode factor combines the basket multiplier ($2.0$) with the angular factor:

Under Uniform Load Method (ULM) rules:

• Basket Hitch at 0° to 45° to vertical: $M_{combined} = 2.0 \times (1.4 / 2) = 1.4$
• Basket Hitch at 45° to 60° to vertical: $M_{combined} = 2.0 \times (1.0 / 2) = 1.0$

Under Trigonometric Method rules:

$M_{combined} = 2.0 \times \cos\theta$

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4. Double Wrap Hitches for Loose Bundles

When lifting loose items—such as bundles of steel rebar, scaffolding tubes, timber posts, or polished stainless pipes—standard hitches are hazardous. A single choked hitch or basket hitch contacts only a portion of the bundle perimeter, allowing smooth internal pipes to slide out or telescope when the bundle flexes.

To secure loose materials safely, riggers must execute Double Wrap Hitches:

Double Wrap Choked Hitch ($M = 0.8$)

The sling is passed around the load a full $360^\circ$ (making one complete wrap) before reeving through the eye to form the choke.

• Mechanism: Provides full $360^\circ$ radial grip around the entire bundle perimeter. As tension increases, the full wrap exerts uniform radial clamping pressure, preventing inner members from sliding out.
• Mode Factor: $M = 0.8$ (applied to $WLL_{inline}$), but provides 100% security against telescoping.

Double Wrap Basket Hitch ($M = 2.0$ Vertical)

The sling makes one full $360^\circ$ wrap around the load object, with both ends extending vertically to the hook.

• Mechanism: Combines the double capacity of a basket hitch ($M = 2.0$) with positive 360-degree frictional clamping.

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5. Using Fewer Legs Than Marked & Endless Sling Ratings

Multi-Leg Slings with Reduced Legs in Use

If a multi-leg sling is used with fewer than its marked number of legs attached to the load, the marked assembly WLL no longer applies—the rating must be reduced. The simple safe rule is to rate the assembly by the number of legs actually in use: a 4-leg sling lifting on only 2 of its legs is treated as a 2-leg sling (ULM factor $1.4$ for $0^\circ$–$45^\circ$ to vertical), and a 3-leg sling used on 2 legs is likewise treated as a 2-leg sling. Unused legs must be hooked back or secured so they cannot snag on the load or structure.

Endless Slings

The slinging factor for an endless chain or wire rope sling assumes it is used in choke hitch, whereas the standard rating for an endless textile sling assumes a straight pull. In all cases it is assumed that, at the points of attachment to both the lifting appliance and the load, the radius around which the sling passes is large enough to avoid damage to the sling.

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Comprehensive Hitch Mode Factor Summary Table

The table below summarizes standard Hitch Mode Factors ($M$) across common sling materials operating in clean, un-damaged conditions:

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Step-by-Step Worked Numerical Examples

Worked Example 1: Effective Capacity of Synthetic Flat Web Sling

Problem: A rigger must lift a $3.2\text{-tonne}$ smooth machinery shaft using a flat synthetic web sling rated at $WLL_{inline} = 2.0\text{ tonnes}$. Compare the allowable lifting capacity of the sling when rigged in:

1. A single standard choked hitch ($M = 0.8$).
2. A single vertical basket hitch ($M = 2.0$).
3. Determine which configuration is safe for the $3.2\text{-tonne}$ load.

Solution:

1. Choked Hitch Capacity: $WLL_{choked} = 2.0\text{ t} \times 0.8 = 1.6\text{ tonnes}$ Evaluation: $1.6\text{ t} < 3.2\text{ t} \implies$ UNSAFE / SEVERE OVERLOAD.

2. Vertical Basket Hitch Capacity: $WLL_{basket} = 2.0\text{ t} \times 2.0 = 4.0\text{ tonnes}$ Evaluation: $4.0\text{ t} \ge 3.2\text{ t} \implies$ SAFE.

3. Conclusion: The vertical basket hitch provides sufficient capacity ($4.0\text{ t}$), whereas the choked hitch is unsafe. To prevent the shaft from slipping sideways in a single basket, two basket hitches spaced apart or double-wrapped must be used.

Worked Example 2: Acute Angle Choke Calculation on Pipe Bundle

Problem: A bundle of loose steel pipes weighing $2.4\text{ tonnes}$ is lifted using two wire rope slings in choked hitches. Due to a tight bundle diameter, the choke point pulls down tight, resulting in an angle of choke of $75^\circ$. Each wire rope sling has a straight inline $WLL_{inline} = 2.0\text{ tonnes}$. Calculate the total effective WLL of the 2-sling arrangement and verify safety.

Solution:

1. Determine Choke Derating Factor: From the choke angle table, an angle of choke of $75^\circ$ ($60^\circ$–$89^\circ$ bracket) carries a derating factor of $0.60$.
2. Calculate Effective WLL per Sling: $WLL_{choked\_leg} = WLL_{inline} \times M = 2.0\text{ t} \times 0.60 = 1.20\text{ tonnes per leg}$
3. Calculate Total Assembly Capacity (2 Slings Parallel): $WLL_{total} = 2 \times 1.20\text{ t} = 2.40\text{ tonnes}$
4. Conclusion: The total capacity ($2.40\text{ t}$) exactly equals the bundle weight ($2.40\text{ t}$). The lift is at $100%$ capacity. However, because this leaves zero safety margin for dynamic shock loading, best practice requires substituting a double wrap choke or using larger wire rope slings.