3.1 Sling Angles, Sling Types & Leg Tension Calculations

Sling Types, Construction, and Material Selection

Industrial millwrights rely on three primary categories of rigging slings: synthetic web slings, wire rope slings, and alloy steel chain slings. Selecting the correct sling requires understanding the mechanical properties, environmental limits, and rated capacities of each material.

Synthetic Web Slings

Synthetic web slings are lightweight, flexible, and non-marring, making them ideal for lifting polished shafts, painted machinery, and delicate components. They are primarily manufactured from two synthetic polymers:

• Nylon: Offers high strength and elasticity (absorbing shock loads well). However, nylon is severely degraded by acids and bleaching agents, while resisting alkalis. Nylon slings must never be used in acidic environments or exposed to acid vapors.
• Polyester: Features lower stretch for precise load positioning and excellent resistance to acids. However, polyester is degraded by alkalis (caustics) and concentrated sulfuric acid.

Both synthetic sling materials have a maximum operating temperature limit of 90°C (194°F), and a minimum of -40°C (-40°F), per OSHA 1910.184 guidance and ASME B30.9. Exposure to temperatures above this threshold or to direct flame will melt or char the fibers. Extended exposure to ultraviolet (UV) radiation breaks down synthetic fibers, leading to strength loss; slings exhibiting fading, stiffness, or solar degradation must be removed from service. All synthetic slings must feature a permanently legible capacity tag stating Working Load Limit (WLL) for vertical, choker, and basket hitches. If the tag is missing or unreadable, the sling must be removed from service immediately.

Wire Rope Slings

Wire rope slings provide high strength, cut resistance, and temperature durability. They are typically constructed from Extra Improved Plow Steel (EIPS) or Extra Extra Improved Plow Steel (EEIPS) wire rope, standardly configured as 6x19 or 6x37 class wire rope. The core of the wire rope affects its flexibility and thermal resistance:

• Independent Wire Rope Core (IWRC): Provides higher strength, crush resistance, and a maximum operating temperature of 200°C (400°F).
• Fiber Core (FC): Made of natural or synthetic fibers, offering superior flexibility but lower crushing resistance and a reduced temperature limit of 82°C (180°F).

Wire rope sling eye terminations should be formed using a Flemish eye (mechanically spliced eye where the strands are unlaid, looped back, and interwoven) secured with a pressed metal sleeve. A simple mechanical press splice without a Flemish interweave relies solely on sleeve friction and is far less safe if damaged. Thimbles must be inserted into eyes to prevent severe wire deformation and pinching when connecting to shackles or hooks.

Alloy Steel Chain Slings

Chain slings are used in severe duty environments involving high temperatures, sharp edges, and heavy abrasion. Crucial Rule: Only Grade 80 and Grade 100 heat-treated alloy steel chains are approved and certified for overhead lifting. Carbon steel chains—such as Grade 30 (Proof Coil), Grade 43 (High Test), and Grade 70 (Transport chain)—lack the necessary ductility, elongation properties, and toughness, and are strictly prohibited for overhead lifting.

Alloy chain slings can withstand temperatures up to 200°C (400°F) without capacity reduction. At elevated temperatures between 200°C and 315°C (400°F to 600°F), Grade 80/100 chain capacity is de-rated by 10%; between 315°C and 400°C (600°F to 750°F), capacity is de-rated by 20%. Chain slings must be inspected link-by-link for stretch (elongation exceeding 5% of original length warrants discard), wear in the link balance/reach points (wear exceeding 10% of nominal wire diameter requires removal), nicks, gouges, or heat discoloration.

Sling Hitches and Hitch Efficiency Factors

The way a sling is attached to a load determines its effective load capacity. Riggers utilize three fundamental hitch configurations:

Choker Hitch Angle Reductions

When a choker hitch is applied, the choke angle (the angle formed where the sling body passes through the choke eye) impacts sling capacity. If the choke angle is 120° or greater, the standard 0.75 choke factor applies. However, if the choke angle is pulled tight (forced choke):

• Choke angle 90° to 117°: Capacity de-rated to 65% of vertical WLL.
• Choke angle 60° to 89°: Capacity de-rated to 55% of vertical WLL.
• Choke angle 30° to 59°: Capacity de-rated to 49% of vertical WLL.

To prevent sharp bending of the choke eye, a double-wrap choker hitch (wrapping the sling 360° around the load before choking) should be used on loose bundles, smooth pipe, or round stock to grip the load securely and prevent slipping.

Sling Angle Multiplier and Leg Tension Calculations

When multi-leg slings (2-leg, 3-leg, or 4-leg bridles) or basket hitches are used, the sling legs are inclined at an angle to the horizontal plane. As the sling angle relative to horizontal decreases (the sling spreads wider), the tension in each sling leg increases dramatically.

Sling Angle Factor (SAF) Formula

The Sling Angle Factor (SAF), also known as the Load Angle Factor (LAF), is defined mathematically as:

$\text{SAF} = \frac{L}{H} = \frac{1}{\sin(\theta)}$

Where:

• L = Length of the sling leg
• H = Vertical height from the load attachment point to the apex of the hook
• θ = Angle formed between the sling leg and the horizontal plane of the load

Leg Tension Calculation Formula

To calculate the total tension acting on each individual leg of a multi-leg sling carrying a balanced load, use the formula:

$\text{Leg Tension} = \left( \frac{\text{Total Load}}{N} \right) \times \text{SAF} = \left( \frac{\text{Total Load}}{N} \right) \times \frac{L}{H}$

Where N is the number of effective load-bearing legs. Note: On a rigid, non-flexible 4-leg bridle, only 2 legs are assumed to carry the load at any given moment due to manufacturing tolerances and slight load tilt (N = 2).

Sample Calculation

A millwright must lift a machine component weighing 12,000 lbs using a 2-leg wire rope sling. Each sling leg is 10 feet long (L = 10 ft), and the vertical height from the hook to the connection points is measured at 5 feet (H = 5 ft).

1. Calculate Sling Angle Factor: $\text{SAF} = \frac{L}{H} = \frac{10}{5} = 2.0$
2. Calculate Horizontal Angle: $\sin(\theta) = \frac{H}{L} = 0.500 \implies \theta = 30^\circ$
3. Calculate Leg Tension: $\text{Tension} = \left( \frac{12,000\text{ lbs}}{2} \right) \times 2.0 = 6,000 \times 2.0 = 12,000\text{ lbs per leg}$

Notice that at a 30° horizontal sling angle, each leg carries tension equal to the full 12,000 lb weight of the load! Total sling tension across both legs equals 24,000 lbs.

Critical Sling Angle Limit

Safety Mandatory Rule: Rigging slings at horizontal angles below 30° is strictly prohibited. At angles below 30°, tension skyrockets toward infinity, risking catastrophic sling failure. Millwrights must always aim for a horizontal sling angle of 60° or greater whenever headroom permits.

D/d Ratio for Wire Rope Slings and Corner Protection

When a wire rope sling is bent around a curved object—such as a shackle pin, crane hook, or cylindrical load—the outer wires undergo severe tension while inner wires are compressed. This bending stresses the rope structure and reduces its effective lifting capacity.

The severity of bending is evaluated using the D/d ratio, where:

• D = Diameter of the pin, hook, or load curvature around which the sling is bent
• d = Nominal diameter of the wire rope sling

$\text{D/d Ratio} = \frac{D}{d}$

For example, if a 3/4-inch (d = 0.75 in) wire rope sling is wrapped around a 1.5-inch (D = 1.5 in) shackle pin, the ratio is D/d = 1.5 / 0.75 = 2. The sling's Working Load Limit must be de-rated to 65% of its nominal tagged capacity.

Softeners and Corner Protectors

Rigging slings must never contact sharp edges, flange corners, or rough machined surfaces. Corner protectors, split pipe sleeves, heavy rubber pads, or aluminum softeners must be placed between the sling and load corners. Softeners protect synthetic slings from cutting and prevent wire rope slings from exceeding minimum D/d bend radii or developing permanent kinks.