Rigging Safety: Sling Angles, WLL, Safety Factors & Inspection

Sling Angle Factor: Why Geometry Multiplies Tension

When a single sling lifts a load vertically, the tension in the sling equals the load weight. When two slings form a bridle hitch (V-shape), the tension in each sling depends on the angle between the sling and the horizontal plane. The sling angle factor formula is: T = W / (N × sin(θ)), where T is the tension in each sling, W is the total load weight, N is the number of sling legs, and θ is the angle each sling makes with the horizontal.

For a two-leg bridle lifting 10,000 lb:

• θ = 90° (vertical): T = 10,000 / (2 × 1.0) = 5,000 lb per sling
• θ = 60°: T = 10,000 / (2 × 0.866) = 5,774 lb per sling
• θ = 45°: T = 10,000 / (2 × 0.707) = 7,071 lb per sling
• θ = 30°: T = 10,000 / (2 × 0.500) = 10,000 lb per sling

At a 30-degree sling angle, each sling in a two-leg bridle carries the entire load weight. At angles below 30 degrees, the tension exceeds the load weight. This is why ASME B30.9 and most rigging standards prohibit sling angles below 30 degrees from horizontal. At 15 degrees, each sling carries nearly twice the load weight (T = 19,318 lb for a 10,000 lb load). At 5 degrees, the tension approaches infinity. Shallow sling angles are inherently dangerous and must be avoided through longer slings, taller crane positioning, or spreader bars.

The design factor (inverse of sling angle factor) is sometimes expressed as a fraction of vertical rated capacity:

• 60°: Use 86.6% of vertical WLL
• 45°: Use 70.7% of vertical WLL
• 30°: Use 50.0% of vertical WLL

Rigging charts posted in shops and printed on sling tags list the rated capacity at each standard angle. Always verify the sling angle before lifting. If the load is wider than expected or the crane hook is lower than planned, the sling angle decreases and the tension increases. Measure or estimate the angle before every lift.

Working Load Limit and Design Safety Factors

The Working Load Limit (WLL) is the maximum load that may be applied to a sling in a specific configuration (vertical, choker, or basket hitch at a specified angle). It is NOT the breaking strength. The WLL is the breaking strength divided by the design safety factor. The safety factor varies by sling type and is established by ASME B30.9:

• Wire rope slings: Safety factor = 5.0. A wire rope sling with a breaking strength of 25,000 lb has a WLL of 5,000 lb in vertical hitch.
• Alloy steel chain slings: Safety factor = 4.0. A chain with a breaking strength of 40,000 lb has a WLL of 10,000 lb.
• Synthetic web (nylon/polyester) slings: Safety factor = 5.0. A web sling rated at 6,400 lb has a minimum breaking strength of 32,000 lb.
• Synthetic roundslings: Safety factor = 5.0.
• Metal mesh slings: Safety factor = 5.0.

These safety factors account for dynamic loading (shock from sudden starts, stops, or snatch loads), degradation from use and environmental exposure, and uncertainty in load weight estimation. They do NOT account for sling damage from cuts, abrasion, corrosion, or overloading. A sling that has been damaged has an unknown capacity and must be removed from service regardless of its rated WLL.

Never exceed the WLL of any component in the rigging assembly. The weakest link determines the capacity of the entire system. A 10,000 lb chain sling connected to a 5,000 lb shackle has a system WLL of 5,000 lb. Always check the WLL of shackles, hooks, eyebolts, and connection hardware in addition to the sling itself.

Hitch Types: Vertical, Choker, and Basket

The three basic hitch configurations change the sling's effective capacity:

• Vertical hitch: One end of the sling is attached to the crane hook, the other to the load. The sling is loaded in straight tension. The rated capacity equals the WLL (100% of vertical rating).
• Choker hitch: The sling wraps around the load and one end passes through the eye of the other end, forming a noose. The choker reduces the effective capacity to 75% to 80% of the vertical WLL because the choke point creates a bending stress in the sling that reduces its tensile capacity. For wire rope slings, the choker rating is typically 75% of vertical. For synthetic slings, it is 75% to 80%.
• Basket hitch: The sling passes under the load with both ends attached to the crane hook, forming a U-shape. In a true vertical basket (both legs vertical), the sling carries twice the vertical WLL because the load is shared between two legs. In practice, basket hitches are rarely perfectly vertical, so the actual capacity is 2 × WLL × sin(θ), where θ is the angle of the sling legs from horizontal.

Double-wrap choker hitches and other advanced configurations are used for specific load shapes and control requirements. A double-wrap choker provides more friction and load control than a single-wrap but reduces capacity further. Tag line control is essential on any load that could spin or swing during lifting.

Load control is as important as load capacity. A basket hitch on a smooth cylindrical load (pipe, shaft, tank) allows the load to slide unless the slings are positioned to cradle the center of gravity. If the center of gravity is not centered between the attachment points, the load tilts when lifted. In extreme cases, the load slides out of the basket entirely. Use nylon sling protectors on sharp edges: a sharp corner can cut through a synthetic sling under load in seconds.

Sling Inspection Criteria: When to Remove from Service

OSHA requires sling inspection before each use ("each day" per 1926.251(a)(6)) and a periodic thorough inspection by a competent person. Sling inspection is not a cursory glance; it is a systematic check for specific damage modes that vary by sling type.

Wire rope slings must be removed from service if any of the following are found:

• Ten randomly distributed broken wires in one rope lay, or five broken wires in one strand in one rope lay
• Severe corrosion of the rope or end fittings
• Kinking, crushing, birdcaging, or other distortion of the rope structure
• Evidence of heat damage (discoloration, loss of lubricant)
• End fittings that are cracked, deformed, or worn
• Hooks opened more than 15% of the normal throat opening or twisted more than 10 degrees from the plane of the unbent hook

Alloy steel chain slings must be removed if:

• Any link is bent, stretched, or elongated (a 5% increase in link length means the chain has been overloaded)
• Excessive wear (normally allowed wear is 10% to 15% reduction in diameter at the wear point)
• Cracks in any link or component (any crack, no matter how small, is cause for immediate removal)
• Nicks or gouges that reduce the cross-section
• Evidence of heat damage (discoloration, loss of temper)

Synthetic web slings must be removed if:

• Acid or caustic burns
• Melting or charring of any part of the sling surface
• Snags, punctures, tears, or cuts that expose the core yarns
• Broken or worn stitching in load-bearing splices
• Distortion of fittings (hooks, rings, links)
• Faded, illegible, or missing sling identification tag (a sling without a legible tag must be removed from service)

Damaged slings must be destroyed or rendered unusable, not just "set aside." A damaged sling that is placed in a discard pile will eventually find its way back into service unless it is cut up or physically destroyed. Mark damaged slings with spray paint, cut them, and dispose of them immediately.

Lift Planning: The Steps Before Steel Leaves the Ground

Every crane lift should be planned before it is executed. For routine lifts (repetitive, low-risk, within 75% of crane capacity), a simple pre-lift checklist is sufficient. For critical lifts (loads over 75% of crane capacity, lifts over personnel, multiple crane lifts, loads near structures), a formal written lift plan is required.

The minimum lift planning steps:

• Determine the load weight: Weigh the load if possible. If not, calculate the weight from dimensions and material density. Add the weight of all rigging hardware (slings, shackles, spreader bars, lifting beams). Never estimate; always calculate or verify. A "500 lb" piece of equipment often turns out to be 800 lb when you include the skid, fluid fill, and attached accessories.
• Determine the center of gravity: The hook must be positioned directly above the center of gravity for the load to lift level. If the CG is not at the geometric center (asymmetric loads, partially filled vessels), the sling lengths or attachment points must be adjusted to compensate. An off-center lift causes the load to tilt and can cause side-loading on the crane boom.
• Select the rigging: Choose slings, shackles, and hardware rated for the load at the planned sling angle, with the appropriate hitch type. Every component must have a WLL equal to or greater than the calculated tension. Check the condition of all hardware before use.
• Check the crane capacity: The crane's load chart gives the rated capacity at each radius (distance from the center of rotation to the hook). As radius increases, capacity decreases dramatically. Verify the lift radius, boom length, and configuration against the load chart. The crane capacity must exceed the total lifted weight (load + rigging) at the maximum radius the load will travel during the lift.
• Identify hazards: Overhead power lines, underground utilities, personnel in the lift zone, wind conditions, ground bearing capacity under outriggers, and swing path obstructions. Establish a controlled access zone around the lift area and assign a signal person.

Communication during the lift uses standard hand signals per ASME B30.5 or radio communication between the operator and the signal person. Only one person gives signals to the operator. If anyone signals "stop," the operator stops immediately, no exceptions. The lift resumes only after the stop signal is resolved and the designated signal person gives the "all clear."