Rigging Load Calculator

Sling angle is the angle between the sling leg and the horizontal. As the sling angle decreases (slings become more horizontal), the tension in each sling leg increases significantly. The sling tension = Load / (Number of legs x sin(angle)). At 90 degrees (vertical), each leg carries Load/N. At 60 degrees, tension increases by a factor of 1/sin(60) = 1.15. At 30 degrees, tension doubles to 1/sin(30) = 2.0x. ASME B30.9 recommends minimum 45-degree sling angle.

ASME B30.9 is the standard for slings used in rigging operations. It specifies design factors, angle factors, inspection requirements, and operating practices for wire rope slings, chain slings, web slings, and round slings. It requires that slings be selected with adequate rated capacity for the specific configuration and angle used, and mandates inspection before each use.

ASME B30.9 requires rigging slings to have a minimum design factor of 5:1 (5 times the rated working load limit before breaking). The working load limit (WLL) is the maximum load the sling is rated for in a straight pull (vertical hitch). When used in a choker or basket hitch, or at less than 90 degrees, the effective WLL is reduced. Never use a sling with a load exceeding the WLL adjusted for configuration and angle.

Vertical hitch: sling is attached directly from load to hook in a straight lift. Efficiency = 100% of WLL. Choker hitch: sling wraps around the load and through itself, forming a noose. Efficiency = 75-80% of WLL. Basket hitch: sling forms a loop under the load with both ends attached to the hook. Efficiency = 200% for balanced vertical (2x WLL) but reduces with angle. Always verify the configuration factor with the sling manufacturer's data.

Total hook load = weight of load + weight of rigging hardware (slings, shackles, hook block, spreader beam). The rigging hardware can be significant for heavy lifts, sometimes 5-15% of the net load. The crane must be rated for the total hook load at the lift radius. Never confuse the net load (what you are moving) with the gross hook load (net load + all rigging weight).