The working load limit (WLL) is the maximum load that a sling, shackle, hoist or other lifting equipment is rated by its manufacturer to handle in normal service.
The working load limit (WLL) is the maximum load that a piece of lifting or rigging equipment is designed to support in general service, as set by the manufacturer. It applies to slings, shackles, hooks, eye bolts, hoist rings, chain, wire rope, hoists and other gear used to lift, pull or secure loads. The WLL is marked on the equipment or its tag and must never be exceeded.
The WLL is well below the force that would actually break the equipment. Manufacturers divide the minimum breaking strength by a design factor, sometimes called a safety factor, to arrive at the WLL. Common design factors are around 4 to 1 for alloy chain slings and 5 to 1 for wire rope and synthetic slings, with some hardware rated at 5 or 6 to 1. The margin allows for wear, minor damage, dynamic loading and uncertainty about the exact weight of the load. It is not spare capacity for routine use.
Related terms include rated capacity, which ASME standards use for slings, and safe working load (SWL), an older term that many standards now avoid because it can suggest that loads up to that figure are always safe.
When a sling leg is at an angle, the tension in the leg is greater than its share of the load. At 60 degrees from horizontal, each leg of a two-leg bridle carries about 1.15 times its share. At 45 degrees, it is about 1.41 times, and at 30 degrees, about 2 times. Sling angles below 30 degrees from horizontal are generally avoided.
A choker hitch reduces a sling's capacity compared with a vertical hitch, typically to around 75 to 80 percent for many sling types, and further if the choke angle is sharp. A basket hitch can increase capacity when the legs are vertical, but angle effects still apply.
OSHA's sling standard for general industry, 29 CFR 1910.184, requires slings to be marked or labeled with their rated capacities and prohibits loading them beyond those capacities. Construction rigging is covered by 29 CFR 1926.251 and the crane rules in 1926 Subpart CC. The ASME B30 standards, including B30.9 for slings and B30.26 for rigging hardware, set detailed requirements for ratings, marking, inspection and removal from service.
A maintenance crew needed to lift a 4,000-pound pump with a two-leg chain sling rated at 3,000 pounds per leg for a vertical hitch. Low headroom meant the legs would hang at about 30 degrees from horizontal. At that angle, each leg would carry about 4,000 pounds, well above its rating.
The rigging supervisor caught the problem during the lift plan review. The crew used a spreader beam to keep the sling legs vertical and checked that every component, including shackles and the beam, had a WLL above the load it would carry. The lift was completed without overloading any part of the rigging and with the area below clear to prevent a dropped object hazard.
No. The breaking strength is the force at which the equipment fails. The WLL is that strength divided by a design factor, so it is much lower.
No. OSHA and ASME require slings and many hardware items to show their rated capacity. Equipment with a missing or unreadable tag should be removed from service until it is identified and re-tagged by the manufacturer or a qualified person.
The capacity of a rigging assembly is limited by its weakest component after accounting for angle and hitch. Every part, from the hook to the shackle to the sling, must be rated for the load it will carry.
Part of SafetyIQ's EHS glossary: plain-English definitions of workplace health and safety terms.
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