What is Working Load Limit (WLL) in rigging?

Working Load Limit is the fundamental rating system for rigging components in the United States. Every wire rope sling, synthetic sling, chain sling, shackle, hook, master link, and piece of rigging hardware is assigned a WLL by the manufacturer based on standardized testing and design criteria. The WLL is the upper boundary for planned loading in normal service. It is required to be marked on the component and it must be legible for the component to remain in service.

WLL vs. Breaking Strength and the Design Factor

The WLL and the minimum breaking strength of a rigging component are not the same value. The design factor - also called the safety factor - is the ratio between the minimum breaking strength and the WLL. For wire rope slings under ASME B30.9, the design factor is 5:1, meaning the minimum breaking strength is at least five times the WLL. For rigging hardware such as shackles under ASME B30.26, the design factor is typically 4:1 or 5:1 depending on the component type and its application category. For alloy chain slings, ASME B30.9 specifies a 4:1 design factor.

The design factor exists to absorb dynamic loading, shock loading, fatigue over repeated use cycles, manufacturing variability, and the effects of rigging geometry. A component being used at exactly its WLL in a clean vertical hoist has consumed its full rated capacity but retains the design-factor margin against failure. A component loaded at 110% of WLL has breached the operational limit and is operating against part of the design factor, leaving reduced margin against failure from any additional dynamic load.

Rigging Angle De-Rating

A sling's WLL is established for a specific hitch type and rigging angle. For wire rope and synthetic slings used in basket or bridle configurations, the effective load on each sling leg increases as the included angle between the sling legs increases (the sling becomes more horizontal). This effect is quantified by the horizontal tension factor, which ASME B30.9 tabulates for common sling angles. At a 60-degree horizontal angle (30-degree angle from vertical), the tension in each sling leg is double the vertical pull. A rigger who ignores this de-rating and uses the vertical WLL for a near-horizontal sling leg is exceeding the WLL by a factor that increases with the angle of departure from vertical.

WLL in Lift Plan Calculations

Every rigging plan must verify that the WLL of every component in the rigging system, after de-rating for rigging angle and hitch type, exceeds the actual load share assigned to that component. The governing limit for the rigging system is the single lowest-rated component in the configuration, after de-rating. The rigging plan must also verify that the combined system capacity is sufficient for the total load. A load plan that only checks the crane's rated capacity without independently verifying the rigging WLL chain is incomplete, because a rigging failure can occur well below the crane's structural or tipping limit if the rigging is undersized or incorrectly assembled.