8.3 Three- and Four-Leg Assemblies and Load Sharing
Key Takeaways
Do not divide weight by leg count without a valid load-sharing model.
A turnbuckle changes length; it is not automatically an equalizer.
Use the manufacturer’s assembly rating and approved imbalance analysis.
Leg count does not establish equal force
A multi-leg sling assembly combines three or more legs into a rated system. Its capacity depends on its manufacturer’s rating basis, angles, component sizes and permitted conditions. Dividing payload weight by the visible number of legs is not a general capacity method.
Differences in effective length, pick elevations, stiffness, load geometry and CG affect sharing. A rigid four-point load may place greater force in some legs while others carry little. A flexible load may redistribute through deformation, but flexibility does not prove equal sharing or protect every attachment. The actual body and suspension system must be considered.
Catalogs commonly rate certain three- and four-leg assemblies on a specified load-sharing basis, often three effective legs for their defined conditions. That rating is not a universal guarantee that every field load shares across three legs. Conversely, it is incorrect to assert that ASME always requires every rigid four-leg bridle to carry the entire load in exactly two legs. Determine the applicable assembly rating and the approved load-sharing model.
Read the assembly information
Find the rated angle, configuration, material, leg size and components in the tag or documentation. A four-leg rating may be lower than four times a single-leg rating. An individual replacement leg can invalidate the original assembly assumptions if its length, stiffness, fittings or rating differ.
| Information needed | Reason |
|---|---|
| Assembly WLL and angle | Establishes the rated combined configuration |
| Individual leg suitability | Prevents a highly loaded leg exceeding its own limit |
| CG and pick coordinates | Determines potential imbalance |
| Effective leg lengths | Influences which legs take tension |
| Equalizing device instructions | Establishes whether and how redistribution is permitted |
Do not transfer a table for one sling family or manufacturer to another. Chain assemblies, wire-rope bridles and textile assemblies can have different fittings and rating details. The fact that two assemblies have four legs does not make their capacities interchangeable.
What equalizing means
An equalizing device provides a designed means of redistributing force or movement under its specified conditions. Examples can include an approved sheave arrangement or other engineered system. An adjustable leg merely changes length; a turnbuckle or shortener does not automatically make all legs carry equally during the lift.
A load cell measures force where it is installed. Several readings can help verify a planned distribution, but measurement does not itself create an equalizer. The plan must specify acceptable forces, the measurement method and a safe response to imbalance. A reading near zero in one leg is evidence to investigate, not permission to multiply another leg’s rating by the original count.
A rated-assembly example
Assume a manufacturer supplies a four-leg assembly rated 30,000 lb for the specified angle and conditions. The proposed payload plus carried lower equipment is 26,000 lb. Comparing these totals addresses the assembly’s rated condition only if the actual angle, CG and configuration satisfy the instructions. It does not establish suitability if the load is eccentric outside the permitted assumptions.
If a qualified plan explicitly analyzes two effective legs at a given geometry, use that model for the calculated demand. If the approved system supports three equal effective shares, use that stated model. In an exam, the supplied assumption controls the arithmetic; do not invent an extra leg to obtain a convenient lower answer.
For illustration, a 24,000-lb load with an explicitly given three-equal-share model at 60 degrees has 8,000-lb vertical share per effective leg and approximately 9,238-lb leg tension. Under a two-equal-share model, each vertical share is 12,000 lb and tension is approximately 13,856 lb. The different result comes from the stated support model, not a new law of trigonometry.
Installation and verification
Make sure master links, intermediate links and hooks can accommodate the complete assembly without crowding, crossed legs or binding. Use the manufacturer’s collector arrangement. Intermediate links can improve connection fit, but they do not by themselves equalize every lower load.
Check that legs are untwisted, appropriate adjustment devices are correctly engaged and the intended picks can carry their forces. During the trial lift, observe seating, attitude and unintended slack from a safe location. A taut-looking leg may carry little compared with another; appearance alone cannot establish its measured tension.
Lower to stable support before changing lengths or connections. Tightening a suspended turnbuckle or moving a chain shortener under load can release or transfer force unpredictably. Adjust only through the approved unloaded procedure or an explicitly designed and approved system.
The examination trap is a claim of automatic load sharing. Reject both casual division by four and an unsupported universal division by two. Use the rated assembly, actual geometry, CG information and applicable qualified-person analysis to establish the permitted demand.
Sources: CCO reference booklet, Crosby sling assembly information.
Which statement about a rigid load in a four-leg bridle is defensible?
All four legs always carry exactly one quarter
ASME universally requires exactly two legs to carry all weight
A level load proves identical tension in all legs
Sharing must be established from the rated assembly and actual geometry; leg count alone is insufficient
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