Often times it is necessary to quickly approximate the capacity of a chain sling or hook. In addition to carefully inspecting (see below) the rigging component in question, estimates of safe working capacities can be made using two simple formulas*.
Manufacturers provide capacity tables for new rigging components. These should be consulted for accurate values, and appropriate safety factors must be considered based on project conditions. The only true determination of lifting capacity is through proof testing.
The Simple Math For Chains*
A rule of thumb for the estimated capacity of a single chain link is to measure the link thickness (D) at the observed smallest point, and use this simple formula to determine approximate safe working capacity:
Safe Working Capacity (in tons) = 8 x D x D
Where D is the smallest thickness or diameter of any given link in inches
Example: For a chain link with a measured thickness of 5⁄8 inch (0.625 in),
Safe Working Capacity = 8 x 0.625 x 0.625 = 3.125 tons or 6,250 pounds
The Simple Math For Hooks*
A rule of thumb for the estimated capacity of a steel hook is to measure the minimum thickness (D) at the point where the inside of the hook starts its arc, and use this simple formula to determine approximate safe working capacity:
Safe Working Capacity (in tons) = D x D
Example: For a value of D equal to 11⁄8 inch (1.125 in),
Safe Working Capacity = 1.125 x 1.125 = 1.27 tons or 2,530 pounds
Inspecting Chains And Hooks
Rigging equipment must be inspected at a minimum in accordance with the required periodic interval specified by OSHA for the particular component under consideration. Chains and hooks that are in frequent use for say, heavy and continuous hoisting, should receive more frequent inspection. Particular attention must be given to the small radius fillets that form the neck of hooks to determine any change from the originally fabricated inside arc. Each link and hook must be observed for cracks, sharp nicks or cuts, small dents, worn surfaces, or any other distortions.
* Reference: Department of the U.S. Army, Field Manual Number 5-125, Rigging Techniques, Procedures, and Applications, Chapter 3, pp. 3-2 – 3-3, Washington, D.C. October 3, 1995