80 RIGGING, HOISTING, and SIGNALING PRACTICES
RATED LOAD TERMINOLOGY A particularly confusing aspect of the rigging and hoisting industry is the variation in terminology for the amount of force that a component may be subjected to and still maintain the appropriate margin of safety. No single term stands out as an industry standard, though “rated load” is used for slings and rigging components by ASME, which is a major contributor to industry regulation. Also, there is a slight preference for “rated capacity” in the context of cranes and hoists. Therefore, these are the two terms chosen for use in this book. Other common terms include “load rating,”“load capacity,” “weight rating,” “weight capacity,” “working load limit,” and “safe working load.”These all have the same definition and, when encountered in the field and in publications, can be treated as equivalent. Note, however, that “breaking strength,” also known as “ultimate strength,” has a very different meaning. This is the force under which the component is expected to fail and must never be used as an allowable sling or equipment load. If ever in doubt regarding the meaning of a similar term for a rigging component, always consult with the manufacturer.
A safety factor is the ratio of a component’s breaking strength to its rated load. This is also known as the design factor. See Figure 6-5. Chain slings use a minimum safety factor of 4, while other types of slings use a minimum safety factor of 5. Therefore, the breaking strength is four to five times greater than the rated load. For example, if a safety factor of 5 is used for a rigging assembly with a breaking strength of 5000 lb, the rated load of the rigging assembly is 1000 lb (5000 lb ÷ 5 = 1000 lb). Note that the rated loads of slings with permanently attached end fittings, such as hooks or rings, are for the entire sling assembly.
Safety Factor
BREAKING STRENGTH RATING WIRE ROPE
APPLIED LOAD AT BREAKING STRENGTH
MARGIN OF SAFETY
WIRE ROPE
BREAKING STRENGTH
RATED LOAD
All sling identification tags must specify at least one rated load, though this information may not be explicitly labeled.
Figure 6-5. The safety factor provides a large margin of safety between a component’s breaking strength and the load weight it is rated to support.
Breaking strength is the tension at which a material is expected to break. The breaking strength of a particular material is determined by subjecting multiple samples to destructive load testing and averaging the results. However, the breaking strength value cannot be used directly as a safe value for lifting. There would be no margin of safety to compensate for slight manufacturing variations, underestimated load weight, sling age, or other weakening conditions. It is too likely that the forces will exceed the material’s breaking strength, which can lead to failure.
Standard safety factors are only valid for steady loading, which requires slow and gradual hoisting and load travel. Dynamic loading involves rapid motions, such as quick changes in direction, load shifting, and load swinging, and should always be avoided. However, if the possibility cannot be avoided, such as when lifting in windy environments, the rigging equipment’s rated load should be derated to approximately 60% of its specification in order to maintain a sufficient operating safety factor.