Sling Strength & Design Factors
The ASME B30.9 standard titled "Slings" defines design factor as "a ratio between nominal or minimum breaking strength and the rated capacity of the sling." Simply put, dividing the breaking strength of a sling by the designated design factor results in the Working Load Limit (WLL). Two important terms from the definition of design factor are "nominal strength" and "minimum breaking strength." Nominal is a term that relates to breaking strengths published by the Wire Rope Technical Board. This is the minimum breaking strength that a wire rope can have. A wire rope may break at a value greater than nominal, but never less.
So, design factor is a number that when divided into the least amount of force required to break a sling results in the WLL for that sling.
Now let's move to a subject of constant concern; sling failure. When and how does a sling fail? Cuts, crushing, heat, caustics, excess loading, are just a few of the more common ways to damage and/or cause a sling to fail. So then, how does a sling fail when the applied load is less than the WLL? It may seem obvious, but the answer rests in our knowledge of breaking strength and design factor.
It only stands to reason that any minor damage would naturally result in a reduction of the sling's WLL, right? Wrong! The WLL of a sling never changes, and the design factor never changes. What changes is the sling's breaking strength -reducing with each minor damaging incident.
It would be a major mistake to reduce a sling's WLL because of existing damage. So users must diligently inspect all rigging gear prior to use, making sure that no damage exceeds manufacturer's specifications or regulatory allowances.
The objective is to remove slings from service long before breaking strength is reduced where it would be equal to or less than the sling's maximum working load limit.
Crane Tech's Rigging Inpector program identifies the rejection criteria for slings and rigging gear. Every sling is new only once. From that point forward it's a test of the inspector's knowledge and proper discretion of removing slings from service before they become a hazard.
Don't get caught thinking your slings have more strength than they actually have, or that design factor is a safety factor that allows for overloading. An investment in rigging gear inspector training pays real dividends.
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