If breaking strength refers to the maximum force a product can be subjected to once without failure, then the working load limit refers to the force that the same product should be subjected to. A product’s working load limit, often abbreviated to “WLL,” is typically only a fraction of its breaking strength. Therefore, if a shackle has a breaking strength of 15,000 pounds, it should regularly see forces far lower to ensure maximum safety and longevity. However, there’s no set correlation between MBS and WLL for all materials; each material utilizes a different ratio, often referred to as the product’s “safety factor” or “factor of safety.”
What Is Safety Factor (SF)?
A product’s safety factor (SF) is the material’s breaking strength compared to its WLL. For example, most shackles have a safety factor of 6:1, meaning a shackle with a 30,000-pound breaking strength has a 5,000-pound working load limit. However, these values aren’t necessarily regulated.
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The Cordage Institute-which generates standards for rope use-recommends a safety factor between 5:1 and 12:1; however, most winch lines and straps have a safety factor of only 4:1 to 3:1, while some military equipment only features a 2:1 safety factor!
You can also use a product’s safety factor to derive its working load limit from its breaking strength. To do so, divide the product’s MBS by its SF; for example, if a steel cable has a 25,000-pound MBS and a 5:1 SF, divide 25,000 by 5 to net its 5,000-pound WLL.
Breaking Strength Vs. Working Load Limit
Now that we understand the key values centered around recovery products’ capacity, let’s tackle the primary differences between breaking strength and working load limit, namely how you should consider and implement each.
Breaking Strength
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Beginning with breaking strength, it’s important to remember that you should never use a product at or near its MBS, except in an absolute emergency scenario when all other options have been exhausted; if you’re about to lose your Jeep down a shear hillside, go ahead and test out the limits of your strap, as long as everyone’s out of harm’s way.
Also, once you subject a piece of gear to force near its breaking strength, even if there aren’t any visible signs of stress or failure, it’s time to retire it. Remember, a product is only designed to withstand its MBS once without failing; any subsequent uses can result in catastrophic failure! When selecting your recovery gear, including tow straps, shackles, and winch lines, select a product with an MBS that’s well above the force it’ll be subjected to. For a 5,000-pound Jeep JL, opt for gear with a breaking strength in the multiple tens of thousands, taking into account its safety factor and working load limit.
Working Load Limit
Feel free to repeatedly use a strap, rope, or shackle at its working load limit! A product’s WLL is the true value you should consider when selecting your recovery gear. This number is well under the product’s MBS, ensuring the product can reliably be subjected to its WLL for an indefinite number of uses without fatiguing.
When selecting your recovery gear, we recommend selecting products with a working load limit that’s at LEAST 1.5 times your vehicle’s gross vehicle weight rating, so a 5,000-pound Jeep should use products with no less than a 7,500-pound WLL. Simple, right?
Bottom Line
Alright, if you’ve made it this far, let’s wrap up everything we’ve learned today:
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Breaking strength (MBS) and working load limit are incredibly different measurements. Breaking strength refers to the force that a product fails at or the minimum force a product must be able to withstand once without failing.
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A product’s working load limit (WLL) is the force that it’s rated to withstand routinely, for extended periods, without fatiguing. WLL is the value you should consider when selecting your recovery equipment.
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Safety Factor (SF) is the ratio of a product’s breaking strength to its working load limit. EX: a 5:1 safety factor means a product’s breaking strength is five times higher than its working load limit.
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When selecting a product, multiply your vehicle’s Gross Vehicle Weight Rating (GVWR) by 1.5, netting the minimum WLL. For a 5,000-pound Jeep, opt for recovery gear with no less than a 7,500-pound WLL.
If you understand these values, you’re halfway to a safe recovery! And remember, when selecting your gear, prepare for any possibility, and plan for every situation, but ensure that safety is the tool you rely on most often!
Source: https://t-tees.com
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