Working Load Limits – Why So Low?

We have had many questions over the years on properly determining the working load limit for cargo control devices like the Original BEDNET® Cargo Restraint.

The Federal Motor Vehicle Safety Administration defines WLL this way: “The working load limit (WLL) of a tie-down, associated connector or attachment mechanism is the lowest working load limit of any of its components (including tensioner), or the working load limit of the anchor points to which it is attached…”

US DOT FMVSA Regulation § 393.108

This means the weakest point on any assembly is used to determine the working load; the stronger components are irrelevant in this initial determination.

So we start with the smallest tensile rating for any component in our device

WLL is typically calculated by dividing the ultimate break strength of the weakest component by a safety factor: for tie-down applications is typically 3 or more, for sling applications, it is 4 or more.

We divide that smallest tensile by a safety factor of 3 or more to get our WLL (BEDNET® uses 3 1/3)

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Let’s look at three examples where we divide the lowest rating by 3 to derive WLL

A tractor-trailer freight cam strap

• E-Clip 8000 lbs tensile
• Cam Buckle 2500 lbs tensile
• Polyester Webbing 5600 lbs tensile

2500lbs / 3= 833 lbs WLL

A tractor-trailer ratchet strap

• E-Clip 8000lbs tensile
• Ratchet 5000 lbs tensile
• Polyester Webbing 10000 lbs tensile

5000 lbs / 3 = 1733 lbs WLL

Original BEDNET® Cargo Restraint

• Steel Hook 2000 lbs Tensile
• Quick Tite Buckle 1500 lbs tensile
• Polyester Webbing 5600 lbs tensile

1500 lbs / 3 = 500 lbs WLL

BEDNET® uses a slightly higher safety factor of 3 1/3 to derive a WLL of 450 lbs

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This post was originally published on Jan. 27, 2014, and has since been updated.