Understanding Working Load Limit

By Brian J. Riker

There is a design factor on everything we use from the shopping bag used to bring groceries home to the wires that carry the electricity to our television. Towing and recovery equipment is not any different. Engineers spend countless hours designing and testing various designs to make sure they are safe when used as intended. Problems happen when we use things in a way the engineers did not intend us to.

Every piece, regardless of how seemingly insignificant, is critical to the overall success of the process. From using properly rated fasteners to having the correct micron rating on our hydraulic filter, each individual component is designed for a specific purpose, and working together they safely accomplish what we ask daily.

The most important thing for a tower to understand about safe working load limits and design factor is they are only a reliable measure of capability when using the product as intended by the manufacturer. This is why there are different ratings on rigging when used in a straight line vs. wrapped around an object in a basket or choker. When used in a way inconsistent with the manufacturer’s intended use, a product will be damaged and may fail at a load significantly lower than the WLL. This is also why shock loading a chain or cable is a huge no-no. Engineers can’t accurately anticipate the loads imposed during an uncontrolled event such as shock loading so they can’t design for it. Shock loads often exceed the WLL and must always be avoided.

We must never intentionally use our equipment outside the working load limit. DO NOT count on the safety factor. It is a common misconception that all chains have a 4:1 safety factor, meaning that a 5/16” Grade 70 (transport) chain with a WLL of 4,700 pounds (taken from the FMCSR table in 393.108) is actually good for 18,800 pounds, or 4x it’s WLL. This is false!

Most often this is described in training classes as the paper clip effect, bending it once or twice won’t break it but doing so repeatedly will cause a failure. Please keep in mind, once overloaded just one time -even unintentionally- the damage has already been done and the product may fail even at a much lower force than the WLL. To be clear, I do not advocate for ever exceeding the WLL of a product no matter the reason or circumstances.

Although there are US and international standards that govern chain, cable and straps (along with many other products) regarding how working load limits are calculated using design factor and ultimate failure/breaking strength, each manufacturer applies these differently depending on which trade association standards they are following and their own internal testing processes.

In the United States it is ultimately up to the end user to determine if a product is fit for the intended use which leaves us as towers on the line for our own safety. Always be sure you fully understand the ratings on every piece of equipment you use before you begin any job.

The FMCSA publishes a table with their acceptable working load limits for unmarked components based upon material of construction. This table, and other applicable regulations on WLL relevant to on-highway transportation can be found in 40 CFR Part 393.

OSHA, via various standards in 29 CFR 1910 and 1926, requires adherence to the American Society of Mechanical Engineers standards for WLL as well as proof testing of any custom designed hooks or attachments. In these same standards, OSHA clearly publishes design criteria that overhead lifting devices, i.e., cranes, must adhere to and how these requirements play into safe working load limits.

Ultimate failure ratings should never be used by anyone other than the manufacturer to validate their designs and purchasers when comparing product values for purchasing. Ultimate failure rating NEVER plays a role in rigging or lifting plans.

These concepts apply to everything we use in towing, not just our chains, straps and wire ropes. From the truck chassis to the wrecker body installed on it, everything is designed to work within certain limits. I often see wreckers and carriers overloaded because of the concept of “if it fits it ships” or “the truck lifted it so I am good to go.” Please take the time to learn about your specific equipment and what it is intended to do.

A wrecker will only safely perform as rated if you are using it within the design and consistent with the manufacturer’s recommendations.

Training, continuing education and understanding the specific pieces of equipment you are using is critical to your safety and success. We are professionals and as such we must know each tool in our toolbox and how to use it properly. Gone are the days of pulling until something moves or breaks and then attacking it again with a bigger chain. Today we have the educational materials and engineering available to help us understand the forces at work while doing any job.

You must know these numbers before putting your first piece of rigging on something, finding out you are overloaded after you have something lifted, even if it is only an inch off the ground, is the wrong time to figure out what is next. By then you have already overloaded your rigging or equipment and the damage is done.

Bottom line, when selecting equipment it is important to understand the different ratings and how they apply to your intended use. Not all manufacturers use the same rating methods so make sure you are comparing apples to apples when making your selection. If in doubt ask them to explain what they mean by an unusual term, or how they calculated their rating if it is way out of line with other similar products.