There is a design factor on everything we use from a plastic grocery bag to the wires that carry electricity to a television. Towing and recovery equipment isn’t any different. Engineers spend numerous hours researching and testing various designs to make sure they are safe when used as intended. Problems happen when we don’t use things as the engineers intended.
I’ve had the privilege of working with several engineers on various projects throughout my career, including some not related to towing at all. One of my hobbies is live audio and stage lighting, which has allowed me to design more than 100 shows throughout the past three decades. In that process, I learned that every piece, regardless of how seemingly insignificant, is critical to the overall success of the process.
The same holds true within our industry. From using properly rated fasteners to having the correct micron rating on our hydraulic filters, each individual component is designed for a specific purpose, and working together they safely accomplish what we ask daily.
So, what is “design factor”? In simple terms, it is what a part or assembly (piece of equipment) is designed to withstand in normal service. For rigging and towing equipment, design factor is calculated as the safe working load limit (WLL) plus a factor of safety. Typically expressed in a ratio, this calculation should never be used in planning a job. It is a good measure to use when comparing products for purchase as it can be an indicator of quality and built-in safety to account for the unintentional overloading of a product.
Most often this is described in training classes as the paperclip 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—damage has already been done and the product may fail even at a much lower force than the WLL. (I do not advocate for ever exceeding the WLL of a product no matter the reason or circumstances.)
The most important thing for a tower to understand about safe WLLs 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 WLL. 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 lbs. (taken from the FMCSR table in 393.108) is actually good for 18,800 lbs., or four-times its WLL. This is false!
Although there are U.S. and international standards that govern chain, cable, and straps (along with many other products) regarding how WLL 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 the 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.
Many companies have published their ultimate failure limits in an attempt to show the durability of their product. Although intended as a marketing campaign, it has caused towers to think they can use their rigging way beyond what it is capable of safely doing. 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.
If you are exceeding working load limits on any of your equipment, please stop immediately and replace the equipment … especially rigging. Once you have knowingly exceeded the WLL of a product, you have weakened it and it will fail, often when being used well below its WLL rating. Never exceed the safe WLL of any part of an assembly. The old adage “a chain is only as strong as its weakest link” is perfectly applicable and soberingly true in this instance.
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.
Also, just because you may be in a state that grants axle weight exemptions, it does not mean that there will not be consequences from overloading your truck’s axles or you are somehow immune from having a safety-critical failure. Just because you have an underlift that is rated to pick 50,000 lbs. does not mean that you can safely pick up that much and still be within the capacities of your truck chassis, rear axle, tires, or braking system. Same with your boom; a wrecker will only safely perform as rated if you are using it within the design and consistent with the manufacturer’s recommendations.
We have all seen the pictures of wreckers on their side after a failed recovery attempt. Many of these incidents have happened while the truck was being used within its design capabilities, but the rigging was not, or the truck was not set up properly so even though the actual weight being moved was lower than the rating of the wrecker, it was unable to support it because it was not set up in a stable manner.
Training, continuing education, and understanding the specific pieces of equipment you are using are 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 educational materials and engineering available to help us understand the forces at work while doing any job.
This is especially true for those of us that are venturing into the crane world with our rotators. We can’t just show up and pick something up, it must be properly planned to ensure success. Crane operators don’t guess. They plan for the forces being applied at each stage of the lift. Load indicators on our boom or load cells in the line are nice to have, but they should only be used to verify our calculations are correct—not to determine how much something weighs!
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. The damage is done.
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.
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