As part of this series on rope, we will get into the materials, technology and other specifics that work together to create our rigging and climbing lines. I would like to look at a concept that not only applies to rope, but applies to all of our other gear as well.
One question that most people have heard or asked is, “How long will this rope last?” It isn’t specific to rope; The same question is asked about saddles, carabiners, rope bags and many other products. It does seem to be a common question when discussing rope. The tough, yet truthful part about this question is, there isn’t a definitive answer. It is a loaded question with a lot of “what-ifs” and variables.
Cycles to Failure may be a term new to you, and if it isn’t, then hopefully this will at least get you thinking more about it while on the job. A quick search for “Cycles to Failure” using your favorite search engine can reveal a wealth of information. Detailed research, diagrams, illustrations and a lot of verbiage come up that may or may not make any sense. I however, was able to find a description from Professor Dan Dubina for Cycles to Failure. From that read, Professor Dubina looks to have high levels of experience in Structural Engineering. Although he seems to be dealing with and focusing on structures, the concept still applies to our equipment and I think he states it nicely.
“The fatigue life of a member or of a structural detail subjected to repeated cyclic loadings is defined as the number of stress cycles it can stand before failure.”
In the same presentation he makes another point that can be applied to our gear:
“The physical effect of a repeated load on a material is different from a static load.”
Every product has a limited lifespan when used. Some products last longer than others and we have a direct impact on how long the product will last. There is no difference when it comes to our gear, and one could argue it is even more important considering what we use our gear for.
It is easy to differentiate the importance of this concept when applied to our cell phone charger and our climbing line.
A cell phone charger failing is at most an inconvenience, while a climbing line failure could result in death. A simple yet great analogy of this concept is found when applying it to a metal close hanger. You can bend it several times and it will bend but not fail. The bent area is certainly visible, but again it has not failed. Continue bending it back and forth and failure is imminent. This failure is a visual representation of the Cycles to Failure of the hanger. Our equipment is very much like the close hanger, it never forgets what it has been subjected to.
How do we apply this concept to rope? I feel as though I see more rope failures than failures of other pieces of gear such as carabiners, saddles, or other gear. This is why I wanted to bring this concept up and apply it to rope. We tend to take our ropes for granted at times, especially ones used for rigging.
A rope has a given tensile strength provided by the manufacturer. The tensile is what the rope is capable of holding before it fails. This is where we need to pay attention to the cycles.
We will use 10,000 lbs. as our tensile for this example.
When new, the rope is good for one pull or load to 10,000lbs. If you load your rope right out of the bag to the 10,000 lbs. and it does not fail, then it has done what it was designed to do. If you load it again, and it fails, then it has reached its failure based on cycles. By pushing the rope to its limits, we have created a very short cycles to failure scenario. This is where WLL (Working Load Limit) or SWL (Safe Working Load) come into the equation.
For climbing and rigging, I tend to go with a 10:1 safety factor. Some people go with a 5:1 safety factor for rigging. I just like to keep it consistent and play it extra safe. Basically, if my rope is rated for 10,000 lbs., the WLL is 1,000 lbs., using a 10:1 safety factor. The idea is that I can greatly extend the life of my rope by working within the WLL. By doing so, I have extended my Cycles to Failure.
I have also improved my ability to predict how the rope is going to behave, because I am not working it to its maximum. I can be confident that the pieces I am rigging will not cause the rope to fail.
On the other side of the spectrum, if we pull the rope to 10 lbs. repeatedly, we can make it fail. But it would take millions of cycles on a certain point of the rope to see failure. This is where selecting the correct rope for the job is crucial and making sure that we pay attention and understand the intended capabilities of the rope. Yes, the tensile strength is important as this is how we arrive at the WLL.
However, we need to be aware that it is our job as the end user of the rope to make sure we are using it correctly. If you know you are looking to rig out massive pieces, then look at a ¾” rigging line. Don’t force the ½” rope to do something it is not designed to do by working way above its WLL. Or if all you have is a ½” rigging line, do not be afraid to just take smaller pieces!
Earlier, it was said that our equipment never forgets.
Many times, rope failures are said to be a defect of the rope because the piece that caused it to break was small (this word small is very subjective). I am not disputing the rope failed on a “smaller” piece, however I think it is worth a look into the history of the rope to see where the cycles to failure were significantly impacted.
What size was the previous piece that the rope was used to rig out? Did you use the rope to pull over trees the day before using heavy machinery?
You may have used the rope for months without issue when the pieces were consistently within the WLL. But the one large piece that shock loaded the rope a few weeks back is what decreased the cycles to failure. The rope has had hundreds of cycles on it within the WLL and that one extreme shock load is what ultimately began the failure of the rope. Our equipment always remembers, and we should, too.
What this all comes down to is doing our job and going home at the end of the day. Simply selecting the rope with the highest tensile strength is not always necessary. Larger diameter ropes are more expensive, heavier, and take up more real estate than its smaller counterparts. I am not saying larger ropes do not have a place in our industry. I am just saying that the larger ropes are not always the answer.
We should not rely on any of our equipment to be extended beyond its capabilities, then see a failure and simply say it is defective. We have a responsibility to ourselves, our coworkers, our families, and our customers to be accountable for our work. We cannot perform our work without our equipment. We also have the responsibility to our equipment. If it is responsible for keeping us in the tree and rigging pieces safely to the ground, I think it is worth reciprocating that responsibility by using it within its limits.
Climb safe. Cut safe.