Through the last couple of posts, we have looked at several factors that affect ropes.
We first looked at rope construction and the different varieties that are available for us to use; and from a high level how each type of construction is applicable on the job.
We then took a turn into the world of Cycles to Failure and how this determines the lifespan, performance, and ability of the rope. Cycles to Failure is an important concept and one that is applied to all of our gear. There is another, more obvious part of rope construction that certainly comes into play, and is very important when understanding ropes and their uses: materials.
We are past the days of using Manila ropes, and although they are still manufactured and used widely, they are not present on tree care job sites. All of our climbing lines, rigging lines, prusik cordage and winch lines use synthetic fibers in their construction. Some of these fibers are very common, while others are less common and much more expensive. The fibers used play a very important role in how the rope performs.
The most widely used fibers in climbing and rigging lines are nylon and polyester. Looking at the two types of fibers, it would be difficult to distinguish one from the other, but once the small fibers are combined to make strands and then braided into a rope, the differences in the two are what make ropes perform the way they do.
Out of the two fibers, nylon is only slightly stronger, but does not perform as well in wet environments as the fibers weaken when wet. However, it does perform well when it comes to elongation associated with dynamic loads.
On the other hand, polyester does fine with wet environments, but does not perform as well with dynamic loads when compared to nylon ropes. Both materials perform well with abrasion resistance, UV protection and both resist rot. We could geek out on the small differences between these materials, but from a high-level perspective, let’s keep it simple.
With the simple differences listed above, and what we have discussed previously about construction types, how does this come into play with our climbing lines and rigging lines? The type of construction and materials used are key factors in the functionality of the rope and should be part of the decision making process.
Most people look directly at tensile strength and base their choice of a rope solely on this factor. Although this is a factor in selecting the right rope for the job, it is not the only factor to be considered. When looking at tensile strength, it is important to use a safety factor of 5:1 or 10:1 to get the Working Load Limit (WLL).
Look at two ropes, and compare the tensile strengths of each. Even if one is rated 3,000 lbs. higher than the other, when you break it down to the Working Load Limit, it is only a 300 lbs. difference (using a 10:1 safety factor). Going back to Cycles to Failure would direct us that we should be working within that limit to prolong the life and performance of the rope. Yes, tensile strength is important, but it should not be the only reason to purchase a rope.
Let’s compare two very popular rigging lines and see how these two rigging lines with the same type of construction perform differently due to the materials used.
Stable Braid rigging line by Samson Rope has been around for a long time and has proven to be a workhorse over the years. It is a double braid with a polyester jacket and core. It is best used with blocks or aerial friction devices with a friction lowering device at the base of the tree.
Yale Dynasorb has been around long enough to also prove its worthiness in the tree care industry and like Stable Braid is available in multiple diameters. Dynasorb is also a double braid rigging line, and uses polyester for the jacket, but utilizes nylon in the core.
On paper, the WLLs for each rope are similar. Both are double braid rigging lines, both handle well, and both are great with knots. So what is the difference? The core materials used are where the main difference comes about. Because Dynasorb uses nylon in the core, Dynasorb can mitigate the dynamic loads associated with many rigging scenarios. This is accomplished due to the nylon core and the elongation characteristics of the material. Many people refer to this as “stretch” and speak of it like it is a bad trait of a rope. In some situations, it could be a disadvantage, but in other rigging scenarios it may very well be a positive attribute.
Absorbing dynamic forces is great, so why not use this rigging line for everything? One time I would not want to use Dynasorb is while using a 5:1 mechanical advantage kit as I would need to pull quite a bit to remove the “stretch” from the line before applying the force to the object I am looking to pull or move.
When left with a short spar, and the ground crew is left with short runs on the portawrap, the climber may appreciate the dynamic energy absorption Dynasorb provides. Even on longer runs with larger pieces, the rope could soften things up for the climber and ground person running the portawrap. But if you are using this rope above a house, keep in mind that it will absorb dynamic loads, but it elongates to do so. Be vigilant when rigging over structures.
Also, remember that just because it is designed to absorb dynamic loads, it is not free from being subject to Cycles to Failure. The rope remembers everything!
Again, it is all about getting the right rope for the job. One other thing to keep in mind, regardless of what rigging line you currently have, you can increase the capabilities of the rigging line by adding in redirects throughout the canopy. The more line in the system, the better capable the rigging line is at absorbing dynamic forces. It also helps spread the load throughout the canopy, which is often beneficial.
Applying this to climbing lines, you would find that climbing lines with a nylon core may elongate more or you may experience a little more bounce on ascent. This will ultimately come down to personal preference in climbing line selection. If the rope seems a bit “stretchy” or bouncy when using it in a Moving Rope System, this characteristic will likely be magnified if using it in a Stationary Rope System.
Ultimately, choosing the right rope for the job is not determined only by the tensile strength. Take into account your climbing and rigging set ups. Look at the different construction of ropes and make certain they are compatible with your set up. Keep in mind Cycles to Failure – and that rope, just like every piece of gear, remembers everything. Store it properly and take care of your rope. We expect it to do its job so we should make it a priority to take care of it. As always, when in doubt replace it! And it is never a bad thing to just take smaller pieces.
Climb safe. Cut safe.