Dynamic Energy in Arborist Rope | Custom Cordage
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One of the most misunderstood aspects of rope selection is the disconnect between breaking strength and a ropes ability to absorb dynamic loads. Many people wrongly assume that the stronger a rope’s rating the harder it is to break. That is not the case as a rope can be parted if called upon to carry a load greater than its breaking strength or, if asked to absorb a dynamic load greater than its energy absorption capability.
Everyone is used to talking about a rope’s breaking strength but almost no one describes a rope’s energy absorption capability. This is obtained by studying a stress strain curve of load vs. elongation. The area described under the stress strain curve pertains to force acting through distance (or the work required to break it).
On the previous page you will find the bar graphs of our ropes’ dynamic characterists. The first shows each types’ working energy absorption which describes how much energy each will absorb before reaching its working load, which in the case of rigging ropes is 1/5 or 20% of its breaking strength. The more work the rope can do getting to 20% of breaking strength the longer it will last. A very stiff rope, with little or no elongation, gets to its working load without doing much work and quickly becomes loaded beyond its safe working load, regardless of how strong it is. These ropes are poor choices for rigging with the single exception where stretch cannot be tolerated, such as working with a zip line over a roof with limited clearances.
The second graph is of the lines ultimate energy absorption. This represents the amount of dynamic loading a line will take before it parts. If you subtract the working energy absorption from the ultimate it tells you how much reserve energy you have to play with. *See supporting graph on bottom of previous page.
A further complication is that these numbers are expressed as foot lbs/lb of rope in tension. So to calculate the rigged lines’ energy capabilities you need to know the length of the line that will bear the load.
A couple of examples may help:
Example 1 – We will use 5/8 diameter Double Esterlon line rigged into a tree with a block in such a way that 25 ft. of line is required to arrest a 500 lb section of trunk falling 5 ft. From the Double Esterlon specification table and energy graph we will need its weight of 13.7 lbs/100 ft or .137 lbs/ft, its green working energy absorption maximum of 544 ft lb per lb of rope in use, and its maximum recommended working load of 3,400 lbs.
First, we will calculate the ft lbs of energy needed to arrest the 500 lb trunk section falling 5 ft. The simple equation of the weight multiplied by the fall will get the result within 1%, so
• 500 lb x 5 ft = 2500 ft lbs.
Next, we will calculate the lines energy absorption capacity for a 25 foot length
• 25 ft x 544 ft/lb x .137 lb/ft = 1863 ft lbs.
From these two calculations we can see that in this scenario the maximum recommended energy absorption is exceeded by 637 ft lbs or 34% (2500 ft lbs / 1863 ft lbs). We can also estimate the load reached in the line multiplying the maximum recommended working load by 134% or
• 3400 x 1.34 = 4,556 lbs.
To illustrate the importance of energy capacity of ropes we will take a look at using a high energy absorption line.
Example 2 – We will substitute a 5/8 diameter Polydyne. Same diameter, but very different energy capacity. Doing the same calculations with Polydyne’s physicals we get the following:
• 500 lb x 5 ft. = 2,500 ft lbs. required
• 25 ft x 1040 ft/lb x .133 lb/ft = 3,458 ft lbs. capacity
In this case, we have reserve energy absorbing capacity of 958 ft lbs and the peak load in the line is estimated at:
• (2500/3458) x 3600 lbs = 2,602 lbs.
The more area in the stress strain graphs (green working and red ultimate) the higher the ropes ability to absorb dynamic loads.