Dynamic Energy in Arborist Rope

Because of the wide range of rope use, exposure to the several factors affecting rope behavior and the degree of risk to life and property involved, it is impossible to make blanket recommendations as to working loads. However, to provide guidelines, working loads are tabulated for rope in good condition with appropriate splices, in noncritical applications and under normal service conditions.

A higher working load may be selected only with expert knowledge of conditions and professional estimate of risk, and if the rope has not been subject to dynamic loading or other excessive use; if the rope has been inspected and found to be in good condition, and is to be used in the recommended manner: and if the applications do not involve elevated temperatures, extended periods under load or obvious dynamic loading, such as sudden drops, snubs or pickups. For all such applications, consult Yale.

Many uses of rope involve serious risk of injury to personnel or damage to valuable property. This danger is often obvious, as when a heavy load is supported above one or more workers.

An equally dangerous situation occurs if personnel are in line with a rope under tension. Should the rope fail, it may recoil with lethal force. Persons should be warned against the serious danger of standing in line with any rope under tension. In all cases where such risks are present, or there is any question about the loads involved or the conditions of use, the working load should be substantially reduced. Minimum breaking strength is based on test data of new, unused rope and is a value not greater than two standard deviations below the mean.

Dynamic Loading Voids Normal Working Load

Normal working loads are not applicable when rope is subject to significant dynamic loading. Instantaneous changes in load, up or down, in excess of 10% of the line’s rated working load constitute hazardous shock load and would void the normal working loads.

Whenever a load is picked up, stopped or swung, there is an increased force due to such dynamic loading. The more rapidly actions occur, the greater the increase will be. In extreme cases, the force put on the rope may be two, three or even more times the normal load involved and may result in the rope parting. Examples could be picking up a tow on a slack line or using a rope to stop a falling object. Therefore, in all dynamic applications, working loads as given do not apply.

Users should be aware that dynamic effects are greater on a low elongation, high-modulus rope such as Aramid and lesser on a higher-elongation, nylon-based product. Dynamic effects are greater on a shorter rope than on a longer one. The working load ratios listed contain provision for very modest dynamic loads. This means, however, that when the working load has been used to select a rope, the load must be handled slowly and smoothly to minimize effect and avoid exceeding provision for it.

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.