Understanding the relationships between rigging rope Break Strengths (BS), Safety Factors (SF) and Working Load Limits (WLL) is standard practice in arboriculture (you can find more information here and here).
However, tree riggers often overlook the relevance of the safety factor, or working load factor, when estimating the value they will get from their new rigging line.
The higher the working load factor (SF), the greater the number of lifts you'll get before failure of the rope (yes, all ropes do fail, eventually). The lift numbers compound.
Assume that you have seven identical rigging lines, each with a 3,000 kg break strength capacity, and you work these ropes daily, with each rope lifting a different load, as shown in the table below.
The table shows that higher working loads relative to the rigging line break strength significantly reduce the rope's service life. For example, if you have a 3,000 kg break strength, you can lift a 2,800 kg load about five times before the rope fails. But if you lifted a 500 kg load of the same rope, you can do it 1,000 times before failure.
Increasing the working load factor (SF) directly correlates to the number of lifts you'll get out of the rope. That is high cycles of use before failure, compounding the effect and ensuring better value for money from your rigging line purchase.
Treetools has the luxury of owning an in-house break testing machine, and we break gear on a daily basis. The figures in the chart ring true for ropes and hardware. Higher safety factors make the job safer and extend the working life of your gear. Period.
*Relative values only. The higher the working load factor, the greater the service life, and the lower the replacement factor. Thus, a working load factor also directly reflects an economic factor. If you are consistently lifting the same weight, then the more substantial the rope, the higher the working load factor, and the longer the rope will last. Thanks to Samson Ropes for the original data.
