This article covers exponential deterioration in wire rope, and how if it is not spotted, can quickly lead to sudden rope failure.
By the end of this article, you should understand exponential deterioration in wire rope, how to spot it, and the relevant actions to take.
At a fundamental level, a rope failure can only occur when the load acting on a rope exceeds the actual breaking load (ABL) of that rope.
Therefore, only two things can cause a rope to fail:
In most cases, failure will be caused by a decrease in ABL rather than increasing load.
Diagram illustrating the point of failure in wire rope as relating to the ABL and the load.
The factors that contribute to a reduction in ABL are:
For the purpose of this article, we will define a sudden rope failure as a rope failure that occurs without prior warning.
As the condition of a wire rope deteriorates, the likelihood of further deterioration increases. Therefore, the speed of deterioration increases. This acceleration is known as exponential deterioration.
A chart taken from the Rope IQ software showing exponential deterioration in wire rope.
Consider the features in the above chart. We can see:
The takeaway from this chart is that the rope has gone from a very safe condition to a dangerous one within two months. The rope would likely be in terrible condition before the next inspection if the usual 6-month inspection period was employed.
When making rope decisions, it is essential to not only look at the last inspection but to also look at previous inspection history. Without doing so, you may not spot exponential deterioration, which is endemic in the causes of sudden rope failures.
It is necessary to utilise an inspection framework that allows you to quantify the condition of the rope, to spot exponential deterioration. The framework could be ISO 4309, or the use of more quantitative methods of inspection such as NDT.
If you spot exponential deterioration, you may need to ramp up the frequency of your inspections to ensure the rope does not reach a dangerous condition.
Another slightly more expensive option is to use a continuous monitoring technique.
The purpose of this article is to highlight how some 'sudden' rope failures are not sudden whatsoever and are sometimes highly predictable.
With proper analysis of inspection data, and by taking a 'time-line' approach to interpreting results, the probability of the occurrence of a sudden rope failure can be reduced dramatically.
Thanks for reading this far! In this article, we have covered exponential deterioration in wire rope, how to spot it, and some actions to take to deal with it.
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