Some of you were probably skiing a few weeks ago. Hopefully nothing bad happened like the cables of the skilift failing.
Those cables are actually rather prone to failure but luckily the cables are designed to withstand a lot and there are easy methods to check the cables and replace them if necessary, before any real dangerous situations occur.
First lets go over the different ways the cables can fail. The most obvious one is of course broken wires, which happens rather frequently, but because the rope actually is made from several smaller ropes twisted together as you can see in figure 1
Other possibilities include but are not limited to: birdcage (the outer layers of the rope becoming longer than the inner ones, as seen in figure 2), loose wires, loop formations, nodes which can be seen in figure 3 and many others
These problems are all caused by either fatigue or fluctuations in temperature.
To reduce the amount of time needed, the wires/ropes are tested while still in place in the ski lifts, and only the faulty ones are removed, but how do they check them?
The first step is a visual inspection, most often at the spot where the most wear will occur, the pulley wheel. Broken wires as in figure 1 are easy to spot in this case.
Next is measuring the diameter of the rope. This happens with a simple caliper as we’ve probably all used one before. If the diameter is reduced, either in the whole rope or only in some sections, this is an indicator for loose wires or possible birdcages. The diameter can reduce a bit, but the more it shrinks, the closer the wires will be together and the more friction can occur between them, leading up to failure once more.
A simple trick to test for loose wires is to try and pry a screwdriver into the rope. If you manage to twist a screwdriver in between the strands, you know something’s wrong and a closer inspection is necessary.
If all above tests indicate something wrong a final test is performed: electromagnetic wire rope examination. (this test is also performed periodically)
This tested can detect internal fractures and confirm the wire should be replaced or not.
The small, portable device can be seen in figure 4, but how does it work?
The apparatus is placed over the wire rope and either the wire is ran through or the machine is lead across the whole rope. The principle of this testing method is that of induction: the inspection equipment carries an AC current and because the wire rope is made out of a conducting element the changing magnetic field generated by the current interacts with the eddy currents induced within the cable. By measuring variations in the flowing current or a shift in either phase or magnitude of the eddy currents, faults can be detected because a crack in a wire strand causes different induced currents.
Thank god for non-destructive engineering to provide us with the tools to make sure our ski lifts are safe (and of course every other application that uses wire ropes) This is once again a great example of how ingenious solutions and methods can make our lives safer and our equipment more reliable.Would you go up the mountain otherwise?