Amazing Magnetic Testings

Ski lifts of any kind be they T-bar, chair, gondola and cabin lifts must be yearly inspected as per tough regulations.

Non destructive tests are carried out from beginning May up to October by certified operators (Level 1) or certified inspectors (Level 2), quite often mountain dwellers with an additional training in “acrobatic conditions” work.

These tests are much tougher than those on the roller coasters or other “high-risk merry-go-rounds”. They make ski lifts much safer, which can be checked with the ratio number of passengers/accidents with or without wounded people or deaths.

The main NDT methods used than are:
• For the cables: an electromagnetic method using Hall-effect probes.
• For all the other parts: Eddy currents (ET), penetrant testing (PT) and magnetic testing (MT).

As an example welds on the pylons are checked in situ using MT with colour contrast products.

Nowadays these pylons are made of hot-dip galvanized steel. Therefore, a contrast paint is sprayed then, when touch-dry, a black magnetic ink is sprayed. An electromagnet induces the magnetization, rarely a permanent magnet (should no electric supply be available).

After test, the white contrast paint remains on the weld beads and HAZ (heat affected zones). The local technical authorities of the ski lifts may ask the service companies in charge of MT to remove this paint. It is a long and boring task, hence a main reason to use a peelable paint. Anyway, this layer’s thickness shall be 50 µm (0.002”) maximum for performing an MT: might as well say that it shall be removed before the next inspection unless one may use the layer of the previous year as a contrast background, if enough is left!

A small anecdote just for fun: one of us was questioned by a service company in the ‘80s. The guy would like to know whether he could perform an MT on a cable-car’s parts covered with a light colored paint 200 µm (0.008”) thick. The answer: a clear “no”, this is too thick to perform any MT.

Paint was not removed, MT was performed. In our best knowledge, no incident or accident occurred later.

Let us come back to our pylons. Some are covered with a light colored paint layer whose thickness is small enough to allow for the inspection without removing it and without need for any contrast paint.

It happened that on such a pylon, an inspection mandatory every decade displayed an impressive indication all the weld long. This experienced man wiped out the indication with a rag, then performed again the test putting the electromagnet poles in an other position. Same impressive indication.

He had an idea - a very peculiar one: he took a screwdriver from his tool box, put the tip on the weld area … and pushed the screwdriver. You may believe it or not - but believe it, it IS a true story: the screwdriver went far into what was supposed to be a weld without any effort! In fact, this area, built 20 years ago, had not been welded! And the mandatory inspection ten years later had not detected any discrepancy!

We may guess that, fortunately, safety margins are calculated well above the needs…but at least two people did not do their job properly!

In the '60s a UK Company had a motto: "For every surface treatment problem, there is a product xxxxxx" (trademark we do not display).

Engineers and Commercial people in this Company had made a "translation": "With every surface treatment product xxxxxx (trademark we do not display), there is a problem".

This anecdote is there only to remind everyone that problems met in workshops may be due to the suppliers/manufacturers as well as to the users.

Our idea in these documents is NOT to target anyone, but on the contrary to bring to your knowledge some interesting cases which may prevent you to duplicate the same mistakes while performing Penetrant Testing (PT) or Magnetic Testing (MT).

All the ministories you will read are TRUE. We think they will be helpful:
• First as examples of specific technical --or non-technical-- requirements or peculiar problems.
• Second to let you see that the problems do not always come where you think they should come from.
• Third so that users feel free to ask for help from people (the experts) who may know more than they do.