Wednesday, October 22, 2014

Dealing With Really Strong Rare Earth Magnets

In the video this week, I solved a technical problem for some librarians, building a very simple jig out of 2x4 and conduit to separate some extremely powerful magnets.  At the time, I was working very intuitively...and it turned out fine.  Actually, this is the only Handyman Kevin video so far that I was able to record in a single take.

I thought it would be interesting to actually look at how an engineer would deal with this problem.  I once flunked out of a pretty good engineering school and I have actually spent a few years working as an engineering technician, so I definitely know enough to be dangerous.   This digression does raise an interesting point, though, which I would like to emphasize.  Even though we handyman types spend 99% of our careers working by intuition and rules of thumb, there does come a time when you need to stop and think about theory a bit.  It can be the difference between whether that bearing wall collapses and takes the client's house (and maybe you) with it or not.

But anyway, back to magnets.

A (very cursory) look around the web indicates that magnets this size have a pull force around 200 lbs.  Remember, though, that the force between two magnets is twice that much.  Actually, since there were three magnets in the stack, it was even more.  How much more?  A quick trip around the web reveals some disagreement between physicists about the exact formula.  Engineers, however, seem to do well enough applying the inverse square law to magnets separated by distance. Unfortunately, I haven't been an engineer for a long time.  My Googling skills are top notch, however, and a few moments later I found this magnet calculator from K&J Magnetics.  After fiddling around a bit, I was pretty sure that the force on the holding the end magnet to our stack of three magnets was around about 240 lbs.

Fortunately, we didn't have to tear it straight off; we could slide it off sideways--shear force, in other words.  One thing I did remember from engineering school, is that the static friction force between two magnets is equal to the coefficient of friction between them, times the normal force pressing them together.  A little more Googeling showed that the coefficient of friction between these sort of magnets is about 0.2.  Therefore, the amount of shear force we needed was about 240 lbs x 0.2 = 48lbs.  This was a little more than I could generate with my bare hands, mainly because there isn't a good way to get leverage.  With our scissor-type separator jig, though, this was no problem.

The scissor jig uses leverage for a mechanical advantage, trading distance for force. The force required out at the end of the handles is only about 1/3 as much as at the magnets, or 16 lbs.  Even a beat-up handyman like me can put 16 lbs on the ends of some 2x4s.  

To there's your lesson for the week:  engineering mechanics applied to 2x4 technology.  Perhaps next time we will delve into more high tech materials.  A rock, perhaps, or some bailing wire.


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