Sunday, 2 December 2012

Magnetic Couplings - Important Information


One of the best ways to kill a good idea is to not define standards early on, so that everyone does it slightly differently, and no two systems can work together. Examples include VHS & Betamax, mobile phone chargers, and many more. There is one critical parameter of the McBogle (also known as DOGRF) Mk.2 coupling that I haven't yet defined and will now attempt to do so. For two couplings to mate correctly, they must both have their N and S poles in the same orientation, so that on mating, N attracts S and S attracts N. Having made about a dozen of these couplings now to an arbitrary standard, I have measured which are the N and S poles and will define this as the McBogle Mk.2 magnetic coupling standard. In that way, any couplings you make should work with anyone else's, and vice versa.

The tools you need are an ordinary magnetic compass (and no, a GPS receiver won't work unless it includes a magnetic compass), and a bar magnet (one of the magnets you are using for the coupling will do). Glue your magnet to the end of a non-magnetic stick with the line between N and S in line with the stick. (If you are using the same magnets I did, then one of the two flat faces is glued to the end of the stick.)

Point the magnet end of the stick at the compass, and move it all the way round the compass. If the red (N) end of the compass needle points to your stick, then we will say that your stick ends in a N pole. (I'm not sure whether this is the normal convention, but that doesn't matter provided that everybody follows the method exactly as I have described it.) If the red (N) end of the compass needle points away from your stick, then your stick ends in a S pole. Mark your stick with either S or N at the magnet end, and the opposite at the other end. You can now use the magnet end of your stick to check the polarity of any coupling or magnet, bearing in mind that N and S are attracted, and N and N or S and S are repelled. Couplings can also be checked by ensuring they mate correctly with a known good one. A known good one can also be used to hold a new pair of magnets in position while they are glued to their "pipes". I will try to add some photos to make this clearer.

And which way round should the coupling magnets be? When looking at the end of a vehicle on the rails with a coupling fitted, the mating face of the right-hand coupling magnet should have its N pole toward you.

Finally, a VERY IMPORTANT SAFETY WARNING. If you bought your magnets from a reputable dealer, you will have been given some dire warnings. It is worth reiterating that these tiny magnets can cause life-threatening injuries if accidentally swallowed. So keep them in their original packaging until assembled, and don't leave them laying on the work bench. And don't fit them to anything that could be used by children. If you pass them to anyone else, also pass on the warnings too. It's all common sense really, but so easy to forget or ignore, with potentially serious consequences.