Neodymium magnets question

[BanjomanParticipant @banjoman]

Hello all,

I wanted to ask whether anyone here has knowledge or experience of using small neodymium magnets (like for example these: **LINK**) on board a model ship, and in particular if there are any potential problems with introducing their comparatively strong magnetic fields into an environment where there are also electronics and radio systems at work?

I am specifically thinking about using this kind of magnets to secure the (removable) wheelhouse roof on my Eilean Mòr puffer build:

Given that this sits some distance away from the innards of the boat, where radio and electronics live, and the quite small magnets I'd like to fit (2×1 or 2×2 mm, I'd say), I don't particularly expect any problems, but thought I might as well ask for some advice before going to the trouble and expense of getting hold of such magnets.

Many thanks in advance

/Mattias

Edited By Banjoman on 05/11/2014 09:15:09

[BanjomanParticipant @banjoman]

Any potential problems to be aware of?

[Malcolm FraryParticipant @malcolmfrary95515]

The only electronic components that have any chance of being affected are Hall effect devices, reed switches and relays, and this is because they are the only components that react to a steady magnetic field, and they have to be very close to the magnet. A fixed magnet can have no effect on radio, so no worries.

[Dave MilbournParticipant @davemilbourn48782]

Mattias

I've used strong magnetic cupboard catches for this purpose and had no trouble. My schoolboy physics tells me that a permanent magnet is only a problem when it's moving near a conductor, when it will induce a current – but only while it's moving. These aren't moving, and they are TINY!

Dave M

[ashley needhamParticipant @ashleyneedham69188]

It may be best to get slightly less-strong but larger magnets as there is not much gluing surface of the very small ones and they can pull off the surface too readily. OR too powerfull a magnet means you have to apply a lot of force to the superstructure/hatch or whatever to pull it off and damage it.

Unless you want to go out in gale force conditions I would stick (pun) to "just enough" magnetic force.

Ashley

[Geoff SleathParticipant @geoffsleath41411]

No problem at all. They're commonly used in aeromodelling to secure hatches etc and the consequences of radio problems are far more likely to be terminal than in model boats. The magnetic battery hatch on my foamie Mk24 Spitfire is so strong you have to keep your fingers out of the way as it snaps shut and the receiver antennae are just below it.

As Dave says, currents are induced when a conductor is moved through a magnetic field but really it just isn't a problem at all in any case.

Geoff

[BanjomanParticipant @banjoman]

Warmest thanks to all of you for these equally helpful and reassuring answers!

Ashley, I had indeed thought about the potential problem of the available gluing surface, but as the installation area is rather limited, I think that smaller size shall have to be given priority. I plan to drill into the top of the four corner posts (which measure +/- 3,5×3,5 mm) of the wheelhouse and secure the magnets into these holes with epoxy, which is the adhesive recommended by what looks like a serious magnet supply company, Magnet Expert Ltd (**LINK**). In order not to make the magnetic joints too strong, I am at this stage planning to just set small steel nail heads into the corresponding underside of the roof frame. Or possibly the other way round (in which case the principle still applies).

Given the pull of these very small magnets, in the region between 100 and 150 grams each, I'm thinking that the forces involved should be neither too weak nor, hopefully, too strong. The wheelhouse roof is already to some extent secured by plywood tabs sticking down into the top of the wheelhouse, but these tabs are quite shallow and I want something that, although not resistant to a Force 8 wind or summat of that ilk, will at least put up enough resistance to prevent a normal breeze from catching under the eaves of the roof and depositing it on the pond surface …

Again, many thanks to everyone!

/Mattias

[Malcolm FraryParticipant @malcolmfrary95515]

To allow relays to release reliably, a small air gap is left between armature and yoke, and ensured by the presence of a not magnetic piece. Commonly a brass stud of a few thou thickness, on occasion a slip of plastic. Sliding the magnet past whatever it is gripping is a good option. With older type magnets, a direct pull away would reduce the power, but sliding past reduces the chance of the magnet remaining fixed to the target plate while becoming detached from its mount.

[BanjomanParticipant @banjoman]

Hi Malcolm,

Many thanks for this suggestion — I had indeed already been toying with airgap ideas!

Unfortunately, sliding the roof off won't be an option, as the tabs securing it laterally when in place will first have to be lifted free of thir slots by about 1,5 mm, which will inevitably mean first breaking the magnetic bond by a a straight lift. As soon as the magnets I've ordered arrive I therefore plan to carry out a couple of tests off the model first, before installing any magnets in the wheelhouse structure, to get a hands on idea of the forces involved.

/Mattias

[Armando LoniParticipant @armandoloni78860]

I use them on all of my boats – mounting them in a countersunk hole, as you suggest. One slight problem I've found is that the external coating of the magnets can come off with time – the field strength is not affected but the exposed surface of the underlying magnetic alloy can subsequently oxidise, becoming a bit rough and allowing pieces to come off. Hence, cotaing them with paint/varnish or eopxy is a good idea once installed.

[BanjomanParticipant @banjoman]

Thanks Armando! As a matter of fact, mine arrived on Monday and I installed them yesterday evening, more or less in the way described earlier in this thread. However, I found that the 150 gram pull of a single 2×2 mm N42 magnet was on the weak side for my needs. Given the limited surface area available on the top of the wheelhouse corner posts, it was not a realistic option to use any much larger diameter magnets, either. Although I could have just managed to fit 3×3 mm ones (which would have provided some 290 grams of pull each) I hadn't bought any of those, so I instead decided to use what I had to hand by putting multiple magnets into each corner post.

The single ones were thus carefully pried out, and the holes drilled down to a depth of 10 mm, i.e. deep enough to fit a pile of magnets five deep.

According to the FAQ on the First4Magnets website (**LINK**), doubling up identical magnets will also double the magnetic force, but only as long as the depth of the pile does not exceed its diameter, i.e. putting two 10×2 disc magnets on top of each other provides the same pull as a single 10×4 one. However, this apparently no longer holds true once the depth becomes greater than the diameter, as in my case with a pile of 2 mm diameter by 10 mm deep.

The FAQ further says that a certain increase still occurs, but no longer at the rate of doubling up. It does not say what that other rate might be, but if one were to assume for the sake of argument that the decrease were a halving of force for each doubling of the depth beyond equal to the diameter, a staple of five 2×2 150 gram pull magnets would give 150+75+37.5+18.75+9.375=290.625 grams.

I have no idea whether this formula holds true, but at least I'd say that from a simple feel-by-hand testing of the different forces involved I think it reasonable to assume that my 10 mm staples provide a pull of some 300 grams or so each, or at least something in that ballpark.

In any case, I also countersunk four 2,3×2,3 mm steel nail heads in the corners of the roof, which is now rather more secure but easy enough to remove by simple hand force when needed.

I will certainly also take your recommendation ad notam, Armando, and shall varnish the tops of the magnets forthwith.

/Mattias

Edited By Banjoman on 12/11/2014 12:51:22

Edited By Banjoman on 12/11/2014 12:54:10

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