Kirby coupler magnets on the surface

[Will Vale]

Hi all,

 

I'm intending to use Brian Kirby's automatic couplings on my modern image layout. But unfortunately I didn't find out about them until I'd laid the track. I really don't want to lift it, and I can't easily fit under-board magnets since the board substrate is 50mm Styrofoam. What I was hoping to do was use small cylindrical or cuboid Neodymium magnets (from eBay) fitted between the sleepers - that way all I have to do is drill a hole where I want one, and I can test them out without even doing that. It should also be easy to make larger uncoupling sites by placing two magnets in the gaps between three sleepers.

 

What I'm trying to find out if whether this will work, and what size magnets would be best? Ideally whatever I use will be strong enough to sit a couple of mm below the sleeper tops so they can be ballasted over cleanly. I've found some 3mm cylinders in various sizes from 3mm deep to around 10mm, similar sizes for blocks. After that they get larger so more surgery will be required. Another option would be to cut out sleepers and the cork and place disc magnets, but the thin ones seem like less work.

 

If no-one's tried this then I'll just get a mix of sizes and have a go, but if anyone has I'd love to hear your experiences and recommendations.

 

I've had a good search around on new and old RMWeb via Google's cache and I don't think this has been covered (what I found seemed to be mainly about large under-board Ne magnets).

 

Many thanks!

 

Will

[34theletterbetweenB&D]

I have used some high field strength ceramic block magnets 6X6x8mm under the sleepers, and these work very well. They are oriented so that the field lines loop up above rail level in a plane parallel to the vertical plane of the rails, so that the piece of iron on the coupler is drawn into the magnetic circuit. Worth inspecting the field patterns of whatever magnets you purchase using iron filings, to see what the optimum orientation is.

[Will Vale]

I have used some high field strength ceramic block magnets 6X6x8mm under the sleepers

 

Thanks for the reply! That's a bit bigger than I was hoping for, but I'll put something similar sized into the order and I can try it out. The iron filings tip is great, I'll have to get some. It's like being back in school

 

Will

[34theletterbetweenB&D]

Don't worry so much about the size of the magnets, these were simply convenient items left over from an industrial project. The point about imaging the field lines with iron filings, is that to get the most out of any given magnetic field in this sort of application, the magnetic field should be arranged such that the action of the moving iron takes it to a position where it provides a shorter circuit for the field.

[Will Vale]

I'm afraid I'm trying to imagine exactly what that means. With a bar magnet, the field lines run through the core and out of each pole, then loop back around the outside to the other pole, is that right?

 

So would making a shorter path mean

 

a. having the bar parallel to the rails (and the staple) or

 

b. (given that the staple is at an angle when the coupler is disengaged) to have the bar vertical and space the magnets so that the staple outer ends sit above the pole centres?

 

From your explanation thinking b is correct - the tail of the disengaged staple would maybe give a shorter path around the end of the magnet than the air, provided it was mainly interacting with the field on one side, but I really don't know for sure. a would seem to make sense only if the staple moved vertically down without tilting?

 

Sorry, a bit clueless here. I studied maths not engineering, and even if you go for applied maths it tends not to cover useful stuff like this

 

Will

[34theletterbetweenB&D]

You have also fallen foul of the terrible 'text aliasing as emoticon' bug!

 

With your bar magnet example: the field lines in air are a curve; you want the piece of ferromagnetic metal to be pulled into a position where it is a chord inside the curve, thus shortening the magnetic circuit.

[Will Vale]

I did indeed - I wasn't expecting list item b to be Pacman in sunglasses. It took me about 3 attempts to edit it out, as well.

 

Thanks for your excellent explanation, that confirms what I was thinking and is a really neat example of clarity through brevity as well. I'll try and report back when I get something working,

 

 

[Will Vale]

My magnets arrived last week - sent very promptly indeed from the USA.

 

Experimenting, I reckon the 10mm diameter/3mm deep discs are the best bet - I can remove a sleeper, dig out the cork and mount the magnet to the MDF base, then replace the sleeper. My hooks lifted every time this way, but I didn't see any drag on the axles, which I did notice when using smaller magnets (10x3x3mm bars) between the sleepers.

 

A couple of other points of interest:

 

1) For my test, I soldered the staples to the coupler hooks. So far, this works really well - neat and quick despite my usually cack-handed soldering efforts. I'll need to re-blacken or paint the hooks though since I had to sand off part of the blackening to make the joint.

 

2) When propelling a wagon across a magnet, the coupler arm on the propelled wagon will lift and drop again with a clear "tick-tick" sound. Is this normal? I noticed that it was more pronounced when I had some of the staple metal in the vertical plane (soldered to the coupler arm's descending tail) which I think is a magnetic thing - when the vertical point passes the centre of the field it becomes a better short path than the horizontal arm, returning the coupler sharply to the horizontal position.

 

So far I'm a happy bunny - I like how amenable the original recipe is to modification, and how easy it was to get something working.

 

Cheers,

 

Will