Magnetic Wheel Design Fundamentals

[Disco_Wheels]

Hello good Ladies and Gentlemen,

For an educational robot I'm building, I need to make a magnetic wheel arrangement and need some technical advice on design.

Sincere thanks to anyone who is able to help, I'm more software oriented and my high school physics has escaped me.

Basics:

-I have some Neodymium ring magnets (72mm dia x 13mm width and 32.75mm dia hole) that I need to use.
-The magnets are magnetised along the 13mm (width) direction (they are not radially magnetised)
-The magnets are rated to ~60kg pulling strength (assumed on the flat face pane).

Questions:

-If I make a sandwich using two of these magnets, with a non-ferrous spacer between them and steel disks as the "bread" on each side; will that be the optimal arrangement to direct the magnetic field outwards to make them function as strongly as possible as magnetic wheels? (I'm assuming that the magnets should be arranged in the sandwich so as to repel each other).

-If the above is true, what formula should I use to calculate the thickness of the end disks and non-ferrous spacer?

Thanking you all in advanced for any help or guidance you could give.

 

[MagMike]

You'll get better performance if your 'sandwich' is made of two steel (magnetic) discs/plates and one magnet between the steel.

There isn't a direct equation/formula that can be used to optimize the steel thickness, but it'll be less than the thickness of the magnet.

Please be careful! That is a big & strong magnet.

 

[Disco_Wheels]

Thanks MagMike!

So there's no benefit to using 2 magnets in the sandwich?

Or if I do use 2 magnets, then you say it's better if it's a "club sandwich" with a steel (ferrous) spacer in the middle of it all instead of a non-ferrous one and steel disks on both sides?

Yes the magnets are crazy strong, already exploded 2 of them by carelessly leaving them close(ish) on the bench together: Slam...shatter...ball of very magnetic fragments stuck together

 

[EdStainless]

After you build a wheel you will need to test it some.
You want the magnetic strength very near saturation in the steel pieces.
And you may need to machine the outside 'rims' after assembly and bonding in order to get them to match up well.
You are going to have to use a non-ferromagnetic hub inside this so that you don't shunt the field.
Likewise you want the hole in the steel a little larger than the hole in the magnets.

It took about 10 years after I left the magnet business for the last fragments of metal to work out of my fingers. The worst was holding a magnet down with your thumb in a pulse magnetizing coil and having the magnet disintegrate when you pulsed it.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagMike]

If you want to use two magnets per wheel, use the following construction:
Layer 1: Steel
Layer 2: Magnet
Layer 3: Steel
Layer 4: Magnet
Layer 5: Steel

The North pole of the magnet in Layer 2 should point at the North pole of the magnet in Layer 4. They will be in repulsion, but the steel spacer in between them should counteract the repulsion forces. You'll have to experiment for optimum arrangement.

 

[MagBen]

Are you making a sandwich for two wheels as a system, or, one magnet one wheel using the non-magnetic spacer to separate wheels, and the two magnets stay in repulsion? It all depends the arrangement and dimensions, the steel may or may not help. One of biggest concerns is to shunt the magnetic field and make the situation much worse when improperly using steels.

 

[Disco_Wheels]

Hi Guys,

Thanks for the details and suggestions.

The arrangement I'm shooting for is per the image attached. For reference of scale: The wheels are about 72mm dia.

Very interesting what EdStainless says about the larger holes in the steel end plates. I wasn't aware of that at all.

The shaft itself I'm planning on making from Stainless (310/316/318 - I don't think 304 will be the best based on it being partially ferro-magnetic(?)) or Aluminium, if its going to be strong enough and sufficiently fracture resistant.

At the moment I'm making some draft designs based on all of the above.

Thanks again for everyone's feedback and advice!

 

[EdStainless]

If you can find some Nitronic 40 or 50 rod for axles that would be good.
The material is strong and very non-ferromagnetic. (search for Aquamet)

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[MagBen]

OP: how will this work? do you want these 4 wheel spin, by what? It doesnot seem to me an optimized design. A solid steel plate here could shunt field; The sandwich structure won't allow you to get 1+1=2, you may even get 1+1 < 1, if your intension is to get a max field between two wheels.

 

[Disco_Wheels]

Thanks EdStainless, I've not heard of that material before.

I'll check it out and see how I go.

Thanks again all, very greatly appreciated