Magnetic tablet stand

[joshua1000]

Hello all,

I'm trying do design a magnetic tablet stand. Something like iOmounts, but I'm going the DIY way about it. I'm pretty crafty with Inventor and got all the proportions the way I'd like them to be but now I've come down to choosing the magnets.
My desing is as illustrated in the attached jpg;
Green - metal sphere
Light Grey - a distancer, I'm considering printing this 3D (plastic)
Red - a magnet with two functions
Pink - a thin metal plate, stuck on my tablet

As far as the two functions of the magnet are concerned:
Its primary function would be to keep the plastic part attached to the metal ball. Its secondary function to pull the metal plate (which would be separated by a tiny gap, to avoid any scratchmarks on the surfaces) and hold the tablet in place.

My problem:
I can find formulae all over the internet about cylindrical magnets and steel plates and such, but I cannot for the life of my find out how to calculate the force in this particular case. Also, would this even be possible without the sphere being magnetised.

Thanks in advance, for any help offered.

Aljosa

P.S.: I like the idea they have at iOmounts, but don't like the price. So I thought I'd try the DIY way and on a budget at that. That's why I can't just order some random magnets and try to work it out. Hope anyone can give me a pointer or two.

 

[joshua1000]

Forgot to include the aforementioned JPG file

 

[EdStainless]

As long as the sphere and plate are both ferromagnetic this could work.
I see two issues, the total amount of force, and the division of force between the two joints.
To minimize slipping you may want the surfaces slightly rough.
You probably want to put pole pieces on your magnet.
Look at the picture, a rectangular magnet with the N on the visible end and the S on the far end. Now put steel plates on the ends that are taller than the magnet. On the sphere side they could even be shaped to allow them to be closer to the surface. Now the air gaps are smaller and the forces are higher.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[joshua1000]

Not quite sure I understand. And it seems I forgot to mention that the JPG is a cross-section. Sorry about that.
I've inluded another JPG, this time it shows what the whole thing should look like. Just for reference, that a 7.7" inch tablet, but I'd like to make this very modular. I plan on getting a 10" tablet at some point in the future and would this to work in that case and also with a regular smartphone.
To minimize slipping: The thin metal plate stuck on the tablet would actually sit in a protrusion (hope I got this right) in the distancer. I'm guessing that would be enough to keep it in place. But for this to work I need the magnet to be strong enough.
Also, I'm having some trouble thinking of a good material for both the steel sphere and the metal plate. Inox (NiCr) steels and Aluminium are not magnetic as far as I know, but I wouldn't like to see corrosion on the stand. It's tough designing something like this without the proper background. I am just finishing my bachelor's in mechanical engineering, though this isn't my field.
I hope I'm making sense.

My theory is as follows:
The magnetic force between the magnet and the sphere would be greater than the force between the plate and the magnet, because the plate is much thinner/has much less volume. Please correct me if I am wrong. And the plastic distancer would move and rotate freely (but not easily) around the sphere, making the stand adjustable in everywhichway. Really, just google iOmounts for a more visual explanation.

Thanks for the help so far!!

Cheers,
Aljosa

 

[MagBen]

The whole design is great, while the magnetic theory is pretty simple.

using ferritic stainless steel (4xx series) for plate and ball if corrosion is a concern. The real challenge seems to control the forces between the magnet and the thin plate, and between magent and the ball. I guess you want to have it higher for the latter. Changing the gaps, the length of your magnet, the type of magnet, and pseudo contact area (lessly relatd to the mass of the plate) etc. will change the forces. you can play all these parameters. More professionally, you may make two legs (or just a slot in the middle of magnet) of magnet facing the ball side to make a more close circuit to increase the force between magnet and the ball.

 

[EdStainless]

If you build a magnetic circuit like a U with a magnet as the base, magnetized left to right, and steel poles pieces on the sides, you will find that when you measure force the holding power at the pole pieces is much higher than just for the bare magnet.
This can be done with disc magnets also. Picture a heavy steel cup that the magnet fits into with some OD clearance (1/4"). And then a steel post from the exposed side of the magnet. The edge of the cup and post are flush, or in your case form a spherical surface.
In either case you need the steel parts thick enough to not be magnetically saturated, but thin enough to have a fairly high field.
This would also let you use less expensive magnets with lower strength and use pole pieces to focus it where you need it.
Using a 4xx SS for the ball might be a good option.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MagBen]

ED, you got the idea which is to make a close magnetic circuit and so to take full advantage of flux (less leakage). My previously mentioned 2 legs may only apply to AlNiCo magnet, on which one can even put 4 or 6 alternative poles (NSNS...), no steel pole pcs are needed.

 

[joshua1000]

Thanks for being so helpful!
So...if I'm getting this right, I'm at the point where I should be thinking about a prototype. I guess I should be making a modular design with the possibily of changing the gap size if I understand you correctly?
My original idea was to go with a simple cylindrical magnet (neodymium, grade N4X, somewhere around 30mm/1.2" in diameter and 5-8mm/0.2-0.3" in height), that I could order online (preferably in the EU) and a plastic distancer I could easily print in 3D.

Just to clarify: Could I be expanding the gap with nonmagnetic materials (like plastic or nonferromagnetic steel)? Would that change the magnetic flux in any way?

I don't understand the heavy steel cup explanation completely, could you please break it down for me again?

Another comment on the design:
The sphere is meant to be very modular - not just for mounting on a pole, but on a car dash, table, wall, etc... Anywhere I could drill a hole and stick a bolt through. Basically my plan is to make all of the things I own look like some evil metal ball race has taken everything over.

 

[joshua1000]

Hi again,

I have a further question.
First a bit of an update:
I've extracted two Nd magnets from an old hard drive and successfully built a prototype that is strong enough to hold regular (~120g) smartphones.
To simulate a thin metal plate I used pieces of olfa knife blades stuck behind the battery cover, so the phone looks stock from the outside and has no magnetic parts inside. Although I noticed that the blades seem a bit magnetized after prolonged exposure to the magnets, but I understand that this should go away after a while?
A single magnet provides enough force to hold the smartphone in place, but I've yet to road test this (I made a car dash mounted stand out of this). I might be needing two, although the magnetic force feels a bit excessive when two magnets are in place.
I can't test if the force doubles with two magnets, although I might get around to that - it feels like it does.
So, does stacking the magnets on top of each other have the same effect as if it were only one magnet with the same height as the stacked ones?
If so, I might have to downsize the magnet in my designs, it seems to be that ~15-20mm height ahould be enough for the weight (350g) of my tablet (combined with a 0.6mm metal plate on the tablet and a 1.5mm air gap between).

Thanks for the help so far.