Exploring magnetic gears, with video

[Evan]

I posted a thread a while ago about a new type of magnetic transmission. Since then I have been intrigued by the idea of non contact magnetic gears. I decided to make a test stand that I can use to try out some of these configurations.






In the first photo the pinion to driven gear spacing is set large to better show the parts. Normally it would run with the minimum possible gap for maximum torque. Several thing about this have surprised me. There is very little cogging action in the drive. The torque transmission capability is considerable. Below the slip torque the drive is positive, no slip and quite stiff.

The ratio is set by the number of poles, not the relative diameters of the pinion and driven gear. In this demo there are 2 poles (N-S) on the pinion and 14 on the driven gear. This gives a ratio of 7 to 1. The pinion makes one complete revolution as the driven gear moves the distance between magnets on it's perimeter.

While the best name for this sort of drive seems to be "magnetic gears" there is little real similarity to purely mechanical gears with meshing teeth other than the obvious visual aspect. Ratio is independent of diameter. Torque is independent of ratio.

This is a first effort and I am surprised at how well it works. There does seem to be something analogous to a pitch circle and it seems to be somewhere in space past the OD of each gear. This pair of gears operates much more smoothly with a large gap than it does with a very small gap. It may be important what the spacing is of the magnets on the driven gear, more experiment is called for. Also, these "gears" have no pole pieces to concentrate the fields. One thing is already obvious, the driven gear may be placed in any planar alignment up to and including a right angle drive. More experiments...

I used Lexan for the test stand to minimize unwanted magnetic interaction and because it makes it much easier to see what is going on.

To really appreciate how this works you need to watch the short video. It shows the gears operating at speed and at a very slow crawl so you can see the relationship between the pinion and driven gear rotation.

1.7 megabyte
http://www3.telus.net/metalshopborealis/magear.wmv

[hornluv]

That's pretty cool, Evan. I noticed a slight speeding up and slowing down of the gears when run at slow speeds, presumably because of the magnetic fields interacting. Is that obvious at high speeds as well or does momentum cover it up. Also, aren't you at all concerned with having powerful magnets that close to your computer?

[Lew Hartswick]

Does the "timing" get out when a sudden stop occurs? Due to the
inertia of the driven one. That is the only possible "gotcha" I can see.
...lew...

[dp]

That particular design looks like it would have some noticeable cogging because of the variable gap between the driver's single magnet surfaces and the driven wheel. This would grow worse as the load increases and finally it would begin to skip. But - the possibilities for this open coupled power transmission are interesting. Unlike gears, there's no reason the wheels cannot have a good deal of overlap which would result in interactions at two nodes. There's also no particular reason there need be a single ring of magnets on a wheel which would introduce a third node, and a forth if each wheel had two rings each.

[Evan]

The most surprising thing is the lack of cogging. If you turn it very slowly by hand the large gear smoothly follows the rotation of the pinion. There is a noticeable difference in the torque which produces a torque ripple but not visible cogging. I suspect that when I make another gear today using more magnets and a different spacing that will also go away. There are so many possible configurations I don't know where to start. For instance, can I have a 2 to one reduction from similar size wheels if one has twice as many poles? I guess probably.

Lew, the timing does get scrambled if the drive slips.

Also, aren't you at all concerned with having powerful magnets that close to your computer?

No. I don't use magnetic media except the hard drives and they have a pair of super magnets inside. The screens are TFT LCD so they are not affected.

[Evan]

Here is a new configuration which has much greater torque. This is now more than a toy as this configuration can do useful work. It has a holding force of about 20 to 24 inch ounces and the torque value is a bit under that.

This uses a smaller driven wheel with 8 magnets. The magnets are arranged with alternating pole directions instead of all the same. The ratio has become half what it was and is now 4 to 1. I still haven't tried it with pole pieces.



Here is another short clip showing it lifting an object on the output shaft that extends from the back side of the jig.

http://www3.telus.net/metalshopborealis/magear2.wmv

[Doc Nickel]

Now, I'm certainly no expert with magnets, but I thought those disc-type NIB "super" magnets had their poles in the flat faces. If that's the case, wouldn't a "gear" train like this be stronger if the discs were mounted face-out?

Or do you need the "thinner" part of the field to make this work?

Next up, how about bar-type elements? I recall seeing some NIB types 3/8" square and 2" long- one presumes that with more... field "area" for want of a better term, the transmittable torque would be just that much greater.

And that makes me wonder if you could reduce the "cogging" even further by having two "gears" on common shafts, offset one spacing.

And interesting project. Looking forward to more.

Doc.

[Evan]

They do have the poles on the faces. For this sort of application both poles of each magnet must be brought into play at once and to do that placing them as shown accomplishes that. Rectangular magnets would certainly be better for some configurations. I don't have any on hand though.

[edit]

I will be trying them on the flat on larger wheels with more poles. That will require the use of pole pieces.

[wierdscience]

Interesting,any idea what the diffrence is in mechanical loss between it and a geared transmission of the same ratio and output?


Looks promising,if you scale it up and use some of these you could replace the tranny in the Rover

http://www.emovendo.net/magnet/6-x-4-x-2-block.html

[darryl]

finally, a decent fridge magnet.

Magnetic gears, very interesting. It occurs to me that since magnetic materials can be magnetized and de-magnetized, that one could create his own pattern of magnetism in a ring or disc, and tailor the number of 'teeth' to suit the application. I have de-magnetized nibs by heating, then remagnetized them both with other magnets and with electromagnets. Since many materials have a preferred direction of magnetization you have to stay with that, but the polarity of magnetization can be reversed. This means that an ordinary disc magnet can be modified to have a multiple of poles spaced around it's surface. Something for suitably certified experimenters to play with.

I'm reminded of the high-ratio gearing setup, I think it's called harmonic gearing. Such a setup has a lot of teeth in contact at one time, and extending this concept to magnetic gearing would mean that a high ratio could be achieved with a very strong torque transfer capability.

There's also the application where a container of sorts has to be completely isolated from another space, and where a set of mixer blades has to be spun, or similar. I've seen magnetic coupling used for this, and with todays strong magnets it might broaden the range of applications for isolated actuation. Where you previously had to use a shaft with seals, you could get away with not needing either. Could be useful in flow control valves, for instance.

There are a large number of shapes available in very small sizes with these neodymium magnets, so some very compact mechanisms could be made up. I would bet that a nifty planetary gearing setup would be useful.

Just throwing in some ideas.