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View Full Version : Spacing Out With CDROM Cans: The Strong Magnet Spacer Approach


timocharis
12-14-2005, 06:25 PM
After I posted a thread on winding a basic 17-turn motor, a number of people asked for tips on placing the magnets.

I know of three techniques: using a template, using something (rubber bands, plastic, paper clips) of the correct width between each magnet, and the Strong magnet spacers available from www.strongrcmotors.com.

I've done all three, and I'm pretty much a convert to using the Strong spacer. Maybe if there's enough interest, I'll cover tricks for the other techniques too.

For this example bell I'll be using N50 curved mags designed specifically for these bells and sold by Don Armstrong. He also has spacers for flat magnets, 20mm bells, GBx, you name it. They're generally about $3.

Before you start, gather up your magnets of either pole and mark the outer edge, leaving the other polarity blank. This will help you keep track of what you're doing. At this stage, I keep them stuck to the bell and pop one in to make sure the cardboard spacer is putting it at the right height. Yup!

But first, a note about yet another problem: the new N50 magnets (use them!) tend to "creep" toward the front of the bell, almost no matter what you do. This is particularly true of the curvies (more on that in a bit). You actually want them as far from the front of the can as possible. The front of the can is just clearance for the wire; the back of the stator should be flush with the back of the can, as should the back of the magnets).

The only cure I've come up with is using a bit of paper card stock as a spacer in the front of the bell. It should be about 3-1/8" long and about a millimeter wide. Too wide is better than too narrow, since you can jam the magnets into it (it's paper, right?). Thickness is not very important, but it should be absorbent and flexible. I find the header cards from Dubro Mini Hinges are perfect (they're 3-1/8" wide! No measuring).


If you look closely in this first picture, you can see the little cardboard ring.

timocharis
12-14-2005, 06:25 PM
First drop the spacer in. I prefer to work without the screw in it because I have hammy fingers. Don suggests leaving it in (partly so you can raise it a bit higher).

Then put your six marked magnets in every other slot. That should be fairly easy, as they're somewhat distant from each other. Then start putting in the other six.

This will be a bit harder, since the magnet you're trying to put in will "jump" toward one side or the other if you're not precise in your placement (and I'm not).

timocharis
12-14-2005, 06:26 PM
When this happens to you, a pair of needle-nose pliers can come in handy. Don also uses a wooden skewer, and toothpicks have been known to do the job. A handy bit of carbon rod.

If you get it straight and centered, it will sort of just pop into the slot.

Normally I would hold the spacer in place while doing this, but I'm also trying to shoot the picture at the same time. Try it; it ain't easy!

timocharis
12-14-2005, 06:27 PM
Should end up looking about like this.

timocharis
12-14-2005, 06:27 PM
Retracting the spacer is a cinch. Just turn in the supplied screw and pull it gently out.

timocharis
12-14-2005, 06:28 PM
On the back is a wood "lift" with a nice big hole in the middle that misses the flange in the middle of the bell. It's also threaded, so you can stick the screw into it.

timocharis
12-14-2005, 06:29 PM
Then you can stick the spacer back in and run it all the way up and down the can. This will take out any unevenness in the spacing (the magnets can still be slightly tilted).

For the truly anal, raise the spacer up about halfway to the top, then rotate the screw lightly. This will even up the spacing between the magnets just a hair more (since they could be anywhere in their alloted slot).

timocharis
12-14-2005, 06:30 PM
I don't trust my eyes to get the magnet level exactly right, so at the end I like to turn the can over and gently press it to a flat surface. After that, the mags should be solidly seated into the paper and all at very close to the same height.

Don't twist the can when you do this!

timocharis
12-14-2005, 06:30 PM
The tension among the magnets, and their attraction to the flux ring, will hold them neatly in place as long as you're reasonably careful about handling the can. This is a bad time to drop it.

Now you'll need to glue them in place. I like to run a small drop of CA between each set, letting it bleed into their edges and under them a hair. The excess will run down into the paper spacer and turn it into a solid magnet grabbing monster. This is a good thing.

Set it aside and let it dry.

timocharis
12-14-2005, 06:31 PM
I'm a compulsive CDROM motor part tryer outer. These didn't seem to be run-of-the-mill curvies (which usually don't have quite the right air gap), so I was intrigued.

In later email, Don told me they are in fact custom designed and ordered by him, which explains why they fit so well.

But does that mean anything to the pilot? If my first impression running this can is any indication, the answer is: yup.

I tested it on a 14-turn 24awg 22.7mm stator that I have previously run with N50 flat magnets and N50 curved magnets I was able to acquire, but that were not a precise fit. By comparison, I'm getting obviously better performance (based on my subjective opinion while flying the plane) and probably better efficiency (based on mah put back in / time flown).

I think they're great, one of the easiest and best hop-ups I've done.

And yes, I paid for my parts!


Dave North
December 2005