Velocivixen

I've been rehabbing a Raleigh Twenty with a Sturmey Archer GH6 dynohub. The bike has the original headlight as well as fender mounted taillights. This is my 4th GH6 rehab, and I'm aware not to separate the outer magnet from the inner armature without a "keeper" otherwise the magnet will become immediately demagnetized and unusable.

"Keepers" are rare and I happened upon some info posted by Jobst Brandt discussing use of a spare armature to use as a keeper. My magnet/armature has grit and rust between them and would like to get it out. I've pushed the armature out (not all the way) a little and used a wire brush on a Dremel, but it's still gritty. Some have suggested WD40 or using sandpaper and slip it between the two to work off the grit.

Anyway, thoughts about using an old armature as a keeper? Anyone have an armature hanging around that you don't want? I don't want to sacrifice my usable dynohubs.

Thanks.

Slash5

Take this with a heap of salt cause I don't know what I'm talking about but this is an interesting read. From the sounds of it, just removing the armature to swap it will have serious effects - I gather you need to push the old armature out with the replacement. Their idea of clamping in a steel vice sounds interesting.

(1) What is a 'keeper', and (2) do I need to use one if I take the armature out of my rotating-coil magneto?

bikamper

I've always just used the largest 3/4" drive socket that would fit but I see no reason why the spare arm wouldn't work.

Velocivixen

Thanks for the link to the article, not sure it applies to me. @bikamper - not sure about what you mean using 3/4" drive socket.

Have you seen an armature & magnet on a Sturmey Archer dynohub? They're large.

rccardr 

So, on tangent here, but does anyone know why removing the armature would immediately demagnetize the magnet? Anyone with actual experience with that happening? Curious because I actually have a LOT of experience with ceramic magnets & armatures & DC power, and don't understand the physics of why that would (or how it could) happen.

bikamper

So are 3/4" drive sockets.;-). My largest 2 1/4". And I've converted all of my gh6 to run LEDs after rebuilding. To simplify, any large piece of steel that will fit inside the magnet will do the trick. Your old arm should fill the bill.

steelbikeguy

The wikipedia page mentions that magnetized iron has a low coercivity value, meaning that it takes a small applied field to magnetize the material, but by the same token, it takes little for it to demagnetize. Increasing the gap in the magnetic path reduces the magnetic flux, which apparently doesn't reinforce the magnet's own field as much. This is where I'm getting into unfamiliar territory. I understand that the flux decreases when the increased air gap increases the reluctance of the magnetic path, but I'm not sure why that causes demagnetization. It seems that it just makes it harder for all of the small magnetic domains to reinforce each other.

Modern magnet material such as alnico or ceramic magnets or especially rare earth magnets have much higher values for coercivity, so they aren't significantly weakened by having a higher reluctance magnetic path.

Somewhere, I've read about people retrofitting rare earth magnets into these SA dynohubs. Probably an interesting project, but it's likely to be easier to just lace a modern hub dynamo into the wheel.

Steve in Peoria

Slash5

That link I provided has a great explanation plus some actual tests of the impact of not providing a keeper. The loss is instant.

Velocivixen

All the technical info makes my head spin. All I know is that loss of magnetism for this particular design is instantaneous and you need a "keeper" that is a very tight fit to the magnet - just like the armature.

rccardr 

Ah, magnetized iron. That explains it.

Narhay

I may be on my own here but I've had a few GH6 dynohubs. One hub I removed the magnet without a keeper before knowing any better. Upon testing with the same headlight with the other non-removed magnets there was no discernible difference in output.

RobbieTunes

I agree, as if I have a clue what this is about......
Interesting. If I ever buy a dyno hub, I'm never taking it apart.

You people amaze me. When I get my bottom bracket assist motor, I'll come here for advice....

RobbieTunes

You started it. Bad girl. Utterly destroyed my male know-it-all ego, but I'll recover. Neat subject.

Velocivixen

So let me get this right. You completely separated the center armature from the magnet? Like one was over here and one was over there? Because everything, including original Sturmey Archer repair manuals say not to separate, even for a second without a keeper.

Well anyway, there are enough folks around telling me of their woes for not using a keeper. I've got my sites on something that I think will work. I'll keep you posted.

As an aside, I weighed this Raleigh 20" (406) steel rim with GH6 dynohub with no tire or tube and this little bugger weighs a whopping 4 lbs. 12 oz.! This is the wheel on my Twenty.

rccardr 

Interesting. I'll have to look up a diagram for these to see how they work.

Narhay

Yes. I saw some rust between the armature and magnet. Pulled both out, went at it with a brass wire drill. I may have had dinner before putting it back together and my light lights up just fine. Now I could have been very lucky or I am used to these headlights not lighting anything up but I am currently using a set that I know was not pulled apart and I couldn't tell the difference.

gna

I think Gentleman Cyclist Merchandise had a dynohub keeper for a time. I don't see it, but you could email and ask if they are still available. I'll ask him myself this weekend, if I remember.

JohnDThompson 

It's not "instantaneous" but it can happen quickly -- these Dynohubs don't use modern, rare-earth magnets that hold their field well without support. "Gentleman Cyclist" offered reproduction keeper rings; perhaps they now have more available?

Velocivixen

@gna & JohnDThompson - I am in communication with The Gentleman Cyclist. Thanks.

I am very eager to see the back side of the armature. I have scoured the Internet looking for images of the wired side of the armature. Perhaps I will soon find out.

JohnDThompson 

A slow, rainy day and a rustbucket Dynohub have coincided to give me an opportunity to take some pictures:





HTH…

Velocivixen

Thanks! If you lived in Oregon with all the rain we have, you'd be in serious trouble. What does it look like between the silver outer plate and the armature? Can you get it apart?

JohnDThompson 

The metal flanges of the outer plate just extend over the wire coils inside. They do not appear to be intended to be dis-assembled. There is/(was) a paper ring between the armature and the magnet, but that totally disintegrated when the armature was separated from the magnet. Not sure how essential that piece is.

Superbe69

Slide a 2 3/4" round wire brush into the magnet and you're home free. Had mine out for 45 minutes while I rebuilt the hub and cleaned the armature. Was even able to clean the magnet since it was secured with the pressure of the bristles. Total cost for the part, $6.00 at the local hardware store and if I had looked around in my tool boxes I probably already owned one.
Make sure it's 2 3/4" a smaller one will fall out and a larger one COULD crack the magnet. Voltage reading after showed improvement so I know it worked for me. Good Luck.