Frank Krygowski <frkrygow@sbcglobal.net> wrote:
> On 7/31/2022 11:35 AM, bob prohaska wrote:
>> I'm re-gluing a loose magnet inside a Shimano DH-3N20 dynamo hub
>> and having trouble removing magnetic debris. It's in the form of
>> fine powder, mostly rust particles. They're too fine to pick out,
>> but there's enough to potentially interfere with getting the
>> magnet seated so it clears the armature.
>>
>
> You might also take a sharp steel scribe, or even a steel nail ground
> down to a point. Attach the strongest magnet you have to the shank. The
> pointy end will be magnetized and let you pick up particles in the
> corners. Maybe that will be sufficient.

I really wanted that idea to work, but no luck. Even with a good
disk seek magnet the debris stuck in the dynamo. I ended up dry
wiping the surfaces to be glued and crossed my fingers. So far it
looks good. The clearance inside the hub is bigger than I thought,
more than ten mils. Maybe it was much ado about nothing.

The hub runs smoothly and generates power, so I'll declare victory
and retreat for now.

Next time the hub comes apart I'll give the sticky putty method
a try. The debris seems to accumulate on the magnet faces and
armature poles. A little oily residue will do no harm there.

As an aside, has anybody tried re-magneting a shimano hub? The
factory magnet appears to be ferrite. It's reasonably strong,
but NeBFe bar magnets should be a big improvement. The size
would be about 30 mm long, 4 mm wide and no more than 3 mm thick.
A quick check on Amazon found nothing that shape, let alone with
the right field orientation.

Thanks to all who replied!

bob prohaska

On 8/1/2022 2:30 PM, bob prohaska wrote:
> Frank Krygowski <frkrygow@sbcglobal.net> wrote:
>> On 7/31/2022 11:35 AM, bob prohaska wrote:
>>> I'm re-gluing a loose magnet inside a Shimano DH-3N20 dynamo hub
>>> and having trouble removing magnetic debris. It's in the form of
>>> fine powder, mostly rust particles. They're too fine to pick out,
>>> but there's enough to potentially interfere with getting the
>>> magnet seated so it clears the armature.
>>>
>>
>> You might also take a sharp steel scribe, or even a steel nail ground
>> down to a point. Attach the strongest magnet you have to the shank. The
>> pointy end will be magnetized and let you pick up particles in the
>> corners. Maybe that will be sufficient.
>
> I really wanted that idea to work, but no luck. Even with a good
> disk seek magnet the debris stuck in the dynamo. I ended up dry
> wiping the surfaces to be glued and crossed my fingers. So far it
> looks good. The clearance inside the hub is bigger than I thought,
> more than ten mils. Maybe it was much ado about nothing.
>
> The hub runs smoothly and generates power, so I'll declare victory
> and retreat for now.
>
> Next time the hub comes apart I'll give the sticky putty method
> a try. The debris seems to accumulate on the magnet faces and
> armature poles. A little oily residue will do no harm there.
>
> As an aside, has anybody tried re-magneting a shimano hub? The
> factory magnet appears to be ferrite. It's reasonably strong,
> but NeBFe bar magnets should be a big improvement. The size
> would be about 30 mm long, 4 mm wide and no more than 3 mm thick.
> A quick check on Amazon found nothing that shape, let alone with
> the right field orientation.

Taking things in sort of reverse order: No, I've never heard of anyone
re-magnetizing a Shimano hub dynamo. I remember Jobst had built a
fixture to do that with the old Sturmey-Archer Dynohubs. Those would
supposedly de-magnetize immediately if disassembled without a keeper. I
made my own keeper to work on mine.

I'm curious about how a fancier magnetic material would work. I've
thought about that for the old Dynohub, but it works fine for me as is,
since this bike needs little beyond a "be seen" light.

But modern bike dynamos are usually engineered fairly carefully to
self-regulate pretty well, putting out ~0.5 Amp and ~6V over a wide
range of speeds. I wonder if a stronger magnet might ruin that self
regulation.

I'm also curious about how your hub lost its magnet in the first place!

--
- Frank Krygowski