Quality of Carbon brushes?

[LJ.]

How do you know if your dynamo brushes are of the correct hardness? Is there a 'penknife scrape test' one can do or do you just have to fit them and either, expect to have to clean them regularly if they are too soft or suffer arcing (spelling?) if they are too hard?

I am finding my M21 60 watt long dynamo is having to be regularly cleaned and its becoming a nuisance. I've often wondered about cutting up some old brushes from burnt out motors that I have. Has anyone done this? Success?

And who reliably sells *Good* long lasting brushes if home made ones are not the way to go?

[a101960]

L.J.

You need to buy from a dynamo repair specialist.  Dave Lindsley, Tony Cooper, and Sean Hawker are just some of the suppliers that spring to mind. A rough test to determine if a brush is to soft is to try writing with it. However also be aware that there are some brushes around that are to hard. I don?t know how you can test for that.

John

[bsa-bill]

not sure how a brush can be too hard John unless it gouges out a track in the slip ring in which case maybe it's more the spring, however as usual I look forward  to being informed

[a101960]

Bill,

There are degrees of hardness due to the manufactoring process. It is a complex business (baking time, pressure, and formulation). There was aproblem a while back where grooving happened due to hard brushes. As ever the problem lies with where the parts are sourced from. Some classic bike spares suppliers are not always technically aware. I agree with you that spring pressure could be an issue.

John

[metalflake11]

 Brushes that are too hard are also too brittle and tend to fracture. I have no idea how to test for that, other than some sort of crush test on a random brush in a batch they are making?

[bsa-bill]

Thanks guys, the grooving I had thought about but not heard of any cases, brittleness now that I had not considered

[BSA_54A10]

Bosh brushes are too hard for Lucas magnetos.
Found that one out the expensive way.
Brushes that are too soft will wear quickly and leave a lot of graphite dust inside the maggy so you will be forever cleaning it out and putting up with misfires due to arcing.

[wilko]

Why aren't Bosch brushes too hard for Bosch maggies? 

[bsa-bill]

Why aren't Bosch brushes too hard for Bosch maggies? 

Could be down to the springs Bosch use Wilko and the material in the slip rings,  not trying to causes an international incident but the most trouble I ever had with sparks was with a Bosch distributor on a Ford Fiesta, under warranty the garage had three goes to cure it (poor performance , missing ), I (I take a bow at this point) eventually suspected the thing was moving in the clamp, so five minute with a file to enlarge the gap in the clamp and it was cured.

[BSA_54A10]

Lucas slip ring contacts and commutators are softer than Bosh ones.
Lucas springs press harder than Bosh ones.
Horses for courses.
Hard brushes pressing against hard contacts will run for longer before needing replacement