[Milsurplus] Dynamotor brushes

[Brenda Gentry]

  The composition and manufacturing techniques for carbon brushes is 
very wide and varied for exact applications. Operation at high altitude 
is a serious concern because of wear, dust production, and the 
possibility of causing a communtator flashover. For ground use, these 
are not much of a concern. The brushes were not pure carbon, there were 
additives to reduce wear and give stable current transfer at differing 
temperatures. I have many decades of experience with these, most of it 
in the elevator craft where electrical stability and long wearing are 
paramount. Even if you use your equipment a few hours every day, you 
should not have any problems with re-shaping brushes to fit.

    B.Gentry, KA2IVY

On 7/5/25 8:05 PM, Charlie L. wrote:
> One thing about using dynamotors with your milsurp, they last a very 
> long time on the ground, compared to  operating at up to 30K feet in 
> unpressurised planes where  they arced like the dickens up there.  A 
> contact I have who restores electrically operated turrets for WWII AC 
> like the B17, said he has never replaced the brushes in the amplidynes 
> that power those turret motors.  That also brings up a question as to 
> their composition.  I wonder if those brushes intended for high 
> altitude, had a different make up than those used in motors that 
> stayed at sea level?  Maybe a harder carbon formula to last over 
> several missions  from England to Germany and back?  Is there any data 
> on how long the brushes did last in the ETO and how often they were 
> changed?  Perhaps they left the radio gear off most of the time, but 
> the intercom system had a dynamotor powered amplifier, BC347,  that 
> had to be running all the time.
>
> Charie, W4MEC in NC
>
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