[ARC5] BC-454-B Dymamotor Question

[J. Forster]

FWIW, I think WW II dynos are absolutely remarkable. My guess is one could
easily be operable after 1000 years in storage. Yes, they wear in use, but
with proper care they are damn near indestructable.

Take that, latest iToy!

-John

================


> On 18 Jan 2013 at 10:45, J. Forster wrote:
>
>> Ken,
>>
>> Agree on all points. However, I've never seen the commutator insulator
>> segments get too high. It probably does happen as many manuals have
>> directions for 'undercutting' the insulation after refinishing the
>> commutators.
>
> Right, John.
>
> I first learned about "undercutting" the insulation from an ancient (1920s
> ?)
> book on automobile maintenance. I used that info to fix both the generator
> and the starter motor in my first car, a 1930 Model "A" Ford. When I
> bought
> that car, for $50.00 in about 1960; it was much older than I was.
>
> What happens is that since the metal that makes up the armature segments
> is actually somewhat softer, and a lot wider, than the insulator, the
> insulator
> wears down at a slower rate than the metal.
>
> Or, when turning a very badly worn armature in a lathe, the insulator
> segments become equal in height. Those must be cut down a few
> thousandths to prevent brush bounce.
>
> The first big indication of that sort of problem is excessive sparking
> between
> the armature and the brushes as the brush "jumps" from one metal segment
> to the next when it rides over the protruding insulator segment.
>
> However, undercutting the armature insulation in a dynamotor would be
> much more difficult, since the dynamotor's armature so much smaller than
> an auto generator, and thus the insulator segments are much thinner and
> would require a very thin blade to do the job properly. For an auto
> generator
> or starter motor, we used a broken off hacksaw blade, which was almost
> exactly the correct thickness.
>
> In the case under discussion, and after thinking about it, I am much more
> inclined to think that one or more of the old brushes is simply stuck.
> Since no
> armature is perfectly round, and no set of brush-holders is perfectly
> aligned,
> the brush is simply not touching the armature at one or more specific
> places
> as the armuature rotates.
>
> I am also very suspicious of the bearings' condition: IMHO, no old
> dyamotor
> should ever be run before the bearings and brushes have been very
> thoroughly checked out and serviced, or replaced if necessary.
>
> One other thing one should look for is the small wire that connects the
> brush
> to the end-clip. This wire runs through the tension spring. If that wire
> is
> broken or missing, the current must flow through the spring. These were
> never designed for this, and will usually overheat and lose their tension,
> allowing the brush to "bounce" or even in extreme cases, preventing it
> from
> making contact with the armature.
>
> In my case, I chose out of my collection of DM-34s, the worst possible one
> I
> had. This one had been under water. After doing minimal work on it,
> including cleaning and lubricating the bearings, it works fine, except
> that it is
> very mechanically-noisy. Obviously at least one of the bearings has rusted
> balls in it. I am going to replace those asap. All four of the brushes
> were
> good, and were loose, although one had been improperly installed and the
> small clip on the end of the tension spring had been bent.
>
> One thing I found very interesting in my "research" on the DM-34s I have
> here is that the manual states unequivocally that experience has proven
> that
> replacing bearings in the field resulted in very much reduced operating
> life
> and it was stated flatly that if a DM-34 needed bearing-work, it was to be
> sent
> back to the factory.
>
> I have been trying to figure out why this was the case.
>
> vy 73,
>
> Ken W7EKB
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