[Boatanchors] DM-28 Dynamotor Restoration Questions

[Robert Downs]

In any dynamotor (except possibly one made to convert 12 to 24 VDC), the
smaller pair of brushes are for the HV or generator end and the larger ones
for the LV or motor end.  The brushes are usually stamped with a + or a - on
one side.  And the convention seems to have been that the polarity symbol
went on top.  In any case, when reinstalling used brushes, it is quite
important that they go back in exactly as they came out.

In the event that you are replacing worn out brushes with new ones, the
maintenance manual usually has a paragraph explaining how to bed or seat new
brushes to the commutators. And if the commutator is worn enough to require
turning and undercutting, do that before you seat the new brushes.

If the bearings are either sealed or shielded, the only successful way to
re-lubricate them is usually to destructively remove the seal or shield and
just leave it out.  Most of the time, if the original bearings are sealed,
the manual will probably say that the lubrication is good for the life of
the bearing.  But the people who wrote the manuals didn't expect the
dynamotor to still be running after 70+ years.  So just disregard that
statement.  Or locate and buy new bearings. Finally, if you expect to re-use
the original bearings, do NOT remove them from the armature or rotor.  The
bearings are usually a press fit onto the rotor.  You can install them
without pushing on the outer race.  But in most cases you can't remove them
without doing it.  Which is almost guaranteed to damage them by a process
that used to be called "brinelling".  

Robert Downs

-----Original Message-----
From: boatanchors-bounces at mailman.qth.net
[mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of
whitebear1122 at comcast.net
Sent: Friday, April 3, 2020 20:22
To: 'David Stinson'; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] DM-28 Dynamotor Restoration Questions

Hi Dave, I'm still learning about a dynamotor.  I just discovered that there
are two sizes of brushes in that dynamotor, so it's the smaller sized 0.268"
width brush by 0.402 length (not counting the wire extension.  

Oh I am so relieved to learn this because I sort of accidentally forgot
which way the dynamotor housing was sitting when I removed the brushes. I
marked which brushes came from which side of the housing but forgot to mark
the 'front' of the dynamotor.  A definite 'aw darn' moment.  Now that I see
that the gold colored brush housing only fits the larger brush, I am now out
of the woods.  Whew!

Thanks, Scott


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