[Boatanchors] DM-28 Dynamotor Restoration Questions

[Michael Hanz]

On 4/3/2020 6:15 PM, whitebear1122 at comcast.net wrote:
> I've got the unrestored BC-348R on the bench as I'm eyeballing it for restoration.  I pulled the dynamotor as I would like to use the radio with the original dynamotor if possible.  I disassembled the dynamotor to lubricate the bearings following the instructions here "https://radioblvd.com/bc348_dynamotor.htm" .  It came apart fine.  The bearings shown on the website are for a DM24 where the balls themselves are visible from the outside and available for cleaning.  Radioblvd didn't remove the bearings but instead cleaned them out while attached to the armature. On mine, the balls themselves are not visible.  They might be on the other side of the bearing where I do not have access since it's facing the armature.  This suggests that the press fit bearing needs to be removed from the armature.  I'm concerned here because I've never done this before..  Does anyone here have experience with this dynamotor bearing situation?

I have a bit of experience with them, Scott.  I wrote a distillation on 
the subject at that might answer some questions that come up - 
https://aafradio.org/docs/Dynamotors.html

I went to the radio boulevard article, and it has some helpful ideas and 
excellent photos.  Only one thing concerned me, and that was a 
recommendation to use "wheel bearing grease".  Now, I know ever since 
WWII hams have probably been using wheel bearing grease in dynamotors, 
and if you only use the radio for an hour a week, you'll probably be 
fine.  But a bearing application engineer would recommend something 
different.  Think about it - on a car the speed of a wheel bearing at 
60mph is perhaps 750rpm, and it is supporting perhaps a half ton of 
radial weight.  The dynamotor bearing is supporting perhaps a pound of 
weight and is spinning at 8,000rpm. That's a radically different 
environment, and the reason there are so many grease types is due to the 
diversity of applications.  The best grease for that job - hands down - 
is an electric motor grease, and there are a number on the market.  If 
you want a space age grease, you might try Mobil Polyrex EM Electric 
Motor Bearing Grease, on Amazon for $15 for a lifetime supply.  I like 
the synthetics, and if you want the ultimate, you can get a tube of 
Mobil SHC Polyrex 222 for $27 plus shipping on ebay.

The BC-224/BC-348 dynamotor bearings I have worked on are what I would 
call a "mild press fit" so they have never come off the armature by 
simply pulling on them with your fingers.  I gave a couple of methods in 
the article above.  Your bearings sound like they have shields on both 
sides, which may mean that they are so-called "permanently lubricated" 
bearings.  If so, then I would simply try them out and see if they're 
making noise.


> Can a non-original used brush be put into a dynamotor where the armature is used to the contour or wearing of the 'original brush"?
> If so, does anyone have a brush for the DM28 that I could purchase?  The brushes are stamped AJS.

Like AB5S, I may have one somewhere in the chaos of the storage area.  
If not, my go-to source for a lot of things like this is usually 
McMaster-Carr - https://www.mcmaster.com/motor-brushes

           73,
  - Mike  KC4TOS