[Boatanchors] DM-28 Dynamotor Restoration Questions

[Michael Hanz]

On 4/8/2020 7:49 PM, whitebear1122 at comcast.net wrote:
> No I did not replace the bearings.  I did not know there was such an option.  I took the dynamotor apart to re-grease the bearings using the radioblvd.com site only to find the bearings are either sealed or permanently covered, the ball bearings were not visible or accessible.  Before removing the armature I was able to manually spin it so the 70 year old grease hadn’t hardened at least.  I dribbled some Mobile 1 into the front and back of both bearings as that was the only thing I could do.
>
> There are replacement bearings out there?

Yes, though you'll have to carefully measure the bearings and check with 
OEM manufacturers to see if there is a modern match.  I don't have a 
cross reference for the BC-348 bearings handy.

> I lightly tapped the bearings/armature out of the end caps with no difficulty.  Reassembly seemed fine.  I reinstalled the bearings into the end caps, attached the end caps to the main housing, then lightly tapped on one end (use a socket) until both sides appeared equal distant.  Placed the shim in on each side, then the gasket and cover.  The armature was free wheeling until I put the bushings back in.  Is there more intricate detail that I neglected?

Possibly.  If you missed a shim, or tapped everything in without doing 
one more step, you may have preloaded the bearings too much. That will 
make them tight.  Fortunately, the solution if a small tap on each end 
of the _axle_ of the armature with a small drift. That will provide a 
tiny bit of end clearance and should alleviate the forcing the balls up 
onto the side of the bearing groove.  I mentioned this in one paragraph 
of the dynamotor refurb article on aafradio.org:

"Preloading the bearings is entirely another discipline - the purpose is 
to provide the right running clearances. Common approaches include screw 
adjustments, lots of little ...argggh... brass or steel shim washers, 
and large preload nuts. Preloading isn’t always critical (thank 
goodness), so you have to follow your TM carefully on this activity - 
just hope that you don’t have to worry about it. (You did remember to 
record any shims when you took apart the dynamotor, didn’t you?) The 
command set dynamotors don't require preloading - just a small end 
clearance (.010" or so). I've seen them run fine with as much as an 
eighth of an inch of slop, but that can cause wear of the end cage bores 
if left to go on too long.

As you assemble the dynamotor you may notice the armature tighten up and 
not spin freely. This is generally because you’re putting a (temporary) 
preload on bearings which may not be designed to be preloaded. Once the 
long "end cage" retaining screws are tight, tap each armature end 
lightly with a small mallet (or hammer with a piece of wood or plastic) 
to center the bearings. Without the brushes the armature should turn 
quite freely by hand. If it doesn’t, take it apart and find out why 
not...could be a cocked bearing, a preload pressure caused by a 
forgotten shim, or a burr in the end cage bearing bore. The bearing 
retaining plates go on then, usually with two small screws in each 
plate. There’s sometimes a little paper gasket inside the plate, but you 
shouldn’t need it with a good grease like the SHC-100. The gasket was 
placed there to reduce dripping with the wide variation of viscosities 
that WWII greases exhibited going from cold to hot, not to mention their 
separation into liquid and solid when they were left sitting for long 
periods. No need to remove them unless they were damaged in disassembly, 
but I wouldn’t go out of my way to make a replacement unless you’re 
really into internal originality. If you did much more than polish the 
commutators a little with sandpaper, you’ll need to break in the 
dynamotor with a couple of hours of running at no load /with the end 
bells off/ (assuming non-vented end bells) before putting it into full 
operation. This allows the brushes to adjust to their new home and 
increases the contact surface area. It also helps to dry out the 
interior, a step which is essential if the dyno hasn't been used in 
years. Neglecting to do this could cause brush overheating and failure. 
It is also the time to check for dynamic preload. While the dynamotor is 
running, slack off on the two screws retaining the end plate in the 
spider. (Be careful*...*there is high voltage there.) Listening to the 
note that the dynamotor will be singing, gradually tighten the cap back 
up to full tightness. With too much end play, you will often experience 
an increased vibration and sometimes a slowing of the rotational speed. 
That will normally be the case when the screws are /loosened/, contrary 
to logic. If that happens as you snug up the screws, it's a sign that 
you don't have enough shims in the mix."


73,

  - Mike  KC4TYOS