If your fan has "oilite" bearings, a porous sintered bronze material,
the bearings will only stay free for a short time after a reoiling.
This is because the pores in the oilite get clogged up with hardened
oil, and metal particles. This keeps the oilite from providing a
continuous (though tiny) amount of oil on the shaft. Oilite works just
like a wet sponge does. It leaves a smear of oil on everything it
touches.

One good way to rejuvenate oilite bearings is to remove them from
their housings, and wash them thoroughly in lacquer thinner. After
they dry, set them in a small puddle of light motor oil, and heat
to 150F (NOT OVER A FLAME!) a light bulb makes a good heat source.
When they cool, they will suck up a proper load of the oil, and will
be good to go for a long, long time.

One other thing to note, the shaft to bearing clearance must be small,
a few thou for a 1/4 inch shaft. If the clearance is too much, the
shaft will rattle about in the bearing and make lots of noise. Usually
on a worn out motor, with oilite bearings, the shaft will be what has
worn.

-Chuck, WA3UQV

wrote:
....[snip]....
If I'm going to start up a dynamotor that hasn't been run in a long
time, does this one drop of oil in the bearings sound sufficient? Is
....[snip]....


I recently put a 30-year-old brand-new squirrel-cage fan into service
(I bought it in the early 1970's and never got around to installing it),
and it was pretty "stiff" until I dripped about 10 drops of light oil
into each of its two "oil here" holes. After that, it would start on
"high" but didn't want to start on "low"; another 10 drops in each hole
and it seems to be working fine now.

--Myron.