Hi Clare,
Here are some thoughts on the subject, for what little they're worth:
So far, I have not yet run into *any* WWII dated manuals for the
avionics gear that mention sealed bearings in the dynamotors. That
doesn't mean there aren't any out there - just that they are not common.
My understanding is that the sealed bearings came out of the
technology explosion that the war generated, but did not reach
'commodity' status until the 1950s. From what I've read, the wear rate
on prewar natural rubber seals for even small, slow speed shafts made it
impractical until synthetic polymers were developed that could take
higher surface speeds without undue erosion.
To compound the confusion over the wartime dynos, I have seen a number
of the more common dynamotors (ART-13 and BC-348 come to mind) that were
retrofitted by depots well into the 1960s with replacement bearings
having integral seals. Some of the postwar manual versions mention
this, as well. These are commonly available now and have the advantage
of having modern lubricants already in them. A good bearing house can
help you with replacements if you can give them the necessary dimensions
off the bearing - the number on the bearing may or may not have a cross
reference, so dimensions and operating speed are what you need. They
are amazingly inexpensive - but enjoying challenges as most of us do,
relubricating the original bearings is an interesting exercise in
craftsmanship.
The sealed bearings also have the advantage of retaining the lighter,
more volatile components of the lubricant for much longer than open
bearings. These are the fractions that evaporate the earliest, but over
30 or 40 years even sealed bearings can lose their "soup". Not sure if
that's what happened to your PU-7, but that's what I would surmise.
I'd be somewhat cautious about lubricating the old bearings with modern
greases until I had made sure it was designed for the application. The
lubricants that actually do the work are of less concern than the
thickener that suspends the lubricant in a form that is useful. I
*know* the Mobil SHC 100 is good - it is designed from the start for
small, high speed ball bearings. It's not the only one, obviously.
Since you didn't mention a spec, I *cannot* say that about the Valvoline
- it may be fine, but just about every grease has claims that it is
"high temperature" so that's not much help. Here's the problem - the
churning action of small balls and grease in a bearing at 6,000 rpm can
create a surprising amount of heat, and the type of soap used as a
thickener to suspend the actual lubricant can cause premature failure of
the bearing if it isn't designed for that kind of environment.
The procedure for checking is pretty simple - run the dyno for about an
hour after relubing (no load is fine) and if the bearing cages are
uncomfortably warm to the touch, you have the wrong lube (or possibly
too much) in there. Most people put too much lubricant in the bearings.
I bought a pair of special open spindle bearings for my lathe last
summer, and because I wasn't inclined to spend another $550 to replace
them, I ordered them pre-lubricated. There was surprisingly little
grease in them, with plenty of gaps between the balls. These bearings
normally last a good twenty years in daily 8 hour shift service without
lubrication, so it's another data point for less is better...the
guideline is 1/3 full for most of these bearings.
Finally, I'm not sure what kind of tub of grease you bought from Mobil,
but I just went downstairs to check on a couple of tubes of the Mobilith
SHC 100 that is used in many OEM motor ball bearings, and there is
certainly a small amount of free lubricant at the top after 3 years year
of storage in the vertical position. Not sure why this might be
considered a problem, though...the comparatively large amounts in a tube
exacerbate the separation and you will see little or none of it in an
actual bearing, at least in my experience.
Any grease ever made experiences a certain amount of separation of the
lubricant and matrix, and those with lighter weight base oils tend to
separate more easily. The heavy lubes used for slow speed automotive
wheel bearings show very little separation, for example, but the lube is
too thick to properly lubricate small high speed ball bearings - you
need something in the ISO 100 viscosity range for that. Storage
conditions have something to do with your Mobil experience, but it's not
clear the Valvoline is the type of grease designed for small motor ball
bearings.
Anyway, those are my thoughts - use 'em or lose 'em as it moves you. :-)
Best 73,
Mike
Clarence M. Owens wrote:
> I have two questions about the military's use of sealed ball bearings in
> rotating equipment like dynamotors and inverters. I've started going
> through all of my dynamotors cleaning and regreasing the open ball
> bearings. I'm using a Valvoline synthetic grease rated for very high
> temperatures. Looks like good stuff and it did not separate at all in the
> tub, unlike the Mobil synthetic grease I have.
>
> First, when did they switch from open to sealed bearings? I'm suspecting
> it was late in or some time after WW II since the only two units I've found
> with sealed bearings are a T-195 dynamotor and a PU-7/AP 400Hz inverter. I
> have the T-195 dynamotor but the PU-7 was something I played with when I
> was in college.
>
> Second, what's the general thought about how well the sealed bearings hold
> up in use after these many years? The T-195 dyno seems to run OK but I've
> only run it for a few minutes so far. When I had the PU-7 back in 1960 or
> so it ran for a few minutes and ate its bearings. I won't be changing the
> T-195 dyno bearings on the if come but am curious about experiences with
> using these old high speed sealed bearings after long storage.
>
> Thanks,
>
> Clare Owens N2RJB
>
>
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