[ARC5] Dynamotor Maintenance

[J. Forster]

I am puzzled by your statement that Graphite is abrasive, especially since
it is widely used as a dry lubricant.

Messy, for sure, but abrasive?

-John

==============



> On 1/19/2013 8:12 PM, Robert Eleazer wrote:
>> My copy of TM-11-850-N, dated 4 Feb 1943, says:
>>
>>   "The bearings of these dynamotors are of the sealed, ball-bearing
>> type, packed with grease at the time of manufacture.  They will
>> require the following additional lubrication: One drop of light
>> lubricating oil at each bearing at the end of 500 hours of operation.
>> Any oil in excess of this amount will do more harm than good."
>
> Ah!  The whole story emerges.  Okay, but that's a *long* way from saying
> the dynamotor bearings were *only* lubricated with a drop of oil every
> 500 hours.  They started out with grease in them.
>
> Three things to keep in mind:
>
> 1) The thickener, or binder, that oil is mixed with to make the
> substance called "grease" doesn't wear out.  Whether it is a soap based
> thickener, or one of the more modern products, it does not lose its
> ability to do its job of entraining oil.
>
> 2)  What *does* happen is that the lighter components of the lubricating
> oil evaporate over time.  You end up with a thickening of the lubricant
> simply because those lighter components aren't there any more.  The
> normal heat of running exacerbates the evaporation rate.  Those end bell
> covers are *not* airtight.  They breath.  Put your nose down near the
> dyno when it has reached operating temperature and you can smell those
> lighter components that are outgassing.  Putting a drop of oil in the
> bearings essentially replaces those parts that have evaporated and
> reuses the binder already charged in the bearing during manufacture.
>
> 3)  Clearly this was a non-sustainable maintenance policy.  While the
> binder doesn't wear out, and you can renew the viscosity level of the
> lubricating oil that does the essential work in the bearing, you cannot
> keep contaminants out, and carbon dust from the brushes is one of the
> most abrasive contaminants around.  I once worked for a guy in
> Albuquerque that had a lathe behind the shop in an open shed.  It was an
> old thing that had seen better days, but we were making things out of
> graphite that we didn't dare let near any of the normal shop equipment,
> because it would take out the precision of those machines in a
> heartbeat.  The poor old thing (and its operator) were covered with
> black abrasive dust by the end of each day, and eventually it just wore
> itself to pieces (the lathe, not the operator...)  That same thing would
> happen to dynamotor bearings that simply had lubricants going in without
> somehow removing the built up contaminants...thus the universal move to
> careful cleaning and repacking with new grease.
>
>> In contrast, Tech Order No 08-10-24, dated 12 June 1936, covering the
>> BC-224-A, has instructions for removing the end bells of the dynamotor,
>> inspecting the brushes, and dressing up the commutator.  And after
>> servicing the commutator the ball bearings are to be thoroughly cleaned
>> and packed with Keystone No.44 grease or Air Corps grade No. 295.
>>
>> A friend who was a maintenance officer for a P-38 recon unit in WWII
>> explained that the prewar Air Corps had some different attitudes than
>> were common during the war.
>
> My guess is that someone eventually came to their senses on the BC-312
> etc. bearing lubricant procedure and standardized on a maintenance
> policy that the majority of design engineers had already recognized as a
> sustainable process to minimize costs over the long haul.  I have always
> been an advocate of a drop or two of Mobil I engine oil for *short* term
> testing of an old dynamotor.  That does not mean I would ever support
> the technique for long term bearing health.  And yet I still read
> accounts of old Clem Hambone doing just that and claiming it "works jes'
> fine."
>
> As John is fond of saying...YMMV,
>
> -  Mike  KC4TOS
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