[ARC5] Dynamotor Maintenance - graphite

[Mike Hanz]

Apropos the subject of graphite dynamotor brushes, I found an 
interesting website in the UK that focuses on motor brushes. There is a 
great pdf on the assessment of commutator and brush appearance that I 
think I may add as a reference to the dynamotor article -

<http://www.engineeringcarbonproducts.com/reports/Assessing the 
Performance of Carbon Brushes.pdf>

is the link.  Well worth downloading the 260kB document if anyone ever 
has a dynamotor or generator to deal with.

     73,
  - Mike


On 1/19/2013 11:15 PM, J. Forster wrote:
> 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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