[ARC5] Dynamotor Maintenance

[Mike Hanz]

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