[SEL] Oil Question

[John Culp]

Jim, this typically happens when the oil is working in boundary layer 
lubrication conditions. The metal parts are separated by a film of oil, 
but the microscopic peaks of the metal surfaces protrude through the 
oil and come in contact with each other, causing friction and tiny 
pieces of metal breaking off and going into suspension in the oil. 
Those suspended particles look black. They also act as oxidizing 
catalysts producing pigmented hydrocarbon derivatives, and if the 
moving parts have much pressure on them, the opposing peaks weld 
together momentarily and break apart producing localized hot spots that 
can produce free carbon and metallic carbides. Really hot.

A graduate of Bob's Quantum School of Mechanics might be able to 
mathematically explain the intense local concentration of heat in 
relatively cool running bearings (think of the application of the total 
frictional heat dissipation over the vanishingly tiny part of the total 
area that's actually occupied by these tiny peaks and you'd be on the 
right track), but John Fankhauser's "Magic" explanation works quite 
well enough.

John

On Jun 18, 2007, at 8:30 PM, JIM KIRKES wrote:

> I was recently at a show next to a hot air engine with lots of 
> linkage.  The owner and I got to talking and we wound up with a oil 
> question.
> when the linkage is oiled it later weeps out black even though their 
> is no combustion nearby and it is in a clean location.  The things 
> that were lubed were a steel pin through a brass link. Why does the 
> oil turn black when dirt does not appear to be a factor?
>
> Jim and Diane Kirkes


John Culp
Bristol, Tennessee, USA