Lubricating oil life at elevated temperatures

[jimhokie]

We use Mil-Spec grade 2190-TEP lubricating oil in numerous machinery bearing applications. I've heard a general "rule of thumb" that the service life of this oil is halved for every 18 to 20 degrees above 150 degrees F. Does anyone know of any test data or other more formal information that can back up this rule of thumb?

Thanks in advance for pointing me in the right direction.

Jim

 

[BigInch]

I've heard the same, but using 10 Cº, so that would fall within 18-20ºF

"Make everything as simple as possible, but not simpler." - Albert Einstein (1879-1955)
***************

http://virtualpipeline.spaces.live.com/

 

[25362]

This is an excerpt from the internet:

The concept is based on the Arrhenius Rate Rule, named for 19th Century Swedish chemist Svante Arrhenius. Heat increases both the collision rate of molecules and the activation energy of the reaction. The higher activation energy helps overcome the barrier (or natural resistance) molecules have to chemical reactions.
With oils, the chemical reaction that typically causes base oil degradation and additive depletion is oxidation.
The activation energy required to induce oxidation in oil is high compared to other chemical reactions. The presence of contaminants such as water and certain metal particles in the oil can considerably speed up the process, that is, increasing the activation rate. For most in-service mineral oils with typical contaminants, the activation energy for oxidation corresponds to a doubling for every 10 degrees C temperature increment.

 

[IRstuff]

The original of this paper might prove interesting:

http://www.thefreelibrary.com/Oxidation+kinetics+in+beltcoat+compound.-a0142639622

Nonetheless at 100ºC, the Arrhenius equation does yield about a factor of 2 degradation for a 10ºC change.

TTFN

FAQ731-376

 

[25362]

If you wish a mathematical demonstration apply the following Arrhenius formula for the rate of reaction k_i:

k₁ = A e^-E/RT₁
k₂ = A e^-E/RT₂

Now take E ~ 80000 J/mol
R= 8.314 J/(mol.K)
A = constant
T₁ = = 100^oC = 373 K
T₂ = 383 K
and divide both equations

k₁/k₂ = e^{(80000/8.314)(1/383-1/373)} = 0.51

meaning that a 10^oC increase doubled the rate.

It is only a "grosso modo" approximation but it gives an idea of the effect of temperature on the rate of oil degradation.

 

[jimhokie]

Thanks very much for not only the answer to my question, but an excellent science lesson as well. As an M.E., I was never exposed to the Arrhenius equation in school. Additional thanks to 25362 for the latin vocabulary lesson--I'll use that in the future!

 

[romke]

the idea of a doubling of the oxidation rate with every 10 deg C above 100 deg C is indeed a rule of thumb and should be regarded as that. in real world applications there are other factors involved that may well change the rate of oxidation quite a bit. on the one hand the use of antioxidants that (at least for a certain time) prevent oxidation, on the other hand materials used in the machinery that can be acting as a catalyst. especially copper tubing can work as a catalyst and therefore should be avoided as much as possible. i have seen numerous examples where oil life was severely shortened due to the use of copper tubing, especially in compressors and turbines. the oil oxidation is also speeded up due to impurities in the oil like water, wear debris, airborne dust etc.

 

[jimhokie]

Thank you for the additional insight. That's interesting about copper being a catalyst. We don't use any copper tubing, but the oil ring is an aluminum bronze.

 

[romke]

aluminumbronze is rather harmless in comparison to copper tubing, also the surface in contact with the oil problably is far smaller. copper is also used in rollerbearingcages and ususally does not pose a problem there. what you should avoid though is copper tubing to transport oil at higher temperatures. that may shorten the useful oil life quite a bit.

see:

http://commonwealthoil.wordpress.com/2008/10/08/all-about-oxidation-of-oil-products/

 

[jimhokie]

I was wondering if the smaller surface area may be offset by the fact that it rides on the shaft, and at every start-up, there may be near metal-to-metal rubbing briefly until the oil ring rotates enough to bring oil up to slide on, possibly releasing copper wear particles. Probably not much of a concern given the residual oil on the shaft/ring, and the very light weight of the ring.

Thanks!