Why focus on TAN for hydraulic and gear oil rather than TBN?

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So I've fallen into an information hole this morning (nerd sniped somehow, I don't remember what started it), and I ended up on the Blackstone website ordering a test kit and eventually clicked through to their Do I Need a TBN? article where it mentions this:
Scientifically speaking, the TBN is one of two “neutralization number” tests run on oils. The TAN (Total Acid Number), which is used for hydraulic and gear oil, is the other.
That last sentence confused me. So, googling around and using the search function here, I got to this thread:
But I'm still at a loss for explaining the prior confusion; why do hydraulic and gear oil applications need to test for TAN as opposed to TBN? Wouldn't these applications still need a basic solution rather than an acidic one in order to prevent surface corrosion? Or do hydraulic and gear oils work differently than motor oils? Or conversely, is it the case that base additives don't matter as much in hydraulic and gear oils, and all we're really interested in is the acidity of the solution, hence the TAN test preference?
 
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Cause most basic hydraulic and gear oils do not have any or much TBN. Not much to measure.
Some high end specialty oils will have some TBN tho.
 

jonlandrum

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Well only a engine produces acids as part of combustion. Transmissions and PS and differentials do not.
I would assume the heat from a gear box would still help convert at least some of the hydrocarbons to an acidic hydrocarbyl species. Although I do see that it wouldn't be as rapid due to the heat difference compared to an ICE. Or is that heat not sufficient? I know a lot of heat is required, but I don't know how to define "lot" in this case.
 
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Actually its the relationship between the two that's important and that relationship needs to start from a sample baseline of the oil you actually use.

That's because OEMs don't give out their exact recipes so all you have is the baseline TBN to gauge from and the TAN can often be above zero from the start because some additives work against each other from the point of blend. ( the longer that oil sits in the drum in the lube room- those values are changing)

You need to know both- track and trend for truly meaningful data.

On the why ( gear and hydraulic)- those systems are normally much more loaded than anything on a vehicle and depend more on critical tolerances, hardness, dimension/geometry and surface finish so oxidation has a greater damage potential.

To dig much deeper, there would have to be a specific application to gauge where the TAN/TBN readings mean in relation to oil and machine monitoring.
 

jonlandrum

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That explains a lot, thank you. So it's not that those applications are aiming for a specific acidity because they need it, but rather that you're monitoring the inevitable acidity as it forms due to the work load being exerted on the fluids. And since they operate under tighter tolerances than, say, an automotive differential, it's much more important to stay on top of it, whereas my Dana 44 may get a fluid change around 100k miles or so just because I'm one to perform preventative maintenance.
 
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to add trans and gears are not assaulted by fuel, water, dirt ingress or not much they just get hot. Some are acidic when new
I think condeming limits on trans and gear are still around TAN <2.0
 
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