Quote:
Originally Posted by greglepore
...No such thing as lifetime lube. Period. The largest reason is that a trans can shed a fair amount of metal and still be fine...for a while. If you get the metal out you extend the life of the trans. There's no way to know you need to change the fluid without changing the fluid.
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Well yes and no. I have seen multiple examples of gearboxes never being drained and when properly sealed they last a very long time. There are several ways to get metal of the gear and into the oil:
Abrasive Wear: from particulate that plows, gouges, cuts the surface. This typically happens upon start-up when the metal surfaces of the gears are touching with minimal oil film to separate them. Only under full operation is there a complete separation so there is a very minimal opportunity for that sort of wear to occur.
External contamination is typically to blame. If you looked under a scope you would see what look like nail clippings. Sometimes there is also rubbing wear which is considered normal wear and that occurs with new components - some call it break-in wear. Those look like platelets. None of these you can see with the bare eye.
Surface Fatigue: from very hard particulate that is forced between the two surfaces and due to such high localized pressure will actually contribute to a crack or fissure on the surface. This is referred to as spalling. The crack will propagate and the surface becomes compromised. Particles are typically spherical in shape. You can also get pitting from particles as well as Brinelling which is due to a dis-harmonic producing atypical vibrations. I some cases (but not in diffs or trans) you can get hydrogen embritterment. Again, not that common in over the road applications but worthy of mention. Surface fatigue is typically due to external contamination (sand) but it can happen with wear particles due to surface contact.
Adhesive Wear: from the two surfaces coming in contact under really high load without the oil film. In such cases even the anti-wear or extreme pressure agents in the oil can't help much. In this scenario you will have scuffing, galling, and even seizing of the gear faces. If the force is ridiculously large the gear teeth will cold well and the set is seized. Its rare but I have seen it. Normally the force is greater then the weld point and chunks of metal break off and tumble around creating havoc and despair. If you have a compromise of the oil film you can get adhesive wear and the particles have a particular shape. This is considered severe sliding wear and they have a plate shape.
There is also chemical wear (fretting, corrosion, static corrosion, rust) but if you have a closed system that would not normally happen. It is possible to get cavitation corrosion which is an implosion of micro air pockets. This is due to pressure differences but the likelihood of that happening on a trans or a diff is remote. Maybe micro-dieseling but even then, the formula of the oil has a stout amount of anti-foam so its rare - could happen if there is water but again, a closed system wouldn't allow it.
Now why is it good to know all these failure modes? Because if you know why it fails maybe you can work towards prevention and that is exactly what I have been doing with several large OEMs that make heavy equipment including engines and what have you. I have helped several of these start a wear debris library. Pulling oil samples form lots of different stages and various models under various conditions goes a long way in designing in maintainability. I also work with several major oil companies as well as several independents doing the same but also looking at used oil condition and chemistry. Now why would an oil company want to develop an oil that would stay in the box forever? Because they all know that if that is not the goal then their competition will achieve it.
I have seen diesel engine oil formulas achieve >100,000 miles between changes - this is from a few different angles, from the oil formulas and the OEM (tolerances, metallurgy, designs) and filtration. I always thought that if an oil company built engines or an engine company formulated oil it would happen faster. To be that vertically integrated is really tough. They do have some great partnerships out there which is very encouraging.
So, how do you really know when to change your oil? You have to test it. No other way.