Lubrication science
Why oil ages: oxidation, heat, shear, water and additive loss
Quick answer
The processes that take an oil from new to worn out, what they leave behind and the sign to watch for.
Causes and effects
| Cause | What happens | Sign to watch |
|---|---|---|
| Oxidation | Oxygen reacts with the oil at heat and with metal as a catalyst. Acids, sludge and varnish form. | Rising TAN and viscosity, darker oil, burnt smell |
| High temperature | Speeds oxidation and can crack the oil. A common rule of thumb is that oxidation rate roughly doubles for every 10 °C rise. | Dark oil, varnish on hot surfaces |
| Shear | Polymer chains in multigrade and some hydraulic oils are cut by high shear. | Falling viscosity |
| Water | Rusts parts, breaks down additives and promotes oxidation. | Cloudy oil, rising water content, rust |
| Dirt and wear metal | Metal particles catalyse oxidation. Dirt abrades surfaces. | High particle count, rising wear metals |
| Additive depletion | Additives are used up protecting the oil and the surfaces. | Falling TBN, zinc or phosphorus; foam; slow water separation |
| Air entrainment | Air bubbles speed oxidation and cause foaming and cavitation. | Foam, noise, spongy controls |
What it costs
- Lower film strength and higher wear.
- Sludge and varnish that block filters and passages and stick valves.
- Seal damage and leaks.
- Higher running temperature and energy use.
- Unplanned stops.
Slowing it down
- Keep the oil cool: clean coolers, correct oil level, no over-filling.
- Keep it clean and dry.
- Use the right oil with the right antioxidant package.
- Do not stretch drain intervals without oil analysis to back it up.
- Keep records so the next sample has a history.
Takeaway. Oil ages through heat, oxygen, water, dirt and use. Keeping it cool, clean and dry does more to extend its life than any additive.
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