Maintenance Chemicals / 4 min
Oil Change Timing and Used Oil Analysis: When to Change During Maintenance
Oil changes in machinery are typically scheduled by fixed operating hours (calendar-based maintenance). Yet an oil's chemical condition varies significantly with operating temperature and contamination level. Used Oil Analysis (UOA) reveals exactly when an oil needs changing, backed by scientific data.
Published by: Rotek Technical Content Team
Content is published based on manufacturer technical documentation and equipment requirements as the basis for final selection. This is general guidance and does not replace equipment-specific manufacturer approval.
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- - Visual Inspection: Does Oil Color Matter?
- - What Does a Change in Viscosity Indicate?
- - Oxidation and Total Acid Number (TAN)
- - Wear Metals and Predictive Maintenance
Visual Inspection: Does Oil Color Matter?
Many operators perceive darkening oil color as degradation, but chemical additives can naturally darken oil as they heat up. As with diesel engine oils, detergent additives trap soot, darkening the oil. Darkening alone doesn't indicate degradation — lab analysis is the definitive answer.
What Does a Change in Viscosity Indicate?
The most critical physical parameter in oil analysis is kinematic viscosity. A rise of more than 10-15% over fresh oil (thickening) signals excessive oxidation and contamination; a drop in viscosity (thinning) indicates solvent, fuel, or the wrong/thinner oil has mixed in.
Oxidation and Total Acid Number (TAN)
Over time, oil reacts with air and heat and oxidizes. During oxidation, the oil forms acidic byproducts. Total Acid Number (TAN) measures the amount of acid in the oil, in mg KOH/g. TAN exceeding critical limits warns that metals in the system will start to corrode.
Wear Metals and Predictive Maintenance
Oil analysis reveals not just the oil's condition but the machine's internal health. Detecting high iron (Fe) in a spectrometer test signals gear wear; detecting high copper (Cu) signals that bronze bearings or pump plates are beginning to fail.
Frequently Asked Questions
Must an oil change follow the manufacturer's hour rating exactly?
Manufacturer hour ratings are given for general averages (ideal conditions). If your machine runs in a very hot, dusty environment under heavy load, you may need to change oil earlier than the manufacturer's estimate; if it runs in a clean, stable environment with synthetic products, you may be able to change it later. Lab analysis should make the final call.
What is TAN (Total Acid Number)?
TAN is a critical value measuring how acidic industrial oils such as hydraulic and gear oils have become over time through oxidation. A high TAN exceeding limits shows the oil has lost its properties and will trigger corrosion/rust in the machine.
Why does silicon (Si) show up in an oil analysis?
High silicon (Si) levels in a used oil analysis are the clearest sign that dust, sand or soil (silica) has entered the machine from outside — pointing to damaged air filters (breathers) or oil seals.
Can grease analysis be performed?
Yes, but it's much harder than with liquid oils and requires specialized lab techniques. For critical and very large bearings (for example wind-turbine main bearings), samples are taken and analyzed to check the wear metals inside the grease.