Understanding Oil Analysis (Tribology)

1. Definition: What is Oil Analysis?

Oil Analysis (also known as Tribology) is a proactive maintenance and condition monitoring technique that involves the laboratory analysis of a lubricant’s properties, suspended contaminants, and wear debris. A small sample of oil is taken from a machine and sent to a lab, which performs a series of tests to provide a detailed report on the health of both the oil and the machine it lubricates.

The principle is that the oil is the “lifeblood” of the machine. Just as a blood test can reveal a great deal about human health, an oil analysis report can provide very early warning of developing mechanical failures and contamination issues.

Oil analysis is highly complementary to vibration analysis; each technology can often confirm the findings of the other and detect problems that the other might miss.

2. The Three Pillars of Oil Analysis

A comprehensive oil analysis report typically focuses on three key areas:

a) Fluid Properties (Oil Health)

This part of the analysis assesses the condition of the lubricant itself to determine if it is still fit for service. Key tests include:

• Viscosity: The most important property of a lubricant. A change in viscosity can indicate oil degradation, contamination with the wrong oil, or fuel dilution.
• Acid Number (AN) / Base Number (BN): AN measures the level of acidic byproducts of oxidation. BN measures the reserve alkalinity in engine oils, which is needed to neutralize acids. These tests determine the remaining useful life of the oil.
• Oxidation and Nitration: Measures the chemical degradation of the oil due to heat and air exposure.

b) Contamination (Contaminant Analysis)

This section identifies the presence of harmful contaminants that can accelerate wear and degrade the oil.

• Particle Count: Measures the overall cleanliness of the oil according to ISO 4406 cleanliness codes. A high particle count is a major cause of abrasive wear.
• Water Content: Water is a highly destructive contaminant that promotes rust, corrosion, and oil degradation. It is often measured in parts per million (PPM).
• Silicon (Dirt): The presence of silicon is a clear indicator of dirt or sand ingress, often from a leaking seal or poor air filtration.

– Coolant/Glycol: The presence of elements like sodium and potassium can indicate a coolant leak into the oil, which is a very serious condition.

c) Wear Debris Analysis (Machine Health)

This is the most powerful part of the analysis for predictive maintenance. It involves identifying and quantifying the microscopic metal particles that have worn off of internal machine components.

• Elemental Spectroscopy (ICP or XRF): This test measures the concentration (in PPM) of various metallic elements. Each element points to a specific component. For example:
  • Iron (Fe): Indicates wear of gears, shafts, or housings.
  • Copper (Cu): Indicates wear of bronze cages, bushings, or brass coolers.
  • Chromium (Cr): Indicates wear of piston rings or rolling-element bearings.
  • Lead (Pb) & Tin (Sn): Indicates wear of journal bearings.

By trending the levels of these wear metals over time, a sudden increase can provide a very early warning of a component beginning to fail, often long before it becomes detectable by other means.

4. The Importance of Proper Sampling

The entire value of oil analysis depends on obtaining a clean, representative sample. Samples should be taken from a live oil line while the machine is running, from a point located upstream of any filters. This ensures that the sample reflects the true condition of the oil circulating within the machine.

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