The Benefits of Incorporating Oil Analysis into Maintenance

Oil Parameters in Lubrication Management and Factors Affecting Their Changes – Part 2

Understanding oil analysis to maximize machine performance and determine optimal replacement timing

In industrial sites where a wide variety of machinery and heavy equipment operate, lubricants such as gear oil, hydraulic oil, and engine oil are indispensable. Each lubrication point requires an oil grade specifically suited to its operating conditions, including load, speed, and environment. Therefore, the first step in maximizing machine performance is selecting the right oil that matches the machine type and its operating conditions.

However, oil gradually deteriorates as the equipment operates. Even if the appropriate oil is used initially, it will lose its original properties over time. This makes timely oil replacement essential. But how exactly does oil degradation occur? And how can we determine the extent of that degradation?

At many sites, oil change intervals are determined by operating hours or mileage. Yet by performing oil analysis, it is possible to determine a more accurate and appropriate replacement timing.

Building on the previous article, this piece explains in as much detail as possible which parameters are evaluated in oil analysis and what each one reveals

Parameters and Benefits of Oil Analysis

Oil analysis is much like a health checkup for oil. Just as our medical checkups include parameters such as blood pressure, electrocardiograms, and blood tests, oil analysis also includes various test parameters. Moreover, just as medical institutions follow the same procedures for the same type of examination, each oil analysis parameter is governed by strict testing methods defined by standards such as JIS (Japanese Industrial Standards) and ASTM (American Society for Testing and Materials). 

In Japan, you can look up JIS standard details online by searching for their standard numbers. Upon reviewing them, you’ll find that they specify every detail — from the dimensions of measuring instruments and testing temperatures to data recording intervals and operation methods. By performing oil analysis in accordance with these standardized procedures, it is possible to assess the oil’s condition objectively and consistently. 

So, what kinds of parameters are included in oil analysis, and what do they actually mean?
Although the topic can get quite technical, let’s briefly review each parameter to understand its significance.

Viscosity Measurement (JIS K2283 and ASTM D7279)

Viscosity measurement is the most common and fundamental parameter in oil analysis. The oil’s viscosity is measured at 40 °C and 100 °C (or both), and the results are compared with the viscosity range specified for the particular oil grade to assess any significant deviation. If the oil has deteriorated considerably due to shear or oxidation, abnormalities are often detected through this test. 

However, because some forms of degradation increase viscosity while others decrease it, there are cases where the oil has degraded substantially yet the viscosity remains within the normal range. Therefore, viscosity measurement alone is insufficient to fully characterize the oil’s condition. What other methods can be used to quantify the extent of oil degradation?

Total Acid Number (JIS K2501 and ASTM D664)

　Total Acid Number (TAN) is a method used to quantify the amount of acidic components in an oil. In this test, the oil is neutralized with an alkaline reagent, potassium hydroxide (KOH), and the result is expressed as the mass (mg) of KOH required to neutralize 1 g of oil. As the base oil in lubricants oxidizes, it generates small amounts of organic acids. Therefore, oils that have undergone greater oxidation generally show higher TAN values. 

In practice, however, additives can also react with alkalis, and oxidation of the base oil does not always result in the formation of acids. For this reason, while TAN measurement is helpful as a rough indicator of oil oxidation, it does not directly determine whether an oil is “good” or “bad” based solely on its value. Instead, changes in oil condition are better identified by tracking TAN trends over multiple measurements or by comparing results with those of new oil.

Key Question: Is there a way to observe oil oxidation directly while eliminating the influence of additives? That is where FT-IR analysis, introduced next, comes into play.

FT-IR analysis

　FT-IR analysis can be simply described as a method in which infrared light is applied to a lubricant, and its composition is inferred from how strongly the lubricant absorbs the infrared radiation. When a lubricant oxidizes, specific molecular structures are formed within the base oil. These structures absorb infrared light at specific wavelengths. Therefore, if the absorption at those wavelengths is greater than that of the new oil, it can be concluded that oxidation has progressed.

Understanding Infrared Absorption

When explained this way, FT-IR analysis may sound like a technique based on complex phenomena, but infrared absorption is observable in everyday life. A familiar example is the way a kotatsu warms the human body, which also relies on infrared absorption. Water molecules absorb infrared radiation efficiently, and the absorbed energy is converted into heat. By emitting infrared radiation, a kotatsu allows water molecules in the human body to absorb it, thereby warming the body. The reason the fabric placed under a kotatsu does not warm up as much as the human body is that, compared to water, fabric absorbs infrared radiation less effectively, resulting in less energy being converted into heat.

Because water absorbs infrared radiation so effectively, FT-IR analysis can also be used to detect moisture contamination in lubricating oils (in fact, FT-IR is often said to be more accurate for detecting water content than for evaluating oxidation). In addition, it can be used to monitor the depletion of specific additives, making it a highly versatile analytical method compared with those previously introduced. However, FT-IR analysis requires comparing infrared absorption spectra between new and used oil. As a result, the analysis cannot be performed when a sample of new oil is unavailable, and its accuracy can vary with the type of oil being analyzed.

What other methods are available to measure the conwater present of lubricating oil? One such method is moisture content measurement, which is introduced next.

Water Content Measurement (JIS K2275-3 and ASTM D6304-C)

Water content measurement, as the name suggests, determines the amount of water present in an oil. If emulsification has progressed and the water and oil phases are no longer well separated, this condition can be detected by this test. While water is generally undesirable in lubricants, its impact varies with the type of oil. In particular, detecting water in hydraulic oil is a serious issue.

1. Voltage Application – A voltage is applied to the solution to generate iodine
2. Reaction with Water – The iodine reacts with water, and the water is gradually consumed
3. Reaction Completion – Once all the water has been consumed, the iodine no longer reacts
4. Calculation – The amount of water is then calculated from the total electric charge applied up to that point.

This gets quite technical, but water content in oil is typically measured using the Karl Fischer method. The Karl Fischer method determines the amount of water by exploiting the reaction between iodine generated by electrolysis and water. Using this method, it is possible to accurately measure water content even when the oil has become emulsified and separation from water is difficult.

Water content measurement is a method for determining the amount of water that has entered an oil. While water is undoubtedly one of the contaminants that can significantly affect lubrication performance, solid contaminants have a more direct impact on wear. What methods are available to quantify the amount of solid contaminants present? That brings us to the following topic: contamination level measurement.

Contamination Level Measurement

This method counts the number of fine particles contained in the oil. By doing so, it is possible to roughly assess the total amount of contaminants, such as gear wear debris, sand and dirt entering from the outside, and carbon particles formed by heat. Contamination level measurement is widely used in oil analysis and evaluation. Because it does not require extensive or complex equipment, portable measuring devices that can be used directly in the field are also commercially available.

There are two types of contamination level indices: the ISO code and the NAS code. However, since the NAS code is no longer widely used, this section focuses only on the ISO code. The ISO code is expressed by three numbers separated by slashes, such as “24/22/20.” It indicates the quantity and size distribution of solid contaminants present in 1 mL of an oil sample. From left to right, the three numbers correspond to the number of particles in 1 mL of the sample that are 4 µm or larger, 6 µm or larger, and 14 µm or larger, respectively. The relationship between each code number and the corresponding particle count is shown in the image below.

Understanding ISO Code Values

For example, the previously mentioned value “24/22/20” indicates that, in 1 mL of the sample, there are 80,000–160,000 particles larger than 4 μm, 20,000–40,000 particles larger than 6 μm, and 5,000–10,000 particles larger than 14 μm. Because changes in particle counts are directly linked to wear debris generation and contaminant ingress, some plants set targets to keep contamination below a defined threshold as part of their lubrication management practices. In addition, regularly measuring contamination levels allows monitoring whether the lubrication condition remains normal or if abnormalities are developing.

　The use of 4 μm, 6 μm, and 14 μm as reference points in contamination measurements is because particles of these sizes are more likely to cause wear and clogging. Among them, particles around 6 μm in size are considered particularly prone to causing wear. But can you picture just how small 6 μm is?

To put this in perspective, the average hair thickness in Japanese people is about 70 μm. This means that particles likely to cause wear are smaller than one-tenth the thickness of a human hair—far too small to be seen with the naked eye. Of course, if the contamination level is extremely high, differences may become visible in the oil’s appearance. However, given how tiny the particles involved in contamination are, it is difficult to accurately judge contamination levels based on appearance alone. As a result, even if the oil looks clear and clean, neglecting to change it can lead to situations where the oil is actually contaminated to a degree that warrants replacement.

While measuring contamination levels is a highly practical method for understanding factors such as wear and the ingress of foreign matter, it focuses solely on particle counts. As a result, it does not reveal what the contaminants themselves actually are. To identify the nature of those contaminants, elemental analysis is a helpful method; it is introduced next.

Elemental Analysis (ASTM D5185)

Elemental analysis, as the name suggests, examines the types and amounts of elements present in lubricating oil, providing clues to help identify the nature of detected contaminants. For example, if a high Fe (iron) level is detected, it may indicate increased iron content in the oil due to wear of iron-based components such as gears. Likewise, elevated Si (silicon) levels may indicate the ingress of sand or dirt. In addition, even substances such as refrigerants—which are not detected as solid contamination particles—may still be identified through their elemental signatures, such as Na (sodium).

Fe(Iron) – Indicates wear of iron-based components such as gears

Si (Silicon) – May indicate ingress of sand or dirt from external sources

Na (Sodium) – Can identify substances like refrigerants not detected as particles

In this way, elemental analysis is an excellent analytical method that provides many valuable insights for lubrication management. However, its main drawback is that it requires specialized equipment and can be quite costly. While contamination level measurements can be performed on-site using relatively simple devices, elemental analysis is a different matter. The most commonly used technique for elemental analysis is ICP optical emission spectrometry (ICP-OES). This method involves spraying the oil into a plasma and identifying the types and quantities of elements based on the wavelengths and intensities of the emitted light, which requires dedicated, highly precise instrumentation. As a result, in some cases, it may actually be more economical to replace the oil outright rather than perform elemental analysis simply to assess its condition.

Conclusion

In this article, we explained the key components of general oil analysis in as much detail as possible. Each analytical method has its own strengths and limitations, and many issues cannot be fully understood from a single analysis alone. Therefore, it is essential to combine multiple analyses and track how the results change over time.

For those responsible for lubrication management, conducting oil analysis can often be challenging due to time and resource constraints. However, why not start by performing oil analysis on only the most heavily loaded or critical equipment?

By doing so, you may be able to accurately assess equipment condition, extend asset life, and optimize oil change intervals.