Understanding Oil Cleanliness: The Basis For Equipment Reliability

Maintaining oil cleanliness in any industrial machine is one of the most important factors influencing productivity and equipment effectiveness.

But, how can you measure cleanliness and ensure that your oil is a clean as it needs to be?

In this week’s tip, we’ll define oil cleanliness, detail the importance of measuring particle contaminants in the oil, and offer guidelines to ensure oil meets the levels of cleanliness for all types of applications. Next week, we’ll talk about how you can implement a plan for maintaining oil cleanliness.

Defining oil cleanliness codes

Oil cleanliness is the level of particle contamination in the oil caused by both external and internal sources. External sources include dirt or dust, while internal sources result from mechanical wear such as abrasion, fatigue, adhesion and erosion.

To determine the cleanliness of an in-service oil, you must measure the amount of particulate matter within the oil. This tactic is rated against different standards, including ISO 4406 or NAS 1638. ISO 4406 cleanliness is classified by a three-number code, e.g. 18/16/13—based on the number of particles greater than 2 µm, 5 µm and 15 µm respectively—in a known volume of fluid. In Figure 1 we can see a cleanliness rating of 18/16/13, which means that there were 1300 - 2500 particles > 2 µm, 320 - 640 particles > 5 µm, and 40 - 80 particles > 15 µm in size.

Note that the NAS 1638 cleanliness standard was originally developed in the U.S. but does not have much industrial application on a wide scale. As shown in Figure 2 (above), NAS 1638 is comprised of fluid cleanliness classes with a maximum amount of particle counts for designated ranges.

Minimizing particle ingression in the system

In the lubricant business, ISO 4406 or NAS 1638 would be defined as a performance method for particle counting. Particle counts are performed in several ways, but the most utilized method is to pass oil through a light sensor. As particles move over the light, it generates a shadow that gets measured, and then the shown particles are counted. This type of counting and measurement is governed by ISO 11500.

There is a manual method for counting particles, and the result of this method is governed by ISO 4407. It is necessary to manually obtain the right particle count of lubricants that contain dispersed additives because dispersed additives inhabit anti-formants or solid forms of antioxidants, which can adversely impact performance.

Monitoring for proper oil cleanliness

When it comes to monitoring cleanliness, the application type dictates the level of cleanliness required for the operating fluid. Figure 3 shows the typical cleanliness ratings for components by ISO 4406.

Here are some key guidelines to ensure proper oil cleanliness:

  • Target cleanliness codes should be based on the most sensitive component in the lubrication system, especially when using a central reservoir to supply multiple
  • The oil choice should pass readily through the filtration required. Some oils are less filterable than
  • If using a non-petroleum-based fluid or water-based lubricant such as water glycols, use a target ISO Cleanliness Code one value lower than in the table (see Figure 1).
  • If equipment operates under frequent cold starts, excessive shock or vibration, or if the component is critical to system reliability, use a target ISO Cleanliness Code two values lower than in the table (see Figure 1).

The above guidelines can help ensure that you choose an oil that meets the level of cleanliness for your application.

Additional guidelines:

  • Many remote gearboxes and bearings do not have filtration. In that case, it is best to maintain system cleanliness with oil changes and occasional portable
  • Do I need to pre-filter my oil? Oil typically comes out of a packaging plant with an average ISO 4406 cleanliness of 19/17/14 (see Figure 1 for ratings reference). If you add this oil to a hydraulic system, for example, the overall particle count of the system fluid will increase until the installed filters clean up the oil, which could take a few operating hours.

You should not assume that the OEM installed filters, or the ones purchased by local buyers, are correct for your personal application. Be mindful that OEMs and buyers operate on a cost basis, and, as a result, lower quality filters often get into the workpiece.

  • in the latest version of  ISO 4406  4,> 6 и> 14 (for optical particle counters),previously, in accordance with the old version ISO 4406, only 5 and 15 μm were counted, 2 μm, very rarely, usually not taken into account (does not affect the operation of the equipment)

  • Rick, You wrote: "ISO 4406 cleanliness is classified by a three-number code, e.g. 18/16/13—based on the number of particles greater than 2 µm, 5 µm and 15 µm respectively." Is this really true and not >4, >6 and >14 respectively?

  • - Thanks, Rick! In one USEFUL article, more recently, the question arose of determining the class of liquid purity at the input control.